,._ ·_/ 

DEPARTMENT OF THE INTERIOR 

FRANKLIN K. LANE, Secretary 

UNITED STATES GEOLOGICAL SURVEY · 

GEORGE OTIS SMITH, Director 

Professional Paper 114 

GEOLOGY AND ORE DE_POSITS 

OF THE 

YERINGTON DIST_RICT, NEVADA 

BY 

• ADOLPH KNOPF 

WASHINGTON 

GOVERNMENT PRINTING OFFICE 

1918 
._,., .. .,... ... '· 

_.! ~ 1 • ~ :. •• ~ .... 

\, :'. 
·CONTENTS. 

. Preface, by F. L. Ransome ...................... :. 

Outline of report ................................ . 

Pttrt I. General features .................•....•.. 

Geography ........................ · ......... . 

Field work .................................. . 

Bibliography ............................... . 

. History of mining .......................... . 

Production .................................. . 

General geology ............................ . 

'l'riassic rocks . . ............ : ........... . 

Character ........................... . 

Age ............................... . 

Volcanic rocks ...................... . 

Andesites ...................... . 

Soda rhyolite-felsite (keratophyre). 

General features ............. . 

Petrography ...... : .......... . 

Sedimentary rocks ................... . 

Garnetites and allied rocks .......... . 

Character and distribution ....... . 

Origin of the metamorphic rocks .. 

Gypsum ........................ .' .. . 

Correlation of the Trias!3ic rocks ...... . 

Cretaceous(?) rocks ..................... . 

General relations . .' ....... : ......... . 

Major ~ntn~~io~s .................... . 

Granochonte ................. , .. . 

Quartz monzo~1ite .............. ·.· 

·General features ............. . 

Scapolitic alteration ........ . 

Minor intrusions .................... . 

Aplite .......................... . 

Quartz monzonite porphyry.· .... . 

Occurrence and character .... . 

Petrographic details ......... . 

'fertiary rocks ........................... . 

The section in general. .............. . 

Conglomerate at the base of the. Tertiary section ........ : ............. . 

Latite series .......................... ~ 

Latite vitrophyre ............... . 

Quartz latites ............... · .... . 

Rhyolite .. · ......................... ~ 

Andesite. breccia ................... . 

Hornblende andesite ................ . 

Conglomerate ...................... . 

Basalt ... : ......................... . 

Correlation of the Tertiary rocks ..... . 

Quaternary deposits ............... , ..... . 

Geologic structure ............... · ........ . 

Page .. 

5 Part I. General features-Continued. 

7 Gener;,tl geology-Continued. 

9 Epitome of the. geologic history of the 

9 district .....•......................... 

9 Part II. The ore bodies ......................... . 

9 

11 

12 

12 

12 

12 

12 

13 

13 

13 

13 

14 

16 

16 

16 

17 

18 

19 

19 

19 

General features ............................ . 

Minerals of the ore deposits .................. . 

Primary ·minerals ....................... . 

Secon'dary minerals .......... ~ .......... . 

Contact-metamorphic ore deposits ........... . 

Geologic environment ................... . 

Composition of the rocks inclosing the 

ore deposits ....................... . 

Relation of the ore deposits to faulting 

and brecciation ................... . 

Form and dimensions of the ore deposits. . . 

Metamorphism by the ore-forming solutions 

:Mineral transformations ............. . 

Composition of the ore-forming solutions ............................. -

Paragenesis ............................. . 

Time of ore deposition .................. . 

Origill. and classification ........... : ..... . 

Oxidation and sulphide enrichment ..... . 

Fissm-e veins and other deposits ............. . 

20 Part III. Mines and prospects .............. · .... . 

20 Mines an'd prospects on contact-metamorphic 

20 

2'0 

21 

21 

21 

22 

22 

22 

23. 

23 

23 

23 

23 

25 

25 

26 

26 

26 

27 

27 

27 

28 

deposits ...... ~ ........ : . ~ .......... · ...... . 

Bluestone :mine ................ · ........ . 

Mason Valley mine ...................... . 

History and de;velopment ............ : 

Geologic featm-es ... ._, ............... . 

_ Malachite mine ............ .' .......... · .. . 

McConnell mine ........ : ............... . 

Western Nevada mine ................... . 

Greenwood prospect ........... , ........ . 

Ludwig mine ........................... . 

General features ..................... . 

Details of the geology ................ . 

The ores ............................ . 

Gypsum deposit .........•............ 

Douglas Hill mine ........................ . 

Casting Copper mine ..................... . 

Mines and prospects on deposits in igneous 

rocks .................................... . 

Montana-Y Jrington mine ................. . 

Empire-Nevada mine .................... . 

Jim,Beatty prospect. .................... . 

Terry and McFarland prospect ........... . 

Blue Jay mine .......................... . 

Index ... · .................. : ................... : .. 

3 

Pago. 

29 

31 

31 

31 

31 

33 

34 

34 

34 

35 

38 

39 

39 

41 

41 

43 

44 

47 

49 

50 

50 

50 

51 

51 

52 

55 

55 

57 

58 

58 

58 

59 

GO 

62 

62 

63 

64 

64 

64 

65 

65 

65 

67 
ILLUSTRATIONS. 

PLATE I. Geologic map of the Yerington district, 

Page. 

_ Nev ..•........ _ .. _ .... _ .... _ . . . In pocket. 

II. A, Garnetized Halobia; B, Photomicro-

- graph of latite vitrophyre. . . . . . . . . . . . . 16 

III. Geologic sections across the. Yerington 

district; Nev. _ . ___ .. __ .... __ . __ .. _ . . . 28 

IV. A, Specimen showing junction of unreplaced limestone with portion wholly 

replaced by andradite; B, Garnetized, 

silicified, and pyritized porphyry from 

the Ludwig mine .... _. ____ ........... 36 

· V. A, Garnetite breccia in which the fragments have been partly replaced by 

epidote, Bluestone mine; B, Gangue of 

copper ore, in which andradite is interstitial between pyroxene crystals, 

Western Nevada milie.. . . . . . . . . . . . . . . 40 

FIGURE ~- Index map showing the location of the 

Yerington district, Nev......... . . . . . 10 

· 2. Geologic section extending N. 71°. W. 

from the summit of Douglas HilL . . . . 17 

4 

FIGURE 3. Columnar section of the Tertiary rocks 

in the Yerington district ........... __ 

4. Section through the glory-hole tunnel of 

the McConneli mine .... _ ... _ .· _ ..... 

5: Geologic section through tunnel No. 4, 

Mason Valley mine ............. _ . __ . 

6. Inclosure of an idiomorphic cross section of lamellar pyroxene in a clinopinacoidal section of pyroxene ...... . 

7. Geologic map of the area surrounding 

the Mason Valley mine ............ . 

8. Plan of the McConnell mine, showing 

the geologic relations ............. : . 

9. Geologic section acr~ the Ludwig lode 

at the vertical shaft .......... _ ....... . 

10.· Geologic section across the Ludwig lode, 

320 feet southwest of the vertical 

shaft_ ........... -- . : .............. __ 

11. Areal geology of the Douglas Hill mine. 

12. Diagrammatic map of the surface geology 

at the Casting Copper mine ......... . 

Page. 

29 

37 

41 

53 

56 

59 

60 

63 

63 
PREFACE. 

By.F. L. RANSOME. 

Y cringtOJl" is one of the smaller and less pro-· 

duct:ivc copper districts of the West, its· entire 

output frorp. the beginning of mining operations 

being less than a third of the annual output of 

the mines at Bisbee, Ariz. Nevertheless, its deposits are exceptionally ifiteresting to the student of ore deposition and differ from those of 

the larger and more productive districts of Utah, 

Nevada, New Mexico, and Arizona in ·being of 

contact-metamorphic origin and only slightly 

modified by enrichment. 

The relation of ore depos~tion to contactmetamorphic. action has been discussed by Mr. 

Knopf in a keen and suggestive way. He 

shows that in this district, probably in early 

Cretaceous time, there were two plutonic intrusions, the first of granodiorite and the second 

of quartz monzonite. These intrusions effected 

intense contact metamorphism of the invaded 

rocks, both igneous and sedimentary. A notable contribution in the report is the recognition 

of a time interval, marked by dike intrusions 

and fissuring, between this first metamorphism 

and the ore deposition, which nevertheless took 

place at high temperatures and was accompanied by the formation of garnet and other silicates characteristic of contact metamorphism 

of the pneumatolytic type. It is clearly shown 

that much iron and silica were introduced into 

the ·limestones during the first metamorphism· 

and that some of these same constituents and 

large quantities of copper and sulphur were 

added during ore deposition. 

The fact that many of the ore bodies are not 

actually at the contacts between the plutonio . 

·rocks and the invaded rocks leads to an interesting discussion of the essential characteristics 

of contact-metamorphic deposits. The conclusion is reached that mineral composition and 

associations are of more weight than mere proximity in deterniining the classification of a 

deposit. 

In describing the relation of enrichment to 

the adjacent limestone in the Lud~ig mine 

Mr. Knopf makes the important suggestion 

that supergene sulphide enrichment is likelr to 

be an extremely wasteful process. This conclusion, which accords with that presented ·in 

my own paper on the ·copper deposits of the 

Ray and Miami districts, Ariz./ indicates that 

estimates of the thickness of rock eroded from 

above an ore body in the course of enrichment, 

based on a comparison between the copper 

contents of pro tore and. ore, may be far under. 

the mark. 

It is impracticable in a brief preface to call 

attention to all the interesting features in the 

following pages. The report is an excellent 

example of a well-balanced, thorough study 

of a small mining district, in which the essential facts have been discriminatingly observed, 

the conclusions ably drawn, and the whole 

pre$ented without superfluous or unrelated 

detail. 

1 U.S. Geol. Survey Prof. Paper 115 (in preparation). 

5 

. . .. ~ . ,. . - . ' . 

~ . . . . \ . 

,; . \,.· 

: .· ~'. 

~. ,." • ~ .J ~ • • ) ' • ~ • 

·.I . . !·; ~ .. ·:1 ~ :. : 

·OUT.LINE ·OF REPORT.' I : 

-.---. 

The Yerington district; which next t9. Ely i~ l of appro~imately upper . ¥ioc;~n~· age. The 

Nevada's 'rriost productive copper district, is ·in midd'Ie subdivisi~n consi~ts ~f. a· series of .andesthe west-central part of the State, 50 m,iles ite flows resting in places on, the ~roded edges . 

southeast of Reno. Before 1912 it did not pro- of t~e rhyolites. The uppern;:t.o~~ subdivisi~Hl 

duce largely, but since the1i, to the end of 1917, consists of subangular conglo~~rate overlain by 

it has produced 61,200,000 pounds of c'opper. basalt, which caps the prominent mesas in the 

The rocks of the district are divided natu- southern part of the district. The ba~alt has 

rally into two strongly contrasted group~, sepa- been slightly tilted since it was erupted and has 

rated· by a conspicuous unconformity--an been much faulted; in fact, the. present relief 

older, Mesozoic group, to which the ore deposits of the range has resulted large.ly from postare restricted, and a younger group, consisting basaltic faulting. · · · 

of Tertiary volcanic rocks. The oldest rocks, The principal ore bodies consist of pyrite and 

of Triassic age, comprise ai1desites, ·soda chalcopyrite in· a gangue of pyroxene, garnet, 

rhyolite-felsites, an·d limestone; with subordi- and epidote. They attain 800 feet in length 

nate quartzite, shale, and gypsum. They ag- and 1qo feet in width; these dimensions, how~ · 

gregate at least 8,000 feet in thickness, of which ever, are exceptional and as a rule are more 

volcanic rocks n1ake up 3,200 feet. They were· applicable to .the ore-bearing zones, in which 

invaded in post-Triassic time, probably early in the ore bodies are commonly small fractions of 

the Cretaceous, by a medium-;-grained . basic the associated masses of barren garnet-pyrox~ne 

granodiorite ru1d shortly after by a coarse- rock. In the Mason Valley mine, for example, 

grained, roughly porphyritic quartz monzonite. masses of barren garnet rock, in places 200 feet 

These intrusions intensely metamorphosed the thick, tinderlie the productive part of the orerocks they invaded: they caused the andesites bearing zone. The known vertical range of 

and felsites adjacent -to them to recrystallize mineralization in the district, in the present 

and converted large volumes of the limestone stage of development, is 950 feet. 

into calcium silicate ~ocks, among which dense The ore deposits have resulted from the reheavy brown garnetites predominate. Prob- placement of comparatively pure limestdnes 

ably one-half of the Triassic area consists of and are of the contact-metamorphic type. 

rocks made up of garnet, wollastonite, and They are clearly related to faulting, and inasallied silicates. After this metamorphism nu-:- much as the replacement of the limestone ex~ 

merous dikes and bosses of quartz monzonite tended qutward from fault fissures the ore 

porphyry were injected. Faul~ing then en- bodies, as a rule, are lodelike in form. The ore 

sued, breaking and displacing the dikes, and tends to occur on the limestone side of the orealong son1e of the faults formed at this time bearing zone, though this rule is not without 

metalliferous solutions rose and produced the notable ex~eptions. As the ore deposits .are 

copper deposits to which the district owes its situated on or near faults and as these faults 

economic importance. . commonly separate dissimilar rocks, such as 

· The Tertiary rocks rest with marked uncon- limestone from felsite, andesite, or stratified 

formity on the Mesozoic rocks. They are·domi- calc-silicate rocks, it follows from these cirnantly volcanic and are at least 7,000 feet thick. cumstances, together with the lodelike form and 

They fall into three major subdivisions, which the tendency of the ore to occur on the limeare separated by two well-marked unconformi- stone side of the ore-bearing .zones, that the 

ties. The lowest subdivision consists of quartz most systematic way of exploring for ore is to 

latito, rhyolite, and andesite breccia; it is drift in the limestone parallel to the ore zone 

tilted at angles as groat as 60°; and it probably and to crosscut back at intervals to the ore 

is tho correlative of the .Esmeralda formation, zone. 

7 
8 OUTLINE OF ·REPORT. 

The ore body at the Bluestone mine, consist- exception)-on limestones that were not siliing essentially of chalcopyrite and epidote, cated by the metamorphism of the first period. 

differs from the others of the. district'in that it The primary .ores of the district are essenwas formed by the replacement not of lime- tially unenriched by later sulphides, supergene 

stone but .of brecciated garnetite. and; other .covellite and chalcocite being oil. the whole of 

contact-metamorphic rocks of the first period of minor importance. The production- has come 

metamorphism. Its features serve to empha- largely from ores of this kind from the contactsize the f:!epa~atenes~ and distinctpess of the, metamorphic deposits, and _the. average tenor 

two p~riods of metamorphism.distingu~shable of the ores has ranged from f.75. to 6 per cent 

in the distric't, bbth of wh~ch occurred, ho-\v:ever, of copper. . 

'during th~ same general epoch of 1ritrusion~an · · ~ypes of ore deposits other than the cpntact- . 

earlie_r, widespr~ad nietamo_rphism m.arked by. metamorphic. are sparll;lgly represented in the 

th~ addition ·or the_iln;nlens~ .. qp.antities of silica distri~t. and are of s"ti,bm;dinate econoric ~mand iron to the:meta~or.p:g.osed limes.tone~, and porta:Q.ce. They are. exemplified· by the ir~ega later~ iocalized metamorphism marked b.y the. ula~ bodies of rich_ cuprite ore in quartz mon:-

addition <;>f ~ilica, iron, copper; and sulphur. zonite po~phyry at· the Empire-Nevada mine 

The second metamorphism, which-produced the and by the :fi~sure veins in granodiorite and 

or~ bodies, acted,· as a rule/ only on pure lime-. quartz monzonite porphyry, as at the Montana-: 

stones (the. Bluestone or~ p9dy beillg ;_~h_e. one Yerington mme. . . 
• t 

G~OLOGY ·AND ORE DEPOSITS OF THE-YERINGTON DISTRICT, NEVADA .. 

PART I. GENERAL FEATURES. 

GEOGRAPHY. Th . . . .. 1 e _range nses abruptly abov·e the floor of 

The Yerington district is· in Lyon eounty, Maso,n Valley, i~s front . i~ not~hed by sharp 

Nev., 50 miles southeast of Reno ... (Sec:-t:fig. 1.) .. canyons,· and a series.,~f. alluvial cones extendIt takes its name from Yermgton, the county ing from the_.mouths of these eanyons spread 

seat, a prosperol.1s agricultural_town in 'M:jtson o:ver. t]le valley floor. The range is made up 

Valley. '1\vo sn1aller towns, 0! more recent ?f a series of ridges that overlap one another, as 

origin and d¢pendent almost wholly on the I~ well shown along the road that crosses the 

mining industry, ·are situated in the district- r~nge through Mason. Pass. on the way from 

Mason on the east side of the range and Ludwig ¥ason Valley to LudWig, :the. crossing being 

on the west side. These two towns an· con- made not ~ransver~ely qut at a narrow angle 

nected by the Nevada Copper Belt r ; ;·oad- to the trend of the range. 

with Wabuska, on the Southerr ... :_:-- · · ~em, Mason Valley is from 4'to 12 miles wide. A 

and with the smelter at Thompson, 2 .nile~ few ·miles south of the ·area shown in Plate I 

from Wabuska. the East and West forks ~f Walker River unite 

As ~sed in this report, the term "Ye~iugton. to form the main river, which -flows northward 

district" is restricted to the area shown on through the valley and then s~utheastwarcl into 

Plate ~' within which are the chief copper- Walker Lake, ~5 ~es east of Y ering~o~. !he 

producing mines of the region. The district water of ~~e. r1ver IS largely used for rrngat10n,. 

includes a small part ·of the Sirigatse Range, ~ an~ the villey sup:port_s aflo~rishing ~gri~ulture. 

n01:thward-trending barren desert range sepa-. _Snnths Y alley, on th~ west side of the Smgatse 

rating Mason Valley on the east from Smiths Range, Is a ~road; and expanse, at whos_e north 

Valley on the west. D. T. Smith 1 ascertained end, northwest of Ludwig, is a playa lake that 

that the Indian name of therange is"Singatse," g_enerall~ becomes dry early in September. 

but neither this nor any other name np- ~e climate of the Yerington district is that 

pears to be locally known or used. SmiL~ ~·nd typical of the Great Basin region. Owing to 

Ransome termed it the Singatse p; , .tl- the aridity and to its moderate elevation, the 

thoughSpurr 2 hadearlierappliedtoil . ....~.~·.:., 1e Singatse Ra~ge is·without running water-in 

"Smith Valley Range." On the t~pogr."····.ic fact,·the range is so dry that there is only one 

map of the Yerington district recently madr- j ;r · small spring within it. · · · 

the United States Geological Survey, whi<)t . forms the base of Plate I, "Singatse.Range" is FIELD WORK. 

employed, and this name is used here. TJ:.·\ Field work was begun on July 3 and comrange averages 4 miles in width and rises pleted on September 11, 1914. During this 

1,500 feet above the valleys. The hi. work the wnter ·was ably assisted by E. L. 

summit, Packhorse, southwest of Lur . Jones, jr. 

reachesanaltitudeof 6,547feet·. Thealtitud~- -- . BIBLIOGRAPHY. 

' thoe range i~ insufficient. to support a scatteA:~r.-~\ : The nl,linberof publications-on the Yerington 

gr?wth of pi~ne~, s~ch ~sIS common on the s.um~ ·. ~listri~t is not large. The only previous sys- · 

nnts of the _ .... 6 hei ranges of the Great Basm. 1 tematiC a~co~nt of the geology is that by Ran1 Smith, D. T., Geology of the upper region of the main Walker River . -;orne, whiCh IS based upon a week's reconnaisNov.: California Univ. Dept. Geology Bull., vol. 4, p. 3, 1904. ' 1 d d 2 Spurr, J. K, Geology of Nevada south of the fortieth pamllel: u.s. : -mnce evote mainly to examinations of the 

Geol. S~rvey Bull. 208, p. 117, 1903. 1 ·nines. 

9 
10 GEOLOGY AND ORE DEPOSITS OF YERINGTON DISTRICT, NEV. 

-~ . 

_:selinont . 

9 2f5 5,0.; ,. 7,!5 l90MILES 

FIGURE 1.-Index map showing the log9;tl~n of the Yerington district, Nev. 

'~ 
. ·HISTORY OF MINING. . 11 

1903. Spurr, J. E., Descriptive geology of Nevada south 

of tho fortieth parallel and adjacent· portions of 

California: U. S. Geol. Survey Bull. 208, pp. 117-· 

120. . 

Gives a few scattered observations concerning 

mainly the southern part of the Singatse Range. 

1905. Smith, D. T., The geology of the upper region of the 

· main Walker River, Nev.: California Univ. Dept. 

Geology Bull., vol.· 4, pp. 1-32. 

~L'he geology of the upper region of the main 

Walker Hiver, of which the Yerington district is 

a small part, is descriped and a reconnaissance 

map is presented. 

1907. Jennings, E. P., The genesis of the copper deposits 

of Yerington, Nev.: Canadian Min. Inst. Jour., 

vol. 10, pp. 257-260. 

Recognizes clearly the contact-metamorphic 

oi1gin of the deposits and their dependence on 

fissuring, but the supporting evidence is not presented convincingly. 

1909. Jennings, E. P., 1'he localization of values in ore 

bodies and the occurrence of shoots in metalliferous deposits: Econ. Geology, vol. 4, pp. 255-

257. ' 

Practically a restatement of the earlier paper on 

the genesis of the Yerington copper deposits. 

1909. Hansomc, F. L., The Yerington copper district, 

Nev.: U.S. Geol.· Survey Bull. 380, pp. 99-119. 

Outlines the broader features of thegeology of 

the district, gives a section across the range, and 

describes the ore deposits and the principal mines. 

1910. Carpenter, J. A., The Yerington copper district: 

Min. and Sci. Press, vol. 101, pp. 4-9. · 

~L'his valuable paper deals mainly with the state 

of development of the principal mines of the district in 1910. · 

1911. Jennings, E. P., Se~ondary copper ores of the Lud-

. wig inine, Yerington, Nev.: Canadian Min. Inst. 

Jour., vol. 11, pp. 463-466. · 

1912. ·Hogers, A. F., The occurrence and origin of gypsum 

and anhydrite at the Ludwig mine, Lyon County, 

Nev.: Econ. Geology, vol. 7, pp. 185-189. 

Gives a section at the mine, showing a bed of 

anhydrite, 450 feet thick, interstratified between 

quartzite and limestone. The anhydrite has 

been hydrated to gypswn to a depth of 400 feet o.n 

the hang~-wal1 side. 

IDSTORY OF MINING. 

Mining in the Yerington district dates back 

11.t least as fn.r as 1895, when attempts were 

made to work the oxidized copper ore at the 

Ludwig mine. Prior to l907, however, oper-

.·ations·th;roughout the district were intermittent 

and never attained much importance, and as 

a matter of fact the district did not begin .to 

yiold much copper until after 1912. The most 

important of the oarly activities appears to 

have boon tho iniJ1ing of natural bluestone to 

supply the reduction works at Virginia City 

during the period when the great· bonan~a ore 

bodie~ of the Con1stock lode were bemg extr~cted. The bluestone occurred iJ1 the outcrops of the principal ore bodies in: considerable 

quantities, and many thousand . tons .were 

mine~. to furnish the copper sulphate used in 

the Washoe process of amalgamating the silver 

ores of the· Comstock lode. A smelter was 

built.at ~~dwig in the early days, but it was not 

successful. At the Bluestone mine, on the east 

side of ihe range, a smelter was built in 1900, 

but it was not operated long, and similarly the 

smelter: at Y eri.ngton station did not make any 

notable output. These early attempts were 

concerned chiefly with the oxidized ores, which 

were naturally richer than the sulphide ore but 

occurred in comparatively small quantity .. In 

the absence of transportation facilities the 

larger bodies of primary ore, essentially un~nriched and of low grade, were not attractive. 

Interest in the disti'ict revived about 1907: 

Some of tlie larg~st deposits were acquired by 

energetic companies with adequate capital, and 

a campaign of active exploration and development was begun. The Nevada Copper Belt 

Railroad,· 41 miles long, was built into the district from Wabuska, on the Southern Pacific 

system; it was commenced in August, 1909, 

and completed to Ludwig. in September, 1911. 

A smelting plant, eventually increased to a 

capacity of 1,800 tons a day, was constructed 

by the Mason Valley :Mines Co. at Thompson, 

2 miles north of Wabuska. Its. situation here 

at 'the north end of the arable part of the valley, together. with the fact that the prevailing 

wind is from the south, in _large measure precludes the danger of serious difficulty with the 

agricultural interests of Mason Valley on account of damage by smelter fumes. The plant 

·began operations on January 6,, 1912,. and a 

considerable output was soon attained, largely 

from the ores of the Yerington. district. At 

first a matte containing approximately 40 per 

cent of copper was produced,· which was 

shipped to the Garfield smelter near Salt Lake 

City for conversion, but lat.er two converters 

were added to. the plant, and early in 1914 the 

production· of blister copper was· commenced. 

The smelter was operated continuously until 

October 20, 1914, when it was shut down, owing 

to the disastrous effect of the outbreak of 

the European war ?n the price. of copper. Tn 
12 GEOLOGY AND ORE J?'EPOSITS OF YERINGTON DISTRICT, NEV. 

spite of the recovery of the copper market and 

the high prices of ·. nietaJs that ruled during 

1915, the· smelter remained idle, as did also 

the Ma8on Valley· :inii:i.e, from which it had 

drawn ·a large part of its supply of' ore. Elsewhere in the district;· ·however, ·mining was 

stimulated by the great rise· in the price of 

copper, and rriuch ·activity ·prevailed. · 

In February, 1917, ·the sinelter ·again began 

operations, and a large output was· made _from 

the ores of the Bluestone, Mason Valley, and 

N ev~da-Douglas mines. · 

PRODUCTION. 

The production of the ·Yerington ·district 

prior to 1905 is not accurately asc;ertainable 

but probably did not exceed 1,060,000 pounds 

of copper, mainly as natural bluestone and 

other oxidized ores. The total mine production 

of copper since 1905, as obtained from the files 

.of the United States Geological _Survey, is 

61,193,800 pounds.· This copper was recovered 

from or was'contaiiled in 886,624 tons of ore, the 

average yield being therefore approximately 

3.45 per cent of copper. The output of gold 

and silver incidental to the recovery of copper 

has been insignificant. 

Copper produced in the Yerington district, Ner., 1905-1917 . . 

Mine production. 

0 ( ) Copper re tons . (pound's). 

1905 .............. -.------ .... -- 1906 .... -.. -.. -.- ·-----.-.--.--- 1907 .. -- .. ---- .... -.----.:-.-.-. 1908 .... · .... -.-.-----.---------- 1911 .......... -.-.--.-.---- ... -. 1912 .. -.... -.-.-- :. ---.--------.- 1913 .... · ... --.- .. -.-- ... -.-.-.-. 1914 .... ---- .. -.-.--.-.-.- .... -. 1915 ...... -.--- ... ---------- _·_·_. 1916 ... -....... ----:- .... -..... -- 1917 .... -- .. -- __ ·_-- .. -----------

1,382 

2,501 

1,074 

360 

120 

239,606 

206,558 

102,467' 

4,736 

19,820 

308,000 

a Preliminary estimate. 

GENERAL GEOLOGY.. 

TRIASSIC ROCKS. 

CHARACTER. 

294,000 

869,000 

297,000 

101,000 

2,800 . 17,058,000 

15,106,000 

7,274,000 

701,000 

3, 491,000. 

a16,000,000 

The oldest rocks of the Y ~rip.gton ·.district . 

are Triassic andesite, f.elsite; lim.estone, quartz-. 

ite, and subordinate shale.· Ne~r Ludwig they 

incl:ude a thick bed of anhyd~ite, .whose 011t-: 

. crop is al.tered to gypsum. They form a nar-

_ro'Y"; irregular belt extending across the portion 

of the range west of Mason. They have .been 

intruded by granodiorite and quartz monzonite, and as a result they have been highly metamorphosed,- some by simple recrystallization 

·and others by addition of new material. The 

limestones especiilly were altered ·by· pneumatolysis, and as the resultant rocks are composed solidly of garnet or kindred silicates, it 

has been found necessary to map large areasin fact, about one-half of the total exposure of 

Triassic rocks-under a separate symbol as 

garnetites and allied rocks. . 

The rocks dip from 70° to 90°; on the east 

flank of the range they dip 70° W. and on the 

west flank 70° E. as a Iille, but the structure 

is probably not synclinal, as the rocks differ 

widely in character and sequence on the two 

sides of the range. They are cut by ·many 

faults, which are of at least thr~e different ages 

and some of which are of large stratl.graphic · 

·displacement. These features, combined with 

the profound contact metamorphism, have 

rendered it impossible to establish fully the 

stratigraphic . sequence. A thickness of at 

lea8t 8,000 feet of rock, however, appears to 

be repres·ented. · 

The Triassic rocks are a small remnant of a 

formerly extensive formation; they have been 

left isolated in an area now occupied mainly 

by granitic rocks of probably early Cretaceous 

age and by Tertiary volc.anic rocks. They are, 

however, the rocks of main economic interest,. 

because the limestones of the formation inclose 

the principal ore bodies of the district. 

AGE. 

Smith 1 found some evidence indicating that 

the rocks are of Triassic age, and Jones -

2 found 

Triassic fossils in place near the Ludwig mine. 

During the present· investigation fossils were 

found on the south side of the road near the 

Malachite mine, where they ·are abundant but 

poorly preserved and mainly of one species •. 

They occur at two horizons, in black shales interstratified with limestone separated by about .. 

150 feet of felsite, but are more abundant at 

the upper horizon. The shales are overlain 

mainly by massive limestone, which· includes .. 

l.S.mith, D. rr .; op. cit .. ,'pp. ~io: · · 2 Jones, J. C., The origin of the anhydrite at the Ludwig mine, Lyon 

County,_Nev.: Econ. Geology _vol. 7;,p. 4_()0, 1912. 
GENERAL. GEOLOGY. 13 

somo alternations of .limestop.e and fel~ite .. Malachit_e :n;line goes, is found to .show the 

T. W. Stanton repol"ts on th~ fossils collected·: following. f~ature~: The feldspar _phenocrysts 

hero as follows: , are ·andesine (Ab55An45); th~y ar~ more or less 

9215. J.Jot B. Malachite mine; Yerington district, ·Nev.: 

Daone1la sp. 

Daonella? sp. 

. Triassic. 

9.217. No. 51-J. Near Malachite mine: 

Daon'ella sp. 

Ammonite; imprint too obscure for generic determination. 

Triassic. 

9218.· No. 52-J. Near Malachite mine: 

Daonella sp. 

Ammonite i.niprint; possibly a Ceratites. 

Triassic. 

Imprints of similar. fossils were found in 

shrue halfway between the Malachite and 

Mason Valley :qllnes and in sh8le 1,500 feet 

north of the McConnell mine. A well-preserved 

Halobia, a typical. Triassic fossil, wholly garnetized, was found in the garnetites west. of 

tho McConnell mine. It is conchJ.ded fron1. 

this distribution of Triassic fossils that all the 

progranitic rocks of the district are of T!iassic 

~& . 

VOLCANIC ROSJKS. 

ANDESITES. 

· Andesites forn'l. a prmni.nent 'belt extending 

along tho.front of tho range from Mason southw&·d. They occur also along the summit 

south of tho direct· road between Mason and 

Ludwig and as. imaulted blocks near the Western Nevada mine. 

The andesites are moderately dark· rocks 

that display son1e petrographic diversity. The 

most common variety is probably that carrying 

abundant phenocrysts of . plagioclase and. 

hornblende or pyroxene, both somewhat inconspicuous and blurred through secondary alteration. So1no of the andesite differs from the 

rest because of its numerous large phenocrysts of tabular plagioclase. Both massive 

porphyries and pyroclastic breccias occur in 

the andesite series, which is therefore clearly 

of extrusive origin. Dacite, characterized by 

numerous large, prominent corroded quartz 

phenocrysts associated with the feldspars, is 

intercalated in the andesites in moderate 

amount. 

Under the microscope a typical andesite, 

from the canyon up which the road to the 

intergrown wi_th amphibole and 9ther secondary 

'minerals, and in consequence most of th~m no 

lon,ge,r stand out sh_arply f~on1 the groundmass. 

The :fe;mic phe~ocrysts-whether t}fey . were 

hqrnblende or pyro~ene originally is now not 

~scertainable-hav:e recrystallized to . irregular 

aggr:egates ·of light-green amphil;>Ole. The 

groundrriass.is fine grained and contaip.s e:venly 

scattered through it innumerable prisms of the 

light-green amphibole and minute grains of 

magnetite. . 

.The andesites _that occur near the i~trusive. 

bodies of quartz n10nzonite have been recrystallized. The effect most obvious on c~sual 

inspection is that they have become darker 

and appear.fresher. Moreover, the porphyritic. 

structure· has become obscure .. In the lower 

part of the canyon through which passes the 

road to the Western Nevada mine the andesites 

have recrystallized so thoroughly that they 

have a decidedly dioritic appearance, but the 

retention of pyroclastic structure, as revealed 

on weather-etched surfaces, still gives a clue to 

their extrusive· origin. Under the microscope 

the thermally altered andesites are found to 

consist mainly of .plagioclase and hornblende. 

The hornblende is evenly scattered throughout 

the rocks, is more or less spongiform, and is 

evidently secondary .. The original porphyritic 

structure h~s. been obscured partly by the 

recrystallization of the fernie phenocrysts and 

partly· by the growth of multitudes of minute 

hornblende prisms in the cores of the plagioclase 

phenocrysts. · 

. The maximum indicated thickness of the andesite series is _2,000 f~et. Immediately above 

the andesites are felsites. As the andesites are 

intr1.1ded · by felsite sills, and as extrusive 

felsites and felsite br~ccias are interstratified 

with the overlying fossiliferous l'riassic limestones, it is clear that the an9esites are the 

lowest member of. the local Triassic section. 

SODA RHYOLITE-FELSITE (KERATOPHYRE). 

GENERAL FEATURES. 

Soda rhyolite-felsites make up an important 

element in the Triassic section. They are 

characterized by their light color, aphanitic 
GE.OLOGY AND ORE. DEPOSITS -OF YERtNGTON DISTRICT, NEV. 

texture/'and obscurely. porphyritic habit .. As ,sporadic corroded crystals of quartz, .but su·ch 

they carry but few· phenocrysts arid as: these· quartz-bearing felsites ·are co~paratively rare. 

are generally inconspicuous; the felsites closely In the intrusive felsites the texture· is microresemble fine-grained· sedimentary rocks rang- crystalline and the phenocrysts ~re more com-. 

