Uranium Potential and Geology of the Challis Volcanics of the Basin Creek-Yankee Fork Area, Custer County, Idaho. Final Report

摘要

Uranium deposits in the area are confined to veins in porphyritic and equigranular phases of quartz monzonite of the Idaho batholith and to stratabound lenses and rolls in a carbonaceous arkosic unit, less than 200 feet thick, that unconformably overlies the batholith and is the lowest stratigraphic unit of the Challis Volcanics. Uranium deposits do not occur in other Challis volcanic units. Geological mapping demonstrates that the area is at the edge of a major eruptive center that built a 100-cubic-mile volcanic edifice. Early pyroclastic activity in the Eocene deposited up to 1,000 feet of water-laid tuff and volcanic sandstones that overlie, in part, the uraniferous arkoses. Glass shards in the tuffs and sandstones are devitrified and carbonaceous material is locally abundant but uranium content of these beds is low. Voluminous lava flows of andesite and dacite were discharged after the early eruptions and cover the area to a maximum thickness of 5,000 feet. They are capped by a 1,000-foot sequence of air-fall and ash-flow tuffs and lapilli tuff from rhyolite to quartz latite in composition. The tuffs were erupted from numerous small vents that are aligned along the northwest, north and northeast trends of faults. These vents were later plugged by late-stage domes and dikes of rhyolite and, rarely, basalt. Shallow, low-grade uranium deposits might be discovered in the quartz monzonite phases but the chief potential lies in the discovery of subsurface bedded deposits in the arkoses beneath the Challis Volcanics cover. A number of areas might contain uranium deposits that are larger than those mined but the discovery of these concealed deposits will be difficult by most conventional prospecting techniques. (ERA citation 04:038206)