Petrographic and Scannin 

Microscope Studies of 

Roberts Mountains and P""n:~~:l'J'l :;~§~{~f~~~;~F>1~~~·~~~ 

Petrographic and Scanning Electron 

Microscope Studies of Samples from the 

Roberts Mountains and Popovich Formations, 

Carlin Mine Area, Eureka County, Nevada 

By AUGUSTUS K. ARMSTRONG, WILLIAM C. BAGBY, 

CHARLES EKBURG, and JOHN REPETSKI 

A study of transmitted-light photomicrographs, 

scanning electron microscope micrographs, and Xray diffractograms of unaltered, altered, and 

mineralized samples from the Roberts Mountains 

and Popovich Formations in the Carlin mine area, 

Nevada 

U.S. GEOLOGICAL SURVEY BULLETIN 1684 
DEPARTMENT OF THE INTERIOR 

DONALD PAUL HODEL, Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1987 

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 

Petrographic and SEM studies of samples from the Roberts 

Mountains and Popovich Formations, Carlin mine area, 

Eureka County, Nevada. 

U.S. Geological Survey Bulletin 1684 

Supt. of Docs. No.: 119.3:1684 

1. Petrology-Nevada-Carlin mine region. 2. 

Photomicrography. 3. Gold ores-Nevada-Carlin mine 

region. 4. Roberts Mountains Formation (Nev.) 5. 

Popovich Formation (Nev.) I. Armstrong, Augustus K. II. 

Series. 

QE75.B9 No. 1684 557.3 s 86-600202 

[QE445.N3] [553.4'1'0979332] 
CONTENTS 

Abstract 1 

Introduction 1 

Acknowledgments 3 

Results 3 

Roberts Mountains Formation: unaltered district samples 3 

Roberts Mountains Formation: altered Carlin mine samples 3 

Type I 3 

Type II 6 

Type III 6 

Another classification 7 

Conodont analysis 8 

Popovich Formation: Carlin mine samples 8 

Summary and discussion 8 

References cited 10 

FIGURES 

1. Index map showing location of the Carlin mine, north-central Nevada 2 

2. Contour map and photograph from Carlin mine, showing some sample 

locations 4 

3-7. X-ray diffractograms of: 

3. Unaltered samples of the Roberts Mountains Formation, Carlin mine 

area 6 

4. Type-1 altered unoxidized samples of the Roberts Mountains 

Formation from the Carlin mine 6 

5. Type-11 altered, oxidized samples of the Roberts Mountains 

Formation from the Carlin mine 7 

6. Type-III altered samples of jasperoids from the Carlin mine 7 

7. Silty limestone of the Popovich Formation from the Carlin mine 8 

8. Schematic characterization of types of alteration in the Carlin mine area 9 

9-16. SEM micrographs and transmitted-light photomicrographs of: 

9. Unaltered rocks from the Roberts Mountains Formation southwest of 

the Carlin mine 12 

10. Altered rock type I from the Roberts Mountains Formation, Carlin 

mine 13 

11. Altered rock type I from the Roberts Mountains Formation, Carlin 

mine 14 

12. Altered rock types I and II from the Roberts Mountains Formation, 

Carlin mine 15 

13. Altered rock type II from the Roberts Mountains Formation, Carlin 

mine 16 

14. Altered rock types I, II, and Ill from the Roberts Mountains 

Formation, Carlin mine 17 

15. Altered rocks from the Popovich Formation, Carlin mine 18 

16. Altered rocks from the Popovich Formation, Carlin mine 19 

17. Colored transmitted-light photomicrographs of altered rock types I 

and II from the Roberts Mountains Formation, Carlin mine 21 

18. Colored transmitted-light photomicrographs of altered rocks from the 

Roberts Mountains and Popovich Formations, Carlin mine 23 

m 

Petrographic and Scanning Electron 

Microscope Studies of Samples from the 

Roberts Mountains and Popovich Formations, 

Carlin Mine Area, Eureka County, Nevada 

By Augustus K. Armstrong, William C. Bagby, Charles Ekburg1

, 

and john Repetski 

ABSTRACT 

The Carlin sediment-hosted, disseminated gold 

deposit in northern Nevada has been a major gold 

producer in the United States for more than 2.0 years. 

The major host formation for gold is the upper part of 

the Roberts Mountains Formation. Minor gold also 

occurs in the overlying Popovich Formation and 

unnamed siliceous {western) assemblage rocks. Our 

studies of samples of these rocks from the Carlin mine 

area included transmitted-light microscopy, scanning 

electron microscopy, and energy-dispersive X-ray 

analysis. The megascopically unaltered Roberts 

Mountains Formation within the Carlin district 

consists of arenaceous, argillaceous limestone and 

dolomitic limestone with fossil fragments of 

echinoderms, brachiopods, and ostracodes. 

Reconnaissance samples from the Roberts Mountains 

Formation in the Carlin mine are categorized into 

three types on the basis of microscopic hydrothermal 

alteration textures: CO dark-gray, organically rich 

arenaceous dolomite containing pyrite and illitic 

material; (Il) oxidized, light-yellow to orange and tan 

arenaceous rock composed of goethite-stained illitic 

material, quartz, and minor calcite; and {Ill) oxidized, 

reddish-brown jasperoid. 

INTRODUCTION 

The Carlin ore body is a sediment-hosted, 

disseminated gold deposit located in northern Nevada, 

northwest of Carlin (fig. 1). It was the first of seven 

known major gold deposits of this type to be 

discovered in the Lynn-Railroad trend, now commonly 

referred to as the Carlin trend, which was originally 

1carlin Gold Mining Company, Carlin, Nevada. 

noted by Roberts (1966). This gold trend is defined by 

a series of sediment-hosted, disseminated gold deposits 

aligned in a northwesterly direction. 

Host formations in the Carlin trend consist of 

the Roberts Mountains Formation, Popovich 

Formation, Vinini Formation, and Webb Formation; 

minor amounts of gold are also present in porphyritic 

dikes and granodioritic plutons. Although the gold 

occurs in several different formations, actual host 

lithologies within the formations are similar. They are 

characterized as either silty, argillaceous carbonate 

rocks or calcareous siltstones. 

