Subduction of aseismic ridges and associated deformation as important controls on porphyry copper formation.

Sjors Timmermans (3922219) Master thesis Faculty of Geosciences Utrecht University First supervisor: Dr. Douwe J.J. van Hinsbergen Second supervisor: Dr Simon Jowitt

Abstract Porphyry copper deposits are the world’s most important source of copper, molybdenum and rhenium. Their formation is widely associated with subduction of oceanic lithosphere, but the exact influence of the subduction-related parameters on ore formation is still subject to a fierce discussion. Many authors have tried to link the present-day tectonic and geochemical setting of porphyry deposits to their environment of formation, but they have never been able to test their hypotheses and be conclusive. In this paper, a plate kinematic reconstruction of the Andes has been used, in which flat-slab segments, as well as bathymetric features, are plotted and I tried to link these to the formation of porphyry deposits. Further, the exhumation rate throughout the Andes is taken into account, to determine if the occurrence of porphyry deposits is exhumation or location-driven. Based on the results of this reconstruction and a similar research by Butterworth et al. (2016), it is concluded that a combination of 5 parameters is necessary. These are relatively old subducting oceanic crust (>25 Myr), a slightly oblique convergence angle for the subducting slab (~15°), a relatively fast convergence rate of ~10 cm/yr, a large distance (>2000 km) from the trench edges (Butterworth et al., 2016) and the subduction of aseismic ridges. This subduction leads to deformation of the overriding plate, such that the ascent of magma is impeded and enrichment of the magma with chalcophile and siderophile elements can take place in MASH zones. As the deformation and the stress-state of the overriding plate are important for the formation of porphyry deposits, far-field effects could play an important role in the formation of porphyry deposits as well.

Introduction Porphyry copper deposits are the world’s most important source of copper, molybdenum and rhenium, with eight of the top 10 and 19 of the top 25 global copper resources formed by porphyry copper deposits (Mudd et al., 2013). Copper is contributing to infrastructure, technology and lifestyle and as the demand for copper is still increasing, it is important to be able to discover new reserves (Mudd et al., 2013). Because of their importance, porphyry copper deposits were studied and described by many authors. However, the exact geodynamic controls on the generation of these deposits remain controversial. Previous researches (e.g. Wilkinson, 2013 and references therein) have described the chemical and tectonic circumstances in which porphyry copper deposits are found at present. Based on these observations they proposed several possible settings in which porphyry copper deposits are formed, but these specific subduction-related parameters have only been qualitatively inferred from the deposits. This way they only managed to link periods of ore deposition to possible geodynamic environment, but they have never been able to test their hypotheses with a plate kinematic reconstruction. In the first proposed setting it is argued that the subduction of aseismic ridges is the most important process (Cooke, Hollings, & Walshe, 2005; Rosenbaum et al., 2005). This subduction of aseismic ridges is often proposed to result in a shallowing of the slab dip (Gutscher et al., 2000). The second proposed setting is the transition from flat-slab subduction to normal subduction (James and Sacks, 1999; Cooke et al., 2005; Hollings et al., 2005). To test these hypotheses, I created a reconstruction of the deforming Andes and plotted the known porphyry deposits, the location and timing of flat slabs, the subducting aseismic ridges and the subducting Chile rise spreading ridge (based on reconstructions of e.g. Gutscher et al. (1999), Yáñez et al. (2001), Rosenbaum et al. (2005), Ramos & Folguera (2009), Ray et al. (2012) and Skinner & Clayton, 2013). This way, I could determine the correlation between these geodynamic features and the formation of porphyry coppers quantitatively. Furthermore, this gave me an opportunity to determine if there is a relation between the distance to the trench and the formation of porphyry deposits, and if so, what could be the reason for these relations. If a correlation between geodynamic processes and porphyry formation is found, the reconstruction in combination with the age of deposits could increase the knowledge about the timing and spatial relationships with respect to the geodynamic processes. By separate plotting of normal and giant deposits, I could determine if specific processes could be coupled to the size as well (appendix 3). Next to these features that could be responsible for the formation of porphyry deposits, the rate of exhumation along the Andes is determined as well (Schildgen et al., 2007; Ruiz et al., 2009; Fosdick et al., 2013; Carrapa & Decelles, 2015). By comparing this rate to the locations of porphyry deposits, I was able to determine if the presence of porphyry deposits is formation-related, or that they are formed along the complete Andes, but not exposed in the regions in which no porphyry deposits are found. Next to this study, another quantitative analysis on the geodynamic controls on porphyry formation is done by Butterworth et al. (2016). They used a plate tectonic model of the Andes, including the spatio-temporal distribution of the ocean floor at the subducting plate. Based on their reconstructions, they applied a statistical analysis to determine the tectonic parameters that result in porphyry copper formation. However, they did not argue extensively why these tectonic parameters combined lead to the formation of porphyry deposits and they did not take bathymetry and flat-slab segments into account. In this paper I will try to argue which tectonic parameters are important and why they lead to the formation of porphyry ores. If a specific setting in which always porphyry copper deposits are formed can be determined, this could help in the search for new copper reserves. To this end, the Andes is chosen for several reasons. First of all, the Andes are known for the amount and the size of the porphyry deposits. The three biggest known porphyry copper deposits in the world are found in the Andes (Cooke et al., 2005). Secondly, both the United States Geological Survey and the Geological Survey of Canada have created outstanding databases containing the porphyry copper deposits in the Andes. Thirdly, a lot of research focused on the deposits in the Andes, to determine the parameters and processes that are important for the formation of porphyry ores has been done yet (e.g. Fuller, 1990; Skewes & Stern, 1995; Gutscher et al., 1999; Gutscher et al., 2000; Kay & Mpodozis, 2001; Oyarzun et al., 2001; Garrido et al., 2002; Reich et al., 2003; Cooke et al., 2005; Hollings et al., 2005; Rosenbaum et al., 2005; Groves & Bierlein, 2007; Sillitoe, 2010; Wilkinson, 2013; Bertrand et al., 2014; Borba et al., 2016). At last, a lot of research has been done on the tectonic setting of the Andes and the relation between aseismic ridges, flat slabs and the magmatism of the Andes (Pilger, 1981; Jordan et al., 1983; LeFevre & McNally, 1985; Gutscher et al., 1999; James & Sacks, 1999; Gutscher et al., 2000; Yáñez et al., 2001; Michaud et al., 2009; Ramos & Folguera, 2009; Ramos, 2010; Ward et al., 2013; Antonijevic et al., 2015; Charrier et al., 2015)

