DEPARTMENT OF THE INTERIOR

HUBERT WORK, Secretary

UNITED STATES GEOLOGICAL SURVEY

GEORGE OTIS SMITH, Director 

Bulletin 762

GEOLOGY AND ORE DEPOSITS

OF THE

ROCHESTER DISTRICT, NEVADA

BY

ADOLPH KNOPF

WASHINGTON

GOVERNMENT PRINTING OFFICE 

1924
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CONTENTS

Preface, by F. L. Ransome____________________________________________

Outline of the report__________________________________________

Part I. General features_____________________________ 

Geography. ___________ __________________.______.

Situation of the district ________________________

Physical features _______________- _________________ 1

Climate and vegetation____ ________________ 3

History of mining______________________________ _ 4

Output of silver and gold_______ ________________ fi

Geologic investigations of the district-^_________________ 7

Bibliography____________________________________ !)

Geology of the Star Peak Range______________________ 9

Part II. General geology of the district____________________ 13

Triassic rocks _______________________________ 13

Introductory outline________________________- 13

Rochester trachyte____________________________. 14

General features____ ______ 14

Petrography______________________________. 14

Thickness and age________ _ ________ 17

Dumortieritized trachyte _____ _____________ 18

Keratophyres________________ _______________ 20

Occurrence and character___________________. 20

Petrographic features__ __________________ 20

Relation of keratophyres to the Rochester trachyte____ '-2

Age and correlation_________________________ 22

Nenzel rhyolite breccia_____ _ _ __________ 23

Occurrence and character. _ _ _________ 23

Stratigraphy and structure____________________ 23

Rhyolite tuffs___________'.__________________ 26

Weaver rhyolite_______________ _____ 20

Occurrence and character __ 2U

Petrography_______ _ 27

Tuffaceous shales ____ _ 28

Limestone____________ __ 29

General features____ __ 20

Age__________________________________ 29

Relation of limestone and volcanic rocks _ 30

Metadiorite _______ - 31

Post-Triassic, probably late Jurassic igneous rocks __ 32

Aplite______________ - 32

General features_____ ; >2

Petrography_____ 32

Genetic relations____ --_ 3$

Granite porphyry __ '>*

Tertiary (?) rocks 35

Pliocene (?) alluvial-cone detritus ___- 35

Occurrence and character - 35

Age__ 36

Basalt______________ - 36

Quaternary alluvium - 37

in
IV CONTENTS

Part II. General geology of the district Continued. Page

Geologic structure ___ ____ _______________ 38

Folding and faulting__________________________ 3S

Schistosity____________________________'___ 40

Physiographic history__________________________,__ 41

Part III The ore deposits___________________________ 43

General features _______________________________ 43

Relation of ore deposits to water level_________________ 43

Silver veins__________________________________ 44

Distribution and general character_________________ 44

Nature of the ore________________-_ _______ 43

Enrichment by supergene argentite________________ 40

Wall-rock alteration_________________________ 50

Gold veins___________'_____________________ 51

General character.. _____:___________________ 51

Nature of the gold ore-forming solutions_____________ 52

Other mineral deposits__________________________ 53

Cinnabar _______________________________ 53

Miscellaneous deposits_______________________ 54

Genesis of the ores of the district___:________________ 55

Placers ________________ _________________ 58

Part IV. Mines and prospects_________________________ 59

Silver mines and prospects_________ ______________ 59

Eochester mine_____________________________ 59

Bonghton & Hackley mine_ ____________________ 64

Nenzel Grown Point property_________ ________ 65

Buck & Charley mine ______ ____ _________ 66 

Octopus mine_______ ___ ________________ . 67

Raven prospect____________ ________________ 68

Abe Lincoln mine__ --__ _ 68

Kaiser Bill prospect_______________ _________ 68

Nevada Packard mine , 69

Gold mines and prospects 74

Lincoln Hill mine__ 74

Oro Fino mine____ ___ ____ _ 75

Hagan prospect________________________ 75

Index ______________ - - 77

ILLUSTRATIONS

Page

PLATE I. Geologic map of the Rochester district _ __ _ 22

II. A, B, Silver-bearing stockwork at the Nevada Packard mine_ 46

III. A, Incipient replacement of sphalerite by supergene argentite;

B, Advanced replacement of sphalerite by supergene argentite 47

IV. Map of Nevada Packard mine workings-____ __ 72

FIGURE 1. Index map showing the location of the Rochester district__ 2

2. Diagrammatic section through the south end of the Star

Pe'ak Range____________________________ 12

3. Geologic section through the north end of Nenzel Hill____ 24

4. Geologic section from portal of Boughton & Hackley tunnel

to northeast corner of North Crown Fraction claim________ 65

5. Diagrammatic section through the Nevada Packard mine

. along the line of tunnel A___ 72
PEEFACE

By F. L. RAN SOME

The modern development of the Rochester district dates from the 

later part of the year 1912. Early in 1913 F. C. Schrader was detailed by the United States Geological Survey to make a reconnaissance study of the district, and his brief examination was followed 

by an excellent report, which was published in 1915. The continued productivity of the Rochester mines and the interesting character of the ore deposits made it evident that more detailed geologic 

work was desirable, and after the completion of a topographic map, 

in 1916, Mr. Knopf was instructed to make a geologic study of the 

district early in 1917. The preparation of his report, like many 

other investigations by the Geological Survey then in progress, was 

interrupted by the urgent war demand for information in regard 

to particular mineral resources and was further delayed by Mr. 

Knopf's withdrawal from full-time participation in the work of the 

Survey and his acceptance of a professorship at Yale University. 

His new duties left only a small part of his time available for the 

investigation that he has now satisfactorily completed.

As Mr. Knopf shows, the ore deposits of the Rochester district are 

mmeralogically of unusual character. Dumortierite, a rare aluminum borosilicate, is in some parts of this district extraordinarily 

abundant and is accompanied by tourmaline and andahisite minerals, 

which also are rather unusual in association with silver deposits. The 

boron-bearing minerals were evidently formed at high temperature, 

and Mr. Knopf concludes that there were three distinct stages in 

the deposition of these minerals. The principal silver-bearing veins 

were probably formed later and at lower temperature. The report 

shows that the original vein material was of too low grade to be ore. 

The district owes its economic development to downward enrichment, 

effected largely by the replacement of sphalerite by argentite a 

process that is fully discussed in its appropriate, place. It follows 

that the silver deposits of the Rochester district are not likely to 

extend to great depth. The same conclusion, based apparently on the 

observed decrease in tenor of the ore on the deeper mine levels, 

finds! expression also in the annual report of the Rochester Silver 

Corporation for 1921. Mr. Knopf has given the explanation for 

the observed decrease.
VI PREFACE

The following report, although it deals with only a small area, 

throws considerable new light on the geology of the Humboldt 

Range. The range has long been noted as a collecting ground for 

Triassic fossils, but our knowledge of the beds in which these ancient 

marine shells are found left much to be desired. Mr. Knopf has not 

only supplied some of this deficiency but has shown that the lightcolored felsitic rocks of the Humboldt Range, which in early reports 

were regarded as metamorphosed sediments and later were classed 

generally as rhyolites, include rocks belonging to the group of 

trachytes a group that is rather sparsely represented in North 

America,
OUTLINE OF THE REPORT

Rochester is a silver-mining district in the Humboldt Range of western 

Nevada. Silver was discovered there in 1912, and to the end of 1922 the 

district produced 7,000,000 ounces of silver. It is second in yearly outpuj; among 

Nevada's many silver districts, but it ranks far below Tonopah, the leading 

district.

The prevailing rocks of the district are a thick series of Triassic volcanic 

rocks overlain by limestone of the same age. The volcanic rocks consist of 

felsitic trachyte and keratophyre, aggregating 10,000 feet in thickness, and an 

overlying series of rhyolite, 1,000 feet thick. Above the igneous rocks lies the 

limestone, of which only 2,000 feet is exposed in the district, although elsewhere in the Star Peak Range it apparently attains a far greater thickness.

At or near the end of the Jurassic period this series of volcanic rocks and 

limestone came within the influence of the great revolution and crustal disturbances that affected the western Cordilleran region at that time. A long 

train of events started then, which culminated in the formation of the ore deposits 

of the district. First the rocks were folded into a broad anticline, which constitutes one of the major structural features of the range. This arching of the 

rocks was followed by the intrusion of granitic magma, which within the 

district proper is represented by fine-grained white granite, or aplite. The 

bodies of aplite are comparatively large for rock of this kind, and their size 

implies, therefore, a notable amount of differentiation of the magma from which 

the aplite was derived. The last rocks to be intruded during this period of 

igneous activity are granite porphyry dikes. Thereupon began a period of 

active circulation of highly heated solutions, at first so hot as to be gaseous 

but later liquid. Doubtless these solutions were expelled from the cooling and 

consolidating magma in depth. The minerals produced by these solutions indicate three stages in these so-called post-intrusive processes one in which 

dumortierite (an aluminum borosilicate) is the distinctive mineral that was 

formed, a second in which gold ores having tourmaline-bearing gangue were deposited, and a third in which silver protores having tourmalinic quartz 

gangue were deposited, which shade into silver protores without tourmalinic 

gangue. These three stages, which are interpreted as being successive in time 

and at temperatures successively lower, were discontinuous, as gradations between the deposits formed during the successive stages do not occur.

During the first stage a large body of trachyte on Lincoln Hill was shattered 

and interlaced with veinlets composed of pink dumortierite and quartz. The 

trachyte adjoining the veinlets was intensely altered and converted into an 

aggregate of andalusite and quartz. The ^umortierite-bearing mass thus produced appears to exceed in size any other known. It is notable that no precious 

metals or sulphides were deposited during this stage; possibly the temperature 

of the solutions was too high to admit of their deposition. During the second 

and third stages the gold ores and the silver protores were deposited, and from 

the silver protores were later developed by supergene enrichment the most 

valuable deposits in the district. These will be considered later at greater 

length; only their relation to the post-Jurassic revolution and accompanying 

igneous activity is here emphasized.
VIII OUTLINE OP THE BEPOKT

A long period of erosion succeeded the post-Jurassic revolution. A deposit 

of bouldery alluvial-cone d6bris at least 600 feet thick records the next event 

in the history of the district. It is probably of Pliocene age and resulted from 

the initial uplift of the Star Peak Range, or northern subdivision of the 

Humboldt Range, and the consequent amassing of alluvial cones along the fault 

scarp of the newly formed range. Later this material was covered, in part at 

least, by flows of basalt. By the faulting and tilting of these basalt sheets 

the Humboldt Lake Range, or southern subdivision of the Humboldt Range, was 

produced, as has been so ably demonstrated by Louderback. The Star Peak 

Range was further uplifted, probably at this same time, and gradually attained 

its present altitude. Physiographically the fault-block origin of the range is 

shown by the clean-cut triangularly faceted escarpment along the west front 

and by the fact that the canyons, which are acutely V-shaped at the western 

front of the range, broaden widely upstream a feature shown particularly 

well by Limerick Canyon. During the uplift of the range as a whole a certain 

amount of fragmentation took place, and the movement and readjustment of 

the minor blocks produced fault-trough valleys, the most notable example of 

which is the valley on the east flank of Nenzel Hill. The upward movement on 

the fault surfaces was slow enough to permit small streams to maintain their 

courses across the rising fault blocks. In this way were produced such 

remarkably anomalous stream conditions as those of American and South 

American canyons. South American Canyon cuts through the Black Range at 

right angles in a narrow V-canyon, 2,000 feet deep, and taps the broad, open 

longitudinal valley on the east side of Nenzel Hill. The physiographic conditions are similar to those shown in the rift valleys of the California Coast 

Ranges, though on a smaller scale and with more rugged relief.

The principal ore deposits of the Rochester district are silver-bearing quartz 

veins and stockworks. They belong to the relatively rare class in which the 

ore is valuable chiefly for its content of silver. The silver-bearing quartz veins 

furnish the greater part of the output of the district, and they are most numerous and productive on Nenzol Hill, at the head of Rochester Canyon. The 

st'ockworks are represented only by the deposits at Packard.

The silver-bearing quartz veins are inclosed in the Triassic rhyolite and 

trachyte. The ore consists of quartz that as a rule is exceedingly fine grained. 

The chief silver mineral is finely disseminated argentite. Other metallic minerals are pyrite, sphalerite, galena, tetrahedrite, covellite, and chalcopyrite, but 

in the aggregate the amount of all these minerals is small. Although quartz is 

the only gangue mineral in most of the veins, some contain noticeable quantities of the significant mineral tourmaline.

The average content of the ore now mined is from 10 to 12 ounces of silver 

to the ton. In early days, when the upper portions of the veins were being 

mined, the ore carried from 30 to 60 ounces to the ton. Although the higher 

grade of the ore then mined was in part due to sorting, nevertheless the silver 

content has greatly declined as greater depth has been attained, and the bottom 

of the ore has been -reached. The extreme vertical range of ore is between 600 

and 800 feet. Why the silver content has steadily decreased in the ore of the 

deeper levels has become clearly apparent during the course of the present 

investigation. The argentite, which determines the silver tenor of the ore, and 

hence its commercial value, is of supergene (secondary) origin, having been 

formed as the result of the reaction between the primary sphalerite of the 

unoxidized ore and silver sulphate that had been carried down by descending 

surface water from the outcrops of the oxidizing veins. The amount of argentite consequently diminishes steadily with increasing depth. The source of the
OUTLINE OF THE REPORT IX

silver by which the ore has thus been enriched is doubtless the sparse tetrahedrite of the primary-vein filling. On oxidation this mineral yielded soluble compounds -of silver and copper, and these substances trickling down in solution 

through veins reacted with the primary sulphides, chiefly the sphalerite, and 

produced the super-gene argentite and the more abundant covellite.

The remarkable silver-bearing stockworks at Packard consist of Trlassic 

rhyolite traversed by widely spaced, nearly imperceptible veinlets. The silver 

occurs as finely disseminated cerargyrite (horn silver), which has probably 

been formed at the expense, of supergene argentite. The stockwork ore bodies 

are large shallow concentrations, nowhere extending more than 30 feet below 

the surface.

It is obvious from the preceding account that the silver ores are the result 

of the supergene enrichment of material originally containing so little silver 

as to be commercially valueless. This unenriched vein filling, too lean to be 

ore the protore, to use Ransome's term was formed largely by the replacement of sheeted and shattered zones in rhyolite and trachyte, a mode of origin 

which accounts for the prevailing fine-grained, in places nearly chalcedonylike character of the quartz gangue. In fact, this fine-grained quartz might 

readily suggest that the Rochester veins, though inclosed in Triassic rhyolite 

and trachyte, are of Tertiary age, like so many of Nevada's silver deposits. 

However, the tourmaline-bearing quartz gangue of certain typical silver veins, 

the remarkable faulting of certain veins by tourmaline lodes, and the profound tourmalinization produced by the aplite intrusions all combine to 

prove that the silver veins are genetically related to the aplite and were 

formed soon after its intrusion.

Economically, the most significant fact in connection with the origin of the 

silver ores is that their commercial value is dependent on their content of 

secondary argentite. The deep-level tunnels have cut the veins below the 

zone of enrichment and have thereby determined the maximum downward 

limit of the ground in which it is worth while to explore for ore. Future 

exploration will therefore be restricted to lateral work on the higher levels, 

in which considerable ground still remains to be tested.

98293 24 2

GEOLOGY AND ORE DEPOSITS OF THE ROCHESTER 

DISTRICT, NEVADA

By ADOLPH KNOPF

PART I. GENERAL FEATURES

GEOGRAPHY 

SITUATION OF THE DISTRICT

The Rochester district as defined in this report is an area of 27 

square miles in the southern part of the Star Peak Range, in western 

Nevada (fig. 1). It is in Pershing County, which was created from 

the southern portion of Humboldt County in 1919. The nearest railroad station and shipping point, Oreana, on the main line of. the 

Southern Pacific system^ is 12 miles distant. Formerly a railroad 

known as the Nevada Short Line extended from Oreana to the town 

of Rochester, but this line is now abandoned. Lovelock, the county 

seat, is the nearest large town and lies 25 miles southwest of the 

district.

Rochester, Lower Rochester, and Packard are the only settlements 

in the district. Rochester is at the head of Rochester Canyon, at an 

altitude of 6,300 feet, and is the chief settlement. The town is now 

little more than the place of residence of the employees of the Rochester mine. Lower Rochester is 2 miles farther down the canyon and 

consists of a few houses only. Packard is in the southern part of 

the district and consists of the dwellings of the employees of the 

Packard mine.

PHYSICAL FEATURES

The Star Peak Range, in which the Rochester district is situated, 

is the northern division of the West Humboldt Range. It is separated from the much lower southern division of that range the division sometimes called the Humboldt Lake Mountains by a broad 

depression trending obliquely to the course of the range. This 

depression is shown in detail in Plate I; in geologic publications it 

is referred to as Cole Canyon, but locally this name is given to the 

canyon transecting the Humboldt Lake Mountains a few miles farther south.

1
2 ROCHESTER DISTRICT, NEV.

The West Humboldt Range was so termed to distinguish it from 

the East Humboldt Range, the loftiest mountain mass in Nevada, 

which lies south of Wells. The East Humboldt Range, however, is

FIGURE 1. Index map showing the location of the Rochester district

now generally known as the Ruby Range and the West Humboldt 

simply as the Humboldt Range. According to the Fortieth Parallel 

Survey, the Indian name for the range is Koipato,
GEOGRAPHY 3

The Star Peak Range is nearly 75 miles long and trends meridional^. It rises .steeply" along its regular western front from an 

altitude of 5,000 feet to an average of 8,000 feet and culminates in 

Star Peak, 10,000 feet above sea level. The southern prong of the 

range, which overlaps en echelon the north end of the Humboldt 

Lake Mountains, is called the Black Range; it culminates in Buffalo 

Peak, 8,400 feet above sea level.

In the latitude of the Rochester district the Star Peak Range is 

15 miles wide. The district lies mainly on the western slope of the 

range. Limerick, Rochester, and Weaver canyons, opening upon the 

western front of the range, can be said to determine the main topographic features of the district. None of these canyons sustains 

a permanent stream, the only flowing water being that furnished 

by the melting of the snow during a few weeks in March and 

April. Limerick Canyon,1 the northernmost, affords an easy grade 

across the range and is the site of the road from Lovelock to Unionville and other mining camps that lie on the east side of the range. 

At its mouth the canyon is deep, rugged, and sharply V-shaped, 

but headward it widens into a notably broad, open valley with gently 

sloping sides. Rochester Canyon is a fairly open valley, more open 

than perhaps is generally implied by the term canyon; it heads 

against Nenzel Hill, whose broad, flat summit stands at 7,200 feet 

above sea level. Weaver Canyon lies next south of Rochester 

Canyon. Both of these canyons have notably asymmetric cross sections, the north slopes being long and relatively gentle and the 

south slopes short and steep.

In general, the topography is of considerable and abrupt relief, 

but on the whole it can not be said to be rugged, as most of the ridge 

crests are smooth. The topographic features of the district are 

such as to favor mining by permitting the development of many 

of the veins in depth by means of adit tunnels. Plate I shows the 

topography excellently.

Precipitation in the Rochester district is small and falls mainly 

during the winter as snow. It is insufficient to support any perennial 

streams, but farther north, where the Star Peak Range is higher, 

many of the canyons contain streams running the year round. The 

winters are cold and stormy but can not be said to be severe. Early 

in April the snow disappears from the hillsides, and soon countless 

flowers burst into bloom. In all too short a time the flowers vanish 

and the hillsides quickly take on the parched look of summer, re1 Formerly known as Sacramento Canyon (so referred to in the reports of the Fortieth 

Parallel Survey), but that name has been transferred to the next canyon north.
4 ROCHESTEK DISTRICT, NEV.

lieved faintly, if at all, by the grayish-green stippling, as it were, 

of sagebrush and other desert bushes. The s\immer weather is warm 

and pleasant.

Mountain jumpers, thinly scattered and of scraggly habit, are 

the only trees in the district, and they grow only above an altitude 

of 6.000 feet.

HISTORY OF MINING

During the early sixties of the last century the Star Peak Eange 

was the scene of an intense silver excitement. One of the chief 

goals of the rush that took place at that time was Unionville, on 

the east foot of the range, and among those drawn to it was Mark 

Twain, who has vividly described, in that exuberant volume " Roughing It," the magnificent exaggeration concerning the wealth of 

Humboldt then current in the daily press, the great and blissful 

ignorance of mining, and the general disinclination of the stampeders to do any hard-rock work. It is a marvelously accurate picture that he has drawn, full of life and local color, such as rarely 

gets into the pages of the professed historian.

The earliest mention of the name Rochester appears in the more 

sedate volumes of Raymond's reports. In 1868 the Rochester Co. 

was intermittently working the south extension of the Montana ledge. 

The outcrops of the ledge, which were large and prominent, were 

in Limerick Canyon on the stage road between Lovelock and Unionville 2 and only 5 miles from the railroad. In 1869 the company 

prosecuted its work on the ledge " with great vigor." The Batavia 

Co. placed a steam hoisting works on the Rochester shaft in 1870, 

but as the drift from the shaft failed to cut any " paying quartz " 

work was discontinued.3 In those early days a tunnel several hundred feet long was driven on the north slope of Limerick Canyon 

evidently to cut in depth some of the many quartz veins on the side 

of Lone Mountain. About 1881 gold was found in the gravel at the 

mouth of American Canyon; these placers are only a few miles east 

of the district in fact, the stream heads at Nenzel Hill, the focal 

point of interest in the Rochester district. The placers were actively 

worked until 1895. The Americans, who were the first to work the 

placers, are said to have taken out $1,000,000, and the Chinese, who 

succeeded them, are credited with having taken out considerably more 

gold as much as $10,000,000, according to some accounts, though 

this estimate severely strains one's credulity.4

2 Raymond, R. W., Mineral resources of the States and Territories west of the Rocky 

Mountains for 1868, p. 125, 1869.

3 Idem for 1870, p. 137, 1872.

* Ransome, F. L., Notes on some mining districts in Humboldt County, Nev.: U. S. Geol. 

Survey Bull. 414, p. 12,, 1909.
HISTORY OF MINING 5

About 1905 an old prospector from the Black Hills, "Hutch" 

Stevens, located a large group of claims on the hill now known as 

Nenzel Hill, against which Rochester Canyon heads on the west 

and American Canyon on the east. His camp was at the head of 

American Canyon, and while returning to this camp from Spring 

Valley, on the east side of the range, he perished during a snowstorm of the winter of 1908-9. His claims thus came into the 

possession of his heirs, one of whom, a niece, was the wife of Joseph 

F. Nenzel.

Other prospectors were in the district at this time. In 1909 they 

drove some tunnels on the Plainview group of claims to prospect a 

gold-bearing silver vein in the rhyolite on the summit of the ridge 

forming the northward extension of Nenzel Hill and hardly a mile 

north of the locality where the great discovery was to be made 

in 1912. The Limerick group of claims on the northwest slope of 

Nenzel Hill also was located in 1909.

In June, 1912, Nenzel discovered rich silver ore on one of the 

many claims inherited from Stevens. The ore was found in a quartz 

vein, subsequenty called the West vein, which crops out at the head 

of Rochester Canyon in the prominent crags on the west brow of 

the mountain, since known as Nenzel Hill. The silver occurs in the 

ore in an inconspicuous form, as finely disseminated argentite, and 

it is doubtless due to this inconspicuousness that the value of the 

deposit remained so long unknown, although it occurs in a region 

that has been more or less continuously prospected since the sixties' 

of the last century and is not far from a well-used route of travel.

Soon after he had discovered ore Nenzel made a small shipment, 

mainly from talus or float, which gave surprisingly high returns. 

Some leases were granted, and the output for the year aggregated 

144 tons of ore, averaging $32.67 a ton.5 The favorable results 

becoming known, a great rush set in near the end of the year and 

the beginning of 1913. More than 2,000 people flocked to Rochester 

Canyon, and three towns were laid out Rochester, at the head of 

the canyon, Lower Rochester, 2 miles down the canyon, and Central 

Rochester, between the two. Central Rochester soon disappeared, 

but the others have continued. At the present time the combined 

population does not exceed 200 or 300. The discovery of dry 

placers at the head of Limerick Canyon led to a stampede there and 

the founding of Panama, but this boom soon collapsed.

Development on Nenzel Hill proceeded rapidly, favored by the 

easy construction of adit tunnels, and in 1913 the output of the district jumped from the $4,704 of 1912 to $477,487.6

* U. S. Geol. Survey Mineral Resources, 1912, pt. 1, p. 798, 1913. 

8 Idem, 1913, pt. 1, p. 827, 1914.
6 ROCHESTER DISTRICT, NEV.

In this year a railroad, known as the Nevada Short Line, was 

extended from Oreana,7 on the Southern Pacific system, up Rochester Canyon, terminating at the portal of the Rochester mine, the 

principal mine in the district. During the first two years of the life 

of the camp all the ore mined was shipped to the smelters for treatment. Under such conditions ore worth less than $22 a ton that is, 

ore carrying less than 35 ounces of silver to the ton could not be 

shipped with profit.8 In 1915 the two principal producers the 

Rochester Mines Co. and the Nevada Packard Mines Co. built 

their own reduction plants, and thenceforth the bulk of the ore 

produced in the district has been treated locally. As a result ore 

containing much less than 35 ounces of silver to the ton could be 

worked profitably, and in fact ore carrying as little as 8 ounces 

to the ton has been treated at a profit. The decline of the boom 

and the building of the milling plants cut down the freight carried 

by the Nevada Short Line, and that road, which had become exceedingly unpopular on account of its unreliable service, became 

defunct toward the end of 1917. The same year saw a vigorous 

attempt to boom a town site at Packard, but this scheme failed. 

