Hydrochemistry and H‐O‐C‐S Isotopic Geochemistry Characteristics of Geothermal Water in Nyemo‐Nagqu, Tibet

Abstract

AbstractNyemo‐Nagqu, Tibet, is rich in high‐temperature geothermal resources. The geothermal fields in Yangbajain and Yangyi as well as 11 unexplored geothermal fields along the geothermal belt from Nyemo to Nagqu were systematically investigated and the hydrochemistry data were collected from the whole field. Meanwhile, H‐O‐C‐S isotope data were obtained for the new fields, and H‐O isotope data for the Yangbajain and Yangyi fields. A comparison of the Nyemo‐Nagqu geothermal fields with those in the Yangbajain area shows that the types of high‐temperature geothermal water are dominated by Cl‐Na and Cl·HCO3‐Na, while the types of medium‐high‐temperature geothermal water are dominated by HCO3‐Na. The concentrations of Li, F, SiO2, and HBO2 in the geothermal water are positively correlated with Cl content, indicating possible mixing with magma water. The reservoir temperatures range from 90 to 270°C by geothermometers. Slight drifting of 18O was recorded at the Dongweng and Nyingzhong high‐temperature geothermal fields, while more significant drifting was recorded at Gulu. The geothermal water is mainly replenished by atmospheric precipitation. The low tritium contents (<1 TU) of the geothermal water from Nyingzhong, Gulu, and Luoma indicate that it is mainly replenished by sub‐modern (prior to 1952) water, while the high tritium content (8.4 TU) in Yuela implies modern water replenishment. Other geothermal fields are replenished by a mix of sub‐modern fresher water. The isotopic data in this study show that the carbon and sulfur in the geothermal water originates mainly from sediment leaching, with some of the carbon and sulfur having a deep origin.