Modeling Mineral Equilibrium in the Pinatubo Geothermal System

Abstract

The equilibrium state between present-day geothermal fluids and hydrothermal mineral phases was tested for the Pinatubo geothermal field. Observed hydrothermal minerals in Pinatubo are typical of neutral to alkaline fluids. Important hydrothermal minerals found in the field include clay minerals (montmorillonite, kaolinite/dickite, interlayered clays, illite), chlorite, epidote, quartz, pyrite, with minor actinolite, calcite and sphene. Calculated mineral saturation indices for the well discharges in the Pinatubo geothermal system showed that the reservoir fluids are oversaturated with actinolite, anhydrite, chlorite, epidote, hematite, magnetite, and pyrite. Chemical equilibrium between the present day fluids and hydrothermal minerals was not attained as indicated by a spread of temperature in log (Q/K) plot and appearances of some minerals that have not been reported.