The Geology and Geochemistry ofCenozoic Topaz Rhyolites

from the Western United States

Eric H. ChristiansenDepartment of Geology

University of IowaIowa City, Iowa 52242

Michael F. SheridanDonald M. Burt

Arizona State UniversityTempe, Arizona 85287

SFEE'It':' FAFE.,205

© 1986 The Geological Society of America, Inc.All rights reserved.

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Published by The Geological Society of America, Inc.3300 Penrose Place, P.O. Box 9140, Boulder, Colorado 80301

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Library of Congress Cataloging-in-Publication Data

Christiansen, Eric HThe 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, Stratigraphic­Crenozoic. 5. Geology-West (U.S.) I. Sheridan,Michael F. II. Burt, Donald M., 1943- . m. Title.IV. Series: Special paper (Geological Society ofAmerica); 205.QE462.R4C48 1986 552'.2 86-273ISBN 0-8137-2205-5

Contents

Acknowledgments v

Abstract I

Introduction ;................................................ 3

Cenozoic topaz rhyoUtes from the western United States 31. Thomas Range, west-central Utah 32. Spor Mountain, west-central Utah 103. Honeycomb Hills, west-central Utah ........................•......... 134. Smelter Knolls, west-central Utah 145. Keg Mountain, west-central Utah 156. Mineral Mountains, western Utah 157. Wah Wah Mountains and vicinity, southwestern Utah and

southeastern Nevada 178. Wilson Creek Range, southeastern Nevada 199. Kane Springs Wash, southeastern Nevada......... . . 19

Topaz rhyolites in the eastern Great Basin: A summary 2110. Cortez Mountains, north-central Nevada 2111. Sheep Creek Range, north-central Nevada 2312. Jarbidge, northern Nevada '" 2413. Blackfoot lava field, southeastern Idaho 2514. Elkhorn Mountains, western Montana 2615. Little Belt Mountains, central Montana 2716. Specimen Mountain, north-central Colorado 2917. Chalk Mountain, central Colorado 3018. Nathrop, central Colorado 3119. Silver Cliff-Rosita, central Colorado 3220. Tomichi Dome, central Colorado 3421. Boston Peak, central Colorado. . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . .. 3522. Lake City, southwestern Colorado 36

Topaz rhyolites in Colorado: A summary 3723. East Grants Ridge, west-central New Mexico ....•...................... 3724. Black Range, southwestern New Mexico 3925. Saddle Mountain, eastern Arizona 4126. Burro Creek, western Arizona 41

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

iii

iv Contents

Principal characteristics of topaz rhyolites 43Distribution and ages .....................................•......... 43Mode of emplacement 44Mineralogy 46

Fe-Ti oxides and titanite 46Feldspar 47Mafic silicates 48

Geochemistry and differentiation trends 50Isotopic composition 59Magma-tectonic setting 59Ore deposits 61

Beryllium 61Climax-type molybdenum deposits 62Tin ' 63Uranium 64Fluorite 64

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

Petrogenetic modelfor topaz rhyolites 69

References cited 74

Acknowledgments

. This work was partially supported by U.S. DOE Subcontract #79-270-E from BendixField Engineering Corporation. Additional support was provided by Arizona· State Univer­sity, the University of Iowa, the U.S. Geological Survey, and the National Aeronautics andSpace Administration (grant NAGW-537). A large number of people have helped with thenew 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 ofC.Craddock and L. Gregonis are greatly appreciated. We are also indebted to the authors ofmany of the articles cited herein for helpful discussions and for recording the presence of topazin the rhyolites they have studied.

v

Geological Society of AmericaSpecial Paper 205

1986

The Geology and Geochemistry of Cenozoic Topaz Rhyolitesfrom the Western United States

ABSTRACT

High-silica, topaz-bearing rhyolites of Cenozoic age are widely distributed acrossthe western United States and Mexico. Topaz rhyolites are characteristically enriched influorine (>0.2 wt%) and contain topaz crystallized during post-magmatic vapor-phasealteration. In the United States, their ages span much of the Cenozoic Era (50 to 0.06Ma). Their emplacement followed or was contemporaneous with calc-alkaline and ba­saltic magmatism in the Basin and Range province, along the Rio Grande rift, and inMontana, and coincided with episodes of extensional tectonism in these regions.

Nearly all topaz rhyolites extruded as small, endogenous lava domes with or with­out lava flows; no topaz-bearing ash-flow tuffs have yet been identified with certainty inthe western United States. Most domes are underlain by a precursory blanket of non­welded 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 extrusionssuggest 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 of these phasesmay form during vapor-phase crystallization. Magmatic phenocrysts include sanidine(ca. Orso), quartz, sodic plagioclase (usually oligoclase), and F- and Fe-rich biotite inorder of usual abundance. Fe-rich hornblende or clinopyroxene occur in a few lavas.Common magmatic accessory minerals include magnetite, ilmenite, zircon, apatite, allan­ite, 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 atthe lower end of this range) and at variable oxygen fugacities. Titanite-bearing lavascrystallized above the NNO buffer under oxidizing conditions. Most others appear tohave crystallized near the QFM oxygen buffer. For individual complexes, temperaturescorrelate negatively with F-content.

All topaz rhyolites are high-SiOz rhyolites with elevated F, Na, K, Fe/Mg and lowTi, Mg, Ca, and P. Samples with F concentrations of about 1% have notably lower Siand higher AI and Na than other topaz rhyolite glasses. Most glasses from topaz rhyo­lites are metaluminous, but many appear to be slightly peraluminous. Fluorine concen­trations in glasses range from slightly less than 0.2 to more than 1.0 wt%, and F/ Cl ratiosare high (3 to 10) compared to F-rich peralkaline glasses «3). Topaz rhyolites arecharacteristically 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 of most topaz rhyolitesare 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 byincreasing F, U, Cs, and other incompatible elements. Titanite-bearing rhyolites haveprominent middle REE depletions. Initial Sr-isotope ratios range from 0.705 to over0.710.

Geochemical trends at individual complexes are interpreted as arising from frac­tional 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 of the trace element patterns.Zircon, apatite, and a REE-rich phase (allanite, monazite, or titanite) were minor butimportant fractionating phases. No liquid-state fractionation is required to explain thegeochemical trends. The high F content and FICI ratios of topaz rhyolite melts may havemodified phase relations so as to produce Na and AI enrichments for highly evolvedmagmas.

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, chem­istry, and style of emplacement, topaz rhyolites bear resemblance to the rhyolitic stocksassociated with Climax-type Mo deposits, and some may be surface manifestations ofsuch deposits.

In their chemical composition and mineralogy, topaz rhyolites are distinct from bothperalkaline rhyolites and calc-alkaline rhyolites with which they may be spatially andtemporally associated. Some of the compositional differences between topaz rhyolitesand peralkaline rhyolites may be attributed to the relative effects of F and CI, on meltstructure and phase relationships in their parental magmas. The F/CI ratios of the meltor its source may determine the alumina saturation of the magma series. Topaz rhyolitesare 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 fromboth peralkaline and calc-alkaline rhyolite suites. Topaz rhyolites are similar to otheraluminous rhyolites erupted in bimodal associations with basalt in the western UnitedStates. They may be the equivalent of the topaz-bearing ongonites of central Asia.

Topaz rhyolites from the western United States are not the eruptive equivalents ofS-type granites; we liken them to the highly evolved, non~peralkaline,and F-rich anoro­genic grnnites. Topaz rhyolites appear to have evolved from partial melts of a residualfelsic granulite source in the lower or middle crust of the Precambrian continent. Ac­cording to the proposed model, the passage of contemporaneous mafic magmas throughthe crust produced necessarily small volumes of partial melts as a result of the decompo­sition of small amounts of F.;rich biotite that persisted in a high-grade metamorphicprotolith. An extensional tectonic setting allowed these small batches of magma to risewithout substantial mixing with contemporaneous mafic magmas. Subsequent fractiona­tion led to their extreme trace element characteristics.

Topaz Rhyolites 3

INTRODUCTION

For decades petrologists have been concerned with the roleof volatiles (principally HzO, cOz, sOz, HzS, BZ03, HCI, andHF) in the genesis and evolution of igneous rocks. Even in fluid­undersaturated magmas, volatiles playa key role in determiningthe physical properties, crystallization histories, and emplacementmechanisms of magmas. Studies of their role can be pursuedthrough theoretical, experimental, and analytical methods. Rhyo­lites that contain topaz (AlzSi04FZ) appear to form a distinctivegroup of silicic lavas with high fluorine concentrations. The oc­currence of fluorine-rich volcanic rocks provides the opportunityto examine the effect of fluorine on the mineralogy, geochemicalevolution, and physical nature of natural rhyolitic magmas. Theintent of this report is to document these geologic and petrologiccharacteristics as a basis for ongoing efforts to determine theorigin and evolution of fluorine-rich silicic magmas (e.g., Christ­iansen et al. 1983a; Ruiz et al. 1985; Kovalenko and Kovalenko1984; Pichavant and Manning 1984; Dingwell et al. 1985) and todetermine the nature of the ore deposits associated with them(e.g., Burt et al. 1982; Burt and Sheridan 1981).

The occurrence of topaz lining vugs and cavities in rhyoliticlavas from Colorado and Utah was first reported in the nine­teenth century (Smith 1883; Simpson 1876). More recent investi­gations, summarized here, have shown that topaz-bearing lavasare widespread in the western United States and that they containother minerals uncommon in silicic volcanic rocks (e.g., beryl,gamet, pseudobrookite, and bixbyite) that reflect the uniquechemistry and origin of these rhyolites. The lavas also containunusually high concentrations of incompatible lithophile ele­m.ents (e.g., Be, Li, U, Th, Sn, Ta, Rb) and fluorine. Informationabout these distinctive rocks is scattered in the literature on thegeology of the western United States.

During a study of uranium mineralization associated withfluorine-rich volcanic rocks (Burt et al. 1980), it became obviousthat topaz rhyolites are surprisingly similar to one another in theirmode of emplacement, mineralogy, major and trace elementchemistry, and tectonic setting. These features are summarizedhere.

CENOZOIC TOPAZ RHYOLITES FROM THEWESTERN UNITED STATES

The distribution of Cenozoic topaz rhyolites in the westernUnited States is shown in Figure 1 where the occurrences arenumbered in their order of discussion (generally clockwisearound the Colorado Plateau, starting in west-central Utah). Wehave visited most of the localities described in this report (allUtah occurrences; Sheep Creek Range, Jarbidge, and KaneSprings Wash, Nevada; Burro Creek, Arizona; both New Mexicooccurrences; Nathrop, Chalk Mountain, and Tomichi Dome,Colorado; Blackfoot lava field, Idaho; and the Elkhorn Moun­tains, Montana). Complete results of our new findings are pre­sented here. For each locality, we have summarized pertinentinformation about 1) the geologic setting and emplacement of the

rhyolites; 2) their petrography and mineralogy; 3) the major­element, trace-element, and isotopic composition of the lavas; 4)the nature of ore deposits associated with them; and 5) wherepossible, the volcano-tectonic setting as revealed by contempo­raneous magmatism and tectonism.

Many of the data on mineralogy, elemental and isotopiccomposition, and mineralization are summarized in the figuresand tables in the last part of the report. The reader is referred tothese summaries in the descriptions of each occurrence. Volcanic .rock classification in this report follows that of the lUGS and isbased on KzO plus NazO and SiOz concentrations (TAS dia­gram; LeMaitre 1984). Where informative, in parentheses wehave also included the original rock name used by the authors.

1. Thomas Range, west-central Utah

The best-known topaz rhyolites are those from the ThomasRange in west-central Utah (Figure 2). The occurrence of topazin rhyolitic lavas from the Thomas Range has been known formore than a century (Simpson 1876: 325-326). Because of theoccurrence of topaz in the lavas and the presence of U, Be, and Fdeposits in the vicinity, these rhyolites have received considerableattention in the literature. The most recent comprehensive studyof the area is that by Lindsey (1979, 1982). Turley and Nash(1980), Bikun (1980), and Christiansen et al. (1984) have exam­ined the petrology of the lavas.

The Thomas Range consists of a group of coalesced lavaflows and domes that were erupted from at least 12 separate vents6 to 7 Ma (Lindsey 1979). Eruptive episodes, as described byBikun (1980), commenced with the emplacement of a series ofpyroclastic flows, minor air-fall sheets, and pyroclastic surgeunits, and were terminated by the effusion of rhyolite lavas.Welded ash-flows occur within the tuffs, but more commonly theignimbrite units are thin (3 to 4 m) and unwelded. Fused tuffs(Christiansen and Lipman 1966) 1 to 2 m thick occur in thetephra immediately below some lava flows. Flow breccias, con­sisting mostly of vitrophyre blocks up to 2 m in diameter, areusually found at the base of the lavas. The breccia grades upwardinto flow-banded rhyolite, commonly with numerous lithophy­sae. The volume of rhyolitic eruptives in the Thomas Range isabout 50 km3.

Rhyolites from the Thomas Range (the Topaz MountainRhyolite; Lindsey 1982) contain up to 20% phenocrysts, but mostsamples are crystal-poor felsites or obsidians. (The mineralogy ofthe rhyolites is summarized in Tables 8 and 9). Sanidine (Or4s toOr6S), quartz and plagioclase (AnlO to Anzs) occur in almost allsamples. Biotite of variable Fe/Mg occurs in most (Figures 31and 32), whereas spessartine-almandine gamet, ferro-augite, andFe-rich hornblende occur as magmatic minerals in a few samples.Accessory minerals include zircon, fluorite, allanite, Fe-Ti oxides,and, in at least one instance, fluorine-bearing titanite (Turley andNash 1980; Christiansen et al. 1984). Fe-Ti oxide and two­feldspar geothermometry indicate that the Topaz MountainRhyolite crystallized at temperatures between 630 and 790°C at

4 Christiansen, Sheridan, andBurt

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20. Tomichi Dome, Colorado21. Boston Peak, Colorado22. Lake City, Colorado23. Grants Ridge, New Mexico24. Black Range, New Mexico25. Saddle Mountain, Arizona26. Burro Creek, Arizona

10. Cortez, Nevada11. Sheep Creek Range, Nevada12. Jarbidge, Nevada13. Blackfoot lava field, Idaho14. Elkhorn Mtns, Montana15. Little Belt Mtns, Montana16. Specimen Mtn, Colorado17. Chalk Mountain, Colorado18. Nathrop, Colorado19. Silver Cliff, Colorado

Figure 1. Locations of known Cenozoic topaz rhyolites in the western United States, The numbers referto the localities listed below and described in the text. Open circles without numbers show locations ofsome 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 Precam­brian craton in the western United States. The solid line represents the outcrop limit of Precambrianrocks (King 1977), the dashed line represents the edge of the craton inferred from Sr-isotope composi­tion ofMesozoic granitoids (Kistler et al. 1981; Armstrong et al. 1977), and the dash-dot line as inferredby Nd-isotope composition of Mesozoic and Cenozoic granitoids (epsilon Nd = -7; Farmer andDePaolo 1983, 1984).1. Thomas Range, Utah2. Spor Mountain, Utah3. Honeycomb Hills, Utah4. Smelter Knolls, Utah5. Keg Mountains, Utah6, MineralMountains, Utah7. Wah Wah Mountains, Utah8. Wilson Creek Range, Nevada9. Kane Springs Wash, Nevada

Topaz Rhyolites 5

Location in Utah

G Older volcanic rocks(30-42 Ma)

~ Sedimentary rocks

o 3kmI I I I

Scale

LEGEND

Quaternary alluvium

Topaz Mtn Rhyolite(6 Ma)

Spor Mtn Formation(21 Ma)

D

!-I

Figure 2. Generalized geologic map of the southern part of the Thomas Range, Utah (after Lindsey1979; Christiansen et al. 1984a). Both the Spor Mountain Formation and the Topaz Mountain Rhyolitecontain topaz in rhyolitic lavas. Numbers indicate samples analyzed by Christiansen et aI. (1984).

fairly low oxygen fugacities (QFM; Figure 30; Turley and Nash1980; Christiansen et al. 1980). Topaz occurs in lithophysal cavi­ties and in the devitrified groundmass of many lava flows. Nomagmatic topaz (e.g., in glass) has been identified. Other vapor­phase minerals in lavas from the Thomas Range include quartz,alkali feldspar, beryl, bixbyite, pseudobrookite, hematite, spessar­tine garnet, and cassiterite.

The average compositions of samples from the ThomasRange are given in Table 1. The compositions of felsites andvitrophyres are similar but felsites have higher KINa ratios thantheir corresponding vitrophyres. The analyses show high Si, K,and Na and low Ti, Mg, Ca, and P typical. of topaz rhyolites.Fluorine ranges from 0.2 to 0.5% in vitrophyres. Most of the lavasare diopside-normative if calculated on a fluorine-free basis. Thetrace element geochemistry of the lavas is typical of topaz rhyo­lites with generally high and covarying concentrations of U:, Th,Rb, Li, Be, and Ta (Table 2 and Figure 3). The rare earth element(REB) distributions in the vitrophyres are similar to other topazrhyolites with relatively large negative Eu-anomalies and heavyRBB (HRBB) enrichments that are correlated with F content andother chemical indexes of differentiation (REB patterns are illus­trated in Figure 40a). Light REE (LREE) abundances declinewith increasing evolution. Sr-isotope ratios (0.707 to 0.712;Table 3) show that the Thomas Range lavas are moderately

radiogenic, which is consistent with a crustal origin for the paren­tal magmas.

Christiansen et al. (1984) presented a quantitative model forthe geochemical evolution of these lavas based on the fractiona­tion of observed phenocrysts from rhyolitic magmas. Major andtrace element geochemistry demand an interpretation that in­volves about 70% crystallization of the most mafic rhyolite ana­lyzed to produce the most evolved rhyolite, even though the Si02content increases by only 2.5% across the series. Both major­element mass-balance calculations and Rayleigh fractionationmodels, using the distribution coefficients of Hildreth (1977) andCrecraft et al. (1981), suggested that fractionation ofsanidine (45to 50%), quartz (30%), plagioclase (15 to 20%), biotite (3%), andFe-Ti oxides (1%) were the principal fractionating phases. Inaddition, the observed changes in trace elements (La, Hf, Zr, andLu) led to estimates of 0.04% each of allanite and zircon in theremoved mineral phases. The observed P depletion implies that0.06% of the cumulate mineral assemblage was apatite. Minordiscrepancies for Y, Nb, Ta, and Th could be explained by thefractionation of extremely small quantities of REB-rich phos­phates (not yet observed in the vitrophyres) and titanite.

Crystallization near the minimum in the simple ternary gran­ite system should produce differentiates whose major elementchemistry 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 %)

HoneycombThomas Range Spor Mountain Hills

1 2 3 4 5 6 7 8ave. 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 75.0Ti02 0.10 0.03 0.17 0.04 0.10 0.01 0.13 0.04 0.05 0.01 0.03 tr. 0.04A1203 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 0.07CaO 0.80 0.42 0.77 0.07 0.85 0.08 0.96 0.35 0.61 0.10 0.34 11.1 0.62Na20 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 4.46P205 0.00 0.00 0.00 0.00 0.01 0.01 0.02 0.02 0.00 0.00 0.03 0.03 0.00F 0.28 0.08 0.21 0.04 0.29 ---- 0.29 ---- 1.14 0.35 0.77 8.00 0.95Cl ---- ---- 0.06 0.01 ---- ---- ---- ---- 0.14 ---- ---- 0.01 0.07

Rhyolitic lava (Staatz and Carr 1964).Low-silica phase of Honeycomb Hills rhyolite(Turley and Nash 1980).

8. Average of 2 rhyolite lavas (Christiansen etal 1980).

1. Average of 11 rhyolites (Christiansen et al. 1984). 6.2. Average of 4 rhyolite lavas (Turley and Nash 1980). 7.3. Average of 3 analyses representing 5 rhyolites

(Shawe 1966).4. Average of 7 rhyolite lavas (Staatz and Carr 1964).5. Average of 11 rhyolites (Christiansen et al. 1984).

TABLE 1. (CONTINUED)

e

Wilson Kane SheepSmelter Mineral Creek Springs Creek

Knolls Mountains Wah Wah vicinity Range Wash Range Jarbidg9 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 75.3Ti02 0.04 0.01 0.08 0.02 0.07 0.04 0.08 0.02 0.04 0.02 <0.2 0.12 0.16A1203 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 0.20CaO 0.96 0.61 0.45 0.04 0.52 0.18 0.74 0.35 0.43 0.15 0.42 0.52 0.34Na20 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 5.44P205 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).10. Average of 5 rhyolites from domes (Evans and

Nash 1978).11. Average of 7 early Miocene rhyolites

(Christiansen 1980; Best et al. 1981).12. Average of 8 Pliocene rhyolites (Christiansen

1980; Best et al. 1981).

13. Average of 5 rhyolite lavas (Barrott 1984;written communication 1985).

14. Kane Spring Wash topaz rhyolite (Novak 1984).15. Rhyolite lava (Christiansen et al. 1980).16. Rhyolite lava (Christiansen, unpublished

analysis.)

Topaz Rhyolites 7

TABLE 1. (CONTINUED)

Elkhorn Specimen ChalkChina Cap Mountains Little Belt Mtns. Mountain Mountain Nathrop

17 18 19 20 21 22 23 24 25 26 27ave. 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 77.5Ti02 0.12 0.016 0.07 0.0 0.08 0.02 0.03 0.06 0.11 ---- 0.09 0.08 0.08 0.07A1203 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 0.35FeO 0.42 0.125 0.49 0.16 0.27 0.20 ---- 0.30 0.10 ---- ---- ---- 0.23 0.25MnO 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 0.04CaO 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 0.43Na20 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 4.4P205 0.01 ---- 0.01 0.01 0.03 0.00 0.01 ---- ---- 0.01 0.01 0.01 0.00 0.00F 0.45 0.073 ---- ---- ---- ---- 0.34 ---- ---- ---- 0.55 ---- 0.21 ----Cl 0.04 0.001 ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ----

17. Average of 6 analyses (Dayvault et al 1984).18. Average of 3 rhyolite lavas (Smedes 1966).19. Rhyolite sill at Yogo Peak (Pirsson 1900).20. Rhyolite stock at Granite Mountain (Witkind 1973).21. Rhyolite stock at Granite Mountain (Rupp 1980).22. Average of 4 rhyolitic lavas (Wahlstrom 1944).

23. "Effusive" rhyolite (Cross 1886).24. Rhyolite vitrophyre (Christiansen et al. 1980).25. Devitrified rhyolite (Christiansen et al. 1980).26. Average of 2 vitrophyres (Van Alstine 1969).27. Devitrified groundmass of rhyolite (Carmichael

1963) .

TABLE 1. (CONTINUED)

d

Tomichi Boston Lake Grants BlackSilver Cliff Dome Peak City Ridge Range

28 29 30 31 32 33 34 35ave. 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 0.88Ti02 0.06 ---- 0.08 0.05 0.08 0.006 0.07 0.01 0.19 0.09 ' 0.07 0.18 0.02A1203 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* 0.07FeO 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 0.06CaO 0.22 0.78 0.65 0.12 0.34 0.05 0.38 0.10 0.78 0.60 0.30 0.53 0.19Na20 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 0.07P205 0.02 ---- 0.04 0.02 0.06 0.06 0.02 0.005 0.05 0.03 0.01 0.03 0.01F 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 anMnO 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 0.60Na20 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 thanP205 0.01 0.01 0.01 two samples.F 0.18 0.03 0.30Cl 0.04 0.01 ----

8 Christiansen, Sheridan, and Burt

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

WilsonThomas Spor Honeycomb Smelter Mineral Wah Wah Creek

Range Mountain Hills Knolls Mtns Mountains Range Cortez1 2 3 4 5 6 7 8 9 10 11 12 13 14

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

BeSrBa

6.528

102241

636

16*27*63*

11200

nd

1315nd

612

2830

1352

ndnd

911 71*

1561419

32

Cr 2.0Co ndCuGa 34

3.40.4

48 53*

3.2 2.6

80 65

3.2

25

1.70.3

45

1311.1

50

Sc 2.0 2.6 3.7 3.9 2.4 1.1 1.4Y 58 49 116 85* 120 105 42 90 175 18 74 214* 134 75Zr 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 40Mo 24 3.4 3.8 14Sn 30 25 65 12

HfTaPb

ThU

5.55.6

54.821.6

31

4911

6.726

6737.1

56*

61. 812.4

5.5

40

26.8151

6.1

30

30.416.7

70 50

2622

7.6

42

57.415.0

37

37

8..17.7

5920.4

16.3

47.311. 7

50

F 4150Cl

2025631

100001370

7970075 80

19000 7200957

4100 46001230

1. Average of 5 rhyolite vitrophyres (Christiansen 7.et a1. 1984). 8.

2. Average rhyolite (Turley and Nash 1980). 9.3. Average of 3 rhyolite vitrophyres (Christiansen 10.

et a1. 1984). 11.4. Average devitrified rhyolite. Analyses with *

are semi-quantitative emission spectrometry 12.analyses (Lindsey 1979).

5. Low-silica phase of Honeycomb Hills rhyolite 13.(Turley and Nash 1980). 14.

6. Rhyolite (Turley and Nash 1980).

4 SM-29-206 CsSM-61a U

Be

Lu

3 Li

Vb TaRb Tb" Th

2 Mn

1

0.5 Fe Hf" NdCe

Mg Lap Tl Co

0.1 Sr Eu

Pegmatitic inclusion (Christiansen et al. 1980).Rhyolite vitrophyre (Christiansen et al. 1980).Average vitrophyre (Turley and Nash 1980).Average dome-related obsidian (Evans and Nash 1978).Average rhyolite vitrophyre (Christiansen et al.1980; Christiansen 1980).Average rhyolite. Analyses with * are semi­quantitative (Keith 1980).Average of 5 rhyolite lavas (Barrott 1984).Average of 2 devitrified rhyolites.

Figure 3. Enrichment factor diagram showing evolutionary trends in therhyolites of the Thomas Range, Utah, (thick line) derived by comparingan incompatible element-poor and an incompatible element-rich speci­men. The samples are from lavas presumed to be cogenetic. Enrichmentfactors for the Bishop Tuff (Hildreth 1979) are shown with thin lines andare similar in magnitude and direction (except for Sc and Sm) to thosefor these rhyolite lavas.

Topaz Rhyolites 9

TABLE 2. (CONTINUED)

Sheep LittleCreek China Belt Silver Tomichi Boston Black BurroRange 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 100Rb 380 275 493 304 176 289 580 378 680 348 451Cs 7.7 9.3

BBeSr 40

BaCrCo

CuGa 55Sc

Y 110Zr 154Zn

Nb 43MoSn

HgHfTa

42

286

14.612.8

<12

<22

176125

<1013

<0.5

7.5

15.6'

461. 981. 90

101.3

111

0.1

1375

835

4.45.1

611

36

2

34101

5316

23

93

35

7.5

63

96

12*

36*

126

600

16925

19

16

65

2.70.42

431.7

105192

39

25

7.83.3

75

29

12114

10

Pb 35Th 50U

F 2800Cl

6050 49

24

4450377

15.3

3375

4334.016.2

1700

33

5.9

1350

5.4 13

1399 2954

47*5940

100 1000

8.2

5200

2132.4

8.1 12.9

3800 3800 1845397

15.

16.

17.18.19.

20.

21.22.

Devitrified rhyolite (Christiansenet al. 1980; and unpublished data).Average rhyolite (Christiansen unpublisheddata).Average rhyolite (Dayvault et al. 1984).Rhyolite (Rupp 1980).Obsidian (Zielinski et al. 1977; andChristiansen unpublished data). Christiansenet a1. (1980) report U (16 ppm) and Th (33 ppm).Average of 3 hydrated rhyolite glassesMutschler et al. in press) •Average of 3 rhyolites (Ernst 1980).Average of 6 rhyolites (Ernst 1980).

23. Average of devitrified rhyolites except U and Thconcentrations from 3 marginal vitrophyres(Steven et al.1977). Zielinski (1978) reportsU (40, 26, 43, and 41 ppm). Analyses with *are semi-quantitative.

24. Average of 21 rhyolites (Ernst 1981).25. Vitrophyre. (Christiansen unpublished data).

Zielinski (1978) reported the U concentrations.26. Devitrified rhyolite (Christiansen unpublished

data) •27. Vitrophyre (Correa 1980; Christiansen unpublished

data) •28. Average rhyolite vitrophyre (Moyer 1982; Burt

et al. 1981).

The differentiates would, nonetheless, have widely varying traceelement characteristics. Crystal settling seems to be an unlikelymechanism of crystal fractionation; a more plausible methodwould be the fractionation model described by McBirney (1980)and Huppert and Sparks (1984) that involves wall crystallization,the generation of a buoyant evolved liquid, and its consequentupward escape to produce a vertically stratified magma chamber.

In contrast to the nearby Spor Mountain rhyolites, no eco­nomic mineralization has been found associated with the youngerTopaz Mountain Rhyolite. Bikun (1980) attributes this lack ofmineralization to lower magmatic concentrations of lithophileelements and to their retention in the spherulitically-devitrifiedrhyolite lavas ofthe Thomas Range.

The Thomas Range lies in the central portion of the DeepCreek-Tintic mineral belt (Shawe and Stewart 1976; Stewart etal. 1977b), an east-trending zone of basement highs, Cenozoicvolcanic centers and associated mineralization (Figure 4). Likethe Pioche mineral belt to the south, it is expressed as a series ofaeromagnetic highs. Cenozoic magmatism along the belt (Lindseyet al. 1975; Lindsey 1982) began about 42 Ma with the eruptionof a calc-alkaline sequence of intermediate-composition lavas, ashflows, and small intrusions. Oligocene (38 to 32 Ma) volcanismin the Thomas Range region was more silicic and is representedby several ash-flow tuffs that emanated from collapse calderas.An 11 m.y.lull in magmatic activity preceded the eruption of theSpor Mountain Formation, which also contains topaz (see

10 Christiansen, Sheridan, and Burt

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

Sample No. Rb(ppm)

Sr(ppm)

AgeMa

Analystl

SM-6lcSl1-62bSM-29-206

MR-l

WW-6bWW-9STC-4

DRS-155-62DRS-149-62

184.6371.8433

188.4

381. 4626596

631. 7627.2

79.3458.60

2.5

37.47

18.1920.210.9

38.3819.44

Thomas Range, Utah6.740 0.70938

18.38 0.71338495 0.75141

Mineral Range, Utah14.56 0.70616

Wah Wah Range, Utah60.78 0.7163589.7 0.72752

159 0.75485

Cortez, Nevada47.71 0.7181093.65 0.72843

0.708790.711740.7071

0.70606

0.705990.71220.7092

0.708000.70862

6.36.36.3

0.5

12.012.020.2

14.914.9

EHCEHCLJ

EHC

EHCLJLJ

EHCEHC

IZ-l

RT-MCTjR-lTjR-2

358

294178352

28

357016.1

Sheep Creek Range, Nevada37 0.71577

Jarbidge, Nevada24.3 0.719497.36 0.71217

63.5 0.72397

0.7085

0.71420.71060.7101

13.8

15.415.415.4

LJ

LJLJLJ

NAT-2

72L-47K

HC-8

318.8

281

350

3.24

112

3.7

Nathrop, Colorado288 0.83433

Lake City, Colorado7.23 0.7073

Black Range, New Mexico270 0.82203

0.71410.7080

0.7054

0.71580.7108

29.330.8

18.5

27.729.0

EHC

PWL

LJ

15a15b

529523

Little Belt Mountains, Mont~na

8.51 179.9 0.8341 0.70948.91 169.8 0.8269 0.7092

48.848.8

ZPZP

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 wereformed after about 10 Ma including the eruption of the TopazMountain Rhyolite 6-7 Ma. Although the rhyolites of theThomas Range were not emplaced in a strictly bimodal volcanicfield with contemporaneous mafic and silicic lavas, they are partof this regional sequence of basalt or basaltic andesite (Figure 5)and high-silica rhyolite. Mafic lavas with ages of about 6 and 1Ma are exposed at Fumarole Butte 23 kIn to the west (Petersonand Nash 1980; Best et al. 1980).

2. Spor Mountain, west-central Utah

The topaz rhyolite exposed around· the margins of SporMountain in west-central Utah is related to the largest commer­cial source of beryllium known in North America. The minerali­zation occurs in an altered pyroclastic deposit cogenetic with a 21Ma rhyolite flow (Lindsey 1982). The most recent studies of therhyolite and the mineral deposits include those of Lindsey (1977,1982), Bikun (1980), and Christiansen et al. (1984).

Topaz Rhyolites 11

NV UT

40° CDc0N-CD:J

39°0()-CDc.OlCilE0

38°' "-CD«

,,- ...- Aeromagnetic High

okm

,

80 160

Mineral Belts

OU -Oquirrh - U i nt aDT -Deep Creek Tintic

P -Pioche01 -Delamar-Iron Springs

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

Spor Mountain consists of a block of tilted and intricatelyfaulted lower and middle Paleozoic sedimentary rocks that arechiefly carbonates (Figure 2). Numerous, relatively small rhyoliteplugs, dikes, and breccia pipes have intruded the sequence. Thepre-volcanic surface was disrupted by northeast-trending ridgesand valleys, perhaps formed by faulting (Williams 1963). Post­eruption basin-and-range faulting has further complicated thestructure making it difficult to estimate the number of vents in­volved. Lindsey (1979) identified at least three major vents. The

eruptions commenced with the emplacement ofa series of ignim­brites, pyroclastic air-fall sheets, and pyroclastic-surge units, andwere terminated by the extrusion of lavas over the tuff. The tuffcontains lithic inclusions of dolomite (altered to fluorite near thetop of the tuff) and older volcanic rocks that were entrained fromthe country rock as the pyroclastic material moved through thevent. Locally, the tuff is absent and the lava rests on Paleozoicsedimentary rocks, but where present the tuff reaches a thicknessof almost 100 m (Williams 1963; drill core information). A

12 Christiansen, Sheridan, and Burt

.1140' 113° 112°I-!I+--------+-----~~-r__40°

Figure 5. Distribution of Miocene and younger rhyolite or mafic lavaassociations in western Utah. Rhyolites are unpatterned; ages (in Ma) arein bold-face numbers. Mafic flows are stippled; ages are in smallernumbers (Best et aI. 1980; Best et aI. 1985; Evans and Steven 1982);basalt (0), andesite-trachyandesite (x), potassic mafic lavas (opentriangles).

variation diagrams) and by sanidine rims on sieve-textured calcicplagioclase cores (see,for example, Hibbard 1981).

The Spor Mountain rhyolite is generally phenocryst-rich (20to 40%). Major phases include sanidine (Orso to Or60), smokyquartz, plagioclase (AnlO to An13), and aluminous Fe- and F-richbiotite (Figures 31 and 32). Magmatic accessories include Fe-Tioxides, zircon, fluorite, and allanite. The groundmass of felsiticsamples is granophyric, probably as a result of the thickness andslow cooling of the flow. The groundmass consists of alkali feld­spars, silica minerals, fluorite, topaz, and biotite or hematite.Topaz also occurs in miarolitic cavities that surround the maficinclusions. Two-feldspar geothermometry indicates the pheno­crysts in the rhyolite equilibrated at 680°C (Table 4) and thecomposition of the biotites suggest equilibration near the QFMoxygen buffer (log f02 = -18 to -19; Figure 30; Christiansen etal. 1980). The mafic inclusions contain plagioclase (AnSO-30Ab4S-600r4-11), augite (Ca37Mg3sFezs), titaniferous magnetite,and ilmenite in a quench-textured matrix of needle-like pyroxene.Sanidine rims (Or60Ab34An6) on plagioclase suggest tempera­tures of 910°C, while co-existing microphenocrysts of Fe-Ti ox­ides yield equilibration temperatures of 1100 to 1200°C and f02near the QFM buffer.

