Petrografia do testemunho de sondagem n4ws 1495, das formações ferríferas bandadas, Serra Norte, Carajás-PA

Abstract

Carajás iron deposits belong to Carajás Formation, a thick (100-400m) banded iron formation and laminated (jaspilite), located in Pará State, northern Brazil. This almost continuous formation, outcrops for at least 260km in 60 deposits, along three main ridges, São Félix, East and Carajás. Latter is a sinformal structure sub-divided into Serra Norte and Serra Sul. Numerous works have treated jaspilite petrography, highlighting all its primary and diagenetic sedimentary characteristics. However, few studies use petrography to discuss the paleoenvironment and iron ore origin. Among point of thought that more discuss paleoenvironment concern about formation of banded iron rocks in Shallow Platform environment, due to geochemical homogeneity of seawater that would prevail cyclical conditions of jaspelites formation. And the point of thoughts on the genesis of iron ore would be by hipogenic changing and supergene alteration, despite advocate different origins assume that dissolution of silica was an active agent during the formation of deposit. This work has objective to study petrologically the drill core rocks to characterize paleoenvironments sedimentation of FFB's Carajás, and to evaluate the diagenetic processes that affected jaspilites after its deposition. For this, we analyzed 43 samples Mine drill core N4WS 1495, which were studied in the literature surveys and laboratory analysis. With emphasis, for microscopic description and analysis in electron microscope (SEM) in order to observe features that evidenced preservation and alteration of rock, such as textures, structures, dissolution cavities, substitution and formation of new minerals to differentiate sin depositional and post depositional processes. Based on the progression of rock alteration degree, following categories were individualized: jaspilite not changed, jaspilite little Changed, Changed jaspilite, jaspilite Breccia, mafic igneous rock with texture preserved, Mafic Rock Strongly Amended and jaspilite Hornfels. Observed structures are characterized by diagenetic features attributed to difference in composition of webs and lithostatic pressure which causes compression, dehydration and lithification sediment. These bands, saturated with water have different densities and mechanical behavior, causing various deformations intensities. Carajás rocks of formation have features as compositional banding; intrabands displacement; folds; thinning and breakup; interpenetrative contacts; pods; excavation and filling; pent and extreme homogeneity mineralogical composed mainly of hematite (1, 2 and 3), magnetite, maghemite and goethite indicate depositional control in the chemical precipitation of sediments that originated Carajás BIF. Silica would be close to saturation levels in seawater, rushing as gel, influenced by seasonal weather conditions. Iron would be derived from hydrothermal leaching pile of basaltic Parauapebas formation. Oxygen in the atmosphere, in the Archean, would be at levels such that there would be a large reservoir of soluble iron in the deep part of sea. Upwelling of groundwater in deep portions of the primitive ocean, far from the coast and in the absence of clastic sediments would lead to deposition of iron by oxidation as oxy-hydroxide. Despite occur in the same place and depositional be precipitated chemicals, iron and silicon have different controls. For iron Eh would oxidant and for silicon, the supersaturation would contribute to the precipitation. These controls do not interfered each other, such as iron intake, hydrothermal leaching, variation in evaporation rate or the rate of supply of water poor in silica. These variations would be responsible both for banding as the difference between the thickness of the bands, as well as by small variations in mineralogy.