GEOLOGICAL SURVEY CIRCULAR 245

GEOLOGY AND PEGMATITES OF PART 

OF THE FOURMILE AREA 

CUSTER COUNTY 

SOUTH DAKOTA

By Andrew J. Lang, Jr. and Jack A. Redden
UNITED STATES DEPARTMENT OF.THE INTERIOR 

Douglas MeKay, Secretary

GEOLOGICAL SURVEY 

W. E. Wrather, Director

GEOLOGICAL SURVEY CIRCULAR 245

GEOLOGY AND PEGMATITES OF PART OF THE FOURMILE AREA 

CUSTER COUNTY, SOUTH DAKOTA

By Andrew J. Lang, Jr. and Jack A. Redden

This report concerns work done on 

behalf of the U. S. Atomic Energy 

Commission and is published with 

the permission of the Commission

Washington, D. C., 1953

Free on application to the Geological Survey, Washington 25, D. C.
CONTENTS

Page

Abstract.................................... 1

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

Descriptive geology.......................... 1

Metamorphic rocks..................... 1

Quartz-mica schist unit............ 1

Amphibolite unit.................. 2

Amphibolite................. 2

Lime-silicate rock........... 4

Cordierite-biotite

schist.................... 4

Microcline-biotite

schist.................... 4

Quartz-mica schist.......... 4

Quartzite................... 4

Quartz-mica-feldspar schist unit..... 4

Amphibolite dikes ................ 4

Pegmatites............................ 5

Homogeneous pegmatites .......... 5

Heterogeneous pegmatites.......... 5

Quartz veins .......................... 11

Sedimentary rocks ..................... 11

Deadwood formation (?)

(Upper Cambrian) ............... 11

Alluvium ........................ 11

Structure ................................. . 11

Folds and linear structures ............. 11

Faults and lineaments .................. 11

Structural relation between pegmatites

and country rock..................... 12

Metamorphism.............................. 12

Economic geology ........................... 12

Pegmatites ............................ 12

Beryl .......................... . 13

Mica ............................ 13

Potash feldspar ................. . 13

Gold ................................... 14

Placer deposits................... 14

Lode deposits .................... 14

Mines and prospects ......................... 14

Pegmatite 12-B (Big Spar No. 1 mine).... 14

Plagioclase-quartz-tourmaline

pegmatite (wall zone) .......... . 14

Plagioclase-quartz-perthite

pegmatite (intermediate zone).... 14

Quartz- perthite- plagioc la s e 

pegmatite (core) ................ 14

Economic minerals ............... 14

Pegmatite 14 .......................... 14

Quartz-plagioclase-muscovite

pegmatite (border zone).......... 15

Plagioclase-quartz-perthite

pegmatite (wall zone) ............ 15

Perthite- plagioc lase-quartz 

pegmatite (core) ................ 15

Economic minerals................ 15

Page

Mines and prospects Continued

Pegmatite 24 ......................... 15

Plagioclase-quartz-perthite

pegmatite (border and wall

zones)....................... . 15

Quartz-perthite-plagioclase

pegmatite (core)................ 15

Economic minerals .............. 15

Pegmatite 53 ......................... 15

Plagioc lase- quartz-per thite

pegmatite (border and wall

zones)........................ 16

Quartz-plagioclase-perthite

pegmatite (intermediate zone).... 16 

Quartz-perthite pegmatite -

(core) ........................ . 16

Economic minerals .............. 16

Pegmatite 54.......................... 16

Quartz-plagioclase-muscovite 

 pegmatite (border zone)........ 16

Plagioclase-quartz-perthite

pegmatite (wall zone) ........... 16

Quartz-perthite-plagioclase

pegmatite (core) ............... 16

Economic minerals .............. 16

Pegmatite 57 (Silver Top lode mine)..... 17

Plagioclase-quartz-perthite

pegmatite (border and wall

zone} ........................ 17

Perthite-quartz-plagioclase

pegmatite (intermediate zone).... 17 

Quartz-perthite-muscovite

pegmatite (core) ............... 17

Economic minerals .............. 17

Pegmatite 65.......................... 17

Quartz-plagioclase-muscovite

pegmatite (border zone) ........ 17

Quartz-plagioclase-perthite

pegmatite (wall zone) ........... 18

Quartz- plagioc lase -perthite

pegmatite (intermediate zone).. 18 

Quartz-perthite pegmatite (core)... 18 

Economic minerals .............. 18

Pegmatite 67 (Dorothy lode mine) ....... 18

Plagioclase-quartz-muscovite

pegmatite (border and wall

zone)......................... 18

Perthite-quartz-muscovite

pegmatite (intermediate zone)... | 18 

Perthite-quartz-plagioclase

pegmatite (core)............... 18

Economic minerals .............. 18

Pegmatite 68 (Highland lode, John Ross 

mine) ............................ . 19

Economic minerals .............. 19

Literature cited ............................ 20

iii
ILLUSTRATIONS

Page

Plate 1. Geologic map and section of the Fourmile area, Caster County, S. Dak..............Insidp back cover

Figure 1. Fourmile area, Custer County ........................................ . ............ . 2

2. Map of the southern Black Hills ...................................................... 3

TABLES

Page

Table 1. Mineralogy of homogeneous pegmatites ............................................... 6

2. Mineralogy of heterogeneous pegmatites ............................................. . 10

iv
GEOLOGY AND PEGMATITES OF PART OF THE FOURMILE AREA 

OUSTER COUNTY, SOUTH DAKOTA

ABSTRACT

The Fourmile area, Custer County, S. Dak., is 

underlain by pre-Cambrian metamorphic rocks that 

surround the granitic core of the Black Hills. The 

main structure in the area is the upright limb of an 

overturned anticline that plunges about 30° S. 10° E.

Three units of metamorphic rocks are described 

that have a total thickness of at least 7, 700 feet. The 

oldest of these units, a quartz-mica schist, is more 

than 6, 500 feet thick. The overlying unit, about 200 feet 

thick, is composed of thin beds of amphibolite and hornblende schist, lime-silicate rock, cordierite-biotite 

schist, microcline-biotite schist, and other types of 

rocks. The youngest unit, a quartz-mica-feldspar 

schist, is more than 1,000 feet thick. The presence 

of kyanite, staurolite, cordierite, and sillimanite in 

the rocks indicates that they have been subjected to 

high-grade metamorphism.

About 420 pegmatites were mapped in the quartzmica-schist and the quartz-mica-feldspar schist. A 

few thin pegmatites in the third unit were not mapped. 

Most of these are concordant with the schistosity and 

relict (?) bedding of the enclosing metamorphic rocks. 

They are as much as 250 feet thick and range from 10 

to 2, 600 feet in length. Nine pegmatites are zoned and 

classified as heterogeneous. The remainder -are homogeneous and are poorly zoned. The major constituents 

are plagioclase, quartz, perthite, and muscovite. The 

accessory minerals are tourmaline, apatite, garnet, 

and biotite. Beryl was observed in 15 pegmatites. The 

heterogeneous pegmatites contain cqmmercial deposits 

of potash feldspar, mica (sheet and scrap), and (beryl.

INTRODUCTION

A part of the Fourmile area, about 2 miles southwest of Custer, S. Dak. (fig. 1), was mapped in the 

summers of 1948-49 as part of'the beryllium program 

of the Geological Survey, carried out on behalf of the 

Division of Raw Materials of the U. S. Atomic Energy 

Commission. About 7. 8 square miles were mapped 

in sees. 19, 20, 27, 28, 29, 30, 32, 33, T. 3 S., 

R. 4E., and sec. 24, T. 3 S., R. 3 E. A. J. Lang, Jr., 

mapped the eastern 6. 2 square miles and J. A. Redden 

mapped the remaining area to the west. Mapping was 

done on aerial photographs enlarged to a scale, 1:12,000.

Although additional mapping was done to the west 

and south by the Geological Survey during the summers 

of 1950-51, only the work on the area mapped under 

the auspices of the Atomic Energy Commission is reported here. Therefore certain conclusions in this 

report should not be considered final and may be revised upon completion of the field work.

The general geology of the Black Hills has been 

described by Newton and Jenny (I860) 1 , Van Hise (1890), 

Darton and Paige (1925), and Connolly and O'Harra 

(1929).

The Fourmile area is about 6 miles southwest 

of the center of the main mass of granite and pegmatite 

that surrounds Harney Peak in the southern Black Hills 

uplift (fig. 2). Many pegmatites and quartz veins intrude the folded pre-Cambrian metamorphic rocks 

that make up most of the bedrock. Paleozoic formations crop out as cuestas about 3 miles to the west.

The area has a mature topography with a^maximum. 

relief of -700 feet. It is drained by French Creek and 

its intermittent tributaries, Ruby Creek and Crow 

Creek. A thin mantle of alluvium covers the creek 

valleys.

The bedrock is well exposed in the western part 

but not in the eastern part of the area. Pegmatites 

and quartz veins are very resistant to weathering and 

generally project above the weathered country rock.

DESCRIPTIVE GEOLOGY 

Metamorphic rocks

The metamorphic rocks are divided into three 

major units: a quartz-mica schist, a unit predominantly amphibolite, and a quartz-mica-feldspar schist. 

The exposed thickness of these three units is at least 

7, 700 feet.

Graded bedding in arkose layers within the quartzmica-feldspar schist, and the relative position of 

schistosity and bedding indicate that the beds are 

vertical. The oldest unit is the thick quartz-mica schist; 

the youngest is the quartz-mica-feldspar schist. The 

thin amphibolite unit is intermediate in age and contains many different rock types.

Quartz-mica schist unit

About three fourths of the area all except a 

northwest-southeast trending belt along the southern 

limits of the region mapped is underlain by the quartzmica schist. The bottom of the unit is not exposed in 

the mapped area. The known exposures indicate that 

the unit is at least 6, 500 feet thick.

Most of the unit consists of light- to dark-gray, 

fine-grained quartz-mica schist containing about 

70 percent quartz and 25 percent mica. Muscovite

1 See Literature cited.
R.3E. R.4E. R.5E.

4 Mites

Figure 1. Four mile area, Custer County, S. Dak.

generally exceeds biotite in abundance. The accessory 

minerals include plagioclase, tourmaline, graphite, 

sphene, apatite,' zircon, rutile, and magnetite.

Thin beds of mica-rich schist containing 50 to 

95 percent mica occur within the more massive quartzmica schist. In these beds biotite generally exceeds 

muscovite in abundance. The biotite-rich beds commonly containl small garnet crystals.

Many ellipsoidal structures of lime-silicate rock 

occur within the quartz-mica schist. The ellipsoids 

have a short axis ranging from 1 to 6 inches, an 

intermediate axis ranging from/4 to 12 inches, and a 

long axis ranging from 12 to 60 inches. They contain 

quartz, garnet, diopside, calcite, epidote, plagioclase, 

hornblende, biotite, and sphene. Biotite and hornblende tend to concentrate in the outer part of the 

ellipsoids. The plagioclase /composition ranges from 

Angg to Angg. Runner and Hamilton (1934) have de-- 

scribed these structures as metamorphosed calcareous 

concretions. Although the ellipsoids;have been found 

throughout the unit, they have a tendency to concentrate 

in certain zones.

Amphibolite unit

A unit of interbedded rocks of various types, 

called the amphibolite unit, overlies the quartz-mica 

schist. The width of the outcrop ranges from 150 to 

1, 500 feet, depending "on the degree of folding and 

topography. The true thickness is probably not less 

than 100 feet and not more than 200 feet.' The unit

contains amphibolite, lime-silicate rock, cordieritebiotite schist, microcline-biotite schist, quartz-mica 

schist, and quartzite. Many intermediate rock types 

represent gradations between the amphibolite, limesilicate rock, cordierite-biotite schist, and microcline-biotite schist.

