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 e ncompassing hydrothermal systems at the Long Valley caldera, Californi a 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-r ay spectrometry and fission-track radiography. In the lateral-flowing hydrothermal system at the Long Valley caldera, where temperatures ran ge from 140 to 200 degrees C, U is concentrated to 20 ppm in Fe-rich z ones of varved tuff and to 50 ppm with Fe-rich mineral phases in tuff fragments of a calcite-cemented breccia. U-series disequilibrium in so me of these samples suggests mobilization/deposition of parent U and/o r its daughters. In the vapor zone of the Valles caldera's hydrotherma l system (temperature similar to 100 degrees C), the concordance of hi gh U, low Th/U and decreasing whole-rock O-isotope ratios suggests tha t U was concentrated in response to hydrothermal circulation when the system was formerly liquid-dominated. In the underlying present-day li quid-dominated zone (temperature to 210 degrees C), U, up to several t ens of parts per million, occurs with pyrite and Fe-oxide minerals, an d in concentrations to several percents with a Ti-Nb-Y-rare earth mine ral. In the Valles system's outflow zone, U is also concentrated in Fe -rich zones as well as in carbonaceous-rich zones in the Paleozoic sed imentary 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. Relat ively high concentrations of U occur in springs representative of wate r recharging the Valles caldera's hydrothermal system. In contrast, co nsiderably lower U concentrations occur in hot waters (> 220 degrees C ) and in the system's outflow plume, suggesting that U is concentratin g in the hotter part of the system. The Long Valley and Valles observa tions indicate that U and Ra are locally mobile under hydrothermal con ditions, 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.