The Bajo de la Alumbrera and Agua Rica Cu–Au (Mo) porphyry deposits of Argentina: Genetic constraints on ore formation and sources based on isotope signatures

• The lead isotopic signature of igneous rocks from Agua Rica and Bajo de la Alumbrera suggest similar Pb source for these rocks.
• Sulfides of Bajo de la Alumbera are somewhat more radiogenic when compared with the Miocene igneous rocks, indicating that some crustal Paleoproproterozoic Pb could have been incorporated during the mineralization process.
• Genetic relation between Bajo de la Alumbrera and Agua Rica deposits are suggested, this paper provides data supporting that they are distinguished mainly by the mineralization process and different degree of crustal component in the mineralization.

Important copper-gold (± molybdenum) porphyry deposits occur in the northwest region of Argentina, part of the Central Andes. This paper provides new isotope information on two of these deposits, Bajo de la Alumbrera and Agua Rica, the latter having an epithermal overprint event. The two deposits are genetically associated with the Miocene Farallón Negro Volcanic Complex. Whole rock and sulfide samples were analyzed for Pb, Sr and Nd isotopes to characterize the sources of magma and mineralization of both deposits. Sr and εNd data made it possible to divide the samples into three distinct groups. Most samples are slightly differentiated, have OIB (Ocean Island Basalts) isotope characteristics, and show some Nd fractionation. Lead isotope also shows distinct groups with an enrichment trend in radiogenic lead. In both deposits, the sulfides are more radiogenic than the host rocks, but are clearly related to them because all plot on a single trend. A crustal contribution to the deposits is suggested on the basis of lead isotope signatures. The similarity on the Pb isotope signatures suggests the same magmatic fluid source, although at Agua Rica the crustal component is more evident than at Bajo de la Alumbrera, possibly because of the assimilation of Paleozoic country rocks at Agua Rica.

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