DEPARTMENT OF THE INTERIOR

HUBERT WORK, Secretary

U. S. GEOLOGICAL SURVEY 

GEORGE OTIS SMITH, Director

Bulletin 765

GEOLOGY OF THE REGION AROUND LEAD

SOUTH DAKOTA

\

AND ITS BEARING ON THE 

HOMESTAKE ORE BODY

BY

SIDNEY PAIGE

WASHINGTON

GOVERNMENT PRINTING OFFICE 

1924
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CONTENTS

Page

Introduction__ ________ 1

Geography _ 2

Geology___________________________ _________ 3

Broad relations____________ 3

Pre-Cambrian geology of the Black Hills_____ __________ 5

Previous work____________:_________________ 5

Broad relations Of the pre-Cambrian rocks _ _ ____ 6

Character of the pre-Cambrian rocks_______________ 6

Metamorphosed sedimentary rocks________________ 7

Igneous rocks ___ 11

Amphibolite___________________________ 11

Granite and pegmatite. 12

Structural geology- _ __ 14

Pre-Cambrian geology in the vicinity of Lead_ _ ___ 14

Stratigraphy 14

Petrography___________. ___ __________ 15

Barren rocks _________ _ _________ 15

Ore-bearing rocks_____________ ________ 16

Chemical analyses of rocks, minerals, and mine water_____ 20

Structure______________ ________________ 25

General features _____ ___ _________ 25

Structure shown on mine levels_____________!__ 29

Mineralization__________ 42

Sulphides ___-___ _ 43

Magnetite _____________________________ 46

Gold_________________________________ 46 

Differences between the Tertiary gold ores in Cambrian

dolomites and ore of the Homestake lode______ ___ 47

Fossil placer deposits in Cambrian conglomerates______ 52

Search for new ore bodies _ 55

Index______ _____ _____ _ __ 57

in
ILLUSTRATIONS

PLATE I. Geologic map of the vicinity of Lead, S. Dak.__ _____________ 14

II. A, Carbonate ore from noi'th end of De Smet cut on surface; B, 

Dolomitic iron-bearing carbonate rock from the 300-foot level 

at B. & M.shafts. _____________________________________ 18

III. A, Highly metamorphic carbonate rock from the 400-foot level 

west of the main fault; B, Cummingtonite rock from the Independence ledge west of the fault.__________________________ 18

IV. A, Chlorite schist associated with small ore body on the 400-foot 

level near B. & M. shafts; B, Ore from the Caledonia ore body 

on the 400-foot level... ____________________________ 18

V. A, Almost complete replacement of a carbonate rock by cummingtonite, green mica, biotite, and chlorite; B, Banded carbonate rock from the 500-foot level of the Incline ore body. _ _ _ 18

VI. A, Carbonate quartz rock making up part of the Old Abe ore 

body on the 700-foot level; B, Carbonate bed near the Terra 

shaft on the 300-foot level________°________________________ IS

VII. A, Silicate band in carbonate rock from the footwall of the 

De Smet ore body on the 900-foot level; B, Ore ledge on the 

1,000-foot level near footwall shear zone.___________________ 19

VIII. Map of 300-foot level__________ ____ ______________ 30

I..- A, Pyrrhotite ore from the 900-foot level; B, Pyrrhotite ore... __ _ 44

X. A, Arsenopyrite cut by pyrite; B, Pyrrhotite, pyrite, and arsenopyrite.________________________________________________ 44

XI. A, Sheared sulphides; B, Arsenopyrite cut by pyrrhotite________ 44

.FIGURE 1. Map of 200-foot level..._____________________ 29

2. Map of 400-foot level.._____________________________ 31

3. Map of 500-foot level.._______________________... 33

4. Ma.p of 600-foot level..__________________________________ 34

5. Map of 700-foot level.__.____._________________ 35

6. Mapof 800-foot level____________ _ _ ___________________ 37

7. Map of 900-foot level......_--___----__-----_------------ 38

8. Map of 1,000-foot level________________________________ 41

9. Pyrite deposited on the rounded edges of arsenopyrite crystals. 43

10. Relation of pyrite and pyrrhotite to arsenopyrite--___________ 44

11. Relation of pyrite and gold to pyrrhotite _-_-_-__-________ 45

12. Pyrite formed during two periods of deposition.____________ 46

13. Grains of gold at contact of pyrrhotite with crystals of arseno-

 pyrite in dark chlorite__--___-_____--_-_-_-____________ 48

14. Grain of gold and accompanying pyrrhotite filling crack in

crystal of arsenopyrite in mass of dark chlorite--.__________ 48

15. Grain of gold with pyrrhotite in iron-magnesium carbonate and

very coarse grained transparent quartz ___________________ 49

16. Small grains of gold with a little pyrite and pyrrhotite, in 

scattered carbonate, with coarse-grained transparent quartz 

and arsenopyrite-_______--__-___________-_-_-_-.______ 49

17. Gold with quartz in mass of hornblende..___________________ 50

18. Small grain of gold with transparent quartz and pyrrhotite in

mass of chlorite---_--_--___----_-___-__-________--_-__ 50

19. Telluride containing bismuth in contact with fine gold_. _____ 51

IV
GEOLOGY! OF THE EEGION AEOUND LEAD, S. DAK., AND 

ITSiBEAEING ON THE HOMESTAKE OEE BODY

By SIDNEY PAIGE

INTRODUCTION

The following account of the geology in the vicmity of the Homestake ore body sets forth part of the results of a more general study 

of the geology and mineral resources of the Black Hills of South 

Dakota. 1 The first systematic geologic investigation of the Homestake mine was undertaken by J. D. Irving and S. F. Emmons, and 

the results of their studies were published in 1904. 2 This investigation did not include the deeper workings of the ^omestake mine, 

for in 1899, during the progress of the work, the managers withdrew permission to study them.

In 1912, while I was engaged in a general examination of the preCambrian rocks of the Black Hills, I obtained permission from Mr. 

T. J. Grier to make a rapid underground reconnaissance, to discover evidence in support of a new hypothesis regarding the origin 

of the ore a hypothesis based on surface studies of the pre-Cambrian stratigraphy and structure. The result of this reconnaissance 

was published in a short paper in 1913.3 The managers did not desire further work at that time. Although brief, this examination 

served to support the hypothesis formulated from a study of the 

surface, and the facts presented and ideas advanced were in some 

respects so at variance with those set forth by Emmons and Irving 

that it was desirable to make a more thorough examination should 

opportunity permit. This opportunity was afforded through the 

courtesy of the superintendent, Mr. B. C. Yates, in the spring of 

1920, it being understood that the study would be concerned solely 

with the " nature and origin of the ore body." The present paper is 

therefore confined strictly to that subject. It is much to be desired

'Darton, N. H., and Paige, Sidney, U. S. Geol. Survey Geol. Atlas, Black Hills folio 

(No. 219) (in press).

a Irving, J. D., Economic resources of the northern Black Hills, with, contributions by 

S. F. Emmons and T. A. Jaggar, jr.: U. S. Geol. Survey Prof. Papei- 26, 1904.

8 Paage, Sidney, Pre-Cambrlan structure of the northern Black Hills,. S. Dak., and its 

bearing on the origin of the Homestake ore body : Geol. Soc. America Bull, vol. 24, pp. 

293-300, 1913.

1
2 GEOLOGY OP KEGION" ABOUND LEAD, S. DAK.

that at some future date a more detailed account of this, mine, perhaps the greatest gold mine in the world, may be published.

I wish to acknowledge the courtesy of the engineering staff and 

the substantial assistance rendered, in the work by Messrs. L. B. 

Wright and Joseph Hosted, geologists for the compa-ny. After 

the completion of my underground examination of six weeks in 

1920 Mr. Wright and Mr. Hosted completed the detaileld mapping 

of the mine. This work extended over a period of two 1; years. It 

was possible, therefore, to check my, generalized maps \vith their 

detailed maps, and it has been a pleasure to acknowledge their 

cooperation by attaching their names to the mine and surface maps 

published in this report.

GEOGRAPHY

The Homestake mine is at Lead (pronounced leed), in western 

South Dakota, in the northern Black Hills. The "hills" embrace 

an oval area 125 miles long and 60 miles wide, with the longer dimension trending nearly northwest. They rise several thousand 

feet above the plains. Abundant rainfall, forests, "parks," and 

streams make th& Black Hills an oasis in this semiarid region. They 

are carved from a dome-shaped uplift of the earth's crust and consist largely of rocks which are older than those that form the surface of the Great Plains. They rise abruptly from the plains, 

although the flanking ridges are of moderate height. The salient 

features are the encircling Hogback Kidge, which constitutes the 

outer rim of the hills; next the Red Valley, a depression that extends completely around the uplift; next a limestone plateau with, 

infacing escarpment; and finally a central area of high ridges culminating in the precipitous crags of Harney Peak, whose crest 

reaches an altitude of 7,216 feet. Two branches of Cheyenne River 

nearly surround the hills and receive many tributaries from them.

Because much of the area consists of high forested ridges not well 

adapted to agriculture the Black Hills support only a small population. The principal stimulus to settlement has been mining, and this 

industry has built up the neighboring cities of Lead and Deadwood, 

the former with a population of 5,013 and the latter with 2,403, 

according to the census of 1920. Most of the mining is nearer to 

Lead than to Deadwood, and the large Homestake mine is in the city 

of Lead. Farther south Ouster, a village of 595 persons, and Hill 

City, with 308, are sustained mainly by lumber and ranching interests in the surrounding country, though some mining has been carried on for. yeats. There are many small farms in the numerous 

" parks " among the hills.
BEOAD RELATIONS 3

The plains country adjoining the Black Hills is largely occupied 

by ranches, most of which are devoted to cattle raising. There are 

several near-by towns and small villages. Eapid City, with a population of 5,777, is the seat of Pennington County and a growing railroad 

center. Here is the State School of Mines, and near by is the Eapid 

City Indian School. Sturgis, with a population of 1,250, is the 

trading center for an extensive ranch country but is partly sustained 

by the United States Army garrison at Fort Meade, 2 miles east* 

of the town. Spearfish, with a population of 1,254, is the largest 

town on the north side of the hills. Whitewood, Hermosa, and 

Fairburn are smaller places.

The Black Hills and the adjoining country are traversed by several lines of railroad. The Chicago, Burlington & Quincy Railroad 

passes from south to north through the center of the hills to Deadwood and Lead and has a branch to Spearfish. The Chicago & 

Northwestern Railway extends along the east side of the hills, has 

a terminus at Lead, and a connecting line from Pierre to Rapid 

City. A branch of the Chicago, Milwaukee & St. Paul Railway also 

reaches the hills at Rapid City. Several smaller railroads run 

through parts of the hills.

GEOLOGY

Although the present report is concerned largely with geologic 

events that occurred in. pre-Cambrian time, a very brief outline of 

the broader features of .geologic structure of the Black Hills is of 

interest, particularly because intrusions of porphyry in Tertiary time 

have disturbed the structure of the pre-Cambrian beds and, it is 

believed, have supplied some gold to the pre-Cambrian ore deposit.

The Black Hills uplift is an irregular dome-shaped anticline with 

its longer dimension trending nearly northwest. The hills, which 

are surrounded by a wide expanse of almost horizontal beds, are 

formed of a nucleus-of Algonkian crystalline rocks around which 

there is upturned a nearly complete sequence of sedimentary formations ranging in age from Upper Cambrian to latest Cretaceous, all 

dipping away from the central mass of ancient rocks. There are also 

in the hills extensive overlaps of the Tertiary deposits that form 

much of the adjoining plain. The region affords most excellent opportunities for the study of stratigraphic relations and variations. 

Many of the rocks are hard, and in such rocks the streams flowing 

out of the central mountain area have cut canyons and gorges in the 

walls of which the formations are extensively exhibited. The structure along the sides of the uplift is that of a monocline dipping 

toward the plains. The oldest Paleozoic sedimentary rocks consti-
4 GEOLOGY OF BEGION AROUND LEAD, S. DAK.

tute the escarpment facing the crystalline-rock area, and each stratum 

passes beneath a younger one in regular succession outward toward 

the margin of the uplift. The Paleozoic and Mesozoic sedimentary 

formations consist of a series of thick sheets of sandstone, limestone^ 

and shale, which are all essentially conformable in attitude. The 

overlapping Tertiary deposits extend across the edges of these older 

rocks. The stratigraphy presents many points of similarity to 

>that of the Rocky Mountains in Colorado and Wyoming but shows 

numerous distinctive local features.4

Tertiary igneous rocks are abundant in the northern part of the 

Black Hills. They occur as a series of separate intrusive masses extending northwestward across the northern part of the structural 

dome from a point near Bear Butte, on the east, to Missouri Buttes, 

on the west, a distance of about 65 miles. At several places there 

are small areas of lava and volcanic breccia.

Within this area streams have cut deeply into the Paleozoic and 

pre-Cambrian rocks, and the structural relations of the intrusive 

masses are unusually well displayed. The shapes of these masses 

have clearly been influenced by the attitude of the rock beds they 

have invaded. Thus the intrusions within nearly vertical pre-Cambrian schist have the form of dikes and stocks. On the. other hand, 

the many sheets and irregular laccoliths that cut the overlying 

Paleozoic sediments and are exposed in all stages of denudation conform more or less perfectly to the bedding planes of the sedimentsor have caused these relatively flexible rocks to rise as domes.

Most of the intrusions lie within an oval area about 20 miles long> 

which trends nearly southeast and extends from Spearfish Canyon 

to a point about 5 miles east of Elk Creek post office. The western 

part of the area, about 10 miles in greatest breadth, surrounds the 

city of Lead. Outlying intrusive masses on the northwest are Crow 

Peak and Citadel Rock; Custer Peak, on the south, and Bear Butte, 

on the northeast, also stand more or less isolated.

The roughly oval shape of this intrusive area is characteristic 

of the other intrusive area lying to the west. These intrusive masses 

are localized in areas where sedimentary strata have been forced up

* For a detailed account of the geology of the Black Hills, see Darton, N. H., Geology 

and underground water resources of the central Great Plains: U. S. Geol. Survey Prof. 

Paper 32, 433 pp., 1905 : Preliminary description of the geology and water resources of the 

southern half of the Black Hills and adjoining regions in South Dakota and Wyoming: 

U. S. Geol. Survey Twenty-first Ann. Rept., pt. 4, pp. 489-599. 1901; Geology and water 

resources of the northern portion of the Black Hills and adjoining regions in South 

Dakota and Wyoming: U. S. Geol. Survey Prof. Paper 65, 1909; Jaggar, T. A., jr., Laccoliths of the Black Hills: U. S, Geol. Survey Twenty-first Ann. Rept., pt 3, pp. 163-303, 

1901; Darton, N. H., and others, U. S. Geol. Survey Geol. Atlas, Oelrichs, Newcastle* 

Edgemont, Sundance, Aladdin, Devils Tower, and Belle Fourche folios (Nos. 85,, 107, 108, 

127, 128, 150, 164), 1902-1907; Paige, Sidney,, and Darton, N. H., U. S. Geol. Survey 

Geol. Atlas. Black Hills folio (No. 219) (in press).
PRE-CAMBRIAN GEOLOGY OF THE BLACK HILLS 5

by igneous rock that is, the areas containing intrusive masses are 

coextensive with the areas of pronounced local deformation.

