The Geology and Geochemistry of Cenozoic Topaz Rhyolites from the Western United States

Eric H. Christiansen Department of Geology University of Iowa Iowa City, Iowa 52242

Michael F. Sheridan Donald M. Burt Arizona State University Tempe, Arizona 85287

SFEE'It':' FAFE., 205 © 1986 The Geological Society of America, Inc. All rights reserved.

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Christiansen, Eric H The geology and geochemistry of Cenozoic topaz rhyolites from the western United States.

(Special paper; 205) Bibliography: p. 1. Rhyolite-West (U.S.) 2. Topaz. 3. Ore­ deposits-West (U.S.) 4. Geology, Stratigraphic­ Crenozoic. 5. Geology-West (U.S.) I. Sheridan, Michael F. II. Burt, Donald M., 1943- . m. Title. IV. Series: Special paper (Geological Society of America); 205. QE462.R4C48 1986 552'.2 86-273 ISBN 0-8137-2205-5 Contents

Acknowledgments v

Abstract I

Introduction ;...... 3

Cenozoic topaz rhyoUtes from the western United States 3 1. Thomas Range, west-central Utah 3 2. Spor Mountain, west-central Utah 10 3. Honeycomb Hills, west-central Utah ...... •...... 13 4. Smelter Knolls, west-central Utah 14 5. Keg Mountain, west-central Utah 15 6. Mineral Mountains, western Utah 15 7. Wah Wah Mountains and vicinity, southwestern Utah and southeastern Nevada 17 8. Wilson Creek Range, southeastern Nevada 19 9. Kane Springs Wash, southeastern Nevada...... 19 Topaz rhyolites in the eastern Great Basin: A summary 21 10. Cortez Mountains, north-central Nevada 21 11. Sheep Creek Range, north-central Nevada 23 12. Jarbidge, northern Nevada '" 24 13. Blackfoot lava field, southeastern Idaho 25 14. Elkhorn Mountains, western Montana 26 15. Little Belt Mountains, central Montana 27 16. Specimen Mountain, north-central Colorado 29 17. Chalk Mountain, central Colorado 30 18. Nathrop, central Colorado 31 19. Silver Cliff-Rosita, central Colorado 32 20. Tomichi Dome, central Colorado 34 21. Boston Peak, central Colorado...... 35 22. Lake City, southwestern Colorado 36 Topaz rhyolites in Colorado: A summary 37 23. East Grants Ridge, west-central New Mexico ....•...... 37 24. Black Range, southwestern New Mexico 39 25. Saddle Mountain, eastern Arizona 41 26. Burro Creek, western Arizona 41

Other "topaz rhyolite" occurrences 42 Other Cenozoic occurrences, western United States 42 Mexican topaz rhyolites '...... 42 Precambrian topaz rhyolites -...... 42

iii iv Contents

Principal characteristics of topaz rhyolites 43 Distribution and ages ...... •...... 43 Mode of emplacement 44 Mineralogy 46 Fe-Ti oxides and titanite 46 Feldspar 47 Mafic silicates 48 Geochemistry and differentiation trends 50 Isotopic composition 59 Magma-tectonic setting 59 Ore deposits 61 Beryllium 61 Climax-type molybdenum deposits 62 Tin ' 63 Uranium 64 Fluorite 64

Comparison with other types ofrhyolitic rocks 64 Calc-alkaline rhyolites 64 Peralkaline rhyolites 66 Aluminous bimodal rhyolites 67 Ongonites 67

Petrogenetic modelfor topaz rhyolites 69

References cited 74 Acknowledgments

. This work was partially supported by U.S. DOE Subcontract #79-270-E from Bendix Field Engineering Corporation. Additional support was provided by Arizona· State Univer­ sity, the University of Iowa, the U.S. Geological Survey, and the National Aeronautics and Space Administration (grant NAGW-537). A large number of people have helped with the new analytical work presented in this report. They include D. McRoberts, M. Druecker, J. Edie, J. V. Bikun, B. Correa, K. Evans, A. Yates, R. Satkin, K. Hon, D. Lambert, C. E. Hedge, K. Futa, A. Bartel, D. R. Shawe, J. S. Stuckless, L. Jones, R. T. Wilson, W. Rehrig, G. Goles, and G. Pine. The technical reviews by W. Nash and W. Hildreth, and editorial assistance of C. Craddock and L. Gregonis are greatly appreciated. We are also indebted to the authors of many of the articles cited herein for helpful discussions and for recording the presence of topaz in the rhyolites they have studied.

v

Geological Society of America Special Paper 205 1986

The Geology and Geochemistry ofCenozoic Topaz Rhyolites from the Western United States

ABSTRACT

High-silica, topaz-bearing rhyolites of Cenozoic age are widely distributed across the western United States and Mexico. Topaz rhyolites are characteristically enriched in fluorine (>0.2 wt%) and contain topaz crystallized during post-magmatic vapor-phase alteration. In the United States, their ages span much of the Cenozoic Era (50 to 0.06 Ma). Their emplacement followed or was contemporaneous with calc-alkaline and ba­ saltic magmatism in the Basin and Range province, along the Rio Grande rift, and in Montana, and coincided with episodes of extensional tectonism in these regions. Nearly all topaz rhyolites extruded as small, endogenous lava domes with or with­ out lava flows; no topaz-bearing ash-flow tuffs have yet been identified with certainty in the western United States. Most domes are underlain by a precursory blanket of non­ welded tephra. A few are small, shallowly emplaced intrusive plugs. Volumes of rock «1 to 100 km3) in individual complexes composed of 1 to many separate extrusions suggest that the lavas were erupted from small to medium sized magma bodies. In addition to topaz, these rhyolites also contain garnet, bixbyite, pseudobrookite, hematite, and fluorite in cavities or in their devitrified groundmasses. All ofthese phases may form during vapor-phase crystallization. Magmatic phenocrysts include sanidine (ca. Orso), quartz, sodic plagioclase (usually oligoclase), and F- and Fe-rich biotite in order of usual abundance. Fe-rich hornblende or clinopyroxene occur in a few lavas. Common magmatic accessory minerals include magnetite, ilmenite, zircon, apatite, allan­ ite, and fluorite. Titanite and REE-rich phosphates have been identified in a few lavas. The rhyolites crystallized over a wide temperature interval (850 to 600°C, with most at the lower end of this range) and at variable oxygen fugacities. Titanite-bearing lavas crystallized above the NNO buffer under oxidizing conditions. Most others appear to have crystallized near the QFM oxygen buffer. For individual complexes, temperatures correlate negatively with F-content. All topaz rhyolites are high-SiOz rhyolites with elevated F, Na, K, Fe/Mg and low Ti, Mg, Ca, and P. Samples with F concentrations of about 1% have notably lower Si and higher AI and Na than other topaz rhyolite glasses. Most glasses from topaz rhyo­ lites are metaluminous, but many appear to be slightly peraluminous. Fluorine concen­ trations in glasses range from slightly less than 0.2 to more than 1.0 wt%, and F/ Cl ratios are high (3 to 10) compared to F-rich peralkaline glasses «3). Topaz rhyolites are characteristically enriched in incompatible lithophile elements (Rb, U, Th, Ta, Nb, Y, Be, Li, and Cs). Elements compatible in feldspars (Sr, Eu, Ba), ferromagnesian minerals (Ti, . Co, Ni, Cr), and zircon (Zr, Hi) are depleted. The REE patterns ofmost topaz rhyolites are almost flat (La/YbN = 1 to 3) and have pronounced negative Eu anomalies (Eu/Eu* = 0.01 to 0.02). Both of these parameters decrease with differentiation as indicated by increasing F, U, Cs, and other incompatible elements. Titanite-bearing rhyolites have prominent middle REE depletions. Initial Sr-isotope ratios range from 0.705 to over 0.710. Geochemical trends at individual complexes are interpreted as arising from frac­ tional crystallization of an initially more "mafic" rhyolite with about 0.2% fluorine. Extensive fractionation of sanidine, quartz, plagioclase, biotite, and Fe-Ti oxides (in

1 2 Christiansen, Sheridan, and Burt

proportions consistent with their modes) produced much ofthe trace element patterns. Zircon, apatite, and a REE-rich phase (allanite, monazite, or titanite) were minor but important fractionating phases. No liquid-state fractionation is required to explain the geochemical trends. The high F content and FICI ratios oftopaz rhyolite melts may have modified phase relations so as to produce Na and AI enrichments for highly evolved magmas. Topaz rhyolites are intimately related to economic deposits of lithophile elements (i.e. Be, U, F, Li, and Sn). The volcanic rocks were probably ore- and, in some cases, fluid-sources for these mineral deposits. In their age, tectonic setting, mineralogy, chem­ istry, and style ofemplacement, topaz rhyolites bear resemblance to the rhyolitic stocks associated with Climax-type Mo deposits, and some may be surface manifestations of such deposits. In their chemical composition and mineralogy, topaz rhyolites are distinct from both peralkaline rhyolites and calc-alkaline rhyolites with which they may be spatially and temporally associated. Some of the compositional differences between topaz rhyolites and peralkaline rhyolites may be attributed to the relative effects of F and CI, on melt structure and phase relationships in their parental magmas. The F/CI ratios ofthe melt or its source may determine the alumina saturation ofthe magma series. Topaz rhyolites are distinguishable from calc-alkaline rhyolites by lower Sr, Ba, and Eu, and higher F, Rb, U; and Th. The usually low La/Yb ratios of topaz rhyolites distinguish them from both peralkaline and calc-alkaline rhyolite suites. Topaz rhyolites are similar to other aluminous rhyolites erupted in bimodal associations with basalt in the western United States. They may be the equivalent ofthe topaz-bearing ongonites of central Asia. Topaz rhyolites from the western United States are not the eruptive equivalents of S-type granites; we liken them to the highly evolved, non~peralkaline,and F-rich anoro­ genic grnnites. Topaz rhyolites appear to have evolved from partial melts of a residual felsic granulite source in the lower or middle crust of the Precambrian continent. Ac­ cording to the proposed model, the passage ofcontemporaneous mafic magmas through the crust produced necessarily small volumes ofpartial melts as a result ofthe decompo­ sition of small amounts of F.;rich biotite that persisted in a high-grade metamorphic protolith. An extensional tectonic setting allowed these small batches of magma to rise without substantial mixing with contemporaneous mafic magmas. Subsequent fractiona­ tion led to their extreme trace element characteristics. Topaz Rhyolites 3

INTRODUCTION rhyolites; 2) their petrography and mineralogy; 3) the major­ element, trace-element, and isotopic composition of the lavas; 4) For decades petrologists have been concerned with the role the nature of ore deposits associated with them; and 5) where of volatiles (principally HzO, cOz, sOz, HzS, BZ03, HCI, and possible, the volcano-tectonic setting as revealed by contempo­ HF) in the genesis and evolution of igneous rocks. Even in fluid­ raneous magmatism and tectonism. undersaturated magmas, volatiles playa key role in determining Many of the data on mineralogy, elemental and isotopic the physical properties, crystallization histories, and emplacement composition, and mineralization are summarized in the figures mechanisms of magmas. Studies of their role can be pursued and tables in the last part of the report. The reader is referred to through theoretical, experimental, and analytical methods. Rhyo­ these summaries in the descriptions of each occurrence. Volcanic . lites that contain topaz (AlzSi04FZ) appear to form a distinctive rock classification in this report follows that of the lUGS and is group of silicic lavas with high fluorine concentrations. The oc­ based on KzO plus NazO and SiOz concentrations (TAS dia­ currence of fluorine-rich volcanic rocks provides the opportunity gram; LeMaitre 1984). Where informative, in parentheses we to examine the effect of fluorine on the mineralogy, geochemical have also included the original rock name used by the authors. evolution, and physical nature of natural rhyolitic magmas. The intent of this report is to document these geologic and petrologic 1. Thomas Range, west-central Utah characteristics as a basis for ongoing efforts to determine the origin and evolution of fluorine-rich silicic magmas (e.g., Christ­ The best-known topaz rhyolites are those from the Thomas iansen et al. 1983a; Ruiz et al. 1985; Kovalenko and Kovalenko Range in west-central Utah (Figure 2). The occurrence of topaz 1984; Pichavant and Manning 1984; Dingwell et al. 1985) and to in rhyolitic lavas from the Thomas Range has been known for determine the nature of the ore deposits associated with them more than a century (Simpson 1876: 325-326). Because of the (e.g., Burt et al. 1982; Burt and Sheridan 1981). occurrence of topaz in the lavas and the presence of U, Be, and F The occurrence oftopaz lining vugs and cavities in rhyolitic deposits in the vicinity, these rhyolites have received considerable lavas from Colorado and Utah was first reported in the nine­ attention in the literature. The most recent comprehensive study teenth century (Smith 1883; Simpson 1876). More recent investi­ of the area is that by Lindsey (1979, 1982). Turley and Nash gations, summarized here, have shown that topaz-bearing lavas (1980), Bikun (1980), and Christiansen et al. (1984) have exam­ are widespread in the western United States and that they contain ined the petrology of the lavas. other minerals uncommon in silicic volcanic rocks (e.g., beryl, The Thomas Range consists of a group of coalesced lava gamet, pseudobrookite, and bixbyite) that reflect the unique flows and domes that were erupted from at least 12 separate vents chemistry and origin of these rhyolites. The lavas also contain 6 to 7 Ma (Lindsey 1979). Eruptive episodes, as described by unusually high concentrations of incompatible lithophile ele­ Bikun (1980), commenced with the emplacement of a series of m.ents (e.g., Be, Li, U, Th, Sn, Ta, Rb) and fluorine. Information pyroclastic flows, minor air-fall sheets, and pyroclastic surge about these distinctive rocks is scattered in the literature on the units, and were terminated by the effusion of rhyolite lavas. geology of the western United States. Welded ash-flows occur within the tuffs, but more commonly the During a study of uranium mineralization associated with ignimbrite units are thin (3 to 4 m) and unwelded. Fused tuffs fluorine-rich volcanic rocks (Burt et al. 1980), it became obvious (Christiansen and Lipman 1966) 1 to 2 m thick occur in the that topaz rhyolites are surprisingly similar to one another in their tephra immediately below some lava flows. Flow breccias, con­ mode of emplacement, mineralogy, major and trace element sisting mostly of vitrophyre blocks up to 2 m in diameter, are chemistry, and tectonic setting. These features are summarized usually found at the base of the lavas. The breccia grades upward here. into flow-banded rhyolite, commonly with numerous lithophy­ sae. The volume of rhyolitic eruptives in the Thomas Range is CENOZOIC TOPAZ RHYOLITES FROM THE about 50 km3. WESTERN UNITED STATES Rhyolites from the Thomas Range (the Topaz Mountain The distribution of Cenozoic topaz rhyolites in the western Rhyolite; Lindsey 1982) contain up to 20% phenocrysts, but most United States is shown in Figure 1 where the occurrences are samples are crystal-poor felsites or obsidians. (The mineralogy of numbered in their order of discussion (generally clockwise the rhyolites is summarized in Tables 8 and 9). Sanidine (Or4s to around the Colorado Plateau, starting in west-central Utah). We Or6S), quartz and plagioclase (AnlO to Anzs) occur in almost all have visited most of the localities described in this report (all samples. Biotite of variable Fe/Mg occurs in most (Figures 31 Utah occurrences; Sheep Creek Range, Jarbidge, and Kane and 32), whereas spessartine-almandine gamet, ferro-augite, and Springs Wash, Nevada; Burro Creek, Arizona; both New Mexico Fe-rich hornblende occur as magmatic minerals in a few samples. occurrences; Nathrop, Chalk Mountain, and Tomichi Dome, Accessory minerals include zircon, fluorite, allanite, Fe-Ti oxides, Colorado; Blackfoot lava field, Idaho; and the Elkhorn Moun­ and, in at least one instance, fluorine-bearing titanite (Turley and tains, Montana). Complete results of our new findings are pre­ Nash 1980; Christiansen et al. 1984). Fe-Ti oxide and two­ sented here. For each locality, we have summarized pertinent feldspar geothermometry indicate that the Topaz Mountain information about 1) the geologic setting and emplacement ofthe Rhyolite crystallized at temperatures between 630 and 790°C at 4 Christiansen, Sheridan, and Burt

r-·-· .. ~--._.-.-. \ ----.--­. \ \ \ 15 . \ ~ '. - ~ ...... -'' ..... ' ", ..... 14 • 0" , , . °t ••••••••: ',:- I : I o '0.······ . ••.....•. 0 : I 12 " .. "" d' .. , ....•. o 11 I ! : , : ", i~ .. I .1 , 1 16 .I 10 3. ~. 5 c"\: ,' . r. ·17 : 2 4 21 . \ '\ I/ 18 \ ''\. J 8' t '.. ·6 20 " ".,..~~./ ,'. . 19 " . 22 ., . \ ...... , .. , . .. , ...... \ , \ ..... ~" o ) .: ·23 " ~,..... ­ 26

o 24 ·25 : "-. ,: .. .. : """ _._._.J.- .-:"'...... 0 \ 0 '0 '...... 0

Figure 1. Locations of known Cenozoic topaz rhyolites in the western United States, The numbers refer to the localities listed below and described in the text. Open circles without numbers show locations of some of the peralkaline rhyolites that are approximately contempciraneons with the topaz rhyolites (Noble and Parker 1974). Also shown are several approximations of the western edge of the Precam­ brian craton in the western United States. The solid line represents the outcrop limit of Precambrian rocks (King 1977), the dashed line represents the edge of the craton inferred from Sr-isotope composi­ tion ofMesozoic granitoids (Kistler et al. 1981; Armstrong et al. 1977), and the dash-dot line as inferred by Nd-isotope composition of Mesozoic and Cenozoic granitoids (epsilon Nd = -7; Farmer and DePaolo 1983, 1984). 1. Thomas Range, Utah 10. Cortez, Nevada 20. Tomichi Dome, Colorado 2. Spor Mountain, Utah 11. Sheep Creek Range, Nevada 21. Boston Peak, Colorado 3. Honeycomb Hills, Utah 12. Jarbidge, Nevada 22. Lake City, Colorado 4. Smelter Knolls, Utah 13. Blackfoot lava field, Idaho 23. Grants Ridge, New Mexico 5. Keg Mountains, Utah 14. Elkhorn Mtns, Montana 24. Black Range, New Mexico 6, MineralMountains, Utah 15. Little Belt Mtns, Montana 25. Saddle Mountain, Arizona 7. Wah Wah Mountains, Utah 16. Specimen Mtn, Colorado 26. Burro Creek, Arizona 8. Wilson Creek Range, Nevada 17. Chalk Mountain, Colorado 9. Kane Springs Wash, Nevada 18. Nathrop, Colorado 19. Silver Cliff, Colorado Topaz Rhyolites 5

LEGEND D Quaternary alluvium Topaz Mtn Rhyolite (6 Ma) Spor Mtn Formation (21 Ma) G Older volcanic rocks (30-42 Ma) ~ Sedimentary rocks

Location in Utah

o 3km IIII Scale

Figure 2. Generalized geologic map of the southern part of the Thomas Range, Utah (after Lindsey 1979; Christiansen et al. 1984a). Both the Spor Mountain Formation and the Topaz Mountain Rhyolite contain topaz in rhyolitic lavas. Numbers indicate samples analyzed by Christiansen et aI. (1984). ! -I

fairly low oxygen fugacities (QFM; Figure 30; Turley and Nash radiogenic, which is consistent with a crustal origin for the paren­ 1980; Christiansen et al. 1980). Topaz occurs in lithophysal cavi­ tal magmas. ties and in the devitrified groundmass of many lava flows. No Christiansen et al. (1984) presented a quantitative model for magmatic topaz (e.g., in glass) has been identified. Other vapor­ the geochemical evolution of these lavas based on the fractiona­ phase minerals in lavas from the Thomas Range include quartz, tion of observed phenocrysts from rhyolitic magmas. Major and alkali feldspar, beryl, bixbyite, pseudobrookite, hematite, spessar­ trace element geochemistry demand an interpretation that in­ tine garnet, and cassiterite. volves about 70% crystallization of the most mafic rhyolite ana­ The average compositions of samples from the Thomas lyzed to produce the most evolved rhyolite, even though the Si02 Range are given in Table 1. The compositions of felsites and content increases by only 2.5% across the series. Both major­ vitrophyres are similar but felsites have higher KINa ratios than element mass-balance calculations and Rayleigh fractionation their corresponding vitrophyres. The analyses show high Si, K, models, using the distribution coefficients of Hildreth (1977) and and Na and low Ti, Mg, Ca, and P typical. of topaz rhyolites. Crecraft et al. (1981), suggested that fractionation of sanidine (45 Fluorine ranges from 0.2 to 0.5% in vitrophyres. Most of the lavas to 50%), quartz (30%), plagioclase (15 to 20%), biotite (3%), and are diopside-normative if calculated on a fluorine-free basis. The Fe-Ti oxides (1 %) were the principal fractionating phases. In trace element geochemistry of the lavas is typical of topaz rhyo­ addition, the observed changes in trace elements (La, Hf, Zr, and lites with generally high and covarying concentrations of U:, Th, Lu) led to estimates of 0.04% each of allanite and zircon in the Rb, Li, Be, and Ta (Table 2 and Figure 3). The rare earth element removed mineral phases. The observed P depletion implies that (REB) distributions in the vitrophyres are similar to other topaz 0.06% of the cumulate mineral assemblage was apatite. Minor rhyolites with relatively large negative Eu-anomalies and heavy discrepancies for Y, Nb, Ta, and Th could be explained by the RBB (HRBB) enrichments that are correlated with F content and fractionation of extremely small quantities of REB-rich phos­ other chemical indexes of differentiation (REB patterns are illus­ phates (not yet observed in the vitrophyres) and titanite. trated in Figure 40a). Light REE (LREE) abundances decline Crystallization near the minimum in the simple ternary gran­ with increasing evolution. Sr-isotope ratios (0.707 to 0.712; ite system should produce differentiates whose major element Table 3) show that the Thomas Range lavas are moderately chemistry is not dramatically different from their parent magmas. 6 Christiansen, Sheridan, and Burt

TABLE 1. MAJOR ELEMENT COMPOSITION OF TOPAZ RHYOLITES FROM THE WESTERN UNITED STATES (IN WEIGHT %)

Honeycomb Thomas Range Spor Mountain Hills 1 2 3 4 5 6 7 8 ave. S.D. ave. S.D. ave. S.D. ave. S.D. ave. S.D. ave.

Si02 75.9 0.33 76.28 1.14 76.6 0.22 76.46 1. 03 74.2 0.82 73.66 63.6 75.0 Ti02 0.10 0.03 0.17 0.04 0.10 0.01 0.13 0.04 0.05 0.01 0.03 tr. 0.04 A1203 12.7 0.17 12.42 0.21 12.4 0.21 12.53 1. 09 13.5 0.48 14.34 11.1 13.6 Fe203 1. 07* 0.19 0.47 0.33 0.82 0.09 0.91 0.37 1. 29* 0.24 0.34 0.25 0.98* FeO ------0.46 0.28 0.29 0.07 0.24 0.12 ------1. 90 0.43 ---- MnO 0.06 0.01 0.04 0.00 0.05 0.01 0.04 0.01 0.06 0.02 0.07 0.03 0.06 MgO 0.14 0.07 0.08 0.00 0.16 0.08 0.18 0.11 0.11 0.06 0.13 0.05 0.07 CaO 0.80 0.42 0.77 0.07 0.85 0.08 0.96 0.35 0.61 0.10 0.34 11.1 0.62 Na20 3.78 0.27 3.48 0.19 3.33 0.19 3.34 0.41 3.95 0.56 3.86 3.64 4.60 K20 4.92 0.27 4.95 0.49 5.10 0.11 4.91 0.38 4.86 0.52 4.76 4.00 4.46 P205 0.00 0.00 0.00 0.00 0.01 0.01 0.02 0.02 0.00 0.00 0.03 0.03 0.00 F 0.28 0.08 0.21 0.04 0.29 ---- 0.29 ---- 1.14 0.35 0.77 8.00 0.95 Cl ------0.06 0.01 ------0.14 ------0.01 0.07

1. Average of 11 rhyolites (Christiansen et al. 1984). 6. Rhyolitic lava (Staatz and Carr 1964). 2. Average of 4 rhyolite lavas (Turley and Nash 1980). 7. Low-silica phase of Honeycomb Hills rhyolite 3. Average of 3 analyses representing 5 rhyolites (Turley and Nash 1980). (Shawe 1966). 8. Average of 2 rhyolite lavas (Christiansen et 4. Average of 7 rhyolite lavas (Staatz and Carr 1964). al 1980). 5. Average of 11 rhyolites (Christiansen et al. 1984).

TABLE 1. (CONTINUED)

Wilson Kane Sheep Smelter Mineral Creek Springs Creek Knolls Mountains Wah Wah vicinity Range Wash Range Jarbidge 9 10 11 12 13 14 15 16 ave. S.D. ave. S.D. ave. S.D. ave. S.D. ave. S.D.

Si02 75.84 loll 76.5 0.29 76.1 1. 05 76.2 0.90 75.4 0.50 76.7 77.6 75.3 Ti02 0.04 0.01 0.08 0.02 0.07 0.04 0.08 0.02 0.04 0.02 <0.2 0.12 0.16 A1203 12.56 0.39 12.7 0.11 12.7 0.29 12.3 0.70 13.2 0.22 13.2 12.5 12.9 Fe203 0.12 0.09 0.35 0.16 1.13* 0.19 1.16* 0.27 1.28* 0.19 0.89* 1.56* 1. 60* FeO 0.99 0.09 0.28 0.11 ------MnO 0.04 0.01 0.08 0.02 0.08 0.03 0.09 0.02 0.04 0.01 ---- 0.04 0.02 MgO 0.08 0.04 0.16 0.12 0.10 0.04 0.09 0.06 0.04 0.02 0.12 0.09 0.20 CaO 0.96 0.61 0.45 0.04 0.52 0.18 0.74 0.35 0.43 0.15 0.42 0.52 0.34 Na20 3.79 0.20 4.30 0.16 3.90 0.56 3.76 0.25 4.75 0.12 3.84 3.00 4.32 K20 4.77 0.-08 4.77 0.14 4.83 0.26 4.53 0.60 4.70 0.15 4.60 5.20 5.44 P205 0.00 0.00 0.02 0.01 ------<0.05 0.02 ---- F 0.72 0.07 0.41 ---- 0.32 ---- 0.42 ------0.49 0.28 ---- Cl 0.10 0.03 ------0.12 ------0.05 ------

9. Average of 4 rhyolites (Turley and Nash 1980). 13. Average of 5 rhyolite lavas (Barrott 1984; 10. Average of 5 rhyolites from domes (Evans and written communication 1985). Nash 1978). 14. Kane Spring Wash topaz rhyolite (Novak 1984). 11. Average of 7 early Miocene rhyolites 15. Rhyolite lava (Christiansen et al. 1980). (Christiansen 1980; Best et al. 1981). 16. Rhyolite lava (Christiansen, unpublished 12. Average of 8 Pliocene rhyolites (Christiansen analysis.) 1980; Best et al. 1981). Topaz Rhyolites 7

TABLE 1. (CONTINUED)

Elkhorn Specimen Chalk China Cap Mountains Little Belt Mtns. Mountain Mountain Nathrop 17 18 19 20 21 22 23 24 25 26 27 ave. S.D. ave. S.D. ave. S.D.

Si02 76.4 0.53 77.3 1.4 74.7 76.2 76.51 77.0 0.81 74.94 75.3 75.8 76.6 77.5 Ti02 0.12 0.016 0.07 0.0 0.08 0.02 0.03 0.06 0.11 ---- 0.09 0.08 0.08 0.07 A1203 12.9 0.42 13.63 1.2 14.5 13.7 13.81 12.6 0.55 14.82 13.1' 12.7 12.9 12.5

Fe203 0.46 0.133 1. 00 0.45 0.51 0.23 0.43* 0.97 0.38 0.56* 0.64* 0.76* 0.40 0.35 FeO 0.42 0.125 0.49 0.16 0.27 0.20 ---- 0.30 0.10 ------0.23 0.25 MnO 0.06 0.003 0.14 0.19 0.25 0.25 ------0.18 0.10 0.06 0.01 0.07

MgO 0.2 ---- 0.03 0.04 0.06 0.10 0.03 0.05 0.03 0.37 0.22 0.05 0.05 0.04 CaO 0.52 0.048 0.34 0.29 loll 0.50 0.29 0.42 0.37 0.84 0.61 0.41 0.43 0.43 Na20 4.21 0.118 3.59 0.42 3.50 4.55 4.61 4.04 0.42 4.00 4.26 4.35 4.20 4.5

K20 4.50 0.105 4.70 0.66 5.00 4.24 4.14 4.55 0.35 4.56 4.97 4.54 4.70 4.4 P205 0.01 ---- 0.01 0.01 0.03 0.00 0.01 ------0.01 0.01 0.01 0.00 0.00 F 0.45 0.073 ------0.34 ------0.55 ---- 0.21 ---- Cl 0.04 0.001 ------

17. Average of 6 analyses (Dayvault et al 1984). 23. "Effusive" rhyolite (Cross 1886). 18. Average of 3 rhyolite lavas (Smedes 1966). 24. Rhyolite vitrophyre (Christiansen et al. 1980). 19. Rhyolite sill at Yogo Peak (Pirsson 1900). 25. Devitrified rhyolite (Christiansen et al. 1980). 20. Rhyolite stock at Granite Mountain (Witkind 1973). 26. Average of 2 vitrophyres (Van Alstine 1969). 21. Rhyolite stock at Granite Mountain (Rupp 1980). 27. Devitrified groundmass of rhyolite (Carmichael 22. Average of 4 rhyolitic lavas (Wahlstrom 1944). 1963) .

TABLE 1. (CONTINUED)

Tomichi Boston Lake Grants Black Silver Cliff Dome Peak City Ridge Range 28 29 30 31 32 33 34 35 ave. S.D. ave. S.D. ave. S.D. ave. S.D. ave. S.D.

Si02 77.0 75.7 75.9 0.37 75.6 0.32 76.2 0.13 76.2 1.22 74.7 77.7 0.88 Ti02 0.06 ---- 0.08 0.05 0.08 0.006 0.07 0.01 0.19 0.09 ' 0.07 0.18 0.02 A1203 13.0 13.9 13.6 0.35 13.6 0.25 13.4 0.22 13.8 0.72 13.7 12.0 0.81

Fe203 0.78 0.70 0.44 0.27 1. 61 0.26 1.00* 0.08 1. 29* 0.43 0.87 1.18* 0.07 FeO 0.15 0.30 0.27 0.06 0.14 0.04 ------0.48 ------MnO 0.14 0.17 0.20 0.04 ------0.12 0.03 0.09 0.02 0.06 0.06 0.01

MgO 0.i4 0.15 0.10 0.07 0.13 0.09 0.06 0.04 ------0.37 0.07 0.06 CaO 0.22 0.78 0.65 0.12 0.34 0.05 0.38 0.10 0.78 0.60 0.30 0.53 0.19 Na20 3.96 3.99 3.45 0.51 4.05 0.39 4.15 0.26 2.34 0.99 4.96 3.30 0.17

K20 4.10 4.30 5.14 0.69 4.38 0.04 4.53 0.10 4.85 0.25 4.50 4.67 0.07 P205 0.02 ---- 0.04 0.02 0.06 0.06 0.02 0.005 0.05 0.03 0.01 0.03 0.01 F 0.18 ---- 0.14 0.06 0.14 0.05 0.52 ---- 0.10 0.05 ---- 0.38 ---- Cl ------,

Burro Topaz 28. Average of 2 rhyolites (Phair and Jenkins 1975) • Creek Rhyoli te 29. "Pitchstone" (Cross 1896) • 36 37 30. Average of 5 rhyolite glasses (Mutshler et a1. 1985) • ave. S:D. 31. Average of 3 rhyolites (Ernst 1980) • 32. Average of 6 rhyolites (Ernst 1980) • 33. Average of 21 rhyolites (Ernst 1981) • 34. Average of rhyolite pumice and glassy lava and (Baker Si02 75.6 0.42 76.0 and Ridley 1970) • Ti02 0.04 0.01 0.06 35. Average of 3 rhyolitic lavas (Correa 1980) • A1203 12.7 0.16 13.0 36. Average of 9 rhyolite vitrophyres (Moyer 1982) • 37. Modal values of histograms in Figure 35. Fe203 0.79* 0.12 1. 0* FeO ------Note: All analyses recalculated H20 and CO2 free. Fluorine and MnO 0.09 0.02 0.06 chlorine concentrations only reported for vitrophyres.

MgO 0.09 0.06 0.06 * FeTotal reported as Fe203. CaO 0.71 0.12 0.60 Na20 4.25 0.32 4.00 ---- Not reported.

K20 4.47 0.37 4.80 S.D. - 1 standard deviation reported for averages of more than P205 0.01 0.01 0.01 two samples. F 0.18 0.03 0.30 Cl 0.04 0.01 ---- 8 Christiansen, Sheridan, and Burt

TABLE 2. TRACE ELEMENT COMPOSITION (IN PPM) OF TOPAZ RHYOLITES FROM THE WESTERN UNITED STATES

Wilson Thomas Spor Honeycomb Smelter Mineral Wah Wah Creek Range Mountain Hills Knolls Mtns Mountains Range Cortez 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Li 50 37 80 336 201 134 590 245 115 65 Rb 423 369 1010 602 1051 1400 1025 441 333 634 564 617 665 Cs 11.3 56 24.4 19.6 21.3 50 21

Be 6.5 10 63 16* 11 13 6 28 13 9 Sr 28 22 6 27* 200 15 12 30 5 nd 11 71* 14 32 Ba 41 63* nd nd 2 nd 156 19 Cr 2.0 3.4 3.2 2.6 3.2 1.7 13 Co nd 0.4 0.3 11.1 Cu Ga 34 48 53* 80 65 25 45 50

Sc 2.0 2.6 3.7 3.9 2.4 1.1 1.4 Y 58 49 116 85* 120 105 42 90 175 18 74 214* 134 75 Zr 129 126 110 97* 50 90 46 60 110 87 140 151* 172 95 ------~------Nb 53 64 109 122* 50 80 145 45 52 40 92 125* 90 40 Mo 24 3.4 3.8 14 Sn 30 25 65 12

Hf 5.5 6.7 5.5 6.1 7.6 8..1 Ta 5.6 26 7.7 16.3 Pb 31 56* 40 30 70 50 42 37 50

Th 54.8 49 67 61. 8 26.8 30.4 26 57.4 37 59 47.3 U 21.6 11 37.1 12.4 151 16.7 22 15.0 20.4 11. 7

F 4150 2025 10000 79700 19000 7200 4100 4600 Cl 631 1370 75 80 957 1230

1. Average of 5 rhyolite vitrophyres (Christiansen 7. Pegmatitic inclusion (Christiansen et al. 1980). et a1. 1984). 8. Rhyolite vitrophyre (Christiansen et al. 1980). 2. Average rhyolite (Turley and Nash 1980). 9. Average vitrophyre (Turley and Nash 1980). 3. Average of 3 rhyolite vitrophyres (Christiansen 10. Average dome-related obsidian (Evans and Nash 1978). et a1. 1984). 11. Average rhyolite vitrophyre (Christiansen et al. 4. Average devitrified rhyolite. Analyses with * 1980; Christiansen 1980). are semi-quantitative emission spectrometry 12. Average rhyolite. Analyses with * are semi­ analyses (Lindsey 1979). quantitative (Keith 1980). 5. Low-silica phase of Honeycomb Hills rhyolite 13. Average of 5 rhyolite lavas (Barrott 1984). (Turley and Nash 1980). 14. Average of 2 devitrified rhyolites. 6. Rhyolite (Turley and Nash 1980).

SM-29-206 Cs 4 U Be SM-61a Lu 3 Li Vb Ta Figure 3. Enrichment factor diagram showing evolutionary trends in the Rb Tb" Th rhyolites of the Thomas Range, Utah, (thick line) derived by comparing 2 Mn an incompatible element-poor and an incompatible element-rich speci­ men. The samples are from lavas presumed to be cogenetic. Enrichment factors for the Bishop Tuff (Hildreth 1979) are shown with thin lines and 1 are similar in magnitude and direction (except for Sc and Sm) to those for these rhyolite lavas.

Fe Hf 0.5 " Nd Ce Mg La p Tl Co 0.1 Sr Eu Topaz Rhyolites 9

TABLE 2. (CONTINUED)

Sheep Little Creek China Belt Silver Tomichi Boston Black Burro Range Jarbidge Cap Mtns. Nathrop Cliff Dome Peak Lake City Grants Ridge Range Creek 15 16 17 18 19 20 21 22 23 24 25 26 27 28

Li 96.5 177 96 16 107 167 75 185 160 23 100 Rb 380 275 493 304 176 289 580 378 680 348 451 Cs 7.7 9.3

B 14.6 Be 12.8 10 6 12* 19 6 7 Sr 40 42 <12 7.5 1.3 11 23 7.5 126 5 5

Ba <22 15.6' 1 36 93 63 600 29 Cr 11 2.7 Co 0.1 0.42 Cu 2 Ga 55 43 Sc 1.7 Y 110 176 13 34 105 121 Zr 154 286 125 75 101 169 192 14 Zn 25 Nb 43 46 83 53 35 96 36* 39 Mo <10 1. 98 5 1 Sn 13 1. 90 6 16 25 10 Hg <0.5 Hf 4.4 7.8 Ta 5.1 3.3 Pb 35 60 43 33 47* 21 Th 50 50 49 34.0 59 32.4 U 24 15.3 16.2 5.9 5.4 13 40 8.2 8.1 12.9

F 2800 4450 3375 1700 1350 1399 2954 100 1000 5200 3800 3800 1845 Cl 377 397

15. Devitrified rhyolite (Christiansen 23. Average of devitrified rhyolites except U and Th et al. 1980; and unpublished data). concentrations from 3 marginal vitrophyres 16. Average rhyolite (Christiansen unpublished (Steven et al.1977). Zielinski (1978) reports data). U (40, 26, 43, and 41 ppm). Analyses with * 17. Average rhyolite (Dayvault et al. 1984). are semi-quantitative. 18. Rhyolite (Rupp 1980). 24. Average of 21 rhyolites (Ernst 1981). 19. Obsidian (Zielinski et al. 1977; and 25. Vitrophyre. (Christiansen unpublished data). Christiansen unpublished data). Christiansen Zielinski (1978) reported the U concentrations. et a1. (1980) report U (16 ppm) and Th (33 ppm). 26. Devitrified rhyolite (Christiansen unpublished 20. Average of 3 hydrated rhyolite glasses data) • Mutschler et al. in press) • 27. Vitrophyre (Correa 1980; Christiansen unpublished 21. Average of 3 rhyolites (Ernst 1980). data) • 22. Average of 6 rhyolites (Ernst 1980). 28. Average rhyolite vitrophyre (Moyer 1982; Burt et al. 1981).

