INSIGHTS ON PROCESSES OF GEOGENIC URANIUM RELEASE INTO GROUNDWATER FROM WASTE-ROCK WEATHERING EXPERIMENTS UNDER NEUTRAL-PH CONDITIONS (Invited Presentation)

Groundwater uranium (U) concentrations in the Dawson Range, Yukon, northwest Canada, can reach hundreds of µg/L under baseline conditions as a result of weathering of granite, gneiss, and schist bedrock. In the present study, we combine baseline environmental monitoring data and weathering experiments conducted on waste rock from a proposed gold mine in the Dawson Range to identify hydrogeological and geochemical processes explaining naturally elevated U. Although not an economic target at this deposit, U is modestly enriched to a median abundance of 3.7 µg/g in the deposit. Weathering experiments show that dissolved Ca concentrations and sorption-site availability are the dominant controls on U mobility under well-buffered neutral-rock drainage conditions, with U sorption limited by formation of calcium-carbonato-uranyl complexes. Rocks characterized by a high degree of initial oxidative weathering therefore released less U, because of higher sorption-site availability and low Ca release. Conversely, the highest U loadings occurred in experiments conducted on fresh rock where sulfide-mineral oxidation led to dissolution of carbonate minerals and Ca release through acid-buffering reactions. Key outcomes are that U release is more strongly controlled by rock carbonate-mineral content that by rock U content, and that U release can be further promoted when carbonate-bearing rock that also contains minor amounts of sulfide minerals is exposed to oxidative weathering.