NSW 2113 Australia. 2Marine Geology Research Lab., University of Toronto, Toronto, Ont. A basement of Eocene-Oligocene arc crust is being thinned by extension, with rapid sedimentation on the tilted blocks. Present-day submarine igneous activity, with which hydrothermal activity in the EMB is associated, has arc geochemical affinities and is related to subduction of the Solomon Sea Plate at the New Britain Trench. These are scattered along the crest of a volcanic ridge formed by fissure eruption of very fluid, mostly aphyric andesite, dacite and rhyodacite lava.
Variable salinity supports other evidence of subsurface phase separation.
Trace element and isotopic compositions of the deposits suggest an important component of magma-derived fluids and metals. Mafic dacites at the crests of the high knolls are extensively brecciated and altered, and carry widespread disseminations and stockworks of pyrite and covellite as well as localised chimneys, mounds and sulfide breccias. Suzette has exposures of both andesite and dacite, and a field of actively venting, chalcopyrite-rich massive sulfide chimneys. The latter are associated with mounds of finely laminated to coarsely banded sulfidic sediments. The nature of an intense hydrothermal plume above Susu Knolls suggests additional venting lower on the slopes than has so far been discovered.