A fine silver-milling plant costing $300,000 was erected at this site 

by the Rochester Combined Mines Co. to treat the ore from a body 

publicly announced to be 130 feet wide and carrying $30 to the ton 

in silver. After operating a few weeks the mill was permanently 

shut, and in 1922 it was torn down and the material shipped to 

Candelaria, where it has been reassembled to treat the silver ore 

of that district.

Apex litigation threatened the principal producer, the Rochester 

Mines Co., in 1917, but fortunately was averted by compromise and 

consolidation, the eventual outcome being the formation of the 

Rwhester Silver Corporation in 1920. The efficient management that 

came into control as the result of these changes soon succeeded in 

putting the mine on a profitable basis, and the amount of dividends 

disbursed since the reorganization has exceeded that of the entire 

previous history of the district.

OUTPUT OF SILVER AND GOLD

The Rochester district produced to the end of 1922 more than 

7,000,000 ounces of silver and 60,000 ounces of gold of a total value of 

$7,000,000. The annual output of silver and gold is shown in the 

following table, which has been compiled from Mineral Resources

7 Renamed for a time Nixon by the railroad company, renamed Nen,zel at the time of 

the Rochester boom, 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. 

328, 1914.
OUTPUT OF GOLD AND SILVER

of the United States. The sudden drop from the average value of 

$30 a ton of ore during the first three years of the life of the camp 

to the average of $10 a ton for the later years coincides with the beginning of operations by the two principal reduction plants in the 

district. After the plants began working little ore was shipped out of 

the 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.......................................

1914. ......................................

1915.......................................

1916.......................................

1918. ................ ......................

1919......................................

1920......................................

1921..... .__.._._._..._..... _..._-...__.__-

1922.......................................

1923........ ............................

Ore (short 

tons)

144 

16, 152 

14,499 

26,665 

67, 992 

88,371 

95, 747 

103, 662 

75,048 

87, 628 

116,455 

91,374

Silver (fine 

ounces)

6,850 

701, 395 

621,833 

663,791 

816, 620 

799, 865 

810,974 

667, 161 

620, 046 

667, 084 

800,238 

664, 714

Gold

$491 

52, 350 

71,769 

105,407 

81,228 

152,855 

177, 120 

117,899 

130, 715 

176, 144 

107, 033 

124, 326

Value

Average 

per ton

$32.67 

29.47 

28,69 

16.61 

9.12 

9.20 

10.36 

8.35 

10.75 

9.61 

7.79 

7.32

Total

$4,704 

476,014 

416,045 

443,091 

620, 151 

813, 369 

992, 154 

865,297 

807, 043 

843, 273 

907, 271 

669,403

More detailed and homogeneous data are given in the subjoined 

table, which has been assembled from the annual reports for 1920 

and 1921 of the chief producer, the Rochester Silver Corporation.

Year

1918.......

1Q1Q

1920.......

1921. ......

Ore

(short 

tons)

42, 820 

55, 181

Content (ounces 

per ton)

Silver

10.49 

10.29 

12.63 

11.63

Gold

0.141 

.105 

.139 

.151

Recovery (ounces)

Per ton

Silver

7.86 

7.85 

10.37 

9.53

Gold

0.126 

.092 

.128 

.138

Total

Silver

444, 187 

526, 023

Gold

5,4C9 

7,701

Value of metals 

recovered

Average 

.per ton

$13.90 

. 12. 42

Total

$595, 351 

685, 246

GEOLOGIC INVESTIGATIONS OF THE DISTRICT

The earliest systematic geologic work in this region was that done 

by Arnold Hague and Clarence King, of the Fortieth Parallel Survey. They mapped the areal geology of the Star Peak Kange on a 

scale of 4 miles to the inch and determined the structure. Although 

their conclusions have been considerably modified by subsequent 

studies, the large permanent result of their work is that they have 

shown that the range is built chiefly of a thick series of Triassic rocks
8 ROCHESTER DISTRICT, NEV.

flexed into a broad anticline. As one of the main traveled roads of 

the region crosses the range by way of Limerick Canyon, which is 

in the Kochester district as defined in the present report, they became 

somewhat familiar with the geology of this part of the range.

In 1902 G. D. Louderback intensively studied the Humboldt Range 

as an example of a fault-block range, devoting himself specially to 

the southern division, or Humboldt Lake Range. His work in the 

Star Peak Range, or northern division, established the significant 

fact that the so-called Archean granite nucleus of the range is in 

reality an intrusive mass of post-Triassic age, in all probability contemporaneous with the great intrusions in the Sierra Nevada a conclusion, as is well known, that has since been found to apply to many 

other of the ranges of the Great Basin.

. Important paleontologic investigations were made by J. P. Smith 

during five seasons from 1902 onward, at the remarkably prolific fossiliferous locality known as Fossil Hill, a few miles east of the 

Rochester district, and by J. C. Merriam, who exhaustively studied 

the ichthyosaurs entombed in the Middle Triassic limestone of that 

locality.

F. L. Ransome, in 1908, made a reconnaissance examination of the 

mining districts of the Star Peak Range, crossing the range by way 

of the road through Limerick Canyon. His report is the main source 

of information on the geology and ore deposits of the range. His 

brief investigation showed that the stratigraphy as determined by the 

Fortieth Parallel Survey was in need of drastic revision. He proved 

that the thick Koipato formation, of Triassic age, is not of sedimentary origin but is composed of volcanic rocks, chiefly rhyolites. 

As his work was not of detailed character, he naturally included 

among the rhyolites the felsitic trachyte of the present report.

In 1913, shortly after the discovery of the silver-bearing veins on 

Nenzel Hill, 'J. C. Jones briefly described the ore deposits and the 

geology of the new camp of Rochester, then at the height of its 

boom. In the spring of the same year F. C. Schrader examined the 

district in a reconnaissance way and prepared an excellent report, 

which appeared in 1915.

Jn 1916 T. P. Pendleton made the topographic map of the district 

that forms the base for Plate I. On the completion of this excellent 

base map a detailed examination of the district was made by the 

writer in the early part of 1917, from February 20 to May 30. Cold 

stormy weather, with frequent snowfall, and the unusual lateness of 

the advent of spring considerably hampered the work. A brief 

visit in August, 1919, allowed the writer to examine some of the 

newer developments in the district. During the War the preparation 

of this report was put aside for work connected with the Geological
GEOLOGY OF THE STAR PEAK RANGE 9

Survey's activities on war minerals. Its subsequent completion has 

in part been delayed by the writer's withdrawal in 1920 from fulltime Government service.

BIBLIOGRAPHY

The following is a list of the principal papers bearing on the 

Rochester district:

1864. Gabb, W. M., Paleontology: California Geol. Survey, vol. 1, pp. 19-35,1864.

1877. Hague, Arnold, Descriptive geology: U. S. Geol. Expl. 40th Par. Kept.,

vol. 2, pp. 713-731.

Meek, F. B., Paleontology: U. S. Geol. Expl. 40th Par. Kept., vol. 4, pt. 1, 

pp. 99-129, pis. 10, 11.

1878. King, Clarence, Systematic geology: U. S. Geol. Expl. 40th Par. Kept, 

vol. 1, pp; 268-278, 294-295, 346-347.

1904. Louderback, G. D., Basin Range structure of the Humboldt region: Geol. 

Soc. America Bull., vol. 15, pp. 289-346.

1905. Hyatt, Alpheus, and Smith, J. P., The Triassic cephalopod genera of 

America: U. S. Geol. Survey Prof. Paper 40, pp. 21-23,> pis. 22-25.

1908. Merriam, J. C., rJJriassic Ichthyosauria: California Univ. Mem., vol. 1, 

No. 1, pp. 18^19.

1909. Ransome, F. L., Notes on some mining districts in Humboldt County, 

Nev.: U. S. Geol. Survey Bull. 414, pp. 31-47.

1913. Jones, J. C., Geology of Rochester, Nev.: Min. and Sci. Press, vol. 106, 

pp. 737-738.

1914. Schrader, F. C., The Rochester mining district, Nev.: U. S. Geol. Survey

Bull. 580, pp. 325-372.

Smith, J. P., The Middle Triassic invertebrate faunas of North America: 

U. S. Geol. Survey Prof. Paper 83, pp. 6-11.

GEOLOGY OF THE STAR PEAK RANGE

As the Rochester district is only a small fraction of the Star Peak 

Range, its geology will perhaps be more clearly understood if the 

broader features of the range are first sketched. Although the geology of the rang© as a whole has not yet been adequately determined, 

it is known well enough to give at least the geologic setting of the 

Rochester district.

The oldest rocks in the range form a bedded succession of lava, 

tuff, and breccia, more or less schistose and aggregating about 10,000 

feet in thickness. They consist of light-colored felsitic trachyte, 

keratophyre of andesitic and greenstone appearance, and rhyolite. 

The rhyolites overlie the trachytes and keratophyres and as a rule 

are conspicuously porphyritic. This assemblage was named the 

Koipato group by the geologists of the Fortieth Parallel Survey, but 

they did not recognize its volcanic origin. It is in all probability of 

Middle Triassic age.

Overlying these volcanic rocks conformably is the Star Peak 

" group " as named by the Fortieth Parallel Survey. It was esti-
10 ROCHESTER DISTRICT, NEV.

mated to be 10,000 feet thick, and this appears to be its probable order 

of magnitude. The Star Peak formation, as it is now termed, was 

believed by King 9 to consist of " an alternation of three great limestone zones and three interposed quartzite zones," but the work of 

J. P. Smith and of the present writer has shown that the interposed 

zones consist mainry, if not wholly, of volcanic rocks.

Smith 10 gives the following columnar section of the West Huuiboldt Range, which, as just indicated, differs radically in lithology 

from the section of the Star Peak formation given in the Fortieth 

Parallel Survey report, and it differs equally in sequence.

Columnar section of the West Humboldt Range

Lower Jurassic 

(Lias).

Unconformity 

Upper Triassic.

«> Middle Triassic.

C

_o

ta

a £

M

PLI 

t*

a 

m

Koipato formation.

Pseudomonotis 

zone.

Daonella zone.

Carries Arietites.

Pseudomonotis subcircularis zone (slates, with Rhabdoceras and 

Balorites). Thickness about 900 feet. ^

Siliceous and tuffaceous beds, without fossils. Thickness 

unknown.

i Massive limestones, probably corresponding to the Hosselkus 

(Upper Triassic) limestone of the California section. Thickness about 2,000 feet.

Siliceous and tuffaceous beds without fossils, partly rhyolites, and other volcanic flows. Thickness 1,000 to 2,000 

feet.

Slaty limestones and tufl beds, hard at the top and grading 

over into shaly limestones and calcareous shales at the bottom. Thickness 1,000 to 1,500 feet. The lower 200 feet 

contain nearly all the fossils, which belong to the fauna of 

Ceratites trinodosus.

Siliceous beds, tuffs, graywaekes, and igneous rocks, of unknown thickness. Without fossils.

Smith was apparently unaware that Eansome n in 1909 had showm 

that the Koipato formation is dominantly of volcanic composition. 

The " siliceous beds " listed under the Koipato are consequently with 

little doubt the felsitic trachytes that m-ake up the preponderant part 

of that formation. Kansome also showed that the " siliceous beds " 

of the Star Peak are in part, at least, volcanic rocks.

The Middle Triassic limestones have yielded the remains of remarkable huge Ichthyosauria (marine "fish-lizards"), which were

9 King, Clarence, Systematic geology: U. S. Geol. Bxpl. 40th Par. Kept., vol. 1, pp. 

268-278, 347, 1878.

10 Smith, J. P., The Middle Triassic marine invertebrate faunas of North America: 

U. S. Geol. Survey Prof. Paper 83, pp. 8-9, 1914. Smith's presentation of the stratigraphic results of his important work is unfortunately all too brief.

u Ransome, F. L., Notes on. some mining districts in Humboldt County, Nov. : U. S. 

Geol. Survey Bull, 414, p. 32, 1909.
11

obtained at Saurian Hill and Fossil Hill,12 on the east flank of Black 

Range, 011 the divide between South American Canyon and Troy 

Canyon, 3 or 4 miles east of the Rochester district. At the same 

locality Smith collected 111 species of ammonites and 19 species of 

other organisms.

The Jurassic appears to consist mainly of slates, which exceed 

2,000 feet, in thickness.13 King thought that the Jurassic rests on the 

Triassic with " perfect conformity," but Louderback and Smith say 

that it' overlies the Triassic unconformably. The unconformity is 

apparently not very great, for, according to Louderback, " some deformation and erosion probably took place at the end of the Triassic, 

but of comparatively limited magnitude, for the Jurassic is distributed to Hbout the same extent that the Triassic is." 14

The Triassic and Jurassic rocks have been folded into a broad anticline whose axis strikes north and plunges southward, thus causing the west limb of the anticline to strike at an angle of 30° with 

the meridional trend of the range itself. As has long been recognized and generally accepted, this folding took place in post-Jurassic 

time, most probably at the end of the Jurassic, contemporaneously 

with the revolution that affected the Sierra Nevada region at this 

time the great orustal disturbance that will here be termed the 

Jurasside revolution.15 In the Star Peak Range, as in many of the 

ranges of the Great Basin and the Sierra Nevada, this revolution 

was accompanied by the intrusion of granite and related rocks. Unlike some of the other ranges, the Star Peak Range shows but a 

small amount of granite so far exposed by erosion. This igneous 

activity, according to Ransome,10 was followed by the deposition of 

a series of antimonial silver ore bodies a conclusion that admittedly 

rested on slender evidence. However, the evidence at Rochester is 

convincingly clear that there the mineralization took place soon 

after the intrusion of the granitic rocks. The lean silver-bearing 

veins that were then formed have subsequently been enriched to 

workable grade by the action of descending surface waters during 

the long erosion to which the region was subjected in post-Jurassic 

time.

The broader geologic features of the Star Peak Range are shown 

in generalized form in Figure 2.

The next younger formation in the range is a coarse bouldery 

deposit, which evidently represents a series of ancient alluvial cones

13 Merriam, .7. C., Triassic Ichthyosauria: California Univ. Mem., vol. 1, pp. 18-19, 1908.

"King, Clarence, op. cit., pp. 269, 294-29-5.

34 Geol. Soc. America Bull., vol. 15, p. 335, 1904.

u This usage conforms to that employed by R. T. Chamberlln to denote the folding or 

strong diastrophic movements that took place either at or near the end of a geologic 

period (Diastrophism and the formative processes: Jour. Geology,, vol. 22, p. 317, 1914).

50 Ransome, F. L., U. S. Geol. Survey Bull. 414, p. 46, 1909.
12 ROCHESTER DISTRICT, NEV.

that were amassed in front of a young growing range. They are 

Pliocene in age or possibly Quaternary. They were partly covered 

by flows of basalt, as is well shown in the north end of the Humboldt 

Lake Mountains. Although these basalt sheets are most extensive in 

the Humboldt Lake Kange, where in fact they form a widespread 

capping, they also occur in the Star Peak Range, but only in small 

areas on its east flank bordering the valley. The basalt sheets of the 

Humboldt Lake Range were broken by longitudinal normal faults 

and tilted eastward. By this faulting and tilting, as proved by 

Louderback,17 the range was given its relief and topographic expression; it was blocked out essentially in the form we now'see it. 

The surface of the main north-south fault forms the abrupt westward-facing escarpment, and the surface of the basalt Sheets, which 

at the north end of the range dip 12° E., forms the gentle east

FIGURE 2. Diagrammatic section through the south end of the Star Peak Range. 1, Triassic limestone, probably Middle Triassic; 2, Weaver rhyolite; 3, trachytic and keratophyric lava, breccia, and tuff; 4, granite porphyry; 5, aplite; 6,. massive Triassic 

limestone, probably Upper Triassic

slope of the range. The physiographic evidence fully supports this 

conclusion concerning the origin of the range. In regard to the 

origin of the Star Peak Range we are limited to physiographic 

criteria, because of the absence of the basalt sheets, but the evidence 

of the land forms in support of its fault-block origin is so strong 

as to be compulsory. The second and major uplift of the Star 

Peak Range, which is thus indicated by the physiographic evidence, 

doubtless took! place contemporaneously with the faulting that 

blocked out the Humboldt Lake Mountains. In Recent time faulting has recurred along the west front of the range and has produced the fresh scarp that traverses the alluvium at the base of the 

range. This fault extends continuously for more than 100 miles, 

and the scarp is from 10 to 29 feet or more high.18

17 Louderback, G. D., Basin Range structure of the Humboldt region: Geol. Soc. America 

Bull., vol. 15, pp. 3l2r-316, 1904.

18 Russell, I. C., Geological history of Lake Lahontan : U. S. Geol. Survey Mon. 11, p. 

277, 1885.
PART II. GENERAL GEOLOGY OF THE DISTRICT

TRIASSIC ROCKS

INTRODUCTORY OUTLINE

Triassic rocks, chiefly of volcanic origin, make up the larger part 

of the bedrock of the Rochester district.

The oldest rocks are felsitic trachytes, comprising lavas, breccias, 

and tuffs, here named the Rochester trachyte. They aggregate 5,000 

feet in thickness. With them are associated keratophyres, partly as 

a thick series of flows and pyroclastic rocks and partly in such intimate association with the trachytes as to form belts of alternating

trachyte and keratophyre.

Conformably overlying the trachytes and keratophyres is another 

volcanic series, which is essentially of rhyolitic composition. The 

lowest member of this sequence is the Nenzel rhyolite breccia, of 

variable thickness but 600 feet thick at a maximum. It is of main 

economic interest, because the most productive silver veins crop out 

in it. Rhyolite tuffs, of sandstone-like appearance but in reality 

composed of shards of glass, overlie the Nenzel rhyolite breccia conformably but being of lenticular habit are not everywhere present. 

Above the Nenzel rhyolite breccia and the tuffs lies the Weaver 

rhyolite, consisting chiefly of flows that aggregate 720 feet in thickness. The rhyolite lavas of this formation have highly distinctive 

characters, such as abundant prominent phenocrysts of quartz and a 

widely prevalent content of quartz nodules and geodes resulting from 

the filling of spherulites that originally were hollow. Some lenses 

of tuffaceous shale are intercalated between the flows of Weaver 

rhyolite.

The assemblage of trachytes, keratophyres, and rhyelites, with their 

breccias and tuffs, makes up the Koipato group of the Fortieth 

Parallel Survey in fact, it was recognized by the geologists of that 

Survey that in Limerick Canyon this " group " attains its greatest 

thickness.19 It is of Middle Triassic age at least, but it may be older, 

though this is regarded as highly improbable.

Limestones overlie the volcanic rocks in the western part of the 

district. They have been faulted down against the Weaver rhyolite. 

They are part of the Star Peak group of the Fortieth Parallel Survey, but as neither Koipato nor Star Peak is a secure stratigraphic 

term and as not enough work justifying their redefinition has been

*> U. S. Geol. Expl. 40th Par. Kept., vol. 1, p. 270, 1878.

13
14 ROCHESTER DISTRICT, NEV.

done, those terms are not employed in this report. The limestone in 

the western part of the Rochester district contains marine fossils, 

which indicate that it is of Middle or Upper Triassic age. Comparison with the columnar section given by Smith suggests that it is of 

Middle Triassic age.

ROCHESTER TRACHYTE

GENERAL FEATURES

A light-colored felsite carrying small inconspicuous phenocrysts 

of microcline is the prevailing rock in Rochester Canyon. It is well 

exposed on the ridges inclosing the canyon and forms the lower slope 

of Nenzel Hill, where it is penetrated by extensive mine workings. 

It extends northward to Gold Mountain and eastward far beyond 

the boundaries of the area mapped. It is the most widespread of the 

Triassic volcanic rocks.

This volcanic formation comprises lavas, breccias, and tuffs. As a 

whole they are. remarkably similar and monotonously alike. More 

than half of the assemblage, as shown by a detailed study of the 

excellent sections along i^he ridges bordering Rochester Canyon,, consists of breccias and tuffs. As the fine-textured tuffs are extraordinarily difficult to distinguish from the massive felsite, it is probable 

that the pyroclastic portion of the formation considerably exceeds 

the massive portion. In places the tuff beds are clearly lenticular 

and are intercalated in the breccias and thus supply a means for 

determining the strike and dip of the formation. At many places 

they fail, however, and the monotonous similarity of the formation 

as a whole and the absence of key layers of breccia or of distinctive 

lava sheets makes it impossible to determine the local structure. 

None of the breccias are coarse, for the fragments of which they 

are composed are rarely as much as 6 inches in diameter and are 

generally less than an inch.

The lava sheets are commonly unfoliated, but the breccias and tuffs 

are distinctly though roughly schistose, with more or less sericitic 

gloss on the foliation surfaces.

PETROGRAPHY

Felsite is a highly appropriate field name for these volcanic rocks. 

They are exceedingly fine grained, and the phenocrysts they contain 

are as a rule sparse, small, and not readily discerned. Feldspar, 

which is commonly striated but which under the microscope proves 

to be microcline, doubtless derived from sanidine as the result of the 

djmamic metamorphism the rocks have undergone, is the only 

porphyritic constitutent in the great bulk of the series. A few 

minute sporadic phenocrysts of quartz occur in some of the upper
TBIASSIC ROCKS 15

members, but they hardly affect the generalization that this thick 

pile of volcanic rocks consists of felsites that carry inconspicuous 

phenocrysts of microcline only. It is the absence of quartz phenocrysts that specially contrasts the trachytes with the overlying 

Nenzel rhyolite breccia and Weaver rhyolite.

The breccias and tuffs consist of angular fragments of felsite of 

the same kind as that of the lava sheets. The lack of variety in the 

constituent material enhances the essential uniformity of the series. 

Dark minerals, such as biotite or hornblende, or vestiges of dark 

minerals have not been found in any of the lavas, breccias, and tuffs. 

In color the felsites on fresh fracture are generally a rather pure 

white. Rarely they are blackish or bluish gray. On weathered surfaces the felsites lavas, breccias, and tuffs alike are characteristically buff or brownish yellow, owing to the oxidation of the finely 

disseminated pyrite introduced into them during the mineralization 

of the district.

The felsitic trachyte of the belt extending from Nenzel Hill north 

to Gold Hill is distinguished for its remarkable streakiness and flow 

banding. Some of the most highly flow-banded felsite consists of an 

alternation of bluish-black and cream-colored bands, a fourth to half 

an inch thick, more or less curly and contorted. Spherulitic structure also is common and has become strikingly accentuated in the 

mineralized areas by the processes of alteration accompanying the 

mineralization. In places the trachyte is vuggy from the prevalence 

of gas cavities, and in the area of mineralization these cavities have 

been filled or lined with zinc blende, galena, pyrite, chalcopyrite, and 

quartz, as is well shown in the Pitt and Friedman tunnels. In addition to the streakiness and flow banding, many of the lavas are flow 

layered, breaking up into thin layers, an inch or so thick, which were 

produced by flowage.

The flowage structure is most conspicuous in the belt extending 

from Sunflower and Nenzel hills to Gold Mountain, but it can be 

found throughout the areal extent of the Rochester trachyte, although elsewhere generally far less pronounced.

The best-preserved material of this formation occurs on Gold 

Mountain, in the northern part of the district. It is nearly normal 

rock without a trace of foliation and without the secondary sericite 

and pyrite so commonly developed in the trachyte throughout most 

of the district. It is regarded as representative of the formation as 

a whole in so far as a single 'specimen is likely to be, and the remainder of the formation probably does not depart widely from it in 

composition. It is clearly the unmineralized, unaltered equivalent 

of the flow-banded felsite on the lower western slope of Nenzel Hill, 

into which the productive silver veins extend in depth and which is 

deeply penetrated by the Pitt and Friedman tunnels.
16 BOCHESTEK DISTBICT, NEV.

The trachyte from Gold Mountain selected for chemical analysis 

was obtained from the upper spherulitic portion of a streaky, highly 

flow-banded lava sheet. It is a dark-gray rock carrying numerous 

slender phenocrysts of microcline, not over a millimeter or two in 

length, many of which have a fairly glassy luster. The groundmass 

consists of spherulites about 3 millimeters in diameter, some of which 

are obscurely banded. Under the microscope phenocrysts of microcline are seen scattered through a matrix composed wholly of spherulites. Crystals of microcline generally form nuclei around which the 

spherulites have grown. The spherulites are dusted with minute 

black particles, doubtless oxide of iron, and it is due to this pigmentation that the trachyte is darker than the great bulk of allied rocks 

in the district. The spherulites are only feebly birefringent, the 

interference tints being generally not higher than iron-grays, and in 

many the birefringence is barely perceptible. They are evidently 

composed of feldspar, as the index of refraction is markedly less than 

that of Canada balsam. They are built of fibers that give parallel 

extinction, some of which have positive and others negative elongation, although the predominant effect on inserting the gypsum plate 

is that of positive elongation ; manifestly the feeble birefringence of 

the spherulites is due in part to the compensatory effect of superposed 

fibers of opposite optical elongations. As the following chemical 

analysis of the rock indicates, the spherulites must consist almost 

wholly of potassium feldspar. They are concentrically banded, 

though as a rule there are not more than two zones. The accessory 

minerals are abundant oxides of iron (apparently hematite and magnetite) and minor amounts of apatite, titanite, and zircon. No ferromagnesian minerals or traces of them are recognizable. The only 

alteration product is quartz, occurring in a few minute veinlets and 

as local, incomplete replacements of some of the microcline phenocrysts.