The Roberts Mountains Formation is part of the 

early Paleozoic autochthonous eastern carbonate 

assemblage of Roberts and others (1958). The Roberts 

Mountains Formation ranges in age from Silurian to 

Devonian, whereas the overlying Popovich Formation 

is Devonian. The Roberts Mountains Formation was 

studied regionally by Mullens (1980) to identify its 

stratigraphic and petrologic characteristics and, in 

particular, how these features may relate to ore 

deposition. Recently, however, it became apparent 

that a modern petrographic study of specimens from 

this formation from the Carlin mine and neighboring 

area might disclose additional information about 

favorability of these rocks as host rocks for ore. We 

used scanning electron microscopy coupled with 

energy-dispersive X-ray analysis and transmitted-light 

microscopy to examine the mineralogical and textural 

characteristics resulting from diagenesis, 

metamorphism, and hydrothermal alteration associated 

with gold deposition in these rocks. The Carlin openpit mine was used for these investigations because of 

the deep exposures available and the substantial data 

base that already existed for the deposit (Hardie, 1966; 

Hausen, 1967; Evans, 1974; Radtke, 1985). 

Sampling in July 1982 of host rocks in and 

outside of the Carlin pit was not based on detailed 

alteration mapping, nor were the samples collected for 

the purpose of systematic characterization of 

alteration. Instead, sampling was directed toward 

providing a suite of rocks for microscopic examination 

1 
2 

116°30' 

Dee mine~ 

ELKO COUNTY 

--,-------- ~~~ 

~,1 ~ 0 0 

u u 

=: < 

z =: ~,1 ~ 

:5 ~ I 

I 

I 

I 

I 

I 

I 

I 

I 

I ~~ 1 ou"Q ,~~ 

I 

0 

0 

Gold Strike mine 0 

"Y. 

Blue Star mine X ~ ~ 

Carlin mine-....... ;z. 

Unaltered samples-x en 

2 

~ 2 

:::> 

0 

~ 

<(' 

cr 

0 

cr 

<(' 

u 

en 

:::> 

,_ Maggie Creek mineX~Gold Quarry 

mine 

5 10 MILES 

5 10 1 5 KILOMETERS 

Figure 1. Index map showing location of the Carlin mine in north-central Nevada. 

AREA OF MAP 
of textures. Pit sampling (fig. 2) of oxidized and 

unoxidized rocks was directed toward collecting 

representative samples from both ore and non-ore 

zones (identified by blast hole assay flags). Southwest 

of the Carlin pit, we collected hydrothermally 

unaltered, weathered rocks that are representative of 

host formation textures before hydrothermal 

alteration. 

Several samples from the Roberts Mountains and 

Popovich Formations were taken for conodont 

analysis. These samples were from both the main body 

of the Carlin ore zones and from a locality 4 km south 

of the mine; they were meant to (1) confirm the dates 

of the respective formations and (2) yield, by the 

conodont color alteration index (CAl), minimum 

postdepositional host-rock temperature values as an 

additional constraint on reconstructing the thermal 

history of these rocks. 

Carbonate rocks were stained for dolomite, 

calcite, and iron-rich carbonate minerals using the 

methods described by Friedman (1959) and were 

classified according to Dunham (1962). Scanning 

electron microscope (SEM) and energy-dispersive Xray analyzer (EDX) systems were those described by 

Welton (1984). X-ray diffractograms were made at the 

following settings: 35 KV, 15 MA; chart speed, 1 

in/min; scan speed, 2° 8; tungsten tube. 

This study of alteration textures and mineralogy 

at the Carlin mine raises certain points that must be 

addressed in genetic modeling of the deposit. This is 

by no means an exhaustive study and is not meant to 

be. It is, however, a presentation of textural 

characteristics of altered rock at the Carlin deposits. 

Acknowledgments.--We thank Robert Oscarson 

for his patient help with the scanning electron 

microscope. We thank Paul Russell for running the Xray diffractograms. We thank R.T. Lierman for 

processing the conodont samples. 

RESULTS 

Roberts Mountains Formation: unaltered district 

samples 

Samples of the megascopically unaltered Roberts 

Mountains Formation were collected about 4 km 

southwest (40°52'30" N., 116°20'00" W.) of the Carlin 

mine. Peloidal wackestone samples are composed of 

10- to 15- J..liD calcite crystals, 30- to 70- J..lffi grains of 

detrital quartz, and minor amounts of detrital 

feldspar. These samples contain various amounts of 5-

to 50-J..lm clear, iron-free dolomite rhombs and 10- to 

50-J..lm crystal agregates of euhedral pyrite. Fossil 

brachiopod and echinoderm fragments have been 

replaced by chalcedony, which is a common diagenetic 

feature in sedimentary rocks. Dolomite samples are 

composed of subhedral, 25- to 50-J..lm rhombs of 

dolomite crystals and minor 50- to 100-J..lm detrital 

quartz grains. Rare detrital monazite and framboidal 

pyrite 1 to 3 J..lm in diameter are also present in these 

samples (fig. 9). 

The relatively large calcite crystals, 10 to 15 J..lm 

in diameter, in the wackestones and lime mudstones 

indicate thermal metamorphism. Three of these 

thermally metamorphosed samples, 82E-R12 (USGS 

fossil locality 11023-SD), 82E-R15 (11024-SD), and 

82E-R17 (11025-SD) were dissolved to extract 

conodonts to determine the depositional age of the 

sample and the minimum temperature to which they 

were exposed during their subsequent history. 

Conodonts from specimen 82E-R17 are indicative of a 

Middle or early Late Silurian (Wenlockian or early 

Ludlovian) for the depositional age of the sample. 

Conodonts from each of these samples have color 

alteration indices (CAl) of about 5.5. A CAl in this 

range indicates that the enclosing strata were once 

heated to about 350 to 450 °C, depending on total time 

of 10,000 to 100,000,000 years at the elevated 

temperature, according to the CAl time-temperature 

correlation chart of Epstein and others (1977). X-ray 

diffractograms of unaltered samples 82E-R17 and 82ER18 indicate that calcite and quartz were the 

dominant minerals. lllitic material2 is noticeably 

absent (fig. 3). 

