A shear-hosted Au-Cu-Bi metallogenic event at ~1660 Ma in the Tennant Creek goldfield (northern Australia) defined by in-situ monazite U-Pb-Th dating

Abstract

High-grade Au, Cu and Bi ores in the Tennant Creek goldfield have been mined from hydrothermal magnetite- and/or hematite-rich ironstone bodies. Less well known is a style of Au-Cu-Bi mineralisation hosted by quartz vein systems within shear zones outside ironstones. We report ion probe (SHRIMP) U-Pb-Th ages of 1659 ± 13 Ma and 1659 ± 15 Ma for hydrothermal monazite associated with this mineralisation style at the Orlando East Au-Cu-Bi deposit and Navigator 6 Au prospect, respectively. The dated monazite at Orlando East is intergrown with Cu and Bi sulfides and Au in quartz-chlorite veins within a shear zone. At Navigator 6 the monazite occurs both in late-stage chalcopyrite-bearing quartz-hematite veins and within earlier-formed sheared chloritic breccia that also contains anomalous Au.Although the full significance of the ~1660 Ma monazite ages has yet to be determined, the results indicate the introduction of Au, Cu and Bi in quartz vein systems at ~1660 Ma, or remobilisation of earlier, ~1850 Ma, main-stage Au-Cu-Bi from mineralised ironstones and into quartz veins 10 s–100 s of metres from the initial mineralisation. In either case, hydrothermal fluids capable of transporting significant concentrations of ore metals were present in the Tennant Creek goldfield at ~1660 Ma, synchronous with deformation. A key implication is that rock sequences up to ~200 million years younger than the ironstone-hosted Au-Cu-Bi deposits may now be considered prospective for a second stage of Au ± Cu ± Bi mineralisation. The results also validate non-ironstone shear-hosted deposits in the Tennant Creek region as an alternative exploration target to the well known high-grade but low-tonnage Tennant Creek ironstone-hosted deposits.