The Hpmestake Gold Mine, An Early Proterozoic 

lron-F<;,>rmation-Hosted Gold Deposit, 

Lawrepce County, South Dakota I 

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Library of Congress Cataloging-in-Pubrication Data 

The Homestake Gold Mine : an early Proterozoic iron-formation-hosted gold 

deposit, Lawrence County, South Dakota I by Stanton W. Caddey ... [et al.]. 

p. em. - (Geology and resources of gold in the United States ; ch. J ) 

(U.S. Geological Survey bulletin ; 1857-J) 

Includes bibliographical references. 

Supt. of Docs. no.: I 19.3:1857-J 

1. Gold ores-South Dakota-Lawrence County. 2. Homestake Mine 

(S.D.) 3. Geology, Stratigraphic-Proterozoic. I. Caddey, Stanton W. 

II. Series. Ill. Series: U.S. Geological Survey bulletin; 1857-J. 

QE75.B9 no. 1857-J 

[TN423.S8] 

557 S-dc20 

[553.4'1 '0978391] 91-13722 

CIP 
Chapter J 

The Homestake Gold Mine, An Early Proterozoic 

Iron-Formation-Hosted Gold Deposit, 

Lawrence County, South Dakota 

By STANTON W. CAD DEY, RICHARD L. BACHMAN, 

THOMAS J. CAMPBELL, ROLLAND R. REID, and 

ROBERT P. OTTO 

U.S. GEOLOGICAL SURVEY BULLETIN 1857 

GEOLOGY AND RESOURCES OF GOLD IN THE UNITED STATES 

DANIEL R. SHAWE and ROGER P. ASHLEY, Scientific Editors 

L.M.H. CARTER, Technical Editor 
CONTENTS 

Abstract Jl 

Introduction J2 

Location, history, and production J2 

Acknowledgments J3 

Geologic setting JS 

Regional tectonic and stratigraphic setting JS 

District geology JS 

Mine stratigraphy and petrology J9 

Introduction J9 

Poorman Formation JlO 

Lower unit of the Poorman Formation (Yates unit, informal) JlO 

Interpretation Jll 

Upper unit of the Poorman Formation J12 

Carbonate-rich schist J12 

Sericite-carbonate-quartz phyllite J12 

Biotite-quartz-car~onate phyllite Jl3 

Graphitic quartz-sericite phyllite J13 

Interpretation J14 

Homestake Formation JlS 

Siderite-dominant phyllite JlS 

Grunerite-dominant schist JlS 

Miscellaneous iron-formation rock types J16 

Chloritic biotite-sericite..'quartz-carbonate phyllite (transitional 

Homestake Formation) J16 

Interpretation J17 

Ellison Formation J17 

Quartzite J17 

Quartz-mica schist J18 

Sericite-quartz phyllite J18 

Biotite-quartz phyllite J18 

Amphibolite J18 

Interpretation J18 

Metamorphism of the mine rocks J20 

Prograde metamorphism J21 

Mafic rocks J21 

Pelitic to semipelitic rocks J22 

Iron-formation J23 

Retrograde metamorphism J24 

Interpretation J25 

Structural geology J26 

Deformational events J26 

Early Dla J26 

Middle Dla J31 

Late Dla J32 

Latest Dla J32 

Early Dlb J34 

Contents Ill 
Middle D 1 b J34 

Late Dlb J34 

Interpretation J36 

Ore deposit geology J36 

Ore ledges J36 

Ore bodies and ore mineralization J39 

Ore mineralogy J47 

Pyrrhotite J47 

Arsenopyrite JSO 

Pyrite J52 

Gold J53 

Other minerals J53 

Tertiary-age mineralization J53 

Major and trace element geochemistry J53 

Retrogressive hydrothermal alteration J56 

Chlorite alteration J56 

Carbonate alteration J56 

Bleached zones and sericite alteration JSS 

Distribution of hydrothermal alteration effects JSS 

Quartz veins J61 

Stage I J61 

Stage II J61 

Stage Ill J63 

Interpretation J63 

Discussion J64 

References cited J65 

FIGURES 

Jl. Map showing generalized geology of the Black Hills uplift, South Dakota and 

Wyoming J3 

J2. Diagram and graph of annual gold production, ore-ledge discovery, and 

significant historical events, Homestake mine, Lead, S. Dak. J4 

J3. Schematic diagram illustrating stratigraphy of the Homestake mine area J6 

J4. Generalized geologic map of the eastern part of the Lead dome of the Black 

Hills uplift, South Dakota J7 

J5. Generalized geologic cross section through 33 Stope, Main Ledge Reference 

Line, Homestake mine JS 

J6. Geologic map of 2600 level, Homestake mine, showing significant fold 

structures and ore ledges J9 

J7. Plot of Mg:(Mg+Fe+2) versus Ca (cation percent) for selected gruneritecummingtonite-series amphiboles from the Homestake mine JlO 

J8. Jensen cation plot for mafic metavolcanic rocks of the Poorman Formation 

(Yates unit) and Ellison Formation, Homestake mine J12 

J9. AFM diagram for metabasalt samples from the Poorman Formation (Yates unit) 

and Ellison Formation, Homestake mine J12 

JIO. KTP diagram for metabasalt samples from the Poorman Formation (Yates unit) 

and Ellison Formation, Homestake mine J13 

IV Contents 
111. Geologic map of 2600 level, Homestake mine, showing upper greenschist-lower 

amphibolite facies and surface trace of garnet isograd J20 

112. ACF diagram for lower amphibolite facies metamorphism of Poorman and Ellison Formations mafic igneous rocks, Homestake mine J21 

113. A'KF diagrams for mineral assemblages from the Poorman and Ellison 

Formations, Homestake mine J21 

114. Projected AFM diagrams for upper greenschist facies and lower amphibolite 

facies mineral assemblages for Homestake Formation, Homestake mine J22 

115. Geologic map of 2600 level, Homestake mine, showing D1a structural 

domains J28 

116. Schematic diagram showing D 1 a fold events at Homestake mine J29 

117. Generalized geologic map of 2600 level, Homestake mine, showing lower 

hemisphere, equal-area stereonet with 288 poles to folded foliations of late S1a 

from the Lead anticline J30 

118. Iron-formation fold profile section of the Gentle Annie structure (east limb of 

the Lead anticline), 2600 level J31 

119. Sketches of late F1a (sheath) and early Fib minor folds in Poorman and Ellison 

Formations in the eastern ledge area of the Lead anticline J33 

120. Structural sketches in profile sections of minor folds in latest Dla shear zones, 

western ledge area on the Poorman anticline J34 

121. Generalized geologic map of part of 6800 level, Homestake mine, western ledge 

area, showing lower hemisphere, equal-area stereonet with 110 poles to 

transposed late S1a foliation J35 

122. Plan view vertical projection of selected horizontal slices of the Caledonia, 9 

Ledge, and 17 Ledge ore bodies to a horizontal plane, showing the geometric 

elements of an ore ledge J37 

123. Plan view vertical projection of selected horizontal slices of Main Ledge ore 

bodies to a horizontal plane J38 

124. Plan view vertical projection of selected horizontal slices of 9 Ledge ore bodies 

to a horizontal plane, showing geometric elements of an ore ledge and the upper 

greenschist-lower amphibolite facies transition zone J40 

125. Geologic map of 2600 level, showing upper greenschist-lower amphibolite 

facies metamorphic transition zone, D 1 a structural domains, and ore centroid 

trend J41 

126. Histogram of ledge gold production, showing average (prerecovery) grade and 

cumulative gold production plus ore reserves, Homestake mine J42 

127. Isometric view of Main Ledge on 1850, 2450, and 3050 levels J43 

128. Geologic map of "D" limb of 9 Ledge on 5600, 5750, 5900, and 6050 

levels J44 

129. Face sketch, 7 Ledge, 1550 level, 58-61 stope, Homestake mine J45 

130. Face sketch, 13 Ledge, 6350 level, 55 stope, west "B" limb, Homestake 

mine J46 

131. Geologic map showing gold grades for 2450 level, Main Ledge J48 

132. Geologic map showing gold grades for "D" limb of 9 Ledge, 5750 level J49 

133. Photographs of typical ore specimens, Homestake mine, including pyrrhotite 

blebs in grunerite- and siderite-rich layers interbedded with chert, layered 

pyrrhotite interlayered with biotite-rich chert and siderite, and coarse-grained 

arsenopyrite within a chlorite-rich siderite matrix JSO 

Contents V 
J34. Photomicrographs showing typical occurrences of arsenopyrite and replacement 

of arsenopyrite by pyrrhotite JSl 

J35. Photomicrographs showing occurrence of gold with arsenopyrite and 

pyrrhotite J52 

J36. Diagram and graph showing distribution of gold, arsenic, total sulfide, and 

chlorite in drill hole 16802A, 21 Ledge, 7400 level JSS 

J37. Geologic map showing distribution of rock types in Main Ledge, 2450 

level J59 

J38. Geologic map showing distribution of rock types in "D" limb of 9 Ledge, 5750 

level J60 

J39. Geologic map of 2450 level, Main Ledge ore body, showing quartz vein 

distribution J62 

TABLES 

Jl. Modal mineral percentages in thin sections of representative Poorman Formation, 

Homestake mine Jll 

J2. Major and minor element geochemistry of Poorman Formation units, Homestake 

mine J13 

J3. Modal mineral percentages in thin sections of representative Homestake 

Formation, Homestake mine Jl6 

J4. Major, minor, and trace element geochemistry of barren Homestake Formation 

units, Homestake mine J17 

J5. Modal mineral percentages in thin sections of representative Ellison Formation, 

Homestake mine J19 

J6. Major and minor element geochemistry of Ellison Formation units, Homestake 

mine J19 

J7. Structural deformation history in the Homestake mine and vicinity J27 

J8. Major, minor, and trace element geochemistry of mineralized Homestake 

Formation samples J54 

J9. Mineral alteration history related to significant deformation events J57 

VI Contents 
GEOLOGY AND RESOURCES OF GOLD IN THE UNITED STATES 

The Homestake Gold Mine, An Early Proterozoic 

Iron-Formation-Hosted Gold Deposit, 

Lawrence County, South Dakota 

By Stanton W. Caddey, 1 Richard L. Bachman, Thomas J. Campbell, 

Rolland R. Reid, and Robert P. Otto 

Abstract 

The Homestake mine in the northern Black Hills, South 

Dakota, is the largest iron-formation-hosted gold deposit 

known. It has produced 1,101 metric tons (35.4 million troy 

ounces) of gold from 124.9 million metric tons of ore milled; in 

1988 the mine produced 12.1 metric tons of gold. The deposit 

was discovered in 1876, and the mine has operated 

continuously to the present day. Ore is currently mined from 

depths as great as 2,438 meters. Gold is the major commodity 

produced, along with a minor silver byproduct. The gold:silver 

ratio averages 5:1; base metal content is negligible. 

The Homestake gold deposit is hosted within quartzveined, sulfide-rich segments of an Early Proterozoic, 

carbonate-facies iron-formation in a sequence of originally 

calcareous, pelitic to semipelitic, and quartzose rocks. Strata 

that contain the Homestake deposit were complexly deformed 

by a series of tight isocl ina I and sheath fold events, and 

synchronous, extensive ductile and ductile-brittle shearing. 

Mine area rocks have been subjected to upper greenschistlower amphibolite facies metamorphism. Early-stage ductile 

shearing appears to have controlled fluid movement in the 

system, thereby controlling thermal energy and metamorphic 

processes. Observed prograde metamorphism in the district is 

therefore characterized as dynamothermal. Intrusion of granite 

similar to the 1.72 Ga Harney Peak Granite in an area 

northeast of the mine postdated regional prograde metamorphism, and appears to have been contemporaneous with 

later stages of semibrittle deformation. 

Gold mineralization took place almost exclusively in the 

Homestake Formation, an iron-formation consisting of siderite 

Manuscript approved for publication February 5, 1991. 

1 All authors, Homestake Mining Company, U.S.A., Lead, SD 

57754. 

and (or) grunerite schist in productive sections of the Homestake mine. The underlying Poorman Formation is composed 

of an upper sequence of sericite- to biotite-dominant 

carbonate-quartz and graphitic phyllites and a lower 

voluminous · sequence of hornblende-plagioclase schist of 

tholeiitic affinity. The overlying Ellison Formation consists of 

interbedded quartzitic, pelitic to semipelitic, and tuffaceous 

strata. 

Nine ore ledges, or plunging fold structures, have 

produced gold. From east to west across the mine area, they 

are Caledonia, Main, 7, 9, 11, 13, 17, 19, and 21 Ledges. Ore 

ledges are synclinal fold forms composed of a series of 

subordinate anticlines and synclines. Interconnecting anticlinal fold forms (even-numbered ledges) are less complex 

structurally and contain locally anomalous gold, but are 

generally barren. Each ore ledge contains a varied number of 

ore bodies that are disconnected, discrete entities. Overall, ore 

bodies constitute less than 3 percent, by volume, of the 

Homestake Formation in the mine area. 

Ore bodies are relatively undeformed, tabular- to pipeshaped segregations of quartz, siderite, chlorite, pyrrhotite, 

arsenopyrite, and minor pyrite. Ore bodies have developed 

within dilated segments of late-stage ductile-brittle shears, and 

they display sharp, well-defined shear contacts with adjacent 

country rock. Gold grades in ore bodies abruptly diminish 

across ore-body contacts from greater than 6.20 grams per ton 

to less than 0.02 grams per ton, commonly within 0.5 meter. 

At least three stages of mineral alteration took place, two 

of which were synmetamorphic and predated gold 

mineralization and a third synchronous with gold 

mineralization. The pre-gold alteration stages were 

characterized by carbon remobilization, and extensive 

potassium, magnesium±iron, and carbonate metasomatism. 

Ore-stage hydrothermal alteration was retrogressive, 

producing extensive chlorite, siderite, and sericite replacement 

in and adjacent to the ore bodies. 

Homestake Gold Mine, South Dakota Jl 
Gold mineralization at Homestake, even though not 

dated directly, is regarded to have taken place during Early 

Proterozoic time. Zircon (U-Th-Pb) dating of tuffaceous 

sedimentary rocks within the Ellison Formation has indicated 

an age of 1.97 Ga, providing a relative time for Homestake 

Formation deposition. A 1 .84 Ga age provides a good 

approximation of regional dynamothermal metamorphism and 

major regional ductile deformation. We favor a post-peak 

metamorphic age (<1.84 Ga) and epigenetic origin for the 

Homestake gold deposit. Emplacement of granite similar to the 

1.72 Ga Harney Peak Granite and associated deformation 

appear to have occurred in large part following gold 

mineralization, although data suggest a strong genetic 

association of the gold event with early stages of evolution of 

the granite. 

INTRODUCTION 

Location, History, and Production 

The Homes take gold mine is the largest gold producer 

from a single deposit outside South Africa and the largest 

deposit of its type in the world. Owing to the tremendous 

size of the gold system, the geologic model of this largely 

stratabound deposit is known worldwide as Homestaketype. The Homestake mine (fig. Jl) is within the Whitewood mining district in the town of Lead, in Lawrence 

County, western South Dakota, at the north end of the Black 

Hills uplift, at lat 44°22' N., long 103°45' W. 

The climate of the Dakota region is temperate, 

characterized by short, warm summers (June-September) 

and long, cold, harsh winters. Elevation in the mining 

district area is about 1,650 m (5,400 ft) above sea level. 

In 1874, the Custer expedition discovered placer gold 

in French Creek, near the present village of Custer, in the 

southern Black Hills. When word of the presence of gold in 

the Black Hills reached the outside world, the gold rush was 

on. In 1875, placer gold was discovered in Deadwood 

Creek, about 5 km east of what was to become the city of 

Lead. The Homestake lode, source area of placer gold, was 

discovered in 1876, at what is now referred to as Main 

Ledge (Open Cut surface mine) and Caledonia ore zones. 

Early mining activities (pre-1936) in the Homestake 

lode followed the Main Ledge and Caledonia ore zones 

(ledges) underground (down-plunge), utilizing shrinkage 

and timber stoping methods (without backfilling), which 

were replaced by drawhole mining (an uncontrolled blockcave system) until previously mined areas began caving to 

the surface. Today, mechanized cut-and-fill (MCF) and 

vertical crater retreat (VCR) mining methods are employed. 

Nine productive ore ledges are known in the mine. Annual 

gold production, ore-ledge discovery history, and 

significant historical events are presented in figure J2. 

The Homestake deposit has been mined continuously 

since discovery in 1876 (with the exception of the 

J2 Geology and Resources of Gold in the United States 

1942-1945 (World War ll) closure that resulted from 

government directive L-208). Current mining has reached a 

depth of 2,440 m (8,000 ft) below surface. An excellent 

historical review of the Homestake mine era is given by 

Fielder (1970). 

The Homestake deposit in addition to gold yields 

silver as a minor byproduct; base metal concentrations are 

very low. The Homestake mine has produced 1,1 01 t 

(metric tons; 35.4 million troy oz) of gold from 124.9 

million t of ore milled from 1878 through 1988 (Homestake 

internal reports). In 1988, 2.2 million t of ore was mined and 

milled from eight active ore ledges, yielding 12.1 t of gold. 

Of this production, 1.4 t of gold was mined from the Open 

Cut surface mine. Underground and surface ore reserves are 

estimated at 22.8 million t averaging 5.97 g Au/t (Homestake Mining Company, 1988). 

In addition to the Homestake deposit, the Lead area 

contains significant gold-bearing deposits of Tertiary age in 

Phanerozoic · rocks, including in the Cambrian Deadwood 

Formation and Mississippian Pahasapa Limestone, and 

locally in Tertiary alkalhcalcic sills, dikes, laccoliths, and 

stocks. Tertiary-age ores came from parts of the Deadwood 

Formation (Irving and others, 1904; Allsman, 1940). 

According to Shapiro and Gries (1970), recorded 

production is 52.9 t (1.7 million oz) gold and 112.0 t (3.6 

million oz) silver. However, Norton (1989) indicated a 

production of about 2.3 million oz gold from the Deadwood 

in the Bald Mountain mining district. 

Mined deposits consisted generally of replacements 

in dolomitic units that are transected by vertical fractures. 

The close spatial association of ore bodies to Tertiary 

intrusions led the earliest workers (Irving and others, 1904) 

to propose a Tertiary age for these deposits. Tertiary 

deposits differ from the Early Proterozoic iron-formationhosted gold deposit of the Homestake mine not only in age 

and host rock, but also in style of deposit and mineralogy. 

