Elemental geochemical characteristics of coal and genesis of pyrite in a certain Coal Mine, Xinjiang, China

In order to reveal the occurrence characteristics and genetic mechanisms of pyrite in the low-sulfur coal, in this study, by taking the coal seams of a certain Coal Mine in the eastern Junggar Coalfield, Xinjiang as research object, using analytical techniques including the polarized light microscopy, scanning electron microscopy with energy-dispersive X-ray spectroscopy(SEM-EDS), X-ray fluorescence(XRF), inductively coupled plasma mass spectrometry (ICP-MS), and electron probe microanalysis (EPMA), we have systematically investigated the geological characteristics of the coal mine, the quality, mineralogical, and geochemical characteristics of the coal, and occurrence characteristics of pyrites in the coal, and particularly have discussed genetic mechanism of pyrite in the coal. The results demonstrate that pyrite and kaolinite are dominant minerals within the coal. Pyrite occurred in forms of granular aggregates, vein, and disseminated grains. The coal generally has low contents of trace elements. The rare earth element (REE) distribution patterns of the coal samples have positive Eu, Y, and Ce anomalies, indicating a type of LREE enrichment. The research suggests that this coal seam was formed in a terrestrial swamp environment. The freshwater influx, coal-forming plants, and subsequent hydrothermal fluid jointly supplied iron and sulfur sources of the coal. However, due to the insufficient supply of sulfur and iron and the weak oxidizing conditions, pyrite only developed locally within the coal seam. Granular pyrites were formed during the early diagenetic stage. The later hydrothermal leaching process resulted in the pervasive development of hematite-containing pores on surfaces of those granular pyrites. Vein-like pyrites were formed in the metazoic stage by the migration and precipitatation of fluids containing iron and sulfur along the pore-fracture system of the coal seam.