· ing from cherts to dense quartzites.. They re- mon and more readily distingui~hable, so that 

semble ·also the apl;lanitic ·calc-silicate rocks the igneous origin of these rocks is far more 

that commonly· result from contact-metamor- .. easily recognizable. · 

phic ·action-rocks that are logically to be Under the microscope the massive felsitesexpected in an area so highly metamorphosed .that is, the flows and intrusives-all appear to 

by igneous intrusions as the .. Yerington district. be essentially similar. They show sporadic 

In· the field, therefore, the felsites are generally phenocrysts of albite, or of sodic oligoclase, 

discriminable with difficulty from a variety of embedded in a groundmass that is generally 

rocks· of widely different origins. ·Fortunately, cryptocrystalline. Biotite or hornblende aphowever, certain of the· felsites reveal their pear not to have been present. A secondary 

volcanic origin either by a conspicuous flow mica, probably sericite, occurs. as minute fibers· 

banding or by a breccia structure that is devel- in- some of the felsites. Further details are 

oped by the we~;tthering of exposed surfaces. presented in conne.ction with. the felsites that: 

Notwithstanding the diversity of structure . have been chemically analyzed. . 

thus locally shown on weathered surfaces, the The breccias, which are composed· of angular 

felsites have the striking peculiarity that Oi?- fragments as much as several inches inlength, 

fresh fracture all are of structureless flintlike or are not as a rule· discriminable fron1 the lavas 

cherty appearance. on fresh fracture. · This variety of pyroclastic 

The main bulkofthefelsites-lies immediately rock_ is the more common jn the region, ·but 

above the top of the andesites. The greatest near the Mason Valley mine somewhat different 

thickness is about 1,200 feet, or at least felsites explosive products of the felsitic eruptions 

strongly preponderate in a belt of this thick- were found. The;1 occur as thin strata of 

ness, as shown along the crest of the ridge quartz.:.feldspar tuff inclosed in the massive 

extending east from the Mason Valley mine: brown garnetite that forms the predominant 

They occur in smaller thicknesses in the over;_ country rock southeast of the office of the mine~ 

lying limestone series. Among the more nota.:. The tuff is a_ white rock made up largely of 

ble of these minor belts is the felsite breccia feldspar but containing quartz particles someforming the footwall of the ore-bearing zone what larger than the average grain of the rock; 

at the Mason Valley· mine. . the general megascopic effect is that of an 

The most prominent development of. the .aplite porphyry. The. clastic origin becomes 

felsites is th~ thick belt already mentioned, ·app·arent under the microscope, however, and 

which extends along the east flank of the range~ the rock is seen to be made up of albite, quartz, 

In this belt they are generally unaltered by and sporadic fragments ·of .a trachytoidal 

dynamic or contact metamorphism. The fel- volcanic material composed of albite. 

sites occurring in the area of highly metamor-. The intrusive felsites, as previously menphosed rocks extending from. the. McConnell tioned, are sufficiently coarse in grain. to be 

mine to the _summit of the range have been termed microcrystalline. A snow-white felrecrystallized,. and finely flaky._ biotite has beeri site of this kind, containing scattered phenocaused to grow in them, so that th,eir recogni- crysts of feldspar, occurs on the divide south 

tion ·becomes difficult; some, indeed, dosely of Ludwig; ·it shows. under the nucroscope 

resemble aplite. crystals of oligoclase-albite, somewhat more 

PETRQG RAPHY. sodic than Ab86An141 . set in a groundmass 

The felsites include flows, tuffs, breccias, and whose components are well .individualized. 

intrusive rocks. In· color they range from In tp.is respect 'it contrasts notably with the 

snow-whit~ to grayish green. 'The effusive and extrusive felsites, but otherwise it is quite 

pyroclastic m~mbers are in the main aphanitic similar.· The groundmass consists of quartz 

rock$. carrying a few: obscure inconspicuou.s ·and feldspar, probably albite, forming a 

phenocrysts of feldspar. Some· contain small mosaic whose structure ranges between allotrio-
··; .. 15. 

morphic and panidiomorphic granular.. Titan- minerals~· ·chlorite (in part possibly· pseudoite,. apatite, 1nagnetite, and zircon .. .occur morpho;us after biotite or hornblende), epidote, 

sparsely, and the secondary minerals epidote ·and calcite are minor .secondary minerals. 

and actinolite are present in s1nall amounts, The groundmass, consisting of irregularly 

mainly in partial replace1nent of the p~eno- interlocking aggregates, averages 0.005 millicrysts. meter in grain. Study. of the groundmass 

A rock that is very similar but is rich in immersed in oils of known refractive indices· 

phenocrysts of albite-an albitophyre, as it raises· some· interesting queries. The .minimay be called~occurs ·on the ridge east of the mum .index. of all. particles is found to .exceed 

Western Nevada n1ine. 1.529-in other words, no orthoclase is present, 

One complete and two partial analyses of although the subjoined. calculation of the 

the felsites have been n1ade by R. C .. Wells in chem~cal. analysis o.f the :1felsite .. calls for. 27 

the .laboratory of ·the Uni~ed State:s ·~eo- per cent of orthoclase: . Grains whose indices 

logical Survey. They show somewhat greater . range between approximately 1.530 and 1.536 

diversity .. than was expected but bring out ·make up ·practically the whole of_ the groundwell the highly siliceous, soda-rich character mass; in liquid of index 1.544 a few grains of · 

of the. lavas and their poverty in iron ·.and quartz can be detected, but the amount is 

magnesia. totally disproportionate to that calculated 

from the analysis. The computed composition 

Analyses of fels?:teJJ (l:eratophyres) from the Yering.ton IS given in the following table: district, Nev. 

[R. C. Wells, analyst.] . 

;~k,:::.::::.:::::::::::::: FeO ................... · ... . 

M~g-_-_-_-_-_-_-_-_-_-::: .:.::::::: ~ ~: 

~~0~: -_-_-::: : : : : : : : : : : : : : ·: : : 

:E£20- -- ~ --. --------------- JJ20 +- --------------. -----

1 

72. 73 

14:42 

. 73 

. 94 

. 32 

. 87. 

2 3 

77. 17 70. 18 a 13. 84 a 18. 43 

1.47 . 2.55 

6. 05 7. 46 . 38 . 44 

~~EHHH~·.~~::~~:. BaO ....................... . 

4.66 

4.50 

. 36 

. 36 

. 22 

. 04· 

.19 

. 07 

. 01 

. 09 :::::·::::1::.:::::: 

100.51 98. 91 1 99. 06 

a Includes some Fc20a, etc. 

1. Malachite mine, :nortl). of road. 

2. :Mason Valley mine, from cutting on road' between 

office and portal of tunnel No. 3; 

3. Douglas mine. 

The felsite obtained near the :Malachite 

mine (No. 1) is a fli.ntlike rock of conchoidal 

fr'acture, carrying sparse phenocrysts of glassy 

plagioclase one-tenth of an inch or less in 

length. Under the n1icroscope it proves to be 

holocrystalline, probably owing to devitrification. 'Ple phenocrysts ai:e seen to be albite 

near Ab02A.n8 in composition. Octahedrons of 

magnetite 0.02 millimeter in size are common, 

and· apatite and titanite are the· other accessory 

.ff!.ineral composition of felsites. 

Quartz ...................... -·- Orthoclase·(mol.) ........... ·_ .. Albite (mol.) ................ . Anorthit~ (mol.) ........... -... . M_agn~tite ................... . T1tamte ...................... . Apatite ............ : ........ . Calcite ...................... . Chlorite ..•................... 

1 2 3 

26.60 37.30 19.63 

26.78 2.23 2.61 

39. 45 51. 55 63. 30 

1.95 7. 23 12.66 

1. 16 . ------ ~ - -------- .59 ----- .••.. ------ .15 ----- -·· .. ------ . 40 ·- ---··· -------- 2.03 -------.------.-

99.11 98.31 98.20 

The felsite from the vicinity of the Mason 

V~lley mine. (No. 2) is. a dense snow-white 

rock without phenocrysts; ·under the microscope it is seen to consist wholly of an 'intiInately interlocking aggregate of minute grains. 

The refractive indices of all the grains exceed 

1.529, and some of the grains· have indices. 

exceedilig 1.544; hence, in connection with the 

chemical analysis, it is clear that the felsite 

consists largely of albitic feldspar plus some 

quartz. · 

The felsite :froin Douglas Hill (No. 3) is· 

also a dense rock without phenocrysts but is 

. charactm:ized. by a well-1narked . banding.· In 

thin section it is found to co~sist of a closely 

intergrown aggregate of colorless, untwil1ned, 

weakly birefringent grains that average 

0.02 millimeter .in diameter. · Apatite, leucoxene ( ~) ,· and actinolite in radial groups occur· 
16 GEOLOGY AND OR.E DEPOSITS OF YERINGTON DISTRICT, NEV • 

as. m1nor constituents. Examination. of. . .the. 

. crushed rock in . oils of known indices shows 

that the indices of all particles exceed 1.529 

and are notably less than .1.544, the minimu1n 

index of quartz. The quartz . mdicated by 

the computed mineral composition is therefore 

not presen.t. The three felsites that have been 

chemically analyzed. and critically examined. 

as to .their mineral composition . accordingly 

· f$uggest that sodium-aluminum silicates more 

siliceous than albite are present, a conclusion 

similar to that which Mugge reached long ago 

in. his classic studies-on the keratophyres of 

W estphalia.1 

SEDIMENTARY ROCKS. 

· .. The sedimentary rocks consist principally of 

limestone and quartzite. The larger masses of 

these rocks have been mapped separately (Pl. I, 

in pocket) . . The limes tones as a rule are 

light .colored, massive, and coarsely crystalline. 

Some thinly bedded black limestone, 200 feet 

thick, occurs east of the Mason Valley mine, 

however, and extends southward to the 

Malachite mine. Shale is not common in the 

district, the principal occurrence being the 

fossiliferous _beds near the south workings of 

the Malachite mine . 

. ·Quartzite. is ·exposed at th~ upper edge of 

the alluvium in the town of Ludwig, where it 

forms the .lowermost member of the local 

Triassic section·. It is . separated from the 

overlying rocks 'by a strike fault. A larger 

mass, surro~ded by granodiorite, occurs on 

the hill northwest of Ludwig. It is a vitreous 

quartzite, which the microscope shows is composed wholly of ·interlocking quartz grains. 

In the absence of ariy positive evidence bearing on its age the quartzite is provi~ionally 

mapped with theTriassic rocks. 

. GARNETITES AND ALLIED ROCKS: 

CHARACTER AND DISTRmUTION. 

. Highly metamorphic rocks-the so-called 

lime-silicate rocks ~~make up about half the 

area underlai~ by. the Triassic.' Rocks com-

.1 Miigge, 0., Untersuchungen ueber·die ·~Lenneporphyre" in Westfalen und den angre~enden Gebieten: Neues Jahrb., Beilage Band 8, . p. 616, 1893, . 

. 

2 There .is no wholly satisfactory name for rocks of this kind. Flett 

and others of the Geological Survey of Great Britain call them calc, 

silicate hornfelses; the writer has elsewhere (Tungsten deposits. of northwestern lnyo County, Cal.: U.S. Geol. Survey Bull. 640, p. 233, 1916) 

suggested that this term be shortened to calo-hornfelses. 

. posed. wholly of brown garnet, referred to in 

this ·report. as garnetites, are most _abundant, 

but many other varieties occur~ such.as those 

cqnsisting of epidote, clinozoisite, and vesuvianite, singly or together, or such as. those 

. making. up the thick white strata composed 

chiefly of wollastonite, ·as at the Bluestone and 

Ludwig mines. 

.·The areas in which rocks of this kind pre-

. dominate are shown separately on the geologic 

map (Pl. I). · Garnetite and allied rocks, 

. however, also occur to some extent interstratifi,ed with the limestone in . the areas 

where limestone prevails. 

In places felsite and felsite. tuff are intercalated with. the calc-hornfelses, and it is accordingly difficult in the field to distinguish 

these t~o kinds of rock. The intercalated 

felsites have not uncommonly been metamorphosed to solid aggregates of fine-grained 

epidote. 

The brown garneti tes are well shown on the 

hill south of the office of the Mason Valley 

mine. They are heavy;- dense, massive rocks 

compo~ed wholly of microcrystalline garnet. 

As indicated by its refractive index (1.81), the 

garnet ·is a variety half_way between grossularite and andradite. Similar garnetites; still 

retaining stratification, form the hill west of 

the· McConnell mine. Under the microscope 

only minute a_mounts of pyroxene, quartz, and 

calcite are found to be associated with the 

preponderant ga~net composing the rock. 

The index of this garnet also proves to be 1.81. 

A garnetite from. this locality, noteworthy in 

containing a garnetized fossil, was partly 

analyzed in the laboratory of ·the Geological 

Survey. 

The analysis given below clearly confirms 

the optical determination· that ·the garnet is a 

variety .about halfway between grossularite 

and andradite. 

Partial analysis of garnetite from hill near the McConnell 

mine. 

(W. B. Hicks, analyst.] 

Si02 ••••• : •.••••••••• , ••••••••••••• · ••• 40.89 

Al20 3 •••••••••••••••••••••••••••••••• 7. 63 

Fe20 3 (total iron) ..................... 15.39 

MgO ...•....•..•.... · ............ ~ ...• 1. 26 

CaO ..................... -............ 30 61 

A stadia traverse was made across the strike 

of the r?cks on Douglas Hill, and specimens 
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 114 PLATE II 

A. GARNETIZED HALOBIA. 

B. PHOTOMICROGRAPH OF LATITE VITROPHYRE. 

lb, Labradorite; p, pyroxene; b, biotite. 
GENERAL GEOLOGY. 17 

were taken from those rooks that differed per- ,in column 3, on p. 15, came from this belt) 

ceptibly to the· unaided eye; the specimens and aphanitic hornfelses. A hornfels of this 

were exru.nined microscopically and the . ac- belt-a heavy even-grained aphanite of grayishcompanying section (:fig. 2) was drawn. It is green color-proves in thin section to consist 

probable that if 1nore specimens had been of vesuvianite, diopside, and guartz, which is 

collected, more vru.·ieties of lime-silicate rocks partly · idiomorphic against the other two 

would have been discrim.inated microscopically. minerals. Near the top of the section there 

At the base of t~e sectio:q is a much brecciated occur garnetites which are composed solidly 

quartzite (a); above this is the coarse .lime- of microcrystalline garnet; as determined by 

stone (b) that inclose~ the Ludwig lode; and the immersion method, the refractive index 

above this are lime-silicate rocks and felsites, of the garnet is 1. 75, showing that the garnet 

aggregating more than 1,500 feet in thickness. is 'nearly pure grossularite·. 

Although these rocks are highly met8morphic, 

they are well stratified and retain their general 

aspect of sedimentary rocks. The fine-grained 

rock near the base of the belt of white liniesilicate rocks (c) consists chiefly of wollastonite, 

with accessory plagioclase (Ab73 An27)', diopside, 

ORIGIN OF THE METAMORPHIC ROCKS. 

The fact that ~he appearance of normally 

stratified rooks is preserved by · the dense 

· aphanitic wollastonite rocks and garnetites 

suggests. that large amounts of silica, ferric 

E. 

FtOURE 2.-0oolog!c section extending N. 71 o W. fro~ the sU:JD.lllit of Douglas Hill, Nev. a, Quartzite, highly brecciated; b, limestone; c, white 

lhno-silicato rocks; d, bluck liine-silicute rocks; e, aphartitic tremolite rock, probably including some felsites; /, black epidote vesuvianite 

·rocks; g, fels!tes, limo-silicate rocks, and gametites; h, fault breccia ccmen_tcd by andradite; i, andradite rock. 

and ·pyrite. The faintly banded ap4anitic 

hornfels (e) resembles to the eye exa~tly the 

banded felsites of the . region; U.nder the 

microscope,. however, it proves tp consist 

largely of slender fibers of tremolite scattered 

through a .feebly birefringent g;ranular aggregate, which probably consists of plagioclase, as· 

its n1axin1un1 refractive index, .determi:ned by 

the iinmersion method, is 1.544. The uppermost belt of black hornfelses (j) on casual 

exanlination resembles a seri~s of thin-bedded 

black lin1estones; 1:>ut on closer inspection it is 

found to ~onsist of heavy aphanitic rock. In 

thin section such rock is seen to be made up 

largely of vesuvianite, of index ·1.71, and 

epidote of a pale variety near clinozoisite. 

The black color of the rock is d~e to very 

abundant ru.1d finely disseminated carbonaceous 

matter. This belt of black rocks is overlain 

by felsites (the felsite whose analysis is given 

38330°-18-2 

oxide, and possibly other substances were 

added_ during metamorphism and that the 

volu~es of the . original strata have remained 

unchanged. This conclusion appears to be 

confirmed by the discovery of a completely 

garnetized 'fossil in the stratified g~rnetites near 

the McConnell1nine.1 . This fossil was identified 

as Halobia sp. by T. W. Stanton, who commented o:r;t its preservation. in undistorted 

form. (See Pl. II, A.) The petrographic. description.· and c~emical analysis of the garnet 

matrix of the Halobia.is given on page 16, and 

from these it appears that the garnet is a 

variety lpgh in t~e andradite molecule. Mani1 Fossils preserved· iii tact in silicatcd contact-metamorphic limestones are not uncommon; Lindgren (Mineral deposits, p. 671, 1913) 

says that "at Tres Hermanas, N. Mex:, near .. the co~tact with granite 

porphyry, Spirifcrs and Fenestellas have been perfectly preserved in the 

garnctized rock." Goldschmidt (Die Kontakt-metamorpboso im Kristianiagebiet, p.327,1911) found that in some of the contact-altered limestones. thick-walled brachiopod shells have been completely altered to 

wollastonite. 
18 GEOLOGY AND ORE DEPOSITS .OF YERINGTON DISTRICT, NEV. 

festly the garnetization of the Halobia and. its GYPSUM. 

matrix took place without change in volume or 

increase of porosity; and it must therefore have _A·b~d. of gyp~um 450 feet in maximum thickinvolved the addition of large amounts of ferric ness a~d suffiCiently large to be shown on t~e 

.iron, silica, and probably alumina to the meta-_ ge_ol_ogH~ ~ap (Pl. I] occurs ne~r the ~udw1g 

motphosed liniestone from ·an extraneous _m~e. I~ Is underlam by quartz1~~ a~d Is ov.ersource. In view of 'the considerable areal ex- lam by.lrmestone about 170 feet thiCk, whiCh 

tent of rocks of this kind in the Yerington dis- separa~es it .from the Lud_wig lode~ These · 

trict this great accession of material becomes geologic relatiOns are' shown m figures 9 (p. 59) 

one 'of the most noteworthy features of the and 1~ (p. 6?). ~ ~mall mass of intrusi:e 

geology of the district. . granodiOrite l~es w1thu1 200 .feet of the mam 

Inclosed in the lime..;silicate rocks are irregu- outcrop o_f ~he gypsum bed. 

lar masses of limestone which as the field The origm of the· gypsum has aroused conevidence discloses are ~·residuai from inter- s_iderable interest. Ransome 2 sugg~sted that 

bedded strata th~t were .not wholly replaced it ~esulte~ from t~e alteration of IU:O~s~one by 

.by silicates. Incomplete replacement of this am.d solu.twns deriv~a fr~m the ox1d1zmg suikind is well shown in a belt of thin-bedded p~des of the Ludwig ore body. Later develblack limestone 200 feet thick near .the Mason. opment work at the mine show~cl, however, 

Valley mine. (See fig. 7, p. 53.). The northern that the gypsum· passes into anhydrite in 

extension of the belt is compl~tely metamor- depth, a~d -Roge~ 3

• established conclusively 

phosed to· silicates, but toward the south the that the g!Psum or1gmated from the anhydrite 

silicates diminish abruptly and :finger out by hydratiOn. It was assumed by Rogers that 

irregularlY- in stringers penetrating . the un- the allhydrite was originally formed during the 

altered limestone.· evaporation of an inland sea. Jones, 4 howAlong the contact of the massive limestone ever, advanced the hypothesis that the. anhybelt and the calcium silicate rocks southwest drite resulted from the dehydration.of a bed of 

of Douglas Hill blocks and slabs of unsilicated gypsltir19riginally laid down during the depolimestone are commonly inclosed in· rocks com- si.tion "of the Triassic rocks. He based ·his exposed solidly of silicates·; that this relation is plan~tion on the fact _that the Triassic at other 

not due to faulting that -.occurred after the places in Nevada ·where it has not been afperiod of silication is shown by .such ·feat~res:. J~cted by intrusive igneous rocks con tams inas the corroded and. embayed boundaries of: t~rstra~,i:fi.ed beds of gyps~m; but a·t Ludwig, 

the limestone blocks and by the tonguelike where. the ·Triassic is cut by granodiorite, 

masses of silicates penetrating the limestone. anhydrite occurs, which is very probably 

Along this contact there occur also breccias of therefore metamorphosed gypsum. If gypsum 

limestone in which the limestone fragments are is heateH at atmospheric pressure to 130° C. 

surrounded by networks of tremolite fibers. it loses an· its water. and becomes converted 

The widespread profound metamorphism into anhydrite; at higher pressures the . terndescribed in the preceding paragraphs occurred perature of dehydration would be raised,· in 

shortly after the intrusion of the granodiorite accordance· wfth the law of Le Chatelier. Now 

and ·quartz monzonite, and, as pointed out in the rocks associated w~th the anhydrite have 

detail on page 43, it almost wholly preceded been profoundly altered by thermal metamorthe intrusion of the quartz . monzonite por- phism, as is described on pages 16-17 and shown 

phyry. It led to the development. of large in figure 2, and this leaves little doubt that 

masses of aphanit~c or microcrystalline silicate the-anhydrite, if originally gypsum, was heated · 

rocks. It is referred to in this report as _"the to a temperature high enough to effect its 

metamorphism of- the :firs.t period" to distin- dehydration, notwithstanding its burial.to conguish it "from the later, more localized, and _____ ....;.· ______ ...:._ _____ _ · · 2Ransome, F; L., The.Yerington copper district,·Nev.: U.S. Geol. widely different metamorphism accompanymg survey Bull. aso, pp. 112-113, 1909. 

the formation of th.e copp_ er ores of the districtl a Rogers; A. F., The occurrence and origin of gypsum and anhydrite at the Ludwig mine, Lyon County, Nev.: Econ. Geology, vol. 7, pp. 

1 A similar relation between regional contact metamorphism and later 185-189, 1912; see also Notes on the occurrence of anhydrite in the United 

deposition of contact-metamorphic ore occurs -in the Darwin district, States: School of Mines Quart., vol. 36, pp. 128-130, 1915. 

Cal. (See Knopf, Adolph, The Darwin silver-lead mining district, Cal.: 4 Jones, J. C;, The origin of the anhydrite at the Ludwig mine, Lyon 

U.S. Geol. Survey Bull. 580, pp. 1-18, 1915.). · County, Nev.: Econ: Geology, vol. 7, pp. 400-402 ,1912. 
GENERAL GEOLOGY. 19 

siderable depth. Ob,7 iously, however, no decisiYe evidence can be adduced in this area of 

intrusion, metamorphism, and faulting. to establi~h whether the bed was originally laid 

down as gypsum or as anhydrite. 

The deposit was for~erly utilized as a commercial source of gypsum, as is described_ on 

page 62. 

CO:B.H.:E.LATION 01" THE TlU.ASSIC ROCKS. 

The Star Peak formation overlies the Koipato 

and consists, as determined by Hague, _of 

"broad alternating zones of quartzites and 

limestones,'' roughly estimated as. 10,000 feet 

thick According to Ransome, the stratigraphy 

and lithology of the formation are in need· of 

detailed study. The. formation is of Middle 

and Upper Triassic age, as established by 

Gabb~ Meek, Hyatt and Sri:rith, and J. C. Merriam. The Triassic of the I-Iw.llboldt R~ge 

The 'rrinssic rocks of Nevada were divided also contains thick lenses of gypsum,S' such as 

by Ilaguo into two formations. The lower o£ occur also in the Yerington district. 

these was called tho Koipato and was described The Triassic rocks of Nevada, as will be 

as consisting ·of quart~ites and "felsitic por- seen from the foregoing statements, attain a 

phyroids," aggregat1ng roughTy. 6,000 ·feet in great thickness, possibly as much as 16,000 

thickness. The porphyries were thought to be feet, of which 6,000 feet is dominantly of 

n1etamorphic sedimentary rocks.· It was shown igneous character. The locality nearest to the 

by H.ansome, 1 however, that the Koipato is Yerington district at 'Yhich Triassic fossils 

dominantly of igneous· origin. "It consists of occur is in the Pine Nut Mou~ntains, 15 mil(:)s 

volcanic flows, n1ostly rhyolitic but including northwest of the district. :Here Whitney 6 and 

also andesit~c lava!;), associated with tuffu, con-. later ·spurr 7 found fossils in shaly limestone, 

glomeru.tes, grits, and lin1estones." The hiter but not much is known about. the rocks of this 

work of Jones 2 and Schrader 3 in the ·recently locali~y. -The Triassic probably occurs, acdiscovered Rochester ·district, in the Humboldt cording to Hill,S at many places in western 

H.ange, has confirmed this statement and has Nevada. In eastern California, in the Inyo 

shown the wide textural variety of rhyolites Range, t~e Triassic appears in large volume; 

included in the Koipato formation. · ·it includes here at least 4,500 feet of andesitic 

One of the more coarsely crystalline "por- lavas, tuffs, and breccias.9 - The igneous rocks 

phyroids" fr01n Cottonwood Canyon, in the are interleaved With marine limestones whose 

I:Itunboldt Range, was analyzed for the For- fauna -has been carefully studied by. J. P. 

tieth Parallel Survey, 4 as follows: · Smith and determined to be of Middle Triassic 

age. Lindgren10 has assembled the information 

showing that the Triassic period was one of 

volcanic activity along the whole Pacific 

coast from Inyo County; Cal., to the Alaska 

Range. 

Analysis ofrhyolitejrom the Elnmboldt Range, Nev. 

[M. H.. ·woodwt~rd, nnnlyst) 

Si02 •••••••••••••••••••• · ••••••• _. ••• 

Al20a ......... ~ ................. : .. 

Fe20 3 ••••• .' •••••••••••••••••••••••• 

CaO .............................. . 

MgO .... : .......................... . 

Na20 ..... · ........................ . 

K20 ................................ . 

Loss on ignition ..................... . 

74. 74 

14. 14' 

. 79 

1. 51 

. . 39 

. 92 

5. 29 

1.88 

99. 66 

CRETACEOUS ( 1) ROCKS. 

GENERAL RELATIONS. 

The· Triassic rocks, after being folded, were 

invaded by a succession of igneous rocks, 

which, named in the order of their intrusion, 

are basic granodiorite, quartz monzonite, aplite, . 

and quartz monzonite porphyry. These post5 Louderback, G. D., Basin Rang~ structure of tho Humboldt region: 

Geol. Soc. America Bull., vol. 15, p. 295, 1904. 

a Whitney, J.D., California Geol. Survey, vol.1, p. 459, 1865. · 

This analysis shows that. the rhyolite is a 

nol'lnal potassic variety, and in this respect it 

differs very considerably from ·the soda:-rich 

rhyolites in the Triassic of the Yerington 

district. 7 Spurr, J .. E., Descriptive geology of Nevada south of tho fortieth 

parallel and adjacent portions of California: U. S. Geol. .survey Bull. 208, 

• Rnnsomc, F. L., Notes on some mining districts in Humboldt County, pp. 122-123, 1903. 

Nov.: U. S. Oool. Survey Bull. 414, p. 32, 1909. a Hill, J. M., Some mining districts in northeastern California imd 

t'Jonos, J. C., Geology ofH.ochestor, Nev.: Min. nnd Sci. Press; vol.106, · northwest~rn Nevada:· U.S. Oeol. Survey Bull. 594, p. 20, 1915. · 

pp. 737-738, 1913. . . 9 Knopf, Adolph, Mineral resources of the Inyo and White mountains, 

a Schrndor,. F. C., Tho H.ochostor mining district, Nov.: U.S. Qeol. Cal.: U.S. Geol. Survey Bull.540, pp. 87-88,1914. 

Survey Bull. 580, pp. 314-338, 1915. 10 Lindgren, Waldemar, Igneous geology of tho Cordilleras,in Problems 

• U. S. Goo!. Expl. 40th Par. H.ept., vol. 2, p. 722, 1877. of American Geology, pp. 2~252, Yale University, 1915. 
20 GEOLOGY AND ORE DEPOSITS OF YERINGTON DISTRICT, NEV. 

Triassic intrusives are presumably of early 

Cretaceous age, like those of the .Sierra Nevada, 

not many miles to the west. 

The granodiorite and quartz monzonite are 

·areally the most important. Although no· great 

interval of time separated their intrusions, 

these two rocks are chemically distinct, the. 

quartz monzonite being much ·more siliceous 

and considerably. lower iri iron, magnesia, and 

lime. If they were derived from a common 

magma "this implies a notable differe_ntiation, 

but no trace of the hypothetically· resultant 

basic differentiate occurs in the district. · 

The invaded sedimentary rocks were· intensely metamorphosed as a result of the 

intrusions, but whether this metamorphism is 

·due to the granodiorite or to the quartz monzonite, or in part to both, coUld not be deter-

. mined. It is certain, however, that the quartz 

monzonite porphyry, the last member ~f the 

intrusive sequence, was injected after the 

metamorphism was effected. This matter; 

which is important· in the genesis of the . ore 

deposits, is discussed in detail on pages 43-44. 

MAJOR INTRUSIONS. 

GRANODIORITE. 

Granodiorite is the oldest granular intrusive 

rock in the district. · It is most widely exposed 

in the area northeast of Ludwig, extending 

across the range to Mason Valley, and is quite 

homogeneous throughout its area. It is a gray 

medium-grained equigranular rock, showing no 

megascopic quartz, and resembles a diorite. 

Under the microscope plagioclase (Ab67An3s) 

is seen to be the dominant constituent; it 

occurs as tabular crystals which are alined 

roughly· parallel, thus giving the granodiorite 

a distinctive structure. -Hornblende and biotite, occurring in about equal quantities, are 

the ferromagnesian minerals. Microcline and 

quartz are subordinate components, the quartz 

being perhaps just sufficiently abundant .to 

warrant naming the rock a granodiorite. The 

accessory minerals are magnetite, which is 

relatively abundant, titanite, apatite, and 

xenotime n).- . In the following table is given 

an analysis. of the. granodiorite, made on a 

specimen from an altitude of 5,540 feet on the 

road west of the Blue.stone mine, together with 

analy'ses of the .quartz monzonite that intrudes 

the granodiorite. 

Analyses of granodiorite and quartz. monzonite from the 

Yerington district, Nev. 

~\?a~~~ : : : : : : : : : : : : : : : : : : : ~ Fe2

0 3• ___ •• __ • __ • _______ • __ . 

Feo ___________________ . _____ _ 

g~::::::::::::::::::::::l K2

0 _______________________ _ 

H20-----------------------

H20+----------------------

rr8~::::::::: ~-::::::::::::: 002----------------.--------

MnO PzOs---- ------------------- ______________________ _ 

BaO,--------------------,-

SrO ... ---------------------

1 

57. 90 

16.38 

3.02 

3.66 

3.13 

6.33 

4. 10 

2.52 

.18 

. 89 

1.12 

None. . 08 

. 42 

.09 

.06 

.08 

2 

68.35 

14. 71 

1. 69 

1. 47 

1.11 

3.11 

4.28 

3.54 

. 31 

. 46 

. 57 

None. 

.14 

. 20 

.04 

. 06 

. 07 

3. 

67. 70 

15.95 

1. 53 

1.14 

1.23 

2. 70. 

4.83 

3. 74 

.19 

. 36. 

. 23. 

99.96 100.11 99.60 

a Undetermined. 

1. Granodiorite. George Steiger, analyst. 

2. Quartz monzonite. George Steiger, analyst. 

3. Quartz monzonite. D. T. Smith, analyst; California Univ. De~t. Geology !.3ull., vol. 4, p. 20, 1905. 

The chemical analysis confirms the deduction- . 

drawn from the mineral comp·osition that the 

rock is a basic end member of the granodiorite 

group; indeed,· the rock might equally well" be 

termed a tonalite or. diorite. The rock closely 

resembles the granodiorite !rom Donner Pass, 

in the Sierra ·N evada. 1 

QUARTZ MONZONITE. 

GENERAL FEATURES. 

Quartz monzonite .appears in large volume 

southeast of Ludwig and occupies. the entire 

width of the range.· It probably makes up 

much of the range south of the mapped area, 

for it is prominently exposed in the lower part 

of the canyon of West Walker River, where 

this stream cuts through the range. 

The intrusive relation of the quartz monzonite to the granodiorite is clearly shown 

northwest of the Bluestone mine, where 

apophyses of quartz monzonite penetrate the 

granodiorite. · The contact between the two 

rocks ·is sharp: the quartz monzonite 'maintains its character unchanged within half an 

inch of the bound.ary, but here it has a salic 

selvage, narrow dikes of which_ cut the gra~odiorite. This absence of contact chilling leaves 

1 Lindgren, Waldemar, Granodiorite and other intermediate rocks: 

Am. Jour. Sci.,4tliser.; vol. 9, p. 273, 1900. 
GENERAL GEOLOGY. 21 

little doubt that the quar.tz monzonite was 

intruded soon after the granodiorite. 

The quartz monzonite is a coarsely granular 

aggregate consisting of plagioclase, ·<?rthoclase, 

quartz, hornblende, and biotite. It is characterized especially'by large imperfect crystals 

of pearl-gray orthoclase, which give it a rude· 

subporphyritic aspect. The plagioClase is white 

and contrasts plainly with the pe.arl-gray 

orthoclase; it is easily seen that the two kinds 

of feldspar occur in about equal quantities, 

and· as a rule the rock is therefore readily 

recognizable as a quartz monzonite. It differs 

fro In the granodiorite mos't obviously in its 

coarser grain, its roughly phenocrystic ortho-· 

clase, and its megascopic ,quartz. Areally the 

quartz monzonite is nearly uniform in structure and mineral composition·. The most 

noteworthy departure from this uniformity_ 

is in the tips of the apophyses that penetrate the 

lime-silicate rocks southeast of Ludwig; here 

the . quartz monzonite grades into coarse 

pegmatite, which commonly shows local development of graphic structure and is in places 

slightly tourmaliniferous. 

