Fluid geochemical constraints on the heat source and reservoir temperature of the Banglazhang hydrothermal system, Yunnan-Tibet Geothermal Province, China

Abstract Longling, located in the Yunnan-Tibet Geothermal Province of China, is characterized by the wide distribution of hydrothermal areas, among which the Banglazhang system is the most geothermally active and marked by intensive hydrothermal activities. There is a lack of neutral Cl waters at Banglazhang, and the geothermal waters are mainly Na-HCO 3 type with low but comparable SO 4 and Cl concentrations. The SiO 2 concentrations of the Banglazhang geothermal waters indicate equilibration in a shallow reservoir with a temperature around 150 °C. While dilution by Mg-rich cold groundwaters complicates geothermometry, the Na-K geothermometer suggests that this shallow reservoir is fed by geothermal fluid from a deeper reservoir above 200 °C. The geophysical investigations made in the neighboring regions of Banglazhang indicate that the Banglazhang system is likely to be located above the margin of a comparatively shallow-seated magma chamber. However, the hydrochemical and O, H and S isotopic evidence doesn't imply a mixing of magmatic fluid with Banglazhang geothermal waters. The geochemical scenario below Banglazhang is quite different than some typical magmatic-hydrothermal systems in China (such as Yangbajain and Rehai) where the geochemistry of geothermal waters are strongly affected by the input of magmatic fluid. The efforts made in this study might be suggestive for identifying the geothermal and geochemical origin of a large number of unexplored high-temperature hydrothermal systems in remote Yunnan and Tibet of China whose heat sources and geological conditions are unclear.