Detection and mapping of hydrothermally altered rocks in the vicinity of the Comstock Lode, Virginia Range, Nevada, using enhanced Landsat images

Abstract

The Virginia Range, immediately southeast of Reno, Nev., consists mainly of flows, breccias, and turfs of Miocene age. Most of these volcanic rocks are of intermediate composition; rhyodacite is the most common rock type. Basalt, rhyolite and rhyolite tuff, and tuffaceous sedimentary rocks of Miocene and Pliocene age also cover substantial areas in the range. Pre-Tertiary metasedimentary, metavolcanic, and granitic rocks are exposed in scattered inliers, mostly along the southern and eastern margins of the range. Several large areas and many small areas within the volcanic pile were subjected to hydrothermal alteration during and after the period of intermediate volcanic activity. Economic precious metal mineralization is spatially and temporally associated with the hydrothermal alteration in several areas. The most important deposit is the Comstock Lode, which produced 192 million troy ounces of silver and 8.3 million troy ounces of gold from epithermal veins (Bonham, 1969). The hydrothermally altered rocks include silicified, advanced argillic, montmorillonite-bearing argillic, and propylitic types. The first three types typically contain pyrite, and some propylitic rocks contain pyrite as well. Supergene oxidation of these pyritic rocks produces limonitic bleached rocks. The term 'limonite,' as used here, refers to any combination of the minerals hematite, goethite, and Jarosite. Where vegetation cover is sparse to moderate, these limonitic rocks are readily identified on Landsat images enhanced by the color-ratio composite technique developed by Rowan and others (1974), so the altered areas can be mapped. About 30 percent tree cover (here mainly pinyon pine) is sufficient to change the spectral signature of individual picture elements (pixels) enough so that limonitic materials can no longer be uniquely identified. As in all other areas where this technique has been applied, limonitic unaltered rocks with intermediate to high albedos have the same appearance on the color-ratio composite as limonitic altered rocks. This problem represents the most important limitation to the use of enhanced Landsat images for detection and mapping of hydrothermally altered rocks. Reflectance spectra of altered and unaltered rocks taken in the field in the Virginia Range show that most altered rocks have a conspicuous absorption band near 2.2 ?m produced by clay minerals or alunite, whereas unaltered rocks have no features in this spectral region. Thus spectral information for selected bands in the 1.1-2.5 ?m region may allow discrimination between limonitic altered and limonitic unaltered rocks (Rowan and others, 1977; Abrams and others, 1977; Rowan and Abrams, 1978). Another potential limitation is loss of spectral information on slopes with low effective sun angle. Although a minor problem in the Virginia Range, loss of information sufficient to preclude identification of limonitic altered rocks occurs with effective sun angle lower than 20-25 degrees. Thus, even at moderate latitudes substantial parts of areas with high topographic relief may be lost to observation.