by Bruna de Freitas, Independent Exploration Expert Geologist and MSc in Mineral Resources
The fast improvement of geophysical tools in recent decades has transformed mineral research at the same rate, allowing prospective ore deposits to be identified without the lengthy and expensive on-site exploration procedure. Many geophysical methods are available for surveys and quick estimating of large areas during prospecting and mineral exploration work.
Advancements in geophysical data processing, depth inversion, and estimation techniques can determine the lateral extension and depth of anomalies with good assertiveness. These advancements have avoided high and unnecessary costs, which often face the impossibility of implementing an undertaking that will not guarantee a return. Therefore, the methods must be applied appropriately and have their surveys planned correctly and consistently with the local geology.
Geophysics and its artifices have increased assertiveness in rock drilling programs, reducing costs, and simplifying the characterization of mineralized bodies and structures. When a commodity concentration shows a potential economic value and sufficient volume to pay for the entire extraction, the geophysics practices have become increasingly valuable and competent, allowing inefficient methods to be abandoned in favor of more efficient ways.
Geophysics has enabled correct planning of positive and negative holes during resource estimates, which is critical during mining exploration. In the implementation phase of a mine and during its operation, geophysical profiling methods have brought significant advantages, such as the precise determination of contact depths and geological structures, given the inefficiency of total rock recovery during drilling. Accurate knowledge of contact depths and geological structures has relevant implications for determining mineralized zones and surveying geomechanical parameters.
Analogous to the prospecting in mineral research, geophysics has been a great resource in defining areas for dams and tailings piles, and during the observation of the behavior of these earthworks, which suffer due to the seasonal climatic variations, still, geophysics has greatly supported environmental monitoring in the stages that follow a mining life cycle.
Airborne geophysics is one of several aspects of geophysics methods applied to mineral exploration. It uses data collected from aircraft to characterize large areas with mineral potential, both at the preliminary recognition level, aiming at the subsequent application of classic geophysical methods in selected regions, and the level of detail, aiming to improve the information available in previously studied areas.
Geophysics can be used to interpret surface and depth geological features. Studies can be developed to define contacts, tectonic structures, lithology, and mineralizations by analyzing geological and geophysical data.
The most used geophysical methods in airborne geophysics are magnetometry and gamma-spectrometry; when performed by airplanes, these fly over the region of interest at altitudes that generally vary between 150 and 300 m (492 and 984 ft) above the ground. Detecting minerals with several physical and/or chemical properties, such as density, electrical conductivity, magnetic susceptibility, and radioactivity, can indicate mineralized zones through airborne geophysical methods.
Mineral occurrences and deposits of gold, copper, chromium, molybdenum, tin, nickel, platinum, tungsten, and iron are associated with specific lithologies and hydrothermal alteration systems that can be identified by airborne geophysics, whether by magnetism, gamma-spectrometry, or gravimetry.
Furthermore, many deposits have extensive hydrothermal alteration zones that airborne geophysical methods can detect. The potassic, sericitic, propylitic, argillic alterations, and silicification zones can present different behaviors in response to airborne geophysical data, mainly due to the aid of the gamma spectrometric signatures of the minerals in the alteration halos.
The effectiveness of geophysics methods for application in the mineral exploration sector is associated with the recognition of anomalies and mineralized zones, which can be very efficient if there are occurrences of magnetometric structures and good gamma spectrometric responses.
Airborne geophysics costs
When we compare its costs versus benefits and the importance of the results for the mining company to better understand where to extract the ore, geophysics becomes a cheaper tool to better understand the subsoil with the need to drill a smaller number of holes, immensely more expensive, or drilling holes that are more accurate for mineralization. Furthermore, without geophysics, these drilling holes would be based only on the outcropping rocks in that region and not on the chemical and physical parameters of the rocks in the subsurface, which could significantly increase the cost of drilling, as this would be more imprecise.
How do we obtain airborne geophysical images applied to mining?
Geophysics makes it possible to investigate the subsurface, that is, to make measurements carried out using modern equipment on or close to the surface. These surveys are often carried out using small aircraft coupled to specific geophysical sensors. Currently, technologies are being developed for the use of drones in their execution, reducing costs and improving the resolution of this type of image.
Before the process, the flight lines are planned in the area to be executed. Different products can be generated from these data depending on the processing and filtering. Each map can be used for prospecting many ores, and its resolution depends on the spacing of the executed flight lines.
Airborne geophysical image processing is increasingly required
These images are being used increasingly, as they greatly facilitate geological mapping, helping to delimit targets to be investigated in the field and thus providing greater assertiveness to find certain differentiated features on the ground that may indicate some ore or rock that may contain ore.
What is the main advantage of processing airborne geophysical images?
The main advantage is that the processing will present a targeted result for the ore that intends to prospect in the region. This way, the resulting map will indicate relevant points on the terrain that the geologist must visit to verify its characteristics. This will make the geologist more assertive during geological mapping, reducing costs and increasing efficiency.
The larger the survey area, the more indicated this type of procedure is performed, as in an extensive area, mapping can be time-consuming and inaccurate if the ore is concentrated only in a specific location on the ground.
The region of São Félix do Xingu (SFX) is located in the Amazon Craton, State of Pará, Brazil. Geologically, the area is located in the eastern part of the Central Amazon province, close to the western limit of the Carajás Archean Mineral Province and is considered a promising area for the mineralization of epithermal gold and porphyry and base metals. The SFX region is composed mainly of andesites to rhyolites (ca. 2.0–1.88 Ga), generally undeformed and affected only by very low-grade metamorphism. These units are considered part of the significant Uatumã Magmatic Event, consisting of several intermediate to felsic magmatism events generated in continental magmatic arcs, with a final orogenic event (ca. 1.87 Ga), including intrusive volcanic, volcaniclastic, plutonic rocks, and sedimentary sequences. Several Au-Ag mineralizations of high, intermediate, and low sulfidation and Au-Cu-(Mo) porphyry were recognized in these units over 400 km (249 mi) from the Tapajós Province to the SFX region, encouraging research in this critical area.
However, due to its high prospective potential, the region lacks more geological details and a limited database. Using airborne geophysical techniques for difficult access areas, emphasizing mineral prospecting was essential for characterizing new domains and creating target areas in the region of interest. Through predictive techniques, using gamma-spectrometric and magnetometric data, it was possible to develop favorability maps for exploratory targets. With geophysical and magnetometric data integration, regions of high favorability were highlighted by applying relationships between channels and structural complexity analysis techniques of the area.
The result, published in the shape of a master’s degree thesis by the author at the University of São Paulo, demonstrates that both methodologies effectively identified promising regions for mineral prospecting. There was a total of six areas highlighted as favorable, two classified as ‘High priority’, two as ‘Moderate priority’, and two as ‘Low priority’. Areas with potential mainly for Au, Cu, Fe, Pt, Ni and Sn were defined based on the correlation between the magnetic data, where zones of significant geological discontinuities can host mineralization, and between the gamma spectrometric data, characterizing regions of interest based on geostatistics and the study from areas with anomalous concentrations of potassium, uranium, and thorium.
In short, geophysics has become a mandatory tool in mineral research work and mining. Geophysics will bring a great return to the mining enterprise if used consciously and coherently. For this, an interpretation consistent with geology and understanding its limitations is fundamental.
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