Age and origin of the Cu (Au-Mo-Ag) Salobo 3A ore deposit, Carajás Mineral Province, Amazonian Craton, northern Brazil

Abstract

The Salobo 3A polymetallic Cu (Au-Mo-Ag) deposit, located in the southeastern part of the Amazonian Craton, northern Brazil, is hosted by an Archean metavolcano-sedimentary sequence of Salobo Group. The copper mineralization consists of bornite-chalcocite and bornite-chalcopyrite disseminated with magnetite. In this paper we attempt to date the mineralization episodes and to characterize the fluid sources. The stepwise Pb leaching technique applied to chalcocite, tourmaline, chalcopyrite and magnetite yielded 207Pb-206Pb ages of 2705 ± 42 Ma, 2587 ± 150 Ma, 2427 ± 130 Ma, and 2112 ± 12 Ma, respectively. The older age is interpreted as the timing of primary mineralization. Ages of 2.58 Ga and 2.42 Ga are considered to be related to tectonic reactivation of the Itacaiúnas Belt, and the younger age, obtained on late magnetite crystals, suggests the presence of Paleoproterozoic hydrothermal episode. Sm-Nd and Rb-Sr ages of 2.4 and 2.1 Ga were obtained for schists from the Salobo Group and chloritized gneisses of the Xingú Complex, respectively. The strong radiogenic Pb isotopic compositions of all analyzed minerals suggest a continental environment for the mineralizations. Our geochronological results are in agreement with a primary syngenetic origin for the Salobo mineralization, with late remobilization into brittle-ductile structures during a policyclic evolution of the Carajás-Cinzento strike-slip system. Low-grade metamorphism and anorogenic granitic plutonism affected the area at ca. 2.1 and 1.8 Ga, respectively.