Under the microscope the potassium feldspar 

of the quartz monzonite proves to be. mainly 

perthitic microcline. The characteristic plaid 

twinning of inicrocline is rather unequally 

developed, and so~e of the feldspar resembles 

orthoclase but is doubtless microcline, in· conformity to the rule that microperthite is an 

"unmi""<ed" solid solution in which the orthoclase has inverted to microcline during the 

"unmi"'{ing. " 1 The ·plagioclase is an oligoclase .of the composition Ab75An25, a composition found to prevail in specimens from .widely 

separated localities. ·Quartz is prominent, and 

biotite and hornblende in about equal amounts 

are the clark minerals. Myrmekite (a peculiar 

intergrowth of quartz and microcline) is a subordinate constituent,. and magnetite, titanite, 

apatite, and zircon are the accessory minerals. 

Epidote is a minor secondary constituent. 

A chemical analysis of the quartz monzonite 

is. given in column 2 on. page 20; it fully confu·ms the mineralogic evidence in showing that 

the quartz monzonite differs notably in composition from the granodiorite. The content 

of potassa is son~ewhat lower than the megascopic evidence suggested, but this discrepancy 

1 Warren, C. H., A 'quantitative study of certain perthitic feldspars: 

Am. Acad. Arts and Sci. Proc., vol. 51, p.l51, 1915. · 

appears to be due to ·the fact revealed by the 

microscope that the potassium feldspar is microperthitic. The specimen analyzed was obtained near Mickey Pass, and for comparison 

an earlier analysis of a specimen obtained by 

D .. T. Smith about 1! miles southwest of this 

locality is given (column 3); it agrees Closely 

with the later analysis. 

In chemical composition the granodiorite 

and quartz monzonite of the yerington district resemble those of the Sierra Nevada, 

especially in their dominantly sodic character. 

SCAPOLITIC ALTERATION. 

The quartz monzonite is profolllidly scapolitized at two localities--one west of the McConn~ll mine and the other on the 6~225-fqot 

hill east of Ludwig. The second occurrence is 

·the more noteworthy of the two. The quartz 

monzonite of this locality, which is several 

hundred feet from the contact, is traversed by 

a system of sc~polite and epidote veins, and 

the intervening rock is scapolitized and epidotized on a large scale. The rock contains 

veinlike masses as much as 2 feet wide that 

consist solidly of white scapolite, which is in 

radial columnar groups attaining s. inches in 

length. The adjoining wall rock, where it is. 

not highly epidotized, resembles a coarsegrained gabbro, but und.er the microscope it 

proves to consist of scapolite, pjrroxene, epidote, quartz; titanite, and apatite. Scapolite 

predominates; it is subhedral, and its refractive indices as determined by immersion in 

oils (w 1.55 and E slightly less than 1.54) show 

that it is p:r:actically a pure sodium scapolite. 

None _of the original minerals of the quartz 

·monzonite remain, except some quartz, and 

even this is in greatly reduced amount. 

The scapolitic and epidotic alteration, it is 

clear, did not take place around blocks of sediment8lry rock engulfed in the quartz monzonite but was determined in large measure by 

a system of steeply dipping joints along which 

the metamorphosing solutions flowed. 

MINOR INTRUSIONS. 

APLITE. 

Aplite is not common in the district. It· is 

practiqally absent from the granodiorite and 

quartz monzonite areas but occurs as sporadic 

dikes in the metamorphic rocks which they invaded. In places it is difficult to distinguish 
. 22 .GEOLOGY AND ORE DEPOSITS OF YERINGTON DISTRICT, NEY . 

in the field aplites from felsites that have been A certain range of .chemical composition of 

thermally metamorphosed; arid for this· reason the porphyries is indicated by the range in the 

the amount of aplite shown in Plate I to occur abundance of quartz phenocrysts in different 

southwest of· the Bluestone mine is possibly dikes. In some the prevalence of rounded ditoo large. Although aplite occurs in the dis- hexahedrons of ·quartz is the most obvious 

trict; pegmatite does not, except for the very .macroscopic character; in: others, however, 

_minor· amount in the ends of the quartz mon- quartz phenocrysts are so rare that they can be 

zonite apophyses. The aplite that cuts the found only after the most searehing.inspection. 

large apophysis of quartz monzonite penetrat- These differences suggest that the varieties · 

ing the granodiorite north of. the B.luestone practically devoid of quartz phenocrysts may 

mine is a snow-:-whlte fine-grained rock wholly be granodiorite porphyries that were injeeted 

devoid ·of dark minerals. I.t is rudely porphy- shortly after the intrusion of the granodiori-te 

titie through the occurrence of quartz in small and before the intrusion of the quartz monzosubhedral phenocrysts. Microscopically it nite; and that those dikes rich in quartz phenoproves to consist of orthoclase, oligoclas·e, and · crysts are quartz monzonite porphyries that 

.quartz. An aplite dike occurring in the gar- were injected after the intrusion of the gr.eat 

netites. a~d wollastonite rocks near the Blue- mass of quartz monzonite. The crucial.test of 

stone mine is found under the microscope to be this hypothesis is in the intersection of the two 

composed o.f _albite and quartz. kinds of. porphyries, but although dikes are 

QUARTZ MONZONITE PORP:S:YRY. 

OCCURRENCE AND CHARACTER. 

common in the district none were found to intersect. Furthermore, as there appears to be a 

gradation in degrees of prevalence of quartz 

Dikes and stocks of quartz monzonite por- phenocrysts in different dikes, it was felt to be 

phyry are common in the district. They cut rather. too fine a distinetion for field use to 

all the pre-Tertiary rocks, inclusive of the attempt to discriminate between granodiorite 

aplites that followed the quartz monzonite in- porphyries and quartz monzonite. porphyry, 

trusion. The dikes are particularly abtindant . and. so all were mapped as quartz monzonite 

in the granodiorite northeast of Ludwig, where porphyry. · 

they are noteworthy for their branching and The porphyries differ from the granodiorite 

gangliform character. They range from varie- and quartz monzonite in the absence of biotite 

ties so highly erowded with phenocrysts as to and, as shown by the microscope, in the absence 

closely resemble granitic rocks to porphyries of magnetite, which is an abundant accessory 

having an aphanitic groundmass. The por- mineral in the granular rocks. They thus show 

phyries of the stocks and the larger dikes are· an impoverishment· in iron and magnesian 

the most nearly granular, but even in dikes minerals, indicating that at the time they wen~ 

that are only 10 feet wide and t:r;averse calc- irijected the parent magma. had begun to. 

silicate rocks the phenocrysts predolJlinate differentiate, as indeed is indicated by another 

over the groundmass. · · line of evidence, in that the porphyry dikes 

A common feature of all the dikes is that the intersect aplite dikes that followed the intruorthoclase occurs as large porphyritic crystals sion of the ·quartz monzonite. 

of pearl-gray color. Plagioclase phenocrysts 

·are numerically more abundant but are"'much · PETROGRAPHIC DETAILS. 

smaller. Hornblende is the sole ferromag- Two porphyries; representing the extreme 

nesian mineral; ·it is usu·any altered, commonly varieties in respect to the abund~nce of quartz 

to epidote. Many of the plagioclase pheno- phenocrysts, have been selected for special 

crysts also are pseudomorphously altered to description. One of these was obtained from a 

epidote; in fact, no material was foup.d in any mass of poiphyry intrusive into the quartz 

of the many dikes of the district that was j monzonite southeast of ·Mickey Pass. I~ is a 

wholly free from epidotization and as altera- white rock that on casual inspection appears to 

tion of this kind is commonly associated with be coarsely granular but. on cioser observation 

~he copper metallization, it arouses the sus..: is seen to be highly porphyritic, a feature most 

picion that the dikes afforded paths of escape apparent from the abundant corroded dihexafor mi~eralizing solutions. hedrons of quartz. It carnes sporadic large 
GENERAl,. GEOLOGY. 23 

phenocrysts of orthoclase, innumerable st.riated 

crystnls of glassy plagioclase, and lesser 

a.mounts of hornblende: The individual crystals of orthoclase are from 8 to 2 7 times as large 

as tho individual plagioclase crystals, but as 

tho phenocrysts of plagioclase far outnumber 

those of orthoclase the a1nounts of orthoclase 

and plagioclase present are roughly the same. 

At the contact with the quartz monzonite this 

ahnost granular porphyry has a notably chilled 

border. The resulting porphyry is characterized by its smaller phenocrysts, which amount 

to roughly half the bulk of the rock, and its 

aphanitic grounclmass. Under the microscope 

mass is an imperfect micrographic intergrowth 

of quartz and orthoclase. Titanite and apatite; occurring as comparatively large crystals, 

are the accessory niinerals. Although this porphyry is macroscopically widely different from· 

the other one, the microscopic analysis appears 

to minimize the difference ·in chemical composition that would be assumed to exist between 

th~m: In. one the qU:a~tz i~ all in the groundmass and in the other it is mainly in the 

phenocrysts. 

TER~IARY ROCKS. 

THE SECTION IN GENERAL. 

tho norn1al porphyry is found. to carry pheno- The Tertiary period is represented in the 

crysts of orthoclase, oligoclase, quartz, and Yerington district by three major s.ubdivisions, 

hornblende in· a groundmass that forms not which are· separated by two well-marked lmover 15 per cent of the bulk of the rock. The conformities. The . rocks ~re probably all 

·orthoclase contains idiomorphic inclusions of younger. than middle Miocene. The lowest 

plagioc~ase and titanite, as well as irregular member consists chiefly of rhyolite with some 

patches of quartz suggestive of the "unmixing" intercalated fluvial ·and lacustral sediments 

of an originally homogeneo~s solid solution; the tilted at angles of. 15 ° to 65 °; the middle me~­

oligoclase don1inates numerically and has the ber consists of andes~te; a;9-_d the upper memb_er 

c01nposition Ab70 An30 ; the quartz crystals are consists of basalt in horizontal sheets capping 

magn1atically corroded; and the hornblende is :a conglomerate. The Yerington section is thus 

palo green, reedy, and anhedral. The ground- dominantly volcanic, and it aggregates roughly 

1nass is an irregular intergrowth. of .quartz and 7,000 feet in thickness. The columnar section 

oligoclase, tho oligoclase being in places poorly is shown in figure 3. 

idio'morphic. Titanite and apatite are the 

·accessory minerals; titanite is the more a bun- CONGLOMERATE AT THE BASE OF THE TERTIARY 

dant and occurs as crystals nearly as large as SECTION. 

the hornblende prisms. 

'fhe other porphyry selected for special 

description occurs in the hanging-wall·country 

rock of t~o Ludwig lode. It carries scattered 

crystals of pearl-gray orthoclase ranging ftom 

12 to 15 n1illimeters in length, innumerable 

plagioclase crystals averaging about 4 millin1eters, and euhedral prisms of hornblende 10 

1nillimetors lo~g. So well does the groundmass blend with the rest of the rock that the 

general effect is that of a somewhat porphyritic 

n1onzonite. When examin.ed lmdor the microscope, however, phenocrysts and groundmass 

are found to be sharply contrasted and, as 

n1easured·by the Rosiwal method, to be roughly 

equal in amount. The ·plagioclase, as in the· 

other. porphyry, is an oligoclase of the composition Ab70 An30 ; some of the crystals are 

completely altered to epidote. The porphyry 

contains .sporadic individuals of q:uartz which 

are highly corroded and very small, not exceeding. 0.6 n1illimoter in length. The ground-

. At the base of the· Tertiary ·section there is 

generally a conglomerate resting on granodiorite. It consists of cobbles and of boulders as 

much as 6 feet in diameter, all of which are 

well rounded but are unsorted and unstratified. 

These. features suggest that th~ conglomerate is 

probably of fluvial origin. Although consisting 

mainly of andesite cobbles, ·it contains sporadic 

boulders of granitic rocks and siliceous sediments. Boulde~s qf granodiorite 6 feet in 

diameter were see11. ·The conglomerate is at 

most 150 feet thick It is persistent enough to 

be mapped,. and ~ts distribution is shown on· 

Plate I (in pocket). 

LATITE SERIES. 

LATITE VITROPHYRE. 

A· series of lavas, characterized by. their 

columnar structure and dark-red or brown color 

on weathered surfaces, overlie the conglomerate. 

The basal member of this series is & highly dis-. 

tinc~iye flow of_ black porpP,yritic glass, whic~ 
, 24 GEOLOGY AND ORE DEPOSrrS OF YERINGTON DISTRICT, NEy. 

attains a maximum thickness of 100 feet. It 

rests on the conglomerate where that member 

is present, but elsewhere it lies directly on ·the 

granodiorite. It occurs at widely separ~ted · 

localities and invariably in the same stratigraphic position and· IS consequently a good 

horizon ni.arker. 

Andes·i~_br~ccia . 

Volcanic grlt 

FIGURE 3.-Columnar section of the Tertiary rocks in the Yerington 

district. 

The glass flow has at some"localiti~s an excel-· 

· lent columnar structure. The columns, which 

doubtless originally stood vertical, now dip at 

angles of about 30° E. as a resul~ of the westward tilting of the latite series. ·The glas~ flow 

attains its greatest thickness, roughly 100 feet, 

near the Montana-Yerington mine; toward the 

south it gradually decreases in thickness, and 

west of Mason it is only 6 to 8 feet thick. On 

account of its thinne~s the glass flow has been 

mapped with the overlying series of quartz 

latites. 

. The vitrophyre is strikingly fresh, being 

coated only with a thin film of decomposition 

products. It consists of a black lustrous glass 

·carrying nll!llerous crystals of plagioclase and 

splendent. foils of biotite. Under the micro-

. scope the vitrophyre shows a marked fluidal 

structure, which in its· swirls incloses many 

phenocrysts, mainly of plagioclase·, biotite, and 

pyroxene. The plagioclase, of the composition Ab5oAn50, predominates; it· is generally 

more or less angular, but some of it is partly 

e:m:bayed,. Sanidine occurs sparsely in some · 

thin sections but fails in others, and it is 

evidently a rare constituent of the vitrophyre. 

Biotite, commonly bent, is the chief ferromagnesian mineraL Both augite and an 

orthorhombic pyroxene, which is either bronzite or enstatite, . occ~r. Hornblende, magnetite, and apatite are accessory minerals. The 

glass is hlghly flow streaked, deep-brown glass 

alternating with light, almost colorless streaks, 

as may be partly seen in Plate II, B; it is completely isotropic, and.its index is found by the 

immersion method· to be 1.50. The microscope fully confirms the megascopic impression 

that the rock is ideally fresh. 

The~ analysis of a specimen obtained northeast of Mason, given below in column 1 ~ was 

made in. · the laboratory of the Geological 

Survey: · 

Analyses of lavas from the Yerington and Tonopah districts. 

(George Steiger, analf-lt.] 

Si02 •••••••••.•••••••••••••••••••••••• 

Al203 ... -.... -.... - -............... . Fe20a··········· ._ .................... . FeO .•............•................. 

~;8_·_·.-.·.-.-.- .·;: ~: ~ ~ ~ ~::: ~ ~:::::::::: .: : 

·N3.:10 ............................... . 

K20 ................... ·~----· · · ····· H.,O- ............................. . 

n;o+. ·····~··········· ......... -·.· Ti02····························· ... . Zr02 •••••••••••• · ••••••••••••••••••••• 

·002 •••••••••••••••••••••••••••••••••• 

P205······························· .. 1\-lnO ................................ . nao ................................ . SrO ............................... . 

l 2 

66.03 

14.98 

l. 65 

1.67 

. 99 

2. 62 

3. 60 

4.34 

. 44 

2.46 

. 48 

None. . 07 

.11 

. 07 

.11 

. 08 

71.71 

14.00 

l. 06 .. 51 

. 43 

2.25 

3.21 

4.41 

.44 

l. 38 

. 28 

. 07 

99. 70 1 99. 75. 

l. Biotite-pyroxene latite vitrophyre, Yei-ingtondistrict. 

2. Dacitic rhyolite, Brougher Mountain, Tonopah. 
GENERAL GEOLOGY. 25 

The high potassa content is probably the 

most noteworthy fact disclosed by the analysis 

and shows that the vitrophyre falls into the 

latite group; the silica, however, is somewhat 

higher than in typical latites, corresponding 

rather with that of the rocks in recent years 

termed quartz latite. As only a small part of 

the potassa is contained in the biotite and the 

amount of ~anidine in the ·vitrophyre is insignificant, the bulk of the potassa is locked up in· 

the glass grounclmass. 

The analysis of the vitrophyre corresponds 

more closely with that of the Brougher Clacitic 

rhyolite (colum1~ 2), as Spurr terms it in his 

latest report on the geology of Tonopah, 1 than 

with that of' any other lava in that district. 2 

It agrees closely in lime, soda, and potassa, but 

the Brovgher :z:ock, which in the nomenclature 

here employed would be called· quartz latite, is 

son1ewhat higher in silica. 

QUARTZ LATITES. 

A distinctive series of lavas overlies_ the glass 

.flow or, where the glass is absent, the conglomerate at the base of the Tertiary sectio:J?.. These 

lavas are characterized by their brown color 

and well-marked columnar structure. They 

are highly porphyritic, containing numerous 

crystals of plagioclase, quartz, mid biotite; in 

the field they were therefore called dacites. In 

some of the lower flows the quartz phenocrysts 

are spa-Tse or lacking, and consequently some of 

these flows resen1ble andesites.· Toward the 

top of the series the quartz phenocrysts become 

abundant and sanidine appears and becomes 

prominent; in short, there is a complete gradation into the rhyolites of the overlying series. 

The boundary between them and the rhyolites 

.drawn on the geologic map (Pl. I) is th~refore 

purely arbitrary. · 

The lavas of this group are best shown along 

the east front of the range, in the northern part 

of the area n1apped. Th~ columnar structure 

is especially conspicuous here, ~d as the lavas· 

weather dark red and brownish red the series, 

viewed in the large, is of basaltic aspect. The 

colum:ris, of which part are hexagonal and part 

are of other perimeters, are 10 feet long and dip 

15° E. The lavas aggregate several hundred 

feet in thickness here, 500 feet being probably 

1 Spurr, J. E., Geology and oro deposition at Tonopah, Nov·.: Econ. 

Geology, vol.lO, p. 740, 1915. . 

2 Spurr, J. E., Geology of tho Tonopah mining district, Nev.: U. S. 

Goo!. Survey Prof. Paper 42, p. 57, 1905. 

the maximum, but they thin toward the south; 

near :Mason only the basal glass flow is left, and 

this itself, as before noted, is but a few feet 

thick. They occur, however, on the west 

flank of the range. 

. A specimen tt1,ken from a flow immediately 

above· the biotite-pyroxene latite vitJ•ophyre 

prov:es microscopically to be a porphyry whose 

groundmass is a glass of markedly fluxional 

structure. Plagioclase (Ab6oAI140) predominates 

among the phenocrysts. Biotite is common. 

Quartz phenocrysts are rare, and sanidine in 

large sporadic crystals is .still less common. In 

the flows higher in the sequence the microscope · 

verifies the observation that the quartz and 

sanidine phenocrysts are abundant and shows 

· that plagioclase is rare. In view of these 

features the lavas are called quartz latites. 

RHYOLITE. 

Rhyolite is the dominant Tertiary volcanic 

rock of the district. It makes up a thick series 

·of tuffs, breccias, and lavas in which the pyroclastic members predominate. They are lightcolored rocks, white, ash-gray, and pink being 

· the. principal colors. As a .rule they -are con-· 

spicuously porphyrit~c and carry numerous 

phenocrysts of quartz, sanidine, and piotite. 

They are quite uniform in mineral composition, and save ·a few scattere"d phenocrysts of 

plagioc~ase they carry none but the three 

mirierals mentioned. 

The rhyolites clip 15°-65~ W., averaging 

about 45° w., and they trend nearly parallel 

to the range. . Their attitude is readily determinable, as thin well-bedded volcanic grits are 

interstratified with them from place to place.· · 

A thin layer of such grits, containing considerable silicified wood, occurs 1,800 feet above the 

base of the rhyolites in the section west of 

Mason, and a belt of grits, 430 feet thick, occurs 

at the top of the section, near the Bluestone 

mine. The upper belt contains a conglomeratic 

bed 30 feet thick made up of a great variety of 

pebbles, including quartz and andesite, which 

range from angular to well rounded. The 

thickness. of the rhyolite series, including the 

upper belt of grits, is about 4,000 'feet. 

Besides the grits mentioned, some thin lenses 

of coarse .conglomerate, indicati~e of stream" 

washes, are intercalated in the rhyolites. One 

of the "more notable of these occurs a~ about 
26 GEOLOGY AND ORE DEPOSITS OF YERINGTON ·DISTRJCT, NEV. 

the horizon of the lower grit; it consists of wellroimded cobbles and boulders 6 feet in· maximum diameter. The boulders comprise andesite, quartzite (or felsite 1), and granite. Much 

r~yolite occurs between the boulders, but this 

is in angular frag-ments. These features clearly 

indicate the filling of ·a stream channel, in 

which the fragments .. that have been carried 

so.me distance .from their sources, as is obviously true of the andesite, granite, and quartzite, are well rounded, whereas the material of 

local derivation is likely to be angular. 

A co~spicuous member of the rhyolite series 

·is the tuff that forins the ridge just west of the 

Mason Valley mille. It is made U:p of fragments ·of rhyolite and crystals of quartz, sanidine, and sporadic biotite in a dense matrix 

which the microscope shows is composed larg~ly 

of g~ass sherds. The . structure is typically 

vitroclastic, as defined by Pirsson. 

ANDESITE BRECCIA. 

half its bulk. Hornblende in stout prisms is 

the most conspicuous component to the unaided 

eye, but under the microscope the feldspar 

(labradorite) isseentq dominate strongly. The 

groundmass is glassy and contains innumerable 

minute crystals of feldspar. Biotite occurJ 

sparingly together with the hornblende In 

some of the andesite. 

HORNBLENDE ANDESITE. 

· A succ~ssion of andesite flows rests in places 

on the eroded edges of the rhyolites. These flows 

.dip 25°. E., whereas the underlying rhyolites dip 

45° W. They are probably best exposed OJ;! 

the east scarp of the 5,995-foot hill northeast 

of Ludwig; the section here consists of a basal 

breccia, about 100 feet thick, overlain by :five 

thin sheets of. columnar lava. Pn th~ southwest side of the 6,100-foot p·eak the basal breccia is . underlain by a thin conglomerate, as 

shown by.the occurrence of well-rounded cobbles. The . thickness of the andesite series 

A coarse andesite breccia overlies the rhyo~ appears not to exceed 300 feet. 

lites conformably_ near the Bluestone. mine. · A distinctive andesite predominates. It is· 

Above the grits near the top of· the rhyolite: a gray lava characterized. by numerous slender 

s~ries is a white rhyolite ~uff overlain by a prisms of hornblende in roughly parallel orienbiotite ~ndesite tuff. The. andesite tuff is a tation and averaging 0.2 inch in length. It 

light-gray rock containing numerous dark por- carries no feldspar· phenocrysts. Under the 

phyritic fragments (probably andesite), angular microscope the· hornblende phenocrysts are 

crystals of p~agioclase (labradorite), ~nd bio- seen to. be embedded in a holocrystalline 

· tite ·flakes, commonly in unbroken hexagonal fluidal groundmass composed largely of f~ldspar · 

crystals. rhe matrix proves under the ~icro- prisms near Ab50An50 in composition, together 

scope to be. composed of finely commmuted with subordinate augite and accessory magglass sherds. Like other vitroclastic rocks in netite and apatite. This rock may be termed 

the district, it is firmly lithi:fie~. a hornblende ·andesite, to distinguish it from· 

The co~rse andesite breccia, comprising frag- other andesites in the di~trict, though that 

ments as . much as. seve:r:al feet in diameter, term is commonly given to andesites carrying 

r~sts above the.biotite andesite tuff. It makes both plagioclase and hornblende phenocrysts. 

up the main part of the formation, which at- The "hornblende andesite occurs as dikes in· 

. tains a thickness of· 1,:400 feet. The formation many of the rocks of the di~trict. It cuts the 

occurs only in· a small ar~a betw~~n the ~ason quartz. monzonite porphyry near Yerington 

Valley and Bluestone mmes; ongmally It· was _station, is intruded along a fault separating 

doubtless ·somewhat .thicker, as its upper limit quartz monzonite from limestone in the 

is the strong fault that has brought the Ter- McConnell mine, and is fairly ·common as 

tiary rocks dowp. against the . Triassic. Lo- thick dikes cutting the rhyolites. 

cally, however, some rhyolite overlies it, but 

as shown in the lower tunnel of the Bluestone 

mille, 130 feet from the portal, this contact is 

highly sheared and slickensided, so that the 

relation of the overlying rhyolite to the. andes:-

ite breccia is obscure. 

The andesite of the breccia is a gray highly 

porphyritic variety, the phenocrysts forming 

CONGLOMERATE. 

The next younger member of the Tertiary 

sequence is a _mass of loosely cemented gravel 

or conglomerate, represented chiefly by the 

deposits. occurring beneath the basalt-capped 

mesas southwest of Mason. They comprise 

a wide variety of rocks, inclucfing much rhyolite, 
GENERAL GEOLOGY. 27 

and are subangular and poorly sorted. Their tiop. that. accumulated in an area where volgreatest thiclmess is 300 feet. canic aggradation pre~ominated~ The ~ge 

A body of similar material occurs. east of of the Esmeralda formatiOn has recen.tly been 

the Mason Pass road, not far from the Martha shown by Merriam 1 to be approximately 

Washington prospect. It is composed largely upper Miocene. The correlation proposed is 

of subangular fragn1ents of ·d~fferent var~eties suggeste~l (1) by the similarity of stratigraphic 

of andesite; granitic rocks are fairly common, relations: the rhyolites are younger than a 

but rhyolite is scarce. Boulders as much as preexisting andesite series, as is true of the 

several feet in dimneter occur in it. Prospect Esmeralda formation, which overlies uncon- , 

pits show it to be unshingled, unsorted, and formably a series of andesitic lavas and is itself 

unstratified. unconformably overlain by later lavas; 2 (2) 

BASALT. by a certain amount of li~hologic resemblance: 

much rhyolite tuff and andesite (latite) tuff 

Basalt is the youngest igneous rock in the occur in the Esmeralda; 3 (3) .by the fact that 

district. It caps the high mesas in the southern the degree of tilting and amount of faulting of 

part of the area mapped and is generally under- the two formations are of the same order; and 

lain. by gravels. It forms a superposed succes- (4) by the paleontologic evidence. 

sion of nearly horizontal flows, which aggregate No fossils were found in the ·rhyolitic series 

200 feet in thiclmess. It has been slightly during the present examination, but some 

tilted since it was erupted and has been much fragments of vertebrate bones had been obtained 

faulted; in fact, t4e present relief of the range by Smith 4 10 miles east of Yeririgton, in rocks 

has resulted largelyfi·om postbasaltic. faulting. that he ·regarded as belongirig to the same 

'll1e basalt is a dark rock showing to the formation. The fossils were originally reunaided eye sporadi~ small phenocrysts of ported on hy W. J. $in clair, 5 who pronounce<.! 

·pyroxene only. Mi?roscopically. it proves .to them to be ''.probably late Miocene or Pliobe composed essentially of calmc labradorite cene." Since. this determination was made 

and augite: The labradorite predominat~s; · knowle.dge of the Great Basin faunas has in- · 

~he large~ crystals are ,euhedral, but as there creased greatly, and it seemed desirable to ·reIs a contmuous gradatiOn between these and vie\v the evidence in the light. of -the new inthe smallest tl~e. structure a:ppro.ac~es the. formation. Prof. J. C. Merriam has kindly 

seriate porphyritic. The augite IS m two reexamined the original collection and reports 

generations, the earlier c.rystals as. phenocrysts, as follows: o ' · 

about the size of the largest feldspars, and the 

later as small prisms and grains lying interstitially between the smaller feldpsars. Olivine 

occurs sporadically in SID:all grains, but most 

of it has altered to iddingsite. Magnetite is 

a minor accessory mineral. 

CORJ.'tELATION OF THE TERTIARY ROCKS. 

The a.ndesitic detritus in· the fluvial conglomerate at the base of the local Tertiary section shows that the eruption of the volcanic 

rocks now occurring in the Yerington district 

had been preceded in earlier Tertiary time by 

the eruption of an andesitic series. · 

·The rhyolites, including the underlying 

quartz latites and the glass flow and the conformably overlying andesite· breccia, probably 

correspond in age to the Esmeralda formation., 

which consists of lacustral beds, tuffs, and in:-

tercalated terrestrial deposits. They appear 

·to represent a phase of the Esmeralda formaThe collection consists of a few fragments and the 

g~eater part of an ankle bone. The fragments are indeterminable. The ankle bone represents a large camel, 

which seelllB to me to be specifically different from the 

large species represented by an ankle bone from the 

Cedar Mountain or Esmeralda beds. One could not 

safeiy state more than that the formation from which 

these bones .came is somewhere between middle :Miocene 

and middle Pliocene. 

QUATERNARY DEPOSITS. 

The Quaternary deposits consist of gravel, 

sand, and silt. The ·gravel is angular and 

unsorted and occurs in a ·series of alluvial 

1 M:erriam, J. C., Tertiary vertebrate fatma from the Cedar M:Otmtain. 

region of western Nevada: California Univ. Dept. G~ology Dull., vol. 9, 

pp. 165-172, 1916. . . . 

2 Duwalda, J. P., Tertiary mammal beds of Stewart and lone valleys 

in west-central Nevada: California Univ. Dept. Geology Dull., vol. 8, 

p. 348, 1914.' 

3 Turner, H. W., Contribution to the geology of the Silver Peak quadrangle, Nevada: Geol. Soc. America Dull., vol. 20, pp. 254-255, 259, 1910. 

c Smith, D. T., California Univ. Dept. Geology Dnll., vol. 4, p. 8, 1905. 

5 Idem, p. 10. 

o Letter of Nov. 9, 1916. 
28 GEOLOGY AND ORE DEPOSITS OF YERINGTON DISTRICT, NEV. 

cones along both flanks of the range; latera)J.y 

it grades into the silts of the valley bottoms. 

Thick deposits of clean sand of obscure origin 

occur toward the heads of some of the canyons, notably in Sand Canyon, where sand is 

present up to altitudes of 5,600 'feet. These 

deposits are now _deeply dissected. . . 

No Lahontan beach lines occur in the dis-

,trict. The elevation of the Lahontan ·beach 

around Walker Lake,. as determined from the 

position shown on Plate XV of Russell's 

monograph 1 and from the altitudes in the 

Hawthorne quadrangle, is approximately 4,250 · 

feet, which is somewhat lower than Russell's 

own estimate-4,378 feet. 2 As the lowest 

altitude in the mapped area· of the Yerington 

district is 4,350 feet, the absence of strand 

lines conforms with the probabilities. 

GEOLOGIC 'STRUCTURE. 

The geologic· structure of the' district is 

rather intricate in detail. This is especially 

true of the area underlain by the Triassic limestones and associated rocks-the area th~t is 

of main economic interest. The Triassic consists of steeply tilted rocks, greatly faulted, 

extraordinarily metamorphosed, and intruded 

·by numerous igneous rocks. Upon the erodecl. 

surface of these older rocks rests a series of 

Tertiary rocks, do~inantly of volcanic origin. 

. These are of broadly simple structure, though 

examination soon discloses tha·t they conceal 

a rather eventful tectonic history. (See Pis. 

I a.~d III.) 

Three periods of faulting at least can be 

recogniz.ed~one accompanying the intrusion of 

the Cretaceo-us igneous rocks, and two in the 

Tertiary; of these the last is post basaltic. 

The earliest faUlting that can be dated approximately is that which occurred soon after 

the iutrusion of the quartz monzonite porphyry 

dikes,. though it is probable that some faulting 

accompanied the intrusion of "the granodiorite 

and quartz monzonite. Many of the dikes are 

broken by faults, ·and along some of these 

faults contact-metamorphic ·silicates have 

formed, as is well shown, for example, near the 

boa:r:ding house at the Malachite mine, where 

masses 20 feet wide composed of garnet; epidote, and lamellar pyroxene occur along the 

1 Russell, I. C., Geological history of Lake Lahontan: U. S. Geol. 

Survey Mon. 11, 1885. · 

2ldem, p. 101. 

fault contact between -limestone and felsite. 

The silicates replace both limestone and-felsite 

but mainly the limestone. Contact-metamorphic silicates occur along other faults, many of 

which were first recognized during the field 

work by the fa.ct that theporphyry dikes abut 

against them. It is noteworthy that the ends 

of some of the dikes abutting upon the faults 

have been profoundly epidotized or garne_tized. 

Copper:-bearing minerals occur. in some of the 

belts of silicates along the faults, ·and such 

faults were thus places of ore deposition; but 

this phase of the matter is discussed in more 

detail elsewhere in this report. The development ·of garnet and allied silicates, minerals 

that form only at relatively high temperatures, 

proves, then, that these faults were formed 

shortly after the injection of the dikes t_hat they 

displace. 

· 'The .displacement can rarely be determined, 

as the severed ends of the dikes on opposite 

sides of the faults as a rule can not be found. 

This would appear to ·indicate that the faults 

are generally of considerable magnitude. 

Some, in fact, clearly represent large Q.isplace-

·ments, inasmuch as rocks widely separated 

stratigraphically have been brought into coh-

. tact, as near the Western Nevada mine, where 

andesite has been brought against n1assive 

limestone . 

During the Tertiary period faulting occurred 

on a large scale, certainly at two different times. 

and probably at three different times. The 

earliest faulting coincided with the tilting of the 

rhyolites. It was during this deformation 

that the rhyolites were faulted down against 

the pre-Tertiary rocks. As shown by the 

structure sections in Plate III, these faults involve vertical displacements of 5,000 to 10,000 

feet; but owing to the lack of close, detailed 

study of the rhyolite area north of Mickey. Pass 

it can not be affirmed that displacements of 

this total magnitude were effected along single 

fault surfaces. One of the most prominent 

faults of this set is well shown at the Mason Valley and Bluestone mines, and it is cut by the 

lowermost tunnel of the Bluestone mine, where 

it dips 75° E. Its throw is undoubtedly several thousand feet. The faults of this age 

·strike not only parallel to the range but also at 

right angles to its trend; ·they dip from 25° to 

75°. 
U.S. GEOLOGICAL SURVEY 

I 

grd } qmp 

----------------~------- ~-- 4 ,0 00 · .._ ______ _ 

SECTI ON A -A' 

B 

SECTION .H -;:]' 

Bluestone mine 

! 

c 

---------- --------- ·-----------· ·---·--'-"------

SECTION !: - C ' 

PROFE!\S O~AJ, PAPER 114 P L ; Ill 

11\ille 

l ,OOO 0 1,000 :0::,000 ;},000 

1918 

-=o....::~~--""-""'-"'--- ---~···-~_- ·· ·· ·~~ ~J. ~ ................. ......... ~. ··· ······ ·· ···· · · ·· · · ·· · · ·~·~::J .o oo ' 

Vol can ic gri ts and 

yongl omerate 

GEOLOGIC SECTIONS ACROSS THE YERINGTON DISTRICT, NEVADA 

QUATE RNARY 

[~] GrM·el, sand, 

and silt 

SEDIMENTARY ROCKS 

TERTIARY ~-'----, Upper Miocene 

m· Pliocene 

I TFc ! 

