Petrological, mineralogical, and geochemical compositions of Early Jurassic coals in the Yining Coalfield, Xinjiang, China

Abstract

The petrological, geochemical, and mineralogical compositions of 10 coal seams (Early Jurassic) from an exploratory borehole (Hole ZKJ502) in the Jieliangzi mining area of the Yining Coalfield, Xinjiang Uygur Autonomous Region, China, were investigated, using optical microscopy and field emission-scanning electron microscopy in conjunction with an energy-dispersive X-ray spectrometer (FE SEM-EDS), as well as X-ray powder diffraction (XRD), X-ray fluorescence (XRF), and inductively coupled plasma mass spectrometry (ICP-MS). The Yining coals are of subbituminous to highly volatile C bituminous in rank (vitrinite reflectance, average 0.49%) and are characterized by low sulfur content (average 0.45%), low ash yield (average 4.10%), high volatile matter (average 38.82%), high proportion of inertinite (average 49.5%), and low concentrations of most trace elements. Relative to the upper continental crust, the rare earth elements and yttrium in the Yining coals are mainly characterized by light and heavy REY enrichment. The minerals in the Yining coals are dominated by quartz, kaolinite, and siderite, followed by pyrite, bassanite, and hexahydrite, along with small proportions of illite, calcite, dolomite, gorceixite, goyazite, ankerite, marcasite, diaspore, florencite, hematite, and millerite. The similarity between the Yining coals and the magmatic rocks from the peripheral mountain area of the basin in terms of the Al2O3/TiO2 ratio, the REY enrichment type, the REY distribution patterns, and the Eu anomalous fractionation suggest that the sediment source region was mainly Carboniferous volcanic rocks (andesite and basalts) on peripheral mountains. The acid conditions and lowered water table during peat accumulation may have caused detrital minerals to be leached, leading to the relatively low ash yield. Another reason for the low ash yield may be the simple coalfield structure and poor groundwater migration channels in the rocks and coal seams in the central zone of the basin, which would limit the minerals carried by the groundwater into coals. A number of authigenic minerals (quartz, kaolinite, siderite, and gorceixite) occur as cell-fillings and elevated concentrations of Li, Be, Sc, V, Cr, Co, Ni, Cu, Zn, Ga, Rb, Zr, Nb, Mo, Cs, Hf, W, Pb, Th, and REY occur in Nos. 26 and 27 coals, indicating that there was an injection of a small-scale solution.