Magmatic fluid input controlling the geochemical and isotopic characteristics of geothermal waters along the Yadong-Gulu rift, southern Tibetan Plateau

Abstract

Geothermal resources, regarded as sustainable green energy, are attracting increasing attention. The southern Tibetan Plateau possesses the most abundant intermediate-high geothermal resources in China and has enormous potential for geothermal exploration. Geothermal waters from the Yadong-Gulu Rift (YGR), which is considered as the most active geothermal belt of the plateau, are studied based on hydrochemical and isotopic characteristics to understand the spatial variations of fluid origin, evolution, and circulation mechanism as well as heat sources. The hydrochemical characteristics of cold springs and surface waters are consistently characterized by Ca-HCO3 dominated (Ca-HCO3 and Ca-HCO3·SO4) waters while those of geothermal springs present extensive variability with Na-Cl dominated (Na-Cl·HCO3, Na-HCO3·Cl, Na·Ca-Cl·HCO3) and Na-HCO3 waters. The main geochemical processes controlling the geothermal springs are combinations of silicate weathering, carbonate dissolution, and cation exchange. The characteristics of trace alkali metals and B indicate they undergone common/similar deep process and the potential input of magmatic degassing. The δD and δ18O values suggest that cold springs and surface waters are recharged by meteoric water, and geothermal springs are replenished by meteoric water, snow-melt, and magmatic fluid. The mass balance model reveal that dominant carbon sources for geothermal waters can be deeply derived CO2 (i.e., magmatic source and metamorphic source), contributing from 77% to 99%. Based on empirical chemical geothermometers, Si-Enthalpy mixing model, and geothermometrical modelling, the calculated reservoir temperatures are highly variable (137-312 °C). The circulation depths of geothermal waters are 2.45-6.85 km. A conceptual model is proposed to reveal the deep groundwater circulation.