INTRODUCTIONThe variety of modern seafloor hydrothermal settings characterized adequately in three dimensions will be greatly expanded by drilling below an active vent field associated with felsic magmatism at a convergent plate margin. Subsurface wall rock alteration and mineral deposition processes, fluid pathways, and sources of metals and ligands for this latter environment are expected to differ significantly from those at basaltic midocean ridges previously tested by the Ocean Drilling Program (ODP) during Legs 106, 139, 158, and 169. The differences profoundly influence chemical and energy fluxes in the global ocean, and on a practical level they are highly relevant to the increasingly difficult problem of maintaining mankind's mineral resource inventories.
Felsic volcanic sequences and their associated intrusions, presumed to have erupted in convergent margin (island arc) settings, have long been recognized as especially prospective for a range of valuable ore styles, including massive sulfide deposits rich in both base and precious metals and porphyry copper-gold deposits. Understanding how such ore bodies were created in the past, by deciphering the interplay between igneous, structural, hydrothermal, and hydrologic processes in a close modern analog of such a setting, will improve the capability of future exploration geoscientists to recognize favorable signals of prospectivity in ancient sequences.
The western margin of the oceanic Pacific plate displays numerous convergent segments or subduction zones. Most of these zones show evidence of seafloor hydrothermal activity at one or more sites in their vicinity (Fig. 1). The Manus Basin in the Bismarck Sea north of Papua New Guinea is the first location other than a midocean spreading axis where hydrothermal "chimney" deposits and associated vent fauna have been discovered (Both et al., 1986). This site, now called Vienna Woods, on the basaltic Manus spreading center, near the apex of a wedge of backarc oceanic crust (Fig. 2). In contrast, eastern Manus Basin has a more complex geological construction involving creation of continental crust, and it accordingly shows closer affinities to ancient ore body settings. It contains the PACMANUS hydrothermal field, discovered in 1991 (Binns and Scott, 1993), where the host volcanic sequence is conspicuously siliceous. Now thoroughly surveyed, the PACMANUS area is where Leg 193 will address the issues raised above.
As well as elevated hydrothermal temperatures, Leg 193 faces technical challenges such as bare rock commencements in rugged volcanic terrain and the uncertain drilling characteristics of vitreous and/or altered dacitic lavas. These constraints have been adjudged by ODP to be worth confronting, given the exceptional opportunity to address the prime scientific objectives.
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