Hydrothermal Alteration of Oxygen Isotope Ratios in Quartz Phenocrysts, Kidd Creek Mine, Ontario

King, Elizabeth M., Barrie, C. Tucker, and Valley, John W. (1997) Hydrothermal alteration of oxygen isotope ratios in quartz phenocrysts, Kidd Creek mine, Ontario: Magmatic values are preserved in zircon. Geology, v. 25, n. 12, p. 1079-108

Abstract

Zircons from rhyolites hosting the Kidd Creek volcanogenic massive sulfide deposit preserve magmatic oxygen isotope ratios (d18O = 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 d18O and heterogeneous (d18O = 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 d18O elevation. In contrast to the variability of quartz, the d18O zircon values do not vary regardless of the degree of magnetic susceptibility. The d18O 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 d18O values of quartz and whole rock at Kidd Creek are a good monitor of hydrothermal exchange and are 5‰ higher than the magmatic value which is preserved in zircon due to its refractory nature.

Figure 2. Histogram of average d18O zircon values from plutonic and volcanic rocks from granite-greenstone belts of Superior Province (King, 1997) and volcanic samples from Kidd Creek area. Large range in values seen in plutonic rocks is related to chemical heterogeneities (see text). VSMOW is Vienna standard mean ocean water.
Figure 3. Plot of d18O values for zircon (Zrc), quartz, whole-rock (WR), and "calculated quartz" (see text). d18O (WR) is analyzed from sample PN77-22. Range of calculated quartz values is based on range of d18O (Zrc) from all four samples at 900 C. For samples with multiple analyses, all data and associated error bars fall within symbol. SMOW is standard mean ocean water.