Hydrothermally-altered coal from the Daqingshan Coalfield, Inner Mongolia, northern China: Evidence from stable isotopes of C within organic matter and C-O-Sr in associated carbonates

Igneous intrusions can bring hydrothermal fluids, metals, and ligands to the surrounding host sequences, changing their physical, textural and chemical features to some extent depending on the degree of thermal exposure. However, regarding hydrothermally-altered coals, previous studies have predominately focused on variations in coal quality, chemistry and mineralogy, despite the growing research on stable and radioactive isotopes within coal-bearing sequences. The Pennsylvanian coal from the Daqingshan Coalfield, Inner Mongolia, China, was thermally affected by a granitic intrusion in the eastern part of the coalfield during the Yanshanian Orogeny, between the Late Jurassic and Early Cretaceous. The investigated Hailiushu, Datanhao, and Adaohai mines in present study are distributed in Daqingshan Coalfield from west to east. The Adaohai Mine is located to the southwest of the intrusion point, at a distance of approximately 50 km. The Hailiushu and Adaohai mines are about10 km apart, and the Datanhao Mine is located between them. With decreasing distance to the intrusion, there is an increase in vitrinite reflectance values (from 0.84% through 1.17% to 1.58%) and a decrease in the volatile matter content (from 44.57% through 31.31% to 19.60%) in the studied three coal mines. The present study revealed that the westernmost Hailiushu coals have the most negative δ¹³C_org values (from −24.8 ‰ to −23.5 ‰; −24.3 ‰ on average). In contrast, with the distance from the intrusion decreases, the δ¹³C_org values of coals from the Datanhao mine range from −24.8 ‰ to −23.5 ‰ (−24.2 ‰ on average). The δ¹³C_org values of the easternmost Adaohai coals range from −23.8 ‰ to −22.8 ‰, (−23.3 ‰ on average), about 1 ‰ less negative than those in the Datanhao and Hailiushu coals. It is not just the maceral composition that causes the difference in organic carbon isotopes, but the loss of ¹²C-enriched volatiles due to the thermal effects associated with the igneous intrusion. This hypothesis was further supported by Rock-Eval analysis on the organic matter: the average T_max of the Hailiushu, Datanhao, and Adaohai coals increases but generation of hydrocarbons and pyrolysable carbon decrease as approaching the intrusion. Moreover, fracture/cleat-filling carbonates are more abundant in the Adaohai coals than in the Datanhao and Hailiushu coals. The δ¹³C_PDB (from −14.2 ‰ to 1.7 ‰) and δ¹⁸O_PDB (from −16.9 ‰ to −7.0 ‰) of these vein carbonates all indicate a hydrothermal origin. Hydrothermal fluids derived from igneous intrusions is also responsible for the elevated ⁸⁷Sr/⁸⁶Sr ratios (0.711392–0.717643) in the carbonate mineral in the thermally altered coals.