Large-scale fluid circulation in the oceanic crust is known to be important in controlling hydrothermal and geochemical processes in a variety of geophysical settings. In some active spreading centers, hydrothermal fluid flow is thought to be a natural analog of ancient ore-forming systems for massive sulfides. In order to produce these ore deposits, large volumes of fluid must circulate through the system, requiring high permeabilities inherent in porous and fractured lavas (Fisher, 1998). In the Manus Basin, located behind an active island arc near Papua New Guinea, volcanic-hosted massive sulfide deposits are currently forming (Fig. F1). Because felsic volcanic host rocks dominate the PACMANUS hydrothermal field, it offers a modern analog for ancient volcanic-hosted massive sulfide deposits (Binns and Scott, 1993).

Intrinsic permeability plays an integral factor in controlling the fluid flow patterns that determine the spatial distribution, lateral extent, and concentration of ore deposits. Previous studies of active hydrothermal systems examined basalt-hosted and sediment-hosted mineralized settings. Leg 193 of the Ocean Drilling Program (ODP) at PACMANUS undertook the first drilling of a felsic volcanic-hosted system, providing new data regarding the physical properties and alteration features of volcanic rock from this complex hydrothermal environment. Here we present an analysis of core-scale permeability and porosity measurements of felsic volcanic rock samples from the actively venting PACMANUS hydrothermal field and compare them to rock characteristics. Permeability data can be used to help interpret the nature of fluid flow in the region and to understand the impact of hydrothermal flow on the ore-forming process.