Evidence for an Early-Middle Jurassic fluid event constrained by Sm-Nd, Sr isotopes, rare earth elements and yttrium in the Bowen Basin, Australia

The Late Permian coal measures in the Bowen Basin, Australia, are extensively mineralized with calcite as cleat infilling and veins. This study utilised strontium and samarium-neodymium isotope analyses as well as rare earth element and yttrium (REY) composition of vein calcites, from two boreholes in a faulted and intruded area within the Jellinbah Thrust Zone, to investigate the origin and timing of calcite precipitation. All samples showed low Sr isotopic ratios that overlapped the range typical of mantle materials, suggesting that fluids were derived from the mantle. The calcite veins in core from Dingonose 14, with two exceptions, exhibit very similar Sr isotopic ratios yet have large variations in REY patterns and a wide range in Sm-Nd ratios. These samples defined an isochron corresponding to an age of 174 +/- 11 Ma, which may record the timing of fluid mobilization from the deeply buried part of the foreland basin during the Early and Middle Jurassic, prior to subsequent extension and uplift in the Late Cretaceous. This fluid event may be a response to the Gondwana supercontinent break-up during the Early-Middle Jurassic. Various thermal and chemical conditions resulted in the different REY patterns of calcite in Dingonose 14 core. Calcite veins in Summit 3 core showed extremely similar REY patterns and Sr isotopic ratios but different Sm-Nd isotopic compositions relative to Dingonose 14 core, with no Sm-Nd isochron able to be resolved. The structural geology of the Summit 3 locality is complex, having overturned beds and large veins of prismatic quartz present in addition to calcite, and potential for multiple periods of veining to have occurred. Hence the lack of Sm-Nd isotopic equilibrium for the Summit 3 samples is most likely due to multiple fluid events.