Mineralogical and geochemical anomalies of late Permian coals from the Fusui Coalfield, Guangxi Province, southern China: Influences of terrigenous materials and hydrothermal fluids

The Late Permian coal in the Fusui Coalfield of southern China is characterized by high Fe-sulfide and organic sulfur contents (2.60 and 2.94%, respectively). Trace elements including Zr (354μg/g on average), Hf (9.18μg/g), Y and rare earth elements (REY, 302μg/g), Li (97.9μg/g), and Cs (7.02μg/g) are significantly enriched in these coals. In addition to Hg and Se enrichment in the roof and floor of the coal, fluorine, Mo, and U are enriched in the roof; the floor is rich in Cl, S, Fe, Pb, and Cd. Compared to the upper continental crust, REY in the parting and coal bench samples are characterized by heavy-REY and light-REY enrichment, respectively; the coals, partings, and host rocks (roof and floor) have negative Eu anomalies. The coal benches have higher ratios of U/Th, Yb/La, Nb/Ta, and Zr/Hf, and more abundant heavy rare earth elements than their adjacent partings. These geochemical anomalies are attributed to the composition of terrigenous materials derived from the Yunkai Upland, multi-stage (syngenetic and epigenetic) hydrothermal fluid activities, and intensive leaching and re-distribution of lithophile elements from partings to the underlying coal benches. Both the organic and sulfide sulfur are also derived mainly from the hydrothermal fluids rather than the marine influence.The minerals in the samples studied, including kaolinite, quartz, and REE-bearing minerals, are of both terrigenous and hydrothermal origin. Al-oxyhydroxides, crystalline FeSO₄(OH), water-bearing Fe-oxysulfate, and goyazite were derived from the hydrothermal activity. Kaolinite, quartz, REE-bearing minerals, and apatite from the sediment source region were also subjected to destruction by hydrothermal fluid leaching.