Overprinting Events from Crosscutting Veins and Mineral Assemblages in Ulubelu Geothermal Field, Indonesia

Abstract

The distribution of hydrothermal minerals associated with the Ulubelu geothermal system of Indonesia records several episodes of hydrothermal alteration during the history of the system. X-ray diffraction and petrographic analyses identify six alteration zones composed of characteristic secondary mineral assemblages: smectite; smectite with kaolin; illite/smectite with epidote; epidote with adularia; and chlorite/smectite with epidote and adularia. Four episodes of overprinting are recorded by fluid-inclusion data and crosscutting relationships observed between the various alteration minerals. The first stage is represented by early quartz-bearing veins at depths between 514 and 1195 m with fluid-inclusion homogenisation temperatures of 210? to 230? C. The second stage is characterised by two episodes of prehnite-quartz vein formation with homogenisation temperatures of 240? to 260?C. A third stage is recognized as an argillic-phyllic overprinting between the depths of 600 and 1052 m that may be due to the descent of acidic steam condensate along major faults. The last stage of alteration is dominated by the deposition of young calcite-quartz veins at 893m and 1100m with fluid-inclusion homogenisation temperatures ranging from 200? to 230?C. These relationships record the thermal and chemical evolution of the geothermal system from initial heating to temperatures of 260?C, the formation of clay minerals from acidic fluids, and cooling to present reservoir temperatures (230?C).