The SuSu Knolls Hydrothermal Field, Eastern Manus Basin, Papua New Guinea: An Active Submarine High-Sulfidation Copper-Gold System
2014
SuSu Knolls comprises three steep-sided conical volcanic peaks, standing on a N-NW-trending ridge in the eastern Manus basin,
a complex zone of convergence between the major Indo-Australian and Pacific plates. The knolls consist of three porphyritic
andesite-to-dacite domes, each 1.0 to 1.5 km in diameter, with crests ranging from 1,150 to 1,520 m below sea level. An intense
hydrothermal plume, with peak transmission anomalies in excess of 40%, originates from SuSu Knolls, and associated hydrothermal
venting, with fluid temperatures exceeding 300°C and sulfide mineralization, has been detected on the crests of all three
edifices. The most important of these is the 2.47 million metric ton (Mt) Solwara 1 copper-gold deposit on Suzette Knoll,
slated for mining by Nautilus Minerals. The porphyritic dacite is commonly strongly altered by reaction with acidic fluids.
Fragments of sulfide mineralization, collected as part of the seafloor talus surrounding the crest of North Su and South Su,
are characterized by an assemblage of pyrite-(fukuchilite)-enargite ± covellite-chalcopyrite. By contrast, there are actively
venting chimneys on the crest of Suzette that are typically zoned, with chalcopyrite-pyrite-tennantite inner zones and barite-dominated
outer zones. The knolls are covered by black sulfidic sediments that contain up to 2.4 wt % Cu and 3.2 ppm Au.
Primary feldspars have been obliterated by the hydrothermal activity that ranges from incipient to intense and which is characterized
by natroalunite, alunite (North Su only), cristobalite, tridymite, and rare quartz and kaolinite. Most volcanic rocks exhibit
a patchy surficial coating of native sulfur, which may also fill vesicles. No altered rocks were dredged from Suzette because
of the thick sulfidic sediment cover. Mass-balance calculations for altered and unaltered rock from North and South Su show
that major and trace lithophile elements are depleted in the altered rocks, except for Ti, Al, Si, and Zr (all near-immobile),
in concert with the destruction of the primary minerals and removal of primary components by acidic fluids. Sulfur and chalcophile
trace elements (i.e., As, Au, Ba, Cd, Cu, Mo, Pb, and Se), typically associated with magmatic Cu-Au mineralization, are enriched
by orders-of-magnitude in both leached and mineralized rock. Sulfide and native sulfur δ 34 S values from all three domes range from −7.4 to +0.4‰, indicating a magmatic component for the sulfur. A small group of ore
elements (i.e., Ag, Bi, In, Sb, and Zn) are strongly enriched in mineralized breccias and depleted in the altered dacites,
suggesting redistribution during alteration.
The presence of advanced argillic alteration, the paragenesis of the Fe-Cu-As sulfide assemblage and presence of native sulfur,
together with the sulfur isotope evidence for magmatic input into the hydrothermal fluid at SuSu Knolls, are consistent with
a submarine high sulfidation magmatic Cu-Au hydrothermal system. Furthermore, the sulfide assemblage pyrite-enargite (±covellite-chalcopyrite)
observed at North and South Su, and orders-of-magnitude enrichment of chalcophile elements in both altered and mineralized
volcanic rocks, relative to unaltered volcanic rock, are also characteristic of subaerial high sulfidation epithermal mineralization.
The SuSu Knolls hydrothermal field is a good example of a modern, high sulfidation, Cu-Au submarine hydrothermal system.
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