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Protracted Magmatic-Hydrothermal History of the Río Blanco-Los Bronces District, Central Chile: Development of World's Greatest Known Concentration of Copper

By
Juan Carlos Toro
Juan Carlos Toro
1
Anglo American Chile, Gerencia de Exploraciones, Av. Pedro de Valdivia 291, Providencia, Santiago, Chile
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Javier Ortúzar
Javier Ortúzar
2
Anglo American Exploration (Australia), Suite 1, 16 Brodie-Hall Drive, Bentley, 6102, Western Australia, Australia
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Jorge Zamorano
Jorge Zamorano
1
Anglo American Chile, Gerencia de Exploraciones, Av. Pedro de Valdivia 291, Providencia, Santiago, Chile
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Patricio Cuadra
Patricio Cuadra
3
CODELCO-Chile, División Andina, Sta. Teresa 513, Los Andes, Chile
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Juan Hermosilla
Juan Hermosilla
3
CODELCO-Chile, División Andina, Sta. Teresa 513, Los Andes, Chile
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Cristian Spröhnle
Cristian Spröhnle
4
Compañia Minera Doña Ines de Collahuasi, Av. Baquedano 902, Iquique, Chile
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Published:
January 01, 2012

Abstract

The Río Blanco-Los Bronces copper-molybdenum porphyry district in the late Miocene to early Pliocene magmatic arc of central Chile is currently being mined by state mining company CODELCO (Río Blanco) and Anglo American Sur (Los Bronces). Combined annual production in 2011 was nearly 450,000 metric tons (t) of copper plus by-product molybdenum. With Anglo American's recent high-grade discoveries in the district (3.7 billion tons (Bt) grading 0.7% Cu at San Enrique-Monolito, and 4.5 Bt grading 0.9% Cu at Los Sulfatos) adding more than 65 Mt of fine copper to the mineral inventory, the district now ranks as the world's largest by contained metal, with more than 200 Mt of copper.

Volcanic and volcaniclastic rocks of the Abanico and Farellones Formations represent premineralization host rocks ranging in age between 22.7 ± 0.4 and 16.8 ± 0.3 Ma (U-Pb dating of zircons). The bulk of the copper-molybdenum porphyry endowment is related to evolution of the San Francisco batholith, a large (200 km2) granodiorite-dominated complex with U-Pb zircon ages between 16.4 ± 0.2 to 8.4 ± 0.2 Ma.

Three geologic domains are defined in the district on the basis of rock types, structural breaks, and age determinations: the Los Piches-Ortiga block in the west, the San Manuel-El Plomo block in the center, and the Río Blanco-Los Bronces-Los Sulfatos block in the east. These geologic domains are younger progressively to the east, with most of the known copper endowment on the easternmost (Río Blanco-Los Bronces-Los Sulfatos) block. Intrusive and hydrothermal activity in the Los Piches-Ortiga block spanned ∼ 2.5 m.y., from 14.8 ± 0.1 to 12.3 ± 0.1 Ma. Although these events apparently did not produce high-grade copper deposits, silver-bearing veins associated with a high sulfidation hydrothermal system are present in the block. To the east, in the San Manuel-El Plomo block, a series of magmatic-hydrothermal systems developed during a ∼ 3-m.y. period between 10.8 ± 0.1 and 7.7 ± 0.1 Ma. These events also apparently failed to generate high-grade copper systems. Magmatic-hydrothermal activity in the eastern Río Blanco-Los Bronces-Los Sulfatos block, hosting virtually all of the copper endowment recognized in the district, spanned a ∼ 4-m.y. period from 8.2 ± 0.5 to 4.31 ± 0.05 Ma.

Copper endowment in the district is associated with vertically continuous breccia bodies and quartz-veinstockworked porphyries. Hydrothermal assemblages follow a characteristic vertical and lateral zonation pattern. Remnants of high sulfidation and/or advanced argillic assemblages (quartz-enargite-tennantite-galenasphalerite-gypsum-anhydrite with dumortierite-pyrophyllite-alunite) and peripheral sericite-illite reflect preservation of shallow levels, whereas K-silicate alteration assemblages (biotite-K-feldspar-albite) are present in association with chalcopyrite-bornite and chalcopyrite-pyrite at depth. Between the mineralized bodies, a distal assemblage of hydrothermal chlorite-epidote-specularite-pyrite predominates. At Río Blanco-Los Bronces, igneous and/or hydrothermal- and hydrothermal-cemented breccias developed in intimate association with porphyry phases. At shallow depths, the hydrothermal breccia cement generally comprises quartz-sericitetourmaline, and contains pyrite > chalcopyrite/molybdenite. At deeper levels the breccia cement is predominantly biotite-K-feldspar containing bornite-chalcopyrite-molybdenite. These progressively grade outward into chalcopyrite-pyrite-dominated zones and ultimately to pyrite-dominated zones. Within the Río Blanco-Los Bronces-Los Sulfatos block, the upper presence of the K-silicate assemblage varies from ∼ 3,000 m above sea level (a.s.l.) in the poorly telescoped northern area (Río Blanco), to ∼ 4,000 m a.s.l in the highly telescoped southern area (Los Sulfatos). These differences may reflect varying rates of synmineral structural exhumation, or varying depth of porphyry emplacement along the Río Blanco-Los Bronces-Los Sulfatos structural corridor.

Key factors contributing to the copper productivity in the district are considered to reflect both far-field tectonic conditions, and district-scale structural controls. Following the last significant phase of volcanism documented in the district (∼ 16.8 Ma), a temporally discrete period of peak compression and rapid exhumation, between ∼ 6 to 3 Ma, affected the central Chilean Andes. This period of uplift relates to flat-slab subduction of sea floor containing the Juan Fernandez Ridge into the Chile Trench and overlaps part of the emplacement history of the Río Blanco-Los Bronces-Los Sulfatos block (8.2–4.31 Ma). The lack of contemporaneous volcanism and concomitant tectonic uplift are interpreted to reflect a state of increased horizontal crustal compression due to shear coupling of the downgoing slab. By suppressing volcanism, these conditions are considered to promote the retention of magma in the deep crustal environment, where higher pressures promote greater solubility of magmatic volatiles and higher temperatures may promote longer lived magma chambers by slowing fractionation processes. Under such conditions, the potential is enhanced for increased amounts of metals and volatiles by addition of fresh batches of magma to the deep magma chamber. At the district scale, closely spaced (2 km) structures that control the position of the porphyry and breccia bodies in the Río Blanco-Los Bronces-Los Sulfatos block appear to have focused long-lived, multistage magmatic-hydrothermal activity within a narrow structural corridor, contributing to the development of large, high-grade porphyry/breccia systems.

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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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