GEOLOGICAL SURVEY CIRCULAR 534

Gold Geochemical Anomaly 

in the Cortez District 

Nevada
Gold Geochemical Anomaly 

in the Cortez District 

Nevada

By R. L. Erickson, G. H. Van Sickle, H. M. Nakagawa, 

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

GEOLOGICAL SURVEY CIRCULAR 534

Washington 1966
United States Department of the Interior

STEWART L. UDALL, Secretary

Geological Survey

William T. Pecora, Director

First printing 1966 

Second printing 1968

Free on application to the U.S. Geological Survey, Washington, O.C. 20242
Gold geochemical anomaly in the Cortez district, Nevada

By R. L Erickson, G. H. Van Sickle, H. M. Nakagawa, J. H. McCarthy, Jr., and K. W. Leong

Abstract

An area in the Cortez district, Nevada, previously 

established to be anomalous in arsenic, antimony, 

and tungsten has been found to be anomalous also in 

mercury and gold. Samples from narrow quartz veins, 

calcite veins, and shear zones in partially silicified 

limestone in the lower plate of the Roberts thrust 

fault (Cortez window) contain as much as 3.4 ounces 

gold per ton. The richest samples are from an outcrop, about 100 feet long, surrounded by gravels. Their 

economic significance is yet to be established.

INTRODUCTION

The geochemical association of arsenic, mercury, 

antimony, and tungsten with gold has been recognized 

at several low-grade open-pit mines such as Getchell, 

Bootstrap, Carlin, and Gold Acres mines in northcentral Nevada (Erickson and others, 1964b; Engineering and Mining Journal, 1966). Geochemical prospecting in the Cortez district revealed anomalous 

amounts of arsenic, antimony, and tungsten and led 

to analysis of samples for mercury and gold. Development by Lakin and Nakagawa (1965) of a sensitive 

chemical method for rapid determination of gold in 

rocks and soils and by Vaughn and McCarthy (1964) 

of a mercury sniffer made such analysis possible in 

the field during geochemical prospecting.

The Carlin and Bootstrap mines in the Tuscarora 

Mountains, Elko and Eureka Counties, and the Gold 

Acres mine in the Shoshone Range, Lander County, 

are associated with windows in the Roberts thrust, 

a major structural feature of north-central Nevada 

along which a number of mining districts are situated 

(Roberts, 1960). Gold ore is chiefly in the thrust zone 

at Bootstrap and Gold Acres and is in and below the 

thrust zone at Carlin (Peter N. Lancar, Newmont's 

Carlin gold mine: Soc. Mining Engineers Fall Mtg. 

Rocky Mtn. Minerals Conf., Phoenix, Ariz.,Oct. 1965). 

At the Getchell mine in the Osgood Mountains, Humboldt County, gold occurs in the high-angle Getchell 

fault zone, a zone that appears unrelated to the Roberts thrust (Hotz and Willden, 1964).

In early 1966 surface-rock and drill-hole samples 

collected in previous geochemical studies in the Cortez 

district were analyzed for gold by Nakagawa, using 

both chemical and atomic-absorption methods. Anomalous amounts of gold 5-14 ppm (parts per million), 

equivalent to 0.145-0.41 ounce per ton were found in 

two surface rock samples; 2-8 ppm (0.06-0.23 oz per 

ton) gold was found in heavy-mineral concentrates of 

rotary-drill cuttings from two shallow holes drilled 

near the boundary of an arsenic-antimony-tungsten 

anomaly. These results prompted additional fieldwork 

in May 1966, which is reported here.

Analyses of newly collected samples showed gold 

concentrations as high as 3.4 ounces per ton. The samples are from narrow quartz veins, calcite veins, and 

shear zones in partially silicified limestone of Silurian 

and Devonian age in the lower plate of the Roberts 

thrust (Cortez window) near the Crescent fault about 

4 miles north of Cortez, Nev. (fig. 1). The highest 

gold assays discovered in this investigation occur in 

samples from an outcrop, about 100 feet long, consisting of altered Devonian limestone near the east 

end of an arsenic-antimony-tungsten geochemical 

anomaly previously reported by Erickson, Masursky, 

Marranzino, Oda, and Janes (1964a).

