Oxygen isotope composition and tourmaline mineral chemistry of the Coso and Darajat geothermal systems

At the Coso geothermal system, near China Lake, California, clay mineralogy and the extent of oxygen depletion in whole rock samples and mineral separates have been determined for a suite of samples from wells 33A-7, 68-6 and 73-19 along the high temperature Western Flank. The local zones of maximum oxygen depletion in each well correspond closely with the depths of current production zone. This association of discrete zones of high permeability indicates that the extent of oxygen depletion serves as a guide to the extent of fluid-rock interaction and permeability in the reservoir rock. The chemical compositions of two separate tourmaline populations from well DRJ-S1 at Darajat, Indonesia have been determined. These data, in combination with petrologic observations, are used to improve our understanding of the evolution of the Darajat vapor-dominated geothermal system.