Currently, the number of rocky caves in Brazil exceeds four thousand. However, the number of researches on this subject is still low, mainly with respect to the caves genesis. Consequently, the knowledge about the subject is still little developed. This research aims to understand the main factors responsible for the formation of caves associated with Jaspilite in N4 plateau, at Carajás Speleological Unit, and additionally positioning the formation of the caves in the evolution of the relief and the ore. The research was based on petrographic and geochemical analyses of Jaspilite with varying degree of weathering, seeking to understand the mineralogical, textural and geochemical evolution in four subgroups: A) Fresh Jaspilite (JF); B) Little Altered Jaspilite (JPA); C) Very Altered Jaspilite (JMA) and D) Iron Ore (MN). These analyses were fundamental to understand silica removal processes, as well as iron enrichment in the alteration profile. The main petrographic feature of the Fresh Jaspilite is the minerals components consisting mainly of hematite-1, magnetite and rare maghemite, in addition to silica minerals such as jasper and chert; Fresh Jaspilite shows no signs of dissolution of the silica and the rock banding is preserved. The Little Altered Jaspilite is characterized by the appearance of secondary minerals, such as goethite and hematite-2; there are signs of dissolution of the silica evidenced by the presence of micro-dissolution cavities. In the Very Altered Jaspilite, magnetite crystals are totally replaced by secondary minerals of hematite-2, and hematite-3 in low occurrences; dissolution cavities are more abundant in this lithotype. The Iron Ore represents the most advanced stage of weathering, characterized by the marked presence of hematite-3 and practically no silica; as a consequence, the banding is practically absent. Chemical analyses are totally in agreement with the petrography. Fe and Si in Fresh Jaspilite account for more than 98% of the total, with SiO2 content ranging from 42.61 to 62.51% and Fe2O3t from 35.92 to 56.48%. Whereas in Iron Ore there was a great loss of SiO2 with a variation of 2.75 to 0.51% and Fe2O3t with a significantly high content ranging from 94.35 to 97.71%. In relation to the trace elements, they show a slight decrease in Fresh Jaspilite to the Iron Ore, indicating the mobility of these elements in the alteration profile, with the exception of Zn and Pb, which showed a small decrease in the content towards the alteration products. In the Fresh Jaspilite, the mean of ΣETR is 6.7 ppm, passing to 15.2 ppm in Iron Ore, demonstrating the concentration of these elements to the top of the alteration profile. The genesis of the study caves is initially related to chemical processes of dissolution and leaching of silica. This dissolution is caused by the percolation of fluids of meteoric origin, channeled through the jaspilite banding itself, together with the inclination of the layers, which served as preferred paths for these solutions. Subsequently, erosion processes in sub-surface accelerated the reduction of volume, with collapse of the residual material and consequent formation of the caves. The initial processes that acted in the formation of the caves are the same ones that conditioned the formation of the friable ore, because as the silica is leached forming the micro-dissolution wells, the iron (Fe2O3t) is relatively concentrated, reaching a content up to 97%. The chemical, mineralogical and textural evidences indicate that the fluids that transformed the Fresh Jaspilite in Iron Ore and, in parallel generated the caves, are of meteoric origin. The dissolution does not act alone, other factors also collaborate, channeling meteoric water to preferential channels of dissolution and erosion, as examples: structural, hydrological, lithostratigraphic factors, abatement and sealant rock.
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