Roberts Mountains Formation: altered Carlin mine 

samples 

Samples from the Roberts Mountains Formation 

in the Carlin mine may be divided into the following 

categories: type I is dark-gray, organically rich 

arenaceous dolomite containing pyrite and illitic 

material; type II is oxidized, light-yellow to orange and 

tan arenaceous rock composed of goethite-stained 

illitic material, quartz, and minor calcite; and type III 

is oxidized, reddish-brown jasperoid. 

In hand specimens, type-1 rocks are medium-dark 

to dark gray and finely laminated; they contain mud 

chips, worm burrows, and weakly developed cross 

laminations. These very subtle sedimentary structures 

are well preserved. Framboidal pyrite is preserved and 

common. An X-ray diffractogram of a specimen of 

this rock type (82E-1 +27) shows the rock to be 

composed principally of quartz, dolomite, calcite, and 

illitic material (fig. 4A). The X-ray diffractogram for 

sample 82E-1 +35 indicates that quartz, calcite, and 

illitic material are the dominant minerals (fig. 4B). 

Type-1 rocks typically contain clear dolomite 

rhombs (30-70 J..lm) in which zoning is defined by ironrich and iron-poor layers. These zoned dolomite 

rhombs are commonly corroded (fig. 12F) and occur in 

a matrix of 5- to 10-J..lm iron-rich dolomfte rhombs with 

detrital quartz grains, illitic material, and abundant 

pyrite (figs. 10; 11; 12A-C; 17 A-E). The pyrite in 

these samples generally occurs as fine spheroidal 

2The term "illitic material" is used in this text as a 

general petrologic term to refer to clay-sized 

micaceous minerals with a 10-A component (see 

Srodon and Eberl, 1984). No oriented X-ray 

diffractograms were made to positively identify the 

clay fraction. 

3 
18,500 

19,000 

19,500 

A 

\ 

I 

I 

I 

0 1000 2000 

I I I 

0 500 

3000 4000 

I I 

1000 

EXPLANATION 

____ Outer edge of bench 

------Toe of slope of bench 

6620 

82E-R40 

Elevation of bench surface in 

feet above sea level 

Sample location and number 

0 ..:.8.::2.::E:....-R....:..::.2~ Direction of sample traverse 

and sample number 

5000 FEET 

I' 

1500 METERS 

Figure 2. Sample locations within the Carlin mine pit. A, Contour map of the Carlin pit from Carlin Mine 

Company, late July 1982. Grid established by Carlin Mine Company. B, North wall of Carlin pit showing geology 

and some sample locations, 1982. -

4 
8 

Dike (Cretaceous) 

Popovich Formation (Devonian) 

I DSrm I Roberts Mountains Formation 

(Devonian and Silurian) 

Figure 2.--Continued 

EXPLANATION 

~ Limestone (Silurian and Ordovician) 

Chert and shale (Silurian and 

Ordovician) 

Carbonaceous limestone and 

shale • 82E-R1 

Contact 

Fault zone -Arrows indicate 

direction of relative movement 

Roberts Mountains thrust fault 

Sample location and number 

5 
aggregates 4-10 um long composed of tiny (1-um) 

euhedral cubes of pyrite (figs. 12C, 14b E). 

Carbonaceous material occurs along bedding planes. 

Vugs and cavities in some samples are lined by 

euhedral quartz crystals as long as 3 mm. The 

surfaces of these quartz crystals are coated by late 

sphalerite and arsenopyrite; in some cases these 

sulfides occur as inclusions in the quartz (figs. 11, 12~ 

m. Although small fractures and cavities are filled by 

spar calcite, these samples have much greater porosity 

and permeability than do the fresh, unaltered samples 

of the Roberts Mountains Formation collected 4 km 

southwest of the mine. 

Type II 

Typically, type-II rocks contain a pale yellowishbrown clay and are quartz rich. Notably, samples of 

this type are composed of abundant illitic material, 

detrital quartz grains, and limonite (a combination of 

goethite, hematite, and jarosite as a stain). Fractures 

and cavities commonly are filled by late crystalline 

>- 80 len 

z 

~ 60 

~ 

w 

> i= 

<( 

_J 

w 

a: 20 

>- 80 len 

z 

~ 60 

~ 

w 

~ 40 

<( 

_J 

w 

a: 20 

cc 

36 32 

36 32 

28 24 20 16 12 8 

DEGREES 29 

A 

28 24 20 16 12 8 

DEGREES 29 

8 

Figure 3. X-ray diffractograms for unaltered samples 

of the Roberts Mountains Formation from south of 

Carlin mine. Q, quartz; cc, calcite. ~ 82E-R17. 

Wackestone thermally metamorphosed to temperatures 

between 350 and 450 °C. !!, 82E-R18. Arenaceous 

peloidal fossiliferous wackestone. 

6 

quartz or calcite (fig. 17F). X-ray diffraction analysis 

indicates that rocks of this type are composed of illitic 

material, quartz, and late calcite (fig. 5). 

Subtle sedimentary structures are preserved in 

the type-11 samples. These structures include 

millimeter-scale cross bedding, fine laminations, mud 

clasts, rip-up clasts, and small worm burrows. Some 

samples have abundant ostracode casts that are 

preserved by replacement silica. Laminations of darkgray, carbonaceous, iron-rich dolomite-bearing bands 

alternate with oxidized, yellowish-brown bands in some 

type-11 samples. 

SEM micrographs show that type-11 rocks are 

highly porous (fig. 13). The pores are partly filled by 

clay and subhedral quartz crystals. Detrital grains 

include quartz and zircon (fig. 12_w. 

