AGE AND COOLING HISTORY OF GOLD DEPOSITS AND HOST ROCKS IN THE WILLOW CREEK MINING DISTRICT, TALKEETNA MOUNTAINS, SOUTH-CENTRAL ALASKA

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 50°C/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 ~280°C by 68.8 Ma and below ~200°C 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 ~300°C 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.