Critical ingredients of IOCG mineralisation in the Eastern Fold Belt of the Mount Isa Inlier: insights from combining spatial analysis with mechanical numerical modelling

Current understanding of the critical processes in the formation of iron oxide copper-gold (IOCG) deposits in the Mount Isa Inlier, Northwest Queensland is based on more than 20 years of research on these Proterozoic hydrothermal systems and their environs. One of the most popular models for the formation of these deposits involves magmatic fluids derived from the post-metamorphic (1550 to 1500 Ma) Williams-Naraku batholith granites, and either mixing with one or more external fluid sources or reaction with favourable wallrocks to form Fe- (commonly magnetite) rich alteration zones that contain vein stockwork, breccia, dissemination or replacement style mineralisation. There is also a potential link between mineralisation and widespread mafic intrusive activity, which spans the entire range of known mineralisation ages. The majority of the copper and copper-gold mineralisation in the Eastern Fold Belt is hosted within late structures of the Isan orogeny (D3 and D4), many of which exhibit strike-slip movement, and these have been associated with the localisation of the bulk of mineralisation in the area. Numerical modelling using a discrete clement technique is employed here to examine the response of a fault system in the Eastern Fold Belt to an applied stress regime. Modelled areas of combined low minimum principal stress (σ3) and high mean stress (σm)show the best correlation with deposits, but these areas do not clearly correspond to specific fault orientations or configurations. Rather, the models produce complex zoning of stress anomalies in response to the partitioning of stress across complex fault blocks, and the interaction between more competent felsic intrusive bodies, less competent metasedimentary rocks, and the fault and rock boundary complexities. The models are consistent with mineralisation occurring (or being remobilised from earlier concentrations) during a major phase of regional fluid flow facilitated by a complex fault array, late during the evolution of the Isan Orogeny, and synchronous with the waning stages of emplacement of the Williams Batholith. In combination with numerical modelling, and to allow the investigation of the considerable range of other potential geological controls on IOCG mineralisation, a GIS was developed enabling evaluation of the current understanding of critical ingredients, and the statistical ranking of the relative importance of the parameters, to gain new insights into potential controls currently unrecognised or considered less important. A 'weights of evidence' (WOFE) approach was used due to the data-rich nature of the region, and the uncertainty about the role played by particular ingredients such as mafic and felsic intrusives. Contrast patterns were ranked from best to worst predictor. Rockchip geochemistry, including copper (>249 ppm) and gold (>0.11 ppm) arc the strongest predictors, with proximity to the Corella Formation contacts the next highest predictor of copper-gold occurrences. Aero-magnetic highs are also a strong predictor, followed by north-south and northeast to east-northeast oriented faults, and proximity to mafic intrusions, lithology, gravity and fault bends associated with the same faults. New geological or exploration models for IOCG deposits in the Mount Isa Inlier must incorporate the critical ingredients highlighted in this study: (I) the significant role of the Corella Formation - Soldiers Cap contact in localising faulting, fluid flow, and juxtaposing lithologies of contrasting rheology, (2) stress partitioning, with areas that localise high strain, with low minimum principal stress being significantly more likely to have localised and focussed mineralising fluids during late stage to post Williams Batholith emplacement, (3) north-south and northeast to east-northeast oriented structures, and (4) the potential role of mafic intrusives in providing a rheological contrast and/or a potential source of sulphur facilitating copper-gold deposition. Posterior probability maps for predicting the prospectivity of the region and the distribution of copper-gold ±iron oxide deposits in the Mount Isa Inlier were produced, combining numerical results and WOFE results