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Chapter E

Gold in Porphyry Copper Systems

Gold in the Bingham District, Utah 

By EDWIN W. TOOKER

Gold in the Butte District, Montana 

By EDWIN W. TOOKER

Gold in the Ely (Robinson) Copper District, 

White Pine County, Nevada

By LAURENCE P.JAMES

The Tomboy-Minnie Gold Deposits at Copper Canyon, 

Lander County, Nevada

By TED G. THEODORE, STEPHEN S. HOWE, 

and DAVID W. BLAKE

U.S. GEOLOGICAL SURVEY BULLETIN 1857

GEOLOGY AND RESOURCES OF GOLD IN THE UNITED STATES

DANIEL R. SHAWE and ROGER P. ASHLEY, Scientific Editors 

LM.H. CARTER, Technical Editor
DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, JR., Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L Peck, Director

Any use of trade, product, or firm names in this publication is for descriptive 

purposes only and does not imply endorsement by the U.S. Government.

UNITED STATES GOVERNMENT PRINTING OFFICE: 1990

For sale by the

Books and Open-File Reports Section

U.S. Geological Survey

Federal Center

Box 25425

Denver, CO 80225

Library of Congress Cataloging-in-Publication Data

Gold in porphyry copper systems.

p. cm. (Geology and resources of gold in the United States ; ch. E) (U.S. 

Geological Survey Bulletin ; 1857)

Includes bibliographical references.

Contents: Gold in the Bingham District, Utah / by Edwin W. looker Gold in 

the Butte District, Montana / by Edwin W. looker Gold in the Ely (Robinson) 

Copper District, White Pine County, Nevada / by Laurence P. James The 

Tomboy-Minnie deposits at Copper Canyon, Lander County, Nevada / by Ted 

G. Theodore, Stephen S. Howe, and David W. Blake.

1. Gold ores West (U.S.) 2. Copper ores West (U.S.) 

3. Geology-West (U.S.) I. Tooker, Edwin Wilson, 1923- . II. Series. 

III. Series: U.S. Geological Survey bulletin ; 1857. 

QE75.B9 no. 1857-E 

[TN423.A5] 

557.3 s-dc20

[553.4' 1 '0978] 89-600379

CIP
CONTENTS

Gold in the Bingham district, Utah

Abstract El 

Introduction E2 

Production and reserves E3 

Geologic setting E4

Stratigraphy E5

Structure E7

Intrusive rocks E8

Alteration E8

Age of ore formation E9

Metallization environment E10 

Gold deposits in the Bingham district E10

Dispersed porphyry gold Ell

Vein and replacement gold £12

Skarn gold E14

Placer gold E14

Disseminated gold E14 

Conclusions E14 

References cited E15 

Appendix 1990 update E16

Gold in the Butte district, Montana

Abstract E17 

Introduction E17 

Production and reserves E17 

Geologic features E19

Regional setting E19 

District setting E20

Host rocks E20

Structures E21

Evolving environment of ore deposition E22

Zonal distribution of Main Stage base and precious metals E25 

Precious metals in Butte ores E25 

Conclusions E26 

References cited E26 

Appendix 1990 update E27

Contents III
Gold in the Ely (Robinson) copper district, White Pine County, 

Nevada

Abstract E28 

Introduction E28

Acknowledgments E30 

Geologic setting E32

Structure and igneous rocks E32

Hydrothermal alteration E33 

Ore deposits E33

Characteristics of gold ores E37

Ore fluids E40 

Conclusions E40 

References cited E41 

Appendix 1990 update E42

The Tomboy-Minnie gold deposits at Copper Canyon, 

Lander County, Nevada

Abstract E43 

Introduction E43

History of reserves and mine development E43

Ore processing E45

Metal production E45

Significance of skarn-associated gold systems E46 

Geologic setting of the Copper Canyon deposits E46 

Tomboy-Minnie gold deposits E47 

Metal zoning and Au:Ag ratios E50 

Studies of fluid inclusions and stable isotopes of sulfur E52 

Conclusions E54 

References cited E54

FIGURES

El. Map showing location of Bingham mining district, Utah, and of section A-A' 

of figure E4 E2

E2. Graph comparing relative cumulative amounts of gold produced from the 

four types of gold deposits during their respective eras of production, and 

estimates of available (or inferred) reserves E5

E3. Map showing surface projections of dispersed, vein and replacement, skarn, 

and placer ore bodies in the Bingham district E6

E4. Northeast-trending cross section A-A', Oquirrh Mountains, from Soldier 

Canyon to the Bingham mining district E7

E5. Section of Paleozoic rocks exposed in the Bingham allochthon in the Bingham district E8

E6. Map showing general geologic setting of the Bingham mining district E9

IV Contents
E7. Sketch showing inferred paths of Sevier foreland thrust nappe movement

against the craton promontory, Bingham Canyon quadrangle E10 

E8. Plan map showing distribution of types of silicate alteration of intrusive

rocks at Bingham Ell 

E9. North-south cross section B-B' of figure E3 through the BP Minerals

America (Kennecott) deposit in the Bingham district, showing distribution

of host rocks and sulfide mineral zones Ell 

E10. Map of western Montana, showing location of the Butte mining district and

its regional geologic setting E18 

Ell. Generalized map showing location of main mining shafts and open pits,

principal lithologies, and vein structures of the Butte mining district E21 

E12. Generalized plan of the 2800 level of the Butte district, Montana, showing

the main vein, fault, and dike systems in Butte Quartz Monzonite, and the

Main Stage mineral zones E23 

E13. Generalized north-south cross section through the Butte district, vicinity of

the Berkeley pit, showing location of molybdenum dome, domain of

pervasive biotitic alteration, the Main Stage vein zone, supergene chalcocite

enrichment blanket, and zone of surface oxidation E24

E14. Map showing location of the Ely (Robinson) mining district, Nevada E29 

E15. Generalized geologic map of the Ely district, Nevada, showing localities of

dated igneous rock samples E30

E16. Map of Ely district showing intensely altered ground, copper- and goldmineralized areas, and mine locations E34 

E17. Cross section through the northwest end of the Tripp porphyry copper pit,

Ely district, southwest of New Ruth, Nevada E36 

E18. Generalized geologic map of the Star Pointer disseminated gold

deposit E38

El9. Generalized cross section of the Star Pointer gold deposit E39 

E20. Geologic sketch map of Copper Canyon area, Nevada E44 

E21. Generalized geologic map and cross section of Tomboy-Minnie area,

Nevada E48 

E22. Idealized northeast-trending cross section through the Tomboy-Minnie

deposits E49 

E23. Map showing zonal distribution of metals and Au:Ag ratios in the Copper

Canyon area E51 

E24. Geologic sketch map of the Tomboy-Minnie deposits showing areas that

include Au:Ag ratios > 1 E53

TABLES

El. Production and average grade of gold deposits, Bingham mining district,

Utah E3

E2. Gold production, Bingham district E4

E3. Gold produced from types of ore mined, Bingham district E4 

E4. Average modal composition of Bingham stock intrusives E8 

E5. Geochronology of intrusives, alteration, mineral deposition, and volcanic

rocks E10 

E6. Amount of base and precious metals recovered in vein and replacement

ores through 1964 compared with that in dispersed intrusive ores E12

Contents
E7. Grades of byproduct gold in nonporphyry ores, 1909-1946 E12

E8. Average grade by level in a bedding fissure ore body, Lark mine E13

E9. Typical grades of gold in lead-zinc replacement ore shoots in the

Lark mine E13 

E10. Cumulative gold and associated metal production, Butte mining district,

Montana E19 

Ell. General sequence of igneous, hydrothermal, and structural events in Butte

mining district E20 

E12. Summary of geologic attributes of ore zone domains in Butte mining

district E24 

E13. Gold and silver content (recovered) of copper ores, Ely, Nevada E37

VI Content*
GEOLOGY AND RESOURCES OF GOLD IN THE UNITED STATES

Gold in Porphyry Copper Systems

Gold in the Bingham District, Utah

By Edwin W. looker

Abstract

The Bingham mining district, Utah, is a major United 

States producer of gold as a byproduct of copper and 

molybdenum sulfides dispersed in an intrusive quartz monzonite porphyry stock and in skarns in adjoining carbonate 

sedimentary rocks. The district has also produced substantial 

amounts of gold, silver, lead, and zinc in poly metallic veins 

and bedded replacements of carbonate-rich sedimentary 

rocks in and peripheral to the intrusive body. Mining of the 

gold-bearing lead-silver lodes and the gold placers began in 

1863. Production from the porphyry deposit started in 1906; 

that from the skarn ores began in 1979. The average grade of 

dispersed gold is believed to be in the range of 0.31- 

0.62 grams (0.01-0.02 troy ounce) per ton; the grade of skarn 

gold is as high as 1.5 grams (0.045 ounce) per ton. The gold 

production from the Bingham district exceeds 603 metric 

tons (19.4 million troy ounces), and the estimated district's 

reserve of gold is well in excess of 505.6 metric tons. 

Sediment-hosted disseminated primary gold discovered 

recently in an area north of the Bingham open pit mine is 

expected to begin production in 1989. A grade of 2.4 grams 

per ton has been identified, and a reserve of 12.4 metric tons 

(400,000 ounces) has been announced.

The Bingham ore deposits occur mainly in the allochthonous folded upper plate of the Bingham nappe, one of the 

foreland Sevier thrust lobes that were emplaced from the 

southwest onto a basement promontory aligned with the 

east-west-trending Uinta-Cortez axial zone. This zone is the 

locus of a number of intrusive porphyry stocks, dikes, and 

sills, and extrusive rocks in western Utah. The main stratigraphic host is the Upper Pennsylvanian Bingham Mine 

Formation of the Oquirrh Group, which consists of cyclically 

interbedded medium- to thick-bedded silica- and carbonatecemented quartz it e, calcareous sandstone, and arenaceous,

Manuscript approved for publication August 1, 1989.

fossiliferous, cherty, argillaceous, and dense limestones. 

Prominent close-spaced northeast-trending faults, which 

resulted from tension by arching during thrust emplacement of the overturned Bingham syncline and an adjoining 

broad anticlinal fold, are the locus of the extensive porphyry 

and vein deposits in and peripheral to the stock.

The district is centered on the composite porphyritic 

and equigranular quartz monzonitic, hydrothermally altered, 

mineralized Bingham stock and associated later intrusive 

dikes and sills, and extrusive volcanic rocks. The ores are 

zoned about the central Bingham stock: The dispersed 

deposit has a central low-grade core containing magnetite, 

and successively a molybdenite zone low in copper, a 

bornite-chalcopyrite-gold higher-grade zone, a pyrrtechalcopyrite zone, a pyrite zone, and an outermost lead-zinc 

zone. The central porphyry ores were formed from hot 

(400-600 °C), highly saline, mantle-derived hydrothermal 

fluids under a pressure of about 800 bars. Rock shattering by 

boiling produced breccia pipes. The outer vein and 

replacement deposits in the sedimentary sequence formed 

at lower temperatures (about 300 °C) as a result of mixing of 

magmatic and meteoric waters. The dispersed gold occurs 

mainly in a network of close-spaced thin fractures in the 

central porphyry body; locally the gold is disseminated in the 

altered fracture blocks in the copper-rich zone as the native 

metal and as telluride minerals. Gold reportedly was enriched 

in the upper parts of the porphyry intrusion. Skarn gold 

deposits also are associated with high-grade copper ores. 

Poly metallic vein gold deposits occur as siliceous gold-silver 

fissures; gold is associated with copper, lead, and zinc ores, 

generally in enriched ore shoots. Native gold is rare in the 

peripheral lead-zinc ores. Replacement gold also occurs 

along bedding fissure zones in small local shoots. Native(?) 

gold is disseminated in altered carbonate-rich (dolomitic) 

rocks at the neighboring Barneys Canyon deposit.

Gold in Porphyry Copper Systems E1
INTRODUCTION

The Bingham mining district, one of the major 

producers of gold in the United States, is located mainly 

in the headwaters of Bingham Canyon on the east side of 

the Oquirrh Mountains, about 32 km southwest of Salt 

Lake City, Utah (fig. El). The Bingham (originally West 

Mountain) district broadly includes the open pit copper 

mine centered on a quartz monzonitic composite porphyritic and equigranular intrusion in Bingham Canyon, 

the underground skarn and polymetallic base and 

precious metal vein and replacement deposits in the 

carbonate rocks peripheral to the intrusion, and the now 

exhausted placers. Sediment-hosted disseminated gold 

has been discovered recently by Kennecott (now known 

as BP Minerals America) in the greater Bingham area. 

This richly mineralized Bingham mining region covers an 

area of more than 16 km2 and includes parts of several 

adjacent drainage basins.

Early development of the district was hampered by 

its remote location and by economic conditions. The 

discovery of the lead-silver lodes by the Bingham 

brothers was reported in local newspapers in 1860.

112°

WASATCH FAULT

EXPLANATION

Strike-slip fault Arrows show direction 

of movement; dotted where concealed

Thrust fault Dashed where inferred/- 

teeth on upper plate

Anticline Axial arrow shows plunge 

Syncline

Figure E1. Location of Bingham mining district and other 

mining districts in the Oquirrh Mountains area, Utah, and of 

section A-A' of figure E4.

George Ogilvie "rediscovered" the deposit in 1863, and, 

in collaboration with Col. P.E. Connor and soldiers of the 

U.S. Army stationed in the Utah territory, located the 

first lode mining claims and organized the West 

Mountain mining district. Gold placers in thick gravels 

along Bingham Creek were discovered in 1864. The 

original mining claim was patented in 1877. In a review of 

the early history of the district, James (1978) noted that 

early mining was sporadic owing to transportation and 

smelting difficulties. The advent of the railroad to Bingham Canyon in 1873 encouraged mining briefly, but the 

relatively low prices of lead, zinc, and silver of that period 

discouraged mining activities for a time. By 1896 nearsurface high-grade copper sulfide lodes were being 

mined underground, and until 1971 gold continued to be 

an important byproduct of the copper-lead-zinc-silver 

vein and manto replacement deposits peripheral to the 

intrusion (Hunt, 1924; Rubright and Hart, 1968; U.S. 

Bureau of Mines Minerals Yearbooks, 1961-1981).

After 1900, the technology was developed to mine 

and recover low-grade copper dispersed in porphyritic 

rocks and enclosing sedimentary rocks, and necessary 

smelters were built nearby. In 1906 open pit mining for 

copper began and byproduct gold was recovered during 

milling of the ore. By 1911 the small mining enterprises 

(once numbering 21) were beginning to be consolidated, 

ultimately to form three major-operations; in 1970, when 

UV Industries, Inc. (formerly U.S. Smelting, Refining, 

and Mining Co.) ceased operations, that number was 

reduced to two, Kennecott and The Anaconda Minerals 

Company. In the mid-1970's development was considered feasible for the copper and the byproduct gold in 

skarn ores in the metamorphosed carbonate rocks 

adjacent to the northwestern side of the Bingham stock, 

underlying the Carr Fork and main Bingham Canyon 

areas (Atkinson and Einaudi, 1978). Limited production 

was begun at Carr Fork in 1979; however, low prices and 

a world oversupply of copper temporarily discouraged 

production of byproduct gold from the skarn ores. 

Continued unfavorable economic conditions forced the 

close of Kennecott's Utah Copper open pit mine in 1985, 

which was the last remaining operation in the district. 

Although down, the Bingham Canyon mine was not out, 

and limited production resumed in 1987 concurrent with 

a massive modernization program for its mining and 

milling facilities (Skillings Mining Review, 1985). In 

1986, Kennecott, then a subsidiary of SOHIO (Standard 

Oil Company, 1986), announced the discovery of a 

substantial deposit of low-grade sediment-hosted disseminated gold in Barneys Canyon, an area about 6.5 km 

north of the open pit mine (see fig. E7).

The purpose of this report is to consolidate and 

review the scattered published data on the occurrence 

and magnitude of the often-overlooked several overlapping types of gold deposition in the Bingham district.

E2 Geology and Resources of Gold in the United States
Specific details on the geologic environment and 

resources of byproduct gold in the dispersed ore body 

have not been readily available until recently, and some 

conclusions may be inferred from other scattered 

published records. The gold deposits at Bingham are 

unique with respect to their size, but are not unlike 

subvolcanic porphyritic granitic pluton-centered vein and 

replacement deposits in the Cordilleran region and 

beyond. The Ely, Nevada, and Battle Mountain, Nevada, 

deposits (described elsewhere in this chapter) are, in 

part, comparable deposit types.

I am pleased to acknowledge the important geologic contributions by staffs of Kennecott, the Anaconda 

Minerals Company, and U.V. Industries during the past 

125 years, which form an important basis for this report. 

Critical reviews by A. Jaren Swensen, Kennecott, Bingham Canyon, Utah, and R.D. Rubright, West Jordan, 

Utah, are appreciated; however, geologic interpretations 

herein remain my responsibility. The contributions of 

U.S. Geological Survey personnel and colleagues, 

particularly MacKenzie Gordon, Jr., R.C. Douglass, and 

WJ. Moore in geochronology and stratigraphic interpretation of formational units, are also gratefully 

acknowledged.

PRODUCTION AND RESERVES

Until recently gold production and reserve data for 

the Bingham district could only be estimated because of

proprietary restrictions on their publication. About 

603 metric tons (19.4 million oz) of gold have been 

produced mainly from four types of deposits (fig. E2). 

About 2.3 metric tons (75,000 oz) of placer gold was won 

from the gravels in Bingham Canyon creek and its 

tributaries (table El and fig. E3) in the early days of the 

district. The first four decades of production from polymetallic vein and replacement deposits averaged nearly 

0.28 metric ton (9,000 oz) per year (table E2). Grade and 

tonnage of typical ores mined and the gold recovery from 

various types of vein and replacement ores are shown in 

table E3. More than 75 metric tons (2.4 million oz) of 

gold was produced as a byproduct of copper-lead-zincsilver ores in the vein and replacement deposits in 

carbonate and intrusive rocks ringing the porphyritic 

stock; the last of these mines was closed in 1971. The 

advent of mining of the dispersed copper and molybdenum ores resulted in a substantial increase in annual 

production of gold from the district (table E2), and for 

many years this byproduct source has been the second 

most abundant domestic source of gold, whose 

production totals nearly 526 metric tons (16.9 million 

oz). In the three years (1982-1984) preceding closing of 

the Utah Copper mine in 1985, dispersed gold production averaged 0.179 metric tons (5,755 oz), as well 

as 163,000 metric tons copper, 74.6 metric tons (2.4 

million oz) silver, and 1,800 metric tons molybdenum 

metal per year (British Petroleum Company, 1986). In 

addition, byproduct platinum group metals, uranium,

Table E1. Production and average grade of gold deposits, Bingham mining district, Utah

Type of ore Years Production of gold 

(1000 g) (1000 oz)

Average

grade 

(oz Au/ton)

References

Placer

Dispersed in 

porphyry.

Total 1 

Vein and replacement 

peripheral to 

porphyry.

Total 

Skarn in limestone

GRAND TOTAL 

1864-1935

1904-1961 

1962-1980

1982-1986

1863-1964 

1965-1971

1979-1981 2

2,333 75

351,468 11,300

155,517 5,000

19,160 616

528,478 16,990

74,648 2,400 

not available

>74,648 >2,400 

1,244 40

Johnson (1973).

0.010 Stowe (1975), Hatnmond (1961), 

and U.S. Bureau of Mines 

(1961-1981).

0.013 British Petroleum (1986).

0.02 Rubright and Hart (1968).

0.05-0.03 Atkinson and Einaudi (1978), 

W.J. Garmoe (oral commun., 

1982, 1987).

>604,370 M9.430

^Reserves of 335.04 metric tons (10.8 million oz) gold inferred minable by open pit in Bingham Canyon 

deposit.

o

''Production did not reach planned level before shut down; reserves of about 143 metric tons (4.6 

million oz) of gold are inferred in the Carr Fork and Bingham Canyon skarn deposits.

Gold In Porphyry Copper Systems E3
Table E2. Gold production, Bingham district (Butler, 1920)

Average

Years Production recovery Principal ores 

(oz Au) (oz/year) mined

1865-1904 338,000

1904-1917 1,177,000

8,700 Vein and replacement ores.

90,500 Dispersed, vein, 

and replacement 

ores.

selenium, tellurium, silver, and rhenium were also 

recovered. The latest forecast estimate for production, 

following plant modernization in late 1988, is 70,000 

short tons of ore per day, from which 168,000 short tons 

of refined copper, 8.087 metric tons (260,000 oz) of gold, 

62.207 metric tons (2.0 million oz) of silver, and 3,600 

metric tons of molybdenum per year will be produced 

(Mining Journal, 1987). Skarn gold, also a byproduct of 

copper-molybdenum-silver mining, amounted to 1.244 

metric tons (40,000 oz) during its initial 3 years 

(1978-1981) of production at the Carr Fork deposit, 

prior to its premature closure in 1982 (Cameron and 

Garmoe, 1987).

A large reserve of gold in the Bingham district, 

until recently in large part undisclosed, is now estimated 

to be at least 505.6 metric tons (16.3 million oz). Placers 

were considered exhausted in the early 1940's (Johnson, 

1973). Byproduct gold resources in the United States 

mine are considered minimal, but gold reserves in the 

Lark mine were calculated to be at least 0.653 metric ton 

(21,000 oz) when these mines were closed (R.D. 

Rubright, written commun., 1983). Reserves of byproduct dispersed gold in the Utah Copper ore body are 335 

metric tons (10.8 million oz). The ores also include 4.9 

million metric tons copper, 2,317.36 metric tons (75.5 

million oz) silver, and 350,000 metric tons molybdenum

(British Petroleum Company, 1986, p. 112). Skarn ore 

reserves in the Carr Fork mine are estimated to be about 

31 metric tons (1 million oz) gold (Cameron and Garmoe, 1987); a larger reserve present in the underground 

mine to be developed by BP Minerals America, which 

includes a northward extension of the Carr Fork deposits 

(fig. E3), is estimated at 127 metric tons (4,127,500 oz) 

gold, 2.28 million metric tons copper, 1,694 metric tons 

(54,462,000 oz) silver, and 243,000 metric tons molybdenum (British Petroleum Co., 1986, p. 113). A reserve 

of at least 12.441 metric tons (400,000 oz) of gold was 

reported in an announcement of the discovery of a 

carbonate-hosted disseminated gold deposit in the Barneys Canyon area (Standard Oil Co., 1986, p. 40; British 

Petroleum Co., 1986, p. 114). Additional drilling underway and projected by BP Minerals America and 

American Barrick Resources (USA) undoubtedly will 

increase this reserve.

GEOLOGIC SETTING

In the Oquirrh Mountains thick sections of miogeoclinal carbonate-rich sedimentary rocks of late Paleozoic age make up the folded upper plates of three thrust 

fault nappes that were emplaced over the Precambrian 

craton platform during the Sevier orogeny of Cretaceous 

age (Tooker, 1983). These rocks were intruded and 

altered by subvolcanic granitoid intrusions of Tertiary 

age (Moore, 1973b). The structural and geochemical 

environment at Bingham became the site of deposition of 

gold, silver, and base metals during the late stages of this 

igneous event. In late Tertiary time, tensional listric 

faults in the eastern Basin and Range province tilted the 

Oquirrh Mountain block eastward, and exposed rocks 

subsequently eroded to the present surface.

