DOFJ_V/10461--'941

D_92 017913 UniversitofyNevada Reno

Intense C_tm.mafi,om' "£'heT_beofr MaMountagmaticinandMagmatoHydro_erm_-t.herm_dAEc'vent:iviw Anat the SouthwesteWidespreadrm Nevada Volcamc Field

A the._submittedinparti_aelnt ofthe,requiremeforms thedegreeof Master ofSdence in Geology.

DISCLAIMER

Thisreportwas prepareasd an accountof worksponsoredby an agencyoftheUnitedStates Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsi- bility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process di,_closed, or represents that its use wot_r..'not infringe privately owned rights. Refer- enee herein to any specific commercial product, process, or _ervice by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply /ts endorsement, recom- mendation, or favoring by the United States Government or any agency thereof, The vie,,,,_ and opinions of authors expresszxt herein do not necessarily state or reflect those of the United States Government or any agency thereof.

Ma= Roy Jackson. lr. May 1988

DISTRIBUTION OF THIS DOCUMENT IS UNI.IMITh'D i

The thesis of Mac Roy _ar.kson,Jr. m_pn)ve_

" --- i_B Thesis Advisor

Dep_=e=ch...... =/ - '

i tJ]F" 11 I ...... i ...... ,_11 . )dlI)mmi_NR_li) Dean, Graduate School

Ut:hm_y__ Ram)

._ Ackaowledgements

I am _Fate_ forthesupportand helpprovidedby many incUvidu_Is.I am indebtedto D. Noble,L Lzrsonand S. Weiss,my co-workersinTask #3 of the

Yucca Mountain Project, for conm'buting information, insights, suggestions and assistance. E. McKee, of the U.S. Geological Survey, arranged for and supervised the K-,_" age determinations crudal to t.tris study. Galli Exploration Associates,

Cordex Exploration Company, H. G. Mining, Inc. and SAGA Exploration Company provided irfformation and property, access; D. Fanzting, P. Galli, D. Finn, G. Austin,

D. Odt, D. Crossland, J. NIarr, D. Spicer and J. Greybeck were especially helpful. J,

Mart gave me ,'m excellent tout of the Sterling mine, and D. Finn provided informa- tion on and tours of the Thompson, Silicon and Mayflower mines. D. Spicer allowed ready access to his properties and provided information and much-needed spare ti_'es. This research was funded by Nevada's Nuclear Waste Project Office through the Center for Neotectonic Sn!dies at the University of Nevada-Reno.

Finally, I owe a great deal to my family for their interest in my geological activities, and to Jeannie for her encouragement and patience. Abstract

Eruptionof the RainierMesa and Ammonia Tanks Members 'F'trnber ..... Mounta_ Tuffatabout11.5and 11.3Ma, respectiverelysult, eindformationofthe TimberMou.utain('I'M)ca.ldera;n_ K-At agesshow thatvolcaniswmithinand

around the TM caldera continued for about 1 m.y. after collapse. Some TM age magmatic activity took piace west and southeast of the TM caldera in the Beatty- Bullfrog Hills and Shoshone Mountain areas, suggesting that volcanic activity at the TM caldera was an intense expression of an areally ¢,,aensive magmatic system ac- tive from about 11.5 to 10 Ma. Epithermal Au-Ag, Hg and fluorite mineralization and hydrothermal alteration are found in both within and surrounding the Timber

Mountain-Oasis Valley caldera complex. New K-Au"ages date this hydrothermal ac- tivity between about 13 and I0 Ma, largely between about 11.5 and I0 Ma, suggest- ing a genetic relation of hydrothermal activity to the TM magmatic system. iv

TableofContents ...... •...... Page Acknowledgements...... ii . Absrcaa ...... iii

Table of Contents ...... • e e ob eo • eo • eeeeeeeoeeeoe oeeoee 4eeee_eeeeosmeoeeeeoeeeoe iv

Introduction•e ...... eeoc ee eeeeee_eeweeeeoe_ eeeeeeeee eeJeeeeeooo eeee • eeeee eeeeeeQo_eeeeeeeeoee • eeeoomeeele oeee_eeeoeeeoeeeee_ I

VolcaniHistc ory...... 1 Radiometricagesofvolcaniunc /ts...... 5 Older volcanic units ...... 6

RhvoZite of Calico Hills ...... 8 Wahmortie and Salyer Formations ...... 8 Paintbrush Tuff ...... 8 Timber Mountain Tuff ...... 9

Post-collapse Timber Mountain volcanism ...... 10 Lavasandtuftofs the BeartT-Bull_ogHillsarea ...... II They/Canyon andStonewalFllatTufts...... 12 AlterationandMineralization...... 13

BullfxogHillsandOasisMounta/narea ...... 13 NorthernandeasternBareMountain...... 14

Siliconand Thompsonmines ...... 16 Transvaal Hills, Claim Canyon cauldronsegmentmarginandYuccaMountain...... 16 Calico Hills, Homsilver mine atWahmonie andMine Mountain ...... 17

Age of HydrothermaAll teratioandMineralization...... 19 Discussion...... ° ...... _.__ . 22

Reladon of hydro_erm_.. aaivitt7o magmauc acuv,t7 ee ...... eeoeeeeeo_o_e eoes_eeeeeeeeeee4 )e eeeo_e_e oee.oeeo4ee4.ee.tet_be_oeMJ,ee 22 Possfble relation of hydrothermal and volcanic activity to a deep magmatic sys'_m ...... 25 ., Relation of hydrothermal activity to the evolution of the Timber Mountain caldera ...... 27

Comparison to the rimingof epithermal mineral/zadon associated with other calderas ...... 28

Comparison ro the duration and timing of hydrothermal activity in other adulaxia-scricite-qcp¢systems ...... 29 Possiblerelationof hydrothermalactivitytodetachmenfat ulting...... 30 Conclusions ...... 32 References ...... 32

Appendix I: Radiometric Ages of Volcanic Units of the SWNVF ...... 40

Appendix 2: Description of K-At Dated Samples ...... 0,2 Tab_

Table I. Published K-At ages .forthe Ammonia Tanks Member of the Timber Mountain Tuff ...... 6

Table2. New K-Atage determinationson volcaniroc cks...... II

Table 3. Similar caldera histories ...... 27

Figure I.Map showingcaldera complexes, calderaasnddistn'butionofvolcanic roe_ in the southwestern Nevada volcauk field ...... 2

Figure 2. Map showing the generalized ution of hydrothermal alteration and mineral- izafiom calderas, ca.l.der-asegments and . I0-11.5Ma rhyolite lava_ in the areaof the T'maberMountztn-Ossis Valley caldera complex ...... 3 vi

Figure 3. Summa.,7 of the volcanic su'atigraphy . of the southwestern Nevada volcamcfield ...... 4

Figure4. Agesofhydrothermaalheradonand m/ne_don intheareaoftheT'unber Mountain-OasVia£s 1eycaldercoa mplexand , agesofvolcanicactivityattheT'tmber Mouataia-Oasis Valley caldera complex and Beatty-BuUtrog Hills area ...... 20 1

Introduction

The southwes_m'a Nevada volcanic field (SWNVF'), located about 100 m/les northwest of Las Vegas, was the site of numerous middle to late Miocene ¢_/der_- formiag, ash-flow eruptions ('Fig. I). Many are_ of Mdrothermal alteration and miuera/_l/on are found in the area of the T'umber Mountain-Oa_ Valley caldera complex _[g. 2). The Bullf:rogHills, Beatty and Bare Mountain area ts c_n'endy thefoc_ of precio_-mer2mdiamg aad exploratiaonctiviwv/ith, two activeAu mines a_d two recently a.uaouaced discoveries of 3! a.ud 0.15 tm_ion ouaces Au

CBoo_, 1988; D?'e, 1988). The pu_#ose of u_,s shady was to dete,".-.._=e_h: ;iu-'_:_ and possible genetic reladon of hydrothermal akerzdon and miner_.tizadon to vot- ca_sm sad ca2dera evotution m the S\VN'VF. This researchpresented, more comple:ely here, was previously su.mma_ed by lackson ez aL (1988).

Volcanic Hh_ory

M._ocenevolcanic rocks of the S_ were deposited on deformed and lo- ca21ymem.morphosed Precambrian and Paleozoic sedimentary rocks (Cornwall 1972; Can', 1984; Robinson, 198.5). Precambrian and P_/eozckrocks, exposed at

J Bare Mouamim the Cameo H_ and _e Bullfrog _ are neartandem _s

(Fig. 2) sad are locally min_ Scott (1986, 1988) proposed that a regional de- taclm_em fault, related to mid- to late Tenim7 regional _ (Mc,_e., 1971;

Noble, 1972; McKce and Noble, 1986), _.pamte_ Precamlx_ and Paleozoic sedimentary rocks from overl_ Miocene voh=,nic rocks. The volcanic m_gruphy of the 5_;NVF is _mmar_d inHgure 3. Byers et al. (1976a), Car: et al. (1986), Chris_en et al. (1977) and Noble et aL (1984) pro- videdetailedisod a._omofthevolcanicmtigraphysad ge_c h_toryof _e

Crater Fiat-prospector Pass caldera complex, the Ttmber M(mnmLn-OasisValley

caldem complex, the Stonewall Moumam caldera complex and the Black Mountai_ • _ c_dcr'a. Pom._um-argon ages of _lca_c _m of d2e somhw_m Nevada vol- canic field are reported by K.i.sfler(1968) and Marvin el_al. (1970). The Timber Mountain-Oasis Valley caldera complex has been mapped in decml (el., Byers et i l|i llm i i i ili, i ,, ,,,,, iii ii i ...... i • ii i iii

i k /,, "_',,,,-_

OOt.OPltlll.O _ Anna OP ,,eG. I

1 "

ST_N[',_¢(.I. MO_JNTAIN |L. ACX tJOIJNI'AIN GAI, 0|AI¢ _t OOSIOI,_N C A t,,_ | ;li A _ / .I \ \ ®u_'_'_. / /.,, " " _. / ,. ".

• 7,#L " k

N

_'_I ,,_

__ y # I i _ll'r Sly| "_ /-_ I .. _..,-" I

20 _lqArilq _I,,,AI' • _' I [ lllllOIPE -C T0 li _&|8 MIt,ES CAi.Olllq A COIdPtAIX % . \%

- °%,,,,

Fig. L Map showing calder-a complexes, ¢lllderm _id _on of volcanic rocksLn the souchwes:emNevada voicamc field. D:uh_x__ed line sbov_ approximazeLimitofvolcanicroc_ _ Le field (mock"cd bom Can"e_aL, 1984).

7)4" "T IlMC --THIRSTY CANYON TUF_ 8- ._

-_MYOLITEOF OIISlOIAN8Ul"I"E g" "=9 TIMBER MTN.-.OA$1S VALLEY CALl:ERA COMPLEX (TMOV) BEATTY-BULLFROG HILLS (88H) 10- "UlCROGRAN*TE RING DIKES LAVA.=.SS RHYOLITE OF 3URTON UTN...... -I0 T!_BER UOUNT_=N rUT= _ _ TUFF OF CUTOFF =OAO......