The average major-element composition of the Spor Moun­tain rhyolite is presented in Table 1. Although generally similar inits characteristics to other topaz rhyolites, the Spor Mountainlavas contain lower SiOz (72 to 74%) and higher AlZ03 (13 to14%) and FeZ03 (1.2 to 1.5%) than others. In spite of slightlylower SiOz contents, the low concentrations of P, Ti, and Mg,and high concentrations of F and Na suggest an "evolved" com­position, an observation corroborated by the extreme enrich­ments of incompatible trace elements. Relative to the rhyolitesfrom the Thomas Range, samples of the Spor Mountain rhyoliteplot nearer the Ab apex of a normative Q-Ab-Or ternary diagram(Figure 36). This is consistent with their F-rich compositions andManning's (1981) experiments. His work shows that increased Fdisplaces residual melts in the water-saturated granite system to­ward the albite corner. Although devitrified samples are generallycorundum normative, the glasses have normative diopside sug­gesting that alkalies were lost during devitrification (Christiansenet al. 1984). The trace element composition (Table 2) of thevitrophyres bears out the distinctive nature of the Spor Mountainrhyolite. It contains very high concentrations of U, Th, Rb, Ta,Nb, Be, Li,Y, Ga, Pb, and Sn. A typical REE pattern is shown inFigure 40b. It shows a deep Eu-anomaly, indicative of feldsparfractionation and the relatively flat REE pattern typical of manytopaz rhyolites. Based on vitrophyre-felsite comparisons, sub­aerial crystallization resulted in the loss of25 to 50% of the F andU in the glasses. Be losses may have been on the order of75% dur­ing devitrification (Bikun 1980). Many other elements (includingRb) do not appear to have been released by devitrification.

The tuff beneath the Spor Mountain rhyolite is the host formajor Be and minor U, F, Li, Mn, Zn, Nb, and Sn mineralization(Lindsey 1977; Bikun 1980). The Be-mineralization is strongestin· the upper few meters of the tuff where BeO concentrations

40

60

)}.

10-8~ (J

6-7 ~a~ ~ (f)X5.3,6.0

21 ~ :,,, 1.0~::..

3.4 6.1

110.3 -Delta

km

miles

4.~

oo

<c

~z

0.4

~.tj{~: ,::/12:';, 0.1,0.2 390

0.9 "',! - Filmore

2.3 ··0~0.4

1.0~2.5e :

~_. 0.5 '~" Il

0.3 .~.!:" Cove Fort 24.

0.5- .8[1' 1.0 u&~8~SV181e20-22~'f::23 7.9 1.1 lQ 12. •o .'!i 13 ~ __

7.6 b. 8 :eo '10: 9-11 c;;,;: 23 21.1 5 :':~".,

<I ~ 50' :';'~;.:: 7 2' 6.4 . 0 ·f:-·

~:~" • 5.4 °I-I~~'d_---::=----~t_-------_;__ 38

central welded zone is developed in the thicker sections of tuff(Williams 1963). Bikun (1980) has suggested that in places itmay be a result of the accumulation of vitrophyric bombs insteadof compactional welding. Although the tuff has been called"water-laid" because of its stratified appearance, no textures in­dicative of an epiclastic origin have been recognized (Bikun1980). However, in the valley that separates Spor Mountain fromthe Thomas Range,a uranium-mineralized lens of tuffaceoussandstone and limestone conglomerate occurs beneath the tuff atthe Yellow Chief uranium mine (Lindsey 1978). A breccia zoneis exposed at the base of the rhyolite lava in several of the open­pit beryllium mines. This breccia is interpreted as an over-riddenapron of talus that accumulated at the front of the moving flow(Bikun 1980). The rhyolite lava has a maximum known thicknessof 300 m (Williams 1963). Dark mafic inclusions with globularand contorted lensoidal shapes are found in the lava. Their tex­tures and chemistry led Christiansen et al. (1981) to suggest that amore mafic magma was injected into the magma chamber of theSpor Mountain rhyolite shortly before its eruption. The mafic(trachyandesite) inclusions themselves show signs of pre-eruptionmixing with the rhyolite in their bulk chemistry (linear trends on

Topaz Rhyolites

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

TemperatureLocation Method Range (OC) n Reference

Thomas Range, UT 2 Feldspar 690 - 790 4 Turley and Nash 19802 Feldspar 630 1 Christiansen et al. 1980Fe-Ti oxides 725 1 Turley and Nash 1980

Spor Mountain, UT 2 Feldspar 680 - 690 5 Christiansen et al. 1980E.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 1980Fe-Ti oxides 665 1 Turley and Nash 1980

Mineral Mountains, UT 2 Feldspar 620 - 770 3 Evans and Nash 1978Fe-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. Christiansenunpublished data

Apatite-bio 590 1 E.H. Christiansenunpub li shed data

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

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

13

reach 1% (Griffitts and Rader 1963; Lindsey 1977, 1982; Bikun1980). Mineralization is associated with dolomite clasts altered tofluorite and opal in association with feldspathic alteration of thematrix (Williams 1963; Lindsey 1977). A thick zone containingLi-bearing clays underlies the ore zone. Bikun (1980) and Burt .and Sheridan (1981) suggest that the ore deposits were formed asBe, V, P, and other elements were released from the lava bygranophyric crystallization and then concentrated in the upperpart of the tuff by meteoric fluids at fairly low temperatures.Beryllium ore (bertrandite) coprecipitated with fluorite formedby reaction of this fluid with the carbonate lithic inclusions. Somesupport for this model is provided by V-Pb ages of opal nodulesin the beryllium tuff. One of these zoned nodules has ages thatdecrease outward from 20.8 ± 1.0 Ma to 8.2 Ma (Ludwig et al.1980). These results suggest that the mineralized nodules formedat the same time as the rhyolite erupted, but continued to grow(perhaps episodically) by the deposition of opal from groundwater. In contrast, Lindsey (1977) and Williams (1963) sug­gested that hydrothermal fluids rose along fractures from an un­exposed pluton, intersected the porous tuff and deposited Be andP below the lava. On Spor Mountain, uraniferous fluorite with­out associated beryllium minerals occurs in breccia pipes formedby partial venting of the rhyolitic magma. As mentioned above, asedimentary V-deposit (Yellow Chiefmine) occurs in an epiclas­tic conglomerate that locally underlies the tuff.

The volcanic history of the Spor Mountain/Thomas Range

region is summarized by Lindsey et al. (1975), Shawe (1972),and Lindsey (1982) and is reviewed in the section describing theThomas Range. The episode of rhyolitic volcanism at SporMountain maybe related to the initial development of the Basinand Range province in a back- or intra-arc setting oflithosphericextension (see, for example, Zoback et al. 1981; Eaton 1984a).The evidence for magma mixing suggests that magmatism mayhave been bimodal. The episodic recurrence ofsilicic magmatismalong the Deep Creek-Tintic belt suggests that it may be a pro­found flaw in the continental lithosphere-perhaps an intracon­tinental transform of the type proposed by Eaton (1979);Regardless of its origin, the Deep Creek-Tintic belt is very similarto the more southerly Pioche belt in its characteristics and history.Mid-Cenozoic igneous activity commenced about 10 Ma earlieralong the northern belt (Stewart et al. 1977b), but the later devel­opment of both regions was very similar and characterized by theeruption ofbimodal suites of mafic lavas and topaz rhyolites afterabout 22 Ma.

3. Honeycomb Hills, west-central Utah

Honeycomb Hills, the western-most topaz rhyolite occur­rence along the Deep Creek-Tintic trend, lies 30 km west of theThomas Range. Studies of the geology of the Honeycomb Hillsand t~eir rhyolites include those of Turley and Nash (1980),Hogg (1972), McAnulty and Levinson (1964), Shawe (1966),

14 Christiansen, Sheridan, and Burt

and Erickson (1963). The Honeycomb Hills have attracted atten­tion primarily as a result of low-grade beryllium and rare-alkalimineralization in tuffs associated with a small topaz-rhyolitedome complex emplaced 4.7 Ma (Lindsey 1977; Turley andNash 1980).

Two rhyolite domes with a total volume of less than 0.5km3 form the Honeycomb Hills. Eruption of the domes waspreceded by the deposition of a lithic-rich tuff. The tuff is slightlyaltered and contains macroscopic fluorite. The rhyolite has abasal vitrophyre that is exposed locally but it generally consists offelsitic rhyolite with highly contorted flow foliation. The rhyoliteapparently intrudes unrelated older (Miocene or early Pliocene)trachyandesitic (shoshonitic) and dacitic lavas and tuffs (Hogg1972). Paleozoic sedimentary rocks were encountered at about50 m in a hole drilled between the two hills (McAnulty andLevinson 1964).

The rhyolites from the Honeycomb Hills contain up to 40%phenocrysts of sanidine (Orso to Or60), smoky quartz, plagioclase(AnlO), and F- and Fe-rich biotite (Turley and Nash 1980; Fig­ures 31 and 32). Two-feldspar geothermometry yields a tempera­ture of 605°C (Turley and Nash 1980). Magmatic accessoriesinclude fluorite and traces of Fe-Ti oxides. Topaz, fluorite, andbiotite occur as devitrification products within the groundmass ofthe rhyolites. The lava also contains globular inclusions of topaz­and fluorite-rich material. The textures in these inclusions rangefrom pegmatitic to aplitic. They may represent fragments of acogenetic granite pluton at depth. Inclusions of the metamorphicbasement (quartzite, metaconglomerate, and meta-arkose) andinclusions of the surrounding mafic lavaS also occur in therhyolite.

The compositions ofseveral samples from Honeycomb Hillsare given in Table 1. Fluorine contents in vitrophyres range from0.95 to 1.2% (Christiansen et al. 1980). The rocks are slightlyricher in Al and Na, and have less Si than many topaz rhyolitesbut are similar to those from Spor Mountain, Utah. These com­positional characteristics probably resulted from the effect ofhighF in the magma. As noted above, Manning (1981) has shownthat the effect of F is to push the water-saturated minimum in thegranite system toward the albite apex as the quartz field expands,producing more sodie and aluminous residual melts. The singlemajor-element analysis reported by Turley and Nash (1980) isvery unusual (Table 1). It contains 11.1% CaO, 8.0% F, and only63.6% SiOz-reportedly due to large amounts of fluorite in thegroundmass. Perhaps this analysis represents one of the globularinclusions described earlier. Some trace-element analyses are alsoavailable for samples from the Honeycomb Hills (Table 2). Con­sistent with their higher fluorine contents, the Honeycomb Hillsrhyolites appear to be more "evolved" than those of the ThomasRange. Analysis 5 is the low-silica sample analyzed by Turleyand Nash (1980) and analysis 6 is for a "normal" rhyolite forwhich no major element data have been published. The othertrace-element analyses (7 and 8) represent a pegmatitic inclusionand a rhyolite respectively (Christiansen et aI. 1980). These rhyo­lites contain high Rb, Li, and Ubut are depleted in Th (about 30

ppm) compared to other topai rhyolites, especially if comparedto those with approximately 1%F such as the Spor Mountainrhyolite. The silica-poor sample contains 151 ppm U and highMo, while the pegmatitic inclusion contains anomalous Li (590ppm) and Sn (65 ppm). REE patterns for the rhyolite are similarto those from Spor Mountain (Figure 40c).

Low grade Be, Li, Cs, and Rb mineralization occurs in thepyroclastic deposit beneath the western dome. The mineralizationoccurs in aim thick zone about 1 m below the base of the lavaand is reported to have been developed when ascending mag­matic fluids were blocked by the impermeable rhyolite (Mc­Anulty and Levinson 1964).

4. Smelter Knolls, west-central Utah

The youngest (3.4 Ma by K-Ar method; Armstrong 1970;Turley and Nash 1980) of the topaz rhyolites identified on theDeep Creek-Tintic trend consists of a rhyolite dome and flowcomplex at Smelter Knolls located about 25 km west-northwestof Delta. The geology of Smelter Knolls is described by Turleyand Nash (1980).

The rhyolite complex atSmelter Knolls is 5 km in diameterand contains about 2.2 km3 oflava. Basal vitrophyres and flowbreccias are exposed locally. No pyroclastic deposits related tothe lava are exposed. The lavas were apparently erupted throughthick alluvial deposits related to the development of the basin andrange topography of the area.

The rhyolite from Smelter Knolls generally contains 15 to20% phenocrysts set in a devitrified flow-banded matrix. Quartz,sanidine (Orso to Or6S), plagioclase (AnlO to Anzo), Fe- andF-rich biotite (Figures 31 and 32), and Fe-Ti oxides (ilmenite israre) occur in all samples. Accessory phases include zircon, fluor­ite, and allanite. Topaz occurs in the devitrified groundmass andalong with hematite in miarolitic cavities. Fe-Ti oxide and two­feldspar geothermometry indicate that the phenocrysts crystal­lized at low temperature (660 to 685°C; Table 4) and at f02 nearthe QFM oxygen buffer (Figure 30; log f02 = -19.9 bars).

The average composition of four samples analyzed by Tur­ley and Nash (1980) is presented in Table 1. The rocks are verysimilar to other topaz rhyolites. Fluorine ranges from 0.65 to0.78% in vitrophyres; a single felsite shows the common fluorinedepletion that accompanies devitrification (F = 0.5%). Devitrifica­tion also produced depletions of U, CS,and Sb. As expected, therhyolites are enriched in incompatible lithophile elements (Table2). The REE patterns (Figure 40d) are also similar to those fromother topaz rhyolites with low La/LuN (1.60), La/CeN (1.06)and Eu/Eu* (0.02).

The two major rhyolite hills at Smelter Knolls are separatedby a normal fault that stretches northward for about 8 km. Faultsof this orientation are common in the area. Mafic volcanism inthe immediate vicinity has K-Ar ages of 0.31 Ma (tholeiitic ba­salt, 48% SiOz: Turley and Nash 1980) and 6.1 Ma (basalticandesite, 57% SiOz: Turley and Nash, 1980) thus bracketing theeruption of the rhyolite. A low-shield volcano composed princi-

Topaz Rhyolites 15

pally of basaltic andesite developed 1 Ma (Peterson and Nash1980). It lies 12 km to the north and is centered on the fault trendnoted earlier. It appears that the tectonic-magmatic setting issimilar to other Great Basin rhyolites that were emplaced duringlate Cenozoic east-west extension and during episodes of maficvolcanism.

5. Keg Mountain, west-central Utah

Rhyolite flows and domes in the Keg Mountain area(termed McDowell Mountains on some maps) have beenmapped as part of the Topaz Mountain Rhyolite (Erickson1963). A brief description of them is included here, as Shawe(1972: 72) implies they bear topaz in lithophysae. However,Erikson (1963) and'Lindsey et al. (1975) report no topaz fromthe Topaz Mountain Rhyolite at Keg Mountain. In our visits tothe Keg Mountains we have not yet identified any topaz-bearingrhyolites.

Erickson (1963) identified atleast two vents and estimatedthat about 15 km3 of rhyolitic volcanic rocks are preserved in theKeg Mountain area. The emplacement of the rhyolites was sim­ilar to other domes and flows with an upward sequence of lithic­rich tuff, basal vitrophyre and/or flow breccia, and flow-bandedrhyolite. The rhyolite lavas attain maximum thicknesses ofabout60 m near their vents. The underlying tuffs accumulated to sim­ilar thicknesses and thin toward the margins of the flows. Thelavas are 8 Ma (Lindsey et a!. 1975).

Most of the lava is felsitic and flow-banded with less than10% phenocrysts of quartz, sanidine, plagioclase, and sparse bio­tite (Erickson 1963). Hematite and possibly topaz occur in litho­physae within the rhyolite.

The geochemistry and mineralogy of the tuff associated withthe rhyolites has been the subject of several investigations todetermine the role of alteration on the mobility of Be and V(Lindsey 1975; Zielinski et a!' 1980), but the lavas remain poorlystudied. The tuffs contain abundant clasts derived from oldervolcanic rocks (some as large as 5 m in diameter) that wereprobably included in the ash as it explosively vented to the sur­face. Broken phenocrysts of quartz, sanidine, plagioclase, andbiotite constitute 5 to 15% of the tuff. Magnetite, hematite, titan­ite, hornblende, augite, zircon, fluorite(?), and apatite occur insmaller amounts, but may be xenocrystic.·The average trace ele­ment composition of the tuff is given in Table 2. The relativelyhigh concentrations of Sr and Ba and low concentrations of Ga,Nb, Y, Th, and V suggest that if the rhyolites are topaz-bearingthey are relatively poor in incompatible lithophile elements com­pared to others from the western Vnited States.

The so-called "alkali"rhyolites of the Keg Mountain areaare deposited on an older sequence of (apparently topaz-free)rhyolitic lavas (10 Ma), ash-flow tuffs (32 to 30 Ma), andintermediate-composition lavas and breccias (38 to 39 Ma)(Shawe 1972; Lindsey et a!. 1975). The temporal magmatic andtectonic development appears to be similar to that in the nearbyThomas Range.

6. Mineral Mountains, western Utah

In the Mineral Mountains of western Vtah topaz-bearingrhyolite domes occur discontinuously along the crest of the range(Evans and Nash 1978; Lipman et a!. 1978b). The MineralRange is a typical basin and range horst that consists predomi­nantly of a multiple-phase granitic pluton of Tertiary age (V-Pbzircon age 25 Ma: Aleinikoff et a!. 1985). The Roosevelt HotSprings Known Ge~thermal Resource Area (KGRA) is locatedalong one of the western range-front faults.

Three sequences of rhyolite lavas exist in the Mineral Range(Lipman et a!. I978b). The general distribution of the lavas isshown in Figure 6. The oldest episode of volcanism (7.9 Ma; alldates are by K-Ar method) is represented by a single dome (Trd)on the western flank of the range. Quaternary' obsidian flows(0.77 Ma) several kilometers long and up to 80 m thick wereerupted shortly before the most viscous topaz-bearing domes(0.58 to 0.53 Ma). At least 11 domes were erupted. The domesusually have a basal vitrophyre (5 to 10 m thick) that in placesforms part ofa flow breccia. The vitrophyre grades upward into adevitrified, flow-banded rhyolite. A frothy mantle ofperlite formsa carapace around parts of some domes. Lithophysae and othergas cavities occur in flow interiors. In contrast to the obsidianflows, several of the domes overlie pumice-rich pyroclastic depos­its that represent the vent-opening eruptions. The domes· rangefrom 0.3 to I km in diameter and are up to 250 m high.

The rhyolite domes are crystal-poor with 2 to 10% pheno­crysts of sanidine, quartz, oligoclase, and sparse biotite. Allanite,titanite, zircon, Fe-Ti oxides, and apatite are accessories. Topazoccurs in vugs along with pseudobrookite and specular hematiteas a result of crystallization from a vapor phase (Evans and Nash1978).

Chemical analyses of rhyolites from the domes show thatthey are typical of topaz rhyolites elsewhere (Tables I and 2). Thefluorine content of the vitrophyres from the domes ranges from0.1 to 0.4%. Relative to the slightly older obsidian flows, thedomes are enriched in f, Na, Mn, Rb, Th, Nb, HREE, and Znand are depleted in K; Ca, Fe, Ti, Ba, Sr, Zr, light REE (Lathrough Tb), and Eu. Temperatures, derived from Fe-Ti oxides(Evans and Nash 1978) and O-isotope geothermetry (Bowman eta!. 1982) are consistently lower in the F-rich domes than in theolder obsidian flows (650 versus 760°C). The dome lavas alsocrystallized at lower oxygen fugacities (log f02 ranges from -16.8to -18.2). These and other observations led Evans and Nash(1978) to suggest that the magma in the domes was a differentiateof the obsidian-flow magmas. The REE pattern of the rhyolitesfrom the Mineral Mountains is different from most other topazrhyolites (Figure 40e). The principal difference lies in the rela­tively low concentrations of the middle REE (Nd through Tm);in this regard they are similar to the patterns of the Lake City,Colorado, topaz rhyolites. This depletion is probably the result oftitanite fractionation. A sample from one of the older lavas (theflow of Bailey Ridge; Figure 6) was analyzed for its Sr-isotopecomposition (Table 3). Its'ratio (0.7062 ± 0.0003) indicates that

16 Christiansen, Sheridan, and Burt

Qac'.

3I

2I

(I)

Z

<I­

Z

:)

o Tg

:E

KM

T~\

Tg

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III

I

-J

«Qac

0::

wz

:E

38°30'

38°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) andhot-spring deposits (Qhl) occur along a range-front fault (bold line). A Tertiary rhyolite (Trd) lies in theextreme ,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/Srratio like those typical of granulite facies metamorphic rocks.This interpretation is consistent with the O-isotopic compositionof the lavas and domes (8180 = 6.3 to 6.9 %0; Bowman et al.1982), which also suggests a crustal source of high metamorphicgrade. Both lines of isotopic evidence probably preclude the deri­vation of the rhyolites from metasedimentary rocks in the Proter­ozoic age basement. No metalliferous ore deposits are known tobe associated with the rhyolites of the Mineral Mountains.

The Mineral Mountains are part of a broad region of thewestern United States that experienced late Cenozoic extensionand bimodal volcanism. Extensive fields of contemporaneous ba­salt and rhyolite (Figure 5) are located north and east of the range(Haugh 1978; Clark 1977; Crecraft et al. 1981; Hoover 1974;Evans and Steven 1982). Most ofthe fields appear to be less than2.5 Ma. The near contemporaneity of basalt and rhyolite volca­nism in the Mineral Range area and the inclusion of vesiculatedbasalt "xenoliths" in the obsidian flows implies a close geneticconnection between the magmas (Evans and Nash 1978). Therhyolites of the Mineral Mountains lie along the .Pioche­Marysvale mineral belt of Shawe and Stewart (1976), which isdescribed more fully below. The distribution of volcanism maybe controlled by fractures that parallel this trend.

7. Wah Wah Mountains and vicinity, southwestern Utahand southeastern Nevada

Numerous fluorine-rich rhyolitic lavas have been identifiedin the southern parts of three ranges- the Wah Wah Mountains,the Needle Range, and the White Rock Mountains of southwest­ern Utah and adjacent Nevada (Christiansen 1980; Best et al.1985). Interest in these lavas has been spurred by the discovery ofa porphyry molybdenum deposit in the southern Wah WahMountains (Tafuri and Abbott 1981; Keith 1980, 1982) and bythe occurrence of numerous small deposits of uranium in andnear the rhyolites (Christiansen 1980).

The distribution of the rhyolites is shown in Figure 7. Twoepisodes of rhyolitic magmatism have been delineated-one at 22to 18 Ma and a second at about 12 Ma (Rowley et al. 1978b;Best et al. 1985). The rhyolites occur as isolated plugs withoutsignificant pyroclastic deposits (e.g., Observation Knoll), as iso­lated domes or flows with underlying pyroclastic breccias andtuffs (e.g., the rhyolites at the Staats Mine and at the Tetons) andas groups of coalesced domes and flows with interlayered tephradeposits (e.g., the Broken Ridge and Steamboat Mountain areas).The tuffs generally consist of thin pyroclastic flow, surge andminor air-fall units. Explosion breccias near some vents containabundant lithic inclusions of the local country rock. Evidence ofmixing between rhyolitic and mafic magmas before eruption isfound in tuffs from the southern Needle Range (Christiansen1980). Vitrophyres a few meters thick are present at the bases ofsome flows. Felsitic, flow-banded lavas with abundant vapor­phase cavities in their upper portions are typical.

The topaz rhyolites can usually be distinguished from nearly

contemporaneous fluorine-poor rhyolites by their low crystalcontent (less than 5%) and the presence ofsmoky quartz. A few ofthe older topaz rhyolites are phenocryst-rich (e.g., the rhyolite ofthe Staats mine). The major phenocrysts in both age groups arethe same and include sanidine, quartz, sodic plagioclase, andsparse Fe-rich biotite. A few samples contain Fe-rich hornblende.Magmatic accessories include zircon, allanite, apatite, fluorite,and Fe-Ti-Mn oxides. Two-feldspar geothermometry indicatesthat some of the topaz rhyolites crystallized at temperaturesaround 650°C (Table 4). Topaz, fluorite, alkali feldspars, hema­tite, bixbyite, and silica minerals line gas cavities and occur in thedevitrified matrix ofsome lavas. Red beryl (up to 1 cm in diame­ter) occurs in the eastern Wah Wah Mountains (Ream 1979).Vapor-phase garnet occurs in some topaz rhyolites but it crystal­lized as a magmatic phase in the tuff ofPine Grove erupted froma developing (molybdenum-mineralized) intrusive system in thesouthern Wah Wah Mountains (Keith 1980, 1982). This tuff isdistinct in age (24 Ma) and chemistry from the younger topaz­bearing lavas (Keith 1980, 1982). Although the composition ofthe rhyolitic portion of the tuff ofPine Grove lies within the rangeofall topaz rhyolite compositions, it is enriched in Ba, Sr, Sc, andAl and depleted in F, Zr, Rb, Nb, Ta, Th, U, Yb, Y, and Morelative to the topaz rhyolite lavas of SW Utah. The late "LouRhyolite," which cuts the Pine Grove stock, is nonetheless geo­chemically identical to other topaz rhyolites from SW Utah(Keith 1982). Its age is not known.

The chemistry of samples from southwestern Utah demon­strates their similarity to other topaz rhyolites (Table 1). Nosystematic differences in the compositions of the two age groupsof topaz rhyolites have yet been demonstrated. Fluorine concen­trations in vitrophyres range from 0.2 to 05%. The trace elementchemistry is summarized in Table 2, and shows the typical en­richments of U, Th, Rb, Ta, Cs, and Li. REE patterns for topazrhyolites from this region are typical of others with low La/LuN,La/CeN, and Eu/Eu* (Figure 40t). Initial Sr-isotope ratios rangefrom 0.706 to 0.712 (Table 3).

Extensive alteration of rocks at the surface is associated withthe fluorine-rich rhyolites in the Wah Wah Mountains. Largeareas of jasperoid (Lindsey and Osmonson 1978) and alunite(K-Ar age 15 Ma; Best et al. 1985) occur in this region. Fluoriteand uranium were deposited in tuffs and in the intrusive marginof the topaz rhyolite at the Staats mine (Christiansen 1980; Lind­sey and Osmonson 1978; Whelan 1965). Other uranium pros­pects associated with the rhyolites are shown in Figure 7. Goldand silver have been recovered from quartz-carbonate-fluoriteveins in the Stateline area of the southern White Rock Moun­tains. These deposits are temporally related to the older episode ofrhyolitic magmatism (Keith 1980). The previously mentionedporphyry molydenum deposit at Pine Grove in the Wah WahMountains, which bears topaz in the alteration zones (Tafuri andAbbott 1981), is slightly older (24 versus 20 Ma) than the firstepisode of topaz rhyolite magmatism (Keith 1980).

The tectonic and magmatic history of the region is outlinedby Rowley et al. (1978b, 1979) and Best et al. (1980, 1985). The

18 Christiansen, Sheridan, and Burt

Rhyolite flows & intrusions(Miocene & Pliocene)

Granite & quartz monzonite(Miocene)

+

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Horse

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•o

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Figure 7. Regional distribution of Miocene and Pliocene rhyolites (many of which contain topaz) andOligocene and Miocene intrusive rocks in southwestern Utah and eastern Nevada (after Christiansen etal. 1980; and Best et al. 1985). The locations ofuranium prospects and of the topaz-bearing molybdenitedeposit at Pine Grove are also shown. Crystal-poor rhyolite lavas in the western Needle Range, at DeadHorse Reservoir, and at Thermo Hot Springs contain no topaz and are chemically distinguished from thetopaz rhyolites by their low concentrations of Rb, Y, and Nb, and by high concentrations of Sr. Basinand range fault-block mountains are outlined with solid lines.

volcanic development of the region was dominated by eventsalong the east-trending Pioche mineral belt of which the BlueRibbon lineament is one element (Rowley et al. 1978b). Thisfeature coincides with a broad aeromagnetic high and·is markedby numerous volcanic and intrusive centers of Oligocene to Hol­ocene age. Calc-alkaline volcanism (andesite-dacite-rhyolite)began in the early Oligocene (about 32 Ma) and continued untilthe Miocene (until at least 19 Ma in the Black Mountains: Row­ley 1978). This volcanism produced widely scattered, partly clus­tered, composite volcanoes with andesitic to dacitic lavas alongthe belt. The intervening lowlands were covered by widespreaddacitic to rhyolitic ash flows erupted from collapse calderas. Theolder F-rich rhyolite domes (20 to 18 Ma) are nearly contempo­raneous with the latter portion of this episode but were accom­panied by the eruption of trachyandesite (62 to 54% SiOz) lavasin the Wah Wah Mountains, forming a bimodal suite there (Fig-

ure 42; Keith 1980; Best et al. 1985; Christiansen and Wilson1982). High-K andesites erupted along the belt from 25 to 21 Ma(Best et al. 1980, 1985). Fluorine-poor, crystal-rich rhyolite lavaswere also erupted during this time beginning about 22 Ma. Agenetic relationship between the F-rich and F-poor rhyolites hasnot been established. A Miocene "lull" in volcanic activity in theeastern Great Basin was followed by renewed bimodal magma­tism that began about 13 Ma ago (Best et al. 1980). Mafic lavas(trachybasalts to trachyandesites) were again accompanied by theeruption of the younger topaZ-bearing rhyolites in the southernWah Wah-Needle Range area and by topaz-free rhyolite else­where (e.g., Dead Horse Reservoir, Thermo Hot Springs). In theWah Wah Mountains, mafic lavas and rhyolite were not eruptedfrom the same centers but their ages correspond closely (Figure5). The two episodes of topaz rhyolite volcanism are reminiscentof those to be described in New Mexico and Colorado. The

Topaz Rhyolites 19

Pioche belt is paralleled by NE-trending faults (some of whichmay have strike-slip movement) and a conjugate NW-trendingset. Best et al. (1985) suggest that the tectonism along the trendmay be the result of relaxation of the Cretaceous compressionthat produced regional thrust sheets. Alternatively the tectonicand magmatic events may be localized by a lithospheric fractureproduced or reactivated by differing rates of extension north andsouth of the Pioche belt (Rowley et aI. 1978b).

8. Wilson Creek Range, southeastern Nevada

Barrott (1984) reports the presence oftopaz in rhyolite lavasthat are exposed in the northern part of the Wilson Creek Rangenear Rosencrans Knolls. The lavas are part of a strongly bimodalsequence of rhyolitic ash flows and basaltic lavas that were em­placed over a short period of time about 23 Ma. The topaz­bearing lavas (K-Ar age 22.6 ± 0.9 Ma; all ages are from Barrott,1984) cap knolls underlain by thick sequences of unwelded lithictuffs. Interstratified with this lower tuff section is a prominentrhyolitic welded tuff (K-Ar age 23.4 ± 0.9 Ma) and a series oftrachybasalt lavas (K-Ar age 23.9 ± 1.0 Ma).

The rhyolite lavas are geochemically similar to other topaz~

bearing lavas in their uniformly high Si02 contents (75 to 76wt%), and their low concentrations of Ti02, Fe203, MgO, andCao (Table 1). No fluorine analyses have been reported Aslightly older (23.4 ± 0.9 Ma K-Ar) welded tuff also consists ofhigh-silica rhyolite that is only slightly enriched in Ti, Fe, Mg, andCa relative to the rhyolite lavas. The trace element differencesbetween the two rhyolites is more striking. The topaz-bearinglavas are significantly enriched in Rb (1 to 3 times), Y (2 times),and Nb (2 to 3 times) relative to the zoned(?) tuff. Likewise, thelavas are depleted in the feldspar-compatible elements Ba (¥.!) andSr (lh). The trace element composition for five analyses (Barrott1984) is summarized in Table 2. The genetic association of thetwo rhyolite units has not been adequately examined but theirclose association in space and time suggests that they may beco-genetic.

The rhyolitic ash flows and early topaz-bearing lavas of thenorthern Wilson Creek Range are part of a Miocene series ofpotassic mafic lavas and rhyolite with no intermediate composi­tions known to be contemporaneous. This early Miocene vol­canic field developed within the bounds of a huge Oligocenecaldera complex-the largest of these calderas was the IndianPeak caldera which formed upon the eruption of several thou­sand cubic kilometers of dacitic magma (Best et aI. 1985). Theyounger high silica tuff may have been erupted from a trap-doorcaldera and has a much smaller volume (less than 20 km3; Bar­rott 1984). The Miocene volcanic rocks were erupted about thesame time as the bimodal sequence of potassic mafic lavas andrhyolites described from the southern end of the Wah Wah andNeedle Ranges. However, they are distinct from these lavas inseveral important ways. 1) The rhyolites of the northern WilsonCreek Range are not a part of the more southerly volcanic belt,which includes topaz rhyolites of southwestern Utah. They lie on

the northern fringe of the aeromagnetic high which marks thePioche mineral belt (Fig. 4). 2) The mafic part of the bimodalsuite consists of more mafic lavas-trachybasalts with up to 2.4%K20 at 50% silica. Farther east, lavas as mafic as basalt do notappear until much later (8 to 9 Ma; Fig. 5). 3) The topaz rhyolitesin the Wilson Creek Range are about 3 Ma older than the oldesttopaz-bearing lavas in the Wah Wah Mountains. 4) The rhyolitelavas may be cogenetic with earlier voluminous ash flows.

9. Kane Springs Wash, southeastern Nevada

An unusual association of topaz rhyolite lavas with the de­velopment of a mildly peralkaline caldera system has been re­ported by Novak (1984). The Kane Springs Wash caldera (Noble1968) is located in southeastern Nevada and lies at the extremesouthwestern part of the ENE-trending aeromagnetic ridge of theDelamar-Iron Springs mineral belt (Figure 4).

The following description of the evolution of the KaneSprings Wash volcanic center is taken from Novak (1984). Theeruption oftrachytic lavas 14.2 Ma (all ages are by K-Ar method)was the first recorded event at the volcanic center. A 19 by 13 kmcaldera (Figure 8) collapsed slightly later (14.1 ± 0.2 Ma) as aresult of the eruption of several compositionally zoned (comen­ditic or fayalite rhyolite to trachyte) ash flows that had a totalvolume of over 130 km3. Immediately following collapse, theeruption of trachyte lavas formed a cumulodome on the floor ofthe caldera. High-silica rhyolites (with ferroedenite) and basalticto trachyandesitic lavas were then erupted, marking a fundamen­tal change in the magma chemistry to non-peralkaline composi­tions. These units have K-Ar ages that are indistinguishable fromthose of the other post-caldera volcanic rocks. The caldera expe­rienced no structural resurgence. More trachyandesite lavas andseveral biotite rhyolite domes are the last eruptive products of theKane Springs Wash center. Two of the biotite rhyolite domes liein the southwestern part of the caldera's moat and contain vapor­phase topaz. Novak (1984) reports a K-Ar (sanidine) age of 13.2± 0.2 Ma for one of these domes, about 900,000 years youngerthan the comenditic ash-flow eruptions. The largest dome isabout 2 km across and is underlain by an initially erupted blanketof tephra. Novak interprets the topaz rhyolites to be cogeneticwith the early trachytes and comendites and suggests that theywere derived from a residual pocket of magma within the solidi­fying sub-caldera chamber. Another topaz rhyolite dome occurs10 km to the SE, where it plugs the vent of a post-caldera tra­chyte. Small patches of variably potassic olivine basalt (OJ to2.3% K20) cap sections both inside and outside the caldera andhave K-Ar ages of 12.7 to 11.4 Ma.

Important differences in the mineralogy exist between theearly rhyolites and the younger biotite and topaz rhyolites. Novak(1984) reports that the biotite rhyolites contain both sanidine(Or6o) and plagioclase (Ab76) while the ferroedenite and earliercomenditic rhyolites contain only sodic sanidine or anorthoclase.In addition, the topaz rhyolites contain quartz, Fe-rich biotite,ferroedenite, and fayalite, which is rarely reported in other topaz-

20 Christiansen, Sheridan, andBurt

Caldera, Nevada

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Kane Springs Wash

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km(

o!