The differentiates would, nonetheless, have widely varying trace The Thomas Range lies in the central portion of the Deep element characteristics. Crystal settling seems to be an unlikely Creek-Tintic mineral belt (Shawe and Stewart 1976; Stewart et mechanism of crystal fractionation; a more plausible method al. 1977b), an east-trending zone of basement highs, Cenozoic would be the fractionation model described by McBirney (1980) volcanic centers and associated mineralization (Figure 4). Like and Huppert and Sparks (1984) that involves wall crystallization, the Pioche mineral belt to the south, it is expressed as a series of the generation of a buoyant evolved liquid, and its consequent aeromagnetic highs. Cenozoic magmatism along the belt (Lindsey upward escape to produce a vertically stratified magma chamber. et al. 1975; Lindsey 1982) began about 42 Ma with the eruption In contrast to the nearby Spor Mountain rhyolites, no eco­ of a calc-alkaline sequence ofintermediate-composition lavas, ash nomic mineralization has been found associated with the younger flows, and small intrusions. Oligocene (38 to 32 Ma) volcanism Topaz Mountain Rhyolite. Bikun (1980) attributes this lack of in the Thomas Range region was more silicic and is represented mineralization to lower magmatic concentrations of lithophile by several ash-flow tuffs that emanated from collapse calderas. elements and to their retention in the spherulitically-devitrified An 11 m.y.lull in magmatic activity preceded the eruption of the rhyolite lavas ofthe Thomas Range. Spor Mountain Formation, which also contains topaz (see 10 Christiansen, Sheridan, and Burt

TABLE 3. Sr AND Pb ISOTOPIC COMPOSITION OF TOPAZ RHYOLITES FROM THE WESTERN UNITED STATES

Sample No. Rb Sr Age Analystl (ppm) (ppm) Ma

Thomas Range, Utah SM-6lc 184.6 79.34 6.740 0.70938 0.70879 6.3 EHC Sl1-62b 371.8 58.60 18.38 0.71338 0.71174 6.3 EHC SM-29-206 433 2.5 495 0.75141 0.7071 6.3 LJ

Mineral Range, Utah MR-l 188.4 37.47 14.56 0.70616 0.70606 0.5 EHC

Wah Wah Range, Utah WW-6b 381. 4 18.19 60.78 0.71635 0.70599 12.0 EHC WW-9 626 20.2 89.7 0.72752 0.7122 12.0 LJ STC-4 596 10.9 159 0.75485 0.7092 20.2 LJ

Cortez, Nevada DRS-155-62 631. 7 38.38 47.71 0.71810 0.70800 14.9 EHC DRS-149-62 627.2 19.44 93.65 0.72843 0.70862 14.9 EHC

Sheep Creek Range, Nevada IZ-l 358 28 37 0.71577 0.7085 13.8 LJ

Jarbidge, Nevada RT-MC 294 35 24.3 0.71949 0.7142 15.4 LJ TjR-l 178 70 7.36 0.71217 0.7106 15.4 LJ TjR-2 352 16.1 63.5 0.72397 0.7101 15.4 LJ

Nathrop, Colorado NAT-2 318.8 3.24 288 0.83433 0.7141 29.3 EHC 0.7080 30.8

Lake City, Colorado 72L-47K 281 112 7.23 0.7073 0.7054 18.5 PWL

Black Range, New Mexico HC-8 350 3.7 270 0.82203 0.7158 27.7 LJ 0.7108 29.0

Little Belt Mountains, Mont~na 15a 529 8.51 179.9 0.8341 0.7094 48.8 ZP 15b 523 8.91 169.8 0.8269 0.7092 48.8 ZP

Note 1: EHC-Eric H. Christiansen, analyst at USGS, Denver. Rb, Sr, and isotope ratios by mass spectrometry and isotope dilution. LJ -Lois Jones, analyst at Conoco, Ponca City. Rb by XRF; Sr and isotope ratios by mass spectrometry and isotope dilution. ZP -Zell Peterman, analyst at USGS, Denver. Rb, Sr, and isotope ratios by mass spectrometry and isotope dilution (Marvin et al. 1973). PWL-from Lipman et al. (1978a).

Decay constant for Rb=1.42 x 10-11/y •

below). Scattered centers of rhyolitic and basaltic lavas were 2. Spor Mountain, west-central Utah formed after about 10 Ma including the eruption of the Topaz Mountain Rhyolite 6-7 Ma. Although the rhyolites of the The topaz rhyolite exposed around· the margins of Spor Thomas Range were not emplaced in a strictly bimodal volcanic Mountain in west-central Utah is related to the largest commer­ field with contemporaneous mafic and silicic lavas, they are part cial source of beryllium known in North America. The minerali­ of this regional sequence of basalt or basaltic andesite (Figure 5) zation occurs in an altered pyroclastic deposit cogenetic with a 21 and high-silica rhyolite. Mafic lavas with ages of about 6 and 1 Ma rhyolite flow (Lindsey 1982). The most recent studies of the Ma are exposed at Fumarole Butte 23 kIn to the west (Peterson rhyolite and the mineral deposits include those of Lindsey (1977, and Nash 1980; Best et al. 1980). 1982), Bikun (1980), and Christiansen et al. (1984). Topaz Rhyolites 11

NV UT

40° CD c 0 N -CD :J 0 39° ()

CD -c. Ol Cil E 0 38°' "- «CD ,,- ...- Aeromagnetic High

km , o 80 160 Mineral Belts

OU -Oquirrh - U i nt a DT -Deep Creek Tintic P -Pioche 01 -Delamar-Iron Springs

Figure 4. Index map of eastern Nevada and western Utah showing the location of east-trending structural, mineral, igneous, and aeromagnetic lineaments (modified from Rowley et al. 1978b). Note the corresopndence of the locations of topaz rhyolites in Utah (filled circles) with the location of the major lineaments-the Deep Creek-Tintic (DT), the Pioche-Marysvale or Pioche (P), and the Delamar­ Iron Springs (D!) mineral belts of Shawe and Stewart (1976).

Spor Mountain consists of a block of tilted and intricately eruptions commenced with the emplacement ofa series ofignim­ faulted lower and middle Paleozoic sedimentary rocks that are brites, pyroclastic air-fall sheets, and pyroclastic-surge units, and chiefly carbonates (Figure 2). Numerous, relatively small rhyolite were terminated by the extrusion of lavas over the tuff. The tuff plugs, dikes, and breccia pipes have intruded the sequence. The contains lithic inclusions of dolomite (altered to fluorite near the pre-volcanic surface was disrupted by northeast-trending ridges top ofthe tuff) and older volcanic rocks that were entrained from and valleys, perhaps formed by faulting (Williams 1963). Post­ the country rock as the pyroclastic material moved through the eruption basin-and-range faulting has further complicated the vent. Locally, the tuff is absent and the lava rests on Paleozoic structure making it difficult to estimate the number of vents in­ sedimentary rocks, but where present the tuff reaches a thickness volved. Lindsey (1979) identified at least three major vents. The of almost 100 m (Williams 1963; drill core information). A 12 Christiansen, Sheridan, and Burt

variation diagrams) and by sanidine rims on sieve-textured calcic .1140' 113° 112° I-!I+------+-----~~-r__40° plagioclase cores (see,for example, Hibbard 1981). The Spor Mountain rhyolite is generally phenocryst-rich (20 10-8 ~ (J to 40%). Major phases include sanidine (Orso to Or60), smoky 6-7)}. ~a quartz, plagioclase (AnlO to An13), and aluminous Fe- and F-rich 4.~ ~ ~ (f)X5.3,6.0 21 ~ :,,, 1.0 biotite (Figures 31 and 32). Magmatic accessories include Fe-Ti c< ~::.. oxides, zircon, fluorite, and allanite. The groundmass of felsitic ~ 3.4 6.1 samples is granophyric, probably as a result of the thickness and z slow cooling of the flow. The groundmass consists of alkali feld­ 110.3 -Delta 0.4 spars, silica minerals, fluorite, topaz, and biotite or hematite. o miles 40 ~ Topaz also occurs in miarolitic cavities that surround the mafic o km 60 .tj{~: , inclusions. Two-feldspar geothermometry indicates the pheno­ ::/12:';, 0.1,0.2 390 crysts in the rhyolite equilibrated at 680°C (Table 4) and the 0.9 "',! - Filmore composition of the biotites suggest equilibration near the QFM 2.3 ··0~0.4 oxygen buffer (log f02 = -18 to -19; Figure 30; Christiansen et 1.0~2.5e : al. 1980). The mafic inclusions contain plagioclase (AnSO-30 _. 0.5 '~" Il Ab4S-600r4-11), augite (Ca37Mg3sFezs), titaniferous magnetite, 0.3 .~.!:" Cove Fort 24. and ilmenite in a quench-textured matrix of needle-like pyroxene. u&~8~SV181e Sanidine rims (Or60Ab34An6) on plagioclase suggest tempera­ 0.5- .8[1'~1.0 20-22~'f::23 7.9 1.1 lQ 12. • tures of 910°C, while co-existing microphenocrysts of Fe-Ti ox­ o .'!i 13 ~ __ ides yield equilibration temperatures of 1100 to 1200°C and f0 7.6 b. 8 : 2 eo '10: 9-11 c;;,;: 23 21.1 5 :':~"., near the QFM buffer. sandstone and limestone conglomerate occurs beneath the tuff at rhyolite. It contains very high concentrations of U, Th, Rb, Ta, the Yellow Chief uranium mine (Lindsey 1978). A breccia zone Nb, Be, Li,Y, Ga, Pb, and Sn. A typical REE pattern is shown in is exposed at the base of the rhyolite lava in several of the open­ Figure 40b. It shows a deep Eu-anomaly, indicative of feldspar pit beryllium mines. This breccia is interpreted as an over-ridden fractionation and the relatively flat REE pattern typical of many apron of talus that accumulated at the front of the moving flow topaz rhyolites. Based on vitrophyre-felsite comparisons, sub­ (Bikun 1980). The rhyolite lava has a maximum known thickness aerial crystallization resulted in the loss of 25 to 50% ofthe F and of 300 m (Williams 1963). Dark mafic inclusions with globular Uin the glasses. Be losses may have been on the order of 75% dur­ and contorted lensoidal shapes are found in the lava. Their tex­ ing devitrification (Bikun 1980). Many other elements (including tures and chemistry led Christiansen et al. (1981) to suggest that a Rb) do not appear to have been released by devitrification. more mafic magma was injected into the magma chamber of the The tuff beneath the Spor Mountain rhyolite is the host for Spor Mountain rhyolite shortly before its eruption. The mafic major Be and minor U, F, Li, Mn, Zn, Nb, and Sn mineralization (trachyandesite) inclusions themselves show signs of pre-eruption (Lindsey 1977; Bikun 1980). The Be-mineralization is strongest mixing with the rhyolite in their bulk chemistry (linear trends on in· the upper few meters of the tuff where BeO concentrations Topaz Rhyolites 13

TABLE 4. GEOTHERMOMETRY OF TOPAZ RHYOLITES FROM THE WESTERN UNITED STATES

Temperature Location Method Range (OC) n Reference

Thomas Range, UT 2 Feldspar 690 - 790 4 Turley and Nash 1980 2 Feldspar 630 1 Christiansen et al. 1980 Fe-Ti oxides 725 1 Turley and Nash 1980 Spor Mountain, UT 2 Feldspar 680 - 690 5 Christiansen et al. 1980 E.H. Christiansen unpublished data

Honeycomb Hills, UT 2 Feldspar 605 2 Turley and Nash 1980 Smelter Knolls, UT 2 Feldspar 630 - 685 3 Turley and Nash 1980 Fe-Ti oxides 665 1 Turley and Nash 1980

Mineral Mountains, UT 2 Feldspar 620 - 770 3 Evans and Nash 1978 Fe-Ti oxides 650 - 780 3 Evans· and Nash 1978

Wah Wah Mountains, UT 2 Feldspar 650 1 Christiansen et al. 1980

Chalk Mountain, CO Fe-Ti oxides 830 1 E.H. Christiansen unpublished data Apatite-bio 590 1 E.H. Christiansen unpublished data

Nathrop, CO 2 Feldspar 650 1 E.H. Chr istiansen unpublished data

Notes: 2 Feldspar geothermometry using equations of Stormer (1975) recalculated 100 bars. Fe-Ti oxides calculated using recalculation method of Stormer (1983) and solution model of Spencer and Lindsley (1981). Apatite-biotite geothermometer after Ludington (1978).

reach 1% (Griffitts and Rader 1963; Lindsey 1977, 1982; Bikun region is summarized by Lindsey et al. (1975), Shawe (1972), 1980). Mineralization is associated with dolomite clasts altered to and Lindsey (1982) and is reviewed in the section describing the fluorite and opal in association with feldspathic alteration of the Thomas Range. The episode of rhyolitic volcanism at Spor matrix (Williams 1963; Lindsey 1977). A thick zone containing Mountain maybe related to the initial development of the Basin Li-bearing clays underlies the ore zone. Bikun (1980) and Burt . and Range province in a back- or intra-arc setting oflithospheric and Sheridan (1981) suggest that the ore deposits were formed as extension (see, for example, Zoback et al. 1981; Eaton 1984a). Be, V, P, and other elements were released from the lava by The evidence for magma mixing suggests that magmatism may granophyric crystallization and then concentrated in the upper have been bimodal. The episodic recurrence ofsilicic magmatism part of the tuff by meteoric fluids at fairly low temperatures. along the Deep Creek-Tintic belt suggests that it may be a pro­ Beryllium ore (bertrandite) coprecipitated with fluorite formed found flaw in the continental lithosphere-perhaps an intracon­ by reaction ofthis fluid with the carbonate lithic inclusions. Some tinental transform of the type proposed by Eaton (1979); support for this model is provided by V-Pb ages of opal nodules Regardless of its origin, the Deep Creek-Tintic belt is very similar in the beryllium tuff. One of these zoned nodules has ages that to the more southerly Pioche beltin its characteristics and history. decrease outward from 20.8 ± 1.0 Ma to 8.2 Ma (Ludwig et al. Mid-Cenozoic igneous activity commenced about 10 Ma earlier 1980). These results suggest that the mineralized nodules formed along the northern belt (Stewart et al. 1977b), but the later devel­ at the same time as the rhyolite erupted, but continued to grow opment of both regions was very similar and characterized by the (perhaps episodically) by the deposition of opal from ground eruption of bimodal suites of mafic lavas and topaz rhyolites after water. In contrast, Lindsey (1977) and Williams (1963) sug­ about 22 Ma. gested that hydrothermal fluids rose along fractures from an un­ exposed pluton, intersected the porous tuff and deposited Be and 3. Honeycomb Hills, west-central Utah P below the lava. On Spor Mountain, uraniferous fluorite with­ out associated beryllium minerals occurs in breccia pipes formed Honeycomb Hills, the western-most topaz rhyolite occur­ by partial venting of the rhyolitic magma. As mentioned above, a rence along the Deep Creek-Tintic trend, lies 30 km west of the sedimentary V-deposit (Yellow Chief mine) occurs in an epiclas­ Thomas Range. Studies of the geology of the Honeycomb Hills tic conglomerate that locally underlies the tuff. and t~eir rhyolites include those of Turley and Nash (1980), The volcanic history of the Spor Mountain/Thomas Range Hogg (1972), McAnulty and Levinson (1964), Shawe (1966), 14 Christiansen, Sheridan, and Burt

and Erickson (1963). The Honeycomb Hills have attracted atten­ ppm) compared to other topai rhyolites, especially if compared tion primarily as a result of low-grade beryllium and rare-alkali to those with approximately 1%F such as the Spor Mountain mineralization in tuffs associated with a small topaz-rhyolite rhyolite. The silica-poor sample contains 151 ppm U and high dome complex emplaced 4.7 Ma (Lindsey 1977; Turley and Mo, while the pegmatitic inclusion contains anomalous Li (590 Nash 1980). ppm) and Sn (65 ppm). REE patterns for the rhyolite are similar Two rhyolite domes with a total volume of less than 0.5 to those from Spor Mountain (Figure 40c). km3 form the Honeycomb Hills. Eruption of the domes was Low grade Be, Li, Cs, and Rb mineralization occurs in the preceded by the deposition of a lithic-rich tuff. The tuff is slightly pyroclastic deposit beneath the western dome. The mineralization altered and contains macroscopic fluorite. The rhyolite has a occurs in aim thick zone about 1 m below the base of the lava basal vitrophyre that is exposed locally but it generally consists of and is reported to have been developed when ascending mag­ felsitic rhyolite with highly contorted flow foliation. The rhyolite matic fluids were blocked by the impermeable rhyolite (Mc­ apparently intrudes unrelated older (Miocene or early Pliocene) Anulty and Levinson 1964). trachyandesitic (shoshonitic) and dacitic lavas and tuffs (Hogg 1972). Paleozoic sedimentary rocks were encountered at about 4. Smelter Knolls, west-central Utah 50 m in a hole drilled between the two hills (McAnulty and Levinson 1964). The youngest (3.4 Ma by K-Ar method; Armstrong 1970; The rhyolites from the Honeycomb Hills contain up to 40% Turley and Nash 1980) of the topaz rhyolites identified on the phenocrysts of sanidine (Orso to Or60), smoky quartz, plagioclase Deep Creek-Tintic trend consists of a rhyolite dome and flow (AnlO), and F- and Fe-rich biotite (Turley and Nash 1980; Fig­ complex at Smelter Knolls located about 25 km west-northwest ures 31 and 32). Two-feldspar geothermometry yields a tempera­ of Delta. The geology of Smelter Knolls is described by Turley ture of 605°C (Turley and Nash 1980). Magmatic accessories and Nash (1980). include fluorite and traces of Fe-Ti oxides. Topaz, fluorite, and The rhyolite complex atSmelter Knolls is 5 km in diameter biotite occur as devitrification products within the groundmass of and contains about 2.2 km3 oflava. Basal vitrophyres and flow the rhyolites. The lava also contains globular inclusions oftopaz­ breccias are exposed locally. No pyroclastic deposits related to and fluorite-rich material. The textures in these inclusions range the lava are exposed. The lavas were apparently erupted through from pegmatitic to aplitic. They may represent fragments of a thick alluvial deposits related to the development of the basin and cogenetic granite pluton at depth. Inclusions of the metamorphic range topography of the area. basement (quartzite, metaconglomerate, and meta-arkose) and The rhyolite from Smelter Knolls generally contains 15 to inclusions of the surrounding mafic lavaS also occur in the 20% phenocrysts set in a devitrified flow-banded matrix. Quartz, rhyolite. sanidine (Orso to Or6S), plagioclase (AnlO to Anzo), Fe- and The compositions of several samples from Honeycomb Hills F-rich biotite (Figures 31 and 32), and Fe-Ti oxides (ilmenite is are given in Table 1. Fluorine contents in vitrophyres range from rare) occur in all samples. Accessory phases include zircon, fluor­ 0.95 to 1.2% (Christiansen et al. 1980). The rocks are slightly ite, and allanite. Topaz occurs in the devitrified groundmass and richer in Al and Na, and have less Si than many topaz rhyolites along with hematite in miarolitic cavities. Fe-Ti oxide and two­ but are similar to those from Spor Mountain, Utah. These com­ feldspar geothermometry indicate that the phenocrysts crystal­

positional characteristics probably resulted from the effect of high lized at low temperature (660 to 685°C; Table 4) and at f02 near F in the magma. As noted above, Manning (1981) has shown the QFM oxygen buffer (Figure 30; log f02 = -19.9 bars). that the effect of F is to push the water-saturated minimum in the The average composition of four samples analyzed by Tur­ granite system toward the albite apex as the quartz field expands, ley and Nash (1980) is presented in Table 1. The rocks are very producing more sodie and aluminous residual melts. The single similar to other topaz rhyolites. Fluorine ranges from 0.65 to major-element analysis reported by Turley and Nash (1980) is 0.78% in vitrophyres; a single felsite shows the common fluorine very unusual (Table 1). Itcontains 11.1% CaO, 8.0% F, and only depletion that accompanies devitrification (F = 0.5%). Devitrifica­ 63.6% SiOz-reportedly due to large amounts of fluorite in the tion also produced depletions of U, CS,and Sb. As expected, the groundmass. Perhaps this analysis represents one of the globular rhyolites are enriched in incompatible lithophile elements (Table inclusions described earlier. Some trace-element analyses are also 2). The REE patterns (Figure 40d) are also similar to those from available for samples from the Honeycomb Hills (Table 2). Con­ other topaz rhyolites with low La/LuN (1.60), La/CeN (1.06) sistent with their higher fluorine contents, the Honeycomb Hills and Eu/Eu* (0.02). rhyolites appear to be more "evolved" than those of the Thomas The two major rhyolite hills at Smelter Knolls are separated Range. Analysis 5 is the low-silica sample analyzed by Turley by a normal fault that stretches northward for about 8 km. Faults and Nash (1980) and analysis 6 is for a "normal" rhyolite for of this orientation are common in the area. Mafic volcanism in which no major element data have been published. The other the immediate vicinity has K-Ar ages of 0.31 Ma (tholeiitic ba­ trace-element analyses (7 and 8) represent a pegmatitic inclusion salt, 48% SiOz: Turley and Nash 1980) and 6.1 Ma (basaltic and a rhyolite respectively (Christiansen et aI. 1980). These rhyo­ andesite, 57% SiOz: Turley and Nash, 1980) thus bracketing the lites contain high Rb, Li, and Ubut are depleted in Th (about 30 eruption of the rhyolite. A low-shield volcano composed princi- Topaz Rhyolites 15

pally of basaltic andesite developed 1 Ma (Peterson and Nash 6. MineralMountains, western Utah 1980). It lies 12 km to the north and is centered on the fault trend noted earlier. It appears that the tectonic-magmatic setting is In the Mineral Mountains of western Vtah topaz-bearing similar to other Great Basin rhyolites that were emplaced during rhyolite domes occur discontinuously along the crest of the range late Cenozoic east-west extension and during episodes of mafic (Evans and Nash 1978; Lipman et a!. 1978b). The Mineral volcanism. Range is a typical basin and range horst that consists predomi­ nantly of a multiple-phase granitic pluton of Tertiary age (V-Pb 5. Keg Mountain, west-central Utah zircon age 25 Ma: Aleinikoff et a!. 1985). The Roosevelt Hot Springs Known Ge~thermal Resource Area (KGRA) is located Rhyolite flows and domes in the Keg Mountain area along one of the western range-front faults. (termed McDowell Mountains on some maps) have been Three sequences of rhyolite lavas exist in the Mineral Range mapped as part of the Topaz Mountain Rhyolite (Erickson (Lipman et a!. I978b). The general distribution of the lavas is 1963). A brief description of them is included here, as Shawe shown in Figure 6. The oldest episode of volcanism (7.9 Ma; all (1972: 72) implies they bear topaz in lithophysae. However, dates are by K-Ar method) is represented by a single dome (Trd) Erikson (1963) and'Lindsey et al. (1975) report no topaz from on the western flank of the range. Quaternary' obsidian flows the Topaz Mountain Rhyolite at Keg Mountain. In our visits to (0.77 Ma) several kilometers long and up to 80 m thick were the Keg Mountains we have not yet identified any topaz-bearing erupted shortly before the most viscous topaz-bearing domes rhyolites. (0.58 to 0.53 Ma). At least 11 domes were erupted. The domes Erickson (1963) identified atleast two vents and estimated usually have a basal vitrophyre (5 to 10 m thick) that in places that about 15 km3 of rhyolitic volcanic rocks are preserved in the forms part ofa flow breccia. The vitrophyre grades upward into a Keg Mountain area. The emplacement of the rhyolites was sim­ devitrified, flow-banded rhyolite. A frothy mantle ofperlite forms ilar to other domes and flows with an upward sequence oflithic­ a carapace around parts of some domes. Lithophysae and other rich tuff, basal vitrophyre and/or flow breccia, and flow-banded gas cavities occur in flow interiors. In contrast to the obsidian rhyolite. The rhyolite lavas attain maximum thicknesses of about flows, several of the domes overlie pumice-rich pyroclastic depos­ 60 m near their vents. The underlying tuffs accumulated to sim­ its that represent the vent-opening eruptions. The domes· range ilar thicknesses and thin toward the margins of the flows. The from 0.3 to I km in diameter and are up to 250 m high. lavas are 8 Ma (Lindsey et a!. 1975). The rhyolite domes are crystal-poor with 2 to 10% pheno­ Most of the lava is felsitic and flow-banded with less than crysts of sanidine, quartz, oligoclase, and sparse biotite. Allanite, 10% phenocrysts of quartz, sanidine, plagioclase, and sparse bio­ titanite, zircon, Fe-Ti oxides, and apatite are accessories. Topaz tite (Erickson 1963). Hematite and possibly topaz occur in litho­ occurs in vugs along with pseudobrookite and specular hematite physae within the rhyolite. as a result of crystallization from a vapor phase (Evans and Nash The geochemistry and mineralogy ofthe tuff associated with 1978). the rhyolites has been the subject of several investigations to Chemical analyses of rhyolites from the domes show that determine the role of alteration on the mobility of Be and V they are typical oftopaz rhyolites elsewhere (Tables I and 2). The (Lindsey 1975; Zielinski et a!' 1980), but the lavas remain poorly fluorine content of the vitrophyres from the domes ranges from studied. The tuffs contain abundant clasts derived from older 0.1 to 0.4%. Relative to the slightly older obsidian flows, the volcanic rocks (some as large as 5 m in diameter) that were domes are enriched in f, Na, Mn, Rb, Th, Nb, HREE, and Zn probably included in the ash as it explosively vented to the sur­ and are depleted in K; Ca, Fe, Ti, Ba, Sr, Zr, light REE (La face. Broken phenocrysts of quartz, sanidine, plagioclase, and through Tb), and Eu. Temperatures, derived from Fe-Ti oxides biotite constitute 5 to 15% of the tuff. Magnetite, hematite, titan­ (Evans and Nash 1978) and O-isotope geothermetry (Bowman et ite, hornblende, augite, zircon, fluorite(?), and apatite occur in a!. 1982) are consistently lower in the F-rich domes than in the smaller amounts, but may be xenocrystic.·The average trace ele­ older obsidian flows (650 versus 760°C). The dome lavas also ment composition of the tuff is given in Table 2. The relatively crystallized at lower oxygen fugacities (log f0 2 ranges from -16.8 high concentrations of Sr and Ba and low concentrations of Ga, to -18.2). These and other observations led Evans and Nash Nb, Y, Th, and V suggest that if the rhyolites are topaz-bearing (1978) to suggest that the magma in the domes was a differentiate they are relatively poor in incompatible lithophile elements com­ of the obsidian-flow magmas. The REE pattern of the rhyolites pared to others from the western Vnited States. from the Mineral Mountains is different from most other topaz The so-called "alkali"rhyolites of the Keg Mountain area rhyolites (Figure 40e). The principal difference lies in the rela­ are deposited on an older sequence of (apparently topaz-free) tively low concentrations of the middle REE (Nd through Tm); rhyolitic lavas (10 Ma), ash-flow tuffs (32 to 30 Ma), and in this regard they are similar to the patterns of the Lake City, intermediate-composition lavas and breccias (38 to 39 Ma) Colorado, topaz rhyolites. This depletion is probably the result of (Shawe 1972; Lindsey et a!. 1975). The temporal magmatic and titanite fractionation. A sample from one of the older lavas (the tectonic development appears to be similar to that in the nearby flow of Bailey Ridge; Figure 6) was analyzed for its Sr-isotope Thomas Range. composition (Table 3). Its'ratio (0.7062 ± 0.0003) indicates that 16 Christiansen, Sheridan, and Burt

'. Qac Tg

38° (I) 30' Z

< I­ Z

:) o Tg :E

I I I I

-J « Qac 0:: w z

:E

T~ \

o 2 3 I I I 38° corral * Probable vent KM 22'L-----...... ,..----'~------'---'-----...... ,..~ 30" Figure 6, Generalized geologic map ohhe central Mineral Mountains, Utah (after Lipman eta!' 1978a). Topaz-b!laring rhyolite domes (Qrd) are underlain by rhyolitic tephra (Qrp). Topaz-free obsidian flows (Qrl) generally lie directly on the granite of the Mineral Mountains (Tg). Surficial deposits (Qac) and hot-spring deposits (Qhl) occur along a range-front fault (bold line). A Tertiary rhyolite (Trd) lies in the extreme ,southwest comer of the map. Stars indicated vents for the rhyolite domes. Topaz Rhyolites 17 it was derived from a crustal source with a relatively low Rb/Sr contemporaneous fluorine-poor rhyolites by their low crystal ratio like those typical of granulite facies metamorphic rocks. content (less than 5%) and the presence ofsmoky quartz. A few of This interpretation is consistent with the O-isotopic composition the older topaz rhyolites are phenocryst-rich (e.g., the rhyolite of of the lavas and domes (8 180 = 6.3 to 6.9 %0; Bowman et al. the Staats mine). The major phenocrysts in both age groups are 1982), which also suggests a crustal source of high metamorphic the same and include sanidine, quartz, sodic plagioclase, and grade. Both lines of isotopic evidence probably preclude the deri­ sparse Fe-rich biotite. A few samples contain Fe-rich hornblende. vation ofthe rhyolites from metasedimentary rocks in the Proter­ Magmatic accessories include zircon, allanite, apatite, fluorite, ozoic age basement. No metalliferous ore deposits are known to and Fe-Ti-Mn oxides. Two-feldspar geothermometry indicates be associated with the rhyolites of the Mineral Mountains. that some of the topaz rhyolites crystallized at temperatures The Mineral Mountains are part of a broad region of the around 650°C (Table 4). Topaz, fluorite, alkali feldspars, hema­ western United States that experienced late Cenozoic extension tite, bixbyite, and silica minerals line gas cavities and occur in the and bimodal volcanism. Extensive fields of contemporaneous ba­ devitrified matrix ofsome lavas. Red beryl (up to 1 cm in diame­ salt and rhyolite (Figure 5) are located north and east of the range ter) occurs in the eastern Wah Wah Mountains (Ream 1979). (Haugh 1978; Clark 1977; Crecraft et al. 1981; Hoover 1974; Vapor-phase garnet occurs in some topaz rhyolites but it crystal­ Evans and Steven 1982). Most ofthe fields appear to be less than lized as a magmatic phase in the tuff ofPine Grove erupted from 2.5 Ma. The near contemporaneity of basalt and rhyolite volca­ a developing (molybdenum-mineralized) intrusive system in the nism in the Mineral Range area and the inclusion of vesiculated southern Wah Wah Mountains (Keith 1980, 1982). This tuff is basalt "xenoliths" in the obsidian flows implies a close genetic distinct in age (24 Ma) and chemistry from the younger topaz­ connection between the magmas (Evans and Nash 1978). The bearing lavas (Keith 1980, 1982). Although the composition of rhyolites of the Mineral Mountains lie along the .Pioche­ the rhyolitic portion of the tuff of Pine Grove lies within the range Marysvale mineral belt of Shawe and Stewart (1976), which is ofall topaz rhyolite compositions, it is enriched in Ba, Sr, Sc, and described more fully below. The distribution of volcanism may Al and depleted in F, Zr, Rb, Nb, Ta, Th, U, Yb, Y, and Mo be controlled by fractures that parallel this trend. relative to the topaz rhyolite lavas of SW Utah. The late "Lou Rhyolite," which cuts the Pine Grove stock, is nonetheless geo­ 7. Wah Wah Mountains and vicinity, southwestern Utah chemically identical to other topaz rhyolites from SW Utah andsoutheastern Nevada (Keith 1982). Its age is not known. The chemistry of samples from southwestern Utah demon­ Numerous fluorine-rich rhyolitic lavas have been identified strates their similarity to other topaz rhyolites (Table 1). No in the southern parts of three ranges- the Wah Wah Mountains, systematic differences in the compositions of the two age groups the Needle Range, and the White Rock Mountains of southwest­ of topaz rhyolites have yet been demonstrated. Fluorine concen­ ern Utah and adjacent Nevada (Christiansen 1980; Best et al. trations in vitrophyres range from 0.2 to 05%. The trace element 1985). Interest in these lavas has been spurred by the discovery of chemistry is summarized in Table 2, and shows the typical en­ a porphyry molybdenum deposit in the southern Wah Wah richments of U, Th, Rb, Ta, Cs, and Li. REE patterns for topaz Mountains (Tafuri and Abbott 1981; Keith 1980, 1982) and by rhyolites from this region are typical of others with low La/LuN, the occurrence of numerous small deposits of uranium in and La/CeN, and Eu/Eu* (Figure 40t). Initial Sr-isotope ratios range near the rhyolites (Christiansen 1980). from 0.706 to 0.712 (Table 3). The distribution of the rhyolites is shown in Figure 7. Two Extensive alteration of rocks at the surface is associated with episodes of rhyolitic magmatism have been delineated-one at 22 the fluorine-rich rhyolites in the Wah Wah Mountains. Large to 18 Ma and a second at about 12 Ma (Rowley et al. 1978b; areas of jasperoid (Lindsey and Osmonson 1978) and alunite Best et al. 1985). The rhyolites occur as isolated plugs without (K-Ar age 15 Ma; Best et al. 1985) occur in this region. Fluorite significant pyroclastic deposits (e.g., Observation Knoll), as iso­ and uranium were deposited in tuffs and in the intrusive margin lated domes or flows with underlying pyroclastic breccias and of the topaz rhyolite at the Staats mine (Christiansen 1980; Lind­ tuffs (e.g., the rhyolites at the Staats Mine and at the Tetons) and sey and Osmonson 1978; Whelan 1965). Other uranium pros­ as groups of coalesced domes and flows with interlayered tephra pects associated with the rhyolites are shown in Figure 7. Gold deposits (e.g., the Broken Ridge and Steamboat Mountain areas). and silver have been recovered from quartz-carbonate-fluorite The tuffs generally consist of thin pyroclastic flow, surge and veins in the Stateline area of the southern White Rock Moun­ minor air-fall units. Explosion breccias near some vents contain tains. These deposits are temporally related to the older episode of abundant lithic inclusions of the local country rock. Evidence of rhyolitic magmatism (Keith 1980). The previously mentioned mixing between rhyolitic and mafic magmas before eruption is porphyry molydenum deposit at Pine Grove in the Wah Wah found in tuffs from the southern Needle Range (Christiansen Mountains, which bears topaz in the alteration zones (Tafuri and 1980). Vitrophyres a few meters thick are present at the bases of Abbott 1981), is slightly older (24 versus 20 Ma) than the first some flows. Felsitic, flow-banded lavas with abundant vapor­ episode of topaz rhyolite magmatism (Keith 1980). phase cavities in their upper portions are typical. The tectonic and magmatic history of the region is outlined The topaz rhyolites can usually be distinguished from nearly by Rowley et al. (1978b, 1979) and Best et al. (1980, 1985). The 18 Christiansen, Sheridan, and Burt

o. <8) Ql Cl c as a: Ql 'C + Ql Ql Z

Spar Horse Resevolr Q~ ~/ . ~Cottonwood Creek

~nto\springS Thermo• Hot Springs o Uranium prospects

Rhyolite flows & intrusions (Miocene & Pliocene)

~ Granite & quartz monzonite L:::..:..:..:.J (Miocene)

Granodiorite (Oligocene)

• i o 25 kilometers

Figure 7. Regional distribution of Miocene and Pliocene rhyolites (many of which contain topaz) and Oligocene and Miocene intrusive rocks in southwestern Utah and eastern Nevada (after Christiansen et al. 1980; and Best et al. 1985). The locations of uranium prospects and of the topaz-bearing molybdenite deposit at Pine Grove are also shown. Crystal-poor rhyolite lavas in the western Needle Range, at Dead Horse Reservoir, and at Thermo Hot Springs contain no topaz and are chemically distinguished from the topaz rhyolites by their low concentrations of Rb, Y, and Nb, and by high concentrations of Sr. Basin and range fault-block mountains are outlined with solid lines. volcanic development of the region was dominated by events ure 42; Keith 1980; Best et al. 1985; Christiansen and Wilson along the east-trending Pioche mineral belt of which the Blue 1982). High-K andesites erupted along the belt from 25 to 21 Ma Ribbon lineament is one element (Rowley et al. 1978b). This (Best et al. 1980, 1985). Fluorine-poor, crystal-rich rhyolite lavas feature coincides with a broad aeromagnetic high and·is marked were also erupted during this time beginning about 22 Ma. A by numerous volcanic and intrusive centers of Oligocene to Hol­ genetic relationship between the F-rich and F-poor rhyolites has ocene age. Calc-alkaline volcanism (andesite-dacite-rhyolite) not been established. A Miocene "lull" in volcanic activity in the began in the early Oligocene (about 32 Ma) and continued until eastern Great Basin was followed by renewed bimodal magma­ the Miocene (until at least 19 Ma in the Black Mountains: Row­ tism that began about 13 Ma ago (Best et al. 1980). Mafic lavas ley 1978). This volcanism produced widely scattered, partly clus­ (trachybasalts to trachyandesites) were again accompanied by the tered, composite volcanoes with andesitic to dacitic lavas along eruption of the younger topaZ-bearing rhyolites in the southern the belt. The intervening lowlands were covered by widespread Wah Wah-Needle Range area and by topaz-free rhyolite else­ dacitic to rhyolitic ash flows erupted from collapse calderas. The where (e.g., Dead Horse Reservoir, Thermo Hot Springs). In the older F-rich rhyolite domes (20 to 18 Ma) are nearly contempo­ Wah Wah Mountains, mafic lavas and rhyolite were not erupted raneous with the latter portion of this episode but were accom­ from the same centers but their ages correspond closely (Figure panied by the eruption of trachyandesite (62 to 54% SiOz) lavas 5). The two episodes of topaz rhyolite volcanism are reminiscent in the Wah Wah Mountains, forming a bimodal suite there (Fig- of those to be described in New Mexico and Colorado. The Topaz Rhyolites 19

Pioche belt is paralleled by NE-trending faults (some of which the northern fringe of the aeromagnetic high which marks the may have strike-slip movement) and a conjugate NW-trending Pioche mineral belt (Fig. 4). 2) The mafic part of the bimodal set. Best et al. (1985) suggest that the tectonism along the trend suite consists of more mafic lavas-trachybasalts with up to 2.4% may be the result of relaxation of the Cretaceous compression K20 at 50% silica. Farther east, lavas as mafic as basalt do not that produced regional thrust sheets. Alternatively the tectonic appear until much later (8 to 9 Ma; Fig. 5). 3) The topaz rhyolites and magmatic events may be localized by a lithospheric fracture in the Wilson Creek Range are about 3 Ma older than the oldest produced or reactivated by differing rates of extension north and topaz-bearing lavas in the Wah Wah Mountains. 4) The rhyolite south of the Pioche belt (Rowley et aI. 1978b). lavas may be cogenetic with earlier voluminous ash flows.