Type III 

The third type of sample from the Roberts 

Mountains Formation collected from within the mine 

100 

>- 80 

Ien 

z 

w 60 I-

~ 

w 

> 40 i= 

<( 

_J 

w 

a: 20 

36 32 28 24 20 16 12 8 

DEGREES 29 

A 

Occ Q 

100 

>- 80 Ien 

z 

w 60 I-

~ 

w 

> 40 i= 

<( 

_J 

w 

a: 20 

24 20 16 12 8 

DEGREES 29 

8 

Figure 4. X-ray diffractograms for type-I altered, 

unoxidized samples of the Roberts Mountains 

Formation from the Carlin mine. Q, quartz; I, illitic 

material; cc, calcite; D, dolomite; M, muscovite. ~ 

82E-1 +27. Silty limestone. !!, 82E-1 +35. Wackestone. 
are silicified rocks (jasperoids). Fresh surfaces of the 

jasperoids are medium-dark-gray, whereas weathered 

surfaces range from very pale orange to dark yellowish 

orange to light brown. Most jasperoids are highly 

fractured, and many of the fractures and cavities are 

coated by iron oxides. The jasperoids typically contain 

euhedral quartz crystals that are coated by fine grains 

of iron oxide (fig. 14_b _m. This silicic alteration is a 

metasomatic replacement of silty, fossiliferous 

dolomite, as indicated by pseudomorphs and relict 

textures of fossils preserved from the original 

carbonate rocks of the Roberts Mountains Formation. 

Very delicate fossil shells are preserved either as silica 

or as carbonate casts or molds of ostracodes, 

brachiopods, and echinoderms. Representative X-ray 

diffractograms are shown in figure 6. 

Another classification 

Samples of rock of these three types may also be 

characterized in terms of Radtke's (1985) 

classification of alteration and ore at the Carlin 

>- 80 t-

(j) 

z 

~ 60 

~ 

w 

~ 40 

< .....1 

w 

a: 20 

>- 80 t-

(j) 

z 

w 

t-

~ 

w 

~ 40 

< .....1 

w 

a: 20 

Q 

36 32 

36 32 

Q 

28 24 20 

DEGREES 29 

A 

Q Q 

28 24 20 

DEGREES 29 

8 

16 12 8 

16 12 8 

Figure 5. X-ray diffractograms for type-II altered, 

oxidized samples of the Roberts Mountains Formation 

from the Carlin mine. Q, quartz, I, illitic material; cc, 

calcite; D, dolomite. _b 82E-2+2. Quartz, illitic 

material. !!, 82E-l+22. Quartz, calcite. 

mine. Radtke identified two types of ore: oxidized 

and unoxidized. The unoxidized ore was further broken 

into five types "on the basis of differences in mineral 

content, chemical composition, and associations of the 

gold" (Radtke, 1985, p. 49). Samples from type I fall 

into Radtke's normal unoxidized ore. Although we 

have no gold assays on the samples analyzed by SEM, 

the alteration textures identifed suggest that the rocks 

we studied were affected by epigenetic hydrothermal 

solutions. For this reason, we feel these samples are 

examples of Radtke's (1985) normal unoxidized ore and 

that they represent variations in alteration textures 

that are typical of this type of ore. Samples from both 

type II and type III fall into Radtke's oxidized ore 

classification. The intense bleaching and the mineral 

composition of the type-II samples suggest that they 

are examples of Radtke's acid-leached rocks from the 

oxidized zone (1985, p. 95). The oxidation of jasperoid 

samples in type III is limited to fractures and appears 

to be late stage. This suggests that type-III samples 

represent oxidized versions of Radtke's siliceous, 

unoxidized ore (1985, p. 54). 

>- 80 t-

(j) 

z 

~ 60 

~ 

w 

~ 40 

< .....1 

w 

a: 20 

>- tii5 

80 

~ 60 t-

~ 

w 

~ 40 

< .....1 

w 

a: 20 

36 32 

36 32 

Q 

28 24 20 16 12 8 

DEGREES 29 

A 

28 24 20 16 12 8 

DEGREES 29 

8 

Figure 6. X-ray diffractograms for type-III jasperoid 

samples from the Carlin mine. Q, quartz; I, illitic 

material; cc, calcite; D, dolomite. ~ 82E-1 +20C. !!, 

82E-1+6A. 

1 
Conodont analysis 

Eight samples of the Roberts Mountains 

Formation from the bottom of the Carlin mine pit 

were checked for conodonts. Of these samples, only 

three contained conodonts (USGS fossil localities 

11026-SD, 1102 7-SD, and 1102 8-SD; from map 

localities 82E-1+26, 82E-1+42, and 82E-2+15, 

respectively), all of which indicate a Late Silurian 

age. The conodont samples show a variable degree of 

thermal alteration in this part of the mine. Two of the 

samples (11026-SD and 11027-SD) show elements 

having CAl of 4, indicating that the host-rock was 

heated to about 150 to 250 °C. The third sample 

(11028-SD) yielded conodont elements exhibiting a 

range of CAl values of 5 to 7. Mixed CAl values in a 

single collection commonly are found in mineralized 

zones. Sample 11026-SD is bleached and oxidized and 

sample 11027-SD is unoxidized, yet conodonts in both 

yield CAl values of 4. This suggests that bleaching of 

organic matter is not responsible for the mixed CAl 

values in sample 11028-SD which is partially 

bleached. An explanation for the mixed values is that 

fluids affecting the host rock of sample 11028-SD 

could have varied between 300 and 500 °C at different 

times and for different durations resulting in mixed 

CAl values between 5 and 7. It is unlikely that fluids 

in this temperature range were associated with gold 

mineralization. 

Popovich Formation: Carlin mine samples 

Samples from the Popovich Formation (Hardie, 

1966) were collected from above its contact with the 

Roberts Mountains Formation along a traverse up the 

north wall (Popovich Hill) of the Carlin pit (fig. 2). 

Samples from near the contact show different degrees 

of hydrothermal alteration (fig. 18~ _!b Q). These 

samples are typically dark gray and carbonaceous, and 

they contain abundant pyrite and detrital quartz 

grains. Some samples contain iron-rich dolomite 

rhombs. Vugs and fractures are partly filled with spar 

calcite (fig. 18.!2_, ~). Scanning electron micrographs 

reveal a matrix of calcite between the dolomite 

rhombs (fig. 15C). 

Samples from about 2 m above the contact with 

the Roberts_ Mountains Formation are carbonaceous, 

peloidal, arenaceous wackestone with ostracode, 

brachiopod, and echinoderm fragments. These rocks in 

turn are overlain by nonferroan dolomites that contain 

relict structures of brachiopod and ostracode shells 

(fig. 18F). 

