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

Abstract

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