40AR/39AR AND RE-OS GEOCHRONOLOGY OF THE PEBBLE COPPER CU-AU-MO PORPHYRY DEPOSIT, SOUTHWEST ALASKA

The Pebble Copper deposit (59°53’54’’ N, 155°17’44’’ W) represents a large Cu and Au resource (1 billion tonnes with 0.30% Cu and 0.34 g/t Au, with a 54 million tonne core of 0.54% Cu and 0.46 g/t Au) within a belt of southwest Alaska Cu and Au deposits. We present precise ⁴⁰Ar/³⁹Ar igneous and alteration ages and a Re-Os mineralization age from the Late Cretaceous Pebble Copper Cu-Au-Mo porphyry deposit and the nearby Tertiary Sill epithermal Au-Ag deposit.

Pre-mineralization rocks proximal to Pebble include diorite and biotite-magnetite pyroxenite. ⁴⁰Ar/³⁹Ar plateau ages of hornblende and biotite from these rocks fall between 95.0 and 96.1 Ma. Biotite from post-mineralization andesite adjacent to the deposit has an ⁴⁰Ar/³⁹Ar age of 84.1±0.3. Additionally, we dated vein K-feldspar from the Sill deposit, a latite-hosted, high grade epithermal Au-Ag prospect, which yielded an ⁴⁰Ar/³⁹Ar age of 46.1±0.1 Ma.

Within the deposit, hydrothermal biotite and K-feldspar, both associated with copper mineralization, record ⁴⁰Ar/³⁹Ar ages of 88.5±0.3 Ma and 86.0±0.2 Ma, respectively. Molybdenite from the center of the deposit yields a Re-Os age of 89.5 ± 0.3 Ma. This range of ages (~3-4 m.y.) is longer than the duration of typical porphyry hydrothermal systems. This is most likely due to recharging of the hydrothermal system, in terms of temperature and activities of the relevant ions, by multiple related intrusions evident from diamond drilling and mapping. Regardless, the ages provide good control on the timing of alteration and mineralization and, combined with the pre- and post-mineralization ages given above, will be useful in the development of local and regional exploration models.