L____l 

Fluvial 

conglomerate 

__A__ ____________________ -., 

Garnetite 

~ and allied rock 

T RIASSIC 

Uypsun1 

IGNEOUS RO CKS 

T ERTIARY 

Pliocene ( ?) -----------...,. 

UppPr M1:ocene o1· Pl·iocene 

Andesite Andesite breccia Rhyolite Quurtz lutite 

POST-TR IASSIC (EARLY CRETACEOUS?) 

Quartz rrwn- 7.0nite porphyry 

·------, 

Granodiorite 
GENERAL GEOLOGY. 29 

The fnult that separates the granodiorite 

from the garnetites at the Bluestone mine and 

continues to the Western Nevada, where it 

separates the quartz monzonite fro~ the metamorphic limestones, probably was formed at 

about this. time. It is certainly younger than 

the faults that displace the porphyry dikes, as 

· it cuts off these faults. 

Faulting occurred also after the extrusion of 

the hornblende andesite. A small mass of 

andesite along the west flank of the range north 

of Ludwig has been faulted .against the rhyolites; at one place the fault surface fOr!flS a 

cliff fnce, 15 feet high and 50 feet long, trending 

N. 35° E. and dipping 75° W. (See Pl. II,I, section B-B'.) This displacement may mark a 

separate epoch of faulting but may possibly 

coincide with that discussed in the next paragraph. 

Severe faulting took 

place after the basalts 

. were erupted. The present relief of the portion 

of · the range south of 

Mason is rather obviously 

due to postbasaltic faulting. The basalt that caps. 

the range is broken by a 

series of step faults forming a succession of treads; 

the base of the lowermost, 

tered near the fault plane, and t}le overlying 

limestone is highly brecciated foi more than 

20 feet from the fault plane. The nline workings disclose· tha.t the dip slip is at least 150 

feet. 

EPITOME OF THE GEOLOGIC HISTORY OF THE 

DISTRICT. 

The oldest geologic records of the district 

date back to the Triassic period. At this time 

4,000 feet of andesitic and dacitic lavas, breccias, and tuffs were erupted, which were succeeded by a peculiar series of soda-rich felsite 

lavas and .tuffs. Marine. conditions then prevailed, and limestones mainly were laid down, 

interrupted, however, by occasional eruptions 

of felsite. At one time during the Triassic a 

o,._ -:--'---.J...-----'----'-'~90 F.eet 

just south of Sand Can- FIGURE 4.-Section through the glory-hole tunnel of the McConnell mine. a, Limestone; b, basalt; 

c, garnet rqck and ore; d, Triassic andesite· .. 

yon, is at an altitude of 

5,000 feet, the base of the middle tread at 

. 5,500 feet, and the base of the highest at.5,800 

feet. 

Sand Canyon is a rift valley resulting from 

the downfaulting of a block between two 

parallel faults trending at right angles to those 

that have determined the front of the range. 

The faults are about 1,000 feet apart, and the 

rockS lying between then1 are extremely 

brecciated. 

. Although the postbasaltic faults are mainly 

normaJ.. faults, s11-ch as those 8iong the front 

of the railge south of Sand Cany9n, just described, reverse faulting has occurred to some 

extent. This is best shown near the McConnell nline, and the fault plane itself is well exposed in the upper tunnel. (fig. 4). It. is 

sharply defined, dipping 54° · W.; the basalt, 

which forms the footwall, is considerably shatbed of gypsum, as much as 450 feet thick, was 

deposited. · 

These rocks 'vere folded, proha~ly in the beginning of Cretaceous time, and were intruded, . 

probably early in the Cretaceous, by large 

masses of granod~orite and quartz tnonzonite. 

These intrusions were followed by others that 

brought in- tninor amounts of aplite and considerable amolints of quartz tnonzonite porphyry. Faulting· occurred during and immediately after this epoch of intrusion, and 

as a sequel to the intrusive activity the ore 

bodies ·were formed. 

A long period of erosion then ens.ued, during 

whi;ch the" Triassic· rocks were in· places entirely 

removed and the granitic rocks were laid bare 

over wide areas. Volcanic eruptions followed, 

probably in the later part of the :Miocene, and 

buried the fluvial d~posits tha.t had accumu-
30 GEOLOGY AND ORE DEPOSITS OF YERINGTON DISTRICT, NEV. 

·lated on the· surface· eroded· upon the older After the deposition of these rocks a period 

rocks. The relief of this surface on which the of pronounced diastrophism set in, the most 

Tertiary rocks accumulated appears to have vigorous in this part of N eyada during Terbeen moderate, though on account of . the tiar.y time. The rhyolites were tilted at angles 

small exposures and the profound faulting to of 25 ° to 65 ° and were greatly faulted, the rocks 

which the rocks of the region have been sub- being displaced stratigraphically to the extent 

jected this conclusion is not firmly established. of thousands of feet. Erosion ensued, and sub-

,.At any rate, ~he streams that flowed on this sur- sequently andesites were erupted, now resting 

face appear to have been powerful, as is indi- at low angles upon the worn edges of the rhyocated by the well-roupded boulders, 6 feet in lites. This eruptive outburst was extremely .. 

diameter, occurring in their gravels. moderate and is represent~d by only a few 

The volcanic activity began with the effusion hundred feet of lava. 

of a distinctive flo~ of black glass, a latite After the ande.sitic extrusion erosion again 

vitrophjre; this flow was succeeded by quartz began. Rather extensive deposits of grave~s 

latites, which are especially characterized by accunlulated, in places. as much as 300 feet 

thick; their unsorted, subangular ·character their columnar structure; and upon these were 

piled about 4,000 feet of rhyolites and 1,

400 suggests that they are of fanglomerate origin. Eruptive forces, ·the final manifestation of 

feet of andesite breccia. From time to time. volcanisn1 in the district, once more becan1e 

during the general period of eruptive activity active, and a series of basalt flows was poured 

fluvial and lacustral con,ditions prevailed; the out. The basalts, although nearly horizontal, 

most noteworthy of thes~ interruptions led to have been considerably-faulted, and this period 

the deposition of 430 feet of lake beds, which of vigorous diastrophis1n is held to mark the 

occur at the top of the rhyolites and underlie beginning of Quaternary time. The faulting· 

the andesite·breccia. This succession of rocks, that took place at that time blo.cked out the 

beginning with the vitrophyre at the base of Singatse Range largely as we see it now. 

the section and including tbe andesite breccia at Since this faulting -the deep c.anyons opening 

the top, appears to correspond to the Esmeralda .. o~t upon Mason Valley have be_en excavated, 

formation, which is now believed to be of late and the material thus derived has built up a 

~ocen~. age. series of alluvial cones fronting the range. 
PART II. THE OR~ BODIES. 

GE~RAL FEATURES. 

The ninin ore bodies of the Yerington district 

£Lre copper-bearing deposits characterized by a 

gnngue of .either pyroxene, garnet, or epidote 

or n mixture of these minerals.· Chalcopyrite 

i~ the chief copper-bearing mineral; pyrite is 

comn.1only associated with it, ·but no other 

pri1nary sulphides occur in the district. 

The deposits belong to the contact-metamorphic group. The. primary ore is essentially unenriched by later sulphides, for supergene 

covellite and chalcocite, although occurring 

locally, are as n rule not abundant. 

Ore bodies of this kind have yielded the 

bulk o~ the metal output. The average tenor 

of the ore mined has ranged from 2. 7 5 to. 6 per 

cent of copper, with gold and silver present in 

traces only. Among the representatives of 

this group are the ore bodies of the Bluestone, 

Mason V nlley, Ludwig, Douglas Hill, Casting 

Copper, and McConnell mines. 

Other types of the copper deposits are sparingly represented in the district and as a whole 

are of minor economic importance. They are 

exemplified by the in·egular bodies ~f rich 

cuprite ore in quartz monzonite porphyry at 

tho Empire-Nevada mine and by the fissure 

veins in quartz monzoni to and quartz monzonite POI'Phyry. 

MINERALS OF THE ORE DEPOSITS. 

PRIMARY MDlERALS. 

A.ctinolite.-A finely fibrous amphibole is 

common in some of tho ore bodies, notably in 

that of the Mason Valley and to a lesser extent 

tho McConnell and West01~n Nevada mines. 

Under tho microscope it is palo green and 

feebly pleochroic, and it is therefore identified 

as actinolite. In some of the ore of the Mason 

Vnlley mine pyrite ·is molded around fibers of 

tremolite, or the tremolite ·"cuts!' the pyrite; 

which of those two explanations is the correct 

one it is impossible to decide, though tho 

former appears the more probable. 

Albite.-Albite was detected microscopically 

as a minor component of some of the gangue in 

the Blu~stone mine, where it occurs tog~ther 

with pyToxene, garnet, epidote, calcite, pyrite, 

tremolit~:, and apatite. 

Andradite._:_The garnet associated with' the 

ores proves invariably to be an andradite. It 

varies widely in color, from pale amber to 

amber-brown, grayish ·green, and resin-yellow. 

Crystals of verj different shades occur close 

together in t.he same hand specimens, and conformably with what Goldschmidt found in the 

.Christiania district, color .proves not to be a 

!eliable index as to whether the garnet is grossularite or andradite. The garnets as a rule are 

partly faceted, the dodecahedral habit being 

.almost universal, but trapezohedral faces are 

occasionally to be found. .-

To determine approximately the composition 

of the garnets, numerous specimens were e~­

amined by the immersion method; they were 

found to have refractive indices ranging from 

1.83 to 1.87 .. Those of index 1.86 predominate, 

corresponding to a garnet in the grossulariteandradite series containing about 80 per cent 

of the andradite molecule. Under the microscope the garnets are commonly found to show 

a wide raJ;lge of optical anomalies. The garnets 

may either be divided into six ~irefringent 

sectors or the _narrow outer zones may be 

birefringent, which is prob_ably the.most common type, or they may show a highly complex 

structure. The maximum birefringence is 0.009. 

Tl?-e garnet rock of the Casting Copper mine 

. consists largely of resin-yellow garnet. Under 

the microscope the deep-yellow garnet, whose 

refractive index is 1.86, is found to be completely isotropic. A paler garnet, whose index 

is L~3, is also present, in part peripheral to the . 

deeper-colored garnet and in part interstitial. 

It is colll:plexly built, is strongly birefringent, · 

.and in places has ·a well-marked parting. 

Accessory minerals are calcite, quartz, chlorite, 

apparently primary, magnetite, and apatite. 

Apatite.-Apatite is an accessory mineral in 

all the contact-metamorphic-ores. It occurs as 

small euhedral prisms embedded in the silicates 

and in the quartz. It is more abundant at the 

Bluestone mine than elsewhere in the distrfct. 

Biotite. -An extremely. close grained black 

rock occurring~ in the footwall of the Mason 

31 
0 

GEOLOGY AND ORE DEPOSIT'S OF Y_ERINGTON-J?ISTRICT, NEV. 

Valley mine proved to be a felsite tuff whose 

. clastic structure has been veiled by the metasomatic development of finely flaky biotite. 

Oalcite.-:-Calcite is common as in_, intersti-

. tial filling in the contact-metamorphic ores. 

The garnet, pyroxene, and allied silicates, as 

well as the quartz and albite, are i:Uvariably 

idiomorphic against it. 

Ohalcopyrite.-Chalcopyrite ·is the sole· primary copper-bearing mirieral found in the 

Yerington district and as such is the ultimate 

source of all the copper in the primary,· oxidized, 

and enriched ores that are mined in the district. 

The chalcopyrite that is inclosed· in an andradite gangue alters commonly during oxidation 

directly to copper pitch ore. In ore enriched 

by supergene sulphides chalcopyrite has altered 

· · to covellite or chalcocite, while the associated 

pyrite has remained intact. -

. · · ·Ohloiite~~hlorite has been noted as a minor 

constituent of ·the ores of the. McCoD.nell and 

Western Nevada ·mfues. 

Epidote.-The silicate epidote occurs most 

abundantly at the Bluestone mine·, where it 

forms the larger ·part of the gangue. ·It has 

originated there principally through replacement of garnet and pyroxene rocks formed dur-

. ing the epoch of _metamorphism following the 

· ~ quartz monzonite intrusion. Epidote is also 

a m·inor constituent in· the pyroxenic and gar-

. netiferous ores ·of the district, in which it is 

."·coeval witn1.the other silicates: The intense 

. .: i ~ : •.• ·pledchloism' a.hd high refractive indices indicate 

." ·.· tha~ :the 'epi~ote ·in these deposits· is a variety 

.. higli iri.;'fehic iron. · 

· · ·· · · · Garnet.~artiet is extremely coinmon. Two 

varieties may ~e distinguished-a fine-grained. 

massive variety commonly near grossularite in 

·composition and a coarse-grained varie~y com-

. ·rrionly: showing crystalline facets and near an:.. 

dradite in composition; the la'tter is the variety 

associated .with the ores,· as described in preceding parag:ra phs. 

Hematite.---Finely · micaceous hemat~te is 

associated with ·the chalcopyrite in the Montana-Yerington vein. 

Magnetite·.-Magnetite. is a rare constituent 

·of the Y erih.gton ores· and as ·a rule 'is detected 

oiily with the microscope. 

Pyroxene~ =----Garnet and · pyroxene are the 

chief ganglie minerals of the contact-metamorphic bi'e. · The relative ·proportions of the 

two vary widely, however, in different deposits; 

in the Mason Valley ore body the gangue is 

prill.cipally pyroxene, and in the Douglas Hill 

ore body it is almost wholly andradite. The 

pyroxene is a graFish-green variety of lamellar 

habit. It ~s coarsely crystalline and commonly 

forms radial groups 2 or 3 inches in dian1eter ,· 

though some as much as 10 inches across are 

occasionally found. 

A carefully selected specimep. of pyroxene 

from ··the 300~foot level of the Mason Valley 

mine·was analyzed by R. C. Wells in the laboratory ·of .·the Geological Survey. Microscopic 

examination showed it to contain a small 

amount of calcite and a trace of pyrite. Calculation of the aria lysis discloses. that the pyroxene is a variety almost exactly halfway :illtermediate ill composition between diopside and 

hedenbergite. Its extinction angle is 45° and 

its maximum refractive index as determined by 

the immersion method is 1.71. 

Analysis of pyroxene from the Mason Valley mine. 

(R. C. Wells, analyst.] 

~i92----------- -- . -. --.... --.... --.. - 50. 59 

Al20 3 __ •• _ • _ •• __ • _ • _ •• __ ••• _ •• _ • , _ _ _ • 11 

Fe20 3 •••• _ ••• ____ •••• ___ •••• _ • _ : __ • ·1. 18 

FeO _ ..... _ .... · ....... __ ..... ___ ... _ 13. 05 

MgO .... __ .-... ___ ..... ___ ... ___ ... ·. 9.19-

CaO .. ___ .. __ ... ____ .. _ ~ _ : _ .... _ .. __ .- 23. 03 

Na20. __ .. __ .:. ___ ... ___ ... _. _. _ ... _ . 15 

K 20 _ ...... : _ .. _ ....... _ ... : ....... _ . 16 

H20-... __ .... ____ .... __ .... ___ ... _ . 71 

H20+ ... __ .... __ ·_ ..... __ ..... __ .... _ . 58 

Ti02 • _. ·.---···-·-···-·--··---·-··-- .13 

Zr02 • __ • _ • ___ · _ •••• ___ ••• ___ • __ •• _ • _ • 02 

C02 • ___ • ___ ••••• __ •••• __ -.· ••• ·• _ ••• _ 1. 52 

P20 5 • _ •• ___ ••••••••••••• ·_ •• ___ ••••• Trace . 

s. -------.---- .. -.-- .. -.- ... ---. ·--- . 02 

Cr20 3 •••• : ........... - •••• - •••••••• • •• None. 

V20a----------·--. : . ..... _ .. _ .. _ .... Trace. 

MnO _ ... ___ ... ~ _ ..... _ .... ·- . _ ..... _ . 26 

BaO .. _ .. _ . _ ..... _ ..... _ . ~ ... _ .- . . . . . None . 

100. 70 

Computed composition of material analyzed. 

Diopside (CaMgSi~0 ). ______________ •• 44.73 

. Hedenbergite (CaFeSi20 6) ..... ___ .... 45. 74 

Alkali pyroxenes: 

K20.Al203.4Si02 -.--.--- ... -- .. - . 44 

K20.Fe20 3 .4Si02.-.---- .. ---.--- . . 50 

N~O.Al .4Si0 .-.---.-.--.-- -·- • 93 

Mg0.Fe20 3 .Si02 .- ••••• -- •••• -------- 1. 04 

Calcite .. __ . ___ . ___ ...... ___ .. _ ~ . . . . . 3. 50 

Quartz .. .'~ ___ ... __ ._ ... _._ ... _ ....... · 2. 04 

Titanite. ____ .... __ .... _._ .. _. __ ...... . 40 

Pyrite .... _.-.- .. __ ................ :.. . 04 

99.36 
',['HE ORE. BODIES; 33 

Pyrite.-Pyrite and chalcopyrite are the the district-a hydrous copper silicate, evionly two primary sulphides in the district. dently of .colloidal ~rigin. Under the microTheir relative proportions vary widely in dif- scope it is' found that probably several different deposits: In the Mason Valley mine, "for ferent minerals are grouped under the 'ter·m 

exmnple, the pyrite averages about twice the "chrysocolla" and that the origjnal ·colloid has 

chalcopyrite in amount, but in· the Douglas gener.ally become crystalline: ·.tSo:J:Q.e chrysoHill mine it occurs in :r;tot more .than traces. colla from 'the D(?ugHts Hill mine ·proves to 

Quartz.-Quartz occurs from place to place be . a cryptocrystalline aggregate .. haying a 

as a mln.or constituent of the contact-meta- ~efractive index of 1.47 and a- barely percep-· 

morphic ores. It incloses idiomorphic garnet, tible birefringence. Other ·chrysocolla, howpyroxene, epidote, and apatite. ever, forms brightly birefrii.tgent ·spherUlites,. 

Tourmaline.-Tourmaline is an accessory whose fibers are· elongated. optically positively 

epigenetic 1ni1,1eral in the biotitized felsite tuff and whose indices are both below 1.54. The 

OCC\lrring in the footwall of the Mason Valley ·copper pitch ore, which is regar,ded as of colore body. · . loidal origin like the chrysocolla, ~xcept that 

Zircon.-Zircon is fo~d as a residual mineral ferric hydrate was precipitated· along with 

in the silicified, garnetized, and pyritized dikes the copper and silicon hydrates, is an isoof quartz monzonite porphyry that occur in tropic substance; in this respect the relation 

some of the ore bodies, notably the Ludwig between chrysocolla and copper pitch ore 

lode. resembles that between chalcedony ~nd ferrif-· 

SECONDARY MINERALS. erous opals, in. that the entai1gled fe~ric-. com-

.Azurite.-The blue copper carbonate azu- ·pound has _prevented_ the. crystallization of the 

rite occurs rarely in the oxidi;z;ed ores. of the colloid. 2 · · . 

district. Under high powers of the microscope the· 

Brochantite.-Brochantite, the basic sul:- chrysocolla, especi8lly that occurring in emphate of copper, occurs as small glassy emerald- . bayments in andradite, shows a brecciated 

green prisms in vugs in the oxidized ore of 'the structure, consisting of multitudes of cri.rved 

Douglas Rill mine. · fragments, which have evidently resulted from 

Ohalcanthite.-Chalcanthite, or bluestone, a8 ·the burating of colloidal membranes by osmotic 

it is generally known in the·- district, formerly ·forces.3 · · • · · 

occurred· in minable qu~tities in the out- Oopper._:.N ativ_e copper ·occurs sparingly in 

crops of most ·of the ore .bodies. It 'is still the oxidized ores of . the district. Iri 'the. 

common in the outcrops at ·the Bluestone. Ludwig mine it is hitimately associated with 

mine, where it forms networks· of · veirilets cuprite iri ore from the 600-foot to the 800-foot 

tra~ersing oxidized gypsiferous material. The levels . 

. veinlets have a cross fibrous structure.· Copper pitch ore.-A brown or brownish-red 

Ohalcedony.-Chalcedony occurs in· sm8ll substance ·of pitchy luster and conchoidal. 

quantity in the- siliceous gossan of the Ludwig fracture is common in the oxidized ores of the 

lode. district. It is locally spoken of as "copper 

Ohalcocite.-As a supergene sulphide chal- oxide" but is .p~rhaps. better termed copper 

cocite is found in small quantity in the dis- ·pitch .ore·, a substance which Von Kobelllong 

trict. It has formed a1:most wholly at the ago showed is a mixture of chrysocolla and 

expense of chalcopyrite and is. generally ·asso_. limonite. Mixtures of cupric and fenic hy-· 

ciated with gypsum. In polished sections it droxides are also included under this term. 

prov.es to be a porous blue_ variety. Rogers 1 According to present conceptions .cop.per pitch 

has presented a photomicrograph of a pol-· oreisdoubtlessacolloidalcomplex9fhydroxides· 

ished section showing chaicop.yrite from the of copper, iron, and silicon and is of variable· 

Ludwig mine partly altered to chalcocite. ·:composition. Copper pitch ore from· the· 

Ohrysocolla.-The term chrysocolla is here 'Greenwood prospect reacts strongly for copper 

used to designate the bluish-green opal-like · · · · · ' 2 Liesegang, R. E., Geologische Diffusionen, p. 100, 1913. , 

substance that occurs· in th~ oxidized ores ·of I 3 Liesegang, R. E., Ein Membrantriiminer-Achat: Centralbl. MineraJogie;·1912, pp. 65-67. Knopf, Adolph, Wood tin in the Terti!iryrhyolites 

1 Rogers, A. F., Min. and Sci. Press, vol.109, p.683, 1914. ofnorthem Nevada: Ecori. Geology, vo1.11; pp. 657-661, 1916. · · 

38330°-18-3 
34 GEOLOGY AND ORE DEP_OSIT'S OF YERINGTON DISTRICT, NEV. 

and iron; examined in oils it pr:oves to he a 

completely isotropic substance having a refractive index of 1.68 .. It is a clear deep brown, 

but it is so strongly colored that. only the 

minutest particles and the edges of the larger 

grains are ti'anslucEm t. · 

Oovellite.-Covellite is common, "though not 

abundant, as a supergene sulphide which has 

been precipitated 1argely at the expense of the 

but is distinctly subordinate to the· copper 

silicates, the chrysocolla and the copper pitch 

ore. 

Opal.-A small amount of_ opal ~as found 

microscopically to occur in the siliceous outcrop 

of the Ludwig lode. 

CONTACT-METAMORPHIC ORE DEPOSITS. 

GEOLOGIC ENVIRONMENT. 

original chalcopyrite in the Ore. It has COMPOSITION OF THE ROCKS INCLOSING THE ORE 

developed in part parallel to what is probably DEPosiTs. 

the pyramidal cleavage of chalcopyrite. Under The principal copper deposits of the district, 

high powers of th~ metallographic microscope characterized by their pyroxene, garnet, or 

the plates of covellite are seen to have exceed- epidote gangue, have, with one notable excepingly irregular edges. . tion, been formed by the replacement of lime- . 

Ouprite.-Cuprite is most abundant in the stones. The Bluestone ore body, the excepore of the Empire-Nevada mine, forming small tion alluded to, consists essentially of chalcomasses in fractured quartz monzonite por- pyrite in an epidote gangue. and was produced 

phyry. It is disseminated through the veins by the replacement of brecciated garnetite and 

and· bodies of copper pitch ore of the Ludwig allied silicate rocks . 

. inine, which are inclosed in the limestone The limestone beds in which· the ore bodies 

footwall of the lode. It extends down in this d·evelopedareshownip.thefollowingparagraphs 

mine to the lowest level at a vertical depth of to have been relatively pure calcite rocks. 

675 feet. Their chemical_composition is a matter of some 

Gypsum. -Gypsum is extremely common in theoretical interest. Lmdgren and others have 

the oxidized ores, occurring not only in veinlets presented evidence to show that ore bodies of 

but also in· plates thoroughly disse~ated · this type have commonly formed in limestones. 

through the ores. During the replacement of of extreme purity. Leith and Mead, 4 however, 

calcite by chrys·ocolla the calcite near the consider the .evidence thus-adduced to be doubtadvancing chrysocolla . becomes filled with ful and inadequate, inasmuch as the specimens 

plates of gypsum,. ~s is well shown at the chosen for analysis represented only single 

Ludwig mine, but the chrysoco~a itself is free -strata, and they emphasize the fact that to obfrom gypsum, showing that the gypsum .has tain evidence of conclusive value samples must 

but a transient existence in this process. be· taken across the full width of the ore zones. 

The clear transparent· variety of gypsum Analyses of the limestone in the ore-bearing 

called selenite is common in the Ludwig mine; zone of the Mason V a)ley mine of theY erington 

it occurs down to the· 600-foot level, where it district are available that in larg~ measure meet 

forms in the vugs in the quartzose ore of the Leith's ·requirement. The limestone was used 

"selenite stope" fine prisms as. much as 8 as smelter flux and hence was carefully analyzed 

inches long. for. lime and silica but less carefully for other 

Libethenite.-Libethenite, a hydrous phos- constituents. A glory hole 200 feet long, .60 

phate of copper, was· noted by RaJ.?.sonie 1 to feet wide, and 75 feet deep was opened up. 

occur sporadically ill the oxidized ore of' the The limestone is mainly in beds ranging froin 

Luclw!g mine in crystalline aggregates resem- 5 to 10 feet in thickness but includes some 

bling malachite. Smith 2 had earlier cliscov- thinly banded varieties. The beds strike north 

erecl this mineral at the Blue J ~y mine, east and clip 70° or ·more westward; the longer ·axis 

of the town of Yerington, and Schaller 3 has of the glory hole is parallel to the·strike of the 

described the crystals found. strata. In column 1 of the subjoined table is 

~Malachite.-The copper carbonate malachite given an averageof seven analyses of limestone 

O<?curs in all the oxidized or:e~ in small amounts from the stope, and in column 2 an average of 

1 Ransome, F. L., U.S. Geol. Survey'Bull.380, · p:113, 1909. four analyses of lim_ estone_from the'' new stope" • 2 Smith, D. T:, California Univ. Dept. Geology Bull., vol. 4, p. 31, 1905. 

a Schaller, w. T., U. S. Geol. Survey Bull. 262, pp. 14Q-143, 1905. • Leith, C. K., and Mead, W. J., Metamorphic geology, p. 148, 1915. 
THE OR.E BODIES •. 35 

situated in the immediate hanging-wall country · 

rock of the ore-bearing zone; from this stope 

lin1estone flux is now being obtained. The 

inclividual analyses do not vary widely from the · 

average figures. 

.Analyses of _limestone from Mason Valle'I.J mine. 

CaO ................................ . 

M~~::. ·. ·. · ... ·::: : : : : : : : : : : : : : : : : : : : : : : : ~~~~:·. •. ·. ·. ·. ·. ·. ·. ·.: : : : : : : : : : : : : : : : : : : : : : : 

C02 (theoretic) ..•..... .-: ........ : ... . 

1 2 

46.9' 

2.3 

9.1 

4.0 

1.1 

39.4 

48.5 

.. 6 

5. 7 

2. 2 

2.3 

38. 7 

102. 8] 98. o 

In these analyses, according to the smelter 

manager, the lime and silica· were carefully 

determined, as these are the constituents 

of importance in calculating the furnace 

charge, and the determinations of the other 

oxides are of nominal value only. The defective summations of the analyses are doubtless 

due to this imperfect determination of the 

minor constituents. 

The analyses concur in showing that the 

limestones of the ore~bearing zone and of the 

hanging wall are relatively pure; they are 

slightly siliceous but ar.e low in magnesia, 

alun1ina, and iron. · 

Two specimens of limestqne obtained from 

the south end of the ore zone on _the ·No. 3 

level of. the Mason Valley mine were analy~ed 

in the laboratory of the ·united States Geological Survey. The great ore lens on the 

south end of this level extends obliquely 

across strata of limestone of different colors, 

ranging f:.;om light bluish white to dark gray 

or black. These beds carry a few fibers of 

tremolite and spora'dic cubes of pyrite. The 

limestone represented by coluinn 1 of the subjoined table, taken 20 feet from the edge of 

the ore body, is a dark-gray, almost black 

crystalline limestone; under the microscope 

it proves to consist ,preponderantly of calcite 

grains, with accessory tremolite and pyrite. 

The limestone· represented by column 2, 

taken from rock immediately adjoining the 

solid andradite of the periphery of the ore 

body,' is a light-gray, rather dense limestone; 

under the nlicroscope it proves to consist 

essentially of calcite and a few;scattered fibers 

of tremolite .. · 

Analyses of limestone from Mason Valley mine. 

[W. C. Wheeler, analyst.] 

1 2 

CaO ................................ . 52. 12 51,63 

Mb~: : ::::. ·:. : : ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ · ~ : : : : 

Al2 0 3 •••••••••••••••••••••••••••••••• 

. 84 3. 17 

3.04 1.80 

1.39 .14 

Fe20 3 (total) ....................... . C02 (theoretic) ..................... . . 64 .17 

41.87 44.02 

99.90 100.93 

These analyses show that the two specimens 

selected are somewhat purer than the average· 

as found by the smelter determinations. The 

analyses are of the same general purport, 

however, and confirm the statements made as 

to the essentially nonmagnesian character of 

the limestone of the ore-bearing zone and their 

low· content of silica, alumilla, and iron. 

The ore bodies at . the other mines in the 

district have formed in coarse calcite rocks,· 

which without doubt are of the same general 

character as the limestones analyzed. 

RELATION OF THE ORE DEPOSITS TO FAULTING AND 

BRECCIATION. 

The ore deposits are situated on or near fault 

contacts. Commonly the faults separate dissimilar rocks, such as limestone from felsite 7· • 

andesite; or stratified calc-silicate rocks. That· 

the faulting occurred before the ore bodies 

were formed ·is established by the following 

structural relations: (1) The faults displace 

the quartz monzonite porphyry dikes, and the 

severed ends of the dikes and the fault drag 

from the dikes have been J?etamorphosed by 

the ore-forming solutions: the brecciated quartz 

monzonite porphyry at the Bluestone ore body, 

which is. an epidote-chalcopyrite deposit, is 

profoundly epidotized and pyritized; and the. 

porphyry inclosed in the Ludwig lode is 

garnetized, silici:fieq, and pyritized. (2) The. 

calc-silicate rocks, such as the fine-grained 

grossularite rocks near the Douglas Hill miner 

have beell: brecciated, and the shattered mass · 

has been recemented .by the coarsely granular 

andradite that was formed at the time the ore· 

was deposited; at the Casting Copper. mine a 

series of thin-bedded black garnetites and allied rocks abut at an angle of 45 ° upon a belt. 

of crystalline limestone; the garnetites were· 

broken ·along the contact, and the angul~ 
36 .GEOLOGY. AND· ORE DEPO.SITS OF YERINGTON DISTRICT, NEV. 

black fragments are inclosed in the yellow an- sive limestone ~as been solidly replaced by 

dradite that forms the gangue of the ore body· coarsely granular resin-yellow andradite for 

These significant structural relations, pointing over 400 feet along its contact with the stratito the considerable faulting that intervened :fied garnetites. The ·garnetites were shattered 

between the time of intrusion of "the quartz al9ng the contact, and the angular fragments 

monzonite and the deposition of the ores and of dense black rocks that resulted therefrom 

proving the dependence of ore deposition on are . now outlined by the thin veinlets . of 

fissuring, will now be considered in some detail. euhedral yellow andradite that traverse the 

The relation of the ore deposits ~o faulting breccia. 

and brecciation is shown most strikingly at Some of the ore deposits, as the Bluestone, 

the Douglas Hill and Casting Copper mines; Mason Valley, and Western Nevada, are situnor is elsewhere shown so. clearly the distinct- rtted. at the contact ·of the Triassic rocks with· 

ness of the ore-forming mineralization as a ·bhe quartz monzonite or the granodiorit.e. 

separate event that took place after the meta- This contact; however, proves to be faulted; 

morphism accompanying the intrusion of the in fact, the granodiorite at the Mason Valley 

quartz monzonite. The ·inineralogically com- mine is a narrow fault wedge lying between 

plex series of calc-silicate rocks on. the west the ore zone and Tertiary rhyolites to the 

flank of Douglas Hill has been described; north. The time of the faulting, except that 

toward the top of the hill these rocks consist it is later than the intrusion of the quartz 

of exceedingly dense, fine-grained garnetites, · monzonite porphyry dikes, is unknown, as is 

composed almost wholly of a·nearly pure grossu- also the amount of displacement. The other 

larite, as shown by its index of 1.75. The sum- garnetiferous pre deposits of the distric.t are at 

mit of the hill consists of an immense mass of some distance froni the la.rge intrusive bodies; 

andradite in which the ore occurs. as irregular that farthest distant-the Douglas Hill, which 

lenses. This great body of andradite resulted is characterized by its extraordinlll'ily extensive 

from the replacement of a massive limestone_ mass of andradite-is 2,500 feet from the 

lying above the stratified series of aphanitic nearest surface exposure of quartz monzonite. 

garnetites. The footwall of the ore zone is a The limestones adjoining the faults along 

belt of remarkable breqcia about 20 fe~t thick. which the ore-forming solutions rose were 

This breccia is composed of sharply angular shattered, anrl there is some evidence that the 

fragments of· the aphanitic garnetites inclosed shattering facilitated or gov~rned the replacein a cementing matrix of coarse, euhedral yel- ment of-the liillestone by the mineralizing solu-

.low garnet. The index· of this garnet proves tions. In th~ Ludwig ore body, which is lodeto be 1.87 and determines the garnet to be a like in form, the limestone was extremely 

.·nearly pure andradite. The garnets of the broken. and crushed by dynamic disturbance, 

two periods of. metamorphism show, ·then, in forniing breccias as much as _80 to 100 feet 

this grossularite hornfels cemented by andra- thick. The garnetiferous quartz-pyrite ore, 

dite their maximal possible contrast. The carrying coarse calcite as a minor component, 

andradite cement carries a little interstitial forms a .large low-grade shoot 50 to 100 feet 

quartz, apatite, and chalcopyrite. wide .. Toward the footwall ·side of the ore 

The dependence of ore deposition on fissur- body, where it grades into unreplaced limeing and the evidence of two distinct periods ot stone and where the mineralizing activity 

garnetization are clearly shown at the Casting was presumably less intense, the result of the 

Copper mine. The ore-bearing zone lies along selective action of the solutions becomes ap-

. the fault contact of a stratified series of bla.ek parent. Here the :finely crushed limestone 

garnetites striking at an angle of 45 ° against that _formed the matrix of the larger angular 

massive limestone, as is shown in figure 12 fragments of the breccia was replaced by py-

(p. 63). The garnetites .are dense, heavy fine- rite; and thus_a limestone bre·ccia cemented by 

grained rocks. Under the microscope they are .pyrite resulted. In the same way the brecciseen to ·be composed wholly of garnet, which is ated .cond1tion of the quartz monzonite porclouded with carbonaceous inclusions, a feature phyry occurring in the Bluestmie ore body has 

that accounts for their black color.· The mas-· been strikingly emphasized by the selective 
U. S." GEOLOGICAL SURVEY PROFESSIONAL PAPER 114 PLATE IV 

A. SPECIMEN SHOWING JU CTION OF UNREPLACED LIMESTONE WITH PORTIO 

ANDRADITE. 

an, Andradite; s, sulphides (ohalcopyrile and pyrite) ; Is, limestone; de, decarbonized limestone. 

