Integração de dados de geofísica aérea aplicada à geologia e à prospecção mineral no distrito esmeraldífero de Itabira-Ferros, Quadrilátero Ferrífero, MG.
Leila Marcia Mendes Carvalho
The Itabira-Ferros area is located in the Quadrilátero Ferrifero region, State of Minas Gerais, and it is known for abundant emerald occurrences. The area is characterized by volcanic metasedimetary sequences of the greenstone belt type (Rio das Velhas Super group) and paleoproterozoic iron formations of Minas Super Group, inserted in granitic bobies (Borrachudos Unit). Emerald genesis is associated to the interaction of pegmatitic fluids with mafic-ultramafic rocks of Rio das Velhas Super group. The present work present data processing, with the purpose of studying and analyzing airborne geophysical information and the in integration with geological data, in order to determine zones prone to carry emerald mineralization. The results obtained with the interpretations of this airborne survey were individually analyzed and integrated among them. The final result of this study has two main specific objectives: the first illustrates the application of geophysical data to the geological and structural mapping and the second, to the mineral prospecting. In the first case evidence is shown of the use of geophysics derived information in the support of geological mapping and structural characterization. In the second, is shown how airborne geophysics can help in the characterization of exploration targets. From this study potential areas for mineralization are identified. In gamma-ray spectrometry, the total count image lead to the definition of a framework of major units covering the area. The analysis of the distribution of the K, U, and Th elements, used together with ternary RGB and CMY images, lead to the characterization of units and structures. The processing of airborne magnetic data lead to images of the anomalous magnetic field, amplitude of analytical signal, magnetic field reduced to the pole, and analytical signal inclination. These images conducted to the design of a framework with emphasis on the crystalline basement of the region, with the definition of shallow and deep units. The integration of magnetic and gamma-ray spectrometric units lead to the definition of 27 distinct geophysical units. Most of these units are correlates to known geological units, however some of them have no correlation with mapped units in the area. Main magnetic and gamma-ray spectrometric interpretes structures are correlated. This pattern seems to indicate that the main gamma-ray spectrometric lineaments are related to magnetic structures. These, on the other hand, are related to tectonic structures present on the study area. The main magnetic structure cuts the study area at a 70 angle to the gamma-ray structures, associated with the outcroping lithologies. Brittle structures, related to pressure relief in the directions E-W and N-S, are noticed in association with gamma-ray lineaments. Analysis and interpretation of the mentioned images suggests the presence of three deformation events. The lineament D1 represents the preferential direction of alignment, NE-SW. In the central region of the area, there is a sigmoidal feature that, probably, may reflect the Pedra Branca shear zone, with dextral movement. Lineament D2 cuts lineaments in the E-W direction, indicating, probably, a transcurrent system of faults with a sinistral movement, indicated by D1 lineament being displaced by D2 faults, as observed in central and northern portions of the area. Other brittle structures are present in the N40W direction. The maps with interpretation of geophysical units and structures show that mineralization in the region occurs in deeply faulted areas, specially associated to the border of the Pedra Branca shear zone, reflex of ductile displacement in the area. The main unit containing emerald mineralization is unit SGRv2, Rio das Velhas Supergroup. Known mineralizations form a belt, with a general NW-SE trend, between two faults, in the same direction, and the cities of João Monlevade and Dores de Guanhães. Emerald mineralizations remain to the east of the Pedra Branda shear zone, directly associated to NE-SW structures. Other beryl mineralizations (acqua marine and other beryls) are associated to the central portion of the shear zone or to other N-W and E-W features cutting the area. Image resulting from the integration of airborne magnetic and gamma-ray data indicates region with potential mineralization for emerald. Potential areas without spatial association with known occurrences suggest the enlargement of the exploration potential for gems for the region.
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