Mineralogy and geochemistry of the Late Triassic coal from the Caotang mine, northeastern Sichuan Basin, China, with emphasis on the enrichment of the critical element lithium

Comprehensive research on the concentrations, modes of occurrence, and origin of critical elements in coal is significant from an economic point of view. Several previous investigations have attributed the host of critical elements such as Li, Ga, Nb, Ta, Zr, and Hf in coals to aluminosilicate minerals derived from terrigenous input, while Li of hydrothermal origin has rarely been found in coals. The Late Triassic coal of the Caotang mine, northeastern Sichuan Basin, China of this study is a high-ash, medium-sulfur, low volatile bituminous coal. The dominant minerals in these coals are illite, calcite, kaolinite, and quartz, with minor chlorite, sulfate minerals, paragonite, pyrite, and anatase, along with traces of phosphate minerals. In comparison with the world hard coals and Chinese common coals, the critical element Li is ∼20-times higher in the Caotang coal. The Li concentrations (range 89.1–520 μg/g, 291 μg/g on average) and the abundance of chlorites in coal low-temperature ashes (LTAs) show a strong correlation (r = 0.88), indicating that chlorites are the dominant host of Li in the coal. The chlorite minerals in this study belong to three types: cookeite, chamosite, and a chlorite mineral of intermediate composition between them. Both the kaolinite-cookeite assemblage with a vermicular texture and Li-bearing chlorite occurring as fracture-fills, indicate that they formed by interaction of kaolinite with Li-rich solutions or directly precipitated from Li-bearing solutions during diagenetic or epigenetic processes. Based on geochemical indicators such as Al₂O₃/TiO₂ ratios, rare earth elements and yttrium (REY) distribution patterns, and the relationship between Al₂O₃/TiO₂ and Zr/TiO₂ and Nb/Yb ratios, the source-area compositions of Caotang coals are felsic-intermediate igneous rocks from the southeast Qinling thrust system, Longmenshan thrust belt, Hannan Upland, and Micangshan-Dabashan Uplift. The kaolinite-paragonite assemblage with authigenic minerals such as chamosite, anatase, calcite, barite, pyrite, quartz, and REY-rich minerals, as well as the REY enrichment patterns, suggest that the Caotang coals may have been subjected to a multi-stage injection of hydrothermal fluids. The Caotang coal is considered to be a promising source of the critical element Li. Furthermore, the concentrations of the critical elements Ga, Nb, and Ta in the coal increase its potential economic significance even further.