GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 242-2
Presentation Time: 8:20 AM


COCKER, Mark D. and ORRIS, Greta J., U.S. Geological Survey, GMEG, 520 N. Park Avenue Ste 355, Tucson, AZ 85719

Economic potash-bearing salt deposits are characterized by numerous closely spaced evaporite cycles containing limestone, anhydrite, salt, potash- and magnesium-salts. The potash mineralization may average 13 to 26% K2O or higher over mining intervals of one to several meters and may be capped by thin clay seams. Total mineralized areas may be several hundred to tens of thousands of square kilometers. Tonnages may range up to several hundred billion metric tons of K2O. Thicknesses, grades and tonnages of these ore bodies result from repeated mineralization events due to multiple episodes of marine invasion, evaporation, and increasing brine salinities in large continental or marine rift and intracratonic basins. The potash-bearing cycles are generally closely spaced near the upper part of a thick section of salt and deposited over relatively short time spans ranging from a few thousand to several hundred thousand years. There may be many evaporite cycles in a salt basin, but numerous closely spaced potash cycles are necessary to provide sufficient potash to form a minable deposit. Origins of cyclic potash mineralization depend on scales varying from seasonal or diurnal fluctuations in local weather conditions to global sea-level changes.

The world’s largest known source of potash is contained within 4 members in the upper 100 m of the Givetian Prairie Evaporite in the Elk Point Basin in Saskatchewan. Each member is the culmination of a larger evaporite cycle several tens of meters thick and consists of 4 to 20 smaller evaporite cycles of halite + sylvinite or carnallitite + clay. Within each cycle K2O grades may range up to 44 % from basal halite to the upper sylvinite with combined overall grades of 20.3 to 26.0 % K2O over mined intervals of 3.35 to 3.7 m. Each potash member may range from 59,000 to 139,000 km2 in extent. Mining since 1962 has produced over 1.5 trillion metric tons of potash ore or nearly 400 Mt of K2O.

Other examples of major potash-bearing deposits include: the Famennian Pripyat Basin in Belarus with 60 cycles; the Aptian Basins in Brazil and West Africa with at least 133 potash-bearing cycles; the Cisuralian Solikamsk Basin in Russia with up to 23 cycles; the Ochoan Delaware Basin in New Mexico with 12 cycles; the Permian Paradox Basin in Utah with 18 cycles and the Lopingian Zechstein Basin in northern Europe with 7 cycles.