Distribution of porphyry copper deposits along the western Tethyan and Andean subduction zones: Insights from a paleotectonic approach
Introduction
Assessing the most favorable areas for mineral prospecting has always been a major concern for exploration geologists. The spatial approach of mineral resources predictivity focuses on the geological context of ore deposits and on distinct parameters that control their distribution, from district to continental scales, defined from geology, tectonic structures, geophysics and geochemistry (e.g.Carranza, 2011, Cassard et al., 2008) but also geodynamics and paleogeography (Scotese et al., 2001). It is an upstream phase of prospection campaigns, the goal of which is to guide exploration strategy by predicting a priori the most favorable areas.
Porphyry Cu deposits were studied and described by many authors (see the reviews by e.g.Seedorff et al., 2005, and Sillitoe, 2010). They are closely linked to their geodynamic surroundings and are most often associated with calc-alkaline and adakitic magmatism in subduction zones (e.g.Burnham, 1979, Cline and Bodnar, 1991, Thiéblemont et al., 1997). These deposits result from a dual melting process with: 1) an initial melting in the metasomatized mantle wedge, above the subducting oceanic slab, which generates relatively oxidized and sulfur-rich mafic magmas with incompatible chalcophile or siderophile elements (such as Cu or Au), and 2) a secondary melting by injection of dykes and sills in the MASH (Melting, Assimilation, Storage, Homogeneization) zone of the lower crust, yielding a crustal- and mantle-derived hybrid magma, with a high content of volatile and metalliferous elements, and a density that is low enough to allow its upward migration through the crust (Richards, 2003, Richards, 2011). They are generally associated with plutonic apexes of granitic bodies (e.g.Burnham, 1979, Cloos, 2001, Guillou-Frottier and Burov, 2003, Shinohara and Hedenquist, 1997) emplaced in the upper crust of the overriding plate (usually 1–4 km depth). Ore grades are often low, but volumes can be huge, which can possibly make them very large deposits (e.g. El Teniente, Chuquicamata or Rio Blanco-Los Bronces, all in Chile, with 78.6, 65.2 and 52.4 Mt of copper respectively; Jébrak and Marcoux, 2008). In addition, porphyry Cu deposits can yield valuable new-technology metals, such as rhenium which is used in strong high-temperature resistant alloys and often produced as by-product of molybdenum (e.g.Berzina et al., 2005, Melfos et al., 2001).
For more than 40 years, authors have demonstrated relationships between tectonics and mineralizing processes (e.g.Sillitoe, 1972, and compilation by Wright, 1977). The new paradigm of plate tectonics, along with numerous metallogenic studies, allowed proposals of new genetic models linking the lithosphere and mantle dynamics to the occurrence of deposits (e.g.Barley et al., 1998, Bierlein et al., 2006, Kerrich et al., 2005, Mitchell and Garson, 1981, Sawkins, 1984, Tosdal and Richards, 2001). Although the close relationship between porphyry Cu deposits and subduction zones is well established, there is, however, no consensus on which subduction parameters primarily control the genesis of porphyry deposits. This is not surprising since, following decades of seismic tomography and modeling studies, distinct modes of lithosphere deformation have been suggested and the number of physical parameters controlling the subduction process has continuously increased (slab density, mantle viscosity, slab to mantle viscosity ratio, etc.). The way the subducted lithosphere behaves beneath the overriding plate appears to depend not only on these physical properties but also on plate features at the surface (plate velocity, slab dip angle, amount of retrograde motion, varying ages along trench, etc.). Deep subducting lithosphere behavior is also controlled by plate motion and plate layout at the surface (Yamato et al., 2009). One objective of this study, rather than promoting a single parameter as key to ore formation, is to investigate what control a single selected process, subduction dynamics, has on formation of porphyry Cu deposits.
