CENOZOIC AND MESOZOIC PORPHYRY COPPER DEPOSITS OF THE CHILEAN AND PERUVIAN ANDES

The Andes are the world's largest Cordilleran-type orogen, and have formed in response to long-term subduction of oceanic lithosphere beneath the South American continent. The Palaeozoic history of the western South American margin was a time of terrane accretion, with the Arequipa and Chilenia terranes added to the continental margin. During the Mesozoic, continental growth was dominated by arc volcanism and sediment accumulation in back arc basins east of the arc. The Cenozoic history was dominated by continued arc volcanism and sedimentation interspersed with the formation of giant porphyry and epithermal ore deposits.

Porphyry copper mineralization in the central Andes represents small disturbances within the protracted history of subduction (>200 million years). There have been at least seven major porphyry mineralising events: Early Cretaceous Cu-Mo, Middle Cretaceous Cu-Mo, Paleocene Cu-Mo, Eocene-Oligocene Cu-Mo-(Au), Middle Miocene Cu-Au and Late Miocene – Pliocene Cu-Mo and Cu-Au-Mo metallogenic belts. The three most richly-endowed porphyry Cu-Mo provinces were the Eocene-Oligocene province of northern Chile, the Late Miocene-Pliocene central Chile province and the Paleocene province of southern Peru and northern Chile. Major porphyry Cu-Au and related high sulfidation epithermal Au deposits formed in Northern Peru and North-Central Chile during the Middle Miocene.

The last 50 m.y. of Andean tectonics has seen periods of normal subduction (subduction angles of ~30°) interspersed with periods of flat subduction and magmatic lulls. The present-day southern margin of the flat slab segment in central Chile corresponds broadly with the position of the three giant porphyry Cu-Mo deposits, and the eastern flexure coincides with the Argentinian porphyry Cu-Au-Mo systems. The giant Miocene porphyry copper-gold and high sulfidation gold deposits of northern Peru overlie the now completely subducted Inca Plateau. Subduction of aseismic ridges, seamount chains and oceanic plateaus appears to have been the tectonic trigger for porphyry ore formation in the Andes and elsewhere around the circum-Pacific in the last 20 m.y. These tectonic perturbations can promote flat slab subduction, crustal thickening, uplift and erosion coincident with the formation of well-endowed porphyry mineral provinces.