ACCRETIONARY TECTONICS AND METALLOGENY OF SE AUSTRALIA AND NW NORTH AMERICA: A COMPARISON OF STYLES AND MINERAL DEPOSIT TYPE DISTRIBUTION

The Lachlan orogen in SE Australia and the northern part of the Cordilleran orogen of North America define major Phanerozoic accretionary continental margins now preserved within the Circum-Pacific region. Mineral deposits in both areas are diagnostic of ores that developed within such a complex tectonic setting. However, varying processes of tectonism along the margins during the general seaward growth of the orogenic systems resulted in differences in the spatial and temporal distribution of deposit types.

The Lachlan developed along the Gondwanan pericratonic terrane margin. Outboard of a massive Cambro-Ordovician turbidite fan upon subducting oceanic rocks, concurrently active subduction zones led to development of an oceanic arc/accretionary system that was added to the margin in Devonian-Carboniferous time. Significant orogenic gold deposits (Bendigo, Ballarat) formed as the fan was deformed in the Ordovician. Simultaneously, porphyry Cu-Au deposits (Cadia, Parkes) developed in the oceanic arc and melts evolved into Sn-W-Mo systems (Ardlethan) by Early Silurian. Late Silurian-Early Devonian rifting and bimodal volcanism during crustal thinning below the accretionary system led to formation of Cu-Au and Pb-Zn sedimentary rock-hosted deposits (Cobar Basin) and polymetallic VMS deposits (Woodlawn, Captains Flat). The onset of outboard terrane accretion in Middle-Late Devonian led to a second, more seaward period of orogenic gold deposition (Hill End).

In contrast to the Lachlan, miogeoclinal facies are recognized along the Paleozoic margin of North America. Devonian-Carboniferous rifting in these rocks included development of giant Ag-Pb-Zn shale-hosed deposits (Red Dog, Selwyn Basin) and subordinate polymetallic VMS (Ambler). Collision and accretion of both pericratonic and exotic oceanic terranes between 185-50 Ma to form Alaska and westernmost Canada included emplacement of early Paleozoic VMS (Niblack) and skarns (Kasaan Peninsula); Late Triassic VMS of Wrangellia (Greens Creek); Jurassic porphyry/epithermal vein systems (Kerr, Sulphurets); and Late Cretaceous VMS (Beatson). Deformation during syncollisional orogenesis led to episodic formation of orogenic and intrusion-related gold deposits (Nome, Fairbanks, Juneau) that young towards the continental margin.