Geology of Majuba Hill 

Pershing County, Nevada

GEOLOGICAL SURVEY BULLETIN 1046-1

This report concerns work done on behalf 

of the U. S* Atomic Energy Commission 

and is published with the permission of 

the Commission

Geology of Majuba Hill 

Pershing County, Nevada

By ALBERT F. TRITES, Jr., and RALPH H, THURSTON

CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

GEOLOGICAL SURVEY BULLETIN 1046-1

This report concerns work done on behalf 

of the U. S. Atomic Energy Commission 

and is published with the permission of 

the Commission " -

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1958
UNITED STATES DEPARTMENT OF THE INTERIOR 

FRED A. SEATON, Secretary

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director

For sale by the Superintendent of Documents, U. S. Government Printing Office 

Washington 25, D. G.
CONTENTS

Page

Abstract______________________________________ 183

Introduction._____________________________________________________ 183

History and production________________________________________ 185

Mine workings ________________________________________________ 186

Geology_______________________________________ 186

Sedimentary rocks_____________________________________________ 187

Argillite___-----_-__-___--___-________________.________ 187

Quartzite___________________________________ 187

Limestone..---___________________________________________ 188

Igneous rocks________________________________________ _________ 188

Prerhyolite dikes and sills__ ________________________________ 188

Rhyolitic rocks__-_________________________________________ 188

Earlier rhyolite______________________________________ 189

Rhyolite porphyry___________________________________ 189

Later rhyolite_________-___--__________-__-___-_______- 190

Breccias__________________________________________________ 190

Origin. ____________________________________ 192

Talus, alluvium, and fanglomerate___-__--_J_______^______-______ 193

Structure_______________________________________ 193

Faults_________________________________________ 193

Fault breccias__-_-__________________________---__-________ 195

Hydrothermal alteration__________________________________________ 195

Ore deposits._____________________________________________________ 196

Copper_________________________________________ 196

Tin______________________________________________________ 200

Uranium._________-__-_-______----___________---_---__-____ 201

Suggestions for prospecting.________________________________ 202

Summary_______________________________________________________ 203

Literature cited.__________________________________________________ 203

ILLUSTKATIONS

[Plates In pocket]

PLATE 7. Geologic map and section of Majuba Hill.

8. Geologic map of surface above Middle adit level.

9. Geologic map of Upper and Middle adit levels and sections through

Copper, No. 153, and Myler stopes. 

' 10. Geologic map of lower adit level.

11. Composite level and assay map.

Page

FIGURE 25. Index map showing location of Majuba Hill___________ 184

in

CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

GEOLOGY OF MAJUBA HILL, PERSHING COUNTY, NEVADA

By ALBERT F. TRITES, JK., and RALPH H. THTJRSTON

ABSTRACT

Majuba Hill is a complex plug of rhyolitic rocks of Tertiary (?) age in the central part of the Antelope Range, Pershing County, Nev. The plug intrudes sedimentary rocks of Triassic(?) age, which consist chiefly of argillite with smaller 

amounts of quartzite and impure limestone. It is about 5,000 feet in diameter 

and has apophyses that extend beyond its margin. The rhyolitic rocks are of 

three varieties that are here called earlier rhyolite, rhyolite porphyry, and later 

rhyolite. Five main varieties of intrusive breccia occur in dikes and irregular 

masses cutting the older rhyolitic rocks. The rhyolitic rocks of the plug and the 

intruded sedimentary rocks have been silicifled, sericitized, and tourmalinized.

Three principal preore faults are exposed in the Majuba Hill mine. The largest of these, the Majuba fault, strikes N. 30° W., dips 54°-84° SW., and has a 

displacement of about 155 feet.

Copper and tin are the principal ore metals at Majuba Hill. From 1916 to 1949 

slightly more than 27,000 tons of copper ore was produced from the Majuba Hill 

mine. About 350 tons of tin ore also has been shipped from this mine and small 

quantities of lead-silver and arsenic-silver ore have been produced from properties nearby. Uranium is locally associated with the copper and tin deposits in 

the Majuba Hill mine but has not been mined.

INTRODUCTION

Majuba Hill is in the Antelope (or Cedar) mining district, Pershing 

County, Nev., in the central part of the Antelope Range. The hill is 

about 20 miles west of Imlay, Nev., which is on the Southern Pacific 

Railroad and on Highway 40 (fig. 25); the hill is shown on the nor%- 

centralpart of the Lovelock, Nev., quadrangle 1° topographic map.

The Antelope Range is near the western margin of the Great Basin 

and is about 40 miles east of the Sierra Nevada. The range trends 

almost north and is about 22 miles long and 10 miles wide. It is 

flanked on the east and west by deep, alluvium-filled valleys. Majuba 

Hill, a prominent landmark in the range, rises to an altitude of 6,886 

feet. i

183
184 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

The principal known ore deposits at Majuba Hill are in sec. 2, T. 32 

N., R. 31 E., Mount Diablo base line and meridian (pi. 7), in the vicinity of the Majuba Hill mine. The deposits contain copper, tin, and 

uranium. A prospect in sec. 34, T. 33 N., R. 31 E., yielded a small

Fxoir&B 25. Index map showing location of Majuba Hill, Pershing County, Nev.
GEOLOGY OF MAJUBA HILL, PERSHING COUNTY, NEVADA 185

amount of arsenic-silver ore during the early 1920's. Lead-silver ore 

has been produced from the Last Chance mine of the Majuba Fresno 

Mining Company, in sec. 1, T. 32 N., K. 31E.

The tin and copper deposits at the Majuba Hill mine have been studied by Smith and Gianella (1942), and briefly examined in 1942 by 

James Gilluly and L. K. Page (written communication) and in 1945 

by J. M. Wiese (written communication) of the U. S. Geological Survey. The U. S. Bureau of Mines conducted a drilling and drifting 

program in the deposit in 1943 (Matson, 1948). A brief reconnaissance examination of the mine for uranium was made in 1948 by Donald Wyant (written communication) of the U. S. Geological Survey.

Between January and June 1949 the writers made a more detailed 

study of the uranium deposits in the area as a part of the uranium investigations carried out by the U. S. Geological Survey on behalf of the 

Division of Kaw Materials of the U. S. Atomic Energy Commission.

The geologic map and section by Smith and Gianella (1942), on a 

scale of 1 inch to 1,000 feet, were used as a base (pi. 7), and only the 

main area of intrusive rocks was remapped. A network of triangulation stations was established as a base of resection, stadia, and pace 

surveys. The underground workings were mapped on a scale of 1 inch 

to 20 feet, using Brunton bearings and chaining distances. The fieldwork was under the direction of Thurston; Trites prepared the manuscript and is thus responsible for many of the conclusions. The geologic maps and sections (pis. 7-10) were prepared by Thurston, assisted by Trites; most of the petrographic, mineralogic, and radiometric studies were made by Trites. Charles Milton identified several 

minerals from the Majuba Hill mine. Chemical analyses were made 

in the laboratories of the U. S. Geological Survey.