8. Wilson Creek Range, southeastern Nevada 9. Kane Springs Wash, southeastern Nevada

Barrott (1984) reports the presence oftopaz in rhyolite lavas An unusual association of topaz rhyolite lavas with the de­ that are exposed in the northern part of the Wilson Creek Range velopment of a mildly peralkaline caldera system has been re­ near Rosencrans Knolls. The lavas are part of a strongly bimodal ported by Novak (1984). The Kane Springs Wash caldera (Noble sequence of rhyolitic ash flows and basaltic lavas that were em­ 1968) is located in southeastern Nevada and lies at the extreme placed over a short period of time about 23 Ma. The topaz­ southwestern part of the ENE-trending aeromagnetic ridge ofthe bearing lavas (K-Ar age 22.6 ± 0.9 Ma; all ages are from Barrott, Delamar-Iron Springs mineral belt (Figure 4). 1984) cap knolls underlain by thick sequences of unwelded lithic The following description of the evolution of the Kane tuffs. Interstratified with this lower tuff section is a prominent Springs Wash volcanic center is taken from Novak (1984). The rhyolitic welded tuff (K-Ar age 23.4 ± 0.9 Ma) and a series of eruption oftrachytic lavas 14.2 Ma (all ages are by K-Ar method) trachybasalt lavas (K-Ar age 23.9 ± 1.0 Ma). was the first recorded event at the volcanic center. A 19 by 13 km The rhyolite lavas are geochemically similar to other topaz~ caldera (Figure 8) collapsed slightly later (14.1 ± 0.2 Ma) as a bearing lavas in their uniformly high Si02 contents (75 to 76 result of the eruption of several compositionally zoned (comen­ wt%), and their low concentrations of Ti02, Fe203, MgO, and ditic or fayalite rhyolite to trachyte) ash flows that had a total Cao (Table 1). No fluorine analyses have been reported A volume of over 130 km3. Immediately following collapse, the slightly older (23.4 ± 0.9 Ma K-Ar) welded tuff also consists of eruption of trachyte lavas formed a cumulodome on the floor of high-silica rhyolite that is only slightly enriched in Ti, Fe, Mg, and the caldera. High-silica rhyolites (with ferroedenite) and basaltic Ca relative to the rhyolite lavas. The trace element differences to trachyandesitic lavas were then erupted, marking a fundamen­ between the two rhyolites is more striking. The topaz-bearing tal change in the magma chemistry to non-peralkaline composi­ lavas are significantly enriched in Rb (1 to 3 times), Y (2 times), tions. These units have K-Ar ages that are indistinguishable from and Nb (2 to 3 times) relative to the zoned(?) tuff. Likewise, the those of the other post-caldera volcanic rocks. The caldera expe­ lavas are depleted in the feldspar-compatible elements Ba (¥.!) and rienced no structural resurgence. More trachyandesite lavas and Sr (lh). The trace element composition for five analyses (Barrott several biotite rhyolite domes are the last eruptive products of the 1984) is summarized in Table 2. The genetic association of the Kane Springs Wash center. Two of the biotite rhyolite domes lie two rhyolite units has not been adequately examined but their in the southwestern part of the caldera's moat and contain vapor­ close association in space and time suggests that they may be phase topaz. Novak (1984) reports a K-Ar (sanidine) age of 13.2 co-genetic. ± 0.2 Ma for one of these domes, about 900,000 years younger The rhyolitic ash flows and early topaz-bearing lavas of the than the comenditic ash-flow eruptions. The largest dome is northern Wilson Creek Range are part of a Miocene series of about 2 km across and is underlain by an initially erupted blanket potassic mafic lavas and rhyolite with no intermediate composi­ of tephra. Novak interprets the topaz rhyolites to be cogenetic tions known to be contemporaneous. This early Miocene vol­ with the early trachytes and comendites and suggests that they canic field developed within the bounds of a huge Oligocene were derived from a residual pocket of magma within the solidi­ caldera complex-the largest of these calderas was the Indian fying sub-caldera chamber. Another topaz rhyolite dome occurs Peak caldera which formed upon the eruption of several thou­ 10 km to the SE, where it plugs the vent of a post-caldera tra­ sand cubic kilometers of dacitic magma (Best et aI. 1985). The chyte. Small patches of variably potassic olivine basalt (OJ to younger high silica tuff may have been erupted from a trap-door 2.3% K20) cap sections both inside and outside the caldera and caldera and has a much smaller volume (less than 20 km3; Bar­ have K-Ar ages of 12.7 to 11.4 Ma. rott 1984). The Miocene volcanic rocks were erupted about the Important differences in the mineralogy exist between the same time as the bimodal sequence of potassic mafic lavas and early rhyolites and the younger biotite and topaz rhyolites. Novak rhyolites described from the southern end of the Wah Wah and (1984) reports that the biotite rhyolites contain both sanidine Needle Ranges. However, they are distinct from these lavas in (Or6o) and plagioclase (Ab76) while the ferroedenite and earlier several important ways. 1) The rhyolites of the northern Wilson comenditic rhyolites contain only sodic sanidine or anorthoclase. Creek Range are not a part of the more southerly volcanic belt, In addition, the topaz rhyolites contain quartz, Fe-rich biotite, which includes topaz rhyolites of southwestern Utah. They lie on ferroedenite, and fayalite, which is rarely reported in other topaz- 20 Christiansen, Sheridan, and Burt

i N

Kane Springs Wash

Caldera, Nevada

o km 5 ! (

Figure 8. Generalized geologic map of the Kane Springs Wash caldera, southeastern Nevada (after Novak 1984). Map symbols: Tb = basaltic lava flows; Trt = topaz rhyolite lava domes; Tr = post-caldera rhyolite lavas and associated tephra; Ttr =interstratified post-caldera trachyandesite lavas and rhyolitic ash-flows; Tts =post-caldera extrusive trachyte and syenite dome complex; Tkw =intracaldera exposure of zoned (trachyte to rhyolite or comendite) Kane Wash tuff which is associated with collapse of the caldera; extra-caldera rocks are stippled; *= vents fur rhyolite domes. bearing rhyolites (Table 8). Accessory phases include magnetite, than Cl in silicate (especially hydrous) minerals (cf. Kovalenko et ilmenite, and zircon. Earlier rhyolites either contained no hydrous al. 1984). Nonetheless, the eruption of comendite and attendant phases (some of the ash flows) or ferroedenite without biotite. loss of a Cl-rich vapor from the residual melt could leave a To date, little geochemical information has been published non-erupted residue with a higher FICI ratio. Subsequent frac­ for the volcanic rocks of Kane Springs Wash. Novak (1984) tionation of this magma might produce a metaluminous·rhyolite reported one major-element analysis of the topaz rhyolite show­ enriched in incompatible elements. Such a scenario was outlined ing that it is indeed a typical metaluminous high-silica rhyolite by Christiansen et al. (1983a) for the generation of some meta­ (Table 1). Compared to the comenditic ash flows, the topaz luminous rhyolite magmas. rhyolite is enriched in F, AI, Ca, and K, but it is depleted in CI, The Kane Springs Wash center is one of a number of peral­ Fe, and Ti. A vitrophyre from the topaz rhyolite dome contains kaline volcanic centers in the Great Basin of Nevada and adjacent 0.49% F compared to 0.34%F in a glassy comendite. More impor­ parts of California and Oregon (Noble and Parker 1974). How­ tantly, the F/CI ratio of the comendite (2) is much lower than ever, the volcanic field lies on an east-west belt of predominantly that in the younger topaz rhyolite (10), even though both units metaluminous volcanic rocks ·of late Cenozoic age (Figure 4). are enriched in fluorine and are presumed to be co-magmatic. A The segment of the belt in eastern Nevada is paralleled by the less F-rich ferroedenite rhyolite has 0.17% F and an FICI ratio of Pahranagat shear system (Tschantz and Pampeyan 1970), which slightly less than 3 as well. It is doubtful that the closed-system shows late Cenozoic strike-slip movement. Basin and range fault­ fractionation ofa parental comendite could produce an aluminous ing developed several million years after the Kane Springs Wash melt or elevate F/CI ratios. Fluorine should be more compatible volcanism. Topaz Rhyolites 21

Eastern Great Basin Chronologie Summary

Age Basalt K-mafic Rhyolite m.y. B. Andesite Calc-Alkaline Comments o T x ;( t ;; x E~W. Extension ~ >< ..( x 10 n 1 \ - i'" NE-SW Extension Miocene ~Iull" 20 Pt. Colorado Plateau ( u uplift )1\ Rhyolite of Pine Grove ,! 30 .r"\,...... , .c.. :5 I ~ ) 40 ~

I, Figure 9. Schematic representation of magmatic and tectonic activity in the eastern Great Basin of Nevada and Utah. There appear to be several separate eruption episodes for topaz rhyolites (x) in this region: 1) contemporaneous with potassic mafic lavas and waning calc-alkaline magmatism in western Utah and southeastern Nevada which peaked about 20 Ma. (This volcanism was accompanied by NE--:SW faulting along the Pioche-Marysvale mineral belt and the formation ofa topaz-bearing Climax­ type Mo deposit at Pine Grove.); and 2) a younger series of topaz rhyolites that are contemporaneous with basalts and basaltic andesites in western Utah and southern Idaho. (Tectonism accompanying this last episode was produced by E-W extension and normal faulting.)

Topaz rhyolites in the eastern Great Basin: A summary tions from intermediate/silicic to potassic basaltic lavas is similar to that observed in Colorado but postdates it by about 6 to 10 Two groups of topaz rhyolites can be discerned in the east­ Ma. ern Great Basin of Nevada and Utah (Figure 9). The division is based on the age and nature of the associated magmatism. An 10. Cortez Mountains, north-central Nevada early Miocene group, associated with K-rich mafic lavas or calc­ alkaline intermediate composition (dacites and rhyodacites) lavas Several plugs and flows of Miocene rhyolite occur in the and tuffs, was erupted 23 to 18 Ma ago. This group is represented southern end of the Cortez Mountains in north-central Nevada by the older topaz rhyolites of the Wah Wah Mountains vicinity; (Gilluly and Masursky 1965). Wells et al. (1971) were the first to the lavas of the northern Wilson Creek Range, the rhyolite at report that the rhyolites contained topaz in "vesicles." The rhyo­ Spor Mountain, and the Be-rich Sheeprock granite (Christiansen lites have been studied in several investigations concerned with et al. 1983b). A late Miocene to Pleistocene group, associated the age and origin of Au-Ag mineralization at the nearby Buck­ with potassic basaltic magmatism, was erupted after about 13 horn and Cortez mines (Wells et al. 1971; Rye et al. 1974). Ma. Intermediate composition lavas and tuffs are very small in The rhyolites crop out over an area of about 3 km2 and volume, but include the andesites of the area around the Mineral occur in slightly dissected domes with pronounced flow banding Mountains, Utah, and the trachytes of Kane Springs Wash, Ne­ (Figure 10). Topaz rhyolites from the vicinity of Horse Canyon vada. These magmatic episodes are separated by an apparent have K-Ar ages of 15.3 Ma (Wells et al. 1971) and 14.5 Ma Miocene "lull" in volcanic activity in the extreme eastern Great (Armstrong 1970). They intrude or overlie slightly older flows of Basin. Both groups of rhyolitic rocks are compositionally similar basaltic andesite (16.3 Ma) and the Ordovician Vinini Formation and were erupted during apparent crustal extension. Most of the (siltstone, shale, and sandstone). Similar bimodal associations of rhyolites occur in EW-belts nearly parallel to the presumed direc­ Miocene rhyolite and basaltic andesite are common in north­ tion of extension. The change in the dominant magma composi- central Nevada (e.g., Shoshone Range and Sheep Creek Range). 22 Christiansen, Sheridan, and Burt

Cortez - Buckhorn, Nevada 012, , , i km N Tba

BUCKHORN + MINE

Tv

Figure 10. Generalized geologic map of the Buckhorn area in the southern Cortez Mountains, Nevada (after Rye et aI. 1974). Miocene topaz-rhyolite lavas and domes (Tr) were ernpted in close proximity to slightly older flows of basaltic andesite (Tba). Map symbols: Qts = undivided Quaternary and Tertiary sedimentary rocks; Tr = Miocene rhyolites; Tv = mid-Tertiary volcanic rocks; Ii = Jurassic intrusive rocks; Pu = undivided Paleozoic sedimentary rocks.

The rhyolites are generally phenocryst-poor (less than 3%) are similar in their Pb-isotopic composition to geochemically felsites with a few lithic fragments. The phenocrysts are smoky distinct Oligocene volcanic rocks in the Cortez Mountains. Thus, quartz, plagioclase and sanidine accompanied by sparse biotite. the radiogenic Pb isotope ratios may have arisen by contamina­ Miarolitic cavities are common and are lined with topaz, fluorite, tion of both the rhyolitic magmas and the older Oligocene mag­ and silica minerals (Wells et aI. 1971). mas by the same crustal reservoir. The Pb-isotope ratios may not No published information exists for the major element geo­ be inherited from their sources. chemistry of the Miocene rhyolites. The average concentrations The Miocene rhyolites of the Cortez Mountains appear to be of some trace elements are given in Table 2. Their Rb-rich nature spatially and temporally associated with Au, Ag, and Hg mineral­ indicates that they are similar to other topaz rhyolites. In addition, ization at Horse Canyon and at the Buckhorn mine, which sug­ Wells et aI. (1971) state that they also contain high concentra­ gested to Wells et aI. (1971) that they were the source of the tions of Sn (20 ppm) and Be (10 ppm). Initial strontium-isotope ore-forming fluids. In contrast, on the basis of low D/H ratios in ratios determined on two felsites with relatively high Sr concen­ the alteration minerals, Rye et aI. (1974) demonstrated that me­ trations are similar to those from the Thomas Range (Table 3; teoric water was the principal ore-fluid. Meteoric waters may 0.7086 ± 0.0002; 0.7080± 0.0002 calculated at 14.9 Ma) and have been heated by the rhyolitic magmatism creating a small are consistent with the involvement of a crustal component in geothermal system. The Au and Ag mineralization occurs in veins their generation. Lead isotope ratios for two samples of the rhyo­ controlled by NNW-trending faults. lite (Rye et aI. 1974) fall within the lower portion of the Area II The Miocene volcanism in this area is apparently related to field delimited by Zartman (1974). However, they have lower the development of the Cortez (or Nevada) rift (Mabey et aI. 87Sr/86Sr and 207Pb/ 204Pb than many other felsic Area II rocks 1978; Stewart et aI. 1975). The rift coincides with a prominent that show evidence in terms of high initial 87Sr/86Sr, aeromagnetic high that trends NNW across north-central Ne­ 207Pb/ 204Pb, and 208Pb/ 204Pb ratios and low epsilon Nd (D. E. vada, and is marked by voluminous basaltic andesite and rhyolite Lee, unpublished data; Stacey and Zartman 1978; Farmer and with minor basalt (Figure 11). Rifting postdates Oligocene erup­ DePaolo 1983) for a significant sedimentary or metasedimentary tions of calc-alkaline rhyodacite to rhyolitic ash flows. The rift component in their sources. Nonetheless, these Nevada rhyolites merges to the north with the Snake River Plain-Columbia River have distinctly higher uranogenic Pb isotope ratios than the topaz volcanic fields (Figure 12). Mabey et aI. (1978) suggest that the rhyolites from Lake City, Colorado, which suggests that the Cortez rift was produced by the initial Cenozoic extension in this sources of the Cortez rhyolites are older or had higher U/Pb part of the Basin and Range province. It is parallel to anp con­ ratios. However, it is important to note that the topaz rhyolites temporaneous with several other northwest-trending extensional Topaz Rhyolites 23

-----,--,---- II l I I [ I '. \, '" ,,- I I ~ Daa

~ Tr

Imm~~~ Tba lI u o I KM

Figure 12. Map showing the distribution of Miocene (20 to 10 Ma) volcanic rocks and tectonic features of the northwestern United States (after Davis 1980). The relationship of the Cortez rift (C-R) to the western limit of the Precambrian continental crust (heavy dashed line) is shown along with other NW-trending tectonic features: DS =feeder dikes for the CRE = Columbia River Basalt; WSR =western· Snake

II River Plain graben; M = Monument dike swarm; OV = Orevada rift o 50 (Rytuba and McKee 1984). The subduction-related volcanic arc (CR = km Cascade Range) active at the time was located near the continental margin. The topaz rhyolites of northern Nevada were erupted during this Figure 11. Generalized geologic map of north-central Nevada showing period of NE-SW extension and basaltic volcanism and are contempo­ the distribution of 13.8 to 16.3 Ma lava flows and rhyolite domes (after raneous with peralkaline rhyolites in NW Nevada, NE California, and Stewart et al. 1975). The volcanic rocks are concentrated along the SE Oregon. Cortez rift that opened 15 to 16 Ma. The locations of topaz rhyolites in the Sheep Creek and Cortez Mountains are shown. Also shown are the traces of the Golconda (GT) and Roberts Mountain (RMT) thrusts has been identified in the southern Sheep Creek Range near the (Stewart 1980) that may mark the approximate site of the rifted margin old Izenhood Ranch, which lies north of Battle Mountain (Fries of the Precambrian continent. Map symbols: Qa - Quaternary alluvium; Tr - Miocene rhyolite domes and flows; Tba - Miocene basaltic andesite, 1942). The rhyolites are the hosts for cassiterite/wood tin miner­ locally including andesite, basalt, and dacite of uncertain age; U - un­ alization similar to that found in the Taylor Creek Rhyolite in divided Tertiary, Mesozoic and Paleozoic rocks. Bold lines are normal New Mexico. faults. Miocene rhyolites (ca. 14 Ma; Stewart et al. 1977a) extend nearly continuously from the Nevada-Oregon border south­ features in the northwestern United States, including the western southeast for 150 km in a zone about 40 km wide along the Snake River Plain graben and the vents for the Columbia River Cortez rift (Figure 11). The rhyolites were apparently emplaced basalts (Figure 12). The continuity of the rift is interrupted by as coalesced domes and lava flows like those in the Thomas younger northeast-trending faults produced after reorientation of Range, Utah. Individual domes and flows cover from 3 to 100 the regional stress-field about 10 Ma (Zoback and Thompson km2(Stewart et al. 1977a). In the Sheep Creek Range, the rhyo­ 1978). The Cortez rift appears to have formed near the presumed lite intrudes a thick section of 14.8 Ma basaltic andesite (59% eastern boundary of the Precambrian crystalline basement (Fig­ Si02) and is overlain by younger (10 Ma) lavas of olivine basalt ure 1), which also appears to limit the distribution of topaz rhyo­ (48% Si02) (Stewart et al. 1977a). The rhyolites are part of a lites (Christiansen et al. 1983a). broadly contemporaneous (14 to 16 Ma) series of basalts and basaltic andesites that occur along the Cortez rift. 11. Sheep Creek Range, north-centralNevada The topaz-bearing phase of the rhyolite, near Izenhood Ranch, is a crystal-rich (25 to 35%), flow-banded felsite. It is not Farther north along the Cortez rift, another topaz rhyolite known iftopaz is restricted to a single dome or a group of domes. 24 Christiansen, Sheridan, and Burt Northern Great Basin Chronologie Summary

Age Basaltic or Calc-Alkaline Comments m.y. B. Andesite Rhyolite o x 0 DevelopmentT of Snake River Plain

10 I II - E- W extension x 1 ~g ~~ Cortez Rift opens 0 (NE-SW extension) 20 \J

\ 30 Calc-alkaline Volcanism N. Nevada/UT.

40 Compression ends

Figure 13. Schematic representation of magmatic and tectonic activity in the northern Great Basin. x = topaz rhyolite age; 0 = peralkaline rhyolite age (Noble and Parker 1974; Rytuba and Conrad 1984). The eruption of the rhyolites followed the decline of calc-alkaline intermediate to silicic volcanism of the Oligocene. The topaz rhyolites appear to be closely associated with the opening of the Cortez rift, the development of the Snake River Plain, and the eruption of basalt and basaltic andesite. Peralkaline rhyolites were erupted contemporaneously, but they are chemically and for the most part spatially distinct. Compiled from sources cited in the text and from Stewart and Carlson (1976). The principal phenocrysts are quartz, oligoclase, and sanidine incrustations in fissures produced by the cooling of the lava. with biotite, zircon, titanite, apatite, and Fe-Ti oxides as accesso­ Meteoric waters mobilized by the hot rhyolites were important ries. The vapor-phase mineralogy as described by Fries (1942) ore fluids, but magmatic fluids released during devitrification consists of topaz, pseudobrookite, sanidine, silica minerals, fluor­ probably produced the initial mobilization of Sn. Boiling would ite, garnet, (described as andradite but it is spessartine­ be expected at such a shallow level. almandine), and possibly cassiterite. No pyroclastic deposits are The magmatic and tectonic setting of the rhyolitic volcanism exposed near the Izenhood Ranch locality and they are rare in the is identical to that in the Cortez Mountains. Lithospheric exten­ southern Sheep Creek Range (Stewart et al. 1977a). sion (NE-SW) along the Cortez rift was accompanied by the The felsitic phase of the lava at Izenhood Ranch is composi­ eruption of rhyolite and basaltic andesite. Along the rift zone tionally similar to other topaz rhyolites (Table 1 and 2). How­ basalt is minor in Nevada (but more voluminous in Oregon). ever, it is higher in Fe, Zr, Y, Ga, and Sr than most and toward Judging from the age of the volcanic rocks, the rift formed 15 to the low end ofJhe observed range for Rb (compare Figures 35 16 Ma in a back-arc environment (cf. Snyder et aI1976). Peral­ and 38). Our preliminary studies of the chemical composition of kaline rhyolites (Noble and Parker 1974), generally erupted from other rhyolite lavas in the area suggest that they have similar calderas, are also typical of this period (less than 20 Ma) in the compositions. The devitrified lava is fairly rich in uranium (12 Great Basin of Nevada (Figure 13). For example, 16 Ma comen­ ppm; Christiansen et al. 1980); presumably the magma contained ditic ash flows were erupted from the McDermitt caldera com­ even more, because uranium is generally lost during devitrifica­ plex (Rytuba and McKee 1984), which developed on the western tion of fluorine-rich lavas (Bikun 1980; Christiansen 1980). The flank of the northern Cortez rift. initial 87Sr/86Sr ratio of one sample is 0.7085 (Table 3). 12. Jarbidge, northern Nevada Fries (1942) describes wood-tin and cassiterite in veinlets from the lava. The similarity of the mineral assemblage associated The Jarbidge Rhyolite is a widespread volcanic unit in with the tin mineralization to that developed in miarolitic cavities northern Nevada along the southern margin of the Snake River suggests that the ores originated as fumarolic or pneumatolytic Plain (Coats 1964). Itcorrelates with similar rhyolitic lavas as far Topaz Rhyolites 25

east as the Utah line and as far south as the East Humboldt Range Springs, Idaho. D. R. Shawe (oral communication, 1982) and (Coats et al. 1977). Topaz has been identified in a single thin Dayvault et al. (1984) report that topaz occurs as a devitrification section of the rhyolite (Coats 1964). product in at least one of the domes, China Cap (called Middle The rhyolite was emplaced as a series of volcanic domes and Cone on some maps). The rhyolites erupted during the develop­ flows that are underlain locally by flow-breccias and pyroclastic ment of the predominantly basaltic Blackfoot lava field. Arm­ units. The country rocks through which it was erupted are slightly strong et al. (1975) report K-Ar ages of 0.04 ± 0.02 Ma (whole older volcanic rocks. Coats et al. (1977) report K-Ar ages for two rock), 0.08 ± 0.04 Ma (sanidine), and -0.1 ± 0.1 Ma (whole samples of Jarbidge Rhyolite as 16.8 and 15.4 Ma. The Jarbidge rock) for three specimens collected from these domes. Leeman Rhyolite is overlain by the voluminous Cougar Point Tuff (12.2 and Gettings (1977) report concordant K-Ar, hydration rind, and Ma; Coats and Stephens 1968), which is related to the develop­ thermoluminescence ages of 50,000 years, and G. B. Dalrymple ment of the Snake River Plain. (cited in Pierce et al. 1982) reports a 61,000 ± 6,000 year age for The lavas contain phenocrysts of quartz, sanidine, and sanidine from the rhyolite at China Hat. These ages make this oligoclase-andesine. The presence of small amounts of pigeonitic rhyolite the youngest known topaz bearing rhyolite. Three other clinopyroxene and hornblende as well as the absence of biotite small rhyolite domes occur about 25 km north of China Hat in distinguish the Jarbidge Rhyolite from most other topaz rhyolites the similar Willow Creek lava field. S. H. Evans (written com­ in the western United States. Accessory phases include garnet munication, 1980) reports K-Ar (sanidine) ages of 1.56 ± 0.06 (apparently magmatic), zircon, apatite, and Fe-Ti oxides (Coats and 1.28 ± 0.15 Ma for two specimens from these rhyolites. 1964). The Blackfoot lava field lies in a NW-SE-trending Tertiary The average of three analyses of the Jarbidge Rhyolite are graben flanked by mountain ranges that expose Paleozoic and presented in Tables 1 and 2. These rhyolites are similar to those Mesozoic sedimentary rocks. Mansfield (1927) presents the most from the Sheep Creek Range in their high concentrations of Fe, complete geologic description of the region. Three rhyolite domes K, and Zr relative to other topaz rhyolites. Nonetheless, these (China Hat, China Cap, and North Cone) lie on a NE-trending features are typical of bimodal rhyolites ofthe Snake River Plain line transverse to the graben structure. Vents for basaltic lavas are and northern Great Basin (Wilson et al. 1983). The Jarbidge also aligned along this trend. China Hat is the largest of the domes

,'; Rhyolite is also lower in Rb (275 ppm) than most topaz rhyolites. and has a maximum dimension of 2 km parallel to the lineament; Analyses presented by Coats et al. (1977) suggest that F (less than it rises to a height of almost 300 m above the basaltic lavas. Two 500 ppm) and Be (less than 5 ppm) are low in these lavas. The smaller bodies of rhyolite occur as islands within the reservoir, scarcity of topaz is indicative of its lack of enrichment in incom­ one of which has a K-Ar age of 1.4 Ma. (S. H. Evans, cited in patible lithophile elements. Initial 87Sr/86Sr ratios for three sam­ Feisinger et al. 1982). The domes are older than the basalts ples of the rhyolite collected by R. T. Wilson range from 0.7101 exposed at the surface, but inclusions of still older basalt and to 0.7142 (Table 3); ratIos that are typical of Cenozoic rhyolites andesite are found in the rhyolite lava (Mansfield 1927) and in from north central Nevada and the Snake River Plain (Wilson et tephra (air fall and base surge deposits) beneath China Hat. al. 1983; Leeman 1982a). Therefore, the rhyolites formed in the midst of the mafic magma­ The Jarbidge Rhyolite is the host for Au-Ag mineralization tism. Mabey and Oriel (1970) identified small positive aeromag­ that yielded about $10 million in these commodities before 1942 netic anomalies associated with the rhyolite domes and a (Granger et al. 1957). The mineralization is found in epithermal compound gravity low over the portion of the lava field where quartz-adularia veins. the rhyolites occur. Based on an interpretation of the magnetic The Jarbidge Rhyolite is part of a bimodal assemblage of high associated with the basalts, they suggest that the basalts are silicic (dacitic to rhyolitic) and basaltic volcanic rocks (Coats about 1000 m thick. From the gravity expression of the field, 1964) that was erupted during the formation of the western Mabey and Oriel (1970) proposed that a caldera centered on the Snake River Plain graben (Armstrong et al. 1975; Leeman rhyolite domes collapsed after extrusion of the basaltic lavas, or, 1982a). The lavas are approximately the same age as the basalts alternatively, the anomalies were caused by a buried granitic and rhyolites in southwestern Idaho and the Columbia River intrusion. An interpretation more consistent with the tectonic Basalt ofIdaho and Oregon (Figure 12). The Cortez rift, 100 km setting (typified by late Cenozoic basin and range faulting) and to the west, was developing at about the same time. These rocks volcanological style (rhyolite domes with many small low shield may be magmatic products ofthe initial NE-SW extension of the volcanoes and cinder cones in a plains-style basalt field) is that northern Basin and Range province (cf. Zoback et al. 1981). the lava field formed over a small sediment-filled graben. The Later faults that cut the Jarbidge Rhyolite (and other correlative low-density basin fill could produce the negative gravity anomal­ volcanic units) are north-trending and block out the present ies without invoking caldera collapse, which is rarely associated mountain ranges of northern Nevada (Figure 13). with the eruptions of small, isolated rhyolite domes. Mabey and Oriel (1970) interpret the structures of nearby Gem Valley injust 13. Blackfoot Lava Field, southeastern Idaho such a manner. There, a slightly elongate gravity low is centered A group of five small rhyolite domes occurs near the south on the principal basaltic shield volcano of the Gem Valley lava end of the Blackfoot Reservoir, about 15 km north of Soda field. 26 Christiansen, Sheridan, and Burt

The rhyolites of the Blackfoot lava field are petrographically lava field rhyolites nor in topaz rhyolites in general (with the similar to one another. The domes consist of phenocryst-poor, possible exception of the Jarbidge Rhyolite, see above). Smith generally devitrified, flow-banded lava. Groundmass textures and Christiansen (1980) relate the formation of the Snake River range from glassy to granophyric. Small phenocrysts of resorbed plain to the passage of the North American plate over a mantle quartz, oligoclase, biotite, and Fe-Ti oxides are ubiquitous. Apa­ plume, while Christiansen and McKee (1978) relate its develop­ tite and zircon are common accessory minerals. Dayvault et al. ment to transform-style accommodation of E-W extensional (1983) identified euhedra ofthorite in glassy specimens as well as faulting proceeding at different rates to the north and south of the allanite, epidote, oxyhornblende, and unidentified grains of a Ce­ plain. Because of the location of the Blackfoot lava field on the Th phosphate (monazite?) and a Nb-Y-Ti-Fe oxide. Tridymite, floor of a fault-bounded graben, an extension of the Basin and quartz, hematite, and topaz occur in lithophysae from China Cap. Range province to the south, we prefer to place the development The average major and trace element composition of six of the Blackfoot field into a context of basin-and-range extension, specimens analyzed by Dayvault et al. (1983) are presented in bllSalt intrusion, and bimodal basalt-rhyolite volcanism. Tables 1 and 2. The major element composition of these rhyolites is indistinguishable from other topaz rhyolites with high Si and 14. Elkhorn Mountains, western Montana alkalies and low Ti, Fe, Mg, and Ca. Although none of the specimens analyzed were obsidians, most contain glass in their A group of Oligocene topaz rhyolites has been identified in grounclmass. As a result, fluorine concentrations are quite high and near the northern Elkhorn Mountains of western Montana. (3500 to 5800 ppm) and may be close to magmatic concentra­ Chadwick (1978) has included them in the Helena volcanic field tions. Chlorine concentrations are uniformly low (130 to 580 (Figure 14). The Elkhorn Mountains are bounded on all sides by ppm) yielding characteristically high FICI ratios (>9). The halo­ major faults and were probably uplifted during the mid-Cenozoic gen ratios may have been disturbed by devitrification with the (Smedes 1966). preferential loss of Cl. Trace element concentrations in the rhyo­ The rhyolites in the Elkhorn Mountains were emplaced as lites of the Blackfoot lava field are also typical of other topaz isolated intrusive plugs, dikes, and small domes and lava flows. rhyolites with elevated concentrations of Be (> 10 ppm), Li (>80 Some of the extrusive rhyolites overlie cogenetic pyroclastic de­ ppm), Sn (>10 ppm), Y (> 150 ppm), U (> 15 ppm), and Th (27 posits. Inclusions of the Butte Quartz Monzonite, the dominant to 60 ppm). Feldspar compatible elements such as Ba and Sr are phase of the Cretaceous Boulder batholith, occur in the pyroclas­ strongly depleted. tic deposits erupted from centers within the mountains (Smedes No known mineralization is related to the topaz rhyolites of 1966). Most of the rhyolite masses are less than 1 km across; four the Blackfoot lava field. were mapped by Smedes (1966). Chadwick (1978) obtained a The Blackfoot lava field is centered about 60 km SE of the K-Ar age of 35.8 Ma on a sample from the topaz-bearing rhyolite margin of the eastern Snake River plain in the northern Basin and at Lava Mountain. Two other similar rhyolite flows from the area Range province. Armstrong et al. (1975) outline the temporal have been dated and have K-Ar ages of 37.3 and 36.9 Ma development of late Cenozoic volcanism in southern Idaho. They (Chadwick 1978; Figure 14). point out that a time-transgressive series of rhyolitic ash-flow tuffs Most of the rhyolite is felsitic, flow-banded, and phenocryst­ were emplaced across southern Idaho. Volcanism began in poor. Obsidian is pres~rved along the margins of some dikes. southwestern Idaho approximately 15 Ma and migrated to the Smoky quartz, sanidine, and sodic plagioclase{An10 to An12) are northeast to its present culmination in the Yellowstone area in the principal phenocrysts. The grounclmass is usually spherulit­ Wyoming. Basaltic volcanism was initiated in the wake of the ically devitrified or altered by vapor-phase crystallization. Topaz caldera-forming eruptions and has continued intermittently, creat­ (up to 30 mm long), along with quartz and fluorite, line miarolitic ing the Snake River plain. Some of these plains-style (Greeley cavities and lithophysae (Smedes 1966). 1982) basaltic lavas were erupted from long fissures, but others Smedes (1966) reports whole-rock chemical analyses of form low-shield volcanoes capped by small craters less than 1km several phases of the rhyolite from the Helena volcanic field that across. The Blackfoot lava field is similar to the Snake River plain demonstrate their chemical similarity to other topaz rhyolites in in several important ways, including its age and the style of the western United States (Table 1). Two samples of "soda rhyo­ basaltic volcanism, but the development of rhyolite domes late in lite" from the Elkhorn Mountains, which may be from the topaz­ the history of a basaltic field is atypical of the Snake River Plain, bearing units, contain an average of 8.9 ppm Uand 37.6 ppm Th although several such domes formed in the central part of the (Tilling and Gottfried 1969). Greenwood et al. (1978) report that eastern plain. Likewise, no early ash-flow volcanism can be tied the topaz rhyolite from Lava Mountain contains high amounts of to the Blackfoot field. Perhaps most importantly, the rhyolites of F (0.2 to 0.5%), Sn (10 to 50 ppm), Be (1 to 20 ppm), Nb (30 to the Snake River plain proper are generally pyroxene andlor 100 ppm) and moderate amounts ofMo (5 to 10 ppm). fayalite rhyolites, with moderately high Fe and Zr contents and Silver-bearing galena, sphalerite, fluorite, and quartz occur high KINa ratios that show high equilibration temperatures in veinlets cementing a brecciated phase of the rhyolites at Lava (Leeman 1982a; Hildreth 1981; Hildreth et al. 1984; Wilson et al. Mountain (Smedes 1966). However, most of the Ag, Au, and Pb 1983); none of these characteristics are found in the Blackfoot mineralization in the rocks of the Boulder batholith appears to be Topaz Rhyolites 27

~CRATER MTN. Late Cenozoic Volcanism 7.8U FIELD , ~ In SW Montana MONT 0 37.3 ANA }D cf36.9 Helena VOLCANO BUTTE I77.l Basalt - 0 1'\ '" ~~ _ BASALT ~ ICLa 29 ~"ON l.:.,:) -: ~,:f(; X:vQ -_ 29.1 r\€.\..\) 3'-4~~" ~~~~ LiNEAA:- Rhyollite ',' , (9 _ 1VtENT 0 ,L a v a _ Sample dated in m.a. 37 Mountain _ o Rhyollite • Basalt XButte J;;l Trachyandesite

o ,,! 5,0 KM

X Bozeman

32.7 34.4 N\'" 30.3 ",,\~G\,/ ~5.4 URNS • c\"t \..\) 10 .-, HEPBSA 38.9,>: Dillon r\€. Q (j> 8.4 :~ALTS .4.0 ~ \BEAVERHEAD " 1.9 t::l ~__If:l.~o,:..:.?~MT _ I FIELD ~',' 0 04.0 ~'UPPER ON I Wy \...... _T () 22.9. '0," tA~~iC\)[£)1::' ',::, - ',M· ' ~2.0), , ,.I, , .. ' 10 ) " ~ {-f\ :. ',,", : ::.' Y ELLOW S TON E '\ I ,I. .' \ ,....tZ:2:7 ~ I ~/---f _/SRP I .

