Mechanisms of lithium and cesium enrichment in the Semi-Dazi geothermal field, Qinghai-Xizang Plateau: insights from H–O–Li–Sr isotopes

Abstract

Hot springs in the southern Qinghai-Xizang Plateau show anomalous lithium (Li) and cesium (Cs) enrichment, but the mechanisms driving this enrichment remain poorly constrained. Using multi-isotope tracers (H, O, Li, Sr), we investigate the Semi-Dazi geothermal field, which hosts the Plateau’s highest recorded geothermal Cs concentrations. The system comprises two geographically separated geothermal areas: Semi and Dazi, spaced ~ 15 km apart, displaying distinct hydrogeochemical signatures. Semi hot springs show significantly higher Li (34.2 to 35.6 mg/L) and Cs (49.8 to 52.7 mg/L) concentrations than Dazi (Li: 11.4 to 21.1 mg/L; Cs: 21.5 to 37.7 mg/L). Isotopic contrasts further differentiate the areas: Semi exhibits elevated δ⁷Li (1.53 to 1.91 ‰) and lower ⁸⁷Sr/⁸⁶Sr (0.739 to 0.741), whereas Dazi shows δ⁷Li values of − 0.25 to 1.24 ‰ and ⁸⁷Sr/⁸⁶Sr ratios of 0.742 to 0.759. Two key processes govern enrichment: (1) atmospheric recharge infiltrates Li–Cs-rich strata, where high-temperature water–rock interactions (217 °C at Semi and 197 °C at Dazi reservoirs) mobilize these elements into geothermal waters; (2) phase separation during ascent causes differential steam loss (Semi: 24%, concentration factor 1.32; Dazi: 8 to 21%, 1.08 to 1.26). Secondary processes (cold water mixing, conductive cooling, mineral adsorption) further modify surface hot springs geochemistry. Semi-Dazi geothermal field illustrates how a shared geothermal system can yield chemically distinct fluids from separate reservoirs characterized by differing hydraulic connectivity and circulation pathways. Geyserite deposits and high reservoir temperatures suggest that a crustal partial melt layer adds extra heat, intensifying water–rock reactions. The occurrence of Li–Cs-rich springs on the Plateau is intrinsically linked to elevated concentrations of these elements in underlying crustal source rocks and spatially associated with deep, extensive fault systems, particularly at fault convergences. These findings underscore the necessity of multi-isotope models for interpreting geothermal Li–Cs anomalies in continental collision zones, with implications for strategic mineral exploration.