The geology of the Cortez quadrangle is described 

by Gilluly and Masur sky (1965), and the reader is 

referred to their report and to reports on the geochemical investigations (Erickson and others, 1961, 

1964a) for a more complete discussion of the geology 

and geochemistry of the area.

Elwin Mosier, Howard Knight, Arthur Hubert, and 

Theodore Roemer, all of the U.S. Geological Survey, 

participated with the authors in this short but intensive 

field study.

RESULTS OF INVESTIGATION

For this investigation 238 rock samples were collected from 95 localities near the Crescent fault about 

4 miles north of Cortez (fig. 1). Analytical results 

(table 1) are given only for those samples that, by 

at least one analytical method, contain significant 

amounts of gold (0.3 ppm, or 0.09 oz per ton). To check 

the gold values determir ed.by colorimetric and atomicabsorption methods, 27 samples were analyzed by fire 

assay by C. O. Parker and Co., Denver, Colo. The results show good agreement (table 1) in view of the fact 

that only 1- and 2-gram samples were used in the 

colorimetric and atomic-absorption methods. Sample 

localities and localities with significant amounts of

1
gold are shown in figure 1. Sample localities and areas 

of anomalous concentrations of arsenic, antimony, 

tungsten, and mercury are shown in figures 2, 3, 4, 

and 5, respectively.

The highest gold assays were found in samples 

from locality 722 (fig. 1), an exposure of altered limestone that, as previously mentioned, is east of the 

area of the strong arsenic-antimony-tungsten anomaly. 

The limestone, which forms a dip slope on the east 

side of a ridge, strikes N. 5° E., dips 65° E., and is 

exposed for about 100 feet along strike. The base of 

the slope is covered by talus. The limestone is dark 

orange brown in overall appearance and contains nearly 

vertical white calcite veins and calcite-cemented 

breccia zones as much as 10 inches wide that strike 

westward into the ridge. Quartz stringer sup to 2 inches 

wide are parallel or subparallel to a brick-red shear 

zone, 8 inches wide, that strikes N. 75° W. and dips 

30° SW. Samples 722 through 722-K (table 1) are 

selected samples from the talus; all others are from 

narrow veins and shear zones in the limestone. All 

veins, shear zones, breccia zones, and fractures sampled at this locality are gold bearing. The type of 

material probably constitutes less than 10 percent of 

the outcrop area. We have no data at present on the 

gold content of limestone that is not sheared, brecciated, or veined.

Dark-brownish-black jasperoid masses that crop 

out discontinuously on the west side of the ridge above 

locality 722 (fig. 1, table 1) contain much smaller but 

nonetheless anomalous amounts of gold. Gravel deposits cover the area between these jasperoid masses 

and locality 722 and may conceal gold-bearing altered 

rocks.

At sample localities 601, 600, 700, 701, and 702 

(fig. 1), a conspicuous brick-red breccia zone, about 

8 feet wide, in skarn and altered limestone strikes 

N. 10° W. and dips steeply east. Gold was detected 

only at locality 600 and in only 2 of 21 samples collected 

there.

At locality 606 gold occur sin a zone of dark-maroon 

jasperoid and breccia that contains white calcite pods 

and stringers. No attempt was made to tracethis jasperoid zone along strike.

Preliminary mineralogic work on one sample 

(722-D) suggests that most of the gold occurs as very 

tiny disseminated specks and fine lacework in oxidized 

pyrite crystals. The largest fragment of gold observed 

in this sample is only a few microns thick but measures 

200 microns in largest dimension.

