Mochamad Nukman, Rofiqul Umam, Rahmat Nawi Siregar, Ulum Marwa, Suharno Suharno, Hirotaka Takahashi
The Sipoholon geothermal area in North Sumatra, Indonesia, is located within a tectonically active pull-apart basin in the central segment of the Sumatra Fault System, providing an excellent setting to investigate isotopic depletion and lithium (Li) enrichment in a structurally controlled geothermal system. This study evaluates the origin, evolution, and geochemical behavior of thermal waters using an integrated approach combining major ions, trace elements (Li and B), and stable isotopes (δ18O and δ2H). The hydrochemical data show systematic relationships between Na-Cl and B-Cl and a narrow salinity range, arguing against significant seawater or direct magmatic water input and supporting a dominantly meteoric origin. Piper diagram analysis identifies three hydrochemical facies, namely Mg–HCO₃ (dominant), Na–HCO₃, and Ca-Cl, reflecting spatial variability in fluid evolution and water–rock interaction. Lithium and boron concentrations show positive correlations with Na, indicating progressive mobilization during subsurface circulation. Under the relatively low-temperature and shallow-reservoir conditions of the system, these trends are interpreted to reflect both primary silicate alteration and secondary clay-related processes, with Li more reliably recording cumulative water–rock interaction and residence time. Stable isotope compositions show marked δ2H depletion (down to −73‰) and variable δ18O shifts, particularly in Group I springs located within the pull-apart basin. These isotopic features suggest prolonged circulation and thermal re-equilibration in shallow reservoirs. Overall, the combined isotope and trace-element evidence indicates that Li enrichment in the Sipoholon system is most consistent with structurally controlled fluid retention, repeated circulation, and progressive geochemical evolution of heated meteoric water. These findings highlight the role of structurally controlled geothermal system in promoting trace-element enrichment and isotopic modification in tectonically controlled geothermal systems. © 2026 The Author(s).
Geophysics Study Program, Departement of Physics, Faculty of Mathematics and Natural Sciences, Gadjah Mada University, Sekip Utara, Yogyakarta, 55281, Indonesia; Faculty of Life and Environmental Sciences, University of Tsukuba, Ibaraki, Tsukuba, 305-8572, Japan; Center for Research in Radiation, Isotopes, and Earth System Sciences (CRiES), University of Tsukuba, Ibaraki, Tsukuba, 305-8572, Japan; Department of Physics, Institut Teknologi Sumatera, Jl. Terusan Ryacudu, Lampung, South Lampung, 35365, Indonesia; Department of Geological Engineering, Faculty of Engineering, Gadjah Mada University, Sekip Utara, Yogyakarta, 55281, Indonesia; Department of Geophysical Engineering, Faculty of Engineering, University of Lampung, Lampung, Bandar Lampung, 35145, Indonesia; Research Center for Space Science, Tokyo City University, 3-3-1 Ushikubo-Nishi, Tsuzuki-Ku, Kanagawa, Yokohama, 224-8551, Japan; Institute for Cosmic Ray Research (ICRR), The University of Tokyo, Chiba 5 Chome-1-5 Kashiwanoha, Kashiwa, Chiba, 277-0882, Japan; Earthquake Research Institute, The University of Tokyo, Tokyo 1 Chome-1-1 Yayoi, Bunkyo City, Tokyo, 113-0032, Japan