Salty Matters

The Blog is written by me, John Warren. Once every three or four weeks or so I will post an article or two on an evaporite topic that has piqued my interest. On the Saltwork Publications webpage (under "the Works") there is a growing library of pdfs and epubs based on these blogs. These articles on the website have much higher resolution extractable graphics in than in the blog. There is also a link to this set of pdfs and epubs on the home page (www.saltworkconsultants.com).

Volatile evaporite interactions with magma Part 1 of 3: Indications of hydrated salts?

John Warren - Sunday, February 10, 2019

Introduction

Direct and indirect interactions between magma and evaporites at a regional scale are neither well documented, nor well understood. Mostly, this is because little or no salt remains once the high-temperature interactions have run their course; instead, there is a suite of indirect geochemical and indicator-mineral assemblages (Warren, 2016). Aside from the presence of what can be ambiguous indicator suites, some hard-rock geologists with a career working in igneous and metamorphic terranes may not be well versed in textures indicative of the former presence of sedimentary evaporites, nor their varying volatility, nor their meta-evaporitic and meta-igneous siblings.

The term pyrometasomatic encompasses some, but not all, of the types of salt-magma interaction and reactions that occur when evaporites and molten magmas of different types are nearby. Styles of evaporites interactions with magma are a spectrum, with two endmember situations; 1) orthomagmatic (salt-assimilative and internal to the magma), and 2) paramagmatic (salt-interactive and external to the magma). Both encompass outcomes that can include a variety of substantial ore deposits (Warren, 2016; Chapter 16). Only in situations where igneous sills and dykes have intruded salt masses, with contacts preserved, can direct effects of magma-salt interaction be documented. Even then, determining the timing of the evaporite igneous interaction can be problematic; one must ask if the chemistry and texture indicate, 1) syn-igneous emplacement, or 2) post-emplacement alteration and deeply circulating groundwater flushing, or 3) a combination.

Orthomagmatic and paramagmatic evaporite associations are distinct from occurrences of primary igneous/magmatic anhydrites, which precipitate from sulphate-saturated melts. Igneous anhydrite forms independently of any sedimentary evaporite assimilation, as seen, for example, in anhydrite crystals crystallised in trachyandesitic pumice erupted from El Chichón Volcano in 1982, or in dacitic pumices erupted from Mount Pinatubo in 1991 and in acidic lavas in the Yanacocha district of northern Peru (Luhr et al., 2008; Chambefort et al., 2008). These evaporite assimilations are also distinct from fumarolic anhydrite, which precipitates where groundwaters and sulphur-bearing magmatic fluids interact, as on Usu Volcano, Hokkaido, and many central American and Andean volcanoes such as El Laco (Zimbelman et al., 2005). Likewise, they are distinct from the anhydrite precipitates (white smokers) in and below submarine vents across numerous mid-oceanic ridges (Humphris et al., 1995). See Warren 2016 (Chapter 16) for more geological detail on these non-evaporite-igneous anhydrite occurrences.

Cooking with salt (thermal decomposition of hydrated versus non-hydrated salts)

Perhaps the most critical factor controlling the local intensity of magmatic interaction with an evaporitic country rock is whether or not the sedimentary evaporite assemblage, in proximity to an igneous heat source, contains abundant hydrated salts, such as gypsum, polyhalite or carnallite. Hydrated evaporite salts, when interacting with the igneous realm, are highly volatile and likely to decompose. They tend to release their water of crystallisation at temperatures many hundreds of degrees below the melting points of their anhydrous counterparts (Table 1).


In contrast, anhydrous salts, such as halite beds intruded by igneous dykes or sills, are much less reactive. At a local scale (measured in metres) with respect to an intrasalt-igneous interaction, there are a number of documented thermally-driven alteration styles, typically created by the intrusion of dolerite dykes and sills into cooler halite, or the outflow of extrusive igneous flows over cooler halite beds (Knipping and Herrmann, 1985; Knipping, 1989; Grishina et al, 1992, 1998; Gutsche, 1988; Steinmann et al., 1999; Wall et al., 2010). Hot igneous material interacts with somewhat cooler anhydrous salt masses, typically halite or anhydrite, to create narrow but distinct heat and mobile fluid-release envelopes(Figure 1), also reflected in the resulting recrystallised inclusion-modified salt textures (Figure 2).


Based on studies of inclusion chemistry and homogenization temperatures in fluid inclusions in bedded halite near intrusives, it seems that the extent of the influence of a dolerite sill or dyke in bedded salt is marked by fluid (brine)-inclusion migration. This is evidenced by the disappearance of chevron structures and consequent formation of clear (sparry) recrystallised halite, with a new set of higher-temperature brine inclusions located at intercrystal or polyhedral intersections. Such a migration envelope is documented in bedded Cambrian halites intruded by end-Permian dolerite dykes in the Tunguska region of Siberia (Figure 2; Grishina et al., 1992). There, as a rule of thumb, an alteration halo extends up to twice the thickness of the dolerite sill above the sill and almost the thickness of the sill below (Figure 1).

Four inclusion type associations were found in halite as a function of the ratio of the distance of the sample from the intrusion contact (d) to the thickness of the intrusion (h), i.e. d/h (Figure 2). Chevron structures with aqueous inclusions progressively disappear as d/h decreases; the disappearance of chevrons occurs at greater distances above than below the intrusive sill. At d/h < 5 above the sill, a low-density CO2 vapour phase appears in brine inclusions, at d/h < 2 H2S-bearing liquid-CO2 inclusions appear, sometimes associated with carbonaceous material and orthorhombic S8, and for d/h < 0.9, CaCl2, CaCl2.KCl and nCaCl2.n MgCl2 solids occur in association with free water and liquid CO2 inclusions, with H2S, SCO, and Sg. The d/h values marking the transitions outlined above are lower below the sills than above. The water content of the inclusions progressively decreases on approaching the sills, whereas their CO2 content and density increase. Carnallite, sylvite and calcium chloride can occur as solid inclusions in the two associations nearest to the sill for d/h<2. Carnallite and sylvite occur as daughter minerals in brine inclusions. The presence of carbon dioxide is taken to indicate fluid circulation and dissolution/recrystallisation phenomena induced by the basalt intrusions. The origin of carbon dioxide is likely related to carbonate dissolution during magmatism (see Salty Matters, Oct 31, 2016).


In some shallow locations, relatively rapid magma emplacement can lead to linear breakout trends outlined by phreatomagmatic or phreatic explosion craters. Such phreatic explosion craters have been imaged on the Tertiary seafloor horizons in parts of the North Sea (Figure 3; Wall et al., 2010). The dykes were emplaced into Paleozoic and Mesozoic sediments and have a common upper termination in Early Tertiary sediments. The dykes are part of the British Tertiary volcanic province emplaced some 58 Ma. These dykes are characterised by a narrow 0.5–2 km wide vertical disturbance of seismic reflections that have linear plan view geometry. Negative magnetic anomalies directly align with the vertical seismic disturbance zones and indicate the presence of underlying igneous material. Linear coalesced collapse craters are found above the dykes. The collapse craters formed above the dyke due to the release of volatiles at the dyke tip and resulting gaseous expansion and subsequent volume loss. According to Wall et al. (2010), the larger craters likely formed due to explosive phreatomagmatic interaction between magma and pore water. The linearly aligned collapse craters can be considered an Earth analogue to Martian pit chain craters.

A phreatic eruption, also called a phreatic explosion, ultravulcanian eruption or steam-blast eruption, occurs when magma heats ground or surface water and is a separate but related occurrence to a phreatomagmatic eruption. A phreatomagmatic deposit typically contains solid inclusions of magmatic (igneous) material, whereas debris tied to a phreatic deposit does not, but ties to the effects of juvenile and deeply circulated hydrothermal waters. Extreme temperatures associated with an emplaced magma (anywhere from 500 to 1,170 °C) can cause a near-instantaneous phase change to steam, so forming a phreatomagmatic deposit. That is, rapid heating results in an intense explosion made up of steam, water, ash, rock, and volcanic bombs. During the eruption of Mount St. Helens, hundreds of steam explosions preceded the1980 Plinian eruption of the volcano core. Many authors argue a less intense geothermal event results in a mud volcano, but there are many other active mud volcanoes worldwide that tie to compactional overpressure unrelated to any magma emplacement (Warren et al., 2011). As the published interpretation of aligned phreatic breakout structures illustrated in Figure 3 is based on seismic without well control, the explosion mechanism may be solely phreatic heating or phreatomagmatic.


Deposits of phreatic eruptions (as contrasted with a phreatomagmatic eruption) typically include steam and rock fragments without the inclusion of fragments derived from liquid magma, lava or volcanic ash. The temperature of the phreatic fragments can range from cool to incandescent. So if molten magma is present, the resulting explosive debris deposit is typically classified as a phreatomagmatic eruption. These eruptions can create broad, low-relief craters called maars. In contrast, phreatic explosions lack debris derived from molten (igneous) material, but emplacement can be accompanied by carbon dioxide or hydrogen sulfide gas emissions. CO2 can asphyxiate at sufficient concentration; H2S is a broad spectrum poison. A 1979 phreatic eruption on the island of Java killed 140 people, most of whom were overcome by poisonous gases. Phreatic eruptions, even if the deposit lacks igneous rock fragments, are typically classed as a type of volcanic eruptions because a phreatic eruption can force juvenile fluids to the surface. But when a phreatic explosion is related to an igneous feature intersecting an evaporite bed, the resultant textures show a contrast between heating of anhydrous and hydrous salts


Hydrous salt interactions in Germany

Textures created by an igneous intrusion into a variably-hydrated evaporite succession can be studied in the dyke-and sill-intruded halite levels exposed in the walls of potash mines of the Werra-Fulda district of Germany (Figure 4; Steinmann et al., 1999; Schofield et al., 2014). There, the Permian Zechstein salt series contains two important potash salt horizons (2-10m thick), which are mined at a depths ≈ 800 m, from within a 400m thick halite host (Figure 4a). In the later Tertiary, basaltic melts intruded these Zechstein evaporites, but it seems only a few dykes reached the Miocene landsurface. The basaltic melt ties to regional volcanic activity, some 10 to 25 Ma. Basalts exposed in the halite-dominant portions of the mine walls are typically subvertical dykes, rather than sills. The basaltic intervals intersect the salt over zones up to several kilometres wide (Figure 4b). However, correlations of individual dyke swarms, either between different mines, or between surface and subsurface outcrops is difficult.


From a paleogeographic perspective, the Werra-Fulda Basin is situated in a southern embayment of the European Zechstein Basin. It contains cyclic evaporites of the Werra Formation (Z1). In the Neuhof area, the evaporites of the Zechstein are underlain by siliciclastic rocks of the Permian Rotliegend interval. The higher Zechstein-cycles (Z2 – Z7), on top of the Werra Formation, consist of a siliciclastic succession with intercalated limestone and anhydrite layers (Strauch et al., 2018; Beer and Barnasch, 2018). The Werra Formation is dominated by rock salt with a thickness up to 300 m.

Two potash seams (Seam Hessen and Seam Thüringen) separate the rock salt of the Werra Formation into three distinct units (Figure 4b). Lower, Middle and Upper Werra rock salt). Seam Hessen mainly consists of hard salt (kieserite, sylvite, halite and anhydrite). It is overlain by several, potash mineral-bearing horizons which show a strong vertical and lateral heterogeneity and consist of kieserite, sylvite, carnallite, halite and anhydrite. Internally, three separate units are identified within the potash Seam Hessen (Figure 4). The “Wurmsalz”, a hard salt with up to four strongly folded anhydritic clay bands represents the lower part of Seam Hessen. The middle part consists of massive, kieserite-rich hard salt with abundant sylvite lenses (“Flockensalz”). The “Bändersalz”, a banded hard salt which is typically intercalated with brownish, halitic layers occurs in the upper part of Seam Hessen. Potash Seam Thüringen usually occurs around 50 m below Seam Hessen. Its lower part is dominated by a well-bedded hard salt with intercalated rock salt. Its upper part consists of a variety of rock types including carnallite, sylvite and hard salt.

In the Fulda region the thermally-driven release of water of crystallisation within particular Zechstein salt beds intersecting igneous dykes creates thixotropic or subsurface “peperite” textures in hydrated carnallitite ore layers, where heated water of crystallisation escaped from the hydrated-salt lattice. Dehydration-driven loss of mechanical strength focuses zones of magma entry into particular horizons in the salt mass, wherever hydrated salt layers were intersected (Figure 5b verses 5c). In contrast, dyke and sill margins are much sharper and narrower in zones of contact with anhydrous salt intervals (Figure 5a, d; Schofield et al., 2014).

Accordingly, away from the immediate vicinity of the direct thermal aureole, heated and overpressured dehydration waters can enter a former Zechstein carnallite halite bed, and drive the creation of extensive soft sediment deformation and peperite textures in the previous hydrated layer (Figure 5c, d). Mineralogically, sylvite and coarse recrystallised halite dominate the salt fraction in the peperite intervals/beds. These deformed beds formed within a hydrated salt bed and so differ from the conventional notion of volcanic peperites indicating water-saturated sediment interactions with very shallow dyke or sill emplacements.

Sylvite in these altered zones is a form of dehydrated carnallite, not a primary-textured salt. In the Fulda region, such altered zones and deformed units can extend along former carnallite layers to tens or even a hundred or more metres from the dyke feeder. Ultimately, the deformed potash bed passes laterally out into the unaltered bed, which can retain abundant inclusion-rich primary chevron halite and carnallite (Figure 5c versus 5d). That is, nearer the basalt dyke, the carnallite is transformed mainly into inclusion-poor halite and sylvite, the result of recrystallisation combined with incongruent flushing of warm saline fluids mobilised from the hydrated carnallite crystal lattice as it was heated and decomposed in response to nearby dyke emplacement. During such Miocene salt alteration/thermal metamorphism in the Fulda region, NaCl-rich diagenetic and juvenile fluids were mixed with fluids originating from thermally-mobilised crystallisation water in the carnallite as it converted to sylvite.

Nearer the basalt dyke, the carnallite is largely transformed into inclusion-poor halite and sylvite, the result of incongruent flushing of warm saline fluids mobilised from the hydrated carnallite crystal lattice as it was heated by dyke emplacement. During Miocene salt alteration/thermal metamorphism in the Fulda region, NaCl-fluids were mixed with fluids originating from thermally-mobilised crystallisation water in the carnallite, as it converted to sylvite. This brine mixture altered the basalts during post-intrusive cooling, an event which numerical models suggest was quite rapid (Knipping, 1989): a dyke of less than 0.5 m thickness probably cooled to temperatures less than 200°C within 14 days of dyke emplacement.

Worldwide, igneous dykes intersecting salt beds tend to widen to become sills in two zones: 1) along evaporite units within the halite mass that contain hydrated salts, such as carnallite or gypsum (Figure 5b, c) and, 2) where rising magma has ponded and so created laccoliths at the upper or lower halite contact with the adjacent nonsalt strata, or against a salt wall (Warren, 2016). The first alteration of the hydrated salt layer is a form of mineral alteration and recrystallisation in response to a pulse of released water/steam as dyke-driven heating forces the dehydration of hydrated salt layers. The second alteration is often folding and fluid-like disaggregation of the former, now dehydrated, layer in response to the mechanical strength contrast at a hydrated-nonhydrated salt-bed contact (Warren, 2016).

Surface expression of hydrated bedded salts interacting with magma in Dallol, Ethiopia

Local potash ores typify thermal sump depressions in the Dallol and Musley areas (Figure 6a, b, c, 7) where a similar set of subsurface destabilisation processes occurred when rising magma reached the levels of hydrated salts (kainite and carnallite beds) in the Houston Formation of the Danakhil depression fill (see Warren 2016 and Bastow et al. 2018 for more detailed discussion of the potash stratigraphy). To attain these hydrated salt levels the rising dyke swarm had passed relatively passively through the Lower Rocksalt Formation (Salty Matters, April 29, 2015). Emplacement of the magma/dykes into hydrated evaporites below the vicinity of what is now the Dallol Mound would have mobilised and deformed the hydrated potash salt level, converting carnallite to sylvite, kainite to bischofite and lesser kieserite, as well as creating widespread cavities filled with rising pressured volatiles carried by MgCl and KCl brines. Pressurisacreation of a cavernous network filled with volatiles at the level of the Houston Formation would have aided in forming the four-way dip closure now seen on the exposed and eroding salt beds that make up much of the Dallol Mound surface.


Once these hydrothermal cavities dissolved and breached the way to surface, the feeder brines cool and precipitate prograde salts such as halite, sylvite and bischofite. Such destabilisation has likely accommodated the emplacement of a basaltic sill at the level of the potash salts, in turn driving the uplift of the lake beds above this region outlined by the centripetal dips of the Dallol Mound. Mound-related uplift and hydrothermal activity then stimulate the formation of natural areas of ground collapse, sulphurous and acidic springs and fumaroles, along with the creation of water-filled chimneys and doline sags, filling with various hydrothermal salts, in the vicinity of the volcanic mound  (Figure 6).

That is this type of potash in the Dallol Mound region is hydrothermally reworked from the uplifted equivalents of the Houston Formation. Even today this hydrology is precipitating carnallitite (associated with bischofite and minor kieserite) in various hydrothermal brine pools atop and around the Dallol Mound, such as the carnallite-dominant Crescent deposit (Figure 7). These hydrothermal salts owe their origins to daylighting of pressurised fluid systems and cavities.


The last pressurised phreatic explosion crater formed in 1926. They were created by the volatile products of hydrated salt layers (Houston Fm) where these salts had come into contact with thermal aureoles or actual lithologies of newly emplaced dykes that had penetrated the underlying halite section. Volcanic rock fragments and other igneous debris have yet to make it to the surface in the Dallol Mound region, although active volcanic mounds and flows do cover the saltflat surface tens of kilometres to the south (Erte Alle ) and north. Based on the analogy exposed within the Zechstein-hosted potash mines of the Fulda region of Germany, it is likely that as well as creating at-surface brine pools, this hydrothermal dyke-related hydrology locally converts most subsurface carnallitite to a disturbed sylvinite bed at the level of contact with the Houston Fm.

Implications

It seems a "one-size-fits-all" model does not characterise magmatic interactions with massively bedded evaporites. Instead, there is a mineralogical control to the intensity of the interaction and the depth of thermal influence of recrystallisation and mobilisation textures. When a dyke-swarm intersects halite or anhydrite the thermally-driven recrystallisation and fluid migration halo is more limited, as outlined in Figure 1 and Figure 5a, d.

In contrast, when a dyke swarm intersects an interval containing hydrous salts such kainite, carnallite or gypsum, the heating drives the expulsion of the bound-water at decomposition temperatures much lower than the salts melting point (Table 1). Such hydrous-salt intervals devolatise, fluidise and flow, with the effects of the heating halo extending much further away from the heat source, driven in part by steam-driven hydrofracturing. On cooling, the resulting mineralogy in the highly-deformed bed is dominated by the anhydrous form of the devolatised salt, as in the sylvite unit after carnallite as seen in potash seams adjacent to dykes in the Fulda Region (Figures 5b, c).

Closer in to the heat source, the basalt that has moved in along the hydrous potash beds show abundant peperite textures (Figure 5c; Schofield et al., 2014). Actually, this is a unique form of peperite that is tied to beds of hydrous evaporite. It forms outside the usual scenario envisaged for peperite whereby molten igneous material interacts with wet sediment, with the water in the wet sediment held in interparticle pores.

The classic definition of a peperite is that it is a "genetic term applied to a rock formed essentially in situ by disintegration of magma intruding and mingling with unconsolidated or poorly consolidated, typically wet sediments. The term also refers to similar mixtures generated by the same processes operating at the contacts of lavas and other hot volcaniclastic deposits with such sediments" (Skilling et al. 2002).

In the case of the bedded hydrous salt intervals, before the intrusion of the igneous heat source, there was little to no free water, other than occasional brine inclusions in associated halite chevrons. What makes these hydrous-salt peperites interesting is that it is the igneous heating drives a mineralogic transformation in the hydrous salts that makes the formerly "dry" salt bed become "wet" sediment.

Before our work in the Fulda region (Schofield et al., 2014), the nature of igneous interactions with evaporites was understood to be mainly that documented by studies in areas with intrusives interacting with thick anhydrous halite and anhydrite beds. The heating haloes were seen as driving recrystallisation and brine migration over limited lateral distances of a few metres. However, the potash seam interactions in the Fulda region show this alteration distance can be much greater (hundreds of metres) id hydrous salt layers are heated.

The surface geology in the Dallol Mound region of Ethiopia shows an even more impressive set of igneous dyke hydrated salt interactions (Warren, 2016). There the potash interval known as the Houston Formation is a tens-of-metres thick section of hydrated salts below the upper halite unit and atop the lower halite. When the rising igneous dyke swarm rose to the level of Houston Formation, it drove a broad linear devolatisation zone in the dyke-heated alteration halo. This, in turn, forced the formation of the closed anticlinal uplift structure that is the Dallol mound. The release of MgCl2 during volatisation also explains phreatic breakout features that are outlined by at-surface collapse dolines with their hot (104-108°C) brine lakes and unusual bischofite (MgCl2) precipitates. Likewise, the same set of processes explains the occurrences of metres to tens of metres thick bischofite intervals that are intersected in cores in some of the potash exploration wells in the vicinity of Dallol Mound (pers. obs). These are likely cavity fill deposits formed as a byproduct of kainite and carnallite devolatisation sourced at the level of Houston Formation.

This set of more mobile brine fluid escape features has implications for nuclear waste storage in halite successions where a storage cavity may be in proximity to an interval of hydrous evaporite salts. Halite-hosted purpose-built caverns in thick evaporite intervals are one of the safest places in the world to store waste but perhaps not in parts of the salt succession that entrain beds of hydrous salts such as carnallite or kainite (Warren, 2017).

References

Bastow, I. D., A. D. Booth, G. Corti, D. Keir, C. Magee, C. A.-L. Jackson, J. Warren, J. Wilkinson, and M. Lascialfari, 2018, The development of late-stage continental breakup: Seismic reflection and borehole evidence from the Danakil Depression, Ethiopia: Tectonics, v. 37.

Beer, W., and L. Barnasch, in press, Werra-Fulda-Becken, SDGG- Monography.

Chambefort, I., J. H. Dilles, and A. J. R. Kent, 2008, Anhydrite-bearing andesite and dacite as a source for sulfur in magmatic-hydrothermal mineral deposits: Geology, v. 36, p. 719-722.

Grishina, S., J. Dubessy, A. Kontorovich, and J. Pironon, 1992, Inclusions in salt beds resulting from thermal metamorphism by dolerite sills (eastern Siberia, Russia): European Journal of Mineralogy, v. 4, p. 1187-1202.

Grishina, S., J. Pironon, M. Mazurov, S. Goryainov, A. Pustilnikov, G. Fonderflaas, and A. Guerci, 1998, Organic inclusions in salt - Part 3 - Oil and gas inclusions in Cambrian evaporite deposits from east Siberia - A contribution to the understanding of nitrogen generation in evaporite: Organic Geochemistry, v. 28, p. 297-310.

Gutsche, A., 1988, Mineralreaktionen und Stotransporte an einem Kontakt Basalt-Hartsalz in der Werra-Folge des Werkes Hattorf: Unpubl. diploma thesis, thesis, Georg-August-Universita, Gottingen.

Humphris, S. E., P. M. Herzig, D. J. Miller, J. C. Alt, K. Becker, D. Brown, G. Brugmann, H. Chiba, Y. Fouquet, J. B. Gemmell, G. G., M. D. Hannington, N. G. Holm, J. J. Honnorez, G. J. Iturrino, R. Knott, R. Ludwig, K. Nakamura, S. Petersen, A. L. Reysenbach, P. A. Rona, S. Smith, A. A. Sturz, M. K. Tivey, and X. Zhao, 1995, The internal structure of an active sea-floor massive sulphide deposit: Nature, v. 377, p. 713-716.

Knipping, B., 1989, Basalt intrusions in evaporites: Lecture Notes in Earth Sciences (Springer-Verlag), v. 24, p. 132 pp.

Knipping, B., and A. G. Hermann, 1985, Mineralreaktionen und Stoff transporte an einem Kontakt Basalt-Carnallitit im Kalisalzhorizont Thüringen der Werra-Serie des Zechsteins: Kali und Steinsalz, v. 9, p. 111-124.

Luhr, J. F., 2008, Primary igneous anhydrite: Progress since its recognition in the 1982 El ChichÛn trachyandesite: Journal of Volcanology and Geothermal Research, v. 175, p. 394-407.

Schofield, N., I. Alsop, J. Warren, J. R. Underhill, R. Lehné, W. Beer, and V. Lukas, 2014, Mobilizing salt: Magma-salt interactions: Geology, v. 42, p. 599-602.

Skilling, I. P., J. D. L. White, and J. McPhie, 2002, Peperite: a review of magma–sediment mingling: Journal of Volcanology and Geothermal Research, v. 114, p. 1-17.

Steinmann, M., P. Stille, W. Bernotat, and B. Knipping, 1999, The corrosion of basaltic dykes in evaporites: Ar-Sr-Nd isotope and rare earth elements evidence: Chemical Geology, v. 153, p. 259-279.

Strauch, B., M. Zimmer, A. Zirkler, S. Höntzsch, and A. M. Schleicher, 2018, The influence of gas and humidity on the mineralogy of various salt compositions – implications for natural and technical caverns: Advances in Geoscience, v. 45, p. 227-233.

Wall, M., J. Cartwright, R. Davies, and A. McGrandle, 2010, 3D seismic imaging of a Tertiary Dyke Swarm in the Southern North Sea, UK: Basin Research, v. 22, p. 181-194.

Warren, J. K., 2016, Evaporites: A compendium (ISBN 978-3-319-13511-3): Berlin, Springer, 1854 p.

Warren, J. K., 2017, Salt usually seals, but sometimes leaks: Implications for mine and cavern stabilities in the short and long term: Earth-Science Reviews, v. 165, p. 302-341.

Warren, J. K., A. Cheung, and I. Cartwright, 2011, Organic Geochemical, Isotopic and Seismic Indicators of Fluid Flow in Pressurized Growth Anticlines and Mud Volcanoes in Modern Deepwater Slope and Rise Sediments of Offshore Brunei Darussalam; Implications for hydrocarbon exploration in other mud and salt diapir provinces (Chapter 10), in L. J. Wood, ed., Shale Tectonics, v. 93: Tulsa OK, AAPG Memoir 93 (Proceedings of Hedberg Conference), p. 163-196.

Zimbelman, D. R., R. O. Rye, and G. N. Breit, 2005, Origin of secondary sulfate minerals on active andesitic stratovolcanoes: Chemical Geology, v. 215, p. 37-60.

 

Brine evolution and origins of potash - primary or secondary. Ancient potash ores: Part 3 of 3

John Warren - Monday, December 31, 2018

Introduction

In the previous two articles in this series on potash exploitation, we looked at the production of either MOP or SOP from anthropogenic brine pans in modern saline lake settings. Crystals of interest formed in solar evaporation pans and came out of solution as: 1) Rafts at the air-brine interface, 2) Bottom nucleates or, 3) Syndepositional cements precipitated within a few centimetres of the depositional surface. In most cases, periods of more intense precipitation tended to occur during times of brine cooling, either diurnally or seasonally (sylvite, carnallite and halite are prograde salts). All anthropogenic saline pan deposits examples can be considered as primary precipitates with chemistries tied to surface or very nearsurface brine chemistry.

In contrast, this article discusses ancient potash deposits where the chemistries and ore textures are responding to ongoing alteration processes in the diagenetic realm. Unlike the modern brine pans where brines chemistries and harvested mineralogies are controllable, at least in part, these ancient deposits show ore purities and distributions related to ongoing natural-process overprints.



Table 1 lists some modern and ancient potash deposits and prospects by dividing them into Neogene and Pre-Neogene deposits (listing is extracted and compiled from SaltWork® database Version 1.7). The Neogene deposits are associated with a time of MgSO4-enriched seawaters while a majority of the Pre-Neogene deposits straddle times of MgSO4 enrichment and depletion in the ocean waters.

Incongruent dissolution in burial

Many primary evaporite salts dissolve congruently in the diagenetic realm; i.e., the composition of the solid and the dissolved solute stoichiometrically match, and the dissolving salt goes entirely into solution (Figure 1a). This situation describes the typical subsurface dissolution of anhydrous evaporite salts such as halite or sylvite. However, some evaporite salts, typically hydrated salts, such as gypsum or carnallite, dissolve incongruently in the diagenetic realm, whereby the composition of the solute in solution does not match that of the solid (Figure 1b). This solubilisation or mineralogical alteration is defined by the transformation of the "primary solid" into a secondary solid phase, typically an anhydrous salt, and the loss of water formerly held in the lattice structure. The resulting solution generally carries ions away in solution.


More than a century ago, van't Hoff (1912) suggested that much subsurface sylvite is the result of incongruent solution of carnallite yielding sylvite and a Mg-rich solution. According to Braitsch (1971, p. 120), the incongruent alteration (dissolution) of carnallite is perhaps the most crucial process in the alteration of subsurface potash salts and the formation of diagenetic (secondary) sylvite.

Widespread burial-driven incongruent evaporite reactions in the diagenetic realm include the burial transition of gypsum to anhydrite (reaction 1)

CaSO4.2H2O --> CaSO4 + 2 H2O ... (1)

and the in-situ conversion of carnallite to sylvite via the loss of magnesium chloride in solution (reaction 2)

KMgCl3.6H2O --> KCl + Mg++ + 2Cl- + 6H2O ...(2)

Typically, a new solid mineral remains, and the related complex solubility equilibrium creates a saline pore water that may, in turn, drive further alteration or dissolution as it leaves the reaction site (Warren, Chapters 2 and 8). Specifically for ancient potash, reaction 2 generates magnesium and chloride in solution and has been used to explain why diagenetic bischofite and dolomite can be found in proximity to newly formed subsurface sylvite. Bischofite is a highly soluble salt and so is metastable in many subsurface settings where incongruent dissolution is deemed to have occurred, including bischofite thermal pool deposits in the Dallol sump in the Danakhil of Ethiopia (Salty Matters, May 1, 2015). In many hydrologically active systems, solid-state bischofite is flushed by ongoing brine crossflow and so help drive the formation of various burial dolomites. Only at high concentrations of MgCl2 can carnallite dissolve without decomposition.

Laboratory determinations

In the lab, the decomposition of carnallite in an undersaturated aqueous solution is a well-documented example of incongruent dissolution (Emons and Voigt, 1981; Xia et al., 1993; Hong et al., 1994; Liu et al., 2007; Cheng et al., 2015, 2016). When undersaturated water comes into contact with carnallite, the rhombic carnallite crystals dissolve and, because of the common ion effect, small cubic KCl crystals form in the vicinity of the dissolving carnallite. As time passes, the KCl crystals grow into larger sparry subhedral forms and the carnallite disappears.

Carnallite’s crystal structure is built of Mg(H2O)6 octahedra, with the K+ ions are situated in the holes of chloride ion packing meshworks, with a structural configuration similar to perovskite lattice types (Voigt, 2015). Potassium in the carnallite lattice can be substituted by other large single-valence ions like NH4+, Rb+, Cs+ or Li(H2O)+, (H3O)+ and Cl- by Br- and I-. These substitutions change the lattice symmetry from orthorhombic in the original carnallite to monoclinic.

When interpreting the genesis of ancient potash deposits and solutions, the elemental segregation in the lattice means trace element contents of bromide, rubidium and caesium in primary carnallite versus sylvite daughter crystals from incongruent dissolution can provide valuable information. For example, in a study by Wardlaw, (1968), a trace element model was developed for sylvite derived from carnallite that gave for Br and Rb concentration ranges of 0.10–0.90 mg/g and 0.01–0.18 mg/g, respectively. In a later study of sylvite derived by fresh-water leaching of magnesium chloride under isothermal conditions at 25 °C. Cheng et al. (2016), defined a model whereby primary sylvites precipitated from MgSO4-deficient sea water, gave Br and Rb concentration ranges of 2.89–3.54 mg/g and 0.017–0.02 mg/g, respectively (no evaporation occurred at saturation with KCl). In general, they concluded sylvite derived incongruently from carnallite would contain less Br and more Rb than primary sylvite (Figure 2; Cheng et al., 2016).


Subsurface examples

The burial-driven mechanism widely cited to explain the incongruent formation of sylvite from carnallite is illustrated in Figure 3 (Koehler et al., 1990). Carnallite precipitating from evaporating seawater at time 1 forms from a solution at 30°C and atmospheric (1 bar) pressure, and so plots as point A, which lies within the carnallite stability field (that is, it sits above the dashed light brown line). With subsequent burial, the pressure increases so that the line defining carnallite-sylvite boundary (solid dark brown line) moves to higher values of K. By time 2, when the pressure is at 1 Kbar (corresponds to a lithostatic load equivalent to 2-3 km depth), the buried carnallite is thermodynamically unstable and so is converting to sylvite + solution (as the plot field now lies in the sylvite + solution field (Figure 3). If equilibrium is maintained the carnallite reacts incongruently to form further sylvite and MgCl2-solution. Thus, provided the temperature does not rise substantially, increasing pressure as a result of burial will favour the breakdown of carnallite to sylvite. However, as burial proceeds, the temperature may become high enough to favour once again the formation of carnallite from sylvite + solution (that is the solution plot point from A moves toward the right-hand side of the figure and back into the carnallite stability field).


Sylvite, interpreted to have formed from incongruent dissolution of primary carnallite, is reported from the Late Permian Zechstein Formation of Germany (Borchert and Muir, 1964), Late Permian Salado Formation of New Mexico (Adams, 1970), Early Mississippian Windsor Group of Nova Scotia (Evans, 1970), Early Cretaceous Muribeca Formation of Brazil and its equivalents in the Gabon Basin, West Africa (Wardlaw, 1972a, b; Wardlaw and Nicholls, 1972; Szatmari et al., 1979; de Ruiter, 1979), Late Cretaceous of the Maha Sarakham Formation, Khorat Plateau, Thailand and Laos (Hite and Japakasetr, 1979), Pleistocene Houston Formation, Danakil Depression, Ethiopia (Holwerda and Hutchinson, 1968), and Middle Devonian Prairie Formation of western Canada (Schwerdtner, 1964; Wardlaw, 1968) (See Table 1).

This well-documented literature base supports a long-held notion that there is a problem with sylvite as a primary (first precipitate) marine bittern salt, especially if the mother seawater had ionic proportions similar to those present in modern seawater (see Lowenstein and Spencer, 1990 for an excellent, if 30-year-old, review). We know from numerous evaporation experiments, that sylvite does not crystallise during the evaporation of modern seawater at 25°C, except under metastable equilibrium conditions (Braitsch, 1971; Valyashko, 1972; Hardie, 1984). The sequence of bitterns crystallising from modern seawater bitterns was illustrated in the previous Salty Matters article in this series (see Figure 1 in October 31 2018).

Across the literature documenting sylvite-carnallite associations in ancient evaporites, the dilemma of primary versus secondary sylvite is generally solved in one of three ways. Historically, many workers interpreted widespread sylvite as a diagenetic mineral formed by the incongruent dissolution of carnallite (Explanation 1). Then there is the interpretation that some sylvite beds, perhaps associated with tachyhydrite, were precipitated in the evaporite bittern part of a basin hydrology that was fed by CaCl2-rich basinal hydrothermal waters (Explanation 2: see Hardie, 1990 for a good discussion    of this mechanism). Then there is the third, and increasingly popular explanation of primary or syndepositional sylvite at particular times in the chemical evolution of the world oceans (MgSO4-depleted oceans).


Changes in the relative proportions of magnesium, sulphur and calcium in the world’s oceans are well supported by brine inclusion chemistry of co-associated chevron halite (Figure 4). Clearly, there are vast swathes of times in the earth’s past when the chemistry of seawater changed so that MgSO4 levels were lower than today and it was possible that sylvite was a primary marine bittern precipitate (see Lowenstein et al., 2014 for an excellent summary).

In my opinion, there is good evidence that all three explanations are valid within their relevant geological contexts but, if used exclusively to explain the presence of ancient sylvite, the argument becomes somewhat dogmatic. I would say that that, owing to its high solubility, the various textures and mineralogical associations of carnallite/sylvite and sulphate bitterns found in ancient potash ore beds reflect various and evolving origins. Ambient textures and mineralogies are dependent on how many times and how pervasively in a potash sequence’s geological burial history an evolving and reactive pore brine chemistry came into contact with parts or all of the extent of highly reactive potash beds (Warren, 2000; 2010; 2016).

In my experience, very few ancient examples of economic potash show layered textures indicating primary precipitation on a brine lake floor, instead, most ancient sylvite ores show evidence of at least one episode of alteration. That is, various forms and textures in potash may dissolve, recrystallise and backreact with each other from the time a potash salt is first precipitated until it is extracted. The observed textural and mineralogical evolution of a potash ore association depends on how open was the hydrology of the potash system at various stages during its burial evolution. The alteration can occur syndepositionally, in brine reflux, or later during flushing by compactional or thermobaric subsurface waters or during re-equilibration tied to uplift and telogenesis. Tectonism (extensional and compactional) during the various stages of a basin’s burial evolution acts as a bellows driving fluid flow within a basin, so forcing and speeding up the focused circulation of potash-altering waters.

 

A similar, but somewhat less intense, textural evolution tied to incongruent alteration is seen in the burial history of other variably hydrated evaporite salts. For example, CaSO4 can flip-flop from gypsum to anhydrite and back again depending on temperature, pore fluid salinity and the state of uplift/burial. Likewise, with the more complicated double salt polyhalite, there are mineralogical changes related to whether it formed in a MgSO4 enriched or depleted world ocean and the associated chemistry of the syndepositional reflux brines across extensive evaporite platforms (for a more detailed discussion of polyhalite see Salty Matters, July 31, 2018). Kainite-kieserite-carnallite also show evidence of ongoing incongruent interactions. This means that, as in gypsum/anhydrite/polyhalite or kain ite/kieserite sequences, there will be primary and secondary forms of both carnallite and sylvite that can alternate during deposition, during burial and any deep meteoric flushing and then again with uplift. In Quaternary brine factories these same incongruent chemical relationships are what facilitate the production of MOP (sylvite) from a carnallitite feed or SOP from kainite/kieserite/schoenite feed (see articles 1 and 2 in this series).

 

To document the three end-members of ancient sylvite-carnallite decomposition/precipitation we will look at three examples; 1) Oligocene potash in the Mulhouse Basin where primary sylvite textures are commonplace, 2) Devonian potash ores in western Canada, where multiple secondary stages of alteration are seen, and 3) Igneous-dyke associated sylvite in east Germany where thermally-driven volatisation (incongruent melting) forms sylvite from dehydrated carnallite.


