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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Lithium in saline settings

John Warren - Sunday, July 30, 2017

 

Introduction

Historically, until the electronic revolution, lithium located near the top of the periodic table, was of little economic interest. With an atomic number of 3 and an atomic weight of 6.9, lithium is exceptionally small and light, with a high charge/radius ratio. Estimates of the average crustal abundance of lithium vary, but it is likely to be approximately 17–20 parts per million (ppm). In igneous rocks, the abundance is typically 28–30 ppm, but in sedimentary rocks, it can be as high as 53–60 ppm (Evans, 2014; Kunasz, 2006).Lithium-prone hypersaline brines can attain values as high as 6000 ppm, but such high levels are unusual.

Lithium compounds are consumed in the production of ceramics, lubricants, glass, and primary aluminium (Figure 1). Its high specific heat capacity makes lithium ideal in heat transfer technology where it is used in welding and metallurgical applications. Its light weight and its high electrochemical potential (it is most electronegative metal known) and its high electrical conductivity make it amenable to battery applications requiring low weight and high storage potential.

Compared to nickel metal hydride batteries, the type of battery currently powering most hybrid electric vehicles, Li-ion batteries are lighter, less bulky, and more energy efficient. Lithium batteries have three times the energy of nickel hydride at one-third of the weight, and they operate at very low temperatures with a longer battery life. The use of Li-ion batteries in electric cars and electronic devices has increased the global demand for lithium, a trend that is likely to continue. Currently, lithium‐rich saline brines are the most economically recoverable Li source on the planet. (Kesler et al., 2012; Grosjean et al., 2012).


Economically recoverable forms of lithium occur in four types of deposits (Table 1; Figure 1)

(1) Pegmatites,

(2) Continental saline brines

(3) Hydrothermally altered saline lacustrine clays,

(4) Geothermal and basinal brines.

Figure 2 plots know occurrences of lithium in saline deposits. There are four main clusters of hypersaline brine-related lithium occurrences, 1) Andean Altiplano, 2) Tibetan Highlands, 3) Qaidam Basin and 4) Playa brines in the Basin and Range of the south-west USA. Then there are the lesser volumes, as yet un-economic lithium accumulations, associated with lacustrine clays near Hector, California and in the Jadar Valley, Serbia. Basinal (oilfield) brines are known to entrain elevated levels of lithium in the Smackover Fm, USA and the Fox Creek region of Canada. Geothermal brines below the Salton Sea can also contain elevated levels of lithium.


Lithium‐rich continental brine sources account for about three‐fourths of the current world lithium production with the remainder from pegmatites (Figure 3; U.S. Geological Survey, 2017). Only the geology of hypersaline brine sources and associated saline sediment hosts, basin brines and clay replacements are discussed in detail in this article. For information on pegmatites and oilfield brines the interested reader is referred to Garrett (2004), Kesler et al.(2012) and Evans (2014). All three articles contain a broader discussion of occurrences of lithium raw materials and their processing.

In natural brines, most lithium salts are highly soluble and tend to stay in solution until lithium concentrations approach and exceed 6000 ppm. Lithium can be to be absorbed by saline playa clays at lower concentrations, as in the hectorite beds in Clayton Playa, Nevada or hydrothermally from hypersaline saline waters as formed jadarite nodules in the Jadar Valley, Serbia. Actual lithium carbonate precipitates are highly soluble and so very rare in sedimentary basins; lithium carbonate (zabuyelite) is a natural precipitate from the high-altitude hypersaline waters of Lake Zabuye on the Tibetan Plateau.


Lithium carbonate brines

Production from hypersaline pore brines in South American salars dominates current world lithium brine production, with Chile and Argentina producing some two-thirds of the documented world brine production (Figure 2). Chile has emerged as the largest lithium carbonate producer from a lake brine, largely through the exploitation of brines in Salar de Atacama. China and Argentina are the other main producers of lithium from saline lake brines (Figure 3).

