Potash Geology 

Potash and ocean Chemistry across Deep Time

Within Phanerozoic oceanic chemistries, there are well-documented changes (cycles) in the proportions of magnesium and sulphate (Lowenstein et al., 2001, 2003). These varying ionic proportions in seawater define whether or not MgSO4 salts were typical marine bittern salts. Work by Kovalevich 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 worldwide (Lowenstein et al., 2001, 2003a), 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 bittern salts when concentrated, and the latter does. This has led to the notion of times of MgSO4-enriched oceans (as in the Neogene and the Permian) and to much longer times of MgSO4-depleted oceans across the Phanerozoic (Figure 2a). The background chemistry of this marine dichotomy (MgSO4-depleted bitterns versus MgSO4-enriched bitterns) is simple, and can be related to brine evolution observations published by Hardie more than 30 years ago (Hardie, 1984).

Phanerozoic seawater A) Secular variation in the amounts of Ca and SO4 in seawater for the last 600 my estimated from fluid inclusions in marine halites (vertical bars), compared to predicted seawater secular variations. The horizontal line around 20 mml/kg H2O is the approximate divide between MgSO4-enriched and MgSO4-depleted seas. Also plotted are the temporal distributions in the primary mineralogies of Phanerozoic nonskeletal carbonates (calcite and aragonite) and periods of MgSO4-free and MgSO4 bitterns. For Mg/Ca, the grey plot bars are from halite inclusions measurements but the Mg/Ca curve is from marine carbonate data (after Lowenstein et al., 2001, 2003a; with Paleozoic MgSO4 boundary change after Holt et al., 2014). B) Chemical divides and the evolution of three major brine types in the framework of oceanic chemistry (in part after Hardie, 1984).

That is, a marine brine’s evolutionary pathways are determined by the ionic proportion chemistry during the precipitation of relatively insoluble evaporitic carbonates (aragonite, high-magnesium calcite, or low-magnesium calcite, dolomite) and then by its constituent chemistry as it attains gypsum saturation. These precipitation stages define the carbonate and gypsum divides. Hence, when we look at marine-fed potash deposits in a framework of geological time, there are times when the bittern suites contain enriched levels of MgSO4 salts in the bittern assemblage (as in Neogene and Permian deposits) and times when the bittern suite is depleted in MgSO4 salts.

Accordingly, a Phanerozoic potash salt series defines two distinct time-related groupings of brines and associated minerals: MgSO4-rich and MgSO4-poor. All potash deposits rich in MgSO4 accessories are composed of some combination of gypsum, anhydrite, halite, polyhalite, kieserite, kainite, carnallite and bischofite. This group of MgSO4-entraining salts typifies Quaternary salts in the Dallol depression of Ethiopia, the Miocene assemblages of Sicily and the Ukraine, as well as some Permian potash deposits in Europe and Russia and the remnants of late Neoproterozoic salt deposits. Potash deposits free of, or deficient in, MgSO4 minerals are dominated by some combination of halite, carnallite, and sylvite with rare sulphate, bitterns. They typify much of the Phanerozoic rock record, other than Neogene and Permian potash deposits. This MgSO4-depleted group constitutes the majority of known reserves of exploited potash, which are dominated by Devonian evaporites in Canada. Potash salts have not been recovered from Precambrian sediments, other than in minor amounts of intercrystalline cement in the latest Neoproterozoic evaporites of the Ara Salt Group of Oman and the Hanserian Evaporite Group (HEG) of the Nagaur-Ganganagar basin in India/Pakistan. But the accessible portion of the latter, as in the Khewra salt mine, have been extensively altered by deep meteoric circulation during Neogene upliftˆ

Potash occurrence and quality at the worldscale

The solubility of potash salts controls how and where beds are economic at the extraction scale, but are the same sets of processes, namely dilution and reprecipitation, also significant in controlling potash texture and distribution at the basin scale, and which tectonic associations are likely to be optimal for the development of basins that entrain relatively undeformed, mineable, potash horizons?

