Petrology and mineralogy of the Cu Au fertile, Archaean Burns Porphyry Intrusive Complex, Yilgarn craton, Western Australia

Hydrous, oxidised (sanukitoid-like) intrusions of the Burns Porphyry Intrusive Complex (BPIC) and their high-Mg basalt country rocks host the recently discovered Burns Cu Au deposit, in the Yilgarn craton, Western Australia. The BPIC is an aggregation of steeply-dipping, quartz-poor, amphibole-feldspar porphyry intrusions, with ubiquitous porphyritic textures and local flow textures in the groundmass, suggesting a shallow level of emplacement (1–3 km). The SiO 2 content of the BPIC, calculated on an anhydrous basis, ranges from 57.9 to 67.2 wt%. A representative porphyry sample gave a U Pb zircon age of 2663 ± 7 Ma (2σ). Amphibole phenocrysts in all porphyry intrusions formed in a subjacent magma chamber, at depths of approximately 7 km, based on amphibole geobarometry. Amphibole-dominated enclaves in the BPIC intrusions are interpreted as samples of cumulate rocks, formed at the margins of the subjacent magma chamber. Whole-rock geochemical and mineralogical data indicate that hornblende crystallized in the absence of plagioclase in the subjacent magma chamber, which comprised compositionally distinct components of alkalic, calc-alkaline and fractioned (dacitic) magma. The modally dominant feldspar phenocrysts in the BPIC porphyries crystallized in situ at the shallower crustal level of emplacement (1 to 3 km depth), but did not fractionate from the parent magma to any great extent. The average initial ƐNd of apatite = +2.8, showing that the BPIC magmas were derived from a depleted,-mantle-like source. Results of apatite and zircon analyses show that the BPIC magmas were hydrous, oxidised, and contained significant amounts of S and Cl, in common with Archaean sanukitoid-like magmas, and Phanerozoic magmas having genetic links to Cu Au mineralization. The thermal anomaly formed by the BPIC promoted the circulation of external hydrothermal fluids, which produced extensive sodic-calcic and sodic alteration in the BPIC and high-Mg basalt country rocks. These hydrothermal events, which generated only minor changes to whole-rock compositions, were a precursor to the more restricted but stronger hydrothermal alteration associated with Cu Au mineralization. • The Burns Porphyry Intrusive Complex (BPIC), associated with Cu Au mineralization, is a shallow level, steeply-dipping aggregate of multiple intrusions that span the SiO 2 range 57.9 to 67.2 % SiO 2 . • The low-SiO 2 group intrusions have a potassic and alkalic character, whereas the main group intrusions (60.5–66.0 % SiO 2 ) formed from mixed alkalic and calc-alkalic magmas, and the high-SiO 2 group intrusions formed from relatively fractionated dacitic magma. • Whole-rock and mineral composition data, and the presence of cumulate-textured, amphibole-rich enclaves in the BPIC indicate crystallization of hornblende and suppression of plagioclase crystallization in a subjacent magma chamber at 7 to 8 km depth. • Accessory mineral compositions show that the BPIC magmas were hydrous and oxidised, in common with Archean sanukitoid magmas and Phanerozoic magmas that generate Cu Au mineralization.