Chemical compositions and B-isotope systematics of rivers-to-terminal salt lakes in the Qaidam Basin (western China) and their implications for borate mineralization

The Qaidam Basin of western China hosts numerous salt lakes, some of which are rich in boron (B) resources. These salt lakes are separated by a central uplift and receive inflow from the Kunlun Mountains to the south and from the Qilian Mountains to the north, defining the South and North systems, respectively. The B concentrations of river water in both systems are significantly higher than those of the world’s major rivers; consequently, the salt lakes are expected to be enriched in B due to the discharge of high-B rivers. However, although the Da Qaidam and Xiao Qaidam salt lakes in the North System contain considerable solid borate resources, no solid borates are found in the South System despite its river water flux being much greater. In this study, samples from various water bodies—including river water, salt lake brines, and fault-related Ca-rich spring waters—were collected and analyzed for chemical compositions and δ11B values. The results, combined with literature data, suggest that B-enrichment in river waters is primarily driven by hot-spring brines from mountainous areas (distinct from Ca-rich spring waters within the basin). In the South System, the input of Ca-rich brines reduces the B concentration relative to total dissolved solids (TDS), typically to <270 mg/L, which is unfavorable for B-enrichment and borate precipitation. Thermodynamic modeling and evaporation experiments indicate that salt lake brines from neither system can precipitate borates readily. Nonetheless, the dilution of highly concentrated brines in certain North System lakes can promote borate precipitation by increasing pH values, as supported by thermodynamic calculation. Ultimately, our results demonstrate how the coupling of tectonic hydrochemistry and hydrological fluctuations shapes the distribution of B resources in hyper-arid basins.