The exploration of precious minerals has always been an activity that depended a lot on the physical efforts of old-time explorers and, more recently, of geologists armed with a hammer for collecting rock samples This is a situation that is beginning to change and to incorporate that which involves the latest advances in technology. The use of satellites and the techniques of digital image processing are some of these new tools, as the work developed at the Geosciences Institute (IG) of the State University of Campinas (Unicamp) shows. By way of the images of two satellites, Landsat and Terra, both launched by the North American Space Agency (NASA), the team led by Alvaro Penteado Crósta has developed a remote sensorial method that is applied to the exploration of minerals that identifies locations favorable towards the occurrence of important commercial minerals, such as gold, silver and copper.
“The advantage of the new method lies in the preliminary delimitation of the areas favorable towards the occurrence of minerals, without the necessity of a detailed knowledge of the region’s geology, which in the majority of cases doesn’t exist, which gives a much lower cost and a much reduced timescale” says Crósta. It is a way of not having to crisscross large areas, observing and manually collecting rock samples, an expensive and time consuming task.
The new method makes use of satellite images in the visible spectrum, like a photograph, and infrared, invisible to the human eye. It was tested in partnership with the Canadian mining company, Iamgold, which intends to expand its activities in the region of Patagonia, in Argentina, where there is not much available geological information. One of the company’s directors is a Brazilian and, in 1998, he sought out Crósta, who has developed a digital processing technique of satellite images for mineral exploration during his doctorate thesis at the end of the 1980s, based on the remote sensorial technology that existed at that time. “The use of sensorial techniques to identify beforehand areas with metallic minerals represents a major economy for the companies that know, in advance, the exact areas where they should sent geologists in order to develop more detailed work, to collect samples for the chemical analysis of the level of metals, and, if the levels were to be confirmed, to carry out drillings to locate the reserves” says Crósta.
The partnership with the Canadian company served to perfect the processing methods previously developed, by making use of images of the sensor Thematic Mapper (TM) of the Landsat-5 satellite. The project was carried out in the Province of Rio Negro, in the north of Patagonia, a region favorable to the use of satellite images for geology because the climate is arid and the vegetation sparse. The researchers initially located diverse areas related to the occurrence of gold and silver, starting from the Landsat images. In around 50 identified areas, 42 revealed the presence of gold. “This is an excellent index of targeting, some 84%” celebrates Crósta, also the director of the IG of Unicamp. For all of the locations pointed out, the analysis and the drillings done by the company over a two year period showed the presence of gold and silver close to the locality of Los Menucos. But the final results indicated that the levels of gold were not sufficiently high for commercial exploration.
Shortly after the conclusion of this regional work with the Landsat TM, a new multi-spectrum sensor was launched aboard the Terra satellite, given the name Aster, Advanced Spaceborne Thermal Emission & Reflection Radiometer, developed by NASA and by the Japanese Space Agency. This is one of the five sensors for image capture that are installed in Terra, launched in December of 1999 as part of NASA’s Earth Observation System (EOS) program, which also included the satellite Aqua. The program’s objective is the analysis of various environmental and climatic parameters of the planet.
The Aster sensor captures the reflected solar energy and thermal energy, and this is related to the temperature emitted by the Earth’s surface, by way of 14 spectral bands, represented by electromagnetic wavebands running from the visible region to various widths of infrared waves. The Landsat uses the same technique, but the captured information is restricted because it works off only seven bandwidths. The Aster’s information bases itself on electromagnetic wave bands that are thinner and serve, among other applications, to identify in detail different mineral characteristics.
Knowing the characteristics of the area of Los Menucos, by way of a study previously carried out, conscious of its potential for development and the test of the processing techniques fitting into the new Aster images, the Unicamp researchers laid out a project for the application of the available techniques brought about by the new sensor.
The team made up of Crósta and Carlos Roberto de Souza Filho, along with the doctorate student Diego Fernando Ducart, also obtained the support of the Iamgold company for the development of their project. “They gave us logistic support for the development of the field work, necessary to validate the results obtained with the employment of the techniques developed for the Aster images.” The Brazilian researchers also had the collaboration of the National University of Rio Cuarto, in Argentina, by way of professor Jorge Coniglio, an ex-postgraduate student at IG-Unicamp.