Amphibolite. Amphibolite occurs predominantly 

in the lower portion of the unit, but is also associated 

with lime-silicate rocks near the middle.

Locally^ the amphibolite in the lower portion of 

the unit is in excess of 50 feet thick, but this probably 

represents an exaggerated thickness due to folding 

or flowage. The amphibolite associated with the limesilicate rocks is generally less than 30 feet thick, and 

in some places is missing entirely.

The lower amphibolHe is schistose and rather 

fine-grained. It is dark green and is commonly stained 

with iron. The dominant mineral is hornblende, although considerable plagioclase and quartz are also 

present. The plagioclase forms from 5 to 30 percent 

of the rock, and its composition ranges from 'An22 to 

Angg. Sphene, apatite, graphite, magnetite, and 

tourmaline are the most common accessory minerals: 

zoisite, garnet, chlorite, biotite, diopside, and calcite are present in a few specimens.

The upper amphibolite is more variable in 

composition and appearance than the lower amphibolite. It contains zones rich, respectively, in 

biotite; biotite and plagioclase; calcite; calcite and
R.3E. R.4E. R.5E. R.6E.

G

Fa

Figure 2. --Map of the southern Black Hills, South Dakota.
plagioclase; and cummingtonite and tourmaline. All 

these types of amphibolite have poorly developed 

schistosities.

The biotite- and biotite-plagioclase-rich amphibolites contain stubby green hornblende crystals more 

than 0. 2 inch in diameter. These crystals are relatively unoriented and appear to "knit" the biotiterich matrix together.

The calcite in the calcareous amphibolite is 

disseminated throughout the rock in variable amounts 

up to 50 percent, and also occurs in veinlets and 

thin layers that apparently are beds. Thin lenses of 

quartzite are associated with the layers of calcite. 

Hornblende generally occurs as thin needles having a 

well-developed lineation a sharp contrast to the 

hornblende in the biotite-bearing amphibolites.

Schistose cummingtonite-tourmaline amphibolite 

occurs only at the Newark gold mine near quartz veins 

in biotite-bearing amphibolite and may represent an 

alteration of the original rock. The presence of graphite 

in this rock is also indicative of a secondary origin 

associated with the quartz veins.

Plagioclase from the upper amphibolite unit 

ranges in composition from Angg to AngQ. The most 

calcic plagioclase is concentrated in the calcite-bearing 

amphibolites that also contain accessory diopside.

Lime-silicate rock. Lime-silicate rock, containing alternating light and dark layers ranging from 

0. 2 inch to 12 inches in thickness, forms a unit of 

variable thickness near the middle of the amphibolite 

unit. The dark-colored layers are composed chiefly 

of fine-grained biotite and a small amount of hornblende and quartz. A few dark layers contain cordierite. 

The light-colored layers are coarser grained and have 

a distinct greenish color. They consist of diopside, 

calcite, microcline, plagioclase, scapolite, sphene, 

and hornblende. The diopside, microcline, and 

scapolite form grains as much as 1 centimeter in 

diameter, but the other minerals are fine-grained. 

The plagioclase (An^-go) tends to concentrate along 

contacts with the dark layers.

Cordierite-biotite schist.- A layer of cordieritebiotite schist less than 40 feet thick, occurs near the 

top of the unit. The schist has a dark gray-brown 

groundmass that encloses light-colored equidimensional 

grains of cordierite that have an average diameter of 

1 millimeter. The grains of cordierite are surrounded 

by a network of biotite flakes and fine-grained microcline, that cause the schistosity to be obscure 1 except 1 

in those specimens that contain abundant oriented mica. 

The schist contains about 40 percent cordierite, 35 percent biotite, 10 percent microcline, and 10 percent 

plagioclase. In some places the schist contains as 

much as 10 percent graphite.

Microcline-biotite schist. - Microcline-biotite 

schist in a layer less than 40 feet thick, is associated 

with cordierite-bearing schist, and locally grades 

vertically and laterally into the cordierite-rich rock. 

This schist is dark grayish-black and is very fine 

grained, averaging 0.02 millimeter in grain size. 

The average composition is about 60 percent microcline, 25 percent biotite, 10 percent muscovite, and 

less than 5 percent plagioclase. Much-of the schist

contains graphite that makes up as much as 10 percent of the rock.

Quartz-mica schist. A few thin layers of quartzmica schist with variable amounts of accessory minerals 

occur within the amphibolite unit. At a few outcrops, 

the schist contains fuchsite and cordierite. In other 

small outcrops, kyanite and sillimanite occur in quartz-i 

biotite-feldspar schist.

Quartzite. Quartzite beds near the upper and 

lower contacts of the amphibolite unit range in thickness 

from 5 to 30 feet. Most of the quartzite is light gray 

to flesh colored. Quartz grains, ranging from 0.02 

to 0. 5 millimeter in diameter, make up about 90 percent of the rock. Much of the quartzite contains bluishgray bands rich in graphite. The other accessory minerals are mica, tourmaline, zircon, and rutile. Locally, 

the quartzite is granulated and has a sugary texture.

Quartz-mica-feldspar schist unit

The uppermost unit is a quartz-mica-feldspar 

schist. Locally it contains small quantities of quartzmica schist, garnet-staurolite schist, and lime-silicate 

ellipsoids. The total thickness is unknown, but it 

probably exceeds 1,000 feet.

The quartz-mica-feldspar schist is fine-grained 

and generally light to dark gray. It contains about 

65 percent quartz, 20 percent mica, and 15 percent 

feldspar. Biotite and muscovite are equal in abundance. 

The feldspar consists predominantly of plagioclase 

(about Anso) with minor amounts of microcline. Near 

the contact with the underlying amphibolite unit the 

schist contains as much as 30 percent feldspar. Recrystallized quartz and feldspar grains as much as 

4 millimeters in diameter are visible in metamorphosed 

coarse-grained arkosic beds near this contact. Part of . 

the schist contains small, slightly flattened pebbles, a 

quarter of an inch thick and three-quarters of an inch 

long, consisting of quartz and feldspar. Biotite lenticles, 

1 millimeter thick, occur in the more feldspar-rich 

schist.

The feldspar content decreases upward away from 

the contact with the amphibolite unit. Beds of quartzmica schist are interbedded with the quartz-mica-feldspar schist farther from the contact. As the feldspar 

content decreases, the mica content increases and the 

grain size becomes smaller.

A bed of schist, less than 40 feet thick, that is 

rich in biotite, staurolite, and garnet, occurs about 

300 feet stratigraphically above the lower contact of 

the unit. Dark layers within the bed have as much as 

60 percent of biotite. Euhedral crystals of staurolite, 

as much as 1 inch long, locally constitute 30 percent 

of the rock. The garnet crystals average 1 millimeter 

in diameter. A small quantity of kyanite was found in 

one outcrop. The remainder of the rock is chiefly 

' quartz, and feldspar.

Amphibolite dikes

'Two small bodies of massive amphibolite that 

apparently cut across the schistosity of the Ruby Creek 

schist are interpreted as being metamorphosed mafic 

dikes. The dikes have an average thickness of 20 feet 

and a length of about 100 feet. They are composed
mainly of hornblende and plagioclase (An3o) grains, 

averaging about 1 millimeter in diameter, that are 

intergrown, giving the rock a speckled appearance that 

differs from the amphibolite of the amphibolite unit. 

The plagioclase characteristically shows reverse zoning. 

An indistinct foliation is visible near the outer edges 

of the dikes.

Pegmatites

About 420 pegmatites were mapped in the area: 

the only unmapped ones were those less than 1 foot 

thick. The length of a single pegmatite is as much as 

2, 600 feet and the thickness as much as 250 feet. Of 

the mapped pegmatites 411 are concordant sill-like 

bodies and of these 394 are less than 20 feet thick. 

All are in the quartz-mica schist and the quartz-micafeldspar schist.

Following the terminology of Johnston (1945), 

the pegmatites of the Fourmile area are divided into 

the following groups: (1) homogeneous pegmatites or 

those that are essentially uniform in texture and mineralogy from wall to wall, and (2) heterogeneous pegmatites or those that contain distinct textural and mineralogic units. It is probable that all gradations exist 

between homogeneous and heterogeneous pegmatites.

Homogeneous pegmatites

Except for nine, the mapped pegmatites were 

considered homogeneous pegmatites. These are similar 

in their mineralogy and texture to the pegmatitic phases 

of the granitic rock near Harney Peak, northeast of 

Custer. Distinct textural and mineralogic units in 

these pegmatites are confined to a fine-grained selvage 

at the contact, that has a thickness commonly less than 

3 inches and a grain size less than one-eighth inch. 

A gradual increase in grain size toward the center is 

noticeable in many of the homogeneous pegmatites; 

others have no noticeable textural changes.

Most of the homogeneous pegmatites contain 

plagioclase, quartz, perthite, and muscovite in decreasing order of abundance. All homogeneous pegmatites more than 20 feet in thickness are composed 

of plagioclase, quartz, and perthite. The variability 

in the proportions of these minerals in the smaller 

pegmatites is shown in table 1. Many neighboring 

pegmatites have similar compositions, textures, and 

shapes, and are designated as pegmatite groups (pi. 1, 

table 1). Plagioclase (An4 to An^) ranges from 15 

to 50 percent; quartz, 15 to 45 percent; perthite, 0 to 

40 percent; muscovite, 1 to 15 percent; and tourmaline, 

1 to 5 percent. In most homogeneous pegmatites the 

plagioclase content tends to decrease and the perthite 

content to increase toward the center.

In some pegmatites, as nos. 23, 49, and 50 

(table 1), more than half of the perthite is in graphic 

granite. Most of the perthite and graphic granite is 

coarser grained than the other minerals. The largest 

crystals, as much as 5 feet in diameter, are graphic 

granite. Finer grained quartz and plagioclase have 

an average grain size of about half an inch. Plagioclase-* 

rich pegmatites, such as nos. 19, 41, and 72 (table 1), 

have an average grain size that is considerably smaller 

than the perthite-rich pegmatites. The perthite and 

graphic granite content varies considerably in different 

parts of a single homogeneous pegmatite.

These pegmatites have a wide range in size and 

shape. Most are thin, sill-Hke bodies. The largest, 

no. 20 (pi. 1), has a length of 2, 350 feet and a thickness 

of 100 feet. One pegmatite in group 4 (pi. 1) is 2, 600 feet 

.long, but is less than 15 feet thick.

Heterogeneous pegmatites

Heterogeneous pegmatites in the Fourmile area 

contain as many as 4 distinguishable zones using the 

terminology of Cameron, and others (1949), may be 

called border, wall, intermediate, and core zones. 

Zones are defined as successive snells, complete or 

incomplete, that reflect to varying degrees the shape 

and structure of the pegmatite body. Where ideally 

developed they are concentric about an innermost zone 

or core. Border zones commonly have the same 

general composition as the wall zone. The zones in 

pegmatites of the Fourmile area contain plagioclase, 

quartz, and muscovite; quartz, perthite, and plagioclase; 

and perthite and quartz (table 2). The relative order 

of abundance of these minerals varies, but the assemblages correspond to the generalized'assemblages 

(1), (3), and (4) of Cameron, and others (1949).

The border zone occurs at the contact of the pegmatite and the country rock. It is fine-grained, averaging slightly less than a quarter of an inch in grain size. 

