Data for igneous intrued coal

Description

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 had focused on the coal quality and element variation affected by igneous intrusion but evidence of hydrothermal alteration is absent. The Pennsylvanian coal in in the Daqingshan Coalfield, Inner Mongolia, China, was thermally affected by a granitic intrusion near the east of coalfield during the Yanshanian Orogeny between the Late Jurassic and Early Cretaceous. With decreasing distance eastwards from the intrusion, there is an increase in vitrinite reflectance values (from 0.84% to 1.58%); a decrease in the coal H/C (from 0.072 to 0.052) value and the content of volatiles (from 44.64% to 21.65%) in comparison with the unaltered coal. The present study revealed that the organic carbon δ 13C values of the Daqingshan coal samples range from -24.8‰ to -22.8‰. The westmost Hailiushu coals have the least negative δ 13Corg values (from -24.8‰ to -23.5‰; -24.3‰ on average), while those of the Datanhao coals (from -24.8‰ to -23.5‰; -24.2‰ on average) and Adaohai coals which have the most positive δ 13Corg (from -23.8‰ to -22.8‰; -23.3‰ on average), are all slightly more positive than the Hailiushu coals. Excluding such factors such as the paleogeography and paleoenvironment, it is not just the maceral content that causes the difference in organic carbon isotopes, but the loss of 12C-rich volatiles due to the thermal effects from the igneous intrusion. This hypothesis was further supported by Rock-Eval analysis on the organic matter: the trend of Rock-Eval Pyrolysis for the Hailiushu, Datanhao, and Adaohai coals follows the trend of increasing distance from the granitic intrusion (e.g., average Tmax value of Hailiushu, Datanhao, and Adaohai coals is 435.25℃, 460.40℃, and 489.20℃ respectively). Moreover, minerals of hydrothermal origin, i.e., ammonium illite and aluminum bearing minerals (boehmite and diaspore) were detected only in the Datanhao and Adaohai coals. The fracture/cleat-filling carbonates are more abundant in the Adaohai coals than in the Datanhao and Hailiushu coals. The presence of these epigenetic minerals which were generally reported from high rank coals further supports the effect of high-temperature hydrothermal fluids. The δ 13CPDB (from -14.2‰ to 1.7‰) and δ 18OPDB (from -16.9‰ to -7‰) of these vein carbonates all indicate a hydrothermal origin. High 87Sr/86Sr ratios (0.711392 - 0.717643) of hydrothermal fluids brought by enriched-87Sr granite flowing through the coals not only altered the mineral components but also was responsible for the 87Sr/86Sr in carbonates. This study has a potential wide significance of revealing maturity and chemical compositions of organic matter in sediments without limitation to coal.

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