The large Geco, Canada, and small Palmeiropolis, Brazil, Precambrian metamorphosed volcanogenic massive sulfide deposits have several features in common, including footwall hydrothermal alteration zones formed mainly by anthophyllite-cordierite-biotite rocks. These rocks are interpreted to be basalts that had undergone hydrothermal alteration during ore deposition and subsequently were metamorphosed to upper amphibolite facies. Geco has a large volume of these rocks but no associated unaltered protoliths in contrast to Palmeiropolis where recognizable protoliths are intercalated with altered rocks. Chemically, the anthophyllite-cordierite-biotite rocks with their strong depletions of alkalies and enrichments of magnesium, aluminum, and ore metals resemble the chlorite zones that underlie many nonmetamorphosed volcanogenic massive sulfide deposits. The delta 18 O value for Geco amphibolite outside the mine area is 7.6 per mil, whereas the alteration zone has values between 3.6 and 9.4 per mil. The rocks having the lowest delta 18 O values chemically resemble those of primary chlorite alteration zones from nonmetamorphosed volcanogenic massive sulfide deposits. The host amphibolite from Palmeiropolis has whole-rock delta 18 O values between 5.6 and 7.9 per mil. The altered (anthrophyllite-biotite) rocks have similar delta 18 O values (5.2-7.9 per mil) and these are much higher than those commonly reported for chlorite zones associated with other volcanogenic massive sulfide deposits. Because no isotopic distinction exists between host and alteration zone rocks at Palmeiropolis, rehomogenization by pervasive fluid flow during metamorphism is suggested. The difference in isotope behavior between the two deposits may reflect the volume of altered rocks present. The small size of the alteration zone and the close spatial at Palmeiropolis facilitated isotopic reequilibration. At Geco, the much larger alteration zone was an effectively closed isotopic system which retained its distinctive low delta 18 O signature through high-grade metamorphism. Oxygen isotopes may be a useful exploration tool in highly metamorphosed terranes if used with caution. Large areas affected by hydrothermal alteration and, consequently, large orebodies such as Geco, may be identifiable using this technique. However, areas characterized by small hydrothermal cells, such as Palmeiropolis, are likely to have undergone isotopic reequilibration during high-grade metamorphism and would be undetectable.