Tertiary deposits are characteristically silver-rich (gold:silver ratios generally below 1:1; Shapiro and Gries, 1970), 

and have significant lea4-zinc. concentrations; and a few 

contain wolframite-group minerals and tellurides. Pyrite is 

the dominant sulfide gangue. For additional information on 

Tertiary ore deposits, the reader is referred to Irving and 

others (1904), Allsman (1940), Noble (1950), and Shapiro 

and Gries (1970). 

This report is restricted to the Homestake gold 

deposit, although gold is also known in Precambrian rocks 

from other parts of the Black Hills uplift. Some of the data 

presented are based on previous work and are credited 

appropriately. Additional new information has been 

generated by the authors over the past 6 years as a 

continuing study during exploration, development, and 

mining of the deposit. 
.. _____ ---__ MO!'{TAN~ ___________ _ 

WYOMING 

jSpearfish 

Wgr TINTON~: 0 

DOME 

Sundance 

I 

LEAD t.J~~.-~r ~ 

EXPlANATION 

Cenozoic and Mesozoic 

I~ I Undifferentiated 

Tertiary intrusive rocks 

undifferentiated 

Paleozoic 

0 Undifferentiated 

Early Proterozoic 

§a Undifferentiated 

Iron-formation 

r Granite 

Late Archean 

I w:. I Undifferentiated 

Granite 

Contact 

Pz 

Newcastle 0 

0 

I 

GzMz 

DOME 

0 Edgemont 

30 KILOMETERS 

I 

0 Rapid City 

Figure J1. Generalized geology ofthe Black Hills uplift, South Dakota and Wyoming, modified from DeWitt and others 

(1986). 

Acknowledgments 

Inasmuch as it would be virtually impossible to list all 

of the many geologists who have contributed to the geology 

of the Homestake mine over the past century, we list some 

of those who have made major contributions through 

publications or ore discoveries. Pioneering greats such as 

Donald McLaughlin, John Gustafson, James Noble, 

Clarence Kravig, James Harder, and Archie Slaughter, 

through their discoveries, expanded the Homestake mine, 

which revealed the truly world-class gold deposit that it is 

today. Publications by most of these individuals are noted in 

the references. We are grateful to Homestake Mining 

Company for permission to publish this report and for 

continuing support in research. 

Homestake Gold Mine, South Dakota J3 
CALEDONIA .,

1

,.

8

-

7

-

6

------.,...--------t 

MAINLEDGE .. ------+------------~~-----~~ 

7 LEDGE 

9 LEDGE 

11 LEDGE 

13 LEDGE 

17 LEDGE 

19 LEDGE 

21 LEDGE 

r--

(!) 

.. Currently producing 0 

N 

Y'} "0 

u) 0 

::::io Cl 

Eq 0 OLO Cl .:::(") c: "OY'} -~ 

Ceased production 

• Discovered 

Cll 0 enUl ~ co 0 co Cll:::l E u 0 ~......, 21,000,000 

700,000 

c: "0 s: Cll 0 0 "0 ::J '5 18,000,000 "0 

e ..r::. 

en 

-~ = 

Co a: s 15,000,000 <{ 

"0 w 

"§ >-

a: s w 

12,000,000 a. 

Cl 

...J 

0 

(.!) 

9,000,000 u.. 

0 

600,000 "0 

Cll 

a: E Cll 

<{ E w Cll 

>- 500,000 a. a: ,; w en a. c: 

0 0 

....J 400,000 -~ 

0 > 0 (.!) c: 

u.. .~ 

0 Cl 

en 300,000 ~ w OE u en 

~ 

6,000,000 <{ 

a: 

z 

::::> 

0 200,000 >- 0 (.!) 

a: 

~ 

100,000 3,000,000 

1878 1888 1898 1908 1918 1928 1938 1948 1958 1968 1978 1988 

YEAR 

Figure J2. Annual gold production, ore-ledge discovery, and significant historical events, Homestake mine, Lead, S. 

Oak. Cumulative gold production through 1988 is 35,399,664 troy oz (1, 101,053,449 g). 

)4 Geology and Resources of Gold in the United States 
We are particularly grateful to R.E. Green and J.A. 

Anderson for their support, valuable comments, and 

guidance that made many results of this paper possible. We 

appreciate the valuable contributions made by the Homestake mine Geology Department consisting of G.C. Nelson, 

L.F. Meier, C.J. Hall, J.R. Church, J.H. Rogers, Jr., K.M. 

Hart, R.L. Brumbaugh, D.M. V ardiman, and G.G. Hamilton. Contributions from the Exploration Division were 

made by A.G. Hoffman, R.D. Nutsch, D.S. Sneyd, K.L. 

Triscori, R.M. Bradsky, K.E. Marlowe, T.L. Hruska, E.L. 

Caddey, and T .M. Rizzi. Appreciation is expressed to Ed 

DeWitt, D.R. Shawe, C.H. Thorman, J.A. Kerswill, J.A. 

Redden, and R.M. Honea for their excellent reviews and 

suggestions. We appreciate the support of C.G. Cunningham and the many valuable discussions with participants 

who attended the 1986 lUGS/UNESCO Workshop on 

Mineral Deposit Modeling--Gold in Precambrian Greenstone Belts, Belo Horizonte, Brazil. 

GEOLOGIC SETTING 

Regional Tectonic and Stratigraphic Setting 

The Precambrian core of the Black Hills uplift (Darton and Paige, 1925) in western South Dakota is exposed in 

an elongate dome approximately 85 km long and 35 km 

wide (fig. Jl). Rocks within the Black Hills uplift range in 

age from Archean to Holocene and have been described 

most recently by Redden and Norton (1975), DeWitt and 

others (1986), and Redden and French (1989). Archean 

granitic rocks at least 2.5 Ga are known in the Little Elk 

Creek (Zartman and Stem, 1967; Gosselin and others, 1988) 

and Bear Mountain areas (Ratte and Zartman, 1970) in 

South Dakota, where they constitute a minor part of the 

Precambrian core. Early Proterozoic sedimentary and 

volcanic rocks are regionally metamorphosed from greenschist to upper amphibolite facies, range in age from 2.20 to 

1.87 Ga (Redden and others, 1990), and make up most of 

the Black Hills core. Protoliths of the Early Proterozoic 

metasedimentary rocks consisted of clastic sediments, black 

shale, iron-formation, and volcanic rocks. 

The Archean and Early Proterozoic rocks were 

regionally deformed and metamorphosed prior to 

emplacement of the 1. 72 Ga (Redden and others, 1990) 

Harney Peak Granite, the youngest Proterozoic rock in the 

Black Hills uplift. Structural and metamorphic studies at the 

Homestake mine indicate that regional metamorphism was 

syndeformational and characterized by a dominant north- to 

northwest-trending regional foliation in the Lead area 

(unpublished Homestake reports). The 2.5 Ga Little Elk 

Granite southeast of Lead is overprinted by this later 

northwesterly foliation (Zartman and Stem, 1967). Zartman 

and Stem (1967) tentatively dated this fabric at 1.84 Ga by 

whole-rock Rb/Sr methods. The 1.84 Ga age likely 

represents the timing of Lead area regional metamorphism. 

Contact metamorphic effects associated with the Harney 

Peak Granite (1.72 Ga) overprint this earlier regional 

metamorphic fabric in the southern Black Hills. 

The Precambrian rocks of the Black Hills were 

uplifted and eroded by about 530 Ma. The resulting 

unconformity surface may represent nearly a billion years of 

time. During a period of relative tectonic quiescence, 

Paleozoic and Mesozoic sediments were deposited on this 

surface. The Black Hills area was uplifted again during the 

Laramide (65-60 Ma) into its present domal configuration. 

After uplift, a west-northwest-trending belt of early Tertiary 

(60-50 Ma) alkali-calcic to alkalic stocks, laccoliths, dikes, 

and sills intruded Precambrian, Paleozoic, and Mesozoic 

rocks of the northern Black Hills. Renewed erosion exposed 

the Precambrian core of the uplift. During the Oligocene, a 

veneer of terrestrial sediments was deposited over much of 

the Black Hills. Subsequent erosion produced the 

topography that characterizes the present-day terrain. 

District Geology 

The Whitewood mining district (known colloquially 

as the Lead district) is located on the north end of the 

regional uplift of the Black Hills (fig. J1 ), within the eroded 

core of an early Tertiary dome known as the Lead window 

(67 km2 in area). The Tertiary Cutting stock, 1.2 km wide 

by 2.5 km long, is exposed near the center of the window. 

Emplacement of the Cutting stock, containing numerous 

dikes and sills, caused a swelling of the Early Proterozoic 

and Phanerozoic rocks forming the dome. The core of the 

dome contains Tertiary intrusive rocks of alkali-calcic 

composition, along with Early Proterozoic metasedimentary 

and metavolcanic rocks. Although Early Proterozoic rocks 

have been divided into six formations (Dodge, 1942; Noble 

and Harder, 1948), as shown in figure J3, only three 

formations important to the Homestake mine area will be 

discussed in detail here: (1) the Poorman Formation, 

comprising a lower unit of voluminous metatholeiite termed 

herein Yates unit (unpublished Homestake reports), 

overlain by an upper unit of heterogeneous, calcareous, 

pelitic to semipelitic phyllite; (2) the Homestake Formation, 

constituting essentially a carbonate-facies iron-formation; 

and (3) the Ellison Formation, constituting quartzite and 

pelitic to semipelitic phyllite. These rocks are intricately 

folded and metamorphosed (Noble and others, 1949). 

Geology of the eastern part of the Lead district is shown in 

figure J4; general geology of the Homestake mine is shown 

in cross section and plan view (2600 level) in figures J5 and 

J6, respectively. 

Early workers who delineated and described these 

units appear to have tacitly assumed that the strata are 

upright (Gustafson, 1933; Noble and Harder, 1948). 

Sedimentary structures indicative of facing (younging) 

Homestake Gold Mine, South Dakota JS 
HOLOCENE 

TO 

OLIGOCENE 

unconformity 

------ EOCENE 

TO 

PALEOCENE 

ORDOVICIAN 

AND 

CAMBRIAN 

EARLY 

PROTEROZOIC 

Gravel and clay deposits (0-70m) 

Intrusive rocks 

Rhyolite, trachyte, phonolite sills and 

dikes (0-300 m) 

Deadwood Formation 

Shale, dolomite, limestone, flat-pebble 

conglomerate, sandstone and con· 

glomerate (0-1 50 m) 

v 

v v 

v v v v 

v 

Grizzly Formation (Xg)-Metagraywacke 

with interbedded sericite-biotite 

schist (1,000 m) 

Flag Rock Formation (Xf)-lnterbedded 

biotite-sericite schist, graphitic phyllite. iron-formation, and hornblende· 

plagioclase schist ( 1,600 m) 

Northwestern Formation (Xn)-Biotite· 

quartz-sericite-garnet schist (0-

1,300 m) 

Ellison Formation (Xe)-Well-banded 

to mas.sive sericite-biotite phyllite 

and schist with interbedded impure 

quartzite and minor amphibolite 

(1,500 m) 

Homestake Formation (Xh)-Grunerite· 

siderite schist and chert (iron-forma· 

tion) with chlorite-rich and biotite· 

rich phyllite or schist. Locally sulfide 

rich (0-125 m) 

Poorman Formation (Xp)-Thin, well· 

banded. sericite-biotite carbonate· 

bearing phyllite, graphitic phyllite 

(200-1,000 m) 

Yates unit (Xpy)-Hornblende-plagio· 

clase schist (600-1,200 m) 

Figure J3. Schematic diagram illustrating stratigraphy (not to scale) of the Homestake area. 

J6 Geology and Resources of Gold in the United States 
Xe 

EXPlANATION 

...-----. Cenozoic 

Tw I Oligocene White River Group 

- sedimentary rocks 

r , ~ ~ ... . , "Ti , , ·-,, . ... I ' t \ I 

Tertiary intrusive rocks 

Paleozoic 

Mississippian and Cambrian 

sedimentary rocks 

Early Proterozoic 

Xg Grizzly Formation 

Xf Flag Rock Formation 

Xn Northwestern Formation 

Xe Ellison Formation 

0hj Homestake Formation 

I Xp I Poorman Formation 

Contact 

•••••••••••••• Isograd 

------- Fault 

- #-~ Overturned anticline-Arrow shows plunge 

N 

t 

Xe 

1 KILOMETER 

Figure J4. Generalized geologic map of the eastern part of the Lead dome of the Black Hills uplift, South Dakota. Modified from 

Noble and others (1949). Section A-A' shown in figure j5. 

Homestake Gold Mine, South Dakota J7 
,., 

\ ~-

\ ' \ ', \ ' 

\ ~' \ \I \ I \I \ I 

\\1 \ \ 

SOUTHWEST \ \ \ \ I 

A \ \\ \ \ \ 

\ \I 

I \ \ \ I \ 

\ \ \ 

\ \ ' \ \ ' 

\ \ \ 

Xe 

2600 LEVEL 

(800.73 m elevation) 

4850 LEVEL 

( 114.93 m elevation) 

300 METERS 

6800 LEVEL 

(-4 79.43 m elevation) 

300 METERS 

Xp 

EXPLANATION 

,,: :~ Ii ',:~:I Tertiary intrusive rocks 

,....----..., Early Proterozoic 

NORTHEAST 

A' 

+ + Xn I Northwestern Formation + + + 

I Xe I Ellison Formation 

KHNH/{J Homestake Formation+ 

+ + + + + 

+ + 

+ + 

+ + 

~ Poorman Formation + + + + + 

Yates unit + + + 

Ore body + + + 

Contact 

Figure j5. Generalized geologic cross section A-A' through 33 Stope, Main Ledge Reference Line, Homestake mine. Position 

of section is shown in figure ]4. 

18 Geology and Resources of Gold in the United States 
N 

t 

Xe 

Xe 

EXPLANATION 

Xe Ellison Formation 

~ Homestake Formation 

I Xp I Poorman Formation 

(ti~~~tfl Yates unit 

Contact 

Figure J6. Geology of 2600 level of the Homestake mine, 

showing significant fold structures and ore ledges. 

direction, however, are rarely preserved in these formations. 

Current workers have noted graded bedding in the Ellison 

Formation, but the position of this graded bedding in the 

limbs of isoclinal folds results in ambiguity. In recent years, 

unpublished facing data from interbedded and deformed 

basalt pillows in the Flag Rock Formation near the Homestake mine suggest that the stratigraphic sequence is indeed 

upright, confirming thoughts of earlier workers. 

Early Proterozoic rocks of the Lead district have been 

metamorphosed to middle greenschist facies in the 

southwest and to middle amphibolite facies in the northeast 

(fig. J4). Regional metamorphism was centered around a 

thermal high located northeast of and below the current 

mine. This thermal high and related metamorphism 

developed synchronously with early ductile deformation at 

approximately 1.84 Ga. A sequence of prograde metamorphic isograds (first appearance of metamorphic index 

minerals, that is, garnet and staurolite) within these metamorphic facies has been established on the surface by 

mapping (Noble, 1939). The garnet isograd as mapped by 

Noble (1939) transects the Homestake mine at the surface 

near the west limb of the Lead anticline (fig. J4). An Early 

Proterozoic granite at Crook Mountain (Bachman and 

others, 1990) was penetrated by diamond drilling northeast 

of the mine. Contact metamorphic effects of the granite 

emplacement overprinted preexisting regional metamorphic 

fabric. 

This granite, here named the Crook Mountain Granite 

for the prominent topographic feature 11 km northeast of 

the Homestake mine designated as the type . area, is 

texturally, geochemically, and mineralogically similar to the 

1. 72 Ga Harney Peak Granite located in the southern Black 

Hills. Rb/Sr whole-rock dating is consistent with an Early 

Proterozoic age, but owing to disturbances indicated by 

rubidium loss, an absolute age could not be determined. The 

granite has a pegmatitic texture. The CIPW norm is 

corundum normative; plagioclase composition indicated by 

the norm is An10• A peraluminous nature, low CaO content, 

and presence of minor coarse-grained tourmaline combined 

with a K:Rb ratio of 160 indicate an intermediate 

differentiate from a more primitive granite akin to the 

Harney Peak. Because the Crook Mountain Granite is 

known only in the subsurface in drill holes, its extent and 

form are poorly understood. 

The Crook Mountain Granite was emplaced into 

rocks that were previously metamorphosed (at 1.84 Ga) to 

amphibolite facies. These rocks are characterized by the 

assemblage muscovite+ biotite+quartz+ gamet+sillimanite+ 

potassium feldspar. Petrographic evidence suggests that 

sillimanite and potassium feldspar formed during contact 

metamorphism near the granite, and the contact metamorphism overprinted the preexisting regional metamorphic 

fabric. 

MINE STRATIGRAPHY AND PETROLOGY 

Introduction 

Early Proterozoic stratigraphic units encountered 

within the Homestake mine, from oldest to youngest, are the 

Poorman, Homestake, and Ellison Formations. Each 

fonnation comprises its own unique set of strata and 

environment of deposition; however, all protoliths and 

contact relations are not readily inferred, as they are masked 

by postsedimentary metamorphic, structural, and hydrothermal alteration effects. These three processes · have 

obliterated most sedimentary structures, modified bulk 

chemistry to varied degrees, complicated contact relations 

between rock types and formations, and modified unit 

thicknesses. Owing to the intensity of folding and shearing 

of the mine rocks, calculated stratigraphic thicknesses for 

these units represent a tectonic average or range. Poorman 

and Ellison Formations average 1,500 and 400 m thick, 

Homestake Gold Mine, South Dakota J9 
respectively. The Homestake Formation generally ranges 

from 0 to 50 m thick and locally attains 125m in thickened 

fold hinges (figs. J5 and J6). 

Descriptions of the Poorman, Homes take, and Ellison 

Formations are based largely on intensive structural, rock 

type, and petrographic studies completed by Homestake 

geologists in recent years. The Poorman Formation rock 

type descriptions are taken largely from T.J. Campbell 

(written commun., 1989). 

The term "graphite" is used in the following 

discussions inasmuch as graphite should be the only stable 

carbon phase at metamorphic conditions of middle greenschist through middle amphibolite facies (Ed Duke, oral 

commun., Dec. 12, 1989). Graphite has been confirmed by 

X-ray diffraction methods on three rock samples. In 

addition, both grunerite and cummingtonite are found in the 

Homestake Formation. Their chemical compositions (fig. 

J7) fall along the line separating the two compositional 

fields as given in the classification of Leake (1978). 

Petrographic studies throughout the mine show that the 

grunerite composition is the more common of the two. 