Neogene potash

First, let us consider the some documented Quaternary potash deposits (Table), these systems can be subdivided into those where the potash minerals occur naturally at the present day landsurface, namely, Salar Gorbea-ET, Quinghai (Dabaxum)-BWk, and Chott el Djerid-BWh. Aridity in these settings ranges across polar tundra, to desert steppe, to hot dry desert. In all three cases, the modern potash precipitate is carnallite, not sylvite, and it is either precipitated as ephemeral efflorescences, or occurs as a karst fill cement in a bedded halite host. In no case is Quaternary potash a marine-sourced mineral, in only two regions (Qinghai and Danakil) is the potash present in sufficient volumes to be exploited, and in neither case is the potash present as extensive at-surface beds. Instead, it occurs in discontinuous karst-influenced cement clusters, hosted in primary-textured halite beds.

In all the other listed Quaternary examples there is no natural precipitate of potash, it is anthropogenically produced in salt factory facilities operating as brine concentrators, with inflow brine feeds derived either from modern lake brines or recovered by the solution mining of ancient evaporites. Mineralogically and topographically, there is a distinct schism in Quaternary potash associations. Muriate of potash is the dominant phase produced, or expected to be produced, in the Dead Sea and the Danakil depressions. Both deposits are forming in extreme endorheic lows on the Earth’s surface (some 415 and 115m metres below sea level, respectively), both are in adiabatic deserts in regions of severe subsidence, both have brine inflows with chemistries and ionic proportions substantially influenced by the dissolution of older marine evaporites. The slightly milder BSh (hot desert steppe) setting of the southern Dead Sea versus the BWh (hot arid desert) setting of the Danakil, which encompasses the hottest place (year-round) on the Earth’s surface, may explain why carnallite is a natural at-surface precipitate in the Danakil and a manufactured salt in the desert steppe climate of southern Dead Sea pans.

In contrast to the two listed muriate of potash occurrences, for the SOP examples, the various combinations of mixed sulphate of potash salts are the precipitated precursor salts in the briney lake sediments, as well as being the manufactured product. In Lop Nur, for example, the natural lake evaporites are nonmarine assemblages of mirabilite-glauberite-polyhalite-bloedite-gypsum-halite. The evaporitic stages of the lake fill in Lop Nur contain massive amounts of glauberite and polyhalite compared to the other salts present, and their predominance, is indicative of pervasive back-reactions, as is the presence of very minor amounts of carnallite and sylvite (Ma et al., 2010; Dong et al., 2012). In the Great Salt Lake, the SOP is produced by processing a slurry of kainite and schoenite that is the prime winter precipitate after halite has formed in the summer in the earlier concentrator pans. In Quill Lake, Canada, SOP is produced by the mixing of MgSO4-rich lake brines with sylvite trucked to the lake processing plant. In the Moab pans, muriate of potash is the product after solar evaporation, dye injection and slurry processing. The brine feed to the solar pans is not a Quaternary lake brine, but is derived from the pumped-in waters of the Colorado River, which are heated, pumped into, and flow through the flooded workings of the former Cane Creek Potash Mine, before being piped into the at-surface solar pans. In Salar de Atacama there are two potash brine processing streams in the evaporation pans, both are fed from a brine field located in the nearby natural sump: a) One of the pan processing streams produces a KCl precipitate (sylvite and carnallite) as it is concentrated to a lithium chloride brine stage; b) The other potash processing stream in the pans precipitates SOP with boric acid as a manufactured byproduct. The hyperarid climate at Atacama facilitates year-round solar-driven production of the salts. Both the Moab and Atacama occurrences and all the other SOP Quaternary examples listed in the Table are situated in cool arid (BWk) or desert steppe (BSk) climates, or, in the case of the Great Salt Lake and Quill lake, even milder temperate seasonally arid climates. None of the SOP climates is as extreme as the MOP forming areas in the Danakil and the Dead Sea. Yet these two, and all of the Quaternary lacustrine settings manufacturing SOP, have hydrologies that are fed nonmarine brine inflow with ionic constituents very different from modern seawater. If we look at Quaternary potash occurrences worldwide, it is clear that all modern potash is a nonmarine salt, as are many of the other economic salt-based feedstocks to the chemical industries of the world (salt cake, soda ash, borates, CaCl2 and lithium brines – Warren 2010). That is, to state the obvious, all Quaternary potash producing environments are defined by markedly nonmarine sets of environments of deposition. We should not use them as same-scale analogues for ancient marine-associated potash occurrences. Phanerozoic potash salt beds comprise two distinct associations: MgSO4-rich and MgSO4-poor (Table).

