Radiometric ages of igneous rocks and hydrothermal uranium-fluorine veins in the eastern source area of the Mount Belknap Volcanics near Marysvale, Utah, show that igneous activity occurred over a time interval of about 9 million years and that hydrothermal uranium deposits were formed about midway through the interval, coincident with the last igneous activity in the Central mining area. Intrusive igneous activity in the Central mining area began about 28 m.y. ago with emplacement of the quartz monzonitic Central Intrusive of the Bullion Canyon Volcanics and was followed by a hypabyssal granitic stock, eruption of the Red Hills Tuff Member, and emplacement of dikes, all consisting of the Mount Belknap Volcanics, finally ending about 18 m.y. ago. Hydrothermal uranium mineralization occurred with the last event in this sequence and is genetically associated with emplacement of glassy rhyolite dikes between 18 and 19 m.y. ago. A 207 Pb/ 204 Pb- 235 U/ 204 Pb isochron age of 19.0 + or - 3.7 m.y. was obtained for whole-rock uranium-fluorine vein samples. Application of a technique using hydrothermal quartz in the veins as a natural external detector for fission tracks from adjacent pitchblende gave an apparent age of 16.5 + or - 4.3 m.y., consistent with the other geochronologic data.

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