Petrologic Evidence for Boiling to Dryness in the Karaha-telaga Bodas Geothermal System, Indonesia

Karaha-Telaga Bodas is a recently discovered geothermal system located adjacent to Galunggung Volcano in western Java. Drilling to depths of 3 km has documented an extensive vapor-dominated zone and a deeper liquid reservoir with measured temperatures up to 350C and salinities of 1-2 weight percent. The vein parageneses record the evolution of a high temperature liquid-dominated system that developed before the onset of vapor-dominated conditions. The change to a vapor-dominated regime was marked by the deposition of chalcedony and quartz. Fluid inclusions trapped in quartz and the mineral relationships indicate that the pressure gradients at the start of this change greatly exceeded hydrostatic and that much of the chalcedony was deposited at temperatures >250C. The deposition of chalcedony at these temperatures implies catastrophic depressurization and boiling of the reservoir liquids. As pressures within the reservoir declined, downward percolating steam condensate deposited anhydrite, calcite, fluorite, pyrite, and chlorite. At depths greater than 800 m, boiling off of the condensate resulted in increasing salinities with increasing temperature. Hypersaline fluids with salinities of 31 weight percent NaCl equivalent were trapped at 300C in anhydrite. Rock surfaces were subsequently coated with scales of NaCl, KCl, FeClx, and Ti-Si-Fe as the condensate boiled to dryness. The age of chalcedony deposition is constrained by C dating of lakebeds from a depth of 988.8 m. These deposits, which predate the formation of chalcedony and the carbonate and propylitic veins, yielded an age of 5910 + 76 years BP. The depressurization and boiling that caused deposition of the chalcedony could have been triggered by massive slope failure of the volcanic edifice ~4200 years ago. This event produced the crater at Kawah Galunggung. The low salinities of the modern reservoir fluids imply that mixtures of meteoric water and condensate are now recharging the system.