Abstract

The Beaverlodge area in Saskatchewan, Canada, hosts numerous fault-controlled uranium deposits that are geochemically and structurally complex because of multiple deformation events. Field, petrographic, and geochemical data indicate there are six distinct styles of uraninite mineralization, with five temporally distinct mineralization events. The earliest two styles of mineralization are hosted in cataclasite and veins that formed at ca. 2.29 Ga from 300°C fluids derived from retrograde metamorphic processes during late stages of the early Paleoproterozoic Arrowsmith orogen. The second style of mineralization is restricted to the albitized granite of the Gunnar deposit and formed between ca. 2.3 and 1.9 Ga from reducing magmatic-hydrothermal fluids near 300°C. However, uranium associated with this metasomatic style of mineralization is overprinted by the more significant breccia-vein and minor volcanic-type mineralization, thus the Gunnar deposit contains three styles of mineralization. Reactivation of major fault zones resulted in breccia-type mineralization, the dominant mineralizing event, at ca. 1.85 Ga, from Ca-Na-F-rich fluids at about 330°C during the postpeak Thelon-Taltson orogen and early stages of the Trans-Hudson orogen. Inversion followed by subsidence resulted in formation of the Martin Lake basin, a successor basin filled with ca. 5 km of arkose, conglomerates, siltstone, and alkaline mafic flows and sills that formed during back-arc extension following peak Trans-Hudson orogeny. Volcanic-style mineralization at 1.82 Ga. resulted from late exsolution of magmatic fluids from the alkaline mafic dikes. Overlying the Martin Lake basin are outliers of the Athabasca Formation associated with the last stage of uranium mineralization, at ca. 1.62 Ga, from oxidizing basinal brines near 230°C. Subsequent erosion of the Athabasca and Martin Lake basin rocks and weathering of the deposits resulted in secondary uranium minerals and late veins.

Multiple stages of deformation and fluid overprinting suggest that uranium mineralization record progressively higher levels in the crust. Each successive event overprinted to varying degrees the previous events. Although the Beaverlodge area represents several distinct mineralizing events, the major event is the breccia-type that occurred at 1.85 Ga. The other styles of mineralization are minor, and their presence complicates both exploration and our understanding of ore deposit genesis in the area.

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