Geology and genesis of the Kingswood porphyry Cu-Au prospect, NSW
Creators: name: Dobbin, FH
Publisher: name: University of Tasmania
Year: 2024

description: Kingswood is a porphyry Cu-Au prospect within Magmatic Resources’ Myall project, located ~85 km north of Parkes, NSW. It is hosted by the Narromine Igneous Complex (NIC) of the Junee-Narromine Volcanic Belt, part of the broader Macquarie Arc. The NIC hosts the Phase 1 Narromine volcanics, plutonic rocks of the Phase 2 Monzogabbro – Monzodiorite – Monzonite (MMMz) suite, and tonalite and quartz monzonite stocks and pipes, and andesite dykes, all previously correlated with the Middle Ordovician Phase 3 Copper Hill suite.Magmatism, alteration, and mineralisation at Kingswood can be divided into four stages. The trachybasalt is the oldest lithology at Kingswood and is interpreted to be part of the Early Ordovician Narromine volcanics. A large monzodiorite pluton of the MMMz suite intruded the trachybasalt and was associated with the development of local magmatic breccias during the pre-mineralisation stage. A tonalite porphyry that intruded the monzodiorite produced widespread sodic – calc-potassic alteration and minor Cu-Au-Mo mineralisation during the early-mineralisation stage. The syn-mineralisation stage was associated with the emplacement of the quartz monzonite porphyry and feldspar porphyry. Magmatic-hydrothermal breccias formed at this time, along with potassic and propylitic alteration assemblages. Felsic dykes intruded prior to the development of phyllic alteration. Strong copper (potassic – phyllic alteration related) and gold (potassic related) mineralisation developed at this stage and overprinted early-stage mineralisation, occurring as breccia cement, veins, and sulfide disseminations. The system was then intruded by late-mineralisation stage mafic dykes, and locally overprinted by argillic alteration.U-Pb zircon dating of the monzodiorite pluton returned two ages (463.6 ± 1.2 Ma and 452.9 ± 2.0 Ma). The younger age is interpreted to be the result of lead loss, and the older age the timing of monzodiorite emplacement, corresponding to previous results for the MMMz suite. The early-mineralisation event at Kingswood has been constrained by Re-Os dating of an early molybdenite vein at 454.3 ± 1.9 Ma. This vein is interpreted to be associated with the emplacement of the tonalite porphyry and is consistent with Phase 3 magmatism defined by Copper Hill suite intrusions from other calc-alkaline porphyry prospects in the Macquarie Arc. The syn-mineralisation stage commenced with the emplacement of the quartz monzonite, which returned two U-Pb zircon ages (455.8 ± 1.5 Ma and 442.8 ± 2.2 Ma). The older age is interpreted to be inheritance of older zircons, and the younger age is the preferred age of crystallisation. This constrains the quartz monzonite and syn-mineralisation stage at Kingswood to Phase 4 Macquarie Arc magmatism.The Phase 1 trachybasalt and broader Narromine volcanics are shoshonitic units, which are equivalent to Phase 1 Macquarie Arc magmatism and similar to the Nelungaloo Volcanics in the Northparkes district. The monzodiorite and MMMz suite are high-K calc-alkaline, which geochemically and temporally (emplacement ca. 464 Ma) indicates that the monzodiorite is associated with Phase 2 Macquarie Arc magmatism. The tonalite porphyry, quartz monzonite, and feldspar porphyry are all medium-K calc-alkaline units. This, combined with the geochronology results, suggests that the tonalite porphyry is associated with the Copper Hill suite, however the quartz monzonite and feldspar porphyry are too young, and instead define a medium-K calc-alkaline event during Phase 4 magmatism in the Macquarie Arc.The age and magmatic affinities of the Kingswood porphyry Cu-Au system has implications for exploration in both the Kingswood district and the Macquarie Arc. Kingswood contains mineralising phases associated temporally with Phase 3 (similar to Copper Hill, Cargo, and Cowal), and Phase 4 (similar to Cadia and Northparkes) magmatism. Kingswood is the first documented calc-alkaline Phase 4 related porphyry system, and the third documented mineralised magmatic-hydrothermal breccia pipe in the Macquarie Arc (along with Two-Thirty and Boda). The highest Cu-Au grades at Kingswood are located where early-stage mineralisation has been overprinted by syn-mineralisation phyllic alteration. Follow-up drilling of these areas, typified by the contacts between the tonalite porphyry and quartz monzonite porphyry, and delineation of the roots of the magmatic-hydrothermal breccias, is recommended to fully delineate the extents of mineralisation at Kingswood.