Figure 14. Distribution of post 40 Ma volcanic rocks in southwestern Montana (after Chadwick 1978, 1981). The topaz rhyolite lavas and domes ofLava Mountain in the Elkhorn Mountains and other rhyolites in the Helena and Beaverhead volcanic fields are about 10 Ma older than dated basalts from the rest of the region but may be part of a bimodal (basalt-rhyolite) suite associated with the inception of block faulting in the region (Chadwick 1981), Younger volcanic fields include those at Yellowstone and the SW-trending Snake River Plain (SRP).

older than the rhyolitic magmatism (Smedes 1966). The Bald northernmost boundary of basin and range faulting and may have Butte molybdenum prospect (Rostad 1978) is located about 25 acted as a transform allowing lithospheric extension and block­ km north of the topaz rhyolite locality, but it is older (about 48 faulting to proceed south of it (Reynolds 1977). The tectonic and Ma) and is more likely related to magmatism of the type de­ magmatic history of the region is summarized in Figure 15. scribed below from the Little Belt Mountains. The rhyolites of the Helena volcanic field are part of a 15. Little Belt Mountains, centralMontana broadly bimodal assemblage of basaltic and silicic lavas that were erupted in southwestern Montana after about 40 Ma (Chadwick The oldest Cenozoic topaz rhyolites we know of (about 50 1978, 1981). Individual fields do not generally contain both Ma) are exposed in the Little Belt Mountains ofcentral Montana magma types, but basalt flows of uncertain age (Pliocene to Oli­ (Weed 1900; Pirsson 1900; Witkind 1973). The mountainous gocene) do occur in the Helena field (Greenwood et al. 1978). terrain exposes several major alkaline and calc-alkaline plutons This volcanic episode postdates dominantly calc-alkaline inter­ that may be genetically related to the volcanic cover (Witkind mediate to silicic magmatism 45 to 55 Ma (Armstrong 1978) and 1973). may mark the initiation of extensional tectonics and basin subsi­ The topaz rhyolites and their less fluorine-enriched counter­ dence in western Montana (Pardee 1950; Chadwick 1978). parts occur as sills and cylindrical plugs (Figure 16). Topaz has Chadwick (1978) suggests that the Helena and other volcanic been descnbed from a sheeted-sill complex that intrudes Cam­ fields lie along the Montana lineament (or Lewis and Clark line) brian sedimentary rocks near Yogo Peak (Pirsson 1900) and that marks the northern boundary of a Precambrian basement within the rhyolitic "bysmalith" (a forcefully emplaced plug that province (Weidman 1965), This lineament coincides with the has pushed the overlying strata up along one or more circumfer- 28 Christiansen, Sheridan, and Burt Montana Chronologie Summary

Age m.y. Basalt Calc-Alkaline Rhyolite Comments 20 Beginning of A Miocene "lull"

30 f \ 1 I II II x 40 Block-faulting begins (?)

( 50 Mo-mineralized \ \] plutons

Last Regional thrust sheets 60

Figure 15. Schematic representation of Cenozoic magmatic and tectonic activity in western Montana. Topaz rhyolites (x) were erupted during basalt-rhyolite volcanism and during older high-K calc-alkaline and alkaline magmatism of central Montana. Compiled from sources cited in the text. ential faults) at Granite Mountain (Pirsson 1900; Witkind 1973). but an intermediate-composition stock of about the same age is The rhyolite at Granite Mountain has a K-Ar age of 48.8 ± 2 Ma. related to Ag-Pb mineralization. Minor quantities of molybdenite The rhyolite at Granite Mountain is described as being dense and scheelite occur in fissure veins within the stock (Witkind and fine-grained with a groundmass of quartz, alkali feldspars, 1973). The Climax-type(?) Big Ben molybdenite deposit (Olmore biotite, and Fe-Ti oxides. Witkind (1973) classified topaz, along 1979) is located less than 20 km to the west and has an age of with albite, as phenocrystic. However, in view of the apparently 49.5 Ma (Marvin et al. 1973) making it approximately the same devitrified nature of the rocks and its typical development else­ age as the topaz-bearing Granite Mountain rhyolite, as well as the where, the topaz is probably the result of growth from a vapor Bald Butte Mo-prospect in the Elkhorn Mountains. phase. The rhyolite sills at Yogo Peak are also fine-grained but The topaz rhyolites in the Little Belt Mountains are part of a contain granules of tourmaline in addition to topaz in the complex assemblage of plutonic and volcanic rocks that were ail groundmass (Pirsson 1900)~both are probably the result of the emplaced between 54 to 48 Ma (Marvin et aI. 1973). The plutons exsolution of vapor. and laccoliths are composed of felsic (70 to 72% SiOz) to inter­ Pirsson (1900) and Witkind (1973) report chemical anal­ mediate (61 to 67% SiOz) composition rocks. Syenite, shonkinite, yses ofthe topaz rhyolites (Table 1). The sill analyzed by Pirsson and lamprophyre (in places mixed with rhyolite in composite may have been slightly altered; it contains slightly more AI, Mg, dikes) are also exposed in small bodies (Witkind 1973). In Mon­ Ca, and P than most topaz rhyolites. The rhyolite of Granite tana and Wyoming, Eocene magmatism was common 54 to 45 Mountain is typical of its class but has slightly lower Ti and Ma (Armstrong 1978; Figure 15). Calc-alkaline and alkaline in­ higher Mg than other topaz rhyolites. Rubidium is enriched termediate to silicic volcanic rocks and subjacent plutons are the (about 525 ppm) and Sr depleted (about 9 ppm) in this phase as principal expressions of this activity. Lipman (1981) and Snyder well (Witkind 1973), which is consistent with other occurrences. et al. (1976) relate the magmatism to subduction of oceanic Rupp (1980) reports that a specimen from Granite Mountain lithosphere near the continental margin. However, Armstrong contains 3400 ppm F and is likewise enriched in Rb, U, and Nb (1978) suggests that intra-arc rifting and basin sedimentation may but not Sn (Table 2). As for most other topaz rhyolites Ba and Sr have been contemporaneous with this episode, which is correla­ are extremely depleted. Initial Sr-isotope ratios of two samples tive with the Challis volcanism of Idaho and arc-type volcanism from Granite Mountain average 0.7093 (t = 48.8 Ma) (Marvin et and graben formation in northern Washington (Davis 1980). al. 1973). Monger and Price (1979) and Ewing (1980) also suggest that the No mineralization is directly associated with the rhyolites, subduction-related volcano-plutonic arc formed nearer the con- Topaz Rhyolites 29

o 5 IIII II km

MPu

Little Belt Mtn., MT

EXPLANATION TERTIARY INTRUSIVE ROCKS U:::';r;} Rhyolitic/granitic -' - Intrusions ~)-;1 Q lame and /. syenite intrusions

PRE-CENOZOIC ROCKS Mesozoic-Paleozoic sedimentary rocks pC sedimentary rocks

pC igneous and metamorphic rocks

Figure 16. Generalized geologic map of the Little Belt Mountains, Montana (after Marvin et al. 1973), showing the locations of topaz rhyolites (x) and granites relative to alkaline intrusive rocks of the same age. The location of the Big Ben molybdenite prospect is also shown. tinental margin on a strike-slip faulted terrane. Thus, the Eocene the core of Specimen Mountain as an intrusive rhyolite plug that volcanism in Montana may have been "back-arc" in nature, in formed shortly after the emplacement of the upper rhyolite lavas. keeping with the extensional faulting and the apparent tectonic All of the volcanic rocks overlie or intrude Precambrian gneiss, setting of other topaz rhyolites. granite, and pegmatite. Fragments of these rock types also occur as inclusions in the vent agglomerates. A series of arcuate faults 16. Specimen Mountain, north-central Colorado cut the plug and flows-apparently formed as the rhyolite plug collapsed back down its conduit shortly after emplacement (see, Wahlstrom (1941) first reported topaz from gas cavities in for example, Corbett 1966, 1968). These relationships suggest rhyolite lava flows from Specimen Mountain on the crest of the that the rhyolite flows and plug are part ofa small dome complex. Front Range in Rocky Mountain National Park. From Wahl­ The entire complex contains about 1.5 km3 of material. strom's (1941, 1944) descriptions it appears to be similar in its The topaz-bearing lavas generally show conspicuous flow mineralogy, chemistry, and emplacement history to other topaz banding and are locally rich in lithophysae. Quartz and sanidine rhyolites. are the only reported phenocrysts (Wahlstrom 1944). The rhyo­ The rhyolite lavas of Specimen Mountain (Figure 17)· are lite plug contains biotite, oligoclase, magnetite, and rare resorbed underlain by a thick (lOOs of meters) sequence of pyroclastic hornblende in addition to quartz and sanidine. No topaz is re­

I " deposits (apparently composed of fall, flow, and breccia units). ported from this phase of the complex. Wahlstrom (1944) in­ I These in tum overlie a basal complex of trachyandesite (quartz cludes the rhyolites and trachyandesites, with phenocrystic latite) lavas and pyroclastic deposits. Wahlstrom (1944) describes plagioclase, amphibole, biotite, and augite, as part of the same 30 Christiansen, Sheridan, and Burt

105· 50' predominant volcanic rocks in the region are poorly studied rhy­ olitic ash-flow tuffs. Their temporal and chemical relationships to Specimen Mountain. CO the topaz rhyolites at Specimen Mountain are unknown. Like­ wise, the age ofan older series ofbasalts(?) and trachyandesites is not established (Eocene to Oligocene) but they represent only a small volume of the Cenozoic effusive rocks (Corbett 1968). The Never Summer stock, a zoned granodiorite to quartz monzonite intrusion that outcrops about 5 km west of Specimen Mountain, is also the same age (± 1Ma) as the silicic volcanic rocks (Corbett 1968).

17. Chalk Mountain, central Colorado

Chalk Mountain is located on the west side of a narrow valley that separates it from the Climax molybdenite deposit in Lake County, Colorado (Figure 18). Cross (1886) first described topaz and garnet from cavities within this rhyolitic plug. Based on o 1 magnetic and gravity anomalies, Tweto and Case (1972) sug­ ,I, gested that the Chalk Mountain stock and the stock at the Climax Km mine are both apophyses ofa batholith that underlies this portion 40.25' L-._...... L --l of the Colorado mineral belt. Alternatively, Chalk Mountain may Figure 17. Generalized geologic map of Specimen Mountain, Colorado be the downfaulted upper portion of the mineralized intrusive (after Wahlstrom 1944). Map symbols: Trp = rhyolite plug; TrZ = topaz­ system (R. P. Smith, oral communication 1982). It lies on the bearing rhyolite lava; Trt = rhyolitic tuff and lava; Tl = trachyandesite west side of the Mosquito fault, which separates it from the (quartz latite) lavas; pCg Precambrian gneiss and granite. Climax deposits. In spite of this provocative suggestion, little information has been published regarding the rhyolite of Chalk Mountain. Although most references to Chalk Mountain term the volcanic sequence and suggests that the rhyolites were derived rhyolitic mass a stock, Cross (1886) describes it as extrusive in from them by crystal fractionation. It is difficult to test this hy­ origin. The rhyolite outcrops over an area of about 4 km2. pothesis in the absence of more chemical information or precise The rhyolite contains large phenocrysts of sanidine and ages for the trachyandesites and the younger rhyolites. Ifthe topaz smoky quartz. Andesine (oligoclase in some) and biotite are set in rhyolites were indeed derived from the trachyandesites, this com­ an aphanitic groundmass that contains topaz, magnetite, rare il­ plex may be unique among U.S. occurrences. However, a promi­ menite, and apatite. Topaz also occurs in drusy quartz-lined cavi­ nent Si02 gap (64.5% to 75.8% Si02 in nonhydrated samples) is ties along with garnet, sanidine, biotite, and Fe oxides (Cross and obvious on variation diagrams. Wahlstrom (1944) also notes Hillibrand 1885; Cross 1886; Pearl 1939). New analyses (by disequilibrium features in the trachyandesites-ealcic plagioclase electron microprobe) of biotite from the Chalk Mountain rhyolite in intermediate composition lava and resorbed hornblende and show that it contains intermediate Fe/(Fe + Mg) ratios (ca. 0.5; biotite-possibly suggesting that an even more mafic magma Figure 31). Fluorine concentrations in the biotite are not excep­ mixed with a rhyolitic magma before eruption to produce the tionally high (averaging about 0.5 wt%) and moderate FICI ra­ trachyandesite. These trachyandesites are chemically similar to tios are typical (log FICI clusters around 1; Figure 32). In both of the magmatic inclusions in the Spor Mountain rhyolite in Utah, these respects the biotites are more similar to those from the which show similar evidence of magma mixing. Mo-mineralized Pine Grove stock in southwestern Utah (Keith Chemically, the topaz rhyolites are virtually indistinguisha­ 1982) than to biotites from the hydrothermally altered rocks of ble from others in the western United States and have high Si, K, the Mo deposit at nearby Henderson, Colorado (Gunbw et al. and Na and low Ti, Mg, and Ca (Table 1, compare with Figure 1980). The compositions of abundant magnetite phenocrysts are 35). Analyses of the topaz-free rhyolite plug are similar but more uniform eXusp .ca. 0.9) and, when combined with analyses of variable. For example, in the plug, Si02 ranges froni 68.9% to sparse ilmenite, indicate that high oxygen fugacities prevailed dur­

77.8% and Al203 ranges from 11.7% to 15.7%, perhaps as a result ing crystallization of the Chalk Mountain rhyolite (log f0 2 about of mixing or alteration. The dacites have potassic intermediate -10.3 at 830°C). This is about 3.5 log units above the QFM compositions (64% Si02). oxygen buffer (Figure 30). Apatite-biotite geothermometry, as The rhyolites of Specimen Mountain are contemporaneous formulated by Ludington (1978), gives sub-magmatic tempera­ with a variety of dacites and rhyolites (Corbett 1968) that were tures (-590°C) for apatite inclusions in biotite. emplaced between 27 and 28 Ma in north-central Colorado. An analysis of the Chalk Mountain rhyolite published by Although the topaz rhyolite of Specimen Mountain is a lava, the Cross (1886) shows that the rhyolite is similar to other topaz Topaz Rhyolites 31

Volcanic Center Approximate Age Magma Type 0 (Ma) 39 Deer Peak 38-32 andesite-trachyte (Iatite) Tomichi Dome 38 topaz rhyolite Bonanza volcanic field 38-33 andesite-dacite-rhyolite Mount Aetna caldera (MA) 36 rhyolite San Juan volcanic field 35-26 andesite-dacite-rhyolite 39 Mile volcanic field 34 andesite 18 Waugh Mtn basalt Nathrop Volcanics 29 topaz rhyolite @ Silver Cliff-Rosita 32-26 andesite-trachyte-rhyolite, topaz rhyolite Chalk Mountain 27 topaz rhyolite Buffalo Peaks andesite Cripple Creek 29 phonolite Hillside <29 andesite-trachyte (Iatite)

~ ~~SILVERCLIFF' •. e • ROSITA . San Juan . " Volcanic Field '. ioo~~ ~Deer ~)eak 38 ~ 0,).

San Luis Valley '.\~~ ~'.:

Figure 18. Locations oftopaz rhyolites (solid dots) in central Colorado in relation to other middle to late Cenozoic volcanic fields (from Epis and Chapin, 1975). The volcanic fields are listed in their approxi­ mate order of development. Age and magma composition references are given in the text.

rhyolites (Table 1). The principal differences lie in slightly higher 18. Nathrop, central Colorado A1203, MnO, and MgO contents than in most topaz rhyolites (compare Figure 35). The validity of the analysis is difficult to The Nathrop Volcanics lie on the west side ofthe Mosquito assess as no modern analyses of samples from Chalk Mountain Range about 10 km north of Salida, Colorado. These rhyolites have been published. were first reported to contain topaz and garnet by Smith (1883) Chalk Mountain contains small molybdenite occurrences and Cross (1886). Van Alstine (1969) and Schooler (1982) and small amounts of silver ore have been removed from the further described their geology. rhyolite contacts (Pearl 1939). The Chalk Mountain rhyolite is In general, the volcanic rocks rest directly on Precambrian 27 ± 1.9 Ma (Tweto and Case 1972) and is one of numerous gneissic quartz monzonite and form low isolated hills along the rhyolitic plugs, stocks, and dikes adjacent to the approximately front of the Mosquito Range (Figure 19). At Ruby Mountain, 30 Ma Climax stock (White et al. 1981), and is probably 00­ where topaz and spessartine garnet are found in lithophysae, magmatic with this fluorine-rich rhyolite porphyry. An older pumiceous tuff, and breccia (ca. 30 m thick), with fragments of series (Late Cretaceous) of diorite to granodiorite plutons appear rocks from the Precambrian basement, are overlain by perlite (ca. to be the only other igneous rocks in the area. The mid-Tertiary 35 m thick) that apparently forms the basal vitrophyre to a cap­ rhyolites are cut by the Mosquito high-angle normal fault, which ping rhyolite lava (ca. 100 m thick). The lava is flow banded with is part of the northward extension of the Rio Grande Rift. The lithophysal and spherulitic textures. Sugarloaf Mountain is also initial tectonism associated with the development of the rift in underlain by a basal tephra and breccia unit. Scott (1975) sug­ Colorado occurred 26 to 30 Ma (Tweto 1979) and was con­ gests that all the rhyolite vented from Bald Mountain (Figure 19). temporaneous with the magmatism at Chalk Mountain. However, the field study of Schooler (1982) demonstrates that 32 Christiansen, Sheridan, and Burt

new Sr-isotope analysis of a whole-rock sample suggests that its initial 87Sr/86Sr ratio is relatively high (0.714 ± 0.0060, Table Qal 3). The large uncertainty is a result of the high Rb/Sr ratio and uncertainty of its age. (29.3 ± 1.5 Ma was used-the oldest of three ages reported by Van Alstine 1969. Other K-Ar ages are 29.1 ± 0.9 and 28.0 ± 0.8 Ma.) Additionally, the effects of even Arkansas small amounts of upper crustal contamination are readily appar­ Valley ent in rocks with low Sr content (3.2 ppm) like the Nathrop Graben rhyolites. For example, 1%assimilation of a component with 300 ppm Sr and an 87Sr/86Sr ratio ofO.nO (values not unrealistic for the Precambrian country rocks at 30 Ma) would double the Xgd present Sr content and elevate the initial ratio from 0.706 to 0.713. Any conclusions based on this isotopic analysis must be tempered by these facts. The Nathrop Volcanics are spatially associated with fluorite deposits that formed in a near-surface hot-spring environment at temperatures of 119° to 168°C (Van Alstine 1969). Van Alstine EXPLANATION assigns the deposits a post-Miocene age and does not relate their formation to the fluorine-rich rhyolite volcanism. IQal! Quaternary alluvium o 2km I I The topaz rhyolites from Nathrop are located near the west­ ~ Nathrop volcanics (28-29 m.y.) rhyolite ern margin ofthe Thirtynine Mile volcanic field (Figure 18; Epis -+-+-+- Dike M'i:;<] Wall Mountain Tuff (36 m.y.) \""~" rhyodacite and Chapin 1968). Rhyodacitic to rhyolitic ash flows are the - Fault IXgdl Precambrian granodiorite oldest volcanic units in the field. They were probably erupted and quartz diorite from centers west of the Thirtynine Mile volcanic field (e.g., the 36 Ma Wall Mountain Tuff; Chapin and Lowell 1979). The Figure 19. Simplified geologic map of the Nathrop Volcanics from cen­ Thirtynine Mile volcanic field consists predominantly of andesitic tral Colorado (after Scott 1975; Van Alstine 1969; and Schooler 1982). lavas and breccias with minor amounts of basaltic lavas, diorite The rhyolites near Nathrop include lavas (cross-hatched) and tuffs (solid plugs, and rhyolitic dikes (Epis and Chapin 1968; Epis et al. color). Three vents probably exist near Nathrop and a fourth at Bald Mountain. 1979). Presumably, the volcanism was related to the develop­ ment of a composite volcano about 34 Ma. A rhyodacitic ash vents also existed at Sugarloaf Mountain, Nathrop Butte, and flow was erupted at about the same time as the Nathrop Volcan­ Ruby Mountain. ics (the 29 Ma old Gribbles Park Tuff; Steven 1975) and overlies The rhyolite lavas contain sparse phenocrysts of sanidine some of the intermediate composition lavas. The topaz rhyolites (Or6oAb38), oligoclase (Or8Ab81; yielding a two-feldspar tem­ of Nathrop (K-Ar age 28-29 Ma; Van Alstine 1969) thus appear perature of 630°C), smoky quartz, and traces of biotite (Van to be part of an andesite-rhyodacite-rhyolite series of calc-alkaline Alstine 1969). The groundmass contains chlorite, topaz, magne­ nature. Nonetheless, strongly alkaline volcanism at Cripple Creek tite, and fluorite. Lithophysal cavities in the lava contain topaz, (75 km east) is contemporaneous (Steven 1975). A marked spessartine garnet, sanidine, silica minderals, magnetite, hematite, change in the nature of the magmatism occurred about 18 Ma opal, and calcite. when the "andesite" of Waugh Mountain was erupted in the The major and trace element chemistry ofthe Nathrop rhyo­ southern part of the Thirtynine Mile volcanic field (Wobus et al. lite has been reported by several investigators; analyses are pre­ 1979). These lavas are similar in age and composition to "silicic sented in Table 1. The major element chemistry is similar to all alkalic basalts" of a post-Oligocene episode of bimodal basalt­ topaz rhyolites with high Si, K, Na, Fe/Mg, and F and low rhyolite volcanism (Lipman and Mehnert 1975; 133). Tweto concentrations of Ca, Ti, and Mg. Zielinski et al. (1977) report (1979) estimates that the Arkansas Valley graben, adjacent to the three trace element analyses of separate phases of the lava (Table Nathrop rhyolites, began to subside approximately 28 Ma. The 2) and show that the vitrophyre is enriched in Mo and the litho­ apparent superposition of the rhyolite dome complex on some of phile elements U, Th, Be, Li, and Nb relative to most rhyolites; the range-front faults (Figure 19) suggests that the tectonism and these values are typical of topaz-bearing varieties (Christiansen et volcanism may have been nearlyconcutrent. The tectonic and al. 1980). The REE concentrations are shown diagramatically in magmatic activity of central Colorado are summarized in Figure Figure 40g where the Nathrop rhyolite is compared to a calc­ 20. alkaline rhyolite from Summer Coon volcano (Oligocene age from the San Juan volcanic field; Zielinski and Lipman 1976). 19. Silver Cliff-Rosita, central Colorado Although the pattern is reminiscent of those from other topaz rhyolites, it has higher La/LuN and Eu/Eu* than most. A single The Silver Cliff and nearby Rosita volcanic fields are 10- Topaz Rhyolites 33 Colorado Chronologie Summary

Age Calc-Alkaline m.y. Basalt Alkaline A-D-R Rhyolite Comments o

Renewed uplift II ranges ,I 10 r-- 'I ~ , Miocene ·'u/l"

x IMOPIulons 20 "--- Spanish Peaks l) intrusions

Iulons + { (iIMO P 30 Rio Grande Rift (y) ~ opens 40

Figure 20. Schematic representation of magmatic and tectonic activity in southern Colorado. The ages of topaz rhyolites (x) show that they were erupted in at least two episodes-one contemporaneous with high-K calc-alkaline magmatism (andesite-dacite-Iow silica rhyolite) and the other group contempo­ raneous with basalt (as used by Lipman and Menhert 1975) and high silica rhyolite. The ages of molybdenum-mineralized stocks (Mo plutons) of the Climax-type are also shown. The term calc­ alkaline is used here to denote igneous rock series whose members contain greater than about 60% Si02 and generally display continuous silica variation diagrams. Basalt or basaltic andesite is used where no intermediate composition rocks are observed. cated along the northeastern side of the Wet Mountain Valley west of Silver Cliff (Figure 21), produced a more diverse group of graben (Figure 18). Cross (1896) first reported topaz and garnet rocks and spanned a much longer period of time. Early eruptions from rhyolitic lavas from these volcanic fields. Subsequent inves­ (32 to 29 Ma) of smaIl volumes of andesite and rhyolitic tephra tigations by Siems (1968), Kleinkopf et aI. (1979), and W. N. from the Deer Peak volcanic center (Figures 18 and 21) were Sharp (1978) have demonstrated that the volcanism in the two followed by extrusions of (rhyo-)dacitic lavas. The rhyolitic pro­ areas occurred contemporaneously 32 to 26 Ma. The rhyolites in ducts of this early phase of volcanism are grouped together as Tro both fields have been related to poorly documented collapse and the non-rhyolitic rocks as Tmo in Figure 21. After a 1 to 2 calderas-an unusual mode of occurrence for topaz rhyolites. Ma lull a younger sequence oftrachyte and trachyandesite (latite) Both complexes are small; neither covers much more than lavas (Tmy) was accompanied by the eruption of topaz rhyolite 30km2. lavas (27 to 26 Ma). (Most of the mafic lavas in both sequences Acording to Siems (1968) the initial eruptions at Silver Cliff have potassic affinities.) Siems (1968) described this center as an were pyroclastic eruptions that emplaced a thick sequence of incompletely developed resurgent caldera, but documents no non-welded rhyolitic tuff and breccia on the Precambrian base­ large-scale collapse or structural resurgence. W. N. Sharp (1978) ment. Apparently these deposits accumulated in a subsiding basin cites the formation of dikes and fissures as evidence for resur­ or were preserved by caldera collapse (Figure 21; Siems 1968). gence. The duration of the activity and the interlude between the Tephra accumulations exceed 600 m as exposed in mine shafts; older and younger volcanic events suggests that two perhaps low residual gravity anomalies suggest they are contained in a unrelated volcanic cycles are represented at Rosita. (The dates are trough-shaped graben (W. N. Sharp, cited in Scott and Taylor from Scott and Taylor 1975; and W. N. Sharp 1978.) 1975). Extrusion of rhyolite domes and flows with basal vitro­ Modern analyses (phair and Jenkins 1975) show that the phyres closed the volcanic cycle at Silver Cliff. The lavas are 40 rhyolites from Silver Cliff-Rosita are very similar to. all other to 50 m thick and have ages of 27 to 26 Ma. topaz rhyolites in their major constituents (Table 1). Two rhyolite The volcanic center at Rosita, centered about 8 km south- specimens have an average U content of 20 ppm and Th content 34 Christiansen, Sheridan, and Burt

Silver Cliff/Rosita, Colorado 108 105 104 0 105 20' \~ ,\ 0 Specimen Mtn sc ~) 127 28 + f • ) 40 ~ ~ 40

/Chalk Mtn OHenderso~ ~ (27-28)\( -9 38 IT r~: ~::o::~ 0 ~Chmax" \\ 39

Boston Peak - ~ LNathrop "\ R~und MIn r \~28-29) Tomichi Dome _ (38)

• ~ Sitv", Cliff (26) 38 38 o 5 I "" La~~8~lty ~ ~\ \ km \y \\ Figure 21. Generalized geologic map of Silver Cliff-Rosita area, Colo­ rado (after W. N. Sharp 1978). Topaz-bearing rhyolitic lava flows and dome (Trf) are underlain by tephra and sedimentary rocks (Trt) that I~; accumulated in a small NW-trending graben at Silver Cliff (8C). Rhyo­ /\ I lites surround a central core of older (Tmo) and approximately contem­ 108 107 106 104 poraneous (Tmy) mafic volcanic rocks (andesite, trachyandesite, and Figure 22. Faults with major Neogene movement in the area of the Rio trachyte) at Rosita (R). Vents (*) and mineralized breccia pipes (+) are Grande rift in Colorado (after Tweto 1979), compared to the location of also shown. Both volcanic centers are located near a fault (dashed) that topaz rhyolites (filled circles) described in the text. The ages of the separates asediment-(QTs)-filled graben from Precambrian gneisses and rhyolites are shown in parentheses. The Rio Grande rift began to form 30 granitic rocks (peg) of the Wet Mountains. A portion of the Deer Peak to 27 Ma (Eaton 1979; Tweto 1979), about the same time as the topaz­ volcanic center is also shown in the SE comer of the map. rhyolite magmatism was initiated in the region. The locations of topaz­ bearing molybdenite deposits (0) younger than 30 Ma (White et aI. 1981) are shown for comparison. of 31 ppm. Trace element analyses of vitrophyres reported by Mutschler et al. (1985) show the rhyolites to contain 850 to 1200 ppm fluorine and relatively low concentrations of incompatible Pliocene time (Scott and Taylor 1975). Apparently this tectonic elements like Rb (<250 ppm) and Li (<25 ppm). The rhyolites episode occurred shortly after the development of several vol­ are nonetheless enriched in Nb and depleted in Zr (-100 ppm), canic centers on the eastern side of the graben (Scott and Taylor Sr «20 ppm), and Ba «60 ppm) in common with other topaz 1975)-Silver Cliff-Rosita (andesite to rhyolite, 32 to 26 Ma), rhyolites. Many analyses (Cross 1896; Mutshler et al. 1985) of Deer Peak (andesite to trachyandesite, 38 to 32 Ma), and Hillside felsitic specimens have high K20/Na20 ratios that suggest (andesite to trachyandesite, less than 29 Ma). The locations of alteration. these fields are shown in Figure 18. The tectonism and magma­ Deposits of hypogene and supergene silver, gold, lead, zinc, tism ofthe Wet Mountains appear to be part ofthe development and copper are associated with the Tertiary volcanic rocks. In the of the Rio Grande rift system in Colorado (Tweto 1979; Eaton Silver Cliff district the deposits are cavity fillings or replacements 1979) and are contemporaneous with the eruption ofother topaz in the lavas and tuffs. The most productive mines are breccia rhyolites in Colorado (compare Figure 22). pipes that formed within the Precambrian gneisses; their relation­ ship to the volcanic cycle is unclear but their similarity to vent 20. Tomichi Dome, central Colorado facies rocks suggest that they are also related to the younger rhyolites. In the Rosita Hills district, mineral deposits occur in Tomichi Dome is located about 35 km east of Gunnison, fIssure veins and along faults in all of the volcanic units. Colorado. Stark (1934) reported topaz from the rhyolite as part The Wet Mountain Valley is a tectonic basin that began to of a study of heavy minerals from Tertiary "intrusions" in central form in late Cretaceous or early Eocene times. However, major Colorado. The geology of the rhyolite (Figure 23) is described by uplift of the flanking ranges occurred in early Miocene to late Stark and Behre (1936) and briefly by Ernst (1980). Topaz Rhyolites 35

Tomichi Dome, Colorado

o 1 2 ! I ! krn Tertiary rocks t~;:H?ll Massive rhyolite

~ t,, .:j Spherulitic rhyolite

• Summit -- -- Fault

Figure 23. Generalized geologic map of Tomichi Dome, Colorado (after Stark and Behre 1936).

As its name suggests, the rhyolite caps a domical mountain others of this group they contain relatively low concentrations of that rises about 600 m above a generally flat region. The rhyolite U (4 to 7 ppm) and Rb «300ppm). Felsites contain about 0.17 was emplaced through Cretaceous sedimentary rocks (sandstone, wt%F. shales, and limestones). The initial eruptions resulted in the em­ F. E. Mutschler (written communication, 1983) reports a placement of a poorly-exposed basal explosion breccia and tuff whole-rock K-Ar age of38 Ma for Tomichi Dome. This is 9 Ma' over 200 m thick. Ernst (1980) identified a breccia pipe (ca. 500 older than any other topaz rhyolite in Colorado and makes it m across) on the northeastern margin of the rhyolite. Fragments contemporaneous with the calc-alkaline magmatism of the Bo­ of sedimentary rocks and Precambrian granite are included in the nanza and San Juan volcanic fields to the south (Varga and Smith breccia pipe and in the pyroclastic deposits. Overlying the tuff is a 1984; Lipman et aI. 1976). phenocryst-poor rhyolite lava flow or dome. The lower 300 m of lavas have spherulitic textures and the upper part is a denser 21. Boston Peak, central Colorado equigranular rock with a granophyric groundmass and prominent flow banding. The exposed dome is 2 to 3 km in diameter and the Three vent complexes of topaz rhyolite are exposed at Bos­ volume of rhyolitic rock exposed is approximately 3 to 4 km3. ton Peak (Ernst 1980), which lies about 40 km northwest of A sill, composed of a rock similar to that in the main mass, Tomichi Dome in Gunnison County, Colorado (Figure 22). All intrudes shales at the base of the dome. The sill is 6 to 10m thick of the rhyolite plugs are small; the largest is only about 800 m and a thermal contact aureole extends away from it for several across. No bedded pyroclastic deposits are associated with the meters into country rock. emplacement of the Boston Peak plugs but a breccia pipe is The principal phenocrysts are biotite, sanidine, oligoclase, exposed adjacent to one of the rhyolite vents. Angular to sub­ and smoky quartz. The matrix consists of these same minerals rounded fragments of underlying sedimentary units and Precam­ and some glass. Magnetite, zircon, and apatite are magmatic ac­ brian granite are included in the breccia pipe. cessories. As devitrification products, topaz and garnet occur in The rhyolites contain phenocrysts of resorbed quartz, sani­ irregular clots, and biotite crystals occur in radiating groups in the dine, albite (An 3 to 6), biotite, zircon, and Fe-Ti oxides. A upper part of the lava. Garnet, of unspecified association, is also vitrophyre is preserved at the margin of one of the vents, other­ present in the basal tuffaceous sequence. Small quantities of wise the phenocrysts are contained in fine-grained, flow-banded, hornblende, ilmenite, and titanite were found in heavy mineral and felsitic matrix. Lithophysae with concentric shells of quartz separates but were not observed in thin section (Stark 1934). and topaz are common; zeolites flil some of the cavities. Fluorite Ernst (1980) analyzed eight specimens from Tomichi Dome crystals occur in fractures in the phenocrysts and, along with for major and trace element concentrations. An average of two topaz, garnet, and tourmaline, are post-magmatic. Muscovite, analyses of the upper topaz-bearing part of the dome is given in probably as an alteration product, occurs as small grains in the Table 1 and 2. In many ways the analyses are typical of other groundmass of two of the rhyolite plugs. topaz rhyolites from the western United States, but compared to The average of six analyses (Ernst 1980) of rhyolites from 36 Christiansen, Sheridan, and Burt

the three vents at Boston Peak are presented in Tables 1 and 2. The major element compositions of these rhyolites are in all ways Lake City, Colorado like their counterparts elsewhere; they show none of the composi­ \ tional "anomalies" ofthe Tomichi Dome rhyolites. A vitrophyric \ specimen contains 0.51% F; felsites contain 0.12 to 0.49% F. In \ ,... accord with these relatively high F concentrations, the rhyolites Te o 5 have relatively high concentrations of incompatible trace ele­ L...L..L...L.J. ments, e.g., Li (90 to 270 ppm), Rb (390 to 820 ppm), U (7 to 24 km ppm; highest in the vitrophyre), and Nb (35 to 160 ppm). The feldspar-compatible elements, Ba (<75 ppm) and Sr «10 ppm), are strongly depleted. The age of the rhyolites is not known, but they lie on a NW-trend that includes the rhyolites at Mt. Emmons and Red­ well Basin (17 Ma-a molybdenum mineralized rhyolite complex that bears topaz; Thomas and Galey 1982; J. E. Sharp Te 1978), Treasure Mountain dome (12.5 Ma; Obradovich et aI., 1969), Round Mountain (14 Ma; Cunningham et aI. 1977), and Tomichi Dome (38 Ma Mutschler, written communication 1983). With the probable exception of Tomichi Dome, these plugs appear to be part of a post 20 Ma bimodal suite of basalt and high-silica rhyolite typical of southwestern Colorado (Figure 20).

