Geochemical characteristics and source mechanism of geothermal water in Tethys Himalaya belt

Geothermal resources are very abundant in Tibet. A very active geothermal zone called the Tethys Himalaya geothermal belt has been developed in the southern part of the Tibetan Plateau. This belt is one of the most intense geothermal zones in modern as well as in ancient period in China's mainland, accounting for over 80 percent of the geothermal resources in Tibet. Through field investigations and sampling analyses for 10 typical hot springs from the geothermal area, the hydrochemical characteristics and source mechanisms are discussed. According to the thermal reservoir temperature, the hydrochemical type and the concentration of typical rare and dispersed elements dissolved in the water, the hot springs can be classified into two types:one type includes Kawu, Qucangang, Chabaquzhen and Gudui hot springs, their thermal reservoir temperatures are higher than 120℃ and they belong to NaCl-HCO₃ type; some rare and dispersed elements such as Li, B and As are obviously enriched. The other type includes Xinqin, Zhegu and Quguo hot springs, their geothermal reservoir temperatures are relatively low (60-110℃); these springs show lower concentrations of elements of Li, B and As with water chemistry dominated by Ca-Na-HCO₃ and Na-HCO₃ type. The high concentration of water chemical composition in the first group is closely related to the deeper water circulation and the higher thermal reservoir temperature, and abnormal enrichment of Li, B and As in the hot springs are more likely to be related to the source of residual magmatic fluids. In contrast, the second group of hot springs mainly denotes a shallower water circulation depth and frequent cold groundwater replenishment and mixing. The formation and evolution of chemical compositions of water are mainly related to water/rock interactions.