Salty Matters

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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.

References

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Casas, E., 1992, Modern carnallite mineralisation and Late Pleistocene to Holocene brine evolution in the nonmarine Qaidam Basin, China: Doctoral thesis, State University of New York at Binghampton.

Casas, E., T. K. Lowenstein, R. J. Spencer, and P. Zhang, 1992, Carnallite mineralization in the nonmarine, Qaidam Basin, China; evidence for the early diagenetic origin of potash evaporites: Journal of Sedimentary Petrology, v. 62, p. 881-898.

Duan, Z. H., and W. X. Hu, 2001, The accumulation of potash in a continental basin: the example of the Qarhan Saline Lake, Qaidam Basin, West China: European Journal of Mineralogy, v. 13, p. 1223-1233.

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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.

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Holt, R. M., and D. W. Powers, 2011, Synsedimentary dissolution pipes and the isolation of ancient bacteria and cellulose: Geological Society America Bulletin, v. 123, p. 1513-1523.

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Lowenstein, T., B. Kendall, and A. D. Anbar, 2014, Chapter 8.21. The Geologic History of Seawater, Treatise on Geochemistry (2nd Edition), Elsevier, p. 569-621.

Lowenstein, T., and F. Risacher, 2009, Closed Basin Brine Evolution and the Influence of Ca–Cl Inflow Waters: Death Valley and Bristol Dry Lake California, Qaidam Basin, China, and Salar de Atacama, Chile: Aquatic Geochemistry, v. 15, p. 71-94.

Lowenstein, T. K., 1988, Origin of depositional cycles in a Permian ''saline giant''; the Salado (McNutt Zone) evaporites of New Mexico and Texas: Geological Society of America Bulletin, v. 100, p. 592-608.

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Stable isotopes in evaporite systems: Part II - 13C (Carbon)

John Warren - Thursday, May 31, 2018

 

Introduction

13C interpretation in most ancient basins focuses on carbonate sediment first deposited/precipitated in the marine realm. Accordingly we shall first look here at the significance of variations in 13C over time in marine carbonates and then move our focus into the hypersaline portions of modern and ancient salty geosystems. In doing so we shall utilize broad assumptions of homogeneity as to the initial distribution of 13C (and 18O) in the marine realm, but these are perhaps oversimplifications and associated limitations need to be recognized (Swart, 2015)

In the next article we shall look at the utility of crossplots of carbon and oxygen isotopes. Stable oxygen isotope values (d18O) crossplotted with respect to carbon isotope values (d13C) from, the same sample creates one of the most widely applied proxies used to infer palaeo-environmental conditions (depositional and diagenetic) in Holocene and ancient carbonate sediments. This is in large part due to kinetic fractionations that occur during evaporation (Leng and Marshall, 2004). It has long been known that as any liquid evaporates, the residual fluid becomes enriched in the less abundant heavy isotope(s) (see Horton et al., 2016 for detailed discussion).

Interpreting 13C

Over the Phanerozoic the standard paradigm for interpreting variations in variations in 13C values from modern and ancient marine carbonate is based on an integration of our understanding of the carbon cycle with the following arguments. Most of the carbon in Earth’s near-surface systems is stored in sedimentary rocks with only about 0.1% in living organisms and the atmosphere-hydrosphere (Figure 1). Oxidized carbon occurs primarily as marine carbonates and reduced carbon as organic matter in sediments. In the carbon cycle, CO2 from the oceans and atmosphere is transferred into sediments as carbonate carbon (Ccarb) or organic carbon (Corg), the former of which monitors the composition of the oceans (Figure 1). The cycle is completed by uplift and weathering of sedimentary rocks and by volcanism, both of which return CO2 to the atmosphere.


