Highty Radioactive Topaz Rhyolites of the Toano Range, Northeastern Nevada

American Mineralogist, Volume 77, pages 1067-1073, 1992

Highty radioactive topaz rhyolites of the Toano Range,northeastern Nevada

JoN.q.rHA.NG. Pnrcn, SrnprrnN B. C,q.sron Nevada Bureau of Mines and Geology, University of Nevada, Reno, Nevada 89557-0088,U.S.A. Dl.vrn M. Mrr-r,Bn U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, U.S.A.

Ansrnlcr Highly radioactive topaz rhyolites of Miocene age are describedfrom a new locality in the Toano Range of northeasternNevada. Quartz phenocrystsare blackenedby radiation damage. High magmatic Th and U concentrations (87 and 46 ppm, respectively) allow the rhyolites to be readily distinguished on aeroradiometric maps. In addition to quartz, the rhyolites contain phenocrystsof sanidine, plagioclase,biotite, titanomagnetite,and rare fayalite. Vitrophyres have severalunusual features:micropheno- crysts of octahedral fluorite, rod- and worm-shaped microlite of iron oxide, and microlite offayalite. Vapor-phasecavities in devitrified rocks contain topaz and quartz. The rhyolites occur as lava flows and pyroclastic deposits that originated from at least two vents. Emplacement of a nonwelded lithic tuff with topaz-rhyolite composition pre- ceded the lava flows. Two groups of lavas are defined by petrographic diferences and trace-elementcompositions. Rocks from the group with relatively largequartz phenocrysts and smaller, less abundant biotite phenocryststend to be more enriched in some incompatible trace elements,notably Th and REE, than the other group. Rocks that have undergone various amounts of postemplacementcrystallization are locally enriched in elementsthat apparently were transported in the vapor phase,notably As, Sb, Au, and Hg. Becausemagma degassing,hydration of vitrophyres, vapor-phase crystallization, and devitrification can affect the rock composition, true magmatic abundancesof these elementsare uncertain.

INrnolucrroN Tertiary sedimentary and volcanic rocks (sandstones, limestones, and re- This study was undertakento characterize,petrograph- tuffaceous sandstones, siltstones, Ts, by Coats, 1987)and the Toano ically and chemically, rhyolites of the Toano Range in worked tuffmapped as pluton (Miller et al., 1990), a Cretaceousmus- northeastern Nevada. The two groups of rhyolites that Springs granite (Fie. we can map and recognize in the field on the basis of covite l). plugs that were mapped by differencesin biotite content and size of quartz pheno- We have studied the two (1987) from which lava flows were erupted crysts are chemically distinct. We report some unusual Glick and (Fig. vents may be buried beneath the flows. petrographicfeatures in theserhyolites and compare their l). Other plugs Proterozoic and Paleozoic metamor- chemical compositions with those of other topaz rhyo- The occur in phic rocks, mostly quartzite and phyllite lites. Topaz rhyolites are typically more enriched in cer- and sedimentary limestone and dolomite. Columnar joints tain incompatible trace elementsthan are other high-sil- but with some plug have intersectionsthat ica rhyolites. The more incompatible-element enriched on the south side of the small plunge horizontally, indicating cooling along a varieties of topaz rhyolites are the sourcesof elementsin locally Precambrian metamorphic rocks. some ore deposits,notably Be and Sn (Christiansenet al., vertical contact with north side of this plug contains nu- 1986). Upon examination of the aeroradiometric maps The rhyolite on the quartzite. Pyroclastic rocks, including of Duval (1988),we suspectedthat the rhyolites of the merous blocks of pumice-rich occur at the base of the craggy Toano Rangemight be particularly radioactive topazrhy- welded tuffs, peak large plug. Contacts betweenthe py- olites. They are, indeed, some of the most Th- and U-rich comprising the the plug are obscure. The large plug topaz-bearingrhyolites that have been described.Proffitt roclastic rocks and vertical extent of 170 m. The lava flows et al. (1982) recognizedthe anomalously high U content has an exposed thickness of 50 m (near locality 37, of these rocks but did not identifr them as topaz rhyo- attain a maximum presently over an area ofapprox- Iites. Fig. l) and are exposed imately 40 km'z. Original erupted volume was probably Frnr,n nnr,.q.:tIoNs on the order of 2 km3. Rhyolites occur in the Toano Range as lava flows, py- Lithic tufi whose pumice is chemically and mineralog- roclastic rocks, and plugs. The flows overlie a packageof ically similar to the lavas, occursin the north-central part 0003-o04x/92109l 0- l 067$02.00 t067 1068 PRICE ET AL.: RADIOACTIVE RHYOLITES

