Mercury stable isotope compositions in magmatic-affected coal deposits: New insights to mercury sources, migration and enrichment

Mercury (Hg) is commonly present at trace levels in coal deposits. Geological processes, particularly magmatic intrusion and hydrothermal cycling in coal-bearing strata can significantly increase Hg levels in coals. However, the effects of thermal stress, magmatic components, magmatic-hydrothermal fluids, low-temperature hydrothermal fluids on Hg enrichment are confounded in a magmatic-affected coal basin. Here, we demonstrate the use of stable Hg isotopes to understand the controlling factors on Hg enrichment in two well-known Chinese coal deposits affected by magmatic activities: Wulantuga low-temperature hydrothermal-altered coal deposit and Wolonghu magmatic-intruded coal deposit. The Wulantuga No. 6 coal seam is extremely enriched in Hg, varying from 0.37 mu g/g in middle coal benches to 40-90 mu g/g in the upper most and lowest coal benches. Approximately 2.5 parts per thousand variation in mass dependent fractionation (MDF, -3.07 parts per thousand to-0.58 parts per thousand in delta Hg-202) is observed. delta Hg-202 vs. 1/[Hg] diagram suggests a mixture of three Hg end-members: original hydrothermal fluids, fractionated hydrothermal fluids and native coal-forming materials. In addition, Wulantuga coals all show positive mass independent fractionation (MIF) of odd isotopes, up to 0.87 parts per thousand in Delta Hg-199 and 0.66 parts per thousand in Delta Hg-201. The Delta Hg-199/Delta Hg-201 ratios range from 1.3 to 3.3, suggesting that a large fraction of photodegraded methylmercury was likely incorporated into the coal deposit. The magmatic-intruded Wolonghu No. 8 coal seam is only slightly to intermediately enriched in Hg, varying from 0.10 mu g/g to 0.40 mu g/g. The coals near the contact sill (<= 5 cm, -1.81 parts per thousand to -1.52 parts per thousand) have similar delta Hg-202 as the magmatic sill rocks (-2.05 parts per thousand to -1.80 parts per thousand), both with insignificant MIF values. Coals far from the contact sill (>= 15 cm) have delta Hg-202 values varying from -3.40 parts per thousand to -1.61 parts per thousand and Delta Hg-199 varying from 0.26 parts per thousand to 0.58 parts per thousand, which are significantly enriched in light and odd Hg isotopes compared to coals near the contact sill. Delta Hg-199 and Delta Hg-201 in Wolonghu coals far from the contact sill resemble those of meteoritic waters and seawater in both magnitude and Delta Hg-199/Delta Hg-201 ratio. Our two case studies show significant MDF and MIF of Hg isotopes, which inform on Hg migration and sequestration in the magmatic rocks and coal deposits, and on the roles of meteoric water and hydrothermal fluid circulation.