GEOLOGICAL SURVEY CIRCULAR 475 

MINERALIZATION ASSOCIATED 

WITH A MAGNETIC ANOMALY 

IN PART OF THE ELY 

QUADRANGLE, NEVADA 

MINERALIZATION ASSOCIATED WITH AI MAGNETIC ANOMALY IN 

PART OF THE ELY QUADR~GLE, NEVADA 

By Arnold L. Brokaw, Garland B. $ott, Don R. Mabey, 

Howard McCarthy, and Ufeana Oda 

I 

GEOLOGICAL SURVEY C!RcfLAR 475 

I 

Washington 196 
United States Department of the Interior 

STEW AR.T .L. UDALL, Sscu'I'AltY 

Geological Survey 

1HOMAS B. NOLAN, DillBCTOR 

Free on application to the U.S. Geological Survey, Washington 25, 0. C. 
MINERALIZATION ASSOCIATED WITH A MAGNETIC ANOMALY IN 

PART OF 1HE ELY QUADRANGLE, NEVADA 

By 

Arnold L. Brokaw, Garland B. <);ott, Don R. Mabey, Howard McCarthy, and Uteana Oda 

Abstract 

Geologic mapping in the Ely quadrangle, ;Nevada, 

in 1958 to 1960, revealed structuralirregular~ties and 

rock alteration suggesting that the Ward Mountain area 

of the Egan Range contains concealed intrusive rocks 

and possibly accumulations of metals of e~onomic 

value. An aeromagnetic survey by the U. S. Geological Survey tested this theory and a large rpagnetic 

anomaly, similar to one near Ruth, Nev., was 1outlined 

in the southwest quarter of the quadrangle; Subsequently, a geochemical study tested the possibility 

that the magnetic anomaly reflects a buried rinineralized intrusive. 

The Egan Range is a major north-trendi~gmountain range in east-central Nevada. The central part 

of the range, which is the subject of this repbrt, lies 

south-southwest of Ely between Steptoe Valley on the 

east and White River Valley on the west (fig,. 1). The 

average altitude of the crest of the range in this area 

is about 10,000 feet above sea level, and the highest 

peak is 10,933 feet. Near Ruth the altitude decreases 

to about 7,000 feet. The floors of the adjoining valleys 

are about 6,000 to 6,500 feet above sea level. 

Paleozoic sedimentary rocks of the ra:qge are a 

sequence of dolomite, limestone, shale~ and sandstone 

ranging in age from Ordovician to Permian. These 

rocks are overlain in places along the east¢rn flank 

by early Tertiary sedimentary rocks and by somewhat 

younger volcanic rocks. Gravity data indicat~ several 

thousand feet of Cenozoic sedimentary rock~ (valley 

fill) in Steptoe Valley but generally less in Wh~te River 

Valley. 

Small quartz monzonite dikes of early ~ertiary 

or Late Cretaceous age intrude the Paleozoiqrocks on 

the east flank of Ward Mountain, and simil~r intrusive rocks are exposed near Ruth. 

The pre-Tertiary rocks have been complexly 

folded and displaced by thrust and high-ang\e faults, 

whe!eas the Tertiary rocks have been only ·slightly 

folded and tilted. 

An aeromagnetic survey of the central p~rt ofthe 

Egan Range was flown at an elevation of 111,500 feet 

along north-south lines spaced about 2 mil~s apart. 

The resulting magnetic data are shown by cotJ.tours on 

figure 1. 

A positive magnetic anomaly extends over most 

of the range from Ruth south to the edge of ~he map. 

Two local positive anomalies are superimposed on the 

more extensive anomaly. The northern an~maly is 

centered over the south edge of the Robinson mining 

district, where disseminated copper deposits occur in 

an east-trending zone of metamorphosed sedimentary 

rocks and altered monzonite porphyry. The nearsurface igneous rocks associated with the copper deposits can account for only part of the measured 

northern anomaly, and a major part of it must be 

caused by a buried igneous body of greater areal extent. 

The soutllern magnetic anomaly is ·centered over 

the west edge of the Egan Range where the range is 

composed of non-magnetic Paleozoic sedimentary 

rocks. The magnetic anomaly is interpreted as due to 

a large mass of igneous rocks underlying the thickest 

part of the Egan Range and the eastern edge of White 

River Valley. The zone of highm~gnetic intensity connecting this anomaly with the anomaly near Ruth suggests that the igneous bodies believed to produce the 

two local anomalies are connected at depth. 

The geologic features (fig. 2) of the general area 

of the southern magnetic anomaly tend to confirm this 

interpretation. In gross aspect this part of the Egan 

Range appears to be a simple tilted fault block similar to many other ranges in Nevada and Utah. In detaii, however, the structure is more complex. The 

sedimentary rocks have been cut by numerous faults 

that displace the beds from a few tens of feet to several 

thousand feet and divide the range into several smaller 

structural blocks. 