Oligocene Potash, Mulhouse Basin France

Moving backwards into deep time, this 34 Ma deposit contains some of the first indications of well preserved primary marine-fed sylvinite (MOP) textures exemplified by laterally-continuous mm-scale alternations of potash and halite layers and lamina (Figure 5a-c). Interestingly, all solid-state potash deposits laid down in the post-Oligocene period contain increased proportions of MgSO4 salts, making them much more difficult to economically mine and process (see Table 1 and Salty Matters, May 12, 2015)

From 1904 until 2002, potash was conventionally mined in France from the Mulhouse Basin (near Alsace, France). With an area of 400 km2, the Mulhouse Basin is the southernmost of a number of Lower Oligocene evaporite basins that occupied the upper Rhine Graben, which at that time was a narrow adiabatic-arid rift valley (Figure 6a). The graben was a consequence of the collision between European and African plates during the Paleogene. It is part of a larger intracontinental rift system across Western Europe that extended from the North Sea to the Mediterranean Sea, stretching some 300 km from Frankfurt (Germany) in the north, to Basel (Switzerland) in the south, with an average width of 35 km (Cendon et al., 2008). The southern extent of the graben is limited by a system of faults that place Hercynian massifs and Triassic materials into contact with the Paleogene filling. Across the north, a complex system of structures (including salt diapirs) put the basin edges in contact with Triassic, Jurassic and Permian materials. In the region of the evaporite basins, the Paleogene fill of the graben lies directly on the Jurassic basement. The sedimentary filling of this rift sequence is asymmetrical with the deeper parts located at the southwestern and northeastern sides of the Graben (Rouchy, 1997).


Palaeogeographical reconstructions place the potential marine seaway seepage feed to the north or perhaps also southeast of the Mulhouse Basin, while marginal continental conglomerates tend to preclude any contemporaneous hydrographic connection with Oligocene ocean water (Blanc-Valleron, 1991; Hinsken et al., 2007; Cendon et al., 2008). At the time of its hydrographic isolation, some 34 Ma, the basin was located 40° north of the equator. Total fill of Oligocene lacustrine/marine-fed sediments in the graben is some 1,700m thick. The saline stage is dominated by anhydrite, halite and mudstone. The main saline sequence is underlain by non-evaporitic Eocene continental mudstones, with lacustrine fossils and local anhydrite beds. Evaporite bed continuity in the northern part of the basin is disturbed by (Permian-salt cored) diapiric and or erosional/fault movement. Consequently, these northern basins are not considered suitable for conventional potash mining (Figure 6a).

The Paleogene fill of the basin is divided into 6 units; a pre-evaporitic series, Lower Salt Group (LSG), Middle Salt Group (MSG), an Upper Salt Group (USG - with potash), Grey Marls Fm., and the Niederroedern Fm (Figure 7; Cendon et al., 2008). The LSG and lower section of the MSG are interpreted as lacustrine in origin, based on the limited palaeontologic and geochemical data. However, based on the presence of Cenozoic marine nannoplankton, shallow water benthic foraminifera, and well-diversified dinocyst assemblages in the fossiliferous zone below Salt IV, Blanc-Valleron (1991) favours a marine influence near the top of the MSG, while recognising the ambiguity of marine proportions with brackish faunas. Many marine-seepage fed brine systems have salinities that allow halotolerant species to flourish in marine-fed basins with no ongoing marine hydrographic connection (Warren, 2011). According to Blanc-Valleron and Schuler (1997), the region experienced a Mediterranean climate with long dry seasons during Salt IV member deposition.


In detail, the Salt IV member is made up of some 210 m of evaporitic sequence, with two relatively thin potash levels (Ci and Cs). The stratigraphy associated with this potash zone is, from base to top (Figure 7):

S2 Unit: 11.5 m thick with distinct layers of organic-rich marls, often dolomitic, with dispersed anhydrite layers.

S1 Unit: 19 m thick, evenly-bedded and made up of alternating metre-scale milky (inclusion-rich) halite layers, with much thinner marls and anhydrite layers. Marls show a sub-millimetric lamination formed by micritic carbonate laminae alternating with clay, quartz, and organic matter-rich laminae. Hofmann et al. (1993a, b) interpreted these couplets as reflecting seasonal variations. Anhydrite occasionally displays remnant swallowtail ghost textures, which suggest that at least part of the anhydrite first precipitated as subaqueous gypsum. Halite shows an abundance of growth-aligned primary chevron textures, along with fluid-inclusion banding suggesting halite was subaqueous and deposited beneath shallow brine sheets (Lowenstein and Spencer, 1990).

S Unit: Is 3.7 m thick and consists of thin marl layers and anhydrite, similar to the S2 Unit, with a few thin millimetric layers of halite.

Mi Unit: With a thickness of 6 m, it is mostly halite with similar characteristics to the S1 Unit. Sylvite was detected in one sample, but its presence is probably related to the evolution of interstitial brines (Cendon et al., 2008).

Ci Unit (“Couche inférieure”): Is formed by 4 m of alternating marls/anhydrite, halite, and sylvite beds (Figure 7).

The Ti unit consists of alternating beds of halite, marl and anhydrite. The top of the interval is the T unit, which is similar to the S unit and consists of alternating beds of marl and anhydrite. Above this is the Ms or upper Marl, near identical to the lower marl Mi. The Mi is overlain by the upper potash bed (Cs), a thinner, but texturally equivalent, bed compared to the sylvinitic Ci unit.

Thus, the Oligocene halite section includes two thin, but mined, potash zones: the Couche inferieure (Ci; 3.9m thick), and Couche superieure (Cs; 1.6m thick), both occur within Salt IV of the Upper Salt group (Figures 5, 7).

Both potash beds are made up of stacked, thin, parallel-sided cm-dm-thick beds (averaging 8 cm thickness), which are in turn constructed of couplets composed of grey-coloured halite overlain by red-coloured sylvite (Figure 5b). Each couplet has a sharp base that separates the basal halite from the sylvite cap of the underlying bed. In some cases, the separation is also marked by bituminous partings. The bottom-most halite in each dm-thick bed consists of halite aggregates with cumulate textures that pass upward into large, but delicate, primary chevrons and cornets. Clusters of this chevron halite swell upward to create a cm-scale hummocky boundary with the overlying sylvite (Figure 5c; Lowenstein and Spencer, 1990).

The sylvite member of a sylvinite couplet consists of granular aggregates of small transparent halite cubes and rounded grains of red sylvite (with some euhedral sylvite hoppers) infilling the swales in the underlying hummocky halite (Figure 5b,c). The sylvite layer is usually thick enough to bury the highest protuberances of the halite, so that the top of each sylvite layer, and the top of the couplet, is flat. Dissolution pipes and intercrystalline cavities are noticeably absent, although some chevrons show rounded coigns. Intercalated marker beds, formed during times of brine pool freshening, are composed of a finely laminated bituminous shale, with dolomite and anhydrite.

The sylvite-halite couplets record combinations of unaltered settle-out and bottom-nucleated growth features, indicating primary chemical sediment accumulating in shallow perennial brine pools (Lowenstein and Spencer, 1990). Based on the crystal size, the close association of halites with sylvite layers, their lateral continuity and the manner in which sylvite mantles overlie chevron halites, the sylvites are interpreted as primary precipitates. Sylvite first formed at the air-brine surface or within the uppermost brine mass and then sank to the bottom to form well-sorted accumulations. As sylvite is a prograde salt it, like halite, probably grew during times of cooling of the brine mass (Figure 8a). These times of cooling could have been diurnal (day/night) or weather-front induced changes in the above-brine air temperatures. Similar cumulate sylvite deposits form as ephemeral bottom accumulations on the floor of modern Lake Dabuxum in China during its more saline phases.


The subsequent mosaic textural overprint seen in many of the Mulhouse sylvite layers was probably produced by postdepositional modification of the crystal boundaries, much in the same way as mosaic halite is formed by recrystallisation of raft and cumulate halite during shallow burial. Temperature-based inclusion studies in both the sylvite and the halites average 63°C, suggesting solar heating of surface brines as precipitation took place (Figure 8b; Lowenstein and Spencer, 1990). Similar high at-surface brine temperatures are not unusual in many modern brine pools, especially those subject to periodic density stratification and heliothermometry (Warren 2016; Chapter 2).

Mineralogically, potash evaporites in the Mulhouse Basin in the Rhine Graben (also known as the Alsatian (Alsace) or Wittelsheim Potash district) contain sylvite with subordinate carnallite, but lack the abundant MgSO4 salts characteristic of the evaporation of modern seawater. The Rhine graben formed during the Oligocene, via crustal extension, related to mantle upwelling. It was, and is, a continental graben typified by high geothermal gradients along its rift axis. In depositional setting, it is not dissimilar to pree-120,000-year potash fill stage in the Quaternary Danakil Basin or the Dead Sea during deposition of potash salts in the Pliocene Sedom Fm. The role of a high-temperature geothermal inflow in defining the CaCl2 nature of the potash-precipitating brines, versus a derivation from a MgSO4-depleted marine feed, is considered significant in the Rhine Graben deposits, but is poorly understood and still not resolved (Hardie, 1990; Cendón et al., 2008). World ocean chemistry in the Oligocene is on a shoulder between the MgSO4-depleted CaCl2-rich oceans of the Cretaceous and the MgSO4-enriched oceans of the Neogene (Figure 4).


Cendón et al. (2008) conclude brine reaction processes were the most important factors controlling the major-ion (Mg, Ca, Na, K, SO4, and Cl) evolution of Mulhouse brines (Figure 9a-d). A combined analysis of fluid inclusions in primary textures by Cryo-SEM-EDS with sulphate- d34S, d18O and 87Sr/86Sr isotope ratios revealed likely hydrothermal inputs and recycling of Permian evaporites, particularly during the more advanced stages of evaporation that laid down the Salt IV member. Bromine levels imply an increasingly concentrated brine at that time (Figure 9a). The lower part of the Salt IV (S2 and S1) likely evolved from an initial marine input (Figure 9b-d).

Throughout, the basin was disconnected from direct marine hydrographic connection and was one of a series of sub-basins formed in an active rift setting, where tectonic variations influenced sub-basin interconnections and chemical signatures of input waters. Sulphate-d34S shows Oligocene marine-like signatures at the base of the Salt IV member (Figure 9c, d). However, enriched sulphate-d18O reveals the importance of synchronous re-oxidation processes.

As evaporation progressed, other non-marine or marine-modified inputs from neighbouring basins became more important. This is demonstrated by increases in K concentrations in brine inclusions and Br in halite, sulphate isotopes trends, and 87Sr/86Sr ratios (Figure 9b, c). The recycling (dissolution) of previously precipitated evaporites of Permian age was increasingly important with ongoing evaporation. In combination, this chemistry supports the notion of a connection of the Mulhouse Basin with basins situated north of Mulhouse. The brine evolution eventually reached sylvite precipitation. Hence, the chemical signature of the resulting brines is not 100% compatible with global seawater chemistry changes. Instead, the potash phase is tied to a hybrid inflow, with significant but decreasing marine input.

There was likely an initial marine source, but this occurred within a series of rift-valley basin depressions for which there was no direct hydrographic connection to the open ocean, even at the time the Middle Salt Member (potash-entraining) was first deposited (Cendon et al., 2008). That is, the general hydrological evolution of the primary textured evaporites in the Mulhouse basin sump is better explained as a restricted sub-basin with an initial marine-seepage stage. This gradually changed to ≈ 40% marine source near the beginning of evaporite precipitation, with the rest of hydrological inputs being non-marine. There was a significant contribution of solutes from recycled, in part diapiric, Permian evaporites, likely remobilised by the tectonics driving the formation of the rift valley (Hinsken et al., 2007; Cendon et al., 2008). The general proportion of solutes did not change substantially over the time of evaporite precipitation. However, as the basin restriction increased, the formerly marine inputs changed to continental, diapiric or marine-modified inputs, perhaps fed from neighbouring basins north of Mulhouse basin. As in the Ethiopian Danakhil potash-rift, it is likely brine interactions occurred both during initial and early post-depositional reflux overprinting of the original potash salt beds.


West Canadian potash (Devonian)

The Middle Devonian (Givetian) Prairie Evaporite Formation is a widespread potash-entraining halite sequence deposited in the Elk Point Basin, an early intracratonic phase of the Western Canada Sedimentary Basin (WCSB; Chipley and Kyser, 1989). Today, it is the world’s predominant source of MOP fertiliser (Warren, 2016). The flexure that formed the basin and its subsealevel accommodation space was a distal downwarp to, and driven by, the early stages of the Antler Orogeny (Root, 2001). Texturally and geochemically the potash layers in the basin show the effects of multiple alterations and replacements of its potash minerals, especially interactions between sylvite and carnallite in a variably recrystallised halite host.

Regionally halite constitutes a large portion of the four formations that make up the Devonian Elk Point Group (Figure 10): 1) the Lotsberg (Lower and Upper Lotsberg Salt), 2) the Cold Lake (Cold Lake Salt), 3) the Prairie Evaporite (Whitkow and Leofnard Salt), and 4) the Dawson Bay (Hubbard Evaporite). Today the remnants of the Middle Devonian Prairie Evaporite Formation constitute a bedded unit some 220 metres thick, which lies atop the irregular topography of the platform carbonates of the Winnipegosis Fm. Extensive solutioning of the various salts has given rise to an irregular thickness to the formation and the local absence of salt (Figure 11a).


The Elk Point Group was deposited within what is termed the Middle Devonian “Elk Point Seaway,” a broad intracratonic sag basin extending from North Dakota and northeastern Montana at its southern extent north through southwestern Manitoba, southern and central Saskatchewan, and eastern to northern Alberta (Figure 11a). Its Pacific coast was near the present Alberta-British Columbia border, and the basin was centred at approximately 10°S latitude. To the north and west the basin was bound by a series of tectonic ridges and arches; but, due to subsequent erosion, the true eastern extent is unknown (Mossop and Shetsen, 1994). In northern Alberta, the Prairie Evaporite is correlated with the Muskeg and Presqu’ile formations (Rogers and Pratt, 2017).

Hydrographic isolation of the intracratonic basin from its marine connection resulted in the deposition of a drawndown sequence of basinwide (platform-dominant) evaporites with what is a uniquely high volume of preserved potash salts deposited within a clayey halite host. The potash resource in this basin far exceeds that of any other known potash basin in the world.


Potash geology

Potash deposits mined in Saskatchewan are all found within the upper 60-70 m of the Prairie Evaporite Formation, at depths of more than 400 to 2750 metres beneath the surface of the Saskatchewan Plains. Within the Prairie Evaporite, there are four main potash-bearing members, in ascending stratigraphic order they are: Esterhazy, White Bear, Belle Plaine and Patience Lake members (Figure 11b). Each member is composed of various combinations of halite, sylvite, sylvinite, and carnallitite, with occurrences of sylvite versus carnallite reliably definable using wireline signatures (once the wireline is calibrated to core or mine control - Figure 12; Fuzesy, 1982).

The Patience Lake Member is the uppermost Prairie Evaporite member and is separated from the Belle Plaine by 3-12 m of barren halite (Holter, 1972). Its thickness ranges from 0-21 m and averages 12 m, its top 7-14 m is made up halite with clay bands and stratiform sylvite. This is the targeted ore unit in conventional mines in the Saskatoon and Lanigan areas and is the solution-mined target, along with the underlying Belle Plaine Member, at the Mosaic Belle Plaine potash facility. The Belle Plaine member is separated from the Esterhazy by the White Bear Marker beds made up of some 15 m of low-grade halite, clay seams and sylvinite. The Belle Plaine Member is more carnallite-prone than the Patience Lake member (Figure 12). It is the ore unit in the conventional mines at Rocanville and Esterhazy (Figure 11b) where its thickness ranges from 0-18 m and averages around 9 m. In total, the Prairie Evaporite Formation does not contain any significant MgSO4 minerals (kieserite, polyhalite etc.) although some members do contain abundant carnallite. This mineralogy indicates precipitation from a Devonian seawater/brine chemistry somewhat different from today’s, with inherently lower relative proportions of sulphate and lower Mg/Ca ratios (Figure 4).

The Prairie Evaporite Fm. is nonhalokinetic throughout the basin, it is more than 200 m thick in the potash mining district in Saskatoon and 140 m thick in the Rocanville area to the southeast (Figure 11a; Yang et al., 2009). The Patience Lake member is the main target for conventional mining near Saskatoon. The Esterhazy potash member rises close to the surface in the southeastern part of Saskatchewan near Rocanville and on into Manitoba. This is a region where the Patience Lake Member is thinner or completely dissolved (Figure 11b). Over the area of mineable interest in the Patience Lake Member, centred on Saskatoon, the ore bed currently slopes downward only slightly in a westerly direction, but deepens more strongly to the south at a rate of 3-9 m/km. Mines near Saskatoon are at depths approaching a kilometre and so are nearing the limits of currently economic shaft mining.

The main shaft for the Colonsay Mine, which took IMC Global Inc. more than five years to complete through a water-saturated sediment column, finally reached the target ore body at a depth of 960 metres. Such depths and a southerly dip to the ore means that the conventional shaft mines near Saskatoon define a narrow WNW-ESE band of conventional mining activity (Figure 11c). To the south potash is recovered from greater depths by solution mining; for example, the Belle Plaine operation leaches potash from the Belle Plaine member at a depth of 1800m.

The Prairie Evaporite typically thins southwards in the basin; although local thickening occurs where carnallite, not sylvite, is the dominant potash mineral (Worsley and Fuzesy, 1979). The Patience Lake member is mined at the Cory, Allan and Lanigan mines, and the Esterhazy Member is mined in the Rocanville area (Figure 11c). Ore mined from the 2.4 m thick Esterhazy Member in eastern Saskatchewan contain minimal amounts of insolubles (≈1%), but considerable quantities of carnallite (typically 1%, but up to 10%) and this reduces the average KCl grade value to an average of 25% K2O. The converse is true for ore mined from the Patience Lake potash member in western Saskatchewan near Saskatoon, where carnallite is uncommon in the Cory and Allan mines. The mined ore thickness is a 2.74-3.35 metre cut off near the top of the 3.66-4.57metre Prairie Lake potash member. Ore grade is 20-26% K2O and inversely related to thickness (Figure 12). The insoluble content is 4-7%, mostly clay and markedly higher than in the Rocanville mines.


A typical sylvinite ore zone in the Patience Lake member can be divided into four to six units, based on potash rock-types and clay seams (Figures 12, 13a; M1-M6 of Boys, 1990). Units are mappable and have been correlated throughout the PCS Cory Mine with varying degrees of success, dependent on partial or complete loss of section from dissolution. Potash deposition appears to have been early and related to short-term brine seaway cooling and syndepositional brine reflux. So the potash layering (M1-M6) is cyclic, expressed in the repetitive distribution of hematite and other insoluble minerals (Figure 13). Desiccation polygons, desiccation cracks, subvertical microkarst pits and chevron halite crystals indicate that the Patience Lake member that encompasses the potash ore was deposited in and just beneath a shallow-brine, salt-pan environment (Figure 13b; Boys, 1990; Lowenstein and Spencer, 1990; Brodlyo and Spencer, 1987; pers. obs).

Clay seams form characteristic thin stratigraphic segregations throughout the potash ore zone(s) of the Prairie Evaporite, as well as disseminated intervals, and constitute about 6% of the ore as mined. For example, the insoluble minerals found in the PCS Cory samples are, in approximate order of decreasing abundance: dolomite, clay [illite, chlorite (including swelling-chlorite/chlorite), and septechlorite, quartz, anhydrite, hematite, and goethite. Clay minerals make up about one-third of the total insolubles: other minor components include: potassium feldspar, hydrocarbons, and sporadic non-diagnostic palynomorphs (Figure 13; Boys, 1990).

In all mines, the clays tend to occur as long continuous seams or marker layers between the potash zones and are mainly composed of detrital chlorite and illite, along withauthigenic septechlorite, montmorillonite and sepiolite (Mossman et al., 1982; Boys, 1990). Of the two chlorite minerals, septechlorite is the more thermally stable. The septechlorite, sepiolite and vermiculite very likely originated as direct products of settle-out, syndepositional dissolution or early diagenesis under hypersaline conditions from a precursor that was initially eolian dust settling to the bottom of a vast brine seaway. The absence of the otherwise ubiquitous septechlorite from Second Red Beds west of the zero-edge of the evaporite basin supports this concept (Figure 9, 10).


Potash Textures

Texturally, at the cm-scale, potash salt beds in the Prairie Evaporite (both carnallitite and sylvinite) lack the lateral continuity seen in primary potash textures in the Oligocene of the Mulhouse Basin (Figure 14). Prairie potash probably first formed as syndepositional secondary precipitates and alteration products at very shallow depths just beneath the sediment surface. These early prograde precipitates were then modified to varying degrees by ongoing fluid flushing in the shallow burial environment. The cyclic depositional distribution of disseminated insolubles as the clay marker beds was possibly due to a combination of source proximity, periodic enrichment during times of brine freshening and the strengthening of the winds blowing detritals out over the brine seaway. Possible intra-potash disconformities, created by dissolution of overlying potash-bearing salt beds, are indicated by an abundance of residual hematite in clay seams with some cutting subvertically into the potash bed. Except in, and near, dissolution levels and collapse features, the subsequent redistribution of insolubles, other than iron oxides, is not significant.

In general, halite-sylvite (sylvinite) rocks in the Prairie Evaporite ore zones generally show two end member textures; 1) the most common is a recrystallised polygonal mosaic texture with individual crystals ranging from millimetres to centimetres and sylvite grain boundaries outlined by concentrations of blood-red halite (Figure 14a). 2) The other end member texture is a framework of euhedral and subhedral halite cubes enclosed by anhedral crystals of sylvite (Figure 14b). This is very similar to ore textures in the Salado Formation of New Mexico interpreted as early passive precipitates in karstic voids.

Petrographically, the halite-carnallite (carnallitite) rocks display three distinct textures. Most halite-carnallite rocks contain isolated centimetre-sized cube mosaics of halite enclosed by poikilitic carnallite crystals (Figure 14c); 1) Individual halite cubes are typically clear, with occasional cloudy crystal cores that retain patches of syndepositional growth textures (Lowenstein and Spencer, 1990). 2) The second texture is coarsely crystalline halite-carnallite with equigranular, polygonal mosaic textures. In zones where halite overlies bedded anhydrite, most of the halite is clear with only the occasional crystal showing fluid inclusion banding.

Bedded halite away from the ore zones generally retains a higher proportion of primary depositional textures typical of halite precipitation in shallow ephemeral saline pans (Figure 14d; Brodylo and Spencer, 1987). Crystalline growth fabrics, mainly remnants of vertically-elongate halite chevrons, are found in 50-90% of the halite from many intervals in the Prairie Evaporite. Many of the chevrons are truncated by irregular patches of clear halite that formed as early diagenetic cements in syndepositional karst.

In contrast, the halite hosting the potash ore layers lacks well-defined primary textures but is dominated by intergrown mosaics. From the regional petrology and the lower than expected Br levels in halite in the Prairie Evaporite Formation, Schwerdtner (1964), Wardlaw and Watson (1966) and Wardlaw (1968) postulated a series of recrystallisation events forming sylvite after carnallite as a result of periodic flushing by hypersaline solutions. This origin as a secondary precipitate (via incongruent dissolution) is supported by observations of intergrowth and overgrowth textures (McIntosh and Wardlaw, 1968), collapse and dissolution features at various scales and timings (Gendzwill, 1978; Warren 2017), radiometric ages (Baadsgaard, 1987) and palaeomagnetic orientations of the diagenetic hematite linings associated with the emplacement of the potash (Koehler, 1997; Koehler et al., 1997).

Dating of clear halite crystals in void fills within the ore levels shows that some of the exceptionally coarse and pure secondary halites forming pods in the mined potash horizons likely precipitated during early burial, while other sparry halite void fills formed as late as Pliocene-Pleistocene (Baadsgard, 1987). Even today, alteration and remobilisation of the sylvite and carnallite and the local precipitation of bischofite are ongoing processes, related to the encroachment of the contemporary dissolution edge or the ongoing stoping of chimneys fed by deep artesian circulation (pers obs.).


Fluid inclusion studies support the notion of primary textures (low formation temperatures in chevron halite in the Prairie evaporite and an associated thermal separation of non-sylvite and sylvite associated halite (Figure 15; Chipley et al., 1990). Most fluid inclusions found in primary, fluid inclusion-banded halite associated with the Prairie potash salts contain sylvite daughter crystals at room temperature or nucleate them on cooling (e.g. halite at 915 and 945 m depth in the Winsal Osler well; Lowenstein and Spencer, 1990). In contrast, no sylvite daughter crystals have been observed in fluid inclusions outlining primary growth textures from chevron halites away from the potash deposits.

The data illustrated as Figure 15 clearly show that inclusion temperatures in primary halite chevrons are cooler than those in halites collected in intervals nearer the potash levels. Sylvite daughter crystal dissolution temperatures from fluid inclusions in the cloudy centres of halite crystals associated with potash salts are generally warmer (Brodlyo and Spencer, 1987; Lowenstein and Spencer, 1990). Sylvite and carnallite daughter crystal dissolution temperatures from fluid inclusions in fluid inclusion banded halite from bedded halite-carnallite are the hottest. This mineralogically-related temperature schism establishes that potash salts occur in stratigraphic intervals in the halite where syndepositional surface brines were warmer. In the 50° - 70°C temperature range there could be overlap with heliothermal brine lake waters. Even so, these warmer potash temperatures imply parent brines would likely be moving via a shallow reflux drive and are not the result of primary bottom nucleation (in contrast to primary sylvite in the Mulhouse Basin). Whether the initial Prairie reflux potash precipitate was sylvite or carnallite is open to interpretation (Lowenstein and Hardie, 1990).


Fluid evolution from mineral and isotope chemistry

Analysis of subsurface waters from various Canadian potash mines and collapse anomalies in the Prairie Evaporite suggest that, after initial potash precipitation, a series of recrystallising fluids accessed the evaporite levels at multiple times throughout the burial history of the Prairie Formation (Chipley, 1995; Koehler, 1997; Koehler et al., 1997). Likewise, the isotope systematics and K-Ar ages of sylvite in both halite and sylvite layers indicate that the Prairie Evaporite was variably recrystallised during fluid overprint events (Table 2; Figure 16). These event ages are all younger than original deposition (≈390Ma) and likely correspond to ages of various tectonic events that influenced subsurface hydrology along the western margin of North America.


Chemical compositions of inclusion fluids in the Prairie Evaporite, as determined by their thermometric properties, reveal at least two distinct waters played a role in potash formation: a Na-K-Mg-Ca-Cl brine, variably saturated with respect to sylvite and carnallite; and a Na-K-Cl brine (Horita et al., 1996). That is, contemporary inclusion water chemistry is a result in part of ongoing fluid-rock interaction. The ionic proportions in some halite samples are not the result of simple evaporation of seawater to the sylvite bittern stage (Figure 17a; Horita et al., 1996). There is a clear separation of values from chevron halites in samples from the Lanigan and Bredenbury (K-2 area) mines, which plot closer to the concentration trend seen in halite from modern seawater and values from clear or sparry halite. The latter encompass much lower K and higher Br related to fractionation tied to recrystallisation. Likewise, the influence of ongoing halite and potash salt dissolution is evident in the chemistry of shaft and mine waters with mine level waters showing elevated Mg and K values, (Figure 17b; Wittrup and Kyser, 1990; Chipley, 1995). What is more the mine waters of today show  substantial overlap with waters collected more than thirty years ago (Jensen et al., 2006)


This notion of ongoing fluid-rock interaction controlling the chemistry of mine waters is supported by dD and d18O values of inclusion fluids in both halite and sylvite, which range from -146 to 0‰ and from -17.6 to -3.0‰, respectively (Figure 18). Most of the various preserved isotope values are different from those of evaporated seawater, which should have dD and d18O values near 0‰.

Furthermore, the dD and d18O values of inclusion fluids are probably not the result of precipitation of the evaporite minerals from a brine that was a mixture of seawater and meteoric water. The low latitude position of the basin during the Middle Devonian (10-15° from the equator), the required lack of meteoric water to precipitate basinwide evaporites, and the expected dD and d18O values of any meteoric water in such a setting, make this an unlikely explanation. Rather, the dD and d18O values of inclusion fluids in the halites reflect ambient and evolving brine chemistries as the fluids in inclusions in the various growth layers were intermittently trapped during the subsurface evolution of the Prairie Formation in the Western Canada Sedimentary Basin. They also suggest that periodic migration of nonmarine subsurface water was a significant component of the crossflowing basinal brines throughout much of the recrystallisation history (Chipley, 1995).

Prairie carnallite-sylvite alteration over time

Ongoing alteration of carnallite to sylvite and the reverse reaction means a sylvite-carnallite bed must be capable of gaining or losing fluid at the time of alteration. That is, any reacting potash beds must be permeable at the time of the alteration. By definition, there must be fluid egress to drive incongruent alteration of carnallite to sylvite or fluid ingress to drive the alteration of sylvite to carnallite. There can also be situations in the subsurface where the volume of undersaturated fluid crossflow was sufficient to remove (dissolve) significant quantities of the more soluble evaporite salts. Many authors looking at the Prairie evaporite argue that the fluid access events during the alteration of carnallite to sylvite or the reverse, or the complete leaching of the soluble potash salts was driven by various tectonic events (Figure 16). In the early stages of burial alteration (few tens of metres from the landsurface) the same alteration processes can be driven by varying combinations of brine reflux, prograde precipitation and syndepositional karstification, all driven by changes in brine level and climate, which in turn may not be related to tectonism (Warren, 2016; Chapters 2 and 8 for details).

In the potash areas of the Western Canada Sedimentary Basin, the notion of 10-100 km lateral continuity is a commonly stated precept for both sylvinite and carnallitite units across the extent of the Prairie Evaporite. But when the actual distribution and scale of units are mapped based on mined intercepts, there are numerous 10-100 metre-scale discontinuities (anomalies) present indicating fluid ingress or egress (Warren, 2017).


Sometimes ore beds thin and alteration degrades the ore level (Section A-A1-A2), other times these discontinuities can locally enrich sylvite ore grade (B-B1; Figure 19). Discontinuities or salt anomalies are much more widespread in the Prairie evaporite than mentioned in much of the potash literature (Figure 19). Mining for maintenance of ore grade shows that unexpectedly intersecting an anomaly in a sylvite ore zone can have a range of outcomes ranging from the inconsequential to the catastrophic, in part because there is more than one type of salt anomaly or “salt horse" (Warren, 2017).


Figure 20 summarises what are considered the three most common styles of salt anomaly in the sylvite ore beds of the Prairie Evaporite, namely 1) Washouts, 2) Leach anomalies, 3) Collapse anomalies. These ore bed disturbances and their occurrence styles are in part time-related. Washouts are typically early (eogenetic) and defined as... “salt-filled V- or U-shaped structures, which transect the normal bedded sequence and obliterate the stratigraphy” (Figure 20a; Mackintosh and McVittie, 1983, p. 60). They are typically enriched in, or filled by, insoluble materials in their lower one-third and medium-coarse-grained sparry halite in the upper two thirds. Up to several metres across, when traced laterally they typically pass into halite-cemented paleo-sinks and cavern networks (e.g. Figure 20b). Most washouts likely formed penecontemporaneous to the potash beds they transect, that is, they are preserved examples of synkarst, with infilling of the karst void by a slightly later halite cement. They indicate watertable lowering in a potash-rich saline sump. This leaching was followed soon after by a period of higher watertables and brine saturations, when halite cements occluded the washouts and palaeocaverns. Modern examples of this process typify the edges of subcropping and contemporary evaporite beds, as about the recently exposed edges of the modern Dead Sea. As such, “washouts” tend to indicate relatively early interactions of the potash interval with undersaturated waters, they may even be a part of the syndepositional remobilisation hydrology that focused, and locally enriched, potash ore levels.

In a leach anomaly zone, the stratabound sylvinite ore zone has been wholly or partially replaced by barren halite, without significantly disturbing the normal stratigraphic sequence (clay marker beds) which tend to continue across the anomaly (Figure 20b). Some loss of volume or local thinning of the stratigraphy is typical in this type of salt anomaly. Typically saucer-shaped, they have diameters ranging from a few metres up to 400m. Less often, they can be linear features that are up to 20 m wide and 1600m long. Leach zones can form penecontemporaneously due to depressions and back-reactions in the ore beds, or by later low-energy infiltration of Na-saturated, K-undersaturated brines. The latter method of formation is also likely on the margins of collapse zones, creating a hybrid situation typically classified as a leach-collapse anomaly (Mackintosh and McVittie, 1983; McIntosh and Wardlaw, 1968).

Of the three types of salt anomaly illustrated, leach zone processes are the least understood. Historically, when incongruent dissolution was the widely accepted interpretation for loss of unit thickness in the Prairie Evaporite, many leach anomalies were considered metasomatic. Much of the original metasomatic interpretation was based on decades of detailed work in the various salt mines of the German Zechstein Basin. There, in an endemic halokinetic terrane, evaporite textures were considered more akin to metamorphic rocks, and the term metasomatic alteration was commonly used when explaining leach anomalies (Bochert and Muir, 1964, Braitsch, 1971). In the past two decades, general observations of the preservation of primary chevron halite in most bedded evaporites away from the potash layers in the Prairie Evaporite have led to reduced use of notions of widespread metamorphic-like metasomatic or solid-state alteration processes in bedded evaporites. There is just too much preserved primary texture in the bedded salt units adjacent to potash beds to invoke pervasive burial metasomatism of the Prairie Evaporite.


So how do leach anomalies, as illustrated in Figure 20b, occur in nonhalokinetic settings? One possible explanation is given by the depositional textures documented in anomalies in the Navarra Potash Province (Figure 21). There, the underlying and overlying salt stratigraphy is contiguous, while the intervening sylvite passed laterally into a syndepositional anomaly or “salt horse” created by an irregular topography on the salt pan floor prior to the deposition of onlapping primary sylvinite layers (see Warren 2016, 2017 for detailed discussion)

On the other hand, in halokinetic situations (which characterises much Zechstein salt) solid-state alteration via inclusion related migration in flowing salt beds is a well-documented set of texture-altering processes (diffusion metasomatism). Most workers in such halokinetic systems would agree that there must have been an original stratiform potassium segregation present during or soon after deposition related to initial precipitation, fractional dissolution and karst-cooling precipitation. But what is controlling potassium distribution now in the Zechstein salts is a recrystallised and remobilised set of textures, which preserve little or no crystal-scale evidence of primary conditions (Warren, 2016; Chapter 6). The complex layering in such deposits may preserve a broad depositional stratigraphy, but the decimetre to metre scale mineral distributions are indications of complex interactions of folds, overfolds, and disaggregation with local flow thickening. We shall return to this discussion of Zechstein potash textures in the next section dealing with devolatisation of hydrated salts such as carnallite. in zones of local heating

Collapse zones in the Prairie Evaporite are characterized by a loss of recognizable sylvinite ore strata, which is replaced by less saline brecciated, recemented and recrystallized material, with the breccia blocks typically made of the intrasalt or roof lithologies (Figure 20c), so angular fragments of the Second Red beds and dolostones of the Dawson Bay Formation are the most conspicuous components of the collapse features in the Western Canada Sedimentary Basin. When ore dissolution is well developed, all the halite can dissolve, along with the potash salts, and the overlying strata collapse into the cavity (these are classic solution collapse features). Transitional leached zones typically separate the collapsed core from normal bedded potash. Such collapse structures indicate a breach of the ore layers by unsaturated waters, fed either from below or above. For example, in the Western Canada Sedimentary Basin, well-developed collapse structures tend to occur over the edges and top Devonian mud mounds, while in the New Mexico potash zone the collapse zones are related to highs in the underlying Capitan reef trend (Warren, 2017). Leaching fluids may have come from below or above to form collapse structures at any time after initial deposition. When connected to a water source, these are the subsurface features that when intersected can quickly move the mining operation out of the salt into an adjacent aquifer system, a transition that led to flooding in most of the mine-lost operations listed earlier.

Identifying at the mine scale the set of processes that created a salt anomaly in a sylvite bed also has implications in terms of its likely influence on mine stability whatever decision is made on how to deal with it as part of the ongoing mine operation (Warren, 2016, 2017). Syndepositional karst fills and leach anomalies are least likely to be problematic if penetrated during mining, as the aquifer system that formed them is likely no longer active. In contrast, penetration or removal of the region around a salt-depleted collapse breccia may lead to uncontrollable water inflows and ultimately to the loss of the mine.

Unfortunately, in terms of production planning, the features of the periphery of a leach anomaly can be similar if not identical to those in the alteration halo that typically forms about the leached edge a collapse zone. The processes of sylvite recrystallisation that define the edge of collapse anomaly can lead to local enrichment in sylvite levels, making these zones surrounding the collapse core attractive extraction targets (Boys 1990, 1993). Boundaries of any alteration halo about a collapse centre are not concentric, but irregular, making the prediction of a feature’s geometry challenging, if not impossible. The safest course of action is to avoid mining salt anomalies, but longwall techniques make this difficult and so they must be identified and dealt with (see Warren 2017).


Cooking sylvite: Dykes & sills in potash salts

 

In addition to; 1) primary sylvite and 2) sylvite/carnallite alteration via incongruent transformation in burial, there is a third mode of sylvite formation related to 3) igneous heating driving devolatisation of carnallitite, which can perhaps be considered a form of incongruent melting (Warren, 2016). And so, at a local scale (measured in metres to tens of metres) in potash beds cut by igneous intrusions, there are a number of documented thermally-driven alteration styles and thermal haloes. Most are created by the intrusion of hot doleritic or basaltic dykes and sills into cooler salt masses, or the outflow of extrusive igneous flows over cooler salt beds (Knipping, 1989; Grishina et al, 1992, 1998; Gutsche, 1988; Steinmann et al., 1999; Wall et al., 2010). Hot igneous material interacts with somewhat cooler anhydrous salt masses to create narrow, but distinct, heat and mobile fluid-release envelopes, also reflected in the resulting salt textures. At times, relatively rapid magma emplacement can lead to linear breakout trends outlined by phreatomagmatic explosion craters, as imaged in portions of the North Sea (Wall et al., 2010) and the Danakhil/Dallol potash beds in Ethiopia (Salty Matters, May 1, 2015).

Based on studies of inclusion chemistry and homogenization temperatures in fluid inclusions in bedded halite near intrusives, it seems that the extent of the influence of a dolerite sill or dyke in bedded salt is marked by fluid-inclusion migration, evidenced by the disappearance of chevron structures and consequent formation of clear halite with a different set of higher-temperature inclusions. Such a migration envelope is well documented in bedded Cambrian halites intruded by end-Permian dolerite dykes in the Tunguska region of Siberia (Grishina et al., 1992).

Defining h as the thickness of the dolerite intrusion in these salt beds, and d as the distance of the halite from the edge of the intrusion, then the disappearance of chevrons occurs at greater distances above than below the intrusive sill. For d/h < 0.9 below the intrusion, CaCl2, CaCl2, KCl and nCaCl2, mMgCl2 solids occur in association with water-free and liquid-CO2 inclusions, with H2S, SCO and orthorhombic or glassy S8. For a d/h of 0.2-2 above the intrusion, H2S-bearing liquid-CO2 inclusions are typical, with various amounts of water. Thus, as a rule of thumb, an alteration halo extends up to twice the thickness of the dolerite sill above the sill and almost the thickness of the sill below (Figure 22).

