Alteration and mineral paragenesis of the McArthur River and Fox Lake uranium deposits, Athabasca Basin: a new model for the formation of unconformity-related uranium deposits

Abstract

The McArthur River uranium mine in the Athabasca Basin, Canada, hosts the largest high-grade uranium deposit in the world, accounting for 12% of global uranium production in 2015. McArthur River is a Proterozoic unconformity-related uranium deposit, with ore bodies located in the P2 fault zone at the unconformity between conglomerate and metamorphic basement rock, and up to 120 m below the unconformity in basement rock. Oxidized basinal fluids are considered the most likely medium for transporting dissolved UO22+ to the unconformity. The reductant responsible for precipitating UO2 is unknown. Core logging and sample collection were followed by bulk compositional analysis. Optical microscopy, scanning electron microscopy, and electron microprobe analysis were used to establish the mineral paragenesis. Here, a new model for the reduction of uranium at McArthur River is proposed. The observations indicate that the P2 reverse fault is characterized by the abundant presence of fault-hosted pre-ore pyrite veins. The formation of these veins resulted in the illitization of feldspar in the basement rocks. Oxidizing, high-salinity Na-Mg-Ca chloride basinal fluids penetrated into the basement along the P2 fault and oxidized the pre-ore pyrite veins. Pyrite veins were replaced by Fe3+-bearing sudoite. Fe-sudoite altered pyrite shows extreme sulfur isotope fractionation consistent with oxidation of pyrite by thiosulfate disproportionation. It is hypothesized that the reaction of basinal fluids with pre-ore pyrite was responsible for the reduction of UO22+ at McArthur River. A comparative study of the nearby Fox Lake unconformity-related uranium deposit was performed to evaluate the general applicability of the genetic model developed for McArthur River. The Fox Lake uranium deposit is a complex-type unconformity-related uranium deposit 10 km west of McArthur River. The mineralization is located along the C-10 fault zone at the unconformity between the basal conglomerate of the Manitou Falls Formation and metamorphic basement rock. The Fox Lake investigations included extensive drill core logging and systematic sampling. Bulk compositional analyses were performed to complement core logging. A paragenesis was established using optical and scanning electron microscopy, and electron microprobe analysis. Petrographic and sulfur isotope investigations show that the large amount of pre-ore pyrite occurring in sandstone above the C-10 fault was oxidized by thiosulfate disproportionation and replaced by Fe-sudoite and a second generation of pyrite. The evidence suggests that the oxidation of pre-ore sulfide minerals by basinal fluids resulted in the precipitation of uraninite. These results are consistent with the genetic model proposed for McArthur River.