Sulfur Isotope Geochemistry Indicating the Source of Dissolved Sulfate in Gonghe Geothermal Waters, Northwestern China

The Gonghe region is characterized by the most active geothermal manifestation in Qinghai Province, northwestern China. The geothermal waters from three hydrothermal areas (Qiabuqia, Guide and Xinghai) of Gonghe have different hydrochemical and isotopic compositions. Nevertheless, most geothermal water samples fall into the area of evaporitic gypsum defined by the range of its δ34S-sulfate values and a few samples plot right on the border dividing the gypsum area and the atmospheric precipitation area, indicating that the formation of sulfate in Gonghe geothermal waters is commonly attributed to the dissolution of gypsum and/or anhydrite in host rocks, with a possible minor contribution of atmospheric precipitation. Moreover, the sulfate in Guide geothermal waters may originate partly from the oxidation of H2S at near-surface oxidizing environment. Sulfur isotopes are proved to be good indicators for identifying the geochemical genesis of geothermal water.