EVOLUTION OF A MINERALIZED GEOTHERMAL SYSTEM, VALLES CALDERA, NEW-MEXICO

The 20-km-diam Valles caldera formed at 1.13 Ma and had continuous pos tcaldera rhyolitic eruptions until 0.13 Ma. Hot springs and fumaroles are surface manifestations of a hydrothermal reservoir (210 degrees-30 0 degrees C; 2-10 X 10(3) mg/kg Cl) that is most extensive in fracture d, caldera fill tuffs and associated sedimentary rocks, located in spe cific structural zones. Fluids are composed of deeply circulating wate r of (primarily) meteoric origin that have a mean residence time in th e reservoir of 3 to 10 k.y. The only component of clearly magmatic ori gin is anomalous He-3 although it is possible that some magmatic water , carbon, and sulfur is contributed to present hydrothermal fluids. Ho st rocks show intense isotopic exchange with hydrothermal fluids. Alte ration assemblages are controlled by temperature, permeability, fluid composition, host-rock type, and depth. A generalized distribution fro m top to bottom of the system consists of argillic, phyllic, propyliti c, and calc-silicate assemblages. Typical alteration minerals in phyll ic and propylitic zones are quartz, calcite, illite, chlorite, epidote , and pyrite, whereas common vein minerals consist of the above minera ls plus fluorite, adularia, and wairakite. Argentiferous pyrite, pyrar gyrite, molybdenite, sphalerite, galena, chalcopyrite, arsenopyrite, s tibnite, barite, and tetradymite (?) have been found at various depths in the Valles system. Fluid inclusion studies show that these mineral assemblages formed from liquid water (0-5.5 wt % NaCl equiv) at tempe ratures ranging from 175 degrees to 310 degrees C, depending on locati on in the system. Fluid inclusion studies also show that the top of th e liquid-dominated zone has descended, leaving behind a low-pressure v apor cap. Dating of spring deposits, core samples of vein minerals, an d altered host rocks by K-Ar, U-Th, U-U, and paleomagnetic methods ind icates that the hydrothermal system was created at about 1.0 Ma and ha s been continuously active to the present. However, these dates in com bination with geologic and fluid inclusion evidence suggest that the v apor zone formed at about 0.5 Ma, after a breach of the southwest cald era wall drained widespread intracaldera lakes and lowered the hydraul ic head on the hydrothermal reservoir. Although petrologic and geophys ical evidence indicates that residual pockets of melt still reside in the pluton beneath the caldera, the size of the hydrothermal system ha s shrunk since initial formation. Thus, the Valles caldera contains a mature hydrothermal system that remains hot, that contains a classic g eothermal configuration, and that displays many analogies to epitherma l mineral deposits exposed in older volcanic environments.