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The basic tool for geologists in all disciplines is a map depicting the distribution and identity of rock units exposed at the earth’s surface. Using these maps, economic geologists search for metal and petroleum deposits; hydrogeologists look for ground water; structural geologists classify faults as active or inactive. With its high spatial resolution, and bands covering a wide part of the electromagnetic spectrum, ASTER will provide data that will greatly improve geologists’ abilities to produce more accurate geologic maps at a fraction of the cost of conventional ground-based methods. Cuprite, Nevada The Cuprite Mining District, located in west-central Nevada, is one of a number of alteration centers explored for precious metals. Cambrian sedimentary rocks and Cenozoic volcanic rocks were hydrothermally altered by acid-sulfate solutions at shallow depth in the Miocene, forming three mappable alteration assemblages: 1) silicified rocks containing quartz and minor alunite and kaolinite; 2) opalized rocks containing opal, alunite, and kaolinite; 3) argillized rocks containing kaolinite and hematite. A general picture of the alteration is shown in Figure 1, combining bands 4,6, and 8 in RGB and processed to increase the color saturation.
Figure 1. Cuprite Mining District Nevada displayed using ASTER SWIR bands 4-6-8 as a RGB composite. Area covered is 15 x 20 km. Red-pink areas mark mostly opalized rocks with kaolinite and/or alunite; the white area is Stonewall Playa; green areas are limestones; and blue-gray areas are unaltered volcanics. Data from the SWIR region were processed to surface reflectance by EDC and image spectra were examined for known targets at Cuprite. Evidence of SWIR crosstalk was apparent, making the data difficult to use for spectral analysis using direct comparisons with library or field spectra. To reduce the crosstalk artifacts, a spectrum of Stonewall Playa was used as a bright target, resampled to the ASTER wavelengths, and divided into the SWIR reflectance data. Library spectra were compiled for minerals known to occur at Cuprite; they were then resampled to ASTER SWIR wavelengths. These spectra were used with a supervised classification algorithm, Spectral Angle Mapper, to map similar spectral occurrences in the SWIR data. The result of this classification is shown in Figure 2.
Figure 2. Spectral Angle Mapper classification of Cuprite SWIR data. blue=kaolinite; red=alunite; light green=calcite; dark green=alunite+kaolinite; cyan=montmorillonite; purple=unaltered; yellow=silica or dickite. When this map was compared with more detailed mineral classification produced from AVIRIS data, the correspondence was excellent. The resampled library spectra are shown in Figure 3 compared with ASTER image spectra extracted from 3x3 pixel areas.
Figure 3. ASTER image spectra (left) and library spectra(right) for minerals mapped at Cuprite.
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