COPPER ISOTOPE SIGNATURES AS AN EXPLORATION TOOL AT THE PEBBLE DEPOSIT, ALASKA

The Pebble Cu-Au-Mo porphyry deposit formed at about 90 Ma and is located in southwest Alaska. The total resource is 10.8 Gt which contains 80.6 Glb Cu, 5.6 Glb Mo and 107.4 Moz Au. Mineralization formed at about 90 Ma and occurs in a West Zone exposed at surface and an East Zone concealed by younger rocks. This study uses copper isotopes fractionation measured in minerals and surface waters to understand high temperature mineralization processes and current weathering processes. Sulfide minerals have measured δ⁶⁵Cu values ranging from 0.78‰ to 2.28‰ (hypogene West), 0.02‰ to 1.55‰ (hypogene East), -3.49‰ to 1.88‰ (oxide West), and -5.04‰ to 1.27‰ (supergene West). The mineralization results show a relationship of the hypogene minerals to elevation whereas δ⁶⁵Cu values become depleted farther from the hypogene source. Shallow groundwaters are enriched in δ⁶⁵Cu above the deposit and become progressively depleted in ⁶⁵Cu away from the deposit. Patterns measured in the copper isotope composition of both the minerals and surface waters demonstrate the potential use of copper isotope fractionation in exploration geology as well as to understand the copper sources to the environment.