TECTONIC SETTING OF SYNOROGENIC GOLD DEPOSITS OF THE PACIFIC-RIM

More than 420 million oz of gold were concentrated in circum-Pacific s ynorogenic quartz lodes mainly during two periods of continental growt h, one along the Gondwanan margin in the Palaeozoic and the other in t he northern Pacific basin between 170 and 50 Ma. These ores have many features in common and can be grouped into a single type of lode gold deposit widespread throughout elastic sedimentary-rock dominant terran es. The auriferous veins contain only a few percent sulphide minerals, have gold:silver ratios typically greater than 1:1, show a distinct a ssociation with medium grade metamorphic rocks, and may be associated with large-scale fault zones. Ore fluids are consistently of low salin ity and are CO2-rich. In the early and middle Palaeozoic in the southe rn Pacific basin, a single immense turbidite sequence was added to the eastern margin of Gondwanaland. Deformation of these rocks in southea stern Australia was accompanied by deposition of at least 80 million o z of gold in the Victorian sector of the Lachlan fold belt mainly duri ng the Middle and Late Devonian. Lesser Devonian gold accumulations ch aracterized the more northerly parts of the Gondwanan margin within th e Hodgkinson-Broken River and Thomson fold belts. Additional lodes wer e emplaced in this flyschoid sequence in Devonian or earlier Palaeozoi c times in what is now the Buller terrane, Westland, New Zealand. Mino r post-Devonian growth of Gondwanaland included terrane collision and formation of gold-bearing veins in the Permian in Australia's New Engl and fold belt and in the Jurassic-Early Cretaceous in New Zealand's Ot ago schists. Collision and accretion of dozens of terranes for a 100-m .y.-long period against the western margin of North America and easter n margin of Eurasia led to widespread, latest Jurassic to Eocene gold veining in the northern Pacific basin. In the former location, Late Ju rassic and Early Cretaceous veins and related placer deposits along th e western margin of the Sierra Nevada batholith have yielded more than 100 million oz of gold. Additional significant ore-forming events dur ing the development of North America's Cordilleran orogen included tho se in the Klamath Mountains region, California in the Late Jurassic an d Early Cretaceous; the Klondike district, Yukon by the Early Cretaceo us; the Nome and Fairbanks districts, Alaska, and the Bridge River dis trict, British Columbia in the middle Cretaceous; and the Juneau gold belt, Alaska in the Eocene. Gold-bearing veins deposited during the La te Jurassic and Early Cretaceous terrane collision that formed the pre sent-day Russian Far East have been the source for more than 130 milli on oz of placer gold. The abundance of gold-bearing quartz-carbonate v eins throughout the Gondwanan, North American and Eurasian continental margins suggests the migration and concentration of large fluid volum es during continental growth. Such volumes could be released during or ogenic heating of hydrous silicate mineral phases within accreted mari ne strata The common temporal association between gold veining and mag matism around the Pacific Rim reflects these thermal episodes. Melting of the lower thickened crust during are formation, slab rollback and extensional tectonism, and subduction of a slab window beneath the sea ward part of the forearc region can all provide the required heat for initiation of the ore-forming processes. (C) 1998 Elsevier Science B.V . All rights reserved.