Figure 8. Generalized geologic map of the Kane Springs Wash caldera, southeastern Nevada (afterNovak 1984). Map symbols: Tb = basaltic lava flows; Trt = topaz rhyolite lava domes; Tr = post-calderarhyolite lavas and associated tephra; Ttr =interstratified post-caldera trachyandesite lavas and rhyoliticash-flows; Tts =post-caldera extrusive trachyte and syenite dome complex; Tkw =intracaldera exposureof zoned (trachyte to rhyolite or comendite) Kane Wash tuff which is associated with collapse of thecaldera; extra-caldera rocks are stippled; *= vents fur rhyolite domes.

bearing rhyolites (Table 8). Accessory phases include magnetite,ilmenite, and zircon. Earlier rhyolites either contained no hydrousphases (some of the ash flows) or ferroedenite without biotite.

To date, little geochemical information has been publishedfor the volcanic rocks of Kane Springs Wash. Novak (1984)reported one major-element analysis of the topaz rhyolite show­ing that it is indeed a typical metaluminous high-silica rhyolite(Table 1). Compared to the comenditic ash flows, the topazrhyolite is enriched in F, AI, Ca, and K, but it is depleted in CI,Fe, and Ti. A vitrophyre from the topaz rhyolite dome contains0.49% F compared to 0.34%F in a glassy comendite. More impor­tantly, the F/CI ratio of the comendite (2) is much lower thanthat in the younger topaz rhyolite (10), even though both unitsare enriched in fluorine and are presumed to be co-magmatic. Aless F-rich ferroedenite rhyolite has 0.17% F and an FICI ratio ofslightly less than 3 as well. It is doubtful that the closed-systemfractionation ofa parental comendite could produce an aluminousmelt or elevate F/CI ratios. Fluorine should be more compatible

than Cl in silicate (especially hydrous) minerals (cf. Kovalenko etal. 1984). Nonetheless, the eruption of comendite and attendantloss of a Cl-rich vapor from the residual melt could leave anon-erupted residue with a higher FICI ratio. Subsequent frac­tionation of this magma might produce a metaluminous·rhyoliteenriched in incompatible elements. Such a scenario was outlinedby Christiansen et al. (1983a) for the generation of some meta­luminous rhyolite magmas.

The Kane Springs Wash center is one of a number of peral­kaline volcanic centers in the Great Basin of Nevada and adjacentparts of California and Oregon (Noble and Parker 1974). How­ever, the volcanic field lies on an east-west belt of predominantlymetaluminous volcanic rocks ·of late Cenozoic age (Figure 4).The segment of the belt in eastern Nevada is paralleled by thePahranagat shear system (Tschantz and Pampeyan 1970), whichshows late Cenozoic strike-slip movement. Basin and range fault­ing developed several million years after the Kane Springs Washvolcanism.

Topaz Rhyolites

Eastern Great Basin Chronologie Summary

21

Colorado Plateauuplift

hyolite of Pine Grove

Agem.y.o

10

20

30

40

Basalt K-mafic RhyoliteB. Andesite Calc-Alkaline

T x;(;; x

~ ><..( n x

\ -i'"

Pt.( )1\ u

R

,!.r"\,......,

.c..:5

I ~ )~

Comments

tE~W. Extension

1NE-SW Extension

Miocene ~Iull"

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

Topaz rhyolites in the eastern Great Basin: A summary

Two groups of topaz rhyolites can be discerned in the east­ern Great Basin of Nevada and Utah (Figure 9). The division isbased on the age and nature of the associated magmatism. Anearly Miocene group, associated with K-rich mafic lavas or calc­alkaline intermediate composition (dacites and rhyodacites) lavasand tuffs, was erupted 23 to 18 Ma ago. This group is representedby the older topaz rhyolites of the Wah Wah Mountains vicinity;the lavas of the northern Wilson Creek Range, the rhyolite atSpor Mountain, and the Be-rich Sheeprock granite (Christiansenet al. 1983b). A late Miocene to Pleistocene group, associatedwith potassic basaltic magmatism, was erupted after about 13Ma. Intermediate composition lavas and tuffs are very small involume, but include the andesites of the area around the MineralMountains, Utah, and the trachytes of Kane Springs Wash, Ne­vada. These magmatic episodes are separated by an apparentMiocene "lull" in volcanic activity in the extreme eastern GreatBasin. Both groups of rhyolitic rocks are compositionally similarand were erupted during apparent crustal extension. Most of therhyolites occur in EW-belts nearly parallel to the presumed direc­tion of extension. The change in the dominant magma composi-

tions from intermediate/silicic to potassic basaltic lavas is similarto that observed in Colorado but postdates it by about 6 to 10Ma.

10. Cortez Mountains, north-centralNevada

Several plugs and flows of Miocene rhyolite occur in thesouthern end of the Cortez Mountains in north-central Nevada(Gilluly and Masursky 1965). Wells et al. (1971) were the first toreport that the rhyolites contained topaz in "vesicles." The rhyo­lites have been studied in several investigations concerned withthe age and origin of Au-Ag mineralization at the nearby Buck­horn and Cortez mines (Wells et al. 1971; Rye et al. 1974).

The rhyolites crop out over an area of about 3 km2 andoccur in slightly dissected domes with pronounced flow banding(Figure 10). Topaz rhyolites from the vicinity of Horse Canyonhave K-Ar ages of 15.3 Ma (Wells et al. 1971) and 14.5 Ma(Armstrong 1970). They intrude or overlie slightly older flows ofbasaltic andesite (16.3 Ma) and the Ordovician Vinini Formation(siltstone, shale, and sandstone). Similar bimodal associations ofMiocene rhyolite and basaltic andesite are common in north­central Nevada (e.g., Shoshone Range and Sheep Creek Range).

22 Christiansen, Sheridan, and Burt

Cortez - Buckhorn, Nevada

iN

Tv

012, , ,km

Tba

BUCKHORN +MINE

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 toslightly older flows of basaltic andesite (Tba). Map symbols: Qts = undivided Quaternary and Tertiarysedimentary rocks; Tr = Miocene rhyolites; Tv = mid-Tertiary volcanic rocks; Ii = Jurassic intrusiverocks; Pu = undivided Paleozoic sedimentary rocks.

The rhyolites are generally phenocryst-poor (less than 3%)felsites with a few lithic fragments. The phenocrysts are smokyquartz, plagioclase and sanidine accompanied by sparse biotite.Miarolitic cavities are common and are lined with topaz, fluorite,and silica minerals (Wells et aI. 1971).

No published information exists for the major element geo­chemistry of the Miocene rhyolites. The average concentrationsofsome trace elements are given in Table 2. Their Rb-rich natureindicates that they are similar to other topaz rhyolites. In addition,Wells et aI. (1971) state that they also contain high concentra­tions of Sn (20 ppm) and Be (10 ppm). Initial strontium-isotoperatios determined on two felsites with relatively high Sr concen­trations are similar to those from the Thomas Range (Table 3;0.7086 ± 0.0002; 0.7080± 0.0002 calculated at 14.9 Ma) andare consistent with the involvement of a crustal component intheir generation. Lead isotope ratios for two samples of the rhyo­lite (Rye et aI. 1974) fall within the lower portion of the Area IIfield delimited by Zartman (1974). However, they have lower87Sr/86Sr and 207Pb/204Pb than many other felsic Area II rocksthat show evidence in terms of high initial 87Sr/86Sr,207Pb/204Pb, and 208Pb/204Pb ratios and low epsilon Nd (D. E.Lee, unpublished data; Stacey and Zartman 1978; Farmer andDePaolo 1983) for a significant sedimentary or metasedimentarycomponent in their sources. Nonetheless, these Nevada rhyoliteshave distinctly higher uranogenic Pb isotope ratios than the topazrhyolites from Lake City, Colorado, which suggests that thesources of the Cortez rhyolites are older or had higher U/Pbratios. However, it is important to note that the topaz rhyolites

are similar in their Pb-isotopic composition to geochemicallydistinct Oligocene volcanic rocks in the Cortez Mountains. Thus,the radiogenic Pb isotope ratios may have arisen by contamina­tion of both the rhyolitic magmas and the older Oligocene mag­mas by the same crustal reservoir. The Pb-isotope ratios may notbe inherited from their sources.

The Miocene rhyolites of the Cortez Mountains appear to bespatially and temporally associated with Au, Ag, and Hg mineral­ization at Horse Canyon and at the Buckhorn mine, which sug­gested to Wells et aI. (1971) that they were the source of theore-forming fluids. In contrast, on the basis of low D/H ratios inthe alteration minerals, Rye et aI. (1974) demonstrated that me­teoric water was the principal ore-fluid. Meteoric waters mayhave been heated by the rhyolitic magmatism creating a smallgeothermal system. The Au and Ag mineralization occurs in veinscontrolled by NNW-trending faults.

The Miocene volcanism in this area is apparently related tothe development of the Cortez (or Nevada) rift (Mabey et aI.1978; Stewart et aI. 1975). The rift coincides with a prominentaeromagnetic high that trends NNW across north-central Ne­vada, and is marked by voluminous basaltic andesite and rhyolitewith minor basalt (Figure 11). Rifting postdates Oligocene erup­tions of calc-alkaline rhyodacite to rhyolitic ash flows. The riftmerges to the north with the Snake River Plain-Columbia Rivervolcanic fields (Figure 12). Mabey et aI. (1978) suggest that theCortez rift was produced by the initial Cenozoic extension in thispart of the Basin and Range province. It is parallel to anp con­temporaneous with several other northwest-trending extensional

Topaz Rhyolites 23

Farther north along the Cortez rift, another topaz rhyolite

11. Sheep Creek Range, north-centralNevada

-----,--,----I Il II [I '.

\,'" ,,-

II

~

KM

oI

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 thewestern limit of the Precambrian continental crust (heavy dashed line) isshown along with other NW-trending tectonic features: DS =feederdikes for the CRE = Columbia River Basalt; WSR =western· SnakeRiver Plain graben; M = Monument dike swarm; OV = Orevada rift(Rytuba and McKee 1984). The subduction-related volcanic arc (CR =Cascade Range) active at the time was located near the continentalmargin. The topaz rhyolites of northern Nevada were erupted during thisperiod of NE-SW extension and basaltic volcanism and are contempo­raneous with peralkaline rhyolites in NW Nevada, NE California, andSE Oregon.

has been identified in the southern Sheep Creek Range near theold Izenhood Ranch, which lies north of Battle Mountain (Fries1942). The rhyolites are the hosts for cassiterite/wood tin miner­alization similar to that found in the Taylor Creek Rhyolite inNew Mexico.

Miocene rhyolites (ca. 14 Ma; Stewart et al. 1977a) extendnearly continuously from the Nevada-Oregon border south­southeast for 150 km in a zone about 40 km wide along theCortez rift (Figure 11). The rhyolites were apparently emplacedas coalesced domes and lava flows like those in the ThomasRange, Utah. Individual domes and flows cover from 3 to 100km2(Stewart et al. 1977a). In the Sheep Creek Range, the rhyo­lite intrudes a thick section of 14.8 Ma basaltic andesite (59%Si02) and is overlain by younger (10 Ma) lavas of olivine basalt(48% Si02) (Stewart et al. 1977a). The rhyolites are part of abroadly contemporaneous (14 to 16 Ma) series of basalts andbasaltic andesites that occur along the Cortez rift.

The topaz-bearing phase of the rhyolite, near IzenhoodRanch, is a crystal-rich (25 to 35%), flow-banded felsite. It is notknown if topaz is restricted to a single dome or a group of domes.

Daa~ Tr

Imm~~~ Tba

lI u

50I I

okm

Figure 11. Generalized geologic map of north-central Nevada showingthe distribution of 13.8 to 16.3 Ma lava flows and rhyolite domes (afterStewart et al. 1975). The volcanic rocks are concentrated along theCortez rift that opened 15 to 16 Ma. The locations of topaz rhyolites inthe Sheep Creek and Cortez Mountains are shown. Also shown are thetraces of the Golconda (GT) and Roberts Mountain (RMT) thrusts(Stewart 1980) that may mark the approximate site of the rifted marginof the Precambrian continent. Map symbols: Qa - Quaternary alluvium;Tr - Miocene rhyolite domes and flows; Tba - Miocene basaltic andesite,locally including andesite, basalt, and dacite of uncertain age; U - un­divided Tertiary, Mesozoic and Paleozoic rocks. Bold lines are normalfaults.

features in the northwestern United States, including the westernSnake River Plain graben and the vents for the Columbia Riverbasalts (Figure 12). The continuity of the rift is interrupted byyounger northeast-trending faults produced after reorientation ofthe regional stress-field about 10 Ma (Zoback and Thompson1978). The Cortez rift appears to have formed near the presumedeastern boundary of the Precambrian crystalline basement (Fig­ure 1), which also appears to limit the distribution of topaz rhyo­lites (Christiansen et al. 1983a).

24 Christiansen, Sheridan, and Burt

Northern Great Basin Chronologie Summary

x 0

-I I

~~x~g

0

\J

\

Agem.y.o

10

20

30

40

Basaltic orB. Andesite

Calc-AlkalineRhyolite

Comments

TDevelopment of

Snake River Plain

IE- W extension

1Cortez Rift opens

(NE-SW extension)

Calc-alkalineVolcanism

N. Nevada/UT.

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). Theeruption of the rhyolites followed the decline of calc-alkaline intermediate to silicic volcanism of theOligocene. The topaz rhyolites appear to be closely associated with the opening of the Cortez rift, thedevelopment of the Snake River Plain, and the eruption of basalt and basaltic andesite. Peralkalinerhyolites were erupted contemporaneously, but they are chemically and for the most part spatiallydistinct. Compiled from sources cited in the text and from Stewart and Carlson (1976).

The principal phenocrysts are quartz, oligoclase, and sanidinewith biotite, zircon, titanite, apatite, and Fe-Ti oxides as accesso­ries. The vapor-phase mineralogy as described by Fries (1942)consists of topaz, pseudobrookite, sanidine, silica minerals, fluor­ite, garnet, (described as andradite but it is spessartine­almandine), and possibly cassiterite. No pyroclastic deposits areexposed near the Izenhood Ranch locality and they are rare in thesouthern Sheep Creek Range (Stewart et al. 1977a).

The felsitic phase of the lava at Izenhood Ranch is composi­tionally similar to other topaz rhyolites (Table 1 and 2). How­ever, it is higher in Fe, Zr, Y, Ga, and Sr than most and towardthe low end ofJhe observed range for Rb (compare Figures 35and 38). Our preliminary studies of the chemical composition ofother rhyolite lavas in the area suggest that they have similarcompositions. The devitrified lava is fairly rich in uranium (12ppm; Christiansen et al. 1980); presumably the magma containedeven more, because uranium is generally lost during devitrifica­tion of fluorine-rich lavas (Bikun 1980; Christiansen 1980). Theinitial 87Sr/86Sr ratio of one sample is 0.7085 (Table 3).

Fries (1942) describes wood-tin and cassiterite in veinletsfrom the lava. The similarity of the mineral assemblage associatedwith the tin mineralization to that developed in miarolitic cavitiessuggests that the ores originated as fumarolic or pneumatolytic

incrustations in fissures produced by the cooling of the lava.Meteoric waters mobilized by the hot rhyolites were importantore fluids, but magmatic fluids released during devitrificationprobably produced the initial mobilization of Sn. Boiling wouldbe expected at such a shallow level.

The magmatic and tectonic setting of the rhyolitic volcanismis identical to that in the Cortez Mountains. Lithospheric exten­sion (NE-SW) along the Cortez rift was accompanied by theeruption of rhyolite and basaltic andesite. Along the rift zonebasalt is minor in Nevada (but more voluminous in Oregon).Judging from the age of the volcanic rocks, the rift formed 15 to16 Ma in a back-arc environment (cf. Snyder et aI1976). Peral­kaline rhyolites (Noble and Parker 1974), generally erupted fromcalderas, are also typical of this period (less than 20 Ma) in theGreat Basin of Nevada (Figure 13). For example, 16 Ma comen­ditic ash flows were erupted from the McDermitt caldera com­plex (Rytuba and McKee 1984), which developed on the westernflank of the northern Cortez rift.

12. Jarbidge, northern Nevada

The Jarbidge Rhyolite is a widespread volcanic unit innorthern Nevada along the southern margin of the Snake RiverPlain (Coats 1964). It correlates with similar rhyolitic lavas as far

Topaz Rhyolites 25

,';

east as the Utah line and as far south as the East Humboldt Range(Coats et al. 1977). Topaz has been identified in a single thinsection of the rhyolite (Coats 1964).

The rhyolite was emplaced as a series of volcanic domes andflows that are underlain locally by flow-breccias and pyroclasticunits. The country rocks through which it was erupted are slightlyolder volcanic rocks. Coats et al. (1977) report K-Ar ages for twosamples of Jarbidge Rhyolite as 16.8 and 15.4 Ma. The JarbidgeRhyolite is overlain by the voluminous Cougar Point Tuff (12.2Ma; Coats and Stephens 1968), which is related to the develop­ment of the Snake River Plain.

The lavas contain phenocrysts of quartz, sanidine, andoligoclase-andesine. The presence of small amounts of pigeoniticclinopyroxene and hornblende as well as the absence of biotitedistinguish the Jarbidge Rhyolite from most other topaz rhyolitesin the western United States. Accessory phases include garnet(apparently magmatic), zircon, apatite, and Fe-Ti oxides (Coats1964).

The average of three analyses of the Jarbidge Rhyolite arepresented in Tables 1 and 2. These rhyolites are similar to thosefrom the Sheep Creek Range in their high concentrations of Fe,K, and Zr relative to other topaz rhyolites. Nonetheless, thesefeatures are typical of bimodal rhyolites of the Snake River Plainand northern Great Basin (Wilson et al. 1983). The JarbidgeRhyolite is also lower in Rb (275 ppm) than most topaz rhyolites.Analyses presented by Coats et al. (1977) suggest that F (less than500 ppm) and Be (less than 5 ppm) are low in these lavas. Thescarcity of topaz is indicative of its lack of enrichment in incom­patible lithophile elements. Initial 87Sr/86Sr ratios for three sam­ples of the rhyolite collected by R. T. Wilson range from 0.7101to 0.7142 (Table 3); ratIos that are typical of Cenozoic rhyolitesfrom north central Nevada and the Snake River Plain (Wilson etal. 1983; Leeman 1982a).

The Jarbidge Rhyolite is the host for Au-Ag mineralizationthat yielded about $10 million in these commodities before 1942(Granger et al. 1957). The mineralization is found in epithermalquartz-adularia veins.

The Jarbidge Rhyolite is part of a bimodal assemblage ofsilicic (dacitic to rhyolitic) and basaltic volcanic rocks (Coats1964) that was erupted during the formation of the westernSnake River Plain graben (Armstrong et al. 1975; Leeman1982a). The lavas are approximately the same age as the basaltsand rhyolites in southwestern Idaho and the Columbia RiverBasalt ofIdaho and Oregon (Figure 12). The Cortez rift, 100 kmto the west, was developing at about the same time. These rocksmay be magmatic products of the initial NE-SW extension of thenorthern Basin and Range province (cf. Zoback et al. 1981).Later faults that cut the Jarbidge Rhyolite (and other correlativevolcanic units) are north-trending and block out the presentmountain ranges of northern Nevada (Figure 13).

13. Blackfoot Lava Field, southeastern Idaho

A group of five small rhyolite domes occurs near the southend of the Blackfoot Reservoir, about 15 km north of Soda

Springs, Idaho. D. R. Shawe (oral communication, 1982) andDayvault et al. (1984) report that topaz occurs as a devitrificationproduct in at least one of the domes, China Cap (called MiddleCone on some maps). The rhyolites erupted during the develop­ment of the predominantly basaltic Blackfoot lava field. Arm­strong et al. (1975) report K-Ar ages of 0.04 ± 0.02 Ma (wholerock), 0.08 ± 0.04 Ma (sanidine), and -0.1 ± 0.1 Ma (wholerock) for three specimens collected from these domes. Leemanand Gettings (1977) report concordant K-Ar, hydration rind, andthermoluminescence ages of 50,000 years, and G. B. Dalrymple(cited in Pierce et al. 1982) reports a 61,000 ± 6,000 year age forsanidine from the rhyolite at China Hat. These ages make thisrhyolite the youngest known topaz bearing rhyolite. Three othersmall rhyolite domes occur about 25 km north of China Hat inthe similar Willow Creek lava field. S. H. Evans (written com­munication, 1980) reports K-Ar (sanidine) ages of 1.56 ± 0.06and 1.28 ± 0.15 Ma for two specimens from these rhyolites.

The Blackfoot lava field lies in a NW-SE-trending Tertiarygraben flanked by mountain ranges that expose Paleozoic andMesozoic sedimentary rocks. Mansfield (1927) presents the mostcomplete geologic description of the region. Three rhyolite domes(China Hat, China Cap, and North Cone) lie on a NE-trendingline transverse to the graben structure. Vents for basaltic lavas arealso aligned along this trend. China Hat is the largest of the domesand has a maximum dimension of 2 km parallel to the lineament;it rises to a height of almost 300 m above the basaltic lavas. Twosmaller bodies of rhyolite occur as islands within the reservoir,one of which has a K-Ar age of 1.4 Ma. (S. H. Evans, cited inFeisinger et al. 1982). The domes are older than the basaltsexposed at the surface, but inclusions of still older basalt andandesite are found in the rhyolite lava (Mansfield 1927) and intephra (air fall and base surge deposits) beneath China Hat.Therefore, the rhyolites formed in the midst of the mafic magma­tism. Mabey and Oriel (1970) identified small positive aeromag­netic anomalies associated with the rhyolite domes and acompound gravity low over the portion of the lava field wherethe rhyolites occur. Based on an interpretation of the magnetichigh associated with the basalts, they suggest that the basalts areabout 1000 m thick. From the gravity expression of the field,Mabey and Oriel (1970) proposed that a caldera centered on therhyolite domes collapsed after extrusion of the basaltic lavas, or,alternatively, the anomalies were caused by a buried graniticintrusion. An interpretation more consistent with the tectonicsetting (typified by late Cenozoic basin and range faulting) andvolcanological style (rhyolite domes with many small low shieldvolcanoes and cinder cones in a plains-style basalt field) is thatthe lava field formed over a small sediment-filled graben. Thelow-density basin fill could produce the negative gravity anomal­ies without invoking caldera collapse, which is rarely associatedwith the eruptions of small, isolated rhyolite domes. Mabey andOriel (1970) interpret the structures of nearby Gem Valley injustsuch a manner. There, a slightly elongate gravity low is centeredon the principal basaltic shield volcano of the Gem Valley lavafield.

26 Christiansen, Sheridan, and Burt

The rhyolites of the Blackfoot lava field are petrographicallysimilar to one another. The domes consist of phenocryst-poor,generally devitrified, flow-banded lava. Groundmass texturesrange from glassy to granophyric. Small phenocrysts of resorbedquartz, oligoclase, biotite, and Fe-Ti oxides are ubiquitous. Apa­tite and zircon are common accessory minerals. Dayvault et al.(1983) identified euhedra ofthorite in glassy specimens as well asallanite, epidote, oxyhornblende, and unidentified grains ofa Ce­Th phosphate (monazite?) and a Nb-Y-Ti-Fe oxide. Tridymite,quartz, hematite, and topaz occur in lithophysae from China Cap.

The average major and trace element composition of sixspecimens analyzed by Dayvault et al. (1983) are presented inTables 1 and 2. The major element composition ofthese rhyolitesis indistinguishable from other topaz rhyolites with high Si andalkalies and low Ti, Fe, Mg, and Ca. Although none of thespecimens analyzed were obsidians, most contain glass in theirgrounclmass. As a result, fluorine concentrations are quite high(3500 to 5800 ppm) and may be close to magmatic concentra­tions. Chlorine concentrations are uniformly low (130 to 580ppm) yielding characteristically high FICI ratios (>9). The halo­gen ratios may have been disturbed by devitrification with thepreferential loss of Cl. Trace element concentrations in the rhyo­lites of the Blackfoot lava field are also typical of other topazrhyolites with elevated concentrations of Be (> 10 ppm), Li (>80ppm), Sn (>10 ppm), Y (> 150 ppm), U (> 15 ppm), and Th (27to 60 ppm). Feldspar compatible elements such as Ba and Sr arestrongly depleted.

No known mineralization is related to the topaz rhyolites ofthe Blackfoot lava field.

The Blackfoot lava field is centered about 60 km SE of themargin of the eastern Snake River plain in the northern Basin andRange province. Armstrong et al. (1975) outline the temporaldevelopment of late Cenozoic volcanism in southern Idaho. Theypoint out that a time-transgressive series of rhyolitic ash-flow tuffswere emplaced across southern Idaho. Volcanism began insouthwestern Idaho approximately 15 Ma and migrated to thenortheast to its present culmination in the Yellowstone area inWyoming. Basaltic volcanism was initiated in the wake of thecaldera-forming eruptions and has continued intermittently, creat­ing the Snake River plain. Some of these plains-style (Greeley1982) basaltic lavas were erupted from long fissures, but othersform low-shield volcanoes capped by small craters less than 1kmacross. The Blackfoot lava field is similar to the Snake River plainin several important ways, including its age and the style ofbasaltic volcanism, but the development of rhyolite domes late inthe history of a basaltic field is atypical of the Snake River Plain,although several such domes formed in the central part of theeastern plain. Likewise, no early ash-flow volcanism can be tiedto the Blackfoot field. Perhaps most importantly, the rhyolites ofthe Snake River plain proper are generally pyroxene andlorfayalite rhyolites, with moderately high Fe and Zr contents andhigh KINa ratios that show high equilibration temperatures(Leeman 1982a; Hildreth 1981; Hildreth et al. 1984; Wilson et al.1983); none of these characteristics are found in the Blackfoot

lava field rhyolites nor in topaz rhyolites in general (with thepossible exception of the Jarbidge Rhyolite, see above). Smithand Christiansen (1980) relate the formation of the Snake Riverplain to the passage of the North American plate over a mantleplume, while Christiansen and McKee (1978) relate its develop­ment to transform-style accommodation of E-W extensionalfaulting proceeding at different rates to the north and south of theplain. Because of the location of the Blackfoot lava field on thefloor of a fault-bounded graben, an extension of the Basin andRange province to the south, we prefer to place the developmentof the Blackfoot field into a context of basin-and-range extension,bllSalt intrusion, and bimodal basalt-rhyolite volcanism.

14. Elkhorn Mountains, western Montana

A group of Oligocene topaz rhyolites has been identified inand near the northern Elkhorn Mountains of western Montana.Chadwick (1978) has included them in the Helena volcanic field(Figure 14). The Elkhorn Mountains are bounded on all sides bymajor faults and were probably uplifted during the mid-Cenozoic(Smedes 1966).

The rhyolites in the Elkhorn Mountains were emplaced asisolated intrusive plugs, dikes, and small domes and lava flows.Some of the extrusive rhyolites overlie cogenetic pyroclastic de­posits. Inclusions of the Butte Quartz Monzonite, the dominantphase of the Cretaceous Boulder batholith, occur in the pyroclas­tic deposits erupted from centers within the mountains (Smedes1966). Most of the rhyolite masses are less than 1 km across; fourwere mapped by Smedes (1966). Chadwick (1978) obtained aK-Ar age of 35.8 Ma on a sample from the topaz-bearing rhyoliteat Lava Mountain. Two other similar rhyolite flows from the areahave been dated and have K-Ar ages of 37.3 and 36.9 Ma(Chadwick 1978; Figure 14).

Most of the rhyolite is felsitic, flow-banded, and phenocryst­poor. Obsidian is pres~rved along the margins of some dikes.Smoky quartz, sanidine, and sodic plagioclase{An10 to An12) arethe principal phenocrysts. The grounclmass is usually spherulit­ically devitrified or altered by vapor-phase crystallization. Topaz(up to 30 mm long), along with quartz and fluorite, line miaroliticcavities and lithophysae (Smedes 1966).

Smedes (1966) reports whole-rock chemical analyses ofseveral phases of the rhyolite from the Helena volcanic field thatdemonstrate their chemical similarity to other topaz rhyolites inthe western United States (Table 1). Two samples of "soda rhyo­lite" from the Elkhorn Mountains, which may be from the topaz­bearing units, contain an average of 8.9 ppm Uand 37.6 ppm Th(Tilling and Gottfried 1969). Greenwood et al. (1978) report thatthe topaz rhyolite from Lava Mountain contains high amounts ofF (0.2 to 0.5%), Sn (10 to 50 ppm), Be (1 to 20 ppm), Nb (30 to100 ppm) and moderate amounts ofMo (5 to 10 ppm).

Silver-bearing galena, sphalerite, fluorite, and quartz occurin veinlets cementing a brecciated phase of the rhyolites at LavaMountain (Smedes 1966). However, most of the Ag, Au, and Pbmineralization in the rocks of the Boulder batholith appears to be

Topaz Rhyolites

~CRATER MTN. Late Cenozoic Volcanism

~7.8U FIELD ,

In S W MontanaMONT 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:- 0 Rhyollite',' , (9 _ 1VtENT

, L a v a _ Sample dated in m.a.37 Mountain _

27

XButte

X Bozeman

o

o Rhyollite• BasaltJ;;l Trachyandesite, , ! 5,0KM

32.7 34.4 N\'"30.3 ",,\~G\,/ ~5.4 URNS

• c\"t \..\) 10 .-, HEPBSA38.9,>: 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 ELL OW STON 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 of Lava Mountain in the Elkhorn Mountains and otherrhyolites in the Helena and Beaverhead volcanic fields are about 10 Ma older than dated basalts fromthe rest of the region but may be part of a bimodal (basalt-rhyolite) suite associated with the inception ofblock faulting in the region (Chadwick 1981), Younger volcanic fields include those at Yellowstone andthe SW-trending Snake River Plain (SRP).

older than the rhyolitic magmatism (Smedes 1966). The BaldButte molybdenum prospect (Rostad 1978) is located about 25km north of the topaz rhyolite locality, but it is older (about 48Ma) and is more likely related to magmatism of the type de­scribed below from the Little Belt Mountains.

The rhyolites of the Helena volcanic field are part of abroadly bimodal assemblage ofbasaltic and silicic lavas that wereerupted in southwestern Montana after about 40 Ma (Chadwick1978, 1981). Individual fields do not generally contain bothmagma types, but basalt flows of uncertain age (Pliocene to Oli­gocene) do occur in the Helena field (Greenwood et al. 1978).This volcanic episode postdates dominantly calc-alkaline inter­mediate to silicic magmatism 45 to 55 Ma (Armstrong 1978) andmay mark the initiation of extensional tectonics and basin subsi­dence in western Montana (Pardee 1950; Chadwick 1978).Chadwick (1978) suggests that the Helena and other volcanicfields lie along the Montana lineament (or Lewis and Clark line)that marks the northern boundary of a Precambrian basementprovince (Weidman 1965), This lineament coincides with the

northernmost boundary ofbasin and range faulting and may haveacted as a transform allowing lithospheric extension and block­faulting to proceed south of it (Reynolds 1977). The tectonic andmagmatic history of the region is summarized in Figure 15.

15. Little Belt Mountains, central Montana

The oldest Cenozoic topaz rhyolites we know of (about 50Ma) are exposed in the Little Belt Mountains ofcentral Montana(Weed 1900; Pirsson 1900; Witkind 1973). The mountainousterrain exposes several major alkaline and calc-alkaline plutonsthat may be genetically related to the volcanic cover (Witkind1973).

The topaz rhyolites and their less fluorine-enriched counter­parts occur as sills and cylindrical plugs (Figure 16). Topaz hasbeen descnbed from a sheeted-sill complex that intrudes Cam­brian sedimentary rocks near Yogo Peak (Pirsson 1900) andwithin the rhyolitic "bysmalith" (a forcefully emplaced plug thathas pushed the overlying strata up along one or more circumfer-

28 Christiansen, Sheridan, andBurt

Montana Chronologie Summary

A

f \1 II II I

x

(\ \]

Last

Agem.y.20

30

40

50

60

Basalt Calc-Alkaline Rhyolite Comments

Beginning ofMiocene "lull"

Block-faultingbegins (?)

Mo-mineralizedplutons

Regional thrustsheets

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-alkalineand alkaline magmatism of central Montana. Compiled from sources cited in the text.

ential faults) at Granite Mountain (Pirsson 1900; Witkind 1973).The rhyolite at Granite Mountain has a K-Ar age of48.8 ± 2 Ma.

The rhyolite at Granite Mountain is described as being denseand fine-grained with a groundmass of quartz, alkali feldspars,biotite, and Fe-Ti oxides. Witkind (1973) classified topaz, alongwith albite, as phenocrystic. However, in view of the apparentlydevitrified nature of the rocks and its typical development else­where, the topaz is probably the result of growth from a vaporphase. The rhyolite sills at Yogo Peak are also fine-grained butcontain granules of tourmaline in addition to topaz in thegroundmass (Pirsson 1900)~both are probably the result of theexsolution of vapor.

Pirsson (1900) and Witkind (1973) report chemical anal­yses of the topaz rhyolites (Table 1). The sill analyzed by Pirssonmay have been slightly altered; it contains slightly more AI, Mg,Ca, and P than most topaz rhyolites. The rhyolite of GraniteMountain is typical of its class but has slightly lower Ti andhigher Mg than other topaz rhyolites. Rubidium is enriched(about 525 ppm) and Sr depleted (about 9 ppm) in this phase aswell (Witkind 1973), which is consistent with other occurrences.Rupp (1980) reports that a specimen from Granite Mountaincontains 3400 ppm F and is likewise enriched in Rb, U, and Nbbut not Sn (Table 2). As for most other topaz rhyolites Ba and Srare extremely depleted. Initial Sr-isotope ratios of two samplesfrom Granite Mountain average 0.7093 (t = 48.8 Ma) (Marvin etal. 1973).

No mineralization is directly associated with the rhyolites,

but an intermediate-composition stock of about the same age isrelated to Ag-Pb mineralization. Minor quantities ofmolybdeniteand scheelite occur in fissure veins within the stock (Witkind1973). The Climax-type(?) Big Ben molybdenite deposit (Olmore1979) is located less than 20 km to the west and has an age of49.5 Ma (Marvin et al. 1973) making it approximately the sameage as the topaz-bearing Granite Mountain rhyolite, as well as theBald Butte Mo-prospect in the Elkhorn Mountains.

The topaz rhyolites in the Little Belt Mountains are part ofacomplex assemblage of plutonic and volcanic rocks that were ailemplaced between 54 to 48 Ma (Marvin et aI. 1973). The plutonsand laccoliths are composed of felsic (70 to 72% SiOz) to inter­mediate (61 to 67% SiOz) composition rocks. Syenite, shonkinite,and lamprophyre (in places mixed with rhyolite in compositedikes) are also exposed in small bodies (Witkind 1973). In Mon­tana and Wyoming, Eocene magmatism was common 54 to 45Ma (Armstrong 1978; Figure15). Calc-alkaline and alkaline in­termediate to silicic volcanic rocks and subjacent plutons are theprincipal expressions of this activity. Lipman (1981) and Snyderet al. (1976) relate the magmatism to subduction of oceaniclithosphere near the continental margin. However, Armstrong(1978) suggests that intra-arc rifting and basin sedimentation mayhave been contemporaneous with this episode, which is correla­tive with the Challis volcanism of Idaho and arc-type volcanismand graben formation in northern Washington (Davis 1980).Monger and Price (1979) and Ewing (1980) also suggest that thesubduction-related volcano-plutonic arc formed nearer the con-

Topaz Rhyolites

o 5I I I I I I

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-Paleozoicsedimentary rocks

pC sedimentary rocks

pC igneous andmetamorphic 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 sameage. The location of the Big Ben molybdenite prospect is also shown.

29

I "I

tinental margin on a strike-slip faulted terrane. Thus, the Eocenevolcanism in Montana may have been "back-arc" in nature, inkeeping with the extensional faulting and the apparent tectonicsetting of other topaz rhyolites.

