GEOLOGICAL SURVEY CIRCULAR 588 

Geochemical Anomalies in 

the Swales Mountain Area 

Elko County, Nevada 

Geochemical Anomalies in 

the Swales Mountain Area 

Elko County, Nevada 

By K. B. Ketner, J. G. Evans, and T. D. Hessin 

G E 0 L 0 G I C A L S U R V E Y C I R C U L A R 588 

Washington J 968 
United States Department of the Interior 

STEWART L. UDALL, Secretary 

Geological Survey 

William T. Pecora, Director 

Free on application to the U.S. Geological Survey, Washington, D.C. 20242 
CONTENTS 

Page 

Abstract ......................................................................... . 1 Structure ....................................................................... . 

Page 

4 

4 

4 

Introduction ................................................................. . 1 

1 

4 

Rock alteration ............................................................. . 

Mineralization .............................................................. . Stratigraphy ................................................................. . Suggestions for prospecting ..................................... . 13 

Intrusive rocks ............................................................. . References cited ......................................................... . 13 

FIGURE 1. 

2. 

3. 

4-10. 

ILLUSTRATIONS 

Page 

Index map of Elko County, Nev., and adjacent areas .......................................................................... 1 

Geologic map of part of Swales Mountain quadrangle showing mineralized areas, sample locations, and drill sites ............................................................................. ................. .................................. 2 

Diagram showing drill holes in Swales Mountain area ...................................................................... 5 

Maps of Swales Mountain area showing concentrations, in samples of mineralized rock, of4. Gold .................................................................................................................................................. 6 

5. Silver ................................................................................................................................................ 7 

6. Molybdenum .................................................................................................................................... 8 

7. Mercury ............................................................................................................................................ 9 

8. Copper ............................................................................................................... .............. .................. 10 

9. Lead .................................................................................................................................................. 11 

10. Zinc .................................................................................................................................................. 12 

iii 

GEOCHEMICAL ANOMALIES IN THE SWALES MOUNTAIN AREA 

ELKO COUNTY, NEVADA 

By K. B. KETNER, J. G. EVANS, and T. D. HESSIN 

ABSTRACT 

The Swales Mountain area, Elko County, Nev., contains a window in the upper plate of the Roberts Mountains thrust. Upper plate rocks, which consist of bedded 

chert and shale of Ordovician age, are assigned to the 

Vinini Formation. Lower plate rocks are carbonaceous 

silty limestone, probably the Roberts Mountains Formation of Silurian age, and an unnamed limestone unit of 

Devonian to earliest Mississippian age. Upper and 

lower plate rocks are intruded by Tertiary monzonite 

porphyry and quartz porphyry stocks and dikes. Contact zones around the igneous rocks contain some gold, 

silver, molybdenum, copper, lead, and zinc; but the 

areas of anomalous metal occurrence are small, and 

only weakly anomalous zones were found in three drill 

holes. 

INTRODUCTION 

Swales Mountain, about 18 miles northwest 

of Elko, Nev., is the most southerly peak in 

the Independence Range (fig. 1). Paleozoic 

rocks exposed in the area consist of both upper 

and lower plate rocks of the Roberts Mountains thrust fault. These are cut by two kinds 

of igneous rock, and small mineral deposits are 

exposed along the igneous contacts. 

The general geologic similarity between formations exposed at Swales Mountain and those 

of the Carlin mine 13 miles to the west prompted the U.S. Geological Survey in 1967 to map 

in detail the rocks of the area and to assess its 

mineral potential as part of the Heavy Metals 

program. The present report gives the results 

of surface and subsurface sampling. 

STRATIGRAPHY 

Rocks of the upper plate of the Roberts 

Mountains thrust, which belong to the widespread Vinini Formation of Ordovician age, 

consist principally of bedded chert and shale. 

The chert of the Vinini Formation is hard 

1 

black rock which is composed almost entirely 

of quartz and which generally occurs in beds 

a few inches thick. The shale is relatively soft 

black to dark-gray or tan rock that is finely 

bedded. 

