During Ocean Drilling Program (ODP) Leg 193, a series of holes was drilled in the PACMANUS hydrothermal field in the Manus Basin to characterize the volcanic architecture, structural and hydrologic properties, and mineralization and alteration patterns of a felsic-hosted hydrothermal system. The goals were to better understand factors that govern the nature and location of mineral deposition, to seek evidence concerning fluid and metal sources, and to investigate subsurface microbial life (Binns, Barriga, Miller, et al., 2002).

The Manus Basin in the eastern Bismarck Sea is a fast-opening (~10 cm/yr) backarc basin bound by the inactive Manus Trench to the north and the active New Britain Trench to the south (Fig. F1). The eastern Manus Basin is characterized by a series of isolated hydrothermal fields including the PACMANUS hydrothermal field located near the crest of the 500- to 700-m-high Pual Ridge.

In this report we present the results of postcruise X-ray diffraction analyses of whole-rock samples from two sites analyzed by three different principal investigators. The sites can be separated into two groups based on their geographic locality. Site 1188 was drilled in the Snowcap hydrothermal site, representing a diffuse venting area, and Site 1189 was drilled in the Roman Ruins site, containing numerous actively high-temperature venting chimneys (Fig. F2). Hydrothermal alteration varies with depth and is complicated by overprinting relationships. The main alteration features are summarized in Binns, Barriga, Miller, et al. (2002).