IMPLICATIONS OF NEW SHRIMP U-PB ZIRCON CRYSTALLIZATION AGES FOR THE ORIGIN OF CARBONATE REPLACEMENT AND SKARN MINERALIZATION IN THE WESTERN FORTYMILE MINING DISTRICT, EAST-CENTRAL ALASKA

Epigenetic base- and precious-metal prospects in the Mount Veta area of the Fortymile district are the focus of recent industry exploration and a study by the USGS. New field, petrologic, and isotopic data for intrusive rocks and spatially-associated Zn-Pb-Ag-Cu-Au prospects in the district provide key constraints on the metallogeny and geologic evolution of the district. The proximity of Late Triassic and Early Jurassic intrusions to both calc-silicate skarns and carbonate replacement prospects previously suggested a genetic relationship between mineralization and early Mesozoic magmatism. Our recently published U-Pb zircon ages of 210 ± 3 and 187 ± 3 Ma for intrusions at the LWM and Fish prospects, respectively, would appear to support this hypothesis. However, Pb isotopic data for sulfides from six Fortymile prospects are more radiogenic than K-feldspar Pb ratios from Triassic and Jurassic intrusions. Sulfides plot in fields defined by mid- to Late Cretaceous plutons, yet no igneous bodies of that age were known near the Fortymile prospects. We report here five new Triassic-Jurassic and five Cretaceous SHRIMP-RG U-Pb zircon ages (in Ma) for intrusive rocks in the district: 214 ± 2 for Bt granodiorite near Mitchell; 190 ± 2 for Hbl monzonite near West-LWM; 186 ± 2 for two variants of megacrystic Mount Veta syenite; 185 ± 2 for a Cpx tonalite dike near Eva; 110 ± 2 for leucogranite at VABM Corner; 109 ± 1 for Qtz-Fs porphyry SW of Iron Creek; 98 ± 1 for Qtz-Pl porphyry in headwaters of Kechumstuk Creek; and 69 ± 1 for granites adjacent to Oscar and to West-LWM. Identification of mid- and Late Cretaceous leucocratic intrusions near these prospects, and recognition of three additional sulfide-bearing Qtz-Fs porphyry stocks during our 2009 mapping, suggest linkages between Zn-Pb-Ag-Cu-Au mineralization and Cretaceous magmatic-hydrothermal systems and associated fluids possibly related to the NE-trending Kechumstuk fault system in the area. This fault system parallels the Black Mountain tectonic zone, ~50 km to the west, which controlled emplacement of 110-95 Ma intrusions and associated Au mineralization. Other possibly related magmatic and mineralization events are W-Mo- and Cu-Mo-porphyries and rhyolite calderas of both 103 and 70 Ma ages that partially surround the Mount Veta area at a radial distance of ~50 km.