Mineralogical and geochemical compositions of the Pennsylvanian coal in the Hailiushu Mine, Daqingshan Coalfield, Inner Mongolia, China: Implications of sediment-source region and acid hydrothermal solutions

Abstract

This paper investigates the mineralogical and geochemical compositions of the Pennsylvanian coal in the Hailiushu Mine, Daqingshan Coalfield, neighboring previously-reported Al (Ga, REE) ore deposits (including the Adaohai Mine in the same coalfield and deposits in the Jungar Coalfield), using optical microscopy, field emission-scanning electron microscopy, X-ray fluorescence, and inductively coupled plasma mass spectrometry. The mineralogical and geochemical compositions in the coal were primarily controlled by the sediment-source region during peat accumulation, and by epigenetic acid hydrothermal solutions. The Hailiushu coal was deposited in a sub-depression (intermontane basin) in the inner part of the orogenic belt, with a sediment-source region composed mainly of Cambrian–Ordovician strata and Archaean metamorphic rocks. The minerals in the coal from the Hailiushu Mine dominantly consist of kaolinite, with minor amounts of quartz, sulfide and selenide minerals (including chalcopyrite, selenian galena, galena, sphalerite, clausthalite, and siegenite), aluminophosphates, and rhabdophane. The coal is enriched in SiO2 (17.05% on average), TiO2 (0.60%), Al2O3 (13.71%), Zr (289μg/g), Hf (7.09μg/g), and to a lesser extent, F, Sc, V, Cu, Ga, Se, Y, Nb, Mo, Cd, Sn, La, Ta, W, Hg, Pb, Bi, and Th. Titanium largely occurs in the kaolinite. Elements such as Cu, Se, Sn, Hg, Pb, and Bi in the coal mainly occur in sulfide and/or selenide minerals. Zirconium, Hf, and Nb were largely derived from the sediment source region. The substitution of Ti for Al in kaolinite, the corrosion of previously-formed zircon, anatase, and quartz, and the enrichment of middle rare earth elements in the coal were caused by the epigenetic acid hydrothermal solutions.