Samples of the Popovich Formation from the 

north wall of the mine are arenaceous calcitic 

dolomites and argillaceous dolomites (fig. 7). These 

rocks contain abundant pyrite as replacement of fossil 

fragments, including ostracodes, echinoderms, and 

tentaculites (fig. 18E). The rocks are carbonaceous, 

and fractures and voids are filled by calcite. Detrital 

quartz grains 40 to 70 flm in size are common. Barite 

is also a common mineral that occurs as interstitial 

grains between calcite crystals and is probably 

epigenetic (fig. 16£.). The majority of samples from 

8 

the Popovich Formation have undergone much less 

alteration than those from the Roberts Mountains 

Formation. 

Four samples from the Popovich Formation along 

the north-wall traverse in the Carlin mine pit were 

checked for conodonts. Only one sample (USGS 10816-

SD) from near the bottom of the pit at the base of the 

Popovich Formation yielded conodonts most likely of 

Devonian age. The CAl of conodonts from this sample 

is between 5 and 6, indicating that the host rock was 

heated to 300 to 350 °C, assuming a duration of 

heating of 10,000 years to 100 m.y. (CAl timetemperature chart, Epstein and others, 1977). 

SUMMARY AND DISCUSSION 

The Roberts Mountains Formation, 4 km southwest of the Carlin mine, consists of fossiliferous, 

arenaceous lime mudstones and wackestones that have 

been subjected to a thermal event in the 350- to 

450 °C range during some part of their history. This 

thermal event caused the micritic calcite crystals to 

grow to about 15 l.lm in diameter. These rocks are 

composed of nonferroan dolomite and calcite and have 

very low porosity and permeability. There is no 

indication that the rocks have been subjected to the 

same hydrothermal fluids as those that altered similar 

rocks in the Carlin mine pit. 

Carbonate rocks of the Roberts Mountains 

Formation in the Carlin mine pit show the effects of 

epigenetic hydrothermal alteration. This alteration is 

exemplified by porosity greater than that of the 

unaltered samples collected southwest of the mine. 

The Carlin mine rocks are organically rich, unoxidized 

dolomites composed of 30- to 100-l.lm rhombs of zoned 

ferroan dolomite, microcrystalline quartz, illitic 

material, and minor amounts of iron, zinc, and arsenic 

sulfides. In the mine pit, some of the carbonate rocks 

of the Roberts Mountains Formation have undergone 

an oxidizing event that removed most of the carbonate 

100 

Q, CC, I 

>- 80 

Iu; 

z 

w 60 I-

~ 

w 

> 

t= 

<{ 

...J 

w 

a: 20 

DEGREES 28 

Figure 7. X-ray diffractogram for silty limestone 

sample of the Popovich Formation from the Carlin 

mine, 82E-23B. Q, quartz; I, illitic material; cc, 

calcite; D, dolomite. 
minerals and organic material from the rock and that 

oxidized the sulfide minerals to limonite. The oxidized 

rocks are extremely porous and are composed of silica, 

illitic material, and a late-stage, void-filling spar 

calcite. The jasperoid stage of alteration represents 

silica replacement and filling of all of the void spaces, 

producing a very nonporous rock. A schematic 

summary of the types of alteration is given in figure 8. 

The Popovich Formation is also an arenaceous 

limestone, but it is not as strongly altered as the 

carbonate rocks of the Roberts Mountains Formation 

in the Carlin mine. 

The different alteration effects between the 

Roberts Mountains Formation and the Popovich 

UNALTERED 

Fossiliferous, dolomitic, 

arenaceous, argillaceous 

limestone 

Partially silicified organic 

material between carbonate 

crystals and detrital sand 

and clay minerals 

:z- to 6-J,Lm calcite crystals; 

1 5-J,Lm calcite crystals if 

heated to 300-350 °C; 

low-Mg calcite, some Fe-free 

dolomite 

Very low porosity and 

permeability 

Fe-poor carbonate rocks 

Silicification of some fossil 

fragments by diagenetic 

chert 

Sedimentary pyrite 

Sedimentary clay minerals 

Possible detrital sedimentary 

sulfides 

Type I 

Organically rich arenaceous 

Fa-dolomite having pyrite and 

some illitic material 

Enriched organic material 

between dolomite rhombs, 

pyrite, and clay minerals 

2- to 1 0-J,Lm ferroan dolomite 

rhombs; 30- to 1 00-J,Lm, 

clear, ferroan-zoned euhedral 

dolomite rhombs. Introduced 

pyrite and black organic 

material 

High porosity and permeability 

Introduced metasomatic Fe; 

ferroan-rich dolomites 

Hydrothermal silicification of 

matrix; quartz crystal growth 

into cavities and voids 

Hydrothermal pyrite 

introduced into matrix 

Introduction of illitic material 

Introduction of Fe-, As-, and 

Zn-sulfide minerals 

Formation apparently reflect different original 

lithologies of the two formations. The Roberts 

Mountains Formation is a thin- to medium-bedded, 

arenaceous, slightly argillaceous, bioclastic peloidal 

limestone and dolomite that has a possible inherent 

porosity and permeability. Thin 1- to 5-cm partings of 

shale and silty argillite are present between the 

carbonate beds. The Popovich Formation, in contrast, 

is considerably more massive and contains arenaceous 

micrites and pelletoidal lime-mudstones and 

wackestones that are impermeable. 

Certain key characteristics of alteration are 

noted in this study. (1) Barite is present with sulfide 

minerals in unoxidized, weakly altered rock from the 

ALTERED 

Type II 

Oxidized arenaceous illitic 

rock containing quartz and 

minor calcite; no dolomite 

No organic material; bleached 

illitic and siliceous rock, 

low in carbonates; very 

vuggy and porous 

lllitic material, quartz-filled 

voids, late-stage spar calcite; 

no pyrite 

High porosity and permeability 

Introduction of Fe-poor spar 

calcite into fractures and 

cavities 

Silicification by small quartz 

crystals into voids and 

space between crystals of 

illitic material 

Pyrite oxidized to limonite and 

hematite 

lllitic material predominates 

over microcrystalline quartz 

Oxidation of Fe-, As-, and 

Zn-sulfide minerals 

Type Ill 

Oxidized jasperoid 

No organic material; all 

jasperoid (Si0 2 n•H 20); no 

porosity 

Original rock totally replaced 

by silica 

Very low porosity and 

permeability 

Introduction of spar calcite 

into fractures and voids 

Replacement of carbonate 

minerals by silica, resulting in 

total silicification of rock 

Oxidized to limonite and 

hematite 

lllitic material in part replaced 

by microcrystalline quartz 

Figure 8. Schematic characterization of alteration of limestone from the Roberts Mountains Formation in the 

Carlin mine area. 