The writers are indebted to Mr. E. J. Myler, owner of the Majuba 

Hill mine, for assistance and many favors during this study.

HISTOEY AND PEODITCTION

The uranium mineral torbernite (hydrous phosphate of copper and 

uranium) was reported in the copper deposit at Majuba Hill by Smith 

and Gianella (1942, p. 55) who state that many crystals were found 

in the Lower adit, 1,800 feet from the portal. The principal uranium 

deposit at the Majuba Hill mine was exposed between 1942 and 1945 

during the mining of copper ore from the Copper stope.

Since about 1907 tin and copper minerals have been known to be 

present at Majuba Hill. Smith and Gianella (1942, p. 49) reported 

that in 1907 a single piece of cassiterite-bearing float was found on the 

hill although its source was not discovered. However, the Majuba 

Hill copper deposit was located in 1907 by A. J. McCauley, of Imlay
186 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

(Matson, 1948, p. 2) and later was acquired by C. A. Copley and A. L. 

Gilmet. No work of importance was undertaken until 1914, when the 

property was operated by the Mason Valley Mines Company. This 

company has done most of the exploration work at the Majuba Hill 

mine, and between 1916 and 1918 produced about 4,000 tons of ore 

that contained 12 percent copper (Smith and Gianella, 1942, p. 41). 

In 1917 the Mason Valley company discovered a tin deposit underground during a search for copper ore.

The Mason Valley Mines Company ceased work in 1918 and, except 

for a small amount of work by A. J. McCauley who in 1928 shipped 

two cars of ore averaging 14 percent copper (Vanderburg, 1936, p. 10), 

the Majuba Hill mine remained idle until early in 1941 when the Freeport Sulphur Company obtained an option (Smith and Gianella, 1942, 

p. 41). This company conducted development work and core drilling 

to explore the tin-bearing zone but finally abandoned the project in 

October 1941.

The mine remained idle from 1941 until May 1942 when E. J. Myler 

acquired the property under a bond and lease arrangement with 

Harvey Reber who then owned the mine. In October 1942, after some 

exploratory work, Myler assigned a lease to J. O. Greenan and G. W. 

Kerr of Reno, Nev. Between October 1942 and May 1945, Greenan 

and Kerr mined about 23,000 tons of ore that had an average grade of 

about 4 percent copper (Matson, 1948, p. 1). The ore was purchased 

at the American Smelting and Refining Company smelter at Garfield, 

Utah, Greenan and Kerr also mined about 350 tons of ore that contained 2-4 percent tin (Matson, 1948, p. 1). This tin ore was milled 

at a tungsten concentrating plant at Toy, Nev., and was sold to the 

Metals Reserve Company at Battle Mountain, Nev. Greenan and 

Kerr relinquished their lease in May 1945. From May 1945 to May 

1949, Myler mined a small quantity of copper ore.

MINE WORKINGS

The Majuba Hill mine workings are on three levels (pis. 9 and 10) at 

altitudes of 5,774, 6,250, and 6,451 feet. The Lower adit is 2,000 feet 

long and the Upper adit is about 100 feet long. The Middle adit workings have a total length of about 2,000 feet; almost all the ore produced 

by 1949 came from this level. A raise connects the Middle adit to the 

Upper adit.

GEOLOGY

' The Antelope Range is made up of a thick section of argillite, limestone, and quartzite of Triassic(?) age. The Triassic(?) section is 

intruded by granitic stocks and plugs of late Mesozoic age, by rhyolitic
GEOLOGY OF MAJTJBA HILL, PERSHING COUNTY, NEVADA 187

rocks of Tertiary (?) age and by sills and dikes of andesite, dacite, 

iatite, and diorite of pre-Tertiary (?) age. The andesite, dacite, latite, 

and diorite are all older than the rhyolitic rocks of Tertiary (?) age. 

The sedimentary rocks also are cut by veins of quartz, most of which 

are barren and concordant with the bedding.

Majuba Hill, a conspicuous peak in the Antelope Range, is a complex plug about 5,000 feet in diameter that is composed of intrusive 

rhyolitic rocks and many diverse types of intrusive breccias. Repeated igneous intrusion and explosive activity have produced a very 

complex sequence of rocks. Irregular dikes of rhyolite porphyry 

extend for several thousand feet into the sedimentary rocks that surround the plug.

SEDIMENTARY BOOKS

The oldest rocks of Majuba Hill are metamorphosed sedimentary 

rocks of Triassic(?) age. These rocks are at least 5,000 feet thick 

and consist of more than 90 percent argillite and a few beds of quartzite and impure limestone. The beds strike northeast and dip from 

50° SE. to 50° NW. (pi. 7). No fossils have been found in the sedimentary rocks; a Triassic age has been inferred by Smith and 

Gianella (1942, p. 42-43) on the basis of lithologic similarity to 

Triassic rocks in the Eugene Mountains, the next range east of the 

Antelope Range.

ARGILLITE

Argillite crops out over most of the area and is in contact with, or 

forms inclusions in, the intrusive rhyolitic rocks and breccias on the 

flanks of Majuba Hill. The argillite is thin bedded and ranges from 

dark blue to light gray. It is composed principally of clay minerals 

and quartz and subordinately of mica and scattered crystals of pyrite. 

In places the argillite has been altered to brown or red slate, phyllite, 

and hornfels for distances as much as 100 feet from intrusive rocks.

QTTARTZITE

Lenticular beds of brown to buff, finely granular, micaceous, metamorphic quartzite constitute less than 1 percent of the sedimentary 

rocks. Individual beds range in thickness from a few inches to 6 feet 

and are seldom traceable along the strike for more than 100 feet. The 

beds weather to a reddish buff and may be distinguished easily from 

other rocks of the vicinity. The quartzite is made up principally of 

interlocking grains of quartz and a small number of grains of chert. 

The quartz grains have an average size of about 0.2 mm and have 

an undulating extinction. In quartzite beds near the rhyolite mass, 

tourmaline has been introduced and replaces some of the quartz grains.

457381 58 2
188 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

LIMESTONE

A few beds of gray medium-grained argillaceous limestone, as much 

as 3 inches thick, were intersected in the Lower adit level of the Majuba Hill mine. One 6-inch bed of calcite-epidote rock near a rhyolite 

porphyry dike is believed to represent an altered limestone bed.

IGNEOUS BOOKS

The igneous rocks of Majuba Hill (pi. 7) consist of prerhyolite dikes 

and sills, and intrusive rhyolitic rocks of Tertiary (?) age. The writers 

have divided the rhyolitic rocks into three varieties on the basis of the 

abundance and sizes of phenocrysts. These are, from oldest to youngest, earlier rhyolite, rhyolite porphyry, and later rhyolite.

