Approximately 13,700 tons (12,400 tonnes) of oxidized uranium ore, averaging about 0.1 percent U2O8, was mined during 1966 and 1967 from the lower unit of the Deweesville Sandstone Member of the upper Eocene Whitsett Formation, from depths of 75 to 90 feet (23-27 m). The mine is in the Karnes County uranium area, 3 miles (5 km) east of Falls City, Tex. Meta-autunite, Ca(UO2)2(PO4)22-6H2O, and meta-tyuyamunite, Ca(UO2)2(PO4)23-5H2O, were identified in samples of the ore. The host rock is light-colored, medium-grained, well-sorted feldspathic sandstone that contains fossil wood and Ophiomorpha. It also contains clinoptilolite, cristobalite, and montmorillonite that probably formed as alteration products of volcanic grains in the original sediment. The host rock was deposited in a beach environment near or on a delta formed at the mouth of a stream channel oriented roughly normal to the beach. The Deweesville can be traced for many miles on the surface and represents in general the point of maximum transgression in a transgressive-regressive cycle. The upper unit of the Deweesville, which overlies the host rock, is now exposed in the mine and contains, in ascending order, storm-beach, shoreface, beach, and tidal-flat facies. The shape of the ore body suggests that it originated as an unoxidized ore roll. The ore-bearing fluids may have entered through porous fluvial rock extending updip. Plant material, abundant in the original host sediment, probably supplied reductant necessary for the roll formation.