Analysis of trachyte from Gold Mountain, Rochester district 

[R. C. Wells, analyst]

SiOs ___________________________ _ 67. 01 

A12 03 ____ ___ - 15. 97 

Fe203 __________ - I 2. 54 

FeO _____________________________ _ . 44 

MgO ____________ __ Trace. 

CaO _________ - 19 

Na2 0 ____________ - 1. 92 

K2 0_-_ .- __ ___-_ 11. 42

TiO2 __________________ __.. __. . . 39

100. 16
TKIASSIC ROCKS - 17

The immediately striking feature of the analysis is the high percentage of K2 0 nearly three times that of the average rhyolite, 

which is 4.09 per cent.20 There are in Washington's tables 21 but 

four analyses of rocks termed rhyolite, quartz porphyry, or felsite 

that contain as much K,0 as this trachyte. The silica content is 

slightly higher than it would be in the absolutely fresh rock, on account of the presence of a small quantity of secondary quartz.

In view of the absence of quartz phenocrysts in the rock from 

Gold Mountain, the extremely high content of K2 0, and the relatively low SiO2 that is, low for a rhyolite it will be termed a 

trachyte. In the norm system of classification it falls into class I, 

order 5, rang 1, subrang 2. This subrang, which is unnamed, is 

represented by only one analysis that of a pegmatite from Broken 

Hill, Australia,22 which, however, should be excluded, as it is 

an altered mineralized rock.

THICKNESS AND AGE

The thickness of the Rochester trachyte exposed in the Rochester 

district is of the order of 5,000 feet. Roughly one-half of this 

thickness, as already mentioned, consists of breccias and tuffs.

The most favorable section for measuring the thickness of the 

formation is that extending westward from Sunflower Hill along 

the ridge crest on the south side of Rochester Canyon. It crosses 

the belt of trachyte approximately at right angles; the strike of the 

formation is N. 30° W., and the dip averages 30° W. From the 

measured 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 how much correction can not be determined, because the amount of faulting is not ascertainable, owing 

to the lack of key horizons. That 5,000 feet represents the order of 

magnitude of the thickness, however, is indicated by the following 

consideration. The trachyte in the measured section is part of the 

western limb of the great anticline of the Star Peak Range, and the 

eastern limb, which is best exposed in the ridges extending eastward from Gold Mountain (east of the mapped area), displays a 

thickness of trachyte of the same order of magnitude. That this 

agreement is a mere coincidence appears improbable.

The apparent thickness of the trachyte along the ridge bounding 

the north side of Rochester Canyon is 6,000 feet. In neither section 

is the base of the trachyte exposed, nor has it been found elsewhere, 

so that 5,000 feet appears to be a conservative estimate for the thickness of the formation.

ao Daly, R. A.,, Igneous rocks and their origin, p. 19, 1913.

21 Washington, H. S., U. S. Geol. Survey Prof. Paper 99, pp. 308-309, 1917.

"Idem, p. 269.
18 ROCHESTER DISTRICT, NEV.

The Rochester trachyte is the preponderant element in the Koipato 

group as defined by King, of the Fortieth Parallel Survey. It was 

considered to be of Lower Triassic age but, as will be shown subsequently, is most probably Middle Triassic.

DUMORTIERITIZED TRACHYTE

Dumortierite, an aluminum borosilicate (Al8HBSi3020 ), occurs 

in extraordinary abundance on the west flank of Lincoln Hill. In 

the amount of this comparatively rare mineral that it contains this 

area, so far as is now known, is without a peer anywhere. The 

dumortierite is prevailingly pink and lavender, the blue color usually 

thought to be characteristic of the mineral being very rare. Kich 

cobalt-blue dumortierite occurs in places with the deep-pink variety, 

and the combination produces a gorgeously colored rock. The dumortierite-bearing area is underlain largely by felsitic trachyte breccias, which strike N. 20° W. and dip 25° W. The dumortierite is distributed through these rocks in brilliantly colored splotches of red, 

pink, and lavender, but more commonly it occurs as a close-spaced 

network of pink veinlets. The area that contains this dumortieritic 

rock is outlined in Plate I; its boundary is, of course, not hard and 

fast, for the amount of dumortierite fades out gradually toward the 

borders. In much of this area hardly a cubic yard of the rock is without a veinlet of dumortierite, and in many places the veinlets are so 

abundant that they average an inch apart. They range in thickness from a small fraction of an inch up to 6 inches. They appear 

to follow no system or systems, though perhaps they are more commonly normal to the bedding.

The thinner veinlets consist chiefly of pink dumortierite with 

some quartz; in places they contain also a little brown tourmaline. 

The thicker veinlets consist more largely of quartz than the thinner 

ones, and some of this quartz is noteworthy because of its fine rose 

color. Between the rose quartz and the pink dumortierite all stages 

of gradation due to intergrowth of the two minerals can be followed 

in hand specimens. Under the microscope the transition can be seen 

more readily and what appears to be clear rose quartz to the unaided 

eye is found to inclose numerous hairlike fibers of dumortierite, 

many of which have the ultra-microscopic diameters of the thinnest 

sillimanite needles, which, indeed, they resemble indistinguishably. 

Where the dumortierite needles become too numerous the rose quartz 

loses its translucence. This new variety of rose quartz thus certainly owes its color to inclosure of capillary fibers of pink dumor-
TRIASS1C ROCKS 19

tierite. 23 The lavender-colored dumortierite, according to W. T. 

Schaller, gives a decided test for titanium. The titanium is thought 

to be present as Ti2 03 , replacing A12 03 .

That the development of the dumortierite in the felsite was' accompanied by other profound changes is evident in the field, but to determine what these changes are requires the aid of the microscope. 

Some silvery-white mica in small scales is in places apparent to the 

unaided eye, as well as a light-colored vitreous mineral that has a 

fairly good cleavage. Under the microscope the dumortieritized 

felsites are seen to have undergone a drastic alteration: all volcanic 

textures have been completely obliterated, and the rocks consist of 

four minerals in variable proportions quartz, andalusite, dumortierite, and sericite. The quartz is the most abundant constituent 

and forms a fine-grained mosaic. The andalusite is generally second 

in abundance; it forms anhedrons as much as half an inch in length,

many of which are markedly skeletal and inclose much quartz. The 

dumortierite is commonly in radiate groups of sillimanite-like 

needles or of fibers that have brushlike ends resembling sillimanite 

brushes. The thicker needles show a red, lavender, or blue pleochroism parallel to the c-axis and paler tones transverse to it. Where 

both dumortierite and tourmaline occur together, as in some of the 

veinlets, they contrast in pleochroism and absorption very markedly. 

The fine-grained micaceous mineral generally present in subordinate 

quantity is believed to be sericite, though some may be pyrophyllite 

(HAlSi2 06 ), which has indistinguishably similar optical properties. 

It has in part grown at the expense of the andalusite.

The extensive development of dumortierite and associated minerals is clearly due to the action of boron-bearing gases, which have 

undoubtedly escaped from-an underlying granite magma and permeated the trachytes as they traveled upward through a closely 

spaced network of fractures. These emanations were evidently rich 

in silicon, aluminum, and boron and deficient in alkalies and sulphides. The" development of andalusite by this pneumatolysis is 

noteworthy, though perhaps not surprising in view of the close resemblance of andalusite and dumortierite, especially in composition. 24 The metasomatic origin of andalusite appears to be rare. A

23 A preliminary account of the occurrence of the dumortierite in the Rochester district 

and the possible utilization of the rose quartz as a semiprecious stone was given by the 

writer in Schaller, W. T., Gems and precious stones: U. S. Geol. Survey Mineral Resources 

1916, pt. 2, p. 893,, 1919. J. C. Jones had earlier (Geology of Rochester, Nev.: Min. and 

Sci. Press, vol. 106, p. 737, 1913) identified the dumortierite as pink tourmaline (rubellite). Dumortierite ia most easily distinguished from tourmaline by its cleavage and by 

its absorption, which Is strongest parallel to- its elongation.

M Schaller, W. T., Dumortierite: U. S. Geol. Survey Bull. 262, p. 110, 1905. The occuricnce of dumortierite in Washington described by Schaller has tho same mineral association as that in Nevada namely, quartz, andalusite, .dumortlertie, and muscovitp hut its 

genesis is unknown.
20 ROCHESTER DISTRICT, NEV.

remarkable example from the Inyo Eange, Calif., has been described, where andalusite has formed in enormous quantity, making 

up a mass more than 300 feet wide, which has resulted from the replacement of ia volcanic porphyry.25 Andalusitic alteration of Tertiary lavas, an alteration that evidently took place under conditions 

somewhat different- from those indicated for the Inyo and Rochester 

occurrences, has been described fry Butler,20 who pointed out that 

alteration of this kind had not previously been recorded.

KERATOPHYBES 

OCCURRENCE AND CHARACTER

The slopes of Limerick Canyon consist largely of a series of altered 

volcanic rocks of keratophyric composition. They comprise tuffs, 

breccias, and lava flows, some of which are amygdaloidal. As a rule 

they are porphyritic, owing to the presence of phenocrysts of white 

striated feldspar, and as these feldspars are embedded in a darkish 

groundmass, the rocks wholly resemble andesite. Inasmuch as 

under the microscope the porphyritic crystals invariably prove to 

be albite, the rocks are termed keratophyres.

The dip of the keratophyre flows and breccias is prevailingly 

30° W., though locally they stand vertical. The apparent thickness 

of the keratophyres on the north slope of Limerick Canyon, possible duplication by faulting being disregarded, is 4,500 feet. Southward they thin abruptly, so that on the ridge between Limerick and 

Rochester canyons they are much thinner. It must be pointed out 

that although boundaries are drawn on the geologic map (PI. I) 

delimiting the keratophyre belt, yet a few sheets of keratophyre are 

intercalated in the adjacent trachyte, both eas^t and west of the main 

belt where it crosses the divide between Limerick and Rochester

canyons.

Smaller areas of keratophyre occur in the Black Range, east and 

southeast of Nenzel Hill.

PETROGRAPHIC FEATURES

The keratophyres contain numerous phenocrysts of albite, which 

are set in a fine-grained groundmass of rather dark color. A few 

have also phenocrysts of quartz, and such varieties may be termed 

quartz keratophyres. Crystals of albite as much as half an inch 

in length have been etched in relief on the weathered surfaces of

some of the pyroclastic members. Most of the keratophyres are

________/______________,_____________

25 Knopf,, Adolph, An andalusite mass in the pre-Cambrlan of the Inyo Range, Calif.: 

Washington Acad. Sci. Jour., vol. 7, pp. 549-552, 1917.

29 Butler, B. S., Geology and ore deposits of the San Francisco and adjacent districts, 

Utah: U. S. Geol. Survey Prof. Paper 80, pp. 78-82, 1913.
TRIASSIC BOOKS 21

roughly or even rather thoroughly schistose, and this is especially 

true of the tuffs and breccias. The considerable alteration, both 

dynamic and pneumatolytic, that these rocks have undergone interposes an insuperable obstacle to their satisfactory petrographic 

study.

However, of the many specimens examined microscopically all 

but a few contain albite phenocrysts only, so that they fall readily 

into the class of keratophyres. The phenocrysts of albite evidently 

resisted metamorphism more effectually than the other constituents 

and have survived both the dynamic disturbance (though some are 

cracked and broken) and the Jurasside pneumatolysis, even though 

the groundmass has been drastically altered. But under intense 

pneumatolytic attack some of the albite phenocrysts have been replaced by aggregates of biotite and calcite. What ferromagnesian 

minerals were originally present was determinable in only one rock 

examined a quartz keratophyre from the summit of the 6,648-foot 

peak at the east margin of the district. It is the best-preserved 

keratophyre found and is massive and of grayish-green color. It is 

a porphyritic rock of dacitic aspect, containing phenocrysts of 

feldspar and quartz and chloritic pseudomorphs. Under the microscope the porphyritic minerals are seen to be albite (Ab90An10 ), 

microcline, and quartz. Chlorite pseudomorphs are abundant, some 

of which clearly are after pyroxene, for square sections with truncated corners are common. 27 The groundmass is fine grained and 

obscured by alteration products. This keratophyre is the only one 

found that contains phenocrysts of potassium feldspar along with 

those of albite and is therefore not typical of the series as a whole. 

The groundmasses of the keratophyre are generally indeterminable, 

but in a fragment inclosed in one of the pyroclastic breccias it could 

be seen that the groundmass is salic, consisting largely of albite in 

faint fiuidal arrangement.

The keratophyres were richer in ferromagnesian constituents than 

the trachytes; hence their darker color and andesitic appearance. 

During dynamic metamorphism the ferromagnesian minerals were 

c altered to chlorite, especially in the pyroclastic keratophyres, which 

thereby became chloritic schists, and subsequently by pneumatolysis 

the chorite was altered to biotite and concomitantly biotite was developed more or less promiscuously throughout the rock, even replacing the albite phenocrysts.

In many places pneumatolytic action Has been particularly intense. 

Tourmaline is extremely common in such places, rather evenly 

scattered as innumerable black needles through the rocks thus affected, and biotite is unusually abundant. f

27 It is interesting to recall that Rosenbusch (Elemente der Gesteinslehre, 3d ed., p. 345, 

1910), in defining the keratophyres, gives diopside as the characteristic ferromagnesian 

phenocryst.
22 ROCHESTER DISTRICT, JSTEV.

RELATION OF KERATOPHTRES TO THE ROCHESTER TRACHYTE

The keratophyre contains in places some intercalated trachyte and 

banded trachyte tuff, proving that these different volcanic rocks were 

erupted contemporaneously. Where the two kinds are associated, 

as near Limerick Pass, the keratophyre is more highly schistose than 

the trachyte. Farther west in Limerick Canyon the keratophyre 

and trachyte form a distinct belt in which they alternate so regularly 

that it was desirable to map the belt as a separate entity. (See 

PL I.) The belt is not of uniform width but widens northward; the 

thickness of the alternating series is 900 feet on the summit of the 

ridge on the north side of Limerick Canyon, where the volcanic 

rocks stand practically vertical. Single sheets of keratophyre are as 

much as 100 feet thick. Near the top of the succession at this place 

is a keratophyre amygdaloid which is spotted white with' numerous 

phenocrysts of albite and amygdules of quartz; it is fairly schistose, 

and its foliation planes shimmer from innumerable tiny flakes of 

biotite.

There are a few narrow belts at various places in the Rochester 

trachyte that appear to be of mixed origin, being due to a commingling of trachyte and keratophyre tuff. Through subsequent 

shearing and the development of secondary minerals it is impossible 

to determine certainly the origin of these rocks. They are darker 

than the f elsites and somewhat lighter in color than the keratophyres, 

but their difference from either is not sharply defined.

AGE AND CORRELATION

The keratophyre eruptions were clearly contemporaneous with 

those of the trachytes, and the keratophyres are therefore of Triassic 

age. The close association of these two varieties appears to imply a 

very notable magmatic differentiation during the Triassic volcanism, 

whereby the potassa was largely concentrated in one magma and the 

soda in another.

Keratophyres occur in other Triassic areas of Nevada. They are 

abundant in the Yerington copper district, where they are lightcolored felsites with inconspicuous phenocrysts of albite. 28 Curiously 

enough, these rocks at Yerington do not in the least resemble in 

appearance the keratophyres of the Rochester district but they do resemble exactly the trachytes. In the Simon silver-lead district of 

Cedar Mountain, north of Tonopah, highly porphyritic quartz keratophyre is associated with limestone of Middle Triassic age. 29

28 Knopf, Adolph, Geology and ore deposits of the Yerington district,. Nev.: U. S. Geol. 

Survey Prof. Paper 114, pp. 13-16, 1918.

29 Knopf, Adolph, Ore deposits of Cedar Mountain, Mineral County. Nev. : U. S. Geol. 

Survey Bull. 72-5, pp. 363-364, 1921.
U. S. GEOLOGICAL SURVEY BULLETIN 762 PLATE I

118 I3'00" R. 33 £-

EXPLANATION 

SEDIMENTARY ROCKS

.§ Coarse alluvial-cone oc

Granite porphyry

Dumortieritized trachyte

Qal ^P^SWpw

Trachyte and keratophyre

in alternating sheets

Faults

(U, upthrow; D. downthrow)

deposits; location approximate

Strike and dip of bedded 

rocks

118 13'00" R. 33 E

Topography by T P. Pendletcm 

Surveyed in 1916

GEOLOGIC MAP AND SECTION OF THE ROCHESTER DISTRICT,

PERSHING COUNTY, NEV.

S cale

Geology by Adolph Knopf 

Surveyed in 1917

i. Mile

sooo 1000 aooo Feet

i Kilometer

Cooxtoxu? interval 25 feet 

i' is m&fuT, sea, level

Independence 

Hill

-A- ,-,

Section alon4 line A~A

5,000
TRIASSIC ROCKS 23

In conclusion, it appears that the Middle and Upper Triassic were 

times of great igneous activity in Nevada, during which large volumes of widely diverse volcanic rocks were erupted. Andesite, 

dacite, trachyte, rhyolite, keratophyre, and quartz keratophyre are 

the principal kinds. The maximum aggregate thickness of these 

rocks is of the order of 10,000 or 15,000 feet.

NENZEL RHYOLITE BRECCIA 

OCCURRENCE AND CHARACTER

The rock whose bold craggy outcrops form the summit of Nenzel 

Hill is here termed the Nenzel rhyolite breccia. It is of particular 

interest because the silver veins first discovered crop out in it, and 

these veins have yielded the bulk of the silver that the district has 

produced.

The most prominent member of the formation is generally devoid 

of bedding, though in a few places it contains fine-grained banded 

layers a few inches thick, which in the best exposure were seen to 

represent lenticular tuff beds pinching out within a length of 40 

feet along the strike. Because of its massive outcrops the Nenzel 

rhyolite breccia closely resembles a lava, and this resemblance is 

enhanced by the fact that quartz and feldspar crystals are distributed 

evenly through it, like phenocrysts in a porphyry. That it is made 

up of ejected volcanic fragments, however, is indicated by the angular pieces of felsite scattered through it, though the breccia nature is 

generally obscure because the fragments resemble the matrix in which 

they are embedded.

The Nenzel breccia is generally stained from the oxidation of the 

secondary pyrite contained in it, but where it is absolutely unoxidized 

it is pure white. The component fragments are generally under an 

inch in diameter, though in places blocks as much as 16 inches in 

diameter occur. The microscope confirms the conclusions drawn from 

the field evidence as to the pyroclastic origin of the rock and its 

rhyolitic composition. The feldspar crystals and fragments prove 

without exception to be the potassium variety and somewhat perthitic. 

The rock fragments comprise felsites of various kinds microspherulitic trachyte and so on and rhyolite carrying quartz phenocrysts.

STRATIGRAPHY AND STRUCTURE

The best section of the Nenzel rhyolite breccia is that on the north 

end of Nenzel Hill; it is clearly exposed and displays a greater lithologic diversity than any other, and the stratigraphic relations are 

fairly well shown. (See fig. 3.) The lowermost member of the 

formation consists of a series of well-stratified breccias carrying as

98293 24 3
24 ROCHESTER DISTRICT, NEV.

a notable feature an abundance of euhedral quartz crystals; these 

beds strike north and dip 35° E. They are 50 feet thick, but their 

full thickness is not shown here, as they are faulted against the 

underlying felsite along a north-south fault dipping 75° E. The 

breccia beds are turned up so that they dip 55° or more to the east 

adjacent to the fault, which is therefore clearly a normal fault. Farther south along the strike of the beds, south of the Boughton & 

Hackley tunnel, the indicated thickness is approximately 250 feet, 

although the base of the series is not satisfactorily exposed. The 

uppermost bed of breccia is rather coarse, containing boulders as 

much as 15 inches in diameter.

Above the stratified breccias lies a highly distinctive bed, a conglomeratic breccia or subangular conglomerate, which is 10 feet 

thick. It consists largely of felsite detritus plus white rhyolite that 

is dotted with quartz phenocrysts; much of this material is sharply

w.

1500 FEET

FIGURE 3. Geologic section through, the north end of Nenzel Hill. 1, Trachyte, roughly 

schistose; 2, trachyte tuffs and breccias; 3, trachyte flows with subordinate tuff; 4, 

bedded rhyolite breccia; -5, conglomerate; 6, rhyolite breccia, essentially without bedding ; 7, well-bedded tuffs, resembling sandstones; 8, alluvium

angular, but some is well rounded. This characteristic bed can be 

traced only a short distance southward, nearly to the Transportation tunnel of the Rochester mine, but it extends northward for a 

mile, to the Plainview prospect. It appears locally on the east flank 

of the ridge also, as at Sage Hen Spring, where its presence is due 

to a number of strike faults that would not be detectable but for 

this bed.

Above the conglomerate is 25 feet of stratified tuff and breccia 

in beds ranging from 3 inches to 3 feet in thickness. Some of the 

beds are identical in appearance with the massive facies of the formation; others consist of white microcrystalline rocks devoid of the 

quartz crystals that are characteristic of the formation in general.

The massive member of the formation, whose outcrops form the 

crags on Nenzel Hill, rests on the bedded tuffs and breccias. It is 

without a trace of bedding, but its fragmental origin is here obvious, especially of the basal part. Fragments 16 inches in diameter 

are the largest that were seen. The total thickness of breccia above
TEIASSIC ROCKS 25

the conglomerate in the section shown in Figure 3 is 350 feet. The 

breccia is overlain by a well-stratified tuff that to the eye resembles a 

sandstone. The contact is a locus of weak mineralization and is 

doubtless a minor fault. Elsewhere the normal superposition of the 

sandstone-like tuff on the massive breccia is excellently shown, as 

on the summit of Nenzel Hill and on its south slope below the 

C P X shaft.

In summary, the Nenzel rhyolite breccia at its best exposure is 350 

feet thick and incloses a conglomerate bed near the middle of the 

section. If allowance is made for the portion faulted out near the 

base of the section, the total thickness may be estimated at 600 feet.

As the formation is traced away from the type section marked 

changes in its lithology become apparent. Toward the south the 

conglomerate disappears, as already mentioned, and furthermore it 

becomes impossible to discriminate the bedded breccias that lie 

below the horizon of the conglomerate from the massive breccia 

above that horizon. The result is that the massive member has apparently increased greatly in thickness, and this increased thickness 

of resistant breccia has produced the unusually bold outcrops on the 

southern brow of Nenzel Hill. Toward the north the formation 

thins progressively: under the summit of the 6,893-foot peak it is not 

more than 30 feet thick and the' intercalated conglomerate bed is 

only 3 inches thick. One of the flows of the Weaver rhyolite, a 

vuggy lava that is sporadically spherulitic, directly overlies the 

Nenzel rhyolite breccia here, the sandstone-like tuff that caps the 

breccia on Nenzel Hill having pinched out. There is some flow 

breccia at the contact of the lava with the underlying explosion 

breccia, so that it is difficult to distinguish the two rocks. Farther 

north the formation thicknes again, and at the Plainview prospect 

the conglomerate member is once more 10 feet thick.

The apparent great thickness of the Nenzel rhyolite breccia northeast of Nenzel Hill, on the east flank of the ridge more than 1,000 

feet is due to repetition by faulting. At least three strike faults 

were detected, but these have not been shown on the map.

The Nenzel rhyolite breccia is somewhat unsatisfactory as a key 

rock, owing to the fact that breccias of similar appearance are intercalated at various horizons in the succession of lavas that make up 

the Weaver rhyolite. None of these other breccias, however, was 

found to inclose the characteristic conglomerate. Nevertheless, faults 

can not be invoked offhand to explain what at first thought might 

appear to be anomalous occurrences of breccias; but in the areas 

shown on Plate I the faults demanded by the stratigraphy to account 

for the distribution of the breccias, as those at the south end of Sage 

Hen Flat, are clearly exposed in prospect pits.
26 ROCHESTER DISTRICT, NEV.

In conformity with the extensive faulting that the formation has 

undergone, the dip is highly variant: it is 35° E. in the type section 

at the north end of Nenzel Hill, horizontal in the Rochester mine 

above the Transportation tunnel, 30° E. on the summit of Nenzel 

Hill, 15° E. between the C P X shaft and the portal of the C P X 

tunnel, and 70° E. at the foot of the east slope of Nenzel Hill.

RHYOLITE TUFFS

Resting conformably on the Nenzel rhyolite breccia is a series of 

tuffs whose even, regular, sharply defined bedding is their most 

prominent feature. This clearly marked bedding has in fact led 

them locally to be called sandstones.

They are well exposed at various places on the summit of Nenzel 

Hill. At the Crawford shaft on this hill they exceed 70 feet in 

thickness, but as they are cut off on the east by a fault their full 

thickness is not known. The tuffs, as already shown in the description of the Nenr?l rhyolite breccia, evidently accumulated as 

lenticular deposits; consequently the Nenzel breccia is not everywhere overlain by these well-stratified tuffs but in places is covered 

by the basal flow of the Weaver rhyolite.

The beds range in thickness from an inch to a foot or two. In 

grain the tuffs range from those so fine as to have a conchoidal 

fracture resembling that of argillite to those that might be termed 

fine-grained breccias.

