Mt. Pancar geothermal system structure assessed from gravity and hydrogeological interpretations

Abstract

Mt. Pancar geothermal prospect, which is categorized as a low-to-moderate geothermal system, is located in Bogor district, West Java, Indonesia. Thermal activity in this area is recognized by the existence of some surface manifestations, including altered rocks, warm ground, and three neutral pH hot springs in Kawah Merah (T = 45 - 68°C), Kawah Hitam (T = 49 - 58°C), and Kawah Putih (T = 47-55°C). There is no indication of thermal activity on the summit of Mt Pancar. Geologically, Mt. Pancar is associated with a Quartenary volcano that overlying the Tertiary marl and clayey shale of Jatiluhur formation. The prospect area is mainly affected by the sinistral movement of Baribis thrust forming the NE-SW and NW-SE faults. Gravity survey and hydrogeological study were conducted to image subsurface geological structure associated with the hydrothermal system in Mt. Pancar. Gravity survey was carried out using Scintrex CG-5 gravimeter. A total of 236 gravity stations were acquired with 250 m spacing and distributed along vicitiny of Mt. Pancar and manifestations. The bouger anomaly of Mt. Pancar shows a good agreement with the surface geological features. A low bouger anomaly is detected in the center of investigation area that could be correlated with the alteration rock beneath Mt. Pancar. Resistivity model derived from 3-D AMT inversion supports this result by indicating the low resistivity layer below Mt. Pancar. The pattern of the high gravity anomaly corresponds to the fault in the western and eastern parts of Mt. Pancar. The occurence of Kawah Merah and Kawah Hitam hotsprings in the eastern part could be controlled by the NE-SW normal fault as supported through 2-D gravity forward modeling. However, Kawah Putih hotspring is more likely controlled by topography and lithological boundary. The hydrogeological conceptual model was then constructed based on the geological structure assessment of gravity data incorporated with the geology and geochemical data. The hotsprings are likely situated in the outflow zone, while an upflow zone is located around Mt. Pancar as the possible heat source of the hydrothermal system in this area.