The Marysvale volcanic field consists of two contrasting assemblages of rocks, an older calc-alkalic assemblage erupted between 35 and 21 m.y. ago from coalescing volcanoes, and a younger bimodal basalt-rhyolite assemblage of heterogeneous lava flows and ash-flow tuffs erupted throughout later Cenozoic time. The Mount Belknap Volcanics, 21 to 16 m.y. old, are the largest accumulation of alkali rhyolite in the bimodal assemblage; they were erupted concurrently from two source areas about 21 km apart in and just east of the northern Tushar Mountains. Products from the two source areas intertongue complexly. The Mount Belknap magma was anomalously radioactive, and vitrophyres from several different localities average about 14 ppm U. Most of the known uranium deposits and occ rrences in the Marysvale volcanic field are associated with the Mount Belknap Volcanics.

Uranium deposits associated closely with igneous centers are epitomized by the hydrothermal uranium-molybdenum-bearing veins in the Central mining area, 6 km north of Marysvale, in the eastern source area of the Mount Belknap Volcanics. The veins are localized in a small area of highly fractured ground believed to mark the surface expression above a hidden intrusive that potentially may host a porphyry-molybdenum deposit. Fluorine-rich hydrothermal fluids at 150°C and having low pH and f₀₂ permeated the broken rocks. At the deepest levels exposed, the fluids and wall rocks interacted to form kaolinitic alteration products and to deposit uraninite, coffinite, jordisite, molybdenite, umhoite, fluorite, quartz, and pyrite in open fractures. The fluids were prog essively oxidized at higher levels, and sooty pitchblende and fluorite were the predominant vein minerals deposited. In the highly oxidizing environment at the top of the system, uranium phosphate minerals were deposited by combining either primary or secondary uranium from the vein systems with phosphate derived by leaching apatite from the wall rocks. Some of these oxidized minerals may be of hypogene and some of supergene origin.

In contrast, the Mount Belknap caldera in the western source area was filled to overflowing with uranium-bearing ash-flow tuffs and lava flows. These rocks were widely altered by postcaldera steaming and hydrothermal activity. Much of the rock uranium was dissolved and incorporated into the hydrologic regime. Some of this mobilized uranium was redeposited in favorable environments within the caldera, but much seems to have been transported elsewhere. Some of the fugitive uranium may have been redeposited across redox fronts in sedimentary fills in adjacent basin-range valleys.

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