Potash salts are a typical part of a brine evaporation series, crystallizing at the higher concentration or bittern end, either at the surface (primary salts) or in the shallow subsurface (secondary salts). Today, bedded accumulations of primary potash evaporites are a relatively rare occurrence. Extremely high solubility of most potash salts means they accumulate in highly restricted, some would say highly continental, modern depositional settings. Wherever Quaternary potash does occur naturally, as in the playas of the intermontane Qaidam Basin in China and in the Danakil Depression in the Afar Rift of Africa, carnallite - MgCl2.KCl.6H2O, not sylvite KCl, is the dominant potash salt. This has led some to postulate that carnallite is the archetypal primary marine potash phase, while sylvite is a secondary diagenetic mineral formed by incongruent dissolution of carnallite. Others have argued that ancient sylvite was sometimes a primary precipitate, deposited by the cooling of highly saline surface or nearsurface brines and from seawater with ionic proportions different to those of today (brine evolution).
Phanerozoic potash salt series define two distinct time-related groupings of brines and associated minerals: MgSO4-rich and MgSO4-poor. 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. No potash salt was recovered in Oman (inferred from wireline log), while the dominant potash mineral cored in the HEG is polyhalite, making it less attractive as an exploration target for fertiliser manufacture in that part of the world.
All potash deposits rich in MgSO4 accessories are composed of some combination of gypsum, anhydrite, 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 the Ukraine and Sicily, as well as some Permian potash deposits in Europe and Russia and the few Eocambrian deposits known to exist. Potash deposits free of, or poor in, MgSO4 minerals are dominated by some combination of halite, carnallite, and sylvite and typify much of the Phanerozoic rock record. This MgSO4-depleted group constitutes the majority of known reserves of exploited potash, which are dominated by Devonian evaporites in Canada.
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