Economic geology of southern Saskatchewan potash mines

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Canadian commercial potash deposits are located in the southern Saskatchewan area of a Middle Devonian evaporite basin that extends across western Canada and into adjacent areas of northeastern Montana and western North Dakota. There are eleven mines in Saskatchewan that annually produce approximately 23 Mt of KCl from halite-sylvite ore associated with the 60 m thick upper interval of the Prairie Evaporite Formation. Of these, eight mines are conventional underground operations at 900–1100 m depths. Solution mining is used at three sites where the ore is buried to 1500 m depth or where this methodology was a necessary remedial action responding to flooding of an underground dry mine. Two new underground mines are currently under construction.The Prairie Evaporite Formation accumulated during the Middle Devonian along an evaporating equatorial inland sea that extended onto the Laurentia paleocontinent. Development of a 400 km long reef at the northern seaward end barred the basin, permitting accumulation of up to 200 m of halite-dominated evaporite beds across north-central Alberta, southern Saskatchewan and into adjacent areas of the United States. Late cycle potassium-rich brines concentrated within the southern Saskatchewan sub-basin, resulting in accumulation of potash ore zones as the uppermost interval of the 100–200 m thick halite-anhydrite deposit. Early diagenetic processes resulted in widespread concentration of sylvite-rich beds upon leaching of Mg chloride from carnallite-rich deposits, resulting in commercially attractive potash ore.Subsequent post-burial dissolution of halite-dominated beds occurred across large areas of Alberta and southern Saskatchewan, including portions of the potash mining districts. Natural dissolution collapse-subsidence structures up to 10 s km long may have permitted some mining areas to be in contact with basin brines. Brines may have also mixed with descending groundwater sourced from Cretaceous strata up-section if the carbonate seals of the overlying Dawson Bay Formation were compromised by fracturing. Mining methodologies are designed to minimize the mine-scale subsidence upon ore removal and limit potential fracturing of overlying Dawson Bay limestone beds. Brine seeps into the underground mines vary from controllable nuisances to catastrophic flooding of underground workings. Remedial actions such as grouting generally control the seeps, but uncontrollable ingress has resulted in one underground operation being converted into a solution mine.