PRERHYOLITE DIKES AND SILLS

Dikes and sills of andesite, dacite, latite, and diorite of preTertiary(?) age are most abundant east and south of Majuba Hill. 

Outside the area mapped, fine-grained porphyritic to coarse-grained 

granite intrudes the sedimentary rocks. All of these igneous rocks 

form short lenticular sills that range in length from 1 to over 100 feet; 

they have an average thickness of 15 feet. Fresh surfaces of these 

rocks are grayish green to green; weathered surfaces are light tan or 

brown. A blocky to slightly platy jointing is well developed in them. 

Sericite, chlorite, calcite, and quartz occur as alteration products in 

many of these rocks. They are all believed to be older than the rhyolitic rocks of the Majuba Hill plug; fragments of them are contained 

as xenoliths in some of the rhyolitic rocks and many of the breccias of 

the plug.

RHYOLITIC ROCES

The rhyolitic rocks of Tertiary (?) age, exclusive of the rhyolitic 

breccias, in order of their intrusion, are: earlier rhyolite, rhyolite 

porphyry, and later rhyolite. The distinguishing features of the three 

varieties are given below:

Grain size of Size of Abundance of

groundmass phenocrysts phenocrysts

Rock (mm) (mm) (percent)

Earlier rhyolite______________ 0. 02 0. 1-4 0 2-10

Ehyolite porphyry_______ ____ .05 . 2-5. 0 20-40

Later rhyolite______________ .05 .5 2-10

All the rhyolitic rocks are silicified and sericitized; the earlier rhyolite and the rhyolite porphyry are tourmalinized in the central parts of 

the Majuba Hill plug.
GEOLOGY OF MAJUBA HILL, PERSHING COUNTY, NEVADA 189 

EARLIER RHYOLITE

The earlier rhyolite forms large outcrops on the crest of the hill and 

has been penetrated by the three adits of the Majuba Hill mine. The 

earlier rhyolite is cut by the later rhyolite, rhyolite porphyry, and 

many of the breccias and occurs as inclusions in both the later rhyolite 

and breccias. It contains inclusions of argillite and quartzite, as much 

as 4 inches wide, especially near its contacts with the sedimentary 

rocks.

The earlier rhyolite is a light-gray rock, but it is light tan or brown 

where weathered, nearly white to light gray where it is silicified or 

sericitized, and mottled gray and black where it is intensely tourmalinized. From 1 to 7 percent of the rock is quartz phenocrysts, 1 to 3 percent sanidine phenocrysts, and less than 2 percent plagioclase phenocrysts. The phenocrysts range in length from 0.1 to 4 mm, the average 

being about 2 mm. The quartz phenocrysts are characteristically welldeveloped, doubly terminated crystals. The groundmass consists 

mainly of quartz, sanidine, and sericite and has an average grain size 

of about 0.02 mm.

RHYOLITE PORPHYRY

The rhyolite porphyry forms an irregular mass within the plug of 

Majuba Hill where it constitutes about 15 percent of the total igneous 

rock exposed. Sills and dikes of rhyolite porphyry occur in the surrounding argillite. Some of the larger dikes (pi. 7) are wider in the 

argillite than in the plug.

The contacts of the rhyolite porphyry with the argillite and the 

earlier rhyolite are sharp, and the rhyolite porphyry shows only a 

slight amount of chilling. These contacts are irregular and dip vertically or steeply to the southeast. Where the rhyolite porphyry is in 

contact with some of the larger masses of breccias, it fingers into the 

breccia, forming an irregular contact zone.

The rhyolite porphyry contains xenoliths of argillite, earlier rhyolite, and rhyolite-bearing breccias. A few rhyolite porphyry dikes are 

separated from the earlier rhyolite by rims of rhyolite-bearing breccias.

The freshly exposed rhyolite porphyry is light gray; it is white or 

very light gray where bleached, or dark gray to mottled gray and black 

where tourmalinized. The rock contains 20 to 40 percent phenocrysts, 

which are 0.2 to 5 mm wide and are mainly quartz with smaller quantities of sanidine and plagioclase. These phenocrysts are set in a 

groundmass of quartz, sanidine, and plagioclase having a grain size of 

about 0.05 mm. Many of the quartz phenocrysts are present in the 

fine-grained groundmass as double bipyramidal crystals. The rhyolite 

porphyry is distinguished from the earlier rhyolite by a larger number 

of phenocrysts and a slightly coarser groundmass.
190 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

LATER BHYOLITE

The later rhyolite crops out over a large area along the southeastern 

margin of the rhyolitic plug where it contains large xenoliths of earlier 

rhyolite and breccia, but has not been penetrated in the Majuba Hill 

mine workings. All observed contacts between the later rhyolite and 

other rocks dip 80° or more. The later rhyolite has been fractured by 

closely spaced joints which have produced a bricklike rubble surface on 

many exposures.

The typical unaltered rock is white or light yellow on both fresh and 

weathered surfaces and is generally lighter in color than the other 

rhyolitic rocks in the area. The later rhyolite has smaller phenocrysts 

than either the earlier rhyolite or the rhyolite porphyry. The phenocrysts in the later rhyolite are of quartz, sanidine, and plagioclase. 

They average 0.5 mm in diameter and constitute from 2 to 10 percent 

of the rock. The groundmass has a grain size of about 0.05 mm.

Much of the later rhyolite has been sericitized; secondary quartz has 

been introduced locally, but silicification is less characteristic of the 

later rhyolite than of the earlier rhyolite and the rhyolite porphyry. 

Although tonrmalinized xenoliths of the earlier rhyolite and rhyolite 

porphyry are present in the later rhyolite, the later rhyolite has not 

been tourmalinized.

BRECCIAS

Dikelike bodies of breccias composed of chaotic mixtures of rubbly 

material occur throughout the rhyolitic plug. These breccias contain 

fragments of the sedimentary rocks and of both the earlier rhyolite and 

rhyolite porphyry. The fragments range in size from a fraction of an 

inch to 9 feet; fragments of rhyolitic rocks are commonly rounded, 

but those of argillite are sharply angular and of smaller sizes. Some 

of the more salient features of the breccias are listed below:

1. The breccias occur both as regular and irregular dikes and as irregular 

masses at dike intersections. The dikes range in length from 10 to more than 

1,000 feet and in width from 4 inches to 60 feet.

2. The breccias trend in diverse directions and dip nearly vertically. Some 

of the dikes, especially those near the outer rim of the plug, have an arcuate 

or circumferential pattern. The breccia dikes near the center of the plug have 

a predominant N. 30°-40° W. trend and are parallel or almost parallel to the 

Majuba fault, a large postrhyolite structure exposed in the workings of the Majuba Hill mine. Many other breccia dikes within the plug trend N. 5°-15° E., 

or N. 70°-85° B. roughly at 45° to the major trend.

