St119 ~~~~~~~~~~~~~~~st70Ko// 9>X/e,'n /idPf,O/) }~~~~~~~ - I-- XlenAa Geology of Bullfrog Quadrangle and Ore Deposits Related to Bullfrog Hills Caldera, Nye County, Nevada and Inyo County, California By HENRY R. CORNWALL and FRANK J. KLEINHAMPL SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY GEOLOGICAL SURVEY PROFESSIONAL PAPER 454-J Prepared in cooperation with- the Nevada Bureau of Mines' UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1964
Cenozoic rocks and volcanism. .... ......Monolithologic breccia.....Titus Canyon Formation of Stock and Bode (1935).Bullfrog Hills caldera and associated rocks .. _._
Description of the volcanic rocks.Ash flowns.----Ash fio ------------------------~~--Air-fall tuffs ....Rhyolite flows and Intrusives ...Basalt flows and intrusives .Latite flows and intrusiveJ .......
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Cenozoic rocks and voleanism-ContinuedBullfrog Hills caldera and associated rocks-Con.
Chemical composition of the rocks.Older gravels.Recent alluvium .
Structure.Tectonic deformation.
Relation to Walker Lane and Las Vegas Valleyshear zone ......
PLArz 1. Geologic map and sections of the Bullfrog quadrangle, Nevada-California.2. Columnar section of Paleozoic rocks on Bare Mountain.3. Columnar section of Titus Canyon Formation of Stock and Bode (1935), 1 mile southeast of Daylight Pass,
Funeral Mountains.4. Generalized geologic map of the Bullfrog Hils and Yucca Mountain ealderas.5. Columnar section of rocks in the Bullfrog Hills ealdera near Beatty.6. Mine workings of the Daisy duorspar mine.7. Geologic map of levels 3, 5, 6, and 8, Daisy fluorspar mine.S. Geologic map of levels 9 to 13, Daisy luorspar mine.9. Longitudinal and crow sections of the Daisy fluorspar mine.
Fiauzs 1. View of Bonanza King and Nopah Formations in Carrara Canyon, Bare Mountain.2. View of Eureka Quartzite, Ely Springs Dolomite, Roberts Mountains Formation, and Lone Mountain Dolomite
in Chuckwalla Canyon, Bare Mountain- ...... ... -3. Rolling hills underlain by Daylight Formation, with small overlying remnant of monolithologic breccia, Grape-
vine Mountains .-------------------------------4. View of the Titus Canyon Formation of Stock and Bode (1935) near Daylight Pass, Funeral Mountains .5. Devitrification zone of ash flow (welded tuff) Fluorspar Canyona.. . . .
9. Rhyolltic vitrophyre breccia at base of flow, upper part of Bullfrog Hills volcanic sequence ...--10. Variation diagrams of oxides versus differentiation Index of volcanic rocks In the Beatty area .-_- _-_.l1. Map of major tectonic features of southwestern Nevada ..... .-------12. Geologic plan of the Vanderbilt bentonite mine ....... :. -------------------
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SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY
GEOLOGY OF BULLFROG QUADRANGLE AND ORE DEPOSITS RELATED TO BULLFROGHILLS CALDERA, NYE COUNTY, NEVADA, AND INYO COUNTY, CALIFORNIA
By Hait R. ComvrwA and FRANK J. KLRIXHAxPL
ABSTRACT
The Bullfrog quadrangle In southern Ny County. Nev.,and eastern Inyo County. Calif., east of Death Valley, isunderlain by Tertiary volcanic rocks, mainly In the northernpart. and by Paleozole sedimentary rocks, mainly In thesouthwestern part.
The volcanic rocks are believed to have been derived froma caldera, measuring 10 by 13 mites, that covers The northernhalf of the quadrangle Rhyolitle ash flows (welded tuffs)and air-fall tuffs predominate In the volcanic sequence, whichalso Includes extrusions and Intrusions of rhyollte, latite, ba-salt, and basanite. The volume of ash flows (83 cubic miles)Is roughly equal to the estinated subsidence in the caldera,and It is probable that the violent eruption of these rocksresulted to the subsidence of the caldera because the erup-tion removed magma from an underlying chamber. Agrabenlike structure extends northeaft tangentially from thesouth rim of the caldera and may also have resulted fromsubjacent removal of magma during the volcanic activity.
The older rocks range from Precambrian through Missis-sipplan; the section is fragmentary because of poor exposuresand intense deformation. These rocks have been folded andIntensely faulted, mainly by bedding-plane thrusts that movednortheast or southwest This deformation to considered to berelated to the development of the Las Vegs Valley shearzone, which probably posses diagonally across the center ofthe quadrangle from southeast to northwest. Monolithologlebreccia of dolomite and limestone In the southwestern partof the quadrangle probably formed by landslide of the frontof the main thrust Tertiary to Recent basin-range faultshave moderately disrupted the rocks.
Ore mineralization appears to be concentrated near themargins of the subsidence structuree The three known mostpromising gold-silver deposits are located on the east rim ofthe Bullfrog Hills caldera. A substanal fluorspar depositand a small mercury deposit ar just south of the grabenlikestructure extending east from the alders acrom the northend of Bare 'Mountain. A smll bentonite deposit is also justsouth of this struture near the calder The ore depositsare In fracture zones, which probably served an channels foremanations from underlying magma bodiea
IETRODUCTION
The Bullfrog quadrangle is in southern Nye County,Nev., and eastern Inyo County, Calif. (fig. 11). TheBullfrog Hills are in the northern half of the quad-rangle. The Grapevine and Funeral Mountains, sepa-rated by Boundary Canyon, occupy the southwesternquarter; the town of Beatty is at the eastern edge.The geology of this quadrangle was mapped in co-
operation with the Nevada Bureau of Mines duringthe period 1958-81. The Bare Mountain quadrangle,which adjoins the Bullfrog on the east, was alsomapped by the present authors (Cornwall and Klein-hampl, 1960a, b; 1961b).
Published references to the geology of the Bullfrogquadrangle include an areal reconnaissance by Ball(1907), a detailed study of the geology and ore depositsof the Bullfrog district by Ransoms and others (1910),and a description of mammal-bearing sedimentarybeds of early Oligocene age by Stock and Bode (1935).
Members of the U.S. Geological Survey identifiedfossils collected by the authors during fieldwork.Cambrian faunas were identified by Helen Duncan,C. A. Nelson, and A. R. Palmer; Ordovician, by R. J.Ross, Jr.; Oligocene, by D. W. Taylor.
PR.CAMBRAN ROCKS
Rocks of older Precambrian age crop out in a smallarea near the center of the quadrangle (pl. 1) in sec 13,T. 12 S., R. 45 E. Quartz-muscovite-feldspar gneissand quartz-muscovite-biotite schist have been intrudedby irregular bodies of gneissic pegmatitic granite. Thegneiss is light gray and is composed mainly of quartzand feldspar, whereas the schist is nearly black andcontains abundant biotite and muscovite with lesseramounts of quartz and a little feldspar. The graniteis nearly white, very coarse grained, and consistsmainly of quartz and feldspar with minor muscovite.
PALEOZOIC ROCKS
Cambrian rocks crop out in the southwestern partof the Bullfrog quadrangle in the Grapevine and Fu-neral Mountains (pl. 1). Smaller exposures of theseand other Paleozoic rocks occur near the middle,northeast comer, and east-central margin of the quad-rangle. Most or all of the Paleozoic rocks identifiedon Bare Mountain (Cornwall and Kleinhampl,1961b), except for the Fluorspar Canyon Formationof Devonian age, have been found in the Bullfrogquadrangle, but due to the limited exposures and com-plex deformation, the section revealed here is frag-mentary and far from complete. The section that was
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n2 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY
measured on Bare Mountain (Cornwall and Klein-hampl, 1961b) is shown on plate 2. Detailed de-scriptions of the Paleozoic rocks will not be givenhere except where they differ from the section alreadydescribed on Bare Mountain (Cornwall and Klein-hampl, 1961b).
DAYLTGHT FORIh.TION
The Daylight Formation crops out over a wide areain the southwestern part of the Bullfrog quadrangle,and also near the east-central and northeast margins.The formation is here named for the extensive expo-sures of it in the southwestern part, where DaylightPass is located (sec. 36, T. 13 S., R. 46 E.). A typi-cal section of the Daylight Formation measured onBare Mountain is shown on plate 2. This formationwas first studied on Bare Mountain and was originallyconsidered to be a part of the Johnnie( 1) Formation(Cornwall and Kleinhampl, 1960b, 1961a, b); butmore recent investigations by the present writers andC. A. Nelson, A. R. Palmer, and J. H Stewart indi-cate that a correlation with the Wood Canyon For-mation is more probable. This problem will be dis-cussed below.
The Daylight Formation consists predominantly ofelastic rocks. Micaceous shale and siltstone are mostabundant, but interbedded quartzitic sandstone bedsare common. Limestone and dolomite beds as thickas 50 feet occur at intervals in the formation, particu-larly in the upper and lower parts, and an arkosicconglomeratic quartzite as thick as 300 feet occursnear the middle of the formation.
The shale and siltstone range in color from olivegray to various shades of green and brown. Theserocks are commonly schistose with wavy parallel,laminated to very thin bedded stratification, and platysplitting. Cleavage may be parallel to the stratifica-tion, but quite commonly it is not. The quartziticsandstone and conglomerate range in color from al-most white to gray and brown and have differingdegrees of purity and induration. Stratification isboth parallel and cross stratified, laminated to thinbedded, with platy to slabby splitting. As a groupthese rocks are commonly fine grained and poorlysorted with moderately well rounded quartz grains anda silty matrix of quartz, sericite, biotite, calcite, anddolomite.
Limestone and dolomite beds in the Daylight For-mation are light to moderate brown and grayish yellowor orange. The beds commonly have a elastic textureand may grade laterally or vertically into quartzosesandstone. In the upper part of the Daylight Forma-tion are one to three closely spaced beds of yellow to
brown, oolitic to pisolitic limestone and dolomite. Thecoarser grained oolitic and pisolitic beds commonlycontain abundant archaeocyathids and pelmatozoandebris. The oolitic limestone or dolomite is a per-sistent marker zone; it occurs about 800 feet below thetop of the Daylight Formation in the unfaulted BareMountain section, but in the Bullfrog quadrangle theoverlying Corkscrew Quartzite (see below) is faultedover the Daylight Formation and most of or all theshale and siltstone above the oolite may be missing(pl. 1). The oolite is distinguished by a separate pat-tern on plate 1.
Another unit in the Daylight Formation that is dis-tinguished on plate 1 is the light-gray conglomeraticarkosic quartzite, roughly 800 feet thick, which occursnear the middle of the formation (pl. 2). This is apersistent unit and it, like the oolitic limestone higherup in the section, has served as a reliable marker inworking out the structural complexities of the area.
In the upper 500 feet of the Daylight Formation arequartzitic sandstone beds interbedded with siltstone,and some of the sandstone beds contain vertical rods,perpendicular to the bedding, that have been identifiedas ScouithAu.
AhG AND CORM<ION
At Daylight Pass the oolitic limestone shown on themap has yellowish-gray to green shale units as thick as30 feet above and below it, and these contain the olenel-lid trilobites Ncvadia and NevadeUa, which are indic-ative of Early Cambrian age (C. A. Nelson and A. R.Palmer, written communications, 1959, 1960). Similarfomsil-bearing shales border the oolite in the exposureswest of Daylight Pass at least as far as the edge ofthe quadrangle, but southeastward the shale pinchesout within a mile and the oolite is bordered by nonfos-siliferous siltstone. At Bare Mountain the beds bor-dering the oolite are siltstone and nearly barren offossils, although C. W. Merriam (oral communication,1960) has found a few fragments tentatively identifiedas olenellid trilobites.