Proposals for porphyry deposit formation Wilkinson (2013) suggested four key triggers for the formation of porphyry deposits. The first is characterized by a cyclical enrichment of magmas with water and metals in the deep crust. This cyclical enrichment is argued to take place in a MASH (melting, assimilation, storage and homogenization) zone at the mantle-crust transition, as described by Hildreth & Moorbath (1988). In this zone mixing of deep-crustal and sub-crustal magmas takes place leading to the formation of calc- alkaline or adakitic magmas, which are closely associated with porphyry Cu deposits (e.g. Richards, 2009; Sillitoe, 2010; Bertrand et al., 2014; Butterworth et al., 2016). The enrichment in volatiles and the generally high oxidation state is considered to be favourable for the formation of porphyry deposits (Lowell & Guilbert, 1970; Cooke et al., 2005; Sillitoe, 2010; Richards, 2011; Kovalenker et al., 2016; Tang et al., 2016). To have cyclical enrichment in MASH zones, it is important that the magma is trapped for a several million years (Richards, 2003; Wilkinson, 2013). This is usually associated with a compressional environment, in which faults that could provide escape ways are closed. Due to this period of volcanic inactivity, sulphur, water and metals are kept in the magmatic reservoir and the volume of this reservoir can increase. The second trigger is the oxidation state of the sulphur in the melt. During the enrichment in the MASH zone, a sulphur-saturated melt has to be oxidized, so that the sulphur is present as sulphate. As a result, chalcophile and siderophile elements (like copper and gold) behave incompatibly and thus remain in the melt (Richards, 2003, 2016). Eventually, the resulting intermediate magma becomes buoyant enough to migrate upwards trough the crust towards shallow magma chambers, in which the formation of porphyry deposits typically takes place (Lowell & Guilbert, 1970; Cloos, 2001; Iko et al., 2001). Another possible system in which the melt can be enriched in metal content, is if the melt is under-saturated in sulphur. This is usually the case in post-subduction melting processes, in which no influx of sulphur is present. The residual sulphides from fractionation of arc-magmas will likely dissolve in the S-undersaturated silicate melt. As siderophile and chalcophile elements have a strong affinity for the sulphide phase (Richards, 2009, 2011, 2013; Sun et al., 2013; Wilkinson, 2013), these sulphides will be rich in metals, which will result in an enriched melt (Richards, 2011). If only small amounts of residual sulphides are present, the original melt will be relatively little depleted in relatively abundant chalcophile elements like copper and molybdenum, but strongly depleted in the sparse siderophile elements like gold. In this case, the second phase of melting could lead to the formation of porphyry Cu-Au deposits (Richards, 2011). If abundant residual sulphides are present, the second phase of melting could lead to porphyry Cu(-Mo) deposits, as the sulphides retain significant amounts of these elements, but the sparse gold would be diluted to low concentrations due to the greater volume of sulphides (Richards, 2011). The third trigger is an efficient transfer of metals into hydrothermal fluids that are exsolved from the magma. This takes place at shallow depths if enough water is present in the melt and the melt is reduced resulting in sulfide-rich hydrothermal fluids. These hydrogen sulphide-rich hydrothermal fluids can strip the magma efficiently from the chalcophile and siderophile metals (e.g., Fe, Cu, Au, Mo), as these have a strong affinity for the sulphide phase (Richards, 2009, 2011, 2013; Sun et al., 2013; Wilkinson, 2013). Fourth, localized processes have to trigger the precipitation of the ores in the crust. An important role in these precipitation processes can be played by the host rock. Impermeable host rocks may prevent the dispersion of metal-rich hydrothermal fluids. If this impermeable host rock is fractured by the hydrothermal fluids, this could lead to a concentrated flow and deposition of high- grade deposits in these fractures. Furthermore, a mafic host rock could play an important role in the precipitation of chalcophile and siderophile elements. Mafic rocks are rich in mafic minerals hosting ferrous iron, which could act as a reductant. This will reduce the melt and hydrothermal fluids, resulting in the formation of sulphides. These sulphides can concentrate the chalcophile and siderophile elements to form fertile hydrothermal fluids, which can produce porphyry deposits (Wilkinson, 2013). At last, focusing of fluid flow through a narrow window and can lead to concentration of mineralization. Due to the fast expansion of the fluid after this window, a steep thermal gradient is created and the hydrothermal fluid cools rapidly. As solubility decreases with a decreasing temperature, this leads to rapid, concentrated deposition of ore minerals (Wilkinson, 2013). To create favorable circumstances for the deposition of porphyry copper in the Andes, two geodynamical settings were proposed. The first proposed setting is that the formation of porphyry copper deposits is closely associated with subduction of ridges, seamount chains or oceanic plateaus beneath continental arcs (Cooke et al., 2005). These subducting slabs are relatively buoyant and the perturbations could lead to crustal thickening, uplift and erosion and adakitic magmatism coeval with the formation of porphyry deposits. This adakitic volcanism could be the result of a flattening of the subduction angle. In this case, the subducting slab remains in a certain pressure-temperature window, allowing the slab to melt and to form adakitic magmas (Gutscher et al., 2000). Furthermore, the subduction of relatively buoyant perturbations favors crustal faulting, which creates space to form the porphyry deposits and these slabs could act as a source of metals (Bertrand et al., 2014). Especially aseismic ridge subduction is proposed as an important mechanism, although the subduction of other bathymetric features like oceanic plateaux is proposed as well (e.g. Bierlein & Pisarevsky, 2008). Volatiles from the ridge may cause metasomatism of the mantle, while the subduction of metalliferous sediments can result in increased sulfur flux. Furthermore, dehydration of the slab may occur, leading to an increasing fluid flux. Again, rapid uplift and erosion, due to the compressional regime, result in rapid decompression. This leads to efficient extraction and transport of magmatic- hydrothermal fluids, which can lead to the formation of porphyry deposits (Cooke et al., 2005) The second geodynamical setting is the transition from flat-slab subduction to normal subduction. During flat slab subduction, hydration of the lithosphere above the flat slab and thickening of the crust above the zone of flat-slab subduction take place (Cooke et al., 2005; Hollings et al., 2005). This hydration leads to advective cooling and a very low heat flow, resulting in a volcanic null (James & Sacks, 1999). Once the slab returns to normal subduction, the interaction of hot asthenosphere and hydrated lithosphere material leads to wet melting of both mantle and crust. However, at first this melt consists of mainly mafic mantle material that has a larger density than the overlying felsic crust (James & Sacks, 1999). Due to mixing of this mafic melt with increasing volumes of felsic melt of the overlying continental crust, eventually the mixture of mafic and felsic magma becomes buoyant enough to rise towards the surface. During this mixing in a MASH zone, enrichment of the magma takes place with sulphur and metals from the mafic magma. The resulting intermediate magma is characterized by a relatively high oxidation state (Richards, 2009). Eventually, during emplacement of the melt in the shallow crust, rapid uplift and erosion, due to the compressional regime, result in rapid decompression. This leads to efficient extraction and transport of magmatic-hydrothermal fluids, which can lead to the formation of porphyry deposits (Cooke et al., 2005). Furthermore, flat-slabs play an important role in style of volcanic activity. For the formation of porphyry ore deposits, it is important that the magmas are emplaced intrusive, as extrusive volcanism leads to the loss of volatiles and metals from the enriched magmas (Rosenbaum et al., 2005). Flat-slab subduction promotes intrusive volcanism in several ways. At first, the hydration of crustal lithosphere increases the felsic input in the melt. As felsic magma has a higher viscosity than mafic magma, this decreases the rate of ascent. Next to this, the cooling period of felsic magma is shorter (Crisp, 1984). At last, flat-slab subduction leads to crustal thickening (Cooke et al., 2005), such that the magma has more time to cool before reaching the surface (Crisp, 1984). Both geodynamical settings occur in compressional regimes. This is favourable for the formation of porphyry deposits, because compression hinders magma ascent through the upper crust, such that water, sulphur and metals remain in the melt and larger magma chambers can form (Cooke et al., 2005; Sillitoe, 2010; Butterworth et al., 2016;). In these magma chambers the cyclical enrichment of the magmas with chalcophile and siderophile elements can take place (MASH zones, (Hildreth & Moorbath, 1988)) and eventually the hydrothermal fluids are restricted to a smaller area, leading to a more concentrated deposition of the copper and gold (Sillitoe, 1998). The increased size of the magma chambers is not necessarily a requirement for the formation of porphyry deposits, as long as a MASH zone can form, but an increased size of the magma chamber could lead to an increase in size of the porphyry deposit (Sillitoe, 2010; Butterworth et al., 2016; Richards, 2016). However, as shown by Gürer et al. (2015), a compressional regime in the Andes does not always lead to a magma chamber in which enrichment can take place, as magma could reach the surface via thrust faults and extensional structures in the upper crust.