In a series of autoclavation laboratory experiments, Fabricius and Rose-Hampton (1988) found that; 1) at atmospheiric pressure carnallite melts incongruently to sylvite and hydrated MgCl2 at a temperature of 167.5°C. 2) the melting/transformation temperature increase to values in excess of 180°C as the pressure increases (Figure 23).


A similar situation occurs in the dyke-intruded halite levels exposed in the mines of the Werra-Fulda district of Germany (Steinmann et al., 1999; Schofield, et al., 2014). There the Herfa-Neurode potash mine is located in the Werra-Fulda Basin in the Hessian district of central Germany (Figure 24a). The targeted ore levels consist of the carnallite-rich Kaliflöz Hessen (K1H) and Kaliflöz Thüringen (K1Th) intervals, which form part of the Zechstein 1 (Z1) bedded Werra salt succession (Warren, 2016). In the mine the K1H and K1Th units range in thickness from 2 m to 10 m, are generally subhorizontal and occur at a depth of 650–710 m below the present-day surface.


In the later Tertiary, basaltic melts intruded these Zechstein evaporites as numerous sub-vertical dykes, but only a few dykes attained the Miocene landsurface. Basaltic melt production was related to regional volcanic activity some 10 to 25 Ma. Basalts exposed in the mine walls, where it cuts non-hydrous units of halite or anhydrite, are typically subvertical dykes, rather than subhorizontal sills. The basalts are phonolitic tephrites, limburgites, basanites and olivine nephelinites. Dyke margins are usually vitrified, forming a microlitic limburgite glass along dyke edges in contact with halite (Figure 24b; Knipping, 1989). At the contact on the evaporite side of the glassy rim, there is a cm-wide carapace of high-temperature salts (mostly anhydrite and ferroan carbonates). Further out, the effect of the high-temperature envelope is denoted by transitions to clear halite, with higher temperature fluid inclusions (Knipping 1989). All of this metre-scale alteration is an anhydrous alteration halo, the halite did not melt (melting temperature of 804°C), rather than migrating, the fluid driving recrystallisation was mostly from entrained brine/gas inclusions. The dolerite/basalt interior of the basaltic dyke is likewise altered and salt soaked, with clear, largely inclusion-free halite typically filling vesicles in the basalt.

Heating of hydrated (carnallitic) salt layers, adjacent to a dyke or sill, tends to drive off the water of crystallisation (chemical or hydration thixotropy) at much lower temperatures than that at which anhydrous salts, such as halite or anhydrite, thermally melt (Figure 24c; Table 3). For example, in the Fulda region, the thermally-driven release of water of crystallisation within carnallitic beds creates thixotropic or subsurface “peperite” textures as carnallitite alters to sylvinite layers. These are layers where heated water of crystallisation escaped from the hydrated-salt lattice. Dehydration-driven loss of mechanical strength focuses zones of magma entry into particular subhorizontal horizons in the salt mass, wherever hydrated salt layers were present. In contrast, dyke and sill margins are much sharper and narrower in zones of contact with anhydrous salt intervals and the intrusive is sub-vertical to steeply dipping (Figure 24b versus 24c).

Accordingly, away from the immediate vicinity of the direct thermal aureole, heated and overpressured dehydration waters can enter carnallite halite bed, and drive the creation of extensive soft sediment deformation and peperite textures in hydrated layer (Figure 24c). Mineralogically, sylvite and coarse recrystallised halite dominate the salt fraction in the peperite intervals of the Herfa-Neurode mine. Sylvite in the altered zone is a form of dehydrated carnallite, not a primary-textured salt. Across the Fulda region, such altered zones and deformed units can extend along former carnallite layer to tens or even a hundred or more metres from the dyke feeder. Ultimately, the deformed potash bed passes back out into the unaltered bed, which retains abundant inclusion-rich halite and carnallite (Schofield et al., 2014).

That is, nearer the basalt dyke, the carnallite is largely transformed into inclusion-poor halite and sylvite, the result of incongruent flushing of warm saline fluids mobilised from the hydrated carnallite crystal lattice as it was heated by dyke emplacement. During Miocene salt alteration/thermal metamorphism in the Fulda region, NaCl-fluids were mixed with fluids and gases originating from thermally-mobilised crystallisation water in the carnallite, as it converted to sylvite. This brine/gas mixture altered the basalts during post-intrusive cooling, an event which numerical models suggest was quite rapid (Knipping, 1989): a dyke of less than 0.5 m thickness probably cooled to temperatures less than 200°C within 14 days of dyke emplacement.

The contrast in alteration extent between anhydrous and hydrous salt layers shows alteration effects are minimal wherever the emplacement temperature of the magma is below that of the anhydrous salt body as it is next to a basalt dyke. If this is the mechanism driving entry of igneous-related volatiles (gases and liquids) into a salt body, then the distribution of products (including CO2) will be highly inhomogeneous and related to the minerally of the salt unit adjacent to the intrusive. Worldwide, dykes intersecting salt beds tend to widen to become sills in two zones: 1) along evaporite units within the halite mass that contain hydrated salts, such as carnallite or gypsum (Figure 24c) and, 2) where rising magma has ponded and so created laccoliths at the upper or lower halite contact with the adjacent nonsalt strata or against a salt wall (Figure 22 vs 24). The first is a response to a pulse of released water as dyke-driven heating forces the dehydration of hydrated salt layers. The second is a response to the mechanical strength contrast at the salt-nonsalt contact.

In summary, sylvite formed from a carnallite precursor during Miocene salt alteration/thermal metamorphism in the Fulda region, NaCl-fluids were mixed with fluids originating from thermally-mobilised crystallisation water in the carnallite, as it converted to sylvite. This brine mixture altered the basalts during post-intrusive cooling, an event which numerical models suggest was quite rapid (Knipping, 1989): a dyke of less than 0.5 m thickness probably cooled to temperatures less than 200°C within 14 days of dyke emplacement.

How do we produce potash salts?

 

Over this series of three articles focused on current examples of potash production, we have seen there are two main groups of potash minerals currently utilised to make fertiliser, namely, muriate of potash (MOP) and sulphate of potash (SOP). MOP is both mined (generally from a Pre-Neogene sylvinite ore) or produced from brine pans (usually via processing of a carnallitite slurry). In contrast, large volumes of SOP are today produced from brine pans in China and the USA but with only minor production for solid-state ore targets. Historically, SOP was produced from solid-state ores in Sicily, the Ukraine, and Germany but today there are no conventional mines with SOP as the prime output in commercial operation (See Salty Matters, May 12, 2015).

The MgSO4-enriched chemistry of modern seawater makes the economic production of potash bitterns from a seawater-feed highly challenging. Today, there is no marine-fed plant anywhere in the world producing primary sylvite precipitates. However, sylvite is precipitating from a continental brine feed in salt pans on the Bonneville salt flat, Utah. There, a brine field, drawing shallow pore waters from saltflat sediments, supplies suitably low-MgSO4 inflow chemistry to the concentrator pans. Sylvite also precipitates in solar evaporator pans in Utah that are fed brine circulated through the abandoned workings of the Cane Creek potash mine (Table 1).

Large-scale production of MOP fertiliser from potash precipitates created in solar evaporation pans is taking place in perennially subaqueous saline pans of the southern Dead Sea and the Qaidam Basin. In the Dead Sea, the feed brine is pumped from the waters of the northern Dead Sea basin, while in the Qaidam sump the feed is from a brine field drawing pore brines with an appropriate mix of river and basinal brine inputs. In both cases, the resulting feed brine to the final concentrators is relatively depleted in magnesium and sulphate. These source bitterns have ionic proportions not unlike seawater in times of ancient MgSO4-depleted oceans. Carnallitite slurries, not sylvinite, are the MOP precipitates in pans in both regions. When feed chemistry of the slurry is low in halite, then the process to recover sylvite is a cold crystallisation technique. When halite impurity levels in the slurry are higher, sylvite is manufactures using a more energy intensive, and hence more expensive, hot crystallisation technique. Similar sulphate-depleted brine chemistry is used in Salar Atacama, where MOP and SOP are recovered as byproducts of the production of lithium carbonate brines.

Significant volumes of SOP are recovered from a combination of evaporation and cryogenic modification of sulfate-enriched continental brines in pans on the edge of the Great Salt Lake, Utah, and Lop Nur, China. When concentrated and processed, SOP is recovered from the processing of a complex series of Mg-K-SO4 double salts (schoenitic) in the Odgden pans fed brines from the Great Salt Lake. The Lop Nur plant draws and concentrates pore waters from a brine field drawing waters from glauberite-polyhalite-entraining saline lake sediments.

All the Quaternary saline lake factories supply less than 20% of the world's potash; the majority comes from the conventional mining of sylvinite ores. The world's largest reserves are held in Devonian evaporites of the Prairie Evaporite in the Western Canada Sedimentary Basin. Textures and mineral chemistry show that the greater volume of bedded potash salts in this region is not a primary sylvite precipitate. Rather the ore distribution, although stratiform and defined by a series of clay marker beds, actually preserves the effects of multiple modifications and alterations tied to periodic egress and ingress of basinal waters. Driving mechanisms for episodes of fluid crossflow range from syndepositional leaching and reflux through to tectonic pumping and uplift (telogenesis). Ore distribution and texturing reflect local-scale (10-100 metres) discontinuities and anomalies created by this evolving fluid chemistry. Some alteration episodes are relatively benign in terms of mineralogical modification and bed continuity. Others, generally tied to younger incidents (post early Cretaceous) of undersaturated crossflow and karstification, can have substantial effects on ore continuity and susceptibility to unwanted fluid entry. In contrast, ore textures and bed continuity in the smaller-scale sylvinite ores in the Oligocene Mulhouse Basin, France, indicate a primary ore genesis.

What makes it economic?

Across the Quaternary, we need a saline lake brine systems with the appropriate brine proportions, volumes and climate to precipitate the right association of processable potash salts. So far, the price of potash, either MOP or SOP, and the co-associated MgSO4 bitterns, precludes industrial marine-fed brine factories.

In contrast, to the markedly nonmarine locations of potash recovery from the Quaternary sources, almost all pre-Quaternary potash operations extract product from marine-fed basinwide ore hosts during times of MgSO4-depleted and MgSO4-enriched oceans (Warren, 2016; Chapter 11). This time-based dichotomy in potash ore sources with nonmarine hosts in the Quaternary deposits and marine evaporite hosted ore zones in Miocene deposits and older, reflects a simple lack of basinwide marine deposits and appropriate marine chemistry across the Neogene (Warren, 2010). As for all ancient marine evaporites, the depositional system that deposited ancient marine-fed potash deposits was one to two orders of magnitude larger and the resultant deposits were typically thicker stacks than any Quaternary potash settings. The last such “saline giant” potash system was the Solfifera series in the Sicilian basin, deposited as part of the Mediterranean “salinity crisis,” but these potential ore beds are of the less economically attractive MgSO4- enriched marine potash series.

So, what are the factors that favour the formation of, and hence exploration for, additional deposits of exploitable ancient potash? First, large MOP solid-state ore sources are all basinwide, not lacustrine deposits. Within the basinwide association, it seems that intracratonic basins host significantly larger reserves of ore, compared to systems that formed in the more tectonically-active plate-edge rift and suture association. This is a reflection of: 1) accessibility – near the shallow current edge of a salt basin, 2) a lack of a halokinetic overprint and, 3) the setup of longterm, stable, edge-dissolution brine hydrologies that typify many intracratonic basins. Known reserves of potash in the Devonian Prairie evaporite in West Canadian Sedimentary Basin (WCSB) are of the order of 50 times that of next largest known deposit, the Permian of the Upper Kama basin, and more than two orders of magnitude larger than any other of the other known exploited deposits (Table 1).

Part of this difference in the volume of recoverable reserves lies in the fact that the various Canadian potash members in the WCSB are still bedded and flat-lying. Beds have not been broken up or steepened, by any subsequent halokinesis. The only set of processes overprinting and remobilising the various potash salts in the WCSB are related to multiple styles and timings of aquifer encroachment on the potash units, and this probably took place at various times since the potash was first deposited, driven mainly by a combination of hinterland uplift and subrosion. In contrast, most of the other significant potash basins listed in Table 10 have been subjected to ongoing combinations of halokinesis and groundwater encroachments, making these beds much less laterally predictable. In their formative stages, the WCSB potash beds were located a substantial distance from the orogenic belt that drove flexural downwarp and creation of the subsealevel sag depression. Like many other intracratonic basins, the WCSB did not experience significant syndepositional compression or rift-related loading.

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Brine evolution and origins of potash ore salts: Primary or secondary? Part 1 of 3

John Warren - Wednesday, October 31, 2018

Introduction

There is a dichotomy in mineralogical associations and precipitation series in both modern and ancient potash ore deposits. Interpretations of ancient potash ore mineralogies across time are generally tied to the evolution of the hydrochemical proportions in modern and ancient oceans. We have already discussed this in previous Salty Matters articles and will not repeat the details here (see August 10, 2015; July 31, 2018).

At times in the past, such as in the Devonian and the Cretaceous, the world ocean was depleted in Mg and SO4 relative to the present-day ocean (Figure 1a). In the relevant literature, this has led to the application of the term MgSO4-depleted versus MgSO4-enriched oceans. In terms of brine evolution, this is related to the gypsum divide, with the term MgSO4-enriched used to describe the ocean chemistry of today and other times in the past, such as in the Permian, when MgSO4 bittern salts typify co-precipitates with sylvite/carnallite (Figure 1b).


The validity of the ocean chemistry argument is primarily based on determinations of inclusion chemistries as measured in chevron halites (Figure 1a; Lowenstein et al., 2014). Inclusions in growth-aligned primary halite chevrons are assumed to preserve the chemical proportions in the ambient oceanic brine precipitating the halite. That is, the working assumption is that pristine aligned-halite chevrons have not been subject to significant diagenetic alteration once the salt was deposited and permeability was lost due to ongoing halite cementation in the shallow (eogenetic) subsurface realm.

The same assumption as to the pristine nature of chevron halite is applied to outcomes of biological experiments where Permian archaeal/halobacterial life has been re-animated using ancient salt samples (Vreeland et al., 2000).

Primary potash ore?

But does the same assumption of pristine texturing across time also apply to the halite layers associated with the world’s potash ores? In my experience of subsurface potash ores and their textures, I have rarely seen primary-chevron halite interlayered with potash ore layers of either sylvite or carnallite. An obvious exception is the pristine interlayering of chevron halite and sylvite in the now-depleted Eocene potash ores of the Mulhouse Basin, France (Lowenstein and Spencer, 1990). There, the sylvite layers intercalate at the cm-scale with chevron halite, and the alternating layering is thought to be related to precipitation driven by temperature fluctuations in a series of shallow density-stratified meromictic brine lakes (documented in the third Salty Matters article).

More typically, ancient potash ore textures are diagenetic and indicate responses to varying degrees of dissolution, brine infiltration and alteration. The simpler styles of brine infiltration consist of a background matrix dominated by cm-dm scale chevron halite layers that have been subject to dissolution and karstification during shallow burial. Resultant cm-dm scale voids typically retain a mm-thick selvedge of CaSO4 lathes and needles, followed by fill of the remaining void by varying amounts of sparry halite, carnallite and sylvite. This type of texture dominates Quaternary stratoid potash layers in the southern Qaidam Basin in China and Cretaceous carnallite-rich layers in the Maha Sarakham Fm in NE Thailand and southern China (Warren, 2016). Then there are the even more altered and recrystallised, but still bedded, textures in the potash ore zones of Devonian Prairie Evaporite of western Canada (Wardlaw, 1968) and potash layers in the Permian Basin in west Texas and New Mexico (Lowenstein, 1988; Holt and Powers, 2011). Beyond this level of diagenetic texturing are the flow-orientated and foliated structural textures of the Permian potash ores in potash mines in the diapiric Zechstein evaporites of Germany and Poland, the Kungurian diapirs of the Cis-Urals of Russia and the Devonian diapirs of the Pripyat Basin.

And so, herein lies the main point of discussion for this and the next two Salty Matters articles, namely, what, where and when is(are) the mechanism(s) or association(s) of hydrochemical mechanisms that sufficiently concentrate or alter a brine’s chemistry to where it precipitates economic levels of a variety of potash salts, as either muriate of potash or sulphate of potash. Notably, there are no Quaternary-age solid-state ore systems that are mined for potash.

In this article, we look at the main modern brine systems where muriate of potash (MOP) is produced economically by solar evaporation (Salar de Atacama, Chile; Qarhan sump, China; and the southern Basin of the Dead Sea). In the second article we will focus on sulphate of potash (SOP) production in Quaternary saline sumps (Great Salt Lake, USA and Lop Nur, China). In the third article we shall discuss depositional and diagenetic characteristics of solid-state potash ores some of the world’s more substantial deposits (e.g. Devonian of western Canada) and relate the observations of ancient potash texture to time-based evolution of potash precipitating brines, and subsequent alteration or the ore textures, which are typically driven by later cross-flushing by one or more pulses of diagenetically-evolved brines.


Potash from brine in Salar de Atacama (MOP in a simple near-uniserial set of brine concentration pans)

Potash production in Salar de Atacama is a byproduct of the output of lithium carbonate from shallow lake brines pumped into a series of solar concentration pans (Figure 2). The inflow feed to the concentrator pans comes from fields of brine wells extracting pore waters from the salt nucleus facies across the central and southern part of the Atacama saltflats (Figure 3a,b). However, Atacama pore brines are not chemically homogeneous across the salar (Alonso and Risacher, 1996; Risacher and Alonso, 1996; Carmona et al., 2000; Pueyo et al. 2017). The most common primary inflow brines to the Atacama sump are sulphate-rich (SO4/Ca > 1), but there are areas in the salt flat at the southern end of the playa, such as those near the Península de Chépica, where pore brines are richer in calcium (SO4/Ca < 1- Figure 4). These brines also contain elevated levels of lithium (Figure 3c; Risacher et al., 2003).


Ion proportions in the natural salar inflows and pore waters are dominated by sodium and chloride, followed by potassium, then magnesium then sulphate in the more saline regions of the salar sump (Figure 4; Lowenstein and Risacher, 2009). In addition, owing to the progressive reduction of porosity with depth, driven mainly by diagenetic halite cementation, the pore brine in the upper 40 meters of the salar sediment column accumulates by advection in the area of greatest porosity, i.e., in this top 40 m of sediments of the salt flat at the southern end of Atacama (Pueyo et al., 2017). When pumped from the hosting salar sediments into the concentrator pans, the final brines contain elevated levels of lithium chloride (≈ 6000 ppm). These lithium-enriched acidic waters are then pumped to a nearby industrial plant and processed to obtain lithium carbonate as the main commercial product.

In a benchmark paper, Pueyo et al. (2017) document the brine evolution and products recovered in the solar pans of Rockwood Lithium GmbH (Figure 2b; formerly Sociedad Chilena del Litio) in the Península de Chépica. There, a bittern paragenesis of salts precipitates that is mostly devoid of magnesium sulphate salt due to the low levels of sulphate attained in the various concentator pans via widespread precipitation of gypsum in the early concentrator pans (Figures 4, 5).


The depletion of sulphate levels in the early concentrators is done via artificial manipulation of ionic proportions in the feeders. Without alteration of the ionic proportion in halite-stage brines, the evaporation of the saltflat brine feeds, which are rich in sulfate, would result in assemblages that, in addition to potassic chlorides, would contain contain problematic magnesium sulfates (such as schoenite, kainite, glaserite as in Great Salt Lake). The presence of such sulphate salts and ions in the liquor feeding the lithium carbonate plant would complicate the lithium carbonate extraction process. So the aim in the Atacama pans is to remove most of the sulphate via constructing a suitably balanced chemistry in the early concentrator brine stage (compare ionic proportion in sulphate between early and end-stage bitterns, as illustrated in Figures 4 and Figure 5a).


Such a sylvite/carnallite brine paragenesis, sans sulphate (as seen in Figure 5), is similar to that envisaged as the feed chemistry for ancient Mg-sulphate-free marine potash deposits (Braitsch, 1971). That is, as the brines pass through the concentrators, with successive pans transitioning to higher salinities, the potash salts carnallite and sylvite precipitate, without the complication of the widespread magnesium sulphate salts, which complicate the processing of modern marine-derived bitterns. Such MgSO4 double-salts typify SOP production in the Ogden Salt flats, with their primary feed of sulphate-rich Great Salt Lake waters. Relative proportions of sulphate are much higher in the Great Salt Lake brine feed (see article 2 in this series).


Once the balance is accomplished by mixing a Ca-rich brine from further up the concentration series, with the natural SO4-brine in an appropriate ratio, the modified brines are then pumped and discharged into the halite ponds of the saltwork circuit (ponds number 17 and 16, as seen in Figures 5 and 6). In these ponds, halite precipitates from the very beginning with small amounts of accessory gypsum as brines are saturated with both minerals. Subsequently, the brines are transferred to increasingly smaller ponds where halite (ponds 15 and 14), halite and sylvite (pond 13), sylvite (ponds 12, 11 and 10), sylvite and carnallite (pond 9), carnallite (pond 8), carnallite and bischofite (pond 7), bischofite (ponds 6, 5 and 4), bischofite with some lithium-carnallite [LiClMgCl26H2O] (pond 3), and lithium-carnallite (ponds 2 and 1) precipitate. The brines of the last ponds (R-1 to R-3), whose volumes undergo a reduction to 1/50th of the starting volume, are treated at the processing factory to obtain lithium carbonate as the main commercial product.

As documented in Pueyo et al. (2017), the average daily temperature in the Salar de Atacama ranges between 22 °C in February and 8 °C in July, with a maximum oscillation of approximately 14 °C. Wind speed ranges daily from< 2 ms−1 in the morning to 15 ms−1 in the afternoon. Rainfall in the area of the salt flat corresponds to that of a hyperarid desert climate with an annual average, for the period 1988–2011, of 28 mm at San Pedro de Atacama, 15.1 mm at Peine and 11.6 mm at the lithium saltworks, in the last case ranging between 0 and 86 mm for individual years. The adjoining Altiplano to the east has an arid climate with an average annual rainfall of approximately 100 mm. The average relative humidity in the saltpan area, for the period 2006–2011, is 19.8% with a maximum around February (27%) and a minimum in October (15%) and with a peak in the morning when it may reach 50%. The low relative humidity and the high insolation (direct radiation of 3000 kWh m−2 yr−1) in the salt flat increase the efficiency of solar evaporation, giving rise to the precipitation and stability of very deliquescent minerals such as carnallite and bischofite. The average annual evaporation value measured in the period 1998–2011, using the salt flat interstitial brine, is approximately 2250 mm with a peak in December–January and a minimum in June–July. This cool high-altitude hyperarid climatic setting, where widespread sylvite and carnallite accumulates on the pan floor, is tectonically and climatically distinct from the hot-arid subsealevel basinwide desert seep settings envisaged for ancient marine-fed potash basins (as discussed in the upcoming third article in this series).

MOP from brine Dabuxum/Qarhan region, Qaidam Basin, China

The Qarhan saltflat/playa is now the largest hypersaline sump within the disaggregated lacustrine system that makes up the hydrology of Qaidam Basin, China (Figure 7a). The Qaidam basin sump has an area of some 6,000 km2, is mostly underlain by bedded Late Quaternary halite. Regionally, the depression is endorheic, fed by the Golmud, Qarhan and Urtom (Wutumeiren) rivers in the south and the Sugan River in the north, and today is mostly covered by a layered halite pan crust. Below, some 0 to 1.3m beneath the playa surface, is the watertable atop a permanent hypersaline groundwater brine lens (Figure 7b).


The southern Qaidam sump entrains nine perennial salt lakes: Seni, Dabiele, Xiaobiele, Daxi, Dabuxum (Dabsan Hu), Tuanjie, Xiezuo and Fubuxum north and south lakeshore (Figure 7). Dabuxum Lake, which occupies the central part of the Qarhan sump region, is the largest of the perennial lakes (184 km2; Figures 7b, 8a). Lake water depths vary seasonally from 20cm to 1m and never deeper than a metre, even when flooded. Salt contents in the various lakes range from 165 to 360 g/l, with pH ranging between 5.4 and 7.85. Today the salt plain and pans of the Qarhan playa are fed mostly by runoff from the Kunlun Mountains (Kunlun Shan), along with input from a number of saline groundwater springs concentrated along a fault trend defining an area of salt karst along the northern edge of the Dabuxum sump, especially north of Xiezuo Lake (Figure 8a).


The present climate across the Qaidam Basin is cool, arid to hyperarid (BWk), with an average yearly rainfall of 26 mm, mean annual evaporation is 3000–3200 mm, and a yearly mean temperature 2-4° C in the central basin (An et al., 2012). The various salt lakes and playas spread across the basin and contain alternating climate-dependent evaporitic sedimentary sequences. Across the basin the playa sumps are surrounded by aeolian deposits and wind-eroded landforms (yardangs). In terms of potash occurrence, the most significant region in the Qaidam Basin is the Qarhan sump or playa (aka Chaerhan Salt Lake), which occupies a landscape low in front of the outlets of the Golmud and Qarhan rivers (Figure 7a, b). Overall the Qaidam Basin displays a typical exposed lacustrine geomorphology and desert landscape, related to increasing aridification in a cool desert setting. In contrast, the surrounding elevated highlands are mostly typified by a high-alpine tundra (ET) Köppen climate.


Bedded and displacive salts began to accumulate in the Qarhan depression some 50,000 years ago (Figure 9). Today, outcropping areas of surface salt crust consist of a chaotic mixture of fine-grained halite crystals and mud, with a rugged, pitted upper surface (Schubel and Lowenstein, 1997; Duan and Hu, 2001). Vadose diagenetic features, such as dissolution pits, cavities and pendant cements, form wherever the salt crust lies above the watertable. Interbedded salts and siliciclastic sediments underlying the crust reach thicknesses of upwards of 70m (Kezao and Bowler, 1986).

Bedded potash, as carnallite, precipitates naturally in transient volumetrically-minor lake strandzone (stratoid) beds about the northeastern margin of Lake Dabuxum (Figure 8a) and as cements in Late Pleistocene bedded deposits exposed in and below nearby Lake Tuanje in what is known as the sediments of the Dadong ancient lake (Figure 8b). Ongoing freshened sheetflow from the up-dip bajada fans means the proportion of carnallite versus halite in the evaporite unit increases with distance from the Golmud Fan across, both the layered (bedded) and stratoid (cement) modes of occurrence.

At times in the past, when the watertable was lower, occasional meteoric inflow was also the driver for the brine cycling that created the karst cavities hosting the halite and carnallite cements that formed as prograde cements during cooling of the sinking brine (Figure 9). Solid bedded potash salts are not present in sufficient amounts to be quarried, and most of the exploited potash resource resides in interstitial brines that are pumped and processed using solar ponds.

Modern halite crusts in Qarhan playa contain the most concentrated brine inclusions of the sampled Quaternary halites, suggesting that today may be the most desiccated period in the Qarhan-Tuanje sump recorded over the last 50,000 years (values in the inset in figure 9 were measured on clear halite-spar void-fill crystals between chevrons). Inclusion measurements from these very early diagenetic halite show they formed syndepositionally from shallow groundwater brines and confirm the climatic record derived from adjacent primary (chevron) halite. The occurrence of carnallite-saturated brines in fluid inclusions in the diagenetic halite in the top 13 m of Qarhan playa sediments also imply a prograde diagenetic, not depositional, origin of carnallite, which locally accumulated in the same voids as the more widespread microkarst halite-spar cements.

Today, transient surficial primary carnallite rafts can accumulate along the northern strandline of Lake Dabuxum (Figure 9; Casas, 1992; Casas et al., 1992). Compositions of fluid inclusions in the older primary (chevron) halite beds hosting carnallite cements in the various Qarhan salt crusts represent preserved lake brines and indicate relatively wetter conditions throughout most of the Late Pleistocene (Yang et al., 1995). Oxygen isotope signatures of the inclusions record episodic freshening and concentration during the formation of the various salt units interlayered with lacustrine muds. Desiccation events, sufficient to allow halite beds to accumulate, occurred a number of times in the Late Quaternary: 1) in a short-lived event ≈ 50,000 ka, 2) from about 17 - 8,000 ka, and 3) from about 2,000 ka till now (Figure 9).

The greatest volume of water entering Dabuxum Lake comes from the Golmud River (Figure 7b). Cold springs, emerging from a narrow karst zone some 10 km to the north of the Dabuxum strandline and extending hundreds of km across the basin, also supply solutes to the lake. The spring water discharging along this fault-defined karst zone is chemically similar to hydrothermal CaCl2 basin-sourced waters as defined by Hardie (1990), and are interpreted as subsurface brines that have risen to the surface along deep faults to the north of the Dabuxum sump (Figure 9, 10; Spencer et al., 1990; Lowenstein and Risacher, 2009). Depths from where the Ca–Cl spring waters rise is not known. Subsurface lithologies of the Qaidam Basin in this region contains Jurassic and younger sediments and sedimentary rock columns, up to 15 km thick, which overlie Proterozoic metamorphic rocks (Wang and Coward, 1990).


Several lakes located near the northern karst zone (Donglin, North Huobusun, Xiezhuo, and Huobusun) receive sufficient Ca–Cl inflow, more than 1 part spring inflow to 40 parts river inflow, to form mixtures with chemistries of Ca equivalents > equivalents HCO3 + SO4 to create a simple potash evaporation series (this is indicated by the Ca-Cl trend line in Figure 10a). With evaporation such waters, after precipitation of calcite and gypsum, evolve into Ca–Cl-rich, HCO3–SO4-poor brines (brines numbered 5, 7-12 in figure 11a).


Dabuxum is the largest lake in the Qarhan region, with brines that are Na–Mg–K–Cl dominant, with minor Ca and SO4 (Figure 10d, 11a). These brines are interpreted by Lowenstein and Risacher (2009) to have formed from a mix of ≈40 parts river water to 1 part spring inflow, so that the equivalents of Ca ≈ equivalents HCO3 + SO4 (Figure 10b). Brines with this ratio of river to spring inflow lose most of their Ca, SO4, and HCO3 after precipitation of CaCO3 and CaSO4, and so form Na–K–Mg–Cl brines capable of precipitating carnallite and sylvite (Figure 11a). This chemistry is similar to that of ancient MgSO4-depleted marine bitterns (Figure 1)

The chemical composition of surface brines in the various lakes on the Qarhan Salt plain vary and appear to be controlled by the particular blend of river and spring inflows into the local lake/playa sump. In turn, this mix is controlled geographically by proximity to river mouths and the northern karst zone. Formation of marine-like ionic proportions in some lakes, such as Tuanje, Dabuxum and ancient Dadong Lake, engender bitterns suitable for the primary and secondary precipitation of sylvite/carnallite (Figures 10b-d; 11a). The variation in the relative proportion of sulphate to chloride in the feeder brines is a fundamental control on the suitability of the brine as a potash producer.

Figure 11b clearly illustrates sulphate to chloride variation in pore waters in the region to the immediate north and east of Dabuxum Lake. Brine wells in the low-sulphate area are drawn upon to supply feeder brines to the carnallite precipitating ponds. The hydrochemistry of this region is a clear indication of the regional variation in the sump hydrochemistry (Duan and Hu, 2001), but also underlines why it is so important to understand pore chemistry, and variations in aquifer porosity and permeability, when designing a potash plant in a Quaternary saline setting.

Compared to the MOP plant in Atacama, there as yet no lithium carbonate extraction stream to help ameliorate costs associated with carnallite processing. Lithium levels in the Qaidam brines, whilee levated, are much lower than in the Atacama brine feeds. Regionally, away from the Tuanje-Dadong area, most salt-lake and pore brines in the Qaidam flats are of the magnesium sulphate subtype and the ratio of Mg/Li can be as high as 500. With such brine compositions, the chemical precipitation approach, which is successfully applied to lithium extraction using low calcium and magnesium brines (such as those from Zabuye and Jezecaka Lake on the Tibetan Plateau and in the Andean Altiplano), would consume a large quantity of chemicals and generate a huge amount of solid waste. Accordingly, brine operations in the Qarhan region are focused on MOP production from a carnallitite slurry using extraction techniques similar to those utilised in the Southern Dead Sea. But owing to its cooler climate compared to the Dead Sea sump, the pond chemistry is subject to lower evaporation rates, higher moisture levels in the product, and a longer curing time.

Potash in the Qarhan region is produced by the Qinghai Salt Lake Potash Company, which owns the 120-square-kilometer salt lake area near Golmud (Figure 7). The company was established and listed on the Shenzhen Stock Exchange in 1997. Currently, it specialises in the manufacture of MOP from pore brines pumped from appropriate low-sulphate regions in the lake sediments (Figure 12). The MOP factory processes a carnallite slurry pumped from pans using a slurry processing stream very similar to the dual process stream utilized in the pans of the Southern Basin in the Dead Sea and discussed in the next section.

The final potash product in the Qaidam sump runs 60-62% K2O with >2% moisture and is distributed under the brand name of “Yanqiao.” With annual production ≈3.5 million tonnes and a projected reserve ≈ 540 million tonnes, the company currently generates 97% of Chinese domestic MOP production. However, China’s annual agricultural need for potash far outpaces this level of production. The company is jointly owned by Qinghai Salt Lake Industry Group and Sinochem Corporation and is the only domestic producer of a natural MOP product.

Dead Sea Potash (MOP operation in the Southern Basin)

The Dead Sea water surface defines what is the deepest continental position (-417 m asl) on the earth’s current terrestrial surface. In the Northen Basin is our only modern example of bedded evaporitic sediments (halite and gypsum) accumulating on the subaqueous floor of a deep brine body, where water depths are hundreds of metres (Warren, 2016). This salt-encrusted depression is 80 km long and 20 km wide, has an area of 810 km2, is covered by a brine volume of 147 km3 and occupies the lowest part of a drainage basin with a catchment area of 40,650 km2 (Figure 13a). However, falling water levels in the past few decades mean the permanent water mass now only occupies the northern part of the lake, while saline anthropogenic potash pans occupy the Southern Basin, so that the current perennial “Sea” resides in the Northern Basin is now only some 50 km long (Figure 13b).


Rainfall in the region is 45 to 90 mm, evaporation around 1500 mm, and air temperatures between 11 and 21°C in winter and 18 to 40°C in summer, with a recorded maximum of 51°C. The subsiding basin is surrounded by mountain ranges to the east and west, producing an orographic rain shadow that further emphasises the aridity of the adjacent desert sump. The primary source of solutes in the perennial lake is ongoing dissolution of the halokinetic salts of the Miocene Sedom Fm (aka Usdum Fm) a marine evaporite unit that underlies the Dead Sea and approaches the surface in diapiric structures beneath the Lisan Straits and at Mt. Sedom (Garfunkel and Ben-Avraham, 1996).

A series of linked fractionation ponds have been built in the Southern Basin of the Dead Sea to further concentrate pumped Dead Sea brine to the carnallite stage (Figure 13). On the Israeli side this is done by the Dead Sea Works Ltd. (owned by ICL Fertilisers), near Mt. Sedom, and by the Arab Potash Company (APC) at Ghor al Safi on the Jordanian side. ICL is 52.3% owned by Israel Corporation Ltd.(considered as under Government control), 13.6% shares held by Potash Corporation of Saskatchewan and 33.6% shares held by various institutional investors and the general public (33.64%). In contrast, PotashCorp owns 28% of APC shares, the Government of Jordan 27%, Arab Mining Company 20%, with the remainder held by several small Middle Eastern governments and a public float that trades on the Amman Stock Exchange. This gives PotashCorp control on how APC product is marketed, but it does not control how DSW product is sold.

In both the DSW and APC brine fields, muriate of potash is extracted by processing carnallitite slurries, created by sequential evaporation in a series of linked, gravity-fed fractionation ponds. The inflow brine currently pumped from the Dead Sea has a density of ≈1.24 gm/cc, while after slurry extraction the residual brine, with a density of ≈1.34 gm/cc, is pumped back into the northern Dead Sea basin water mass. The total area of the concentration pans is more than 250 km2, within the total area of 1,000 km2, which is the southern Dead Sea floor. The first stage in the evaporation process is pumping of Dead Sea water into header ponds and into the gravity-fed series of artificial fractionation pans that now cover the Southern Basin floor. With the ongoing fall of the Dead Sea water level over the past 60 years, brines from the Northern Basin must be pumped higher and over further lateral distances. This results in an ongoing need for more powerful brine pumps and an increasing problem with karst dolines related to lowered Dead Sea water levels. Saturation stages of the evolving pan brines are monitored and waters are moved from pan to pan as they are subject to the ongoing and intense levels of natural solar evaporation (Figure 13b, c; Karcz and Zak, 1987).

The artificial salt ponds of the Dead Sea are unusual in that they are designed to trap and discard most of the halite precipitate rather than harvest it. Most other artificial salt ponds around the world are shallow pans purpose-designed as ephemeral water-holding depressions that periodically dry out so that salts can be scrapped and harvested. In contrast, the Dead Sea halite ponds are purpose-designed to be permanently subaqueous and relatively deep (≈4m). Brine levels in the ponds vary by a few decimetres during the year, and lowstand levels generally increase each winter when waste brine is pumped back into the northern basin.

As the Dead Sea brine thickens, minor gypsum, then voluminous halite precipitates on the pan floor in the upstream section of the concentration series, where the halite-precipitating-brines have densities > 1.2 gm/cc (Figure 13c). As the concentrating brines approach carnallite-precipitating densities (around 1.3 gm/cc), they are allowed to flow into the carnallite precipitating ponds (Figure 13c). Individual pans have areas around 6-8 km2 and brine depths up to 2 metres. During the early halite concentration stages, a series of problematic halite reefs or mushroom polygons can build to the brine surface and so compartmentalise and entrap brines within isolated pockets enclosed by the reefs. This hinders the orderly downstream progression of increasingly saline brines into the carnallite ponds, with the associated loss of potash product.


When the plant was first designed, the expectation was that halite would accumulate on the floor of the early fractionation ponds as flat beds and crusts, beneath permanent holomictic brine layers. The expected volume of salt was deposited in the pans each year (Talbot et al., 1996), but instead of accumulating on a flat floor aggrading 15-20 cm each year, halite in some areas aggraded into a series of polygonally-linked at-surface salt reefs (aka salt mushrooms). Then, instead of each brine lake/pan being homogenized by wind shear across a single large subaqueous ponds, the salt reefs separated the larger early ponds into thousands of smaller polygonally-defined inaccessible compartments, where the isolated brines developed different compositions (Figure 14). Carnallitite slurries crystallised in inter-reef compartments from where it could not be easily harvested, so large volumes of potential potash product were locked up in the early fractionation ponds (Figure 14a, b). Attempts to drown the reefs by maintaining freshened waters in the ponds during the winters of 1984 and 1985 were only partly successful. The current approach to the salt reef problem in the early fractionation ponds is to periodically breakup and remove the halite reefs and mushrooms by a combination of dredging and occasional blasting (Figure 14c).