--------"Gr-u-nerite-is-used to represent amphiboles of the gruneritecummingtonite series. In earlier reports on the Homestake 

deposit (Noble and Harder, 1948; McCarthy, 1976), 

sideroplesite was described as the diagnostic carbonate 

mineral of the Homestake Formation. However, sideroplesite is a varietal term for siderite (magnesian siderite; Roberts 

and others, 1990, p. 786) containing as much as 7 wt. 

percent magnesium. Therefore, only the term siderite is 

used in this report. The term sericite is used throughout most 

of the following rock descriptions in reference to finegrained muscovite; no other white micas are known to exist 

in Lead area rocks. 

An age of 1.97 Ga has been established from zircons 

extracted from a metatuff unit in the Ellison Formation 

(Redden and others, 1990). The metatuff unit lies directly 

above the upper contact of the Homestake Formation and 

provides an approximate age for the Homestake. 

Poorman Formation 

Hosted and Wright (1923) and Gustafson (1933) 

described the Poorman as dominantly a gray to black, 

banded to laminated, micaceous phyllite that is ankeritic 

and locally garnetiferous. We note that the Poorman can be 

subdivided into lower and upper units based on distinct 

differences in rock type. The lower unit of the Poorman 

comprises amphibolite (interpreted as tholeiitic metavolcanic deposits), and the upper unit of the Poorman 

consists of a carbonate-rich rock, sericite- and biotite-rich 

carbonate and quartz-bearing phyllite, and graphitic phyllite. Other subordinate rock types are also present. 

The Poorman has generally been described as greater 

than 600 m thick (Hosted and Wright, 1923; Noble and 

Harder, 1948). However, recent diamond drilling within the 

J10 Geology and Resources of Gold in the United States 

1.0 

0.9 -

MAGNESIOCUMMINGTONITE 

0.8 -

c\l 0.7 

+ Cl) 

w.. 0.6 r-

+ 

t:n 0.5 6 r- CUMMINGTONITE 

0.4 O'l 

~ 0.3 

r- . . . . . . •: . . . . ... . 0.2 .. . rGRUNERITE 

0.1 rl I l I 

0.01 0.02 0.03 0.04 0.05 

Ca, IN CATION PERCENT 

Figure j7. Plot of Mg:(Mg+Fe+2) versus Ca (cation percent) 

for selected grunerite-cummingtonite-series amphiboles from 

the Homestake mine. Petrographic studies of amphiboles in 

iron-formation across the mine show that grunerite 

composition is most abundant. Diagram modified from Leake 

(1978); data from T.J. Campbell (unpublished Homestake 

report). 

mine and to the north indicates that the Poorman is 

generally 1,500 m thick (measured from northeast to 

southwest); the observed base is at the center of an isocline. 

Lower Unit of the Poorman Formation 

(Yates Unit, Informal) 

The lower unit of the Poorman Formation, and the 

lowest exposed unit, is amphibolite. Diamond drilling 

revealed a large volume of this unit north of the Homestake 

mine and in the core of the Lead anticline. The main body 

of the unit is hidden under Paleozoic and Tertiary cover 

north of Lead; it forms an elongate north-south-trending 

mass, and its southern margin plunges south into the mine 

workings where it is observed in the Yates Shaft and 

Number 4 Winze areas. The unit extends along strike from 

north to south of Lead for at least 10 km. This amphibolite 

unit is significant enough that we have denoted it in this . 

report as the Yates unit, an informal unit of the Poorman 

Formation. It is characteristically a dark-green, massive to 

faintly banded, fine-grained, and moderately well lineated 

hornblende-plagioclase schist. Ubiquitous, 2-mm- to 2-

cm-thick, white calcite bands and veinlets are also 

characteristic. Minerals composing the Yates unit, in 

decreasing order of abundance, are ferro-magnesio-hornblende, oligoclase-andesine, calcite, dolomite, ankerite, and 

trace amounts of ilmenite, magnetite, titanite, "leucoxene," 

pyrrhotite, and pyrite (table J1 ). Hornblende exists as 0.1- to 

4-mm, subhedral, prismatic, subnematoblastic to randomly 
Table jl. Modal mineral percentages in thin sections of representative Poorman Formation, Homestake mine 

[Data from unpublished Homestake reports. Chemical data on table J2 are for different samples than shown here. Trace amounts of unusual 

minerals are not shown. HPS, hornblende-plagioclase schist (Yates unit); HBCS, hornblende-biotite-carbonate schist; CS, carbonate-rich schist; 

GQSP, graphitic quartz-sericite phyllite; SCQP, sericite-carbonate-quartz phyllite; BQCP, biotite-quartz-carbonate phyllite; X, <1 percent] 

~ <\l -Cl.l ~ u c ·;n 

2 0 Cl.l ·a ·on X 

·:;: 0 N 

Rock 0 <\l u 0 Cl.l 0.. :J c N type t: Cl.l 0 ~ :J Cl.l Cl.l ~ u <\l -o ..D s ~ ~ .2 ~ Cl.l 

:J ~ c ..r: ] c 0 0 C) ] -o .fG Cl.l 0.. ~ ..r: Q) Cl.l ~ X ::0 E ~ ~ ~ ..r: u ~ § E c ·;:: c c <\l ·u ..r: u 0 c c 0 0 

-ro :J 0 ~ :§ .fG u I C m :.0 ~ ::; ~ -o d; 00 ·- ~ ·a. ~ l5 I z co (j) u.. ~ D (9 ::( c r2 F LJ..J N 

HPS 2 84 12 

HPS 4 77 18 X X 

HPS 5 75 20 

HBCS 40 40 

cs 33 20 3 

GQSP 38 4 18 

GQSP 38 9 28 

GQSP 28 10 30 

SCQP 30 27 

SCQP 18 45 

SCQP 22 5 30 X 

BQCP 35 56 

BQCP 25 55 15 

oriented grains. Anhedral, 0.05- to 0.2-mm, untwinned 

grains of plagioclase are interstitial to hornblende and 

commonly occur as poikilitic inclusions in the amphibole. 

In polished section, pyrrhotite is seen to replace ilmenite 

and magnetite, and all three are replaced by Tertiary-age 

pyrite. Minor to moderate amounts of biotite and chlorite 

are found locally. 

In greenschist-facies metamorphic rocks, actinolitic 

hornblende and albite are present instead of ferro-magnesiohornblende and oligoclase-andesine. Amphiboles are in 0.1-

to 3-mm, subhedral to euhedral, prismatic grains. Subhedral, less than 1-mm grains of plagioclase, carbonate 

minerals, and accessory minerals are interstitial to hornblende. Locally, the Yates is partially to totally replaced by 

the assemblage chlorite-calcite-biotite. 

Strata intercalated with the schist include graphitic 

quartz-sericite phyllite, crudely banded grunerite-bearing 

iron-formation without sulfides (eastern mine), coarsegrained amphibole-bearing units, and chert containing 

minor stilpnomelane, graphite, and pyrrhotite. 

~ ·.;::; 

2 

0 

~ c 

Cl.l 

E 

-

1 

X 

X 

5 

Location ~ 

·~ ~ ~ ~ ·~ ~ ~ .-§ 0.. ..r: .fG ~ 0 c 0.. .fG ·u -E c 0.0 <\l .:,L. ~ ~ <\l 

~ -o c ~ >. >. ~ Vi <( u Ci.... < Ci.... 

1 X 3800 level, Yates Shaft area 

X 1 4100 level, Yates Shaft area 

X 4850 lev~l , Yates Shaft area 

15 7700 level, No. 6 Winze 

3 9 30 2 4100 level, Yates Shaft area 

11 12 13 9 8000 level, 21 Ledge 

9 15 1 8000 level, 19 Ledge 

20 10 12 4850 level, 15 Ledge 

2 40 X 4100 level, Ross Shaft area 

1 35 X 1 4850 level, 4 Winze area 

3 27 2 1 6800 level, near Main Ledge 

3 6 4850 level, 15 Ledge 

X 5 7700 level, 6 Shaft area 

Interpretation 

Hornblende-plagioclase schist, or amphibolite of the 

Yates unit, forms the lithologic base of the known Poorman. 

The protolith, as determined from geochemistry and field 

relations (conformable contacts, various intercalated 

sediments, structures interpreted as relict pillows), was a 

mixture of tholeiitic basalt, tuffaceous and (or) epiclastic 

debris, and subsequently reworked material. The chemical 

composition of three hornblende-plagioclase schist samples 

in table J2 indicates a basalt protolith. Of 18 samples plotted 

in figure J8, all but three fall within the compositional range 

of tholeiitic basalt and all but one fall within the iron 

tholeiite field. Three samples have higher MgO values and 

are tentatively interpreted as basaltic komatiite. Samples 

plotted on an AFM diagram (fig. J9) are entirely within the 

tholeiitic field, as defined by Irvine and Baragar (1971). 

Geochemical data for amphibolite samples of the Yates 

from the Lead area are similar to available data on amphibolite throughout other sectors of the Black Hills (Norby, 

Homestake Gold Mine, South Dakota )11 
1984; Tapper, 1984), but differ somewhat in having slightly 

lower Ti02 values. 

Because tholeiitic basalt is known in both continental 

and oceanic settings, the hornblende-plagioclase schist 

samples were plotted in a KTP diagram, as used by Pearce 

and others (1975), to discriminate between continental and 

oceanic basalt (fig. J1 0). All but three (possibly four) 

samples from the Poorman Formation plot in the oceani'c 

basalt field. Tholeiitic basalts are common features of 

Archean greenstone belts worldwide. Komatiite sequences, 

deep in the greenstone belts, are commonly cited as possible 

source terrains for remobilized gold in Archean gold 

deposits categorized as epigenetic (Groves and others, 

1987). The voluminous nature of the Yates unit at Lead 

suggests possible greenstone affiliation of earliest Proterozoic rocks, not fully appreciated from limited surface 

exposures. Iron-tholeiites of the Yates, and possibly other 

iron-tholeiites of similar Early Proterozoic rocks in the 

northern Black Hills, may represent basalts related to a 

rifting event and development of a back-arc or possibly 

intracontinental basin as suggested by Redden and others 

(1990). The Poorman, Homestake, and Ellison metasedimentary rocks are interpreted as infillings of the 

northern part of the basin during Early Proterozoic (1.97 

Ga) contemporaneous with a period of rift failure or 

allochthon development. 

Upper Unit of the Poorman Formation 

Carbonate-rich Schist 

Directly overlying the lower unit of the Poorman (the 

Yates unit) is a carbonate-rich schist of the upper unit of the 

Poorman. Locally the carbonate-rich schist is absent or thin 

(FeO + Fez03 + TiO:J 

Fe-THOLEIITE 

\ BASALT 

0 

\ 0 • 

\ 8 • 

-~ ~. 

• 

EXPLANATION 

• Poorman Formation 

o Ellison Formation 

BASALTIC 

KOMATIITE 

0 • 

Figure J8. Jensen cation plot for mafic metavolcanic rocks of 

the Poorman Formation (Yates unit) and Ellison Formation, 

Homestake mine. Data are calculated in cation percent. Field 

boundaries from Jensen (1976). 

J12 Geology and Resources of Gold in the United States 

/ 

/ 

/ 

F 

THOLEIITIC 

EXPLANATION 

• Poorman Formation 

o Ellison Formation 

0 

oo •1. 

~0 0 

CALC-ALKALINE 

AL_ ____________________________________ _JM 

Figure j9. AFM diagram for metabasalt samples from the 

Poorman Formation (Yates unit) and Ellison Formation, Homestake mine. Field boundaries from Irvine and Baragar (1971 ). 

at the Yates contact; in some areas it is interbedded with the 

Yates. The carbonate-rich schist is less than 1 m to 30 m 

thick, and is slightly graphitic, fine grained, light gray · to 

gray, and moderately well foliated although locally massive. 

In places, nearly white chert layers as much as 2 em thick 

are interbedded with the carbonate-rich layers. Quartz, 

biotite, and sericite are moderately abundant (table Jl). 

Alternating carbonate-, sericite-, and biotite-bearing layers 

generally define bands that range from 3 mm to 2 em wide. 

Proportions of carbonate minerals (calcite; minor dolomite 

and ankerite), sericite, biotite, and quartz are varied. Quartz 

commonly shows a high modal abundance (±30 percent). 

Sericite-Carbonate-Quartz Phyllite 

The dominant carbonate-rich, semipelitic rock of the 

upper unit of the Poorman is a sericite-carbonate-quartz 

phyllite (table Jl), which is present wherever complete 

sections are cut by the mine workings. It ranges in thickness 

from a few tens of meters to 170 m. Its characteristics are 

somewhat varied, but generally the rock is very fine to fine 

grained with thin ( <1 mm) to medium-wide ( <1 em), 

moderately well to very well developed alternating lightgray to very dark gray layers; these layers are graphite poor 

and graphite rich, respectively. Graphite content ranges 

from 1 to 7 percent, averaging 4 percent overall by volume 

of the sericite-carbonate-quartz phyllite. Thin ( <1 mm), 

discontinuous streaks and lenses of pyrrhotite oriented 

parallel to layering compose 1-2 percent (by volume) of the 

rock; pyrite is present locally. Sericite is the dominant 

mineral, followed by varied proportions of carbonate 

minerals, quartz, and biotite. Carbonate minerals are calcite, 

dolomite, and ankerite; two carbonates may be present in 
Table J2. Major and minor element geochemistry of Poorman Formation units, Homestake mine 

[Unpublished Homestake data. Chemical data are for samples different from those shown on table Jl. All values in weight percent; samples analyzed by 

atomic absorption and ICP methods, Skyline Labs, Wheatridge, Colo. C02 determined by gas evolution. Leaders ( --), not determined). HPS, 

hornblende-plagioclase schist; CS, carbonate-rich schist; HBCS, hornblende-biotite-carbonate schist; GQSP, graphite-quartz-sericite phyllite; SCQP, 

sericite-carbonate-quartz phyllite; BQCP, biotite-quartz-carbonate phyllite} 

Rock type Si02 Al20 3 fe20 3 

HPS 48.90 13.10 1.30 

HPS 49.30 14.40 3.15 

HPS 51.20 13.00 2.10 

cs 46.30 9.70 2.96 

HBCS 36.40' 10.90 2.04 

GQSP 45.00 7.60 12.20 

SCQP 46.80 7.50 .73 

BQCP 63.30 13.40 1.21 

0 

0 

CONTINENTAL 

FeO MgO CaO 

10.30 6.80 10.30 

10.20 6.90 10.70 

11.50 6.10 8.80 

5.50 6.40 10.20 

10.00 3.70 16.30 

3.60 4.50 

2.30 6.60 11.60 

7.30 2.90 .13 

EXPLANATION 

• . Poorman Formation 

o Ellison Formation 

Figure Jl 0. KTP diagram for metabasalt samples from the 

Poorman Formation (Yates unit) and Ellison Formation, Homestake mine. Field boundary from Pearce and others (197 5). 

the same sample. Chlorite is present locally in trace 

amounts; other trace accessories include plagioclase, zircon, 

titanite, and clinochlore (variety of leuchtenbergite). Gamet 

is sparsely distributed, and is found mostly in the eastern 

ledges. 

Biotite-Quartz-Carbonate Phyllite 

The biotite-quartz-carbonate phyllite (table Jl), 

commonly found close to Homestake Formation, is 

characterized by a high biotite content, and varied 

proportions of carbonate minerals, sericite, and quartz. It 

Na20 K20 MnO Ti02 P20s 

elemental C02 S(tot) 

2.40 0.54 0.210 0.93 0.06 0.37 0.68 

2.70 .22 .210 1.00 .07 <0.05 .60 

2.40 .32 .230 1.30 .09 .58 .45 

1.20 1.90 .170 .43 .26 1.70 7.10 

2.00 1.80 .220 2.50 .55 <0.02 11.40 

.15 2.70 .130 .24 .02 7.20 6.39 

.56 3.00 .040 .11 2.20 8.80 

.50 5.10 .078 .42 .04 .28 1.37 

ranges in thickness from a few meters to 30 m. Like the 

sericite-carbonate-quartz phyllite, the rock is well foliated 

and thin (1 mm) bedded to medium (1 em) bedded, with 

alternating, well-developed brown-gray and dark-gray 

layers; the difference in color is due to variations in graphite 

concentrations. Vein quartz and chert bands less than 1 em 

thick are locally present. Trace to minor amounts of chlorite 

are found in some areas, and the amount -of pyrrhotite 

streaks ranges from a trace to 2 percent (by volume). 

Chlorite concentration generally increases close to the 

Homestake Formation. Minor amounts of garnet and tremolite are locally present, the former predominantly in the 

eastern ledges. 

Graphitic Quartz-Sericite Phyllite 

The graphitic quartz-sericite phyllite (table Jl) 

generally is found in the upper part of the Poorman beneath 

the Homestake Formation, but it is found interbedded with 

other Poorman units as well. This phyllite, colloquially 

termed "graphitic phyllite," is found as layers from less than 

1 m thick to sequences more than 30 m thick, especially in 

the western ledges. In highly sheared areas of the mine, 

graphitic phyllite occurs as transgressive zones. 

Overall, the graphitic phyllite is dark gray to black, 

and very fine grained, and it exhibits moderately well 

developed banding that is locally disrupted. A lepidoblastic 

texture is observed in thin section; however, foliation is 

generally obscured in hand specimen by the high graphite 

content. The rock contains quartz as grains interstitial to the 

graphite-sericite-biotite-ankerite matrix. In higher concentrations, graphite imparts a greasy feel to the rock. 

Homestake Gold Mine, South Dakota J13 
Sulfide concentration ranges from a trace to more than 60 

percent (by volume); pyrrhotite and pyrite are present and 

pyrrhotite is dominant. Pyrrhotite is in two forms, streaks 

and blebs. The streaky variety is present as less than 0.1- to 

5-mm-thick and 1-mm- to 2-cm-long aggregates that are 

oriented parallel or subparallel to foliation. Irregular blebs 

or masses of pyrrhotite are also common and range from 2 

mm to several centimeters thick. Pyrrhotite blebs locally 

contain fragments of quartz veins and enclosing phyllite. 

Pyrrhotite blebs replace streaks and are associated with 

quartz veins. The pyrrhotite-rich zones containing irregular 

blebs are commonly anomalous in gold; values reach as 

much as 0.5 g/t. Pyrite is partly intergrown with and partly 

replaces both varieties of pyrrhotite, generally occurring as 

euhedral, 0.5- to 1-cm cubes. 

Interpretation 

The upper unit of the Poorman is a carbonate-rich 

metasedimentary rock dominated by calcite and ankerite, 

and containing a significant pelitic component along with 

minor dolomite (tables J1 and J2). Carbonate is abundant in 

sericite-carbonate-quartz phyllite and graphitic quartzsericite phyllite as well as in the carbonate-rich schist. 

