Couraças ferruginosas e solos associados em diferentes ambientes do Quadrilátero Ferrífero, Minas Gerais, Brasil

Abstract

The litolithic variability, coupled with a complex geomorphological development, makes the region of the Quadrilátero Ferrífero (QF) have a rich diversity of soil cover. Among these, the soils derived from rocks with high iron content such as Itabiritos and iron crusts stand out for their unicity. This research is dedicated to the study of the formation and transformation of iron crusts of the QF, with the central objective of analyzing how these iron crusts are formed directly from rocks, such as itabirites, and how they evolve into soils with high iron content. The areas of investigation were chosen from field incursions and assistance of a base cartography, being defined two areas: the western edge of the Sinclinal Moeda and the Hogback of the Pedra Rachada. In order to characterize the physical constituents of the Serra da Moeda soil, a method was developed for the dispersion of particles with high Fe2O3 content, concluding that due to genetic characteristics of the material itself, a method that provides higher vibration for the particles that do not promote too much abrasion is best suited for obtaining safe results from fine fractions such as silt and clay. The formation of oxisols rocks with high iron content within the QF is due to the accumulation of colluvial material at the foot of the slope where the development of erosive amphitheaters in the mid-high slope facilitates the mechanical destabilization of the top iron crusts and increases the amount of surface transported materials from of concentrated flows. Thanks a detailed study in Pedra Rachada, northern portion of the QF, the following morphological variations were found in the front: F1 - itabirito not altered; F2 - Structured placoidal iron crust; F3 - structured placoidal iron crust with zones of plates in fragmentation; F4 and F5 - fragmented iron crust with disorganized plates and F6 - canga. On the reverse only fragmented breastplates with disorganized plaques are present, and can be differentiated into R1 - placoidal iron crust with incipient fragmentation and, fragmented iron crust with fine plates (R2), medium (R3) and thick (R4) disorganized. The association of the morphological variations allowed to recognize the presence of three facies of genetically associated alteration: facies isalteritic iron crust, facies fragmented iron crust and facies canga. Its formation and evolution are closely associated with lithostructural and geomorphological conditions. When analyzing the micro aggregates of the soil with high iron content it could be noted that morphologically the breast nodules present a ferruginous inner nucleus and a ferro-alumino alteration cortex. Considering the mineral chemical composition, the values found indicate the presence of hematite and goethite as the main minerals of these internal nuclei and aluminous goethite in the cortex. When analyzing the aggregates, the same compositional variation and zoning of the harnesses is not observed. The genesis of these microaggregates seems to bear witness to a more complex trajectory than in other soils with this structure. This is evidenced by the existence of distinct organizations that may be incorporated in this category and by the fact that these soils differ from the others by eminently ferruginous composition.