I 

I 

Gold I 

Ac_resl 

m1ne 1 • 

·~ ·~ l~ 

I \ 

~d1~fifct \ __________ --l----------~ 

0 25 50 75 MILES 

r ~~-.. Elko County 

~ . :~ \ 

~ I ., : 

',., ~ ' '·..j . NEVADA 

FIGURE 1.-Index map of Elko County, Nev., and 

adjacent areas. 
Rocks of the lower plate of the thrust, which 

consist principally of limestone, are divided 

into two units. The lower unit, which is probably the Silurian Roberts Mountains Formation, 

is exposed on the banks of Camp Creek in the 

NW1,4 sec. 4 (fig. 2). The limestone is dark 

gray to black, is distinctly bedded, and contains 15 to 40 percent silt. The beds range in 

40 

55' 

thickness from a fraction of an inch to several 

feet, but the thicker beds are more typical. 

Drill hole 2 in the NW%, sec. 4 penetrated 

more than 2,500 feet of nearly flatlying beds of 

this unit without reaching its base. Throughout 

this thickness the appearance and composition 

of the unit are strikingly uniform. 

FIGURE 2._Geologic map of part of the Swales Mountain quadrangle, T. 35 N., R. 53 E., Nevada, showing 

mineralized areas, sample locations, and drill sites. Geology based on mapping by J. G. Evans in 1967. 

2 
The upper unit in the lower plate sequence 

is Devonian and earliest Mississippian in age. 

Because this unit has not been described or 

named in previous studies it will be called 

simply the upper limestone in this report. It is 

probably the same unit that is called the 

Popovich limestone in the Carlin mine area 

(Hardie, 1966). A 900-foot-thick sequence of 

the upper limestone is very well exposed on 

the north bank of Camp Creek in the center of 

sec. 4. It consists partly of dark-gray limestone 

resembling that of the underlying Roberts 

EXPLANATION 

D 

Quaternary and Tertiary 

tuffaceous sandstone and 

conglomerate; also 

landslide debris 

Monzonite Quartz porphyry porphyry 

Tertiary intrusive rocks 

Mississippian to Silurian 

limestone. (Lower plate of 

Roberts Mountains thrust) 

Ordovician Vinini Formation 

(Chert and shale; upper 

plate of Roberts 

Mountains thrust) 

Contact 

High-angle fault 

. . . Thrust fault 

0 

Cenozoic and Paleozoic 

rocks 

Area of altered and 

mineralized rock showing 

sample locality 

® 

Drill hole 

1 MILE 

FIGURE 2.-Continued. 

3 
Mountains Formation and partly of a distinctive sequence of alternating sandy and shaly 

beds. The sandy beds are brown or tan and 

consist of sand-size grains of carbonate and 

quartz. The shaly beds are gray and consist of 

fine-grained limestone and clay. 

Beds of loosely consolidated sandstone and 

conglomerate of Tertiary and Quaternary age 

cover the lower slopes east and west of Swales 

Mountain. Many of the sandy beds consist partly of volcanic ash. Andesite and latite lava flows 

and ash beds overlie the Vinini Formation 

north of Swales Mountain. 

INTRUSIVE ROCKS 

Intrusive roc.ks in the Swales Mountain area 

consist mainly of monzonite porphyry and 

quartz porphyry. A stock of monzonite form.s 

the south end of Swales Mountain in sec. 18, and 

a thick dike of monzonite extends from the 

NW%, sec. 16 northward through sec. 5. Most of 

the other intrusive rocks shown in figure 2 are 

qua~tz porphyry. The two types of roc.k are 

easily distinguished : the monzonite is rather 

dark and is spotted with black biotite; the 

quartz porphyry is light colored and is spotted 

with crystals of clear quartz. Geologic relations 

and radiometric data indicate that the intrusive 

rocks are of Tertiary age. 

STRUCTURE 

The Roberts Mountains Formation and the 

upper limestone unit are separated from the 

overlying Vinini Formation by a thrust fault. 

A thrust fault such as this one, which separates 

younger limestones from overlying older siliceous rocks, in this region is generaUy called the 

Roberts Mountains thrust fault (Roberts and 

others, 1958). Originally the thrust fault 

probably lay almost horizontally in the Swales 

Mountain area, but now both lower plate and 

upper plate rocks and the thrust fault that 

separates them are folded into an arch or 

anticline that trends north. The association of 

intrusive rocks with the anticline suggests that 

forcible e·mplacement of igneous rocks may 

have caused the anticlinal structure. The Roberts Mountains Formation and the upper limestone unit are exposed along the axis of this 

anticline, and they underlie the Vinini Formation at unknown depths on the flanks of the 

anticline. 

