TECTONIC SETTING AND LITHOGEOCHEMICAL CHARACTERISTICS OF METAVOLCANICS AND METABASITE SILLS OF THE DELTA DISTRICT, EAST-CENTRAL ALASKA RANGE, ALASKA

The Delta District hosts more than 40 volcanogenic massive sulfide (VMS) deposits within a deformed and metamorphosed assemblage of volcano-sedimentary rocks and mafic sills. The district is in the east-central Alaska Range approximately 300 km southeast of Fairbanks, Alaska, near the southern margin of the Jarvis Creek Subterrane (JCT), a unit within the Yukon-Tanana Terrane (YTT). The Devonian-Mississippian YTT is an accreted, metamorphosed assemblage of volcano-sedimentary rocks bounded by major right-lateral strike-slip fault systems; the Tintina Fault to the north, and the Denali Fault to the south. Within the district, JCT has been informally subdivided into two NW-trending litho-stratigraphic assemblages; the “outboard” Kimball Belt, and the “inboard” Delta Schist Belt (DSB). DSB has been further subdivided into five structural-stratigraphic “series.” From structurally highest to lowest, they are: (1) Tok River; dominantly meta-sediments with minor marble; (2) Drum; relatively thin metamorphosed felsic volcanics with minor meta-sediments; (3) Tiger; relatively thick dacitic metavolcanics with minor interbedded metasediments; (4) Lagoon; dominantly metamorphosed carbonaceous and calcareous sediments; and (5) Tushtena Pass; chloritic- and carbonate-rich schists after interbedded volcanics, calc-arenites, and siltstones. Extensive whole rock geochemical analyses of DSB metavolcanics indicate that formed in a calc-alkaline Late-Devonian volcanic arc environment. Mafic (metabasite) sills comprise more than 10% of the overall litho-stratigraphic section. They occur throughout the district but are particularly concentrated in the Lagoon series where they are spatially associated with stratigraphic horizons containing VMS mineralization. However, field evidence indicates that the metabasites post-date VMS mineralization. Based on whole rock analyses, the metabasites classify as metaluminous, low potassium, magnesium-enriched tholeiites. In contrast to the metavolcanics, the immobile element discriminate plots indicate that the sills formed in a transitional rift tectonic setting. The sills may be related to a Triassic rifting event that has been documented in other parts of the Alaska Range.