22. Lake aty, southwestern Colorado

An ENE-trending line of topaz-bearing rhyolite plugs, sills and laccoliths is exposed in the northwestern portion of the San ~ Post-caldera mafic laval ~ EXPLANATION Juan volcanic field (Steven et aI. 1977). The trend starts about 5 Rhyolite plugs (18-19 m.y.l km north of Lake City, Colorado. The belt of intrusions extends I@ I Granitic Intrusive Rocks .'.._Caldera for 20 km and consists of about 10 separate bodies that are all :...... wall r:fs;l Sunshine Peak Tuff described as being mineralogically and chemically similar to one ~ (22m.y.) another. The geologic setting, mineralization potential, and trace Older ash flows and element chemistry of these rhyolites are reported by Steven et aI. lavas (30-26 m.y;) (1977), and isotopic analyses (Pb, Sr) of one plug are presented Early intermediate composition by Lipman et aI. (1978a). The petrology of these rhyolites and the lavas arid tuffs (35-30) rocks of the Lake City caldera are presently being studied by R. A. Zielinski and K. Hon of the U.S. Geological Survey. Figure 24. Generalized geologic map of the Lake City area, southwestern The rhyolites (Figure 24) were emplaced as discordant in­ Colorado (after Steven et al. 1977; and Lipman et al. 1978). Plugs of trusions, generally less than 1 km across, within the volcanic and topaz rhyolite (cross-hatched) are nestled within the rim of the Uncom, sedimentary fill of the Uncompahgre caldera, which formed 28 pahgre caldera (formed 28 Ma), but are much younger (18 to 19 Ma). Ma. All of the intrusions are small, with maximum dimensions of The rhyolites are also younger than the Silverton/San Juan (28 Ma) and the Lake City (22.5 Ma) calderas and probably represent a distinct 1 to 2 km; Although none of the rhyolites are reported to be volcanic episode in this area. extrusive, the preservation ofmarginal vitrophyres and the forma­ tion of lithophysae suggests that they were emplaced at very shallow levels. K-Ar dating indicates they are 18.5 Ma (Lipman et aI. 1978a); at least 8 Ma younger than other dated topaz Ernst (1981) reports chemical an,alyses ofthe rhyolite plugs rhyolites in Colorado. (Table 1) that show them to be typical high-silica rhyolites. Many The rhyolites are generally devitrified and light gray in color. of the samples have high Kz0/NazO, indicating post-magmatic Reported phenocrysts include abundant quartz and sodic sanidine redistribution of alkalies; no major-element analyses of vitro­ along with sparse biotite and oligoclase. Titanite is a prominent phyres have been published. A variety of trace element analyses microphenocryst (R. A. Zielinski, oral communication, 1982) reported by various investigators is given in Table 2. Ernst along with apatite, zircon, and sparse Fe-Ti oxides (Ernst 1981). (1981) reports that the rhyolites have relatively high Sr (ave. 126 Small crystals of topaz and fluorite occur within some cavities ppm) and Ba (600 ppm) for topaz rhyolites, but his analyses also and along fractures. demonstrate an enrichment in Rb and Li. Semi-quantitative anal- Topaz Rhyolites 37

yses (Table 2) also show that the rhyolites are enriched in other Topaz rhyolites in Colorado: A summary incompatible trace elements: Be (7 to 20 ppm), Nb (20 to 80 ppm), Pb (30 to 70 ppm), Sn (up to 10 ppm), Y (up to 20 ppm), Although geochemically similar, there appear to be at least and Mo (up to 15 ppm). Delayed neutron activation analyses two separate groups of topaz rhyolites in Colorado (Figure 20)­ show that the rhyolites are also enriched in U (10 to 42 ppm) and an older Oligocene group emplaced before about 25 Ma (repre­ Th (26 to 64 ppm). Fluorine concentrations, even in "vitro­ sented by Specimen Mountain, Chalk Mountain, Nathrop, Silver phyres," range erratically from 500 to 1300 ppm, values that are Cliff-Rosita, and possibly Tomichi Dome, which is 38 Ma) and a much lower than for most other topaz rhyolites. Vitrophyres have younger Miocene group emplaced after about 22 Ma (repre­ higher values of U, Be, and Mo than their devitrified equivalents sented by the Lake City group and perhaps the plug at Boston and Steven et al. (1977) suggest that these elements were released Peak). The Oligocene group is contemporaneous with the waning from the lavas during cooling and crystallization in the form of stages of generally calc-alkaline volcanism (characterized by ex­ halogen complexes. The REE patterns for the topaz rhyolites of tended Si02 variation diagrams) in the San Juan Mountains and Lake City display prominent middle REE depletion (R. A. Zie­ other centers on the flanks of the Rio Grande rift. Their distribu­ linski and K. Hon, written communication 1982) that may have tion coincides with the locus of Neogene tectonism as they pre­ resulted from the fractionation of titanite. These patterns are sim­ ceded or were contemporaneous with the development of the rift. ilar to those of the titanite and topaz-bearing rhyolites from the The topaz-bearing stocks at the Climax and Henderson molybde­ Mineral Range, Utah. The Sr- (0.7054) and Pb- isotopic compo­ num mines are probably part of this group. The younger group of sition of these rhyolites are relatively unradiogenic (Table 3) and topaz rhyolites were erupted as part ofa clearly bimodal group of imply that they may have arisen from a source with relatively low basaltic (or basaltic andesite) lavas and rhyolitic lavas and tuffs. -Rb/Sr, Th/Pb, U/Pb and Th/Pb ratios. Lipman et al. (l978a) The locations of the younger rhyolites are not limited to the rift suggest that the source for these rhyolites (and other similar proper, instead they occur on the western side of the rift. In Miocene-Pliocene rhyolites) is in amphibolite-facies lower crust addition, they are not clearly associated with faulting related to because ofthe inferred low Th/U ratio of the source. Low Th/U the still active Rio Grande rift (Tweto 1979). The rhyolitic stocks ratios are atypical of some, but not all, exposed granulite-facies at Mount Emmons and Redwell Basin (topaz-bearing Climax­ metamorphic terranes. type molydenite deposits; J. E. Sharp 1978; Thomas and Galey All of the young Lake City rhyolites are anomalously ra­ 1982) are included in this younger group. dioactive (scintillometer readings over them are about two times that found over their hosts-Steven et al. 1977) and they have 23. East Grants Ridge, west-central New Mexico been extensively prospected for uranium. In 1960, a small quan­ tity of uranium ore was removed from a supergene deposit near Grants Ridge is a discontinuous basalt-capped mesa that one of these plugs (Steven et al. 1977). Much of the mineraliza­ slopes southeast towards Grants, New Mexico. The eastern part tion in the San Juan Mountains is associated with the emplace­ of Grants Ridge is underlain by a rhyolite dome complex that ment of young (less than 22 Ma) silicic magmas that are grossly contains topaz and garnet in lithophysae (Kerr and Wilcox similar to the Lake City rhyolites but generally topaz-free (Lip­ 1963). The volcanism is related to the development of the ande­ man et al. 1976). sitic composite volcano at Mount Taylor, which is centered about . The initial eruptions (35 to 30 Ma) in the San Juan volcanic 12 km to the northeast (Figure 25). field formed clusters of central volcanoes composed ofandesitic to The initial volcanic activity at Grants Ridge is represented rhyodacitic lavas and breccias. Large ash-flow eruptions of more by a rhyolitic tuff. The tuff includes black obsidian bombs and silicic (about 72% Si02) magmas occurred 30 to 26 Ma, at the large xenoliths of Precambrian(?) granitic rocks. Precambrian same time as the Rio Grande rift was developing farther east (Fig­ rocks are not exposed at the surface and must lie several 1OOOs of ures 20 and 22). Eruptions of minor volumes of andesitic to dacitic meters deep in the vicinity of Grants Ridge (Thaden et al. 1967). lavas accompanied resurgence of the calderas. About 20 to 25 Ma The tephra probably formed a low tuff cone and are comprised of (early Miocene) volcanism in the San Juan Mountains had pyroclastic flow, surge, and fall deposits. Separate perlite and changed to a bimodal assemblage of basalt (or basaltic andesite) felsite domes rose through the tuff; both contain topaz and garnet. and "high-silica alkali rhyolite" (Lipman 1981). The topaz rhyo­ The perlite dome is intimately flow banded and Kerr and Wilcox lites at Lake City are included in this latter group. The only caldera (1963) suggest that hydration occurred under magmatic condi­ in the San Juan volcanic field to develop during this episode was tions and is not the result oflow temperature hydration of obsid­ the Lake City caldera (22.5 Ma) with which the topaz rhyolites ian at the surface. The lava dome consists of a felsitic rhyolite (18.5 Ma) are spatially associated. It is important to note that the core surrounded by a collar or rind ofobsidian and perlite. Flow topaz rhyolites of the San Juan Mountains are younger than other banding is well-developed in all phases of the body and suggests dated topaz rhyolites from Colorado (26 to 38 Ma versus 18.5 Ma) that the dome has a concentric internal structure. Lithophysae, and they do not occur within the Rio Grande rift system (Figure some 10 cm in diameter, are common in the outer part of the 22). In addition, contemporaneous mafic lavas are alkali basalts, dome. Rhyolite from the dome has K-Ar dates of3.2 Ma (Bassett not andesites or their derivatives (Lipman and Menhert 1975). et al. 1963) and 3.3 Ma (Lipman and Mehnert 1980). The rhyo- 38 Christiansen, Sheridan, and Burt

pTr

pTr

Qb r---.---I'l-~R a Cubero

35~----~""':';";'~-_J-_------_"""L.-I""'------""

o 10 20 I I I K II o,m e t e r s Figure 25. Generalized geologic map of volcanic rocks in the region surrounding Grants Ridge, New Mexico (from Lipman and Mehnert 1980). The inset shows the relationship of Grants Ridge (GR) and the Mount Taylor volcanic field to the Raton (R)-8pringerville (8) or Jemez lineament of New Mexico and Arizona; late Cenozoic volcanic fields in the region are outlined. Map symbols: Qb - Holocene and Pleistocene tholeiitic basalt and the alkalic Zuni Canyon basalt flow; Qbt - Pleistocene alkalic basalts of the high mesas surrounding Mt. Taylor; Qa - Pleistocene andesites, basalts, and pyroclastic flows of Mt. Taylor; QTr - Pleistocene and Pliocene rhyolite domes and tuffs including the topaz-bearing rhyolite at East Grants Ridge; Tb - Pliocene basalt; pTr - pre-Tertiary sedimentary rocks.

lites are partially covered by slightly younger olivine basalt flows (1978) reports that the uranium contents of the lavas at Grants that contain inclusions of the rhyolite. Several scoria cones rep­ Ridge range from 7 to 8 ppm, relatively low concentrations for resent the final activity at the basalt vents. Although the basalt topaz-rhyolite lavas. Baker and Ridley (1970; citing P. Pushkar) extrusions have not been dated, similar mafic lavas that form part report that a dacite from Mt. Taylor has an initial 87Sr/86Sr ratio of the Mount Taylor volcanic field have ages that range from 2.9 of 0.7193 and believe that it was derived by mixing of mafic to 1.6 Ma (Lipman and Mehnert 1980). magma with a rhyolitic magma (represented by the domes and The rhyolites at East Grants Ridge are all phenocryst-poor tuff) derived by partial melting of the crust. but contain small phenocrysts of quartz, sanidine, and sodic pla­ No known mineralization is associated with the rhyolites. gioclase. Devitrification of glass has produced a light-grey felsitic The Cenozoic volcanism is unrelated to the large uranium depos­ groundmass in most of the dome. Vapor-phase crystallization of its of the Grants region that are Late Jurassic to mid-Cretaceous garnet, topaz, and tridymite occurred in lithophysae that are pres­ in age (Brookins 1980). ent in obsidian, felsite, and some phases of the perlite dome. Mount Taylor consists of an andesite composite cone with Baker and Ridley (1970) report an average of two rhyolite early high-K basaltic andesites (trachytes) or dacites (4.44 Ma; analyses from the Mt. Taylor volcanic field, but do not give the Lipman and Mehnert 1980) and rhyolitic tuffs exposed in a cen­ locations of the samples. The average is similar to other topaz tral amphitheater. The cone is composed of porphyritic andesite rhyolite analyses reported in Tables 1 and 2 and shows the char­ lavas, and developed 2.4 to 2.9 Ma. The surrounding mesas are acteristic depletion ofTi, Mg, and Ca. Our trace element analyses capped by a differentiation series of high-K basalts to andesite also show that the Grants Ridge rhyolites are similar to other (alkali basalts to trachytes; Crumpler 1980) that developed con­ (r I topaz rhyolites with enrichments ofF (0.5% in a vitrophyre), Li, temporaneously 4.3 to 1.5 Ma (Lipman and Mehnert 1980; Lip­ !' Rb, and Sn. Baker and Ridley (1970) report two rhyolite analyses man et al. 1979). The rhyolites and basalts of Grants Ridge are I. for Rb (602,537 ppm) and Sr (nd., 2 ppm) that are in accord part of the peripheral volcanism. The development of the Mt. with these analyses and typical of other topaz rhyolites. Zielinski Taylor volcanic field was concurrent with minor north-northeast Topaz Rhyolites 39 faulting in the area. Lipman and Mehnert (1980) relate the devel­ opment of the volcanic field to activity on the east-northeast­ trending Jemez Lineament or Springerville-Raton zone (Figure 25). Volcanism of similar age occurred elsewhere along this belt that stretches from southwestern Arizona to northeastern New Mexico.

24. Black Range, southwestern New Mexico

The Taylor Creek Rhyolite is a topaz-bearing lava that crops out in the northern Black Range of southwestern New Mexico lIT] Calc-alkalic rhyolite (Fries 1940; Fries et al. 1942; Ericksen et al. 1970; Lufkin 1976, 1977; Correa 1980). The rhyolite has been extensively studied in part because it contains low-grade concentrations of tin as cassit­ erite and wood tin. The Black Range lies on the eastern margin of the Mogollon-Datil volcanic field that was active from about 40 to 20 Ma (Elston and Bornhorst 1979). The Taylor Creek Rhyolite, which has a volume of 130 km3 (Rhodes 1976), was emplaced as a series of endogenous domes (5 50, to 15 separate vents may exist). Pyroclastic material that pre­ KM ceded the lava eruptions is exposed at the present margins of the lavas and presumably underlies the lavas as well. Locally, an autoclastic breccia forms the base of the flows. Flow-banding is Figure 26. Tectonic map of southwestern New Mexico showing the well-developed and reveals large folds that show the internal relationship of the topaz-bearing Taylor Creek Rhyolite to the major structure of the domes and lava flows. The rhyolite has K-Ar Cenozoic faults and calderas in the region (Elston 1978, 1984; Ratte et aL 1984). Mid-Tertiary calderas are indicated by dashed lines and nor­ (sanidine) ages of 24.6 Ma (Elston 1978) and 27.7 Ma (Ratte et mal faults by heavy solid lines. The Mogollon caldera (M) was the al. 1984) from the same locality. In any case, the extrusion of the source of the Cooney Tuff and appears to have formed 34 Ma. The Gila domes probably spanned a considerable length of time. Accord­ Cliff Dwellings caldera (GCD) formed 30 to 29 Ma and may have been ing to Ratte et al. (1984) the Taylor Creek Rhyolite may repre­ the source of the Davis Canyon and/or Shelley Peak Tuffs. The large Bursum caldera (B) formed 29 to 28 Ma and was the source of the sent ring-fracture volcanism of the Bursum caldera (source of the Bloodgood Canyon and Apache Springs Tuffs. The relationship of the Bloodgood Canyon and Apache Springs Tuffs), which formed Taylor Creek Rhyolite (24.6 to 27.7 Ma) to these calderas is open to 29-28 Ma. About 30 m of Bloodgood Canyon Tuff overlie the question. Taylor Creek Rhyolite (Ratte et al. 1984) discounting the sugges­ tion that the Taylor Creek Rhyolite is the devolatilized post­ textures are the most common, spherulitic devitrification textures eruption residue of the Bloodgood Canyon magma chamber as are also present. The upper parts of the flows are vapor-phase suggested by Rhodes (1976). Nonetheless, it is just as likely to be altered and contain the most abundant gas cavities. unrelated to the development of any of the large calderas (T. The Taylor Creek Rhyolite is chemically similar to most Eggleston, oral communication 1985). The distribution of the topaz rhyolites (Table 1) with high Si, K, Na, and Fe/Mg and lava and the location of some of the calderas are shown in low Mg, Ca, Ti, and P (Correa 1981). Vitrophyres from the flow Figure 26. contain up to 0.4% F and are enriched in Sn and Rb (Table 2). Most of the Taylor Creek Rhyolite is composed of devitri­ Eggleston and Norman (1984) mistakenly reported that the lavaS fied and vapor-phase altered lava. It contains 20 to 40 percent were Cl-rich: The Taylor Creek Rhyolite contains less D, Ta, and phenocrysts of quartz and sanidine with lesser amounts ofplagio­ Th than some of topaz rhyolites from western Dtah (Figure 35), clase. Biotite and ferrohornblende are present in most samples; but is still enriched when compared to other high-silica rhyolites ferroaugite (Mg24Ca3SFe41) occurs in biotite and hornblende-free from the region. In addition, it is decidedly rich in Y (>100 ppm). flows (Correa 1980). Accessory phases include zircon, titanite in The REE pattern of the rhyolite (Figure 40b) has a deep Eu some specimens, Fe-Ti oxides (mostly titaniferous magnetite), anomaly (Eu/Eu* = 0.07), low La/LuN (2.4), and low La/CeN and fayalite(?) (Lufkin 1976, 1977). Vapor-phase minerals that (Correa 1981), typical of topaz rhyolites in general. Eggleston occur in miarolitic cavities and veinlets include quartz, alkali and Norman (1984) report that the delta 180SMOW value for the feldspar, hematite, bixbyite «(Mn,Feh03), pseudobrookite rhyolite is 8 permil, indicating a crustal history for the magmas' (Fe2TiOs), cassiterite, topaz, monazite, fluorite, (Lufkin 1976), source. A single Sr-isotope analysis of a whole rock sample of the garnet (Fries et al. 1942), and beryl (Kimbler and Haynes 1980). Taylor Creek Rhyolite suggests that its initial 87Sr/86Sr is very Inclusion-ridden topaz crystals in miarolitic cavities at Round high (0.71583 ± 0.0028; Table 3). The usefulness of this result is Mountain reach lengths of 2 to 3 cm. Although granophyric questionable both because the sample contains only 3.7 ppm Sr 40 Christiansen, Sheridan, and Burt New Mexico Chronologie Summary

Age Basaltic Calc-Alkaline Comments m.y. Basaltic Andesite Rhyolite o 'i. Mt Taylor \ X Intraplate Block Faulting Jemez 10

20 A I Questa I~ Back-arc Extension x + 30 Rio Grande Rift opens

40 ?

Figure 27. Schematic representation of magmatic and tectonic activity in New Mexico (after Elston and Bornhorst 1979). Topaz rhyolites (x) appear to have erupted in two episodes-one contemporaneous with the basaltic andesite suite of Elston and Bornhorst and the other contemporaneous with a younger basalt to andesite suite that includes the andesites and alkalic basalt to trachyte suite at Mt. Taylor. The age ofthe Questa molybdenite and peralkaline volcanism there are also shown (0).

(thus it could have been easily contaminated at the magmatic also associated with topaz rhyolites in Nevada and Mexico (see stage) and because the sample was a hydrated vitrophyre. (Hy­ below). No known F, Be, or U mineralization is directly aSso­ dration may radically change the Sr content and hence Sr­ ciated with the Taylor Creek Rhyolite. However, F, Be, Fe, W, isotopic composition of ·vitrophyres: e.g., Hargrove 1982). A Sn, and U mineralization occurs in skarns and along faults near detailed Br-isotope investigation of the Taylor Creek rhyolite is Iron Moutnain, east of the Black Range (Jahns 1944; Hillard being conducted by D. Norman and T. E. Eggleston (oral com­ 1969). A specimen from the aplitic intrusions associated with the munication 1985). skarns has a K-Ar age of 29.2 Ma (Chapin et al. 1978) indicating Tin mineralization in the form of cassiterite and wood-tin that these magmas are approximately the same age as the Taylor veinlets (Lufkin 1976, 1977) deposited as a result of fumarolic Creek Rhyolite. activity is common in the upper parts of the Taylor Creek Rhyo­ The mid- Terti~ry magmatism of the Mogollon-Datil prov­ lite (Correa 1980; Eggleston and Norman 1984). The mineraliza­ ince has been interpreted to be the product of three overlapping tion is restricted to the flanks of intensely vapor-phase magma suites (Elston and Bornhorst 1979; Figure 27). A calc­ recrystallized zones just below the carapace of the domes~ Fluid alkaline andesite to rhyolite suite (43 to 29-28 Ma) produced inclusion and oxygen isotope studies indicate that quartz and composite volcanoes and slightly younger compositionally zoned topaz crystallized from saline magmatic fluids (8180 = +6 to +10 silicic ash-flow tuffs. This volcanism·appears to have been con­ %0) at temperatures over 600°C (Eggelston and Norman 1984) temporaneous with subduction of oceanic lithosphere at the and miarolitic cassiterite-hematite-quartz was deposited at continental margin. Back-arc (or intra-arc) extension led to the temperatures above 500°C (Rye et al. 1984); probably also froni development of a bimodal basaltic andesite and high-silica rhyo­ magmatic fluids derived by degassing of the rhyolite lavas. Late lite suite that extended from 30 to 19 or 18 Ma. The third stage is cassiterite crystallized at 150 to 200°C from boiling fluids with represented by flows of tholeiitic and alkalic basalt and high-silica calculated 8180 of ~6 to 0 %0 (Eggleston and Norman 1984). rhyolite with ages ranging from 19 Ma to less than 1 Ma. This The wood-tin depositing fluids appear to have contained a large suite was erupted during a period ofblock faulting. A somewhat meteoric component. Placer deposits of Sn derived from the lavas different picture was presented by Ratte et al. (1984).They group have been mined on a small scale. Wood-tin mineralization is the high-silica rhyolitic ash flows and lavas ofElston's and Born- Topaz Rhyolites 41 horst's intermediate stage with the oldest suite, interpreting them '0 '" ' '", as being the first erupted parts of zoned (dacite-rhyolite-high silica '" ,,'"","< : '" I 0",'" rhyolite) magma chambers. Ratte et al. (1984) suggests that the I !f1,I "ring-fracture" rhyolites were also erupted from these zoned N D', Negro Ed chambers. These authors place voluminous post-caldera andesites , ,, in a "fundamentally basaltic" suite (26 to 23 Ma) but also recog­ ,, nize a bimodal suite of basalt and high-silica rhyolite that ap­ peared about 19 Ma, long after the eruption of the Taylor Creek p€ Tvsp

Rhyolite. Rhodes (1976) and Ratte et al. (1984) suggested that a ~TVSP 3.4 37' 30" composite granitic batholith underlies the volcanic field. The topaz rhyolites may have evolved by fractional crystallization and Old() () LU::;;: dehydration of one of these magma bodies following collapse of >10­ one of the large calderas (Rhodes 1976, suggests the Gila Cliff l..-I ::c,>< < o 1 kilometer 0'< Dwelling caldera and Ratte et al. 1984, suggest the Bursum cal­ Tvs :::;1>- dera). However, Correa (1980) speculates that the Taylor Creek Figure 28. Geologic sketch-map of the volcanic geology near Burro Rhyolite may have arisen from a separate magma body not di­ Creek, Arizona (after Burt et al. 1981b). The topaz-bearing rhyolite rectly related to these calderas. dome at Negro Ed (Tvst) is underlain by a pyroclastic unit (Tvsp). Other small rhyolite domes (Tvs) in the area contain vapor-phase garnet and 25. Saddle Mountain, eastern Arizona are underlain by cogenetic tephra (Tvsp). The rhyolites lie on a Precam­ brian basement of gneisses and granites (pC). Normal fault and sense of movement shown as dashed line. Information about the topaz rhyolite near Saddle Mountain in the Galiuro Mountains of southeastern Arizona is sketchy. Anthony et al. (1977) state that topaz, together with pseudo­ brookite, spessartine garnet, and bixbyite, occurs in a rhyolite in garnets are spessartine-almandine solid solutions (Figure 34) with the Winkelman area. The topaz rhyolite was not located during a up to 3000 ppm fluorine (analyzed by ion chromatography and brief visit· to the area. The rhyolite nearest Saddle Mountain electron microprobe). They may represent the "almandine" oc­ (which is formed by resistant andesitic lavas) is described by currence at the southern end of the Aquarius Range mentioned Krieger (1968) as an intrusive plug. Biotite from the rhyolite has by Anthony et al. (1977). a K-Ar age of 61 Ma. Younger (ca. 24 Ma) rhyolitic lavas occur An average chemical composition of vitrophyres from 3 to 5 km northeast of Saddle Mountain. garnet- and topaz-bearing lavas is given in Table 1 and trace element analyses in Table 2 (Moyer 1981). The average is typical 26. Burro Creek, western Arizona of topaz rhyolites. Fluorine concentrations range from 1400 to 2600 ppm; chlorine concentrations are 4 to 5 times lower. Rhyolitic lavas with lithophysae containing garnet and/or Although a number of lithophile-element deposits (Be, W, topaz occur in eastern Mohave County, Arizona (Burt et al. 1981; V) occur in the region" most are associated with anorogenic Moyer 1981). Topaz and garnet occur in a mesa identified as granites of Precambrian age (Anderson 1983). However, Burt et "Negro Ed" on the 7lf2 minute quadrangle of the same name al. (1981) have suggested that the fluorine-rich rhyolites or sim­ (V.S. Geological Survey 1980). The hill is formed by the rem­ ilar rocks may have been the source some of the uranium depos­ nants ofa flat-topped rhyolite lava (Figure 28). A basal vitro­ ited to the south at the Anderson mine (Sherborne et al. 1979). phyre (l to 3 m thick) is locally disrupted and forms part of a The topaz rhyolites of Burro Creek are part of a more or less flow-produced breccia. Locally, a thin (less than about 50 m bimodal assemblage of late Cenozoic volcanic rocks from west­ thick in places thinning to about 1 m) pyroclastic deposit and/or ern Arizona (cf. Suneson and Lucchitta 1983). In the vicinity of explosion breccia with fragments of the Precambrian country Burro Creek, Moyer (1981) has delineated a series of low-silica rock is exposed at the base of the complex. The dome is more rhyolite lavas (ca. 70% SiOz) and sub-alkaline basalts to basaltic than 200 m high and 2 km across. Several other small domes in andesites (48 to 56% SiOz) that are broadly contemporaneous the area contain vapor-phase garnet in lithophysae or along more with, but slightly younger than, the aphyric (topaz-bearing) lavas. coarsely crystalline flow bands. The ages of the rhyolites are not Tuffs emplaced during dome-forming eruptions are interlayered known, but they are probably late Miocene or Pliocene. Mesa­ with basaltic lavas in the region. A period oflate Cenozoic listric capping basaltic lavas in the area have K-Ar ages of 8 to 9 Ma normal faulting was followed by high-angle normal faulting be­ (Shafiqullah et al. 1980). tween 14 and 7 Ma in western Arizona (Suneson and Lucchita The rhyolites are all phenocryst-poor (3 to 5%) with sparse 1983). The magma-tectonic setting of this region appears to be phenocrysts of quartz, sanidine, oligoclase, biotite, and Fe-Ti ox­ similar to that found elsewhere in the Basin and Range province ides. Spherulitic devitrification is the most common groundmass where topaz rhyolites occur in association with basaltic lavas and texture in the felsites. Moyer (1981) reports that the vapor-phase extensional faulting. 42 Christiansen, Sheridan, and Burt

OTHER "TOPAZ RHYOLITE" OCCURRENCES al. 1980). The topaz rhyolites were erupted from 32-27 Ma during the climax ofthe mid-Tertiary calc-alkaline magmatism of Several other occurrences of topaz in or associated with the Sierra Madre Occidental (Huspeni et al. 1984; Cameron et volcanic rocks have been reported that: 1) are not sufficiently al. 1980). In the states of Durango and Zacatecas, the tin-bearing documented to warrant separate discussion; 2) are not Cenozoic varieties occur as subvolcanic plugs and as extrusive domes and in age; or 3) are not from the western United States. Brief discus­ lava flows. They intrude other volcanic units near the margins of sions of these "other" occurrences are presented here for com­ calderas. Some are covered by crystal-rich rhyolitic ignimbrites pleteness and for comparative purposes. that may have erupted shortly after the emplacement of the lavas, leading some authors to suggest that the Sn rhyolites are directly Other Cenozoic occurrences, western United States associated with the caldera-related rhyolites (Huspeni et al. 1982; Ruiz et al. 1985). Our own field observations and those of Trimble and Carr (1976) report "topaz(?)" in heavy mineral others (e.g. Ludington et al. 1984) suggest that this interpretation separates from the tuffof Arbon Valley along the southern mar­ requires further study. gin of the Snake River Plain in Idaho. A topaz locality in south­ Most of the F-rich lavas are phenocryst-poor with pheno­ central Idaho is shown on Shawe's (1976) distribution map of crysts of quartz, plagioclase (AnlO-20), sanidine (OrSO-70), and topaz rhyolites in the United States. At this locality, topaz oc­ traces of ferroaugite and Fe- and F-rich biotite (Pan 1974; Hus­ curred with fayalite, allanite, chevkinite(?) (D. R. Shawe, oral peni et al. 1984, Ruiz et al. 1985). Crystallization temperatures communication 1982) and zircon in a stream sediment concen­ are relatively low (650 to 780°C) as determined by two-feldspar trate. The stream drains Tertiary rhyolites (rhyolites of Magic geothermometry (Pan 1974; Huspeni et al. 1984). One notable Reservoir) on the north side of the Snake River Plain and part difference from topaz rhyolites in the United States may be the of the Idaho batholith and thus the topaz may not be from a presence of fayalite (Ympa and Simmons 1969; Pan 1974) in rhyolite at all. Leeman (1982a, b) reports that the rhyolite dome some tin rhyolites (though not necessarily those that bear topaz). at Moonstone Mountain (which is drained by the sampled In rhyolites with moderate fluorine contents (0.1 to 0.2%) the stream) is a high-silica rhyolite lava with 0.2% fluorine. Perhaps exchange of Al for Fe in the vapor-phase mineralogy stabilizes this 3 to 6 Ma rhyolite is the source of the topaz. Other F-rich garnet over fayalite, and biotite is generally the stable Fe-bearing rhyolite domes occur near Magic Reservoir. However, Bennett magmatic phase. (1980) reports that the 44 Ma Dismal Swamp stock (which Huspeni et al. (1984) and Ruiz et al. (1985) report that the intrudes the southern part of the Idaho Batholith) contains topaz F-rich lavas are geochemically similar to topaz rhyolites from the and beryl and is anomalously radioactive. It is not known if this western United States. Like their counterparts to the north, the Mo-mineralized intrusion has a rhyolitic phase, but such plutons Mexican tin rhyolites are high-Si02, metaluminous to slightly could also be a source of the topaz. Blixt (1933) reports topaz and peraluminous lavas, with marked depletions ofTi, Mg, Ca, and P fluorite of uncertain paragenesis associated with gold mineraliza­ (Table 5). Many of the analyses reported have anomalously high tion in the North Moccasin Mountains of central Montana. Van K20/Na20 ratios (> 1.5) similar to but higher than those found Alstine (1969,1974) reports the occurrence oftopaz in the upper in tin rhyolites from the Black Range, New Mexico, and the part of a devitrified rhyolitic (68 to 73% Si02) ash flow from near Sheep Creek Range, Nevada. The role of post-magmatic pro­ Poncha Springs in central Colorado. The vitrophyre of the ash cesses, such as alkali metasomatism during devitrification, needs flow is chemically dissimilar to topaz rhyolites and contains only to be examined, but "fresh vitrophyres" show similar ratios. Fluo­ 0.08% fluorine. The ash flow appears to correlate with the 36 Ma rine concentrations in two vitrophyres were 2000 and 3000 ppm; Wall Mountain Tuff mentioned earlier. no CI analyses were reported. Their trace element concentrations are indistinguishable from topaz rhyolites in the western United Mexican topaz rhyolites States (Figure 41), with the typically high concentrations of Rb, Cs, Ta, Th, U, and variable enrichment of Sn (<20 ppm). REE The youngest volcanic sequence in the Sierra Madre Occi­ patterns show deep Eu anomalies and low La/YbN ratios. In dental of Mexico contains numerous topaz rhyolites (e.g. Foshag their high Y·(>100 ppm) and total REE content they are most and Fries 1942; Pan 1974; Huspeni et al. 1984; Ludington et al. similar to the Sn-mineralized varieties in the Black Range, New 1984; Ruiz et al. 1985). The rhyolites commonly contain tin Mexico, and the Sheep Creek Range, Nevada. Initial 87Sr/86Sr mineralization like that described from the Black Range in New ratios of the tin rhyolites range from 0.7054 to 0.7075 (Huspeni Mexico (e.g. Ypma and Simmons 1969; Huspeni et al. 1984; et al. 1984; Ruiz et al. 1985) and are slightly higher than asso­ Duffield et al. 1984). ciated calc-alkaline (i.e. F-poor) volcanic rocks. Most of the occurrences lie in a southeast-trending belt ex­ tending from Durango to near Mexico City (Figure 29) that Precambrian topaz rhyolites parallels the main Tertiary volcanic belt of the region. The rhyo­ lites overlie mid-Tertiary andesitic to rhyolitic lavas and tuffs Topaz rhyolite magmatism in not strictly a Cenozoic phe­ (Foshag and Fries 1942; Ympa and Simmons 1969; Cameron et nomenon in western North America. Topaz (and fluorite) occurs Topaz Rhyolites 43

o,'-__----'--1()0__--'-200__...... J Miles

Figure 29. Distribution of topaz rhyolites in Mexico (compiled from sources cited in text). 1- America­ Saporis, Durango; 2 - Cerro de los Renidos, Durango; 3 - Fresnillo, Zacatecas; 4 - Pinos, Zacatecas; 5 ­ Guadalcazar, San Luis Potosi; 9 - Hacienda Sauceda, Guanajuato; 10 - San Felipe, Guanajuato; 11 ­ TIachiquera, Guanajuato; 12 - Leon, Guanajuato; 13 - Tepuxtepec, Guanajuato; and 14 - Apulco, Hidalgo. Small unnumbered dots correspond to rhyolites with Sn mineralization which may also contain topaz. For the most part the F-rich lavas occur along the eastern margin of the Tertiary calc-alkaline volcanic belt of the Sierra Madre Occidental. Also shown are the location of the presently active volcanoes (stars).

in Precambrian rhyolites of the Keewatin District in the Distribution and ages Northwest Territories of Canada (Le Cheminant et al. 1981). The anomalously radioactive rhyolites are part of a bimodal Topaz rhyolites are widespread in western North America magmatic suite related to an early Proterozoic rift. Topaz has also and their occurrence closely coincides with the limit oflate Ceno­ been found in heavy mineral concentrates from a Precambrian zoic extensional faulting (Christiansen et al. 1983a). In the United meta-rhyolite tuff from the Flying W Ranch area, west of Young, States, their emplacement appears to have spanned most of the Arizona (Conway 1976). The origin of the topaz is equivocal. It Cenozoic Era (Table 6). Their isotopic ages range from 50 Ma may be metasomatic and related to younger beryl mineralization (LIttle Belt Mountains, Montana) to 0.06 Ma (Blackfoot lava in nearby quartz veins. field, Idaho), although all but 3 are younger than 30 Ma. In Mexico, isotopic ages of topaz rhyolites cluster at 27 to 32 Ma PRINCIPAL CHARACTERISTICS OF (Huspeni et al. 1983; Ruiz et al. 1985). TOPAZ RHYOLITES Most known topaz rhyolites in the western United States lie within the eastern and southern Basin and Range province and From the occurrences reviewed above, all Cenozoic topaz along the Rio Grande rift and thus appear to surround the Colo­ rhyolites from the western United States appear to be remarkably rado Plateau. As far as is known, no topaz rhyolites occur in the similar in terms of their mode of emplacement, mineralogy, western Great Basin region of California, Nevada, or Oregon, in chemistry, associated ore deposits, and volcanic-tectonic setting, spite of this area's contemporaneous bimodal (basalt-rhyolite) despite their wide distribution and diversity of ages. These sim­ volcanism and extensional faulting. No topaz rhyolites have been ilarities are reviewed below. The implications of these characteris­ identified to the west of the initial 87Sr/86Sr = 0.706 line as tics are also considered in companion papers (Christiansen et al. determined for Mesozoic plutonic rocks (Figure 1; Kistler 1983; 1983a; Burt et al. 1982). Armstrong et al. 1977) or Cenozoic silicic volcanic rocks (Wilson 44 Christiansen, Sheridan, and Burt

TABLE 5. AVERAGE COMPOSITION OF TIN RHYOLITES FROM NORTHERN MEXICO.

