Different phase lithium isotopic geochemistry of clay minerals in Qinghai-Tibet plateau lithium-rich salt lakes: implications for lithium enrichment mechanisms and resource potential

Lithium is a strategically important critical metal primarily hosted in brines and hard-rock deposits. Growing demand has prompted increasing attention to lithium in lacustrine clay sediments as a potential complementary host. The Qaidam Basin supplies ∼ 80% of China’ s lithium from brines, yet faces increasing supply pressure. Anomalous lithium enrichment has been reported in coeval lacustrine clays of the secondary Mahai Basin. This study investigates phase-specific lithium distribution and isotopic signatures (δ 7 Li) in Mahai Basin lacustrine clays using XRD and Tessier sequential extraction, combined with major/trace element and lithium isotope analyses of different phases. XRD results show that silicon-bearing minerals comprise ∼ 61% and clay minerals ∼ 26–27% of the bulk mineralogy. Lithium is overwhelmingly hosted in the silicate phase (89.5%). δ 7 Li values vary markedly among phases: water-soluble (−18.79 to + 26.99‰), ion-exchangeable (−16.44 to + 19.12‰), and silicate (+0.28 to + 3.65‰). The clays formed under cold, arid conditions dominated by physical weathering of felsic igneous rocks. Elevated water-soluble Li relative to ion-exchangeable Li is attributed to competitive adsorption inhibition. The water-soluble δ 7 Li signature is consistent with regional brines, and calculated fractionation factors (0.973–0.995) align with lithium adsorption onto montmorillonite. Silicate δ 7 Li values fall within the range of upper continental crust, indicating dominant continental weathering input. Integration with Sr and B isotopes reveals multi-source lithium contributions from silicate weathering of surrounding orogens, fluvial input (Yuka River), and deep Ca-Cl fluids. These findings clarify lithium sources, phase partitioning, and the role of competitive adsorption in lithium mobility, providing a scientific basis for understanding lithium enrichment processes in lacustrine clay sediments of salt-lake systems.