Hydrogeochemical mechanism of Li-Cs-rich thermal springs in the Karakoram Fault, Qinghai-Xizang Plateau: New insights from multiple stable isotopes

Abstract

High lithium (Li) and cesium (Cs) concentrations in the Qinghai-Xizang Plateau thermal springs pose environmental and health challenges, but their origins and enrichment mechanisms remain unclear. This study focuses on the Sogdoi geothermal field, located along the southern Karakoram Fault, to investigate these processes. Multi-isotope analyses (H, O, Li, Sr) reveal that Li and Cs predominantly originate from the host rocks, especially granitoids and meta-sedimentary rocks, rather than from magmatic fluids. We propose a two-stage enrichment process mechanism that includes water-rock interactions and steam loss (~22 %) as water ascends. The geothermal field contains three distinct spring groups, all sourced from a common parent geothermal fluid at ~7 km depth with temperatures around 200 °C. Geochemical variations across groups reflect different degrees of conductive cooling, steam loss, and cold water dilution. Group I (Li: 45.21-51.93 mg/L, Cs: 27.37-29.39 mg/L, ⁸⁷Sr/⁸⁶Sr: 0.717-0.725, δ⁷Li: 1.45-1.74 ‰) represents deep fluids with minor dilution. Group II (Li: 32.16-39.98 mg/L, Cs: 28.72-33.91 mg/L, ⁸⁷Sr/⁸⁶Sr: 0.710-0.712, δ⁷Li: 1.64-1.79 ‰) and Group III (Li: 21.49-22.37 mg/L, Cs: 6.56-6.79 mg/L, ⁸⁷Sr/⁸⁶Sr: ~0.709, δ⁷Li: 4.22 ‰) show increasing dilution effects. Our findings indicate that Sr isotopes are highly sensitive to cold water mixing, while Li isotopes demonstrate greater resistance but are more susceptible to mineral adsorption. The binary Li-Sr isotope mixing model is more effective than single-isotope models in discerning mixing effects. It is crucial to select samples that are minimally affected by cold water intrusion to accurately trace fluid sources and their evolution. The anomalous Cs concentrations across groups are likely due to adsorption processes, as evidenced by decreasing Cs/Na ratios. We propose that deep faults are crucial in forming Li-Cs-rich thermal springs by enabling the deep circulation of water. This study provides insights into the hydrogeochemical mechanisms of Li and Cs in geothermal fluids, contributing to resource exploitation and environmental management.

Keywords: Enrichment mechanism; Karakoram Fault; Li-Cs-rich thermal springs; Qinghai-Xizang Plateau.