areasurrounded by Tertiary sedimentaryrocks and airfall tuffs (betweenlocalities 48 and 49, too small to be mapped on Fig. l).

Prrnocupruc cHARACTERISTIcs The rhyolites contain phenocrysts,in decreasingabun- dance,of blackened(deeply smoky) quaftz, sanidine,plagioclase, biotite, titanomagnetite (generally oxidized to hematite + ilmenite), and rare fayalite (positively identified as a phenocryst phase in only one of 44 thin sec- tions). Total phenocryst content is approximately 150/0, and qtartz accounts for approximately 600/oof the phenocryst volume. Quartz phenocrysts are anhedral and commonly resorbed, whereas sanidine, plagioclase,and biotite phenocrystsare subhedral to the euhedral. Fresh devitrified rocks have a light gray to medium light gray groundmasscomposed primarily of quartz and alkali feldspar. Vitrophyres locally contain micropheno- crysts of octahedralfluorite (Fig.2a) and fayalite (Fig. 2b) plus rod- and worm-shaped, flow-aligned microlite of iron oxide. Some microlite sampleshave been positively identified as hematite, both optically (from their red internal reflection) and by quantitative energydispersive analysis with a scanningelectron microscope.Iron oxide microlite lacking red internal reflection is probably magnetite, and all of the hematite microlite samples may be oxidized magnetitegrains. We identified small crystalsof pyrite in one sample of black, hydrated vitrophyre. Fig. l. Geologic sketch map of the outcrop extent of topaz Although magmatic fluorite has been recognized in rhyolites of the Toano Range, Nevada (modified from Glick, some other topaz rhyolites, fayalite is rare in theserocks 1987). Two identified plugs are outlined in bold lines on the (Christiansen et al., 1986). In rhyolites of the Toano easternpart ofthe map. Folds and faults in Paleozoicand Pre- Range, fayalite is most readily visible in vitrophyres. Its cambrian rocks and minor normal faults in Tertiary rocks are pure (Fer,.Mrroor- not shown for simplicity. Symbols indicate the types of samples composition as nearly fayalite, that have been petrographicallyexamined; numbers, without the CaoorMgo oor)SiOo, was determined by quantitative energy J89 or J90 prefix, identifu samplesfor which chemical analyses dispersive analysis with a scanningelectron microscope. are reported in Table 1. Solid circles : vitrophyres from the It occursas discrete,pale greenmicrolite in some samples small plug and petrographically similar flows (samplesJ89-8, and as small crystals attached to iron oxide microlites J9O-12,and J90-39); open circles : devitrified rhyolite from the (Fig. 2b) in others. This texture suggeststhat the fayalite small plug and petrographicallysimilar flows (samplesJ90-10, crystals nucleatedon the iron oxide microlites. J90-33,J90-37, J90-38, and J90-41);open triangles: devitrified Biotite occurs in several forms: as brown- and, gener- rhyolite from the large plug and petrographically similar flow ally smaller, green-pleochroicphenocrysts; as brown grains (J90-42,J90-43b, and J90-48); closedtriangle : pyroclastic unit in glomerocrystswith plagisslase,alkali feldspar, qrtartz, from base oflarge plug (J90-44); closed square : pumice from nonwelded tuff in northern outcrop area (J90-49). and traces of zircon; as brown inclusions in quartz and feldspars;and as generally $een-pleochroic grains inter- grown with quartz and feldspar in the groundmassof de- of the study area (Fig. l). This nonwelded tuff overlies vitrified rocks. In some devitrified rocks, clots of green tilted tuffaceous sedimentary rocks along a nearly hori- groundmass biotite surrounding magnetite phenocrysts zontal contact. Lithic clasts include Precambrian and Pa- suggestthat thesebiotite grains nucleatedat a site ofhigh leozoic metamorphic rocks and the Cretaceous muscovite Fe content. Some samplescontain feldspar-quartzdevit- granite. Because the tuff lacks welding and flattening of rification spherulesthat appear to have nucleated on small pumice blocks, we interpret it to be an aidall tuff. The biotite phenocrysts. tuff is composed of upper and low€r lithic-rich units and Topaz occurs as euhedral crystals as long as 5 mm in a middle pumice-rich unit; cumulative thickness is less vapor-phasecavities, generally with quartz. We recognize than or equal to 30 m. Thickness and clast size in the tuff topaz in samples from the southwestern and northern decrease northward and indicate an eruptive site south of outcrop areasbut not from the easternoutcrop area.Most the outcrop, perhaps at the mapped plugs. A different vapor-phasecavities are lined with euhedralquartz, with- lithic tuff, one containing only clasts of rhyolite petro- out topztz. graphically similar to the small plug, crops out in a small At least two