Lithium in salar brines of the Andes

Salar de Atacama lies on the Tropic of Capricorn at an altitude of 2,300 m in the Desierto de Atacama, some 200 km inland from Antofagasta. In its more central portions this salt-encrusted playa contains a massive halite unit (nucleus) that is more than 900 m thick, with an area ≈ 1,100 km2. Fringing saline muds, with an area ≈ 2,000 km2, surround the nucleus (Figure 4a, b).

The current salt crust atop this halite nucleus contains a sodium chloride interstitial brine that is rich in Mg, K, Li, and B ((Figure 4c; Figure 5; Alonso and Risacher, 1996). Lithium contents of the pore brine range from 200-300 ppm in the marginal zone, some 500-1,600 ppm in the intermediate zone and 1,510-6,400 ppm in the salt nucleus). The nucleus zone averages 4,000 ppm lithium and is asymmetric with respect to the salar centre due to the sump offset via ongoing faulting. Main inflows to the salar drain volcanic formations of the Andean Highlands located to the east of the basin.


Salts dissolved in inflow waters have a double origin. Weathering of volcanic rocks supplies K, Li, Mg, B and, to a lesser extent, Na and Ca. Leaching of ancient halokinetic evaporites sourced in a mother salt layer beneath and piercing the volcanic formations provides additional amounts of Na, Ca, Cl, SO4 to the most saline inflow waters. The mass-balance of the upper halite nucleus in the salar shows a strong excess of NaCl with respect to the bittern solutes Mg, K, Li, B. According to Alonso and Risacher (1996), this suggests that the nucleus did not originate from evaporation of inflow waters similar to the present groundwaters. Rather, the excess of NaCl is due to NaCl-rich inflow waters that formerly drained the Cordillera de la Sal, a Tertiary-age evaporitic ridge along the western rim of the present-day salar (Figure 4b).


Although annual salt accretion rates in a salar salt nucleus facies in an Andean salar can be as high as 5-6 cm/year (Ruch et al., 2012), the average sedimentation rate of halite in the Atacama lake centre is ≈ 0.1 mm/year, based on the age of an ignimbrite interbedded with the salt. This slow aggradation rate implies a climatic setting of long dry periods and inactivity alternating with short wet periods during which large amounts of water, and so large amounts of salt are first recycled and then accumulate in the halite crusts of the basin sump. The lack of peripheral lacustrine deposits and the high purity of the Atacama salt also suggest that the main salt unit is not the remnant of an ancient deep saline lake, but originated mostly from evaporation of waters supplied by subsurface and subterranean saline seeps.


Once Li-rich lake brines are pumped to the surfac,e they flow into a series of evaporation pans where three main economic products (halite, potash and lithium salts) are recovered. To achieve this controlled salt-series production, the brines are first pumped from 30 metre deep boreholes that penetrate the porous salt nucleus layer into a series of solar evaporation ponds (Figure 6a). Over the successive passage through the concentrator pons, liquors are concentrated by a factor of 25, generating a final brine strength of 4.3% Li (Figure 6b). During evaporation and processing for production of halite, potassium chloride and potassium sulphate from lithium precursor brines, the ion ratios are continuously monitored and adjusted to avoid the precipitation of a lithium potassium sulphate salt. This combination of solar concentration and brine processing, proceeds as follows:

1) Sodium chloride (common salt) precipitates first. If required, this salt can be scrape-harvested as a by-product.

2) At the appropriate level of concentration, the brine is transferred to a second set of ponds in which a mixture of sodium chloride (salt) and potassium chloride (potash, in the form of sylvinite) is precipitated. These salts usually are harvested and the two components separated in a flotation plant.

3) The remaining brine is piped to another set of evaporation ponds where it remains until the concentration increases to 6000 ppm Li (essentially the saturation point of lithium chloride - saturated brines typically show a green colour as visible in Figure 6b). Ripe brine is then transferred to a recovery plant where impurities such as magnesium and boron are removed. When soda ash (sodium carbonate) is added to the ripe brines, lithium carbonate drops out. Brine with low magnesium levels is the preferred feed brine as this makes for simpler processing.