Although these igneous rocks exhibit considerable diversity in 

color and texture, and although the rhyolite, quartz monzonite, 

monzonite, phonolite, and grorudite families are represented, all are 

chemically related. They were intruded about the same time 

geologically, from a common magma.

PBE-CAMBRIAN GEOLOGY OF THE BLACK HILLS

The nature of the Homestake ore body will be more clearly understood if the general character and structure of the pre-Cambrian 

rocks of the Black Hills as a whole are briefly described.

PREVIOUS WORK 6

The first published mention of the existence of pre-Cambrian rocks 

in the Black Hills appeared in 1859.6 In this paper F. V. Hayden 

recognized the pre-Cambrian age of the granite and metamorphic 

rocks and correlated them with those of the Lake Superior region. 

Again in 1862, 1863, and 1872 he briefly described their nature. 

Winchell 7 in 1875 also described a series of mica slates and mica 

schists below the " Primordial sandstones." The first report, that 

treated in detail of these rocks, however, was that of Newton and 

Jenney,8 published in 1880. This report, in view of the short time 

spent in exploration and the breadth of the field covered, is remarkable for its thoroughness, accuracy, and generally scientific viewpoint. Though some of the conclusions of these authors regarding 

the pre-Cambrian rocks have been modified by subsequent work, 

it is evident that they clearly understood the nature of these rocks. 

Writing at a time when speculation regarding the nature of granite 

was rife, Newton set forth concisely the facts that established the 

intrusive character of the granite in the Black Hills, though a later 

observer, W. O. Crosby,9 disputed his conclusion.

Newton did not recognize the complex folding which had affected 

the rocks; but this is not surprising, for the dips are for the most 

part steep, many of the strata are monotonously alike, and it is only 

by detailed work that the folding can be made out. The basis for

B For summary of the literature covering the Algonkian rocks of the Black Hills, see 

Van Hise, C. R., and Leith, C. K., Pre-Cambrian geology of North America: U. S. Geol. 

Survey Bull. 360,, pp. 727-733, 1909.

6 Hayden, F. V., Acad. Nat. Sci. Philadelphia Proc., vol. 10, pp. 139-158, 1859.

'Winchell, N. H.. Geological report on the Black Hills of Dakota: Chief Eng. U. S. 

Army Rept. for 1874, vol. 2, pt. 2, pp. 1131-1172.

8 Newton, Henry, and Jenney, W. P., Report on the geology and resources of the Black 

Hills of Dakota, with atlas, U. S. Geog. and Geol. Survey Rocky Mtn. Region, 1880.

8 Crosby, W. 0., Geology of the Black Hills of Dakota: Boston Soc. Nat. Hist. Proc.. 

vol. 23, pp. 488-517; vol. 24, p. 11, 1888.
6 GEOLOGY OF BEGION AROUND LEAD, S. DAJK.

his division of the rocks into an older and a younger series, however, 

is not clear, even if considered on purely lithologic grounds, and it 

is interesting to note that the results of petrographic work by J. H. 

Caswell for the same report did not support the division. Caswell 

found the rocks of the " two series" essentially alike.

A number of other workers supported Newton's dual division of 

younger slates and older schists, but the correctness of this view was 

questioned when Van Hise 10 visited the field in 1890. Van Hise 

recognized the folded attitude of the rocks, the cleavage induced by 

the folding, and the secondary foliation produced by the granitic 

intrusion. He found no basis for a division into an older and a 

younger series. He went too far, however, in his assertion that the 

prominent structural features previously taken as bedding are secondary and in his restriction of the schists to the vicinity of the 

granite and to the northern hills, where late porphyries have invaded the rocks.

Later workers, particularly Irving and Jaggar, have described the 

pre-Cambrian rocks and the economic resources of the northern hills, 

though they undertook no systematic areal mapping of the preCambrian formations.11

BROAD RELATIONS OF THE PRE-CAMBRIAN ROCKS

The pre-Cambrian rocks are exposed in an oval area about 60 

miles long and 25 miles wide at the widest part, trending northwest 

from a point 2 miles south of Pringle, in Custer County, to a point 

3 miles beyond Lead, in Lawrence County. The area is the highest 

in the Black Hills dome, and in most of it the Paleozoic and younger 

rocks have been completely worn away. In its extreme northern

part, however, near Lead and Deadwood, there remain many small 

inliers of Paleozoic strata, and the Deadwood-Lead pre-Cambrian 

area is separated from the southern pre-Cambrian area by a narrow 

neck of Cambrian sediments and late intrusive porphyry.

CHARACTER OF THE PRE-CAMBRIAN ROCKS

The pre-Cambrian rocks of the Black Hills may be divided into 

three general classes metamorphosed sediments, metamorphosed 

diorite or gabbro, and essentially unmetamorphosed granite and 

pegmatite. The metamorphosed sedimentary rocks form the bulk

10 Van Hise, C. R., The pre-Cambrian rocks of the Black Hills: Geol. Soc. America Bull., 

Tol. 1,, pp. 203, 244, 1890.

11 Irving, J. D., A contribution to the geology of the northern Black Hills: New York 

Acad. Sci. Annals, vol. 12, pt 9, pp. 187-340, 1899. Jaggar, T. A., Jr., The laccoliths of 

the Black Hills: U. S. Geol. Survey Twenty-first Ann. Rept., pt. 3, pp. 171-303, 1901. 

Irving, J. D., Economic resources of the northern Black Hills pt. 1, General geology, by 

T. A. Jaggar, Jr.: U. S. Geol. Survey Prof. Paper 26, pp. 13-41, with geologic map, 1904.
PRE-CAMBBIAN GEOLOGY OF THE BLACK HILLS 7

of the assemblage. They comprise the metamorphic equivalents of 

common types of sedimentary strata conglomerate, grit, sandstone, 

shale, and limestone. The diorite and gabbro are represented by 

amphibolite derived from intrusive stocks and sheets. The granite 

is locally gneissoid.

Inasmuch as no general map of the Black Hills accompanies this 

report, the description that follows will take no account of localities 

and the distribution of the sediments will be considered only in the 

vicinity of the Homestake mine. The main purpose of this description is to emphasize the originally sedimentary character of the 

bulk of the pre-Cambrian schists.

METAMORPHOSED SEDIMENTARY ROCKS

Conglomerate occurs at various horizons in the series and grades 

upward, downward, and along the strike into sediments of finer 

texture (arkosic grit). These conglomerates have been deposited as 

lenses or channel fillings of various lengths and thicknesses and contain waterworn pebbles and boulders, some of which are a foot or 

more in diameter. The pebbles are predominantly white quartz, 

but blue quartzite, banded quartz and iron oxide, contorted schist, 

black slate, and cross-bedded siliceous schist or sandstone are also 

represented among them. The matrix is mainly quartzitic but contains much feldspar, mica, and chlorite. The conglomerate where 

fresh has a distinctly greenish cast, owing to its micaceous and 

chloritic minerals. The beds have been compressed and distorted 

by crushing, flowage, and recrystallization, and the pebbles and 

boulders have been squeezed into lenticular forms, whose longest 

diameter parallels the dip of the beds. The feldspathic and clayey 

material within the matrix has been converted in great part into the 

micaceous minerals sericite and chlorite.

Coarse arkosic grit is especially abundant and grades into conglomerate on the one hand but into finer-grained sediments on the 

other. It was composed originally of various proportions of feldspar and quartz. The feldspar is now to a considerable degree 

altered to sericite and quartz, and small quantities of chlorite have 

 developed. The chlorite proves that iron and magnesia were present 

in the original sediment. That the quartz grains have been mashed 

is plainly discernible with the microscope. They are roughly lenticular. Much of the debris in this coarse arkose is angular and suggests somewhat rapid accumulation. The sericite, which is abundant, determines the plane of foliation of the rock and may be seen 

in places wrapped around the more rigid grains of feldspar and 

quartz. These arkosic rocks have when fresh a distinctly greenish 

line, due to abundant chlorite, but weathered surfaces are red, yellow,
8 GEOLOGY OF EEGION AROUND LEAD, S. DAK.

or light brown. In color they contrast sharply with the dark slate 

and 9gray wacke described below. The arkose grades imperceptibly 

into finer-grained quartzitic schist, which also is characterized by 

light colors on weathered surfaces.

Large areas are underlain by graywacke, a metamorphic felclspathic sandstone, which grades on the one hand into nearly pure 

quartzite and on the other into slate or schist. 'Although many of 

these rocks have medium-grained texture they are sufficiently resistant to erosion to form rugged hills. The graywacke comprises rocks 

of many textural gradations, from those in which the individual 

grains are plainly visible to those in which the grains can hardly 

be seen. They merge therefore on the one hand into grit and on 

the other into slate and schist. The beds change their texture along 

the strike as well as across the bedding.

Besides the massive quartzite associated with the graywacke there 

are many minor beds of quartzite within the schist series. In general individual beds may divide along the strike and be replaced 

by two or more beds, or they may either thicken or thin. Because 

of such changes and because of the squeezing and tearing to which 

the beds have been subjected their outcrops are neither regular nor 

continuous. Nevertheless they are sufficiently continuous to prove 

of great importance in unraveling the intricate structure of the rocks 

involved.

All the quartzites are lithologically very much alike. They are 

dark gray to blue-black and of vitreous luster when freshly fractured. Almost everywhere they contain vein quartz in networks of 

veins and veinlets and in great or small masses. It is noteworthy 

that the deposition of secondary silica by which these veins were 

formed was more extensive in quartzite beds than elsewhere. This 

may be due in part to the solution and redeposition during metamorphism of silica originally present in the quartzite.

Under the microscope the quartzite is seen to be made up of 

quartz, a little sericite, and an appreciable amount of carbonaceous 

matter. The carbonaceous or graphitic matter is arranged in lenslike films, surrounding aggregates of interlocking quartz grains, 

each aggregate representing a crushed and recrystallized quartz 

fragment. This graphitic material gives the rock its dark color.

The slate and schist form a group of rocks characterized by generally good cleavage and fine' grain. Their colors range through 

light hues of gray or green, darker gray, light and dark brown, to 

black. The very fine grained varieties may have a silky luster and 

wavy cleavage surfaces. In many places a perfect cleavage cutting 

across bedding planes may be noted, although the cleavage is generally 

parallel to the bedding. Where soft rocks are folded and crinkled,
PRE-CAMBRIAN GEOLOGY OF THE BLACK HILLS 9

the cleavage will cross the bedding planes on and; near the axial 

plane of each fold. These rocks consist of quartz, feldspar, sericite, 

biotite., chlorite, carbonate, garnet, and iron oxide. Staurolite and 

tourmalinfe occur in places but are not abundant. Differences in 

original composition and texture and in the degree of metamorphism 

to which the beds have been subjected account for the various kinds 

of schist and slate. Where particular beds are of coarse texture and 

where metamorphism has not been thorough, signs of clastic origin 

are plainly visible. Individual grains of quartz and feldspar remain 

unbroken, set in a matrix of sericite, chlorite, feldspar, and quartz, 

containing scattered grains of iron oxide. Such unbroken grains on 

further deformation have been crushed, recrystallized, and drawn 

out into lenslike forms, and ultimately they can be recognized only 

by the carbonaceous matter that surrounds them.

Biotite-sericite-quartz-feldspar schists are common rocks. The 

biotite may be very scanty or very abundant. The composition of 

the rock may vary noticeably from bed to bed within a few feet 

or may remain the same through a considerable thickness. In many 

specimens the mashed recrystallized grains are plainly visible. In 

others the relatively large fragments are lacking, the original bed 

having been more even textured. In places biotite schist shows with 

striking clearness the clastic nature of the bed from which it was 

derived. The grains of feldspar and quartz may be seen in various 

stages of crushing, and the biotite is alined with the schistosity of 

the rock and is entirely secondary. Here and there biotite has 

been developed after deformation had ceased, as is evident where the 

long axis of the biotite crystal crosses the direction of the. schistosity, 

also where the biotite cr}7 stals are spongelike and poikilitically inclose the other constituents of the rock. Garnets, mostly small but 

some as much as a quarter of an inch in diameter, are present in 

many biotite schists. Few of them are deformed. Apparently they 

have developed later than the schistosity of the rock and have replaced the biotite and quartz or feldspars. Some are solid crystals 

with sharp crystal faces and others are spongelike crystals which 

poikilitically inclose the remaining minerals. Tourmaline is present 

in some of the schists and is usually a contact-metamorphic mineral, for in the granite it is abundant.

Carbonate rocks, though forming but a very small part of the 

pre-Cambrian assemblage, are found at many horizons throughout 

it and are important both stratigraphically and economically. The 

carbonate rocks in places are fine grained, fairly pure, and generally 

light colored, showing various hues of yellow, brownish white, or 

blue. By the addition of quartz or aluminous material they grade 

into the fine-grained arkosic schist. Where originally mixed with
10 GEOLOGY OF REGION AROUND LEAD, S. DAK.

fine aluminous sediment they have become sericite phyllite; by the 

admixture of sand they grade rather abruptly into siliceous schist. 

On weathered surfaces these rocks usually assume darker hues of 

brown and yellow. In many places, too, fine ridges of siliceous 

material stand out on such surfaces. These ridges are especially 

useful as a means of recognizing the rocks in the field. In general 

the carbonate rocks are massive rather than schistose, so that the 

effect of metamorphism on them is less conspicuous than on the more 

sandy beds. Locally, however, impurities that are present in layers 

have altered to sericite and chlorite, and the rock cleaves readily 

along the planes of these layers. The carbonate beds differ in thickness and in persistence along the strike.

At many places the schist derived from impure calcareous or dolomitic sediments, which contained clay and sand, carbonaceous 

matter, and iron, bears little resemblance to the original sedimentary 

rock from which it was derived, and only a practiced eye will detect its nature. Where carbonaceous matter and silica were the 

principal impurities, the schist is graphitic, is dark blue to black, 

and may easily be mistaken for a graphitic slate, but on weathered 

surfaces the fine ridges of silica that stand out in relief give to the 

rock a characteristic appearance.

Banded quartz-hematite beds are developed at several places. 

These beds grade along the strike and across it into the other rocks 

of the series. They are not confined to a single horizon but occur 

either below, above, or partly associated with the carbonate rocks. 

The beds consist of black specular hematite and crystalline quartz 

in alternating layers a fraction of an inch to a few inches in thickness. In many places abundant chlorite and green mica are also 

present. The hematite is in places interlayered with carbonate and 

may be crumpled and broken. Indeed, it is difficult or impossible to 

decide in some places whether a conglomerate or a breccia is present. 

Many of these rocks are certainly breccias. The perfect banding has 

been destroyed, the quartz layers are discontinuous, and in cross 

section the rock presents the appearance of a flattened-pebble conglomerate. It is believed, however, that this coarse clastic appearance of the beds is generally due to brecciation, the stiff quartz 

layers having parted under the stress of close folding and having 

been drawn out into lenslike bodies. This is certainly true at some 

places where the brecciation of the rock has not been masked by 

sufficient pressure and folding to give the fragments that rounded 

form so deceptive in other places. The broken pieces may be seen 

only a short distance removed from one another and still angular 

enough and of such shape as to prove their former continuity. The 

field relations of these rocks indicate that they are sediments. The 

iron may well have been present originally as ferrous carbonate.
PRE-CAMBRIAN GEOLOGY OF THE BLACK HILLS 11 

IGNEOUS ROCKS

The schist is invaded by pre-Cambrian igneous rocks of two periods an earlier group of dioritic intrusives, now highly metamorphosed to amphibolite, and a later group of unmetamorphosed 

granite and pegmatite.