The colitic limestone contains, as was mentionedabove, sparse to abundant archaeocyathids and pelma-tozoan debris. A few of the larger fragments wereidentified by Helen Duncan (written communication,1959) as Etfmophylum sp., and some of the smallerdebris was probably derived from such archaeocya-thids as Archeocyatkuu or Protopharetra. Cystidplates were identified in the pelmatozoan debris. Somesmall algal masses were also seen.
This sequence of elastic rocks, here designated theDaylight Formation, was originally correlated by thepresent writers (Cornwall and Kleinhampl, 1960b,1961a, b) with the Johnnie Formation because of its
BULLFROG QUADRANGLE AND BULLFROG CALDERA, NEV.-CALIF. J.3
close lithologic resemblance to that formation in thetype locality in the northern Spring Mountains (Nolan1929, p. 461-43; B. C. Burchfiel L) and elsewhere. TheDaylight Formation is also similar lithologically tothe upper part of the Wood Canyon Formation (Nolan,1929, p. 463-404), but the middle and lower parts of theWood Canyon Formation tend to be coarser grainedthan comparable parts of the Daylight Formation. Ac-cording to Nelson and Palmer (written communica-tion, 1981), Nevadia, which occurs in the upper part ofthe Daylight Formation, has also been found in theupper part of the Wood Canyon Formation in theNopah Range, 75 miles southeast of the Bullfrog quad-rangle; in the Desert Range, 80 miles to the east; andin the Spring Mountains, 50 miles to the southeast.
Oolitic limestone and dolomite beds in the upperpart of the Wood Canyon Formation (A. R. Palmer,written communication, 1961) contain archaeocyathidsand pelmatozoan debris as do similar beds in the Day-light Formation. Oolite beds in the upper part of theJohnnie Formation, on the other hand, do not containsuch organic debris More work will have to be doneto determine whether the Daylight Formation is cor-relative with the Johnnie or Wood Canyon Formation,but present indications favor a correlation with theWood Canyon.
COKSCREW QV.&UTZ=
The Corkscrew Quartzite forms the major unit in athrust plate that has overridden the Daylight Forma-tion in the southwestern part of the Bullfrog quad-rangle at the south end of the Grapevine Mountains.Corkscrew Quartzite is also thrust over the DaylightFormation in the northeastern corner of the quad-rangle, and a small exposure occurs near the middleof the quadrangle on the west edge of Bullfrog Moun-tain.
The Corkscrew Quartzite is here named for expo-sures on the east flank of Corkscrew Peak (se. 5 and8, T. 14 S., I. 48 E.) in the southwestern corner ofthe Bullfrog quadrangle. About 1,200 feet of thequartzite are exposed there (sec. A-A', pL 1). OnBare Mountain an unfaulted section of the quartzitehas a measured thickness of 1,140 feet (pl. 2). TheCorkscrew Quartzite is a vitreous, pure quartzite thatweathers to gray, pink, and reddish or purplish brown.It is fine to medium grained, and locally conglomer-atic, containing quartz and sparse red jasper pebbles.The rock is poorly to moderately well sorted, thinlylaminated to thin bedded, and exhibits a slabby to
'Burftel I. C.. 1961. Strut. ad fldrfoapb7 of the fteemRa&Dp qtuadat , N4 County, Nevada: New Haim Com.. YTaV Ul-veralt7. Pi D. theds, P. 23-30.
massive splitting. Crossbedding is conspicuous in thelower part; stratification in the upper part tends to beobscure. Adjacent to the thrust fault that underliesthe Corkscrew Quartzite in the Bullfrog quadrangle,the quartzite is brecciated; this zone, as much as 30feet thick, is commonly nearly white, in contrast tothe typical pink or reddish color of the normal rock,probably due to bleaching by aqueous solutions thatmoved along the permeable zone. A similar zone ofbrecciation and bleaching of the basal part of the Stirl-ing Quartzite above the Johnnie thrust in the north-ern Spring Mountains has been described by Nolan(1929, p. 46667).
AGs A" COmRasATION
No fossils have been found in the Corkscrew Quartz-ite in the Bare Mountain or Bullfrog quadrangles, buttrilobites indicative of an Early Cambrian age (C. A.Nelson and A. R. Palmer, written communications,1959, 1960; Cornwall and Kleinhampl, this paperand 1961b) have been found in the underlying Day-light and overlying Carrara Formations. Thus, thequartzite itself must be Lower Cambrian.
The Corkscrew Quartzite in the Bare Mountain andBullfrog quadrangles was earlier correlated tenta-tively with the Stirling Quartzite (named by Nolan,1929, p. 463) by the present writers (Cornwall andKleinhampl, 1960b, 1981a, b). This correlation wasbased on the fact that it, like the type Stirling, is athick quartzite overlying what was then considered tobe typical Johnnie Formation. The character of theCorkscrew Quartzite is also rather similar to at leastthe upper part of the Stirling Quartzite. Much of thelower part of the Stirling Quartzite, however, iscoarser grained and more conglomeratic than theCorkscrew Quartzite
As was discussed above, the Daylight Formationunderlying the Corkscrew Quartzite is now consideredby the present writers as probably correlative with theWood Canyon Formation (Nolan, 1929, p. 483-484).The Wood Canyon Formation, as designated by Nolanin the type area in the Spring Mountains, has a quartz-ite approximately 20 feet thick overlying it. In theNopah and Resting Springs ranges Hazzard (1937, p.309) included a correlative quartzite, which is 160 feetthick there, in the upper part of the Wood CanyonFormation and named it the Zabriskie quartzite mem-ber. If, as the present writers are now inclined to be-lieve, the Daylight Formation is indeed equivalent tothe Wood Canyon Formation as defined by Nolan, thenthe Corkscrew Quartzite may be a considerablythickened section of the Zabriskie quartzite memberof Hazzard. This correlation was first suggested byA. R. Palmer (written communication, 1959). The
J4 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY
Corkscrew Quartzite is 1,000 to 1,200 feet thick, where-as the maximum thickness; of* definitely knownZabriskie is less than 200 feet except at Eagle Moun-tain, 50 miles southeast of the Bullfrog quadrangle,where a thickness of 420 feet was measured (J. H.Stewart, oral communication, 1961).
The two quartzites are very similar lithologically;they are pure, vitreous, generally fine to mediumgrained, but locally conglomeratic. Another strikingsimilarity is that both have prominent Scolithmw eitherin or near the base. Scolithum occurs in the basal partof the Zabriskie, but it occurs below the main body ofthe Corkscrew in quartzitic beds in the upper part ofthe Daylight Formation.
CA.LBRAA FORXA&T02T
The Carrara Formation crops out only in the south-west and northeast corners of the Bullfrog quadrangle,where it conformably overlies the Corkscrew Quartz-ite. The upper contact is not exposed and complexfaulting precludes measurement of a reliable section.The rocks are, for the most part, similar to the forma-tion at the type locality on Bare Mountain (pl. 2;Cornwall and Kleinhampl, 1961b).
The formation consists of interstratified shale andlimestone with minor amounts of sandstone and silt-stone. Shale predominates in the lower part of theformation and limestone in the upper part. The totalthickness measured on Bare Mountain is 1,785 feet,and most of the major units occurring there are pres-ent in the two areas of outcrop in the Bullfrog quad-rangle. The base of the formation is an abrupttransition from the underlying Corkscrew Quartzite;it consists of alternating beds of sandstone, siltstone,shale, and limestone. Above this are several hundredfeet of greenish-, yellowish-, to brownish-gray shalewith interbeds of gray and orange limestone, at thetop of which is a massive, hif-forming, dark-grayalgal (Girvanella) limestone bed, commonly over 100feet thick. Above the limestone are approximately400 feet of gray to brown shale and siltstone, and theseare overlain by 500 to 1,000 feet of brightly colored,alternating white, pink, orange, and brown limestone.The colored limestones are somewhat clayey or silty.
The age of the lower part of the Carrara Formationhas been well established in the Bare Mountain quad-rangle. According to Palmer (written communica-tions, 1959, 1902) and Nelson (written communication,1960), the trilobites Brblolia and Paedumia werefound below the massive limestone containing Girva-nella; and Fremontia, OleneUt&, and Paedunim werefound above it. Fremontia and Bristolia (Nelson,written communication, 1960) were also found in a
small outcrop of the Carrara Formation in the south-western part of the Bullfrog quadrangle. These fos-sils indicate a late Early Cambrian age for the lowerpart of the formation. the upper part of the forma-tion is Early or Middle Cambrian. The Carrara For-mation correlates in faunal and general lithologiccharacter with the sequence of Latham Shale, Chamb-less Limestone, and Cadiz Formation in the Provi-dence Mountains described by Hazzard (1954, p. 30-32), and in the Nevada Test Site (according to therevised nomenclature of Barnes and Palmer, 1961).The Carrara Formation is also approximately equiva-lent to the Bright Angel Shale as defined by Nolan(1929, p. 464) in the northern Spring Mountains, Nev.James McAllister (written communication, 1961) hasextended the Carrara Formation southward into anarea in the Resting Spring Range.
NOPO FOEXATION
At Bare Mountain the Nopah Formation of LateCambrian age overlies the Bonanza King Formationand has a thickness of about 1,900 feet (pl. 2 and fig. 1;Cornwall and Kleinhampl, 1961b) and consists pre-dominantly of gray dolomite but also has a 100-footbasal shale member and locally a dark-gray limestonemember that overlies the shale. In the Bullfrogquadrangle the Bonanza King Formation is absent andthe Nopah Formation crops out only in one smallarea, 1,000 feet across, 2 miles northwest of BullfrogMountain. The rock is entirely dolomite, which isbroadly banded light and dark gray. The dolomite
Frnac 1.-Bonanza King (Cbk) and Nopah (Ca and ns. the latter toshale member) Tormations. view eastward. Canara Canyon. BareMountain. Nev.; sequence is eontinuous along skyline but faultedtoward observer.
BULLFROG QUADRANGLE AND BULLFROG CALDERA, NEV.-CALIF.
contains some chert in nodules and thin lenses andalso has zones in which the dolomite has partly re-crystallized as coarse-grained, white to yellowish-grayflecks, stringers, and irregular masses.
POGONIP GO3UP
The Pogonip Group has a measured thickness onBare Mountain of 1,375 feet (p1. 2; Cornwall andKleinhampl, 1961b) and consists of silty and chertylimestone with interbeds of shale and pure limestone.Diagnostic fossils, including the spongelike form Re-ceptaculites, the brachiopod Orthidiela, and the gas-tropods Paiieria and Macluritee, indicate Early andMiddle Ordovician age. In the Bullfrog quadrangle,small outcrops exposing partial sections, mostly of theupper part of the Pogonip Group, occur south, south-east, and northwest of Bullfrog Mountain (p1. 1). Therock is aphanitic to medium grained, gray, yellow, andbrown silty and shaly limestone. OrthidieUa, Pal-liseria, and Mactwitea (R. J. Ross, written communi-cation, 1960) were found in the limestone. A promi-nent white limestone bioherm (Ross and Cornwall,1961) was found in the Pogonip Group at Bare Moun-tain, but none were seen in the Bullfrog quadrangle.
RU1rZA. QUARTZ=TX
A section of the Eureka Quartzite measured at thenorth end of Bare Mountain on Meiklejohn Peak (pls.1, 2; fig. 2) was found to be 880 feet thick, and a com-plete section that crops out in a small area 1 milenorthwest of Bullfrog Mountain in the Bullfrog quad-rangle has approximately the same thickness. The.quartzite has thin units of sandstone at top and bot-tom, but it is chiefly a vitreous, fine-grained quartzitewith well-rounded and well-sorted grains. Beddingtends to be indistinct, very thin to thick bedded; thecolor is white to grayish orange or brown No fossilsoccur in the Eureka Quartzite in this area but it isconsidered to be Middle Ordovician on the basis ofdiagnostic fauna found in the underlying and over-lying formations here and elsewhere. Figure 2 showsthe Eureka Quartzite and overlying formations asthey occur on Bare Mountain.
