DEVONIAN VOLCANOGENIC MASSIVE SULFIDE DEPOSITS AND OCCURRENCES, SOUTHERN YUKON-TANANA TERRANE, EASTERN ALASKA RANGE, ALASKA

A belt of volcanogenic massive sulfide deposits extends for over 150 k m along the southern margin of the Yukon-Tanana terrane of the eastern Alaska Range. Located north of the Denali fault, the Yukon-Tanana ter rane forms a major basement unit in east-central Alaska. The volcanoge nic massive sulfide deposits are primarily in the jarvis Creek Glacier subterrane, one of the highest structural-stratigraphic levels of the Yukon-Tanana terrane. The Jarvis Creek Glacier subterrane consists of a volcanogenic massive sulfide-bearing metavolcanic rock member and a metasedimentary rock member. The volcanogenic massive sulfide deposit s and enclosing metavolcanic and metasedimentary schists exhibit two p eriods of regional metamorphism and penetrative deformation: an older, Early Cretaceous, amphibolite facies event and a younger, mid-Cretace ous lower greenschist facies event. From northwest to southeast, the m ajor voleanogenic massive sulfide districts are the Bonnifield, Triden t Glacier, and Delta. The Bonnifield district contains massive sulfide deposits in metavolcanic and metasedimentary rocks that are correlate d with the Jarvis Creek Glacier subterrane. The 25-km-long, northwest- trending Trident Glacier district contains several occurrences consist ing of massive pyrrhotite and lesser pyrite, minor chalcopyrite with o r without sphalerite, and galena and arsenopyrite in south-dipping mas sive sulfide pods, lenses, and stringers that parallel the foliation o f the enclosing schists. Host-rock protoliths were mainly andesite, da cite, rhyodacite tuffs and flows, shale, and limestone, with lesser qu artz-rich keratophyre flows and tuffs, volcanic graywacke, and siltsto ne. The Delta district contains at least 35 deposits in an 800-km2 are a. Thirty-one of the deposits define four mineral trends, 5 to 32 km l ong, that are subparallel to the west-northwest striking, southwest-di pping structures and lithologies. The deposits consist of layers and z ones containing varying amounts of massive to disseminated pyrite and pyrrhotite, and lesser amounts of chalcopyrite, galena, sphalerite, an d arsenopyrite. The larger deposits range between 1.0 and 12.4 million metric tons with combined grades of 5.6 percent total metal (Cu, Pb, Zn), and Ag and Au contents of 56 and 1.9 g/metric ton, respectively. Hostrock protoliths are similar to those found in the Trident Glacier district, but some of the deposits are spatially associated with diori te or gabbro sills that exhibit lower greenschist facies metamorphism and deformation. Sulfur isotope values of 26 pyrrhotite separates from the Trident Glacier district occurrences average 7.7 per mil, the mea n value of seven pyrite separates is 8.1 per mil. Forty-one Delta dist rict pyrite separates average 5.9 per mil and ranges of values of the four northwest trending trends overlap. The relative consistency of su lfur isotope values of iron-bearing sulfides in the Trident Glacier di strict, and the apparent lithologic relationship of isotope values in the Delta district, may reflect original-rock sulfur isotope values. A lternatively, the S-34-enriched values may have resulted from the addi tion of reduced seawater sulfate to the hydrothermal fluid.