Table lists a variety of Neogene potash occurrences (Warren, 2016)

Pre-Neogene potash

Ancient potash deposits, rich in MgSO4, are composed of some combination of gypsum, anhydrite, polyhalite, kieserite, kainite, carnallite and bischofite. Potash deposits free of, or poor in, MgSO4 are dominated by some combination of halite, carnallite, and sylvite. This latter group constitutes a majority of the known “in situ” potash reserves of the world, and is very much dominated by the Devonian evaporites of western Canada. Other than minor amounts of sylvite in the halokinetic structures in Late Neoproterozoic-Early Cambrian Ara Salt of Oman, potash salt remnants have not been recognised in Precambrian sediments. In contrast, to the markedly nonmarine locations of potash recovery from the Quatern ary sources, almost all pre-Quaternary potash operations extract product from marine-fed basinwide ore hosts during times of MgSO4 depleted and MgSO4 enriched oceans. This time-based dichotomy in potash ore sources with nonmarine hosts in the Quaternary deposits and marine evaporite hosted ore zones in Miocene deposits and older, reflects the simple lack of basinwide marine deposits since the late Miocene (Warren, 2010). As for all ancient marine evaporites, the depositional system that deposited ancient marine-fed potash deposits was one to two orders of magnitude larger and the resultant deposits were typically thicker stacks than any Quaternary potash settings. The last such “saline giant” potash system was the Solfifera series in the Sicilian basin, deposited as part of the Mediterranean “salinity crisis”. So, what are the factors that favour the formation of, and hence exploration for, additional deposits of exploitable ancient potash? First, they are all basinwide, not lacustrine deposits. Within the basinwide association, it seems that intracratonic basins host significantly larger reserves of ore, compared to systems that formed in the more tectonically-active plate-edge association. This is a reflection of:1) accessibility – near the shallow current edge of a salt basin, 2) a lack of a halokinetic overprint and, 3) the setup of longterm, stable, edge-dissolution brine hydrologies that typify many intracratonic basins. Known reserves of potash in the Devonian Prairie evaporite in West Canadian Sedimentary Basin (WCSB) are of the order of 50 times that of next largest known deposit, the Permian of the Upper Kama basin, and more than two orders of magnitude larger than any other of the other known exploited deposits (Table 10). Part of this difference in the volume of recoverable reserves lies in the fact that the various Canadian potash members in the WCSB are still bedded and flat-lying. Beds have not been broken up or steepened, by any subsequent halokinesis. The only set of processes overprinting and remobilising the various potash salts in the WCSB are related to multiple styles and timings of aquifer encroachment on the potash units, and this probably took place at multiple times since the potash was first deposited, driven mainly by a combination of hinterland uplift and subrosion. In contrast, most of the other significant potash basins listed in the Table have been subjected to ongoing combinations of halokinesis and groundwater encroachments, making these beds much less laterally predictable. In their formative stages, the WCSB potash beds were located a substantial distance from the orogenic belt that drove flexural downwarp and creation of the subsealevel sag depression. Like many other intracratonic basins, the WCSB did not experience significant syndepositional compression or rift-related loading.

Table lists a variety of Pre-Neogene potash occurrences (Warren, 2016)