The PACMANUS hydrothermal system in the eastern Manus Basin is hosted by felsic volcanic rocks and thus provides the opportunity to examine contrasts with basalt-hosted systems at mid-ocean ridges. The accretion of new crust in the eastern Manus Basin differs markedly from mid-ocean-ridge settings, and examination of the consequences for hydrothermal alteration patterns may broaden our understanding of links between magmatism, tectonism, and hydrothermalisms in modern and ancient hydrothermal systems. The vesicular and brecciated nature of lava flows at PACMANUS (Paulick et al., 2004) have consequences for the patterns of fluid flow, the pervasiveness of hydrothermal alteration, and the efficiency of metal leaching by hydrothermal fluid systems. Moreover, the situation of the PACMANUS hydrothermal system behind an active island arc and its felsic volcanic affiliation may make it a close analog for many ancient volcanogenic massive sulfide (VMS) ore environments. Chemical and isotopic compositions of anhydrite (and other hydrothermal precipitates) provide valuable tracers for the nature of hydrothermal fluids (e.g., Mills and Elderfield, 1995; Mills and Tivey, 1999; Teagle et al., 1998b; Coggon et al., 2004). Here we present Sr and S isotope data as well as minor and trace element concentrations of anhydrite separated from Ocean Drilling Program (ODP) Leg 193 drill core samples from basement beneath the PACMANUS hydrothermal field in the eastern Manus Basin.