The silver content of allgo Id-bear ing samples is 

very low (maximum assay, 0.16 oz per ton). Spectrographic analyses show that arsenic is the most abundant metal in the gold-bearing samples; arsenic and 

mercury contents show a direct correlation with gold 

content. Small amounts of antimony and tungsten may 

be present; although antimony and tungsten contents do 

not show a direct correlation with gold content, anomalously high concentrations of antimony and tungsten 

commonly occur in rocks in or adjacent to areas of

gold mineralization. Lead, zinc, and copper are not 

present in significant amounts.

An entirely different suite of metals was detected at 

locality 759 (fig. 1). Here, a light-gray, pink-weathering fine-grained limestone with finely disseminated oxidized pyrite crystals contains 5,000 ppm zinc, 300 

ppm copper, and 20 ppm bismuth. A thin white pyritic 

quartz veinlet cutting the limestone contains 3,OOOppm 

zinc, 500 ppm copper, 300 ppm lead, 70 ppm tin, 200 

ppm bismuth, and 200 ppm antimony. Gold was not detected in these samples.

Caliche-cemented gravel was sampled at 12 localities (fig. 1) in the search for possibleplacer accumulations of gold. Gold content was below the limit of 

detection (0.1 ppm) in all samples.

REFERENCES CITED

Engineering and Mining Journal, 1966, Exploration.. .A 

study in ingenuity: Eng. Mining Jour., 167, 6. 

p. 200.

Erickson, R. L.,Masursky, Harold, Marranzino, A.P., 

and Oda, Uteana, 1961, Geochemical anomalies in 

the upper plate of the Roberts thrust near Cortez, 

Nevada, in Short papers in the geologic and hydrologic sciences: U.S. Geol. Survey Prof. Paper 

424-D, p. D316-D320.

Erickson, R. L., Masursky, Harold, Marranzino, A. P., 

Oda, Uteana, and Janes, W. W., 1964a, Geochemical anomalies in the lower plate of the Roberts 

thrust, near Cortez, Nevada, in Geological Survey 

Research 1964: U.S. Geol. Survey Prof. P aper 

501-B, p. B92-B94.

Erickson, R. L., Marranzino, A. P., Oda, Uteana, and 

Janes, W. W., 1964b, Geoc hemical exploration 

near the Getchell mine, Humboldt County, Nevada: 

U.S. Geol. Survey Bull. 1198-A, 26 p.

GilJuly, James, and Masursky, Harold, 1965, Geology 

of the Cortez quadrangle, Nevada: U.S. Geol. Survey Bull. 1175, 117 p.

Hotz, P. E., andWillden, C. R., 1964, Geology and mineral deposits of the Osgood Mountains quadrangle, 

Humboldt County, Nevada: U.S. Geol. Survey Prof. 

Paper 431. 128 p.

Lakin, H. W.. and Nakagawa, H. M., 1965, A spectrophotometric method for the determination of 

traces of gold in geologic materials, in Geological Survey Research 1965: U.S. Geol. Survey Prof. 

Paper 525-C, p. C168-C171.

Roberts, R. J., 1960, Alinement of mining districts in 

north central Nevada, in Geological Survey Research 1960: U.S. Geol. Survey Prof. Paper 400-B, 

p. B17-B19.

Vaughn, W. W., and McCarthy, J. H., Jr., 1964, An instrumental technique for the determination of submicrogram concentrations of mercury in soils, 

rocks, and gas, in^ Geological Survey Research 

1964; U.S. Geol. Survey Prof. Paper 501-D. 

p. D123-D127.
Table 1. Analyses of rocks from the lower plate of the Roberts thrust, Cortez window, Nevada. 

[Spectrographic analyses oy Elwin Mosier; colorimetric analyses for gold by G. H. Van Sickle; atomicabsorption analyses for gold by H. M. Nakagawa, G. H. Van Sickle, K. W. Leong, and Arthur Hubert; and 

for mercury by W. W. Janes and J. H. McCarthy, Jr.; fire-assay analyses by Charles O. Parker and Co. 

tr., trace; n.d., not determined. Sample localities shown in fig. 1. Multiple samples from one locality 

indicated by letter suffix.]