... TUF-'_ 3F 3RC}@K:_ 3ANY3,_ _EMBE_ _i3.-_J TUF:_ OF ;L.¢UR.CE-LIS ;_ANCH, ,. .= 1 I-- TUF: OF' BUTTONHOOK ,rASH _EMBER LATIT_ J,T qONT,.5_'OSH, _AINE'" -El

"" -;:IAINIER UESA UEMOE_ uJ - AMMONIA T&NKS _AE_3E_ _ (3 < 12- -12 I:)41NTBRUSH TUF ¢ WSVC WAHMONIE FORMA TI(_N., TUFF OF =INYON PAS_ @ .,TIVA CANYON MEMBE,q -- SALYER FORMAT1CN ''",, .... 1 _-- YUCCA k4OUNTAIN _EMBER ...j "". P)AH CANYON k4EMBER _" " --I 3 CFImlICC "''TOPOPAH SPRING k4EMBER 1 SC¢ DIKES ON E FLANK BARE k4TN.. -- RNYOLJTE OF CALICO HILLS I STOCKAOII: WA_l,I TUFF CRATER FLAT TUFI: "".. 14- ...... I _ELTe, D RANGE TUFIW PROW PASS MEMBER -= 4 ...... G/_3(J,$E C,MIYON MEMIER BULLFROG MEMBER ...... TRAM MEMBER

18_ -) G

TUFF OF YUCCA FLAT LJTNIC nIOGE TUFF

1_-- .. TUFt ¢ OF nEDROCK ','ALL_-Y --_ 5

17 _ -17

Fig. 3. Summary of the volcanic stratigraphyof the southwestern Nevada volcanic field. !UGS constant corrected rad_omztricages are plotted with a dash; see Appendix 1 for sourcesof ages. Abbreviationsfor rhyolite [avasin the areaof the Timber Mountain-Oa.sisValley calderacomplex:lavaswhosestradgraphicposition is basedon a K-ha"age- 1) rhyolite of ShoshoneMm., 2) rhyolitesof the T'unber Mountain calderamoat, 3) rhyolite ofPEntaclesRidge, 4) pre-Rainier Mesarhyolite lavas, 5) rhyolite of Delirium Canyon; lwas not radiometricaily dated, reladve strafigraphic position is shown. A) rhyoUteof Beatty Wash, B) rhyoUteof West Cat Canyon, C) pre-Ammonia Tanks rhyolite I_vas. Pre-Rainier Mesa rhyolite lavas are also found in the Beatty-BuUf:rogHiUs area. Volcanic centers are shown in bold; abbreviations include: SMCC - Stonewall Mm. caldera complex, BMC - Black MEn. caldera, SCC - S/dcta Canyon caldem, WS'¢C - Wahmonie-Sa.lyer volcanic center, CFPPCC - Crater Flat-Prospector Pass calderacomplex. 5 al., 1976b; Scott and Bonk, 1984). A brief summary of previous work on the vol- canic evolution of the southwestern Nevada volcanic field, as well as some new in- terpretations relevant to understanding the timing of hydrothermal alteration and mineralization follows.

Radiometric ages of volcanic units

Ages from radiometric dating of volcanic units in the SWNVF are important for understanding and constraining the timing of hydrothermal alteration and min- eralization, as well as volcanic activity,in the SWNVF. Averages of published K-At and fission-track ages of volcanic units in the area of the Timber Mountain-Oasis

Valley caldera complex are given in Appendix 1. Averages may vary significantly depending on which ages are used in calculating the average; this is parti_larly ira. portant in the calculation of the average age for units that have been dated several times and for which there is some variation between individual determinations. For example, in Table 1 are 16 rUGS constant corrected K-Ar age determinations pub- Lishedfor the Ammonia Tanks Member of the Timber Mountain Tuff. The tuff of

Cat Canyon, originally considered a separate unit, has been correlated with the Ammonia Tanks (Byers et al., 1976a), and ages from this unit should be included in the calculation of an average age for the Ammonia Tanks. Marvin et al. (1970) treated the 12.42 Ma age (starred), as an outlier and excluded it from their calcula-

tion of an average age for the Ammonia Tanks. Excluding thi_;age results in an average age of 11.3_ Ma for the Ammonia Tanks, whereas inclusion of the 12.42 Ma

age results in an age of 11.40 Ma. lt is important to note that indi_dual K-Ar ages and averages calculated ft-ore them may vary by several tenths of a million years. In this study the critical evaluation of K-At ages for volcanic units of the SWNVF by Marvin et al. (1970) was closely followed. Kisfler (1968), as well as some other

sources, did not, unfortunately, include values for analytical uncertainty. Potassium- 6

Table I. Published K-At Ages for the Ammonia Tanks Member of the Timber Mountain Tuff.

UNIT SAMPLE MZNERAL AGE (Ma)

Tuff of Cat Canyon Ring Fra=ture 'IXlff Age 7 Biotite Ii II " " Ii[ 181 Age 7 Sanidine I0.85 " " Ii. 221

Lower Cooling unit Age 22 Sanidine 11.36 Age 23 Biotite 12.02 Age 23 Sanidine Ii. 392 Age 23 Sanidine ii. 673

Ammonia Tanks Member Age 17 Biotite 11.16 Age 16 Sal_idine ii.024 Age 1 _ Sanidine ii. 09 `7 Age I._ Biotite II. 48 Age 20 Biotite ii. 64 Age 5 Biotite ].2.42* Age 18 Biotite 11,.30 Age 18 Sanidine 11.50

ges are from Kistler (1968) Age calculated from a second Ar extraction Age from -100, +115 mesh f_action of sample Age 22 Age from -80, +I00 mesh fraction of sample Age 23 Age from -80, +100 mesh fraction of sample Age 16 Age from -65, +100 mesh fraction of sample Age 16 Age considered too old by Marvin et al. '(1970)

argonagesandaveragesofagesona volcamunitc shouldbeevaluatinedconsidera- tior_of thei_"aaa,lyticm]uncertainty,t.herelative stratigraphJcposition of the unit and the radiometricagesof underlyingand overlyingunits.

Old_volcanicunits

Olderunitsofash-flotuffw wereeruptedft'overentswithint.heSleepingButte calderasegmentthe, SilentCanyoucalderaandtheCra.ter_at-ProspectorPass c-alderacomplex(FigsI.md 2),The oldestunitoftheTimberMountain-O_is Va.Uccalderay completxsthecalc.aJktuffaticofRedr(_ckValley,whichCart et al. (1986) believe was erupted from vents within the ,Sleeping Bu_.te caldera segment (Figs. 2 and 3). Although more work is needed, preliminary K-Ar ages (IE.H.McKe¢, D,C. Noble and S.I. Weiss, unpub, data, 1988) suggest that the tuff of Redrock Valley may be several miEion years younger than the currently accepted 16.1 Ma age (Marvin, 1970). The tuff of Rech'ock V_,ey is overlain sequentially by the regionally extensive Lithic Ridge Tuft' and the tuff of Yucca Flat (Cart et al., 1986). Little is known about the origin of these units, but it has been suggested that the Lithic Ridge Tuff was erupted from vents within an older caldera buried under the northwest end of Yucca Mountain (Carret al., 1986). The Crater Flat Tuff consists of three rhyolitic to quartz latitic ash-flow units

believed to have been erupted from vents with irt the Crater Flat Frospector-Pass caldera complex about 14 million years ago (Figs. 2 and 3)(Marvin, i970; Carr, 1986). Cart et al. (1986) suggest that the oldest unit, the Tram Member, was probably erupted from the northern caldera of the compl,ex, the Prospec':or-Pass caldera (Fig. 2). The younger members of the Crater Flat Tuff, the voluminous BuUtrog Member and the overlying Prow Pass Member, are thought to have been erupted from vents within the Bullfrog caldera (Fig. 2)(Cart et al., 1986). At about 13.9 Ma, quartz latite dikes intruded Paleozoic sedimemary rocks at Bare Mountain on the west side of the Crater Nat-Prospector Pass caldera complex. Although the stratigraphic position of these dikes is Dot well known, pettographically and compo-

sitionally similar lavas underlie the Lithic Ridge Tuff in Beatty Wash; therefore, the dikes as'o thought to be older than the Crater Flat and Lithic Ridge Tufts (Cart et al., .986). The Belted Range Tuff and overlying Stockade Wash Tuff were enlpted from vents within the calc-alkalic to peralkaline Silent Canyon Caldera (Figs. 2 and 3) (Noble et al., I968) (Byers ¢t al., 1976a). The upper member of fl'_,eBelted

Range Tuff, The Grouse Caxlyon Member, was dated at 14.1 Ma (Noble, 1968; 8

Mar_n, 1970). StnatigraphicrelationsindicatethattheBeltedRange Tuffis youngerthanrb.Lithic-.. RidgeTuffandolderthantheCrater'FlatTuffand thatthe StockadeWash TuffisyoungerthantheCraterFlatTuffand olderthanThe PaintbrusTuffh (Cartetal.198, 6).

RhyolitoefCalicHillso

At about 13.8 Ma, lavas and tufts of the rhyolite of Calico Hills were erupted from vents in the Calico Hills (Figs. 2 and 3)(Kistler, 1968; Byers, 1976a; 1976b). The rhyolite of Calico Hills is overlain by the Paintbrush Tuff.

Wahmonie and Salyer Formations

A local sequence of calcic to calc-alkalic intermediate lavas and ash-flow and air-fall ruffs was erupted prior to and during early P,'dmbrush volcanism at the Wahmonie.Salyer volcanic center (Figs. 2 and 3)(Poole et al., 1964; Ekren and

Sargent, 1965). At the W',thmonie-S'alyervolcanic center, the dacitic to rhyodacitic Wahmortie Formation overlies the mostly rhyodacitic Salyer Formation (Ek.rcn and Sargent, 1965). The basal ash-fall and the upper rhyodacite portions of the

Wahmonie Formation have been dated at about 1.3.2Ma and 12.8 Ma, respectively (Kistler, 1968).

Paintbrush Tuff

The Paintbru.sh Tuff was erupted between about 13.3 Ma and 12.8 Ma (Figs. 2 and 3)(Byers et al., 1976a; Christiamen et al., 1977). The Paintbrush Tuff consists of five alkall-caleic, rhyolitic to quartz latitic ash-flow units contalr_g few quartz phenocrysts. The Topopala Spring and overlying Pah Canyon Members of the Paintbrush Tuff were erupted from vents within the Claim Canyon cauldron seg- ment. The source of the younger Yucca Mountain and overlying Tiva Canyon 9

Members of the Paintbrush Tuff is disputed. Byers et al. (197r,a) believed that the Yucca Mountain Member, Tiva Canyon Member and tuff of Piuyon Pass were also erupted from vents within the Claim Canyon cauldron segment. Christiansen ct al. (1977), however, thought that these units were erupted from an area overlapping the Claim Canyon cauldron segment but also including at least part of the Oasis Valley caldera segment. The exu'acaldera rhyolite of Delirium Canyon is intercalated with units of the Paintbrush Tuff and was probably related to magmatic activity at the nearby Claim Canyon cauldron segment (Fig. 3)(Warren et al., 1988).

Timber Mountain Tuff

The last caldera-forming, ash-flow eruptions in the Timber Mountain-Oasis Valley caldera complex took place at about 11.5 and 11.3 Ma, respectively, with the eruption of the alkali-calcic, compositionaily zoned, rhyolitic Rainier Mesa and Ammonia Tanks Members of the Timber Mountain Tuff (Fig. 3)(Byers et al., 1976a). There is some debate over the source area of the Rainier Mesa Member.

