Abstract Iron oxide–Cu–Au (IOCG) deposits encompass a range of ore body shapes, including strata-bound replacement ores and hydrothermal breccias. We use the implicit method to make a detailed three-dimensional geological model of a strata-bound IOCG in the Cloncurry District, the E1 Group, to elucidate structural controls on mineralization. This model is compared with the nearby, world-class, Ernest Henry breccia-hosted IOCG deposit. Cu–Au mineralization in the E1 Group occurs as structurally controlled, mainly strata-bound, replacement bodies hosted in metasedimentary and metavolcaniclastic rocks intercalated with barren meta-andesite. Replacement bodies in the E1 Group conform to a series of NNW-plunging folds formed in regional D 2 during peak metamorphism. Folding was followed by local D 3 /regional D 4 shortening, which formed a dextral, transpressional Riedel brittle to ductile system along the regional Cloncurry Fault Zone. Modelling suggests that much of the Cu–Au mineralization is controlled by synthetic R structures associated with this Riedel system. The deformation sequence at Ernest Henry is comparable, but differences in host rock rheology, permeability and fluid pressure may explain the variation in ore body types and total Cu–Au resource between the two deposits. The results carry implications for other districts containing these styles of IOCG mineralization. Supplementary materials: Sup 1: Probability plots of assay data for modelled elements. Plots made in ioGAS software. Power transform applied to y -axes of all elements. Note that Fe, P and S do not follow normal/log-normal distributions. Sup 2: Summary statistics of assay data for modelled elements. A description of the rock type (lithology) codes used in the geological model are available in Sup 3. The 3D models presented in this paper are available as supplementary data online (Sup 4) and may be viewed in the free Leapfrog Viewer program, which can be downloaded from http://www.leapfrog3d.com/ . These supplementary files are available at https://doi.org/10.6084/m9.figshare.c.3729946

Since the discovery of CO 2 fluid inclusions in granulites, the role of fluids in the formation of these rocks has been widely studied. Owing to the complexity of the tectono-metamorphic history of granulite terrains, fluid inclusion data alone are not sufficient. They need to be integrated with geochemical and mineralogical studies done on the same rock samples. A clear understanding of the tectono-metamorphic history of granulite terranes is also indispensable. The widespread occurrence of CO 2 and the later discovered high-salinity aqueous fluid inclusions support the idea that the lower crust underwent fluid flow and that both carbonic and brine fluids played a role in its formation. Both low-H 2 O-activity fluids play a similar role in destabilizing hydrous mineral phases. Furthermore, experimental studies have shown that brine fluids have a much larger geochemical effect on granulites than initially expected. These fluids are far more mobile in the lower crust compared with CO 2 and also have the capability for dissolving numerous minerals. As in the example of the Limpopo Complex, fluid inclusions and many metasomatic features observed in granulite terranes can thus be explained only by large-scale movement of high-salinity aqueous fluids and, to a lesser extent, CO 2 , implying that lower-crustal granulites are not as dry as previously assumed. Similar brines and CO 2 -rich fluids are also found in mantle material, most likely derived from deeply subducted supracrustal protoliths.

Petrological and fluid-inclusion data of high-grade metapelitic gneisses that occur as enclaves and in the immediate surroundings of the 2.612 Ga old Bulai granitoid intrusive are presented in this chapter. The Bulai intrusive is an important time marker in the tectono-metamorphic evolution of the Central Zone of the Limpopo Complex. The host-rock gneisses show one generation of garnet, cordierite, and sillimanite, whereas the enclave gneisses show two different generations of garnet (Grt 1,2 ), cordierite (Crd 1,2 ), and sillimanite (Sil 1,2 ). The first generation defines a gneissic texture, whereas the second generation shows a random mineral orientation. Grt 1 and Crd 1 show a higher Mg content compared with Grt 2 and Crd 2 . Host rock garnet and Grt 1 show K-feldspar micro-veins at the contact with quartz as a result of high-temperature metasomatism. Host rock garnet, Grt 1 , and Grt 2 are zoned and participate in two simultaneously operating reactions: sillimanite + garnet + quartz = cordierite and garnet + K-feldspar + H 2 O = biotite + sillimanite + quartz. The combination of petrographic, geothermobarometric, and fluid-inclusion results shows evidence of two different pressure-temperature (P-T) paths in the enclave and a single P-T path in the host rocks. The decompressional cooling P-T path in the host rock is typical of the country rocks throughout the Central Zone. The high-pressure part of the host-rock P-T path overlaps with the Grt 1 -Crd 1 -Sil 1 P-T path found in the enclave rocks. The second P-T path is calculated from the Grt 2 -Crd 2 -Sil 2 assemblage and is found only in the enclave rocks. The two P-T paths in the enclave rocks can be connected by a sub-isobaric heating event of ~50 °C at 5.5 kbar. This increase in temperature is followed by decompressional cooling but with a lower P-T gradient compared with that of the country rocks caused by the emplacement of the Bulai Pluton. Fluids present during granulite metamorphism include CO 2 and brines. Retrograde infiltration of water in graphite-bearing country rocks under relatively reduced conditions resulted in the formation of a methane-rich fluid.