Search

The eastern Himalayan syntaxis is the easternmost end of southern Tibet and undergoing the continental Indian-Eurasian collision. The occurrence of large amounts of geothermal waters with high temperatures along the regional active faults is a clear manifestation of an abnormal thermal structure of the crust. In order to further complement the distribution characteristics of hydrothermal activity in southern Tibet, geothermal waters were collected and the hydrochemical characteristics and genetic mechanisms were analyzed. In the eastern Himalayan syntaxis geothermal field, geothermal waters show different compositional features. The major ions are Na+, Ca2+ and HCO3−, SO42−, which are mainly controlled by water-rock interaction. Two peculiar hydrochemical characteristics of geothermal waters are proposed: ⅰ) the Cl− concentration of three water samples is abnormally high due to the likely contribution of magmatic water; ⅱ) the other three have a lower Cl− concentration but higher temperature because of flashing steam heating. The positive correlations between B, Li and Cl concentrations indicate that the geothermal waters originate in a parent geothermal liquid. The enthalpy-chloride diagram shows that the Cl− concentration and the enthalpy of the parent geothermal liquid are 283 mg/L and 1732 J/g (matching a temperature of 413.74 °C), respectively. Four cooling processes for the geothermal waters which ascend from reservoirs to surface were suggested, i.e., conductive cooling, adiabatic cooling, mixing with cold groundwater and the combination of these processes. The partial melting derived from crustal anatexis or crust-mantle mixing is the heat source for this geothermal field. The oxygen and hydrogen isotopic ratios indicate that the geothermal waters are meteorically-fed. The meteoric waters infiltrate via brittle tensional small-scale faults. According to the results of hydrochemical analysis, the evolution patterns of geothermal waters have been further summarized into seven patterns. Except for Pattern g, Patterns a to f are associated with the parent geothermal liquid.

A large number of thermal springs are widely distributed along the deep suture zones and faults in the north section of the Sanjiang Orogenic belt in southeast Tibetan Plateau, connecting the two high-temperature hydrothermal areas in southern Tibet and western Sichuan. Whether the genesis of all geothermal water in the north section of the Sanjiang Orogenic belt is consistent is still an unanswered question. In order to fill the research gap of the high temperature geothermal system in this region, in this study, we classified the geothermal springs into three geothermal fields (Jinsha, Lancang, and Nujiang geothermal fields) and attempted to find clues by comparing the hydrochemical and isotopic characteristics of geothermal fluids in the three geothermal fields. The δ2H and δ18O composition of geothermal springs suggest that all thermal springs are replenished by the meteoric water. Low content of trace elements and low degree of oxygen isotope shift indicate that the partial melting of the crust at a depth of about 20 km mainly provide heat for all thermal springs in the geothermal fields through conduction. But due to the difference of tectonic conditions, there are two formation patterns of geothermal waters in the study area, one was controlled by regional faults, the other was controlled by the folded complex. The geothermal waters controlled by regional faults were heated after deep circulation, and the reservoir temperatures calculated by the Silica-enthalpy mixing model are 175–200 °C and 200–225 °C, respectively. But the geothermal waters controlled by the folded complex were heated after shallow circulation, and the reservoir temperature is only 100–125 °C. Upon rising to the surface, the geothermal waters mixed with cold groundwater to different degrees. The dissolution of Na/K-silicates and carbonate rocks controlled the formation of Na-HCO3 and Ca·Mg-HCO3 type water, respectively. But dissolution of halite and gypsum (or anhydrite) in some areas can lead to further diversity of hydrochemical species, resulting in the formation of Na-Cl, Na-HCO3·Cl, Ca·Mg-SO4 and Na-HCO3.SO4 type waters.