GEOCHEMICAL CHARACTERIZATION OF THE KASKANAK BATHOLITH ASSOCIATED WITH THE PEBBLE CU-AU-MO PORPHYRY DEPOSIT, AND OTHER BARREN CRETACEOUS INTRUSIONS IN THE VICINITY: IMPLICATIONS FOR OF THE UNDERLYING CRUSTAL SECTION

The Pebble porphyry Cu-Au-Mo deposit in SW Alaska is a product of Early- to Late Cretaceous volcanic arc subduction which includes mineralization within the Nutzotin-Kluane Belt (130-108 Ma) in southeast Alaska and the Southwest Alaska Range Belt (101-88 Ma). The Pebble deposit is associated with the ~91-89 Ma granodiorite Kaskanak batholith, but other non-mineralizing Late Cretaceous (~99-95 Ma) intrusions in the vicinity include diorite and granodiorite sills, monzodiorite stocks, shoshonite dikes, monzonite porphyries, and associated pyroxenite cumulates.

In the Pebble district, Late Cretaceous magmatism is metaluminous to weakly peralkaline, magnetite-bearing, is high-K calc-alkaline to moderately alkalic in composition, and falls within a narrow range of juvenile whole-rock isotopic compositions (εNd_{(90 m.y.)} = +4.9 - +6.1; ⁸⁷Sr/⁸⁶Sr_{(90 m.y.)} = 0.70329 - 0.70424). This magmatism is also characterized by Sr/Y ratios between 15 - 65, La/Yb ratios between 5 - 18, and N-MORB normalized depletions in Ti, Nb, and Ta; a common feature of hydrated volcanic arc settings. The mineralized Kaskanak Batholith is oxidized, hydrous, and fractionated compared to other barren Late Cretaceous intrusions based on very large V/Sc ratios (15 - 110), zircon Eu and Ce anomalies, an abundance of amphibole >> primary biotite, and greater depletions in MREEs and HREEs.

Xenocrystic zircons in Late Cretaceous intrusions (i.e. 105 Ma < X < 550 Ma; N = 137) are largely sourced from the Kahiltna Flysch based on age distribution, but also include an atypical percentage of Mississippian - Devonian grains, possibly sourced from metasedimentary components of the Kakhonak Complex. Age-resolved Eu and Ce anomalies, and Yb/Gd ratios from these xenocrystic zircons records a history of increased oxidation, hydration, and melt differentiation with the transition of intraoceanic to continental arc subduction during the Jurassic.

These data suggest a relatively homogenized mantle-dominated reservoir source free of radiogenic crustal components with minor mid- to upper crustal contributions from the Kahiltna Flysch and another source; possibly metasedimentary or metavolcanic units of the Late Triassic Kamishak Formation and/or the Cottonwood Bay Greenstones.