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Gold Deposits in Western Pacific Island Arcs: The Magmatic Connection

By
Richard H. Sillitoe
Richard H. Sillitoe
8 West Hill Park, Highgate Village, London N6 6ND, England
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Published:
January 01, 1989

Abstract

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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Contents

Economic Geology Monograph Series

The Geology of Gold Deposits: The Perspective in 1988

Reid R. Keays
Reid R. Keays
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W. R. H. Ramsay
W. R. H. Ramsay
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David I. Groves
David I. Groves
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Society of Economic Geologists
ISBN electronic:
9781629490014
Publication date:
January 01, 1989

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