Table E3. Gold produced from types of ore mined, Bingham district (from Butler, 1920)

Years

1903-1905, 1908-1917

1903-1917

1903-1917

1907-1913, 1916

1910-1911

1912-1913

Types of ore

Siliceous ore

Copper ore

Lead ore

Copper-lead ore

Lead-zinc ore

Zinc

Average ore 

mined 

(tons/year)

2,300

524,666

77,267

207

468

3,931

Average gold 

recovered 

(oz/ton)

0.2

.09

.08

.4

.07

.03

Average gold 

production 

(oz/year)

460

47,200

6,181

83

33

118

Includes rich ore shoots.

E4 Geology and Resources of Gold in the United States
o

O 10 

c/)

UJ

U

§ 8

O 

uu

O 6 

c/)

1 4

PLACERS

POLYMETALLIC VEIN AND 

REPLACEMENT

« 9

PAST PRODUCTION :-zDISPERSED (PORPHYRY) 

COPPER

^^ V'JSESERVESSiS

. .«$ '&& f'-^11^iflf i

SKARN COPPER

1860 1880 1900 1920 1940 1960

YEARS

1980 2000 2020

Figure E2. Comparison of the relative cumulative amounts of gold produced from the four types of gold deposits (solid line) 

during their respective eras of production, and estimates of available (or inferred) reserves, if any, remaining (dashed).

The stratigraphic and structural features of the 

sedimentary hosts, the petrologic and geochemical attributes of intrusive rocks, and the composition of accompanying hydrothermal solutions are detailed in reports 

beginning with BoutwelPs classic report (1905). This 

paper was followed by a number of geologic reports 

resulting from the 1919-1920 Ap&x lawsuits (Beeson, 

1917; Hunt, 1924; Peterson, 1924; and Winchell, 1924), 

and more recent studies sponsored by the mining companies (Hunt and Peacock, 1950; James, Smith, and 

Bray, 1961; James, Smith, and Welsh, 1961; Peters and 

others, 1966; Rubright and Hart, 1968; John, 1978; 

Lanier, John, and others, 1978; Warnaars and others, 

1978; and Atkinson and Einaudi, 1978).

Stratigraphy

The productive ore deposits at Bingham occur in 

allochthonous miogeoclinal cratonal (shelf) rocks 

including the upper part of the Butterfield Peaks Formation and the Bingham Mine Formation in the Bingham nappe, and the herein informally designated Flood 

Canyon and Dry Fork units in the Pass Canyon nappe. 

Strata on the Bingham plate comprise about 2,230 m of 

sedimentary rocks of Pennsylvanian (Desmoinesian to 

Virgilian) age (figs. E4 and E5) that have moved an 

unknown distance northeastward as a thrust nappe 

(Tooker, 1983). The Flood Canyon and Dry Fork

Gold in Porphyry Copper Systems E5
112° 07 30"

40° 32' 

30"

40° 

30'

(Inferred location of 

North ore body)

Lark mine

EXPLANATION 

Placer gold deposits

jj^^j Projection of vein and replacement 

Fswyftl base and precious metal deposits

|»; -v| Generalized projection of skam 

L -**ii*l copper deposits

[i vi ^1 Porphyritic quartz monzonite 

I > A I core of the Bingham stock

PHrzHIj Area of central chalcopyrite 

EHr-EHH] dispersed ore zone

.. " -. Boundary separating an inner copper 

:-.... .- ' from the outer silver-lead-zinc zones

< f Boundary of Utah Copper 

^ open pit mine

^ Projected and existing 

'"^-. stream courses

t f) Anticline, locally overturned

I .{J Syncline, locally overturned

T Fault, ball-bar on downthrown side

-f T* Thrust fault, teeth on upper plate

Figure E3. Surface projections of dispersed, vein and replacement, skarn, and placer ore bodies in the Bingham district, Utah 

(compiled from Rubright and Hart, 1968; Atkinson and Einaudi, 1978; and Johnson, 1973). Approximate line of section B-B' 

of figure E9.

units of Early Permian (Wolfcampian) age originally 

were thrust generally east on the Pass Canyon nappe 

plate prior to emplacement of the Bingham plate. The 

Pass Canyon nappe and Flood Canyon unit are named 

for prominent canyons on the west side of the range that 

empty into Tooele Valley in sections 8 and 6, respectively, T. 3 S., R. 3 W. The Dry Fork unit is named for 

exposures in Dry Fork, a tributary that empties into 

Bingham Canyon in sec. 13, T. 3 S., R. 3 W.

The stratigraphic section in the Bingham nappe is 

composed of the two uppermost formation al units in the 

Oquirrh Group. The Butterfield Peaks Formation 

(figs. E4-E6) contains a cyclically interbedded predominantly carbonate sequence of calcareous quartzite, 

orthoquartzite, calcareous sandstone, and fossiliferous, 

arenaceous, cherty, argillaceous, and fine-grained dense 

limestone beds. The Clipper Ridge Member, the lower 

member of the Bingham Mine Formation, is composed 

predominantly of cyclically interbedded orthoquartzite, 

calcareous quartzite, and quartzose sandstone interbedded with several thick limestone layers. The Markham Peak Member, the upper member of the Bingham 

Mine Formation, is composed of orthoquartzite, calcareous quartzite, calcareous sandstone, and a few thin

fusulinid-bearing arenaceous limestone units (Swensen, 

1975; Tooker and Roberts, 1970).1 Thick limestone units 

in the lower part of the Clipper Ridge Member and 

thinner beds in the Markham Peak Member are the 

principal units that were faulted, cut by stocks, and 

altered to skarns and (or) mineralized adjacent to the 

stocks.

The Barneys Canyon disseminated gold prospect 

occurs in tightly folded, faulted, and sheared rocks of the 

allochthonous Flood Canyon unit of the Pass Canyon 

nappe in sec. 34, T. 2 S., R. 3 W. This area, which 

contains a number of small old mines and prospects, is 

a few kilometers north of the open pit mine porphyry ore 

bodies. The host rocks of Permian age consist of a series 

of light-brown to reddish-brown calcareous quartzite 

beds at the base followed upward by light-gray or tan 

calcareous sandstone, intraformational breccia and interbedded arenaceous limestone, dolomite, dolomitic

JThe top 370 m of the upper member includes strata described 

by Tooker and Roberts (1970) as the upper part of the Markham Peak 

Member, Bingham Mine Formation, and by James, Smith and Welsh 

(1961) as the lower part of the Curry Peak Formation of James, Smith, 

and Welsh (1961) and of Swensen (1975).

E6 Geology and Resources of Gold in the United States
A'

FEET

24,000

14,000

2,000

0

-2,000

Top of Oquirrh Group

LONG RIDGE 

ANTICLINE

**X

 .POLE CANYON 

' .SYNCLINE.. Upper Member 

Lower Member

Faults of northeast system

 .''MIDDLE CANYON 

.' ; SYNCLINE

Upper strand 

Midas Thrust.

- A Soldier 

m Can yon

r^

_ __?_ ? nns

Figure E4. Northeast-trending cross section A-A', Oquirrh Mountains, from Soldier Canyon to the Bingham 

mining district, showing characteristic fold deformation in the Bingham allochthon (modified from looker, 1971). 

Location of section, figure E1.

limestone, calcareous sandstone, and quartzitic sandstone. Sedimentary rocks are cut by altered porphyry 

dikes.

Structure

A series of structural events preceding the 

intrusion of igneous rocks prepared this area for the 

formation of the mineral deposits at Bingham (Tooker, 

1983). The district overlies an inferred buried east-west 

cratonal lineament or suture zone (Uinta-Cortez axis of 

Roberts and others, 1965), which, locally along its length, 

has been a site for occasional uplift during Phanerozoic 

time (Tooker, 1971); it is also the locus of a family of 

Tertiary granitic intrusions (Moore and McKee, 1983) 

that cut Paleozoic and Mesozoic miogeoclinal strata 

overlying the Precambrian craton platform rocks. A 

foreland craton buttress against which the Sevier overthrusts of Cretaceous age were obstructed, deflected, 

deformed, and halted (Tooker, 1983) consisted of an 

uplifted promontory along the Uinta-Cortez lineament in 

the vicinity of Salt Lake City. At least three Sevier thrust 

nappe lobes converged on the buttress located east of the 

northern Oquirrh Mountains (fig. E7; Tooker and 

Roberts, 1988). The main gold-bearing ore deposits 

occur in the Bingham nappe plate, the second lobe to be

emplaced from the southwest along the Midas thrust 

fault2 system. The thrust system produced asymmetrical 

to locally overturned folds in an arc convex to the 

northeast, as shown in figure E6. Segments of folds in the 

Bingham area locally are overturned to the northeast, 

and where the fold curvature is tightest, bedding shears 

and small intraplate thrusts were formed locally. Tension 

within the arc produced a prominent conjugate set of 

steeply dipping northeast- and northwest-trending cross 

faults. Vein, replacement, and skarn deposits occur 

mainly near these cross, thrust, or bedding faults, and 

outward from the intrusions along reactive beds.

The Last Chance and Bingham stocks, and associated plugs and dikes, were intruded into structurally 

disrupted adjoining parts of the Bingham and Pass 

Canyon nappes. Intrusion of the stocks provided local 

dilation of the northeast-trending fault set. The

*The local versus regional characterization of the Midas thrust, 

as defined originally in mapping by James, Smith, and Welsh (1961), 

and the Midas thrust system, required by the geometry implicit in 

regional mapping by Tooker and Roberts (1988), remains unresolved, 

pending release of more detailed underground data. It seems clear, 

however, that the original fault structure shown on the map is a minor 

imbrication in the upper plate of an as yet unexposed more 

fundamental structure, for which I tentatively retain the term basal 

thrust of the Midas system in this report.

Gold in Porphyry Copper Systems E7
T'. i , > rx

3

s

O

cr 

O

EXPLANATION

Conglomerate

_jk__^_^-JLMidas thrust fault

r°

co - 200

DC

H -400

UJ 5

-600

-800 

Vertical scale

Figure E5. Paleozoic rocks exposed in the Bingham allochthon in the Bingham district (modified from Swensen, 1975).

disseminated gold prospect occurs along a northeasttrending fault in rocks of the Pass Canyon nappe and is 

associated with granitic dike rocks.

East-west extension of the region west of the 

Wasatch Range, in what is now called the Great Basin, 

produced listric faults that subsequently tilted the 

Oquirrh Mountains block as much as 25° to the east 

(Atkinson and Einaudi, 1978). Many of these faults, 

particularly those on the west side of the range, resulted 

from local reactivation of the earlier conjugate fault 

system, producing an irregular saw-tooth faulted range 

front.

Intrusive Rocks

The Bingham district is centered on a composite, 

hydrothermally altered and mineralized pluton, the Bingham stock, its unaltered neighbor, the Last Chance stock, 

and associated later intrusive and extrusive bodies (figs. 

E3, E6; Lanier, Raab, and others, 1978). Major intrusive 

phases are composed of equigranular monzonite and 

quartz monzonite porphyry (table E4) (Moore, 1973b;

Table E4. Average modal composition, in percent, of 

Bingham stock intrusives (Lanier, John, and others, 1978)

Monzonite Quartz monzonite porphyry'

Potassium feldspar

Plagioclase

Quartz

Augite

Biotite

Amphibole

Magnetite

Accessory minerals

30

33

7

11

8

7

2

2

Potassium feldspar

Plagioclase

Quartz

Mafic minerals

(mainly

hornblende

and biotite)

32

32

22

14

Total 100 Total 100

Dark gray, fine grained, equigranular. 

o Light gray, coarse grained, 50 percent phenocrysts, mostly altered.

Lanier, John, and others, 1978). The center of 

mineralization was the porphyritic quartz monzonite 

intrusion within the generally equigranular Bingham 

stock. The adjoining unaltered Last Chance stock and 

smaller satellitic pluton bodies may be connected at 

depth. A latite porphyry dike swarm and thin younger 

quartz latite porphyry dikes and plugs cut the stocks 

generally along the northeast trend of cross faults.

Pre-ore mostly barren breccia pipes (not shown) of 

widely varied sizes (as much as 180 m diameter) occur on 

the south and east sides of the Bingham stock (Rubright 

and Hart, 1968). They are nearly vertical, and may 

penetrate to depths of as much as 1,220 m. The pipes are 

composed of rounded to angular fresh and altered 

fragments of most rocks found in the district in a matrix 

of silica and porphyry.

Alteration

A zoned hydrothermal alteration pattern (fig. E8) 

occurs in the intrusive and sedimentary rocks around the 

extremely fractured central quartz monzonite porphyry 

phase of the Bingham stock (Moore and Nash, 1974; 

Lanier, Raab, and others, 1978; Atkinson and Einaudi, 

1978). Rock alteration resulted both from hightemperature, high-salinity fluids accompanying the late 

stages of the intrusion of the equigranular quartz monzonitic (Bingham) stock, and from peripheral lowtemperature, low-salinity fluids related to the influx and 

mixing of meteoric and magmatic waters. The mineralogic consequences also vary with the composition of the 

fresh host rocks.

Wallrock alteration and sulfide mineral zones 

extend outward from the quartz monzonite intrusion 

along its contacts, in fracture zones in the sedimentary 

and intrusive rocks, and in crosscutting fissures. Two 

prominent zones of quartz monzonite alteration

E8 Geology and Resources of Gold in the United States
1 12°07'30"

EXPLANATION

Volcanic rocks (Tertiary)

Intrusive rocks (Tertiary)

PASS CANYON NAPPE 

Dry Fork unit (Lower Permian)

BINGHAM NAPPE

Bingham Mine Formation 

(Upper Pennsylvanian)

Markham Peak Member 

Clipper Ridge Member

Butterfield Peaks Formation 

(Middle Pennsylvanian)

Contact

Fault-Bar-ball ondownthrown side

Thrust fault-Teeth on upper plate

Anticline

Syncline, locally overturned

Figure E6. General geologic setting of the Bingham mining district (modified from looker, 1971).

recognized include a central quartz-orthoclasephlogopite (hydrothermal biotite) potassium silicate 

zone, and an outer actinolite-chlorite-(epidote) propylitic 

alteration zone (Lanier, Raab, and others, 1978). A 

superposed zone of pervasive sericitic alteration of the 

biotitized intrusion along a northeast-trending zone in 

the northern part of the district is not part of the 

concentric alteration pattern (Moore, 1978).

Atkinson and Einaudi (1978) recognized three 

stages in the alteration of the sedimentary rocks. An early 

contact metasomatism of calcareous quartzite and thin 

silty limestone to diopside, and of thick cherty limestone 

to wollastonite and garnet, was accompanied by deposition of trace sulfides. A main-stage alteration of diopside in quartzite to actinolite, and garnetization of 

wollastonite-bearing marble in limestones occurred 

simultaneously with the potassic alteration in the intrusion. These skarn zones host copper sulfides, galena, 

sphalerite, and molybdenite. The late-stage assemblage 

of pyrite, chlorite, smectite, sericite, and calcite replaced 

the calc-silicate minerals. Lead-zinc and gold ores 

formed during this stage, accompanied by arsenicbearing minerals. This late stage was contemporaneous 

with the sericite-pyrite alteration in the monzonite. On 

the basis of alteration mineral assemblages in sedimentary rocks peripheral to the central intrusion, Hunt 

(1957) inferred that much of the lead-zinc-silver 

mineralization was associated with a later hydrogen

metasomatism that produced sericite in igneous rocks, 

clay minerals in calc-silicate sedimentary rocks, and 

carbonate-silica minerals in limestone.

Age of Ore Formation

The deposition of gold and associated base metals 

took place during several pulses of hydrothermal activity 

over a span of about 1 m.y. (table E5), following the 

intrusion of the equigranular monzonite at 39.8 Ma 

(Warnaars and others, 1978; Moore and Lanphere, 1971; 

Moore, 1973a). The occurrence of free gold intimately 

associated with the copper minerals chalcopyrite and 

tetrahedrite (Peters and others, 1966; Cameron and 

Garmoe, 1983), and fine-grained native vein gold (Boutwell, 1905; Rubright and Hart, 1968) suggests that early 

gold deposition may have been followed by subsequent 

redistribution or reintroduction of this metal. Igneous 

activity in and adjacent to the district at Shaggy Peak, 

about 3,500 m to the southeast (not shown), shows that 

this activity continued over a span of 6 m.y. (table E5). 

The age of disseminated gold in Barneys Canyon remains 

to be determined, but that gold also is presumed to have 

been deposited during a low-temperature late phase.

Gold in Porphyry Copper Systems E9
112°07'30" Table E5. Geochronology of intrusives, alteration, mineral 

deposition, and volcanic rocks (Warnaars and others, 1978)

Figure E7. Sketch showing inferred paths of Sevier foreland thrust nappe movement against the craton promontory 

(shaded) in the Bingham Canyon quadrangle. 1, Pass 

Canyon nappe; 2, Bingham nappe; 3, Rogers Canyon 

nappe. Location of the area of Barneys Canyon disseminated 

prospect (B) north of the present Utah Copper open-pit mine 

(A) is shown.

Metallization Environment

The characteristics of hydrothermal fluids that 

altered the host rocks and deposited metals are inferred 

from mineralogic associations and the chemical and 

isotopic composition of fluid inclusions in the rocks 

(Field and Moore, 1971; Moore and Nash, 1974; 

Bowman and others, 1987). These data indicate that high 

temperatures (400-600 °C) and highly saline solutions 

were present in the Bingham stock and adjacent sedimentary rocks, and that the temperature decreased to 

300 °C in the peripheral deposits (Roedder, 1971). Sulfur 

isotopic data suggest that these high-temperature 

solutions were derived from a common mantle source 

(Field and Moore, 1971). An estimate of fluid pressures 

of about 800 bars by Moore and Nash (1974) corresponds to a lithostatic load of about 3 km. The fluid 

inclusions as well as the breccia pipes provide evidence of 

shattering of the rocks and boiling. An increase in mixing 

of hydrothermal solutions with meteoric water occurred 

as the rocks cooled during probably more than one 

episode of recharge by the hot solutions (Field and 

Moore, 1971; Bowman and others, 1987) in a nearRock units Age (Ma)

Shaggy Peak Rhyolite 

Quartz latite porphyry 

Porphyry copper mineralization 

Quartz monzonite porphyry

Monzonite (Last Chance and 

Bingham stocks)

35.0-32.8

38.8±0.3 (average) 

39.8-38.8 

38.8

39.8±0.4

surface environment. Mineralization occurred mainly 

before the emplacement of the quartz latite porphyry, 

which is poorly fractured and weakly altered and 

mineralized only where it cuts strongly fractured and 

mineralized monzonite. The late-mineralized veins may 

represent the waning final stages of hydrothermal activity 

in the peripheral parts of the system. The continuity of 

magmatic and hydrothermal conditions indicated by 

sulfur and lead isotopic data (Field and Moore, 1971; 

Stacey and others, 1967) suggests that potassic alteration 

and initial copper sulfide mineralization were accomplished by highly saline high-temperature solutions. The 

lead in these solutions was derived from the mixing of 

radiogenic leads associated in Tertiary intrusive magma 

and crustal basement rocks (Stacey and others, 1968).

GOLD DEPOSITS IN THE 

BINGHAM DISTRICT

Gold occurs as an important byproduct in three 

main types of primary (and possibly in part remobilized) 

dispersed porphyry, skarn, and vein and replacement 

deposits. The areal distribution of these deposits, shown 

in figure E3, and their relations to the sulfide mineral 

zones centered on the mineralized Bingham intrusion 

(fig. E9) support a cogenetic relationship. Sulfide 

minerals, with which the gold is associated, are distributed in concentric rings (fig. E3) or in inverted 

cup-shaped zones (see cross section fig. E9) centered 

on or draped over the central porphyritic quartz monzonite phase of the Bingham stock (John, 1978). The 

general zoning of sulfide minerals is: (1) a low-grade core 

containing magnetite and less than 0.5 percent sulfides; 

(2) a bordering molybdenite zone that is low in copper 

but grades outward into: (3) a bornite-chalcopyrite highgrade copper-ore zone; (4) a pyrite-chalcopyrite zone; 

(5) a pyrite zone; and (6) an outermost lead-zinc zone. 

Mineralization was most intense in the upper parts of the 

intrusion, where fracturing was greatest. Mineralization 

also occurred farther from the stock, however, along 

bedding in limestones, at contacts of the stock, and in

E10 Geology and Resources of Gold in the United States
EXPLANATION

Porphyritic quartz monzonite phase; 

remainder of stock is equtgranular

Propylitic alteration

Potassic alteration 

Argillic alteration

Sericitic alteration

Unaltered equigranular 

igneous rock

Sedimentary rocks 

Utah Copper open pit boundary

Contact Dashed where approximate

Figure E8. Plan map showing distribution of types of silicate alteration of intrusive rocks at Bingham 

(modified from Lanier, John, and others, 1978; and Moore and Nash, 1974).

B'

Original hill profile

Interior low-grade core

Figure E9. North-south cross section 8-8' through the BP Minerals America (Kennecott) deposit in the Bingham district 

showing distributton of host rocks and sulfide mineral zones (modified from John, 1978). Location of section, figure E3.

cross-fracture systems in the sedimentary rocks and Last 

Chance stock, as indicated in figure E3. The amounts of 

base and precious metals recovered in these zones are 

shown in table E6. Studies of the newly recognized 

disseminated gold have not progressed sufficiently to 

determine whether they are genetically related to the 

central intrusive system.

Dispersed Porphyry Gold

Approximately 75 percent of the dispersed byproduct gold in the copper zone occurs in the shattered 

porphyritic and equigranular monzonite intrusive rocks 

of the Bingham stock in the core of the district; the 

remainder occurs in the adjoining mineralized altered

Gold in Porphyry Copper Systems E11
Table E6. Amount of base and precious metals recovered 

in vein and replacement ores through 1964 compared with 

that in dispersed intrusive ores

[Values in millions of units; vein and replacement ores from Rubright 

and Hart, 1968; ores dispersed in intrusives as approximated from 

values by Hammond, 1961]

Table E7. Grades of byproduct gold in nonporphyry ores, 

1909-1946 (Hunt and Peacock, 1950)

Vein and replacement

Ores dispersed in

Gold 

(oz)

2.4

8

/r

Lead 

(Ibs)

Silver 

(oz)

136.0

69.0

Zinc 

(Ibs)

Copper 

(Ibs)

814.5

1 J , V/H7 1

Mo lybdenum 

(Ibs)

Vein and replacement

ores 4,180.0

Ores dispersed in 

intrusives --- -

1,709.0

451.1

quartzite and carbonate sedimentary rocks. Gold occurs 

mainly as the native metal and also as a telluride in the 

copper-rich zone accompanying pyrite, chalcopyrite, 

minor bornite, and molybdenite (Boutwell, 1905; Peters 

and others, 1966). The mined-out part of the porphyry 

copper body was unusually uniform in composition and 

could be projected down dip. The subtriangular form in 

plan is structurally concordant with the major northeastand northwest-trending faults and the steeply dipping 

bedding (James, Smith, and Bray, 1961).