1 S.D. 2 S.D. 3 S.D. Si02 76.9 0.60 77.0 0.12 75.6 0.71 Ti02 0.07 0.01 0.06 0.0 0.14 0.09 A1203 13.0 0.15 12.8 0.12 12.8 0.38 Fe203* 1.14 0.08 1.14 0.09 1.12 0.19 MnO 0.02 0.00 0.04 0.02 0.06 0.01 MgO 0.13 0.10 0.09 0.06 0.15 0.07 CaO 0.56 0.38 0.30 0.21 0.133 0.37 Na20 2.76 0.10 3.91 0.08 3.73 0.28

K20 5.~5 0.36 4.67 0.05 5.04 0.35 P205 0.01 0.00 0.00 0.01 0.00 0.01

F 0.31 0.21 0.23 0.09 Rb 548 8 807 18 510 42

Zr 73 21 95 6 114 15

U 16 5 17 2 21 5

Ta 4.7 0.2 10 0.1 5.6 0.8 Note: All analyses in weight percent or ppm and recalculated H20, C02 and S02 free. S.D. is one standard deviation. Fluorine analyses from vitrophyres. *Total Fe as Fe203 1. Average of 3 "host-rhyolites" for Sn mineralization at Sombrete, Zacatecas (Huspeni et al. 1984). 2. Average of 4 "host-rhyolites" for Sn mineralization at America-Saporis, Durango (Huspen~ et al. 1984). 3. Average of rhyolites from the Thomas Range, Utah (Christiansen et al. 1984).

et al. 1983). This line is taken by these investigators to mark the Mode ofemplacement westernmost extent of the Proterozoic craton in the western United States. Farmer and DePaolo (1983) suggest from their Nd Nearly all of the topaz rhyolites described in this report were and Sr isotope studies of granitoids from the region that the emplaced as endogenous lava domes with or without lava flows sialic continental margin lies farther inland (where eNd = -7 or (Table 7). Several topaz rhyolites were also emplaced as small 87Sr/86Sr = 0.708; Figure 1), but this datum is not well­ intrusive domes or plugs that may not have vented to the surface constrained in central Nevada. These crustal discontinuities, ex­ (e.g. Lake City, Colorado, Chalk Mountain, Colorado, Little Belt pressed structurally as the Roberts Mountain and Golconda Mountains, Montana, and some occurrences in the Wah Wah thrusts in central Nevada, mark the eastern limit of a series of Mountains, Utah); their fme grain-size, the presence of miarolitic allochthonous or "suspect" terranes composed of ocean-floor or cavities and glassy margins suggest that even they were emplaced island arc crust (e.g. Speed 1979). These terranes may have at very shallow levels. The extrusive rhyolites are generally under­ formed as oceanic crust at the margin of North America during lain by pyroclastic deposits that appear to be remnants of tuff the Paleozoic and early Mesozoic Eras (Oldow 1984). Given a rings formed by base-surge eruptions (cf. Sheridan and Updike crustal origin for the parental magmas of topaz rhyolites and the 1975; Wohletz and Sheridan 1983a). The pyroclastic deposits absence of topaz rhyolites in this region, the young, mafic crust generally consist of a lower, near-vent ("peel-back") breccia that does not appear to have a composition appropriate for the gener­ represents vent-clearing explosions. Breccia fragments may come ation of topaz rhyolites. from as deep as 1 km (e.g. East Grants Ridge, New Mexico). The Topaz Rhyolites 45

TABLE 6. AGES OF CENOZOIC TOPAZ RHYOLITES IN THE WESTERN UNITED STATES

Location Age (Mal Reference

I. Thomas Range, UT 6-7 Lindsey 1981 2. Spor Mountain, UT 21 Lindsey 1981 3. Honeycomb Hills, UT 4.7 Turley and Nash 1980

4. Smelter Knolls, UT 3.4 Turley and Nash 1980 5. Keg Mountains, UT 8 Lindsey et al. 1975 6. Mineral Mountains, UT 0.5 Lipman et al. 1978b

7. Wah Wah Mountains, UT 20-18 Lindsey and Osmonson 1978 12 Best et al. 1985 8. Wilson Creek R, NV 22.6 Barrot 1984 9. Kane Springs Wash, NV 13.4 Novak 1984

10. Cortez Mountains, NV 15 Wells et al. 1971 II. Sheep Creek Range, NV 14 Stevlart et al. 1977 12. Jarbidge, NV 16 Coats et al. 1977

13. Blackfoot Lava Field, ID 0.06 Pierce et al. 1982 14. Elkhorn Mountains, NV 36 Chadwick 1978 15. Little Belt Mtns, MT 50 Witkind 1973

16. Specimen Mountain, CO 28-27 Corbett 1968 17. Chalk Mountain, CO 28-27 Tweto and Case 1972 18. Nathrop, CO 28-29 Van Alstine 1969 19. Silver Cliff-Ros ita, CO 26 Sharp 1978 20. Tomichi Dome, CO 38 F.E. Mutschler unpub. 2I. Boston Peak, CO Ernst 1980

22. Lake City, CO 18.5 Lipman et al. 1978a 23. Grants Ridge, NM 3.3 Bassett et al. 1963 24. Black Range, NM 28 Ratte et al. 1984

25. Saddle Mountain, AZ Anthony et al. 1977 26. Burro Creek, AZ L Cenozoic Burt et al. 1981b

breccia is commonly overlain by stratified pyroclastic-surge units Lavas are generally underlain by a flow breccia (about I m produced during pulsing unsteady eruptions. Some short and thin thick) produced as the flow-front crumbled, slumped, and was (less than I m) lithic-rich ash-flow tuffs probably resulted from overridden by the flow in caterpillar fashion. Rapidly quenched minor collapse of low eruption columns. Mantling ash-fall unitS vitrophyric blocks from the flow front are common in this layer. punctuate the record of explosive volcanism. These features sug­ The volume of magma in individual domes or flows ranges gest that the pyroclastic eruptions were initiated as rising magmas from less than 0.5 km3 to a probable maximum ofabout 10 km3. explosively mixed with groundwater (hydromagmatic eruptions; However, in some cases, fairly large volumes (10 to 100 km 3) of Wohletz and Sheridan 1983b). However, the origin of the driv­ coalesced domes and flows accumulated over short intervals ing volatiles (magmatic versus phreatic) is difficult to establish (about 1 Ma); for example, the Thomas Range, Utah, the Wah without detailed studies of individual complexes (cf. Taylor et al. Wah Mountains, Utah, and the Black Range, New Mexico. 1983). Once the vent was cleared, relatively quiet eruptions of These observations set topaz rhyolite eruptions apart from rhyolite lava proceeded. Lava eruptions may have been caused by the large ash-flow eruptions of high-silica rhyolite that culminate the eruptive degassing of the magma and the evisceration of a in caldera collapse. Volumes of ash flows from calderas are volatile-rich cap of a small magma chamber, or by the restricted commonly 1 to 2 orders of magnitude larger (Smith 1979) than access of ground water to the vent. those from topaz rhyolites. However, eruptions of large volumes The geology of the Mineral Mountains domes in Utah is of magma over geologically short time intervals in the Thomas instructive in this regard in that the F-rich domes have basal Range, Utah, and in the Black Range, New Mexico, suggest that tephra deposits while earlier F-poor lavas lack them. This obser­ some topaz rhyolites may emanate from magma chambers with vation suggests that some explosive volcanism resulted from volumes approaching those of caldera-related plutons. In fact, magmatic differentiation and volatile enrichment (Evans and some topaz rhyolites may have arisen from magma chambers that Nash 1978) ofthe upper part of a rhyolitic magma chamber. The gave rise earlier to large ash-flow sheets (e.g., the Black Range, viscous domes may have been extruded following devolatilization New Mexico, and at Kane Springs Wash, Nevada). Some of of a chamber's cap. these differences are shown schematically in Figure 4K 46 Christiansen, Sheridan, and Burt

TABLE 7. MODE OF EMPLACEMENT OF TOPAZ RHYOLITES

Location Area Emplacement (km2)

Thomas Range, UT 160 Coalesced domes and lava flows with underlying tuffs. Spor Mountain, UT 5 Lava with underlying tuff and isolated dome(?). Honeycomb Hills, UT 1 Lava domes with underlying tuff. Smelter Knolls, UT 10 Isolated lava dome with no tuff exposed. Keg Mountain, UT 30 Partially coalesced domes and lava flows with underlying tuffs. Mineral Range, UT 8 ~1ultiple isolated domes with underlying tuffs. Wah Wah vicinity, UT 75 Coalesced domes and lava flows with underlying tuff1 some isolated plugs and domes (many

Mineralogy Fe-Ti oxides and titanite. Titaniferous magnetite and ilmenite both occur in topaz rhyolites. These phases may be The mineralogy of topaz rhyolites is relatively simple and is altered in devitrified lavas but are commonly unoxidized in vitro­ summarized in Tables 8 and 9. Phenocryst-poor (less than 5%) phyres. The few Fe-Ti oxide analyses that exist indicate that the rocks are the most common, but in some lavas and shallow ilmenites are generally Mn-rich. Table 4 contains the results of Fe­ intrusions the phenocryst content may be as high as 40%. In order Ti oxide geothermometry for topaz rhyolites and shows that most of abundance, sanidine, quartz, and sodic plagioclase are the of these rhyolites crystallized at temperatures between 600 and principal phenocrysts. Biotite is common; hornblende, garnet, and 850°C; mostly in the lower end of that range. Moreover, these clinopyroxene occur in a few samples. Common magmatic acces­ analyses indicate that f02 is commonly low,(Figure 30), near the sories include zircon, apatite, magnetite, ilmenite, allanite, fluo­ QFM oxygen buffer (e.g. the rhyolites of the Thomas Range, rite, and titanite. Spor Mountain, and Smelter Knolls, Utah). However, analyses of Topaz Rhyolites 47

TABLE 8. MAGMATIC MINERALS REPORTED IN TOPAZ RHYOLITES

Location Sa Qz PI Bt Mt Op II Ho Px Fa Gt Zr Al Ap Tt Fl Tz Th

Thomas Range., UT x X X + x X ± ± ± X X X ± + Spor Mountain, UT x X X x x X X X Honeycomb Hills, UT X X X x X X

Smelter Knolls, UT x XXXX X XX X Keg Mountain, UT XXXX Mineral Mountains, UT XXXXX X XXXX

Wah Wah vicinity, UT XXX + X ± XXX ± Wilson Creek Range, NV XXXX Kane Springs Wash, NV X XXXX XX X X

Cortez Mountains, ·NV X XXX Sheep Creek Mountains, NV X XXX X X X X Jarbidge, NV X XX + X ± ± X X X Blackfoot lava field, ID X XX X X ± X X X X Elkhorn Mountains,MT X XX Little Belt Mountains, MT X XX X X ?

Specimen Mountain, CO XX Chalk Mountain, CO XXXXXX ± X XX Nathrop, CO XXXXX X

Silver Cliff, CO XXXX X Tomichi Dome, CO XXXX XX X XX Boston Peak, CO XXXX X xjV X

Lake City, CO XX + X X X XX Grants Ridge, NM XXX Black Range, NM XXX ± X ± ± ? X ± Saddle Mountain, AZ Burro Creek, AZ XXX ± X

X-present in most samples; ±-present in some samples; ?-questionable or uncertain report. XjV­ may be vapor phase.

Sa-sandine; Qz-quartz; PI-plagioclase; Bt-biotite; Mt-magnetite; Op-unidentified opaque mineral; II-ilmenite; Hb-hornblende; Px-pyroxene; Fa-fayalite; Gt-garnet; Zr-zircon; AI-allanite; Ap-apatite; Tt-titanite; Fl-flourite; Th-thorite; and Tz-topaz.

oxides from the Mineral Range, Utah (ca. NNO), and Chalk lites that have prominent middle REE depletions (R. A. Zielinski, Mountain, Colorado (3 log units above NNO), demonstrate that written communication, 1982), which probably indicate titanite some topaz rhyolites crystallized under relatively oxidizing fractionation. Titanite is also reported from the Sheep Creek conditions. Mountains and Jarbidge, Nevada, where it is not known if the In the latter case, these values approach those inferred by rhyolites possess middle REE depletions. It should be noted, Keith (1982) to have existed in the rhyolitic magma chamber that however, that not all topaz rhyolites that bear titanite have mid­ gave rise to the tuff of Pine Grove and to a related Climax-type dle REE depletions. For example, some samples of the Taylor molybdenite deposit in the Wah Wah Mountains ofsouthwestern Creek Rhyolite, New Mexico, and a few of the lavas from the Utah. It is important to note that of all the topaz rhyolites de­ Thomas Range, Utah contain sparse titanite but, as far as is scribed in this report, the Chalk Mountain rhyolite is most known, none of the lavas possess middle REE depletions. It thus intimately related to such a mineral deposit-the Climax deposit appears that there are less and more oxidized topaz rhyolites, a itself. Perhaps this bears out the suggestion of Keith (1982) that situation perhaps analogous to that described for the Proterozoic high oxygen fugacities are important for the generation of anorogenic granites described by Anderson (1983) to consist of Climax-type Mo deposits. an ilmenite- and a magnetite-series. In the absence of detailed studies of Fe-Ti oxides, the pres­ Feldspar. Almost all topaz rhyolites are two-feldspar rhyo­ ence of titanite in several topaz rhyolites may be a mineralogic lites, in contrast to many other bimodal rhyolites-for example, indicator of relatively high f02 (Haggerty 1976). Titanite occurs many of the rhyolites of the Snake River Plain, Idaho (Leeman in both the Chalk Mountain, Colorado, and Mineral Range, 1982a; Hildreth 1981) and the peralkaline rhyolites of the west­ Utah, rhyolites, where independent evidence suggests oxidizing ern Great Basin (e.g., Rytuba and McKee 1984; Conrad 1984; conditions. Titanite also occurs in the Lake City, Colorado, rhyo- Novak 1984; NOble and Parker 1974). In general, one-feldspar 48 Christiansen, Sheridan, and Burt

TABLE 9. DEVITRIFICATION AND VAPOR-PHASE MINERALOGY REPORTED IN TOPAZ RHYOLITES

Location Sa Qz PI Bt Mt Gt FI Tz Bx Ps Hm Be Ct Tm

Thomas Range, UT XX XXX XX XX Spor Mountain, UT XX X X XX Honeycomb Hills, UT XXX XX X Smelter Knolls, UT XX X X Keg Mountain, UT X X ? X Mineral Mountains, UT XX X XX

Wah Wah vicinity, UT X X XXX XX X X Wilson creek Range, NV X X X Cortez Mountains, NV X XX

Kane Springs Wash, NV X Sheep Creek Mountains, NV XX XXXXX X Jarbidge, NV X Blackfoot lava field, ID X X X Elkhorn Mountains, MT X X X Little Belt Mountains, MT X X/M Specimen Mountain, CO X Chalk Mountain, CO XX XX X Nathrop, CO X X X XXX X Silver Cliff, CO X X X Tomichi Dome, CO X XX XXX Boston Peak, CO XX?XXX X X Lake City, CO X X Grants Ridge, NM XX XX Black Range, NM X X XX X XX XX X Saddle Mountain, AZ XXXX Burro Creek, AZ X X XX

x-present in some samples; ?-uncertain identification; X/M-may be magmatic. Bx-bixbyite, Ps-pseudobrookite, Hm-hematite, Be-beryl, Ct-cassiterite, Tm-tourmaline; others as in Table 8.

rhyolites crystallize at higher temperatures than those inferred for ing more magnesian biotites in some lavas. In general, the Altot is F-rich topaz rhyolites. Sanidine in topaz rhyolites is generally lessthan 3 moles per 24 (0, OH, F, Cl). In contrast, biotites from Or40 toOr60. Plagioclase is generally sodic oligoclase, although. two-mica granites of the Basin and Range province and strongly compositions as sodie as calcic albite are found in evolved topaz peraluminous, S-type, granites the world· over generally contain rhyolites like the Spor Mountain rhyolite, Utah. Andesine is more Al than this, a fact indicative ofthe metasedimentary parent­ found in less evolved rhyolites like Chalk Mountain, Colorado. age of.S-type granites (Figure 31). The concentrations of F and Two-feldspar temperatures (Stormer 1975) are shown in Table 4. Cl have been analyzed in relatively few biotites from topaz rhyo­ Two.-feldspar (calculated at 100 b to 1 kb) and Fe-Ti oxide lites. Existing analyses demonstrate that the biotites have high temperatures are generally in good agreement where they have F-contents (up to 5 wt%), Concentrations this high for Fe-rich been analyzed from the same sample. The rhyolites of the Thom­ biotites suggest crystallization at high fHF and at high fHF/fH20 as. Range, Utah, show the broadest temperature range (790 to (10-1 to 10-3 for the Thomas Range Rhyolites; Turley and Nash 600°C); temperature ill negatively correlated with F and other 1980). F/CI ratios in the biotites also suggest crys~llization at incompatible element concentrations. All equilibration tempera­ high fHF/fHGI. On a molar plot of Mg/(Mg+Fe) vs log F/CI tures, as determined by feldspar pairs from other localities, fall (Figure 32), biotites from topaz rhyolites fall in the same compo­ within the lower part of this range. sitional fields as the ilmenite series granites from Japan (Cza­ Mafic silicates. Biotites from topaz rhyolites generally manske et al. 1981) and greisenized Sn-mineralized granites have high Fe/(Fe+Mg)ratios (Figure 31) reflecting the high (Figure 32). Gunow et al. (1980) and Munoz (1984) have shown Fe/(Fe+Mg) of the magma (in many cases, molar Mn and Ti that such high F/CI ratios are also characteristic of the Henderson exceed Mg), and perhaps the prevalence of relatively low f0 2 in molybdenite deposit. Brimhall et al. (1983) and Munoz (1984) these types of magmas. Nonetheless, variable Fe/Mg ratios have have generalized this observation to many other Mo, Sn, W, and resulted from the variaple oxidation states inferred above produc- Be deposits and contrast these deposits with piotites from Cu-rich Topaz Rhyolites 49

-6 ~~- "., - :0... :. -8 - ,- -10 ,- ,- ./ •• .' . ./ -12 ,- ./ C'iI ./ Bishop Tuff 0 / .... ./ 0) -14 -0 ..... -16 o -18 Topa2: rhyolites -20 west-central Utah

-22

600 700 800 900 1000 1100 Temperature ( °C)

Figure 30. Compilation of T-log f02 data for silicic andesites, dacites and rhyolites from western North America (after Ewart 1979, except as noted). The oxygen buffer curves (HM hematite-magnetite; NNO nickel-nickel oxide; QFM quartz-fayalite-magnetite) are for one atmosphere pressure. Large dots are Californian bimodal rhyolites; small dots are for calc-alkaline intermediate to silicic rocks; and the open field is for the Bishop Tuff, California (Hlldreth 1977). The topaz rhyolite field includes rhyolites from the Thomas Range, Smelter Knolls, and Spor Mountain; Utah; large filled circles are for Chalk Mountain, Colorado, and open circles for the Mineral Range, Utah (all from sources cited in text). porphyry deposits which have low FICI ratios when corrected and their implications for the transport of elements in a vapor for Fe-F avoidance (Figure 32). phase remain little studied. However,fluid inclusion and oxygen Clinopyroxene (in high T andF~poor rhyolite; e.g. "mafic" isotope studies of topaz and quartz in rhyolites from the Black Thomas Range, Utah, and Jarbidge, Nevada, specimens), fayalite Range indicate that vapor-phase crystallization-.occurred (Kane Springs Wash, Nevada, and some Mexican tin rhyolites), at a temperature in excess of 600°C (Eggleston and Norman Fe-rich hornblende (Figure 33), or Fe-Mn garnet are found in a 1984). High salinity in the fluid probably reflects fluid boiling. In few specimens from topaz-bearing rhyolites. Fe-enriched mafic addition, the high iron content of bixbyite and the presence of minerals are common in bimodal rhyolite magmas (Ewart 1979) pseudobrookite instead of rutile indicate initial' temperatures of and in anorogenic or A-type granites (e.g., Anderson 1980, over 500°C during crystallization in some cavities (Lufkin 1976). 1983). As for the case of biotite, this association reflects the high Burt (1981) has shown that the vapor-phase mineral assemblage

Fe/Fe+Mg ratios of the magmas and, possibly, the prevalence of is consistent with moderately high fHF and relatively high fo 2• relatively low f02 in these types of magmas. Barton's (1982) exposition of the thermodynamic properties of The vapor-phase mineralogy of topaz rhyolites (Table 9) is topaz suggest that the HF/H20fugacity ratio exceeded 10-3 at diverse and includes topaz, fluorite, spessartine gamet, beryl, Fe­ temperatures above 600°C, stabilizing topaz and K-feldspar Ti-Mn oxides (pseudobrookite, hematite, and bixbyite), silica over muscovite plus quartz. The latter assemblage is more typical minerals, and alkali feldspar. The compositions of these minerals ofgreisens formed at some depth. The general absence of fayalite 50 Christiansen, Sheridan, and Burt

Annite Siderophyllite 3r------...,..~----___,

0.8 f- 2

H () o Chalk MIn .... H • Spor MIn u. OJ K Smelter Knolls o H Honeycomb Hills ...J o Thomas Range

1 2.0 3.0 4.0 o Phlogopite AI atoms/24 (O,OH,F) Eastonite

Figure 31. Compositions of unoxidized biotite from topaz rhyolites from the western United States in terms of molar Fe/(Fe + Mg) and total Al -1 L-_.l-_.l-_..I-_...l-_...J....._...J....._...J....._...J....._-'-_-' relative to ideal end members. Sources of data for topaz rhyolites are 0.1 0.3 0.5 0.7 0.9 given in the text. For comparison the compositions of biotites from the Bishop Tuff, California (RT- Hildreth 1977), from calc-alkaline igneous Mg/Mg+Fe rocks from the western United States (Wender and Nash 1977; Hausel and Nash 1979; Dodge et aI. 1969) and from muscovite-bearing grani­ Figure 32. Compositions of unoxidized biotite in topaz rhyolites from the toids of the western United States (MG-Best et aI. 1974; Lee et aI. western United States in terms of molar Mg/(Fe + Mg) and log F/Cl 1981; Kistler et aI. 1981; Dodge et aI. 1969) are also given. (molar). Symbols are the same as in Figure 31. Sources of the data for topaz rhyolites are given in the text. Other fields shown for comparison are from Keith (1982), Gunow et aI. (1980), Czamanske et aI. (1981), Pargasite Hastingsite Jacobs and Parry (1979), and Parry et aI. (1978). Isopleths of equal F 1.0 enrichment have a positive slope on this diagram. Topaz rhyolites have biotites which are consistently F-rich when "corrected" for their usually high Fe/Mg ratios. In this regard, they are most like biotites from ilmenite-series granites of Japan (Czamanske et aI. 1981) and from u:; granites associated with Mo-Sn-W-Be deposits (Munoz 1984). :i 0.8 q, 9 (as a phenocryst or as a product of vapor-phase crystallization) is o:l' 0.6 notable; it is presumably unstable with respect to F-bearing bio­ .....C'\l III tite. Peralkaline minerals (aegerine, riebeckite, etc.) are likewise e 0 absent. In contrast, the presence of aluminous minerals in miaro­ 0 .... ; ... - .... III , litic cavities, especially topaz and spessartine garnet (Fig. 34), 0.4 / \ S. I Wolf \ some of which is F-bearing (Moyer, 1982), suggests a link be­ Amphiboles sorting out the role of volatiles and melt structure in determining from other anorogenic silicic magmas have similar Fe/(Fe + Mg) ratios partition coefficients in highly silicic magmas (e.g., Mahood and (Wolf River Batholith, Wisconsin: Anderson 1980; and the Pikes Peak Hildreth 1983). batholith, Colorado (+): Barker et aI. 1975). Hornblendes from calc~ alkaline volcanic rocks of the eastern Great Basin are shown as open Geochemistry and differentiation trends circles (Hausel and Nash 1979; Wender and Nash 1977; Keith 1982). The solid line encloses the composi~ions of hornblendes from the Sierra Nevada batholith, California (Dodge et aI. 1969), and the southwest The major element composition range of topaz rhyolites is Japanese batholith (Czamanske et aI. 1981). fairly restricted. All are high-silica rhyolites with high Na, K, F, Topaz Rhyolites 51

Spessartine

x Burro Creek, Arizono o Nathrop, Colorado •. Thomas Range, Utah \ \ t::. Pine Grove, Utah \ o Ely, Nevada \ \ A Canterbury, New Zealand \ Kern Plateau, California \ + \ ---- Pegmatitic garnets \ \ \ \

\XX I

" \XXX \ \X. \ \X

" \ X \ \ \ \ \ \

Pyrope L-- ---"'--'''''''''=<...... Almandine

Figure 34, Compositions of garnet from rhyolitic volcanic rocks in terms of Fe, Mn, and Mg end­ members. Garnets from the rhyolitic tuff of Pine Grove, Utah (Keith 1980), and lavas from Canterbury, New Zealand (Wood 1974), and the Kern Plateau, California (Bacon and Duffield 1981) are magmatic; the rest are Mn-rich vapor-phase garnets (Miyashiro 1955; Cross 1886; Christiansen et al. 1980; Moyer 1982; Pabst 1938). The pegmatitic garnet field is from Miyashiro (1955). and Fe/Mg and low Ti, Mg, Ca, and P (Table 1; Figure 35). (1981) experiments show that the stability field for quartz ex­ These characteristics are typical ofbimodal rhyolites (see below), pands with increasing fluorine content in a water-saturated haplo­ Topaz rhyolites are apparently a subclass ofthis group (a conclu­ granite at 1 kb pressure (Figure 36). It is thus conceivable that sion also justified by their trace element chemistry, Fe-enriched enhanced quartz fractionation could lead to a reversal of normal mineralogy, tectonic setting, and magma associations). The com­ Si02-enrichment during fractional crystallization of a fluorine­ position of a "typical" topaz rhyolite is shown in Table 1 and rich rhyolite. represents modal values from the histograms in Figure 35, Most topaz rhyolites contain 12% to 14% Al203 (Figure 35). All topaz rhyolites thus far identified from the western Uni­ High aluminum contents are found in the evolved low-silica rhyo­ ted States have Si02 concentrations greater than 72% (all Si02 lites noted previously, reflecting the increased proportion of concentrations have been recalculated on an anhydrous basis) feldspar components, especially albite. All topaz rhyolites have and a strong mode exists at 76% (Figure 35). Silica contents vary high concentrations of alkalies ranging between 8% and 10% little with differentiation trends seen in individual dome com­ Na20 plus K20. In general K20/Na20 ratios are greater than plexes. For example, silica ranges from 74.2% to 76.7% in the one (typically about 1.2 to 1.4 by weight). Several topaz rhyolite rhyolites of the Thomas Range, Utah, whereas incompatible ele­ occurrences seem to be typified by K20/Na20 ratios higher than ments such as Rb triple in concentration, As a result silica con­ 1.5-the topaz (and topaz-free) rhyolites of northern Nevada, the centrations are poor indicators of the chemical variability of these Taylor Creek Rhyolite in the Black Range of New Mexico and rhyolitic magmas. In addition, silica contents are actually lower in the Sn and F-rich rhyolites ofMexico's Sierra Madre Occidental. rhyolites that are extremely enriched iIi fluoriIle and incompatible Because only a few vitrophyres have been analyzed from these elements such as the lava at Spor Mountain, Utah. The average areas, it remains to be demonstrated that these high ratios are silica concentration of these samples is about 74%. Other highly magmatic and not the result of alkali-metasomatism during sub­ evolved topaz rhyolites, such as the one at the Honeycomb Hills, aerial crystallization, In d!fferentiation sequences, K generally de­ Utah, and the ongonites discussed below, also show silica con­ clines with advancing concentrations of F and other decidedly tents lower than 76%, Christiansen et al. (1984) interpreted the incompatible elements, while Na increases-a result of the com­ low silica content as the result of crystallization near the min­ bined fractionation ofpotassic sanidine and biotite (Figure 36). In imum in the granite system, with elevated fluorine. Manning's spite of high alkali concentrations, topaz rhyolites are not peralka- 52 Christiansen, Sheridan, and Burt

70

60

50 51°2

40

30

20

10

72 74 76 78 80 0 0.08 0.16 0.24 10 12 14 16 0.6 1.0 1.4 1.8 0.02 0.06 0.10 0.14 0.18

40

K 0 P205 MgO 2 30 CaO Na 2 0 F 20

10

~ n I 0.04 0.20 0.36 0.20 0.60 1.0 1.4 3.0 3.8 4.6 4.0 4.6 5.4 0 0.04 0.08 0.2 0.6 1.0 1.4

Figure 35. Histograms of whole-rock chemical analyses of topaz rhyolites from the western United States (values in wt%). Analyses of 118 samples from 22 locations are represented (sources cited in the text). Analyses were rejected if H20 was greater than 3 wt% or if K20/Na20 (by weight) exceeded 2. All analyses were recalculated to 100% on an H20- and C02-free basis. The vertical scale shows the frequency of each value. line, and the use of the term "alkali rhyolite," reserved by lUGS normative corundum, than it would be otherwise. This crystalli­ usage for peralkaline rhyolites (Streckeisen 1979), should be zation order is not that observed in topaz rhyolites. Fluorite is one abandoned. Use of this term may lead to confusion with the truly of the last phases to crystallize in some extremely F-rich glasses peralkaline rhyolites with which topaz rhyolites are contempo­ and more generally fluorite is post-magmatic. In short, topaz raneous in the western United States. In the lUGS system topaz rhyolites are not strongly peraluminous indicating that they are rhyolites are generally rhyolites or alkali feldspar rhyolites. In­ not the eruptive equivalents of S-type grllnites (e.g., White and deed, many vitrophyres from topaz rhyolites are metaluminous. Chappel 1983). The presence ofgarnet and topaz (absent as vapor-phase ttlinerals The concentrations of CaO (generally <0.8%), Fez03* in peralkaline volcanic rocks) also reflects the aluttlinous charac­ (generally <1%), MgO (generally <0.20%), TiOz (generally ter of topaz rhyolites. Many felsites are slightly peraluminous as <0.12%), and PzOs «0.02%) in topaz rhyolites are relatively low indicated by normative corundum-probably as a result of the and similar to those found in other high-silica rhyolites (Figure loss of alkalies during crystallization. 35). In cogenetic suites, all of these elements correlate negatively We prefer to calculate CIPW norms based on fluorine-free with incompatible elements and with fluorine. Their decreasing analyses. The CIPW scheme ties up Ca with F to form fluorite concentrations can be shown to relate to the fractionation of before Ca is used to form plagioclase, thereby producing a rock modal proportions of plagioclase, biotite, Fe-Ti oxides and apa­ that appears to be more strongly peraluminous, as indicated by tite. A few samples from the Honeycomb Hills and Spor Moun- Topaz Rhyolites 53

Q

•• . L6• LOCATION OF PLOT AREA

D. 0.5 D. D. 1 D. ~. &. D.D. 2 M. D. 3~~: • D. D.

5 *D. D. • 1%

2%*

D. Ab '--_---'>l._---"----"'-----''------'>l.--....:L.--'''--_--''"--_--''-__...l. Or Figure 36. Normative composition of topaz rhyolites from the western United States in terms of quartz (Q), albite (Ab), and orthoclase (Or) compared to experimentally determined ternary minima in: 1) the hydrous granite system at pressures given in kb (PHzO) next to the filled circles (Tuttle and Bowen 1958; Luth et aI. 1964; and Whitney 1975); and 2) the F-bearing granite system at 1 kb (Manning 1981). The numbers next to the stars indicate weight %F in the water-saturated system at 1 kb. tain, Utah and perhaps elsewhere, show Ca-enrichment as the concentrates CI in contrast to F. Meaningful chlorine concentra­ result of the accumulation of post-magmatic fluorite. Topaz rhyo­ tions can only be obtained by analysis of vitrophyres or obsidians. lites have high Fe/Mg ratios and plot in the "tholeiitic" field on Figure 37 shows the fluorine and chlorine concentrations in Si02 versus FeO*/(FeO* + MgO) diagrams such as the one used glassy specimens of topaz rhyolites. In specimens thus far ana­ by Anderson (1983) to discriminate calc-alkaline from tholeiitic lyzed, Cl concentrations remain less than 0.2% and are generally or reduced anorogenic suites. In addition, Fe/Mg ratios increase much lower than this. Important features of topaz rhyolites are with differentiation, driven principally by biotite fractionation as their high F/Cl ratios (greater than about 3) as compared to F­ FeO*IMgObiotite is substantially less than FeO*IMgOmelt. and Cl-rich peralkaline rhyolites, which have lower F/Cl. Manganese concentrations are also low, less than 0.08% Topaz rhyolites are variably enriched in incompatible trace MnO in almost all samples, but in general Mn appears to behave elements (Li, Rb, Cs, Be, U, Th, Y, Nb, Ta, Ga, Pb, Mo, Sn, W, as an incompatible element. MnO increases with differentiation and HREE) and depleted in feldspar-compatible trace elements from 0.05% to 0.08% in the rhyolites of the Thomas Range. (Ba, Eu, and Sr). Zirconium and Hfconcentrations are generally Similar increases of MnO with differentiation of rhyolitic mag­ low as well, consistent with their compatibility with fractionating mas have been noted by many authors (see review in Hildreth zircon. As for other metaluminous rhyolitic magmas (e.g. Hil­ 1981) and appear to relate to limited biotite fractionation. dreth 1979), Zr/Hfratios (typically 25 to 20) decline with differ­ Ofcourse, the most discriminating feature of topaz rhyolites entiation as the result of the greater compatibility of Zr in zircon. as a group is their high fluorine content. Topaz appears as an The concentrations of many elements that are taken up by mafic identifiable vapor-phase mineral in lavas whose vitrophyres con­ silicates (Ni, Co, Cr, and V) are extremely low, but as they lie tain over 0.2% F. Fluorine concentrations of more than 1% are near the detection limits for common analytical metho~ their only known from vitrophyres from Spor Mountain and the concentrations are not well known. Typical concentrations are Honeycomb Hills, both in western Utah. Figure 35 shows listed in Table 2 and illustrated in Figure 38. Using trace element concentrations in felsites and vitrophyres, where most of the low compositions, Pearce et al. (1984) have attempted to interpret the values «0.2%) are from felsites. Comparisons of vitrophyre­ tectonic settings of granitic rocks. In this classification (Figure felsite pairs almost universally show that F is lost during devitrifi­ 39), topaz rhyolites from the western United States consistently cation. Chlorine is also Ibst by devitrification, as no mineral phase straddle the boundary between WPG (within-plate granites, like 54 Christiansen, Sheridan, and Burt

0.8

-eft ~ :;= - 0.6 w Z .,. I--l a0:::: --l I 0.4 .... U ......

0.2

<>

0.2 0.4 0.6 0.8 1. 0 1.2 FLUORINE (wt.%) Figure 37. Fluorine and chlorine concentrations in glassy topaz rhyolites (open diamonds) and peralka­ line rhyolites (closed triangles). A FICl ratio of 1divides oceanic from continental peralkaline rhyolites (Bailey 1980). A F/C! ratio of 3 separates peralkaline rhyolites from topaz rhyolites of the western United States. The composition ofthe Bishop Tuff, California (B: Hildreth 1979) is shown for compari­ son. Data are from Christiansen et al. (1980), Moyer (1982), Dayvault .et al., (1984), Novak (1984), Turley and Nash (1980); Conrad (1984); Mahood (1981); Bailey (1980); Macdonald and Bailey (1973). . the Nigerian Younger granites) and syn-COLG (syn-collisional Thomas Range rhyolites as resulting from the fractionation of granites, like the leuco-granites of the Himalayas). Although these small amounts of allanite (0.04 wt% of the fractionated mineral diagrams point to a certain uniqueness among topaz rhyolites as assemblage). The middle REE depletions noted for the Mineral compared to many other granites, it is unlikely that simple dis­ Mountains and the Lake City rhyolites suggest the fractionation criminant diagrams such as these will provide a unique definition of titanite. of the type or tectonic setting of granites because of the wide Devitrification mobilizes a number of trace elements (U, Sb, variety of crustal and mantle components that are involved in F, CI, Li, Be, perhaps Sn, W, and Mo)judging from lower granite genesis. Note for example the location of the metalumi­ concentrations in vitrophyre-felsite pairs. However, it has not nous Bishop Tuff, California, and the comenditic Tala Tuff, been demonstrated that devitrification, without significant vapor­ Mexico. phase alteration, significantly changes concentrations of many REE patterns (Figure 40) show some variability (perhaps other trace elements, including petrogenetically important ele­ inherited from slightly different source rocks and/or differentia­ ments like Rb, Sr, Ba, Y, Zr, Hf, Nb, Ta, Th, REE, Sc, and Ga. In tion histories) but they generally display low La/CeN, La/YbN (1 fact, for some trace~element and isotope studies, dense felsites may to 3 for most but this ratio may be as high as 12 for rhyolites from be better samples than variably hydrated vitrophyres, as hydra­ Colorado), and Eu/Eu* (0.45 to 0.01 for analyzed specimens). tion may involve significant addition of Sr, Ca, and other ele­ Light REE concentrations generally do not exceed 200 times ments (Hargrove 1982), as well as 0 and H isotope exchange. chondrite values and more typically are near or less than 100 Some of the compositional features of topaz rhyolites are times chondrite. Differentiation trends for the REE are seen in summarized in Figure 41, which compares the chemical composi­ several complexes (Thomas Range, Utah, Mineral Mountains, tion (Table 2) of a variety of topaz rhyolites on normalized Utah,Wah Wah Mountains, Utah, Lake City, Colorado) and geochemical diagrams. Such normalized-concentration diagrams show decreases in LREE and Eu concentrations that are coupled allow the relative concentrations of many elements in a single to increasing HREE and other incompatible element concentra­ sample to be displayed and elemental fractionation is easily por­ tions. Christiansen et al. (1984) have modelled this trend for the trayed. All concentrations have been normalized to those of the Topaz Rhyolites 55

Rb Sr Ba Zr y

.~

0 400 800 0 20 0 100 200 0 80 160 20 100 180

Li Be Nb U Th

o 80 160 o 40 80 o 40 80 120 o 20 40 20 40 60

Figure 38. Histograms of trace element concentrations in topaz rhyolites from the western United States (all values in ppm). Analyses of 30 to 90 samples are represented for each element and come from 19 different localities (references cited in Table 2). The vertical scale represents the relative frequency of each value.

U.S. Geological Survey geochemical reference sample RGM-l (a ofY and Sm relative to geochemically similar elements. Normal­ rhyolite from Glass Mountain, California) as presented in Govin­ ized major element diagrams show the depletions of Fe, Mg, Ti, daraju (1984). RGM-1 was chosen because the concentrations of and Ca typical of most high-silica biotite rhyolites with two feld­ many elements have been accurately determined and because it is spars, including topaz rhyolites. a sub-alkaline high-silica rhyolite grossly similar to many topaz Taken together these compositional features suggest that rhyolites. RGM-1 is an obsidian and the concentrations of ele­ topaz rhyolites are highly differentiated or evolved magmas. ments otherwise mobilized by devitrification and hydration Their common association with less evolved, topaz-free rhyolites should be nearly magmatic. This is particularly important for F, suggests they may have fractionated from more "mafic" composi­ Cl, and U-elements of interest here. RGM-l is reported to have tions. Such evolutionary relationships are discernible in at least a Th/U ratio of2.6 (1515.8) and a FICI of 0.6 (3401540); values 15 of the 26 localities described here. Christiansen et al. (1984) that may be typical of calc-alkaline rhyolitic glasses. The elements have used the extreme depletion of Eu and other compatible are listed in order of increasing c/r2, so that geochemically similar elements to preclude an important role for variable degrees of elements are plotted near one another. Chondrite- or mantle­ partial melting in the observed chemical variability. This is not to normalization was deemed to be inappropriate in this case be­ suggest that a varying proportion of partial melting was not criti­ cause of the tremendous differences between rhyolites and these cal to the generation of topaz rhyolites, but only that the evidence other materials. For example, the FICI ratio of chondritic mate­ of such a process has been obscured by subsequent fractionation rial is approximately 10. , processes. The tremendous enrichments in F, Ta, Nb, Y, Rb, U, Th, . As indicated throughout this discussion, an examination of and HREE and the relative depletion of Zr, Sr, Eu, and Ba are the elemental composition of cogenetic lavas reveals the impor­ easily visualized in these diagrams. Chlorine concentrations in tance of crystal fractionation in the evolution of topaz rhyolites. glasses show up as deep anomalies, indicating the lack of enrich­ Those centers examined in any detail document chemical trends ment relative to F and Rb. Some topaz rhyolites show depletions consistent with crystal fractionation. The correspondence be- 56 Christiansen, Sheridan, and Burt

Or-An-F-H20), consistent with the observed small changes in Si02, A1203, and the alkalies. These variations correlate with 1000 larger changes in trace element concentrations controlled by the syn-COLG fractionation of major phases and by the observed accessory min­ erals (i.e., zircon, allanite, apatite, titanite). The correlation of 400 higher F with lower Si02 and higher Al203 and Na20 in the rhyolites is also consistent with crystallization differentiation of -. more F-rich magmas as predicted by the experimental studies of E c. Manning (1981). In short, although these rhyolites are among the ...... c. most F-rich magmas yet analyzed, we see no compelling evidence .c 100 a: to indicate that convection-driven thermogravitational diffusion 81'hOPT~ WPG (Le. volatile complexing in the melt or Soret diffusion) controlled their evolution (cf. Mahood 1981; Hildreth 1981). The importance of crystal fractionation is strikingly illus­ trated by the correspondence of the trace element patterns of the VAG ORG rhyolites with distinctive accessory mineral phases. (Wolf and Storey (1984) have used this principle in a convincing explana­

10 tion of the trace element zonation patterns seen in highly alkaline 1.0 10 100 magma bodies.) In topaz rhyolites and other aluminous rhyolites the negative Zr (and usually Hf) anomalies and decreasing Zr/Hf Vb +Ta (ppm) ratios are controlled by fractionation of zircon. Fractionation of allanite, and perhaps monazite or chevkinite in some lavas, pro­ duce the typical LREE depletion patterns shown in Figures 40 1000 and 41. In some rhyolites, titanite appears as a microphenocryst Topaz and its fractionation is probably responsible for the Y, and middle Rhyolites REE depletions apparent for lavas from the titanite-bearing rhyo­ syn-COLG lites of the Mineral Range, Utah, Lake City, Colorado, and other areas as noted by the dashed lines in Figure 41. Elements that are ..... E largely incompatible in the major and trace phases become sub­ c. c. stantially enriched. Such elements include both large-U, Th, Rb, ..... 100 .c es, Ta-and small ions-Be, Li. Fluorine also behaves as an a: WPG incompatible element as it is only removed by biotite fractiona­ tion and biotite occurs in small proportions. Moreover, the biotite/ melt partition coefficient for Fis strongly dependent on the Fe/ VAG Mg ratio of the biotite (Munoz 1984) and hence on the fugacity of oxygen. On the other hand, the negative Cl anomaly must be a reflection of the original composition of topaz rhyolite melts,

10 100 because Cl should be more strongly incompatible than F.

