Potassium isotope constraints on brine sources and evolution in Qaidam Basin, Tibetan Plateau

Abstract

The differences in potassium isotopes between two different hydrochemical types (sulfate type and chloride type salt lakes) are very clear. The relative variation in potassium isotopes in chloride type salt lakes is relatively small (−0.57 ‰ to 0.22 ‰), whereas the potassium content is relatively high. Conversely, the range of potassium isotope variation in sulfate type salt lakes is much larger (−0.77 ‰ to 1.31 ‰), indicating that potassium isotopes can not only indicate changes in material sources, but also reflect salt lake brine evolution processes. • The δ 41 K values of the samples significantly differ with regard to [K] among different brine salts and different sample types, ranging from −0.77 ‰ to +1.31 ‰. • The δ 41 K values of brine lakes in the Qaidam Basin display regional distributions. • δ 41 K serves as a powerful tracer not only for indicating changes in sources but also for reflecting brine evolution processes. The genesis and evolutionary pathways of potassium resources in salt lakes form the scientific foundation for sustainable exploration and utilization of these critical mineral reserves. Potassium isotope system (δ 41 K), characterized by significant mass-dependent fractionation, has emerged as an effective tracer for investigating geological processes and material sources. This study presents a comprehensive geochemical investigation of brine salts, intercrystalline brines, and river waters collected from the Qaidam Basin on the northern Tibetan Plateau, China. The ion compositions, hydrochemical characteristics, and potassium isotopic compositions of the samples were analyzed. Analytical results reveal substantial spatial heterogeneity in potassium concentrations ([K + ]) across different sample types, ranging from less than 0.01 × 10 3 mg L −1 to ∼21.12 × 10 3 mg L −1 . A distinct [K + ] hierarchy emerges: river samples < freshwater lakes < semi saline lakes < brine lakes < intercrystalline brines. Concurrent δ 41 K values exhibit significant variability (−0.77 ‰ to + 1.31 ‰), demonstrating systematic correlations with both [K + ] concentrations and basin-scale structural features, as evidenced by comparative analysis with published δ 7 Li and δ 11 B datasets. Multivariate isotopic analysis (δ 41 K-δ 7 Li-δ 11 B) coupled with hydrochemical fingerprinting reveals a polygenetic potassium origin for Qaidam’s brine systems, involving: multiple mixture of rivers, deep groundwater via fractures (oil field brines, hot springs, etc.), and low-temperature weathering processes of K-rich rock. Notably, δ 41 K signatures show systematic differentiation between two major brine types. The δ 41 K values of chloride type salt lakes is relatively small (−0.57 ‰ to 0.22 ‰), whereas those in sulfate type salt lakes is much larger (−0.77 ‰ to 1.31 ‰). The results suggest that δ 41 K is a powerful tracer not only for indicating changes in sources but also for reflecting the evolutionary processes of brine.

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