The Chilean Andes is a typical example of a mountain range developed along a convergent plate margin. The Andean Cordillera is a non-collisional orogen formed over a long-lived, currently active subduction system. The geological evolution of this orogen was represented by the development of successive north–south trending magmatic arcs and related fault systems on the active continental margin. Magmatic arcs migrated inland stepwise after successive stages of compressive deformation associated with crustal thickening. Moreover, the relative abundance of calcalkaline igneous rocks is a distinctive feature of the Chilean Andes, which has contributed in the genesis of multiple ore deposits of hydrothermal origin (i.e. porphyry, epithermal and IOCG systems). Hydrothermal processes related to supra-subduction igneous activity of felsic to intermediate magmas are the main source of Chile’s metallic ore deposits. Most of these hydrothermal deposits are genetically associated with the succession of N-S trending magmatic arcs, which developed since Late Paleozoic to Jurassic times, over a barren accretionary metasedimentary prism (Mpodozis & Ramos 1989). As a result, successive metallogenic belts were formed parallel to such magmatic arcs, ranging from Jurassic age in the west to younger Miocene age in eastern portions of the country.
Late Paleozoic-Early Triassic Belt (~ 298 – 195 Ma)
Pre-Mesozoic mineralization is for the most part unknown in the Chilean Andes, with exception of a porphyry Cu–Mo belt within the northern portion of a Late Carboniferous to Early Triassic magmatic arc that stretches for over 2500 kms from northern Chile to southern Argentina. This Late Palaeozoic–Triassic porphyry Cu–Mo belt was first reported by Sillitoe (1977) while exploring the Argentinean Andes. K–Ar geochronology confirmed ages ranging from 298 to 195 Ma. Late Palaeozoic–Triassic age porphyry Cu-Mo occurrences are interspersed with Cenozoic porphyry Cu–Mo deposits of northern Chile (Camus 2003). Despite systematic exploration works, most of these Late Paleozoic-Early Triassic porphyry exhibit limited ore mineralization and only few of them have demonstrated to be economically viable. The poor mineralization of these porphyry systems is currently understood as a result of significant unroofing of Upper Palaeozoic to Lower Triassic igneous rocks that only left exposure of the roots of these porphyry systems. This suggests that conditions for significant copper concentration were not optimal during Late Palaeozoic to Triassic times (Camus, 2003). Nevertheless, a detailed study of the San Jorge deposit (Reserves of 2 Mt Cu) revealed that preservation of the upper portions of these porphyry Cu–Mo deposits can be found in the Argentinean Andes together with related shallow epithermal vein systems (Williams et al. 1999). From north to south, some of the known Late Paleozoic – Early Triassic porphyry Cu-Mo deposits in the Chilean Andes are Lilian, Tornasol, Lila and El Jardin.
Early Cretaceous Belt (~ 132 – 97 Ma)
Late Jurassic to Early Cretaceous Andean evolution took place under an overall extensional tectonic regime characterized by coeval volcanic arcs and structure-controlled batholitic intrusions. Two other important components of such extensional tectonic regime were the generation of the intra-arc, sinistral, strike-slip Atacama Fault System along the Coastal Cordillera, as well as the formation of back-arc marine carbonate sedimentary basins. This early period of the evolution of the Chilean Andes was strongly dominated by copper mineralization characterized by a number of Late Jurassic and Early Cretaceous volcanic-hosted Cu-Ag manto-type deposits, mesothermal Cu-Au-Ag veins and iron oxide copper–gold (IOCG) deposits. In addition, iron ore-apatite deposits were also formed along the Atacama Fault System in contact zones between Neocomian age volcanic rocks and Early Cretaceous intrusive rocks. This is known as Chile’s Central Iron Belt. Some examples include Mantos Blancos, Manto Verde, Candelaria, Punta del Cobre, Centinela, Montecristo, Andacollo and El Soldado deposits.
Paleocene to Early Eocene Belt (~ 65 – 55 Ma)
Ever since Late Cretaceous times, a compressive tectonic regime characterized by basin inversions and episodic uplifts has controlled the evolution of the Chilean Andes. Magmatic arcs migrated stepwise east (inland) and this new tectonic setting led to the generation of most calcalkaline porphyry Cu–Mo deposits, which continued until Tertiary times. As a result, a belt of Paleocene–Early Eocene porphyry Cu–Mo deposits extends from southern Peru down to 29°30’ S latitude in northern Chile, totaling 12.7 Mt of contained copper in Chile (resources plus production; Camus 2003). However, only those porphyries with a well-developed supergene enrichment blanket such as Lomas Bayas, Cerro Colorado and Spence deposits have proved to be economic. In addition, several low sulphidation epithermal Au-Ag deposits such as El Penon, Faride, San Cristobal and high sulphidation such as El Guanaco occurred within a Paleocene–Early Eocene volcanic belt that extends along the northern portion of Chile’s Precordillera (22°S to 26°S) and have been preserved thanks to the limited extent of denudation in the Atacama Desert.