3. The breccias contain heterogeneous assemblages of well-rounded to angular 

fragments of all rocks cut by them. The longer axes of many of the angular fragments are oriented parallel to the dip of the contact, but the kind of fragments 

does not necessarily conform to the wall rock.
GEOLOGY OF MAJUBA HILL, PERSHING COUNTY, NEVADA 191

4. At least three, and possibly as many as five, different ages of breccias have 

been determined. The formation of all, except perhaps the last, of these breccias 

was followed by the injection of rhyolitic rocks.

5. The contacts of the breccia dikes with the rhyolitic rocks range from regular 

to very irregular. The contacts are relatively sharp where the breccia is in 

contact with argillite or prebreccia rhyolite and gradational where the breccia is 

in contact with postbreccia rhyolite which has sent apophyses into the breccia.

6. Rhyolitic rock has been locally injected into some of the earlier breccias, 

filling the spaces between the fragments. There seems to be a complete gradation between breccias with such a rhyolitic groundmass and rhyolitic rocks with 

abundant xenoliths.

7. Many of the breccias have been silicified, sericitized, and tourmalinized.

Five main varieties of breccia have been distinguished. These five 

varieties, classified according to rock fragments, are: Breccia 1, composed principally of fragments of argillite; breccia 2, composed principally of fragments of earlier rhyolite; breccia 3, composed of fragments of argillite and earlier rhyolite; breccia 4, composed of 

fragments of argillite, earlier rhyolite, and rhyolite porphyry; and 

breccia 5, composed of fragments of earlier rhyolite and rhyolite 

porphyry. The distribution of the breccias is so complex that the individual varieties are not shown on the surface or underground maps.

The order of formation of rhyolitic rocks and breccias of Tertiary 

(?) age at Majuba Hill is believed to be as follows:

1. Breccia 1 (oldest) 5. Rhyolite porphyry

2. Earlier rhyolite 6. Breccia 4

3. Breccia 2 7. Breccia 5

4. BrecciaS 8. Later rhyolite (youngest)

Breccia 1. The argillite breccia occurs as discontinuous segments 

that are as much as 100 feet wide near the contacts of the earlier rhyolite of the plug with argillite wall rock. The rock ranges from white or 

light gray to dark blue or black, and consists of fragments of argillite 

with minor amounts of quartzite, limestone, prerhyolite quartz veins, 

and earlier rhyolite. Earlier rhyolite and rhyolite porphyry have been 

injected locally into breccia 1.

Breccia 2. The fragmental constituents of breccia 2 are subrounded, have a maximum size of 5 inches, and are dominantly of 

earlier rhyolite. Breccia 2 is exposed near the northeast part of the 

summit of Majuba Hill. A large part of breccia 2 has been injected 

by rhyolite porphyry and is intensely silicified and sericitized but not 

tourmalinized.

BrecciaS. Fragments of both argillite and earlier rhyolite compose 

breccia 3, which is the most abundant and widespread breccia at Majuba Hill. At the Majuba Hill mine it forms dikes that cut argillite 

and an interconnecting network that cuts the earlier rhyolite of the 

plug. The breccia is light gray to white and contains fragments of
192 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

argillite and earlier rhyolite in varying proportions. Locally breccia 

3 has been injected by earlier rhyolite and rhyolite porphyry, and the 

rock generally has been silicified, sericitized, and tourmalinized.

Breccia 4- Fragments of argillite, earlier rhyolite, and rhyolite 

porphyry constitute breccia 4. This breccia forms the largest dikes in 

the area and has been cut in all the stopes of the Majuba Hill mine. 

The dikes exposed in the mine workings trend from north to northeast 

and dip from 7° to 80° east or southeast. A prominent outcrop of 

breccia 4 immediately north of the Upper adit is about 195 feet long 

and as much as 75 feet wide. This exposure of breccia is bounded on 

all sides by faults (pi. 8, coordinates 1800 N. and 400 W.). Breccia 4 

has been intensely tourmalinized, as well as silicified and sericitized.

Breccia 6. Fragments of earlier rhyolite and rhyolite porphyry 

predominate in breccia 5, which crops out in an irregular area of about 

2,500 square feet near the center of the plug. The rock is mottled 

black and white, and has weathered to a rubble of rounded fragments. 

Tt has been silicified, sericitized and tourmalinized.

ORIGIN

Many of the breccias are believed to have been formed by gases attendant with the intrusion of the rhyolitic rocks in the plug. Breccia 

1 (argillite breccia) which surrounds much of the plug may have been 

formed during the initial explosive action, and a large number of the 

breccia dikes probably represent material that was pushed ahead of the 

advancing rhyolite. The chaotic nature of many of the breccias and 

the large range in size of fragments further suggest that explosive 

action was responsible for their origin. The lack of gradation between many of the breccias into the argillite country rock and earlier 

formed rhyolitic rocks, with little evidence of plucking along the walls, 

also suggests vigorous activity.

The main body of earlier rhyolite was injected following the initial 

brecciation of the argillite, but the presence of some fragments of earlier rhyolite in breccia 1 indicates that part of this rhyolite had solidified prior to the explosive activity. This rhyolite perhaps assimilated 

many of the shattered, displaced rocks and filled the interstices of much 

of the breccia 1. The vertical direction of flow and gaseous activity is 

suggested by the vertical orientation of xenoliths along the edges of the 

rhyolite dikes and by the nearly vertical walls of the breccia dikes.

Following the injection of the earlier rhyolite, another period of 

gaseous activity may have ensued during which some of the earlier 

rhyolite was brecciated and breccias 2 and 3 were formed. A second 

pulse of rhyolitic magma then injected the rhyolite porphyry in the 

form of dikes and masses which cut the earlier rhyolite and locally 

impregnated the three earlier formed breccias. Apparently a third
GEOLOGY OF MAJUBA HILL, PERSHING COUNTY, NEVADA 193

main period of gaseous activity followed the injection of the rhyolite 

to form breccias containing fragments of rhyolite porphyry (breccias 

4 and 5). This was apparently the last step of violent brecciation activity and was followed by the injection of the later rhyolite on the 

southeast side of the hill.

Burbank (1941, p. 170-178) believes that the passage of hot juvenile 

gases along joints or fissures formed many of the volcanic pipes of the 

Red Mountain district, Colorado. These volcanic emanations blowpiped their way upward, locally brecciating the rock as they moved 

and producing cylindrical-shaped bodies of breccia. While such action may have occurred in the formation of the breccias at Majuba 

Hill, the dikelike shapes of the breccia bodies and their heterogeneousness would seem to negate the importance of such gaseous action.

