Abstract

The Karpinka Lake prospect is a strata-bound uranium deposit hosted by Aphebian meta-sediments located just to the south of the important unconformity-type deposits at Key Lake in northern Saskatchewan. The uranium occurs as disseminated uraninite and brannerite in sillimanite-bearing quartz-feldspar-biotite gneisses which, on the basis of major element data, are interpreted to represent metamorphosed arkoses. Sulfides, predominantly pyrrhotite, occur in amounts up to 10 volume percent in the uraniferous gneisses. These zones are flanked by sulfide-poor gneisses containing over 10 percent titanomagnetite and ilmenite. The uraninite is commonly in contact with sillimanite and the brannerite is commonly rimmed by sulfides. Both minerals have similar U/Th ratios of approximately 35:1.Bulk chemical analyses suggest that the host rocks were leached of sodium prior to metamorphism. The mineralized rocks show higher than background concentrations of copper, molybdenum, and vanadium.U-Pb isotopic analyses of the uraniferous phases indicate that the deposit is at least 1,800 m.y. in age. This age is approximately the same as that of the Hudsonian orogeny and is interpreted to reflect metamorphic resetting of the isotopic clock. The 1,800-m.y. age for the Karpinka Lake mineralization is at least 500 m.y. greater than the oldest dates obtained for the nearby unconformity-type deposits.It is proposed that the Karpinka Lake deposit represents a metamorphosed epigenetic sand-stone-type uranium deposit. A model is presented which involves sulfidation of iron-titanium oxide-rich arkoses and subsequent alteration of these rocks by uranium-bearing solutions. Uraninite is inferred, from its association with sillimanite, to have formed by uranium adsorption on precursor kaolinitc produced by alteration of detrital feldspar or felsic volcanic fragments. Brannerite is considered to have formed by uranium adsorption on titania sponges created by the alteration of iron-titanium oxides.The demonstrable existence of Aphebian strata-bound uranium mineralization and the observation that the Saskatchewan unconformity-type deposits are controlled by faults parallel to the Aphebian gneissosity lend support to previous suggestions that the basement supplied the uranium for the younger unconformity-type deposits.

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