Material from some of the thin beds that are particularly suggestive of sandstone was examined under the microscope. It is a 

white rock which is faintly but distinctly laminated or banded. 

It proves to consist largely of shards of glass now silicified, filaments 

of pumice, and angular bits of orthoclase and quartz. The crescentic 

and pronglike sections of the glass shards and the striated, drawnout structure of the pumice fragments (like bits of pulled candy) 

unmistakably prove that this rock is an old rhyolitic ash. Despite 

the geologic vicissitudes through which this ancient ash has passed 

the vitroclastic texture has been strikingly well preserved.

WEAVER RHYOLITE 

OCCURRENCE AND CHARACTER

The Weaver rhyolite consists chiefly of lava flows. Intercalated 

tuffs and breccias are small in volume, contrasting in this respect with 

the great thickness of pyroclastic rocks associated with the Rochester 

trachyte. The main area of the rhyolite forms a broad, continuous 

belt extending from Nenzel Hill to Packard, and as the formation
TEIASSIC ROCKS 27

is well exposed at the head of Weaver Canyon, it is named the 

Weaver rhyolite. At Packard the rhyolite is of economic significance, 

because the ore bodies of the Packard mine are inclosed in it. Another 

belt extends along the west slopes of Independence Hill and Lincoln Hill, where it is much more schistose than elsewhere.

The successive flows of the Weaver rhyolite are much alike, the 

consequence of which is to emphasize the unity of the formation. 

Certain features that are distinctive of the formation as a whole are 

widely prevalent conspicuous phenocrysts of quartz and feldspar, 

spherulitic structure, and quartz nodules and geodes, which have 

been produced by the partial filling of gas cavities or of hollow 

spherulites with quartz. The variable development of these different features in one and the same flow gives rise to highly differing 

fades. For example, the quartz nodules due to the filling of hollow 

spherulites may locally be so abundant that the rhyolite containing 

them resembles a conglomerate made up largely of quartz pebbles.

The fresh rhyolite is white, but stained reddish-yellow varieties 

are most common. A botryoidal fracture is typical and under the 

microscope is found to be due to the spherulitic structure. Quartz 

phenocrysts are prominent in some flows, and pearly-white feldspar 

phenocrysts in others, but biotite or other dark minerals do riot occur, 

nor do they appear ever to have been present, except in negligible 

quantity in some flows. The obviously porphyritic habit of these 

rocks and especially their prominent phenocrysts of quartz distinguish them from the Rochester trachyte.

The thickness of the Weaver rhyolite is at least 700 feet, but its 

full measure is unknown, for the original top of the formation was 

nowhere seen. The tuffs and breccias that occur in small volume 

in the formation closely resemble the Nenzel rhyolite breccia.. North 

of Nenzel Hill the Weaver rhyolite rests directly on the Nenzel 

rhyolite breccia. Undoubtedly this breccia is the material ejected 

by the initial explosive o'l+.hnvstr that immediately preceded th?. outpouring of tlie succession of flows that make up the Weaver rhyoite.

PETROGRAPHY

The Weaver rhyolite, where fresh, is a pure-white rock, but in 

the mineralized areas it is generally stained yellowish, reddish, or 

buff from the oxidation of the pyrite. In structure it ranges from 

massive that is, without a trace of foliation to conspicuously 

though roughly schistose, with the foliation surfaces heavily plated, 

as it were, with silvery sericite. The rhyolites on the west flank of 

Independence Hill and Lincoln Hill are particularly strongly 

foliated, and their marked foliation is attributable to their being 

situated well down on the western limb of the anticline. In contrast,
28 BOCHESTEK DISTRICT, NEV.

those occupying the main ridge, which are situated at the crown of 

the anticline, as a rule are entirely massive.

The spherulitic crystallization so common in most of the flows has 

given rise, where the spherulites radiate from widely spaced centers, 

to a botryoidal fracture, or, where the spherulites were originally 

hollow and have later been filled with quartz, to quartz nodules or 

spheroids, or to geodes where the filling has been incomplete. Geodes 

of this kind are in places as large as a hen's egg. Both features the 

botryoidal fracture and the quartz-filled hollow spherulites may, 

of course, occur together. The quartz filling is coarse, granular, and 

glassy; and although it forms nodules that may be as much as V/% 

inches in diameter, the smaller nodules, averaging half an inch, are 

more common. In places they are so abundant as to make up half 

the bulk of the inclosing rhvolite. Schistose facies of such rocks O V

closely resemble sheared quartz conglomerate. Where the quartz 

filling is in irregular masses it simulates inclusions of quartzite in 

the rhyolite.

Spherulitic structure, then, is a particularly noteworthy feature of 

the Weaver rhyolite. In fact, certain flows consist entirely of a 

closely packed aggregate of spheroids and ellipsoids of spherulitic 

origin. The spherulites rarely exceed half an inch in diameter. 

Under the microscope most of them are seen to be composed of 

radiating fibers of orthoclase, which show negative elongation. 

Others are made up of radiating fibers of orthoclase and quartz, the 

orthoclase as before showing negative elongation and the quartz 

positive elongation, higher birefringence, and index exceeding 1.54. 

Some spherulites consist of both positively and negatively elongated 

fibers of feldspar. Some of them are concentrically banded, and 

where contiguous spherulites have mutually interfered the outer 

bands have coalesced, producing in thin section lemniscate patterns.

The phenocrysts comprise quartz and microcline, both showing 

marked magmatic corrosion. Biotite is extremely rare and is invariably completely altered. The microcline, which does not show 

pronounced plaid twinning, is perthitic, the albite having separated 

out as well-defined lamellae in some, of the pheiiocrysts. This perthitic microcline evidently represents ancient phenocrysts of sanidine 

that have become " unmixed " that is, the original homogeneous 

solid solution has broken up into microcline and albite. The groundmass, as a rule, is microspherulitic.

In summary, the petrography of the Weaver flows shows them to 

be rhyelites of normal composition.

TUFFACEOUS SHALES

Lentils of tuff and shale occur in the Weaver rhyolite intercalated 

between successive flows. They are generally from 50 to 100 feet thick
TRIASSIC BOCKS ' 29

and exceed 1,000 feet in length. The most extensive area of these 

rocks is that on the east slope of Nenzel Hill, but their apparent 

considerable thickness here is doubtless due to duplication by 

faulting.

The tuff is white where unweathered; in degree of metamorphism 

it varies widety from sericitic phyllit© to massive rock of almost 

felsitic aspect. The shale, or argillite, as some of it may be termed, 

is dark-colored. Some of the shale is slightly calcareous. Both tuff 

and shale are in places notably banded a useful feature, as it allows 

the attitude of the inclosing rhyolite flows to be determined. One 

of the most instructive exposures of such an intercalated mass of 

banded shale is that in the hill southwest of Packard. It is best 

displayed on the southeast side of the hill; the sedimentary bedding 

is plainly apparent, dipping 35° W., and the cleavage cutting the 

bedding at high angles is excellently shown, as is also its refraction

from bed to bed.

The long, narrow belt of tuff and shale at the Packard mine, a 

belt that is of particular economic significance, is bounded on both 

sides by parallel normal faults. Neither the presence of these faults 

nor the extent of the belt would have been determinable, except for 

the numerous exposures made during the course of mining.

LIMESTONE 

GENERAL FEATURES

Limestone occurs in the western part of the district, forming the 

foothill slopes of the Star Peak Range. It is a dark-gray rock, 

which is stratified in beds averaging 3 inches in thickness, those as 

much as 1 foot thick being rare. One or two thin layers of shale 

are intercalated in the limestone and are notable for the innumerable 

prisms of tourmaline they contain. The limestone strikes N". 35° W. 

and dips 25°-40° W. It conforms thus to the strike and dip of the 

underlying Weaver rhyolite, from which, however, it is separated 

by a fault that strikes northwest and dips west, probably at an 

angle of 45°. The limestone shows considerable internal disturbance, 

and in places it has been buckled into small folds that cause local 

eastward dips.

The apparent thickness of the limestone exposed is 2,000 feet, but 

there may possibly be some repetition by faulting, and neither the 

top nor the bottom of the section is observable.

AGE

The limestone along the flank of Independence Hill is shown on 

the map of the Fortieth Parallel Survey as the southernmost exten-
30 KOCHESTEB DISTRICT, NEV.

sion of the Star Peak formation. It contains many fossils of ammonites and belemnites, but as a rule they are poorly preserved. A 

small lot of fossils recently collected from this area was submitted 

to T. W. Stanton, who. reports as follows:

10078. Western part of Rochester district, Humboldt Range, Nev., in the Star 

Peak limestone of the Triassic. Only two fossils are recognizable in the limestone, one a small ammonite not well enough preserved for generic determination, the other a small slender belemnoid probably belonging to the genus 

Atractites. These fossils doubtless belong to the Triassic fauna, but I am 

unable to determine from them whether the rocks are Middle or Upper Triassic.

RELATION OF LIMESTONE AND VOLCANIC ROCKS

Within the mapped area (PL I) the formation mapped as limestone contains practically nothing but calcareous beds, but just south 

of the area it contains thick intercalations of volcanic rocks at several horizons. This assemblage of alternating limestone and igneous 

material is excellently exposed on the west flank and summit of the 

Black Kange. Rhyolite tuff and breccia are interbedded with limestone, and thick belts of roughly schistose, coarse andesite breccia 80 

and andesite amygdaloid 30 nearly half of the bulk of which consists 

of amygdules are intercalated in the limestone. From this locality 

Star Peak fossils were obtained by the Fortieth Parallel Survey.31 

It is thus apparent that the limestone and volcanic rocks are essentially contemporaneous and that Star Peak time was therefore a 

period of active volcanism.

The bedded rocks of the West Humboldt Kange were divided by 

King and Hague into two groups a lower group termed the Koipato, 4,000 to 6,000 feet thick, consisting of argillite and quartzite, 

which in places " remarkably resemble an erupted felsite porphyry," 

and an upper group, termed the Star Peak, 10,000 feet thick, consisting of " an alternation of three great limestone zones and three 

interposed quartzite zones." 32 In 1909 Ransome,33 as a result of a 

rapid reconnaissance of the Humboldt Kange, showed that the Koipato group, which he called the Koipato formation, consists predominantly of rhyolite and associated tuffaceous rocks and that the 

Star Peak greatly needed revision of its stratigraphy and lithology. 

These conclusions have been amply confirmed. The argillites, quartzites, and porphyroids of the Fortieth Parallel Survey are the rocks

80 Field determination ; it may possibly be keratophyre.

81 King, Clarence, Systematic geology: U. S. Geol. Expl. 40th Par. Kept., vol. 1, pp. 

277-278, 1878.

32 Idem, pp 268-278,, 347, 1878. (Hague's usage differs from, that of King; he refers to 

the Koipato as a, " series " and to the Star Peak both as a formation and as a series. 

Idem, vol. 2, pp. 727, 729, 1877.)

33 Ransoms, F. L.. Notes on some mining districts in Humboldt Couaty, Nev.: U. S. Geol. 

Survey Bull. 414, pp. 31-46, 1909.
TRIASSIC BOCKS 31

now termed trachytes, keratophyres, and rhyolites. Furthermore, 

much of the so-called quartzite in the Star Peak formation has likewise proved to be rock of volcanic origin.

The Koipato formation was regarded by King and Hague as of 

Lower Triassic age, and in considering it Triassic they have been 

followed by Ransome, Jones, and Schrader. What is actually known 

bearing on its age is that it lies conformably beneath the Daonella 

zone, of Middle Triassic age, as shown in the columnar section given 

by Smith (p. 10), who is noncommittal as to the age of the Koipato. 

Two considerations point to the conclusion that the formation is of 

Middle Triassic age: (1) The Daonella zone consists of limestone 

interbe'dded with tuff and may therefore represent the less active and 

waning stage of the volca.nism that began during Koipato time; (2) 

volcanism did not break out in the Cordilleran region during the 

Mesozoic until Middle Triassic time. Where the Lower Triassic is

present, as in the Inyo Range of eastern California, it is free from 

volcanic rocks, but during the Middle and Upper, Triassic great volumes of volcanic rock were erupted.34 Moreover, the volcanic rocks 

constituting the Koipato are essentially similar in degree of preservation and metamorphism to those in the overlying series, a correspondence that further supports the conclusion that they are all essentially 

contemporaneous;

METADIORITE

An irregular mass of diorite, which has been altered to metadioite, 

is intrusive into the Triassic limestone just south of Packard Station. 

Similar rock has not been found to occur elsewhere in the district. 

It has metamorphosed the limestone surrounding it, having caused 

numerous prominent crystals and sheaves of actinolite to form, 

without, however, altering the limestone matrix to marble.

The metadiorite is more or less schistose and has obviously been 

considerably metamorphosed, as shown by the amphibole it contains, 

which is in sheaf-like aggregates of fibers. Under the microscope the 

rock is found to be composed chiefly of granulated feldspar, which 

rarely shows albite twinning, actinolite in sheaves and radial groups, 

and chlorite; epidote and titanite are subordinate. It is accordingly 

termed a metadiorite.

On account of its metamorphic condition, the metadiorite is tentatively held to have been intruded before the post-Triassic deformation and therefore during the Triassic igneous activity.

01 Knopf, Adolph, Geologic reconnaissance of the Inyo Range and the eastern slope of 

the southern Sierra Nevada, Calif.: U. .S. Geol. Survey Prof. Paper 110, pp. 47-48,. 58-59, 

1918.

98293 24 4
32 ROCHESTER DISTRICT, NEV.

POST-TRIASSIC, PROBABLY LATE JURASSIC IGNEOUS

ROCKS

APLITE 

GENERAL FEATURES

A fine-grained white granite, a typical aplite, occurs as extensive 

masses in the district. It is particularly abundant at Lone Mountain 

and in Limerick Canyon, and it forms the backbone of Black Range. 

The aplite mass of Black Range is exposed in the gorge of South 

American Canyon, which cuts through the range, revealing it to a 

depth of nearly 2,000 feet; and from the summit of the range, with 

all the clearness of a diagram, the aplite mass can be seen in other 

deep canyons on the eastern flank of the range to pierce the crown 

of the great limestone anticline of which the range is here built. 

All the aplite bodies, whether on the flanks of the anticline or in the 

crown, are crosscutting intrusions, which have exerted no doming 

action on the surrounding rocks.

The aplite is intrusive into limestone, trachyte, and rhyolite and 

is itself intruded by dikes of granite porphyry. It is notably even 

grained in texture and homogeneous in composition. It is generally 

massive, though that in the Black Range is intensely shattered, evidently owing to its nearness to post-Tertiary lines of faulting, such 

as that along the west flank of the range.

A conspicuous feature of all the aplite masses throughout the district is the abundance in them of veins made up largely of black 

tourmaline and quartz. This prevalence of tourmaline attains its 

maximum near Lone Mountain, where there is hardly a cubic yard 

that does not contain a tourmaliniferous vein or veinlet. Furthermore, the keratophyres surrounding the intrusive aplite are intensely tourmalinized, so that it is manifest that here was a focus of 

powerful pneumatolysis.

PETROGRAPHY

s

The aplite is a fine-grained rock, devoid of all dark minerals, and 

is of white or light-gray color. Locally it has r an inconspicuous 

miarolitic structure. Under the microscope the specimen selected for 

chemical analysis, from the Black Range, is seen to be composed 

essentially of potassium feldspar and quartz in sharply defined micrographic intergrowth.' Albite (Ab95An5 ) is a rare pyrogenetic 

constituent, but it occurs in considerable amount as perthite, and 

this is confirmed by the analysis; its segregation during the breaking 

up of the original solid solution has been incomplete. Magnetite is 

a minor accessory. The grain of the rock is uniform, and the largest 

individuals hardly exceeed 1 millimeter in major dimension. The
POST-TEIASSIC IGNEOUS BOCKS 33

rock is fairly fresh, showing only minor sericite and a slight amount 

of kaolin. The chemical composition of the aplite is shown by the 

following analysis:

Analysis of aplite from Rochester district 

IB. C. Wells,, analyst]

Si02____________________________ 77.10 

AlaOa____________________-_____________ 12. 28

Fe2Os__________ ___ ___ 1. 04 

MgO___________________________________ . 04' 

CaO_____________________ ________ _ . 24

Na2O_________________________________ 2. 27 

K2 0___________ ___-___ ___ 6.17

H20-1 7, Ha0+ ]-- - 

Ti02___________________________-_____ . 13

Coj______________________________ __ Truce.

100.03

This analysis has been recast into mineral composition, as follows: 

Orthoclase_____________________________ 36.7 

Albite (AboBAns)__________________________ 20.5 

Quartz____________________________^___ 39.5 

Accessory minerals _______________ _ 3. 3

100.0 

GENETIC RELATIONS

The plutonic rock to which the aplite is genetically related is not 

exposed within the district. The nearest mass of this kind is the 

granite at Rocky Canyon, 5 miles north of the district. From the 

description and chemical analysis of this rock given by Hague 35 it 

is apparent that the rock would now be termed a quartz monzonite.

Analysis of granite from, the Star Peak Range 

[T. M. Drown, analyst]

SiO2_________________________________. 68.58

AhOn_________________________________. 16.51 

FeO _________________________________ 2. 52 

MnO_________________________________ . 11 

CaO _________________________________ 2. 91 

MgO _________________________-__ _. . 47

Na=O_________________________________. 4. 28 

K20___________________________________. 4. 34 

Ignition loss______________________________ .45

100.17

M U. S. Geol. Expl. 40th Par. Kept, vol. 2, p. 715, 1877.
34 ROCHESTER DISTRICT, NEV.

The quartz monzonite, as shown by Louderback,36 is intrusive and 

is of post-Triassic, probably epi-Jurassic age. It forms a small mass, 

4 miles long and 2 miles wide, and is doubtless to be interpreted as a 

stocklike upward extension of a large batholithic body lying at 

greater depth. The extensive bodies of aplite in the Rochester area 

have evidently been injected somewhat farther from the parent 

magma and locus of their origin than is common around most batholiths. Noteworthy also is the complete absence of lamprophyre intrusions, implying that the aplite has not resulted from the splitting 

of a parent magma into two so-called complementary magmas. It 

appears most probable that the aplite magma resulted from fractional crystallization and was subsequently squeezed out from the 

locus of differentiation.

The abundance of aplite associated with many of the Cordilleran 

batholiths is truly astonishing. In the Boulder batholith of Montana aplite occurs as innumerable dikes and as large stocks cutting 

the quartz monzonite, altogether aggregating between 5 and 10 

per cent of the 1,100 square miles of the exposed area of the 

batholith.37 In the Mount Whitney region of the Sierra Nevada 

aplite occurs in enormous abundance in the immense volume of 

quartz monzonite which there makes up the range.88 The great 

escarpment of the Sierra shows, moreover, that the zone1 of aplite 

injection has a vertical extent of at least 6,000 feet. In neither the 

Sierra nor the Boulder batholith is there sufficient lamprophyre (in 

fact, lamprophyre is practically absent) to meet the demands of the 

hypothesis that aplite and lamprophyre are the complementary fractions of the differentiation of a parent magma.

GRANITE PORPHYRY

Dikes and small masses of granite porphyry occur throughout the 

Rochester district, though none, so far as known, occur in the productive part of the district. They are conspicuously porphyritic, 

mainly because of the prominent quartz crystals they contain, though 

in some dikes both quartz and feldspar phenocrysts are prominent. 

As the groundmass of the porphyry is light-colored and aphanitic, 

the rocks resemble the Weaver rhyolite. A point of difference, which, 

however, is not an intrinsic difference, is that the orthoclase phenocrysts of the granite porphyry have commonly been altered to aggregates of minutely scaly biotite. These biotite aggregates weather

86 Louderback, G D., Basin Range structure of the Humboldt region : Geol. Soc. America 

Bull., vol. 15, pp. 317-318, 1904.

87 Knopf, Adolph, Ore deposits of the Helena mining region: U. S. Geol. Survey Bull. 

527, pp. 34-35,, 1913. Billingsley, Paul, and Grimes, J. A., Ore deposits of the Boulder 

batholith, of Montana: Am. Inst. Min. Eng. Trails., vol. 58, p. 289. 1918.

88 Knopf, Adolph, A geologic reconnaissance of the Inyo Range and the eastern slope of 

the southern Sierra Nevada, California: U. S. Geol. Survey Prof. Paper 110, p. 73, 1918.
TERTIARY (?) ROCKS 35

readily to limonite, and the porphyry is commonly dotted with small 

rusty masses of linionite. It can not be affirmed that the porphyry 

ever contained any original ferromagnesian mineral.

The granite porphyry, or alaskite porphyry, as it may possibly be 

called, is the youngest member intruded during the Jurasside igneous 

activity. Its position in the igneous sequence is fixed by the fact 

that in Limerick Canyon a dike, 100 feet wide, cutting the Triassic 

keratophyre can be traced into the aplite, as shown in Plate I, and 

therefore must be younger than the aplite. Near Packard Springs 

a long dike of the porphyry traverses the great mass of aplite there.

Under the microscope the porphyry from the dike in Limerick 

Canyon shows phenocrysts of quartz, orthoclase, and albite, which 

are embedded in a groundmass that, remarkably enough, is partly 

spherulitic. The orthoclase phenocrysts are almost wholly replaced 

by deep-brown, highly birefringent biotite in small flakes. This 

alteration to biotite was evidently effected by solutions at high temperature. The alteration of pyroxene or hornblende to biotite is 

common during contact metamorphism or " high-temperature" vein 

formation, but the conversion of orthoclase to biotite appears to be 

less common. As if to leave no doubt concerning the character of the 

agency that produced the alteration, deep-blue tourmaline is associated with the biotite. Sericite also is developed.

The granite porphyry dikes and masses as a rule are much altered 

by alteration of the orthoclase phenocrysts to pseudomorphs consisting of finely flaky biotite and by serialization in general. The 

granite porphyry was injected after the aplite and before the main 

epoch of tourmalinization and attendant pneumatolysis, hydrothermal activity, and ore deposition. If these energetic tourmalinizing 

solutions originated within still unconsolidated portions of the aplite 

magma, as appears to be a reasonable hypothesis, then they must have 

ascended from great depths, for the aplite into which the granite 

porphyry was injected was cold enough to cause the porphyry 

magma to cool and consolidate so rapidly as to produce a spherulitic 

groundmass. A considerable interval must therefore have elapsed 

between the time of consolidation of the aplite now exposed to view 

and the emission of the tourmalinizing solutions, which were released 

after the intrusion of the granite porphyry into the aplite.

TERTIARY (?) ROCKS

PLIOCENE (?) ALLTJVIAL-CONE DETRITUS

OCCURRENCE AND CHARACTER

Coarse detrital material consisting of angular blocks, some of 

which are as much as 6 feet in diameter, occurs in the western part 

of the district. It appears to be but loosely consolidated. It occurs
36 ROCHESTER DISTRICT, NEV.

at the mouth of Limerick Canyon, where it is maturely dissected to 

a depth of 400 feet, thus forming a series of rounded piedmont hills 

along the front of the Star Peak Eange; it is widespread at the 

mouth of Eochester Canyon and extends continuously up to a point 

high on the east slope of the Humboldt Lake Eange, and it occurs 

at Little Black Knob.

The detritus is evidently the material making up a series of ancient 

alluvial cones. It consists largely of boulders of Triassic trachyte 

and rhyolite derived from the Star Peak Eange, but in places, as at 

Little Black Knob, it consists chiefly of fragments of limestone. 

Only rarely can the exact locality from which the detritus was 

brought be determined, but when boulders of the dumortierite rock 

are found it is at once manifest that the source was Lincoln Hill. 

Such boulders, several feet in diameter, occur high on the flank of the 

Humboldt Lake Eange, 500 feet above the bottom of Eochester 

Canyon. They prove how deeply the alluvial cones have been dissected since they were built.

The thickness of the alluvial-cone detritus is unknown but exceeds 

500- feet. - AGE

That the alluvial-cone detritus is of Pliocene age appears probable 

from the following considerations:

The detritus is overlain by the series of basalt flows that form so 

prominent an element in the architecture of the Humboldt Lake 

Eange. These flows have been faulted and tilted, and it is this faulting and tilting, as is so convincingly demonstrated by Louderback,39 

that gave the range its elevation and essential topographic form. In 

short, the alluvial-cone detritus is older than the range. Since the 

range has come into existence it has been attacked by erosion, and 

deep, widely flaring canyons have been gnawed into it, some of them 

extending almost through it. Because of the diversity of events since 

the amassing of the alluvial-cone detritus the eruption of great 

basalt flows, the blocking out of a mountain range, and the subsequent great erosion of this range it seems reasonable to regard the 

alluvial-cone detritus as at least as old as Pliocene.

Subangular conglomerate, overlain by basalt and having the same 

stratigraphic position and geologic history as that in the Eochester 

district, occurs in the Yerington district, in western Nevada.40

BASALT

Basalt occurs only in the southwestern part of the district, which 

includes a bit of the eastern edge of the Humboldt Lake Eange. It

88 Louderback, G. D., Basin Range structure of the Humboldt region: Geol. Soc. America 

Bull., vol. 15, pp. 304-312, 1904.

40 Knopf, Adolph, Geology and ore deposits of the Yerington district, Nev.: U. S. Geol. 

Survey Prof. Paper 114, pp. 26-27, 30, 1918.
QUATEBNABY ALLUVIUM 37

makes up a number of superposed sheets, which rest on the older 

coarse alluvial detritus and tilted 12° E.

