Correlation of hydrothermal sericite composition with permeability and temperature, Coso Hot Springs geothermal field, Inyo County, California

Petrographic and geochemical analyses of cuttings from six wells in the Coso Hot Springs geothermal field show a systematic variation in the occurrence, texture, and composition of sericite that can be correlated with high permeability production zones and temperature. The wells studied intersect rhyolitic dikes and sills in the fractured granitic and dioritic basement rocks which serve as the reservoir for the geothermal system. Low-permeability non-productive zones in the wells contain coarse-grained compositionally homogeneous primary muscovite. High-permeability production zones are characterized by abundant fine-grained hydrothermal sericite that exhibits a systematic increase in K and Al and decrease in Si with increasing temperature. Calculated activities of the muscovite and pyrophyllite components for the hydrothermal sericites increase and decrease, respectively, with increasing temperature. This trend is similar to that reported for geothermal systems at Roosevelt Hot Springs and the Salton Sea, although sericites from these systems are compositionally distinct from those at Coso. Computed distribution of aqueous species in the geothermal fluids were combined with activity phase diagrams accounting for hydrothermal sericite compositions to determine a pH of 6.1 for the geothermal fluids. These calculations suggest that hydrothermal sericites are in local equilibrium with the geothermal fluid, but primary muscovites in lowmore » permeability zones are metastable. Compositional data on hydrothermal sericite from active geothermal systems at Coso Hot Springs, Roosevelt Hot Springs, and the Salton Sea show that the mineral is sensitive to temperature and water/rock ratios and may be a useful indicator of high permeability zones within a geothermal field.« less