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Book Chapter

Update of the Geologic Setting and Porphyry Cu-Mo Deposits of the Chuquicamata District, Northern Chile

By
Sergio L. Rivera
Sergio L. Rivera
1
Exploraciones Mineras Andinas S.A., Apoquindo 4775, Of. 602, Santiago, Chile
2
Universidad Católica del Norte, Departamento de Ciencias Geológicas, Programa de Magister en Geología Económica, Av. Angamos 1280, Antofagasta, Chile
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Hugo Alcota
Hugo Alcota
1
Exploraciones Mineras Andinas S.A., Apoquindo 4775, Of. 602, Santiago, Chile
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John Proffett
John Proffett
3
P.O. Box 772066, Eagle River, Alaska 99577
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Jaime Díaz
Jaime Díaz
4
Codelco Chile, División Mina Ministro Hales, Calama, Chile
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Gabriel Leiva
Gabriel Leiva
5
Codelco Chile, División RT, Calama, Chile
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Manuel Vergara
Manuel Vergara
6
Codelco Chile, División Chuquicamata, Calama Chile
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Published:
January 01, 2012

Abstract

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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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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