Geological setting The central Andes of southern Peru, Bolivia, Argentina and Chile (between 12°S and 42°S) is an active orogen. The tectonics in this orogen are related to the ongoing subduction of the oceanic Nazca plate beneath the continental South American plate. Due to the along-strike variations in the topographic geometry and subduction angles of the Nazca plate, different styles of crustal deformation are present (Ward et al., 2013). These along-strike variations are mainly the result of the presence and subduction of several aseismic ridges. These ridges are the Inca plateau (since 50 Ma), the Iquique ridge (since 50 Ma), the Juan Fernandez ridge (since 49 Ma) and the Nazca ridge (since 30 Ma) (Gutscher et al., 1999; Yáñez et al., 2001; Ray et al., 2012). Of these ridges, the Inca plateau is not observed at present, since it is completely subducted beneath the South American continent. This ridge has been reconstructed by Gutscher et al. (1999) and Skinner & Clayton (2013). In these reconstructions, the reconstruction of Gutscher et al. (1999) is located ~600 km west of the reconstruction of Skinner & Clayton (2013). By using the present day location of the aseismic ridges, determined by reconstructions of Skinner & Clayton (2013) and Yáñez et al. (2001) and coupling with the motion of the Nazca plate, I could reconstruct the time of onset of subduction (figure 6, table 1). Parts of these subduction zone are associated with segments of flat-slab subduction (figure 1). These flat-slab segments are usually interpreted to have caused cessation of arc volcanism, and the cessation of arc volcanism above the two modern flat-slab segments led to estimated onsets of the Peruvian (11 Ma) and Pampean (12 Ma) flat-slab subduction. These avolcanic provinces define the northern and southern termination of the Central Volcanic Zone (CVZ) of the Central Andes (Ramos & Folguera, 2009; Charrier et al., 2015). The segments with flat slab subduction alternate with segments where flat-slab subduction evolves into normal subduction. This transition is associated with extension and back-arc basaltic volcanism. This can be seen in the Payenia volcanic province (Figure 2). This is a basaltic volcanic plateau of Quaternary age, which is argued to be formed by back-arc volcanism, resulting from lithospheric extension due to steepening of the Payenia flat slab and rebirth of an asthenospheric wedge (Burd et al., 2014; Charrier et al., 2015). Due to both along-strike and across-strike variations in the style of deformation, the Andes have been segmented in several distinct morphotectonic provinces. In the area of the Central Andean Plateau the main provinces from west to east are the forearc, the Western Cordillera, the Altiplano-Puna plateau, the Eastern Cordillera and the Subandes (figure 3). To the south of the Central Andean Plateau, active thick-skinned deformation in the Sierras Pampeanas extends 800 eastwards from the trench, as a result of the Pampean flat-slab subduction (Jordan et al., 1983; Ward et al., 2013). Due to subduction erosion since the Neogene, subsidence in the offshore part of the forearc has led to normal faulting, which continues onshore, where the vertical motions change from subsidence to uplift forming the coastal Cordillera (Ward et al., 2013). The Western Cordillera is defined by a Holocene age volcanic arc consisting of dacitic and andesitic stratovolcanoes. Seismological studies have shown a consistently thick (~70 km) crust below the Central Andes. Compositional differences in subduction-related volcanic rocks suggest that this thickness has been increasing since the mid- Oligocene (Schmitz et al. 1999; Beck & Zandt 2002; Yuan et al. 2002; Mamani et al. 2010). The Altiplano and Puna plateau are characterized by a high elevation with low (Altiplano) to medium (Puna) relief. The transition between these plateaus is formed by the Altiplano-Puna volcanic complex (APVC), which consists of young (~12 – 1 Ma) ignimbrites (de Silva 1989; Kay et al. 2010; Salisbury et al. 2011). The Eastern Cordillera is composed of strongly deformed Paleozoic rocks that have been intruded by Triassic and Tertiary granitoid bodies. The Subandean zone at last forms the transition from the high elevations of the Central Andean Plateau to the foreland plains. This transition represents the transition from thick- skinned deformation, during which both basement rocks and overlying layers deform, to an active thin-skinned deformation front, in which only the rocks on top of the basement deform (Ward et al., 2013). The present-day tectonic regime of the Andes is mainly dominated by the so-called Chile- type setting (Figure 4). This is a mild to strong compressional regime with a well-evolved magmatic arc, fold- and thrust belts and foreland basin formation along a continental margin. This regime is characterized by important deformation resulting from the absolute westward motion of the overriding South- American plate, which started ~50 Ma (Oncken et al., 2006; Schepers et al., 2016). This westward motion is about 7 times larger than the shortening rate at these latitudes (Ramos, 2010). At certain locations the subduction of aseismic ridges in this Chile-type tectonic regime may lead to a stronger coupling between the lower and upper plate and the formation of flat-slab segments (Jordan et al., 1983; Gutscher et al., 2000; Ramos & Folguera, 2009). The formation of flat-slab segments can be the result of the absolute westward motion of South America as well. These appear if the westward motion of the trench is faster than the rollback of the slab bend. By comparing the amount of subducted lithosphere since the onset of flat-slab subduction with the extent of the flat-slab segments, it is implied that subducting lithosphere has been moving through the slab bend and the flat-slab segments do not result from buoyant resistance of the Nazca plate lithosphere against subduction. Because there has not been significant westward rollback of the slab bend for the last ~12 Ma, it is implied that mantle material below the flat-slab segments is trapped and keeps the slab bend at a constant location. Eventually these flat slabs can collapse, if over-pressure of the sub-slab region is released by flow of mantle material through slab-tears (Schepers et al., 2016).

Latitude 40-30 Ma 30-18 Ma 18-13 Ma 13-11 Ma 11-5 Ma 5-3 Ma 3 Ma-present 10-5N - - - Cessation of arc Cessation of arc Cessation of arc Cessation of arc volcanism, volcanism, volcanism, volcanism, associated with associated with associated with associated with flat-slab flat-slab flat-slab flat-slab subduction subduction subduction subduction 5-1N - Formation - - - - - of a couple of porphyry deposits 1 N -2 S - Formation - - - - Flat-slab of a couple subduction, of porphyry cessation of arc deposits volcanism 2-14 S - - (Onset of) Subduction of Subduction of Subduction of Subduction of subduction of the Inca plateau the Inca plateau the Nazca ridge, the Nazca ridge, the Inca and Nazca ridge, and Nazca cessation of cessation of plateau and formation of ridge, formation volcanism, volcanism, Nazca ridge, porphyry of porphyry associated with associated with formation of deposits deposits, flat-slab flat-slab porphyry cessation of subduction subduction deposits volcanism, associated with flat-slab subduction 14-19 S Cessation of Cessation of formation of formation of formation of - - arc arc porphyry porphyry porphyry volcanism, volcanism, deposits; deposits deposits associated associated onset of with flat-slab with flat- volcanism subduction; slab formation of subduction porphyry deposits 19-25 S Formation of - Cessation of Cessation of arc Subduction of Subduction of Subduction of many arc volcanism, volcanism, northeastern northeastern northeastern porphyry associated associated with part of the Juan part of the Juan part of the Juan deposits with flat-slab flat-slab Fernandez Fernandez Fernandez subduction; subduction; Ridge; onset of Ridge; Ridge formation of onset of widespread porphyry subduction of volcanism deposits the Juan around the Fernandez edges of the Ridge; flat-slab formation of segment porphyry deposits around the edges of the flat-slab segment 25-33 S Formation of - Formation of Onset of subduction of subduction of subduction of porphyry porphyry subduction of the Juan the Juan the Juan deposits deposits the Juan Fernandez Fernandez Fernandez Fernandez Ridge; cessation Ridge; cessation Ridge; cessation Ridge; cessation of arc volcanism of arc volcanism of arc volcanism of arc volcanism associated with associated with associated with associated with flat-slab flat-slab flat-slab the onset of subduction; subduction; subduction; flat-slab formation of formation of formation of subduction; porphyry porphyry porphyry formation of deposits deposits deposits porphyry deposits 33-38 S Cessation of arc Cessation of arc Extension and Extension and volcanism, volcanism, back-arc back-arc associated with associated with basaltic basaltic flat-slab flat-slab volcanism volcanism subduction subduction 38-45 S ------45-52 S - - - - - Subduction of Subduction of the Chile Rise the Chile Rise spreading ridge spreading ridge Table 2 Summary of the characteristics of several parts of the Andes through time

