Geochronology, mineralogy, and geochemistry of tonsteins from the Pennsylvanian Taiyuan Formation of the Jungar Coalfield, Ordos Basin, North China

Altered volcanic ashes occur worldwide in coal-bearing sequences. The Carboniferous is one of the important coal-forming periods but the altered volcanic ashes within coals in China of this geological age have not been well studied. In this study, the nature of the partings in the No. 9 coal seam of the Pennsylvanian Taiyuan Formation from the Yaogou Mine, Jungar Coalfield, Ordos Basin, North China, was comprehensively investigated. Fifteen partings were identified as altered volcanic ashes based on the typical features, including thin but persistent bands, sharp contacts with the enclosing coals, vermicular and book-like kaolinite, angular quartz, and euhedral primary zircons, while some muscovite and detrital zircon grains in the samples indicate minor contribution of terrigenous debris. Two U–Pb zircon ages from the altered volcanic ashes at the top and bottom of the No. 9 coal constrain that the Taiyuan Formation in the studied area falls within the Gzhelian Stage of the Late Pennsylvanian. All the altered volcanic ashes were defined as tonsteins due to the relative abundances of kaolinite (> 50%). Furthermore, the TiO₂/Al₂O₃ values suggest that the tonsteins have three magmatic origins, which were mostly felsic, with less intermediate and mafic. The trace elements of the enclosing coals, detrital intra-seam clay beds, floor, and roof were mainly sourced from the felsic-intermediate rocks. However, the elevated concentrations of Li, Ga, Zr, Nb, In, Sn, Hf, Ta, Pb, and Th may be related to the leaching of the tonsteins by (acidic) groundwater or hydrothermal solutions. This is supported by the dominant MREY enrichment patterns with positive Gd anomalies of the coals.