According to Byers et al. (1976a), the Rainier Mesa was erupted from vents within the Timber Mountain caldera and Oasis Valley caldera segment; Christiansen et al. (1977), however, believed the Oasis Valley segment is related to eruption of the Paintbrush Tuff and that the Rainier Mesa Member was erupted solely from vents within the Timber Mountain eaidera (Fig. 2). Shortly after eruption of the Rainier

Mesa Member, the Ammonia Tanks Member was erupted from vents within the Timber Mountain caldera, resulting in renewed subsidence of the T'tmber Mountain caidera (Figs. 2 and 3). Some of the tuff dikes in the Timber Mountain dome are petrologically and compositionaUy similar to the upper part of the Ammonia Tanks Member and may have been vents for the Ammonia Tanks (Byers, 1976a). Compo- sitionally similar lavas preceded eruption of the Rainier Mesa and Ammonia Tanks Members of the Timber Mountain Tuff (Fig. 3); some pre-Rainier Mesa rhyolite lo lavas were erupted west of the Timber Mountain-Oasis Valley caldera complex in the Beatty-BuJJlrog Hills area. The Rainier Mesa and Ammonia Tanks Members of the Timber Mountain Tuff are q_te voluminous, with estimated volumes greater than 1,200 Km. 3 and 900 Km.3, respectively (Byers ct al., 1976a).

Post-collapse Timber Mountain volcanism

New K-At ages indicate that post-collapse volcanism at the Timber Mountain caldera continued for about one million years. Resurgence of the Timber Mountain calde,'a, forming the Timber Mountain dome, took piace early during this period of post-collapse volcartic activity, which included eruption of the intracaldera tuff of Battonhook Wash and tufts of Crooked Canyon members of the Timber Mountain Tuff, intrusion of microgranite ring dikes into the Timber Mountain dome and eruption of some intra- and extracaldera lavas (Fig. 3) (Byers et al., 1976a;

Christian.sen, 1977). Warren et al. (1988) conclude that lavas previously mapped as the rhyolite of Forwm.ile Wash (Byers et al., 1976b) consist of several flows ranging in age from 13.5 to 10.6 Ma (lavas 2 through 5 shown on Figure 3), including two bodies of lava in the Timber Mountain caldera moat dated at 10.6 and 10.7 Ma.

The petrology of these moat lavas indicates that they were erupted from the Timber Mountain magma body (Warren et al., 1988). The intracaldera r.hyolite lavas of Beatty Wash and West Cat Canyon were probably also erupted during post-collapse volcanism at the Timber Mountain caldera (Fig. 3)(Byers et al., 1976a; 1976b). A aew K-A.r age of 10.3 _+0.4 Ma.,obtained in this study, for the youngest un.it of the rhyolite of Shoshone Mountain suggests that eruption of the extraealdera rhyolite of Shoshone Mountain., was related to post-coUapse Timber Mountain magmatic activity, some of which took piace southeast of the T'tmber Mountain ealdera (Figs. 2 and 3)(Table 2). 11

Table 2. New K-At age determinations on volcanic rocks , , _ i SAMPLE# UNIT K O 40Ar* 40Ar*/4OAr Age (wt:.2 %) (mole/g) (%) (Ma)

16 Tuff of Cutoff Rd. 10.4 1.5579xi0 -I0 88.2 10.4 ± 0.3

17 Rhyolite of 5.71 8.4770xi0 -II 50.0 10.3 _+ 0.3 Shoshone Mtn.

18 [hyolite of 5.14 6.4927xi0 -11 71.9 8.8 _+ 0°2 Obsidian Butte

More complete descriptions, including sample locations, are included in Appendix 2 Analyses were made in laboratories of the U.S. Geological Sul-.'ev, Menlo P_rk, Cal_=orni_, under supe._vision of E.H. McKee C_ns_nts: k E = 0.581 x !0 -I0 _r -1, k G = 4.962 x l0 -10 yr- , _K/K(total ) 1.157 x i0-" kmo/k_o.

Lava:, and ruffs of the Bearty-Bullh'og Hills area

,_though sources are not knowu, distribution of units, fades relations and K-

Au" ages indicate that volcanism west of the Timber Mountain-Oasis Valley caldera

complex in the Beatty-Bullfrog Hills area was syuchronous with post-collapse volcanism at the Timber Moumam caldera (Fig. 3). The tu_f of Cutoff Road, dated

in this study at 10.4 _+ 0.3 Ma (Table 2), and the uaderlymg tufts of Fleur-de-Lis

Ranch are thickest in the Oasis Mountain area and thin to the east (Byers et al.,

1976a); although Byers ¢t al. (1976a) beLieved that these units were erupted from

sources within the Oasis Valley caldera segment, vent areas for _ ash-flow units

may be west of the Timber Mountain-Oasis Valley caldera complex, possibly in the

area of the Bull£rog Hills (Fig. 2). The tuff of Cutoff Road and t,/Is of Fleur-de-IAs

Ranch are compositionally similar to the rhyolite lavas of Beatty Wash and rhyolite

lavas of West Cat Canyon, respectively; compositional similarity to these

intracaldera lavas supports the interpretation of Byers et al. (19"7_) that these units

were erupted from vents within the Oasis Valley caldera segment. Rocks mapped as 12 tuff of Cutoff Road within the Timber Mountain caldera in the eastern Transvaal

Hills (Byers et al., 1976b) are much richer in lithic fragments than is the ruff of Cutoff Road exposed in the western Tran,waal Hills and on Oasis Mountain and probably represent a local, intracaldera unit related to the eruption of post-collapse rhyolite lavas in the Timber Mountain caldera moat. The porphyritic latite exposed at the Montgomery-Shoshone mine and the rMolite of Burton Mountain, dated at 10.7 + 0.3 and 10.1 ± 0.4 Ma, respectively (Morion et al., 1977; Marvin and Cole,

1978; Cart, 1984), were erupted from vents in the Beatty-Bullfrog Hills area (Fig. 2). Pre-Rainier Mesa lavas, dated at 11.5 Nta, were also erupted in the Beatty-Bullfrog Hills area (Fig. 3)(Byers et al. 1976a). This 11.5to l0 Ma perio_ of volcanic activity in the Beatty-Bullfrog Hills area demonstrates the presence of some Timber

Mountain magmatic activity west of the Timber Mountain-Oasis Valley caldera complex in the Beatty-Bullfrog Hills area.

Thirsty Canyon and Stonewall Flat tufts

A hiatus in volcanic activity at the SWNVF occurred between about 10 and 8.8 Ma, after which silicic volcanism continued to the north at Obsidian Butte and at the younger subalkaline to peralkaline Black Mountain and Stonewall Mountain calderas (Figs. 1 and 3)(Noble et al., 1984). The rhyolite of Obsi0ian Butte (Fig. 1) was erupted at 8.8 _+0.3 Ma (Table 2), as a local sequence of lava flows. The Thirsty

Canyon Tuff was erupted from vents within the Black Mountain caldera between about 7.8 to 7.5 million years ago (Weiss et aL, iu press). Eruption of the Civet Cat Canyon Member of the Stonewall Flat Tuff from vents within the Stonewall

Mo,retain caldera complex at about 6.3 Ma represents the last caldera-formmg, ash- flow eruption of the SWNVF (Noble et al., 1984; Weiss and Noble, in review). 13

Alteration and Mineralization

Hydrothermal alteration and mineralization has effected rocks over a large area within and surrounding the Timber MountaJn-Oasis Valley caldera complex (Fig. 2). Miner_liTadon, hosted by Tertiary volcanic rocks and neax-caldera Paleozoic sedimentary rocks, is epithermal, generally low m base metals and charac- terized by the deposition of precious metals, Hg and/or fluorite. Quartz, adularia, sericite and calcite are the most common gangue minerals in veins. Adularized and silicified rock, with or without K-mica, generally occurs near structural conduits and/or in deeper levels of epithermal systems. Argillic alteration, alunitization and silicification are found in the higher levels of hydrothermal systems. Although fur- ther characterization is needed, the presence of adularia and sericite, manganese oxides and the absence of enargite suggest that volcanic-hosted epithermal deposits best fit the adularia-sericite type of Heald et al. (1987). The Sterling mine on the east flank of Bare Mountain is a sediment-hosted disseminated Au deposit, and other unrecognized examples may occur in the area. A more detailed description of mineralized and altered areas follows.

Bullfrog [-Iillsand Oasis Mountain area

In the Bulldog Hills and Oasis Mountain area, classic epithermal Au-Ag veins and related disseminated mineralization are hosted by Tertiary volcanic rocks

(Fig. 2)(Ransome, 1907; Ran.some et al., 1910; Cornwall and Kleinhampl, 1964). Gold was discovere,t at the Original BuUfi'ogmine in 1904, and early Au production

in the Bullf:rog Hills continued until the early 1920's (Cornwall and Kleinhampl, 1964; Ransome et al., 1910; Smith et al., 1983). Precious-metal exploration and

mining activity has recently r_.'umed to the BuUfa'ogHiils; the Gold Bar mine is a currently active Au-Ag producer, and DaUhold Resources, Inc. has recently announced a discovery of 3.2 million ounces Au and 3.4 million ounces Ag about 1 .1.4 milesouthof theMontgomery-Shoshonemine (Fig.2)(Booth,1988).Precious- metalmineralizationintheBullfrogHillsgeneralltoy okpiacealongsteeply-d/p- ping,north-trendingnormalfaultsin Tertiaryvolcanicrocks.At theOriginal Bullfrogand Happy Hooliganmines,however,Au-valuesarereportedto occur alonglow-anglefaultsbetweenTertim-voly canicandPaleozoicsedimentarrocksy

(Ransome,1907;Ransome etall.19,10).Theselow-anglestructuresmay be theup- permostof two detachment fault surfaces above a metamorphic core complex in the Bullfrog Hills (McKee, 1983; Maldonado, 1985; 1988), Although some chalcocite and chalcopyrite are found at the Original Bullfrog mine, mineralized rock in the

Bullfrog Hills is generally low in base metals with Au and Ag originally occurring in pyrite (Ransome et al., 1910; Smith et al., 1983). In the Bullfrog Hills and Oasis Mountain area, veins with quartz, adularia, sericite and calcite gangue are com- monJ,y surrounded by adularia-, sericite- and quartz=bearing alteration assemblages near the vein, grading out into kaolinite-bearing assemblages farther from the vein. Although the vertical zonation of alteration in adularia-sericite-type deposits is var- iable (Heald et "al.,1987), the presence of vein adularia and sericite, adularized wall rock, low base-metal values and zones of argillic alteration suggest that miner- alization and alteration e:q:)osedin the Bullfrog Hills are characteristic of the upper pan of the ore zone in volcanic-hosted precious-metal deposits (Buchanan, 1981; Silberman and Berger, 1985).

