ALBERT

Description: The Willow Creek mining district was the third-largest lode gold district in Alaska in the 20 th century, having produced 19 metric tons (t) of gold from vein deposits and another 2 t of gold from associated placer deposits. The district is located in the southern Talkeetna Mountains, north of the Castle Mountain strike-slip fault system. Most gold occurs in widespread mineralized quartz veins hosted by granitic rocks of the Willow Creek pluton. A geochronologic study of granitic rocks, alteration minerals, and adjacent metamorphic rocks was conducted in order to better understand the timing of magmatism, thermochronologic evolution, and age of gold mineralization in the district. New U-Pb zircon dates from the Talkeetna batholith, including the Willow Creek pluton, range from 72.1 to 71.3 Ma. The U-Pb dates are similar to reported U-Late Cretaceous Pb and K-Ar dates from elsewhere in the study area, and tightly constrain the age of plutonic rocks that host the Willow Creek lode gold deposits. Granitic rocks that intrude the Willow Creek pluton along its northwestern margin yield U-Pb dates of 75.7 and 70.8 Ma. The 40 Ar/ 39 Ar thermochronologic data for the Willow Creek pluton and other granitoids reveal rapid postemplacement cooling through hornblende, muscovite, and biotite closure temperatures. Thermochronologic data from K-feldspars document slower cooling rates through lower temperatures, which may be related to slow uplift and exhumation during the Paleocene-Eocene. The data are compatible with a simple tectonothermal history following emplacement of the plutonic rocks, with essentially no evidence of significant thermal disturbance of the rocks after about 64 Ma. Uranium-thorium-lead dating of hydrothermal phosphate minerals (monazite and xenotime) and 40 Ar/ 39 Ar dates from hydrothermal sericite from wall rocks and veins indicate that the main stage of gold mineralization occurred at about 67 to 65 Ma. Combining U-Th-Pb data from monazites from the Independence, Talkeetna, Gold Bullion, and Fern deposits gave an average of 65.4 ± 2.3 Ma; hydrothermal xenotime from the Independence vein also yields a statistically indistinguishable age of 64.5 ± 2.3 Ma. These ages overlap 40 Ar/ 39 Ar dates of 67 to 66 Ma obtained from hydrothermal sericite associated with formation of the gold-bearing veins, and the age of a muscovite from a weakly mineralized pegamatite that cuts the Willow Creek pluton. In contrast to previous studies that suggested that at least some mineralization was Eocene in age, we found no compelling evidence for significant gold mineralization within the district at this time. The 40 Ar/ 39 Ar and K-Ar dates from graphitic white micas from schist south of the Hatcher Pass fault zone yield ages of 61 to 50 Ma that are probably related to uplift and cooling of these Late Cretaceous metasedimentary rocks. The absence of evidence for postemplacement thermal events affecting the plutonic rocks of the Willow Creek district to the north indicates that the schists and granitoids do not share a similar tectonothermal history, and that they have been juxtaposed by subsequent faulting along the Hatcher Pass fault zone.