Quartz and plagioclase are the most abundant minerals 

and muscovite is usually the third most abundant. Pegmatite 24, however, contains a border zone of plagioclase, 

quartz, andperthite. Pegmatite 12-B is a pod within a 

homogeneous pegmatite and does not possess a border 

zone. Tourmaline, apatite, and garnet are the accessory minerals in the border zones.

Wall zones occur inside of the border zone and 

are thicker, ranging from half a foot in no. 65 to 5 feet 

in no. 14. As in the border zone, the two most abundant 

minerals are plagioclase and quartz. The third most 

abundant mineral is commonly perthite, but may be 

tourmaline (no. 12-B) or muscovite (no. 67). Wall 

zones average about half an inch in grain size. They 

contain the same accessory minerals as the border 

zone, and in addition may contain beryl.

Intermediate zones are any zones between the 

wall zone and core. They range in thicknesses from 

a few feet to as much as 20 feet. They contain perthite, 

plagioclase, and quartz in varying proportions. Tourmaline, garnet, muscovite, and beryl may occur as 

accessory minerals. Quartz and plagioclase are finegrained, averaging about 1 inch in diameter, andperthite 

is coarse-grained, averaging 1 foot or more.

The cores of the heterogeneous pegmatites have 

a variable thickness and may be a series of small 

disconnected pods. The two most abundant minerals 

of the cores are quartz and perthite. Plagioclase or 

muscovite may be the third most abundant. Tourmaline, 

apatite, and beryl commonly occur as accessory minerals. Perthite is coarse-grained as in the intermediate zones. Quartz is either massive, or in finegrained aggregates with plagioclase and muscovite.

Heterogeneous pegmatites also contain fracturefilling units that were defined by Cameron, and others 

(1949) as "units, generally tabular, that fill fractures 

in previously consolidated pegmatite. " Pegmatite 

12A (pi. 1) could be classified as heterogeneous because it contains a large fracture filling (?). This
Table 1. Mineralogy of homogeneous pegmatites

No. and 

name of 

pegmatite 

(pl. D1

1

2 (11) 

3 (21) 

4 (12) 

5 (12) 

6 (2k) 

7 

8 (5) 

9 

10 

11 (2)

12a(2) 

13 (2)

15 

16 (s)

17 

18 

19 

20 

21 (3)

Vail rock

Type and 

formation

<)unrtz-mica 

schist.

 do 

 do 

Alteration

 

 

Pegmatite

Relation to 

wall rock

Conformable--

- do 

cutting. 

Sonformabla--

Shape

abular 

 do -

 do 

Internal 

structure

Inzoned -

 do 

~do- -----

__do 

Average 

grain 

size 

inches)

1 to 4

1 to 4 

1 to 4 

1 to 4

1 to 4

1 to 4

1 to 4 

1 to 4

1 to 4 

1 to 4

1 to 4 

1 to 4 

1 to 4 

1 to 4 

1 to 4

Mineralogy

Flagloelase

Parcent i

45

50 

40 

40 

40 

30 

35 

40 

30 

50 

35

45 

40

30 

35

40 

40 

45 

35 

35

Size 

inches

<. 1

1/2 

V2 

1/2 

1/2 

<- 1 

< 1 

<1 

3/4 

1/2 

1/2

1 

3/4

3/4 

<i 1

3/4 

<; l 

<, l 

3/4 

3/4

Perth! ts

Parcentj

15

15 

20 

15 

20 

15 

20 

15 

20 

5 

20

20 

20

25 

20

15 

20 

15 

10 

10

Size 

Inches

4 to 

12

6 

5 

5 

5 

4

4 to 

12

4 to 

12

12

4 to 

12

4 to 

12

12 

12

12

4 to 

12

6

4 to 

12

4 to 

12

6 

4

Graphic 

granite

Percent (

5

5 

5 

5 

5 

5 

5 

5 

20 

5 

10

10 

10

10 

10

5 

10 

5 

20 

20

Size

.nches

4 to 

12

6 

5 

5 

5 

4

It to 

12

tt tO

12 

12

4 to 

12

4 to 

12

12 

12

12

4 to 

12

6

4 to 

12

4 to 

12

12 

12

Quartz

Per-

)cent|

30

25 

30 

35 

30 

40 

30 

35 

25 

30 

25

15 

25

25 

30

35 

25 

30 

25 

25

Size 

inches

C 1

1/2 

1/2 

1/2 

1/2 

<, 1 

<; 1 

<; l 

3/4 

1/2 

1/2

l 

3/4

3/4 

<1

3/4 

-^ 1 

< 1 

3/4 

3/4

Muscovite

Pericanti

3

3 

3 

3 

3 

5 

5 

3 

3 

3 

5

5 

1

3 

2

3 

3 

2 

5 

5

Size

inche:

V2

1/2 

1/2 

1/2 

1/2- 

3/4 

V2 

1/2 

3/4 

1/4 

3/4 

3/4

V2

3/4 

V2

1/2 

1/2 

1/2 

1/2 

1/2

Garnet

Per-

)oent

c 1

c 1 

< 1

<. 1 

< 1 

< 1 

<. 1 

<. 1 

<. 1 

1 

<. 1 

2

< 1

<. 1 

<. 1

< 1

-^ 1

Size

inche!

1/16

1/16 

1/16 

1/16 

1/16 

1/16 

1/16 

1/16 

1/16 

1/16 

1/16 

1/8

1/8

1/16 

1/16

1/16 

1/16

Tourmaline

Per- 

)cent(

1

2 

2 

2 

2 

3 

3 

2 

2 

2 

2

3 

2

3 

2

2 

2 

3 

3 

3

Sice 

inches

1

1 

1 

1 

1 

1 

3/4 

1/2 

1 

V4

V2

1 

3/4

1 

3/4

V2 

1 

V2 

V2 

V2
22 (8)

23

25 (20)

26 (8)

27 (5)

28 (4)

29 (8)

30 (2)

31 (14)

32 (25)

33 (30)

34 (7)

35 (2)

36 (3D

37 (18)

38 (27)

39

40 (2)

41

42 (13)

43 (3)

 -do ______

. do 

. do ________

 ___do ______ 

. do -_- _

luartz-mica- 

-fsldspar 

schist.

licrocline 

porphyroIblaetB.

ilcrocline 

porphyroblasts.

41crocline 

porphyroblasts.

Kicrooline 

porpfcyroblasts.

ization.

 

. do 

. do - 

. do - 

cutting.

 - do - 

i do 

 do 

 do 

 do 

- do -

 do -_--__

 do 

 do '

 do 

 do 

1 to 4

1 to 4

1 to 4

<1

1 to 4

1 to 4 

1 to 4

1 to 4

35

35 

45

30

45

35

35 

30

30 

30

35

35

25

25 

35

25

35

45 

30

30

C 1

1

3/4

3/4

2

3/4

1

1

1/2

1

3/4

1/4

1/2

V2

3A

1/2

1/2

1/2

3/4

1/4

1/2

20

10

5

10

10

10

10

10

30

25

20

10 :

5

10

15

20

20

10

5 :

20

25

i. to 

12

8

5

+ to 

12

+ to 

12

6

12

4

6

5

4

L to 4

4

4

6

4 to 

12

8

12

. to 4

2

6

10

15

15

20

5

10

5

30

5

5 :

25

20 :

10

20

15

10

10

25

15 :

5

y to 

12

12

8

4 to 

12

4 to 

12

10

12

5

4

to 4

6

Lto 4

6

6

6

4 to 

12

12

12

to 4

4

30

30

30

30

25

30

35

25

25

30

25

25

35

35

35

25

35

20

20

40

35

< 1

3/4

3/4

3/4

1/2

1/2

1/2

3/4

1/2

V2

1/2

V4

1/2

V2

1/2

V2

1/2

1/2

1/2

V4

1/2

2

5

3

5

10

10

10

5

5

5

5

5

10

5

5

<1

5

5

10

5

10

<. 1

V2

V2

1/2

3/4

3/4

3/4

1/2

1/2

1/2

3/4

1/8

1

1

3/4

V2

1/2

V2 .

1/2

1/2

1

; 1

<. 1

<.!

<^1

< 1

<.!

<1

<1

V16

<1

1/8

1/16

1/16

1/8

1/16

1/16

1/8

1/16

1/8

1/16

2

2

2

3

2

3

4

3

2

2

2

2

3

3

3

2

3

4

2

3

<. 2

<1

1

V2

1/2

1

3/4

1

1

1/2

1/2

V2

V4

1

1

3/4

V2

1/2

1/2

1/2

1/2

1/2

Y

m

Y

m
Table 1.--Mineralosy of homogeneous pegmatites Continued

No. and 

name of 

pegmatite 

(Pi. D 1

44

45 

46 (5) 

47 (3)

48 (2) 

49 (4) 

50 

51 (2) 

52 (2) 

55 (9) 

56 

58 (5) 

59 (2) 

60 

61 (3) 

62 (2) 

63 (2) 

64 

66 

69 (3) 

70 

71 (2) 

72 

73

74 (5) 

75 (2) 

76

Vail rook

Type and 

formation

ftuartz-mleafeldspar 

schist.

3uartz-mlca

Alteration

 

Pegjnatlte

Relation to 

wall rock

Conformable 

Shape

Tabular 

Internal 

structure

Unzoned 

"do 

trerage 

grain 

size 

inches]

1 to 4

1 to 4 

1 to 4 

1 to 4

1 to 4 

1 to 4 

1 to 4 

1 to 4 

1 to 4-

1 to 4 

<, 1 

1 to 4 

^. 1 

1 to 4 

1 to 4 

1 to 4 

1 to 4 

L to 4 

L to 4 

L to 4 

L to 4 

<. 1 

. to 4

. to 4 

. to 4 

. to 4

Mineralogy

Plagioclase

Parcent i

25

35 

35 

40

35 

30 

35 

35 

25 

35 

35 

30 

25 

30 

20 

35 

30 

30 

25 

35 

35 

35 

45 

35

35 

40 

50

Size 

inches

1/2

<, 1 

1/2 

1/4

1/2 

1/2

<. 1 

1/2 

3/4

1/2 

1/2 

1/4 

3/4 

1/4 

1/2 

<, 1

h 1

1/2 

1/2 

1/2 

1/2 

L/2 

3/4 

1/2

1/2 

1/2 

1

Perthite

Per- 

)cent|

30 ]

30 ] 

20 

15

10 

10 

10 

10 

5 

5 

5 

5 

5 

20 ] 

15 4 

15 k 

15 4 

15 1 

10 

15 

10 

25

20

20

15 

5

Size 

inches

to 4

to 4 

8 

12

10 

10 

6 

8 

10 

8 

8 

8 

4 

to 4 

to 12 

to 12 

to 12 

to 4 

4 

5 

4 

4

3

4 

6 

4

Qraphio 

granite

Per- 

)cent(

20

10 :

5 

5

10 

20 

20 

20 

25 

20 

20 

15 

25' 

5 - 

25 i

5 i

20 

20 

20 

5

20

5

15 

10

Size

inches

Lto 4 

to 4

e

12

10 

14 

10 

12 

14 

12 

14 

12 

6 

tto 12 

to 12

to 12

6 

8 

10 

6

12

8 

12 

8

Quartz

Per- 

)cent (

15

20 

30 

30

30 

25 

25 

25 

35 

30 

30 

40 

30 

35 

30 

40 

40 

40 

30 

20 

27 

25 

40 

20

30 

24 

30

Size

inches

1/2

v 1 

1/2 

1/4

1/2 

l/Z 

^. 1 

1/2 

1/2 

1/2 

1/2 

1/4 

3/4 

V4 

1/2 

t, 1 

c. 1 

1/2 

1/2 

1/2 

1/2 

1/2 

1 

3/4

1 

1/2 

1/2

Muscovite

Per-

)oent(

5

3 

5 

5

10 

10 

5 

5 

5 

5 

5 

5 

10 

5 

5 

5 

5 

15 

10 

4 

4 

5 

15 

3

4 

3 

3

Size

inches

1/2

1/2 

1/2 

1/4

1/4 

1/4 

1/4 

1/4 

1/2 

1/2 

1/2 

1/8 

1 

1/4 

1/2 

<. 1 

1/4. 