AMPHIBOLITE

The amphibolite is a metamorphic rock derived from igneous 

rocks of diorite-gabbro composition. It is of various hues from light 

gray to deep green or nearly black and is for the most part, especially in large intrusions, of massive appearance. At the borders of 

the amphibolite masses, however, the rock is schistose and in many 

thin sills closely resembles chlorite or biotite schist of sedimentary 

derivation. The amphibolite ranges in texture from medium to 

fine grained, usually according to the coarseness of the original 

igneous rock. The coarser-grained varieties show clearly their 

igneous character: hornblende and feldspar are readily observed 

with the naked eye, and the rock is easily recognized as a diorite. 

The finer-grained varieties, if massive, are also in appearance characteristically igneous rocks, as for example the metadiabases; but 

these rocks where very schistose resemble metamorphic sedimentary 

schist.

The amphibolite is composed of amphibole, feldspar, quartz, chlorite, zoisite, epidpte, calcite, and a little apatite. Pyrite may be present. All these minerals, except some of the feldspar and perhaps 

some of the hornblende, are secondary, are products of dynamic 

metamorphism, and are derived in the main from the breaking down 

of feldspar, augite, and hornblende of the original dioritic rocks.

In none of the rocks observed was augite recognized with certainty. 

Metamorphism has everywhere proceeded too far to leave unaltered 

any of this very common constitutent of diorite and diabase. Except 

for this break in the chain, all degrees of alteration may be observed, 

from rocks unquestionably igneous to those in which no igneous 

texture remains.

The essential process in the metamorphism that produced the 

amphibolite consisted in the combined mechanical and chemical 

breaking down of the original minerals and their replacement by 

metamorphic minerals. The borders or cleavage planes of feldspar 

were first attacked by amphibole or chlorite, or zoisite and chlorite 

may have ultimately developed at a number of points within the 

crystal, completely filling the space once occupied by the feldspar. 

Hornblende penetrated feldspar crystals and replaced them by tufts 

of needle-shaped crystals springing from the border of the hornblende, and zoisite appeared at the same stage within the feldspar
12 GEOLOGY OF REGION AROUND LEAD, S. DAK.

mass. An endless variety of patterns was formed. Zoisite may have 

developed in dense aggregates of small grains replacing feldspar, or 

idiomorphic crystals of epidote may have been formed. Granulation 

of feldspar may be observed in many places, and where the rock 

exhibits considerable schistosity zoisite may be arranged in flowing 

lines, or amphibole may be the dominant mineral oriented with the 

schistosity. In some of the rocks hornblende is entirely lacking and 

chlorite takes its place as the dominant metamorphic mineral.

When the folding of the sedimentary beds and the very steep dips 

that prevail in them are taken into account, it becomes evident that 

the parts of the amphibolite now exposed are but cross sections of 

the masses. Only two suppositions are possible as to the origin of 

these amphibolite masses either that they represent lava flows or 

that they represent intrusive sill-like masses and dikes. The weight 

of evidence, both structural and petrographic, supports the second 

supposition. It is possible to account for certain forked masses, not 

mentioned before, as produced by combined surface and subaqueous 

lava flows, but the petrographic character of the rock that takes 

this form is opposed to such an origin. The undoubted crosscutting 

masses of dioritic and diabasic character, in no important way different from narrow sills, point to an intrusive origin, and the presence of many minor intercalated sills favors this view. It is believed, therefore, that the amphibolites are the metamorphosed equivalents of basic intrusive rocks injected into the sedimentary series.

GRANITE AND PEGMATITE

Granitic intrusives occur principally in the southern part of the 

pre-Cambrian area, but there is a small area of gneissoid granite 

crossing meridian 103° 30' 3 miles north of Nemo. Few dikes are 

found north of the latitude of Hill City. Another mass about 2 

miles long on the western border of the area extends about the same 

distance to the north.

The largest mass of granite unmixed with schist lies south of 

Hill City; it is roughly circular and about 10 miles in diameter. 

East, south, southeast, and southwest of it to the Paleozoic boundaries 

much of the country is cut by numberless masses and dikes of 

granite, both large and small.

Surrounding the principal granite mass is a zone of varying width 

in'which the schistosity follows the contact and along which many 

parallel dikes have been injected. The dikes range in width from 

a few feet to several hundred feet and in places extend without 

interruption for a mile or more. Elsewhere, notably south of 

Sylvan Lake, there are zones of intricate intrusion and impregnation; blocks of schist may be seen in every stage of change from
PBE-CAMBRIAN GEOLOGY OF THE BLACK HILLS 13

those clearly recognized as fragments to those having essentially 

the nature of an igneous rock. Where the contact is of this nature 

it is not possible to draw an accurate boundary line between schist 

and granite.

The granite is a muscovite granite carrying an unusual amount of 

tourmaline. Of the feldspars microcline, orthoclase, albite, and oligoclase may be present. Microcline is the most abundant. Muscovite is almost always present, and biotite, though not abundant, 

appears in some places. Tourmaline occurs in crystals and grains 

and may be either blue or brown in thin section. Apatite, magnetite, zircon, and titanite are common accessory minerals, and 

garnets also occur.

In the coarse varieties microcline is perthitically developed with 

albite and intergrown graphically with quartz. The crystals of 

such microcline are large, commonly several inches in length. Albite, oligoclase, or orthoclase may occur as individual crystals. 

Some of the pegmatite dikes carry a great variety of minerals, 

many of them rare.

The finer-grained varieties of the granite are granular rocks of 

similar mineral composition in which quartz, microcline, and muscovite are almost always prominent. A little biotite may be intergrown with muscovite. 

Much of the granite is so extremely coarse that the term pegmatite fits the rock almost as well as granite. This is especially true 

of much of the larger mass surrounding Harney Peak, where except 

at the borders there is very little admixture of schist. It applies 

precisely to most of the dikes that cut the schist at numberless 

places. There are masses, however, particularly in the complexly 

intruded and broken terranes east and southeast of Harney Peak, 

that are more normal, being medium to coarse granite, with a few 

masses of finer grain.

Perhaps the most striking mineralogic characteristic of the 

granite is the almost invariable presence of more or less tourmaline. 

This mineral is found in both coarse and fine grained varieties and 

very abundantly in the pegmatite. In many places it may be 

counted among the major constituents of the rock. It occurs as fine 

grains or as slender prismatic crystals in the finer-grained granite, 

with quartz and feldspar, and may be the only dark constituent of 

the rock. A little biotite or muscovite or both may be present in 

other varieties, or the tourmaline may be absent, and only biotite 

or muscovite may accompany the quartz and feldspar.

The coarse-grained mass around Harney Peak is composed of 

quartz, feldspar, tourmaline, and muscovite. Its texture is notably 

irregular. Feldspar is micrographically intergrown with quartz,

101062 24 2
14 GEOLOGY OF BEGIOX AROUND LEAD, S. DAK.

and individual crystals of the feldspar may attain large size yet be 

literally filled with quartz. Irregular masses of this quartz are 

also abundant but sporadically distributed through the rock. Tourmaline or muscovite or both may occur in groups of large crystals 

or may be disseminated throughout the rock.

A rude layering, visible only on a large scale and due in a broad 

way to the arrangement of the minerals of the rock, may be observed 

in this central mass. It dips outward with the dip of the schist and 

flattens as it approaches the summit of the mass. It is probably 

connected with pressure exerted during intrusion.

STRUCTURAL GEOLOGY

Broadly viewed, the pre-Cambrian rocks of the Black Hills comprise a thick series of slate and schist, for the most part monotonously alike, which strike northwest and generally have steep dips 

to the east, except in the extreme southwestern part of the hills. 

On the south intrusions of granite break through the strata, and 

around the principal mass, which includes Harney Peak, a schistosity parallel with the granite contact is developed.

A closer study, however, shows that the schistosity is for the most 

part parallel to the bedding; that theories is compressed into a 

number of great folds, upon which are innumerable minor isoclinal 

folds; and, finally, that a sufficient number of individual beds can be 

traced to establish the position and nature of the greater axes of 

folding and to reveal the presence of important faults.12 The structure in the vicinity of the Homestake mine is described on pages 

25-42.

PRE-CAMBRIAN GEOLOGY IN THE VICINITY OF I/EAD 

STRATIGRAPHY

It is possible to subdivide the pre-Cambrian metamorphic sedimentary rocks in the vicinity of the Homestake mine into lithologic 

groups and by tracing certain beds to decipher the major folds in 

which they are involved. The boundaries of these subdivisions are 

drawn on Plate I. From east to west, presumably in descending 

order, the schists comprise the following groups: (1) Biotite schist 

and slate, characteristically exposed in the region around Deadwood 

and farther west. (Near the base of this group there are bodies of 

amphibolite schist derived from basic intrusions. These are not 

shown on the map.) (2) A thin layer of quartzite accompanying 

graphitic slate and locally strongly pyritized. This bed has been

13 See Paige, Sidney, and Barton, N. II., U.. S. Geol. Survey Geol. Atlas, Black Hills 

folio (No. 219) (In press).
U. S. GEOLOGICAL SURVEY BULLETIN 765 PLATE I

EXPLANATION

PRE-CAMBRIAN

Quartz- pyrite 

replacement veins 

accompanied by 

quartzite

Cambrian 

sediments

Biotite schist 

and slate

Garnetiferous 

biotite schist

U ndifferentiated 

pre-Cambrian

GEOLOGIC MAP OF THE VICINITY OF LEAD, S. DAK.

By Sidney Paige, L. B. Wright, and Joseph Hosted , * '

Shearzone
PETROGRAPHY 15

traced with minor interruptions from the Cambrian contact on the 

north to a point near Kirk, beyond which I did not determine its 

position with assurance, but it has been further traced by Wright 

and Hosted and appears to swing westward around the nose of the 

Lead anticline. (3) A group of garnetiferous and biotitic schists 

lying directly beneath this quartzite. (4) A number of thin layers 

of quartzite interbedded with slate (75 feet in thickness), which have 

been followed, with interruptions by porphyry and Cambrian cover, 

from the basal Cambrian contact on the north into the Plomestake 

mine, where they are involved in folds with which the ore body is 

associated. (5) A series of argillaceous slate, dolomitic and calcareous slate, and schist, directly beneath these thin quartzites. 

Within this dolomitic series and not far beneath the overlying quartzites are a number of highly metamorphosed beds carrying varying 

amounts of the amphi^ole cummingtonite and in places also garnet. 

Two of these beds (separated by 150 feet of slate) have been followed with interruptions from Bobtail Gulch to and into the Homestake ore body, of which they form a very important part.

PETROGRAPHY

The general petrographic features of these pre-Cambrian rocks 

have already been described somewhat fully. Here, therefore, I 

will mention only briefly those units that are not important as ore 

carriers but will describe in detail the carbonate series, which carries the bulk of the Homestake ore.

BARKEN ROCKS

1. The biotite schist, and slate, which are abundant in the Black 

Hills, are composed of quartz, feldspar, biotite, and sericite in proportions that vary like those of sand and clay in unmetamorphosed 

sedimentary rock. The composition may vary noticeably from bed to 

bed within a few feet or may remain essentially constant through a 

considerable stratigraphic thickness. With a microscope the grains of 

quartz and feldspar may be seen in various stages of crushing, and 

the biotite is alined with the schistosity of the rock. The essential 

thing to note here about these rocks is their generally siliceous 

character. They are not rocks particularly susceptible to replacement by ore-bearing solutions except along open fractures.

2. The quartzite and graphitic slate are grouped together because 

of their prominence at the surface. They are very useful in deciphering structure. The quartzite, though thin, is persistent and is 

associated in all its outcrops with a black graphitic pyritized slate. 

Surticial oxidation of the pyrite has produced a prominent outcrop 

" comb ? ' of red iron oxide and silica known locally as the " iron
16 GEOLOGY OF REGION" AROUND LEAD, S. DAK.

dike." Pyrite is so abundant in some places that it was mined in the 

early days, notably at the Montezuma & Whizzers property, in Deadwood Gulch, and used by the smelters. In places a slate lies above 

the quartzite, in places below. In still other places the quartzite is 

represented by two beds. This irregularity is not surprising, however, for beds of sandstone at many places are conspicuous for their 

variation along the strike. The quartzite and graphitic slate' are 

important in this discussion wholly because of their structural significance. They occur nowhere in the mine proper.

3. The garnetiferous schist, directly beneath this thin quartzite 

and black slate, is in all its broad essentials similar to the biotite 

schist overlying the quartzite and black slate, but it generally carries varying amount of garnets, which are mostly small but may be 

very abundant. Apparently the garnets were developed at a slightly 

later stage than the schistosity of the rocks, and they appear to have 

replaced biotite, quartz, and feldspar. Some are solid crystals with 

sharply defined faces; others are spongelike crystals that inclose the 

other minerals. The composition of the garnets indicates a change 

of sedimentation in the direction of increased iron, magnesia, and 

calcium carbonate, which become so conspicuous in the underlying 

rocks.

4. The narrow band of alternating thin quartzites and slates, about 

75 to 100 feet thick, presents no unusual petrographic features but 

is important structurally, as the group can be traced on the surface 

and recognized in the Homestake mine. These quartzites overlie the 

ore at many places, though in other places apparently the ore horizon stops somewhat below them. The beds, as might be expected 

from their origin, change from place to place in thickness and 

relative position with reference to the included slate bands and 

have been thickened, thinned, and broken by folding. Nevertheless, 

as a group they are persistent and can be used to aid structural 

interpretations where all other evidences of the intricacies of folding 

are lacking.

ORE-BEARING ROCKS

5. The calcareous and dolomitic schists are the rocks that contain 

the ore, and the recognition of the notable amount of carbonate 

in these rocks and the nature of the structure which the beds display 

is the fundamental basis of an adequate understanding of the Homestake ore body. Therefore, a more detailed description of their 

mineralogy follows.

The carbonate series includes many varieties of schist in which, 

besides varying amounts of iron, magnesium, and calcium carbonate, there occur biotite mica, sericite mica, phlogopite (green mica), 

chlorite, amphibole, garnet, and quartz.
PETROGRAPHY 17

In the railroad cut opposite the mouth of Blacktail Gulch, on 

Deadwood Creek, a fairly pure band of the calcareous rock crops 

out, and there are many places in Bobtail Gulch (the small gulch 

on which Terraville is built), as well as in Deadwood Gulch in the 

vicinity of Central City, where the calcareous beds, more or less 

impure, may be observed.

A highly metamorphic bed that crops out on Deadwood Creek 

a short distance below the mouth of Blacktail Gulch and can be 

traced, with interruptions, to the Homestake open cut and thence 

into the mine is peculiar in that -it contains many small flattened 

lenses and bands of sugar-grained quartz set in a compact matrix 

of cummingtonite, the iron-magnesium amphibole, and a lime-ironaluminum garnet. The development of these silicates suggests at 

once that the bed may be the metamorphic equivalent of a sideritic 

dolomite and that the quartz may represent residual chert. A second 

bed similar to this one occurs a short distance lower stratigraphically but can not be traced so far, though in the upper workings of 

the mine and on some lower levels there are certainly more than one 

of these beds.