Methods To test the importance of several geodynamical controls on the formation of porphyry copper deposits, literature is used to narrow down the possible controls (as described in ‘proposals for porphyry deposit formation’). Afterwards, Gplates (https://www.gplates.org) was used to create a reconstruction of the formation of porphyry copper deposits and possible tectonic triggers (supplementary movie 1, figure 6). At first, all known porphyry copper deposits in the Andes which are formed since 83 Ma are plotted in a reconstruction with an assigned age uncertainty of ±3 Myr for the dating (see appendix 1), since for most deposits which have a known error margin, this margin is smaller than 3 Myr (appendix 2). Annen et al. (2006) calculated a minimal ascent rate of 0.05 m/h for intermediate magmas with a low water content (4 weight percent), that are located at only 3.7 km depth, meaning that they are relatively cool. This means that for the hotter, wet melts of the MASH zones, this ascent rate is higher and the magmas reach the shallow crustal magma chambers in less than 1 Myr. As a result we can relate the age of the porphyry deposits with the geodynamic processes at depth from which they result. To reconstruct the original location of formation as precise as possible, deformation of the Andes has to be taken into account. As it is not possible yet to plot deposits in a deforming mesh in GPlates, Poblete (2016, personal communication) divided the Andes into several smaller, rigid blocks. To this end, he used the kinematic restoration of the Andean deformation of Schepers et al. (2016). The porphyry deposits were coupled to these blocks, by assigning them the plate-id of the block in which they are located (supplementary table, appendix 2). This was done by plotting the locations at which the deposits are found at present, so that I was able to determine the plate id of the block in which they are located and assign this to the specific deposit. By moving together with the small, rigid block in which they are found, the porphyry deposits can be traced back to their original location of formation as precisely as possible. At first, to determine if the location of the porphyry deposits is only related to the geodynamical circumstances that lead to their formation, the exhumation rates along the Andes have been compared. These exhumation rates are mainly determined by apatite and zircon fission tracks. The apatite partial annealing zone lies between 60-120°C, while the zircon partial annealing zone lies between 210-270 °C (Ruiz et al., 2009). To determine the exhumation rate, a geothermal gradient between 30 °C km-1 (Carrapa & Decelles, 2015) and 47 °C km-1 (Ruiz et al., 2009) is used to establish the depth at which the apatite or zircon fission tracks were formed. By combining this with the age of formation the exhumation rate could be calculated. Afterwards, the circumstances of porphyry formation were investigated. Because flat slab evolution and the subduction of bathymetric features like aseismic ridges and spreading ridges are often proposed as important features in the formation of porphyry deposits, these were implemented in the reconstruction. To this end, the timing and location of flat slabs, the subduction of aseismic ridges and the subduction of the Chile rise spreading ridge had to be plotted and compared to the timing and location of porphyry deposit formation. These were outlined by several authors (e.g. Gutscher et al., 1999; Yáñez et al., 2001; Ramos & Folguera, 2009; Ray et al., 2012; Skinner & Clayton, 2013). Ramos & Folguera (2009) reconstructed past and present flat slabs. Present day flat slabs were reconstructed based on seismological data, as well as by using the presence of volcanic gaps. Past flat-slab subduction segments were reconstructed based on several criteria. At first a rapid cessation of the magmatic arc was found and no igneous rocks of the age of the flat-slab segment are known in these areas. Furthermore, widespread deformation and crustal thickening, resulting from flat-slab subduction, are found in the Eastern Cordillera. The end of flat-slab subduction is recognized by widespread volcanism, as the hydrated lithosphere on top of flat-slab segments is melted by hot asthenosphere, which rises as a result of slab steepening. The present location of aseismic ridges has been done by several authors (Gutscher et al., 1999; Yáñez et al., 2001; Ray et al., 2012; Skinner & Clayton, 2013). For the reconstruction of the Nazca ridge, the Inca plateau and the Iquique ridge, Skinner and Clayton (2013) assumed that they were formed at the East Pacific Rise. In this case, the Marquesas, Tuamotu and Austral seamounts/plateaus on the Pacific plate are their conjugate features. They used the Earthbyte model by Müller et al. (2008) to reconstruct the location and timing of the formation of these features by coupling them to the Pacific plate. By creating a mirrored feature across the spreading ridge on the Nazca plate and couple their motion to the Nazca plate, they were able to reconstruct the present day location of the Nazca ridge, Inca plateau and Iquique ridge. The Inca plateau has been reconstructed by Gutscher et al. (1999) as well. Their reconstructed location of the Inca plateau is located ~600 km west of location reconstructed by Skinner & Clayton (2013). For the reconstruction of the Juan Fernandez ridge, Yañez et al. (1999) assumed a stationary hotspot and used the absolute plate motion of the Nazca plate and the South American plate to predict the hotspot track. The onset of subduction can be determined by coupling the present day aseismic ridges in our reconstruction to the Nazca plate as well and determine the time of onset by moving back in time. The present day location of the Chile rise spreading ridge is determined by Skinner & Clayton (2013). The part of the Chile rise that is subducted since 6 Ma, has been reconstructed by Lagabrielle et al. (2000). To be able to determine if the absolute plate motion of the South American plate influences the formation of porphyry deposits, the mantle is fixed in the reconstruction. To this end, the hotspotframe of Doubrovine (2012) has been used. In the reconstruction, the porphyry copper deposits were divided into three different groups, based on the classification as described by Singer et al. (2008). These are porphyry copper-gold (Cu- Au) deposits (if the Au/Mo ratio is ≥ 30), porphyry copper-molybdenum (Cu-Mo) deposits (if the Au/Mo ratio ≤ 3) and porphyry copper (Cu) deposits (otherwise). In this classification, the Au concentration is in parts per million and the Mo concentration is in percent. A porphyry deposit in which no molybdenum is present is classed as porphyry Cu-Au if the gold concentration > 0.2 g/t. If no gold is present, but the molybdenum grade is bigger than 0.03%, a deposit is called a porphyry Cu- Mo (Singer et al., 2008). Using this reconstruction, distance between the deposits and the aseismic ridges, the trench and the flat slabs during formation has been measured. The results were plotted versus age and/or latitude. In these graphs, 5 categories are distinguished: next to the previous division in Cu, Cu-Au and Cu-Mo porphyry deposits, the Cu and Cu-Au deposits are split into normal and giant deposits. A porphyry Cu deposit is called giant, if it contains at least 3.162 Mt copper, while a porphyry Cu-Au deposit is called giant if it contains > 100 t gold (Cooke et al., 2005).

