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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 65 (1981)

Issue: 4. (April)

First Page: 769

Last Page: 769

Title: Uranium in Challis Volcanic Field, Idaho: ABSTRACT

Author(s): Peter L. Siems

Article Type: Meeting abstract


Recorded uranium production and known resources in the Challis Volcanics and the Challis-related epizonal silicic plutons of Eocene age are small, but the potential resource is moderately high. Former production was from arkosic conglomerates and sandstones at the base of the Challis Volcanics in Stanley basin and from a scheelite deposit on the contact of the Summit Creek stock. Uranium mineralization also occurs in water-laid rhyodacite pumice-rich tuffs, in the Twin Springs pluton, in the Beaverhead stock, and in shear zones in the Castro granite.

The Challis Volcanics are potash-rich, calc-alkaline rocks and, in general, are not known to be enriched in uranium. However, numerous highly differentiated silicic intrusions that acted, in part, as feeders to the closing phases of Challis volcanism are rich in uranium and thorium. These intrusions range from rhyolite, rhyodacite to granite, and from plugs, domes, dikes, stocks, to batholiths.

Exploration in the Challis field should be concentrated in areas of silicic tuffs and breccias or in arkosic beds that occupy depressions in pre-Challis basement or volcanic-tectonic subsidences. Few such structures are known because detailed mapping of the field is incomplete; one exception is a trap-door graben that contains the uranium-rich arkoses of Stanley basin. Mineralized indicators of uranium are hematite staining, disseminated pyrite, and opaline silica alteration. Radioactive anomalies in present-day valleys can result from the fixation of uranium in organic-rich bogs or from accumulations of coarse detritus from granitic plutons.

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