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 sulfide deposit preserve magmatic oxygen isotope ratios (δ¹⁸O = 5.4‰ ± 0.3‰, n = 11) that are indistinguishable from average values of other plutonic and volcanic rocks of the Superior Province. Oxygen isotope ratios of petrographically pristine, primary quartz phenocrysts, however, are greatly elevated in δ¹⁸O and heterogeneous (δ¹⁸O = 14.2‰ to 16.1‰). Such phenocrysts are not a good monitor of magmatic processes. The presence of healed microcracks in quartz phenocrysts, as seen in cathodoluminescence images, and the variability of quartz oxygen isotope ratios suggest that recrystallization is the process of δ¹⁸O elevation. In contrast to the variability of quartz, the δ¹⁸O zircon values do not vary regardless of the degree of magnetic susceptibility. The δ¹⁸O values of zircon are interpreted to reflect the primitive, magmatic oxygen, consistent with the rhyolite geochemistry, for magmas derived by partial melting of a tholeiitic source. Our data show that the δ¹⁸O values of quartz and whole rock at Kidd Creek are a good monitor of hydrothermal exchange and are 5‰ higher than the magmatic value that is preserved in zircon due to its refractory nature.