Mineralogical characteristics of late permian coals from the Yueliangtian coal mine, Guizhou, southwestern China

Abstract

This paper reports the mineralogical compositions of super-low-sulfur (Yueliangtian 6-upper (YLT6U)) and high-sulfur (Yueliangtian 6-lower (YLT6L)) coals of the Late Permian No. 6 coal seam from the Yueliangtian coal mine, Guizhou, southwestern China. The mineral assemblages and morphology were detected and observed by X-ray diffractogram (XRD), optical microscopy and field-emission scanning electron microscope (FE-SEM) in conjunction with an energy-dispersive X-ray spectrometer. Major minerals in the coal samples, partings and host rocks (roof and floor strata) include calcite, quartz, kaolinite, mixed-layer illite/smectite, chlorite and pyrite and, to a lesser extent, chamosite, anatase and apatite. The Emeishan basalt and silicic rocks in the Kangdian Upland are the sediment source for the Yueliangtian coals. It was found that there are several modes of chamosite occurrence, and precursor minerals, such as anatase, had been corroded by Ti-rich hydrothermal solutions. The modes of occurrence of minerals present in the coal were controlled by the injection of different types of hydrothermal fluids during different deposition stages. The presence of abundant pyrite and extremely high total sulfur contents in the YLT6L coal are in sharp contrast to those in the YLT6U coal, suggesting that seawater invaded the peat swamp of the YLT6L coal and terminated at the YLT6U-9p sampling interval. High-temperature quartz, vermicular kaolinite and chloritized biotite were observed in the partings and roof strata. The three partings and floor strata of the No. 6 coal seam from the Yueliangtian coal mine appear to have been derived from felsic volcanic ash. Four factors, including sediment-source region, multi-stage injections of hydrothermal fluids, seawater influence and volcanic ash input, were responsible for the mineralogical characteristics of the Yueliangtian coals.