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Isotopic studies covering some 200 km2 of the Kidd Creek Volcanic Complex, within about 10 km of the giant Kidd Creek deposit, include the analysis of 395 whole-rock and quartz phenocryst samples for oxygen isotopes and 87 whole-rock samples for hydrogen isotopes. All of the rocks of the Kidd Creek Vol canic Complex are enriched in 18O relative to fresh or even mildly altered equivalents elsewhere, comprising a range for whole rocks of δ18Owhole rock = 6.3 to 15.7 per mil. Mapped distribution of δ18O whole rock values indicates several prominent zones of lower δ18O whole rock values located in the footwall of the Kidd Creek mine sequence and in footwall-equivalent sequences at the Chance deposit. Other zones located elsewhere suggest widespread hydrothermal activity throughout the complex. Broadly conformable zones of relative 18O increase in mafic and rhyolitic rocks, primarily in hanging wall-equivalent sequences, mark waning hydrothermal activity and cooling temperatures. These broad zones are not spatially associated with either the Kidd Creek mine or the Chance deposit, but they are nevertheless related to the evolving hydrothermal activity in the Kidd Creek Volcanic Complex. Isotopic alteration of the crust was the result of long-lived hydrothermal activity (possibly on the order of 10 m.y.) that continued past the period of sulfide mineralization at Kidd Creek. The zones of 18O enrichment are, in many cases, associated with uneconomic but anomalous occurrences of Zn that may represent the manifestation of a cooling hydrothermal system still able to mobilize minor amounts of metal.

The minimum oxygen isotope composition of rhyolitic magma in the Kidd Creek Volcanic Complex inferred from analyses of phenocrysts (δ18Oquartz) was ca. 8.5 per mil due to melting or assimilation of 18O-enriched, possibly low-temperature altered igneous crust. Quartz phenocrysts with δ18Oquartz values as high as 15.4 per mil indicate subsolidus exchange with the rock matrix during regional greenschist facies metamorphism.

Hydrogen isotope studies indicate narrow ranges in δD values for all rock types except several rhyolite flows. A rhyolite flow in the footwall ultramafics, about 1,000 m beneath the Kidd Creek mine, has δD value vs. wt percent HO characteristics that mirror rhyolites emplaced and degassed in very shallow to surficial environments. At least 1 km of subsidence is inferred to have occurred over a short period of time, prior to mineralizing hydrothermal activity at Kidd Creek. An extensional (i.e., rifting) tectonic environment would promote both subsidence of the crust and deep penetration of seawater-derived hydrothermal fluids.

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