Structural and mineralogical controls on the formation of the ‘Inter-lens’ at the Ernest Henry Deposit, Queensland

Abstract

The Ernest Henry Iron-oxide Copper Gold (IOCG) deposit is by far the largest in the Eastern Succession of the Mount Isa Inlier. In the current genetic model, the release of CO2 from fluids sourced from enriched mantle was critical to brecciation and mineralisation. However, a weakly mineralised and brecciated shear zone within the orebody named the ‘Inter-lens’ separates the orebody into two distinct lenses. The Inter-lens was not well reported early in the life of the mine and has not been taken into account in the current ore deposit models. Establishing the relative timing of the Inter-lens structure provides strong geological constraints for the formation of the orebody. In this study, optical petrographic investigations, Scanning Electron Microscopy (SEM) and Mineral Liberation Analysis (MLA) were used to investigate the protolith. Key mineral relationships and textures were assessed to reveal the paragenesis of the Inter-lens. Structural observations in oriented drill core complemented underground mapping of exposures of the Inter-lens to reveal the deformational history of the Inter-lens with respect to the Ernest Henry orebody. The protolith was revealed to be Mount Fort Constantine Metavolcanics that have undergone intense deformation with a metasomatic evolution broadly consistent with the main orebody. Mineralisation stages overprinted tectonic fabrics via veining, replacement and infill, providing direct evidence that the Inter-lens is a pre-mineralisation structure. Preservation of the Inter-lens during brecciation and mineralisation of the Ernest Henry deposit requires that the currently accepted ‘explosive’ breccia model must be revised.