Unlike seawater feeds to conventional marine coastal saltworks producing halite with marine inflow salinities ≈35‰, the inflow brine pumped into the header ponds from the Dead Sea already has a salinity of more than 300‰ (Figure 15). Massive halite precipitation occurs quickly, once the brine attains a density of 1.235 (≈340‰) and reaches a maximum at a density of 1.24 (Figure 13c). Evaporation is allowed to continue in the initial halite concentrator ponds until the original water volume pumped into the pond has been halved. Concentrated halite-depleted brine is then pumped through a conveyance canal into a series of smaller evaporation ponds where carnallite, along with minor halite and gypsum precipitates (Figure 13c). Around 300–400 mm of carnallite salt slurry is allowed to accumulate in the carnallite ponds, with 84% pure carnallite and 16% sodium chloride as the average chemical composition (Figure 6a; Abu-Hamatteh and Al-Amr, 2008). The carnallite bed is harvested (pumped) from beneath the brine in slurry form and is delivered through corrosion-resistant steel pipes to the process refineries via a series of powerful pumps.

This carnallitite slurry is harvested using purpose-specific dredges floating across the crystalliser ponds. These dredges not only pump the slurry to the processing plant but also undertake the early part of the processing stream. On the dredge, the harvested slurry is crushed and size sorted, with the coarser purer crystals separated for cold crystallisation. The remainder is slurried with the residual pan brine and then further filtered aboard the floating dredges. At this stage in the processing stream the dredges pipe the treated slurries from the pans to the refining plant.


On arrival at the processing plant, raw product is then used to manufacture muriate of potash, salt, magnesium chloride, magnesium oxide, hydrochloric acid, bath salts, chlorine, caustic soda and magnesium metal (Figure 16a). Residual brine after carnallitite precipitation contain about 11-12 g/l bromide and is used for the production of bromine, before the waste brine (with a density around 1.34 gm/cc) is returned to the northern Dead Sea water mass. The entire cycle from the slurry harvesting to MOP production takes as little as five hours.

In the initial years of both DSW and APC operations, MOP was refined from the carnallite slurry via hot leaching and flotation. In the coarser-crystalline carnallitite feed, significant volumes of sylvite are now produced more economically in a cold crystallisation plant (Figure 16b). The cold crystallisation process takes place at ambient temperature and is less energy-intensive than the hot crystallisation unit. The method also consumes less water but requires a higher and more consistent grade of carnallite feed (Mansour and Takrouri, 2007; Abu-Hamatteh and Al-Amr, 2008). Both hot (thermal) and cold production methods can be utilized in either plant, depending on the quality of the slurry feed.

Sylvite is produced via cold crystallisation using the addition of water to incongruently dissolve the magnesium chloride from the crystal structure. If the carnallite slurry contains only a small amount of halite, the solid residue that remains after water flushing is mostly sylvite. As is shown in Figure 16b, if the MgCl2 concentration is at or near the triple-saturation point (the point at which the solution is saturated with carnallite, NaCl, and KCl), the KCl solubility is suppressed to the point where most of it will precipitate as sylvite. For maximum recovery, the crystallising mixture must be saturated with carnallite at its triple-saturation point. If the mixture is not saturated, for example, it contains higher levels of NaCl, then more KCl will dissolve during the water flushing of the slurry. Industrially, the cold crystallizers are usually fed with both coarse and fine carnallite streams, such that 10% carnallite remains in the slurry, this can be achieved by adjusting addition of process water (Mansour and Takrouri, 2007).

Successful cold crystallisation depends largely on a consistent high-quality carnallite feed. If a large amount of halite is present in the feed slurry, the resulting solid residue from cold crystallisation is sylvinite, not sylvite. This needs to be further refined by hot crystallisation, a more expensive extraction method based on the fact that the solubility of sylvite varies significantly with increasing temperature, while that of salt remains relatively constant (Figure 16c). As potash brine is hot leached from the sylvinite, the remaining halite is filtered off, and the brine is cooled under controlled conditions to yield sylvite.

Residual brine from the crystallisation processes then undergoes electrolysis to yield chlorine, caustic soda (sodium hydroxide) and hydrogen. Chlorine is then reacted with brine filtered from the pans to produce bromine. The caustic soda is sold, and the hydrogen is used to make bromine compounds, with the excess being burnt as fuel. Bromine distilled from the brine is sold partly as elemental bromine, and partly in the form of bromine compounds produced in the bromine plant at Ramat Hovav (near Beer Sheva). This is the largest bromine plant in the world, and Israel is the main exporter of bromine to Europe. About 200,000 tons of bromine are produced each year.

Residual magnesium chloride-rich solutions created by cold crystallisation are concentrated and sold as flakes for use in the chemical industry and for de-icing (about 100,000 tons per year) and dirt road de-dusting. Part of the MgCl2 solution produced is sold to the nearby Dead Sea Periclase plant (a subsidiary of Israel Chemicals Ltd.). At this plant, the brine is decomposed thermally to give an extremely pure magnesium oxide (periclase) and hydrochloric acid. In Israel, Dead Sea Salt Work’s (DSW) production has risen to more than 2.9 Mt KCl since 2005, continuing a series of increments and reflecting an investment in expanded capacity, the streamlining of product throughput in the mill facilities, and the amelioration of the effects salt mushrooms, and increased salinity of the Dead Sea due to extended drought conditions (Figure 17).

On the other side of the truce line in Jordan, the Arab Potash Co. Ltd. (APC) output rose to 1.94 Mt KCl in 2010 The APC plant now has the capacity to produce 2.35 Mt KCl and like the DSW produces bromine from bittern end brines. Early in the pond concentration stream, APC also has to remove salt mushrooms from its ponds, a process which when completed can increase carnallite output by over 50,000 t/yr. Currently, APC is continuing with an expansion program aimed at increasing potash capacity to 2.5 Mt/yr.


MOP brines and Quaternary climate

As mentioned in the introduction, exploited Quaternary potash deposits encompass both MOP and SOP mineral associations across a range of climatic and elevation settings. This article focuses on the three main MOP producing examples, the next deals with SOP Quaternary producers (Great Salt Lake, USA and Lop Nur, China). Interestingly, both sets of Quaternary examples are nonmarine brine-fed depositional hydrologies. All currently-active economic potash plants hosted in Quaternary systems do not mine a solid product but derive their potash solar evaporation of pumped hypersaline lake brines. For MOP processing to be economic the sulphate levels in the brines held in bittern-stage concentrator pans must be low and Mg levels are typically high, so favoring the precipitation of carnallite over sylvite in all three systems.

In Salar de Atacama the low sulphate levels in the bittern stage is accomplished by artificially mixing a CaCl2 brine from further up the evaporation stream with a less saline more sulphate-enriched brine. The mixing proportions of the two brine streams aims to maximise the level of extraction/removal of CaSO4 in the halite pans prior to the precipitation of sylvite and carnallite. In the case of the pans in the Qarhan sump there is a similar but largely natural mixing of river waters with fault-fed salt-karst spring waters in a ratio of 40:1 that creates a hybrid pore brine with a low sulphate chemistry suitable for the precipitation of both natural and pan carnallite. In the case of the Dead Sea brine feed, the inflowing Dead Sea waters are naturally low in sulphate and high in magnesium. The large size of this natural brine feed systems and its homogeneous nature allows for a moderate cost of MOP manufacture estimated in Warren 2016, chapter 11 to be US$ 170/tonne. The Qarhan production cost is less ≈ US$ 110/tonne but the total reserve is less than in the brine system of the Dead Sea. In Salar de Atacama region the MOP cost is likely around US$ 250-270/tonne, but this is offset by the production of a bischofite stage brine suitable for lithium carbonate extraction.

Outside of these three main Quaternary-feed MOP producers there are a number of potash mineral occurrences in intermontane depressions in the high Andes in what is a high altitude polar tundra setting (Koeppen ET), none of which are commercial (Figure 18a). Similarly, there a number of non-commercial potash (SOP) mineral and brine occurrences in various hot arid desert regions in Australia, northern Africa and the Middle East (Koeppen BWh) that we shall look at in the next article. In the Danakhil depression there is the possibility of a future combined MOP/SOP plant (see Salty Matters April 19, 2015; April 29, 2015; May 1, 2015; May12, 2015 and Bastow et al., 2018). In the Danakhil it is important to distinguish between the current non-potash climate (BWh - Koeppen climate) over the Dallol saltflat in Ethiopia, with its nonmarine brine feed and the former now-buried marine fed potash (SOP)/halite evaporite system. The latter is the target of current exploration efforts in the basin, focused on sediments now buried 60-120m below the Dallol saltflat surface. Nowhere in the Quaternary are such dry arid desert climates (BWh) associated with commercial accumulations of potash minerals.


Climatically most commercial potash brine systems in Quaternary-age sediments are located in cooler endorheic intermontane depressions (BWk, BSk) or in the case of the Dead Sea an intermontane position in the sump of the Dead Sea, the deepest position of any continental landscape on the earth’s surface (-417 msl). The association with somewhat cooler and or less arid steppe climates implies a need for greater volumes of brine to reside in a landscape in order to facilitate the precipitation of significant volumes of potash bitterns (Figure 18a,b).

In summary, all three currently economic Quaternary MOP operations are producing by pumping nonmarine pore or saline lake brines into a series of concentrator pans. The final bittern chemistry in all three is a low-sulphate liquour, but with inherently high levels of magnesium that favors the solar pan production of carnallite over sylvite that is then processed to produce the final KCl product. The brine chemistry in all three examples imitates the ionic proportions obtained when evaporating a ancient sulphate-depleted seawater (Figure 1). The next article will discuss the complexities (the double salt problem at the potash bittern stage when concentrating a more sulphate-enriched mother brine.

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Calcium Chloride (CaCl2), Article 2 of 2: CaCl2 minerals in evaporites

John Warren - Wednesday, May 31, 2017

 

Introduction

Found in association with some highly-saline calcium chloride brines are four naturally occurring CaCl2 minerals with evaporite associations, namely; 1) tachyhydrite CaMg2Cl6.12H2O), 2) antarcticite (CaCl2.6H2O), 3) sinjarite (CaCl2.2H2O) and 4) chlorocalcite (KCaCl3). For any CaCl2 salt to precipitate in a saline setting requires a Ca-enriched mother brine completely depleted in sulphate (Table 1). Otherwise, gypsum precipitates and in so doing removes all calcium from the concentrating brine, well before the bittern stage.

 

Of the four, tachyhydrite is perhaps the most common solid phase, sinjarite or chlorocalcite the least. Bischofite is sometimes a co-precipitate with tachyhydrite and indicative of a mother brine with an elevated MgCl2 content (Figure 1). Even so, tachyhydrite is still a rare bittern mineral phase across the Phanerozoic of Earth, although as we shall see later, the situation may be a little different on the martian surface. Along with carnallite and bischofite, tachyhydrite typifies highly saline marine bittern assemblages in only a few ancient potash-rich evaporite systems. These same systems are also MgSO4 poor, and tend to define times in Earth history of MgSO4-depleted seas. Occurrences include: Silurian potash in Michigan Basin; Carboniferous potash in the Canadian Maritimes and the Paradox Basin; Permian Zechstein-2 Stassfurt potash seams in Stassfurt, Germany; Triassic Saharan potash of North Africa; Cretaceous potash evaporites in Brazil, Gabon and Thailand and Oligocene potash in the Rhine Graben. Tachyhydrite-bearing successions are much less common within a broader MgSO4-poor group of potash deposits (see Warren 2016, chapters 2 and 11; Warren, 2017).


To form antarcticite (CaCl2).6H2O) at the bittern stage, a CaCl2 brine must be depleted in Mg, otherwise tachyhydrite forms, along with carnallite and bischofite. Carnallite, bischofite and tachyhydrite are not part of the bittern suite precipitated from today’s MgSO4-rich oceans, where sulphate and magnesium levels are too high relative to calcium. At times in the past, these three salts were more common precipitates in the bittern stages of ancient CaCl2 or MgSO4-depleted oceans. These tended to be at times when rates of seafloor spreading, and hence volumes of through-circulated seawater, were greater than today (Lowenstein et al., 2014). It seems that most the highly saline calcium chloride brines form in sedimentary basins with a thick evaporite near the base of the sediment fill (see article August 11, 2015). But, not all are basins with highly saline CaCl2 brines are co-associated with bedded tachyhydrite occurrences in the buried and dissolving evaporite units.

Antarcticite

Antarcticite (CaCl2.6H2O) is extremely hygroscopic, colourless with perfect basal cleavage and good to perfect prismatic cleavage, is brittle with a hardness between 2 and 3, and has a specific gravity of 1.715 ± 0.010 (14 °C), and a density of 1.700 g/cm3 (Dunning and Cooper, 1969). The lowest temperature for antarcticite to precipitate is -50 °C and it’s melting point is 30 °C.

There are only two known modern hypersaline saline deposits where the pure calcium chloride mineral antarcticite (CaCl2.6H2O) is present as thin crystal beds and efflorescences; they are, Bristol Dry Lake (BWh) in California and Don Juan Pond (EF) in the Wright Valley, near Lake Vanda in Antarctica (Dunning and Cooper, 1969; Horita, 2009). Both saline lakes possess similar CaCl2-enriched, MgCl2-depleted chemistries and are fed in part by hydrothermal waters. In Bristol Dry Lake the concentration process is driven by solar evaporation, while cryogenesis is the primary driver in Don Juan Pond. Only in Bristol Dry Lake is a CaCl2 brine is commercially extracted from Quaternary sediments. There is also a minor occurrence as isolated small crystals in the Kunteyi Lake of the Tibetan Plateau (Kezao and Bowler, 1986; Zheng and Liu, 2009)


Don Juan Pond, Antarctica

Don Juan Pond is a perennial free-standing water body, approximately 300 m long and 100 m wide, surrounded by glacial moraine deposits in the Wright Valley, Antarctica (Figure 2). It defines the outcrop portion of a liquid water drainage sump that retains a 10-cm-deep CaCl2 brine at the surface (Torri and Ossaka, 1965; Marion, 1997; Burt et al., 2003) (Figure 2). This pool contains some of the saltiest free-standing perennial waters on the earth’s surface, with concentrations ranging up to 40% (by mass; Dickson et al., 2013). Mean annual precipitation the Don Juan Pond region is 5 to 10 cm and air temperatures range from –55°C to +10°C (Marion, 1997). The pond does not freeze because it is a eutectic brine of H2O, CaCl2, and NaCl, with a freezing point of -52°C and a density of approximately 1.4 (Marion, 1997; Burt et al., 2002). Crystals of antarcticite were first found sticking out of the brine at the bottom of the pond, with halite and gypsum fringes at the edges of the pond (Torri and Ossaka, 1965).

Precipitation of antarcticite in this cryogenic environment is the result of the arid climate, ongoing cold temperatures and a CaCl2 brine chemistry (Torri and Ossaka, 1965). The CaCl2 brine is in part derived from springs fed by a deep, confined, basaltic aquifer, with a groundwater similar in composition to the surface brine (Harris et al., 1979; Marion, 1997; Burt et al., 2002). The origin of the CaCl2 is still contentious, but is most likely be related to mineral weathering and periodic deliquescence of calcium chloride held in the sediments(Dickson et al., 2013).


Brine in the Don Juan Pond is predominantly from CaCl2 solutions (Figure 3; Marion, 1997). Only Na of the remaining cations contributes significantly to the overall charge balance. Both Mg and K are present at very low concentrations, and SO4 is usually not detectable. In contrast to low ionic variability in seawater composition, levels of the dominant ions (Ca, Na, and Cl) in the Pond are highly variable (Figure 3). The concentration of the brine varies seasonal, controlled mainly by a complex hydrologic system including groundwater discharge, melting of nearby permafrost, rare precipitation, and evaporation or sublimation (Harris et al., 1979; Dickson et al., 2013). Despite the changes in the absolute concentration of Ca and Cl across an annual cycle, the Ca/Cl molal ratio remained relatively constant over the 23-year sampling period; it varies between 0.432 and 0.506 with an average of 0.462 ± 0.018 (Figure 2; Marion, 1997).

Bristol Dry Lake, California

Bristol Dry Lake is a 155 km2 continental playa located within a closed Basin and Range valley in the Mojave Desert of southern California (Figure 4a). It is the second known location of natural antarcticite and is thought to precipitate from lake brines via a lowering of brine temperatures (Dunning and Cooper, 1969). No antarcticite was identified in the nearby Cadiz or Danby dry lakes. Lake brines first concentrate by solar evaporation beyond halite precipitation Mountains surrounding Bristol Dry Lake are composed mostly of pre-Cambrian and Mesozoic granite, some Palaeozoic carbonates, and Tertiary and Pleistocene volcanic rocks including Quaternary olivine basalts that erupted from nearby Amboy Crater. Alluvial fans are present between the mountains and the lowest parts of the valley (Dunning and Cooper, 1969; Handford, 1982).


Bristol Dry Lake playa contains more than 300 m of interbedded halite and salt-bearing sands, silts and clay. Halite and gypsum are currently deposited from ephemeral bodies of water under evaporative conditions, with a gypsum pavement surrounding more saline halite-saturated sumps in the central lake depression (Figure 4a; Dunning and Cooper, 1969; Handford, 1982; Rosen and Warren, 1990). The primary evaporite minerals at Bristol Dry Lake are gypsum, anhydrite, and halite with rare antarcticite, celestite (SrSO4), calcite, and possibly chlorocalcite (KCaCl3). Bristol Dry Lake is mined for its chloride minerals and has been since the 1900s, and so is covered with pits and trenches (Figure 4b).

Antarcticite was identified in one of these previously excavated trenches approximately 9 kilometres south of Amboy, near the road from Amboy to Twenty-nine Palms. Dunning and Cooper (1969) examined this and other trenches during the winters of 1961, 1962, and 1968. Only one trench had antarcticite. The trench was 3 m deep, 20 m long, and contained a 10 to 25 cm deep pool of CaCl2 brine extending along the length of the trench. At the base of the pit was a massive halite deposit, with acicular groups of antarcticite protruding outward from the halite and within the brine. Antarcticite is not present on the playa surface as no surface brines are present. The only CaCl2 brines exposed to solar radiation are subsurface brines that have seeped into, and are now exposed within, the excavated trenches.


Dunning and Cooper (1969) suggested that CaCl2 brines at Bristol Dry Lake originated from chlorine of volcanic origin interacting with calcium carbonate in surrounding alluvial deposits (Figure 5). Lowenstein et al. (2009) argue that the nearby active magma chamber drives the formation of Ca-Cl brines at elevated temperatures, as well as the thermally-driven transportation of these brines to the surface. Other evidence for the magma chamber is the Amboy crater and its associated recent lava flows, which occur directly North of Bristol Lake. Evaporative concentration of this calcium chloride brine, driven by an arid climate and typical surface temperatures below 30°C, encourage antarcticite precipitation (Dunning and Cooper, 1969).

Sinjarite (CaCl2.2H2O)

Sinjarite is a modern ephemeral precipitate in surficial alluvial fan sediments near Sinjar in Iraq (Figure 6). No natural occurrences of either antarcticite or sinjarite have been documented in ancient salts, except as a volumetrically insignificant mineral phase in brine inclusions in both sediments and igneous rocks (Polozov et al., 2016), even in sedimentary basins containing highly saline CaCl2-rich pore fluids.


Sinjarite is an extremely rare pink-tetragonal-hygroscopic mineral that is highly soluble with 117 g dissolving in 100 g of water at 20 °C (Table 1; Aljubouri and Aldabbagh, 1980). The lowest temperature for sinjarite to precipitate is 42.3 °C, while its melting point is 175.5 °C (Wardlaw, 1972). Theoretical composition of sinjarite is 27.26% Ca, 48.24% Cl, and 24.50% H2O but the actual chemistry for sinjarite present at its type locality in Iraq is 25.84% Ca, 46.64% Cl, 26.55% H2O, and 0.85% Na with 226 ppm K and trace amounts of Mg, Sr, and Fe (Aljubouri and Aldabbagh, 1980). Properties include; good cleavage, average density of 1.81 g/cm3, a very deliquescent habit, hardness of 1.5, vitreous to resinous lustre, white streak, and occurs naturally in massive to granular forms.

Little research has been performed on sinjarite and knowledge of the environment of formation comes from the only place it was discovered. Sinjarite occurs in detritus in a wadi cutting through the Sinjar anticline near the town of Sinjar, Iraq (west of Mosul). Sinjarite precipitates via slow evaporation of groundwater saturated with Ca and Cl ions. CaCl2 in the groundwater solution must be 3.5 times greater than NaCl for sinjarite to precipitate instead of halite. The extremely deliquescent nature of sinjarite means that the mineral is ephemeral and quickly dissolves in wet seasons or changes to the hexahydrite antarcticite when the temperature is less than 30°C (Aljubouri and Aldabbagh, 1980).


CaCl2 salts and liquid water flows on current-day Mars

On the current Martian surface it has been recently proposed that aqueous solutions form seasonally, via the deliquescence of hygroscopic salts in contact with atmospheric water vapor. Regions of these hygroscopic salts are thought to be indicated by occurrences of surface features known as recurring slope linae - RSL (Figure 7a, b; Chevrier and Rivera-Valentin, 2012). Older studies largely focused on perchlorate species as the most likely set of hydroscopic salts driving the periodic formation of RSLs, but another Mars-relevant set of salts, with similar low eutectic temperatures, relevant to ambient conditions with a significant deliquescence potential, are the calcium chloride hydrates, antarcticite and sinjarite. Gough et al. (2016) propose hydrated calcium chloride salts are linked to RSL formation on Mars. They also note deliquescence of CaCl2, not perchlorate, is also known to be responsible for the only terrestrial RSL analog known thus far: these are the seasonal water tracks in the McMurdo Dry Valleys, especially in the vicinity of the Don Juan Pond, as documented by Dickson et al., 2013 (Figure 8).

Soluble regolith salts like perchlorate and calcium chloride salts with low eutectic temperatures are likely to deliquesce at low relative humidity (RH) values at a wide range of temperatures (Gough et al., 2016). Deliquescence is the process by which a solid crystalline phase absorbs water vapor to form a saturated aqueous (liquid) solution. This phase transition from solid salt to liquid brine occurs at a deliquescence relative humidity (DRH), the value of which is specific for each hydration state of each salt and often varies with temperature. A salt is expected to be aqueous whenever the temperature is above the eutectic temperature of the salt and the relative humidity is above the DRH, although at much higher RH values ice may form. The low temperature deliquescence of perchlorates has been extensively studied (Gough et al., 2011, 2014; Nuding et al., 2014; Zorzano et al., 2009). Many perchlorate species have deliquescence relative humidity (DRH) values below 40% RH, and so should deliquesce and be stable or metastable liquids under Martian surface conditions (Chevrier et al., 2009), although slow reaction kinetics may limit their formation (Fischer et al., 2014). Not as much attention has been paid to other soluble, deliquescent salts, especially the calcium chloride salts, that may be present in the martian regolith.

Chlorine has been detected by rovers and landers in every Martian soil sample analyzed to date and is found at similar concentrations (≈0.2 to 1%) in all locations (Glavin et al., 2013). The form of the chlorine (i.e. chloride vs. perchlorate) is unknown at some sites, although measurements of the regolith at the Phoenix landing site confirmed chloride, perchlorate (Hecht et al., 2009) and probably chlorate (Hanley et al., 2012). The identity of the associated cation(s) is also generally unknown, although calcium, sodium, magnesium and potassium are the most likely candidates (Hecht et al., 2009). Regionally and globally widespread chloride deposits have also been detected by orbiting spectrometers (Figure 7c; Osterloo et al., 2010; Keller et al., 2006).

The global distribution of chloride deposits across the Martian surface is similar to that of recurring slope lineae (RSL), prompting hypotheses of a relationship between the two (Figure 7c; McEwen et al., 2011; Stillman et al., 2017). Chevrier and Rivera-Valentin (2012) suggest that CaCl2 is one of the best candidates for the formation of RSL, the narrow, dark features that appear and grow seasonally on Mars and appear to be caused by flowing liquid.


Two hydrated chloride salts, MgCl2 and CaCl2, may be the most appropriate salts because the seasonality of observed RSL formation best matches the seasonality of the melting of ice associated with these salts (Chevrier and Rivera-Valentin, 2012). Specifically, these chloride salts have eutectic temperatures less than or equal to the threshold temperature of 250 K that seems to mark the start of RSL activity (McEwen et al., 2011), but the eutectic temperatures of these salts are not so low that brines on Mars would be permanently liquid. Additionally, calculations show that CaCl2 in particular will cause seasonal melting of water ice throughout the top 20 cm of the martian regolith, therefore providing greater potential fluid flow than other salts (Chevrier and Rivera-Valentin, 2012). More recently, hydrated chloride salts (although not specifically calcium chloride) were observed to be present in RSL but not in surrounding regions (Ojha et al., 2015). It is not generally believed that deliquescence could be fully responsible for RSL formation because the small amount of water vapor in the martian atmosphere would limit the condensed phase water that could form. It is certainly possible, however, that salt deliquescence may be involved in the formation of RSL or in their appearance or behavior (McEwen et al., 2011; Ojha et al., 2015; Dickson et al., 2013; Stillman et al., 2017).

In summary, determining whether liquid water exists on the Martian surface is central to understanding the hydrologic cycle and potential for extant life on Mars (Ohja etal., 2015; Stillman et al., 2017). Recurring slope lineae, narrow streaks of low reflectance compared to the surrounding terrain, appear and grow incrementally in the downslope direction during warm seasons when temperatures reach about 250–300 K (-23°C to +26°C), a pattern consistent with the transient flow of a volatile material. Brine flows (or seeps) associated with seasonal deliquesence of hydrated salts (possibly hydrated calcium chloride salts or perchlorates) are proposed to explain the formation of recurring slope linea. As yet, no direct evidence for either liquid water or actual hydrated salt mineralogies has been found.


Tachyhydrite

Tachyhydrite (CaMg2Cl6.12H2O) (occasionally spelled tachydrite or tachhydrite) is a yellow, transparent to translucent, trigonal-rhombohedral mineral with very high solubility. Pure tachyhydrite is composed of 7.74% Ca, 9.39% Mg, 41.10% Cl, 37.09% O, and 4.67% H (Wardlaw, 1972). Its chemical formula is most accurately expressed as [Mg(H2O)6]2[CaCl6] because the structure consists of Mg(H2O)6 octahedra and CaCl6 octahedra loosely linked together by hydrogen bonds. Physical properties include; good cleavage, vitreous to greasy lustre, white streak, massive habit, bitter taste, deliquescent habit (dissolves or liquefies upon exposure to air), a density of 1.66 g/cm3, a hardness of 2, and it is hygroscopic (Braitsch, 1971).

Tachyhydrite is extremely soluble with 160 g dissolving in 100 g of water at 20 °C, and its solubility increases with increasing temperature (D’Ans, 1961; Wardlaw, 1972). Its hygroscopic and extremely deliquescent nature means at earth surface temperatures a crystal becomes liquid by absorbing moisture in the air. On exposure, tachyhydrite quickly alters to a residue of bischofite (MgCl2.6H2O) and a CaCl2.nH2O phase (Wardlaw, 1972). This is why tachyhydrite is typically documented in natural occurences where a crystal is still encased in halite. Tachyhydrite has a wide thermal stability field from 21.95°C to at least 167°C (Braitsch, 1971; Clark et al., 1980). The lowest temperature for tachyhydrite precipitation is 21.95°C at a concentration of 92.7 mol CaCl2/1000 mol H2O (450 g CaCl2/liter (D’Ans, 1961; Braitsch, 1971; Wardlaw, 1972). Its lowest temperature of formation increases by 0.0162ºC for every atmosphere of pressure (D’Ans, 1961).

Modern tachyhydrite occurrences

Natural tachyhydrite is documented only in a few modern hypersaline settings and it never forms a bed with primary precipitation textures (Figure 9). It is found in greater quantities in some ancient potash deposits. Tachyhydrite occurs seasonally as a minor interstitial cements and efflorescences, along with antarcticite (the hexahydrate form of calcium chloride), in the modern ephemeral halite crusts, atop sabkhas, of the Gavkhoni Playa (BWk), southeast of Isfahan, Iran (Pakzad and Ajalloeian, 2004), in mine wates in the Salar de Pedenales in the andean Altiplano, and perhaps as minor salt effloresecnes in the uppermost parts of the Abu Dhabi sabkha (Wood et al., 2005).

Much of the elevated ionic content of various Iranian playas in the Great Kavir is due to salt dissolution of the crests of nearby at- or near-surface diapirs and namakiers, where the mother salt sourced in halokinetic Miocene marine salts (Warren, 2008). These outcropping diapirs have carried carnallite and sylvite remnants into the namakiers that now reside at the surface (Rahimpour-Bonab and Kalantarzadeh, 2005). Tachyhydrite is also found as white feathery efflorescences within waste piles at an abandoned borate working along the southwestern margin of Salar de Pedernales, Chile (a BWk Koeppen climate location very near the ET climate boundary; Ericksen et al., 1989).

It seems that tachyhydrite’s high solubility means it has little or no preservation potential as a solid salt in any modern at-surface depositional setting; in the Peruvian case, it is a winter precipitate that disappears with the end of winter. In Abu Dhabi it, along with other highly soluble salts in the salty surface efflorescences, can disappear in morning dew (Warren, pers. obs.). In both the Iranian and the Peruvian settings the climate is cool and elevated. It seems the arid desert environments where bedded subaqueous-textured tachyhydrite accumulated in the drawndown Aptian seepage basins of the opening Atlantic were very different to its occasional rare occurrence in the evaporite settings of today.

Cretaceous of Brazil & W. Africa

Lower Cretaceous (Aptian) evaporite deposits in Sergipe, Brazil, and Gabon and Congo in western Africa contain significant amounts of tachyhydrite in halite-carnallite beds, along with other SO4-poor bitterns (Figure 10; Wardlaw, 1972; Borchert, 1977; de Ruiter, 1979; Hardie, 1990; Garrett, 1995; Zhang et al., 2017). These evaporite basins formed during the early Cretaceous rifting of Africa and South America. In each basin, potash-rich halite-dominant evaporites are transitional between older continental pre-rift and rift siliciclastics and younger post-rift marine shales and carbonates (Wardlaw, 1972, Borchert, 1977; de Ruiter, 1979; Szatmari et al., 1979; Hardie, 1990; Garrett, 1996).


The basic salt cycle of the Gabon and Congo basins includes from bottom to top: (1) thin black shale, (2) halite, (3) combination of halite and carnallite (carnallitite, and (4) bischofite and/or tachyhydrite (Figure 10;de Ruiter, 1979; Zhang et al., 2017). Although variations exist, a similar sequence is present in Sergipe, Brazil comprising the Ibura Member of the Muribeca Formation. The Ibura Member includes from bottom to top: (1) halite, (2) carnallite, (3) tachyhydrite, (4) sylvite (KCl) and halite, and (5) anhydrite (CaSO4) (Wardlaw, 1972; Borchert, 1977; Garrett, 1996). Tachyhydrite is located within the central and deepest portions of the Sergipe basins (Wardlaw, 1972; Borchert, 1977; Szatmari et al., 1979).

The origin of the evaporite sequences in Sergipe, Gabon and Congo is controversial with some authors suggesting a marine origin (Wardlaw, 1972; de Ruiter, 1979; Szatmari et al., 1979) or diagenetic origin (Borchert, 1977). More recent literature discussing Sergipe, Gabon, Congo, and other similar tachyhydrite deposits cite a non-marine/hydrothermal origin based on geochemical and textural studies (Lowenstein et al., 1989; Hardie, 1990; Garrett, 1996; El Tabakh et al., 1999). Yet other recent authors indicate a marine source based on its dominance in beds with a CaCl2 oceanic timing (Warren, 2016; Zhang et al., 2017).

The brine that tachyhydrite precipitated from must have been high in Ca because in waters with more SO4 than Ca present, the Ca is used-up by gypsum or anhydrite. Only if the Ca level is greater than SO4 in the early stages of precipitation will Ca stay in the brine allowing for potential precipitation of tachyhydrite (Wardlaw, 1972; Hardie 1990). The CaCl2-rich brine that produced the tachyhydrite and other potash salts is thought to have formed either by the interaction of hot meteoric groundwater with rift-related sediment and bedrock, or an influx of seawater at a time of a MgSO4-depleted ocean. Based on figure 1, Hardie (1990) suggests heated seawater could also have interacted with sediment and/or bedrock to produce a CaCl2 brine. This hot CaCl2 brine was perhaps driven to the surface by thermal circulation (Hardie, 1990). Alternatively, without arguing that the proportions of major ions in seawater change over time, the fact that modern marine water cannot precipitate tachyhydrite precludes it being the dominant mother brine in the Sergipe, Congo, and Gabon Basin evaporite basins,  (Lowenstein et al., 2014; Warren  2016).

Textures recovered in core, and visible in mine walls in Brazil, indicate Sergipe tachyhydrite was a primary bedded precipitate in those circum-Atlantic Cretaceous evaporite basins with stratiform tachyhydrite units in potash beds (Figure 10; Wardlaw, 1972). The Cretaceous world ocean at the time this tachyhydrite precipitated was a MgSO4-free marine bittern. Figure 9 shows tachyhydrite does not occur as a bittern phases in many other potash sequences deposited in basinwide settings at other times of CaCl2 oceans (e.g. the Devonian of Canada, the Silurian of the Michigan Basin). This has led to the argument that stratabound tachyhydrite in potash beds indicates an additional hydrothermal/basinal source for the mother brine. If so, the presence of tachyhydrite is not solely indicative of a MgSO4-depleted marine feed to the basin (Hardie 1990).

Cretaceous occurrences in Thailand

Another significant tachyhydrite occurrence is within slightly older mid Cretaceous (Cenomanian) evaporites of the Maha Sarakham Formation on the Khorat Plateau of northeastern Thailand (Figure 11). The majority of the Maha Sarakham Formation consists of alternating anhydrite and halite, separated by mudstone and siltstone “redbeds”. A 10- to 75-m-thick salt unit overlies a lower halite unit and underlies mudstone red beds of the lower clastic unit. Carnallite is most abundant in this salt unit, but sylvite and halite are also locally abundant. Tachyhydrite is present with halite and carnallite, but not with sylvite and is perhaps concentrated more in former basin centre or sump positions (El Tabakh et al., 1999; Warren 2016). Hite and Japakasetr (1979) estimated that tachyhydrite comprises less than 30% of the total carnallitite deposit, but tachyhydrite can form pure layers up to 16 m thick. Tachyhydrite crystals are euhedral and average 1 mm in size (El Tabakh et al., 1999). Sylvite and halite are also associated with accessory amounts of hilgardite [Ca2BCl(OH)2] and boracite (Mg3ClB7O13).


The origin of the Maha Sarakham Formation evaporites is still contentious; Hite and Japakasetr (1979) advocate a purely marine origin (highly restricted and concentrated sea), Hardie (1990) advocating an exclusively non-marine origin (hydrothermal CaCl2 brines), and El Tabakh et al. (1999) advocate an alternating marine and non-marine origin. They envisage an inland basin that was periodically inundated by the rising sea and at the time of tachyhydrite deposition received an additional influx of hydrothermal CaCl2 brines. Hardie (1990) states that formation of Maha Sarakham evaporites may have been similar to that of tachyhydrite formation in the Cretaceous basins of Brazil and western Africa, although he offers no evidence for the origin of the hydrothermal CaCl2 brines. El Tabakh et al. (1999) suggested that hydrothermal CaCl2 waters entered the restricted marine basin and created the right conditions for tachyhydrite precipitation and they cite contemporaneous granitic intrusions as possible evidence of thermal activity during the time of tachyhydrite formation.

Hydrothermal oceanic CaCl2 brine

Today, large volumes of relatively dilute, oceanic CaCl2 brines form via hydrothermal circulation and thermally-driven alteration of mid-ocean ridge (MOR) lithosphere (see part 1). These CaCl2 waters occur in and near active fracture zones, wherever seawater interacts with labile basalt (oceanic crust) at elevated temperatures and converts the circulating fluid from a Na-Mg-Cl water into a low-salinity Na-Ca-Cl brine, depleted in Mg and SO4. Similar hydrothermally-driven alteration of continental basalts via deeply circulated seawater interactions forms modern CaCl2-rich brine seeps, for example, within the thermally active continental Danakil rift valley (Hardie, 1990).

Tachyhydrite is a 'Swift Water" mineral

There may be an additional factor at work in terms of our recognition of tachyhydrite’s basin-by-basin distribution across the Phanerozoic. That factor is tachyhydrite’s high solubility in most subsurface waters and drilling muds. Even the name, tachyhydrite, was chosen from the Greek tachy hydros for “swift water” to indicate its rapid deliquescence on exposure to atmospheric conditions. This property means that any ground-up remains (rock chips) of tachyhydrite in a return mudstream in a conventional drilling operation typically do not make it to the surface. So, as most oil companies prefer not to core evaporites when drilling for hydrocarbons trapped beneath salt, there may be more tachyhydrite occurrences in deeply buried basin evaporites than the few currently documented.

Ancient tachyhydrite has only been found encased in halite in recovered cores and mine walls, never in drill cuttings, and its presence or absence can be easily misinterpreted in wireline logs, especially when drilling through thick salt masses in the search for oil and gas (companies tend to run a more limited log suite in thick salt units).

 

Tachyhydrite’s lack of shear strength and extremely high solubility makes mining any associated potash beds difficult; it presence leads to head beam and floor stability problems in conventional underground mines in Brazil and West Africa. Holle, a West African potash mine in the 1970s, was never economical and was lost to flooding after a few years of conventional mining and the Brazilian mine has ongoing floor and wall stability problems ties to tachyhydrite-induced heave. Today, solution mining is proposed as a more efficient means to exploit areas of Cretaceous potash associated with tachyhydrite beds, as in Aptian halite-potash associations on both sides of the Atlantic. High levels of tropical humidity mean at-surface potash evaporation pans are not viable in either area, so brine concentration and processing will probably require a more expensive option involving motorised dryers.

 

Not only is tachyhydrite highly soluble, but it is also highly reactive whenever in comes into contact with subsurface bitterns. Based on its occurrence in the Permian Stassfurt series in Germany, D’Ans (1961) suggested that much of the widespread polyhalite found in the CaCl2 brine-rich parts of the Zechstein Basin was a reaction product indicating former tachyhydrite. He noted that, at temperatures higher than room, there is a reaction between gypsum/anhydrite, sylvite and tachyhydrite-bischofite, or their dissolution-related brines, which removes tachyhydrite to form diagenetic polyhalite and calcium chloride brines;

CaMg2Cl6.12H2O+4KCl+8CaSO4--> 2CaSO4Mg(SO4)4(H2O)2 + 5CaCl2 + 4H2O

This was the mechanism suggested by Manheim (1974) to explain the lack of tachyhydrite, and the formation CaCl2-rich brines that characterise hydrothermal pore waters circulating in sediments associated with buried Miocene evaporites along the deep axial trough of the Red Sea.

