GEOLOGICAL SURVEY CIRCULAR 549 

Economic Significance of Revised 

Age Relations of Rocks in the 

Cornucopia Mining District 

Elko County, Nevada 

Economic Significance of Revised 

Age Relations of Rocks in the 

Cornucopia Mining District 

El ko County, Nevada 

By R. R. Coats 

GEOLOGICAL SURVEY CIRCULAR 549 

Revision of geological interpretation and inferred 

possible unmined extensions of silver ore bodies 

Washington 1967 
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 
Abstract -----------------------------------

Introduction ______ --------------------------

CONTENTS 

Page 

1 

"1 

Geology ___________________________________ _ 

References cited -"----------------------------

ILLUSTRATIONS 

Pa

Pa

Figure 1. Index map of Elko County, Nev., showing location of the Cornucopia mining district ----------- 

2. Geologic sketch map of the Cornucopia mining district -------------------------------------- 

III 

ECONOMIC SIGNIFICANCE OF REVISED AGE RELATIONS OF ROCKS IN THE 

CORNUCOPIA MINING DISTRICT, ELKO COUNTY, NEVADA 

By R. R. COATS 

Abstraci 

Recent geologic work in the Cornucopia mmmg 

district, a small silver-gold mining district in northwestern Elko County, Nev., has resulted in significant 

revision of the geological interpretation. Rocks formerly thought to be premineralization in age, but unmineraliz.ed, are now known to be postmineral extrusives, resting unconformably on the altered andesite 

in which the ore bodies are found. Extensions of the 

.known productive veins may be expected at shallow 

depth beneath the younger volcanic rocks, and are 

separated from the mined part of the veins by postmineral high-angle faults that have brought the 

younger volcanic rocks into fault contact with the 

mineralized andesite. Some veins are apparently terminated against premineral faults. 

INTRODUCTION 

The Cornucopia district is a silver-gold mining district in northwestern Elko County, Nev., 

(;fig. 1). According to Granger, Bell, Simmons, 

and Lee (1957, p. 41) it has had a production 

of about $114 million in gold and silver, ninetenths before 1883. The silver-gold ratio by 

117" 115" ··-··-··- --·-,42" 

Figure 1.-Index map of Elko County, Nev., showing 

location of the Cornucopia mining district. 

1 

weight for the latest productior was. about 

68:1; earlier recon:ls are not adequate to permit 

an esUmate of the ratio. 

GEOLOGY 

The only available geological information is a 

brief report by Emmons ( 1910, p. 63-65), who 

visited the district in 1908 after mining had 

ceased and when most of the workings were 

inaccessible. gmn1ons recognized that the ore 

deposits are in altered a:ndesite r.nd that the 

hills "in the main, are capped with rhyolite and 

obsidian." He described the rhyolite as having 

a "g;lassy, microlitic groundmass and phenocrysts of quartz, oligoclase, pyroxere, magnetite 

and a little hornblende." Some phases of the 

rhyolite were said to be vesicular and some 

pumiceous. Emmons noted that tl'~ accessible 

ore deposits are restricted to the andesite, but 

thought the andesite ~ntruded the rhyolite. 

The mines are no more accessible now than 

they were in 1908. In the course of a brief examination of the surfa~e in 1966, I made the 

sketch map presented herewith (fig. 2). Fortunately, recent surficial prospecting disclosed 

critically important information on the rock 

relations. 

The oldest exposed rock is the andesite in 

which the ore deposits occur. The fresher rocks 

examined in thin sections are pyroxene andesites. Most of the clinopyroxene is fresh augite; 

some patches of chlorite and nontronite appear 

to be pseudomorphs after orthopyr'lxene. More 

highly altered rocks are bleached and partly 

replaced by calcite, pyrite, and se""icite. Some 

of these bleached rocks have nests of secondary 

quartz that simulate quartz phenocrysts in hand 

specimen. 

The andesite and the mineral der'lsits that it 

contains are overlain unconformably by a series 
Base enlarged from U.S. Geological Survey 

S.E. ~Wilson Reservoir (1967) 

0 

CONTOUR INTERVAL 20 FEET DATUM IS MEAN SEA LEVEL 

GeoiOIY by R. R. Colts, 1966 

1 MILE 

EXPLANATION 

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UHCO ~FOifltiiTY 

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dacite ~ .. ,.., ..... ... .,...., ..... ........ .. ~rA.-.I•u 

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ADdellite 

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Fault 

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wur• poritto. u •• 

c•rtatt'. utur ..... eat.•fatllt rq~•fl • 

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Strike and dip of bedl 

Figure 2.-Geologic sketch map of the Cornucopia mining district. 

2 
of rhyolitic and rhyodacitic rocks. The lower 

unit of these rocks, which in some places can 

be seen to rest on the andesite, is a biotitic 

rhyolite ignimbrite, the basal part of which is 

buff wnwelded tuff that grades upward into a 

reddish-brown welded phase. Because of lack 

of exposures the maximum thicknrss in the area 

is not known. The minin1um thickne~s is about 

30 feet-measured in an exposure just west of 

the place where the road passes over the saddle 

which is near the center of sec.-18, T. 42 N., R. 

