Hydrochemical evolution and formation mechanisms of geothermal systems across contrasting tectonic settings in Tibet, China

Abstract The Tibetan Plateau lies within the India–Eurasia collision zone and contains abundant geothermal resources with pronounced regional differences in formation mechanisms. Comparisons of geothermal system genesis across Tibetan tectonic units remain limited. Hydrochemical and isotopic analyses, geothermometry, and silica–enthalpy mixing models were applied to Longmu Co, Doma, and Yangbajing. The integrated dataset was used to identify principal recharge sources, assess reservoir water–rock equilibrium, and develop regional conceptual models of geothermal fluid circulation. Longmu Co waters in northern Qiangtang are of the Cl·HCO₃–Na·Ca type, mainly reflecting carbonate and evaporite dissolution. Doma waters in southern Qiangtang are predominantly HCO₃–Na, controlled by water–rock interaction and cation exchange. Yangbajing waters in the Lhasa terrane are Cl·HCO₃–Na, influenced by silicate and evaporite dissolution. Meteoric water supplies all three systems. Longmu Co receives recharge from surrounding mountains; Doma receives distant high-elevation precipitation and snow and ice meltwater. Yangbajing receives precipitation and meltwater from the Nyainqentanglha Mountains. Estimated recharge elevations are 5236–5497, 6240–6362, and 5086–5436 m at Longmu Co, Doma, and Yangbajing, respectively. Corresponding circulation depths are 5047–6806, 6653–7294, and 4867–5345 m. Reservoir temperatures are 145–196, 175–192, and 225–247°C, and cold-water mixing fractions are 51–61%, 57–63%, and 66–69%, respectively. Doma also shows a possible deep magmatic fluid signal. Conceptual models indicate a shared deep thermal background but contrasting heat-source mechanisms. Yangbajing is sustained primarily by intracrustal magma chambers; Doma and Longmu Co reflect mantle heat flow and localized intracrustal melts under differing structural controls. These comparisons inform geothermal exploration and resource assessment in high-elevation collisional orogens.