A common variety of this calcareous schist is characterized by 

chlorite and biotite. This rock exhibits various shades of green 

and has in places a decidedly micaceous appearance and a moderately well-developed cleavage. On casual inspection it shows little 

resemblance to limestone, though it may carry as much as 50 per 

cent of carbonate. A variety noted at a number of places in the 

northern hills is light grayish green and spotted with red iron 

oxide. It is a schistose rock composed of quartz and chlorite, 

throughout which are scattered patches of oxidized ferriferous 

dolomitic carbonate. There is no possibility of this carbonate being 

secondary in origin. It is certainly an integral part of the original 

rock.

An unusually pure carbonate rock, probably a dolomite, occurs in 

Deadwood Gulch just west of the small mill town of Central City.

As may be plainly seen by consulting the geologic map (PI. I) 

the strike of this carbonate series carries it directly into the Homestake mine, and in the following section under " Structure " and in 

the descriptions of the ore bodies its relations to the ore will be 

pointed out. Abundant specimens of ore and wall rock were collected and examined with a microscope, and a number were analyzed 

to prove that the series represented in the ore zone is this carbonate 

series. A description of some of the types found in the mine follow.

At the extreme north end of the ore body, on the surface, in the 

De Smet open cut, a remnant of ore remained undisturbed in the 

trough of a minor synclinal fold. This rock is dark green to black 

and rather massive and micaceous. With the microscope abundant
18 GEOLOGY OF REGION AROUND LEAD, S. DAK.

carbonate can be seen, which on analysis proves to be magnesium 

and iron carbonate. The percentages of lime, magnesia, iron, and 

carbon dioxide soluble in acid were determined as follows: CaO, 

none; MgO, 2.6; Fe2 03 , 23.9; C02 . 7.2. From these figures the percentage composition of carbonates was calculated, thus: CaCO3 , 

none; MgC03 , 5.4; FeC03 ,1.1.6; Fe2 03 (as limonite), 16. Manganese 

carbonate was not present. The carbonate is abundant and, as may 

be seen in Plate II, A, appears as cloudy patches set in a groundmass of quartz and green mica. The border of many of the carbonate patches has altered to a brownish-red mineral, possibly limonite. There is no doubt that this carbonate is an integral and original 

part of the rock, and that this rock was before metamorphism a 

dolomitic or sideritic sandstone. This rock is the ore carrier of 

the De Smet ore body of the Homestake mine.

On the 300-foot level at the B. & M. (Old Abe) shaft a bluish 

banded calcareous schist is made up dominantly of carbonate with 

abundant chl-orite, some mica, some amphibole, and subsidiary 

quartz. (See PL II, /?.) "

An analysis shows the following percentages of carbonates: 

CaCO,, 5.5; MgCO8 , 9.C; FeCO:! , 4.7; also Fe2 08 (as limonite), 28.3. 

Here, as in many other specimens to be described, it is clear that the 

mica and amphibole have replaced carbonate. This rock was originally a dolomitic iron-bearing carbonate rock. This rock is related 

to the small Old Abe ore body, of which the ledge matter consists 

of such materials in varying degrees of replacement by amphibole, 

mica chlorite, and sulphides.

Another specimen collected at the edge of the ore body on the 

300-foot level is a siliceous schist but contains layers of carbonate.

Typifying the more highly metamorphic examples of these carbonate rocks is the " 80-foot " ledge on the 400yfoot level. This 

stratum is west of the main ore body and west of the footAvall 

shear zone and is not shown on the map, but it is typical of the 

highly metamorphic strata found also east of the fault. Here 

there is an abundant development of cummingtonite and green mica, 

enmeshed in a groundmass of quartz and a subsidiary feldspar. 

Residual carbonate is seen with the microscope, and Plate III, /i, 

illustrates clearly the characteristic fan-shaped development of cummingtonite, the iron-magnesium amphibole. It is significant that 

in places the carbonate, like this silicate, proves to be quite free 

of calcium.

Another specimen, illustrated in Plate III, #, from the small 

Independence ledge on the 300-foot level, shows the growth of 

cummingtonite even more clearly than the previous specimen. At 

this locality the Independence ledge is plunging southeastward, as if 

forming the nose of an anticlinal fold. The specimen was collected
U. S. GEOLOGICAL SURVEY BULLETIN 765 PLATE II

A. CARBONATE ORE FROM NORTH KND OF DK SMET CUT ON SURFACE

B. DOLOMITIC IRON-BEARING CARBONATE ROCK FROM THE 300-FOOT 

LEVEL AT B. & M. SHAFTS
U. S. GEOLOGICAL SURVEY BULLETIN 765 PLATE III

A. HIGHLY METAMORl'HIC CARBONATE ROCK FROM THE 400-FOOT LEVEL 

WEST OF THE MAIN FAULT

B. CUMMINGTONITE ROCK FROM THE INDEPENDENCE LEDGE WEST OF

THE FAULT
U. S. GEOLOGICAL SURVEY BULLETIN 765 PLATE IV

A. CHLORITE SCHIST ASSOCIATED WITH SMALL OHE 1SODY 

ON THE 400-FOOT LEVEL NEAR B. & M. SHAFTS

B. ORE FROM THE CALEDONIA ORE BODY ON THE 400-FOOT 

LEVEL
TJ. S. GEOLOGICAL SURVEY BULLETIN 765 PLATE V

A. ALMOST COMPLETE REPLACEMENT OF A CARBONATE ROCK BY 

CUMMINGTONITE, GREEN MICA, BIOTITE, AND CHLORITE

B. BANDED CARBONATE ROCK FROM THE 500-FOOT LEVEL OF THE INCLINE

ORE BODY
U. S. GEOLOGICAL SURVEY BULLETIN TB.i PLATE VI

A. CARBONATK QUARTZ ROCK MAKING UP PART OF THE OLD ABE ORE 

BODY ON THE 700-FOOT LEVEL

R. CARBONATE BKH NKAR THE TKIIRA SHAKT ON THE :100-FOOT LEVEL
U. S. GEOLOGICAL SURVEY BULLETIN- 7fi:> PLATE VII

A. SILICATE BAND IN CARBONATE ROCK FROM THE FOOT- WALL 

OF THE DE SMET OHE BODY ON THE 900-FOOT LEVEL

B. ORE LEDGK ON THE 1,000-FOOT LKVKL NHAH KOOTWALL SHEAR ZONE
PETKOGKAPHY 19

near the western wall. Plate III, #, shows the rather scant patches 

of carbonate interspersed with the quartz between blades of cummingtonite. Inasmuch as at other localities the cummingtonite may 

be seen as bands in carbonate-bearing schist, there is no reason to 

doubt that this almost complete replacement is simply a matter of 

degree. The cummingtonite has replaced not only the carbonate 

but also much quartz.

At the B. & M. shaft on the 400-foot level a chlorite schist associated with the small ore body there developed contains much carbonate with the quartz that makes up the bulk of the rock.' The 

peculiar nodular, spongelike growth of the chloVite is unusual. 

(See PI. IV, A.)

Specimen A from the Caledonia ore body on the 400-foot level, 

showing blades of chlorite in a groundmass of carbonate and quartz, 

is illustrated in Plate IV, B. An analysis showed the following percentages: CaC03 , 0.7; MgC03 , 2.5; FeC03 , 11.6.

Plate V, A, illustrates the massive ore of the Caledonia ore body 

on the 500-foot level. It consists almost wholly of abundant cummingtonite, green mica, and chlorite with quartz.

Here and there throughout the mine a nearly pure layer of carbonate rock may be found interbedded with the more impure beds. 

Such a layer occurs on the 600-foot level near the Deadwood Terra 

shaft, and there is another on the 500-foot level. As is clearly shown 

by a partial analysis, this rock contains much carbonate, the percentages being, CaC03 , 22.7; MgC08 , 16.1; FeC03 , 9.3; MnC03 , 2.

An unusually good example of the carbonate-bearing ledge matter, shown in Plate V, B. was collected from the Incline ore body, 

where there is more structural evidence than in other parts of the 

mine to prove that all the ore bodies are merely metamorphic sedimentary beds. This specimen illustrates with particular perfection a 

banded carbonate and quartzose rock in which some of the carbonate bands are replaced by cummingtonite. In the illustration 

most of the granular gray material is carbonate, most of the white 

is carbonate and quartz, and the bladed mineral is cummingtonite. 

Other parts of the rock sho\v less cummingtonite and more carbonate 

banded with quartz. Analysis gave the following percentages: 

CaCO3 , none; MgC03 , 5.2; FeC03 , 13.4; Fe2 0, none; limonite, 11. 

Calcium is lacking. No better petrographic proof is needed to support the argument that the main ore ledges are carbonate-bearing 

sedimentary beds.

The rocks near the DeadAvood Terra shaft on the 500-foot level 

contain quartz, carbonate, and much carbonaceous matter. Plate 

VI, B, illustrates a purer bed of light-green rock clearly containing 

carbonates and quartz and sericite in large blades. A partial analy-
20 GEOLOGY OF KEGION AROUND LEAD, S. DAK.

sis showed the following percentages: CaC03 , 20.9; MgC03 , 10; 

FeC03 , 16.1.

On the 700-foot level a carbonate-bearing siliceous rock makes up 

at least part of the small ore body mined near the Old Abe shaft. 

What were evidently layers of carbonate interbedded with quartz 

have been pulled apart and are now patches of calcite or dolomite. 

Some of the flowage lines are plainly visible in the photograph reproduced in Plate VI, A.

The footwall of the De Smet ore body on the 900-foot level is a 

light-gray rock of carbonate appearance, showing narrow bands of 

silicate mineral§, such as those pictured in Plate VII, A. The carbonate bands are made up of quartz and carbonate, and carbonate 

is dispersed through the silicate areas. Biotite and green mica ari 

very abundant.

On the 1.,000-foot level a narrow ore body is composed near the 

footwall of a carbonate quartz rock with abundant chlorite. There 

is more carbonate than appears in the accompanying illustration 

(PI. VII, #), for much of it appears white, like the quartz.

CHEMICAL ANALYSES OF ROCKS, MINERALS, AND MINE WATER

A number of analyses of rocks and rock-making minerals and mine 

waters from the Homestake mine have been published.18 Some of 

these are reprinted and discussed below and corroborate my own 

observations.

Analyses of slates from Homestake mine

Pyrite (FeSj) -- ... ... ... ... ... ...

Silica... _.--------.-----------------------

Soda. -__.--- .--.....- .. . - 

18

Trace.

66.08

16.22

5.55

.04

1.88

.36

1.30

5.30

2.90

99.83

19

1,900

3.51

50.36

12.00

15.45

2.46

1.17

.68

5.08

6.98

97.69

20,

200

2.90

63.90

12.84

8.20

2.26

1.32

6.13

21

200

2.20

62.20

12.45

.11

1.63

2.28

7.74

22

200

4.87

54.20

12.20

10.58

.40

1.76

} 2.V8

10.05

23

100

4.61

53.90

12.21

10.32

/ Trace.

\ 4.12

9.35

24

100

4.05

55.20

12.32

11.14

3.25

10.16

« A small proportion of the iron in No. 18, and a considerable proportion in the 9thers, is in the form of 

ferric oxide; some specular hematite occurs in No. 19, and hematite and limonite in the others. All iron 

not combined as FeSa has been reckoned as FeO.

6 Loss on ignition includes some sulphur from the pyrite. In Nos. 20 to 24 some of this loss is no dotibt 

due to water taken up through alteration subsequent to the opening of the mine workings, some of the rock 

sampled having been exposed to moist air for 20 to 30 years.

The analyses given in the foregoing table are noteworthy in that 

lime and magnesia are low and that these rocks are, with the possible 

exception of No. 19, considered wall rocks.

18 Sharwood, W. J., Analyses of some rocks and minerals from the Homestake mine, 

Lead, S. Dak.: Econ. Geology, vol. 6, pp. 729-789, 1911.
CHEMICAL ANALYSES 

Analyses of chlorite from Homestake m\m,e

21

Silica... ............................................

Water................. __ ....... __ .... __ ......

FeAsS. .............................................

40a 41a

Dark, as found

25.13 

24.98 

Not sep. 

32.44 

Not det. 

Trace. 

9.50 

1.02

26.02 

23.76 

1.88 

30.40 

7.99 

.40 

8.73 

.72 

99.90

40b 41b

Excluding 

sulphide

25.38 

25.23

32.76

Trace. 

9.60

26.21 

23.92 

1.89 

30.63 

8.05 

.40 

 8.80

42

Light

24.20 

20.90 

4.27 

32.34 

9.42 

Trace. 

8.39

99.52

43

Calculated 

from formula

27.25 

23.05

32. 45 

9.11

8.14

Sharwoo'd adds that sample No. 42 was associated with magnesian 

siderite in a specimen that was styled " massive ore " by Irving. I 

have no doubt that one of the carbonate rocks is here indicated. 

Chlorite would be a normal metamorphic derivative of such a rock. 

Several analyses of hornblende in the next table are likewise particularly pertinent in this connection.

Analyses of hornblende and asbestos from Homestake mine and of typical

cummingtonite

"silica...........................

Alumina ........................

Wfcf-pr

44a

52.36

1.54

33.76

".45

8.10

.94

M.68

.89

100. 12

Q OQ7

44b

52.77

1.55

34.02

-.45

8.16

.95 -.40

H.70

100.00

3.37

45

50.36

1.86

34.62

.62

9.86

Trace.

.74

Trace.

.73

98.78

3.28

46

45.66

6.87

31.40

9.20

1.04

.50

.73

.14

2.05

47

46.8

» 5.02

33.0

.31

9.50

1.16

2.90

F

51.09

.95

32.07

1.50

10.29

Trace.

.75

Trace.

3.04

99.69

3.42

O

50.74

.89

33.14

1.77

10.31

Trace

.54

Trace.

3.04

100. 43

° Determined in another sample of similar material.

6 Determined by Penfleld tube.

44a. Hornblende from 800-foot level, light brownish white, in masses of small radial aggregates, containing a little pyrrhotite.

44b. Same, eliminating S as FerSs.

45. Asbestos from 1,100-foot level, long-flbered, white, silky, free from quartz.

46,47. Dark impure masses of radiating hornblende, greenish black; appears to contain both chlorite and 

biotito with Iron oxide.

F, O. Analyses of cummingtonite from type locality, Cummington, Mass., by J. L. Smith and O. J. 

Brush.

According to Sharwood, 14 this hornblende was described as tremolite by Irving and Emmons. " In thin section it is practically colorless or may show a faint tinge of green or brown, but it differs from 

tremolite by containing little or no lime and a much larger proportion of iron, while tremolite is essentially a lime-magnesian amphibole."

* Op. dt, p. 746.
22 GEOLOGY OF BEGION AROUND LEAD, S. DAK. 

Analyses of carbonates from Homestake mine

Color of specimen _

Depth. _ . __ feet..

Percentage composition:

Calcium carbonate.

Magnesium carbonate.

Ferrous carbonate.

Manganese carbonate.

Percentage of impurities in sample.

