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Lts Ficez 5Coz7'a1ra ltS FicEz 5coZ7'a1ra he most valuable gem- Gem-quality red beryl on quality red beryl comes white devitri- from thc Wah Wah fied rhyolite Mountains of south- matrix. This western Utah (Fig. 1). sample mea- Red beryl occurs as a secondary sures 9 x 26 mm (O.35 x 1 mineral in topaz rhyolite. Market- in.l. (Photo by able crystals from the Wah Wah Sky Hall.) Mountains have been produced from the Violet Mine, operated on a limited scale since 1976 under the ownership of the Harris family of Delta, UT. From 1989 to 1995. oroduction from the mine amounted to more than $3 million with an inventory of several million dollars of unsold gems (Harris, 1995). In 7994, Kennecott Exploration leased the mine and surrounding claims to de- termine reserves and feasibility of gem recovery. Mining and explora- E.H. Christiansen and tion activity are continuing. J.D. Keith are proles- Previous work on red beryl has s0rs arld T.J. dealt mainly with crystallographic, Thompson is a student, chemical and optical properties of the crystals (Shigley and Foord, Ieparlment of Geohgy, 1984) with only general descriptions Brigham Yrung Univer- of the geology (Ream, 1979). How- sitt Brx 251 I l, Prlvn, ever. the conditions needed for for- mation of red beryl have not been UT 84fi[2 well constrained. It is generally pro- posed that beryl originates by pre- cipitation in fractures and vugs from high-temperature silica rhyolite and andesitic lava flows. gases as they are released from slowly cooled rhyolite Beginning about 23 Ma, the style and composition of lava. Dozens of topaz rhyolite flows and domes, similar volcanism changed to smaller-volume, ash-flow and in composition to the beryl-bearing flow in the Wah Wah dome-forming eruptions of high-silica rhyolite and Mountains. occur across the western United States trachyandesitic lava. These rocks form the Blawn For- (Christiansen et al., 1986). All of these flows devitrified mation of which the beryl-bearing rhyolite is one mem- to form vapor-phase topaz and locally other vapor-phase ber (Best et al., 1987). Abbott et al. (1983) reported a minerals such as bixbyite, garnet, hematite, pseudo-broo- K-Ar age of 22.1+ 0.8 Ma for this flow. It is probably the kite and fluorite. But the occurrence of beryl is rare. oldest topaz rhyolite flow in the Wah Wah Mountains. A Even within the rhyolite flow that hosts the red beryl, the subsequent episode (I2 to 13 Ma) of bimodal productive open pits comprise only a small fraction of magmatism produced the Steamboat Mountain Forma- the surface area of the flow. Apparently, the conditions tion. The rhyolitic part of that is locally topaz-bearing. that favor the formation of red beryl are rarely achieved. Displacement on the late Cenozoic extensional faults that formed the north-trending mountain ranges began Geologic setting about22Ma. The rhyolite flow that hosts the red beryl deposit lies along the eastern flank of the Wah Wah Mountains (Fig. Geology of the Violet Mine 1). Prevolcanic rocks consist of Proterozoic, Paleozoic The rhyolite that hosts the red beryl deposit is part and Mesozoic sedimentary rocks that were folded and of a flow/dome complex that overlies nonwelded tuff of thrust to the east during the Sevier orogeny. Cenozoic cogenetic origin. This rhyolite is exposed over an area of volcanism began about 34 Ma. It consisted mostly of about 9 km2 (3.5 sq miles) (Fig. 1). An unknown addi- larse volume dacitic ash flows and lesser volumes of low- tional portion of the rhyolite is concealed beneath MfNtNG ENGTNEERTNG r FEBRUARY 1997 37 Fte I TXPTANAIION Map of rhyolite lava flow Xat. liavfr o[ B{mstm the that ReswF F6 {12-l? }.1a, hosts red beryl in the Wah Wah = lo$"z ffxI red be.yl rhyoltte Mountains. The map shows the loca- I d ,lb*n tn ( ?$ ilal tion of faults, argillic alteration and Iufl mmls of Bf*n erosional lineaments. = f qwtffi (20-22 |{al ff hdela{ l4ocere Iraben r&gt}ergte lault distinctive elemental compositions gr*e aflt drp of 1stBry lE(klql detlGrts are present in the complex. Appar- ently, several magmas were at least Str&e rxl dip of tert@y compacton fo[at0n partially mixed during eruption and Irfe,red dflectron ol fh&'al emplacement. lranslnrt p(of to eml)lacement of red beryl rhyolte Within the Violet Mine, red beryl crystals occur as well-formed crystals up to several centimeters long. Euhedral beryl crystals are mostly embedded in soft clay. But they may be tightly encased by dense, devitrified rhyolite. This indi- younger mafic lavas and sediments. cates crystal growth by replacement of rhyolite rather The authors'work suggests that the red beryl flow than open-space filling. Beryl tends to form euhedral occupies a northeast-trending graben (Keith et a1.,1994). crystals