Biotite-quartz-carbonate phyllite, and perhaps only local 

parts of it, may be the only true pelitic rock in the Poorman, 

containing 13.4 wt. percent Al20 3 (table J2). Original 

detrital components were very fine grained ( <0.02 mm) silt 

consisting of clay, mica, and quartz grains. The detritus was 

incorporated with varied amounts of chemical precipitates 

in the form of calcitic to ankeritic carbonate and chert in 

addition to tuffaceous material locally. 

The lack of coarse clastics, the presence of abundant 

carbonate, and the rhythmic thinly layered to laminated 

character of the rock indicate a low-energy depositional 

environment with very limited current agitation for the 

sediment. Either a deep marine abyssal plain or a restricted 

shallow to intermediate-depth oceanic basin is a good 

possibility. We favor the shallow to intermediate-depth 

basin environment largely because of ubiquitous graphite 

found in the Poorman and because some of the graphite 

manifests itself in thin ( <1 mm) laminae possibly 

representing algal mats. Much of the graphite, however, 

probably represents a combination of bacteria and algal 

organisms. 

Alternating graphite-rich and graphite-poor layers 

characteristic of most strata composing the upper unit of the 

Poorman may represent seasonal fluctuations that were 

pervasive up until deposition of the Homestake Formation. 

Rare elliptical structures found locally in graphite-rich 

layers of some phyllites may represent sporelike structures 

similar to those described by Bondesen (1970) in the 2.0 Ga 

Early Proterozoic rocks in the Graenseland area of 

southwest Greenland. 

The abundance of microorganisms in the Poorman, 

evident from the pervasive graphite, is not characteristic of 

J14 Geology and Resources of Gold in the United States 

abyssal plain environments. Detrital quartz grains ( <0.02 

mm) are ubiquitous in the upper unit of the Poorman, 

comprising anywhere from 5 to 25 modal percent. Detrital 

quartz grains are not transported for long distances as a 

suspended load into the deep ocean, especially not in the 

abundances just given. In addition, the overlying Ellison 

Formation is currently interpreted as a deltaic deposit (at 

least in the eastern ledges), an unlikely succession if the 

Poorman is interpreted as an oceanic abyssal plain sediment. 

The pelitic component of the original Poorman is 

represented in the equivalent metamorphosed rocks by the 

presence of layer silicates. Probably, clays and extremely 

fine grained micas, along with detrital quartz grains, were 

transported basinward by terrestrial drainages emptying into 

the basin. Upon mixing with saline oceanic waters, the clay 

fraction flocculated and deposited. This fine detritus was 

deposited concomitantly with chemical precipitates forming 

on the basin floor. 

Bottom conditions in the basin must have been 

reducing and euxinic, given the ubiquitous presence of 

graphite and pyrrhotite streaks. The occurrence of graphite 

and its coexistence with carbonate indicate that j02 was low 

and that conditions were close to the C-C02 buffer. Pyrrhotite streaks are interpreted as sedimentary owing to their 

mode of occurrence, widespread distribution, crosscutting 

relations with younger pyrrhotite, and lack of association 

with quartz veins or gold. Their abundance thus suggests 

that jS2 was relatively high throughout most of the basin. 

The presence of chert also indicates bottom conditions 

characterized by low Eh, and relatively low pH and 

temperature. The majority of the sedimentary sulfides 

(pyrrhotite streaks with minor pyrite) probably formed as a 

result of bacteriogenic reduction of seawater sulfate, as 

suggested by sulfur isotope analyses (William Hallager and 

Peter Holland, unpublished Homestake reports). Organic 

material obviously was present in metasedimentary rocks of 

the upper unit of the Poorman throughout sedimentation. 

The organic material may have had a significant influence 

on processes governing the formation of chemical 

precipitates. If the organisms were photosynthetic, they 

might have contributed significantly to the development of 

an oxygenic atmosphere (for example, Roscoe, 1969, p. 83) 

and possibly to the development of iron-formation. 

The restricted nature of the Poorman sedimentary 

basin inferred from preserved structures and from the 

evidence of the physico-chemical conditions of the metasedimentary rocks may have been due to peripheral 

carbonate shoals. The concept of such restriction was 

introduced by Button (1976) for chemical sediments, 

including iron-formation, comprising the Olifants River 

Group of the Transvaal Basin, South Africa. As in the 

Transvaal example, the restrictive barrier in the Poorman 

basin may have been composed of stromatolites or of clastic 

carbonate. The barrier might also be · attributed to a 

combination of chemical precipitation of carbonate and 
growth of stromatolites. That restrictive barriers affected 

iron-formation deposition was suggested long ago by Woolnough (1941), James (1954), and Goodwin (1956). James 

(1954) also introduced the concept of restriction caused by 

anticlinal warps in a basin floor, the scale of which might 

vary. Slight compression at deformation in this particular 

Early Proterozoic rifting environment may have been 

responsible for production of a warp in the development of 

a restricted basin. Such compression seems likely because 

basin closure directions were dominated by east-west 

compressive forces as noted by Redden and others ( 1990). 

During prograde metamorphism some carbonaceous 

material and sulfur were remobilized into shear zones and 

structural traps, which explains the structurally 

transgressive character of some occurrences of graphitic 

phyllite. The source of iron in the Poorman sediments may 

have been (1) a volcanic component already present within 

the basin, and (2) paleo-hot spring activity on the seafloor 

providing not only iron, but possibly silica and carbonate as 

well. 

Homestake Formation 

Mineralogy of the Homestake Formation reflects 

more the metamorphic intensity and extent of devolatilization reactions (that is, amount of H20 and C02) that 

affected the rocks than does mineralogy of the underlying 

Poorman or overlying Ellison Formations. For example, in 

the Homestake in upper greenschist facies, siderite phyllite 

is dominant, whereas in lower amphibolite facies, grunerite 

schist is dominant. Chloritic schist is important as a 

"transitional" phase into Ellison or Poorman Formations 

and locally occurs as discontinuous lenses in the Homestake. Prior to metamorphism, the Homestake may have 

been 20-30 m thick (Noble and Harder, 1948), although 

present thicknesses range from zero to 125 m (the latter in 

thickened fold hinges). Thickness contrast is attributed to 

intense deformation (differential strain) and metamorphism 

( devolatilization). 

Overall, the Homestake is a sequence of carbonatedominated and (or) silicate-dominated rocks, depending 

upon location in the mine. The formation is locally massive 

to thin bedded, marked by abrupt stratigraphic changes. 

Mixtures of iron-carbonate and iron-silicate minerals 

characterize the central mine area. End members of 

exclusively iron-carbonate and iron-silicate exist in the 

extreme western and eastern mine areas, respectively. Thin 

chert beds are commonly interlayered with carbonate- and 

silicate-dominated rocks. Layering (commonly called 

"banding") in the Homestake is characteristic in the western 

ledges but less common in the eastern ledges. In the eastern 

ledges the Homestake is largely massive; layering is 

obliterated (local islands preserved) by grunerite growth 

that accompanied increasing metamorphic intensity. In the 

western ledges, layering is well developed, consisting of 

alternating thin laminae of chert, siderite, and biotite, 

locally with interlayered thin chlorite, sericite, and pyrrhotite. 

Siderite-Dominant Phyllite 

Homestake Formation in which siderite is relatively 

more abundant than grunerite contains siderite, quartz, 

biotite, and locally chlorite as major minerals along with 

varied but subordinate amounts of ankerite and graphite 

(table J3). Siderite occurs in two morphologically distinct 

varieties that are probably related to different events and 

modes of formation. The first siderite type is very fine 

grained and is found in a heterogranular polygonal texture 

with quartz; it characterizes the siderite-dominant phyllite. 

The second type occurs as coarse-grained, postkinematic 

porphyroblasts that crosscut earlier formed minerals; its 

development was related to gold mineralization. Much of 

the Homestake in the upper eastern and middle to upper 

western parts of the mine is dominated by siderite-dominant 

phyllite. Minor amounts of grunerite, stilpnomelane, 

muscovite, and almandine garnet may occur in the phyllite. 

Siderite-dominant rocks are olive green to gray green, 

with layers moderately well to very well developed, 

alternating between 1-5-mm-thick, graphite-rich layers and 

millimeter- to centimeter-thick, graphite-poor or graphitefree layers. Graphite-poor layers are biotite rich and (or) 

chlorite rich. Alternating carbonate-quartz- and biotitechlorite-rich layers also constitute layering. Locally, 

layering is disrupted and exhibits diagonal fractures filled 

by carbonate minerals (siderite and (or) ankerite) and 

quartz. Chert beds are ubiquitous and varied in thickness. 

Locally, chert layering is common at the lower contact with 

the Poorman and rare at the upper contact with the Ellison. 

Grunerite-Dominant Schist 

Iron-formation in which grunerite dominates over 

siderite ranges widely in texture and mode (table J3). The 

most typical example is a variably layered, olive-green, 

fine- to coarse-grained, grunerite+biotite+quartz±siderite± 

chlorite schist. Increasing metamorphic grade brings a 

concomitant reduction in siderite and quartz content. An 

end-member rock type typical of lower amphibolite facies 

metamorphic grade is a grunerite-biotite-quartz schist. This 

rock type dominates the deep eastern part of the mine and is 

fine to coarse grained, dark green, massive to thick banded 

with common minor chert. The grunerite is in 2-mm to 2-cm 

subradial aggregates that lack preferred orientation and 

generate a very competent rock. Hornblende has been 

observed petrographically as a trace component in the deep 

eastern part of the mine. Overall, hornblende in the Homestake is rare. Almandine garnet and chlorite are important 

constituents locally. 

Homestake Gold Mine, South Dakota J15 
Table J3. Modal mineral percentages in thin sections of representative Homestake Formation, Homestake mine 

[Data from unpublished Homes take reports. Chemical data on table J4 are for different samples than shown here. Trace amounts of unusual minerals are 

not shown. GDS, grunerite-dominant schist; SDP, siderite-dominant phyllite; CQS, chlorite-quartz schist; X, <1 percent; *, equivalent mine level 

encountered in drill core. No visible gold present] 

a.> 

(/) 

ro -a.> 2 

2 u c:: "Vi .Q a.> 

X ·a ·:; O'l 0 N 

0 ro u 0 

Rock a.> u Q. ::J 

·= N 0 (/) 2 ~ C3 type t:: a.> :e ::J 2 ~ a.> ro "0 E ·~ ro c:: ::J 2 c:: ..c:: 0 0 ~ 0 :0 0" a.> ·~ a. 2 _Q ..c:: a.> ro .~ a.> ::0 Ci> ·5 E 2 ..c. u 2 § § 0 c:: c:: ·u ..c:: u 0 E c:: 

2 ro '"§ ·~ u 0, c:: :0 ::J ro "0 ro 0 '*' ch ro <( 2 ~ r= ·a. ~ <.9 I z en CfJ u. ~ u <.9 c:: LU 

GOS 4 51 34 5 

GOS 30 56 5 2 

GOS 2 78 6 3 3 6 

GOS 4 38 13 36 

GOS (ore) 40 5 5 40 

GOS 8 75 8 8 1 

SOP (ore) 6 4 4 6 

SOP 24 10 4 X 

SOP (ore) 24 2 

SOP 18 18 

SOP 28 2 24 

SOP (ore) 26 20 

CQS 38 15 36 16 1 

CQS 14 49 

CQS 26 4 37 

Miscellaneous Iron-Formation Rock Types 

Other rocks interbedded with and constituting ironformation include biotite-quartz-siderite phyllite or schist 

and cherty chlorite-quartz-siderite (CQS, table J3) phyllite. 

The biotite-quartz-siderite phyllite or schist is found locally 

as a distinct unit within the Homestake in upper greenschist 

facies rocks, and the cherty chlorite-quartz-siderite phyllite 

constitutes a significant part of the Homestake in the west 

limb of the Poorman anticline. Cherty chlorite-quartzsiderite phyllite also is found within the Poorman Formation 

in the core of the Lead anticline. It represents either 

structurally displaced Homestake or a distinct ironformation horizon in the Poorman that was not subjected to 

higher grades of metamorphism. These units range from 

absent to 10 m thick. 

J16 Geology and Resources of Gold in the United States 

c:: 

0 

f:: 

N 

~ ·.;:; 

2 2 Location 

·~ 0 2 ~ 2 .~ 'i5 .~ '"§ a. ..c. 2 0 

Q; 2 c:: c:: c:: a. ·~ -E a.> 2 a.> O'l ~ ..lw:: ·u (/) ·~ E ro 

~ ~ U5 

"0 

<( 

c:: u 

co a_ > ~ a_ 

4 2 4550 level, Main Ledge 

3 3 1 4550 level, 9 Ledge 

2 6800 level, 21 Ledge 

1 8 6800 level, 21 Ledge 

5 5 7200 level, 9 Ledge* 

X X 8300 level, Pierce 

Structure (Main Ledge}* 

4 72 4 800 level, 7 Ledge 

2 60 1700 level, 7 Ledge 

13 50 11 6650 level, 9 Ledge 

4 60 X 57 50 level, 17 Ledge 

2 41 3 5900 level, 17 Ledge 

6 42 6 6800 level, 21 Ledge 

211 X 800 level, 7 Ledge 

12 6 5600 level, 11 Ledge 

33 6950 level, 21 Ledge 

Chloritic Biotite-Sericite-Quartz-Carbonate Phyllite 

(Transitional Homestake Formation) 

Chloritic biotite-sericite-quartz-carbonate phyllite, 

averaging 1 m thick, is present locally at the upper and 

lower contacts of the Homestake, grading into Ellison or 

Poorman, respectively. Conventionally, this phyllite has 

been assigned to the Homestake, owing to the presence of 

chlorite, although it is transitional with either underlying 

Poorman or overlying Ellison Formations. The rock is very 

fine to fine grained, well foliated, and moderately well to 

well layered, with alternating gray, greenish-gray, and 

dark-gray layers. Gray layers are sericite and carbonate rich; 

greenish-gray layers are chloritic and contain biotite with 

trace amounts of siderite and (or) ankerite. As in other 

rocks, dark-gray layers are graphitic. Gamet is recognized 

locally, and chert content is varied. 
Table j4. Major, minor, and trace element geochemistry of barren Homestake Formation units, Homestake mine 

[Unpublished Homestake data. Chemical data are for samples different from those shown on table J3. Trace element values in parts per million (ppm), gold 

values in parts per billion (ppb), major and minor element values in weight percent. Samples analyzed by atomic absorption and ICP methods, Skyline Labs, 

Wheatridge, Colo. C02 determined by gas evolution. GDP, grunerite-dominant phyllite; SDP, siderite-dominant phyllite] 

Major and minor elements 

Rock type Si02 Al20 3 Fe2 0 3 FeO MgO CaO Na20 K20 

GDP 52.60 2.60 9.35 17.70 4.90 3.60 2.00 1.10 

GDP 54.60 4.00 7.19 23.50 4.30 .17 .09 .71 

GDP 48.50 2.60 8.13 25.10 5.90 1.40 .65 .77 

GDP 48.80 4.40 3.50 24.60 6.10 1.20 .06 1.30 

SDP 48.70 3:20 3.96 23.70 4.40 .79 .05 .57 

SDP 37.30 5.10 28.14 23.90 6.20 1.50 .13 .63 

SDP 48.50 4.80 1.69 24.20 6.00 1.40 .10 .96 

SDP 48.70 5.90 1.90 27.00 6.50 .45 .07 1.20 

Interpretation 

We interpret the iron-formation in the Homestake to 

represent a carbonate-facies chemical sediment (table J4) 

precipitated in a low-energy restricted basin, as discussed 

previously and hypothesized by Bidgood (1977). The iron 

source, as with other iron-formations worldwide, is 

enigmatic. Rye and Rye (1974) suggested a hot spring 

model or seafloor exhalative origin for the Homestake based 

on oxygen and sulfur isotope data. We believe that the onset 

of volcanic exhalations or hot spring activity influenced 

precipitation of iron and may have supplied a portion 

thereof. We suggest that iron probably accumulated under 

reducing conditions in basin waters, having been derived 

largely from deep weathering of iron-rich rocks in an 

adjacent, low-lying land mass. Ferrous iron concentration 

increased to saturation, triggering precipitation. We propose 

that iron precipitation was induced by volcanic exhalative 

activity and evaporative processes that caused changes in 

physico-chemical conditions in the basin. 

Siderite, like other carbonates, is more soluble under 

high C02 activity, attributed to low temperatures 

(Krauskopf, 1967) and the presence of organic matter. It 

follows that siderite solubility is reduced with decreased 

activity of C02 by warming, by increase in pH, or by 

photosynthetic actiVIty of microorganisms, causing 

precipitation. All three mechanisms could have operated 

during deposition of the iron-formation. Discharging vents 

on the seafloor influence heat and pH variation in basin 

waters or brines. Photosynthetic microorganisms, probably 

neritopelagic, may have been present in the basin water and 

could have influenced siderite precipitation. 

Microorganisms use iron to obtain energy, and they serve as 

catalysts to initiate chemical reactions. Warming and 

evaporation would heat basinal waters, causing iron 

Trace elements 

ppb ppm 

MnO Ti02 PzOs 

elemental C02 S(tot) Au Ag Cu Pb Zn 

1.180 0.12 0.22 2.30 1.90 10 0.4 55 <5 15 

.300 .15 .05 <0.02 .30 8 <0.02 <5 <5 20 

1.550 .15 .16 13.50 1.00 51 .6 50 10 25 

.720 .38 .37 .08 1.04 100 <0.2 90 5 25 

.410 .11 .19 <0.02 .73 14 <0.2 100 <5 20 

.630 .15 .33 .08 1.10 30 <0.2 20 <5 60 

.380 .37 .15 .35 .59 18 <0.2 25 <5 35 

.620 .48 .15 .02 .66 21 <0.2 70 10 40 

precipitation. A portion of the iron-formation components 

in the Homestake may have originated by a replacement 

process (sideritization) analogous to dolomitization, 

whereby siderite replaced calcite- or dolomite-bearing 

chemical sediments, as postulated by Button (1976) for 

parts of the Malmani Dolomite in the Transvaal. 

Transitional Homestake Formation (the chloritic biotitesericite-quartz-carbonate phyllite) may represent a metamorphic reequilibration product of iron-formation and semipelitic rocks; metamorphic fluids channeled through rock 

strata of differing competencies promoted chemical 

reactions between two chemically diverse rock types. 

