Reconnaissance characterization of the Willow Creek roll front uranium deposit

Current uranium production in Wyoming takes place by in situ mining of sedimentary roll front type uranium deposits. Although these ore deposits are broadly similar, they vary in age, ore mineralogy, and the reductant that causes the roll-front to form. This study focuses on the Willow Creek mine in the southern Powder River Basin near Pumpkin Buttes. It investigates two main questions: 1) whether the roll-front is presently active, and 2) if the mineralogy, water chemistry, and mechanism of ore deposition are the same at two different mine units four miles apart within the Willow Creek project. To answer these questions, this study applies petrographic analysis, whole rock and groundwater geochemistry, geochemical modeling, stable isotope analysis, and phospho-lipid fatty acid analysis (PLFA).;This study obtained permissive, but not conclusive, evidence that the roll front at Willow Creek is currently active. Geochemical modeling indicated that modern groundwater can precipitate the observed ore-stage minerals at both mine units. PLFA identified living microbial communities in groundwater but could not resolve unambiguously that these include sulfur-reducing bacteria. Similarly, carbon isotopic data neither confirm nor rule out a role for microbial participation in precipitation of ore minerals.;The arkose hosting the roll front at both mine units is almost identical, but the uranium ore mineral suite is different: one is dominated by tyuyamunite and the other by uraninite and coffinite. The distribution of uranium-bearing minerals is heterogeneous at the cm-scale; they are associated with areas free of clay minerals. Stable isotopic compositions indicate that the groundwater at the two sites was recharged at different times in different climatic conditions. The mine site with younger water has higher total dissolved solids and lower pH.;Important findings of this study include the discovery of a large area enriched in vanadium in the barren altered zone behind the roll front, and that Eh and pH strongly control the specific uranium and vanadium-bearing minerals present in the roll front.;Understanding of this Wyoming roll front uranium deposit can be further advanced by geophysical imaging of the roll front structure, TEM identification of ore-stage minerals, and metagenomic identification of bacterial species along with sulfur isotope confirmation of their presence.