Tectonics, petrology and hydrothermal processes in areas of alkaline island-arc volcanoes in the Southwest Pacific: The Taba-Lihir-Tanga-Feni Island Chain, Papua Neu Guinea. Cruise report SONNE 94, 27.02-14.04.1994, Singapore-Rabaul-Manila (English)

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Cruise SO-94 (EDISON: Epithermal Deposits Southwestern Pacific Ocean) of R/V Sonne completed a detailed investigation of the Tabar-Lihir-Tanga-Feni island chain in the New Ireland forearc basin of Papua New Guinea. The objectives of this program were to better understand tectonics, petrology, and ore-forming processes in an area of young, alkaline forearc seamounts. Detailed mapping and sampling was carried out in the vicinity of each of the four island groups of the Tabar-Feni chain. On the southern flank of Lihir island, a group of three volcanic cones ("Edison, Conical, and TUBAF Seamounts") 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/m ). The recovered volcanic rocks consist of fresh alkali-olivine basalts, clinopyroxene-rich basalts (ankaramites), and porphyritic phlogopite basalts. One of the volcanoes has a recently erupted ejecta blanket (recovery of warm ash!), containing abundant mafic to ultramafic xenoliths, ranging from spinel Iherzolite, dunite and harzburgite, to wehrlite, gabbro, anorthosite, serpentinite and amphibolite. This suite appears to represent a unique cross-section of subduction-modified mantle material sampled by volatile-rich primary forearc magma during its ascent to the seafloor. A fourth large volcano ("New World Seamount") northeast of Lihir measures about 2500 m at its base and has a 100 m deep pit crater at about 1200 m depth. The most spectacular findings were the discovery of an active hydrothermal system with associated vent fauna and mineralization at "Edison Seamount", and the recovery of unusual 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 epithermal Ladolam gold deposit on Lihir. The discovery of submarine hydrothermal vents associated with highly alkaline, quartz-undersaturated forearc volcanics represents a significant departure from similar hydrothermal systems associated with MORB- type and felsic calc-alkaline lavas on the mid-ocean ridges and in back-arc basins. The high gold grades in hydrothermal precipitates at "Conical Seamount" may indicate the first example of a shallow- marine epithermal system analogous to those known on land, and has important implications for understanding the origin of large Ladolam-type epithermal gold deposits.