There are two stable carbon isotopes, carbon 12 (6 protons and 6 neutrons) and carbon 13 (6 protons and 7 neutrons). Photosynthetic organisms incorporate disproportionately more CO2 containing the lighter carbon 12 than the heavier carbon 13 (the lighter molecules move faster and therefore diffuse more easily into cells where photosynthesis takes place). During periods of high biological productivity, more light carbon 12 is locked up in living organisms and in resulting organic matter that is being buried and preserved in contemporary sediments. Consequently, due the metabolic (mostly photosynthetic) activities of a wide variety of plants, bacteria and archaea, the atmosphere and oceans and their sediments become depleted in carbon 12 and enriched in carbon 13 (Figure 2)


It is assumed that the carbon isotopic ratio in calcareous shells of marine organisms is in equilibrium with that of seawater. So as more carbon 12 is held in biomass during times of high primary productivity, and increased burial of organic carbon, calcareous (CaCO3) skeletal materials become enriched in carbon 13. In contrast during periods of low biological productivity and decreased burial of organic carbon, for example following mass extinctions, marine calcareous skeletal materials become enriched in carbon 12.

Hence plotting variations in carbon isotopes in marine carbonates and organic matter over time offers a way to trace the growth of the crustal reservoir of reduced carbon (Des Marais, 1997). That is, the relative abundance of carbon isotopes is controlled chiefly by: 1) equilibrium isotopic effects among inorganic carbon species, 2) fractionation associated with the biochemistry of organic matter, and 3) the relative rates of burial of carbonate and organic carbon in sediments (Condie 2016).

Because organic matter preferentially incorporates 12C over 13C, there should be an increase in the 13C/12C ratio (as measured by δ13C) in buried carbon with time, and indeed this is what is observed (Des Marais, 1997; Worsley & Nance, 1989). δ13Corg increases from values < -40‰ in the Archaean to modern values of -20 to -30‰. On the other hand, seawater carbon as tracked with δ13Ccarb remains roughly constant with time, with δ13Ccarb averaging about 0%.

Variation in fluxes over time within the carbon cycle can be monitored by an isotopic mass balance (Des Marais, 1997), whereby;

δin = fcarbδ13Ccarb + forgδ13Corg

δin represents the isotopic composition of carbon entering the global surface environment comprised of the atmosphere, hydrosphere, and biosphere. The right side of the equation represents the weighted-average isotopic composition of carbonate (δ13Ccarb) and organic (δ13Corg) carbon buried in sediments, and fcarb and forg are the fractions of carbon buried in each form (fcarb = 1 - forg). For timescales longer than 100 Myr, δin = -5‰, the average value for crustal and mantle carbon (Holser et al., 1988). Thus, where values of sedimentary δ13Ccarb and δ13Corg can be measured, it may be possible to determine forg for ancient carbon cycles. Higher values of δ13Ccarb indicate either a higher value of forg or a greater negativity of average δ13Corg.


During the Phanerozoic, there are several peaks in δ13Ccarb, the largest at about 110, 280, 300, 400, and 530 Ma (Figure 3). These peaks are widely interpreted to reflect an increase in burial rate of organic carbon (Des Marais et al., 1992; Frakes et al., 1992). This is because organic matter selectively enriched in 12C depletes seawater in this isotope, raising the δ13C values of seawater. In the late Paleozoic (300-250 Ma), the maxima in δ13Ccarb correspond to the rise and spread of vascular land plants, which provided a new source of organic debris for burial (Condie 2106, Berner, 1987, 20 01). Also conducive to preservation of organic remains at this time were the vast lowlands on Pangea, which appear to have been sites of widespread swamps where bacterial decay of organic matter is minimized. The drop in δ13Ccarb at the end of the Permian is not understood. Perhaps, large amounts of photosynthetic O2 generated by Carboniferous forests led to extensive forest fires that destroyed large numbers of land plants in the Late Permian (Condie, 2016). However, the reasons for the oscillations in δ13Ccarb are not yet unequivocally resolved and, as in all sciences, the tenet "...perceived correlation does not necessarily equate to causation"must always be at the forefront in the scientific mindset.


Across the Precambrian and the Phanerozoic, the initiation of glaciation on a global scale, as in the Cryogenian ‘Snowball Earth’, has been interpreted to be dependent on parameters like the latitudinal extent of continents and oceanic circulations (Figure 4; Condie, 2016). The main drive for an onset of global glaciation is believed to be the lowering of atmospheric CO2. It likely also requires a continental landmass to be covering one of the earth's polar positions. More recently, cooling related to an increase in the earth's albedo due to widespread evaporites (saline giants) has been added to the list of possible drivers to the onset of glaciation.