Association or mode

of occurrence.

Approximate molecular ratio:

CaCO3 --------

MgCOs. ---

FeCOs--...----

48

Brownish

white.

200

99.45

Trace.

.55

None.

1.3

(Slate and

quartz) .

As crust

on slate.

1

0

0

49

Creamy.

600

87.50

7.05

5.68

Not det.

6. 2

(Quartz and

slate) .

With coarse

gold in vug

lining.

20

2

1

50

Black.

600

53.05

40. 10

6.85

Trace.

63.5

(Silica, carbon, pyrite).

Hornblende,

pyrite, carbonaceous

matter.

9

8

1

51

Pure white.

700

49.12

18.08

32.47

.11

30

(Pure

quartz) .

Chlorite,

quartz,and

arsenopyrite.

** 7

3

4

52

Creamy.

800

0

22.5

77.5

Trace.

8.6

(Chlorite,

iron oxides,

carbon,

quartz) . Quartz, arsenopyrite,

pyrrhotite,

hematite.

0

2

5

53

Brown.

iinn TtW

1.19

6.93

90.30

1.67

.64

(Arsenopyrite,

quartz,

chlorite) .

Chlorite,

quartz, arsenopyrite.

1

8

80

This tabulation is based on analyses calculated to material soluble in dilute acid.

Several of these analyses are of particular interest. No. 50 was

a black opaque material found in a number of specimens, usually in patches 

filling the spaces between crystallized minerals. It apparently always contains 

carbonates and carbonaceous matter, sometimes evidently graphitized. Magnetite or specularite is sometimes present; the latter is a form easily mistaken for graphitic flakes. Pyrite or pyrrhotite is usually present. The 

sample analyzed was apparently homogeneous, filling spaces between aggregates of radiating crystals of hornblende on the 600 level.

There is no doubt in my mind that this material was an original 

part of the rock. The ore was a replacement of a dolomitic schist.

No. 52 " was associated with quartz, arsenopyrite, and pyrrhotite 

in what is termed ' massive ore' by Irving." Some specimens formed 

rather large masses, black from included scales and plates of specular 

hematite and carbonaceous matter. One such mass contained coarse 

free gold, and fine gold was found in several specimens. This type 

is, to judge by appearances, common on the lower levels, occurring 

in the "massive" and "contorted" ore, in places associated with 

sulphides and chlorite. This rock is a magnesian iron carbonate. 

The cummingtonite is the natural metamorphic product of such a 

rock, and the residual carbonate was the determining factor in 

affording a rock susceptible of replacement by ore-bearing solutions.

No. 53 was brown carbonate from the West crosscut, Independence, 

short 400-foot level. It resembles some specimens of siderite, and 

similar specimens were found in some of the upper workings associated with more or less oxidized and altered ore, in some places
CHEMICAL ANALYSES 23

filling cracks in barren pyrite. The particular sample analyzed was 

a granular aggregate of minute crystals having a texture like that 

of lump sugar and was associated with quartz, chlorite, and arsenopyrite in rock of the average value. The carbonate was seen to be 

strained and shattered; the cracks had been refilled with quartz.

All these observations by Sharwood are fully in accord with my 

own, noted in other places.
Analyses of waters from Homestake mine and n

[Parts per million]

Total solids...... ......... __._.

Silica... ___________________

Alumina _____ _____ ----------- _ _

Lime ______ ___ ______ . ........

Soda ______________________

Potash.. __ --------_._..--. _ ------ __ .

Chlorine. ____ - _ __ . _____ ..--....

Mine waters

Normal

26

510 

12

} i.

62 

111 

27 

10 

130 

11 

100

27

1,228

0 

101 

370

} »

609 

17 

73

28

695

7 

4

0 

35 

61 

/ 121 

\ 147 

162 

 19 

139

29

306 

18

Fire period, spring, 1907

30

2,670

Trace.

28 

338 

442

427

1,115 

8 

347

31

5,790

7.3

49 

475 

450

1,642

2,339 

32 

600

32

3,120

600 

1,458

33

1,500

/

0 Some suspended matter included; the dissolved portion is chiefly silica.

6 These figures represent COa found in residues from evaporation and should be approximately doubled to g

26. 300-foot level, 1909.

27. 1,100-foot level, dry period, 1906. .

28. 1,550-foot level, wet period, 1909.

29. Pump, B. & M. shaft, 1805.

30. Average sample after fire. Contains 0.0017 per cent of gold.

31. 1,250-foot level.

32. 1,100-foot level.

33. Above 300-feet leve

34. 300-foot level, hot w

35. 36. Low water, 1907

37a, 37b. High water, 1

38. November, 1906. 

39a. May, 1911. 

39b. August, 1911.
STRUCTURE 25

' Sharwood says: 15

Analyses of waters from the mine (Nos. 26-29), samples under normal conditions, as compared with water from other sources in the vicinity, show a 

considerably larger proportion of lime and magnesia. During the fire period 

this condition is even more accentuated.. (Note particularly No. 34.) Contrast these analyses with Nos. 35 to 39, taken from the sources from which 

the water was taken to fill the mine at the time of the fire. The unusual 

amounts noted in the Spearfish Creek (Lead city supply) are probably due to 

the proximity of the overlying Paleozoic dolomitic sediments there prevalent.

The. support that these data lend the thesis here set forth is clear.

STRUCTURE 

GENERAL FEATURES

The pre-Cambrian rocks of the Black Hills comprise (1) a great 

thickness of metamorphosed sedimentary rocks; (2) basic igneous 

rocks which invaded others prior to metamorphism and are in consequence metamorphosed to amphibolites; (3) granite invading alike 

the metamorphic sedimentary and igneous rocks.

The Homestake ore bodies occur entirely within the metamorphosed sedimentary rocks, and as has been shown by field and laboratory observation, the idea that any pre-Cambrian igneous rocks 

are involved in their structure can be dismissed. Although metamorphosed basic dikes occur in the general vicinity of the mine, no 

pre-Cambrian dikes were observed in the mine, and none of the ore 

bodies are composed of such rocks. Granite likewise does not occur 

in the mine, though much quartz in veinlets and masses has been 

introduced into the rocks and is, no doubt, an end product of the 

cooling underlying granite. The structure of the Homestake ore 

bodies, therefore, must be explained wholly by reference to those 

principles which govern the metamorphism, folding, faulting, and 

mineralization of a series of sedimentary rocks.

A study of the field relations here and in other regions proves 

conclusively that where rocks are highly compressed at great depths 

a system of folds is evolved which have certain characteristics 

namely, the major folds carry secondary folds upon their flanks, the 

secondary folds carry a tertiary system, the third system carries a 

fourth, and so on. In their large and in their minor characteristics, 

too, all these folds show notable resemblances. The folded beds are 

generally thicker at their crests and troughs than along their flanks. 

Cleavage has been developed across the bedding of the strata at the 

turns of the folds, nearly at right angles to the axial planes. Soft 

beds are more likely to be folded than hard beds, for hard beds may 

rupture and be pulled apart for long distances. All these features

. 16 Op. cit, p. 738.
26 GEOLOGY OF REGION AROUND LEAD, S. DAK.

may be studied on the small folds, some of them may be observed 

on the intermediate folds, and others may be inferred with respect 

to the major folds after field study.

To decipher the intricacies of the folds near Lead it was necessary 

to trace on the surface a number of definite beds and to follow the 

folds of these beds underground in the mine. Although the pre 

Cambrian rocks are invaded by Tertiary porphyry and their outcrops are concealed in places by the Cambrian cover, it proved possible to establish the position of the major folds and of many of 

the minor folds.

The axis of a major anticline passes southward through Central 

City and swings southeastward near Lead. (See geologic map, 

PL I.) West of this major anticline is a major syncline with a southeastward-trending axis. West of this syncline are other major folds, 

the details of which have not been mapped.

The lowest rocks (stratigraphically) exposed at the surface in 

the major anticline are the impure calcareous and dolomitic schists 

and slates in Deadwood Gulch. Here the westward dips on the west 

limb of the anticline may be observed. These west dips give way 

to the prevailing steep easterly dips opposite the De Smet cut, the 

northernmost extension of the Homestake ore body.

On the geologic map (PL f) are represented the outcrops of a narrow band of quartzite and of several metamorphic dolomitic beds. 

These beds with interruptions may be followed from Bobtail Gulch 

to the Homestake open cut and may also be observed in many places 

underground in the mine. They have been traced also by Wright 

and Hosted through the great open cut. Some of the details of the 

folds in which they are involved are shown in the mapping of the 

major syncline, to the west. A shear zone occurs between the major 

anticline and major syncline, and its position is shown on the 

geologic map.

The relative position of the several open cuts of the Homestake 

system of ore bodies (see PL I) is suggestive of the structure that 

has determined their position. For example, the Caledonia ore body 

is formed by a tightly compressed double fold involving massive 

beds of cummingtonite rock. All the details of this folding have 

not been deciphered, but some of them are discussed below. The 

beds exposed in the Caledonia open cut are the same as those that 

may be observed in Deadwood Gulch, and they may be traced with 

interruptions beyond the Caledonia cut to the Clair cut. Thence 

they pass by folds to the Hercules cut. From the Hercules cut a 

prominent layer of cummingtonite schist can be traced directly into 

the great Homestake open cut, where it becomes involved in a number of anticlinal and synclinal folds that determine the shape and 

position of the several remaining ore bodies namely, the Old Abe
STRUCTURE 27

ore body, the Incline ledge, the Pierce ledge, the Main ledge, and 

the De Smet ledge, really an integral part of the Main ledge. (See 

PL I.)

If for a moment all the disturbance produced by the Tertiary 

porphyry intrusion and faulting is forgotten and it is kept in mind 

that the dolomitic schists do not everywhere make ore, the structure 

becomes very clear in its major features. From the Old Abe ore 

body the cummingtonite-bearing beds pass by way of a double fold, 

first synclinal and then anticlinal, into the synclinal fold that makes 

the Incline ore body. Thence the beds pass to the Pierce ore body, 

which is a tightly compressed anticlinal fold, and thence to the Main 

ore body, made up of a number of closely compressed synclines and 

anticlines which on their west side are involved in a shear zone. 

West of this zone these beds again appear in a major syncline and 

at a number of places are mineralized. If the ore bodies are viewed 

as mineralized portions of a series of folded beds of definite stratigraphic position, most of^their characteristics are readily explainable.

Studies underground have corroborated surface observations. Although man}^ places where underground observations might have 

been advantageous were inaccessible, it can be said in general that 

none of the observations it was possible to make failed to support in 

every particular the hypothesis of structure suggested by observations on the surface and set forth above. The underground observations only served to elaborate and make clearer the intricacies of the 

folding which were hidden by reason of surficial cover and igneous 

intrusion. For example, it is always possible underground to determine the all-important point whether the rocks at any particular 

place in the mine are below or above the quartzite horizon. It has 

been found that the ore-bearing beds are invariably either immediately or a short distance stratigraphically beneath this quartzite. 

The beds above the quartzite, chiefly garnetiferous schists, are everywhere barren of ore. The carbonate beds beneath the quartzite, however, particularly the cummingtonite rock, almost invariably accompany the ore in fact, at most places such rock forms the so-called 

ore-bearing ledges. It is true that the chlorite schist and slate associated with the more highly carbonate rocks carry gold and are 

mineralized in places, but it is this calcareous magnesium and iron 

bearing series of metamorphic rocks that contains practically all 

the ore.

If, now, we consider in its simplest terms the theoretical disposition of a series of isoclinally folded beds superimposed upon a major 

anticlinal arch and cut by a shear zone on its west side, as described 

above, what dominant structural characteristics would such a series 

reveal ?
28 GEOLOGY OF BEGION AROUND LEAD, S. DAK.

1. The series should in all places present no stratigraphic anomalies, barring those that might be caused by faulting, intrusion, or 

completely overturned folds.

2. Folded beds would be thickened on the crests of the folds and 

thinned on the flanks.

3. Duplication of beds where several folds were pressed side by 

side would give the appearance of local thickening of that particular 

group of beds.

4. Brittle beds, such as quartzite, would be broken in places, particularly along the attenuated limbs of the folds.

5. The folds would have a characteristic pitch.

6. If the minor folds on such an anticline were tightly compressed 

and overturned and the series were cut by a mine level, as in the 

Homestake mine, observation on this one level alone would not reveal 

the true state of affairs. It would be necessary to determine from 

regional studies the position and nature of the major fold and the 

relations of the minor folds to it.

7. Where such a series of folds was cut by a fault the beds would 

terminate against it, and on the opposite side of the fault a different 

arrangement of beds should be expected, even though the same series 

. of rocks might be present.

8. The minor overturned folds could be detected only by observations at their crests, and here, if the rocks were brittle, such as quartzite, joint planes parallel with the axis of the fold would simulate 

bedding and tend to deceive the observer, and if the rocks were soft, 

such as slate, a good cleavage would be developed across the bedding, also deceiving the observer.

9. The variety and complexity of individual small folds would be 

great: folds would die out and their place be taken by others, the 

attenuated limbs of folds would develop into faults, soft beds would 

in places be squeezed out completely between harder beds, and the 

observer would have to be constantly on his guard against being 

thrown off the track structurally by minor details. But if it is understood that these minor details indicate very clearly what has 

taken place on a large scale, then their usefulness will be great.

A study of the ore bodies from level to level affords abundant 

data supporting all the points enumerated above. These characteristics will be further discussed in the description of the ore bodies.

The major anticline with its many superimposed minor folds is cut 

by a large shear zone, which limits the mine on the west and may 

be regarded as the footwall of the ore body for many levels. West 

of this zone the major structure is quite different. Here the schists 

form a major syncline with isoclinal dips eastward, and on this 

major fold there are numerous minor folds. The rocks involved in 

this syncline are very similar to those in the major anticline just
STRUCTURE 29

east of the shear zone, 

and there is every reason to believe that essentially the same beds 

are present.

The proof of the 

major synclinal structure lies in the convergence to the north of a 

number of beds of 

quartzite and highly 

metamorphosed dolomitic rock which can be 

traced on the surface. 

Their outcrops are represented on the map. 

Crosscuts underground 

disclose these quartzites, 

cummingtonite beds, 

and various schists similar to those east of the 

shear zone, and they 

are in places mineralized.

STKUCTURE SHOWN 

MINE LEVELS

ON

The following descriptions of mine levels 

with maps are presented to clarify and 

illustrate the structural 

relations of the ore 

body. The maps are 

much generalized. No 

mine shafts are shown, 

nor any coordinate 

lines; stoped areas are 

not differentiated. On 

the other hand, the 

maps indicate the position of all the major 

folds, the relation of 

the folds to the shear 

101062 24 3
30 GEOLOGY OF REGION AROUND LEAD, S. DAK.

zone, and the great fragmentation of parts of the ore body by 

porphry.

It should be borne in mine while studying these maps that all the 

ore bodies pitch to the southeast. The position of these ore bodies 

on each level must therefore be imagined as moved slightly to the 

southeast of its position on the next higher level.

The evidence of the. essential structure is assembled for each level, 

in order that a picture may be conceived of the whole. Intricate 

folding and great dislocations by invasions of Tertiary porphyry almost compel such a treatment.

200-foot level. None of the 200-foot level was examined. Most of 

it is now in the open cut, and but little of the remainder is accessible. 

