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

Abstract

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 (δ⁴¹K), 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 × 10³ mg L⁻¹ to ∼21.12 × 10³ mg L⁻¹. A distinct [K⁺] hierarchy emerges: river samples < freshwater lakes < semi saline lakes < brine lakes < intercrystalline brines. Concurrent δ⁴¹K 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 δ⁷Li and δ¹¹B datasets. Multivariate isotopic analysis (δ⁴¹K-δ⁷Li-δ¹¹B) 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, δ⁴¹K signatures show systematic differentiation between two major brine types. The δ⁴¹K 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 δ⁴¹K is a powerful tracer not only for indicating changes in sources but also for reflecting the evolutionary processes of brine.