Print this pageAbstract: Uranium Deposits in Volcaniclastic Rocks Near Mountain City, Nevada
Uranium deposits near Mountain City, Nevada, near the northern boundary of the Basin and Range province, have produced about 4,700 tons of uranium ore since initial discovery in 1957.
The Cretaceous Mountain City biotite-quartz monzonite stock contains uranium-bearing veins, but uranium impregnations in overlying Eocene(?) volcaniclastic rocks are more important. Two rock types are mineralized: tan arkosic conglomeratic "granite wash" with vitric-lithic tuff, and bluish-green vitric-lithic tuff breccia composed mostly of montmorillonite matrix with andesite and charcoal fragments. Ore zones underlie black vitrophyre or andesitic vitrophyric agglomerate lenses which are basal glassy phases of andesitic and dacitic welded tuffs.
Fluvial montmorillonitic devitrified tuff breccia fills channels and depressions in the prevolcanic quartz monzonite erosion surface. Mineralized rock, within hematitic bleach-rock halos, reaches a maximum thickness of 80 ft (25 m) and grades exceeding 2% in some thin zones. Autunite with minor uraninite, carnotite, torbernite, and renardite is associated with pyritiferous coalified wood and fractures. Vanadium, zinc, lead, copper, molybdenum, nickel, and cobalt are anomalously abundant.
Postmineral normal faults have modified some deposits during eastward block tilting.
Possible source rocks include the Mountain City stock, which contains economic concentrations of copper, silver, and gold, and the uranium-rich extrusive Tertiary rocks. Uraniferous solutions possibly migrated along the selectively permeable quartz monzonite-volcaniclastic nonconformity forming deposits by reduction near carbonaceous material or by adsorption on montmorillonite.
AAPG Search and Discovery Article #90961©1978 AAPG Annual Convention and Exhibition, Oklahoma City, Oklahoma