Li-S isotopic constraints on differential enrichment mechanisms of deep brine lithium resources in the Yahu and Mahai areas in Qaidam Basin, Tibetan Plateau

The growing global demand for lithium resources has positioned deep brine lithium deposits in the Qaidam Basin as a focal point for resource exploration. However, the hydrochemical characteristics and source of lithium in these brines remain poorly understood, and the relationship between reservoir heterogeneity and the spatial distribution of lithium enrichment has not been systematically defined. This study focuses on the deep brines from the Yahu and Mahai areas, integrating hydrochemical analysis with lithium and sulfur isotope tracing to elucidate the genetic processes and differential lithium enrichment mechanisms. Results indicate that the fissure-pore brines of the Yahu Anticline exhibit high lithium concentrations (14.04–41.73 mg/L), low δ7Li values (10.24‰–13.93‰), and high δ34S values (44.00‰–46.55‰), reflecting a closed tectonic environment influenced by deep hydrothermal activity and intense bacterial sulfate reduction. In contrast, the sandy gravel–pore brines from the Mahai area display low lithium content (2.45–4.45 mg/L), high δ7Li values (24.18‰–44.50‰), and low δ34S values (12.02‰–16.57‰), indicative of a semi-closed system dominated by surface runoff and shallow brine recharge, as well as clay mineral adsorption. Mineral saturation index and hydrochemical coefficients further confirm intense water–rock interaction in Yahu, versus evaporative concentration and potash salt dissolution in Mahai. These findings establish distinct metallogenic models: Yahu brines represent a fault-controlled, hydrothermal-recharged system with significant lithium potential, whereas Mahai brines, though lithium-depleted, have formed substantial clay-type lithium deposits and possess valuable potassium resources. This study provides critical insights into the differential enrichment mechanisms of lithium in deep brines and offers strategic guidance for future resource exploration and development in the region.