The ore zone consists of a network of small 

fractures a few centimeters apart. Primary copper 

minerals are dispersed principally along thin fractures, 

and locally are also disseminated in the highly fractured 

altered blocks as a replacement of mafic minerals; 

molybdenite is more commonly found in veinlets. A 

projection showing the vertical distribution of ore 

mineral zones (fig. E9) suggests that ore mineral zones 

thin with depth along the sides of the intrusion. A zone of 

oxidation and secondary sulfide enrichment from 60 to 

250 m thick, composed of chalcocite and covellite 

replacements of the primary copper sulfide minerals, has 

been removed by mining. The leached capping contained 

iron and copper oxides, carbonates, and native copper, in 

which Boutwell (1905) reported enrichment of gold. 

Possible formation of any supergene replacement 

minerals between zones of sulfide enrichment and the 

leached capping apparently was inhibited by rapid 

erosion in this area.

During milling, selective flotation is used to 

recover and separate gold from the silicate and sulfide 

minerals of the dispersed porphyry copper deposit. Based 

on published production data, the gold content probably 

averaged somewhat less than 0.31 g (0.01 oz) of gold per 

ton of ore; however, current plans to modernize the mine

Types of ore

Lead ore

Lead-zinc

Lead- (copper )-zinc

Gold-silver siliceous

Grade Au 

(oz/ton)

0.065

.062

.037

.135

Production Au 

(oz)

232,000

607,000

8,000

168,000

Copper ores (excludes 

surface mines)

Total

.072 534,000

1,549,000

are based on an ore grade of 0.47 g (0.015 oz) Au/ton 

(Hillman and others, 1984). An oxidized zone, now 

removed, contained higher values of gold, according to 

Boutwell (1905); it undoubtedly was the source of placer 

gold. A part of the copper-rich zone occurs in altered 

fractured sedimentary rocks bordering the stock, which 

are mainly removed as waste. The values of copper (and 

presumably the accompanying gold) in these rocks drop 

off rapidly in the outer pyrite-rich zone. Some finegrained gold in quartz may have been lost in the tailings.

Vein and Replacement Gold

Gold and silver occur with copper, lead, and zinc 

ores in bedding and crosscutting fissures and in manto 

replacements of limestone beds in the periphery of the 

Bingham district. These deposits have been described 

more fully by Hunt (1924), Hunt and Peacock (1950), 

and Rubright and Hart (1968), and the amount of metals 

recovered is shown in tables E6 and E7. The bedding 

fissures occur mainly along the footwall of limestone 

beds, the result of slippage and brecciation during folding 

and faulting that accompanied thrusting of the allochthon, and during intrusion of the stocks. Thicker 

limestone units may contain more than one bedding 

fissure and locally grade into bedding-replacement 

deposits. Bedding-fault copper and lead-zinc ore shoots 

also commonly occur at intersections with northeasttrending cross faults. Copper ore in the western part of 

the district contained chalcopyrite and pyrite, a sparse 

quartz gangue, and gold and silver. Scattered, erratically 

distributed high-grade gold- and silver-bearing ore 

bodies (shoots) were found at depth in some bedding 

fissures even though, in general, the amount of precious 

metals decreased downward as pyrite and silica increased 

(Rubright and Hart, 1968). Mining as much as 1,370 m 

down dip on a bedding-fissure body in the Lark mine

E12 Geology and Resources of Gold in the United States
Table E8. Average grade by level above and 

below adit (0) level in a bedding fissure ore body, 

Lark mine (Rubright and Hart, 1968)

Depths and

levels Au oz/ton

Table E9. Typical grades of gold in lead-zinc replacement 

ore shoots in the Lark mine (Rubright and Hart, 1968)

+200 to surface 

0 +200 

0

200

500

800 

1200 

1600 

2050 

2500 

2750 

3000 

3250

0.057

.012

.017

.014

.015

.016

.026

.019

.034

.030

.031

.033

.024

(principal mine locations shown on fig. E3), however, 

showed a small average increase in grade of gold with 

depth (table E8).

Mineralized northeast-trending steeply dipping 

crosscutting veins cut bedding fissures and intrusions, 

particularly on the south and southwest margins of the 

Bingham stock. Rubright and Hart (1968) reported that 

ore was produced from 30 named and many unnamed 

fault structures across a 3,000 m zone. Some of these 

fissures have been explored down-dip for more than 

1,500 m, and the distribution of ores in these fissures is 

not appreciably restricted to specific host rocks, as was 

observed in the upper workings (Hunt, 1924). R.D. 

Rubright (written commun., 1983) commented that in 

the U.S. mine more ore came from quartzite- and 

porphyry-hosted fissures than from limestone-hosted 

fissures.

The fissure ores consisted mainly of lead or leadzinc, pyritic copper, and siliceous gold-silver types; the 

lead and lead-zinc ores carried rare native gold. Rubright 

and Hart (1968) reported that the gold content of these 

ores generally increased toward outer fringes and 

decreased with depth as pyrite increased. Measured from 

the center of the porphyry, ores at shallow depth in veins 

were zoned from 1,200 to 1,500 m, a pyrite and cupriferous pyrite zone; 1,500 to 2,100 m, a lead and lead-zinc 

zone; 2,100 to 2,750 m, a zone of less lead and zinc with

Quartzite 1

Limestone

Limestone

Limestone

Average grade 

(oz Au/ton)

0.038

.019

.090

.022

Production 

amount (oz)

5,000

5,000

7,000

880

Near intersection with Midas thrust.

increased gold and silver. Gold of extremely varied, 

sometimes high grade was produced from some of the 

lead-zinc ore shoots on the lower mine levels. The 

siliceous gold-silver fissures in the highest levels of the 

mine contained appreciable copper, lead, and zinc, and 

graded into lead-zinc deposits. Some of the lead-zincbearing fissures of the northeast system were mined for 

gold close to the outer and upper southwestern limits of 

ore deposition. Here, calcite and manganese carbonate in 

gangue minerals occur, particularly where a fissure cuts 

the unaltered Last Chance stock. Old records show that 

high-grade copper ore was also mined in association with 

lead, zinc, gold, and silver. In the Lark mine, on the 

northeast side of the Bingham porphyry, ores also 

occurred at the intersections of the Midas thrust fault 

and crosscutting veins.

Replacement copper and lead-zinc ores occur 

mainly in the thick (60 m plus) limestone beds between 

460 and 610 m from the central part of Bingham stock 

(Rubright and Hart, 1968). Typical grades of gold for 

such deposits are shown in table E9. The lead-zinc 

replacement deposits may occur as far as 1,220 m from 

this center. Replacement of the whole bed occurred 

locally, preserving sedimentary rock textures in sulfide 

minerals. Thinner limestone beds also contained 

irregular local replacement bodies, particularly where 

they are cut by cross faults or where they occur within 

thick zones of highly fractured quartzite. Gold was not 

persistent throughout the replacement ores although 

small local shoots contained from 0.16 to 1.55 g (0.005 to 

0.05 oz) Au/ton accompanied by silver and siliceous 

gangue. Boutwell (1905) reported local high-grade siliceous ore in the oxidized zone of replaced limestones. 

Small amounts of stibnite, realgar, orpiment, cinnabar, 

and pyrite formed during a late stage of mineralization 

that crosscuts sulfide minerals in the U.S. mine, and 

cinnabar occurred as a coating on galena and sphalerite 

in fractured quartzites in the Lark mine. Both of these 

occurrences were about 900 m below the present surface.

The zone of oxidation overlying the vein and 

replacement deposits generally is shallow. Sulfide 

minerals occurred at the surface outcrops, but secondary

Gold in Porphyry Copper Systems £13
minerals locally formed at depth owing to deep circulation of ground water along some fault systems. 

Boutwell (1905) reported that black sulfide ores in the 

zone of oxidation yielded 118.2 g (3.8 oz) Au/ton, 

1,822.6 g (58.6 oz) Ag/ton, and 42.3 percent copper, as 

well as some tellurium.

Skarn Gold

Copper-bearing skarns were formed by contact 

magnesium- and iron-metasomatism of thick calcareous 

strata in the Bingham Mine Formation, mainly along the 

northern and southwestern contacts of the Bingham 

stock. Hunt (1924) reported that copper ores in the 

sedimentary rocks close to the porphyry stock were 

associated with the intensely silicated (skarn zone) 

limestones. These deposits in the Carr Fork mine were 

described in more detail by Cameron and Garmoe 

(1987); earlier studies of the alteration of these rocks are 

by Atkinson and Einaudi (1978) and Reid (1978). Gold, 

averaging 1.55 g (0.05 oz) Au/ton, occurs in garnetized 

limestones that contain ore-grade copper. Higher assays 

of from 3.1 to 71.5 g (0.1 to 2.3 oz) Au/ton occur locally 

in silicified and pyritized skarn (Atkinson and Einaudi, 

1978). Cameron and Garmoe (1983) observed a close 

correlation between copper and gold in all skarn ore 

bodies, and the copper and gold assays increase from 

hanging wall to footwall in any one ore body; the 

structurally higher bodies contain higher values of gold. 

Early pulses of copper mineralization apparently were 

pervasive and enriched in gold compared to late pulses. 

BP Minerals America-owned extensions of these skarn 

deposits underlie an area north of the Utah Copper mine 

(fig. E3) and have been explored underground.

Placer Gold

The now exhausted gold placers of Bingham 

Canyon occurred in bench and stream gravels as much as 

75 m thick (Boutwell, 1905; Johnson, 1973) and were 

located mainly in the areas of the Carr Fork, Upper 

Bingham Canyon, and Bear Gulch branches, downstream to the mouth of the canyon (fig. E3). The source 

of gold is attributed to the oxidized parts of the nonporphyry ores, where it occurred in the honeycombed siliceous gangue. Some of the fine-grained gold 

undoubtedly was also derived from the mineralized 

central porphyry intrusive of the Bingham stock.

Disseminated Gold

An occurrence of disseminated gold in the Barneys 

Canyon area, about 6 km north of the Utah Copper Mine 

(fig. E7), is reported to be amenable to open pit mining

and bulk-leach gold recovery (Standard Oil Co., 1986). 

BP Minerals America has plans to mine the deposit. The 

gold is disseminated in altered carbonate-rich (dolomitic) rocks along the northeast-trending Tooele fault 

(Tooker and Roberts, 1988). The ore zone contains a 

number of small workings and prospect pits, relicts of 

early-day mining. To 1988, an initial reserve of approximately 12.4 metric tons (400,000 oz) of gold has been 

identified, and additional reserves are expected.

CONCLUSIONS

Over a period of 123 years in excess of 603 metric 

tons (19.4 million oz) of gold has been produced mostly 

as a byproduct of base metal mining in the Bingham 

mining district. The gold has come from placers, metal 

dispersed in the monzonite intrusions and adjacent 

sedimentary rocks, veins and replacements, and skarns in 

the peripheral carbonate strata. Reserves in the district, 

those measured in the porphyry and skarn deposits, and 

those inferred in the Barneys Canyon gold prospect, may 

aggregate more than 505.6 metric tons. The discovery of 

the disseminated-type deposit at Bingham opens untested potentially favorable ground for exploration and 

enlargement of the gold reserve in the Bingham district.

The geologic setting for the district is at a favorable 

structural-stratigraphic-igneous rock juncture. Miogeoclinal carbonate-rich shelf deposits composed of reactive 

permeable sedimentary rocks were thrust as nappes 

unknown distances from the west and west-southwest. 

Asymmetrically folded and locally overturned beds in 

which the Bingham productive deposits occur were 

flexed into an arc, and a tensional northeast- and 

northwest-trending cross fault system formed in response 

to the maximum stress in the thrust plate. This fractured 

and potentially reactive zone became the main locus for 

intrusion of quartz monzonitic stocks similar to those 

that were intruded along the trace of a cratonal suture 

zone (Uinta-Cortez axis) in the Wasatch Range, east of 

Bingham (Tooker, 1971); these latter stocks also are 

associated with base and precious metal deposits. Closely 

spaced fissures at Bingham subsequently became conduits into these reactive sedimentary rocks for igneous 

rocks and hydrothermal solutions, as well as deposition 

sites for metals carried by the hydrothermal fluids 

developed in the later stages of the intrusive cycle.

A hydrothermal origin has been proposed for these 

deposits on the evidence of a brecciated, altered subvolcanic porphyritic quartz monzonite stock that vented 

boiling mixed magmatic and meteoric waters. Deposition 

of thermally and mineralogically zoned ores followed. A 

large volume of finely divided gold seems to have been 

deposited with copper and iron along fractures and in 

pores of the central porphyry stock and in adjacent skarns 

in limestones. The grade of this type of gold is in the

£14 Geology and Resources of Gold in the United States
range of 0.31-0.62 g (0.01-0.02 oz) Au/ton, locally as 

much as 1.55 g (0.05 oz) Au/ton in skarns. Equally high 

grade, but volumetrically less abundant, fine to coarse 

generally free gold occurs in the uppermost and distal 

parts of the system that adjoin vein and beddingreplacement deposits in the lead-zinc-sulfide zone. 

Locally, very late gold occurs in rhodochrosite gangue. 

Little gold was deposited in the pyritic zone that 

separates the copper and lead-zinc zones.

These relations suggest the possibility of two (or 

more) generations of gold an early high-temperature 

phase accompanying copper, following brecciation of the 

porphyry, and one or more later (possibly redistribution) 

phases in the waning stages of the hydrothermal system. 

The later phases thus reflect an epithermal low-pressure, 

low-temperature environment of deposition in open 

spaces, coating earlier ore minerals. Disseminated gold, 

discovered along mineralized structures north of the 

porphyry deposit, ultimately may also be considered part 

of this late-stage mineral formation.

Manuscript received by scientific editors December 1984

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Gold in Porphyry Copper Systems E15
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APPENDIX-1990 UPDATE

The Bingham Canyon mine was part of BP Minerals America 

acquired by the RTZ Corp. (London) from BP Petroleum Company in 

1989 (Anonymous, 1989a). RTZ will return the Kennecott name to 

corporate use in the copper mining business in the United States. This 

acquisition will double the RTZ copper output (Anonymous, 1989b). 

The Bingham Canyon open-pit mine currently produces more than 

200,000 t/yr Cu, 350,000 oz/yr Au, 27 Moz/yr Ag, and 4,000 t/yr Mo. 

The estimated remaining life of the deposit is 25 years.

The Barneys Canyon deposit contains a goldrsilver ratio of 1:1 

(Gunter and others, 1990). Anomalous trace elements include arsenic, 

antimony, mercury, thallium, and barium. Minable oxide reserves are

10 Mt (average 0.046 oz Au/t). The Mel-Co deposit 1 mile southwest 

of Barneys Canyon mine is spatially associated with a steeply dipping 

siliciclastic breccia. Minable oxide reserves are 3.1 Mt (average 0.07 oz 

Au/t). Heap leaching operations began in 1989.

Anonymous, 1989a, BP Minerals sale to RTZ to take place in June: 

American Mining Congress Journal, v. 75, no. 6, p. 22.

Anonymous, 1989b, RTZ completes BP appraisal: Mining Journal, 

London, v. 312, no. 8021, p. 406-407.

Gunter, W.L., Hammitt, J.W., and Babcock, R.C., 1990, Geology of the 

Barneys Canyon gold deposit, Bingham Canyon, UT: 119th 

AIME Annual Meeting, Salt Lake City, February 1990, 

Abstracts with Program, p. 38.

E16 Geology and Resources of Gold in the United States
Gold in the Butte District, Montana

By Edwin W. looker

Abstract

The Butte mining district in southwestern Montana was 

for many years among the top 10 producers of gold, primarily 

as a byproduct of copper, silver, zinc, and manganese ores. 

Mining was nearly continuous from 1864 to 1983; open pit 

mining was resumed in 1986. The placer deposits were 

exhausted by 1867, and the silver vein lodes became the 

main productive ores from 1866 to 1888 when the copper 

lodes were discovered. These sulfide veins and later the 

dispersed ores have been the main sources of byproduct 

gold until the present time.

A production figure of 93.311 metric tons (3 million 

ounces) of gold is undoubtedly under-reported because of 

poor records during the earliest years. An inferred reserve of 

6.594 metric tons (212,000 ounces) of gold may be uncertain 

owing to the fact that, because of its very low grade, assays 

of gold in the ores were not obtained.

The district is located in the south-central part of the 

Boulder bat hoi it h, a quartz monzonite (78-73 Ma) intrusive 

into Proterozoic to Cretaceous sedimentary and volcanic 

rocks. Aplite and pegmatite dikes and irregular monzonitic 

masses intrude the batholith. Later quartz porphyry dikes 

(68-64 Ma) and a plug also intruded the batholithic rocks. 

Post-ore igneous activity (48-40 Ma) included the 

emplacement of porphyritic rhyolitic dikes, welded tuff, and 

volcanogenic sediments. The stages of ore formation in the 

batholith host rocks comprise (1) pre-Main Stage veins and 

pervasive alteration in a core zone (64-63 Ma) and (2) the 

upper Main Stage veins (58-57 Ma) all imposed upon three 

successive structural-mineralogical domains centered on the 

core zone. Most of the Butte production has come from 

underground workings in the Main Stage veins. More recent 

production has come from lower grade open pit deposits. 

The ores are distinctively zoned, both vertically and 

horizontally. The upper Main Stage zone structures persist 

downward into a dome-shaped domain of pre-Main Stage 

pervasive biotite alteration and a network of small fractures 

and disseminated copper sulfides. This zone overlaps a 

lower dome-shaped molybdenite domain. The fracture 

system initiated by intrusion of the batholith provided sites for 

quartz porphyry dikes and vein structures that hosted hightemperature (600 °C) hydrothermal pre-Main Stage solutions. 

Very little gold has been reported in this high-temperature 

domain. Much later, lower temperature (300 °C) mixed 

hydrothermal and meteoric solutions produced the rich Main 

Stage veins and extensive argillization of the wall rocks. The

Main Stage ores were zoned outward from a highly altered 

central copper- and iron-bearing sulfide zone containing 

small amounts of gold and silver, an intermediate zone in 

which copper sulfides predominated and which also contained small amounts of precious metals, and a peripheral 

zone containing ores of manganese, zinc, lead, and silver, 

and gold in trace amounts.

INTRODUCTION

Gold has been produced as a primary and byproduct metal from placer, vein, and disseminated deposits in 

the Butte district in southwestern Montana (fig. E10), 

almost continuously from 1864 until 1983, when the last 

mine closed. Open-pit mining was resumed in July 1986. 

Copper, zinc, manganese, and silver have been the main 

metals produced; however, until 1980, Butte remained 

among the top 10 gold-producing districts of the United 

States. This review of the occurrence and magnitude of 

the gold in the Butte deposits was obtained from published sources (Meyer and others, 1968; Sales and 

Meyer, 1950; Klepper, 1973; Miller, 1973; Proffett, 1973; 

Brimhall, 1977, 1980; Brimhall and others, 1984; Weed, 

1903, 1912; Sales, 1913; Perry, 1932; Brown, 1894) and 

U.S. Bureau of Mines (1973-1982). I am indebted to the 

late Charles Meyer, longtime participant in the studies of 

the Butte district as geologist for the Anaconda Minerals 

Company, for his critical review of this text.

PRODUCTION AND RESERVES

Butte, one of the important mining districts of the 

world on the basis of its production and known reserves 

(Miller, 1973), was closed in 1983 because of unfavorable 

metal prices; however, the district was not exhausted, and 

contains substantial base and precious metal reserves 

and resources, which were considered sufficient for 

reopening under new ownership in 1986. Production 

began originally in 1864, when shallow placer deposits 

were discovered. The best placers were worked out by 

1867 after about 2.333 metric tons (75,000 troy oz) of

Gold in Porphyry Copper Systems El7
116° 114" 112° no°

45° 

EXPLANATION

Volcanic rocks (Tertiary)

Miogeoclinal and platform terranes (Mesozoic and Paleozoic) - - 

Underlain by Proterozoic basement rocks

Belt Supergroup (Proterozoic)- - In part allochthonous

Sedimentary, metamorphic, and igneous rocks of Wyoming 

shield terrane (Tertiary to Archean)

Boulder batholith (Cretaceous)

Idaho batholith (Mid-Cretaceous)

Contact-Dotted where inferred 

i Thrust fault-Teeth on upper plate 

Fault-Barrball on downthrown side

Figure E10. Western Montana, showing location of the Butte mining district and its regional geologic 

setting, modified from Meyer and others (1968), Smedes and others (1973), and Bennett (1984).

gold were won. Mining of the silver lodes that cropped 

out nearby on Butte Hill began in 1866. Silver continued 

as the principal metal sought until the discoveries of

poorly exposed copper lodes between 1882 and 1885. 

From that time until the present, copper has remained 

the major district product. Other metals, principally zinc,

£18 Geology and Resources of Gold In the United States
Table E10. Cumulative gold and associated metal production, Butte mining district, Montana

[Meyer and others (1968); Miller (1973); U.S. Bureau of Mines 

(1973-1982); oz, troy ounce; t, metric ton; kg, kilogram; n.r., not 

reported]

Metal

Gold (oz)

(kg)

Copper (t)

Zinc (t)

Manganese (t)

Lead (t)

Silver (kg)

Bismuth (kg)

Sulfuric

acid (t)

Selenium (kg)

Tellurium (kg)

Cadmium (kg)

Arsenic

compound (t)

1880-1964

2,506,253

77,954

7,347,784

2,175,344

1,680,695

379,774

^0, 055, 752

h, 833, 752

1 8,507,660

1 38,032

1 82

h, 848, 322

1 144,447

Through 1972

2,757,710

85,775

8,308,176

2,228,287

1,680,695

387,992

L 621, 076, 393

h, 833, 752

1 8, 584, 252

143,725

107,619

1,953,272

n. r.

Through 1982

2,928,795

91,097

9,034,677

2,226,272

n.r.

387,663

702,423,156

n. r.

n.r.

n.r.

n.r.

n.r.

n.r.

Produced during shorter time, ending in 1964.

manganese, and lead, were actively mined beginning 

in 1915-1916, but their production declined after the 

mid-1960's.

Following the early placer period, most of the 

metal production was from extensive Main Stage vein 

deposits (Meyer and others, 1968). Production from 

veins in the underground mines dropped off in the early 

1950's, as block caving, open pit mining of the disseminated ores, and leaching of dumps and extraction of 

metals from acid mine waters became economic for 

copper production (Miller, 1973, fig. F-l). At one time 

about 45 companies were active in the district, but 

through consolidation and purchase this number was 

reduced to the Anaconda Company, later the Anaconda 

Minerals Division of Atlantic Richfield. These properties 

were sold to the Washington Corp., Missoula, Mont., in 

1985 (Metals Week, 1985, v. 56, no. 37, p. 8), and mining 

by the Montana Resources Company began in mid-1986 

at the East Berkeley pit, immediately east of the deep 

(Anaconda) Berkeley pit (Eng. and Mining Journal, 

1986). A production figure of nearly 93.33 metric tons 

(3 million oz) of byproduct gold, shown in table E10, is a 

compilation of available production data for this district. 

Early records (pre-1882) undoubtedly are underreported, but the order of magnitude of the district as a 

producer of gold is clear from these data.

The gold resources at Butte are not well known at 

present, but they could be substantial if "disseminated" 

gold were recovered from potential deep copper and 

copper-molybdenum resources, not currently economic.

