Evolution of a mineralized geothermal system, Valles Caldera, New Mexico

Abstract

The 20-km-diam Valles caldera formed at 1.13 Ma and had continuous postcaldera rhyolitic eruptions until 0.13 Ma. Hot springs and fumaroles are surface manifestations of a hydrothermal reservoir (210 degrees -300 degrees C; 2-10 X 10 3 mg/kg Cl) that is most extensive in fractured, caldera fill tuffs and associated sedimentary rocks, located in specific structural zones. Fluids are composed of deeply circulating water of (primarily) meteoric origin that have a mean residence time in the reservoir of 3 to 10 k.y. The only component of clearly magmatic origin is anomalous 3 He although it is possible that some magmatic water, carbon, and sulfur is contributed to present hydrothermal fluids. Host rocks show intense isotopic exchange with hydrothermal fluids. Alteration assemblages are controlled by temperature, permeability, fluid composition, host-rock type, and depth. A generalized distribution from top to bottom of the system consists of argillic, phyllic, propylitic, and calc-silicate assemblages. Typical alteration minerals in phyllic and propylitic zones are quartz, calcite, illite, chlorite, epidote, and pyrite, whereas common vein minerals consist of the above minerals plus fluorite, adularia, and wairakite. Argentiferous pyrite, pyrargyrite, molybdenite, sphalerite, galena, chalcopyrite, arsenopyrite, stibnite, 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 temperatures ranging from 175 degrees to 310 degrees C, depending on location in the system. Fluid inclusion studies also show that the top of the liquid-dominated zone has descended, leaving behind a low-pressure vapor cap. Dating of spring deposits, core samples of vein minerals, and altered host rocks by K-Ar, U-Th, U-U, and paleomagnetic methods indicates that the hydrothermal system was created at about 1.0 Ma and has been continuously active to the present. However, these dates in combination with geologic and fluid inclusion evidence suggest that the vapor zone formed at about 0.5 Ma, after a breach of the southwest caldera wall drained widespread intracaldera lakes and lowered the hydraulic head on the hydrothermal reservoir. Although petrologic and geophysical evidence indicates that residual pockets of melt still reside in the pluton beneath the caldera, the size of the hydrothermal system has shrunk since initial formation. Thus, the Valles caldera contains a mature hydrothermal system that remains hot, that contains a classic geothermal configuration, and that displays many analogies to epithermal mineral deposits exposed in older volcanic environments.