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Use of stable mercury isotopes to trace the geological sources of mercury in hydrothermal coals

2018-01-22 71

Coal is a type of organic sedimentary rock of geological interests. Research on the sources of mercury in coals is of great significance for understanding mercury cycling and geological evolution in sedimentary basins. Mercury is generally present as trace levels in coals, ranging from 0.01 to 1.0 ppm. However, for some coals formed under geologically active regions, e.g., hydrothermal coals, their Hg can reach as high as several tens or hundreds of ppm.

The low-temperature hydrothermal fluids could be generated due to the intrusion of magmas into coal-bearing strata, which elute mercury and other mobile elements from the surrounding rocks, and penetrate into the coal seams via faults and fractures. The research group of associate professor Ruoyu SUN found that the coals affected by near-field and far-field magmatic intrusion exhibit significantly positive odd mass isotope independent fractionation (odd-MIF), a feature that is quite different from the coals formed under normal conditions. Two different mercury enrichment pathways have been proposed for the hydrothermal coals: 1) meteoritic water (low mercury, positive odd-MIF) → low-temperature hydrothermal fluids heated by coeval magmas → leaching of eruptive rocksHg enrichment, no odd-MIF) →surface transport and cycling (photo-reductive odd-MIF) → intrusion into coal seams along faults and fractures (Figure A) 2) meteoritic water (low mercury, positive odd-MIF) → stored in underground aquifers → low-temperature hydrothermal fluids heated by post-depositional magmas (Hg enrichment, conserving odd-MIF of meteoritic water) → intrusion into coal seams along faults and fractures (Figure B). The pathway 1 delivers mercury into coals via meteoritic water and rock leaching, in contrast to pathway 2 that only delivers mercury via meteoritic water. Thus, the coals affected by pathway 1 have significantly higher mercury than those affected by pathway 2.

This research is mainly supported by the National Natural Science Funds (41602167; 41773104). The published paper (Zheng, L.#; [Sun, R.#,*]; Hintelmann, H.; Zhu, J.; Wang, R.; Sonke, J. E., Mercury stable isotope compositions in magmatic-affected coal deposits: New insights to mercury sources, migration and enrichment. Chemical Geology, doi.org/10.1016/j.chemgeo.2017.12.032) can be accessed via the following link: https://www.sciencedirect.com/science/article/pii/S0009254117307143.

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