Major hydrogeochemical processes controlling the composition of geothermal waters in the Kangding geothermal field, western Sichuan Province

The Kangding geothermal field, located in western China, is a high-temperature geothermal system potentially rich in geothermal resources. In this study, we employed various hydrogeochemical methods to gain more insight into the heat source and cooling processes involved in forming various thermal springs in the geothermal field. The study showed that though the majority of the samples analyzed were immature in terms of mineral aqueous equilibria, coupling classical geothermometers with the FixAl method enabled more reliable reservoir temperature estimations. A deduction from the silica-carbonate, the chloride-enthalpy, and the silica-enthalpy mixing models indicated that the parent geothermal fluid exists beneath this study area. The thermal waters discharged from the Kangding geothermal field originate in the same deep reservoir; and the parent geothermal fluid has a temperature of about 260 degrees C, with a Cl- concentration of 1056 mg/L. Isotope (delta D and delta O-18) studies confirmed the magmatic heat effect on the parent geothermal fluid. Also, though all the thermal spring waters in the field are derived from the parent fluid, they undergo different cooling processes during ascent to the earth's surface. However, the thermal spring waters in both the Yulingong and Erdaoqiao geothermal sites were mainly formed by the mixing of the parent geothermal fluid and infiltrating groundwater. The thermal spring waters in the Erdaoqiao also had the highest levels of Ca2+, Mg2+ and TIC (total inorganic carbon) due to the presence of carbonate in its geologic stratum. The thermal springs at Zhonggu formed as a result of the CO2 condensate, consisting of snow-melt water and meteoric water, mixing with the deep parent geothermal fluid. We attribute the absence of acid springs in the Kangding geothermal setting to the deep-seated magma chamber and a relatively small concentration of H2S in the deep thermal waters in the area.