Pre-Quaternary (Phanerozoic) potash

Surface-pan operations in Quaternary lacustrine settings account for a little over 10% of the world’s annual potash production. The other 90% comes from the extraction and mining of potash from ancient (Phanerozoic) evaporite successions. Many ancient potash deposits accumulated as the bittern portions of marine-fed saline giants, with the larger examples forming during times of MgSO4-depleted ocean chemistries and most have no modern marine counterpart (Warren, 2010). We shall now look at two examples, Prairie potash (Canada) and Kungurian potash (Russia) that together hold more than 80% of the world's known potash reserves. More details on these and many other potash deposits are discussed in Chapter 11 of Warren (2016).

The Middle Devonian (Givetian) Prairie Evaporite Formation is a potash-entraining halite sequence deposited in the Elk Point Basin, an early intracratonic phase of the Western Canada Sedimentary Basin. Today it is the world’s predominate source of potash. The flexure that formed the foreland basin and its subsealevel accommodation space was a distal downwarp to, and driven by, the early stages of the Antler Orogeny. Devonian halite constitutes a large portion of the four formations that make up the Elk Point Group: 1) the Lotsberg (Lower and Upper Lotsberg Salt), 2) the Cold Lake (Cold Lake Salt), 3) the Prairie Evaporite (Whitkow and Leofnard Salt), and 4) the Dawson Bay (Hubbard Evaporite). Today the remnants of the Middle Devonian Prairie Evaporite Formation constitute a bedded unit some 220 meters thick, which lies atop the irregular topography of the platform carbonates of the Winnipegosis Fm. Extensive solutioning of the various salts has given rise to an irregular thickness to the formation and the local absence of salt (A, B, C).

 

The Elk Point Group was deposited within what is termed the Middle Devonian “Elk Point Seaway,” a broad intracratonic sag basin extending from North Dakota and northeastern Montana at its southern extent north through southwestern Manitoba, southern and central Saskatchewan, and eastern to northern Alberta (A). Its Pacific coast was near the present Alberta-British Columbia border, and the basin was centred at approximately 10°S latitude (D). To the north and west the basin was bound by a series of tectonic ridges and arches; but, due to subsequent erosion, the true eastern extent is unknown (B). In northern Alberta, the Prairie Evaporite is correlated with the Muskeg and Presqu’ile formations. Hydrographic isolation of the intracratonic basin from its marine connection resulted in the deposition of a drawndown sequence of basinwide (platform-dominant) evaporites with what is a uniquely high volume of preserved potash salts deposited within a clayey halite host. The potash resource in this basin far exceeds that of any other known potash basin in the world.

 

Above the Prairie Evaporite Formation is a series of cyclic Devonian limestones, dolomites and evaporites that make up the Dawson Bay, Souris River, Duperow and Nisku Formations. Together the Dawson Bay Formation and overlying Souris River constitute the informally named Manitoba group. The presence of the “Manitoba Group” above any potential mine is important as it entrains two halite beds: (1) the “Hubbard Salt,” which is the uppermost bed of the Dawson Bay, and (2) the “Davidson Evaporite” composed of two halite beds separated by an anhydrite bed. Where present, the evaporites of the Manitoba Group form a flood protection zone, separating the Prairie Evaporite potash mining horizons from the water and brine aquifers present within the overlying Mesozoic sands. Its absence, especially of its halites, implies dissolution and removal processes have been active, so creating potential head and lateral stability difficulties in a conventional underground mine. Lying unconformably on the Dawson Group are the Lower Cretaceous sands of the Mannville Group, which are in turn overlain by younger Cretaceous shales and capped by Quaternary glacial sediments.

