Caracterização do processo de enriquecimento dos fosforitos da jazida de Campos Belos, base da formação Sete Lagoas, grupo Bambuí

Abstract

The Campos Belos’s phosporite is hosted in the Sete Lagoas Formation northwest portion of the São Francisco Craton. It consists of a siliciclastic and carbonate succession with synsedimentary intraclastic breccias and tectonic breccias that were deposited in a peritidal environment. Deposition in the Sete Lagoas Formation is interpreted as a post-glacial transgression which flooded the São Francisco Craton during the Neoproterozoic. Phosphorite is a marine sedimentary rock which, by definition, contains a minimum of 18% P2O5. Despite the recognized sedimentary origin, later remobilization of phosphate linked with metamorphic/tectonic events that drive hydrothermal fluids formed the economic phosphorite. In the study area phosphorite is found mainly in synsedimentary and tectonic breccias with up to 24% P2O5. These breccias occur in all lithofacies, and represent high-energy events such as hurricanes that impacted the platform. These events reworked the previously deposited sediment, and also help to concentrate francolite grains. Hydroxyapatite is the ore mineral and represents the alteration of the original sedimentary francolite. It occurs as an irregular mass under SEM, and can make up as much as 70% of the rock matrix. It is suggested that this concentration is a result of remobilization of P from primary facies by hydrothermal fluids and precipitation in the pores of the rock. The paragenetic association with quartz is very common. The relationship between hydroxyapatite, quartz, clay minerals, and vugs suggests that there was a destabilization of clay minerals followed by a hydroxyapatite and quartz precipitation event. Although there are some features that suggest quartz is replacing hydroxyapatite, it is not possible to define exactly the timing of this relationship. Wavelite represents a late hydrothermal event in the HPO4-3 unsaturated facies. It occurs as fracture filling cements and in vugs with dickite along the wall of the pores. Carbon and oxygen isotope data from carbonates represent burial and hydrothermal conditions, and reflect the diagenetic evolution of the Sete Logoas Formation.