Geological controls on enrichment of Mn, Nb (Ta), Zr (Hf), and REY within the Early Permian coals of the Jimunai Depression, Xinjiang Province, NW China

Abstract

Previous investigations on major and trace elements within the Early Permian coals from Xinjiang Province are very scarce. This paper reports on the geochemistry of the Early Permian coals and associated non-coal rocks in the Jimunai depression, Xinjiang Province, northwestern China. Samples collected from coal exploration boreholes ZK2603, ZK3003, and ZKH1102 as well as the Haerjiao underground coal mine (Nos. HRJ-7 and HRJ-9 Coals) in the Jimunai Depression were analyzed using SEM-EDS, ICP-AES, and ICP-MS. These Early Permian coals reach the rank of semi-anthracite to anthracite and have low-sulfur contents (0.30% on average). In comparison with the average element concentrations of world coals, Zr, Hf, and rare earth elements and Y (REY) are slightly enriched (concentration coefficient (CC) = 2–5, as the ratio of element content in sample vs. that in world coals) and Mn is enriched (CC = 5–10) in the ZK2603, ZK3003, and ZKH1102 coals. Niobium and Ta are highly enriched in the ZK2603 and ZK3003 coals (CC = 10–100). The HRJ-7 and HRJ-9 coals are slightly enriched in Mn (CC = 2–5). The coals have a sediment source region mainly composed of intermediate-felsic compositions from the Saur Volcanic Arc to the south of the Jimunai Depression, which may have provided relatively high geochemical background concentrations of Zr, Hf, Nb, and Ta. Manganese is hosted mainly in ankerite that was derived from epigenetic precipitation from Mn-bearing hydrothermal fluids. Zirconium and Hf in the ZK2603, ZK3003, and ZKH1102 coals are controlled primarily by detrital matter or hydrothermal fluids, the latter of which results in the higher Zr/Hf ratio in the ZK2603 and ZK3003 coals due to the re-distribution of Zr and Hf. Niobium and Ta in the ZK2603 and ZK3003 coals are concentrated chiefly in zircon, and hydrothermal fluid activity caused the release of Nb and Ta from previously-existing inorganic constituents and subsequent incorporation into zircon, resulting in the lower Nb/Ta ratio due to Ta being more easily incorporated into zircon than Nb. REY are hosted primarily within multiple mineral phases (e.g., zircon, phosphates, anatase, clay minerals) and their enrichment results from a combination of detrital matter input and hydrothermal fluid influx, the former of which is evidenced by the occurrence of REY-bearing detrital zircon and anatase, and the latter of which is indicated by the existence of REY-bearing authigenic rhabdophane and anatase and corroded anatase particles, as well as by the positive Gd anomalies and MREY-type or HREY-type REY distribution patterns. © 2019 Elsevier B.V.