Caracterização mineralógica de grafita e gamaespectrometria dos metassedimentos de alto grau metamórfico no sul do estado do Rio de Janeiro: implicações sobre a origem e cristalinidade da grafita

Abstract

In the southern portion of the state of Rio de Janeiro (SE Brazil), flake-type graphite occurrences disseminated in metapelites associated with the Neoproterozoic supracrustal rocks of the Andrelândia Megasequence, linked to the Occidental Terrane of the Ribeira Belt, were analyzed. Graphite is a mineral of significant economic interest, with numerous industrial and technological applications. It is composed of carbon monolayers and is widely used in the refractory, steel, battery, and nanotechnology industries for graphene production. The world's largest graphite producers, India and China, are associated with geological terrains known as khondalitic. These terrains are essentially composed of aluminous rocks, with occurrences of quartzites, gondites, calcsilicate rocks, and amphibolites, metamorphosed under granulite facies conditions. Such lithotypes are recognized in the state of Rio de Janeiro and are named the Marangatu, Palma-Guaçuí, and São Fidélis Khondalitic Belts. These belts contain several occurrences of graphite, Mn, Au, and Zn/Ag. Through the analysis of airborne gamma-ray spectrometry maps and geological-geophysical profiles, it was possible to correlate a signature for the graphite-hosting rock, best defined by the thorium and uranium radioelements. Graphite occurs disseminated within the compositional banding of leucosomes and melanosomes in highly altered sillimanite-garnet-biotite gneiss, exhibiting tabular blade-like habits, tabular with frayed ends, and ragged forms. The crystals were classified according to the granulometric classification for commercial purposes, ranging from <0.075 mm to approximately 2 mm. Graphite concentration via flotation showed low recovery rates. Similarly, X-ray diffraction (XRD) characterization was inefficient due to the high content of impurities floated along with the graphite. Scanning electron microscopy (SEM) and optical microscopy allowed for detailed observation of the mineral phases occurring between the lamellae (cleavages) of the mineral. Raman spectroscopy analysis of the graphite samples enabled the estimation of the mineral's formation temperature, which ranged from 716.3°C to 804.3°C. A high degree of crystallinity was also observed based on the behavior of the band peaks. Stable carbon isotopes showed a δ13C signature between -20.7‰ and -17.3‰, indicating a biological origin, possibly with some contribution from inorganic graphite of uncertain provenance. Khondalitic rocks are metallogenetically favorable for the formation of crystalline flake-type graphite. Studies of gneisses related to these belts may contribute to expanding mineral exploration in high-grade metamorphic terrains in the state of Rio de Janeiro, highlighting a new focus and potential for these areas, considering the economic and strategic relevance of this material to the industry.