GARNET I ZED, SILICIFIED, AND PYRITIZED PORPHYRY FROM THE LUDWIG Mil E. 

g, Garnet; e p, epidote ; py, pyrite. 
THE ORE BODIES. 37 

replacement of the more finely crushed portion and pressures from solutions permeating the 

of the breccia. It was further emphasized by limestone by diffusion or through microscopic 

the fact that the sulphides form margins fractures. . · 

around tho larger fragments of the breccia. From the preceding discussion it appears 

The replacement of fractured and brecciated that the chief ore deposits were localized by 

limestone by andradite- and sulphides is recog- the ascent of the ore-forming solutions along 

nizable along .the periperhy ·of the large ore fault fissures. The presence of limestone 

lens at the south end of the ore zone of the obviously appears to have favorably inMason Valley mine. , The andradite and sui- fluenced the precipitation of ore. The main 

phides have replaced a dark-gray-fine-grained . ore bodies are in the limestone adjoining the 

limestone; the junctio!l between wholly re- faults, but concomitantly with the deposition· 

· plaeod and unreplaced limestone is sharp; and of the ore large bodies of garnet and pyroxene 

the only change shown by the limestone is th~t .were formed, which in amount greatly exceed 

where it adjoins the replaced- rock it is bor- the ore. To find the bodies of ore in this 

dered hy a white band less than h~lf. an ~ch unprofitable envelope is. one of the main 

FIGuRE 5.-0oolog.ic section through tunnel No. 4, M:ason Valley mine. a, Lim~st'one; b, ore; c, garnetiferous rock; d, basic dikes; e, lime-silicate 

rocks with intercalated fclsitcs; /,quartz monzonite porphyry; g, Tertiary rhyolite. 

wide. The limestone· has evidently been decarbonized in this band, but its grain has not 

been coarsened. (See Pl. IV, A.) At the 

McConnell mine, also, there is some evidence 

that the replacement by the mineralizing solutions progressed along .fractures. However, it 

is difficult to obtai~ proof in many of the 

deposits in this district,· as in others, that replacement by garnet, .pyroxene, and allied silicates was governed by the fractured condition 

of the limestone in which they have developed. 

·The conclusion to be drawn from this general 

la.ck of positive evidence is probably that the 

replacement of limestone by garnet arid allied 

silicates is commo:o.ly so thorough that it has 

obliterated the evidence of the premineral 

fracturing. There has consequently been a 

general tendency to regard ore deposits of this 

~nd as having originated at high temperatures 

problems of mining in the district. As a rule, 

not without important exceptions, however, 

the ore tends to occur in the marginal portion 

of the contact-silicate rock where thi~ borders 

the limestone. This tendency shows most 

clearly in the ore .bodies of the Mason Valley 

(fig. 5) and Casting Copper mines. That ore 

is most likely to occur on the limestone s~de·. of 

the contact-metamorphic copper deposits at 

Mackay, Idaho, has been recognized by Umpleby; 1 and in a later paper by the same 

writer 2 this rule is shown to apply to a considerable number of other contact-metamorphic 

deposits.· r_rhe Yerington district broadly sup-. 

p_orts the rule. · 

1 Umpleby, J. B., The genesis of tho Mackay copper deposits, Idaho: 

Econ. Geology, vo1.9,p.321, 1914. 

2 Umpleby, J. B., The occurrence of ore on the limcstonesideofgarnct 

zones: California Vniv. Dept. Geology Bull., vol. 10, pp. 25-37, 

1916. 
38 GEOLOGY AND ORE .DEPOSITS OF YERINGT,ON DISTRJCT, •. NEV. 

FORM AND DIMENSIONS OF THE ORE DEPOSITS. pointing, and some of the deposits· appear 

definitely to have been bottomed. Although 

The o~e deposits that occ~r in fault zones most of the deposits· attain their maximal 

tend to approximate lodes in· form. This dimensions of primary or~ at or near the pres-

.tendency reaches its maximum in the·Ludwig ent surface, others hav~ been found in depth 

ore body, which is dimensionally a typical to be much larger than was indicated by the 

lode.. As the deposits are the result of the. re- surface exposures; of these the Mason Valley is 

placement of limestone by , con~act-silicates, the most conspicuous illustration, showing at 

however, they ·are likely to. depart from the its state of development in 1914 the largest and 

typical lode form from place to place ~ecause most contmuous bodies' of primary ore at 

of irregular embayments -and protuberances depths of 300 to 409 feet below the surthat extend into the adjoining limestone. face. 

Ev~n where the lode form is well developed the . The known vertical range of cupriferous 

copper content is as a -rule distributed errati- rineralization in the district is 950 feet. The. 

cally through the garnet or pyroxene mass that. two points that fix 'the measure of this range 

makes up the main bulk of the lode. Conse- are .the suimn.it of Douglas.Hill, whose altitude 

quently some of the lodes or deposits are better is 5,600 feet, and the ·bottqm leve~ of the·Luddesignated ore-bearing zones. . . ' . wig mine~ whose altitude is 4,650 feet; they ~e ' 

The ore bodies are commonly small fraot10ns so clos~ together that it is practically certam 

of the masses of garnet-pyroxene· rock. As .that this difference in altitude has not been 

already pointed out, the ore tends to occur on ch~nged by postmineral faulting. The vertical 

that side of the garnet-pyroxene bodies which range of 950 feet is, th~n, a minimal_.measure 

adjoins .the limestone. On this side the ore of the vertical distance through which the 

. generally grades abruptly into limeston~;. on ore-forming . ~olutions were able to deposit 

. the opposite side it grades into lean pyritifer-: pyrite and chalcopyrite togeth~r with andraous garnet roc~ al).d' from this into barren-.gar- dite and allied silicates. 

net rock. Such masses of ,barren garnet rock, Concerning the persistence in depth of simin plac~s as much as 200 ~eet thick, u~derlie ilar deposits Lindgren 1 wrote in 1905: 

the productive part of -.the ore zone Ill the Although cases may be easily conceived in which the 

Mason ·Valley mine.. The ·occurrence of the deposits would continue in ·depth and length fo~ several 

ore toward the limestone side of the silicate thousand feet it is far more common to find them Irregular 

masses is well shown in the Mason Valley mine and spotted i~ their mineralization, so that whil~ there is 

(fig. 5), in the Casting Copper . mine, ~nd in no genetic reason why they should not be cont.muous to 

h the ·greatest depth· attainable by mining they will as a the McConnell mine (fig. 8) · On the ot er matter ~ffact often give out when least expected. * * · * 

hand, lenses of ore are erratic~y distriht~ted Few mines on contact deposits have been worked at a 

·throughout the great m~s of an_dra~ite. on greater depth than a few hundred feet. 

Douglas Hill (see fig. 11); and this distribu- This still remains essentially true, except 

tion appears not to be governed by any rule. that mining in recent years has made known a 

The ·ore bodies of the district attain 800 feet few deposits of greater persistence in _depth 

in length and 100 feet in width. These are than was known in 1905 .. The contactmaximal dimensions, and the average are much metamorphic copper deposits on Hetta Inlet, 

smaller, owing to the lenticular habit of the s~utheastern Alaska, have a known vertical 

individual bodies within the ore-bear4tg zones. range of 700 feet;2 at Mackay, Idaho, the ~ange 

As representative of the dimensions of a large is 900 . feet; s and the Marble Bay deposit on 

lens of good-grade ore may be cited the prin- Texada Island, at present the deepest worked 

cipal ore body of the· Casting Copper mine, ·a contact-metamorphic ore body in the world, is · 

large lens 120 feet long, ~75 feet high, and 20 worked at a depth of.1,160 .feet, where good 

feet wide. . ore is exposed. 4 

The greatest vertical depth attained in the ----..,-----------~-=--- b d · f 1 Lindgren, Waldemar, Ore deposition and deep mining: Econ. district is 678 feet, at which large O IeS O Geology, vol. 1, p. 37, 1906. · 

low-grade ore, carrying less_th_an 3 per cent of 2 ~opr, Adolph, u.s. Geol. survey Bull. 480, PP· 99, 101,.1911. 

d I a1 a Umpleby, J. B., Ecori. Geology, vol. 9, p. 313, 1914. 

copper, have . been· develope ·. ll ge~er ' • McConnell, R. G., Texada Island, British Columbia: Canada Geol. 

however, exploration in depth has been disap- survey.Mem. ss, p. 48, .1914. 
THE OR.E BODIES: 39 

METAMOU.PHISM BY THE ORE-FORMING· SOLU- the same type . OCCUrring in other districts. 

TIONS. Apatite is a fairly common accessory mineral 

MmERAL TRANSFORMATIONS. . in th~ _ores formed by the replacement of lime~ 

. The ore-forming solutions in their ascent stone, but it is detectable·only with the ·aid of 

traversed limestone, quartz. monzonite por- the microscope. · . 

phy~·y, felsite, and the rocks formed during the The qua.rtz monzo11ite porphyry dikes show 

first period of n1etamorphism, such as garnetite three different modes of alteration under the. 

and quartz-pyroxene hornfels. They meta- action of the mineralizingsolutions-actinolitimorphosed the limestone most profoundly and ·zati?n, epidptizat~on, .and silicification accom~ 

extensively,. but they also metamorphosed parried by garnetizatwn. All three processes 

strikingly the other kinds of rocks, causing new were ~cco~panied by pyrit~ation. 

minerals to grow in them similar to those _Actmolitized porphyry Is known to occur 

developed in the limestones. only ·as a narrow· dike in the O!e zone· of:the 

The limestones have been replaced mainly Mason Valley mine. It is· a white, highly por- · 

by garnet, pyroxene, .~d sulphides. The phyritic ro~k carrying finely disseminated py- . 

garnet is invariably a calcium-iron variety near . rite. 'l!nder the micr~scope the main change. 

the andradite end of the grossularite-andra~ that ·w:as effe.cted in the porphyry is seen to be_ 

dite series. Son1e of the ·andradite, notably the development of_ha4-like·a~tinolite through-' 

that in the Casting Copper mine, is of pro- out its · grm1~dmass: The actinolite . form~ 

nounced zonal structure, and quantitative ex- numerous sh~·aves .and brushlike aggregates; 

amination shows that the peripheral· zon~ of averaging 0.2 mjllimeter in le:ngth, and furth~r. 

s~ch garnet is somewhat nearer gross'ularite in occurS as c9untless minute· fibers of variou~. 

composition than the core is~ a feature possibly sizes down to the limit of the· resolving power· 

indicating that the ore-forming . solutions o~ the m.fcroscope.. ,N arro:w veiclets of clear· 

changed slightly during the course ·of ore depo- glassy plagioclase··~~~ayerse the porphyry, oecu:-

sition. The pyroxene, as shown by the analysis pying fractures that have not displace~ the.· 

on page 32, is halfway intermediate be.tween phenocrysts, and the new 'feldspars in the· 

diopside. (CaMg(Si03) 2) and hedenbergite ·veinlets have the optical orientation of the 

· (CaFe(Si03) 2). The· ·garnet rocks associated plagioclase phenocrysts in which they are 

with the ore bodies are relatively ·Coarse inclosed. 

grained, and the garnets are more or less In the process o~ epidotization, the r~sults· 

faceted, and in both these respects they differ of which are seen mainly in the Bluestone and 

obviously fro1n the dense aphanitic or micro- Malachite rq.ines, the porphyry was almost comcrystalline garnet rocks of the first period of pletely replaced by epidote, the quartz phenometamorphism. The pyroxene is inva~iably crysts· .alone remaining intact.· Where the 

lamellar and ·forms coarse radial aggregates porphyry has been less profoundly altered, the. 

that are. as 1nuch as 10 inches across. The feldspar and hornblende pheno'crysts have been 

replacen1ent of the limestone by this heden- pseudomorphously replaced by the _epidote. 

bergitic pyroxene indicates that . the ore- The quartz monzonite porphyry was most· 

forming solutions carried abunda:q.t ferrous strikingly transformed by the solutions that' 

iron, as well as ferric iron indicated by the deposited ~he garnetiferous· ore of ·the Ludwig 

developn1ent of ·andradite in the limestone. lode. The porphyry was fractured; through 

Among the accessory minerals formed during the passages thus formed flowed solutions that 

the replacement of the limestone are magnetite deposited andradite, epidote, pyroxene, quartz; 

and apatite. The magnetite occurs only at the calcite,' pyrite, and chalcopyrite; and from· 

Mason ·Valley mine as a macroscopic compo- these veinlets the porphyry was thoroughly 

nent and there only in minute amount; ~lse- metAmorphosed, chiefly by the growth of 

where i.q. the district it occurs only as a rare quart!';, garnet, and pyrite at .the expense o( 

microscopic component.. The practical ab- the pyrogenic minerals. Only the. quartz 

sence of. ma'gnetite and· hematite-..:..minerals phenocrysts,. usually rounded . by magmatic 

generally common in the contact-silicate ores- corrosion, have survived intact;.· in fact, the· 

~istinguishes 'the Yerington ore~ from those ·of transformation of the porphyry has commonly 
.40. GEOLOGY AN~ ?REDEPOSITS OF YERING~ON DISTRICT, NEV. 

been so thorough that the quartz crystals alone There remains to be described the metasogiv.e clues .that. the resultant siliceous pyritic matism effected by the. ore-forming solutions 

.rock was derived .from porphyry. in the limestones ·previously metamorphosed 

. The altered porphyry, whose content of intact py the quartz monzonite intrusion-that is; 

quartz phenocrysts clealj.y indi~ates its igne~us the metasomatic alteration . of grossularite 

origin, proves under the microscope to consist hornfels, or other garnetites, and· of .. quartzlargely of quartz. The quartz phenocrysts are. py:rox~ne hornfels. Alteration of this kind has 

generally unchanged; ·the smoothly rounded ta;ken place on the largest scale in the Blue-

.. outlines of the magmatically corroded crystals stone ore body. .. 

remain intact, even though some of th~ crystals .. The Bl~e$tone ore deposit differs from the 

are surrounded 1:>y broad· .~ureoles of quartz o·thers of the district in that the ore originated 

optically continuous with th{Lt ·of the pheno- through the replacement not of limestone but 

crysts. Some, however, are partly rep~aced of previo~sly formed calc-silicate rocks. The 

by pyrit.e, which is common as elihedral crys- ore body is situated along a·fault that displaces 

tals . throughout the- rest of , the rock. The a quartz monzonite porphyry dike, and the 

·orthoclase and plagioClase phenocrysts have ore has developed through the replacement of 

been so thoroughly.replaced by .q:uartz that t~~ adjacent shattered_ and brecciated _calc-

~o traces of. them . remain .. Garnet,-: sharply silicate rocks~grossularite_hornfels ~nd quartzidiomorphic ~sa rule, occurs sc11ttered through- p;yrox~ne hornfels. ~e mtroduc~10n of the 

·out. the rock, espec1ally in quartz. areas of ore mm_erals chalcopyrite and PJ!Ite was accoarser fabric than the rest of the ground- Companied by the gro~t~ of e~Idote at the 

mass, as shown in Plate IV, B. De.ep-bro~ expen~e -. ?f · t~e preex1st~g mmerals .. The 

Pli~a in finely foliated aggregates is present as ep1d?tiZat10n of t~e brecciated :poq~hyr! has 

a subordinate constituent. Apatite, besides alrea_dy b~en describe~. _The ep1_dot1zatwn ?f 

occurring as large sporadic prisms, obviously the brecmated garnet1te ~s well illustrated m 

-residual from the porphyry, also occurs in Plate V, A, _where the angular fragment~, borinllumerable minute ne~dles, suggestive of dered by e~1dote, are excellently shown; ~the 

addition by metasomatic action. Titanite oc- actual spemmen t~e dark_ ~een of the ep1dote, · · f 1 tal · th 1 ·d of course,. emphasiZes strikmgly the extent to 

curs In a ew arge crys s, as 1n · e 1.tna tere - hi h h" . 1 h · d d d 1 d . Porphyri.es, and · · · w c t 1s _mmera as mva e an _rep ace zrrcon 1s a mmor· a~cessory 

m. al the angular fragments of the brecma. The 

Iner . I b . . all . In- the more highl metamorphosed or- rep acement. so o vwus ma~roscopiC y 1s 

h · · h "d t y d p still more evident under the miCroscope. The 

p yr1es muc ep1 o e an. pyroxene appear 1 - d . ·. · d b inl 

al · th th · t d · t" · rep ace rock 1s foun to e composed rna . y ong w1 e garne , an 1n propor Ion as · . . 

th · al b b d t th lt" of garnet whose refractive mdex of. 1.80 ese mmer s ecome a un an e resu mg . d" · · · · ul · · k b · · 1 diffi It t -d" t" In Icates a compositiOn near gross ar1te; roc ecomes mcreasmg y cu o IS m- · · b d" 0 h · h f t h d 1

. t. pyroxene Is a su or· mate component. t er gu1s rom me amorp ose rmes one. b · f · f fi 1 1 

Th ·fl. b d d f 1 ·t th t f h h recma- ragments CO:Q.Sist o a ne y granu ar 

e ow- an e e s1 e a orms t e ang- bl · f d k f · 11 f th L d · 1 d . d th h b · assem age. o quartz an pyroxene, a roc o 

mg .wa 0 e u wig 0 e m ep . as. een the kind here termed quartz-pyroxene hornfels. 

par~ly replaced '?Y garnet, pyroxene, ~pidote, The garnet shows 'the replacement by epidote, 

pyr1t_e, and calmte. The footwall felsite tuff but it is· traversed py veinl~ts of epidote, 

of the Maso~ Valley ore zone has been alter_ed adjoining· which it is irregularly altered to 

s?mewhat differently. ~uch finely fl.a~y b1~- epidote; in places certain zones of the garnet 

t1te has been develop~d m the tuff, so that It are pseudomorphically altered to ·epidote. 

has _become a fine-grt).~ed black rock. .Tour- Chalcopyrite also occurs as a replacement of 

malme .oc~urs as a minor· accessory mm~ral, garnet and is invariably accompanied ~y epiand th1s ~~.the . only. known. occ~rr~nce of dote~ .. Subordinate components of the ore are 

pneumatolytiC .mmerals associated· With the quartz and calcite; apatite is a minor but conore de.posits of ·the district. The andesite .in· stant accessory mineral. The epidote is rarely 

the footwall_ of . the Mason Valley ore. zone is idiomorphic· except in the veinlets that cons,t)tered by the II.letasomatic growth of pyrite, stitute. the. cement or binding of the ore-bear~g 

epidote, chlorite,. and biotite.. breccia. 
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 114 PLATE V 

A . GARNETITE BRECCIA IN WHICH THE FRAGMENTS HAVE BEEN PARTLY REPLACED BY EPIDOTE,, 

)3LUESTO E MINE. 

ep, Epidote; br, fragments of brecciated garnetite. 

B. GANGUE OF COPPER ORE, IN WHICH ANDRADITE IS INTERSTITIAL BETWEEN PYROXENE CRYSTALS, 

WESTERN NEVADA MINE. 

an, Andradite; pyr, pyroxene. 
THE ORE BODIES. 41 

COMPOSITION OF THE ORE-FORMING SOLUTIONS. gate, the most obvious deductions would be 

The n1etaniorphism effected by the or~-form- that the solutions .were rich in silicon and suling solutions shows that they were charged phur, that they contained sufficient phosphorus 

with · silicon, ferrous and ferric iron, mag- to cause the development of accessory apatite, 

nesium, copper, sulphur, and phosphorus. and that th~y were deficient in aluminum, ferric 

What else they carried remains unknown; that iron, calcium, magnesiu1n, and alkalies. From 

· they carried boron, for instance, is shown by the study of the garnetiz·ed and epidotized por-

. the metasomatic occurrence of tourmaline in phyry it would be concluded that the composithe footwall of the Mason Valley ore zone, .tion of the solutions was. like that indicated by 

but the small amount of tour~1aline may mean the alteration of the limestone, with the imporeither that the solutions were unsaturated tant exception that in reacting with the porwith respect to this mineral or that the tern- phyry the solutions, instead of removing calperatur~ conditions were unfavorable, probably · cium, have added large amounts of this element 

too low, for its formation. The solutions were to the porphyry. The physical-chemical deducmanifestly deficient in calcium, as they removed tion from this ·comparison appears· to be :that 

imn1ense quantities of calcite. . In what form the solutions were saturated with garrtet~ ·hut 

the bulk of this calcite was· dissolved n1ust undersaturated with respect to other c·aldil.'ittt 

ren1ain unknown. That 'it may in part have compounds, such as the chloride; hence,: the 

been carried as calciun1 chloride is a reasonable same solutions were able to replace botli· iimfjhypothesis, ~view of the intensive scapolitiza- stone and porphyry, although the one 'replac·etion shown at some localities by the quartz ment ~eant a subtraction of calcium· and the 

n1onzonite, although, on the other hand, it· other an addition of calcium .. 

must b~ confessed that no scapolite is known 

to be associated. with the ore deposits. For PARAGENESIS. 

every molecule of calcite removed· as chloride . a n1olecule of carbon dioxide would be set free The sulphides and associated silicatesand, proportionally to the p~e~sure existing at ._pyr<_:>xene, andr~dite, amphibole, and epidotethe point of replacen1ent, would aid in the broadly cons~dered, were 

removal of the calcite as bicarbonate. formed essentially at the 

The solutions appear, further, to have been same time. ·Under the microdeficient in sodiun1 and potassium, as is sliown scope seque~~ial relations beby the complete obliteration of the feldspars tween the silicates. are occain the quartz n1.onzonite porphyry and by the sionally observable, in that 

fact that no sericite has been developed in this certain minerals appear to FwuRE 6.-Inclosuro or an 

rock. Biotite however has locally been have continued to grow after idiomorphiccrosssoction . ' . . ' h h d d B or lamellar pyroxene in a formed 1n small quantities. . ot ers a stoppe · ut clinopinacoidal section or 

It is of some interest to compare the com- such relations, clet;tf enough pyroxene. 

position of the ore-forming solutions as deduced in the small areas of certain thin sectwns, 

from their action on li1nestone with their com- do not appear to be supported by any 

position as deduced froni their action on quartz broadly significant evidence ii1 the ore ·deposits 

monzonite porphyry, both ·kinds of rock occur- themselves, and they are th~refore r~garded as 

rmg in the same ore body. From· the meta- evid~nce of minor vagaries in the general course 

somatic replacement of the limestone by of ore deposition. The sulphides appear to 

andradite, pyrite, chalcopyrit~, quartz, and have formed continuously throughout the 

apatite, it would be concluded that the solu- period during which the g~ngue minerals were 

tions were highly charged with silicon, ferrous being deposited. The details. on which these 

and ferric iron, copper, and sulphur and con.:.' conclusions are based are given in the following 

tained sufficient phosphoi·us to develop acces- paragraphs. 

sory apatite; moreover, 'they must have be- The radial and spherulitic pyroxene not uncome highly charged with calcium, derived commonly shows· under the microscope idiofrorn the immense quantities of calcite that they ·morphic sections of one crystal that are 'inclosed 

dissolved. From the study of the silicified in idiomorphic sections of another crystal, :as 

porphyry, essentially. a pyritic quartz aggre- illustrated in figure 6. The relations shown in 
42 GEOLOGY AND ORE DEPOSITS OF YERINGTON DISTRICT, NEV. 

the :figur~ are clearly explicable a~ having silicates than with the calcite. It l.s further 

resulted from unequal rates of growth of two apparent that the sulphides have riot replaced 

different pyroxene crystals growing from con-:- interstitial calcite, because the silicates are 

tiguous spherulitic Cel,lters; had the mineral invariably . idiomorphic against. calcite, and 

whose cross section· is shown been. other than consequently, if the sulphides did replace cal-

. pyroxene, however, it would ~ccording to cite that was formerly interstitial they should 

usage somewhat common-in the study- of ore be molded around idiomorphic garnet and pydeposits· be said to "cut" and replace the roxene; as a matter of fact, however, such a 

pyroxene shown in Clinopinacoidal section. ·relation is rarely or never found. If the sui-

.. The pyroxene in a specimen from thew estern phides replaced interstitial calcite in the garnetNevada mine is sharply idiomorphic, and the pyroxene masses (the calcite being therefore 

associated andradite lies interstitially between essential to their precipitation), it is very 

· the pyrQxene crystals (Pl. V, B); from this remarkable that no sulphides have been derelation it follows that in this particular speci- posited in the limestone beyond the border of 

men the andradite continued to gfow after the the silicate masses. . This· failure of the sul-. 

pyroxene had ceased to grow. Garnet appears phides to occ:ur in the favorable limestone. 

to. have· replaced the earlier-formed pyroxene environment beyond the silicate masses ·is 

to. a _slight extent; but the main bulk of the ·therefore . highly siglli:ficant, and it strongly:· 

evidence; both the field and the microscopic, supports the other evidence of ~he· essential 

points to the essential contemporaneity of the co~temporaneity of sulphides and silicates. 

pyrox.ene and garnet. . · . 'The chalcopyrite is almost invariably anheEpidote, which is -rare -except In· the Blue- dral; the pyrite, however, is generally e11hedral.: 

stone ore· body, is clearly syngenetic with the Cubes of pyrite are inclosed,in-the chalc,opyrite, 

pyroxene and garnet that were deposited by and· as seen under the microscope they show no 

the ore-forming solutions. In the Bluestone sign of fracturing or brecciation. The proore, how~ver, ·it has clearly replac~d the garnet portion: of chalcopyrite to pyrite differs widely 

and pyroxene of the first epoch of meta- in different deposits; in some pyrite is nearly 

morpp:ism; · The replacement is strongly indi- absent, whereas in others it is more than twice 

cated by megascopic evidence, which ·shows it as abundant as the chalcopyrite. The suito have followed the faulting· and brecciation phi des are more closely associated in son1e 

of the replaced rock, and this megascopic evi- deposits with the pyroxene and actinolite, as 

dence is confirmed by the microscope. It in the Mason VaHey ore body, .and in others 

is manifest, .. therefore; that the replacement of .with the andradite, as in the Douglas I-Iill 

the garnet and pyroxene by epidote and sul- deposit. The sulphides are not unqommonly 

phides in the Bluestone ore records an event in embedded in pyroxene, which shows no ti·acethe genesis of the deposit that is of reai geologic of alteration to actinolite, so that it is clear that. 

significance. the pyroxene was not altered to actinolite as a 

Actinolite is most common in the Mason result. of the introduction of the sulphides. In· 

Valley ore deposit, where it occurs irregularly the Bluestone deposit, which differs from the 

distributed through the pyroxenic gangue. · It others of the district ·in that· the ore has redoes not· appear to_ have originated as a result placed garnetite and related hornfelses instead 

of the introduction of the sulphides, nor is there of limestone, the sulphides that were formed by 

any strl;lctural evidence, such ·as faulting or the metasomatic replacement of garnet and 

brecciation of the previously formed ore, to. pyroxene are m variably accompanied by epishow that its origin· was connected· with any dote, which also has replaced the garnet and 

change in- the conditions of ore deposition of pyroxene. This paragenetic relation clearly 

broad geologi(,} significance. 1m plies that ·if during the introduction of the 

The sulphides occur inclos~d in g·arnet, in sulphides the pyroxene was actinolitized the 

pyroxene,- or· in the ·calcite that in places lies sulphides woUld probably not be inclosed in 

·interstitially between the silicates. As the the un{tltered pyroxene. . 

sllica:tes~ strongly predominate ·in the ores of Fibers and prisms of· amphibole· project into 

the dist~ict, it is· clear t_hat on. the whole the or are inclosed by chalcopyrite, but whether 

sulphides=' are_ niore closely ·associated with the this means ·that the amphibole .·"cuts" the-
THE . ORE BODIES. 43 

·sulphide or that the sulphide is later than the 

amphibole and has grown around it is not cle·ar. 

·From similar relations observed in ore from 

Arizona Tolman1 concludes that the amphibole 

is later· than the sulphide it ''cuts," _but this 

conclusion appears to be not more probable 

than the reverse conclusion. It is likely that 

the pyrite bears the same relation to the 

tremolite that the pyroxene in cross section 

· seen in figure 6 bears to the clinopinacoidal 

'sec~ion in that figure. . . 

The facts that veinlets comp'osed of prisms of 

-quartz, dodecahedrons of g·arnet, and sulphides 

break across the structure of stratified garnetites; that veinlets of quartz, calcite, sulphides, 

garnet, pyroxene, and epidote traverse quartz 

· ·monzonite porphyry; and, to take an example 

'on a larger scale, that the Ludwig lode, whose 

gangue, although conf3isting essentially of calcite and subordinate quartz, nevertheless 

·contains notable quantities of andradite all 

~show that the deposition of the silicates, ·per-

·sisted untif qu'artz and calCite ·were. deposited 

simultaneously with them. If, then, irt the 

£:nal stage of ore deposition the silicates, sulphides, quartz, and calcite were deposited 

together, it Is improbable that the silicates 

·would have befm replaced by sulphides, quartz, 

·or calcite in any of the earlier stages of 

-deposition. 

quartz monzonite profoundly metamorphosed 

the ·rocks of the district. It was at this time, 

as was previously shown, that the garnetites 

and calcium-silicate hornfelses were produced. 

After this metamorphism the quartz monzonite 

porphyry dikes were injected, cutting all the 

preexisting rocks · of the district-limestones, 

calcium silicate rocks, granodiorite, and quartz 

monzonite. The .relation of the dikes to the 

metamorphosed limestones-the calcium· sili-

~ate roc~s-l.s of considerable interest. Where 

the porphyries traverse the garnetites and 

other calcium silicate rocks they show chilled 

contact selvages. For example, one of the 

prominent dikes south~ast of Ludwig splits 

'irregularly into branches and incloses masses 

of dense, aphanitic quartz-pyroxene hornfels. 

The porphyry is bordered by a notably chilled 

selv.age, 8 inches wide, within which. it .is 

aphanitic and carries small phenocrysts of 

feldspar _and quartz, 'although the normal rock 

is so crowded with large. crystals as to be 

almos_t granitic in habit. Another dike, which 

is more -than 100 feet. thick,. ·has a chilled 

margin several feet ·wide bordering the garnetite it traverses .. These .observations indicate 

that the. calcium silicate rocks were at a 

temperature low enough' to chill the porphyries 

at the time of intrusion, but whether this 

means a long interval between the time of the 

metamorphism when the calcium silicate rocks 

Til\fE OF ORE DEPOSITION. originated and the intrusion of the dikes is an 

'rhe earlieat plutonic rock to invade the open question, in view of ·the lack of quantitadistrict was the granodiorite; a medium- tive data on the physics of such phenomena. 

grained rock. This was followed by the quartz Although it is firmly established that the ore 

monzonite, a coarse-grained rock, areally the deposition took place after the ill.trusion of the 

most widespread rock in the district. Aplite porphyry dikes, yet the question can be raised 

succeeded the quartz monzonite and· was in whether the dikes are aU of one.pulse of ~truturn followed by an extensive injection of sion. May not ·a pitrticular set of dikes---,that 

quartz monzonite porphyry dikes. Faulting is, a series of granodiorite porphyries-have 

then took place, displacing many of the dikes, followed the irruption of the granodiorite, and 

and the metalliferous solutions. rose along the another series the irruption of the quartz monfaults and formed the ore deposits. zonite ~ If this has happened, it is conceivIt is therefore. important to consider closely able that the faulted and rriineralized dikes are 

the relation of the ore deposits to the porphyry granodiorite porphyries and that their n;rineraldikes and to the whole sequence of. intrusive ization ensued contemporaneously with the 

rocks, for a long lapse ·

0 f time evidently quartz monzonite intrusion and prior to the 

occurred between the consolidation of the injection of the last set of porphyry dikes. Alparent magma and the deposition of the ore. though it can :p.ot be positively affirmed_ that 

'l'he intrusion of· the granodio~{te and ~he there is not a series of grano9-iorite porphyries 

-----------."""'.'-. ________ distinct from a later ser-ies of quartz monzonite 1 ;rotma.n, c.:l~., jr ., Observations on certain, types or chalc?citc a.nd . porphyry dikes . yet . inasmuch as. dikes pet 0 _ thwr characteristic etch patterns: Am. Inst. M:m. Eng. Bull.llO, p. 409, hi . ' ' . · r 1916. . grap cally like those cuttmg the quartz mon-
44 GEOLOGY -AND ORE DEPOSITS OF YERINGTON DISTRICT, NEV. 

