Modes of occurrence and origin of mineral matter in the Palaeogene coal (No. 19-2) from the Hunchun Coalfield, Jilin Province, China

Previous investigations on minerals and elements in coals from northeastern China are very scarce. This paper investigates the mineralogy and geochemistry of the Palaeogene No. 19-2 Coal, parting, and host rocks (roof and floor strata) in the Baliancheng mine, Hunchun Coalfield, Jilin Province, northeastern China. Samples collected from this coalfield were analyzed using low-temperature ashing plus X-ray diffraction analysis, scanning electron microscopy in combination with energy-dispersive X-ray spectrometry, X-ray fluorescence spectrometry, and inductively coupled plasma mass spectrometry. The investigated coal is of subbituminous A to high volatile bituminous rank (0.50-0.56% R-o,R-ran) and has a low sulfur content (0.25% on average). The mineral assemblage of the coal is dominated by kaolinite, illite and expandable clays, and quartz, with a trace of albite. In comparison with low-rank coals worldwide, the No. 19-2 Coal is enriched in Cs, V, and Mo. The minerals and elements in the coal and its roof and floor strata were mainly derived from the Mesozoic (Jurassic-Lower Cretaceous) intermediate-felsic volcanic rocks surrounding the coal basin. The parting within the coal seam is identified as a tonstein (bed of altered volcanic ash) based on its mineral composition, dominantly of vermicular and book-like kaolinite (plus smectite, high-temperature quartz, sanidine, apatite, and ilmenite), and its persistent distribution throughout the coal deposit. Based on these features and its chemical composition, this tonstein is deduced to have been derived from a Paleogene intermediate-felsic volcanic ash, which is characterized by some adaidtic signatures of trace elements, and terrigenous materials. Normalized to the upper continental crust, the investigated samples (particularly the tonstein layer) show positive Eu anomalies inherited from the source rocks. This is in contrast to the coals, host rocks, and tonsteins with intermediate-felsic compositions in many other coal deposits, which are generally characterized by negative Eu anomalies. Geochemical and mineralogical features also indicate that the sedimentary materials of the tonstein, and, to a lesser extent, the floor horizon, were of aeolian origin, but those of most coal plies and roof strata were the result of aqueous deposition.