ELY SpataG DOLOM"
The Ely Springs Dolomite overlies the EurekaQuartzite; on Bare Mountain its thickness is 300 feet(pi. 2; fig. 2). In the Bullfrog quadrangle it crops out1 mile northwest of Bullfrog Mountain in the samelocality as the Eureka Quartzite, described above, andthe thickness there is about 200 feet. It is a dark-grayaphanitic to very fine grained laminated to thin-bedded cherty dolomite. The chert, as anastamosing
lenses, composes 10 percent of the formation and islocally even more abundant. Diagnostic fossils werenot found in the Ely Springs Dolomite in the Bull-frog quadrangle but paleontologic evidence elsewhereindicates a Late Ordovician age.
RoMTS NOUNTAMNS YOUMATION
A complete section of the Roberts Mountains For-mation, 800 to 900 feet thick, crops out in the smallarea 1 mile northwest of Bullfrog Mountain and is verysimilar in general lithology to exposures of the sameformation on Bare Mountain (pl. 2; fig. 2; Cornwalland Kleinhampl, 1901b). The formation consists oflaminated to thin-bedded dolomite and limestone.The lower 200 feet of the formation is dark-gray chertydolomite; above this is approximately 400 feet of dark-gray platy limestone; and at the top is roughly 200 feetof dark-gray dolomite, which grades upward into light-gray dolomite. Pentameroid brachiopods and corals,collected from the upper dolomite, are similar tofauna from the type locality (Merriam and Anderson,1942, p. 1887), which date the formation as MiddleSilurian.
LON1 NOUMTAI DOLOIOTE
Overlying the Roberts Mountains Formation in thearea of Paleozoic rocks 1 mile northwest of BullfrogMountain is a partial section of the Lone MountainDolomite. The dolomite is homogeneous, light gray,pitted, fine to medium grained. The rock is indis-tinctly stratified and massive. No fossils were foundin the formation in this area, but similar lithologyand stratigraphic position indicate a correlation withthe Lone Mountain Dolomite at Bare Mountain (p1.2; fig. 2; Cornwall and Kleinhampl, 1961b) and else-
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J6 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY
where (Nolan and others, 1956, p. 39, 41) where theage is considered to be Late Siluriin and EarlyDevonian.
OTHER PALEOZOIC ROCKS
The Fluorspar Canyon Formation, of Devonian age,which overlies the Lone Mountain Dolomite at BareMountain (pl. 2; Cornwall and Kleinhampl, 1961b) andcorrelates with the Nevada Formation (Merriam, 1940,p. 14-16, 22-25; Nolan and others, 1956, p. 4248), isnot exposed in the Bullfrog quadrangle. The over-lying Meildejohn Formation (pl. 2; Cornwall andKleinhampl, 1961b), however, crops out 1% mileswest of Bullfrog Mountain in an area shown as un-differentiated Paleozoic rocks. The exposures con-sist mainly of brown to black silty claystone and shalewith interbeds of black chert and dark-gray fossilifer-ous dolomite. Fossils consisting of Foraminifera andconodonts collected at the type locality on Bare Moun-tain indicate a Late Mississippian age, and the forma-tion correlates both faunally and lithologically withthe Chainman Shale and Diamond Peak Formationnear Eureka, Nev. (Nolan and others, 1956, p. 56861)and in part with the Eleana Formation at the NevadaTest Site (Johnson and Hibbard, 1957, p. 357-360).
In addition to the formations described above, dolo-mite fragments of the Bonanza King Formation ofMiddle and Late Cambrian age constitute an importantpart of the monolithologic breccia described below.
Small areas of limestones whose true identity is un-certain are shown on plate 1 as undifferentiated Pale-ozoic rocks in the SE% sec. 12, T. 12 S., R. 45 E.,and in the NEI/4 sec. 24, T. 12 S., R. 46 E. In addi-tion, three patches of quartzite that extend to the southmargin of the quadrangle in the southwestern part, aswell as one in the NWy4 sec. 6, T. 80 N., R. 1 E.,are also shown as undifferentiated Paleozoic rocksbecause of uncertain identity.
CENOZOIC ROCKS AND VOLCANISHThe Cenozoic rocks in the Bullfrog quadrangle range
in age from early Tertiary to Quaternary. Most ofthe rocks are Tertiary and consist of terrestrial sedi-mentary rocks of the Titus Canyon Formation ofStock and Bode (1935) overlain by tuffs, welded tuffs,and other volcanic rocks derived from the BullfrogHills caldera. Monolithologic breccia of early Paleo-zoic carbonate rocks underlies the Titus Canyon For-mation and younger volcanic rocks and is probablyearly Tertiary but possibly partly Mesozoic in age.Overlying the volcanic sequence are Quaternary oldergravels in dissected fans and in terraces found highin present canyons and Recent alluvium in the valleys.
KONOLITHOLOGIC BRECCLA
Monolithologic breccia masses, consisting almostentirely of limestone and dolomite of the Carrara andBonanza King Formations, respectively, overlie theDaylight Formation in several areas in the south-western corner of the quadrangle (fig. 3); one of thesemasses, near the south edge of the quadrangle, isoverlain with a sedimentary contact by the Titus Can-yon Formation.' Other similar breccias occur in scat-tered outcrops north of the area mentioned above, andthese breccia masses are overlain by Tertiary tuffand basalt flows. Two small masses of similar car-bonate breccia are interbedded in the tuffs and weldedtuffs of the Bullfrog Hills caldera sequence just westof the center of the quadrangle (secs. 15 and 22, T. 12S., R. 45 E.).
Individual breccia masses are as much as 2,000 feetacross in longest dimension and as thick as 200 feet,and they consist of angular to subrounded fragmentsof limestone or dolomite (fig. 3). Most of the frag-ments are less than 8 feet in diameter, but individualbeds in the thoroughly brecciated masses may, inplaces, be traced without significant offset for 50 feetor more. The matrix consists of finer fragments ofthe same material. Toward the base of some massesthe matrix contains increasing amounts of brownclayey material, and where the breccias overlie theDaylight Formation scattered fragments of quartzitefrom the Daylight Formation are incorporated in thebasal breccia zone.
In considering the origin of the monolithologic brec-cia, it should be noted that most of the brecciamasses occur either as erosional remnants on the Day-light and Crokscrew Formations in the southwesternpart of the quadrangle, or along the contact wherethese early Paleozoic rocks are unconformably over-
Irsas &.-Rolling bills underlain by Daylight Formation. Black pointnear oeuter of photograph Is small remnant of dolomitic monoiltho-logie brecda mass Picture was taken looking east from much largerI cm of monollthologie breccia likewise overlying the Daylight For-mation aear Daylight Pas, Grapevine Mountains, Calif.
BULLFROG QUADRANGLE AND BULLFROG CALDERA, NEV.-CALIF.
lain by the Titus Canyon Formation of Stock andBode (1935) of Oligocene age.
Recent studies by M. W. Reynolds (oral communi-cation, 1962) in the Grapevine Mountains west of theBullfrog quadrangle indicate that there the monolitho-logic breccias are partly interbedded in the TitusCanyon Formation.
The source of the monolithologic breccias was prob-ably a thrust plate (described later) of lower Paleozoicrocks (Corkscrew Quartzite, Carrars, and BonanzaKing Formations) that moved northeastward over theDaylight Formation into the southwest corner of thequadrangle. It is believed that most of the monolitho-logic breccias formed as landslides that slid off thefront of the thrust fault either while it was active orlater during the long erosion period previous to deposi-tion of the terrestrial fanglomerates of the TitusCanyon Formation. Normal faulting along the eastface of the Grapevine Mountains may have also pro-duced scarps from which blocks of carbonate rocksslid off to form monolithologic breccias, but most ofthese faults in the Bullfrog quadrangle offset, andare thus younger than, the Titus Canyon Formation,whereas most of the monolithologic breccias appearto be as old as or older than that formation.
The thrust fault mentioned above is probably relatedin age to the major deformation along the Las VegasValley shear zone (Cornwall and Kleinhampl, 1960a)to the southeast, which Longwell (1960, p. 197) con-siders to be Cretaceous or early Tertiary. The land-slide breccias must have formed during or followingthis period. The youngest monolithologic breccias, aswas mentioned earlier, are interbedded in and thuscontemporaneous with volcanic rocks of the BullfrogHills caldera of probable Miocene age.
The interpretation of the monolithologic carbonatebreccias as landslide masses agrees with the conclu-sions of Longwell (1951), Wright (1955), Grose (1959,p. 1535-1537), Kupfer (1960, p. 197-198), and othersconcerning the origin of similar breccias elsewhere inthe Great Basin. These breccias can be distinguished,because of certain significant diierenceu, from tectonicbreccias along the soles of thrust faults that are alsoprominent in the Great Basin (Noble, 1941, p. 983-977; Noble and Wright, 1954, p. 152; Kupfer, 1980, p.204-208). Kupfer (1960, p. 197-198, 205-206) recog-nized both types of breccias in the Silurian Hills justsouth of Death Valley, Calif., and ably described thedifferences between the two types. The sedimentarybreccias tend to have smaller, more angular fragmentsthan tectonic breccias, and the brecciation is moreintense. Surfaces of movement are common on blocksin tectonic breccias and rare in sedimentary breccias.
TITUS CANYON FORMATION 0F STOCK AND BOSDE(1935)
The Titus Canyon Formation, first defined andmapped by Stock and Bode (1935), extends southeast-ward for 30 miles in a narrow, almost continuous beltalong the California-Nevada border from the northend of the Grapevine Peak quadrangle to the east edgeof the Chloride Cliff quadrangle. These quadranglesadjoin the Bullfrog quadrangle on the west and southrespectively, and the Titus Canyon Formation cropsout in the southwestern part of the Bullfrog quad-rangle along the eastern flank of the Grapevine andFuneral Mountains. Patches of the Titus CanyonFormation, too small to be shown on plate 1, alsooccur in the northeastern corner of the quadrangle,unconformably overlying the Carrara Formation.In the southwestern part of the quadrangle the TitusCanyon Formation lies unconformably above the Day-light Formation, Corkscrew Quartzite, and CarraraFormation.
The Titus Canyon Formation consists of terrestrialconglomerate with interbedded sandstone, siltstone,mudstone, limestone, and tuff. The formation as awhole is lenticular and changes abruptly in thicknessalong the strike from 0 to about 3,000 feet. A section(pl. 3; fig. 4) was measured 1 mile southeast ofDaylight Pass where the formation is 2,700 feet thick.Conglomerate is predominant in the lower half, where-as arkosic and tuftaceous sandstone and siltstone,and air-fall tuft are increasingly abundant upwardabove the middle of the formation.
The conglomerate is characteristically reddishbrown to brown and contains boulders, cobbles, andpebbles of well-rounded, highly polished black andgray chert, and white and brown quartzite, as well as
Flau 4.-Titus Canyon Formation of Stock and Bodo (1935): top offormation La near top of peak where obviously bedded rocks are over-lain by Dassve grm tat looking northeat. 1 mile southeast of
aVulight Pass Funeral Mountaus., qer.
'4,j
J8 SHORTER CONTRIBUTIOINS TO GENERAL GEOLOGY
some gray to black limestone and dolomite, in anarkosic matrix. A few pebbles of brown stony rhyo-lite were also found. Lenticulr~lieds, as much as100 feet thick, of reddish-brown to gray muddy lime-stone occur near the base, just above the middle, andlocally near the top of the Titus Canyon Formation.Some of the limestone beds that occur just above themiddle of the formation are algal. The sandstone,siltstone, and mudstone, which are most abundant inthe upper half of the formation, are arkosic or tuff-aceous, and air-fall tuff beds as much as 50 feet thickare interbedded with them. These rocks are reddish-to yellowish-brown or gray; a few are green. One ofthe green beds is conspicuous because it containsabundant magnetite crystals.
