Researchers from the Geosciences Institute (IG) at Unicamp, in partnership with the Geological Survey of Brazil – CPRM, developed a study that could add greater efficiency to gold mineral prospecting. The work investigated whether hyperspectral remote sensing, that is, the application of high spectral resolution sensors to remotely obtain detailed information about an object on the Earth's surface, can provide valuable data for prospecting some types of gold deposits, an element whose application is essential in areas such as medicine, satellite construction and even photography.
Recently published in the journal Ore Geology Reviews, from the Elsevier group, the study was the result of the dissertation by João Luís Carneiro Naleto, developed during the master's degree in Geosciences at IG, under the guidance of professor Carlos Roberto de Souza Filho, and also included the participation of specialists from the Geological Survey of Brazil Mônica Mazzini Perrotta and Felipe Grandjean da Costa. According to João, the objective of the study was to fill gaps in knowledge about orogenic gold deposits – generated in tectonic plate convergence environments and which correspond to a large part of world gold production – of the hypozonal or amphibolite facies type, even little studied in the light of spectral technologies.
Hypozonal deposits are hosted in metamorphosed terrain at depths greater than 15 km and at temperatures of around 700 o C and are usually less significant than orogenic deposits of the greenschist facies type, formed at temperatures below 450 o C. “ The results demonstrate that these technologies, known to be quicker to respond and, in certain cases, cheaper than some traditional methods, are capable of providing additional key information for gold prospecting in geological and climatic environments similar to the one studied”, he explains.
To carry out the research, data from the Pedra Branca Gold Deposit, located in the Central region of the State of Ceará, in the Troia Massif, was used, an area that corresponds to a segment of the Earth's crust aged between 2,85 and 2,13 billion of years. They were obtained by the Areas of Relevant Mineral Interest (ARIM) program, developed by the Geological Survey of Brazil – CPRM, which throughout 2013 and 2014 obtained aerial hyperspectral images of this region.
The work also established a spectro-mineralogical model for the Pedra Branca deposit through reflectance spectroscopy analyzes on samples collected in the field. This technology is simple and fast to operate, and provides compositional information based on the interaction between electromagnetic energy in the visible to short-wave infrared bands and the surface of materials. Furthermore, cylindrical rock samples extracted from the subsurface (probing core), provided by the mining company Jaguar Mining, provided the basis for obtaining mineralogical information on a detailed scale. “The model aims to contribute to a better understanding of metallogenetic controls and to the identification of mineralogical trends related to mineralizing events, which can be used as prospective guides for this and also for other gold deposits”, says João.
As a result, the study demonstrated that both reflectance spectroscopy and hyperspectral remote sensing are valuable tools for gold exploration in the studied region. At the same time, the maps produced from the digital processing of aerial hyperspectral images support the assumption that target areas for gold prospecting in the region can be identified by data from remote sensors. Furthermore, established mineral guides can be investigated by sensors at multiple scales, such as handheld, profilers, benchtop, airborne, or satellite imagers.
The expectation of the study authors is that the example of application of these methods in the Pedra Branca Gold Deposit region will encourage the inclusion of reflectance spectroscopy and hyperspectral remote sensing in the workflows of exploratory projects. “They could, consequently, benefit from a possible gain in efficiency, a measure that is extremely relevant to the economic viability of small deposits, as is typically the case with orogenic gold deposits in amphibolite facies”, concludes João.