The high initial lithium content of the Atacama brines and the extremely arid setting (3200 mm pan evaporation and <15 mm precipitation) means that only 90 hectares of evaporation ponds are required in one of the current brine operations on the salar, this is only 5% of the area required at Clayton Valley, Nevada with its milder climate and lower Li concentration in the feeder brine (Figure 7). Borate (as perborate) is recovered at levels of 0.84 g/l during lithium extraction at Atacama. Increasing volumes of lithium are also produced by new salar brine processing facilities in nearby Salar de Hombre Muerto, Argentina and Salar de Uyuni, Bolivia. All these salars have lower levels of Li in the primary brine feed than Atacama.

Lithium brines in the USA

Clayton Valley is host to the only commercially producing lithium project in North America, Albemarle’s Silver Peak brine evaporation pond project (Figure 7). Historically, the Clayton Valley playa produced about one-third of the US lithium requirements, but its economic viability suffered from fierce market competition, especially from South America, and a largely depleted brine supply. Originally, the central valley area contained 100–800 ppm Li, and the discovery well at 229 m depth contained 678 ppm when pumped at 450 gpm (Garrett, 2004). The average brine analysis when commercial production of lithium carbonate began in 1966 was about 400 ppm (Figure 7). Since that time the feed concentration of lithium has been slowly declining, and in 1998 the concentration was about 100–300 ppm Li (averaging 160 ppm, Harben and Edwards, 1998).


The Silver Peak Playa has an area of 50 km2 and an elevation of 1300- 1400 m (Figure 7). It lies in the rain-shadow of the Sierra Nevada, with an annual rainfall ≈130 mm and an evaporation rate of ≈1380 mm. Near-surface sediments consist of a mixture of clays (smectite, illite, chlorite, kaolin) and salts (halite and gypsum) and widespread pedogenic calcite. Lithium in the brines is derived from weathering and leaching of volcaniclastics in the Tertiary Esmeralda Formation and Quaternary ash-fall tuffs (Davis et al., 1986). Lithium content is highest on the eastern side of the playa adjacent to the outcropping marls of the Esmeralda Fm. Before it is leached, lithium is held in the clay fraction of the playa sediments and is probably part of the clay structure (hectorite is a widespread but minor component in the Clayton Valley clays - see later)).

Lithium-rich brine feed to the plant averages 0.023% (230 ppm) lithium in a background NaCl concentration of 200,000 ppm, is pumped from depths of 100-300m in the Clayton Valley (Silver Peak) playa via a number of gravel-packed wells. The lithium (and potassium) in the deposit probably originated from hot springs along the Silver Peak Fault, with the current brine composition being a blend of evaporated water from these springs and surface and ground water that drains into the basin (Garrett, 2004). Modern saline spring outflows contain 9280–10,000 ppm Na, 786–826 ppm K and 24–43 ppm Li. Unusually high brine temperatures in some areas of the deposit (up to 44°C at fairly shallow depths ≈ 25m) tend to support a volcanic/geothermal origin for the lithium. Some of the brine feeder wells show elevated levels of radon gas.

Pumped brine progresses through a series of fractionating evaporation ponds (Figure 7; Zampirro, 2004). Lithium concentration in the liquor increases to 6,000 ppm over the course of 12 to 18 months in the solar evaporation pans. When the lithium chloride level reaches optimum concentration, the liquor is pumped to a recovery plant and treated with soda ash to precipitate lithium carbonate, which is then removed by filtration, dried, and shipped.

Lithium from brine, when the Clayton Valley first produced product in the 1970s, was considered a unique deposit. Its operations established the technology and economic viability of lithium recovery from saline brine, which led to the development of brine production from the salars of South America that now dominate world production of lithium from brine.