Northern and ea_em Bare Mountain

At the Daisy, Telluride, Goldspar and Sterling mines on the northern and eastern flanks of Bare Mountain, epithermal Au-Ag, Hg, and fluorite mineralization is hosted by Paleozoic sedimentary rock (Fig. 2)(Cornwall, 1972; Papke, 1979; Smith et al., 1983; Tingley, 1984). Mineralized rock on the north and east flanks of Bare Mountain is low in base metals in comparison to that on the south and west flanks 15

(Tingley, 1984). Although not reported by Tingley (1984), some elevated concentra- tiom of Au, as evidenced by the presence of visible gold at the Gold Ace mine (LT. Larson and MJ. Jackson, unpub, data, 1987), are found on the southwest flank of Bare Mountain. Small Au and Hg discoveries were made in the Bare Mountain dis- trier during the early 1900's (Cornwall, 1972; Smith et al., 1983). The small produc- tion was generally not recorded, although several hundred flasks of Hg were report- edly produced from the Telluride mine area and the Thompson mine (Smith et al., 1983). Fluorspar has been produced at the Daisy mine since 1918, and the Sterling mine is a currently active Au producer. Fluorite and ro.inor amounts of gold and cinnabar occur in veins and as replacement bodies between steeply dipping normal faults in the nose of an anticline in Paleozoic carbonate rocks at the Daisy mine (Cornwall and Kleinhampl, 1961; Papke, 1979; J. Greybeck, pers. commun., 1987). In the Telluride mine area, quartz, opal, alunite, pyrite and subeconomic concentra- tions of Au are found along the not'them range-bounding normal fault between Paleozoic carbonate rocks in the foot'wall and unnamed bedded tuff of the Timber

Mountain Tuff in the hanging wall (Carr and Monsen, 1988; P.P. Ork/ld, pets. commun., 1988); cinnabar is found along faults and within pipe-like breccias in the Paleozoic rocks in the hanging wall (Cornwall and Kleirthampl, 1961; D. Farming,

pers. commun., 1987; D. Finn, pers. commun., 1987). At the Goldspar mine, sub- economic concentrations of Au and fluorite occur in a hydrothermal breccia,

containing clasts of Tertiar__volcanic rock, a quartz latite dike, dated at about 13.9 Ma, and Paleozoic dolomite (Papke, 1979; Smith et al., 1983; T'mgley, 1984). Dis-

seminated gold and minor amounts of fluorite and dnnabar are located along and above a thrust fault in Paleozoic sedimentary rocks at the Sterling mine; ore zones

are found where the thrust plane has been dropped down to form small, north-

trending grabens (Cornwall and Kleinhampl, 1961; Odt, 1983; J. Mart', pers. commun., 1987). Wall rocks at Bare Mounta.i.n are silicified, sericitized and 2.6 azgfltizeThd.e presenceofbodiesofhydrothermabrl ecciaand elevatedconcentra- t.ions of Hg on the northern and eastern flanks of Bare Mountain suggests the exposed rocks were altered and mineralized in the shallow portions of hydrothermal systems.

Silicon and Thompson mines

At the Silicon and Thompson mines, high-level epithermal Hg mineralization is hosted by Paintbrush and Timber Mountain Tuff, which have been altered to alunite-, kaolinite-, quartz-, and opal-bearing assemblages (F!g, 2)(Tingley, 1(),S-• _)

In the early 1900's, small amounts of Hg were mined from the Thompson mine and ceramic-grade silica was produced from altered tuff of the Rainier Mesa Member of the Timber Mountain Tuff at the Silicon mine (Can" et al., 1986; Smith et al., 1983'

Tingley, 1984). Cinnabar is found in altered Paintbrush Tuff (Scott and Bonk, 1984;

Orkild, pers. commun., 1988) at the Thompson mine and less commonly at the

Silicon mine. The Silicon and Thompson mines are situated along a NNW-trending zone of alteration that extends for about 2 miles from a short distance south of the

Thompson mine to north of Beatty Wash. Deposition of Hg and more pervasive alunitization and silicification of tuff seems to have taken place at the intersection of this NNW-trending zone with more easterly-trending faults. Elevated concentra- tions of Hg, and alunite-, quartz- and opal-bearing alteration assemblages are found

in the uppermost portions of adularia-sericite-type volcanic-hosted precious-metal deposits (Silberman and Berger, 1985; Heald et al., 1987).

Transvaal Hills, Clm'm Canyon cauldron segmem margin and Yucca Mountain

In the Transvaal Hills and along the margin of the Claim Canyon Cauldron

segment, local low-level gold anomalies axe found in volcanic rocks within large

areas of predominantly argillically altered rock (Fig. 2)(D. Spicer, pers. commun., 17

1987; M.R. Jackson and S.I. Weiss, unpub, dam, 1987). Quartz-adularia veins and adularized wall rock are found locally along faults in the Transvaal Hills, and are surrounded by more areally extensive alteration zones characterized by the presence of kaoliniteQ, quartz- and alunite-bearing assemblages. These areas of hydrother- really altered rock parallel the margins of the Timber Mountain caldera and the

Claim Canyon cauldron segment and may be related to migration of hydrothermal fluids up calder_ ring fractures (Fig. 2). In the Transvaal Hills, the Timber

Mountain Tuff and the tuff of Cutoff Road are locally altered and in places contain elevated concentrations of .Au: overlying basalt and Thirsrv Canyon Tuff are unal- tered. Along the margin of the Claim Canyon cauldron segment alteration and mineralization extend into the Tiva Canyon Member of the Paintbrush Tuff, the youngest unit found in altered areas along the margin (Byets et al., 1976b).

At Yucca Mountain, drill holes G-I, G-2 and G-3 show that older rafts in the subsurface of Yucca Mountain have been altered to zeolite- and illite-bearing assemblages (Fig. 2)(Spengler et al., 1981; Caporuscio et al., 1982; Maldonado and

Koether, 1983; Vaniman et al., 1988). Fluorite, barite and possibly low concentra- tions of gold are found locally within this zone of altered rock (Spengler et al., 1981;

Caporuscio et al., 1982; Maldonado and Koether, 1983; Broxton et al., 1986).

Geothermometry based on alteration mineral assemblages indicates that paleo-fluid temperatures increase to the north, suggesting lateral migration of hydrothermal fluids south from the nearby Timber Mountain-Oasis Valley caldera complex (Bish,

1987).

Calico Hills, Homsilver mine at Wahmonie and Mine Mountain

In the central portion of the Calico Hills, elevated concentrations of Au and

Ag, base-metal sulfide veins and fluorite axe found locally in Paleozoic sedimentary rocks (Fig. 2)(Orkild and O'Connor, 1970; Quade and Tingley, 1984). Although no 18 production records exist, old workings indicate that a small amount of prospecting and mining has been done in the central portion of the Calico Hills (Quade and

Tingley, 1984). Fluorite veins and replacement bodies are found along a low-angle fault within Paleozoic sedimentary rock (M.R. Jackson and S.I. Weiss, unpub, data,

1987). Volcanic rocks, which have been altered to alunite-, kaolinite-, quartz- and chalcedony-bearing _semblages, overlie these Paleozoic rocks and make up most of the Calico Hills (McKay, 1963; McKay and Williams, 1964). Although alteration seems to be centered on the locally erupte,t rhyolite of Calico Hills in the western portion of the Calico Hills, in the eastern Calico Hills silicification and alunitization extend upward into the Rainier Mesa and Ammonia Tanks Members of the Timber

Mountain Tuff (Orkild, 1968; M.R. Jackson and S.I. Weiss, unpub, mapping, 1987).

At the Hornsilver mine in :ac Wahmonie district, Au and Ag bearing veins are hosted by intermediate lava and ash-flow and air-fall tuff of the Wahmonie

Formation (Fig. 2)(Ekren and Sargent, 1965). Northeast-trending veins containing quartz, adularia, sericite, and calcite gangue are situated in a broad halo of propylitic alteration. An adularia-, sericite- and quartz-bearing alteration assem- blage is found near the veins and locally grades out into kaolinite-bearing rock.

Rich Au-Ag ore was discovered at the Hornsilver mine as early as 1847 and mined during the 192ffs; recent assays as high as 42 oz/ton Au and 1130 oz/ton Ag are reported for ore samples (Quade and Tingley, 1984).

At Mine Mountain, elevated concentrations of Au, Ag, and Hg are found in

Paleozoic sedimentary rocks (Fig. 2)(Orkild, 1968; Quade and Tingley, 1984; S.I.

Weiss, tmpub, data, 1987). Claim notices indicate that exploration of Mine

Mountain began as early as 1928 and, although no records exist, old mine workings

and retorts are evidence for a small amount of production (Cornwall, 1972; Quade

and Tingley, 1984). Elevated values of Au, Ag and Hg are found in north-trending

faults and breccia zones filled with quartz and barite (Quade and Tingley, 1984; S.I. / 19

Weiss, unpub, data, 1988). The Ammonia Tanks Member of the Timber Mountain Tuff is argillized and silicified at the southern end of Mine Mountain (Orkild, 1968; S.I. Weiss, pet's, commun., 1988).

Age of Hydrothermal Alteration and Mineralization _y

The timing of hydrothermal alteration and mineralization in the area of the Timber _" Mountain-Oasis Valley caldera complex is constrained by K-Ar ages obtained on hydrothermal alunite and adularia, as well as by the radiometric and relative ages of ,_ host rocks, i.e. geologic relations. Shown in Fig. 4 are K-Ar ages from hydrothermal alunite and adularia, as well as lower age limits on hydrothermal alteration and mineralization based on geologic relations; detailed K-Ar sample descriptions and

analytical data are included in Appendix 2. Location of K-Ar samples and the generalized distribution of hydrothermal alteration and mineralization with respect to age are shown in Figure 2. Ali of the K-At ages for hydrothermal alteration and mineralization in the area of the Timber Mountain-Oasis VaLleycaldera complex fall between about 13 and 9 Ma (Figs. 2 and 4). Older ages for hydrothermal alteration and rrtineraliza- tion are from the Hornsilver mine at Wahmonie and the Thompson mine (Figs. 2

and 4). The youngest age, 8.7 ± 0.3 Ma, is from the Original BuUfrog mine. Geologic relations and petrographic evidence indicate that some of the K-Ar ages

are too old (Figs. 2 and 4). The bedded tuff stratigraphically above the pre-Rainier Mesa rhyolite lavas and below tile Rainier Mesa Member of the Timber Mountain Tuff is si/icified and argillized at the Telluride mine area; alteration and mineralization at the Telluride mine area is, therefore, younger than about 11.5 Ma,

and the 12.2 ± 0.4 Ma K-At age for alteration and mineraliTation is too old. The Ammonia Tanks Member of the Timber Mountain Tuff and tuff of Cutoff Road are

locally silicified and argillized in the Transvaal Hills; thus, alteration and mineral- 2O ization in the Transvaal Hills is younger than about 11.3 Ma and may be as young as about 10.4 Ma. Based on these geologic reIafiom, the 12.2 ± 0..3 Ma K-At age for _Ite,-anon and m/ne_fion in the Tmmva_ Hms is too old. Petrographic AGE Oil VOLCANIC ACTIVITY AGES OF MYOIIOTHF.,RMAI. AKTIlRATION AND ItlNERAJ_ZATION ,AGE (M&_ . ! e: i'l : . o _ " )" ; } = 1 TMOV IIIIH g_ .;. m _ ., Z A

.,_ ' 2 " ,_.,_.,.,,

::Ii _ _" T _ A A =_ × -I i* ' m _ 11-- , l X i _, A I _ T A A A h,.- " I_ [ A. I1 .. 12" li - :- I i-= T ,i l _ ,T,

I_, '_4 -'- A A

ZZlPlana_ion

-----SAMPL_ I.D.. (A_pendix 2) /7"

_&_ALYTICAL UNCERTAINTY/ ,_'-_ ( _ - Ages that (i_) ...... _ axe _oo old based on geologic or petrograpl_ic /_Q.____GE LrKET- ----A evidence. Based on geologic relations.

