PALEOSUBDUCTION ZONES AND FORMATION OF WORLD CLASS PORPHYRY COPPER DEPOSITS

Many world–class porphyry Cu+/-Mo+/-Au deposits are associated with major crustal structures that are deep seated faults with access to mantle derived magmas and crustal fluids, particularly when they are terrane boundaries (sutures). Furthermore many of these structures are paleo-subduction zones. Southwest North America hosts one of the largest clusters of porphyry copper deposits in the world. The deposits are aligned along northeast trends. One of the most productive of these trends is the Jemez structure which is a Proterozoic suture that has been repeatedly reactivated. Recent seismic refraction data has imaged a paleo-subduction zone along the structure. This structure is coincident with several porphyry copper deposits including the world-class Resolution Cu-Mo deposit.The late Eocene (38Ma) Bingham porphyry Cu-Mo-Au deposit is one of the richest porphyry copper deposits in the world and is located along the Uinta-Cortez axis. The Uinta-Cortez axis is a long-lived geologic structure which was originally formed as a Proterozoic suture. Seismic refraction data across eastern extensions of the structure have also imaged a paleo-subduction zone. Mafic dikes (minettes) associated with the Bingham deposit are evidence that mantle derived magmas have been tapped by this structure. Associated geochemical, isotopic and petrologic data show evidence of mantle metasomatism. Several other world-class porphyry deposits such as Butte, Pebble and Grasberg occur along major terrane boundaries. This supports a model where the suture acts as a conduit for mantle derived arc magmas, and fluids generated from hydrated, metasomatized crust, and facilitates their interaction to form a world-class porphyry Cu+/-Mo+/-Au deposit.