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Abstract

Recent advances in geometallurgy have made nickel laterites a premier target for mining and exploration companies. Parallel to this, a series of advances in remote sensing have become available at very low cost. Two of these advances, namely better topographical data, through NASA’s Shuttle Radar Topographic Mission (SRTM), and multispectral imagery from the ASTER sensor, have been combined to aid nickel laterite exploration in central Brazil.

The Conceição do Araguaia region, located in Pará and extending into Tocantins state, is part of the Neoproterozoic Araguaia fold belt. The target area covers the Quatipuru mafic-ultramafic association, which includes serpentinites (metaperidotites and metadunites), talc schists, tremolite-actinolite schists and small volumes of pillow basalts, phyllites, BIFs, gabbroic and jasperoid rocks. These are enclosed regionally by slate to phyllitic rocks. Several other occurrences of mafic-ultramafic rocks, along with Quatipuru mafic-ultramafic association are interpreted as part of an ophiolitic complex. Low-grade (greenschist) regional metamorphism is dominant. Laterization has been active since the early Tertiary, resulting in an extensive regolith cover over the older rock units.

In the present work, the Quatipuru mafic-ultramafic association was examined for nickel laterite mineralization by data compilation and remotely sensed image processing and interpretation. ASTER’s multispectral visible-shortwave infrared (SWIR) remote sensing capabilities were used to map areas of prospective mineral alteration and key mineral groups. Using spectral libraries for selected nickel-bearing minerals as standards, SWIR and visible-NIR bands of georeferenced mosaiced ASTER scenes were processed by feature-oriented principal component analysis (PCA), and the results converted in mineral-abundance images, based on statistical classification and pseudocoloring. The mineral abundance maps highlight areas most likely to contain minerals of interest.

Processing was performed for the whole region of interest, and for its part in the central scene alone, which covers about 85 percent of the concession areas. In both cases, statistics for PCA were conducted on a subset of the data, minimizing extraneous factors such as large drainages and urban sprawl, and then applied to the whole region.

Mineral abundance maps for the area have been built into a geographic information system (GIS), along with other remotely sensed data, public-domain regional geophysics, geologic, and infrastructure data, scouted geochemistry samples, and a leveled and continuous SRTM digital elevation model. Mapping for the occurrence of mafic-ultramafic rocks was achieved by a combination of PCs 1, 4 and 2 of ASTER bands 2, 4, 5, and 8. Clusters of anomalous contents of selected minerals are draped over the digital elevation model and indicate that the northeast-dipping rock units are covered by a laterized sequence constituting the main exploration targets. This targeting exercise revealed a number of favorable sites that are currently under exploration by mining companies.

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