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Hydrothermal uranium veins are exposed over a 300 m (980 ft) vertical range in mines of the Central Mining area, near Marysvale, Utah. They cut 23 Ma quartz monzonite, 21 Ma granite, and 19 Ma rhyolite ash-flow tuff. The veins formed 18-19 Ma, in an area 1 km (0.6 mi) across, above the center of a composite magma chamber at least 12 × 6 km across that fed a sequence of 21-14 Ma hypabyssal granitic stocks, and rhyolitic lava flows, ash-flow tuffs, and volcanic domes. Intrusive pressure uplifted and fractured the roof; molybdenite-bearing, uranium-rich, glassy dikes were intruded; and a breccia pipe and uranium-bearing veins were formed.
The veins appear to have been deposited near the surface above a concealed rhyolite stock, where they filled high-angle fault zones and flat-lying to concave-downward "pull-apart" fractures. Low pH and f02 hydrothermal fluids at temperatures near 200°C (392°F) permeated the fractured rocks; these fluids were rich in fluorine and potassium, and contained uranium as uranous-fluoride complexes. Fluid-wall rock interaction increased fluid pH, causing precipitation of uranium minerals. At the deepest exposed levels, wall rocks were altered to kaolinite and sericite, and uraninite, coffinite, jordisite, fluorite, molybdenite, quartz, and pyrite (with ^dgr34S near zero per mil) were deposited. The fluids were progressively oxidized higher in the system; iron i the wall rocks was oxidized to hematite, and sooty uraninite and umohoite were deposited.
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