Climate modelling studies imply that CO2 concentrations as low as 100–150 ppm are required to initiate global glaciation (e.g. Liu et al., 2013; Feulner and Kienert, 2014). One potential cause of lowered CO2 is drawdown of CO2 during intense silicate weathering in equatorial regions (Hoffman and Schrag, 2002; Goddéris et al., 2003). Photosynthesis provides another mechanism for CO2 drawdown, via conversion of CO2 to O2 and rapid burial of organic carbon, which is reflected in a positive δ13C excursion for carbonates (Pierrehumbert et al., 2011). Additionally, long term cloud cover (Feulner et al., 2015), fluctuations in atmospheric-ocean heat transport, the earth's albedo, or solar luminosity (Pierrehumbert et al. (2011) are also proposed as potential causes of the onset of glaciation (ice-house mode climate).

In a recent paper, Schmid 2017 focused on the cause of the Bitter Springs carbon isotope anomaly, she argues the cause of the pre-glacial, globally recognised, carbon and oxygen isotope variations in carbonate sediments tied to the Bitter Springs anomaly is a response to widespread fractional evaporation of dissolved CO2. This carbon isotope anomaly ties to a well defined correlation with the distribution of Neoproterozoic evaporite basins. She also shows volcanism occurred during the onset of the Bitter Springs Stage (811–788 Ma) and associated widespread evaporite distribution across Australia.


Schmid (op. cit.) argues that the albedo effect began with of the widespread deposition of Rodinian supercontinent evaporites in very shallow marine to epicontinental sedimentary successions beginning ≈810 Ma, increased siliciclastic redbed weathering. This and continuing evaporite deposition and exposure between ≈780 and 720 Ma drove a worldscale increase in Earth's albedo. Such highly reflective salt deposits defined a saline giant across an area that today covers one-third of the Australia continent. Thus, this and other penecontemporaneous saline giants over the Rodinian supercontinent played a potentially significant role in the onset of atmospheric cooling via a significant increase in albedo (Figure 5). These salt beds occur in periods that typify the onset of local (750 Ma) and then global glaciation (720 Ma).


Schmid (2017) goes on to note that the degree of evaporation in the Bitter Springs group sediments is related to the δ13C signature in variably concentrated waters (Figure 6). That is the Tonian Bitter Springs Group (≈830–750 Ma), within the Amadeus Basin in central Australia consists of thick halite and sulphate evaporite accumulations and associated carbonates. The deposition of halite occurred in shallow marine, lagoon (salina) environment (Gillen Formation), and developed into sulphate-dominated supratidal sabkha during sea level regression (Johnnys Creek Formation). The overall regression was interrupted by a transgressive phase lasting at least 20 Ma and leading to deposition of basin-wide stromatolitic dolostone (Loves Creek Formation). The salinity and high evaporation is reflected in positive δ13C in the intercalated carbonates (+4 to +6‰ VPDB) of the evaporitic units, while the shallow marine stromatolitic incursion of the Loves Creek Formation (−2‰ δ13C) show typical marine carbonate isotopic values (Figure 7).

This salinity controlled isotopic separation supports the observations of Stiller et al. (1985) who noted extreme enrichment of 13C in the dissolved inorganic carbon pool in evaporating brines up with δ13C values of up to + 16.5‰ under natural abiotic, oxic conditions in Dead Sea evaporation ponds (Figure 7). The systematic increase in 13C values in highly evaporated waters from the various bittern ponds of the Dead Sea Saltworks is thought to result from a nonequilibrium gas-transfer isotope fractionation. The process of ongoing evaporation leads to CO2 loss within the evaporative brine as less and less gas can held in solution (see Warren 2016, Chapter 9). CO2 exchange in a concentrating surface brine occurs directly between the water column and air, resulting in direct CO2 loss through evaporation. In a sabkha environment. CO2 is released from the hypersaline groundwater through sediments before being released to air as evaporites may form intrasediment precipitates. Overall, atmospheric CO2 uptake in hypersaline settings fed by shallow marine water is diminished compared to the normal marine settings.