9 
Popovich Formation. This mineral assemblage of 

coexisting pyrite and barite helps constrain the oxygen 

fugacity during hydrothermal alteration. (2) The 

presence of illitic material in altered rocks, and its 

absence in unaltered rocks, helps to constrain the pH 

of fluids during hydrothermal alteration. (3) Ferroan 

dolomite formed during hydrothermal alteration. The 

precipitation and stability of this mineral during 

hydrothermal alteration certainly reflects the iron, 

magnesium, and carbonate content of the 

hydrothermal fluids. (4) Framboidal and euhedral 

pyrite occur in both altered and unaltered unoxidized 

samples. This bimodality of the pyrite population and 

the difficulty of identifying pyrite that is genetically 

associated with hydrothermal alteration from other 

pyrite must be considered in sulfur isotope studies of 

deposits of this type. Additional detailed 

documentation and modeling of this alteration should 

lead to an understanding of the fluid chemistry of gold 

deposition at Carlin. 

The thermal alteration recorded by conodonts 

appears not to have been a regional event. Thermally 

metamorphosed unaltered rocks 4 km south of the 

mine reached the same or even higher temperatures as 

some of the hydrothermally altered samples in the 

mine. However, samples from unnamed Ordovician 

siliceous (western) assemblage rocks correlative with 

the Vinini Formation collected less than 3 km due east 

of the Carlin mine (USGS collections 8826-CO, 8827-

CO, 8828-CO) contain conodonts having CAl values as 

low as 2 to 3, which indicate minimum host-rock 

heating in this area was between 60 and 100 °C. 

The thermal metamorphism and resultant growth 

of large calcite crystals raise the question of how 

much the original rock permeability influenced the 

hydrothermal fluid flow. It is reasonable to assume 

that rocks at the Carlin mine contained large matrix 

crystals of calcite prior to initiation of the 

gold-depositing hydrothermal system. An alteration 

feature noted in the unoxidized but altered rocks (type 

I) from the mine is a porosity greater than that of the 

unaltered rocks. This porosity is a result of, and not 

an antecedent to, hydrothermal alteration. A review 

of the literature on sediment-hosted precious-metal 

deposits (Bagby and Berger, 1985) suggests that sandy 

and silty, finely laminated limestones are more 

favorable gold hosts than massive limestones are. The 

fact that more gold is deposited in laminated 

limestones of the Roberts Mountains Formation than in 

massive limestones of the Popovich Formation 

supports this contention. The growth of calcite during 

thermal metamorphism essentially turns sandy, silty 

limestones into massive, impermeable rocks. Calcitedissolving hydrothermal solutions create permeability, 

and the presence of sand and silt particles in the 

limestone must maintain that porosity. Thus, the fluid 

flow rate, at first slow, is increased and brings more 

gold into the rocks. 

10 

REFERENCES CITED 

Bagby, W.C., and Berger, B.R., 1985, Geologic 

characteristics of sediment-hosted, disseminated 

precious-metal deposits in the western United 

States, .!.!!_, Berger, B.R., and Bethke, P.M., eds., 

Geology and geochemistry of epithermal 

systems: Society of Economic Geologists 

Reviews in Economic Geology, v. 2, p. 169-202. 

Dunham, R.J ., 196 2, Classification of carbonate rock 

according to depositional texture, in Ham, W.E., 

ed., Classification of carbonate rocks--a 

symposium: American Association of Petroleum 

Geologists Memoir 1, p. 108-121. 

Epstein, A.G., Epstein, J.B., and Harris, L.D., 1977, 

Conodont color alteration--an index to organic 

metamorphism: U.S. Geological Survey 

Professional Paper 995, 27 p. 

Evans, J.G., 1974, Geologic map of the Rodeo Creek 

NE quadrangle, Eureka County, Nevada: U.S. 

Geological Survey Geologic Quadrangle Map GQ1116, scale 1:24,000. 

Friedman, G.M., 1959, Identification of carbonate 

minerals by staining methods: Journal of 

Sedimentary Petrology, v. 29, p. 87-97. 

Hardie, B.S., 1966, Carlin gold mine, Lynn district, 

Nevada, in Papers presented at the AIME Pacific 

Southwest Mineral Industry Conference, Sparks, 

Nevada, May 5-7, 1965: Nevada Bureau of Mines 

Report 13, pt. A, p. 73-83. 

Hausen, D.M., 1967, Fine gold occurrence at Carlin, 

Nevada: New York, N.Y., Columbia University, 

Ph.D. dissertation, 166 p. 

Mullens, T.E., 1980, Stratigraphy, petrology, and some 

fossil data of the Roberts Mountains Formation, 

north-central Nevada: U.S. Geological Survey 

Professional Paper 1063, 67 p. 

Radtke, A.S., 1985, Geology of the Carlin gold deposit, 

Nevada: U.S. Geological Survey Professional 

Paper 1267, 124 p., 50 figs., 7 pls. 

Roberts, R.J ., 1966, Metallogenic provinces and 

mineral belts in Nevada, in Papers presented at 

the AIME Pacific Southwest Mineral Industry 

Conference, Sparks, Nevada, May 5-7, 1965: 

Nevada Bureau of Mines and Geology Report 13, 

pt. A, p. 47-72. 

Roberts, R.J., Hotz, P.E., Gilluly, James, and 

Ferguson, H.G., 1958, Paleozoic rocks of northcentral Nevada: American Association of 

Petroleum Geologists Bulletin, v. 4 2, no. 12, p. 

2813-2857. 

Srodon, Jan, and Eberl, D. D., 1984, Illite, in Bailey, 

S. W ., ed., Micas: Mineralogical Society of 

America, Reviews in Mineralogy, v. 13, p. 495-

544. 