Fig. 4. Ages of hydro_ermal slterarion andminera_zarionin the _rea of _e T'u_r Moun_-Ossis V_ey caldem comple= and ages of volcanic activity at the T_mberMoumaimOasisValleycalderscomplex(TMOV)and Bearry-Bu_:rogHills area (BBH). M.o_of _e K-At ages s_ownin Hg. 4 were pzoducedin _ study, also included are. A) an IUGS comr_uzcorrected9.5 Ma K-A_"age for n_ersliz_- tionatthe Montgome,ry-Shcshone mine (Mortonet al.1,977)andB) 10to11Ma K- A.rsgesforillitefros maltererdocksM thesubsn%ceofYuccaMountain(Aronson HoandrmilvB/sh,ermin1987)e.atWAbbreviatiahmon/eons. :PosOt.B-.coll- Oda,volpEinalseczBuaism/Itarot gthmineeTim, berWahmMountainonie - calderiasshowninthediagonaplatternSa. mpLelocationsareplottedon Figure2. 21

examinationrevealedpartiaaldularizationofthesamplefromthealtered13.9Ma quartzlatitdike eattheGoldsparmine,resultinginhydrothermarlecrystallization _d resettingofpotassiuminonlysome ofthefeldspainr thesample.Thus,the 12.9.+.0.4Ma age probablyprovidesan olderage limitforalterationand mineralizationattheGoldsparmine.Eliminatingagesthatarejudgedtobe tooold basedon geologicrelationsandpetrographicstudyit, isapparentthatmostofthe agesforhydrothermalalterationandmineralizationfallbetweenabout11.5and 10 Ma (Fig. 4). Included in this 11.5 to !0 Ma group is hydrothermal alteration and mineral- ization at the E'ullfrog Hills, Oasis Mountain, the Telluride mine area, the Silicon mine, the Transvaal Hills, the Calico Hills and in the subsuHaee of Yucca Mountain

(Figs. 2 and 4). In addition, alteration and mineralization at Mine Mountain is younger than the 11..3Ma Ammortia Tan_ Member of the Timber Mountain Tuff and may well fall within this 1.5m.y. period of widespread hydrothermal activi_,. The timing of hydrothermal activity in some areas is not so well known (Fig. 2). The age of hydrothermal alteration and mineralization is poorly constrained at the Sterling mine, the Goldspar mine and along the margins of the Claim Canyon cauldron and Sleeping Butte segments. Geologic relations strongly suggest that alteration and mineralization at the Sterling mine is yotmger than the 13.9 Ma dikes on Bare Mountain becalttse the dikes are silicified, iron-stained, _ed and con-

rain subeconomic gold values near the mine (M.R. Jackson and S.l. Weiss, unpub. data, 1.987). Alteration and mineralization at the Goldspar mine is also younger than the dikes on Bare Mountain, as fluorite was found in fractures in the argillized

dike exposed in the northern mine pit. As previously mentioned, the 12.9 _+0.4 Ma age for alteration and mineralization at the Goldspar mine is probably provides only

an older age Limit for hydrothermal activity at the Goldspar mine. Alteration and mineralization along the margin of the Claim Canyon Cauldron segment is younger

_l_lll * _p,n , _ll I)llllllllr'l,_[ ,irl, l,i I, ii, i_ i, _,, lr ,llll(i, IIB I rll_lt,llIi, p i illl _ rl_llll, II1[1 'lilr ii I Ii'l_l il[I .... Iltl ' ' III I ill?' * i)pll i]I _]1,, iiitl, ll, ) I, 'I,, , ,_p),,mC..... :"_1 ..... lEI ' 22 than the 12.8 Ma Tiva C,anyon Member of the Paintbrush Tuff, but no suitable material for radiometric dating has yet been found. The age of alteration and mineralization on the southwest flank of Bare Mountain and in the Bailey's Hot Springs area is not known. Bailey's Hot Springs is currently active and surrounding alteration may be very young,

Discussion

Relation of hydrothermal activi_, to magmatic activity

As shown in Figure 4, hydrothermal alteration and mineralization in the area of the Timber Mountain-Oasis Valley ealdera complex is temporally related to pulses of magmatic and volcanic activity. Older periods of hydrothermal alteration and mineralization at the Thompson and Hornsilver mine were probably related to magmatic activity at the Claim Canyon cauldron segment and Wahmonie.Salyer volcanic center, respectively. Based on a single K,Ar determination and the occur- rence of alteration in the Tiva Canyon Member of the Paintbrush Tuff at the Thompson mine, hydrothermal activity at the Thompson mine took place shortly after eruption of the Tiva Canyon Member. Therefore, hydrothermal activity,at the Thompson mine was probably related to nearby magmatic activity at the Claim

Canyon cauldron segment and possibly the Oasis Valley caldera segment (Figs. 2 and 4). Alteration and mineralization at the Hormilver mine at Wahmonie oc- curred within a similar period of time but is more likely related to local interme. diate volea.nism at the Wahmonie volta.tile center between about 13.2 and 12.8 Ma.

In addition, geophysical data suggest that mineralization at W,a.kmonietook piace above and within the upper portion of the granodiorite inm.tsion believed to be co- magmatic with the Wahmonie Formatiou (Ponce, 1981; 198,4; Smith et al., 1981;

Hoover et al., 1982), supporting a genetic relationship between mineralization and local magmatic activity at Wahmortie. Also, hydrothermal alteration predating the 23

Timber Mountain Tuff is present along the Sleeping Butte caldera segment of the

Timber Mountain-Oasis Valley caldera complex (D.C. Noble, pers. commun., 1988) and may be related to an older magmatic event.

The 8.7 _+0.3 Ma age on hydrothermally altered rock at the Original Bullfrog mine is more difficult to interpret (Fig. 4). Although it is possible that the age is too young as a result of loss of radiogenic argon or analytical error, the age may also be correct. If so, hydrothermal activity at the Original Bullfrog mine would be signifi- cantly younger than the main 9.5 and II.0 Ma period of hydrothermal activity in the

Bullfrog Hills synchronous with volcanic activit-v in the Beatty-Bullfrog Hills area

(Fig. 4).

Certainly, the nature and structural control of mineralization at the Original

Bullfrog mine is different from that of other areas in the Bullfrog Hills; ores are comparatively high in base metals and may occur along a proposed upper detach- ment fault between Paleozoic sedimentary rocks and overlying Tertiary volcanic rocks (Maldonado 1985; 1988). Alteration and mineralization at the Montgomery-

Shoshone mine has been dated at 9.5 +_0.2 Ma (Morton et al., 1977) and is the only other age for hydrothermal alteration and mineralization in the southern pan of the

Bullfrog Hills. The two ages suggest that hydrothermal activity in the southern

BuILf:rog Hills may be significantly younger than that in the northern Bull_og Hills.

The 8.8 _*0.3 Ma age for the rhyolite of Obsidian Butte, about 30 miles to the north, suggests the possibility of a younger period of magmatic activity in the SWNVF' that could be related to hydrothermal activity ai the Original Bullfrog and Montgomery- Shoshone mines.

Other evidence for the timing ofalteration and mineralization in the south- em BuUfxog Hills supports the interpretation that it may nevertheless be related to the 10.7 to 10.1 Ma period of magmatic and volcanic activity documented in the area. As discussed in a following section, _drothermal activity in adularia.sericite 24 systems is commonly on the order of a million years younger than spatially related volcanic rocks (Silberman et al., 1972; Heaid et ai., 1987). The 9.5 _+0.2 Ma age for alteration and mineralization at the Montgomery-Shoshone Mine is probably within the limit of azmlytical uncertainty of the 10.0 Ma average age for the rhyolite of Burton Mountain (Can', 1984) and, therefore, is not unreasonably younger than magmatic activity in the Bullfrog Hills. CooLing ages of 11.2 ± 1.1 and 10.5 Ma are reported for Precambrian gneisses exposed within i mile southwest of the Original BulLfrogmine (Fig. 4)(McKee, 1983; Maidonado, 1985), suggesting high heat flow in the southern Bullfrog Hills contemporaneous with Timber Mountain age volcanic and magmatic activity, Therefore, it seems likely that hydrothermal activity at the Original Bullfrog mine occurred during this period of high heat flow, consistent with the interpretation that the 8.7 ± 0.3 Ma age is too young due to analytical error or loss of radiogenic argon. Further dating of alteration and mineralization in the southern Bullfrog Hills, especially along the proposed detachment fault, may clarify this matter.

Perhaps more importantly, the 0.8 Ma difference in the ages obtained from the Original Bullfrog and Montgomery-Shoshone mines suggests that ores at the two mines may have been deposited by different, and perhaps geneticaUy unrelated

hydrothermal systems. Hydrothermal activity at the Original Bullfrog mine may have been detachment-related, whereas that at the Montgomery-Shoshone mine

may have been related to high angle faults. Widespread hydrothermal activity between about 11.5 and 10 Ma took place

during a period of magmatic activity at the S3NNVF that included volcanic activity at the Timber Mountain caldera, the Shoshone Mountain area and the Beatty-BuU£rog

Hills area (Figs. 2 and 4). Most of this hydrothermal alteration and mineralization occurred during the approximately 1 m.y. period of post-collapse volcanism at the Timber Mountain caldera and synchronous volcanism in the Shoshone Mountain 25 and Beatty-Bulltrog Hills areas, suggesting that widespread magmatic heat sources of the Timber Mountain magmatic system drove extensive hydrothermal systems from about 11.5to I0 Ma.

Possible relation of hydrothermal and volcanic activity to a deep magmatic system

Cathles (1977; 1981) showed that the duration of volcanic and hydrothermal activity, at volcanic centers is significantly longer than the amount of time predicted as necessary, to convectively cool related subjacent magma bodies of reasonable size, suggesting that continued episodic intrusion of magma lengthens the cooling time of magma bodies and thus, increases the duration of related volcanic and hydrothermal activity. Silberman et al. (1979) demonstrated that, although there were some intermittent periods of inactivity', hydrothermal activity at Steamboat Springs, Nevada, continued for about 3 m.y. and was probably related to several intrusive events. Similarly, the 7 m.y. life span of volcanic activity in the area of the Timber Mountain-Oasis Valley caldera complex necessitates the periodic intrusion of magma and, as shown previously, hydrothermal activity is related to these pulses of magmatic activity. Rhyolite lavas dated between 11.5 and about 10 Ma are found both within and outside the Timber Mountain caldera (Fig. 2), suggesting that eruption of extra- caldera lavas and volcanic activity at the Timber Mountain caldera were related to the same general magmatic system. Lavas related to caldera volcanism were also erupted over large areas surrounding the Li_e Walker (Slemmons, 1966; Noble et al., 1974) and Silent Canyon calderas (Noble, 1968). During the development of a magmatic system, silicic magmas related to the intrusion of basaltic magmas into the lower crust may migrate upwards along separate pathways, in some cases reaching the surface to form lava flows, domes, or small-volume pyroclastic deposits, or may coalesce into large magma bodies above which calderas may form (Hildreth, 1981). 26

The intense period of hydrothermal activity, extracaldera and caldera volcanism

between 11.5 and 10 Ma may have been related to both the coalescence of a large magma body beneath the Timber Mountain caldera and peripheral intrusion of magmas along separate pathways during the development of a _,egional magmatic system.