The basalt collected is highly vesicular and carries rather inconspicuous phenocrysts of glassy feldspar. Under the microscope the 

phenocrysts are seen to be bytownite (Ab20An80 ) and augite. The 

texture is seriate porphyritic. Basalt from Little Black Knob is 

compact and practically nonporphyritic. The sporadic phenocrysts 

in this basalt also prove to be bytownite (Ab20An80 ).

Olivine, partly altered to iddingsite, occurs in small crystals and 

grains. Augite is restricted to the groundmass, where it occurs in 

extremely minute grains. Both basalts are fresh.

The basalts of the Humboldt Lake Range were particularly studied 

by Louderback,41 because they supply stratigraphic evidence that the 

west slope of the range is a fault scarp. Where seen by him they 

rest on Tertiary rhyolite or older rocks, but at the extreme north end 

of the range, in the Rochester district, they rest on a considerably 

younger formation, the Pliocene (?) alluvial-cone detritus. Because 

they overlie this detritus they are tentatively considered to be of late 

Pliocene age.

QUATERNARY ALLUVIUM

The floors of the canyons and of the valleys upon which the canyons open are covered with angular gravel. Locally the gravel 

contains gold, as in the broad basin at the head of Limerick Canyon. 

The shaft sunk farthest out in the flat in prospecting these goldbearing gravels reached bedrock at a depth of 30 feet. In general, 

however, not much is known about the thickness of the gravel.

Along the sides of the canyons the gravel merges imperceptibly 

into the mantle of loose debris that covers the lower slopes of the 

ridges. On many slopes, especially south slopes, this mantle extends 

up surprisingly far. As a consequence prospect tunnels may have 

to be driven 50 feet or more before they cut bedrock. The extreme 

example of this is seen in the tunnel at an altitude of 6,269 feet 

northeast of Nenzel Hill; the tunnel, which is 200 feet long, does 

not penetrate to solid bedrock. As the slope of the hill is here 18°, 

the thickness of the debris mantle exceeds 60 feet.

During the spring, when the snow is melting, the mantle of debris 

has the consistency of mud and is in a state of flowage toward the 

canyon and valley bottoms. This flowage of rock detritus, or solifluxion, as it has been termed, is the chief process by which erosion 

is now going on in the region.

41 Louderback, G. D., op. cit
38 ROCHESTER DISTRICT,

STRUCTURE 

FOLDING AND FAULTING

The major structural feature of the Star Peak Kange is the broad 

anticline into which the Triassic rocks have been folded. In the latitude of the Rochester district the volcanic rocks form the core of the 

anticline, and the limestone that overlies them stratigraphically appears only in a narrow belt on each side of the range, as shown in 

Figure 2. Farther south, in the Black Range, or southern prong of 

the Star Peak Range, the limestone predominates. The anticlinal 

arch of limestone, spanning the full width of the range, can be seen 

remarkably well from the summit of the Black Range.

The dips of the western limb of the anticline are moderate, 30° W. 

being the average. On the east limb the dips are higher, averaging 

60° E. The massive limestone on the ridge on the north side of 

American Canyon dips as much as 70° E., but this high dip is doubtless in part due to rotation of the limestone in the course of faulting 

that occurred long after the folding, probably while the range was 

being uplifted as a fault block. Though the structure is thus complicated by later faulting and concomitant steepening of dips by 

rotation, it appears probable that the anticline is asymmetric and 

that its axial plane dips steeply westward (75° W.).

In the Rochester district the axis of the anticline crosses near the 

head of Rochester Canyon in a northerly direction. It should pass 

through Sunflower Hill, as is inferable from the dips plotted on 

Plate I, but when a structure section is drawn through the hill 

(section A-A\ in PL I), it is seen that the structure demands a reverse fault along the axial plane of the anticline. The estimated 

displacement of this fault is 4,000 feet. If, however, part of the 

great thickness of the felsitic trachytes forming the western limb 

of the anticline, as shown in the section from Sunflower Hill to Independence Hill, is due to repetition by faulting, the amount of displacement along the axial plane becomes correspondingly reduced. 

Although this fault was sought for on Sunflower Hill, it could not 

be located on the ground because of the uniformity of the felsites 

there. On the north side of Rochester Canyon, however, the fault 

appears to cross the ridge where the Highline road crosses over 

into Limerick Canyon, there being an abrupt change in lithology on 

the two sides of the road. The extension of the fault southward from 

Sunflower Hill is doubtless the easily traceable fault on the south side 

of Weaver Canyon, which brings the Rochester trachyte against 

the Weaver rhyolite and is demonstrated by the mapping to be a 

reverse fault dipping 65° W. The crown of the anticline is here 

clearly broken by this fault. It is the known dip of this fault that
STRUCTURE 6\$

warrants the dip that is given to the fault shown in the structure 

section. .

The anticline appears to pitch southward, as indicated by the distribution of the Weaver rhyolite. Consequently the beds west of 

Sunflower Hill strike northwest and those east of it strike northeast ; they converge southward to the nose of the anticline at Packard. This southward pitch of the anticlinal axis accounts for the 

fact that the limestone that stratigraphically overlies the Weaver 

rhyolite predominates in the south end of the range. Along the 

western flank of the range the strata strike 30° or 35° west of north,, 

and consequently they make an angle of 30° or 35° with the meridional trend of the front of the range.

The simplicity of the broad anticlinal structure of the Star Peak 

Range is impaired within the Rochester area, as has already become 

apparent, by faults and by intrusions. The faults are many and include both longitudinal and transverse faults. Evidence of several 

periods of faulting is recognizable, and the main periods may be 

tentatively classified as (1) that which accompanied the folding at 

or near the end of Jurassic time, (2) that which accompanied the 

opening of the mineral-bearing fissures, and (3) that which accompanied the block-faulting by which the Star Peak Range was 

uplifted near the end of Pliocene time. As the range was uplifted 

during at least two stages separated by a protracted halt, the possibility of a fourth period of faulting is indicated.

The recognition of faulting within the area underlain by the 

Rochester trachyte is practically impossible, because of the essential 

uniformity of that formation. But in the overlying rhyolite sequence, 

where distinctive beds are recognizable, such as a conglomerate bed, 

or sandstone-like tuffs, faulting is readily determinable. Therefore^ 

considerable faulting is shown on the map to occur on Nenzel Hill; 

indeed, it might be said that the amount of faulting detectable at any 

locality is proportional to the number of key horizons.

The section through Nenzel Hill shown in Figure 3 indicates 

further that even more faults occur there than are shown on the 

geologic map.

The earliest faulting appears to have taken place during the 

up-arching of the anticline. Such appears to have been the origin 

of the steep reverse fault on the south side of Weaver Canyon, which 

brings the Rochester trachyte over the Weaver rhyolite.

The faulting by which the vein fissures were opened resulted in 

both normal and reverse faults. Subsequent to the mineralization 

renewal of movement occurred along some of the veins, intensely 

crushing them, and is best illustrated by the East vein. Later than
40 ROCHESTEB DISTRICT, NEV.

this postmineral shattering both normal and reverse faulting occurred, displacing some of the veins.

Extensive faulting occurred at the end of Pliocene time, when the 

range was uplifted as a great block of the earth's crust. Many faults 

ar» demanded by the physiographic features, and some of them can 

be found by detailed geologic mapping. The fault extending along 

the front of Independence and Lincoln hills occupies the position demanded by the physiographic consideration that the west slope of 

the range is a fault scarp. The dip of this fault as indicated by the 

mapping is 45° W.

The best example, however, is in the southern part of the district. 

The Black Range south of Packard Springs (at the edge of the 

area mapped) has clean-cut triangular facets upon its basal spurs, and 

the faulting to which these are due has produced a notable " oversteepening " along the base of the range. The fault thus indicated 

by the physiographic evidence as extending along the foot of the 

range south of Packard Springs passes north of the springs into 

the body of the range. It is there traceable as a fault between the 

aplite and the Rochester trachyte, and as determined from the 

mapping it dips 50° W. Physiographically it produces the terracelike form, or shoulder, that at an altitude of 6,450 feet breaks the 

escarpment's otherwise even slope from the crest to the foot of the 

range.

SCHISTOSITY

The rhyolites, trachytes, and keratophyres are roughly schistose. 

The degree of schistosity varies widely, the rocks ranging from those 

that are almost imperceptibly foliated to those that are fairly well 

foliated. The lavas as a rule are only slightly schistose, but the 

associated pyroclastic beds, especially the finer tuffs, are notably 

foliated, and some of these rocks have been transformed into white 

phyllite or sericite schist.

The schistosity has essentially a regional constancy of direction; 

it strikes parallel to the trend of the volcanic rocks but dips invariably westward, both in the east and in the west limbs of the broad 

anticline into which the rocks of the Star Peak Range have been 

folded. The prevailing dip is 60° W., though on the west limb of the 

anticline the schistosity has a tendency to conform with the bedding 

and accordingly may in places dip as low as 25° W. Where tuffs and 

breccia alternate in thin beds, refraction of the schistosity in passing 

from bed to bed is well shown. Some of the least schistose rocks 

in the district are those on the summit of Nenzel Hill, where even 

the tuffs are massive; consequently it appears probable that they have 

escaped foliation because they occupy the crown of the anticline.

The peculiar relation of the schistosity to the bedding roughly 

parallel to the bedding on the west limb of the anticline and trans-
PHYSIOGRAPHIC HISTORY 41

verse to it on the east limb is noteworthy. If, as has been inferred, 

the anticline is an overturned arch whose axial plane dips 75° W., 

then the observed dip of the regional schistosity (60° W.) agrees 

fairly well with the dip of the axial plane of the anticline an 

agreement that accords with certain theories on the origin of 

schistosity.

PHYSIOGRAPHIC HISTORY

The clues to the earliest history of the present land forms are in 

the deeply dissected alluvial deposits at the entrances of Rochester 

and Limerick canyons. The make up of these deposits, which consist 

of bouldery detritus containing angular blocks as much as 6 feet 

in diameter, shows that they were laid down at a time when the 

near-by country was of considerable relief and the streams that 

flowed from it were of great carrying power in short, the alluvial 

deposits are the cones that were built up at the mouths of the 

canyons that indented the escarpment of a youthful fault-block 

range. They are the evidence of the initial uplift of the Star Peak 

Range; manifestly the southern division of the Humboldt Range 

the Humboldt Lake Range had not yet come into existence, for 

some of the alluvial cones extended over the site of the future 

range.

Subsequently the alluvial cones were partly buried under the succession of basalt flows that were poured out late in Tertiary time. 

The lava sheets now dip 12° E. and form the gentle east slope of the 

northern part of the Humboldt Lake Range, which here overlaps 

the south end of the Star Peak Range. The abrupt west slope of 

the Humboldt Lake Range is a fault escarpment, as has been so 

amply demonstrated by Louderback 42 by the concordance of structural, stratigraphic, and physiographic evidence. During the period 

of faulting by which the Humboldt Lake Range was formed the 

Star Peak Range was again uplifted.

That the Star Peak Range has attained its present altitude by 

two uplifts which were separated by a long halt is best recorded in 

Limerick Canyon. Near its mouth the canyon is a deep, acutely 

V-shaped valley, but upstream it widens, and at its head it has 

opened out to a broad, capacious basin, flat floored and underlain 

by gravel. (See PI. I.) In an accentuated way it thus exhibits 

the feature so characteristic of valleys developed on the scarp flanks 

of fault-block ranges that is, the valley is more mature headward. 

The prominent remnants of a high bedrock terrace along the lower 

part of the canyon, several hundred feet above the canyon floor, indicate that there were two stages in the dissection of the canyon and

43 Louderback, G. D., Basin Range structure of the Humboldt region: Geol. Soc. America 

Bull., vol. 15, pp. 289-346, 1904.
42 BOCHESTEE DISTRICT, NEV.

therefore that a protracted standstill interrupted the uplift of the 

range. When the second uplift commenced the stream began to cut 

down to its present level, and the alluvial cones that had been built 

up along the front of the range as the result of the initial uplift 

began to be eroded. The cones are now maturely dissected to adepth of 400 feet and form a piedmont fringe of rounded hills.

The great block of the earth's crust that was lifted into the zone 

of erosion and sculptured into the Star Peak Range was not elevated as a single unbroken mass but was more or less internally 

fractured by subordinate dislocations. During the uplift of the 

main mass movements occurred along the other faults, as well as 

along the major fault that determines the west front of the range. 

The terrace-like forms on the scarp of Black Range have already 

been mentioned. By movement along the faults within the range 

some remarkably anomalous physiographic features have been produced. The most noteworthy is a rift valley, which is much like 

some of those along the San Andreas and Haywards faults of the 

Coast Ranges of California.43 This rift valley lies east of Nenzel 

Hill and trends north ; it is a relatively broad, open valley of alluviated floor and mature aspect. The northern part of the valley is 

drained by American Creek, and the southern part by South American Creek, but the divide between the two parts is almost imperceptible. The specially noteworthy feature is that the east wall of 

the rift valley is breached by South American Canyon, a sharp 

V-shaped gorge 2,000 feet deep. Evidently the faulting by which 

the valley was formed took place so slowly that the feeble stream in 

South American Canyon was able to maintain its course across the 

relatively rising edge of the fault block on the east side of the rift 

valley. As a result South American Canyon transects the Black 

Range, which here forms the east wall of the rift valley.

The main canyons of the district Limerick, Rochester, and 

Weaver have' notably asymmetric cross sections, the north slopes 

being long and relatively gentle and the south slopes short and steep. 

In the spring the snow on the north slopes melts rapidly, and the 

debris mantle becomes a mass of mud and stones and flows toward the 

canyon floor. This solifluxion appears to be the main factor in the 

processes of erosion now at work. On the south slopes, however, 

owing to the poorer insolation, the snow persists much longer and 

hence a larger portion disappears by evaporation in the dry desert 

atmosphere. This greater evaporation of the snow on the south 

slopes appears thus to be the determining cause in the development 

of the asymmetric cross sections of the canyons.

, A. C., U. S. Geol. Survey Geol. Atlas, San Francisco folio (No. 193), p. 37, 

1914.
PART III. THE ORE DEPOSITS 

GENERAL FEATURES

The ore deposits of chief economic importance in the Rochester district are silver-bearing quartz veins and stockworks. Gold veins 

are of minor importance; and other deposits, carrying quicksilver or

 copper, are, so far as now known, only of geologic interest. The deposits are all inclosed in the Triassic volcanic rocks, chiefly the 

rhyolite and trachyte.

The silver-bearing veins are of the replacement fissure type. They 

occur mainly on Nenzel Hill and yield the larger part of the district's output of precious metal. Their economic value depends on 

the amount of argentite that they contain. As the argentite is of

 supergene origin, it diminishes in quantity downward, and below 

:a moderate depth the veins are unprofitable.

The silver-bearing stockworks occur only at Packard, in the southern part of the district, and have yielded a notable portion of the 

output. Their tenor of silver is largely determined by their content 

of cerargyrite. The stockworks consist of masses of Weaver rhyolite 

that are cut by a widely spaced network of narrow, veinlets. In 

freshly broken ore the veinlets are invisible, but after exposure to 

the weather they become apparent as extremely narrow seams, as is 

shown in Plate II. The stockworks are large bodies of elliptical 

surface area, but at shallow depths, not exceeding 30 feet, they become of too low grade to be workable. The ore mined has averaged 

under 8 ounces of silver to the ton.

The ore of the stockworks is commonly sericitic and more or less 

stained with limonite, but the cerargyrite is so finely disseminated 

as to be generally imperceptible. It is found very rarely as greenish 

waxy films. High-grade ore is indicated by an abundance of small 

pits or cavities that are due to the removal of sulphides, presumably 

sphalerite and pyrite. Much of the ore mined looks like mere country rock and has no visible indication that it is ore. These deposits 

are described in more detail on pages 70-73, under the description of 

the Nevada Packard mine, the only mine on stockworks.

RELATION OF ORE DEPOSITS TO WATER LEVEL

The abrupt relief and the arid climate cause the water table in 

general to lie at considerable depth. As a result water level has been 

reached in only two mines. In the Nevada Packard water level was 

cut in a prospect shaft at a depth of 235 feet below the lowest adit,

43
44 EOCHESTEE DISTEICT, NEV.

whose portal is at an altitude of 5,590 feet. The position of the 

water level here is nearly 300 feet below the bottom of commercial 

ore. At the Buck & Charley mine water level was reached in the 

winze 42 feet below the lowest adit, whose portal is situated near the 

floor of Rochester Canyon: evidently this water represents the underflow of Kochester Canyon. Good ore was found in the winze and 

persists below water level.

On Nenzel Hill the East vein has been explored 1,600 feet below 

the outcrop, as measured on the vein, which dips 35° W., and water 

level has not been reached. In this vein as well as in other veins cut 

by the deep-level tunnels the downward limit of ore sufficiently enriched by argentite ,to be profitably workable has been found. The 

water table, however, lies far below that level. The enrichment, 

therefore, was not determined by the present position of the watertable.

During the existence of the great Quaternary lake known as Lahontan the Humboldt Eange formed a long peninsula jutting intoit.44 Inasmuch as the lake eventually disappeared by desiccation,, 

the water table in the surrounding ranges doubtless sank concurrently during the increasing aridity. This sinking of the water 

table probably accounts in part for the fact that it now stands so 

far below the bottom of the zone of enrichment, but the main reason is probably that the enrichment took place much earlier in the' 

history of the district. It may tentatively be suggested that the enrichment was effected during the initial uplift of the Star Peak. 

Range and that the considerable vertical distance between the bottom of the zone of enrichment and the present position of the 

water table is due to the second and major uplift of the range.

SILVER VEINS 

DISTRIBUTION AND GENERAL CHARACTER

The chief productive silver veins are on Nenzel Hill. They cropout in the Nenzel rhyolite breccia or in the overlying stratified rhyolite tuff of sandstone-like appearance. The outcrops are well exposed 

naturally, in places projecting 3 or 4 feet above the surface, and are 

easily traceable, some for distances exceeding 500 feet. The veins in 

general are narrow, averaging about 3 feet in thickness, though the 

two most productive veins, the East and West veins, average nearer 

6 feet. At the outcrops they dip steeply westward, from 60° to 

nearly 90°. Both the East and West veins, of which more is known 

than of the other veins, flatten notably at depth, dipping 35° W. andl 

locally as low as 15° W. »

"Russell, I. C., Geological history of Lake Lahontan: U. S. Geol. Survey Mon. 11, pL. 

4, 1885.
SILVER VEINS 45

At moderate depth, the veins pass from the Nenzel rhyolite breccia 

into the underlying flows and tuffs of the Rochester trachyte. A 

conspicuously flow-banded f elsite, made up of an alternation of wavy 

bands each a fraction of an inch thick, is the prevailing country rock 

in depth. It has been considerably mineralized, so that most of the 

trachyte making up Nenzel Hill carries finely disseminated pyrite 

and sphalerite, and its gas cavities are lined or filled with quartz, 

sphalerite, pyrite, and galena.

The veins occur along fissures that have generally been produced 

by normal faulting. Along the East vein the displacement parallel 

to the dip amounts to 120 feet. (See p. 62.) A marked sheeting was 

developed parallel to the fissure and at many places has produced 

surfaces that simulate walls, which bound the ore. Much of the 

quartz filling has been formed by replacement of the wall rocks and 

is generally fine grained, a large part of it being almost or entirely 

chalcedony-like in appearance. The-veins are therefore of the replacement fissure type. On account of the prevailing fine-grained 

nature of the quartz the veins on first examination appear to be of 

Tertiary age, or possibly of an analogous type that was formed in 

connection with the eruption of the Triassic rhyolites, but both impressions, as will be shown later, prove to be wrong. Some of the 

veins have been crushed by postmineral movement, and thick gouges 

have been produced along both f ootwall and hanging wall of one and 

the same vein. In its narrower portions a vein may be wholly reduced to a white gouge filled with angular fragments of crushed 

quartz. In places postmineral fissuring has produced gouges in the 

country rock, and as those gouges consist of felsitic fragments they 

resemble exactly the gouges resulting from the crushing of the chalcedonoid quartz veins.

In addition to the veins on Nenzel Hill, which have supplied the 

bulk of the silver that Rochester has produced, other veins of essentially similar character occur in the western part of the district, 

at Lincoln Hill, Lower Rochester, and Octopus Gulch. They are 

of subordinate economic importance but have proved genetically 

more illuminating than those on Nenzel Hill.

The Contact lode at Packard (pp. 71-73) differs markedly from 

the other silver deposits of the district, consisting of sheared, highly 

sericitized rhyolite adjoining a fault; but, like the other deposits, 

it contains ore only where it has been enriched by descending solutions, and the protore carries less than an ounce of silver to the ton.

NATURE OF THE ORE

The ore of the veins on Nenzel Hill is prevailingly composed of 

fine-grained quartz. The only recognizable metallic mineral in most 

of the ore is pyrite, which is scattered through it as minute grains
46 ROCHESTEK DISTRICT, NEV.

in small amount; but there can be seen, especially in the richer ore, 

many small blackish patches, of bluish cast and suffused outline, 

whose presence immediately indicates, to those familiar with the ore, 

that such ore is high in silver. Ore showing these blackish mottlings 

becomes, when simply heated in the forge to redness, coated with 

innumerable bright beads of metallic silver. In thin section the ore 

of this kind is seen to contain scattered particles of a lead-gray 

metallic substance, which is therefore identified as argentite.

In ore from the deeper levels, especially in unoxidized ore, resinous sphalerite is fairly common though nowhere abundant, galena is 

occasionally detectable, and tetrahedrite occurs as minute particles. 

The bluish-black mottlings also occur. In high-grade ore, which is 

characteristically somewhat porous from the partial removal of sulphides by leaching, minute scales of covellite can easily be recognized as lining the pores, and covellite of this kind persists down at 

least to the level of the Friedman tunnel, 800 feet in vertical depth 

telow the outcrop of the veins. In addition to the pyrite, sphalerite, 

galena, tetrahedrite, and covellite already mentioned, chalcopyrite 

becomes apparent in thin sections and polished specimens as a very 

minor constituent, and argentite is seen to form narrow borders 

around grains of sphalerite and tetrahedrite. Covellite is closely 

associated with some of the argentite, but most of the covellite occurs alone as borders around grains of sphalerite.

The ore is commonly deeply stained with limonite, which gives it 

the prevailing rusty color. Jarosite (a potassium-ferric sulphate) 

occurs in some of the ore of the upper levels and appears to have been 

formed as the result of the reaction of ferric solutions on sericite.

In places the oxidized ore contains pure-white, waxy halloysite, 

which checks upon drying. Locally some of the veins where cut 

by the Friedman tunnel are coated by an efflorescence of copper 

sulphate.

The partly oxidized, limonite-stained condition of the quartzsilver ores persists down to the greatest depth attained by mining, 

which on the East vein is 1,600 feet as measured on the vein. Because of the low dip of the vein and the slope of the surface this 

depth is vertically only 300 feet.

ENRICHMENT BY SUPEBGENE ARGENTITE

The tenor of the silver ores of the Kochester district depends 

largely, if not wholly, on their content of argentite. This argentite 

is rarely distinctly visible, and even where visible it does not occur in 

particles that are large enough to be surely identified by the unaided 

eye, though its presence gives rise to a bluish-black mottling of the 

quartz in which it is inclosed. At the Octopus mine, however, it is
U. S. GEOLOGICAL SURVEY ISULLETIN 702 PLATE II

n

SILVEK-BEARING STOCKWOHK AT THE NKVADA. PACKARD MINE
U. S. GEOLOGICAL SURVEY BULLETIN 702 PLATE III

INCIPIENT REPLACEMENT OF SPHALERITE HY SUPERGENE 

ARGKNTITK

It. ADVANCED REPLACEMENT OK SPHALERITE RY SUPERGENE ARGHNTITE
SILVER VEINS 47

present in fairly large blebs of soft, dull-blackish material, which 

on ch«mical examination proves to be pure argentite free from 

admixed copper or lead compounds. It is a rather compact form of 

the so-called sooty argentite.

The argentite, as seen under the microscope, commonly occurs as 

a border around sphalerite and as embayments in the sphalerite. 

It is clearly of later origin than the sphalerite and has formed at 

the expense of that sulphide. An early stage of replacement of the 

sphalerite by argentite is shown in the specimen illustrated in 

Plate III, A, in which the argentite forms a discontinuous fringe 

on the sphalerite. The tendency of projecting points of the sphalerite 

to succumb completely to replacement early in the process is well 

shown. Some argentite has formed also within the interior of the 

sphalerite grains, the solutions that caused this replacement having 

doubtless gained access to the interior by way of the cleavage cracks: 

In Plate III, Z?, is illustrated a more advanced stage of replacement, 

as indicated by the thicker coating of argentite on the sphalerite. 

