Selenium and molybdenum enrichment in uranium roll-front deposits of Wyoming and Colorado, USA

Sandstone uranium (U) roll-front deposits of Wyoming and Colorado (USA) are important U resources, and may provide a terrestrial source for critical accessory elements, such as selenium (Se), molybdenum (Mo), and tellurium (Te). Due to their associated toxicity, Mo-Se-Te occurrences in roll-fronts should also be carefully monitored during U leaching and ore processing. While elevated Mo-Se concentrations in roll -fronts are well established, very little is known about Te occurrence in such deposits. This study aims to establish Mo-Se-Te concentrations in Wyoming and Colorado roll -fronts, and assess the significance of these deposits in an environmental and mineral exploration context. Sampled roll-front deposits, produced by oxidized groundwater transportation through a sandstone, show high Mo-Se content in specific redox zones, and low Te, relative to crustal means. High Se concentrations (up to 168 ppm) are restricted to a narrow band of alteration at the redox front. High Mo content (up to 115 ppm) is typically associated with the reduced mineralized nose and seepage zones of the roll-front, ahead of the U orebody. Elevated trace element concentrations are likely sourced from proximal granitic intrusions, tuffaceous deposits, and local pyritic mudstones. Elevated Mo-Se content in the sampled roll fronts may be regarded as a contaminant in U in-situ recovery and leaching processing, and may pose an environmental threat in ground waters and soils, so extraction should be carefully monitored. The identification of peak concentrations of Mo Se can also act as a pathfinder for the redox front of a roll-front, and help to isolate the U orebody, particularly in the absence of gamma signatures.