Hydrothermal Alteration in the Garung-Banten Geothermal Area, West Java

Abstract

The Garung hydrothermal system is located within a positive gravity anomaly in the northern margin of the Rawa Danu caldera, 125 to the west of Jakarta. The area studied composed of porphyritic andesitebelong tothe Volcano of Pleistocene age. rocks in the area have been studied a polarizingmicroscope, XRD, SEM and fluid inclusion geothermometry. Four distinct alteration types occur. An early event produced by alkali chloride water converted the primary rocks to an illite-chlorite assemblage. This alteration occurred at temperatures between 180 and and at fluid between 4 and 5. The progressive of the water allowed an overprint of onto the illite-chloriteassemblage. Fluid inclusion geothermometry of quartz from this later assemblage indicated the altering fluid was at temperatures between 200 and This suggests erosion as deep as 300 m has occurred. Descending acid sulphate and sulphate-bicarbonatefluids mixed with reservoir fluid to precipitate quartz and form pyrophylliteat low and at temperatures above