Lithium brine in Chinese salt lakes: Zabuye (Zhabei) and Qaidam basins

The lithium brine resource of China is mostly stored in two saline lake regions in high altitude zones, Lake Zabuye region in the Alpine tundra climate zone on the Tibetan Plateau and four salt lakes in the cold arid steppe climate region of the Qaidam Basin on the Mongolian Plateau (Figure 2). Something like 80% of brine lithium resource found in China is contained in the four salt lakes of the Qiadam: Bieletan, DongTaijinaier, XiTaijinaier, and Yiliping (Figure 8; Yu et al., 2013). Zabuye lake on the Tibetan Plateau is probably the most geologically interesting as the Li content of the lake waters are so elevated that it is the only known lacustrine location where lithium carbonate, zabuyelite, is a natural brine precipitate (Figure 9. Nie et al., 2009; Gao et al., 2012).


Qaidam Lakes

Detailed sedimentological and hydrological work in the Qaidam by Yu et al. (2013) has shown that: (1) Some 748.8 tonnes of lithium is transported annually into the lower catchment of the four salt lakes via the Hongshui-Nalinggele river (H-N river in Figure 8), which is the largest river draining into the Qaidam Basin, (2) Li-rich brines are formed only in those salt lakes in the Qaidam that are associated with inflowing rivers with Li concentrations greater than 0.4 mg/l, and (3) the water's Li concentration is positively correlated with elevated levels in both the inflowing river and the associated subsurface brine. Their findings show that long-term input of Li+ from the Hongshui-Nalinggele river controls the formation of lithium brine deposits. They conclude that the source of the lithium in the lake brines is ultimately from hydrothermal fields, where two active faults converge in the upper reach of the Hongshui River. These hydrothermal fields are associated with a magmatic heat source, as suggested by the high Li+ and As3+ content water in geysers in the geothermal field. Based on the assumption of a constant rate of lithium influx, they estimate that the total reserves of lithium in the Qaidam were likely formed since the postglacial period.

Field mapping and coring indicate that lithium reserves in each of the four salt lakes depend on the influx of Li+-bearing water from the H-N River. The data also suggest that during the progradation of the alluvial Fan I, the Hongshui-Nalinggele drained mostly into the Bieletan salt lake, until the Taijinaier River shifted its watercourse to the north and began to feed the salt lakes of the DongTaijinaier, XiTaijinaier and Yiliping salt lakes, while also driving Fan II progradation (Figure 8).

One of the You et al. (2013) major findings in terms of lithium enrichment models is the importance of the contrasting hydroclimatic conditions between the high mountains containing ice caps and the terminal salt lakes. The greater than 4000 m of relief in the watershed enables a massive amount of ions, such as K+, to be weathered and transported, together with detrital material from the extensive, relatively wet alpine regions to the concentration sumps in hyperarid terminal salt lakes, where intense evaporation rapidly enriches the lake water, resulting in evaporite deposition and associated K- and Li-rich brines. It is no surprise that a saline lake at the foot of the nearby Golmud River fan is one of the few places in the modern world where carnallitite is found (Casas and Lowenstein, 1992).


Lake Zabuye

Lake Zabuye is located some 1000 km west of Lhasa, the Tibetan capital, and lies in the ET Köppen high altitude climate zone of the Tibetan Plateau (Figures 2, 9). The lake is perennial, and water levels can vary by metres each year; in 2008 the water level was some 4422 m above sea level. At this level, the lake’s area is approximately 247 km2. Salinity varies from 360 to 440 ‰, depending on seasonal differences in water input and evaporation rate. The volume of lithium product at the lake is currently limited by the sulphate-rich nature of the primary lake brine, prior to concentration in solar pans (Gao et al., 2012).

