Genesis of high-sulfate geothermal water in the Namcha Barwa syntaxis, eastern Xizang

In the Namcha Barwa syntaxis of eastern, high-sulfate geothermal water is extensively developed, presenting significant potential for exploitation and utilization. However, the sources and formation mechanisms of these sulfates remain poorly understood. This study focused on the hot spring and borehole water in the Namcha Barwa syntaxis, aiming to identify the sources of ions in geothermal water. The study first examines the hydrochemical components, followed by comprehensive analyses of strontium, sulfate sulfur, and oxygen isotopes to determine the sources of sulfate in geothermal water In addition, this study investigated the water-rock equilibrium and temperature of deep thermal reservoirs and summarized the genesis of high-sulfate geothermal waters in the Namcha Barwa syntaxis. The geothermal water is characterized as weakly acidic to weakly alkaline-alkaline water, with total dissolved solids ranging from 130 mg/L to 3265 mg/L. The hydrochemical types include HCO3•SO4—Ca•Na, SO4—Ca, and SO4•HCO3—Ca•Na. Water chemistry and strontium isotope results indicate that the dissolution of silicate minerals in gneiss and sulfate in evaporites are key factors influencing the water chemistry components. The \begin{document}${}^{34}{\mathrm{S}}_{{\mathrm{SO}}_4} $\end{document}-\begin{document}${}^{18}{\mathrm{O}}_{{\mathrm{SO}}_4} $\end{document}isotopes suggest multiple \begin{document}${\mathrm{SO}}_4^{2-} $\end{document} sources, including atmospheric precipitation, soil sulfate, pyrite, and gypsum. Hydrogen and oxygen isotopes reveal that the hot spring thermal water is recharged by atmospheric precipitation, with a recharge elevation of 2646 m to 3045 m. Calculations using the silica geothermometer and the silica enthalpy model estimate the deep reservoir temperatures to be approximately 232 °C to 275 °C, with shallow reservoir temperatures around 180 °C and a cold water mixing ratio of 74% to 82%. The study reveals that atmospheric precipitation in this area migrates into deep reservoirs along water-conducting pathways such as fault fracture zones or bedrock fissures. It leaches soil sulfate, metamorphic rocks, and localized salt layers along the way. Once heated at deep reservoirs, it flows upward and ultimately emerges as springs after mixing with infiltrating cold water near the surface. This study establishes a genesis model for high-sulfate geothermal water, providing a scientific basis for developing and utilizing geothermal resources in the Namcha Barwa syntaxis.