Stock and Bode (1935, p. 577-578) have dated theTitus Canyon Formation as early Oligocene on thebasis of mammalian fossils found in the mudstones inthe lower part of the formation. The fauna includesthe horse 3fesohippu=, two types of titanotheres, hy-dracodont rhinoceroses, artiodactyls, and a sciuro-morph rodent. The present writers found fresh-watersnails, identified by D. W. Taylor (written communi-cation, 1960) as Valita and indeterminate Lymnae-idae, in a limestone bed near the top of the formation,and also a fossil tree truck.
The Titus Canyon Formation is overlain conform-ably in the Bullfrog quadrangle, but unconformably tothe northwest and southeast (Stock and Bode, 1935, p.577) by tuffs, welded tuffs, lava flows, and sedimen-tary rocks. The volcanic rocks were probably derivedfrom the Bullfrog Hills caldera.
BULLSEOG ILWZ CALERA ANMD ASSOCIATED ROCKS
The Bullfrog Hills calderm is roughly coextensivewith the Bullfrog Hills (pls. 1, 4). It has been de-scribed by one of the present writers (Cornwall, 1962).The northern and southern margins of the caldera arecovered by alluvium, but the caldera appears to beoval in shape, roughly 18 miles long in a northeasterlydirection and 10 miles across. Inside this area vol-canic rocks form a broad faulted dome and dip out-ward. The caldera and its associated volcanic andand minor sedimentary rocks are probably Miocene inage (Stock and Bode, 1935, p. 572-578).
The volcanic rocks are predominantly rhyolitic tuffsand welded tuffs, and it is believed that these rockswere derived from a rather shallow magma chamber,which collapsed following the eruption of the tuffs andwelded tuffs to form a caldera. Flows and intrusivesof rhyolite, latite, basalt, and basanite are also asso-ciated with the caldera. The caldera is delineated bya peripheral fault zone where bedrock is exposed, but
the recognition of the structure as a caldera is alsobased on several features of the volcanic rocks, includ-ing their type, relative abundance, distribution, andpattern of deformation. The caldera structures havebeen partly modified or masked by erosion, depositionof alluvium, and basin-range faulting.
The nomenclature used here in describing the tuffsand welded tuffs is that adopted by R. L. Smith(1960a) in his excellent review article on ash flows.Smith states, "The basic unit of ash-flow deposits isthe ash flow that is analogous to, or perhaps the sameas, the deposit resulting from the passage of one nu~eardente."
An ash flow may be partly or even completelywelded into a cohesive rock, and this is called awelded ash flow or welded tuft. Commonly there willbe a zonation from densely welded rock in the interiorof an ash-flow sheet (made up of one or more indi-vidual ash flows) to nonwelded tuft at the top andbottom, and this is called a cooling unit. In a simplecooling unit cooling was continuous and uninterrupted;in a compound cooling unit abnormal zonation of thewelded and nonwelded parts indicates disruption of thecooling gradient by later addition of one or more newhot ash flows.
The central part of the caldera is covered predomi-nantly by ash flows; the margins consist predomi-nantly of younger tuffs with intertonguing rhyolite andlatite flows and intrusives. Within the caldera is anorthwest-trending dome, measuring roughly 1 by 4miles, of Precambrian and Paleozoic rocks that wereprobably pushed up into the volcanics at a late stageby re-entry of magma into the underlying chamber.The domal uplift probably extends 2 miles farthernorthwest where another block of highly faulted Pale-ozoic rocks occurs surrounded by pyroclastic rocks ofthe caldera sequence, and the intervening area con-sists of older tuffs and interbedded sedimentary lenses,which are also faulted against the surrounding youngerash flows In these tuffs is an intrusion of rhyolitethat may represent a small protrusion of the intrusivebody that presumably underlies the dome.
-The main fault that marks the rim of the caldera iswell exposed on the southeastern rim (pl. 1, sec.B-B'). The stratigraphic displacement on this faultand on several parallel faults to the east indicatesthat the caldera on the west subsided roughly 4,000feet. Extending northeastward tengentially from thesoutheast margin of the caldera is a series of normalfaults, which have dropped volcanic rocks of theBullfrog Hills caldera sequence downward succes-sively to the northwest. This foundering is probablyrelated to the caldera subsidence in a manner similar
to the graben that extends outward tangentially fromthe Creede caldera in Colorado (Steven and Ratt6,1960, p. B14-B15).
It is the character and distribution of the rocks asdescribed above that, by analogy with similar relationsin more recent unequivocal calderas (see Smith, 1960a,p. 834; Williams, 1941), has led to the conclusion thathere, too, is a caldera.
The ash flows and, in part, other flows and tuffs ofthe Bullfrog Hills caldera extend several miles south-west of the caldera where they overlie the Titus Can-yon Formation of Oligocene age. The volcanic rocksalso extend several miles east of the caldera, wherethey are faulted against Paleozoic rocks on the southand are overlain unconformably by alluvium andyounger tuffs on the north and east (pl. 4).
A columnar section of the volcanic and minor sedi-mentary rocks in the Bullfrog Hills caldera, is givenon plate 5. The thickness of 10,000 feet is probablyclose to the maximum. A series of ash flows, totaling6,000 feet, has five distinctive cooling units each ofwhich has nonwelded and densely welded parts, andthree of which have prominent basal vitrophyrezones. The remainder of the section consists of air-fall tuffs, flows of rhyolite, latite, and basalt, andminor sedimentary lenses. The five cooling units ofthe ash-flow sequence have also been identified in thestructural trough that extends east of the BullfrogHills caldera. The easternmost exposure of thecooling units, 6 miles east of the caldera rim (pl. 4),consists of units 4 and 5. Pyroclastic rocks fromthe Bullfrog Hills caldera in this area are faultedagainst younger ash flows to the east that were prob-ably derived from the Yucca Mountain caldera. The
total volume of ash flows in and around the BullfrogHills caldera is estimated to be .85 cubic miles (36(cu km), which is in the range of deposits known to berelated to calderas (Smith 1960a, p. 819).
DESCRIPTION OF TM VOLCANIC ROCKS
A brief description will be given here of the flows,intrusives, and tuffs associated with the caldera, andthe ash flows will be described in somewhat greaterdetail.
ASH FL4WS
Plate 5 gives a graphic representation of the ash-flow section, probably at its maximum thickness, with-in the Bullfrog Hills caldera. It will be noted thatthere are five cooling units in this pile of ash flows.
Each cooling unit has a densely welded central partand partially welded to nonwelded upper and lowerparts. The densely welded part ranges from 100 to1,300 feet in thickness There is a zone of vitrophyreas much as 30 feet thick at the base of the denselywelded part, overlain by a much thicker zone of devitri-fication (fig. 5), and, near the top, a zone of vaporphase crystallization. These correspond to the zonesin ash flows described by Smith (1960a, p. 830-831).The vitrophyre zones may lens out along the strike toas little as 1 foot and be barely visible, but a glassytransition nevertheless exists between partly weldedtuff below and densely welded, devitrified tuff above.
The nonwelded to partially welded zones of the ashflows, which separate the cooling units, range froma few feet to as much as 500 feet thick. The relationsin the Bullfrog Hills ash flows are illustrated on plate5, where four nonwelded zones intervene between fivecooling units. It will be noted that the boundaries be-tween cooling units 1 and 2, and between 2 and 3, liewithin the nonwelded zone; the boundary is marked bya parting separating two lithologically different tuffs.Between the remaining cooling units the boundary lieseither at the top or bottom of the nonwelded zonewhere basalt flows occur.
Single cooling units in the ash flows may representone flow with the zonation expectable in the cooling ofa single flow. Figure 5 portrays such a flow. Part ofthe cooling units, however, in both the Bullfrog Hillsand Yucca Mountain ash flows, contain more than oneindividual flow. In the case of cooling unit 3 of theBullfrog Hills ash flows, for example, differences inlithology between the lower and upper parts indicatequite certainly that more than one flow is present.The contrast in lithology is illustrated by variations inthe mode (table 1, analyses 4-8). Analyses 4 and 3from the lower part of the unit are similar in theirscarcity of phenocrysts, whereas analysis 6, from theupper part, contains nearly 20 percent phenocrysts.
Fiawna &.-De'vitrifcation soos of an ash low (welded tuE); orieuted.fattened cavities are elongated paraUel to the low plane and reproesent fattened pumice fragments that have dIferentlally eroded outon th weatherdng surface. Fluormpar Canyon. north end of BareMountain. Ne.v
J10 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY
TABLE 1.--Modal analyaes (volume percent) of ash flows in the Bullfrog Hille caldera
Coo -Pooling unit I Cooling Unit 3 Cooling unit 4 Cooling unit Sunit 1 I ,7ws.
litboidol Nonwelded Uthotdal Nonwelded Litholdal Litboldal Nonvelded Glas tone Litholdal Nonwelded Litholdalwelded tufn welded tuft welded welded tuft of welded welded tuff welded
Plagioclase composdtion.. Ante Af-u Ante An= Anse Ant Ane Ano An - - - Ann
As indicated in the preceding discussion, variationsin the lithology of the ash flows are due to: (a) Varia-tions between individual flows in the proportions ofcoarse to fine materials, and in the relative propor-tions of crystals to glass. Accidental lithic fragmentsalso vary from one flow to another. Before welding,the flows were predominantly glass, in the form ofshards, pumice, and dust. (b) Variations in the ratesof cooling within cooling units. Retention of heat inthe interiors of cooling units permitted compaction,welding, deuteric crystallization, and vapor activity.The cooling units are thus zones with variations indensity and porosity, degree of flattening of pumicefragments, color, degree of crystallization, and altera-tion of phenocrysts.
In considering the different original lithologies ofindividual ash flows, the crystal content is probablythe most helpful guide to their identification in theBeatty area. Mackin (1960, p. 0 ) and Williams(1960) have recently correlated a sequence of ash flowsover a considerable area elsewhere in the Great Basinon the basis of phenocryst content. Al the ash flowsstudied here contain phenocrysts, partly broken, asmuch as a mm in length. Quartz, oligoclase or ande-sine, and sanidine are most abundant; biotite andmagnetite are quite ubiquitous in smaller amounts andsmaller crystals. The phenocryst content of the ashflows ranges from 1 to 25 percent.
The modal analyses in table 1 of the Bullfrog Hillsash flows illustrate differences in crystal content thathelp to identify individual ash flows. For example,in cooling unit 1 plagioclase is abundant; cooling unit3 has no quartz; in cooling units 4 and 5 quartz andsanidine are most abundant. Cooling unit 5 is dis-tinguished from cooling unit 4 by its smal but wide-
spread content of basalt fragments, which are notfound in the other units.
Lithologic characteristics that are related to zona-tion of cooling units will be described next. Non-welded tuffs commonly occur at the margins of coolingunits but may be absent at either the top or bottom ofthe unit, as is illustrated in the columnar section onplate 5. The nonwelded tuffs have a relatively lowdensity and high porosity and are light colored-white, gray, or yellowish gray. Undeformed pumicefragments and shards are easily seen (fig. 6A). X-rayanalysis has shown that in several of the nonweldedtuffs the glass has been partly to largely altered toclinoptilolite (a zeolite), but this is considered to bedue to diagenesis that occurred sometime after theemplacement and cooling of the ash flows.
The nonwelded tuffs grade through a zone, normallyless than 80 feet thick, of partially welded tufE into thedensely welded zone. The partially welded tuffs inthe lower parts of cooling units commonly have a pinkcolor that deepens upward toward the densely weldedzone, and pumice fragments are increasingly flattenedupward (fig. 6D).
The densely welded zones of the ash flows are pre-dominantly devitrified lithoidal welded tuff (figs. 5 and60) with a gray to reddish-brown color. Commonlythe rock has a eutaxitic structure with lenticular thinstreaks of light brown in darker brown rocks. Theselenticles are oriented approximately parallel to theflow plane and represent pumice fragments that havebeen flattened from several millimeters to one or less.In the upper parts of the densely welded zone, flatten-ing of the pumice fragments is less pronounced. Thedensely welded zone commonly has a vitrophyre zoneas thick as 80 feet at its base. The glss is brown to
BULLFROG QUADRANGLE AND BULLFROG CALDERA, NEV.-CALIF. ill
black in color, partly perlitic, and has black or browneutaxitic lenticles of collapsed pumice fragments (fig.6B).
