Lithium is used in rechargeable batteries due to its high electrochemical potential, and the rapid development of electric vehicles is exerting increasing pressure on reserves. Currently, pegmatites and brine deposits are the two main Li sources, although volcanic clay-type and carbonate-hosted clay-type that formed mainly in continental or coastal basins are receiving increasing attention due to their large size and potential as future Li sources. However, mechanisms of clay Li enrichment are still debated. Marine black shale at the top of the upper Carboniferous-lower Permian Maping Formation in the Nandan area, northern Guangxi, SW China, is enriched in Li to levels of similar to 1000 ppm. Knowledge of its genetic mechanism would contribute to understanding the geochemical behaviour and enrichment mechanisms of Li in surface environments. The different geological features of the shale, relative to the above clay types, suggest that the Nandan shale is a new type of Li-rich sedimentary rock. Its whole-rock Li contents are positively correlated with K2O and illite contents, indicating that illite (K0.7Al2[(Si,Al)(4)O-10](OH)(2)) is the carrier of Li in the shale, as supported by high shale Al, Si, K, and Li contents in the in situ images. Mineralogical studies have revealed that the Li-rich illite is of detrital origin and was formed during continental weathering. The widespread occurrence of framboidal pyrite and high contents of TOC in the shale indicate that Li-rich shales formed under anoxic depositional conditions. Our findings may provide reference material for the study of the genesis of clay-type Li resources.