There are, however, several features observable on the maps of this 

level worthy of comment. (See fig. 1.) The intrusive porphyry, 

which descends to the bottom of the mine, is here seen separating 

the main ore body from both the Incline and the Pierce ledges, and 

this relation is maintained throughout the mine. This level shows a 

number of folds, which at lower levels appear even more characteristically. A large horse of barren slate (A, fig. 1) separates two 

groups of ore-bearing beds, which converge to the southeast, indicating a sharp anticlinal fold. The syncline shown at B, Figure 1, 

is separated into two parts by a horse of slate entering from the 

southeast, made up of beds overlying the ore. This syncline terminates a short distance to the northwest.

If it is kept in mind that the shear zone so clearly shown on 

lower levels passes acrossl the ends of these folds on this level, a 

simple solution of the structure is obtained.

On this level the beds that make up the Old Abe ledge connect 

with the ledge immediately to the southeast by a double fold, 

though the mineralization was not sufficient to make ore.

The Pierce ledge is shown isolated from the other ledges, cut off 

by porphyry. This ledge (see p. 31) is a plunging anticline, broken 

on the west, on some levels, by a clearly defined fault, and it would 

connect with both the Incline ledge and the Main ledge were it not 

for faulting and the invasion by porphyry.

300-foot level. Much of the 300-foot level (PI. VIII) is now 

inaccessible, particularly the footwall country rock. On this level, 

however, are shown all the ore bodies which, when connected up, 

prove the folded attitude of the ore-bearing beds. Here, for example, the suggested connection of the Old Abe ledge northwestward and northward with the Caledonia is very clear. The 

Caledonia apparently terminates at the north in a very sharp 

synclinal fold.

On this level the plan of the ore body takes on the distinct appearance of a group of connected synclinal and anticlinal folds. 

The main portion of the ore body shows an alternation of barren
U. 6. GEOLOGICAL SURVEY BULLETIN 765 PLATE VIH

Later porphyry

Anticlinal axis 

Synclinal axis 

Pitch of anticline

Position of principal folds 

(generalized)

(Carbonate series occurs iv&s 

of shear zone, our is nor mapped.)

MAP OF 300-FOOT LEVEL 

By Sidney Paige, L. B. Wright, and Joseph Hosted
STRUCTURE 31

.horses and connecting 

ore-bearing ledges. 

This arrangement is 

brought out by the 

dotted lines in Plate 

VIII. The upper of 

these lines (stratigraphically) represents 

the approximate position of the base of the 

barren quartzite series; 

the lower represents 

the approximate base 

of the cummingtonite 

series that is, the beds 

that make ore. The Incline ledge is much 

broken by porphyry. 

The De Smet ledge is 

formed by the single 

limb of a closely appressed syncline cut on 

the west by a shear 

zone. It is probable 

that the Independence 

ledge, before shearing 

occurred, connected 

with the De Smet 

ledge. The connecting 

bed is now drawn out 

andi pulled apart. 

There is evidence to 

support the view that 

the structure of the 

Independence ledge is 

anticlinal.

The porphyry is well 

shown in this level 

shattering the nose of 

the major anticline.

400-foot level. On 

the 400-foot level, although porphyry dikes 

have broken the con-
32 GEOLOGY OF REGION AROUND LEAD, S. DAK.

tinuity of the beds, it is possible to recognize even more clearly, 

many of the major folds in which the ore-bearing rock is involved. 

(See fig. 2.) It is noteworthy that the Caledonia ore body can on 

this level be brought into the scheme of folding. On lower levels 

this relation appears even more clearly. Of the structural features 

shown on the map of this level some are determined by observations, 

others inferred from observations at higher and lower levels.

The overturned anticlinal nose of the Pierce ore body is established 

with certainty by the dips and strikes of the quartzite beds in the 

crosscut just south of the ore body.

The second syncline southeast of the Old Abe ledge that is, the 

syncline that carries the Incline ore body is greatly broken by 

porphyry, but it is one of the most persistent and clearest structural 

features in the mine. Observations show that the ledges on the 

eastern limb of this syncline fail in ore along the strike by decrease 

in gold tenor, but they are not terminated by a fold, like the 

Pierce. They connect with the Old Abe ledge through two minor 

folds. On a deeper level there is almost conclusive evidence that the 

western limb of the Incline ledge connected with the Pierce ledge 

prior to faulting. In this connection the great fragmentation of the 

major anticlinal fold by porphyry intrusions should be observed, and 

the near approach of its south end to the north end of the Pierce 

,ledge.

The Old Abe ore body shows an evident trend toward the ledges 

of the Caledonia ore body. Developments have practically established the fact that the Old Abe ore body is connected with the 

Main ore body, on the other hand, by way of an anticline and a 

syncline.

Thus the entire set of ore bodies is brought into an orderly though 

complex system of folded ledges, broken by the porphyry invasion 

and otherwise disconnected as ore bodies because of lack of mineralization.

The footwall of the Main ore body on this level was inaccessible 

at most places, but enough was seen to make it practically certain 

that the long, narrow northern portion of the De Smet ore body 

on this level is a single bed, sheared along its west side. Such a 

shear accounts for the progressive shortening of this limb as depth 

is attained.

500-foot level. Much of the 500-foot level was inaccessible, but 

the principal features of the structure could nevertheless be made 

out with a good deal of assurance, for they may be coordinated with 

folds on the levels above and below. (See fig. 3.)

The Caledonia ore body is considerably wider here than on higher 

levels. The Incline ledge is still much broken by the invasion of 

porphyry, and the Pierce ledge is still separated from the Main
STRUCTURE 33
FIGURE 4. Map of 600-foot level. (By Sidney Palge, L. B. W
STRUCTURE 35

ledge. The De Smet ledge remains a single bed, interpreted as the 

east limb of a close synclinal fold, the west limb of which has been 

sheared off.

600-foot level. Many of the folds of the ore-bearing beds are very 

<clearly shown on the 600-foot level. (See fig. 4.) The closely 

appressed synclinal fold of the Caledonia ledge is particularly 

<clear. This fold is twisted sharply east at its northern termination.
36 GEOLOGY OF REGION AROUND LEAD, S. DAK.

The footwall slate curves around the ore-bearing bed in perfect conformity.

The Incline ledge is less disturbed by porphyry than on higher 

levels, and thus the syncline that forms its two limbs is more clearly 

recognized. It is more apparent, too, on this level that the anticline 

of the Pierce ledge would join both the Incline ledge and the Main 

ledge were it not separated by an intrusion of porphyry.

The folds of the main ore body are very closely appressed, and it 

is this remarkable repetition of beds that makes this wide ore body 

possible. A contrast is afforded by the very narrow De Smet ledge, 

where only a single bed is involved.

700-foot level. At several places on the 700-foot level significant 

structural features may be observed. (See fig. 5.) For example,, 

the anticlinal fold at the south end of the Pierce ledge may be 

clearly seen on this level. The beds of quartzite overlying the ore 

curve around the ore-bearing bed and plunge southeastward. The 

beds dip isoclinally to the southeast. The twist in the ore body on 

this level is similar to the twist on the 1,000-foot level and is due to 

a similar minor anticline and syncline. The straight west side and 

tapering north end on this level suggest a fault. In this respect 

the two levels, the 700 and the 1,000, are very similar.

The syncline of the Incline ledge presents about the same featuresas on higher and lower levels but is perhaps even more clearly 

shown.

The narrow, compressed syncline of the Caledonia ledge may be 

followed to the point of the fold, which is bent sharply eastward. 

The wall rock curves around the ore with perfect conformation.

The narrowness of the Main ore body at its north end is noteworthy on this level; likewise the certainty that strong shear zones 

are in some measure responsible for this condition. There is much, 

in fact, to suggest that the ore body is here bounded on both sides, 

by faults. Pyrite zones may be observed both in the footwall and 

along the hanging wall at the extreme north end.

At the south end of the Main ore body quartzite beds occur in a. 

crosscut opposite the end of the ore body. As these quartzites belong above and near the ore stratigraphically, and as they are not 

found a little farther north, it is very reasonable to suppose that 

they are faulted out by the principal shear zone passing along and 

near this western footwall.

800-foot level. On the 800-foot level several of the major structural features are very clear. (See fig. 6.) Others, such as the-, 

anticline of the Pierce ore body and the nose of the major anticline 

of the Main ore body, were inaccessible and can simply be interpreted 

by analogy with other levels. The folds of the Main ore body on this
STRUCTURE
38 GEOLOGY OF KEGION ABOUND LEAD, S. DAK.

level are nearly parallel with the footwall fault. Most of the footwall on this level was inaccessible, but quartzites were observed in a 

position on the footwall that suggests that they are cut off by a 

fault. Evidence of this fault or shear zone may be seen on this level

iar to the north, in the Highland crosscut, where brecciated pyrite 

zones appear.

The syncline of the Incline ledge is fully developed on this level. 

There is no development east of it to give any clue as to its connection 

with the ledge southeast of the Old Abe shaft, but it is believed 

to connect by way of an anticlinal fold, either simple or complex,
STRUCTURE 39

'somewhere in the region indicated by the dotted lines on Figure 6. 

As has been shown, this connection is clear on higher levels.

The form of the Incline ore body on this level is especially significant. The two limbs of the fold come together at the north end and 

diverge southward. This is exactly the form to be expected where 

 u horizontal plane (the mine level) cuts a plunging synclinal fold.

The small ore body west of the Incline ledge is separated from the 

Main ore body and from the Incline ledge by porphyry, but undoubtedly it was at one time joined to both. Thus it may be considered 

structurally the northernmost extension of this particular syncline on 

this level.

900-foot level. On the 900-foot level many of the major folds of 

the Homestake system are very clearly illustrated. (See fig. 7.) The 

syncline of the Incline ledge, with two limbs diverging southward, 

may be compared with the Pierce ore body, at the south end of 

which heavy quartzites appear on both sides of the ledge. This 

south end of the Pierce ore body does not show an anticlinal nose so 

perfectly as on the 1,000-foot level, and it is probable that the orebearing rock is leaner on this level at this point and that it was 

not mined out to the point where the wall rock curves around it. At 

the south face of the stope on this level the strike of the cummingtonite rock was still parallel to the walls.

The Incline ledge is bounded on the west by porphyry, which 

separates it from the Main ledge, of which there can be no doubt 

that it is a part. It is reasonable to suppose that the small ore 

body at the point marked A on Figure 7 is a part, and probably the 

northernmost extension, of the Incline ledge, as pointed out for the 

800-foot level. At the extreme east end of the main crosscut traversing the Incline ledge cummingtonite rock appears, separated from 

the Main ledge by schist. This cummingtonite rock is probably the 

same ledge folded over on an anticline.

The twist in the middle of the Pierce ore body is no doubt due 

to the same small double fold so clearly to be seen on the 1,000-foot 

level, but this inference can not be verified, as on the 900-foot level 

the ore body was inaccessible at this point.

The narrow anticline at the south end of the Main ore body is 

indicated by the fact that quartzites curve around and cut off the 

ore at the south, thus forming the anticlinal nose. These quartzites 

may be observed in the main crosscut at the south end of the ore 

body. Naturally, in folding so close as that here involved, where 

for most of the distance on both flanks of a fold the dips are 

isoclinal, it is not easy to locate the precise point where the beds 

turn. But, as should be expected, there is a point on each flank of 

the fold where the dips become vertical, and such dips were observed 

in the mine. A nice corroboration of this hypothesis as to the struc-
40 GEOLOGY OF REGION AROUND LEAD, S. DAK.

ture was obtained by locating quartzites in the trough of the major 

syncline, which lies immediately east of this major anticline. The 

question may well be raised here: Why do the quartzites not follow 

the ore closely at all points ? In the first place, not all the potential 

ore-bearing material has been rich enough to mine; second, variations in the composition of beds from place to place inhibited continuous replacement by solutions; third, there is no reason to expect 

that perfect regularity in replacement would occur.

The trough of this major syncline is clearly outlined by the extensive barren horse that projects northwestward into the Main 

ore body. In a mining operation of this type much barren rock 

must in places be removed and milled in. order to obtain the richer 

portions, and therefore the configuration of the mined-out portions of 

the ore bodies does not in some places conform to the bedding planes 

of the " carbonate " beds. It must also be remembered that mineralization was not always confined to exactly the same horizon but occurred from place to place in a group of beds.

The quartzites on the west side of the Main ore body may be observed in the " draw hole " drifts off the main header, but their continuation to the north is truncated by a pronounced shear zone. 

This shear zone limits the ore at its west side, at the extreme north 

end of the Main body, and can be followed southward almost to the 

quartzites. It is reasonable to believe that the shear has cut off the 

quartzites and is a major structural feature. In fact, this fault or 

group of faults extends from the surface to the lowermost levels.

On higher levels, notably the 500, 600, and 700, there seems to be 

ample warrant for believing that the folds which constitute the 

Main ore body trend into and are cut off by the footwall fault or 

shear zone. On the 800-foot level, however, the configuration of

the mined-out ore suggests that the major folds have swung more 

nearly parallel with the footwall fault, and it is also difficult to explain the configuration on the 900-foot level by any other hypothesis.

The interpretation of structure along the western footwall on this 

level that explains the most facts observed is as follows: The quartzites that may be seen along this wall occupy a sharp syncline, the 

axis of which is indicated on the map. Their termination to the 

north is due to this synclinal structure. The shear zone passes along 

and cuts out part of the western limb of this syncline, and the narrow band of ledge matter shown on the map expresses this fact.

1,000-foot level. On the 1,000-foot level the folds of some parts 

of the ore body are clearly illustrated, but the folding of other parts 

is obscured by shearing. (See fig. 8.) For example, the anticlinal 

nose at the south end of the Pierce ore body is clearly indicated. 

The ore is terminated against a curving wall of schist that pitches 

down to the 1,000-foot level. It is the quartzite series that here
FtGUHD 8. Map of 1,000-foot level. (By Sidney Paige, L. B. 
42 GEOLOGY OF BEGION" ABOUND LEAD, S. DAK.

from the Main ore body on this level, as it does on higher and lower 

levels, and the ore mass marked A on the map may well be a portion 

of the unbroken synclinal trough. The curve of the drift parallel to 

the ore is at least very suggestive of a fold at this place.

In the Main ore body the folds can not be so clearly seen, though 

the weight of supporting evidence from higher levels leaves no doubt 

of their presence. The quartzites, for example, may be observed in 

abundance along the main drift to the south of the Main ore body. 

This is the position in which they should occur if this southern point 

of the ore body is a major anticlinal nose. These quartzites may be 

observed in the successive crosscuts from the " main header " northwestward along the west side of the ore. They are believed to turn 

back and make the trough of a closely squeezed syncline, the axis 

of which is indicated on the map, for there is a narrow ore-bearing 

ledge of the normal cummingto'hite rock west of the Main ledge, and 

it lies west of the quartzites. It is this narrow ledge that strongly 

suggests a faulted, western limb of a syncline with the quartzites 

Jying stratigraphically above it.

A well-defined fault follows the west side of the Pierce ore body. 

It can be traced, as indicated on the map, at least three-fourths of 

the length of the ore body. The fault follows the Pierce ore body 

to lower levels and, it is reasonable to suppose, is in a measure the 

explanation of the severance of this ore body from the Incline ledge. 

