Abstract
The Rio Itapicuru Greenstone Belt (RIGB), located in north-eastern Brazil, is an important gold producing region. It hosts two important mining districts, the Fazenda Brasileiro and the Fazenda Maria Preta, owned by Companhia Vale do Rio Doce (CVRD), plus a number of gold occurrences, all associated with hydrothermal alteration along shear zones. In 1992, Geoscan multispectral scanner data were acquired for exploration purposes over more than 7,500 km² in this greenstone belt. Geoscan AMSS is a 24-band airborne scanner, with 5 meter spatial resolution. Standard image processing techniques, applied immediately after data acquisition, defined some spectral anomalies, particularly due to silicification and oxidation, which could be due either to hydrothermal alteration or to weathering processes. We have reprocessed the Geoscan data over a portion of the RIGB in order to define possible anomalies due to hydrothermal alteration minerals, using the feature-oriented principal selection (FPCS) technique, modified for use with Geoscan data. Laboratory reflectance spectra of residual soils were acquired to establish superficial expression of hydrothermal minerals that occur in sub-surface and to guide image processing and interpretation. This led to the production of mineral component images, in which the spatial distribution of some minerals are shown. Results of the spectral analysis of soil samples showed that goethite may be used as a surface indicator of hydrothermal alteration and that it is not possible to differentiate the alteration zones from surrounding rocks based on the hydroxyl or carbonate spectral features.
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