Results From the databases of porphyry deposits in the Andes (USGS and Geological Survey of Canada, see appendix 2), it becomes clear that there are 2 major periods of porphyry deposit formation (figure 5,6). The first period starts at ~64 Ma and ends at ~29 Ma. From ~42 Ma onwards, the formation rate of porphyry deposits increases. Almost all of these deposits were formed in the Central part of the Andes and are at present found between ~12°S and ~25°S. Next to these deposits, there are 3 known deposits formed between ~49 Ma and ~42 Ma, which are found in the northern part of the Andes. During this period only one deposit (Quebrada del Bronce) is formed south of the Central Andes (at a present latitude of ~35°S). A second period of porphyry formation takes place between ~20 Ma and ~4 Ma. These deposits are spread over the middle part of the Andes. In the most northern part and the most southern part of the Andes no porphyry deposits are found. Most of the deposits were formed in two clusters. These are found just north and south of the Central Andes. The northern cluster is found east of the Nazca ridge and Inca plateau, while the southern cluster is found east of the Juan Fernandez ridge. As described by several authors (Cloos, 2001; Iko et al., 2001; Singer et al., 2008; Wilkinson, 2013; Bertrand et al., 2014; Williamson et al., 2016), porphyry deposits are usually formed in the upper 4 km of the continental crust. This means that only up to 4 km of exhumation has to take place to find the porphyry deposits at the surface. Based on apatite and zircon fission tracks, the exhumation rate for several parts of the Andes has been determined. Ruiz et al. (2009) argued that the average rate of exhumation for SE Peru is 0.17 km Ma-1 for the period between 36 Ma and 15 Ma. If we assume that this steady-state exhumation rate remained constant since 15 Ma, this means that the maximum amount of exhumation in SE Peru has been 6.12 km since 36 Ma. For the Altiplano-Puna plateau, a variable pattern of exhumation has been found (Carrapa & Decelles, 2015, see figure 7). This figure gives the apatite fission track ages that are found at the surface. As they used a geothermal gradient of 30 °C km-1 is assumed, this means that rocks of 2-4 km depth are exposed at the surface. These data agree with an exhumation rate of 0.267 km Ma-1, as determined by Schildgen et al. (2007) by using canyon incision history of the western margin of the Altiplano- Puna plateau. Both SE Peru and the Altiplano-Puna plateau are locations at which porphyry deposits are found. In the Patagonian Andes however, no porphyry deposits have been found. Fosdick et al. (2013) determined an exhumation rate of 0.22 – 0.35 km Ma-1 since 22 Ma in this part of the Andes. This gives a maximum amount of exhumation of 7.7 km for this period.

Figure 5 Tonnages of formed copper in porphyry deposits versus age (A) and latitude (B)

Figure 7 (Carrapa & Decelles, 2015) Isochron map of the Altiplano-Puna plateau based on apatite fission tracks.

By plotting the known porphyry deposits of the Andes in a plate reconstruction (movie 1), I have been able to measure some of the relations between the deposits and geodynamical features influencing the tectonics of the Andes by using the measurement tool in GPlates. This gives an error margin of ± 100 km. These relations are plotted versus the age at which the deposits are formed or the latitude at which they are found, so that I could determine if a clear relation between the geodynamical features and the formation of porphyry deposits is present. At first, I have measured the shortest distance between the deposits and the trench (figures 8 and 9). From these results, a clear trend can be seen. This trend gives a general formation between 200 and 400 km from the trench. However, relatively young deposits (<20 Ma) at a latitude between ~33°S and ~24°S show a couple of deposits that are formed farther away from the trench. Further, between ~64 Ma and ~57 Ma, a cluster of deposits is formed relatively close to the trench (100 – 200 km). These are formed between ~16.5°S and ~27°S. A last striking transition takes place at ~16°S. At that latitude, the deposition distance to the trench goes from very close to the trench to the south, to the average distance to the north.

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Cu Giant Cu Cu-Au Giant Cu-Au Cu-Mo Figure 8 Distance of porphyry deposits to the trench. A porphyry Cu deposit is called giant, if it contains at least 3,162 Mt copper, while a porphyry Cu-Au deposit is called giant if it contains > 100 t gold (Cooke et al., 2005).

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Cu Giant Cu Cu-Au Giant Cu-Au Cu-Mo Figure 9 Distance of porphyry deposits to the trench. A porphyry Cu deposit is called giant, if it contains at least 3,162 Mt copper, while a porphyry Cu-Au deposit is called giant if it contains > 100 t gold (Cooke et al., 2005).

Secondly, I have plotted the different deposits against the distance from the nearest flat slab (Figure 10 and 11). This distance is measured in a straight line between the projections of both the flat slabs and the deposits on the surface. If the deposit is formed on top of the flat-slab segment, the distance is measured as negative. For most flat slabs the deposits are formed throughout the presence of the flat slabs (Figure 10). For the Altiplano slab (~40-18 Ma, (Ramos & Folguera, 2009)), however, the porphyry deposits are only formed during the first ~10 Myr of the flat slab and during the last ~2 Myr of the flat slab, with a period of ~10 Myr in between during which no known porphyry deposits are found. During this period, no volcanic arc has been formed in this region (Ramos & Folguera, 2009). For this flat-slab segment, an error margin of ~9 Ma is present for the onset and termination of the flat-slab subduction (Ramos & Folguera, 2009). In my reconstruction the maximum duration of flat- slab subduction is taken as a reference. Furthermore, for most flat slabs the deposits are formed within 1000 km of the edge of the flat slab, but for the Puna slab (18-12 Ma, (Ramos & Folguera, 2009)) these distances are much larger and up to 2500 km. This distance is so large that we cannot correlate this to the presence of the Puna flat slab. However, Espurt et al. (2008) and Manea et al. (2017) argued that influence of flat slab subduction reaches only to ~150 km from the flat-slab. The Peruvian (11 Ma – present, (Ramos & Folguera, 2009)) and Pampean (12 Ma – present, (Ramos & Folguera, 2009)) flat slab are striking, as the distance of formation from the flat slab edge is very small for these segments. For the Carnegie segment, most porphyry deposits are even formed on top of the flat-slab segment instead of next to it, while for most slabs almost all porphyry deposits form next to it.

Bucamaranga slab Pampean slab Peruvian slab Payenia slab Puna slab Altiplano slab Figure 10 Distance of porphyry deposits to the nearest flat slab. A negative distance means that it is formed on top of the flat slab.

Cu Giant Cu Cu-Au Giant Cu-Au Cu-Mo Figure 11 Distance of porphyry deposits to the nearest flat slab. A negative distance means that it is formed on top of the flat slab. A porphyry Cu deposit is called giant, if it contains at least 3,162 Mt copper, while a porphyry Cu-Au deposit is called giant if it contains > 100 t gold (Cooke et al., 2005).

Thirdly, I have measured the shortest distance between the porphyry deposits and the projection of the subducting aseismic ridges. These ridges are derived from reconstructions by other authors (Gutscher et al., 1999; Yáñez et al., 2001; Ray et al., 2012; Skinner & Clayton, 2013). However, since the location of the Inca plateau in my reconstruction (Skinner & Clayton, 2013) is ~600 km to the east of the location as reconstructed by Gutscher et al. (1999), an uncertainty of ~600 km is present for the few deposits, which are closest to the Inca plateau. As can be seen in figure 12, this gives 2 obvious clusters for the relation between age and distance to the nearest aseismic ridge. It is striking that a linear relationship between age and distance to the aseismic ridge can be found, with a gap of ~10 Myr without any porphyry deposits in between the two clusters (between 30 Ma and 20 Ma). Within the clusters, the different deposit types are mixed. Furthermore, no porphyry deposits formed since 4 Ma are known. From the reconstruction, it can be concluded that the deposits that have been formed since 20 Ma are mostly formed just in front of a subducting aseismic ridge, with a couple formed on top of a subducting aseismic ridge. As seen in figure 12 and appendix 3, the formation on top of a subducting ridge is not related to a specific type of porphyry deposit. distance to aseismic ridge 4500

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Cu Giant Cu Cu-Au Giant Cu-Au Cu-Mo Figure 12 Distance of porphyry deposits to the nearest aseismic ridge. A negative distance means that it is formed on top of the subducting aseismic ridge. A porphyry Cu deposit is called giant, if it contains at least 3,162 Mt copper, while a porphyry Cu-Au deposit is called giant if it contains > 100 t gold (Cooke et al., 2005).

Fourthly, a spreading ridge called the Chile rise is currently subducting beneath the southern part of the Andes. This subduction started at ~12 Ma. As the Chile rise consists of several small segments, the subduction location migrated northwards through time (Gorring et al., 1997; Lagabrielle et al., 2000; Georgieva et al., 2016). In this part of the Andes no porphyry copper deposits have been found yet.

