| Title | The Evolution of a Partially Vapor-Dominated Geothermal System at Karaha-Telaga Bodas, Indonesia: Insights from Mineral Distributions and Fluid Inclusion Measurements |
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| Authors | Joseph N. Moore |
| Year | 2012 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | vapor-dominated, fluid inclusions |
| Abstract | Temperature and pressure surveys, fluid samples, and petrologic analyses of rock samples from deep drill holes at the Karaha – Telaga Bodas geothermal field on the flank of Galunggung Volcano, west Java, have provided a unique opportunity to characterize the evolution of an active volcano-hosted geothermal system. Wells up to 3 km in depth have encountered temperatures as high as 353°C and a weakly altered granodiorite that intruded to within 2 to 3 km of the surface. The intrusion is shallowest where an acid lake overlies a nearly vertical low resistivity structure (less than 10 ohm-m) defined by magnetotelluric measurements. This structure is interpreted to represent a vapor-dominated chimney that provides a pathway to the surface for magmatic gases. Four distinct hydrothermal mineral assemblages document the evolution of the geothermal system and the transition from liquid- to vapor-dominated conditions. The earliest assemblage represents the initial liquid-dominated system generated during emplacement of the granodiorite between 5,910 ± 76 and 4,200 ± 150 years BP. Tourmaline, biotite, actinolite, epidote and clay minerals were deposited contemporaneously at progressively greater distances from the intrusive contact (assemblage 1). At 4,200 ± 150 years BP, flank collapse and the formation of the volcano’s crater, Kawah Galunggung, resulted in catastrophic decompression and boiling of the hydrothermal fluids. This event initiated development of the modern vapor-dominated regime. Chalcedony and then quartz were deposited as the early low salinity liquids boiled (assemblage 2). Both vapor- and liquid-rich fluid inclusions were trapped in the quartz crystals. Liquid-rich fluid inclusions from the southern part of the field record salinities ranging from 0 to 26 weight percent NaCl-CaCl2 equivalent and locally contain fluorite daughter crystals. Temperature-salinity relationships and evidence of boiling suggest that these fluids were progressively concentrated as steam was lost from the system. As pressures declined, CO2-and SO4-rich steam-heated water drained downward, depositing anhydrite and calcite (assemblage 3) in the fractures, limiting further recharge. Fluid inclusions with salinities up to 31 weight percent NaCl equivalent were trapped as the descending water vaporized. The final assemblage is represented by precipitates of NaCl, KCl and FeClx deposited on rock surfaces in portions of the vapor-dominated zone that boiled dry. Vapor-dominated conditions extend over a distance of at least 10 km and to depths below sea level. Deep wells drilled into the underlying liquid-dominated reservoir in the northern and central part of the volcanic ridge produce low salinity fluids representing recent recharge of meteoric and steam-heated water. |