Geology of the Pitangui greenstone belt, Minas Gerais, Brazil: Stratigraphy, geochronology and BIF geochemistry

Based on drillcore description of rocks at the São Sebastião gold deposit and surroundings a new geological map and a detailed stratigraphic column are presented for the Pitangui greenstone belt, which is located northwest of the Quadrilátero Ferrífero, southern São Francisco craton, Brazil. This belt comprises three units: a Lower Unit predominantly of metavolcanic ultramafic and mafic rocks interbedded with thinner metasedimentary intervals of banded iron formations (BIFs), turbiditic metasandstones and metapelites; a Middle Unit of turbiditic metasediments interbedded with metavolcanic intermediary rocks, minor BIFs and quartzites; and an Upper Unit composed of quartzites and polymictic metaconglomerates. Rocks of all three units are metamorphosed under upper greenschist to lower amphibolite facies conditions and are bounded by domes (TTGs and late- to post-tectonic alkaline granitoids). The Pitangui greenstone belt records the geologic evolution of an ancient oceanic basin that evolved from a thick succession of metavolcanic ultramafic and mafic rocks interbedded with metasedimentary intervals of BIFs and turbidites; the latter representing moments of quiescence in volcanic activity. This basin experienced a transient period dominated by andesitic volcanism, turbidite deposition and minor BIF, and a final stage of clastic-only sedimentation at the same time when volcanic activity ceased. Combined U-Pb and Lu-Hf isotope data of detrital zircon from Lower Unit metasandstone revealed a maximum depositional age of 2859 ± 11 Ma. This age is similar to those found in the basal unit of the Nova Lima Group (2.90-2.78 Ga) in the Rio das Velhas greenstone belt, located approximately 100 km southeast. Both belts can be correlated on the basis of similarities in stratigraphy and isotopic ages, indicating that they possibly represent parts of an ancient larger domain. Geochemical data of BIF coupled with stratigraphic analysis place constraints on paleoenvironmental conditions during the development of the Mesoarchean Pitangui greenstone belt. Shale-normalized rare-earth elements (REE + Y) in some BIF samples display negative cerium (Ce) anomalies, which may be interpreted as evidence for the oldest oxygenation pulse documented in Brazil to date (2.86-2.78 Ga). Such anomalies endorse the presence of episodic periods of seawater oxygenation before the Great Oxidation Event.