4 

In the course of mapping the Swales Mountain area some doubt arose as to whether limestone exposed there was actually in the lower 

plate of the Roberts Mountains thrust or 

whether it was an unusually thick body of 

limestone in the upper plate. Three hoi& 

drilled to clarify the structural picture indicate 

the limestone to be in the lower plate. These 

holes were designated as holes 2, 3, and 4 

(figs. 1, 2, 3). Hole 2 was drilled to a depth of 

2,547 feet, entirely within the Roberts Mountains Formation. Hole 3 penetrated limestone 

and skarn to a depth of 861 feet. Hole 4, 

located near Swales Creek (SE%, sec. 28, T. 36 

N., R. 53 E.), started in Vinini Formation in 

the upper plate of the Roberts Mountains 

thrust fault, penetrated the thrust fault at a 

depth of 348 feet, and was extended 200 feet 

into limestone of the lower plate. 

ROCK ALTERATION 

Chert and limestone have been somewhat 

altered close to the borders of igneous intrusive rocks. Some ,chert of the Vinini Formation 

has been recrystallized and bleached to a 

quartzitelike material. Limestone along intrusive contacts has been partly converted to a 

mixture of garnet and pyroxene minerals 

called skarn or tactite. This rock is extremely 

dense, hard, and difficult to drill. Limestone 

alteration at Swales Mountain thus is different 

from that at the Carlin mine, where carbonate, 

in the Roberts Mountains Formation has evidently been extensively leached, leaving a 

bleached, porous, soft residue of silica. 

MINERALIZATION 

Exposed mineralized rock is concentrated 

mainly at the contact of povphyry with upper 

plate chert on the northeast and southeast 

slopes of Swales Mountain (SE%, sec. 6; Elh 

sec. 18) and along contacts between monzonite 

and limestone (SE%, sec. 5; Elh sec. 8; NE%, 

sec. 17). A few samples from these areas. contain as much as several tens of parts per million of gold, several hundred parts per million 

of silver, more than 100 parts per million of 

molybdenum, and thousands of parts per million of lead, zinc, and copper, but most samples 

are leaner. The samples whose metallic contents are shown in figures 4 to 10 were randomly selected from visibly mineralized rock 
Upper 

limestone 

unit 

Roberts 

Mountains 

Formation 

HOLE 3 

} ;l~;~tly anomalous 

in silver, 

~---

FEET 

0 

100 

200 

lead, and 

barium 

Roberts 

Mountains 

Formation 

slightly Zone I 

anomalous 

in gold 

HOLE 2 

Vinini 

Formation 

HOLE 4 

Zone 

slightly 

J 

anomalous in 

Roberts molybdenum Mountains and m r u e c r y thrust "' 

fault ---t----H 

Lower 

plate 

limestone 

Barium 

FIGURE 3.-Drill holes in the Swales Mountain area, showing formations and zones of slight metal concentrations. 

310-089 0-68-2 

5 
10 

15 

~MILE 

Gold, in parts per million 

• 0-0.1 

• 0.1-1 

• More than 1 

FIGURE 4.-Concentrations of gold in samples of mineralized rock, Swales Mountain area, T. 35 N ., R. 53 E. Each 

symbol represents the median value of 4 to 9 samples. See figure 2 for explanation of geologic symbols. 

6 
Silver, in parts per million 

• 1-10 

• 10-70 

• More than 70 

FIGURE 5.-Concentrations of silver in samples of mineralized rock, Swales Mountain area, T. 35 N., R. 53 E. Each 

symbol represents the median value of 4 to 9 samples. See figure 2 for explanation of geologic symbols. 

7 
Molybdenum, in parts per million 

2-5 

• S-10 

• More than 10 

FIGURE 6.-Concentrations of molybdenum in samples of mineralized rock, Swales Mountain area, T. 35 N., R. 53 

E. Each symbol represents the median value of 4 to 9 samples. See figure 2 for explanation of geologic symbols. 

8 
10 

15 

¥.!MILE 

Mercury, in parts per million 

• 0.04-0.200 

• 0.200-0.400 

• More than 0.400 

FIGURE 7.-Concentrations of mercury in samples of mineralized rock, Swales Mountain area, T. 35 N., R. 53 

E. Each symbol represents the median value of 4 to 9 samples. See figure 2 for explanation of geologic symbols. 

9 
Copper, in parts per million 

• 30-500 

• 500-2,000 

• More than 2,000 

FIGURE B.-Concentrations of copper in samples of mineralized rock, Swales Mountain area, T. 35 N., R. 53 E. 

