A concentrically zoned alteration pipe is present at the Amulet "A" Cu-Zn mine in the Archean Abitibi greenstone belt. The pipe consists of a central core zone of so-called "dalmatianite" (=spotted cordierite-anthophyllite rock) surrounded by a zone of biotite-bearing grid-fracture alteration. Study of the dalmatianite indicates that these rocks experienced two episodes of recrystallization. First, the original andesite was completely reconstituted during a hydrothermal event, producing a quartz-chlorite assemblage. This transformation was associated with the formation of the massive sulfide lens and presumably took place in a submarine environment. Later contact metamorphism associated with the intrusion of the nearby Lake Dufault granodiorite resulted in partial recrystallization of the chlorite and quartz to cordierite and anthophyllite.Whole-rock delta 18 O values decrease from about 6 to 10 in most of the Abitibi belt to 5 to 7 in the country rocks surrounding the ore deposit, to values as low as 3.6 in the core of the dalmatianite zone. This oxygen isotopic zoning must have been produced by the hydrothermal activity, because the dehydration reactions associated with contact metamorphism cannot have affected delta 18 O by more than 0.5 per mil. Assuming an alteration temperature of 300 degrees + or - 50 degrees C and a water/rock ratio greater than two, the hydrothermal fluid must have had delta 18 O = 0.5 + or - 1.0. This indicates that the Amulet ore deposit formed from a hydrothermal fluid that had a delta 18 O value similar both to modern seawater and to the fluids which formed the Phanerozoic massive sulfide deposits of the Kuroko and Cyprus types. The apparent constancy of delta 18 O of seawater during the Precambrian and the continued importance of seawater hydrothermal processes are important constraints which must be considered in developing models of the history of the earth.