 

Devonian potash in the Western Canada sedimentary basin

 

Potash deposits mined in Saskatchewan are all found within the upper 60-70 m of the Prairie Evaporite Formation, at depths of more than 400 to 2750 meters beneath the surface of the Saskatchewan Plains. Within the Prairie Evaporite there are four main potash-bearing members, in ascending stratigraphic order they are: Esterhazy, White Bear, Belle Plaine and Patience Lake members (B). Each member is composed of various combinations of halite, sylvite, sylvinite, and carnallitite, with occurrences of sylvite versus carnallite definable with wireline signatures (once calibrated to core or mine control - D). The Patience Lake Member is the uppermost Prairie Evaporite member and is separated from the Belle Plaine by 3-12 m of barren halite. Its thickness ranges from 0-21 m and averages 12 m, its top 7-14 m is made up halite with clay bands and stratiform sylvite. This is the targeted ore unit in conventional mines in the Saskatoon and Lanigan areas and is the solution-mined target, along with the underlying Belle Plaine Member, at the Mosaic Belle Plaine potash facility (C). The Belle Plaine member is separated from the Esterhazy the White Bear Marker beds made up of some 15 m of low grade halite, clay seams and sylvinite, The Belle Plaine Member is more carnallite-prone than the Patience Lake member (Figure). It is the ore unit in the conventional mines at Rocanville and Esterhazy (B) where its thickness ranges from 0-18 m and averages around 9 m. In total, the Prairie Evaporite Formation does not contain any significant MgSO4 minerals (kieserite, polyhalite) although some members do contain abundant carnallite. This mineralogy indicates precipitation from a Devonian seawater/brine chemistry somewhat different from today’s.

 

As early as 1860, salt springs and seepages near the edge of the Elk Point Basin indicated the presence of widespread salt in Western Canada. Rock salt was first sampled in 1907, while potash beds were intersected in Saskatchewan in 1942 during the sinking of oil and gas wells. The potential of commercial grades of potash mineralisation was not recognized until 1946. Canadian potash deposits are among the richest and largest on earth, containing around 5 billion tons of ore in a mineable band up to 50 miles wide, which stretches some 725 km (450 miles) across the province (A). Canada today supplies more than 30% of the world’s annual potash production.

 

The Prairie Evaporite Fm. is nonhalokinetic throughout the basin, it is more than 200 m thick in the potash mining district in Saskatoon and 140 m thick in the Rocanville area to the southeast. The Patience Lake member is the main target for conventional mining near Saskatoon. The Esterhazy potash member rises close to the surface in the southeastern part of Saskatchewan near Rocanville and on into Manitoba. This is a region where the Patience Lake Member is thinner or completely dissolved (B). Over the area of mineable interest in the Patience Lake Member, centred on Saskatoon, the ore bed currently slopes downward only slightly in a westerly direction, but deepens more strongly to the south at a rate of 3-9 m/km. Mines near Saskatoon are at depths approaching a kilometer and so are nearing the limits of currently economic shaft mining. The main shaft for the Colonsay Mine, which took IMC Global Inc. more than 5 years to complete through a water-saturated sediment column, finally reached the target ore body at a depth of 960 meters. Such depths and a southerly dip to the ore means that the conventional shaft mines near Saskatoon define a narrow WNW-ESE band of activity (C). To the south potash is recovered from greater depths by solution mining; the Belle Plaine operation leaches potash from the Belle Plaine member at a depth of 1800m.

 

The Prairie Evaporite typically thins southwards in the basin; although local thickening occurs where carnallite, not sylvite, is the dominant potash mineral. The Patience Lake member is mined at the Cory, Allan and Lanigan mines, and the Esterhazy Member is mined in the Rocanville area (C). Ore mined from the 2.4 m thick Esterhazy Member in eastern Saskatchewan contain minimal amounts of insolubles (≈1%), but considerable quantities of carnallite (typically 1%, but up to 10%) and this reduces the average KCl grade value to an average of 25% K2O. The converse is true for ore mined from the Patience Lake potash member in western Saskatchewan near Saskatoon, where carnallite is uncommon in the Cory and Allan mines. The mined ore thickness is a 2.74-3.35 metre cut off near the top of the 3.66-4.57metre Prairie Lake potash member. Ore grade is 20-26% K2O and inversely related to thickness (D, E). The insoluble content is 4-7%, mostly clay and markedly higher than in the Rocanville mines.