1/4 

1/2 

3/4 

1/2 

1/2 

3/4 

3/4

1/2 

1/2 

1/2

Oarnet

Per-

)cont I

<. 1

<, 1

<, 1 

<, 1 

<. 1

<, 1

1

1

Size

inches

1/16

1/16

1/16 

1/16 

1/16

1/16 

1/4

V2

Tourmaline

Per- 

)cent(

5

2 

3 

3

2 

2 

2 

2 

3 

2 

2 

2 

2 

3 

3 

2 

2 

1 

2 

4 

3 

2 

<, 1 

1

3 

4 

2

Size

inches

1/2

1/4 

1/4 

1/4

1/4 

1/4 

1/4 

1/4 

1/4 

1/4 

1/4 

1/4 

1/2 

1/4 

1/2 

^ 1 

^ 1 

1/4, 

1/2 

1 

3/4 

V2 

1/2 

3/4

1

1 

1
77

78 (9)

79 (6)

80 (3)

feldspar

schist.

1 to 4

1 to 4

1 to 4

1 to 4

15

40

30

25

V2

1/2

1/2

1/2

20

20

15

15

4

3

6

IS

30

5

25

30

10

6

12

20

25

30

20

20

1

V2

1/2

3/4

5

4

3

4

3/4

V2

1/2

3/4

<,!

<. I

^ 1

5

V4

V4

1/2

3/4

4

1

3

4

1

3/4

3/4

1-1/2

The number of Individual pegmatites designated by one number on plate 1 Is given in parentheses.
Table £. Mineralogy of heterogeneous pegmatites

No. and 

name of 

pegmatite 

(Pi. D1

12-B 

(Big Spar 

No. 1)

14

24 

53

54

57 

(Silver Top

65

67 

[Dorothy 

lode).

68

(Highland 

lode ) .

Vail rook'

Type and 

formation

Quartz-mica 

schist.

-- doAlteration

 -

 -

Pegmatite

Relation to 

wall rock

Fod within

granit e 

pegmatite.

ConformableJo nformable 

and locslly 

disconformable.

Shape

Tabular 

 do 

 do 

Internal 

structure

#all aone^ 

interne dial 

zone.

 all zonofall zonefall zonejitermediat 

zone.

"Bull mica' 

unit.

(all zonefall zonejrtermediat 

zone.

Hail zone- 

[ntermediat 

zone. 

Core-^ 

[ntermediat 

zone.

'Bull mica" 

_unit.

(all zoneUpper 

lower r

Werage 

grain 

size 

Inches

H

e

VC

H

C

H

C

F

e 

C

C 

«F

F 

C

F 

a 

H 

C

F 

e 

H 

C

F

B

C 

VC

«F

H 

C

C

Mineralogy

Plagloclase

Percent 

(

5° .

45 

15

60 

35

50 

25

40 

30

1Q

40

25

40 

20

35 

35

55

7

15

15.

50. 

< 5 

15

Size 

.n ones'

1/4

1 

1

1 

 1 

V2 

1

1/2

1

1/2

1/2

1

3/4

1

1/2 

3/4

3/4

1 

3/4

1/2

1 

1 

1

Perthite

Percent

f:

15

5 

10

10 

20

20 

5 

40

15 

15

20

35 

30

10

20 

40

80 

40

< 5 

75 

25

Size

.richest

12 

1

id

3

12

1 

24 

18

2 

24

2

24 

24

2

12 

12

36 

36

24 

36 

36

Oraphle 

granite

Percent 

(1

5 

40

30

10 

25

15

10 

10 '

Size

pshsa.

12 

24

24

14 

24

24

24 

24

Cfeiartz

Percent 

f30 

30 

40

30 

20

30 

40

30 

35 

60 

20

35 

36

30

30 

55

40

40 

50

35

10 

40

25

40 

15 

55

Size

ocbfliO

1/4 

1 

2

1 

1

1/2

1

1/2 

1 

12 

1/2

1/2

1

3/4

1 

12

1/2

3/4 

12

3/4

1 

12

1/2

2 

6 

8

Muscovite

Percent 

( 

5 

5

5

3 

2

6 

3

5 

3 

2 

70

7 

10

5

3 

5

10

5

10

5

2 

5

60

T 

1

1

Size 

nchaa.

1/4 

1 

1/4

1/4 

1/4

3/4 

1/2

1/4 

3/4 

1/4 

1

3/4 

3/4

3/4

1/2 

1/2

1/2

3/4 

1

3/4

3/4 

1/2

3/4

2 

V2 

4

Garnet

Percent 

( 

< 1

< 1

<1 

<1

<1 

1 

<1

<. 1 

< 1

< 1 

< 1

< 1

Size

no.hes

1/8

1/8

V8 

1/8

1/8 

1/16 

1/16

1/8 

1/8

1/8 

1/8

1

Tourmaline

Percent 

('

10

5^ 1

c. 1

2

1

2

1

2

1

3 

2

3 

2

1 

<, 1

1 

1

3

1

Size

nr.hoa

3

1/4 

1/4

V4 

1/4

1/2 

1/4

1/4 

1/4

1/2 

1

1/4 

1/4

1/4 

1/4

1/4 

1/4

3

5

1 Average grain size: F is < 1 Inch; H is 1 to 4 inches; C is 4 to 12 inches; VC is > 12 inches.
fracture filling, however, is distinctly zoned and is 

described separately as no. 12-B. (See Mines and 

prospects.)

A few of the pegmatites listed as homogeneous 

in table 1 contain very small fracture-filling units. 

In all of these the fracture fillings are of such small 

size that the pegmatites were included with the homogeneous group.

Quartz veins

Quartz veins, ranging from a quarter of an inch 

to 6 feet in thickness, are found throughout the area. 

The largest vein is 300 feet long. The larger veins 

are generally concordant, but many of the smaller ones 

are discordant.

In addition to quartz, they contain small quantities 

of microcline, plagioclase, tourmaline, apatite, and 

mica. The thick veins contain only a small amount of 

these other minerals, but a few, less than a foot thick, 

contain as much as 20 percent feldspar.

A concentration of dark minerals occurs in the 

schist wall rock, immediately adjacent to the larger 

quartz veins in the quartz-mica schist unit and the 

quartz-mica-feldspar schist unit. This concentration 

of dark minerals forms an "envelope" around the vein, 

ranging from half an inch to 2 feet thick. Biotite, 

graphite, and tourmaline are the chief constituents, 

but small quantities of chlorite, quartz, apatite, and 

muscovite are also present. All these minerals are 

normal constituents of the schists, but biotite, graphite, 

and tourmaline are much more abundant than in most 

schists of this area.

Sedimentary rocks 

Deadwood formation (?) Upper Cambrian

A small outcrop of quartzitic conglomerate in the 

NWiSEi sec. 33, T. 3 S., R. 4 E. is probably part 

of the Deadwood formation. The reddish-brown ironstained quartzite matrix contains well-sorted, coarse 

sand grains, averaging about half a millimeter in size. 

Well-rounded and undeformed pebbles average about 

three-eighths inch in diameter and have a maximum 

diameter of 1 inch. Nearly all the pebbles are quartzite. 

They are arranged in poorly defined beds approximately 

parallel to the present surface of the ground.

The positively identified Deadwood formation 

that is exposed about 5 miles to the west is very 

similar to rocks in this outcrop in sec. 33. Also, 

pegmatites in the vicinity of this "probable Deadwood 

formation are beveled off at about the same elevation 

as the base of the outcrop. This concordance of pegmatite outcrops probably corresponds to the pre-Cambrian 

erosion surface, and this single outcrop probably is 

all that remains of the Deadwood formation in this area.

Alluvium

A thin cover of gravel is found in the valleys of 

Crow Creek, Ruby Creek, and French Creek. The 

thickest cover, probably less than 20 feet thick, is 

along French Creek. The gravels are composed chiefly

of sand and pebbles, but some pegmatite boulders as 

much as 6 feet in diameter are included.

STRUCTURE

Folds and linear structures

The part of the Fourmile area that has been 

mapped lies on the southwestern limb of a southeastplunging anticline. The axis of the anticline trends 

about N. 35° W., and on the basis of minor superimposed folds and linear structures, plunges about 30°S. 

10° E.

The axis of the major anticline lies slightly 

northeast of the mapped area and can be located on 

aerial photographs of the region to the north and east. 

A syncline paralleling the anticline has its axis about 

2 miles to the southwest of the mapped area. The two 

folds apparently have the same general plunge, as smaller 

folds and elongated lime-silicate ellipsoids at many 

localities throughout area have about the same average 

plunge, 30° S. 10° E. The attitude of the beds on the 

fold limbs is felatively constant, having strikes of 

N. 30° to 60° W. and dips of 25° to 60° SW. Bedding 

and schistosity are generally parallel, but locally 

diverge both in strike and dip. Locally the bedding 

may have a more westerly strike than the schistosity 

that commonly strikes about N. 40° W. In places the 

beds and schistosity have the same strike, but the 

schistosity has a steeper dip, indicating the relative 

movement of the* beds on the limb of the anticline. 

Where the dip of schistosity and beds differ, there is 

commonly a poorer bedding-plane schistosity. Elongated biotite grains and small-scale crenulations along 

the planes of schistosity generally plunge in a direction 

10° to 35° west of the plunge direction of the minor 

folds and have no obvious relation to the general structure 

of the area. A slight change of stress direction during a late stage in the folding or a much later deformation could produce the second lineation.

Faults and lineaments

No major faults have been recognized in the 

area. Minor steeply dipping faults with horizontal 

displacements of less than 30 feet and vertical displacements of less than 5 feet offset several pegmatites. 

These small faults generally are indicated only by 

displacement of the resistant pegmatite outcrop and 

cannot be traced in the metamorphic rocks. Most of 

the faults strike N. 40° E.

Two linear elements, probably surface expressions 

of shear zones, which are readily recognizable on 

aerial photbgraphs, but not so readily recognizable 

in the field, have been called lineaments (pi. 1). The 

lineaments are approximately parallel and strike 

N. 35° E. The western lineament can be traced on 

aerial photographs for about 4 miles and the eastern 

lineament for 6 and possible 9 miles, where it apparently 

disappears under the Paleozoic rocks. They are 

clearly marked on aerial photographs by a shallow 

valley averaging about 200 feet wide. Only a small 

amount of horizontal and vertical movement is indicated by displacement of pegmatites and the contacts 

of the amphibolite unit. As the contacts of the amphibolite unit are located only approximately at the points

11
where the lineaments cross the formation, it is impossible to measure the total displacement accurately. 

It can be inferred, however, from the approximate 

location of the contacts, that the total horizontal 

displacement along each lineament is less than 100 feet. 

Vertical movement, as indicated from displacement of 

individual pegmatites, is very small, if not lacking 

entirely.

