Sm–Nd dating and rare-earth element tracing of calcite: Implications for fluid-flow events in the Bowen Basin, Australia

Late Permian coals and coal measures in the Bowen Basin, Australia, are extensively mineralised with carbonates as cleat fillings and veins. The calcites in coal samples exhibit large variations in rare-earth element (REE) patterns and Sm–Nd ratios with some superchondritic values. Samples enriched in heavy REE (HREE) relative to light REE (LREE) define an Sm–Nd isochron corresponding to an age of 235 ± 15 Ma, which is consistent with the timing of final contractional phase of the Hunter–Bowen orogeny. The corresponding initial εNd is + 1.2 combined with HREE-enriched patterns suggests significant fluid interaction with primitive source rocks, which are consistent with arc-related volcanogenic sediments in the basin. Calcites characterised by HREE depletion relative to LREE are not in Sm–Nd isotopic equilibrium with the former samples that is interpreted to be due to the effect of a later hydrothermal event during the Late Triassic–Early Jurassic. Vein calcites in mudrocks from the coal measures display more pronounced HREE depletion and significant positive EuCN anomalies, which is related to the later hydrothermal event. REE distributions, together with the basin burial history, suggest that physico-chemical environments of fluid-flow events during the two separate major tectonic regimes were significantly different. This study shows that a combined application of Sm–Nd dating and REE geochemical tracing of authigenic carbonate minerals is very useful to constrain the evolution of multiple fluid-flow events in a sedimentary basin with a complex history.