Geochemical and H-O-Sr-B isotope signatures of Yangyi geothermal fields: implications for the evolution of thermal fluids in fracture-controlled type geothermal system, Tibet, China

High-temperature hydrothermal systems are mainly distributed in the north-south graben systems of southern Tibet as an important part of the Mediterranean-Tethys Himalayan geothermal belt in mainland China. As the largest unit capacity and second stable operating geothermal power station in China, Yangyi is the fracture-controlled type geothermal field in the center of Yadong-Gulu Graben. In this paper, hydrogeological and hydrochemical characteristics, isotope composition (& delta;D and & delta;O-18, Sr-87/Sr-86 and & delta;B-11) of borehole water, hot springs, and surface river samples were analyzed. From the conservative elements (such as Cl- and Li+) and & delta;D and & delta;O-18 values, the geothermal water of the Yangyi high-temperature geothermal field is estimated to be of meteoric origin with the contributions of chemical components of the magmatic fluid, which is provided by partially molten granite as a shallow magmatic heat source. According to logging data, the geothermal gradient and terrestrial heat flow value of the Yangyi high-temperature geothermal field are 6.48 celcius/100 m and 158.37 mW m(-2), respectively. Combining the hydrothermal tracer experiment, Sr-87/Sr-86 and & delta;B-11 ratios obtained with gradually decreasing reservoir temperatures from the Bujiemu stream geothermal zone to Qialagai stream geothermal zone, we suggested the deep geothermal waters were mixed with local cold groundwater and then flow northeastward, forming the shallow reservoir within the crushed zone and intersect spot of faults in the Himalayan granitoid. Furthermore, in the process of ascent, the geothermal water is enriched in K+, Na+, and HCO3- during the interaction with underlying Himalayan granitoid and pyroclastic rocks that occur as wall rocks. The detailed description and extensive discussion are of great significance for the further exploitation and utilization of north-south trending geothermal belts in Tibet.

Automated summary

This paper analyzes the hydrogeological and hydrochemical characteristics of the Yangyi high-temperature geothermal field in southern Tibet, mainland China. It estimates that the geothermal water in this field is of meteoric origin with contributions from chemical components of magmatic fluid. The geothermal gradient and terrestrial heat flow values are determined to be 6.48 celcius/100 m and 158.37 mW m(-2), respectively. It suggests that deep geothermal waters mix with local cold groundwater and flow northeastward to form the shallow reservoir within the crushed zone and faults intersection of the Himalayan granitoid.