In the ARCO/Anaconda merger filing (Cox and others, 

1981), the published district copper reserves were 154 

million metric tons at a grade of 0.67 percent copper 

(1,031,800 metric tons copper). Assuming that gold will 

be recovered from this ore in the same ratio as it was 

from copper production between 1973 and 1982, an 

inferred reserve of gold in the district could be on the 

order of 7.558 metric tons (243,000 oz). However, this 

resource figure may well be too high (Charles Meyer, 

oral commun., 1984), owing to the fact that no systematic 

data base exists for recent gold production; gold is 

irregularly distributed in some parts of the district, and 

assays for gold were seldom obtained during mining 

because of gold's relatively low value in terms of total ore 

value. Thus one cannot be certain that the projected 

gold:copper ratio will prove accurate for deeper ores.

GEOLOGIC FEATURES

Regional Setting

The Butte district is located in the south-central 

part of the Boulder batholith, which is intrusive into the 

cratonal platform of North America. The batholith lies 

close to the boundary between the platform and 

Wyoming shield terranes (King, 1976) on the south, and 

at the junction of at least two major craton platform 

lineaments, the west-northwest-trending Lewis and Clark 

(Meyer and others, 1968) and the northeast-trending 

trans-Challis-Great Falls (Bennett, 1984) zones, on the 

north and west. The trans-Challis lineament separates 

the Boulder batholith from the Idaho batholith to the 

west (fig. E10). The shield is composed of Archean 

gneiss, schist, and granite, and has remained a stable 

undeformed structural entity since its formation in the 

Precambrian. The Boulder batholith intruded and 

metamorphosed Proterozoic to Cretaceous sedimentary 

cratonal, miogeoclinal shelf, and volcanic rocks. Where 

exposed, volcanics form the roof of the batholith (Smedes 

and others, 1973). To the north and west, folded Proterozoic Belt Supergroup clastic rocks have been thrust 

northeastward over Paleozoic and Mesozoic formations 

on the Lewis thrust fault.

The Boulder batholith was forcefully emplaced, in 

stages (Klepper, 1973), into an unstable deformed zone 

between the cratonic basement structures, during the 

Laramide orogeny of Late Cretaceous age (80-70 Ma). 

Smedes and others (1973) estimated that the intrusion 

lifted up the eastern part of the batholith roof between 

900 and 3,000 m along a border fault. A roughly 

concordant western margin dips gently westward under 

roof rocks. The north and south margins of the batholith 

lie along complex cross fault zones. Of the individual

Gold in Porphyry Copper Systems El 9
plutons in the batholith, the Butte Quartz Monzonite is 

the largest, and was one of the later intrusive phases 

formed. The most important lead, zinc, and silver 

deposits in this region are located within or along the 

contacts of the Butte Quartz Monzonite. Meyer and 

others (1968) proposed that Butte deposits probably 

were derived from a different unexposed(?) later 

intrusion that activated the large complex copper-rich 

base- and precious-metal sulfide system.

District Setting

Details of the geologic features of the Butte district 

(fig. Ell) and their interpretation are well documented 

by Weed (1912), Sales (1913), Meyer and others (1968), 

Proffett (1973), Brimhall (1977), Roberts (1973), and 

Gustafson (1973). The significant points in the evolution 

of host rocks, ore-bearing structures, and hydrothermal 

events are summarized in table Ell.

Host Rocks

Three groups of igneous intrusives form the district 

host rocks at Butte (Meyer and others, 1968). The oldest, 

the Butte Quartz Monzonite, is medium to coarse 

grained, and is composed of 40-45 percent quartz and 

orthoclase (microperthite) in equal proportions, 36-40 

percent plagioclase, and 15-20 percent hornblende and 

biotite. The second group of intrusives includes aplite 

and pegmatite dikes and irregular monzonite masses 

composed mainly of quartz and potassium feldspar, and 

locally including tourmaline and sulfide minerals. These 

latter intrusives are scattered throughout the quartz 

monzonite. The youngest group, three nearly vertical 

quartz porphyry dikes 3-15 m thick, intrude the quartz 

monzonite, trending west to west-northwest across the 

district. The quartz porphyry of the dikes is composed of 

quartz, feldspar, and sparse biotite phenocrysts in a 

fine-grained groundmass of quartz and alkali feldspar 

(Proffett, 1973). The porphyry is brecciated locally and 

cut by quartz and quartz-molybdenite (pre-Main Stage?) 

veinlets.

An irregular quartz porphyry plug (the Modoc) of 

similar petrographic characteristics is present in the 

northeast part of the district. Breccias associated with 

this mass include quartz porphyry and Butte Quartz 

Monzonite fragments, which contain quartz-molybdenite 

veinlets with potassium-silicate alteration envelopes. The 

matrix of these breccias is "sand" cemented by potassium 

feldspar and biotite. The Modoc plug may have been 

emplaced later than the set of southern quartz porphyry 

dikes during the potassium silicate phase of alteration 

(pre-Main Stage). But like the southern dike set, it is 

everywhere cut by Main Stage veins.

Table E11. General sequence of igneous, hydrothermal, 

and structural events in Butte mining district

[From Meyer and others (1968), Miller (1973), and Proffett (1973)]

Relative age

(Ma) Events

Pleistocene(?) 

& Pliocene(?)

48-40

58-57

63-60

68-64 

70

78-73

Continental fault, Basin-Range

structure; zone 30-90 m wide, northtrending, steeply west dipping; 900- 

m downward displacement on west; no 

hypogene minerals.

Rarus and Middle normal fault systems, 

northeast- and east-trending, 

moderately northwest dipping, 10-100 

m downward displacements in south; 

cut main stage faults; no hypogene 

minerals.

Regional post-ore tilting, >25° NW.

Rhyolite (porphyritlc quartz latlte or 

rhyodacite), east-trending dikes and 

Big Butte intrusive complex, west of 

district.

Main Stage vein systems, complexly

Intersecting; reverse S-curvature 

structural grain; altered wallrocks!

Hanging Wall Steward strikes east, 

dips gently south; banded quartzmolybdenite veins at depth are cut 

by Main Stage sulfides; good ore at 

depth, spotty above; offsets the 

Steward vein system.

Steward trends east-northeast, dips 

steeply southeast; normal displacement of 45 m; wall rocks intensely 

altered; a few ore shoots in upper 

parts; much post-ore displacement.

Blue trends northwest, dips southwest; left lateral displacement of a 

few hundred feet; offsets Anaconda 

system; fault gouge; some post-ore 

movement; prominent ore shoots.

Anaconda trends northeast to eastnortheast with en-echelon gaps; dips 

steeply both to north and south; 

small right-lateral displacements of 

<30 m; horsetail structures on east 

end; wide (10-30 m) mineralized 

zones; major ore producer.

Pre-Main Stage, potassium-feldspar 

alteration of quartz monzonite at 

depth; formation of small vein 

systems and injection of banded 

quartz-pyrite-molybdenite veinlets 

in core zone.

Quartz porphyry dike emplacement.

Crosscutting aplite, pegmatite, and 

granoaplite dikes, mainly on periphery of mineralized-replacement zone.

Butte Quartz Monzonite intrusion.

E20 Geology and Resources of Gold in the United States
112°30'

46°00' -

Alluvium (Quaternary) 

Rhyolite (early Tertiary)

Quartz porphyry

i

Aplite pegmatite (Cretaceous?) 

Quartz monzonite 

Open pit

EXPLANATION

Main Stage veins 

' Post-Main Stage veins

- Contact

 Fault-Bar and ball on downthrown 

side, dotted where inferred

Intermediate mineral zone 

Central mineral zone 

Zone of MoSnveinlets

X Principal mine shafts

1 Travona

2 Emma

3 Steward

4 Kelly

5 Mountain Con

6 Lexington

7 Black Rock

8 Leonard

9 Anaconda

10 Pittsmont shaft

11 Anselmo

Figure E11. Generalized map showing location of main underground mine shafts and open pits, the 

principal geologic lithologies, and vein structures of the Butte mining district (modified from Miller, 1973). 

Map area covers the northwestern part of the city of Butte.

Post-ore igneous activity is recorded by the 

intrusion of porphyritic quartz latite or rhyodacite 

(locally called rhyolite) dikes and the emplacement of 

welded tuff and volcanogenic sediments.

Structures

Meyer and others (1968) inferred that stresses 

accompanying the intrusion of pre-mineralization host

rocks at Butte also initiated a regional fracture system in 

the quartz monzonite that provided pathways for magma 

and fluid migrations in the district. Emplacement of the 

magmas and associated fluids resulted in the formation 

of distinctive structural-mineralogical domains.

Nearly all the Butte production of base and 

precious metals has come from the uppermost of three 

structural domains (fig. E12) characterized by large 

Main Stage veins and by intersecting zones of closely

Gold in Porphyry Copper Systems E21
spaced fractures whose walls are extensively altered 

sequentially into argillic and sericitic zones (described in 

detail by Sales and Meyer, 1950). These vein structures 

contain relatively low temperature (300 °C), zoned basemetal sulfide ore assemblages (Meyer and others, 1968). 

Two principal Main Stage vein systems include the 

Anaconda, a steep east-northeast- to east-trending 

system, and the Blue, a northwest-trending system. Sales 

(1913) recognized a third, less well developed system, the 

steep northeast-trending Steward system; and Meyer and 

others (1968) described a fourth east-trending gently 

south dipping system found in deep workings, the 

Hanging Wall Steward system. These interrelated 

systems form a reverse S-curvature, locally producing the 

well-known horsetail structures of the Butte district. 

Gustafson (1973, fig. J-l) showed that these structures 

occur where the eastern ends of the Anaconda easttrending veins are dissipated along their south sides into 

a network of small southeast-trending ore-bearing veinlets and disseminations, surrounded by highly altered 

wall rocks. The width of the network of these veinlets and 

disseminations varies along strike and with depth. The 

Anaconda system has en-echelon gaps where veins thin 

or disappear, but it has been mineralized over long 

horizontal and vertical distances (Meyer and others, 

1968). The average width of ore shoots is 6-9 m, but 

locally, where the strike changes, some are as wide as 

30 m. The horsetail ore bodies are as much as 18 m wide 

and have been mined to depths greater than 600 m. The 

Blue vein ore shoots are not as wide (1.5-6 m) or as 

persistent laterally (30-730 m) or vertically (180-550 m). 

The Blue system commonly is lower in grade at the 

surface, but a more important ore producer in lower 

levels. Most veins have been displaced along strike, and 

possibly some tensional opening of veins has occurred. 

Meyer and others (1968) believed that the fracture 

pattern occupied by the Main Stage was incipient in 

quartz monzonite before the large Main Stage veins were 

fully opened and filled. Proffett (1973) confirmed that in 

the deeper levels most if not all the important Main Stage 

veins fill fractures that earlier were quartz-molybdenite 

veins. Meyer and others (1968) suggested that some of 

these early veins preceded Main Stage veins by at least 

5 m.y.

Many of the Main Stage fractures continue downward into a second, lower structural domain, which was 

described by Roberts (1973) as a single large outwarddipping, elongate dome-shaped form (fig. E13), roughly 

concentric with the Main Stage ore zonal pattern, and 

which trends southeastward from beneath the Berkeley 

pit to east of the Continental fault zone. This domain is 

characterized by pervasive pre-Main Stage biotitization 

of hornblende in the quartz monzonite along a network 

of small fractures, and the dissemination of chalcopyrite, 

pyrite, and bornite, generally in sites of the former mafic

minerals in the walls of fractures. The original plagioclase, quartz, and accessory minerals are unaltered. The 

biotite alteration-structural domain may represent a 

zone of overlap between a molybdenite dome below, and 

Main Stage vein zone above.

The third (also pre-Main Stage) domain (fig. E13), 

which underlies the pervasive biotitic alteration domain, 

beginning at about 850 m level below the surface, is 

called the molybdenite dome or domain. This domain is 

characterized by an early high-temperature fracturecontrolled quartz-molybdenite vein and a disseminated 

pyrite and chalcopyrite assemblage in the Butte Quartz 

Monzonite. Where the quartz-molybdenite veins split off 

from the Main Stage system, part of which persists to this 

depth, the wallrocks generally show thin rinds of potassic 

alteration. The molybdenite domain is exposed at the 

surface east of the Continental fault, which is believed to 

have an offset (down on the west) of nearly 1,600 m. The 

base of the molybdenite domain has not yet been 

observed.

Following the Main Stage, intrusive rhyolite crosscut the district as a broad dike, and formed a flow-banded 

complex intrusive in the Big Butte area on the west side 

of the district (Proffett, 1973). Rhyolite, welded tuff, and 

volcanogenic sediments on the west side of the district 

record a subsequent regional tilting of 25° to 50° to the 

northwest in some fault blocks.

Later, the generally unmineralized normal faults, 

the Rarus and Middle, were formed, trending northeasterly to easterly across the district. A final structural 

event was north-trending Basin-Range faulting along the 

Continental fault zone on the east side of the district.

Evolving Environment of Ore Deposition

Ore formation, as deduced by Meyer and others 

(1968), resulted from a long-term, perhaps continuous 

series of events, summarized in table E12. Intrusion of 

the Butte Quartz Monzonite pluton initiated a fracture 

system along which subsequent quartz porphyry and 

perhaps an as yet unidentified(?) deep earlier porphyritic 

intrusion were emplaced. These porphyritic intrusions 

generated a high-temperature (600 °C) hydrothermal 

event and deposited a pre-Main Stage quartzmolybdenite assemblage in veinlets, and disseminated 

pyrite and chalcopyrite within potassium-silicate-altered 

vein walls of the molybdenite dome domain. This hydrothermal event may have created the overlying pervasive 

biotitic alteration domain containing pyrite and copper 

sulfide veinlets (Roberts, 1973). Brimhall (1980) considered these porphyry copper-type assemblages as the

E22 Geology and Resources of Gold in the United States
46°00'

EXPLANATION 

Rhyolite dikes 

Quartz porphyry dikes

Butte Quartz Monzonite 

Vein systems

*-^Y Anaconda

^"^xN 

^ \ ^

 . Post-mineral

PERIPHERAL METAL ZONE

/") Outer edge, molybdenite 

'^ veinlets

112*30'_________

X Mine shafts

1 Leonard

2 Anaconda

3 Emma

4 Mountain Con

5 High Ore

INTERMEDIATE METAL ZONE/. "' \X

.- CENTRAL METAL ZONE

/ ...-"V- V;^~7

V /^^-

\

Figure E12. Generalized plan of the 2800 level of the Butte district showing the main vein, fault, and dike systems in Butte Quartz 

Monzonite and the Main Stage mineral zones that surround the central pre-Main Stage zone containing molybdenite veinlets 

(modified from Meyer and others, 1968).

protore for the Main Stage; however, a deeper source for 

some of the base metal sulfides may be required because 

of the huge volumes of base metals, sulfur, and arsenic in 

the Main Stage veins and scant evidence for depletion of 

copper sulfides in the molybdenite dome.

Perhaps 5 m.y. later, possibly during an unrelated 

magmatic intrusion interval during which deep crustal 

faulting was developed (according to Meyer and others, 

1968), a lower temperature (300 °C) hydrothermal Main 

Stage system evolved, which allowed meteoric water to 

penetrate to deep levels. These solutions mixed with 

magmatic fluids and picked up metals from a deep 

source, then moved up through the pre-Main Stage zone, 

where additional metals were added through hypogene 

leaching. Thus, through a process of hypogene alteration, 

oxidation, leaching, and redistribution, the base and 

precious metals were flushed upward into the extensive 

open near-surface fissures. Base and precious metal

veins (mainly composed of chalcocite-enargite-bornitechalcopyrite-pyrite) were formed in the central and 

intermediate metal zones, and zinc, lead, manganese, and 

silver-gold veins developed in the intermediate and 

peripheral metal zones. The vein openings were wider in 

the central ore zone than in the outer ore zones, which 

latter presumably were cooler and farther from the 

source. The solutions produced extensive argillic 

alteration of the central zone vein wallrocks; the amount 

of alteration decreased in peripheral zones. The Main 

Stage veins everywhere cut quartz-molybdenite veinlets 

but also were emplaced along strike of some large 

quartz-molybdenite veins. Sulfur isotopes indicate a 

genetic relation of magmatic-hydrothermal solutions to 

Butte Quartz Monzonite, and that the sulfur probably 

was of crustal rather than subcrustal origin (Lange and 

Cheney, 1971). Sulfur isotopes of early pre-Main Stage 

solution were different from those of the Main Stage.

Gold in Porphyry Copper Systems E23
N

Supergene chalcocite 

enrichment blanket

Main Stage veins

Quartz porphyry 

dikes

Fracture system

MINE LEVELS 

r-1600

Pervasive biotite 

altered zone.k 

pre-Main Stage X^-j

-3800

Quartz-molybdenite^ 

pre-Main Stage 150 300 450 600 METERS 

I I I I

Figure E13. Generalized north-south cross section through the Butte district, vicinity of the Berkeley pit, 

showing location of molybdenum dome, domain of pervasive biotitic alteration, the Main Stage vein zone, 

supergene chalcocite enrichment blanket, and zone of surface oxidation. A zone of intense hypogene wallrock 

alteration generally parallels the vein fractures and porphyry dikes. Modified from Brimhall and Ghiuso (1983).

Table E12. Summary of geologic attributes of ore zone domains in Butte mining district 

[From Meyer and others (1968), Proffett (1973), Roberts (1973), and Miller (1973)]

Ore type Vein type Vein minerals Wallrock alteration Alteration assemblage

Main Stage

base and

precious

metals.

Wide veins; sericite District-wide hypogene

K-Ar age 57.3-58 Ma. zonation; central

copper; intermediate

copper-zinc-silver;

peripheral zinc-leadmanganese-silver( 1 )

(oxide, sulfide, carbonate, and silicate;

sulfate gangue);

temp. 200-350 °C.

Varied, depending on

position in ore

zone; broad where

veins are wide or

closely spaced

(horsetail zone)

in central ore

zone; narrower in

peripheral ore

zone.

Sequence outward

from vein:

sericite, white

argillic, green

argillic, and

fresh quartz

monzonite (see

Meyer and others,

1968, fig. H).

Pre-Main Network of small

Stage veinlets in elongate 

copper domal structure 

sulfides. (overlaps lowest

domain, concentric 

with Main Stage 

zones).

Pre-Main Narrow, discontinuous 

Stage veinlets, varied 

molybden- orientations, 

ite dome.

Quartz, molybdenite, 

chalcopyrite, 

pyrite; temp. 550- 

700 °C.

Fine-grained quartz, 

alkali feldspar, biotite, opaque minerals 

(molybdenite, chalcopyrite, pyrite, magnetite); temp. 600- 

690 °C.

Pervasive biotitic 

alteration "dome": 

plagioclase, biotite, potassium 

feldspar, quartz, 

and accessories; 

not altered.

Host rocks everywhere altered, 

but only in 

narrow zone along 

vein.

Original hornblende 

altered to biotite, quartz, and 

anhydrite; chalcopyrite, pyrite, 

and bornite disseminated in wallrock.

Secondary alkali 

feldspar, quartz, 

sericite, biotite, 

and opaque minerals; 

grades outward to 

fresh host rocks.

E24 Geology and Resources of Gold in the United States
The Main Stage oxygen isotope composition is believed 

to be indicative of hydrothermal solutions containing a 

large component of meteoric origin (Sheppard and 

others, 1969).

More recent supergene enrichment of the veins at 

the surface has produced a chalcocite blanket.

Zonal Distribution of Main Stage Base and 

Precious Metals

As observed by Sales (1913), early in the 

development of the upper parts of mines in the district, 

three nearly concentric zones of Main Stage metals or 

ore minerals occur in the veins (fig. E12): (1) a highly 

altered central zone containing mainly copper and ironbearing sulfide minerals in which small amounts of gold 

and silver also occur; (2) an intermediate zone in which 

copper sulfide predominates but which also contains 

some zinc sulfide ores as well as small amounts of gold 

and silver; and (3) a peripheral zone containing ores of 

manganese, zinc, lead, silver, and traces of gold. The 

metal zones broaden with depth and were capped by 

successive oxidized, leached, and enriched supergene 

blanket deposits. These post-Main Stage supergene 

deposits in the zone of oxidation contained quartz, as well 

as copper, manganese, iron, and silver oxides, but no 

gold. A fourth metal zone encountered in deeper 

workings beneath the central metal zone contains the 

pre-Main Stage quartz-pyrite-molybdenite-chalcopyrite 

assemblage (of the molybdenite dome), and little 

reported gold (Meyer and others, 1968).

PRECIOUS METALS IN BUTTE ORES

Gold is a minor but not insignificant constituent of 

Butte base metal-silver ores. The occurrence of gold is 

believed related to that of the copper sulfides; however, 

gold at Butte is so finely dispersed that its association 

with any specific sulfide mineral has been difficult to 

determine.

Widely varied gold values reported in the early 

literature suggest an irregular distribution of gold in the 

near-surface early-mined Main Stage deposits. Brown 

(1894) reported gold values of 62.2 g (2 oz) Au to 3,110 

g (100 oz) Ag associated with fine-grained pyrite and to 

a lesser extent with galena and sphalerite. By 1897, about 

15.5 metric tons (0.5 million oz) of gold had been 

produced, and Emmons and Tower (1897) commented 

on the waning of placering sources. Abandonment of 

placer mining occurred in part because of large amounts 

of silver alloy in placer gold, which resulted in low bullion 

values. Emmons and Tower (1897) also reported that the 

grade (5 month average) in vein ore was about 1.86 g 

Au/metric ton (0.055 oz Au/ton), with local richer

concentrations in the silver-rich ores. Weed (1912) 

observed that each pound of copper contained 1.16 g 

(0.0375 oz) of silver and $0.0025 (about 0.004 g or 

0.00012 oz) of gold. By 1910, 34.2 metric tons (1.1 

million oz) of gold had been produced. Weed reported 

that the copper ores contained little silver, that the silver 

ores rarely contained copper, but that copper and silver 

ores both contained gold. High-grade silver ores 

contained highest gold values; however, the average gold 

values for all mines during that period was 0.093 g 

Au/metric ton (0.003 oz Au/ton). In the Leonard mine 

some spotty high-grade (as much as 1.06 g Au/metric ton 

(0.03 oz Au/ton)) occurrences of gold were associated 

with the presence of tetrahedrite, a rare mineral at Butte. 

Free gold was rare but did occur in some silver ores, and 

was observed on some galena surfaces. Weed noted the 

common association of gold, tellurium, and selenium in 

smelter returns. Sales (1913) reported that by 1912,38.6 

metric tons (1.24 million oz) of gold had been produced 

from 65 million tons at a grade of 0.017 oz Au/ton. 

Analyses of samples from oxidized parts of productive 

copper veins reported variations in gold content from 

0.16 to 2.2 g Au/metric ton (0.005 to 0.064 oz Au/ton). 

Higher values occurred in silver veins, generally 

associated with tetrahedrite. The location of these ores 

was not specified, but it is presumed that the ores 

represent the district at large. Perry (1932) reported 

cumulative gold production through 1931 as 57.2 metric 

tons (1.84 million oz). Native gold occurred commonly, 

usually as wires, generally associated with chalcocite. 

Gold occurred in ore shoots as rich as $100 (about 155.5 

g Au/metric ton or 4.5 oz Au/ton); all ores contained gold 

and silver, but copper ores contained 10 times as much 

gold and one-third as much silver as zinc ores. Average 

copper ore (4-5 percent Cu) contained 0.31 g Au/metric 

ton (0.01 oz Au/ton) and 62.2 g Ag/metric ton (2 oz 

Ag/ton). Average zinc-lead ores contained 0.25 g 

Au/metric ton (0.007 oz Au/ton) and 264.4 g Ag/metric 

ton (8.2 oz Ag/ton); manganese ores contained negligible 

gold.

