The role of geological fluids on the distribution of lithium in anthracite, an example from the Yangquan Mining District, Qinshui Basin, northern China

Abstract

Coal with abundant kaolinite and illite has become an important target for the exploration of potential coal-hosted lithium deposits. The No.15 coal from the Xinjing Mine, northern China, is rich in kaolinite, illite, and NH₄-illite. This study comprehensively applied multi-scale mineral analysis and in-situ micro-regional characterization methods to analyze the lithium content distribution in clay minerals, while elucidating the genetic mechanisms of clay minerals and their interactions with geological fluids. The genesis of these minerals and their interaction with geological fluids during thermal evolution were analyzed using whole-rock and in-situ geochemical methods. Lithium is enriched in the No.15 coal (42.2 μg/g). The Li concentration in clay minerals in the No.15 coal seam follows a descending order of: cryptocrystalline kaolinite (649.9 μg/g) > detrital kaolinite (366.1 μg/g) > cell-filling kaolinite (94.8 μg/g) > illite (16.1 μg/g) and NH₄-illite (21.1 μg/g). Cryptocrystalline and detrital kaolinite are thus the primary hosts of Li. Whole-rock and mineral geochemical data indicate detrital input as the main source of Li. The similarities in the REE distribution patterns between kaolinite in coal and Yinshan granite suggest that kaolinite, a key weathering product of acidic magmatic rocks in humid sedimentary systems, may have originated from the Yinshan Mountain granite. The presence of seawater and organic acids within coal-bearing strata collectively facilitate the illitization of kaolinite at temperatures of 120–140 °C. The negative correlation between Gd anomalies and Sr/Ba ratios indicates that the interaction between groundwater and seawater has established a geochemical barrier, which facilitates the enrichment of Li in coal.