Variations in the stable isotope composition of mercury in coal-bearing sequences: Indications for its provenance and geochemical processes

Abstract

Mercury (Hg) stable isotope variations provide a potential tool to better understand the provenance and geochemical processes that control Hg occurrences in coal deposits. In this study, we explore the variations in Hg isotope compositions in coal benches of a single coal seam (No. 3-1 coal seam, Daizhuang Coal Mine, Jining Coalfield, Shandong Province), and in successive coals seams of a coal-bearing sequence (No. 1 to No. 11-2 coal seams, Zhuji Coal Mine, Huainan Coalfield, Anhui Province) to assess Hg isotopes as a Hg biogeochemical tracer in coal deposits. Large variations in mass dependent Hg isotope fractionation (MDF) were observed in both Daizhuang and Zhuji coal deposits, with δ202Hg values ranging from −2.34 to −0.25±0.12‰ (2SD, n=8), and from −1.62 to 0.44±0.12‰ (2SD, n=18), respectively. Daizhuang coals showed insignificant mass independent Hg isotope fractionation (MIF) in most samples (−0.04 to 0.12±0.08‰ for Δ199Hg, 2SD, n=8), whereas MIF in the younger Nos. 7-11 coal seams of Zhuji coals (0.06 to 0.22±0.08‰ for Δ199Hg, 2SD, n=7) were positive and significant. Increased trends in both Hg concentrations and δ202Hg values going from older to younger Zhuji coal seams were observed, and were possibly related to variations in coal-forming environment of different coal seams. The significant negative correlations of δ202Hg vs. 1/Hg in the Zhuji coals, and of δ202Hg vs. Δ199Hg in the Daizhuang coals suggest that Hg isotopes can potentially be used to trace organic and inorganic Hg end-members in coal deposits. Natural coke, a metamorphosed form of coal, is on average two-fold enriched in Hg compared to coal, indicating that hydrothermal fluids derived from magmatic intrusions bring Hg into coal deposits. In addition, coke has either distinctly higher (0.70 to 0.91±0.12‰, 2SD, n=2) or lower δ202Hg (−4.00 to −3.47±0.12‰, 2SD, n=3) than corresponding coals, demonstrating that significant Hg MDF occurred when coals were thermally contacted by intruded magmas.