Hydrogeochemical Constraints on an Intracrustal Magma-Driven and Fault Controlled High-Temperature Geothermal System (daggyai) in Southern Tibet

High-temperature geothermal systems in southern Tibet are commonly associated with crustal magmatism and active faulting, but how magmatic heat, fault-controlled circulation, and shallow fluid processes jointly shape their surface hydrochemical signatures remains poorly constrained. Here, we investigate the Daggyai geothermal field on the northern margin of the Yarlung Zangbo Suture Zone using a field-derived dataset of 27 geothermal water samples. In situ physicochemical measurements, major and trace element concentrations, and δD and δ18O compositions were integrated with field structural observations, mineral–fluid equilibrium assessment, geothermometry, Cl-based binary mixing calculations, and isotope reconstruction. The waters comprise 22 alkaline samples (Group 1), three acidic samples (Group 2), and two weakly acidic mixed-type samples (Group 3). Group 1 waters are enriched in Cl, B, Li, and As and preserve the clearest signatures of deep reservoir fluids. Group 2 waters are interpreted as steam-heated waters formed through the condensation and oxidation of H2S-bearing vapor in shallow groundwater, whereas Group 3 records mixing between deep and shallow fluid components. Mineral–fluid equilibrium relationships and Na–K geothermometry indicate a deep reservoir temperature of 252.6–262.5 °C. Reconstructed deep-fluid isotope compositions exhibit pronounced 18O enrichment relative to local meteoric water, consistent with high-temperature water–rock interaction and a magmatic-fluid contribution. These results support a process-based model in which intracrustal magmatic heat sustains the high-temperature reservoir, while deep faults govern meteoric recharge and geothermal-fluid ascent; boiling and near-surface mixing subsequently generate the observed hydrochemical diversity. This study distinguishes deep magmatic and structural controls from shallow geochemical overprinting and provides a transferable framework for the genetic interpretation and resource evaluation of high-temperature geothermal systems in southern Tibet.