Geology of the McClean uranium deposits, northern Saskatchewan

The McClean uranium deposits are in northern Saskatchewan, 11 km west-northwest of the Rabbit Lake mine. Their average grade is 1.8 percent U3O8 and they contain 6,350 tonnes of uranium oxide in seven pods of variable grade and mineral assemblage. They are hosted by both basal Athabasca Group and the regolith developed on the underlying crystalline basement. The major mineral facies are: sulphide, arsenide, 'bleached', and hematite, with the important uranium minerals being coffinite, uraninite and pitchblende. Abundances of carbon and sulphur isotopes are consistent with their derivation from pyritic graphite in basement gneisses. Isotopes of oxygen and hydrogen suggest mineralizing fluids were aqueous and at temperatures of about 200 deg.C. Ar39/Ar40 laser age determination of illite indicates mineralization began about 1300 m.y. ago and that significant hydro-thermal alteration ceased about 1170 m.y. ago though Pb/U data suggest the deposits continued to evolve into the Mesozoic. The deposits are interpreted as the result of oxidizing ground water stripping uranium and other metals from either the Athabasca sandstone, especially the heavy mineral layers, or from the regolith, and precipitating these when encountering a warm reduced fluid emanating from the basement