Welton, J.E., 1984, SEM petrology atlas: American 

Association of Petroleum Geologists, Methods in 

Exploration Series, no. 4, 237 p. 
FIGURES 9-18 
Figure 9. Specimens of unaltered rocks from the Roberts Mountains Formation, from an outcrop 4 km southwest 

of the Carlin mine in the SEl/4, sec. 27, R. 50 E., T. 35 N. ~ Transmitted-light photomicrograph; ~-f, SEM 

micrographs • .b 82E-R16. Arenaceous peloidal lime mudstone that contains 5- to 20-JJm calcite rhombs and some 

clear iron-poor dolomite; brachiopod shell in center of photograph is replaced by calcite. ~ 82E-R13. 

Arenaceous peloidal wackestone. Dark areas are calcite; large white areas are quartz and clay • ..Q, 82E-R18. 

Arenaceous peloidal fossiliferous wackestone. Dark areas are calcite; large light areas are silica. Small amounts 

of clay between calcite crystals. D-b 82E-R16. Q, Small crystal of pyrite and rutile on a calcite surface. b 

Small pyrite framboids on calcite surrounded by white quartz. f, Small grain of a tungsten mineral embedded in 

quartz. 

12 
Figure 10. Specimens of altered rock type I from the Roberts Mountains Formation, Carlin mine, SEM 

micrographs. A. 82E-1 + 19. Nearly euhedral detrital zircon crystal in a matrix of porous authigenic clay 

minerals. Rockshows some evidence of oxidation. B. C, 82E-l +20. B, Illitic material, dolomite, and quartz; note 

size of crystals and porosity. C. Quar·tz, illitic material, and dolomite; note intracrystalline porosity. D. 82E1 +20. Dark-colored calcite surrounded by small crystals of quartz, illitic material, and minor amounts of 

dolomite. E-F. 82E -1 +20. E. Crystallizea epigenetic illitic material filling pores for mea in a dolomite matrix. 

Sample is dark gray, organically rich, and composed of zoned, iron-rich dolomite rhombs, angular detrital quartz 

fragments, and pyrite cubes. F. Crystalline illitic material. Sample is aark gray, organically rich. and composed 

of zoned, iron-rich dolomite rhombs, detrital quartz grains, and pyrite cubes. 

13 
Figure 11. Specimens of altered rock type I from the Roberts Mountains Formation, Carlin mine; SEM 

micrographs of sample 82E-1+31. Clear, 10- to 5D-J.Im subhedral dolomite rhombs in a matrix of iron-rich dolomite 

and detrital quartz. Rock, from an ore zone, is dark gray and organically rich and contains numerous small vugs 

lined with quartz crystals. Views are within one of these v~s. A, Quartz crystals within a vug; some surfaces 

are covered by small sphalerite crystals. B, Center quartz crystal contains an enclosed crystal of arsenopyrite, 

and floor of vug has scattered crystals ofsphalerite. C, Sphalerite crystals on surfaces of quartz crystals. D, 

Enlargement of sphalerite crystals on quartz. E, Enlargement of arsenopyrite crystal within a quartz crystal that 

has a few sphalerite crystals on its surface. f., Detail of sphalerite crystals on surfaces of quartz. 

14 
8 Jjlm 

Figure 12. Specimens of altered rocks type I (A-C) and type II (D-F) from the Roberts Mountains Formation, 

Carlin mine; SEM micrographs. A-C, 82E-1+20B:" Dark-gray, organically rich rock with iron-rich dolomite and 

detrital silt-size quartz grains. A, Cavity containing and floored by calcite with white crystals of arsenopyrite. 

!!_, Enlarged view of cavity wall tllustra ting white arsenopyrite crystals on calcite. C, Pyrite fra mboids in rna trix 

of calcite, dolomite, and clay minerals. D, 82E-1+24. Yellowish-brown, oxidized rock. Quartz and clay matrix 

having extensive porosity formed by interconnected v~s. Small white cluster in center is composed of nonsulfurbearing lead mineral (possibly lead carbonate?). E, F, 82E-1+25. E, lllitic material and detrital crystals of zircon 

and quartz. X, Iron-rich, zoned dolomite rhomb that is partly corroded and preserved in a matrix of silica and 

clays. Note vuggy porosity of matrix and dolomite rhomb. 

15 
F 4!Am 

Figure 13. Specimens of altered rock type II from the Roberts Mountains Formation, Carlin mine; SEM 

micrographs. ~' _!!, 82E-1+2. Intensely altered, yellowish-brown oxidized rock. ~' Silica, illitic material, and 

etched dolomite rhombs, in center of photograph; note porosity. B, Highly porous rock of illitic material and 

quartz. C-£, 82E-1 +22. Intensely altered, yellowish-brown oxidiZed rock. £, Q, Very porous vuggy rna trix 

composed of clay and detrital quartz with authigenic silica overgrowths. _§, Enlarged, detailed view of vuggy 

porous silica and clay rna trix. f., 82E-1 + 23. Vuggy porosity in a rna tr ix of clay and quartz. 

16 
Figure 14. Specimens of all three altered rock types from the Roberts Mountains Formation, Carlin mine; SEM 

micrographs. ~!!, 82E-1 +16. Altered rock type IlL ~Surface of jasperoid. ~Cavity within jasperoid, showing 

subhedral crystals of quartz coated by thin dusting of iron oxide crystals. .Q, 82E-1 +23. Altered rock type II. 

Quartz-rich rock showing quartz and clay matrix with a cavity filled by late sparry calcite. Q.-f, Altered rock 

type I. Q., 82E-1 +33. Organically rich, dark-gray dolomitic arenaceous rock. Epigenetic quartz crystals in matrix 

of dolomite and clay minerals. _!b f, 82E-1 +35. Pyrite framboids in porous matrix of clay, quartz, and iron-rich 

dolomite. Thin sections of this rock show relict outlines of small fragments of echinoderms, ostracodes, and 

brachiopods preserved in dolomite. 

