Near surface lithium isotopic fractionation and lithium enrichment in coal-hosted Li-rich clays from the South China Block

Surficial low-temperature sedimentary clay-type Li deposit is one of three major types of Li deposits in the world; however, its formation and corresponding Li isotope fractionation mechanisms are still unclear. In this study, we demonstrate an unusually high Li concentration (89.1 ppm to 520 ppm) within a Late Triassic coal-hosted clay Li deposit from the Sichuan Basin, China. In-situ mapping of the Li distribution combined with isotopic analysis of Li occurring as clay-hosted ion-exchangeable and structurally bound Li was carried out to investigate low temperature Li migration and enrichment processes near the Earth's surface. The in-situ mapping shows that the dominant host of Li is fracture-fill and vermicular chlorite, while massive illite aggregate and fracture-fill paragonite contains lesser amounts of Li. Kaolinite and chlorite also absorbed a part of Li on surface/interlayer sites. The Li enrichment processes involve a contribution from detrital clastics (aggregate illite and kaolinite), especially felsic pyroclastics (vermicular-kaolinite) from collisional- and subduction setting, as well as the water-rock interactions during peat accumulation/burial stage between the acidic groundwater and primary Li-bearing clays. The primary kaolinite and illite was replaced with chlorite and paragonite accompanied with Li isotopic fractionation (Δ⁷Li: +2.67 ‰ to +15.4 ‰).