Caracterização petrográfica e mineralógica de cavidades desenvolvidas como produtos de alteração de rochas máficas na unidade espeleológica Carajás – sudeste do Pará

Abstract

S11D plateau is inserted into the set of Serra Sul plateaus that is located in the Província Mineral de Carajás in the southeast of Pará State. The region is in the area of Dissected Plateau South of Pará. Plateaus generally exhibit top plan wavy, with dimensions ranging 500-800 m. In the most devastated portion, there is a regional pediplain with quotas ranging 250-350 m. In this portion, there is occurrence of plateaus and isolated hills, with elevations above 500 m, of which Serra Sul é remnants of South American area, developed between Cretaceous and Tertiary Superior. There are a lot of cavities that developed as weathering products of iron formation and associated rocks of Grão Pará Group in this area. This study consisted of mineralogical and petrographic characterization of a cavity (S11D-47) located in Serra Sul, precisely on S11D plateau. Methods used were essentially literature related to the subject, petrography and X-Ray Diffraction (XRD). Application of these methods in rock samples from wall, floor, ceiling of this cavity. Description showed that the rock samples from wall, floor and ceiling of the cavity are made of the same material, which is a reddish yellow staining rock, soil aspect and friable. Locally, centimeter circular, subcircular features were observed filled with a whitish material, with silky texture and gelatinous yellowish material. Pores exhibit irregular contours sometimes interconnected becoming elongated. Mineralogically, the host rock consists of hematite, gibbsite, goethite and gibbsite. Whitish material are of montmorillonite, kaolinite and rancieite, while the yellowish material with gelatinous texture is constituted predominantly of hematite, nissonite, gibbsite and ferrowyllieite. Microscopically, it was observed that hematite is preferably as granular crystals dispersed in a goethite cryptocrystalline plasma and also show up as thin tapes that surround pores edges of gibbsite occurs partially or completely filling the pores and filling existing microfractures. Goethite occurs as a cryptocrystalline plasma. Minerals: montmorillonite, rancieite, nissonite kaolinite and ferrowyllieite were identified only in the XRD analysis, possibly because it is very fine-grained minerals. Textures observed that assembly and mineral rock, suggest that it is product of an intense supergene change, and fractures and structures present in the cavity strongly suggest a genesis area controlled by tectonics.