Some unreplaced nuclei of sphalerite in the coating can still be 

discerned in the illustration. The partial control exerted by the 

cleavage on the course of the replacement, as indicated by certain 

rectilinear boundaries, is also well shown; but on the whole, in this 

grain as well as in all other grains of sphalerite, it is remarkable 

how little the excellent cleavage of the sphalerite has directed or 

controlled the course of replacement. The replacement has proceeded 

centripetally by increase in thickness of the border of argentite. The 

smaller grains of sphalerite originally scattered through the ore have 

in this Avay been wholly transformed into argentite. Covellite is commonly associated with the argentite; but it is far more abundant 

alone, occurring as borders around grains of sphalerite. It forms 

also rims around the galena, tetrahedrite, and chalcopyrite and 

perhaps around some of the pyrite, though covellite as a coating on 

pyrite has not been positively identified. Although grains of pyrite 

are common in the ore, none of them were found to be coated with 

films of argentite, and none, with perhaps insignificant exceptions as 

already noted, are coated with films of covellite in other words, the 

pyrite was practically immune to the reactions that produced the 

nrgentite and covellite, and this immunity is to be attributed to the 

fact that the easily attackable sphalerite was present in abundance. 

It is clearly manifest that the sphalerite reacted much more readily 

to form argentite and covellite than any other hypogene sulphide 

in the ore, except possibly the galena.

It is noteworthy that the boundaries between the hypogene sulphides, such as those betAveen sphalerite and tetrahedrite or chalco-
48 ROCHESTER DISTEICT, NEV.

pyrite, are sharp, clean-cut, and rectilinear as seen in polished sections of ore, whereas the boundaries between argentite and sphalerite 

and those between covellite and sphalerite are highly irregular 

through innumerable minute indentations, and as a result they characteristically appear fuzzy or blurred.

The argentite and the covellite intimately associated with it were 

obviously formed later than the sphalerite, tetrahedrite, galena, and 

chalcopyrite. The soft, pulverulent condition of the argentite, the 

leached appearance of the rich ore, and the marked decrease in the 

quantity of argentite in the deeper levels all combine to prove that 

the argentite was formed by descending solutions that had derived their silver from the oxidation of sulphides in the zone of 

weathering. In short, the argentite, which determines the silver content of the ore, is of supergene origin.

The source of the silver in the supergene argentite appears to 

have been the tetrahedrite in the ore, which is the only hypogene 

mineral that is probably silver-bearing that has been found in the 

ore. Hypogene argentite may occur, but if so it has not been recognized. When the hypogene sulphides pyrite, sphalerite, tetrahedrite, and chalcopyrite were attacked by atmospheric waters as the 

tops of the veins became exposed at the earth's surface, they yielded 

solutions containing sulphuric acid and sulphates of iron, zinc, 

silver, and copper. These solutions moved downward along the 

veins without change of chemical composition until they came into 

contact with unaltered sulphides. Thereupon they began to react 

with the sulphides, especially with the sphalerite, as shown so 

abundantly in the microscopic sections of the ores. The reactions 

that were of chief importance are expressed by the following 

equations:

(1) ZnS+Ag,SO4=AgoS+ZnSO4

(2) ZnS+CuS04=CuS+ZnS04 

and possibly

(3) CuS+Ag2S04=Ag8S+CuSO<

The first two equations give the reactions by which the argentite 

(Ag2S) and the covellite (CuS) were formed at the expense of the 

sphalerite (ZnS). That both reactions took place appears to be 

proved beyond all doubt by the microscopic evidence. Which reaction occurred first ^depended upon the relative concentrations of 

silver and copper ions in the descending solutions.

Because of the far greater abundance of primary copper minerals 

that were being oxidized in the outcrops of the veins, the copper ions 

were present in enormous excess over the silver ions. The solubility
SILVER VEINS 49

jproduct 45 of silver sulphide is 4X10"50 and that of cupric sulphide 

1.2X10'42 , as given by Stieglitz.40 Therefore, from a solution containing both silver and copper ions silver sulphide will be precipitated 

alone until the cupric ion becomes 3.3 X107 as concentrated as the 

silver ion. Cupric sulphide will then be precipitated, with traces 

of silver sulphide. In spite of the enormous excess of cupric ions 

in the descending solutions, it is probably not great enough to prevent

 the silver sulphide from being precipitated first, because the solubility 

product of the silver sulphide is exceedingly small.

On applying these chemical principles to the problem of silver 

'enrichment at Eochester, it appears likely that the process was as 

f olloAvs: Highly dilute solutions of ferric, cupric, and silver sulphates

 were formed in the outcrops of the veins. As these solutions trickled

 downward they gradually reached the primary sulphides, where the 

silver ions combined to form silver sulphide (argentite) with the 

available sulphur ions, which were furnished chiefly by the sphalerite. 

Evidently this reaction took place before the ferric ions were reduced 

"to ferrous ions by the primary sulphides, for no native silver was 

precipitated, as it would have been if there had been an appreciable 

concentration of ferrous ions. As the solutions moved downward 

they gradually became depleted in silver ions, until eventually the

 concentration of the cupric ions relative to the concentration of the

 silver ions became sufficiently high to cause the precipitation of

 cupric sulphide (covellite). Therefore, the zone in which covellite 

was formed should extend farther in depth than the zone in which

 argentite Avas formed. The actual extent of the covellite zone in 

the Rochester veins matches with this conclusion, for supergene 

covellite perists in noteworthy abundance below the zone in which/ 

argentite has formed in economic quantity. That covellite was precipitated instead of chalcocite indicates, as does the absence of 

.metallic silver, that the concentration of ferrous ions in the descending solutions was low.

Because of the steadily progressive lowering of the surface by

 erosion, the zone of oxidation must in the course of time have encroached upon the zone of enrichment. The supergene silver in the 

enriched ore thus attacked must have been redissolved and carried 

farther clown. As it moved doAvnward it must first have descended 

through a zone in Avhich the sphalerite had either all been used up

48 The principle of the constant solubility product " is an extremely convenient condensation, Into a very simple mathematical form, of the main factors involved in the precipitation and solution of difficultly soluble salts, acids, and bases" (Stieglitz, Julius, 

Qualitative chemical analysis, pt. 1, p. 143. 1916). This principle is that in a saturated 

solution of a difficultly soluble salt the product of the concentrations of the ions of the 

salt is constant at a given temperature. Precipitation of the difficultly soluble salt will 

therefore result when the product of the ioa concentrations becomes greater than the

 solubility-product constant for that substance. 

«0p. cit., pp. 224, 212.
50 ROCHESTEK DISTRICT, NEV.

or had been insulated with coatings of supergene covellite. In 

theory the solutions should then have reacted with the covellite according to the equation

CuS+Ag2S04=Aga S+CuSO«

This reaction has been experimentally established by Posnjak. 47 

The copper thus dissolved during this reaction would be carried 

downward and reprecipitated on sphalerite in the lower part of the 

zone of silver enrichment.

Material sufficiently enriched by the deposition of supergene 

argentite to be ore has a maximum vertical range on Nenzel Hill of 

800 feet, or 1,400 feet as measured on the vein. Formerly the vertical range must have been greater, for ore containing supergene 

argentite occurs at the outcrop. During the present cycle of erosion 

the zone from which the silver was leached to supply the zone of 

enrichment has been eroded away from some of the veins, and the 

enriched zone has consequently been exposed at the surface. The 

vertical extent of the portion of the enriched zone thus removed by 

erosion is, of course, not determinable.

During the present cycle of erosion, while the water level has 

stood so low under Nenzel Hill more than 900 feet below the outcrops of the chief veins oxidation has deeply stained the enriched 

ore with limonite derived from the pyrite. That in the course of 

this oxidation any silver has been carried down to form an enriched 

zone at present water level is highly improbable, inasmuch as the 

abundant finely divided supergene covellite below the zone of argentite enrichment would inhibit the silver from moving far downward, 

because of the ease with which it would be precipitated. That such 

an enriched zone is not absolutely impossible, however, is suggested 

by the fact that if the descending silver-bearing solutions moved 

rapidly enough down through the veins, all their silver may not 

have been precipitated until they had been slowed down at or below 

water level.

WALL-BOCK ALTERATION

The wall rocks have been altered by the primary ore-depositing 

solutions, mainly by silicification. In fact, much of the quartz 

gangue of the veins themselves is due to replacement of the country 

rock by quartz; hence the prevailing fine grain of the ore. Where 

the Nenzel rhyolite breccia has been replaced phenocrysts of quartz, 

remain as relicts. In some of the ore, in which shattered and brecciated country rock has been replaced, " ghosts" of the angular 

fragments are still visible, and the various stages of obliteration and

47 Posnjak, Eugene, Determination of cuprous and cupric sulphide in mixtures of one 

another: Am. Chem. Soc. Jour., vol. 36, pp. 2475-2479, 1914.
GOLD VEINS 51

disappearance of the phantom fragments as the result of more 

thorough silicification can readily be followed.

The best megascopic evidence of siliceous replacement is assuredly 

to be seen in those places where quartz veinlets break across the conspicuous flow banding of the Rochester trachyte. The central portion of the veinlets consists of moderately coarse white quartz, and 

this is bordered on each side by an irregular band of dark-gray 

chalcedonoid quartz, which cuts off diagonally the flow bands of 

the trachyte. In general, the microcrystalline quartz formed by the 

replacement of the rhyolite and trachyte is exceedingly like the felsitic groundmasses of those rocks; and it is probable, therefore, that 

the amount of silicification is likely to be overestimated by the eye.

Under the microscope it was found that silicification was the dominant process near the veins and that the development of sericite 

was the main alteration effected at greater distances from the 

veins. The quartz of replacement origin has characteristically an 

extreme variability of grain, ranging from cryptocrystalline to 

moderately granular within fractions of the area of a thin section. 

The microcline phenocrysts of the trachyte were more easily replaced than the groundmass and have commonly been pseudomorphosed by relatively coarse aggregates of quartz. In less highly 

altered rock sericite is abundant, generally as ultra-fine fibers and 

scales. Pyrite and sphalerite are common in the altered wall rocks.

The descriptions of the preceding paragraphs apply especially to 

the veins on Nenzel Hill, which carry no tourmaline. In the tourmaline-bearing silver veins, such as the Octopus and Raven, the ore is 

essentially like that of the nontourmalinic veins, and the metasomatic alteration of the wall rocks is also similar, except that the ore 

in places carries multitudes of minute prisms of black tourmaline 

and that tourmaline has developed in the wall rocks along with the 

quartz, sericite, and sulphides.

GOLD VEINS 

GKENEBAL CHAEACTEB,

The gold-bearing veins of the Rochester district are few and have 

been worked on a noteworthy scale in only one mine. They are quartz 

veins inclosed in the Triassic volcanic rocks. They are all notable 

in that either they contain tourmaline in their gangue or their wall 

rocks contain it.

The most productive mine is the Lincoln Hill, where several narrow quartz veins have been worked. Short shoots of ore high in 

free gold have been extracted.
52 BOCHESTEK DISTRICT, NEV.

The quartz in the gold veins is coarsely granular, in marked contrast with that in the silver veins, which is commonly so fine grained. 

as to be almost like chalcedony in appearance.

Some of the gold-quartz veins are remarkable in that they contain 

abundant coarse microcline. The Hagan lode, which is in keratophyre, consists of quartz, tourmaline, and microcline that shows thecharacteristic plaid twinning under the microscope. The Horseshoe 

vein, which cuts the Weaver rhyolite a mile east of Packard and in 

places is 12 feet thick, contains abundant microcline and silverywhite mica in its gangue of coarse milky- white quartz. The microcline in this vein resembles albite in appearance, but its refractive 

indices and plaid twinning fix its identity. The twinning of the 

microcline in both these veins is not due to dynamic deformation, as 

the associated quartz is uncrushed and optically normal.

Tourmaline-bearing dumortierite-quartz veins, tourmaline-bearing; 

gold-quartz veins, and tourmaline-bearing silver-quartz veins occur 

in close proximity to one another on Lincoln Hill, but there are notransitions between veins of any of the three types, nor are there any 

transitions between them elsewhere in the district.

NATURE OF THE GOLD ORE-FORMING SOLUTIONS

The wall rocks of the gold veins have been altered by the action of 

the ore-depositing solutions, which have caused the development in 

them chiefly of quartz, tourmaline, and pyrite and subordinately of 

sericite. Adjoining the Oro Fino vein the alteration has been sothorough that the original wall rock has been converted into a black 

rock composed wholly of tourmaline and quartz, which forms a selvage 1 to 2 feet thick on each side of the vein. The gangue of the 

Hagan lode is pegmatitic, consisting of a coarse aggregate of quartz. 

microcline, and columnar tourmaline. The evidence is clear that the 

gold ores were deposited from solutions at a very high temperature. 

The microcline in the veins indicates that the temperature was between 400° and 600°. 48

The tourmaline content of the veins and the clear connection between the aplite masses and tourmalinization, so obvious in the 

field, point unmistakably to a genetic relation between the mineralizing solutions and the aplite intrusions. It will be assumed that,. 

in conformity with current doctrine, the mineralizing solutions were 

differentiates or distillates from deep-lying portions of the aplite 

magma.

The gold veins do not grade into the silver veins. This is well 

shown on Lincoln Hill, where both gold and silver veins occur..

, E., Ueber die Alkalifeldspilte : Geol. FSren. Forh., Band 39, pp. 321-184, 

1917.
CINNABAB 53

Moreover, the large area of dumortieritized trachyte interlaced with 

innumerable dumortierite-quartz veins and veinlets is on Lincoln 

Hill, and though this dumortieritization is undoubtedly the earliest 

mineralizing effect of the aplite intrusion, there is no trace of a gradation between this effect and the later ones, as represented by the gold 

and silver veins.

OTHER MINERAL DEPOSITS

A few other mineral deposits occur, which differ widely from the 

gold and silver deposits already described. They include a cinnabar deposit, a jamesonite-quartz vein, a tourmaline copper-bearing 

vein, and a baritic deposit. They do not appear to be of economic 

value, but they are of interest in showing the diversity of mineralization in the district.

CINNABAR

A cinnabar deposit occurs in American Canyon a mile east of the 

area shown in Plate I. A small reduction plant was erected here 

about 1908, and a little quicksilver was produced. An incline sloping 15° NE. and 200 feet long was sunk in the deposits, and considerable drifting was done. This work has failed to disclose the 

shape of the deposit, which is noteworthy for the numerous white 

walls that traverse it at various angles and that for evenness, whiteness, and smoothness rival artificial walls finished with plaster of 

Paris.

The deposit consists of soft white crushed material containing 

numerous fragments of a fairly hard substance of unctuous appearance suggestive of compact sericite. It was originally described by 

Eansome 49 as a kaolinized zone in rhyolite, and under the supposition that the material is kaolin several wagon loads of it were 

shipped out in 1917 to determine its ceramic availability. Under 

the microscope the material of the kind shipped is seen to consist 

of sericite and quartz, and this determination is confirmed by the 

accompanying chemical analysis. The original rock from which the 

sericitic material was derived is a highly flow-banded felsite, typical 

of the Eochester trachyte. Consequently, the analysis of the type 

specimen of the Rochester trachyte is given here again, in order to 

show the drastic changes that accompanied the sericitization. These 

changes comprise chiefly the addition of silica and water and the 

removal of potassa in large quantity.

0 Hansome, F. I/., Notes on some mining districts in Humboldt County, Arlz.: U. S. 

Geol. Survey Bull. 414, p. 37, 1909.
54 EOCHESTER DISTRICT, NEV.

Chemical analyses of sericitized trachyte and unaltered trachyte 

[R. C. Wells, analyst]

Si03 ........... ..........................................__._.___........._..........

A.\i03 .. ....................................................... ......................

TKtOt.. .............................................................................

FeO.. ..............................................................................

MgO....... .........................................................................

CaO

Na2 0. ..............................................................................

K8O.......... ....... ................................................................

HaO........ ........................................................................

HaO+...... .........................................................................

TiOa. -

COa ..... ...... .................. ......................... ............

1

74.58

17.24

.65

Trace.

.18

.27

4.47

] 2.72

.17

100.28

2

67.01

15.97

2.54

.44

Trace.

.19

1.92

11.42

.28

.39

100. 16

1. Highly sericltized trachy ,e from Quicksilver mine, Rochester, Nev.

2. Unaltered trachyte, Gold Mountain, Nev.

With K2 0.2H2 0.3Al2 03 .6Si02 as the formula of sericite, computation of the analysis shows that the material consists of 55.50 per cent 

of quartz, 42.03 per cent of sericite, and 1 per cent of other minerals.

Some specks of cinnabar were found in places scattered through 

the sericitic mass and were the incentive that led to the exploration 

of the deposit. Small siliceous masses or blebs not exceeding a foot 

in length occur sporadically through the deposit. In these the sericite 

is coarse enough to show a silvery luster, and the small druses in them 

contain small fibers of black tourmaline. Under the microscope the 

tourmaline and sericite appear to be contemporaneous, which leads 

lo the unexpected conclusion that this quicksilver-bearing deposit 

was formed under conditions that admitted the formation of tourmaline. The evidence thus indicates that' the quicksilver-bearing 

deposit, like the other metalliferous deposits in the district, is genetically related to the igneous intrusions at or near the end of Jurassic 

time.

MISCELLANEOUS DEPOSITS

A narrow fissure vein in aplite has been partly explored by two 

tunnels at an altitude of 6,675 feet on the west flank of the Black 

Range. The ore contains abundant jamesonite and resinous sphalerite, with traces of galena in a quartz gangue. This vein is the only 

one in the district in which jamesonite has been found. Jamesonite, 

however, was abundant at the Sheba mine, 10 miles north of Rochester, on the east flank of the Humboldt Range. The ore there, presumably formed during the same metallogenetic epoch as the Rochester veins, consisted of white quartz carrying argentiferous jamesonite, 

galena, sphalerite, pyrite, and tetrahedrite.50 Some of the jamesonite

60 Ransome, F. L., Notes on some mining districts in Humboldt County, Nev.: U. S. 

Geol. Survey Bull. 414, p. 43, 1909.
GENESIS OP THE ORES 55

from the Sheba vein, selected with great care and to all appearance 

perfectly homogeneous, was found to contain 6.14 per cent of silver.51

A copper-bearing vein occurs at the shaft above the jamesonitequartz veins. It is in places 3 feet thick, strikes N". 15° W., and 

dips 30° E. It consists of coarse milky-white quartz, slightly 

tourmaliniferous, carrying chalcopyrite, now largely oxidized to copper pitch ore and minor chrysocolla and malachite.

Another mineral deposit differing even more widely than the preceding from the prevailing ore deposits in the Rochester district is 

the baritic vein in South American Canyon, on the south slope of 

the.6,648-foot peak. The prospect opened here shows a deposit that 

contains a little pyrite and chalcopyrite in a gangue of barite and 

subordinate quartz and sericite.

GENESIS OF THE ORES OF THE DISTRICT

The silver-bearing quartz veins were formed mainly by the replacement of sheeted and shattered zones in Triassic rhyolite and 

trachyte. In some of the veins considerable tourmaline was introduced along with the quartz, and contemporaneously the hypogene 

sulphides, pyrite, sphalerite., galena, tetrahedrite, and chalcopyrite.. 

were deposited. The vein filling thus formed was not sufficiently 

high in silver to constitute ore, and its subsequent conversion to ore 

by the deposition of supergene argentite took place by very different 

processes, as described on pages 46-50, where this phase of the 

genesis of the ores is fully discussed.

The lean silver-bearing veins were formed shortly after the intrusion of the large aplite masses and the granite porphyry dikesy 

at or near the end of Jurassic time, and evidently are genetically 

related to the intrusion of the parent magma from which these differentiates were derived. This relation is significant, for the great 

volume of aplite in the region implies that the parent magma from 

which the aplite was derived had already differentiated to a verynotable extent; and as there are no visible masses of complementary 

lamprophyre in the region and it is unlikely that they occur in 

depth, because the post-aplitically injected material is granite 

porphyry, it is probable that the aplite originated by fractional 

crystallization and that the volatile components originally contained 

in the parent magma became concentrated in the aplite fraction. 

This concentration of the volatile matter in the aplite magma, it may 

be noted in passing, did not suffice to cause that magma to consolidate as pegmatite, as might perhaps be expected.

B1 Burton, B. S., Contributions to mineralogy : Am. Jour. Sci., 2d ser., vol. 45, pp. 36-38, 

1868.

98293 24 5
56 ROCHESTER DISTRICT,

Highly heated solutions, doubtless gaseous, were evidently given 

off from the magma that underlay the aplite that is now exposed 

by erosion. The first to be given off appear to have been the hot 

boron-bearing solutions that produced the very remarkable dumortieritization and concomitant development of andalusite on an enormous scale. This intensive activity was not accompanied by the development of any ore or sulphides. Next followed the deposition of 

the ^tounnalinic gold ores. The abundance of tourmaline in some 

of the gold deposits and the coarse pegmatitic character of the ore 

in others, as in the Hagan lode, where the ore consists of quartz, 

microcline, and tourmaline, indicate that high temperatures (between 400° and 600° C.) prevailed during the deposition of the 

gold ores. As deposits of both the dumortieritic and the auriferous 

tourmalinic types occur within the same area, namely, on Lincoln 

Hill, and as they do not grade into each other, the difference between 

them is obviously not due to a zonal arrangement of mineralization 

around a focus of emission of mineralizing solutions. It may represent a superposition of the auriferous tourmalinic type on the dumortieritic type, due to retrogression of the conditions of deposition 

toward the focus, but in view of the lack of lateral gradation anywhere between deposits of the two types, this hypothesis appears 

hardly probable.

The silver veins were presumably formed last and doubtless at 

lower temperatures, as indicated by the fine-grained texture of the 

quartz, the lesser abundance of tourmaline, and the minute acicular 

instead of coarse columnar habit of the tourmaline. The most productive silver veins, those of Nenzel Hill, contain no tourmaline, 

though otherwise identical with the tourmaliniferous silver veins. 

They are evidently the end members of the series of veins formed in 

the district, being those that were deposited in an environment too 

cool to admit of the deposition of tourmaline. An extraordinarily 

significant fact is that certain tourmaliniferous silver veins, such 

as the Octopus, are faulted by very highly tourmalinic veins, proving that while the silver-bearing quartz veins were being deposited 

there was a temporary or local reversion to conditions of higher 

temperature. Such faulting may have facilitated the escape of somewhat hotter solutions by opening pathways to deeper-lying pockets 

of still fluid magma charged with gases. This renewal or recrudescence of higher-temperature conditions may indicate that the 

period of ore deposition was relatively short so short, in fact, as 

to leave the igneous mass with great stores of heat energy. However, the focus of emission, the still fluid magma from which the 

ore-forming solutions were presumably expelled, was situated at 

great depth, for the consolidated aplite had cooled sufficiently for
GENESIS OF THE OEES 57

the subsequently injected granite porphyry to assume a spherulitic 

habit even where it was inclosed in aplite; and the tourmalinization 

and vein formation took place after the injection of the granite 

porphyry.

In recapitulation, there are, then, at least three types of boronbearing deposits of high-temperature origin in the Rochester district. As the members of the several types were nowhere found to 

intersect, their sequence in time was not directly determinable, but 

their internal characters indicate that the three types were formed 

successively, and each at a somewhat lower temperature than the 

preceding. The three types are, in the probable order of decreasing 

temperature, (1) dumortieritic deposits, which carry only sporadic 

tourmaline and no sulphides, (2) tourmalinic gold deposits, and (3) 

tourmalinic silver deposits. A fourth type may possibly be represented by the innumerable coarse quartz veins, notably tourmaliniferous and carrying traces of sphalerite, galena, and pyrite, that 

occur in the aplite, as at Lone Mountain, and in some of the granite 

porphyry. Transitions or gradations between the different types do 

not exist. Furthermore, deposits of all three types may occur within 

a small area, as on Lincoln Hill. In short, these three types are not 

zonally distributed around the intrusive masses or around foci of 

emission of mineralizing solutions.

The conclusion adopted is that the three types of deposits owe 

their origin to solutions of distinctive character, which were given 

off successively by the consolidating magma. In other words, the 

composition of the mineralizing solutions changed abruptly at 

three successive stages, presumably because of internal changes in 

the consolidating magma from which they were issuing.

The discontinuity in the processes of primary ore deposition in 

the Ely district of Nevada led Spencer 52 to the belief that a magma 

undergoing consolidation might pass through critical stages, "on 

either side of which the products of differentiation might be very 

different." This same problem was considered in some detail by 

Hills 53 in his monographs on the great zinc-lead sulphide deposits 

of the Read-Rosebery district, Tasmania, where tourmaline veins 

and zinc-lead sulphide deposits overlap, although both are genetically related to the same intrusion of igneous rock. The evidence 

there led to the conclusion that the overlapping is due to the successive release of magmatic differentiates of distinctive composition 

from the gradually cooling and differentiating magma, each successive differentiate having been at a lower temperature than the 

first.

62 Spencer, A. C'.. Geology and ore deposits o'f Ely, Nev.: U. S. Geol. Survey Prof. Paper 

96, pp. 68-69, 1917.

88 Hills, Loftus, The zinc-lead sulphide deposits of the Read-Rosebery district, pt. 2, 

Rosebery group: Tasmania Geol. Survey Bull. 23, pp. 86-88, 1915.
58 ROCHESTER DISTRICT, NEV.

The principle that successive ore-forming differentiates (" distillates"), each with its own distinctive constituents and characteristics, are given off during successive stages of a progressively cooling magma will be found, it is here suggested, to be widely applicable in the study of ore deposition. As a modifying factor the 

principle of zonal distribution is recognized as having been operative 

in some districts, but of and by itself the principle of zonal variation 

in ore deposition is of very narrow application. The metallic content and composition of the vast majority of ore deposits genetically 

related to igneous rocks are determined primarily by those unknown 

but fundamental factors that determine the origin of petrographic 

provinces and that correlatively determine metallogenic provinces.

