Metallogenic Setting and Temporal Evolution of Porphyry Cu-Mo Mineralization and Alteration in the Delamerian Orogen, South Australia: Insights From Zircon U-Pb, Molybdenite Re-Os, and In Situ White Mica Rb-Sr Geochronology

Abstract

Paleozoic porphyry-style hydrothermal alteration and mineralization has previously been recognized within the Delamerian orogen, South Australia, where porphyry prospects include Anabama Hill, Netley Hill, and Bendigo. However, limited exploration due in part to thick postmineralization cover hinders the understanding of the temporal context, metallogenic setting, and mineral potential of the porphyry systems along the Proterozoic continental margin of Australia. In this study, we have characterized the hydrothermal alteration and mineralization of these porphyry occurrences. Zircon U-Pb, molybdenite Re-Os, and white mica Rb-Sr ages have been determined to constrain the timing for emplacement of magmatic intrusions, precipitation of metal-bearing sulfides, and duration of hydrothermal alteration in the Delamerian orogenic belt. Zircon U-Pb laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses of nine granitoids reveal that the intrusive rocks were emplaced mostly between 485 and 465 Ma, whereas three intrusions at Bendigo have zircon U-Pb ages of 490 to 480 Ma. Molybdenite isotope dilution-negative thermal ion mass spectrometry (ID-NTIMS) Re-Os dating of the four prospects identifies two porphyry Cu-Mo mineralization events at 480 and 470 to 460 Ma, respectively. Nineteen white mica Rb-Sr LA-ICP-MS/MS (tandem mass spectrometers) analyses return an age range between 455 and 435 Ma for phyllic alteration at the Anabama Hill and Netley Hill prospects, whereas intense white mica-quartz-pyrite alteration at Bendigo prospect appears to have developed between 470 and 460 Ma. These geochronologic results indicate that the Delamerian porphyry systems postdated subduction-related magmatism in the region (514–490 Ma) but instead formed within an inverted back-arc regime, where mineralized magmas and fluids ascended along favorable lithospheric-scale structures, probably due to asthenospheric upwelling triggered by mafic delamination. Porphyritic stocks, dikes, and aplites with ages of 470 to 460 Ma are the most likely hosts to porphyry-style mineralization in the Delamerian orogen that appears to have formed simultaneously with the oldest known porphyry systems in the intraoceanic Macquarie arc (e.g., Marsden, E43, and Milly Milly; 467–455 Ma). These results emphasize the significance and potential of Early-Middle Ordovician intrusive systems to host such a type of magmatic-hydrothermal mineralization in the Delamerian orogen.

Using zircon and apatite chemistry to fingerprint porphyry Cu – Mo ± Au mineralization in the Delamerian Orogen, South Australia

“…Molybdenite Re-Os chronology revealed two porphyry Cu-Mo mineralization events at ca. 480 Ma and 470-460 Ma, respectively, with the later prevailing at Anabama Hill, Netley Hill, and Bendigo (Hong et al 2023). Hydrothermal alteration and mineralization in the Delamerian prospects are comparable to typical porphyry Cu-Mo deposits, with Cu-bearing sulfides being hosted by multiple veins, stockworks, and minor breccias.…”

“…Detailed descriptions of hydrothermal alteration, vein stages, and mineralization regarding these prospects can be found in Hong et al (2023), and concise summaries of related intrusive rocks, mineralization ages, hydrothermal alteration, and sulfide associations are listed in ESM2 Table S1. Granodiorite batholiths are the dominated intrusive phases, and diorite porphyry, monzonite, voltage, 15-20 nA beam current, and 5-µm beam diameter during the apatite sessions.…”

“…LA-ICP-MS data reduction was conducted using LADR software (https:// norsci.com/ladr). Representative analytical zircon spots with CL images can be referred to Hong et al (2023).…”

“…1b-d). These intrusions have zircon U-Pb ages ranging from 490 to 465 Ma (Hong et al 2023). Molybdenite Re-Os chronology revealed two porphyry Cu-Mo mineralization events at ca.…”

“…These results highlight a moderate magmatic oxidation state of the Delamerian porphyry systems, which could be inherited from fO 2 state of the parental magmas during post-subduction processes of the Delamerian Orogeny (ca. 514-490 Ma; Foden et al 2006;Foden et al 2020;Hong et al 2023). Ti, and initial U concentrations in zircon by adopting the approach of Loucks et al (2020), irrespective of temperature, water content, and melt composition (Fig.…”