Uranium abundances and distribution in associated glassy and crystalline rhyolites of the western United States

The abundance and distribution of uranium have been determined in 11 units of rhyolitic lava and ash-flow tuff of calc-alkaline and transitional composition from the western United States in order to further evaluate the potential of rhyolitic glass as a source of uranium ores. Samples consist of coexisting obsidians, perlites, and felsites that range in age from Pleistocene to Oligocene. Uranium abundances in analyzed obsidians are 5 to 46 ppm. Obsidians and coexisting perlites have identical (±5%) uranium concentrations, which confirms that little or no uranium is lost during hydration. Felsites show uranium depletions as high as 80% relative to coexisting obsidians and perlites. Combination of this data with the results of earlier work on peralkaline rhyolites indicates that uranium depletion seems to increase with age, with different rates of depletion for calc-alkaline (slowest) and peralkaline (fastest) compositions. Uranium distribution is homogeneous in obsidians, perlites, and spherulites, but inhomogeneous in felsites. Electron microprobe analyses of the least-depleted felsites indicate that uranium is associated with concentrations of Fe-Ti-Mn oxides or is in accompanying accessory minerals. Secondary Fe-Mn oxides in older, depleted felsites are uranium-bearing, especially along fractures or flow layers. Uranium loss from felsites seems to be largely controlled by low-temperature solution over long time periods, with some precipitation in secondary phases.