Abstract

Several geological and chemical factors appear to have contributed to the formation of unique gem-quality red beryl in a topaz rhyolite lava flow in the Wah Wah Mountains, Utah. All beryl crystals in the flow occur exclusively along shrinkage fractures in devitrified rhyolite, rather than in lithophysal pockets, as is often the case for topaz, or in unfractured rock. The flow occupies a graben and perhaps a significant paleo-drainage. Both beryl-bearing fractures and the host rhyolite show an unusual amount of clay alteration compared to other topaz rhyolite flows in the Wah Wah Mountains and elsewhere in the western United States. Hydrothermal alteration does not appear to be associated with volcanic vents, but may be related to incursion of surface water along shrinkage fractures within the cooling flow.

Beryllium was likely transported with fluoride complexes; fluoride concentrations would remain at optimal levels for Be transport only if concentrations of Ca were low. Whole-rock concentrations of CaO within the host rhyolite are very low (<0.01% to 0.18%) compared to other topaz rhyolites while Be concentrations are about average. Beryl growth occurred at temperatures below magmatic values (~300-650°C), but above the temperature of kaolin development (200-300°C) as fluoride-rich vapors, released during devitrification, encountered fractures. If the rhyolite flow was partially bathed in surface water during cooling, a water-rich low-density fluid, or vapor, probably permeated shrinkage fractures during beryl formation. Beryllium-fluoride complexes reacted with alkali feldspar, water, and Fe-Mn oxides along fractures to produce red beryl. Continued equilibration of the flow with surface water at lower temperatures would likely produce a boiling, more acidic fluid capable of producing the kaolin lined fractures and argillic alteration which are commonly present. Consequently, eruption of low-Ca topaz rhyolite lava followed by incursion of surface water into some medial portions of the flow at high temperatures are critical factors that led to formation of beryl in this unique deposit.

No proven mine reserves exist. However, there is no evidence that most or all of the productive fractures have been found and mined. In the main pit, productive fractures occur every few meters. There is no reason to suspect that beryl-bearing fractures of equal gem-quality do not occur within a few meters or tens of meters of the current pit walls or at other locations in the flow. Probable reserves of red beryl may greatly exceed the amount that has been produced.