Hydrochemistry and multi-isotopes for interpreting formation mechanisms of different-type geothermal waters in the Cuona-Woka Rift, southern Tibetan Plateau

Abstract

High-temperature geothermal activities are widely distributed in rift tectonic zones, where significant volumes of geothermal waters with diverse hydrochemical characteristics are exposed. However, it is still unclear whether these geothermal waters have different formation mechanisms, which hinders the efficient exploitation and utilization of geothermal resources. Hence, this study investigates 41 geothermal water samples from the Cuona-Woka Rift (CWR) on the Tibetan Plateau to elucidate their hydrogeochemical evolutions and formation mechanisms. These geothermal waters are distributed along normal faults (N–S) and thrust faults (E–W), with discharge temperatures ranging from 34.0 to 85.5 °C. Self-organizing map classification identifies three distinct hydrochemical groups: Group 1 (Cl-Na and Cl·HCO₃-Na), Group 2 (HCO₃·Cl-Na), and Group 3 (SO₄·Cl-Ca·Na). The δD and δ¹⁸O values indicate that meteoric and snow-melt waters are the dominant recharge sources for geothermal waters, with magmatic water contributions ranging from 18 % to 24 % (Group 1) and 12 % to 21 % (Group 2). The hydrochemical composition is primarily controlled by silicate and carbonate mineral dissolution, gypsum leaching, and cation exchange, with a higher contribution rate than the mixing of magmatic waters. All geothermal waters originate from the same deep sources, with Groups 2 and 3 undergoing mixing with 68 %–88 % and 57 %–70 % shallow cold groundwater, respectively. The significantly enriched trace alkali elements (Li, Rb, and Cs) in Group 1 are attributed to the input of crustal magma melts. Deep reservoir temperatures are estimated at 251–270 °C (Group 1), 226–229 °C (Group 2), and 189–194 °C (Group 3) based on empirical chemical geothermometers, silica-enthalpy mixing model, and geothermometric modeling. The maximum circulation depths are 4.8–5.2 km, 4.3–4.4 km, and 3.5–3.6 km, respectively. Three genesis conceptual models controlled by rift structures are proposed: melt intrusion heating type, hot–cold mixing heating type, and steam heating type. These findings will enhance the understanding of the origin of rift-type geothermal waters and provide valuable insights for the global exploitation and utilization of high-temperature geothermal resources.