VOLCANISM AND HYDROTHERMAL ACTIVITY: THE VIEW FROM GALUNGGUNG VOLCANO, INDONESIA

Petrologic analyses, production data and ¹⁴C dating of samples from the Karaha-Telaga Bodas geothermal field on Galunggung Volcano, west Java, document the rapid evolution of a high-temperature hydrothermal system containing many features of high sulfidation ore deposits. Deep wells, some drilled to 3 km, have encountered a vapor-dominated regime extending below sea level, a deeper, low salinity liquid-dominated reservoir and temperatures up to 353°C. Granodiorite emplaced at depths of 2-3 km provides heat to the system. At the southern end of the field, an acid lake overlies a vapor-dominated chimney defined by magnetotelluric surveys.

Within ~300 m of the granodiorite, contact metamorphism has produced assemblages containing tourmaline, pyroxene, garnet and biotite. At shallower depths, amphiboles, epidote, chlorite and illite dominate the secondary assemblages. These assemblages record the development of a high temperature liquid-dominated geothermal system generated by intrusion of the stock. The age of the granodiorite is constrained to the period between ~5910 + 76 and 4200 + 150 y BP by ¹⁴C dating of charcoal. At ~4200 y BP, flank collapse and the formation of the volcano's crater, Kawah Galunggung resulted in decompression and boiling of the early liquid reservoir. Amorphous silica, chalcedony and quartz precipitated during this event. Fluid inclusion salinities up to 24 wt. % NaCl-CaCl₂ equivalent record the progressive boiling-off of the early liquid-dominated system and the formation of the vapor-dominated regime. As the fluid pressures declined, descending condensate deposited anhydrite and carbonates. Tourmaline, fluorite, apatite and native sulfur precipitated near the vapor chimney, reflecting interactions with magmatic gases. Chlorite and wairakite formed where condensate mixed with the deeper reservoir fluids. Scales of Na, K, and Fe chlorides document the dryout of the vapor-dominated zone.