Genetic mechanism of the Permo−Carboniferous high-grade coal-bearing kaolin in the Datong Coalfield

The petrographical, mineralogical, whole-rock geochemical, and zircon U-Pb geochronological and Lu-Hf isotopic studies were carried out for investigating the provenance and genetic mechanism of the high-grade coal-bearing kaolin within the coal seams of the Permo-Carboniferous Taiyuan Formation in the Xiaoyu mine of the Datong coalfield, Shanxi Province. The coal-bearing kaolin is composed mainly of well-ordered kaolinite with no detrital quartz, feldspars and muscovite. Some samples contain boehmite that were formed through the desilication of kaolinite. The zircons in the coal-bearing kaolin are typically igneous origin with only one age population of ca. 300 Ma, same with the depositional age of the coal, confirming that the coal-bearing kaolin was altered from volcanic ash. Besides, the coal-bearing kaolin commonly contains tabular/columnar and vermicular kaolinite that are also diagnostic feature of alteration from volcanic ash. These coarse-grained kaolinites are thought to be pseudomorphic kaolinite altered from crystal fragments in volcanic ash, such as plagioclase and biotite. The zircon εHf(t) is dominant by a wide range of negative values, indicative of their formation from magma generated by substantial remelting of ancient continental crust, coinciding with that of contemporaneous magmatic rocks formed from continental arc volcanism in the Inner Mongolia Paleo-Uplift at the northern margin of North China. This suggests their cognate relationship. The geochemical fingerprints of trace element of zircons also indicate that they were crystalized from continental arc magma. The northern margin of the North China Craton (NCC) is an Andean-type continental margin in the late Paleozoic. During the coal-accumulating period of the Late Carboniferous to Early Permian in North China, the Paleo-Asian Ocean (PAO) plate was continuously southward subducted beneath the NCC, resulted in a frequent continental arc volcanism. The emitted volcanic ash fell into peat swamps developed in a paralic environment in North China, which were subjected to an intensive leaching and altered to the coal-bearing kaolin.