PETROLOGY, U–PB AGE CONSTRAINTS, AND ORE GENESIS OF THE SHEEP CREEK SEDIMENT-HOSTED ZN-PB-AG-SN PROSPECT IN THE BONNIFIELD MINING DISTRICT, EAST-CENTRAL ALASKA

The Sheep Creek prospect is a stratabound Zn-Pb-Ag-Sn massive sulfide occurrence in the Bonnifield mining district, northern Alaska Range. Deposits and prospects in this district lie near the western end of a belt of Late Devonian to Early Mississippian volcano-plutonic complexes and syngenetic base-metal deposits that extend from east-central Alaska to southern British Columbia. The belt formed along the peri-Laurentian (ancient Pacific) margin of the North American craton. The Sheep Creek prospect is within a quartz-sericite-graphite-chlorite schist unit in Devonian carbonaceous and (or) siliceous metasedimentary rocks. Volcanogenic massive sulfide (VMS) deposits in the Bonnifield district are hosted in felsic metavolcanic rocks (362 ± 2 Ma) that overlie the stratigraphic sequence at Sheep Creek. Felsic metaigneous rocks in underlying units are 372 ± 4 Ma to 366 ± 4 Ma. Sheep Creek is atypical of the other deposits and prospects in the district in (1) having Sn grades up to 1.2 %; (2) being hosted in fine-grained, quartz-rich rocks and quartz-pebble conglomerate that likely originated as chert and chert-clast sediment, respectively; and (3) showing minimal evidence of volcanic components in the host rocks. Pyrite occurs throughout all stages of the ore paragenesis joined by later galena, sphalerite, and minor tetrahedrite. Cassiterite is disseminated in the quartz-mica matrix and forms inclusions within sulfides. A comparison of immobile trace-element proportions for graphitic and siliceous rocks from the Sheep Creek area with those for argillite associated with the felsic metavolcanic host rocks of the Bonnifield VMS deposits indicates a continental volcanic-arc provenance for the former vs within-plate and passive margin provenances for the latter. In contrast to previously published interpretations, we propose that the Sheep Creek prospect has a metasedimentary-exhalative (SEDEX) rather than a VMS affinity. In our model, Zn-Pb-Ag-Sn mineralization formed by syngenetic or early diagenetic processes on or beneath the seafloor, possibly in the shallow-water environment of an outer continental shelf setting. Potential analogues are the Paleozoic SEDEX deposits in the Canadian Selwyn Basin outboard of the Laurentian continental margin.