TALUS, ALLUVIUM, AND FANGLOMERATE

Unconsolidated deposits of talus, alluvium, and fanglomerate of 

Quaternary age overlie much of the bedrock at Majuba Hill. Talus 

occurs on the steeper slopes of the hill itself, covering much of the rhyolitic rocks and grading into fanglomerates near the base of the hill. 

Alluvium occurs in patches along the stream valleys on the north and 

south sides of the hill.

STRUCTURE

The sedimentary rocks of Triassic(?) age strike generally N. 

45°-70° E., and dip 50°-90° NW., except in the southwest quarter 

of the area (pi. 7) where the rocks strike from N. 5° W. to N. 25° E. 

and dip 75° SW. or 60°-90° NW. Locally the rocks are highly 

crumpled and overturned near contacts with igneous rock. The 

argillites are intruded by all the igneous rocks and contain concordant quartz veins.

The dominant structural feature is the rhyolitic plug, roughly 5,000 

feet in diameter, containing dikes and irregular masses of breccia. 

Ehyolite porphyry dikes extend across the earlier rhyolite into the 

argillite to the northeast and southwest beyond the margin of the plug, 

and later rhyolite occurs as an irregular body along the southeast 

margin of the plug.

FAULTS

Three main faults, the Majuba fault, the No. 211 fault, and the Myler 

fault, are exposed in the Majuba Hill mine. The Majuba fault is exposed in each of the three levels (pis. 9 and 10) and may crop out at one 

place on the surface above the mine workings. The fault strikes N. 30° 

W. and 54°-84° SW.; its fault gouge and breccia zone thickens 

from less than 3 feet at the surface and in the Upper adit to 10 feet in
194 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

the Lower adit. According to Smith and Gianella (1948, p. 48), the 

displacement along the Majuba fault is between 30 and 150 feet. They 

say:

In the drift that extends northwestward along the fault from the end of the west 

crosscut on the lowest level * * * flutings show that the hanging wall moved 

obliquely downward and southward at an angle diverging 35° from a line pointing straight down the dip. The amount of displacement in this direction is 

certainly over 30 feet but probably less than 150 feet, as indicated by the contact 

between slate and rhyolite porphyry that is exposed in the footwall, east on the 

east side of the drift.

The writers believe that fault 211 has been displaced by the Majuba 

fault as shown in plate 9. The displacement of this fault measured 

along the Majuba fault is about 155 feet. The Majuba fault has been 

displaced in the Middle adit level by a nearly parallel fault which is 

exposed in drift 201-N and in crosscuts 215 and 217 (pi. 9). The west 

side of the Majuba fault has been moved 70 feet northward relative 

to the east side along the fault. The Majuba fault has apparently 

been terminated at its south end by a cross-fault exposed in drift 

201-S. Although the Majuba fault may have been displaced rather 

than terminated along this cross-fault, such a faulted segment has not 

been found in either drift 201-S or the Middle adit. The Majuba 

fault zone pinches north of crosscut 215 where it continues as a much 

smaller fault beyond the end of drift 201-N.

The Majuba fault is believed to have been formed before the introduction of copper, tin, and uranium minerals, because primary sufides 

have been found in the gouge along the fault. Unbroken crystals of 

tourmaline appear to substantiate this premineralization movement 

along the fault.

Fault 211 is exposed in the Middle adit level about 390 feet from the 

portal, in crosscut 211 about 20 feet east of the adit, and at the head 

of the "G" raise about 53 feet above the adit level. A fault which is 

exposed beneath the Copper stope in the sublevel 43 feet below the 

Middle adit level (pi. 9) is believed to be a segment of fault 211 displaced downward by the Majuba fault.

A fault, called the Myler fault, is exposed only in the Myler stope 

(pi. 9) where it has about 4 feet of gouge and fault breccia. This 

fault strikes N. 5°-20° W. and dips 60°-63° SW. As the Myler 

fault does not occur in crosscut 211, it is inferred to end against fault 

211 about 30 feet above crosscut 211 (section A-A', pi. 9).

Several diversely oriented faults having small displacements are 

widely distributed within the plug. A few of these faults can be 

traced beyond the periphery of the plug, but do not extend for more 

than 100 feet into the surrounding argillites.
GEOLOGY OF MAJTJBA HILL, PERSHING COUNTY, NEVADA 195

Fault movement may have taken place at intervals over a considerable period of time. It is believed that there was movement on all the 

faults after the intrusion of the three rhyolitic rocks and the five breccias. The fault zones are sharply defined in the earlier rhyolite and 

the rhyolite porphyry and are less obvious in dikes and masses of

" l 1 J'Ti breccias. The main component of the movement was vertical, and tne 

principal faulting is believed to have resulted from the readjustment 

of the rocks of the plug after intrusion and explosion had ceased.

FAULT BRECCIAS

Breccias that range in thickness from *4 inch to 10 feet, and 

that contain fragments of all the rocks cut by the faults, are exposed 

along many of the faults at Majuba Hill. Individual breccia fragments range in diameter from yi6 to 8 inches, and most of the 

fragments are sharply angular. Sericite and kaolinite are commonly abundant in the gouge associated with the breccia; quartz 

and tourmaline have been introduced locally.

HYDROTHERMAL ALTERATION

Hydrothermal alteration, associated with the rhyolitic rocks and 

breccias, has affected the rocks in the plug as well as the argillites surrounding the plug. The three major types of alteration, from oldest 

to youngest, are silicification, sericitization, and tourmalinization.

Silicification is believed to have been the first and most widespread 

alteration and was apparently of two or more stages. Granular 

quartz has filled many cavities formed by the leaching of sanidine and 

plagioclase phenocrysts of the rhyolitic rocks; quartz and sericite form 

the matrix of some of the altered rhyolite. Veinlets of later quartz 

locally cut the quartz-sericite groundmass in some of the earlier 

rhyolite.

Sericitization has affected all of the rhyolitic rocks of Majuba Hill. 

Sericite constitutes 30 to 60 percent of the groundmass of many of the 

altered rhyolitic rocks and is abundant in intrusive breccia and in the 

argillite adjacent to rhyolitic rocks. Sericite has replaced many of the 

phenocrysts of plagioclase and sanidine, quartz in feldspar voids, and 

feldspar and quartz of the groundmass.

Tourmalinization has been most intense near the breccias; tourmaline is abundant in the rhyolite porphyry and in the breccias, less 

abundant in the earlier rhyolite, and apparently absent in the 

later rhyolite. Irregular grains, needles, and rosettes of tourmaline, as 

much as 2 mm across, incompletely fill the voids left in the rhyolitic 

rocks by leaching of the feldspar phenocrysts. A mixture of quartz 

and tourmaline completely fills some of these voids. The tourmaline
196 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

in the intrusive breccias forms grains and rosettes as much as 5 inches 

in greatest dimension.