Very little information about the occurrence and 

abundance of gold in deeper workings at Butte was 

published subsequently until Meyer and others (1968) 

indicated that gold is rare at depth in the central, 

intermediate, and peripheral zones. The best grades 

occurred in local ore shoot concentrations in the 

intermediate and peripheral zones of the Main Stage. 

Gold is much less abundant in the pre- and post-Main 

Stage ores. Traces of gold occur in the pyrite-poor 

supergene zone, but not in pyrite-rich parts. Brimhall and 

others (1984) recently have considered the zoning of 

precious metals within base metal sulfides in the central 

zone. They found that the pattern of distribution of gold 

and silver in the sulfides was similar, and they postulated 

that the pattern was controlled by steep faults related to

Gold in Porphyry Copper Systems E25
late-stage porphyry intrusives, with which was associated 

the formation of the Main Stage veins, as well as acidsulfate alteration. In a zone between two steeply dipping 

faults, the precious metals were concentrated locally to 

depths of 1,220 m below the supergene chalcocite 

blanket in a chalcocite-digenite-pyrite Main Stage assemblage. Precious metals outside this zone occur in a 

bornite-chalcopyrite-pyrite assemblage. The AgrAu ratio 

decreases generally upward. Brimhall (1979) and Brimhall and others (1984) believed that gold was introduced 

late in the formation of Main Stage veins, long after 

formation of the pre-Main Stage copper protore, which 

has a very low gold and silver content.

CONCLUSIONS

Resumption of copper production at Butte will 

result in the production of additional byproduct gold. 

The amount of gold remaining in unmined copper ore 

reserves is difficult to assess because of the lack of 

systematic assays for gold in mined ores. As a minor 

constituent of ore, the gold was not worth the cost of 

analyses even though it was recovered (Charles Meyer, 

oral commun., 1984); and although a great deal of 

published geologic, geochemical, and mineralogic data is 

available, information on the occurrence of the gold in 

these deposits is fragmentary. First, the district production of nearly 93.3 metric tons (3 million oz) itself is 

an approximation, owing to under-reporting in the 

earliest days, and lumping of Butte production with other 

production data from more recent times. Virtually no 

data have appeared on the mineralogic occurrences and 

grade of gold mined since 1930; some data indicated the 

grade of early-mined ores as on the order of 0.09-0.93 g 

Au/metric ton (0.003-0.03 oz Au/ton). As a consequence 

of gold's being a minor byproduct, no hard information 

exists about the regularity of distribution of trace 

amounts of gold in recently mined ores. Meyer and 

others (1968) reported that gold is associated with base 

metal sulfides in all Main Stage ore zones; the best 

grades are in local intermediate and peripheral highgrade shoots. Little was reported in pre-Main Stage 

mineralized material, and trace gold was found in the 

supergene enrichment blanket. More recently Brimhall 

and others (1984) have used mineral analysis and 

mathematical analytical methods to determine that 

beneath the Berkeley pit in the central copper ore zone, 

gold occurs in several iron and copper sulfides, is 

structurally controlled by deep-reaching veins, and that 

the Ag:Au ratio generally decreases upward in this part 

of the system, which they consider evidence for strong 

metal zonation upwards.

From these several observations, the status of 

inferred gold reserves, which is based on the copper 

reserve at Butte, must be considered uncertain. The

indication that gold content in sulfide ores decreases 

downward suggests that an estimated inferred reserve of 

7.558 metric tons (243,000 oz) may be overly optimistic. 

Nevertheless, a substantial resource of gold is indicated; 

only the amount is inferred.

Manuscript received by scientific editors December 1984

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Bennett, E.H., 1984, The trans-Challis fault zone A major 

crustal discontinuity in central Idaho: Geological Society 

of America Abstracts with Programs, v. 16, no. 6, 

p. 442.

Brimhall, G.H., Jr., 1977, Early fracture-controlled disseminated mineralization at Butte, Montana: Economic 

Geology, v. 72, no. 1, p. 37-59.

___ 1979, Lithologic determination of mass transfer mechanisms of multiple-stage porphyry copper mineralization 

at Butte, Montana Vein formation by hypogene 

leaching and enrichment of potassium-silicate protore: 

Economic Geology, v. 74, no. 3, p. 556-589.

___ 1980, Deep hypogene oxidation of porphyry copper 

potassium-silicate protore at Butte, Montana A 

theoretical evaluation of the copper remobilization 

hypothesis: Economic Geology, v. 75, no. 3, p. 384-409.

Brimhall, G.H., Cunningham, A.B., and Stoffregen, Roger, 

1984, Zoning in precious metal distribution within 

base metal sulfides: Economic Geology, v. 79, no. 2, 

p. 209-226.

Brimhall, G.H., Jr., and Ghiuso, M.S., 1983, Origin and oreforming consequences of the advanced argillic alteration 

process in hypogene environments by magmatic gas 

contamination of meteoric fluids: Economic Geology, 

v. 78, no. 1, p. 73-90.

Brown, R.G., 1894, The ore-deposits of Butte City: 

Transactions of the American Institute of Mining 

Engineers, v. 24, p. 543-558.

Cox, D.P., Wright, N.A., and Coakley, G J., 1981, The nature 

and use of copper reserve and resource data: U.S. 

Geological Survey Professional Paper 907-F, 25 p.

Emmons, S.F., and Tower, G.W., Jr., 1897, Economic geology 

of the Butte special district: U.S. Geological Survey Butte 

Special Folio, no. 38, p. 3-8.

Engineering and Mining Journal, 1986, Montana Resources 

resumes copper mining at Butte open pit: Engineering 

and Mining Journal, v. 187, no. 9, p. 93-94.

Gustafson, D.L., 1973, Distribution, mineralogy, and structural 

features of the horsetail ore bodies, Butte district, 

Montana, in Miller, R.N., ed., Guidebook for the Butte 

field meeting of the Society of Economic Geologists: 

Butte, Mont., The Anaconda Company, p. J1-J4.

King, P.B., 1976, Precambrian geology of the United States; an 

explanatory text to accompany the geologic map of the 

United States: U.S. Geological Survey Professional 

Paper 902, 85 p.

Klepper, M.R., 1973, Geology of the southern part of the 

Boulder batholith [abs.], in Miller, R.N., ed., Guidebook

E26 Geology and Resources of Gold in the United States
for the Butte field meeting of the Society of Economic 

Geologists: Butte, Mont., The Anaconda Company, 

p. Bl.

Lange, I.M., and Cheney, E.S., 1971, Sulfur isotopic reconnaissance of Butte, Montana: Economic Geology, v. 66, 

no. 1, p. 63-74.

Meyer, Charles, Shea, E.P., Goddard, C.C., Jr., and staff, 

1968, Ore deposits at Butte, Montana, in Ridge, J.D., ed., 

Ore deposits of the United States, 1933-1967 (GratonSales Volume): New York, American Institute of 

Mining, Metallurgical, and Petroleum Engineers, v. 2, 

p. 1363-1416.

Miller, R.N., 1973, Production history of the Butte district and 

geological functions, past and present, in Miller, R.N., 

ed., Guidebook for the Butte field meetings of the 

Society of Economic Geologists: Butte, Mont., The 

Anaconda Company, p. F1-F10.

Perry, E.S., 1932, The Butte mining district, Montana: 16th 

International Geological Congress, Washington, D.C., 

1933, Guidebook 23, 25 p.

Proffett, J.M., Jr., 1973, Structures of the Butte district, 

Montana, in Miller, R.N., ed., Guidebook for the Butte 

field meetings of the Society of Economic Geologists: 

Butte, Mont., The Anaconda Company, p. G1-G12.

Roberts, S.A., 1973, Pervasive early alteration in the Butte 

district, Montana, in Mille., jx.N., ed., Guidebook for the

Butte field meetings of the Society of Economic 

Geologists: Butte, Mont., The Anaconda Company, 

p. HH1-HH8.

Sales, R.H., 1913, Ore deposits at Butte, Montana: 

Transactions of the American Institute of Mining 

Engineers, v. 46, p. 3-109.

Sales, R.H., and Meyer, Charles, 1950, Interpretation of wallrock alteration of Butte, Montana: Quarterly of 

Colorado School of Mines, 75th Annual volume [Mineral 

resources in World affairs], v. 45, no. IB, p. 261-273.

Sheppard, S.M.F., Nielsen, R.L., and Taylor, H.P., Jr., 1969, 

Oxygen and hydrogen isotope ratios of clay minerals 

from porphyry copper deposits: Economic Geology, 

v. 64, no. 7, p. 755-777.

Smedes, H.W., Klepper, M.R., and Tilling, R.I., 1973, The 

Boulder batholith, Montana, in Miller, R.N., ed., 

Guidebook for the Butte field meetings of the Society of 

Economic Geologists: Butte, Mont., The Anaconda 

Company, p. El-El8.

U.S. Bureau of Mines, 1973-1982, Mineral Yearbooks, Area 

Reports Montana, volume 2: pages vary.

Weed, W.H., 1903, Ore deposits at Butte, Montana: U.S. 

Geological Survey Bulletin 213, p. 170-180.

___ 1912, Geology and ore deposits of the Butte district, 

Montana: U.S. Geological Survey Professional Paper 74, 

262 p.

APPENDIX-1990 UPDATE

In 1989, Asarco purchased a 49.9 percent interest 

in a new partnership formed by Montana Resources, 

Inc., to own the latter's Montana copper mining business, 

which includes the former Anaconda properties at Butte, 

Mont. Montana Resources operates the Continental 

open pit mine at Butte, which in 1988 produced 53,200

tons of copper and 14.1 million pounds of molybdenum 

in concentrates. The mine has ore reserves of 468 million 

tons averaging 0.31 percent copper and 0.043 percent 

molybdenum (Anonymous, 1989).

Anonymous, 1989, Asarco buys into Butte, Mont., 

copper operation: American Mining Congress 

Journal, v. 75, no. 6, p. 22.

Gold in Porphyry Copper Systems E27
Gold in the Ely (Robinson) Copper District, 

White Pine County, Nevada

By Laurence P. James1

Abstract

The Ely or Robinson copper district has yielded about 

3 million troy ounces of byproduct gold, recovered during 

milling and smelting of porphyry copper ores. Compared to 

many porphyry copper systems, a relatively high trace-gold 

content is present. Gold appears preferentially concentrated 

in the upper and outer zones of the porphyry system. Small 

ore bodies of considerably higher grade (more than 0.1 

ounce gold per ton where oxidized) have been delineated in 

these peripheral zones. The smaller ore bodies exhibit varied 

degrees of structural and stratigraphic control, and highly 

varied primary sulfide content. Typically they are low in 

copper and silver, and only moderately anomalous in 

arsenic. A disseminated body currently being mined is 

localized in silicified calcareous sandstone, adjacent to the 

principal copper ore trend. Gold ores more distant from the 

porphyry system are mainly structurally controlled and 

typically have a higher tellurium content than the disseminated body.

The district preserves features from zones greatly 

different in depth of formation, subsequently dissected and 

juxtaposed by low-angle faulting. Mineralization and accompanying alteration took place during the cooling of some 

phases of a Cretaceous intrusive quartz monzonite system, 

emplaced into a west-trending, probably south dipping 

regional structure. Virtually all known peripheral gold 

mineralization was localized in the hanging wall of this 

structural zone, in sedimentary rocks of Mississippian to 

Permian age. Controls of gold mineralization included 

proximity to the porphyry system, structures such as faults 

and fractures, and permeable siliceous-calcareous stratigraphic units. Mineralogy of gold-rich copper ores of the 

district differs from many porphyry systems in that bornite 

and enargite zones are lacking. High-grade gold zones are 

unknown in the igneous rocks of the district.

A mid-Tertiary sequence of volcanic rocks, including 

diatremes, tuffs, pyroclastic units, and a garnetiferous rhyolite dome locally cut and conceal copper- and gold-bearing

Japan.

'BHP-Utah Minerals International, Kagoshima-Shi, Kyushu,

deposits. The volcanic rocks bear no relation to genesis of 

primary ores.

INTRODUCTION

The Ely or Robinson mining district lies in White 

Pine County, in the Basin and Range province, eastcentral Nevada (fig. E14). The porphyry copper deposits 

that brought prosperity to the district and the region 

occur in an 11-km-long west-trending belt extending 

from near the town of Ely to west of the mining town of 

New Ruth. The belt crosses a low summit in the northtrending Egan Range, immediately south of U.S. 

Highway 50 (fig. E15).

In 1867, prospectors discovered siliceous, iron-rich 

gossans in Lane Valley, between the present towns of Ely 

and New Ruth (fig. E15). These were first mined for 

precious metals, as were nearby irregular veins (Raymond, 1872, p. 158). Early records (rather incomplete) 

show ore was mined, milled, and smelted as early as 

1873-1877 at the Aultman and Saxton mines near the 

Chainman mine (fig. El 6) and at other small mines 

(Spencer, 1917, p. 93; Read, 1965, p. 267) in Lane Valley. 

The Chainman mine, until recently the most productive 

precious-metals mine in the district, was extensively 

developed by the 1890's. One or more small cyanide 

plants operated intermittently near Lane City until the 

20th century, producing bullion from these and other 

small mines. Lessees reopened some of the mines in the 

1930's and 1970's during periods when the nearby copper 

smelter purchased siliceous flux.

The Giroux Consolidated Mining Company was 

organized in 1903 and the Nevada Consolidated Copper 

Company in 1904 to mine copper. Nevada Consolidated 

started construction of a mill and smelter at McGill, 

27 km northeast of Ely, in 1906, and the first copper from 

the district was produced in 1908. Consolidated Copper 

Mines Company took over operations from Giroux and 

several other mining companies in 1913. Both Nevada 

Consolidated and Consolidated Copper Mines closed

E28 Geology and Resources of Gold in the United States
120'

42'

118' 116' 114' ~i I r

NEVADA

40

Virginia City 

Carson City

112' I 

UTAH

Salt Lake 

City

110'

38

50 100 MILES 

I I____I

50 100 KILOMETERS

36'

Figure E14. Map showing location of the By (Robinson) mining district, Nevada.

down briefly following World War I. Consolidated 

Coppermines Corporation was reorganized in 1922; 

Kennecott Copper Corporation took over Nevada 

Consolidated in 1933, and became the Nevada Mines 

Division of Kennecott in 1943. In 1958 virtually all 

properties in the Ely district were consolidated into a 

single holding by the Nevada Mines Division of Kennecott. (Data are from Lincoln, 1923, and Smith, 1976.)

Descriptions of the early silver-gold mines were 

presented by Spencer (1917), and by mining journals. 

These reports, a few unpublished reports from the 

1930's, and two reports on the distribution of gold, silver, 

tellurium, and mercury (Gott and McCarthy, 1966; 

McCarthy and Gott, 1978) are the only literature on 

these deposits. Spencer (1917, p. 98) estimated a total 

production of $600,000 in gold plus silver prior to 1902. 

As no large silver producers were noted in the district, it 

seems reasonable that only about 22,000 oz of gold was 

produced, if 75 percent of $600,000 in actual production 

is assumed to have been gold at $20.67/oz. The Chainman mine was reworked as a small open pit in the late

1970's, producing gold-bearing high-silica smelter flux. 

Production figures for this period are not available.

To the mid-1980's the largest share of the district's 

gold production has been a byproduct of copper mining 

and smelting. The relatively low grade copper ore in 

porphyry and in altered limestone was mined by both 

underground and open-pit methods. Most of the ore was 

concentrated and smelted at McGill. Gold was recovered 

from canvas-lined launders in the copper sulfide flotation 

plant (mainly in the 1930's) and during electrolytic 

refining of the blister copper, conducted outside Nevada. 

Because of the steady production of copper, the district 

was also the leading gold producer in Nevada in years 

when gold mines were not producing heavily; as of 1960, 

it was third in total gold production in the State, having 

produced an estimated 1,959,659 oz between 1902 and 

1959 (Koschmann and Bergendahl, 1968). Copper 

mining produced about 339 million tons of ore, mainly 

sulfide, between 1908 and 1977 (Wilson, 1978); 

operations ceased in 1978. Latest cumulative gold

Gold in Porphyry Copper Systems E29
115*00'

39»15 f 

103±3 (MHO Hat* 

 onzonlte dike)'

Figure E15 (above and facing page). Generalized geology of the Ely copper mining area, and adjacent goldproducing area to the east, modified from Gott and McCarthy (1966), Brokaw (1967), Brokaw and Barosh (1968), 

Brokaw and others (1973), and Brokaw and Heidrick (1966). Plutons, many of them mineralized with disseminated 

sulfide minerals, and two major sedimentary rock groups are outlined. Faults, including post-ore extensional (BasinRang e-type) faults, are shown. The areas surrounding the plutons consist of complexly faulted Paleozoic sedimentary 

rocks and Tertiary volcanic rocks. Potassium-argon age data for samples are from McDowell and Kulp (1967) and Beat 

(1957).

production figures have not been released, but it is 

estimated from production rates and averages that the 

district produced more than 3 million oz of gold in the 

period 1900-1978. (Appendix provides more data.)

Exploration drilling during the 1960's, 1970's, and 

1980's by Kennecott, the operator of the copper mines, 

and their exploration subsidiary Bear Creek Mining Co., 

delineated bodies of low-grade gold-mineralized rock on 

the flanks of the Ruth porphyry copper ore body. Goldbearing pyrite zones also were identified and drilled in 

the vicinity of the Saxton and other old gold mines of the 

district. Silver King Mines, Inc. leased the Kennecott 

properties in 1985. Currently the Star Pointer ore body, 

which crops out a short distance south-southwest of the

old production shaft for the Ruth copper mines, is 

operated as an open pit mine feeding a cyanide plant. 

Smith and others (1988) described this deposit in detail. 

Reserves at the end of 1986 were stated as 1.65 million 

tons of ore assaying 0.13 oz Au/ton (British Petroleum 

Co., financial summary, 1986) and about 1 million tons of 

low-grade ore assaying 0.03 oz Au/ton (Smith and others, 

1987). Available data indicate a low silver content.

Acknowledgments

Encouragement from and discussions with many 

geologists formerly of Kennecott Copper Corporation,

E30 Geology and Resources of Gold In the United States
114*52'30'

 39MB'

EXPLANATION

Quaternary alluvium

Tertiary volcanic rocks, 

predominantly rhyolite

Cretaceous quartz 

monzonite porphyry

Cretaceous quartz monzonite 

porphyry, altered

Permian and Pennsylvanian 

sedimentary rocks

Pre-Pennsylvanian 

sedimentary rocks

109+3 

(M-K)

Contact

Normal fault Symbol 

on downthrown side

Thrust fault Teeth 

on upper plate

Location of dated 

sample; K-Ar age 

in millions of 

years; (M-K), 

McDowell and Kulp 

(1967); (B). L.H. 

Beal (written 

commun., 1957)

Gold In Porphyry Copper Systems £31
including C.H. Phillips, J.C. Wilson, R.L. Nielsen, I.E. 

Welsh, W.R. Wilson, J.S. Vanderpool, G. Westra, and 

H.D. Kreis are especially appreciated. W.R. Wilson, M. 

Smith, and J. Benham of Silver King Mines, Inc. provided 

a tour of recent developments. Reviews by S.C. Potter, 

R.F. Hardyman, and D.R. Shawe improved this paper. 

Collaboration with A.W. Rose of The Pennsylvania State 

University and Chi-I Huang, now of FMC Corporation, 

aided in the work. Several libraries and private collections yielded old unpublished reports on inaccessible 

mines. The Nevada Mines Division of Kennecott, a 

subsidiary of Standard Oil of Ohio, owner of the 

property, permitted publication of this paper. The writer, 

however, bears responsibility for all conclusions.

GEOLOGIC SETTING

Folded Paleozoic marine sedimentary rocks and 

Cretaceous igneous rocks host the ores of the Ely district. 

The general stratigraphic sequence is as follows:

Permian: Arcturus Formation

Rib Hill Sandstone 

Riepe Spring Limestone 

Pennsylvanian: Ely Limestone 

Mississippian: Chainman Shale

Joana Limestone

Mississippian-Devonian: Pilot Shale 

Devonian: Guilmette Limestone

These rocks are described in detail by Hose and 

others (1976). Pre-ore d6collement faulting juxtaposed 

Permian-Pennsylvanian and pre-Pennsylvanian rocks 

(fig. El5), and technically thinned some units in parts of 

the district. Cretaceous quartz monzonite porphyry 

bodies, with which gold and copper mineralization was 

associated, intrude all these rocks (McDowell and Kulp, 

1967). Intensely altered porphyry and adjacent sedimentary rocks contain the economically mineralized 

ground of the district. A mid-Tertiary sequence of extrusive and locally intrusive volcanic rocks perforates and 

caps the older sedimentary rocks and Cretaceous 

porphyry bodies. No metallic mineralization is known to 

have been associated with the younger igneous rocks.

The sedimentary sequence is generally unmetamorphosed except near the Cretaceous intrusions. 

Where the ore deposits are localized, nearly all preTertiary rocks show evidence of hydrothermal alteration 

(fig. E15). As documented by James (1972; 1976) and 

Westra (1982), post-ore extensional faulting has sliced 

the system into a series of blocks which can be restored to 

a pre-fault configuration. The quartz monzonite intrusions grade upward from a propylitically altered core to a 

potassically altered, strongly mineralized porphyry 

copper system. Figure E17 shows this gradation. The 

potassic zone grades upward successively into a sericitic

zone and then a zone of advanced argillic alteration, 

following the classification of Meyer and Hemley (1967). 

Alteration in adjacent shale included the development of 

extensive hornfels aureoles adjacent to altered intrusive 

rocks, as well as potassic to propylitic alteration similar to 

that in the intrusions. Shale host rock adjacent to fresh or 

propylitically altered intrusions shows considerably fewer 

effects of alteration.

Structure and Igneous Rocks

The porphyry copper bodies and their calc-silicate 

aureoles are cut by low- to moderate-angle Basin-Range 

type faults (fig. E15). These faults disrupted an originally 

elongate, more compact porphyry copper system that had 

a vertical extent of perhaps 1,000 m (James, 1976). A 

remarkable cross section through the mineralized part of 

the system and its weakly altered and pyritized root zone 

is thus exposed. The gold-rich top of the system (fig. E16) 

lies to the east, whereas its deepest part is north of the 

westernmost open-pit mines. With the district restored to 

its pre-Basin-Range geometry, the mineralized porphyry 

system yet retains an east-west elongation.

The fundamental pre-ore structural elements of 

the system are two:

1. A west-trending deep-seated linear structure.

2. A system of low-angle d6collement faults, 

probably contemporaneous with the Sevier orogeny 

(Cretaceous), into which the quartz monzonite bodies 

were intruded. The faults, some of them almost flat, are 

post-Permian and pre-intrusion.

Both structural elements contributed to the east-west 

elongated form of the original pluton(s), and to the 

elongate shape of the narrow precious-metal mineralized 

aureole that surrounds it. Second-order structures of 

multiple azimuths and ages are abundant. The Star 

Pointer gold deposit is localized in the hanging wall of a 

fault that strikes N. 60° E. (fig. E19) and dips 60° NW. 

The strike of the fault is radial to the Ruth porphyry 

copper ore body. Smith and others (1988) described preand post-ore movement on this structure.

