Abstract

Monterrosas, Eliana, Ra[uacute]l, and Condestable are the most important copper mines in the central coast of Peru, between lat 12 degrees 30' and 14 degrees 30' S. Their related geologic setting, close age relation, and similar mineralogy and geochemistry argue in favor of their grouping together as amphibolitic Cu-Fe skarn deposits. In the Lower Cretaceous continental margin, 100 to 120 Ma, incipient rifting accompanied by batholithic gabbrodiorite intrusions at depth gave rise to an elongated trough in which submarine volcanic and sedimentary rocks accumulated. Skarn deposits developed in association with the intrusive activity are characterized by chalcopyrite concentrations with low Ti magnetite and/or pyrite in actinolite gangue; minor constituents are amphiboles of hastingsitic and tschermakitic varieties, apatite, pyrrhotite, and sodic scapolite. Copper ores are fracture controlled and hosted by Albian gabbrodiorite plutons at Monterrosas and Eliana. Volcanogenic strata-bound mantos, as well as disseminated sulfide stringers and minor veins, occur at Ra[uacute]l and Condestable in Hauterivian to Albian volcano-sedimentary rocks. Geochronologic K-Ar data identify a Lower Cretaceous episode of metallogenesis for Ra[uacute]l, Condestable, and Eliana. Geologically related iron deposits of Hierro Acari and Marcona are dated as Lower Cretaceous and Upper Jurassic, respectively.Petrography and major element chemistry of the gabbrodiorite suite at Monterrosas and Eliana indicate a low K tholeiitic character with calc-alkaline affinities and high Na 2 O/K 2 O ratios. Late magmatic and hydrothermal activity developed in the roof zones of the plutons and formed amphibole-magnetite-chalcopyrite deposits. Mineralization at Eliana involved introduction of Cu, Fe, and P along fractures and contacts to form ore shoots. The scapolite alteration halo is characterized by Si, Na, and Ca enrichment. The hydrothermal deposits of Ra[uacute]l formed mainly at subvolcanic levels with a dominant seawater component (Ripley and Ohmoto, 1977, 1979). Geologic correlation with Eliana and Monterrosas indicates that both magmatic and seawater components coincided to form amphibolitic Cu-Fe skarn deposits, which span the plutonic and volcanic environments.Lead isotope compositions of chalcopyrite, pyrite, and galena define a primitive trend for these skarn deposits compared to that of porphyry Cu deposits in southern Peru and Chile and Sn deposits in Bolivia. The isotopic contrast is interpreted as a result of rifting and extreme crustal thinning during the Lower Cretaceous along the continental margin of central and southern Peru. This tectonic and metallogenetic environment continued into northern and central Chile where amphibolitic Fe and Cu deposits of the same age range are known.

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