Each symbol represents the median value of 4 to 9 samples. See figure 2 for explanation of geologic symbols. 

10 
Lead, in parts per million 

• 10-100 

• 100-1,000 

• More than 1,000 

FIGURE 9.-Concentrations of lead in samples of mineralized rock, Swales Mountain area, T. 35 N., R. 53 E. 

Each symbol represents the median value of 4 to 9 samples. See figure 2 for explanation of geologic symbols. 

11 
Zinc, in parts per million 

• 0-1,000 

• 1,000-10,000 

• More than 10,000 

FIGURE 10.-Concentrations of zinc in samples of mineralized rock, Swales Mountain area, T. 35 N., R. 53 E. 

Each symbol represents the median value of 4 to 9 samples. See figure 2 for explanation of geologic symbols. 

12 
at each site. The tonnage of mineralized rock 

cannot be estimated, but the narrow extent of 

surface exposures indicates that it is small. 

Nearly 300 3-inch segments taken at regular 

intervals from drill core were analyzed by 

spectographic and atomic absorption techniques. Barely detectable amounts ( 0.02 to 

0.1 ppm) of gold were found in some samples 

from hole 2 between 1,080 and 1,230 feet below the surface. Silver, lead, and barium were 

found in the top 100 feet of hole 3 in concentrations only slightly greater than those of 

ordinary rock. Mercury, and molybdenum were found in slightly above-normal 

concentrations in hole 4 in the Vinini Formation within 50 feet of the Roberts Mountains 

thrust, and barium was found in above-normal 

concentrations in lower plate limestone within 40 feet of the thrust. The metal contents of 

all samples of drill core are too low to be of 

any immediate economic significance. 

SUGGESTIONS FOR PROSPECTING 

Disseminated gold deposits should be sought 

at Swales Mountain in the kinds. of rocks that 

carry disseminated gold elsewhere. In the Carlin mine (Hardie, 1966) and the Cortez district (Erickson and others, 1966) gold is 

disseminated in the Roberts Mountains Formation. At the Gold Acres mine (Ketner, 1965) 

gold is disseminated in broken rock along the 

Roberts Mountains thrust. The presence of 

small gold deposits in the upper plate rocks at 

Swales Mountain suggests the possibility of 

13 

more extensive deposits in the Roberts Mountains Formation where it lies concealed by 

upper plate rocks or in the broken rock along 

subsurface parts of the Roberts Mountains 

thrust. Exploratory efforts therefore should 

include drilling through the upper plate in 

areas immediately surrounding Swales. Mountain. Unfortunately there is no way of knowing 

how deep the thrust zone lies or how far below 

the thrust the Roberts Mountains Formation 

lies. Both could be very close to the surface 

without revealing their presence in any way. 

Lower plate rocks intruded by igneous rocks 

are concealed beneath alluvium on the east 

flank of the mountain (sees. 4, 9) . Exploration 

of these rocks would require drilling through 

the alluvium east of the area where lower 

plate rocks are exposed. 

REFERENCES CITED 

Erickson, R. L., VanSickle, G. H., Nakagawa, H. M., 

McCarthy, J. H., Jr., and Leong, K. W., 1966, Gold 

geochemical anomaly in the Cortez district, N evada: U.S. Geol. Survey Circ. 534, 9 p. 

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

Nevada: Nevada Bur. Mines Rept. 13, pt. A, p. 

73-83. 

Ketner, K. B., 1965, Economic geology, in Gilluly, 

James, and Gates, Olcott, Tectonic and igneous 

geology of the northern Shoshone Range, Nevada: 

U.S. Geol. Survey Prof. Paper 465, p. 129-144. 

Roberts, R. J., Hotz, P. E., Gilluly, James, and Ferguson, H. G., 1958, Paleozoic rocks of north-central 

Nevada: Am. Assoc. Petroleum Geologists Bull., 

v. 42, no. 12, p. 2813-2857. 

U.S. GOVERNMENT PRINTING OFFICE: 1968 0-310-089