Stable isotope depletion and lithium enrichment due to accumulation processes in a structural controlled geothermal field : Evidence of meteoric water heated at shallow reservoir depths in the Sipoholon geothermal area, North Sumatra, Indonesia

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 (518O and 52H). 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 52H depletion (down to-73 parts per thousand) and variable 518O 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.