Localized alteration of the paintbrush nonwelded hydrologic unit within the exploratory studies facility

The mound-like feature in the lower part of the PTn, composed of highly disrupted strata and open-system alteration were the result of intense water-rock interaction. This feature was formed by fumarolic activity during cooling of the 12.8 Ma Topopah Spring Tuff and prior to deposition of the pre-Pah Canyon bedded tuff. Evidence of vapor phase crystallization, commonly observed in fumaroles, is not pervasive but may have been destroyed by subsequent low-temperature water-rock interaction. The ESF has provided a unique opportunity to view the internal morphology and to sample the compositional variability of this feature, however, outcrop and borehole studies suggest that such localized occurrences of alterative are not uncommon phenomenon within the PTn Hydrology Unit.