Quaternary potash

There is no economic production from a Quaternary potash source by conventional mining anywhere in the world, rather potash is produced by brine processing in modern solar evaporation pans sourced in various saline continental lakes and playas. The world’s largest Quaternary brine factories for MOP currently located at the southern end of the Dead Sea, Israel and Jordan, with a smaller MOP operation in the Dabuxum Lake area of the Qaidam Basin, China (Warren, 2016). In both these operations lake waters are pumped into solar concentrator brine pans, with a carnallitite slurry as the desired product. There are plans for potash recovery from lake brines in the Dankhil Depression, in the Afar region of Africa. An even larger set of surface ponds than those of the southern Dead Sea have been recently constructed for SOP production in Lop Nur region, China. Detailed discussion of these and all the other currently producing potash brine operations, as well as published references, are to be found in Warren (2016)

• Lop Nur, China (sulphate of potash production)

Dead Sea potash (MOP - muriate of potash production)

In the Southern Basin of the Dead Sea a series of linked fractionation ponds have been built to concentrate Dead Sea brine to the carnallite stage (A). On the Israeli side this is done by the Dead Sea Works Ltd. (DSW), near Mt. Sedom, and by the Arab Potash Company (APC) at Ghor al Safi on the Jordanian side. In both brine fields, muriate of potash is extracted by processing carnallitite slurries, created by sequential evaporation in a series of linked, gravity-fed fractionation ponds. 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 area of the concentration pans is more than 130 km2, within the total area of 1,000 km2 that is the Dead Sea floor. The first stage in the evaporation process is pumping of Dead Sea water into header ponds to 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, resulting in a ongoing need for more powerful brine pumps. Saturation stages of the evolving 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 (B).

 

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 halite 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 decimeters 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). As the concentrating brines approach carnallite precipitating densities (around 1.3 gm/cc), they are allowed to flow into the carnallite precipitating ponds (C). Individual pans have areas around 6-8 km2 and brine depths up to 2 meters. During the early halite concentration stages a series of halite reefs or mushroom polygons can build to the brine surface and so compartmentalise and entrap brines within isolated pockets enclosed by the reefs, so preventing the orderly downstream progression of increasingly saline brines into the carnallite ponds, with the associated loss of product. This has been and ongoing problem for both the Israeli and Jordanian saltworks. Reason and methods to deal with the "mushroom" problem as well as the  processing methods for recovering sylvite from the produced carnallitite slurry are discussed in Warren (2016).

 

Potash in the Danakhil Depression (potential MOP and SOP production)

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.

 

What follows is a short summary of my current understanding of potash controls in the Dallol depression it expands on the currently published data as summarised in Warren, 2016. The complete set of information on which this short summary  based on field work by the author during a vist to the region sponsored by BHP Minerals. The parts of the geological story that were given permission to be made public were presented in a series of four Salt Matters articles/blogs on this web site (pdf 1 of 4, pdf 2 of 4, pdf 3 of 4, pdf 4 of 4).

Surficial geology of the northern Danakil depression, shows well-developed outcrop rims defined by an earlier reef and later gypsum phase  as well as the extent of the modern halite pan (Salty Matters article, April 19, 2015). Each zone is located at successively lower elevations in the depression. Also shown is the position of the Dallol volcanic mound, its fringe of uplifted lake sediments, the associated hydrothermal springs and collapse dolines where carnallite and bischofite are precipitating today

Three styles of Quaternary potash salts in the Danakhil basin

1) An initial subaqueous marine-fed setting that deposited widespread bedded potash with significant amounts of potassium sulphates indicating its derivation from seawater with ionic proportions similar to modern seawater bitterns (Salty Matters article, April 29, 2015). This bedded, now tilted potash interval is found in subsurface across much of depression. It sits at the top of the lower salt unit, with a deposition transition downward into clastic subaqueous salt textures. Above it, and atop a likely exposure/alteration surface, are the upper halite unit and the interbedded clays and halites, identical to what is accumulating beneath of modern salt pan.