by metasomatic replacement in other settings as Fluvial sediments are present beneath and above por- well (Beus, 1983). Unlike other localities, the red beryl is tions of the red beryl rhyolite. Streams may have emp- not found in lithophysal cavities. Here, beryl crystals tied into the graben. Compaction foliations of appear to occur exclusively along, or adjacent to, underlying ash flow tuff units dip toward the inferred subvertical fractures in the rhyolite. Not all fractures, graben. The graben is also parallel to the dominant ori- nor all portions of a single fracture, contain beryl. Often, entation of faults and dikes of Blawn age (Fig. 1). the productive zone of any single fracture is less than 3 to The rhyolite has a mineralogy and composition simi- 4 m (10 to 13 ft) high and 30 to 60 m (100 to 200 ft) long. lar to most other topaz rhyolites. It is enriched in incom- The fracture orientations cluster in two groups with azi- patible elements like Rb, Nb, Y and Be (about 20 ppm) muths of about 65" and 140'. Fractures are parallel and and depleted in compatible elements like Ti, Sr and Ba. perpendicular to the inferred trend of the graben. They For example, CaO concentrations (<0.01 to 0.52o/") are may have formed as the flow was channelized by the gra- low, even lower than most topaz rhyolites. Phenocrysts ben. These shrinkage cracks formed during cooling. of quartz, plagioclase, sanidine and sparse biotite are They may have served as the roots of fumaroles that al- present. Both crystal-poor (about 5% phenocrysts) and lowed collection and transport of exsolved vapors and crystal-rich (about 15% phenocrysts) rhyolite make up influent surface water. the red beryl flow. In some areas, the two kinds of lava The productive fractures contain vapor-phase miner- occur as distinct, intermixed portions of strongly flow- als such as bixbyite, tridymite, cristobalite and rare topaz. laminated or brecciated rock. Moreover. lavas with two Most have amorphous silica and clay. Fracture-hosted clays are mostly kaolin with trace amounts of smectite or a more nlixed-layer clay (about 20% illite and about 80% smectite). Both types of bulk clay contain traces of R siderite. Some red beryl crystals oc- cur along irregular planes. These appear to have been once open frac- tures that were sealed by deposition of tridymite or cristobalilte but clay is not obvious. Bixbyite and Mn-he- matite also occur along these sealed fractures. Most of the beryl-bearing frac- tures contain abundant kaolin. However, it is unlikely that beryl Rex Harris, owner of the Violet Mine, works a fracture. Red beryl occurs along iron-stained fractures that are commonly argillized. lt is embedded in devitrified rhyolite. 38 FEBRUARy 1997 x MINING ENGINEERTNG FTG.2 .;r;;;:1:ji;.';11;,. .,:.:,,,r.,1.;,-,;;r:i;tii::jl,:.-'i};,:Ii-,, '.'i.--:1, ..,+, ;,:''i:,..,.1f,.r:i.1,: ru,i.l:j:d:;e.'i Schematic of a cross section showing the sequential de- A Incursion of low-density velopment of red beryl along a shrinkage lracture within I water- rich f luid flow-foliated rhyolite. Step A At shallow depths within I Y the devitrifying rhyolite llow, exsolved- Be fluoride com- --;***-- plexes escape without reacting with feldspar to produce r F racture-peposited beryl. 0ther vapor-phase mineral may be deposited in or bi xby ite, hemati te, near the fracture at this time. ,>- $4n- ,. * topaz, +i SiO2 polymor and kaolin formed concurrently as an equilibrium as- Cl osely-spaced shr inkagej cr acks semblage. Barton (1986) noted that beryl often alters to ',! Exsolved vapors form kaolinite but they form together only at low activi- j ( ties of beryl (along with pegmatitic beryls that are alkali- Devi tr i fi ng presence rich). The of abundant kaolin along many J rhyolite fractures indicates the eventual establishment of a low { pH, water-rich environment after beryl formation. \ Some details of the mineralization sequence can be tcrr Negligible gradient in T, established using cross-cutting and inclusion relation- pH, and fH2O ships. Bixbyite and topaz were among the earliest vapor- phase minerals. They appear to have cocrystallized with K-feldspar and silica minerals during primary devitrifica- tion. Beryl is distinctly later and contains abundant in- lying mafic lavas. However, a significant amount of ero- clusions of quartz, K-feldspar, bixbyite and Mn-hematite, sion may have occur,red before the eruption of the mafic often with textures indicating replacement (Aurisicchio lavas, considering the 9 million years that elapsed. The et al., 1990). In addition, bixbyite is one of the best indi- nature of the flow foliation may also give some indica- cators of potential beryl-bearing fractures. The clays tion of depth. In and near the main pit.
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