ALBERT

Description: 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 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.

Description: The 2.7 m Bikpela chimney differs from sulfide-rich chimneys at PACMANUS, eastern Manus basin, by virtue of high silica content, a stromatolite-like layered structure, and the presence of a large internal collapse cavity. Its interior is dominated by 5- to 10-mm-thick primary laminae of opal-A-dominated fronds clouded by submicroscopic sphalerite inclusions, set between thin subhorizontal growth borders. Larger sphalerites with sulfosalt-galena intergrowths at frond ends project into discoid cavities under the growth borders. A steeper, second-order lamination reflects arrested expansion of the 50-cm-high internal collapse cavity. Underlying the cavity floor an axial breccia pipe contains distorted fragments of laminated interior modified by dissolution and then redeposition of opal and further growth of pyrite and sphalerite. Beneath a thin Mn oxide crust, a hard outer sphalerite-barite carapace ~5 cm thick and devoid of opal is banded parallel to the chimney exterior. At least three stages of chimney growth are evident, beginning with repetitive deposition of opaline frond layers probably developed from microbial mat precursors by overgrowth of hydrothermal opal and fine sphalerite. Next, with rising fluid temperatures, hydrothermal Fe-poor sphalerite with Pb sulfosalts and galena deposited at frond ends and under growth borders and, via lateral diffusion and mixing with seawater, formed abundantly in the outer carapace of the chimney. Passage of hot, more acid fluids into the chimney axial zone then caused dissolution of primary opal and consequent formation of the collapse cavity and underlying breccia pipe, together with deposition of Fe-rich sphalerite and pyrite in second-order fractures and cavities related to the collapse. Finally, conductive cooling during or after this latter stage produced films and spherules of clear opal coating pores and grain margins throughout the structure, apart from in the carapace. Bikpela contains significant gold and silver (avg: 8.7 ppm Au, 250 ppm Ag; n = 9), but it is low overall in Cu (up to 0.5 wt % in the pipe) and Pb (up to 0.7 wt % in the carapace). Its carapace is relatively rich in Zn (up to 54 wt %), Ag, As, and Sb, with one sample containing 0.12 wt % silver.