2) Remobilised potash salts and brines (Salty Matters article, May 1, 2015). There are two main types of remobilised potash; both are related to the circulation of hydrothermal and shallow meteoric fluids that drive alteration fronts whereby groundwaters interact with the original marine-derived potash-sulphate entraining beds to form a variety of potash products including sylvite

 

2a) This group of remobilised potash salts/ore occurs in a sylvinite/carnallite band along the western side of the depression as groundwater alteration units developed where deeply circulating saline meteoric interact with potash-rich marine salt beds that were first deposited below the upper halite unit. This set of secondary potash salts is tied to the incongruent dissolution of  penecontemporaneous carnallitite and kainitite. This type of sylvite is a potential ore interval (via solution mining) and is now best developed as alteration fronts in the subsurface along the western side of the depression. There, it ties to a hydrochemical interface created by the encroachment of the bajada groundwaters into the western margin of the primary potash salt zone.

Away from shallower zones where sylvite forms via alteration, the primary potash interval is mostly as a present as a carnallitite and kainitite association that now dips westward. It is shallow (tens of meters deep) along the eastern side of the depression passing out in buried depths of more than 500 meters further to the south-east, beneath the modern salt pan floor.

2b) This group of potash salts is tied to the circulation of warm hydrothermal circulation cells in the vicinity of Dallol mound. Importantly, a ground visit quickly shows that Dallol Mound is not a volcanic cone, rather it is an anticlinal dome of uplifted and eroded bedded salt, capped and surrounded by hydrothermal crater features typified by karst pools and brine outflows. In the past 50 years, one of these pools formed explosively via the escape of highly pressurised brines, not lavas.  Pool and groundwater sump creation are likely related to emplacement of igneous material at depth, into levels with hydrated salts, especially carnallitite and kainitite. As yet there, has been no breakout of volcanic rock material in the Dallol mound area. Rather the interaction igneous sills and dykes with buried hydrated salt levels drive the release of fluids that can then escape to the surface via a process of hydrofracture to cut across mechanically-weak halites and clays. This same fluid release process may aid in the bed doming and erosion that typifies the Dallol Mound. Ancient equivalents showing fluidization of hydrated potash zones and alteration of carnallite to sylvite are seen in some Permian salt mines in parts of eastern Germany where Eocene basaltic sills and dykes cut through the Zechstein salts (Schofield et al., 2014).

The fact there is not one, but three, associations of potash salts in the Danakhil, each with variable levels of sylvite, carnallite and kainite has important economic implications for the nature of remobilised potash and the creation of potential potash ores in the Dallol Mound area. Inherent depositional and diagenetic complexity means the visually spectacular sylvite-bischofite pools, that are active at the surface today in the vicinity of the Dallol mound, are process-independent of the other two associations of potash salts. Exploration models must separate potential ore targets from the more regional distribution of primary potash beds (kainitite and carnallitite) and from the bajada associated alteration front that explains the other main style of sylvite occurrence in the Danakhil depression (Salty Matters, May 12, 2015).

 

Lop Nur potash, China (muriate of potash production)

Sulphate of potash is and attractive product as it does not salinise the soils and readily as muriate of potash. We have already seen how it is produced from the Ogden flats, and other continental brine lake settings. The most significant lake brine production volume comes from Lop Nur, China

 

Sulphate of potash (SOP) via brine processing (solution mining) of lake sediments and brines is currently underway in the Luobei Hollow region of the Lop Nur playa, in the southeastern part of Xinjiang Province, Western China. The recoverable sulphate of potash resource is estimated to be 36 million tonnes from lake brine. Lop Nur is in the eastern part of the Taklimakan Desert, China’s largest and driest desert, and is in the drainage sump of the basin, it lies some 780 meters above sea level and is in a BSk climate belt. Lop Nur depression first formed in the early Quaternary due to the extensional collapse of the eastern Tarim Platform, and is surrounded and typically in fault contact with the Kuruktagh (to north), Bei Shan (to east) and Altun (to south) mountains.

 

The resulting Lop Nor sump is a large groundwater discharge playa that is the terminal point of China’s largest endorheic drainage system, the Tarim Basin, which occupies an area of more than 530,000 km2. The playa is the hydrographic base level to local and regional groundwater and surface water flow systems, and thus collectively captures all river and subsurface flow originating in the surrounding mountainous regions. The  Lop Nur  region has been subject to ongoing Quaternary climate oscillations, resulting in concentric strandzones on the playa surface over the last few hundred years, and widespread longer term (thousands of years) changes driving deposition of saline glauberite-polyhalite deposits, alternating with more humid lacustrine mudstones.