Prisms of epidote have formed in the alteration of beds of impure 

limestone near the rhyolitic rocks of the plug. One bed of calcite and 

epidote cut by the Lower adit is believed to be such an altered limestone. 

Zoisite is an alteration product in some of the altered argillite and 

breccia. The zoisite is in the form of microscopic prismatic crystals 

that are grouped together in bundles replacing the fragments of the 

breccias and the clay minerals of the argillite.

ORE DEPOSITS

The principal ore deposits at Majuba Hill are those of copper, tin, 

and uranium exposed in the Majuba Hill mine. The arsenic-silver 

deposit in sec. 34, T. 33 JST., R. 31 E., was inaccessible at the tune of the 

writers' visit and will not be discussed in this report. The lead deposit 

at the Last Chance mine was examined for uranium but was not studied 

in detail because no abnormal radioactivity was found.

All samples collected at the Majuba Hill mine contain some gold 

and silver. The gold content was invariably low, ranging from a trace 

to 0.05 ounce per ton; silver ranged from 0.10 to 11.08 ounces per ton 

in individual samples. The highest grade silver-bearing rock is in the 

breccia exposed in the stope 211. Spectrographic analyses of samples 

are shown in the table on pages 198-199.

COPPER

Copper ore has been mined from the Copper, Myler, and No. 153 

stopes of the Middle adit level. Smith and Gianella (1942, p. 54) 

stated that the 4,000 tons of 12 percent copper ore mined in 1915-18 

came from a large stope above and a small stope below the Middle 

adit. In this report the large stope is called the Copper stope, and 

the small stope 43 feet below the Middle adit level is called the sublevel. A large part of the 23,000 tons of copper ore of slightly less than 

4 percent copper produced in 1942-45 (Matson, 1948, p. 1) is believed 

to have been mined from the Copper and No. 153 stopes. A small 

amount of copper ore was produced in 1949 from the Myler stope. 

Mining has left only small remnants of ore in all except the Myler 

stope.

The copper ore bodies have been formed by the enrichment of copper 

in a relatively narrow zone beneath an oxidized zone. The primary 

copper sulfide minerals have been leached near the surface and copper 

has been enriched by supergene processes beneath the oxidized zone. 

The zones of oxidation and enrichment are not well defined. Complete 

oxidation extends to about 60 feet below the surface, but some oxida-
GEOLOGY OF MAJTJBA HILL, PERSHING COUNTY, NEVADA 197

tion extends to the greatest depth of exploration, nearly 1,000 feet. 

The supergene enrichment zone extends from the oxidized zone to a 

maximum depth of slightly more than 200 feet below the surface.

MINERALOGY

Most of the copper ore is composed of chalcocite with smaller 

amounts of chalcopyrite and cuprite. Abundant azurite, brochantite, 

malachite, chalcanthite, and chrysacolla occur locally in the upper 

parts of some of the ore bodies. The principal metallic gangue minerals associated with the chalcocite are arsenopyrite and pyrite; nonmetallic gangue minerals include quartz, iron oxides, and tourmaline. 

Locally chalcocite is associated with cassiterite and metazeunerite.

The hypogene copper sulfide chalcopyrite in association with the 

other hypogene minerals arsenopyrite and pyrite occurs as scattered 

specks in much of the ore. Arsenopyrite is notably abundant in the 

copper- and tin-bearing vein in the Copper stope. Charles Milton 

(1950, written communication) found that the arsenopyrite was 

distinguished readily by its brilliant silvery-white color, hardness, 

and strong anisotropy. Small anhedra of pyrite are widely scattered through the copper ore bodies.

Chalcocite is the most abundant copper mineral in the Majuba Hill 

mine and is of supergene origin. Much of the chalcocite is in fissures 

in chalcopyrite, arsenopyrite, and pyrite. Milton found the chalcocite to be gray with a bluish tint in reflected light and apparently 

isotropic. Smith and Gianella (1942, p. 54) have reported that small 

amounts of covellite are associated with the chalcocite.

The oxidized copper minerals cuprite, azurite, brochantite, malachite, chalcanthite, and chrysacolla occur with the iron oxides in the 

oxidized zone and the upper part of the supergene zone. Cuprite is 

especially abundant in the breccia exposed in the Myler stope. 

Columns of blue chalcanthite crystals have formed efflorescent coatings on many of the walls of the Copper stope, and emerald green 

crystals of brochantite have been deposited in some of the breccias 

and fault gouge.

Scorodite, olivenite, and chalcophyllite have been reported by Smith 

and Gianella (1942, p. 55). The scorodite, a hydrous ferric arsenate, 

occurs locally with the oxidized copper minerals. It is abundant 

in the winze at the Tin stope where it has formed thin green to brown 

coatings on altered earlier rhyolite. Olivenite, a basic copper phosphate, has formed dark-green crusts and microscopic veinlets in the 

rock in many places in the two upper levels. Some of the olivenite 

is in felted masses of extremely small acicular crystals. Larger crystals are bright green under the microscope and are doubly terminated.
198 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

1. Semiquantitive spectrographic analyses ofsam

[Analytical data are visual estimates and indicate these relative orders of magnitude: (10) 10 percent 

percent. "0" indicates element looked for but not found. No thorium was

Field 

No.

T-5

-6

-7

0

-9

-10

-11

-12

-13 

-14

-15 

-16

-17

-18 

-19

-20

-21

-22 

-23

-24

-25

-26

-27

Location

Do .

Raise 211 

Do 

Southwest wall of drift 202__

Northwest wall of Myler 

stope. 

Southeast wall of stope 211. .

Southwest wall of stope 211-.

Do 

South wall of Copper stope- - 

Do ................ ...

Do ..

Do 

Northwest wall of Copper 

stope.

East wall of stope 153 .......

Do ................ ...

Rock

 . _do . 

 ..do 

rhyolite porphyry. 

 ..do 

 ..do 

.....do... .. _.. ......

Copper-bearing breccia 4 ... .

Copper-bearing rhyolite porphyry.

- .do ...... .. . . -

Copper-bearing rhyolite porphyry. 

. .do 

 . .do .-. - .....

vein. 

Copper-bearing breccia 4.. ....

 ..do .

phyry.

Type of 

sample

Grab.. ........

 .do ...

 ..do 

12-foot

channel. 

14-foot

channel. 

8-foot channel.

6-foot channel .

Bulk.......... 

8-foot channel.

15-foot 

channel.

6-foot channel.

3-foot channel. 

10-foot

channel. 

12-foot

channel.

10-foot 

channel.

Bulk..........

. .do ......

