The genesis and isotopic composition of carbonates associated with some Permian Australian coals

Abstract

Siderite and calcite are the two forms of carbonate commonly associated with Permian Australian coals. The former occurs as disseminated spherulites and is a product of the early post-depositional environment. Isotopic measurements show that the CO 2 fixed as siderite did not result from the direct oxidation of photosynthetically derived materials, but rather from the anaerobic fermentation of these. The higher concentrations of calcite are generally found towards the roofs of coal seams and are characterized by isotopic enrichments to δ 13C values of +25‰ PDB. Isotopic exchange between CO 2 and CH 4 within the coal seam is postulated as the mechanism which leads to the formation of isotopically heavy CO 2. At sites along the seam margins where the CO 2 escapes, interaction with circulating metal ions or preexisting calcite results in the deposition of “heavy” calcite. With increasing alteration of coal by thermal metamorphism, the 13C content of calcites and finally siderites decreases so that it more nearly approaches that of the associated coal.