When concentrated, the crystallisation sequence of salts from highly concentrated Zabuye lake brine at 25°C is (Figure 10a; Nie et al., 2009):

halite (NaCl) --> aphthitalite (3K2SO4•Na2SO4) --> zabuyelite (Li2CO3) --> sylvite (KCl) --> trona (Na2CO3•NaHCO3•2H2O) and thermonatrite (Na2CO3•H2O)

The lake’s brine is naturally supersaturated with NaCl and other salts, so millions of metric tons of halite, potash, trona, and other minerals have accumulated on the bottom of the lake in the past few thousand years (Zheng and Liu, 2010). Lithium carbonate and sylvite precipitate, via a combination of brine concentration and cooling, and higher levels of lithium carbonate precipitation in the end brine can be induced by the addition of soda ash, as is done in the South American salars (Figure 10b, c).


The problem with the natural lake chemistry of the Zabuye salt lake is that a lithium sulphate salt Li2SO4.3Na2SO4. 12H2O precipitates naturally in the early stages of the low-temperature evaporation process, so reducing the levels of lithium carbonate in the end-stage brines. If the brine concentration series in the pans can be artificially held at mirabilite concentration, then the amount of lithium lost to the sulphate salt is reduced, so levels of lithium in the end-stage brines improve (Gao et al., 2012).

Zabuye Lake is of significant economic value as it is a new type of exploited saline lacustrine deposit (compared to the salars of South America) in that contains it precipitates lithium and borate salts in addition to significant volumes of potash, halite, natron and Glauber’s salt. Lake waters also retain elevated levels of caesium, rubidium and bromine.

Lithium in minerals soaked in saline brines

Two saline minerals in sedimentary basins known to have significant lithium contents are hectorite and jadarite. Hectorite [Na0.33(Mg,Li)3Si4O10(F,OH)2] is a clay mineral of the smectite group, where the replacement of aluminium by lithium and magnesium is essentially complete. It has a lithium content of more than 1%, a hardness of 1–2 on Mohs scale, and a density of 2–3 kg/m3. To date, an economically viable technology for extracting lithium from hectorite, rather than from brines that enclose some of these clay deposits, has yet to be developed (Evans, 2014). Jadarite [LiNaB3SiO7(OH)], is a newly recognised mineral with up to 5.7% Li and 14.7% B. Jadarite is a white porcellanous borosilicate mineral with a Moh hardness of 4-5, and a density of 2.45 gm/cc. It is associated with borate salts such as colemanite in the Oligocene-Pliocene lacustrine host sediments in its type area in the Jadar Valley in Serbia (Stanley 2007). Hectorite is probably associated with crossflows of moderate salinity hydrothermal waters, while jadarite requires a bath of hypersaline hydrothermal waters to form.

Hectorite

Hectorite has a soft, greasy texture, a candlewax-like appearance and feels like modelling clay when squeezed between the fingers. As a colloid, hectorite’s unique thixotropic properties for emulsion stabilising, gelling, suspending, binding, bodying and disintegrating, means it sells for more than US$2,000 a ton, generally as a lubricant to the oil and gas industry. Associated authigenic clays include stevensite and saponite, and in its type area at Hector California lies adjacent to a colemanite deposit.

Hectorite is mined periodically (not as a lithium source) in its type area, the Hector Mine, near Barstow, California. There, hectorite is the main clay mineral in a sequence of altered volcanic ash beds that are interbedded with lake sediments and travertines along an 8 km fault zone (Figure 11; Ames et al., 1958). The hectorite is thought to have formed through hydrothermal alteration of the ash by saline fluids moving up the fault zone (Sweet, 1980). Lithium-bearing volcanic rocks that probably formed in the same way have also been described from Arizona, and the Clayton and King Valleys in Nevada (Brenner-Tourtelot and Glanzman, 1978; Kesler et al., 2012). Hectorite is not considered to be a prime lithium resource in any of these occurrences. It is, however, considered of co-indicator of the former, or current, presence of Li-rich saline brines and as such is considered a pointer mineral to a possible lithium brine resource.


Hectorite is thought to be a result of the combination of three distinct geological processes: 1) the alteration of volcanic ash or glass; 2) precipitation of authigenic phases from saline lacustrine pore waters; and/or 3) the incorporation of lithium into existing smectite clay deposits (Asher-Bolinder, 1991). To form hectorite, all three processes require an arid environment and are associated with lithium-enriched saline alkaline waters, volcanic rocks and hot springs that can also co-precipitate travertines and fine-grained amorphous silica (Zientek & Orris, 2005).