Structural relation between pegmatites 

and country rock

Except for seven pegmatites of group 33 and 

two of group 11 (pi. 1), all the mapped pegmatites 

are concordant with schistosity and bedding. The larger 

pegmatites may be locally discordant but in general 

outline are concordant. Where outcrops of country 

rock are few or lacking, the attitude of the pegmatites 

is the main clue to the underlying structure (group 4, 

Pi. 1).

Small rolls in the contacts and the ends of pegmatites indicate three general directions of plunge. One 

set of plunge directions parallels that of the" minor folds, 

about S. 10° E.; a second parallels the plunge of the 

biotite lineation, about S. 5° to 35° W.; and a third 

plunges S. 45° W. It appears that the pegmatites were 

generally intruded along structural features formed 

during the folding of the rocks.

METAMORPHISM

The metamorphic rocks of the Fourmile area, 

originally sandstones, shales, arkoses, conglomerates, 

and carbonate-rich sedimentary rocks, have been 

altered by dynamo-thermal metamorphism to schists 

and lime-silicate rocks containing mineral assemblages 

characteristic of high-grade metamorphism. In places 

within a few feet of the pegmatite contacts, minerals, 

such as tourmaline, have been formed by reactions 

between the wall rocks and the solutions escaping from 

the pegmatites.

The quartz-mica schist unit and the quartz-micafeldspar schist unit, originally mainly aluminum-poor 

sediments, now consist predominantly of quartz, mica, 

and feldspar. Garnet occurs only in widely scattered 

thin biotite-rich beds, and staurolite, garnet, biotite, 

and minor kyanite in only one thin bed in the quartzmica-feldspar schist. Lime-silicate ellipsoids, 

originally carbonate beds or concretions, are composed 

of diopside, garnet, and anorthite-rich plagioclase.

The am'phibolite unit, originally consisting of 

predominantly carbonate-rich rocks of different compositions, consists of several rock types containing 

several mineral assemblages, some of which are 

indicative of high-grade metamorphism. These assemblages include hornblende-plagioclase-quartz; 

hornblende-plagioclase-calcite; hornblende-biotitequartz; biotite-muscovite-quartz; microcline-biotitemuscovite-plagioclase; diopside-microcline-plagioclasequartz-hornblende-calcite; cordierite-microclinebiotite-plagioclase; and kyanite-sillimanite-biotitemuscovite-plagioclase-quartz. The last three of the 

above assemblages are considered high-grade metamorphism by Harker (1939).

Probably the metamorphic grade of the entire 

area is on the border between high- and medium-grade 

metamorphism. Evidence for this is found in thin 

sections from the amphibolite unit that have sillimanite 

apparently replacing kyanite and muscovite. The 

absence of sillimanite throughout the area, except in 

the amphibolite unit, is mainly a result of the composition of the different rocks, however, its development 

may have been affected by the presence of abundant 

CC>2 in certain beds in the unit that may have permitted 

the attainment of equilibrium faster there than in the 

quartz-rich rocks.

The plagioclase composition in the metamorphic 

rocks ranges fromAn2Q to Ang5 and is apparently the 

result of the original composition of the rocks. Plagioclase 

from a lime-silicate ellipsoid had a composition of 

about Ang5, whereas plagioclase from quartz-micafeldspar schist, 1 foot from the ellipsoid, contained

An20-

At the contacts of a few of the pegmatites, there 

is a small amount of contact metamorphism, but the 

wall rocks of others do not show any. The general lack 

of metamorphism, either^ in forming new minerals or 

in the coarsening of grain size of original minerals, 

is more striking than the actual contact metamorphism. 

When any contact effect of the pegmatites is evident, 

it usually is the development in the schist along the 

contact of coarser grained, excess tourmaline in 

zones a few inches thick, or at the most, a few feet 

thick. In addition, microcline, albite, and apatite 

apparently have been added to the schist; but, without 

chemical analyses, it is impossible to say whether 

this is true or whether smaller grains of these minerals may have merely recrystallized into coarse,' 

clearly visible crystals. At a few contacts abundant 

tourmaline in the schist indicates the addition of 

boron, as the average schist contains only 1 percent 

or less of tourmaline. Probably a little potash, soda, 

and phosphorus migrated from the pegmatite into the 

wall rock to form the coarser grained microcline, 

albite, and apatite that occur on and near a few of the 

contacts.

EC GNOMIC'GEOLOGY

The mineral deposits of economic interest in 

the Fourmile area are small gold placers, gold-bearing 

quartz veins, and'pegmatite deposits. Pegmatites in 

the Fourmile area have produced potash feldspar, 

sheet and scrap mica, columbite, and beryl. The 

value of the potash feldspar produced exceeds that of 

all other minerals.

Pegmatites

Nearly all the homogeneous pegmatites are noncommercial in relation to 19521 prices and mining 

practices. Many of these contain potash feldspar 

recoverable by hand cobbing, but the grade is too low 

for commercial production. Furthermore, in many 

of the homogeneous pegmatites the potash feldspar 

occurs in graphic granite and contains too much quartz 

to be commercially acceptable.

12
Small quantities of scrap mica, crude sheet mica, 

and beryl occur in many, but were not of economic grade 

at the 1952 market. Many of the larger homogeneous 

pegmatites have potential reserves and could be mined 

upon the development of a successful milling process 

to recover potash feldspar, mica, and beryl.

Heterogeneous pegmatites contain zones rich in 

industrial minerals and, therefore, are more likely 

to be mined. Recoverable quantities of potash feldspar 

occur in the intermediate zones and the core. Beryl 

and mica occur in quantities too small to be mined 

except as by-products of feldspar mining.

Beryl

pegmatites, as nos. 15 and 80 (pi. 1) potential beryl 

producers. Any such milling process would also 

make it possible to recover the fine-grained beryl 

from the beryl-bearing heterogeneous pegmatites and 

from homogeneous pegmatites 13-B and 15. The 

potential beryl reserves of the mapped area would then 

be at least 350 tons.

Mica

Heterogeneous pegmatites 12-B, 14, 53, 54, 56, 

65, 67, and 68 and homogeneous pegmatites 13-B, 15, 26, 

73, 76, 79, and 80 contain beryl (tables 1 and-2). The 

beryl reserves of the heterogeneous pegmatites are 

estimated at about 310 tons. Of this total reserve 77 tons 

is considered recoverable by hand sorting.

Pegmatite 68 contains more recoverable beryl 

than any other heterogeneous pegmatites listed in table 2. 

Pegmatite 12-B has a large total beryl reserve, but 

only 20 percent of the beryl is considered recoverable 

by present mining methods.

Beryl may occur in the wall zone, intermediate 

zone, and core of the heterogeneous pegmatites. The 

average content of beryl in these zones is about 0.1 percent, although pegmatite 68 contains a beryl-bearing 

portion of the core with a grade of 0.8 percent. Pegmatite 14, however, contains a wall zone with a grade of 

only 0.01 percent.

A reliable estimate of the beryl in the homogeneous 

pegmatites is "somewhat difficult to make. The beryl 

is limited to coarser grained pods or segregations. 

The beryl content of one of these "pods" is 0. 3 percent, but the remainder of the pegmatite apparently 

is barren. The quantity of beryl observed in most of 

the homogeneous pegmatites in which beryl was found 

consisted of one or two crystals, commonly less than 

1 inch in diameter. More than 12 small crystals of 

beryl, ranging from 0.15 to 1 inch in diameter were 

seen in pegmatite 80.

Although beryl was found in only seven of the 

homogeneous pegmatites, the authors believe that it 

occurs in most of them. The reasons for the failure 

to observe beryl in the others are probably that the beryl 

occurs in small-size crystals and in anhedral crystals 

that are easily mistaken for quartz and other minerals, 

and that beryl may occur in unexposed interiors of the 

pegmatites. The homogeneous pegmatites probably 

contain about 0.001 percent of BeO. This estimate 

is based on spectrographic analyses of samples collected 

by D. H. Kupfer (personal communication) from the 

granite in the Harney Peak area, which is considered 

the source rock. The estimate of 0.001 percent of 

BeO corresponds to approximately 0.01 percent of 

beryl. An estimate of 0.01 percent beryl may be too 

high, for it is not certain that all of the BeO is contained in the beryl lattice.

The development of a milling process whereby 

fine-grained beryl could be recovered with potash 

feldspar and scrap mica would make large homogeneous

All the heterogeneous and homogeneous pegmatites contain scrap mica. With the exception of nos. 14 

and 67, (table 2) all heterogeneous pegmatites contain 

sheet and punch mica.

Sheet and punch mica occur in the wall zone of 

nos. 24, 54, 57, and 67, and in the intermediate zones 

of nos. 12-B and 53. The percent of sheet and punch 

in the crude sheet in these zones is low, and most of the 

mica reserves will remain unmined. In actual mining 

practice only the richer mica shoots are mined and 

the percent of sheet and punch mica in the mined rock 

is therefore considerably higher. Pegmatites 24 and 

67 (Dorothy lode) are the richest mica-bearing pegmatites in the area.

The other heterogeneous pegmatites contain very 

small quantities of sheet and punch mica. Pegmatite 

12-B (Big Spar No. 1), a feldspar mine, produced 

150 pounds of sheet mica from 5,100 tons of pegmatite. Pegmatite 68 (Highland lode) produced about 

32 pounds of sheet mica, 150 pounds of punch mica, 

and 343 tons of scrap mica from 1938 through 1944. 

With the possible exceptions of pegmatites 24 and 67, 

it is necessary to recover all mica as a by-product 

of feldspar mining.

The sheet and punch mica reserves of the mapped 

area (including pegmatite 68-Highland lode) are estimated 

at 31 tons. Pegmatites 24 and 67 contain nearly onethird the total sheet and punch mica. The heterogeneous 

pegmatites are estimated to contain about 2, 700 tons 

of cobbable scrap mica. Homogeneous pegmatites 

contain only traces of sheet and punch mica. Flotation 

milling could recover very large quantities of scrap 

mica from the homogeneous pegmatites. Pegmatite 20 

and nos. 10 and 8 (pi. 1) are estimated to contain over 

200,000 tons of scrap mica.

Potash feldspar

Very large reserves of potash feldspar are present in the homogeneous and heterogeneous pegmatites 

of the Fourmile area. In most of these, however, the 

quantity of feldspar that could be recovered by present 

mining methods is too small to permit profitable mining. 

In addition, much of the feldspar (perthite) occurs in 

graphic granite and is used only in small quantities 

for glass feldspar.

Pegmatite 67 (table 2) contains the largest recoverable reserves of perthite of any of the heterogeneous pegmatites. The intermediate zone is 140 feet 

long and 8 feet thick and is composed of 80 percent 

perthite, 70 percent of which is cobbable. Pegmatites 

65 and 67 (table 2) have smaller, but substantial reserves of perthite. Pegmatite 14 (table 2) has larger 

reserves of potash feldspar, but over 70 percent is 

in graphic granite.

13
Homogeneous pegmatites nos. 10, 18, and 20 

(pi. 1) were estimated to contain over 1, 500, 000 tons 

of potash feldspar. Most of the feldspar is not cobbable, 

but the flotation process could recover the feldspar 

and scrap mica. In addition, these three pegmatites 

are located within a radius of a quarter of a mile. 

Most of the other homogeneous pegmatites are small 

and widely scattered and would be of little value for 

milling.

Gold

Placer deposits

Dredging has been in operation in the valley of 

French Creek through sees. 19, 20, 28, and 29, 

T. 3 S., R. 4 E. The dredging was restricted to a 

narrow belt less than 100 feet wide along the present channel of French Creek. Figures are not available on the amount of gold recovered.