Terrestrial CaCl2 minerals across time

The high solubility of CaCl2 minerals means that, with the exception of some parts of the Aptian Atlantic Potash association, even in past times of likely CaCl2-rich oceans, calcium chloride minerals are relatively rare as primary-textured occurrences in solid salt beds. On the other hand, as we saw in part 1, CaCl2 brines are commonplace in basinal or formation waters in many Phanerozoic sedimentary basins entraining thick sequences of dissolving ancient salt. For example, since 1914, CaCl2 brines have been extracted from Silurian strata adjacent to Salina Salt of the Michigan Basin USA, yet there are no significant volumes of tachyhydrite documented in the Michigan Basin(Garrett, 2004).

Historically, before micro-inclusion studies of chevron halite showed that the ionic proportions of seawater likely varied across the Phanerozoic, the various CaCl2 basinal brines occurrences in Silurian and Cretaceous strata were explained as an indicator of widespread dolomitisation and other diagenetic reactions, which preferentially extracted magnesium from pore waters. Since then, some authors have argued that CaCl2 enrichments in many ancient basinal brines, including brines in the Detroit group are partial leftovers of primary seawater chemistries (Lowenstein and Timofeeff, 2008). That is, Ca-enriched (MgSO4-depleted) pore brines are indicators of ancient CaCl2 oceans, with the pore brines being remnants from time the enclosing evaporitic and marine sediments were deposited (relict or connate brines).

Others, such as Houston et al. (2011), conclude this is not necessarily so, they agree that there are two end-members typifying highly saline subsurface brines. But they conclude that end-member chemistries relate to either substantial subsurface halite dissolution, or to preservation of early reflux-related seawater. Houston et al. (2011) go on to argue that CaCl2-enriched formation water chemistries from many basins worldwide, including the Michigan Basin, do not support an interpretation of variation in ionic proportions in seawater across the Phanerozoic. They find that CaCl2-rich brines formed either by dissolving bittern salts in the subsurface, or simply lost water in the subsurface after significant rock-fluid interaction had taken place. Water loss might be achieved by interaction with a gas phase at the elevated temperatures of deep burial or, alternatively, water may have been lost to clays. Both these mechanisms would have the effect of dehydrating (concentrating) the brine.

However, whatever the mechanism, it does seem that there is a Quaternary continental/pre-Quaternary marine dichotomy in the nature and distribution of CaCl2 brines and the associated antarcticite/tachyhydrite minerals, much as there is for the world’s potash deposits (MgSO4-enriched versus MgSO4-depleted). On Earth, there are no documented examples of pre-Quaternary bedded antarcticite. The situation may be different on Mars.

Today terrestrial antarcticite either precipitates in the hot-arid Bristol Dry Lake or the icecap-polar setting of Don Juan Pond, neither is associated with brine sourced in seawater or derived by the recycling of older (“connate”) marine evaporite successions. Rather the occurrenceof the calcium chloride minerals is in part a reponse to low temperatures and regolith cycling. Other present-day examples of regions with somewhat enriched levels of CaCl2 surface waters, lack both tachyhydrite and antarcticite precipitates, and contain higher levels of Mg that are tied to deeply circulated marine/hydrothermal waters and variable interactions with MgSO4-enriched marine salts (e.g., Lake Asal and the Dallol Salt Pan). CaCl2 brines of the Dead Sea and the Red Sea show even more elevated levels of MgCl2 as they are derived, at least in part by the fractionated dissolution of bedded and halokinetic Miocene marine evaporites. The Mg-rich clay palygorskite is co-associated with these CaCl2 brines in the subsurface of the Red Sea and the Dallol.

In contrast, tachyhydrite in some ancient marine-fed basinwide evaporite associations is found as somewhat rare, but at times bedded, units in the bittern-rich portions of the halite succession, as in the Cretaceous basinwide evaporites of Brazil, the Congo and Thailand, and as a cementing phase in the Permian Stassfurt 2 in the Zechstein (where it is commercially extracted in association with MgCl2 brines).

All the ancient tachyhydrite examples mentioned above, are associated with the presence of widespread potash salts within adjacent salt beds. However, there are many other even larger and richer ancient potash deposits, such as the intracratonic Alberta basin (Devonian) and the Kama basins (Permian) where no tachyhydrite is documented. It seems that the terrestrial precipitation of bedded tachyhydrite is not just favoured by times of CaCl2 oceans, it also requires additional input from saline hydrothermal/basinal waters. Such settings are most likely in the transition phases in an actively-opening hydrothermally-influenced continental rift as passes into the marine seepage realm at a time when the adjacent ocean was a MgSO4-depleted system.

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Salt, Oil, Gas & Metals: What Drives the Link?

John Warren - Friday, March 31, 2017

Introduction

This article is based on a review written for CSPG Reservoirs and presented orally at the AAPG 2016 meeting in Calgary. It endeavours to give an up-to-date synopsis of how and why ancient salty basins tend to contain elevated levels of oil, gas and metals. It begins with an overview of hydrologies, then places ancient salt bodies in their climatic and tectonic context, and lastly looks at industrial associations and predictors. For brevity, details of many regions, deposits and references are summarised as tables, more complete discussion and referencing can be found in appropriate sections in Warren (2016), or feel free to contact me (jkwarren@saltworkconsultants.com or via www.saltworkconsultants.com) with requests for more comprehensive documentation of particular examples.


Evaporite styles reflect intrabasin brine hydrology

If we accept the definition of an evaporite as “A salt rock originally precipitated from a saturated surface or near-surface brine in hydrologies driven by solar evaporation,” then the greater volume of saline mineral salts in the earth’s sedimentary realm are the product of solar heating of brine. There are other sets of mineral salts in the depositional and diagenetic realm with the same mineral composition as evaporite salts, but these salts result from cryogenic, hydrothermal or burial re-equilibration processes (Warren, 2016). For a water molecule to escape into the vapour phase in an evaporitic setting, and so increase the salinity of an enduring brine body or pore water in the capillary zone of a sabkha, the water molecule must; 1) absorb heat energy, 2) be located near the liquid surface, 3) be moving in the proper direction, 4) have sufficient energy to overcome liquid-phase intermolecular forces and, 5) pass through the surface tension inter-face (Figure 1).


Simple physics of molecular escape (rate and intensity) during solar evaporation essentially controls the potency of re-maining brine. Bed textures and mineralogies entomb evidence of concentration levels in the brine and its hydrological setting/stability (Figures 2, 3). This makes evaporites excellent gauges of climate present and past. In combination, relative brine density and specific heat capacity of adjacent brine masses control density and thermal stratification in saline brine bodies located at or near the earth’s surface (Figure 2). Subsequent burial alteration controls later textural evolution by interactions with regional shallow and deep phreatic crossflows (Warren, 2016).

Specific heat is the amount of heat needed to raise one gramme of a substance by 1 °C. For a given amount of heat input, a unit volume of hypersaline water will show a greater increase in temperature than a less salty water (Figure 2c). Given the same degree of insolation, this means density-stratified water bodies tend to be heliothermic, with the lower, denser bottom brine layer being warmer than the somewhat less saline, less dense, upper water layer (Figure 2a). The level in any water column where a marked change in temperature occurs is the thermocline, and in a density-stratified brine mass corresponds to the halocline, also termed a chemocline (Figure 3a). The combination of high temperature, high salinity and lower oxygen levels in the lower brine mass in a heliothermal brine system means only a specialised biota can survive there, often with specialised bacterial populations living in waters just above a saline thermocline. A modern example is the purple sulphur-oxidising community flourishing immediately above the halocline in Lake Mahoney, British Columbia. Fluctuating salinity and nutrient levels endemic to many evaporite depositing regions encourage preservation of elevated levels of organic matter in a variety of hypersaline settings past and present (“feast and famine” associations; Warren, 2011).

As any brine concentrates, its density increases (Figure 2b). The overlying water body must be holomictic for bottom-nucleated salts to accumulate across the subaqueous floor of a brine mass and for dense, saturated brine to sink (reflux) into underlying sediments (Figure 3a). Holomixis means a near homogenous distribution of brine density, temperature, and salinity throughout the brine mass, with internal mixing being ongoing and mostly maintained by wind movement. In contrast, a meromictic brine body is internally stratified, with a lower more-saline, denser, warmer water mass separated across a halocline from an upper, less saline, less-dense, cooler water mass. A longterm halocline hinders chemical or physical changes in the underlying denser waters, so shutting down bottom nucleation, as well as slowing and ultimately stopping brine reflux. A permanently stratified system is ectogenic, while a brine column that is temporarily stratified is endogenic.

Over decades, saline water masses can change from ectogenic to endogenic. In February 1979, salinity equalisation drove the mixing of upper and lower water masses in the Dead Sea, resulting in a holomictic water body. Since then, aside from short episodes of storm-flood-driven freshening of the upper water mass in 1980 and 1994 (meromixis), the Dead Sea has been holomictic, and halite has been accumulating on the deep lake floor (Gertman and Hecht, 2002). Before 1979, the Dead Sea had been a stratified system for at least 400 years and only pelagic carbonate laminites with minor gypsum, not halite, accumulated on the deep lake floor, beneath a 370-380 m deep brine column.


Holomixis permits deposition of a coherent salt layer across the whole basin floor, beneath both shallow and deep brine columns. Density stratification allows evaporitic salts to crystallise only in the upper water mass or at the upper brine - lower brine interface, so bottom nucleation tends to occur on the shallower lake floor, where it lies above the halocline (Figure 3a). That is, long-term (ectogenic) column stratification mean bottom nucleation of salts can only occur where the upper salt-saturated brine mass intersects the sediment bottom, with a pelagic settling of salts occurring deeper out in the depositional basin, as in the Dead Sea prior to February 1979 (Figure 3a). The bottom growth of crystals cannot occur on a deep bottom located beneath a density-stratified system, as there is no mechanism to drive ongoing supersaturation in the lower water mass. For the same reason, constant brine reflux driving sinking of a dense brine into sediments beneath the floor of the evaporite basin can only proceed if significant regions of the overlying brine mass are holomictic. Deposition of capillary salts (sabkha deposits) occurs in subaerial settings, wherever the saline capillary zone inter-sects the land surface (Figure 3b).

When salts are accumulating beneath a holomictic brine mass, textures in bottom nucleates is controlled by the stability of the overlying brine column (Figure 3b). When the overlying column is deep (>30-100m) then, other than areas on the deep bottom of local phreatic spring-fed outflows, there is no general hydrochemical mechanism to drive fluctuations in bottom-brine chemistry. The resulting deep bottom precipitates tend to be monomineralogic crystal clusters, possibly encased by re-transported material washed in from the shallower surrounds (Figure 3a, 3b). In contrast, when the overlying brine column is shallow (<30m and typically <5-10m) then the chemistry and stability of the brine varies on a shorter term (daily-weekly) basis, so more layered bi-mineralogic bottom-nucleates can accumulate as layered to laminated salt beds. In addition, all evaporite sediments can be reworked by bottom currents, with similar textures to those that characterise siliciclastic and mechanically-modified carbonate sediments (Figure 3b).

"Now" versus "then" in evaporite deposition

Uniformitarianism is an essential tenet of geological understanding. Yet, when we look at evaporite volumes and depositional settings across deep time, we see that the diversity of modern evaporite analogues is constrained by a deficit in two conditions, specifically; 1) the current lack of greenhouse eustasy; contemporaneous atmospheric conditions and sea levels are controlled by the earth’s current icehouse climate mode and have been for the last 10-12 million years, and 2) the current lack at the plate-edge scale of marine seepage into large hydrographically-isolated oceanic sump basins (Warren, 2010). Both situations circumscribe different hydrologies and eustasies compared to continental-fed in-flows that typify the world's current larger evaporite basins. Today, and across the Quaternary, the largest and thickest salt stacks, with areal extents up to 10,000 km2 and thickness up to 900 m, tend to precipitate in the lower parts of suprasealevel intermontane lacustrine sumps located in tectonically active parts of continental interiors, such as Salar Atacama and Salar di Uyuni in the Andean Altiplano (Figure 4a). Most ancient evaporites are marine-fed and were deposited in huge hydrographically-isolated subsealevel marine-seepage sumps located in intracratonic basins or within rifts or compressional sutures. Often, the areal extents of these ancient systems were more than 250,000 km2, this is more than two orders of magnitude larger than any Quaternary evaporite deposit. Bedded (pre-halokinetic) thicknesses could be more than a kilometre.


Ancient marine saline giants (megahalites and megasulphates) accrued in either of two plate-scale settings, which at times merged into one another, namely; 1) Platform evaporites (Figure 5) and 2) Basinwide evaporites (Figure 6). The first major contrast with nonmarine continental dominance in Quaternary evaporite settings is the fact that platform evaporites require greenhouse eustasy, the second is that basinwide evaporites require tectonically- and hydrographically-isolated widespread subsealevel depressions, typically found along plate edges with continent-continent proximity (Figure 5).

Neither condition is present on the current earth surface. For basinwides, suitable hydrologic conditions were last present during the Messinian Salinity Crisis in the Mediterranean region, and platform evaporite settings were last present on earth across large parts of the Middle East carbonate platform during the Eocene (Tables 1, 2). There is a third group of ancient evaporite deposits; it encompasses all nonmarine lacustrine beds past and present (Table 3). This group has same-scale modern-ancient counterparts, unlike ancient marine platform and basinwide evaporites (Figure 4a; Warren, 2010, 2016).


Platform evaporites

Are made up of stratiform beds, usually <50 m thick and composed of stacked <1 to 5 m thick parasequences or evaporite cycles, with a variably-present restricted-marine carbonate unit at a cycle base (Table 1). Salts were deposited as mixed evaporitic mudflat and saltern evaporites, sometimes with local accumulations of bittern salts. Typically, platform salts were deposited in laterally extensive (>50-100 km wide), hydrographically-isolated, subsealevel marine-seepage lagoons (salterns) or evaporitic mudflats (sabkhas and salinas). These regions had no same-scale modern coun-terparts and extended as widespread depositional sheets across large portions of hydrographically isolated marine platform areas, which passed seaward across a subaerial seepage barrier into open marine sediments (Figure 5). In marine margin epeiric settings, such as the Jurassic Arab/Hith and Permian Khuff cycles of the Middle East or the Cretaceous Ferry Lake Anhydrite in the Gulf of Mexico, these platform evaporites are intercalated with shoalwater marine-influenced carbonate shelf/ramp sediments, which in turn pass basinward across a subaerial sill into open marine carbonates. Landward they pass into arid zone continental siliciclastics or carbonate mudflats.


Platform evaporite deposition occurred in both pericontinental and epicontinental settings, at times of low-amplitude 4th and 5th order sealevel changes, which typify greenhouse eustasy (Figure 5; Warren, 2010). Platform evaporites also typify the saline stages of some intracratonic basins. Platform evaporites cannot form in the high-amplitude, high-frequency sealevel changes of icehouse eustasy. The 100m+ amplitude oscillations of icehouse times mean sealevel falls off the shelf edge every 100,000 years, so any evaporite that had formed on the platform is subaerially exposed and leached. Fourth order high-amplitude icehouse eustatic cycles also tend to prevent laterally-continuous carbonate sediment barriers forming at the top of the shelf to slope break and so icehouse evaporite systems tend not to be hydrographically isolated (drawdown) at the platform scale. Rather icehouse eustasy favours nonmarine evaporites as the dominant style, along with small ephemeral marine-margin salt bodies, as seen today in the bedded Holocene halites and gypsums of Lake Macleod in coastal West Australia.

Ancient platform evaporite successions may contain halite beds, especially in intracratonic basinwide settings, but periplatform settings, outside of intracratonic basins, are typically dominated by 5–40 m thick Ca-sulphate beds intercalated with normal-marine platform carbonates (Table 1). The lateral extent of these epeiric platform sulphate bodies, like the Middle Anhydrite Member of the Permian Khuff Fm. of Saudi Arabia and the UAE, with a current area of more than 1,206,700 km2, constitute some of the most aerially-extensive evaporite beds ever deposited.


Basinwide evaporites

Are made up of thick evaporite units >50–100 m thick made up of varying combinations of deepwater and shallow wa-]ter evaporites (Table 2). They retain textural evidence of different but synchronous local depositional settings, including mudflat, saltern, slope and basin (Figures 6). When basinwide evaporite deposition occurs, the whole basin hydrology is evaporitic, holomictic, and typically saturated with the same mineral phase across vast areas of the basin floor, as in the Dead Sea basin today. The Dead Sea has a more limited lateral scale than ancient basinwides but currently has halite forming simultaneously as; 1) decimeter-thick chevron-dominated beds on the saline-pan floor of the shallow parts around the basin edge in waters typically less than 1-10 metres deep, and 2) as coarse inclusion-poor crystal meshworks of halite on the deep basin floor that sits below a halite-saturated brine column up to hundreds of metres deep (Figure 3a). Ancient basinwide successions are usually dominated by thick massive salt beds, generally more than 100-500 m thick. Deposits are made up of stacked thick halite beds, but can also contain substantial volumes of thick-bedded Ca-sulphate and evaporitic carbonate, as in the intracratonic basinwide accumulations of the Delaware and Otto Fiord Basins (Table 2).

Owing to inherent purity and thickness of the deposited halite, many halite-dominant basinwide beds are also remobilized, via loading or tectonics, into various halokinetic geometries (Hudec and Jackson, 2007). Some basinwide systems (mostly marine-fed intracratonic settings) entrain significant accumulations of marine-fed potash salts, as in the Devoni-an Prairie Evaporite of western Canada. In contrast, all Quaternary examples of commercial potash deposits are accumulating in continental lacustrine systems (Warren 2016; Chapter 11).

Basinwide evaporite deposits are the result of a combination of tectonic and hydrological circumstances that are not currently active on the world’s surface (Figure 4b). They were last active in the Late Miocene (Messinian), in association with soft-suture collision basins tied to the Alpine-Himalaya orogenic belt, and in Middle Miocene (Badenian) basins developed in the early rift stages of the Red Sea. Basinwide systems will be active again in the future at sites and times of appropriate plate-plate interaction, when two continental plate edges are nearby, and the intervening seafloor is in or near a plate-edge rift or suture and is both subsealevel and hydrographically isolated (Figure 6).


Lacustrine (nonmarine) evaporites

Quaternary continental playa/lacustrine are constructed of stratiform salt units, with the greater volume of saline sediment accumulating in lower, more-saline portions of the lacustrine landscape. Beds are usually dominated by nodular gypsum and displacive halite, deposited in extensive evaporitic mudflats and saltpans with textures heavily overprinted by capillary wicking, rather than as bedded bottom-nucleated layers on the subaqueous floors of perennial brine lakes (Figure 3b; Ruch et al., 2012). In ancient counterparts, the total saline lacustrine thickness ranged from meters to hundreds of meters, with lateral extents measured in tens to hundreds of kilometres (Table 3; Figure 4a). Lacustrine salt beds are separated vertically, and usually surrounded by, deposits of lacustrine muds, alluvial fans, ephemeral streams, sheet floods, eolian sands, and redbeds. As today, ancient lacustrine salts accumulated in endorheic or highly restricted discharge basins, with perennial saline water masses tending to occur in the drainage sumps of steep-sided drainage basins (Warren, 2010, 2016). Saline lake basins accumulating gypsum, or more saline salts like halite or glauberite, typically have a shallow water table in peripheral saline mudflat areas and so are dominated by continental sabkha textures. Nearby is the lowermost part of the lacustrine depression or sump where deposition is typified by ephemeral ponded brine pan deposits, rather than permanent saline waters.

Saline lacustrine mineralogies depend on compositions of inflowing waters, so depositional sumps in regions with non-marine ionic proportions in the feeder inflow, accumulate thick sequences of nonmarine bedded salts dominated by trona, glauberite, and thenardite. In contrast, nonmarine areas with thalassic (seawater-like) inflows tend to accumulate more typical sequences of halite, gypsum, and anhydrite.


Across the Quaternary, less-saline perennial saline-lake beds tend to occur during more humid climate periods in the same continental-lacustrine depressions where saline-pan beds form (e.g., Lake Magadi, Great Salt Lake, Lake Urmia). On a smaller scale, in some modern saline lake basins, parts of the lake floor can be permanently located below the water surface (Northern Basin in the Dead Sea or Lake Asal). In some modern saline sumps dominated by mudflats, a perennial saline lake water mass is located toward the edge of a more central salt-flat zone, forming a perennial water filled “moat” facies surrounding a seasonally desiccated saline pan (as in Salar, de Atacama, Salar de Uyuni, Lake Magadi, Lake Natron). These permanent to near-permanent saline water “moat” regions are typically created where fresher inflows encounter saltier beds of the lake centre, dissolve them, and so form water-filled peripheral depressions. Bottom sediment in the moats tend to be mesohaline carbonate laminites, which can contain TOC levels as high as 12%.

High-water stage perennial saline lacustrine sediments tend to be carbonate-rich or silica-rich (diatomaceous) laminites. Ancient examples of large saline lacustrine deposits made up of alternating humid and desiccated lacustrine units include the Eocene Green River Formation of Wyoming and the Permian Pingdiquan Formation of the Junggar Basin, Chi-na (Table 4). Evaporites deposited in a suprasealevel lacustrine basin (especially Neogene deposits) have numerous same-scale Quaternary analogues, unlike the more voluminous ancient marine platform and basinwide evaporites (Figure 4a).


Salt punches above its weight, but why? (facilitator for economic accumulations of oil, gas and metals)

In terms of total mass in sedimentary basins, the proportion of evaporite across the world’s Phanerozoic basins is rarely more than 2% (Figure 7; Ronov, 1980). Today we have comprehensive documentation that salt horizons, their brines, associated dissolution and alteration conduits control significant economic associations of oil, gas and metals (Warren, 2016):

  • 50% of world’s carbonate reservoirs (seals, traps and source rocks)
  • All the world’s supergiant oil and gas fields in thrusts (seals and structural traps).
  • All supergiant sedimentary copper deposits (halokinetic brine focus)
  • 50% of world’s giant SedEx deposits (halokinetic brine focus).
  • 80% of giant MVT deposits (sulphate-fixer & brine interface)
  • World’s largest Phanerozoic Ni deposit (meta-igneous – Noril’sk).
  • Many larger IOCG deposits (meta-evaporite, brine and hydrothermal).
  • This enrichment runs counter to a proportion of 2% of the world's Phanerozoic sediments. The exact why or how of these associations is still not well understood. Most geologists working with oil, gas or metal buildups in a salt-rich basin will come to have a suspicion, and for some, the conviction, that salt or its subsurface alteration plays a role in defining the position or enrichment level of the commodity of interest. Evaporite masses in the subsurface, especially if halite-dominant, enable both physical and chemical alterations, which tend to improve economic prospectivity. The unique properties of salt in the diagenetic realm tend to facilitate, focus and stabilise processes that lead to elevated levels of accumulations of various commodities (Figure 8). That is, evaporites in a basin tend to enhance the volume of oil, gas or metals in an accumulation, but are not necessarily the direct cause of the accumulation/precipitation.


    Evaporite-hydrocarbon association

    When the reservoired hydrocarbon below a salt seal is oil, little or no leakage can take place through a laterally continuous evaporite. Even when the reservoired hydrocarbon is methane, little or no loss occurs, even by diffusion (Ehgartner et al., 1998). The much greater efficiency of evaporite seals, compared to shales, is clearly seen in the total hydrocarbon volumes held back by the two lithologies. Total worldwide shale sediment volume in the Earth's sedimentary crust is more that an order of magnitude greater than that of evaporites, yet the split between reservoired hydrocarbons below a shale or an evaporite seal is roughly 50:50 (Grunau, 1987). Likewise, typical volumes of salt-sealed giant fields dis-covered in the last decade are much grander than that held in mudrock-sealed systems (Bai and Yu, 2014).

    Of the 120 giant oil and gas fields discovered in the period 2000 - 2012 some 54.6 % are hosted in marine carbonates and 12% in lacustrine carbonates, meaning less than a third of new giant discoveries are in siliciclastic reservoirs (Figure 9a). Some 56% of these oil and gas giants have an evaporite seal, with 82% of the marine carbonates having an evaporite seal and 91% of the lacustrine carbonates having an evaporite seal (Figure 9b, c). Clearly, carbonate reservoirs with evaporite seals constitute most of the giant oil and gas discoveries across the period 2000-2012, and the proportions of this association are likely to increase in conventional discoveries across the next decades (Bai and Yu, 2014).


    Of these 120 oil and gas giants, one is a megagiant field with a recoverable reserve of 50 Bboe (Billion barrels oil equivalent) or more, and 6 are supergiant fields with recoverable reserves of 5 Bboe or more. The seven largest fields are: Galkynysh (aka Yolotan or Osman) gas field in the Ama-Darya Basin of Turkmenistan with an original 2P reserve of 67.1 Bboe, making it likely the second largest known gas field in the world; Kashagan oil field (18.1 Bboe) in the North Caspian basin; Kish 2 gas field in the Arabian Basin (Iran): Lula (previously known as Tupi), Franco and Libra oil fields in the Santos Basin of offshore Brazil; and Sulige gas field (5.7 Bboe) in the Ordos Basin, China. In this listing, the geology of Galkynysh is not yet reliably published, but of the remaining 6, only Sulige does not have an evaporite association.

    Typical bedded evaporite seals, especially beds composed of monomineralogic assemblages such as massive nodular anhydrite or massive halite and have measured entry pressures more than 3000 psi. Most impure evaporite beds have entry pressures greater than 1000 psi, as do many evaporite-plugged reflux dolomites. Contrast this with most shales, which tend to be water-bearing (mostly bound and structural water), with typical entry pressures between 900 and 1500 psi (Sneider et al., 1997). Although such shales are respectable seals, over time shale allows substantial diffu-sive leakage of methane and even liquid hydrocarbons via inherent microporosity, less so if the shales are organic-rich. Even ignoring halite's ability to reanneal and flow under stress, any evaporite seal has much lower intrinsic permeability than shale, and this helps maintain its seal integrity. With permeabilities of 10-7 md, a hydraulic gradient of 0.01, and a porosity less than  0.01, a brine would take somewhere between 3 and 30 million years to flow 1 metre into an unfractured halite seal. For anhydrite, which has permeability some 100 times higher than halite, a brine would take between 30,000 and 300,000 years to flow 1 metre into the seal (Beauheim and Roberts, 2002).


    Evaporites are excellent longterm seals to substantial hydrocarbon columns. This applies to any siliciclastic or carbonate reservoir adjacent to bedded or halokinetic salty seals (Figure 10). The exemplary ability of bedded evaporites to act as seals holding back massive hydrocarbon columns is clearly seen in the Middle East, where Ghawar, the world’s largest oil field, is sealed by bedded anhydrites of the Jurassic Arab D Formation and the overlying Hith Anhydrite seal. This evaporite seal association in Ghawar holds back an estimated remaining reserve of more than 100-200 billion barrels. Bedded platform anhydrites also seal Safaniya, the world’s largest offshore field, also in Saudi Arabia, with estimated reserves of more than 25-30 billion barrels of oil and 5 billion cubic feet of natural gas. Likewise, Permian platform anhydrites are the regional seal to North Field in offshore Qatar, the world’s largest single gas field (non-associated gas) with more than 500 tcf of reserves (Alsharhan and Nairn, 1997). This gas is reservoired and sealed in the evaporitic dolomites of the Permian Khuff Formation and recent announcements by the Qatari Government have postulated more than 900 tcf of certified non-associated gas sealed in the North Field structure.

    In terms of physical processes, buried near-pure halite beds and masses (rock salt) in the sedimentary realm are rheologically unusual compared to other nearby sediments, in that at geological time scales rock salt can set up deformation responses that mimic Newtonian fluid responses. At the same time, intercrystalline textures in the flowing salt mass maintain seal integrity, even as local crystals dissolve and reprecipitate. That is, down to depths of 6-8 km rock salt flows and maintains seal integrity, while adjacent non-salt sediments tend to fault and fracture. The ability of bedded salt to seal (Figure 10a) and of flowing salt to create and seal supra-, intra- and sub-salt reservoirs is well documented (Figure 10b).

    Dense evaporitic brines can pass through adjacent or underlying sediments both at the time the bedded salts are accumulating, or later as a subsurface salt mass dissolves. For example, chemical responses, created within the set-up hydrology of widespread salt deposition and early burial, drive brine reflux, moving magnesium-rich modified seawater brines into and through the underlying carbonate sediments. This hydrology crafts broad reflux dolomite haloes, with local burial anhydrite patches and overall improves intercrystalline connectivity, at least until the dolomite reservoir becomes overdolomitised (Figure 11). Once this happens, all effective polyhedral porosity is lost in the dolomite and the reservoir potential is destroyed. Later dissolution brines can create hydrothermal waters capable of leaching and burial dolomitization, as in the burial-dolomite reservoirs of the Western Canada Sedimentary Basin (Davies and Smith, 2006).


    Organic-hydrocarbon association

    As long ago as the middle of last century, Weeks (1961) emphasised the importance of evaporites as seals to many of the world's major hydrocarbon accumulations. He also pointed out that many of the cycles of deposition that involve organic-rich carbonate marls or muds also end with evaporites. Clearly, in evaporitic settings, there is an association with Type I-II hydrogen-prone kerogens in mesohaline source rocks, and this is related to the ability of halotolerant photosynthetic algae and cyanobacteria to flourish in periodically mesohaline waters. Such kerogens tend to be oil-prone rather than gas prone and typified by long-chain hydrocarbons (Warren, 2011).

    Much of the mesohaline organic matter preserved in evaporitic carbonates, and the resulting source rocks, originated as planktonic blooms (pelagic “rain from heaven”) or from the benthic biomass (“in situ” accumulations). Such organics typically settled out as seriate pulses of organic matter (often pelleted and laminated) that sank to the bottom of a layered brine column. Each pulse was tied to a short period when surface brines were diluted and halotolerant producers (mostly cyanobacteria and algae) flourished in the freshened lit zone. That is, laminated mesohaline mudstones that constitute most evaporitic source rocks reflect biological responses to conditions of “feast or famine” in vacillatingly-layered brine bodies (Figure 12; Warren, 1986, 2011).


    Worldwide, studies of schizohaline evaporitic basins have shown that organic-rich mesohaline sediments can accumulate beneath ephemeral surface brines in salterns, or in basin and slope settings in both marine and continental settings (Figure 12; Kirkland and Evans, 1981; Oehler, 1984; Warren, 1986, 2011; Rouchy, 1988; Busson, 1992). The most prolific accumulations of organics in ancient evaporitic settings tend to be laminated micritic carbonates deposited be-neath intermittently stratified moderately-saline (mesohaline) anoxic water columns of varying brine depth.

    There are three, possibly four, major mesohaline density-stratified settings where organic-rich laminites (source rocks) accumulate in saline environments that are also associated with, or evolve into, evaporite deposits (Warren, 2011):

    1) Basin-centre lows in marine-fed evaporitic drawdown basins (basinwide salts).

    2) Mesohaline intrashelf lows atop epeiric evaporitic platforms.

    3) Saline-bottomed lows in perennial underfilled saline lacustrine basins.

    4) Closed seafloor depressions in halokinetic deepwater marine slope and rise terrains.

    The Metal-evaporite association

    In addition to the evaporite hydrocarbon association, there is an association of evaporite settings with the larger of the known MVT, Sedex, Stratiform Sedimentary Copper and some IOCG associations (Figure 13; see Warren 2016, Chapter 15 and 16 for detailed case histories and models). The role of evaporites in focusing metalliferous ore accumulations is two-fold; 1) In solution (halite-dominant precursor) they can act as chloride-rich metal carriers and 2) Locally, as beds or masses (especially of CaSO4), their dissolution products, especially if trapped, can supply sulphur (mostly as bacteriogenic or thermogenic H2S) and also set up chemical interfaces that act as foci in the setup of brine mixing conditions suitable for precipitation of metal sulphides or native elements. Hence, most evaporite-associated ore systems tend to be epigenetic, rather than syngenetic. Subsurface salt beds and masses are merely the solid part of a large ionic recycling system; dissolved metals are another part, and zones of mixing between the two are typically sites where ores tend to accumulate. Halokinesis steadies the position of a redox interface, tied to a salt dissolution brine halo, and enables an extended phase of focus to a metal precipitative (redox) interface at a stable location in subsurface earth-space (Warren, 2016).


    At the world-scale, evaporite-associated metalliferous systems are driven by plate tectonics. Halite-dominated sequences, deposited in the drawdown basin centres, tend to dissolve in burial, and so supply chloride ions to the brine system. Salt beds that are thick enough tend to flow and so focus the upward, and centripetal passage of basinal and hydrothermal fluid flows. Dissolving gypsum or anhydrite beds, typically deposited higher on the basin platform or diagenetically accumulated along salt dissolution edges and touchdowns can supply sulphur, via bacterial or thermochemical sulphate reduction, while simultaneously focusing metalliferous brine flows into the precipitation interface.

    When the chemistries of the dissolving salt beds and the metal carriers interact so that redox fronts, salinity contrasts, and other precipitative interfaces are set up, an ore deposit can form. Thus, in base and precious metal exploration within evaporitic terranes, we are ultimately searching for those parts of a subsurface ionic cycling system where the salt dissolution, salt beds and metal systems have interacted to create economic levels of metalliferous precipitates.

    If salt is more than a seal, then......

    From a time in the 1950's and 1960's when evaporites were mostly seen as seals, knowledge systems developed over the next five decades now allow us to do much more with respect to predictive industrial geology, centred on the following evaporite facts:

    1) The distribution of evaporite depositional textures maps paleotopography across the underlying carbonates and siliciclastics. So, depositional signatures preserved in the seal can be used to map reservoir quality trends in terms of reflux dolomite intensity, anhydrite cement-patch distributions and zones of subaerial diagenesis

    2) Thick beds of halite tend to accumulate in particular plate tectonic settings. Beds deposited in plate-edge saline sumps tend to flow via sediment loading and extension, without later superimposed tectonic stresses. In contrast, bed deposited in intracratonic settings tend to require later externally-imposed tectonic stress in order to flow. Hydrocarbon trapping geometries are tied to particular styles of salt tectonics

    3) Mesohaline source rock distribution is related to evaporite basin architecture and likely fluid escape pathways, which in turn are related to seal type and timing of salt dissolution or halokinetic withdrawal.

    4) Potash ore quality controls are related to the timing of various brine crossflows generated during deposition, mesogenesis and telogenesis.

    5) Positions of likely base metal and copper accumulations relate to set-ups of dissolution-related of redox interfaces, mesogenetic cross-flows, and in some cases, halokinetic geometries. At the plate tectonic scale, these accumulations occur at particular hydrological interfaces within the basin architecture.

    References

    Alsharhan, A. S., and A. E. M. Nairn, 1997, Sedimentary Basins and Petroleum Geology of the Middle East: Amsterdam, The Netherlands, Elsevier Science B. V., 942 p.

    Bai, G., and Y. Xu, 2014, Giant fields retain dominance in reserves growth: Oil & Gas Journal, v. 112, p. 44-.

    Busson, G., and et al., 1992, Basins paleogenes saliferes de l'Est de la France (Valence, Bresse et Haute-Alsace) Translated Title: Paleogene salt basins of eastern France, Valence, Bresse and Haute-Alsace: Geologie de la France, v. 1, p. 15-64.

    Davies, G. R., and L. B. Smith, 2006, Structurally controlled hydrothermal dolomite reservoir facies: An overview: Bulle-tin American Association Petroleum Geologists, v. 90, p. 1641-1690.

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    Hay, W. W., A. Migdisov, A. N. Balukhovsky, C. N. Wold, S. Flogel, and E. Soding, 2006, Evaporites and the salinity of the ocean during the Phanerozoic: Implications for climate, ocean circulation and life: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 240, p. 3-46.

    Jiang, L., C. F. Cai, R. H. Worden, K. K. Li, and L. Xiang, 2013, Reflux dolomitization of the Upper Permian Changxing Formation and the Lower Triassic Feixianguan Formation, NE Sichuan Basin, China: Geofluids, v. 13, p. 232-245.

    Kirkland, D. W., and R. Evans, 1981, Source-rock potential of an evaporitic environment: Bulletin American Association of Petroleum Geologists, v. 65, p. 181-190.

    Oehler, J. H., 1984, Carbonate source rocks in the Jurassic Smackover trend of Mississippi, Alabama, and Florida, in J. G. Palacas, ed., Petroleum geochemistry and source rock potential of carbonate rocks, v. 18: Tulsa, Oklahoma, American Association of Petroleum Geologists, Studies in Geology, p. 63-69.

    Pilcher, R. S., B. Kilsdonk, and J. Trude, 2011, Primary basins and their boundaries in the deep-water northern Gulf of Mexico: Origin, trap types, and petroleum system implications: Bulletin American Association Petroleum Geologists, v. 95, p. 219-240.

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    Seawater chemistry (1 of 2): Potash bitterns and Phanerozoic marine brine evolution

    John Warren - Tuesday, August 11, 2015

    The significance of evaporites as indicators of the chemical evolution of seawater across time and in relation to potash bitterns is considered in the next two Salty Matters articles. This article focuses on Phanerozoic seawater chemistry, where actual salts are widespread and the proportions of potash bittern salts are a useful pointer to the chemical makeup of the mother brine. Throughout both articles, the term “lower salinity” refers to marine brines with salinities between one and ten times that of ambient seawater. The second article considers seawater chemistry based on Precambrian evaporites, where much of the evidence of mother brine composition comes from salt pseudomorphs, rather than remnants of actual salts. In the second article we shall see that atmospheric conditions in the Early Precambrian were reducing and hotter than today, so that seawater was more saline, warmer, anoxic, with higher levels of calcium and bicarbonate compared to Phanerozoic seawater. Gypsum (CaSO4.2H2O), which requires free sulphate, was a rare precipitate during concentration of Archean seawater. Changing atmospheric proportions of CO2, CH4 and O2 meant sodium carbonate salts were significant lower-salinity early Archean marine-brine precipitates. Yet today, sodium carbonate salts, such as trona (NaHCO3.Na2CO3), nahcolite (NaHCO3) and shortite (2CaCO3.Na2CO3) cannot precipitate from a brine with the ionic proportions of modern seawater. The presence of sodium carbonate salts in any evaporite succession across the Phanerozoic is a reliable indicator of a nonmarine mother brine (Figure 1).


    A Phanerozoic dichotomy: evolving marine potash bitterns

    Consistently across the last 550 million years, halite and gypsum (mostly converted to anhydrite in the subsurface) are the dominant lower-salinity marine salts. But potash-bittern evaporite associations plotted across the same time framework define two end-members (Figure 1):

    1) Sulphate-enriched potash deposits, with ores typically composed of halite (NaCl) with carnallite (MgCl2.KCl.6H2O) and lesser sylvite (KCl), along with varying combinations of MgSO4 salts, such as polyhalite (2CaSO4.MgSO4.K2SO4.H2O), kieserite (MgSO4.H2O) kainite (4MgSO4.4KCl.11H2O) and langbeinite (2MgSO4. K2SO4); and

    2) Sulphate-depleted potash deposits are composed of halite with sylvite and carnallite, and entirely free or very poor in the magnesium-sulphate salts. The sulphate-depleted association typifies more than 65% of the world’s exploited Phanerozoic potash deposits. Sylvite ores with this association have properties that are easier to process cheaply (Warren 2016; see also blog 4 of 4 in the Salty Matters Danakhil articles).

