Unusual boron isotopic value and hydrochemical characteristics of thermal springs indicating magmatic fluids upwelling along Cuona-Sangri rift in the Tibet (China)

Abstract

Geothermal systems located in southern Tibet exhibit a considerable amount of potential energy and mineral resources. Thermal springs from Cuona Sangri Rift (CSR; China) are Cl-Na type and exhibit notable concentrations of minor elements (B, Li, Rb, and Cs) and unusually low δ¹¹B values. This study aims to explore the source(s), and physico-chemical processes controlling the chemical and isotopic features of the thermal springs in relation with the active Cuona-Sangri rift zone (CSR). In order to trace the origin of both water and solutes in the CSR springs, we characterized the major and minor element compositions, along with the stable isotope ratios (δ²H-δ¹⁸O, δ¹¹B), of the spring water. The anomalous abundance of B, Li, Rb, and Cs is possibly related to magmatic fluid input rather than just dependent on water-rock interactions. According to boron isotopes, the magmatic fluid component accounts for approximately 1.74 % to 8 % of the thermal water. A conceptual model is proposed to elucidate the intricate patterns of geothermal system deep groundwater circulation controlled by the deep fault zone.