Intrusion-related versus orogenic gold, Western Lachlan Orogen, Tasmanides, Australia

Long-lived accretionary orogens like the Tasmanides of eastern Australia have typically undergone a series of compressional and extensional episodes with multiple magmatic and hydrothermal events that shape its rheological, compositional and metallogenic character. In the state of Victoria, the Western Lachlan Orogen (WLO) of the Tasmanides is home to one of the world’s largest gold provinces. Multi-million-ounce orogenic gold deposits of Ordovician age occur in the Bendigo and Stawell zones, whilst the oldest known gold mineralisation in the Melbourne Zone is Devonian with the largest deposits associated with a mafic to intermediate dyke swarm. Widespread Devonian magmatism in the WLO resulted in goldfields spatially associated with granites with some older orogenic gold deposits overprinted by Devonian magmatism (e.g. Maldon). This study, driven by the underexplored Tabberabbera Zone (potential long-strike extension of the Bendigo Zone according to the Lachlan Orocline model), addresses those challenges.

Eight gold deposits in the WLO of Victoria are revisited. Five are spatially associated with, and in the hornfels surrounding, the Devonian Beechworth and Yackandandah I-type granites in the northern Tabberabbera Zone (Barambogie, Twist Creek, Bon Accord, Homeward Bound and Happy Valley). Others include Haunted Stream near the Dead Bird Suite in the southern Tabberabbera Zone, Golden Mountain in the hornfels of the Devonian S-type Strathbogie Granite in the central Melbourne Zone and the classic orogenic style at Gill Reef in the world-class Bendigo Goldfield in the north central Bendigo Zone. A macro- to micro-scale approach is used, integrating micro-XRF MAIA mapping, automated mineralogy, EPMA-CL mapping, and apatite U-Pb geochronology.

At Golden Mountain, gold is disseminated in the cordierite-bearing Strathbogie Granite and hosted in fault structures in the surrounding hornfels with no Bi and Te identified. Gold mineralisation is hosted as free gold in quartz veins at Barambogie, Twist Creek and Bon Accord. These veins host apatite with U-Pb ages that are coeval with the Devonian intrusions. Occurrences of Bi and Te, lollingite, Ti in quartz and the thermal U-Pb reset of the detrital apatite suggest this gold mineralisation is intrusion-related. In contrast, at Haunted Stream, the abundance of sulfides, gold locked in pyrite and a lack of Bi and Te indicate an orogenic style. Apatite in quartz veins at Haunted Stream yield a Jurassic age suggesting reactivation of the nearby Haunted Stream Fault during the breakup of Gondwana. Apatite in low-temperature Al-rich quartz veins at Happy Valley and Gill Reef yield Triassic and Carboniferous ages respectively. Gold mineralisation at Happy Valley is not associated with the Yackandandah Granite. Instead, rare Triassic magmatism in eastern Victoria and the occurrence of weak Bi suggest a likely distal magmatic signature.  At Gill Reef, the Carboniferous hydrothermal apatite may result from late stage magmatic-related activity of the Upper Devonian Harcourt Granodiorite. Apatite located in arsenopyrite-bearing stylolitic veinlets suggest a potential gold remobilisation/enrichment event during the emplacement of the intrusion, like the Maldon deposit. New U-Pb geochronology of apatite in quartz veins and the hornfels integrated with mineralogical observations have been crucial in unravelling the cryptic gold metallogeny of the WLO.