PLACERS

Gold was discovered in 1913 in the gravel that occupies the broad 

basin at the head of Limerick Canyon. Several miners were successful at that time in winning about $1,100 in placer gold during 37 

days' work.54 In the long run, however, it was found that the 

returns did not exceed poor wages, and the excitement soon subsided. 

The total output was roughly $10,000, of which $7,374 was obtained 

in 1914. 55

The placers were worked by sinking shafts to bedrock and drifting on bedrock, chiefly near the upper edge of the alluvial detritus, 

where the gravel is thin. The deepest shaft is 30 feet deep. The 

gravel taken out was either treated in dry washers or was hauled 

a mile to water to be washed. The gravel is wholly angular but is 

not coarse. It is said to carry from 3 to 5 cents to the pan. Bedrock was found to contain depressions or runways, and in mining 

these headward the gold content improved. In this way the Hagan 

lode was discovered, for the pay streak was found to head against 

a gold-bearing lode in place. (See p. 76.)

The lode is a tourmaline-bearing quartz deposit carrying coarse 

gold. Tourmaline-quartz stringers are common in the volcanic rocks 

and granite porphyry on both sides of Spring Valley Pass and were 

doubtless the source of much of the placer gold formerly mined at 

Fitting or Spring Valley and American Canyon.

64 U. S. Geol Survey Mineral Resources, 1913, pt. 1, p. 827, 1914. 

66 Idem, 1914, p. 68-5, 1916.
PART IV. MINES AND PROSPECTS

SILVER MINES AND PROSPECTS 

ROCHESTER MINE

The bulk of the silver that the Rochester district has produced 

has come from the veins on Nenzel Hill, chiefly from two the East 

and West veins. There are many other veins in the hill, and these 

were formerly held by various companies, but this divided ownership led to controversy and finally threatened disastrous litigation. 

The two principal veins were being worked by the Rochester Mines 

Co., and its title to the most productive vein, the East vein, was 

threatened in 1917 simultaneously by the Rochester Elda Fina Mining Co. and the Rochester Merger Mines Co. Fortunately a compromise was reached, and the holdings of these companies were 

merged under one ownership by forming the Rochester Silver Corporation, incorporated in 1920, after an attempt to consolidate 

these companies, together with the Nenzel Crown Point Mining 

Co. and the Rochester Combined Mines, as the Rochester Nevada 

Silver Mines Co. had failed. The capitalization is $2,000,000 in 

2,000,000 shares of $1 each, a considerable reduction from the sum 

total of the capitalizations of the three constituent companies.

The Rochester Mines Co., incorporated in 1912, paid only one dividend, $80,000, which was distributed in 1918. Its successor, the 

Rochester Silver Corporation, paid its initial dividend on June 20, 

1921, amounting to $88,536; and two more dividends were paid that 

year, bringing the total for 1921 to $177,523.95. In 1922 $141,646 

was distributed in dividends.

The ore is treated in a 160-ton cyanidation mill, situated 2 miles 

down Rochester Canyon from the mine. The mill was built by the 

Rochester Mines Co. in 1915, and the ore was delivered to it from 

the mine, known as the Rochester mine, by the Nevada Short Line 

Railroad. The service, however, was expensive and unreliable, and 

eventually an aerial tramway was built, which went into operation 

in April, 1917. Power is supplied by the Nevada Valleys Power Co. 

from the Lahontan dam, a Government irrigation project 85 miles 

distant.

The veins are opened in depth by three long adit tunnels. The 

chief of these is the Friedman tunnel, 1,600 feet long, whose portal

59
60 EOCHESTEE DISTEICT, NEV.

is at an altitude of 6,427 feet, between 700 and 800 feet below the 

outcrops of the East p and West veins. The West vein was the site 

of Nenzel's original discovery, and this vein, with the more productive East vein, have so far yielded most of the silver that has come 

out of Nenzel Hill.

In 1920 the Kochester Silver Corporation milled 42,820 tons of 

ore, from which $595,351 in bullion was obtained. The ore contained 

12.63 ounces of silver and 0.139 ounce of gold to the ton, of which 

82.1 per cent of the silver and 91.7 per cent of the gold was won. In 

former years the ore was of considerably higher grade: for example, 

during 1913, the first year of large-scale mining, the Big Four lease 

on the West vein yielded 5,308 tons of ore carrying 60 ounces of 

silver to the ton, and the Codd lease 9,304 tons of ore carrying 30 

ounces to the ton. This high-grade ore was mined above the 300-foot 

level. From that time the grade of the ore rapidly declined, and 

during the years 1918 to 1920 it averaged from 10 to 12 ounces to the 

ton. Numerous veins were cut by the Friedman tunnel, the so-called 

I5 360-foot level, but their silver content proved disappointing. The 

report of the corporation for 1920 says: " On the whole, our work 

from this lowest level of the mine was decidedly unfavorable and 

showed the veins to have become unpayable at this depth."

The main haulage way of the Kochester mine is the Friedman tunnel, from whose portal an aerial tram extends to the mill. In the 

early history of the mine the upper levels were worked mainly 

through the Transportation tunnel, or 500:foot level, whose portal is 

at an altitude of 6,900 feet. From this level the Codd winze was 

sunk on the East vein, dipping 35° W., and the 650, 700, and 800 

foot levels were turned off from it. These workings were eventually 

connected with the Friedman tunnel below, and all ore was thereafter sent to the surface through this tunnel. At the 900-foot level 

the Codd winze cut a large fault, which terminated the downward 

extension of the East vein in the winze, stopped for a number of years 

downward exploration of the East vein, and rendered doubtful any 

decision as to which of the veins cut by the Friedman tunnel was the 

downward extension of the East vein. By 1919, however, the faulted 

segment had been found.

The West vein crops out on the west side of Nenzel Hill. At its 

south end it is just under the brow of the summit. It has been 

stoped to the surface continuously for several hundred feet and was 

found to range in width from 3 to 10 feet. It strikes N. 45° E. and on 

the average dips 75° W. At the northeast end it reaches the summit, 

and here some of the croppings are intact, projecting 3 or 4 feet 

above the surface. They consist of extremely fine grained quartz, 

much resembling chalcedony. The silver in this ore occurs in minute
SILVER MINES AND PKOSPECTS 01

suffused points of argentite. The country rock inclosing the West 

vein is the Nenzel rhyolite breccia, which dips 15° E.; it is here 

extremely massive and resistant.

Below the 250-foot level the dip of the West vein flattens notably 

in fact, in the south end of the drift on the 250-foot level the dip 

decreases abruptly from 55° W. to 15° W. In 1917 the West vein was 

regarded as worked out, and little work had been done on it below 

the 250-foot level, although it contained here one of the best blocks 

of ore in the mine, and no work had been done below the 500-foot 

level. Later, however, under new management, a fine ore shoot was 

found on the 500, 600, and 700 levels. The corporation's report for 

1920 says: " Our drift north on the 500 level was carried through 

for a connection with a drift off the Transportation tunnel for 

ventilation and disclosed at this point that if the former operators 

had driven two rounds more on their drift they would have opened 

this splendid shoot of ore."

The ore of the West vein is identical in appearance with that of 

the East vein. It is prevailingly oxidized and hence is stained with 

limonite. Less oxidized ore carries finely disseminated pyrite 

together with some zinc blende and contains bluish-black mottlings 

due to mixtures of covellite and argentite. In places the vein is 

crushed and has gouges on both foot and hanging walls.

The East vein, as its name implies, is east of the West vein. 

Its great importance first became apparent underground from the 

development work done there, and for a time considerable uncertainty existed as to where the East vein apexed, whether within 

the boundaries of the Rochester Mines Co. or within the neighboring 

claims. In 1917 two shafts were sunk the Emma shaft, on the 

Elda Fina claim of the Elda Fina Mining Co., and the C P X shaft, 

on the Crown Point Extension claim of the Rochester Merger 

Mines Co. to establish apex rights and hence ownership to the East 

vein.

No large amount of ore has been mined on the East vein above the 

250-foot level, but raising has been done to establish the continuity 

of the vein to the surface. At first the E-ochester Mines Co. holed 

through to the surface on a vein dipping 80° W. This vein is readily 

traceable on the surface for 150 feet, ranges from 6 to 18 inches in 

thickness, and is tightly frozen to both walls. It proved to be a 

hanging-wall spur of the East vein, which crops out prominently 

42 feet east of the outcrop of its spur.

The East vein has a strong outcrop of quartz, which is traceable 

with certainty only for 330 feet on the surface. On the north it 

branches, one branch trending N. 10° W. and the other N. 20° E. 

Blocky, well-stratified tuff resembling sandstone lies between the 

branches. At the outcrop the East vein dips 60° W., but at a mod-
62 ROCHESTER DISTRICT, KEY.

erate depth the dip flattens to 35°, which it maintains with great regularity. At the outcrop due east of the so-called Apex shaft the vein 

is inclosed in well-stratified tuff dipping 35° E.; at a vertical depth 

of 70 feet it passes into the Nenzel rhyolite breccia; and between the 

Transportation level and the 600-foot level it passes into the underlying Rochester trachyte.

The vein occupies a zone of fissuring produced by a normal fault, 

on which the displacement now measurable is 120 feet. This displacement is best seen in winze 302, sunk on the vein from the 500- 

foot level to the 650-foot level south of the Codd incline: the hanging 

wall of the vein from one level to the other is Nenzel rhyolite breccia dotted with prominent quartz crystals, whereas the footwall is 

flow-banded felsite of the Rochester trachyte. How much of this 

displacement is due to premineral movement and how much to postmineral movement appears to be indeterminable.

The prevailing ore is a highly oxidized breccia (due to postmineral crushing) consisting of angular fragments of fine-grained 

quartz and silicified country rock in a clayey limonitic paste. Generally the limonite colors the whole mass a rusty red, but where 

it is absent the ore is white. Much of the quartz is nearly as compact and fine grained as chalcedony and is mainly the result of the 

replacement of rhyolite breccia in the higher levels of the mine and 

felsite (Rochester trachyte) in the lower levels. In unoxidized ore 

finely disseminated pyrite and sphalerite occur in some abundance 

and galena and tetrahedrite occur rarely. The high-grade ore has a 

leached appearance, due to the partial removal of these sulphides, and 

the resultant pores are lined with covellite or, as the microscope 

shows, a mixture of covellite and argentite. These darkish sulphides give the ore a characteristic appearance and indicate at once 

to those familiar with the mine the unusual grade of the ore.

Spangles of native silver are found occasionally, even as deep as the 

800-foot level. Masses of waxy white halloysite occur locally in the 

ore and testify to rearrangements effected by waters of atmospheric 

origin.

A marked sheeting was developed parallel to the East vein and 

manifestly facilitated the silicification of the rocks and their conversion to ore. This sheeting makes it unsafe to regard any one wall 

on either side of the vein as the footwall or the hanging wall, for in 

many places the present operators have found ore above what the 

previous operators considered to be the hanging wall, and they have 

also found ore below what had been considered to be the footwall.

In 1917 extraordinary importance was attached to a white gouge 

associated with the East vein along much of its course. It is commonly pure white, though locally striped red and white, and consists 

of angular fragments of rhyolite, whose corners have been somewhat
SILVER MINES AST) PROSPECTS 63

rubbed off by attrition, embedded in a kaolinic matrix. Locally it is 

2 feet or more thick. It occurs in some places on the footwall, in 

others on the hanging wall of the East vein; in some places it occurs 

in the middle of the vein, and in others it is separated from the vein 

by 6 feet of hard rhyolitic breccia, as on the 250-foot level of the 

C P X shaft, which was sunk on the white gouge in order to establish 

apex rights to the East vein. The gouge has evidently been produced 

by postmineral movement along a fissure that in places coincided with 

the East vein and in places diverged from the vein. The gouge can 

not be regarded as peculiar to the East vein, for identical gouges 

occur in the Rock tunnel and the Nenzel Crown Point tunnel.

At certain intersections of the East vein with what are either 

hanging-wall spurs or more steeply dipping veins the gouge of the 

East vein cuts through the steeper vein at the junction, showing that 

the postmineral movement has been chiefly parallel to the East vein. 

At such intersections unusual quantities of good ore were stoped 

out, but such ore did not generally persist far up the steeper vein. 

The reason for the failure of the ore to extend up the steeper vein 

is probably that the ore was due to downward enrichment and the 

meteoric water percolating down the steeper vein was temporarily 

halted or impounded at the intersection by the nearly impervious 

gouge of the East vein, and therefore more of the silver in solution, 

having more time to react with the zinc blende and other sulphides, 

was precipitated as argentite.

The East vein has been stoped for a length of 500 to 800 feet. It 

reaches a maximum of 10 feet, but probably 5 feet is a fair average. 

In places it splits, inclosing horses of massive country rock as much 

as 12 feet thick, and both branches have well-defined walls, and both 

have suffered postmineral crushing. On the south the vein splits and 

becomes unprofitable; north of its intersection with the West vein it 

has not been found to carry ore. In places the vein intersects more 

steeply dipping veins or sends off branches into the hanging wall and 

footwall. Probably both independent veins and spurs or branches 

are represented. An example of the first kind is doubtless the footwall vein that was higher in gold than the East vein; it was stoped 

from the 1,000-foot level to the 800-foot level and was from 360 to 

400 feet long. It is a steep quartzose vein, dipping 70° W., and has 

less gouge than the East vein.

The East and West veins intersect on the 500-foot level 80 feet 

north of the Transportation tunnel. The average divergence in 

strike is 40°, the East vein striking N. 5° E. and the West vein N. 

45° E. The West vein is about 20° steeper than the East vein, and 

its abrupt steepening above the 250-foot level throws the intersection far to the north in the upper levels. The gouge of the East
64 ROCHESTER DISTRICT, NEV.

vein cuts through the gouge of the West vein, showing that the last 

postmineral movement was along the East vein, but the displacement, if any, is not clearly exposed.

In exploring the East vein in depth by means of the Codd 

incline, the vein was found to be cut off by a fault at the 900-foot 

level. This fault proved to be normal, and it strikes N. 8° E. and 

dips 80° E. The displacement is 45 feet, and the fault drag contained enough ore to make stoping for a considerable height profitable. A fair tonnage of ore was obtained from the East veil) below 

the 900 fault, as it is called, but the grade was below the average. It 

is thought that the ore in this pa-rt of the mine has been practically 

all taken out.59 This impoverishment in depth had been clearly indicated several years before by the low silver content of the numerous 

veins cut by the Friedman tunnel (or 1,360-foot level). Long drifts 

have been run both northward and southward from the Friedman 

tunnel. A winze was sunk on the East vein from the level of the 

Friedman tunnel for several hundred feet (as measured on the vein), 

the dip 'being 30° W., though for large stretches it is as low as 15°. 

The vein filling is as deeply oxidized here as on the higher levels, 

but no ore was found. Water level was not reached, even at this 

considerable vertical depth. The results of the exploratory work on 

the Friedman tunnel level have shown, as already mentioned, that 

the veins have become unprofitable at this depth. As but little lateral 

development was done in the earlier history of the mine, much 

ground remains to be explored on the upper levels.

BOUGHTON & HACKLEY MINE

The Boughton & Hackley mine is on the west flank of Nenzel 

Hill, and the portal of the tunnel by which it was developed is at an 

altitude of 6,841 feet, or 356 feet above the portal of the Pitt tunnel. 

The mine is now part of the property of the Rochester Silver Corporation.

The vein is cut by the tunnel about 70 feet below the outcrop; it 

dips 50° W., and in places it has been stoped to the surface. The 

hanging-wall country rock is finely banded trachyte; the footwall 

country rock is Nenzel rhyolite breccia. This relation means that 

the vein was formed along a reverse-fault zone, for the trachyte 

normally underlies the Nenzel rhyolite breccia. Twenty feet west 

of the vein a normal fault of large displacement, dipping 70° E., is 

exposed in the tunnel. This fault must cut off the Boughton & 

Hackley vein 15 feet below the tunnel level and doubtless accounts 

for the failure to find the vein in the Pitt tunnel below, which was 

expressly driven to cut the vein in depth. The geologic relations are 

shown in Figure 4.

66 Rochester Silver Corporation, report for year 1920, 1921.
SILVER MINES AND PROSPECTS

NENZEL CROWN POINT PROPERTY

65

The principal workings on the Nenzel Crown Point Co.'s property are on the east side of Nenzel Hill. A shaft at an altitude of 

6,896 feet was sunk 200 feet on the Big Chief vein. Later an adit, 

whose portal is at an altitude of 6,784 feet, was driven to the shaft, 

and a crosscut was driven westward 1,200 feet; near its end this 

crosscut was connected with the Pitt tunnel, 300 feet below, which 

penetrates Nenzel Hill from the west flank for a distance of 1,800 

feet.

The 1,200-foot crosscut is entirely in Nenzel rhyolite breccia, except for the first 25 feet. The fault known as the Jones fault is well

200 200- 400 FEET

PIGDHH 4. Geologic section from portal of Boughton & Hackley tunnel to northeast corner 

of North Crown Fraction claim. 1. Rochester trachyte; 2, Nenzel rhyolite breccia: 3. 

conglomerate member of Nenzel rhyolite breccia; 4, well-bedded tuffs resembling sandstone

shown in the crosscut, where it strikes N. 80° E. and dips 40° N., 

and it reappears in the adit. It has a notable gouge, and the rock in 

the fault zone has been reduced to a rubble 5 feet thick. It is a normal fault, as can be seen on the summit of Nenzel Hill, where it displaces the sandstone-like tuff. The fault will be fchmd to displace 

all the veins that have been cut in the crosscut, but as the character 

of the displacement is known, the faulted segments can be readily 

discovered.

The Big Chief vein is vertical near the surface but dips 70° E. 

in depth. It averages about 3 feet in thickness. The ore is that 

typical of Nenzel Hill, consisting of fine-grained and cryptocrystalline quartz showing bluish-black patches and sporadic specks of
66 ROCHESTER DISTRICT, NEV.

pyrite. Though no silver mineral can be recognized, silver appears 

in countless small beads all through the ore after the ore has been 

brought to a red heat in the forge. Chemical tests show that the 

silver is present as argentite.

At the surface the Compressor vein (or vein No. 1) lies 50 feet 

west of the Big Chief; in the crosscut it is 85 feet west of the Big 

Chief. It dips 60° W. and is from 3 to 4 feet thick. A number of 

other veins have been cut. Vein No. 5 strikes N. 30° E. and dips 60°. 

It consists of 2^ feet of fine-grained quartz, somewhat cellular, 

containing limonite and sericite. The more compact quartz carries 

a little pyrite, with some zinc blende and covellite. Vein No. 10 

strikes N. 40° E. and dips 30° W.; it is 2 feet thick and has a welldefined gouge on its hanging wall. Among the more important 

veins cut is the Zero vein, on which some drifting has been done. 

The vein dips 60°-70° W., is quartzose, and at most is 3 feet thick. 

It has been postminerally shattered but on the whole has no welldefined gouge. On the north the vein is cut off by a fault striking 

N. 30° W. and dipping 45° W.

In general, at the time of examination little development work 

had been done on most of the many veins intersected by the long 

crosscut.

BUCK & CHARLEY MINE

The Buck & Charley mine is on the south side of Rochester Canyon 

a short distance below Lower Rochester. The veins were discovered 

in 1912. The developments consist of adits, the lowest of which is 

only a few feet above the floor of the canyon. A winze has been 

sunk from the lowest or main adit, and two levels have been turned 

off from the winze, the lower of which is 112 feet below the main 

adit. Water level was reached about 42 feet below the main adit, 

and the mine while being worked in 1919 made 80 gallons of water 

a minute.

To the end of 1917 the mine had produced $47,000. The principal 

shoot of ore was extracted by J. H. Watters under lease. The 

smelter returns Show that the ratio of silver to gold was remarkably 

constant about 1 ounce of silver to every 0.01 ounce of gold. The 

highest-grade shipment, of 35 tons, carried 134 ounces of silver to 

the ton.

The main output has come from a steep vein trending roughly 

N. 40° E. The wall rock is a hard felsite of the Rochester trachyte, 

which near the vein carries considerable pyrite and zinc blende. 

The shoot of ore in this vein was from 40 to 100 feet long and 1.5 

to 2 feet wide. It plunges to the south, at angles ranging from 

70° to 50°. On the bottom level, according to Mr. Myron Warner,
SILVER MINES AND PROSPECTS . 67

who in 1919 operated the mine under a lease, the ore shoot is 55 

feet long, of which 25 feet is $100 ore over a width of 16 inches.

The ore is a highly silicified felsite netted with quartz veinlets 

carrying pyrite, zinc blende, and subordinate galena. It has been 

somewhat altered by descending solutions, which have specially attacked the zinc blende, producing covellite and doubtless also the 

rich silver sulphide, argentite.

OCTOPUS MINE

The Octopus mine is in the western part of the district, not far 

from Lower Rochester. The vein was discovered in 1913 by Sam 

O'Connell, of Lovelock. The output up to 1917 is reported to be 

$28,000.

The mine is opened by two drift tunnels, the lower of which is 

90 feet vertically below the upper. From the lower level a winze 

was sunk 115 feet on the vein and some drifting was done. The 

water level has not been reached in any .of the workings. The 

country rock is felsite (Rochester trachyte).

The vein strikes N. 10° W. and dips 40° W. It ranges in thickness from a few inches to 6 feet and averages nearly 3 feet. The 

hanging wall is notably well defined, even, and regular. The vein 

has a strong hanging wall gouge in fact, the whole vein on the 

upper level has been largely reduced to the condition of a gouge. 

The vein is displaced at 200 feet from the portal of the upper 

tunnel by a steep fault trending N. 50° W., the offset measured 

along the fault being 24 feet, and a short distance farther in the 

vein is again faulted, along a fault parallel to the other, this offset 

being 40 feet. The first fault fissure is occupied by a tourmalinequartz ledge, the tourmaline forming as much as one-half the ledge 

matter, which locally carries pyrite and sporadic galena. In the 

second fault the filling is a soft gray rock 3 feet thick carrying 

scattered prisms of tourmaline. These faults have also been found 

on the level driven from the bottom of the winze.

The ore of the Octopus vein is shattered felsite that has been 

silicified and metallized. It is fairly tourmaliniferous at the portal 

of the upper tunnel and sparsely so at other places. It is prevailingly oxidized, but on the lower level and in the winze sunk from this 

level sulphides appear. They consist chiefly of deep-brown or black 

sphalerite, together with galena and pyrite. Such sulphide-bearing 

material is low in silver, containing about 2 ounces to the ton.

The ore shoot is 200 feet long on the upper level, where it is reported to average 13 ounces of silver and $2 in gold to the ton over 

a width of 4% feet; it is shorter on the lower level, 60 feet carrying 

20 ounces of silver to the ton and an additional 86 feet carrying 8
68 ' BOCHESTEE DISTRICT, NEV.

ounces to the ton. Picked specimens of rich ore, stained with limonite and other oxidation products, show notable quantities of soft, 

sooty argentite. In places it forms masses an inch long and a 

quarter of an inch thick. The Octopus mine is the only place in the 

district where the argentite occurs in such large masses, where it can 

readily be recognized and easily identified. Some of the rich ore 

shipped in early days by lessees, which averaged 80 ounces of silver 

to the ton, doubtless contained much of this soft argentite.

The Eaven prospect is at an altitude of 6,147 feet on the southeast slope of Lincoln Hill. The country rock is a faintly flow-banded 

felsite carrying small porphyritic crystals of microcline; it is one 

of the flows of the Rochester trachyte. The vein strikes 1ST. 30° 

E., dips 80° W., and ranges from 1 to 2^ feet in thickness. The ore 

is oxidized and is a somewhat vuggy fine-grained sugary quartz, in 

many places containing multitudes of minute prisms of black tourmaline. The country rock is laced with quartz veinlets and is 

tourmalinized and pyritized.

Small shipments of ore carrying silver and gold have been made.

ABE LINCOLN MINE

The Abe Lincoln mine, owned by the Rochester Lincoln Hill Gold 

& Silver Mining Co., is on the north side of Lincoln Hill at an 

altitude of 6,346 feet. The output is said to amount to $15,000.

The main opening at the time of visit in 1919 was a tunnel 135 

feet long, which had attained a depth of 85 feet below the old workings on the surface. The vein strikes northeast, dips 55° W., and 

ranges from 1 to 3 feet in thickness. The ore is silicified pyritic 

felsite (trachyte carrying inconspicuous phenocrysts of microcline); 

in places it contains sporadic aggregates of tourmaline fibers. It 

shows postmineral crushing, oxidation, and leaching. It is reported 

that the vein assays $8.70 in gold and 7 ounces in silver to the ton 

across a width of 3 feet. Ore sacked for shipment is said to average 

75 ounces of silver and $10 in gold to the ton. Inspection of such 

ore shows that it has been enriched by supergene sulphide.

KAISER BILL PROSPECT

The Kaiser Bill prospect is in Limerick Canyon, at an altitude of 

5,775 feet, a few hundred feet north of the road up the canyon. It 

has been explored by a vertical shaft 100 feet deep. The ore body is 

a narrow quartz vein inclosed in aplite. It lies between well-defined 

wavy walls, but only a few inches of the vein filling is considered 

ore a coarse quartz carrying sparse zinc blende. The aplite adjacent to the vein is more or less shattered, penetrated by quartz-tour-
SILVER MIXES AXD PEOSPECTS 69

maline veinlets, and pyritized. Six tons of ore has been shipped and 

is said to have averaged $25 a ton, mainly in silver.

