Shallow-water (5–10 m) hydrothermal venting in a nearshore coral reef environment at Ambitle Island in the Tabar-Feni island arc, east of Papua New Guinea, occurs as focused discharge of boiling fluid from discrete ports 10–15 cm in diameter, and as dispersed discharge of diffuse bubble streams that issue through the sandy mixed carbonate-volcaniclastic sea floor. Abiotic aragonite and microcrystalline ferroan, low-Mg calcite, interlaminated with Fe-oxyhydroxides, are the prominent hydrothermal precipitates. Geochemical attributes of aragonite (δ18O, δ13C, and fluid inclusions) suggest that cements formed from a solution with salinities <5‰ at temperatures of ∼100 °C, with probable contribution of hydrothermal CO2. Sr isotope ratios in abiotic (hydrothermal) aragonite (∼ 0.704 15) are similar to those in island-arc basalt and denote considerable subsurface water-rock interaction of meteoric water derived from the adjacent volcanic island. The Sr isotope ratio of a coral sample (0.707 46) collected adjacent to a vent portal suggests coral growth within a mixed seawater-hydrothermal environment.