Alternatively, the distribution of calderas and volcanic and hydrothermal activity in a volcanic field may be related to a single large magma body. The twin subsidence and resurgence features of the Yellowstone caldera are believed to have

occurred above separate accumulations of magma at the top of a single large magma body (Smith and Christiansen, 1980; Christiansen, 1982). Similarly, episodic eruption of petrographically distinct ash=flow sheets in the San Juan volcanic field

are thought to be related to differentiation of magmas in separate cupolas after movement of a subjacent 31 by 62 mile magma body to shallow crustal levels (Steven and Lipman, 1976; Lipman et al., 1978). Byers et ai. (1975a) showed the possible presence of a batholith beneath the Timber Mountain-Oasis Valley caldera complex. In more recent work, Byers et al. (in review) and Warren et al. (1988) summarize and present evidence that the 7 m.y. history of volcanic activity, in the area of the Timber Mountain-Oasis Valley caldera complex may be related to the evolution of a single large magma body. Intense volcanic and hydrothermal activity between 11.5 and I0 Ma took piace over a 25 by 35 mile area extending beyond the

margins of the Timber Mountain=Oasis Valley caldera complex (Fig. 2) and may have been related to this large intrusion. Whether related to an individual intrusive

body or intntsion of separate magmas and local coalescence of larger magma bod- ies, this intense episode of widespread volcanic and hydrothermal activity had to be _ related to the development of an equally intense magmatic system, during which large amounts of magma migrated upwards through the crust.

i 27

l= ±= dn.ssisc_eme of _Id=m e_,olu_io-of Smi_ and Batty (1968), hych-o_crma/a_vi_/mayoccurtt_rou_ou_ks =tlde_c/clebu_becom_ mosz _,_,idsmaRsr cn/derco= llaps_e tlmwaningstagesofvolcn_¢a_vi_. As show_ m F_,.R'e4,_e timingofl_/_ermal ac_wiD,at _e T'umberMountain cn/de__ r/Rsgenen/model Some _ll-smdieyod uagercn/denshavevolcnnic aadhyciror.he:'=_/historiesanzlogousto_e T'u::bMouer =m/n c=/de:-Ja.s.;show= Table#3,skn//_I r/l_'or_yea.rpenodsofpos:-¢o_apsevolc:tnzn/:d:om_-:pora.

neous hy_-othe._!zciv/U ar_ reportedfor the Long V_eZ, ValIes _d Y_;/ovs=o, R¢c!i...r_.I= addiiongold, g c,.uT-:beL_gt_y dsposkedduringpose-

co[lapshye /ro_h_r'-,acm/-/viwates Ye[lows:ome(SLll_zoeaad BoRhaa_ 198_), C_A/abozas (Grunder st aL, I987) m_dMoroa calderas (Km._ and S,_w'a'd.,1987). Pos_-collao_o_s¢¢."mal andvolami_¢._/vir_ac±_ TunberMoumam c=ider=

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C.A.LD_"dA COLZA2SZ V0LCAMZSM ACTI'7_TY

L_nq Valley "Z 0.7 Ma To 0.05 Ma _z"_tn1:2.y ac_:ive

Valles _ I.Z Ma To < 0.01 Ma ?? C%IZ'_tnv.lyac_.ve

Yellowsv.one 3 0. _3 Ma _o 0 o07 Ma _.Iz_r,1:ly acv.£ve

_a:__Z_yev. al. (ZeTa), _ ,rcal.. (_S85) Do,all ev. _. (Z9_8), Sm.it.tramd _£l_y (1968) Ct_z'£sv.£ansemand _lan_ (L972], SmA_:tan(__isv.£ansen (ZeSt)

' took pla_ over a larger ¢¢tzac_der4are_than runny of dacsemodem analogues. However, hot springsat rb, e Yellowstonecalderaare _lsosituatedover aIarg¢e.x='=- 28 caldera area, having been found as far as 25 miles from the caldera margin (Smith and Christiansen, 1980). As previously discussed, the l_ge areal dism'bution of hydrothermal activity associated with the Timber Mountain and Yellowstone calderas may be related to the presence of a single large magma body, the margir_sof which probably extend beyond the caldcra, or the regional intrusion of separate magma bodies.

Comparison to the timing of epithermal mineralization',associated with other calderas

Although not always related to the caldera cycle, epithermal mineralization has been found m association with other calderas (e.g., Elston, 1978; Ryruba, 1981; Sillitoe and Benham, 1984). Some studies of caldera-related epithermal deposits show that mineralization took place during post-collapse magmatic activity, similar to the timing of hydrothermal alteration and mineralization at the Timber Mountain J, caldera. At the Lake City ealdera, base- and precious-metal mineralization is spa- tinily and temporally associated with post-collapse ring-fracture i.rtrusions and domes (Steven et al., 1974; Men.hen et al., 1980; Slack, 1980). Mercury mineraliza- tion at the McDermitt caldera took piace during a period of post-collapse magma- mm that lasted for several million years (McKee, 1976; Noble et al., in press). Silver mineralization took piace shortly after post-coLlapse magmatic activity at the Nevada Pormgueza volcanic center in Peru (Noble and McKee, 1982). In these examples, like the Ttmber Motmmin caldera, significant post-collapse magmatic activity may have enhanced the development of hydrothermal systems du.dng the post-collapse period of the caldera cycle. Mineralization associated with some ealde as, however, did not take piace during the post-collapse period of magmatic activity. Caldera structures may be im- portant conduits for later hydrothermal systems unrelated to volcanic and magmatic activity of the caldera cycle. For example, mineralization associated with the 29

Platoro and Summitwflle calderas in the San Juan volcanic field took piace about 5 m.y. after the end of volcanic activity at the calderas and is thought ,.o be related to younger intrusions not related to the caldera cycle (Mehnen et NJ.,1973; Steven et al.,1974).

Comparison to the duration and timing of hydrothermal activity in other adularia-sericite-type systems

In adularia-sericite-type volcanic-hosted precious-metal deposits, as found in a detailed study of mineralization at Bodie, California (Silberman et al., 1972),

hydrothermal activity commonly begins at the culmination of volcanic activity and ,,, continues for over a million years (Heald et al., 1987). As at Bodie, mineralization

at the Timber Mountain-Oasis Valley caldera complex took place during the waning stages of volcanic and magmatic activity, but the detailed dating of various stage veins of a fossil hydrothermal system necessary to determine the duration of hydrothermal activity was not undertaken in this study. Given the average 1.25 m.y. life span suggested for hydrothermal systems (Silberman, 1983; Noble and

Silberman, 1984), it is possible that hydrothermal activity continued for longer periods of time after the _,10 Ma culmination of volcanic activity at the Timber Momatam-Oasis Valley caidera complex than indicated by K-At dates produced in this study.

Although there are notable exceptions (e.g., Round Mountain, NV (Shawe et al., 1986; Sanders, 1987) and Silverton, ID ('F_, 1975)), Heaid et al. (1987)

believe that a time gap of at least a million years between the age of the host rock and the age of mineralization is characteristic of adularia-sericite-type deposits. For some areas of alteration and mineralization at the T'tmber Mountain-Oasis Valley ealdera complex, e.g., the Transvaal Hills, Calico Hills and Telluride Mine area, a

time gap does exist between the age of the host rock (i.e., the older age limit shown by the angle bracket in Figure 4) and the age of hydrothermal activity. The time 3O gaps shown in Figure 4 are exaggerated because analytical uncertainty is not shown for the ages of the host rock_, and in many of the areas alteration is also found in younger, undated rocks. However, in no case was alteration and mineralization found to be over a million years younger than the age of the youngest host rock. lt should be noted that although alteration and mineralization in the Timber Mountain-Oasis Valley caldera complex is temporally related to volcanic and mag-

matic activity, the Ira-ge',,4' areal extent and significant lateral flow characteristic of adularia-sericite-t'ype hydrothermal systems (Heald et al., 1987) may result in alter- ation and mineralization of older host rocks not genetically related to the hydrothermal activity, creating an apparent time gap between related volcanic and hydrothermal events. In addition, young magmas, related to hydrothermal activity, may simply not reach the surface.

Possible relation of hydrothermal activity to detachment faulting

Widespread magmatic and hydrothermal activity between about 11.5 and 10.0 Ma, the Timber Mountain magrnato-thermal event, occurred during a period of regional crustal extension in the Basin and Range province, which began about 18 Ma (McKee, 1971; Noble, 1972; McKee and Noble, 1986). Although Maldonado (1985, 1988) and Cart and Monsen (1988) disagree about the age of faulting, they present evidence for late Miocene, west-northwest extension above two detachment faults that extend from a break-away zone on the northwest side of Bare Mountain westward to the Bullfrog Hills and Grapevine Mountains. The lower detachment fault is believed to be between metamorphosed Proterozoic rocks and overlying unmetamorphosed Paleozoic rocks; the upper detacament is thought to separate Paleozoic rocks from overlying Tertiary volcanic rocks (Can" and Monsen, 1988; Maldonado, 1988). Although Scott (1986; 1988) believes that related detachment faulting occurred beneath Yucca Mountain between 13 and 10 Ma, Carr (1988) 31 maintains that detachment faulting is not found east of Barc Mountain and suggests that the Timber Mountain-Oasis Valley caldera complex is located on a pull-apart at a major right-step of the Walker Lane Belt. Although the age and nature of extension and detachment faulting in the surrounding area is not yet completely understood, it is clear that the Timber Mountain=Oasis Valley caldera complex is located in an area of complex extensional tectonics.

Hydrothermal activity, from about 11.0 to 9.5 Ma and possibly continuing to about 8.7 Ma, in the proposed upper plate of the detachment system may provide evidence for the age of detachment faulting in the Bullfrog Hills. The occurrence of mineralization in theBullfrog Hills along moderately to steeply-dipping faults re- lated to extension in Tertiary volcanic strata above the upper detachment fault sug- gests that movement on the detachment may have begun prior to hydrothermal activity between about ll.Sand 9.5 Ma. Underground descriptions ofthe Original

Bullfrog and Happy Hooligan mines (Ransome, 1907; Ransome et al., 1910) suggest that precious metal values at these mines occurs along the proposed upper detach- merit fault in the Bullfrog Hills; if so, at least some movement along the detachment fault took piace prior to mineralization in these areas. In this case, if the younger age for alteration and mineralization at the Original Bullfrog mine is accurate, detachment faulting in the Butlf:rog Hills began before about 8.7 =+0.3 Ma. FIorian

Maldonado (pers. commun., 1988), however, believes that the upper detachment fault truncates mineralization at the Original Bullfrog mine; if so, and if the radio- metric age for hydrothermal alteration at the Bullfrog Mine is accurate, at least some detachment faulting took place after about 8.7 _+0.3 Ma. Further constraints on the age of detachment faulting in the Bullfrog Hills may result from additional work on the timing and structural control of hydrothermal alteration and mineral- ization. 32

Conclusions

Following voluminous caldera-forming eruptions at the Timber Mountain caldera, post-collapse volcanismcontinued for about 1 'millionyears. This post-col- lapse volcanic activity took place not onlywithin the Timber Mountain caldera, but also west and southeast of the caldera in the Beatty-BuUtrogHills and the Shoshone Mountain areas. Widespread hydrothermal activity took place in the Bullfrog-Hills and Oasis Mountain, Bare Mountain, Yucca Mountain, and Calico Hills areas dur- ingthis approximately 1 million year period of widespread, post-collapse magmatic activity,suggesting that hydrothermal activity and associated mineralization were genetically related to magmas of the Timber Mountain magmatic system. During development of the Timber Mountain magmatic system,magmasmay have intruded the crust over a large area, some coalescing to form a large magma body beneath the Timber Mountain-Oasis Valleycaldera complex and some erupting as lavas west and southeast of the caldera complex. Hydrothermal alteration and mineralization in the Bullfrog Hills seems to occur in pan along faults related to extension above the proposed, upper detachment fault; thus, some extension in the Bullfrog Hills may have taken place prior to the 11.0-9.5Ma period of intense hydrothermal activity in the Bullfrog Hills.

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Spengler, R.W., Byers, F.M., and Warner, .I.B., 198L Stratigraphy and structure of volcanic rocks in drill hole USW G-l, Yucca Mountain, Nye County, Nevada: U.S. Geological Survey Open- File Report 81.-1349,50 p.

Steven, T.A., Luedke. R.G., and Lipmam P.W., 1974, Relation of mineralization to calderas in the San Juan volcanic field, southwestern Colorado: U.S. Geological SurveyJournal of Research, v.2, p. 405.a09.

