Summary:
Much of southern Saskatchewan is underlain by the Prairie Evaporite Formation, a layered sequence of salts and anhydrite which contains one of the world’s largest deposits of potash. The potash extracted from the predominantly sylvinite ore has its main use as a fertilizer.
The 100 m to 200 m thick Prairie Evaporite Formation is overlain by between 400 and 500 m of Devonian carbonates, followed by approximately 100 m of Cretaceous sandstone, and between 400 and 500 m of Cretaceous shales and more recent Pleistocene glacial tills to surface; it is underlain by Devonian carbonates (Fuzesy, 1982). The Phanerozoic stratigraphy of Saskatchewan is remarkable in that units are flat-lying and relatively undisturbed over very large areas.
Potash mineralization in this region of Saskatchewan is predominantly sylvinite, which is comprised mainly of the minerals sylvite (KCl) and halite or rock salt (NaCl), with trace carnallite (KMgCl3 · 6H2O) and minor water insolubles. Potash fertilizer is concentrated, nearly pure KCl (i.e. greater than 95% pure KCl), but ore grade is traditionally reported on a % K2O equivalent basis. The “% K2O equivalent” gives a standard measurement of the nutrient value of different potassium-bearing rocks and minerals. To convert from % K2O equivalent tonnes to actual KCl tonnes, multiply by 1.58.
There are three mineable potash members within the Prairie Evaporite Formation of Saskatchewan. Stratigraphically highest to lowest, these members are: Patience Lake, Belle Plaine, and Esterhazy.
The Allan potash deposit lies within the Patience Lake Member of Prairie Evaporite Formation. There are two potash seams named A Zone and B Zone within this Member; at present, only the A Zone is being mined at Allan. Some test mining has been carried out in the B Zone, but no mining is done in this layer at present. Neither the Esterhazy nor the White Bear Potash Members are present in the Allan area.
Potash mineralization at Allan, Cory Lanigan, and Vanscoy occurs at about 1,000 meters depth below surface. The A Zone is approximately 3.35 meters thick and occurs near the top of the Prairie Evaporite Formation salts. Salt cover from the ore zone to overlying units is approximately 12 meters in the Allan, Cory, and Vanscoy areas.
Salt cover from the top of the A Zone mining horizon at Lanigan is approximately 7 meters thick, while the salt cover from the top of the B Zone mining horizon to overlying units is approximately 14 meters thick. The Belle Plaine Member is present in varying degrees at Allan, Cory, Lanigan, and Vanscoy. The Esterhazy Member is present in varying degrees at Allan and Lanigan.
Potash mined at Rocanville lies within the Esterhazy Member of the Prairie Evaporite Formation. The potash zone at Rocanville is approximately 2.4 meters thick and occurs near the top of the Prairie Evaporite Formation. Salt cover from the ore zone to overlying units is approximately 30 meters. The Belle Plaine Member is present but not well-developed in the Rocanville area.