GEOLOGY AND ORE DEPOSITS OF THE ROCHESTER DISTRICT, NEVADA

7/28/2019 GEOLOGY AND ORE DEPOSITS OF THE ROCHESTER DISTRICT, NEVADA

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D E P A R T M E N T O F T H E I NTERI ORH U B E R T W O R K , Secretary

UNITED S T A T E S G E O L O G I C A L S U R V E YG E O R G E O T I S S M I T H , Director

Bulletin 762

GEOLOGY AND ORE D E P O S I T SO F THE

ROCHESTER DISTRICT, NEVADAB Y

A D O L P H K N O P F

W A S H I N G T O NG O V E R N M E N T P R INT ING OFFICE

1924


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ADDITIONAL COPIESO F THIS P U B L I C A T I O N M AY B E P R O C U R E D FROM'

T H E S U P E R I N T E N D E N T O F D O C U M E N T SG O V E R N M E N T P R I N T I N G O F F I C E

W A S H I N G T O N , D . CA T

15 CENTS PER COPY


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C O N T E N T SPreface , by F . L. Ransome____________________________________________Outl ine of the r e p o r t _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _Part I. General features_____________________________Geography. ___________ __________________.______.Situation of the district ________________________Physical features _______________- ________________ 1Cl ima te an d vegetation____ ________________ 3History of mining______________________________ 4Output of silver and gold_______ ________________ f iGeologic inves t igat ions of the district-^_________________ 7Bibliography____________________________________ ! )

Geology of the Star Peak R a n g e______________________ 9Part II. General geology of the district____________________ 1 3Triass ic rocks _______________________________ 1 3Introductory outline________________________- 13Roches ter trachyte____________________________. 14General features____ ______ 14Petrography______________________________. 1 4Thickness an d age________ _ ________ 1 7Dumort ier i t ized trachyte _____ _____________ 18Keratophyres________________ _______________ 20Occurrence an d character___________________. 2 0Petrographic features__ __________________ 2 0Relat ion of keratophyres to the Roches ter trachyte____ '- 2Age an d correlation_________________________ 22Nenzel rhyoli te breccia_____ _ _ __________ 23Occurrence an d character. _ _ _________ 2 3Stra t igraphy an d structure____________________ 23Rhyolite tuffs_____________________________ 2 6Weaver rhyolite_______________ _____ 2 0Occurrence an d character __ 2 UPetrography_______ _ 2 7Tuffaceous shales____ _ 28Limestone____________ __ 2 9General features____ __ 20Age__________________________________ 2 9Relat ion of l imes tone an d volcanic rocks _ 30Metadiori te _______ - 3 1Pos t -Triass ic , probably late Jurassic igneous rocks __ 3 2Aplite______________ - 3 2Genera l features_____ ; > 2Petrography_____ 3 2Genetic relations____ --_ 3 $Grani te porphyry __ '> *Tertiary (? ) rocks 3 5Pliocene (? ) alluvial -cone detritus ___- 35Occurrence an d character - 3 5Age__ 3 6Basalt______________ - 3 6

Quaternary alluvium - 3 7in


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IV CONTENTSPart II. Genera l geology of the district Continued. PageGeologic structure___ ____ _______________ 3 8Folding an d faulting__________________________ 3SSchistosity____________________________'___ 4 0

Physiographic history__________________________,__ 4 1Part III T he ore deposits___________________________ 43General features _______________________________ 4 3Relat ion of ore deposits to water level_________________ 43Silver veins__________________________________ 4 4Dist r ibut ion and genera l character_________________ 4 4Nature of th e ore________________-_ _______ 4 3E n r i ch m en t by supergene argentite________________ 4 0Wall-rock alteration_________________________ 5 0Gold veins___________'_____________________ 5 1Genera l character.. _____ :___________________ 5 1Nature of the gold ore- forming solutions_____________ 5 2Other mi n e ra l deposits__________________________ 5 3C i n n a b a r _______________________________ 5 3Miscel laneous deposits_______________________ 5 4Genesis of the ores of the district___:________________ 5 5Placers ________________ _________________ 5 8Part IV . Mines an d prospects_________________________ 5 9Silver m i n e s an d prospects_________ ______________ 5 9Eoches t e r mine_____________________________ 5 9Bonghton & Hackley mne_ ____________________ 64Nenzel Grown Poin t property_________ ________ 65Buck & Charley mine ______ ____ _________ 66Octopus mne_______ ___ ________________ . 67Raven prospect____________ ________________ 68A be Lincoln mne__ --__ _ 68Kaise r Bill prospect_______________ _________ 68Nevada Packard mne , 69Gold mines an d prospects 7 4Lincoln Hill mne__ 7 4Oro Fino mne____ ___ ____ _ 7 5Hagan prospect________________________ 7 5Index ______________ - - 7 7

ILLUSTRATIONSPageP L A T E I. Geologic m ap of th e Roches te r district _ __ _ 22II. A, B, Silver-bearing stockwork at the Nevada Packard mne_ 4 6

III. A, Incipient r ep lacement of sphaleri te by supergene argentite;B, Advanced r ep lacement of sphaleri te by supergene argent i te 4 7IV . M ap of Nevada Packard m i n e workings-____ __ 7 2F I G U R E 1 . Index map showing the location of the Roches te r district__ 22 . Di ag r am m a t i c section through the south en d of th e StarPe'ak Range____________________________ 123 . Geologic section through the nor th en d of Nenzel Hill____ 2 44 . Geologic section from portal of Boughton & Hackley tunne lto nor theas t corne r of North C r o w n Frac t ion claim________ 655 . D i a g ra mma t i c sect ion through th e Nevada Packard m i n e. along th e l ine of t unne l A___ 7 2


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PEEFACEB y F . L. RAN S O M E

The modern development of the Rochester district dates from thelater part of the year 1 9 1 2 . Early in 1 9 1 3 F. C . Schrader w as detailed b y the United States Geolog ica l Survey to make a reconna i s s ance study of the district, an d his brief examination w as fol lowedb y an excel len t report, w h i ch w as published in 1 9 1 5 . The c o n tinued productivity of the Rochester m i n e s and the interesting character of the o re deposits m a d e it evident that m o r e detailed geologicwork w as des i rable , and after the completion of a topographic map,in 1 9 1 6 , M r. Knopf w as instructed to make a geolog ic study o f thedistrict early in 1 9 1 7 . The preparation of his report, like manyother investigations b y the Geolog ica l Survey then in progress, w asinterrupted by the urgent war demand for information in regardto particular mineral resources and w as further delayed b y M r.Knopf's withdrawal from full-time participation in the work of theSurvey and his acceptance of a professorship at Yale University.His n ew duties left only a s m a l l part of his time available for theinvestigation that he has n ow satisfactorily comple ted .A s M r . Knopf shows , the o re deposits of the Rochester district aremmeralogically of unusual character. Dumortierite, a rare a luminum boros i l ica te , is in s o m e parts of this district extraordinarilyabundant and is acco m pan i ed b y tourmaline and andahisite minerals,whic h also are rather unusual in associa t ion with silver deposi ts . Theboron-bearing minerals w e re evidently formed at high temperature,and M r. Knopf concludes that there w er e three distinct s tages inthe deposition of these minerals. The principal silver-bearing veinsw e re probably formed later and at l o w er temperature. The reports h o ws that the original vein material w as of too lo w grade to be ore.The district o w e s its e c o n o mic d e ve lop me n t to downward enrichment,effected largely b y the replacement of sphalerite by argentite aprocess that is fully discussed in its appropriate, place. It fol lowsthat the silver deposits of the Rochester district are not likely toextend to great depth. The s a m e conc lus ion , based apparently on theobserved decrease in tenor o f the o re on the deeper m i n e levels ,finds! express ion also in the annual report of the Rochester SilverCorporation for 1 9 2 1 . M r. Knopf has given the explanation forthe obse rved decrease .


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VI PREFACE

The fol lowing report, although it deals with only a smal l a rea ,throws considerable n ew light on the geology o f the HumboldtR a n g e . The r ange has long been n o ted as a collect ing ground forTr iass i c fossils, b ut o ur knowledge o f the beds in which these anc i en tm a r i n e shells are found left m u c h to be desired. M r. Knopf has n o tonly supplied s o m e o f this deficiency but has shown that the light-colored felsitic rocks o f the Humboldt R a n g e , which in ear ly reportswere regarded as metamor p hosed sed iment s an d later were c lassedgenera l ly as rhyolites , include rocks be lon g in g to the group oftrachytes a gro up that is rather sparsely represen ted in NorthAmer i ca ,


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OUTLINE O F THE REPORTRoches te r is a s i lver -min ing district in the Humbold t Range of wes te rnNevada. Silver w as discovered there in 1 9 1 2 , an d to the en d of 1 9 2 2 thedistr ict produced 7 , 000 , 000 ounces of silver. It is second in yearly outpuj; a m o n gNevada ' s m a n y silver districts , but it ranks far below Tonopah, the leadingdistr ict .T he prevail ing rocks of the district are a thick series of Triass ic volcanicrocks overlain by l imes tone of the s a m e age. T he volcanic rocks consi s t offelsitic t rachyte and keratophyre , aggregat ing 10 , 000 feet in thickness , an d anoverlying series of rhyolite, 1 , 0 0 0 feet thick. Above th e igneous rocks lies thel imestone, of which only 2 , 000 feet is exposed in the dis t rict , al though elsewhere in th e Star Peak R a n g e it apparently attains a far greater thickness.A t or n e a r the end of the Jurassic period this ser ies of volcanic rocks an dl imes tone c a m e within the influence of the g r ea t revolution an d crus tal disturbances that affected the western Cordi l leran region at that t ime . A longtrain of events started then, which culminated in the form at ion of the ore depositsof the distr ict . First the rocks were folded into a broad ant icline , which const itutes one of the ma jo r structural features of the r ange . This arching of the

rocks w as followed by the intrusion of grani t ic m a g m a , which within thedistr ict proper is represented by f ine-grained whi te gran i te , or aplite. T hebodies of aplite are comparatively large for rock of this kind, an d their sizeimplies, therefore, a notable am ou n t of differentiation of th e magma f rom whichthe aplite w as derived. T he last rocks to be intruded during this period ofigneous activity a re gran i t e porphyry dikes. Thereupon began a period ofactive circulation of highly heated solutions , at first so hot as to be gaseousbut later liquid. Doubt less these solut ions were expelled from the cooling an dconsol idating m a g m a in depth. T he minera l s produced by these solutions in dicate three s tages in these so-called post- intrusive processes one in whichdumort ie r i te (an aluminum borosi l icate) is the dis t inct ive m i n e r a l that w asformed, a second in which gold ores having tourmal ine-bear ing gangue were deposited, an d a third in which si lver protores having tourmal in ic quar tzgangue were deposited, which shade into silver protores without tourmal in icgangue. These three s tages , which are in terpreted as being successive in t imean d at t empera ture s successively lower, were discontinuous, as gradat ions between the deposits formed during the successive s tages do n o t occur.During the first s tage a large body of t rachyte on Lincoln Hill was shatteredan d interlaced with veinlets composed of pink dumort ie r i te and quar tz . T het rachyte adjoining th e veinlets w as in tensely altered an d converted into anaggregate of andalusi te an d quartz. T he ^umortierite-bearing m a s s thus produced appears to exceed in size an y other known . It is notable that no preciousmeta l s o r sulphides were deposited during this stage; possibly the t empera tu reof the solut ions was to o high to a d mi t of their deposition. During the secondan d third s tages the gold ores an d the s i lver protores were deposited, an d fromth e silver protores were later developed by supergene enr ichment the m o s tvaluable deposits in the district. These wil l be considered later at greaterlength; only their relat ion to the post -Jurassic revolution an d accompany ingigneous activity is here emphasized.