Lode deposits

Many of the larger quartz veins in the area contain a small amount of gold and apparently are the 

source of the placer gold. Prospect pits and shafts 

are located on most of the larger veins, but it is 

probable that the gold content of the veins exposed in 

the prospect pits is too low to warrant further development. In 1938 Allsman (1940) sampled the Newark 

mine and obtained assays of $3. 00 per ton for the 

quartz veins and $5.00 per ton for the schist immediately adjacent to the vein. Most of the prospect pits 

are on quartz veins along the amphibolite unit. Whether 

or not the gold content is greater in these quartz veins 

than in those in the quartz-mica schist unit and the 

quartz-mica-feldspar schist unit is not known.

MINES AND PROSPECTS 

Pegmatite 12-B (Big Spar No. 1 mine)

Pegmatite 12-B (Big Spar No. I mine) is in the 

SWi sec. 21, T. 3S., R. 4 E. (pi. 1). It is owned 

by the Consolidated Feldspar Co., of Trenton, N. J., 

which operated the mine for feldspar intermittently 

from 1943 to 1948. The mine workings consist of a 

single large open-cut, 85 feet long, 25 feet wide, and 

averaging 30 feet deep.

This pegmatite is described by J. W. Adams 

(1953?), who visited the property in June 1945. This 

report includes a brief description of the mine workings 

and the geology of the pegmatite.

Pegmatite 12-B is a well-zoned, lenticular fracture 

filling in the southwest part of pegmatite 12-C. Pegmatite 12-C is poorly zoned and is composed of 45 percent plagioclase, 30 percent quartz, 15 percent perthite, 

and 5 percent muscovite. It is at least 250 feet in 

length and as much as 60 feet in thickness. The general trend of pegmatite 12-C is N. 45° W., and the 

average dip is 45° SW. Pegmatite 12-B has an exposed 

length of 140 feet and a thickness of 50 feet. It strikes 

N. 35° W. and dips to the southwest. The apparent 

plunge is to the southwest.

Plagioclas e- quartz- tourma line 

pegmatite (wall zone)

The wall zone, averaging 1 foot in thickness, is 

composed of an estimated 50 percent plagioclase, 

30 percent quartz, 10 percent tourmaline, 5 percent 

muscovite, and accessory apatite and garnet. Black 

tourmaline crystals as much as 12 inches in length 

and 4 inches in diameter locally may form as much 

as 80 percent of the rock.

Plagioclase-quartz-perthite 

pegmatite (intermediate zone)

The intermediate zone is 90 feet long and 15 feet 

thick. It is composed of an estimated 45 percent 

plagioclase, 30 percent quartz, 20 percent perthite, 

5 percent muscovite, and accessory beryl, tourmaline, 

apatite, and garnet. Cream-colored perthite crystals 

average 3 feet in length. About 20 percent of the 

perthite is in graphic granite. Quartz and plagioclase 

are fine-grained, averaging about 1 inch in diameter. 

Light-greenish muscovite books average 1 inch in 

diameter. Beryl crystals, about 2 inches in length, 

occur in the outer part of the intermediate zone.

Quartz- perthite- plagioclase 

pegmatite (corej

The core of the pegmatite is 50 feet long and 

20 feet thick. It is composed of an estimated 40 percent quartz, 40 percent graphic granite and perthite, 

15 percent plagioclase, 5 percent muscovite, and 

accessory tourmaline, apatite, and garnet. The zone 

contains massive quartz and large cream-colored 

perthite crystals as much as 3 feet in length. The 

remaining quartz, plagioclase, and muscovite are 

fine-grained.

Economic minerals

Beryl occurs in the intermediate zone as lightgreen crystals. The' maximum length was about 

8 inches. An area of 160 square feet contained 0.1 percent beryl on the basis of a mineral measurement. 

About 2 tons of beryl was recovered during feldspar 

mining.

Most mica produced from the Big Spar No. 1 

mine is scrap quality. About 150 pounds of sheet 

mica, of which 74 percent was no. 2 quality and 

26 percent no. 2 inferior, was recovered during World 

War II. This sheet mica was mined from the intermediate zone.

The intermediate zone contains about 15 percent 

recoverable potash feldspar and most of the past 

production came from this zone. The potash feldspar 

in the core is in graphic granite and, therefore, is 

not acceptable for grinding at the present time.

Pegmatite 14

Pegmatite 14 is in the SWi sec. 21, T. 3 S., 

R. 4 E. (pi. 1). The mine workings include two opencuts: a large cut 45 feet long, 10 feet wide, and 8 feet 

deep, and a smaller one 15 feet long, 6 feet wide, and 

3 feet deep.

14
The pegmatite is a tabular body about 450 feet 

long and 30 feet thick. It is exposed as a dip slope 

that strikes N. 55° W. and dips 45° SW. The approximate plunge is 45° S. 40° W. The pegmatite is 

crudely zoned, containing a border zone of fine-grained 

quartz, plagioclase, and muscovite-, a wall zone of 

plagioclase, quartz, and perthite, and a core of 

perthite, plagioclase, and quartz.

Quartz-plagioclase-muscovite 

pegmatite (border zone)

A thin border zone, averaging 1 inch in thickness, 

occurs at the contact between pegmatite and wall rock. 

It consists of fine-grained quartz-plagioclase-muscovite 

pegmatite about one-eighth inch in diameter. Most 

of the zone on the hanging wall has been removed by 

erosion.

Plagioclase-quartz-perthite 

pegmatite (wall zone)

The wall zone, averaging 5 feet in'thickness, 

consists of 60 percent plagioclase, 30 percent quartz, 

5 percent perthite, 3 percent muscovite, and less 

than 2 percent tourmaline, garnet, biotite, apatite, 

and beryl. The average grain size of the zone is

1 inch. Yellowish-green beryl crystals are unevenly 

distributed throughout the zone.

Perthite-plagioclase-quartz 

pegmatite (core7

The core is exposed for a length of 420 feet and 

an average width of 15 feet. It consists of 40 percent 

perthite, 35 percent plagioclase, 20 percent quartz,

2 percent muscovite, and 3 percent tourmaline, arsenopyrite, biotite, garnet, and apatite. The average 

grain size of the core is larger than that of the wall 

zone. Large perthite crystals as much as 4 feet in 

length are surrounded by finer grained plagioclase 

and quartz, the average grain size of which is 1. 5 inches. 

About 75 percent of the perthite is graphically intergrown with quartz. Other perthite crystals contain 

inclusions of garnet and tourmaline. Large tourmaline 

crystals, as much as 2 feet in length and 4 inches in 

diameter, are associated with massive quartz. Muscovite books are very subordinate throughout the core 

and have an average diameter of less than 1 inch.

Economic minerals

Beryl crystals in the wall zone average a quarter 

of an inch in diameter and three-quarters of an inch 

in length. In an area of 27 square feet a mineral count 

indicated 0.01 percent beryl. About 20 percent of the 

beryl is cobbable.

The wall zone contains an estimated 3 percent 

of scrap mica.' The larger core contains about 2 percent. The core contains 40 percent of potash feldspar, 65 percent of which is in graphic granite. The 

quality of the potash feldspar is further lowered by 

the presence of inclusions of tourmaline and garnet..

Pegmatite 24

Pegmatite 24 is in the SWi sec. 21, T. 3 S., 

R. 4E. (pi. 1).

The pegmatite is a tabular body, striking 

N. 45° W. and dipping 44° SW. The approximate 

plunge is about 36° S. 24° E. It measures 270 feet, 

in length and has an average thickness of 10 feet.

<

Three zones have been recognized in this pegmatite: a border zone and a wall zone of plagioclase, 

quartz, and perthite, and a core of quartz, perthite, 

and plagioclase.

Plagioclase-quartz-perthite pegmatite 

(border and wall zonesj

Plagioclase-quartz-perthite pegmatite has a 

grain size about a quarter |of an inch that makes up a 

2-inch thick border zone.

The wall zone, about 1 foot thick, consists of 

an estimated 50 percent plagioclase, 30 percent quartz, 

10 percent perthite, 5 percent muscovite, and less 

than 3 percent tourmaline, apatite, and garnet. The 

average grain size is half an inch, although muscovite 

books, as much as 4 inches in diameter, and tourmaline 

crystals, as much as 5 inches in length, are present. 

Medium-grained cream-colored perthite crystals 

average about 3 inches in diameter.

Quartz-perthite-plagioclase 

pegmatite (core)"

The core is composed of most of the visible 

pegmatite. It is 260 feet in length and averages 6 feet 

in thickness. The estimated mineral composition is 

40 percent quartz, 30 percent perthite, 25 percent 

plagioclase, 3 percent muscovite, and 2 percent 

tourmaline, apatite, and garnet. Small, 1-inch 

muscovite books occur with quartz and plagioclase 

of similar size. Blocky perthite crystals, as much 

as 3 feet in diameter, are in a fine-grained matrix 

consisting of all the other minerals of the zone. The 

perthite crystals average 1 foot in diameter. About 

25 percent of the perthite is in graphic granite.

Economic minerals

Scrap mica occurs in both the wall zone and 

core. The wall zone on the hanging wall contains an 

estimated 3 percent of ruby-colored crude sheet mica 

in books about 3 inches in diameter. A small quantity 

of sheet mica occurs elsewhere in the pegmatite.

The wall zone contains 10 percent potash feldspar, mostly too fine grained to be cobbed. Some 

feldspar, however, could be mined from the core. 

The quantity would be small, unless the core thickens 

with depth, as appears likely from the exposed pegmatite contacts.

Pegmatite 53

Pegmatite 53 is in the NEi sec. 29, T. 3 S., 

R. 4 E. (pi. 1). The mine workings consist of a 

single open-cut 54 feet long, 20 feet wide, and 6 feet 

deep.

The pegmatite is a tabular body with a strike 

length of 200 feet and an average thickness of 15 feet, 

strikes N. 30° W., and dips 40° SW. The approximate 

plunge is 35° S. 27° W. It contains a border zone

15
of plagioclase, quartz, and muscovite; a wall zone of 

plagioclase, quartz, and perthite; and a discontinuous 

core of quartz and perthite.

Plagioclase-quartz-perthite pegmatite , 

(border and wall zonej

A thin discontinuous border zone of fine-grained 

plagioclase-quartz-muscovite pegmatite has a maximum 

thickness of 2 inches and a grain size of a quarter of 

an inch.

A wall zone, about 1 foot thick, consists of 

40 percent plagioclase, 30 percent quartz, 20 percent 

perthite, 5 percent muscovite, and 3 percent tourmaline, 

garnet-, and apatite. The average grain size is half 

an inch, but muscovite books as much as 2 inches in 

diameter are present.

Quartz-plagioclase-perthite pegmatite 

(intermediate zone)

An intermediate zone, 12 feet thick and 155 feet 

long, makes up the bulk of the pegmatite. It consists 

of 35 percent quartz, 30 percent plagioclase, 30 percent perthite, 5 percent muscovite, and accessory 

tourmaline, garnet apatite, and beryl. About 80 percent of the perthite is in graphic granite. The grain 

size of the entire zone is about 2 inches. Light-pink 

perthite and graphic granite crystals average 2 feet 

in length, but are as much as 4 feet long and 2 feet 

thick. Quartz and plagioclase are fine-grained. Most 

of the muscovite occurs as aggregates of small, light 

yellow-green books. Pale-yellow crystals of beryl, 

as much as 9 inches in length and 5 inches in diameter, 

are unevenly distributed throughout the zone.