16. Specimen Mountain, north-central Colorado

Wahlstrom (1941) first reported topaz from gas cavities inrhyolite lava flows from Specimen Mountain on the crest of theFront Range in Rocky Mountain National Park. From Wahl­strom's (1941, 1944) descriptions it appears to be similar in itsmineralogy, chemistry, and emplacement history to other topazrhyolites.

The rhyolite lavas of Specimen Mountain (Figure 17)· areunderlain by a thick (lOOs of meters) sequence of pyroclasticdeposits (apparently composed of fall, flow, and breccia units).These in tum overlie a basal complex of trachyandesite (quartzlatite) lavas and pyroclastic deposits. Wahlstrom (1944) describes

the core of Specimen Mountain as an intrusive rhyolite plug thatformed shortly after the emplacement of the upper rhyolite lavas.All of the volcanic rocks overlie or intrude Precambrian gneiss,granite, and pegmatite. Fragments of these rock types also occuras inclusions in the vent agglomerates. A series of arcuate faultscut the plug and flows-apparently formed as the rhyolite plugcollapsed back down its conduit shortly after emplacement (see,for example, Corbett 1966, 1968). These relationships suggestthat the rhyolite flows and plug are part ofa small dome complex.The entire complex contains about 1.5 km3 of material.

The topaz-bearing lavas generally show conspicuous flowbanding and are locally rich in lithophysae. Quartz and sanidineare the only reported phenocrysts (Wahlstrom 1944). The rhyo­lite plug contains biotite, oligoclase, magnetite, and rare resorbedhornblende in addition to quartz and sanidine. No topaz is re­ported from this phase of the complex. Wahlstrom (1944) in­cludes the rhyolites and trachyandesites, with phenocrysticplagioclase, amphibole, biotite, and augite, as part of the same

30 Christiansen, Sheridan, and Burt

105· 50'

Specimen Mountain. CO

o 1, I ,

Km40.25' L-._......L --l

Figure 17. Generalized geologic map of Specimen Mountain, Colorado(after Wahlstrom 1944). Map symbols: Trp = rhyolite plug; TrZ = topaz­bearing rhyolite lava; Trt = rhyolitic tuff and lava; Tl = trachyandesite(quartz latite) lavas; pCg Precambrian gneiss and granite.

volcanic sequence and suggests that the rhyolites were derivedfrom them by crystal fractionation. It is difficult to test this hy­pothesis in the absence of more chemical information or preciseages for the trachyandesites and the younger rhyolites. Ifthe topazrhyolites were indeed derived from the trachyandesites, this com­plex may be unique among U.S. occurrences. However, a promi­nent Si02 gap (64.5% to 75.8% Si02 in nonhydrated samples) isobvious on variation diagrams. Wahlstrom (1944) also notesdisequilibrium features in the trachyandesites-ealcic plagioclasein intermediate composition lava and resorbed hornblende andbiotite-possibly suggesting that an even more mafic magmamixed with a rhyolitic magma before eruption to produce thetrachyandesite. These trachyandesites are chemically similar tothe magmatic inclusions in the Spor Mountain rhyolite in Utah,which show similar evidence of magma mixing.

Chemically, the topaz rhyolites are virtually indistinguisha­ble from others in the western United States and have high Si, K,and Na and low Ti, Mg, and Ca (Table 1, compare with Figure35). Analyses of the topaz-free rhyolite plug are similar but morevariable. For example, in the plug, Si02 ranges froni 68.9% to77.8% and Al203 ranges from 11.7% to 15.7%, perhaps as a resultof mixing or alteration. The dacites have potassic intermediatecompositions (64% Si02).

The rhyolites of Specimen Mountain are contemporaneouswith a variety of dacites and rhyolites (Corbett 1968) that wereemplaced between 27 and 28 Ma in north-central Colorado.Although the topaz rhyolite of Specimen Mountain is a lava, the

predominant volcanic rocks in the region are poorly studied rhy­olitic ash-flow tuffs. Their temporal and chemical relationships tothe topaz rhyolites at Specimen Mountain are unknown. Like­wise, the age ofan older series of basalts(?) and trachyandesites isnot established (Eocene to Oligocene) but they represent only asmall volume of the Cenozoic effusive rocks (Corbett 1968). TheNever Summer stock, a zoned granodiorite to quartz monzoniteintrusion that outcrops about 5 km west of Specimen Mountain,is also the same age (± 1Ma) as the silicic volcanic rocks (Corbett1968).

17. Chalk Mountain, central Colorado

Chalk Mountain is located on the west side of a narrowvalley that separates it from the Climax molybdenite deposit inLake County, Colorado (Figure 18). Cross (1886) first describedtopaz and garnet from cavities within this rhyolitic plug. Based onmagnetic and gravity anomalies, Tweto and Case (1972) sug­gested that the Chalk Mountain stock and the stock at the Climaxmine are both apophyses ofa batholith that underlies this portionof the Colorado mineral belt. Alternatively, Chalk Mountain maybe the downfaulted upper portion of the mineralized intrusivesystem (R. P. Smith, oral communication 1982). It lies on thewest side of the Mosquito fault, which separates it from theClimax deposits. In spite of this provocative suggestion, littleinformation has been published regarding the rhyolite of ChalkMountain. Although most references to Chalk Mountain term therhyolitic mass a stock, Cross (1886) describes it as extrusive inorigin. The rhyolite outcrops over an area of about 4 km2.

The rhyolite contains large phenocrysts of sanidine andsmoky quartz. Andesine (oligoclase in some) and biotite are set inan aphanitic groundmass that contains topaz, magnetite, rare il­menite, and apatite. Topaz also occurs in drusy quartz-lined cavi­ties along with garnet, sanidine, biotite, and Fe oxides (Cross andHillibrand 1885; Cross 1886; Pearl 1939). New analyses (byelectron microprobe) of biotite from the Chalk Mountain rhyoliteshow that it contains intermediate Fe/(Fe + Mg) ratios (ca. 0.5;Figure 31). Fluorine concentrations in the biotite are not excep­tionally high (averaging about 0.5 wt%) and moderate FICI ra­tios are typical (log FICI clusters around 1; Figure 32). In both ofthese respects the biotites are more similar to those from theMo-mineralized Pine Grove stock in southwestern Utah (Keith1982) than to biotites from the hydrothermally altered rocks ofthe Mo deposit at nearby Henderson, Colorado (Gunbw et al.1980). The compositions of abundant magnetite phenocrysts areuniform eXusp .ca. 0.9) and, when combined with analyses ofsparse ilmenite, indicate that high oxygen fugacities prevailed dur­ing crystallization of the Chalk Mountain rhyolite (log f02 about-10.3 at 830°C). This is about 3.5 log units above the QFMoxygen buffer (Figure 30). Apatite-biotite geothermometry, asformulated by Ludington (1978), gives sub-magmatic tempera­tures (-590°C) for apatite inclusions in biotite.

An analysis of the Chalk Mountain rhyolite published byCross (1886) shows that the rhyolite is similar to other topaz

Topaz Rhyolites 31

38

~ ~~SILVERCLIFF' •.

e • ROSITA .. "

'. ioo~~ ~Deer ~)eak~ 0,).

'.\~~ ~'.:

Volcanic Center Approximate Age Magma Type0 (Ma)

39 Deer Peak 38-32 andesite-trachyte (Iatite)Tomichi Dome 38 topaz rhyoliteBonanza volcanic field 38-33 andesite-dacite-rhyoliteMount Aetna caldera (MA) 36 rhyoliteSan Juan volcanic field 35-26 andesite-dacite-rhyolite39 Mile volcanic field 34 andesite

18 Waugh Mtn basaltNathrop Volcanics 29 topaz rhyolite

@ Silver Cliff-Rosita 32-26 andesite-trachyte-rhyolite,topaz rhyolite

Chalk Mountain 27 topaz rhyoliteBuffalo Peaks andesiteCripple Creek 29 phonoliteHillside <29 andesite-trachyte (Iatite)

San JuanVolcanic Field

San Luis Valley

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

rhyolites (Table 1). The principal differences lie in slightly higherA1203, MnO, and MgO contents than in most topaz rhyolites(compare Figure 35). The validity of the analysis is difficult toassess as no modern analyses of samples from Chalk Mountainhave been published.

Chalk Mountain contains small molybdenite occurrencesand small amounts of silver ore have been removed from therhyolite contacts (Pearl 1939). The Chalk Mountain rhyolite is27 ± 1.9 Ma (Tweto and Case 1972) and is one of numerousrhyolitic plugs, stocks, and dikes adjacent to the approximately30 Ma Climax stock (White et al. 1981), and is probably 00­

magmatic with this fluorine-rich rhyolite porphyry. An olderseries (Late Cretaceous) of diorite to granodiorite plutons appearto be the only other igneous rocks in the area. The mid-Tertiaryrhyolites are cut by the Mosquito high-angle normal fault, whichis part of the northward extension of the Rio Grande Rift. Theinitial tectonism associated with the development of the rift inColorado occurred 26 to 30 Ma (Tweto 1979) and was con­temporaneous with the magmatism at Chalk Mountain.

18. Nathrop, central Colorado

The Nathrop Volcanics lie on the west side of the MosquitoRange about 10 km north of Salida, Colorado. These rhyoliteswere first reported to contain topaz and garnet by Smith (1883)and Cross (1886). Van Alstine (1969) and Schooler (1982)further described their geology.

In general, the volcanic rocks rest directly on Precambriangneissic quartz monzonite and form low isolated hills along thefront of the Mosquito Range (Figure 19). At Ruby Mountain,where topaz and spessartine garnet are found in lithophysae,pumiceous tuff, and breccia (ca. 30 m thick), with fragments ofrocks from the Precambrian basement, are overlain by perlite (ca.35 m thick) that apparently forms the basal vitrophyre to a cap­ping rhyolite lava (ca. 100 m thick). The lava is flow banded withlithophysal and spherulitic textures. Sugarloaf Mountain is alsounderlain by a basal tephra and breccia unit. Scott (1975) sug­gests that all the rhyolite vented from Bald Mountain (Figure 19).However, the field study of Schooler (1982) demonstrates that

32 Christiansen, Sheridan, and Burt

Figure 19. Simplified geologic map of the Nathrop Volcanics from cen­tral Colorado (after Scott 1975; Van Alstine 1969; and Schooler 1982).The rhyolites near Nathrop include lavas (cross-hatched) and tuffs (solidcolor). Three vents probably exist near Nathrop and a fourth at BaldMountain.

vents also existed at Sugarloaf Mountain, Nathrop Butte, andRuby Mountain.

The rhyolite lavas contain sparse phenocrysts of sanidine(Or6oAb38), oligoclase (Or8Ab81; yielding a two-feldspar tem­perature of 630°C), smoky quartz, and traces of biotite (VanAlstine 1969). The groundmass contains chlorite, topaz, magne­tite, and fluorite. Lithophysal cavities in the lava contain topaz,spessartine garnet, sanidine, silica minderals, magnetite, hematite,opal, and calcite.

The major and trace element chemistry ofthe Nathrop rhyo­lite has been reported by several investigators; analyses are pre­sented in Table 1. The major element chemistry is similar to alltopaz rhyolites with high Si, K, Na, Fe/Mg, and F and lowconcentrations of Ca, Ti, and Mg. Zielinski et al. (1977) reportthree trace element analyses ofseparate phases of the lava (Table2) and show that the vitrophyre is enriched in Mo and the litho­phile elements U, Th, Be, Li, and Nb relative to most rhyolites;these values are typical of topaz-bearing varieties (Christiansen etal. 1980). The REE concentrations are shown diagramatically inFigure 40g where the Nathrop rhyolite is compared to a calc­alkaline rhyolite from Summer Coon volcano (Oligocene agefrom the San Juan volcanic field; Zielinski and Lipman 1976).Although the pattern is reminiscent of those from other topazrhyolites, it has higher La/LuN and Eu/Eu* than most. A single

new Sr-isotope analysis of a whole-rock sample suggests that itsinitial 87Sr/86Sr ratio is relatively high (0.714 ± 0.0060, Table3). The large uncertainty is a result of the high Rb/Sr ratio anduncertainty of its age. (29.3 ± 1.5 Ma was used-the oldest ofthree ages reported by Van Alstine 1969. Other K-Ar ages are29.1 ± 0.9 and 28.0 ± 0.8 Ma.) Additionally, the effects of evensmall amounts of upper crustal contamination are readily appar­ent in rocks with low Sr content (3.2 ppm) like the Nathroprhyolites. For example, 1%assimilation of a component with 300ppm Sr and an 87Sr/86Sr ratio ofO.nO (values not unrealistic forthe Precambrian country rocks at 30 Ma) would double thepresent Sr content and elevate the initial ratio from 0.706 to0.713. Any conclusions based on this isotopic analysis must betempered by these facts.

The Nathrop Volcanics are spatially associated with fluoritedeposits that formed in a near-surface hot-spring environment attemperatures of 119° to 168°C (Van Alstine 1969). Van Alstineassigns the deposits a post-Miocene age and does not relate theirformation to the fluorine-rich rhyolite volcanism.

The topaz rhyolites from Nathrop are located near the west­ern margin of the Thirtynine Mile volcanic field (Figure 18; Episand Chapin 1968). Rhyodacitic to rhyolitic ash flows are theoldest volcanic units in the field. They were probably eruptedfrom centers west of the Thirtynine Mile volcanic field (e.g., the36 Ma Wall Mountain Tuff; Chapin and Lowell 1979). TheThirtynine Mile volcanic field consists predominantly ofandesiticlavas and breccias with minor amounts of basaltic lavas, dioriteplugs, and rhyolitic dikes (Epis and Chapin 1968; Epis et al.1979). Presumably, the volcanism was related to the develop­ment of a composite volcano about 34 Ma. A rhyodacitic ashflow was erupted at about the same time as the Nathrop Volcan­ics (the 29 Ma old Gribbles Park Tuff; Steven 1975) and overliessome of the intermediate composition lavas. The topaz rhyolitesof Nathrop (K-Ar age 28-29 Ma; Van Alstine 1969) thus appearto be part ofan andesite-rhyodacite-rhyolite series ofcalc-alkalinenature. Nonetheless, strongly alkaline volcanism at Cripple Creek(75 km east) is contemporaneous (Steven 1975). A markedchange in the nature of the magmatism occurred about 18 Mawhen the "andesite" of Waugh Mountain was erupted in thesouthern part of the Thirtynine Mile volcanic field (Wobus et al.1979). These lavas are similar in age and composition to "silicicalkalic basalts" of a post-Oligocene episode of bimodal basalt­rhyolite volcanism (Lipman and Mehnert 1975; 133). Tweto(1979) estimates that the Arkansas Valley graben, adjacent to theNathrop rhyolites, began to subside approximately 28 Ma. Theapparent superposition of the rhyolite dome complex on some ofthe range-front faults (Figure 19) suggests that the tectonism andvolcanism may have been nearlyconcutrent. The tectonic andmagmatic activity of central Colorado are summarized in Figure20.

The Silver Cliff and nearby Rosita volcanic fields are 10-

19. Silver Cliff-Rosita, central Colorado

Xgd

-+-+-+- Dike

- Fault

o 2kmI I

EXPLANATION

IQal! Quaternary alluvium

~ Nathrop volcanics (28-29 m.y.)rhyolite

M'i:;<] Wall Mountain Tuff (36 m.y.)\""~" rhyodacite

IXgdl Precambrian granodioriteand quartz diorite

Arkansas

Qal

Valley

Graben

Topaz Rhyolites 33

Colorado Chronologie Summary

Comments

Miocene ·'u/l"

Spanish Peaksintrusions

Renewed upliftranges

Iulons

Iulons +Rio Grande Rift

~ opens

II, Ir--

'I~ ,

x IMOP"---

l)

{(iIMO P

( )y

Calc-AlkalineBasalt Alkaline A-D-R Rhyolite

40

20

30

10

Agem.y.o

Figure 20. Schematic representation of magmatic and tectonic activity in southern Colorado. The agesof topaz rhyolites (x) show that they were erupted in at least two episodes-one contemporaneous withhigh-K calc-alkaline magmatism (andesite-dacite-Iow silica rhyolite) and the other group contempo­raneous with basalt (as used by Lipman and Menhert 1975) and high silica rhyolite. The ages ofmolybdenum-mineralized stocks (Mo plutons) of the Climax-type are also shown. The term calc­alkaline is used here to denote igneous rock series whose members contain greater than about 60% Si02and generally display continuous silica variation diagrams. Basalt or basaltic andesite is used where nointermediate composition rocks are observed.

cated along the northeastern side of the Wet Mountain Valleygraben (Figure 18). Cross (1896) first reported topaz and garnetfrom rhyolitic lavas from these volcanic fields. Subsequent inves­tigations by Siems (1968), Kleinkopf et aI. (1979), and W. N.Sharp (1978) have demonstrated that the volcanism in the twoareas occurred contemporaneously 32 to 26 Ma. The rhyolites inboth fields have been related to poorly documented collapsecalderas-an unusual mode of occurrence for topaz rhyolites.Both complexes are small; neither covers much more than30km2.

Acording to Siems (1968) the initial eruptions at Silver Cliffwere pyroclastic eruptions that emplaced a thick sequence ofnon-welded rhyolitic tuff and breccia on the Precambrian base­ment. Apparently these deposits accumulated in a subsiding basinor were preserved by caldera collapse (Figure 21; Siems 1968).Tephra accumulations exceed 600 m as exposed in mine shafts;low residual gravity anomalies suggest they are contained in atrough-shaped graben (W. N. Sharp, cited in Scott and Taylor1975). Extrusion of rhyolite domes and flows with basal vitro­phyres closed the volcanic cycle at Silver Cliff. The lavas are 40to 50 m thick and have ages of 27 to 26 Ma.

The volcanic center at Rosita, centered about 8 km south-

west of Silver Cliff (Figure 21), produced a more diverse group ofrocks and spanned a much longer period of time. Early eruptions(32 to 29 Ma) of smaIl volumes of andesite and rhyolitic tephrafrom the Deer Peak volcanic center (Figures 18 and 21) werefollowed by extrusions of (rhyo-)dacitic lavas. The rhyolitic pro­ducts of this early phase of volcanism are grouped together as Troand the non-rhyolitic rocks as Tmo in Figure 21. After a 1 to 2Ma lull a younger sequence of trachyte and trachyandesite (latite)lavas (Tmy) was accompanied by the eruption of topaz rhyolitelavas (27 to 26 Ma). (Most of the mafic lavas in both sequenceshave potassic affinities.) Siems (1968) described this center as anincompletely developed resurgent caldera, but documents nolarge-scale collapse or structural resurgence. W. N. Sharp (1978)cites the formation of dikes and fissures as evidence for resur­gence. The duration of the activity and the interlude between theolder and younger volcanic events suggests that two perhapsunrelated volcanic cycles are represented at Rosita. (The dates arefrom Scott and Taylor 1975; and W. N. Sharp 1978.)

Modern analyses (phair and Jenkins 1975) show that therhyolites from Silver Cliff-Rosita are very similar to. all othertopaz rhyolites in their major constituents (Table 1). Two rhyolitespecimens have an average U content of 20 ppm and Th content

34 Christiansen, Sheridan, and Burt

38

39

40

104

104105

106107

108

108

\~ ,\) 0 Specimen Mtn

~ f 127•28

)

~ ~/Chalk Mtn OHenderso~ ~(27-28)\( -9

38 IT r~: ~::o::~0 ~Chmax" \ \

Boston Peak - ~ LNathrop "\R~und MIn r \~28-29)

Tomichi Dome _(38)

• ~ Sitv", Cliff (26)

La~~8~lty ~ ~\ \ \y \\I~; /\ I

40

38

Figure 22. Faults with major Neogene movement in the area of the RioGrande rift in Colorado (after Tweto 1979), compared to the location oftopaz rhyolites (filled circles) described in the text. The ages of therhyolites are shown in parentheses. The Rio Grande rift began to form 30to 27 Ma (Eaton 1979; Tweto 1979), about the same time as the topaz­rhyolite magmatism was initiated in the region. The locations of topaz­bearing molybdenite deposits (0) younger than 30 Ma (White et aI.1981) are shown for comparison.

Pliocene time (Scott and Taylor 1975). Apparently this tectonicepisode occurred shortly after the development of several vol­canic centers on the eastern side of the graben (Scott and Taylor1975)-Silver Cliff-Rosita (andesite to rhyolite, 32 to 26 Ma),Deer Peak (andesite to trachyandesite, 38 to 32 Ma), and Hillside(andesite to trachyandesite, less than 29 Ma). The locations ofthese fields are shown in Figure 18. The tectonism and magma­tism of the Wet Mountains appear to be part of the developmentof the Rio Grande rift system in Colorado (Tweto 1979; Eaton1979) and are contemporaneous with the eruption of other topazrhyolites in Colorado (compare Figure 22).

20. Tomichi Dome, central Colorado

Tomichi Dome is located about 35 km east of Gunnison,Colorado. Stark (1934) reported topaz from the rhyolite as partof a study of heavy minerals from Tertiary "intrusions" in centralColorado. The geology of the rhyolite (Figure 23) is described byStark and Behre (1936) and briefly by Ernst (1980).

o 5I ""

+sc

km

Figure 21. Generalized geologic map of Silver Cliff-Rosita area, Colo­rado (after W. N. Sharp 1978). Topaz-bearing rhyolitic lava flows anddome (Trf) are underlain by tephra and sedimentary rocks (Trt) thataccumulated in a small NW-trending graben at Silver Cliff (8C). Rhyo­lites surround a central core of older (Tmo) and approximately contem­poraneous (Tmy) mafic volcanic rocks (andesite, trachyandesite, andtrachyte) at Rosita (R). Vents (*) and mineralized breccia pipes (+) arealso shown. Both volcanic centers are located near a fault (dashed) thatseparates asediment-(QTs)-filled graben from Precambrian gneisses andgranitic rocks (peg) of the Wet Mountains. A portion of the Deer Peakvolcanic center is also shown in the SE comer of the map.

Silver Cliff/Rosita, Colorado

1050

20'

of 31 ppm. Trace element analyses of vitrophyres reported byMutschler et al. (1985) show the rhyolites to contain 850 to 1200ppm fluorine and relatively low concentrations of incompatibleelements like Rb (<250 ppm) and Li (<25 ppm). The rhyolitesare nonetheless enriched in Nb and depleted in Zr (-100 ppm),Sr «20 ppm), and Ba «60 ppm) in common with other topazrhyolites. Many analyses (Cross 1896; Mutshler et al. 1985) offelsitic specimens have high K20/Na20 ratios that suggestalteration.

Deposits of hypogene and supergene silver, gold, lead, zinc,and copper are associated with the Tertiary volcanic rocks. In theSilver Cliff district the deposits are cavity fillings or replacementsin the lavas and tuffs. The most productive mines are brecciapipes that formed within the Precambrian gneisses; their relation­ship to the volcanic cycle is unclear but their similarity to ventfacies rocks suggest that they are also related to the youngerrhyolites. In the Rosita Hills district, mineral deposits occur infIssure veins and along faults in all of the volcanic units.

The Wet Mountain Valley is a tectonic basin that began toform in late Cretaceous or early Eocene times. However, majoruplift of the flanking ranges occurred in early Miocene to late

Topaz Rhyolites 35

Tomichi Dome, Colorado

• Summit

- - -- Fault

t, ~ :j Spherulitic rhyolite, .

2!

1I

o!

krnTertiary rocks

t~;:H?ll Massive rhyolite

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

As its name suggests, the rhyolite caps a domical mountainthat rises about 600 m above a generally flat region. The rhyolitewas emplaced through Cretaceous sedimentary rocks (sandstone,shales, and limestones). The initial eruptions resulted in the em­placement of a poorly-exposed basal explosion breccia and tuffover 200 m thick. Ernst (1980) identified a breccia pipe (ca. 500m across) on the northeastern margin of the rhyolite. Fragmentsof sedimentary rocks and Precambrian granite are included in thebreccia pipe and in the pyroclastic deposits. Overlying the tuff is aphenocryst-poor rhyolite lava flow or dome. The lower 300 m oflavas have spherulitic textures and the upper part is a denserequigranular rock with a granophyric groundmass and prominentflow banding. The exposed dome is 2 to 3 km in diameter and thevolume of rhyolitic rock exposed is approximately 3 to 4 km3.

A sill, composed of a rock similar to that in the main mass,intrudes shales at the base of the dome. The sill is 6 to 10m thickand a thermal contact aureole extends away from it for severalmeters into country rock.

The principal phenocrysts are biotite, sanidine, oligoclase,and smoky quartz. The matrix consists of these same mineralsand some glass. Magnetite, zircon, and apatite are magmatic ac­cessories. As devitrification products, topaz and garnet occur inirregular clots, and biotite crystals occur in radiating groups in theupper part of the lava. Garnet, of unspecified association, is alsopresent in the basal tuffaceous sequence. Small quantities ofhornblende, ilmenite, and titanite were found in heavy mineralseparates but were not observed in thin section (Stark 1934).

Ernst (1980) analyzed eight specimens from Tomichi Domefor major and trace element concentrations. An average of twoanalyses of the upper topaz-bearing part of the dome is given inTable 1 and 2. In many ways the analyses are typical of othertopaz rhyolites from the western United States, but compared to

others of this group they contain relatively low concentrations ofU (4 to 7 ppm) and Rb «300ppm). Felsites contain about 0.17wt%F.

F. E. Mutschler (written communication, 1983) reports awhole-rock K-Ar age of38 Ma for Tomichi Dome. This is 9 Ma'older than any other topaz rhyolite in Colorado and makes itcontemporaneous with the calc-alkaline magmatism of the Bo­nanza and San Juan volcanic fields to the south (Varga and Smith1984; Lipman et aI. 1976).

21. Boston Peak, central Colorado

Three vent complexes of topaz rhyolite are exposed at Bos­ton Peak (Ernst 1980), which lies about 40 km northwest ofTomichi Dome in Gunnison County, Colorado (Figure 22). Allof the rhyolite plugs are small; the largest is only about 800 macross. No bedded pyroclastic deposits are associated with theemplacement of the Boston Peak plugs but a breccia pipe isexposed adjacent to one of the rhyolite vents. Angular to sub­rounded fragments of underlying sedimentary units and Precam­brian granite are included in the breccia pipe.

The rhyolites contain phenocrysts of resorbed quartz, sani­dine, albite (An 3 to 6), biotite, zircon, and Fe-Ti oxides. Avitrophyre is preserved at the margin of one of the vents, other­wise the phenocrysts are contained in fine-grained, flow-banded,and felsitic matrix. Lithophysae with concentric shells of quartzand topaz are common; zeolites flil some of the cavities. Fluoritecrystals occur in fractures in the phenocrysts and, along withtopaz, garnet, and tourmaline, are post-magmatic. Muscovite,probably as an alteration product, occurs as small grains in thegroundmass of two of the rhyolite plugs.

The average of six analyses (Ernst 1980) of rhyolites from

36 Christiansen, Sheridan, and Burt

Figure 24. Generalized geologic map of the Lake City area, southwesternColorado (after Steven et al. 1977; and Lipman et al. 1978). Plugs oftopaz rhyolite (cross-hatched) are nestled within the rim of the Uncom,pahgre caldera (formed 28 Ma), but are much younger (18 to 19 Ma).The rhyolites are also younger than the Silverton/San Juan (28 Ma) andthe Lake City (22.5 Ma) calderas and probably represent a distinctvolcanic episode in this area.

\\

\,...Te

EXPLANATION

.'.._Caldera:....... .... wall

Older ash flows andlavas (30-26 m.y;)

Early intermediate compositionlavas arid tuffs (35-30)

o 5L...L..L...L.J.

km

Te

Lake City, Colorado

~ Post-caldera mafic laval~ Rhyolite plugs (18-19 m.y.l

I@ I Granitic Intrusive Rocks

r:fs;l Sunshine Peak Tuff~ (22m.y.)

Ernst (1981) reports chemical an,alyses of the rhyolite plugs(Table 1) that show them to be typical high-silica rhyolites. Manyof the samples have high Kz0/NazO, indicating post-magmaticredistribution of alkalies; no major-element analyses of vitro­phyres have been published. A variety of trace element analysesreported by various investigators is given in Table 2. Ernst(1981) reports that the rhyolites have relatively high Sr (ave. 126ppm) and Ba (600 ppm) for topaz rhyolites, but his analyses alsodemonstrate an enrichment in Rb and Li. Semi-quantitative anal-

the three vents at Boston Peak are presented in Tables 1 and 2.The major element compositions of these rhyolites are in all wayslike their counterparts elsewhere; they show none of the composi­tional "anomalies" of the Tomichi Dome rhyolites. A vitrophyricspecimen contains 0.51% F; felsites contain 0.12 to 0.49% F. Inaccord with these relatively high F concentrations, the rhyoliteshave relatively high concentrations of incompatible trace ele­ments, e.g., Li (90 to 270 ppm), Rb (390 to 820 ppm), U (7 to 24ppm; highest in the vitrophyre), and Nb (35 to 160 ppm). Thefeldspar-compatible elements, Ba (<75 ppm) and Sr «10 ppm),are strongly depleted.

The age of the rhyolites is not known, but they lie on aNW-trend that includes the rhyolites at Mt. Emmons and Red­well Basin (17 Ma-a molybdenum mineralized rhyolitecomplex that bears topaz; Thomas and Galey 1982; J. E. Sharp1978), Treasure Mountain dome (12.5 Ma; Obradovich et aI.,1969), Round Mountain (14 Ma; Cunningham et aI. 1977), andTomichi Dome (38 Ma Mutschler, written communication1983). With the probable exception of Tomichi Dome, theseplugs appear to be part of a post 20 Ma bimodal suite of basaltand high-silica rhyolite typical ofsouthwestern Colorado (Figure20).

22. Lake aty, southwestern Colorado

An ENE-trending line of topaz-bearing rhyolite plugs, sillsand laccoliths is exposed in the northwestern portion of the SanJuan volcanic field (Steven et aI. 1977). The trend starts about 5km north of Lake City, Colorado. The belt of intrusions extendsfor 20 km and consists of about 10 separate bodies that are alldescribed as being mineralogically and chemically similar to oneanother. The geologic setting, mineralization potential, and traceelement chemistry of these rhyolites are reported by Steven et aI.(1977), and isotopic analyses (Pb, Sr) of one plug are presentedby Lipman et aI. (1978a). The petrology of these rhyolites and therocks of the Lake City caldera are presently being studied by R.A. Zielinski and K. Hon of the U.S. Geological Survey.

The rhyolites (Figure 24) were emplaced as discordant in­trusions, generally less than 1 km across, within the volcanic andsedimentary fill of the Uncompahgre caldera, which formed 28Ma. All of the intrusions are small, with maximum dimensions of1 to 2 km; Although none of the rhyolites are reported to beextrusive, the preservation ofmarginal vitrophyres and the forma­tion of lithophysae suggests that they were emplaced at veryshallow levels. K-Ar dating indicates they are 18.5 Ma (Lipmanet aI. 1978a); at least 8 Ma younger than other dated topazrhyolites in Colorado.

The rhyolites are generally devitrified and light gray in color.Reported phenocrysts include abundant quartz and sodic sanidinealong with sparse biotite and oligoclase. Titanite is a prominentmicrophenocryst (R. A. Zielinski, oral communication, 1982)along with apatite, zircon, and sparse Fe-Ti oxides (Ernst 1981).Small crystals of topaz and fluorite occur within some cavitiesand along fractures.

Topaz Rhyolites 37

yses (Table 2) also show that the rhyolites are enriched in otherincompatible trace elements: Be (7 to 20 ppm), Nb (20 to 80ppm), Pb (30 to 70 ppm), Sn (up to 10 ppm), Y (up to 20 ppm),and Mo (up to 15 ppm). Delayed neutron activation analysesshow that the rhyolites are also enriched in U (10 to 42 ppm) andTh (26 to 64 ppm). Fluorine concentrations, even in "vitro­phyres," range erratically from 500 to 1300 ppm, values that aremuch lower than for most other topaz rhyolites. Vitrophyres havehigher values of U, Be, and Mo than their devitrified equivalentsand Steven et al. (1977) suggest that these elements were releasedfrom the lavas during cooling and crystallization in the form ofhalogen complexes. The REE patterns for the topaz rhyolites ofLake City display prominent middle REE depletion (R. A. Zie­linski and K. Hon, written communication 1982) that may haveresulted from the fractionation of titanite. These patterns are sim­ilar to those of the titanite and topaz-bearing rhyolites from theMineral Range, Utah. The Sr- (0.7054) and Pb- isotopic compo­sition of these rhyolites are relatively unradiogenic (Table 3) andimply that they may have arisen from a source with relatively low

-Rb/Sr, Th/Pb, U/Pb and Th/Pb ratios. Lipman et al. (l978a)suggest that the source for these rhyolites (and other similarMiocene-Pliocene rhyolites) is in amphibolite-facies lower crustbecause of the inferred low Th/U ratio of the source. Low Th/Uratios are atypical of some, but not all, exposed granulite-faciesmetamorphic terranes.

All of the young Lake City rhyolites are anomalously ra­dioactive (scintillometer readings over them are about two timesthat found over their hosts-Steven et al. 1977) and they havebeen extensively prospected for uranium. In 1960, a small quan­tity of uranium ore was removed from a supergene deposit nearone of these plugs (Steven et al. 1977). Much of the mineraliza­tion in the San Juan Mountains is associated with the emplace­ment of young (less than 22 Ma) silicic magmas that are grosslysimilar to the Lake City rhyolites but generally topaz-free (Lip­man et al. 1976).

. The initial eruptions (35 to 30 Ma) in the San Juan volcanicfield formed clusters of central volcanoes composed ofandesitic torhyodacitic lavas and breccias. Large ash-flow eruptions of moresilicic (about 72% Si02) magmas occurred 30 to 26 Ma, at thesame time as the Rio Grande rift was developing farther east (Fig­ures 20 and 22). Eruptions of minor volumes ofandesitic to daciticlavas accompanied resurgence of the calderas. About 20 to 25 Ma(early Miocene) volcanism in the San Juan Mountains hadchanged to a bimodal assemblage of basalt (or basaltic andesite)and "high-silica alkali rhyolite" (Lipman 1981). The topaz rhyo­lites at Lake City are included in this latter group. The only calderain the San Juan volcanic field to develop during this episode wasthe Lake City caldera (22.5 Ma) with which the topaz rhyolites(18.5 Ma) are spatially associated. It is important to note that thetopaz rhyolites of the San Juan Mountains are younger than otherdated topaz rhyolites from Colorado (26 to 38 Ma versus 18.5 Ma)and they do not occur within the Rio Grande rift system (Figure22). In addition, contemporaneous mafic lavas are alkali basalts,not andesites or their derivatives (Lipman and Menhert 1975).

Topaz rhyolites in Colorado: A summary

Although geochemically similar, there appear to be at leasttwo separate groups of topaz rhyolites in Colorado (Figure 20)­an older Oligocene group emplaced before about 25 Ma (repre­sented by Specimen Mountain, Chalk Mountain, Nathrop, SilverCliff-Rosita, and possibly Tomichi Dome, which is 38 Ma) and ayounger Miocene group emplaced after about 22 Ma (repre­sented by the Lake City group and perhaps the plug at BostonPeak). The Oligocene group is contemporaneous with the waningstages of generally calc-alkaline volcanism (characterized by ex­tended Si02 variation diagrams) in the San Juan Mountains andother centers on the flanks of the Rio Grande rift. Their distribu­tion coincides with the locus of Neogene tectonism as they pre­ceded or were contemporaneous with the development of the rift.The topaz-bearing stocks at the Climax and Henderson molybde­num mines are probably part of this group. The younger group oftopaz rhyolites were erupted as part ofa clearly bimodal group ofbasaltic (or basaltic andesite) lavas and rhyolitic lavas and tuffs.The locations of the younger rhyolites are not limited to the riftproper, instead they occur on the western side of the rift. Inaddition, they are not clearly associated with faulting related tothe still active Rio Grande rift (Tweto 1979). The rhyolitic stocksat Mount Emmons and Redwell Basin (topaz-bearing Climax­type molydenite deposits; J. E. Sharp 1978; Thomas and Galey1982) are included in this younger group.