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VIII OUTLINE OP THE BEPOKTA long period of erosion succeeded the post - Jurass ic revolution. A depositof bouldery alluvial -cone d6bris at leas t 600 feet thick records the next eventin th e history of the district. It is probably of Pliocene age an d resulted fromthe initial uplift of the Star Peak R ange , o r northern subdivis ion of theHumbold t R ange , and the consequent a m a s s i n g of alluvial cones a long the faultscarp of the newly fo rmed range. Later this material w as covered, in part atleast, by flows of basal t . B y the faulting an d tilting of these basalt sheetsthe Humbold t Lake Range , or southern subdivision of the Humbold t Range , w asproduced, as has been so ably demons t r a t ed by Louderback. T he Star PeakRange was further uplifted, probably at this s a m e t ime , an d gradual ly attainedi ts present al t i tude. Physiographical ly the fault-block origin of the r an ge isshown by the clean-cut triangularly face ted escarpment along the w e s t frontan d by the fact that the canyons , which a re acutely V-shaped at the wes te rnfront of the range, broaden widely upstream a feature shown particularly

well by Limer ick Can yon . During the uplift of the r ange as a whole a cer ta ina m o u n t of f ragmenta t i on took place, an d the m o v e m e n t an d read jus tmen t ofthe m i n o r blocks produced fault- trough valleys, the m o s t notable example ofwhich is the valley on the eas t flank of Nenzel Hill. T he upward m o v e m e n t onthe fault surfaces w as slow enough to permi t sma l l s t r e am s to main ta i n theircourses across the r ising fault blocks. In this way were produced suchr emarkab ly anomalous s t ream condit ions as those of Amer ican an d SouthAmer ican canyons . South Amer ican C a n y o n cuts through the Black Range atright angles in a nar row V-canyon , 2 ,000 feet deep, an d taps the broad, openlongi tudinal valley on the eas t side of Nenzel Hill. T he physiographic condit ions a re s imi lar to those shown in the rift valleys of the Cal i fornia C o a s tR anges , though on a smal ler scale an d with more rugged relief.T he principal ore deposits of the Roches ter district are silver-bearing quartzveins and stockworks. They belong to the relatively rare class in which th eore is valuable chiefly fo r its con ten t of silver. T he silver-bearing quartz veinsfurni sh the greater part of the output of the district, an d they are mos t numer ous and productive on Nenzol Hill, at the head of Roches te r Can yon . T hest 'ockworks are represen ted only by the deposits at Packard.T he silver-bearing quar tz veins are inclosed in the Triass ic rhyol ite andtrachyte. T he ore consis ts of quar tz that as a rule i s exceedingly fine gra ined.T he chief silver m i n e r a l is finely disseminated argent i te . Other metall ic m i n erals are pyrite, sphalerite, galena , tetrahedrite, covellite, and chalcopyri te , butin the aggregate the a m o u n t of a ll these minera l s is small . Although quartz isthe only gangue m i n e r a l in m o s t of the veins, s om e conta in no t iceable quant i t ies of th e s ignif icant m i n e r a l tourmal ine .T he average conten t of th e ore n ow m i n e d is from 1 0 to 12 ounces of silverto the ton. In ear ly days , when the upper port ions of the veins w e r e beingmined , the ore carried from 3 0 to 6 0 ounces to the ton. Although the highergrade of the ore then mined w as in part due to sor t ing , nevertheless the s ilvercon ten t has great ly declined as greater depth has been at ta ined, and the bottomof the ore has been -reached. The ex tr em e vertical r ange of ore is be tween 60 0an d 8 0 0 feet. W hy the silver content has s teadily decreased in the ore of thedeeper levels has become clearly apparent during the course of the presen tinvest igation. T he argent i te , which determines the s i lver tenor of the ore, an dhence its commercial value, is of supergene ( secondary) origin , having beenformed as the result of the reaction between the primary sphaler i te of theunoxidized ore an d silver sulphate that had been carr ied down by descendingsurface wa t e r from the outcrops of the oxidizing veins. T h e a m o u n t of argentite consequently diminishes s teadi ly wi th increas ing depth. T he source of the


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OUTLINE O F THE REPORT IXs i lver by whi ch the ore has thus been enr iched is doubt less the sparse tetrahe-drite of the primary-vein filling. O n oxidat ion this mineral yielded soluble compounds - o f s i lver an d copper, an d these substances trickling down in solut ionthrough veins reacted with the primary sulphides, chiefly the sphalerite, andproduced the super-gene argentite an d the m o r e abundant covelli te.T he remarkable s i lver -bear ing s tockworks at Packard cons i s t of Trlassicrhyolite traversed by widely spaced, nearly im percept ible vein le ts . The silveroccurs as finely disseminated cerargyrite (horn silver), which has probablybeen f o r m e d at the expense , of supergene argentite. T he s t ockwork o re bodiesare large shallow concentrations, n o w h e r e extending m o r e than 3 0 feet belowthe surface.It is obvious from the preceding accoun t that the silver ores are the resultof the supergene enrichment of material originally containing so little silveras to be commercia l ly valueless . This unenriched vein filling, too lean to beore the protore , to use R a n s o m e ' s term was fo rmed largely b y the replacement of sheeted an d shattered zones in rhyolite and trachyte, a m o d e of originwhich accounts for the prevailing f ine-grained, in places nearly chalcedony-like character of the quartz gan gue . In fact, this f ine-gra ined quartz m i g h treadily sugges t that the Rochester veins , though inclosed in Triassic rhyoliteand trachyte, are o f Tertiary age, like so m a n y of Nevada's s i lver deposi ts .However , the tourmaline-bearing quartz g a n g u e of certain typical s i lver veins ,the remarkable faulting of certain veins by tourmaline lodes, and the profound tourmalinization produced by the aplite intrusions all combi n e toprove that the si lver veins are gene t ica l ly related to the aplite and w e r efo rmed soon after its intrusion.Economica l ly , the m o s t s ign i f ican t fact in connec t ion with the origin of thesilver ores is that their commerc i a l va lue is dependen t on their content ofsecondary argentite. T he deep-level tunnels have cut the veins below thezone of enrichment and have thereby determined the m a x i m u m d o w n w a r dlimit of the g r o u n d in which it is worth while to explore for ore . Futureexploration will therefore be restricted to lateral w o r k on the higher levels,in whi ch cons ide rab le g round still remains to be t es ted .

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G E O L O G Y A N D O R E D E P O S I T S O F T H E R O C H E S T E RD I S T R I C T , N E V A D AB y ADOLPH KNOPF

PART I. GENERAL FEATURESGEOGRAPHY

SITUATION O F THE DISTRICTThe Rochester district as defined in this report is an area of 2 7square mi le s in the southern part of the Star Peak Range, in western

Nevada (fig. 1 ). It is in Pershing County, whic h w as created fromthe southern portion of Humboldt County in 1 9 1 9 . The nearest railroad station and shipping point, Oreana, on the main line of. theSouthern Pacific sys tem^ is 1 2 mi le s distant. Formerly a railroadk n o w n as the Nevada Short Line extended fromOreana to the townof Roches te r , but this line is n ow a ba n d on e d . Lovelock, the countysea t , is the nearest large town and lies 2 5 mi le s southwest of thedistrict.Roches te r , Lower Roches te r , and Packard are the only settlementsin the district. Rochester is at the head of Rochester C a n y o n , at analtitude of 6 , 3 0 0 feet, and is the chief settlement. The t o w n is n owlittle m o r e than the place of re s idence of the employees of the Rochester m i n e . Lower Rochester is 2 mi le s farther d o w n the c a n y o n andcons i s t s of a few houses only . Packard is in the southern part ofthe district and cons is t s of the dwellings of the employees of thePackard m i n e .

PHYSICAL FEATURESThe Star Peak Range, in whic h the Rochester district is situated,

is the northern division of the West Humboldt R a n g e . It is separated from the m u c h lo w e r southern division of that range the divis i o n so m e t i m es called the Humboldt Lake Mountains by a broaddepress ion trending obl iquely to the course of the range. Thisdepress ion is sh o w n in detail in Plate I; in geo log ic publications itis referred to as Col e C a n y o n , but locally this name is given to thec a n y o n transecting the Humboldt Lake Mountains a few miles farther south.

1


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2 ROCHESTER DISTRICT, NEV.The Wes t Hu m b o l d t R a n g e w as so t e r m e d to distinguish it fromthe East Hu m b o l d t R a n g e , the loftiest moun ta in m a s s in Nevada ,which lies south of Wells . T he East Humboldt R a n g e , however , is

FIGURE 1. Index m ap showing the location of the Rochester districtn ow genera l ly k n o w n as the R u b y R a n g e an d the W e s t Humboldts imply as the Humboldt R ang e . Accor d in g to the Fortieth ParallelSurvey , the Indian n a m e for the r a n g e is Koipato ,


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G E O G R A P H Y 3The Star Peak Range is nearly 7 5 miles long and trends meridi

onal . It rises .steeply" a long its regular western front from analtitude of 5 , 0 0 0 feet to an average of 8 , 0 0 0 feet and culminates inStar Peak, 1 0 , 0 0 0 feet above sea level. The southern prong o f therange, which overlaps en echelon the north e n d of the HumboldtLake Mountains, is called the Black Range; it culminates in BuffaloPeak, 8 , 4 0 0 feet above sea level.In the latitude of the Ro c h e s t e r district the Star Peak Range is1 5 m i l e s wide . The district lies mainly on the western slope of therange. Limerick, Roches te r , and Weaver c a n yo n s , opening upon thewestern front of the range, ca n b e said to determine the main to pographic features of the district. N o n e of these c a n y o n s sustainsa permanent stream, the only f lowing water b e in g that furnishedb y the melting of the snow during a few weeks in March andApril. Limerick C a n y o n , 1 the northernmost, affords an easy gradeacross the range and is the site o f the road from Lovelock to Union-ville and other mining c a m p s that lie on the ea s t side of the range.At its mouth the c a n y o n is deep, rugged, and sharply V-shaped ,but headward it wide n s into a notably broad, open valley with gentlysloping s ides . Rochester C a n y o n is a fairly open va l ley , more openthan perhaps is generally implied b y the term canyon; it headsagainst N e n ze l Hill, whose broad, flat summit stands at 7 , 2 0 0 feetabove sea level. Weaver Canyon lies next south o f RochesterC a n y o n . Both of these c a n y o n s have notably a s y m m e t r i c cross sections, the north slopes being long and relatively gentle and thesouth slopes short and s teep.