types of rhyolite are petrographically dis- PRICE ET AL.: RADIOACTIVE RHYOLITES 1069 tinguishable,both in plugs and flows. The smaller of the two plugs and most of the lava flows contain relatively large (up to 6 mm in diameter) quartz phenocrystsand biotite microphenocryststhat are generally too small to be recognizedin hand specimen.The larger plug and petrographically similar flow contain smaller (generally0.5- 2.5-mm and rarely up to 3.5-mm) quartz phenocrysts and biotite phenocrystsup to 2 mm long. The grain-size distinction between the two types of rhyolite is evident in the percentageof quartz phenocrysts greater than 1 mm in diameter: 35-650/ofor the rhyolite with largequartz crystals vs. 14-360/ofor the rhyolite with small crystals. The small-quartz, large-biotite flows have been mapped only in scatteredoutcrops northwest ofthe large plug (tri- anglesin Fig. l). Most outcrops are rhyolite that contains large quartz phenocrysts and lacks macroscopic biotite (circlesin Fig. 1). The pyroclasticrocks that crop out adjacentto the large plug contain quartz and biotite phenocrystsof the same size as phenocrystsin that plug. In thin section, some of the resorbed quartz phenocrystsin the pyroclastic rocks appear to be broken. The nonwelded, lithic tuff in the north-central part of the study area contains a phenocryst assemblagesimilar to the topaz rhyolite flows: resorbedqtartz, sanidine,plagioclase,and brown-pleochroic biotite. The quartz in this rock is somewhat smoky but not blackened as in the topaz rhyolites. Large biotite suggestthat the tuffwas erupt- Fig. 2. Photomicrographof (a) a fluorite microphenocryst ed from the vent marked by the large plug. and (b) fayalitemicrolite samplesattached to iron oxide mi- crolitesamples in glass,sample J89-8. View in a is perpendicular CrrnMrc.ql ANALYSES to a (1I l) triangularface on a fluoriteoctahedron, with the un- Analyses of 14 rocks from this area are reported in derlying(1 11) face forming a six-pointedstar in two dimensions. Table l. Care was taken in the field to remove any ob- The edgeofthe octahedronis 8 pm long.These octahedral crys- vious weathering rind from rocks sampled for chemical tals do not occurin all samplesof vitrophyre;they havebeen analyses. Sample preparation and analyses were per- recognizedfrom the southern,central, and northern outcrop ar- (solid in Fig. 1).In b the centraliron oxiderod is 44 formed by commercial laboratories (Bondar-Clegg for eas circles long.In this sample,fayalite appears to havenucleated on most analyses,XRAL Activation Servicesfor Au) using rrm magnetite.In someglass samples, fayalite occurs as independent quality assuranceprocedures established for the Geo- microphenocrystsnot attachedto iron-oxidemicrolite. Both chemical Sampling and Characterization Program at the fluoriteand fayalite were confirmed by energydispersive analysis Nevada Bureau of Mines and Geology. Analysesof stan- with a scanningelectron microscope. dard referencematerials and control samplesare reported by Bonham er al. (1990). The following analytical techniques were used: multiple acid total digestion plus direct current plasma spec- dicate that the reproducibility of the Au analysesis +0.5 troscopy for major oxides; gravimetric analysis for loss ppb. on ignition; Leco technique for total S; cold vapor atomic At least two of the analyzedsamples are not represen- absorption spectrophotometry for Hg; multiple acid di- tative of magmatic compositions.Sample J90-41 is a gestionand MIBK extraction followed by atomic absorp- breccia that contains fragrnents ofPaleozoic quartzite and tion spectrophotometryfor Tl; multiple acid total diges- perhaps other sedimentary rocks in a matrix of rhyolite; tion plus atomic absorption spectrophotometry for Be; it is not included in the plots illustrating chemical com- multiple acid total digestion plus inductively coupled parisons.Sample J90-49, pumice from the nonweldedtufi plasma-atomic emission spectroscopyfor Li, V, Cr, Co, contains abundant secondary calcite. Sample J90-48, a Ni, Cu, Zn, Ga, Sr, Y, Nb, Cd, Sn, Ba, Pb, and Bi; in- devitrified flow, is distinctly higher in silica and lower in strumental neutron activation analysisfor Sc, As, Se, Br, KrO than other samples and may have undergone con- Rb, Zr, Mo, Ag, Sb, Te, Cs, La, Ce, Sm, Eu, Tb, Yb, Lu, siderable vapor-phase alteration. With these exceptions Hf, Ta, W, Ir, Th, and U; and neutron activation followed and the exception of sample J90-9, the Cretaceousmus- by radiochemical separation by fire assay for Au. Our covite granite, other samples in Table I are representa- quality-assurancedata on replicate control samples in- tive of the topaz rhyolites of the Toano Range. 1070 PRICE ET AL.: RADIOACTIVE RHYOLITES