Locality 

and 

Sample 

Nos. Description

Fire assay analyses

Ounces per 

ton

Au | AS

Parts per 

million

Au | Ag

Atomic absorption 

analyses in parts 

per million

Au I Hg

Colorimetric 

analyses In parts 

per million

Au

Spectrographic analyses In 

parts per million

As Sb | W

600-C 

600-E

606

606-A 

606-B

608

630-A

631

632

712

722 

722-A

722-B

722-C

722-D 

722-E

722-F 

722 -G

722-H 

722-1

722 -J

722-K 

722-L 

722 -M

722 -N 

722-0 

722 -P

722 -Q 

722 -R 

722-S

72U 

72U-A

726

7U9 

7U9-A

750

755

Iron- rich material, yellow brown 

in prominent, nearly vertical

Limestone, brick- red, leached;

Jasperoid, yellow brown; contains

Jasperoid, gray; weathers

Limestone breccia, brown; cemented

Limestone, silicified; contains 

seams of white calcite and ironrich material; weathers dark

 . DO 

Limestone, red-brown, altered 

Limestone, red-brown, partially

Limestone, red-brown, with some 

faint green stain, leached; 

contains white calcite stringers - 

Limestone, red-brown; cut by 

1.5-in. white quartz vein with 

dark-brown iron oxide ribbon

Limestone, red-brown, leached;

Calcite vein, white; with some

Like 722-B 

Til me stone, red-brown; veined with

Limestone, brown; partially 

silicified 

Breccia zone, red, calciteCalcite, white; veins In limestone 

Shear zone, brick- red; 8 in. wide 

Quartz vein, white; 2 in. wide;

Jasperoid, gray; weathers

Quartz seam, gray and red;

Limestone, gray, partially silicified; with coarsely crystalline

Limestone, gray; weathers brown - 

Breccia zone in limestone, calciteLimestone, dark-gray, silicified 

Limestone, dark-gray, partially

Like 72U-A;. float piece 

Jasperoid, brown, slightly vuggy;

0.12 

n.d.

.18 

n.d. 

n.d.

n.d.

n.d.

n.d.

n.d.

1.7U 

1.96

1.60

3. to 

2.98

.Ok 

.02

1.12 

.29

.08

.06 

.12 

3-1

 3U 

 30 

.90

.ou

.05 

.01

.10

n.d. 

n.d.

n.d.

n.d. 

n.d.

n.d.

n.d.

tr. 

n.d.

.10 

n.d. 

n.d.

n.d.

n.d.

n.d.

n.d.

.08 

.10

.10

.16 

.12

.10 

.10

.11* 

tr.

tr.

tr. 

tr. 

.16

.1U 

.12 

tr.

tr. 

.15 

tr.

tr.

n.d. 

n.d.

n.d.

n.d. 

n.d.

n.d.

n.d.

U.I tr. 

n.d. n.d.

6.1 3.U 

n.d. n.d. 

n.d. n.d.

n.d. n.d.

n.d. n.d.

n.d. n.d.

n.d. n.d.

n.d. n.d

59 2.7 

67 3.U

5U 3-U

116 5.5 

101 k.l

l.U 3.U 

.7 3-U

38 U.8 

9-9 tr.

2.7 tr.

2.0U tr. 

U.I tr. 

K)5. U 5-5

11.56 U.8 

10.2 U.I 

30.6 tr.

1.36 tr. 

1.7 5-1 

.3U tr.

3'.U tr.

n.d. n.d. 

n.d. n.d.

n.d. n.d.

n.d. n.d. 

n.d. n.d.

n.d. n.d.

n.d. n.d.

