20 Gemstone and Semiprecious Minerals and Host Rocks in the Western United States

Thursday, 9 May 2002
8:00 AM - 11:00 PM

TOPAZ RHYOLITE HOSTED RED BERYL IN THE WAH WAH MOUNTAINS, UTAH: A GENETIC MODEL AND MINE UPDATE

Paper number: 20-1
Program: THEME SESSIONS
Time: 10:00 AM – 10:20 AM

Authors

Abstract

Gem-quality red beryl in the Wah Wah Mountains of southwestern Utah formed as a post-magmatic mineral in a topaz rhyolite lava flow. Unlike topaz, this beryl occurs along fractures in devitrified rhyolite. The flow occupies a graben and perhaps a significant paleo-drainage. Beryl occurs at medial depths within the eroded flow. An 40Ar/39Ar age of 20.06 Ma for this flow was obtained on sanidine. Both beryl-bearing fractures and the host rhyolite show an unusual amount of argillic alteration compared with other topaz rhyolite flows. Alteration may be related to the incursion of surface water along shrinkage fractures within the flow. Whole-rock concentrations of Be (about 20 ppm) within the rhyolite are average compared with other topaz rhyolites, but CaO contents are very low (less than 0.3 percent). Low Ca activities may have prevented fluorite precipitation and allowed Be transport as fluoride complexes. Beryl probably grew at subsolidus temperatures (300 to 650 C), but above the temperature of kaolin development (200 to 300 C) as fluoride-rich devitrification vapors encountered fractures and mixed with vapors derived from surface water flowing through the fractures. Beryllium fluoride complexes reacted with water vapor, silica minerals, alkali feldspar, and Fe-Mn oxides in approximately modal proportions to produce red beryl. Continued reaction of the cooling flow with surface water at lower temperatures probably made a boiling, more acidic fluid that produced the argillic alteration. Eruption of low Ca topaz rhyolite followed by entrance of oxidized surface water into medial portions of a devitrifying, and still hot, lava flow appear to be critical factors that led to red beryl formation.

Substantial reserves have been identified at the Ruby Violet mine by both drilling and underground workings. Ownership and mineral rights to these reserves is unclear due a recent default in payment schedule to the Harris family. This situation will be explored and explained in detail.