Geodynamics and metallogeny of the Lachlan Orogen

Abstract

The Palaeozoic Lachlan Orogen of eastern Australia is an accretionary orogen made up of structurally thickened oceanic successions, including turbidites from deep‐sea fans, andesitic volcanics from remnant island arcs, forearc sediments and slices of oceanic crust. Accretion of these units has been inferred to occur by collapse of a marginal basin during double divergent subduction within a complex southwest Pacific‐style oceanic setting. The small basin closed behind, and in the backarc position to, a major, long‐lived subduction system off Gondwanaland in the Palaeozoic. Stepwise deformation and metamorphism from Late Ordovician through Early Carboniferous times formed three distinct geological subprovinces that have interacted with each other through time and space. Metallogeny relates directly to these three provinces and their position within this tectonic framework. In the Western Subprovince, Ordovician turbidites host major lode Au deposits (e.g. Bendigo and Ballarat, central Victoria) because of structural thickening and metamorphism within a coupled thick turbidite wedge ‐ oceanic crustal layer above an inferred flat, west‐dipping subduction zone. In the Central Subprovince a Late Ordovician to Early Silurian high‐T metamorphic belt hosts Sn–W ± Mo skarn/greisen deposits as part of a more evolved, differentiated magmatic arc associated with northwest‐trending, east‐dipping subduction beneath the Tabberabbera Zone. In the Eastern Subprovince, porphyry Cu–Au deposits (e.g. Goonumbla and Lake Cowal, central‐northern New South Wales) formed within an Ordovician oceanic island arc, and volcanic‐hosted massive‐sulfide deposits (e.g. Captains Flat and Woodlawn, eastern New South Wales) formed in younger Silurian forearc sediments. Both deposit types are related to the outboard, long‐lived, west‐dipping subduction zone off Gondwanaland.