GSA 2020 Connects Online

Paper No. 148-1
Presentation Time: 1:35 PM

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


SKIERSZKAN, Elliott Karl1, DOCKREY, John W.2, MAYER, K. Ulrich1, MCBETH, Joyce M.3, BONDICI, Ibi4 and BECKIE, Roger D.5, (1)Earth, Ocean and Atmospheric Sciences, University of British Columbia, 2207 Main Mall room 2020, Vancouver, BC V6T 1Z4, Canada, (2)Lorax Environmental Services Ltd., 2289 Burrard Street, Vancouver, SK V6J 3H9, Canada, (3)Geology, University of Regina, 3737 Wascana Parkway, Regina, SK S4S 0A2, Canada, (4)Canadian Light Source, 44 Innovation Blvd, Saskatoon, SK S7N 2V3, Canada, (5)Department of Earth, Ocean and Atmospheric Sciences, University of British Columbia, Vancouver, SK, Canada

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.