Devitrification of the densely welded lithoidal zonehas resulted in the development of spherulitic or axio-litic intergrowths of cristobalite or quartz, and sanidine(Smith, 1960b, p. 152; Ross and Smith, 1961, p.36). In addition to devitrification of the glass, vaporsescaping upward through the flows have depositedcristobalite, tridymite, and sanidine in pore spacesand have altered the glass and phenocrysts.
The phenocrysts of sanidine tend to have achatoyant luster in the devitrified zone whereas theyare clear in the more quickly chilled glass margins,and biotite that is fresh green or black in the glassyparts has been altered to reddish brown or completelydestroyed in the lithoidal parts of the ash flows.
This vapor phase activity, as suggested by Smith(1960a, p. 832), requires a porous matrix and was mostintense in the upper part of the densely welded zone.
AmPFALL TuFFS
Air-fall tuffs are abundant in the lower and upperparts of the pyroclastic sequence of the Bullfrog Hillscaldera. The tuffs are predominantly massive butpartly bedded and range in color from white to brownor locally green. The tuffs contain fragments asmuch as 50 mm in diameter of pumice, glass shards,rhyolitic felsite, and locally basalt. They also containscarce to abundant partly broken crystals, less than2 mm in longest dimension, of quartz, sanidine, albite,oligoclase, and biotite. The groundmass consists offinely comminuted pumiceous glass and glass shards.
The tuffs are locally intensely silicified, opalized,and argillized probably by thermal springs. Theglassy parts of the tuffs are also partly altered toclinoptilolite, probably by diagenesis.
RHRYOLTZT FLOWS A.nD XNTRUU'VU
Flows and intrusions of rhyolite occur predomi-nantly near what are considered to be the margins ofthe caldera. The rhyolite bodies are partly vitrophy-ric and partly felsitic. The color ranges from gray toblack in the vitrophyre and gray to reddish brown inthe felsite. Most of the vitrophyre is perlitic (fig. 7A).The felsite is fine grained to aphanitic (fig. 8A)and partly spherulitic (fig. 7B). The phenocryst con-tent of the rhyolite bodies is commonly less than 5 per-cent but may be as much as 15 percent. Thephenocrysts, as long as 3 mm, are quartz, sanidine,oligoclase, biotite, magnetite, and hematite. Two in-trusive bodies have anorthoclase rather than sanidine.
The vitrophyre tends to occur most abundantly
along the margins of the flows and intrusives, and themarginal zones may also be brecciated (fig. 9).
BASALT FWOWS A"D IIRUSIVAS
Basalt occurs in the Bullfrog Hills in dikes and ir-regular intrusions, and as flows above cooling units 3,4, and 5, and near the top of the pyroclastic sequence(pl. 5). Other basalt dikes and flows occur in thepyroclastic rocks several miles east of the BullfrogHills caldera. A quartz basalt flow is the youngestunit in the Bullfrog Hills volcanic section (pl. 5), andanalkime basanite occurs locally, probably as analteration facies of the basalt flows above cooling units3 and 5.
The basalts are dark gray to black, fine grained, andporphyritic; the phenocrysts, as long as 1 mm, arelabradorite and olivine partly altered to bow lingite,antigorite, and iddingsite. Analysis 20, table 2, rep-resents one of the basalt flows. The groundmass is anintergrowth of labradorite or andesine, augite, mag-netite, hematite, and a little biotite. The groundmassis trachytic in some bodies and is partly brown glasscontaining skeletal magnetite crystals in others.
The quartz basalt, represented by analysis 19, table2, contains phenocrysts, as much as 15 mm acrose, ofandesine and quartz in a fine-grained groundmass oflabradorite or andesine, quartz, augite, pigeonite,magnetite, hematite, and a little biotite.
The two basalt flows above cooling units 3 and 5grade abruptly, either horizontally or vertically, intoanalcime basanite. The basanite (analysis 22, table2) has phenocrysts, as much as 4 mm across, of olivine,pigeonite, and augite in an aphanitic, nearly blackgroundmass of analcime, plagioclase (Ans..o), pyrox-ene, magnetite, and a little biotite. The analcime,identified by X-ray, is partly interstitial to the otherminerals. The basalt in the flows that contain basa-nite is very similar texturally and mineralogically tothe basanite, including the presence of rather abun-dant pigeonite as well as augite. The only differenceis the absence of analcime. It is not known whatcaused the local development of analcime, but it mayhave formed as a late stage primary precipitate, oras a deuteric alteration where volatile constituentslocally accumulated.
LAT17T5 FLOWS A-D INtRUSIEES
Porphyritic latite was erupted late in the volcanicsequence of the Bullfrog Hills caldera. It forms ratherextensive flows as thick as 200 feet near the east rimof the caldera and intrudes the ash flows and tuffsin the western part of the caldera.
J12 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY
A B 8
0 D
FIGUR 8.
? ROTOMICROGRUPRU 01 4BE MLWWS
BULLFROG QUADRANGLE AND BULLFROG CALDERA, NEV.-CALIF. J13
The intrusive latite and the interiors of the flowsare massive reddish- to grayish-brown to black rock;the flow tops, as thick as 10 feet, are vesicular andreddish brown; the basal zones of the flows, 10 to 25feet thick, are commonly brecciated, and locally theflows have a black glass zone, 1 to 5 feet thick at thebase. Phenocrysts as much as 3 mm across are com-monly arranged in clusters of plagioclase (Anos..,),augite, pigeonite, magnetite, and biotite (fig. 8B).The groundmass is microcrystalline, partly trachytic,and consists of intergrown sanidine, oligoclase, mag-netite, and hematite. Analyses 17 and 18 representtwo of the flows.
CURNICAL CoMlpoSTXON OF TUB ROCKS
The volcanic rocks of the Bullfrog Hills calderarange from basanite and basalt to latite, rhyodacite,and rhyolite. The rocks are probably hypabyssal inorigin and occur as flows, ash flows (welded tuffs),tuffs, and intrusives. The rhyolitic rocks quantita-tively far exceed the other types and include the ashflows, most of the tuffs, and the majority of the flowsand intrusives.
Chemical analyses are given in table 2 of 22 volcanicrocks in the Beatty area and of four related tuffs andash flows from the same pyroclastic field in the Ne-vada Test Site, 35 miles northeast of Beatty (Wilcox,1958). Four of the analyses are old and were made inconnection with a study of the Bullfrog mining district(Ransoms and others, 1910). The remainder are new.All were made by analysts of the U.S. Geological Sur-vey, as were the spectrographic analyses shown intable 2. Norms and modes are also shown for most ofthe rocks
The differentiation index of Thornton and Tuttle(1958, 1960) has been calculated from the norms, andthe oxides have been plotted against this index infigure 10. A semilogarithlic graph was used andthe differentiation index was plotted on the logarith-mic coordinate. The graph plotted in this way showsgreatest detail in the rhyolitic range, where the ma-
jority of the analyses lie, and yet permits inclusion ofthe basaltic rocks on the same graph in a reasonablespace pattern.
The trend lines for the 5,000 analyses in Washing-ton's tables as contoured by Thornton and Tuttle(1960, p. 674-679) are also shown for six of the oxidesin figure 10. Where the analyses from the Beattyarea have a nearly linear arrangement, they followthe trend lines rather closely. This is true for SiO2,which increases with increasing D.I. (differentiationindex), and also for CaO and MgO, which decreasewith increasing D.L TiO, and FeO + FeO also followgood trends, decreasing with increasing D.I., whereasNaO and K 30 increase as D.I. increases, but withsome scattering of points.
It will be noted that the air-fall ana nonwelded tuffsamples plot more or less as a group and are sep-arated from the other rocks for several of the oxides.The tuffs are markedly low in NaO and AIA03 withrespect to the trend for the other rocks in the D.I.range in which they lie. The tuffs are also slightlylow in FeO+FeO,, CaO, and TiO,. It was deter-mined by X-ray analysis that the glass in these tuffshas been altered to clinoptilolite. Changes in the bulkcomposition of the rocks probably occurred duringthis alteration and may account for their somewhatabnormal composition.
Minor c.Zment.-Quantitative spectrographic anal-yses were obtained for 18 of the 28 volcanic rockslisted in table 2. Only a brief discussion of the resultswill be given here. The amounts of the minor ele-ments in each rock have been compared with theestimates of Vinogradov (1958) for the same rock typein the whole earth's crust.
The abundance of the following minor elements involcanic rocks of the Beatty area is roughly compa-rable to Vinogradov's estimate: B, Be, Ga, Mo, Nb,Pb, Sc, V. The rocks in the Beatty area are low inCo, Cr, Cu, Li, and Ni, and they are high in La, Y,Yb, and Zr. The La content of the Beatty latite
MAzArToN JOB AduU 6
A. Nonwelded basal zone of ash Sow. Dark patches are accidental felsite inclusions. The large medium gray patch is anundeformed pumice fragment Quartz crystals, partly broken Into small fragments, are white. Matrix consists of glassdust and angular shards. Plain light, x 20.
B. Vitrophyre zone that overiles the basal nonwelded tone t A. The rock if almost entirely medium, and dark-brown glasswith schlieren oriented more or less parallel to the flow plane. One Incompletely fused flattened pumice fragment(speckled gray) can be seen near the middle of the picture. Scattered white crystals are sanidine. Plain light, X 20.
C. Devitriled densely welded titholdal zone of same ash flow as in A. Groundmass of microcrystalline eutaxitic schtleren.elongated more or less parallel to the 9ow plane, but deflected around crystals and fragments. Dark fragment is glass.White crystals are sanidine and anorthoclase. Plain light, X 20.
D, Partly welded tuff just below the vitrophyre zone of an ash flow. Pumice fragments (speckled gray) are flattenedparallel to the Blow plane. Grounda consists of glass dust (black), amail pumice fragments (gray), and angularglass shards (white and Ught gray). Crystals (white) are quartz, plagloclase, and sanidine. Plain light. X 20.
J14 SHORTER CONyRIBUTIONS TO GENERAL GEOLOGY
A BFr an 7.-Photomlcrographs of perlitic and spherunltle rftoltt
bodies. A, Perlite from the margn of a rbyolits Intrusion, Rock Irmostly peritic glass. but It contains seversl subrounded crystals ofquartz (white). plagloclase (white), and sanidine (slightly darker
ray than the glass). Plain light. x ISI. B, pherulites from themargin of a wbyollte Blow. Rock consists of spherullt.s of rartoussizes and scattered crystals (white). Large erystal I ol4goclaseesmall crystal to quartz. Crossed rdeots. X 20.
.. - f . -.
A BFioas S.-Photomicrographs of felsitic rocks A. UhyoUte porphyry
intrusive. Rectangular white latches are anorthoclase phenocrysts.Small white streaks re amarolitic cavities filed with quart sandanorthocla. Groundman (dark ray) consists of a very fine Inter-growth of quarts and feldspar speckled with hematite. 'Plain light
X I5. 3. LatBte porphyry low. Phenocryat of oligodase (white).augite (ray, In or adjacent to oligoclase).and altered blotite (black)tend to be clustered together. The rondmass Is very One pained.slightly trachytic. and consists of ntergrown oligoclase. K-feldsar.and magnetite. Plain light. X 15.
BULLFROG QUADRANGLE AND BULLFROG CALDERA, NEV.-CALIF. J15
Fwovis 9.-RbyoUtte vitrophyre breccia at base of Bow In the upperpart of the Baulfrog Hlls volcanic sequence. Rainbow Mountain. Bul-trog quadrmagle Nevada.
and basalts is 3 to 10 times Vinogradov's figure forthe same rocks. Ba and Sr are nearly equal to Vino-gradov's figure in the rhyolites, but in the latite andbasalts they are 2 to 7 times higher. F is aboutaverage in the rhyolites of the Beatty area, but it is3 times higher than Vinogradov's estimates in thelatite and basalt.
