A Method to Delineate Hydrothermally Altered Areas by Remote Sensing

Abstract

The hydrothermally altered areas by fumarolic activity at Steamboat Springs, Nevada, USA were discriminated using the characteristics of the spectral absorption of iron from 0.45 um to 0.60 μm and OH-bond absorption around 2.2 μm. For several years, the importance of using 2.2 μm spectral characteristics for finding highly altered areas by clay minerals has been pointed out. The ratio of 1.6 μm/2.2 μm has also been used to delineate clayish altered areas. This ratio, however, is high not only at the altered areas but also at the vegetated areas, and the distinction of clayish altered areas from vegetated areas by using only this ratio is rather difficult.
Some attempts were performed to distinguish hydrothermally altered areas from unaltered areas as well as from vegetated areas by using the spectral characteristics of OH-bond absorption around 2.2 μm. From detailed analyses of remote sensing data of the Steamboat Springs area, it has become clear that the radiance difference from 1.65 μm to 2.2 μm may be an important index to discriminate hydrothermally altered areas from unaltered areas and vegetated areas.