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Tectonic setting of porphyry Cu – Au mineralisation in the Ordovician – Early Silurian Macquarie Arc, Eastern Lachlan Orogen, New South Wales

2007; Taylor & Francis; Volume: 54; Issue: 2-3 Linguagem: Inglês

10.1080/08120090701221672

1440-0952

R. A. Glen, Andrew J. Crawford, David R. Cooke,

earthquake and tectonic studies

Abstract Available age data enable the recognition of four groups of porphyries that were emplaced during the ∼50 million year punctuated history of the earliest Ordovician to earliest Silurian intra-oceanic Macquarie Arc in the Eastern subprovince of the Lachlan Orogen. These porphyries were not emplaced during steady-state subduction. Porphyry groups 1 – 3 formed during critical events in the evolution of the arc related to interruptions and resumptions of arc activity. They are pre-accretionary in character, in that they formed in the arc that for most of the Ordovician lay on the Gondwana Plate, above a west-dipping subduction zone, and separated from the Gondwana margin by the Wagga Basin. In contrast, Group 4 porphyries are syn-accretionary and were emplaced into tilted and deformed volcanic and volcaniclastic packages during an extension or relaxation event in the multiphase Early Silurian Benambran Orogeny, attributed to the accretion of the arc and collapse of the former backarc basin. The Lachlan Transverse Zone is a major arc-normal corridor that favoured emplacement of many porphyries commonly, but not exclusively, shoshonitic in chemistry. All porphyry groups are potentially mineralised, with Groups 3 and 4 known to contain world-class Cu – Au deposits. Key words: Lachlan OrogenMacquarie ArcOrdovicianporphyry copper gold depositsSilurian Acknowledgements This research was funded by an ARC SPIRT grant. We thank our sponsor companies for their financial and logistical support: Alkane Ltd, Goldfields Ltd, Homestake Ltd, Newcrest Mining Ltd, and North Ltd. We thank the rest of the Macquarie Arc team for discussions, David Forster for peer review, Phil Seccombe and Ian Tedder for formal reviews and Ian Percival for editorial handling, all of which improved the quality of the final paper. RAG publishes with permission of the Deputy Director-General, New South Wales Department of Primary Industries—Mineral Resources. AJC and DRC acknowledge the support of the Centre for Ore Deposit Research, an ARC Special Research Centre/Centre of Excellence.