Behavior of Minerals and Trace Elements during Natural Coking: A Case Study of an Intruded Bituminous Coal in the Shuoli Mine, Anhui Province, China

The effects of thermal alteration by an igneous intrusion on the organic matter and inorganic constituents of a coal seam in the Shuoli mine, Anhui Province, China, have been investigated using reflected light microscopy, X-ray diffraction analysis (XRD), X-ray fluorescence spectrometry (XRF), inductively coupled plasma mass spectrometry (ICP-MS), an electron microprobe, and a scanning electron microscopy system equipped with an energy-dispersive X-ray spectrometer (SEM-EDS). A total of 11 coal benches were collected from the profile (numbered as SI5-1 to SI5-11), all of which were found to be metamorphosed from a distance of 80 cm below the sill to the sill/coal contact; benches Sl5-8 to Sl5-11, which are in direct contact with the sill, were completely converted to natural coke. The maximum paleo-temperature inferred from R-o,R-max of the natural coke is estimated to be 1000 degrees C. The minerals formed by the molten magma invasion are dominated by veins of mixed-layer illite/smectite (I/S). However, mixed-layer I/S of terrigenous origin is also present as lenses or thin layers. The mixed-layer I/S shows an increasing degree of illitization upward from the bottom. in addition to abundant mixed-layer I/S, the molten magma invasion also resulted in the precipitation of nontronite, albite, quartz, pyrite, and anatase, which mainly occur as fracture or pore fillings. Compared to the ash of the unaltered coal, the SiO2/Al2O3 ratio (average of 1.4), and the percentages of K2O (average of 4.3%), Na2O (average of 0.8%), and Fe2O3 (average of 3.2%) are higher in the ashes of altered coal and natural coke. The K2O and Na2O contents increase from the bottom to the top of the seam. Trace elements, including Be, F, Zn, As, Rb, Sr, Cs, Hg, Tl, Bi, Th, and U, exhibit a marked enrichment in the natural coke. Among these elements, the enrichment of Be, F, Rb, Sr, Cs, Th, and U is associated with the formation of molten magma-related minerals, such as mixed-layer I/S. High concentrations of Zn, As, Hg, Tl, and Bi in the natural coke are attributed to pyrite, which was related to the veins of mixed-layer I/S. The concentration of REY (rare-earth elements and yttrium) in the thermally altered coals is observed to be in the range of 267-980 mu g/g (ash basis). The sill and all the coal samples are mainly characterized by a LREY-rich type (normalized to Cl-chondrite). The sill shows a typical europium anomaly with an Eu/Eu* value of 0.26. However, Eu/Eu* in the profile distinctly changes from weak negative to strong negative from the bottom of the seam toward the sill. The systematic fractionation change in Eu/Eu* in the profile is primarily attributed to changes in the nature of the intrusive molten magma.