TABLE1. Chemical compositions of igneous rocks from the Toano Range, Elko County, Nevada

Sample no. J89-8 J90-12 J90-39 J90-10 J90-33 J90-37 J90-38 J90-41 J90-42 J90-43b J9o-48 J9o-44 J9o-49 J9o-9 Oxides(c/o, normalized, volatile free) sio, 76.4 76.5 76.7 77.9 76.3 77.3 76.4 80.4 77.6 77.6 79.5 76.7 71.7 75.4 Tio, 0.11 o 10 0.07 0.10 0.09 0.07 't2.3 0.07 0.12 0.08 0.08 0.06 0 05 0.04 0 09 Al,o3 12.9 12.5 12.5 12.O 12.2 12.1 8.8 11.6 11.9 10.2 12.1 12.2 't4.5 Fe,O.,d 1.15 142 1.31 1.30 1.42 1.15 1.30 1.22 1.16 119 1.15 1.28 1.25 0.36 MnO 0.03 0.03 0.02 0 03 0.03 0.02 0.03 0.04 0.03 0.03 0.o2 0.04 0.04 0.01 MgO 0.18 0.06 0.06 0 03 0.10 0.03 0.02 0.19 0.04 0.05 0.11 0.04 0.23 0.08 CaO 0.66 0.66 1.07 0.13 0.95 0 26 0.s4 1.67 0.29 0.22 1 35 0.19 5.98 0.44 Na,O 318 3.21 2.81 3.46 3.49 3.39 2.18 0.69 2.14 2.73 2.94 2.88 2.O7 4.35 K.O 5.29 5.41 5.43 4.79 5.27 5.40 7.36 6.59 6.98 6.19 4.36 6.50 6.33 4.48 P,Ou 0.20 0.10 0.06 0.24 0.07 014 0.03 0.27 0.05 0.09 0.33 0.17 0.11 0.26 LOr 3 38 3.00 4 50 0.88 0.84 0.63 0.88 2.21 0.70 0.55 1.61 0.59 7.85 0.88 Total 97.4 98.3 99.7 97.9 1000 98.8 99.4 9s.8 99.1 99.2 98.9 98.1 98.8 98.5 Traceelements (ppm) Li 36 25 110 70 120 120 110 44 78 93 73 140 37 <1 Be 27 21 26 17 19 24 18 20 12 20 13 26 24 4.3 F s900 4700 4900 2200 2400 3100 1900 800 2000 1800 600 2200 1000 1400 400 300 200 <200 200 200 <200 200 600 200 200 200 200 <200 Sc 2.7 2.5 2.7 2.2 2.7 2.4 2.4 2.5 4.3 4.9 4.1 3.6 3.8 1.3 <'l <1 <1 <1 19 7 3 10 1 <1 29 <1 3<1 3 <1 <1 <1 1 <1<17 1<1 <1 <1 22 Co