    The sulphate-enriched group of ancient potash salts contains a bittern mineral suite predicted by the evaporation and backreaction of seawater with proportions similar to modern marine brine. In contrast, the sulphate-depleted group of bittern salts must have precipitated from Na-Ca-Mg-K-Cl brines with ionic proportions quite different from that of concentrated modern seawater. The separation between the two bittern associations is defined by brine evolution across the gypsum divide. That is, once gypsum (CaSO4.2H2O) and halite (NaCl) have precipitated in the lower salinity spectrum, are the remaining brines enriched in sulphate or calcium (Figure 1)? The greater suitability for potash utilisation of the sulphate depleted bitterns makes understanding and hence predicting occurrences of the sulphate-depleted association in time and space a useful first-order potash exploration tool.

    Why the dichotomy?

    In the older literature dealing with Phanerozoic salt chemistry, MgSO4-depleted potash evaporites were often explained as diagenetically-modified marine evaporite brines, thought to result from backreactions during burial diagenesis of normal marine waters (Borchert, 1977; Dean, 1978; Wilson and Long, 1993). If so, then the mother seawater source across the Phanerozoic had ionic proportions like those of today, but diagenetically altered via; a) dolomitisation, b) sulphate-reducing bacterial action, c) mixing of brines with calcium bicarbonate-rich river water, or d) rock-fluid interaction during deep burial diagenesis. As another option, Hardie (1990) suggested MgSO4-depleted potash bitterns formed by the evaporative concentration of sulphate-depleted nonmarine inflow waters seeping into an evaporite basin via springs and faults. Such springs were sourced either from CaCl2-rich hot hydrothermal brines or via cooling of deep basinal brines. Such fault-fed deeply-circulating CaCl2 brines source the various springs feeding the Dead Sea, the Qaidam Basin, the Salton Sea and the Danakil Depression. In all these cases, the elevated salinities of inflow waters are related, at least in part, to the dissolution of buried evaporites. Upwelling of brines in these regions is driven either by thermally-induced density instabilities, related to magma emplacement, or by the creation of tectonically-induced topographic gradients that force deeply-circulated basinal brines to the surface. Ayora et al. (1994) demonstrated that such a deeply-circulating continental Ca–Cl brine system operated during deposition of sylvite and carnallite in the upper Eocene basin of Navarra, southern Pyrenees, Spain.

    Today, a more widely accepted explanation for SO4-enriched versus SO4-depleted Phanerozoic potash bitterns, is that seawater chemistry has evolved across deep time. Background chemistry of the marine potash dichotomy is simple and can be related to brine evolution models published by Hardie more than 30 years ago (Hardie, 1984). He found that the constituent chemical proportions in the early stages of concentration of any marine brine largely controls the chemical makeup of the subsequent bittern stages. These ionic proportions control how a brine passes through the lower salinity CaCO3 and gypsum divides (Figure 1). That is, a marine brine’s bittern make-up is determined by the ionic proportions in the ambient seawater source. It determines the carbonate mineralogy during the precipitation of relatively insoluble evaporitic carbonates (aragonite, high-magnesium calcite, or low-magnesium calcite) which in turn controls its constituent chemistry as it attains gypsum saturation. These two stages are called the CaCO3 and gypsum divides. Hence, the chemical passage of a bittern is controlled by the ionic proportions in the original ambient seawater. The CaCO3 divide kicks in when a concentrating seawater brine attains a salinity around twice that of normal seawater (60‰). The gypsum divide occurs when brine concentrations are around 4-5 times that of normal seawater (140-160‰). Normal seawater has a salinity around 35-35‰ and the various potash bittern salts precipitate when concentrations are around 40-60 times that of the original seawater (Figure 2 – lower part).

    As seawater concentrates and calcium carbonate mineral(s) begin to precipitate at the CaCO3 divided then, depending on the relative proportions of Ca and HCO3 in the mother seawater, either Ca is used up, or the HCO3 is used up. If the Ca is used up first, an alkaline brine (pH>10) forms, with residual CO3, along with Na, K, SO4 and Cl, but no remaining Ca (Figure 1). With ongoing concentration this brine chemistry will then form sodium bicarbonate minerals, it cannot form gypsum as all the Ca is already used up. Such an ionic proportion chemistry likely defined oceanic waters in the early Archaean but is not relevant to seawater evolution in the Proterozoic and Phanerozoic, as evidenced by widespread gypsum (anhydrite) or pseudomorphs in numerous post-Archean marine-evaporite basins. At higher concentrations, early Archean marine brines would have produced halite and sylvite bittern suites, but with no gypsum or anhydrite (Figure 1).

    If, instead, HCO3 is used up during initial evaporitic carbonate precipitation, as is the case for all Phanerozoic seawaters, the concentrating brine becomes enriched in Ca and Mg, and a neutral brine, depleted in carbonate, is formed. Then the ambient Mg/Ca ratio in a concentrating Phanerozoic seawater will control whether the first-formed carbonate at the CaCO3 divide is aragonite (Mg/Ca>5) or high-magnesium calcite (2>Mg/Ca>5), or low-Mg calcite(Mg/Ca>2). The latter Mg/Ca ratio is so low it is only relevant to concentrating Cretaceous seawaters. Elevated Mg/Ca ratios favouring the precipitation of aragonite over high Mg-calcite typify modern marine seawater brines, which have Mg/Ca ratios that are always >5 (Figure 2). At lower salinities, modern marine brines are Na-Cl waters that with further concentration and removal of Na as halite evolve in Mg-SO4-Cl bitterns (Figure 2)

     

    The next chemical divide reached by concentrating marine Phanerozoic brines (always depleted in HCO3 at the carbonate divide) occurs when gypsum precipitates at around 4-5 times the concentration of the original seawater (Gypsum divide in Figure 1). As gypsum continues to precipitate, either the Ca in the brine is used up, or the SO4 in the brine is used up. If the Ca is depleted, a calcium-free brine rich in Na, K, Mg, Cl, and SO4 will be the final product and the diagnostic bittern minerals will include magnesium sulphate minerals. This is the pathway followed by modern seawater bitterns. If, however, the sulphate is used up via gypsum precipitation, the final brine will be rich in Na, K, Mg, Ca, and Cl. Such a sulphate-depleted brine precipitates diagnostic potassium and magnesium chloride minerals such as sylvite and carnallite. If calcium-chloride levels are very high, then diagnostic (but uncommon) minerals such as tachyhydrite (CaCl2.2MgCl2.12H2O), and antarcticite (CaCl2.6H2O) can precipitate from this brine. But both these assemblages contain no sulphate bittern minerals, making potash processing relatively straightforward (Warren, 2016). In Phanerozoic marine salt assemblages, tachyhydrite, which is highly hygroscopic, is present in moderate quantities only in Cretaceous (Aptian) marine sylvite-carnallite associations in the circum-Atlantic potash basins and the Cretaceous (Albian) Maha Sarakham salts of Thailand, along with its equivalents in Laos and western China. The CaCl2-entraining bittern mineral assemblages of these deposits imply ionic proportions of Cretaceous seawater differ from those of today.

    Inclusion evidence

    Based on a study of brine inclusion chemistry preserved in halite chevrons, from the Early Cretaceous (Aptian, 121.0–112.2 Ma) of the Sergipe Basin, Brazil, the Congo Basin, Republic of the Congo, and the Early to Late Cretaceous (Albian to Cenomanian, 112.2–93.5 Ma) of the Khorat Plateau, Laos and Thailand, Timofeeff et al. (2006) defined a very different chemical makeup for Cretaceous seawater, compared to that of today. Brine proportions in the fluid inclusions in these halites indicate that Cretaceous seawaters were enriched several fold in Ca, depleted in Na and Mg, and had lower Na/Cl, Mg/Ca, and Mg/K ratios compared to modern seawater (Table 1). 


    Elevated Ca concentrations, with Ca>SO4 at the gypsum divide, allowed Cretaceous seawater to evolve into Mg–Ca–Na–K–Cl brines lacking measurable sulphate. Aptian seawater was extreme in its Ca enrichment, more than three times higher than present day seawater, with a Mg/Ca ratio of 1.1–1.3. Younger, Albian-Cenomanian seawater had lower Ca concentrations, and a higher Mg/Ca ratio of 1.2–1.7. Cretaceous (Aptian) seawater has the lowest Mg/Ca ratios so far documented in any Phanerozoic seawater from fluid inclusions in halite, and lies well within the range chemically favourable for precipitation of low-Mg calcite ooids and cements in the marine realm.


    Likewise, a detailed analysis of the ionic make-up of Silurian seawater using micro-inclusion analysis of more than 100 samples of chevron halite from various Silurian deposits around the world was published by Brennan and Lowenstein (2002), clearly supports the notion that ionic proportions in the world’s Silurian oceans were different from those of today (Figure 3). Samples were from three formations in the Late Silurian Michigan Basin, the A-1, A-2, and B Evaporites of the Salina Group, and the Early Silurian in the Canning Basin (Australia) in the Mallowa Salt of the Carribuddy Group. The Silurian ocean had lower concentrations of Mg, Na, and SO4, and much higher concentrations of Ca relative to the ocean’s present-day composition (Table 1). Furthermore, Silurian seawater had Ca in excess of SO4. Bittern stage evaporation of Silurian seawater produced KCl-type potash minerals that lack the MgSO4-type late stage salts formed during the evaporation of present-day seawater and allowed sylvite as a primary precipitate. In a similar fashion, work by Kovalevych et al. (1998) on inclusions in primary-bedded halite from many evaporite formations of Northern Pangaea, and subsequent work using micro-analyses of fluid inclusions in numerous chevron halites (Lowenstein et al., 2001, 2003), shows that during the Phanerozoic the chemical composition of marine brines has oscillated between Na-K-Mg-Ca-Cl and Na-K-Mg-Cl-SO4 types. The former does not precipitate MgSO4 salts when concentrated, the latter does (Figure 3). A recent paper by Holt et al. (2014), focusing on chevron halite inclusions from various Carboniferous evaporite basins, further refined the transition from the Palaeozoic CaCl2 high Mg-calcite sea into a MgSO4-enriched aragonite ocean of the Permo-Carboniferous, so showing CaCl2 oceanic chemistry (and sylvite-dominant bitterns) extend somewhat further across the Palaeozoic than previously thought (Figure 4).

     

    More recent work has shown varying sulphate levels in the Phanerozoic ocean rather than Mg/Ca variations are perhaps more significant in controlling aragonite versus calcite at the CaCO3 divide and the associated evolution of MgSO4-enriched versus MgSO4-depleted bittern suites in ancient evaporitic seaways than previously thought. Bots et al. (2011) found experimentally that an increase in dissolved SO4 decreases the Mg/Ca ratio at which calcite is destabilized and aragonite becomes the dominant CaCO3 polymorph in an ancient seaway (Figure 5). This suggests that the Mg/Ca and SO4 thresholds for the onset of ancient calcite seas are significantly lower than previous estimates and that Mg/Ca levels and SO4 levels in ancient seas are mutually dependent. Rather than variations in Mg/Ca ratio in seawater being the prime driver of the aragonite versus calcite ocean chemistries across the Phanerozoic, they conclude sulphate levels are an equally important control.


    Mechanisms

    There is now convincing inclusion-based evidence that the chemistry of seawater has varied across the Phanerozoic from sulphate-depleted to sulphate-enriched, what is not so well understood are the various worldscale processes driving the change (Figure 4). Spencer and Hardie (1990) and Hardie (1996) argued that the level of Mg in the Phanerozoic oceans has been relatively constant across time, but changes in the rate of seafloor spreading have changed the levels of Ca in seawater. This postulate is also supported in publications by Lowenstein et al. (2001, 2003). Timing of the increase of Ca in the world’s oceans was likely synchronous with a decrease in the SO4 ion concentration, which at times was as much as three times lower than the present.

    Simple mixing models show that changes in the flux rate of mid-oceanic hydrothermal brines can generate significant changes in the Mg/Ca, Na/K and SO4/Cl ratios in seawater (Table 1). Changes of molal ratios in seawater have generated significant changes in the type and order of potash minerals at the bittern stage. For example, Spencer and Hardie’s (1990) model predicts that an increase of only 10% in the flux of mid-ocean ridge hydrothermal brine over today’s value would create a marine bittern that precipitates sylvite and calcium-chloride salts, as occurred in the Cretaceous instead of the Mg-sulphate minerals expected during bittern evaporation of modern seawater. Such Ca-Cl potash marine bitterns correspond to times of “calcite oceans” and contrast with the lower calcium, higher magnesium, higher sulphate “aragonite oceans” of the Permo-Triassic and the Neogene (Figure 3; Hardie, 1996; Demicco et al., 2005).

    Ocean crust, through its interaction with hydrothermally circulated seawater, is a sink for Mg and a source of Ca, predominantly via the formation of smectite, chlorite, and saponite via alteration of pillow basalts, sheeted dykes, and gabbros (Müller et al., 2013). Additional removal of Mg and Ca occurs during the formation of vein and vesicle-filling carbonate and carbonate-cemented breccias in basalts via interaction with low-temperature hydrothermal fluids. Hence, changing rates of seafloor spreading and ridge length likely influenced ionic proportions in the Phanerozoic ocean and this in turn controlled marine bittern proportions.

    According to Müller et al., 2013, hydrothermal ocean inputs are and the relevant ionic proportions in seawater are driven by supercontinent cycles and the associated gradual growth and destruction of mid-ocean ridges and their relatively cool flanks during long-term tectonic cycles, thus linking ocean chemistry to off-ridge low-temperature hydrothermal exchange. Early Jurassic aragonite seas were a consequence of supercontinent stability and a minimum in mid-ocean ridge length and global basalt alteration. The breakup of Pangea resulted in a gradual doubling in ridge length and a 50% increase in the ridge flank area, leading to an enhanced volume of basalt to be altered. The associated increase in the total global hydrothermal fluid flux by as much as 65%, peaking at 120 Ma, led to lowered seawater Mg/Ca ratios and marine hypercalcification from 140 to 35 Ma. A return to aragonite seas with preferential aragonite and high-Mg calcite precipitation was driven by pronounced continental dispersal, leading to progressive subduction of ridges and their flanks along the Pacific rim.

    Holland et al. (1996), while agreeing that there are changes in ionic proportion of Phanerozoic seawater and that halite micro-inclusions preserve evidence of these changes, recalculated the effects of changing seafloor spreading rates on global seawater chemistry used by Hardie and others. They concluded changes in ionic proportions from such changes in seafloor spreading rate were modest. Instead, they pointed out that the composition of seawater can be seriously affected by secular changes in the proportion of platform carbonate dolomitised during evaporative concentration, without the need to invoke hydrothermally driven changes in seawater composition. In a later paper, Holland and Zimmermann (2000) suggest changes in the level of Mg in seawater were such that the molar Mg/Ca ratio of the more saline Palaeozoic global seawater (based on dolomite volume) was twice the present value of 5.

    Using micro-inclusion studies of halites of varying ages, Zimmermann (2000a, b) has proposed that the evolving chemistry of the Phanerozoic ocean is more indicative of changing volumes of dolomite than it is of changes in the rates of seafloor spreading . Using halite inclusions, she showed that the level of Mg in seawater has increased from ≈38 mmol/kg H2O to 55 mmol/kg H2O in the past 40 million years (Figure 6). This increase is accompanied by an equimolar increase in the level of oceanic sulphate. Over the longer time frame of the Palaeozoic to the present the decrease in Mg/Ca ratio corresponds to a shift in the locus of major marine calcium carbonate deposition from Palaeozoic shelves to the deep oceans, a change tied to the evolution of the nannoplankton. Prior to the evolution of foraminifera and coccoliths, some 150 Ma, the amount of calcium carbonate accumulating in the open ocean was minimal. Since then, a progressively larger portion of calcium carbonate has been deposited on the floor of the deep ocean. Dolomitization of these deepwater carbonates has been minor.

     

    In a study of boron isotopes in inclusions in chevron halite, Paris et al. (2010) mapped out the changes in marine boron isotope compositions over the past 40 million years (Figure 7). They propose that the correlation between δ11BSW and Mg/Ca reflects the influence of riverine fluxes on the Cenozoic evolution of oceanic chemical composition. Himalayan uplift is a major tectonic set of events that probably led to a 2.5 times increase of sediment delivery by rivers to the ocean over the past 40 m.y. They argue that chemical weathering fluxes and mechanical erosion fluxes are coupled so that the formation of the Himalaya favoured chemical weathering and hence CO2 consumption. The increased siliciclastic flux and associated weathering products led to a concomitant increase in the influx levels of Mg and Ca into the mid to late Tertiary oceans. However the levels of Ca in the world’s ocean are largely biologically limited (mostly by calcareous nannoplankton and plankton), so leading to an increase in the Mg/Ca ratio in the Neogene ocean.

     

    a study of CaCO3 veins in ocean basement, utilising 10 cored and documented drilled sites, Rausch et al. (2013) found for the period from 165 - 30 Ma the Mg/Ca and the Sr/Ca ratios were relatively constant (1.22-2.03 mol/mol and 4.46-6.62 mmol/mol respectively (Figure 8). From 30 Ma to 2.3 Ma there was a steady increase in the Mg/Ca ratio by a factor of 3, mimicking the brine inclusion results in chevron halite. The authors suggest that variations in hydrothermal fluxes and riverine input are likely causes driving the seawater compositional changes. They go on to note that additional forcing may be involved in explaining the timing and magnitude of changes. A plausible scenario is intensified carbonate production due to increased alkalinity input to the oceans from silicate weathering, which in turn is a result of subduction-zone recycling of CO2 from pelagic carbonate formed after the Cretaceous slow-down in ocean crust production rate. However, world-scale factors driving the increase in Mg in the world’s oceans over the past 40 million years are still not clear and are even more nebulous the further back in time we look.

     

    Changes in Phanerozoic ocean salinity

    As well as changes in Mg/Ca and SO4, the salinity of the Phanerozoic oceans shows a fluctuating but overall general decrease from the earliest Cambrian to the Present (Figure 9; Hay et al. 2006). The greatest falls in salinity are related to major extractions of NaCl into a young ocean (extensional continent-continent proximity) or foreland (compressional continent-continent proximity) ocean basins (Chapter 5). Phanerozoic seas were at their freshest in the Late Cretaceous, some 80 Ma, not today. This is because a substantial part of the Mesozoic salt mass, deposited in the megahalites of the circum-Atlantic and circum-Tethyan basins, has since been recycled back into today’s ocean via a combination of dissolution and halokinesis. Periods characterised by marked decreases in salinity (Figure 9) define times of mega-evaporite precipitation, while periods of somewhat more gradual increases in salinity define times when portions of this salt were recycled back into the oceans (Chapter 5).


    The last major extractions of salt from the ocean occurred during the late Miocene in the various Mediterranean Messinian basins created by the collision of Eurasia with North Africa. This was shortly after a large-scale extraction of ocean water from the ocean to the ice cap of Antarctica and the deposition of the Middle Miocene (Badenian) Red Sea rift evaporites. Accordingly, salinities in the early Miocene oceans were between 37‰ and 39‰ compared to the 35‰ of today (Figure 9). The preceding Mesozoic period was a time of generally declining salinity associated with the salt extractions in the opening North Atlantic and Gulf of Mexico (Middle to Late Jurassic) and South Atlantic (Early Cretaceous) and the earliest Cambrian oceans also had some of the highest salinities in the Phanerozoic. Recently, work by Blättler and Higgins (2014) utilising Ca isotopes studies of selected Phanerozoic evaporites has confirmed the dichotomous nature of Phanerozoic ocean chemistry that was previously defined by micro-inclusion studies of chevron halite (Figure 3).

    So what?

    In summary, based on a growing database of worldwide synchronous changes in brine chemistry in fluid inclusions in chevron halite, echinoid fragments, vein calcites at spreading centres and Ca isotope variations, most evaporite workers would now agree that there were secular changes in Phanerozoic seawater chemistry and salinity. Ocean chemistries ranged from MgSO4-enriched to MgSO4-depleted oceans, which in turn drove the two potash endmembers What is not yet clear is what is the dominant plate-scale driving mechanism (seafloor spreading versus dolomitisation versus uplift/weathering) that is driving these changes.

    In terms of marine bitterns controlling favourable potash ore associations, it is now clear that the variation in ionic proportions in the original seawater controls whether or not potash-precipitating bitterns are sulphate enriched or sulphate depleted. A lack of MgSO4 minerals as co-precipitates in a sylvite ore makes the ore processing methodology cheaper and easier (Warren, 2016). Understanding the ionic proportion chemistry of Phanerozoic seawater is a useful first-order exploration tool in ranking potash-entraining evaporite basins across the Phanerozoic.

    References

    Ayora, C., J. Garciaveigas, and J. Pueyo, 1994, The chemical and hydrological evolution of an ancient potash-forming evaporite basin as constrained by mineral sequence, fluid inclusion composition, and numerical simulation: Geochimica et Cosmochimica Acta, v. 58, p. 3379-3394.

    Blättler, C. L., and J. A. Higgins, 2014, Calcium isotopes in evaporites record variations in Phanerozoic seawater SO4 and Ca: Geology, v. 42, p. 711-714.

    Borchert, H., 1977, On the formation of Lower Cretaceous potassium salts and tachyhydrite in the Sergipe Basin (Brazil) with some remarks on similar occurrences in West Africa (Gabon, Angola etc.), in D. D. Klemm, and H. J. Schneider, eds., Time and strata bound ore deposits.: Berlin, Germany, Springer-Verlag, p. 94-111.

    Bots, P., L. G. Benning, R. E. M. Rickaby, and S. Shaw, 2011, The role of SO4 in the switch from calcite to aragonite seas: Geology, v. 39, p. 331-334.

    Brennan, S. T., and T. K. Lowenstein, 2002, The major-ion composition of Silurian seawater: Geochimica et Cosmochimica Acta, v. 66, p. 2683-2700.

    Dean, W. E., 1978, Theoretical versus observed successions from evaporation of seawater, in W. E. Dean, and B. C. Schreiber, eds., Marine evaporites., v. 4: Tulsa, OK, Soc. Econ. Paleontol. Mineral., Short Course Notes, p. 74-85.

    Demicco, R. V., T. K. Lowenstein, L. A. Hardie, and R. J. Spencer, 2005, Model of seawater composition for the Phanerozoic: Geology, v. 33, p. 877-880.

    Hardie, L. A., 1984, Evaporites: Marine or non-marine?: American Journal of Science, v. 284, p. 193-240.

    Hardie, L. A., 1990, The roles of rifting and hydrothermal CaCl2 brines in the origin of potash evaporites: an hypothesis: American Journal of Science, v. 290, p. 43-106.

    Hardie, L. A., 1996, Secular variation in seawater chemistry: an explanation for the coupled secular variation in the mineralogies of marine limestones and potash evaporites over the past 600 m.y.: Geology, v. 24, p. 279 - 283.

    Hay, W. W., A. Migdisov, A. N. Balukhovsky, C. N. Wold, S. Flogel, and E. Soding, 2006, Evaporites and the salinity of the ocean during the Phanerozoic: Implications for climate, ocean circulation and life: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 240, p. 3-46.

    Holland, H. D., J. Horita, and W. Seyfried, 1996, On the secular variations in the composition of Phanerozoic marine potash evaporites: Geology, v. 24, p. 993-996.

    Holland, H. D., and H. Zimmermann, 2000, The Dolomite Problem Revisited: Int. Geol. Rev., v. 42, p. 481-490.

    Holt, N. M., J. García-Veigas, T. K. Lowenstein, P. S. Giles, and S. Williams-Stroud, 2014, The major-ion composition of Carboniferous seawater: Geochimica et Cosmochimica Acta, v. 134, p. 317-334.

    Kovalevych, V. M., T. M. Peryt, and O. I. Petrichenko, 1998, Secular variation in seawater chemistry during the Phanerozoic as indicated by brine inclusions in halite.: Journal of Geology, v. 106, p. 695-712.

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    Paris, G., J. Gaillardet, and P. Louvat, 2010, Geological evolution of seawater boron isotopic composition recorded in evaporites: Geology, v. 38, p. 1035-1038.

    Rausch, S., F. Böhm, W. Bach, A. Klügel, and A. Eisenhauer, 2013, Calcium carbonate veins in ocean crust record a threefold increase of seawater Mg/Ca in the past 30 million years: Earth and Planetary Science Letters, v. 362, p. 215-224.

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    Danakil potash: K2SO4 across the Neogene: Implications for Danakhil potash, Part 4 of 4

    John Warren - Tuesday, May 12, 2015

    How to deal with K2SO4

    In this the fourth blog focusing on Danakhil potash, we look at the potash geology of formerly mined Neogene deposits in Sicily and the Ukraine, then compare them and relevant processing techniques used to exploit their K2SO4 ore feeds. This information is then used to help guide a discussion of processing implications for potash extraction in the Danakhil, where kainite is the dominant widespread potash salt. As seen in the previous three blogs there are other potash mineral styles present in the Danakhil, which constitute more restricted ore fairways than the widespread bedded kainaite, these other potash styles (deep meteoiric -blog 2 of 4 and hydrothermal - blog 3 of 4), could be processed to extract MOP, but these other potash styles are also tied to high levels of MgCl2, which must be dealt with in the brine processing stream. The most effective development combination is to understand the three occurence styles , define appropriate specific brine processing strams and then combine the products in an single processing plant and then produce sulphate of potash (SOP), rather the Muriate of Potash (MOP), as SOP has a 30% price premium in current potash markets.

    Kainite dominated the bedded potash ore feed in former mines in the Late Miocene (Messinian) sequence in Sicily and the Middle Miocene (Badenian) sequence in the Carpathian foredeep], Ukraine. Kainite also occurs in a number of potash deposits in the Permian of Germany and Russia. In Germany a combination of mined sylvite and kieserite is used to manufacture sulphate of potash (SOP). Interestingly, Neogene and the Permian are times when world ocean waters were enriched in MgSO4 (Lowenstein et al., 2001, 2003). In contrast, much of the Phanerozoic was typified by an ocean where MgSO4 levels were less. It is from such marine brine feeds that most of the world’s larger Phaneorzoic (SOP) potash ore deposits were precipitated (Warren, 2015). SOP is also produced from Quaternary Lake brines in China and Canada (see cryogenic salt blog; 24 Feb. 2015).

    SOP in Messinian evaporites, Sicily

    A number of potash mines on the island extracted kainitite from the late Miocene Solofifera Series of Sicily (Figure 1). The last of these mines closed in the mid-1990s, but portions of some are maintained and are still accessible (eg Realmonte mine). The halite-hosted potash deposits are isolated ore bodies within two generally parallel troughs, 115 km long and 5- 10 km wide, within the Caltanissetta Basin (Figure 1). They are separated by a thrust-related high 11-25 km wide and capped by the limestones of the “Calcare di Base”. Kainite is the dominant potash mineral in the mined deposits. Across the basin, ore levels constitute six layers of variable thickness, with a grade of 10%-16% K2O (pure kainite contains 18.9% K20), with very little insoluble content (0.4%-2.0%).

    At the time the potash was deposited there was considerable tectonic activity in the area (Roveri et al. 2008, Manzi et al., 2011). Host sediments were deposited in piggy-back basins some 5.5 Ma atop a series of regional thrusts, so the ore layers have dips in the mines ranging up to 60° (Figure 2). Little if any of the limestone associated with the deposits was converted to dolomite, nor was the thick Messinian gypsum (upper and lower units), encasing the halite /kainitite units, converted to anhydrite, it remains as gypsum with well preserved depositional textures. However, the elevated salinities, and perhaps temperatures, required for kainite precipitation means anhydrite micronodules, observed in some ore levels, may be primary or syndepositional. A lack of carnallite, along with isotopic data, indicates that when the deposits were formed by the evaporation of the seawater, salinities did not usually proceed far past the kainite crystallization point (in contrast to Ethiopia where carnallite salinities typify the later stages of kainitite deposition)..

     

    The largest Sicilian ore body was at Pasquasia, to the west of Calanisseta, covering a 24 km2 area at depths of 300-800 m (Figure 1). There were five ore beds at Pasquasia, all with highly undulating synclinal and anticlinal forms. The Number 2 bed was the thickest, averaging perhaps a 30-m thickness of 10.5% to 13.5% K2O ore. The Pasquasia Mine was last operational from 1952 to 1992.

     

    Ore geology remains somewhat more accessible at the former Realmonte mine, near the town of Agrigento. There, four main depositional units (A to D from base to top) typify the evaporite geology. As at Pasquasia, kainitite was the targeted ore within a Messinian evaporite section that has total thickness of 400-600 m. As defined by Decima and Wezel, 1971, 1973; Decima, 1988, Lugli, 1999, the Realmonte mine section is made up of 4 units (Figure 2a):

    - Unit A (up to 50 m thick): composed of evenly laminated grey halite with white anhydrite nodules and laminae that pass upward to grey massive halite beds.

    - Unit B (total thickness ≈100 m): this potash entraining interval is dominated by massive even layers of grey halite, interbedded with light grey thin kainite laminae and minor grey centimetre-scale polyhalite spherules and laminae, along with anhydrite laminae; the upper part of the unit contains at least six light grey kainite layers up to 18 m-thick that were the targeted ore sequence. Unlike the Danakil, carnallite does not typify the upper part of this marine potash section. The targeted beds are in the low-angle dip portion of a thrust-folded remnant in a structural basin (Figures 2b, 3).

    - Unit C (70-80 m thick): is made up of white halite layers 10-20 cm thick, separated by irregular dark grey mud laminae and minor light grey polyhalite and anhydrite laminae (Figure 3).

    - Unit D (60 m thick): is composed of a grey anhydritic mudstone (15-20 m thick), passing up into an anhydrite laminite sequence, followed by grey halite millimetre to centimetre layers intercalated with white anhydrite laminae.


    According to Lugli, 1999, units A and B are made up of cumulates of well-sorted halite plate crystals, up to a few millimeters in size. Kainite typically forms discrete laminae and sutured crystal mosaic beds, ranging from a thickness of few mm to a maximum of 2 m, intercalated and embedded within unit B (Garcia-Veigas et al., 1995). It may also occur as small isometric crystals scattered within halite mosaics. Kainite textures are dominated by packed equant-granular mosaics, which show possible pressure-dissolution features at some grain boundaries. The associated halite layers are dominantly cumulates, which show no evidence of bottom overgrowth chevrons, implying evaporite precipitation was a “rain from heaven” pelagic style that took place in a stratified permanently subaqueous brine water body, possibly with a significant water depth to the bottom of the permanent lower water mass.

    Only the uppermost part of potash bearing portion of unit B shows a progressive appearance of large halite rafts along with localized dissolution pits filled by mud, suggesting an upward shallowing of the basin at that time. In many parts of the Realmonte mine spectacular vertical fissures cut through the topmost part of unit B at the boundary with unit C, suggesting desiccation and subaerial exposure at this level (Lugli et al., 1999).

    The overlying unit C is composed of cumulates of halite skeletal hoppers that evolve into halite chevrons illustrating bottom growth after foundering of the initial halite rafts. Halite layers in unit C show numerous dissolution pits filled by mud and irregular truncation of the upper crystal terminations, implying precipitation from a nonstratified, relatively shallow water body. Palaeo-temperatures of the brine that precipitated these halite crystals are highly variable from 22 to 32°C (Lugli and Lowenstein, 1997) and suggest a shallow hydrologically unstable body of water, unlike units A and B.

    The bromine content of halite increases from the base of unit A to the horizons containing kainite (layer B) where it obtains values of up to 150 ppm. Upwards, the bromine content decreases once more to where at the top of Unit C it drops below 13 ppm, likely indicating a marked dilution of the mother brine. The dilution is likely a consequence of recycling (dissolution and reprecipitation) of previously deposited halite either by meteoric-continental waters (based on Br content; Decima 1978), or by seawater (based on the high sulphate concentration and significant potassium and magnesium content of fluid inclusions; Garcia-Veigas et al., 1995).

    As in the Danakhil succession, evaporite precipitation at Realmonte began as halite-CaSO4 interlayered succession at the bottom of a stratified perennial water body, which shallowed and increased in concentration until reaching potash kainite saturation. In Sicily, this was followed by a period of exposure and desiccation indicated by the presence of giant megapolygonal structures. Finally, seawater flooded the salt pan again, dissolving and truncating part of the previous halite layers, which was then redeposited under shallow-water conditions at the bottom of a nonstratified (holomitic) water body (Lugli et al., 1997, 1999).

    Unlike Ethiopia, the Neogene kainite deposits of Sicily were deposited in a thrust “piggy-back” basin setting and not in a rift sump (Figure 2b). Mineralogically similar, very thick, rift-related, now halokinetic, halite deposits of Midddle Miocene age occur under the Red Sea’s coastal plain between Jizan, Saudi Arabia (where they outcrop) to Safaga, Egypt, with limited potash is found in some Red Sea locations at depths suitable for solution mining (Notholt 1983; Garrett, 1995). Potash-enriched marine end-liquor brines characterise Red Sea geothermal springs, implying a more sizeable potash mass may be (or once have been) present in this region. Hite and Wassef (1983) argue gamma ray peaks in two drill hole logs in this area suggest the presence of sylvite, carnallite and possibly langbeinite at depth.

    K2SO4 salts in Miocene of Ukraine

    Miocene salt deposits occur in the western Ukraine within two structural terranes: 1) Carpathian Foredeep (rock and potash salt) and (II) Transcarpathian trough (rock salt) (Figure 4a). These salt-bearing deposits differ in the thickness and lithology depending on the regional tectonic location (Czapowski et al., 2009). In the Ukrainian part of Carpathian Foredeep, three main tectonic zones were distinguished (Figure 4b): (I) outer zone (Bilche-Volytsya Unit), in which the Miocene molasse deposits overlie discordantly the Mesozoic platform basement at the depth of 10-200 m, and in the foredeep they subsided under the overthrust of the Sambir zone and are at depths of 1.2-2.2 km (Bukowski and Czapowski, 2009); Hryniv et al., 2007); (II) central zone (Sambir Unit), in which the Miocene deposits were overthrust some 8-12 km onto the external part of the Foredeep deposits of the external zone occur at depths of 1.0-2.2 km; (III) internal zone (Boryslav-Pokuttya Unit), where Miocene deposits were overthrust atop the Sambir Nappe zone across a distance of some 25 km (Hryniv et al., 2007).


    The Carpathian Foredeep formed during the Early Miocene, located north of emerging the Outer (Flysch) Carpathians. This basin was filled with Miocene siliciclastic deposits (clays, claystones, sandstones and conglomerates) with a maximum thickness of 3 km in Poland and up to 5 km in Ukraine (Oszczypko, 2006). Two main evaporite bearing formations characterise the saline portions of the succession and were precipitated when the hydrographic connection to the Miocene ocean was severely reduced or lost (Figures 4, 5): A) Vorotyshcha Beds, dated as Late Eggenburgian and Ottnangian, some 1.1-2.3 km thick and composed of clays with sandstones, with exploitable rocksalt and potash salt interbeds. This suite is further subdivided into two subsuites: a) A lower unit, some 100-900 m thick with rock salt beds and, b) An upper unit, some 0.7-1.0 km thick, with significant potash beds, now deformed (Hryniv et al., 2007).The Stebnyk potash mine is located in this lower subset in the Boryslav-Pokuttya Nappe region, close to the Carpathian overthrust); B) Tyras Beds of Badenian age reach thicknesses of 300-800 m in the Sambir and Bilche-Volytysa units and are dominated by salt breccias and contain both rock and potash salts. Thicknesses in the Bilche-Volytsya Unit range from 20-70 m and are made up of a combination of claystones, sandstones, carbonates, sulphates and rock salts with little or no potash.


    Hence, potash salts of the Carpathian Foredeep are related either to the Vorotyshcha Beds located in the Boryslav–Pokuttya zone, or to the Tyras Beds (Badenian) in the Sambir zone (Figure 5). These associations range across different ages, but have many similar features, such as large number of potash lenses in the section, mostly in folded-thrust setting, and owing to their likely Neogene-marine mother brine contain many sulphate salts, along with a high clay content. Accordingly, the main potash ore salts are kainite, langbeinite and kainite–langbeinite mixtures. Hryniv et al. (2007) note more than 20 salt minerals in the Miocene potash levels and in their weathering products. Bromine contents in halites of the Carpathian Foredeep for deposits without potash salts range from 10 to 100 ppm (on average 56 ppm); in halite from salt breccias with potash salts range from 30 to 230 ppm (average 120 ppm); and in halite from potash beds ranges from 70 to 300 ppm (average 170 ppm). In the ore minerals from the main potash deposits, bromine content ranges are: a) in kainite 800–2300 ppm; b) in sylvite 1410–2660 ppm; and c) in carnallite 1520–2450 ppm. This is consistent with kainite being a somewhat less saline precipitate than carnallite/sylvite (Figure 6).


    The brines of Vorotyshcha and Tyras salt-forming basins (based on data from brine inclusions in an investigation of sedimentary halite, listed by Hyrniv et al. (2007), are consistent with mother brines of the Na–K–Mg–Cl–SO4 (MgSO4-rich) chemical type (consistent with a Neogene marine source). Inclusion analysis indicates the temperature of halite formation in the Miocene basin brines in Forecarpathian region was around 25°C. During the potash (Kainite) stages it is likely these solutions became perennially stratified and heliothermal so that the bottom brines could be heated to 40-60°C, more than double the temperature of the brine surface layer (see Warren, 2015 for a discussion of the physical chemistry and the various brine stratification styles). During later burial and catagenesis the temperatures preserved in recrystallised halites are as high as 70°C with a clear regional tectonic distribution (Hryniv et al. (2007).

    Maximum potash salt production was achieved under Soviet supervision in the 1960s, when the Stebnyk and Kalush mines delivered 150 x 106 tonnes of K2O and the “New” Stebnyk salt-works some 250 x 106 tonnes as K2SO4 per year.


    Stebnyk potash (Figure 7a)

    The potash salt deposit in the Stebnyk ore field occurs within the Miocene (Eggenburgian) Vorotyshcha Beds (Figures 4, 5). Salt-bearing deposits in the Stebnyk area were traditionally attributed to two main rock complexes (Lower and Upper Vorotyshcha Beds) separated by terrigenous (sandstones and conglomerates) Zahirsk Beds (Petryczenko et al., 1994). More recent work indicates that the Zahirsk Beds belonged to a olistostrome horizon (a submarine slump, interrupting evaporite deposition) and there are no valid arguments for subdividing the Vorotyshcha Beds into two subunits (Hryniv et al., 2007).

