Alkali Metasomatism and Fossil Geothermal Activity: Wah Wah Springs Tuff, Utah

Variable degrees of widespread alkali metasomatism have been detected within Wah Wah Springs Tuff samples collected in southwestern Utah. Highly altered samples show (1) strong periodic enrichment trends for alkali metals (except Na) at the expense of alkaline earth elements (except Ba), and (2) complete to partial replacement of andesine phenocrysts by K-feldspar: Mildly altered samples show (1) similar but less extreme enrichments and depletions compared to highly altered samples, and (2) no megascopic evidence of alteration. Cs is the most sensitive of the elements enriched by the alteration, followed by Rb and K20. Greater tendency toward fixation of heavy alkalis at low temperatures accompanying the addition of K-feldspar and the reduction of andesine explains. relative alteration trends. The lack of prvximal mineralization and widespread nature of alkali metasomatism associated with minor pmpylitic alteration suggests fossil geothermal activity in this part of Utah.