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 Studies of submarine hydrothermal vents over the past 20 years have confirmed that extensive sulfide chimney fields and mounds are created along active rift zones in open-ocean and back-arc basin settings (see Fornari and Embley, 1995; Hannington et al., 1995). These modern deposits represent the surficial expressions and upflow zones of hydrothermal systems thermally driven by intrusion of magma into the oceanic crust. Comparisons have been made with ancient volcanogenic massive sulfide (VMS) orebodies, some associated with ophiolites, but there are few places on the modern sea floor where the “stockwork” zone has been exposed or studied in detail. In addition, ancient deposits and their associated volcanics are commonly metamorphosed and deformed, so that original structural, mineralogic, and geochemical characteristics have been severely modified or obliterated. Here, we present a review, based on detailed sea-floor observations and sampling, of the geology and petrochemistry of the eastern Galapagos spreading center (EGSC) at ∼86° W in the Pacific Ocean where an extinct hydrothermal system (Galapagos fossil hydrothermal field), including sulfide mounds, chimneys, and an underlying stockwork zone, are intimately associated with fresh and altered lavas. Data for major elements, lithophile trace elements, Sr, Pb, and O isotopes are presented to demonstrate the cogenetic nature of the diverse assemblage of oceanic lavas. Using this framework, the behavior of the trace base metals and sulfur during magmatic evolution is evaluated. We also provide detailed field, mineralogic and chemical descriptions of two areas of massive sulfide mineralization within the Galapagos fossil hydrothermal field (GFHF). The physical and chemical relations between magmatism and hydrothermal mineralization indicate that there is a synergy between these geologic processes, which may be of general application.