Dimensiones fundamentales de la sostenibilidad en la dirección de proyectos

The central Andean margin was largely non-volcanic through the Oligocene and lower Miocene, although extension occurred in the southern Andean arc and forearc at 28-25 Ma. This affected the entire Andean margin south of 18ºS (Jordan et al., 2001) and may have been a response to changes in athenospheric flow caused by acceleration of the Nazca plate velocity (Tebbens and Cande, 1997). It is plausibly the event in which the Central Valley and Preandean Depression basins developed to

Table 2.1. Tectonic and metallogenic events recognised in the Central Andes, including summaries

of key geological relationships that have been used to define each period of tectonism. References are indexed in order of appearance (top left to bottom right):

approximately their modern extent. In the Late Oligocene, flat subduction conditions prevailed in response to strong interplate coupling (Oncken et al., 2006) that arose from rapid convergence (Somoza, 1998) driven by acceleration of the westward drift of the South American continent (Silver et al., 1998), and enhanced by the drying climate and lower trenchward sediment flux (Lamb and Davis, 2003). Related shortening and uplift are recorded as unconformities in central (Moquequa Fn.) and southern Peru (Tacazá Fn.), and also in the Bolivian Altiplano, and collectively define the Aymara Tectonic Phase (25-18 Ma; Table 2.1, Sandeman et al., 1995). Facilitated by the flat subduction regime (Creager et al., 1995), clockwise rotation of northern Chile about the Arica Bend occurred between the Eocene and Early Miocene (MacFadden et al., 1995; Arriagada et al., 2000). Bending was a geometric necessity of bulk shortening gradients along the arc (Oncken et al., 2006), with maxiumum shortening measured in the axis of the orocline (Allmendinger et al., 2003). Along-arc shortening gradients were accomodated by major arc-parallel strike slip fault zones, recorded, for example by post-Paleocene sinistral throw of ~37 km on the Domeyko FS (Dilles, 1997; Tomlinson and Blanco, 1997b; Maksaev and Zentilli, 1999; Campbell et al., 2006). Arc-parallel strike-slip shear was associated with subsidence along intermontaone basins such as the Preandean Depression (Scheuber et al., 1994; Tomlinson and Blanco, 1997b).

Extensive erosion of the uplifted Domeyko Arc and foreland through this period is recorded by the Pampa de Tamarugál (northern Chile) and Llanuras Costeranas (southern Peru) pediplains (Mortimer and Saric, 1975; Sillitoe et al., 1968; Sandeman et al., 1995; Reutter et al., 1996; Digert et al., 2003). Semi-arid climate and high denudation rates in the Domeyko Cordillera promoted the formation of supergene enriched mineralisation blankets over the unroofed Paleogene porphyry and

epithermal deposits (Alpers and Brimhall, 1989; Sillitoe and McKee, 1996; Bouzari and Clark, 2002; Arancibia et al., 2006a). At the same time, copper leached from the regolith of these deposits was transported laterally in the groundwater and deposited as exotic copper oxide- and copper silicate-facies concentrations in active terrestrial gravels (Munchmeyer, 1994) via a bacteriogenic mechanism (Nelson et al., 2009). Deposits of this type are spatially associated with the major porphyry copper deposits of the Domeyko Cordillera, including Mina Sur (Chuquicamata), Huinquintipa

(Collahuasi), and Damaina (El Salvador).

Andesitic arc volcanism was re-established along a broad front east of the Preandean Depression at ~17 Ma and has continued in that position to the present day (Fig. 2.5: Western Cordillera). This sequence is intercalated with major felsic pyroclastic units that extend at least 50 km west of the arc front (Naranjo and Puig, 1984; Vergara and Thomas, 1984; Scheuber et al., 1994; Kay et al., 2003) although the calderas from which they are thought to originate are commonly located on the continent-ward (Bolivian) side of the arc (Francis and Baker, 1978; Hildyard et al., 2001; Lindsay et al., 2001; Richards and Villeneuve, 2002; Milner et al., 2003).

