Geologic and metallogenetic patterns in the Archean and Early Proterozoic of Bahia State, Eastern Brazil

Abstract

The territory of Bahia State is part of the Sao Francisco structural province. The basement of the province is characterized by granite-greenstone and high grade metamorphic terrains developed during the Archean and Early Proterozoic. The Early Archean is represented by small tonalitic-granodioritic and enderbitic-charuockitic nuclei 3,500 Ma old. Several portions of gneissic-migmatitic and granulitie terrains have ages of 3,000 Ma and Rb/Sr isochronic values of 2,700 Ma that represent the principal phase of the Jequie geotectonic cycle. During the Early Proterozoic, extensive areas of the basement of the Sao Francisco province underwent intensive remobilization, through granitization and K-metasomatism related to the Transamazonico geotectonic cycle (2,200-1,800 Ma). Besides greenstone belts the granite-greenstcne terrains contain volcano-sedimentary (greenstone belt like) and platform sequences developed since the Archean to the Early Proterozoic. The high grade metamorphic terrains contain remnants of supracrustal sequences and basic-ultrabasic complexes. The metallogenic Framework of Bahia shows that more important mineral deposits such as Au, Cr , Cu, Fe-Ti-V, Pb-Zn-Mn, magnesite and tale are related to Archean and Early Proterozoic evolution. Gold deposits are located in volcanic rocks of greenstone belt environments, and in conglomerates of Witwatersrand type. Economic deposits of Cr, Cu, Fe-Ti-V are associated with mafic-ultramafic complexes in high grade metamorphic terrains. Pb-Zn and Mn mineralizations occur in volcano-sedimentary (greenstone belt like) sequences and the platform sequences contain large deposits of magnesite and talc.