In general, the topography is of cons ide rab le and abrupt relief,but o n the whole it cannot b e said to b e rugged, asmost of the ridgecres t s are sm o o t h . The topographic features of the district a resuch as to favor mining by permitting the de v e lo pme n t of m a n yof the veins in depth by m e a n s of adit tunne ls . Plate I s h o w s thetopography excellent ly.

Precipitation in the Roches te r district is small and falls mainlyduring the winter as s n o w . It is insuffic ient to support a n y perennialstreams, but farther north, wh e r e the Star Peak Range is higher,many o f the c a n y o n s contain streams running the year round. Thewinters are cold and stormy but can not be said to b e severe. Earlyin April the snow disappears from the hillsides, and soon count lessf lowers burst into b l o o m . In a ll too short a time the f lowers vanishand the hillsides quickly take on the parched look o f s u m m e r , re -

1 Formerly k n o w n as Sacramento C a n y o n (s o referred to in the reports o f the FortiethParallel Survey), but that name has been transferred to the next can y o n north.


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4 ROCHESTEK DISTRICT, NEV.

lieved faintly, if at all, b y the gr ay i sh -g r een stippling, as it were ,o f sagebrush an d other deser t bushes. T he s \ i m m er wea the r is warman d pleasant .Moun ta in jumpers, thinly sca t t e red and o f sc ragg ly habit , a rethe only t rees in the district, and they grow only above an altitudeo f 6 .000 feet.HISTORY OF MINING

During the early sixties o f the last century the Star Peak Eangew as the scene o f an in tense silver exci tement . O n e o f the chiefgoals o f the rush that took place at that t i m e w as U nionvi lle , onthe eas t foot o f the r ang e , an d a m o n g those drawn to it w as MarkT w a i n , w ho has vividly described, in that exuberant volume " Roughing It," the magni f i cen t exaggera t ion conce rn ing the weal th ofH u m b o l d t then current in the daily press , the great and bl issfuli g no rance o f m in ing , and the general dis incl inat ion o f the s t a m -peders to do an y hard-rock work. It is a marvelously accurate picture that he has drawn , full o f life an d local color, such as rarelygets in to the pages of the professed his tor ian .T he ear l ies t m e n t i o n o f the n a me Roc he s t e r appears in the m o r esedate volumes o f Raymon d ' s r ep or t s . In 1 8 6 8 the Roches te r C o.w as intermittently wor k in g the south extens ion of the M o n t a n a ledge.The outcrops of the ledge, which were l a r ge an d p r omin en t , werein Limer i ck C a n y o n on the s tage r oad between Lovelock an d U n i o n ville 2 an d only 5 miles from the railroad. In 1 8 6 9 the c o m p a n yprosecuted its work on the ledge " with great vigor." The BataviaC o. placed a s team hoisting works on the Roches te r shaft in 1 8 7 0 ,but as the drift f r om the shaft failed to cu t an y " paying quartz "work w as discont inued. 3 In those early days a tunnel severa l hundred feet long w as driven on the north slope o f Limer i ck C a n y o nevidently to cut in depth s o m e of the m a n y quartz veins on the sideo f L o n e Moun t a i n . A b o u t 1 8 8 1 gold w as found in the gravel a t themo uth o f Amer i can C a n y o n ; these placers are only a few mi les eas to f the district in fact , the s t ream heads at N enze l Hill, the focalpoint o f interest in the Rochester district. T he placers w er e activelyworked until 1 8 9 5 . T he Ame r i c a n s , w ho were the first to w o r k theplacers , are sa id to have taken out $1 ,000 ,000 , and the Chinese , w hosucceeded them, are credited with having taken o ut cons iderably m o r egold as m u c h as $10 ,000 ,000 , accord ing to so m e accounts , thoughthis es t imate severely strains one ' s credulity. 4

2 Raymond, R. W. , Mineral resources o f the States and Territories west of the R o c k yMountains for 1 8 6 8 , p. 1 2 5 , 1 8 6 9 .3 Idem for 1 8 7 0 , p. 1 3 7 , 1 8 7 2 .

* R a n s o m e , F. L., Notes on s o m e mining districts in Humboldt County, Nev.: U . S. Geol.Survey Bull. 4 1 4 , p. 12, , 1 9 0 9 .


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HISTORY O F MINING 5A b o u t 1 9 0 5 an o ld prospector from the Black Hills, "Hutch"Stevens , located a large gr oup of c la ims on the hill n ow k n o w n asNenzel Hill, against which R o ch es t e r C anyo n heads on the w e s tan d A m e r i c a n C a n y o n on the east . His c a m p w as at the head ofA m e r i c a n C a n y o n , an d while returning to this c a m p from SpringVal ley , on the eas t side of the r ang e , he perished during a s n ow s to r m o f the win te r of 1 9 0 8 - 9 . His cla ims thus c a m e into thepossession of his heirs , on e o f w h o m , a niece, w as the wife o f JosephF. Nenzel .Other prospectors were in the district at this t ime . In 1 9 0 9 theydrove so m e tunnel s on the Pla inview group of cla ims to prospect agold-bear ing silver vein in the rhyolite on the s u m m i t of the r idge

forming the northward extension of Nenzel Hill an d hardly a milenorth of the locality where the great discovery w as to be m a d ein 1 9 1 2 . T he Limer ick group of claims on the nor thwes t slope ofNenzel Hill also w as located in 1 9 0 9 .In June, 1 9 1 2 , Nenzel discovered rich silver ore on on e of them a n y cla ims inher i ted from Stevens . T he ore was found in a quartzvein, subsequenty called the Wes t vein, which crops o ut at the headof Roches ter C a n y o n in the p r o m i n e n t crags on the w es t brow of

the m o u n t a i n , s ince k n o w n as Nenzel Hill. T he silver occurs in theore in an inconspicuous fo rm , as finely dis semina ted argen t i t e , an dit is doubtless due to this inconspicuousness that the value of thedeposi t r ema in ed so lo ng unkno wn , al though it occurs in a reg ionthat has been m o r e o r less cont inuously prospected s ince the sixties'of the last cen tury and is n o t far from a well -used route of travel .Soon after he had discovered ore Nenzel m a d e a smal l sh ipment ,main ly f rom talus or float, which gave surprisingly high re turns .Some leases were granted, and the output for the year agg r ega t ed1 4 4 tons of ore, aver ag in g $32 .67 a ton . 5 T he favorable resultsbeco m i ng k n o w n , a great rush se t in near the end of the year an dthe beg in n in g of 1 9 1 3 . Mo r e than 2 , 000 people flocked to RochesterC a n y o n , and three t o w n s were laid out Rochester, at the head o fthe canyon , L o w e r Roches ter , 2 miles down the c a n yon , an d CentralRoches ter , be tween the two. C e n t r a l Roches te r soon disappeared,but the others have cont inued. At the p resen t t ime the co m b i nedpopulation does n o t exceed 2 00 or 3 0 0 . T he discovery of dry

placers at the head of Limer i ck C a n y o n le d to a s t ampede there andthe founding of Panama, but this boom soon collapsed.Deve lo pmen t on Nenzel Hill proceeded rapidly, favored by theeasy const ruct ion of adit tunnels , an d in 1 9 1 3 the output of the district jumped f rom the $4 ,704 of 1 9 1 2 to $ 4 7 7 , 4 8 7 . 6* U . S. Geol. Survey Mine ra l Resources, 1 9 1 2 , pt . 1 , p. 7 9 8 , 1 9 1 3 .8 Idem, 1 9 1 3 , pt . 1 , p. 8 2 7 , 1 9 1 4 .


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6 ROCHESTER DISTRICT, NEV.In this year a railroad, known as the Nevada Short Line, w as

extended from Oreana,7 on the Southern Pacif ic sy s t e m , up Rochester C a n y o n , terminating at the portal of the Rochester m i n e , theprincipal m i n e in the district. During the first tw o years of the lifeof the c a m p all the o re mined w as shipped to the smelters for treatm e n t . Under such conditions o re worth less than $ 2 2 a ton that is ,ore carrying less than 3 5 ounces of silver to the ton could not beshipped with profit. 8 In 1 9 1 5 the tw o principal producers theRochester M i n e s C o. and the Nevada Packard M i n e s Co. builttheir ow n reduction plants, and thenceforth the bulk o f the oreproduced in the district has been treated locally. As a result o recontaining m u c h less than 3 5 ounces of silver to the ton could bew o r k e d profitably, and in fact ore carrying as little as 8 ouncesto the ton has been treated at a profit. The decl ine of the boomand the building of the milling plants cut d o w n the freight carriedb y the Nevada Short Line, and that road, wh ic h had b e c o m e exceeding ly unpopular on a c c o u n t of its unreliable serv ice , b e c a m edefunct toward the en d of 1 9 1 7 . The s a m e year saw a vigorousattempt to b o o m a town site at Packard, but this s c h e m e failed.A fine silver-milling plant costing $ 3 0 0 , 0 0 0 w as erec ted at this siteb y the Rochester C o m b i n e d M i n e s C o. to treat the ore from a bodypublicly announced to b e 1 3 0 feet wide and carrying $3 0 to the tonin s i lver. After operating a few weeks the mill w as permanentlyshut, and in 1 9 2 2 it w as torn d o w n and the material shipped toCandelaria, wh e r e it has been r e a s s e mb le d to treat the silver o reof that district.

Apex litigation threatened the principal producer, the RochesterM i n e s Co. , in 1 9 1 7 , but fortunately w a s averted b y c o m p r o m i s e andconsolidation, the eventual o u t c o m e being the formation of theRwhester Silver Corporation in 1 9 2 0 . The efficient management thatc a m e into control as the result of these changes soon succeeded inputting the mine on a profitable bas i s , and the amount of dividendsdisbursed s ince the reorganization has exceeded that of the entireprevioushistory of the district.

