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

Oxygen and hydrogen isotope analyses were carried out on authigenic clay mi nerals from Late Permian coal measures of the Bowen Basin (Australia). In t he northern Bowen Basin, the oxygen isotope compositions of the mixed-layer illite/smectite show significant irregular variations with respect to dept h, which parallel the changes in the extent of the illitisation reaction an d are interpreted as reflecting changes in water/rock ratio in turn related to permeability. The delta(18)O and delta D values of illite-smectite and kaolinite in the northern Bowen Basin and the calculated fluid isotopic com position (delta(18)O = -3%, to +1%; delta D = -70% to -90%) in equilibrium with these clays are considerably lower than those typically reported for d eeply buried sedimentary basins. These stable isotope data, together with r elatively high inferred palaeotemperatures (up to 235 degrees C) and abnorm ally high geothermal gradients are consistent with a hydrothermal origin fo r clay mineral formation in the northern Bowen Basin. The hydrothermal syst em is interpreted to be a result of the Late Triassic extensional tectonic regime, which developed in large parts of eastern Australia and affected th e northern part of the Bowen Basin. In the southern Bowen Basin, by contras t, clays are more enriched in O-18 and deuterium, which is explained by low er temperatures (in the shallow coal measures) and a significant enrichment in the fluid isotopic composition (delta(18)O = -3.6% to +5.6%, delta D = -66% to -35%) under low water/rock ratio conditions, especially in deeper p elitic rocks. (C) 2000 Elsevier Science B.V. All rights reserved.