Anomalies of rare metals in Lopingian super-high-organic-sulfur coals from the Yishan Coalfield, Guangxi, China

The origin and modes of occurrence of rare metals in coal (e.g., rare earth elements, U, Se, Re, Ge, Ga, and Nb) have attracted much attention in recent years because such investigations can provide fundamental information for assessment of their economic significance. This paper investigates the abundance, modes of occurrence, and origin of rare metals in three Lopingian coals (LL5-K3, CG3-K6, and CG1-K7 Coals) from the Yishan Coalfield, Guangxi Province, southern China. The coals of this coalfield are preserved within carbonate successions and are characterized by super-high-organic sulfur (SHOS) contents (mostly 6-9%) and an elemental assemblage enriched in U-Se-Mo-V-Re. Two different hydrothermal solutions were responsible for the geochemical anomalies in the Yishan coals. One was responsible for the U-Se-Mo-VRe enrichment in the three coal seams while the other was a high-temperature solution responsible for the positive Eu anomalies and the elevated rank (R-o,R-ran = 2.05%) of the LL5-K3 coal. The U, Se, Mo, V, and Re in the CG3-K6 and CG1-K7 coals mainly have an organic association, but the same elements in the LL5-K3 coal have a mixed organic -inorganic affinity, attributed to high-temperature leaching in the early coalification stage. In addition to highly elevated concentrations of U-Se-Mo-V-Re, the average concentration of oxides of the rare earth elements and Y in the LL5-K3 coal (1387 ppm on average, ash basis) is higher than the cut-off grade for industrial deposits of these elements and thus the coals also have potential as raw materials for the recovery of rare earth elements. (C) 2017 Elsevier B.V. All rights reserved.