Effects of igneous contact metamorphism on the mineral and toxic trace element composition of low-sulfur coal in the Huaibei Coalfield, eastern China

Abstract

It is not uncommon for coalfields to be affected by igneous intrusions, but the formation mechanism of mineral and chemical composition in thermally altered coals (TACs) during the rock-coal contact metamorphism is not yet fully understood, although there have been a number of studies. The present study reports effects of a mafic/intermediate sill (porphyrite) on the mineralogy and geochemistry of low-sulfur (total sulfur content 0.24%–0.56%, dry basis) coals in the Zhangzhuang Mine of the Huaibei Coalfield, eastern China. The porphyritic sill intruded along the coal seam roof. Towards the intrusion, the coal random vitrinite reflectance values increase from 1.38% (for the unaltered coal) to 5.19% (for the TAC). Similarly, the moisture content and ash yield of coals increase closer to the intrusion, whereas the volatile matter yield, nitrogen, hydrogen, and sulfur contents tend to decrease. The intrusion-related hydrothermal affected zone in coal is estimated as thick as 100-cm, especially in the zone of 0-10-cm concentrated below the sill. Minerals present in the TACs consist mainly of clay minerals (kaolinite, illite, and chamosite), carbonate minerals (Fe-containing dolomite, ankerite, and calcite), and quartz. Most of these minerals are suggested to be epigenetic, forming under intrusion-related hydrothermal influence. The enrichment of Mn, Zn, and Sr in the TACs is closely related to carbonate minerals directly precipitated from hydrothermal fluids, whereas the enrichment of Ba, Cr, Ni, Rb, Tl, Cu, and Pb associated with clay minerals that might form indirectly due to the clay alteration and adsorption. The abnormal enrichment of Hg in the coal closest to the intrusion may be caused by its adsorption onto carbonate minerals. The depletion of Be and Co in the TACs is principally associated with organic matter affected by igneous intrusion. The concentrations of V and U are unaffected, which is mainly related to clay minerals with minor portion associated with organic matter.