OUTPUT OF SILVER AND GOLDThe Rochester district produced to the e n d of 1 9 2 2 more than7 , 0 0 0 , 0 0 0 ounces of silver and 6 0 , 0 0 0 ounces of gold of a total value of$ 7 , 0 0 0 , 0 0 0 . The annual output of silver and gold is s h o wn in the

following table, wh ic h has been compi led from Mineral Resource s7 Renamed for a time Nixon b y the railroad c o m p a n y , renamed Nen,zel at the time o fthe Rochester b o o m , and now called by its original name Oreana. The Post Office Department has steadfastly maintained the original name Oreana.8 Schrader, F . C ., The Rochester mining district, Nev. : U . S. Geol. Survey Bull . 580 , p .3 2 8 , 1 9 1 4 .


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OUTPUT O F G O L D AND SILVERof the United Sta te s . The sudden drop from the average value of$3 0 a ton of o re during the first three years of the life of the c a m pto the average of $1 0 a to n for the later years coinc ides with the beginning of operations by the tw o principal reduction plants in thedistrict. After the plants b e g a n working little ore w as shipped out ofthe district, and selective mining and sorting have been discontinued.

Silver and gold produced from deep mines in the Rochester district, Nev.,191S-.W2S

Year

1912.......................................1913.......................................1 9 1 4 . ......................................1915.......................................1916.......................................1 9 1 8 . ................ ......................1919......................................1920......................................1921..... .__.._._._..._..... _..._-...__.__-1922.......................................1923........ ...........................

O re (shorttons)

1 4 416, 1521 4 , 4 9 92 6 , 6 6 567 , 99 28 8 , 3 7 195 , 74 71 03 , 6627 5 , 0 4 887 , 62 81 1 6 , 4 5 59 1 , 3 7 4

Silver (fineounces)

6 , 8 5 0701 , 3 956 2 1 , 8 3 36 6 3 , 7 9 1816 , 6 20799 , 8 658 1 0 , 9 7 4667 , 1 6 1620 , 04 6667 , 0848 0 0 , 2 3 8664 , 714

Gold

$4915 2 , 3 507 1 , 7 6 91 0 5 , 4 0 78 1 , 2 2 81 5 2 , 8 5 5177 , 12 01 1 7 , 8 9 91 3 0 , 7 15176 , 144107 , 033124 , 3 2 6

Value

Averageper ton

$32 .672 9 . 4 72 8 , 6 91 6 . 6 19 . 1 29 . 2 01 0 . 3 68 . 3 51 0 . 7 59 .617 . 7 97 . 3 2

Total

$ 4 , 7 0 44 7 6 , 0 1 44 1 6 , 0 4 54 4 3 , 0 9 16 2 0 , 151813 , 3 699 9 2 , 1548 6 5 , 2 9 78 0 7 , 0 4 38 4 3 , 2 7 3907 , 2 716 6 9 , 4 0 3

M o r e detailed and h o m o g eneo us data are given in the subjoinedtable , whic h has been a s sem b led from the annual reports for 1 9 2 0and 1 9 2 1 of the chief producer, the Rochester Silver Corporation.

Y e a r

1918.......1Q1Q1920.......1 9 2 1 . ......

O re(shorttons)

42 , 8 2055 , 1 8 1

Content (ouncesper ton)

Silver

10 .4910 .291 2 . 6 31 1 . 6 3

Gold

0 . 1 4 1. 1 0 5. 1 3 9. 1 5 1

Recovery (ounces)

Per to nSilver

7 . 8 67 . 8 51 0 . 3 79 . 5 3

Gold

0 . 1 2 6. 0 9 2. 1 2 8. 1 3 8

TotalSilver

4 4 4 , 187526 , 023

Gold

5 , 4 C 97 , 7 0 1

Value of metalsrecovered

Average.per ton

$ 1 3 . 9 0. 12 . 4 2

Total

$ 595 , 3 516 8 5 , 2 4 6

GEOLOGIC INVESTIGATIONS OF THE DISTRICTThe earliest systematic geologic work in this region w as that do neb y Arnold Hague and Cla r e n c e King, of the Fortieth Parallel Survey. They mapped the areal geo logy of the Star Peak Kange on ascale of 4 miles to the i nch and determined the structure. Although

their conc lus ions have been considerably modif ied by subsequen ts tudies , the large permanent result of their work is that they havesh o w n that the range is built chiefly of a thick series of Triassic rocks


18/92

8 ROCHESTER DISTRICT, NEV.flexed into a broad ant icl ine. A s one of the main traveled roads ofthe region crosses the range b y w a y o f Limer i ck C a n y o n , which isin theKochester district as defined in the presen t report, they becam es o m e w h a t familiar with the geology o f this part o f the r an ge .In 1 9 0 2 G. D . Louderback in tensively studied the Humboldt R a n g eas an example o f a faul t -b lock range , devot ing h imsel f specially tothe southern division, o r Humboldt Lake R a n g e . His w o r k in theStar Peak R a n g e , o r northern division, establ ished the s ignif icantfact that the so-called Archean granite nucleus o f the range is inreality an int rus ive m a s s of post -Tr ia ss i c age , in a ll probability cont emp or an eous w ith the great i n t rus ions in the Sierra Nevada a conclusion, as is well k n o w n , that has s ince been found to apply to m a n yother of the ranges o f the Great Bas in .. Important paleontologic invest igat ions were m a d e by J. P. Smithduring five seasons from 1 9 0 2 o n w a r d , at the remarkably prolific fos-si l iferous locality k n o w n as Fo ss i l Hill, a few mi le s eas t o f theRoches ter district, an d by J. C . M e r r i a m , w ho exhaust ively s tudiedthe ichthyosaurs en tombed in the Middle Triassic l imes tone of thatlocali ty.F . L. Ra n s ome , in 1 9 0 8 , m a d e a reconna i s sance examina t ion of them i n i n g districts o f the Star Peak Ran g e , c r o s s in g the range b y w a yo f the road through Limer i ck C a n y o n . His report is the m a i n sourceo f information on the geology an d o re deposits o f the r ange . Hisbrief invest igat ion showed that the stratigraphy as dete rmined by theFortieth Parallel Survey w as in need of drastic revision. He provedthat the thick Koipato fo rma t i o n , o f Triass ic age , is not of sedim e n t a r y origin but is composed o f volcanic rocks , chiefly rhyolites.A s his w o r k w as not of detailed character , he naturally includedam on g the rhyoli tes the felsi t ic trachyte o f the pre sen t report .In 1 9 1 3 , shortly after the discovery o f the s i lver -bear ing veins onNenze l Hill, 'J. C . Jones briefly described the ore deposits an d thegeology of the n ew c a m p of Rochester , then at the height of itsb oom . In the spring of the s a m e yea r F. C . Schrader ex amined thedistrict in a r econnaissance w ay an d prepared an excellent report,which appeared in 1 9 1 5 .

Jn 1916 T. P. Pendle ton m a d e the topographic m ap o f the districtthat fo rms the base for Plate I. O n the complet ion of this excellentbase m ap a detailed examina t ion o f the district w as m a d e by thewriter in the early part o f 1 9 1 7 , from February 20 to M ay 30 . Colds to rm y w ea the r, with f requent snowfal l , an d the unusua l la teness o fthe advent o f spring considerably hampered the work. A briefvisi t in Augus t , 1 9 1 9 , allowed the writer to e xa min e s om e o f thenewer deve lopment s in the district . During the War the preparationof this report w as put aside for wor k connected with the Geologica l


19/92

G E O L O G Y O F THE STAR PEAK RANGE 9Survey ' s activi ties on w a r min er a l s . Its subsequent comple t ion hasin part been delayed by the writer's wi thdrawal in 1 9 2 0 f rom full -t i m e G o v e r n m e n t service.

BIBLIOGRAPHYT he fol lowing is a list of the principal papers bea r i ng on theRoches te r district:18 64 . Gabb, W. M., Paleontology: Cal i forn ia Geol. Survey, vol. 1 , pp. 1 9 - 3 5 , 1 8 6 4 .18 7 7 . Hague, Arnold, Descript ive geology: U . S . Geol. Expl. 40th Par . Kept.,vol. 2 , p p . 7 1 3 - 7 3 1 .M e e k , F . B ., Paleontology: U . S. Geol . Expl. 40th Par. Kept., vol. 4 , pt. 1 ,pp . 9 9 - 1 2 9 , pis. 1 0 , 1 1 .1 8 7 8 . King, Clarence, Systematic geology: U . S. Geol. Expl. 40th Par. Kept,vol. 1 , pp ; 2 6 8 - 2 7 8 , 2 9 4 - 2 9 5 , 3 4 6 - 3 4 7 .190 4 . Louderback, G. D., Bas in R an ge structure of th e Humbold t reg ion : Geol.Soc. Amer ica Bull., vol. 15 , pp. 289 -346 .190 5 . Hyatt , Alpheus, an d Smith , J. P., T he Triass ic cephalopod genera ofA me r i c a : U . S . Geol. Survey Prof. Paper 40 , pp. 2 1 - 2 3 , > pis. 2 2 - 2 5 .190 8 . Merr i am , J. C ., rJJriassic Ichthyosauria: Cal i forn ia Univ . Mem., vol. 1 ,No. 1 , pp. 18^19 .190 9 . Ransome , F . L., Notes on s o m e mining distr icts in Humbold t County ,Nev.: U . S . Geol. Survey Bull. 414 , pp. 3 1 - 4 7 .1 9 1 3 . Jones , J. C ., Geology of Rochester , Nev. : Min. an d Sci. Press , vol. 10 6 ,pp. 737 -738 .1914 . Schrader , F . C ., The Rochester mining district , Nev.: U . S. Geol. SurveyBull. 580 , pp. 325 -372 .Smith , J. P., The Middle Triass ic inver tebrate faunas of Nor th Amer ica :U . S . Geol. Survey Prof. Paper 8 3 , pp. 6 - 1 1 .

GEOLOGY OF THE STAR PEAK RANGEA s the Roches te r district is only a sma l l fraction of the Star Peak

R a n g e , its geology will perhaps be m o r e clearly unders tood if thebroader features o f the r a n g e are first sketched. Although the geology of the rang as a whole has no t y et been adequately dete rmined ,it is k n o w n well enough to give at least the geologic setting of theRoches te r district .T he oldest rocks in the range form a bedded succession o f lava,tuff, an d breccia , m ore or less schistose an d aggregating abo u t 10 ,000feet in thickness. They consis t of l ight-colored felsi t ic t rachyte ,keratophyre of andesi tic an d greens tone appearance , and rhyol i te .The rhyoli tes overl ie the t rachytes an d keratophyres an d as a ruleare conspicuously porphyritic. This assemblage w as n a m e d theKoipato group by the geologists of theFortieth Parallel Survey , butthey did not recognize its volcanic or igin . It is in a ll probabi l i ty ofMiddle Triass ic age.

