Radiogenic geothermal fluid from Bangka Island, Indonesia: Genesis and reservoir temperature estimation

The Bangka geothermal field is the most representative radiogenic geothermal system in Indonesia which is characterized by widespread hot spring manifestations and high radiogenic heat production. However, the origin of hot water and temperature reservoir estimation are poorly understood. Therefore, this study aimed to (1) characterize the geochemical composition of radiogenic geothermal fluids in Bangka Island, (2) classify fluid types and reservoir temperatures using conventional and multicomponent geothermometer, as well as (3) evaluate fluid genesis and mixing process. The results showed two fluid categories; immature-carbonate waters (Pemali, Terak, Keretak, and Nyelanding) and partially equilibrated-chloride waters (Permis and Dendang). Based on multicomponent geothermometer, the estimated reservoir temperatures range from 88 °C to 111 °C, with lower values corresponding to immature carbonate water which affected by meteoric water. This was affected by meteoric water and higher values, which reflected chloride water in partial equilibrium with granitic host rocks.Mineral saturation indices (SI) reinforced this distinction: carbonate waters were undersaturated with respect to anhydrite, dolomite, gypsum, and sepiolite. However, chloride waters were supersaturated with minerals such as aragonite, calcite, chalcedony, and quartz. These results outlined two distinctive geochemical pathways; carbonate waters which were interpreted from geochemical signatures to reflect meteoric dilution and mineral dissolution, alongside chloride waters reflecting deeper reservoir including mineral precipitation. Additionally, the results provided a scientific groundwork for the sustainable development of medium-low enthalpy radiogenic geothermal systems in Indonesia.