Uranium-beryllium-fluorine mineralization at Spor Mountain in western Utah accompanied basin-range faulting and alkali rhyolite volcanism after major episodes of caldera-related volcanism had ended. Volcanism began about 42 m.y. ago with eruption of intermediate-composition flows, breccias, and tuffs from small central volcanoes, and culminated with eruption of intermediate-composition ash flows and subsidence of the Thomas caldera about 39 m.y. ago. Intermediate-composition volcanism was accompanied by base- and precious-metal mineralization. Eruption of rhyolitic ash flows 38 to 32 m.y. ago largely filled the Thomas caldera; some of these eruptions caused subsidence of the Dugway Valley cauldron. Alkali-rhyolite volcanism, basin-range faulting, and uranium-beryllium-berylliumfluorine mineralization began at Spor Mountain about 21 m.y. ago, at least U m.y. after the last cauldron subsidence. Most faulting and mineralization had ended by 6 to 7 m.y. ago, when voluminous alkali rhyolite was erupted in the Thomas Range.

Extensional tectonism was the probable cause of both alkali-rhyolite volcanism and uranium-beryllium-fluorine mineralization at Spor Mountain. Vents developed along basin-range faults and fault intersections at 21 m.y. and 6 to 7 m.y. ago, and mineralizing fluids rose through a plumbing system of vents and faults after eruption of tuff and alkali rhyolite 21 m.y. ago. Mineralizing fluids invaded faults in Paleozoic rocks and deposited uraniferous fluorite; they pervaded dolomite clast-rich tuff, which is interleaved between relatively impermeable strata, and deposited uranium in the structures of fluorite and opal and beryllium in bertrandite. Precipitation of uranium and beryllium occurred in response to breakdown to beryllium fluoride, uranium fluoride, and uranium-silica complexes as fluorite and silica were precipitated from cooling fluids. Uranium of magmatic origin in glassy tuff and that added by hydrothermal fluids was remobilized by ground water to form secondary concentrations in tuff and tuffaceous sandstone; such concentrations comprise minable deposits at the Yellow Chief mine.

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