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Large, globally documented negative δ13C excursions that occur across the Permian-Triassic transition are thought to represent the isotopic fingerprint of considerable releases of isotopically light carbon into the atmosphere, with the largest of these excursions (as much as −22.2‰) measured in bulk organic matter in coals and carbonaceous shales from Antarctica. Previous studies have attributed the carbon isotopic excursion (CIE) at the Permian–Triassic boundary to the release of CH 4 from organic-rich rocks intruded by dikes and sills associated with the Siberian Traps flood basalts (Russia) and Longwood-Bluff (New Zealand) intrusions. However, any assessment of paleoatmospheric conditions based on this or other δ13C org excursions must consider the multiple controls on isotopic composition, including maceral content and post-depositional diagenetic and maturation processes. Permian-age coal seams and associated sediments in Antarctica were extensively altered after burial by localized high heat flow and, in some cases, by contact metamorphism associated with Jurassic dikes and sills. Levels of organic maturation reach anthracite, meta-anthracite, and even graphite; such intense thermal alteration had the potential to change the carbon isotopic composition of the bulk organic matter (δ13C org). To evaluate the Permian–Triassic isotopic excursion in Antarctica, it is important to understand the inherent variability in δ13C org in Permian-age coal and organic-rich sedimentary rocks prior to the CIE; thus, 128 samples representing pre-boundary coals and shales were selected from the United States Polar Rock Repository for analysis. Petrographic (maceral analysis and vitrinite reflectance), geochemical (C, H, N), and carbon isotopic analyses were performed. Based on the presence of coarse-grained circular and fine-grained lenticular mosaic in the anisotropic cokes found in samples in close proximity to intrusions, the rank of the samples prior to intrusion is estimated to have been medium volatile bituminous (∼1.2% R max). These highly altered coals and cokes can have random vitrinite reflectances (R r) exceeding 6%. Subsequent to intrusion, alteration beyond the contact aureoles continued due to high heat flow, producing high rank coals with R r > 2%. Values of δ13C org range between −26.5 and –21.1‰ (5.4‰ difference). Significant relationships (at the 1 and 5% levels) exist between δ13C org and the level of organic maturity, organic carbon (C org) content (i.e., size of the carbon reservoir), and total inertinite content, suggesting variability due to these factors should be considered when assessing isotopic excursions and interpretation of Permian-Triassic atmospheric conditions. These findings highlight the importance of incorporating organic petrography into δ13C org studies. • Late Permian Antarctic samples are bituminous to meta-anthracite in rank and coke. • δ13C org values for these samples ranges from −26.5 to −21.1‰ (5.4‰ difference). • δ13C org correlates with C org content, maturation level, and maceral content. • Organic petrography should be incorporated into carbon isotope excursion (CIE) studies. • Considering multiple variables, CIEs should be interpreted with caution. [ABSTRACT FROM AUTHOR]

Highlights: [•] This study has established the usefulness of optical microscopy as well as SEM study for better study of heat-altered coals. [•] Differentiation of microtextures and microstructures and contact metamorphic associations were studied. [•] Anisotropy proved supplementary tool to SEM and enabled study of minerals and baked coal in 3-D view. [•] Through this study, combined maceral–mineral analysis is helpful in coal geological studies. [Copyright &y& Elsevier]

The study of coals affected by magmatic intrusions is important from a geological point of view, in order to understand how heat and magmatic fluids could affect the organic and inorganic fractions as well as the properties of the coals. These fluids may lead to an enrichment of trace elements and heavy metals of environmental concern or serve as unconventional sources of critical elements. Therefore, the objective of this study is to investigate the organic and inorganic fractions of Peñarroya-Belmez-Espiel Basin (PBEB) (Spain) coals in order to verify whether changes of the petrography, chemical composition, mineralogy, and inorganic geochemistry have taken place. The PBEB corresponds to a Carboniferous intramontane basin filled with a Middle Mississippian to Lower-Middle Pennsylvanian sedimentary succession with coal-bearing units. Two main successive depocentres (extended from NW to SE) were recognized and in which bituminous and anthracite coals, as well as natural cokes occur. Petrographic analysis, ultimate and proximate analysis, scanning electron microscopy with energy dispersive X-ray spectrometry (SEM-EDS) analysis, inductively coupled plasma-optical emission spectrometry (ICP-OES) and inductively coupled plasma mass spectrometry (ICP-MS) analysis were carried out for the purpose of this study. The sampling consisted of a selected set of nine samples of bituminous coals, ten samples of anthracites and three samples of natural cokes. The results show that the organic fraction of bituminous and anthracite coals is mainly made up of vitrinite. Although the minerals identified in the two depocentres vary, they mainly consist of quartz and clay minerals in both cases. Authigenic kaolinite and epigenetic mineralization related to thermal events that occurred in the basin were observed in the anthracites. Natural coke shows isotropic and fine mosaic textures, and signs of epigenetic mineralization. The trace element content of the coal samples was compared with the values established for world hard coals and an enrichment of Cs, in bituminous and anthracite coals, and of Hg in natural cokes, was observed. The occurrence of REY in bituminous coals is related to the detrital sedimentary source, whereas in anthracite the epigenetic and thermal effects may have also been involved. In natural cokes, the REY content is probably associated with the original coal. Mercury enrichment in natural cokes may be associated with magmatic fluids and explained by Hg volatilization, its capture in natural coke pores and subsequent precipitation. • Petrographic and geochemical characterization of coals and natural cokes of PBEB. • Effects of a magmatic intrusion in coal-bearing sequences of PBEB. • Influence of magmatic fluids in Hg enrichment/depletion in natural cokes. • Organic and/or inorganic Hg association in natural cokes. [ABSTRACT FROM AUTHOR]