Precipitated carbonates modern salinas and sabkhas are mainly aragonite, and formed in association with such evaporative brine, are consistently13C enriched, as seen in nearby Solar Lake and Sabkha Gavish (Figure 2; Stiller et al., 1985; Schidlowski et al., 1984). In a similar fashion, Palaeoproterozoic interbedded shallow marine carbonates, redbeds and evaporites have values up to δ13C + 17.2‰ (Melezhik et al., 1999). Permian and Triassic (Schmid et al., 2006a) redbeds and evaporite sequences also have 13C-rich carbonates (up to +7‰) and enrichment is partly attributed to evaporation and associated CO2 loss (Beauchamp et al., 1987). In modern oceans, atmospheric CO2 is consumed by biological activity and carbonate production originates from mainly marine organisms, leading to near atmospheric to organic negative δ13C signatures in the precipitated sediment(Andersson, 2013).

If increasing salinity leads to unfavourable conditions for photosynthesising organisms to survive (Lazar and Erez, 1992), carbonate through to bittern precipitation becomes increasingly abiotic and evaporation driven, especially at the upper end of the evaporation series. The loss of Ca during evaporation of a brine, via aragonite and calcium sulphate precipitation, leads to an increase in Mg/Ca ratio and an increase in residual brine density. This can result in primary dolomite precipitation or widespread reflux dolomitisation (Schmid et al., 2006, Warren 2000, 2016.

In summary, the typical δ13C signature in normal marine carbonate sediment across much of geological time centres around 0 ‰ and ranges between a few parts per mille on either side of the zero line reflecting precipitation by calcifying and photosynthesising organisms (e.g. algae), while abiotic, evaporation induced carbonates tend to have δ13C values above +1‰. More positive δ13C values (+4 to +6‰) tend to typify dominantly abiotic carbonates (and local methanogenic carbonates with even more positive values) and support the notion of evaporation-driven 13C-enrichement in times of widespread evaporitic epeiric and basinwide carbonates. In the Precambrian, widespread marine stromatolitic units such as, algal Loves Creek Formation reflects δ13C values for biogenic carbonate precipitation under shallow marine, non-hypersaline conditions. The change from a shallow hypersaline lagoon towards evaporitic mudflats and salterns suggests an increase in aridity and continentality/hydrographic isolation, with associated more positive δ13C values.

Implications for some types of 13C anomaly

The Bitter Springs Group chemostratigraphy has been correlated globally and the negative excursion was named previously after this unit (Bitter Springs Stage anomaly). However, the mechanism of evaporation-driven fractionation of δ13C is different from the commonly proposed inorganic-organic carbon fractionation, and challenges the views on interpreting global chemostratigraphic anomalies or excursion and their cause. Evaporite basins covered vast regions worldwide prior to the Sturtian glaciation, e.g. the Australian evaporites would have covered a third of the continent. The light surface of evaporites and associated carbonates would have had a high albedo and effectively cause less surface heat absorption. This subsequently would have triggered temperature decrease on a continental and possibly global scale. The Schmid paper hypothesises that the deposition of evaporites worldwide would have contributed to global cooling starting ≈100 Ma prior to Snowball Earth and would have played an important role in the onset of global glaciation.

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Aeolian Gypsum and Saline Pans - an indicator of climate change

John Warren - Friday, June 30, 2017

Introduction

Evaporites deposited as aeolian dunes are not commonplace in Quaternary successions and not yet documented in any pre-Quaternary succession (Table 1). These eolian deposits are deposited above the water table in a vadose setting, generally in a degrading playa or salt lake hydrology. Consequently, there is an inherent low preservation potential for this style of evaporite; most documented examples are less than a few tens of thousands of years old.

 

Even though relatively rare as an evaporite type, the presence of eolian evaporites, usually as gypsum dunes or lunettes with associated soils and saline mudflats, does indicate particular climatic and hydrological conditions. Eolian gypsum deposits may have possible counterparts in the Martian landscape (Szynkiewicz et el., 2010).

