Stable isotope geochemistry of authigenic clay minerals from Late Permian coal measures, Queensland, Australia: implications for the evolution of the Bowen Basin

Oxygen and hydrogen isotope analyses were carried out on authigenic clay minerals from Late Permian coal measures of the Bowen Basin (Australia). In the northern Bowen Basin, the oxygen isotope compositions of the mixed-layer illite/smectite show significant irregular variations with respect to depth, which parallel the changes in the extent of the illitisation reaction and are interpreted as reflecting changes in water/rock ratio in turn related to permeability. The δ18O and δD values of illite–smectite and kaolinite in the northern Bowen Basin and the calculated fluid isotopic composition (δ18O=−3‰ to +1‰; δD=−70‰ to −90‰) in equilibrium with these clays are considerably lower than those typically reported for deeply buried sedimentary basins. These stable isotope data, together with relatively high inferred palaeotemperatures (up to 235°C) and abnormally high geothermal gradients are consistent with a hydrothermal origin for clay mineral formation in the northern Bowen Basin. The hydrothermal system is interpreted to be a result of the Late Triassic extensional tectonic regime, which developed in large parts of eastern Australia and affected the northern part of the Bowen Basin. In the southern Bowen Basin, by contrast, clays are more enriched in 18O and deuterium, which is explained by lower temperatures (in the shallow coal measures) and a significant enrichment in the fluid isotopic composition (δ18O=−3.6‰ to +5.6‰, δD=−66‰ to −35‰) under low water/rock ratio conditions, especially in deeper pelitic rocks.