Revisiting the thermally metamorphosed coals of the Transantarctic Mountains, Antarctica

Abstract

Petrographic and geochemical data for Late Permian coals and carbonaceous shales from the Transantarctic Mountains, Antarctica (source: Polar Rock Repository, PRR), were used to evaluate maturation levels and assess the effects of contact metamorphism. Coals were evaluated for locations in the Southern Transantarctic Mountains, the Central Transantarctic Mountains, and South Victoria Land, including samples from the Buckley, Mt. Glossopteris, and Queen Maud formations and the Weller Coal Measures. These formations have been intruded by sills and dikes of the Jurassic Ferrar Group (177–183 Ma) associated with the breakup of Gondwanaland. Proximate (129 samples), total sulfur (69) analyses, vitrinite reflectance analysis (92), and petrographic composition (34) were determined. One third of the samples have >50% ash yields (dry basis). A subset of samples (87) with <50% ash (dry basis) was treated with 6 N HCl to remove any carbonates formed during the intrusive events. Acid treatment did not significantly reduce ash, suggesting silicates are a major component; sulfur contents of 1–2% (dry basis) decreased to <0.8% reflecting gypsum dissolution (as confirmed by XRD). Volatile matter (VM) contents (dry, ash-free or daf basis) for samples with <50% ash range from 3 to 43%. Based on VM, samples range from high volatile bituminous to anthracite; however, reflectance analysis indicates anthracite to meta-anthracite, with some reflectances >7%. Thus, VM does not reveal true rank of the Antarctic coals. Vitrinite reflectance (Rr) typically surpasses that of inertinite. The VM–Rr relationship for these coals does not follow that of coals matured by normal burial maturation, but more closely follows that of intruded coals. Coke textures, including isotropic coke and anisotropic mosaics, vacuoles, pyrolytic carbon, and coked bitumen are observed, indicating alteration by contact metamorphism and providing insights to the rank of the coal at the time of intrusion. Coarse-grained circular and fine-grained lenticular mosaic textures suggest coal rank at the time of intrusion was medium volatile bituminous coal (maximum vitrinite reflectance ~1.2%). This would imply a burial depth by time of intrusion of ~5–5.5 km (assuming 25 °C/km) or ~ 4 km (assuming 34 °C/km). Modern-day background reflectance levels of ~2.5% Rr indicate continued post-intrusion maturation, possibly due to exposure to higher regional heat flow. Coals and carbonaceous shales from the Polar Rock Repository (PRR) can provide reliable petrographic and maturation data (using reflectance) to help decipher the burial history for various parts of the Transantarctic Mountains. However, geochemical data must be used with caution due to the high original inorganic content, and possible formation of gypsum and changes in VM during long-term storage. HCl-treatment removes some of the neoformed minerals, but samples should be treated extensively with HCl-HF to remove all silicate minerals prior to proximate and ultimate analysis to ensure more reliable data.