Lithium enrichment and modes of occurrence in carboniferous–permian coals from western Shandong, North China: constraints from TIMA and TOF-SIMS

The Carboniferous-Permian No. 3 coal seam in the Luxi coalfield, western Shandong Province, North China, was investigated to clarify lithium enrichment, mineral hosts, and potential provenance controls. Coal quality, mineral assemblages, elemental geochemistry, petrography, TIMA, and TOF-SIMS mapping were integrated for coal and associated gangue samples. The coal is low-ash, ultra-low-sulfur, medium- to high-volatile, medium-rank bituminous coal. Among the analyzed critical elements, lithium shows the strongest enrichment relative to world coal averages, with an average concentration coefficient of 4.78 and a maximum of 21.45. TIMA results indicate that kaolinite dominates the inorganic fraction, with subordinate pyrite, hematite/magnetite, and chlorite. Petrographic observations and Al2O3/TiO2 ratios greater than 21 point to substantial input from felsic volcanic and terrigenous detrital sources. TOF-SIMS images and element correlations show that lithium is mainly associated with aluminosilicate phases rather than organic matter or pyrite. However, the weak correlation between lithium and bulk kaolinite abundance suggests that lithium may occur in several clay components, including illite or mixed-layer clays and possibly Li-bearing chlorite/cookeite-like phases. We propose that lithium enrichment in the Luxi coal measures reflects a coupled process involving felsic volcanic ash or detrital supply, fluid-mediated leaching and redistribution in a peat-swamp to paralic setting, and diagenetic incorporation into clay minerals. The results support the potential of Luxi coal and gangue as unconventional lithium-bearing materials, while emphasizing that resource evaluation should also consider grade heterogeneity, mineral hosts, processing pathways, and selective separation costs.