Crrnnarc,q.I-coMpARrsoNS of the rhyolites of the Toano Rangeare exceptionallyrich (Fig. with quartz The rhyolites of the Toano Range are chemically sim- in U, Th, and K 4, Table l). Rocks large phenocrysts ilar to other topaz rhyolites. They are all high-silica rhy- and sparse,small biotite phenocrysts(from olites. Like other topaz rhyolites, those in the Toano Range the small plug and petrographicallysimilar flows) tend to are enriched in Li, Be, F, Zn, Rb, Y, Nb, Cs, and heavy have higher Th concentrationsthan other rocks. The av- REE, Ta, Th, and U (seeTable l) relative to most other erageTh content ofseven large-quartz rhyolites is 87 + rhyolites. They fall into the middle range of topaz rhyo- 9 ppm. Assuming that some U may have been leached lites from Utah in terms of Nb and Rb (Fig. 3), but some from the somewhat oxidized. devitrified rocks. the three PRICE ET AL.: RADIOACTIVE RHYOLITES 107I

140 + 120 + ++ Smallplug and petrographicallysimilar flows 120 + + + o r00 \2"- o 100 o 3O E 80 o_ 80 o_ _o z _c OU

xx 40 + SporMountain 40 Largeplug and petrographicallysimilar rocks x ThomasBange 20 500 1000 1500 2000 20 345618 Rb(ppm) KzO (%) Fig. 3. Comparisonof Nb andRb in rhyolitesof the Toano Range,Nevada (solid circles), with topazrhyolites from theTho- Fig.4. Comparisonof Th andKrO in rhyolitesof theToano 1. masRange (crosses) and Spor Mountain, Utah (plussigns), from Rangewith topazrhyolites from Utah. Symbolsas in Figure (crosses) Christiansenet al. (1984). Topaz rhyolitesfrom the ThomasRange and Spor Mountain,Utah (plussigns), are from Christiansenet al. (1984). KrO valuesare recalculated to a volatile-freebasis. vitrophyres probably yield a better averagemagmatic U content:46+ 2ppm. deposits. One explanation for this difference is that the Rhyolites of the Toano Range have flat REE patterns vapor phase,which precipitatedtopaz and quartz in vugs, with pronounced negative Eu anomalies. These patterns presumably during devitrification, transported and pre- are characteristicof topaz rhyolites (Fig. 5). With the ex- cipitated these elements. The devitrified rocks are en- ception of sample J90-37, rocks with large quartz phe- riched, relative to the averageofthe three analyzedvitro- nocrysts tend to have higher REE concentrations than phyres,by as much as 37x in As, 14x in Hg, llx in those with small quartz phenocrysts. The rhyolites are Au, and lOx in Sb. As an alternative explanation, it is only mildly peraluminous (Table l), as are most topaz possiblethat the diflerencesbetween devitrified rocks and rhyolites(Christiansen et al., 1986).In contrast,the nearby Cretaceous muscovite granite is strongly peraluminous and has distinctly different trace element contents (Tablel; Leeet al., l98l). 't00 The one analyzedpyroclastic rock, a pumice-rich weld- o ed tuff (J90-44 in Table 1), is chemically similar to the 'f ! flow rocks of the nearby large plug (samplesJ90-42 and c J90-43b).Pumice from the nonweldedlithic o tuff(J90-49), O located 6 km north of the large plug and occurring be- -o-4) neath the rhyolite flow, is also chemically similar. It is E a enriched in Li, Be, Rb, Cs, heavy REE, Ta, T1, Th, and a 12 U, as are the topaz rhyolites. 1 The chemical analysesconfirm the petro$aphic obser- v vation of at least (1) two magma compositions: a more LaCeSmtuTb\bLu evolved type with relatively large quartz phenocrysts (samplesJ89-8, J90-10,J90-12, J90-33, J90-37, J90-38, Fig. 5. Rare earth element concentrations in selectedsam- and J90-39) probably erupted from a vent occupied by ples:large open circles: devitrified rhyolite from flow in western the small plug, and (2) a less evolved type (samplesJ90- outcrop areaand petrographicallysimilar to the small plug (sam- 42, J90-43b,J90-44, and J90-48) erupted from a vent ple J90-33); small open circles: devitrified rhyolite from flow occupied by the large plug. in northern outcrop area and petrographicafi similar to the small plug (sampleJ90-37); open triangles : devitrified rhyolite from It is noteworthy that As and Sb are enriched only in the large plug (J90-42). For comparison, solid lines are samples the devitrified rocks and are uniformly low in hydrated SM-35 and SM-61a, topaz rhyolites from Spor Mountain and vitrophyres and pyroclastic rocks (Fig. 6). The devitrified the Thomas Range,respectively (from Christiansenet al., 1984). rocks are also locally enriched in certain other elements, The Eu anomaly for the Toano samples,which is shown as equal notably Au and Hg (see Table l). These four elements to that for Spor Mountain, is uncertain; none of the rhyolite are characteristicpathfinder elementsfor epithermal gold samplesexceeds the detection limit (2 ppm). r072 PRICE ET AL.: RADIOACTIVE RHYOLITES