NEVADA PACKARD MINE

The Nevada Packard mine, owned by the Nevada Packard Mines 

Co., is in the southern part of the district, on the edge of the foothills. The company owns four claims aggregating 80 acres on a 

spur locally known as Packard Ridge. Ore was discovered on the 

property in December, 1912. For a time the chief production was 

made by lessees, who are said to have taken out $100,000. Later the 

company erected a cyanide plant having a daily capacity of 100 tons, 

which began operations in December, 1915. Electric power is obtained from the Nevada Valleys Power Co., which transmits it from 

the Lahontan dam, 85 miles distant.

The mine is opened by a series of drift tunnels and a number of 

large open cuts that are worked by the glory-hole method. The lowest tunnel, the 200-foot level, is the main haulageway, and all ore is 

dropped to this level and trammed to the mill.. A shaift was sunk 

from a point near the face of the 200-foot level to. a depth of 235 feet, 

where the water level was cut. The intention was to continue to a 

depth of 300 feet, crosscut westward, and explore the downward 

extension of the Contact lode.

The ore is crushed first in a gyratory crusher and then passed 

through Garfield rolls, where it is reduced to %-inch size, then sent 

through ball mills, and finally cyanided. Seventy per cent of the 

silver is extracted before the pulp reaches the agitators. The total 

extraction now averages 90 per cent. When the ore treated carried 

all its silver in the form of chloride the extraction averaged as high 

as 94.9 per cent.57

To August 1,1919, there had been milled 114,211 tons of ore, from 

which was obtained 845,514 ounces of silver. The gold content of 

the ore is small: in 1918, for example, 30,555 tons of ore yielded 

240,411 ounces of silver and 347 ounces of gold, or in the ratio of 

1 ounce of gold to 7,000 ounces of silver. Three dividends have been 

paid to date, amounting to $110,646.

The rocks at and near the Packard, mine are members of the formation termed in this report the Weaver rhyolite. They consist of 

an alternation of massive rhyolite, schistose breccia, pure-white sericite phyllite (which has resulted from the metamorphism of finegrained tuff), and soft shaly white tuff. The massive rhyolite, representing a succession of lava flows, predominates, but it appears 

even more abundant than it actually is because it crops out promi57 Thomson, H. G., Construction and operation of the Nevada Packard mill: Min. and 

Sci. Press, vol. 113, p. 384, 1917.
70 EOCHESTEE DISTRICT, NEV.

nently, whereas the soft beds of tuff are covered by the float from 

the hard rhyolite. The massive rhyolite consists of successive flows, 

of which a variety mottled in blue and white, locally known as the 

" hard blue." and a variety containing numerous quartz-filled, originally hollow 'spherulites are most common. The tuffs are banded, 

and they include some intercalated beds of shale. They are of special significance because their stratification is readily ascertainable, 

and so they serve to determine the structure of the rocks. Furthermore, one of the main ore bodies the Contact ore body occurred 

along a fault where a particular belt of tuff formed the hanging 

wall. The rhyolite flows with their intercalated tuffs strike 1ST. 5° E. 

and dip 15°-30° W., but the schistosity dips at a considerably higher 

angle 60°. Consequently, in determining the structural relations 

of the ore bodies this schistosity must be carefully discriminated 

from the bedding. In the mine itself and in its near vicinity dips 

of about 15° W. prevail; but a few hundred feet west of the chief 

workings the dip is 22° W., as is clearly shown in the Kromer tunnel.

As seen from the settlement of Packard, a prominent line of crags 

crosses the brow of Packard Ridge. It has been locally named the 

" dike," but on examination close at hand it is obvious that the crags 

are not the outcrop of a dike, and they are found to be far less regular than they seem when viewed from a distance. They form an 

irregular zone, roughly 100 feet wide, bordered on the east by a tuff 

belt, about 100 feet wide as measured on the surface. This is probably a slightly silicified fault zone, which is possibly the southward 

extension of the fault by which the Rochester trachyte is faulted 

against the Weaver rhyolite on the ridge on the south side of Weaver 

Canyon. (See PI. I.) Schrader 58 thought that the fault on Packard Ridge was the main factor in the localization of the ore on 

Packard Ridge, but subsequent development work has not borne out 

this opinion in fact, tunnels driven under the " dike " in the belief 

that it is a locus of mineralization have so far failed to disclose 

anything of value. The tuffs in the belt east of the " dike " were not 

found in the crosscuts under the " dike " and therefore must have 

been cut off by a fault, providing that they dip west, a condition 

which could not be verified at the time of examination. If they dip 

east, then it is fairly certain that the " dike " represents the southward extension of the fault previously mentioned.

The ore bodies are of two kinds lodes, exemplified by the Contact ore body, and stockworks, which have furnished the bulk of the 

ore. In both the lodes and the stockworks the ore is essentially 

similar.and consists of rhyolite carrying disseminated cerargyrite 

(horn silver), which is so finely distributed as to be only rarely

68 Schrader, F. C., The Rochester mining district, Nev.: U. S. Geol. Survey Bull. 580, 

pp. 335, 351-352, 1914.
SILVER MINES AND PEOSPECTS 71

perceptible to the eye. The rhyolite in the lodes is roughly schistose 

and highly sericitic. Some of the tuff beds have been so thoroughly 

altered to sericite that they are now compact waxy sericite schists. 

Sericite is the only conspicuous gangue mineral that was formed during the mineralization; quartz is practically absent. In the ore of 

the stockworks sericite is far less common.

The Contact lode is a zone of mineralization along a fault that 

trends N. 30° E. and dips 45°-60° W. This fault has brought 

a belt of soft tuff down against the rhyolite. The normal undisturbed dip of the tuff is 15° W., but near the fault surface it has 

greatly steepened. These features are most plainly shown in the 

upper tunnel of the Rochester Combined mine, whose portal is in 

well-stratified, somewhat banded gray tuff. Naturally, when the 

base of the tuff was reached in the downward progress of mining, 

the sedimentary contact of the tuff on the underlying rhyolite was 

found to diverge abruptly from the fault surface and the fault 

was found to continue downward in the rhyolite.

The west contact of this tuff belt is clearly exposed in only one 

place in a tunnel some 400 feet northwest of cut 23. It is alsoo a 

fault contact, striking N. 30° E. and dipping 60° W., which like the 

east contact has been mineralized. A small body of ore on this 

contact, reported to be 200 tons carrying 25 ounces of silver to the 

ton, was sfcoped out to the surface by lessees.

Along the south end of the lode the ore occurred in the sheared, 

rhyolite adjoining the fault, but none occurred in the tuff hence the 

name Contact ore body and the fact that the two dissimilar rocks 

join against the fault was thought to have determined the localization of the ore. However, as drifting proceeded northward the 

base of'the belt of tuff was found to rise, owing to the differing 

strikes and dips of the fault and the tuff, and ore was found below 

the belt of tuff in the sheared rhyolite on both sides of the fault. 

If anything, then, the tuff was in reality unfavorable to the formation of ore, for where it is not present ore formed on both sides of 

the fault.

A number of valuable ore shoots were stoped out along a length 

of 600 feet on the Contact lode. The ore was of higher grade than 

that obtained from the stockworks, and some of it averaged 13 

ounces to the ton. The shoots were as much as 250 feet long; and o

where widest were 40 feet across, but they had a small vertical range, 

about 50 feet. Some of them did not extend to the surface, as the 

material in them was not of ore grade above a depth of 40 feet. As 

the bottom of the ore shoots was reached the ore became more difficult to cyanide, and the extraction fell off to 83 per cent, indicating 

that part of the silver was locked up in a less amenable compound

98293 24 6
72 ROCHESTER DISTRICT, NEV.

than the chloride, doubtless the sulphide. The mineralized lode matter below the ore shoots consists of sericitized rhyolite that carries a 

small quantity of finely disseminated pyrite, zinc blende, and a trace 

of galena and contains only 0.5 ounce of silver to the ton. The vertical distribution of the mineralization in the Contact lode shows, 

therefore, a zone of impoverishment near the surface, a zone of enrichment below it, and a zone of unenriched mineralized material, the 

protore, far too lean to work, extending below the enriched zone. 

The ore in the zone of enrichment, which when formed consisted of 

argentite, was later converted into the chloride.

Good ore was found in the Contact lode as far as the north end 

line of the Nevada Packard mine, and a small amount in the exten-

-20o_ ''-

FIGURE 5._Diagrammatic section through the Nevada Packard mine along the line of 

tunnel A. 1, Massive rhyolite; 2, soft gray tuff, with some interbedded shaje; 3, compact sericite schist; 4, schistose rhyojite tuff and breccia; 5,, ore; 6, leanly mineralized 

lode matter (protore)

sion into the adjoining ground of the Combined mine, which was the 

only ore found during the great fiasco connected with the exploitation

of that mine.

The structural relations of the Contact lode are shown in Figure 5, 

which is a section drawn along the line of tunnel A. A section 

drawn across the lode somewhat farther north would show the base 

of the tuff belt at a higher level and ore occurring in the subjacent 

rhyolite on both sides of the fault.

The stockwork deposits are mined by large open cuts. There is 

little to indicate that much of the material mined is ore, as it does 

not differ perceptibly from the general run of country rock. However, certain of the ore bodies after being subjected to weathering 

for a time show that the country rock is traversed by a widely spaced
ROCHESTER COMB,NED MlN£s
SILVER MINES AND PROSPECTS 73

network of narrow veinlets, as is illustrated in Plate II. The veinlets 

are fine seams^ which in ore newly exposed to the weather are not 

apparent but which on oxidation become visible. Good ore is indicated by the development of coarser sericite and by the presence of 

numerous small iron-stained cavities formed as the result of the removal of pyrite and other sulphides by weathering. Assaying, however, is the only reliable method of determining what is or is not ore.

The stockworks are roughly elliptical in plan, the major axis 

trending N. -25° E. The ore in all of them gives out at a depth! 

of 30 feet or less. The fracture planes of the ore are deeply stained 

by a deep-blue bloom, possibly vivianite, but this staining, although 

conspicuous, is of no economic significance. The stockworks occur 

both east and west of the so-called dike, but the largest quantity of 

ore has so far been found west of the " dike."

The Kromer Hampton open cut was one of the first from which 

ore was mined on the Nevada Packard property. It is in an extremely hard mottled blue and white massive rhyolite, known locally 

as the " hard blue." The fact that this rock is ore runs counter to 

all ideas as to what ore should look like., The pit is 150 feet long 

and 100 feet wide and attains an extreme depth of about 30 feet. The 

position of the Kromer Hampton cut relative to the workings on the 

Contact lode is shown in Plate IV. A fault trending N. 50° E. and 

dipping 60° NW. extends southwestward from the cut and converges 

with the Contact lode at the C stope. The 100-foot level is drifted 

along this fault, but no ore occurs along it.

The A cut is 250 feet northeast of the Kromer Hampton cut, and 

much ore was obtained from it in early days. It is 250 feet long and 

75 feet wide, but it is rather shallow. The A tunnel was driven under 

this cut, and a raise was put through to the bottom of the cut, 90 feet 

above. No ore was found in the tunnel or raise.

In 1919 the mainstay of the mine was the Margrave cut, a large 

elliptical pit trending N. 25° E. The ore from it averaged $8 a 

ton in silver. Like the others, however, it proved to be shallow, and 

it was worked out by the end of the year.

The lessees are reported to have shipped in early days from opencut workings carload lots of ore running from $30 to $60 a ton. Such 

high-grade ore was clearly the result of selective mining and careful 

sorting, for the open cuts have yielded by bulk mining ore that has 

 averaged slightly under 8 ounces of silver to the ton. In addition to 

the ore, there is an immense amount of rock carrying several ounces 

of silver to the ton in fact, the striking feature of Packard Ridge 

is the widespread dispersion of the silver and its concentration in 

sporadic shallow bodies of ore.
74 ROCHESTER DISTRICT, NEV.

GOLD MINES AND PROSPECTS 

LINCOLN HILL MINE 

The Lincoln Hill mine is on the northwest flank of Lincoln HilL 

It is opened by two adits, of which the lower, at an altitude of 6,190 

feet, is the principal haulageway. It intersects the veins approximately 80 feet below their outcrops. The ore is hauled from the 

portal of the lower adit by wagon to a small milling plant in Rochester Canyon. The mine is reported to have produced $80,000 in 

gold and silver up to 1917. The gold bullion produced is of low 

grade, ranging in fineness from 0.530 to 0.600, owing to the considerable proportion of silver it contains.

The country rock is a quartz trachyte or rhyolite containing 

numerous conspicuous porphyritic crystals of orthoclase and a few 

inconspicuous crystals of quartz; in places it is faintly flow banded. 

Three narrow quartz veins have been worked, which are known as 

the Peerless, the Dike, and the Spur.

The Peerless vein furnished most of the output prior to 1917. It 

has been stoped for more than 200 feet along its strike, averaging 

6 inches in thickness.

The Dike vein is peculiar in that the quartz ore that is mined occurs along both contacts of a dike locally termed the " tourmaline 

porphyry." This dike consists of a whitish-gray fine-grained rock 

specked with fibers and columns of black tourmaline. It strikes N. 

40° W. and dips 70° E. Because of the thorough hydrothermal 

alteration of this dike, its original nature remains obscure, but it 

appears to have been of aplitic composition. In places secondary 

biotite has formed in it, but more generally it has been highly sericitized, and the tourmaline prisms formed at the same time give the 

rock its apparent porphyritic texture.

The ore is a milky quartz, in places carrying some tourmaline. 

Coarse gold is common, yielding ore that runs several hundred dollars to the ton, and is of primary origin. Sulphides are relatively 

sparse; black zinc blende predominates, followed by pyrite and by a 

very minor amount of galena. The wall rocks are pyritized and 

tourmalinizecl. The ore was localized in shoots that averaged a few 

inches in thickness.

The Spur vein branches off from the Dike vein, striking N. 70° W. 

and dipping 35° E. It averaged about 6 inches in thickness, and thequartz was frozen to both walls. At its junction with the Dike vein, 

it is said, there was 2 feet of ore averaging $400 a ton.

In drifting south on the Dike vein on the upper level a fault was 

cut, which gave much trouble before the lost segment of the vein 

was found. The fault trends N. 30° E. and dips steeply north, nearly 

vertical. The fault zone is 4 feet wide and is thoroughly crushed.
GOLD MIKES AND PEOSPECTS 75

The crushed breccia where first cut contains east of the vein much

 quartz, seemingly torn off from the Dike vein, and therefore naturally suggested that the severed segment of the vein lay farther east 

in other words, the fault was thought to be a normal fault. Drifting for 200 feet eastward along the fault failed to disclose the vein, 

however, and it was then discovered that fragments of the distinctive " tourmaline porphyry " occur in the fault drag west of the 

vein. This clue was acted on, and the lost segment was soon found. 

The fault thus proved to be of the reverse kind, and the misleading 

quartz drag was then perceived to have been torn off from the Spur 

vein, which, along the line of the fault on the upper level, is 45 feet

 east of the Dike vein. The displacement of the Dike vein as measured 

on the lower level is 42 feet.

High-grade ore was found in the segment south of the fault. The 

mining of such narrow high-grade shoots, whose thickness is generally less than 6 inches, yielded only a small tonnage, and in 1919 the 

policy of breaking a greater width of ore of lower grade was adopted. 

It was reported that 4 feet of $15 ore was being extracted.

OBO FINO MINE

The Oro Fino mine is in the western part of the district, on Gold 

Ridge between Rochester and Limerick canyons. The vein was 

discovered long before the present camp of Eochester came into 

existence, and it is said to have yielded considerable high-grade 

gold ore in 1879.59 It was later abandoned, but in recent years it has 

been relocated by U. W. Harwood and associates, who have renamed 

it the Rico Nevada. It was stoped for 200 feet along its strike, 

and some of the old inclines attain a depth of 120 feet on the vein.

The vein consists of coarse white quartz, in places carrying masses 

of acicular black tourmaline. It ranges from 3 to 5 feet in width and 

lies between well-defined walls of fine-grained rhyolite breccia. The 

portion of the vein lying against the footwall has been severely 

crushed over a width of 15 inches. The ore carries no visible metallic 

minerals. It is reported that the gold tenor is spotted that the ore 

can be sorted to run $50 a ton, but that other ore apparently similar 

will carry after sorting only $8 a ton.

HAGAN PROSPECT

The Hagan prospect is in Limerick Canyon near Spring Valley 

Pass. It is on the Mary McKinney claim, owned by A. F. Hagan. 

In 1917 it was under lease and bond, and some gold was being mined.

"» Schrader, F. C., U. S. Geol. Survey Bull. 580, p. 357, 1914.
76 ROCHESTEK IHSTKLCT, NBV.

The lode was discovered as the result of dry placer operations. 

It is at the apex of a thin alluvial fan, the gravel of which is 

gold bearing. The placers, as can be plainly seen from the workedout ground, headed against the lode on the Mary McKinney claim, 

and consequently the gold tenor of the gravel in the shallow gulch 

improved headward, for the gold was not so widely scattered as it 

was farther down on the alluvial fan, where it was first found.

The lode has been opened up by 200 feet of tunnels and drifts. 

The footwall rock is a brownish iron-stained sparsely porphyritic 

rock, which under the microscope in found to be a quartz keratophyre.

The ore is in a fine-grained white rock, probably a trachyte, as 

it contains phenocrysts of orthoclase in a fine-grained groundmass, 

and the ore body consists of this white rock cut by stringers and 

veins composed of quartz, microcline, and tourmaline. The gold, 

which is coarse, generally occurs embedded in masses of limonite that 

have resulted from the oxidation of arsenopyrite. In preparing 

the ore for shipment, the material as mined is screened through a 

%-inch mesh and all but the screenings is rejected. The screenings are reported to carry $20 a ton in gold. The aplite contains 

innumerable cubes of oxidized pyrite, and the whole mass of rock 

that is, the mineralized aplite plus the quartz stringers traversing 

it is'said to average $4 a ton in gold.

A strong quartz vein, also carrying coarse feldspar and tourmaline 

like the stringers. in the Hagan lode, has been uncovered several 

hundred yards northeast of the Hagan lode on the same group of 

claims. Some chalcopyrite occurs in the vein. Pockets high in gold 

occur in the adjoining highly altered wall rocks.

Early in 1922 it was reported that a 20-ton amalgamation plant 

had been built on the property and water brought in by a pipe line 

6,000 feet long.60

80 Mln. and Sci. Press, vol. 123, p. 943, 1921.
INDEX

Page

Alluvial-cone detritus, age of..,.-._.--.._-._ 36 

occurrence and character of...... .. 35-30

Alluvium, nature and occurrence of..._ 37 

Alteration of wall rock... ...... 50-51

Andalusite, development of........ . 19-20

Aplite, analysis of...._...... .. .. 33

general features of.... ......... 32

genetic relations of,_ ... . 33-34 

petrography of...... 32-33

Argentite, occurrence and origin of....... 46-50

suporgene, replacement of sphalerite by,

plates showing.............__ 47

Barite, deposit of......... .. 55

Basalt, nature and occurrence of....___.- 36-37 

Bibliography of the district_________ 9 

Boughton & Hackley mine, description of... 64 

Buck & Charley mine, description of ___- 66-67

Cinnabar, deposit of.....---...._--.--.-- 53-54

Climate ofthe district....................... 3-4

Contact lode, features of.._..______ 70-72 

Copper, deposit of...._ ... .. 55

Daonella zone, nature and place of.____. 31 

Decrease of silver content of the ores in

depth ......................... vm-ix

Deformation of the rocks, outline of..... __ vin

Drown, T. M., analysis by._.._ _.. 33

Dumortierite, development of... _.. __ 18-19,53

genesis of._________. __.___. 56,57

Enrichment of silver ores, process of. .... 46-50

Faulting, results of....................... 12,39-40

Felsite, petrography of.....__.._____. 14-17

Folding, effects of..................... 38-39

Fossils, collection and study of...___ 8,9,10-11

Genesis of the ores : . 55-58 

Geography ofthe district .-..-. 1-4

Gold, production of.... .---..- 6-7 

Gold mines and prospects, descriptions of... 74-76 

Gold ores, genesis of . 56,57 

Gold veins, general character of... 51-52 

nature of solutions that traversed .. 52 

Granite porphyry, nature and occurrence of. 34-35

Hagan prospect, description of.. .. 75-76 

Hague, Arnold, early survey by. 7-8,9 

History of mining.. . 4-6 

Humboldt Range, name and divisions of.... 1-2

Investigations, early...__...... . ... 7-9

Jamesonite, deposit of..... ....... 54-55

Jones, J. C., earlier survey by......... .. 8,9

Page 

Kaiser Bill prospect, description of.......... 68-69

Keratophyres, age and correlation of_...... 22-23

occurrence of.._.______.___ 20 

petrography of....____.__.___ 20-21

relation of, to the Rochester trachyte. _ 22 

King, Clarence, early survey by.____.. 7-8,9 

Kromer Hampton cut, working of_____ 73

Limerick Canyon, features of__.___.... 3

Limestone, age of.______________ 29-30

general features of..___________ 29

relation of, to intercalated volcanic rocks. 30-31

Lincoln, Abe, mine, description of______ 68

Lincoln Hill mine, description of.._____ 74-75

Louderback, G. D., early survey by____ 8,9

Magma, distillates given off from______ 57-58 

Map, geologic, of the district... _______ 22 

Mark Twain, description of a mining camp

by............................... 4

Merriam, J. C., study of fossils by_____ 8,9 

Metadiorite, nature and occurrence of.._.. 31 

Mineralization of the rocks, outline

of..................._.... vn,vni-ix

Minerals in gold veins.__________'... 51-52 

in miscellaneous deposits.________ 53-55 

in silver veins.._____________ 45-46

Nenzel, Joseph F., discovery by...____.. 5 

Nenzel Crown Point property, description of. 65-66 

Nenzel rhyolite breccia, occurrence and

character of......__... ..... 23

stratigraphy and structure of......__. 23-26

Nevada Packard mine, description of.___ 69-73 

map of workings of. __.__...__.. 72 

silver-bearing stockwork at, plates showing............ .... ..... 46

Octopus mine, description of..._______ 67-68 

Ore deposits, general features of........._.. 43

relation of, to water level........ ..... 43-44

Ores, deposition of, from a cooling magma... 57-58

genesis of....._..._ _... ..... 55-58 

Oro Fino mine, description of._._... 75 

Outline of the report __ . . vn-ix

Pendleton, T. P., map made by............. 8

Physical features of the district 1-3

development of.. _.. 41-42

Placers, discovery and working of.....__-. 4,58

Publications, earlier...__ _ . 9

Ransome, F. L., early survey by 8,9

Preface by.................. . v-vi

Raven prospect, description of.. 68

77
78 INDEX

Page 

Replacement of sphalerite by argentite...... 47-50

Rhyolite, Weaver, occurrence and character

of..................... ....... 26-27

Weaver, petrography of._________ 27-28 

tuffaceous shales in..____........ 28-29

Rhyolite tuffs, description of. _____.... 26

Rochester Canyon, features of______... 3 

Rochester mine, description of......._..... 59-64

Rochester Mines Co., early operations by... 4,6 

Rochester Silver Corporation, organization

, of.,.........--...-...... ... 59

Rochester trachyte, analysis of....... _. 16-17,54

andalusite in..____________._ 19-20 

dumortierite in___..__..._..__.._.__ 18-19 

general features of...___..____... 14 

petrography of....__________-_ 14-17

relation of keratophyres to...... ___.. 22

thickness and age of.__......._..... 17-18

Schistosity, degree and direction of.__._ 40-41 

Schrader, F. C., earlier survey by.._..__ 8,9 

Sericite, occurrence and development of..... 53-54

Settlements in the district.___.._._ . 1,5 

Silver, decrease of, in depth......_._.. vin-ix

production of________.____ 6-7

Silver mines and prospects, descriptions of... 59-73

Silver veins, distribution of._________ 44-45

enrichment of...._______.____ 46-50

genesis of-....__- -_. _-- ... 55,56-57

minerals in_________________ 45-46 

nature of........._..___........ 43,44-45

PageSituation of the district_.-.....-....-.-.... 1

Smith, J. P., cited........................... 10

study of fossils by....... 8, 9-

Stanton, T. W., cited........................ 30

Star Peak Range, analysis of granite from... 3£ 

features of__......_____...___ 3

geology of..........__........---..--. 9-12

Stockworks, silver-bearing, nature' and occurrence of..-.--.--._----...-.--.. 43

Structure of the rocks__-...__-...--.--- 38-41 

Surveys, early.........-.......... ........ 7-9'

Tourmaline, presence and development of... 51 

Trachyte, analysis of._..._______ 16-17,54 

Triassic period, rocks of...........__.... 13-31

Vegetation of the district..___............ 3-4-

Volcanic rocks, relation of, to intercalated

limestone....-.-----...-.-.-..... 30-31

Wall rock, alteration of __-...__.... 50-51

Water table, relation of the ore deposits to.. 42-44

Weaver Canyon, features of.... 3-

Weaver rhyolite, occurrence and character of. 26-27

petrography of_._____. ______ 27-28

tuffaceous shales in._.... _ 28-29'

Wells, R. C., analyses by. . ... 16,33,54

West Humboldt Range, columnar section of. 10

features of _. __. __________.. 1-2

o