Over the Quaternary and across the Australian continental interior, increased aridity is expressed by episodes of dune reactivation, lake basin deflation with eroded sediment accumulating downwind in transverse dunes or lunettes (Bowler, 1973; Fitzsimmons et al., 2007), Deposition is tied to increased dust mobility (Hesse and McTainsh, 2003) and reduced river discharge and channel size (Nanson et al., 1995). Such responses to increasing landscape aridity in saline groundwater sumps are seen in most arid to semi-arid regions of the world where water tables are falling, usually driven by increasing aridity.

This article focuses on eroded subaerial evaporites as a response to increasing aridity, especially the formation of gypsum dunes and lunettes (Table 1; Figure 1).


Gypsum dune styles and saline pans

Figure 1 and Table 1 plot documented occurrences of eolian gypsum across the world, overlain on a Koeppen climate base (Figure 1a). Figure 1b plots the latitudinal occurrences of documented gypsum dunes versus elevation and Koppen climate type. Figures 1c and 1d plot the detail of these same occurrences for the USA and Australia, where individual deposits are better documented. At the worldscale, there is an obvious tie to the world's desert belts with occurrences consistently situated in regions of the cool dry descending cells of northern and southern hemisphere Hadley cells (positions indicated by light blue rectangles in Figure 1b - See also Salty Matters article from Jan. 31, 2017). Many occurrences are also situated in Late Pleistocene to Holocene climate transition zones, marked by aridification at the transition from Late Pleistocene to Holocene climates, and in many case tied to transitions from perennial saline lakes and mega-lakes to continental saltflats to dunes and interdunal pans, An example of a quartz sand erg association (downwind of a gypsiferous strandzone) is seen in the transition area into the southern Kallakoopah Pans from the northern margin of Lake Eyre, Australia and its megalake precursor (Figure 2).


At the local scale, gypsum dunes generally occur downwind or atop a saline pan or playa that is, or was, recently subject to a lowering of its lacustrine watertable. In many situations the elongation of individual pan shapes line up in an orthogonal direction to the dominant wind and so also show an eolian control, like the associated gypsum dune position and alignment (Figure 3). Wind-aligned lakes and sumps and oriented-pans are much more numerous with a broader climatic range than gypsum dunes (Goudie and Wells, 1995; Goudie et al., 2016). When present, eolian bedforms associated with oriented pans lacking evaporites are dominated by clay pellets or quartz sand.


Many of the pan edge dunes show crescent shapes and so are termed lunettes. (Figure 3; Bowler, 1973). Lunette sediments range in composition from quartz-rich to sandy clay, gypsiferous clay to nearly pure gypsum. Pure quartz dune lunettes likely formed under lake-full conditions, and so show a distinct hydrology from that of the clay pellet or gypsum-rich varieties, which form by deflation of subaerially-exposed adjacent lake floors. The flocculation of suspended clays into pellets requires some degree of salinity but is less than that required to precipitate gypsum.

Lunette sediments range in composition from quartz-rich, sandy clay, through gypseous clay to nearly pure gypsum. Pure quartz dunes formed under lake-full conditions and are distinct from that of the clay and gypsum-rich varieties, which formed by flocculation and deflation from adjacent subaerially exposed lake floors. (Bowler, 1986). Gypsum and pelleted clay dunes (lunettes) line the edges of many salt lakes and playas in southeastern, southern and southwestern Australia; Prungle Lakes and Lake Fowler (gypsum lunettes), Lake Tyrell (clay lunette with occassional gypsum enrichment) and Lake Mungo (quartz sand lunette). All these lunettes are lake or pan-edge relicts from the Late Pleistocene deflationary period, when the lacustrine hydrology changed from perennial water-filled lakes to desiccated mudflats. Likewise, there are gypsum dunes in deflationary depressions in Salt Flat Playa and the Bonneville/Great Salt Lake region of Utah (Figure 4; Table 1).


Internal sedimentary structures in many of these lake-edge gypsum dunes or lunettes show tabular cross beds with consistent bedform orientation. Many lack abundant trough or festoon cross beds, suggesting consistent wind directions (Jones 1953; Bowler, 1973, 1983). Grain constituents clearly indicate deflation of former lake sediments, which were mostly vadose prior to deflation and passage into the dunes (Figure 4).

