Permian potash (Kungurian), Upper Kama, CIS-Urals

The largest potash occurrence currently mined in Russia is in the Upper Kama Basin in the Perm region of Russia, some 250 km north of the town of Perm, adjacent to the Kama River and the Ural Mountains. This deposit is second in size only to the Saskatchewan deposits in Canada (although perhaps also smaller than the under-explored Nepskoye potash district in Siberia to the west). In Russia, the region is often called the Ural or Verkhnekamskoye potash ore district. The potash is part of the Iren Fm (Kungurian- Permian) and covers about 3,000 km2 near the shallow edge of a somewhat larger megasulphate basin that is the upper part of Cis-Ural Trough (B, C). In the potash ore district, there are 13 potentially mineable beds at depths of 200-500m. Ground collapse and water influx are ongoing environmental problems in areas of the Kama Basin where the exploited intervals are within a few hundred meters of the landsurface; as seen in the Solikamsk 2 mine collapse in 1995 and the collapse in the 3rd Berenzki mine in 1986 (Chapter 13, Warren, 2016).

Historically, the deposit’s major extraction areas have been centred about the towns of Solikamsk. Bereznki and Romanovo). Annual capacity in the region is around 850,000 tonnes/year, and total mineable reserves are thought to be over 3.8 billion tonnes. The two largest potash producers in the area, both semi-privatised state enterprises, merged in 2011 to create a US$ 15 billion enterprise. The Joint Stock Company (JSC) “Uralkali”, headquartered in the city of Bereznki, operates in the southern part of the potash deposit, and the Joint Stock Company “Silvinit”, headquartered in Solikamsk, operates in the northern area.

The Upper Kama potash deposit lies in the central part of the Solikamsk (Solikamskaya) depression of the Pre-Ural foreland basin. This horizontally bedded evaporitic formation, up to 500 m thick, is composed predominantly of halite with substantial interbedded sylvinite, carnallite, clay, and anhydrite layers (A). Regionally, the lower rocksalt is 250-400 m thick and is overlain by the potash-hosting sequence with a thickness of 30-125 m (B, C). The Iren Fm. evaporites were deposited after the rise of the Ural Mountains in the late Carboniferous as a result of the continental collision between the East-European platform and West-Siberian plate. The evaporite formation is underlain by 2–3 km of clastic Proterozoic rocks, a Devonian reef sequence, a dominantly carbonate Carboniferous sequence, and Lower Permian sediments of up to 1 km in thickness. The Lower Permian molasse (flysch) wedge, consisting of conglomerate, sandstone and argillite beds, underlies the eastern part of the potash deposit (C). The uppermost 80–120 m of evaporite formation contains economic deposits of potash salt, which include 13 sylvinite and carnallite mineable beds and interbed halite layers. 

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The potash zone overlies a thick halite deposit, and most recovered ores are medium grade (16-23% K2O). Four sylvite-rich beds (referred to as A, Red I, Red II and Red III) in the lowest part of productive strata are the principal mineable beds with typical ore grades in the medium quality range of 16-23% K2O (J). Nine carnallite beds (namely B, V, G, D, E, Zh, Z, I, K) along with halite interlayers lie on top of the four beds of the sylvinite layer and form a carnallite zone that ranges in thickness from 38 to 80 m. A halite layer some 20 m thick, termed the overlying rock salt, covers the potash beds, unless breached or dissolved. The Middle Permian terrigenous-carbonate complex with multiple aquifer layers overlies evaporites. This aquifer system typically overlies and locally penetrates the evaporite strata with water pressures of up to 3.5 MPa, while the ore deposit depth varies from 75-450 m.

Six of the 13 potash beds are intermittent over 15-35% of this area, and the principal bed that is mined, Red (KrII), has now been depleted across significant portions of the defined ore district. The ore zone dips to the west and is lost via complete dissolution to the east along a halite basin edge that has captured the Glukhaya Vil’va River (C). Even so, the remaining mineable reserves of K2O in the ore district are still estimated to be more than 2 billion tonnes.

