Salty Matters

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

John Warren - Wednesday, October 31, 2018

Introduction

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

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


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

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

Primary potash ore?

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

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

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

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


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

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


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

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


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


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


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

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

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

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


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


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


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

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

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

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

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

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


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


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

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

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

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

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

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

Dead Sea Potash (MOP operation in the Southern Basin)

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


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

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

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

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

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


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


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

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


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

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

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

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

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

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

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


MOP brines and Quaternary climate

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

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

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


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

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

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

John Warren - Wednesday, May 31, 2017

 

Introduction

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

 

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


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

Antarcticite

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

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


Don Juan Pond, Antarctica

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

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


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

Bristol Dry Lake, California

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


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

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


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

Sinjarite (CaCl2.2H2O)

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


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

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


CaCl2 salts and liquid water flows on current-day Mars

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

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

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

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


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

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


Tachyhydrite

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

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

Modern tachyhydrite occurrences

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

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

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

Cretaceous of Brazil & W. Africa

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


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

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

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

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

Cretaceous occurrences in Thailand

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


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

Hydrothermal oceanic CaCl2 brine

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

Tachyhydrite is a 'Swift Water" mineral

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

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

 

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

 

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

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

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

Terrestrial CaCl2 minerals across time

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

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

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

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

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

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

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

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

John Warren - Tuesday, August 11, 2015

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


A Phanerozoic dichotomy: evolving marine potash bitterns

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

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

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

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

Why the dichotomy?

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

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

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

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

 

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

Inclusion evidence

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


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


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

 

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


Mechanisms

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

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

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

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

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

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

 

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

 

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

 

Changes in Phanerozoic ocean salinity

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


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

So what?

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

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

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

John Warren - Tuesday, May 12, 2015

How to deal with K2SO4

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

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

SOP in Messinian evaporites, Sicily

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

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

 

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

 

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

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

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

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

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


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

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

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

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

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

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

K2SO4 salts in Miocene of Ukraine

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


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


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


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

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


Stebnyk potash (Figure 7a)

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

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

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

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

Kalush potash salt geology (Figure 7b)

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

Conventional processing streams for manufacture of SOP and MOP

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

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

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

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

    followed by water-leaching of the schoenite intermediate

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


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

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

    Can Danakhil potash be economically mined?

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

    Conventional mining

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

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

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

    Can the Danakhil potash be solution mined?

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

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

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

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

    And so?

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

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

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

    References

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

    John Warren - Wednesday, April 29, 2015

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


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

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


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

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

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


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

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

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

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

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

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

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

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

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

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

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

    References

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

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

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

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

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

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

    John Warren - Sunday, April 19, 2015

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

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

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

    Dallol Physiography

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

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

    Climate

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


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

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


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


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

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

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

    EVAPORITE DEPOSITIONAL PATTERNS IN OUTCROP

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

     

    Alluvial Fan fringe (Bajada) 

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

     

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

    Chemical sediments outcropping in the depression

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


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

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

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


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


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

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

    References

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

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

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

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

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

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

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

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

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


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