, zonite were highly mineralized ·during the· pe-· phos~d, and the pseudoniorphic replacement of 

riod of ore depo~ition, mani~estly· the ores were an undistorted Halobia in typical rock of this 

·formed after the las~ of the porphyry dikes had kind indicates that the garnet resulted not 

been injected. This is most clearly shown at from recrystallization of the impurities in the 

the Ludwig mine. Here the two extreme vari- limestone but from the accession of rl.ew rnaants· of the porphyry dike series are present. terial, principally silica and iron. In view of 

A thick dike of porphyry, characterized by its the extent pf the metamorphosed limestone, it 

almost granitoid habit and lack of quartz is ·evident that silica and iron were added in 

phenocrysts, cuts the hanging-wall country large quantities, but notwithstanding this large 

.rock, terminating against ·the hanging-wall addition, no copper or sulphur were added at 

fault; In the lode itself occurs ·an, aphanitic this time: ·In places the quartz monzonite is 

porphyry whose quartz phenocrysts are its traversed by veins of scapolite as much as 2 

most prominent features; it carries also large feet_ thick and is itself extensively altered to 

phenocrysts of orthoclase and similar crystals scapolite near the veins; but t.his scapolitizaof plagioclase, and it is clearly a quartz mon- tion is the only record that chlorine or any other 

zonite porphyry similar to· those porphyries pneumatolytic agent was active at this stage 

rich in phenocrysts of quartz that are known in the igneous history of the district.. The into cut the quartz monzonite. This porphyry trusion of the .quartz monzonite was succeeded 

h.as been garnetized, silicified, and PTI"itized in. by that of aplite in minor amount and by the 

many places so thoroughly as to be almost be- injection ·of numerous quartz monzonite por-

. yond recognition. There is._little doubt, there- phyry dikes. Faulting then ensued, as is 

.fore, that· >the deposition of the garnetiferous shown by the dislocation of the dikes, and along 

ores-the contact-metamorphic ore bodies- certain of the fault fissures metalliferous solu~ 

occurred _after the injection of the quartz mon- tions rose and formed the ore bodies of conzonite- porphyry dikes. No intrusions geneti- tact-metam·orphic type. A long _interval of 

. cally related to the quartz ~onzonite magma time, therefore, separated the intrusion of the 

succeeded the injection of the quartz monzonite .quartz monzonite and the formation of the ore. 

porphyry dikes. If the ·ores are genetically During this interval the consolidating quartz 

connected with the quartz monz.onite magma, monzonite magma differentiated to some exas. they are believed to be, then their deposi- tent, as the aplite and the porphyry series 

tion was the last manifestation of its hypogene prove; the porphyries, however, do not differ 

energy, which was postponed until long after greatly from the parent m&gma, being only · 

the consolidation of the quartz monzonite now somewhat more salic. 

exposed to view. · Th.e .ore. deposits, as shown by the minerals 

ORIGIN AND CLASSIFICATION. 

The genesis of the ore deposits, although 

anticipateq. · in the previous descriptions, will 

now be .recapitulated and attention will be 

called .to the theoretical bearing of certain 

features of the ore bodies. 

Granodiorite, followed by quartz monzonite 

in far larger amount, invaded the region probably in early Cretaceous time. These intru-

·sions ·profoundly metamorphosed the invaded 

rocks: they caused the andesites and felsites 

adjace:r;tt .to them to recrystallize. and in part 

to become schistose, and they transformed the 

limestones into _aphanitic or ·microcrystalline 

rocks composed mainly of garnet or wollastonite.. Dense, ·heavy garnetites probably pre-

·do:r;ninate among the limestones thus metamorcomposing them, were formed at high temperatures. As the component garnets are generally birefringent, it is probable, from some 

experi:n;t.ental work by H. E. Merwin, that the 

temperatures did not, however, exce_ed 600° C.1 

The ore-forming solutions were therefore at 

high temperatures, but whether they were gaseous or liquid can not be. positively affirmed 

because of lack of data on the concentrations 

of the various constituents in· the solutions. 

Doubtless water strongly predominated. The 

critical temperature of pure water is 370° C.; 

the presence of th~ gaseous· constituents C02 

and H2S would lower the critical temperature/ 

but the presence of the difficultly volatile com1 Wright, C. w.; Geology and ore deposits of Copper Mountain and 

Kasaan Peninsula, Alaska: U. S. Geol. -Survey Prof. Paper 87, p. 108, 

1915 .. 

· 2 Nernst, W., Theoretische Chemic, 7th ed., p. 115, 1913. 
T:H~ ORE BODIES. 45i 

pounds would raise the critical temperature. 1 "recrystallization hypothesi~'!. in its appliWhat was the balance between the~e two op- cability to the Yerington .district, because so 

posing effects in the ore-forming solutions many features of the ore deposits absolutely 

under discussion no one can tell, but the prob- negative its adoption, -even as a tentative exable strong predominance of water makes it . planation. Probably the most powerful single 

likely that their critical temperature was near argument against the · recrystallization hy3700 C. If, fi!' appears probable, a higher tern- pothesis as applied to tllis district -lies in the 

peratl.\re than this prevailed ~uring the synthe- facts that all the impurities in the- limestones 

sis of the garnet and pyroxene, the solutions recrystallized· during the first epoch of meta-

. were therefore gaseous. The physical condi- morphism (and even during this recrystalliza- . 

· tion of such solutions has been a matter of con- tion much iron and silica were added to the 

siderable discussion among geologists, but. that recrystalliz~d. limeston-es)· ~nd that the ore 

it is of any geologic sig~ificance does not seem bodies and their .great masses of associated 

to have been shown. As long ago as 1879 garnet and pyroxene were formed by replace-

.IIannay and I-Iogarth determined that sub- ment and, with one excepti~:m, by the replacestances would act above their critical tempera- ment of relatively pure limestones. The single 

tures · as solvents for difficultly volatile com,- exception, the Blueston~ ore body, affords if 

pounds, and confirmatory evidence has been ·possible, even stronger evidence against the 

since accum-ulating. 2 The geologically im- applicability of the recrystallization hypotheportant substance water has, hdwever, not yet sis, as the ore of this deposit was formed by the 

been experimentally investigated in tllis ;regard, replacement of brecciated garnetite and quartzbut the geologic evidence leaves little doubt pyroxene hornfels. 

that water conforms in its behavior with the The significance of such iron-bearing silicates 

substances experiment~lly investigated and is as andradite and hedenbergite in contacta solvent above its critical temper.ature. metamorphic deposits has been amply emphaA notable feature of the ore-forming :rp.eta- sized in recent years. It appears to have been 

morphism is that some ore bodies, or large first. recognized by Lacroix, 4 who described a 

portions of them, consist almost wholly of contact deposit of magnetite-a deposit that, 

pyroxene plus some sulphide,· whereas others he says, is of a high theoretic importance, as it 

consist essentially of andradite. A mass that strongly indicates the acces~ion .. of substance 

is composed of diopside-hedenbergite (a silicate from the granite magma. · The silicates accom:.. 

of calcium, magnesium, and ferrous iron con- parrying the magnetite are iron-bearing varitaining 50 per cent ·of silica) m~st differ widely eties. Lacroix says: 

in chemical composition from a mass composed · The garnet is ·not grossularite but melanite; the pyroxof andradite (a silicate of calcium and ferric ene is not light-co.lored diopside but a diopside near hedeniron containing 36 per cent of silica). That bergite; and the amphibole, instead of being faintly tinted ·· 

these widely differing bodies of silicate rock in pale green in thin section, is extremely deep gree-p. and · . 

have formed within. the same limestone strata is a highly ferriferous hornblende .. 

appears to oppose insuperably any attempt to Later work, especially· by ·Kemp and Lindapply the theoret~cal views of Leith ~to account gren, has abundantly confirmed Lacroix's estifor the silicate masses of the Yerington district. mate of ·the theoretic importance of these 

If, in accordance with these views, the deposits paragenetic relations. 

had resulted largely ·from elimination of the Probably the .most noteworthy fact concalcite in the limestones and recrystallization nected with the occurrence of the a~1dra.dite in 

of the residual impurities, then the chemic"al the Yerington district, aside from its general 

composition· of the various silicat~ masses association with the ores,· conformably with 

throughout the district should show a consid- what is now recognized as the rule that the 

erable degree of uniformity. It is perhaps garnet of contact-metamorphic ore deposits 

hardly necessary to ·apply this . test to the is generally andradite, is that along certain 

1 Dooko, ll. K, Grundlagon dcr physikalischcn-chcmischen Petrographic, pp. 261-263, 1915. 

2 Idem, pp. ·261-268. 

• a I..oith, C. K., UocrystalJization of limestone at igneous contacts: 

Am. I.nst. Min. Eng. Trans., vol. 48, pp. 209-214, 1915. 

faulted c'ontacts the andradite forms the binding material of breccias made up of angular 

c Lacroix, A., Le granite des Pyrenees et scs phenomenes do contact: 

Services carte geol. France Dull. 71, pp. 9-11, 1900. 
46 GEOLOGY AND ORE DEPOSITS OF .YERINGTON DISTRICT, NEV. 

fragments of grossularite rock. or allied contact per and sulphur, it was rather localized and 

rocks formed during the first period of .meta- resulted in ore deposition. · 

morphism. · . . At this point' the questions arise: Are the 

The distribution of the '-'contact deposits" pyroxenic and garnetiferous copper deposits of' 

throughout the district independent of dose the Yerington district properly called contactproximity to the intrusive rocks is due to the metamorphic ore bodies? What, in fact, are 

thermal history of the ,dist:dct. The long the distinctive features of contact-metamorphic 

interval separating the initial intrusion of the ore deposits? Three criteria for distinguishing 

granodiorite from the final injection of the deposits of this kind are generally recognized as 

qUtartz monzonite porphyry dikes caused the valid-(1) that the deposits commonly occur in 

invaded rocks to become highly heated and limestone at or near its contact with an intru--

temperature gradients to flatten. 1 Conditions sive igneous rock; (2) that the sulphides are 

had therefore become favor~ble to the devel- associated with contemporaneously formed oxopment of ''contact deposits" at considerable ides, chiefly of iron; and (3) that the metallifdistances from the igneous contact. Superim- erous minerals are associated with contact-

. posed upon this favorable condition was an- metamorphic silicates, among which garnet, 

other, possibly even more favorable in this pyroxene, epidote, and amphibole predominate. 

respect, namely, that the ore-forming solutions When actual usage is examined, however, 

· ascended along fault channels, thereby extend-: It is found that the third criterion~ the presence 

ing to long distances from the igneous contacts of "contact" silicates-is the sole diagnostic 

·the temperature conditions necessary to form feature., Visible· nearness to an irruptive rock 

garnet, pyroxene, and allied silicates. The is not necessary, though opinions will differ as 

maximum distance from the nearest visible to what constitutes' 'nearness." For example, 

granular igneous rock at which an andradite the copper deposits of Phoenix, British Comass has formed is 2,500 feet. lumbia, consisting of chalcopyrite, pyrite, and 

As the ore bodies originated at high temper- hematite in a gangue of epidote and andradite, 

atures, they are clearly linked genetically to are from 1 to 2 miles distant from the nearest ex-

. the igneous intrusion, but whether they were posure of the granodiorite batholith· to .which 

deposited by magmatic emanations or by they are genetically related; 2 but this distance 

meteoric waters stimulated by the he~t of the appears to be the greatest at which ore bodies· 

intrusion the evidence does not show so of this. kind are known to occur. Moreover,. 

clearly. The structural complexity of the according to Lindgren/ ''Some ore deposits of 

. rocks, however, would appear to have been un- the contact-metamorphic type occur at a· disfavorable -to the establishment of a systematic tance from. any intrusive .rock to which .their 

circulation of meteoric waters. The conclu- origin could be attributed "-a. st.atement of . 

sion is thus reached that the ore deposits are itself clearly implying that ore deposits of the 

causally connected with igneous rocks,· if not contact-metamorphic type ·are . recognizable 

. consanguineous, and that .they were not formed otherwise than by their nearness to intrusive 

until oonsiderable time had elapsed after the masses. Of the seven types of deposits that 

intrusion of the quartz monzonite. FurtJ:!er- Lindgren groups under the contact-metamormore, it follows that if the depos~ts 'Yere. phic class many representatives are known in 

formed by magmatic.solutions, as is regarded which the sulphides are not associated with 

the more probable, the two distinct periods of magnetite or hematite. The one distinctive 

metamorphism, which have been discussed feature of the contact-metamorphic group, 

under the designations "metamorphism of the then, is the association of metalliferous minerals 

first period" and "ore-forming . metamor- · with contact silicates. These silicates are as 

phism," show that ·material was supplied by a rule characterized chemically by the domithe magma or magmas at two separate and nance of calcium among the positive radicles; 

distinct times; during the first metamorphism they are the mineral witnesses of the fact tha't 

the addition of materifil was rather widespread the ore deposits containing them were formed 

in its scope, and during the later, which was 

characterized especially by the addition of cop-. 

1 Daly, R. A., Igneous rocks and theirorigin, pp. 198-:199, 1914. 

2 LeRoy, 0. E., The geology and ore deposits of Phoenix, Boundary 

district, British Columbia: Canada Geol. Survey Mem. 21, p. 67, 1912. 

s Lindgren, Waldemar, Mineral deposits, p. 675, 1913. 
THE ORE BODIES. 47. 

:J" 

at high. temperature m a limestone environ- throw previously mentioned. It occurs where 

ment. the faul~ traverses limestone, but where the 

Coniact-metamorphic ore deposits may be fault traverses granite or quartz porphyry 

regarded as a special facies of one of the two greisen occurs, consisting of muscovite, quartz, 

forms of contact metamorphism recogniz~d by pyrite, and locally much specular hematite. In 

petrologists, namely, as a facies of pneuma to.:. short, the greisen and the contact-metamorphic 

lytic contact metamorphism,. as distinguished ore bodies were formed by the same solutions; 

from normal contact metamorphism such as is they are therefore genetically coordinate, but 

typified by the classic example of Barr-Andlau. this close kinship is not denoted qy the termi- · 

The special facies is of course due to the occur- . nology usually employ~d; and this leads again 

renee of sulphur and of heavy metals such as to the previously expressed conclusion that the 

copper in the·metamorphosing,solutions. Now, distinguishing features of contact-metamorphic 

the characteristic features of pneumatolytic ore deposits result from their formation at 

contact action are that it takes place ~fter the high temperature in a limestone environment, 

normal metamorphism has been effected and and that the sole criterion diagnostic· of these 

that it is localized along fractures and :fissures, deposits is the presence of contact silicates, 

instead of forming a continuous aureole of which are generally of calcic composition. 

metamorphic rock encircling the intrusive Fr.om the foregoing· discussion it appears 

rock. . These relations have long been specially neces~ary. to conclude that the Yerington deemphasized by Lacroi.x,t Rosenbusqh, 2 and posits are· clearly members of the contactother petrologists and may be considered as metamorphic ciass .. The special features of 

well-established laws of _petrology. They are broader import as to the theories of ore deposiparticularly well illustrated in the Christiania tion that they illustrate exceptionally well are 

region, as recently described by Goldschmidt.3 (1). the long time that intervened between the 

In this region the prieumatolytic contact meta- formation of the ore deposits and the conmorphism prodticed num~rous argentiferous solidation of the magma on. which they are 

zinc-lead ore deposits having a gangue of andra~ genetically dependent; (2) the great amount of 

elite and fluorite. These deposits are sit~ated material, prmcipally iron. arid silica, that was· 

along faults, ·the fault at Glomsrudkollen; for added to the sedimentary rocks during this 

example, having a throw between 1,000 and interval, without traces of .ore deposition; and 

2,000 meters. 4 (3) the genetic dependence of the contactAmong economic geologists there ·appears 'to metamorphic ore on fault fissures, along which 

be some hesitation to classify the deposi.ts of the fractured or brecciated limestone has been 

this kind that occur along fissures as typically metasomatically replaced by garnet, pyroxene, 

contact metamorphic. According to Krusch,5 and sulphides. 

where fissure .fillings have resu1 ted from con tactmeta.morphic agencies, the distinction between 

contact deposits and fissure deposits becomes 

difficult. That the attempted distinction is 

rather artificial is shown, for example, by a 

consideration of the geologic relations of' the 

contact-metamorphic sphalerite deposit at 

Glomsrudkollen, just referred to. The ore of 

this deposit consists of sphalerite, pyrrhotite, 

and pyrite in a gangue of fluorite and andra-

.dite, .and it is localized along the fault of latge 

1 Lacroix, A., Le granite des Pyrenees et ses phenomffiles de contact: 

Services carte gcol. l~rance Bull. 71, pp. 62-67, HJOO. 2 Hosonbuscl~, H., Mikroscopischo · Physiographic dcr massigen Ge· 

stci.nc, 4th cd., vol. 2, pt. 1, pp. 123-127, 1907. 8 Golclschmidt, V. M:., Die Kontaktmctamorphoso im Kristian.lagebiet, 

pp. 70, 86, 108, 1911. 

• Idom, p. 88. . 6 Krusch, P., Dio Classification der Erzlagerstatten von Kupferberg 

in Schlesien: Zeit s~br. prakt. (leologie, Jabrg. 1901, p. 228. 

OX~ATION AND SD:LPHIDE ENRICH¥ENT .... 

. The ores in genera~ are not oxidized extensively or to great depths. Residual sulphides, 

in fact, commonly occur in the outcrops of the 

oxidized ores. Enrichment by supergene sulphides has not been important. The bulk of' 

the copper output of the district has come 

from primary ores. Water level has not been 

reached in any of the mines except the Ludwig, 

where it was struck at a vertical depth of 500 

feet. · 

The notable features of the .oxidized ores are 

the prevalfmce of gypsum, chalcanthite (bluesto-ne)' .chrysocolla, and copper pitch ore. 

Chalcanthite was formerly abundant enough 

to be mined to supply the amalgamating millsof the Comstock lode; it is even now common 
48 GEOLOGY AND ORE DEPOSITS. OF YERI-NGTON DISTR.ICT, NEV. 

in the oxidized ores yet remaining, especially 

of the Bluestone mine. Its prevalence is of 

course a direct result of the existing arid climate. The oxidized ores are permeated with 

gypsum, which in places forms ·plates as m~ch 

as an inch in diameter and a quarter of an inch 

thick. Narrow veinlets of gypsum are also 

common.· The gypsum in the- oxidized ores 

was derived from the action of sulphuric acid 

on the interstitial calcite of the gangue. and 

probably to some extent from the action of the 

acid on the andradite, which, as Bergeat 1 

has shown, is easily decomposed by dilute 

sulphuric acid. Its prevalence proves that 

throughout the history of the ore bodie_s the 

downward movement of oxidizing water has 

·been sluggish-so sluggish as to be unable t.o 

remove so soluble a compound as gypsum. 

This imperfect lixiviation in the zone of oxidation is clearly due to the general imperviousness 

of the massiv~ garnet-pyroxene ores. . 

Chrysocolla and the related copper pitch ore 

are relatively abundant among the oxidized 

compounds. ·Their pr(;)valence is due to two 

factors; first,_ the andradite, easily decomposable by weak sulphuric acid, furnished abundant colloidal sili~a, and, second, the rapid 

neutralization of the -sulphuric acid (in which 

chrysocolla and copper pitch ore ·are readily 

soluble) by the calcite ~nd the andradite of 

the ore allowed their ready precipitation. 

The ores have not been notably enriched 

by secondary (supergene) sulphides, and moreover, the zone of enrichment is not sharply 

separated from the zone of oxidation. The 

supergene sulphides occur as a rule as· soft, 

sooty bluish-black. or black material. They 

have formed chiefly at th~ expense of the chalcopyrite originally present, and in some ore 

this action has gone so far that the chalcopyrite has wholly disappeared, leaving- _the pyrite 

essentially intact. This selective replacement 

·is common throughout the district and is perhaps the most noteworthy feature of sulphide 

enrichment that the district exemplifies. Covelli te and chalcocite are the two sulphides 

formed during enrichment. They are invariably .as~ociated closely with gypsum, which not 

only occurs as plates io.tergrown with them but 

also penetrates them in veinlets. It is found as 

f~r down as enrichment extends. 

1 Bergeat, Alfred, Der Granodiorit von Concepcion del Oro ini Staate 

Zacatecas (Mexiko) und seine Kontaktbildungen: Neues Jahrb., 

B eilage Band 28,- p. 534. 1909. · 

The covellite, as may be seen on. polished 

sections of ore, has tended to replace the chalcopyrite along crystallographic directions, 

forming triangular and rhombic patterns; 

obvious even to the unaided eye.1 Possibly 

these directions are those of. the pyramidal 

cleavage (201) of the chalcopyrite, which Dana 

says is sometimes distinct; but etching with 

potassium cyanide failed to bring out the 

cleavage, so that this supposition is unverified. 

In general, however, the covellite traverses the 

chalcopyrite ill networks of minute veinlets; it· 

is from some of these that branches. extend 

parallel to the supposed cleavage. 

Chalcocite, like the cov.ellite of the district, 

has for~ed chiefly at the expense ·of the pri...: 

mary chalcopyrite. It was more abundant 

at.the Ludwig mine than in any other deposit 

of the district. 

The ~onditions of oxidation and sulphide 

enrjchment iR the Ludwig lode differ rather 

markedly from those_ prevailing in other deposits of the district. These differences have 

resulted from its lode form, its siliceous character, and its greater perviousness, due to post-· 

mineral shattering. · Owing to this combination 

·of circumstances the lode .has been. oxidized 

and leached down to water level, "at a vertical 

depth of about 500 feet. Below the 'leached 

zone is the chalcocitized zone, at its maximum 

not much over 100 feet in vertical extent and 

·averaging much less than this. In view of the 

large_ quantity of highly leached gossan above 

water level, the amount of chalcocite formed 

was disproportionately small. This meager 

enrichment is explicable as due to the fact that 

th~ lode, which dips 70~ E., is underlain by 

limestone. ·The acid copper-bearing solutions 

formed by the oxidation of the primary pyrite 

and ·chalcopyrite flowed in large measure into 

the footwall, their ingress into which was facilitated by the pos.tmineral faulting and shattering. Once· in this calcitic environment the 

acid was :neutralized and the copper was. precipitated . as carbonates, copper-pitch ore, 

chrysocolla, and cuprite. The ore thus deposited shows a r.ather well defined layering 

parallel to its iri.closing walls, and much .of it. 

plainly fills solution cavities in the limestone. 

On the other hand, the local replacement . of 

1 A. F. Rogers (Secondary sulphide enrichment of copper orcs with 

special reference to microscopic study: Min. and. Sci. Press, ·vol. 109, 

p. 684, fig. 5, 1914) has described a similar replacement of chalcopyrite 

by covellite in ore from the Rambler mine, Wyo. 
THE ORE BODIES. 49 

lime~tone by chrysocolla is remarkably well . ore in quartz monzonite porphyry are being 

shown. In conclusion, the relations shown at worked, but the primary · sulphic!es from 

the Ludwig mine suggest ~hat had not the lode., whi~h these have been derived have not so far 

been underlain by limestone large amounts of been disclosed. 

copper would have been dispersed· and· not . In the granodiorite hills east of the town of 

rcprecipitated and so would have been lost be- Yerington, an ·area not shown in the map 

yond recovery," and that therefore the. process accompanying ·this report,. occur a group of 

of supergene sulphide enrichment is iikely to be fissure veins in which chalcopyr~te and· pyrite 

extremely w~steful of copper. are associated with specular· hematite in a 

FISSURE VEINS AND OTHER DEPOSITS.> 

In addition to the . contact-metamorphic 

. deposits a few fissure veins and related deposits 

occur in the. district, but. none' of thenl have 

yet attained economic prominence. The only 

fissure vein that was being worked in .1914 

was the l\1ontana-yerington, in which the 

ore bodies .. occur in a crushed and foliated 

zone in granodiorite. The ore consists chiefly 

of chalcopyrite with minor amounts of euhedral 

pyrite, associated with finely disseminated, 

plates of specular hen1a ti te. At the EmpireNevada mine scattered bunches of c~prite 

gangue of quartz.·. Representative of these 

are the New Yerington and the Butte & Yerington; at the Black Rock prospect the chalcopyrite. is inclosed in quartz that carries 

much magnetite .intergrown with specularite. 

A few narrow quartz veins are inclosed in 

the quartz monzonite in the southwest corner 

of the mapped area, a few miles sbuth of 

Ludwig. They carry pyrit~ and chalcopyrite 

and in one a minor amount of galena-the 

only occurrence. of a primary sulphide in the· 

district other than pyrite and chalcopyritebut the value. qf these ores is in their gold 

content .. 

. ~ .. . .. ,.\ 
PART IlL MINES AND PROSPECTS. ~ 

MINESAND.PROSPECTS ON CONTACT-METAMOR- diopside and· tremolite.. Underlying these are 

P~IIC DEPOSITS. · darker-colored lime-silicate rocks, mainly dense 

BLUESTONE MINE. fine-grained garnetites, which· constitute the 

The Bluestone mine, the oldest in the dis- colllitry rock in .the immediate neighborhood 

trict, is 1! miles west of Masori, at an altitude of the mine. Associated with them, however, 

of 5,400 feet. · In the seventies it supplied are a few strata of qua~tzite-like appearance, 

natural bluestone to the am~lgamating mills which are found under the microscope to be 

on the Comstock .lode. La.ter :a small smelter composed of quartz and pyroxene. These. 

was built near the mine and WaS Tun· for a lime-silicate rocks form a belt about 1,000 feet 

time on oxidized ore, but the high cost of fuel, wide at the niin.e, which is bounded on both. 

which had to be hauled from the railroad at sides by faults-one on the west· separating 

W abuska, soon brought operations to a close. it from granodiorite and· one on the east 

In recent years work has been restricted separating· it from rhyolite. ·(See Pl. III; 

mainly to exploration and development of section 0-C'.) Both fault contacts are well 

the ore. In 1917 the mine b~came produc- .shown in the main adit; that between the 

tive, supplying 1,000 tons of ore a day to the rhyolite and lime-silicate rocks is 210 feet 

smelter .at Thompson. · , from the portal and dips 77° E., and that 

The mine is opened by two adits. The between the lime-silicate rocks and the grano..: 

lower or main adit, whose portal is at an diorite is near the face of the. adit and dips 

altitude of 5,151 feet, entering fron1 the east, 35° E. These faults converge northward, . 

undercuts the ore-bearing zone at a depth of 300 causing the lime-silicate belt to narrow abruptly 

feet, 780 feet from the portal. The principal in that direction. 

development, however, is on the upper or One of the quartz monzonite porphyry dikes 

100-foot level, which connects with the surface is fairlywellexposed at the "head frame shaft.'' 

through an adit entering from the. west at an It is at least 50 feet thick, is ~ighly shattered, 

· altitude of 5,370 feet. · This level consists of and locally is so thoroughly epidotized as not to 

·an irregular ri.etwor~ of drifts and crosscuts be easily recognizable. It. is cut off by a 

intersecting the ore body. Considerable wo;rk transverse fault and displaced somewhat over 

has also been done on the 200-foot level, but 100 feet to the east. This fault appears to have 

comparatively little on the three .levels· or been the principal channel along which ore-· 

sublevels below this. The total development forming solutions ascended. 

work aggregates about 7,000 linear feet. The ore body in plan, as shown on the 100-

The ·rocks in the vicinity of the Bluestone foot level, is roughly elliptical. The major 

mine consist principally of highly metamor- axis, which trends a'bout N. 30° W., is 400 feet 

phosed limestones,. the so-called calc-silicate ·in length, and the minor axis is 200 feet in 

hornfelses, which on the west have been faulted length. The area thus circumscribed is, howagainst granodiorite. Two thick dikes of ever, not uniformly mineralized, and 'p.arts of 

quartz monzonite porphyry intersect the lime- it are in fact of too low grade to be ore. The 

silicate rocks, and an aplite dike occurs a northwest end of the ore-bearing area coinshort distance north of the mine. cides · with the fault along which the quartz 

The details summarized in the preceding mo!l!l'<onite porphyry dike has been displaced, 

·paragraph will now _be presented. White and here the ore formed a ·rich lodelike shoot, 

lime-silicate rocks, interstratified with beds of a8 stoped out in drift 106. 

white marble, make up the north end of the · The ore-bearing zone is separated on the 

ridge extending north from the mine. They footwall side from the barren country rock 

strike N. 85° E. arid dip 70° N. They con- by a well-defined fissure, locally known as -

sist largely of wollastonite, with subordinate "the footwall," which trends. N. 15° W. and 

50 

• 
MINES AND PROSPECTS .. 51 

dips 35° E., steepening to 45°. on the 200- and destroying it. The end result of intensive 

foot level. The ore does not, however, extend action of this kind was chalcopyrite-epidote 

everywhere to the footwall. Toward the ore. 

hanging wall the copper contep.t gradually. The outcrop of the Bluestone 9re body 

diminishes, but some of the' hanging-wall was· largely covered with soil and hence 

country rock is said to average 0.75 per cent inconspicuous. · The barren capping is thin, 

of copper for 200 feet beyond the limits of however, nowhere exceeding 10 feet. The 

the com1nercial ore. The country rock of the surface workings disclose notable quantities 

footwall zone, of the hanging-wall zone, and of blues tone· and gypsum. Both of these minin fact in all barren portions of the mine erals ramify through the oxidized material in 

differs in appearance from that of . the ore- veinlets characterized by cross-fiber ~tructure .. 

bearing parts in that the barren rock is com- Unal~ered nuclei of chalcopyrite, :P.owever, 

posed of silicates that are lighter colored than are present~ the oxidized ore, and in general 

the epidote that is characteristically associated the depth to which oxidation has progressed 

with the ore. As verified under the microscope is inconsiderable, in few places extencl.i.J?-g to 

the barren rock is made up chiefly of fine- the 100-foot level. Enrichment by supergene 

grained massive garnet.· . sulphides is unimportant; sporadically the chalThe ore ·consists ·of chalcopyri~e inclosed copyrite has been in part altered to steely chalin a gangue consisting predominantly of co cite, but in general the small amount of enrichfinely granular epidote with subordinate quartz ment that has occurred has produced only sooty 

and calcite. Owing to th,e prevalence of films on the chalcopyrite. . 

epidote the ore is. of dark-greenish <?olor, a The Bluestone ore body attains its maximal 

·feature that distinguishes the ore-bearing rock dimensions on the 100-foot level; it is confrom the barren garnetite. The chalcopyrite siderably smaller on "the 2~0-foot level,· and · 

occurs as grains and irregular small masses; in th_e lower workings the me~allization, which 

pyrite is absent from the main ore body but is mainly pyrite, is clearly confined to narrow 

app~ars in the low-grade material of the fissures traversing dense, massive garnetite. 

hanging-wall country rock. The ore shows in The quantity of ore blocked out is estimated 

places an obscure breccia structure,. which by the manager as 1,000,000 tons, averagmg 

is due to the fact ~hat s:uch ore is composed of 2.5 per cent of copper. 

angular fragments of garneti~e, of quart~­

pyroxerie hornfels, or .of porphyry,. which 

have been cemented by chalcopyrite, epidote, 

quartz, and pyroxene. The breccia structure 

is particularly .well displayed near· drift 121, 

where the quartz ·monzonite porp4yry dike 

has been crushed and the angular fragments 

thus produced have been epidotized and sur:.. 

rounded by sulphides. The breccia structure 

has in this way been strikingly emphasized. 

That the epidotization ·and the deposition 

of chalcopyrite took place together is obvious; 

and .if in other parts of the mine the breccia 

structure is generally less evident than in drift 

121 it is probably due to the obliteration of 

the garri.etite fragments through replacement 

by epidote. Some of the ore, however,· incloses 

fragments of garnetite, and· when examined 

under the microscope clearly shows not only 

that epidote along with the other rrlinerals 

:fills the fractur·e, but that the ·epidote has 

grown a~ the expense of the g~rnet, invading 

. MASON VALLEY MINE. 

IDSTORY ;AND DEVELOPMENT. 

The Mason Valley mine is 1! miles west of 

Mason. Between 1870 and 1875 the mine 

flirnished considerable bluestone to the mills 

on the .Comstock lo9-e.· No large output of 

copper ore was made, however, until after the 

mine was acquired by the Mason Valley Mines 

Co. Active development hi this company 

began in March, 1907, but copper was not produced unt~l afte.r the·smelter, built by the company at Thompson, 16 miles from the mine, 

was blown, on January 6, 1912. The development work don~ by the company aggregated 

23,107 feet at the end of 1914.. · 

The mine yielded 98,912 tons of ore in 1912, 

114,854 tons in 1913, and 75,038 tons in 1914. 

The total ·production aggregates 288,900 tons; 

which averaged .2. 75 per cent of copper. AD.· 

estimate of the ore reserves available in 1910 

was made· by the company's engineer as fol-
GEOLOGY AND ORE DEPOSJTS OF YERINGTON DISTRICT, NEV. 

lows:1 1,074,635 tons," assaying 3.95 per cent of 

copper; 92,000 tons, assaying 2.38 per cent of 

copper; and ·100~000 tons, assaying 1.90 per 

cent of copper. 

On account of the break in the price of copper during the early months of ~he European 

war, the mine and smelter were forced to shut 

down :in the middle of October, 1914. They 

remained idle during 1915 ·and 1916 but reopened in 1917. 

· The min~ is developed principally by. crosscut tunnels, the upper one of which cuts the 

ore zone at a depth of 9o feet. . The: main 

tmmel; known as No. 4, is at ari altitude of 

5,096 feet; it cuts the ore at 1,200 feet from 

the portal and attains a depth of 410 feet below the outcrop. The ore is hauled out of the 

nline by electric traction and delivered to an 

aerial tramway, 6,250 feet long, which connects the niine with the railroad at Mason. A 

minor level, 70 feet below No. 4 tunnel level, 

has also been opened. The ore zone. has 

further been explored by diamond drilling. 

The shrinkage stope system is the method of 

mining employed; the ground stands well, and· 

little tirilber is required. · 

GEOLOGIC FEATURES. 

The ore bodies of the ·Mason Valley mine 

consist of irregular lenses of pyrite and chalcopyrite in a· gangue of pyroxene and garnet. 

These ienses occur in a garnetized zone which 

lies between a· footwall of felsite tuff-breccia 

and andesite, and ·a hanging wall of limestone. 

The ore-bearing zone is about 1,200 feet long 

and from 150 to 375 feet wide. It is terminated on the north by faults, which have 

dropped a wedge of granodiorite between ·the 

~re zone and the.. Tertiary rhyolites on the 

north. 

Th~ geol<?gy in the vicinity o~ the mine is 

complex, as may be· seen from inspection of 

figure 7, in which the distribution of the rocks 

is shown in considerable detail; its complexity 

is due to the profound metamorphism that has 

taken place here arid to the severe faultfugthat has occurred. during several epochs of 

disturbance. The most impressive evidences 

of metamorphism are the large areas of limestone and felsite that have been converted 

completely into garnet and epidote rocks. 

1 Mason Valley Mines Co. First Ann. Rept., for the year ended Dec. 31, 

1910, 1911. 

The most important rocks in the vicinity of 

the · Mason Valley mine are crystalline limestones, ranging from fine to coarse grained. 

