Subsurface Hydrothermal Alteration in the Kamojang Geothermal Field, West Java, Indonesia

The presently vapor-dominated Kamojang geothermal field is located in West Java (Indonesia). The field lies in high terrain, 1500 m above sea level, and is hosted by volcanic rocks. Samples from seven wells were kindly made available for this study, namely from KMJ-7, KMJ-10, KMJ-11, KMJ12, KMJ-53, KMJ-59, and CHR-1 drilled to depths ranging from 536 – 1804 m. The reservoir is hosted by interbedded pyroclastic and lavas of andesite composition with some basaltic andesites and lacustrine tuffs. The field is now vapor-dominated but the hydrothermal minerals show that the rock-altering fluid were dominantly liquid. There are 2 hydrothermal mineral assemblages present, namely those produced by “acid” and “neutral “ pH fluids. The “acid” mineral assemblage which occupies shallow levels (down to 100 – 300 m) consists of kaolin, smectite, alunite, quartz, cristobalite, and pyrite. The altering fluid was of acid sulphate type formed due to the oxidation of H2S. The “neutral pH“ mineral assemblage occupies deeper levels and comprises varying proportion of quartz, adularia, albite, epidote, titanite, wairakite, laumontite, calcite, siderite, hematite, pyrite, anhydrite, smectite, chlorite, illite, and interlayered clays. The altering fluid was liquid of near neutral pH, and chloridesulphate type. In general, the present-day measured downwell temperatures are lower than those indicated by the alteration mineralogy (i.e. epidote, wairakite, laumontite, and clays) by about 10-50 C and fluid inclusion homogenisation temperatures (by about 60 C). This suggests cooling, although some of the measured downwell temperatures may be too low. Permeability has decreased due to mineral deposition, but tectonic activity has helped prolong it by reopening vertically permeable features. Hydrothermal mineral parageneses and vein crosscutting relationships in the deeper levels (below the “acid” assemblage zone), for example vein filled with quartz only and quartz+wairakite-calciteanhydrite cut by vein of quartz-pyrite, suggest that although this field has undergone at least three hydrothermal episodes, the composition of the altering fluid was always of near neutral pH.