    There are multiple salt-bearing series in the Stebnyk deposit (Figure 4b) and their total thickness ranges up to 2,000 m in responses to intensive fold thickening and overthrusting of the Carpathians foredeep. Intervals with more fluid salt mineralogies were compressed and squeezed into the centers of synclinal folds, to form a number of elongate lens-shape ore bodies (Figure 4b). These bodies are often several hundreds meters wide and in mineable zones occur at the depth of 80-650 m, typically at 100-360 m.

    The lower part of the Vorotyshcha Suite (Beds) in the Stebnyk Mine area is composed of a salt-bearing breccia, with sylvinite or carnallitite interclayers typically in its upper parts, as well as numerous blocks of folded marly clays (Bukowski and Czapowski, 2009). Above this is the potash-bearing ore series , some 10-125 m thick and, composed of beds of kainite, langbeinite and lagbeinite-kainite with local sylvinite and kieserite (Hryniv et al., 2007). The potash interval is overlain by a rock salt complex some 60 m thick (Koriń, 1994).

    The Stebnyk plant is now abandoned and in disrepair. In 1983 there was a major environmental disaster (explosion) at a nearby chemical plant (in the ammonia manufacture section), which was supplied chemical feedstock by the mine. No lives were lost, but damage at the plant, tied to the explosion, released some 4.6 million cubic metres of thick brine from an earthen storage dam into the nearby Dniester River. At the time this river was probably the least environmentally damaged by industrial operations under Soviet administration. The spill disrupted water supplies to millions of people along the river, killed hundreds of tons of fish, destroyed river vegetation and deposited a million tons of mineral salts on the bottom of a 30-mile-long reservoir on the Dniester. Stebnik is located in the Ukrainian province of Lvov. Staff members at the United States Embassy at the time seized on the name to dub the incident ‘’Lvov Canal,’’ after the Love Canal contamination in the United States.

    Kalush potash salt geology (Figure 7b)

    Thickness of Miocene (Badenian) deposits near the Kalush Mine is around 1 km (Figures 4a). Two local salt units (beds) are distinguished within the Tyras Beds: the Kalush and Holyn suites, which constitute the nucleus of Miocene deposits of Sambir Unit (Figure 5). Beds have been overthrust and folded onto the Mesozoic and Middle to Upper Miocene molasse sediments of the outer (Bilche-Volytsya) tectonic unit (Figure 4b). The Kalush Beds are 50-170 m thick, mostly clays, with sandstone and mudstone intercalations,. In contrast the Holyn beds are more saline and dominated by clayey rock salts (30-60% of clay), salty clays and claystones (Koriń, 1994). Repeated interbeds and concentrations of potash salts up to several meters thick within the Holyn beds define a number of separate potash salt fields in the Kalush area (Figures 4b, 5). Such salt seams are dominated by several MgSO4-enriched mineralogies: kainite, langbeinite-kainite, langbeinite, sylvinite and less much uncommon carnallite and polyhalite. These polymineralogic sulphate ore mineral assemblages are co-associated with anhydrite, kieserite and various carbonates. The potash ore fields typically occur in tectonic troughs within larger synclines, usually at depths of 100-150 m, to a maximum of 800 m.

    Conventional processing streams for manufacture of SOP and MOP

    To date the main natural sulphate salts that have been successfully processed to manufacture sulphate of potash (SOP) are;

  • Kainite (KCl.MgSO4.3H2O) (as in Sicily - potash mines are no longer active)
  • Kieserite (MgSO4.H2O) (as in Zechstein, Germany - some potash mines active)
  • Langbeinite (K2SO4.2MgSO4) (as in Carlsbad, New Mexico - active potash mine)
  • Polymineralic sulphate ores (as in the Stebnyk and Kalush ores, Ukraine - these potash mines are no longer active)
  • All the processing approaches deal with a mixed sulphate salt or complex sulphate brine feed and involve conversion to form an intermediate doublesalt product, usually schoenite (or leonite at elevated temperatures) or glaserite. This intermediate is then water-leached to obtain SOP.

    For example, with a kainite feed, the process involves the following reactions:

    2KCl.MgSO4.3H2O --> K2SO4.MgSO4.6H2O + MgCl2

    followed by water-leaching of the schoenite intermediate

    K2SO4.MgSO4.6H2O --> K2SO4 + MgSO4 + 6H2O


    In Sicily in the 1960s and 70s, the Italian miners utilized such a solid kainitite ore feed, from conventional underground mining and leaching approaches. The various Italian mines were heavily government subsidized and in terms of a free-standing operation most were never truly profitable. The main kainitite processing technique used in Sicily, is similar in many ways to that used to create SOP from winter-precipitated cryogenic salt slurries in pans that were purpose-constructed in the North Arm area of in Great Salt Lake, Utah (Table 1; see Warren, 2015 for details on Great Salt Lake operations). The Italian extraction method required crushing and flotation to create a fine-sized kainite ore feed with less than 5% NaCl. This product was then leached at temperatures greater than 90°C with an epsomite brine and converted into a langbeinite slurry, a portion which was then reacted with a schoenite brine to precipitate potassium chloride and epsomite solids, which were then separated from each other and from the epsomite brine. A portion of the potassium chloride was then reacted with magnesium sulphate in the presence of a sulphate brine to create schoenite and a schoenite brine. This schoenite brine was recycled and the remaining potassium chloride reacted with the schoenite in the presence of water, to obtain potassium sulphate and a sulphate brine.

    The processing stream in the Ukraine was similar for the various Carpathian ore feeds, which “out-of-mine-face” typically contained around 9% potassium and 15% clay and so were a less pure input to the processing stream, compared to the typical mine face product in Sicily. Like Sicily, schoenite was the main intermediate salt. Ore was leached with a hot synthetic kainite solution in a dissolution chamber. The langbeinite, polyhalite and halite remained undissolved in the chamber. Salts and clay were then moved into a Dorr-Oliver settler where the clays were allowed to settle and were then moved to a washer and discarded. The remaining solution was crystallized at the proper cation and anion proportions to produce crystalline schoenite. To avoid precipitation of potassium chloride and sodium chloride, a saturated solution of potassium and magnesium sulfate was added to the Dorr-Oliver settler. The resulting slurry of schoenite was filtered and crystals were leached with water to produce K2SO4 crystals, which were centrifuged and recycled and a liquor of potassium and magnesium sulfates obtained. The liquid phase from the filter was recycled and added to the schoenite liquor from obtaoned by vacuum crystallization. Part of the schoenite liquor was evaporated to produce crystalline sodium sulfate, while the magnesium chloride liquid end product was discarded. The slurry from the evaporation unit was recycled as “synthetic kainite.” This process stream permitted the use of the relatively low quality Carpathian ore and produced several commercially valuable products including potassium sulfate, potassium-magnesium sulfate, potassium chloride, sodium sulfate and magnesium chloride liquors. Being a Soviet era production site, the economics of the processing was not necessarily the main consideration. Rather, it was the agricultural utility of the product that was paramount to the Soviet state.

    Can Danakhil potash be economically mined?

    For any potash deposit (MOP or SOP) there are three approaches that are used today to economically extract ore (Warren 2015): 1) Conventional underground mining. 2) Processing of lake brines 3) Solution mining and surface processing of brines. Historically, method 1 and 2 have been successfully conducted in the Danakhil Depression, although method 1) was terminated in the Dallol area by a mine flood.

    Conventional mining

    To achieve a successful conventional underground MOP potash mine any where in the world, ideally requires (Warren, 2015): 1) A low dipping, laterally continuous and consistently predictable quality ore target, not subject to substantial changes in bed dip or continuity. 2) An ore grade of 14% K2O or higher, and bed thickness of more than 1.2 m. 3) Around 8-m of impervious salt in the mine back or roof, although some potash mines, such as the Boulby mine in the UK are working with < 2 meters of salt in the back (but there the extraction is automated and the access roads approach the target ore zone from below). 4) An initial access shaft that is vertical and typically dug using ground freezing techniques to prevent unwanted water entry during excavation. 5) A typical ore depth in the range 500-1100 metres. Shallower mines are subject to unpredictable water entry/flooding and catastrophic roof collapse, as in the Cis-Urals region (see Solikamsk blog). Mines deeper than 1000-1100 metres are at the limit of conventional mining and the salt surround is subject to substantial creep and possible explosive pressure release outbursts (as in some potash mines in the former East Germany). 6) At-surface and in-mine conditions not subject to damage by earthquakes, water floods or volcanism.

    During the feasibilty phase of the Parsons Mining Project it became evident that the halite material overlying the Sylvinite Member was porous and that there was no adequate hydrologic protection layer above the Sylvinite Member. In my mind, this is further evidence of the hydrologic access needed to convert carnallite to sylvite along the bajada front (see previous blog). In any event the absence of a hydrologic protection layer above the Sylvinite Member means that conventional underground mining is not feasible for this type of potash. In addition, given the tectonic instability of the Danakhil Depression it is likely that no underground conventional mine is feasible in the hydrologically, seismically and hydrothermally active setting, which is the Danakhil depression, even if planning to exploit the deeper widespread kainitite beds (>350-450m)

    Some explorers in the Danakhil depression, especially on the Eritrean side are proposing to use surface or open-pit mining (quarrying) approaches to reach and extract/processing shallow ore salts. For this approach to be successful requires the shallow potash targets to be above regional groundwater level. Depths to the different ore targets on the Ethiopian side of the depression range between 45m and 600m and almost all lie below the regional water. Also, to access the mineralised material a large volume of variably water-saturated overburden would need to be removed. Even if areas with ore levels above the water table do exist on the Ethiopian side, the whole of the Danakhil sump is subject to periodic runoff and sheetflooding, sourced in the western highlands. Open pit areas would be regularly flooded during the lifetime of the pit, resulting in a need for extensive dewatering. For these reasons, and the possibility of earthquake damage, open pit mining is likely not feasible.

    Can the Danakhil potash be solution mined?

    To achieve this, brines extracted from different mineralogical levels and ore types will need to be individually targeted and kept as separate feeds into dedicated at-surface processing streams. On the Dallol surface, there are numerous sites that are suitable for pan construction, the climate is suitable for natural solar concentration as the region is typically dry, flat and hyperarid. If the potash zones in the Dallol depression are to be economically exploited via solution mining it will likely first require an understanding of the geometries of the 3 different forms of potash, namely; 1) Bedded kainitite-carnallitite (widespread in the depression), 2) Diagenetic sylvite via incongruent dissolution (focused by deep meteoric mixing and the bajada chemical interface along the western margin. 3) Hydrothermal potash (largely found in the vicinity of Dallol mound). Next, in order to have known-chemistry feedstocks into a SOP chemical plant, it will require the appropriate application of extraction/solution mining chemistries for each of these deposit styles. This would involve the construction of dedicated brine fields and the pumping of shallow Dallol brines (mostly from <200-250m below the surface) into a series of mineralogically-separated at-surface solar concentrator pans. 

    There are some subsurface aspects that need to be considered and controlled  in a solution mining approach in the Danakhil. The first is the possibility of uncontrolled solution cavity stoping (for example where a solution cavity blanket layer is lost due to cavity intersection with an unexpected zone of high permeability). If cavity shape is not closely monitored (for example by regular downhole sonar scans) and controlled, this could ultimately lead to the collapse of the land surface atop regions of shallow evaporites (<150-200 below the surface). As we saw in blog 3, doline collapse is a natural process in the Dallol Mound region, as it is any region of shallow soluble evaporites in contact with undersaturated pore waters. Ongoing solution via interaction with hydrothermal waters has created the colorful brine springs that attract tourists to the Dallol Mound region. But a operator does not want new dolines to daylight in their brine field, as environmental advocates would quickly lay blame at the feet of the brinefield operator. For this reason, the region in the vicinity of the Dallol Mount (eg the “Crescent deposit”) should probably be avoided.

    Most modern brinefield operators prefer a slowly-dissolving targeted salt bed that is at least 400-500m below the land surface (Warren, 2015). This broadens and lessens the intensity of the cone of ground collapse above the extraction zone and so lessens the possibility of catastrophic surface collapse. Use of a diesel rather than air blanket during cavity operation is also preferred because of potential porosity intersections at the base of the Upper Rock Salt (URF) contact (see blog 2 in the Danakhil blogs) Appropriate deeper potash beds in the Danakhil are laterally continuous beds of kainitite with lesser carnallitite. Drilling to date has identified little sylvite or bischofite in these widespread layers. This simplifies the mineral input chemistry in terms of a kainite target further out in the saltflat with a sylvite or sylvite bischofite operation closer toward the western margin, but there are no currently active solution mines solely targeting a kainite ore anywhere in the world.

    This leads to another consideration with a solution mining approach in the Danakhil depression, and that is that there are no existing brine technologies that can deal economically with high concurrent levels of magnesium and possibly-elevated sulphate levels in a recovered brine feed. The third consideration is reliably predicting the occurrence of, and avoiding, any metre- to decametre-scale brine-filled cavities that the drilling has shown are not uncommon at the sylvinite-bischofite-carnallite level in the Dallol stratigraphy along the Bajada chemistry zone. Intersecting and slowly dewatering such large brine cavities may not lead to at-surface ground collapse, but if not identified could create unexpected variations in the ionic proportions of brine feeds into the solar concentrators (for example drilling has identified subsurface regions dominated by bischofite, which is one of the most soluble bittern salts in the Danakhil depression - see Ercospan 2010, 2011 for drill result summaries).

    And so?

    So, at this stage, there are encouraging possibilities for economic recovery of both MOP and SOP from solution brines pumped to chemistry-specific solar pans in the Danakhil. Processing chemistry will require further site-specific studies to see which of the current known methods or their modification is economically feasible for SOP and perhaps combined SOP and MOP manufacture in the hyperarid climate of the Danakhil, as is being currently done by Allana Potash. It is also possible that a new processing stream chemistry could to be developed for the Dallol brines, in order to deal with very high concurrent levels of MgCl2 (widespread bischofite beds), or develop new or modify existing processing streams that target kainitite at depth. Similar K2SO4 brine processing chemistries have been applied in pans of the margins of the Great Salt Lake. But there salt pan processing was in part seasonally cryogenic, something that the Dallol climate certainly is not, so it is likely modified or new approaches to year-round pan management will be required.

    Any future potash operation in the Danakil will have to compete in product pricing with well established, high-volume low cost producers in Canada, Belarus and Russia (Figure 8). Today, establishing a new conventional underground potash mine is associated with setup costs well in excess of a billion dollars (US$). The costs are high as the entry shaft to a conventional underground mine must be completed without water entry and is usually done via ground freezing. This is the approach currently underway at BHP’s MOP Jansen Mine in Saskatchewan, Canada. Because of the very high costs involved in underground entry construction, and the well established nature of the competition, the proved amount of ore for a conventional mine should be sufficient for at least 20 years of production (subject to a given mill size, mill recovery rate for a given ore depth and the density and origin of salt “horses”). Kogel et al. (2006) states any potash plant or mill should be at capable of least 300,000 t K2O per annum in order to compete with a number of established plants with nameplate capacity in excess of 1 Mt.

    In contrast, the shallow nature of a Danakhil potash source means cheaper access costs, while a solution well approach makes for much cheaper and shorter approach times for brine/ore extraction, providing suitable economic brine processing streams are available (Figure 8). Potash is a mine product where transport to market is a very considerable cost proportion in terms of an operation's profitability. The location of the Danakhil gives it a low-cost transport advantage as a future supplier to the ever-growing agricultural markets of Africa, India and perhaps China. And finally, a potassium sulphate product has a 30% cost premium over a muriate of potash (KCl) product.

    References

    Bukowski, K., and G. Czapowski, 2009, Salt geology and mining traditions: Kalush and Stebnyk mines (Fore-Carpathian region, Ukraine): Geoturystyka, v. 3, p. 27-34.

    Czapowski, G., K. Bukowski, and K. Poborska-Młynarska, 2009, Miocene rock and potash salts of West Ukraine. y): Field geological-mining seminar of the Polish Salt Mining Society. Geologia (Przegląd Solny 2009), Wyd. AGH, Kraków, 35, 3: 479-490. (In Polish, English summary).

    Decima, A., J. A. McKenzie, and B. C. Schreiber, 1988, The origin of "evaporative" limestones: An Example from the Messinian of Sicily: Journal of Sedimentary Petrology, v. 58, p. 256-272.

    Decima, A., and F. Wezel, 1973, Late Miocene evaporites of the central Sicilian Basin; Italy: Initial reports of the Deep Sea Drilling Project, v. 13, p. 1234-1240.

    Decima, A., and F. C. Wezel, 1971, Osservazioni sulle evaporiti messiniane della Sicilia centromeridionale: Rivista Mineraria Siciliana, v. 130–132, p. 172–187.

    Garcia-Veigas, J., F. Orti, L. Rosell, C. Ayora, R. J. M., and S. Lugli, 1995, The Messinian salt of the Mediterranean: geochemical study of the salt from the central Sicily Basin and comparison with the Lorca Basin (Spain): Bulletin de la Societe Geologique de France, v. 166, p. 699-710.

    Garrett, D. E., 1995, Potash: Deposits, processing, properties and uses: Berlin, Springer, 752 p.

    Hite, R. J., and A. S. Wassef, 1983, Potential Potash Deposits in the Gulf of Suez, Egypt: Ann. Geol. Survey Egypt, v. 13, p. 39-54.

    Hryniv, S. P., B. V. Dolishniy, O. V. Khmelevska, A. V. Poberezhskyy, and S. V. Vovnyuk, 2007, Evaporites of Ukraine: a review: Geological Society, London, Special Publications, v. 285, p. 309-334.

    Koriń, S. S., 1994, Geological outline of Miocene salt-bearing formations of the Ukrainian fore-Carpathian area (In Polish, English summary): Przegląd Geologiczny, v. 42, p. 744-747.

    Lowenstein, T. K., L. A. Hardie, M. N. Timofeeff, and R. V. Demicco, 2003, Secular variation in seawater chemistry and the origin of calcium chloride basinal brines: Geology, v. 31, p. 857-860.

    Lowenstein, T. K., M. N. Timofeeff, S. T. Brennan, H. L. A., and R. V. Demicco, 2001, Oscillations in Phanerozoic seawater chemistry: Evidence from fluid inclusions: Science, v. 294, p. 1086-1088.

    Lugli, S., 1999, Geology of the Realmonte salt deposit, a desiccated Messinian Basin (Agrigento, Sicily): Memorie della Societá Geologica Italiana, v. 54, p. 75-81.

    Lugli, S., and T. K. Lowenstein, 1997, Paleotemperatures preserved in fluid inclusions in Messinian halite, Realmonte Mine (Agrigento, Italy): Neogene Mediterranean Paleoceanography, 28–30 September 1997, Erice. Abstract volume, 44–45.

    Lugli, S., B. C. Schreiber, and B. Triberti, 1999, Giant polygons in the Realmonte mine (Agrigento, Sicily): Evidence for the desiccation of a Messinian halite basin: Journal of Sedimentary Research Section A-Sedimentary Petrology & Processes, v. 69, p. 764-771.

    Manzi, V., S. Lugli, M. Roveri, B. C. Schreiber, and R. Gennari, 2011, The Messinian "Calcare di Base" (Sicily, Italy) revisited: Geological Society of America Bulletin, v. 123, p. 347-370.

    Notholt, A. J. G., 1983, Potash in Developing Countries, in R. M. McKercher, ed., Potash '83; Potash technology; mining, processing, maintenance, transportation, occupational health and safety, environment, p. 29-40.

    Oszczypko, N., P. Krzywiec, I. Popadyuk, and T. Peryt, 2006, Carpathian Foredeep Basin (Poland and Ukraine): Its Sedimentary, Structural, and Geodynamic Evolution, in J. Golonka, and F. J. Picha, eds., The Carpathians and their foreland: Geology and hydrocarbon resources, The American Association of Petroleum Geologists Memoir, v. 84, p. 293-350.

    Petryczenko, O. I., G. M. Panow, T. M. Peryt, B. I. Srebrodolski, A. W. Pobereżski, and K. W.M., 1994, Outline of geology of the Miocene evaporite formations of the Ukrainian part of the Carpathian Foredeep (In Polish, English summary): Przegląd Geologiczny, v. 42, p. 734-737.

    Roveri, M., S. Lugli, V. Manzi, and B. C. Schreiber, 2008, The Messinian Sicilian stratigraphy revisited: new insights for the Messinian salinity crisis: Terra Nova, v. 20, p. 483-488.

    Warren, J. K., 2015, Evaporites: A compendium (ISBN 978-3-319-13511-3) Released August 2015: Berlin, Springer, 1600 p.

    Danakhil Potash; Ethiopia - Modern hydrothermal and deep meteoric KCl, Part 3 of 4

    John Warren - Friday, May 01, 2015

    So far we have discussed the modern salt pan geology of the Danakhil (Part 1 of 4) and the initial subaqueous setting for widespread bedded potash, now in the subsurface, mostly as a kainitite bed (Part 2 of 4). In this blog we will discuss examples of potash in the Danakhil where remobilised salts and brines are related to the circulation of hydrothermal and meteoric fluids have facilitated localised reworking of potash to the surface (part 3 of 4). These fluids are related to the thermal anomalies created by the emplacement of the Dallol mound and the chemical front created by the encroachment of the Bajada along the western margin of the saltflat. Notably, we shall see the Dallol Mound is not a volcanic cone, rather it is an anticlinal dome of uplifted and eroded bedded salt, capped and surrounded by hydrothermal crater features typified by karst pools and brine outflows. Its creation is likely related to emplacement of igneous material at depth but, as yet there, has been no breakout of volcanic rock material in the mound area. This has important economic implications for the nature of remobilised potash and the creation of potential potash ores in the Dallol Mound area, these cosiderations are separate from the regional distribution of primary potash beds (kainitite and carnallitite) that were discussed in the previous blog.


    Thermal brine springs and potash occurrences near Dallol mound

    Today, hot springs supply and maintain a number of hydrothermally-fed brine pools and brine filled karst lakes in various depressions both atop and near the regional anticlinal salt mound or salt dome, sometimes called Dallol Mountain (Figure 1). As it only rises some 60 metres from the surrounding surface (-81 m versus -120 m) the term mountain is a misnomer. The highly dissected and eroded slope of bedded halite that is the southwest margin of Dallol mound shows the various springs are active in a region of uplifted and eroded bedded evaporite that defines the Dallol mound (Figure 2a). For example, brine springs still supply a small carnallite deposit known as the Crescent deposit located near the uplifted black halite beds that define Black Mountain and located 1.5 km southwest of Dallol mound (Figure 2b). This potash ore is the result of hydrothermally-driven groundwater activity, likely related to the emplacement of the Dallol Mound. The uplift-related thermal hydrology has broken up the mineralogical continuity of the nearsurface evaporite beds including the equivalents to the potash-rich Houston Fm.


    The Black Mountain potash deposits caught the attention of the Houston-based  Ralph M. Parsons company in 1954 where, according to Holwerda and Hutchinson, 1968, potash mining had previously already taken place at the Crescent carnallite/sylvite deposit. Earlier extraction had involved, amongst other techniques, flooding of salt pans around a continuously flowing hot spring, followed by harvesting of potash-rich salts, once natural deliquescence had flushed most of the highly soluble MgCl2 from the system. A concession was obtained Parsons linked to obligations to investigate the various potash deposits in the area, some of which were tied to actual outcrops of potash salts. The Parsons Company set up its base on Dallol Mountain at a site previously occupied by the Italian mining community, which had operated in the first few decades of last century (Figure 2a; the modification and reuse of older salt brick buildings is still evident on the ground today). As well, Parsons Co. constructed airstrips on Dallol Mountain and in the Musley area. They drilled more than 300 holes in order to better understand the the distribution of the potash beds. Drilling operations in 1959-1961 led to the delineation of the small localized "Crescent" carnallitite deposit in the vicinity of Black Mountain . This was followed by the discovery of the much larger (>80 million tonnes) "Musley" sylvite deposit near the base of the Ethiopian Highlands, some 5km W of Dallol, and extending at least 10km in a N-S orientation. A 92m vertical shaft and a total of 805m of drives were made in this deposit, but all work was stopped in 1967 after rapid influx of water into the conventional mine killed a number of workers. The political tensions in the area at the time probably also played a part in preventing mining activity in the following years.

    Holwerda and Hutchinson (1968) argue that geographical location of the main "Musley" sylvite strata is directly west of Dallol Mound and at the base of the highlands. This, and the fact that sylvite is an alternation product that consistently overlays the carnallite strata and thickens (although discontinuously) along the western margin (see drill hole intersections published in Ercosplan, 2011), suggests that the potash enrichment was produced by selective leaching of MgCl2 from a carnallite precursor, driven by phreatic run-off waters sourced in the Ethiopian highlands. My own observations and plotting of enrichment fairways (using published Ercosplan 2010, 2011 data) confirms Holwerda and Hutchinson’s inferences. If diagenesis, not primary precipitation, is the prime mechanism of sylvite creation in the Musley region, then the regional sylvite control/distribution for this style of enrichment is related to a subsurface meteoric/groundwater phreatic overprint that parallels the encroaching bajada edge. It is a separate ore fairway to the more regional easterly dipping bedded kainitite/carnallitite trend.

    Waters in some of the active brine-filled hydrothermal craters and dolines can locally have temperatures of more than 100°C and when waters cool they precipitate varying combinations of halite, carnallite and bischofite. The brines are so saturated with salts that if a stick is thrust into a boiling brine pool and removed it is immediately covered by layer of carnallite or bischofite and halite (Figure 2b, c). The same pools are also rich in FeCl2, sulphur and manganese, which explains the spectacular bright green, red-orange and yellow colours of many of the saline mineral assemblages precipitating in and about these active spring-formed pools. Occasional intense storm-driven sheetfloods can drive renewed activity in the various springs in vicinity of the mound, as happened in the recent floods of February 2011, when the intensity of water circulation and the areal extent of the pools greatly increased. After the same storm flood, a natural collapse doline tens of metres across formed on the western depression margin. Clearly, the local hydrothermal/karstic enhancement style of bittern enrichment is a separate process set for potash enrichment compared to the widespread earlier deposition of marine-fed subaqueous kainite. Hence, it contrasts with the much more widespread set of depositional/early diagenetic processes that laid down the bulk of the bedded potash association that is the Houston Fm. in the Danakhil Depression (as discussed in the previous Danakhil blog).

    What is the Dallol Mound and what drives its uplift hydrology?

    Despite the widespread misconception that the Dallol mound is a lava cone, Mount Dallol is not a volcanic-centered feature on the Danakhil landscape. A visit to the area reveals no observable volcanic products (lava, ashfall or scoria) on the surface on or near the Dallol mound. This is so even in the region of the most recent phreatic activity in 1926 where a 30 m-diameter phreatic (explosion? or daylighting hydrothermal karst) crater formed, hosted in salt beds (Figure 2b). All the rocks associated with this cavity and its formative event are not volcanic. This means the mechanism that created the Dallol Mound is unlike the magmatic events that created the world famous Erte Ale volcanic cone, with its distinctive longterm active magma lake and located some 80 km to the south of Dallol and still in the Danakhil depression. Instead, the Dallol mound crest is made up of uplifted and eroded halite and potash beds soaked in a thermal hydrology that breaks out on the lake surface as a number of hot bubbling sulphurous brine pools. This is also true of the off-mound crater that formed in 1926 near Black Mountain and still retains bubbling brines with present temperatures ~65-70 °C. Nearby “Black Mountain” is a small area of dark coloured bedded and recrystallised halite, it is not a primary volcanic feature.

    As a sedimentologist visiting the area, I wondered at why the Dallol mound features had ever been called volcanic cones, hornitos, or maars (as they are widely described in the literature). To use such genetic terms in a geologically correct fashion I would like to put my hand on a piece of volcanic debris (lava, pumice, scoria or ash) in any of the craters before I call the Dallol mound a volcanic cone. And yet, many workers in the published literature dealing with the Dallol area are happy to do this. I am not saying there is no influence of magmatic heating in forming Dallol Mound, only that molten volcanic rock has yet to surface in the immediate Dallol region. Hence it is unlike the many actual volcanic cones, maars and hornitos to the south and north and this is an significant observation as it deals with mechanism of local potash enrichment. I will argue in the next section that this is because Dallol Mound is a salt uplift feature or dome capped by phreatic cone/ hydrothermal karst structures and all related to the migration of molten magma into more deeply buried salt beds, which contain hydrated salts at the level of the Houston Fm and perhaps even deeper buried hydrated salt layers (see blog 2).

    Darrah et al (2013) and Detay (2011) argue that the 30m diameter 1926 crater and other nearby pools on the Dallol saltflat in the vicinity of the Dallol mound are the result of a phreatic explosions, tied to the increasing gas pressure in superficial hydrothermal reservoirs atop a deeper mass of molten rock. The mound is a landscape feature indicative of deep dyke/sill intrusion that did not surface. According to Holwerda and Hutchinson (1968) this yet-to-daylight dyke complex explains the linear orientation of the mound, its pools and other karst/erosion features on the salt flat surface in vicinity of the Dallol mound. That is, the various Dallol hot springs typically consist of 30-40m diameter circular to sub-circular ponds, initially formed by explosive vapor eruptions, to form at-surface circular features, which are widely termed maars, although I would prefer to call them "maar-like." A “maar” is defined in the AGI Glossary of Geology as “a low relief, broad volcanic crater formed by multiple shallow explosive eruptions. It is surrounded by a crater ring, and may be filled by water. Type occurrence is in the Eifel area of Germany.” Given the lack of a volcanic crater rim the Dallol Mound and adjacent brine-filled cavities are not really maars, nor are they hornitos. They will likely evolve into such features, but in their current state better considered brine-filled fumaroles or solfateras or even better, hydrothermal karst cavities that have daylighted. Once the cavities have broken out onto the salt flat surface, these circular (possibly-explosive) features can continue enlarge due to ongoing rise of undersaturated waters and so evolve into expanding hydrothermal karst pools or they can be partially to completely filled with saline precipitates (with no volcanic products derived from molten igneous rock materials).


    So, instead of at-surface volcanic products such as lava and ashfall, most of the superficial precipitates/sediments observed in and around the various on- and off-structure Dallol brine pools are evaporite salts, along with some remnants of older clay-sediments. Brine fluids in various hot spring pools in the Dallol area (in the Dallol “hill” crest and the “Crescent” region near Black mountain, and in the “Boiling Lake” region south of the mound) are typically multi-coloured warm/hot ponds (Figure1, 3; Gebresilassie et al., 2011). The various pools are extremely salty (>500g/L), can be highly acidic (sometimes with a pH approaching 0.5), and gas-rich (as evidenced by steady, vigorous bubbling of gases). According to Darrah et al. (2013) the Dallol “salt dome” fluids and associated hot springs are hypothesized to result from the interaction between hot mantle fluids or basalt dyke injections with evaporite deposits at unknown depths. However, direct observations of the volumes of pool waters and the vigour of the outflow are known to increase after the occasional heavy rain event, as happened in February, 2011. Hence, it is unclear if sulfur-rich gases and the low pH brine fluids provide evidence of the interaction of hot mantle fluids with the evaporites (as inferred by Darrah et al., 2013) or the pool waters are, at least in part, related shallower ongoing hydrothermal/karst interactions with more deeply circulated meteoric waters sourced in the 1000-m high adjacent rift highlands.

    Why hydrated salts are important in some salt-hosted thermal systems: a Permian Zechstein analog

    Most published volcanogenic-related studies of the Dallol Mound have not considered the effects of hydrated salt layers in a situation of rising molten rock, where the country rock contains beds of hydrated evaporites such as kainite or carnallite. This situation is exposed in the dyke-intruded halite-carnallite levels in the mines of the Werra-Fulda mine district of Germany (Schoefield et al., 2013; Warren, 2015). There, the Permian Zechstein salt series contains two important potash salt horizons (2-10m thick), which are mined at a depths ≈ 800 m from within a 400m thick halite host (Figure 4a). In the later Tertiary, basaltic melts intruded these Zechstein evaporites, but only a few dykes reached the Miocene landsurface. Basaltic melt production was related to regional volcanic activity some 10 to 25 Ma. Basalts exposed in the mine walls are typically subvertical dykes, rather than sills. These basaltic intervals can crosscut the salt over zones up to several kilometres wide (Figure 4b). However, correlations of individual dyke swarms, between different mines, or between surface and subsurface outcrops, is difficult.


    The basalts are phonolitic tephrites, limburgites, basanites and olivine nephelinites. Dyke margins in contact with halite are usually vitrified, forming a microlitic limburgite glass along dyke edges (Knipping, 1989). At the contact on the evaporite side of the glassy rim there is a cm-wide carapace of high temperature salts (mostly anhydrite and ferroan carbonates). Further out, the effect of the high temperature envelope is denoted by transitions to clear halite, with higher temperature fluid inclusions (Knipping 1989). All of this centimetre to metre-scale alteration is an anhydrous alteration halo, the salt did not melt (halite’s melting temperature is 804°C), rather than migrating, the fluid driving recrystallisation was largely from local movement of entrained brine inclusions. The dolerite/basalt interior of the basaltic dyke is likewise altered and salt soaked, with clear, largely inclusion-free halite typically filling vesicles in the basalt.

    Worldwide, dykes intersecting salt beds tend to widen to become sills in two zones: 1) along evaporite units within the halite mass that contain hydrated salts, such as carnallite or gypsum and, 2) where rising magma has ponded and so created laccoliths at the upper or lower halite contact with the adjacent nonsalt strata or against a salt wall (Warren, 2015). The first is a response to a pulse of released water as dyke-driven heating forces the dehydration of hydrated salt layers. The second is a response to the mechanical strength contrast at the salt-nonsalt contact. The first is what is observed in the Fulda region and is also likely relevant to the formation of the Dallol Mound and its remobilised potash-precipitating brines.

     

    In such subsurface regions, the heating of hydrated salt layers (such as carnallite or kainite), adjacent to a dyke or sill, drives off the water of crystallisation (chemical or hydration thixotropy) at a much lower temperatures than that at which anhydrous salts, such as halite or anhydrite, thermally melt (Table 1). In the Fulda region the thermally-driven release of water of crystallisation within particular Zechstein salt beds creates thixotropic or subsurface “peperite” textures in carnallitite ore layers, where heated water of crystallisation escaped from the hydrated-salt lattice. Dehydration-driven loss of mechanical strength focuses zones of magma entry into particular horizons in the salt mass, wherever hydrated salt layers were intersected (Figure 4c verses 4d). In contrast, dyke and sill margins are much sharper and narrower in zones of contact with anhydrous salt intervals (Figure 4b; Schofield et al., 2014).

    Accordingly, away from immediate vicinity of the direct thermal aureole, heated and overpressured dehydration waters can enter a former Zechstein carnallite halite bed, and drive the creation of extensive soft sediment deformation and [1]peperite textures in the former hydrated layer (Figure 4d, e). Mineralogically, sylvite and coarse recrystallised halite dominate the salt fraction in the peperite intervals/beds. These are evaporite-related beds formed within a hydrated salt bed and so differ from the common notion of volcanic peperites indicating water-saturated sediment intercations with very shallow dyke or sill emplacements. Sylvite in these altered zone is a form of dehydrated carnallite, not a primary-textured salt. In the Fulda region, such altered zones and deformed units can extend along former carnallite layers to tens or even a hundred or more metres from the dyke feeder. Ultimately, the deformed potash bed passes laterally out into the unaltered bed, which retains abundant inclusion-rich primary chevron halite and carnallite (Figure 4d versus 4e). That is, nearer the basalt dyke, the carnallite is largely transformed into inclusion-poor halite and sylvite, the result of incongruent flushing of warm saline fluids mobilized from the hydrated carnallite crystal lattice as it was heated by dyke emplacement. During Miocene salt alteration/thermal metamorphism in the Fulda region, NaCl-fluids were mixed with fluids originating from thermally-mobilised crystallisation water in the carnallite, as it converted to sylvite.

    In the Dallol depression I think it is highly likely that a similar set of destabilisation processes occurred when rising dyke magma reached the levels of hydrated salts (kainite and carnallite beds) in the Houston Formation of the Danakhil fill, after passing relatively passively through the Lower Rocksalt Formation (see the previous blog). Emplacement of the magma/dyke into  hydrated evaporites in the vicinity of what is now the Dallol mound would have mobilised and deformed the hydrated salt level, converting carnallite to sylvite, kainite to bischofite and lesser kieserite, as well as creating widespread cavities filled with pressured volatiles carried by MgCl and KCl brines. Once these hydrothermal cavities dissolve their way to surface, the feeder brines can cool and precipitate as prograde salts such as halite, sylvite and perhaps bischofite. Such destabilisation would have accommodated the emplacement of a basaltic sill at the level of the potash salts, in turn driving the uplift of the lake beds above this region. Mound-related uplift and hydrothermal activity then drives the formation of natural regions of ground collapse, sulphurous and acidic springs and fumaroles, along with the creation of water-filled chimneys and doline sags, filling with various hydrothermal salts, in the vicinity of the volcanic mound.

    Implications for Potash distribution in the Danakhil Depression

    The discussion of potash mineral-forming processes in this and the previous blog clearly underlines a trichotomy in the way potash has accumulated in halite host-beds across the Danakhil Depression. The most widespread form of potash in the Danakhil Depression is as a primary evaporite bed, composed of primary marine kainitite precipitates with a carnallite cap (Houston Formation). Across the western side of the depression this easterly dipping bed is now buried beneath 30-150 m of overburden salts. It likely precipitated as a marine seepage-fed bittern layer, at a time the Danakhil depression was hydrographically isolated from a direct surface connection with the Red Sea. Its brine hydrology was dominantly subaqueous and not unlike that of modern Lake Asal in Djibouti, although it was more saline than Asal in the subaqueous potash sump areas. Thus, the Danakhil potash bed (Houston Fm) formed sometime ago, its formative hydology is no longer present in the depression and it may be as old as Pliocene or more likely early to mid Pleistocene. There has been sufficient time for this bed to tilt toward the east. The unit is underlain by the subaqueous Lower Rocksalt Formation (LRF) and subsequently overlain by the Upper Rocksalt Formation (URF). Both these halite formations do not entrain primary potash beds. The LRF contains numerous CaSO4 layers, while the URF contains clayey laminite beds and locally hosts regions of remobilised potash salts. The URF evolves upward into the saltflat/ephemeral lake hyperarid hydrology that typifies the modern depression.

    More localised forms of potential potash ore typify occurrences in the Dallol and Musley areas (Figure 2a). There potash in the Dallol Mound region is hydrothermally reworked from the uplifted equivalents of the Houston Formation. Even today this hydrology is precipitating carnallitite (associated with bischofite and minor kieserite) in various hydrothermal brine pools atop and around the Dallol Mound, such as the carnallite-dominant Crescent deposit (Figure 2b). These hydrothermal salts owes their origins to daylighting of pressurised fluid systems and cavities. They were created by the volatile products of hydrated salt layers (Houston Fm) where these salts had come into contact with thermal aureoles or actual lithologies of newly emplaced dykes that had penetrated the underlying halite section. Actual molten volcanic rock has yet to make it to the surface in the Dallol Mound region, although active volcanic mounds and flows do typify the saltflat surface tens of kilometres to the south (Erte Alle ) and north. Based on the analogy exposed within the Zechstein-hosted potash mines of the Fulda region of Germany, it is likely that as well as creating at-surface brine pools, this hydrothermal dyke-related hydrology converts any carnallitite to a sylvinite bed at the level of contact with the Houston Fm. 