Overlying these volcanic rocks conformably is the Star Peak" group " as named by the Fortieth Parallel Survey. It was esti-


20/92

1 0 ROCHESTER DISTRICT, NEV.mated to be 1 0 , 0 0 0 feet thick, and this appears to b e its probable orderof magnitude. The Star Peak formation, as it is n o w termed, w asbelieved b y King 9 to co ns i s t of " an alternation of three great l imes to ne zones and three interposed quartzite zones , " but the work ofJ. P. Smith and of the present writer has s how n that the interposedzones cons i s t m a i n r y , if not wholly, of volcanic rocks .

Smith 1 0 gives the following columnar sec t ion of the West Huui-boldt Range, which , as just indicated, differs radically in lithologyfrom the sect ion of the Star Peak formation given in the FortiethParallel Survey report, and it differs equally in sequence .

Columnar section of the West Humboldt RangeLower Jurassic(Lias).

Unconfo rmi ty

Upper Triassic.

>Middle Triassic.

C_ otaaMP L It*am

Koipato formation.

Pseudomonotiszone.

Daonella zone.

Carries Arietites.

Pseudomonotis subcircularis zone (slates, with Rhabdoceras an dBalorites). Thickness abou t 9 0 0 feet.^Siliceous an d tuffaceous beds, without fossils. Thicknessu n k n o w n .

i Massive limestones , probably corresponding to th e Hosselkus(Upper Triassic) l imes tone of th e California section. Thickness abou t 2 , 0 0 0 feet.Siliceous an d tuffaceous beds without fossils, partly rhyo-lites, an d other volcanic flows. Thickness 1 , 0 0 0 to 2 ,000feet.Slaty l imes tones an d tufl beds, hard at th e to p an d gradingover into shaly l imes tones an d calcareous shales a t the bott o m . Thickness 1 , 0 0 0 to 1 , 5 0 0 feet. T he lower 2 0 0 feetconta in n early al l th e fossils, which belong to the fauna ofCeratites trinodosus.Siliceous beds, tuffs, graywaekes, and igneous rocks, o f unknown thickness. Without f o s s i l s .

Smith w as apparently unaware that Eansomen in 1 9 0 9 had showmthat the Koipato formation is dominantly of volcanic compos i t ion .The " sil iceous beds " listed under the Koipato are consequen t ly withlittle doubt the felsitic trachytes that m-ake up the preponderant partof that formation. K a n s o m e also s h o we d that the " s i l iceous beds "of the Star Peak are in part, at leas t , volcanic rocks .The Middle Triassic l im e s ton e s have yielded the remains of remarkable huge Ichthyosauria (marine "fish-lizards"), which we r e

9 King , Clarence , Sys tema t i c geology: U . S. Geol. Bxpl. 4 0 t h Par. Kept., vol. 1 , pp.2 6 8 - 2 7 8 , 3 4 7 , 1 8 7 8 .1 0 Smith , J. P., T he Middle Triassic m a r i n e invertebrate faunas of North America:U. S. Geol. Survey Prof . Paper 8 3 , pp. 8 - 9 , 1 9 1 4 . Smith's presentation of the strati-graphic results of his important work is unfortunately a ll to o brief.u R a n s o m e , F . L., Notes on . s o m e mining districts in Humboldt County, Nov. : U . S.Geol. Survey Bull, 4 1 4 , p. 3 2 , 1 9 0 9 .


21/92

1 1obta ined at SaurianHill and Foss i l Hill,1 2 on the eas t flank of BlackR a n g e , 0 1 1 the divide between Sou th Amer ican C a n y o n an d Tro yC a n y o n , 3 or 4 miles eas t o f the Roches ter distr ict . At the s a m elocality S m i t h collected 111 species of a m m o n i t e s an d 1 9 species o fother orga n i s ms .T he Jurassic appears to cons i s t m a i n l y o f slates , which exceed2 ,000 feet, in thickness . 1 3 King thought that the Jurassic rests on theTr iass ic with " perfect con fo r mi ty , " but Louderback an d S m i t h saythat it' overlies the Tr iass ic unco nfo rmab ly . T he un con fo r mi ty isapparently not very grea t , for , according to Louderback , " s o me de fo rmat ion an d erosion probably took place at the en d of the Tr iass ic ,but of comparat ively l imi ted magn i tude , for the Jurassic is distributed to Hbout the s a m e exten t that the Tr iass ic is." 1 4T he Triass ic an d Jurassic rocks have been folded into a broad ant i cline whose axis str ikes north and p lunges sou thward , thus causing the w e s t l imb of the ant icl ine to strike a t an ang le of 3 0 withthe mer id ion a l trend of the range itself. A s has long been recognized an d general ly accepted, this folding took place in post -Jurassict ime , m o s t probably at the en d of the Jurassic , con temporaneous lywith the revolution that affected the Sierra Nevada reg ion at thistime the great orustal dis turbance that will here be t e r m e d theJurasside revolution. 1 5 In the Star Peak R a n g e , as i n m a n y of ther an ges of the Great B a s i n a n d the Sierra Nevada , this revolutionw as accompan ied by the in trusion o f granite an d related rocks. U n like s o me of the other ranges , the Star P e a k R a n g e shows but as ma l l a m o u n t o f granite so far exposed by eros ion. This igneousactivi ty, according to R a n s o m e , 1 0 w as followed by the deposi t ion ofa series o f an t imon ia l silver ore bodies a conclusion that admi t tedlyres ted on s lender evidence. However , the evidence at Roches ter isconvincingly c lear that there the minera l iza t ion took place soonafter the intrusion of the granitic rocks. T he lean s i lver -bear ingveins that were then fo r med have subsequently been enriched toworkable grade by the act ion of descending surface wa te r s duringthe long erosion to which the reg ion w as subjected in post -Jurassict ime.The b ro ade r geologic features of the Star Peak R a n g e a r e shownin general ized form in Figure 2 .T he nex t y o u n g e r fo rmat ion in the r a n g e is a coarse boulderydeposit , which evidently represents a series of an c ien t alluvial cones

1 3 Merriam, . 7 . C ., Triassic Ichthyosauria: California U n i v . M e m . , vol. 1 , pp . 18-19, 1 9 0 8 ."King, Clarence, op. cit. , pp . 2 6 9 , 2 9 4 - 2 9 - 5 .3 4 Geol. Soc. America Bull. , vol. 1 5 , p. 3 3 5 , 1 9 0 4 .u This usage conforms to that em p l o y ed by R . T. Chamberlln to denote the folding orstrong diastrophic movements that took place either at or near the end of a geologicperiod (Diastrophism and the formative processes: Jour. Geology,, vol. 2 2 , p. 3 1 7 , 1914).5 0 R a n s o m e , F . L., U . S. Geol. Survey Bull . 4 1 4 , p. 4 6 , 1 9 0 9 .


22/92

12 ROCHESTER DISTRICT, NEV.

that were am as sed in front o f a y o u n g g r o w i n g range . They a rePliocene in age or possibly Quaternary. They were partly coveredby flows of bas al t, as is well shown in the north en d of the HumboldtLake Mounta ins . Although these basalt sheets a re m o s t extensive inthe Humboldt Lake K a n ge , where in fact they form a widespreadcapping , they also occur in the Star Peak R a n g e , but only i n s ma l lareas on its east flank bordering the valley. The basalt sheets o f theHumboldt Lake Range were broken by longitudinal normal faultsan d tilted eas tward . By this faulting an d tilting, as proved b yLouderback , 1 7 the range w as given its relief an d topographic express ion ; it was b locked o ut essent ially in the form w e n o w ' s e e it.The surface of the m a i n nor th -sou th fault f o r m s the abrupt westward-facing esca rpmen t , an d the surface of the basalt Sheets, whicha t the north en d of the range dip 1 2 E., f o r m s the gent le eas t

FIGURE 2. Diagrammatic section through the south en d of the Star Peak R a n g e . 1 , Tri-assic limestone, probably Middle Triassic; 2 , Weaver rhyolite; 3 , trachytic and kera-tophyric lava, breccia, and tuff; 4 , granite porphyry; 5 , aplite; 6 , . massive Triassiclimestone, probably Upper Triassicslope o f the range . The physiographic evidence fully supports thisconclusion conce rn ing the or igin of the range . In regard to theor igin of the Star Peak R a n g e w e are l imi ted to physiographiccriteria, because o f the absence of the basalt sheets , but the evidenceof the land forms in support o f its faul t -b lock or igin is so strongas to be compulsory . T he second an d major uplift of the StarPeak R a n g e , which is thus indicated b y the physiographic evidence,doubtless took! place contemporaneously with the faulting thatblocked out the Humboldt Lake Moun t a i n s . In Re c e n t t ime faultin g has recurred a long the w e s t front of the range and has produced the f resh scarp that t raverses the alluvium at the base o f therange . This fault extends cont inuously for m o r e than 10 0 miles ,an d the scarp is from 10 to 2 9 feet or m o r e high. 1 8

1 7 Louderback, G. D ., Basin Range structure o f the Humboldt region: Geol. Soc . AmericaBull., vol. 1 5 , pp. 3 l 2 r - 3 1 6 , 1 9 0 4 .1 8 Russell, I. C ., Geologica l history o f Lake Lahontan : U . S. Geol. Survey M o n . 1 1 , p.2 7 7 , 1 8 8 5 .


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P A R T II. GENERAL GEOLOGY OF THE DISTRICTTRIASSIC ROCKS

INTRODUCTORY OUTLINETr iass ic rocks, chiefly of volcanic or ig in , m a k e up the larger parto f the bedrock of the Roches te r distr ict .T he oldest rocks are fels i t ic t rachytes , compris ing lavas , breccias ,an d tuffs, here n a m e d the Rochester t rachyte . They agg r ega t e 5 , 000feet in thickness . With them a r e associated keratophyres , partly asa thick ser ies of flows an d pyroclas t ic rocks an d partly in such intim a t e associat ion with the t rachytes as to form belts of alternatingtrachyte and keratophyre.C on fo rm ab ly ove rly in g the trachytes an d keratophyres is ano thervolcanic series , which is essentially of rhyoli t ic composi t ion . T hel owe s t me mbe r o f this sequence is the Nenze l rhyolite breccia , ofvariable thickness but 60 0 feet thick at a m a x i m u m . It is of m a i neconomic in teres t , because the m o s t productive silver veins crop o utin it . Rhyoli te tuffs, of sandstone- l ike appearance but in realitycomposed of shards of glass , overlie the Nenzel rhyolite breccia confo r mab ly but be ing o f lent icular habit are not everywhere present .Above the Nenze l rhyolite breccia an d the tuffs lies the W e a v e rrhyol i te , consis t ing chiefly of flows that agg r ega t e 7 2 0 feet in thickness . T he rhyoli te lavas of this fo rmat ion have highly distinctivecharacters , such as abundan t p ro minen t phenocrys ts of quar tz an d awidely prevalent con ten t of quar tz nodules an d geodes resulting from

the fil l ing of spheruli tes that original ly were hollow. Some lenseso f tuffaceous shale are in tercala ted be tween the flows o f Weaverrhyolite.T he assemblage of t rachytes , keratophyres , and rhyel i tes , wi th theirbrecc ias an d tuffs, ma k e s up the Koipato group o f the FortiethParallel Survey in fact , it w as recognized by the geologis t s of thatSurvey that in Limer i ck C a n y o n this " group " attains i t s grea tes tthickness . 1 9 It is o f Middle Triass ic age a t least , but it m ay be older,though this is regarded as highly improbable .Limes tones over l ie the volcanic rocks in the western part of thedistr ict . They have been faulted d own against the W eaver rhyol ite .They are part o f the Star Peak group of the Fortieth Parallel Survey, but as ne i ther Koipato n or Star Peak is a secure stratigraphicterm an d as n o t eno ug h wo rk justifying their redefinit ion has been

