Geology and undiscovered resource assessment of the potash-bearing Pripyat and Dnieper-Donets Basins, Belarus and Ukraine

Abstract

Undiscovered potash resources in the Pripyat Basin, Belarus, and Dnieper-Donets Basin, Ukraine, were assessed as part of a global mineral resource assessment led by the U.S. Geological Survey (USGS). The Pripyat Basin (in Belarus) and the Dnieper-Donets Basin (in Ukraine and southern Belarus) host stratabound and halokinetic Upper Devonian (Frasnian and Famennian) and Permian (Cisuralian) potash-bearing salt. The evaporite basins formed in the Donbass-Pripyat Rift, a Neoproterozoic continental rift structure that was reactivated during the Late Devonian and was flooded by seawater. Though the rift was divided, in part by volcanic deposits, into the separate Pripyat and Dnieper-Donets Basins, both basins contain similar potash‑bearing evaporite sequences. An Early Permian (Cisuralian) sag basin formed over the rift structure and was also inundated by seawater resulting in another sequence of evaporite deposition. Halokinetic activity initiated by basement faulting during the Devonian continued at least into the Permian and influenced potash salt deposition and structural evolution of potash-bearing salt in both basins. Within these basins, four areas (permissive tracts) that permit the presence of undiscovered potash deposits were defined by using geological criteria. Three tracts are permissive for stratabound potash-bearing deposits and include Famennian (Upper Devonian) salt in the Pripyat Basin, and Famennian and Cisuralian (lower Permian) salt in the Dnieper-Donets Basin. In addition, a tract was delineated for halokinetic potash-bearing Famennian salt in the Dnieper-Donets Basin. The Pripyat Basin is the third largest source of potash in the world, producing 6.4 million metric tons of potassium chloride (KCl) (the equivalent of about 4.0 million metric tons of potassium oxide or K 2 O) in 2012. Potash production began in 1963 in the Starobin #1 mine, near the town of Starobin, Belarus, in the northwestern corner of the basin. Potash is currently produced from six potash mines in the Starobin area. Published reserves in the Pripyat Basin area are about 7.3 billion metric tons of potash ore (about 1.3 billion metric tons of K 2 O) mostly from potash-bearing salt horizons in the Starobin and Petrikov mine areas. The 15,160-square-kilometer area of the Pripyat Basin underlain by Famennian potash-bearing salt contains as many as 60 known potash-bearing salt horizons. Rough estimates of the total mineral endowment associated with stratabound Famennian salt horizons in the Pripyat Basin range from 80 to 200 billion metric tons of potash-bearing salt that could contain 15 to 30 billion metric tons of K 2 O. Parameters (including the number of economic potash horizons, grades, and depths) for these estimates are not published so the estimates are not easily confirmed. Historically, reserves have been estimated above a depth of 1,200 meters (m) (approximately the depths of conventional underground mining). Additional undiscovered K 2 O resources could be significantly greater in the remainder of the Fammenian salt depending on the extents and grades of the 60 identified potash horizons above the USGS assessment depth of 3,000 m in the remainder of the tract. Increasing ambient temperatures with increasing depths in the eastern parts of the Pripyat Basin may require a solution mining process which is aided by higher temperatures. No resource or reserve data have been published and little is known about stratabound Famennian and Frasnian salt in the Dnieper-Donets Basin. These Upper Devonian salt units dip to the southeast and extend to depths of 15–19 kilometers (km) or greater. The tract of stratabound Famennian salt that lies above a depth of 3 km, the depth above which potash is technically recoverable by solution mining, underlies an area of about 15,600 square kilometers (km 2 ). If Upper Devonian salt units in the Dnieper-Donets Basin contain potash-bearing strata similar to salt of the same age in the Pripyat Basin, then the stratabound Famennian tract in the Dnieper-Donets Basin could contain significant undiscovered potash resources. The Cisuralian evaporite sequence in the Dnieper-Donets Basin consists of 10 evaporite cycles with the upper 3 cycles containing potash-bearing salt (mainly as sylvite and carnallite) in several subbasins and polyhalite in the sulfate bearing parts of the identified tract. The area of the Cisuralian tract is 62,700 km 2 . Potash-bearing cycles are as much as 40 m thick. One subbasin is reported to contain 794 million metric tons of “raw or crude” potash-bearing salt which could contain 50 to 150 million metric tons of K 2 O, depending on the grade. Undiscovered potash resources in the remainder of this permissive tract may be significantly greater. Depths to the Permian salt range from less than 100 to about 1,500 m. Undiscovered resources of halokinetic potash-bearing salt in the Dnieper-Donets Basin were assessed quantitatively for this study by using the standard USGS three-part form of mineral resource assessment (Singer, 2007a; Singer and Menzie, 2010). Delineation of the permissive tract was based on distributions of mapped halokinetic salt structures. This tract contains at least 248 diapiric salt structures with a total area of 7,840 km 2 that occupies approximately 8 percent of the basin area. The vertical extent of these salt structures is hundreds of meters to several kilometers. This assessment estimated that a total mean of 11 undiscovered deposits contain an arithmetic mean estimate of about 840 million metric tons of K 2 O in the halokinetic salt structures of the Dnieper-Donets Basin for which the probabilistic estimate was made.