 

Presently, the Lop Nur playa lacks a longterm surface inflow and is characterized by desiccated saline mudflats and polygonal salt crusts. The upward capillary flux from the shallow groundwater helps to maintain a high rate of evaporation in the depression and drives the formation of a metre-thick ephemeral halite crust that covers much of the depression. Historically, prior to the construction of widespread irrigation systems in the upstream portion of the various riverine feeds to the depression and the diversion of water into the Tarim-Kongqi-Qargan canal, brackish flood waters periodically accumulated in the Lop Nur depression. After the diversion in flow, the terminal desiccation led to the formation of the concentric shrinkage shorelines, that today outline the “Great Ear” region of the Tarim Basin.

 

Climate is extremelyr arid; average annual rainfall is less than 20 mm and the average potential evaporation rates are ≈3500 mm/y). The mean annual air temperature is 11.6°C; highest temperatures occur during July (>40°C) and the lowest temperatures occur during January (<20°C). Primary wind direction is NE. The Lop Nor basin experiences severe and frequent sandstorms; the region is well known for its wind-eroded features, including many meso-yardangs along the northern, western and eastern margins of the Lop Nur salt plain.

 

Salinity and chemical composition of modern groundwater brine varies little in the ‘‘Great Ear” area and appears not to have changed significantly over the last decade. Dominant river inflow waters to the Lop Nor basin are of the Na-Mg-Ca-SO4-Cl-HCO3 type. In contrast, the sump region is characterized by highly concentrated groundwater brines (≈350 mg/l) that are rich in Na and Cl, poor in Ca and HCO3 + CO3, and contain considerable amounts of Mg, SO4 and K, with pH ranging from 6.6 to 7.2. When concentrated, the brine is saturated with respect to halite, glauberite, thenardite, polyhalite and bloedite.

 

Groundwater brines in the northern sub-depression, the Luobei region, where potash is produced in a series of concentrator pans, is similar in chemistry and salinity to the Great Ear area. There, K-rich mother brines, that also contain MgSO4, occur in pores of a widespread subsurface glauberite bed, with a potassium content of 1.4% (Liu et al., 2008). These are the source brines pumped in to a large field of concentrator pans to produce sulphate of potash.

 

Brine chemical models, using current inflow water and groundwater brine chemistries and assuming an open system hydrology, show good agreement between theoretically predicted and observed minerals in upper parts of the Lop Nor basin succession. Such modeling does not explain the massive amounts of glauberite (Na2SO4.CaSO4) and polyhalite (K2SO4MgSO4.2CaSO4.2H2O) recovered in a 230 m deep core (ZK1200B well) from the Lop Nor basin. Hydrochemical simulations assuming a closed system at depth and allowing brine reactions with previously formed minerals imply that widespread glauberite in the basin formed via back reactions between brine, gypsum and anhydrite and that polyhalite formed by diagenetic reaction between brine and glauberite. Diagenetic textures related to recrystallization and secondary replacement are seen in ZK1200B core, they include gypsum-cored glauberite crystals and gypsum replacing glauberite. Such textures indicate significant mineral-brine interaction during crystallization of glauberite and polyhalite.

 

Mineral assemblages predicted from the evaporation of Tarim river water match closely with natural assemblages and abundances and, in combination with a model that allows widespread backreactions, can explain the extensive glauberite deposits in the Lop Nor basin. It seems that the Tarim river inflows, not upwelling hydrothermal brines, were the dominant source throughout the lake history and that layered distribution of minerals in the cored sediments documents the evolving history of inflow water response to wet and dry periods in the Lop Nor basin. The occurrence of abundant glauberite and gypsum below 40 m depth, and the absence of halite, polyhalite and bloedite in the same sediment suggests that the brine underwent incomplete evaporation in the wetter periods Figure). In contrast, the increasing abundance of halite, polyhalite and bloedite in the top 40 m of core from the ZK1200B well indicate relatively dry periods, where halite precipitated at lower evaporative concentrations (log = 3.15), while polyhalite and bloedite precipitated at higher evaporative concentrations (log = 3.31 and 3.48 respectively). Following deposition of the more saline minerals the lake system once again became more humid, until the anthropogenically induced changes in the hydrology over the last few decades.

 

The Lop Nur potash recovery plant and pan system, located in the LuoBei Hollow (inset), utilises a brine source where the potash brine is reservoired in intercrystalline and vuggy porosity in a thick glauberite bed. This makes the Lop Nur system unique in that it is the first large-scale example of brine commercialisation for potash recovery from a continental playa system with a non-KCl brine target

 

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