Major epithermal and porphyry gold mineralisation occurred during two stages

Figure 2.6. Map of the key tectonic elements of the western South American convergent margin, showing the position of the Peru-Chile trench, the major aseismic ridges and plateaux of the Nazca Plate, the 150 km depth contour of the Benioff Zone and the locations of active volcanoes (triangles). Selected major mineral deposits and belts (pink) of the Miocene metallogenic epoch are shown (square = Cu-Au porphyry, diamond = Cu-Mo porphyry, star = epithermal; circle = skarn). Eastward steps in the Benioff contour indicate slab flattening and coincide spatially with the subduction of the Inca Plateau-Nazca Ridge in Peru and the Juan Fernandez Ridge in Chile. These zones have little or no modern volcanism and host the bulk of Miocene mineralisation. The Iquique Ridge has yet to be begin subduction. Diagram compiled from Jordan et al. (1983), Sasso and Clark (1998), Gutscher et al. (1999; 2000), Cooke et al. (2005), and Rosenbaum et al. (2005).

during the early and late Miocene in the central Chilean Maricunga and El Indio belts (Fig. 2.6: 20-6 Ma: considered one belt by Mpodozis and Kay, 2003), and in central and northern Peru (27º-29ºS, Fig. 2.4: Noble and McKee, 1999). Major porphyry-related tin-silver-(bismuth) mineralisation formed inboard of the main arc, in the Bolivian Cordillera Oriental during part of the same period (23-12 Ma: Sillitoe et al., 1998). Two discrete phases of mineralisation in the central Chilean belt migrated southward in time between 24-12 Ma, against the plate convergence vector (Vila et al., 1991; Kay et al., 1999; Bissig et al., 2000; Mpodozis and Kay, 2003) whereas those in Peru proceeded ahead of the margins of the subducting Nazca Ridge (Rosenbaum et al., 2005). Two of the largest deposits formed during the Quechua Tectonic Phase (Antamina, 9.8 Ma; Love et al., 2000; Yanacocha, 8.2-13.6 Ma; Longo, 2006). Regolith copper-enrichment processes that had occurred from the upper Eocene to lower Miocene did not affect the Neogene deposits because the establishment of the Antarctic ice cap and the northward progression of the cold Humboldt Current caused a regional climate change to hyperarid conditions in the Andes during the Lower - Middle Miocene (Alpers and Brimhall, 1988, Carrizo et al., 2007; 19-13 Ma: Rech et al., 2006).

Neogene magmatism in the Western Cordillera was interrupted by two compressive tectonic phases at 9-8 Ma (Quechua Tectonic Phase) and 5-4 Ma (Daguita Tectonic Phase: Jordan and Alonso, 1987; Scheuber et al., 1994; Sandeman et al., 1995). These events may have been related to transfer of Pacific oceanic spreading from the Galapagos to the East Pacific Rise (Herron, 1972; Herron et al., 1979; Mammerickx et al., 1980). However, the Central Andean plateau has been sustained by ongoing shortening throughout the Neogene, such that these events represent bursts of accelerated shortening superimposed on the broader plateau orogeny (e.g., Oncken et al., 2006). Both phases occurred under near-orthogonal arc convergence (Pardo-Casas and Molnar, 1987). They primarily caused reactivation of pre-existing structures in western parts of the orogen such as the Domeyko Cordillera (Scheuber et al., 1994; Muñoz and Charrier, 1996) and domal and epeiric uplift in the ‘Western’ Cordillera and western Altiplano (e.g., Schildgen et al., 2007) that was facilitiated by thin-skinned thrusting along the eastern orogen margin (e.g., Müller et al., 2002). Estimates of Neogene net vertical displacements are 2.0 - 3.5 kilometres (Wilkes

and Goerler, 1994; Farias et al., 2003; Garzione et al., 2006; Schildgen et al., 2007). Accounting for the likely Paleogene and post-Miocene uplift and erosion, these Late Miocene-early Pliocene tectonic events therefore generated 50 - 70% of the modern Andean physiographic relief.

