Abstract:

Study area is located in the Nimu—Naqu hightemperature geothermal belt, within the MiddleNorth section of the DangxiongYangbajain—Duoqingcuo active tectonic belt. Eleven unexploited geothermal fields are selected for this study, the hydrochemistry, hydrogen and oxygen isotope data, geothermal gases and their isotope data are collected and gathered from the study area, and the comparison with shallow and deep reservoirs in Yangbajain are conducted, in order to improve the study of genetical mechanism of the hightemperature geothermal systems and provide guidance for exploration and drilling in the area. Based on the analytical data of hydrochemistry of geothermal fluids, the chemical types of the high temperature thermal water are mainly of Cl-Na or Cl·HCO₃-Na type, and the lowtemperature thermal water is mainly of HCO₃-Na type. There is likely a positive correlation between Li, F, SiO₂, HBO₂ and Cl, which indicates the possible mixing of magmatic water. From stable isotope data, the thermal water showed mainly meteoric recharge. The thermal water show slight “18O drift” in Dongweng, Ningzhong hightemperature geothermal fields. The phenomenon is more obvious in Gulu geothermal field than the others. The tritium contents are less than 1 TU in Ningzhong, Gulu and Luoma, indicating mainly submodern water recharge, the trtium content at Yuela is 8.4 TU, which can be inferred to modern water recharge. The underground thermal water from other geothermal fields is the hybrid water recharge of submodern with recent water. The geothermal gases are mainly composed of carbon dioxide and nitrogen; there is a good negative correlation between the contents of carbon dioxide and nitrogen. Judging from the carbon isotopes and helium isotopes from carbon dioxide, combining with comprehensive index analysis, the source of the geothermal gases is mainly derived from the crust. However, the monitored R/Ra values are greater than 1 in Ningzhong and Yuzhai,showing that there must be a significant release of mantle material. The proportion of mantle helium ranges from 2.73 to 30.93 percent. Finally, the comprehensive study of the genesis model for typical, high-temperature geothermal field in the study area, will provide an important scientific basis for the further research as well as the development and utilization of the geothermal fields.

Key words: Key words: Tibet, geothermal fluids, gases, mantle fluids, genesis