100 of the western United Statesis uncertain. Cenozoic magmatism in northeasternNevada consistedof extrusion of A late Eoceneto Miocene andesitesto rhyolites associated with subductionor early extensionor both (Stewart,I 980; Miller, 1984;Seedorfl l99l) and a bimodal basalt-rhyolite suite associatedwith Basin and Range extensional E tectonics. Additionally, the SnakeRiver Plain 150 km to the north produced voluminous rhyolite and basalt asso- e10 A al, ciated with the eastward migration of the Yellowstone hot spot. Coats (1987) mapped rhyolites of the Toano Range as o a part of his unit Trr, the youngest rhyolitic rocks of the region. The Toano rocks are, however, unlike rocks at ta oo other outcrops of Tr3 that we have examined in the region, which consist of many types: more-or-less normal .1 1 10 rhyolite lava flows, areally extensive rhyolite flows that resemble ash-flow in pattern, Sb (ppm) tuffs outcrop and rheo- morphic ash-flow tuffs. Miller (1984) reported an age of Fig. 6. As and Sbin rhyolitesofthe ToanoRange, Nevada. 12.9 + 0.4 Ma (K-Ar date on mixture of plagioclaseand Symbolsas in Figure4. sanidine) for rhyolite of the Toano Range. This age is similar to those of many rhyolite lavas in nearby ranges. This regional outpouring of rhyolite is consistent with a vitrophyres plus pyroclastic rocks are the result of differ- genetic relationship to hot spot passageassociated with encesin magma degassing.The pyroclastic rocks may be development of the Snake River Plain. Topaz rhyolites low simply becauseof degassingduring eruption. The vit- associatedwith bimodal volcanism of the Snake River rophyres, which occur on the margin of the small plug Plain include the Jarbidge Rhyolite in northeastern Ne- and at the baseof the lava flow related to the small plug, vada, which only locally contains topaz (Coats, 1964; may representan early volume of magma that lost more Leeman, 1982; Christiansenet al., 1986),and possibly gas than later magma that filled the vent and capped the the Quaternary China Cap dome in the Blackfoot lava flows. field in southeasternIdaho (Armstrong et al., 19751,Day- Becauseof its low boiling point, Hg is clearly a volatile vault et al.,1984:'Christiansen et al., 1986).Alternative- element. The other anomalouselements are not generally ly, the rhyolites of the Toano Range may be a newly consideredto be volatile, but recent studies have dem- recognizedpart of the large province of extension-related onstrated that many elements are transported in vapor topaz rhyolites scatteredthroughout westernNevada and phasesassociated with different types ofcooling lava (cf. easternUtah (Christiansenet al., 1986). Stoiber and Rose, 1970, 1974; Keith et