HYDROTHERMAL FLUID EVOLUTION IN THE GEOTHERMAL AREA OF PANTAR-GUNUNG SIRUNG, ALOR REGENCY, EAST NUSA TENGGARA PROVINCE

• Main Author: Amir, Alfauzan
• Format: Theses
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/53643
• Institution: Institut Teknologi Bandung

The research area is located in the Geothermal Area of Pantar-Gunung Sirung, Pantar Tengah District, Alor Regency, East Nusa Tenggara Province. Geological exploration has been carried out in this geothermal area and found geothermal manifestations in the form of hot soil, hot spring, fumaroles, and solfatara. Geothermal system of the Pantar-Gunung Sirung is associated with Sirung Volcano, has age of Quaternary and has the potential to be developed for power plants. This research used primary data about 13 core rock samples from the temperature slope well drilling of PSR-1 with a depth interval of 668 m. The methodology that has been used in this study includes petrological analysis, petrography, fluid inclusion analysis, and X-Ray Diffraction (XRD). Based on the results of the analysis, it is expected to conclude the evolutionary conditions of hydrothermal fluids in the study area. The lithology in the PSR-1 well is divided into 6 rock units, including the Brown Lapilli Stone Unit (3 - 24 m), the Tuff Breccia Unit (24 - 170 m), the Lapili Tuff Unit (170 - 204 m), the Gray Lapili Stone Unit (204 - 250 m), Andesite Breccia Unit (250 - 480 m), and Green Lapilli Stone Unit (480 - 670 m). Alteration in the PSR-1 well is grouped into 3 alteration zones, namely, the Oxidation Zone (3 - 24 m), the Smectite-Kaolin-Quartz-Calcite Zone (24 - 480 m), and the Illite-Sericite-Quartz-Chlorite-Anhydrite Zone. (480 - 668 m). The fluid inclusion analysis of the quartz vein in the PSR-1 well, showed the homogenization temperature of 270 oC, melting temperature of -2.4 oC, eutectic temperature of -11.4 oC, and salinity of 4.24 wt% NaCl eq. The evolution of hydrothermal fluids in the Pantar-Gunung Sirung Geothermal Region is divided into three phases, the first phase is meteoric infiltration of water was absorbed deeper than 700 m. This water was heated by the volcanic activity of Mount Sirung into a hydrothermal fluid with temperature about 220 - 310 oC which is indicated by the presence of illite and sericite. At a depth of 668 m, the hydrothermal fluid underwent a boiling process, this is indicated by the presence of lattice bladed textured quartz vein containing two-phase inclusions. The homogenization temperature of the fluid inclusions indicated that the hydrothermal fluid had a higher temperature than 270 oC. In the second phase, hydrothermal fluid then flowed upwards and cooled to temperature about 100 - 160 oC which is indicated by the presence of smectite, kaolin, and cristobalite at a depth of 24 to 480 m. In this phase the hydrothermal fluid mixed with condensate water which is rich in HCO3, this is indicated by the presence of alteration paragenesis, namely calcite veins formed after quartz veins. The third phase of hydrothermal fluid continued to flow upwards and had temperature decreased about below 60 oC, indicated by the presence of geothermal manifestations on the surface in the form of bicarbonate type Airmama Hot Springs.