Late Eocene – Early Oligocene Belt (~ 43 – 31 Ma)
The most important metallogenic epoch took place during Late Eocene–Early Oligocene, when enormous porphyry Cu–Mo deposits were formed along the Domeyko Cordillera of northern Chile. Together, these deposits account for the largest copper concentration in the world, totaling near 220.5 Mt of copper (resources plus production; Camus 2002). Despite the fact that the mineralizing period lasted between 43 to 31 Ma, 60% of copper resources were accumulated between 34 to 31 Ma (Camus 2002). Late Eocene–Early Oligocene porphyry Cu–Mo deposits are also spatially associated with a major intra-arc, strike-slip and shear fault system known as the Domeyko Fault System, which developed during a period of NE oblique subduction, transpression, crustal thickening and denudation of the Domeyko Cordillera (Maksaev & Zentilli 1999). Classic examples of this belt include Collahuasi, Chuquicamata, El Salvador, Escondida, and Esperanza.
Early to Intermediate Miocene Belt (~ 23 – 12 Ma)
Most epithermal Au-Ag deposits in Chile are related to early Miocene volcanism that developed in the easternmost section of the Chilean Andes (extending to the east into Argentinean and Bolivian territories). Most deposits are of high-sulphidation epithermal type, have structurally controlled orebodies and occur along extensive zones of argillic alterated rocks. Some examples include La Coipa, El Tambo, Pascua Lama, El Indio, Sancarron, Pimenton, and Veladero. In addition, Miocene porphyry Au deposits occurs in close spatial and temporal association with high-sulphidation epithermal Au-Ag deposits along the Maricunga Belt (27–28°S), commonly at the core of eroded volcanoes (Vila & Sillitoe 1991). Some examples of this belt include Lobo-Marte, Refugio, Cerro Casale, Santa Cecilia and Cavancha.
Late Miocene – Early Pliocene Belt (~ 7 - 2.5 Ma)
The last major metallogenic epoch in the Chilean Andes was the formation of enormous porphyry Cu–Mo deposits in central latitudes of Chile which together account for an estimated 183 Mt of copper resources plus production (Camus 2002). This youngest metallogenic epoch was followed by successive stages of compressive deformation, crustal thickening and denudation during Miocene and Early Pliocene periods. Hydrothermal ore precipitation was once again genetically associated with the cease of igneous activity along the western slope of the Andes, prior to a 50 km eastward arc migration.
Classic examples of this belt include Los Pelambres, Río Blanco–Los Bronces and El Teniente deposits.
To conclude, the world’s largest and richest porphyry Cu–Mo deposits occur in the northern portions of the Chilean Andes and have allowed Chile to become not only the leading copper-producing country (5,764 kMT of copper were produced in 2015, 30% of annual global copper production) but also the second largest producer of molybdenum (52,579 MT in 2015) as a by-product of the copper exploitation.
Despite the fact that Chile’s overall mineral wealth lies mainly in these world class Cu–Mo porphyry deposits, it is important to acknowledge other significant metallic ore occurrences, such as epithermal Au-Ag, iron oxide-copper-gold (IOCG), volcanic-hosted manto-type Cu-Ag, iron ore–apatite, mesothermal Cu-Au-Ag- vein and minor skarns Cu-Ag-Au deposits.
CAMUS, F. 2003. Geología de los sistemas porfíricos en los Andes de Chile. Servicio Nacional de Geología y Minería, Santiago.
Maksaev V., Townley B., Palacios C. & Camus F. (2007). Metallic ore deposits. In: MORENO, T. & GIBBONS, W. (ed) The Geology of Chile. The Geological Society, London, 181–201.
MPODOZIS, C. & RAMOS, V. A. 1989. The Andes of Chile and Argentina. In: ERICKSEN, G. E., CAÑAS, M. T. & REINEMUND, J. A. (eds) Geology of the Andes and its Relation to Hydrocarbon and Energy Resources. Circum-Pacific Council for Energy and Hydrothermal Resources, American Association of Petroleum Geologists, Houston, Texas, Earth Science Series, 11, 59–90.
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SILLITOE, R. H. 1977. Permo-Carboniferous, Upper Cretaceous, and Miocene porphyry copper-type mineralization in the Argentinean Andes. Economic Geology, 72, 99–109.
WILLIAMS, W. C., MEISSL, E., MADRID, J. & DE MACHUCA, B. C. 1999. The San Jorge porphyry copper deposit, Mendoza, Argentina: a combination of orthomagmatic and hydrothermal mineralization. Ore Geology Reviews, 14, 185–201.