SUBDUCTION ZONES AND PORPHYRY COPPER DEPOSITS

The association of porphyry copper deposits with subduction zones has been recognized since the earliest days of the plate tectonics revolution. However, the observation that most deposits are either Cu with Mo or Cu with Au is not explained by the standard subduction generates a volcanic arc model. The Cu+Mo deposits found in the Andes or southwest North America are associated with long-term subduction and arc volcanism whereas, the Cu+Au deposits found in New Guinea, the Philippines, and British Columbia are associated with episodes of collisional tectonism (subduction zone jamming and slab breakoff). The dichotomy in secondary metal content can be explained in terms of the primary source of the magmas. In subduction zones, steady magma generation is primarily due to water fluxing of the asthenosphere, with a component of melt sourced from subducted sediment with high Mo/Au ratio. In collisional setting, most magma generation is due to adiabatic decompression melting of asthenosphere that upwells into the gap that forms as the subducted slab breaks off. In this situation, a component of melt also comes from the lithospheric mantle, which commonly has a high Au/Mo ratio due to prior fertilization by small degree partial melts and metasomatizing fluids.

In both subduction settings, ore formation is localized in both time and space. Factors such as magma flux to the crust, chamber shape, wallrock thermal gradients, fluid exsolution depth, and coeval tectonism are important. Optimal conditions appear to be where stock geometry and magma flux result in steady heat-loss-driven bubbling at depths of 6 km or more. Giant deposits can form where stocks emanate from batholithic chambers. Strike-slip tectonic movements, as active today in the Cu+ Au province of the Philippines, may be the most critical factor as the right rate of extension fracturing of the hot rock above a cupola can enable the throttled discharge of Cu-rich fluid that can form orebodies.