~ Individual volcanic centers need to be examined in more ,,(p+Nb (ppm) detail to test these conclusions regarding the geochemistry and Figure 39. Trace element discriminant diagrams for granitic rocks differentiation of topaz rhyolites. Ofparticular importance would (Pearce et al. 1984) showing the compositions of topaz rhyolites from the be studies of the halogen· compositions of lavas and/or tuffs that western United States, the Bishop Tuff (Hildreth 1977), and the Tala Tuff (Mahood 1981). VAG = volcanic arc granites; ORG = ocean ridge maybe cogenetic with topaz rhyolites. Changes in F/Cl ratios granites; WPG =within plate granites; and syn-COLG =syn-collision could be monitored in this way. Another fruitful avenue of re­ granites. Topaz rhyolites clearly overlap the within-plate and collisional search would be to examine in more detail the relationship of fields. accessory mineral assemblages and compositions with the trace element patterns of their host "liquid." Befbre unique geochemi­ tween major and trace element models for the evolution of the cal models can be constructed, mineral/liquid partitioning needs Thomas Range rhyolites was noted above, and involved the frac­ to be studied, but has thus far been completely neglected for the tionation of sanidine >quartz >plagioclase »biotite >bxides rhyolites described here. V. I. Kovalenko and others (e.g. 1978 >> apatite>zircon>allanite; The extreme depletion offeldspar­ and 1984) have been examining crystal/liquid partitioning in compatible elements is thus explained as are enrichments in a ongonites. Studies of the geochemical effects of devitrification variety·of incompatible trace elements. Crystallization appears to have thus far been cursory. The mineralogic controls on this have occurred close to the mi~imum in the granite system (Q-Ab- process need to be determined. Experimental studies are called Topaz Rhyolites 57

a Thom•• Range. UtAh 5""-61-- ..... _ 100

SM·2~:z.06 --...-_------... 10

b ------.. r----='SPc:;O':.:M::..:.n:::••.::;uT'--_- ~ 100 ...... -- ~ ... ------_ ... ----.._-. Black R.ngc.NM

...... Nathrop.CO ...• 10 Figure 40. Chondrite-normalized REE patterns for topaz rhyolites and related rocks from the western United States. Concentrations normalized c to 0.83 times concentrations in Leedey chondrite (Masuda et al. 1973). Honeycomb Hill.. Utah a. Thomas Range, Utah (Christiansen et al. 1984a). Sample SM-61

100 contains relatively low concentrations of incompatible trace elements (e.g. 5 ppm U) while SM-29-206 contains high concentrations of these elements (e.g. 19 ppm U). La/LuN and Eu/Eu* are negatively correlated with U content and apparently decrease as a result of differentiation of

10 the parental magma. HREE concentrations increase with differentiation. The lava from which SM-61 was collected appears to be topaz-free, but d Christiansen et al. (1984a) postulate that it is genetically related to the topaz-bearing rhyolites of the Thomas Range. Smelter KnoU. Utah b. Specimens from Spor Mountain, Utah (Christiansen et al. 1984), 100 Black Range, New Mexico (HC-8; Correa 1980), and Nathrop, Colo­ rado(Zielinski 1977) cover the range of REE concentrations seen in topaz rhyolites from the western United States. c. Honeycomb Hills, Utah (Turley and Nash, 1981). The low Si, high 10 Ca and F sample described in the text has a very similar pattern. d. Smelter Knolls, Utah (average of three analyses; Turley and Nash 1980). e. Mineral Mountains, Utah, topaz rhyolites (average of samples from 100 e two domes) compared with less differentiated but probably co-genetic Minerai Mount.lna. U,... lava (Bailey Ridge flow) (Lipman et al. 1978a). Depletion of light and middle REE, and especially Eu, with differentiation is apparent. Yb and Lu show slight enrichment. 10 f. Wah Wah Mountains, Utah (Christiansen et al. 1980). Samples WW­ 9 and WW-41 are late Miocene rhyolites from the Broken Ridge area; STC-4 is an early Miocene rhyolite from the plug at the Staats (U-F) Wah W.h Mountalna mine. ------... 100 g. Nathrop, Colorado, topaz rhyolite (Zielinski 1977) compared to calc­ alkaline rhyolite REE pattern from Summer Coon volcano in the nearby San Juan volcanic field (Zielinski and Lipman 1976). These rhyolites are not co-genetic. Note the lower La/Yb ratio and deeper Eu anomaly of 10 -WW·9 the topaz rhyolite. These features are typical ofcomparisons between ___ a WW'41 topaz rhyolites and nearby, often nearly contemporaneous,calc-alkaline .. STC-4 rhyolites.

100 ~---­ g

Nalhrop; lopal ~"':I~~;"-""""'''' ---., Southw••tern Colorado \ I \ 10 \,, \I \ , "11

0.1 ~-:'-_-..I._---J__....L-:'--L-L--' --JL.....L..:J l. c. Nd Sm [u Gd Tb Dy Vb L ... 58 Topaz Rhyolites

Topaz Rhyolites Mexican Tin Rhyolites

10 10

5 5

~ I ::;;I ::;; (!l (!l 0: ....0: 11> 11> C. c. 0.5 E E 0.5 (/)'" (/)'"

0.1 0.1

F CI Rb Sa Sr Eu La Sm Yb Y Th U Zr Nb Ta a F CI Rb Sa Sr Eu La Sm Yb Y Th U Zr Nb Ta c

Figure 41. Concentrations of selected trace elements in rhyolitic rocks Peralkaline Rhyolites normalized to those in RGM-l, a rhyolite obsidian from Medicine Lake volcano, California, and U.S. Geological Survey geochemical reference (Govindaraju 1984). The elements are listed in order of their relative field strength, increasing to the right. a) Topaz rhyolites from the western 10 United States typically show negative CI, Ba, Sr, Eu, and Zr anomalies on such diagrams. Titanite-bearing rhyolites commonly show negative 5 Y anomalies shown with dashed lines; titanite-free lavas show no nega­ tive Yanomalies. b) "Tin" rhyolites from Mexico (vertical bars; Huspeni et al. 1983), which commonly have vapor-phase topaz as well, have trace element compositions that largely overlap with those of topaz rhyolites from the western United States (shaded). Chlorine concentra­ tions from glassy specimens have not yet been reported. c) The trace element patterns of peralkaline rhyolites (Civetta et al. 1984; Mahood 1981; Conrad 1984; Christiansen et al. 1980) are strikingly different 11> C. from those of topaz rhyolites (shaded) and generally have small negative E 0.5 (/) (or in some cases even positive) CI anomalies. In addition, they generally '" have higher REE contents, but marked negative Th and U spikes are distinctive. The absence of negative Zr anomalies is the result of the enhanced solubility of zircon in peralkaline melts. • PantelJeria o Sierra La Primaveva 0.1 • McDermitt

b F CI Rb Sa Sr Eu La Sm Yb Y Th U Zr Nb Ta Topaz Rhyolites 59 for to determine the role of fluorine on melt properties and phase ThlU ratios (>5), many, especially felsic granulites, do not (see relationships and to constrain the values ofintensive properties of compilation in Iyer et al. 1984). For the Lake City rhyolites, each system. Lipman et al. (1978b) have interpreted the Pb isotope data to indicate derivation from an amphibolitic lower crustal source Isotopic composition with ThlU ratio of about 3.6. Another important feature of the Pb-isotopic composition of both the Cortez and Lake City rhyo­ Initial strontium isotope ratios for 18 samples from 10 local­ lites is their similarity to chemically and temporally distinct rocks ities are reported in Table 3. These analyses indicate that initial in the same region. It appears that Pb isotope regionalization 87Sr/86Sr ratios for topaz rhyolites range from 0.7055 to about noted by Zartman (1974) and others extends to these silicic vol­ 0.712. Considerable uncertainties exist in the initial 87Sr/86Sr canic systems as well. because of poorly known ages for some of the samples, coupled Integrated isotopic (Pb, Sr, Nd, 0) and petrologic studies of with their extremely high RblSr ratios. The effects of recalcula­ topaz rhyolites are of paramount importance for determining the tion to different ages are shown for the rhyolites from Nathrop, geochemical nature of the protolith of topaz rhyolites. Given the Colorado, and the Black Range, New Mexico. These isotope probable crustal origin of topaz rhyolites, these studies might be ratios are suggestive of an important crustal component in the informative of crustal structure and composition when examined protolith of topaz rhyolites. Using approximate crustal province on a regional scale, but studies of single volcanic complexes are ages (Farmer and DePaolo 1984), these ratios correspond to needed before the role of crustal contamination versus source crustal sources with relatively low RblSr ratios. For example, an inheritance can be fIrmly established. initial 87Sr/86Sr of 0.709 from the eastern Great Basin corre­ Magma-tectonic setting sponds to a RblSr ratio in the source of about 0.07 (assuming separation of the crust 2.2 Ga ago from a bulk-earth reservoir The nature of igneous rock associations and contemporane­ with RblSr ratio ofO.029-parameters from DePaolo and Was­ ous tectonic activity gives some clues about the generation of serburg 1977); a ratio of 0.706 implies a source RblSr of 0.04. magmas. For example, lithospheric subduction at continental Initial ratios between 0.709 and 0.7055 from Colorado and New margins is consistently associated with concurrent calc-alkaline Mexico (with crustal ages of 1.7 to 1.8 Ga) imply RblSr ratios of magmatism. However, any attempt to delineate the magma­ 0.08 to 0.04. These elemental ratios are consistent with the hy­ tectonic setting of topaz rhyolites in the western United States is pothesis that topaz rhyolites are derived from high-grade limited by our incomplete understanding of the complex evolu7 metamorphic rocks, perhaps of granulite grade (see compilation tion of Cenozoic tectonism across the region. in Pettingill et al. 1984). Moreover, these values are lower than Extensional tectonism appears as a common denominator in would be expected if the rhyolites were derived from metasedi­ almost all areas where topaz rhyolites were erupted in the western mentary materials of similar Proterozoic ages. For example, United States. Episodes of topaz rhyolite magmatism coincide Farmer and DePaolo (1984) estimated that the RblSr ratios for with periods of lithospheric extension; 1) in the eastern Great the sources of strongly peraluminous (S-type) granitoids in the Basin where basin and range faulting may have begun as early as western Cordillera range from 0.3 to 0.05. These aluminous gran­ 21-20 Ma (Rowley et al. 1978a) and then was renewed under a itoids are thought to be derived from a middle crustal different stress orientation about 10 Ma which has persisted to the metasedimentary source (e.g. Lee and Christiansen 1983; Farmer present (Zoback et al. 1981); 2) along Nevada's Cortez rift that and DePaolo 1983). Upper crustal contamination of cogenetic opened 16 Ma (Stewart et al. 1975); 3) in Montana where block topaz rhyolites may be indicated by variable initial Sr-isotope faulting began about 40 Ma (Chadwick 1978) and intra- or back­ ratios for samples from the Thomas Range, which belong to a arc graben formation may havebegun as early as 50 Ma (Arm­ coherent suite as judged from trace and major element geochem­ strong 1978); 4) along the Rio Grande rift and its northern istry (Christiansen et al. 1984). extension into Colorado, which initially developed about 30 Ma Oxygen isotope ratios for topaz rhyolites are available only (Eaton 1979; Elston and Bornhorst 1979); and 5) in western from the Mineral Mountains, Utah, (Bowman et al. 1982) and Arizona where block faulting was underway by about 9 Ma Lake City, Colorado, (R. A. Zielinski, written communication, (Sunneson and Luchitta 1983). Several groups of topaz rhyolites 1982). These bear out the suggestion of a crustal source in their lie on possible continental transform zones that developed as a low to moderate values (8 180 =6.3 to 6.9 %0 for the Mineral result ofdifferential extension rates (Wah Wah Mountains, Kane Range and 7 to 10 %0 for Lake City). Springs Wash, Spor Mountain, Elkhorn Mountains). Litho­ Interpretation of the Pb-isotopic data in such a manner is spheric extension occurred in back- or intra-arc and post-arc equivocal as the· two topaz rhyolites thus far examined (Lake environments (Eaton 1979, 1984a; Elston and Bornhorst 1979). City, Colorado, and Cortez, Nevada) do not show elevated The intimate association ofextensional tectonics and topaz rhyo­ 208Pb/204Pb ratios. Elevated thorogenic Pb isotope ratios are lite magmatism in the western United States implies a strong commonly taken to indicate derivation from granulitic rocks be­ genetic connection, but the nature or existence of extension con­ cause of the presumed depletion of U relative to Th during met­ current with topaz rhyolite magmatism in Mexico needs to be amorphism. Although many granulite terranes do show high clarifIed. 60 Christiansen, Sheridan, and Burt

TABLE 10. MAGMATIC ASSOCIATIONS OF TOPAZ RHYOLITES

Calc-alkaline Suite Bimodal Basalt-Rhyolite Suite Alkaline Suite

Andesitic to dacitic lavas Generally potassic basalts Alkaline to peralkaline and rhyolitic ash flows of tholeiitic or alkaline tuffs, lavas, intrusions associated with granodioritic affinity. Alkalic basalts may also intrusions. Continuous Si02 occur. Trachytes variation diagrams. present.

Chalk Mountain, CO Thomas Range, UT Little Belt Mountains,MT Nathrop CO Spor Mountain, UT Silver Cliff, CO Specimen Mountain, CO Smelter Knolls, UT Kane Springs Wash, NV Tomichi Dome, CO Honeycomb Hills, UT Grants Ridge, NM Black Range, NM Wah Wah Mountains, UT Sierra Madre Occidental L. Pliocene Mexico Mineral Mountains, UT Cortez, NV Probable Jarbidge, NV E. Miocene Wah Wah Mtns, UT Sheep Creek Mtns, NV Wilson Creek Range, NV Blackfoot lava field, ID Elkhorn Mountains, MT Lake City, CO Boston Peak, CO Burro Creek, AZ Climax-type Mo deposits Climax-type Mo deposit Henderson, CO Mt. Emmons, CO Climax, CO Pine Grove, UT

The magmatic associations of topaz rhyolites are less broadly contemporaneous with volcanic rocks that show ex­ straightforward. Following Lipman et al. (1972) and Christiansen tended and continuous SiOz-variation diagrams, but lack basaltic and Lipman (1972), the magma-tectonic evolution of the western compositions. Some may be part of a "transitional" group United States may be divided into two fundamentally different associated with the change from subduction-related magmatism stages. An early suite ofsubduction-related calc-alkaline magmas to extension-related magmatism (White et al. 1981; Bookstrom was time transgressive across the western United States and pro­ 1981). In addition, the early Miocene rhyolites of the Wah Wah duced eruptions of andesitic lavas, dacites, and rhyolites; the latter and Needle Ranges in Utah form a restricted bimodal mostly occur as ash-flow tuffs associated with caldera collapse. (trachyandesite-rhyolite) association within the mountain range, After about 20 Ma, the Basin and Range region experienced the but consideration of volcanism op. a broader scale (southwestern eruption of bimodal suites of basalt and rhyolite associated with Utah) could allow them to be grouped with the calc-alkaline lithospheric extension. Elsewhere, the temporal relationships are magmatic association. different, but the magmatic and tectonic products of each associa­ The second group consists of topaz rhyolites clearly in bi­ tion can generally be identified. In the southern Rocky Moun­ modal association with more mafic rocks-the fundamentally tains, Elston and Bornhorst (1979) defined an episode transitional -basaltic group of Christiansen and Lipman (1972). The composi­ between these two associations, which is typified by eruptions of tional variety of late Cenozoic "basalt" makes the definition of basaltic andesite and high-silica rhyolite in a modified back-arc this group somewhat arbitrary. The mafic members range in extensional environment. We include these rocks with those of a composition from basalt to andesite and are variably alkaline. We predominantly calc-alkaline character (cf. Ratte et al. 1984). have also included the Oligocene rhyolites ofthe Elkhorn Moun­ Topaz rhyolites appear to have been produced in all of these tains in the group because of their association with basaltic lavas igneous associations. In addition, several topaz rhyolites are con­ and the apparent absence of intermediate composition rocks. temporaneous with the emplacement of distinctively alkaline A third type of magmatic association includes the alkaline magmas. Thus we have tentatively identified three principal rocks associated with topaz rhyolites at Kane Springs Wash vol­ magma associations in which topaz rhyolites are found; 1) inter­ canic center, Nevada (trachyte to peralkaline rhyolite); Silver mediate to silicic calc-alkaline volcanic suites; 2) bimodal basalt­ Cliff-Rosita, Colorado (andesite, trachyte, rhyolite); and in the rhyolite associations; and 3) alkaline to (silica-saturated) peral­ Little Belt Mountains, Montana (trachybasalt to trachyte; quartz kaline suites in which trachytes (or their intrusive equivalents) are monzonite to syenite). The Grants Ridge rhyolites in New Mex­ important. Table 10 shows these groups. Reference to Figure 42 ico are associated with the construction of an "andesitic" volcano may be helpful in this regard. at Mount Taylor, which is surrounded by alkali basalt to trachyte In some cases, the definition of the calc-alkaline group is lava flows, and could be placed with the second group. problematic. The Oligocene topaz rhyolites of Colorado are all In spite of the vagaries of any such classification, it is ob­ grouped with the calc-alkaline association because they are vious that topaz rhyolites are associated with a variety of igneous Topaz Rhyolites 61

a e 5 6 6 . • 0 4 4 4

2 2 2

U 0 0 b . . 5 5 6 8 ~ 4 4 4 3 2 2 ~ ~ 2 Q N N +' +' +' ~ ~ ~ Specimen Mtn U 0 0 C 0 0 k N_6 N 5 N 5 ::s:: ::s:: ::s::

4 4

2

0 011"-_-+-_-+-_-+-""""1 d h 6 .. 5 5 Q ~+ 4 4

2 2

55 55 75 ll'5 55 55 75 55 65 75 SiOZ (ivt/.) Si02 (wtiO 5 i 02 (wt7.)

Figure 42. Potassium versus silica variation diagrams for mafic to intermediate composition volcanic rocks associated with topaz rhyolites from the western United States. The original sources of the data are given in the description of the individual occurrences. Representative analyses of topaz rhyolites are indicated by crosses, other volcanic rocks as open boxes. Most topaz rhyolites occur in strongly bimodal associations with variably potassic mafic rocks (e.g. basalt or basaltic andesite). Topaz rhyolites asso­ ciated with trachytic magmas do not show strong SiOz gaps and have variation diagrams which extend to mafic rocks (e.g. Little Belt Mountains, Kane Springs Wash, Grants Ridge/Mt. Taylor). A few, like those in the Black Range (and the Oligocene of Colorado, not shown), have intermediate to silicic calc-alkaline trends which include topaz rhyolites at their high silica ends. Lines are those used by Ewart (1979) to define low-K, interrnediate-K, and high-K rock series. rocks. Indeed, they show no consistent spatial or temporal rela­ (Table 11). The marked magmatic· enrichment of these same tionship to a single magma series from which they could be elements in topaz rhyolites strongly suggests that.the ore elements derived by differentiation. We suggest that none of these more were derived from the rhyolites or their intrusive relatives in the mafic magmas are parental to the F-rich rhyolites discussed here. case of Climax-type Mo-W deposits (Burt and Sheridan 1980; Instead, the observation that topaz rhyolites are associated with a Burt et al. 1982). Other types of mineralization (alunite, Hg, variety of more mafic magma suites suggests that the rhyolites Au-Ag) are spatially and temporally associated with some topaz have a thermal relationship to the more mafic magmas. The rhyolites. The association ofthese deposits with the rhyolites may residence of these mafic magmas in the crust may have provided rely more on magmatic heat content and volcanologic style for the thermal energy required for melting to produce magmas par­ their generation than on any particular compositional feature of ental to topaz rhyolites. topaz rhyolites. A variety of these ore deposit environments are schematically indicated in Figure 44. Ore deposits Beryllium. The most important ore deposit directly asso­ ciated with a topaz rhyolite is the beryllium deposit at Spor Mineralization associated with topaz-rhyolite magmatism Mountain, Utah. It is currently (1985) the only important source generally consists of F, Be, Li, Cs, U, Sn, Mo(?), and W(?) of Be in North America. Bertrandite (Be4Siz07) occurs in the 62 Christiansen, Sheridan, and Burt

TABLE 11. MINERALIZATION ASSOCIATED WITH TOPAZ RHYOLITES --~-

Location Mineralization

Thomas Range, UT Spor Mountain, UT Be, U, Li, F Honeycomb Hills, UT Be, Rb, Cs, Li

Smelter Knolls, UT Keg Mountain, UT Mineral Mountains, UT

Wah Wah Mtns. vicinity, UT U, F, Hg, Au, Ag, Mo(?) Wilson Creek Range, NV Kane Springs Wash, NV 0 36 •••• 30~028 Cortez Mountains, NV Au, Ag, Hg o Sheep Creek Mountains, NV Sn 17~~ 24 o Jarbidge, NV Au, Ag 00 0 • 0 0 Blackfoot lava field, ID • Elkhorn Mountains, MT Ag, Pb, Zn, Mo(?) 023 Little Belt Mountains, MT Mo

Specimen Mountain, CO Chalk Mountain, CO Mo, W Nathrop, CO F(?)

Silver Cliff/Rosita, CO Ag, Au, Pb, Zn, Cu Tomichi Dome, CO Boston Peak, CO

Lake City, CO U Grants Ridge, NM Black Range, NM Sn

Saddle Mountain, AZ Figure 43. The distribution and ages (Ma) of granitic Climax-type por­ Burro Creek, AZ phyry molybdenum deposits (open circles) and prospects (x) (Westra and Keith 1981; White et al. 1981), compared to the locations of topaz rhyolites (filled circles) in the western United States. upper part of a tuff beneath a rhyolite lava flow. Likewise, ber­ ened the alteration process in these intrusions to the formation of trandite is probably the source of the Be-mineralization at the topaz-bearing lithophysae. Garnet, another aluminous mineral Honeycomb Hills, Utah (Lindsey 1977). On the other hand, beryl common in topaz rhyolites, is found in other molybdenum depos­ occurs in a number of other topaz-bearing lava flows including its or prospects (e.g. Pine Grove, Utah: Keith 1980; Mt. Hope, some lavas in the Thomas Range, Utah, the Wah Wah Moun­ Nevada: Westra 1982; and Henderson, Colorado: Gunow et al. tains, Utah, and in the Taylor Creek Rhyolite in the Black Range 1980). The similarity ofbiotite compositions, which are sensitive of New Mexico. In each case, beryl occurs within intensely devi­ indicators of volatile fugacities, in both types of magmatic systems trilled lavas. Late magmatic beryl segregations also occur in the has already been noted in terms of their F and CI contents and topaz-bearing Sheeprock granite of west-central Utah (Williams ratios. 1954). The association of beryllium mineralization with topaz 2) The spatial distribution and ages of the major known rhyolites strongly suggests that the magmatic enrichment of Be in Climax-type Mo deposits are illustrated in Figure 43. Their loca­ the rhyolites is important in the genesis ofthe deposits. Indeed, Be tions are shown superimposed on the distribution of topaz rhyo­ concentrations average about 60 ppm in glassy specimens of the lites. The spatial and temporal correspondence of both types of rhyolite from Spor Mountain, a value about 20 times that found magmatism in Colorado, Montana, and Utah have been noted in in an average granite. the individual descriptions and suggest some sort of genetic link Climax-type molybdenum deposits. The association of exists between these distinctive groups of rocks. topaz-rhyolite magmatism and "Climax-type" Mo-W deposits 3) The tectonic setting of both types of magmatism appears has been noted by Burt and co-authors (1980,1982), Westra and to be in continental rifts or in zones of back-arc extension (e.g. Keith (1981), White et al. (1981), and probably many others in Sillitoe 1980). Both magma types a.re emplaced in the upper crust industry. By way ofjustifying this correlation we note the follow­ as relatively small stock-like intrusions that explosively vent to ing similarities. the surface to emplace relatively small volumes of tuff and/or 1) Topaz rhyolites are mineralogically similar to the igneous lava (J. E. Sharp 1978; Keith 1982). and metasomatic rocks associated with Climax-type Mo deposits. 4) Multiple intrusion/extrusion episodes are apparent for Most molybdenite deposits associated with granitic (or rhyolitic) both types of magmas. rocks cOl1tain topaz in their mineralized zones. Burt (1981) lik- 5) The chemical similarity of the magmas involved is shown Topaz Rhyolites 63

Dome 4

Pyroclastic Deposit Pre-existing Sediment

Figure 44. Schematic cross-section showing the hypothetical structure ofa small rhyolite dome complex and some ofthe types of mineralization possibly associated with topaz rhyolite volcanism (after Burt et al. 1982). 1 = hQt springs deposits (Ag, Au, W, Mn, etc.) within rhyolite or associated volcanic rocks; 2 = clastic sedimentary rocks beneath tuffs (D); 3 = mineralized pyroclastic deposits (Be, D, F, Li, Cs, etc.); 4 =fractured and flow-banded lavas (Sn); 5 =vent and contact breccias (F, D, etc.); 6 =base and precious metal veins (Ag, Pb, Zn, Au, Sn, W, etc.); 7 = mineralized breccia pipes (Mo, Ag, Au, F); 8 = stockwork porphyry deposits (Mo, Sn, W, etc.); 9 = fluorite-rich skarn and/or sulfide-rich replacement ore bodies in non-eruptive environment (Sn, W, Be, etc.); and 10 =greisen-bordered veins in non-eruptive envi­ ronment (Sn, W, Cu, Zn, Be, etc.).

in Table 12. The granites of Climax-type Mo-systems and topaz occur in the Sheep Creek Range, Nevada, and in the Black rhyolites share their major characteristics: high Si, K, Na, and F Range, New Mexico, are similar to the numerous small Sn­ and low Ti, Fe, Mg, and Ca (cf. Figure 35). The principal differ­ deposits of Mexico. Burt and Sheridan (1984) and Duffield et al. encesbetween the rocks lie in the lower concentrations of P and (1984) conclude that the high temperature vapor-phase deposi­ Mg in topaz rhyolites. Differences in the Na and K contents are tion of cassiterite (and topaz) results from the extraction of Sn probably not meaningful because most Climax-type intrusions from the rhyolitic glass or lava. The halogens appear to be effec­ have experienced some potassic alteration. The trace element tive complexing agents for Sn (Manning 1981b; Jackson and signature of topaz rhyolites (high D, Be, Sn, Li, Nb, Rb, and F Helgeson 1985). Breccias and other permeable zones (i.e. flow and low Sr, Ba, and Ti) is typical of Mo-related systems as well bands and fractures) in the upper parts of these lavas are favora­ (Westra and Keith 1981; Keith 1980; Mutschler et al. 1981; ble locations for the accumulation, decompression, and cooling of White et al. 1981). metal-bearing vapors or fluids, resulting in the common associa­ The similarities in distribution, age, tectonic setting, mode of tion of vapor-phase features increasing in abundance toward the emplacement, chemistry, and mineralogy of fluorine-rich subalk­ tops of rhyolite domes and flows. Low-temperature remobiliza­ aline rhyolites (with topaz or garnet) and Mo-mineralized tion of Sn by circulating meteoric waters may lead to the deposi­ "rhyolite" stocks leads us to conclude that the eruption of topaz tion of wood-tin in narrow veinlets in cooling and flow fractures. rhyolites may be a surface manifestation of a potentially ore­ The small size of such deposits will probably prevent their suc­ forming intrusive system. cessful exploitation in this country. Nonetheless, erupted topaz Tin. The small deposits of cassiterite and wood tin that rhyolites may be indicators of topaz granites that may develop 64 Christiansen, Sheridan, and Burt

TABLE 12. AVERAGE COMPOSITION OF SILICIC INTRUSIONS RELATED TO Mo-DEPOSITS decades (e.g., Peters 1958) that fluorite deposits are more com­ mon in the eastern part of the American Cordillera. With the new 1 S.D. 2 S.D. 3 S.D. understanding of the accretionary history of the continental crust of western North America, it is clear that fluorite deposits (as well Si02 76.1 1.04 75.8 2.23 75.6 0.71 as Be, Sn, and Climax-type Mo) are restricted to terranes under­ Ti02 0.11 0.11 0.28 0.73 0.14 0.09 lain by Precambrian craton. Paleozoic and Mesozoic accreted 12.7 0.46 13.4 0.82 12.8 0.38 A1203 terranes consisting offragments ofocean floor and island arcs are Fe203 0.84 0.36 0.71 0.83 1.12* 0.19 FeO 0.55 0.26 0.42 0.38 not typified by fluorite deposits; instead, an association with Au MnO 0.06 0.05 0.04 0.03 0.06 0.01 and Hg mineralization is indicated. Christiansen and Lee (1985) MgO 0.46 0.23 0.24 0.23 0.15 0.07 have shown that the granitoids of the northern Great Basin show CaO 0.74 0.32 0.66 0.45 0.83 0.37 differences in F-content that correlate with their location; grani­ Na20 3.30 0.38 3.67 0.48 3.73 0.28 toids in accreted terranes are F-poor, whereas granitoids rooted in K20 5.08 0.60 4.85 0.51 5.04 0.35 P20 5 0.09 0.44 0.05 0.06 0.00 0.01 Precambrian sial are variably enriched in F. They interpret this F 0.21 0.21 0.11 0.11 0.23 0.09 difference as resulting from different F concentrations in the crustal component of their parent magmas. These disparate Note: All analyses in weight percent and recalculated observations point to the Precambrian continental crust as the H20, C02 and S02 free. S.D. is one standard deviation. ultimate source of the F (and probably Be, Sn, and Mo as well) in *Total Fe as Fe203 the ore deposits and in topaz rhyolites. 1. Average of 13 unaltered ore-related granite and rhyolite porphyries from Mo deposits (Mutschler et al. 1981). COMPARISONS WITH OTHER TYPES OF 2. Average of 50 granite and rhyolite porphyries from RHYOLITIC ROCKS unmineralized stocks near Mo deposits (Mutschler et al. 1981). 3. Average of 14 rhyolite lavas from the Thomas Range, Utah (Christiansen et al. 1984). The geochemical distinctiveness of topaz rhyolites is clearer when contrasted with other types of rhyolitic volcanic rocks. In western North America, topaz rhyolites are nearly contempo­ raneous with calc-alkaline rhyolites and peralkaline rhyolites. economic deposits of Sn (or W) in greisens or skarns (e.g., East The calc-alkaline rhyolites are part of an early to mid-Cenozoic Kemptville, Nova Scotia: Richardson et al. 1982; and Anchor suite of intermediate .to silicic composition (e.g., Lipman et al., Mine, Tasmania: Groves 1972). 1972). The peralkaline rhyolites are part of a late Cenozoic bi­ Uranium. Small, generally sub-economic, deposits of ura­ modal suite of basalts and rhyolites (Christiansen and Lipman nium are associated with many topaz rhyolites. The enrichment 1972). In spite of their close temporal and spatial association with of U in topaz rhyolites probably accounts f<>r this association. these silicic rocks, topaz rhyolites are distinct from both. Notable examples include the rhyolites at Spor Mountain, Utah, in the Wah Wah Mountains, Utah, and those near Lake City, Calc-alkaline rhyolites Colorado. The Be tuff member of the Spor Mountain Formation contains low-grade U (and Th) mineralization that overlaps the Calc-alkaline rhyolites are the silicic representatives of the Be ore zone (Lindsey 1982; Bikun 1980). The U occurs in fluorite orogenic magma series characterized by a lack of iron-enrichment and opal. Accumulations of U (as uranophane and weeksite) also during its differentiation. Calc-alkaline rhyolites are typically as­ occm in a small lens of non-volcanic conglomerate at the base of sociated with andesitic volcanism on continental margins overly­ the Be tuff. The· U was probably leached by groundwater from ing subduction zones. They generally occur as small domes or the U-rich tuff and is not associated with enrichments of Be or Li. lava flows associated with composite volcanoes or calderas but In the Wah Wah Range, U occurs in small altered horizons with may form voluminous ash-flow sheets. Large volumes of high-K Fe~oxides and in conformable lenses with pyroclastic deposits calc-alkaline rhyolite were erupted during the mid-Cenozoic of beneath topaz-bearing lavas (Christiansen 1980). the western United States. Fluorite. Fluorite deposits are closely associated with topaz Ewart (1979) has reviewed the chemistry and mineralogy of rhyolites from Spor Mountain, the Wah Wah Mountains, and the silicic orogenic volcanic rocks, including the calc-alkaline Needle Range, Utah, and perhaps with the Nathrop Volcanics of series. He points out that the calc-alkaline rhyolites generally central Colorado. Fluorite occurs in calcic rocks (sedimentary contain phenocrysts of plagioclase, Mg-augite, Mg-hypersthene, carbonates or intermediate composition volcanic rocks) spatially Ca-Mg hornblende, Mg-biotite, Fe-Ti oxides, and occasionally associated with topaz rhyolite vent complexes and in the Spor olivine. High-K varieties contain quartz and sanidine. Zircon, Mountain district in tuff-lined breccia pipes. A hint to the origin apatite, titanite, and allanite are notable accessory minerals. Al­ of the F. enrichment of topaz rhyolites and the generation of though generally not fluid-saturated before eruption, the common fluQrite deposits in general lies in their distribution, as reiterated presence ofhornblende and biotite in these rhyolites indicates that most recently by Eaton (1984b). It has been known for several they are relatively hydrous. The T-fo2 relationships for some Topaz Rhyolites 65

TABLE 13. COMPARISON OF CALC-ALKALINE RHYOLITES WITH TOPAZ RHYOLITE

1 2 3 4 5 6 Utah Colorado Guatemala Talasea Taupo Topaz Rhyolite

Si02 76.0 77 .1 74.4 75.3 74.2 76.0 Ti02 0.3 0.19 0.12 0.27 0.28 0.13 A1203 13.0 14.3 11. 9 l2.G 13.3 12.8

Fe203* 1.2 0.42 0.83 2.55 1. 89 1.07 MnO 0.04 0.04 0.07 0.07 0.05 0.06 MgO 0.05 0.19 0.2 0.24 0.28 0.10

CaO 1.1 2.7 0.8 1. 25 1. 59 0.74 Na20 3.1 4.6 3.5 4.02 4.24 3.73 K20 4.4 4.4 4.3 3.82 3.18 5.00 P205 0.02 0.05 0.05 0.00 Trace elements (ppm)

Zr 100 59 150 145 126 Rb 85 124 55 107 450 Sr 300 350 109 200 106 20

Ba 1500 2000 1135 645 859 41 Th 24 6 14 12 49 U 8 2 4 3 19

--- Not reported.