23. East Grants Ridge, west-central New Mexico

Grants Ridge is a discontinuous basalt-capped mesa thatslopes southeast towards Grants, New Mexico. The eastern partof Grants Ridge is underlain by a rhyolite dome complex thatcontains topaz and garnet in lithophysae (Kerr and Wilcox1963). The volcanism is related to the development of the ande­sitic composite volcano at Mount Taylor, which is centered about12 km to the northeast (Figure 25).

The initial volcanic activity at Grants Ridge is representedby a rhyolitic tuff. The tuff includes black obsidian bombs andlarge xenoliths of Precambrian(?) granitic rocks. Precambrianrocks are not exposed at the surface and must lie several 1OOOs ofmeters deep in the vicinity of Grants Ridge (Thaden et al. 1967).The tephra probably formed a low tuff cone and are comprised ofpyroclastic flow, surge, and fall deposits. Separate perlite andfelsite domes rose through the tuff; both contain topaz and garnet.The perlite dome is intimately flow banded and Kerr and Wilcox(1963) suggest that hydration occurred under magmatic condi­tions and is not the result oflow temperature hydration of obsid­ian at the surface. The lava dome consists of a felsitic rhyolitecore surrounded by a collar or rind of obsidian and perlite. Flowbanding is well-developed in all phases of the body and suggeststhat the dome has a concentric internal structure. Lithophysae,some 10 cm in diameter, are common in the outer part of thedome. Rhyolite from the dome has K-Ar dates of3.2 Ma (Bassettet al. 1963) and 3.3 Ma (Lipman and Mehnert 1980). The rhyo-

38 Christiansen, Sheridan, and Burt

pTr

Qb

pTr

aCubero

r---.---I'l-~R

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

oI

10I

20I

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 lineamentof 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 andtuffs including the topaz-bearing rhyolite at East Grants Ridge; Tb - Pliocene basalt; pTr - pre-Tertiarysedimentary rocks.

lites are partially covered by slightly younger olivine basalt flowsthat contain inclusions of the rhyolite. Several scoria cones rep­resent the final activity at the basalt vents. Although the basaltextrusions have not been dated, similar mafic lavas that form partof the Mount Taylor volcanic field have ages that range from 2.9to 1.6 Ma (Lipman and Mehnert 1980).

The rhyolites at East Grants Ridge are all phenocryst-poorbut contain small phenocrysts of quartz, sanidine, and sodic pla­gioclase. Devitrification of glass has produced a light-grey felsiticgroundmass in most of the dome. Vapor-phase crystallization ofgarnet, topaz, and tridymite occurred in lithophysae that are pres­ent in obsidian, felsite, and some phases of the perlite dome.

Baker and Ridley (1970) report an average of two rhyoliteanalyses from the Mt. Taylor volcanic field, but do not give thelocations of the samples. The average is similar to other topazrhyolite analyses reported in Tables 1 and 2 and shows the char­acteristic depletion ofTi, Mg, and Ca. Our trace element analysesalso show that the Grants Ridge rhyolites are similar to othertopaz rhyolites with enrichments ofF (0.5% in a vitrophyre), Li,Rb, and Sn. Baker and Ridley (1970) report two rhyolite analysesfor Rb (602,537 ppm) and Sr (nd., 2 ppm) that are in accordwith these analyses and typical of other topaz rhyolites. Zielinski

(1978) reports that the uranium contents of the lavas at GrantsRidge range from 7 to 8 ppm, relatively low concentrations fortopaz-rhyolite lavas. Baker and Ridley (1970; citing P. Pushkar)report that a dacite from Mt. Taylor has an initial 87Sr/86Sr ratioof 0.7193 and believe that it was derived by mixing of maficmagma with a rhyolitic magma (represented by the domes andtuff) derived by partial melting of the crust.

No known mineralization is associated with the rhyolites.The Cenozoic volcanism is unrelated to the large uranium depos­its of the Grants region that are Late Jurassic to mid-Cretaceousin age (Brookins 1980).

Mount Taylor consists of an andesite composite cone withearly high-K basaltic andesites (trachytes) or dacites (4.44 Ma;Lipman and Mehnert 1980) and rhyolitic tuffs exposed in a cen­tral amphitheater. The cone is composed of porphyritic andesitelavas, and developed 2.4 to 2.9 Ma. The surrounding mesas arecapped by a differentiation series of high-K basalts to andesite(alkali basalts to trachytes; Crumpler 1980) that developed con­temporaneously 4.3 to 1.5 Ma (Lipman and Mehnert 1980; Lip­man et al. 1979). The rhyolites and basalts of Grants Ridge arepart of the peripheral volcanism. The development of the Mt.Taylor volcanic field was concurrent with minor north-northeast

(rI

!'I.

Topaz Rhyolites 39

faulting in the area. Lipman and Mehnert (1980) relate the devel­opment of the volcanic field to activity on the east-northeast­trending Jemez Lineament or Springerville-Raton zone (Figure25). Volcanism of similar age occurred elsewhere along this beltthat stretches from southwestern Arizona to northeastern NewMexico.

24. Black Range, southwestern New Mexico

The Taylor Creek Rhyolite is a topaz-bearing lava that cropsout in the northern Black Range of southwestern New Mexico(Fries 1940; Fries et al. 1942; Ericksen et al. 1970; Lufkin 1976,1977; Correa 1980). The rhyolite has been extensively studied inpart because it contains low-grade concentrations of tin as cassit­erite and wood tin. The Black Range lies on the eastern marginof the Mogollon-Datil volcanic field that was active from about40 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 (5to 15 separate vents may exist). Pyroclastic material that pre­ceded the lava eruptions is exposed at the present margins of thelavas and presumably underlies the lavas as well. Locally, anautoclastic breccia forms the base of the flows. Flow-banding iswell-developed and reveals large folds that show the internalstructure of the domes and lava flows. The rhyolite has K-Ar(sanidine) ages of 24.6 Ma (Elston 1978) and 27.7 Ma (Ratte etal. 1984) from the same locality. In any case, the extrusion of thedomes probably spanned a considerable length of time. Accord­ing to Ratte et al. (1984) the Taylor Creek Rhyolite may repre­sent ring-fracture volcanism of the Bursum caldera (source of theBloodgood Canyon and Apache Springs Tuffs), which formed29-28 Ma. About 30 m of Bloodgood Canyon Tuff overlie theTaylor Creek Rhyolite (Ratte et al. 1984) discounting the sugges­tion that the Taylor Creek Rhyolite is the devolatilized post­eruption residue of the Bloodgood Canyon magma chamber assuggested by Rhodes (1976). Nonetheless, it is just as likely to beunrelated to the development of any of the large calderas (T.Eggleston, oral communication 1985). The distribution of thelava and the location of some of the calderas are shown inFigure 26.

Most of the Taylor Creek Rhyolite is composed of devitri­fied and vapor-phase altered lava. It contains 20 to 40 percentphenocrysts of quartz and sanidine with lesser amounts ofplagio­clase. Biotite and ferrohornblende are present in most samples;ferroaugite (Mg24Ca3SFe41) occurs in biotite and hornblende-freeflows (Correa 1980). Accessory phases include zircon, titanite insome specimens, Fe-Ti oxides (mostly titaniferous magnetite),and fayalite(?) (Lufkin 1976, 1977). Vapor-phase minerals thatoccur in miarolitic cavities and veinlets include quartz, alkalifeldspar, hematite, bixbyite «(Mn,Feh03), pseudobrookite(Fe2TiOs), cassiterite, topaz, monazite, fluorite, (Lufkin 1976),garnet (Fries et al. 1942), and beryl (Kimbler and Haynes 1980).Inclusion-ridden topaz crystals in miarolitic cavities at RoundMountain reach lengths of 2 to 3 cm. Although granophyric

lIT] Calc-alkalic rhyolite

50,

KM

Figure 26. Tectonic map of southwestern New Mexico showing therelationship of the topaz-bearing Taylor Creek Rhyolite to the majorCenozoic faults and calderas in the region (Elston 1978, 1984; Ratte etaL 1984). Mid-Tertiary calderas are indicated by dashed lines and nor­mal faults by heavy solid lines. The Mogollon caldera (M) was thesource of the Cooney Tuff and appears to have formed 34 Ma. The GilaCliff Dwellings caldera (GCD) formed 30 to 29 Ma and may have beenthe source of the Davis Canyon and/or Shelley Peak Tuffs. The largeBursum caldera (B) formed 29 to 28 Ma and was the source of theBloodgood Canyon and Apache Springs Tuffs. The relationship of theTaylor Creek Rhyolite (24.6 to 27.7 Ma) to these calderas is open toquestion.

textures are the most common, spherulitic devitrification texturesare also present. The upper parts of the flows are vapor-phasealtered and contain the most abundant gas cavities.

The Taylor Creek Rhyolite is chemically similar to mosttopaz rhyolites (Table 1) with high Si, K, Na, and Fe/Mg andlow Mg, Ca, Ti, and P (Correa 1981). Vitrophyres from the flowcontain up to 0.4% F and are enriched in Sn and Rb (Table 2).Eggleston and Norman (1984) mistakenly reported that the lavaSwere Cl-rich: The Taylor Creek Rhyolite contains less D, Ta, andTh than some of topaz rhyolites from western Dtah (Figure 35),but is still enriched when compared to other high-silica rhyolitesfrom the region. In addition, it is decidedly rich in Y (> 100 ppm).The REE pattern of the rhyolite (Figure 40b) has a deep Euanomaly (Eu/Eu* = 0.07), low La/LuN (2.4), and low La/CeN(Correa 1981), typical of topaz rhyolites in general. Egglestonand Norman (1984) report that the delta 180SMOW value for therhyolite is 8 permil, indicating a crustal history for the magmas'source. A single Sr-isotope analysis of a whole rock sample of theTaylor Creek Rhyolite suggests that its initial 87Sr/86Sr is veryhigh (0.71583 ± 0.0028; Table 3). The usefulness of this result isquestionable both because the sample contains only 3.7 ppm Sr

40 Christiansen, Sheridan, and Burt

New Mexico Chronologie Summary

Agem.y.o

10

20

30

40

Basaltic Calc-AlkalineBasaltic Andesite Rhyolite

'i.\ X

AI

I~x

?

Comments

Mt TaylorIntraplate Block

FaultingJemez

QuestaBack-arcExtension

+Rio Grande Rift

opens

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

(thus it could have been easily contaminated at the magmaticstage) and because the sample was a hydrated vitrophyre. (Hy­dration may radically change the Sr content and hence Sr­isotopic composition of ·vitrophyres: e.g., Hargrove 1982). Adetailed Br-isotope investigation of the Taylor Creek rhyolite isbeing conducted by D. Norman and T. E. Eggleston (oral com­munication 1985).

Tin mineralization in the form of cassiterite and wood-tinveinlets (Lufkin 1976, 1977) deposited as a result of fumarolicactivity is common in the upper parts of the Taylor Creek Rhyo­lite (Correa 1980; Eggleston and Norman 1984). The mineraliza­tion is restricted to the flanks of intensely vapor-phaserecrystallized zones just below the carapace of the domes~ Fluidinclusion and oxygen isotope studies indicate that quartz andtopaz crystallized from saline magmatic fluids (8180 = +6 to +10%0) at temperatures over 600°C (Eggelston and Norman 1984)and miarolitic cassiterite-hematite-quartz was deposited attemperatures above 500°C (Rye et al. 1984); probably also fronimagmatic fluids derived by degassing of the rhyolite lavas. Latecassiterite crystallized at 150 to 200°C from boiling fluids withcalculated 8180 of ~6 to 0 %0 (Eggleston and Norman 1984).The wood-tin depositing fluids appear to have contained a largemeteoric component. Placer deposits of Sn derived from the lavashave been mined on a small scale. Wood-tin mineralization is

also associated with topaz rhyolites in Nevada and Mexico (seebelow). No known F, Be, or U mineralization is directly aSso­ciated with the Taylor Creek Rhyolite. However, F, Be, Fe, W,Sn, and U mineralization occurs in skarns and along faults nearIron Moutnain, east of the Black Range (Jahns 1944; Hillard1969). A specimen from the aplitic intrusions associated with theskarns has a K-Ar age of29.2 Ma (Chapin et al. 1978) indicatingthat these magmas are approximately the same age as the TaylorCreek Rhyolite.

The mid-Terti~ry magmatism of the Mogollon-Datil prov­ince has been interpreted to be the product of three overlappingmagma suites (Elston and Bornhorst 1979; Figure 27). A calc­alkaline andesite to rhyolite suite (43 to 29-28 Ma) producedcomposite volcanoes and slightly younger compositionally zonedsilicic ash-flow tuffs. This volcanism·appears to have been con­temporaneous with subduction of oceanic lithosphere at thecontinental margin. Back-arc (or intra-arc) extension led to thedevelopment of a bimodal basaltic andesite and high-silica rhyo­lite suite that extended from 30 to 19 or 18 Ma. The third stage isrepresented by flows of tholeiitic and alkalic basalt and high-silicarhyolite with ages ranging from 19 Ma to less than 1 Ma. Thissuite was erupted during a period ofblock faulting. A somewhatdifferent picture was presented by Ratte et al. (1984).They groupthe high-silica rhyolitic ash flows and lavas ofElston's and Born-

Topaz Rhyolites 41

Figure 28. Geologic sketch-map of the volcanic geology near BurroCreek, Arizona (after Burt et al. 1981b). The topaz-bearing rhyolitedome at Negro Ed (Tvst) is underlain by a pyroclastic unit (Tvsp). Othersmall rhyolite domes (Tvs) in the area contain vapor-phase garnet andare underlain by cogenetic tephra (Tvsp). The rhyolites lie on a Precam­brian basement of gneisses and granites (pC). Normal fault and sense ofmovement shown as dashed line.

garnets are spessartine-almandine solid solutions (Figure 34) withup to 3000 ppm fluorine (analyzed by ion chromatography andelectron microprobe). They may represent the "almandine" oc­currence at the southern end of the Aquarius Range mentionedby Anthony et al. (1977).

An average chemical composition of vitrophyres fromgarnet- and topaz-bearing lavas is given in Table 1 and traceelement analyses in Table 2 (Moyer 1981). The average is typicalof topaz rhyolites. Fluorine concentrations range from 1400 to2600 ppm; chlorine concentrations are 4 to 5 times lower.

Although a number of lithophile-element deposits (Be, W,V) occur in the region" most are associated with anorogenicgranites of Precambrian age (Anderson 1983). However, Burt etal. (1981) have suggested that the fluorine-rich rhyolites or sim­ilar rocks may have been the source some of the uranium depos­ited to the south at the Anderson mine (Sherborne et al. 1979).

The topaz rhyolites ofBurro Creek are part of a more or lessbimodal assemblage of late Cenozoic volcanic rocks from west­ern Arizona (cf. Suneson and Lucchitta 1983). In the vicinity ofBurro Creek, Moyer (1981) has delineated a series of low-silicarhyolite lavas (ca. 70% SiOz) and sub-alkaline basalts to basalticandesites (48 to 56% SiOz) that are broadly contemporaneouswith, but slightly younger than, the aphyric (topaz-bearing) lavas.Tuffs emplaced during dome-forming eruptions are interlayeredwith basaltic lavas in the region. A period oflate Cenozoic listricnormal faulting was followed by high-angle normal faulting be­tween 14 and 7 Ma in western Arizona (Suneson and Lucchita1983). The magma-tectonic setting of this region appears to besimilar to that found elsewhere in the Basin and Range provincewhere topaz rhyolites occur in association with basaltic lavas andextensional faulting.

Old() ()

LU::;;:>10­< <::c,>0'<:::;1>-

3.4 37' 30"

Tvsp

~TVSP

l..-Io 1 kilometer

'0'"'",

'"0",

Negro Ed

p€

N

Tvs

'" '

!f1'" ","<,,'" :I

II,

D',,,,,,

horst's intermediate stage with the oldest suite, interpreting themas being the first erupted parts ofzoned (dacite-rhyolite-high silicarhyolite) magma chambers. Ratte et al. (1984) suggests that the"ring-fracture" rhyolites were also erupted from these zonedchambers. These authors place voluminous post-caldera andesitesin a "fundamentally basaltic" suite (26 to 23 Ma) but also recog­nize a bimodal suite of basalt and high-silica rhyolite that ap­peared about 19 Ma, long after the eruption of the Taylor CreekRhyolite. Rhodes (1976) and Ratte et al. (1984) suggested that acomposite granitic batholith underlies the volcanic field. Thetopaz rhyolites may have evolved by fractional crystallization anddehydration of one of these magma bodies following collapse ofone of the large calderas (Rhodes 1976, suggests the Gila CliffDwelling caldera and Ratte et al. 1984, suggest the Bursum cal­dera). However, Correa (1980) speculates that the Taylor CreekRhyolite may have arisen from a separate magma body not di­rectly related to these calderas.

26. Burro Creek, western Arizona

Information about the topaz rhyolite near Saddle Mountainin the Galiuro Mountains of southeastern Arizona is sketchy.Anthony et al. (1977) state that topaz, together with pseudo­brookite, spessartine garnet, and bixbyite, occurs in a rhyolite inthe Winkelman area. The topaz rhyolite was not located during abrief visit· to the area. The rhyolite nearest Saddle Mountain(which is formed by resistant andesitic lavas) is described byKrieger (1968) as anintrusive plug. Biotite from the rhyolite hasa K-Ar age of 61 Ma. Younger (ca. 24 Ma) rhyolitic lavas occur3 to 5 km northeast of Saddle Mountain.

25. Saddle Mountain, eastern Arizona

Rhyolitic lavas with lithophysae containing garnet and/ortopaz occur in eastern Mohave County, Arizona (Burt et al. 1981;Moyer 1981). Topaz and garnet occur in a mesa identified as"Negro Ed" on the 7lf2 minute quadrangle of the same name(V.S. Geological Survey 1980). The hill is formed by the rem­nants of a flat-topped rhyolite lava (Figure 28). A basal vitro­phyre (l to 3 m thick) is locally disrupted and forms part of aflow-produced breccia. Locally, a thin (less than about 50 mthick in places thinning to about 1 m) pyroclastic deposit and/orexplosion breccia with fragments of the Precambrian countryrock is exposed at the base of the complex. The dome is morethan 200 m high and 2 km across. Several other small domes inthe area contain vapor-phase garnet in lithophysae or along morecoarsely crystalline flow bands. The ages of the rhyolites are notknown, but they are probably late Miocene or Pliocene. Mesa­capping basaltic lavas in the area have K-Ar ages of 8 to 9 Ma(Shafiqullah et al. 1980).

The rhyolites are all phenocryst-poor (3 to 5%) with sparsephenocrysts of quartz, sanidine, oligoclase, biotite, and Fe-Ti ox­ides. Spherulitic devitrification is the most common groundmasstexture in the felsites. Moyer (1981) reports that the vapor-phase

42 Christiansen, Sheridan, and Burt

OTHER "TOPAZ RHYOLITE" OCCURRENCES

Several other occurrences of topaz in or associated withvolcanic rocks have been reported that: 1) are not sufficientlydocumented to warrant separate discussion; 2) are not Cenozoicin age; or 3) are not from the western United States. Brief discus­sions of these "other" occurrences are presented here for com­pleteness and for comparative purposes.

Other Cenozoic occurrences, western United States

Trimble and Carr (1976) report "topaz(?)" in heavy mineralseparates from the tuff of Arbon Valley along the southern mar­gin of the Snake River Plain in Idaho. A topaz locality in south­central Idaho is shown on Shawe's (1976) distribution map oftopaz rhyolites in the United States. At this locality, topaz oc­curred with fayalite, allanite, chevkinite(?) (D. R. Shawe, oralcommunication 1982) and zircon in a stream sediment concen­trate. The stream drains Tertiary rhyolites (rhyolites of MagicReservoir) on the north side of the Snake River Plain and partof the Idaho batholith and thus the topaz may not be from arhyolite at all. Leeman (1982a, b) reports that the rhyolite domeat Moonstone Mountain (which is drained by the sampledstream) is a high-silica rhyolite lava with 0.2% fluorine. Perhapsthis 3 to 6 Ma rhyolite is the source of the topaz. Other F-richrhyolite domes occur near Magic Reservoir. However, Bennett(1980) reports that the 44 Ma Dismal Swamp stock (whichintrudes the southern part of the Idaho Batholith) contains topazand beryl and is anomalously radioactive. It is not known if thisMo-mineralized intrusion has a rhyolitic phase, but such plutonscould also be a source of the topaz. Blixt (1933) reports topaz andfluorite of uncertain paragenesis associated with gold mineraliza­tion in the North Moccasin Mountains of central Montana. VanAlstine (1969,1974) reports the occurrence oftopaz in the upperpart ofa devitrified rhyolitic (68 to 73% Si02) ash flow from nearPoncha Springs in central Colorado. The vitrophyre of the ashflow is chemically dissimilar to topaz rhyolites and contains only0.08% fluorine. The ash flow appears to correlate with the 36 MaWall Mountain Tuff mentioned earlier.

Mexican topaz rhyolites

The youngest volcanic sequence in the Sierra Madre Occi­dental of Mexico contains numerous topaz rhyolites (e.g. Foshagand Fries 1942; Pan 1974; Huspeni et al. 1984; Ludington et al.1984; Ruiz et al. 1985). The rhyolites commonly contain tinmineralization like that described from the Black Range in NewMexico (e.g. Ypma and Simmons 1969; Huspeni et al. 1984;Duffield et al. 1984).

Most of the occurrences lie in a southeast-trending belt ex­tending from Durango to near Mexico City (Figure 29) thatparallels the main Tertiary volcanic belt of the region. The rhyo­lites overlie mid-Tertiary andesitic to rhyolitic lavas and tuffs(Foshag and Fries 1942; Ympa and Simmons 1969; Cameron et

al. 1980). The topaz rhyolites were erupted from 32-27 Maduring the climax ofthe mid-Tertiary calc-alkaline magmatism ofthe Sierra Madre Occidental (Huspeni et al. 1984; Cameron etal. 1980). In the states ofDurango and Zacatecas, the tin-bearingvarieties occur as subvolcanic plugs and as extrusive domes andlava flows. They intrude other volcanic units near the margins ofcalderas. Some are covered by crystal-rich rhyolitic ignimbritesthat may have erupted shortly after the emplacement of the lavas,leading some authors to suggest that the Sn rhyolites are directlyassociated with the caldera-related rhyolites (Huspeni et al.1982; Ruiz et al. 1985). Our own field observations and those ofothers (e.g. Ludington et al. 1984) suggest that this interpretationrequires further study.

Most of the F-rich lavas are phenocryst-poor with pheno­crysts of quartz, plagioclase (AnlO-20), sanidine (OrSO-70), andtraces of ferroaugite and Fe- and F-rich biotite (Pan 1974; Hus­peni et al. 1984, Ruiz et al. 1985). Crystallization temperaturesare relatively low (650 to 780°C) as determined by two-feldspargeothermometry (Pan 1974; Huspeni et al. 1984). One notabledifference from topaz rhyolites in the United States may be thepresence of fayalite (Ympa and Simmons 1969; Pan 1974) insome tin rhyolites (though not necessarily those that bear topaz).In rhyolites with moderate fluorine contents (0.1 to 0.2%) theexchange of Al for Fe in the vapor-phase mineralogy stabilizesgarnet over fayalite, and biotite is generally the stable Fe-bearingmagmatic phase.

Huspeni et al. (1984) and Ruiz et al. (1985) report that theF-rich lavas are geochemically similar to topaz rhyolites from thewestern United States. Like their counterparts to the north, theMexican tin rhyolites are high-Si02, metaluminous to slightlyperaluminous lavas, with marked depletions ofTi, Mg, Ca, and P(Table 5). Many of the analyses reported have anomalously highK20/Na20 ratios (> 1.5) similar to but higher than those foundin tin rhyolites from the Black Range, New Mexico, and theSheep Creek Range, Nevada. The role of post-magmatic pro­cesses, such as alkali metasomatism during devitrification, needsto be examined, but "fresh vitrophyres" show similar ratios. Fluo­rine concentrations in two vitrophyres were 2000 and 3000 ppm;no CI analyses were reported. Their trace element concentrationsare indistinguishable from topaz rhyolites in the western UnitedStates (Figure 41), with the typically high concentrations of Rb,Cs, Ta, Th, U, and variable enrichment of Sn (<20 ppm). REEpatterns show deep Eu anomalies and low La/YbN ratios. Intheir high Y·(>100 ppm) and total REE content they are mostsimilar to the Sn-mineralized varieties in the Black Range, NewMexico, and the Sheep Creek Range, Nevada. Initial 87Sr/86Srratios of the tin rhyolites range from 0.7054 to 0.7075 (Huspeniet al. 1984; Ruiz et al. 1985) and are slightly higher than asso­ciated calc-alkaline (i.e. F-poor) volcanic rocks.

Precambrian topaz rhyolites

Topaz rhyolite magmatism in not strictly a Cenozoic phe­nomenon in western North America. Topaz (and fluorite) occurs

Topaz Rhyolites

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

Miles

Figure 29. Distribution of topaz rhyolites in Mexico (compiled from sources cited in text). 1- America­Saporis, 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 alsocontain topaz. For the most part the F-rich lavas occur along the eastern margin of the Tertiarycalc-alkaline volcanic belt of the Sierra Madre Occidental. Also shown are the location of the presentlyactive volcanoes (stars).

43

in Precambrian rhyolites of the Keewatin District in theNorthwest Territories of Canada (Le Cheminant et al. 1981).The anomalously radioactive rhyolites are part of a bimodalmagmatic suite related to an early Proterozoic rift. Topaz has alsobeen found in heavy mineral concentrates from a Precambrianmeta-rhyolite tuff from the Flying W Ranch area, west of Young,Arizona (Conway 1976). The origin of the topaz is equivocal. Itmay be metasomatic and related to younger beryl mineralizationin nearby quartz veins.

PRINCIPAL CHARACTERISTICS OFTOPAZ RHYOLITES

From the occurrences reviewed above, all Cenozoic topazrhyolites from the western United States appear to be remarkablysimilar in terms of their mode of emplacement, mineralogy,chemistry, associated ore deposits, and volcanic-tectonic setting,despite their wide distribution and diversity of ages. These sim­ilarities are reviewed below. The implications of these characteris­tics are also considered in companion papers (Christiansen et al.1983a; Burt et al. 1982).

Distribution and ages

Topaz rhyolites are widespread in western North Americaand their occurrence closely coincides with the limit oflate Ceno­zoic extensional faulting (Christiansen et al. 1983a). In the UnitedStates, their emplacement appears to have spanned most of theCenozoic Era (Table 6). Their isotopic ages range from 50 Ma(LIttle Belt Mountains, Montana) to 0.06 Ma (Blackfoot lavafield, Idaho), although all but 3 are younger than 30 Ma. InMexico, isotopic ages of topaz rhyolites cluster at 27 to 32 Ma(Huspeni et al. 1983; Ruiz et al. 1985).

Most known topaz rhyolites in the western United States liewithin the eastern and southern Basin and Range province andalong the Rio Grande rift and thus appear to surround the Colo­rado Plateau. As far as is known, no topaz rhyolites occur in thewestern Great Basin region of California, Nevada, or Oregon, inspite of this area's contemporaneous bimodal (basalt-rhyolite)volcanism and extensional faulting. No topaz rhyolites have beenidentified to the west of the initial 87Sr/86Sr = 0.706 line asdetermined for Mesozoic plutonic rocks (Figure 1; Kistler 1983;Armstrong et al. 1977) or Cenozoic silicic volcanic rocks (Wilson

44 Christiansen, Sheridan, and Burt

TABLE 5. AVERAGE COMPOSITION OF TIN RHYOLITESFROM NORTHERN MEXICO.

1 S.D. 2 S.D. 3 S.D.

Si02 76.9 0.60

Ti02 0.07 0.01

A1203 13.0 0.15

Fe203* 1.14 0.08

MnO 0.02 0.00

MgO 0.13 0.10

CaO 0.56 0.38

Na20 2.76 0.10

K20 5.~5 0.36

P205 0.01 0.00 0.00 0.01

0.21

77.0 0.12

0.06 0.0

12.8 0.12

1.14 0.09

0.04 0.02

0.09 0.06

0.30 0.21

3.91 0.08

4.67 0.05

3.73 0.28

5.04 0.35

0.15 0.07

0.133 0.37

1.12 0.19

0.06 0.01

21 5

5.6 0.8

75.6 0.71

0.14 0.09

12.8 0.38

0.00 0.01

0.23 0.09

510 42

114 15

2

6

0.1

18

17

10

95

807

4.7 0.2

73 21

16 5

0.31

548 8Rb

U

Zr

Ta

F

Note: All analyses in weight percent or ppm andrecalculated H20, C02 and S02 free. S.D. is onestandard deviation. Fluorine analyses from vitrophyres.

*Total Fe as Fe203

1. Average of 3 "host-rhyolites" for Sn mineralization atSombrete, Zacatecas (Huspeni et al. 1984).

2. Average of 4 "host-rhyolites" for Sn mineralization atAmerica-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 thewesternmost extent of the Proterozoic craton in the westernUnited States. Farmer and DePaolo (1983) suggest from their Ndand Sr isotope studies of granitoids from the region that thesialic continental margin lies farther inland (where eNd = -7 or87Sr/86Sr = 0.708; Figure 1), but this datum is not well­constrained in central Nevada. These crustal discontinuities, ex­pressed structurally as the Roberts Mountain and Golcondathrusts in central Nevada, mark the eastern limit of a series ofallochthonous or "suspect" terranes composed of ocean-floor orisland arc crust (e.g. Speed 1979). These terranes may haveformed as oceanic crust at the margin of North America duringthe Paleozoic and early Mesozoic Eras (Oldow 1984). Given acrustal origin for the parental magmas of topaz rhyolites and theabsence of topaz rhyolites in this region, the young, mafic crustdoes not appear to have a composition appropriate for the gener­ation of topaz rhyolites.

Mode ofemplacement

Nearly all of the topaz rhyolites described in this report wereemplaced as endogenous lava domes with or without lava flows(Table 7). Several topaz rhyolites were also emplaced as smallintrusive domes or plugs that may not have vented to the surface(e.g. Lake City, Colorado, Chalk Mountain, Colorado, Little BeltMountains, Montana, and some occurrences in the Wah WahMountains, Utah); their fme grain-size, the presence of miaroliticcavities and glassy margins suggest that even they were emplacedat very shallow levels. The extrusive rhyolites are generally under­lain by pyroclastic deposits that appear to be remnants of tuffrings formed by base-surge eruptions (cf. Sheridan and Updike1975; Wohletz and Sheridan 1983a). The pyroclastic depositsgenerally consist of a lower, near-vent ("peel-back") breccia thatrepresents vent-clearing explosions. Breccia fragments may comefrom as deep as 1 km (e.g. East Grants Ridge, New Mexico). The

Topaz Rhyolites

TABLE 6. AGES OF CENOZOIC TOPAZ RHYOLITES IN THEWESTERN UNITED STATES

Location Age (Mal Reference

I. Thomas Range, UT 6-7 Lindsey 19812. Spor Mountain, UT 21 Lindsey 19813. Honeycomb Hills, UT 4.7 Turley and Nash 1980

4. Smelter Knolls, UT 3.4 Turley and Nash 19805. Keg Mountains, UT 8 Lindsey et al. 19756. Mineral Mountains, UT 0.5 Lipman et al. 1978b

7. Wah Wah Mountains, UT 20-18 Lindsey and Osmonson 197812 Best et al. 1985

8. Wilson Creek R, NV 22.6 Barrot 19849. Kane Springs Wash, NV 13.4 Novak 1984

10. Cortez Mountains, NV 15 Wells et al. 1971II. Sheep Creek Range, NV 14 Stevlart et al. 197712. Jarbidge, NV 16 Coats et al. 1977

13. Blackfoot Lava Field, ID 0.06 Pierce et al. 198214. Elkhorn Mountains, NV 36 Chadwick 197815. Little Belt Mtns, MT 50 Witkind 1973

16. Specimen Mountain, CO 28-27 Corbett 196817. Chalk Mountain, CO 28-27 Tweto and Case 197218. Nathrop, CO 28-29 Van Alstine 1969

19. Silver Cli ff-Ros ita, CO 26 Sharp 197820. Tomichi Dome, CO 38 F.E. Mutschler unpub.2I. Boston Peak, CO Ernst 1980

22. Lake City, CO 18.5 Lipman et al. 1978a23. Grants Ridge, NM 3.3 Bassett et al. 196324. Black Range, NM 28 Ratte et al. 1984

25. Saddle Mountain, AZ Anthony et al. 197726. Burro Creek, AZ L Cenozoic Burt et al. 1981b

45

breccia is commonly overlain by stratified pyroclastic-surge unitsproduced during pulsing unsteady eruptions. Some short and thin(less than I m) lithic-rich ash-flow tuffs probably resulted fromminor collapse of low eruption columns. Mantling ash-fall unitSpunctuate the record of explosive volcanism. These features sug­gest that the pyroclastic eruptions were initiated as rising magmasexplosively mixed with groundwater (hydromagmatic eruptions;Wohletz and Sheridan 1983b). However, the origin of the driv­ing volatiles (magmatic versus phreatic) is difficult to establishwithout detailed studies of individual complexes (cf. Taylor et al.1983). Once the vent was cleared, relatively quiet eruptions ofrhyolite lava proceeded. Lava eruptions may have been caused bythe eruptive degassing of the magma and the evisceration of avolatile-rich cap of a small magma chamber, or by the restrictedaccess of ground water to the vent.

The geology of the Mineral Mountains domes in Utah isinstructive in this regard in that the F-rich domes have basaltephra deposits while earlier F-poor lavas lack them. This obser­vation suggests that some explosive volcanism resulted frommagmatic differentiation and volatile enrichment (Evans andNash 1978) of the upper part of a rhyolitic magma chamber. Theviscous domes may have been extruded following devolatilizationof a chamber's cap.

Lavas are generally underlain by a flow breccia (about I mthick) produced as the flow-front crumbled, slumped, and wasoverridden by the flow in caterpillar fashion. Rapidly quenchedvitrophyric blocks from the flow front are common in this layer.

The volume of magma in individual domes or flows rangesfrom less than 0.5 km3 to a probable maximum ofabout 10 km3.

However, in some cases, fairly large volumes (10 to 100 km3) ofcoalesced domes and flows accumulated over short intervals(about 1 Ma); for example, the Thomas Range, Utah, the WahWah Mountains, Utah, and the Black Range, New Mexico.

These observations set topaz rhyolite eruptions apart fromthe large ash-flow eruptions of high-silica rhyolite that culminatein caldera collapse. Volumes of ash flows from calderas arecommonly 1 to 2 orders of magnitude larger (Smith 1979) thanthose from topaz rhyolites. However, eruptions of large volumesof magma over geologically short time intervals in the ThomasRange, Utah, and in the Black Range, New Mexico, suggest thatsome topaz rhyolites may emanate from magma chambers withvolumes approaching those of caldera-related plutons. In fact,some topaz rhyolites may have arisen from magma chambers thatgave rise earlier to large ash-flow sheets (e.g., the Black Range,New Mexico, and at Kane Springs Wash, Nevada). Some ofthese differences are shown schematically in Figure 4K

46 Christiansen, Sheridan, and Burt

TABLE 7. MODE OF EMPLACEMENT OF TOPAZ RHYOLITES

Location Area(km2)

Thomas Range, UT 160

Spor Mountain, UT 5

Honeycomb Hills, UT 1

Smelter Knolls, UT 10

Keg Mountain, UT 30

Mineral Range, UT 8

Wah Wah vicinity, UT 75

Wilson Creek R, NV 20

Kane Springs Wash, NV 4

Cortez Mountains, NV 2

Sheep Creek Mtns, NV 50

Jarbidge, NV ?

