Abstract

Twenty-five Pb isotope determinations are presented on galena, chalcopyrite, pyrite, chalcocite, and cerussite from 15 mineral deposits in the copper, iron, and polymetallic provinces of central and southern Peru. The results are evaluated in comparison with the Pb isotope signatures of potential source materials which include plutonic rocks of the Coastal batholith, Precambrian basement rocks of the Arequipa massif, as well as midocean ridge basalts (MORB) of the Nazca plate, and Pacific Ocean pelagic-metalliferous sediments subducted at the Peru-Chile trench.Several types of mineral deposits are recognized in the copper province which roughly coincides areally with exposures of the Early Jurassic to Paleogene Coastal batholith. Amphibolitic Cu-Fe skarns with the magnetite-pyrite-chalcopyrite mineral association define an array with a Pb isochron age of 3.2 Ga in the 207 Pb/ 204 Pb versus 206 Pb/ 204 Pb covariation diagram. Mineral deposits of the copper veins in granitoids association at the Cata-Canete and Cobre-Acari districts plot along the same array. The isochron age of 3.2 Ga is too old to have chronological significance. The array of points is therefore interpreted as the product of mixing between two end members. Enriched subcontinental mantle (produced by mixing in the subduction zone of pelagic and/or metalliferous sediments and material from a MORB-like source) and supracrustal sedimentary materials are the most likely end members.The Pb isotope compositions of disseminated porphyry Cu-Mo deposits are virtually identical to those of the batholithic rocks with which they are genetically linked. Thus the El Molino prospect has high Pb ratios, typical of plutons near the Lima-Arequipa segment boundary in the Coastal batholith (Mukasa, 1986a). The Pb content of the plutons is chiefly from mixtures of a depleted, MORB-like component and subducted pelagic-metalliferous sediments. The Cerro Verde deposit, like its host rocks, has low 206 Pb/ 204 Pb values most easily explained by mixing of materials derived from the Precambrian basement with those from the subduction-generated enriched subcontinental mantle keel.Deposits of the auriferous vein association and the unclassified Unturos-Chacaya galena and pyrite deposit show a wide Pb isotope scatter in the Pacific Ocean sediment field, as well as both above and below it. Similarly, galena and chalcopyrite from the polymetallic province show major Pb isotope heterogeneities.Extensive metal scavenging from local supracrustal materials occurred demonstrably. Accordingly, metal sulfides along Andean-type plate margins do not have a unique source.

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