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

Luiza Carneiro De Rezende,Sérgio Luis Lima De Moraes Ramos,Alexandre De Oliveira Chaves

doi:10.1590/2317-4889202120200083

Luiza Carneiro De Rezende, Sérgio Luis Lima De Moraes Ramos + Show 1 more

Open Access

https://doi.org/10.1590/2317-4889202120200083

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Abstract

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 recorded 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.

Highlights

Graphite and diamond are the polymorph occurrence of native carbon on the nature (Harlow 1998)
Graphite can be formed through maturation and metamorphism of biogenic carbonaceous material (CM); as precipitation from C-O-H fluids; mantle-derived; and through reduction of carbonates (Simandl et al 2015)
The graphite formed by biogenic CM undergoes a progressive and irreversible process called graphitization (Buseck and Beyssac 2014)

Summary

Introduction

Graphite and diamond are the polymorph occurrence of native carbon on the nature (Harlow 1998). Graphite can be formed through maturation and metamorphism of biogenic carbonaceous material (CM); as precipitation from C-O-H fluids; mantle-derived; and through reduction of carbonates (Simandl et al 2015). The graphite formed by biogenic CM undergoes a progressive and irreversible process called graphitization (Buseck and Beyssac 2014). It occurs in temperature and pressure of burial metamorphism, and decreases the hydrogen-to-carbon (H/C) and oxygen-to-carbon (O/C) ratios, transforming disordered and non-crystalline CM in crystalline graphite (Kwiecińska and Petersen 2004) (Fig. 1). Kerogen maturation in terms of H/C and O/C atomic ratios

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