Point and imaging spectroscopy investigations on the Pedra Branca orogenic gold deposit, Troia Massif, Northeast Brazil: Implications for mineral exploration in amphibolite metamorphic-grade terrains

Abstract

The Pedra Branca gold deposit is a recent discovery conducted by private exploration companies in the Troia Massif of the northern Borborema Province, Northeast Brazil. It comprises an orogenic gold deposit hosted by the amphibolite-facies Serra das Pipocas greenstone belt. Airborne hyperspectral images (ProSpecTIR-VNIR-SWIR system) covering the Pedra Branca deposit as well as spectroscopic data (ASD-Fieldspec and SisuCHEMA instruments) from representative rock samples in the area were used in this study to investigate the spectro-mineralogical patterns of alteration minerals at both district and deposit scales, respectively. The spectroscopic data were integrated with geological information from the deposit, and also with X-ray powder diffraction and petrographic analyses, in order to reveal alteration footprints that could guide gold exploration targeting in the region. The results indicate the occurrence of a main hydrothermal calc-silicate alteration in ferromagnesian host rocks, consistent with proximal alteration assemblages found in amphibolite-facies gold deposits. White mica compositional trends suggest a later hydrothermal alteration that overprinted the pre-existing mineral assemblages and crystallized Al-rich white mica in distal zones and Al-poor white mica in the ore zone. Reflectance spectroscopy analyses of outcrop samples indicate that well-ordered kaolinite occurs as a weathering product of Al-poor white mica-bearing samples, and therefore could be used as a proxy to mineralization at the deposit scale. Mineral maps produced by processing airborne hyperspectral images, combined with soil geochemical anomalies, further support the notion that well-ordered kaolinite can be used as a proxy to mineralization in weathered ferromagnesian host rocks. Target areas for gold exploration can be defined by airborne hyperspectral data based on the identification of spectral mixtures of nontronite and well-ordered kaolinite. The mineral and physicochemical trends identified in this study set an important baseline for mineral exploration of amphibolite-facies orogenic gold deposits in greenstone belt terranes. The mineral guides defined here can be sensed by VNIR-SWIR hyperspectral sensors at multiple scales using portable, core scanning, wall-imaging, airborne or satellite-borne instruments, respectively.