Abstract The concentrations of incompatible elements in felsic volcanic rocks can be correlated with the ore metal ratios of associated volcanogenic massive sulfide (VMS) deposits in various localities worldwide. The felsic rocks are predominantly dacitic to rhyolitic in composition with a calc-alkalic affinity comprising variable proportions of bimodal volcanic rocks thought to have formed in a back-arc extensional environment. They can be categorized into low, mid, and high Th rhyolites. The low Th rhyolites comprise <15 percent of the bimodal suites in the Abitibi (Noranda area). They contain low concentrations of incompatible elements and low Th/Th* (<3) and La n /Yb n (<3). The mid Th rhyolites make up about 15 to 80 percent of the bimodal suites in the Hokuroku and Abitibi (Kidd Creek, Kamiskotia, Matagami, and Manitouwadge) areas. They have medium Th concentrations and moderate Th/Th* (3–8) and La n /Yb n (2–6). The high Th rhyolites constitute >65 percent of the bimodal suites in the Bathurst Mining Camp and the Iberian Pyrite Belt and have high concentrations of incompatible elements as well as high Th/Th* (>8) and La n /Yb n (>4). The enrichment of K and trace elements, such as Pb, Th, and light rare earth elements (LREE), in felsic volcanic rocks is accompanied by an increased Pb content and Pb/Zn ratio in associated VMS deposits, except for some Archean Abitibi deposits. The high Th rhyolites host Pb + Zn-rich deposits with high Pb/Zn ratios (0.3–0.5) in the Bathurst Mining Camp and the Iberian Pyrite Belt. The mid Th rhyolites are associated with Cu + Zn ± Pb deposits with moderate Pb/Zn ratios (0.1–0.3) in the Hokuroku district and some parts of the Abitibi belt (Kidd Creek and Matabi). The low Th rhyolites underlie the Cu-rich VMS deposits with low Pb/Zn ratios (<0.1) in other parts of the Abitibi belt (Noranda). Based on these compositional correlations, three types of VMS deposits can be recognized in felsic volcanic rocks, each having distinct magmatic and tectonic settings. The Noranda-type VMS deposits formed in a back-arc basin behind an immature island arc, which is similar to the Manus back-arc basin in the western Pacific. The low concentrations of incompatible elements in their low Th rhyolites may be related to a depleted source region below a mafic-dominant immature island-arc crust. The Kuroko-type deposits are associated with the mid Th rhyolites that probably originated in a moderately enriched source within a mature island arc, comparable to the Taupo-Havre-Lau back-arc basins in the south Pacific. The relatively high Pb/Zn ratios in the Hokuroku felsic rocks and Kuroko deposits, compared to those of Archean mid Th felsic rocks and associated VMS deposits, are possibly related to relatively large proportions (30–50%) of a felsic component in the mature island-arc crust. The Bathurst-type deposits occur in the high Th rhyolitic terranes where the felsic magmas probably resulted from the partial melting of enriched, felsic-dominant continental crust. They are related to an intracontinental back-arc environment analogous to the Okinawa trough in the northwestern Pacific. The compositional correlations between felsic volcanic rocks and associated VMS deposits imply a genetic relationship. It is speculated on the basis of results from experimental, isotope, and melt and/or fluid inclusion studies that felsic magmas, by degassing ore metal-rich magmatic fluids, contribute large quantities of ore metals to the formation of VMS deposits, in addition to providing heat to drive the hydrothermal convection.

Abstract The structural history of part of the Purcell Mountains of British Columbia is interpreted in the light of the discovery of the Precambrian age of the Hellroaring Creek stock. This stock, the first Precambrian granitic intrusion recognized in situ in the Canadian Cordillera, cuts Moyie (Purcell) Intrusions and verifies their Precambrian age. The Sullivan ore body, one of the world’s largest deposits of lead and zinc, is younger than a low-grade regional metamorphism of its host metasedimentary rocks but, on the basis of potassium-argon dating of a lamprophyre, is probably no younger than 765 million years. The lead-isotope content of the Sullivan deposit is homogeneous and indicates a single mineralization. Deposits of the East Kootenay district contain two main types of lead; a relatively nonradiogenic variety and a markedly radiogenic type. The irdistribution is unrelated to geography and to major structural trends, out there is an apparent relation between isotopic composition and specific combinations of structure and host rock. Deposits that are structurally conformable with strata of the Aldridge Formation probably belong to the relatively nonradiogenic lead group, although there is no relation between isotopic compositions and occurrences in the Aldridge Formation i n general. Deposits believed to be genetically related to the Moyie Intrusions in which they occur are probably characterized also by lead of the relatively nonradiogenic group, a correlation that is in harmony with their proposed origin. Another correlation is between isotopic composition and size: small deposits, with the significant exception of those probably related to Moyie Intrusions, are likely to be of the markedly radiogenic type, whereas the commercial lead production of the district has come from deposits of the relatively nonradiogenic type. If any single lead was common to a ll the deposits of the dist rict its presence in many of them, especially the smaller ones, has been masked by additions of more radiogenic lead.