Geochemical constraints on magmatic and metallogenic processes: Iskut River Formation, volcanogenic massive sulfide-hosting basalts, NW British Columbia, Canada

Abstract

Volcanic rocks of the Jurassic Iskut River Formation (IRF) in northwestern British Columbia (Canada) host several volcanogenic massive sulfide (VMS) deposits, including the exceptionally high-grade Eskay Creek Ag–Au–Cu–Pb–Zn deposit. The IRF comprises voluminous pillow basalt (>90%), minor rhyolite, and sedimentary rock of late Early to early Middle Jurassic age, filling a series of sub-basins along a 300 by 50 km north-trending belt. Two geochemically distinct types of tholeiitic basalts interfinger; both resemble back-arc basin basalts formed from the melting of asthenospheric and sub-arc mantle sources. Group 2 basalts are more enriched in light rare-earth elements, Ba, K, Sr, Th, and U, and have lower positive ε Nd values than group 1 basalts (+3.2 to +6.3 versus +6.9 to +8.4, respectively). The compositional differences between group 1 and group 2 basalts are interpreted to result from crustal contamination in group 2. Group 1 basalts are most common in the southern part of the IRF belt where they are closely associated with the Eskay Creek, Bonanza, and Hidden Creek (Anyox) VMS deposits. Group 2 basalts are most abundant in the northern half of the belt and are not associated with exploited mineral deposits. The lack of crustal contamination in group 1 basalts indicates that they formed from rapidly ascending magma in an advanced rift setting and were associated with high heat flow that drove hydrothermal circulation. Group 1 and group 2 basalts are reliably discriminated by Ta/Th <2.5 in the former and >2.5 in the latter. This geochemical criteria can therefore be used as an exploration tool to identify VMS permissive sub-basins and (or) stratigraphy in the IRF.