Highly mineralized, potassically altered quartz 

monzonite porphyry is locally intruded by unmineralized 

quartz monzonite. K-Ar ages on biotite from the 

mineralized quartz monzonite porphyry average 110 Ma; 

the K-Ar age on hornblende from the unmineralized 

quartz monzonite is 103 Ma (McDowell and Kulp, 1967, 

p. 907). The ages of initial intrusion and mineralization 

appear to be approximately the same; and mineralization 

had ceased before intrusion of unmineralized quartz 

monzonite a few million years later. The Star Pointer 

gold deposit lies in the general vicinity of both intrusive 

rock types, but it most probably predated the unmineralized quartz monzonite.

£32 Geology and Resources of Gold in the United States
Hydrothermal Alteration

The siliceous dolomitic limestone units (notably 

the thick Ely Limestone) adjacent to the mineralized 

porphyry bodies record mineralogically and economically 

spectacular hydrothermal alteration. Adjacent to propylitically altered and non-altered intrusive rock, a zone of 

fine-grained diopside and minor green-tinged andraditegrossularite garnet extends less than 1 m from the 

contact. Pyrite grains are present at one locality as are 

scheelite crystals. Adjacent to potassically altered and 

mineralized quartz monzonite bodies, the Ely Limestone 

contains a wide aureole of colorful calc-silicate skarn (or 

tactile) minerals, veined by quartz and sulfide minerals. 

James (1976) referred to this aureole as the tactile zone. 

Above the tactile zone, adjacent to allered porphyry 

containing predominanlly quarlz, sericile, and pyrile, 

limeslone is replaced by fine-grained quartz, green saponile clays, and pyrile. A surficial oxidation of Ihis rock 

type produced jasperoid-like silica bodies. Few published 

dala compare gold conlenl of calc-silicale rocks wilh 

jasperoids and olher silicified rocks wilhin areas mined 

for copper.

Wilh some restoralion of posl-ore faulting, and 

assumplion of a generally homogeneous siliceous magnesian limestone in contact wilh a single major inlrusion 

of porphyry, a generalized labulalion of primary 

alleralion assemblages in limeslone and porphyry can be 

made:

In porphyry In adjacent limestone Position

Advanced argillic?

Quartz-sericite: 

strongly silicified, 

pyrite-rich.

Relict biotite in strongly 

veined silicified 

porphyry.

Biotite-orthoclase 

alteration 

assemblage: 

secondary biotite and 

orthoclase veining in 

less silicified porphyry; plagioclase 

relicts rimmed by 

orthoclase.

Propylitic assemblagerelict plagioclase and 

chlorite; local areas 

of intense magnetite 

metasomatism.

Almost totally unaltered 

equigranular porphyritic monzonitequartz monzonite.

Advanced argillic: zunyite and gibbsite in 

aluminous rocks. 

Local gold-bearing 

silica bodies.

Silica-pyrite zone: 

quartz, pyrite, copper 

sulfide minerals, and 

nontronite replace 

much limestone.

Relict skarn minerals, 

nontronite-saponite, 

epidote, and siderite.

Skarn or tactile zone: 

massive garnet, diopside, and magnetite; 

veins of actinolite 

sulfide minerals, 

quartz and 

nontronite-saponite.

Quartz-magnetite rock, 

geometry not well 

known; low content 

of sulfide minerals.

Bleached limestone 

with narrow aureole 

of anhydrous skarn 

minerals.

Near-surface, largely 

eroded or mined.

Extends well below 

supergene zone. 

Disseminated gold in 

calcareous sandstone.

Hypogene copper± 

gold ore.

Hypogene ore extends 

well below supergene 

zone.

Beneath known ore.

Beneath ore and distant 

from mineralized 

porphyry.

Einaudi (1982) presenled a comparison of Ihis zoning 

wilh olher skarn deposils.

Presence of an advanced argillic alleralion 

assemblage (including gibbsile, zunyile, kaolinile, and 

abundanl pyrile) in several gold-rich deposils above 

silicic inlrusive bodies has been noled, for example at 

Summilville, Colorado (Sleven and Rall6, 1960), and 

Chin-Qua-Shih, Taiwan (Folinsbee and olhers, 1971; 

Yen, 1976). In bolh these deposits, ihe gold conlent of 

veins sharply decreases with deplh, as advanced argillic 

alleralion fealures disappear. Silliloe (1983), Carlile and 

Kirkegaard (1985), and James (1984) described Ihe 

relalionship of gold-rich enargile-advanced argillic 

assemblages lo deeper or adjacent porphyry coppermineralized bodies in Ihe Philippines and Indonesia.

Weslra (1982, p. 956-964) described quartzsericile, biolile-orlhoclase, and calc-silicale hornfels 

alleralion of Ihe Chainman Shale adjacenl lo mineralized 

porphyry in Ihe Rulh pit. He described the easl (Rulh) 

end of Ihe dislricl as a cupola formed alop a large slock. 

Garneliferous, sulfide-bearing skarns occur in Ihe Joana 

Limeslone, and Pilol Shale has been converted lo hornfels. Jasperoid breccias, noled in sillslone-shale horizons 

of Ihe Pilol Shale near gold deposils elsewhere in the 

region, have not been idenlified.

The rocks wilh the highest gold values at Ihe Rulh 

porphyry deposil are near the top of the present exposures, in silica-rich rocks in Ihe vicinity of advanced 

argillic mineral occurrences. Erosion and pyrile decomposilion have deslroyed some of Ihe original fealures. 

The dislribulion of Irace gold peripheral to the Rulh 

deposil (fig. E16; Goll and McCarthy, 1966) is much 

more strongly anomalous lhan adjacenl to porphyry 

deposits in Ihe weslern parl of Ihe dislricl.

ORE DEPOSITS

The melamorphic-metasomatic aureole surrounding Ihe porphyry inlrusions has yielded between 80 and 

87 million Ions of copper ore (Bauer and olhers, 1964, 

and my projeclions), mainly from Ihe lactite zone. The 

gold content of this ore has not been reporled separalely from lhal produced from porphyry ores. The 

melamorphic-melasomalic aureole (fig. E17) consists of 

several facies of alteration types, cut by mulliple stages of 

veining. In the Veleran, Tripp, and al leasl Ihe deeper 

half of Ihe easl end of Ihe Liberty pit, skarns contained 

economic quantities of copper, as described by Bauer 

and others (1964) and by James (1976). Bauer and others 

(1964) noled lhal ore-grade malerial in allered limeslone 

"exlends up lo 500 feet [150 m] from Ihe porphyry 

conlacts, but the average is probably 200 feet [60 m]." In

Gold in Porphyry Copper Systems £33
Figure E16 (above and facing page). Ely district, showing intensely altered ground, copper- and gold-mineralized 

areas, and mine locations. Line of section locates figure E17.

places, ore-grade primary copper in limestone occurs 

adjacent to much lower grade mineralized porphyry. In 

contrast, Westra (1982) determined that only insignificant tonnages of ore-grade material exist in the 

150-210-m-wide skarn aureole in the Ruth pit. The few 

nonproprietary data available on gold content of 

limestone ores indicate erratic, or very complex, variation 

of gold content of mineralized limestone.

The gold content of mineralized porphyry given in 

the reports just cited generally ranges from 0.15 to 0.5 

ppm or 0.004 to 0.015 oz Au/ton. Substantially higher 

values are reported in calcareous Rib Hill Sandstone 

atop mineralized porphyry. At least local modification of 

gold distribution near the surface, by supergene 

enrichment and leaching, or residual enrichment, is 

suspected. Copper was strongly enriched here. The

behavior of gold in the acid supergene environments near 

oxidizing pyritic porphyry bodies is not understood.

The Star Pointer gold deposit (figs. E18, E19) is 

believed to be hosted in one of the thick calcareous 

quartz-sandstone units in the Rib Hill Sandstone, at least 

700 ft above its base (Smith and others, 1988). Faultcontrolled hydrothermal breccia is associated with ore 

(Durgin, 1989). Gold shows preferential concentration in 

strongly silicified sandstone locally containing as much as 

a few percent black specularite or carbonlike material of 

uncertain origin. No visual correlation of mineralized 

sandstone with calc-silicate minerals is observed.

A number of pyritic gold occurrences in the eastern 

part of the district are hosted by much older strata. 

Structure and general proximity to intrusions are probable ore controls.

E34 Geology and Resources of Gold In the United States
U4«52'30'

39'15'

EXPLANATION

Generalized boundary of intensely altered area within 

which most rocks are silicified and enriched in iron 

(from Gott and McCarthy. 1966; in part from Spencer. 1917)

Generalized area within which mineralized samples contain 

0.5 percent Cu or more (from Gott and McCarthy. 1966)

Generalized area within which mineralized samples contain 

0.00875 oz Au/ton or more (0.3 ppm Au or more) (from Gott 

and McCarthy. 1965)

Mine 

Shaft 

Open pit

Gold in Porphyry Copper Systems £35
190?.prem(nesurface .......--"

Former porphyry

.v.V."""

. ''Former \ 

( jasperoid;

Copper ore with 

byproduct gold 

mined 1950-1970

- 6800

- 6600

- 6400

Figure E17 (above and facing column). Cross section 

through the northwest end of the Tripp porphyry copper pit, 

southwest of New Ruth, Nev., showing nature of intrusion 

and related altered rock that comprise the mineralized 

porphyry and skarn of the district. Section is viewed N. 42° 

W.; it passes immediately northwest of the Old Glory Shaft, 

an early copper mine described by Spencer (1917); no 

vertical exaggeration. Extrapolatbn of data from Bauer and 

others (1964) indicates that about 40 million tons of ore was 

produced from this pit before closure (owing to slope failures) 

in 1969-1970. The section, modified from James (1972), is 

based on drill-hole data and mapping during mining, 

1968-1970. Contacts and units queried where uncertain. In 

Cretaceous quartz monzonite porphyry, q/s is quartz-sericite 

alteration assemblage, almost all of hypogene origin; 

contains quartz veinlets and 5-10 percent pyrite; q/o is 

transition between quartz-sericite alteration assemblage and 

biotite-orthoclase alteration assemblage; b/o is biotiteorthoclase alteration assemblage (flooding and veining), with 

2-3 percent pyrite and about 1 percent chalcopyrite; p/h is 

propylitic alteration assemblage, possibly of a younger 

intrusive phase, in which plagioclase and hornblende are 

little altered. In Pennsylvanian Ely Limestone, w is wollastonite that occurs as a selvage against tactile alteration 

zone; n is transition between silica-pyrite and tactile alteration 

zones and also is a selvage against quartz-sulfide veinlets; 

tactile alteration zone assemblage varies with dolomite 

content of the host unit and degree of local iron or 

magnesium metasomatism; in the Tripp pit the assemblage 

is normally diopside plus quartz, veined by actinolite plus 

quartz, plus sulfide minerals or magnetite; g is andradite 

garnet plus quartz and sulfide minerals or magnetite, a 

variant of the tactile zone most common in the Liberty pit.

EXPLANATION 

Cretaceous quartz monzonite porphyry

Permian Rib Hill Sandstone

Permian Riepe Spring Limestone Pyritized

Pennsylvanian Ely Limestone

B888 Silica-pyrite alteration zone in Ely Limestone

Q

Nontronite or iron saponite after actinolite 

and diopside in Ely Limestone

|||p| Tactite alteration zone in Ely Limestone 

Mississippian Chainman Shale

Contact Queried where uncertain 

Fault Dashed where projected

E36 Geology and Resources of Gold in the United States
Table E13. Gold and silver content (recovered) of copper ores, Ely, Nevada

[Calculated from metal production figures. Recovery by flotation process in 1920's approximated 70 percent of the gold, and 90 percent of the 

copper (U.S. District Court testimony, no. F-157, in Parsons, 1957). In later decades, when more oxidized and low-grade ores from limestone were 

processed, copper recovery is known to have decreased. S (1947), unpublished 25th annual report of Consolidated Coppermines Corporation, New 

York, 1947, 20 p.; **, direct smelting ores; no mill recovery loss incurred; ***, average grade produced from mine; - - -, no data]

Mine(s) and (or) Au Ag Cu Cu:Au 

Year(s) ore type (oz/ton) (oz/ton) (percent) ratio Sources and comments

1909-1910 Porphyry ores

1908-1935 All mines; mainly 

porphyry.

1943 Emma Nevada,

Morris, Brooks, 

underground, 

mainly porphyry.

1943 Richard (850-ft 

level).

1943 Taylor

1943 Tonopah (West

Liberty pit), 

porphyry.

1947 Brooks, Morris,

mainly porphyry.

1947 Tonopah (West

Liberty pit), 

porphyry.

1947 Richard, underground.

19?? Consolidated 

Coppermines 

(probably Emma, 

Morris, Tonopah).

1986-87 Star Pointer gold

0.00096-0.019 0.037-0.045 2 (about)

.0093

.01

.002**

.047**

.006

.009

.004

.003**

.0175***

.031

.020

.016**

1.318**

.015

.018

.015

.035**

.043***

,13 Low

1.11

.824

26,000 Spencer (1917), "20 to 40 

cents in gold"; also, 

production to 1915 

(Spencer, 1917, table, 

p. 98).

31,700 Bateman (1935, p. 309).

74,900 S (1947, p. 6). Average 

copper recovery was 

87.8 percent, gold 

69.5 percent, silver 

61.2 percent.

4.73*** 627,740 S (1947),

4.99*** 28,350 S (1947). Minor tungsten 

also produced.

.74*** 33,630 S (1947). West end, 

Liberty pit.

.79*** 23,320 S (1947, p. 10). Average 

copper recovery was 

79.7 percent, gold 52 

percent, silver 55 

percent.

.68*** 45,120 S (1947).

4.35*** 39,500 S (1947).

(low) S (1947). Average of 15 

samples assayed from a 

composite of mill-head 

samples collected over 

a period of 8 days, per 

court records.

Low Smith and others, 1988. 

Copper "slightly 

enriched relative to 

fresh country rocks"; 

Au:Ag=4:l in cyanide 

mill solution.

Characteristics of Gold Ores

The character of the porphyry ores and that of 

adjacent calc-silicate copper ores, which have yielded 

most of the gold in the district, have been well described 

by Bauer and others (1966), Fournier (1967), and James 

(1976). Chalcopyrite is the main primary mineral, and is

replaced by chalcocite and locally by covellite. The 

distribution of gold within these ores can be calculated 

only from production data, which include losses 

attendant to milling of ore. Table E13 presents available 

data on gold content. Hose and others (1976) stated that 

the average gold content of ores in the Ely district is 

0.0076 oz Au/ton, apparently of recoverable gold.

Gold in Porphyry Copper Systems E37
EXPLANATION

Dump material and 

Quaternary alluvium

Cretaceous monzonite 

porphyry Stipple shows 

mineralized porphyry

Permian Arcturus Formation

Permian Rib Hill SandstoneDark gray is silicified 

rock (cross section only); 

crpsshatch shows 

mineralized sandstone

Pennsylvanian Ely Limestone

Dump

0 100 METERS 

Contour interval 25 fee

Approximate contact 

Arrow shows dip

Fault Dashed where 

projected; arrow shows dip

Drill hole (cross section only)

Figure E18. Generalized geology of the Star Pointer disseminated gold deposit (modified from Smith and others, 

1988). Cross section is shown on figure E19.

The most significant variations in gold content 

(based on published data) are between the typical open 

pit and underground porphyry-calc-silicate ores and the 

ores from the Richard mine area. The Richard and Alpha 

underground mines, located south of the west end of the 

belt of mineralized plutons (figs. E15, E16) produced 

high-grade oxide, carbonate, and native copper ores. The 

main ore minerals were delafossite (CuFeO2 ) and 

azurite. As indicated in table E13, these ores were 

unusually low in gold. These deposits are believed to 

have been fossil exotic copper occurrences, emplaced

where paleodrainage from the main porphyry bodies 

entered favorable sedimentary rock units. Correlation of 

data presented by Kesler (1973, p. 107) with data from 

areas mined in 1968-1970 indicates that rock from the 

Tripp-Veteran pit area probably assayed higher in gold 

than rock from the east end of the Liberty pit.

The principal porphyry-calc-silicate ore deposits of 

copper at Ely are generally high in gold compared to the 

Laramide (Late Cretaceous-early Tertiary) porphyry 

copper bodies of the southwestern U.S. (Kesler, 1973). 

Copper deposits at Bisbee, Ariz., of Jurassic age and at

E38 Geology and Resources of Gold in the United States
Projected outline 

of open pit.

7400

-7200

7000

-6800

6600

6400

6200

Figure E19. Generalized cross section showing interpreted geology of the Star Pointer gold deposit (modified from Smith and 

others, 1988). Data mainly from drill holes by Kennecott Copper Corporation and Silver King Mines Company. Symbols 

explained on figure E18.

Bingham, Utah, of Oligocene age both have high gold 

contents, also. Both these deposits also were emplaced 

in calcareous sedimentary sequences. The reasons for the 

high gold contents of these deposits remain speculative. 

Possibilities include a host rock of greater regional 

(metallogenic) gold content, an evolution of the porphyritic igneous rocks more conducive to gold 

concentration, and a more alkaline environment for gold 

transport allowing a higher gold content in mineralizing 

fluids.

The mineralogy of gold in the copper ores at Ely is 

not well understood. Microscopic examination of several 

dozen polished sections of massive and disseminated 

copper ore failed to reveal gold, but many higher grade 

gold ores worldwide also rarely show metallic gold in 

random section. As table E13 indicates, all the reported 

gold was recovered during flotation, where the intent was 

to recover chalcopyrite and chalcocite and depress the 

maximum amount of pyrite. Metallurgists also noted 

minor gold concentrations at low spots in the mill circuit

(gravity concentration of free gold). Eilers (1914) noted 

that the metallic copper produced from the Ely ores in 

the early years of operations had a very low tellurium 

content, compared with that obtained at Bingham, Utah, 

of 5.5 Ibs/ton. On the basis of these relations, gold in the 

primary ores probably occurs within chalcopyrite and as 

fine-grained free gold; that which remained encapsulated 

in quartz or pyrite or for other reasons did not "float" 

with the copper sulfide minerals was not recovered. The 

copper content of most ores from the center of the 

district is more than 23,000 times the gold content (table 

E13). Veinlets containing free gold were not recognized 

during mining at Ely, as they have been in some porphyry 

districts.

The small ore bodies outside the zone of major 

copper deposits are less well exposed for study. Some, 

such as at the Taylor mine north of the Tripp pit area 

(table E13), have a high content of copper sulfide 

minerals. Most lie to the east of the copper-rich part of 

the district, and are thoroughly oxidized at the surface.

Gold in Porphyry Copper Systems E39
Mining company records indicate very minor production 

of gold from small vein systems west and south of the 

open pit mines. Samples of oxidized veinlels, jasperoids, 

and silica boxworks contained anomalously high amounts 

of gold, silver, tellurium, and mercury, notably from 

areas as far east as the western edge of the town of Ely 

(Gott and McCarthy, 1966). Ores from the Black SpolFarnsworth mines in this eastern area, localized in Joana 

Limestone just beneath Chainman Shale, have no known 

base metal content. The recorded production and 

reserves of these mines are of bedded deposits as much 

as 6 m (20 ft) thick that contain 0.12 oz Au/lon (4 ppm 

Au) and 5.8 oz Ag/ton (200 ppm Ag).

At the Chainman-Aultman-Joana group of mines 

west of the Black Spot-Farnsworth mines and on the 

south side of Lane Valley, ore is localized in the basal 

part of the Chainman Shale near its contact with the 

underlying Joana Limestone. The mineralized bed, which 

does not crop out, dips shallowly to the south. The 

limestone is strongly impregnated with limonite, and 

exhibits mineralized karsllike features. In the Chainman 

and other mines, the ore above water level was "porous, 

honeycombed quartz, which in places was so soft as to be 

almost sand***undoubtedly formerly containing sulfides" (unpublished report by H. Krumb on Chainman 

Consolidated Mining Company, Ely, Nevada, to William 

Boyce Thompson, February 6,1913). These gossan ores, 

shipped as flux to smelters, locally carried from 0.5 to 

5 oz Au/ton. Spencer (1917, p. 172) reported samples 

collected by the mine operators that contained 0.04- 

0.14 oz Au/ton and from 20 to 50 percent Fe. A 1907 

unpublished company map shows the bottom, or 240-ft, 

level as having "an excess of sulphides." The adjacent 

Revenue mine had similar gold-silver gossan ore in a bed 

of the Joana Limestone. The Aultman mine was noted 

for a relatively high silver content (>2 oz Ag/ton).

The Saxton gold mine lies between the Kimbley pit 

and the Chainman mine (fig. E16; Spencer, 1917). It is in 

Mississippian-Pennsylvanian sedimentary rocks near the 

south contact of a pyritized argillized monzonite pluton. 

The intrusion contains no known chalcopyrite ore, but it 

appears to contain local chalcocite perhaps of supergene 

origin. Sandy beds in the Chainman Shale and adjacent 

limestone have been thoroughly replaced by white quartz 

and pyrite, accompanied by gold, in the footwall of a 

d6collement or thrust fault. Old slopes follow a N. 70° E. 

fracture zone. Most production came from a high-grade 

oxidized spongy silica (unpublished 1931 report by E.N. 

Pennebaker on gold deposits on properties of Consolidated Coppermines Corporation). Coarse subhedral 

pyrite is the major sulfide mineral noted in primary 

protore here.

Gold produced from the McDonald-Ely (Robust) 

mine was from a vertical pyrite-rich vein and from 

jasperoids; this mine was also noted for base metal

showings (Spencer, 1917, p. 176). The mineralized 

ground here occurs near a small granitic stock characterized by large pink orthoclase phenocrysts. Adjacent to 

one of the gold-copper veins, silicate minerals in 

limestone include micas.

According to A.V. Heyl (oral commun., 1985) a 

small amount of high-grade gold ore (about 0.25 oz 

Au/ton) was produced in about 1956 from a breccia pipe 

in Ely Limestone at the Midnight mine east of New Ruth. 

The siliceous gold-bearing core of the pipe was surrounded by a shell containing oxide-zinc minerals.

Exploration drilling for copper in deeper Mississippian and Devonian rocks beneath the porphyry system 

has failed to reveal substantial gold deposits.

Ore Fluids

Fluid inclusions in garnet, pyroxene, and quartz 

from the Ely copper deposits were studied by Huang 

(1976) and Huang and others (1978) using a heating 

stage. Fluid inclusions in minerals from the tactile zone 

have homogenization temperatures of 450-600 °C, and 

are dominated by liquids. Crosscutting quartz-sulfide 

veinlets surrounded by argillically altered rock showed a 

wide range of salinities, as much as 42 weight percent 

NaCl equivalent, and varied temperatures, suggesting 

multiple generations of veining. Silicified limestone 

overlying the calc-silicate rocks of the tactile zone 

contains vapor-rich inclusions, suggesting lhal boiling 

occurred here. No dala were oblained on advanced 

argillic alteration minerals. Goss (1983), studying fluid 

inclusions within and below the Rulh deposil, found little 

or no evidence of early magma-related fluids in deep 

unaltered igneous rocks and minor associated quartz 

veins.

Huang (1976) and Huang and others (1978) also 

analyzed carbon and oxygen isotopes from the zone 

peripheral to ihe porphyry copper bodies. In Iraverses 

inward from unaltered limestone to marble, they 

delermined a systematic decrease in 518O, suggesting the 

effecl of magmalic waler. The S18O values are higher for 

calcile lhan for coexisling quartz, suggesting isolopic 

disequilibrium.