 ..do 

Si02

10

10

10

10

10

10

10

10

10 

10

10 

10

10

10 

10

10

10

10 

10

10

10

10

10

AlaOs

10

10

10

10

10

10

10

10

10 

10

10 

10

10

10 

10

10

10

10 

10

10

10

10

10

FeaOa

10

10

10

1.0

1.0

10

10

10

10

10

10 

10

10

10 

10

10

10

10 

10

10

10

10

10

Chalcophyllite, a hydrous copper arsenate of variable composition, 

occurs locally with malachite and azurite in the Middle adit level. The 

mineral has a platy habit and superficially resembles metazeunerite; 

the crystals have an excellent basal cleavage. Milton found the chalcophyllite to be optically negative, uniaxial, with moderate birefringence and drab greenish-yellow and bright bluish-green dichroism. 

The clinoclase, a copper arsenate, at the Majuba Hill mine has been 

described by Gianella (1946). The mineral is associated with malachite, azurite, olivenite, and hydrated oxides of iron; it occurs in dark 

blue-green radiating crystals as much as 6 mm long, as tufts of small 

crystals, and as drusy incrustations in fractures in tourmalinized 

rhyolite.
GEOLOGY OF MAJUBA HILL, PERSHING COUNTY, NEVADA 199

flea, Majuba Hitt mine, Pershing County, Nev.

or more: (1.0) 1 to 10 percent; (0.1) 0.1 to 1 percent; (0.01) 0.01 to 0.1 percent; and (0.001) 0.001 to 0.01 

observed in any of the samples. Analyses by the U. 8. Geological Survey.]

CaO

0.1

.1

.1

.1

.1

.1

.1

.1

1.0

0

0

.1

.1

.1

.1

0

0

0

.1

0

1.0

.1

0

MgO

1.0

1.0

.1

1.0

1.0

1.0

1.0

1.0

1.0

1.0

1.0

1.0

1.0

1.0

1.0

1.0

1.0

1.0

.1

.1

1.0

1.0

1.0

Ag

0.1

.1

.1

.01

.01

.01

.01

.001

.01

.1

.001

.01

.001

.001

.001

.001

.001

.001

0

.01

.01

.01

.001

BsOs

1.0

1.0

1.0

1.0

1.0

1.0

1.0

1.0

.1

.1

.1

.1

.1

.01

.01

.01

.01

.01

.1

1.0

.1

.1

.01

BijOa

0.01

.01

.01

.001

.001

.01

.01

0

0

.001

0

0

0

0

0

0

0

0

0

.001

0

0

0

CojOi

0

0

0

0

0

0

0

0

.01

0

.01

.01

0

0

0

0

0

0

0

0

.001

.001

.001

CrsOs

0

0

0

.001

0

0

0

.001

.01

0

.001

0

0

0

0

0

0

0

0

0

0

0

0

Cu

1.0

1.0

1.0

.1

.1

1.0

1.0

.1

1.0

1.0

1.0

1.0

1.0

1.0

1.0

1.0

1.0

1.0

.1

1.0

1.0

1.0

1.0

MnO

0.1

.1

.1

.1

.1

.1

.1

.1

.01

.01

.01

.01

.01

.01

.01

.01

.01

.01

.01

.01

.01

.001

.01

MoO

0.001

.001

.001

0

.001

.001

.001

.001

.01

.001

.001

.001

.001

0

.001

.001

.001

.001

.001

0

.01

.001

0

PbO

0

0

0

0

0

0

0

0

.01

0

0

0

0

0

0

0

0

0

0

0

0

0

0

SnOs

0.1

1.0

1.0

.01

.1

.01

.01

.01

.01

.01

.01

.01

.001

.001

.001

.001

.001

.001

.001

.1

.01

.001

.001

TiOa

0.1

.1

.1

1.0

1.0

1.0

1.0

1.0

.1

.1

.1

.1

.01

.01

.01

.01

.01

.01

.01

.1

.1

.1

.01

Va05

0.01

.01

.01

.1

.01

.01

.001

.01

.001

.001

.001

.001

0

.001

0

0

.001

.001

.001

.001

.001

.001

.001

ZnO

0

0

.001

.001

0

.001

.001

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

ZrOj

0.1

.1

.1

.1

.1

.1

.01

.1

.01

.01

.01

.01

.01

.001

.001

.001

.001

.001

.001

.001

.001

.001

.001

ORIGIN

Smith and Gianella (1942, p. 55) believed that the normal fault 

{Majuba fault) localized the deposition of the copper ore. The 

primary sulfide mineral, chalcopyrite, was deposited with arsenopyrite 

and pyrite along fissures in the faulted rhyolites and as replacements 

of rhyolite and breccia. The writers agree that the Majuba fault was 

the largest structure controlling the deposition of the copper deposits. 

Small faults, such as the two low-angle faults in Myler stope, are believed to have localized smaller ore bodies. The primary sulfides were 

replaced by secondary copper minerals, especially chalcocite, which 

enriched the copper-bearing rock to ore grade.

BESEBVES

Nearly all the known copper ore bodies have been mined out at the 

Majuba Hill mine. A small tonnage of relatively low-grade ore is
200 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

believed to be present in the vicinity of the Myler stope, and a very 

small tonnage of higher grade ore is present in a uraniferous tin- and 

copper-bearing vein in the Copper stope.

TIN

The 350 tons of 2 to 4 percent tin ore produced in 1942-45 (Matson, 

1948, p. 1) was mined from a tin shoot exposed on the Middle adit level 

(pi. 9). Two composite samples of the tin-bearing rock collected from 

the Tin stope by the writers contained 0.41 and 0.78 percent tin (pi. 11). 

This is believed to represent the largest tin deposit in the mine. The 

uranif erous tin- and copper-bearing vein in the Copper stope contains 

about 0.47 percent tin; mineralized rhyolite and breccia in stope 211 

(pis. 9 and 11) contain from 0.01 to 1.14 percent tin; samples of mineralized rhyolite and breccia collected elsewhere in the mine contained 

much lower grade tin, from 0.001 to 0.10 percent.

MINERALOGY

Cassiterite (tin dioxide) is the only tin mineral that has been found 

at Majuba Hill. It is found in the oxidized zone as well as in the 

primary sulfide zone and the zone of supergene enrichment of copper. 

Cassiterite is most conspicuous in the tin shoot exposed in the Tin stope 

(pi. 9), but has also been observed in mineralized breccias and rhyolitic 

rocks elsewhere in the mine. The Cassiterite in the tin shoot is associated with quartz, sericite, iron oxides, tourmaline, and fluorite. Some 

Cassiterite is present in the copper sufide ores in which it is associated 

with chalcocite, chalcopyrite, pyrite, and arsenopyrite.

Euhedral crystals of dark-brown to yellow-brown Cassiterite, less 

than 0.1 inch wide, are most abundant in tourmaline-bearing breccia 

in which many of the crystals rest on tourmaline prisms. Some pores 

left in the earlier rhyolite and rhyolite porphyry by the leaching away 

of the feldspar phenocrysts are partly filled by cassiterite and tourmaline. Smith and Gianella (1942, p. 49) noted that generally the 

quartz, tourmaline, sericite, and cassiterite were intimately intergrown, 

but that locally cassiterite-quartz veinlets cut through tourmaline 

crystals.