* > U . S . Geol. Expl. 40th Par. Kept. , vol. 1 , p. 2 7 0 , 1 8 7 8 .13


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done , those t e r m s a re n o t employed in this report . T he l imes tone inthe weste rn part of the Roches te r district conta ins m a r i n e fossils ,which indicate that it is of Middle or Upper Triass ic age. C o m p a r i son with the c o lumn a r section given by Smith sugges ts that it is o fMiddle Triass ic age.ROCHESTER TRACHYTE

G E N E R A L FEATURESA l ight -colored felsite carrying sm a l l inconspicuous phenocrys t sof microc l ine is the prevailing rock in Roches te r C a n y o n . It is wellexposed on the r idges inclos ing the c a n yon an d fo r m s the l ower slope

of Nenze l Hill, where it is penetrated by extens ive m i n e workings .It extends northward to Gold Moun t a i n a n d eas tward far b e yon dthe boundar ies of the area mapped. It is the m os t w idespread of theTriass ic volcanic rocks.This volcanic fo rmat ion comprises lavas, breccias , an d tuffs. A s awhole they are . r emarkab ly s im i l a r an d m on o ton ou s l y alike. Morethan half of the a s semblage , as shown by a detailed s tudy of theexcellent sect ions a long i ^ h e r idges bordering Roches te r Canyon , , consis ts o f breccias an d tuffs. A s the fine- textured tuffs are extraordinarily difficult to dis t inguish from the mass ive felsite, it is probablethat the pyroclast ic portion of the formation considerably exceedsthe mass ive port ion . In places the tuff beds a re clearly lenticularan d are intercalated in the breccias an d thus supply a m e a n s forde t e rmin ing the strike an d dip of the forma t ion . At m a n y placesthey fail, however , an d the m on o ton ou s similarity of the fo rmat ionas a whole an d the absence o f key layers of brecc ia or of dist inct ivelava sheets m a k e s it imposs ib le to dete rmine the local s tructure.N o n e of the breccias a re coarse , for the f r ag men t s of which theyare composed are rarely as m u c h as 6 inches in diame te r an d aregenera l ly less than an inch.T he lava sheets are c o m m o n l y unfolia ted , but the breccias an d tuffsare dist inctly though roughly schistose, with m o r e o r less sericiticgloss on the foliat ion surfaces.

PETROGRAPHY

Fels i te is a highly appropriate field n a m e for these volcanic rocks.They are exceedingly fine grained, an d the phenocrys t s they conta inare as a rule sparse , smal l , an d not readily discerned. Feldspar,which is c o m m o n l y striated but which under the microscope provesto be microcl ine , doubtless derived f rom san id ine as the result of thed j m a m i c me t a morp h i s m the rocks have undergone , is the onlyporphyritic constitutent in the great bulk of the series. A fe wminute sporadic phenocrysts o f quartz occur in s o m e of the upper


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T B I A S S I C R O C K S 15m e m b e r s , but they hardly affect the generalization that this thickpile of volcanic rocks cons is t s of fels i tes that carry inconsp icuousphenocrysts of m ic roc l i n e only . It is the absence of quartz pheno-crysts that special ly contrasts the trachytes with the overlyingN e n z e l rhyolite breccia and Weaver rhyolite.The breccias and tuffs cons i s t of angular fragments of fels i te ofthe s a m e kind as that of the lava sheets. The lack of variety in theconstituent material enhances the es sen t ia l uniformity of the ser ies.Dark minerals, such as biotite o r hornblende, or ves t iges of darkminerals have not been found in an y of the lavas , brecc ia s , and tuffs.In color the felsites on fresh fracture are generally a rather purewhi te . Rarely they are blackish or bluish gray. O n weathered surfaces the felsites lavas, brecc ia s , and tuffs alike are characteristically buff or brownish yel low, o w i n g to the oxidation of the finelydisseminated pyrite introduced in to them during the mineralizationof the district.

The felsitic trachyte of the belt extending from N e n z e l Hill northto Gold Hill is distinguished for its remarkable streakiness and flowbanding. S o m e of the m o s t highly f low-banded felsite cons is t s of analternation of bluish-black and c ream-colored bands, a fourth to halfa n i n c h thick, m o r e o r less curly and contorted. Spherulitic structure also is c o m m o n and has b e c o m e strikingly accentuated in themineralized a reas b y the processes of alteration accompanying themineralization. In places the trachyte is vuggy from the prevalenceof gas cavit ies , and in the area of mineralization these cavi t ies havebeen filled or lined with zinc b lende , ga lena , pyrite, chalcopyrite, andquartz, as is wel l shown in the Pitt and Friedman tunnels. In addition to the streakiness and flow banding, many of the lavas are flowlayered, breaking up into thin l aye rs , an inch or so thick, which w e r eproduced b y f lowage.The f lowage structure is m o s t conspicuous in the belt extendingfrom Sunf lower and N e n z e l hills to Gold Mountain, but it can befound throughout the areal extent of the Rochester trachyte, a lthough e lsewhere generally far less p ron ou n c e d .

The best-preserved material of this formation occurs on GoldMountain, in the northern part of the district. It is nearly normalrock without a trace of foliation and without the secondary sericiteand pyrite so c o m m o n l y developed in the trachyte throughout m o s tof the district. It is regarded as representative of the formation asa whole in so far as a single ' specimen i s l ikely to be, and the remainder of the formation probably does not depart widely from it incompos i t ion . It is clearly the unmineralized, unaltered equivalentof the f low-banded felsite on the l o we r western slope of N e n z e l Hill,in to which the productive silver veins extend in depth and w hic h isdeeply penetrated b y the Pitt and Friedman tunnels.


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16 B O C H E S T E K D I S T B I C T , N E V .The trachyte from Gold Mountain selected for c h e m i c a l analysisw as obtained from the upper spherulitic portion of a streaky, highlyf low-banded lava sheet . It is a dark-gray rock carrying numerous

slender phenocrysts of microc l ine , not over a millimeter or two inlength, many of w hi c h have a fairly glassy luster. The groundmasscons i s t s of spherulites about 3 millimeters in diameter, s o m e of whichare obscure ly banded . Under the mic roscope phenocrysts of m i c r o cline are seen scattered through a matrix c o m p o s e d wholly of spherul i tes. Crystals of microcline generally form nucle i around which thespherulites have grown. The spherulites are dusted with minuteblack particles, doubtless oxide of iron, and it is due to this pigmentation that the trachyte is darker than the great bulk of allied rocksin the district. The spherulites are only feebly birefringent, theinterference tints being generally not higher than iron-grays, and inmany the birefringence is barely perceptible. They are evidentlyc o m p o s e d of feldspar, as the index of refraction is markedly less thanthat of Canada b a l s a m . They are built of fibers that give parallelextinction, s o m e of which have positive and others negative e longation, although the predominant effect on inserting the gypsum plateis that of positive elongation manifestly the feeble birefringence ofthespherulites is duein part to the compensatory effect of superposedfibers of opposite optical e longa t ions . As the following c he m i c a lanalysis of the rock indicates, the spherulites must cons i s t almostwholly of potassium feldspar. They a re concentrically banded,though as a rule there are not m o r e than tw o zones. The accessoryminerals are abundant oxides of iron (apparently hematite and m a g netite) and minor amounts of apatite, titanite, and zi rcon . N o fer-romagnesian minerals or traces of them are recognizable . The on lyalteration product is quartz, occurring in a few minute veinlets andas local , incomplete replacements of s o m e of the microc l ine phenocrys t s .

Analysis of trachyte from Gold Mountain, Rochester district[R. C . Wells, analys t ]

S iO s ___________________________ 7 . 01A1 2 0 ____ __ 5 . 9 7Fe 0 __________ - . 5 4FeO_____________________________ . 4 4M gO ____________ _ race.C aO _________ 1 9Na2 0____________ . 9 2K0_-_ .-_ ___-_ 1 1 . 4 2TiO___________________.. __. . 3 9

1 0 0 . 1 6


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T K I A S S I C R O C K S - 17The immediately striking feature of the analysis is the high perc e n t a ge of K2 0 nearly three t im e s that of the average rhyolite,which is 4 .09 per cent . 2 0 There are in Washington's tables 2 1 but

four ana lyses of rocks termed rhyolite, quartz porphyry, or felsitethat contain as m u c h K,0 a s this trachyte. The silica content isslightly higher than it would b e in the absolute ly fresh rock, on account of the pre sence of a s m a l l quantity of secondary quartz.In view of the absence of quartz phenocrysts in the rock fromGold Mountain, the ext remely high content of K 0, and the relatively low S iO 2 that is , lo w for a rhyolite it will b e termed atrachyte. In the norm sy s t em of classif icat ion it falls into c lass I,order 5 , rang 1 , subrang 2 . This subrang, which is unnamed, isrepresented b y only on e analysis that of a pegmatite from BrokenHill, Australia,2 2 which , howeve r , should be excluded, a s it isan altered mineralized rock.

THICKNESS A N D A G EThe thickness of the Rochester trachyte exposed in the Rochester

district is of the order of 5 , 0 0 0 feet . Roughly one-half of thisthickness, as already mentioned, cons is t s of brecc ia s and tuffs.

The m o s t favorable sec t ion for measuring the thickness of theformation is that extending westward from S u n f lo we r Hill alongthe ridge crest o n the south side o f Rochester C a n y o n . It crossesthe belt o f trachyte approximately at right angles; the strike of theformation i s N. 3 0 W ., and the dip averages 3 0 W . From themeasured width of the belt, the thickness of the formation is calculated to be about 5 , 000 feet. This calculation is subject to correction for faulting, but ho w m u c h correction can not be determined, because the amount of faulting is not ascertainable, o w i n gto the lack of ke y horizons . That 5 , 0 0 0 feet represents the order ofmagnitude of the th ickness , however , is indicated b y the followingconsideration. The trachyte in the measured sec t ion is part of thewestern l imb of the great anticline of the Star Peak Range, and theeastern l imb , w hic h is b e s t exposed in the ridges extending eas t ward from Gold Mountain (east of the mapped area), displays athickness of trachyte of the s a m e order of magnitude. That thisagreement is a m e r e coincidence appears improbable.

