Epithermal Mineral Zonation in an Active Island Arc: the Bacon-Manito Geothermal Systems, Philippines

Abstract

The Bacon-Manito geothermal field exhibits chemical zonation of thermal features that istypical of island arc andesite hydrothermal systems. Zonation of hydrothermal minerals in the vadose zone in the production wells reflects this chemical ' distribution of surface fluids. The interface between the vadose zone and the chloride reservoir r'luids is marked by distinct changes in hydrothermal minerals: the influx of anhydrite and calcite, and the transition of kaolinite to chlorite and cristo-balite to quartz. The hydrothermal mineralogy and fluid inclusion data indicates that the epithermal region of theeastern sector of Bac-Maw approximated " boiling point with depth" conditions in the past. Cooling of the system and downward percolation of cool acid sulphate and neutral sulphate- bicarbonate fluids from the vadose zone in to the hot chloride reservoir has caused precipitation of quartz, anhydrite and calcite infrsctures, and produced a 80@m sealed cap on a neutral chloride reservoir, locally trapping two phase, gas rich pockets.