USING MINERAL ASSEMBLAGES AND COMPOSITION TO CHARACTERIZE THE KARAHA-TELAGA BODAS GEOTHERMAL SYSTEM

The Karaha-Telaga Bodas geothermal system, located on the flank of the active basaltic-andesitic Galunggung volcano in West Java, represents a system that has evolved from liquid- to vapor-dominated. This study focuses on propylitic mineral assemblages from drill core samples (core T-2, max depth = 1,383 m, bottom T of 321 ⁰C). All samples contain euhedral to subhedral plagioclase, which may be variably altered. Other minerals found in the core include: illite, quartz, pyrite, calcite, ilmenite, epidote, chlorite, magnetite, potassium feldspar, calcic amphiboles, titanite, chalcopyrite, rutile, chloroapatite, barite, and fluorite.

Petrographic analysis has been used in conjunction with EDS to identify the fine-grained and complex mineralogy in the samples. Despite significant alteration in the rock, plagioclase is present throughout the sequence. Epidote and chlorite are present in the shallower samples (to depths of 980 m), and calcic amphibole is present in deeper samples (depths > 1249 m). The transition from epidote and chlorite to calcic amphibole with depth may be related to an increase in temperature. These samples from the T-2 core are characterized by a sub-assemblage of either plagioclase+epidote+chlorite or, with depth, plagioclase+calcic amphibole. Quantitative chemical analysis was performed by using EPMA on samples from the following depths: 396 m, 888 m, 961 m, 980 m, 1,249 m, and 1,378 m. The mole fraction of anorthite (Xan) of the plagioclase cores increases generally with depth, containing nearly An-end-member plagioclase in some cases, except for the deepest sample (Xan as ranges, with increasing depth): 0.44-0.86, 0.78-0.86, 0.72-0.93, 0.83-0.95, 0.86-0.97, and 0.56-0.73, respectively. Where epidote exists, there is no discernable trend in the mole fraction of pistacite (Xps=0.14-0.35) with depth; the variation in the epidote composition in any sample is approximately equivalent to the range in composition within the entire core. The wide range in mineral composition within and between samples may be indicative of variable amounts of alteration reaction progress down to the sub-mm scale.