2009 Portland GSA Annual Meeting (18-21 October 2009)

Paper No. 10
Presentation Time: 10:55 AM

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


DUSEL-BACON, Cynthia, USGS, 345 Middlefield Rd., MS 901, Menlo Park, CA 94025, ALEINIKOFF, John N., USGS, Denver Federal Center, MS 963, Denver, CO 80225, DAY, Warren C., US Geological Survey, MS 911, Denver, CO 80225, O'NEILL, J. Michael, USGS, Denver Federal Center, MS 964, Denver, CO 80225, SLACK, John F., USGS, National Center, MS 954, Reston, VA 20192 and WINTZER, Niki E., USGS, 904 W. Riverside Ave., Rm. 202, Spokane, WA 99201, cdusel@usgs.gov

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.