Stable (delta C-13 and delta O-18) and radiogenic Sr-87/(86) Sr isotopic data have been used to investigate the origin of cleat dawsonite (NaAlCO3(OH)(2)) in the Late Permian Wittingbam Coal Measures of the Upper Hunter region in the Sydney Basin, New South Wales. The delta C-13(PDB) values have a narrow range (- 1.7 parts per thousand to +2.4 parts per thousand), with an average of +0.3 parts per thousand, suggesting a magmatic source for the carbon. In contrast, delta O-18(SMOW) values have a wide range (+ 13.6 parts per thousand to + 19.8 parts per thousand), and decrease systematically with decreasing distance from a major intrusion. This systematic variation reflects establishment of localised hydrothermal cells. Water-rock interaction between fluids associated with these hydrothermal cells, and Rb-poor volcaniclastic detritus in the coal measures, produced mantle-like Sr-87/Sr-86 (0.705032 to 0.706464) in the dawsonite. (c) 2005 Elsevier B.V. All rights reserved.