Geochemistry of thermal fluids in the Yangbajain Area (Tibet/China)

The valley containing the Yangbajain geothermal field is an asymmetric graben running NNE-SSW, filled with glacial and fluvioglacial continental deposits. This tensile structure is bordered by two mountain ranges: (1) the Nyainquentanglha Range to the NW reaches a high altitude and is formed almost completely of metamorphic rocks, partially covered by Quaternary deposits; (2) the Tang mountains to the SE are characterized both by metamorphites and by sedimentary rocks and volcanites. The general outline is characterized by a nucleus of thermal outflows in the central part of the study area, where all deep wells and the most important manifestations are located, and which has a fairly homogeneous Na-K-Cl-HCO3 composition. These waters are recharged at 5000 m a.s.l. and have an underground residence time that certainly exceeds 30 years. The thermal fluids rise up along a front of a few kilometres on the graben's northern escarpment. In this area mixing could occur at different rates, between regional shallow waters (Ca-Mg-HCO3 type) and deep fluids (Na-K-Cl-HCO3 type). The resulting system is affected by further mixings, mainly along the borders of the actual productive area. The gas composition, varying from the prevailing CO2 to N2, confirms the previously suggested scheme. Both isotopic and chemical geothermometers indicated approximately 200 deg. C, a temperature slightly higher than that measured in the geothermal wells. Two other thermal areas have been recognized SW and NE of the presently exploited field. Na-K-HCO3 water and mainly CO2 gas are present in the southwest and can be considered as a secondary part of the main geothermal field. On the other side, the fluid outflowing in the northeastern area does not seem to be connected with the exploited geothermal system. (author). 18 refs, 5 figs, 10 tabs