Genetic Significance of Oxygen and Hydrogen Isotope Variations at the Kidd Creek Volcanic-Hosted Massive Sulfide Deposit, Ontario, Canada

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 δ18O values (9.7-12‰). Zones of higher δ18O 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 δ18O 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 δ18O 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 δ18O (2.7-4.1‰) and higher δD (-47 to -41‰) values than chlorite from metamorphic veins (δ18O = 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 δ18O ∼ 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 δ18O ∼ 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 18O-enriched country rock; it may have contained up to 20 percent magmatic hydrothermal water.