Steven, T.A., and Lipman, P.W., 1976, Calderas of the.San Juan volcanic field, southwestern Colorado: U.S. Geological Scrvey Professional Paper 958, 35 p.

Ti,giey, J.V., 1984, Trace element associations in mineral deposits, Bare Mountain (Fluorine) mining district, southern NYOCounty, Nevada: Nevada Bureau of Mines and Geology Report 39, 28 P.

Vaniman, D., Bi.sh, D., Broxton, D., Bye_ F., Carlt_ B., Levy, S., and Chlpera, S., 1988, Mineralogy- petrology and geochemical characteristics of the volcanic rocks at Yucca Mountain: Geological Society of America Abstracts _th Programs, v. 20, p. 240.

Warren, R.G., MeAowelL F.W., Byers, F.M., Brtmon, D.E., Can', W./., and Orkild, P.P., 1988, Episodic leaks _om Timber Mountain caldea-a:new evidence from rhyolite lavas of Fortymile Canyon, southwe,gern Nevada volcanic field: Geological Society of America Abstracts with Programs, v. 20, p. 241.

Weiss, S.I., Noble, D.C., and McKee, E.I-L, in pre_ Paleomagnetic constraints on the timing of eruption of the Pahute Mesa and Trial Ridge Members of the Thirsty Canyon Tttff: Journal of Geophysical Research. 29

Weiss, S.I., and Noble, D.C., in review, Stonewall Mountain volcanic center, southern Nevada: strar./graphic,su'uctural,and facies relations of outflow sheets, near-venttufts, and i_trac.a/dera units: Journal of Geophysical Research. 4O

APPENDIX 1

Average IUGS Constant Corrected Radiometric Ages of Volcanic Units of the SWNVF U ZT XaZ FZ NCZS (Ma) Stonewall Flat Tuff Cive_ Cat Canyon Member 6.3 Noble et al., 1988 Spearhead Member 6.3 Kistler, 1968; Noble et al., 1984 ; 1988

Thirsty Canyon Tuff Gold Flat Member 8.0 _+ 0.6 Marvin et al. , 1970 Rocket Wash Member 7.7 Kistler, 1968

Rhyolite of Obsidian Butte 8.8 _+ 0.3 This paper

Microgranite Ring Dikes 9.9 _+ 0.8 Marvin and Timber Mtn. Dome Dobson, 1979

Rhyolite of Burton Mtn. I0.i Marvin and Cole, 1978; Carr, 1984

Rhyolite of Shoshone Mtn. 10.3 ± 0.4 This paper

Tuff of Cutoff Road 10.4 _+ 0.3 This paper

Rhyolites of Timber I0.6, i0.7 Warren et Mtn. Caldera Moat al., 1988

Tuff of Buttonhook Wash i0.7 Kistler, 1968

Latite at Montgomery- 10.7 + 0.3 Morton et Shoshone Mine al°, 1977

Rhyolite of Pinnacles Ridge i1.3 Warren et al. , 1988

Timber Mountain Tuff Ammonia Tanks Member 11.3 Kistler, 1968

Rainier Mesa Member 1105 Marvin et al. , 1970

Pre-Rainier Rhyolite Lavas 11.5 Kistler, 1968

Rhyolite of Delirium Canyon 12.6 Warren et al. , 1988 41

U_IT aSZ _FZ_NCES (Ma) Paintbrush Tuff Tuff of Chocolate Mountain 12.9 Marvin et (Tiva Canyon Member) al., 1970

Tiva Canyon Member 12.8 Marvin et al. , 1970 ; Kistler, 1968

Topopah Spring Member 13.3 Marvin et al. , 1970

Wahmonie Formation Rhyodacite Lava 12.8 Kistler, 1968 Basal Ash Fall 13.2 Kistler, 1968

Rhyolite of Calico Hills 13.8 Kistler, 1968

Bare Mountain 13.9 Carr, 1986 Quartz Latite Dikes

Belted Range Tuff Grouse Canyon Member 14oi Marvin et al. , 1970 ; Noble et al. , 1968

Tuff of Crater Flat Bullfrog Member 14.3 _+ 0.4 Marvin et al. , 1970

Tuff of Redrock Valley 16oi _+ 0.6 Marvin et al. , 1970 42

APPENDIX 2

Description of K-Ax Dated Samples

I. Off,na/BullfroMineg K-At Adu/arized welded ash-flow tufts of the Lithic Ridge Tuff cut by veinlets con- taining quartz ,a_d adu/aria, Origina/Bullfrog mine (SW/4 SE/4 Sec. 12, T12S, R45E; 36 54.06 N, 116 53.1'W',, Bullf:ro, Hills 15--quad, N o. NV). A__a/data: K_O = 12.4 %,_0Ar = _.5497 x I0 "'u mole/g, _Arq/_0Ar = 49.5 %. Collecte6by: S.I. Weiss and M.R. Jackson, Univ. of Nevada-Reno. Ana/yzed by: E.H. McKee, U.S. Geol. Survey. Comment: The Lithic Ridge Tuff is older than the 14.3 z 0.4 Ma Bullfrog Member of the Crater Flat Tuff Marvia et ai., 1970; Byers et al., 1976a). Ransome (1907) and Ransome et ai. 1910) report that alteration and minerai_ation at the Origin a/Bullfrog mine was controlled by a low-angle fault zone; as proposed by Ma/donado (1985; 1988), this low-angle fault may be a detachment fault. Sample Mineralogy: quar_ 55%; adu/aria, 30%; a/bite, 11%; montmorillinoid (nontronite.?), 3%; FeOx (ox/di.zed pyrite), I%. (adularia) 8.7 _+0.3 Ma

2. Mayflower Mine K-At Adularized tuff, Mayflower mine (SE/4 NE/4 Sec.ll, T11S, R46E; 36 59.73'N, 116 47.3'W; BuLlfrog Hills 13' quad, Nye Co.,, N_._nalytical data: I¢_0 = 13.71%, _'A.r = 1.9745 x 10"_umoles/gram, "uAr/_Ar = 78.7 %. Collected by: D. Finn, Univ. of Nevada-Reno. Analyzed by: E.H. McKee, U.S. Geol. Survey. Comment: This sample was collected from the dump at the shaft of the Mayflower mine. The shaft at the Mayflower mine is collared in siiicified and aduiarized tuff of Buttonhook Wash Member of the Timber Mountain Tuff (Cornwall and KleinhampL 1964; Beck, 1984). Sample mineralogy: quartz, 81%; adu/aria, 18%; FeOx, 1%. (adularia) 10.0 ± 0.3 Ma

3. Northern Bullfrog Hills K-At Adularized tuff (W/2 NE/4 See.5, TllS, R47E; 370.80'N, 11645_53'W; ringdale Nev. 7.5',_tuad, Nye0_o.,,Ny). Analytical data: I_O = 11.19 %, = 1.7741 x 10"Lu-mole/& "JAr /'_Ar = 65.7 %. Collected by: D. Finn, Univ. of Nevada - Reno. Ana_,ed by: E.H. McKee, U.S. Geol. Survey. Comment:. This sample was collected from a dump at the north end of a N20F_,,7YWfault in uncorrelated, ash-flow tuff. Alteration along the fault is zoned from a quartz-, adularia- and sericite-bearing assemblage near the fault to kaolinite-bearing rock about 200' away from the fault. Sample Mineralogy: quartz, 75%; adu/aria, 21%; Rlire/Muscovite, 4%. (adularia) 11.0 ± 0.4 Ma

4. Oasis Mountain K-At Adu/arized Ammonia Tanks Member of the Timber Mountain Tuff, from Oasis Mountain (NW/4 NW/4 Sec.29, T10S, R47E; 37 2'36"N, 116 44'56"W; _ty Canyon SW 7_5' quad, Ny_,,Co,,._vr). Ana._t/ca/data: .K,20 = 5.32 %, _MJAA t..- _g 8.1468 x I0"£_. mole/g, _ AI/ _A..Ar = 43.4 %. Colleffted by: M.R. lackson, Univ. of Nevada-Reno. Analyzed by: E.H. McKee, U.S. GeoL Survey. Comment:. Adu/arized Ammonia Tanks Member of the Timber Mountain Tuff was collected along a north-trending fault, flanking the west side of the central valley in Oasis Mountain(Byers et aL,1976a). The Ammonia Tanks Member of the Ttmber Mountain Tuff has an IUGS constant corrected age of 11.3 Ma 42

(Kisfler, 1968). ArgilLization and silicification extend up into the overlying rulf.sof Fleur-de-Lis Ranch and tuff of Buttonhook Wash on the east side of the valley. Sample mineralogF: quartz, 67%; adularia, 26%; muscovite, 5%; FeOx, 2%. (aflularia) 10.6 _+0.3 Ma 5. TelluridMine e K-At Silicified and alunitized gravel, NE flank of Bare Mountain in the Telluride mine area (NW/4 NW/4Sec.18, T12S, R48E; 36 53.86'N, 116 39.5'W; Bare .o- 15'3.9508quad,x 10Nye"11 moleCo., /g,NVJUAr. /.UArA._'ca/ = 26.dat6 a:%. CK.aO311ec=tedby22:.5M.R.%, Jackson, Univ. of"Nevada-Reno. Analyzed by: E.H. McKee, U.S. Geol. Survey. Comment: Sample was collected fl'om the hang:ing wall of the northern range- bounding fault at Bare Mountain (Cornwall and Kleinhampl, 196I). The fault _ mineralized at depth, containing elevated concentrations of gold (D. arming, pers. commun., 1987). Altered hanging wall gravels are intercalated with ash-flow and air.fall ruffs, which have been correlated with the unnamed, bedded ruff beneath the 11.5 Ma Rainier Mesa Member of the Timber Mountain Tuff (Cart and Monsen, 1988; Orkfld, pers. commun., 1988). Sample mineralogy: quartz, 60%; alun/te, 30%; halloyslte, 10%. (alunite) 12.2 + 0.4 Ma 6. Telluride Mine K-At Alunite clasts in a matrix supported breccia, NE flank of Bare Mountain in ,the Telluride mine area (SW/4 NW/4 See.18, T12S, R48E; 3653.66N, 11629.5'.W; Bare Mtn. 15) quad, Ny: C .,,NV'). Analytical data: K,,O = 10.65 %, '_Ar = 1.7212 x 10"_°mole/g, _Arq/4°Ar = 71.3 %. Collectgd by: M.R. Jackson, Univ. of Nevada-Reno. Analyzed by: E.H. McKee, U.S. Geol. Survev. Comment: Clasts of pure alunite were collected from a pipe-like brec- cia within the Roberts Mountain Formation several hundred yards south of 3- MJ-118 and in the footwall of the range-bounding, normal fault on the north flank of Bare Mountain (Cornwall and Kleinhampl, 1961; Scott and Bonk, 1984). Sample mineralogy: alunite, 100%. (alunite) 11.2 t 0.3 Ma