Porphyry invasion also played a part in this break. Oil the 900-foot 

level the west side of the Pierce ledge (except at the north end and 

the extreme south end) was inaccessible, but porphyry at the north 

end separates the Incline ledge from the Pierce along lines suggesting that the porphyry invasion was influenced by or followed thisfault.

MINERALIZATION

It has been shown above that the Homestake ledges form part of a 

series of folded carbonate-bearing schists, and that the form of the 

ore bodies arises from this fact. It will be shown below that the ores 

are replacement sulphide lodes, and the essential facts of their 

mineralogy will be described and illustrated. It is also of practical 

importance and scientific interest to determine the age of the ore 

deposit. Is it pre-Cambrian, Tertiary, or partly both? Systematic 

search for new ore bodies should be guided to a considerable degree 

by the decision reached. Evidence will be presented, as the ores 

are described, to show that the ore deposit was formed in pre-Cambrian time and probably enriched during Tertiary time a satisfactory conclusion when it is remembered that this deposit is perhaps 

unique in point of genesis, and the mine is among the greatest gold 

mines of the world.
MINEBALIZATION 43

SULPHIDES

All the ores of the Homestake lode carry sulphides, and generally 

in the best ore the sulphides are abundant. In the order of their 

introduction they are arsenopyrite, pyrrhotite, and pyrite. Gold is 

associated with each of these minerals that is, it occurs in them or 

in gangue minerals near by.

The sulphides have replaced portions of the carbonate schist 

.series. Thus the deposit can not be described as a vein nor as a 

series of sheeted veins. It is distinctly a replacement lode. (See 

Pis. IX, J., B; X, B.) The sulphides in the main conform to the 

schistose structure of the rock. They follow bedding planes and 

lines of schistosity, and inference based wholly upon their general 

appearance would be that they had been intensely compressed and1 

folded with the gangue minerals. Although such an inference would be only partly 

true, it can be shown to be 

warranted, in a measure at 

least, for evidence suggests 

that the sulphides were introduced before the final stages 

of the metamorphism.

There is abundant evidence 

that arsenopyrite was the first 

sulphide to be introduced. Its 

typical occurrence is illustrated 

in Plates X, A, and XI, B. 

In both illustrations it may be 

plainly seen, first, that arsenopyrite characteristically develops with crystal faces, replacing the body of the rock 

irrespective of the minerals that constitute it, and in Plate X, A, it is 

particularly clear that subsequent compression and movement have 

broken these crystals and by attrition rounded their edges. On 

these rounded edges and in cracks in the crystals pyrite has been 

deposited. (See fig. 9.) Nowhere was arsenopyrite observed cutting pyrite.

In one specimen pyrrhotite bears precisely the same relations 

to arsenopyrite that pyrite bears in the other specimen. Nowhere 

was arsenopyrite observed cutting pyrrhotite. It follows, therefore, 

that pyrrhotite is also later than arsenopyrite. In some large arsenopyrite crystals the arsenopyrite incloses numerous small masses of 

pyrrhotite. (See fig. 10.) These masses of pyrrhotite may be

Pyrite

FIGURE 9. Pyrite deposited on the rounded* 

edges of arsenopyrite crystals, indicating 

the later origin of the pyrite
44 GEOLOGY OF REGION ABOUND LEAD, S. DAK.

regarded either as poikilitically inclosed in and earlier than the 

arsenopyrite, as contemporaneous with the arsenopyrite, or as of 

later introduction. With the pyrrhotite are small masses of dark 

silicate. They also may have either of the above-stated three relationships. But, as shown above, there is evidence that compression 

of the schists continued after the introduction of arsenopyrite. 

Further evidence lies in the tiny cracks that traverse arsenopyrite 

crystals and that are filled with dark silicates and quartz. (See fig. 10.) 

These indicate that solutions were very active, and that silicate 

formed while arsenopyrite crystals were being broken, or just after

Arsenopyrite 

Pyrrhotite

Pyrite 

Gold

FIGURE 10. Relation of pyrite and pyrrhotite to arsenopyrite. The arsenopyrite is 

replaced by pyrrhotite and dark silicates in small irregular masses and cracks

that event. If this was so, then the probabilities are that both the 

pyrrhotite and some of the dark silicate are later than the arsenopyrite.

These facts seem to prove that arsenopyrite was introduced at 

a late stage of the metamoi>phism of the schists and was partly 

deformed and that shortly afterward pyrrhotite and pyrite were 

introduced.

It may be readily shown that pyrite is later than pyrrhotite. 

Pyrite replaces pyrrhotite and occurs as veinlets cutting pyrrhotite 

and filling angles in groups of pyrrhotite crystals. In Figure 11 some 

of these relations are illustrated. Numerous grains of pyrite occur
TJ. S. GEOLOGICAL SURVEY BULLETIN 765 PLATE IX

A. PYRRHOTITE ORE FROM THE 900-FOOT LEVEL

Note pyrite crystals in pyrrhotile. The pyrrholite follows the schistosity of the rock and 

is later than the large mass of quartz

B. PYHRHOTITE ORE
U. S. GEOLOGICAL STIEVEY BULLETIN 765 PLATE X

A. ARSKNOPYHITK CUT HY I'YIUTE

B. PYRRHOTITK, PYHITK, AND AHSKNOPYRITK (a)
U. S. GEOLOGICAL SURVEY BULLETIN 765 PLATE XI

A. SHEARED SULPHIDES

a, Sheared pyrrholite; b, mixture of slule and pyrrhofile

B. AHSKNOPYKITK CUT BY PYHRHOTITE
45 MINERALIZATION

This pyrrhotite. replacing borders, their upon gold with associated 

proves practically position a such in pyrite and gold of association 

pyrrhotite. the than later introduced was pyrite the that 

pyrite. of generations two of discovery the is interest special Of 

first the of grains pyrite cutting observed were pyrite of Veinlets 

is it as Inasmuch 12. Figure in illustrated are These generation. 

Cambrian in deposited were pyrite of quantities great that known 

Pyrite |=j Gold I 

Gangue Eg?fl Pyr-hotite 

pyrrhotite to gold and pyrite of 11. Relation FIOUBB 

concrete find to gratifying is it lode, Homestake the overlying beds 

ore. Homestake the in generation second the of evidence 

the that view the supporting strongly evidence further is There 

of zone large A time. pre-Cambrian in occurred mineralization first 

been has It body. ore Main the of side west the follows shearing 

shear this that structure geologic on otiscussion the in above shown 

occurs It age. pre-Cambrian of is and folding close to related is zone 

syncline. major a against abuts anticline appressed closely a where 

101062 24 i
DAK. S. LEAD, AROUND BEGION OP GEOLOGY 46

slate of zone mashed a 900-foot, the particularly levels, several On 

corresponding position a in observed be may wide feet 2 places in 

mashed of feet 2 this Within zone. shear this of position the to 

sulphides, the deformed has shearing and folding intense slate 

are pyrrhotite of Masses gold. the also and pyrite, and pyrrhotite 

and state divided finely a in out drawn and edges their at sheared 

a proves whole The similarly. suffered has Gold slate. with mixed 

com- severe under movement 

is occurrence The pression. 

A. XI, Plate in illustrated 

indi- are as forces such No 

in- operated have here cated 

this in time later or Tertiary 

is it contrary, the On region. 

stresses, tensional that probable 

time. that at dominant were 

MAGNETITE

magnetite of occurrence The 

several in abundantly rather 

as clearly and ore of specimens 

than later introduced mineral a 

evidence further affords pyrite 

the of age pre-Cambrian the of 

mag- The mineralization. first 

masses fringing as occurs netite 

pyrite around crystals and 

angles reentrant filling grains, 

f ., ° ' ? 

crystals^ pyrite or groups m 

Pyrite

pyrite, of Veins 

generation second 

Gold Ell 

Pyrrhotite [\ t; 

crys- replacing or cutting J an( periods two during formed 12. Pyrite FIGURE 

/ ? * 6 deposition of 

has Magnetite pyrite. of tals 

lode, Homestake the overlying ores Tertiary the in noted been not 

invasion granitic the with associated mineral common a is it but 

I veins. quartz in and pegmatite in both hills, southern the of 

therefore and ores Homestake the in association this of as it regard 

deposits- the of age pre-Cambrian the of evidence confirmatory as 

GOLD

about and free, occurs ores Homestake the in gold the of Most 

sulphide of sections Polished amalgamation. by caught is cent per 70 

the in be may This'gold gold free show invariably almost ore 

sur- cleavage the along example, flakes for thin as minerals, gangue 

im as or, carbonate; in or quartz, in be may it or chlorite; of faces 
MINEBALIZATION 47

many places, it may be more closely associated with sulphides. It 

has been noted in arsenopyrite, in and on the borders of pyrite grains 

and abundantly replacing or accompanying pyrrhotite.

In Figure 11 is illustrated the occurrence of gold associated with 

pyrite and replacing pyrrhotite. To a noteworthy extent the gold 

occurs at the borders of the pyrite grains but in the pyrrhotite.

Gold in pyrrhotite with a gangue of quartz occurs in veinlets cutting arsenopyrite, also gold and pyrrhotite later than but associated 

with arsenopyrite at the borders. Gold in veinlets cutting pyrrhotite or between pyrrhotite and pyrite is illustrated in Figure 10.

A number of drawings copied from Sharwood also illustrate the 

occurrence of gold. (See figs. 13-18.) It seems probable from these 

illustrations that gold came in with both pyrrhotite and pyrite, in 

pre-Cambrian time, and it will be shown below that probably some 

very fine gold, possibly associated with telluride, was introduced 

in Tertiary time, enriching the pre-Cambrian deposit.

The introduction of the sulphides and gold in pre-Cambrian time 

is believed to have been directly related to the last stages of meta- 

 morphism of the schists and connected with the invasion of granite. 

The statement that the sulphides appear in a certain sequence expresses their dominant relationships. It is probable that no great 

time interval separated the introduction of the several sulphide? 

and the gold that some gold was introduced with pyrrhotite, and 

more later with pyrite. Quartz also was no doubt introduced at 

several stages, or continuously, and the silicates that make up the 

body of the.rock were no doubt undergoing change throughout a 

considerable period, while sulphides were being deposited.

DIFFERENCES BETWEEN THE TERTIARY GOLD ORES 16 IN CAMBRIAN 

DOLOMITES AND ORE OF THE HOME8TAKE LODE

Inasmuch as the question arises, Are not the Homestake ores of 

Tertiary age? it is important to point out that there are striking 

differences between the gold ores of Tertiary age that replace the 

Cambrian dolomites overlying the Homestake lode and the ores of 

the Homestake lode proper. These differences are of two sort's 

differences in mineralogy, emphasized by the need of different 

metallurgic treatment, and differences in silver-gold ratio of the 

bullion. The Tertiary ores, occurring in certain areas where vertical 

fractures traverse the Cambrian dolomites and sandstones, are 

pyritic replacement deposits carrying varying but minor amounts of 

arsenopyrite. Both pyrrhotite and magnetite, the former so important in the Homestake lode, the latter shown to be later than 

pyrite in the Homestake ores, are absent from the Tertiary ores.

18 For a very complete description of the Tertiary siliceous ores see Irving, J. D., op. clt.
48 GEOLOGY OF EEGION ABOUND LEAD, S. DAK.

0.5H

1.0MMrU_04 INCH

FIGURE 13. Grains of gold at contact of pyrrhotite with crystals of arsenopyrite in dark 

chlorlte. (After Sharwood)

2.5 MM.

FIGUKE 14. Grain of gold and accompanying pyrrhotite filling crack in crystal of arsenopyrite in mass of dark chlorite. Much pyrrhotite present in. irregular masses, and 

some pyrite in compact grains. No quartz observable. (After Sharwood)
MINERALIZATION 49

MM. LL-.05 INCH

FIGURE 15. Grain of gold with pyrrhotite in iron-rnanganese carbonate (CA) and very 

coarse grained transparent quartz (Q). "Massive ore." (After Sharwood)

0.5

1.0

rrO

--.01

--02

-;03

--.04

MM.LLOSINCH

FIGURE 16. Small grains of gold with a little pyrite and pyrrhotite, in scattered carbonate, with coarse-grained transparent quartz and arsenopyrite. (After Shanvood)
50 GEOLOGY OF REGION AROUND LEAD, S. DAK.

1.0-

MM.

FIGURE 17. Gold with quartz in mass of hornblende. Hornblende in radiating aggregates,, at this point invaded and largely replaced by quartz, with occasional pyrite and 

scattered masses of iron-magnesium carbonate. (After Sharvvood)

.Orr1

1.0 --.

MM.

L.05 

INCH

FIGURE 18. Small grain of gold with transparent quartz (Q) and pyrrhotite to mass of 

chlorite (C). Much pyrrhotite present in very irregular masses, and arsenopyrite 

showing crystal outlines and small inclosures of pyrrhotite. (After Sharwood)
MINERALIZATION 51

The gold of the Cambrian ores (except the conglomerate ore, discussed further below) is " so fine [fine grained] that no gold could 

be obtained in the pan, an almost universal characteristic of these 

ores." 17 This is quite the opposite from the condition in the Homestake mine, where much gold 70 per cent, more or less is readily 

caught by direct amalgamation.18 The explanation of this difference 

probably rests on the fact that in the ores of Tertiary age the gold 

occurs as a telluride, whereas in the pre-Cambrian ores at least 70 

per cent is free gold. F. E. Smith 19 has shown that tellurium is an

PYRITE

FIGURE 19. Telluride containing bismuth in contact with free gold. (After Sharwpod)

abundant constituent of the Tertiary ores and suggests that sylvanite 

is probably the telluride present. Typical samples yielded as high 

as 8.426 ounces of tellurium to the ton, and the analysis of nine different samples showed the following average percentages: Tellurium, 59.97; gold, 7.64; silver, 32.39. At the Dacy mine the percentages obtained " approximate the composition of sylvanite " 

tellurium, 61.20; gold, 36.27; silver, 2.53.

In the Homestake ores tellurides are rare, though they have been 

noted. (See fig. 19.) It is possible that the portion of the gold 

caught by cyaniding may be present partly in the form of tellurides.

17 Devereux, W. B., The occurrence of gold In the Potsdam formation, Black Hills, 

Dakota: Am. Inst. Min. Eng. Trans., vol. 10, p. 475, 1881. 

18 Sharwood, W. J., op. cit, p. 775.

18 Smith, F. R.,, The Potsdam ores of the Black Hills of South Dakota: Am. Inst. Min. 

Eng. Trans., vol. 27, pp. 404-428, 1897.
52 GEOLOGY OF EEGION AROUND LEAD, S. DAK.

Notable differences are mentioned by Sharwood 20 between the 

silver-gold ratio of Tertiary ores in the vicinity of the Homestake 

lode and the silver-gold ratio of the Homestake ore. Ratios of 5j 

10.58, 11.4, and 4.1 to 1 were noted in the Tertiary ores, whereas in 

the Homestake ores the average is about 0.215 to 1 for bullion caught 

on the plates and about 0.433 for bullion recovered by cyaniding.

The facts set forth above clearly support the view that the first 

mineralization of the Homestake lode occurred in pre-Cambrian 

time. It is very probable, however, that some enrichment of the lode 

took place in Tertiary time. The evidence for this enrichment,, 

though meager, may be summarized as follows:

Tellurides, though rare, have been noted in the Homestake ores. 