OLDB3 GRAV=
Older gravels, probably Pleistocene in age andcharacterized by the variable sorting of the detritus,occur in the south and northeastern parts of the Bull-ftog quadrangle. These gravels are old dissectedfans, which have been eroded and partly covered byRecent alluvium. These dissected fans consist of rel-atively fine detritus, cobbles, and boulders. The up-per surfaces are strewn with boulders, mostly lessthan 3 feet across, derived from the various types ofbedrock in the adjacent areas. In the south part ofthe quadrangle, the boulders on the fans are quartzitefrom the Corkscrew and Daylight Formations, dolo-mite and limestone from the Bonanza King and Car-ram Formations, and welded tuff and basalt from theTertiary volcanic rocks. In the northeastern part ofthe quadrangle, the dissected fans are strewn withthe same types of boulders, but rhyolite derived fromwelded tuffs and intrusives predominates.
Two of the dissected fan remnants in the southernpart of the quadrangle had been intruded by basalt(pl. 1) prior t? dissection and the surfaces adjacent tothese areas are strewn with basalt boulders.
ECENTt ALLUVWuX
The Amargosa Desert and Sarcobatus Flat in thesoutheastern and northwestern parts of the quad-
rangle, respectively, are mostly underlain by Recentalluvium composed of gravel, sand, and silt. Thealluvium has encroached on the Funeral and Grape-vine Mountains in the southwestern part of the quad-rangle, and on the Bullfrog Hills in the northern part.Large areas of the alluvium are covered by smoothdesert pavement broken by the gullies of ephemeralstreams.
STRUCTURE
The structural features of the rocks in the Bullfrogquadrangle, like those in the Bare Mountain quad-rangle, are due both to tectonic deformation and tovolcanic activity. The Paleozoic rocks have been in-tensely folded and faulted by a major tectonic orogeny(Cornwall and Kleinhampl, 1960a), probably in theCretaceous and early Tertiary, that also producedmajor thrust faults and right-lateral strike-slip faultsin the Las Vegas Valley shear zone (Longwell, 1960)southeast of Bare Mountain. The volcanic rocks inthe northern parts of the two quadrangles have beendeformed by the catastrophic subsidence (Cornwall,1962) of the edifices from which these rocks wereerupted to form the Bullfrog Hills caldera, probablyin the Miocene, and the Yucca Mountain graben,probably in the Pliocene (pl. 4). During the Tertiaryand Quaternary the whole region has been subjectedto basin-range normal faulting.
TECTONIC DEJORATIONIn the southwestern part of the Bullfrog quadrangle
a flat thrust plate of Corkscrew Quartzite and overly-ing carbonate rocks of the Carrara and Bonanza KingFormations moved northeastward over the older Day-light Formation. Erosional remnants of the thrustplate, consisting mostly of Corkscrew Quartzite, arescattered over the area (pl. 1). This is essentially abedding-plane fault and stratigraphic displacementhas for the most part not been great; the sole of thefault lies near the base of the Corkscrew Quartzite, andin most places it overlies the upper part of the Day-light Formation. TIe Johnnie thrust in the northernSpring Mountains (Nolan, 1929, p. 46-471) shows sim-ilar relations. Movement along the fault has beensufficient to thoroughly brecciate the basal 20 to 30 feetof the Corkscrew Quartzite immediately above thefault. Percolating group water has subsequentlybleached the normally reddish-brown rock.
Underneath the thrust fault the incompetent Day-light Formation has been folded with axes trendingnorthwest at right angles to the direction of movementof the thrust plate. The Daylight Formation has beenrather thoroughly fractured and faulted, and the basalpart of the formation is also faulted against a massivequartzite (shown as four patches of Paleozoic rocks,undifferentiated, on pL; 1) near the south boundary of
J16 SHORTER CONTRIBU'TIONS TO Q.ENERAL GEOLOGY
DIFFERENTIATION INDEX
90OIFFERENTIATION INDEX
9098 50 0 98 50 0
OI-
801
4o 0
S1O.
3 Welded tuft (ash flow)* Nonwelded tuft (ash flow) xa Air-fall tuft Nx Lava flow Na Intrusive rocks x
Oxide trend for the 5000 analyses R\of Washington's tables (after xThornton and Tuttle. 1960)
i
i
i
II
j12.12 L O Welded tuft (ash flow)
0 Nonwelded tuft (ash flow)10 L * kir-fill tuft
x Lava flow8 L = Intrusive rocks
_ Oxide trend for the 5000 analysesof Washington's tables (after
6 ! Thornton and Tuttle. 1960)
4 _
!2| -
OF 12z t v i x _X_-ss
CaO/
XI
/X
.0, 0x
xf
2C
it
if
14
13
W ICauIC
I-M
4
2
C
' x '-1x
Z - K- r -e S
I- i~
2I 0 2 e "t xx
t~~~~~~~~~ ~ I I I ' ! i i
I.
zI'
I'
~~~~i--- - - -K K
I Neo0
21\
I 0
K20 ~~~~~A\
0L O ; I I I I !.l' I I
0~~~~~;. 20 -X N
1.50 x x
a e ~ A
Q4° i
O.S ~ (a ,,, 'L
.0-
10 -
48-
2 -
0o
MgO
/
I I I I . ... I I I . I. I I..
frtoun 10.-Variation diaarms of oxide yarsus diterentlation Index of volcanic rocks in the Beatty area. Nevada.
the quadrangle. This fault, like the thrust fault be-neath the Corkscrew Quartzite, is generally rather flatand may also be a thrust fault. Thus the shaly Day-light Formation probably occurs as an incompetentplate between two massive quartzites.
Just east of the Bullfrog quadrangle on Bare Moun-tain, the Paleozoic rocks have been intensely deformed
by flat thrust faults and north-trending right-lateralstrike-slip faults (Cornwall and Rleinhampl, 1960 a, b,1961b). The oldest thrust plates moved southwest,and younger overlying plates moved south or south-east. The rocks on the east sides of the strike-slipfaults have moved south relative to those on the westsides.
RELATION TO WALMR LANZ AND LAS VEGAS VALLEYSHEAB zoNE
The Las Vegas Valley shear zone, a major trans-current lineament with right-lateral displacement esti-mated at about 25 miles (Longwell, 1960, p. 197; B. C.Burchfiel 2 1961, p. 134-135), has been traced north-west to the west end of the Spectre Range quadrangle(fig. 11). Bare Mountain is only 25 miles northwestof there, and it is most likely that the shear zone passesnorthwestward near Bare Mountain either on the eastin Crater Flat or on the west across the AmargosaDesert. The general east-west strike of the beds onBare Mountain (Cornwall and Kleinhampl, 1961b)may be due to drag along the shear zone, similar to thesituation farther southeast in Las Vegas Valley (fig.11), and may thus indicate proximity to the shearzone.
Gianella and Callaghan (1934, p. 3, 18-19) sug-gested that the Las Vegas Valley shear zone may bepart of a major lineament that extends 200 milesnorthwest to the area of Cedar Mountain, Nev., andLocks and others (1940, p. 522-523) named this linea-ment the Walker Lane for the valley at Goldfield,which was the route used by the explorer Walker.This valley, in which there are indications of a right-lateral shear zone at Goldfield according to Locke,extends south to the Bullfrog quadrangle along Sar-cobatus Flat and thence into Amargosa Desert.
Recent mapping by B. C. Burchfiel' (1961, pis. 10,14) indicates that the Las Vegas Valley shear zonestrikes nearly due west at the west border of theSpectre Range quadrangle (fig. 11). If this is thecase, the shear zone probably passes south of BareMountain and thence northwestward across the Amar-gosa Desert and the Bullfrog Hills in the Bullfrogquadrangle as shown in figure 11. Burchfiel 2 (1961,p. 136-140) further suggests that a major branch ofthe shear zone may continue northwestward just southof Silver Peak, 60 miles northwest of the Bullfrogquadrangle, and thence into California north of theWhite Mountains, but geologic mapping in this area,described below, does not, favor this interpretation.
J. P. Albers and J. HF Stewart (oral communica-tion, 1962) have recently completed geologic mappingof Esmeralda County, Nev., and their data indicatethat the shear zone may extend, as shown in figure11, northwestward from the Bullfrog quadrangle to-ward Coaldale and thence northwestward up theSoda Spring Valley, where a right-lateral fault witha displacement of 4 miles has been postulated by Fer-guson and Muller (1949, pl. 1 and p. 14, 29). It is now
Burchftel B. C.. 1961. Structure and utrattzraphy of the SpectreRange quadrangle. Nye County, Nevada: New Haven. Conn.. Yale Unt-versity. PhL D. thesi-.
considered more likely (Albers, oral communication,1963) that the shear zone extends northward up Sar-cobatus Flat past Goldfield to connect with the CedarMountains fault zone near Tonopah, as suggested byGianella and Callaghan (1934) and Locke and others(1940).
The right-lateral transcurrent fault along the eastside of the White Mountains in the valley west of Sil-ver Peak, considered by Burchfiel 2 (1961, p. 136-140)to be a possible extension of the Las Vegas Valleyshear zone, has been recently studied by B. H. McKee(oral communication, 1962). According to McKeethis fault can be traced southeastward into Death Val-ley and down the east side of Death Valley where itis known as the Furnace Creek fault (Jennings, 1958).
It thus appears that there are two major right-lateralfault zones in this region (fig. 11). One, the LasVegas Valley shear zone, extends northwestward pastBeatty up Sarcobatus Flat, and possibly into SodaSpring Valley, or to Tonopah and thence northwest-ward to the Cedar Mountains. The other, the FurnaceCreek fault, extends northwestward along Death Val-ley and thence along the east side of the White Moun-tains.
BASIN-RANGE FAULTING
Basin-range normal faults have probably been de-veloping in the Bullfrog quadrangle from Tertiary toRecent time. One Recent fault along the east-centralborder of the quadrangle (pl. 1) and extending intothe Bare Mountain quadrangle has displaced Recentalluvial fans by as much as 40 feet, with the west orvalley side down.
Older normal faults, trending more or less north-south, have moderately disrupted the Paleozoic andTertiary rocks in the southwestern part of the quad-rangle. One such fault underlies Boundary Canyonand another is in the next canyon to the west thatruns south from Willow Spring. A northwest-strikingfault has dropped a small graben of Titus CanyonFormation of Stock and Bode (1935) into the olderDaylight Formation at the south border of the map,southeast of Boundary Canyon. All of these faultshave downward displacement on the west side.
Other normal faults of the basin-range type, due toregional subsidence, may occur in the volcanic rocksof the Bullfrog Hills caldera. The faults there arenormal, and due to subsidence, but most of the de-formation is probably related to the local collapse ofthe Bullfrog Hills caldera.
VOLCANC DICJ NATION
The Bullfrog Hills caldera has been described above(p. J84J15), and only a brief review of the principal
SHORTER CONTRIBUTIONS TO GENERAL GEOLOGYJ18
Direction of movement of thrust plates !
inob iL-Usa of the major tectoulc features of southwestern NevadaL Generalized tectonic features In the Las Vegas VaIUey fromLoUgwel (1960, P. 194. fig. 2), and In the Spectke Ra E xtadrange from D. C. Durehfl (unpublished data). Other data fromGlanells and Caflaghan (1934. V. 3- Us. 1 and 3). Locke end others (1940. pL 1), Ferguson End Muller (1149). Seanings (1958).
- . P. Albers (oral eommunkstlon. 1962). and E. IL McKee (oral communlcatlon. 1962).
structural features will be given here The calderameasures approximately 10 by 18 miles and is elong-ated in a northeasterly direction. The rocks in thecaldera, mainly ash flows and air-fall tuffs, have theform of an intricately faulted dome, developed sub-sequent to the initial collapse, with the ash flows andtuffs dipping outward toward the rim and normalfaults commonly dipping inward (pl.4, section A-A').
