Influence of localised igneous activity on cleat dawsonite formation in Late Permian coal measures, Upper Hunter Valley, Australia

摘要

Stable (δ~(13)C and δ~(18)O) and radiogenic ~(87)Sr/~(86)Sr isotopic data have been used to investigate the origin of cleat dawsonite (NaAlCO_3(OH)_2) in the Late Permian Wittingham Coal Measures of the Upper Hunter region in the Sydney Basin, New South Wales. The δ~(13)C_(PDB) values have a narrow range (-1.7per thousand to +2.4per thousand), with an average of+0.3per thousand, suggesting a magmatic source for the carbon. In contrast, δ~(18)O_(SMOW) values have a wide range (+13.6per thousand to +19.8per 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 ~(87)Sr/~(86)Sr (0.705032 to 0.706464) in the dawsonite.