The apparent th ickness of the trachyte along the ridge boundingthe north s ide of R o c h e s t e r C a n y o n is 6 , 000 feet . In neither sec t ionis the b a s e of the trachyte exposed, nor has it b e e n found e lsewhere ,so that 5 , 0 0 0 feet appears to be a conservative estimate for the thickn e s s of the formation.

a o Daly , R. A . , , Igneous rocks and their origin , p. 1 9 , 1 9 1 3 .2 1 W as h ing ton , H. S., U . S. Geol. Survey Prof . Paper 99, pp. 3 0 8 - 3 0 9 , 1 9 1 7 ."Idem, p. 2 6 9 .


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The Roches ter trachyte is the preponderant e lemen t in the Koipatogroup as defined by K i n g , o f the Fortieth Parallel Survey . It w asconsidered to be of L o w e r Triassic ag e but , as will be shown subsequent ly , is m o s t probably Middle Triass ic .DUMORTIERITIZED TRACHYTE

Dumort ie r i t e , an a lumin um borosi l icate (A l 8 HBSi 3 02 0 ), occursin extraordinary abun dan ce on the w e s t flank of Lincoln Hill. Inthe a m o u n t o f this compara t ive ly rare m i n e r a l that it con ta in s thisarea , so far as is n ow k n o w n , is without a peer anywhere . T hedumortierite is prevailingly pink and lavender , the blue color usuallythought to be characteristic of the m i n e r a l being very rare . Kichcobal t -b lue dumortierite occurs in places with the deep-p ink variety ,and the combina t ion produces a gorgeously colored rock. The du-mort ie r i t e -bear ing area is underlain largely by felsitic trachyte breccias , which strike N . 2 0 W . and dip 2 5 W . T he dumortierite is distributed through these rocks in brilliantly colored splotches of red,pink , and lavender , but m o r e c o m m o n l y it occurs as a close-spacedn e t w o r k o f pink veinlets. T he area that con ta in s this dumortieriticrock is outl ined in Plate I; its bo unda ry is , of course , not hard an dfast , for the a m o u n t o f dumortierite fades out gradually toward theborders . Inm u c h o f this area hardly a cubic yard o f the rock is without a veinlet o f dumor t ie r i t e , an d i n m a n y places the veinlets are soabundant that they average an inch apart. They range in thickness from a s m a l l fraction o f an inch up to 6 inches. They appearto follow n o sys tem o r sys tems , though perhaps they are m o r e c o mm o n l y n o r m a l to the bedding .T he thinner veinle ts cons is t chiefly o f pink dumort ier i te withs o m e quartz; in places they conta in also a little b ro wn to urma l i ne .T he thicker veinlets cons is t m o r e largely of quartz than the thinnerones , an d s o me o f this quartz is n o tewor thy because of its fine rosecolor. Between the rose quartz and the pink dumortierite a ll s tagesof gradation due to intergrowth of the tw o mine ra l s can be followedin hand spec imens . Under the microscope the transition can be seenm o r e readily an d w h a t appears to be clear rose quartz to the unaidedeye is fo und to inclose n umer ous hairlike fibers o f dumor t i e r i te ,m a n y o f which have the ul t r a -microscopic d iameter s of the thinnestsi l l imani te needles, which, indeed, they re semble i nd i s t ingui shab ly .Wher e the dumortierite needles b e c om e too n umer ous the rose quartzloses i t s t ranslucence. This n ew variety o f rose quartz thus certainly owes its color to inclosure of capillary fibers of pink dumo r -


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T R I A S S 1 C R O C K S 19tierite. 2 3 The lavender-colored dumortierite, according t o W . T .Schaller, gives a decided test for titanium. The titanium is thoughtto b e present as Ti 2 0 , replacing A1 2 0 .

That the d e ve lop me n t of the dumortierite in the felsite was ' accom panied b y other profound ch ang es is evident in the field, but to determine what these changes are requires the aid of the microscope .S o m e silvery-white m i ca i n s m a l l scales is in places apparent to theunaided eye, as wel l as a light-colored vitreous mineral that has afairly g o o d cleavage. Under the microscope the dumortieritizedfels i tes are seen to have undergone a drastic alteration: a ll volcanictextures have been comple te ly obliterated, and the rocks cons i s t offour minerals in variable proportions quartz, andalusite, dumortierite, and ser ici te. The quartz is the m o s t abundant constituentand forms a f ine -gra ined mo s a i c . The andalusite is generally secondin abundance; it forms anhedrons as m u c h as half an i nch in length,many of which are markedly skeletal and inc lose much quartz. Thedumortierite is c o m m o n l y in radiate groups of sillimanite-likeneedles or o f fibers that have brushlike ends resembling sillimanitebrushes . The thicker needles show a red, lavender, o r blue pleo-chroism parallel to the c-axis and paler tones transverse to it. Whereboth dumortierite and tourmaline occur together, as in s o m e of theveinle ts , they contrast in pleochroism and absorption very markedly.The fine-grained micaceous mineral generally present in subordinatequantity is believed to be ser ic i te , though s o m e m ay b e pyrophyllite(HAlSi 2 0 ), which has indistinguishably similar optical properties.It has in part grown at the expense of the andalusite.

The extensive d e ve lop me n t o f dumortierite an d associa ted m i n erals is clearly due to the ac t ion of boron-bearing gase s , which haveundoubtedly escaped from-an underlying granite magma and permeated the trachytes as they traveled upward through a closelyspaced network o f fractures. These emanations w e re evidently richin s i l icon , aluminum, and boron and deficient in alkalies and sulphides. The" dev e lo pm en t o f andalusite b y this pneumatolysis isnoteworthy, though perhaps not surprising in view o f the close re s e m b l a n c e of andalusite and dumortierite, especially in compos i t ion . 2 4 The metasomatic origin of andalusite appears to be rare. A

2 3 A preliminary account of the occurrence o f the dumortierite in the Rochester districtand the possible utilization o f the rose quartz as a semiprecious stone was given by thewriter in Schaller, W . T . , G em s and precious stones: U . S. Geol. Survey Mineral Resources1 9 1 6 , pt. 2 , p. 893 , , 1 9 1 9 . J. C . Jones had earlier (Geo logy o f Rochester, Nev.: M i n . andSci. Press, vol. 1 0 6 , p. 7 3 7 , 1 9 1 3 ) identified the dumortierite as pink tourmaline (rubel-lite). Dumortierite ia most easily distinguished from tourmaline by its cleavage and byits absorption, which Is strongest parallel to- its elongation.M Schaller, W . T ., Dumortierite: U . S. Geol. Survey Bull . 2 6 2 , p. 1 1 0 , 1 9 0 5 . The occur-icnce o f dumortierite in Washington desc r ibed by Schaller has tho same mineral as soc ia tion as that in Nevada namely, quartz, andalusite, .dumortlertie, and muscovi tp hut itsgenesis is unknown.


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remarkable example from the Inyo Eange, Calif., has b e e n descr ibed, where andalusite has formed in e n o r m o u s quantity, makingup a m a s s more than 3 0 0 feet wide , which has resulted from the re placement of i a volcanic porphyry. 2 5 Andalusitic alteration of Tertiary lavas, an alteration that evidently took place under conditionss o m e w h a t different- from those indicated for the Inyo and Rochesteroccur rences , has been descr ibed fry Butler,2 0 w ho pointed out thatalteration of this kind had not previously been recorded.

KERATOPHYBESO C C U R R E N C E AND C H A R A C T E R

The slopes of Limerick Canyon co ns i s t largely of a ser ies of alteredvolcanic rocks of keratophyric compos i t ion . They co m pr i s e tuffs,b recc ia s , and lava f lows, s o m e of which are amygdaloidal. A s a rulethey are porphyritic, owing to the presence of phenocrysts of whitestriated feldspar, and a s these feldspars are e m b e d d e d in a darkishgroundmass, the rocks wholly r e s e m b l e andes i te . Inasmuch a sunder the microscope the porphyritic crystals invariably prove tobe albite, the rocks are termed keratophyres.

The dip of the keratophyre flows and breccias is prevailingly3 0 W ., though locally they stand vertical. The apparent thicknessof the keratophyres on the north s lope of Limerick C a n y o n , possible duplication b y faulting being disregarded, is 4 , 5 0 0 feet . Southward they thin abruptly, so that o n the ridge be tw een Limerick andRochester c a n y o n s they are much thinner. It must b e pointed outthat although boundaries are drawn on the geolog ic map (PI. I)delimiting the keratophyre belt, yet a few sheets of keratophyre areintercalated in the adjacent trachyte, both eas^t and w e s t of the mainbelt w h er e it crosses the divide b e tw e e n Limerick and Rochesterc a n yon s .

Smaller areas of keratophyre occur in the Black Range, east andsoutheast of Ne n ze l Hill.

PETRO G RAPHIC F E A T U R E SThe keratophyres contain numerous phenocrysts of albite, w hic h

are set in a fine-grained groundmass of rather dark color . A fewhave also phenocrysts of quartz, and such varieties may b e termedquartz keratophyres. Crystals of albite as much as half an i nchin length have been etched in relief on the weathered surfaces ofs o m e of the pyroclastic m e m b e r s . M o s t of the keratophyres are________/______________,_____________

2 5 Knopf, , Adolph, A n andalusite mass in the pre-Cambrlan of the Inyo Range, Calif.:Washington Acad. Sci. Jour., vol. 7, pp. 549-552, 1917.2 9 Butler, B. S. , Geology and ore deposits of the San Francisco and adjacent districts,Utah: U. S. Geol. Survey Prof . Pap e r 80, pp. 78-82, 1913.