Blackfoot Lava Field, In 4

Elkhorn Mountains, MT 5

Little Belt Mtns, MT 5

Specimen Mountain, CO 8

Chalk Mountain, CO 4

Nathrop, CO 3

Silver Cliff, CO 12

Tomichi Dome, CO 5

Boston Peak, CO 1

Lake City, CO 10

Grants Ridge, NM 6

Black Range, NM 100

Saddle Mountain, AZ ?

Burro Creek, AZ 4

Emplacement

Coalesced domes and lava flows withunderlying tuffs.

Lava with underlying tuff and isolateddome(?).

Lava domes with underlying tuff.

Isolated lava dome with no tuff exposed.

Partially coalesced domes and lava flowswith underlying tuffs.

~1ultiple isolated domes with underlyingtuffs.

Coalesced domes and lava flows withunderlying tuff1 some isolated plugs anddomes (many <lkm2).

Lava flows with underlying lithic tuffs anda prominent welded ash-flow sheet.

Isolated intra-caldera dome with underlyingtuff.

Isolated domes or plugs with no tuffexposed.

Coalesced dome/flow complexes with littleassociated tuff.

Coalesced dome/flow complexes (?) withlittle tuff.

5 isolated domes with underlying tuffs.

Isolated plugs and lava domes, some withtuff.

Isolated plug (bysmalith) and sills.

Isolated lava flow/plug with underlyingtuff.

Isolated intrusive stock or extrusive plug.

3 isolated domes with lava flow andunderlying tuff.

Coalescing flows overlying thick tuff insmall subsidence structure.

Isolated lava dome with underlying tuff;sill.

Isolated flow-banded plugs and brecciapipe1 no tuff exposed.

Isolated intrusive plugs, dikes and sillsnear margin of older unrelated caldera.

Perlite and lava dome intruding tuff, shortlava flows.

Coalesced lava dome complexes withassociated tuff.

?

MUltiple isolated dome/flow complexes eachwith an underlying tuff.

Mineralogy

The mineralogy of topaz rhyolites is relatively simple and issummarized in Tables 8 and 9. Phenocryst-poor (less than 5%)rocks are the most common, but in some lavas and shallowintrusions the phenocryst content may be as high as 40%. In orderof abundance, sanidine, quartz, and sodic plagioclase are theprincipal phenocrysts. Biotite is common; hornblende, garnet, andclinopyroxene occur in a few samples. Common magmatic acces­sories include zircon, apatite, magnetite, ilmenite, allanite, fluo­rite, and titanite.

Fe-Ti oxides and titanite. Titaniferous magnetite andilmenite both occur in topaz rhyolites. These phases may bealtered in devitrified lavas but are commonly unoxidized in vitro­phyres. The few Fe-Ti oxide analyses that exist indicate that theilmenites are generally Mn-rich. Table 4 contains the results of Fe­Ti oxide geothermometry for topaz rhyolites and shows that mostof these rhyolites crystallized at temperatures between 600 and850°C; mostly in the lower end of that range. Moreover, theseanalyses indicate that f02 is commonly low,(Figure 30), near theQFM oxygen buffer (e.g. the rhyolites of the Thomas Range,Spor Mountain, and Smelter Knolls, Utah). However, analyses of

Topaz Rhyolites

TABLE 8. MAGMATIC MINERALS REPORTED IN TOPAZ RHYOLITES

47

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

Thomas Range., UTSpor Mountain, UTHoneycomb Hills, UT

xxX

XXX

XXX

+xx

x

Xx

X ± ± ± XX

XX

X ± +XX

Smelter Knolls, UTKeg Mountain, UTMineral Mountains, UT

x X X X XX X X XX X X X X

X

X

X X X

X X X X

Wah Wah vicinity, UTWilson Creek Range, NVKane Springs Wash, NV

X X X + X ±X X X XX XXXX X X X

X X X

X

±

X X XX X XX X +

Cortez Mountains, ·NV XSheep Creek Mountains, NV XJarbidge, NV X

Blackfoot lava field, ID XElkhorn Mountains,MT XLittle Belt Mountains, MT X

X XX XX X

X

X

XX

X

X

± ±

±

XX

X

X

X

X

X

XX

?

X

X XX X X X X X ±X X X X X

Specimen Mountain, COChalk Mountain, CONathrop, CO

Silver Cliff, COTomichi Dome, COBoston Peak, CO

Lake City, COGrants Ridge, NMBlack Range, NM

Saddle Mountain, AZBurro Creek, AZ

X X X XX X X XX X X X

X X + XX X XX X X ±

X X X ±

X

X

X

X

X

X X

± ± ?

X

XxjV X

X

X

X XX

X X

X X

±

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

Sa-sandine; Qz-quartz; PI-plagioclase; Bt-biotite; Mt-magnetite; Op-unidentified opaquemineral; 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 ChalkMountain, Colorado (3 log units above NNO), demonstrate thatsome topaz rhyolites crystallized under relatively oxidizingconditions.

In the latter case, these values approach those inferred byKeith (1982) to have existed in the rhyolitic magma chamber thatgave rise to the tuff of Pine Grove and to a related Climax-typemolybdenite deposit in the Wah Wah Mountains of southwesternUtah. It is important to note that of all the topaz rhyolites de­scribed in this report, the Chalk Mountain rhyolite is mostintimately related to such a mineral deposit-the Climax deposititself. Perhaps this bears out the suggestion of Keith (1982) thathigh oxygen fugacities are important for the generation ofClimax-type Mo deposits.

In the absence of detailed studies of Fe-Ti oxides, the pres­ence of titanite in several topaz rhyolites may be a mineralogicindicator of relatively high f02 (Haggerty 1976). Titanite occursin both the Chalk Mountain, Colorado, and Mineral Range,Utah, rhyolites, where independent evidence suggests oxidizingconditions. Titanite also occurs in the Lake City, Colorado, rhyo-

lites that have prominent middle REE depletions (R. A. Zielinski,written communication, 1982), which probably indicate titanitefractionation. Titanite is also reported from the Sheep CreekMountains and Jarbidge, Nevada, where it is not known if therhyolites possess middle REE depletions. It should be noted,however, that not all topaz rhyolites that bear titanite have mid­dle REE depletions. For example, some samples of the TaylorCreek Rhyolite, New Mexico, and a few of the lavas from theThomas Range, Utah contain sparse titanite but, as far as isknown, none of the lavas possess middle REE depletions. It thusappears that there are less and more oxidized topaz rhyolites, asituation perhaps analogous to that described for the Proterozoicanorogenic granites described by Anderson (1983) to consist ofan ilmenite- and a magnetite-series.

Feldspar. Almost all topaz rhyolites are two-feldspar rhyo­lites, in contrast to many other bimodal rhyolites-for example,many of the rhyolites of the Snake River Plain, Idaho (Leeman1982a; Hildreth 1981) and the peralkaline rhyolites of the west­ern Great Basin (e.g., Rytuba and McKee 1984; Conrad 1984;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 X X X X X X X X XSpor Mountain, UT X X X X X XHoneycomb Hills, UT X X X X X X

Smelter Knolls, UT X X X XKeg Mountain, UT X X ? XMineral Mountains, UT X X X X X

Wah Wah vicinity, UT X X X X X X X X XWilson creek Range, NV X X XCortez Mountains, NV X X X

Kane Springs Wash, NV XSheep Creek Mountains, NV X X X X X X X XJarbidge, NV X

Blackfoot lava field, ID X X XElkhorn Mountains, MT X X XLittle Belt Mountains, MT X X/M

Specimen Mountain, CO XChalk Mountain, CO X X X X XNathrop, CO X X X X X X X

Silver Cliff, CO X X XTomichi Dome, CO X X X X X XBoston Peak, CO X X ? X X X X X

Lake City, CO X XGrants Ridge, NM X X X XBlack Range, NM X X X X X X X X X X

Saddle Mountain, AZ X X X XBurro Creek, AZ X X X X

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 forF-rich topaz rhyolites. Sanidine in topaz rhyolites is generallyOr40 toOr60. Plagioclase is generally sodic oligoclase, although.compositions as sodie as calcic albite are found in evolved topazrhyolites like the Spor Mountain rhyolite, Utah. Andesine isfound in less evolved rhyolites like Chalk Mountain, Colorado.Two-feldspar temperatures (Stormer 1975) are shown in Table 4.Two.-feldspar (calculated at 100 b to 1 kb) and Fe-Ti oxidetemperatures are generally in good agreement where they havebeen analyzed from the same sample. The rhyolites of the Thom­as. Range, Utah, show the broadest temperature range (790 to600°C); temperature ill negatively correlated with F and otherincompatible element concentrations. All equilibration tempera­tures, as determined by feldspar pairs from other localities, fallwithin the lower part of this range.

Mafic silicates. Biotites from topaz rhyolites generallyhave high Fe/(Fe+Mg)ratios (Figure 31) reflecting the highFe/(Fe+Mg) of the magma (in many cases, molar Mn and Tiexceed Mg), and perhaps the prevalence of relatively low f02 inthese types of magmas. Nonetheless, variable Fe/Mg ratios haveresulted from the variaple oxidation states inferred above produc-

ing more magnesian biotites in some lavas. In general, the Altot islessthan 3 moles per 24 (0, OH, F, Cl). In contrast, biotites fromtwo-mica granites of the Basin and Range province and stronglyperaluminous, S-type, granites the world· over generally containmore Al than this, a fact indicative ofthe metasedimentary parent­age of.S-type granites (Figure 31). The concentrations of F andCl have been analyzed in relatively few biotites from topaz rhyo­lites. Existing analyses demonstrate that the biotites have highF-contents (up to 5 wt%), Concentrations this high for Fe-richbiotites suggest crystallization at high fHF and at high fHF/fH20(10-1 to 10-3 for the Thomas Range Rhyolites; Turley and Nash1980). F/CI ratios in the biotites also suggest crys~llization athigh fHF/fHGI. On a molar plot of Mg/(Mg+Fe) vs log F/CI(Figure 32), biotites from topaz rhyolites fall in the same compo­sitional fields as the ilmenite series granites from Japan (Cza­manske et al. 1981) and greisenized Sn-mineralized granites(Figure 32). Gunow et al. (1980) and Munoz (1984) have shownthat such high F/CI ratios are also characteristic of the Hendersonmolybdenite deposit. Brimhall et al. (1983) and Munoz (1984)have generalized this observation to many other Mo, Sn, W, andBe deposits and contrast these deposits with piotites from Cu-rich

Topaz Rhyolites 49

:0... :.

.' .••

~~- -".,-

Topa2: rhyoliteswest-central Utah

o

-22

-18

-20

-6

-8

-10,-

,-,-

./

./

-12 ,-./ Bishop TuffC'iI ./

0 /...../

0) -140 ...- ..

-16

600 700 800 900 1000 1100

Temperature ( °C)

Figure 30. Compilation of T-log f02 data for silicic andesites, dacites and rhyolites from western NorthAmerica (after Ewart 1979, except as noted). The oxygen buffer curves (HM hematite-magnetite; NNOnickel-nickel oxide; QFM quartz-fayalite-magnetite) are for one atmosphere pressure. Large dots areCalifornian bimodal rhyolites; smalldots are for calc-alkaline intermediate to silicic rocks; and the openfield is for the Bishop Tuff, California (Hlldreth 1977). The topaz rhyolite field includes rhyolites fromthe Thomas Range, Smelter Knolls, and Spor Mountain; Utah; large filled circles are for ChalkMountain, Colorado, and open circles for the Mineral Range, Utah (all from sources cited in text).

porphyry deposits which have low FICI ratios when correctedfor Fe-F avoidance (Figure 32).

Clinopyroxene (in high T andF~poor rhyolite; e.g. "mafic"Thomas Range, Utah, and Jarbidge, Nevada, specimens), fayalite(Kane Springs Wash, Nevada, and some Mexican tin rhyolites),Fe-rich hornblende (Figure 33), or Fe-Mn garnet are found in afew specimens from topaz-bearing rhyolites. Fe-enriched maficminerals are common in bimodal rhyolite magmas (Ewart 1979)and in anorogenic or A-type granites (e.g., Anderson 1980,1983). As for the case of biotite, this association reflects the highFe/Fe+Mg ratios of the magmas and, possibly, the prevalence ofrelatively low f02 in these types of magmas.

The vapor-phase mineralogy of topaz rhyolites (Table 9) isdiverse and includes topaz, fluorite, spessartine gamet, beryl, Fe­Ti-Mn oxides (pseudobrookite, hematite, and bixbyite), silicaminerals, and alkali feldspar. The compositions of these minerals

and their implications for the transport of elements in a vaporphase remain little studied. However,fluid inclusion and oxygenisotope studies of topaz and quartz in rhyolites from the BlackRange indicate that vapor-phase crystallization-.occurredat a temperature in excess of 600°C (Eggleston and Norman1984). High salinity in the fluid probably reflects fluid boiling. Inaddition, the high iron content of bixbyite and the presence ofpseudobrookite instead of rutile indicate initial' temperatures ofover 500°C during crystallization in some cavities (Lufkin 1976).Burt (1981) has shown that the vapor-phase mineral assemblageis consistent with moderately high fHF and relatively high fo2•

Barton's (1982) exposition of the thermodynamic properties oftopaz suggest that the HF/H20fugacity ratio exceeded 10-3 attemperatures above 600°C, stabilizing topaz and K-feldsparover muscovite plus quartz. The latter assemblage is more typicalofgreisens formed at some depth. The general absence of fayalite

50 Christiansen, Sheridan, and Burt

Siderophyllite

o

H

2

H

3r-------------...,..~----___,

()....u.OJo

...J

4.0

Eastonite

1

o Chalk MIn• Spor MInK Smelter KnollsH Honeycomb Hillso Thomas Range

3.0

AI atoms/24 (O,OH,F)

Annite

0.8 f-

2.0

Phlogopite

Figure 31. Compositions ofunoxidized biotite from topaz rhyolites fromthe western United States in terms of molar Fe/(Fe + Mg) and total Alrelative to ideal end members. Sources of data for topaz rhyolites aregiven in the text. For comparison the compositions of biotites from theBishop Tuff, California (RT - Hildreth 1977), from calc-alkaline igneousrocks from the western United States (Wender and Nash 1977; Hauseland Nash 1979; Dodge et aI. 1969) and from muscovite-bearing grani­toids of the western United States (MG-Best et aI. 1974; Lee et aI.1981; Kistler et aI. 1981; Dodge et aI. 1969) are also given.

Figure 33. Composition of hornblende in terms of molar Fe/(Fe + Mg)and octahedra1 Al relativeto ideal end members. The filled circle repre­sents hornblendes from one lava flow in the Thomas Range. Amphibolesfrom other anorogenic silicic magmas have similar Fe/(Fe + Mg) ratios(Wolf River Batholith, Wisconsin: Anderson 1980; and the Pikes Peakbatholith, Colorado (+): Barker et aI. 1975). Hornblendes from calc~

alkaline volcanic rocks of the eastern Great Basin are shown as opencircles (Hausel and Nash 1979; Wender and Nash 1977; Keith 1982).The solid line encloses the composi~ions of hornblendes from the SierraNevada batholith, California (Dodge et aI. 1969), and the southwestJapanese batholith (Czamanske et aI. 1981).

0.90.70.50.30.1-1 L-_.l-_.l-_..I-_...l-_...J....._...J....._...J....._...J....._-'-_-'

Mg/Mg+Fe

Figure 32. Compositions ofunoxidized biotite in topaz rhyolites from thewestern United States in terms of molar Mg/(Fe + Mg) and log F/Cl(molar). Symbols are the same as in Figure 31. Sources of the data fortopaz rhyolites are given in the text. Other fields shown for comparisonare from Keith (1982), Gunow et aI. (1980), Czamanske et aI. (1981),Jacobs and Parry (1979), and Parry et aI. (1978). Isopleths of equal Fenrichment have a positive slope on this diagram. Topaz rhyolites havebiotites which are consistently F-rich when "corrected" for their usuallyhigh Fe/Mg ratios. In this regard, they are most like biotites fromilmenite-series granites of Japan (Czamanske et aI. 1981) and fromgranites associated with Mo-Sn-W-Be deposits (Munoz 1984).

Geochemistry and differentiation trends

The major element composition range of topaz rhyolites isfairly restricted. All are high-silica rhyolites with high Na, K, F,

(as a phenocryst or as a product of vapor-phase crystallization) isnotable; it is presumably unstable with respect to F-bearing bio­tite. Peralkaline minerals (aegerine, riebeckite, etc.) are likewiseabsent. In contrast, the presence of aluminous minerals in miaro­litic cavities, especially topaz and spessartine garnet (Fig. 34),some of which is F-bearing (Moyer, 1982), suggests a link be­tween F and Al in an escaping vapor-phase after eruption. Thiscomigration may be an extension of their association in alumino­silicate melts proposed by Manning et al. (1980) and Christiansenet al. (1983a).

Additional detailed studies of magmatic and vapor-phasemineral compositions could help to further constrain the values ofthe intensive parameters that prevailed during cooling and crystal­lization of topaz rhyolites. Mineral chemistry could also help todocument the role of fluorine in silicate melts and minerals. Stud­ies of phenocryst/melt partitioning are as yet lacking for topazrhyolites from the United States. Such studies would be useful forsorting out the role of volatiles and melt structure in determiningpartition coefficients in highly silicic magmas (e.g., Mahood andHildreth 1983).

1.0

Hastingsite

0.8

Ferroedenite

... - ....; ,/ \

I Wolf \I River \,Massif }

,-\ //

'-; "

0.60.4

Fe/Fe+ Mg

0.2

Edenite

0.0

Pargasite

1.0

u:; 0.8:iq,9o:l' 0.6C'\l.....IIIe 00....

0.4III

S.<E

0.2

Topaz Rhyolites 51

Spessartine

\\

\\

\\

\\

\\

\\

\XX

I

" X X\ X\ .

\ X\

\ X

" X\\

\\

\\

\

x Burro Creek, Arizonoo Nathrop, Colorado• . Thomas Range, Utaht::. Pine Grove, Utaho Ely, NevadaA Canterbury, New Zealand

+ Kern Plateau, California---- Pegmatitic garnets

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

Figure 34, Compositions of garnet from rhyolitic volcanic rocks in terms of Fe, Mn, and Mg end­members. 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; Moyer1982; Pabst 1938). The pegmatitic garnet field is from Miyashiro (1955).

and Fe/Mg and low Ti, Mg, Ca, and P (Table 1; Figure 35).These characteristics are typical ofbimodal rhyolites (see below),Topaz rhyolites are apparently a subclass of this group (a conclu­sion also justified by their trace element chemistry, Fe-enrichedmineralogy, tectonic setting, and magma associations). The com­position of a "typical" topaz rhyolite is shown in Table 1 andrepresents modal values from the histograms in Figure 35,

All topaz rhyolites thus far identified from the western Uni­ted Stateshave Si02 concentrations greater than 72% (all Si02concentrations have been recalculated on an anhydrous basis)and a strong mode exists at 76% (Figure 35). Silica contents varylittle with differentiation trends seen in individual dome com­plexes. For example, silica ranges from 74.2% to 76.7% in therhyolites of the Thomas Range, Utah, whereas incompatible ele­ments such as Rb triple in concentration, As a result silica con­centrations are poor indicators of the chemical variability of theserhyolitic magmas. In addition, silica contents are actually lower inrhyolites that are extremely enriched iIi fluoriIle and incompatibleelements such as the lava at Spor Mountain, Utah. The averagesilica concentration of these samples is about 74%. Other highlyevolved topaz rhyolites, such as the one at the Honeycomb Hills,Utah, and the ongonites discussed below, also show silica con­tents lower than 76%, Christiansen et al. (1984) interpreted thelow silica content as the result of crystallization near the min­imum in the granite system, with elevated fluorine. Manning's

(1981) experiments show that the stability field for quartz ex­pands with increasing fluorine content in a water-saturated haplo­granite at 1 kb pressure (Figure 36). It is thus conceivable thatenhanced quartz fractionation could lead to a reversal of normalSi02-enrichment during fractional crystallization of a fluorine­rich rhyolite.

Most topaz rhyolites contain 12% to 14% Al203 (Figure 35).High aluminum contents are found in the evolved low-silica rhyo­lites noted previously, reflecting the increased proportion offeldspar components, especially albite. All topaz rhyolites havehigh concentrations of alkalies ranging between 8% and 10%Na20 plus K20. In general K20/Na20 ratios are greater thanone (typically about 1.2 to 1.4 by weight). Several topaz rhyoliteoccurrences seem to be typified by K20/Na20 ratios higher than1.5-the topaz (and topaz-free) rhyolites of northern Nevada, theTaylor Creek Rhyolite in the Black Range of New Mexico andthe Sn and F-rich rhyolites of Mexico's Sierra Madre Occidental.Because only a few vitrophyres have been analyzed from theseareas, it remains to be demonstrated that these high ratios aremagmatic and not the result of alkali-metasomatism during sub­aerial crystallization, In d!fferentiation sequences, K generally de­clines with advancing concentrations of F and other decidedlyincompatible elements, while Na increases-a result of the com­bined fractionation ofpotassic sanidine and biotite (Figure 36). Inspite of high alkali concentrations, topaz rhyolites are not peralka-

0.6 1.0 1.4 1.8 0.02 0.06 0.10 0.14 0.1810 12 14 16

Christiansen, Sheridan, and Burt

72 74 76 78 80 0 0.08 0.16 0.24

52

70

60

5051°2

40

30

20

10

40

MgOK2 0 P205

30CaO Na 2 0

F20

10

~ nI

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 UnitedStates (values in wt%). Analyses of 118 samples from 22 locations are represented (sources cited in thetext). 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 thefrequency of each value.

line, and the use of the term "alkali rhyolite," reserved by lUGSusage for peralkaline rhyolites (Streckeisen 1979), should beabandoned. Use of this term may lead to confusion with the trulyperalkaline rhyolites with which topaz rhyolites are contempo­raneous in the western United States. In the lUGS system topazrhyolites are generally rhyolites or alkali feldspar rhyolites. In­deed, many vitrophyres from topaz rhyolites are metaluminous.The presence ofgarnet and topaz (absent as vapor-phase ttlineralsin peralkaline volcanic rocks) also reflects the aluttlinous charac­ter of topaz rhyolites. Many felsites are slightly peraluminous asindicated by normative corundum-probably as a result of theloss of alkalies during crystallization.

We prefer to calculate CIPW norms based on fluorine-freeanalyses. The CIPW scheme ties up Ca with F to form fluoritebefore Ca is used to form plagioclase, thereby producing a rockthat appears to be more strongly peraluminous, as indicated by

normative corundum, than it would be otherwise. This crystalli­zation order is not that observed in topaz rhyolites. Fluorite is oneof the last phases to crystallize in some extremely F-rich glassesand more generally fluorite is post-magmatic. In short, topazrhyolites are not strongly peraluminous indicating that they arenot the eruptive equivalents of S-type grllnites (e.g., White andChappel 1983).

The concentrations of CaO (generally <0.8%), Fez03*(generally <1%), MgO (generally <0.20%), TiOz (generally<0.12%), and PzOs «0.02%) in topaz rhyolites are relatively lowand similar to those found in other high-silica rhyolites (Figure35). In cogenetic suites, all of these elements correlate negativelywith incompatible elements and with fluorine. Their decreasingconcentrations can be shown to relate to the fractionation ofmodal proportions of plagioclase, biotite, Fe-Ti oxides and apa­tite. A few samples from the Honeycomb Hills and Spor Moun-

Topaz Rhyolites

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) thehydrous granite system at pressures given in kb (PHzO) next to the filled circles (Tuttle and Bowen1958; Luth et aI. 1964; and Whitney 1975); and 2) the F-bearing granite system at 1 kb (Manning1981). The numbers next to the stars indicate weight %F in the water-saturated system at 1 kb.

53

tain, Utah and perhaps elsewhere, show Ca-enrichment as theresult of the accumulation of post-magmatic fluorite. Topaz rhyo­lites have high Fe/Mg ratios and plot in the "tholeiitic" field onSi02 versus FeO*/(FeO* + MgO) diagrams such as the one usedby Anderson (1983) to discriminate calc-alkaline from tholeiiticor reduced anorogenic suites. In addition, Fe/Mg ratios increasewith differentiation, driven principally by biotite fractionation asFeO*IMgObiotite is substantially less than FeO*IMgOmelt.

Manganese concentrations are also low, less than 0.08%MnO in almost all samples, but in general Mn appears to behaveas an incompatible element. MnO increases with differentiationfrom 0.05% to 0.08% in the rhyolites of the Thomas Range.Similar increases of MnO with differentiation of rhyolitic mag­mas have been noted by many authors (see review in Hildreth1981) and appear to relate to limited biotite fractionation.

Of course, the most discriminating feature of topaz rhyolitesas a group is their high fluorine content. Topaz appears as anidentifiable vapor-phase mineral in lavas whose vitrophyres con­tain over 0.2% F. Fluorine concentrations of more than 1% areonly known from vitrophyres from Spor Mountain and theHoneycomb Hills, both in western Utah. Figure 35 showsconcentrations in felsites and vitrophyres, where most of the lowvalues «0.2%) are from felsites. Comparisons of vitrophyre­felsite pairs almost universally show that F is lost during devitrifi­cation. Chlorine is also Ibst by devitrification, as no mineral phase

concentrates CI in contrast to F. Meaningful chlorine concentra­tions can only be obtained by analysis of vitrophyres or obsidians.Figure 37 shows the fluorine and chlorine concentrations inglassy specimens of topaz rhyolites. In specimens thus far ana­lyzed, Cl concentrations remain less than 0.2% and are generallymuch lower than this. Important features of topaz rhyolites aretheir high F/Cl ratios (greater than about 3) as compared to F­and Cl-rich peralkaline rhyolites, which have lower F/Cl.

Topaz rhyolites are variably enriched in incompatible traceelements (Li, Rb, Cs, Be, U, Th, Y, Nb, Ta, Ga, Pb, Mo, Sn, W,and HREE) and depleted in feldspar-compatible trace elements(Ba, Eu, and Sr). Zirconium and Hf concentrations are generallylow as well, consistent with their compatibility with fractionatingzircon. As for other metaluminous rhyolitic magmas (e.g. Hil­dreth 1979), Zr/Hf ratios (typically 25 to 20) decline with differ­entiation as the result of the greater compatibility of Zr in zircon.The concentrations of many elements that are taken up by maficsilicates (Ni, Co, Cr, and V) are extremely low, but as they lienear the detection limits for common analytical metho~ theirconcentrations are not well known. Typical concentrations arelisted in Table 2 and illustrated in Figure 38. Using trace elementcompositions, Pearce et al. (1984) have attempted to interpret thetectonic settings of granitic rocks. In this classification (Figure39), topaz rhyolites from the western United States consistentlystraddle the boundary between WPG (within-plate granites, like

54 Christiansen, Sheridan, and Burt

0.8

-eft~:;=- 0.6

w .,.ZI--l

0::::a--l

0.4 ..I ..U .. .. ..

0.2

0.2 0.4 0.6 0.8 1. 0FLUORINE (wt.%)

<>

1.2

Figure 37. Fluorine and chlorine concentrations in glassy topaz rhyolites (open diamonds) and peralka­line rhyolites (closed triangles). A FICl ratio of 1 divides oceanic from continental peralkaline rhyolites(Bailey 1980). A F/C! ratio of 3 separates peralkaline rhyolites from topaz rhyolites of the westernUnited States. The composition ofthe Bishop Tuff, California (B: Hildreth 1979) is shown for compari­son. 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-collisionalgranites, like the leuco-granites of the Himalayas). Although thesediagrams point to a certain uniqueness among topaz rhyolites ascompared to many other granites, it is unlikely that simple dis­criminant diagrams such as these will provide a unique definitionof the type or tectonic setting of granites because of the widevariety of crustal and mantle components that are involved ingranite genesis. Note for example the location of the metalumi­nous Bishop Tuff, California, and the comenditic Tala Tuff,Mexico.

REE patterns (Figure 40) show some variability (perhapsinherited from slightly different source rocks and/or differentia­tion histories) but they generally display low La/CeN, La/YbN (1to 3 for most but this ratio may be as high as 12 for rhyolites fromColorado), and Eu/Eu* (0.45 to 0.01 for analyzed specimens).Light REE concentrations generally do not exceed 200 timeschondrite values and more typically are near or less than 100times chondrite. Differentiation trends for the REE are seen inseveral complexes (Thomas Range, Utah, Mineral Mountains,Utah,Wah Wah Mountains, Utah, Lake City, Colorado) andshow decreases in LREE and Eu concentrations that are coupledto increasing HREE and other incompatible element concentra­tions. Christiansen et al. (1984) have modelled this trend for the

Thomas Range rhyolites as resulting from the fractionation ofsmall amounts of allanite (0.04 wt% of the fractionated mineralassemblage). The middle REE depletions noted for the MineralMountains and the Lake City rhyolites suggest the fractionationof titanite.

Devitrification mobilizes a number of trace elements (U, Sb,F, CI, Li, Be, perhaps Sn, W, and Mo)judging from lowerconcentrations in vitrophyre-felsite pairs. However, it has notbeen demonstrated that devitrification, without significant vapor­phase alteration, significantly changes concentrations of manyother trace elements, including petrogenetically important ele­ments like Rb, Sr, Ba, Y, Zr, Hf, Nb, Ta, Th, REE, Sc, and Ga. Infact, for some trace~element and isotope studies, dense felsites maybe better samples than variably hydrated vitrophyres, as hydra­tion may involve significant addition of Sr, Ca, and other ele­ments (Hargrove 1982), as well as 0 and H isotope exchange.

Some of the compositional features of topaz rhyolites aresummarized in Figure 41, which compares the chemical composi­tion (Table 2) of a variety of topaz rhyolites on normalizedgeochemical diagrams. Such normalized-concentration diagramsallow the relative concentrations of many elements in a singlesample to be displayed and elemental fractionation is easily por­trayed. 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

20 40 6020 40o40 80 120o40 80o80 160o

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 19different localities (references cited in Table 2). The vertical scale represents the relative frequency ofeach value.

U.S. Geological Survey geochemical reference sample RGM-l (arhyolite from Glass Mountain, California) as presented in Govin­daraju (1984). RGM-1 was chosen because the concentrations ofmany elements have been accurately determined and because it isa sub-alkaline high-silica rhyolite grossly similar to many topazrhyolites. RGM-1 is an obsidian and the concentrations of ele­ments otherwise mobilized by devitrification and hydrationshould be nearly magmatic. This is particularly important for F,Cl, and U-elements of interest here. RGM-l is reported to havea Th/U ratio of2.6 (1515.8) and a FICI of 0.6 (3401540); valuesthat may be typical ofcalc-alkaline rhyolitic glasses. The elementsare listed in order of increasing c/r2, so that geochemically similarelements are plotted near one another. Chondrite- or mantle­normalization was deemed to be inappropriate in this case be­cause of the tremendous differences between rhyolites and theseother materials. For example, the FICI ratio of chondritic mate­rial is approximately 10.

The tremendous enrichments in F, Ta, Nb, Y, Rb, U, Th,and HREE and the relative depletion of Zr, Sr, Eu, and Ba areeasily visualized in these diagrams. Chlorine concentrations inglasses show up as deep anomalies, indicating the lack of enrich­ment relative to F and Rb. Some topaz rhyolites show depletions

ofY and Sm relative to geochemically similar elements. Normal­ized major element diagrams show the depletions of Fe, Mg, Ti,and Ca typical of most high-silica biotite rhyolites with two feld­spars, including topaz rhyolites.

Taken together these compositional features suggest thattopaz rhyolites are highly differentiated or evolved magmas.Their common association with less evolved, topaz-free rhyolitessuggests they may have fractionated from more "mafic" composi­tions. Such evolutionary relationships are discernible in at least15 of the 26 localities described here. Christiansen et al. (1984)have used the extreme depletion of Eu and other compatibleelements to preclude an important role for variable degrees ofpartial melting in the observed chemical variability. This is not tosuggest that a varying proportion of partial melting was not criti­cal to the generation of topaz rhyolites, but only that the evidenceof such a process has been obscured by subsequent fractionation

, processes.. As indicated throughout this discussion, an examination ofthe elemental composition of cogenetic lavas reveals the impor­tance of crystal fractionation in the evolution of topaz rhyolites.Those centers examined in any detail document chemical trendsconsistent with crystal fractionation. The correspondence be-

56 Christiansen, Sheridan, and Burt

tween major and trace element models for the evolution of theThomas Range rhyolites was noted above, and involved the frac­tionation of sanidine >quartz >plagioclase »biotite >bxides>> apatite>zircon>allanite; The extreme depletion offeldspar­compatible elements is thus explained as are enrichments in avariety·of incompatible trace elements. Crystallization appears tohave occurred close to the mi~imum in the granite system (Q-Ab-

~,,(p+Nb (ppm)

Figure 39. Trace element discriminant diagrams for granitic rocks(Pearce et al. 1984) showing the compositions of topaz rhyolites from thewestern United States, the Bishop Tuff (Hildreth 1977), and the TalaTuff (Mahood 1981). VAG = volcanic arc granites; ORG = ocean ridgegranites; WPG =within plate granites; and syn-COLG =syn-collisiongranites. Topaz rhyolites clearly overlap the within-plate and collisionalfields.

Or-An-F-H20), consistent with the observed small changes inSi02, A1203, and the alkalies. These variations correlate withlarger changes in trace element concentrations controlled by thefractionation of major phases and by the observed accessory min­erals (i.e., zircon, allanite, apatite, titanite). The correlation ofhigher F with lower Si02 and higher Al203 and Na20 in therhyolites is also consistent with crystallization differentiation ofmore F-rich magmas as predicted by the experimental studies ofManning (1981). In short, although these rhyolites are among themost F-rich magmas yet analyzed, we see no compelling evidenceto indicate that convection-driven thermogravitational diffusion(Le. volatile complexing in the melt or Soret diffusion) controlledtheir evolution (cf. Mahood 1981; Hildreth 1981).

The importance of crystal fractionation is strikingly illus­trated by the correspondence of the trace element patterns of therhyolites with distinctive accessory mineral phases. (Wolf andStorey (1984) have used this principle in a convincing explana­tion of the trace element zonation patterns seen in highly alkalinemagma bodies.) In topaz rhyolites and other aluminous rhyolitesthe negative Zr (and usually Hf) anomalies and decreasing Zr/Hfratios are controlled by fractionation of zircon. Fractionation ofallanite, and perhaps monazite or chevkinite in some lavas, pro­duce the typical LREE depletion patterns shown in Figures 40and 41. In some rhyolites, titanite appears as a microphenocrystand its fractionation is probably responsible for the Y, and middleREE depletions apparent for lavas from the titanite-bearing rhyo­lites of the Mineral Range, Utah, Lake City, Colorado, and otherareas as noted by the dashed lines in Figure 41. Elements that arelargely incompatible in the major and trace phases become sub­stantially enriched. Such elements include both large-U, Th, Rb,es, Ta-and small ions-Be, Li. Fluorine also behaves as anincompatible element as it is only removed by biotite fractiona­tion and biotite occurs in small proportions. Moreover, the biotite/melt partition coefficient for Fis strongly dependent on the Fe/Mg ratio of the biotite (Munoz 1984) and hence on the fugacityof oxygen. On the other hand, the negative Cl anomaly must be areflection of the original composition of topaz rhyolite melts,because Cl should be more strongly incompatible than F.