al., l98l; Os- karsson,1981; Greenlandand Aruscavage,1986; Low- AcKNowLEDGMENTS ensternet al., l99l). Symondset al. (1987),who exam- We thank Jim Sjoberg,U.S. Bureau of Mines, and Martin Jensen,Mackay ined condensates,sublimates, and incrustations from School of Mines, for their assistancein identifying phaseswith scanning fumaroles at Merapi, an andesitic stratovolcano in In- electron microscopes.Hal Bonham and Dick Meeuwig provided helpful comments on an earlier version of the manuscript. The reviews of Wen- donesia, demonstrated that significant amounts of Au, dell Dufreld, Howard Wilshire, Don Burt, and W.P. Nash improved the As, and, to a lesserextent, Sb and many other elements manuscript. Partial financial support was provided by the Mining Coop- can be transported in a magmatically derived vapor phase. erative Fund, State ofNevada, for cooperative research between the Ne- Given that magma degassingcan vary with time and that vada Bureau of Mines and Geology and the U.S. GeologicalSurvey. hydration of vitrophyres, vapor-phase crystallization, and RrrnnnNcns crrED devitrification can alter the chemical compositions of Armstrong, R.L., Leeman, W.P., and Malde, H.E. (1975) K-Ar dating, rocks, the magmatic abundancesof those trace elements Quaternary and Neogene volcanic rocks ofthe Snake River Plain, Ida- that are capableofbeing transported in significant quan- ho. American Journal of SciencE,27 5, 225-251. tities in a vapor phase, such as Hg, Au, As, and Sb, are Bonham, H.F., Jr., Garside, L.J., Hsu, L.C., Desilets, M., Price, J.G., uncertain. Additional work, including analysesof these Lechler, P.J., and Davis, D. (1990) Geochemical sampling and char- elements in melt inclusions trapped in phenocrysts, as acteizatiot of the Preble Formation. Nevada Bureau of Mines and Geology Open-File Repon, 1-C93. reported by Lowenstern et al. (1991) for Cu, is neededto Christiansen,8.H., Bikun, J.V., Sheridan, M.F., and Burt, D.M. (1984) determine true magmatic compositions. Geochemical evolution oftopaz rhyolites from the Thomas Range and Spor Mountain, Utah. American Mineralogist, 69,223-236. Rnr,,l,rroNsrrrp ro orHER RHyoLITES Christiansen,E.H., Sheridan, M.F., and Burt, D.M. (1986) The geology geochemistry rhyolites from western Rhyolites of the Toano Range part and ofCenozoic topaz the United are of the latest States.Geological Society ofAmerica SpecialPaper 205,82 p. seriesof magmatic events in northeasternNevada (Stew- Coats, R.R. (1964) Geology of the Jarbidge quadrangle,Nevada-Idaho. art, 1980), but their relationship to other topaz rhyolites U.S. GeologicalSurvey Bulletin ll4l-INI,24 p. PRICE ET AL.: RADIOACTIVE RHYOLITES 1073

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