Richard H. Sillitoe, 1989. "Gold Deposits in Western Pacific Island Arcs: The Magmatic Connection", The Geology of Gold Deposits: The Perspective in 1988, Reid R. Keays, W. R. H. Ramsay, David I. Groves
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This analysis of the western Pacific gold province, from Japan in the north to North Island, New Zealand, in the south, takes hccount of 56 principal gold deposits. All the deposits were emplaced during the last 25 Ma as integral parts of volcanoplutonic arcs, which were constructed during or immediately following episodes of subduction. Extension affected all or parts of several arcs at the times of gold mineralization and led to the generation of alkalic or weakly bimodal magmatic suites.
The gold deposits comprise porphyry-type stockworks, contact metasomatic skarns, and the unique Porgera deposit, all related intimately to intrusions; volcanic-hosted epithermal deposits of acid sulfate and adularia-sericite types; and a single kuroko-type massive sulfide deposit. Preliminary data suggest that the type of gold deposit is dependent, to some degree at least, on the composition of the host magmatic suite. At least 75 percent of western Pacific gold is contained in intrusion-related deposits and in epithermal deposits confined to porphyry copper systems. In these porphyry systems, acid sulfate-type gold deposits are commonly located above the mineralized stocks whereas adularia-sericite-type deposits tend to occur distally around the stocks. Many of the gold deposits are located in volcanic centers, with flow-domes and/or maar-diatreme systems acting as hosts for a number of epithermal deposits.
Gold in the intrusion-related deposits was introduced largely with copper as a component of hot magmatic hydrothermal brines. In contrast, cooler and more dilute meteoric hydrothermal fluids were instrumental in the final concentration of the epithermal gold. However, it is proposed that the gold contents of the meteoric hydrothermal fluids were contributed directly during admixture with magmatic brines or volatiles, and/or indirectly as a result of partial remobilization of magmatic hydrothermal gold ore or protore. Upgrading of magmatic hydrothermal preconcen-trations may be a fundamental factor in the generation of giant epithermal gold deposits. The geologic evidence, supported by the results of a few laboratory studies, favors direct and/or indirect provision of gold by magmas rather than its being scavenged from diverse gold-poor country rocks during leaching in subaerial or submarine geothermal systems.
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When the price of gold rose from about $200 (U.S.) an ounce in 1979 to nearly $700 an ounce by the end of the same year, the gold rush of the 1980s was under way. Gold production in the western world rose dramatically; from 1981 to 1986 production increased by 300 to 1,282 metric tons per year. Annual production may reach 1,500 to 1,600 metric tons by 1990 (Woodall, 1988). The major contributors to the increased stream of gold have been Australia, Canada, Brazil, and the United States together with other circum-Pacific countries. The increased price of gold and new methods of extraction have allowed many older deposits to be reopened, but the most important factor has been the high success level of exploration. This success has resulted in large part from the application of new genetic models and from the development of new exploration techniques.
There are hundreds of thousands of reported gold occurrences around the world. The majority are alluvial placers, but large numbers of bedrock occurrences have also been discovered. Most of these occurrences prove to be very small and are relatively unimportant in the overall world production level. Most mined gold has come from a small number of giant deposits, which were found by prospectors. It is becoming increasingly clear, however, that the discovery of giant deposits in the future will involve more than the sharp eyes and persistence of the old prospector. The use of sound geologic principles, and exploration programs based on those principles, is what the future holds. An example can be seen in the successful search for gold deposits in the South Pacific. There, exploration models have been based on principles developed in the study of modern geothermal systems. Giant deposits such as Lihir and Porgera have been the reward. Another example is the giant copper-gold-uranium deposit at Olympic Dam, South Australia, discovered beneath 300 m of cover using an exploration program based on models developed by Western Mining Corporation geologists for Zambian copper belt-type deposits.
Gold deposits are widely dispersed throughout many geologic settings and in virtually all kinds of rocks, but they do not seem to have formed at a uniform rate throughout geologic history. On the contrary, two very distinct metallogenic periods have been defined. The first is the Archean era, when most of the great deposits in greenstone belts were formed and the vast Witwatersrand basin deposits in