MESOZOIC FRAMEWORK FOR MAGMATISM, METALLOGENY, AND FAULTING IN THE WESTERN FORTYMILE MINING DISTRICT, EAST CENTRAL ALASKA—SHRIMP U-PB ZIRCON GEOCHRONOLOGY, WHOLE-ROCK GEOCHEMISTRY, AND LEAD ISOTOPES

Epigenetic base- and precious-metal prospects in the western Fortymile mining district, east-central Alaska, are spatially associated with NE-trending steep faults and igneous rocks. We report petrographic, geochemical, geochronologic, and isotopic data for Mesozoic igneous rocks that are proximal to Zn-Pb-Ag mineral prospects. These data are essential to understanding the metallogeny and mineral potential of east-central Alaska and adjacent Yukon and help constraint the tectonic history of the northern Cordillera.

Twenty-eight new SHRIMP U-Pb zircon ages define four phases of magmatism: Late Triassic (216–208 Ma); Early Jurassic (191–181 Ma); mid-Cretaceous (112–94 Ma); and Late Cretaceous (70–66 Ma). Whole-rock geochemistry indicates calc-alkalic arc compositions and progressively more evolved compositions through time. The Early Jurassic Mount Veta intrusion and coeval dikes at the LWM carbonate replacement (CR) prospect formed as synkinematic bodies emplaced during NW-directed crustal shortening. The NE-striking Kechumstuk fault zone transects the area and shows sinistral and normal displacement. We propose that emplacement of ~95 Ma hypabyssal bodies and 68–66 Ma felsic dikes and a stock within the fault zone was synchronous with fault displacement and that fault displacement was a far-field effect of dextral translation along precursors to the Denali and Tintina fault systems, which bound the region.

Pb-isotope compositions of igneous feldspars from Late Triassic, Early Jurassic, and Late Cretaceous igneous rocks plot below the average upper crustal growth curve for this part of the Cordillera, suggesting derivation from more juvenile sources. Feldspars from mid-Cretaceous rocks plot above the growth curve, indicating derivation from more evolved sources. The juvenile isotopic ratios for Late Cretaceous feldspars suggest an input of mantle material. This mantle input may have been related to asthenospheric upwelling resulting from the sinking of an inactive inner subduction zone of the previously accreted Wrangellia Composite terrane, after an outer subduction zone was established. Comparison of Pb-isotope compositions of igneous feldspars and sulfides from CR and skarn prospects and their spatial proximity to dated felsic intrusions indicates a 68–66 Ma age for mineralization.