Origin of lithium-rich salt lakes on the western Kunlun Mountains of the Tibetan Plateau: Evidence from hydrogeochemistry and lithium isotopes

Abstract

Lithium-rich salt lakes have attracted much attention as important lithium resources. The source and enrichment mechanisms of lithium in salt lakes are fundamental to uncovering the genesis of brine-type lithium deposits. However, the origin of lithium in salt lakes has been controversial, especially for lithium-rich brines associated with hard rock lithium ore has not been concerned by scholars. In this study, for the first time, we investigated the lithium isotope and hydrochemical composition characteristics of a lithium-rich salt lake (Kushui Lake) and a series of surrounding water samples in the lithium pegmatites district of West Kunlun, Xinjiang, China. Hydrochemical composition shows that different samples in the basin are rich in Li, Na, and K and poor in Mg and Ca. The water chemistry of the Kushui River, the largest river within the catchment, changes from carbonate type to sulfate subtype through evaporation. The precipitate of sulfate and chloride minerals is also gradually increasing. In addition, Li, B, Sr, Rb, Cs, U, Ti, V, Mn, Cu, Zn, Cr, Co, Ni, and REE are abnormally enriched in the rivers and lakes of the basin, especially in the spring water, which is several times higher than in the rivers. Likewise, dissolved Li and δ7Li showed ranges of 0.01–287.86 mg/L and 3.28–16.44 ‰, respectively. These data were used to determine the origin of lithium and its enrichment process in salt lakes. The results show that the source of Li in Kushui Lake is closely related to the surrounding felsic volcanic rocks, pegmatite-type lithium ore and deep fluids, whereas the water–rock interaction in the deep crust and the chemical weathering in the supergene environment play an influential role during the formation of Li-rich fluids. In addition, the spring samples in the northwestern of Kushui Lake have inherited the lithium isotopic signature of lithium-bearing source rocks from the water–rock reaction process in the deep crust, allowing us further to isolate the source of Li in the target waters. With the movement of water, lithium exhibits a trend of continuous enrichment under the influence of evaporation and mixing. At different stages of evaporation, the Li/Na ratio in water in this catchment shows a trend of increasing, decreasing, and increasing due to the adsorption of 6Li by secondary minerals and the crystallization of Na. It suggests that strong evaporation under arid conditions is the main mechanism responsible for the rapid enrichment of Li in salt lakes. Moreover, the positive relationship between Li content and the δ7Li value of rivers along the discharge path indicated that the adsorption of 6Li by secondary minerals still existed even at a minor watershed scale, resulting in the fractionation of lithium isotopes in river waters. This study may contribute to a deeper understanding of lithium sources and their enrichment processes in Li-rich salt lakes. In addition, it is also beneficial to search for new lithium resources based on the source relationships among the various lithium reservoirs.