Petrology, geochemistry, U-Pb Zircon ages and structure of the Yiddah Porphyry Cu-(Au-Mo) prospect of the Late Macquarie Arc, NSW

The Yiddah porphyry Cu-(Au-Mo) system of central-southern New South Wales is contained within the Ordovician-Silurian Goonumbla-Trangie Volcanic Belt of the Macquarie Arc, 12km north of the village of Barmedman and 2 km east of the Gilmore Fault Zone. It is one of several small and poorly understood porphyry-related systems contained within a ~ 40 km north-northwest striking region known as the Rain Hill district. Petrography (microscopy, XRD, ion microprobe), whole rock geochemistry (XRF, ICP-MS), geophysical imagery and geochronology (U-Pb SHRIMP zircon dating) have been used to describe the geology and produce a petrogenetic model of the Yiddah system. This study shows that emplacement of the Yiddah porphyry Cu-(Au-Mo) system occurred during the early Silurian (433.8 ± 6.4 Ma) with the intrusion of a number of oxidised amphibole-feldspar porphyritic stocks of previously unrecognised primary monzodiorite shoshonitic affinity. These mineralised stocks evolved from a contemporaneous (439.2 ± 6.4 Ma) and genetically related sub-equigranular monzodiorite granitoid located to the west and beneath the mineralised zone. Geophysical, petrographic and geochemical findings indicate that this granitoid represents the northern extension of the Rain Hill Monzodiorite identified in the southern Gidginbung Volcanics. Mineralised stocks were intruded into two steeply eastward dipping volcaniclastic units; a low-K calc-alkaline basaltic volcaniclastic and a high-K calc-alkaline andesitic volcaniclastic. The Yiddah porphyry system has an early, central chlorite-magnetite alteration zone containing disseminated and quartz-seam hosted chalcopyrite and molybdenite, a similarlymineralised overlying chlorite-sericite zone and a further overlying lesser-mineralised sericitic zone. Beneath the chlorite-magnetite zone, propylitic style alteration is developed within the basaltic volcaniclastics and the Rain Hill Monzodiorite, neither of which are significantly mineralised. Post-emplacement deformation relating to movement of the Gilmore Fault Zone generated sub-greenschist facies metamorphism at temperatures of ~ 300oC as constrained by chlorite geothermometry. The shear zone-related deformation manifests itself as a strong north-northwest trending foliation, a pervasive ‘regional-propylitic’ overprint and the minor mobilisation of chalcopyrite. All of the intrusive samples analysed possess trace element signatures consistent with subduction-related tectonic settings with a possible crustal component (moderate LIL/HFS element ratios and Ta-Nb depletion). Shoshonitic magmatism is therefore interpreted to be the result of either; (1) latest Macquarie Arc activity, whereby incorporation of an increased flux of Gondwanan-derived subducted sediment occurred, or (2) postcollisional collapse immediately following accretion of the arc onto eastern Gondwana, analogous to the configuration currently observed in the Northern Taiwan Volcanic Zone (Luzon Arc). The recognition of late Macquarie Arc shoshonitic affinity magmatism within the Gidginbung Volcanics is significant as it provides a genetic link between the Rain Hill Cu-(Au-Mo) porphyry systems and the world class Cadia and Goonumbla porphyry districts; thus improving prospectivity in this relatively under-explored portion of the Macquarie Arc.