Geochemical characteristics of the Permian coals in the Junger-Hebaopian mining district, northeastern Ordos Basin, China: Key role of paleopeat-forming environments in Ga-Li-REY enrichment

Abstract In order to study the trace element enrichment mechanism, this paper reports new petrology, geochemistry, and mineralogy data on the Permian coals of the Junger mining district and the Hebaopian mining district, northeastern Ordos Basin. In comparison with hard coals worldwide, the P2s1 coals are enriched in Be and the P1t3 coals are enriched in Zr in the Junger mining district, while the P2s1 coals in the Hebaopian mining district are enriched in Li, Zr, and Hf. The provenances of the enriched elements in the Permian coals from the Junger and Hebaopian mining districts are the Middle Proterozoic moyite in the Yinshan Oldland and the Benxi Formation bauxite. The paleopeat-forming environments of the Junger-Hebaopian mining district, northeastern Ordos Basin, were reconstructed in detail using the nature of the macerals, geochemical indicators, and mineralogical characteristics, with special emphasis given to the key role of paleopeat-forming environments in Ga, Li, and REY enrichment by comparing the variations in the Ga, Li, and REY contents to the paleopeat-forming environment characteristics. In the Junger-Hebaopian mining district, the higher REY concentrations mainly occur in coals derived from Neuropteris and Lepidodendron, and the coals formed in relatively alkaline, suboxic to anoxic, brackish or seawater influenced environments with strong hydrodynamics, while Li enrichment occurs in coals formed from sphenopsids and formed in acidic, brackish water influenced, relatively anoxic environments with strong hydrodynamic conditions. In addition, an acidic, freshwater, suboxic sedimentary environment with weak hydrodynamic conditions and Neuropteris and Lepidodendron peat-forming vegetation is beneficial to the enrichment and accumulation of Ga and REY. Although there is no direct evidence to support this hypothesis, a sequence of events that could have influenced the trace-element contents of the studied coals is proposed based on the general trace-element geochemistry of paleopeat-forming environments. This theory provides valuable insight into the selection of appropriate mining targets for these beneficial metal mineral resources (Ga-Li-REY) in other coal-bearing basins.