Geoquímica isotópica (δ13c, δ18o e 87sr/86sr) e elementar das rochas carbon·ticas associadas a fosforitos na regi„o de Fortaleza de Minas, Minas Gerais - Brasil

Abstract

The phosphoritic-carbonate succession in the Fortaleza de Minas region, in southwestern Minas Gerais, was significantly affected by the Brasiliano orogeny and present great tectonic complexity resulting from the action of the Passos Nappe in the context of the southern Brasília Orogen. This tectonic, characterized by low-angle deformation, resulted in phenomena such as the juxtaposition and overlapping of lithological units and stratigraphic inversion, which complicates the precise establishment of the chronostratigraphic positioning of these rocks. In this context, the region hosts the important carbonate-phosphate deposit of Pratápolis. This deposit, associated with carbonate rocks and underlain by the Campos Gerais Domain (Archean-Paleoproterozoic), has estimated reserves of 5×10⁷ tons with an average grade of ~10% P₂O₅, producing about 400,000 tons/year of fertilizer. Its characteristics suggest a possible correlation with other carbonate-phosphate deposits on the edge of the Brasília Orogen, such as those of Arraias-Campos Belos (TO-GO) and Irecê (BA), both of which can be correlated to the Bambuí Group. In this study, investigations were carried out to characterize three carbonate sections located approximately 5 km southwest of the MV mining area. The analytical approach included: (1) petrography; (2) whole-rock geochemistry (major, trace, and rare earth elements) using lithium metaborate fusion and a four-acid attack; (3) selective leaching of carbonate phases with 0.5N HNO₃; (4) total organic carbon analysis; and (5) isotopic (δ¹³C, δ¹⁸O) and radiogenic (⁸⁷Sr/⁸⁶Sr) analyses. The results reveal that the diagenetic evolution promoted intense dolomitization and recrystallization, with evidence of hydrothermal interaction that likely caused phosphorus remobilization. These processes are corroborated by: (i) depletion of heavy rare earth elements (HREE) and Sr; (ii) negative δ¹⁸O values (-9.26‰); (iii) contrasts in the rare earth and yttrium (REY+Y) patterns between whole-rock samples and leached material. While the whole-rock analyses show a significant detrital contribution (with attenuation of Y anomalies), the leached samples exhibit Y/Ho ratios (34-46) compatible with seawater and estuarine contribution. The isotopic data from the Central quarry show a trend of δ¹³C from slightly negative values at the base (-0.94‰) to positive at the top (+4.37‰), a pattern similar to that observed in the Sete Lagoas Formation. The ⁸⁷Sr/⁸⁶Sr ratios (~0.7076) in samples with higher Sr content (>900 ppm) reinforce the possible association with Neoproterozoic postglacial deposits, such as those of the Sete Lagoas Formation and its correlative units.