They are. the rocks that have been altered into 

ore in the ore-bearing zone. They strike about 

north and dip 70° or more to the west. · Below 

them lie felsites and lime-silicate rocks, principally garnetites; and below the garnetites 

is thin-bedded black limestone, which strikes 

N. 25° E. and dips 70° W~ The garnetites are 

dense, heavy brown rocks· of fine grain; under 

the microscope they are found to - consist 

wholly of garnet, whose- refractive index (1.81) 

indicates it to be a variety halfway intermediate between grQssularite and andradite. 

. Igneous rocks are abundant near the mine 

and comprise a considerable number of types. 

The· oldest are the felsites of Triassic age. A 

belt of felsite tuff-:breccias, 100 feet wide, 

forms tP.e footwall ~ong the greater part of 

the ore· zone. Part of the breccia is compo~ed 

of sharply angular fragments of felsite, averaging a quarter of an inch.in diameter and containing small sporadic phenocrysts of quartz. 

The breccias range in color from white to 

black, and where the fragmental .structure is. 

not apparent it is impossible to establish the 

identity of the rocks without the aid of the 

microscope. A notable feature. of the breccia 

belt is the profound metamorphism. it has 

undergone· from place to place. Large 

amounts of it· have been converted· to ·finegrained yellowish-green rocks composed solidly 

of epidote. In places it is obvious that this· 

transformation has spread out from fractures. 

Toward the ~outh end of the belt the action 

was particularly intense, and the felsite breccia 

was converted into a garnet-epidote rock over 

an area of several thousand square feet. 

Other felsites occur below the footwall breccia.. Thin beds of breccia and tuff are intercalated in the garnetite and other lime-silicate. 

rocks. A belt of felsite is well exposed on· the 

-road between the office and the powder maga;.. 

zine; it is a snow:-white dense variety, which 

resembles a dense :fine-grained quartzite, but 

its analysis, given on page 15, proves its igneous 

origin. 

An irregular mass of porphyry, here provisionally termed andesite, occ~ also in the footwall of the ore zone. Its irregular shape suggests that it may _be _intrusive, but careful 

search failec:l to revear any evidence tending to 
MINES AND PROSPECTS. 53 

prove or disprove this possibility. The andes-'. A belt of granodiorite occurs at the north end 

ite ·carries numerous phenocrysts of plagio- of the ore zone, where it is faulted between the 

clas~ in an aphanitic groundmass; it is pyri- pre-Tertiary rocks on the south and the Tertized. and at the s1uface is considerably oxi- tiary rhyolites ·on the· north. Segments of the 

dized. Underground this rock is exposed in quartz monzonite porphyry dikes that followed 

the footwall crosscut of No. 4 level, where it is the granodiorite intrusion .ai·e prominent at 

found to be a moderately dark porphyry show- the mines and in the surrounding area and 

LEGEND 

SEDIMENTARY ROCKS 

-

Limestone u iii 

f5~~ ~ Lime-silicate rocks ~ Jll'incipally gametite (contains some inter- ce/a ted fl!lsit:e) 

IGNEOUS ROCKS 

Rhyolite 

Qua,.tz ~ monzonite po,.phy,.y 

~~f:iW&J~~j Granodio,.ite 

Felsite 

B Fault (De3hed line indicates probvble fault) 

m 

Str-ike and dip • 

N 

I 

50EL:::::r:=:::J:=:==::e:::3o:::::========S:JOO FEET 

FIGURE 7.-Geologic map of the area surrounding the l\1ason Valley mine. 

ing abundant crystals of andesine; it is heavily 

pyritized, with the development of epidote, 

chlorite, and biotite. The ru:idesite clearly 

came into -its present position before the mineralization took pliwe·, and the most .'prol:>able 

conjecture is that it represents a phase of the· 

Triassic volcanic activity. · 

afford registers of some of the faulting t!J.at has 

.taken .place. The quartz monzonite porphyry 

clike·that crops out strongly in the ·gully east of 

the incline tramway is cut by fouT small basic 

dikes, and similar· dikes are rather commonly 

observed undergroun-d, especially in the north 

end of the ore zone. They range in thickness 
54 GEOLOGY AND ORE. DEPOSITS OF YERINGTON DISTRICT, NEV • 

. ~-:-

from 6 inches to 6 feet. They are later than by secondary sulphides ·has taken place, chiefly 

the ore, which they cut, and show a notable by the development of covellite. .Oxidation 

tendency to split into branches. They appear and sulphide enrichment hav~ been restricted 

to be of no especial significance· and constitute to zones of postmineral fissuring; they have 

so much waste rock in the ore. They are prob- been retarded by the presence of calcite and the 

ably underground equivale.rits -of the basalt imperviousness of the pyroxene gangue. Offflows of the district·and are consequently of late setting the presence of the calcite, however, is 

Tertiary age. the prevalence of pyrite, which has yielded sufThe most prominent igneous rocks are the :ficient sulphuric acid to sulphatize the calcium 

rhyolites, which because of their brilliant color- carbonate. This reaction accounts 'for the high 

ing and sharply eroded forms make a conspicu- . gypsum content of the oxidized ore; and the 

ous element of the surface features at the mine. abundance of this comparatively solul:>le mineral 

They are faulted against the rocks of the ore- proves the sluggish and ineffective lixiviation 

bearing zone; they are of Tertiary age and of the oxidized zone. Tl?-e presence of chalhence of :m,uch later origin than the ore deposits. canthite in minable quantities is of cour.se a 

They are therefore of no· economic importance testimony of the extreme aridity of the recent 

in the exploration of the ore bodies. (post-Lahontan) climate. 

The ore consists of chalcopyrite and pyrite The ore-bearing zone trends north and dips 

in a gangue of pyroxene, garnet, and. calcite. steeply to the west; it is 1,200 feet long and 

The pyroxene is by far the dominant gangue from 250 to 375 feet wide. On the north it is 

mineral, making up more than 75 per cent of faulted against granodio:r;ite, which lies as a 

the ore; it is a grayish-green. variety. halfway narrow wedge between it :and the Tertiary 

intermediate in. composition between diopside rhyolites. The footwall of the ore zone is the 

and hedenbergite, as shown by its analysis on felsite tuff-breccia;. its dip from the surface 

page 32. It is lamellar in habit and commonly down to No .. 4level averages 70° W., but below 

forms radial groups 2 or 3 inches in diameter, this levei it either flattens considerably or is 

though groups 10 inches across are observable displaced by a fiat-lying fault. The hanging 

here and there. The garnet, which proves to wall is marked by a zone of fissuring and crushbe andradite, appears to occur principally on ing, dipping 75° W.; in places the shattere<;l 

the edges of the ore lenses. It is notably true and broken limestone attains a width of 50 feet .. 

that the great masses of barren lime-silicate rock The general structural relations at the mine are 

associated with the ore b9dies are highly garnet- shown in figure 5c (p. 37), which is a section· 

iferous; in fact, they are essentially garnetites. through tunnel No: 4, making an angle of 60° 

Thicknesses of over 100 feet of such barren with the trend of the ore-bearing zone. The 

garnetite are shown in the footwall crosscuts quartz monzonite porphyry dike is cut at right 

on No. 3 level. - angles to the line of the section, so it is shown 

Other minerals occurring in the sulphide ore with its norillal dip; the two basic dikes, howare actinolite, calcite, chlorite, quartz, and mag-. ever, make large angles with the line of the 

netite. The actinolite forms the dense dark- section, and their dips are consequently .shown 

green gangue rather characteristic of the Mason flattened to half their real .amount. A belt 

Valley ore; the calcite is invariably present of felsites, 80 feet wide as exposed in tunnel 

as a minor in~erstitial component; the chlorite No. 4, lies immediately under the footwall but 

. and quartz are recognizable only under the is not shown separately in the figure.· 

microscope; and the magnetite is an inJinitesi- The general northerly. strike of the. footwall 

mal constituent. of the ore-bearing zone is broken near its south 

The oxidized ore cmisists of cuprite, native end by a fault striking N. 40° W., whereby 

copper, azurite, malachite, and chalcanthite, the southern extension of the ore zone was distogether with gypsUm. and limonite and partly placed about 125 feet southwestward, as deter· 

. decomposed pyroxene and garnet. Oxidation mined by the mapping of the surfa~e geology. 

has locally extended as deep as No.·3level, but (See fig. 7.) This faulting occurred before the 

the principal bodies of oxidized ore occur nearer primary ~ineralization took place, and this 

the surface. Sulphides, in fact, occur in. the fact has a bearing on the exploration for new 

outcrop. ·some· small amount of ·enrichment ore bodies, as is. pointed out on pages 35-37. 
MINES AND PROSPECTS. 

Postmineral disturbance has occurred . along 

the footwall fissure of _the ore zone and has produced considerable crushing, which is well 

shown Qn No. 1 level and the 170-foot sublevel 

below it. This crushing has. facilitated the 

oxidation to greater 'depth and thoroughness 

than is CQlllillOn in the mine and accounts for 

the occuirence of high:-grade oxidized ore found 

. in the so-called east ore body. 

The ore zone is made up of the ore bodies, 

pyroxene and garnet masses, and limestone. 

The ore ~odies on the hole are situated ne.arer 

the hanging-wall fissure; below them -that is, 

toward the footwall~are great masses of barren 

garnet rock, attaining thicknesses as great as 

200 feet. These relations are well shown in 

· figure 5; the positions of ore and garnet rock 

are shown only for the different levels, a~ lack 

of ·data did not admit· of their representation 

for the intervening· space. 

The ore bodies are irregul_ar masses and 

lenses of irregular copper content. They grade 

into lean pyroxene rock, or into barren garnet 

In recent years an ore body was discovered 

in crosscutting east from the .lime stope above 

No. 1 tunnel level, which is called the east ore 

body. Being somewhat closer to. the felsite 

footwall than the ore- bodies occurring in other 

parts of the mine, i.t Jies somewhat to the east 

. of them. A footwall crosscut was· driven· east 

from plug 373· for· .a distanbe exce~ding 200 feet 

on No. 3 level, all in garnetiferous rock, in the 

endeavor to prove the downward extension of 

the east ore body. In this part of the mine the 

footwall of the ore. zone is displaced about 125 

feet to the southwest by a premin·eral fault; 

therefore, inasmuch as in the r~~t of the mine 

the ore tends to occur between the limestone 

and the garneti.ferous zone that lies on the footwall, the ore zone in this part of the mine ought. 

here to· bend rather abruptly southwest'Yard. 

Accordingly, crosscuts. driven westward from 

thisfoQtwall crosscut will explore a considerable 

block of ground in which masses of ore are lik~ly 

,to occur. · MALACHITE MINE. 

rock, or into limestone. The transition from The ·Malachite mine a9.joins the Mason 

ore to opure limestone is as a rule extremely Valley· mine on the south, and its workings 

abrupt. The limestone adjoining the ore connect with those of the 300-foot level of the 

shows no coarsening of its· grain; in this respect Mason Valley mine. The geology of the 

it contrasts with mos~ ore deposits of si~ar vicinity of the portal of the main tunnel is 

type, in which proximity to ore is indicated shown in figure 7. The fault cutting off the 

by the increasing granularity of the. inclosing quartz monzonite porphyry dike at the main 

limestone. · Many of the ore lenses are of . tunnel is well exposed at the surface and shows 

great size; and as the ground stands well after that the porphyry has been altered largely to 

extraction of the ore large chambers have epidote for a distance of 20 feet from the fault. 

resulted, some being .100 feet long, 30 to 50 The principal ore .shoot has mineralogic fenfeet wide, and 50 to 60 feet high. . The largest tures like those of the ore in the Ma.Son Valley' 

ore bodies occur on No. 3 and No. 4 levels; on mine, .consisting of pyrite and chalcopyrite in 

No. 3 level the ore is practically continuous a gangue of grayish-green lamellar pyroxene, 

for a length of 800 feet. The copper content with· subordinate brown garne.t. 

is erratic, however, and it is obvious that, as In addition to .the ore found in the southern 

in most other contact-metamorphic· ore de- extension of the Mason Valley ore zone, some 

posits, it would be difficult to estimate tp.e cop- bodies of cupriferous lime-silicate ore have 

per content with Inuch accuracy in advance of been explored that occur on the low ridge ori 

exploitation and extraction. The. ore so far t4e east side of the road to the McConnell1nine .. 

developed on the 4 70~foot level is mainly lean They are situated along· the coritact of .felsite 

garnet-pyroxene rock. . and. gray lin1estone. 

The main outcrops of the Mason Valley ore 

bodies are near the nQrth end of the ore-bearing 

zone. Toward the south end outcrops of ore 

or other indications of ·ore are insignificant, 

yet as shown by the underground development 

large ore bodies occur below what on the surface 

is nothing but pure lin1estone .. · 

McCONNELL MINE. 

The McConnell miRe,'. owned by the McConnell Mines Co., is on the east flank of the range, 

about a mile southwest of the Mason Valley 

mine. The main workings consist of a l~rge 

open cut or· glory hole, ~he· ore being taken out· 
56 GEOLOGY AND ORE DEPOSITS-oF-YERINGTON DISTRICT, NEV. 

through ·two tunnels, the lower of which rocks.. The andesites ·are sep~rated' from the 

attains a ·depth of abop.t 100 feet below the· garnet mass, in which the ore body is inclosed, 

croppings. A vertical . shaft 400 feet deep by a fault trending east and dipping 65 o S. 

has bee:J;l sunk and levels turned at depths of They are fine-grained black rocks of 0 bscurely . 

200 a:r;td 400 feet. . · porphyritic character and have been considProduction began in ¥arch, 1912, at the erably recrystallized as a resUlt o~ thermal 

rate ·of 50 tons daily and continued until metamorphism. · Under the microscope they 

November,. 1913. The output aggregated are found to consist larg.ely of plagioclase and 

~4,100 tons of ore, averaging slightly more hornblende. As seen in the mine workings 

than 3 per emit of copper. The ore carries no they show in addition the effects of intense 

gold nor silver: crushing. and shearing, .and in· consequence of 

The n1ine is situated in ·the thick belt of the fissile structure they have assumed they 

. limesto-ne extending southward from the Mason are locally known as shales. · · · 

FIGURE 8.-Plan of the McConnell mine, showing .the geologic re-lations. The heavy dashed line represents the outcrop of the body. 

Valley mine and is a few hundred feet east of 

the quartz monzonite. The hill west of the 

mine, covering the area lying between the 

mine and the quartz monzonite,. has been· COJ;lverted. solidly into garnet. In this massive 

garnetite was found -the I:lalobia-like fossil 

whose presence indicates a Triassic age for the 

rocks. . . 

The -rocks in the imme-diate vicinity of the 

mine are limestone,. Triassic andesite,. and Tertiary basalt. (See fig. 8.) Intense faulting 

of premineral as well ·as· postmineral origin 

has complicated the mutual relations of these 

. A portion of the basalt capping that ove~lies 

the mesa east of the McConnell mine has been 

faulted against the limes~one in proximity to 

the· ore body. The faults .are well exposed in 

the upper and lower tunnels. .The fault seen 

in the upper turinel is a sharply defined plane 

trending N. 15° W ... and dipping 54° W. 

The basalt (the footw:all country rock) is considerably shattered near the contact, and the 

overlying limestone is brecciated for more than 

20feet from the contact. Thefaultshowson the 

200-foot level also, but ·here it has passed 

through the basalt, and the andesites adjoin 

the limestone. These relations are ·shown in 

figure 4 (p~ 29.). The fault is a reverse fault, 

whose dip slip is at least ~50. feet. The 

appearance of the andesites on the 200-foot 

level means either that the dip slip greatly 

exceeds 150 feet, or that the move1nent alo·ng 

·the fault plane had a large horizontal com~ 

ponent. On the surface the shifting of the · 

contact of ande-site and lin1estone by this fault. 

amounts to · 500 or 600 feet. The side of ·a 

separate . mass of basalt is partly exposed in 

the south wall of the glory hole. It is better 

shown on the level of the lower tunnel, where 

it is found to be. a-brecciated mass at least 180 

feet long and 30 feet thick: It is bounded on 

the north by a steep fault and on the south by 

a fault dipping 70° S. Later than this faulting is a vertical fault trending N. ~0° E., whose 

slickensided surfaces are conspicuous features 

of the McConnell glory hole. .The displacement along this fault is small, however. . 

The ore body is elliptical in plan.. At the 

outcrop the major axis measured 400 feet and 

the minor axis 80 feet; on ·the level of the 

upper tunnel, which attains an average depth 

of about 40 feet. below the outcrop, the din1en-
MINES AND . PROSPECTS. I 57 

sions were respectively 250 feet and 50 feet. 

Relatively little ore has so far been found on 

the lower levels. 

The primary ore consists of chalcopyrite 

and pyrite in a ·gangue composed mainly of 

amber and pale-brown garnet. A light-green 

amphib.ole, pyroxene, and calcite occur as subordinate compon~nts. The croppings were 

highly oxidized, and the copper was present 

chiefly in the form of chrysocolla and malachite. 

Partly altered nuclei of chalcopyrite are present, however, in otherwise strongly oxidized 

ore. As a ruie the oxidized ore is .thoroughly 

pern1eated with gypsum," and this feature is 

especially marked ill the vicinity of residual 

chalcopyrite, where there are not uncomnionly 

plates of gypsum, as much as an inch in diame-

. ter and one-quarter of an inch thick, which 

have grown in the body of the ore. The area 

of garnet rock is considerably larger than the 

outcrop of the ore body, which is situated between the limeston~ and the barren part of 

the garnet.mass. 

Th~ origin of the McConnell ore body appears 

to. be connected with the fault that separates 

the andesites from the limestone ·belt. In 

places the limestone shows that the garnet and 

amphibole developed along fractures; and it.is 

probabl~ that the metalliferous solutions 

ascended .along the fault and· replaced the· 

brecciated limestone adjoining it. Genera.lly 

the replacement has been so complete that 

little evidence of its mode of operation remains. 

The Red mine, which belongS to the McConnell Mines Co., is a few hundred yards southeast 

of the MeConnell mine. Here was worked a body 

of oxidized ore, which was inclosed in limestone 

at the quartz monzonite contact. Intense 

crushing produced by postniineral dyn.amic 

disturbance has affected the deposit .. The end 

of the lower tunnel follows a well-defin~d fault 

striking north and dipping. 70° E. · This is the 

fault that separates the quartz monzonite from· 

the litnestone; but a dike of andesite characterized by lustrous hornblende prisms has· 

been intruded for some 'distance along the fault. 

Under the . microscope the andesite . shows 

phenocrysts of hornblende in a fluidal groundmass of slender plagioclase prisms; and these 

fea.tures leave little doubt .as to the identity of 

this rock with the h9rnblende andesit'e that is 

common as a surface flow rock in the northern 

part. of the district. Subsequent. movement 

along the fault surface has profoundly sheared 

the dike, and much of the shiny gouge, characteristic of -the mine, is sheared dike rock that 

has "been stained deep red by the infiltration 

of iron oxide. 

WESTERN NEVADA MINE. 

The Western Nevada mine·is at the head of 

a narrow canyon in the east flank of the Singatse 

Range, near the south end of the mineral belt. 

The mine has never shipped any ore., although 

a considerable quantity has been extracted· and 

thrown on the dump. In March, 1915, tb,e 

property was merged with that of the NevadaDouglas Copper Co., forming the NevadaDouglas. Consolidated Copper Co., and active 

development and exploitation were contem:-

plated. . 

In addition to a number of short adits near 

the outcrops, the rrii:Q.e is opened by two 

principal adits, of which the lower, or· main 

adit, 140 feet below the upper, attains a vertical depth. of 235 feet below the ~op of the hill 

in which the ore bodies lie. This adit is-1,000 

feet long· and strikes N .. 50° W.; froni it an ·extensive system of drifts extend~ northwa-rd, exploring an area of p:froxenic ·and garnetiferous 

.rocks in which the principal ore shoot is inclosed. F:rom this level a winze, 300 feet deep, 

was sunk, and the ore .shoot was crosscut on 

the "300-foot" and "400-foot" levels. 

The oi·e bodies .and their associated silicate 

rocks occur .in a thick belt of limestone, faulted 

on the ·west against quartz monzonite. A 

block of the Triassic.andesites has been faUlted 

against ,the limestone at the n:rine. The main 

adit, commencing .in these andesites, C\lts 

tln·ough 300 feet of them, .then. through limesilicate .roc~s, and . again through andesites, 

finally penetrating.the quartz monzonite. The. 

last few hu,ndred. feet of the . adit is in very 

thoroughly crushed andesite .(locally called 

"shale"). The cause of this crushed condition 

becomes plain from a consideration of. the surface geology, frmn which it appears that the 

.course of the adit trends at a narrow angle with 

the fault contact of the limestone and the andesites. The adit is now caved at 830 feet ftom 

the portal. Ransome, ·who examined· the mine 

when the fac~ of the adit was accessible, ·states 

that the quartz monz9nite contact is due to 

faulting, the. fault plane ·dipping 54 ° E.. Lc-

()Ording to t~e safi?.e authority the hanging wall 
58 (}EOLOGY AND ORE DEPOSITS OF YERINGTON DISTRICT, NEV. 

is much broken, but no mineralization has 

· taken place along the fault. About 100 feet 

east of the fault there is a fissure trending nearly 

north which is partly filled by a quartz ·vein 

carrying pyrite but no constituents of value. 

This vei;n. dips 35° E. It is probably younger 

than the cupriferous ore bodies in the limestone, 

·but its age was not certainly determined. 

Numerous scattered bodies of ore, generally 

from 10 to 20 feet long and trending parallel 

to the stratification of the inclosing limestone, 

are shown in the sUrface <;uts and winzes at the 

summit of the hill. These. ore masses are most 

abundant :g.ear the saddle overlain by the horizontal sheet of Tertiary basalt. The copper 

ore is associated with radial lamellar pyroxene, 

with garnet, or with both minerals; it is as· a 

rule highly oxidized, consisting of gossany 

material netted with veinlets · of chrysocolla 

and more or less gypsiferous. One outcrop 

shows garnet and other silicates extending con~ 

tinuously for a length of 100 feet and expanding 

at one place to a width of 20 feet. 

The ore shoot explored on the level of the 

main adit is about 200 feet long and trends 

· N. 15°-E. It ranges from 2 to 6 feet in width 

and averages from 2 to 3 per cent of copper. 

Some enrichment by supergene sulphides has 

ta:Ken place, and chalcanthite is even now in 

process of formation by the feeble percolation 

of ·descending solutions. . Toward the north this 

sh<?ot is cut off by a fault along which an unusually fine example of a limestone attrition 

· conglomerate has been produced. . 

The ore consists of chalcopyrite and pyrite 

embedded in.an intergrowth of pyroxene and 

garnet with interstitial calcite. The pyroxene 

is dark grayish green, occurs in radial lamellar 

groups or in dense fine-grained aggrega_tes, and 

is probably the predominant gangue mineral. 

The garnet, which is of the andradite variety, 

ranges from brown to amber and commonly 

shows crystal .faces of dodecahedral . and 

tr~pezo.hedral habit. Locally these minerals 

have altered to chlorite, amphibole, and serpentine, but this action has not been extensive or 

of economic significance. . · · 

GREENWOOD ·PROSPECT. 

The Greenwood prospect is. 1 mile ~ortheast 

of Ludwig,. at an altitude of. 6,000 feet. The 

principal developments consist of an inclined 

shaft, which.is cut by a tunnel150 feet below. 

The general country rock .is garnetite, which is 

traversed by a thick dike of quartz monzonite 

p'orphyry. The dike makes a rather abrupt 

bend in its course through. the property, as 

shown in Plate I; and near this bend bunches 

of chrysocolla ·and copper pitch ore. with 

subordinate malachite are found on both 

sides of the dike. The primary sulphide is 

chalcopyrite, but this has mostly. been oxidized 

to the minerals mentioned; the gangue is 

mainly andradite garnet, with mmor quartz 

and epidote. 

LUDWIG MINE. 

GENERAL FEATURES. 

Th~ Ludwig mine,· around which the small 

settlement of Ludwig has grown, is on the west 

slope of the Singatse Range at the edge 'of 

Smiths Valley. The mine was opened in 

1865 on oxidized copper orelying in the footwall of the primary deposit, and a sh~ow 

tunnel was driven for 500 feet on the strike. 

Considerable bluestone was produced for the 

reduction works on . the · Comstock lode. In 

1907 the mine was acquired by the NevadaDouglas Copper Co., which commenced eD;ergetic operations in both the primary and the 

derived ores. The production from December, 

1911, to August 1, 1914, was 125,689 tons of 

ore averaging 6 per cent of copper. 

During the later part of 1914 experimentation was actively in progress to devise methods ' 

to treat the ores of the Nevada.:.Douglas Copper Co.'s mines-the Ludwig, Do~glas Hill, 

and Casting Copper-by a leaching process. 

The plan under trial was designed to utilize the 

heavy pyritic ore of the primary ore body of 

the Ludwig mine to furnish the sulphuric acid 

required to leach the oxidized ores of this and 

the other mines. As a result of this experimentation a mill having a daily capacity of 250 

tons was b~~lt at Ludwig during 1915. 

The mine is worked principally through an 

inclined shaft situated near the north end of 

the lode. This attains a vertical depth of 678 

feet. Eight levels extend from it along the 

strike of the lode, and the lowest level is known 

as the "800." An old vertical shaft 400 feet 

deep serves as an auxiliary entran.ce and in 

1914 was used in exploiting the secondary ores 

occurring in the limestone footwall of the 

primary deposit. · . 

The Ludwig deposit is essentially a replace~ 

ment lode developed along the faulted. contact 
MINES AND PROSPECTS. 59 

of a ~assive limestone· and an overlying series 

of garnetites and felsites. The fissuring alongwhich the ore-forming solutions rose coincided 

approximately with the beddll)g plane between 

the footwall limestone and ·the dense garnetite 

that forms the hanging wall of the deposit': 

The salient features of the lode are well shown 

at the vertical shaft-the dense garnetite hanging wall, the bold quartzose iron-stained crop:-

pings, the footwall of white limestone in which 

occur more or less discrete pyritic lenses, and 

the irregular bodies of copper pitch ore in this 

footwall limestone. 

accessory minerals practically proves that the 

quartzose gossan ·is the oxidized equivalent 

of the silicified.and pyritized porphyry. It is 

believed that the po!phyry, which by its 

alteration gave rise to the quartzose gossan, is 

mainly fault material dragged into .the zone of 

fissuring. 

The quartz outcrop formerly contained much 

cubical pyrite, as is shown by the many casts 

of this mineral and tl,le prevalence of iron 

oxides. 

The footwall of the lode is a coarse crystalline· white li:mestone, which is approximately 

FIGURE 9.-Goologic section across tho Ludwig lode at tho vertical shaft, slightly generalized. ·a, Quartzite; b, gypsum; c, anhydrite; d, · 

limestone; e, quartzose gossan; /, garnetito and other lime-silicate rocks; g, porphyry; h, heavy sulphide ore; i, felsite. 

DE'l'An,s m· 'l'HE .GEOLOGY. 170 feet· thick. It is underlain by a gypsum 

'"fhe prominent iron-stained quartz outcrop bed, from .which, as seen undergrou~d, it is 

of the lode extends 600 feet south of the separated· by- a fault breccia containing a 

vertical shaft, ranging in width from 20 to .60 considerable proportion of quartzite fragments. 

feet. The quartz mass is fine grained, whoily This breccia . is locally known as the ''conunlike the quartz·o~ veins that fill fissures, and glomerate," probably because of ·the partly 

its general appearance strongly suggests the rounded c01idition of the limestone fragme,nts 

silicified porphyry that is found on the lower it contains.· So sev~re has the dynamic dislevels of the mine. Under the microscope the turbance been that blocks of limestone have 

siliceous material of the outcrop is found to . been. split in two, arid. gouge filled with small 

consist mainly o~ quartz, together with. a little a.rigular fragments of quai·tzite has been 

chalcedony and opal and the accessory minerals squeezed into the .fissure. The footwall limegarnet, apatite, and zircon. As will be seen stone, although on casual inspection apparently 

from the description of the altered porphyries a single massive bed~ contains some thin strata · 

fou:q.d underground the p:resence of these three. of dark-gray limestone, and these serve as 
60 GEOLOGY AND ORE DEPOS~TS OF YERINGTON DISTRICT, NEV. 

registers of the remarkable internal disturbance to which the main stratum has been sub-: 

jected. Reverse faults, with displacements 

ranging between 5 and 6 feet, are shown, and 

sharply angular fragments of the_ dark-gray 

limestone are embedded in the white limestone 

·at considerable distances fr·om their place of 

derivation. 

The hanging wall of the lode, as seep. at the 

surface and on the upper levels,· consists of 

:fine-grained lime-silicate rocks resulting from 

the metamorphism of a series of thin-bedded 

limestones. Their mineral makeup is 1.1ot 

readily recognizable by the unaided eye, but 

under the microscope they are found to be 

composed principally of garnet. A specimen 

from the 100 level is a dense aphanitic garnetite composed almost wholly of a garnet 

intermediate between grossularite and andradite. . In the 700 and 800 levels the hangingwall rock as seen near the shaft is a banded 

felsite, more or less altered by pyritization 

and the development of . garnet, epidote, and 

pyroxene.· 

The fault along which the lode occurs trends 

parallel to the formation and dips on the average 70o_ E. The average dip is apparently a 

little steeper than that of the .stratification, 

which ranges from 60° to 65° E., and this supposition is strengthened by the fact that the 

lithologic character of the hanging wall changes 

in depth. The fault is well shown underground 

along barren stretches of the lode, along which 

the limestone lies directly against the garnetite hangmg wall. It is excellently exposed 

on the 100 level, north of the vertical shaft, 

where it can be seen that the movement on 

the fault surface was mainly horizontal. The 

limestqne in·· proximity to the fault at many 

places in. the mine is trem~ndously shattered 

and brecciated, forming breccias as much as 

80 to 100 feet thick. 

Some postmineral faulting has affected the 

lode. At the vertical shaft a fault cutting 

the lode at right angles ~nd dipp~g 70°-80° 

N. has displaced the ·lode about 50 feet hori-. 

zontally squtheast .. 

Porphyry dikes· are common at the mine. 

One of these, which is over ·100· feet thick,. 

forms the hanging wall of the :fissure north of 

. the shaft;· its course is shown on the geologic 

map (PL I), which shows also the inter·esting 

faulting to which it has been subjected. This 

porphyry . contains phenocrysts of pearl~gray 

orthoclase, ·of plagioclase, and of hornblende. 

In .the. mine workings are found dikes of this 

character, and also porphyries especially chf,Lracterized by the prevalence of rounded pheno-

~rysts of quartz. The dikes of both kinds are 

much shattered. A short distance .south of 

the south end of the quartz outcrop of the lode 

a large· fr~gment of shattered porphyry, evidently fault drag, is inclosed in the fault zone 

along which the .lode originated. 

THE ORES. 

The iron-stamed cellular siljceous matter 

of the outcrop extends down to the 500 and 

-------1-tOO-F'OOTI..EV£1.. 

----1-200-F'OOTI..EV£1.. 

---'--I-300-FOOTI..£V£t. . 

'----+-400-rOOT I..EV£1. 

---+soo-roor t.cvcL 

700-FOOT /.EVE/. 

800-FOOT I..CV£1.. 

·o 3oofeet 

FIGURE 10.-Geologic section across the Ludwig lode 320 feet southwest of the vertical shaft. a, Lirr..cstonc; b, quartzose gossa.D.; c, 

garnetite; d, porphyry; e, heavy sulphide ore;/, pyritized and silicified 

quartz porphyry; g, limestone fault breccia. 

600 levels with ·little .change of character. 

Along dip and strike, however, the dimensions 

of the gossan are markedly irregular, as shown 

in two cross sections of the lode, .320 feet apart 

(figs. 9 and 10). · 

The oxidized ores occur principally in the 

footwall limestone in extremely irregular bodies 

ranging from circular pipes to fairly persistent 

tabular masses. The ore consists largely of a 

reddish-brown aphanitic substance, copper 

pitch ore. It averages abou;t ] 5 per cent of 
MINES AND PROSPECTS. 61 

copper.' . Chrysocolla and cuprite occur in 

minor amounts ·and are the only definite copper minerals recognizable 'in the ore now seen. 

Formerly carbonates were abundant. The ore 

shows a rather well-defined layering parallel 

to the inclosing walls, and much of it plainly 

fills solution cavities in the limestone. The 

main occurrence of ore . of this. kind is in the 

footwull limestone in the vicini.ty of the vertical shaft-that is, near the postmineral cross 

fault already mentioned. 

The copper of this secondary ore was· evidently derived mainly from the siliceous gossan 

during its oxidation. The solutions that originated dru·ing this process tended to migrate 

into the footwall of the lode, and this tendency 

· was augmented 1:>y the cross faultiJ:tg and .the 

shattering that accompanied this faulting. 

Once in this calcitic environment the copper 

was eventually precipitated as carbonates an~ 

clll'ysocolla. 

· Chalcocitized ore :fhst appeared on the 500. 

level, the depth at which the water level was 

reached. Stopes on ore of this kind were 

worked· on the 600 level, b-.;tt. below this the 

ore is mainly of primary origin. The secondary sulphide has formed chiefly at the expense 

of the chfilcopyrite in the primary ore .. Gypsum is common in the enriched ore. A no-: 

table example of this is the "selenite stope" 

·On the 600 level, where the gypsum is crystallized in fine long prisms, which attain exceptionally a length of 8 inches. 

Although primary ore predominates belo:w 

the 600 level, some· bodies of i·ich oxidized ore 

have been found even in the bottom level, 

·Consisting- of coarsely crystaliine cuprite partly 

mi.-xed with native copper. They appear to 

have been localized ft:long exceptionally favor-

.able channels for downward-moving waters. 

The primary ore, as seen on the 700 and 800 

.levels, consists largely of pyrite in a gangue of· 

quartz, calcite, and andradite gar~et. Chalcopyrite is a subordinate .constituent, but in 

:general no copper-bearing mineral is recognizable by the unaided eye. The ore is re-

·ported to· average 2· per cent· copper, and a 

body of this grade ranging from 50 to 100 

feet wide Is exposed on the 800 level. In 

this body the garnetiferous quartz-pyrite ore, 

·carrying coarse calcite as :a rriinor constituent, 

:grades toward the footwall into a heavy pyrite 

ore with calcitic matrix; The pyrite has 

obviously replaced the cement of the limestop.e 

breccia that resulted from the faulting which 

took place along the contact ·of the llmestone 

and the overlying rooks. The character of 

the primary ore differs somewhat from place 

to place. On the 700 -level the ore consists 

of pyrite in large cubes, some as much as atl" 

inch in diameter, inclosed in coarse ·white 

quartz with which is mingled ·some sparry 

calcite. In addition to these varieties· of ore 

there occur large pyritic masses derived from 

the mineralization of quartz porphyry dikes. 