The new technique identifies on the images the spectral signature of the minerals, which is the manner in which the Earth’s surface materials interact with electromagnetic radiation. Each type of rock interacts with the Sun’s radiation, reflecting or absorbing determined lengths of waves, transforming the results into a type of digital fingerprint of this material. These minerals are formed by the action of water at extremely high temperatures or of vapor, whose circulation is related to volcanic processes. “Thermal waters, together with vapor, at the same temperature that removes and transports the metals from the rocks that are at depths and deposit them in a concentrated form much closer to the surface, as well modify the existing rocks and create the minerals of hydrothermal alteration. In this manner, the presence of these minerals on the surface can be used as an indicator of the occurrence of metallic minerals beneath the surface” explains Crósta.
“With the Aster it was possible to advance more in the detailing of the mineralogical composition of the hydrothermal alteration, a phenomenon that arises in this region of ancient volcanic activity” stated professor Crósta. The hydrothermal alteration is the denomination of the minerals that are associated with the occurrence of the metals.
Once the various minerals of the hydrothermal alterations have been identified, by way of the application of image processing techniques that selectively extract only the information of interest within the enormous quantity of other information contained in the Aster images, they are transposed to maps that show the zones of hydrothermal alterations superimposed on the images of the topographical relief also captured by the same sensor. The cartographic composition results in a projection with three dimensional characteristics of the region.
The final images do not show in a direct manner the gold, copper or whatever other metal, but exhibit the present of the alteration minerals such as: kaolinite, illite, alunite and silica. These are indirect indicators of the eventual presence of precious metals. “We extract from the image the spectral signature related to the chemical composition and to the structure of minerals that are on the surface of the land” says Crósta. Many materials on the Earth’s surface, including minerals, possess distinct spectral signatures, which are captured by the multi-spectral sensors and used for analysis.
“In the case of Los Menucos, we identified, using the new image processing technique, the locations where the various minerals from hydrothermal alteration related to the occurrence of gold occur and afterwards we went out to verify the results in the field” says Crósta. The second study for the confirmation of the mapping method of hydrothermal alterations using Aster images took place in the foothills of the Andes, in the southern Quellaveco region of Peru and the north of Chile, in an already known copper mine that was in its initial exploration phase. The colors found on the map, such as red, indicate the presence of alteration minerals known as quartz (silica) and illite of high crystallinity. They function as a signal that identifies the presence of copper in the researched area. “In this presentation, the redder the image of the area, the greater is the possibility of finding copper” explains Crósta.
In this research with multi-spectral images it is of fundamental importance to make use of the analytical technique of reflectance spectroscopy. For this to happen, the Reflectance Spectroscope belonging to the Geosciences Institute itself was made use of (see Pesquisa FAPESP Nº 86). With this instrument the spectral description of the rock samples for mineral identification are carried out and a comparison made with the results obtained starting from the multi-spectral images. These studies also include an analysis of mineral crystallinity, a factor that can give indications of the temperatures in which the hydrothermal alteration minerals were formed, important information in the identification of deposits of precious metals.
The project also resulted in a master’s degree thesis, a doctorate thesis and publications
In international scientific magazines in this area. The next steps for the research group are already laid out. They are leaving Patagonia and the foothills of the Andes, appropriate locations for the use of remote sensors for mineral exploration because they have good conditions of surface exposure of rocks and sparse vegetation – contrary to that of the Amazon, where the vegetable cover makes the identification of mineral reserves by this method difficult – and are going off to the Brazilian Northeast, more precisely to the Borborema Ridge, between the states of Rio Grande do Norte and Paraíba. Here there is a large concentration of pegmatite, rocks that produce gems for ashlars and jewelry, such as aqua marine, tourmaline and various others, as well as various types of industrial minerals. “In the case of the pegmatite, the occurrence also has a relationship with the circulation of thermal waters, and, indeed, with the presence of hydrothermal alteration minerals in the rocks, which allows studies using Aster’s multi-spectral images.”
Evaluation of the multi-spectral images from the Advanced Spaceborne Thermal Emission and Reflection Radiometer sensor (nº 02/07978-1); Modality Regular Line of Research Assistance; Coordinator Álvaro Penteado Crósta – Unicamp; Investment R$ 66,925.75 and US$ 21,450.00 (FAPESP)