Eruptive Styles and Nature of the Volcanic Edifice Associated with the Solwara 1 Seafloor Massive Sulfide Deposit, East Manus Basin, Bismarck Sea

Abstract:

Basaltic to rhyolitic active submarine volcanism in the East Manus Basin (EMB) of the Bismarck Sea is associated with extension leading to backarc basin formation. The EMB hosts seafloor massive sulfide (SMS) occurrences, the most significant being Solwara 1, a copper-gold SMS ore deposit, located 1520mbsl, at the intersection of the Tumai and Bugave volcanic ridges. The nature of the edifice hosting Solwara 1, however, is a topic of debate. Whether it is a rifted fragment of the larger and adjacent North Su volcano, or is a separate volcanic edifice produced by eruptions along the Tumai Ridge is unknown. The approach here is to combine remotely operated vehicle (ROV) video footage, volcanic rock samples obtained from drill core and dredging, and high resolution bathymetric and backscatter data from multibeam echo sounder surveys to improve understanding on the eruptive styles and origin of the volcanic edifice, and then to provide constraints on the petrogenetic relationships between magmatism and mineralization.

Approximately 3km² of ROV footage over Solwara 1 have so far been examined. Exposed volcanic rock is not always visible because of thick volcaniclastic and/or pelagic sediment cover, but coherent lavas including pillow lavas, and volcanic breccias are observed. Limited whole-rock geochemistry indicates the volcanic rocks along the ridge range in composition from basaltic andesite to dacite. Pillow lavas are visible close to the ridge summit and pillow lava breccias are more abundant at lower elevations. The observed extent of pillow lavas, unrecognized in literature, implies low volumetric effusive eruptions most likely from vents along the ridge crest. These pillow lava occurrences contrast with the interpretation of the adjacent North Su volcano as an andesitic dome where exposed volcanics are dominantly in situ blocky jointed lava and hyaloclastite and where small explosive eruptions have likely produced the volcaniclastic sediment blanketing the ridge. The preliminary data indicate the Tumai Ridge, which hosts Solwara 1, has a wider compositional variety of magmas and different eruptive styles, and is unlikely to be a rifted fragment of the North Su volcano.

Further analysis of the data is currently underway to refine the volcanic facies interpretation to guide petrological investigations.