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Book Chapter

Cenozoic Tectonics and Porphyry Copper Systems of the Chilean Andes

By
Constantino Mpodozis
Constantino Mpodozis
Antofagasta Minerals, Apoquindo 4001, Piso 18, Santiago, Chile
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Paula Cornejo
Paula Cornejo
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Published:
January 01, 2012

Abstract

Subduction under South America has been active for the past 550 m.y. but large porphyry copper deposits were essentially emplaced during the Paleocene (60–50 Ma) in southern Peru, and mid-Eocene-early Oligocene (43–32 Ma) and late Miocene-Pliocene (10–6 Ma) in north and central Chile. Although the tectonic setting of the Paleocene porphyry deposits is still poorly understood, those of the northern Chile Eocene-Oligocene belt were emplaced along the margin-parallel Domeyko fault system, where active compressional and/ortranspressional deformation and block rotations took place during the formation of the Bolivian orocline. Eocene-early Oligocene oroclinal bending was a consequence of differential tectonic shortening focused along a mechanically weak zone of the Central Andean crust inherited from the Paleozoic. Deformation occurred during an episode of accelerated westward absolute motion of the South American plate, which coincided with very high rates of oceanic crust production in the eastern Pacific. The slow South American-Farallon convergence rates recorded for the Eocene-Oligocene suggest, however, that strong interplate coupling existed during that time. This permitted the transfer of horizontal stresses and large-scale deformation of the Andean margin, creating a favorable scenario for the generation and emplacement of porphyry copper magmas along the Domeyko fault system.

The younger, Miocene-Pliocene porphyry copper deposits of central Chile-Argentina were emplaced in a different setting, after the initiation of compressional deformation within a volcano-tectonic depression (Abanico basin) that evolved during another, late Oligocene to early Miocene, period of increased East Pacific oceanic crust production. Nevertheless, in contrast to the Eocene-Oligocene situation in northern Chile, the relatively stationary position of the South American plate compared to the mantle reference frame and weak interplate coupling that permitted rapid subduction, increased volcanism, and overriding plate extension. Tectonic inversion of the basin and compressional deformation along with crustal thickening and mountain building began at around 20 m.y. ago as interplate coupling increased when the westward motion of South America accelerated and the Nazca-South America convergence velocity decreased in the mid-Miocene. Compression was accompanied, as during the Eocene-Oligocene in northern Chile, by slab shallowing and increased fore-arc subduction erosion.

In both cases, the largely structurally controlled, syn- to post-tectonic porphyry copper deposits are associated with long-lived magmatic systems that were active for more than 10 m.y. In northern Chile, the deposits occur as parts of discrete intrusive clusters that comprise a suite of precursor plutons emplaced during multiple events since the Cretaceous. Porphyry copper mineralization is linked to multistage, amphibole-bearing intrusions of intermediate composition derived from hydrous, oxidized magmas with adakitic geochemical signatures. These intrusions appeared when crustal thickness increased to a critical threshold in the course of deformation. Production of magmas with high metal-carrying capacity was fostered as fluids were liberated when amphibole became unstable and was destroyed as the crust thickened. At the same time, source regions within the mantle were contaminated by hydrated fragments of fore-arc continental crust, as the result of enhanced subduction erosion during peaks of compressional deformation.

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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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