Overturned folds and thrust faults mapped within 

and beyond the the limits of the area shown in figure 2 

indicate that thrust-faulting was important in the early 

development of the range. Along Rowe Creek, below 

the outcrop belt of the Chainman Shale, the be~· are 

overturned and dip to the west on the limb of a recumbent fold that is overturned to the east. The attitude of 

the fold indicates that a thrust plate moved to the east 

over the rocks now exposed. The small klippen of 

Devonian rocks 1 and3milessouthofRowe Creek may 

represent remnants of this plate. 

Just north of Rowe Creek and near the center of 

the southern magnetic anomaly (shown in detail in 

fig. 3), the rocks are cut by the Rowe Creek fault, 

which displaces the rocks several thousand feet in a 

left-lateral sense. The fault is terminated on the east 

by a northwest-trending reverse fault. This reverse 

fault appears to die out to the southeast in the Chainman Shale, although its counterpart or continuation may 

be one of the strong faults that cut the Pennsylvanian 

and Permian rocks to the southeast. Similarly, the off1 
2 

EXPLANATION 

~'2400 

~ 

MPJntlfic. con_toLJr.s 

t:tncl fli~hf /1nes 

Exfenf of Paleozoic 

sedimentary rocks in 

-the £,jan Range 

Igneous intrusive 

rocks 

0 

N E V A D A 

I 

5 Miles II 

Figu. re !.--Aeromagnetic map ~f the centlal Egan Range. Values are total ma.gnetic intensity relative to an arbitrary datum. Contour interval is 10 and 50, gammas. Flight level is 11,500 feet above sea level. 

Magnetic surv_ey was flown and compil d under the supervision of J. L. Meuschke. Geology is generalized 

from Guidebook to the geology of ea t-central Nevada, Intermountain Association of Petroleum Geol-. 

ogists, 11th Annual Field Conference, 1960, held with Eastern Nevada Geological Society. 
114 ° 55' 

R.62 E. R.63 E. 

0 2 MILES 

~---'-.. 

Contact ~--- '0 

Fault 

....... ~. 

Thrust fault 

Dashed where approximately located Dashed where approximately located. U, Sawteeth on upper plate 

upthrown side, D, downthrown side 

T. 

13 

N. 

r--------. 

I I 

I I 

L---------1 

3 

Area of geochemical anomalies 

Figure 2.--Generalized geologic map of the southwest part of the Ely quadrangle, Nevada. Qal, alluvium 

(Quaternary); Tv, volcanic rocks, and Tsp, Sheep Pass Formation of Winfrey (1958) (Tertiary}; TKm, 

monzonite porphyry (Tertiary or Cretaceous); PP, Permian and Pennsylvanian, undifferentiated; Me, 

Chainman Shale (Mississippian); Mj, Joana Limestone, and Mjj, Joana jasperoid (Mississippian}; D, 

Devonian, undifferentiated; SO, Silurian and Ordovician, undifferentiated; Oep, Eureka Quartzite and 

Pogonip Group (Ordovician). The Joana jasperoid is stippled. Geology by A. L. Brokaw and D. R. Shawe. 
4 

39 

05' 

··--. 

0 2 MILES 

Figure 3.--Aeromagnetic map, southwestpart of Ely quadrangle. Contour interval 10 gammas. 

T. 

14 

N. 

T. 

N. 

T. 
T.14 N. 

T.14N. 

~I 

. ./· 

R.62 E. ·R.63 E. 

12 

.--·· 

o'~~-'e ............. · a&9..<:>_.-:----

~7· 

13 

SILVER 

8 

/ ___ _...,.· 

~ 

~~o tiliilililiill 

>50 10-50 

R.62E. R.63E. 

=~/ 12 

. .---·· 

. ./ 

~\·~· 13 

·--~~~~-· ~ 

10,000 parts 

per million 

>300 

COPPER 

FTIITSTI 

tiliiliililill 

30-70 

5-10 <5 

D 

<30 

0 

T.14 N. 

T.14 N. 

I 

100-:7000 

ARSENIC 

LilllliiliJ 

~ 

30-IOO 

R.62 E. R.63 E. 

·· . ._.;-· .. __,-.·· 

=_,;!1 12 

. ./· 

. .---

·----· 

\\ II 

~ 

~~\\ 

II 

II 

II 

" II 

II 

,, 

~ 

~ 

MOLYBDENUM 

8 

D 

<30 

8 

5 

{ 

/ 

{ 

/ 

>200 I!I~D 100-2oo. . 50-100 <50. 

2 MILES 

Figure 4.--General distribution, in parts per million, of silver, arsenic, copper, and molybdenum in jasperoids and in iron oxides in vugs and fractures. 
6 

R.62 E. R.63 E. R.62 E. R.63 E. 

