Abstract

The epigenetic Tony M vanadium-uranium orebody in south-central Utah is hosted in fluvial sandstones of the Morrison Formation (Upper Jurassic). Although the deposit is mined for uranium, vanadium has a higher average abundance in the ore. Thus, the geochemistry of vanadium in the orebody was studied to characterize ore-forming processes within the inferred ground-water flow regime. Measurements of the relative amounts of V (super +3) and V (super +4) in ore minerals show that V (super +3) is more abundant. Thermodynamic calculations show that vanadium was more likely transported to the site of mineralization as V (super +4) . The ore formed as V (super +4) was reduced by hydrogen sulfide, followed by hydrolysis and precipitation of V (super +3) in oxide minerals (e.g., montroseitc or paramontroseite) or chlorite. Uranium was transported as uranyl ion (U (super +6) ), or some complex thereof, and reduced by hydrogen sulfide, forming coffinite. Detrital organic matter in the rocks served as the carbon source for sulfate-reducing bacteria. It was this bacteriogenic H 2 S that reduced the metals in the mineralization process.Possible sources for the V and U in this deposit have been identified previously. Vanadium most likely was derived from the dissolution of iron-titanium oxides, which liberated Fe as well as V. A zone of titanium-rich remnants is observed updip and up the hydrologic gradient from the deposit (M. Goldhaber and R. L. Reynolds, unpub. data). Uranium probably was derived from the overlying Brushy Basin Member of the Morrison Formation (Northrop, 1982). A preliminary age date for the deposit of 115 Ma (K. Ludwig, 1986, pers. commun.) indicates that the ore formed after deposition of the Brushy Basin Member.Previous studies have shown that the ore formed at the density-stratified interface between a basinal brine and dilute meteoric water. The mineralization processes described above occurred within the mixing zone between these two fluids. Stable isotope analyses of ore-stage dolomite show a progressively heavier carbon and oxygen isotope signature with increasing depth through an ore horizon, consistent with the two-solution interface model.

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