Porphyry-type deposits and plate tectonics: The link between aseismic ridge subduction and metallogenesis

Category Tectonic & Seismotectonic
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Rosenbaum, Gideon
Holding Date 11 October 2008

The formation of porphyry-style ore deposits is controlled by magmatic–hydrothermal processes within the subduction arc. However, such deposits are not evenly distributed along strike of the arc, nor through the temporal history of an arc. Rather, they are geographically clustered and formed during relatively short periods of time. This suggests that the mechanism responsible for the occurrence of porphyry-related ore deposits is possibly controlled by internal interactions within the subduction system and their associated tectonic and magmatic processes.
Plate tectonic reconstructions reveal that a large number of Miocene ore deposits in the Peruvian Andes are spatially and temporally linked to the subduction of the Nazca and Inca ridges beneath the South American plate. The reconstruction shows a rapid metallogenic response to the arrival of an anomaly at the subduction trench, indicated by clusters of ore deposits situated within the proximity of the zone of the subducting Nazca and Inca ridges. It seems that the impingement of these aseismic ridges at ~15 Ma had an immediate response in the formation of ore deposits in northern Peru. Furthermore, subsequent southward migration of the zone of metallogenic activity follows the lateral migration of the zone of ridge subduction. This zone of mineralisation coincides with a migrating gap in active volcanism (but not in plutonism).
On the scale of the entire Andean belt, it is recognised that almost all of the largest, young (<18 Ma) Cu and Au deposits are located in areas lacking active volcanism and where the dip of subduction is relatively flat. Furthermore, giant ore deposits in central Chile coincide with the timing and location of ridge subduction associated with the arrival of the Juan Fernandez hotspot chain at the subduction zone. Kinematic reconstruction shows that the southward younging of metallogenic activity in this region roughly follows the migration of aseismic ridge subduction. The Iquique Ridge in northern Chile does not correspond with occurrences of young (<18 Ma) giant ore deposits. This is explained by plate reconstruction, showing that the Iquique ridge has started to be subducted very recently (<2Ma). Therefore, any deposits that might have been produced are unlikely to be exposed at the surface.
It is concluded that there is a spatial and temporal link between ridge subduction and ore deposit formation, with high metal fluxes concentrated in zones of ridge subduction. The plate kinematic approach has the potential to be applied to other areas of ridge subduction, in which there is mature datasets on ore deposits, and may be expanded to include palaeo-subduction systems. Thus, the recognition of tectonic patterns associated with ridge subduction can provide insights into predictive mineral discovery by enhancing our ability to predict the position of large mineral deposits in other modern and ancient orogens.