| Summary: | Uranium (U) contamination in groundwater from geogenic sources affects water quality globally. Here, we use a multifaceted isotopic and geochemical approach to elucidate U sources and controls on geogenic U release to groundwater and surface water at a prospective subarctic gold deposit in Yukon, Canada, that is characterized by permafrost, fractured bedrock, and cold (<2 °C) groundwater. X-ray absorption spectroscopy, sequential extractions, and micro X-ray fluorescence mapping show extensive subsurface oxidation and solid-phase U present in its hexavalent and mobile form. Limited 238 U/ 235 U isotope fractionation and predominance of U(VI) in rocks suggest U(VI) sorption–desorption is the main driver of U mobilization. Groundwater U concentrations are appreciable (median 38 μg/L, range 1.2–535 μg/L) and are explained by high-alkalinity, Ca-rich groundwater produced from oxidative weathering of sulfide and carbonate-mineralized structures around the deposit. Minor 238 U/ 235 U isotope fractionation in groundwater indicates that limited U(VI) reduction occurs beneath permafrost despite groundwater redox conditions below Fe(III) and S(VI) reduction, and groundwater ages inferred from 3 H and 14 C to be on the order of thousands of years. The complexation of U as uranyl–calcium–carbonate complexes and the resilience of these complexes to U(VI) reduction contributes to high U(VI) mobility under cold groundwater conditions. This study provides insight into processes and time scales of U transport in subarctic groundwater at a pivotal time when hydrogeochemical changes may be anticipated in cold regions worldwide due to permafrost degradation.
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