Mississippian Anthracites in Guangxi Province, Southern China: Petrological, Mineralogical, and Rare Earth Element Evidence for High-Temperature Solutions

Minerals and elements in coal are derived from the processes associated with peat accumulation and rank advance, as well as possibly a range of epigenetic processes, and thus can provide information on the depositional conditions and geologic history of coal beds and coal-bearing sequences, as well as perhaps on regional tectonics. This paper investigates the petrology, mineralogy, and geochemistry of anthracites and their host rocks (partings, roof, and floor strata) from two coal deposits (Hongmao and Luocheng) in Guangxi Province, southern China. Samples collected from these deposits were analyzed using optical microscopy, scanning electron microscopy plus energy-dispersive X-ray spectrometry, low-temperature ashing, X-ray diffraction, X-ray fluorescence spectrometry, and quadrupole-based inductively coupled plasma mass spectrometry. The anthracites from the two deposits are medium- to high-sulfur coals. The rare earth elements in the Luocheng anthracites are characterized by distinct positive Eu anomalies and medium- and heavy-REE enrichment types. The minerals in the Hongmao anthracites are characterized by the occurrence of K-illite, paragonite, and ammonian illite, which, however, are absent in the Luocheng anthracites. The paragonite, ammonian illite, and chlorite resulted from the alteration of pre-existing kaolinite by hot solutions containing Na+ NH4+, and Fe2+ respectively. The partings in the Luocheng anthracite seams contain pyrophyllite that was formed by reactions between kaolinite and quartz during injection of high-temperature solutions. The mineralogical and geochemical characteristics of the anthracites in the two deposits indicate that the coals were subjected to high temperature ( > 250 degrees C) solutions that also elevated the coal rank to anthracite grade. The assemblage of K-illite-paragonite-NH4-illite in the coals of the Hongmao deposit was formed under greater hydrothermal but less tectonic influence compared to the Luocheng deposit; by contrast, the pyrophyllite-illite-chlorite assemblage of the Luocheng deposit reflects a greater tectonic but lesser hydrothermal influence.