Individual volcanic centers need to be examined in moredetail to test these conclusions regarding the geochemistry anddifferentiation of topaz rhyolites. Of particular importance wouldbe studies of the halogen· compositions of lavas and/or tuffs thatmaybe cogenetic with topaz rhyolites. Changes in F/Cl ratioscould be monitored in this way. Another fruitful avenue of re­search would be to examine in more detail the relationship ofaccessory mineral assemblages and compositions with the traceelement patterns of their host "liquid." Befbre unique geochemi­cal models can be constructed, mineral/liquid partitioning needsto be studied, but has thus far been completely neglected for therhyolites described here. V. I. Kovalenko and others (e.g. 1978and 1984) have been examining crystal/liquid partitioning inongonites. Studies of the geochemical effects of devitrificationhave thus far been cursory. The mineralogic controls on thisprocess need to be determined. Experimental studies are called

100

Topaz

Rhyolites

ORG

10010

VAG

1000 syn-COLG

400

-.Ec.

81'hOPT~c........c 100a: WPG

VAG

101.0 10

Vb +Ta (ppm)

1000

syn-COLG

.....Ec.c......

100.c

WPGa:

Topaz Rhyolites 57

--5""-61 ..... _

aThom•• Range. UtAh

100

10

100

10

100

10

100

10

SM·2~:z.06

------- ..

....................................

-WW·9

___ a WW'41

.. STC-4

~---­

Nalhrop; lopal ~"':I~~;"-""""''''---.,\I\

\,,\ I\ ,"11

--...-_------- ...

b

r----='SPc:;O':.:M::..:.n:::••.::;uT'--_- ~ 100

-- ~ ... ------_... ----.._-.Black R.ngc.NM

................................Nathrop.CO ...• 10

cHoneycomb Hill.. Utah

dSmelter KnoU.Utah

100

10

eMinerai Mount.lna. U,...

Wah W.h Mountalna

100------------- ...

10

g

Southw••tern Colorado

Figure 40. Chondrite-normalized REE patterns for topaz rhyolites andrelated rocks from the western United States. Concentrations normalizedto 0.83 times concentrations in Leedey chondrite (Masuda et al. 1973).a. Thomas Range, Utah (Christiansen et al. 1984a). Sample SM-61contains relatively low concentrations of incompatible trace elements(e.g. 5 ppm U) while SM-29-206 contains high concentrations of theseelements (e.g. 19 ppm U). La/LuN and Eu/Eu* are negatively correlatedwith U content and apparently decrease as a result of differentiation ofthe parental magma. HREE concentrations increase with differentiation.The lava from which SM-61 was collected appears to be topaz-free, butChristiansen et al. (1984a) postulate that it is genetically related to thetopaz-bearing rhyolites of the Thomas Range.b. Specimens from Spor Mountain, Utah (Christiansen et al. 1984),Black Range, New Mexico (HC-8; Correa 1980), and Nathrop, Colo­rado(Zielinski 1977) cover the range of REE concentrations seen intopaz rhyolites from the western United States.c. Honeycomb Hills, Utah (Turley and Nash, 1981). The low Si, highCa and F sample described in the text has a very similar pattern.d. Smelter Knolls, Utah (average of three analyses; Turley and Nash1980).e. Mineral Mountains, Utah, topaz rhyolites (average of samples fromtwo domes) compared with less differentiated but probably co-geneticlava (Bailey Ridge flow) (Lipman et al. 1978a). Depletion of light andmiddle REE, and especially Eu, with differentiation is apparent. Yb andLu show slight enrichment.f. Wah Wah Mountains, Utah (Christiansen et al. 1980). Samples WW­9 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)mine.g. Nathrop, Colorado, topaz rhyolite (Zielinski 1977) compared to calc­alkaline rhyolite REE pattern from Summer Coon volcano in the nearbySan Juan volcanic field (Zielinski and Lipman 1976). These rhyolites arenot co-genetic. Note the lower La/Yb ratio and deeper Eu anomaly ofthe topaz rhyolite. These features are typical of comparisons betweentopaz rhyolites and nearby, often nearly contemporaneous,calc-alkalinerhyolites.

Vb L ...Sm [u Gd Tb DyNdc.0.1 ~-:'-_-..I._---J__....L-:'--L-L--' --JL.....L..:J

l.

58 Topaz Rhyolites

Mexican Tin Rhyolites

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

5

10

0.1

c

Topaz Rhyolites

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

10

5

0.1

a

I~

I::;; ::;;(!l (!l0: 0:....

11>11>

C. c. 0.5E 0.5

E

'" '"(/) (/)

10

5

11>C.E 0.5

'"(/)

0.1

b

Peralkaline Rhyolites

• PantelJeria

o Sierra La Primaveva

• McDermitt

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

Figure 41. Concentrations of selected trace elements in rhyolitic rocksnormalized to those in RGM-l, a rhyolite obsidian from Medicine Lakevolcano, California, and U.S. Geological Survey geochemical reference(Govindaraju 1984). The elements are listed in order of their relativefield strength, increasing to the right. a) Topaz rhyolites from the westernUnited States typically show negative CI, Ba, Sr, Eu, and Zr anomalieson such diagrams. Titanite-bearing rhyolites commonly show negativeY anomalies shown with dashed lines; titanite-free lavas show no nega­tive Yanomalies. b) "Tin" rhyolites from Mexico (vertical bars; Huspeniet al. 1983), which commonly have vapor-phase topaz as well, havetrace element compositions that largely overlap with those of topazrhyolites from the western United States (shaded). Chlorine concentra­tions from glassy specimens have not yet been reported. c) The traceelement patterns of peralkaline rhyolites (Civetta et al. 1984; Mahood1981; Conrad 1984; Christiansen et al. 1980) are strikingly differentfrom those of topaz rhyolites (shaded) and generally have small negative(or in some cases even positive) CI anomalies. In addition, they generallyhave higher REE contents, but marked negative Th and U spikes aredistinctive. The absence of negative Zr anomalies is the result of theenhanced solubility of zircon in peralkaline melts.

Topaz Rhyolites 59

for to determine the role of fluorine on melt properties and phaserelationships and to constrain the values of intensive properties ofeach system.

Isotopic composition

Initial strontium isotope ratios for 18 samples from 10 local­ities are reported in Table 3. These analyses indicate that initial87Sr/86Sr ratios for topaz rhyolites range from 0.7055 to about0.712. Considerable uncertainties exist in the initial 87Sr/86Srbecause of poorly known ages for some of the samples, coupledwith their extremely high RblSr ratios. The effects of recalcula­tion to different ages are shown for the rhyolites from Nathrop,Colorado, and the Black Range, New Mexico. These isotoperatios are suggestive of an important crustal component in theprotolith of topaz rhyolites. Using approximate crustal provinceages (Farmer and DePaolo 1984), these ratios correspond tocrustal sources with relatively low RblSr ratios. For example, aninitial 87Sr/86Sr of 0.709 from the eastern Great Basin corre­sponds to a RblSr ratio in the source of about 0.07 (assumingseparation of the crust 2.2 Ga ago from a bulk-earth reservoirwith RblSr ratio ofO.029-parameters from DePaolo and Was­serburg 1977); a ratio of 0.706 implies a source RblSr of 0.04.Initial ratios between 0.709 and 0.7055 from Colorado and NewMexico (with crustal ages of 1.7 to 1.8 Ga) imply RblSr ratios of0.08 to 0.04. These elemental ratios are consistent with the hy­pothesis that topaz rhyolites are derived from high-grademetamorphic rocks, perhaps of granulite grade (see compilationin Pettingill et al. 1984). Moreover, these values are lower thanwould be expected if the rhyolites were derived from metasedi­mentary materials of similar Proterozoic ages. For example,Farmer and DePaolo (1984) estimated that the RblSr ratios forthe sources of strongly peraluminous (S-type) granitoids in thewestern Cordillera range from 0.3 to 0.05. These aluminous gran­itoids are thought to be derived from a middle crustalmetasedimentary source (e.g. Lee and Christiansen 1983; Farmerand DePaolo 1983). Upper crustal contamination of cogenetictopaz rhyolites may be indicated by variable initial Sr-isotoperatios for samples from the Thomas Range, which belong to acoherent suite as judged from trace and major element geochem­istry (Christiansen et al. 1984).

Oxygen isotope ratios for topaz rhyolites are available onlyfrom the Mineral Mountains, Utah, (Bowman et al. 1982) andLake City, Colorado, (R. A. Zielinski, written communication,1982). These bear out the suggestion of a crustal source in theirlow to moderate values (8 180 =6.3 to 6.9 %0 for the MineralRange and 7 to 10 %0 for Lake City).

Interpretation of the Pb-isotopic data in such a manner isequivocal as the· two topaz rhyolites thus far examined (LakeCity, Colorado, and Cortez, Nevada) do not show elevated208Pb/204Pb ratios. Elevated thorogenic Pb isotope ratios arecommonly taken to indicate derivation from granulitic rocks be­cause of the presumed depletion of U relative to Th during met­amorphism. Although many granulite terranes do show high

ThlU ratios (>5), many, especially felsic granulites, do not (seecompilation in Iyer et al. 1984). For the Lake City rhyolites,Lipman et al. (1978b) have interpreted the Pb isotope data toindicate derivation from an amphibolitic lower crustal sourcewith ThlU ratio of about 3.6. Another important feature of thePb-isotopic composition of both the Cortez and Lake City rhyo­lites is their similarity to chemically and temporally distinct rocksin the same region. It appears that Pb isotope regionalizationnoted by Zartman (1974) and others extends to these silicic vol­canic systems as well.

Integrated isotopic (Pb, Sr, Nd, 0) and petrologic studies oftopaz rhyolites are of paramount importance for determining thegeochemical nature of the protolith of topaz rhyolites. Given theprobable crustal origin of topaz rhyolites, these studies might beinformative of crustal structure and composition when examinedon a regional scale, but studies of single volcanic complexes areneeded before the role of crustal contamination versus sourceinheritance can be fIrmly established.

Magma-tectonic setting

The nature of igneous rock associations and contemporane­ous tectonic activity gives some clues about the generation ofmagmas. For example, lithospheric subduction at continentalmargins is consistently associated with concurrent calc-alkalinemagmatism. However, any attempt to delineate the magma­tectonic setting of topaz rhyolites in the western United States islimited by our incomplete understanding of the complex evolu7tion of Cenozoic tectonism across the region.

Extensional tectonism appears as a common denominator inalmost all areas where topaz rhyolites were erupted in the westernUnited States. Episodes of topaz rhyolite magmatism coincidewith periods of lithospheric extension; 1) in the eastern GreatBasin where basin and range faulting may have begun as early as21-20 Ma (Rowley et al. 1978a) and then was renewed under adifferent stress orientation about 10 Ma which has persisted to thepresent (Zoback et al. 1981); 2) along Nevada's Cortez rift thatopened 16 Ma (Stewart et al. 1975); 3) in Montana where blockfaulting began about 40 Ma (Chadwick 1978) and intra- or back­arc graben formation may havebegun as early as 50 Ma (Arm­strong 1978); 4) along the Rio Grande rift and its northernextension into Colorado, which initially developed about 30 Ma(Eaton 1979; Elston and Bornhorst 1979); and 5) in westernArizona where block faulting was underway by about 9 Ma(Sunneson and Luchitta 1983). Several groups of topaz rhyoliteslie on possible continental transform zones that developed as aresult of differential extension rates (Wah Wah Mountains, KaneSprings Wash, Spor Mountain, Elkhorn Mountains). Litho­spheric extension occurred in back- or intra-arc and post-arcenvironments (Eaton 1979, 1984a; Elston and Bornhorst 1979).The intimate association of extensional tectonics and topaz rhyo­lite magmatism in the western United States implies a stronggenetic connection, but the nature or existence of extension con­current with topaz rhyolite magmatism in Mexico needs to beclarifIed.

60 Christiansen, Sheridan, and Burt

TABLE 10. MAGMATIC ASSOCIATIONS OF TOPAZ RHYOLITES

Calc-alkaline Suite

Andesitic to dacitic lavasand rhyolitic ash flowsassociated with granodioriticintrusions. Continuous Si02variation diagrams.

Chalk Mountain, CONathrop COSpecimen Mountain, COTomichi Dome, COBlack Range, NMSierra Madre Occidental

Mexico

ProbableE. Miocene Wah Wah Mtns, UTWilson Creek Range, NV

Climax-type Mo depositsHenderson, COClimax, COPine Grove, UT

Bimodal Basalt-Rhyolite Suite

Generally potassic basaltsof tholeiitic or alkalineaffinity.

Thomas Range, UTSpor Mountain, UTSmelter Knolls, UTHoneycomb Hills, UTWah Wah Mountains, UT

L. PlioceneMineral Mountains, UTCortez, NVJarbidge, NVSheep Creek Mtns, NVBlackfoot lava field, IDElkhorn Mountains, MTLake City, COBoston Peak, COBurro Creek, AZ

Climax-type Mo depositMt. Emmons, CO

Alkaline Suite

Alkaline to peralkalinetuffs, lavas, intrusionsAlkalic basalts may alsooccur. Trachytespresent.

Little Belt Mountains,MTSilver Cliff, COKane Springs Wash, NVGrants Ridge, NM

The magmatic associations of topaz rhyolites are lessstraightforward. Following Lipman et al. (1972) and Christiansenand Lipman (1972), the magma-tectonic evolution of the westernUnited States may be divided into two fundamentally differentstages. An early suite ofsubduction-related calc-alkaline magmaswas time transgressive across the western United States and pro­duced eruptions of andesitic lavas, dacites, and rhyolites; the lattermostly occur as ash-flow tuffs associated with caldera collapse.After about 20 Ma, the Basin and Range region experienced theeruption of bimodal suites of basalt and rhyolite associated withlithospheric extension. Elsewhere, the temporal relationships aredifferent, but the magmatic and tectonic products of each associa­tion can generally be identified. In the southern Rocky Moun­tains, Elston and Bornhorst (1979) defined an episode transitionalbetween these two associations, which is typified by eruptions ofbasaltic andesite and high-silica rhyolite in a modified back-arcextensional environment. We include these rocks with those of apredominantly calc-alkaline character (cf. Ratte et al. 1984).

Topaz rhyolites appear to have been produced in all of theseigneous associations. In addition, several topaz rhyolites are con­temporaneous with the emplacement of distinctively alkalinemagmas. Thus we have tentatively identified three principalmagma associations in which topaz rhyolites are found; 1) inter­mediate to silicic calc-alkaline volcanic suites; 2) bimodal basalt­rhyolite associations; and 3) alkaline to (silica-saturated) peral­kaline suites in which trachytes (or their intrusive equivalents) areimportant. Table 10 shows these groups. Reference to Figure 42may be helpful in this regard.

In some cases, the definition of the calc-alkaline group isproblematic. The Oligocene topaz rhyolites of Colorado are allgrouped with the calc-alkaline association because they are

broadly contemporaneous with volcanic rocks that show ex­tended and continuous SiOz-variation diagrams, but lack basalticcompositions. Some may be part of a "transitional" groupassociated with the change from subduction-related magmatismto extension-related magmatism (White et al. 1981; Bookstrom1981). In addition, the early Miocene rhyolites of the Wah Wahand Needle Ranges in Utah form a restricted bimodal(trachyandesite-rhyolite) association within the mountain range,but consideration of volcanism op. a broader scale (southwesternUtah) could allow them to be grouped with the calc-alkalinemagmatic association.

The second group consists of topaz rhyolites clearly in bi­modal association with more mafic rocks-the fundamentally

-basaltic group of Christiansen and Lipman (1972). The composi­tional variety of late Cenozoic "basalt" makes the definition ofthis group somewhat arbitrary. The mafic members range incomposition from basalt to andesite and are variably alkaline. Wehave also included the Oligocene rhyolites of the Elkhorn Moun­tains in the group because of their association with basaltic lavasand the apparent absence of intermediate composition rocks.

A third type of magmatic association includes the alkalinerocks associated with topaz rhyolites at Kane Springs Wash vol­canic center, Nevada (trachyte to peralkaline rhyolite); SilverCliff-Rosita, Colorado (andesite, trachyte, rhyolite); and in theLittle Belt Mountains, Montana (trachybasalt to trachyte; quartzmonzonite to syenite). The Grants Ridge rhyolites in New Mex­ico are associated with the construction ofan "andesitic" volcanoat Mount Taylor, which is surrounded by alkali basalt to trachytelava flows, and could be placed with the second group.

In spite of the vagaries of any such classification, it is ob­vious that topaz rhyolites are associated with a variety of igneous

75

Q ~+

55 65

5 i 02 (wt7.)

4

2

011"-_-+-_-+-_-+-""""15

61

6 .• 0

4

2

0

6

4

~ 2N+'~ Specimen Mtn

00 kN 5::s::

4

7555 55

Si02 (wtiO

Topaz Rhyolites

6e

4

2

0 ..5 8 ~4

3

2~

N+'~

0 5N::s::

4

2

0

5h..

7555 55

SiOZ (ivt/.)

d

2

4

ll'5

6

5a

4

2

U

5b

4

Q2

+'~

U0 CN_6

::s::

Figure 42. Potassium versus silica variation diagrams for mafic to intermediate composition volcanicrocks associated with topaz rhyolites from the western United States. The original sources of the data aregiven in the description of the individual occurrences. Representative analyses of topaz rhyolites areindicated by crosses, other volcanic rocks as open boxes. Most topaz rhyolites occur in strongly bimodalassociations with variably potassic mafic rocks (e.g. basalt or basaltic andesite). Topaz rhyolites asso­ciated with trachytic magmas do not show strong SiOz gaps and have variation diagrams which extendto mafic rocks (e.g. Little Belt Mountains, Kane Springs Wash, Grants Ridge/Mt. Taylor). A few, likethose in the Black Range (and the Oligocene of Colorado, not shown), have intermediate to siliciccalc-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 rela­tionship to a single magma series from which they could bederived by differentiation. We suggest that none of these moremafic magmas are parental to the F-rich rhyolites discussed here.Instead, the observation that topaz rhyolites are associated with avariety of more mafic magma suites suggests that the rhyoliteshave a thermal relationship to the more mafic magmas. Theresidence of these mafic magmas in the crust may have providedthe thermal energy required for melting to produce magmas par­ental to topaz rhyolites.

Ore deposits

Mineralization associated with topaz-rhyolite magmatismgenerally consists of F, Be, Li, Cs, U, Sn, Mo(?), and W(?)

(Table 11). The marked magmatic· enrichment of these sameelements in topaz rhyolites strongly suggests that.the ore elementswere derived from the rhyolites or their intrusive relatives in thecase of Climax-type Mo-W deposits (Burt and Sheridan 1980;Burt et al. 1982). Other types of mineralization (alunite, Hg,Au-Ag) are spatially and temporally associated with some topazrhyolites. The association of these deposits with the rhyolites mayrely more on magmatic heat content and volcanologic style fortheir generation than on any particular compositional feature oftopaz rhyolites. A variety of these ore deposit environments areschematically indicated in Figure 44.

Beryllium. The most important ore deposit directly asso­ciated with a topaz rhyolite is the beryllium deposit at SporMountain, Utah. It is currently (1985) the only important sourceof Be in North America. Bertrandite (Be4Siz07) occurs in the

62 Christiansen, Sheridan, and Burt

TABLE 11. MINERALIZATION ASSOCIATED WITHTOPAZ RHYOLITES

Saddle Mountain, AZBurro Creek, AZ

Figure 43. The distribution and ages (Ma) of granitic Climax-type por­phyry molybdenum deposits (open circles) and prospects (x) (Westraand Keith 1981; White et al. 1981), compared to the locations of topazrhyolites (filled circles) in the western United States.

--~-

023

30~028

17~~o24

00 0• 0 0

•

0 36 ••••o

Mineralization

U

Sn

Au, Ag, HgSnAu, Ag

Ag, Pb, Zn, Mo(?)Mo

Mo, WF(?)

Ag, Au, Pb, Zn, Cu

U, F, Hg, Au, Ag, Mo(?)

Be, U, Li, FBe, Rb, Cs, Li

Location

Wah Wah Mtns. vicinity, UTWilson Creek Range, NVKane Springs Wash, NV

Lake City, COGrants Ridge, NMBlack Range, NM

Specimen Mountain, COChalk Mountain, CONathrop, CO

Silver Cliff/Rosita, COTomichi Dome, COBoston Peak, CO

Blackfoot lava field, IDElkhorn Mountains, MTLittle Belt Mountains, MT

Smelter Knolls, UTKeg Mountain, UTMineral Mountains, UT

Cortez Mountains, NVSheep Creek Mountains, NVJarbidge, NV

Thomas Range, UTSpor Mountain, UTHoneycomb Hills, UT

upper part of a tuff beneath a rhyolite lava flow. Likewise, ber­trandite is probably the source of the Be-mineralization at theHoneycomb Hills, Utah (Lindsey 1977). On the other hand, beryloccurs in a number of other topaz-bearing lava flows includingsome lavas in the Thomas Range, Utah, the Wah Wah Moun­tains, Utah, and in the Taylor Creek Rhyolite in the Black Rangeof New Mexico. In each case, beryl occurs within intensely devi­trilled lavas. Late magmatic beryl segregations also occur in thetopaz-bearing Sheeprock granite of west-central Utah (Williams1954). The association of beryllium mineralization with topazrhyolites strongly suggests that the magmatic enrichment of Be inthe rhyolites is important in the genesis of the deposits. Indeed, Beconcentrations average about 60 ppm in glassy specimens of therhyolite from Spor Mountain, a value about 20 times that foundin an average granite.

Climax-type molybdenum deposits. The association oftopaz-rhyolite magmatism and "Climax-type" Mo-W depositshas been noted by Burt and co-authors (1980,1982), Westra andKeith (1981), White et al. (1981), and probably many others inindustry. By way ofjustifying this correlation we note the follow­ing similarities.

1) Topaz rhyolites are mineralogically similar to the igneousand metasomatic rocks associated with Climax-type Mo deposits.Most molybdenite deposits associated with granitic (or rhyolitic)rocks cOl1tain topaz in their mineralized zones. Burt (1981) lik-

ened the alteration process in these intrusions to the formation oftopaz-bearing lithophysae. Garnet, another aluminous mineralcommon in topaz rhyolites, is found in other molybdenum depos­its or prospects (e.g. Pine Grove, Utah: Keith 1980; Mt. Hope,Nevada: Westra 1982; and Henderson, Colorado: Gunow et al.1980). The similarity of biotite compositions, which are sensitiveindicators of volatile fugacities, in both types of magmatic systemshas already been noted in terms of their F and CI contents andratios.

2) The spatial distribution and ages of the major knownClimax-type Mo deposits are illustrated in Figure 43. Their loca­tions are shown superimposed on the distribution of topaz rhyo­lites. The spatial and temporal correspondence of both types ofmagmatism in Colorado, Montana, and Utah have been noted inthe individual descriptions and suggest some sort of genetic linkexists between these distinctive groups of rocks.

3) The tectonic setting of both types of magmatism appearsto be in continental rifts or in zones of back-arc extension (e.g.Sillitoe 1980). Both magma types a.re emplaced in the upper crustas relatively small stock-like intrusions that explosively vent tothe surface to emplace relatively small volumes of tuff and/orlava (J. E. Sharp 1978; Keith 1982).

4) Multiple intrusion/extrusion episodes are apparent forboth types of magmas.

5) The chemical similarity of the magmas involved is shown

Topaz Rhyolites 63

Pre-existing Sediment

Dome

4

PyroclasticDeposit

Figure 44. Schematic cross-section showing the hypothetical structure of a small rhyolite dome complexand some of the types of mineralization possibly associated with topaz rhyolite volcanism (after Burt etal. 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 preciousmetal veins (Ag, Pb, Zn, Au, Sn, W, etc.); 7 = mineralized breccia pipes (Mo, Ag, Au, F); 8 = stockworkporphyry deposits (Mo, Sn, W, etc.); 9 = fluorite-rich skarn and/or sulfide-rich replacement ore bodiesin non-eruptive environment (Sn, W, Be, etc.); and 10 =greisen-bordered veins in non-eruptive envi­ronment (Sn, W, Cu, Zn, Be, etc.).

in Table 12. The granites of Climax-type Mo-systems and topazrhyolites share their major characteristics: high Si, K, Na, and Fand low Ti, Fe, Mg, and Ca (cf. Figure 35). The principal differ­encesbetween the rocks lie in the lower concentrations of P andMg in topaz rhyolites. Differences in the Na and K contents areprobably not meaningful because most Climax-type intrusionshave experienced some potassic alteration. The trace elementsignature of topaz rhyolites (high D, Be, Sn, Li, Nb, Rb, and Fand low Sr, Ba, and Ti) is typical of Mo-related systems as well(Westra and Keith 1981; Keith 1980; Mutschler et al. 1981;White et al. 1981).

The similarities in distribution, age, tectonic setting, mode ofemplacement, chemistry, and mineralogy of fluorine-rich subalk­aline rhyolites (with topaz or garnet) and Mo-mineralized"rhyolite" stocks leads us to conclude that the eruption of topazrhyolites may be a surface manifestation of a potentially ore­forming intrusive system.

Tin. The small deposits of cassiterite and wood tin that

occur in the Sheep Creek Range, Nevada, and in the BlackRange, New Mexico, are similar to the numerous small Sn­deposits of Mexico. Burt and Sheridan (1984) and Duffield et al.(1984) conclude that the high temperature vapor-phase deposi­tion of cassiterite (and topaz) results from the extraction of Snfrom the rhyolitic glass or lava. The halogens appear to be effec­tive complexing agents for Sn (Manning 1981b; Jackson andHelgeson 1985). Breccias and other permeable zones (i.e. flowbands and fractures) in the upper parts of these lavas are favora­ble locations for the accumulation, decompression, and cooling ofmetal-bearing vapors or fluids, resulting in the common associa­tion of vapor-phase features increasing in abundance toward thetops of rhyolite domes and flows. Low-temperature remobiliza­tion of Sn by circulating meteoric waters may lead to the deposi­tion of wood-tin in narrow veinlets in cooling and flow fractures.The small size of such deposits will probably prevent their suc­cessful exploitation in this country. Nonetheless, erupted topazrhyolites may be indicators of topaz granites that may develop

64 Christiansen, Sheridan, and Burt

TABLE 12. AVERAGE COMPOSITION OF SILICIC INTRUSIONSRELATED TO Mo-DEPOSITS

1 S.D. 2 S.D. 3 S.D.

Si02 76.1 1.04 75.8 2.23 75.6 0.71Ti02 0.11 0.11 0.28 0.73 0.14 0.09A1203 12.7 0.46 13.4 0.82 12.8 0.38

Fe203 0.84 0.36 0.71 0.83 1.12* 0.19FeO 0.55 0.26 0.42 0.38MnO 0.06 0.05 0.04 0.03 0.06 0.01

MgO 0.46 0.23 0.24 0.23 0.15 0.07CaO 0.74 0.32 0.66 0.45 0.83 0.37Na2 0 3.30 0.38 3.67 0.48 3.73 0.28

K20 5.08 0.60 4.85 0.51 5.04 0.35P2 0 5 0.09 0.44 0.05 0.06 0.00 0.01F 0.21 0.21 0.11 0.11 0.23 0.09

Note: All analyses in weight percent and recalculatedH20, C02 and S02 free. S.D. is one standard deviation.

*Total Fe as Fe203

1. Average of 13 unaltered ore-related granite andrhyolite porphyries from Mo deposits (Mutschler etal. 1981).

2. Average of 50 granite and rhyolite porphyries fromunmineralized stocks near Mo deposits (Mutschler etal. 1981).

3. Average of 14 rhyolite lavas from the Thomas Range,Utah (Christiansen et al. 1984).

economic deposits of Sn (or W) in greisens or skarns (e.g., EastKemptville, Nova Scotia: Richardson et al. 1982; and AnchorMine, Tasmania: Groves 1972).

Uranium. Small, generally sub-economic, deposits of ura­nium are associated with many topaz rhyolites. The enrichmentof U in topaz rhyolites probably accounts f<>r this association.Notable examples include the rhyolites at Spor Mountain, Utah,in the Wah Wah Mountains, Utah, and those near Lake City,Colorado. The Be tuff member of the Spor Mountain Formationcontains low-grade U (and Th) mineralization that overlaps theBe ore zone (Lindsey 1982; Bikun 1980). The U occurs in fluoriteand opal. Accumulations of U (as uranophane and weeksite) alsooccm in a small lens ofnon-volcanic conglomerate at the base ofthe Be tuff. The· U was probably leached by groundwater fromthe U-rich tuff and is not associated with enrichments of Be or Li.In the Wah Wah Range, U occurs in small altered horizons withFe~oxides and in conformable lenses with pyroclastic depositsbeneath topaz-bearing lavas (Christiansen 1980).

Fluorite. Fluorite deposits are closely associated with topazrhyolites from Spor Mountain, the Wah Wah Mountains, andNeedle Range, Utah, and perhaps with the Nathrop Volcanics ofcentral Colorado. Fluorite occurs in calcic rocks (sedimentarycarbonates or intermediate composition volcanic rocks) spatiallyassociated with topaz rhyolite vent complexes and in the SporMountain district in tuff-lined breccia pipes. A hint to the originof the F. enrichment of topaz rhyolites and the generation offluQrite deposits in general lies in their distribution, as reiteratedmost recently by Eaton (1984b). It has been known for several

decades (e.g., Peters 1958) that fluorite deposits are more com­mon in the eastern part of the American Cordillera. With the newunderstanding of the accretionary history of the continental crustof western North America, it is clear that fluorite deposits (as wellas Be, Sn, and Climax-type Mo) are restricted to terranes under­lain by Precambrian craton. Paleozoic and Mesozoic accretedterranes consisting of fragments ofocean floor and island arcs arenot typified by fluorite deposits; instead, an association with Auand Hg mineralization is indicated. Christiansen and Lee (1985)have shown that the granitoids of the northern Great Basin showdifferences in F-content that correlate with their location; grani­toids in accreted terranes are F-poor, whereas granitoids rooted inPrecambrian sial are variably enriched in F. They interpret thisdifference as resulting from different F concentrations in thecrustal component of their parent magmas. These disparateobservations point to the Precambrian continental crust as theultimate source of the F (and probably Be, Sn, and Mo as well) inthe ore deposits and in topaz rhyolites.

COMPARISONS WITH OTHER TYPES OFRHYOLITIC ROCKS

The geochemical distinctiveness of topaz rhyolites is clearerwhen contrasted with other types of rhyolitic volcanic rocks. Inwestern North America, topaz rhyolites are nearly contempo­raneous with calc-alkaline rhyolites and peralkaline rhyolites.The calc-alkaline rhyolites are part of an early to mid-Cenozoicsuite of intermediate .to silicic composition (e.g., Lipman et al.,1972). The peralkaline rhyolites are part of a late Cenozoic bi­modal suite of basalts and rhyolites (Christiansen and Lipman1972). In spite of their close temporal and spatial association withthese silicic rocks, topaz rhyolites are distinct from both.

Calc-alkaline rhyolites

Calc-alkaline rhyolites are the silicic representatives of theorogenic magma series characterized by a lack of iron-enrichmentduring its differentiation. Calc-alkaline rhyolites are typically as­sociated with andesitic volcanism on continental margins overly­ing subduction zones. They generally occur as small domes orlava flows associated with composite volcanoes or calderas butmay form voluminous ash-flow sheets. Large volumes of high-Kcalc-alkaline rhyolite were erupted during the mid-Cenozoic ofthe western United States.

Ewart (1979) has reviewed the chemistry and mineralogy ofthe silicic orogenic volcanic rocks, including the calc-alkalineseries. He points out that the calc-alkaline rhyolites generallycontain phenocrysts of plagioclase, Mg-augite, Mg-hypersthene,Ca-Mg hornblende, Mg-biotite, Fe-Ti oxides, and occasionallyolivine. High-K varieties contain quartz and sanidine. Zircon,apatite, titanite, and allanite are notable accessory minerals. Al­though generally not fluid-saturated before eruption, the commonpresence ofhornblende and biotite in these rhyolites indicates thatthey are relatively hydrous. The T-fo2 relationships for some

Topaz Rhyolites 65

TABLE 13. COMPARISON OF CALC-ALKALINE RHYOLITES WITHTOPAZ RHYOLITE

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

Rhyolite

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

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

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

Trace elements (ppm)

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

Ba 1500 2000 1135 645 859 41Th 24 6 14 12 49U 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 ofstudies indicate that most orogenic silicic rocks crystallize underrelatively oxidizing conditions-2 to 3 log units above the QFMbuffer (Ewart 1979; Hildreth 1981; Gill 1981). This property isexpressed in the Mg-rich nature of the mafic minerals includingbiotite and hornblende. Calc-alkaline batholithic rocks from theSierra Nevada and intermediate to silicic rocks of western Utahand eastern Nevada have strikingly different compositions ofbio­tites and hornblendes when compared to those found in topazrhyolites from the western United States (Figure 31).

Although there is substantial chemical variation among calc­alkaline rhyolites, they are generally richer in AI, Ti, Fe, Mg, andCa, and poorer in totalll1kalies and F (although data are sparse)than topaz rhyolites (Ewart 1979). In Table 13, five analysesrepresentative of calc-alkaline rhyolites are compared with the"typical" topaz rhyolite composition described above. Few anal­yses of the halogens in glasses from calc-alkaline rhyolites exist,but substantial differences in FICI are indicated. FICI ratios maybe less than 1 in magmas related to subduction processes (e.g.Garcia et at. 1979; Coradossi and Martini 1981). In terms of theirtrace element characteristics, calc-alkaline rhyolites generallyhave lower concentrations of Rb, U, Th, Nb, Ta and other in-

compatible elements, and have higher concentrations of Ba, Sr,and other compatible elements than topaz rhyolites. The K-Thand Th-U concentrations of calc-alkaline volcanic rocks fromwest-central Utah are compared with topaz rhyolites from thesame area in Figures 44 and 45; the enrichment of topaz rhyolitesin U and Th is obvious. Little REE data exists for the suite ofcalc-alkaline rhyolites that preceded the eruptions of topaz rhyo­lites in Utah, New Mexico, and Colorado. The relationshipsshown for the topaz rhyolites from Nathrop, Colorado, and aslightly older calc-alkaline rhyolite from the San Juan volcanicfield may be typical (Figure 40). The topaz rhyolite from Nathropis relatively depleted in HREE compared to other topaz rhyolites,but it is nonetheless enriched compared to the calc-alkaline rhyo­lite from Summer Coon volcano. Likewise the rhyolite fromNathrop shows a striking negative Eu anomaly. These.importantdifferences notwithstanding, the differentiation trends of calc­alkaline rhyolites appear to be similar to those of topaz rhyolites,but the extreme enrichments and depletions noted above are notobserved.

It is generally agreed that rhyolitic magmas may originate byfractional crystallization ofplagioclase, pyroxenes, and Fe-Ti ox­ides from dacite or rhyodacite (e.g. Ewart 1979). Crustal fusion

66 Christiansen, Sheridan, and Burt

-

4

-

-

00

3

I

•

o

••

o00 00

o Q

o 00cP1iPoO

2•

In Th cone. (ppm)

••

In U cone. (ppm)

•••

• ••• 0 •••••. ..-: .\ .. .••

• •• •

2 r------....,.r------'.r---------,

11-

eruptions (Noble and Parker 1974). Peralkaline rhyolitesgenerally contain phenocrysts of anorthoclase or sodic sanidine,quartz, sodic ferrohedenbergite, aenigmatite, and fayalite (Suther­land 1974). Arfvedsonite and riebeckite generally crystallize asdevitrification products. Zircon and apatite are common acces­sory minerals. Fe-Ti oxides mayor may not be present (Nichollsand Carmichael 1969). The anhydrous nature of most Fe-Mgsilicates and the common enrichment in Cl suggests that mostperalkaline rhyolites were not fluid-saturated before eruption(Bailey 1980). Chlorine partitions strongly into hydrous fluidsand would be quantitatively extracted from a saturated magmawhile F prefers to remain in a magma that coexists with a fluid(Burnham 1979). Such a pattern does not appear in peralkalinerhyolites (Figure 37). The common absence of hydrous maficsilicates indicates a low water fugacity andlor high temperature.Mineral geothermometry indicates that crystallization occurs attemperatures generally exceeding 800°C (e.g. Wolf and Wright1981; Conrad 1984; Mahood 1981; Ewart 1981). The Fe-richcharacter of the mafic silicates and estimates of f02 from co­existing ilmenite and magnetite suggest that crystallization occursat low f02 in many silicic peralkaline magmas (between QFMand WM; Ewart 1981; Wolf and Wright 1981; Conrad 1984;Mahood 1981).

