Geology and Magmatic-Hydrothermal Evolution of the Giant Pebble Porphyry Copper-Gold-Molybdenum Deposit, Southwest Alaska

• DOI: 10.2113/econgeo.108.3.437
• Publication Date: 2013-03-07T23:32:58Z
• Authors: J. R. Lang; M. J. Gregory; C. M. Rebagliati; J. G. Payne; J. L. Oliver; K. Roberts
• Keywords: Argillic alteration; Batholith; Diorite; Quartz monzonite; Breccia; Hypogene

The Pebble deposit is located ∼320 km southwest of Anchorage, Alaska. It one the largest porphyry deposits known with a total resource 10.78 billion tons (Bt). comprises East and West zones, which are approximately equal in size, slightly lower grade mineralization center where peripheries two zones merge. zone was discovered by Cominco America 1989 Northern Dynasty Minerals 2005. oldest rock district Jurassic-Cretaceous Kahiltna flysch unit, contains basinal turbidites, interbedded basalt flows, associated gabbro intrusions. These rocks were intruded between 99 96 Ma coeval granodiorite diorite sills, followed shortly thereafter alkalic monzonite intrusions related breccias. Subalkalic hornblende plutons Kaskanak batholith emplaced at ∼90 Ma. Similar, smaller around margins to Cu-Au-Mo mineralization. Re-Os dates on molybdenite 89.7 90.4 A Late Cretaceous volcanic sedimentary ”cover sequence” completely conceals zone, whereas overlain only glacial sediments exposed small outcrop. Eocene subvolcanic occur east southeast unconsolidated widespread. represent hydrothermal centers within single system. extends from surface ∼500-m depth centered four into flysch, diorite, much higher least 1,700-m hosted larger pluton adjacent sills flysch. merge depth. On side deposit, high-grade has been dropped 600 900 m NE-trending graben, remains undelineated Variations hypogene metal ratios reflect multiple stages introduction redistribution. Hornfels formed prior activity most intensely developed Disseminated vein-hosted mineralization, dominated chalcopyrite locally accompanied bornite, potassic alteration shallow part sodic-potassic zone. Illite ± kaolinite overprinted throughout variably redistributed copper gold. High-grade copper-gold advanced argillic controlled synhydrothermal brittle-ductile fault potassic, sodic-potassic, illite Advanced core pyrophyllite chalcopyrite, bounded west an upward-flaring sericite digenite, covellite, trace enargite tennantite. quartz veins introduced additional molybdenum several parts deposit. Grade-destructive quartz-sericite-pyrite forms halo entire yields outward propylitic alteration. quartz-illite-pyrite cap preserved weakly mineralized Mineralization predominately hypogene. thin, incompletely supergene occurs thin leached capping. There no evidence for paleosupergene or leaching below cover sequence Molybdenite high concentrations rhenium Elevated palladium number large, deep-seated magnetic anomalies intersection crustal-scale structures both parallel angles mid-Cretaceous magmatic arc. This setting similar fertile environments northern Chile suggests that southwestern Alaska highly prospective exploration. large size grades may combination vertically restricted, lateral fluid flow induced hornfels aquitards