The zone also contains several pod-shaped micarich aggregates as much as 3 feet long and 2 feet wide. 

These aggregates contain an estimated 70 percent 

muscovite, 20 percent quartz, and 10 percent plagioclase. 

The average grain size is three-quarters of an inch.

Quartz-perthite pegmatite (core)

The core is a series of disconnected irregularshaped pods that are about 4 feet in length and 2 feet 

in thickness. The pods contain 60 percent massive 

quartz and 40 percent perthite. Muscovite books half 

an inch in diameter constitute about 2 percent of the 

zone. The perthite is in light-pink crystals that 

average 1. 5 feet in diameter.

Economic minerals

Beryl is found in the intermediate zone in crystals 

large enough to be recoverable by hand cobbing. Measurements of beryl crystals gave 0.01 percent of beryl 

over an area of 200 square feet.

Scrap mica occurs in the three inner zones of 

the pegmatite. The intermediate zone contains approximately 3. 5 percent of crude sheet mica.

Large reserves of potash feldspar occur in the 

intermediate zone, however, 20 percent is in graphic 

granite and is not acceptable for grinding at the present time. About 40 percent of the core is cobbable 

perthite, but the total quantity is low because of the 

small size of the core.

Pegmatite 54

Pegmatite 54 is in the SWi sec. 20, T. 3 S., 

R. 4 E. (pi. 1). The mine workings consist of a small 

open-cut 10 feet long, 8 feet wide, and 5 feet deep.

The pegmatite is a tabular body 110 feet long and 

10 feet thick, that strikes N. 43° W., and dips 46° SW. 

Its approximate plunge is 42° S. 20° W. A shear zone, 

recognizable on aerial photographs as a part of a 

lineament, strikes N. 49° E. and offsets the pegmatite 

at two points. The horizontal displacement is less 

than 5 feet.

The pegmatite contains three zones: a fine-grained 

border zone of quartz, plagioclase, and muscovite; 

a wall zone of plagioclase, quartz, and perthite; and 

a core of quartz, perthite, and plagioclase.

Quartz-plagioclase-muscovite 

pegmatite (border zone)

The border zone is about three-eighths of an inch 

thick but is only discontinuously exposed along the 

pegmatite contact. It is composed of 50 percent quartz, 

35 percent plagioclase, 10 percent muscovite, and' 

3 percent tourmaline, apatite, and garnet. Muscovite 

occurs as pale-yellow books about one-eighth of an 

inch in diameter. The quartz and plagioclase have an 

average grain size approximately the same as the 

muscovite.

Plagioclase- quartz- perthite 

pegmatite (wall zone)~

The wall zone averages 1 foot in thickness and 

consists of 40 percent plagioclase, 35 percent quartz, 

15. percent perthite, 5 percent muscovite, and less 

than 4 percent tourmaline, apatite, and garnet. The 

grain size is about half an inch. Muscovite books 

are present as much as 1. 5 inches in diameter. Creamcolored perthite crystals are as much as 1 foot in 

length but the average is about 2 inches.

Quartz- perthite- plagioclase 

pegmatite (core)"

The core, 8 feet thick and 105 feet long, consists 

of about 35 percent quartz, 30 percent perthite, 25 percent plagioclase, 10 percent muscovite, and 5 percent 

tourmaline, garnet, beryl, and apatite. The core is 

much coarser in texture than the outer zones. It contains large pink perthite crystals as much as 4 feet 

long, surrounded by finer grained quartz and plagioclase. About half the perthite is graphically intergrown with quartz. Tourmaline crystals and muscovite 

books occur in the outer part of this zone. Beryl 

crystals were found in the fine-grained quartz-plagioclase matrix. The crystals are yellowish green, and 

range from 1 to 5 inches in length and half an inch 

to 3 inches in diameter.

Economic minerals

Beryl occurs in the core in crystals large enough 

to be recovered by hand cobbing. The beryl content 

of the core was estimated at 0. 8 percent on the basis 

of a mineral count over an area of 45 square feet. 

The volume of the core, and hence the beryl-bearing 

rock, however, is small.

16
Most of the mica in the pegmatite is scrap quality. 

The wall zone, however, contains 2. 5 percent of crude 

sheet mica in books ranging from 1 to 3 inches in 

diameter.

Recoverable potash feldspar occurs in the core 

zone, but the percent of potash feldspar is low and 

about half occurs in graphic granite. Feldspar production would be small because of the size of the core.

Pegmatite 57 (Silver Top lode mine)

Pegmatite 57 (Silver Top lode mine) is in the 

SWisec. 20, T. 3 S., R. 4 E. (pi. 1). The property 

is owned by R. A. Schull, Custer, S. Dak. The mine 

workings comprise two small open-cuts along the west 

side of the pegmatite.

The pegmatite is a tabular body, 150 feet long 

and 25 feet thick. It is exposed as a dip slope and 

strikes N. 46° W., and dips 50° SW. The plunge of 

the pegmatite is 40° S. 26° W.

The pegmatite contains four zones: a border zone 

of plagioclase, quartz, and muscovite; a wall zone 

of plagioclase, quartz, and perthite; an intermediate 

zone of perthite, quartz, and plagioclase; and a core 

zone of quartz, perthite, and muscovite.

Plagioclase-quartz-perthite pegmatite 

(border and wall zone) '

The border zone is about 3 inches thick and has 

an average grain size of a quarter of an inch.

A wall zone 2 feet thick has a grain size of about 

1 inch. It is composed of 40 percent plagioclase, 

30 percent quartz, 20 percent perthite, and 5 percent 

muscovite. Accessory tourmaline, garnet, biotite, 

apatite, and beryl constitute less than 5 percent of the 

zone. Muscovite is in books as much as 3 inches in 

diameter. Pale-yellow to green beryl occurs in euhedral 

crystals in the inner part of the wall zone. Although 

a few crystals are as much as 3 inches long, the 

average is less than 1 inch. Most of the beryl is 

fractured and altered to a clay-like material. The 

content of beryl in the zone was estimated at 0.1 percent.

Perthite-quartz-plagioclase pegmatite (intermediate zone) 

An intermediate zone, 18 feet'thick and 140 feet 

long, consists of 45 percent perthite, 30 percent quartz, 

20 percent plagioclase, and about 3 percent muscovite. 

Accessory tourmaline, apatite, garnet, and beryl 

constitute about 2 percent of the zone. Coarse-grained 

perthite and graphic granite crystals average 2 feet 

in diameter, and are enclosed in a matrix of quartzplagioclase-muscovite pegmatite with an approximate 

grain size of three-quarters of an inch. Although beryl 

is unevenly distributed throughout the zone, it is'most 

abundant in the outer part. The largest crystal measured was 11 inches long and 2 inches in diameter. Beryl 

was visually estimated at 0.1 percent of the zone.

Quartz-perthite-muscovite 

pegmatite (core7

The core is several disconnected lenticular pods, 

less than 10 feet in length and 5 feet thick, that are 

located near the center of the intermediate zone. The 

pods contain about 55 percent quartz, 40 percent perthite, and 5 percent muscovite. Accessory tourmaline, 

apatite, and beryl constitute iess than 3 percent of 

the pods. Quartz and perthite are coarse-grained, 

but the muscovite is fine-grained, occurring in half 

an inch books. Beryl crystals were observed in two 

pods. The largest crystal was 13 inches long and 

6 inches in diameter.

Economic minerals

Beryl occurs in all the zones. It is generally 

altered in the wall zone and intermediate zone, and 

fresh in the core. Beryl in the wall zone is too fine 

grained to recover by hand cobbing, but most of the 

beryl from the intermediate and core zones could be 

recovered. The beryl content of the entire pegmatite, 

on the basis of measurement of beryl crystals in 

more than 1, 500 square feet of outcrop, was estimated 

at 0.1 percent.

Mica, in most of the pegmatite, is scrap quality. 

The wall', zone, however, contains about 2. 5 percent 

of crude sheet mica in books as much as 3 inches in 

diameter.

The intermediate zone contains from 20 to 25 percent of cobbable potash feldspar. The disconnected 

pods of the core have a similar percentage of recoverable feldspar, but the volume of the core is much 

less than that of the intermediate zone.

Pegmatite 65

Pegmatite 65 is in the NWi sec. 20, T. 3 S., 

R. 4 E. (pi. 1). No prospecting work has been done.

The pegmatite is a tabular body that strikes 

N. 38° W., and dips 35° SW. The plunge is probably 

about 40° S. 26° W. The pegmatite is exposed for 

180 feet along the strike and has an average thickness 

of 10 feet. '

The pegmatite contains at least four zones: a 

thin border zone of quartz, plagioclase, and muscovite; 

a wall zone of quartz, plagioclase, and perthite; an 

intermediate zone of quartz, plagioclase, and perthite; 

and a core consisting of disconnected pods of quartz 

and perthite.

Quartz-plagioclase-muscovite 

pegmatite'(border zone)

A border zone 1-inch thick occurs at the pegmatite contact. It consists of fine-grained (averaging 

less than a quarter of an inch) quartz-plagioclasemuscovite pegmatite.

17
Quartz-plagioclase-perthite 

pegmatite (wall zone)

The wall zone is 0. 5 foot thick and contains approximately 40 percent quartz, 35 percent plagioclase, 

10 percent perthite, 10 percent muscovite, and less 

than 3 percent tourmaline, beryl, and apatite. The 

grain size of the wall zone is about half an inch. 

Perthite crystals, having an average diameter of 

2 inches, are enclosed in a fine-grained matrix of 

the other minerals of the zone. The grain size of 

the muscovite is about a quarter of an inch. Finegrained light-green beryl crystals are intergrown with 

quartz and plagioclase.

Quartz-plagioclase-perthite pegmatite 

^intermediate zone)

The intermediate zone is exposed for 175 feet 

along the strike and has an average width of 8 feet. 

It contains an estimated 40 percent quartz, 35 percent 

plagioclase, 20 percent perthite, 5 percent muscovite, 

and accessory tourmaline and beryl. Cream- to lightpink perthite occurs in crystals in finer grained quartz 

and plagioclase. Pale yellowish-green muscovite 

books and light-green beryl crystals range from half 

an inch to 2 inches in diameter.

Quartz-perthite pegmatite (core)

The apparent core is a series of disconnected 

pods, as much as 4 feet thick, consisting of an estimated 

50 percent quartz, 40 percent perthite, 10 percent 

muscovite, and accessory beryl. It can not be determined whether the pods coalesce into a single core at 

depth. Perthite crystals as much as 2 feet long are 

found in the core. Muscovite is concentrated along the 

periphery of the quartz-perthite core in books about 

3 inches in diameter. Beryl crystals average 3 inches 

in length.

Economic minerals

An estimated 0.1 percent of beryl occurs in the 

two inner zones of the pegmatite. The beryl in the 

wall zone can be recovered only by milling, but 30 percent of the beryl in the intermediate zone and 25 percent of the beryl in the core can be hand cobbed.

Mica in the wall and intermediate zones is scrap 

quality. A few sheet mica books, 3 inches in diameter, 

occur along the periphery of the core.

Pegmatite 67 (Dorothy lode mine)

Pegmatite 67 (Dorothy lode mine) is in the SWi 

sec. 27, T. 3 S., R. 4 E. (fig. 2). It is owned by 

Consolidated Feldspar Corp., Trenton, N. J. The 

mine workings consist of an open-cut 100 feet long, 

20 feet wide, and about 30 feet deep. This pegmatite 

was described in a Geological Survey report by W. E. Hall 

(1953?), who visited the property October 31, 1942. 