    Then there is the deep-meteoric alteration system that is altering the kainitite/carnallitite of Houston Fm into sylvinite, it is active along the deep meteoric alteration front located at the irregular interface between the downdip end of the Musley Fan and the updip portion of the Houston Fm. This diagenetic mechanism formed the Musley potash deposit, defined and exploited by the Parsons Company operations and documented in Holwerda and Hutchison (1968). Variations on this deep-meteoric alteration theme likely extend south and north of the Musley fan, wherever the active phreatic hydrology of the bajada located at the foot of the Ethiopian Highlands interacts and interfingers with the updip edge of the easterly dipping Houston Formation.

    Once again there is no "one-size-fits-all) model for economic potash understanding (Warren, 2010, 2015). Even in what is probably the youngest known marine-fed potash system in the world, the original potash mineralogy and distribution has been altered and locally upgraded via diagenetic interactions with hydrothermal or deep-meteoric fluids. Predicting ore distributions in this, and all potash systems worldwide, requires an understanding of formative process evolution through deep time, and not just the simple application of a layer-cake primary stratigraphic model. 

    References

    Carniel, R., E. M. Jolis, and J. Jones, 2010, A geophysical multi-parametric analysis of hydrothermal activity at Dallol, Ethiopia: Journal of African Earth Sciences, v. 58, p. 812-819.

    Darrah, T. H., D. Tedesco, F. Tassi, O. Vaselli, E. Cuoco, and R. J. Poreda, 2013, Gas chemistry of the Dallol region of the Danakil Depression in the Afar region of the northern-most East African Rift: Chemical Geology, v. 339, p. 16-29.

    Detay, M., 2011, Le DALLOL revisité: entre explosion phréatomagmatique, rifting intra-continental, manifestations hydrothermales et halocinèse: LAVE. Liaison des amateurs de volcanologie européenne, v. 151, p. 7-19.

    ERCOSPLAN, 2010, Techical report and current resource estimate: Danakhil Potash Deposit, Afar State, Ethiopia: Project Reference: EGB 08-024.

    ERCOSPLAN, 2011, Preliminary Resource Assessment Study, Danakhil Potash Deposit, Afar State, Ethiopia: G & B Property: Project Reference: EGB 10-030.

    Gebresilassie, S., H. Tsegab, and K. Kabeto, 2011, Preliminary study on geology, mineral potential, and characteristics of hot springs from Dallol area, Afar rift, northeastern Ethiopia: implications for natural resource exploration: Momona Ethiopian Journal of Science, v. 3, p. 17-30.

    Holwerda, J. G., and R. W. Hutchinson, 1968, Potash-bearing evaporites in the Danakil area, Ethiopia: Economic Geology, v. 63, p. 124-150.

    Knipping, B., 1989, Basalt intrusions in evaporites: Lecture Notes in Earth Sciences (Springer-Verlag), v. 24, p. 132 pp.

    Schofield, N., I. Alsop, J. Warren, J. R. Underhill, R. Lehné, W. Beer, and V. Lukas, 2014, Mobilizing salt: Magma-salt interactions: Geology.

    Warren, J. K., 2010, Evaporites through time: Tectonic, climatic and eustatic controls in marine and nonmarine deposits: Earth-Science Reviews, v. 98, p. 217-268.

    Warren, J. K., 2015, Evaporites: A compendium (ISBN 978-3-319-13511-3) Released August 2015: Berlin, Springer, 1600 p.

    ------------------------- 

    [1] Peperite is a sedimentary rock that contains fragments of igneous material and is formed when magma comes into contact with wet water-saturated sediments. 

    Danakhil Potash, Ethiopia: Beds of Kainite/Carnallite, Part 2 of 4

    John Warren - Wednesday, April 29, 2015

    The modern Dallol saltflat described in the previous blog defines the upper part of more than 970 metres of halite-dominated Quaternary evaporites that have accumulated beneath the present salt pan of the Northern Danakhil. The total sequence is made up of interbeds of halite, gypsum, anhydrite and shale with a potash succession separating two thick sequences of halite (Figure 1; Holwerda and Hutchison, 1968; Augustithis, 1980). At depths of more than 35-40 meters, and deepening to the east, this km-thick subcropping Quaternary halite-dominated fill contains one, and perhaps two or more, potash beds. For a more detailed description of the upper part of the fill the reader is referred to the previous blog and Chapter 11 in Warren, 2015.


    Bedded Pleistocene evaporites may underlie the entire Danakil depression, but younger lava flows of the Aden Volcanic Series and alluvium washed in from the surrounding bajada obscure much of the older Pleistocene sedimentary series across much of the southern part of the depression beyond Lake Assale). Potash exploration drilling and core recovery is concentrated in the accessible parts of the northern Danakhil rift, where the saltflat facilitates vehicle access, compared with the lava-covered regions south of Lake Assale. The most recent volcanic activity affecting the known potash region was the emplacement of the Dallol Mound, which has driven local uplift of the otherwise subsurface potash section to where it approaches the surface in the immediate vicinity of the mound (Figure 2a).

    Away from the Dallol volcanic mound the upper potash bed beneath the saltflat lies at a depth of 38-190 metres. A lower inferred potash bed likely occurs at depth along the eastern end of the saltflat, but this second bed is inferred from high API kicks in gamma logs run in deeper wells, no solid salt was recovered (Holwerda and Hutchison, 1968). The upper proven potash bed is now the target zone for a number of minerals companies currently exploring for potash in the region. Regionally, both potash units dip east, with the deepest indicators of the two units encountered by the drill in a single well on the eastern side of the saltflat at depths of 683 and 930 m, respectively (Figure 2: Holwerda and Hutchison, 1968). The likely Quaternary age of the potash units, the marine brine source, explains the high magnesium content of the potash bittern salts, as modern seawater contains high levels of Mg and SO4.


    My study of core that intersected the potash interval and that is sandwiched between the Lower and Upper Halite units shows both the lower and the upper halite units retain pristine sedimentary textures, with features and vertical successions that indicate distinct hydrologies during their deposition (Figure 3). There is no textural evidence of halokinetic recrystallization in halites any of the studied cores and published seismic also indicates consistent dips in the evaporites . Most of the textures in the cored potash interval indicate a subaqueous density-stratified environment, with brine reworking of the upper part of primary kainitite, carnallitite units. Perennial subaqueous, density-stratified brines also typify the hydrology of the Lower Halite unit, albeit with somewhat lower salinities tan those precipitating the bitterns (Figure 3). The brine that precipitating the Upper Rocksalt Formation was shallower and more ephemeral. The following paragraphs summarise my core-based observations and interpretations that led to this interpretation of the evolving brine hydrology.

    The Lower Rocksalt Formation (LRF) is dominated by bottom-growth-aligned subaqueous halite textures and lack of siliciclastic detritus, unlike the Upper Rocksalt Formation (Figure 3). Halite textures in the LRF lack porosity and dominated by coarsely crystalline beds made up of cm-scale NaCl-CaSO4 couplets dominated by upward-pointing halite chevrons and mantled by thin CaSO4 layers (Figure 3). This meromictic-holomictic textural association passes up into the upper part of the LRF with cm-scale proportions alternating of less-saline to more-saline episodes of evaporite precipitation decreasing, indicating an “on-average” increasingly shallow subaqueous depositional setting as one approaches the base of the kainitite unit. The combination of bottom-nucleated and cumulate textures in the LRF are near identical to those in the halites in the kainitite interval in the Messinian of Sicily (see later). 

    The laminated Kainitite Member is also a subaqueous unit with layered cumulate textures (Figure 3), it was likely deposited on a pelagic bottom beneath a shallow body of marine-fed bittern waters, which never reached carnallite saturation. Above this are the variably present carnallitic Intermediate and Sylvinitite members and the overlying Halite marker beds in turn overlain by the Upper Halite unit. All retain pristine textures indicating mostly subaqueous deposition, soon followed by varying exposure and reaction with shallow phreatic brines moving across the top of Kainitite member. This shallow phreatic brine crossflow drove syndepositional mineral alteration and collapse in the upper part of the kainitite and carnallitite units.


    The potash-entraining interval between the URF and LRF is called the Houston Formation has been drilled and cored extensively by explorers in the basin, showing it is consistently between 15 and 40 metres thick (Figure 1). Stratigraphically, it consists of lower Kainitite Member (4-14m thick) atop and in depositional continuity with the LRF (more than 500m thick) (Figure 3). The Kainitite Member is fine-grained, laminated, locally wavy-bedded, containing up to 50% kainite cumulates in a cumulate (non-chevron) halite background, along with small amounts of a white mineral that is likely epsomite. It is overlain by what older literature describes as the “Carnallitite Intermediate unit” (3-25 m thick). More recent potash exploration drilling has shown all the members that constitute the Intermediate Carnallitite Member is not always present within the Dallol depression. Mineralogically is at best considered as variably developed (Figure 3). Its lower part is a layered to laminated carnallite-halite mixture with some kieserite, anhydrite and epsomite. This can pass up or laterally into kainitite with sylvite. Above the Intermediate Member is the 0-10m thick Sylvinite Member containing 20-30% sylvite, along with polyhalite and anhydrite (up to 10%). Typically the sylvinite member shows primary layering disturbed by varying intensities of slumping and dissolution. Often the upper part of a carnallite unit (where present) also shows similar evidence of dissolution and reprecipitation.

    Cores through the sylvinite member and parts of the upper carnallitite member sample a range of recrystallization/flow/slump textures, rather than primary horizontal-laminar textures. Beneath the sylvinite member, the variably-present upper carnallitite member contains a varied suite of non-commercial potash minerals that in addition to carnallite include, kieserite, kainite (up to 10% by volume) and polyhalite, along with minor amounts of sylvite. Minor anhydrite is common, while rinneite may occur locally, along with rust-red iron staining. Sylvite is more abundant near the top of the carnallitite member and its proportion decreases downward, perhaps reflecting its groundwater origin. Kainite is the reverse and its proportion increases downward. The sylvinite member and the carnallitite member also show an inverse thickness relationship. Bedding in the carnallitite member is commonly contorted with folded and brecciated horizons interpreted as slumps. The base of the carnallitite member is defined as the level where carnallite forms isolated patches in the kainite before disappearing entirely.

    Drilling in the past few years has clearly show that in some parts of the evaporite unit, located nearer to the western side of the basin, the lower and upper carnallite units are separated by thick bischoftite intervals (Figures 2b, 3). The bischofite is layered at a mm-cm scale and with no obvious breaks related to freshening and exposure, implying it too was deposited in a perennially subaqueous or phreatic cavity setting (Pedley et al., in press).

    The potash/bischofite interval passes up into a slumped and disturbed halite-dominated unit that is known as the “Marker Beds” because of the co-associated presence of clay lamina and bedded halite, along with traces of potash minerals (Figure 3). This unit then passes up into the massive Upper Rock Salt unit across an unconformity at the top of the halite “Marker Beds.” Bedded, and at times finely laminated cumulate textures in the various magnesian bittern units, are used by many to argue that the kainitite and the lower carnallitite members are primary or syndepositional precipitates.

    Three types of potash-barren zones can occur within it and are possibly related to the effects of groundwaters and solution cavity cements within the carnallitite unit, perhaps precipitated before the deposition of the overlying marker halites. Barren zones in the Sylvinite member are regions where: a) the entire sylvinite bed is replaced by a relatively pure stratiform halite, along with dispersed nodules of anhydrite, b) zones up to 23 m thick and composed of pure crystalline halite (karst-fill cements?) that occur patchily within the sylvinite bed and, c) potash-depleted zones defined by coarsely crystalline halite instead of sylvinite. Bedding plane spacing and layering and some slumping styles in the halite in styles a and b are similar to that in the sylvite bed. Contact with throughflushing freshened nearsurface and at-surface waters perhaps created most of the barren zones in the sylvinite. Fluid crossflow may also have formed or reprecipitated sylvite of the upper member, via selective surface or nearsurface leaching of MgCl2 from its carnallite precursor (Holwerda and Hutchison, 1968; Warren 2015). Due to the secondary origin of much of the sylvite in the Sylvinite member, the proportion of sylvite decreases as the proportion of carnallite increases, along with secondary kieserite, polyhalite and kainite.

    The kainite member is texturally distinctive and is composed of nearly pure, fine-grained, dense, relatively hard, amber-coloured kainite with ≈ 25% admixed halite (Figure 3). Core study shows the lamina style remained flat-laminar (that is, subaqueous density-stratified with periodic bottom freshening) as the mineralogy passes from the LRF up into the flat-laminated kainitite member (Figure 4: Warren, 2015; Pedley et al., in press). Throughout, the kainitite unit shows a cm-mm scale layering, with no evidence of microkarsting or any exposure of the kainitite depositional surface. That is, the Kainitite Member is a primary depositional unit, like the underlying halite and still retains pristine evidence of its dominantly subaqueous depositional hydrology. The decreased proportion of anhydrite in the Kainitite Member, compared to the underlying LRF, indicates a system that on-average was more saline than the brines that deposited the underlying halite. The preponderance of MgSO4 salts means the Kainitite unit like the underlying LRF formed by the evaporation of seep-supplied seawater.

    This situation differs from the present “closed basin” hydrology of the Danakil Depression which typifies the URF and the overlying Holocene succession (Hardie, 1990; Warren, 2015).

    Units atop the primary laminated textures of the kainitite, lower carnallitite and bischofite members (where present) tend to show various early-diagenetic secondary textures (Figure 4). It seems much of the sylvinite and upper carnallitite member deposition was in shallow subsurface or at-surface brine ponds subject to groundwater crossflows and floor collapse, possibly aided by seismically-induced pulses of brine crossflow. In addition, this perennial density-stratified brine hydrology was at times of holomixis subject to brine reflux and the brine-displacement backreactions that typify all evaporite deposition, past and present (Warren 2015).

    The observation of early ionic mobility in potash zone brines in the Danakil depositional system is also not unusual in any modern or ancient potash deposit. It should not be considered necessarily detrimental to the possibility of an extensive economically exploitable potash zone being present in the Danakil Depression. Interestingly, all the world’s exploited potash deposits, including those in the Devonian of Canada and Belarus, the Perm of the Urals and the potash bed of west Texas, show evidence of syndepositional and shallow burial reworking of potash (Warren, 2015). Early potassium remobilization has created the ore distributions in these and other mined potash depositsTextures and mineralogies in the Upper Rocksalt Formation (URF) define a separate hydrological association to the marine-fed LRF and Houston Formation (Table 4). Compared to the LRF, the URF has much higher levels of depositional porosity, lacks high levels of CaSO4, and has high levels of detrital siliciclastics. This is especially so in its upper part, which shows textural evidence of periodic and ongoing clastic-rich sheetflooding and freshening (Figure 4). It was deposited in a hydrology that evolved up section to become very similar to that active on today’s halite pan surface. The URF contains no evidence of salinities or textures associated with a potash bittern event and is probably not a viable exploration target for solid potash salts.

    Above the URF is a clastic unit with significant amounts of, and sometimes beds dominated by, lenticular gypsum and displacive halite. The unit thickens toward the margins of the depression (Figure 2). The widespread presence of diagenetic salts indicates high pore salinities as, or soon after, the saline beds that stack into the clastic unit were deposited. Some of these early diagenetic evaporite textures are spectacular, as seen in the displacive halite recovered in a core from the lower portion of the clastic overburden, some 45 m below the modern pan surface (Figure 3).

    What is clear from the textures preserved in the potash-rich Houston formation and the immediately underlying and overlying halites is that they first formed in a subaqueous-dominated marine-fed hydrology (Figure 4), which evolves up section into more ephemeral saltpan hydrologies of today (see the previous blog). The potash interval encapsulated in the Houston formation has primary mineralogical associations that are derived by evaporation of Pleistocene seawater (kainitite, carnallitite). In contrast the sylvite section in the Houston tends to form when these primary mineralogies are altered diagenetically perhaps soon after deposition but, especially, when hydrothermal waters circulated through uplifted beds of the Houston Formation, as is still occurring in the vicinity of the Dallol Volcanic Mound. Or where the chemical/meteoric interface associated with the encroachment of the bajada sediment pile drove incongruent dissolution of carnallite along the updip edge of the Houston Fm (as we shall discuss in the next blog). 

    References

    Augustithis, S. S., 1980, On the textures and treatment of the sylvinite ore from the Danakili Depression, Salt Plain (Piano del Sale), Tigre, Ethiopia: Chemie der Erde, v. 39, p. 91-95.

    Hardie, L. A., 1990, The roles of rifting and hydrothermal CaCl2 brines in the origin of potash evaporites: an hypothesis: American Journal of Science, v. 290, p. 43-106.

    Holwerda, J. G., and R. W. Hutchinson, 1968, Potash-bearing evaporites in the Danakil area, Ethiopia: Economic Geology, v. 63, p. 124-150.

    Pedley, H. M., J. Neubert, and J. K. Warren, in press, Potash deposits of Africa: African Mineral Deposits, 35TH International Geological Congress (IGC), Capetown (28 August to 4 September 2016).

    Warren, J. K., 2015, Evaporites: A compendium (ISBN 978-3-319-13511-3) Released August 2015: Berlin, Springer, 1600 p. 

    Danakhil Potash, Ethiopia: Is the present geology the key? Part 1 of 4

    John Warren - Sunday, April 19, 2015

    Geology of potash in the Danakil Depression, Ethiopia: Is the present the key?

    The Danakhil region, especially in the Dallol region of Ethiopia, is world renowned for significant accumulations of potash salts (both muriates and sulphates), and is often cited as a modern example of where potash accumulates today. What is not so well known are the depositional and hydrological dichotomies that control levels of bittern salts in the Pleistocene stratigraphy that is the Danakhil fill. Geological evolution of the potash occurrences in the Dallol saltflat and surrounds highlights the limited significance of Holocene models for potash, when compared to the broader depositional and hydrological spectra preserved in ancient (Pre-Quaternary) evaporite deposits (see Warren, 2010, 2015 for a more complete analysis across a variety of evaporite salts).

    Across the next four blogs, I shall discuss the geological character of the Danakhil fill and the controls on potash in the depression via four time-related discussions; A) Current continental fan - saltflat hydrology that typifies present and immediate past deposition in the depression (Danakhil Blog1). B) A time in the latest Pleistocene when there was a marine hydrographic connection exemplified by a healthy coralgal rim facies (probably ≈ 100,000 years ago, and a subsequent drawndown gypsum rim facies. Both units are discussed in this blog, (Danakhil Blog1), and C) a somewhat older Pleistocene period when widespread potash salts were deposited via a marine seepage fed hydrology (Danakhil Blog2). Then, within this depositional frame, we will consider D) the influence of Holocene volcanism and uplift driving remobilisation of the somewhat older potash-rich evaporite source beds into the Holocene hydrology (Danakhil Blog3) and finally how this relates to models of Neogene marine potash deposition (Danakhil Blog4). These observations and interpretations are based in large part on a two-week visit to the Dallol, sponsored by BHP minerals, and focused on the potash geology of the region. 

    Dallol Physiography

    The Danakhil Depression of Ethiopia and Eritrea is an area of intense volcanic and hydrothermal activity, with potash occurrences related to rift magmatism, marine flooding, and deep brine cycling. The region is part of the broader Afar Triple Junction and located in the axial zone of the Afar rift, near the confluence of the East African, Red Sea and Carlsberg rifts (Figure 1a; Holwerda and Hutchison, 1968; Hutchinson and Engels, 1970; Hardie, 1990). The depression defines the northern part of the Afar depression and runs SSE parallel to the Red Sea coast, but lies some 50 to 80 km inland, and is separated from the Red Sea by the Danakil Mountains. The fault-defined Danakil Depression is 185 km long, up to 70 km wide, with a floor that in the deeper parts of the depression is more than 116 meters below sea level. It widens to the south, beginning with a 10 km width in the north and widening up to 70 km in the south (Figure 1a). In the vicinity of Lake Assele, the northern portion of the Danakil is known as the Dallol Depression and has been the focus for potash exploration for more than a century and is in the deepest region of the depression with elevations ranging between 50m to 120m below sealevel (Figure 1b, c). Shallow volcano-tectonic barriers, behind Mersa Fatma, Hawakil Bay and south of the Gulf of Zula, prevent hydrographic (surface) recharge to the depression. Marine seepage is not occurring at the present time, but likely did so at the time the main potash unit was precipitated. Lake Assele (aka Lake Kurum) with a water surface at -115m msl should not be confused with Lake Asal (-155 msl), located 350 km to the southeast of the Danakil. Asal an active marine-fed hydrographically isolated lacustrine drawdown system, which today is depositing a combination of pan halite and subaqueous gypsum in the deepest part of the Asal-Ghubbat al Kharab rift (Figure 1a; Warren, 2015).

    Today the halite-floored elongate saltpan, known as the Dallol saltflat, occupies the deepest part of the northern Danakil Depression, extending over an area some 40 km long and 10 km wide (Figure 1b, c). The pan’s position is asymmetric within the Danakil Depression; it lies near the depression’s western edge, some 5km from the foot of the escarpment to the Balakia Mountains and the Ethiopian Highlands, but some 50 km from the eastern margin of the depression, which is in Eritrea. The Dallol saltpan and adjacent Lake Assele today constitute the deepest continental drainage sump in the Afar depression (Figure 1b, c). The area, located east and northeast of the main modern Dallol saltpan depression, is mostly an extensive gypsum plain (Bannert et al., 1970). As we shall see, the gypsum pavement, and its narrower equivalents on the western basin flank, defines a somewhat topographically higher (still sub-sealevel) less-saline, lacustrine episode in the Dallol depression history fill. To the south of the Dallol salt pan, bedded Pleistocene evaporites may underlie the entire Danakil depression, but younger lava flows of the Aden Volcanic Series in combination with alluvium washed in from the surrounding bajada obscure much of the older Pleistocene sedimentary series in southern part of the depression beyond Lake Assele (Figures 1a).

    Climate

    In terms of daily and monthly temperatures, the Dallol region currently holds the official record for highest average, year-round, monthly temperatures; in winter the daily temperature on the saltflat is consistently above 34°C and in summer every day tops 40°C, with some days topping 50°C (Figure 2; Oliver, 2005). These high temperatures and a lack of rainfall, typically less than 200 mm each year, place the Dallol at the hyperarid end of the world desert spectrum and so it lies at the more arid end of the BWh Köppen climate zone (Kottek et al., 2006; Warren, 2015).


    History of extraction of salt products and their transport (Table 1)

    Using little-changed extraction and transport methods, salt (halite) has been quarried by local Afar tribesmen for hundreds of years. First, using axes, a crust of pan salt is chopped into large slabs (Figure 3a). Then workers fit a set of sticks into grooves made by the axes. Next, working the stick, workers lever slabs of bedded salt, which is cut into rectangular tiles of standard size and weight, called ganfur (about 4kg) or ghelao (about 8kg). Tiles are stacked, tied and prepared for transport out of the depression on the backs of camels and donkeys (Figure 3b). Around 2,000 camels and 1,000 donkeys come to the salt flat every day to transport salt tiles to Berahile, about 75 km away. Previously, salt tiles were carried via camel train to the city of Mekele, some 100 km from the Danakil. Mekele, located in the Ethiopian highlands is known as the hub of Ethiopia’s former “white gold” salt trade and still today is known as the “old” salt caravan city. Today, the salt caravans walk the extracted salt to Berahile, located some 60 km from Mekele. From there, trucks transport the salt to Mekele. Each truck can transport up to 350 camel salt loads. From the Mekele salt market, Dallol salt blocks are transported and sold to all parts of Ethiopia for use mainly as table salt or as an add-on in animal feed. The lifestyle of the miners and the camel trains is likely to change in the next few decades as sealed roads are now under construction that will link Mekele to Dallol.


    Once potash (sylvite and carnallite) was discovered in the Dallol region in 1906, an Italian company by the name of Compagnia Mineraria Coloniale (CMC) established the first mining operation. In 1918 a railway was completed from the port of Mersa Fatma to a termination some 28 km from Dallol (Table 1). Rail construction took place from 1917-1918, using what was then the British and French “military-standard” 600 mm rail-gauge Decauville system. "Decauville" rail construction used ready-made sections of small-gauge track and so the trackway was rapidly assembled; <2 years to complete more than 50 km of track. Once completed, the railway transported extracted potash salt from the "Iron Point" rail terminal near Dallol, via Kululli to the port. Potash production is said to have reached some 50,000 metric tons in the 1920s, extracted from an area centred on the Crescent Deposit, which is located near the foot of uplifted lake beds on the southern flanks of Mt Dallol. However, significant salt production had ceased by the end of the 1920s, as large-scale mines in Germany, the USA, and the USSR began to supply the world market with cheaper product. Unsuccessful attempts to reopen potash production were made in the period 1920-1941. Between 1925-29 some 25,000 tons of sylvite were shipped by rail from the Dallol, with a product that averaged 70% KCl. After World War II, the British administration dismantled the railway and removed all traces of it. In 1951-1953, the Dallol Co. of Asmara sold a few tons of product from the Dallol.


    The potash concession title was transferred to the American “Ralph M. Parsons Company” (Parsons) at the end of the 1950s. Parsons initiated the first systematic exploration for potash in the Danakil depression and drilled more than 250 exploration holes during their 9-year evaluation campaign. Major potash resources were confirmed a few km west of Mount Dallol, in a mineralized zone that was named the “Musley” Deposit. Following on from positive exploration results, they began an engineering study to investigate potential processing and mining methods for the Musley Deposit and subsequently in October 1965 sank a shaft into the orebody. They installed underground mine facilities and established a pilot processing plant on surface, to investigate recovery from the bulk samples collected from the underground workings. They envisaged developing the Musley Deposit as a conventional room-and-pillar operation and to this end developed six underground drifts totalling some 805 m in length. Unconfirmed reports suggest that an influx of water flooded the mine (possibly triggered by a seismic event) and after failed attempts to solve the water problem, the activities Parsons ceased activities in 1968. As of end 2014, some salt block buildings built by the Italian and other companies still partially stand as ruins, along with rusting equipment.

    Based on the previous work conducted by Parsons, a German potash producer, Salzdetfurth AG (SAG), began a new exploration campaign in the Danakil Depression in 1968 and 1969. In addition to their work in the Dallol depression, SAG drilled a number of wells in a concession south of Lake Assale, and conducted a geological mapping campaign as far north as Lake Badda, on the border with Eritrea. SAG’s exploration work away from the known Dallol deposits did not prove fruitful as they drilled only one drill hole that reached the potash level. This drill hole, located approximately 25 km to the southeast of Mount Dallol, intersected a kainitite bed, with no sylvinite intersection. The SAG concession was returned to state authorities of Ethiopia. Subsequent drilling by other explorationists in this region has confirmed the deepening of the kainitite level to the southeast of Dallol and the lack of sylvinite at greater depths.

    Since the dismantling of the railway, there has been no high-volume transport system to carry potash product the Red Sea coast. Currently, the Ethiopian Government is constructing all-weather roads from Dallol to Mekele and Afdera When complete this road system will facilitate transport of future potential potash product from the Dallol to Afdera, from where existing roads provide access to Serdo and from there to the seaport of Tadjoura in Djibouti (Figure 1a). This section requires an addition 30 km of all-weather road to be completed to the coast and will facilitate cost-effective transport of potash product to the large agricultural markets of India and China. The transport distance to the Eritrean coast from Dallol is much shorter, but political considerations mean such a route is not a viable option at the present time.

    EVAPORITE DEPOSITIONAL PATTERNS IN OUTCROP

    Surficial sediment distributions outline classic drawdown facies belts in the Dallol region, with a wadi-fed alluvial fan fringe passing down dip into sandflats (local dune fields), dry mudflats (with springs), saline mudflats and ephemeral to perennial brine pans of Lake Assele (Figure 1b). The fans, especially along the western margin of the depression are indented or locally covered by a mostly younger succession of constant-elevation marine, biochemical and evaporitic sediments fringes or “bathtub rim” facies (Figure 4).

     

    Alluvial Fan fringe (Bajada) 

    Pleistocene alluvial/fluvial beds, exposed by local uplift, deflation and ongoing watertable lowering, outcrop about updip edges to the salt-crusted parts of the northern Danakil, and form low flat-topped plateaus or mesas on the plain. These mesas define the tops of alluvial fans aprons, which are heavily dissected and eroded by occasional storm runoff and rainfall. This fan fringe contains relatively fresh water lenses in a desert setting that is one of the world’s harshest (Kebede, 2012). Most of the depositionally active fans line the western margin of the basin and many of the downdip fan edges occur slightly up dip a still-exposed gypsum pavement (Figure 5a), showing depositional equilibration largely with an earlier higher lake stage, while others, such as the Musley fan, have flowed across cut into the gypsum pavement level and now feed water and sediment directly into the edges of the saltflat that defines the lower parts of the depression (Figure 4). Watercourses of the fans that have dissected earlier wadi (bajada) deposits as well as the earlier lacustrine gypsum and limestone pavements so create excellent windows into the stratigraphy of these units. Fan avulsion is indicated by palaeosol layers exposed by downcutting of younger streams (Figure 5b, c).

     

    The Musley fan characteristics are well documented by current and previous potash explorers in the basin as these permeable gravels and sands store a reliable water source for potential solution mining/ore processing in the Musley area and so has been cored by a number of proposed water wells. Internally, the fan is composed of interfingering layers and lenses of sand, gravel and clay (paleosols), with highly porous intervals in the sand and gravels (Figure 5b, c). Depth to the watertable varies from >2m to 60m, and salinities from 760 ppm to more than 23,400 ppm. The principal source of recharge is flash flooding, originating in Musley Canyon, which drains the Western Escarpment, along with minor inflows from the adjacent uplifted volcanic block and local highly intermittent rains (Figure 1a). Of six potential water wells drilled in the fan by the Ralph M. Parsons Company in the 1960s, four returned water of good quality (<2000 mg/l), while the other two had waters with salinities in excess of 20 g/l. Pumping test data indicate average transmissivity of the water-bearing beds around 870m2/day, with salinities in the fan increasing from west to east, approaching the saltflat.

    Chemical sediments outcropping in the depression

    Overall, surface sediment patterns in the Danakil depression define a depositional framework of brine drawdown, related to basin isolation from an earlier hydrographic (at surface) marine connection to the Red Sea, followed by stepped evaporative drawdown. This is indicated by fringing topographically-distinct belts or rims of now inactive coralgal carbonates and gypsum evaporites (aka “bathtub ring” patterns) that cover earlier Pleistocene and Neogene clastics (Figures 3a, e, Figure 4). These “rims” of marine limestone and subsequent gypsum were followed by today’s drawdown saline-pan halite-dominant hydrology (Figures 4, 7a-c). The current hydrological package of sediments encompassing the current drawdown episode lies atop and postdates the Pleistocene potash-hosting Lower Halite Formation in the depression and is probably equivalent to the uppermost part of the clastic overburden facies, as illustrated in the drilled and cored portions of the depression stratigraphy. As we shall discuss in the next blog, only the uppermost portion of the recovered core stratigraphy has equivalents in current depression hydrology (Figure 6). 


    In earlier work, some authors interpreted the fringing belts, especially the exposed coralgal reef belt, as being possibly of Pliocene or even Miocene age. However, when one looks at the stratiform nature of the outcrop trace of both the reef belt and the gypsum belt, and the carapace nature of its depositional boundaries in the field, it is immediately apparent they must be younger (Figure 5a, c; Figure 7). Both the reefal and gypsum belts track horizontal hydrological intersections with the landscape, in what is an ongoing volcanogenic and tectonically active depression. When the reefal belt image is overlain by a DEM it shows the reef belt is consistently at sea level (Figure 1c). If the outcrops of the reef belt and the gypsum pavement were older than late Pleistocene or Holocene, then ongoing episodes of tectonism and volcanism would have modified the elevations of the two outcrop belts in the landscape, as is seen in Miocene redbed outcrops. These underlying and centripetal Miocene sections clearly show the influence of ongoing tilting and tectonism and hence why the flat-lying tops to the reef and gypsum belts imply a late Pleistocene-Holocene (Figure 5d).

    That is, the topographic distribution of the top of the reef facies, which lies within a metre or two of current sea level, implies that the Danakil depression had a relatively recent connection to the Red Sea. The pristine preservation of aragonitic corals and sand dollars in the adjacent marls suggest the connection was either related to the penultimate interglacial (around 104, 000 years ago) or to an early Holocene transgression into the depression. Bannert et al. (1970) assign a C14 age of 25.4-34.5 ka to this formation. However, we consider this is unlikely as DEM overlay levelling shows the reef rim, wherever it outcrops, lies within a meter of current sealevel. World eustacy clearly shows that sealevel was more than 50-60m below its present level some 25,000-30,000 years ago. A 25-35 ka determination of the reef rim would require the whole basin was subject to a single basinwide wide vertical uplift event that did not fragment or disturb the lateral elevation of the rim.

    The coralgal reef terrace indicates normal marine water were once present in the Dallol depression, while the occurrences of the stratiform gypsum pavement are consistent with a former arid lake hydrology at a somewhat lower elevation than the reef rim (Figures 1c, 5a). Like the reef rim, the gypsum pavement fringe defines a consistent elevation level or surface, most clearly visible along the western margin of the present salt flat. It is the result of gypsum deposition during a period of drawdown associated with brine level stability, subsequent to the isolation of the depression from its former “at-surface” marine connection. During this time gypsum accumulated as a stack of subaqueous aligned gypsum beds, along with a series of gypsiferous tufas and rhizoconcretions in zones about the more marginward spring-fed parts of the gypsiferous lake margin (Figure 7d-f). The evolution from marine waters that deposited the reefs and adjacent echinoid-rich lagoonal marls at a higher level in the depression (the lit zone) into a more saline seepage-fed system, with no ongoing marine surface connection to the Red Sea is indicated by the diagenetic growth of large lenticular (“bird’s-beak”) gypsum crystals within the marine marls and the dominant subqueous bottom-nucleated textures in the gypsum beds. In a similar way, the now-outcropping subaqueous-gypsum drawdown rim deposits, located at higher elevations than current saline pan levels typify other drawdown saline lakes in the Afar region, such as Lake Asal in Djibouti, all such occurrences indicate an earlier, somewhat less saline, hydrological equilibrium level (Warren, 2015).


    Active today is the lowest parts of the Dallol saltflat is an ephemeral saltpan hydrology indicated by bedded salt crusts dominated by megapolygonal crusts made up of aligned-chevron halite stacks separated by mm-cm thick mud layers . This current pan hydrology is associated with even greater drawdown levels compared to the former gypsum-dominant hydrology (Figure 8). Current deposits, made up a series of stacked brine-pan salt sheets,  are still quarried as a renewable resource by the local tribesmen (Figure 3). These modern brine flats accumulate pan halite whenever the Lake Assele brine edge (strandline) is periodically blown back and forth over the modern brineflat. It driven by southerly winds, which are frequent in the annual weather cycle, and can move thin sheets of brine kilometres across the pan in a few hours (Figure 1a, Figure 8). Superimposed on this southerly supply of brine is an occasional land-derived sheetflood event, driven by rare rainstorms and the deposition of silt-mud layers from water sheets sourced from the adjacent wadi belt. This ephemeral brineflat hydrology is stable with respect to the current climate (groundwater inflow ≈ outflow). It means the current brineflat of the northern Danakil low is  accumulating bedded pan salt at an even lower topographic level in the basin than the surrounding gypsum pavement, so implying today’s halite-dominant pan beds form under more arid conditions (less humid, more drawndown) than that of the gypsum pavement.


    This stepped (reef to gypsum to halite) late-Pleistocene-early Holocene hydrology, captured in the modern surficial geology of the Dallol Depression, likely postdates a somewhat wetter (humid) climatic period indicated by the widespread deposition of a clastic overburden unit, atop the Upper Halite Formation (UHF; Figure 6). That is, the modern hydrology in present-day Lake Assale, and the adjacent saline mud flats of the Dallol pan, is not the same hydrology as that which precipitated the massive salt of the Upper Halite Formation (UHF). A potash-free halite unit extensively cored beneath the present clastic-dominated saline pan (to be discussed in the next blog). Texturally and hydrologically the depositional system of stacked salt crusts, which dominates the upper part of the UHF in the cored wells, is similar to today's halite-dominated passage from the salt flat into the subaqueous Lake Assale. However, as we shall see, a wetter moister period, dominated by sheetfloods and higher amounts of clastics, separates the two hydrological events in all the cored wells. Today's outcrop geology of alternating saltpan and clastic beds are a different to marine-fed seepage hydrology formed the Lower Halite Formation (LHF), with its potash bittern cap (Houston Formation). 

    Most importantly there is no evidence of primary potash deposition in the modern lake/ pan hydrology of the Dallol saltflat. It is clear that the world-famous bedded potash (mostly kainitite) units of the Danakhil accumulated in a bittern hydrology that is not present in today's Dallol depositional hydrology (Blog 2). As we shall see, Holocene potash only occurs in the vicinity of the Dallol Volcanic Mound, where uplift has moved older, formerly buried, potash beds into a more active hydrothermal hydrology (Blog 3).

    References

    Bannert, D., J. Brinckmann, K. C. Käding, G. Knetsch, M. KÜrsten, and H. Mayrhofer, 1970, Zur Geologie der Danakil-Senke: Geologische Rundschau, v. 59, p. 409-443.

    Ercosplan, 2011, Resource Report for the Danakhil Potash Deposit, Afar State Ethiopia, comissioned by Allana Potash. Document EGB 11-008.

    Hardie, L. A., 1990, The roles of rifting and hydrothermal CaCl2 brines in the origin of potash evaporites: an hypothesis: American Journal of Science, v. 290, p. 43-106.

    Holwerda, J. G., and R. W. Hutchinson, 1968, Potash-bearing evaporites in the Danakil area, Ethiopia: Economic Geology, v. 63, p. 124-150.

    Hutchinson, R. W., and G. G. Engels, 1970, Tectonic significance of regional geology and evaporite lithofacies in northeastern Ethiopia: Philosophical Transactions of the Royal Society, v. A 267, p. 313-329.

    Kebede, S., 2012, Groundwater in Ethiopia: Features, numbers and opportunities, Springer.

    Kottek, M., J. Grieser, C. Beck, B. Rudolf, and F. Rubel, 2006, World Map of the Köppen-Geiger climate classification updated: Meteorologische Zeitschrift, v. 15, p. 259-263.

    Warren, J. K., 2010, Evaporites through time: Tectonic, climatic and eustatic controls in marine and nonmarine deposits: Earth-Science Reviews, v. 98, p. 217-268.

    Warren, J. K., 2015, Evaporites: A compendium (ISBN 978-3-319-13511-3) Released August 2015: Berlin, Springer, 1600 p.


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