The most important chemical features of peralkaline rhyo­lites relative to other rhyolites are high Fe, Mn, Ti, F, and Cl,along with low Al and Ca (Table 14). They are distinct fromtopaz rhyolites in each of these characteristics except their gener­ally high fluorine content.

The F and Cl content of peralkaline rhyolites is compared tothat of topaz rhyolites in Figure 37. Peralkaline rhyolites haveFICI of less than 3 and are easily distinguished from topaz rhyo­lites on this diagram. The affinity of fluorine for continental set­tings has been pointed out by Bailey (1980), who showed that a

o1--....... L-1 ......11..... ..-l FICI ratio of 1 divides oceanic from continental peralkaline2 3 4 5 rhyolites. Schilling et al. (1980), however, have shown that glasses

from tholeiitic mid-ocean ridge basalts have high FICI ratios(averaging 8.5), but at much lower concentrations than those dis­cussed here. F ranges from 150 to 400 ppm. Plume-type magmas,associated with oceanic peralkaline rocks, have lower FI Cl ratios.

Extreme enrichments and depletions of certain trace ele­ments characterize both peralkaline and topaz rhyolites, but per­alkaline rhyolites from the western United States generally havelower concentrations of Rb, V, Th, Ta, and Ba and higher con­centrations of Zr, Hf, Nb, and Zn (Figure 41; Christiansen et al.1983a). The contrast between the two types of magmas is mostclearly seen in the nature of the negative Th-V "anomalies" andabsence of negative Zr anomalies in peralkaline rhyolites whencompared to subalkaline rhyolites. Peralkaline rhyolites also lackor have small negative Cl anomalies. In addition, peralkalinerhyolites generally have higher concentrations of LREE than doF-rich aluminous rhyolites. As a consequence they have steeperchondrite-normalized REE patterns (Christiansen et al. "1983a).Peralkaline rhyolites show differentiation trends (indexed by in­creasing (NazO +KzO)1Alz03 and incompatible trace elements)

5 _-----r-.----...,.r-------,

21

.CJI:oCJ

..c::Eo-I: 3 ~-

.CJI:oCJ

oN

:::.:::I:-

-

Figure 45. Geochemical comparison of Cenozoic volcanic rocks fromwest-central Utah. (a) Logarithmic plot of K20 versus Th (b) Logarith­mic plot of Th versus U. Open circles = late Tertiary topaz rhyolites;closed circles = mid-Tertiary calc-alkaline rhyolites and rhyodacites.Data are from Lindsey (1982) and Christiansen et al. (1980).

Peralkaline rhyolites contain a molecular excess of NazO +KzO over Alz03, expressed as normative acmite (for F- andCl-free analyses). They are most easily recognized by the presenceof sodic pyroxenes or amphiboles as phenocrysts or as vapor­phase minerals. Many peralkaline rhyolites in the Great Basinwere erupted during the late Cenozoic in large caldera-forming

and assimilation may also play an important role in the differenti­ation of silicic orogenic magmas (e.g. Myers and Marsh 1981;Grove et al. 1982; Hildreth 1981).

Peralkaline rhyolites

Topaz Rhyolites 67

TABLE 14. COMPARISON OF PERALKALINE RHYOLITES WITHTOPAZ RHYOLITE

Comendites (1) Pantellerites(l) TopazRhyolite (2)

average range average range average

Si02 74.0 69.4-75.0 71. 2 67.4-74.9 75.6Ti02 0.21 0.09-0.87 0.37 0.14-0.65 0.14A1203 11.6 10.2-13.4 9.11 6.30-11. 3 12.8Fe203 1. 25 0.40-3.22 2.38 0.44-5.60 1.12*

FeO 1. 88 0.80-3.70 4.52 1.60-6.73MnO 0.08 0.01-0.17 0.21 0.03-0.36 0.06MgO 0.04 0.0-0.22 0.09 0.0-0.75 0.15

CaO 0.36 0.0-1.12 0.45 0.06-2.04 0.83Na20 5.35 3.99-6.39 6.44 4.68-7.83 3.73K20 4.46 3.49-4.98 4.40 3.39-4.90 5.04

P205 0.02 0.0-0.08 0.05 0.0-0.28 0.00F 0.37 0.06-0.76 0.30 0.11-1. 30 0.33Cl 0.24 0.05-0.41 0.28 0.06-0.82 0.06

* Total Fe as Fe203

1. Macdonald 1974a.2. Average of 11 rhyolite lavas from the Thomas Range

(Christiansen et al. 1984; Turley and Nash 1980, forCl) .

that differ markedly from topaz rhyolites. Trachyte to pantelleritetransitions are marked by decreasing AI, Ca, Ba, Sr, Mg, Sc, Ti,Ni, and Co that correlate with increasing Na, Cl, Mn, Fe, Zn, Hf,Zr, Ta, Y, Nb, REE, U, Th, Rb and occasionally Eu and P(Macdonald and Bailey 1973; Noble et al. 1979; Civetta et al.1984). Differentiation trends involving trachytes and comenditesare similar to those characteristic of more aluminous magmaswith decreasing Fe, Ti, and P; but Zr, Hf, and the REE (excludingEu) remain incompatible (e.g., Ewart 1982; Conrad 1984). Someof these characteristic trace element features can be explained bythe high solubility of Zr in peralkaline melts and the consequentlack of zircon fractionation. In a similar fashion, the absence ofstable REE-rich aluminosilicates like allanite, and perhaps phos­phates like monazite as well, may explain the high concentrationsof LREE in fractionated peralkaline rhyolites.

Silicic peralkaline rocks occur predominantly in continentalrift environments or rift-like settings (Macdonald 1974b) and areprominent members of bimodal volcanic suites. Peralkaline rhyo­lites also occur in oceanic islands and late orogenic suites but thecommon feature linking all of the geologic environments is litho­spheric extension. We have shown that this strong associationwith extension is also typical of topaz rhyolites.

Peralkaline rhyolites can be derived by fractionation of al­kali basalt through an intermediate trachytic composition. Mostquantitative major and trace element models invoke fractionationof plagioclase, andlor ternary alkali-feldspar, Fe-rich pyroxene,magnetite, apatite, and olivine from trachyte (or high-alkali da­cite) to produce a vertically zoned chamber with comendite orpantellerite residing in the upper part of the chamber (Barberi etal. 1975; Civetta et al. 1984; Parker 1983; Middlemost 1981;Bevier 1981; Souther and Hickson 1984; Conrad 1984). Co-

mendite to pantellerite transitions are rarely observed. In contrast,Hildreth (1981) invokes the action ofan "extraordinarily halogen­rich" flux· released from crystallizing basalt to produce partialmelting of the lower crust or an earlier accumulation of under­plated gabbro to yield the parental magmas for peralkalinerhyolites.

Aluminous bimodal rhyolites

Ewart (1979) established that (non-peralkaline) rhyolites ofbimodal associations are distinctive from most orogenic rhyolitesin their mineralogy and chemistry. He notes that they havestrongly Fe-enriched mafic silicates and appear to have crystal­lized at temperatures in excess of about 800°C and at low f02

(between QFM and NNO) as compared to calc-alkaline rhyo­lites. In addition, Ewart points out that these rhyolites generally.exhibit fractionated trace element patterns-Ba, Sr, Cr, Ni, and Vare depleted and Nb, Pb, and La are enriched. Although weregard topaz rhyolites as part of this group, a variety of rhyolitetypes exists within it. At one extreme lie the high temperature,pyroxene (and commonly one-feldspar) rhyolites of the SnakeRiver Plain region (Hildreth 1981; Hildreth and Christiansen1984; Leeman 1982a; Wilson et al. 1983). These rhyolites aretypified by anhydrous mafic silicates such as pyroxene and fayal­ite. At another extreme lie two-feldspar rhyolites with low equili­bration temperatures and biotite as the principal mafic phase, asin topaz rhyolites. The bimodal rhyolites of the Coso Range,California (Bacon et al. 1981) and at Twin Peaks, Utah (Crecraftet al. 1981), are chemically similar to topaz rhyolites. Notablecontrasts between the two groups include the relatively highKINa, Zr, Fe, and Ti of the first group, coupled with less extremeenrichments of incompatible trace elements, including F. Mostinvestigators derive the parental magmas for bimodal rhyolites bypartial melting of the (lower) continental crust (Hildreth 1981;Leeman 1982a; Ewart 1982; Christiansen et al. 1983a). Subse­quent fractionation (near the minimum in the granite system) ofplagioclase, alkali feldspar, quartz, biotite or pyroxene, Fe-Ti ox­ides, and accessories (apatite, zircon, allanite, and monazite) leadsto the characteristic compositions of these high-silica rhyolites.Fractionation may occur enroute to the surface or in (relativelyshallow) magma chambers. These rhyolites have a close thermal,but not chemical, relationship to contemporaneous basalts thatappear to have provided the heat for crustal melting.

Ongonites

Several authors (Burt and Sheridan 1981; Turley and Nash,1980; Christiansen et al. 1983a) have suggested that topaz rhyo­lites are similar to the so-called ongonites that occur in Mongoliaand the Trans-Baikal region of the U.S.S.R. (Kovalenko andKovalenko 1984). Ongonites are defined as topaz-bearing"quartz keratophyres." They occur in subvolcanic dikes, stocks,and as lava flows with underlying pyroclastic depositS (Kova­lenko et al. 1971; Kovalenko and Kovalenko 1976; Kovalenko etal. 1979). The primary minerals of ongonites are albite, potas-

68 Christiansen, Sheridan, and Burt

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

100

E 800-0- 60.....0Z

40

30

20

a

200

100

+ - ......

200 300 400 600 800 1000 2000

Rb(ppm)

Figure 46. Rb-Nb-Ta concentrations in ongonites from central Asiacompared to other rhyolitic rocks from western North America. a) Loga­rithmic plot ofRb and Nb. b) Logarithmic plot ofTa and Nb. Arrowsshow differentiation sequences in topaz rhyolite dome complexes (WW=Wah Wah Mountains; SM =Spor Mountain; TR =Thomas Range)and from the Coso Range, California, rhyolites which are not known tocontain topaz (COSO - Bacon et al. 1981). The compositions of4 ongo­nites that do not lie in the same field as others are shown with crosses.The average compositions of three types of orogenic rhyolites from thewestern United States (Ewart 1979) are also shown (open circles), alongwith 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 nearOngon, Mongolia, which contains columbite. The fractionation of thismineral may have buffered evolving liquid compositions to Nb concen­trations of approximately 60 ppm in much the same manner that zirconsaturation controls Zr concentrations. Data for topaz rhyolites and ongo­nites are from sources cited in text.

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

60 80 10030 40201053210 L.- ---I:....-....:.-......._-L----I'--..L..-..L..-.L.....1~ ..L._.._ ____J'______L_ _L_...................L....L....I

1

100~

80

"60Ongonites I-E Ic.

/c.- ..,.,.0 40 --- --Z ~~

~

30 .<,.fl>o

::.Q\~

20

b Ta(ppm)

Topaz Rhyolites

TABLE 15. AVERAGE MAJOR ELEMENT COMPOSITION OFONGONITES FROM CENTRAL ASIA

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

Mongolia Mongolia USSR Mongolia Utah

Si02 71. 4 74.4 73.0 74.1 73.2Ti02 0.09 0.10 0.05A1203 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 0.58MnO 0.18 0.05 0.09 0.05 0.06

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

K20 3.34 -3.67 4.14 4.60 4.86P205 0.07 0.05 " 0.05 0.04 0.00F 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 (Kovalenkoand 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.

69

sium feldspar, and quartz. Micas (biotite, muscovite, or lithium­phengite) occur as phenocrysts and in the groundmass along withtopaz. Accessory minerals include fluorite, gamet, zircon, Fe-Tioxides, columbite-tantalite, cassiterite, Li-phosphates, pyrite, andsometimes tourmaline. The relative importance of magmatic ver­sus vapor-phase crystallization in developing this mineralogy isunclear; but the relatively high Rb/Nb ratios in ongonite (Figure46) suggest that columbite may be a fractionating magmaticphase. Kovalenko and Kovalenko (1976) also regard topaz andmica to be magmatic, but the samples do not appear to be glasses.

The average chemical composition of four ongonites isshown in Tables 15 and 16. Although there are some obviousdifferences between topaz rhyolites and these ongonites (ongo­nites have higher Al and P and lower Si), we feel that theirsimilarities are greater. Figures 41 and 46 show the overall sim­ilarity in the chemical features of ongonites and topaz rhyolitesfrom the western United States. Ongonites are markedly enrichedin F, Li, Rb, Cs, Nb, Ta, Be, and other incompatible lithophileelements and are depleted in Zr, Ba, Sr, and Eu just like thefluorine-rich rhyolites described here. Kovalenko et al. (1983)published the REE concentrations of one ongonite from Mongo­lia (Figure 47) that shows a familiar negative Eu anomaly andextreme HREE enrichment, so much so that LaN/YbN is lessthan one. Overall, the REE are depleted when compared withtopaz rhyolites. Based on phenocryst/matrix partition coefficientsand .experimental studies; Kovalenko (1977), Kovalenko et al.(1978), and Antipin et al. (1980a,b) have suggested that theseextreme geochemical features are the result of protracted frac-

tional crystallization of crustally-derived magmas with 0.2. to0.5% F. Expansion of the stability field of quartz by elevatedfluorine contents leads to Na and Al enrichment with Si depletionduring crystal fractionation (Manning 1981) as noted for the SporMountain rhyolite, Utah. The simultaneous fractionation ofquartz, feldspars, and REE-rich accessory minerals seems to berequired (Kovalenko et al. 1983). Some fluid-phase transport ofF and fluorophile elements to the upper parts of evolving magmachambers may have aided in their differentiation. However, toenrich the melt in these elements by this process would requirethe volume of the fluid to exceed the volume of melt by up toseveral hundred times. The suggested origin by fractional crystal­lization is strengthened by the association ofongonites with largegranitic massifs from which they may have differentiated. Theirspatial and temporal association with basaltic lavas and a descrip­tion of a mixed (basalt-ongonite) magma (Kovalenko et al. 1975)suggest that ongonites, like topaz rhyolites, are products ofbimo­dal magmatic processes. Regarded as the volcanic analogs of Li-Frare-metal granites, ongonites are associated with Wand othertypes of rare-metal mineralization (Mo, Li, and F).

PETROGENETIC MODEL FOR TOPAZ RHYOLITES

Based on the information reviewed here, Christiansen et al.(1983a) have formulated a petrogenetic model that accounts forthe principal features of topaz rhyolites. We summarize thismodel in the context of the nature and composition of the crustalsource of topaz rhyolites; the net power input, as represented by

70 Christiansen, Sheridan, and Burt

TABLE 16. AVERAGE TRACE ELEMENT CONCENTRATIONS INONGONITES FROM CENTRAL ASIA

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

Mongolia Mongolia USSR Mongolia Utah

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

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

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

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

Note: All analyses in ppm.

1. Average of 53 analyses of the Amazonitov dike (Kovalenkoand 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).

0.1L.-~--'-----'-----'-_L.-~--'-----'-----'_-'----'-----'-----'-_L.-~...J

Ongonite

100

60

~-6 10c0~

06

Ql

C.E11Ien

0.6

La Ce Nd 8m Eu Gd Dy Ho Yb Lu

Figure 47. Rare earth element pattern for a Mesozoic ongonite fromMongolia as reported by Kovalenko et al. (1983).

equilibration with metasedimentary graphite. Likewise, whole­rocks are not strongly peraluminous and in many cases are meta­luminous. Equilibration of igneous melts with muscovite oraluminosilicates produces liquids with 3 to 8% normative corun­dum (Thompson and Tracy 1977; Clemens and Wall 1981). Inaddition, the relatively high temperatures of some lavas (up to

the flux of mafic magma, to the base of the continental crust; andthe nature and magnitude of stress in the lithosphere. Accordingto Hildreth (1981), these are the principal controls on the natureof continental igneous rock associations and their eruption styles.

The distribution of topaz rhyolites in the western UnitedStates points strongly to the importance of a magmatic compo­nen.t derived from the Precambrian craton. of North America. Thenotibn that a distinctive crustal reservoir is the source of theF-enrichment found in topaz rhyolites is supported by the distri­bution of fluorite deposits (Eaton 1984b) and F-rich granitoids(Christiansen and Lee 1985) in. the western United States asdescribed above. In the absence of Precambrian crust in. thenorthwestern part of the Great Basin., topaz rhyolites are notgenerated. Instead, a sUbtly different bimodal rhyolite, emplacedin small extrusive domes with high Na/K ratios and with lowerconcentrations of incompatible trace elements, is widely distrib­uted in. western Nevada and eastern Oregon (Wilson et al. 1983;E. H. Christiansen, in. preparation).

In spite of this inferred crustal origin, topaz rhyolites are notevolved volcanic equivalents of S-type granites derived by partialmelting of pelitic metasedimentary rocks. The rhyolites possessdistinctly lower initial 87Sr/86Sr, 207Pb/204Pb, and 180/160 ra­tios than the S-type granites of the western United States (Wilsonet al. 1983; Lee et al. 1981; Farmer and DePaolo 1983, 1984).The compositions of biotites from topaz rhyolites are distinctlyless aluminoUS than those in. muscovite- or garnet-bearing S-typegranites. The relatively oxidized conditions under which sometopaz rhyolites crystallized (QFM or greater) is inconsistent with

Topaz Rhyolites 71

850°C) and their rise to shallow crustal levels suggest that mus­covite decomposition was not involved in their genesis.

The low to moderate Sr-isotope ratios of topaz rhyolitessuggest that their protoliths had Rb/Sr ratios of 0.04 to 0.08.These are relatively low ratios for a source in the continentalcrust, which is typified by Rb/Sr ratios in excess of 0.2 (Taylor1964). However, such low ratios are typical ofgranulitic terranesthat experienced Rb depletion during metamorphism and/or ana­texis. This sort of protolith is also consistent with the oxygenisotope ratios, but sparse Pb isotope ratios suggest that the Th/Uratio of the protolith must have been "normal" rather than high,as is found in many granulitic terranes. Small amounts of uppercrustal contamination are suggested by the high initial Sr-isotoperatios found at some complexes (e.g. the Thomas Range andNathrop) and suggest caution in attributing measured isotopicvalues to magmatic sources.

Another important indication of a high-grade metamorphicprotolith for topaz rhyolites are their elevated F concentrationsand low F/Cl ratios. Hydrous minerals from high-grade meta­morphic rocks are F-rich. As shown by Holloway (1977; see alsoHolloway and Ford 1975; and Manning and Pichavant 1983),high F/(F+OH} ratios increase the thermal stability ofbiotite andamphibole. Others have shown that F/(F+OH) ratios in hydrousmafic silicates increase with increasing metamorphic grade, ex­tending to granulite facies (Fillippov et aI. 1974; Janardhan et aI.1982; E. R. Padovani, oral communication, 1984) or to the onsetof melting (White 1966). Thus, although the absolute amount ofbiotite may decrease with increasing grade of metamorphism, itprobably becomes more F-rich. The decomposition of smallamounts of F-rich biotite would therefore produce small amountsof aluminous F-rich melt (probably on the order of 0.2 wt% F)that could evolve to produce a topaz rhyolite. Such melts areprobably less viscous than their dry equivalents (Dingwell et aI.1985). It is perhaps noteworthy that scapolites from granulitegrade rocks are Cl-poor relative to those found in amphibolitegrade metamorphic rocks (Hoefs et al. 1981). A depletion of Cl isexpected in granulites whether they are formed by reaction with aC02-rich fluid with consequent dehydration or by the removal ofa silicate melt. In either case CI would preferentially partition intothe escaping fluid/melt. Thus, igneous rocks derived from granu­lites would be expected to have high F/CI ratios as found in topazrhyolites.

Although consistent with a granulitic source, the require­ment that the sources of topaz rhyolites have relatively lowRb/Sr ratios is in sharp contrast to the remarkably high Rb/Srratios of topaz rhyolites themselves. By analogy with high-grademetamorphic rocks, the presumed lower crustal protoliths shouldalso be depleted in other elements characteristically enriched intopaz rhyolites such as U, Th, K, Cs, Li, Be, Nb, Ta, and Y(Collerson and Fryer 1978; Sheraton et al 1984; Condie et aI.1982). This "dilemma" can be resolved if the degree of partialmelting that produces topaz rhyolites is low. Using estimated bulkpartition coefficients for granulitic restite, Christiansen et aI.(1983b) suggest that values of approximately 10% batch partial

melting of a low Rb (30-50 ppm) and U (1.5 ppm) protolith willproduce magmas that could fractionate toward compositions typ­ical of topaz rhyolites such as the Spor Mountain rhyolite. Suchsmall proportions of partial melting are a natural consequence ofwater-undersaturated melting of high-grade metamorphic rockssuch as granulites. The small degree of melting required by in­compatible element enrichments could occur with the completedecomposition of less than about 10% biotite at lower crustalpressures (cf. Clemens 1984; Burnham 1979). The decompositionof biotite and its replacement with residual pyroxene (plus melt)would also lower the bulk partition coefficient between rhyoliticmelt and restite enhancing Rb enrichment in the melt. Thus itappears that the concentrations of Rb, U, and by analogy othertrace elements enriched in topaz rhyolites, need not be higherthan those found in average continental crust. The inferred con­centrations are in fact significantly lower than average for conti­nental crust. Likewise, there is no requirement that the crustalsources of topaz rhyolites contained anomalously low concentra­tions of Sr, Ba, Eu, Ni, and other compatible elements. Instead,the small degrees of partial melting in the lower crust, followedby substantial crystal fractionation (of sanidine, quartz, plagio­clase, biotite, Fe-Ti oxides, and accessories) enroute to the surfaceand in small magma chambers have produced the characteristiccompositional features of topaz rhyolites.

The proposed granulitic nature of their protoliths, their ex­tensional tectonic setting, and their geochemical features implythat topaz rhyolites may be the extrusive equivalents of A-type oranorogenic granites (Loiselle and Wones 1979; Collins et aI.1982). There are two "species" of anorogenic granites that mayhave contrasting origins and evolutionary histories. One type ismetaluminous to slightly peraluminous (analogous to topaz rhyo­lites) and the other is peralkaline (analogous to peralkaline rhyo­lites). In granitic complexes both types may coexist, one intrudingthe other or grading into the other (e.g. the Arabian shield, Stuck­less et aI. 1982; or the Younger granites of Nigeria, Bowden et aI.1984). A similar situation is apparent for the comenditic ash-flowtuffs that preceded the eruption of the topaz rhyolite at KaneSprings Wash, Nevada.

Collins et aI. (1982) and Christiansen et aI. (1983a) haveemphasized the role of F and CI in the evolution of A-typegranites. The fractional crystallization histories of these magmasmay be controlled in part by their characteristic F/Cl ratios. Forexample, Manning et aI. (1980) have suggested that F and Alhave a strong affinity in granitic melts-so much so that Al isremoved from tetrahedral coordination in the aluminosilicateframework of the melt and placed in interstitial sites in octahedralcoordination. This effect may result in the lowering of the activityof aluminum in the melt and coexisting minerals maintaining analuminous composition throughout the fractionation history ofthe magma. In contrast, Cl-rich magmas may experience en­hanced Ca-plagioclase fractionation as a result of Na-CI com­plexes in the melt. This process could lead to the production ofperalkaline rhyolites through the plagioclase effect of Bowen(1928). Any process such as volatile escape, which would signifi-

72 Christiansen, Sheridan, and Burt

CrustMantle

SilicicHybrid Magma

«F/CI)

I. t, ,'. ,\. \

\ . '\\ I ,'. ,. I \ \ \Residual /j.' .. \' \\Reworked,,/ I \ \Crust / /.. .

/' . \

Cry s ta lliz a t ion,Degassing,Contamination of Basaltic Magma

CalderaSystem

d

Pre-eruptionCumulates

Figure 48. Hypothetical cross-sections of magma systems (modified inlarge part from Hildreth 1981), showing a variety of environments inwhich topaz rhyolites are produced. The relative sizes ofvolcanic edificesare exaggerated. a) The intrusion of hot mantle-derived basalt into thecontinental crust may result in partial melting of felsic granulites in thelower to middle crust upon decomposition of hydrous minerals. Exten­sional tectonism and a diffuse focus ofdike injection (caused perhaps bydistributed extension) favor the separate rise and eruption of silicic andmafic magmas. Some silicic magmas may accumulate in small high-levelchambers and experience wall-crystallization (small arrows show thedirection ofmovement for the crystallization front) and vertical stratifica­tion (e.g. Sheeprock granite pluton of west-central Utah; Christiansen etal. 1983b). Periodic eruptions from the top of such an evolving chambermay produce large fields of rhyolite domes (e.g. Thomas Range, Utah)contemporaneous with variably fractionated and/or contaminated maficlavas. Some rhyolites fractionate on their passage through the coolercrust obviating the requirement for their eruption from a sizable shallowmagma reservoir. b) Where the zone of mafic magma injection is well­focused or the rate of injection is high, hybridization of mantle and crustmaterials could be enhanced. Mixing might be unavoidable in such anenvironment. The magmas produced could fractionate along a calc­alkaline basalt-andesite-dacite (BAD) trend in a shallow magma reser­voir feeding a stratocone (gradational to larger caldera-related systemsdescribed below). Alkalic basalts and trachytes also appear to be com­mon in this sort of environment. Examples include the Mt. Taylor vol­canic field, New Mexico, and the volcanic systems at Silver Cliff/Rosita,Colorado. Hybridization should be limited on the flanks of the thermalfocus and partial melting of the crust in these areas could lead to theeruption of topaz-bearing rhyolite lavas under favorable stress regimes(e.g. Grants Ridge). Similar high-silica rhyolites may be produced inadvance of crustal penetration by mafic magmas, or after decline of themafic magma input when crustal temperatures were still high but oppor­tunities for mixing were small. c) In a variety oftectonic environments,large collapse caldera systems may develop as the result of the sustainedinjection or ponding ofmafic magma in the crust. Fractionation of maficmagmas or hybridization of mafic and silicic crustal melts may havepreceded diapiric(?) separation of moderately silicic magmas to shal­lower levels. A strongly modified residual crust composed of restitephases remains in the lower crust (perhaps bearing a mixed mantle andcrustal isotopic signature). Residual hydrous phases should be F-rich.Decomposition of these phases during a later heating event (representedby late basalts) could produce the parental magmas for topaz rhyolites

~­Exten.sionor

Shear

VariablyCont'aminatedBasalt

Ash-flow Sheet

Mafic Lava

Alkaline Lavas

Stock

,Ca.ldera-re la ted

Pluton

Dike Injection

Chamber

Ponding,Differentiation

Contamination

f I

Mantle-derivedBasaltic Magma

)Crystallizing Magma

Stratovolcano

Rhyolite Dome

i~( \AI.\\ I

I', '\ 1\/ " \~eltlng a d Mixing

./' ." '9 9" ..... i~ ... ' ,'9' 'I t ......

, ..". ~ .............

Rhyolite Dome

Gabbro

BasalticMagma

Continued(?) Mafic Magma Input

Partial ) .. - ....

Melting \ ~(Q.,': :>' ~

c

Ash-flow Sheet

Crust

b

Rhyolite

Mantle

Crust

Mantle

(;)

Crust

a

Mantle

Topaz Rhyolites 73

cantly alter the F/Cl ratio of the melt, might change the fractiona­tion path followed by the remaining magma and explain theassociation of both types of anorogenic magmas within one igne­ous complex.

In their major and trace element compositions, topaz rhyo­lites are similar in most respects to the aluminous A-type granitesof southeastern Australia. Notable exceptions are that topaz rhyo­lites are not LREE rich (125 to 200 ppm Ce versus less than 100ppm in topaz rhyolites) or Sc rich (greater than 10 versus lessthan 3 ppm for topaz rhyolites), but topaz rhyolites from thewestern United States contain higher concentrations ofNb (20 to30 ppm versus 30 to 120 ppm in topaz rhyolites). These differ­ences may relate to higher temperatures inferred for Australianexamples (most are hypersolvus one-feldspar granites with rela­tively high Zr concentrations) or to differences in the compositionof their crustal sources. The A-type granites of Australia, theYounger granites of Nigeria, and the Precambrian anorogenicgranites of the southwestern United States (Anderson 1983) arealso F-rich like topaz rhyolites. Subalkaline A-types generallycontain Fe-rich biotite and/or amphibole; fluorite is a commonphase as well.

Anorogenic granites are thought to result from differentia­tion of variably contaminated alkali basalts (Loiselle and Wones1979) or from small degrees of partial melting of "residual"crustal materials from which earlier water-rich magmas had beenremoved during granulite-grade metamorphism. It is this latermodel that we prefer for the origin of aluminous anorogenicrhyolites/granites, but it is difficult to imagine how Cl-rich peral­kaline magmas could come from granulites without the introduc­tion of CI (and possibly other volatiles) from another source(presumably mantle-derived basalts). As pointed out by Collins etal. (1982) and Christiansen et al. (1983a), the protoliths of alum­inous anorogenic granites probably consist of felsic granuliteswith potassium feldspar, plagioclase, clinopyroxene and ortho­pyroxene (after decomposition of biotite, which we presume tobe consumed by the melt forming reaction) and quartz. The

which could then erupt through an olcier caldera-related magma system(e.g. Kane Springs Wash, Nevada, or SW Colorado). Alternatively,residual pockets of silicic melt might be retained in the lower crust to riseand erupt slightly later. Their separation and rise could be induced by achange in stress orientations (e.g. Colorado topaz rhyolites follow volum­inous calc-alkaline magmatism during transition to extensional tectonismand development of the Rio Grande rift) or regional adjustments to theredistribution of mass in the lithosphere following the development ofgranitic batholiths. d) An alternative explanation for the association oftopaz rhyolites with silicic caldera systems holds that topaz rhyolites areproduced by fractionation of the residue left in the magma chamber aftereruption. Such residues might have the high F/Cl ratios typical of topazrhyolites if de-volatitization during an earlier eruption effectively ex­tracted CI in preference to F from a portion of the magma remaining inchamber. Such a process would require the streaming of substantialamounts of water vapor through the unerupted portion of the magmachamber. Examples that could be studied with this process in mindinclude the Kane Springs Wash, Nevada, and Black Range, New Mex­ico, rhyolites that are intimately related to caldera systems.

experimental studies of Naney (1983) on a synthetic granite andgranodiorite suggest that 10w-SiOz rhyolitic melts would coexistwith this biotite-free phase assemblage at 850 to 900°C and 8 kb.At lower pressures the requisite temperature is also lower. Resid­ual accessory minerals are probably zircon and apatite, as indi­cated by their low solubilities in granitic melts (summarized inWatson and Harrison 1.984) and by the low concentrations of Zrand P in aluminous anorogenic rocks. Magnetite, ilmenite, ortitanite may also be important residual phases depending on f02•

Monazite or some other REE-rich phase is probably residual tothe melting process and holds REE content of the melt to accept­able levels.

In short, we suggest that the most important component intopaz rhyolites is derived from felsic granulites of the lower ormiddle crust. According to this model, high heat flow, resultingfrom the emplacement of mafic magmas in or at the base of thecrust, elevated temperatures sufficiently to produce the decompo­sition of hydrous silicates. More mafic magmatism of a variety oftypes is typically associated with the eruption of topaz rhyolites(Table 10). In Figure 48 we have illustrated some of the geologicenvironments in which topaz rhyolites occur.

Topaz rhyolites are commonly found in extensional envi­ronments in association with contemporaneous basalts (Figure48a). Because of density contrasts, hot, mantle-derived basalticmagma may pond at the base of the crust where it differentiatesby· fractional crystallization and assimilation of crustal materials.Partial melting of continental crust occurs when temperaturesbecome high enough to induce the breakdown of hydrous miner­als such as biotite. Small quantities of rhyolitic partial melt wouldbe formed. The character of the erupted mafic magma dependson its original composition and upon the extent of fractionationandinteraction with crust materials. In an extensional stress field,these buoyant melts could rise, fractionate, and erupt to producetopaz rhyolites which are coeval with variably contaminated (po­tassic) basalts-a typical bimodal suite (Christiansen and Lip­man 1972). Mixing of the contrasting magma types is inhibitedby the efficient separation of the silicic magma in an extensionalsetting. Subsequent fractionation in a shallow magma chamber ofmoderate size is indicated for some topaz rhyolites by the erup­tion of moderate volumes of rhyolites over short periods of timeand by the existence of at least one small pluton of Cenozoictopaz-bearing granite. Eruptions from the tops of vertically zonedmagma chambers may explain the strongly fractionated characterof topaz rhyolites. Many other rhyolite domes may not requireshallow magma chambers because fractionation would probablyoccur enroute to the surface.

In an environment where extension is less pronounced orwhere the flux of mantle-derived basalt is strongly focused, hy­bridization of mafic and crust-derived silicic melt might be morecommon (Figure 48b). The magmas rising from this zone couldfractionate to produce basalt-andesite-dacite sequences and stra­tovolcanoes. On the spatial or temporal flanks of the thermal"focus," independent batches of silicic partial melts and variablycontaminated and fractionated basalt could develop and erupt, as

74 Christiansen, Sheridan, and Burt

for example at Grants Ridge, New Mexico, which is related to theMt. Taylor volcanic field.

Some topaz rhyolites are erupted during the development ofoverlapping caldera cycles. This has been interpreted to be thecase for the Mexican topaz rhyolites and for the rhyolites in NewMexico's Black Range. The eruption of voluminous silicic ash­flow tuffs and the formation of collapse calderas suggest theexistence of large shallow-level magma chambers. In examplesfrom the United States, topaz rhyolites are commonly eruptedafter the latest ash-flow tuff in a given area. We suggest twopossible alternatives for the production of topaz rhyolites in thisenvironment. Our preferred explanation calls on the late rise ofsmall pockets of melt trapped in the reworked, residual crust, orthe later generation of small volumes of partial melt of the resid­ual crust (Figure 48c). As described by Hildreth (1981), thismodified, perhaps granulitic, crust is thought to have developedas the result of the injection of mantle-derived magma into thecontinental crust and the subsequent extraction of silicic meltsthat coalesce to form a large magma body. This model is consis-

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tent with the coincident rise of mafic and rhyolitic magmasthrough the sub-caldera magma chamber, indicating that themagma was solidified and susceptible to brittle fracture and thepropagation of dikes. An alternative explanation for the occur­rence of topaz rhyolites in caldera settings, which might apply tothe Kane Springs Wash caldera, invokes fractional crystallizationof the unerupted portion of an ash-flow-producing magmachamber (Figure' 48d). The ash flows and the F-rich rhyolitelavas are seen as being co-genetic in a partially open magmasystem. The volatile saturation and eruption of the early magmaare critical for the development of the high FICI ratios observedin topaz rhyolites relative to earlier magmas. The melt's preferen­tialloss of CI relative to F (Burnham 1979) during volatile exso­lution associated with a large plinian eruption might be able toproduce this change. Subsequent fractional crystallization couldelevate F concentrations to the levels required for the formationof topaz during post-eruption devitrification and vapor-phase al-

i teration of the evolved residual magma.

Bailey, J. C., 1977, Fluorine in granitic rocks and melts: A review: ChemicalGeology, v. 19, p. 1-42.

Baker, I., and Ridley, W. I., 1970, Field evidence and K, Rb, Sr data bearing onthe origin of the Mt. Taylor volcanic field, New Mexico, U.S.A.: Earth andPlanetary Science Letters, v. 10, p. 106-114.

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