The response of organic matter in Pennsylvanian coals in the North China Basin to rapid heating by magmatic intrusion

Changes in organic matter during conventional burial are expected to differ from those induced by rapid, high-temperature heating during magmatic events. This study investigated coal seam profiles in the southern North China Basin directly affected by magmatic intrusion, focusing on the thermal response of organic matter and the protective role of a mudstone barrier. Magma-affected coals differ markedly from the overlying unaltered high-volatile bituminous coal, and are characterized by low ash yield, ultra-low moisture and volatile matter. Samples close to the intrusion locally show abnormal increases in ash yield and volatile matter, interpreted to reflect carbonate mineral formation. The intrusion strongly altered vitrinite, producing abundant pyrolytic carbon and vitrinite-derived coarse and fine cokes, whereas most inertinite retained its original characteristics. Semifusinite shows porous textures, and coking of the vitrinite–semifusinite transition is also observed. Residual organic compounds are dominated by n -alkanes and n -alkylcyclohexanes, with minor amounts of esters, branched alkanes, phenols, and alcohols. Greater molecular diversity is preserved in the mudstone-protected No. 8 coal seam than in the intrusion-contacted No. 9 seam. At the magmatic contact zone, even short-chain n -alkanes undergo pyrolysis. The δ 13 C values vary narrowly in both seams, indicating only limited influence of intrusion on carbon isotopes, although all profiles show a slight negative excursion at 50- to 100-cm from the intrusion. By contrast, δ 15 N values vary widely and are obviously affected by magmatic hydrothermal fluids. This difference is attributed to reactions between organic nitrogen and inorganic nitrogen in coal/hydrothermal fluids at high temperatures, whereas the mudstone barrier buffered isotopic fractionation. Large variations in the ratio of total organic carbon (TOC) to total nitrogen (TN) further indicate enhanced hydrothermal influence in the directly intruded seam. Opposite trends in TOC and TN relative to δ 13 C and δ 15 N in the mid-distance zone suggest migration and accumulation of carbon- and nitrogen-enriched materials from coal/hydrothermal fluids. Based on these findings, a conceptual model for magmatic intrusion into coal seams is proposed.