| Abstract |
Karaha-Telaga Bodas is a partially vapor-dominated geothermal system related to Galunggung Volcano. The gas compositions of fluid inclusions trapped during the evolution of the vapordominated regime were analyzed to determine their sources and the processes that have affected the fluids. Minerals from three core holes were analyzed. Core holes T-2 and T-8, drilled near an active thermal area, penetrated several hundred meters of the vapor-dominated regime. The third well, K-33, is located in the central part of the field, where the steam zone is thin. N2/Ar ratios of quartz-hosted inclusions in all three wells record the flux of magmatic gases during the initial development of the vapor-dominated regime. The trapping of magmatic gases suggests that the emplacement of a high level intrusion triggered the depressurization that led to the boiling off of an early liquid-dominated system. Fluids trapped in younger minerals in T-2 and T-8, and the presence of tourmaline, fluorite and native sulfur in T-2, document the continued flux of magmatic volatiles in the southern part of the field. Younger inclusions in K-33, however, contain gases that were dominantly meteoric in origin. Gas concentrations in T-2 and T-8 commonly range from > 1.5 to 10 mole percent, although values in excess of 20 mole percent were obtained. These gas concentrations can be related to boiling and the accumulation of externally derived volatiles within an expanding vapor-dominated regime. Variations in the gas contents of quartz-, pyrite- and anhydrite-hosted inclusions record the progressive growth of the vapor-dominated regi'me. |