The Macquarie Arc, Lachlan Orogen, New South Wales: its evolution, tectonic setting and mineral deposits

Abstract

The Lachlan Orogen of New South Wales, Victoria and eastern Tasmania is the best understood element of the eastern Australian Tasmanides. The Tasmanides encompass continental growth in a Neoproterozoic passive margin setting and a Paleozoic-Mesozoic active margin in east Gondwana, bounded to the east by the Pacific Ocean. In the Ordovician, the supra-subduction zone element in the Lachlan Orogen is the Macquarie Arc. This comprises one minor and three major belts of mafic to intermediate volcanic and volcaniclastic rocks, limestones and intrusions that, with two hiatuses in magmatism, span the Ordovician and extend into the Early Silurian. The three major belts in central New South Wales are separated by Silurian-Devonian rift basins and are therefore, inferred to have been rifted apart during crustal extension. Chemical and isotopic data suggest the Macquarie Arc is intraoceanic, developed on primitive oceanic crust. Despite this, key features such as its longevity, stacked magmatic phases, little deformation and flanking coeval craton-derived turbidites and black shales with no provenance mixing indicate differences from modern intraoceanic arcs. Paleogeographic setting and magmatic evolution of the Macquarie Arc provided perfect conditions for mainly porphyry-related, rich Au-Cu deposits, in the Ordovician, and especially in the Early Silurian after amalgamation of the arc with its flanking terranes.