K-Ar age of alunite alteration at Red Mountain, Lake City area, western San Juan Mountains, Colorado

The San Juan Mountains of southwestern Colorado are famous for rich vein deposits of base and precious metals and for patterns of conspicuous hydrothermal alteration. Areas of acid-sulfate alteration, characterized by extensive base leaching of volcanic rocks or hypabyssal plutons, are of current interest because of their local association with Au, Ag, and, in some cases, U mineralization (Lipman and others, 1976; Cunningham and others, 1978; Steven and others, 1979). Potential may also exist for recovery of aluminum from altered rocks rich in alunite (Hall, 1978). Alunite has been shown to be suitable for K-Ar dating in other mining districts (Mehnert and others, 1973b; Ashley and Silberman, 1976). We report here two concordant K-Ar ages determined on hydrothermal alunite from Red Mountain, in the Lake City area of the western San Juan Mountains.