His report includes a brief description of the pegmatite, 

the muscovite-bearing units, and the feldspar and mica 

production for September and October 1942.

The pegmatite forms a narrow ridge surrounded 

by quartz-mica schist. It is a tabular body that strikes 

N. 70°E., and dips about 46°SE. The plunge is probably

35° S. 23° E. The pegmatite, as exposed, is 150 feet 

long and averages 25 feet thick.

The pegmatite contains four zones: a border zone 

and wall zone of plagioclase, quartz, and muscovite; 

an intermediate zone of perthite, quartz, and muscovite; 

and a core of perthite, quartz, and plagioclase.

Plagioclase- quartz- muscovite 

"pegmatite (border and wall zones)

The border zone is only a few inches thick and 

contains plagioclase, quartz, muscovite, and accessory 

apatite, tourmaline, and garnet. The grain size is 

about a quarter of an inch.

A wall zone 2 feet thick is composed of an 

estimated 55 percent plagioclase, 35 percent quartz, 

and 5 percent muscovite. Accessory tourmaline, 

apatite, and garnet constitute less than 5 percent of 

the zone. The grain size is about 1 inch.

Perthite- quartz- muscovite pegmatite 

(intermediate zone)

The intermediate zone has a length of 142 feet 

and an average thickness of 8 feet. It is composed of 

80 percent perthite, 10 percent quartz, and 7 percent 

plagioclase. Muscovite, tourmaline, and apatite are 

accessory. White perthite is in crystals as much as 

5 feet in length. These are surrounded by a finegrained matrix of quartz and plagioclase, that averages 

three-quarters of an inch in diameter. Locally, either 

quartz or plagioclase may predominate in this finer 

grained matrix. Tourmaline occurs in the fine-grained 

quartz and plagioclase, and may be found as small 

inclusions in the perthite.

Perthite-quartz-plagioclase 

pegmatite (coreT

The core is 12 feet wide and 125 feet long and is 

composed of an estimated 40 percent perthite, 40 percent quartz, 15 percent plagioclase, and accessory 

muscovite, beryl, and phosphate minerals (lithiophilite-triphylite). The core consists mainly of 

massive quartz and coarse-grained perthite. Perthite 

occurs in crystals as much as 4 feet in length. Finegrained quartz 'and plagioclase form small segregations 

and may represent replacement bodies within the core. 

The beryl ranges from 0. 5 inch to 2 inches in diameter.

The core also contains mica-rich aggregates or 

"bull" mica units. These units are as much as 4 feet 

in diameter and contain about 60 percent of small 

muscovite books, averaging half an inch in diameter. 

The remaining minerals are quartz and plagioclase.

Economic minerals

Beryl occurs in the core zone in crystals up to 

3 inches in diameter, only about 20 percent is large 

enough to recover by hand cobbing. The grade was 

visually estimated at 0.01 percent.

Mica in the intermediate and core zones is scrap 

quality. Books of sheet mica, as much as 7. 5 inches 

in diameter, were mined from the wall zone. The 

crude sheet mica content of the wall zone is visually 

estimated at 2. 5 percent.

18
About 70 percent of the potash feldspar in 

the intermediate zone is recoverable at present. 

The core contains only half as much feldspar 

as the intermediate zone and 80 percent is considered to be recoverable.

Pegmatite 68 (Highland lode, John Ross mine)

Pegmatite 68 (Highland lode, John Ross 

mine) is a well zoned pegmatite in the NE-|- 

sec. 30, T. 3 S., R. 4 E. (fig. 2). The pegmatite has been described in detail by Page and 

Stoll (1953?). The following brief description is 

taken from that report:

"The Highland lode pegmatite is owned by 

the New York Holding Association and is leased 

by John Ross who has operated it intermittently 

for feldspar, beryl, mica, and columbite since 

1938. The mine workings include a large opencut, an inclined shaft, stope, adit, and a small 

prospect pit.

"The pegmatite crops out as a roughly oval 

mass, 220 feet long and 150 feet wide. It 

appears to have a shape similar to a flattened 

tear drop. The hanging wall and footwall converge down the plunge to the southwest until the 

pegmatite is probably only 25 feet thick at the 

elevation of the lower workings.

"Three units have been mapped in the pegmatite. These are a wall zone of albite and quartz,

and a core made up of two units, namely, perthite and quartz and quartz, perthite, and albite 

pegmatite.

"The wall zone consists of albite and quartz 

with pink perthite, black tourmaline, yellowish 

beryl, and muscovite. Perthite crystals are as 

much as 4 feet long, and on the hanging-wall 

side of the pegmatite extend to the contact. They 

are less common on the footwall part of the zone. 

The other minerals have an average grain size 

of about 1 inch.

"Both units of the core zone are considerably 

coarser grained than the wall zone. The perthitequartz pegmatite unit of the core is primarily 

perthite with interstitial quartz intergrown with 

a little albite and muscovite. The quartz-perthite - 

albite pegmatite is predominantly quartz with about 

25 percent perthite. Albite, associated with garnet and tourmaline, replaces many of the perthite 

crystals and also occurs intergrown with quartz, 

beryl, columbite, muscovite, and lithiophilite 

between the perthite crystals."

Economic minerals

The Highland lode is predominantly a feldspar mine as indicated by the following figures 

on past production:

Year

1938

1939

1940

1941

1942

1943

1944

1945-50 

Total

Beryl 

(tons)

8.0

11.0

8.25

26.5

4.0

15-75

5-75

3-0

82.25

Feldspar 

(tons)

560

1,877

3,563

1,880

578

886

 

 

9,344

Scrap 

mica 

(tons)

3-5

3-5

2.0

37-0

240.0

57

 

 

343.0

Sheet 

mica 

(pounds)

 

 

 

8.0

2.0

21-7

1.1

 

32.8

Punch 

mica 

( pounds )

 

 

 

140.0

7-0

 

 

 

147. 5-,

Columbite 

(pounds)

 

 

 

. 

 

100

 

 

100

19
Beryl was produced in the past as a byproduct of 

feldspar mining. Green beryl crystals as much as 

4 feet in diameter and 8 feet in length were found in 

the quartz- perthite-albite pegmatite, but commonly 

the crystals range from 6 to 8 inches in diameter. 

The BeO content of these crystals is about 12. 5 percent. The albite- quartz pegmatite of the wall zone 

has pale-yellow beryl crystals ranging from half an 

inch to 6 inches in length and from a quarter of an 

inch to li inches in diameter. Most of these are not 

recoverable at present.

The beryl- rich pegmatite of the core is a flat 

lenticular body, 6 to 8 feet thick, 150 feet in length 

along the strike, and 75 to 100 feet in length along 

the dip. The body is parallel to the footwall contact 

of the pegmatite. Measurements of beryl crystals 

on exposures of the beryl-rich quartz-perthite-albite 

pegmatite gave values of 0. 7 percent and 1.0 percent 

of beryl. Measurements on the albite-quartz pegmatite gave values of 0. 2 percent of beryl.

The mica is mostly scrap quality, although a 

few pounds of sheet mica were sold during World War II. 

It occurs as "bull" mica aggregates in the quartzp'erthife-albite pegmatite and as book mica, mainly 

scrap quality "A" mica, in the albite-quartz wall zone.

Columbite occurs with interstitial albite in the 

core. A few hundred pounds have been recovered 

during feldspar mining. Specific gravity tests indicate 

that the columbite has a TagOs content of 17 percent 

and a NbgOgi content of 63 percent.

Feldspar has been the main economic mineral 

produced in the past. Past over-all recovery has been 

about 36 percent of the total mined rock. The feldspar 

deposit decreases in grade down the plunge of the pegmatite as a result of the pinching out of the perthitequartz unit. The quartz-perthite-albite unit has considerable recoverable feldspar, but it is doubtful if 

it is rich enough to allow profitable mining solely for 

feldspar.

LITERATURE CITED

Adams, J. W., Big Spar No. 1 mine, in Page, L. R., 

and others, 1953?, Pegmatite investigations 1942- 

45, Black Hills, South Dakota: U. S. Geol. Survey 

Prof. Paper 247. [In press. ]

Allsman, P. T., 1940, Reconnaissance of gold mining 

districts in the Black Hills, South Dakota: U. S. Bur. 

Mines Bull. 427, p. 132.

Cameron, E. N., Jahns, R. H., McNair, A. H., and 

Page, L. R., 1949, Internal structure of granitic 

pegmatites: Econ. Geology, Mon. 2, pp. 14, 20, 61.

Connolly, J. P., and O'Harra, C. C., 1929, The mineral wealth of the Black Hills: South Dakota School 

of Mines Bull. 16.

Darton, N. H., and Paige, Sidney, 1925, Central Black 

Hills: U. S. Geol. Survey Geol. Atlas, folio 219.

Hall, W. E., Dorothy lode prospect, in Page, L. R., 

and others, 1953, Pegmatite Investigations 1942- 

45, Black Hills, South Dakota: U. S. Geol. Survey 

Prof. Paper 247. [ In press. ]

Harker, Alfred,- 1939, Metamorphism: 2d ed., rev., 

E. P. Dutton and Co., New York, pp. 227-229, 

234-235, 256.

Johnston, W. D., Jr., 1945, Beryl-tantal-ite pegmatites of northeastern Brazil: Geol. Soc. America 

Bull., vol. 56, pp. 1024-1025.

Newton, Henry, and Jenney, W. P., 1880, Report on 

the geology and resources of the Black Hills of 

Dakota: U. S. Geog. and Geol. Survey Rocky 

Mountain Region.

Page, L. R., and Stoll, W. C., Highland lode (John 

Ross mine), in Page, L. R., and others, 1953, 

Pegmatite investigations 1942-45, Black Hills, 

South Dakota: U. S. Geol. Survey Prof. Paper 247. 

[In press. ]

Runner, J. J., and Hamilton, R. G., 1934, Metamorphosed calcareous concretions and their genetic 

and structural significance: Am. Jour. Sci., 5th ser., 

vol. 28, pp. 51-64.

Van Hise, C. R., 1890, The pre-Cambrian rocks of 

the Black Hills: Geol. Soc. America Bull., vol. 1, 

pp. 203-244.

20

IMT.-DUP. SEC.. HASH.. D.C. 35/79
UNITED STATES DEPARTMENT OF THE INTERIOR 

GEOLOGICAL SURVEY CIRCULAR 245 PLATE 1

*!«£ 

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Inferred contact 

Limit of outcrop Deadwood formation

Zoned pegmatite Probable fault

ttO

PI agio close-qua rtz-pert hite pegmatite Strike and dip of beds

Strike and dip of foliation

32^36

Strike and dip of foliation 

and plunge of lineation

Thin pegmatite bodies showing dip

Quartz-mica-feldspar schist unit

(Pattern in outcrop area)

Plunge of minor fold

E\^33

Amphibolite unit Plunge of lime-silicate ellipsoid

Groups of pegmatites and number (Tables I- 4-, 

letters designate pegmatites referred to in 

text and tables)

Quartz-mica schist unit 

[Pattern in outcrop area- letter E shows 

location of lime silicate ellipsoids)

Geology by Andrew J. Long and Jack A. Redden, 

1948-49

Contact showing dip, dashed where 

approximately located

Datum /s mean sea level

Small mine or prospect

Quartz-mica-feldspar 

schist unit

Amphibolite unit

Quartz-mica schist unit

,//ss

s/'//Ss''~s /////, s s ; , , ' ,

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GEOLOGIC MAP AND SECTION OF THE FOURMILE AREA, CUSTER COUNTY, SOUTH DAKOTA