Enrichment of critical metals (Li, Ga, and rare earth elements) in the early Permian coal seam from the Jincheng Coalfield, southeastern Qinshui Basin, northern China: With an emphasis on cookeite as the Li host

Abstract

The Qinshui Basin is one of the largest coal-bearing basins and the largest reservoir of commercial coalbed methane in China. Recent studies have also reported a potential coal-hosted Li deposit in the Pennsylvanian coal seam of the Jincheng Coalfield, Qinshui Basin. This study further investigates the mineralogy and inorganic geochemistry of the Permian No. 3 coal and associated rocks of the Jincheng Coalfield, to reveal the modes of occurrence and enrichment origins of critical elements including Li, Ga, and rare earth elements and yttrium (REY). The mineralogy of the coal is dominated by two groups of mineral assemblages, kaolinite/NH4-illite/paragonite and ankerite/chamosite/siderite. At least one intra-seam altered volcanic ash layer was identified in the coal seam. The No. 3 coal is highly enriched in Li, with the Li2O content in the coal ashes averaging 0.11 %, which is above the common cut-off grade of 0.1 % Li2O in claystone deposits. The primary host of Li in the coal seam is cookeite, the presence of which is confirmed by ToF-SIMS analysis, which is of hydrothermal origin. Rare earth elements and yttrium are also enriched in the coal (REY oxides average 919 μg/g, ash basis), occurring mainly as phosphate minerals (florencite and crandallite). The concentration of Ga in the coal (58.3 μg/g on average, ash basis) exceeds the suggested cut-off grade in coal (50 ppm, ash-basis), with diaspore and kaolinite being the primary hosts. Diaspore, mainly occurring as fracture infillings, appears to be of late-stage hydrothermal origin. Lithium, Ga, and REY in the No. 3 coal are highly enriched and the coal is a potential source of these critical metals, making it a target for more comprehensive exploration and potential development. The enrichment origin of Li and REY in the Early Permian coal is similar to that in the Pennsylvanian no. 15 coal of the same coalfield, both of which are likely due to hydrothermal fluids originating from magmatic activity during the Yanshanian movement.