Hydrothermal overprinting of the Li-rich strata deposited in the Mesoproterozoic Wumishan Formation, Hebei Province, North China

Lithium (Li), as a strategic critical metal, plays a pivotal role in the emerging energy landscape, particularly in the context of Li-ion batteries driving the new energy economy. Recently, Li-rich strata (with Li₂O > 0.3 wt%) have been discovered in the Mesoproterozoic Wumishan Formation in Hebei Province, North China, suggesting a prospective Li reservoir. This study investigates these Li-rich strata using geochemical and in-situ micro-analytical techniques to explore the occurrence of Li and the formation mechanism of Li-host minerals, aiming for a comprehensive understanding of the supernormal enrichment of Li. The Li-rich samples are predominantly composed of dolomite and quartz, followed by clay minerals such as illite, interstratified illite–smectite (I/S), and chlorite, with minor amounts of K-feldspar, albite, biotite, calcite, baryte, fluorite and fluorapatite. In-situ analysis and ⁷Li NMR spectroscopy reveal that Li predominantly occupies the octahedral sites within the structures of authigenic illite and I/S, while its absence in clastic illite, clastic chlorite, unaltered K-feldspar, and dolomite. The presence of veined minerals (e.g., fluorite, baryte, and calcite) and a strong positive correlation between Li and F imply that post-depositional hydrothermal fluids have significantly contributed to the formation of Li-host minerals. The paragenesis of these minerals suggests that Li-bearing illite has formed through the hydrothermal alteration of K-feldspar. These Li-bearing illites subsequently transformed into Li-bearing I/S, consisting of illite-rich I/S and smectite-rich I/S, under continuous hydrothermal alteration. Lithium could have been leached from the surrounding carbonate rock and tuff through water–rock interaction and subsequently enriched by post-depositional hydrothermal fluids in specific regions, leading to mineralization. These findings provide valuable insights for targeting exploration of this promising Li resource.