COPPER METALLOGENESIS IN CHILE FROM RE-OS ISOTOPES

Copper mineralization is commonly associated with subduction processes. In Chile, and Peru, for example, large porphyry base metal deposits of the main Cordillera were formed during 65-51Ma, 41-31Ma, and as recently as 16-4Ma. Some of these porphyry copper deposits are anomalously large. In addition to the porphyry copper deposits, other copper deposits known as mantos also occur in the magmatic arc. A fundamental difference between mantos and porphyry copper deposits in Chile is the relative importance of intrusions spatially associated with the mineralization. The porphyry deposits are set within Mesozoic to Tertiary intrusive complexes where large hydrothermal systems with prolonged interaction between fluids in the intrusive magmas and country rocks concentrated metals well above average crustal values. The Chilean manto-type deposits are restricted to volcanic and volcano-sedimentary sequences of Middle-Late Jurassic, Early Cretaceous, Late Cretaceous and Early Tertiary times. Large intrusive centers are not spatially associated with these deposits, although minor intrusions of dioritic composition are common. A central question in the genesis of the copper deposits of the Andes is the source of ore-forming components, in particular the ore-metals. The relative importance of associated magmas, country rocks and underlying basement have been discussed by many but the overall constraints on the question are few. Our studies show a difference between the Os isotopic composition of the porphyry and manto-type deposits and all the values are more radiogenic than expected for mantle-derived Os. These radiogenic values require that some of the Os come from a crustal reservoir with a high Re/Os ratio, such as black shales. The the Os isotopic data indicate that the source for these two types of base metal deposits is different but that both Os reservoirs reside in the crust. Furhermore, the data also show that a strong inverse correlation between the size of a porphyry copper deposit and the source of Os, with the largest deposits having the least radiogenic Os. Finally, combined high resolution dating of porphyry copper deposits, such as El Teniente, show that the large deposit was formed by distinct and discrete hydrothermal systems superimposed in the same district.