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

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.