\I 

~ ... __,..... ... _.,.-.·· ~~" ... __.,.,.- .. ~··· 

II 

\\ 

I) 

-=-oe!J II 8 { II 

12 II / 1\ 

~ 

,__.,...-···..,..,-. \\-

··- . ./ / 

8 

12 _ _____. .. ·.,__.-

. ./ 

~·~·· .. \ ............ ~~-

T.14N. 1 17 

13 T.14 N. 13 

···~·· 

LEAD AND ZINC TIN AND TUNGSTEN 

rill • 

T.14 N. 

>SO 

Lead 

R.62 E. R.-63 E. 

• 1 )lo.s 

f8 

MERCURY 

riTTITITITIJ 

~ 0.2-0.5 

>100 

-Zinc 

8 

~ 

..::::;::::.~ ,, 

II 

II 

II 17 II 

II 

I~ 

D 

<0.2 

0 

{ 

.. / 

I 

10-50 

Tin 

>100 

2 MILES 

L-------------------~~------------------_J 

ANTIMONY 

ffiiTITITITTI 

EilllllliJ 

10-100 

50-300 

Tungsten 

D 

<10 

{ 

/ 

Figure 5.--G~neral distribution~ in parts per million, of lead, zinc, tin, tungsten, mercury, and antimony 

in jasperoids and in iron oxides in vugs and fractures. 
set segment of the Rowe Creek fault maybe one of the 

left-lateral faults that extend across Ward Mountain. 

Where Joana Limestone is cut by the reverse fault and 

related fractures it is altered to jasperoid, and many 

of the minor fractures are filled with secondary calcite and iron-oxide minerals. The exposed lower and 

middle Paleozoic limestones and dolomites also are 

silicified in the areas of structural deformation. 

Some evidence suggests that the rocks near the 

magnetic anomaly, particularly in the vicinity of the 

klippen, have been differentially elevated, forming a 

structural high that might be due to doming related to 

a buried intrusive. The 1,500-foot difference in,elevation between the two klippen shown on the map is more 

than can be accounted for by the known faulting. 

The similarity of the two magnetic anomalies, the 

existence of major ore bodies in the area of the 

northern anomaly and of abandoned small mines on 

Ward Mountain, the presence of minor intrusive 

masses in the areaofWardMountain,andthe presence 

of jasperoid, all suggested that the area of the southern 

anomaly may also contain significant ore deposits. A 

geochemical study was therefore made. 

Semiquantitative spectrographic analyses for 30 

elements, including Cu, Ph, Mo, Ag, Sn, and W, were 

made in the field by U. Oda, M. DeValliere, and W. W. 

Janes, using a truck-mounted spectrograph. As, Sb, 

and Zn were determined by field-type chemical 

methods of analysis (Ward, Lakin, Canney, and others, 

in press). Hg analyses were made spectrographically 

by Paul Barnett using a large sample charge in speciai 

electrodes. 

The analytical results show that anomalous 

amounts of Ag, Hg, Cu, Ph, Zn, Mo, Sb, W, and Sn are 

present in jasperoid and iron-rich fracture fillings in 

the limestones and dolomites along and adjacent to 

major faults directly over the magnetic anomaly 

(figs. 4 and 5). The Rowe Creek fault and the northwesttrending fault that is roughly parallel to the outcrop 

belt of the Joana Limestone show the highest concentrations of metals and probably were the principal 

conduits for metal-bearing solutions. Ba, Cr, Fe, Mn, 

and Ti also are present in abnormally high concentrations in the anomalous area. Unaltered limestones and 

calcite veins are not appreciably enriched in any of the 

metals except perhaps mercury and arsenic. 

The high concentration of metals along the faults 

and over the magnetic anomaly may be a leakage halo 

from metal deposits in and near the postulated concealed intrusive. The quartz monzonite dikes on the 

east side of the area are probably connected at depth 

with this postulated intrusive. The extensive alteration 

of limestone to jasperoid suggests that similar dikes 

are present at depth in the Rowe Creek area, under 

the geochemical anomalies. The depth to the intrusive 

rocks is not known; however, the low mobility of some 

of the elements in a limestone environment suggests 

that the intrusive may be at a relatively shallow depth. 

7 

Depth analyses of the geophysical data indicate 

that the top of the mass causing the major magnetic 

feature (fig. 1) is about 3,000 feet above sea level or 

3,000 feet below the surface of White River Valley. 

Details of the magnetic field were not well defined by 

this survey, but the gradients associated with the 

southern magnetic high indicate that parts of the mass 

are nearer the surface than others. Its surface is 

probably irregular and may be quite shallow in some 

places. 

References 

Ward, F. N., Lakin, H. W., Canney, F. C., and others, 

1962, Analytical methods used in geochemical exploration by the U. S. Geological Survey: U. S. 

Geol. Survey Bull. 1152, in press. -

Winfrey, W. M., Jr., 1958, Stratigraphy, correlation, 

and oil potential of the Sheep Pass formation, 

east-central Nevada, in Am. Assoc. Petroleum 

Geologists Rocky Mtn:-section, Geol. Rec., 8th 

Ann. Mtg., p. 77-82. 

INT.DUP.,D.C.62-17625