HYDROTHERMAL ALTERATION OF OXYGEN-ISOTOPE RATIOS IN QUARTZ PHENOCRYSTS, KIDD CREEK MINE, ONTARIO - MAGMATIC VALUES ARE PRESERVED IN ZIRCON

Zircons from rhyolites hosting the Kidd Creek volcanogenic massive sul fide deposit preserve magmatic oxygen isotope ratios (delta(18)O = 5.4 parts per thousand +/- 0.3 parts per thousand, n = 11) that are indis tinguishable from average values of other plutonic and volcanic rocks of the Superior Province, Oxygen isotope ratios of petrographically pr istine, primary quartz phenocrysts, however, are greatly elevated in d elta(18)O and heterogeneous (delta(18)O = 14.2 parts per thousand to 1 6.1 parts per thousand). Such phenocrysts are not a good monitor of ma gmatic processes, The presence of healed microcracks in quartz phenocr ysts, as seen in cathodoluminescence images, and the variability of qu artz oxygen isotope ratios suggest that recrystallization is the proce ss of delta(18)O elevation, In contrast to the variability of quartz t he delta(18)O zircon values do not vary regardless of the degree of ma gnetic susceptibility, The delta(18)O values of zircon are interpreted to reflect the primitive, magmatic oxygen, consistent with the rhyoli te geochemistry, for magmas derived by partial melting of a tholeiitic source, Our data show that the delta(18)O values of quartz and whole rock at Kidd Creek are a good monitor of hydrothermal exchange and are 5 parts per thousand higher than the magmatic value that is preserved in zircon due to its refractory nature.