Hydrogeochemical Evolution Mechanism of Carbonate Geothermal Water in Southwest China

Geothermal water resources enriched in carbonate rocks in southwest China, and the geothermal water type and evolution mechanism are two controlling factors for the exploitation of geothermal water resources in this area. In this study, 17 geothermal water samples were collected from southwest China, and XRD, SEM, hydrogen and oxygen isotopes, correlation analysis, mineral saturation index method, and reverse geochemical simulation were combined to uncover the types and evolution mechanism of geothermal water.The results show that the main chemical types of geothermal water in the area are SO4·HCO3-Ca·Mg type water, followed by Cl-Na、Cl.SO4-Na.Ca type water, and the origin of geothermal water is atmospheric precipitation recharge, with its elevation of 564.87-1447.44 m. The main mineral composition of thermal reservoir is dolomite, followed by calcite, gypsum, halite and a small amount of clay minerals and quartz. During the process of hot water runoff, there is a strong water rock interaction with the surrounding rocks of the thermal reservoir. Ca2+, Mg2+, SO42−, HCO3− are mainly derived from the dissolution of carbonate minerals and gypsum salts. Especially, Mg2+ mainly comes from the dissolution of dolomite, HCO3− is from the dissolution of dolomite and calcite, Ca2+ is controlled by the dissolution of dolomite, calcite and gypsum, and SO42− mainly comes from the dissolution of gypsum. In addition, the abnormally high Na+, Cl−, and TDS in some geothermal waters in the area is mainly controlled by the cation exchange reaction of clay minerals or clay-like minerals in the salt-forming basins and thermal reservoirs of Sichuan, northwest of the study area. PHREEQC inverse modeling verifies that dolomite, gypsum, and halite-NaCl dissolution and partial weak cation exchange reactions were the main hydrogeochemical reactions. The research results provide a reference for the study of the hydrogeochemical evolution mechanism of geothermal water in carbonate rock areas, contributing to the rational development, utilization and protection of geothermal water resources in the study region.