The Ocean Drilling Program (ODP) has, over the last 10 years, dedicated four expeditions to investigations into the subsurface nature of three different manifestations of seafloor hydrothermal systems. Studies of cores recovered from the hydrothermal mound in the Atlantic discovered during the Trans-Atlantic Geotraverse cruise (Humphris, Herzig, Miller, et al., 1996; Herzig, Humphris, Miller, and Zierenberg, 1998) explored the interaction between mid-ocean-ridge basalt (MORB) and hydrothermal fluid, while expeditions to Juan de Fuca Ridge (Davis, Mottl, Fisher, et al., 1992; Mottl, Davis, Fisher, and Slack, 1994; Fouquet, Zierenberg, Miller, et al., 1998; Zierenberg, Fouquet, Miller, and Normark, 2000) and to Escanaba Trough (Fouquet, Zierenberg, Miller, et al., 1998; Zierenberg, Fouquet, Miller, and Normark, 2000) examined hydrothermal processes in marine environments with high sedimentation rates. These expeditions demonstrated that the chemistry of seawater (Butterfield et al., 1994; Tivey et al., 1998; James et al., 1998) and the ocean crust (Kurnosov et al., 1994; Honnorez et al., 1998; Humphris et al., 1998; Stuart et al., 1999) are both intimately related to hydrothermal circulation. Additional interest in the ODP expeditions was fueled by the understanding that each of the systems investigated host mineral deposits that are analogous to ore deposits on land (Duckworth et al., 1994; Krasnov et al., 1994; Hannington et al., 1998; Zierenberg et al., 1998) and that they support unique seafloor and subseafloor biological communities (Reysenbach et al., 1998; Cragg et al., 2000; Summit et al., 2000). As a result of each of these expeditions, hypotheses relative to the formation and evolution of ancient ore deposits were either validated or revised (e.g., Duckworth et al., 1994; Humphris et al., 1995; Hannington et al., 1998; Herzig et al., 1998; Zierenberg et al., 1998).
The fourth expedition in this series was targeted at the PACMANUS (named after participants in the discovery team, Papua New Guinea, Australia, and Canada and the location of the expedition) hydrothermal system in the eastern Manus Basin (Fig. F1A) on top of Pual Ridge (Fig. F1B). This volcanic edifice is 1–1.5 km wide, 20 km long, and rises 500–600 m above the surrounding ocean floor (Fig. F1C). Previous dredge and submersible-based sampling demonstrated that it consists mainly of dacite and rhyodacite lava flows (Binns and Scott, 1993; Binns et al., 1996b; Waters and Binns, 1998). Trace element and isotopic analyses of the high-silica rhyodacite of Pual Ridge (Binns et al., 1996b; Woodhead and Johnson, 1993) suggest that the felsic lavas may have a geochemical affinity to the subareal volcanic rocks of New Britain. The descriptions of this area are the result of several expeditions including PACMANUS (Franklin 1991, 1993, 1996, and 1997), EDISON-I (Sonne, 1994), ManusFlux (Yokosuka, 1995), BIOACCESS (Natsushima, 1996, 1998), and KODOS'99 (Onnuri, 1999) cruises. The primary objective of ODP Leg 193 was to combine studies of the volcanology, structure, hydrology, mineralization, and microbiology to develop a comprehensive model for this hydrothermal system. This would allow us to compare PACMANUS, a felsic-hosted system, to MORB-hosted systems and ancient ore bodies.
Felsic volcanic sequences associated with convergent margins (island arcs) may host economic deposits of massive sulfides including base and precious metal mineralization. PACMANUS represents an environment where surface sampling revealed elevated concentrations of Au, Ag, and Pb (Herzig and Hannington, 2000). Our goal was to delve into the subsurface to determine the extent of and controls on mineralization in this environment.
Despite the extensive seafloor exposure of high-temperature sulfide chimneys and diffuse hydrothermal fluid reaction zones, subsurface mineralization was rare, indicating a very young mineralizing system (Shipboard Scientific Party, 2002a). However, in every hole that penetrated more than a few meters into the subsurface, pervasive hydrothermal alteration indicates long-lived active hydrothermal circulation. Shipboard reconnaissance studies indicated that each sampled site had distinct mineralogical and geochemical characteristics suggesting the possibility of separate sources for lavas or at least distinct evolutionary trends.