ORIGIN

Smith and Gianella (1942, p. 47) suggested that the tin and copper 

deposits were formed either during two separate periods of mineralization or during two distinct phases of a single protracted period of mineralization. They suggested that the cassiterite was formed before 

the sulfide minerals during a late stage of the quartz-sericite-tourmaline mineralization, and that the cassiterite is slightly younger than 

the tourmaline. They believed the tin ore shoot to have been displaced
GEOLOGY OF MAJTJBA HILL, PERSHING COUNTY, NEVADA 201

by the Majuba fault. James Gilluly and L. K. Page (written communication) agreed with Smith and Gianella that the cassiterite was 

younger than the tourmaline but considered both tin and copper to be 

of post-Majuba fault age and concluded that no faulted segment of the 

tin shoot exists.

The present writers agree with Smith and Gianella and with Gilluly 

and Page that the cassiterite seems to be localized in rubbly, vuggy 

breccia, especially near the Majuba fault. They also agree with Gilluly and Page that the tin mineralization was post-Ma juba fault in 

age. The general paragenetic sequence of tourmaline, cassiterite, and 

sulfide minerals was confirmed by the field observations of the writers. 

However, the tourmaline found coating sulfide minerals and cassiterite 

along the edges of vugs indicates that the period of its deposition may 

have overlapped the period of deposition of the sulfide minerals and 

cassiterite, or that it may have been deposited in periods before and 

after the cassiterite and sulfide minerals were formed. The tourmaline seems to be related to the. breccias and may have been formed in 

more than one stage since each of the intrusions of breccia was probably followed by its own supply of volatiles.

RESERVES

Only remnants of the tin ore shoot have been left in the Tin stope 

(pi. 9). Small reserves of lower grade tin remain in the tin- and copper-bearing vein in the Copper stope and in mineralized rhyolite and 

breccia in stope 211. Production of tin will depend upon the possible 

discovery of new deposits. A small amount of exploration on the 

Middle adit along the Majuba fault to the northwest of the present 

workings might be justified.

URANIUM

The only significant uranium deposit at Majuba Hill is contained 

in a 3-foot copper- and tin-bearing vein in the south part of the Copper 

stope in the Majuba Hill mine (pis. 9 and 11; coordinate 1700 N, 350- 

400 W.). The vein is estimated to have 4an average grade of 0.30 percent U3O8 and the wall rock, for a width of 15 feet, is estimated to have 

an average grade of 0.10 percent U3O8. The vein contains, in addition 

to the uranium, 0.47 percent tin and 12.7 percent copper. Lower grade 

uranium-bearing material, containing from 0.004 to 0.04 percent U3O8> 

occurs in altered rhyolite porphyry at the extreme south end of drift 

202 (pis. 9 and 11). A sample of uraniferous limonitic fault gouge in 

crosscut 216 contains 0.016 percent U3O8 (pis. 9 and 11). Most of the 

other rhyolitic rocks and breccias sampled contained only 0.003-0.006 

percent U3O8. Eadon, the radioactive gaseous daughter product of
202 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

uranium is found in crosscut 215 of the Middle adit level (pis. 9 and 11) 

and near the end of the Lower adit (pis. 10 and 11) during periods of 

low barometric pressure. Eadon, therefore, suggests the presence of 

uranium in the vicinity of these workings.

MINERALOGY

Metazeunerite is the only uranium-bearing mineral found by the 

writers at Majuba Hill. It occurs as bright green platy crystals associated with chalcocite, pyrite, arsenopyrite, and cassiterite in the 

copper- and tin-bearing vein in the Copper stope, and locally with 

secondary copper minerals, cassiterite, tourmaline, and iron oxides in 

lower grade rock. No primary mineral of uranium has yet been found.

ORIGIN

The secondary uranium mineral, metazeunerite, is believed to have 

been derived from the oxidation of a primary uranium mineral, either 

uraninite or coffinite, which was deposited with the primary sulfide 

minerals. The concentration of the metazeunerite with chalcocite suggests that the primary uranium mineral was leached in the oxidized 

zone and the uranium moved downward to combine with the copper to 

form the secondary uranium mineral in the zone of supergene 

enrichment.

RESERVES

The uranium reserves in the Majuba Hill mine are small, and 

;are limited to the uraniferous copper- and tin-bearing vein in the Copper stope.

SUGGESTIONS FOR PROSPECTING

Some areas in the Majuba Hill mine are favorable for a small exploratory program for uranium. This exploration could be done either 

by diamond drilling or by drifting and crosscutting. The following 

suggestions for prospecting in the mine are presented:

1. The area south of drift 202 in the Middle adit level (pi. 9) appears favor- 

.able for finding the downward continuation of the uraniferous vein exposed 

in the Copper stope. ,

2. The ground in the vicinity of crosscut 215 (pi. 9) may contain a uranium 

deposit responsible for the radon in the crosscut.

3. The area near crosscut 216 (pi. 9) may contain a uraniferous deposit, as 

suggested by the uranium minerals in the 3-inch gouge in the fault in this 

 crosscut.

4. The area near the end of the Lower adit (pi. 10) might be explored for the 

source of the radon.
GEOLOGY OF MAJTJBA HILL, PERSHING COUNTY, NEVADA 203 

SUMMARY

The deposition of the ore minerals of copper, tin, and uranium 

followed the intrusion of the rhyolitic rocks and breccias and also the 

major faulting at Majuba Hill. The largest ore bodies have been 

deposited in the brecciated rocks along the Majuba fault. Only small 

reserves of copper, tin, and uranium are in sight, and production will 

depend on the finding of new reserves.

LITERATURE CITED

Burbank, W. S., 1941, Structural control of ore deposition in the Ked Mountain,

Sneffels, and Telluride districts, Colorado: Colorado Sci. Soc. Proc., v. 14,

p. 141-261. 

Gianella, V. P., 1946, Clinoclasite from Majuba Hill, Nev.: Bcon. Geology,

v. 31, no. 5, p. 259-260. 

Matson, E. J., 1948, Investigation of Majuba Hill copper-tin mine, Pershing

County, Nev.: U. S. Bur. Mines Kept, of Invest. 4378. 

Smith, W. C., and Gianella, V. P., 1942, The tin deposit of Majuba Hill, Pershing

County, Nev.: U. S. Geol. Survey Bull. 931-C, p. 39-55. 

Vanderburg, W. O., 1936, Reconnaissance of mining districts in Pershing County,

Nev.: U. S. Bur. Mines. Inf. Circ. 6902.

U. 8. eOVERNHEHT PRINTING OFFICE: 1958