51 E. The ignimbrite is overlain, possibly unconformably, by a dark, partly glassy, pigeonite 

rhyodacite containing much xenocrystic material. This rock fm111s the summit of Silver Peak 

and the surface of the long north-trending 

ridge immediately south of the mine shafts 

shown on the map. 

The relations of the volcanics of Silver Peak 

to the mineralized andesite are critically important to the interpretation of the geology. 

Because of the prevalence of faulting a:nd the 

ready disintegration of the tuffaceous lower 

portion of the ignin1brite, exposures showing 

the depositional contact are few. The most accessible exposure is in some artificial cqts about 

0.4 mile cast and 0.45 mile north of the southwest corner of sec. 18. As the n1ap reveals, 

f~ulting is prevalent in the Cornucopia area. 

Some of these faults display characteristics 

indicating that at least some of the movement 

on them is premineral; others may also be in 

part premineral, but either cut the mineral deposits, or displace the contact of the volcanics 

of Silver Peak and the mineralized andesite. 

An example of a fault that is probably in 

part, at least, premineral is marked "B" in 

figure 2. At the time of my visit, it was reported by one of the persons engaged irn exploration that the Leopard vein, which has 

been extensively worked under the n1ine dumps 

shown on the map, has been traced northward 

to the point where it is cut off by a silicified 

fault-vein trending rnorthwest (B in fig. 

2). A branch of this fault may be exposed in 

the portrul of an adit on the south side of the 

gully that bounds the mined block on the north. 

The fault at the portal has a strike about N. 

10° W. and dips about 70° southwest. It also 

shows extensive silification. If, as the silicification along it suggests, this fault, or these 

3 

faults, are premineral in age, th~ likelihood of 

finding a continuation of the Leopard ore body 

northeast of them is not great, a.lthough ore is 

kmown to occur in that area. 

The vokanics of Silver Peak may be observed 

in fault contact with the mineralized andesite in 

a series of cuts about 0.4 mHe east of the southwest corner of sec. 18. The buff, poorly consolidated, tuffaceous phase of the ignimbrite forms 

the hanging wall of a fault (A in fig. 2) that 

trends northwest and dips from 36° to 60° 

southwest. The fault trace is marked by a 

brick-red gouge that contains fragments of 

quartz vein material up to 4 inches in diameter. 

In the gully to the east of this point, severaL.. 

cuts expose massive buff rhyolitE. tuff. Obscure 

bedding in the tuff dips west about 45 °. This 

steep westerly dip is thought to be caused by 

drag on a normal fault that must pass just 

east of the cuts and follows the course of the 

north-trending guUy. 

The geologic evidence that suggests that the 

rhyolite and rhyodacite are postandesite is 

confirmed by the petrographic evidence. The 

buff tuffaceous lower part of the Hotite rhyolite 

ignimbrite has a substantial content of clear 

unaltered sanidine phenocrysts, green euhedral 

biotite tablets, phenocrysts of quartz and 

plagioclase, and equant fragment~ of vesicular 

black glass. It is not likely that th~ biotite, sanidine, and glass would have survived unaltered 

while the adjacent andesite was b~ing intensely 

propylitized and sericitized .. 

The lack of mineralization ir. the rhyolite 

and rhyodacite can thus be explained by the 

fact that they are younger thar the andesite 

and the mineralization. The faults that separate the rhyolite and rhyodacite from the mineralized andesite in the mine area are postmineral; the entire ridge that lies southwest of the 

area ·of active mining is part of a block, doW!Ilthrown with respect to the blocks lying to the 

north and east. As the available exposures suggest that the most important ore--bearing veins 

of the district are cut off by ther~ postmineral 

normal faults, extensions of these veins may 

exist beneath the rhyolite and rlcyodacite. Unfortunately, the thicknesses of th~ rhyolite arnd 

rhyodacite units are not well kr0wn both because an unknown amount of postdepositional 

erosion has occurred and because original 
thickness varied g-reatly frmn place to place 

due to deposition of the rhyolite and rhyodacite 

units uporn an irregular surface. Hence, it is 

not possible to give a precise estimate of the 

throw of the fault on the northeast boundary 

(A in fig. 2) of the ridge block that lies south 

of the mine shafts. However, if the tuff of the 

rhyolite and rhyodacite unit, exposed west of 

the road in the west half of sec. 18, corresponds 

to the tuff exposed in the hanging wall of this 

fault, then the buried upper surface of the min4 

eralized andesite may be only a few scores of 

feet below the present groUind surface, and concealed veins may offer attractive exploration 

targets. 

REFERENCES CITED 

Emmons, W. H., 1910, A reconnaissance of some mining camps in Elko, Lander, and Eureka Counties, 

Nevada: U.S. Geol. Survey Bull. 4C3 .. 130 p. 

Granger, A. E., Bell, M. M., Simmons, G. C., and Lee, 

Florence, 1957, Geology and mineral resources of 

Elko County, Nevada: Nevada Bl,r. Mines 'Bull. 

54, 190 p.