At last, the absolute plate motion of the South American plate is considered. As follows from the reconstruction of Schepers et al. (2016), in combination with the hotspot frame of Doubrovine (2012), this absolute plate motion is very similar for the complete Andes during the period 70-20 Ma. Only in the last 20 Myr, deformation in the Central Andes results in slightly smaller westward motion compared to the northern and southern part of the Andes.

Discussion To be able to determine the main triggers for the formation and preservation of porphyry deposits, it is important to combine the exhumation data with the geodynamical triggers that follow from the plate kinematic reconstruction. The exhumation data can be used to determine if the presence of the known porphyry deposits is due to a localized exposure, or due to a localized formation. In case of a formation trigger, the distance to bathymetric features, in combination with geological and geochemical implications determined by other authors, could give a better understanding of the necessary circumstances for the formation of porphyry deposits.

Exhumation or location-driven? As seen in figure 7 the rate of exhumation can vary significantly within a relatively small area of the Andes (Carrapa & Decelles, 2015). At the Altiplano-Puna plateau, this has resulted in the presence of porphyry deposits of two age clusters (~38-30 Ma and ~15-10 Ma, see appendix 2, 3, reconstruction). The average rate of exhumation in this area, however, is 0.267 km Ma-1 (Schildgen et al., 2007; Carrapa & Decelles, 2015). This is similar to the average rate of exhumation in the Patagonian Andes, which is 0.22-0.35 km Ma-1 (Fosdick et al., 2013). Although the exhumation rate of the Patagonian Andes is similar to the exhumation rate in the Altiplano-Puna plateau, there are no porphyry deposits found in the Patagonian Andes. This implicates that the exhumation rate is not important for the presence of porphyry deposits. But because porphyry deposits are formed at a depth of 1-4 km (Cloos, 2001; Iko et al., 2001; Singer et al., 2008; Wilkinson, 2013; Bertrand et al., 2014; Williamson et al., 2016), the exhumation rate is important for the preservation of porphyry deposits and to bring the deposits to the surface. Since the exhumation rate is similar between regions with and without porphyry deposits, the possible triggers for porphyry formation will be discussed. To this end, at first the distance between the location of porphyry deposits and the trench will be described.

Distance of porphyry deposits to the trench As seen in figures 8 and 9, most porphyry deposits form within 200-300 km from the trench. This correlation is most likely the result of the pressure-temperature conditions at which both the mantle wedge and parts of the slab can melt due to hydration by slab-derived fluids. Due to the melting of this mantle wedge, combined with melting of the overlying lower crust, MASH zones can be formed at the bottom of the crust (Hildreth & Moorbath, 1988). However, for young deposits between ~33°S and ~24°S the horizontal distance can become as large as 600 km. From the reconstruction we can conclude that this is contemporary with the formation of flat slabs in this area (Skinner & Clayton, 2013) and the arrival of the NE-SW directed part of the Juan Fernandez ridge (Yáñez et al., 2001). An analytic solution for the thermal structure of a cold slab moving a fixed distance through hot asthenosphere shows that temperature is independent of the slab dip (Gutscher et al., 2000). This means that flat slab subduction leads to a shallower melting farther away from the trench compared to normal subducting slabs. As a result, the location of MASH zones and eventually the distance from the trench at which porphyry deposits form is dependent on the slab dip. Except for this correlation, flat slabs do not seem to be a prerequisite for the formation of porphyry deposits. This is tested by plotting the porphyry deposits against the distance to the nearest flat slab edge (Figures 10 and 11). To find new porphyry deposits, it is thus important to take the present and past slab dip in a certain geodynamical setting into account. Beneath all of Peru and Chile the subduction takes place at a relatively shallow dip of 30° or less (Pilger, 1981), so the distance of porphyry deposits to the trench in the Andes is relatively big.

Importance of flat slabs As seen in figure 10, most porphyry deposits are formed throughout the period of existence of flat slabs. One exception to this is the Altiplano slab. For this slab no porphyry deposits formed throughout the second half of its existence until the last ~2 Myr. During the presence of this flat slab, no volcanic arc is formed in this area. As discussed before, this is conducive for the formation of porphyry deposits. Because the Altiplano slab is associated with a volcanic null during the time that no porphyry deposits are formed, we can conclude that this is not the only prerequisite for the formation of porphyry deposits. For the Payenia and Bucamaranga slab only two porphyry deposits are found, so for these flat-slab segments there is no correlation with the porphyry deposit formation as far as we know. These flat-slab segments are the only flat slabs formed since the onset of subduction of the known aseismic ridges, which are not on top or in front of a subducting aseismic ridge. However, only the deposits that are closest to the Peruvian and Pampean slabs are all formed above or close to (edge of) the flat slab. For the Puna and Altiplano slab, the deposits are up to 2500 km away from the flat slab segments, which means that these deposits are definitely not related to the flat-slab segments. Cooke et al. (2005) and Hollings et al. (2005) proposed that flat-slabs are an important requirement for the formation of porphyry deposits. During the flat slab subduction, the overlying crust becomes hydrated. They propose that steepening of the slab leads to interaction of hot asthenosphere and hydrated mantle and crust, which leads to wet melting. As the flat-slab subduction leads to thickening of the overlying crust, this magma is stalled at the base of the crust and a MASH zone can form. During the melting, more crustal material is added the MASH zone, so the magma becomes more felsic and the density becomes small enough to rise towards the surface. However, the influx of the hot asthenosphere and the hydration of the overlying mantle take place on top of and at the edge of the flat-slab segment and Espurt et al. (2008) and Manea et al. (2017) argued that influence of flat slab subduction reaches only to ~150 km from the flat-slab. As most porphyry deposits form > 150 km away from the flat-slab segments, it is not likely that flat slabs play an important role in the formation of porphyry deposits. Furthermore, it follows from my reconstructions, that giant deposits are not preferentially formed on top or close to a segment of flat-slab subduction (appendix 3, supplementary movie 2), although flat slabs could improve the circumstances related to the formation of porphyry deposits. However, as discussed before, the location of past and present flat slabs could be important for determining the location of porphyry deposits, because a smaller slab dip results in the formation of MASH zones and thus porphyry deposits further away from the trench (Gutscher et al., 2000).

Influence of aseismic ridges As can be seen in figure 12, there are two obvious clusters of porphyry deposits in relation to the distance to the nearest aseismic ridge. As all different categories of deposits are mixed throughout the whole graph, there is no strong correlation between the distance to an aseismic ridge and the kind of porphyry deposit. From the reconstruction, it becomes clear that the cluster of deposits forming since 20 Ma is closely related in space to the subduction of aseismic ridges (the Nazca ridge, Inca plateau and Juan Fernandez ridge). These deposits all form from a few million years before the onset of subduction of aseismic ridges beneath the South American plate. The porphyry deposits around the Nazca ridge and Inca plateau formed from 20 Ma onward, while the formation of porphyry deposits related to the subduction of the Juan Fernandez ridge starts around 15 Ma. Based on kinematic reconstruction of the Nazca ridge as reconstructed by Skinner & Clayton (2013), subduction started at ~18 Ma. The subduction of the Inca plateau started between 16 Ma (based on the kinematic reconstruction of the Inca plateau as reconstructed by Skinner & Clayton (2013)) and 12 Ma ((Gutscher et al., 1999), based on their kinematic reconstruction and the location of the conjugate Marquesas plateau). For the Juan Fernandez ridge, a large uncertainty in onset of subduction is found. Yáñez et al. (2001) and Garrido et al. (2002) argue that onset of subduction took place at ~24 Ma, based on a plate motion prediction and a volcanic quiescence around 20 Ma. However, by combining their reconstruction of the location of the Juan Fernandez ridge at present with the motion of the Nazca plate, an onset of subduction at ~13 Ma is found (this study). Because most of the porphyry deposits form in front of the subducting aseismic ridges (to the east, see figure 6) instead of on top of them, it is not likely that their formation is the result of a metallogenic response to the arrival of the aseismic ridges as suggested by Cooke et al. (2005) and Rosenbaum et al. (2005). Instead, due to the buoyancy and topography of the aseismic ridge, the subduction of the ridge will be impeded during arrival at the trench. This causes a strongly compressional environment impeding the ascent of magmas trough the crust, so that enrichment of these magmas in a MASH zone can take place (Hildreth & Moorbath, 1988). This means that rather than the composition of the subducting ridge, the deformation and stress state of the overriding plate as a result of the subduction of aseismic ridges are the main control on porphyry ore formation. As a result, it may be important to include far-field effects into the research on porphyry formation. Since the cluster of deposits formed between ~64 Ma and ~29 Ma is formed at > 2000 km from the subducting aseismic ridges, it is unlikely that their formation is the result of the subduction of the known aseismic ridges. However, a clear clustering of these deposits (both giant and normal) can be observed at the Altiplano-Puna plateau (see figure 6). These deposits cannot be related to the subduction of the known aseismic ridges, as the oldest known aseismic ridges (Inca plateau and Iquique ridge) are only formed at 50 Ma (Gutscher et al., 1999). In the area of the present-day Altiplano-Puna plateau, a flat slab segment has been present from 40 Ma – 18 Ma (Altiplano slab, (Ramos & Folguera, 2009)). A possible explanation for this cluster is the subduction of older aseismic ridges, which cannot be determined precisely.

