VAPOR PHASE ORIGIN OF SPLENDID MINERALS IN VUGS IN VOLCANIC ROCKS

Occurrences of splendid crystals in rhyolite are the red beryl in the Wah Wah Range, Utah, the garnets of Garnet Hill, Ely, Nevada, and the topaz, red beryl, hematite, garnet, pseudobrookite, and bixbyite of Topaz Mountain, Thomas Range, Utah. The Thomas Range consists of 6-7 Ma topaz rhyolite flows with a length of ~ 12 miles and width of ~ 6 miles. It is the type location for topaz rhyolites, red beryl, and several mineral species. The minerals occur in vugs and along fractures and in alteration zones. After eruption of the lava fluorine-rich vapor moved through the still cooling lava flows. The vapor dissolved Si and Al and transported Be, Mn and Fe. As this vapor was transported upward it cooled, expanded, and reacted with rhyolite (quartz and feldspar) when it entered into or created vugs in the rock. Many of the best topaz crystals have a point of attachment to the wall of the vug. The area of the crystal near the point of attachment contains included rhyolite and quartz. The best crystals become clear and almost inclusion free in sections growing into the vug.

Contrasting this is the small (~1 mile) diameter rhyolite of Garnet Hill, Nevada. It has a simple mineralogy of garnet, rare sanidine, and calcite. The garnets from this site occur in vugs and along fractures in the rhyolite, most have a broad attachment area with the rhyolite and are heavily included with rhyolite. A few are nearly perfect crystals. Many are ½ of a crystal growing on rhyolite. Some are just 1-2 complete faces with 4-5 partial faces. The Garnet Hill rhyolite is not a topaz rhyolite. Perhaps chlorine-rich vapor was a transporter of Fe, Mn, Al, and Si to the vugs and fracture zones.

Unique in the world is the red beryl deposit in the Wah Wah Mountains. The red beryl is several orders of magnitude larger than the red beryl found in the Thomas Range. At the Wah Wah site the red beryl occurs in an alteration zone that appears to be a late-stage event involving heated groundwater and alteration of the rhyolite.

High temperatures have been considered to be part of the conditions for vapor phase minerals forming at Ely and the Thomas Range. Lower temperatures may be indicated for the red beryl deposit in the Wah Wah site. Both the Thomas Range and Wah Wah Mountains are potential sites for deep exploratory drilling and coring for potential mineral deposits and pegmatites with giant splendid crystals.