The same association of processes explains the lithium-rich hectorite clays in King Valley (Nevada Lithium prospects) Nevada. There, layers of hectorite occur in a sequence of sedimentary and tuffaceous rocks in moat sediments along the western side of the McDermitt caldera (Figure 12; Kesler et al., 2012). Volcanic activity at the McDermitt caldera complex has yielded precise 40Ar/39Ar ages of 16.5 to 16.1 million years ago and was characterised by extrusion of early metaluminous and peralkaline rhyolite, followed by the eruption of a voluminous ignimbrite with peralkaline rhyolite to metaluminous dacite compositions (Carew and Rossi, 2016). After collapse, the central part of the caldera complex was the site of resurgence, and a moat-like lake formed between this resurgent dome and the caldera walls. The lake was the site of deposition of volcaniclastic sediments that now form a nearly continuous ring within the caldera and host the various hectorite lenses(Figure 11).


Hectorite layers ranging from 1 to 90 m in thickness and have been recognised over a length of about 20 km. Individual layers or groups of layers extending for several km and are annotated as stage 1-5 lenses. The Stage 1 lens of the Lithium Nevada deposit (informally known as the King Valley deposit) has proven and probable reserves of 50 million tonnes, averaging 0.312% Li (Carew and Rossi, 2016). As in the type area in the Hector Mine in California, hectorite in the various lenses is the main lithium-bearing clay mineral in a sequence of altered volcanic ash beds. These ash beds are interbedded with saline lake sediments and travertines, and are hosted in the sedimentary moat facies adjacent to an 8 km fault zone. That is the hectorite formed through hydrothermal alteration of volcaniclastic ash in regions where moderately saline hydrothermal fluids moved up a fault zone.

Hectorite clays are also found in the Sonora Lithium Project, 11 km south of Bacadehuachi in the Sonora state of north-west Mexico. The resource statement, in an April 2016 report, lists 839,000 tonnes of contained lithium in the indicated category and a further 515,000 tonnes in the inferred category, within two distinct lacustrine clay units situated below basaltic caprocks (Pittuck and Lepley, 2016). A pre-feasibility study has been completed, which proposes a two-phase open-pit mine with lithium carbonate processing facility and a mine life of 20 years. A pilot plant has also been constructed, and discussions have commenced regarding possible off-take agreements.

None of these hectorite occurrences are currently mined as a lithium resource.

Jadarite

Jadarite was discovered in 2007 by Rio Tinto and the Jadar deposit, near the town of Loznica, and at that time was estimated to contain an inferred resource of 125.3 million tonnes at a weighted average of 1.8% Li2O, in addition to an inferred resource of boron minerals. Jadarite has so far only been identified in significant amounts within the 20-km long Jadar Basin of Serbia. The Jadar Basin entrains oil shales, dolomicrites, pyroclastic sediments and evaporites which are believed to have accumulated in an intermontane lacustrine environment.


The jadarite occurs both in massive form, several metres thick, and also as small nodules within a fine-grained carbonate matrix (Stanley, et al., 2007). At the main Jadar deposit, a layer containing nodular colemanite (Ca2B6O115H2O) overlies three separate layers or lenses containing jadarite LiNaB3SiO7(OH). Jadarite likely formed via a hydrothermally-facilitated interaction between saline brine and clastic/evaporitic sediment, either in a tuffaceous or clay host (Kesler et al., 2012).

In May 2017, Rio Tinto announced that the Jadar area contains one of the largest lithium deposits in the world, lifting their estimate for Lower Jadar's deposits to 138 million tonnes. Extraction is scheduled to begin in 2023, with a projected underground exploitability of 50 years. As of June 2017, construction of a mine has not begun. A jadarite processing plant is also planned next to the mines, that plant will process the ore into lithium carbonate and boric acid.


