Update of the Geologic Setting and Porphyry Cu-Mo Deposits of the Chuquicamata District, Northern Chile
Sergio L. Rivera, Hugo Alcota, John Proffett, Jaime Díaz, Gabriel Leiva, Manuel Vergara, 2012. "Update of the Geologic Setting and Porphyry Cu-Mo Deposits of the Chuquicamata District, Northern Chile", 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 Chuquicamata district of northern Chile contains > 130 million metric tons (Mt) Cu in resources and past production. The mineralization occurs in various types of Eocene to early Oligocene porphyry Cu systems hosted by Paleozoic and Triassic volcanic and Triassic granodiorite rocks. Emplacement of the deposits occurred during E-W-directed contraction, crustal shortening, and uplift related to the Incaic orogeny between 38 and 31 Ma. The N-S-oriented West Fissure is the most important structural feature of the district, with significant postmineral, left-lateral displacement of 30 to 35 km estimated by regional mapping, although it remains to be confirmed by displaced alteration-mineralization features at the individual deposit scale.
The West Fissure, across which both the geology and mineralization differ at any given point, divides the district into two domains. The Eastern Block comprises Paleozoic metamorphic and intrusive complexes overlain by Permian and Triassic volcanic and sedimentary rocks and intruded by Triassic granodiorite. These are overlain in the north by Cretaceous continental sedimentary and volcanic rocks and in the south by Eocene to Miocene sedimentary rocks of the Calama Basin, the lowermost units of which show evidence of syntectonic deposition. The Eastern Block contains the Chuquicamata and Radomiro Tomic deposits, both hosted by the Chuqui Porphyry Complex, a N-NE-oriented, 14- × 1.5-km megadike intruded into Triassic volcanic and intrusive rocks, and with a SHRIMP U-Pb age of ∼ 36 Ma. Hypogene mineralization at Chuquicamata occurs mainly in the East porphyry, the dominant phase of the porphyry complex and does not show a close relationship to smaller, later porphyry bodies with SHRIMP U-Pb ages of ∼ 34 Ma. Much of the Cu was introduced early (∼ 34–35 Ma; 40Ar/39Ar) during potassic alteration, which comprises a large low-grade body containing biotitized hornblende. Within this large low-grade body higher Cu grades occur as bornite and other Cu-bear-ing sulfides, without pyrite, in intense potassic alteration halos (K-feldspar-sericite) along early fractures. This was followed by introduction of quartz-molybdenite veins, with an Re-Os age of ∼ 35 Ma. Sericite-quartz-pyrite alteration, with advanced argillic alteration near veins, is later, as shown by crosscutting relationships, and returns 32 to 31 Ma 40Ar/39Ar ages. Veins with high sulfidation Cu-bearing sulfide-pyrite assemblages are part of this later stage, which overprinted and sulfidized the earlier Cu-bearing sulfides. Similar early and late mineralization occurs at Radomiro Tomic, but there the late-stage veins are of lesser importance.
The Western Block comprises Paleozoic metamorphic complexes, Permian and Triassic volcanic and sedimentary rocks, and Triassic granodiorite in the south, overlain by Jurassic carbonates and continental sedimentary rocks. In the north, Cretaceous and Lower Tertiary volcanic and continental sedimentary rocks overlie the Jurassic unit. Jurassic to Eocene strata in the central to northern parts of the Western Block hosts the Eocene Los Picos and Fortuna batholiths. The Mina Ministro Hales deposit, at the eastern edge of the Western Block, is associated with Eocene porphyries (∼ 39–35 Ma SHRIMP and LA-ICP-MS U-Pb ages) that intrude wall rocks similar to those at Chuquicamata. Mina Ministro Hales is characterized by mineralization similar to that at Chuquicamata, but late-stage sericitic and advanced argillic alteration (∼ 32 Ma 40Ar/39Ar ages) are of greater importance, especially at the shallower levels. The Toki Cluster deposits, farther to the west, are associated with swarms of small porphyries thought to be late-stage phases of the ∼ 38 Ma Fortuna granodiorite batholith. The mineralization consists of bornite and/or chalcopyrite, commonly with magnetite but little or no pyrite, in A-type veins accompanying potassic alteration. The strongest mineralization is in and near the earliest porphyries and is truncated by the next younger porphyries, which in turn are cut by similar but less intense mineralization and still younger, less-mineralized porphyries. Late pyrite-sericite alteration occurs mostly on the peripheries of these deposits and carries little Cu.
A favorable combination of desert climate and morphotectonic evolution resulted in the formation and preservation of significant supergene enrichment and/or oxidation. The potassic zones at Chuquicamata, Radomiro Tomic, and the Toki Cluster underwent in situ oxidation. Supergene enrichment blankets formed in quartz-sericite-pyrite alteration at Chuquicamata and Mina Ministro Hales. Supergene processes also resulted in lateral migration of Cu and formation of the Mina Sur exotic deposit. From an exploration perspective, the history of the district demonstrates how geologic observations and interpretation have played a key role in development of the resource base. Only the Chuquicamata oxide Cu mineralization cropped out, and original open-pit development in 1912 was followed by exploration and evaluation of the giant, high-grade enrichment blanket from the 1930s onward. District-scale exploration resulted in discovery of Radomiro Tomic in the 1950s, Mina Sur in the 1960s, Mina Ministro Hales in the 1990s and the Toki Cluster at the beginning of this century, none of which were exposed at surface. The Chuquicamata district, now producing ∼ 900,000 t Cu/yr, has been operating for 100 years and retains a substantial resource base that will enable it to continue for many years to come.
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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.