Mineralogy and elemental geochemistry of pyrite coal-balls, Datong Coalfield, Shanxi Province, China: Implications for differentiation mechanism

The pyrite coal-balls found in No.9 coal of Taiyuan Formation in Dongzhouyao Mine of Datong Coalfield were selected to investigate its mineralogy and elemental geochemistry using optical microscope observation, scanning electron microscopy combined with an energy-dispersive X-ray spectrometer (SEM-EDX), inductively coupled plasma mass spectrometry analysis (ICP-MS), laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS), and electron probe micro-analysis (EPMA). Due to the high content of pyrite, the major-element oxides in the coal-ball samples were mainly Fe2O3, with an average of 62.68 %. LA-ICP-MS results showed that coal-balls were enriched in Tl (CC = 32.79), Pb (CC = 9.22), and Mo (CC = 2.07), while the coals in the same layer were enriched in Ga (CC = 5.31), Tl (CC = 3.66), and Li (CC = 2.27). Affinities to S or Fe and the variation of elemental concentration in the environment during the formation of coal-balls were the primary factor of the elemental differentiation between coals and coal-balls, as well as within coal-balls. The coal-balls formed in four stages in a marine-continental transition environment, with Fe from terrestrial sources and S from marine phases, and its physical origin is basically consistent with that of some Carboniferous-Permian coals in the Datong Coalfield. According to the formation sequence and geochemical characteristics, four periods of pyrite coal-balls evolution have been proposed.