ABSTRACT
The PACMANUS (Papua New Guinea-Australia-Canada-Manus) hydrothermal site in the Manus backarc basin of Papua New Guinea is notable for its distinctly siliceous volcanic host rock (dacite) and for the fact that its massive sulfide chimneys are particularly rich in copper and gold relative to those at typical basalt-hosted hydrothermal fields in midocean and backarc spreading centers. Its geological and tectonic setting at a convergent margin is effectively destined to become continental crust—hence it is a closer analog of ancient ore body environments than other modern hydrothermal fields drilled by the Ocean Drilling Program, all of which have been at divergent plate margins.Geochemical and isotopic studies of seabed samples from the PACMANUS field imply an important role for magmatic sources of metals and mineralizing fluids. The predominance of metals leached from wall rocks by circulating seawater in high-temperature reaction zones adjacent to magma bodies—the well-established model for better-studied spreading ridge hydrothermal systems—might not apply for systems at convergent margin or island arc settings. Further clarification of such differences and of their implications for both fundamental and applied science issues demands information from the third dimension.
Leg 193 at the PACMANUS area represents a start toward satisfying this need. We will examine the internal anatomy of the system—volcanic architecture; lateral and vertical variability in wall rock alteration and sulfide mineralization patterns, including possible subsurface massive sulfide horizons; volcanological and structural controls on fluid pathways; and the relationship between all processes involved. This expedition will also provide the first tests for the presence of a deep biosphere in a convergent margin hydrothermal system, with special interest attached to subsurface hyperthermophilic microbes.
Four "bare rock" sites are planned for achieving our scientific objectives. Two sites are located at outflow zones of the hydrothermal system at, or close to, sites of focused and diffuse venting, respectively. One site is at a likely seawater inflow zone for the system, and one site is at a "background" position for providing comparisons between mineralized and unmineralized settings.