Mineral matter in the Pennsylvanian coal from the Yangquan Mining District, northeastern Qinshui Basin, China: Enrichment of critical elements and a Se-Mo-Pb-Hg assemblage
Yongjie Hou, Shifeng Dai, Victor P. Nechaev, Robert B. Finkelman, Hongdong Wang, Shaowei Zhang, Shaobo Di
Abstract
The Qinshui Basin, the largest reservoir of commercial coalbed methane in China, has attracted much attention and several studies have focused on the mineralogy and element geochemistry of coals in this basin. However, the sources of minerals and elements, particularly those of high-ash coals in the northeastern basin were yet unidentified. This study investigated mineralogy and geochemistry of the Pennsylvanian No. 8 Coal (including the roof strata and partings) from the Xinjing mine, northeastern Qinshui Basin, Shanxi Province, China. The Xinjing Coal is a low-ash, medium‑sulfur semi-anthracite. The minerals in the studied coals are mainly represented by kaolinite, illite, calcite, ankerite, siderite, dolomite, and pyrite. The Xinjing Coal is characterized by enriched Li, Zr, and Hg, and slightly enriched F, Ga, Ge, Se, Sr, Mo, Hf, and Pb. The diagrams of Al2O3-TiO2, Zr/TiO2-Nb/Y, as well as the REY (rare earth elements and Y) distribution patterns show that the sediment source region of the studied deposit is represented by the old granitoides, sedimentary rocks and bauxites of the Yinshan Upland located to the north of Qinshui Basin. In this study, three geological factors (sediment source regions, injections of hydrothermal fluids, and seawater) are identified as the dominant influences on geochemical and mineralogical compositions. Elevated concentrations of critical elements such as Li and Ga are dominantly derived from the sediment source region. Authigenic minerals including sulfides, quartz, carbonates, phosphates, and clausthalite, as well as the significant variations of Se-Mo-Pb-Hg assemblage across the coal seam, indicate that the Xinjing coal probably has been subjected to a multi-stage hydrothermal activities. Relatively high total sulfur contents (2.3% on average), framboidal pyrite, Sr/Ba (3.3) and Th/U values (2.2) suggest that seawater is another important geological factor affecting the Xinjing coal.