Geological controls on the distribution of REY-Zr (Hf)-Nb (Ta) enrichment horizons in late Permian coals from the Qiandongbei Coalfield, Guizhou Province, SW China

Abstract

The abnormal enrichment of the REY (rare earth elements and yttrium)-Zr (Hf)-Nb (Ta) assemblage in Late Permian coals occurring in some coalfields in SW China has been extensively investigated, but the geological controlling factors of this assemblage are still under debate. This study reports on the mineralogical and geochemical features of coals and noncoal rocks collected from the Yudai and Jinqi coal mines within the Qiandongbei Coalfield in Guizhou Province to elucidate the geological controls on the distribution of REY-Zr (Hf)-Nb (Ta) enrichment horizons. The studied coals show rather consistent clay mineral assemblage of kaolinite, together with varying amounts of quartz, gypsum, pyrite, and calcite. Notably, the coals are anomalously enriched in REY, Nb, Ta, Zr, and Hf and enriched in other incompatible elements (e.g., Ga, Th, and U). Phosphates (e.g., florencite) and, to a lesser extent, zircon are the primary carriers of REY. Zirconium and Hf are mainly incorporated into zircon and then anatase, the latter of which is a predominant carrier of Nb and Ta. The detrital materials originate from a mixture of the erupted felsic volcanic ash resembling a peralkaline rhyolite composition and the eroded materials of Emeishan Large Igneous Province (ELIP) basalts. The studied coals owe their highly enhanced concentrations of the REY-Zr (Hf)-Nb (Ta) assemblage to the input of felsic volcanic ash, wherein the highly acidic aqueous solutions allowed these elements to become mobile and be redistributed. Both geological processes governed the distribution of REY-Zr (Hf)-Nb (Ta) enrichment horizons. The highly acidic solutions were possibly derived from the oxidation of peat bogs and volcanism-related acidic rain. The concentrations of REY, Zr, and Nb in the coal ashes exceed their corresponding cut-off grades; thus, these coals can be considered highly promising raw material sources for the REY-Zr (Hf)-Nb (Ta) assemblage.