Using calculated fractionations for water, Huang 

and others (1978) determined a temperalure of 660-360 

°C for formalion of quarlz-magnetite pairs from the 

clay-sulfide veinlets. They also found, in the tactile zone 

of ihe Veleran pit, thai calculated 618 O compositions of 

ore fluids decreased regularly from +10.5 per mil al Ihe 

cenler of Ihe inlrusion to + 7 per mil al ils periphery.

CONCLUSIONS

Subslantial amounts of gold have been produced 

from the Ely district because very large tonnages of

E40 Geology and Resources of Gold in the United States
copper ore with minor gold content have been produced, 

and because much of the contained gold was recoverable 

with the copper. Gold mines in the periphery of the 

district, most probably lying in the outer zones of the 

same copper system, have to the late 1980's yielded a 

smaller production, although gold grades in these zones 

are actually higher than in the porphyry copper deposit 

itself. Rocks from a few leached, silicified outcrops may 

have produced high-grade silver gold ores for early 

miners, but were most significant as indicators of copper 

ores, concealed below. Unknown quantities of goldbearing capping rock were stripped off during copper 

mining. A potential remains for discovery of deposits 

similar to the Star Pointer ore body.

The Ely copper deposits differ from many porphyry 

and skarn systems in the United States in their higher 

gold content, greater age, and elongate form. Gold 

distribution within the Ely system approximates that of 

other gold-rich porphyry systems with overlying advanced 

argillically altered, high-sulfide ore zones, described by 

Sillitoe (1983). Gold deposition accompanied a major 

enrichment in sulfur, copper, and probably iron and 

silica, in and adjacent to the apex of the cooling quartz 

monzonite body. Westra (1979, 1982) and Huang and 

others (1978) suggested that hydrothermal fluids, of 

magmatic origin during early stages of ore formation, 

entered fractured but still hot porphyry beneath the west 

end of the district, and passed upward and eastward 

toward the top of the system. A cap of argillically altered 

silicified rock containing a substantial concentration of 

gold probably characterized the upper and outlying parts 

of this large hydrothermal system.

Manuscript received by scientific editors May 1985

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JA., 1966, Porphyry copper deposits in the Robinson 

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J.N., 1964, Origin of the disseminated ore in metamorphosed sedimentary rocks, Robinson mining district, 

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Beal, L.H., 1957, Wall-rock alteration in the western portion of 

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V., 1971, Chinkuashih A gold-pyrite-enargite-barite 

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Ety porphyry copper deposit by utilization of fluid 

inclusions: University Park, Pa., The Pennsylvania State 

University M.S. thesis, 79 p.

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silver, tellurium, and mercury in the Ety mining district, 

White Pine County, Nevada: U.S. Geological Survey 

Circular 535, 5 p.

Hose, R.K., Blake, M.C., Jr., and Smith, R.M., 1976, Geology 

and mineral resources of White Pine County, Nevada: 

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Huang, Chi-I, 1976, An isotopic and petrologic study of the 

contact metamorphism and metasomatism related to 

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Pennsylvania State University Ph. D. thesis, 188 p.

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inclusions and isotopic gradients as guides to ore at Ety, 

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Geochemists, 7th International Geochemical Exploration Symposium, Golden, Colorado, p. 43-44.

Gold in Porphyry Copper Systems E41
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___ 1984, Geochemical and geological exploration for goldcopper deposits, in southern Luzon, the Philippines 

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in porphyry copper deposits: Economic Geology, v. 68, 

p. 106-112.

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78, no. 2, p. 348-352.

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mineable precious metal deposits of the western United 

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APPENDIX-1990 UPDATE

A 1,200 ton/day cyanide mill in the district became operational 

Oct. 1,1986. The mill, currently owned by Alta Gold Co. in partnership 

with Echo Bay Group, produced 123,416 oz Au through March 31, 

1989, including production from heap leaching operations begun in 

1988. Silver King Mines, Inc.-Pacific Silver Corp. (1989) reported 

proven and probable diluted reserves of minable gold ore at the Star 

Pointer, plus a number of other smaller deposits within its landholdings in the district, to be 2,250,000 tons of 0.089 oz/ton mill ore 

and 43,750,000 tons of 0.016 oz/ton heap leach ore. These reserve 

figures include a calculation for dilution resulting from waste that will 

have to be processed with the ore. Some dump material, removed 

mainly from the Ruth pit area, has been determined to be of teachable 

grade. Durgin (1989) noted that additional tonnages of low-grade

copper-gold porphyry ore are not presently economic, and are not 

considered as reserves.

Durgin (1989) described several of the smaller deposits included 

in the above figures. The recently opened Northwest Ruth deposit, at 

the northwest corner of the former Ruth copper pit, is in Riepe Spring 

Limestone and Rib Hill Sandstone. Here, intrusive breccia invaded a 

northeast-trending complexly faulted zone. Rather erratic silicification 

was controlled by faults and favorable lithologies. Gold commonly is 

coarse (to 0.02 mm) and not everywhere associated with silica. In 

contrast to Star Pointer gold ore, pyrite and fluorite are locally 

abundant, silver content is much higher, and some gold is present in 

quartz veinlets. Limited data show lead, copper, and zinc exceed 0.1 

percent, selenium and tellurium are as high as 50 ppm, and arsenic 

ranges between 50 and 100 ppm (Durgin, 1989).

Silver King Mines, Inc.-Pacific Silver Corp., 1989, Prospectus/Proxy 

statement: Salt Lake City, p. 60-66.

E42 Geology and Resources of Gold in the United States
The Tomboy-Minnie Gold Deposits at 

Copper Canyon, Lander County, Nevada

By Ted G. Theodore, Stephen S. Howe, and David W. Blake1

Abstract

The Tomboy-Minnie gold deposits are part of a middle 

Tertiary porphyry copper system whose central porphyry 

body is centered at Copper Canyon. Gold-silver ores in the 

Tomboy-Minnie deposits occur mostly in a pyrrhot'rte- and 

pyrhe-rich basal 30-m-thick sequence of altered calcareous 

conglomerate in the Middle Pennsylvanian Battle Formation. 

The Tomboy-Minnie deposits are distal contact-metasomatic 

gold deposits that are characterized by actinolite- and 

chlorite-dominant assemblages ,n marked contrast to the 

skarn, potassium silicate, and phyllic assemblages that 

characterize copper-gold-silver deposits closer to the central 

porphyry stock. Introduction of gold occurred penecontemporaneously with replacement of early diopside alteration 

assemblages by actinolite and chlorite. Metals are zoned 

strongly in the system: A proximal copper-gold-silver zone 

grades outward to a gold-silver zone (in which lie the 

Tomboy-Minnie deposits), which in turn is succeeded by a 

distal lead-zinc-silver zone. The entire mineralized system 

contained a minimum of about 3.3 million ounces of gold 

before large-scale mining operations began.

INTRODUCTION

Mining in the Battle Mountain mining district, 

about 19 km southwest of the town of Battle Mountain, 

Nev., spans a period of more than 120 years, from 1866 to 

the present. However, the first large-scale attempt to 

mine base and precious metals by open-pit methods was 

begun by Duval Corporation in 1967. Prior to that time, 

the Copper Canyon underground mine (fig. E20) had 

been operated sporadically between 1917 and 1955 

(Roberts and Arnold, 1965). Placer gold was discovered 

in Copper Canyon in 1912, and intermittent small-scale 

placer operations were carried on into the early 1940's. 

From 1944 to 1955, Natomas Company operated a 

dredge on the alluvial fan at the mouth of Copper

Battle Mountain Gold Exploration, Battle Mountain, Nev.

Canyon and reportedly produced 100,000 oz of gold 

(Johnson, 1973, p. 37-38). The closely spaced TomboyMinnie deposits, first described by Blake and others 

(1978), were placed into operation as copper reserves 

declined in the nearby West ore body (Theodore and 

Blake, 1975; 1978).

History of Reserves and 

Mine Development

Ore reserves in Duval's East ore body (fig. E20) in 

Copper Canyon prior to the 1967 start-up included 

13,875,000 tons containing 0.79 percent Cu, along with 

0.025 oz Au/ton and 0.47 oz Ag/ton (Sayers and others, 

1968, p. 56). The deposit has since been mined out 

(Theodore and Blake, 1975). The West ore body was 

developed subsequent to the depletion of the East ore 

body, and contained approximately 4 million short tons of 

ore at grades generally similar to those in the East ore 

body (Theodore and Blake, 1978). The Tomboy-Minnie 

deposits contained an estimated 3,900,000 short tons of 

ore grading 0.09 oz Au/ton and 0.28 oz Ag/ton 

(Anonymous, 1981). The Tomboy-Minnie deposits were 

mined out during late 1982, and precious-metal mining 

operations were shifted to an area immediately 

surrounding the Independence Mine (fig. E20), which is 

referred to as the Northeast Extension deposit just north 

of the East ore body.

In 1981, Duval Corporation announced discovery 

of a large gold-silver skarn ore body, the Fortitude, just 

north of the West ore body in the Copper Canyon area. 

Initial mining reserves for the Fortitude deposit were 

stated to be 16 million tons containing 2.4 million oz of 

gold and 9.2 million oz of silver (Anonymous, 1981). 

Subsequent reserve studies for the Fortitude deposit 

predicted that the same amount of precious metals was 

contained in approximately 12 million tons (Wotruba and 

others, 1986).

Gold In Porphyry Copper Systems E43
117°08'

Copper Canyon 

underground mine

Battle 

Mountain

Reno Area of 

figure E20 .Tomboy-Minnie. . . 

deposits '.'.'.'.

Figure E20 (above and facing page). Geologic sketch map of Copper Canyon area, Lander County, 

Nevada.

E44 Geology and Resources of Gold in the United States
EXPLANATION

Qal

QTb

v''T.a avi

Pe

PPap

::::PPp:::::

Pb

Dsc

 h

LIST OF MAP UNITS

Alluvial deposits

Basalt flows

Caetano Tuff

Altered granodiorite porphyry of Copper Canyon

Quartz diorite

Edna Mountain Formation

Antler Peak Limestone

Pumpernickel Formation

Battle Formation

Scott Canyon Formation

Harmony Formation

Contact 

Fault

CORRELATION OF MAP UNITS

^^ ^ M^H Thrust fault Sawteeth on upper plate

-"-- - -111 Area of more than 1 percent dispersed iron sulfides

- BioII»»»" Area of strongly developed potassic alteration 

Chiefly secondary biotite

-1 ' --1 Area of more than 2 percent dispersed iron sulfides

GOLCONDA 

ALLOCHTHON

}!

X 0) }

° c 

<° % r- U 

§.!

uj O

Unconformity

ROBERTS MOUNTAINS 

ALLOCHTHON 

(De Witt Thrust)

ROBERTS MOUNTAINS

ALLOCHTHON 

(fault not exposed)

On January 1, 1985, the board of directors of 

Pennzoil Company, parent of Duval Corporation, 

announced the formation of an independent company, 

Battle Mountain Gold Company, as a spin-off to the 

shareholders of Pennzoil Company. This announcement 

occurred subsequent to a previous announcement by 

Pennzoil Company that all of Duval's metal-mining 

operations were up for sale (Epler, 1985). In 1986, Battle 

Mountain Gold Company confirmed existence of a gold 

deposit in Copper Basin northeast of Copper Canyon, termed the Surprise (Argall, 1986). Estimated 

reserves for the Surprise Mine include 160,000 oz of gold 

in 1.75 million tons of ore (Anonymous, 1986), and 

production from the Surprise Mine commenced during 

the third quarter of 1987 (Anonymous, 1987).

Ore Processing

Milling of copper-gold-silver ores, mostly from the 

East and West ore bodies at Copper Canyon (Theodore 

and Blake, 1975; 1978), continued to 1977, when minable 

copper-gold-silver ores there were exhausted. Copper

production from 1967 to 1975 in the Battle Mountain 

mining district ranked third in the State behind 

operations at Yerington and Ely, both presently (1988) 

suspended. Leaching operations in the Battle Mountain 

district, centered mostly in the Copper Basin area, 

resulted in the largest production of copper in the State 

of Nevada in 1981 (Lockard and Schilling, 1983). The 

mining operation at Copper Canyon shifted to the 

processing of gold-silver ores in January 1979, initially in 

large part from highly sulfidized replacement ore bodies 

at the Tomboy-Minnie deposits (Blake and Kretschmer, 

1983; Theodore and others, 1986). Existing plant 

facilities at Copper Canyon were modified at this time to 

include cyanide leach and carbon-in-pulp adsorption 

sections for processing of gold-silver ores (Jackson, 

1982).

Metal Production

Metal production from the district to 1961 

included 150,000 oz of gold, 2.1 million oz of silver, 

15,000 tons of copper, 5,000 tons of lead, and 1,500 tons

Gold in Porphyry Copper Systems E45
of zinc. Duval's production from both milling and leachprecipitation operations at Copper Canyon and nearby 

Copper Basin for the period 1967-1974 was 102,082 tons 

of copper (Theodore and Blake, 1975, p. C2).

During 1980 and 1981, production of gold from ore 

bodies in the Copper Canyon area contributed significantly toward making Nevada the leading gold-producing 

State (Lockard and Schilling, 1983; Lucas, 1982). Goldsilver production in 1980-1984 from the Copper Canyon 

area included the following (Prospectus, issued by Battle 

Mountain Gold Company, July 12, 1985):

1980 1981 1982 1983 1984 

(Tons and oz expressed in thousands)

Tons ore milled

Stripping ratio

Mill feed (oz

Au/ton)

Recovery factor,

Au (pet.)

Oz gold recovered

Oz silver recovered

1,068

3.95:1

0.073

87

69

21

1,234

6.31:1

0.064

85

66

39

1,400

9.32:1

0.059

85

71

92

1,291

12.06:1

0.072

87

80

307

1,231

17.26:1

0.071

85

73

357

In addition, through December 1984, recovery of 

precipitates of copper from leach dumps at both Copper 

Canyon and Copper Basin continued at a rate of approximately 2,400,000 Ib copper per year.

Estimated full-scale production from the Fortitude 

mine of 150,000 oz of gold per year was anticipated to be 

reached in 1985. Production in 1985 from the Fortitude 

mine actually amounted to 220,000 oz of gold and 

647,000 oz of silver (Argall, 1986), and in 1986, 259,000 

oz of gold and 964,000 oz of silver (Northern Miner, Feb. 

23, 1987, p. 24).

Significance of Skarn-Associated 

Gold Systems

Gold skarn systems in the Battle Mountain mining 

district, including the Tomboy-Minnie distal contactmetasomatic gold deposits, and gold skarns in the McCoy 

mining district, approximately 30 km south of Copper 

Canyon, define a cluster of such deposits that are similar 

in age, 35-38 Ma, but hosted by rocks in different 

tectonic blocks. The gold skarns in these districts seem to 

have developed in successively higher tectonic blocks 

near the southern limit of the cluster as it is presently 

known.

The Fortitude and McCoy gold deposits are parts 

of two world-class gold skarn systems (Orris and others, 

1987). The importance of this gold-mineralized environment in north-central Nevada is evidenced further by the 

announcement by Echo Bay Mines that it had discovered 

an additional 4 million oz of gold at its Cove deposit, 

which apparently occurs in Triassic limestone and

conglomerate on the fringes of its McCoy gold skarn 

deposit (Echo Bay Mines, Special Report to Stockholders, December 23,1987). Thus, the Battle Mountain 

and McCoy mining districts probably contained about 10 

million oz of gold in known deposits prior to the onset of 

large-scale mining operations. For comparison, along a 

72-km stretch of the northwest-trending Carlin mineralized belt, which occurs approximately 50 km east of the 

Battle Mountain mining district, about 27 million oz of 

gold are known to occur in 21 sediment-hosted gold 

deposits (The Northern Miner, November 16, 1987, p. 

A28).

GEOLOGIC SETTING OF THE 

COPPER CANYON DEPOSITS

A geologic sketch map of the Copper Canyon area, 

based on mapping by Roberts (1964), Theodore and 

Blake (1978), and Wotruba and others (1986), shows the 

location of the copper-gold-silver West and East ore 

bodies and the gold-silver deposits of the Northeast 

Extension, the Tomboy-Minnie and the Fortitude 

deposits (fig. E20). All these deposits consist of 

replacement-disseminated sulfide ore, within Paleozoic 

sedimentary rocks.

The Paleozoic sedimentary and minor volcanic 

rocks of the Copper Canyon area underwent several 

stages of deformation, including major thrust faulting 

during the middle Paleozoic Antler orogeny and early 

Mesozoic Sonoma orogeny, and Basin-and-Range block 

faulting and intrusive activity during the Tertiary 

(Roberts, 1964). Rocks belonging to the Early Mississippian Roberts Mountains thrust system include 

sandstone, shale, and minor limestone of the Upper 

Cambrian Harmony Formation, which have been thrust 

along the DeWitt thrust over chert, minor volcanics, and 

limestone of the Devonian Scott Canyon Formation. The 

DeWitt thrust is an imbricate fault related to the Roberts 

Mountains thrust system. Disconformably overlying the 

Harmony Formation are coarse clastic rocks and limestone of the autochthonous Antler sequence, which 

includes the Middle Pennsylvanian Battle Formation, the 

Pennsylvanian to Permian Antler Peak Limestone, and 

the Permian Edna Mountain Formation. These formations of the Antler sequence host the bulk of the precious 

and base metals at Copper Canyon. Above the Antler 

sequence are chert and argillite of the Pennsylvanian to 

Permian Pumpernickel Formation, which were transported tectonically eastward into this area along the 

Golconda thrust fault during the early Mesozoic Sonoma 

orogeny (Silberling and Roberts, 1962; Speed, 1977).

Precious and base metal deposits in the Copper 

Canyon area of the Battle Mountain mining district are

E46 Geology and Resources of Gold in the United States
genetically and spatially related to a middle Tertiary 

(Theodore and others, 1973) altered granodiorite porphyry that has intruded the sequence of Paleozoic 

sedimentary and volcanic rocks (fig. E20). The contact 

metamorphic aureole associated with this stock extends 

several hundred meters into its wallrocks. Most base and 

precious metal ore at Copper Canyon occurs as 

replacement and disseminated sulfides in originally 

calcareous rocks that have been metamorphosed and 

metasomatically altered to various calc-silicate assemblages including skarn (Theodore and Blake, 1978). Ore 

occurs locally in noncalcareous rocks as disseminated 

sulfide minerals, veinlets, and fissure veins. Hydrothermal silicates and sulfide minerals are distinctly zoned 

about the stock, a relation that contributed significantly 

toward the recognition and development of additional 

ore reserves in the Copper Canyon area.

During late Eocene or early Oligocene time the 

porphyritic granodiorite stock intruded the sequence of 

Paleozoic sedimentary and volcanic rocks at Copper 

Canyon. The stock contains phenocrysts of quartz, 

plagioclase, potassium feldspar, and biotite set in a 

mosaic, microaplitic groundmass of quartz, potassium 

feldspar, and biotite. Detailed petrographic studies of 

this stock show that its modal composition overlaps the 

quartz monzonite-granodiorite compositional field of 

Bateman (1961). Locally rare hornblende phenocrysts 

occur in deep parts of the stock. Potassic alteration 

assemblages that include widespread secondary biotite 

are predominant throughout most of the altered granodiorite porphyry of Copper Canyon.

TOMBOY-MINNIE GOLD DEPOSITS

The geology of the Tomboy-Minnie gold deposits is 

relatively simple (fig. E21). Although unconformable 

elsewhere in the district, bedding in both the Battle 

Formation and the underlying Harmony Formation 

strikes generally north and dips about 30° W. in the 

Tomboy pit. Shale and siltstone of the Harmony 

Formation have been converted to brown biotite hornfels 

during emplacement of the porphyritic granodiorite of 

Copper Canyon, whereas the fine-grained matrix of 

sandstones (including quartzarenite, subarkose, and 

litharenite) was recrystallized and silicified to clay 

minerals, white mica, and secondary quartz. In the 

Copper Canyon area, the Battle Formation consists of 

three members with a total thickness of about 250 m 

(Wotruba and others, 1986). A relatively small, complexly faulted, downdropped sliver of the lower member 

of the Battle Formation is exposed in the Minnie pit. A 

much greater thickness of the lower member of the 

Battle Formation is exposed in the Tomboy pit, where it 

is as much as 100 m thick (Roberts, 1964; Theodore and 

Blake, 1975). The lower member of the Battle Formation

in the Tomboy pit may be divided into two units (not 

shown on fig. E21) based on primary lithology, and 

hydrothermal metallization. Gold mineralization was 

restricted mainly to a basal 30-m-thick unit of calcareous 

conglomerate that is replaced by calc-silicate-mineral 

assemblages with a high content of sulfide minerals. 

Typically, this basal sequence is dark greenish black in 

contrast to the lighter colored rocks of the Battle 

Formation higher in the lower member. The upper part 

of the lower member contained less calcareous material 

and upon metallization yielded a much lower sulfide 

content. In the Tomboy-Minnie deposits, most of the 

basal 30-m-thick sequence of the conglomerate in the 

Battle contains subangular to subrounded, frameworksupported clasts of chert, quartzite, and lesser limestone. 

Thin limy siltstone and shale beds are also interbedded 

with the basal calcareous conglomerate. Generally, the 

upper 70 m of the Battle Formation in the Tomboy pit is 

conglomerate with a matrix of quartzose sandstone made 

of tightly interlocked grains of quartz and lithic 

fragments. Locally, near the top of the Battle, there are 

several beds that are generally less than 1 m thick and 

consist almost entirely of actinolite-tremolite.

In the Tomboy-Minnie deposits an early diopside 

assemblage is replaced successively by actinolite- and 

chlorite-bearing assemblages, each of which was 

accompanied by iron sulfide minerals and free gold. 

Veins are absent from the ore zones in the basal 30 m of 

conglomerate in the Battle. The matrix, formerly 

calcareous and in places showing traces of relict 

carbonate, now consists of either (1) an actinolitedominant assemblage that also includes quartz, plagioclase, sphene, minor chalcopyrite and epidote, and traces 

of potassium feldspar and apatite; or (2) chloritedominant assemblages containing quartz, clays, pyrrhotite and (or) pyrite, chalcopyrite (minor), epidote, 

sphene, and relict actinolite and diopside. Galena and 

sphalerite are abundant in shaly parts of the deposit, and 

are complexly intergrown with pyrrhotite and minor 

chalcopyrite, yielding a "banded" aspect to metasomatized rocks. Textural relations in chlorite-pyrite-rich ores 

suggest that the pyrite, which contains rare microscopic 

blebs of galena, is paragenetically somewhat later than 

pyrrhotite, sphalerite, early-stage galena, chalcopyrite, 

and arsenopyrite (trace).

A north-trending granodiorite porphyry dike crops 

out along the east wall of the Minnie open pit and is 

shown schematically in a cross section through the 

deposits (fig. E21). North of the Minnie pit, this dike is 

well exposed in a roadcut, and dips steeply to the west. 