Spreading ridges In the Andes subduction zone, only one subducting spreading ridge (the Chile rise) is found. The subduction of a spreading ridge leads to the formation of a slab window (Georgieva et al., 2016). This window leads to near-edge volcanism, by which the asthenospheric magmas can reach the surface (Gorring et al., 1997; Lagabrielle et al., 2000; D’Orazio et al., 2003; Georgieva et al., 2016), instead of being enriched in a MASH zone. As a result, no porphyry deposits has been found close to the subducting spreading ridge and I argue that the subduction of spreading ridges is not an important prerequisite for the formation of porphyry deposits. As no porphyry deposits are found close to the ridge, the subduction of spreading ridges could have destroyed the potential for porphyry formation. However, because the Chile rise is the only subducting spreading ridge in the Andes, and this part of the Andes does not host porphyry deposits that have been formed before the subduction of this spreading ridge, we cannot conclude if the subduction of spreading ridges destroys the potential of ore formation.

Previous research Recently, Butterworth et al. (2016) performed a similar research, in which they employed plate tectonic models to simulate the plate tectonic regimes acting along the South American trench for the past 200 Myr. Eventually, they did a statistical analysis to determine the most important tectonic conditions for the formation of porphyry ore deposits. From this analysis, they concluded that a combination of four parameters is important for the formation of porphyry deposits. These are (1) relatively old subducting seafloor (> 25 Myr), (2) a slightly oblique convergence angle for the subducting slab (~15°), (3) a relatively fast convergence rate of ~10 cm/yr and (4) a large distance (>2000 km) from the trench edges. The relatively old seafloor is important, as seafloor thickens with age and at the same time the sediment load increases. Due to the thickening of both the seafloor and the sediment load, the amount of porphyry forming minerals present increases. This is in agreement with the observation by my reconstruction, that no porphyry deposits form around subducting spreading ridges. Next to this, the subduction angle is important, as complete perpendicular subduction leads to melting at greater depths, at which no mixing with felsic, crustal material can take place and the melt will not be able to ascent to shallow magma chambers. If the subduction obliquity is too large on the other hand, melting will take place too shallow and the melt will be too felsic. As mafic melt provides the mix with volatiles, sulphur and metals, this will not lead to porphyry formation either. The fast convergence rates are suggested to be important for porphyry formation if they are followed by a rapid deceleration (Bertrand et al., 2014; Butterworth et al., 2016), as this leads to an increase in partial melt and the possibility for magmas to ascent to the upper crust. At last, a location far from the trench edge appears to be positively correlated to porphyry formation as a result of increased rollback of the subducting plate. This leads to increased inflow of hot asthenosphere, which results in an increase of the quantity of melt (Stegman et al,, 2006; Butterworth et al., 2016). In their analysis, however, Butterworth et al. (2016) did not include the influence of subduction of spreading ridges, aseismic ridges and flat-slab subduction, although these are often proposed to be of great importance for the formation of porphyry deposits.

Chemical variability As can be concluded from the plots of tectonic triggers versus age or latitude (figure 8,9,11 and 12), the tectonic triggers are not responsible for the differences in grade of the main resource metals (copper, gold and molybdenum). This implicates that the difference is the result of variations in the chemical composition of the magmas from which the porphyry deposits are formed. Richards (2009) argued that the formation of porphyry Cu-Au deposits is often related to a second stage of melting. If moderate amount of sulfide are present near an oxidized, sulfate-rich melt, chalcophile and siderophile elements like copper and gold will preferentially partition into this sulfide phase. However, as the volume of the sulfide is relatively small compared to the volume of the melt, this will hardly effect the concentration of the relatively abundant copper in the silicate melt. The less abundant gold, however, will be depleted in the magma and enrich the residual sulphides. If the fertile residue is subject to a second stage of melting, the Au-rich residual sulphide may redissolve and form a magma with relatively high Au/Cu ratios. This magma has the potential to form porphyry Cu-Au deposits (Richards, 2009). For the formation of porphyry Cu-Mo deposits, especially the depth of the magma chamber is important. If this is relatively deep, the water saturation content of the magma is relatively high. Consequently, to reach water saturation a lot of crystallization has to take place. As Mo is incompatible in a crystallizing melt, its concentration will increase in the residual melt. When saturation is achieved, Mo will concentrate even more in the H2O fluid-phase and thus in the porphyry deposit, because of its positive partition coefficient (Robb, 2013).

