Distribution of potential hydrothermally altered rocks in central Colorado derived from Landsat Thematic Mapper data: A geographic information system data set

Abstract

As part of the Central Colorado Mineral Resource Assessment Project, the digital image data for four Landsat Thematic Mapper scenes covering central Colorado between Wyoming and New Mexico were acquired and band ratios were calculated after masking pixels dominated by vegetation, snow, and terrain shadows. Ratio values were visually enhanced by contrast stretching, revealing only those areas with strong responses (high ratio values). A color-ratio composite mosaic was prepared for the four scenes so that the distribution of potentially hydrothermally altered rocks could be visually evaluated. To provide a more useful input to a Geographic Information System-based mineral resource assessment, the information contained in the color-ratio composite raster image mosaic was converted to vectorbased polygons after thresholding to isolate the strongest ratio responses and spatial filtering to reduce vector complexity and isolate the largest occurrences of potentially hydrothermally altered rocks.

Automated mapping of mineral groups and green vegetation from Landsat Thematic Mapper imagery with an example from the San Juan Mountains, Colorado

“…The long-wavelength edge of this absorption extends into the visible, absorbing blue and some green light and creating the characteristic red, orange, and yellow colors of such minerals. This ratio is similar to the ASTER 2/1 ratio described by Rockwell (2012), and is commonly used to highlight ferric iron minerals (Knepper, 1989(Knepper, , 2010Rockwell, 1989). …”

“…Only the reflective bands were used for this study, and have a ground instantaneous field of view (GIFOV, or ground pixel size), of 30 meters. Band ratios, which highlight variation in reflectance between wavelength regions measured by the various bands, or channels, of the sensor, have been the simplest and most widely used analysis method applied to TM data (Crippen, 1989;Knepper, 1989Knepper, , 2010Rockwell, 1989).…”

“…Landsat ratio results can be spurious in areas of low surface albedo, including those with poor solar illumination caused by terrain shadow (Knepper, 2010). Such areas are masked from analysis using thresholds of Landsat TM band 5 determined through examination of many scenes from different climates, land cover, and terrain relief.…”

“…2), and the Landsat 5/7 ratio is used to highlight this broad mineral group (Knepper, 1989(Knepper, , 2010Rockwell, 1989). Carbonate minerals such as calcite and dolomite, Fe-OHbearing minerals such as epidote, and Mg-OHbearing phyllosilicate minerals such as chlorite, talc, and serpentine ("CEC" mineral group) have SWIR absorptions at slightly longer wavelengths that are only partially covered by the bandpass of Landsat band 7 ( fig.…”

Aeromagnetic and spectral expressions of rare earth element deposits in Gallinas Mountains area, Central New Mexico, USA

“…Band ratio 4/2 is useful for detecting ferric (Fe 3+ ) iron oxides such as hematite and blue spectral regions (Rockwell, 1989;Knepper, 1989;. Thus, band ratio 4/2 commonly enhances the reddish color from ferric iron minerals (Sabins, 1999;Dehnavi et al, 2010;Shalaby et al, 2010;Knepper, 2010) even if they are not present in high concentration. Area of ferric iron mineral occuring as coatings or disseminated on surfaces of hydrothermally altered rocks, sedimentary rocks and mafic related regolith at low grade can be identified with this ratio (Rockwell, 2013;Dehnavi et al, 2010).…”