Mineralization of REE-Y-Nb-Ta-Zr-Hf in Wuchiapingian coals from the Liupanshui Coalfield, Guizhou, southwestern China: Geochemical evidence for terrigenous input

Abundances, modes of occurrence, enrichment, and origin of critical elements in coal and coal-bearing sequences have attracted much attention in recent years. Previous investigations have attributed REE-Y-Nb-Ta-Zr-Hf mineralization in coals and in intra- and inter-seam tuffs in coal-bearing sequences to alkali volcanic ash eruptions and subsequent hydrothermal alteration. In this study, two major minable coal seams (D101 and W407) that are located in the lowermost and uppermost Wuchiapingian Longtan Formation in the Liupanshui Coalfield, Guizhou, southwestern China, were investigated in order to better understand the mechanism of REE-Y-Nb-Ta-Zr-Hf enrichment in these coals. Elevated concentrations of REE-Y-Nb-Ta-Zr-Hf in the D101 coal are dominantly due to the alkaline volcanic material forming the top of the Emeishan Large Igneous Province (ELIP), which served as a terrigenous source region for the coal basin and to airborne volcanic ash, which occurs as an intra-seam tonstein within the coal seam. Both of the two source materials were subsequently subjected to groundwater leaching, resulting in redistribution of the rare earth elements and Y (REY), leading to an uneven distribution of REY and Zr along the studied coal-seam sections. The average REY oxides in the D101 and W407 coals are 2682 and 1260 ppm (both on ash basis), respectively, and the outlook coefficients are 0.79 and 1.15 respectively, indicating that both ashes from the two coals are potential raw sources for REY.