Ellison Formation 

The Ellison Formation consists of phyllite, quartzmica schist, and quartzite. Noble and Harder (1948) 

subdivided the Ellison into upper, middle, and lower 

"members" on the basis of surface mapping; however, rocks 

of the formation are intermixed, and such distinctions 

within the Homestake mine are not practical. Ellison 

thickness was given as 1,500 m (Noble and Harder, 1948), 

although indicated thicknesses in the mine area generally do 

not exceed 400 m. Apparent Ellison thickness in the western 

part of the district approaches 1 ,500 m, but owing to 

stratigraphic and structural ambiguities the ·1,500-m value is 

not considered an original stratigraphic thickness. 

Quartzite 

Quartzite (table J5) is fine grained, gray to dark gray, 

massive to faintly bedded, and moderately well sorted, 

containing well-rounded grains. Graded bedding and 

contained rock fragments are rare. Foliation is poorly 

Homestake Gold Mine, South Dakota J17 
discernible owing to the paucity of layered silicates. Most 

quartzite units are impure and contain subordinate sericite, 

biotite, and plagioclase. Based on the classification of 

McBride (1963) and Folk (1968), Ellison quartzite is best 

termed a subarkose, or rarely a quartz arenite. Minor 

accessories include microcline, ankerite, titanite, 

tourmaline, ilmenite, rutile, xenotime and (or) monazite, 

pyrite, and pyrrhotite streaks. In cross section, quartzite 

units are lenticular bodies. Individual strata are 0.3 to 4 m 

thick, and composite beds are several tens of meters thick. 

Long-axis dimensions are not precisely known, although a 

quartzite bed west of the mine extends for at least 1,000 m 

along strike. Lenticular quartzites are interbedded with 

other Ellison rocks. 

Quartz-Mica Schist 

Quartz-mica schist is mineralogically similar to 

quartzite but contains more mica and garnet, particularly in 

rocks of the garnet grade of metamorphism. The quartzmica schist is moderately well foliated; layering is poorly 

developed and locally absent. Layers where present range in 

thickness from 1-2 em to several centimeters. Color varies 

from light gray to tan to pale brown, depending on relative 

abundance of sericite and biotite. Quartz-mica schist is 

generally dominated by sericite; locally, biotite is abundant. 

Individual schist units are characteristically more extensive 

(laterally and vertically) than quartzite. They are locally 

interbedded with quartzite and mica-dominated phyllites. 

Sericite-Quartz Phyllite 

Sericite-quartz phyllite is very fine to fine grained, 

light gray to gray, and well foliated, and has 1-cm- to 

greater than 5-cm-thick layers that are poorly developed 

(table J5). Sericite-quartz phyllite is discontinuous, is 

commonly interbedded with quartzite, and is as much as 30 

m thick. Sericite content ranges from 35 to 70 percent (by 

volume); quartz, ankerite, and biotite are subordinate. Trace 

to minor amounts of garnet, graphite, tourmaline, and 

chlorite exist locally. 

Biotite-Quartz Phyllite 

The biotite-quartz phyllite is very fine to fine grained, 

light brown, thick bedded to massive, and contains 1-3-

mm-thick, local graphite-bearing layers (table J5). This unit 

is discontinuous and is as much as 100 m thick. Interstitial 

ankerite is minor but ubiquitous. Sericite and tourmaline are 

locally present. Content of 1-4-mm, anhedral to euhedral, 

pink to red, almandine or spessartitic almandine garnet 

ranges from a trace to more than 15 percent (by volume). 

Carbonate minerals are generally a minor component but 

)18 Geology and Resources of Gold in the United States 

locally may constitute a significant part of the rock. Sulfide 

minerals (pyrrhotite streaks, pyrite) are generally absent, or 

present in trace amounts. 

Amphibolite 

Amphibolite (table J5) occurs as discordant to concordant, fine- to coarse-grained, dikelike bodies and as 

concordant, fine-grained bodies interbedded with other 

rocks. These units are lenticular and locally are 65 m thick. 

The coarse-grained discordant bodies are interpreted as 

metagabbro (Dodge, 1942). Concordant, fine-grained, interbedded amphibolite strongly resembles the Yates unit of the 

Poorman. Chemical analysis of one representative sample is 

given in table J6. Contact relations and textures of the 

concordant units suggest probable altered mafic volcanic 

rocks (figs. J8, J9, and JIO). 

Interpretation 

Protoliths of the phyllite, schist, and quartzite of the 

Ellison Formation have been interpreted as shale, impure 

siltstone, and sandstone, respectively (De Witt and others, 

1986). Redden and others (1990) interpreted the Ellison as 

a shallow-water shelf quartzite correlated to deeper water 

turbidite and quartzite deposits in the Rochford area. Based 

on recent detailed underground mapping, key surface 

outcrops, and petrography, we interpret the majority of 

Ellison in the mine area as a deltaic sequence. Specifically, 

the rock types and their distribution probably represent a 

lobate or constructive-type delta (Fisher and others, 1969) in 

which fluvial processes dominated. Selected chemical 

analyses for major rock types appear in table J6. 

Pelitic to semipelitic and psammitic Ellison strata are 

characterized by their medium to thick, poorly developed to 

moderately well developed bedding, and some massive 

quartzite and quartz-mica schist units. Relict sedimentary 

structures preserved in lenticular quartzite units include 

moderately well rounded to well-rounded grains exhibiting 

moderate to good sorting, graded bedding, and rare crossbedding; local scour surfaces are developed in underlying 

phyllite. Most original sedimentary structures, however, 

have been destroyed by metamorphism and structural 

deformation. Conglomerate beds in the Ellison are rare. 

Ellison rock types and their geometric distribution 

can be correlated to specific deltaic environments, as 

discussed in Broussard (1975). Graphite-rich phyllites may 

be ascribed to marsh, backswamp, and meander-loop 

channel environments of delta-plain facies. Associated 

sericite- and biotite-rich quartz-phyllites may represent 

levee and tidal-flat deposits in the same facies. Carbonatebearing phyllites in proximity to these rock types may have 

been evaporite deposits within tidal flats. Quartz-mica 

schist units of large areal extent are interpreted as deltafront facies. Extensive sericite- and biotite-rich quartz 

phyllites may represent prodelta and offshore marine facies. 
Table JS. Modal mineral percentages in thin sections of representative Ellison Formation, Homestake mine 

[Data from unpublished Homestake reports. Chemical data on table J6 are for samples different from those shown here. Trace amounts of unusual 

minerals are not shown. SQP, sericite-quartz phyllite; BQP, biotite-quartz phyllite; QMS, quartz-mica schist; X, <1 percent] 

Q) 

(/) 

ctl -Q) 2 

2 u c ·c;; .Q Q) 

>< ·a ·:;; Cl 0 N 

0 co u 0 ~ 

Rock Q) u 0. ::::::1 .s: N '2 Location 0 (/) 

type t Q) ::::::1 2 ~ u 2 ctl "0 .0 E 2 2 Q) 

.~ Q) 0 ·~ ::::::1 2 c ..c '§ .2 c 0 0 2 ~ 2 ~ 0'" "0 a. ·~ Q) a. 2 ..c Q) co Q) Q) ·.;::; 2 2 ·g >< ::c E 2 ..c u a> E § .~ 0 c ..... Q) ..c ·~ 2 0 

"5 c c ·u ..c u 0 2 c 0 c c a. ·~ -E c co 2 ·g u E Q:; "0 Q) Cl ~ ~ Q) 

2 0 cb ·~ I c :.c ::::::1 co ~ co a, Cl ·- co ~ (/) ·~ c ~ F ·c. E co 

~ "0 c co > :::2: <.9 I z co U) LL :::2: u <.9 <( UJ N :::2: U5 <( u 0... 4: 0... 

Quartzite 83 3 5 X 3 3 4550 level, 11 Ledge 

Quartzite 95 X X X X X X X 6500 level, Main Ledge 

Quartzite 91 X 5 X X X X 6800 level, 9 Ledge 

QMS 60 20 15 4 1 X X X X 5900 level, 13 Ledge 

SQP 30 X 65 5 X X X 2600 level, east of Yates Shaft 

SQP 30 20 50 6800 level, Main Ledge 

SQP 12 15 70 3 6800 level, 13 Ledge 

SQP 35 35 30 X 8000 level, 15 Ledge 

BQP 45 45 10 X X X X X 2600 level, east of Yates Shaft 

BQP 23 30 3 35 10 6500 level, Main Ledge 

BQP 40 45 15 X 6800 level, 9 Ledge 

Amphibolite 72 4 20 X 3 X Drill hole north of Lead, S. Oak. 

Table J6. Major and minor element geochemistry of Ellison Formation units, Homestake mme 

[Unpublished Homestake data. Chemical data are for samples different from those shown on table JS. All values in weight percent. Samples analyzed by 

atomic absorption and ICP methods, Skyline Labs, Wheatridge, Colo. C02 determined by gas evolution. SQP, sericite-quartz phyllite; BQP, biotite-quartz 

phyllite] 

Rock type Si02 Al 20 3 Fe20 3 FeO MgO CaO 

Quartzite 86.30 5.30 1.13 1.30 0.69 0.59 

SQP 58.90 16.20 1.84 6.70 2.20 .19 

BQP 60.90 12.30 1.13 5.80 3.40 3.10 

Amphibolite 40.20 12.70 1.07 11.10 4.70 9.40 

Thin-banded to laminated phyllites of the Ellison may in 

part be offshore marine facies developed on and above the 

Homestake upper contact and are difficult to distinguish 

from similar phyllites in the Poorman at its upper contact 

with the Homestake. Lenticular quartzite units distributed 

throughout the Ellison, especially within quartz-mica schist 

units, are believed to represent bar finger complexes of 

delta-plain and delta-front facies. 

Na20 K20 MnO Ti02 P20 5 

elemental C02 S (tot) 

0.06 1.90 0.065 0.13 0.02 0.39 0.75 

.59 4.70 .340 .58 .04 1.10 .60 

.68 4.10 .940 .47 .05 .03 1.50 

2.80 1.90 .180 2.00 .42 .32 6.00 

Deposition of the Ellison deltaic sequence was 

probably initiated by major orogenic activity well inside the 

adjacent landmass, the probable source of the clastic debris. 

Progradation of the delta complex consisted of slow 

inundation of at least part of the restricted basin with clastic 

material. Influx of clastics may have served to extinguish 

exhalative activity on the basin floor and terminate 

production of iron-formation. Thus the youngest uppermost 

Homestake Gold Mine, South Dakota Jl9 
Poorman may be the time equivalent of different parts of the 

Homestake, or of the oldest lowermost Ellison. 

METAMORPHISM OF THE MINE ROCKS 

Upper greenschist and lower amphibolite facies rocks 

of a typical Barrovian sequence (Tilley, 1925) are 

encountered in the Homestake mine area and related to an 

Early Proterozoic (1.84 Ga) regional dynamothermal metamorphic event that preceded the emplacement of the 

(probably) 1.72 Ga Crook Mountain Granite (Bachman and 

others, 1990). Typical metamorphic assemblages of the 

principal minerals in the stratigraphic units are listed in 

tables J1, J3, and J5, and depicted in A'KF, ACF, and AFM 

diagrams (figs. J12, J13, and J14). Thompson projections 

(Thompson, 1957) were not used for the Poorman and 

Ellison because requisite analytical data are insufficient. 

Assemblages are dominated by carbonate rocks (tables 

Jl-J6) that contain a strong pelitic component. Rocks other 

than iron-formation, amphibolite, and garnet-biotite-bearing 

semipelitic rocks do not contain definitive assemblages for 

facies assignment; they are classified on the basis of 

neighboring rocks. 

Little is known about metamorphic parameters in the 

complex metamorphic terrain of the Lead area, especially 

concerning the iron-formation in the Homestake. However, 

research is currently underway by Homestake's Exploration 

staff in conjunction with the South Dakota School of Mines 

and Technology to define parameters such as temperature, 

pressure, fluid composition, fluid paths, and mineral 

reactions. Preliminary temperature determinations from 

garnet-biotite and ankerite-siderite geothermometry across 

the Lead area by Kath (1989) show values ranging from 

about 370 °C in the west (upper greenschist facies) to 

approximately 630 °C in the northeast (middle amphibolite 

facies). Initial pressure estimates by R. Kath (written 

commun., 1990), using the plagioclase-biotite-garnetmuscovite geobarometer of Ghent and Stout (1981), range 

from 3.5 to 4.9 kb in semipelitic rock samples. 

A sequence of prograde metamorphic isograds (first 

appearance of index minerals garnet and staurolite) has been 

established by surface mapping in the Homestake mine area 

(Noble, 1939; this report, figs. J4, J5, and Jl1). The garnet 

isograd has been extrapolated into the subsurface in the 

mine area. Although an isograd has been established in the 

mine itself for garnet (figs. J4, 15, and J11), some very fine 

grained ( <2 mm) pink garnet in the western ledges is 

manganese-bearing (spessartitic almandine) and not used as 

an indicator of lower amphibolite facies or the garnet 

isograd. For mapping purposes, coarse (>3 mm) red garnet, 

generally of almandine composition, in iron-formation and 

in metapelites has been used to establish the garnet isograd. 

From west to east in the mine area, upper greenschist 

facies rocks grading into lower amphibolite facies rocks 

J20 Geology and Resources of Gold in the United States 

N 

t 

Garnet isograd 

'\'\!(surface trace) 

\ 

,, 

\ "'1:) \ 

0 \ ~ l 

~ l 

~ 

Xe 

UPPER 

GREENSCHIST 

FACIES 

EXPlANATION 

Xe I Ellison Formation 

~ Homestake Formation 

Xp I Poorman Formation 

I 

("') 

::D 

m 

m 

"' 

l> 

::D 

m 

l> 

Xe 

LOWER 

AMPHIBOLITE 

FACIES 

Composite of upper greenschistlower amphibolite facies 

transition zone (siderite and 

grunerite coexist in Xh) 

1 KILOMETER 

Figure )11. Geology of 2600 level, Homestake mine, 

showing upper greenschist-lower amphibolite facies and 

ground surface trace of garnet isograd. 

form a metamorphic transition zone that averages 800 m 

wide (fig. 111). The transition zone strikes, on average, N. 

20° W. and dips nearly vertical. The transition zone is 

defined mineralogically by the coexistence of siderite (fine 

grained) and grunerite in iron-formation. Within the Yates 

unit (metatholeiite) of the Poorman, the transition zone is 

thought to be defined by the coexistence of hornblende and 

actinolitic hornblende in addition to oligoclase-andesine and 

albite. The upper greenschist-lower amphibolite facies 

transition zone is structurally bounded east and west by 

early ductile shear zones, and it is contained within a 

structural domain dominated by late upright folds. (See 

section, "Structural geology.") Ductile shears appear to 

have exerted strong control on position of the transition 

zone. The surface trace of the garnet isograd as mapped by 

Noble (1939) lies near the west margin of the transition 

zone. A retrograde metamorphic event is also recorded in 
Plagioclase 

and (or) 

epidote 

A 

Kyanite 

Anthophyllite 

Calcite Cummingtonite 

c~------------------~~L-----~F 

Hornblende 

Figure )12. ACF diagram for lower amphibolite facies metamorphism of mafic igneous rocks of the Poorman and Ellison 

Formations, Homestake mine. Rectangle indicates location of 

average tholeiitic basalt according to Winkler (1979). Tielines 

define possible stable mineral-assemblage fields. 

Homestake mine rocks and is largely manifested as a biotite 

halo developed within and peripheral to the iron-formation 

in the Homestake. This retrograde event occurred soon after 

the prograde thermal peak. 

Prograde Metamorphism 

Mafic Rocks 

Mafic rocks in the mine are dominated by Yates unit 

metatholeiite (hornblende-plagioclase schist) of the Poorman Formation. Mineral assemblages within the Yates unit 

can be used as an indicator of metamorphic grade because 

of widespread distribution of the unit across the core of the 

Lead anticline where the unit is encompassed by the garnet 

isograd. The Yates unit lies largely within the lower 

amphibolite facies, which is characterized by ferromagnesian hornblende and plagioclase, the latter in the 

compositional range of oligoclase and andesine (T.J. Campbell, written commun., 1990). Subordinate accessory 

minerals include epidote-group minerals, carbonates, 

quartz, ilmenite, and magnetite. Minor parts of the Yates 

unit that occur in upper greenschist facies terrain consist of 

actinolitic hornblende, albite-oligoclase, carbonates, and 

epidote group minerals (T.J. Campbell, written commun., 

1990). A possible metamorphic reaction (unbalanced) to 

account for the assemblages observed in the Yates unit at 

upper greenschist-lower amphibolite facies transition is: 

actinolite+albite+ankerite±chlorite = hornblende+andesine+ 

H20+C02• 

The mineral fabric of the Yates unit is judged to be a 

particularly important indicator of the degree to which 

lower amphibolite facies metamorphic grade was attained. 

A' 

Muscovite 

+Quartz 

±Ankerite 

±Albite 

±Carbon 

K L-------------------------------~~~F 

Biotite 

A 

A' 

Muscovite 

B 

Actinolite 

+Quartz 

±Ankerite 

±Albite 

±Carbon 

±Pyrrhotite 

Biotite 

Hornblende 

Figure )13. A'KF diagrams for mineral assemo1ages rrom me 

Poorman and Ellison Formations, Homestake mine. A, upper 

greenschist facies; 8, lower amphibolite facies. Data are not 

available for Thompson projections (Thompson, 1957). 

Tielines define possible stable mineral-assemblage fields. 

Hornblende and plagioclase are intergrown in a somewhat 

granoblastic fabric in which all the plagioclase is clearly 

metamorphic (equant, unzoned, untwinned, 0.1-mm grains). 

No relict igneous plagioclase persists. Locally, biotite is 

abundant, replacing hornblende along certain layers. An 

ACF diagram indicating the mineral assemblage for the 

Yates unit and Ellison Formation amphibolites is given in 

figure J12. The formation of hornblende from orthopyroxene, clinopyroxene, and (or) olivine is one of the 

major transformations of basaltic rock during prograde 

metamorphism in the presence of water (Winkler, 1979). 

Reactions forming hornblende must have been essentially 

Homestake Gold Mine, South Dakota J21 
FeO 

-0.1 

-D.2 

-0.3 

-D.4 

A 

FeO 

-0.1 

-D.2 

-D.3 

-0.4 

B 

0.1 

0.6 

0.5 

0.6 

0.5 

0.9 

0.8 

0.7 

0.6 

Biotite 

0.5 

MgO 

MgO + FeO (Moles) 

0.9 0.9 

Biotite 

0.5 

0.8 

0.7 

MgO 

MgO + FeO (Moles) 

0.5 

0.4 

0.3 

0.2 

0.1 

MgO 

-0.1 

1.0 

-D.2 

-D.3 

-D.4 

1.0 

Figure j14. Projected AFM diagrams for A, upper greenschist facies, and B, lower amphibolite facies mineral 

assemblages for Homestake Formation, Homestake mine. 