In the Tethys belt it is widely accepted that the genesis of many types of mineralization is closely linked to the geodynamic context (e.g.De Boorder et al., 1998, Lescuyer and Lips, 2004, Lips, 2007). Neubauer et al. (2005) and Loiselet et al. (2010a) have shown the strong impact of the geometry and dynamics of the eastern Mediterranean subduction on the distribution of porphyry and epithermal deposits in the Carpathian and Aegean regions. Similarly, in the Andes numerous studies have suggested specific relationships between subduction parameters and the occurrence of porphyry Cu deposits: conditions of flat-slab subduction (Billa et al., 2004, Kay and Mpodozis, 2001), stress relaxation and transtensional structures (Richards et al., 2001). In particular, the convergence configuration between the subducting and the overriding plates (velocities and obliquity) would dictate how mineralized bodies emplace in the shallow crust (Tosdal and Richards, 2001). Rosenbaum et al. (2005) have suggested that subduction of topographic anomalies (ridges and plateaus) triggered the formation of ore deposits. According to Cooke et al. (2005), topographic and thermal anomalies on the subducting slab could trigger the formation of giant porphyry deposits. All these studies clearly show that past subduction history and, in particular, the convergence parameters have to be accounted for when genesis of porphyry Cu deposits is studied.
To identify relationships between mineralization and geodynamic processes, it is, thus, necessary to place the mineralization within the geodynamic framework that prevailed at the time of its genesis. It is a necessary step to better understand the relationships between the mineralization itself and its environment (plate boundaries, tectonic structures, stress and strain regimes, geology, etc.). This would, in turn, help identify criteria that are favorable to its genesis. The present study aims at better understanding of the geodynamic parameters, in terms of plate kinematics and slab dynamics, that could favor the genesis of porphyry Cu deposits in subduction contexts. For this, we have focused our analysis on two mineralized subduction zones: the western Tethyan suture and the Andean subduction zone. We have adopted a paleotectonic approach, which has been little used so far in the field of metallogeny, to study past geodynamic contexts and plate kinematic patterns. This approach is coupled with results from laboratory experiments to assess the 3D slab dynamics and its possible relationships with plate kinematics and deposit genesis.
Section snippets
Dynamics and deformation of the subducting lithosphere
Dynamics of subduction zones is governed by the balance between driving forces (i.e. slab pull, ridge push), resisting forces (i.e. viscous shear and viscous resistance in the mantle) and other external forces due to the large-scale mantle flow or to density contrasts created by phase transitions in the mantle (e.g.Billen, 2008, Heuret and Lallemand, 2005, Husson, 2012). Relative magnitude of these forces determines surface plate kinematics, including the possibility of trench retreat or
The western Tethyan subduction zone
The present Tethyan suture was built through accretion of micro continents and arcs during convergence between the Africa, India and Eurasia plates, which progressively closed the Tethyan Ocean. This accretionary system extends over 5000 km between the collisional fronts of Apulia, to the west, and the Himalayan collision to the east. Numerous studies propose tectonic reconstructions that describe the Mesozoic–Cenozoic evolution of the Tethyan region, such as geodynamic models from Dercourt et
The Andean subduction zone
The Andean margin results from the eastward subduction of the Nazca plate beneath South America, at convergence rates that amount to several cm/yr but are not constant through time (Pardo-Casas and Molnar, 1987). According to seismic tomography signatures (e.g.Engdahl et al., 1995, Liu et al., 2003), subduction history and geometry of the Andean subduction zone seem much simpler than those of the Tethyan subduction zone. The varying subduction angle (from flat subduction zones in central Peru
Discussion
The present study shows that four Cretaceous or younger clusters of porphyry Cu deposits along the western Tethyan and Andean margins were emplaced in relatively similar kinematic contexts. To explain the observations presented above, we propose a simple geodynamic model, based on the impact of the plate convergence rate on the melting processes and stress regimes that would favor the formation of porphyry Cu deposits. This model is thus composed of two phases:
- 1)
a high rate of convergence, which
Conclusion
Despite their different geodynamic regions and subduction context, we evidenced four clusters of porphyry Cu deposits – two of upper Cretaceous–Paleocene and Oligo-Miocene age in the Aegean–Balkan–Carpathian region (closure of the western Tethys), and two of Eocene-lower Oligocene and Miocene age along the Andes (subduction of the Nazca plate) – that were emplaced in relatively similar kinematic contexts. These contexts are characterized by: 1) a relatively fast convergence rate that could have
Acknowledgements
This study was done within the framework of the scientific research activity at BRGM and was fully funded by its Research Division. We wish to thank our colleagues Laurent Bailly, Daniel Cassard, Laurent Jolivet and Armel Menant, whose discussions and remarks helped improve this work. We also wish to warmly thank Jeremy P. Richards, whose constructive review and comments also greatly helped improve this work. We thank our colleague John Douglas for improving the English in our manuscript (we
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