Saskatchewan’s potash deposits are part of a vast potash‐bearing region extending across southern Saskatchewan into Manitoba, Montana and North Dakota. The potash occurs near the top of a thick sequence of halite and anhydrite that constitute the Upper Middle Devonian Prairie Evaporite Formation in Saskatchewan. This formation typically occurs at a depth of 400 to 2,700m below ground and attains a maximum thickness of 215m. The ore called sylvinite consists of a mixture of halite, sylvite and carnallite, and variable amounts of insoluble minerals. It averages between 27 and 32% K2O.
The Prairie Evaporite
Bedded and laterally extensive evaporite beds containing deposits of halite, sylvite and carnallite form the Prairie Evaporite Formation. The thickness of the Prairie Evaporite Formation ranges between 0m at the edges of the Elk Point Basin to over 215m in the centre and is divided into three Members. From top to bottom these are:
Patience Lake Member(PLM): The uppermost member of the Prairie Evaporite Formation member with potash potential. The top 7 to 14m (25 to 45 ft) of the Patience Lake Member is typically halite with clay bands. The sylvite‐rich beds within this unit (sylvinite) are mined using conventional underground mining techniques in the Saskatoon and Lanigan areas of Saskatchewan and by solution mining techniques at Mosaic’s Belle Plaine potash mine.
Belle Plaine Member(BPM): The Belle Plaine Member underlies the Patience Lake and is separated from it by barren halite beds. Potash from the Belle Plaine is mined using solution mining techniques at Mosaic’s Belle Plaine mine.
Esterhazy Member(EM): The Esterhazy Member is separated from the Belle Plaine Member by the White Bear Marker Beds, a sequence of clay seams, low‐grade sylvinite beds, and halite. The Esterhazy Member is mined using conventional underground techniques at the PCS Esterhazy and Rocanville potash mines and by solution mining techniques at Mosaic’s Belle Plaine potash mine. The Esterhazy potash beds are typically thin and of low grade so may not be as desirable as the upper two members for solution mining. The potash beds are underlain by halite and could be partially recovered during sump emplacement when the cavern is developed.
The typical sylvinite interval within the Prairie Evaporite Formation consists of a mass of interlocked subhedral to euhedral sylvite crystals that range from pink to translucent and which may be rimmed by greenish‐grey clay or bright red iron insolubles, with minor intercrystalline halite randomly disseminated throughout the interval. Local large greater than 2.0–2.5 cm) cubic translucent to cloudy halite crystals may be present within the sylvite groundmass and overall, the sylvinite ranges from a dusky brownish red color (lower grade, 23%–27% K2O grade with an increase in amount of insolubles) to a bright, almost translucent pinkish‐orange color (high grade, +30% K2O grade). The intervening barren beds typically consist of brownish red, vitreous to translucent halite with minor sylvite and increased insolubles content.
The potash ore is called sylvinite which consists of a mixture of halite, sylvite), carnallite and an insoluble clay‐like material that largely consists of anhydrite with lesser quartz, dolomite and clay minerals. Sylvite is the principal ore mineral. Iron oxide staining is pervasive and ranges from pale flesh to blood red in colour.
A. Halite (NaCl) occurs in thick beds with fine to coarse grained crystals and range in color from clear to brown to grey. The crystal size has been observed to increase towards the tops of the beds.
B. Sylvite (KCl) is the principal ore mineral and occurs as centimetre‐sized crystals that range in colour from white to pink and light orange. Crystals are typically anhedral, although subhedral crystals have been observed.
C. Carnallite (KMgCl3∙6H2O) ranges in grain size from 0.5 cm to larger than 10 cm with colours ranging from clear white to red to local occurrences of black carnallite. Carnallite is associated with sylvinite or halite at the edges of the salt members.
D. Insoluble components (insolubles) occur as fine disseminations in the evaporite salts and intercalated with salt beds that are up to 1m thick. The colour varies from brown to grey. Insolubles have been identified as clays, anhydrite, gypsum, dolomite, quartz, hematite and micas or illites. Workers have had difficulty tracing insolubles on a regional basis, but they can be traced on local scales within a mine site or exploration property.
The potash mineralization forms stratabound to stratiform, laterally‐extensive beds with a medium‐ to coarsely‐crystalline mineralogy. The ore minerals, sylvite and carnallite, may occur together or separately. They are present within the halite as scattered crystals, thin‐layered crystal concentrations or thick potassium‐rich beds. Halite is always the dominant mineral. Mineral distribution is determined by a cyclic style of deposition with the central part of each member composed of halite and sylvite. Laterally there is a sharp change where carnallite appears. This is related to the secondary leaching and remobilization of Mg chloride brines.