Granite-bauxite provenance of abnormally enriched boehmite and critical elements (Nb, Ta, Zr, Hf and Ga) in coals from the Eastern Surface Mine, Ningwu Coalfield, Shanxi Province, China

Abstract

Critical elements in coal have attracted much attention in recent years as some coals and coal combustion products may provide an alternative source of these. This paper provides mineralogical and geochemical data on the No. 4 coal seam, the major minable coal from the Eastern Surface Mine, Ningwu Coalfield, Shanxi Province, northern China. It also discusses the provenance of mineral matter, the origin of aluminum minerals (boehmite), carbonates (calcite and ankerite) and phosphates (goyazite) and the potential economic significance of critical elements (e.g., Al, Nb, Ta, Zr, Hf, and Ga). Mineralogically, the studied coal samples are characterized by high kaolinite (64.7% on average, low-temperature ash basis) and boehmite (24.1%) contents, followed by carbonates (calcite, ankerite, siderite, and dolomite), titanium oxides (anatase and rutile), goyazite, quartz, and apatite. In comparison with average values for Chinese coals, the studied coal samples are characterized by high Al2O3 contents (mean 11.82% on a whole-coal basis and mean 49.49% on an ash basis). Compared with world hard coals, the No. 4 Coal is enriched in Ga, Se, Zr, Nb, La, Ce, Hf, Ta and Pb. In addition to Al, critical elements including Nb, Ta, Zr, Hf, and Ga in this coal have an economic significance. The Al2O3/TiO2 vs. Nb/TiO2, Al2O3/TiO2 vs. Zr/TiO2, Zr vs. Nb, Hf vs. Ta, and REY distribution patterns (REY, rare earth elements plus Y), indicate that the provenance of mineral matter (e.g., boehmite and critical elements Nb, Ta, Zr, Hf and Ga) in the No. 4 Coal is a number of sediment-source regions including the Neoarchean granites of the Yinshan Oldland, and the Paleoproterozoic granites and Benxi Formation bauxites (Late Carboniferous, to the northeast of the coal basin) of the northern Trans North China Orogen. Carbonate minerals (calcite and ankerite) are of epigenetic origin while the phosphate mineral (goyazite) was syngenetically deposited in the peat swamp during a period of a low water table.