Caracterização isotópica e cristalográfica das grafitas da Faixa Orós, nordeste do Brasil

Abstract

In recent decades, natural graphite from organic carbon has become increasingly important from an academic and technological point of view, due to its numerous applications in these areas. As a result, several techniques have been refined to understand the characteristics of these materials. Organic graphites are known as the result of the transformation of organic matter subjected to regional or contact metamorphism. Therefore, it is recognized that carbonaceous materials are sensitive to temperature and that their structural ordering is the result of a progressive increase in metamorphism from greenschist to granulite facies. Because these materials occur naturally in restricted places, that is, in environments with sedimentation enriched in organic debris and/or carbonaceous sediments, they have been widely used as geosuture markers, basin paleoenvironment and, due to their irreversibility in the face of low temperatures, also are used as a natural geothermometer. Based on stable carbon isotope data, the source and content of carbon contained in graphite deposits located in the regions of Solonópole and Orós, both located along the Orós Belt, Borborema Province, were determined. Through Raman spectroscopy analysis, the crystallinity of the graphites was determined and from this the formation temperatures of the graphites and consequently of the host rocks were calculated. The isotopic results obtained indicated that the carbon source of the Orós Belt graphites originated from organic matter from terrestrial aquatic environments. These data associated with the lithotypes that occur along this belt corroborate the understanding that it is a terrestrial aquatic paleoenvironment of low energy or very calm, where sedimentary deposition in the deepest regions occurred extremely slowly, forming strata or layers of silt and/or clays enriched in organic matter. These sedimentary strata are currently represented by thick pelitic packages flanked by quartzite and marble, which extend for long kilometers along the Orós Belt, hosting extensive graphite lenses. The graphite formation temperatures were calculated from natural geothermometers and ranged from 520.44°C and 519.27°C for Solonópole and Orós, respectively. These temperatures not only represent the temperature at which graphites are formed, but also the lithotypes that host them, as well as regional rocks. The mineral assemblage associated with the temperatures calculated from the Raman spectroscopy data represents an amphibolite facies metamorphism, which has also been presented in many previous studies. Both the graphites from the Solonópole and Orós deposits, despite both being microcrystalline, present a high degree of crystallinity and small deformations/defects in their crystallographic structure.