Genesis of Caves at Serra Norte, Carajás, Pará based in petrographic and geochemical studies of Banded Iron Formations

Abstract

This research aims to understand the main factors responsible for the formation of caves associated with jaspilite in N4 plateau, at Carajás Speleological Unit, Pará state. The study was based on petrographic and geochemical analyses of jaspilite with varying weathering degrees, seeking to understand the mineralogical, textural and geochemical evolution of four categories: 1) fresh jaspilite (JF); 2) least altered jaspilite (JPA); 3) very altered jaspilite; 4) iron ore (MN). In transition from fresh jaspilite to iron ore, the intense leaching of SiO₂ (62.51 to 0.51%) happens, as well as the consequent enrichment of Fe₂O₃t (56.48% to 97.71%). Chert and jasper are leached, causing the formation of dissolution microcavities until the complete removal of the siliceous band (roughly 50–60% in vol.). On the other hand, there is a neoformation of two hematite varieties, Hm-2, and Hm-3, besides goethite. The research verified that the caves’ genesis is initially related to chemical processes of dissolution and leaching of silica, which were caused by the percolation of fluids of meteoric origin, channeled through the jaspilite banding and made more intense by the inclination of the layers, which served as preferred paths for these solutions. Processes of underground erosion with collapse of the residual material accelerated the growth of the open spaces in the jaspilite with the consequent formation of caves. We interpret that the initial processes that acted in the formation of the caves are the same that conditioned the formation of the friable ore of the studied mine.