The dike is composed of quartz, plagioclase, and biotite 

phenocrysts set in a microaplitic groundmass of potassium feldspar, quartz, plagioclase, and minor biotite. This

Gold in Porphyry Copper Systems E47
 Tomboy / :::::::::::>:::::>:

mine ^[ ' ' ' ' xOutline 

of ore bodies

200 METERS

A 9

 v-

:x:N/:x»: ::.:::.:-x/^ 

:>--/:;gx;x-:;x;:: :Tag^

Figure E21. Generalized geologic 

map and cross section A-A' of 

Tomboy-Minnie area, Lander 

County, Nevada. Tag, altered gran6000' odi° rite °f Copper Canyon; PPp, 

Elev. Permian-Pennsylvanian Pumpernickel Formation; Pb, Pennsylvanian Battle Formation; -Ch, Cambrian Harmony Formation. Heavy 

lines, faults; dashed where inferred. 

Sawteeth on upper plate of thrust. 

Dip of faults and strike and dip of 

bedding are shown. On cross 

section arrows show direction of 

movement on faults.

E48 Geology and Resources of Gold in the United States
dike is similar petrographically to the potassically altered 

granodiorite of Copper Canyon. Locally, however, the 

dike has been enriched in gold and silver in its more 

altered parts.

The Tomboy-Minnie gold deposits occur within an 

alteration zone, termed transitional by Blake and others 

(1978), that contains more than 4 volume percent total 

sulfide minerals, in contrast to rocks outside this zone, 

which have generally 2 volume percent or less total 

sulfides. Alteration in the deposits resulted from a 

combination of early, mostly isochemical, thermal metamorphism followed by metasomatism. Thermal effects 

resulted in the calcareous basal conglomerate of the 

Battle Formation being converted to a quartz + diopside 

+ epidote hornfels assemblage. Sparse garnet replaced 

some limestone clasts. Secondary quartz is generally 

present, except in thin lenses that consist entirely of 

actinolite-tremolite. Actinolite-dominant assemblages 

and chlorite-plus-minor-clay assemblages were formed 

contemporaneously with sulfide mineralization and the 

ore-forming stage. Some chlorite occurs as a replacement of earlier formed calc-silicate minerals. Epidote is 

more common near the base of the Battle Formation 

than higher in the sequence; epidote is also concentrated 

along fractures in the underlying Harmony Formation 

directly below the unconformity. Hydrothermal alteration in the Harmony Formation caused development of 

clay adjacent to some veinlets, replacement of phyllosilicate matrix by clay, and introduction of very sparse 

quartz-sulfide veinlets. Limited studies of these veinlets 

show that they contain sulfide-mineral assemblages 

similar to the replacement-disseminated assemblages in 

the overlying Battle Formation.

Premineral and postmineral faults were important 

in the development of these deposits. The Minnie deposit

has been offset approximately 200 m below the level of 

the Tomboy deposit along a series of northwest-striking, 

east-dipping postmineral normal faults (fig. E21). A 

west-dipping, northwest-striking reverse fault has offset 

these east-dipping, low-angle normal faults. Just west of 

the Tomboy deposit, a nearly vertical north-striking fault 

locally has dropped the Battle Formation about 30 m to 

the west. Many minor northwest-striking, high-angle 

faults and fractures of small displacement are premineral throughout both deposits, and they served to 

enhance access of hydrothermal solutions and the 

deposition of sulfide minerals.

Sulfide content in these ore bodies ranges from 10 

to more than 50 volume-percent and is mostly pyrrhotite 

and pyrite. Locally, however, marmatite and galena are 

abundant in podlike replacement bodies shown 

schematically in figure E22. Figure E22 is a N.-45°-E. 

schematic section through the deposits prepared from 

geologic and assay data obtained from 19 percussion drill 

holes along the section line. Samples from each 1.5- or 

3-m (5- or 10-ft) drill-hole interval were analyzed for 

silver, gold, copper, lead, and zinc. However, only silver, 

gold, and zinc assays are shown, and the lower limits of 

assays used to depict the distribution of each metal are 

based on ore-grade calculations and background geochemistry for deposits elsewhere in the district; they are 

0.10 oz Ag/ton, 0.050 oz Au/ton, and 500 ppm Zn. Other 

sulfide minerals in the Tomboy-Minnie deposits, in 

decreasing abundance, include marcasite (replacing pyrrhotite), chalcopyrite, and arsenopyrite. Trace arsenopyrite occurs in rare quartz veinlets in the deposits. Chalcopyrite locally increases with an increase in pyrrhotite 

and is most abundant near the base of the Battle 

Formation.

ELEVATION,

FEET 

6,000-,

TOMBOY MINE

MINNIE MINE

5,500-

\

IU111JAU>0.050 oz/ton 

CHAG>0.10 oz/ton 

[HiZN>500 ppm

Figure E22. Idealized northeast-trending cross section through the Tomboy-Minnie deposits showing rocks 

containing >0.05 oz Au/ton, >0.10 oz Ag/ton, and >500 ppm Zn. Data taken from 19 percussion drill holes 

along the trace of the cross section. Pb, Pennsylvanian Battle Formation; -Ch, Cambrian Harmony Formation.

Gold in Porphyry Copper Systems E49
Gold has not been observed in microscopic 

examination of the Tomboy-Minnie deposit ore. However, statistical evaluation of assays of drill-hole samples 

suggests that gold is closely associated with pyrrhotite 

and pyrite. Free gold, generally less than 0.05 mm 

diameter, was recovered during the milling of ore from 

these deposits; this relation suggests that some gold 

occurs in the native state as discrete grains together with 

pyrrhotite and pyrite. Free gold in the East ore body was 

in larger particles than that mined in the Tomboy and 

Minnie deposits, and typically was very strongly associated with sulfide-bearing quartz veins. Silver in the 

Tomboy and Minnie deposits correlates closely with 

galena. Studies at DuvaPs laboratory in Tucson, Ariz., 

have shown that native silver has exsolved from galena. 

Silver content also may be related, in part, to overall 

sphalerite content.

The Tomboy and Minnie gold deposits in Copper 

Canyon have not been dated. Based on the general 

zonation of these gold deposits relative to the altered 

granodiorite porphyry of Copper Canyon, we believe 

the time of gold deposition of the deposits to be similar 

to that of the East ore body, which was determined as 37 

Ma, using the potassium-argon method on fine-grained 

secondary biotite in the Battle Formation (Theodore and 

others, 1973).

METAL ZONING AND AU:AQ RATIOS

Roberts and Arnold (1965) recognized a zonal 

distribution of metals in the Battle Mountain district. At 

Copper Canyon, their zonal distribution of metals has 

been modified on the basis of more recent data. The 

zonal distribution of metals based on assays of samples 

from open-pit mining operations and drill holes is 

shown in figure E23. Nearly all the data reflect 

metallization hosted by the Battle Formation, except for 

that associated with the andradite- and diopside-rich 

skarn assemblages in the West ore body (Theodore and 

Blake, 1978). The metals are zoned strongly outward 

from the altered granodiorite porphyry of Copper 

Canyon. A proximal copper-gold-silver zone gives way 

outward to a gold-silver zone, which in turn is succeeded 

by a distal lead-zinc-silver zone. The granodiorite 

porphyry at the center of these metal zones contains 

anomalous but subeconomic amounts of copper, 

molybdenum, gold, and silver. The zone containing 

economic concentrations of copper, gold, and silver at 

the West and East ore bodies is located within the 

mapped outer limit of potassic alteration (Blake and 

others, 1978). However, the West ore body also shows 

significant modification of early andradite- and diopsidebearing assemblages by tremolite- and actinolite-bearing 

assemblages. Sulfide minerals in the copper-gold-silver 

zone include pyrrhotite, pyrite, chalcopyrite, and lesser

quantities of sphalerite, galena, marcasite, and arsenopyrite. The gold-silver zone typified by the Fortitude, 

Northeast Extension, and Tomboy-Minnie deposits is 

situated between the copper-gold-silver and lead-zincsilver zones. The Tomboy-Minnie deposits, however, are 

also associated spatially with some locally high concentrations of lead and zinc. The precious metal zone results 

from an especially high concentration of gold- and silverbearing sulfides, mostly pyrrhotite and pyrite, that are 

disseminated and had replaced the calcareous matrix and 

lenses of the lower Battle Formation. The outermost 

zone contains a significantly lower overall content of 

sulfide minerals and is dominated by galena and sphalerite typically as veins. In addition, some major northstriking, pre-metallization faults at Copper Canyon 

affected the overall distribution of metals by providing 

conduits that allowed metal-depositing fluids to move 

beyond the limits of the replacement zones of 

mineralization.

Metal zoning of the deposits on a local scale 

apparently occurs at the Tomboy deposit, where proximal high concentrations of gold are followed outwards by 

increased abundances of zinc and silver (fig. E22). In 

addition, such a zoning pattern of increased zinc and 

silver towards the northwest is a reversal of the overall 

zoning pattern that surrounds the altered granodiorite 

porphyry of Copper Canyon. This zoning pattern was 

established by prior geochemical studies of the TomboyMinnie deposits (Blake and Kretschmer, 1983) which 

showed that gold assays of samples from the first interval 

of drill holes and soil samples best outlined the deposits; 

lead and zinc concentrations increased in a zone 

peripheral to each deposit. Because the Tomboy-Minnie 

deposits may be considered to be distal contactmetasomatic gold deposits, these deposits should exhibit 

their own metal zonation (Sillitoe, 1983, p. 54). Also, it is 

possible (as we will discuss further) that these two 

precious metal deposits might be genetically related to a 

hypabyssal igneous body separate from the main 

intrusive body at Copper Canyon.

Figure E22 diagrammatically shows that gold 

concentrations greater than 0.050 oz Au/ton are 

restricted to the high sulfide-bearing rocks near the base 

of the Battle Formation. The thickest section of goldbearing conglomerate occurs under the ridge crest near 

the original Tomboy discovery and in the tectonically 

depressed fault blocks of the Battle Formation in the 

Minnie deposit. In the Tomboy deposit, this gold-bearing 

conglomerate extends down dip for about 200 m, 

following the unconformity that separates the Battle and 

Harmony Formations. At 200 m, the zone of high gold 

mineralization thins out and grades into a zone that 

shows zinc mineralization and that contains more than 

500 ppm Zn. Here the zinc mineralization formed pod-

£50 Geology and Resources of Gold in the United States
117°08'

40° 

33'

Lower and Upper 

Fortitude 

ore bodies

Independence

^Ruton c 

Copper Canyon

Au:Ag 

Ratio

500 METERS 

_J

O.02

Tomboy-Minnie 

deposits

Figure E23. Zonal distribution of metals and Au:Ag ratios in the Copper Canyon area.

like bodies and replacement lenses that slightly overlap 

the gold zone. Overall contents of sulfide minerals in the 

thin distal parts of the gold zone are less than that in the 

thick parts of the gold zone. Sphalerite also occurs in the 

Harmony Formation below the main gold zone at the 

Tomboy deposit. At the Minnie deposit, zinc mineralization was extensive in the Harmony Formation, 

where sphalerite is localized primarily in the breccia 

along northwest- and northeast-striking faults.

The outermost metal zone recognized at the 

Tomboy-Minnie deposits is represented by silver assays 

greater than 0.10 oz Ag/ton. Sphalerite and galena also

occur within this zone. In the Tomboy deposit, silver 

mineralization occurred in an 18- to 25-m-thick tabular 

body at the base of the Battle Formation. As was the case 

with the gold and zinc zones, the silver and zinc zones 

show a slight overlapping. A remnant of the silver zone is 

preserved in the Minnie deposit; all three metals 

apparently are zoned vertically in one and the same fault 

block of Battle Formation.

The three metal zones described at the TomboyMinnie deposits are characterized also by significantly 

different abundances of silver, lead, and zinc. Average 

values for each metal zone are:

Gold in Porphyry Copper Systems E51
Au zone 

Znzone 

Agzone

Ag

(oz/ton)

0.06 

0.11 

0.22

Pb 

(ppm)

40.0 

160.0 

800.0

Zn

(ppm)

155.0 

1,855.0 

2,320.0

The high lead and zinc contents of the silver zone reflect 

isolated pods and replacement lenses of sphalerite and 

galena that commonly have low pyrite and pyrrhotite 

contents.

Differences in gold:silver ratios in the district, as 

compiled from assays of drill-hole samples, ore reserves, 

and past production, also reflect metal zoning within the 

Copper Canyon porphyry copper system and its precious 

metal deposits. The average Au:Ag ratios for the metal 

zones are: copper-gold-silver zone, 0.066; gold-silver 

zone, 0.530; and lead-zinc-silver zone, 0.020 (fig. E23). 

An average Au:Ag ratio for the Copper Canyon stock is 

0.057, closely comparable to that for the copper-goldsilver zone situated immediately adjacent to the stock.

The distal Tomboy-Minnie deposits exhibit a 

zoning of metals similar to that at the proximal deposits 

of the Copper Canyon system, but the overall gold:silver 

ratio is higher. Data for the Tomboy-Minnie area were 

taken from the first assay interval from more than 200 

percussion drill holes. Both ore bodies are outlined 

clearly by the gold:silver ratio in samples from the 

percussion drill holes (fig. E24). However, Au:Ag ratios 

> 1 in assays of drill-hole samples are a better indication 

of the surface projection of ore than gold assays only of 

samples from the first interval of percussion holes, as 

described in Blake and Kretschmer (1983, fig. 5). A final 

Au:Ag ratio of >1 was determined from metalproduction data for both deposits which differs somewhat from the announced pre-production ratio determined from percussion drill holes.

These Au:Ag ratios in the Copper Canyon precious 

and base metal deposits, including the Tomboy-Minnie 

gold occurrences, compare well with those described by 

Boyle (1979). He concluded that the Au:Ag ratios for 

gold-bearing skarn deposits elsewhere are quite varied 

and can range from 0.005 to 10 (Boyle, 1979, table 43, p. 

202). The Au:Ag ratios in the several zones at Copper 

Canyon are certainly more restricted, ranging from 0.020 

to 1, but most importantly, they appear to be characteristic of metal zones recognized earlier in the district.

Metal zoning within the Tomboy-Minnie deposits 

reflects protracted deposition of gold, zinc, and silver 

primarily within a favorable stratigraphic unit. This 

sequence of metal deposition may represent a contactmetasomatic occurrence related to a nearby igneous 

body separate from the main intrusive mass of the 

system. The thickest part of the gold zone, shown in 

figure E22, generally coincides with rocks that have

undergone intense retrograde hydrothennal alteration, 

fracturing, brecciation, and faulting. Although no 

igneous rocks have been recognized in the Tomboy 

deposit, a north-striking, silicified granodiorite porphyry 

dike crops out in the east wall of the pit at the Minnie 

deposit (fig. E21). This dike contains anomalous gold. 

The overall zoning pattern and local structural preparation of the rocks in the Tomboy-Minnie deposits suggest 

that such mineralization could have been related to a 

heat source somewhere at depth near the granodiorite 

dike.

STUDIES OF FLUID INCLUSIONS AND 

STABLE ISOTOPES OF SULFUR

The results of standard fluid-inclusion and sulfur 

isotope studies of selected samples from the Tomboy 

gold deposit are included in a report by Theodore and 

others (1986). A brief summary of some of the more 

important conclusions of that study is presented here. As 

we have described, the introduction of gold occurred 

penecontemporaneously with the replacement of earlier 

diopside alteration assemblages by actinolite-tremoliteand chlorite-dominant assemblages. Temperatures 

ranged widely during these changes in silicate mineralogy, decreasing from about 500 °C during the earliest 

hydrosilicate stages to about 220 °C during the final 

stages. Preliminary fluid-inclusion studies suggest that 

CaCl2-rich boiling fluids first circulated there; these were 

apparently followed by a vapor-dominant stage. Eventually highly saline, late-stage fluids possibly were 

responsible for much of the introduction of gold. Fluidinclusion relations in the Tomboy deposit suggest that 

fluids displaying highly diverse chemistries and wideranging temperatures circulated repeatedly through the 

rocks as the porphyry system centered at Copper Canyon 

evolved. Apparently some of the earliest epigenetic fluids 

to circulate through the deposit, fluids that were trapped 

as secondary fluid inclusions in detrital fragments of 

quartz in the Battle Formation, were boiling at temperatures in the range of 400 to 500 °C. Apparently they 

contained appreciable amounts of CaCl2 : as much as 

about 25 weight percent CaCl2-equivalent has been 

determined by Theodore and others (1986) to occur in 

some liquid + vapor fluid inclusions. In fact, these fluid 

inclusions are abundant and relatively large (typically 

from 15 to 90 fjaa in largest dimension), and are the 

predominant fluid-inclusion type in many microdomains 

studied in the deposit.

The early-stage, CaCl2 -dominant fluids apparently 

were followed by a vapor-dominant, intermediate stage 

in the range 320-400 °C, wherein the circulating fluids 

show progressively increasing abundances of dissolved

E52 Geology and Resources of Gold In the United States
0 200 METERS 

1 I I I I

Figure E24. Geologic sketch map of the Tomboy-Minnie deposits showing areas 

that include Au:Ag ratios s 1 in the first assay interval in percussion drill holes 

(crosshatched pattern). These data indicate the positions of the ore bodies (dotted 

outlines). Tag, altered Tertiary granodiorite porphyry of Copper Canyon; Pb, 

Pennsylvanian Battle Formation; - h, Cambrian Harmony Formation. Light lines, 

contacts; dashed where inferred. Heavy lines, faults; dashed where inferred. Sawteeth on upper plate of thrust.

NaCl and KC1. During this stage, the fluids circulating 

through the deposit also contained minimal amounts of 

carbon dioxide, as shown by fluid inclusions that contain 

liquid carbon dioxide at room temperature. The bulk of 

the actinolite in the deposit probably was deposited 

during this stage, roughly 320-400 °C.

The fluid-inclusion studies of Theodore and others 

(1986) reinforce the suggestion that the heat source 

associated with mineralization at the Tomboy deposit

must be closer than the altered granodiorite porphyry of 

Copper Canyon that crops out almost 1 km to the 

north-northwest. An apophysis of altered granodiorite 

porphyry may occur at depth somewhere in the general 

area of the Tomboy-Minnie deposits. Further, the 

relation of fluids boiling at times in the range 400-550 °C 

at the Tomboy and fluids boiling at approximately 

350-375 °C during the ore-forming stage(s) at the East 

ore body described by Nash and Theodore (1971),

Gold in Porphyry Copper Systems E53
together with the fact that both ore bodies are approximately the same elevation and occur in the same tectonic 

block, suggests that the interface between boiling and 

nonboiling fluids during the early stages of mineralization at the Tomboy must have been at a significant 

depth below the deposit. This depth may have been as 

much as 2.2 km below the paleosurface based on 

preliminary calculations that use 450 °C as a representative temperature for boiling, mostly CaCl2 -bearing, 

aqueous solutions. Indeed, the model for porphyry 

systems such as this probably should include a 

gradational interface between boiling fluids above and 

nonboiling fluids below that drapes umbrellalike across 

the system; the interface might have deep lobate 

extensions, possibly controlled by premineral faults, near 

the gold-enriched margins of the system.

The sulfur isotopic compositions of 35 sulfide 

mineral separates from the Copper Canyon deposits, 10 

from the West ore body, 9 from the East ore body, and 16 

from the Tomboy ore body, were measured by Theodore 

and others (1986) in order to constrain the source(s) of 

sulfur in the deposits and possibly on the temperature of 

mineralization. The 634S values for all sulfide minerals 

range from +1.1 to +5.3 per mil, except for a galena 

sample from the Tomboy ore body with a very depleted 

value of-5.2 per mil. Neglecting this galena sample, the 

average 634 S values of the sulfide minerals from the East 

and Tomboy ore bodies are within 0.5 per mil of each 

other and are about 2 per mil heavier than the average 

value for the sulfide minerals from the West ore body. 

The East and Tomboy deposits are certainly at more 

distal portions of the system than the West ore body, and 

they may contain a larger component of heavy sulfur 

derived from syngenetic-diagenetic sulfur in sedimentary 

rocks and (or) pre-Tertiary evaporite or sulfate deposits. 

Pyrite enriched in 834 S from the Roberts Mountains 

Formation and bedded barite of Devonian age as heavy 

as + 56 per mil are known from central Nevada (Rye and 

Dhmoto, 1974; Rye and others, 1978). Still, the narrow 

range of 834S values for the East and Tomboy ore bodies 

suggests that the contribution of heavy, crustal sulfur was 

relatively small and highly homogenized. Dissolution and 

reprecipitation of preexisting sulfides and sulfates do not 

appear to have been an important process in the 

formation of the Copper Canyon deposits. These data 

suggest that sulfur mostly from a magmatic or deepcrustal source was carried by hydrothermal fluids as 

aqueous H2S to the West, East, and Tomboy deposits.

CONCLUSIONS

Copper skarn ores in many porphyry systems may 

contain unrecognized high gold-low copper zones that 

may have been overlooked previously because of their 

high contents of seemingly barren pyrrhotite and pyrite.

Gold mineralized rock at the Tomboy-Minnie deposits 

formed as a result of introduction of large amounts of 

iron sulfide minerals, mostly pyrrhotite and pyrite, 

together with gold during retrograde, hydrosilicatesulfide stages marginal to copper-bearing skarn. The 

introduction of gold occurred penecontemporaneously 

with the replacement of early diopside alteration 

assemblages by actinolite-tremolite- and chloritedominant assemblages. Temperatures of deposition 

ranged widely, decreasing from about 500 °C during the 

earliest phases of the hydrosilicate stages to about 220 °C 

during the final stages. Probably the gold-rich ore at the 

Tomboy-Minnie deposits formed at about the same time 

as the copper-gold-silver ores in the East ore body; an 

actinolite-tremolite assemblage also is a common 

accessory silicate assemblage in the East ore body. All 

these deposits at Copper Canyon bear many similarities 

to porphyry-related copper skarns elsewhere (Einaudi 

and others, 1981). However, the porphyry copper system 

at Copper Canyon belongs to a 'Vallrock" end-member 

of this deposit type, wherein all the ore occurs in the 

country rock surrounding a genetically associated intni" 

sive body (Titley, 1972).

The Tomboy-Minnie deposits are included by 

Orris and others (1987) with their gold-skarn class of 

deposits. Relative to 31 gold skarns worldwide, the ores 

at the Tomboy-Minnie are significantly greater than the 

median tonnage (400,000 t) and somewhat less than the 

median grade (3.54 g/t versus 5.0 g/t).

Many porphyry copper systems that include 

considerable "wallrock" components historically have 

yielded significant amounts of gold. For example, 

production of gold between 1904 and 1972 for the 

Bingham, Utah, deposits has amounted to approximately 

8 million oz (Gilmour, 1982). The grade of gold at 

Bingham up to 1972 was 0.0064 oz Au/ton in the 

disseminated copper zone (Gilmour, 1982), whereas the 

grade in some of the copper skarns there (Highland Boy 

Mine) was about 0.07 oz Au/ton (Einaudi, 1982). The 

Ely, Nev. deposit produced about 3 million oz of gold 

(D.P. Cox and D.A. Singer, unpub. data, 1984). The 

system at Copper Canyon contained a minimum of about 

3.3 million oz gold initially. However, the overall grade of 

gold at Copper Canyon was about 0.025 oz Au/ton, and 

that of the gold-silver zone deposits was about 0.1- 

0.15 oz Au/ton and locally in excess of these values 

(Wotruba and others, 1986).

Manuscript received by scientific editors May 1984

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E54 Geology and Resources of Gold in the United States
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&U.S. GOVERNMENT PRINTING OFFICE: 1990-773-047/06061 Gold In Porphyry Copper Systems E55



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