Tielines define possible stable mineral-assemblage fields. 

complete, based on ubiquitous hornblende in the Yates unit 

throughout the Lead area. The overwhelming abundance of 

hornblende, indicative of chemical equilibrium, suggests 

that prograde metamorphism may have taken place over a 

relatively long time. Grain size of the Yates unit, smaller 

than that of the tholeiitic basalt proto lith, indicates a greater 

crystal-surface area available for fluid-rock interaction 

favoring recrystallization and equilibration. Additionally, 

much of the Yates unit chemistry was either directly or 

indirectly affected by extensive ductile shearing that 

J22 Geology and Resources of Gold in the United States 

facilitated recrystallization and equilibration. Relatively 

uniform hornblende compositions, implying large-scale 

redistribution of elements, also suggest isochemical 

conditions within and adjacent to ductile shears transecting 

the Yates unit. Such redistribution is supported by the 

observations of Beach ( 1980) in rocks of the Lewis ian 

complex in Scotland. 

The coexistence of hornblende and oligoclaseandesine in the Yates unit of the eastern mine area 

corresponds to grunerite-almandine-garnet assemblages in 

iron-formation. Therefore, the hornblende-plagioclase 

assemblage provides an estimate for the eastern boundary of 

the metamorphic transition zone. 

Pelitic to Semipelitic Rocks 

Metamorphic grade in phyllitic rocks of the upper 

unit of the Poorman, and to a lesser extent in the Ellison, is 

difficult to ascertain. This uncertainty exists largely because 

fine-grained graphite disseminated throughout most of the 

phyllites inhibited reactions that form minerals indicative of 

metamorphic grade. Another factor that deterred metamorphic reactions is the presence of abundant carbonate 

(which deforms plastically, reducing porosity) and finegrained white mica (muscovite). Most Poorman phyllites 

cannot be termed pelites based on bulk composition; the 

rocks contain less than 18 weight percent Al20 3 (table J2) 

and are referred to herein as semipelites. The deficiency of 

alumina in most Poorman rocks combined with factors 

deterring mineral reactions precludes the formation of 

garnet, an indicator of metamorphic grade. Other minerals 

indicative of metamorphic grade in more aluminous pelitic 

rocks (> 18 wt. percent Al20 3), such as chloritoid, stilpnomelane, and spessartitic almandine, are rare in the Poorman. 

The typical Poorman assemblage of muscovite+ 

biotite+carbonate+quartz is stable (or metastable) in both 

upper greenschist and lower amphibolite facies. 

Locally, where they contain almandine garnet, phyllites of the Ellison Formation (quartz-mica schist, sericitequartz phyllite, and biotite-quartz phyllite) are indicative of 

metamorphic grade. Otherwise, mineral assemblages are 

similar to those of the Poorman: sericite+biotite+ 

quartz±carbonate. (Figure J13 is A'KF diagrams for mineral 

assemblages in· phyllites of the Poorman and Ellison 

Formations.) Bulk compositions of Ellison phyllites are 

largely pelitic (not reflected in chemical analyses in table 

J6) to semipelitic (table J6), and indicator minerals of 

metamorphic grade are present (table J5), but their 

distribution is not well known. A possible metamorphic 

reaction responsible for almandine garnet formation in 

the Ellison and Poorman is: chlorite+muscovite+quartz 

=almandine+biotite+H20. This is a typical garnet-forming 

reaction in most pelitic rocks, as noted by Thompson and 

Norton (1968), and reflects minerals and textures observed 

in pelitic to semipelitic rocks of the Lead area. 
Remobilized graphite is a common feature recognized 

largely in the western ledges. Based on crosscutting 

relations, remobilization took place late during prograde 

metamorphism and continued well into the retrograde event. 

Its effects are most pronounced in phyllitic rocks controlled 

exclusively by ductile shearing. Within the mine 

environment are some shear zones that contain abundant 

graphite (about 7-25 modal percent) and that are indicative 

of carbon remobilization by metasomatic fluids. Carbon 

mobility can be explained by the following reaction (E.F. 

Duke, oral commun., Dec. 12, 1989): 

C+2H20=CH4+02 

The inferred reaction also is suggested by methanerich and methane-bearing fluid inclusions observed in 

quartz veins and near graphite-bearing phyllites (Homestake 

internal reports). These dark-gray to black graphite-rich 

zones exhibit poorly developed ·or disrupted layering and 

locally obvious shear fabric. Quartz vein stringers and 

pyrrhotite blebs are abundant locally in graphite-rich zones; 

pyrite is a minor constituent but is common in some areas. 

These zones transect earlier developed metamorphic rocks 

or mineral assemblages. A more comprehensive discussion 

is given by Campbell and others (1990). 

I ron-Formation 

Typical mineral assemblages in upper greenschist 

facies of iron-formation of the Homestake Formation 

consist of the equilibrium assemblage siderite+ 

quartz+biotite+ankerite+ferroan clinochlore. The presence 

everywhere of two carbonates in upper greenschist facies of 

the Homestake has been verified in numerous samples by 

X-ray diffraction and electron microprobe techniques. The 

mineral assemblage may also contain varied amounts of 

almandine, stilpnomelane, albite, and chlorite group 

minerals. Figure 114' s AFM diagrams represent this 

assemblage. 

Two chlorite varieties are distinguished and 

interpreted by us to represent prograde metamorphic 

products. (See description of a third variety in the section, 

"Retrogressive hydrothermal alteration.") The two varieties 

treated here are recognized by their optical properties, X-ray 

diffraction patterns, mineral chemistry, mineral textures, 

and distribution; they are best developed in iron-formation, 

but also exist in pelitic to semipelitic rocks. They are as 

follows: 

1. A faintly pleochroic, anomalously birefringent, 

lepidoblastic chlorite, found in small amounts in ironformation and selected semi pelitic rocks west of the garnet 

isograd in upper greenschist facies rocks. This chlorite, 

termed Type I, is considered early metamorphic, possibly a 

relict of precursor lower to middle greenschist facies 

regional metamorphism predating the earliest deformation 

recognized in the mine. 

2. A ubiquitous but volumetrically small chlorite 

component of iron-formation, found as subhedral, randomly 

oriented, platy grains that are pleochroic from green (X) to 

yellow (Z) and exhibit anomalously high birefringence. 

Through X-ray diffraction methods, we have determined 

this chlorite-group mineral, termed Type IT, to be ferroan 

clinochlore (variety of ripidolite). Ferroan clinochlore is 

pervasively widespread, and formed during the thermal 

maximum of prograde metamorphism largely in ironformation. 

It is generally accepted both by past workers (Noble 

and Harder, 1948; Gustafson, 1930, 1933; and Wayland, 

1936) and by present workers that siderite and quartz are 

products of, and probably the major constituents of, the 

prograde metamorphic reactions that produced grunerite. 

Petrographic studies (unpublished Homestake data; Kath, 

1990) indicate that several grunerite-forming reactions may 

have taken place in the iron-formation during prograde 

metamorphism. These reactions are based on mineral 

textures and in part on combined mineral and whole-rock 

chemistry. 

The simplest and possibly most important reaction 

proposed is the following: 

7 siderite+8 quartz+H20=grunerite+7 C02 (1) 

This was one of several grunerite-producing reactions 

proposed by Floran and Papike (1978) for the banded 

Gunflint Iron-formation of the Early Proterozoic Animikie 

Group in northern Minnesota-southern Ontario. The 

following two unbalanced grunerite-forming reactions, one 

involving the production of garnet and the other using 

stilpnomelane as a reactant, have been suggested by Kath 

(1989): 

siderite+quartz+chlorite=grunerite+almandine+C02 (2) 

siderite+stilpnomelane+H20=grunerite+biotite+chlorite+ 

quartz+C02 (3) 

Stilpnomelane is known at Homestake locally in 

lower grade metamorphic rocks (Bidgood, 1977; present 

authors); however, its distribution is not well documented. 

Another possible unbalanced reaction, proposed by Bidgood (1977) and by us, which accounts for a commonly 

observed assemblage, is the following: 

siderite+quartz+chlorite(I)+H20+K=grunerite+biotite+ 

almandine+C02± chlorite(ll) ( 4) 

The proposed reactions 1 through 4 are only a few of 

many that could be responsible for producing the prograde 

metamorphic assemblages observed. The end-member prograde assemblage found in the deep eastern ledges 

comprises grunerite+quartz+biotite+almandine garnet (fig. 

114). These four reactions assume an isochemical system, 

but such a system probably is not entirely likely given the 

complex assemblages and textures observed in many 

Homestake Gold Mine, South Dakota J23 
Homestake Formation rocks. Potassium (K+), C02, and 

water have undoubtedly been introduced at some time 

during the metamorphic/deformational history of the ironformation; and these components, other than C02, have not 

been taken into account in most of the reactions just listed. 

Textural evidence for these reactions consists of 

crosscutting relations between earlier formed and later 

formed minerals. Such relations include grunerite sprays 

transecting locally embayed quartz and siderite grains 

(reaction 1 ). Similar textural relations support reaction 2, 

but in addition euhedral garnet growth is synchronous with 

grunerite. In reaction 3 siderite and stilpnomelane are 

overprinted by radial aggregates of grunerite along with 

metacrysts of biotite and chlorite, but the chlorite may be 

retrograde in some cases. Although reaction 4 is complex 

and tenuous, crosscutting textural relations lend support to 

the proposed reaction. However, quartz abundance in the 

higher grade mineral assemblage suggests an initial excess 

of quartz over that required for the reaction. 

Prograde reactions in the Homestake Formation were 

dominantly devolatilization reactions involving the 

liberation of C02 during the breakdown of the carbonates. 

Liberation of water from pore fluids and dehydration 

reactions also were significant. Elsewhere, prograde metamorphic reactions in the Gunflint Iron-formation, 

Minnesota, that have been interpreted as promoted by 

thermal effects of the Duluth Gabbro emplaced within the 

Gunflint, coincide with a significant stratigraphic thinning 

toward the intrusive contact. This thinning is not believed to 

have resulted from depositional processes, but instead from 

volume loss from devolatilization reactions (Floran, 1975; 

Floran and Papike, 1978). Vaniman and others (1980), 

Labotka and others (1982), and Labotka (1985) made 

similar observations of volume loss during metamorphism 

of iron-formation adjacent to the Archean ~tillwater 

Complex, Montana. In both cases, volatile loss was 

responsible for at least a 20 percent volume reduction of the 

iron-formation. In the Homestake Formation, volume 

reduction is apparent as indicated by thicknesses of ironformation on either side of the garnet isograd on the surface 

(fig. J4) or on either side of the metamorphi~ transition zone 

underground (fig. Jll). The Homestake Formation is thicker 

in upper greenschist facies rocks in the upper western part 

of the mine than it is in lower amphibolite facies rocks in the 

deeper eastern part of the mine (not illustrated). As in the 

Gunflint Iron-formation and Stillwater Complex examples, 

thickness differences are not believed to have been a result 

of the original depositional nature of the Homestake. 

Volume reduction is interpreted to have resulted directly 

from devolatilization reactions and differential strain along 

plunge during prograde dynamothermal metamorphic processes operating in early stages of ductile deformation. (See 

section, "Structural geology.") Additional support is given 

by the drastic reduction of hydrous and volatile mineral 

phases from west to east across the mine. 

J24 Geology and Resources of Gold in the United States 

Retrograde Metamorphism 

Retrograde metamorphic effects, first recognized by 

Noble (1939), overprinted prograde mineral assemblages 

and were superimposed on the upper greenschist-lower 

amphibolite facies transition zone in the mine area. 

Retrograde metamorphic assemblages recognized by Noble 

(1939) are similar to those recognized by us; however, some 

of Noble's retrograde assemblages are actually temporally 

related to a later hydrothermal alteration event synchronous 

with gold mineralization. Based on crosscutting relations, 

retrograde metamorphism was earlier and distinct from the 

gold event associated with retrogressive hydrothermal 

alteration. As noted by Noble and Harder (1948), the effects 

of retrograde metamorphism make the garnet isograd 

difficult to trace along its western margin because of 

incomplete alteration of garnet to chlorite, sericite, quartz, 

carbonate, and iron oxide. The "prochlorite" zone ("white 

chlorite") described by Noble and Harder (1948, p. 

958-959), extends for some distance into rocks of lower 

amphibolite facies east of the mine. This zone, however, has 

not been recognized in the subsurface. 

Retrograde minerals associated with the Homestake 

Formation are primarily biotite, siderite, chlorite, and 

ankerite. Biotite is the most obvious and best documented 

retrograde product; siderite, chlorite, and ankerite are less 

conspicuous. Away from ore zones, chlorite, siderite, and 

ankerite that are products of the retrograde event are 

difficult to distinguish from these same minerals that 

formed during hydrothermal alteration. Altered parts of the 

Yates unit metatholeiite may be related to the retrograde 

event. 

A biotite halo is prevalent around the iron-formation 

in the Homestake Formation throughout most of the mine; 

based on observations made thus far, the biotite halo 

represents an early stage of alteration not related to 

subsequent gold mineralization. We suggest here that the 

biotite halo is a retrograde alteration product formed 

subsequent to the prograde metamorphic event. However, 

some metamorphic fluids generated during the prograde 

event may have reentered the system to produce a biotite 

halo. Retrograde fluid flow was focused primarily along 

rheologically contrasting rocks such as iron-formation and 

enclosing pelitic to semipelitic rocks where ductile shears 

had preferentially developed. However, the western ledges 

with their tight, attenuated fold structures were affected 

differently by ductile shearing owing to rheologic contrasts. 

Whether biotite formation marking the alteration halo was 

the result of potassium metasomatism, magnesium±iron 

metasomatism, or both, is currently unclear. 

Potassium metasomatism alone would be difficult to 

substantiate, except in chlorite-bearing rocks, because the 

majority of the rocks enclosing the Homestake are pelitic to 
semipelitic in composition. The addition of K+ would have 

had little effect unless chlorite was a constituent; moreover, 

biotite replacement of chlorite was rare. 

Given the pervasiveness of biotite, the large 

difference in bulk chemical composition between iron- · 

formation and bounding rock types (tables Jl-J6), and 

competency contrast between respective formations, 

magnesium±iron metasomatism was very likely. 

Attainment of equilibrium conditions between highly 

contrasting bulk chemical compositions was promoted by 

ductile shearing, competency contrasts, and fluids 

channeled through potential conduits. Equilibrium would 

then have been facilitated through the fluid medium by 

metasomatic exchange between magnesium and iron in the 

iron-formation, and aluminum and possibly potassium in 

the enclosing phyllites and schists. The equilibrium reaction 

involved liberation of magnesium and iron from octahedral 

sites in grunerite (lower amphibolite facies) and of octahedrally coordinated metal atoms in siderite or ankerite (upper 

greenschist facies). Aluminum was released from 

octahedral sites in muscovite: chemical gradients were 

established across stratigraphic/formational boundaries 

exchanging magnesium and iron for aluminum with a 

concomitant release of C02 and water from carbonates and 

amphiboles, respectively. Potassium in pelites and semipelites and silica both in pelites and semipelites and in 

iron-formation also were available during dissociation and 

redistribution. A chemical gradient developed with respect 

to K+, causing its migration into the iron-formation, and the 

production of biotite. A silica gradient was negligible. 

Mobilized iron and silica in iron-formation were combined 

with introduced aluminum that had been released from 

pelites and semipelites to form garnet in the iron-formation. 

A reverse process is also possible, whereby aluminum 

liberated in semipelites, along with silica, may have 

combined with introduced iron to form garnet in pelitic to 

semipelitic rocks. 

Fluid migration along units of contrasting 

competence (that is, ductile shears) and bulk chemical 

composition may have induced metasomatism and 

promoted equilibrium conditions through development of 

chemical gradients with respect to aluminum and potassium 

in phyllites, and magnesium and iron in iron-formation. 

Such a process may explain mineral assemblages and 

rock-type distributions found along lower and upper 

contacts of the Homestake, within both the iron-formation 

and the enclosing Poorman and Ellison Formations. Biotite 

is found in all rock types at formational boundaries in the 

upper western ledges and is accompanied by garnet in the 

lower eastern ledges. Based on limited data, the suggested 

fluid migration along ductile shears at contacts may explain 

the apparent increase in modal abundance of dolomite (or 

ferroan dolomite), ankerite, and locally siderite, at the 

expense of calcite and ankerite in the Poorman toward its 

contact with iron-formation. 

Additional biotite alteration took place locally along 

the western sheared margins of metatholeiite of the Yates 

unit. The altered metatholeiite is characterized by biotite, 

calcite, and minor chlorite that replaced hornblendeandesine or locally actinolitic-homblende-albite. The 

alteration was largely a hydration-carbonation reaction 

incorporating the addition of K20 and CaO with a 

concomitant depletion in MgO and possibly Al20 3 and Si02 

(table J2; unpublished Homestake data). Whether this 

alteration was related to retrograde metamorphism and 

synchronous with biotite alteration associated with ironformation, or was the manifestation of a later temporally 

and spatially distinct hydrothermal alteration event is not 

clear. 

Interpretation 

Prograde metamorphic reactions in the Lead area 

were initiated synchronously with transposition of the mine 

rocks from an originally flat lying position to their present 

steeply dipping orientation. The transposition was 

accompanied by a more focused phase of ductile shearing 

creating large-scale shear systems. Prior to ductile shear 

development, metamorphic processes were largely preceded 

by intergranular fluid migration, along microcracks and 

larger scale widely spaced fractures, as described by Yardley (1986) for metamorphic rocks in general. Fractures 

propagated as a wave front away from a heat source. The 

large ductile shear zones served as conduits for fluid flow 

during prograde and retrograde events. Fluid flow regimes 

in the mine area can be compared to several examples 

discussed by Kerrich ( 1986). He indicated that, in general, 

flow regimes in shears follow an order from locally derived 

fluids at low water:rock ratios and at high temperaturepressure conditions during inception of the structures, to 

large volumes of fluid flow along conduits as the structures 

propagate. In the Lead district large shear zones were 

initiated during early tectonism synchronously with prograde metamorphism. High temperature-pressure conditions 

accompanied by low water:rock ratios probably 

characterized the early stages of deformation, and the 

thermal maximum occurred late in ductile deformation. 