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T R I A S S I C B O O K S 21roughly o r even rather thoroughly schistose, an d this is especiallytrue o f the tuffs an d breccias. T he considerable al terat ion , bothd yn a mic and pneumato ly t ic , that these rocks have undergone in terposes an insuperable obstacle to their satisfactory petrographics tudy.However , o f the m a n y specimens examined microscopical ly a llbut a fe w conta in a lbi te phenocrys t s only, so that they fall readilyinto the class o f keratophyres . The phenocrys t s of albi te evidentlyres is ted metamorphism more effectually than the other cons t i tuentsan d have survived both the d yn a mic dis turbance ( though s o m e arecracked an d broken) an d the Jurasside pneumatolys is , even thought he g roun d ma s s has been drastically altered. But unde r in tensepneumato ly t ic attack so m e of the a lbi te phenocrys t s have been re placed by aggrega te s of bioti te an d calcite. What fe r romagnes ianminera l s were or iginal ly pre sen t w as de te rminable in only on e rockexamined a quartz keratophyre from the s u m m i t o f the 6 , 648 - foo tpeak a t the eas t m a r g i n of the district . It is the best -preservedkeratophyre found and is mass ive an d o f grayish-green color. It isa porphyritic rock of dacitic aspect , conta in ing phenocrys t s o ffeldspar an d quar tz an d chloritic pseudomorphs. Under the microscope the porphyritic minera l s a re seen to be albi te (Ab 9 0 A n 1 0 ),microcl ine , an d quartz. Chlori te pseudomorphs are abundan t , s o m eo f which clearly are after pyroxene , for square sec t ions with truncated corne r s a re c o m m o n . 2 7 T he g roun d ma s s is fine grained an dobscured by alteration products . This keratophyre is the only onefound that conta ins phenocrys t s o f potassium feldspar along withthose of albi te an d is therefore n o t typical of the series as a whole.T he groundmasses of the keratophyre are general ly i nde te rminable ,but in a fragment inclosed in on e of the pyroclast ic breccias it couldbe seen that the g roun d ma s s is salic , cons is t ing largely o f albi te infaint fiuidal a r r ang emen t .T he keratophyres were r icher in fe r romagnes ian cons t i tuen t s thanthe trachytes; hence their darker color an d andesi t ic appearance .During dynamic me tamorphi sm the f e r ro mag nes i an m ine ra l s werec al tered to chlori te , especially in the pyroclas t ic kera tophyres , whichthereby b e c a me chloritic schists, an d subsequent ly by pneumatolys i sthe chori te w as altered to biot i te and concomi tan t ly biot i te w as developed more or less promiscuously throughout the rock, even replacing the albi te phenocrysts .In m a n y places pneumato ly t ic act ion Has been particularly in tense .To urma l i ne is ext remely c o m m o n in such places , rather evenlyscat tered as i n n ume ra b le black needles through the rocks thus affected, and bio t i te is unusua l ly abundant . f

2 7 It is interesting to recal l that Rosenbusch ( E lemen t e der Geste inslehre , 3 d ed., p. 3 4 5 ,1 9 1 0 ) , in def ining the keratophyres , gives diopside as the characteristic f e r romagne s i anphenocrys t .


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22 ROCHESTER DISTRICT, JSTEV.

RELATION O F KERATOPHTRES T O THE ROCHESTER TRACHYTEThe keratophyre contains in places s o m e intercalated trachyte and

banded trachyte tuff, proving that these different volcanic r o cks w e r eerupted contemporaneously. Where the two kinds are associa ted ,as near Limerick Pass, the keratophyre is m o r e highly schis tose thanthe trachyte. Farther w e s t in Limerick Canyon the keratophyreand trachyte form a distinct belt in which they alternate so regularlythat it w as desirable to map the belt as a separate entity. (SeeP L I.) The belt is not of uniformwidth but w i dens northward; thethickness of the alternating ser ies is 9 0 0 feet on the summit of theridge on the north side of Limerick C a n y o n , where the volcanicrocks stand practically vertical. Single sheets of keratophyre are asm u c h as 1 0 0 feet thick. Near the top of the success ion at this placeis a keratophyre amygdaloid which is spotted white with' numerousphenocrysts of albite and amygdules of quartz; it is fairly schis tose ,and its foliation planes shimmer from innumerable tiny flakes ofbiotite.

There are a fe w narrow bel ts at various places in the Rochestertrachyte that appear to be of m i x e d origin, being due to a c o m mingling of trachyte and keratophyre tuff. Through sub sequen tshearing and the development of s eco nda r y minerals it is imposs ib leto determine certainly the origin of these rocks . They are darkerthan the felsi tes and s o m e w h a t lighter in color than the keratophyres,but their difference from either is not sharply defined.

A GE A N D C O R R E L A T I O NThe keratophyre eruptions w e re clearly contemporaneous with

those of the trachytes, and the keratophyres a re therefore of Triassicage. The close associa t ion of these two varieties appears to imply avery notable magmatic differentiation during the Triassic v o lcan i sm ,w h er eby the potassa w as largely concentrated in on e magma and thesoda in another.

Keratophyres occur in other Triassic areas of N ev ada . They areabundant in the Yerington copper district, where they are light-colored fels i tes with i nconspicuous phenocrysts of albi te. 2 8 Curiouslyenough , these rocks at Yerington do not in the least re semble inappearance the keratophyres of the Rochester district but they do res e mb le exactly the trachytes. In the Simon silver-lead district ofCedar Mountain, north of Tonopah, highly porphyritic quartz keratophyre is assoc ia ted with l i m es to ne of Middle Triassic age. 2 9

2 8 Knopf, Adolph, Geology and ore deposits of the Yerington district,. Nev.: U . S. Geol.Survey Prof. Paper 1 1 4 , pp . 13-16, 1 9 1 8 .2 9 Knopf, Adolph , Ore deposits of Cedar Mountain, Mineral County. Nev. : U . S. Geol .Survey Bull. 72 -5 , pp . 363-364, 1 9 2 1 .


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U . S. GEOLOGICAL SURVEY BULLETIN 76 2 PLATE I118 I300" R. 33 -

EXPLANATIONSEDIMENTARY ROCKS

. Coarse alluvial-cone oc

Granite porphyry

Dumortieritized trachyte

Qal P SWpwTrachyte and keratophyrein alternating sheets

Faults(U, upthrow; D . downthrow)

deposits; location approximate

Strike an d dip of beddedrocks

118 1300" R. 33 ETopography by T P. Pendletcm

Surveyed in 1916 G EO LO G I C MA P A N D S E C T I O N OF THE ROCHESTER DISTRICT,PERSHING C O U N T Y , NEV.S cale

Geology by Adolph KnopfSurveyed in 1917

i. Milesooo 1000 aooo Feet

i KilometerCooxtoxu? interval25feet

i' is m&fuT, sea, level

IndependenceHll

-A- ,-,

Section alon4 line A~A 5,000


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T RI A SSI C R O C K S 23In conclus ion , it appears that the Middle and Upper Triassic w e r et im e s of great igneous activity in Ne va d a , during w hic h large volu m e s of widely diverse volcan ic rocks we r e erupted. Andesite,daci te , trachyte, rhyolite, keratophyre, and quartz keratophyre are

the principal kinds. The maximum aggregate thickness of theserocks is of the order of 1 0 , 0 0 0 or 1 5 , 0 0 0 feet .NENZEL RHYOLITE BRECCIA

OC C U R R E N C E A N D C H A R A C T E RThe rock whose bold craggy outcrops form the summit of N e n z e l

Hill is here termed the N e n z e l rhyolite brecc ia . It is of particularinterest because the silver veins first discovered crop out in it, andthese veins have yielded the bulk of the silver that the district hasproduced.

The most prominent m e m b e r of the formation is generally devoidof bedding, though in a few places it contains fine-grained bandedlayers a few inches thick, which in the b e s t exposure w e r e seen torepresent lenticular tuff beds pinching out within a length of 4 0feet along the strike. B e c a u se of its m a s s ive outcrops the N e n z e lrhyolite breccia closely r e s e mb le s a lava , and this r e s e mb la n c e i se n ha n c e d b y the fact that quartz and feldspar crystals are distributedevenly through it, like phenocrysts in a porphyry. That it is m a d eup of e jec ted volcan ic fragments, however , is indicated b y the a n g u lar pieces of felsite scattered through it, though the breccia nature isgenerally obscure because the fragments r e s e mb le the matrix in w hic hthey are e mb e dde d .

The Nenze l breccia is generally stained from the oxidation of thesecondary pyrite contained in it, but w he re it is absolute ly unoxidizedit is pure whi te . The c o m p o n e n t fragments are generally under aninch in diameter, though in places blocks a s much as 16 inches indiameter occur . The m ic roscope conf irm s the conc lus ions drawn fromthe field evidence as to the pyroclastic origin of the rock and itsrhyolitic compos i t ion . The feldspar crystals and fragments provewithout except ion to be thepotassiumvariety and s o m e w h a t perthitic.The rock fragments comprise fe l s i tes of various kinds microspher-ulitic trachyte and so on and rhyolite carrying quartz phenocrys t s .

ST R AT I G R AP HY A N D STRUCTUREThe bes t sec t ion of the N e n z e l rhyolitebreccia is that on the northe n d of N e n ze l Hill; it is clearly exposed and displays a greater lith-ologic diversity than an y other, an d the stratigraphic relations are

fairly wel l sho w n . (See fig. 3 .) The l o w e r m o s t m e m b e r of theformation cons is t s of a ser ies of well-stratified brecc ia s carrying as

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2 4 ROCHESTER DISTRICT, NEV.a no tab le feature an abun dan ce of euhedral quartz crystals; thesebeds strike north an d dip 3 5 E. They are 5 0 feet thick, but theirfull thickness is not shown here , as they are faulted against theunderlying felsi te along a nor th - sou th fault dipping 7 5 E. Thebreccia beds are turned up so that they dip 5 5 o r m o r e to the eas tadjacent to the fault, which is therefore clearly a n o r m a l fault. Farther south along the strike o f the beds, south o f the Bough ton &Hackley tunne l , the indicated thickness is approximate ly 2 5 0 feet ,although the base o f the series is n o t satisfactorily exposed. T heuppe rmo s t b ed o f breccia is rather coarse , con t a in ing boulders asm u c h as 15 inches in d iameter .Abo ve the strat if ied breccias lies a highly dist inc t ive bed, a conglomera t i c breccia or subangu la r cong lomera te , which is 10 feetthick. It consis ts largely o f felsite detritus plus white rhyolite thatis dot ted with quartz phenocrys t s ; much o f this material is sharply

w .

1500 FEETFIGURE 3. Geologic sec t ion through, the north end o f Nenze l Hill. 1 , Trachyte , roughlyschistose; 2 , trachyte tuffs and breccias; 3 , trachyte flows with subord ina te tuff; 4 ,bedded rhyolite breccia; - 5 , cong l omer a te ; 6 , rhyolite breccia , essentially without bedding ; 7 , well-bedded tuffs, r e s emb l ing sandstones; 8 , alluviumangular, but s o m e is well rounded. This characteristic bed can betraced only a short distance southward , near ly to the Transportat ion tunnel o f the Roches ter m i n e , but it extends northward for amile , to the Pla inview prospect. It appears locally o n the eas t flanko f the ridge also, as at Sage Hen Spring, where its presence is dueto a n u m b e r o f strike faults that would n o t be detectable but forthis bed.Abo ve the conglomera te is 25 feet o f strat if ied tuff and brecciain beds ranging from 3 inches to 3 feet in thickness. S o m e o f thebeds are ident ical in appearance with the mass ive facies of the formation; others cons i s t o f white microcrys ta l l ine rocks devoid o f thequartz crysta ls that are character is t ic of the formation in general .T he mass ive m e m b e r

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GEOLOGY AND ORE DEPOSITS OF THE ROCHESTER DISTRICT, NEVADA