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

摘要

Available age data enable the recognition of four groups of porphyries that were emplaced during the similar to 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,