Genesis of geothermal water in the Laochang area, eastern Yunnan Province: Constraints from hydrochemistry and C-H-O-S isotopes

The Mile-Shizong Fault Zone in Eastern Yunnan is rich in geothermal resources. However, due to the less research and unclear genetic mechanism, the sustainable development and utilization of geothermal resources in the region has been restricted. Taking the hot spring water and geothermal well water in Laochang area in the north section of the Mile-Shizong Fault Zone as the research object, this paper comprehensively applies the integration of field investigation, hydrogeochemistry and environmental isotope to study the characteristics and genetic mechanism of geothermal water in the study area. The pH values of the geothermal water are between 7.30～8.12 and TDS values are between 224～382 mg/L, which belongs to weakly alkaline fresh water. The hydrochemical types of geothermal water are HCO₃·SO₄－Ca type and HCO₃·SO₄－Ca·Na type. The geothermal water contains high contents of trace components such as Fe, As and Sb, and thus are not suitable for drinking. The δ¹³C value of HCO₃⁻ is between −3.31‰ and −7.79‰, and the calculated δ¹³C value of CO₂ involved in water-rock reaction is between −9.50‰ and −15.68‰, which is mainly from sedimentary organic matter. Ion ratio analysis and sulfur isotope characteristics show that the dissolution of carbonate minerals and gypsum is the main factor controlling ion concentrations of geothermal water. In addition, the oxidation and cation exchange of sulfide ore bodies in the fault zone have a certain impact on the composition of geothermal water. Hydrogen and oxygen isotope characteristic and ¹⁴C dating results show that the recharge origin of the geothermal water is the atmospheric precipitation with low temperature in the late Pleistocene, and the supply elevation is between 1984.9 m and 2283.9 m. The recharge area is located in the mountainous area with the elevation of 1984.9～2283.9 m around the study area. The mixing ratio of cold water calculated by silicon enthalpy equation is between 71.9% and 82.4%. The heat reservoir temperature calculated by silicon enthalpy equation and corrected quartz geothermal temperature scale is between 87.5℃ and 135.7℃. The circulation depth of geothermal water is between 1538.0 m and 2502.0 m. The research results are helpful to improve the research level of geothermal water genesis in the Mile-Shizong low-temperature hot water zone in Eastern Yunnan, and provide theoretical support for the rational development and protection of geothermal resources in the area.