 

 

A typical sylvinite ore zone in the Patience Lake member can be divided into four to six units, based on potash rock-types and clay seams (E). Units are mappable and have been correlated throughout the PCS Cory Mine with varying degrees of success, dependent on partial or complete loss of section from dissolution. Potash deposition appears to have been early and related to syndepositional reflux. So it is cyclic, expressed in the repetitive distribution of hematite and other insoluble minerals. Desiccation polygons, desiccation cracks, microkarst pits and chevron halite crystals indicate that the Patience Lake member that entrains the potash ore was deposited in and just beneath a shallow-brine, salt-pan environment (F).

 

Clay seams form thin stratigraphic layers throughout the potash ore zone(s) of the Prairie Evaporite, as well as disseminated intervals, and constitute about 6% of the ore as mined. Insoluble minerals found in the PCS Cory samples are, in approximate order of decreasing abundance: dolomite, clay [illite, chlorite (including swelling-chlorite/chlorite), and septechlorite], quartz, anhydrite, hematite, and goethite. Clay minerals make up about one-third of the total insolubles: other minor components include: potassium feldspar, hydrocarbons, and sporadic non-diagnostic palynomorphs.

 

Potash salts probably first formed as syndepositional secondary precipitates just beneath the sediment surface and were modified to varying degrees by ongoing fluid flushing in the shallow burial environment. The cyclic depositional distribution of disseminated insolubles was possibly due to a combination of source proximity and the strength of the winds blowing detritals out over the brine seaway. Possible intrapotash disconformities, created by dissolution of overlying potash-bearing beds, are indicated by an abundance of residual hematite in clay seams. Except in, and near, dissolution and collapse features, the secondary redistribution of insolubles, other than iron oxides, is insignificant.

 

Bedded halite away from the ore zones generally retains primary depositional textures typical of halite precipitation in shallow ephemeral saline pans. Crystalline growth fabrics, mainly remnants of vertically-elongate halite chevrons, are found in 50-90% of the halite from many intervals in the Prairie Evaporite. Many of the chevrons are truncated by irregular patches of clear halite that formed as early diagenetic cements in syndepositional karst.

 

In contrast, the halite hosting the potash ore layers lacks well-defined primary textures. Regional petrology and lower than expected Br levels in halites in the Prairie Evaporite Formation indicate a series of recrystallization events formed and reformed sylvite after carnallite and were the result of periodic flushing by hypersaline solutions. This origin as a multi-stage secondary precipitate is supported by observations of intergrowth and overgrowth textures), large-scale collapse and dissolution features, radiometric ages and palaeomagnetic orientations of the diagenetic hematite linings associated with the emplacement of the potash minerals.

Permian (Kungurian) Upper Kama potash in the 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 (H, I). 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 13Warren, 2016).

 

Historically, the deposit’s major extraction areas have been centred about the towns of Solikamsk. Bereznki and Romanovo (H). Annual capacity in the region is around 850,000 tonne/year and total mineable reserves are thought to be in excess of 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 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 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 (G). Regionally, the lower rocksalt is 250-400 m thick and is overlain by the potash-hosting sequence with a thickness of 30-125 m (I). 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 (I). 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.

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 4 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 terrigeneous-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 (I). Even so, the remaining mineable reserves of K2O in the ore district are still estimated to be in excess of 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 overlaying 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 impermeable strata is 70– 90 m. Insolubles are present in the ore succession, mostly as thin centimeter-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.

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 (K). 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 Chapter 13; case histories). 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 recrystallized 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.

 

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 a major tectonic stress and stress from the faults and numerous uplift structures leads to deformation reorientation or overlapping the deformational structures with different trends (I). 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 visually observed only in the non-annealed clay–anhydrite layers. Open fractures are encountered mostly in the sylvinite–carnallite zone. They usually develop with meridianal 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.

 

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