Geochemical and geochronologic investigations into carbonate veins from historical drill cores in undercover Mt Isa, NW Queensland

This study investigates carbonate veins in five drill cores archived at Geological Survey of Queensland from NW Queensland, ~100 km south of Mt Isa, with geochemical and geochronologic approaches in order to characterise origins of fluids and their mineralisation potentials. Carbonate veins are mostly hosted in Proterozoic age (approximately 1800-1650 Ma) supra-crustal rocks, which are inferred from geophysics to be covered by 350 to 2,000 m of younger sedimentary rocks of the Eromanga and Georgina basins. Data regarding the nature of fluid activities is important for comparison between the undercover southern Mt Isa and outcropped Mt Isa mineral district, which is a world-class mineral province.

Multiple-phase carbonate veins (mostly calcite and dolomite) are identified, including late formed brittle veins, pyrite/chalcopyrite mineralisation-bearing carbonate veins, and calcite hosted in crackle breccias. Samples are prepared into one-inch polished mounts and studied with SEM-EDS, and in situ laser ablation ICP-MS for geochemical and U-Pb geochronological studies. Relatively pure calcite phases are also prepared with microdrill for stable isotope C and O analysis. Trace element datasets suggest enrichments in rare earth elements and ytterbium (REE+Y) with distinct negative Eu anomalies. Stable isotopes range ~-5 to – 15 (δ13C_VPDB‰) and ~10 to 25 (δ18O_VSMOW‰). Trace element data, Yb/La and Yb/Ca ratios, and stable isotope signatures imply that these carbonate veins have hydrothermal origins. ²⁰⁶Pb/²³⁸U geochronology data has indicated multi-phase calcite formation ranging from the late Neoproterozoic to Cretaceous. Our results provide a novel dataset to demonstrate the use of carbonate veins in revealing fluid activities in a mineral district and help the future exploration of critical mineral deposits in undercover southern Mt Isa when interpreted against regional structural data and well-documented mineralisation events in the northern Mt Isa district.