Summary

Characteristics that appear to be essential to define a potential lithium carbonate brine resource are; i) an arid climate and, ii) a closed, tectonically active basin, with significant elevation and tectonically activity, which can entrain brines with elevated lithium contents (Figure 15; Bradley et al., 2013; Yu et al., 2013; Warren, 2010, 2016). Sources of lithium can be deeply circulated magmatic or recycled basinal fluids. Magnesium levels in the brine should not be too high as this complicates brine processing during lithium carbonate extraction. A co-occurring potash resource, extractable from the same brine, if present can reduce processing costs.

Another possible requirement—or at least a favourable characteristic—is elevated heat flow, as evident from young volcanoes or hot springs and the associated increase in Li-rich juvenile waters flushing the surrounding drainage basin, as is occurring beneath the Andean Altiplano. Volcanogenic source rocks in the lake drainage, such as felsic, vitric tuffs that have abundant and readily leached lithium are favourable, but perhaps not essential, since lithium is present in most crustal rocks at tens of parts per million. Worldwide all the exploited salt lakes have lithium levels in their lake brines that are well above typical (Figure 14).

Another possible favourable indication of a lithium brine is the existence of hectorite or jadarite in associated clays in the bajada rims.

All known and potential lithium brine deposits are located in arid tectonically active areas, typically in subduction or collision belts with deep-faulted suture systems (Figure 2). At the world scale, lithium-prone saline deposits are latitudinally restricted to cool arid Koeppen climate belts within endorheic brine sumps surrounded by high altitude drainage basins (Figures 2, 15b; Warren, 2010). Borates as evaporite salts are generally tied to the same setting (Warren, 2010).


Active faulting appears to be involved in forming a suitable spring-fed hydrology for all known economic lithium brine basins. Fault-related subsidence also creates accommodation space, without which only a thin veneer of arid basin sediments and brines can accumulate. Thus, a thick basin fill is needed to provide an aquifer of sufficient volume to hold a viable lithium brine resource.

In contrast, saline lakes atop shallow, superficial basins in intracratonic regions such as the Sahara Desert and most inland Australian deserts largely lack active fault control and associated rapid subsidence, and are not known to be prospective for lithium brines.

Salt fills in some lithium basin lacustrine sumps are cut by active intrabasinal faults (known from boreholes and seismic) but have no surface expression due to rapid infill and levelling of the accommodation space by salt precipitation. Significantly, the brine pools in Clayton Valley, Salar de Atacama and the Qaidam sumps are localized along active intrabasinal faults, which also control the distribution of aquifers and influence groundwater movement patterns, as well as the position of maximum stacking of concentrates and brines in the halite nucleus, along with porosity retention levels in the subsurface halite host (Zampirro, 2004; Jordan and others, 2002).

Porosity levels in a host halite aquifer are a major constraint on the potential economics of any salar or salt lake lithium brine resource. Most halite units lose their effective porosity and permeability by depths of 50-60 metres (Warren, 2016; Chapter 1). Thus, most Quaternary lithium brine operations hosted in a halite bed/aquifer will have an economic basement to brine recovery at around this depth. It is unlikely that recovery operations in Salar de Atacama and planned projects in Salar de Uyuni can recover economic brine volumes at much greater depths. There may be a 900m thick halite-dominant succession infilling Salar de Atacama and a number of halite beds to a depth of 120m in Salar de Uyuni, but economic porosities in the halite will likely only be present in the upper portions of the halite fill in both salars.

Postulating likely lithium resources, in a salar of a salt lake, to depths greater than 50-60m should only be done after salar-hydrology-aware drilling has established the presence of economic permeabilities in the hosting halite aquifer and this is likely related to the presence of active faults-. Such measurements require drilling and sampling equipment that facilitates reliable “in-situ” determinations of porosity and permeability in the halite mass and Li measurements in the brine that are related to actual content at the level of measurement, with minimal contamination by waters from outside the measured horizon.

References

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