A thick sequence of late Tertiary volcanic tuff covers portions of the Tushar Plateau in south-central Utah. Rhyolite of similar age fills parts of the Sevier River valley to the east. Mines along the valley produce uranium ore from deposits genetically related to the tuff and rhyolite. The threefold relation of mountain tuff, valley rhyolite, and uranium mineralization has been investigated by field mapping and laboratory study of the Mount Belknap volcanic group.

Field mapping defines changes in the volcanic stratigraphy and shows the interfingering of the regional tuff with the local valley rhyolites. Primary features of compaction, intergradation, and flow folding suggest unusual modes of air-fall aggregation and subsequent gravity flow for the emplacement of some of the volcanic units.

Thin section and clay-mineral investigations yield information about the primary structure of the volcanic units and the chemical and mineralogical reaction to alteration. Zeolites, quartz, and feldspar represent primary alteration features. Variations in kaolinite, montmorillonite, and mixed-layer clay suites reflect differences in the intensity of secondary alteration along fault zones.

Quartz monzonite intrusives and associated ore bodies lie along a major east-west zone of weakness which passes through Marysvale, Utah. Younger fault systems cut across this zone and define the north-south axis of the Tushar Range. Uranium mineralization and related alteration phenomena have formed fissure veins along these younger fault trends.

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