BAKER, Jean M., KEITH, Jeffrey D., CHRISTIANSEN, Eric H., and DORAIS, Michael J., Department of Geology, Brigham Young Univ, Provo, UT 84602,

Red beryl in the Thomas Range of west-central Utah occurs as a vapor phase mineral in rhyolite lava flows similar to most other red beryl occurrences. In Starvation Canyon, the red beryl occurs in an evolved rhyolite; Rb (475 ppm), Nb (60 ppm), Th (75 ppm), F (0.5%), and Be (10 ppm) concentrations show the lava flow is like other topaz rhyolites across the western United States. Beryl formed in a small area in the medial part of the flow and is associated with clay-lined fractures and vapor phase minerals, including bixbyite, pseudobrookite, Mn-hematite, and topaz. The beryl is similar to gem red-beryl in the Wah Wah Mountains, Utah, except for slightly higher Mg and morphology (flat hexagonal tablets seldom exceeding 1.3 cm across). Some have theorized that this unique crystal morphology is the result of the presence of Cs or Rb in the channel sites. Our analyses have not confirmed this suggestion.

The flow contains abundant vapor-phase topaz and electron microprobe analyses show the topaz is fully fluorinated within analytical uncertainty. No evidence was found of variations in the OH/F ratio in topaz intergrown with beryl and in topaz elsewhere in the flow. The presence of hematite, bixbyite, pseudobrookite and Fe3+-rich beryl, as well as a very low magnetic susceptibility (less than 0.1 SI units) suggest the mineralized zone was highly oxidized. The mineral assemblage, including abundant clay, is convincing evidence of surface water interaction during the crystallization of the beryl. Apparently, red beryl was deposited from Be fluoride complexes in a low density fluid released by devitrification as the fluid encountered water in a fracture within the cooling lava flow. These characteristics appear to generally apply to the deposits of red beryl in the Wah Wah Mountains. However, the host rhyolite of the Wah Wah Mountains appears to be less evolved or less rich in F and incompatible trace elements.

Rocky Mountain - 54th Annual Meeting (May 79, 2002)
Session No. 20
Gemstone and Semiprecious Minerals and Host Rocks in the Western United States
Sharwan Smith Center: Starlight Room
8:00 AM-11:00 PM, Thursday, May 9, 2002

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