* Total Fe as Fe203 1. Joy Tuff, Black Glass member (Lindsey 1981) •. 2. Rhyolite from Summer Coon volcano (Zielinski and Lipman 1976). 3. Los Chocoyos ash (Rose et al. 1979). 4. Rhyolite from New Britain (Lowder and Carmichael 1970). 5. Average rhyolitic lava, New Zealand (Ewart and Stipp 1968). 6. Average topaz rhyolite from the Thomas Range, UT (Christiansen et al. 1984; Ba from Turley and Nash 1980).

calc-alkaline rhyolites are shown in Figure 30. A wide variety of compatible elements, and have higher concentrations of Ba, Sr, studies indicate that most orogenic silicic rocks crystallize under and other compatible elements than topaz rhyolites. The K-Th relatively oxidizing conditions-2 to 3 log units above the QFM and Th-U concentrations of calc-alkaline volcanic rocks from buffer (Ewart 1979; Hildreth 1981; Gill 1981). This property is west-central Utah are compared with topaz rhyolites from the expressed in the Mg-rich nature of the mafic minerals including same area in Figures 44 and 45; the enrichment of topaz rhyolites biotite and hornblende. Calc-alkaline batholithic rocks from the in U and Th is obvious. Little REE data exists for the suite of Sierra Nevada and intermediate to silicic rocks of western Utah calc-alkaline rhyolites that preceded the eruptions of topaz rhyo­ and eastern Nevada have strikingly different compositions ofbio­ lites in Utah, New Mexico, and Colorado. The relationships tites and hornblendes when compared to those found in topaz shown for the topaz rhyolites from Nathrop, Colorado, and a rhyolites from the western United States (Figure 31). slightly older calc-alkaline rhyolite from the San Juan volcanic Although there is substantial chemical variation among calc­ field may be typical (Figure 40). The topaz rhyolite from Nathrop alkaline rhyolites, they are generally richer in AI, Ti, Fe, Mg, and is relatively depleted in HREE compared to other topaz rhyolites, Ca, and poorer in totalll1kalies and F (although data are sparse) but it is nonetheless enriched compared to the calc-alkaline rhyo­ than topaz rhyolites (Ewart 1979). In Table 13, five analyses lite from Summer Coon volcano. Likewise the rhyolite from representative of calc-alkaline rhyolites are compared with the Nathrop shows a striking negative Eu anomaly. These.important "typical" topaz rhyolite composition described above. Few anal­ differences notwithstanding, the differentiation trends of calc­ yses of the halogens in glasses from calc-alkaline rhyolites exist, alkaline rhyolites appear to be similar to those of topaz rhyolites, but substantial differences in FICI are indicated. FICI ratios may but the extreme enrichments and depletions noted above are not be less than 1 in magmas related to subduction processes (e.g. observed. Garcia et at. 1979; Coradossi and Martini 1981). In terms of their It is generally agreed that rhyolitic magmas may originate by trace element characteristics, calc-alkaline rhyolites generally fractional crystallization ofplagioclase, pyroxenes, and Fe-Ti ox­ have lower concentrations of Rb, U, Th, Nb, Ta and other in- ides from dacite or rhyodacite (e.g. Ewart 1979). Crustal fusion 66 Christiansen, Sheridan, and Burt

5 _-----r-.----...,.r------, eruptions (Noble and Parker 1974). Peralkaline rhyolites generally contain phenocrysts of anorthoclase or sodic sanidine, quartz, sodic ferrohedenbergite, aenigmatite, and fayalite (Suther­ land 1974). Arfvedsonite and riebeckite generally crystallize as - o devitrification products. Zircon and apatite are common acces­ 00 00 sory minerals. Fe-Ti oxides mayor may not be present (Nicholls o Q O . o 00cP1iPo 00 - and Carmichael 1969). The anhydrous nature of most Fe-Mg CJ silicates and the common enrichment in Cl suggests that most I: o peralkaline rhyolites were not fluid-saturated before eruption CJ (Bailey 1980). Chlorine partitions strongly into hydrous fluids ..c:: • •• and would be quantitatively extracted from a saturated magma Eo- • 0 ••••• while F prefers to remain in a magma that coexists with a fluid I: ~ . ..-: . 3 \ .. . - (Burnham 1979). Such a pattern does not appear in peralkaline - •• rhyolites (Figure 37). The common absence of hydrous mafic • • silicates indicates a low water fugacity andlor high temperature. • • Mineral geothermometry indicates that crystallization occurs at temperatures generally exceeding 800°C (e.g. Wolf and Wright 1981; Conrad 1984; Mahood 1981; Ewart 1981). The Fe-rich 2 I • character of the mafic silicates and estimates of f0 from co­ 1 2 3 4 2 existing ilmenite and magnetite suggest that crystallization occurs

In U cone. (ppm) at low f02 in many silicic peralkaline magmas (between QFM and WM; Ewart 1981; Wolf and Wright 1981; Conrad 1984; 2 r------....,.r------'.r------, Mahood 1981). o The most important chemical features of peralkaline rhyo­ lites relative to other rhyolites are high Fe, Mn, Ti, F, and Cl, . along with low Al and Ca (Table 14). They are distinct from CJ topaz rhyolites in each of these characteristics except their gener­ I:o 11- • • • - ally high fluorine content. CJ • The F and Cl content ofperalkaline rhyolites is compared to o •• • that of topaz rhyolites in Figure 37. Peralkaline rhyolites have N • FICI of less than 3 and are easily distinguished from topaz rhyo­ :::.::: lites on this diagram. The affinity of fluorine for continental set­ I: tings has been pointed out by Bailey (1980), who showed that a - o 1--...... L-1 ...... 11...... -l FICI ratio of 1 divides oceanic from continental peralkaline 2 3 4 5 rhyolites. Schilling et al. (1980), however, have shown that glasses from tholeiitic mid-ocean ridge basalts have high FICI ratios In Th cone. (ppm) (averaging 8.5), but at much lower concentrations than those dis­ Figure 45. Geochemical comparison of Cenozoic volcanic rocks from cussed here. F ranges from 150 to 400 ppm. Plume-type magmas, west-central Utah. (a) Logarithmic plot of K20 versus Th (b) Logarith­ associated with oceanic peralkaline rocks, have lower FI Cl ratios. mic plot of Th versus U. Open circles = late Tertiary topaz rhyolites; Extreme enrichments and depletions of certain trace ele­ closed circles = mid-Tertiary calc-alkaline rhyolites and rhyodacites. Data are from Lindsey (1982) and Christiansen et al. (1980). ments characterize both peralkaline and topaz rhyolites, but per­ alkaline rhyolites from the western United States generally have lower concentrations of Rb, V, Th, Ta, and Ba and higher con­ and assimilation may also play an important role in the differenti­ centrations of Zr, Hf, Nb, and Zn (Figure 41; Christiansen et al. ation of silicic orogenic magmas (e.g. Myers and Marsh 1981; 1983a). The contrast between the two types of magmas is most Grove et al. 1982; Hildreth 1981). clearly seen in the nature of the negative Th-V "anomalies" and absence of negative Zr anomalies in peralkaline rhyolites when Peralkaline rhyolites compared to subalkaline rhyolites. Peralkaline rhyolites also lack Peralkaline rhyolites contain a molecular excess of NazO + or have small negative Cl anomalies. In addition, peralkaline KzO over Alz03, expressed as normative acmite (for F- and rhyolites generally have higher concentrations of LREE than do Cl-free analyses). They are most easily recognized by the presence F-rich aluminous rhyolites. As a consequence they have steeper of sodic pyroxenes or amphiboles as phenocrysts or as vapor­ chondrite-normalized REE patterns (Christiansen et al. "1983a). phase minerals. Many peralkaline rhyolites in the Great Basin Peralkaline rhyolites show differentiation trends (indexed by in­ were erupted during the late Cenozoic in large caldera-forming creasing (NazO +KzO)1Alz03 and incompatible trace elements) Topaz Rhyolites 67

TABLE 14. COMPARISON OF PERALKALINE RHYOLITES WITH mendite to pantellerite transitions are rarely observed. In contrast, TOPAZ RHYOLITE Hildreth (1981) invokes the action of an "extraordinarily halogen­

Comendites (1) Pantellerites(l) Topaz rich" flux· released from crystallizing basalt to produce partial Rhyolite (2) melting of the lower crust or an earlier accumulation of under­ average range average range average plated gabbro to yield the parental magmas for peralkaline rhyolites. Si02 74.0 69.4-75.0 71. 2 67.4-74.9 75.6 Ti02 0.21 0.09-0.87 0.37 0.14-0.65 0.14 A1203 11.6 10.2-13.4 9.11 6.30-11. 3 12.8 Aluminous bimodal rhyolites Fe203 1. 25 0.40-3.22 2.38 0.44-5.60 1.12* Ewart (1979) established that (non-peralkaline) rhyolites of FeO 1. 88 0.80-3.70 4.52 1.60-6.73 MnO 0.08 0.01-0.17 0.21 0.03-0.36 0.06 bimodal associations are distinctive from most orogenic rhyolites MgO 0.04 0.0-0.22 0.09 0.0-0.75 0.15 in their mineralogy and chemistry. He notes that they have CaO 0.36 0.0-1.12 0.45 0.06-2.04 0.83 strongly Fe-enriched mafic silicates and appear to have crystal­ Na20 5.35 3.99-6.39 6.44 4.68-7.83 3.73 K20 4.46 3.49-4.98 4.40 3.39-4.90 5.04 lized at temperatures in excess of about 800°C and at low f02 (between QFM and NNO) as compared to calc-alkaline rhyo­ P205 0.02 0.0-0.08 0.05 0.0-0.28 0.00 F 0.37 0.06-0.76 0.30 0.11-1. 30 0.33 lites. In addition, Ewart points out that these rhyolites generally Cl 0.24 0.05-0.41 0.28 0.06-0.82 0.06 .exhibit fractionated trace element patterns-Ba, Sr, Cr, Ni, and V are depleted and Nb, Pb, and La are enriched. Although we * Total Fe as Fe203 regard topaz rhyolites as part of this group, a variety of rhyolite 1. Macdonald 1974a. 2. Average of 11 rhyolite lavas from the Thomas Range types exists within it. At one extreme lie the high temperature, (Christiansen et al. 1984; Turley and Nash 1980, for pyroxene (and commonly one-feldspar) rhyolites of the Snake Cl) . River Plain region (Hildreth 1981; Hildreth and Christiansen 1984; Leeman 1982a; Wilson et al. 1983). These rhyolites are typified by anhydrous mafic silicates such as pyroxene and fayal­ that differ markedly from topaz rhyolites. Trachyte to pantellerite ite. At another extreme lie two-feldspar rhyolites with low equili­ transitions are marked by decreasing AI, Ca, Ba, Sr, Mg, Sc, Ti, bration temperatures and biotite as the principal mafic phase, as Ni, and Co that correlate with increasing Na, Cl, Mn, Fe, Zn, Hf, in topaz rhyolites. The bimodal rhyolites of the Coso Range, Zr, Ta, Y, Nb, REE, U, Th, Rb and occasionally Eu and P California (Bacon et al. 1981) and at Twin Peaks, Utah (Crecraft (Macdonald and Bailey 1973; Noble et al. 1979; Civetta et al. et al. 1981), are chemically similar to topaz rhyolites. Notable 1984). Differentiation trends involving trachytes and comendites contrasts between the two groups include the relatively high are similar to those characteristic of more aluminous magmas KINa, Zr, Fe, and Ti ofthe first group, coupled with less extreme with decreasing Fe, Ti, and P; but Zr, Hf, and the REE (excluding enrichments of incompatible trace elements, including F. Most Eu) remain incompatible (e.g., Ewart 1982; Conrad 1984). Some investigators derive the parental magmas for bimodal rhyolites by of these characteristic trace element features can be explained by partial melting of the (lower) continental crust (Hildreth 1981; the high solubility of Zr in peralkaline melts and the consequent Leeman 1982a; Ewart 1982; Christiansen et al. 1983a). Subse­ lack of zircon fractionation. In a similar fashion, the absence of quent fractionation (near the minimum in the granite system) of stable REE-rich aluminosilicates like allanite, and perhaps phos­ plagioclase, alkali feldspar, quartz, biotite or pyroxene, Fe-Ti ox­ phates like monazite as well, may explain the high concentrations ides, and accessories (apatite, zircon, allanite, and monazite) leads of LREE in fractionated peralkaline rhyolites. to the characteristic compositions of these high-silica rhyolites. Silicic peralkaline rocks occur predominantly in continental Fractionation may occur enroute to the surface or in (relatively rift environments or rift-like settings (Macdonald 1974b) and are shallow) magma chambers. These rhyolites have a close thermal, prominent members of bimodal volcanic suites. Peralkaline rhyo­ but not chemical, relationship to contemporaneous basalts that lites also occur in oceanic islands and late orogenic suites but the appear to have provided the heat for crustal melting. common feature linking all of the geologic environments is litho­ spheric extension. We have shown that this strong association Ongonites with extension is also typical of topaz rhyolites. Peralkaline rhyolites can be derived by fractionation of al­ Several authors (Burt and Sheridan 1981; Turley and Nash, kali basalt through an intermediate trachytic composition. Most 1980; Christiansen et al. 1983a) have suggested that topaz rhyo­ quantitative major and trace element models invoke fractionation lites are similar to the so-called ongonites that occur in Mongolia of plagioclase, andlor ternary alkali-feldspar, Fe-rich pyroxene, and the Trans-Baikal region of the U.S.S.R. (Kovalenko and magnetite, apatite, and olivine from trachyte (or high-alkali da­ Kovalenko 1984). Ongonites are defined as topaz-bearing cite) to produce a vertically zoned chamber with comendite or "quartz keratophyres." They occur in subvolcanic dikes, stocks, pantellerite residing in the upper part of the chamber (Barberi et and as lava flows with underlying pyroclastic depositS (Kova­ al. 1975; Civetta et al. 1984; Parker 1983; Middlemost 1981; lenko et al. 1971; Kovalenko and Kovalenko 1976; Kovalenko et Bevier 1981; Souther and Hickson 1984; Conrad 1984). Co- al. 1979). The primary minerals of ongonites are albite, potas- 68 Christiansen, Sheridan, and Burt

300 ,.------;;~--___"7I

200

Figure 46. Rb-Nb-Ta concentrations in ongonites from central Asia compared to other rhyolitic rocks from western North America. a) Loga­ 100 rithmic plot ofRb and Nb. b) Logarithmic plot ofTa and Nb. Arrows show differentiation sequences in topaz rhyolite dome complexes (WW 80 + - ...... E =Wah Wah Mountains; SM =Spor Mountain; TR =Thomas Range) 0- ....0- 60 and from the Coso Range, California, rhyolites which are not known to .0 contain topaz (COSO - Bacon et al. 1981). The compositions of4 ongo­ Z nites that do not lie in the same field as others are shown with crosses. 40 The average compositions of three types of orogenic rhyolites from the western United States (Ewart 1979) are also shown (open circles), along 30 with the composition of the Bishop Tuff, California (BT - Hildreth 1977) and the mildly peralkaline Tala Tuff, Mexico (TT - Mahood 1981). Most ongonites with low Nb/Ta and Nb/Rb ratios are from a dike near 20 Ongon, Mongolia, which contains columbite. The fractionation of this mineral may have buffered evolving liquid compositions to Nb concen­ trations of approximately 60 ppm in much the same manner that zircon saturation controls Zr concentrations. Data for topaz rhyolites and ongo­ nites are from sources cited in text. 100 200 300 400 600 800 1000 2000 a Rb(ppm)

200 ,.------r--~---____::>-----____:II

100 ~ 80 Ongonites 60 "I E -c. I c. ..,., / -.0 40 --- Z ~~ -- ~ 30 .<,.fl>o ::.Q\ ~

20

10 L.- ---I:....-....:.-...... _-L----I'--..L..-..L..-.L.....1~ ..L._.._ ____J'______L_ _L_...... L....L....I 1 32 5 10 20 30 40 60 80 100

b Ta(ppm) Topaz Rhyolites 69

TABLE 15. AVERAGE MAJOR ELEMENT COMPOSITION OF ONGONITES FROM CENTRAL ASIA

1 2 3 4 5 Ongon Baga-Gazryan Arybulak Teeg-Uula Spor Mtn. Mongolia Mongolia USSR Mongolia Utah

Si02 71. 4 74.4 73.0 74.1 73.2 Ti02 0.09 0.10 0.05 A1203 16.9 15.3 14.8 13 .5 13.5

Fe203 0.27 0.21 1. 14 1. 20 1. 29* FeO 0.26 0.82 0.54 0.58 MnO 0.18 0.05 0.09 0.05 0.06

MgO 0.20 0.19 0.23 0.20 0.11 CaO 0.34 1. 02 0.54 0.86 0.61 Na20 5.29 4.49 4.17 4.24 3.95

K20 3.34 -3.67 4.14 4.60 4.86 P205 0.07 0.05 " 0.05 0.04 0.00 F 2.01 0.82 0.96 0.52 1. 14

Note: All analyses recalculated H2O-free.

1. Average of 53 analyses of the Amazonitov dike (Kovalenko and Kovalenko 1976). 2. Average of 6 ongonites (Kovalenko and Kovalenko 1976). 3. Average of 9 ongonites (Antipin et al. 1980). 4. Average of 3 volcanic ongonites (Kovalenko et al. 1979). 5. Average of 11 topaz rhyolites (Christiansen et al. 1984).

* FeTotal reported as Fe203. sium feldspar, and quartz. Micas (biotite, muscovite, or lithium­ tional crystallization of crustally-derived magmas with 0.2. to phengite) occur as phenocrysts and in the groundmass along with 0.5% F. Expansion of the stability field of quartz by elevated topaz. Accessory minerals include fluorite, gamet, zircon, Fe-Ti fluorine contents leads to Na and Al enrichment with Si depletion oxides, columbite-tantalite, cassiterite, Li-phosphates, pyrite, and during crystal fractionation (Manning 1981) as noted for the Spor sometimes tourmaline. The relative importance of magmatic ver­ Mountain rhyolite, Utah. The simultaneous fractionation of sus vapor-phase crystallization in developing this mineralogy is quartz, feldspars, and REE-rich accessory minerals seems to be unclear; but the relatively high Rb/Nb ratios in ongonite (Figure required (Kovalenko et al. 1983). Some fluid-phase transport of 46) suggest that columbite may be a fractionating magmatic F and fluorophile elements to the upper parts of evolving magma phase. Kovalenko and Kovalenko (1976) also regard topaz and chambers may have aided in their differentiation. However, to mica to be magmatic, but the samples do not appear to be glasses. enrich the melt in these elements by this process would require The average chemical composition of four ongonites is the volume of the fluid to exceed the volume of melt by up to shown in Tables 15 and 16. Although there are some obvious several hundred times. The suggested origin by fractional crystal­ differences between topaz rhyolites and these ongonites (ongo­ lization is strengthened by the association of ongonites with large nites have higher Al and P and lower Si), we feel that their granitic massifs from which they may have differentiated. Their similarities are greater. Figures 41 and 46 show the overall sim­ spatial and temporal association with basaltic lavas and a descrip­ ilarity in the chemical features of ongonites and topaz rhyolites tion of a mixed (basalt-ongonite) magma (Kovalenko et al. 1975) from the western United States. Ongonites are markedly enriched suggest that ongonites, like topaz rhyolites, are products ofbimo­ in F, Li, Rb, Cs, Nb, Ta, Be, and other incompatible lithophile dal magmatic processes. Regarded as the volcanic analogs of Li-F elements and are depleted in Zr, Ba, Sr, and Eu just like the rare-metal granites, ongonites are associated with Wand other fluorine-rich rhyolites described here. Kovalenko et al. (1983) types of rare-metal mineralization (Mo, Li, and F). published the REE concentrations of one ongonite from Mongo­ lia (Figure 47) that shows a familiar negative Eu anomaly and PETROGENETIC MODEL FOR TOPAZ RHYOLITES extreme HREE enrichment, so much so that LaN/YbN is less than one. Overall, the REE are depleted when compared with Based on the information reviewed here, Christiansen et al. topaz rhyolites. Based on phenocryst/matrix partition coefficients (1983a) have formulated a petrogenetic model that accounts for and .experimental studies; Kovalenko (1977), Kovalenko et al. the principal features of topaz rhyolites. We summarize this (1978), and Antipin et al. (1980a,b) have suggested that these model in the context ofthe nature and composition of the crustal extreme geochemical features are the result of protracted frac- source of topaz rhyolites; the net power input, as represented by 70 Christiansen, Sheridan, and Burt

TABLE 16. AVERAGE TRACE ELEMENT CONCENTRATIONS IN ONGONITES FROM CENTRAL ASIA

1 2 3 4 5 Ongon Baga-Gazryan Arybulak Teeg-Uula Spor Mtn. Mongolia Mongolia USSR Mongolia Utah

Li 1670 186 417 340 80 Rb 1876 842 1024 975 971 Cs 121 6 71 32 56

Be 19 2 20 25 63 Pb 41 35 50 40 Nb 69 56 55 147 120

Ta 67 22 24 30 26 Zr 66 78 83 138 107 Hf 11 11 6 9 7

Mo 1 2 Sn 37 101 71 30 Ba 25 166 69 62 Sr 20 115 32 47 6

Note: All analyses in ppm.

1. Average of 53 analyses of the Amazonitov dike (Kovalenko and Kovlenko 1976). 2. Average of 6 ongonites (Kovalenko and Kovalenko 1976). 3. Average of 9 ongonites (Antipin et al. 1980). 4. Average of 3 volcanic ongonites (Kovalenko et al. 1979). 5. Average of 2 topaz rhyolite vitrophyres (Christiansen et al. 1984).

the flux of mafic magma, to the base of the continental crust; and Ongonite the nature and magnitude of stress in the lithosphere. According to Hildreth (1981), these are the principal controls on the nature 100 of continental igneous rock associations and their eruption styles. 60 The distribution of topaz rhyolites in the western United States points strongly to the importance of a magmatic compo­ nen.t derived from the Precambrian craton. of North America. The ~ -6 10 notibn that a distinctive crustal reservoir is the source of the c 0 ~ F-enrichment found in topaz rhyolites is supported by the distri­ 0 6 bution of fluorite deposits (Eaton 1984b) and F-rich granitoids Ql C. (Christiansen and Lee 1985) in. the western United States as E described above. In the absence of Precambrian crust in. the en11I northwestern part of the Great Basin., topaz rhyolites are not

generated. Instead, a sUbtly different bimodal rhyolite, emplaced 0.6 in small extrusive domes with high Na/K ratios and with lower concentrations of incompatible trace elements, is widely distrib­ uted in. western Nevada and eastern Oregon (Wilson et al. 1983; 0.1 E. H. Christiansen, in. preparation). L.-~--'-----'-----'-_L.-~--'-----'-----'_-'----'-----'-----'-_L.-~...J In spite of this inferred crustal origin, topaz rhyolites are not La Ce Nd 8m Eu Gd Dy Ho Yb Lu Figure 47. Rare earth element pattern for a Mesozoic ongonite from evolved volcanic equivalents of S-type granites derived by partial Mongolia as reported by Kovalenko et al. (1983). melting of pelitic metasedimentary rocks. The rhyolites possess distinctly lower initial 87Sr/86Sr, 207Pb/ 204Pb, and 180/160 ra­ tios than the S-type granites of the western United States (Wilson equilibration with metasedimentary graphite. Likewise, whole­ et al. 1983; Lee et al. 1981; Farmer and DePaolo 1983, 1984). rocks are not strongly peraluminous and in many cases are meta­ The compositions of biotites from topaz rhyolites are distinctly luminous. Equilibration of igneous melts with muscovite or less aluminoUS than those in. muscovite- or garnet-bearing S-type aluminosilicates produces liquids with 3 to 8% normative corun­ granites. The relatively oxidized conditions under which some dum (Thompson and Tracy 1977; Clemens and Wall 1981). In topaz rhyolites crystallized (QFM or greater) is inconsistent with addition, the relatively high temperatures of some lavas (up to Topaz Rhyolites 71

850°C) and their rise to shallow crustal levels suggest that mus­ melting of a low Rb (30-50 ppm) and U (1.5 ppm) protolith will covite decomposition was not involved in their genesis. produce magmas that could fractionate toward compositions typ­ The low to moderate Sr-isotope ratios of topaz rhyolites ical of topaz rhyolites such as the Spor Mountain rhyolite. Such suggest that their protoliths had Rb/Sr ratios of 0.04 to 0.08. small proportions of partial melting are a natural consequence of These are relatively low ratios for a source in the continental water-undersaturated melting of high-grade metamorphic rocks crust, which is typified by Rb/Sr ratios in excess of 0.2 (Taylor such as granulites. The small degree of melting required by in­ 1964). However, such low ratios are typical of granulitic terranes compatible element enrichments could occur with the complete that experienced Rb depletion during metamorphism and/or ana­ decomposition of less than about 10% biotite at lower crustal texis. This sort of protolith is also consistent with the oxygen pressures (cf. Clemens 1984; Burnham 1979). The decomposition isotope ratios, but sparse Pb isotope ratios suggest that the Th/U of biotite and its replacement with residual pyroxene (plus melt) ratio of the protolith must have been "normal" rather than high, would also lower the bulk partition coefficient between rhyolitic as is found in many granulitic terranes. Small amounts of upper melt and restite enhancing Rb enrichment in the melt. Thus it crustal contamination are suggested by the high initial Sr-isotope appears that the concentrations of Rb, U, and by analogy other ratios found at some complexes (e.g. the Thomas Range and trace elements enriched in topaz rhyolites, need not be higher Nathrop) and suggest caution in attributing measured isotopic than those found in average continental crust. The inferred con­ values to magmatic sources. centrations are in fact significantly lower than average for conti­ Another important indication of a high-grade metamorphic nental crust. Likewise, there is no requirement that the crustal protolith for topaz rhyolites are their elevated F concentrations sources of topaz rhyolites contained anomalously low concentra­ and low F/Cl ratios. Hydrous minerals from high-grade meta­ tions of Sr, Ba, Eu, Ni, and other compatible elements. Instead, morphic rocks are F-rich. As shown by Holloway (1977; see also the small degrees of partial melting in the lower crust, followed Holloway and Ford 1975; and Manning and Pichavant 1983), by substantial crystal fractionation (of sanidine, quartz, plagio­ high F/(F+OH}ratios increase the thermal stability of biotite and clase, biotite, Fe-Ti oxides, and accessories) enroute to the surface amphibole. Others have shown that F/(F+OH) ratios in hydrous and in small magma chambers have produced the characteristic mafic silicates increase with increasing metamorphic grade, ex­ compositional features of topaz rhyolites. tending to granulite facies (Fillippov et aI. 1974; Janardhan et aI. The proposed granulitic nature of their protoliths, their ex­ 1982; E. R. Padovani, oral communication, 1984) or to the onset tensional tectonic setting, and their geochemical features imply of melting (White 1966). Thus, although the absolute amount of that topaz rhyolites may be the extrusive equivalents of A-type or biotite may decrease with increasing grade of metamorphism, it anorogenic granites (Loiselle and Wones 1979; Collins et aI. probably becomes more F-rich. The decomposition of small 1982). There are two "species" of anorogenic granites that may amounts of F-rich biotite would therefore produce small amounts have contrasting origins and evolutionary histories. One type is of aluminous F-rich melt (probably on the order of 0.2 wt% F) metaluminous to slightly peraluminous (analogous to topaz rhyo­ that could evolve to produce a topaz rhyolite. Such melts are lites) and the other is peralkaline (analogous to peralkaline rhyo­ probably less viscous than their dry equivalents (Dingwell et aI. lites). In granitic complexes both types may coexist, one intruding 1985). It is perhaps noteworthy that scapolites from granulite the other or grading into the other (e.g. the Arabian shield, Stuck­ grade rocks are Cl-poor relative to those found in amphibolite less et aI. 1982; or the Younger granites of Nigeria, Bowden et aI. grade metamorphic rocks (Hoefs et al. 1981). A depletion of Cl is 1984). A similar situation is apparent for the comenditic ash-flow expected in granulites whether they are formed by reaction with a tuffs that preceded the eruption of the topaz rhyolite at Kane C02-rich fluid with consequent dehydration or by the removal of Springs Wash, Nevada. a silicate melt. In either case CI would preferentially partition into Collins et aI. (1982) and Christiansen et aI. (1983a) have the escaping fluid/melt. Thus, igneous rocks derived from granu­ emphasized the role of F and CI in the evolution of A-type lites would be expected to have high F/CI ratios as found in topaz granites. The fractional crystallization histories of these magmas rhyolites. may be controlled in part by their characteristic F/Cl ratios. For Although consistent with a granulitic source, the require­ example, Manning et aI. (1980) have suggested that F and Al ment that the sources of topaz rhyolites have relatively low have a strong affinity in granitic melts-so much so that Al is Rb/Sr ratios is in sharp contrast to the remarkably high Rb/Sr removed from tetrahedral coordination in the aluminosilicate ratios of topaz rhyolites themselves. By analogy with high-grade framework of the melt and placed in interstitial sites in octahedral metamorphic rocks, the presumed lower crustal protoliths should coordination. This effect may result in the lowering of the activity also be depleted in other elements characteristically enriched in of aluminum in the melt and coexisting minerals maintaining an topaz rhyolites such as U, Th, K, Cs, Li, Be, Nb, Ta, and Y aluminous composition throughout the fractionation history of (Collerson and Fryer 1978; Sheraton et al 1984; Condie et aI. the magma. In contrast, Cl-rich magmas may experience en­ 1982). This "dilemma" can be resolved if the degree of partial hanced Ca-plagioclase fractionation as a result of Na-CI com­ melting that produces topaz rhyolites is low. Using estimated bulk plexes in the melt. This process could lead to the production of partition coefficients for granulitic restite, Christiansen et aI. peralkaline rhyolites through the plagioclase effect of Bowen (1983b) suggest that values of approximately 10% batch partial (1928). Any process such as volatile escape, which would signifi- 72 Christiansen, Sheridan, and Burt

Rhyolite Dome Mafic Lava Ash-flow Sheet Stock

Caldera ) System Crystallizing Magma Pre-eruption Cumulates Silicic (;) I ~ . t Hybrid Magma Exten.sion , ,'. ,\. \ «F/CI) \ . '\ ­or \I ,'. , Shear .I\\\ Partial ) .. - .... Residual /j.' .. \' \\ Dike Injection Melting \ ~(Q.,': ~ Reworked,,/ I\\ :>' Crust / /.. . /' . \ Crust Crust Mantle Ponding, Mantle Basaltic Differentiation Crys ta lliz a t ion, Magma Contamination Degassing, Contamination of Basaltic Magma a f I d Stratovolcano Rhyolite Alkaline Lavas Figure 48. Hypothetical cross-sections of magma systems (modified in large part from Hildreth 1981), showing a variety of environments in which topaz rhyolites are produced. The relative sizes ofvolcanic edifices are exaggerated. a) The intrusion of hot mantle-derived basalt into the continental crust may result in partial melting of felsic granulites in the lower to middle crust upon decomposition of hydrous minerals. Exten­ Chamber sional tectonism and a diffuse focus ofdike injection (caused perhaps by distributed extension) favor the separate rise and eruption of silicic and i~( mafic magmas. Some silicic magmas may accumulate in small high-level \ chambers and experience wall-crystallization (small arrows show the AI.\\ I direction of movement for the crystallization front) and vertical stratifica­ I', '\ 1\ tion (e.g. Sheeprock granite pluton of west-central Utah; Christiansen et /" \~eltlng a d Mixing al. 1983b). Periodic eruptions from the top ofsuch an evolving chamber ./' ." '9 9" ..... i may produce large fields of rhyolite domes (e.g. Thomas Range, Utah) ~ ... ' ,'9' 'I t ...... , ..". ~ ...... contemporaneous with variably fractionated and/or contaminated mafic Crust lavas. Some rhyolites fractionate on their passage through the cooler Mantle Mantle-derived crust obviating the requirement for their eruption from a sizable shallow Basaltic Magma magma reservoir. b) Where the zone of mafic magma injection is well­ focused or the rate ofinjection is high, hybridization of mantle and crust materials could be enhanced. Mixing might be unavoidable in such an b environment. The magmas produced could fractionate along a calc­ Variably alkaline basalt-andesite-dacite (BAD) trend in a shallow magma reser­ Rhyolite Dome Cont'aminated Basalt voir feeding a stratocone (gradational to larger caldera-related systems Ash-flow Sheet described below). Alkalic basalts and trachytes also appear to be com­ mon in this sort of environment. Examples include the Mt. Taylor vol­ canic field, New Mexico, and the volcanic systems at Silver Cliff/Rosita, Colorado. Hybridization should be limited on the flanks of the thermal focus and partial melting of the crust in these areas could lead to the , eruption of topaz-bearing rhyolite lavas under favorable stress regimes Ca.ldera-related (e.g. Grants Ridge). Similar high-silica rhyolites may be produced in Pluton advance of crustal penetration by mafic magmas, or after decline of the mafic magma input when crustal temperatures were still high but oppor­ tunities for mixing were small. c) In a variety oftectonic environments, large collapse caldera systems may develop as the result of the sustained injection or ponding of mafic magma in the crust. Fractionation of mafic magmas or hybridization of mafic and silicic crustal melts may have preceded diapiric(?) separation of moderately silicic magmas to shal­ Crust lower levels. A strongly modified residual crust composed of restite Mantle Gabbro phases remains in the lower crust (perhaps bearing a mixed mantle and crustal isotopic signature). Residual hydrous phases should be F-rich. Continued(?) Mafic Magma Input Decomposition of these phases during a later heating event (represented c by late basalts) could produce the parental magmas for topaz rhyolites Topaz Rhyolites 73 cantly alter the F/Cl ratio ofthe melt, might change the fractiona­ experimental studies of Naney (1983) on a synthetic granite and tion path followed by the remaining magma and explain the granodiorite suggest that 10w-SiOz rhyolitic melts would coexist association ofboth types ofanorogenic magmas within one igne­ with this biotite-free phase assemblage at 850 to 900°C and 8 kb. ous complex. At lower pressures the requisite temperature is also lower. Resid­ In their major and trace element compositions, topaz rhyo­ ual accessory minerals are probably zircon and apatite, as indi­ lites are similar in most respects to the aluminous A-type granites cated by their low solubilities in granitic melts (summarized in of southeastern Australia. Notable exceptions are that topaz rhyo­ Watson and Harrison 1.984) and by the low concentrations of Zr lites are not LREE rich (125 to 200 ppm Ce versus less than 100 and P in aluminous anorogenic rocks. Magnetite, ilmenite, or • ppm in topaz rhyolites) or Sc rich (greater than 10 versus less titanite may also be important residual phases depending on f02 than 3 ppm for topaz rhyolites), but topaz rhyolites from the Monazite or some other REE-rich phase is probably residual to western United States contain higher concentrations ofNb (20 to the melting process and holds REE content of the melt to accept­ 30 ppm versus 30 to 120 ppm in topaz rhyolites). These differ­ able levels. ences may relate to higher temperatures inferred for Australian In short, we suggest that the most important component in examples (most are hypersolvus one-feldspar granites with rela­ topaz rhyolites is derived from felsic granulites of the lower or tively high Zr concentrations) or to differences in the composition middle crust. According to this model, high heat flow, resulting of their crustal sources. The A-type granites of Australia, the from the emplacement of mafic magmas in or at the base of the Younger granites of Nigeria, and the Precambrian anorogenic crust, elevated temperatures sufficiently to produce the decompo­ granites of the southwestern United States (Anderson 1983) are sition of hydrous silicates. More mafic magmatism of a variety of also F-rich like topaz rhyolites. Subalkaline A-types generally types is typically associated with the eruption of topaz rhyolites contain Fe-rich biotite and/or amphibole; fluorite is a common (Table 10). In Figure 48 we have illustrated some of the geologic phase as well. environments in which topaz rhyolites occur. Anorogenic granites are thought to result from differentia­ Topaz rhyolites are commonly found in extensional envi­ tion of variably contaminated alkali basalts (Loiselle and Wones ronments in association with contemporaneous basalts (Figure 1979) or from small degrees of partial melting of "residual" 48a). Because of density contrasts, hot, mantle-derived basaltic crustal materials from which earlier water-rich magmas had been magma may pond at the base of the crust where it differentiates removed during granulite-grade metamorphism. It is this later by· fractional crystallization and assimilation of crustal materials. model that we prefer for the origin of aluminous anorogenic Partial melting of continental crust occurs when temperatures rhyolites/granites, but it is difficult to imagine how Cl-rich peral­ become high enough to induce the breakdown of hydrous miner­ kaline magmas could come from granulites without the introduc­ als such as biotite. Small quantities of rhyolitic partial melt would tion of CI (and possibly other volatiles) from another source be formed. The character of the erupted mafic magma depends (presumably mantle-derived basalts). As pointed out by Collins et on its original composition and upon the extent of fractionation al. (1982) and Christiansen et al. (1983a), the protoliths of alum­ andinteraction with crust materials. In an extensional stress field, inous anorogenic granites probably consist of felsic granulites these buoyant melts could rise, fractionate, and erupt to produce with potassium feldspar, plagioclase, clinopyroxene and ortho­ topaz rhyolites which are coeval with variably contaminated (po­ pyroxene (after decomposition of biotite, which we presume to tassic) basalts-a typical bimodal suite (Christiansen and Lip­ be consumed by the melt forming reaction) and quartz. The man 1972). Mixing of the contrasting magma types is inhibited by the efficient separation of the silicic magma in an extensional setting. Subsequent fractionation in a shallow magma chamber of which could then erupt through an olcier caldera-related magma system moderate size is indicated for some topaz rhyolites by the erup­ (e.g. Kane Springs Wash, Nevada, or SW Colorado). Alternatively, tion of moderate volumes of rhyolites over short periods of time residual pockets of silicic melt might be retained in the lower crust to rise and by the existence of at least one small pluton of Cenozoic and erupt slightly later. Their separation and rise could be induced by a change in stress orientations (e.g. Colorado topaz rhyolites follow volum­ topaz-bearing granite. Eruptions from the tops of vertically zoned inous calc-alkaline magmatism during transition to extensional tectonism magma chambers may explain the strongly fractionated character and development of the Rio Grande rift) or regional adjustments to the of topaz rhyolites. Many other rhyolite domes may not require redistribution of mass in the lithosphere following the development of shallow magma chambers because fractionation would probably granitic batholiths. d) An alternative explanation for the association of occur enroute to the surface. topaz rhyolites with silicic caldera systems holds that topaz rhyolites are produced by fractionation of the residue left in the magma chamber after In an environment where extension is less pronounced or eruption. Such residues might have the high F/Cl ratios typical of topaz where the flux of mantle-derived basalt is strongly focused, hy­ rhyolites if de-volatitization during an earlier eruption effectively ex­ bridization of mafic and crust-derived silicic melt might be more tracted CI in preference to F from a portion of the magma remaining in common (Figure 48b). The magmas rising from this zone could chamber. Such a process would require the streaming of substantial fractionate to produce basalt-andesite-dacite sequences and stra­ amounts of water vapor through the unerupted portion of the magma chamber. Examples that could be studied with this process in mind tovolcanoes. On the spatial or temporal flanks of the thermal include the Kane Springs Wash, Nevada, and Black Range, New Mex­ "focus," independent batches of silicic partial melts and variably ico, rhyolites that are intimately related to caldera systems. contaminated and fractionated basalt could develop and erupt, as 74 Christiansen, Sheridan, and Burt for example at Grants Ridge, New Mexico, which is related to the tent with the coincident rise of mafic and rhyolitic magmas Mt. Taylor volcanic field. through the sub-caldera magma chamber, indicating that the Some topaz rhyolites are erupted during the development of magma was solidified and susceptible to brittle fracture and the overlapping caldera cycles. This has been interpreted to be the propagation of dikes. An alternative explanation for the occur­ case for the Mexican topaz rhyolites and for the rhyolites in New rence of topaz rhyolites in caldera settings, which might apply to Mexico's Black Range. The eruption of voluminous silicic ash­ the Kane Springs Wash caldera, invokes fractional crystallization flow tuffs and the formation of collapse calderas suggest the of the unerupted portion of an ash-flow-producing magma existence of large shallow-level magma chambers. In examples chamber (Figure' 48d). The ash flows and the F-rich rhyolite from the United States, topaz rhyolites are commonly erupted lavas are seen as being co-genetic in a partially open magma after the latest ash-flow tuff in a given area. We suggest two system. The volatile saturation and eruption of the early magma possible alternatives for the production of topaz rhyolites in this are critical for the development of the high FICI ratios observed environment. Our preferred explanation calls on the late rise of in topaz rhyolites relative to earlier magmas. The melt's preferen­ small pockets of melt trapped in the reworked, residual crust, or tialloss of CI relative to F (Burnham 1979) during volatile exso­ the later generation of small volumes of partial melt of the resid­ lution associated with a large plinian eruption might be able to ual crust (Figure 48c). As described by Hildreth (1981), this produce this change. Subsequent fractional crystallization could modified, perhaps granulitic, crust is thought to have developed elevate F concentrations to the levels required for the formation as the result of the injection of mantle-derived magma into the of topaz during post-eruption devitrification and vapor-phase al- continental crust and the subsequent extraction of silicic melts i teration of the evolved residual magma. that coalesce to form a large magma body. This model is consis-

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