Tabular uranium-vanadium mineralization characteristic of the Colorado Plateau occurs in fluvial sandstones of the Salt Wash Member of the Morrison Formation (Jurassic) within the Henry structural basin, south-central Utah. The ore consists of a mineralized interval (MI) of two closely spaced uranium and vanadium-rich zones separated by one barren of uranium but enriched in vanadium. No known stratigraphic feature controls the position of this MI which occurs at successively higher stratigraphic levels toward the interior of the basin. The dominant clay mineral throughout the MI is an unusual vanadium-rich di, trioctahedral chlorite. Laterally continuous with and below the MI, mixed-layer chlorite/smectite and illite/smectite (greater than 75% expandable layers) predominate. Above the MI, kaolinite in sandstone beds and illite/smectite plus kaolinite in bentonitic beds are the dominant authigenic clay minerals. The MI and its unmineralized lateral extensions are bounded, both above and below, by zones rich in authigenic dolomite cement. Petrographic evidence places the dolomite as pre-ore to contemporaneous with ore, and the chlorite contemporaneous with ore. Geochemical and mineralogical data, δ18O to δD values of clay minerals and δ18O to δ13C values of dolomite indicate the presence of an interface between two isotopically and chemically distinct fluids. The lower fluid was typical of closed-basin evaporated brines with a high Mg/Ca ratio and high SO42− content. The upper fluid was meteoric water. Elemental zoning patterns and isotopic data sug-gest that the upper (meteoric) fluid carried the uranium and vanadium to the solution interface, but that ore grade mineraliza-tion occurred only where the brine-meteoric water interface in-tersected horizons with anomalous concentrations of organic matter (dominantly detrital plant debris).

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