© 2012 Society of Economic Geologists, Inc. Special Publication 16, pp. 329–360 Chapter 14 Cenozoic Tectonics and Porphyry Copper Systems of the Chilean Andes CONSTANTINO MPODOZIS† AND PAULA CORNEJO Antofagasta Minerals, Apoquindo 4001, Piso 18, Santiago, Chile 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 conver- gence rates recorded for the Eocene-Oligocene suggest, however, that strong interplate coupling existed dur- ing that time. This permitted the transfer of horizontal stresses and large-scale deformation of the Andean mar- gin, 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 (Aban- ico 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. Tec- tonic inversion of the basin and compressional deformation along with crustal thickening and mountain build- ing 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 associ- ated 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 multi- ple events since the Cretaceous. Porphyry copper mineralization is linked to multistage, amphibole-bearing in- trusions of intermediate composition derived from hydrous, oxidized magmas with adakitic geochemical sig- natures. 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 en- hanced subduction erosion during peaks of compressional deformation. Introduction Basin and Range extension in the Eocene, far inland from the THE STUDY of the tectonic setting of porphyry copper deposits Pacific margin of North America (Kloppenburgh et al., 2010). is fundamental to understanding their genesis (e.g., Sillitoe, Noncollisional porphyry copper deposit examples in sub- 1998; Kay and Mpodozis, 2001; Cooke et al., 2005; Sillitoe duction-related arc settings include those from the Chagai and Perelló, 2005; Richards, 2009, 2011a; Tosdal et al., 2009). belt in Pakistan, the Laramide porphyry copper province of Some Cenozoic porphyry copper deposits are known to have the western United States and northern Mexico (Lang and Ti- formed during or shortly after continent-continent, conti- tley, 1998; Valencia-Moreno et al., 2007; Perelló et al., 2008) nent-island arc, or island arc-island arc collisions in the Hi- and the Central Andes province, which host some of the malayas-Tibet, the Kerman arc in Iran, and Papua New largest known porphyry copper deposits in the world (Camus, Guinea (Solomon, 1990; Zenqiang et al., 2003; Shaifei et al., 2003; Cooke et al., 2005; Sillitoe and Perelló, 2005). 2009). A Paleozoic example of this type of deposit may be The Andes has long been considered as the type example of Oyu Tolgoi in Mongolia (Perelló et al., 2001). In contrast, a noncollisional orogenic system (e.g., Jordan et al., 1983), other large porphyry deposits such as Bingham Canyon in the where subduction of Pacific oceanic crust beneath South western United States formed during the earliest stages of America has been active for the past 570 m.y. (Cawood, 2005). Nevertheless, the largest porphyry copper deposits are † Corresponding author: e-mail, [email protected] the result of anomalous magmatic systems that developed 329 Downloaded from https://pubs.geoscienceworld.org/books/chapter-pdf/3813036/9781629490410_ch14.pdf by guest on 05 August 2020 330 MPODOZIS AND CORNEJO during short periods at specific locations within the Andean Diverse, yet basically subeconomic, porphyry copper deposits orogen. These include the Paleocene to early Eocene (66−52 formed during these events in northern Chile and along the Ma) and middle Eocene to early Oligocene (43−32 Ma) belts Frontal Cordillera in west-central Argentina (Sillitoe, 1977; in southern Peru and northern Chile, and the late Miocene to Sillitoe and Perelló, 2005; Cornejo et al., 2006; Munizaga et early Pliocene (10−5 Ma) porphyry systems in central Chile al., 2008). and contiguous Argentina (Perelló et al., 2003a; Sillitoe and Perelló, 2005). In this contribution, with emphasis on the Jurassic to Early Eocene Tectonics and Chilean belts, we will try to demonstrate how major Cenozoic Metallogeny of the Central Andes tectonic events along the central Andean convergent margin, After the Triassic rifting event, subduction was reestab- prompted by large-scale reorganizations of the global tectonic lished in northern Chile and southern Peru during the Early system, were the main triggers for the formation of large por- Jurassic when a new magmatic arc developed west of the phyry copper deposits. extinct late Paleozoic arc front. Since then, subduction has proceeded uninterrupted to date. Initial Jurassic to Early Pre-Andean History: Cretaceous arc magmatism occurred under extensional con- From Rodinia Dispersal to Pangea Breakup ditions that permitted the formation of a series of intercon- The western margin of South America underwent mag- nected back-arc basins to the east of the main arc, which were matic and tectonic activity at least since the late Neoprotero- progressively filled with marine and continental sedimentary zoic breakup of Rodinia (800−700 Ma), when the separation strata (Mpodozis and Ramos, 1989, 2008). Transpressional of Laurentia from Gondwana produced the opening of the deformation along the arc axis created the intra-arc Atacama proto-Pacific (Iapetus) ocean (Dalziel, 1997). East-directed fault system in northern Chile (Scheuber and González, subduction of newly formed ancestral Pacific crust below 1999) and was accompanied in the Early Cretaceous by the western Gondwana began at ~570 Ma and was fully active emplacement, in northern Chile, of some porphyry copper along the proto-Andean margin by 485 to 465 Ma (Pankhurst deposits at ~140 to 130 Ma (e.g., Antucoya-Buey Muerto, et al., 1998; Cawood, 2005; Chew et al., 2007). Plate conver- 141−139 Ma; Puntillas-Galenosa, 135−132 Ma; Perelló et al., gence in the Central Andes region during the Ordovician to 2003b; Maksaev et al., 2006, 2010). Fast convergence rates Devonian included the progressive collision and accretion of during the global mid Cretaceous superplume event (Larson, a group of tectonostratigraphic terranes of Laurentian and/or 1991) produced an upsurge in volcanism along the Andean Gondwanan affinities (e.g., Ramos et al., 1986; Astini et al., margin, accompanied by intra-arc extension and transtension 1995) against the western South American margin. Terrane which fostered iron oxide-copper-gold (IOCG)−type mineral- amalgamation contributed to the formation of the accre- ization between 120 and 100 Ma in northern Chile and south- tionary Terra Australis orogen, which extended for more than ern Peru (Marschik and Fontboté, 2001; Sillitoe, 2003; Silli- 18,000 km along the Pacific margin of Gondwana from Aus- toe and Perelló, 2005; Chen et al., 2010). Small, low-grade, tralia to South America (Cawood, 2005). The accretionary gold-rich porphyry copper deposits such as Andacollo (104 stage was followed, in the Central Andes, by the buildup of a Ma), Domeyko-Dos Amigos (108−104 Ma), and Pajonales (97 late Carboniferous to Early Permian (320? -280 Ma) supra- Ma) were emplaced under extensional conditions during