Mineralogy and geochemistry of the Palaeogene low-rank coal from the Baise Coalfield, Guangxi Province, China

Some previous studies have investigated the enrichment origin of Sb and Cs in coal, but strong supportive evidence for the sources of the two elements in coal is still absent. The main aims of this study were to provide a better understanding of the geochemical and mineralogical characteristics of the Palaeogene No. 5 Coal from the Zhoujing coal mine, Baise Coalfield, Guangxi Province, southwestern China. The No. 5 Coal has a low rank (0.44% R_o,max; 46.96% volatile matter content), an average ash yield 19.37%, and a medium sulfur content (1.23% on average). The main minerals in the No. 5 Coal are kaolinite, illite, quartz, and pyrite. Bassanite (2CaSO₄•(H₂O)) was identified in the low-temperature ashes in varying proportions (2.7% - 37.6%). The mineralogy of the non-coal samples (partings, roof and floor strata) is similar to that of the coals but chlorite is additionally present and pyrite is largely absent in these non-coal samples. The modes of calcite occurrence in the partings indicates a terrigenous origin for this phase. Compared with average values for world low-rank coals, the No. 5 Coal is distinctively enriched in Sb (29.12 μg/g), Cs (9.71 μg/g), and U (15.61 μg/g). Compared with the average values for world clays, the parting and host rock samples have normal concentrations for most trace elements, with an exception of higher Sb (9.93 μg/g). The elevated Sb and Cs in the coal were derived from the detrital material shedding from the exposed middle Triassic strata, which host notable Au and Sb deposits in the surrounding region. The REY (rare earth elements and Y) distribution patterns for the coal benches, partings, roof and floor strata are generally characterized by enrichment in medium REY and positive Gd anomalies, both of which were due to acidic waters circulating within the coal basin. The mineral compositions and their association indicate that the depositional environment was weakly acidic at the early stage of peat accumulation and then changed to neutral or weakly alkaline in the later stage.