Mobility and depositional controls of radioelements in hydrothermal systems at the Long Valley and Valles calderas

The loci and abundance of U and Th were examined in tuffaceous rocks encompassing hydrothermal systems at the Long Valley caldera, California and the Valles caldera, New Mexico. Aspects of these systems may be analogous to conditions expected in a potential site for a high-level waste repository in welded tuff. Examination of radioelements in core from scientific drill holes at these sites was accomplished by gamma-ray spectrometry and fission-track radiography. In the lateral-flowing hydrothermal system at the Long Valley caldera, where temperatures range from 140 to 200 °C, U is concentrated to 20 ppm in Fe-rich zones of varved tuff and to 50 ppm with Fe-rich mineral phases in tuff fragments of a calcite-cemented breccia. U-series disequilibrium in some of these samples suggests mobilization/deposition of parent U and/or its daughters. In the vapor zone of the Valles calderaʹs hydrothermal system (temperature ~ 100 °C), the concordance of high U, low Th/U and decreasing whole-rock O-isotope ratios suggests that U was concentrated in response to hydrothermal circulation when the system was formerly liquid-dominated. In the underlying present-day liquid-dominated zone (temperature to 210 °C), U, up to several tens of parts per million, occurs with pyrite and Fe-oxide minerals, and in concentrations to several percents with a Ti-Nb-Y-rare earth mineral. In the Valles systemʹs outflow zone, U is also concentrated in Fe-rich zones as well as in carbonaceous-rich zones in the Paleozoic sedimentary rocks that underlie the Quaternary tuff. Th, associated with accessory minerals, predominates in breccia zones and in a mineralized fault zone near the base of the Paleozoic sedimentary sequence. Relatively high concentrations of U occur in springs representative of water recharging the Valles calderaʹs hydrothermal system. In contrast, considerably lower U concentrations occur in hot waters (> 220 °C) and in the systemʹs outflow plume, suggesting that U is concentrating in the hotter part of the system. The Long Valley and Valles observations indicate that U and Ra are locally mobile under hydrothermal conditions, and that reducing conditions associated with Fe-rich minerals and carbonaceous material are important factors in the adsorption of U, and thus can retard its transport in water at elevated temperature.