Utah Geological Association Publication 23 155 GEOLOGICAL AND CHEMICAL CONDITIONS OF FORMATION OF RED BERYL, WAH WAH MOUNTAINS, UTAH JEFFREY D. KEITH! ERIC H. CHRISTIANSEN! AND DAVID G. TINGEY! 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- 6500C), 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 bery1 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. INTRODUCTION Utah (Christiansen and others, 1986,) and in the Black Range, New Mexico (Kimbler and Haynes, 1980), but neither of these The only known occurrence, and most-valued variety, of has produced crystals suitable for faceting. East Grants Ridge, gem-quality red beryl in the world is in the Wah Wah New Mexico and San Luis Potosi, Mexico also may have Mountains, Utah (Shigley and Foord, 1984). Red beryl at this similar occurrences of beryl, but these have not been well locality (essentially "red emerald", although the name emerald documented (Ream, 1979). Most of the marketable crystals is reserved exclusively for green stones) occurs as a vapor­ from the Wah Wah Mountains have been produced from the phase mineral in topaz rhyolite and was precipitated by Violet Mine which has operated on a limited scale since 1976, escaping gases after eruption of rhyolite lava. Only a few under the ownership of the Harris family of Delta, Utah. similar occurrences of rhyolite-hosted beryl have been Production figures for the mine from 1989-1993 are 2.3 documented worldwide. These occur in the Thomas Range, million dollars with an inventory of 1.1 million dollars of 1Department of Geology, Brigham Young University, Provo, UT 84602 156 Cenozoic Geology and Geothermal Systems of Southwestern Utah unsold gems (Rex Harris, written communication). No Weaver, 1980). Tertiary volcanism in the area began about 34 drilling or other exploration techniques have been used to Ma and consisted dominantly of large volume dacitic ash flows determine proven or probable reserves of the mine. However, and lesser volumes of low-silica rhyolite and andesitic lava in April of 1994 Kennecott Exploration leased the mine and flows. Beginning about 23 Ma the style and composition of surrounding claims to determine reserves and feasibility of gem volcanism changed to smaller-volume ash-flow and recovery by other mining and milling techniques. dome-forming eruptions of high-silica rhyolite and Previous work on the occurrence of rhyolite-hosted red trachyandesitic lava flows. This bimodal magmatism is part beryl has dealt mainly with crystallographic, chemical, and of a roughly east-west belt of Miocene igneous rocks and optical properties of the crystals (Nassau and Wood, 1968; mineral deposits stretching from Pioche, Nevada to Marysvale Flamini and others, 1983; Shigley and Foord, 1984; Utah (figure 1). It has been termed the Pioche-Marysvale Aurisicchio and others, 1990; Hosaka and others, 1993) with igneous belt (Rowley and others, 1979), the Pioche mineral only general descriptions of the geology (Ream, 1979). belt (Shawe and Stewart, 1976), the Wah Wah - Tushar belt Recently proposed models of formation for red beryl have (Hilpert and Roberts, 1964), and the Blue Ribbon lineament modified only slightly the original suggestions of Hillebrand (Rowley and others, 1978). (1905); the unusual conditions needed for formation of red The Miocene volcanic activity in the belt can be divided beryl have not been well constrained. It is generally proposed into two distinct episodes; the first episode, from 23 to 18 Ma, that beryl originates in a manner that is similar to other vapor­ formed rhyolitic tuffs and domes and trachyandesitic lava flows phase minerals; namely, by precipitation in fractures and vugs which have been designated the Blawn Formation (figure 1; from high-temperature gases as they are released from slowly­ Best and others, 1987a). A subsequent episode (12 to 13 Ma) cooled rhyolite lava. Dozens of topaz rhyolite flows and of bimodal magmatism produced the Steamboat Mountain domes, similar in composition to the beryl-bearing flow in the Formation, the rhyolitic part of which is often topaz-bearing. Wah Wah Mountains, occur across the western United States The character of the early Miocene magmatism is of (Christiansen and others, 1986). All of these flows, particular concern here, inasmuch as this episode produced the comprising a total area of 100-200 kffi2, devitrified to form beryl-bearing rhyolite flow. Abbott and others (1983) report a vapor-phase topaz and locally other vapor-phase minerals such K-Ar age of 22.1 ±0.8 Ma for the beryl-bearing flow; as bixbyite, garnet, hematite, pseudobrookite, and fluorite, but stratigraphically, it is the youngest member of the Blawn the occurrence of beryl is exceptionally rare. Even within the Formation which is present along the eastern side of the Wah rhyolite flow which hosts the red beryl, the productive open Wah Mountains. The oldest member of the Blawn Formation pits comprise only 0.02% of the surface area of the flow. in the central Wah Wah Mountains is the garnet-bearing tuff Consequently, the conditions that favor the formation of red member which vented from Pine Grove, about 18 km beryl are apparently rarely achieved -- even within the premier northwest of the beryl rhyolite (figure 1). The vent at Pine beryl-bearing flow! Clearly, if a better understanding of the Grove was subsequently filled by rhyolite domes. 40 Arf39 Ar conditions of formation could be had, then exploration for ages for the garnet-bearing tuff member and subsequent domes additional deposits and new mine reserves would be aided. range from 22.9 ± 0.3 Ma to 21.7 ± 0.3 Ma (Keith and others, This report describes the occurrence of red beryl in the 1986). A well-dated regional ash-flow tuff with an age of Wah Wah Mountains, Utah and chemical and geological 22.74 ± 0.05 Ma (Myron Best, oral communication, 1993), factors which may have favored its formation in this particular the Bauers Tuff Member of the Condor Canyon Formation, rhyolite flow over very similar topaz rhyolite flows which are also underlies the beryl rhyolite. This rhyolitic tuff was apparently barren. In addition, it examines the possibility that derived from the Caliente caldera complex 125 km to the additional reserves of gem material may exist within the flow. southwest. Three other dated occurrences of topaz rhyolite Potential exploration guides are also discussed. Topaz occur in the central Wah Wah Mountains. They are the Tetons rhyolites are also genetically related to the large emplaced at 18.3 ± 0.7 Ma (Best and others, 1987a), the Staats bertrandite/phenakite deposits of west-central Utah. pluton at 20.2 ± 0.9 (Rowley and others 1978), and the Lou Conditions which may favor the formation of beryl rather than rhyolite at 19.1 ±0.2 (Keith and others, 1986). These data bertrandite or phenakite will be discussed. suggest that the beryl rhyolite is probably the oldest topaz rhyolite flow in the Wah Wah Mountains. Blawn magmatism is characterized by initial eruptions of GEOLOGIC SETTING rhyolitic tuffs and flows which are high-silica (75-77% SiOz) but not as strongly enriched in lithophile elements as most The rhyolite flow which hosts the red beryl deposit is topaz rhyolites (Keith and others, 1986). These early Blawn located along the eastern flank of the Wah Wah Mountains, rhyolites do not exhibit conspicuous vapor-phase Utah (figure 1). Pre-volcanic rocks in the Wah Wah Mountains mineralization and can be classified as calc-alkaline rhyolites consist of Proterozoic, Paleozoic, and Mesozoic sedimentary (Christiansen and others, 1986) and may have fractionated from rocks that were folded and thrust to the east during the Sevier dacitic precursors (Keith, 1982).