Geodynamical setting of porphyry deposit formation Based on the paper by Butterworth et al. (2016), in combination with the results of my own plate tectonic reconstruction of the Andes, I think that a combination of several factors is important for the formation and location of porphyry deposits. Although several authors suggested that the presence of flat slabs and the transition back to normal subduction is a prerequisite for the formation of porphyry deposits (e.g. Cooke et al., 2005; Hollings et al., 2005), I think these are only important to determine the location of porphyry formation. Since the melting of a subducting slab is not dependent on the slab dip, but on the distance that the slab has traveled through the mantle (Gutscher et al., 2000), a smaller slab dip results in melt farther away from the trench. From my data, it follows that for a large period during the presence of the Altiplano slab, no porphyry deposits are formed. This probably the result of impingement of magma at the base of the crust, due to strong compression related to flat slab subduction. Further, as many porphyry deposits have formed during the initial stage of the Altiplano slab, it is likely that the mantle and crust in this area were depleted. The apparent close correlation between the Peruvian and Pampean flat slabs and porphyry deposits is probably the result of the nearby subduction of the Juan Fernandez ridge. Furthermore, the exhumation rates of the Altiplano-Puna plateau in comparison to the Patagonian Andes are very similar. This shows that the distribution of porphyry deposits is not the result of a specific exhumation pattern. It is important for the formation of porphyry deposits, to have a relatively old subducting seafloor (> 25 Myr), a slightly oblique convergence angle for the subducting slab (~15°), a relatively fast convergence rate of ~10 cm/yr and a large distance (>2000 km) from the trench edges as argued by Butterworth et al. (2016). However, during the period between ~29 Ma and ~20 Ma, the values for these parameters are very similar to the values in the 10 Myr before and after this period (Butterworth et al., 2016), although this is the only period in which no porphyry deposits are formed. This implies that another parameter is important as well. From my reconstructions, I derive that subduction of aseismic ridges plays an important role in the formation of porphyry deposits during the last ~20 Myr. For the cluster of porphyry deposits formed between ~64 Ma and ~29 Ma, there is no clear correlation with aseismic ridges. However, most of these deposits are formed in the area of the Altiplano-Puna plateau. In this region, a flat slab segment (the Altiplano slab, (Ramos & Folguera, 2009) was present from ~40 Ma – ~18 Ma. As flat slab subduction is usually related to the subduction of aseismic ridges (Pilger, 1981; Jordan et al., 1983; James & Sacks, 1999; Gutscher et al., 2000; Cooke et al., 2005; Rosenbaum et al., 2005; Ramos & Folguera, 2009; Ramos, 2010), this could imply the presence of a subducting aseismic ridge during that time. As discussed by James & Sacks (1999) and Antonijevic et al. (2015), the subduction of a ridge is necessary for the formation of a flat slab, but not sufficient. Next to ridge subduction, trench retreat and suction are necessary. This could explain why not all porphyry deposits in this period are formed close to the flat-slab segment. Subduction of aseismic ridges triggers the formation of porphyry coppers due to their buoyancy and topography. As a result the subduction of the ridge will be impeded during arrival at the trench. This causes a strongly compressional environment impeding the ascent of magmas through the crust, so that enrichment of these magmas in a MASH zone can take place (Hildreth & Moorbath, 1988). Eventually, ascent of this fertilized magma to shallow magma chambers can take place in several ways. At first, LeFevre & McNally (1985) and Rosenbaum et al. (2005) argued that impingement of aseismic ridges leads to stronger coupling between the subducting and overriding plate. This would lead to crustal deformation. Faults associated with this deformation could provide a pathway for fertile magmas to travel towards the surface and exsolution of fluids at shallow levels could lead to the formation of hydrothermal systems (Rosenbaum et al., 2005). However, the buoyancy of the ridge can also result in downdip extension along the subducted portion of the downgoing plate, as adjacent portions of the plate subduct more easily (LeFevre & McNally, 1985). This would provide pathways for the fertile magmas as well. Furthermore, no porphyry deposits are formed close to the subducting Chile rise spreading ridge. This has several possible reasons. At first, subduction of a spreading ridge leads to a slab window. This gives the asthenospheric melt the possibility to ascent towards the surface and leads to near-trench volcanism, without being enriched in a MASH zone (Gorring et al., 1997; Lagabrielle et al., 2000; D’Orazio et al., 2003; Georgieva et al., 2016). Furthermore, the oceanic crust subducting at spreading ridges is very young, so it does not contain enough porphyry forming minerals yet (Butterworth et al., 2016). At last, the absolute plate motion of the South American plate very similar between areas with and without porphyry deposits. This means that it is not a distinguishing circumstance for the formation of porphyry deposits. However, as discussed by Schepers et al. (2016) it does play a role in the formation of flat slabs, so it is important for the location of porphyry deposits. Further, the average rollback of the subducting Nazca plate has been smaller than the absolute westward motion of the overriding South American plate since 50 Ma (Schepers et al., 2016). This leads to stronger compression in the overriding plate. As discussed before, this is important for the formation of porphyry deposits. Since, next the previously discussed structural requirements for the formation of porphyry deposits, some chemical requirements have to be met as well, I will discuss these shortly. At first, the melt has to have a mafic origin. This is important to supply enough water, sulphur and metals to the magma (Iko et al., 2001). Furthermore, if the melt is sulphur-saturated, the melt has to be oxidized, so that chalcophile and siderophile elements act incompatible and remain in the melt during the enrichment in the MASH zone and the ascent to shallow magma chambers (Richards, 2003, 2011). If the melt is under saturated with sulphur, porphyry deposits can form, if residual sulphides, which are rich in chalcophile and siderophile elements, are dissolved in the melt (Richards, 2011). In the MASH zone, these mafic magmas have to mix with felsic crustal-derived magmas, such that the density eventually becomes low enough to promote buoyant rise towards shallow magma chambers. After shallow emplacement, the magma has to be accessible for reduced, sulphur-rich hydrothermal fluids in order to be efficiently stripped from the chalcophile and siderophile elements. These hydrothermal fluids eventually form the porphyry deposits.

Conclusion Based on the results of this study, it can be concluded that, based on the porphyry deposits in the Andes, a number of tectonic factors have to be present at the same time to have formation of porphyry deposits. I argue that, next to a relatively old subducting oceanic crust (>25 Myr), a slightly oblique convergence angle for the subducting slab (~15°), a relatively fast convergence rate of ~10 cm/yr and a large distance (>2000 km) from the trench edges, the subduction of aseismic ridges plays an important role. This subduction leads to deformation in the overriding plate, such that the ascent of magma is impeded and enrichment of the magma with chalcophile and siderophile elements can take place in MASH zones. As the deformation and the stress-state of the overriding plate are important for the formation of porphyry deposits, further research could focus on the influence of far-field effect on porphyry deposits. To find the porphyry deposits, it is important to take into account the dip of the subducting slab, as this determines at which distance from the trench the deposits are present. At last, the deposits have to be exhumed at the right rate, such that they are exposed at the surface. As for the Inca plateau, the Nazca ridge and the Juan Fernandez ridge many porphyry deposits are formed just before their subduction beneath South America, while in front of the Iquique ridge no deposits are found, the area east of the Iquique ridge may be of interest to conduct research into the exhumation rate of this area, to determine if the absence of porphyry deposits in this area could be the result of a lack of exposure. This could provide more insight in the relation between aseismic ridge subduction and the formation of porphyry deposits. Acknowledgement I thank dr. Douwe van Hinsbergen for his supervision, the useful discussions about the contents of this thesis and the constructive reviews. Dr. Simon Jowitt is thanked for his supervision and his constructive reviews during the writing of this thesis. Fernando Poblete is thanked for creating the rigid blocks, which were used in this thesis to approach a deforming mesh and for writing the code which is used to plot symbols in GPlates. At last, MSc Gerben Schepers is thanked for creating the reconstruction of a deforming Andes.

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Appendix 1 Tutorial Dot2GPML_v1.0.py script

This script by Poblete (2016) is used to plot point features in GPlates. These features are plotted as symbols, which can be assigned by the user. To this end, a couple of steps have to be taken. 1) Open the command prompt and go to the directory in which the script is stored. The input file that contains at least the latitude, longitude, name, age and plate id should be in the same folder. The input file should have latitude and longitude as the first two columns respectively and the plate id as the last column. 2) Type python Dot2GPML_v1.0.py 3) Follow the instructions in the command prompt. - The input file should be a .txt file - The output file should be a .gpml file - For the feature type, a number should be typed. In order to plot the symbols a symbol file (.sym) should be created and loaded into GPlates. These symbols could represent all features that are available in GPlates. In the symbol file, the exact same names of the features should be coupled to a symbol. - For the purpose of this thesis, the identity is irrelevant. This means that this prompt could be skipped by just pressing enter. - Type the numbers of the columns you want to use for the name, separated by a comma. In this case it is important to note that the numbers of the columns start at 0 instead of 1. - Type the number of the column in which the age is written down. Again it is important to note that the columns start at 0. - Determine the type of time interval in which you want to display the symbols. A) the interval is fixed at 10 Myr for each interval. B) The interval can be personalized (years before and after formation that a feature is shown). In this case the next input is to define the limits of the interval. C) A fixed end time can be given. In this case the next input is to define the desired end time. - The last input is the column in which the plate id is specified. As the plate id is the last column in the input file, the input should be -1. 4) Finish the script by pressing ENTER. To show the symbols in GPlates, the symbol file, as well as the created output file, should be loaded in GPlates.

For creating the symbol file, 4 parameters should be given. These are the feature type, symbol type, symbol size and fill state. The feature type should be one of the standard feature types in GPlates. The symbol type should consist of TRIANGLE, CIRCLE, SQUARE or CROSS. The symbol size should be a positive integer and the fill state should be FILLED or UNFILLED.

Appendix 2

Appendix 3