Fluid geochemistry and geothermometry applications of the Kangding high-temperature geothermal system in eastern Himalayas

Abstract

High-temperature geothermal systems hold an enormous capacity for generating geothermal energy. The Kangding area is a typical high-temperature geothermal field in the Himalayan Geothermal Belt. Hydrogeochemical, gas geochemical and isotopic investigations were performed to identify and qualify the main hydrogeochemical processes affecting thermal water composition, including mixing and degassing, and then to estimate a reliable reservoir temperature. Nine water samples and four geothermal gas samples were collected and analysed for chemical and isotopic components. The results demonstrate the alkaline deep geothermal water is the mixtures of approximately 75% snow-melt water and 25% magmatic water. It is enriched in Na, K, F, Li and other trace elements, indicating the granite reservoir nature. The shallow geothermal water is the mixtures of approximately 30% upward flow of deep geothermal water and 70% meteoric cold water. High concentrations of Ca, Mg and HCO3 indicate the limestone reservoir nature. There is no remarkable oxygen isotope shift in the geothermal water since the rapid circulation is difficult to trigger off strong water-rock interaction. CO2 is the predominant geothermal gas, accounting for more than 97% of total gases in volume percentage. The concentration of CO2 degassing ranged from 0.4 mol L−1 to 0.8 mol L−1 via geothermometrical modelling. As a result, the geothermal water pH increased from 6.0 to 9.0, and approximately 36% of the total SiO2 re-precipitate. The sources of CO2 are the metamorphism of limestone and magmatic degassing based on the composition of carbon isotope. The appropriate geothermometers of Na-K and Na-Li yield reservoir temperature of 280 °C. The geothermometrical modelling, developed to eliminate the effects of CO2 degassing, yields temperature of 250 °C. The silica-enthalpy mixing model yields temperature of 270 °C with no steam separation before mixing.