Assays are reported by Hosted and Wright to be somewhat higher 

for material near porphyry dikes. Replacement deposits of gold 

occur in the Cambrian dolomites overlying the Homestake lode, 

and it is inherently probable that the solutions which passed upward through the vertical fractures that led to these Tertiary ores 

deposited some gold in the pre-Cambrian rocks. A rough measure 

of this amount may possibly be expressed by the proportion caught 

in cyaniding.

FOSSIL PLACER DEPOSITS IN CAMBRIAN CONGLOMERATES

It is desirable to weigh the evidence pointing to the existence of a 

pre-Cambrian ore body cropping out at the time the Cambrian 

conglomerate, sandstone, and dolomite were laid down that is, 

to examine the validity of the statement that fossil placer deposits 

derived from the Homestake lode occur in the basal Cambrian conglomerates.

living,21 following Devereux,22 has presented evidence to show 

that fossil placers actually exist, but an impartial and critical review 

of Irving's statement and the facts that he presents raises doubts not 

as to the probabilities of the case but as to the conclusiveness of the 

argument on the basis of the conglomerate ores alone. The main 

points made by Devereux and Irving will be very briefly presented.

In describing the gold-bearing conglomerates Irving says:

At the base of the series of Cambrian strata, which lie unconformably upon 

the upturned metamorphic schists of the Algonkian, a conglomerate is usually 

present. It varies in thickness from a few inches to more than 30 feet. At 

a few localities it is entirely absent, but in general it is so prominent a 

feature of the series that it has been recognized as a separate formation. 

* * * There are two localities where it attains an unusual thickness. The 

first is known as Cement Ridge and is on the west side of Spearfish

20 Op. cit, pp. 783-784.

m Irvlng, 3. D., and Emmons, S. F., Economic resources of the northern Black Hills: 

U. S. Geol. Survey Prof. Paper 26, pp. 98-100, 1904.

21 Devereux, W. B., The occurrence of gold in the Potsdam formation, Black Hills* 

Dakota: Am. Inst. Min. Eng. Trans., vol. 10, pp. 465-475, 1882.
MINERALIZATION 53-

Canyon; the second is the region in and about Lead. In the latter locality 

only it is auriferous and has yielded large amounts of gold. * * * The 

conglomerate is overlain by cross-bedded beach sands and quartzites, which 

are in turn conformably overlain by the higher members of the Cambrian 

series. It is underlain unconformably by metamorphic schists and slates. 

In these underlying Algonkian rocks is a great mineralized zone of relatively 

indurated character, which strikes approximately N. 34° W. * * * This 

zone was probably a reef in the old Cambrian sea at the time of the deposition; 

of the conglomerates, as it offered more resistance to erosion than the en~ 

compassing schists on account of the hardness imparted to it by pre-Cambrian 

mineralization. * * * The gold-bearing conglomerate occupies depressions 

in the old Algonkian surface but thins out to nothing along the strike of theHomestake lode, where the higher measures of the Cambrian lap over onto 

the mineralized rocks of the Algonkian. * * * Lithologically this auriferous, 

conglomerate is formed of rounded waterworn pebbles of quartz or Algonkian 

quartzite with an intersprinkling of schist fragments which seem to decrease 

in abundance as one proceeds farther from the Homestake lode. It may be 

at once distinguished from the non gold-bearing portions of the basal conglomerate, as it is cemented by either oxide of iron in the weathered portions, 

or by pyrite when it has not suffered alteration. The nonauriferous conglomerate, on the other hand, has always a quartzitic or in rare instances 

a slightly calcareous matrix. The pyritic cement occurs in all the productive 

areas except one, and, as all degrees of oxidation are present, it can beassumed that the matrix of all the gold-bearing conglomerate was once pyrite.. 

Much of the gold in the richest conglomerates is detrital, as proved by its. 

waterworn condition and its concentration near the bedrock. This gold was 

derived undoubtedly from the erosion of auriferous lodes in the Algonkian rocks 

and was mechanically deposited in depressions along the old Algonkian shore 

line. Some of the gold was dissolved by ferric sulphate resulting from the 

oxidation of the pyrite and from this solution was redeposited in thin films 

in the schists below. This has also produced an enrichment of the lowermost: 

layers of conglomerate.

Up to this point it is clear that Irving regards gold in these 

deposits as of detrital origin. His next statement is the first to ca 

doubt upon this hypothesis. He continues:

In addition to these, it is possible that gold was introduced with the pyrite 

that once formed a large portion of the matrix of the pebbles. To determine 

this, careful assays were made in the only productive area now completely 

accessible. * * * It was found that along definite lines in the center of 

the stopes there were values of from $5 to $12 per ton in the conglomerate, 

while at a distance of from 10 to 100 feet from such lines the values sink to 

between $1 and $2 per ton. Iron-stained fractures are found in the roofs of 

these stopes. A possible inference is that the gold which has low and fairly 

uniform values distributed through the mass is of detrital origin^ and that 

the additional values have been introduced into the conglomerate together with 

the pyrite along zones of fracture. This is rendered still more probable by 

the fact that much of the gold does not yield to simple amalgamation, the 

conglomerate being treated largely by the cyanide process. The introduction 

of the pyrite was subsequent to the deposition of the conglomerates, since 

mineralization extends into fractures in the quartz pebbles. The pyrite is 

probably a replacement of the original cementing material, which elsewhere 

consists of quartzose sands.
54 GEOLOGY OF REGION AROUND LEAD, S. DAK.

Intrusions of rhyolite cut the conglomerate in some places. They are much 

mineralized with pyrite, which is thought to be of postintrusive origin. This 

would place the pyrite mineralization subsequent to the rhyolite intrusion.

These gold-bearing conglomerates must have been either of fluviatile or of 

littoral origin. All of the evidence is in favor of the latter view, for (1) the 

rocks immediately above the conglomerates contain marine fossils and were 

unquestionably deposited in an extensive Cambrian sea; (2) the area covered 

by the heavy conglomerates is very small and quickly passes in all directions 

into regions where finer material with marine shells shows that the conditions were marine; hence a land surface sufficient to support a drainage 

extensive enough to deposit such heavy conglomerate could hardly have existed; (3) cross-bedded sands and quartzites were deposited upon the conglomerates with perfect conformity so that they are to be considered as an integral 

part of a typically marine series; (4) conglomerate is absent from the outcrop of the Homestake lode, which must therefore have projected above the old 

pre-Cambrian surface; (5) schist fragments, such as could only have been preserved near the source from which they originated, decrease in abundance as 

one proceeds outward from the Homestake lode. While, therefore, these were 

littoral deposits they were exceptional in that they were not uniformly deposited along the shore but were confined to the vicinity of the outcrop of a large 

gold lode, and the detrital material from that lode was held in irregular depressions in the submarine surface in its vicinity.

To quote directly from Devereux,23

In general we find much the same variations of quantity as are shown in 

the ordinary gravel placer. The local channels referred to show the same 

alternations of rich and poor material, due to different conditions of current, 

and the universal occurrence of the greater part of the gold near the bedrock. 

In general only 5 or 6 feet in thickness will pay for mining and milling. 

* * * The gold has all the characteristics of placer gold and was generally 

what is called shot gold or smooth rounded grains, slightly flattened. I observed one nugget of nearly 3 pennyweights in weight.

The cementing material of the conglomerate was generally oxide of iron, 

and the gold which had lain on bedrock was often attached to the overlying 

boulders by this medium. In general where conditions had been such as to 

allow the subsidence of other materials of high specific gravity, the gold was 

most abundant, ordinarily with large quartz boulders, or with pebbles of 

hematite. The latter were seldom found without gold being attached to 

themselves having a smooth, polished surface. In general the position of 

thQ gold was always such as to point ito its great specific gravity as the 

locating cause, and not to solution or precipitation. * * * The fineness 

of this gold is greater than that of the quartz veins, as is generally the case 

with placer gold.

With the facts thus far seen to be in accord with the position taken, we 

have another source of proof, in that we find in these same conglomerates gold 

which has undoubtedly been precipitated from solution in situ. I have several 

times split open pieces of decomposed t'alcose schist and found in the cleavage 

plane a continuous thin film of gold. Moreover, the schist underlying one of 

these deposits has been found to carry sufficient gold to pay for milling, although after a depth of 10 feet has been obtaitfed it seems to give out. 

Specimens from this locality show the gold in thin flakes, seldom coarse, and 

pan tests show the gold to be fine like chert. On the other hand, where the

a Op. cit., p. 468.
MINERALIZATION 55

slate has been very soft cement gold will be found to have worked down into 

the crevices for several feet. It can be washed out in a pan from the fragments without crushing and then appears like smooth brown gravel, so continuous is the coating of red oxide of iron. In addition we have gold undoubtedly precipitated in the quartzites of this series at a period much more 

recent, and this gold presents none of the characteristics of the cement gold.

It is apparent to the critical reader of Irving's and Devereux's reports that many of the facts presented above suggest strongly that 

placer gold actually existed. These may be summarized as follows:

There is stratigraphic proof that a pre-Cambrian reef existed 

along the outcrop of the Homestake lode. The conglomerates, were 

derived from this reef.

The conglomerates carried coarse gold said to resemble detrital 

gold from other places, and its location was apparently both within 

the conglomerate and below in the bedrock, to which it was carried 

by gravity that is, its position was due to its weight. The gold, 

too, carried less silver than the lode gold.

The gold in the Cambrian formation everywhere except in the 

conglomerates is very finely divided, and most of it can not be recovered by amalgamation. The heavy matrix of pyrite (or limonite 

where oxidized) that accompanies the gold-bearing conglomerates 

suggests that the conglomerates were derived from a pyrite-bearing 

lode, though it is problematic whether all the pyrite would not have 

"been oxidized, and it is probable that after the invasion of these 

rocks by Tertiary volcanic material and the passage through them 

of a certain amount of gold-bearing solutions derived from the 

porphyry, the detrital pyrite may have been introduced.

We may conclude that fossil placers probably did exist and that 

 they were enriched by Tertiary veins.

SEARCH FOR NEW ORE BODIES

The conclusions reached regarding the nature and origin of the 

Homestake ore body can be applied directly in the development of 

the ore body and in the search for new ore bodies. The approximate 

position of the ore-bearing beds, those favorable to ores, can be determined by structural and ^stratigraphic studies on the surface and 

underground. Drilling and workings should be directed according 

ito such studies.

The influence of porphyry as a possible enricher of the preCambrian ore should be a subject of continued study.

To summarize the geologic factors that favorably affect the location of ore:

1. The metamorphic carbonate beds that contain cummingtonite 

have so far proved the best ore carriers. They are overlain by 

.barren quartzites and schists. These overlying beds were unfavor-
56 GEOLOGY OP REGION ABOUND LEAD, S. DAK.

able to ore deposition and may have impounded solutions, thus 

favoring concentration of ore in the immediately underlying carbonate series.

2. Close folding, with the consequent thickening of the favorable 

beds, produces a large body of replaceable rock.

3. Shearing favored the deposition of abundant sulphides, and 

abundant sulphides generally accompany ore.

4. Enrichment is probably greatest near the Tertiary porphyry.
INDEX

Page 

.Acknowledgments for aid 2

 Age of the mineraliaation on the

Homestake lode_ 45-46,47,52 

.Amphibolite In the Black Hills__ 11-12 

Analyses of rocks, minerals, and

waters of the region 20-25 

.Arkoslc grit In the Black Hills__ 7-8 

Arsenopyrlte, cut by pyrite, plates

showing : 44 

mineralization by _______ 43-44

Biotite schist and slate of the re-

.gion, features of 15 

Black Hills, area of exposed preCambrian rocks in ___ 6

features of___________ 2-3, 3-5

metamorphosed pre - Cambrian

sediments of 7-11

pre-Cambrian geology of, previous work on _ 5-6

structure of the pre-Cambrian

rocks in ______ 14

 Carbonate ore, plate showing 18 

'Carbonate quartz rock, plate showing ' 18

 Carbonate rock, almost completely 

replaced, plate showing_________ __ 18

banded, plate showing____ 18 

from Homestake mkje, analyses 

of_____'__________

highly metamorphlc, plate showing_________________

iron-bearing, plate showing____

nature and importance of____ 

near the Terra shaft, plate

showing_ _ __ 18 

silicate band in, plate showing. 19 

'Chlorite from Homestake mine, analyses of_-_ __ 21 

Chlorite schist, plate showing 18 

Conglomerate in the Black Hills 7 

'Oummingtonite, typical, analyses of_ 21 

Cunimingtonite rock, plate showing. 18

Devereux, W. B., cited 51, 54

Geologic map of the region__ __ 14 

'Geology; pre-Cambrian, of the region__________ 14-<66

 Gold, mineralization by___ 43, 45, 46-47 

ore of the Homestake lode compared with Tertiary 

ores 47-52

Page

Granite in the Black Hills__.__ 12-14 

Graywacke in the Black Hills____ 8

Homestake lode, mineralization of 42-55

possible development of 55-56

Homestake mine, examination of__ 1

map of 300-foot level in___ 30

structure of rocks on 200-foot

level________ 30 

300-foot level_______ 30-31 

400-foot level_______ 31-32 

500-foot level______ 32-33, 35 

600-foot level______ 34,35-36 

700-foot level________ 35,36 

800-foot level________ 36-39

900-foot level______ 38,39-40 

1,000-foot level ______ 40-42 

Hornblende from Homestake mine,

analyses of_ 21 

Hosted, Joseph, acknowledgment to_ 2

Irving, J. D., and Emmons, S'. F.,

cited______________ 52-54

Magnetite, mineralization by. 46

Ore, ledge of, plate showing _ __ 19 

from the Caledonia ore body,

plate showing- _ 18 

Ore bodies, new, direction of search

for ____________ 55-56

Pegmatite in the Black Hills 13 

Placers, fossil, in Cambrian conglomerates 52-55 

Pyrite, cutting arsenopyrite, plates

showing 44

mineralization by 43-45

Pyrrhotite, mineralization by _ 43-45

Pyrrhotite ore, plates showing 44

Quartz-hematite in the Black- Hills. 10 

Quartaites of the region, features

of___________ 8,15, 16

Schist in the Black Hills______ 8-9,10 

of the region around Lead, garnetiferous_______ 16 

varieties and composition

of_____________ 16-20 

Sharwood, W. J., cited________ 20-25 

Slate, from Homestake mine, analyses of 20 

in the Black Hills____ 8-9 

of the region around Lead, features of_________ 15,16 

57
58 INDEX

Page

Stratigraphic groups of the region_ 14-15 

Structure of the rocks of the region,

general features of 25-29

shown in the Homestake mine 29-42 

Sulphides, mineralization by_ 43-46

sheared, plate showing_____ 44

Tellurides, occurrence of, in Tertiary ores and in ore 

of the Homestake lode. 51, 52

PageTertiary gold ores compared with 

ore of the Homestake 

lode ___________ 47-52- 

Tourmaline, occurrence of_____ 13-14

Waters, from Homestake mine, analyses of _ 24-25> 

surface, of the region, analyses

of_______-_____ 24-<25 

Wright, L. B., acknowledgment to 2:

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