Geochemical characteristics and the significance of two major coal‐bearing strata claystones from the Datong Coalfield (North China)

This study aims to investigate the significance of the geochemical differences and the factors that control the geochemistry of two major types of coal-bearing stratum claystones (sedimentary kaolinitic claystone [SK-claystone] and tonstein). The SK-claystone and tonstein derived from the Taiyuan Formation of the Upper Carboniferous coal-bearing strata in the Datong Coalfield of North China have been comprehensively analysed using X-ray powder diffractometer, scanning electron microscopy, X-ray fluorescence, and inductively coupled plasma mass spectrometry. The results showed that the SK-claystones and tonsteins exhibit similar SiO2 content; however, the content of Al2O3 in tonsteins is higher than that in SK-claystones, whereas the TFe2O3 and K2O content in tonsteins are lower than those in SK-claystones. These findings are consistent with the mineral composition analysis of the samples. The SK-claystones and tonsteins have different parent rocks, as shown by the Al2O3/TiO2 ratios, diagram of Zr/TiO2 versus Nb/Y, and chondrite-normalized rare earth element patterns. The palaeosalinity indexes (B/Ga and Sr/Ba) and palaeoredox proxies (Mo and U enrichment factors as well as U/Th ratios) indicate that both SK-claystones and tonsteins were formed in freshwater suboxic-oxic environments. The kaolinite in SK-claystones and tonsteins is formed via the deposition or crystallization of aluminium-rich weathering products and alteration of volcanic ash, respectively. In summary, the different parent rocks and diagenetic processes are the main factors differentiating the geochemical characteristics of the two types of claystones. Our study links the different types of elemental enrichment features to the geological genesis of the two types of claystones, indicating that the trace elemental enrichment characteristics can be used as indicators for distinguishing the SK-claystone and tonstein in the Datong Coalfield as well as other similar coalfields.