OXYGEN-ISOTOPE COMPOSITION OF ALTERATION ZONES OF HIGHLY METAMORPHOSED VOLCANOGENIC MASSIVE SULFIDE DEPOSITS - GECO, CANADA, AND PALMEIROPOLIS, BRAZIL

The large Geco, Canada, and small Palmeiropolis, Brazil, Precambrian m etamorphosed volcanogenic massive sulfide deposits have several featur es in common, including footwall hydrothermal alteration zones formed mainly by anthophyllite-cordierite-biotite rocks. These rocks are inte rpreted to be basalts that had undergone hydrothermal alteration durin g ore deposition and subsequently were metamorphosed to upper amphibol ite facies. Geco has a large volume of these rocks but no associated u naltered protoliths in contrast to Palmeiropolis where recognizable pr otoliths are intercalated with altered rocks. Chemically, the anthophy llite-cordierite-biotite rocks with their strong depletions of alkalie s and enrichments of magnesium, aluminum, and ore metals resemble the chlorite zones that underlie many nonmetamorphosed volcanogenic massiv e 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 b etween 3.6 and 9.4 per mil The rocks having the lowest delta(18)O valu es chemically resemble those of primary chlorite alteration zones from nonmetamorphosed volcanogenic massive sulfide deposits. The host amph ibolite from Palmeiropolis has whole-rock delta(18)O values between 5. 6 and 7.9 per mil. The altered (anthophyllite-biotite) rocks have simi lar delta(18)O Values (5.2-7.9 parts per thousand) and these are much higher than those commonly reported for chlorite zones associated with other volcanogenic massive sulfide deposits. Because no isotopic dist inction exists between host and alteration zone rock at Palmeiropolis, rehomogenization by pervasive fluid flow during metamorphism is sugge sted. The difference in isotope behavior between the two deposits may reflect the volume of altered rocks present The small size of the alte ration zone and the close spatial association of nonaltered and altere d rocks at Palmeiropolis facilitated isotopic reequilibration. At Geco , the much larger alteration zone was an effectively closed isotopic s ystem which retained its distinctive low delta(18)O signature through high-grade metamorphism. Oxygen isotopes may be a useful exploration t ool 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, area s characterized by small hydrothermal cells, such as Palmeiropolis, ar e likely to have undergone isotopic reequilibration during high-grade metamorphism and would be undetectable.