2003 Seattle Annual Meeting (November 25, 2003)
Paper No. 115-10
Presentation Time: 1:30 PM-5:30 PM


HARLAN, Stephen S., Dept. of Environmental Science and Policy, George Mason Univ, Fairfax, VA 22030, sharlan@gmu.edu, SNEE, Lawrence W., USGS, Box 25046, MS 974, Federal Center, Denver, CO 80225, VIELREICHER, Richard M., Centre for Global Metallogeny, School of Earth and Geographical Sciences, Univ of Western Australia, Crawley, 6009, nvielrei@geol.uwa.edu.au, GOLDFARB, Richard G., USGS, Box 25046, MS 973, Federal Center, Denver, CO 80225, MORTENSEN, James, Department of Earth and Ocean Sciences, Univ of British Columbia, Vancouver, BC V6T 1Z4, and BRADLEY, Dwight C., USGS, 4200 University Dr, Anchorage, AK 99508

The Willow Creek district, southwest Talkeetna Mountains, is historically the third-largest lode-gold producing district in Alaska, having produced 624,000 oz of gold. Most of the gold occurs in extensive quartz vein arrays hosted by Late Cretaceous granitoid rocks of the Willow Creek pluton. U-Pb data indicates that the pluton was emplaced at ~72.3 Ma. 40Ar/39Ar theromochrologic data suggests that the pluton experienced a relatively simple cooling history. Hornblende dates range from 72.8 to 70.5 Ma and biotite dates from 70 to 67.9 Ma, yielding cooling rates for hornblende-biotite pairs of 190 to 50C/m.y. The Willow Creek pluton was subsequently intruded by non-mineralized biotite-muscovite granite along its northern margin. Zircons from granite near the contact gave a U-Pb date of 70.5 Ma. Biotite, muscovite and K-spar 40Ar/39Ar dates indicate that the granite had cooled below ~280C by 68.8 Ma and below ~200C by 66 Ma. Six 40Ar/39Ar dates from sericite associated with Au-related hydrothermal alteration/vein emplacement in the Willow Creek pluton yield ages of 66.9 to 65.6 Ma. The argon data are thus consistent with a single discrete episode of gold mineralization within the pluton. Calcite-dolomite- and chlorite-geothermometry and sericite geobarometry indicate gold mineralization and alteration occurred under lower greenschist facies conditions, at temperatures of ~300C and pressures of 200-300 MPa. To the south, across the Hatcher Pass fault zone, minor gold mineralization related to discontinuous quartz veins and shear zones in Jurassic (?) schist, gave a muscovite age of 54.1 Ma, providing evidence for economically insignificant Eocene gold mineralization. Muscovite dates from the schists yield apparent ages of 61 to 57 Ma, indicating that the metamorphic rocks experienced a distinctly different thermal history than the plutonic rocks to the north.

2003 Seattle Annual Meeting (November 25, 2003)
Session No. 115--Booth# 28
Economic Geology (Posters) II: Miscellaneous Ore Deposits and Genesis
Washington State Convention and Trade Center: Hall 4-F
1:30 PM-5:30 PM, Monday, November 3, 2003

Geological Society of America Abstracts with Programs, Vol. 35, No. 6, September 2003, p. 235

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