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Magmatic-Hydrothermal-Structural Evolution of the Giant Pebble Porphyry Cu-Au-Mo Deposit with Implications for Exploration in Southwest Alaska

By
James R. Lang
James R. Lang
1
Hunter Dickinson Inc., 15th Floor, 1040 West Georgia Street, Vancouver, British Columbia, Canada V6C 2V6
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Melissa J. Gregory
Melissa J. Gregory
2
Pebble Limited Partnership, P.O. Box 267, Iliamna, Alaska 99606
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Published:
January 01, 2012

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 metric tons (Bt) of mineralized rock divided between the contiguous West and East zones.

The oldest rocks in the Pebble district are Jurassic-Cretaceous Kahiltna flysch, which contains interbedded basalt and associated gabbro intrusions. These were cut between 99 and 96 Ma by coeval granodiorite and diorite sills, followed by alkalic intrusions and related breccias. Subalkalic hornblende granodiorite porphyry plutons were emplaced at ∼90 Ma and include the Kaskanak batholith and smaller stocks related to Cu-Au-Mo mineralization. Porphyry mineralization has been dated between 89.5 and 90.4 Ma by Re-Os on molybdenite. Late Cretaceous volcanic and sedimentary rocks completely conceal the East zone. Eocene volcanic rocks and subvolcanic intrusions occur east and southeast of the Pebble deposit and 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 plugs emplaced into flysch, diorite and granodiorite sills, and alkalic intrusions and breccias. The much higher grade East zone is hosted by the larger East zone granodiorite pluton and adjacent granodiorite sills and flysch and extends to at least 1,700-m depth below surface. The granodiorite intrusions merge with depth. Lower grade, less extensive mineralization occurs in the center of the deposit where the peripheries of the East and West zones converge. On the eastern side of the deposit, faulting dropped high-grade mineralization 600 to 900 m into the NE-trending East graben where the deposit remains open.

Variations in hypogene grade and metal ratios reflect multiple stages of metal introduction and redistribution. Premineralization hornfels formed around the Kaskanak batholith. Early disseminated and vein-hosted Cu-Au-Mo mineralization formed with potassic alteration in the East zone and sodic-potassic alteration in the West zone. In the East zone, potassic alteration is underlain by weakly mineralized sodic-potassic ± calcic alteration. Slightly younger quartz veins introduced additional molybdenum. Illite ± kaolinite alteration overprinted the early alteration assemblages and variably redistributed copper and gold. Late-stage advanced argillic alteration is associated with high-grade Cu-Au mineralization in the East zone; it was controlled by a synhydrothermal brittle-ductile fault zone and comprises a core of pyrophyllite alteration associated with chalcopyrite bounded to the west by sericite alteration with hypogene bornite, digenite, covellite, and trace enargite and tennantite. Copper was removed by quartz-sericite-pyrite alteration that forms a halo to the deposit and yields outward to propylitic alteration. A weakly mineralized quartz-illite-pyrite cap is preserved in the upper central part of the deposit. Weak supergene mineralization is present only in the West zone where the Late Cretaceous cover sequence was eroded. 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.

The Pebble deposit occurs in one of several large, deep-seated magnetic anomalies which occur at the intersection of crustal-scale structures both parallel and at high angles to an arc, which formed in southwest Alaska during the Cretaceous. This setting is similar to fertile porphyry environments in northern Chile and suggests that southwestern Alaska is highly prospective for porphyry exploration.

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Contents

Special Publications of the Society of Economic Geologists

Geology and Genesis of Major Copper Deposits and Districts of the World: A Tribute to Richard H. Sillitoe

Jeffrey W. Hedenquist
Jeffrey W. Hedenquist
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Michael Harris
Michael Harris
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Francisco Camus
Francisco Camus
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Society of Economic Geologists
Volume
16
ISBN electronic:
9781629490410
Publication date:
January 01, 2012

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