Magmatic-Hydrothermal-Structural Evolution of the Giant Pebble Porphyry Cu-Au-Mo Deposit with Implications for Exploration in Southwest Alaska
James R. Lang, Melissa J. Gregory, 2012. "Magmatic-Hydrothermal-Structural Evolution of the Giant Pebble Porphyry Cu-Au-Mo Deposit with Implications for Exploration in Southwest Alaska", Geology and Genesis of Major Copper Deposits and Districts of the World: A Tribute to Richard H. Sillitoe, Jeffrey W. Hedenquist, Michael Harris, Francisco Camus
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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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Geology and Genesis of Major Copper Deposits and Districts of the World: A Tribute to Richard H. Sillitoe
It has been recognized for the past century that copper deposits, in common with those of many other metals, are heterogeneously concentrated in Earth’s upper crust, resulting in areally restricted copper provinces that were generated during several discrete metallogenic epochs over time intervals of up to several hundred million years. Various segments of circum-Pacific magmatic arcs, for example, have total contained copper contents that differ by two orders of magnitude. Each metallogenic epoch introduced its own deposit type(s), of which porphyry copper (and related skarn), followed by sediment-hosted stratiform copper and then iron oxide copper-gold (IOCG), are globally preeminent. Nonetheless, genesis of the copper provinces remains somewhat enigmatic and a topic of ongoing debate.
A variety of deposit-scale geometric and geologic features and factors strongly influence the size and/or grade of porphyry copper, sediment-hosted stratiform copper, and/or IOCG deposits. For example, development of major porphyry copper deposits/districts is favored by the presence of clustered alteration-mineralization centers, mafic or massive carbonate host rocks, voluminous magmatic-hydrothermal breccias, low sulfidation-state core zones conducive to copper deposition as bornite ± digenite, hypogene and supergene sulfide enrichment, and mineralized skarn formation, coupled with lack of serious dilution by late, low-grade porphyry intrusions and breccias. Furthermore, the copper endowment of all deposit types undoubtedly benefits from optimization of the ore-forming processes involved.
Tectonic setting also plays a fundamental role in copper metallogeny. Contractional tectonomagmatic belts, created by flat-slab subduction or, less commonly, arc-continent collision and characterized by crustal thickening and high rates of uplift and exhumation, appear to host most large, high-grade hypogene porphyry copper deposits. Such mature arc crust also undergoes mafic magma input during porphyry copper formation. The premier sediment-hosted stratiform copper provinces were formed in cratonic or hinterland extensional sedimentary basins that subsequently underwent tectonic inversion. The IOCG deposits were generated in association with extension/transtension and felsic intrusions, the latter apparently triggered by deep-seated mafic magmas in either intracratonic or subduction settings. The radically different exhumation rates characteristic of these various tectonic settings account well for the secular distribution of copper deposit types, in particular the youthfulness of most porphyry relative to sediment-hosted stratiform and IOCG deposits. Notwithstanding the importance of these deposit-scale geologic, regional tectonic, and erosion-rate criteria for effective copper deposit formation and preservation, they seem inadequate to explain the localization of premier copper provinces, such as the central Andes, southwestern North America, and Central African Copperbelt, in which different deposit types were generated during several discrete epochs. By the same token, the paucity of copper mineralization in some apparently similar geologic settings elsewhere also remains unexplained.
It is proposed here that major copper provinces occur where restricted segments of the lithosphere were predisposed to upper-crustal copper concentration throughout long intervals of Earth history. This predisposition was most likely gained during oxidation and copper introduction by subduction-derived fluids, containing metals and volatiles extracted from hydrated basalts and sediments in downgoing slabs. As a result, superjacent lithospheric mantle and lowermost crust were metasomatized as well as gaining cupriferous sulfide-bearing cumulates during magmatic differentiation—processes that rendered them fertile for tapping during subsequent subduction-or, uncommonly, intraplate extension-related magmatic events to generate porphyry copper and IOCG districts or belts. The fertile lithosphere beneath some accretionary orogens became incorporated during earlier collisional events, commonly during Precambrian times. Relatively oxidized crustal profiles—as opposed to those dominated by reduced, sedimentary material—are also required for effective formation of all major copper deposits. Large sedimentary basins underlain by or adjoining oxidized and potentially copper-anomalous crust and filled initially by immature redbed strata containing magmatic arc-derived detritus provide optimal sites for large-scale, sediment-hosted stratiform copper mineralization. Translithospheric fault zones, acting as giant plumbing systems, commonly played a key role in localizing all types of major copper deposits, districts, and belts. These proposals address the long-debated concept of metal inheritance in terms of the fundamental role played by subduction-metasomatized mantle lithosphere and lowermost crust in global copper metallogeny.