Sm–Nd studies at Mina III gold deposit, Crixás greenstone belt, central Brazil: implications for the depositional age of the upper metasedimentary rocks and associated Au mineralization

Abstract

The Crixás greenstone belt, central Brazil, is a volcanosedimentary sequence comprised of ultramafic and mafic metavolcanic rocks at the base and detrital and chemical metasedimentary rocks at the top. The sequence is part of the Archean sialic block exposed in the central part of the Neoproterozoic Brası́lia belt and therefore is strongly affected by the Brasiliano orogeny. Important gold deposits, such as Mina III, are mainly associated with carbonaceous and Fe- and Al-rich metasedimentary rocks of the upper unit. Sm–Nd isochron data for the basal metakomatiites and metabasalts indicate an age of 3.00±0.07 Ga. However, Nd isotopes for the upper metasedimentary rocks show T DM model ages of mostly 2.33–2.49 Ga, which indicates that neither they nor the associated Au mineralization are Archean. Also, garnet whole-rock dating for a Au-bearing chlorite–garnet schist yields an age of 505±7 Ma, suggesting that the mineralization is Neoproterozoic. The Nd model ages suggest that the metasedimentary sequence of Crixás represents a thrust sheet of Proterozoic rocks emplaced over the Archean terrains during the Neoproterozoic Brasiliano orogeny.