Devonian potash in the Western Canada sedimentary basin

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 predominant 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 crucial 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 the 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.

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Potash in the Elk Point Basin, western Canada. A) Regional geology showing thickest salt and potash intervals in the Prairie Evaporite and its dissolution edge. B) Cross section of basin showing distribution of potash members. C) Map of potash mines in the basin; ** indicates solution mine with brine field, * solution mine utilising flooded mine shafts(compiled and modified from Worsley and Fuzesy, 1979; Fuzesy, 1982; Boys, 1993; SaltWork® Database version 1.8)

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 recognised 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 kilometre 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 five 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.

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Typical ore grades, thicknesses and wireline characteristics of the potash members in the Upper Prairie Evaporite (after Fuzesy, 1982)

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.

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Mineralogy and textures in the ore section in the Patience Lake member of the Prairie Evaporite Formation in the PCS Cory potash mine. A) Typical mineralogy in cycles (units). B) Contacts in two ore cycles showing early karst outlines (redrawn and modified based on Boys 1990).

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 recrystallisation 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.

For more information on ore quality controls in the Devonian potash deposits of Canada download a relevant Salt Matters article (click here)

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BHP Jansen Mine currently under construction