HR: 11:45h
AN: V52A-06    [Abstracts]
TI: Hydrothermal Input into Volcaniclastic Sediments of the SuSu Knolls Hydrothermal Field, Eastern Manus Basin, Bismarck Sea, Papua New Guinea
AU: * Hrischeva, E H
EM: hrischeva@geology.utoronto.ca
AF: Department of Geology University of Toronto, 22 Russell Street, Toronto, ON M5S 3B1 Canada
AU: Scott, S D
EM: scottsd@geology.utoronto.ca
AF: Department of Geology University of Toronto, 22 Russell Street, Toronto, ON M5S 3B1 Canada
AB: Short sediment cores were examined from the active SuSu Knolls hydrothermal field in the eastern Manus back-arc basin in order to understand the origin of the hydrothermal component in sediments surrounding volcanogenic massive sulfide deposits. Their mineralogical and geochemical composition displays various inputs of intra-basin volcaniclastic, hydrothermal, terrigenous and biogenic components. A 40 cm-thick sediment recovered from the base of a core proximal to the Suzette chimney site consists of blocky nonvesicular to elongate vesicular volcanic glass fragments at different stages of alteration intermixed with pyrite, chalcopyrite, barite, gypsum, atacamite, illite, Fe oxyhydroxide, quartz, cristobalite, plagioclase and alunite. The composition indicates that the sediment was derived from erosion of volcanic edifices and old oxidized chimneys. Geochemical indicators for the mass wasting event are the extremely high concentrations of Cu (up to 2.3%) and Au (up to 3.5 ppm), elevated concentrations of As, Ba, Zn and Fe, as well as a positive Eu anomaly. The strong Cu-Au positive correlation suggests that chalcopyrite and gold-rich chimneys of the Suzette site are the source of hydrothermal detritus. 14C dating of foraminifera points to an approximate age of the beginning of the strongest mass wasting event at about 2050 ybp. This event was interrupted by deposition of a widespread apron of volcaniclastic sediment overlying the SuSu Knolls volcanic rocks. The volcaniclastic sediment consists of dacite fragments with plagioclase and pyroxene microlites, angular grains of Ca-rich plagioclase and clino- and orthopyroxenes, glass shards, cristobalite, aggregates of Si-dominated amorphous material and illite, alunite, pyrite, magnetite and barite. Based on the compositional similarity between the components of the volcaniclastic sediment and plagioclase-pyroxene porphyric dacite lavas building the SuSu Knolls together with the products of their hydrothermal alteration and mineralization, it is suggested that the volcaniclastic sediment originated from violent hydrothermal eruptions at the SuSu Knolls field. The incorporation of a hydrothermal component representing altered volcanics creates elevated concentrations of Au, Cu and Ba. On the other hand, the fast sedimentation of volcaniclastic material could obscure the geochemical signal, and in particular REE patterns, resulting from fallout of particles from a hydrothermal plume in the overlying water column. Deposition of plume derived hydrothermal material proximal to the venting source is inferred for the surface sediments of the cores adjacent to the Suzette site. Indicators are barite aggregates and Cu-containing particles of probable bacterial origin, which account for elevated concentrations of Ba and Cu. The Mn enrichment in the cores far from SuSu Knolls is considered to represent more distant deposition of the particulate fallout.
DE: 1050 Marine geochemistry (4835, 4850)
DE: 3675 Sedimentary petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2005 Joint Assembly