The geology of potash deposits at PCS Cory mine, Saskatchewan
Salt anomalies are a common problem to potash mining in Saskatchewan. The geology of the PCS Cory potash mine in the Patience Lake Member of the Prairie Evaporite Formation was studied to attempt to find indicators of proximity to anomalies. Initially, a basic understanding of normal potash ore was needed to provide a foundation of data for the study. The ore zone was divided into six units, based on potash rocktypes and clay seams. The units may be correlated throughout the mine with varying degrees of success and are mappable. The potash deposition appears to have been cyclic, expressed in the repetitive distribution of hematite and other insoluble minerals. Four potash cycles, each about 1 m thick, make up the lower Patience Lake Member. In the upper Patience Lake Member there are 3 complete potash cycles, and 3 or 4 incomplete potash cycles. The cyclic distribution of disseminated insolubles was possibly due to a combination of source proximity and the strength of winds. The potash salts and associated iron oxides probably formed just beneath the sediment surface. Possible disconformities, created by the dissolution of overlying potash-bearing beds, may be indicated by an abundance of residual hematite in clay seams. Desiccation polygons, desiccation cracks, microkarst pits, and chevron halite crystals indicate that the Patience Lake Member was deposited in a shallow-brine, salt-pan environment. The insoluble minerals found in the PCS Cory samples are, in approximate order of decreasing abundance: dolomite, clays [illite, chlorite (including swelling-chlorite/chlorite), and septechlorite), quartz, anhydrite, hematite, and goethite. Detrital minerals include dolomite, illite (or mica precursors), chlorite, quartz, and fossil fragments. The septechlorite and swelling-chlorite/chlorite are probably authigenic. X-ray peaks and heating data indicate that the chlorites are rich in magnesium. Except in collapse features, the secondary redistribution of insolubles, other than iron oxides, is insignificant. Up to five post-burial facies, including carbonate-mud collapse breccias and bedded sylvinite, may result from postburial leaching. The effects of the leaching that produced the anomalies, range from weak to strong, from selectively preserving delicate laminae and chevron textures, to deforming and destroying salt beds. Good preservation of iron oxides in halite may indicate that the leaching was weak or of short duration. In leach anomalies, salinities increase downward possibly because fluids exited downward. NaC1-saturated fluids tend to follow the chemical gradient provided by potash beds. Common indicators may exist equally for two different types of anomalies. Large blebs (eg. >200 cm2) of sylvite-poor potash cross-cutting units of an incomplete potash cycle near the top of the ore zone, were found about 5 m from a salt anomaly. Anhydrite was more common in an anomaly than in ore. Once into an anomaly, indicators of a major collapse feature may include: stretched clay seams, folded beds, small collapse features (1-20 m), split clay seams with injected salt, and drops in topography >10 m of the marker seams. Any number, or none, of these indicators may be found close to a major collapse feature. The local abundance of anhydrite in ore may indicate the proximity of a salt anomaly.