Application of Remote Sensing to Geobotany to Detect Hydrothermal Alteration Facies in Epithermal High-Sulfidation Gold Deposits in the Amazon Region
Published:January 01, 2009
Teodoro Isnard Ribeiro De Almeida, Carlos Roberto De Souza Filho, Caetano Juliani, Fábio Cardinale Branco, 2009. "Application of Remote Sensing to Geobotany to Detect Hydrothermal Alteration Facies in Epithermal High-Sulfidation Gold Deposits in the Amazon Region", Remote Sensing and Spectral Geology, Richard Bedell, Alvaro P. Crósta, Eric Grunsky
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This work aims to assess geobotanical anomalies related to high-sulfidation gold mineralization in terranes covered by almost virgin tropical rain forests located in the Tapajos gold province, northern Brazil. Landsat-5 TM data were processed using recently developed algorithms tailored to enhance both specific and anomalous vegetation spectral responses. The method consists of a four-stage successive concentration of information and reduction of data dimensionality, as follows: (1) spectral indexes are prepared to enhance responses of the vegetation characteristics, such as pigments and leaf moisture; (2) a principal component analysis (PCA) is applied on the set of indexes; (3) another PCA is applied to the original NIR and SWIR bands (SWIR; TM 4, 5, and 7), thus obtaining a PC1 as a texture-rich albedo image; and (4) a low-pass proportional convolution filter is applied to selected principal component images derived in the second step. The final product yielded through this technique indicates the presence of differentiated vegetation covers. In the study area, field information showed a near-perfect spatial correlation between color classes highlighted in this Landsat image product and hydrothermal alteration facies identified in outcrops. Although the product is unable to provide direct indication of any rock type, minerals, or presence of hydrothermal alteration, it proved suitable to outline areas where geobotanic anomalies associated to mineralogical variations occur. The method and its products are here considered an important aid for exploration planning and follow-up investigations in the field in terranes covered by equatorial forests.
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Remote Sensing and Spectral Geology
Two recent papers, “Utility of high-altitude infrared spectral data in mineral exploration: Application to northern Patagonia Mountains, Arizona,” by Berger et al. (2003), and “Mapping hydrothermally altered rocks at Cuprite, Nevada, using the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), a new satellite-imaging system,” by Rowan et al. (2003), make a distinctive mark on the use of airborne and satellite hyperspectral imaging as an exploration tool.
These two papers deal with imaging of the Earth’s surface using the visible (0.4 μm) to near infrared (2.5 μm) part of the electromagnetic spectrum to map various mineral species. Depending on their structure and molecular bonding, minerals reflect and absorb the electromagnetic spectrum in unique ways. A large group of minerals have distinct electromagnetic signatures that make it possible to identify them from imaging systems that map the range of the electromagnetic spectrum between 0.5 and 2.5 μm.
These papers represent two distinct approaches. The first paper, by Berger et al., discusses the use of the AVIRIS (Airborne Visible Infrared Imaging Spectrometer) scanner, which provides high-resolution reflectance measurements in the spectral domain (224 channels between 0.4 and 2.45 μm) and variable spatial resolution (20 m), dependent on aircraft altitude. The second paper, by Rowan et al., discusses the use of the ASTER satellite scanner, which offers a limited range of spectra at three spatial resolutions (15, 30, and 90 m). ASTER measures reflectance radiation in 3 bands within the 0.52- to 0.86-μm range (visible-near-infrared) at 15-m spatial resolution, and 6 bands between 1.00 and 2.43 μm (short wave infrared) at 30-m spatial resolution. Emitted radiation is measured in 5 bands between 8.125 and 11.650 μm (thermal infrared) with a 90-m spatial resolution.
The main advantage of the AVIRIS sensor is the level of spectral detail, which provides accurate measurements of reflectance and absorption features of minerals that enables detailed mineral mapping. Its main disadvantages, however, are the extensive processing required to make the reflectance spectra useful, and its limited spatial coverage and acquisition cost based on programmed flights. In contrast, the main advantage of the ASTER sensor is that it measures key portions of the visible, near-infrared, and thermal infrared spectra of minerals for large-scale mapping projects, whereas its main disadvantage is that the data represent only portions of the electromagnetic spectrum and some minerals cannot be distinctively mapped. In addition, the lower spatial resolution in the near-and thermal infrared portions of the spectrum makes it more difficult to map at detailed scales.