Evaluation of 0.46- to 2.36-mu m multispectral scanner images of the east Tintic mining district, Utah, for mapping hydrothermally altered rocks

Airborne multispectral scanner images recorded in the 0.46- to 2.36-mu m region for the East Tintic mining district, Utah, were evaluated to determine their usefulness for distinguishing six types of hydrothermally altered rocks from a wide range of sedimentary and igneous rock types. Limestone, dolomite, and argillaceous rocks were of particular interest because these have spectral reflectance features that might make them difficult to separate from some of the altered rocks when these images are used.The laboratory and field evaluation of a color ratio composite image, supported by in situ spectral reflectance measurements and an alteration map compiled from a published map, shows that silicified, argillized, and pyritized rocks can be mapped in detail utilizing an intense OH absorption band centered near 2.2 mu m. This absorption band is absent or weak in most of the unaltered rocks. However, the 2.2-mu m band is also weak in the reflectance spectra representing the other three types of altered rocks--hydrothermal dolomite, calcitic volcanic rocks, and chloritized rocks. Consequently, these altered rocks are not consistently distinguished from unaltered rocks in these images. Furthermore, very pure dolomite spectra have an intense 2.33- mu m CO (super -3) absorption band that is not distinguishable from the 2.2-mu m band in these images; therefore, the pure dolomite exposures were distinguished from the silicified, argillized, and pyritized rocks through field checking. The 2.33-mu m absorption feature is weak in spectra for less pure carbonate rocks. Separation of these altered rocks and local exposures of argillaceous rocks also posed problems because of their mineralogic and hence spectral reflectance similarities.These results are especially important for estimating the potential of the 1.6- and 2.2-mu m bands in the Landsat D thematic mapper for mapping altered rocks. Here, as we found in an earlier study in south-central Nevada, limonite-poor hydrothermally altered rocks that were not consistently distinguished in Landsat multispectral scanner images should be detectable using these two thematic mapper bands in combination with other bands at shorter wave-lengths. However, distinguishing among some hydrothermally altered and unaltered rocks will require other approaches, including analysis of high-spectral resolution measurements in the 2.0- and 2.5-mu m region and multispectral thermal infrared data.