Mineralogical and geochemical responses of coal to igneous intrusion in the Pansan Coal Mine of the Huainan coalfield, Anhui, China

• The characteristics of thick sill in the No. 1 coal were described.
• Coal quality changes induced by igneous intrusion were discussed.
• Sandy mudstone parting is an effective thermal and chemical insulating layer.
• Three stages of hydrothermal fluids in the No. 1 coal were differentiated.
• The origins of elements in the No. 1 coal were investigated.

The Huainan coalfield, where magma is commonly intruded into the coal measures in the northern section, (e.g., the Zhuji, Dingji, Pansan, and Panbei mines), is the largest energy base in eastern China.To investigate the mineralogical and geochemical responses of coal to igneous intrusion, minerals, major and trace elements in sandstone, thermally-altered coal, igneous rock, sandy mudstone, and unaltered coal samples were collected from a representative profile of the No. 1 Coal of the Pansan Coal Mine. The samples were analyzed using an optical microscope, powder X-ray diffraction, scanning electron microscopy in conjunction with X-ray energy dispersive spectroscopy, X-ray fluorescence, inductively coupled plasma atomic emission spectrometry and mass spectrometry. The results indicated that the thick sill transformed from mafic rocks at the bottom, via intermediate rocks in the middle to felsic rocks at the top of the profile. The moisture, total sulfur, and carbon contents of the coal increased, whereas volatile matter, hydrogen, and nitrogen decreased during contact metamorphism caused by igneous intrusion. Epigenetic minerals (i.e., dolomite, quartz, and pyrite) occurred in the thermally-altered coals. Three stages of hydrothermal fluids (i.e., Ca-, Mg-, and Fe-rich; Si-rich; and Fe- and/or H2S-rich solutions) were identified. Iron, Ca, S, Si, Mg, Zn, Cd, and Pb were transported into the thermally-altered coal by hydrothermal fluids. The concentrations of Co and Ni in the thermally-altered coal increased in relation to the increase of ash yields that were caused by contact metamorphism, whereas the B in the coal was volatilized. Manganese is directly related to the intrusive magma. Phosphorus, Ge, and Sr might be introduced into the coal by groundwater; however, K, Na, Ga, and Ba were leached out. Titanium, Sc, Cr, V, Cu, Zr, Nb, and rare earth elements and yttrium (REY) in the coals originated from terrigenous input and were not influenced by igneous intrusion.