Geological characteristics of the Ernie Junior Iron-Oxide-Copper-Gold ore body, Mt Isa Inlier, North West Queensland

Abstract

The Ernest Henry Iron Oxide Copper Gold (IOCG) deposit is situated ~35km NE of Cloncurry, QLD in the Proterozoic Mount Isa Inlier. It is the second largest IOCG deposit in Australia with a pre-mining resource of 167 Mt @ 1.1% Cu and 0.54 ppm Au). The ‘Ernie Junior’ orebody exists between Ernest Henry and the bounding Footwall Shear Zone. This study presents the first characterisation and genetic model for the Ernie Junior ore body. Core logging, petrology, SEM, MLA and 3D modelling are undertaken on representative samples. The Ernie Junior ore body is hosted within a meta-volcanic sequence of fractured and clast-supported breccias. Host rocks have been subject to 1) regional albite alteration, 2) localised potassic alteration and 3) K-feldspar alteration associated with host breccias. Ore is also hosted in later stage veins where stage 3 alteration is present. The infill ore assemblage is comprised of chalcopyrite, gold, and gangue minerals magnetite, calcite, quartz, titanite and barite. High intensity K-feldspar alteration is coincident with brittle deformation. Brecciation is not as well developed in Ernie Junior as it is in Ernest Henry, possibly accounting for its lower grade. Outside the breccia zone, foliations and replacement textures indicate ductile deformation linked to the Footwall Shear Zone. The paragenetic sequence and lateral zonation of Ernie Junior is comparable to that of Ernest Henry providing evidence that the two ore bodies are highly genetically related. Potential exists for further repetitions of Ernest Henry and Ernie Junior mineralisation north of the Footwall Shear Zone and this may be a viable target for future near-mine exploration.