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).