The Turner-Albright sulfide deposit, part of the Josephine ophiolite, formed on and below the seafloor during Late Jurassic volcanism at a back arc spreading center. Ore fluids were probably localized by faults which were active on the seafloor at the time of sulfide deposition. The uppermost massive sulfide formed on the seafloor at hydrothermal vents, similar to sulfide-depositing hot springs on modern spreading centers. The bulk of the sulfide mineralization formed below the seafloor within olivine basalt hyaloclastite erupted near the time of mineralization. Infiltration of hydrothermal fluid into the hyaloclastite altered the rock to quartz+Fe-chlorite and quartz+sericite+Fe-chlorite. Intense alteration resulted in replacement of the protolith by quartz+sulfide. The fluid responsible for the hydrothermal alteration was evolved seawater with low pH and Mg and high Fe. Sulfide Δ34S values average 4.7%0 and indicate contribution of sulfur from both basalt and seawater sulfate. The average value of sulfide Δ34S and the difference between sulfide and contemporaneous seawater sulfate Δ34S values are similar to ophiolite-hosted sulfide deposits in Cyprus. Chlorite-rich hydrothermally altered basalt that Δ18O values of 3.0--6.8%0 due to high-temperature (250¿--350 ¿C) reaction with a seawater-derived ore fluid. Quartz in silicified basalt has higher Δ18O values of 12.3--15.5%0, probably reflecting lower-temperature recrystallization of amorphous silica formed during initial alteration of basalt. Mudstone and clinopyroxene basalt above the sulfide horizons were not altered by the ore-transporting hydrothermal fluid, but these rocks were hydrothermally metamorphosed by altered seawater heated by deep circulation into hot oceanic crust. This subseafloor metamorphism produced a mineral assemblage typical of prehnite-pumpellyite facies metamorphism. Exchange with altered seawater increased the whole-rock Δ18O of the basalts to values of 9.4--11.2%0. ¿ American Geophysical Union 1988