Pacmanus is an active submarine hydrothermal system in the Manus back-arc basin, Papua New Guinea, located at 1,600 to 1,700 m below sea level on the crest of a dacitic volcano. It is inferred to represent a modern analogue of ancient mineralizing sea-floor hydrothermal systems that produced volcanogenic polymetallic massive sulfide deposits. Ocean Drilling Program (ODP) Leg 193 drilled the subsea-floor hydrothermal alteration zone of Pacmanus in a high-temperature discharge area (Roman Ruins) and a low-temperature discharge area (Snowcap), reaching a maximum depth of 380 m below the sea floor. Evidence from short-wavelength infrared (SWIR) spectroscopy, X-ray diffraction (XRD) analyses, calculated normative mineral abundances, and electron microprobe data show that there are substantial variations in the hydrothermal phyllosilicate assemblages within the Pacmanus hydrothermal system. The altered dacite at Roman Ruins contains 10 to 25 wt percent normative chlorite+smectite and up to 15 wt percent normative illite. In contrast, altered dacite below Snowcap contains up to 50 wt percent combined normative illite+paragonite+chlorite+smectite+pyrophyllite. SWIR data show that pyrophyllite is particularly abundant between 50 and 120 and between 220 and 270 m below sea floor, indicating localized interaction of dacite with acidic hydrothermal fluids. Variations in the Na/K ratio of dioctahedral phyllosilicate, determined by electron microprobe and spectral analyses, confirm the presence of a paragonitic component at Snowcap. A model is suggested to explain the differences in the distribution of hydrothermal phyllosilicate assemblages in the two areas in terms of variations in fluid composition and changes to the subsea-floor hydrology in a dynamically evolving volcanic-hydrothermal environment.