GEOCHEMICAL AND ND ISOTOPIC EVIDENCE FOR AN EXTENDING CONTINENTAL-MARGIN ARC/BACK ARC SETTING FOR PERALKALINE RHYOLITE-HOSTED MASSIVE SULFIDE DEPOSITS OF THE BONNIFIELD DISTRICT, EAST-CENTRAL ALASKA

Geochemical and Nd isotopic characteristics of Late Devonian felsic and subordinate mafic metavolcanic rocks associated with Zn-Pb-Ag volcanic-hosted massive sulfide (VHMS) deposits of the Bonnifield mining district provide insight into the tectonic environment and magma sources during submarine hydrothermal mineralization along the ancient Pacific Margin. The largest deposits, WTF and Red Mountain (RM), are hosted by peralkaline metarhyolite of the Mystic Creek member of the Totatlanika Schist. The Anderson Mountain (AM) VHMS deposit, 32 km SW of the Mystic Creek deposits, is exposed on the south limb of an antiform cored by siliceous metasedimentary Healy schist. Metarhyolites hosting WTF and AM have identical SHRIMP U-Pb zircon ages of 363±2 Ma but their trace-element signatures and Sm–Nd systematics differ. Mystic Creek metarhyolite and the underlying alkalic metabasalt (Chute Creek member) both have within-plate, extensional, trace element signatures. Dark grey shales interdigitated with metarhyolite and massive sulfide at WTF and RM show HFSE and LREE enrichment similar to that in Mystic Creek metarhyolite, suggesting a significant peralkaline rhyolitic tuff component in the seafloor sediments, consistent with their deposition in a synvolcanic, extending basin. Elevated eNd_t values for Mystic Creek metarhyolites (-1.0, -1.5, -1.6) and Chute Creek metabasalts (+1.7 and +5.3) indicate an enriched mantle component for both members of the bimodal suite. AM is hosted by metarhyolite of the Wood River assemblage which has a trace-element composition similar to continental crust; associated metabasalt has a weakly developed arc signature. Metarhyolites from AM and the Healy schist have lower eNd_t values (-4.5 and -13.5, respectively) reflecting a large continental crustal component. A continental margin setting is indicated by Proterozoic Nd model ages of 1030–2010 Ma for Bonnifield district metarhyolites and 570–910 Ma for Chute Creek member metabasalt, consistent with the regionally widespread occurrence of Precambrian detrital and inherited zircons. We propose formation of massive sulfides in an extensional (back-arc) basin (Mystic Creek) and an associated volcanic arc (AM) overlying continental crust. Possible modern analogs are the Okinawa Trough and the Woodlark Basin.