On the
southern flank ofLihir Island, a group of three volcanic cones were discovered
at water depths from 1000 - 1500 m. The volcanoes are located in a
narrow zone of recent seismic activity and elevated heat flow (up to 100 mW/m2). The recovered
rocks consist of fresh alkali-olivine basalts, clinopyroxene-rich basalts, and
porphyritic phlogopite basalts. One of the volcanoes has a recently erupted
ejecta blanket (recovery of warm ash!), containing a suite of abundant mafic to
ultramafic xenoliths. These xenoliths appear to represent a unique cross-section
of subduction-modified mantle material sampled by volatile-rich primary fore-arc
magma during its ascent to the seafloor. The most spectacular finding was the
discovery of an active hydrothermal system with associated vent fauna and
mineralization at Edison Seamount, and the recovery of unusually gold-rich (up
to 43 ppm Au) hydrothermal precipitates in 1050 m water depth at the top of
Conical Seamount, only 25 km south of the world-class epithermal Ladolam gold
deposit on Lihir. The discovery of submarine hydrothermal vents associated with
highly alkaline, quartz-undersaturated fore-arc volcanics represents a
significant departure from similar hydrothermal systems on the mid-ocean ridges
and in back-arc basins. The high gold grades in hydrothermal precipitates at
Conical Seamount may indicate the first documented example of a shallow-marine
epithermal system analogous to those known on land, and has important
implications' for the gold potential of submarine arc environments.