Mineralogical and Geochemical Compositions of Ammonian Illite-Enriched High-Rank Coals of the Xingying Mine, Northeastern Chongqing, China

Ammonian illite (NH4-illite)-rich late Permian coals of high rank were discovered in southwestern China. This research reports new mineralogical and geochemical data of 11 bench samples from the adjacent Xingying mine, northeastern Chongqing Coalfield, southwestern China, with an emphasis on the modes of occurrence and origin of NH4-illite. The Xingying coals, with low ash yields and medium sulfur, have a high rank (semianthrite, R-o,R-ran = 3.67%), owing to the plutonic metamorphism. Minerals in the coal consist of NH4-illite and pyrite and, to a lesser extent, jarosite, albite, and anatase, with traces of chamosite, quartz, bassanite, apatite, fluorapatite, florencite, and rhabdophane. Compared with world hard coals, vanadium is significantly enriched with a concentration coefficient (CC) higher than 10; Mo and Pb are enriched (5 < CC < 10); F, Co, Ni, Cu, Ge, Se, Y, Zr, Nb, Ag, Cd, In, Sn, Cs, Sm, Eu, Tb, Dy, Er, Yb, Hf, Bi, and U are slightly enriched (2 < CC < 5) in the Xingying coals. Fluorine in host rocks, including roof, floor, and parting, is significantly enriched. Fluorine concentration in the coal may be increased greatly if the coal is mixed with host rocks during mining activity. Hence, the Xingying coals should be subjected to beneficiation before utilization for the environment and human health. The Al2O3/TiO2 and Eu anomalies demonstrated that the terrigenous materials come from the mafic basalts of the Kangdian Upland. NH4-illite is formed by interaction of pre-existing kaolinite or K-illite with NH4+ released from organic matter under high temperatures during the process of hydrothermal alteration. The authigenic chamosite, albite, quartz, anatase, apatite, fluorapatite, and rhabdophane are also deposited from the hydrothermal solutions. In addition, the Xingying coals are subjected to marine influences. Based on the preliminary evaluation, the Xingying coals cannot be a potential source for critical elements such as rare earth elements and yttrium. This indicates that not all the late Permian coals in southwestern China have economic significance for critical elements.