Paper No. 4
Presentation Time: 9:45 AM
GEOCHEMISTRY AND U-PB ZIRCON GEOCHRONOLOGY OF THE KASKANAK BATHOLITH, PEBBLE PORPHYRY CU-AU-MO DEPOSIT, ALASKA
The Pebble porphyry Cu-Au-Mo deposit in Alaska is one of the world's largest Cu-Au resources. The deposit is associated with the Kaskanak batholith, a ~150 km2
multiphase granodiorite intrusion that is exposed at surface west of the deposit and lies at >600 m depth in the east. The main phase is equigranular, and it transitions to a series of porphyritic cupolas (West and East zones) cross-cut by three distinct porphyry dike sets associated with mineralization. These are: 1) a voluminous granodiorite porphyry in the East zone, 2) quartz-orthoclase granodiorite to granite porphyries in the East zone, and 3) leucocratic quartz granite porphyries in the 38 zone prospect. New SHRIMP-RG and LA-ICP-MS U-Pb analyses of zircon from 32 samples, together with previously published ages, indicate that pre-ore diorite, alkalic monzodiorite and granodiorite dikes and sills intruded at ~98-95 Ma, the main Kaskanak granodiorite at ~91 Ma, and younger mineralized porphyries between ~91-89 Ma (e.g. 90.3 ± 1.0 Ma, 89.4 ± 1.4 Ma; 89.2 ± 1.2 Ma for each porphyry respectively). A syn-ore andesite porphyry dike located SW of the 308 Zone prospect is 90.6 ± 1.5 Ma, and cross-cutting Paleocene andesite and dacite dikes in the East zone are 63.9±1.0 Ma and 65.9±1.4 respectively.
Zircon trace element data show distinctive differences between ore-related porphyries and pre-ore intrusions, but do not discriminates between the granular and porphyry phases of the Kaskanak batholith. Ti-in-zircon geothermometry indicate pre-ore intrusions were hotter (~750-850ᵒ C) than syn-ore intrusions (~700ᵒ C). Zircon CeN/Ce* and EuN/Eu* values are elevated in all granodiorite intrusions associated with mineralization as well as in pre-ore granodiorite sills, which reflects an increase in ƒO2 and H2O content from other pre-ore intrusions.
Pebble intrusions all have primitive isotopic signatures (87Sr/86Sri = 0.703290 - 0.704226; ƐNdi = 4.53 - 6.12; t = 90 Ma) and relatively young depleted mantle model ages (TDM = 355 – 717 Ma) typical of ocean island arc terranes of southern Alaska. The intrusions are interpreted as anatectic melts of mafic and relatively young crust that fractionated olivine, pyroxene, and hornblende to produce the Kaskanak granodiorites. The ~9 Ma of magmatic evolution culminated with late ore-forming melts enriched in Cu, Mo, Au, S, Cl, and H2O.