Geochemistry and Isotopes of Volcanic and Non-Volcanic Geothermal Systems on the Indonesian Island of Java

This study aims to understand the differences in the characteristics of volcanic and non-volcanic geothermal systems on Java Island, specifically in the Central and East Java geological zones. The analysis is done through geochemical studies of anion-cation, isotopes (δ¹⁸O and δD), and rare earth elements. Volcanic systems are centred around the Quaternary volcanic regions, such as Dieng, Ungaran, and Ijen. In contrast, non-volcanic systems are found in the Kendeng Zone, South Serayu, and the Southern Mountains. The research results show that volcanic geothermal systems are dominated by chloride water with high Cl⁻ ion concentrations due to direct interaction with magma. These systems are rich in rare earth elements, such as Sr, Ba, and Eu, reflecting high mineralization potential. In contrast, non-volcanic systems are dominated by bicarbonate and sulfate waters, which form due to tectonic activity and geothermal gradients. Isotope analysis reveals that volcanic systems are influenced by a mix of meteoric, hydrothermal, and magmatic waters, while non-volcanic systems are primarily dominated by meteoric water. These findings reveal different geothermal fluid dynamics in the two systems, reflecting their respective geological characteristics. The novelty of this study lies in the comprehensive comparison of the chemical and isotopic characteristics of volcanic and non-volcanic geothermal systems, as well as the identification of rare earth elements potential in both systems. This research contributes significantly to the diversification of renewable energy, exploration of rare minerals, and the development of sustainable geothermal energy to support national strategies toward net-zero emissions.