Prograde fluid-volatile generation culminated late in ductile 

deformation and was characterized by high fluid fluxes in 

the shear · zones; the high fluid fluxes promoted chemical 

disequilibrium that in tum initiated and then accelerated the 

mineral reactions. Metamorphic conditions progressed from 

a largely internally buffered, essentially closed, and 

possibly isochemical system during early tectonism, to an 

externally buffered and open system during later stages of 

the ductile event as ductile shears propagated. Prograde 

metamorphic reactions, reaction progress, and associated 

pressure-temperature-time (P-T-t) paths were highly 

modified as a result of fluid flow in these shear zones during 

the prograde and subsequent retrograde metamorphic 

Homestake Gold Mine, South Dakota J25 
events. Ductile shear zones also delimited the development 

of the garnet isograd by focusing fluids and heat, buffering 

rocks west of the mine area from higher temperature 

metamorphic effects. Thus, lower to middle greenschist 

facies rocks west of the mine may represent a pri111itive 

regional burial metamorphic event; the regional dynamothermal event at 1.84 Ga had minimal effect on the rocks. 

STRUCTURAL GEOLOGY 

Deformational Events 

Early Proterozoic structural geology in the vicinity of 

the Homestake mine was characterized by two major 

deformational events termed the older event, D 1 a, and the 

younger event, D 1 b. Deformation phases for these events 

include early Dla (regional "plate-scale" shearing and 

folding), middle Dla (flattening event, no lineation), late 

Dla (sheath2 folding with axial fabric and stretching 

lineation), latest Dla (ductile shearing with planar mylonitic 

fabric), early Dlb (upright folding with vertical axial plane 

foliation), middle D 1 b (ductile-brittle shearing 

accompanied by ore-stage mineralization), and late D 1 b 

(post-ore brittle shearing, and crenulation/kink fabric). 

Following conventional terminology, the folding and 

subsequent shear events are defined by planar, folded, and 

linear elements (table J7). Deformation Dl occurred during 

the Early Proterozoic. Deformation phases in Dla are 

believed to have taken place during a single thermal event, 

as deduced from metamorphic assemblages and mineral 

textures. Available data suggest that this thermal event was 

a regional prograde metamorphism that probably was 

initiated during early Dla, reached its maximum in late 

Dla, and possibly continued into latest Dla. Regional 

prograde metamorphism is generally dated at 1.84 Ga 

(Zartman and Stem, 1967). Retrograde metamorphism was 

apparently short lived and was synchronous with latest Dla. 

A subsequent retrogressive hydrothermal event and 

associated gold mineralization were synchronous with 

middle D 1 b, possibly initiated in early D 1 b. Deformation 

associated with the emplacement of the Crook Mountain 

Granite (this report; Bachman and others, 1990) is 

represented by late D 1 b even though early initial phases 

may have coincided with middle Dlb. Folds correlated with 

the successive deformational events are termed Fla, Fl b, 

... n, and related shears and lineations are termed S 1 a, S 1 b, 

... n, and Lla, Lib, ... n, respectively. Some much younger 

2

"Sheath fold" is used in this report to denote a fold of conical 

form generated within a strain regime where the strain release path is 

directed along a single direction and contains a single stretching lineation 

(late Lla) parallel to its fold axis (figs. 116 and J17). 

J26 Geology and Resources of Gold in the United States 

faulting of Laramide and early Tertiary age occurred, but it 

has no bearing on the Early Proterozoic deformation and 

metamorphism and thus has not been studied. 

Noble (1939) generated a structural model at the mine 

involving early folds cut by crossfolds. Our work has 

several points of similarity to Noble's model. Noble (1939, 

p. 214) stated that, "***the directions*** [of crossfolding] 

***range from N. 17° W. to N. 35° W., and the dips of the 

shear planes, all eastward, range from 50 degrees to nearly 

vertical." His data on crossfolds pertain to our early Fl b 

fold sets recognized in the current study. Chinn (1969) 

analyzed Homestake structure and suggested three 

important fold events. His Fl appears to correlate with our 

late Fla; his F2 to our early Fib. Our crenulation cleavage 

and kink folds (late Fl b) may be structures that he described 

as F3. Apparently, neither Noble nor Chinn recognized 

early Fla folds or Dla and Dlb ductile shears. 

Structural geology in the mine area is characterized 

by the overprinting of successive individual deformation 

phases. The complex fold structures and shear zones that 

were produced present a composite of the deformation 

history. The Lead anticline, Lead syncline, and Poorman 

anticline (fig. Jll) are "structural composites" of the total 

deformation. Figure J15 shows the distribution ofdominant 

Dla structural domains relative to the Homestake 

Formation on the 2600 mine level. Domain boundaries are 

transitional and, in detail, commonly overlap. These 

structural domains represent areas where a particular 

deformation phase dominates and various combinations of 

other phases are subordinate. As an example in figure J15, 

the dominant latest D 1 a ductile shearing domain contains 

subordinate components of middle Sla fabric, late Fla 

sheath folds and fabric, and minor Dlb folds and shears. 

Early Dla 

We believe that the early Dla deformation in the 

Lead area (table J7) developed during major transcurrent 

movement, related to the Trans-Hudson orogen, along the 

eastern margin of the Wyoming Archean craton (Bachman 

and Campbell, 1990). The resulting "plate-scale" shear zone 

is expressed in the Early Proterozoic core of the northern 

and central Black Hills as a series of anastomosing ductile 

shear zones that truncate and postdate nappe-style folding 

described by Redden and others ( 1990), and R.R. Reid 

(internal Homestake report, 1983). We suspect the presence 

of transposed minor nappe fold elements in the Lead area, 

although no definitive stratigraphic evidence for larger scale 

nappe fold closures has yet been found in the district. 

Possibly early Fl a folds discussed here may have had their 

origin related to nappe-style folding. If so, these early nappe 

folds are likely to have developed while the rock units were 

flat lying, prior to early Dla deformation. 

Transcurrent ductile shearing during early Dla, 

coincident with initial stages of prograde metamorphism, 
Table )7. Structural deformation history in the Homestake mine and vicinity 

ro "'0 

c: Q) 

0 cnro g>c: Cl>-o co..., w ·-+-'c: ..... -m (/) ·- 0 ~..c: UQ) 

~ rom ..c:·- Ec: Description of deformation ~E "'0 (.) ..... E > roC/l 0 ....,m ,_Q) 

i= ...... - ~0 Q)Cl> Cl>> (folding and shearing) (5C1> .... -a u.. ..... c: ..CCI> ::::lCI> en ...... ...... ci):= t- Q) en:2 Cl 0 

...... 

'I' .~ ~ Post-ore semibrittle kink-shear and kink-fold phase, minor fault ~..a -.:en ~..a ~..a ~..a a:; m,..... m , m,..... m.- m.- displacement of ore bodies, steep to shallow fold plunges, S:t- ....JCl Cl>..o ....Jen ....J u.. ....J .....J -,..... 0 NE.-NW. trend kink-shear surfaces (Stage Ill quartz veins). ctien ....J 

..0 ro Post-upright folding ductile-brittle shear phase, shears develop on Q) ro.- Q) Q) Q) § f'· -..a -o,..... .-en =o..a =o..a :O..o bedding planes and earlier foliation surfaces, shears trend NS.- ~~ Cl> m :-2o "'0,..... "'0,..... -o.-

~en ·- u.. ~u.. £<.9 N. 60° W., predominant dip-slip shearing, drag folds develop, hori- ~ ro ro ~ eN w -ot-- zontal plunge (Stage II quartz veins with ore mineralization). 

>.....: 

..C: II 

g; c: ..... ·- 0 (/) (/) ·- (/)+-' Q) Cl> m ~..a ~~ ~.0 ~..a ~..a c,:U Upright open to isoclinal folding phase, vertical axial-plane foliation roC) rou; ..... ,..... m.:i o.:!:: LLI _sen LLI ~u.. LLI .;:; m developed, NS.-N. 60° W. fold trends, SE. plunge . 

m Q) 

u ....J c::: 

-

0 

N Change of strain and thermal conditions in continuing deformation 

0 

c::: 

w Post-sheath ductile shear phase, ductile shears develop on bedding t- ..... ..... ..... co planes and earlier foliation surfaces, shears trend N. 20° W., pre- 0 Cll m Cl> m Cll m Cll m Et9 Cl>,..... _§Ci) ~u; ~ ..... dominant strike-slip, minor dip-slip shearing, reclined and semi- c::: 'rtlo Cll'<t mu.. :.c~ c... ....J ....J ....J reclined drag folds (Stage I quartz veins) . Developed late, with re- o..-

..._ II trograde metamorphism. 

>- E E .....J m ::::l 

a> E c::: E ·;;;: <( m w ~ E 

Q) m- Sheath folding phase, apparent constriction flow in axial-planar Cl> m :0 m Cl> m Cl> m ~ m ,_ m II\ .§o .§u; j~ "'0,..... m,..... e>E early S1a, axial symmetric fabric, average fold trends N. 35° W., 0 ..... ~en _J.....J ..... Q) plunges SE., axial planes dip NE. a..:§. 

Q) Q) - m "'0,..... :0 m :'S!o "'0,..... Flattening phase, minor flow along the axial plane. ~ ~en m E "'0 (/) m ·- ..... ..c: C>o. 

0 ..... ..... 0 

~E m m ·- ..... Early regional "plate-scale" ductile shear phase, predominant :!:: Q) 

..= E ~m ~m ~m strike-slip shearing, mega-reclined drag folds develop, north- .......... 0 rou; ..... ,..... south trends (Lead-Poorman anticlinorium) . During early D1a ~Cl en w ~u.. deformation Black Hills Proterozoic rocks were transposed to 

their current steeply dipping position. 

Homestake Gold Mine, South Dakota J27 
N 

t 

1 KILOMETER 

I 

EXPLANATION 

Xe I Ellison Formation 

~ Homestake Formation 

Xp I Poorman Formation 

Xe 

Dla STRUCTURAL DOMAINS 

1

:::::::::::::::::::::::::1 Early D1a folding/middle Dla fabric (with 

.· · · · · · · · · · · · ·. moderate late D1a sheath overprint) 

,. ·. ·. ·. ·. · ., Late D1a sheath folding (with strong early 

· · · · · · · · D 1 b overprint) 

1.

:-:-:-:-:-: · :-:-1 Latest D1a ductile shearing (with weak early 

. · · · · · · · ·. D 1 b overprint) 

Figure J15. Geology of 2600 level, Homestake mine, showing 01 a structural domains. 

transposed the originally flat lying Early Proterozoic strata 

into the steeply dipping rocks that characterize the Black 

Hills today. A major component of this model is the 

J28 Geology and Resources of Gold in the United States 

importance of district-scale variations in rock competence. 

Relatively competent masses such as the Yates unit metatholeiite acted as strain buttresses during shearing, focusing 
shearing and folding along margins and in frontal positions 

of the "metatholeiite" (fig. 116, shears not shown). The 

folded iron-formation in the Homestake Formation which 

defines the Lead-Poorman anticlinorium (fig. 115) is 

believed to have developed in a strain shadow created by the 

Yates unit metatholeiite. Folds generated as a result of this 

deformation (early Dla) are effectively shear-induced drag 

folds, isoclinal in form; and they trend north-south in the 

Lead area. Nontransposed early Fla folds plunge eastnortheast down the dip line (reclined folds) in the shear 

foliation plane, indicating a dominant component of strikeslip shearing during fold generation. Drag fold asymmetry 

suggests sinistral slip. 

Small-scale, early F1a drag folds are best preserved in 

the east limb of the Lead anticline and in the adjacent City 

Creek area (fig. 115) where deformational shear strain was 

weakest. Distribution of the early Dla structural domain is 

shown in figure 115. Generally, the east limb of the Lead 

EXPlANATION 

Xe Ellison Formation 

f'::<xh Homestake Formation 

I Xp Poorman Formation 

t::::::::}BY.:.·:::::/J Yates unit 

anticline (fig. 117) and the City Creek area are characterized 

by moderately east dipping foliation (late S 1 a, fig. 118) that 

roughly parallels but locally crosscuts compositional 

layering and an early foliation (SO/early S la) in limb 

positions. Within the mine environment, where late and 

latest D 1 a deformational phases were most intense, linear 

and planar elements related to early Dla are entirely 

transposed. In most cases, early Fla folds are 

indistinguishable from younger folds except where local 

differences in fold orientation and style are noted, or where 

refolded fold profiles are observed. 

Figure 118 shows medium-sized early Fla folds and 

inferred early S 1 a foliation overprinted by late S 1 a foliation 

in profile section (perpendicular to Jate F1a axial 

orientation), developed in Homestake Formation. Figure 

118 is enlarged from the fold labeled "transposed early Fla" 

on the east limb of the Lead anticline in figure 117. The fold 

shows large and small elements both symmetrical about late 

Late F1 a 

sheath fold termination 

East 

West 

Figure J16. Schematic perspective diagram showing 01 a fold events at Homestake mine. 

Homestake Gold Mine, South Dakota )29 
LEAD 

Xe 

tV 

I 

n=288 

ANTICLINE 

Xp 

Orientation of 

fold axes: 

latest F1 a 

EXPLANATION 

FOR STEREONET 

r .............. 'l )i{}/\{ >15 percent 

, ................................. 19-15 percent 

I : . : . : · : · : · : ·13-9 percent 

Xe 

Transposed 

early F1 a 

(area of fig. J18) 

EXPLANATION 

FOR MAP 

Homestake Formation 

Xp I Poorman Formation 

500 METERS 

Figure )17. Generalized geologic map of 2600 level, Homestake mine, showing lower hemisphere, equal-area stereonet with 

288 poles to folded foliations of late 51 a from the Lead anticline (eastern ledge area). Poles contoured at 3, 9, and 15 percent per 

1 percent area. L1 a (+) and L1 b (o) are average stretching lineations and fold axes in late F1 a (sheath fold phase) and early F1 b, 

respectively. Details of structural domains are not shown. 

J30 Geology and Resources of Gold in the United States 
SOUTH 

B 

30 METERS 

NORTH 

B' 

EXPLANATION 

Xe I Ellison Formation 

1'. · ·. ~h. ·. · ·.1 Homestake Formation 

I Xp I Poorman Formation 

---- Contact 

Figure )18. Iron-formation fold profile of the Gentle Annie structure (east limb of the Lead anticline), 2600 level, Homestake 

mine, observed down plunge to east-southeast and along sheath fold axis in figure J17. Late Fl a sheath folding refolded transposed 

early Fl a folds. 

S la. The symmetry and style of folding, in folded early S la 

foliation and early Fla folds in figure 118, indicate the effect 

of superimposed deformation fabrics. Early Fla fold 

structures are not well understood owing to the intensity and 

pervasiveness of subsequent shearing and transposition. 

Middle Dla 

Middle Dla deformation (table J7) is characterized by 

a flattening fabric (middle Sla) that developed in a field of 

coaxial strain. No folds or lineation were produced during 

middle Dla. 

We identified flattening fabric petrographically by 

evaluating three orthogonal thin sections taken from 

oriented specimens, one perpendicular to each fabric axis. 

Flattening fabric of middle Sla apparently overprints and is 

thought to lie parallel to preexisting early S la foliation; 

early Sla fabric commonly is transposed. Mineral grains are 

deformed into oblate spheroids. Where minimal 

overprinting has occurred, these "pancake-shaped" grains 

are not rotated, and they lie parallel to the early S la foliation 

plane, indicating a post-early Dla development. Middle 

Dla flattening fabric is preserved in relatively few domains, 

generally in the eastern mine area and in the adjacent City 

Creek area (fig. 115). Flattening fabric is overprinted in 

various degrees by axially symmetric fabric (late S la 

foliation). In cases of extreme strain, no evidence of the 

flattening phase survived late Dla deformation. 

Flattening fabric is present in the Yates unit metatholeiite and in phyllite and schist of the Poorman and 

Ellison Formations. Iron-formation in the Homes take 

Formation is generally recrystallized and massive in lower 

amphibolite facies and shows little directed fabric at 

microscopic scale. In amphibolitic rocks, flattening fabric is 

expressed by random orientation of hornblende prisms lying 

in the foliation plane. In phyllite and schist, it is expressed 

Homestake Gold Mine, South Dakota )31 
by garnet that forms partly equant and partly oblate 

spheroids parallel to foliation. These garnet crystals contain 

small inclusions of platy quartz grains that are also aligned 

parallel to foliation. Gamet probably grew synkinematically 

during early stages of prograde metamorphism in a field of 

coaxial strain. Additionally, quartz, plagioclase, and pyrrhotite streaks in phyllite and schist, as well as plagioclase in 

Yates unit metatholeiite, have the form of tiny oblate 

spheroids whose principal axes parallel foliation; these 

observations support the interpretation of flattening fabric. 

Late D1a 

Folds generated in late D1a (table J7) are sheathlike 

(conical) in form (fig. Jl9), trending on average N. 35° W., 

with inclined axial planes dipping northeast. Sheath fold 

plunge is varied, averaging 45° SE. A penetrative axial 

symmetric fabric (late S1a foliation) was developed in 

conjunction with sheath folding, as was a distinctive 

stretching lineation (late L1a). Sheath folding (see Park, 

1988) characteristically resulted in marked changes in 

structural profile along fold-axis plunge in iron-formation 

and nearby schist or phyllite (fig. Jl6). Sheath folds develop 

within a field of apparent constriction flow (Ramsay and 

Huber, 1983, p. 172), where stretching lineation (late L1a) 

indicates the direction of tectonic transport. During the 

initial stages of sheath development, minor folds, isoclinal 

in form (fig. Jl9A-C), are generated in response to flow 

along the plunge line. As strain weakens and flow subsides, 

sheath fold terminations are created. Sheath fold 

terminations are closed and elongate structures (figs. Jl6 

and Jl9D) with shallow taper. Fold-axis plunge is varied 

near sheath fold terminations. 

Axially symmetric fabric (late S la foliation) is most 

common in schist and phyllite near iron-formation. It 

overprints and generally destroys early flattening fabric 

(middle S1a) and transposes early F1a folds. In metatholeiite, the fabric is expressed as hornblende prisms with 

parallel c-axes; the resulting lineation is subparallel to 

hinges of late F1a sheath folds. Locally, individual prisms 

are boudinaged, and resulting openings are filled with 

quartz, feldspar, or minor epidote-group minerals. In micarich schist, the axially symmetric fabric (late S1a) is 

expressed by rodded mineral grains, commonly quartz and 

pyrrhotite streaks. Small grains attain the general form of 

prolate spheroids having long axes subparallel to hinges of 

late Fla .sheath folds. Some grains are locally boudinaged, 

and resulting openings are filled with chlorite and quartz. 

