Spatial variations in fluid composition along structures hosting unconformity-related uranium deposits in the Athabasca Basin, Canada: implications for ore-controlling factors

Unconformity-related uranium (URU) deposits in the Athabasca Basin occur in local areas near the intersections of the basal unconformity of the basin and regional-scale, basement-rooted faults. This study examines the ore-controlling factors through analysis of fluid inclusions from syn-mineralization drusy quartz in mineralized and distal (> 500 m away from mineralized zones) barren areas of two major URU-hosting structures: the P2 fault in the eastern, and the Patterson Lake corridor (PLC) in the southwestern, parts of the basin. Microthermometric data indicate that fluids in mineralized zones have salinities ranging from 24.8 to 31.7 wt.% NaCl + CaCl 2 and homogenization temperatures (T h ) from 64 to 227 °C, which are comparable with those from distal barren areas with salinities ranging from 25.2 to 32.5 wt.% NaCl + CaCl 2 and T h from 91 to 213 °C. Laser ablation-inductively coupled plasma-mass spectrometry analyses of individual fluid inclusions indicate that the fluids have elevated concentrations of U in both mineralized zones (0.50 to 109 mg/kg) and distal barren areas (0.32 to 73.8 mg/kg). Bulk fluid inclusion analyses also indicate elevated U concentrations for mineralized zones (0.39 to 1560 mg/kg) and distal barren areas (0.12 to 1.5 mg/kg), and U was detected in fluid inclusions from both mineralized and distal areas with synchrotron X-ray fluorescence mapping. The development of uraniferous fluids with similar geochemistry in distant areas with different basement lithologies supports the hypothesis that U for mineralization was mainly derived from the basin. The presence of uraniferous fluids with similar thermal and compositional characteristics in both mineralized zones and distal barren areas along the same structures with similar lithologies suggests that mixing of U-rich fluids with reductants-carrying fluids played a critical role in ore precipitation; lack or low flux of either or both fluids resulted in lack or poor development of mineralization.