PRIMARY MINERALIZATION OF HEXAVALENT URANIUM IN TERTIARY “ROLL FRONT” DEPOSITS OF WYOMING

Primary uranium mineralization in Tertiary roll front deposits typically includes highly insoluble oxides and silicates of U⁴⁺ (tetravalent uranium minerals uraninite and coffinite, TUMs). Locally some deposits in Wyoming Tertiary basins include areas highly enriched in carnotite group minerals (CGMs) in which U⁶⁺ and V⁵⁺ are principal cations. These minerals have historically been interpreted to be secondary minerals formed by the weathering and oxidation of primary TUMs. Detailed analysis of petrofabrics strongly suggest that the vanadate mineralization is primary in these deposits. Silicate grains are partially or fully replaced by uranium vanadates, coated with clay, then uranium vanadates, and finally vanadates lacking uranium. Calcite is always associated with the uranium vanadates. There is little or no organic matter or pyrite present, and where pyrite does exist there is no significant occurrence of uranium minerals. The ore occurs well below the water table and has not experienced any subaerial weathering; there is no evidence of preexisting TUMs that have been subsequently oxidized. Uranium is everywhere positively correlated with vanadium, indicating uranium is mineralized exclusively as CGMs. Deposits that contain primarily CGMs are uncommon and may lack classic roll front morphology; they most closely resemble the tabular CGM deposits of the Uravan Mineral Belt. CGMs are stabilized relative to TUMs by high Eh and/or high pH. At constant Eh, increase in pH can stabilize CGMs. Low pH fluids transporting V^4+,5+ and U⁶⁺ encountered calcite-bearing sediments; increased pH moved the solutions into the stability field of CGMs, causing primary mineralization of uranium vanadates. Primary CGMs have been shown to be precipitated out of solution in Australian calcretes due to similar mechanisms. In-situ recovery (ISR) uranium mining employs oxidizing fluids to convert primary tetravalent uranium to the highly soluble hexavalent state for extraction so the presence of primary oxidized uranium minerals has important implications for ISR mining processes.