17 
c 101-'m 

Figure 15. Specimens from the Popovich Formation, Carlin mine. ~-.§_, SEM micrographs, E., Transmitted-light 

photomicrograph. ~ ~ 82E-1 +42. Dark-gray limestone. A, Pyrite cube, in center, fills a cavity between quartz 

grains. ~ Typical limestone of the Popovich Formation, composed of large calcite crystals with inclusions of 

small dolomite rhombs and minor amounts of unidentified clay minerals. Sample was collected about 2 m above a 

faulted contact with the highly altered Roberts Mountains Formation. C-f, 82E-1 +44. .£, Calcite matrix with 

rhombs of dolomite and clay minerals. D, Typical surfaces of large calcite crystals and subhedral light-colored 

dolomite rhombs. b Small, 2- to 20-~m calcite crystals with dolomite and quartz. L Ostracode shells, peloids, 

black pyrite cubes, and clear detrital quartz grains. 

18 
A 30~m 

F 4~m 

Figure 16. Specimens from the Popovich Formation, Carlin mine. !_, _!!, transmitted-light photomicrographs; C-.f., 

SEM micrographs. !_, _!!, 82E-R3B. !_, 20- to 100-J.lm dolomite rhombs; calcite fills spaces between dolomite 

crystals. Black pyrite fills voids in fossil mold • ..!!, Abundant relict molds and casts of ostracode and brachiopod 

shells parallel to bedding; black areas are pyrite . .£, 82E-Rl. Calcite with abundant clay and minor quartz. Note 

framboidal pyrite in upper right. D-.f., 82E-R4. Q, Calcite crystal matrix with intracrystalline clay minerals and 

quartz • .§., Clay matrix with framboidal pyrite in center . .f., Mass of barite in center of calcite-rich matrix with 

intercrystalline clay and quartz. 

19 
20 

Figure 17. Specimens from the Roberts Mountains 

Formation; transmitted-light photomicrographs. b ~ 

.Q, b altered rock type I;..£, X, altered rock type II. b 

82E-1 +20B. Unoxidized and organically rich rock that 

contains abundant pyrite crystals. Angular, clear 

quartz grains, brown dolomite crystals, blue fer roan 

dolomite matrix, and dark-gray to black pyrite and 

clay mixture. ~ 82E-1 +32. Organically rich rock that 

has undergone oxidation. Clear grains of detrital 

angular quartz 50 to 75 J.lffi in diameter in matrix of 

dark-brown clay and fine pyrite. Blue-stained f erroan 

dolomite crystals 3 to 10 J.lffi in diameter. ..£, 82E1 +30. Organically rich, dark-colored rock with 

abundant pyrite. Clear, detrital, angular quartz grains 

in rna trix of brown clay and black pyrite crystals. 

Small, 5- to 30- J.lffi crystals of blue-stained, corroded, 

ferroan dolomite. Large dolomite rhomb at center has 

black pyrite crystal in its center. Rhomb is zoned by 

clear dolomite and blue-stained ferroan dolomite. All 

dolomite in this photograph has been subjected to 

dissolution . .Q, 82E-1 +34. Organically rich, unoxidized 

rock. Angular, clear quartz grains 50 to 80 ).lffi in 

diameter; light-brown, iron-free, 30- to 50-J.lm 

dolomite rhombs and blue-stained, iron-rich, 1- to 10-

J.lffi dolomite rhombs. b 82E-1 +30. Organically rich, 

dark-colored rock. Clear to translucent dolomite 

rhombs and dolomite-replaced brachiopod shell; black 

cubes are small pyrite crystals. Small, 1- to 10-).lm, 

blue-stained, iron-rich dolomite rhombs between clear 

dolomite rhombs. Iron-rich dolomite has replaced 

iron-free dolomite. X, 82E-1 +39. Oxidized rock. 

Clear, angular, detrital quartz grains; brown, oxidized 

clay minerals; black iron minerals; and curved red area 

of late-stage alizarin-stained calcite vein. 
E F 

21 
22 

Figure 18. Specimens from the Popovich Formation 

(A ~ D-E) and the Roberts Mountains Formation (C), 

Carlin mine; transmitted-light photomicrographs. b 

82E-1 +42. Organically rich, dark-gray rock collected 

about 1 m above contact with the Roberts Mountains 

Formation. Clear, angular, detrital quartz grains in 

matrix of brown and dark-brown, 50- to 80-J.lm 

dolomite rhombs; some blue-stained 10- J.lffi ferro an 

dolomite rhombs; black of pyrite grains. Some clay 

minerals occur between dolomite rhombs. .!!_, 82E2+21. Dark-gray, organically rich rock near contact 

with Roberts Mountains Formation. Clear, angular, 

detrital quartz grains floating in matrix of 5- to 10-).lm, 

brown and blue-stained fer roan dolomite, some clay 

minerals, and black pyrite grains. .Q, 82E-2+0. 

Bleached and oxidized type II rock of the Roberts 

Mountains Formation. Clear, angular, detrital quartz 

in matrix of brown clay and iron oxides. Alizarin-red 

stained calcite fills voids and fractures in clay. _!2., 

82E-2+21. Same rocks as in B. Clay, organically rich 

dark material, and 2- to 30- irii crystals of blue-stained 

ferroan dolomite. Large crystals of clear silica, at 

center, whose voids are filled by spar calcite that has 

been stained red by alizarin dye. Large brown areas 

are iron oxides. b 82E-R3A. Dark-gray, organically 

rich rock from bench on east wall of Carlin mine pit 

(fig. 2). Large black area is pyrite replacement of 

brachiopod shell, and small} dark-brown to black areas 

are pyrite crystals in matrix of light-brown, 5- to 20-

J.lffi dolomite rhombs and clear (white) detrital quartz 

grains. Calcite crystals are stained red by alizarin 

dye. Clay minerals are between carbonate grains • .L 

82E-R4. Dark-gray, organically rich, dolomitic 

ostracode-, brachiopod-, and echinoderm-bearing 

wackestone from bench on east wall of Carlin mine pit 

(fig. 2). Calcite crystals are weakly stained red by 

alizarin dye. Fossil shells are replaced by quartz 

(white). Angular detrital quartz grains are 40 to 70 ).lm 

in diameter. Black, crystalline pyrite grains are 5 to 

10 J.lm in size. About 10 percent of rock is 5- to 10-J.lm 

dolomite rhombs. 
A 8 

c D 

23 
U.S. Government Printing Office: 1987/787-801