A high resolution gravity survey near McArthur River uranium deposit, Athabasca Basin, Canada

A high resolution gravity traverse (27.2 km-long, 50 m or 100 m station-spacing) across the unconformity-associated McArthur River uranium deposit, Athabasca Basin, Canada was conducted to investigate basement geology and locate alteration zones related to mineralization. In this geological environment, amplitudes of gravity anomalies relevant to uranium exploration (related to offsets of the basement unconformity, basement relief and alteration zones) are typically <1 mGal, but range up to 2 mGal. Alteration near the McArthur River deposit is commonly present as silicification of the basin sandstones, located above basement quartzite units. Many short to intermediate wavelength (250-1800 m) anomalies, amplitudes <0.5 mGal, are identified along the traverse profile, superposed on longer wavelength and larger amplitude (up to 6000 m, and 1.8 mGal, respectively) anomalies. Preliminary modeling of anomalies, constrained by magnetic data, drill-hole logs and density information, and reflection seismic data, indicates a variety of geological contributions. Short wavelength anomalies are attributed to changes in thickness of the glacial overburden, thickening in some places being linked to preferential erosion along an underlying fault. Long wavelength anomalies are explained by variations in basement density and offsets of the basement unconformity. Importantly, from an exploration perspective, most intermediate wavelength positive anomalies are explained by a combination of silicification within the Athabasca Sandstone, and associated basement relief produced by underlying basement quartzite units. Desilicification is also modeled, but limited. The gravity method, apparently, has been successful in identifying alteration zones that may be of significant interest for uranium exploration, because of their association with mineralizing fluids, and because patterns of silicification/desilicification are important for studies of fluid flow.