Geochemistry of coal thermally-altered by igneous intrusion: A case study from the Pansan Coal Mine of Huainan Coalfield, Anhui, Eastern China

To investigate the geochemical characteristics of thermally-altered coals formed by igneous intrusion, a series of roof and floor mudstone, thermally-altered coal, and igneous rock samples were collected from a representative borehole profile of the Pansan No. 4-2 coals of the Huainan Coalfield, Anhui Province, Eastern China. Furthermore, seven unaltered coal samples from the same coalbed were collected from seven coal mines of the Huainan Coalfield for comparison. The minerals were analyzed using X-ray diffractometry and optical microscopy. The major element oxides and trace elements were determined by X-ray fluorescence (XRF), inductively coupled plasma mass spectrometry (ICP-MS), ICP-MS with collision/reaction cell technology, an ion-selective electrode (ISE), and a DMA-80 mercury analyzer. The results indicate that kaolinite and quartz are the dominant minerals in the thermally-altered coals, and pyrite is dominant at the contact zone between the thermally-altered coal and igneous rock. The unaltered Huainan No. 4-2 coals were slightly enriched in only Cr, Se, and W compared to the Chinese coals. However, the thermally-altered coals were slightly enriched in F and V and depleted in Co, Ni, Hg, Sb, Zn, and W compared to the Chinese coals. Although the igneous intrusion reached a thickness of 1.97 m, it had a limited effect on the trace elements of terrigenous origin, i.e., Li, F, Sr, Sn, V, Cr, Co, Ni, Cs, Sc, Ga, Rb, Zr, Nb, Ag, Cd, In, Ba, Hf, Ta, W, Th, U, and rare earth elements and Y (REY). Contact metamorphism resulted the devolatilization and redeposition of coal organic matter, which increased the Be and Ge concentrations of the thermally-altered coals adjacent to the intrusion. Copper, As, Sb, Tl, and Bi were transported by the igneous intrusion and redeposited in the thermally-altered coals. The coals were almost completely stripped of Zn, Se, Mo, Pb, and Hg by the igneous intrusion. •A full geochemical information on trace elements in Huainan coals was presented.•New cases of geochemical response of coal to igneous intrusion were added.•Zn, Se, Mo, Pb, and Hg in coals were completely driven away by igneous intrusion.