Origin and metamorphism of graphite from Formiga, Minas Gerais (Brazil)

During the Paleoproterozoic Era, the Brazilian cratons experienced orogenic events that modified the archean basement and sedimentary successions. In the southern São Francisco Craton, it can be recognized evidence of an orogenic event that happened between Rhyacian and Orosirian periods. It is related to the closure of an oceanic basin at this time, which led to the collision between the Archean Divinópolis and Campo Belo metamorphic complexes. Graphite schist occurs close to the cities of Formiga and Itapecerica (Minas Gerais), located between these complexes. To contribute to the understanding of the origin and metamorphism of the graphite from Formiga, petrographic studies, X-ray diffraction (XRD) and Raman spectroscopy analyses have been done. XRD and Raman methods revealed that the temperatures record-ed by graphite are around 460°C. However, Raman data showed that the crystallite sizes correspond to higher metamorphic grade conditions (amphibolite to granulite facies). Temperatures of 460°C are probably associated with hydrothermal processes along faults in post-collisional stage. The presence of todorokite, a mineral typical of deep-sea Mn nodules formed by microorganisms, in association with graphite from Formiga, suggests a biogenic origin for the graphite occurrence.