8.0 

3-5

5 

1 

 5

1.5

.5

.5

.6

 7

*7

U9

5U

63 

88

U.5 

2.3

78 

11.5

2.6

U.5 

1.25 

111

16.5 

11.0 

31-

.1 

U 

.1

1.6-

1 

 35

.25

.6

.8

.25

U

1.0 

.55

.15 

.20 

.13

.15

.13

.17

.12

.2U

.85 

3

1-7

1.2 

 90

.38

1.6

1.8

2.7

1 

2.8.30

.85 

1 

.90 

3-2

.01 

.08 

.02

.07

.60

.03

.1U

.08 

.07

.06

.06

10 

n

7 

n 

n

n

1

n

fy 

60

60

125 

150

5

1

52 

10

2

2 

3 

160

25 

15 

50

5

2

.0 

.d.

.5

.d. 

.d.

.d.

.75

.5

.d.

 5

.5

 35 

.05

 75 

.35

 75

 75 

.75

.5

3.5

7,000 

3,000

700 

700 

< 500

500

< 500

< 500

< 500

2,000

1,500 

1,500

3,000

3,000 

2,000

500 

3>000

3,000 

5,000

1,000

1,500 

< 500 

2,000

1,000 

1,000 

3,000

< 500 

< 500 

< 500

< 500

< 500 

< 500

1,000

700 

1,000

< 500

500

1,500 

1,000

300 

300 

200

< 100

< 100

150

100

200

< 100

100

< 100

100

150

< 100 

< 100

< 100 

< 100

< 100 

< 100

< 100 

< 100

< 100

< 100

< 100

< 100

< 100 

< 100

< 100

2,000 

< 100

300

200

100

< 100

< 100

300 

150

100 

200 

300

< 50

< 50

< 50

< 50

200

< 50 

< 50

50

< 50 

50

< 50 

< 50

< 50 

50

< 50

< 50 

< 50 

< 50

< 50 

< 50 

< 50

< 50 

< 50 

< 50

< 50

< 50 

< 50

< 50

< 50 

< 50

< 50

< 50

756 Qalcite, white, and iron-stained 

limestone; in shear zone ~- n.d. n.d. n.d. n.d. .1 .09 < 500 150 50
116°37'30

Q

Enlargement of northeast part of map area.
EXPLANATION

Intrusive rocks 

Quart* porphyry and latite

Skarn

Sample locality and number

 6SS >lppmgold

SS22 0.SS to 1 ppm gold

O? 11 <O.S5 ppm gold

Samples underlined reported 

in Table 1

Contact

Dfuhtd where approximately located

Fault

Dashed where approximately located; 

dotted where concealed. U, upthrown side; D, dtvintkrovin Bide

Thrust fault 

Dashed where approximately located; £&, 

saw teeth on upper plate

Siliceous clastic rocks of Ordovician, 

Silurian, and Devonian age

FIGURE 1. Geochemical and geologic map of an area in the north-central part of the Cortez 

Nevada. Geology modified from Gilluly and Masursky (1965). Gold content of underlined sam

in Table 1.
1000 ?000 3000FEET

FIGURE 2. Geochemical map showing distribution of arsenic. Samples of jasperoid, fracture 

shear zones within shaded area contain at least 500 ppm. Solid circles are samples that cont

2,000 ppm.
FIGURE 3. Geochemical map showing distribution of antimony. Samples of jasperoid, fracture fi

shear zones within shaded area contain at least 100 ppm. Solid circles are samples that conta

1,000 ppm.
V. .V

ihpipp^ r^--"N) °t

illy \ -\S

FIGURE 4. Geochemical map showing distribution of tungsten. Samples of jasperoid, fracture fil

shear zones within shaded area contain at least 50 ppm. Solid circles are samples that contain at 

ppm.
116*37'30"

FIGURE 5. Geochemical map showing distribution of mercury. Samples of jasperoid, fracture 

shear zones within shaded area contain at least 0.1 ppm. Solid circles are samples that con

0.5 ppm.