Gypsum dunes are part of a much broader lake-edge eolian sandflat association with the lakes often supplying large volumes of quartzose eolian sediment into adjacent sand seas or ergs (Figure 2; Warren, 2016). As mentioned pan-edge dunes described as ‘lunettes’ have a characteristic crescentic shape, other lake edge dunes may show more linear or longitudinal outlines, sometimes with parts of large sand seas or ergs being fed by the deflation of the salt lake or pan as at the southern edge of the Simpson Desert in Australia where it is in contact with the expanding and contracting edge of (Lake Eyre Figure 2).

Hydrological transitions from downwind evaporite dunes and lunettes

The role of salts, groundwater oscillations and the associated lake water levels/watertables are critical in creating eolian evaporites. Typically, once seasonal drying of an increasing arid lake floor sump begins, remaining surface waters with suspended clay become saline enough for the clay to flocculate and sink to the bottom of the desiccating water mass. If surface water concentration continues and the water surface sinks into the sediments to become a saline water table, then secondary gypsum prisms and nodules grow within the capillary zone of already-deposited sediment. In waters that are increasingly saline but not saturated with gypsum or halite, pelletization can continue to occur in the capillary fringe of clayey surface sediment (Figure 5).


Ongoing seasonal aridity further lowers the watertable in a saline mudflat, so the upper part of the vadose sediment column leaves the top of the capillary zone. It then deflates, leading to an accumulation of sand-sized sediment in adjacent eolian lunettes. If there is a prevailing wind direction, this builds significant volumes of dune sediment in a particular wind-aligned quadrant of the saline pan edge. Whether clay pellets or gypsum crystals are the dominant lunette component depends on the humidity inherent to the pan climate. In hyperarid situations, halite can be an eolian component in the lake hydrology (Salar de Uyuni; Svendsen, 2003).

In some lunettes, the mineralogy changes according to climate-driven changes in the hydrogeochemistry of the lake waters sourcing the lunette. For example in the Lake Tyrell lunette in semi-arid southwest Australia, the sediments in a layer range from clay pellets (75%) and dolomite (25%) in somewhat humid times of deflation to layers, with gypsum making up >90%, indicative of a more arid hydrochemistry. Lunettes associated with the shrinkage and deflation of Late Pleistocene Estancia megalake (New Mexico, USA) show similar variations in the proportions of clay pellet and gypsum sands in lake margin deposits around the edges of up to 120 blowout depressions. These blowouts define the former extent of the shrinking megalake and encompass both shoreline and lunette sands (Allen and Anderson, 2000)

Thus, the presence of an active gypsum lunette-field at a saline pan or playa edge is tied to landscape instability and a change from more humid to more arid conditions. To form a lunette requires a change in climate and an associated change in pan or playa hydrology and it hydrological base level and lake edge water table level, over time frames typically measured in hundreds to thousands of years.

 

Not just sand and dust-sized particles

Coarser than sand-sized gypsum crystals are transported in in lake margin mounds under hyperarid windy conditions that typify ephemeral pans and saline mudflats in parts of the Andean Altiplano and even higher elevations in the alpine tundra climatic zones. Salar Gorbea is a type example for this type of coarse-grained eolian transport (Figure 6; Benison, 2017). Whirlwinds, dry convective helical vortices, can move large gypsum crystals in their passage over the saline muflat. The transported gravel-sized crystals are entrained on the saline pan surface, after they first grew subaqueously in shallow surface brine pools. Once the pools dry up the crystal clusters disaggrate and then are transported as much as 5 km to be deposited in large dune-like mounds.

The dune gravel is cemented relatively quickly by gypsum cement precipitating from near-surface saline groundwater, resulting in a gypsum breccia. This documentation marks the first occurrence of gravel-sized evaporite grains being moved efficiently in air by suspension and provides a new possible interpretation for some ancient breccias and conglomerates, and improves understanding of limits of extremity of Earth surface environments.

 

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