The main fault on the rim of the caldera is exposedat the Maontgomery-Shoshone mine on the southeast
rim (phs. I; 4), where rocks in the caldera on thenorthwest side have been displaced downward about8,500 feet with respect to those on the southeast. Ad-ditional subsidence of about 500 feet has occurredalong several other faults parallel to this fault in azone about 1 mile wide east of the Montgomery-Sho-shone mine (pls. 1, 4).. Extending northeastward from the southeast rim of
the caldera into the Bare Mountain quadrangle (Corn-wall, 1962) is a series of normal faults that have
BULLFROG QUADRANGLE AND BULLFROG CALDERA. N-EV.-CALIF. .il)
dropped the volcanic rocks downward successively tothe northwest (pl. 4). This subsidence is also prob-ably due to withdrawal of magma from an underlyingchamber. These fault blocks are bounded on the southby a north-dipping fault of variable strike and dipalong which the volcanic rocks have apparently sliddown toward the north across underlying Paleozoicrocks. This fault was earlier considered by the pres-ent writers (1961b) to be a thrust fault, before thegeneral pattern of subsidence related to the calderawas recognized. Ransoms (Ransoms and others, 1910,p. 101-102) favored the interpretation of this fault asa thrust, but he considered that normal movementwas also a possibility.
The rocks in the caldera were still further deformedwhen magma re-entered the underlying magma cham-ber at a late stage and pushed the basement of Pre-cambrian and Paleozoic rocks up into the volcanicsequence (p. 33; pi. 1, section B-B').
The youngest extrusive rocks of the Bullfrog Hillsvolcanic sequence are latite and quartz basalt flows,mainly in the eastern part of the caldera. Thesegently dipping flows in part unconformably overliethe older steeply dipping volcanic rocks (p1. 1, sec-tion B-B') but they, too, are deformed, though to alesser degrees, in the same general pattern as theolder rocks. It thus appears that the major part ofthe deformation, owing to the subsidence of the cal-dera, occurred before the latest volcanic event,namely, the extrusion of the latite flows.
ORE DEPOSITS ASSOCIATED WITH TUE CALDERA
The four most prominent known ore deposits in theBullfrog and Bare Mountain quadrangles, as well asthe majority of the smaller deposits, are located alongmarginal faults of the Bullfrog Hills caldera, or nearthe related area of subsidence that extends outwardfrom the southeast rim of the caldera (pls. 1, 4).Three of the deposits, the Montgomery-Soshone, May-flower, and Pioneer (one-half mile north of the May-flower) gold-silver mines, are on the east rim of thecaldera. The fourth, the Daisy fluorspar mine, occursin Paleozoic rocks adjacent to the southern margin ofthe subsidence zone that extends tangentially out-ward from the caldera toward the northeast.
The greatest concentration of mineral prospects,mostly gold-silver, also occur along the east rim ofthe Bullfrog Hills caldera near the Montgomery-Soshone mine. In addition, several small gold de-posits, including the initial discovery of the district,the original Bullfrog mine, occur within the calderaalong the north margin of the area of pre-Tertiary
basement rocks that have been pushed up into thetuffs and ash flows.
Mineral exploration has been carried on actively inthe Bare Mountain and Bullfrog quadrangles since1904 when gold was discovered at the Original Bull-frog mine (Ransome and others, 1910, p. 12) at thesouth end of Bullfrog Mountain. The total productionof gold-silver ore is valued at nearly $2 million. Mostof the ore has come from the Bullfrog district in theBullfrog Hills west of Beatty and most of the miningwas prior to 1910 (Kral, 1951, p. 29).
Fluorspar is the only other mineral commodity thathas been produced in significant amounts in the twoquadrangles under consideration. Since the discoveryof fluorite in 1918, production of somewhat more than100,000 tons has come mostly from the Daisy mine atthe north end of Bare Mountain. In addition to theseminerals, a small production of bentonite, mercury,and pumicite has been recorded.
LDVOHBPA3
Several fluorspar deposits have been explored in thePaleozoic carbonate rocks of Bare Mountain. Thesedeposits have been briefly described by the presentauthors elsewhere (Cornwall and Kleinhampl, 1961b);only those at the north end of Bare Mountain near,and possibly related to, the caldera subsidence struc-ture mentioned above will be discussed here. Partof these deposits have been described by Thurston(1949). By far the largest deposit occurs in the No-pah Formation, a dolomite of Late Cambrian age, atthe north end of Bare Mountain in Fluorspar Canyon(p1. 4). This deposit, at the Daisy mine, is describedin detail below. A small prospect, the Enif, occurs aquarter mile west of the Daisy in the same formation.A shaft was first sunk in the Enif deposit in 1906 ina search for gold; the small amount of fluorite foundwas not of interest at that time. In 1918 the Conti-nental Fluorspar Company found fluorite on the Daisyclaim; 1,300 tons of fluorspar were mined betweenthen and 1922. Another fluorite deposit occurs inlimestone of the Carrara Formation (Lower and Mid-dle Cambrian) three-quarters of a mile south of theDaisy mine.
All the fluorite deposits have similar characteristicsand are undoubtedly related in origin. They occur inhighly deformed and fractured dolomite or limestonenear major faults. The fluorite ranges in color fromwhite to yellow, purple, or nearly black, but most ofit is purple. It is commonly intergrown with yellowor brown clay from the faults with which the depositsare associated. Really large bodies of nearly puremassive fluorite are known only in the Daisy deposit.
J20 0SHORTER CONTRIBUTIONS TO, GENERAL GEOLOGY
DAISY MINE
The Daisy fluorspar mine is by far the largest ofthe known fluorspar deposits in the Bare Mountainarea. It is located at the north end of Bare Mountain5 miles east of Beatty in Fluorspar Canyon, in theNWl,4, sec. 23, T. 12 S., R. 47 E. As mentioned above,the Daisy mine was one of several fluorspar prospectsthat were explored during the period 1919-22 by theContinental Fluorspar Company, headed by J. IrvingCrowell. In 1927 the claims were acquired by J.Irving Crowell, Jr., and the Daisy mine has been ac-tively operated since that time. The deposit has beendeveloped to a depth of over 500 feet and for a horizon-tal length of 900 feet in a maze of workings (pl. 6)that includes 14 levels and sublevels.
GEOLOGIC IfTMG
The Daisy fluorspar deposit occurs in dolomite of theNopah Formation of Upper Cambrian age in an areaof great structural complexity. The Paleozoic rocksin the area have been intensely deformed by repeatedthrust faults, locally to the point of imbrication, andby steeply dipping right-lateral strike-slip -faults.Rhyolitic pyroclastic rocks of probable Miocene ageare faulted against the deformed Paleozoic rocks lessthan a quarter of a mile north of the Daisy mine.The Tertiary rocks are also deformed, but the pat-tern of deformation is quite different from that in thePaleozoic rocks.
As has been discussed earlier (p. J15J17), the prin-cipal deformation of the Paleozoic rocks occurred dur-ing a major period of thrusting and right-lateralstrike-slip faulting along the Las Vegas Valley shearzone, probably in the Cretaceous. The thrust faultsdip gently to moderately north and northeast at thenorth end of Bare Mountain and the major right-lateral strike-slip faults strike nearly north-south.The Miocene pyroclastic rocks are cut by a series ofnortheast-trending normal faults which have, for themost part, dropped the rocks successively toward thenorthwest. This subsidence structure, describedabove, is bounded on the south by an undulating nor-mal fault, variable both in strike and in dip, that sepa-rates pyroclastic rocks from the Paleozoic rocks tothe south in which the fluorite deposits occur. Thisfault was earlier mapped and described as a thrust(reverse) fault by the present writers (Cornwall andKleinhampl, 1961b) before they recognized evidence ofsubsidence in the Tertiary volcanic rocks.
STIVC==Z 0r TEz OBZ Bz1Oss
The shape and distribution of ore shoots in the flu-orite deposit appear to be controlled in large part by
two principal sets of faults. One set strikes roughlynortheast and dips vertically to steeply east. The oreshoots extend along and are bounded laterally byfaults of this set. In detail the individual faults un-dulate quite markedly, both in plan and in section,and the fluorite bodies bounded by such faults ac-cordingly range in thickness from less than 1 foot to amaximum of 80 feet. Examples of these patterns areseen both in the plan maps (pls. 7, 8) and in the sec-tions (pl. 9.) Level 3 on plate 7 and the upper fourlevels on plate 9 are modifications of mapping doneby Thurston (1949). The shoot with the maximumhorizontal thickness of 80 feet occurs on and adjacentto level 6.
The other set of faults that control the distributionof ore strikes more or less northwest and dips gentlyto moderately northeast These faults, like the firstset, undulate markedly in strike and dip. There areonly a few of these faults, but their control on the dis-tribution of ore appears to be quite fundamental. Thethree most important of these faults are best seen inthe sections (pL 9), particularly in section A-A'.These faults do not bound the ore shoots quite asabruptly as is indicated on the somewhat generalizedsections. For example, the fault that limits the baseof an ore shoot in the upper four levels and the top ofa larger shoot in levels 5 through 8 (pl. 9, section A-A')has some ore below and adjacent to it on level 3 andsome above it on level 8. The second of these faultsbounds the bottom of the larger shoot on the interme-diate levels and also limits the base of another shoot atthe southwest end of levels 3 and 5. On levels 5 and 8a little ore occurs below it. A pipelike shoot extendsdownward from this fault to level 18 along the southwinze (pl. 9, section A-A'). It was by following thisshoot, in places not much larger than the winze, thatthe large ore shoot on level 18 was discovered. Thethird important low-angle fault of this set bounds thetop of the shoot on level 18.
These low-angle, northeast-dipping faults definitelycut off the northeast-striking steep faults in places, butelsewhere they appear to bend and branch into thesteep fauts It was not possible to determine the di-rection of displacement along any of the major faults.
The available evidence indicates that most of thefaulting occurred before the fluorite mineralization andthat the zones of fractured rock along the steep, north-east-trending faults served as channels for the oresolutions. The ore shoots are almost everywherebounded by gouge zones of faults, and it appears thatthese impermeable zones restricted the ore solutionsto definite channels where the fractured dolomite wasalmost completely replaced by the fluorite-bearing
solutions. As has been indicated by the breccia patternon the level maps (pis. 7, 8), quite a bit of the ore con-tains scattered angular fragments, and these fragmentsare mostly dolomite as much as a foot in diameter butmostly less than 2 inches. Locally the wall rock islimestone or shale and these may also occur as frag-ments in the ore in such areas. Small fragments ofclayey gouge are also sparingly present, and the fluo-rite itself is locally fragmented. The lower part of thedeposit below level 8 contains less fragmental ore thanthe upper levels. The local presence of fluorite frag-ments indicates some fault movement during the min-eralization process. The matrix of all the brecciatedore is mostly fluorite.
sNATUR oF TsI O3
The ore consists mostly of fine-grained purple flu-orite with seams, lenses, and layers of yellow clayeygouge. The ore is partly banded parallel to the faultsthat bound the shoots. The banding is due to the al-ternation of light-purple fluorite, dark-purple fluorite,and scattered seams and layers of yellow gouge, andalso to the alternation of dense and vuggy layers.Most of the fluorite appears aphanitic, but part of itis visibly crystalline. The aphanitic fluorite actuallyconsists of tiny crystals less than 0.1 mm in diameter.In places the fluorite has a comb structure due to thegrowth of elongated crystals outward from the wallsof cavities.
In the lower levels the fluorite is partly to largelywhite or yellow with a fine granular texture Locallyit tends to be loosely consolidated and may flow likegranulated sugar when struck with a pick. Calcitefissures as much as 3 feet wide are rather common inthis ore, particularly where the ore pinches out alongthe strike. These calcite veins are vuggy, even cav-ernous, and the vugs are lined with yellow or whitefluorite, clear calcite, quartz crystals, and locally finecrystals of cinnabar.