7. Goldspar Mine K-At Adularized quartz latite dike from the Go!dspar mine (Diamond Queen mine), east flank of Bare Mountain (NE/4 NW/4 Sec.5, T13S, R48E; 36 50.41'N, 116 38,.3'W; Bare Mtn. 15',_tuad, Nye Co., ,_V_. Analytical data: K_O = 7.68 %, 4°At = 1.4345 x 10"w moles/gram, _°Ar/_Ar = 58.9 %. C61Iectedby: S.I. Weiss and M.R. Jackson, Univ. of Nevada-Reno. Analyzed by: E.H. McKee, U.S. Geol. Survey. Comment: Adula_r/zed dike was sampled in the north pit of the Goldspar mine where the dike cuts the dolornitic Nopah Formation (Cornwall and Kleinhampl, 1961; Papke, 1979). The quartz latite dikes on the east flank of Bare Mountain have been dated at 13.9 Ma (Can" et al., 1986). Veinlets of fluorspar invade the dike in the nort)-pit, and some Old is associated with the fluorite mineralization at the Goldspar mine ke, 1979; J. Mart, pers. commun., 1987). Sample mineralogy: quartz, ; adularia, 16%; kaolinite, 12%; biotite, 5%; calcite, 2%. (adularia) 12.9 ± 0.4 Ma 8. Silicon Mine K-Ar Alunitized Rainier Mesa Member of the T'tmber Mountain Tuff, Silicon mine (NW/4 NW/4 Sec.30, TllS, R48E; 36 57.51'N, 116 38.6'W; Bare Mtn. 15' 44

quad, Ny._C0.,.brV). Analytical data: K_O - 6.89 %, 40 Ar= 1.1560 x 10"10 mole/g, 4°Ar'/_Ar = 52.3 %. Collected'by: D.C. Noble, M.R. Jackson and D. Finn, Univ. of Nevada-Reno. Analyzed by: E.H. McKee, U.S. Geol. Survey. Comment: The Silicon mine is hosted by the 11.5 Ma Rainier Mesa Member of the Timber Mountain Tuff (Scott and Bonk, 1984; Can" et al., 1986). Sample Mineralogy: quartz, 65%; alunite, 35%. (alunite) 11.6 _+0.4 Ma 9. Thompson Mine K-At Vein containing fine grained alunite and halloysite, Thompson mine (NW/4 SE/4 See. 29, TllS, R48E; 38 56.94'N, 1163_^0'_/; Bare Mtn. 15' .quad, Nye _o., ,b_hr)._,Analytical data: K20 ---6.48 %, 'mAr = 1.20805 x 10"_Umole/g, qOAr /qoAr = 19.6 %. Collected by: D.C. Noble, M.R. Jackson and D. Finn, Univ. of Nevada-Reno. Analyzed by: E.H. McKee, U.S. Geol. Survey. Comment: The Thompson mine is hosted bythe Yucca Mountain Member of the Paintbrush Tuff (Scott and Bonk, 1984; M.R. Jackson and S.I. Weiss, unpub, mapping, 1987). The Yucca Mountain Member is bracketed between average rUGS constant corrected ages of 13.2 Ma for the Toi?opah Spring Member and 12.8 Ma for the Tiva Canyon Member of the Paintbrush Tuff (Kistler, 1968; Marvin et al., 1970). The Tiva Canyon Member is altered west of the Thompson mine. Sample mineralogy: alunite, 85%; halloysite, 15%. (alunite) 12.9 _+0.5 Ma 10. Transvaal Hills K-Ar Veins containing alunite cutting the Rainier Mesa Member of the Timber Mountain Tuff, TransvaaJ Hills (SW/4 SE/4 See.35, T10S, R48E; 37 1'4"N, 116 34'31"W; ThArst_ Canyon SE 7._' quad, Nye _o,0¢ NV). Analytical data: .K,20 = 9.05 %, _Ar = 1.2940 x 10"_umole/g, UAr / Al"= 37.0 %. Collected b)/: S.I. Weiss and M.R. Jack.sort, Univ. of Nevada-Reno. Analyzed by: E.H. McKee, U.S. Geol. Survey. Comment: Separate consisted of coarsely crystalline (1 mm diameter), unusually clear alunite fracture fillings in bleached vitrophyre of the 11.5 Ma Ra_n/er Mesa Member of the Timber Mountain Tuff (Byers et al., 1976b). Sample mineralogy: quartz, 60%; alunite, 35%; kaolinite, 5%. (alunite) 9.9 +_0.4 Ma 11. Transvaal Hills K.Ar Adularized Debris Flow (?) - Rainier Mesa Member (?) of the Timber Mountain Tuff, Transvaal Hills. (NE/4 NW/4 See& TllS, R48E; 37 00'53"N, 116 34'51"W; Tl_rstX Canyon SE 7&' quad, N3/e (_o,, NV). Analytical data: .K20 = 6.27 %, '_Ar = 1.1409 x 10"_°mole/g, mAr/,mAr = 63.7 %. Collected b);:D.C. Noble, S.I. Weiss and M.R. Jackson, Univ. of Nevada-Reno. Analyzed by: E.H. McKee, U.S. Geol_ Survey. Comment" Debris flow intertongues with and overlies the 11.5 Ma Rainier Mesa Member of the Timber Mountain Tuff in the Transvaal Hills (Byers et al., 1976a; 1976b). The Ammonia Tanks Member of the Timber Mountain Tuff (average IUGS constant corrected K- Ar age -- 11.3 Ma) and the tuff of Cutoff Road (new K-Ar age of 10.4 Ma) are also altered in the Transvaal Hills. Sample Mineralogy: quartz, 75%; adularia, 18%; biotite, 4%; illite/muscovite, 3%. (adularia) 12.2 :!:0.3 Ma 12. Calico Hills K-At Alunitized rhyolite of Calico Hills, western Calico Hills (SE/4 NE/4 Sec. 23, T12S, R50E; 36 52.78'N, 116 21.47'W; Topopah Spring 7.5' quad, Nye Co., 45

NV). Analytical data: K_O = 11.2 %, _Ar' = 1.67856 x 10"10 mole/g, /UAr = 73.9 %. Cfllected by: S°I. Weiss and M.R. Jackson, Univ. of Nevada-Reno. Analyzed by: E.H. McKee, U.S. Geol. Survey. Comment: Th2s sample, containing fine grained alunite, was collected from a unit of altered tuff m the rhyolite of Calico Hills (Orldld and O'Connor, 1970). The rhyolite of Calico Hills has an lUGS constant corrected age of 13.8 Ma (Kisder, 1968; Byers et al., 1976a). Sample Mineralogy: alunite, 50%; cristobalite, 50%. (alunite) 10.4 e 0.3 Ma 13. Calico Hills K-Ar Vein containing fine grained alunite and quartz, eastern Calico Hills (SE/4 SW/4 See. 12, T12S, R51E; 36 54.20'N, 116 14.28'W&Mine Mountain 7.5' quad, Ny_^Co_,.NV). Analytical data: K2O = 11.02 %, '_Ar - 1.64906 x 10.)`0 mole/g, _Ar/_°Ar = 87.3 %. Collected by: S.I. Weiss and M.R. Jackson, Univ. of Nevada-Reno. Analyzed by: E.H. McKee, U.S. Geol. Survey. Comment: Collected from a vein cutting the 11.5 Ma Rainier Mesa Member of the Timber Mountain Tuff (Marvin et al., 1970). The 11.3 Ma Ammonia Tanks Member of the Timber Mountain Tuff (Kistler, 1968) is also altered in the eastern portion of the Calico Hills (Orldld, 1968; Byers et al., 1976a). Sample Mineralogy: alunite, 95%; quartz, 5%. (alunite) 10.4 ± 0.3 Ma 14. Hornsilver Mine at Wahmonie K-Ar Adularized intermediate lava of the Wahmonie Formation, Hornsilver mine (NE/4 SE/4 Sec. 18, T13S, R52E; 36 49.21'N, 116 10.0WWL,Sk_IIMountain 7.5',,quad, Nye _o., ,NV_. Analytical data: KoO = 12.96 %, UAr ---2.3657 x 10"LUmole/g, 4°At /_Ar = 59.5 %. Collected by: S.I. Weiss and M.R. Jackson, Univ. of Nevada-Reno. Analyzed by: E.H. McKee, U.S. Geol. Survey. Comment: The basal air-fall and upper portion of the Walamonie Formation have 1-UGS constant corrected ages of 13.2 Ma and 12.8 Ma, respectively (Ekren and Sargent, 1965; Kistler, 1968). Sample Mineralogy: quartz, 47%; adularia, 46%; illite, 5%; FeOx (oxidized pyrite) 1.5%; pyrite, 0.5%. (adularia) 12.6 _.+0.4 Ma 15. Hornsilver Mine at Wahmonie K-Ar Adularized intermediate lava of the Wah.mortie Formation cut by veins of quartz and adularia, Hornsilver mine (NE/4 SE/4 See. 18, T13S, R52E; 36 49.25'N, 116 10.03'W,;S_tll Mountain 7.5_,,quad, Nye,,,Co.a Nif',). Analytical data: KzO = 14.0 %, '_Ar - 2.601"77x 10"_° mole/g, "As/"UAr = 66.5 %. Collecte?dby: SI. Weiss and M.R. Jackson, Univ. of Nevada-Reno. Analyzed by: E.H. McKee, U.S. Geol. Survey,. Comment: The basal air-fall and upper portion of the Walamonie Formauon have lUGS constant corrected ages of 13.2 Ma and 12.8 Ma, respectively (Ekren and Sargent, 1965; Kistler, 1968). Sample M/neralo_: , quartz, 70%; adularia, 26%; illite, 2%; calcite, 1%; FeOx _,oxidizedpyrite), 1%. (adnlaria) 12.9 ± 0.4 Ma 16. Tuff of Cutoff Road K-At Tuff of Cutoff Road, Transvaal Hills (NW/4 Se/4 S¢,e.3, TllS, R48E; 37 00'21"N, 11635'37"W; Thirstx C/rayon SE 7.5',,quad, Ny,_ C,o._,.NV). Analytical data: KeO = 10.4 %, "°Ar - 1.5579 x 10"iUmole/g, '_Ar /"At = 88.2 %. Collected-by: D.C. Noble, S.I. Weiss and M.R. Jackson, Univ. of Nevada-Reno. Analyzed by: E.H. McKee, U.S. Geol. Survey. Comment: An average IUGS eomtant corrected age of 9.8 Ma was calculated from Kistler's 46

K-At data for the tuff of Cutoff Road. The tuff of Cutoff Road is altered Io- caJJyin the Tran,waal Hills (Byers et al., 1976a; 1976b). (sanidlne) 10.4 ± 0.3 Ma 17. RhyoLiteof Shoshone Mountain K-At Upper flow unit of the rhyolite of Shoshone Mountain (Tsrd), Shoshone MOuntain (SW/4 NE/4 Sea 22, TllS, R45E; 3656.98'N, 116 16.70'W; opah Spring 7.5' ql:ad, Nye C9,o _ Analyn'cal data: K,oO = 5.71%, • = 8.47695 x 10"_ mole/g, '_Ar-/"UAr = 50.0 %. CoU'&'tedby: D.C. Noble and S.I. Weiss, Univ. of Nevada-Reno. Amzlyzed by: E.H. McKee, U.S. Geol. Survey. Comment: Unaltered rhyolite of Shoshone Mountain overlies altered tufts in the eastern Calico Hills (Orkild and O'Connor, 1970). (sanidine) 10.3 + 0.3 Ma 18. Rhyolite of Obsidian Butte K-At RhyoLiteof Obsidian Butte, west of Tolicha Peak (SW/4 NE/4 Sec. 22, TllS, R45E; 37 18.42'N, 116 50.90'W; To,licha Peak 15', uad, Ny,' Co.,, NV). Analytical data: K,:O = 5.14 °b, 40At = 6.4927 x 10"_tmole/g,_Ar'/_Ar = 71.9 %. Collectec7 by: D.C. Noble. Analyzed by: E.H. McKee, U.S. Geol. Survey. Comment: This local rhyolite lava was erupted west of ToLicha Peal northwest of the Timber Mountain-Oasis VaUey caldera complex. (sanidine) 8.8 + 0.3 Ma