Mapping of Hydrothermally Altered Rocks using the Modified Spectral Angle Mapper (MSAM) Method and ASTER SWIR Data

  • Kodama, S


The shortwave infrared (SWIR) bands of the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) target OH– and CO32– absorption features in the 2 µm region, in carbonate minerals and hydrothermal alteration minerals such as alunite, kaolinite, and calcite. We modified the Spectral Angle Mapper (SAM) method in order to use ASTER SWIR data to map hydrothermally altered rocks. The new method (Modified Spectral Angle Mapper: MSAM) provides a mineral index that is insensitive to the grain size of minerals and topography. The MSAM method can avoid misidentification due to mixture of the target mineral with different materials such as vegetation. The mineral index estimated from the simulated ASTER SWIR bands using the MSAM method shows greater variation among analyzed minerals than that obtained using the original SAM method. The mineral index images derived using the MSAM method exhibited a clear boundary between areas of contrasting mineralogy in the Cuprite region and Northern Grapevine Mountains region; this result is consistent with a previously published geologic map and hyperspectral data. Further investigation by the MSAM method identified several types of hydrothermal alteration zones associated with porphyry ore deposits near the Yerington District, Nevada. These results demonstrate the usefulness of ASTER SWIR data and the MSAM method in terms of lithologic mapping.