Petrografia e geoquímica de espeleotemas em cavidades naturais, N4E-08 e N4WS-72, em formações ferríferas da região de Carajás – Unidade Geomorfológica Serra Norte

Abstract

The Carajás region is located in southeast of the Pará State, Brazil. It has one of largest Brazilian speleological units, where several cavities were formed in iron ore. In this region landforms are partly characterized by residual plateaus of the south of Pará State, locally named Carajás Mountains, which is divided in North, East, Bocaina, Tarzan, and South Mountains. The plateaus of the Carajás are covered by a thick lateritic iron crust, which is generated of volcanic rocks and banded iron formations (BIF) of Grão Pará Group, and the sedimentary rocks of Águas Claras Formation. The lateritic cover usually shows karst features (caves), which typically occur at the interface jaspilite/lateritic iron crust, what suggesting origin by weathering and degradation of these rock types. This work made a petrographic and geochemical study on samples of jaspilite, lateritic iron crust and speleothems of two caves of the North Mountains, N4E-08 and N4WS-72, aiming to characterize and define the genesis of speleothems in these natural cavities. The speleothem jaspilite speleothem-coraloide consists of amorphous silica, goethite, hematite, and alunite minerals, as well as fragments of jaspilite, which were generated by the degradation along iron formations, through the action of chemical solutions flowing in the banding planes of the rocks, faults, and fractures. The speleothem lateritic iron crust speleothem-coloforme-white crust consists of leucophosphite crystals of coprogenic origin, with the bat guano as the main source of phosphorus.