THE LOST CREEK ROLL-FRONT OF WYOMING: ACTIVE OR FOSSIL DEPOSIT?

Roll-front deposits are important sources of U and whether these deposits are active or fossil is a topic of debate. To investigate this question, we analyzed pre-mining core as well as groundwater samples collected within the last 15 years from the Lost Creek deposit, hosted in the 55-47 Ma Battle Springs Formation in the northern Great Divide Basin, Wyoming. Pyrite and U- bearing minerals are absent and aqueous SO₄^2- and U concentrations (66-121 ppm and 4-194 ppb, respectively) are lower in the oxidized barren altered rock compared to the downgradient ore zone. Calcite is present in small amounts, <1 wt%, in the altered rock. At the interface between the ore zone and oxidizing groundwater, pyrite and U-bearing minerals are being oxidized and dissolved. The concentration of aqueous SO₄^2- and U at the interface and throughout the ore zone increases to 120-269 ppm and 45-844 ppb, respectively. We infer that in this semi-oxic environment, pyrite dissolution produces several metastable sulfur species which disproportionate to H₂S and SO₄^2-. Pyrite appears at the redox interface and increases substantially in abundance in the downgradient reduced zone, up to 0.20 wt%. Calcite shows the same pattern. The abundance of U minerals increases, but to a lesser extent. Lower concentrations of aqueous SO₄^2- and U in the downgradient reduced zone (71-171 ppm and 4-342 ppb, respectively) suggest the precipitation of pyrite and U minerals in the upgradient ore zone. Thus, Lost Creek was geologically active within the last 15 years prior to mining and was dissolving and remobilizing at the interface between oxidizing fluids and the ore body.