Miocene-Pliocene metallogeny in the Andes is spatially associated with the onset of flat slab subduction (Fig. 2.6: Kay et al., 1999; Camus, 2002; Hollings et al., 2005; Cooke et al., 2005; Rosenbaum et al., 2005). Major porphyry Cu mineralization occurred between 12 and 4 Ma in a short belt in central Chile (Fig. 2.6). The time- space distribution of these deposits has been interpreted as younging southward in front of the subducting Juan Fernandez Ridge (Kay et al., 1999; Hollings et al., 2005). However, the age distribution is also bimodal, with the northern group of deposits all formed at approximately 12-10 Ma, and the southern group at 6.5-4 Ma (Toro et al., 2006). Prior to the Miocene, subduction of the Juan-Fernandez Ridge was not associated with a reduction in volcanic productivity, nor with shallow subduction or mineralisation (Trumbull et al., 2006). Therefore metallogeny in this belt may ultimately be controlled in time by the dynamics of the Mio-Pliocene compressive tectonism, and localised in space by deformation relating to the position of the subducting aseismic ridge.

Porphyry Cu-Au and epithermal Au mineralization also formed east of the arc front in the Bajo de la Alumbrera and Famatina districts, western Argentina, during the Late Miocene and Pliocene, respectively (Figs. 2.4 and 2.6: Losada-Calderon and McPhail, 1994; Losada Calderon et al., 1996; Proffett, 2003; Harris et al., 2004). These represent the most recent known mineralisation event in this part of the orogen and appear to comprise a discrete belt of mineralisation inboard of the main arc, rather than implying an eastward step of the arc front (Sasso and Clark, 1998). The Bajo de la Alumbrera district is situated on a proposed basement terrane boundary represented by the NW-trending Culampaja lineament that is also coincident with the El Salvador porphyry district (Salfity, 1985; Richards, 2000). This tectonic and structural relationship may explain why these are two of the more Au-rich porphyry systems in the Andes outside the Maricunga Belt (Shatwell, 1995).

to the present, at around 6 cm per year (Angermann et al., 1999); about half of the rate during inception of the Incaic and Quechua tectonic phases (Pardo Casas and Molnar, 1987). In the Central Andes the continental margin is dominated by near-orthogonal compression, whereas the Southern Andes is primarily a dextral transtensional domain (e.g., Yáñez et al., 2002; Melnick et al., 2006). Of that convergence in the central Andes, as much as 85% is absorbed by westward growth of the central Andean forearc (Clift et al., 2007) and the bulk of the remainder

manifests as shortening in the foreland fold and thrust belts of Bolivia, Argentina and eastern Peru (Norabuena et al., 1998). Elsewhere, crustal deformation is dominated by shortening along foreland fold belts, locally associated with low volume alkalic and adakitic magmatism in back-arc positions. This magmatism is spatially related to subduction of the Nazca Ridge in Peru (e.g., Gutscher et al., 1999: James and Sacks, 1999; Espurt et al., 2007), the Juan Fernandez Ridge in central Chile (Kay et al., 1989; Cahill, 1990; Laursen et al., 2002; Kay et al., 2006), the Carnegie Ridge in Ecuador (Beate et al., 2001) and the actively spreading Chile Ridge in southern Chile (e.g., Gerbault et al., 2005). Calc-alkaline arc magmatism occurs along the arc front of the remainder of the Peruvian and Chilean segments of the South American margin. Magmatism extends into the back arc environment and locally traces long- lived structural corridors (Fig. 2.5: Schreiber and Schwab, 1991; Matteini et al., 2001; Seggiaro, 2006), some of which show evidence of active hydrothermal systems (Richards and Villeneuve, 2002; Richards et al., 2006a). Present rates of erosion (e.g., Barnes and Pelletier, 2006) and likely depths of emplacement (e.g., McInnes et al., 2005) imply that any porphyry mineralisation forming in the southern Central Andes today might be exposed sometime in the next 3 - 7 million years.

Chapter 3.

STRATIGRAPHY, INTRUSIVE HISTORY AND