Abstract Oxygen isotope mapping in rhyolitic rocks hosting the Kidd Creek volcanic-hosted massive sulfide deposit indicates that the ores are associated with a zone of relatively low δ 18 O values (9.7-12‰). Zones of higher δ 18 O values (13-15.8‰) occur 300 to 500 m stratigraphically above ore and are associated with massive rhyolite bodies as the footwall to the orebodies. The δ 18 O values (7.2-11.3‰) of mafic rocks are lower than those of rhyolitic rocks from the strongly silicified zone immediately underlying the massive sulfide bodies. Mafic rocks with the lowest δ 18 O values (<9‰) occur in the core of a diorite sill stratigraphically above the ore zone. Hydrogen isotope mapping indicates that a zone of low δD values (<–40‰) extends at least 500 m stratigraphically below the orebodies. Most chlorite associated with chalcopyrite stringers has lower δ 18 O (2.7-4.1‰) and higher δD (-47 to -41‰) values than chlorite from metamorphic veins (δ 18 O = 5.7-7.8‰; δD = -59 to –45‰). Quartz-chlorite pairs from metamorphic veins indicate temperatures of 370° to 400°C and a metamorphic fluid composition of δ 18 O ∼ 6.7 to 7.0 per mil and δD 17 ± 8 per mil. The isotopic composition of ore-forming fluids is inferred to have been δ 18 O ∼ 3.8 ± 0.5 per mil and δD ∼ –8 ± 5 per mil. The Kidd Creek ore-forming fluids are best interpreted as evolved seawater that exchanged with 18 O-enriched country rock; it may have contained up to 20 percent magmatic hydrothermal water.

Abstract Isotopic studies covering some 200 km 2 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 18 O relative to fresh or even mildly altered equivalents elsewhere, comprising a range for whole rocks of δ 18 Owhole rock = 6.3 to 15.7 per mil. Mapped distribution of δ 18 O whole rock values indicates several prominent zones of lower δ 18 O 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 18 O 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 18 O 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 (δ 18 O quartz ) was ca. 8.5 per mil due to melting or assimilation of 18 O-enriched, possibly low-temperature altered igneous crust. Quartz phenocrysts with δ 18 O quartz 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.

Abstract Tourmaline in the North orebody and altered rocks of the Kidd Creek massive sulfide deposit grew mainly during sulfide deposition; only trace amounts grew during subsequent low-grade regional meta-morphism. Tourmaline retains its original oxygen, hydrogen, and boron isotope compositions, providing a unique record of the origin and evolution of hydrothermal fluids responsible for alteration and mineralization in this deposit. The fluid history at Kidd Creek involved boiling of seawater at ca. 100° to 150°C in shallow recharge zones, followed by heating at depth to ca. 250°C to form a modified seawater fluid; additional heating (>350°C) of the modified seawater, probably accompanied by supercritical phase separation, and water-rock reaction produced further enrichment in 18 O, forming a high-temperature fluid. Tourmaline- and sphalerite-dominated sulfide assemblages precipitated from mixtures of modified seawater and slightly modified seawater over a range of temperatures from ca. 150° to 250°C, whereas tourmaline- and chal-copyrite-dominated sulfide assemblages formed from mixtures of high-temperature fluids and modified seawater at higher temperatures (ca. 275°-425°C). The temperature of the ore-forming fluids varied from 183° to 429°C, based on quartz-tourmaline oxygen isotope fractionations. Cooling and dilution of the hydrothermal fluids were principal factors responsible for sulfide precipitation. Three fluids are identified: (1) slightly modified seawater, with δ 18 O ≈ 0 per mil, δD ≈ 0 per mil, and δ 11 B ≈ 3.0 per mil, and an assumed normal salinity of ca. 3.2 wt percent NaCl; (2) extensively modified seawater with a boiling model-based, estimated maximum salinity of 6.4 to 16 wt percent NaCl and δ 18 O≈4.0 per mil, S D≈20 per mil, and S 11 B≈-3.0 per mil; and (3) a high-temperature, probably supercritical, fluid with a similarly estimated maximum salinity of at least 6.4 to 16 wt percent NaCl and δ 18 O ≈7.0 per mil, S D≈0 per mil, and S 11 B≈-3.0 per mil. The boron isotope composition of slightly modified seawater and the oxygen isotope composition of high-temperature fluid require reaction with rocks previously altered at low temperatures. Mineralizing brines that formed near the surface circulated to greater depths where they were heated to higher temperatures, probably by heat supplied by the ultramafic and felsic dikes and sills in a rifting environment. The high salinities inferred for the hydrothermal fluids were most likely a contributing factor in the development of such a large massive sulfide orebody.