Regionally within the Upper Kama basin there is a general tendency for the potash zone to contain 5-13% more KCl to the south than in the north, and 1-1.5% more KCl in the east than in the west at Solikamsk, and 5-6% more in the east than in the west in the southern district (Garrett, 1995). For example, the four beds of the Red II layer near Solikamsk average 5.2 m with 23% KCl, and the A layer averages 1.4 m with 29% KCl, while near Bereznki the comparable figures are 4.8 m and 32% KCl, and 1.5 m and 43% KCl respectively.

The upper part of the potash deposit along with the overlying marl, halite, clay, anhydrite, gypsum and carbonate beds of the transitional series is left un-mined to form a water-protective barrier above the mine. The average thickness of the impermeable strata is 70– 90 m. Insolubles are present in the ore succession, mostly as thin centimetre-scale parting seams composed of clay and anhydrite. The 1–2 m thick carbonate clay layer (referred to as “Marker Clay”) located in the underlying rock salt, some 20 m below sylvinite zone, is the most stable stratigraphic marker at the Upper Kama deposit.

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Most of the historically mined beds have very gentle dips of from 0-4°, but folds do occur, influencing continuity of strata, as do occasional cavities and numerous barren zones (E). For example, folds in bed V have been found between 2-9 m in height, with a width of 8-30 m and length of 50-150 m.

The salt roof to the ore zone, averaging about 25 m, is a sometimes entirely sub-eroded. In its place is an alternating sequence of originally existing rocksalt, now mostly sub-eroded, and claystone, which changes substantially in thickness (0-80 m). This then passes up to the upper Kungur member, a lime-sand and clay-sand stratum, which is about 25 m thick. Within the mined potash beds, small quantities of trapped brines occur frequently, and there are locally significant quantities of methane and hydrogen. There are also brine-filled poorly documented disturbances in the uppermost portions of the saline formations, the watery residues from former poorly-controlled long-term solution mining of rock salt in the Soviet era (see Salty Matters). The patchy distribution of highest-grade ore and abrupt lateral transitions, are reflective of the shallow nature of the ore zone across the district, so that controlling water entry and preventing or minimising salt-karst related collapse are ongoing problems.

There are major zones of ore impoverishment in the Kama deposit, and the two most common styles are illustrated in K. Likely they formed in; 1) Regions of the basin that were structurally higher during or soon after the potash deposition or perhaps located in zones that were more severely leached when the basin was reflooded, or 2), Areas where there has been later fault or karst-related fluid movement, allowing intruding formation waters to enter the deposit and leach the potash, replacing it with halite. In these barren zones the bedding planes are often missing, and the structure is that of recrystallised salt similar to the salt horses of New Mexico and Carlsbad. Sylvinite is thought to be richer in the areas of the deposit where there was no syndepositional subsidence (or there was even a slight upwarping), and less rich where settling (or downwarping) occurred during deposition. ˆ

The Upper Kama deposit has been subjected to major compressive regional tectonic, gravitational and mining-induced forces. Extensive folding of variable amplitudes and wavelengths is characteristic of evaporite formation of the Upper Kama deposit. Predominantly, tectonic movements in the adjacent orogens governed the folding deformation of the evaporite, so that the main folds in the Upper Kama potash deposit have submeridianal orientations, perpendicular to principal lateral E–W tectonic stress caused by the Uralian Orogen. Locally, a superposition of significant tectonic stress and stress from the faults and numerous uplift structures leads to deformation reorientation or overlapping the deformational structures with different trends. Because of their inherently high plasticity, brittle deformation is not common in the potash zones, but occurs locally when tectonic, gravitational or mine induced stress exceeds the strength of the rock mass.

A number of discrete natural fractures and fracture systems of various scales have been observed, especially in the central part of the Upper Kama deposit. Most of these fractures are effectively healed and can be found only in the non-annealed clay–anhydrite layers. Open fractures are encountered mostly in the sylvinite–carnallite zone. They usually develop with meridional and NW–SE direction. The most intense deformations are seen in the carnallite zone, where very steep folding, tectonic brecciation and blocks of interbed rock salt are present.ˆ

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