These dikes are netted with qu~rtz veinh)ts 

and have been so intensely silicified that only 

the quartz phenocrysts have remained intact~ 

In places they have also been metamorphosed. 

by the development of garnet and epidote, an 

alteration that lias spread out frOJn veinlets 

of quartz, calcite, garnet, pyroxene, and pyrite. 

These features are well shown in the long 

hanging-wall crosscut on the 700 level, and 

on the 600 and 500 levels. The most nearly 

unaltered porphyry was found on the 500 

level ne.ar plug 518, and this .prov~s to be 

quartz monzonite porphyry containing sporadic lm·ge orthoclase crystals, smaller and 

rriore ·abundant plagioclase crystals, and numerous corroded quartz crystals, which are the 

most conspicuous phenocrysts. The conditions shown in the drifts near plug 611 probably 

give a clearer idl:)a of the relation of the porphyry. dikes to the mineralization than can be 

obtained anywhere else in .the 1nine. · A 

number of narrow dikes cut the limestone· here; 

the margin~ of the dikes have been severely 

brecciated, and the breccia is cemented by 

garnetiferous calcite-pyrite:veinlets. The porphyry fragments have been altered beyond 

easy recognition, and it is manifest that where 

brecCiation has been more extensive and the 

miner~lization more profound, as on the 700 

leyel, it would be impossible to discriminate 

between· mineralized limestone and mineralized 

porphyry. The recognition of silioified, pyritized porphyry derived from porphyry that 

c·ontained no quartz phenocrysts, like that of 

the thick dike occurring in the h~ging wall · 

of the lode, ·would be especially difficult. 

Silicified and pyritized porphyry of the kind 

so well shown on the 500, 600, and 700 levels 

is probably· the material from .which the siliceous outcrop of the Ludwig lode was mainly 

derived, as well as the gr'eat bodies of leached 
62 GEOLOGY AND ORE DEPOSITS OF YERINGTON DISTRICT, NEV. 

siliceous gossany material that extend down 

to the 600 level. The pyritic bodies derived 

froni the metamorphism of the porphyry dikes 

are too ]ow in copper to be ore. 

In places the hanging wall of the lode is cut 

by quartz-garnet-sulphide veinlets. At the 

outcrop veinlets composed of quartz prisms 

and garnet dodecahedrons and carrying a little 

chrysocolla traverse the brecciated garnetite 

adjoining the quartz outcrop. Underground 

the penetration of the hanging-wall rocks by 

veinlets· composed of quartz, calqite, garnet, 

and pyrite is well.shown in a number of crosscuts. Near the shaft on the 700 and 800 

l~vels the felsite hanging wall is cut by coarse 

veinlets of quart7., epidote, garnet~ and pyrite. 

Microscopic examination shows that the felsite 

has been partly replaced by garnet, pyrite, 

calcite, epidote, and pyroxene. 

-GYPSUM DEPOSIT. 

which-the main haulageway of the mine-attains a depth of 120 feet below the crest of the 

hill. The production since December, 1911, has 

been 68,905 tons of ore carrying 5 per cent copper. In 1914, however, the mine was idle. At 

the time it was shut down, pending the construction of the leaching plant at Ludwig, the mine 

was furnishing 50 tons of 5 per cent ore-5 

per cent· being the minimum percentage· at 

which it was profitable to ship ore to the 

smelter. 

The ore forms irr~gular lenses in an immense 

mass of andradite garnet, which caps the 

SlPDmit of Douglas Hill. This great bq_dy of 

garnet resulted from the replacement of the 

massive limestone lying above a stratified . 

series of fine-grained garnetites. The footwall ·of the ore zone is a belt of breccia, pro b..:. 

ably 20 feet thic;k. This breccia is composed of 

fragments of a d~nse aphanitic garnetite which 

are cemented by coarse euhedral yellow garnet. 

The large deposit of gypsum occurring on The garnet of the fine-grained rock, as shown 

the property of the Nevada-Douglas Copper by its refractive index (1.75), is a nearly pure 

Co.- has. been extensively quarried just north grossularite;· the garnet of the cement, as 

of the Ludwig shaft. The gypsum, as first shown by its index (1.87), ·is a nearly pure 

·proved by A. F.' Rogers/ has resulted from the andradite.. The breccia structure-that is, 

hydration of anhydrite. This is excellently the inclosure. of sharp angular fragments of 

shown on the 400 level of the Ludwig niine, garnetite in coarsely euhedral garnet-iS as a 

where a crosscut has been ruri. into the deposit rule excellently shown; in places, however, the 

for a distance of 100·feet. The gypsum bed, cement, the.secondary garnet, appears to form 

as previously described, is separated from the an unusually large proportion of the rock mass, 

thick· limestone belt· forming the footwall of but this is undoubtedly due to the replacement 

·the Ludwig lode by a fault breccia containing of limestone that was originally present in the 

innumer.able quartzite fragments; underlying breccia; .Chalcopyrite occurs sporadically in 

this breccia is 10 feet of fine-grained gypsum the cement. 

which· grades rather abruptly into hard, mas- The andradite. of the ore-bearing zone is 

sive, coarsely gra~ular anhydrite. The geo- characteristically coarsely faceted and has a 

logic ·relations are shown in figure 9 (p. 59). notably fatty or adamantine luster;. and in 

The quarry was operated from November, these respects it contrasts strongly with the 

191i, to the middle of Septe_mber, 1912, and fine-grained garnetites, which obviously origi- · 

during that time about 75 tons a day was nated at an earlier epoch of metamorphism. 

shipped to the West~rn Gypsum Co. at Reno, 'Although the ore-bearing.zone consists largely of 

Nev. Since then it has lain idle. The gyp- andradite with locally some epidote and pyroxsum· is reported to have averaged 96 per cent ene, yet the surface exposures as well as the mine 

·pure. · workings disclose sporadic residual masses of 

DOUGLAS HI;LL MINE. pure limestone, though even SOme Of these are 

The Douglas Hill mine, owned by the N e- penetrated by stringers of garnet. · In figure 

vada-Douglas Consolidated Copper Co., is. on 11 the areal geology of the Douglas Hill mine 

the. summit of the ridge known as Douglas Hill, is shown, and this map brings ·out clearly the 

east of Ludwig. The workings consist princi- erratic · distribut~on of the ore lenses in the 

pally of thirteen large pits from which the ore andradite rock, as indicated by the many pits. 

was taken out through 'tunnels, the lowest of ·The ore is predominantly oxidized; chryso1 Econ. Geology, vol. 7, pp. 185-189, 1912. colla and copper pitch ·are the principal min-
MINES AND PROSPECTS. 63 . . . 

erals. Brochantite in small emerald-green garnetites probably iJ?.clude clinOZ9isite-vesuviglassy prisms and malachite occur subordi- amte rocks and allied varieties, as shown in the 

.nately. Chalcopyrite is the ':lnain primary detaile<;l section of the rocks at Luciw~g, but 

sulphide, pyrite occurring in traces only. except for one specimen, which_prov~d to con-

. In places considerable postmineral move- sist, wholly of garnet crowde~ wi_th ·carbonamont has crushed and brecciated the garnet of ceous matter. they were not examined roi,~ro0 sooFeet ~---~------~---~------~---~ 

FIGURE 11.-Aroal geology at the Douglas Hill ~o. 

the ore-bearing zone.· Locally.son1e downward 

sulphide enrichment has taken place, controlled possibly by the· postmineral crushing 

just referred to; the ore known at the mine as 

"spot ore" was an exan1ple of an unusually higP,-

grade chalcocitized ore, carrying as much as 15 

per cent of coppm~. The deeper workings, 

however, show little evidence of. enrichment. 

CASTING COPPER MINE. 

The Casting Copper mine, which is owned by 

the Nevada-Douglas Consolidated Copper Co., 

is just south of the town of Ludwig~ It is 

de,,eloped principally by a shaft 350 feet deep. 

The production up to Augus~ 1, 1914, was 

29,241 tons, averaging 5 per cent of .copper. 

All ore has cmne from above the 200-foot level, 

and the greater part of it has come from the 

large glory hole, which is the prominent 

feature of the surface workings of the n:iine: 

The ore-bearing zone lies along a fault .that 

has caused a series of t?trati:fied black garnetites to abut against. massive limestone, as 

shown in figure '12. The garnetites are dense, 

:fine-grained rocks which are distinctly bedded 

in. relatively thin strata; the .limestone is 

coarsely crystalline and. obscurely bedded. 

The rocks here referred to for co_,nvenience as 

scopically. 

·':['he limestone is the productive rock; it has 

been replaced by coarsely crystalline garnet 

for a length exceeding 400 feet ~long the strike 

of the fault and for a width in places of 150 

feet. The garnet rock of the_:; ore-bearing zone 

is of highly distinctive appearance, consisting 

of notably lustrous resin-yellow andradite garnets, as· a rule distinctly crystallized according 

to the common dodecahedral habit and surrounded by paler'andradite, which is generally 

less distinctly faceted than the resinous garnet. 

A little calcite, 1 or 2 per cent, occurs interstitially between the garnet crystals. Along 

~he fault contact the black garnetites have 

been slightly shattered, and the angular fragments. are outlined by the thin. veinlets of wellcrystallized yellow garnets that surround them: 

the evidence of two distinct periods of garnetization ·is well shown in this mine. The contact 

between the ore-bearing garnet masses· and. the 

massive limestone is highly irregular, and veins 

and bunches of solid garnet are most erratically 

distributed 'through the limestone adjoining 

the main garnet masses.· 

The main ore body consists of a large lens 

which is reported to have been 120 feet long 

and 40 feet wid~ on the upper levels and to 

sooFeet. ~~---~---~~---~ 

FIGURE 12.-Diagrammatic map of the surface geology at tho Casting 

Copper mine: a, Gametite; ·b, andradite of the ore-bearing zone; 

. c, massive limestone. · · · · 

have· averaged 6 per cent <?f copper. At the 

time of visit the stapes on this lens had been 

earried doW!l within 35 feet of the 200-foot 

leyel. The ore body is n.a:rrow here, averaging 

a few feet in width. · The ore consists of partly 

en~iched chalcopyrite inclosed in a gangue of 
·:·64 GEQLOGY AND ORE DEPOSITS OF YERINGTON DISTRICT, NEV. 

alter~d-.,gari_le.t and" pyroxene and· permeated MINES AND PROSPECTS ON DEPOSITS IN 

.:with~ ~-.conside~~bie-:amount of .gypsum. The .IGNEous ROCKS. 

,o:r.e 'i;!OdN .. i~ WnQijy. inclosed in massive lime- MONTANA-YERINGTON MINE. 

s-tope.,~· artd· :Plqclts of limestone occur in· it.. The · . · . . . . lateral drifts· show, as is observable on the 200- .rhe · Mon~ana-Yermg~on rome IS 2!- miles 

foot level, that irregular veins and bunches. of west of Yermgton statwn. Before 1912 not 

euhedral garnet occur in the limestone adjoin- ·:n;tuch develop~e~t work had been do~e, but 

ing the ore body. Although .the ore body· is smce then an mcline 300 ~eet long, sloping 4~ 0 

thus in a limestone environment and is niore S., ~as been sunk, and drifts extended from It 

or less oxidized, copper _qarbonates are never- at .different levels .. In 1914 the main wor~ w~s 

t~el~ss scarce,· and chrysocolln. is the pre- bemg do~e on _the~300-foot level. The ~e IS 

dominant recog~~zable oxidized copper com- report~d to have produced 2,500 tons of_ ore, 

pound, forming "'veinlets on the edge of th.e ~verag.mg 6 per cent of copper and carrying a 

oxidized ore against the limestone. little silver and gold. · · . . 

Sporadic masses of ore lying· in proximity · T~e country rock ~s granodiorite cut by. dikes. 

to the. main ore lens have ·been found on the of quartz monzorute porphyry. The con20.~-foot level. It is a noteworthy feature of glom~ra~e at·· th~ base of the Tertiary section. 

the .ore .hodies in this mine, as it is of those at and the_ overlpng glass flow, succeeded by 

th~ ·~as.9~ .. V ~lley mine, that, although the q~artz latites and rhyolites, occur near the 

workings ~is_clo~e inJ?lense bodies of practically rome. . T~ey have bee~ faulted against_ the 

solid _garne~, the· g~ngue of the ore-bearing granodionte, and the faulting has greatly 

parts of. th<;~se great garnet masses is charac~ ~hattered the belt of r~cks in which the mine is 

terized by :the presence of a considerable pro- situated. ·Th.e vein, as seen on the 300-foot 

portion qf lamellar pyroxene.. level, has a farrly well-defined footwall, striking 

All ~~~ ~-~hows. some sulphide enrichment, N. 80° E. and dipping 55° S. Th~ country 

;g.~p..~:r,Q.ny::f\S a_ ~9ft, sooty_black mineral. . Wher·e rock has_been crushed and roughly foliated, and 

-~~-~~lH~~~~?l~~'n.tlf~\ i s~condary sulphide is com- al~m5 this zone the o~e occ~ ill. short stringers 

J??-<?l}l~.;~~~en,. ;~?:·).e <f?Vellite, which. tends to or sm~ lenses.of so_lid sulphide. Chalcopyrite 

9?~~11 ~-s~ p~~_te.~ · ~f~ng_ p~rallel to the pyramidal predoromates; pyrite occurs principally as 

.c~e:~~il~e o~ ·~he :·cha~_cqpyrite. The enriched ~solat~d elihedral crystals; and the foliated rock . 

ore contains prevailingly a notable. amount of Inclosmg the sulphides carries much· finely 

gypsum. Seconda!'Y. sulppj.des in gypsiferous disseminated flakes of specular hematite. T~e 

gang_u~. extend as de.ep as the 200~foot level. chalcopyrite has been slightly enriched by the 

Although. secondary sulphides have forme.d development of so·oty bluish and blackish 

throughout the ore bodies. much of the. sooty sulphides.· West of the incline the vein is cut 

black mineral associated with the chalcopyrite off by a fault striking N. 20° E. and dipping 

proves to b~ black copper o_xide. 50° E. Some attempt to recover the vein has 

A ~ell-defined ·f~ult surface trending N. 50° been made without success by drifting southE. and dipping 70° NW. determines the north- ward; if, however, the dislocation is a normal 

west face of the glory-:Q.ole. pit; it is excellently fault-that is, if the hanging wall has slipped 

· shown on the 100-foot level. It limits the ore down relatively to the footwall-the faulted 

on the northwest and is a postmineral fault segment of the vein will be found most expedia~parently coinciding closely with. the pre- ~iously by cutti~g thro.ugh the fault and drift-. 

mmeral fault that cuts off the black garnetites mg northward. 

against the massive limestone. 

. 'fhe recognition of . the. fact that ·the oreforming solutions arose ·along a fault contact 

suggests that the ore-bearing zone can be ex~ 

p.lored most ~horoughly by means. of nmp.erous 

crosscuts or drill. holes extending. at right 

angles to levels drjv~n along the faulted edge 

of the black garnetite.s. 

EMPIRE-NEVADA MINE. 

The Empire-Nevada mine is half a mile west 

of -Yerington station, on the Nevada Copper 

Belt Railroad. In 1914 the prop_erty was being 

worked by three separate parties of lessees of 

two :rp.en each. Considerable shallow surf ace 

work has been done in past years, _and it is· 
MINES AND PROSPECTS. 65 

BLUE JAY MINE. probable that several thousand tons of oxidized , 

ore hus been extracted. Five Keystone drill 

holes were put down in the spring of 1914, the 

deepest. 700 feet deep, but th~ results have not 

The Blue Jay mine is 3 . niiles east of the 

town of Yerington, on the west fiank of the 

low range of bills that pi·ojects above the level 

floor of :Mason Valley. · The mine is de:veloped by a shaft 450 feet deep, fr?m which · 

drifts have been cut on the 200 and 400 foot 

levels. In 1914 a tunnel was being driven 

to cut the 'shaft 100 feet below the collar; 

this le~el will give a depth of 150 or 175 feet 

~der the outcrop. 

been disclosed. 

Some .s1nall bodies of quartz 1nonzonite 

porphyry project through the alluvium in tl?-e 

vicinity of the mine. The porphyry, owing to 

the prevalence of phenocryst~ of plagioclase 

(Ab;0An30 ), orthoclase, biotite, and sporadic 

quartz, closely resen1bles a granitic rock, but 

under the microscope it 'is found to have a 

finely granuln:r 'groundinass of orthoclase and 

quartz. Part of the biotite 'is derived_ ~rom· 

hornblende and has been largely chlont1zed. 

The porphyry has been irregularly fractured aiid 

silicified, and the silicification was apparently 

accomp.anied in places by the development of 

secondary biotite. Bunches of ore occur in the 

fractured porphyry and co.nsistlargely of cuprite, 

which is in places altered to 1nalachite and other 

oxidized compounds. Pyrite or other sulphides 

have not yet been. found. 

JIM BEATTY PROSPECT. 

The Jim Beatty prospect is at the southwest 

corner of the area shown on Plate I. The 

workings consist o.f two. tunnels, which enter 

from opposite. sides of a small hill and intersect, 

and open cuts extending for several hundred 

feet. along the vein between .the tunnels. 

Several shipments of ore· have been made, 

a-mounting to a few tons. .A lo~ shipped in 

1914 is said to have carried $65 a ton in gold. 

The ore body, inclosed in quartz monzonite, 

consists of an 8-inch vein containing pyrite, 

chalcopyrite, 1ninor galena, and their oxidation 

products in a gangue of coarse quartz and 

calcite. · 

TERRY & McFARLAND PROSPECT. 

The Terry & :McFarland prospect is near 

tho southwest corner of the area 1napped. 

The principal development work is a tunnel 

380 feet long. This tunnel follows a shear z~ne 

in quartz n1onzo1lite which contains along mo:;;t 

of its course an iron-stained quartz vein ranging fr01n a few inches to 1 foot in thiclmess. 

The vein trends N. 15° W. and dips 70° W. 

Ore on the ·dmnp is coated with druses of 

malachite. The average value of the ore is 

said to be. $8 a ton in gold. 

:38330°-18-5 

· ·The prevailing country rock is ·a Jnediuingrained granodiorite close to quartz diorite 

in c01nposition. It is composed of oligoclase, 

biotite,' augite~ ho~nblende, quartz, and orthoclase and is a s01newhat n1ore ba·sic variety 

of granodiorite than that on the west side of 

:Mason Valley. The ore deposit occurs in a 

crushed zone in the granodiorite, '\vhich at 

the n1ine is 300 to 400 feet wide. , The crushed 

condition· ·of the granodiorite has favored its 

taking on a deeper coat of desert varnish than 

the· massive granodiorite, and the zone is in 

consequence· clearly visible froi:n Yerington 

as a dark band trending nearly at right angles 

across the range of hills. A dike of granodiorite porphyry, 10 feet wide, strikes across 

the shattered zone; at the south margin of the 

zone it ha.s been displaced 20 feet by the fissure 

that separates the n1assive fi·oni the shattered 

granodiorite. 

The porphyry' dike forn1s the footwall of 

the or~ body. Above it is a roughly crescentic outcrop of quartz grading out into 

r~placed granodiorite. A little chalcopyrite 

occurs in tight quartz, but as a rule the sulphides have been leached out. 

Ore from the 200-foot level is a gray rock 

carrying chalcopyrite. ·Under the riucroscope 

it is found to be con1posed largely of orthoclase 

and quartz, with chlorite as 'a nlinor constituent. Chal~opyrite and chalcocite are present, 

and the chalcocite is clearly of secondary origin. 

All stages of replacement, from solid grains 

·of chalcocite representing completely replaced 

grains of chalcopyrite to large particles of 

chalcopyrite peripherally coated with chalcocite, can be observed. Sonic brochantite 

accompanies the chalcocite. . This gray ore · 

appears to represent the extreme phase of 

alteration of the granodiorite by t_he primary 
.-

66 GEOLOGY AND ORE DEPOSITS_ OF Y~RINGTON DISTRICT, NEV.· 

mineralization, plus .a slight alteration accom,-

panying the enrichment by chalcocite. Less 

metamorphosed granodiorite is darker, owing 

to the prevalence of -chlorite; epidote also is 

noteworthy in this· phase; and pyrite occurs 

with the chalcopyrite. . · 

R-ecently a s~all rich shoot of "cuprite" 

ore was found near the collar of the shaft. 

About half of the "cuprite" proves to be chalcocite~ The ore ·consists _of stringers and 

masses of chalcocite ~nd -cuprite traversing 

a grayish or greenish rock which the mic1·q~ 

scope shows to be composed of. quartz whose 

structure indicates its origin by replacement, 

undoubtedly_ -of granodiorite. The chalcocite 

incloses sporadic grains of native copper, with 

which is associated- some brochantite. 

The oxidized ore ·seen by Ransome contained 

chrysocolla, malachite, cuprite, and the rare 

copper phosphate libethenite, previously identified from this deposit by Smith. The chalcocite ore was the objective of the operations 

in 1914, and some high-grade ore,' consisting 

of chalcopyrite almost wholly replaced by 

chalcocite, had been found at the time of 

visit. 
INDEX. 

Page. 

Albite, occurrence of ..................•....... , .... 0 •••••••••• 0.. 31 

Am phi bolo, genesis of ..... 0 0 •••• 0. 0 ••••••••••• o o •••• _ •••• o •••••••• 42-43 

Andesite breccia: natmo nnd occurrence or 0 •• 0 •• .'. 0 ••••••••• 0 •• 0. 26 

Andesite, 'rortiary, natmo and distribution of. . .' .. 0. 0 0 0:.0....... 26 

Trln.sslc, nature and occurrence of.. ..... 0 ••••• 0 ••••• 0. 0...... 13 

Andradite interstitial between pyroxene cryst:als; plate showing.. 40 

occurrence or 0 0 0 •• 0 • 0 0 0 0 •• 0 0 •• 0 • 0 0 0 0 •• 0 0 0 0 •• 0 0 0 0 0 0 0 0 0. 0 0 ° 0 ° 0 0 0 3( 

replacing limestone, plate showing ............... .' ........... · 36 

significance or, in contact metamorphism ..................... 45-46 

Anllydrito, origin of. ........ , ..... : .............................. 18-19 

Apatite, occurrence of.. .......................................... 31,39 

Aplite, nature nnd occurrence or. .................. 00 ............. 21-22 

Actinolite, occurrence and genesis of.............. . ........... 31, 39,42 

Azurite, occurrence of............................................ 33 

Page. 

Garnetite breccia partly replaced by epidote, plate Rhowing ..... ~ 40 

Garnehtcs, cpidotizntion of. ...................... : . . . . . . . . . . . . . . . 40 

nature and distribution of .................................... 16-17 

origin of ...................................................... li-18 

Geography of the district......................................... .9 

Geologic map of the district ................................. In pocket. 

Geologic sections across tho district, plate sho"ing................ 28 

Geology of the district ............ 0 •••••••••••••••••••• , • • • • • • • • • • 12-30 

Glomsrudkollcn, Norway, contact-metamorphic deposits at. 0 •••• • 47 

Granodiorite, nature and distribution or.......................... 20 

metamorphism by .............................. ·: . . .. .. . .. . . 44 

Greenwood prospect, description of.. .............. , ........... 0 •• 58 

Gypsum, origin and occurrence of. ............................ 18-19,34 

prevalence of, in the oxidized orcs ............................ 47-48 

Basalt, natnro and occurrence of.................................. 27 Ilalobia sp., garnctized, description of. ........................... 17-18 

BibHogrnphy of tho district ......................... :............ 9, 11 garnctizod, plato showing..................................... ~6 

Biotite, occurrence of:. ................................ ·.· ....... , 31-32,41 Hematite, occurrence of. ..................... .' ................... 32,39 

Black Hock prospect, oro deposit of .... , .... :.................... 49 Hicks, W, B., analysis by.'........................................ 16 

Bluo Jay mine, description of .................................... 65--{)6 History, geologic, of the district .... : ........................ , .... 29-30 

Bluestone mine, description of .......................... , ........ 50-51 Hornblende andesite, nature and occurrence of .................. : 26 

mineralization 111 •••••••••••••••••••••••••••••••••••••••••• 36, 40, 42 Igneous rocks, Cretaceous, nature o.n<l distribution or. .. ·.· ........ 19-23 

Brecciation, rol:ltion of oro deposit~ to ............................ 35-37 mines on ore deposits in .......................... :........... 64 

Brocbrut~ito, occurrence of.. .............. :....................... 33 See al8o Volcanic rocks, 

Calc-sllicato rocks, replacement of. ........ :...................... 40 Iron oxides, association. of, with contact-motmnorphic dopo~its ... . 46 

Calcite, occurrence of ............................................ . 

C":astlng Coppllr mine, description of ............................. . 

mineralization in ............................................. .36, 38 

63~~ Jim Beatty prospect, description of ..... 0 ••••••••••••••••••••••••• 

Jones, E. L., jr., 'vork of ......................... .' .............. . 

65 

9 

Chaleanthito, occurrence of.. ....... :............................. 33 Keratophyrcs, nature lUld distribution of. ........................ 13-16 

Chalcedony, occurrence of........................................ 33 Koipato formation, nature or .......................... : ..... <.... 19 

Chalcocite, formation and occurrence or. .......................... 3a, 48 

Chalcopyrite, origin and occurrence of. .................... , ...... 32,42 

Chlorite, occurrence of............................................ 32 

Chrysocolla, origin· and occurrence of. ........•.................... 33, 48 

Conglomomtos, nature and occnrronco of ...................... 23, 26-27 

·stream chn.nnol'.fillod by ....... 0 ............. 0 ....... 0 •• .' .... 0 '25-26. 

Contact-metamorphic oro deposits, criteria of ...... 0 ••• 0 •••••••• 0 .' 46-47 

deepest known ... 0 ••• 0 •• 0. ··o 0 •• 0 .'. 0 0 ••• 0 •••••••••••••• 0 ••••• 0 38 

distances or, from intrusive masses ... 0 ••••••••••••••••• 0 ••• 0.. 46 

mines and pro~ poets on .......... , ............................ 50-64 

Contact-metamorphism, pneumatolytic, features of .... 0 0 ••••• 0... 47 

Copper, nativo, occmronco of..................................... 33 

Copper pitch oro, origm and occurrence or ... 0 •••• 0. :·0 ••• 0 ••••• 33~'34, 48 

Covelli to, origin and occurrence of. ............... 0 •••••••• .' ••• 0 •• 34,48 

Cretaceous period, rocks of .................... 0 ••• 0 0 ••••• 0 •• 0 0 ••• 19-23 

Cuprite, o<!curronco of .......... 0 ••••••••••• 0 ••• 0 •••••••••• 0 ••• 0 0. 34 

Dacite, Triassic, occnrronco of................................... 13 

Diastrophism, time and extent or................................ BO 

Douglas Rill mine, description or.. ............................... 6?rli3 

.miuoralizntion in ................... : ...... 0 •••••••••••••••••• 36,38 

Empiro-Novnda mine, description of ....................... ; .. '·. fi4-65 

oro of. ............................................. ,......... 49 

Epidote, origin n.nd occurrence or.. ............................ 32, 39,42 

Erosion, times and extent of. .................................... 29,30 

Faulting, ~mturo and extent of. ..................... 00 ........... 28-2Q 

relation of oro doposi.ts to ..................................... 35-37 

thnos of .................... , ............. : ...... ·.. . .. . .. .. .. . 29, 30 

l!'olsito, metamorphism of ..... :. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 

soda rhyolite, nature and diRtrilmtion of... .................... 13-16 

l!'ield work, record of............................................. 9 

J?issure deposits, formation of. ........................... :....... 47 

]?o~~il, garnctizcd, dcRcription or.. ................................ 17-18 

Fossl.ls, occurrence of ... ~ ............. ~ ........................ 0 •• 12-13 

On lena, occnrronco or............................................. 49 

Garnet;, occnrronco or.. ............ : ........ : ..................... 31,:32 

Lacroix, A., cited .................................. 0 ••••••••••••• ~ 45 

Latitc, quartz, nature and occurrence or ...... 0................... 25 

Latito vitrophyre, nature and occurrence or ............. 0 ••••• ; •• 23-25 

photomicrograph or .•••... ·. 0. 0 ••• 0. 0. 0. 0 0 ••• 0 0 0 0 0 0. 0. 0. 0 0. 0.. 16 

Leith, C. K., recrystallization hypotheSis of...................... 45 

Libcthcnitc, occurrence or..' .......... 0 .... 0...................... 34 

Limestone, composition of ........................................ 34-35 

incomplete replacement of.................................... 18 

junction of, with andradite replacing limestone, plato showing. 36 

Lindgren, Waldemar, cited...................................... 38 

Ludwig mi~c, general features of •........................•....... 58-59 

· geology of. .................................................... 59-C.O 

gypsumin ................................................ 5!J,G1,62 

mineralization in ............. 0. 0 ..... 0 0 ...... : .. 0 0 .. 3G, 38, 39, 48-49 

orcs of ........................................................ 60-62 

McConnell mine, description of. .............................. : ... 55-57 

mineralization in ........ 0 :. • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 37 

Triassic fossil found ncar ....... 0. . . . . . . . . . . . . . . . . . . . . . . . . . . . • 17, 56 

·Magmatic solutions, probable action or ...... ·..................... 4G 

Magnetite, occurrence of.. ........................... ; ........ : ..... 32, 39 

1\!alachito, occurren<:Jo of.......................................... 34 

Malac4ite mine, description of ................................ .' .. 0 55 

Mason VaHey mine, geologic features of..-............... , ......... 52-55 

history or. ................................................... _ 51-52 

mineralization in .•............................ 0 ••••••••••• 36, 37,38 

Merriam, J. C., fossils determined by............................. 27 

Mining, history or.. ............................................... 11-12 

Montana-Yerington mine, description or.. ........................ 49,64 

Opal, occurrence of..... .. .. .. . .. .. .. .. .. .. . . .. . .. . .. .. . . .. .. .. .. . 34 

Ore deposits, ago of.. 0 •••••••••••••••••••••••• 0 ••••• 0 ••••••••••••• • 43-44 

~~~~~~~~:~lt.~~~~~i~: ~::::: :·:::::::::: ::::::: :: :·:: :::::: ::::: 3~~ . 

form and dimensions or..................... .. . .. .. .. .. .. .. .. . 38 

formation or .. 0 .... 0 0 0 0 ..... 0 .... 0 0 0 0 0 0 0 0 0; 0 0 0 0 0 0 0 0 0 0 0 0 0 0 .. 0 0 0 44-47 

general features or 0 0 0 0 0. 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 • 0 0 0 0 0 0 31 

in igneous rockR, mines on ..................................... G4.-66 

67 
68 INDEX. 

Page. 

Oro deposits, minerals or ................................. : ....... 31-34 

occurrence of, adjoining limestone............................ 38 

oxidation in .................................. · ................ 47-49 

relation of, to faulting and brecciation ........................ 35-37 

sulphide enrichment. in ....................................... 47-49 

Outline ofreport •............. ,. ............................... ·... 7-8 

Paragenesis, evidence on .•....................................... 41-43 

Pine Nut Mountains, ·Triassic fossils in .............. ~............ 19 

Porphyry, garnetized, silicified, and pyritizod, plate showing..... 36 

quartz monzonite, nature and occurrence of .................. 22-23 

· transformation of ........................ : ................ 39-40 

Production ofcoppcrin the district................................ 12 

Pyrite, origin and occurrence of ..... ·.· ............................ 33, 42 

Pyroxene, composition and occurrence of......................... 32 

genesis of. ................... ~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 

Page. 

Smith, D. T., analysis by........................................ 20 

Soda rhyolite felsites, nature and distribution of. . . . . . . . . . . . . . . . . . . 13-16 

Solutions, ore-forming, composition of............................ 41 

Star Peak formation, nature of. ....................... ·. . . .. . . . . . . 19 

Stanton, T. W., fossils determined by ............................ 13,17 

Steiger, George, analyses by ............... , ...................... 20,21 

Structure, geologic, of the district ......... : ................... :. . 23-29 

Sulphides, supergene, occurrence of.... . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 

Temperatures, critical, of oro-forming solutions .................... 44-45 

Torry & McFarland prospect, description of .... ·.................. 65 

. Tertiary period, rocks of, correlation· of........................... 27 

rocks of, nature and distribution of. ......................... :. 23-27 

Tourmaline, occurrence of. ........ · ............................... · 33, 41 

Triassic period, rocks of, correlation of.. . . . . . . . . . . . . . . . . . . . . . . . . . . 19 

rocks of, metamorphic ....................... · ..... : ............ lG-19 

Quartz, occurrence of.. .................................... .'. . . . . . 33 • nature of. ........ · ................. :. . . . . . . . . . . . . . . . . . . . . . 12-13 

Quartz sedimentary.... .. . .. .. . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . 16 latitos, nature and occurrence of. ... : ............... ,.:... 25 

Quartz monzonite, metamorphism by............................ 44 volcanic .................................................. 13-16. 

nature and distribution of. ................................... 20-21 

sea politic alteration of........................................ 21 

Quartz monzonite porphyrt", nature and occurrence of. ............ 22-23 

Quaternary period, deposits of. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27-28 

Railroads in the district.......................................... 9, 11 

Ransome, F. L., preface by ............ ·.......................... 5 

Recrystallization hypothesis, objections to .. ; ................... . 45 

Vitrophyre, latite, nature and occurrence of ...................... 23-25 

Volcanic rocks: · 

Tertiary, nature and distribution of ................ : ........ 23-27 

Triassic, nature and distribution of .......................... 13-16 

. . See also Igneous rocks. 

Volcanism, times and extent of.. .............................. : .. 30 

Rhyolite: Wells, R. C., analyses by .......................... , .............. 15,32 

Triassic, analysis of..... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 

Tertiary, nature and occurrence of. ... ; ...................... 25-26 

Wostc;n·Ne_va~a ~ine, description of.: .......................... 57-58 

rmneralizat10n m •.. , ......................................... 36,42 

Sand Canyon, origin of .................. : . . . . . . . . . . . . . . . . . . . . . . . . 29 

Silicates, formation of............................................ 45 

Wheeler, ,V. C., analysis by ............................. :........ 35 

Woodward, M. R., analysis by ... ·.·.............................. 19 

Silicates, recog1:1ition of contact ore deposits through .. ~ ... : ...... 46-47 

.ADDITIONAL COPIES 

OF THIS PUBLICATION.MAY BE PROCURED '!<'ROM 

THE SUPERINTENDENT OF DOCUMENTS 

GOVERNMENT PRINTING OFnCE 

WASHINGTON, D. C. 

AT 

20 CENTS PER COPY 

'\1