X-ray study of the yellow clayey gouge that occursas seams or layers in the fluorite shows that it is mont-morillonite. The shale member of the Nopah Forma-tion that locally is adjacent to ore consists, on theother hand, of illite and very small amounts of mont-morillonite and chlorite. The purple and white vari-eties of fluorite were examined with the X-rayspectrograph and found to be nearly pure except forsmall amounts of iron in the purple fluorite and a traceof iron in the white fluorite. The iron may be due tosmall amounts of limonite in the fluorite sample.
The radioactivity of the Daisy fluorspar deposit hasbeen investigated by Chesterman and ain (in Lover-ing, 1954, p. 91-92). Six channel samples of the
purple fluorite ranged from 0.007 to 0.015 percentequivalent uranium, and a sample of mill concentrateran 0.002 percent. These values, while high enough tomerit consideration, are probably not recoverablecommercially. For comparison, the fluorspar de-posits of the Thomas Range, Utah, which are consid-ered to be high in uranium for this type of deposit,range from 0.003 to 0.33 percent uranium (Staatz andOsterwald, 1956). The highest values are apparentlydue to secondary enrichment of uranium near thesurface.
The present writers, using a scintillation detector,mapped the distribution of radioactivity throughoutthe Daisy mine. The fluorite ore has a radioactivityof one to two orders of magnitude greater than thebarren dolomite wallrock. The highest values werefound in purple ore that contains appreciable claygouge or.occurs adjacent to shale, indicating that theradioactive material is concentrated in the clay.
12O0DCZO1
The total production from the Daisy mine, given intable 3, amounts to 118,000 short tons through 1961.Since 1945, production has been at a rate of about5,000 tons per year. The grade of the ore has rangedbetween 70 and 80 percent CaF, and the average isprobably close to 75 percent. The SiO, content hasaveraged less than 2 percent. The remaining mate-rial in the ore consists of calcite, dolomite, and clay(fault gouge).
The largest tonnage of ore occurs in the shoot thathas its greatest width and length on levels 6 and 8and extends downward at the northeast end of themine to below level 12, but the most promising areafor finding new ore is at the southwest end of the minein the lower levels where a large recently discoveredore shoot is being developed at the present time.
Mr. Crowell, the owner and operator of the Daisymine, has done a remarkable job of following the ir-regular, lenticular ore shoots downward, and of find-ing new shoots. His exploration technique of stayingin ore as much as possible has been very successful.In 1945 an exploration was carried out by the Bureauof Mines (Geehan, 1946) in the Daisy mine area.Twelve diamond-drill holes were drilled but no orewas found. The mine workings were mapped geolog-ically by W. R. Thurston (1949) of the U.S. Geolog-ical Survey as a part of this exploration program.
OIZLXX OF TEZ DXPOSM
The Daisy fluorite depost is believed to be a hydro-thermal deposit from ascending solutions that movedalong permeable zones in the highly deformed andfractured Paleozoic rocks. The ore channels andsites of deposition were determined by the spatial ar-rangement of two sets of faults and fissures, one steepand northeastward trending, the other flat and north-westward trending: Impermeable gouge along themore important faults apparently confined the ore solu-tions to certain zones where the CWFs from the solu-tions almost completely replaced the dolomite andminor limestone. Fluorite was also deposited in cavi-ties that resulted either from brecciation during thedeformation or from solution of the carbonate rocksby the migrating hydrothermal solutions. Calcite andsmall amounts of quartz and cinnabar were also de-posited in cavities.
The ore-bearing solutions were probably related tothe Tertiary volcanism. that erupted large volumesof rhyolite ash flows, tuffs, and flows immediatelynorth of the Daisy mine. The volcanic rocks, as men-
tioned above, are in an area of subsidence that prob-ably was underlain by a magma chamber from whichthe ore solutions may have come.
It was noted above that locally fragments of fluoriteoccur in a fludfite matrix and this indicates deforma-tion during the ore-forming period, but the deforma-tion must have been very minor as the amount of suchbreccia is small. The principal deformation in themine area is clearly related to the thrusting andstrike-slip faulting elsewhere in the Paleozoic rockson Bare Mountain and must be pre-Tertiary becausethe Tertiary rocks were not involved.
GOLD
The early prospectors roamed the Bullfrog Hillsand Bare Mountain looking for gold, and it was foundin 1904 at the Original Bullfrog mine, 7 miles west ofBeatty (Ransome and others, 1910, p. 12; Lincoln,1928, p. 162; Kral, 1951, p. 28). A rush of prospectorsfollowed and by 1905 a number of claims were beingexplored. Most of the gold showings were found inthe pyroclastic rocks of the Bullfrog Hills, but severalprospects were explored in the Paleozoic rocks on BareMountain.
Recorded gold and silver production through 1948amounted to $1,886,000 (Kral, 1951, p. 29). Most ofthe production came before 1910 and from the Mont-gomery-Shoshone mine, which yielded 128,980 tons ofore valued at $1,844,000. The total production fromBare Mountain is not known but small;
The gold deposits in the Bullfrog Hills are in fis-sumres and veins related to normal faults. These de-posits have been described in detail by Ransome(Ransome and others, 1910, p. 90-125) and will onlybe briefly described here. Most of the gold-bearingfissures, including the Montgomery-Soshone, are steepand occur at or near the rim of the Bullfrog Hills cal-dera, and this relationship is probably more than co-incidence. Several other deposits, including theOriginal Bullfrog mine, occur along the north contactof the central domal uplift of basement rocks into theTertiary pyroclastica. This contact is a low-angle,north-dipping fault.
The mineralogy of the gold fissure deposits is simpleand consists of quartz, calcite, and finely disseminatedgold-silver in scattered grains of pyrite. Near thesurface the pyrite has been altered to limonite. TheOriginal Bullfrog mine also contains a little chalcocitethat has been oxidized to malachite and chrysocolla.Cerargyrite has been detected but is apparently notprominent even in rich ore. The production record in-dicates that for the district as a whole the ratio of sil-ver to gold in the ore is 8 to 1 (Lincoln, 1923, p. 162).
BULLFROG QUADRANGLE AND BULLFROG CALDERA, NEV.-CALIF. J23
The mined ore averaged about $10 per ton (Kral,1951, p. 29).
The gold-silver mineralization of most of the fissuresis meager, even though hydrothermal alteration ofthe rhyolite may be pronounced; but in theMontgomery-Soshone mine a sizable body of fracturedrhyolite on the footwall (southeast) side of the3fontgomery-Soshone fault averaged $10 per ton ingold-silver. This ore body occurred near the surfacein a fractured zone near the fault where numerous,nearly vertical, north-striking fissures intersect thenortheast-trending fault (Ransome and others, 1910, p.97). The MIontgomery-Soshone fault is a northwest-dipping, steep normal fault with an apparent down-ward displacement of 3,500 feet on the north side.
Most of the other gold-silver prospects in the Bull-frog Hills are similar to but leaner than theMontgomery-Soshone deposit and occur along steepnormal faults near the rim of the caldera. Two ofthe most promising deposits, other than theMontgomery-Soshone, are the Mayflower and Pioneermines on the northeast rim of the caldera. The May-flower deposit occurs along a fissure or fault thatstrikes N. 50° W. and dips 60° to 650 SW. (Ransomeand others, 1910, p. 124). The Pioneer deposit occurshalf a mile north of the Mayflower mine (just northof the north border of the Bullfrog quadrangle, whichis shown on plate 1) and is said to be "almost identicalto the adjoining Mayflower" (Kral, 1951, p. 39).
One of the gold prospects on Bare Mountain is lo-cated 2 miles northeast of the Daisy fluorspar mine(pl. 4), near the subsidence feature associated with theBullfrog Hills caldera described earlier, and may berelated to it. This mine, called the Harvey (formerlyknown as the Telluride), was first prospected for goldin 1905 and later mined for mercury. It will be de-scribed under quicksilver.
ORIGIN O TNI DhOhTS
It has been pointed out that most of the gold depositsin the Bullfrog Hills district occur either along steepnormal faults near the rim of the Bullfrog Hills cal-dera, or near the domal uplift of basement rocks intothe Tertiary pyroclastic rocks within the calders. Itis probable that the ore-bearing solutions were derivedfrom the magma chamber that presumably existedbeneath the caldera. The mineralization must haveoccurred late in the period of volcanism after thedevelopment of the structures to which the depositsare related. The probable age thus is late Mioceneor Pliocene.
3BRNTONITR
A small bentonite deposit at the Vanderbilt minehas been operated for over 10 years by the Silicates
Corporation; it is located 12 miles south of Beatty(p]. 4). Two bodies of bentonite occur 300 feet aparton the footwall side of a fault that dips 50° W. Mostof the production has come from the larger deposit,which is shown in figure 12.
The bentonite was formed by the alteration of denselywelded and nonwelded tuff of cooling units Nos. 4 and5 respectively of the ash-flow sequence in the BullfrogHills caldera. The bentonite occurs in a zone of in-tense fracturing and faulting and apparently resultedfrom the activity of hydrothermal solutions that mi-grated through this permeable zone. The deposit oc-curs at the south edge of the subsidence zone thatextends eastward from the Bullfrog Hills caldera.
The high-grade bentonite ore is soft and white andhas scattered waxy pink or tan spots which probablyrepresent replaced pumice fragments The originalunaltered rock had rather abundant phenocrysts ofquartz, sanidine, oligoclase, and biotite, and theseare still present in the bentonite ore but the feldsparand biotite are moderately to intensely altered. Inpart the contacts between bentontinte ore and unal-tered welded or nonwelded tuff are sharp, but else-where a zone of moderately altered rock forms atransition. X-ray analysis shows that the bentoniticclay, both the white rock and also the pink fragments,is nearly pure montmorillonite.
QUIClamI
Cinnabar (HgS) was discovered in 1908 at the northend of Meiklejohn Peak (NW1/4 sec. 18, T. 12 S., R.48 E., unsurveyed) in the Bare Mountain quadrangle.Quicksilver production from this property, the Harveymine, was recorded as 72 flasks up to 1943 accordingto Bailey and Phoenix (1944, p. 142). The depositwas mined again for about a year in 1956, but theamount of production is unknown and probably small.The mercury occurs as cinnabar sparsely disseminatedin a lens of chalcedony and opal along a steeply dip-ping fissure in dolomite of the Fluorspar CanyonFormation of Devonian age.
Another small mercury deposit known as the TipTop mine, is located 600 feet north of the Harvey minein the Lone Mountain Dolomite of Silurian age. Pro-duction here is reported as possibly about 100 flasksof quicksilver (Bailey and Phoenix, 1944, p. 144).The cinnabar occurs along a southwest-trending,nearly vertical fault and is in 1- to 2-inch veins andalso disseminated in the gouge.
PUNITM
A moderate amount of pumicite was quarriedaround 1950 from pumiceous tuf located 3 milesnortheast of Beatty (SEY4 sec. 28, T. 11 S., R. 47 E.)
324 SHORTER CONTRIBCTIONS TO GENERAL GEOLOGY
N
10 0 10 20 30 40 FEETI . I I I I I
45~~4
EXPLANATION
Welded tuft
Nonwelded tuft
Ore
50
Contact. showing dip
t40__0
Fault, showing dipDodied where oppmirmoueey
located
30
Strike and dip of beds
MHead of winze
Foot of raiserioaus 12.-4eologto plan of the Vanderbllt bentonite mine, Nyt County, Nov.
near Nevada Highway 95. The pumicite was used tomake lightweight aggregate building blocks accordingto Kral (1951, p. 68).
PERBITE
Several large bodies of perlite have been prospectedbut no production is reported. One occurs in BeattyWash (SW¾4 sec. 25, T. 11 S., R. 47 E.); another is lo-cated in the NE¼4 of sec. 10, T. 12 S., R. 47 E. AUof these perlite bodies are glassy facies of rhyoliteflows or intrusives.
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19Clb, Geology of the Bare Mountain quadrangle. Ne-vada: U.S. GeoL Survey GeoL Quad. Map GQ-157,scale 1: 2,500.
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BULLFROG QUADRANGLE AND BULLFROG CALDERA. NEV.-CALIF.
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