Late S 1 a fabric is also expressed by mica orientations 

in phyllite and schist. In petrographic thin sections normal 

to the lineation, edges of mica flakes exhibit random 

orientations. In orthogonal sections parallel to the lineation, 

mica flakes are seen randomly oriented, from edgewise 

)32 Geology and Resources of Gold in the United States 

flakes to basal sections, reflecting the axial symmetry. 

Micas are not curved or bent, as might be the case if the 

fabric were produced by folding perpendicular to the axis of 

symmetry. 

In domains of axially symmetric fabric (late S1a), 

garnet porphyroblasts are commonly rotated so their planar 

inclusion fabric occurs at all angles to foliation in the 

enclosing matrix of schist or phyllite. From this, we 

interpret garnet as a relic from the flattening phase (middle 

D1a) and suggest that it was rotated during the later Fla 

sheath fold phase. This rotation attests to non-coaxial strain 

character in the late D 1 a strain history. 

Late F1a sheath folds overprint early F1a folds and 

middle S 1 a flattening fabric throughout the central mine. 

How much early F1a fold forms are retained within sheathdominated fold domains is not known. Probably a 

significant portion of the isoclinal component in the mine 

rocks is relict early Fla that has been transposed and 

overprinted by late F1a sheath folds (fig. Jl8). Within the 

mine area sheath folds are best preserved, and they represent 

the dominant fold domain in the Lead syncline (fig. Jl5). 

Sheath folds are overprinted by the latest D1a ductile shear 

phase in the Poorman anticline and a part of the Lead 

anticline. We interpret that peak prograde metamorphism 

was attained late during this deformation phase, probably 

extending somewhat into latest D 1 a. 

Latest D1a 

Deformation associated with latest D1a (table J7) was 

dominated by ductile shearing that formed extensive shear 

zones as much as 1,000 m wide within the mine area (fig. 

J15). Latest D1a shearing, where most intense, generated a 

planar mylonitic fabric (latest S 1 a) with an average foliation 

of N. 18° W., 52° NE. 

Latest D1a shears formed on bedding planes and 

earlier foliation surfaces, to create a complex series of 

smaller anastomosing meso-shears that exist in sets several 

meters wide. These meso-shears are composed of even 

smaller micro-shears that form anastomosing strands around 

competent rock masses. Variations in rock competence 

controlled shear-set geometry. Strain intensity was highly 

varied and was dependent on rheologic contrasts. Ductile 

shear zones are characterized by disrupted macro- and 

micro-sheath fold hinges, well-developed flow fabric, drag 

folds, and high-temperature mylonites (fig. J20). 

We believe that movement within Dla shear zones 

was predominantly strike slip with a minor dip-slip 

component producing reclined and semireclined folds that 

plunge to the northeast and southeast, respectively. Reclined 

and semireclined folds (latest F1a) represent various 

degrees of transposition of early F1a folds and late Fla 

sheath folds. These transposed folds are effectively drag 

folds (latest F1a) formed by ductile shear movement. Figure 
sw NE 

Early S1b-......._ 

Late F1a 

so 

Early F1b 

"'-Early L 1 b 

Early L 1b 

~~'' 

2 METER 2 METERS 

A C 

SW NE 

, /Late 

\ S1a 

sw NE 

\ 

yLate S1a 

0.5 METER 

1 METER 

B D 

Figure J19. Sketches of late F1 a (sheath) and early F1 b minor folds in Poorman and Ellison Formations in the eastern 

ledge area of the Lead anticline, Homestake mine. A, typical late F1 a fold in profile section; 8, meso .. scale late 

F1 a-early F1 b refolded combination in profile section; C, late F1 a-early F1 b refolded combination in plan view-this 

style of fold interference is seen at all scales; 0, meso .. scale late F1 a sheath fold termination in profile section. Profile 

sections viewed up plunge along the fold-axis direction. 

J21 shows effects of ductile shearing on late Fl a fold plunge 

in the Poorman anticline area (6800 level). This event 

coincided with the transition from prograde to retrograde 

metamorphism (table J7) and emplacement of the oldest 

recognized set of quartz veins, that is, veins of Stage I. (See 

section, "Quartz veins.") 

Homestake Gold Mine, South Dakota )33 
A 

NW 

B 

Mylonitic 

shear fabric 

latest S1 a 

NE sw 

0 0.25 METER 

L__j 

0.25 METER 

L__j 

posed late F1 a 

SE 

Figure )20. Structural sketches in profile sections of minor 

folds in latest 01 a shear zones, western ledge area on the 

Poorman anticline, Homestake mine. A, Poorman Formation; 

floating, detached fold hinge. B, Ellison Formation; disharmonic reclined drag folds. Single-barb arrows show direction 

of shear. 

Early Dlb 

Folds generated in early D1b (table J7) are upright, 

are open to isoclinal in form, and have nearly vertical axial 

planes trending on average N. 20° W. (fig. J19B, C). In 

detail, F1 b fold trends range from N.S. to N. 60° W. Fold 

plunge is generally moderate, averaging 35° SE. An axialplane foliation (early S1b) and intersection lineation (early 

L1b) developed in conjunction with folding (fig. 117). The 

foliation and lineation are best developed in phyllite. 

In general, the D1 b deformation was substantially 

weaker than all previous phases. Early F1 b fold domains are 

restricted to selected locales in the mine, chiefly the Lead 

syncline area (fig. J15); but they do exist in the other 

domains as well. In places, early F1 b folds locally refold 

late F1a minor folds, late Sla foliation, and the planar 

mylonitic fabric of latest Dla. They do not substantially 

modify the form oflarge-scale late F1a sheath folds or latest 

D1a ductile shears (not shown). The early F1b overprint on 

late Fla resulted in development of conical folds that locally 

J34 Geology and Resources of Gold in the United States 

modify structural profile along plunge. These conical folds 

are characterized by the presence of two lineations: a 

crossing lineation defined as late L 1 a and a lineation 

parallel to early F1b defined as early L1b (fig. 117). The 

early F1b-parallellineation is due to the intersection of early 

S1b with SO/S1a fabric (if preserved), or the F1b hinge. 

Middle Dlb 

Deformation in middle D1b (table J7) was a late-stage 

ductile-brittle shear phase that is considered a continuation 

of the D1b deformation with strain conditions changing 

from upright folding to reverse shearing. Gold 

mineralization was synchronous with middle D 1 b shearing. 

Middle D 1 b shears formed on bedding planes and 

earlier foliation surfaces. They also consist in part of 

reactivated older shear zones. Middle D1b ductile-brittle 

shears dip 45° NE. to vertical. Asymmetric drag folds 

(middle F1 b) with nearly horizontal axes are developed 

within or adjacent to the shears, indicating reverse shearing 

with northeast sides up. Middle D 1 b ductile-brittle shear 

trends are varied, generally ranging from N.S. to N. 60° W. 

Strike and dip of middle D 1 b ductile shears were controlled 

by the attitude of existing foliation surfaces, which was in 

tum influenced by the attitude of more competent gruneritedominant Homestake Formation and Ellison Formation 

quartzite. Similar to latest Dla shears, middle Dlb shears 

anastomose through the mine. Certain selected segments 

were dilated; many of these dilated shears developed 

synchronously with ore bodies and many coincide spatially 

with ore bodies. As shearing continued, parts of some ore 

bodies were drag folded, sheared, and boudinaged. The 

middle D 1 b shearing event was synchronous with 

retrogressive hydrothermal alteration and emplacement of a 

second set of quartz veins, that is, Stage II veins. (See 

section, "Quartz veins,") 

late Dlb 

Deformation textures characterized by late D1b (table 

J7) are distinctly postmetamorphic and obviously postdate 

gold mineralization; they developed in an environment of 

semibrittle strain and lower temperature. Kink folds (late 

F1b) and associated kink shear surfaces (late S1b) with 

north-south as well as northeast to northwest trends were 

generated in late D 1 b. 

Structures associated with late D 1 b are interpreted to 

have developed in response to injected phases of the Crook 

Mountain Granite (this report; Bachman and others, 1990) 

northeast of the mine. The northeast- to northwest-trending 

kink shear surfaces are steeply dipping and apparently 

formed earlier than the north-south kink set characterized by 

shallow dips (R.R. Reid, 1982, Homestake internal report). 

Kink shears produced minor displacement (a few meters), 
Orientation of 

fold axes: 

POORMAN 

ANTICLINE 

Xe 

Figure )21. Generalized geologic map of part of 6800 level, 

western ledge area, Homestake mine, showing lower 

hemisphere, equal-area stereonet with 110 poles to transposed 

late 51 a foliation (nf) from the 6800 to 8000 levels, contoured 

to 5, 10, and 15 percent per 1 percent area. Tight isoclinal 

folding (late F1 a) is indicated by the strong point concentration 

of fold axes with varied plunge. Great circle defines average 

axial plane of partly transposed late F1 a isoclinal folds with 

N 

Xp 

Xe 

N 

t 

EXPLANATION 

FOR STEREONET 

fii't.~i~~iifJJ.I > 15 percent 

l

·. ·:::::.·::.::j :·:·:·:·:·:·:·:·:·:·:·:j 9-15 percent 

r::::::::~::~:::l3-9 percent 

Partly transposed 

late F1 a and late L 1 a 

EXPLANATION 

FOR MAP 

Xe Ellison Formation 

Homestake Formation 

Xp Poorman Formation 

500 METERS 

parallel limbs. Linear features (nb in western ledges are 

represented by 131 points: 69 minor fold axes and 62 

stretching lineations that are parallel in part to neighboring 

minor fold axes. Lineation attitudes range from strike line to 

dip line. Linear elements lie in plane of foliation. Structures 

related to 01 b not developed in this domain. Details of 

structural domains not shown. 

Homestake Gold Mine, South Dakota J35 
commonly on flat shear surfaces, in the ore bodies. Late 

D 1 b kink fabrics exist in narrow restricted domains that are 

more prevalent in the eastern mine area and adjacent City 

Creek area. A third set of quartz veins (Stage Til) was 

emplaced during this phase of deformation. (See section, 

"Quartz veins.") 

Interpretation 

The tectonic evolution of Early Proterozoic rocks in 

the mine area was complex; the major observed deformation 

was related to dynamics of the Trans-Hudson orogen and its 

subsequent reactivation (Bachman and Campbell, 1990). 

Tectonic events occurring prior to deformation Dla (that is, 

nappe folding) are not considered here because, at least in 

the mine area, they were not substantiated by stratigraphic 

relations. A logical tectonic break existed between the 

generation of recumbent nappe fold structures in flat-lying 

sedimentary rocks and transposition of those sedimentary 

rocks to nearly vertical in a transpressional environment 

during the Dla phase of deformation at 1.84 Ga. 

Movement of the Superior craton northwestward 

relative to the Wyoming craton is a probable mechanism for 

development of transpressional forces that initiated the 

Trans-Hudson orogenic event and early Dla sinistral transcurrent shearing in the Black Hills (Bachman and Campbell, 

1990). This plate movement coincided with plate collision 

along the Great Lakes tectonic zone and the Penokean 

orogen at 1.85 Ga (Sims and others, 1987). Within this 

transpressional environment (southeast-northwest) during 

early Dla, Early Proterozoic rocks were transposed to 

nearly vertical attitudes, and structures such as the LeadPoorman anticlinorium began to develop. In continuing 

deformation during middle and late D 1 a, the stress field 

progressively rotated to an inclined east-west to northeastsouthwest compressional configuration. Ductile shearing of 

latest D 1 a was initiated in response to changing stress 

conditions that occurred late in D 1 a sheath fold 

development. Following latest Dla shearing, we interpret 

that significant active deformation ceased, and was 

followed by a period of quiescence. 

In our view, renewed tectonism related to Dlb was 

initiated in response to reactivation of the Trans-Hudson 

zone as a result of continent-arc collision during the Central 

Plains orogen, active between 1.80 and 1.63 Ga (Bickford 

and others, 1986; Sims and others, 1987). Early Fib folds 

appear to have developed during a simple northeastsouthwest compression; middle S 1 b shears with dominant 

reverse movement indicate a similar stress configuration. 

Generation of late D 1 b kink shears and folds is attributed to 

late-stage granite emplacement (Bachman and others, 

1990). 

J36 Geology and Resources of Gold in the United States 

ORE DEPOSIT GEOLOGY 

The Homestake gold deposit is largely stratabound 

within the Homestake Formation and is mined for gold; 

silver is extracted as a byproduct. Overall, ore bodies make 

up less than 3 percent of iron-formation in the Homes take 

Formation in the mine area. Through 1988, cumulative 

production plus ore reserves was 147.7 million t (metric 

tons) at an average grade of 8.36 g Au/t. 

Main and Caledonia ore ledges were mined initially 

in 1876 where they were exposed at the surface. Main 

Ledge is the largest, and it continues to produce ore from the 

Open Cut and from underground intermittently to the 6800 

level, 3,380 m down plunge from the surface. Levels in the 

mine are referenced in feet below the collar of the Incline 

shaft, at an elevation of 5,227 ft or 1,594 m above sea level. 

Twenty-one Ledge, on the 7400 level, is the deepest 

ore-producing area in the mine. The 8000 level, at which 

active drifting and exploration drilling are taking place, is 

the deepest work area. 

Ore Ledges 

The word "ledge" as used in this report refers to a 

plunging fold structure containing Homestake Formation. 

Where economically mineralized with gold, a ledge is 

referred to as an ore ledge. Nine ore ledges have produced 

gold from the Homestake mine and eight remain producers 

(fig. J2). From east to west across the mine area the nine ore 

ledges are: Caledonia, Main Ledge, 7, 9, 11, 13, 17, 19, and 

21 Ledges (fig. J6). Odd-numbered ledges are synformal 

folds composed of a series of subordinate anticlines and 

synclines. Where ore mineralization was most intense, as in 

Main Ledge on the surface and 9 Ledge on the 3200 level, 

these subordinate fold structures are all ore bearing. Evennumbered ledges are anticlinal fold forms that are less 

complex structurally; they are generally barren but may be 

weakly mineralized. The Independence anticline (6 Ledge, 

located between Main and 7 Ledges and not illustrated) was 

a minor gold producer. 

The mine is divided into two general areas: eastern 

ore ledges and western ore ledges. Eastern ore ledges 

include Caledonia, Main, 7, 9, 11, and 13. Western ore 

ledges include 17, 19, and 21. The orientation of an ore 

ledge reflects the attitude of folded and sheared ironformation in the Homestake, and this attitude varies 

throughout the mine depending on the deformation domain 

in which the ore ledge lies. Eastern ore ledges trend on 

average N. 35° W. parallel to late Fla sheath folds, and they 

lie predominantly within the late Dla sheath fold domain 

(figs. J6 and 115). Two noted exceptions to the trend are 

Caledonia between the surface and 1400 level (fig. J22) and 

Main Ledge below the 6200 level (fig. J23). In these areas 

ore ledge trends are deflected eastward owing to the 
influence of early Dla deformation on the Homestake 

Formation. Eastern ore ledges plunge on average 40° SE. 

Western ore ledges trend on average N. 80° E. 

parallel to latest Fla reclined folds and lie predominantly 

within the latest Dla shear domain (figs. J6, J15). An 

, 

exception to this is 21 Ledge below the 6800 level, where 

the southernmost ore bodies project into the late Dla sheath 

fold domain. These ore bodies accordingly deflect 

southward and trend N. 50° W. (not illustrated). The plunge 

of western ore ledges is varied, ranging between 60° NE. 

'\ 'It 

\ ' ....,, ~ 

' 

N 

t 

Orientation of 

fold axes: 

1000 METERS 

Figure J22. Vertical projection of selected horizontal slices of the Caledonia, 9 Ledge, and 17 Ledge ore bodies to a horizontal 

plane, showing the geometric elements of an ore ledge. 

Homestake Gold Mine, South Dakota J37 
w 

= 

0 

~ 

0 

~ 

~ Q. 

i 

c 

.. 

n 

~ 

s. 

0 

0 

s:: 

:r 

-:r 

I'D 

c: 

= ;: 

Q. 

i 

/-.---

/ - / - _...;- ....... / 

.... /-

/ 

/ 

-~..;/ ................... ,_ ...... 

-~--, . ~ :-; ... ~ ' ...... -----' l-19/ '~.......... ' ., 

I 

' / 

\ 

...... _ .... 

,_, MAIN LEDGE I • 

-.-- _/ __ I'-:' . ' ..--:::-- -:;.- / ... '• .. • - - - - ., -- / - - J.- -/ .. .. • ... ' - .:;;a--•"~~--..; /. -:._.' .. · ... ~ .. ~<.'l_i:/"' .. o - -- '""' ._ -... • - -- ,._ __ - "' 0 

--- ----~---' ,.. .. -/ ---- - "'o -.; .. ,. -? ' -- -/ '-- ".> 0 • 

----- "-' 

.s-..o 

"' "'"' "' 

"'& --- v. .. "'& 

&' "'o &'_,"'"''b-.;~., & 

0

o -.; 

0

0 0 ' .;> "& 0 0 0 L • -.; L L i9,S, 0 L L -.; o,_ 0/ 

'-o 'I. o,_ 0 L 'I. o,_ '- 0/ ~~ y 0/ 0/ L 'I. 0/ 0/ 0/ 

/.> 00 

00 ~t- ~t- 19/ 

19/ 

Orientation of 

fold axes: 

~lateF1a 

earJ y 1=7 b 

19t- (9/ 

(9/ 

0 500 METERS 

~"' Figure j23. Vertical projection of selected horizontal slices of Main Ledge ore bodies to a horizontal plane. Dashed line, boundary of Main Ledge late Fl a sheath folds. 
and 40° SE. Average ore ledge plunge in both eastern and 

western ledges is a function of the fold and shear domain in 

which the ore ledge occurs. 

Composite plan view projections (figs. J22, J23) 

illustrate the distribution of ore bodies within an ore ledge. 

To avoid superposition, the figures show ore bodies on 

selected mine levels; horizontal ore-body slices in the 

figures represent numerous individual ore bodies that are 

discontinuous between mine levels. 

Ore ledges are defined by three geometric elements: 

core, upper tail, and lower tail areas (fig. J22); the core area 

contains the largest and most continuous ore bodies. The 

volume of ore in the core area generally exceeds 50 percent 

of the total ore in the ore ledge. The vertical height (mine 

level to level) occupied by a core area ranges from 200 m 

(13 Ledge) to more than 800 m (Main Ledge). The mine 

level within a core area containing the largest crosssectional ore area is termed the ore ledge centroid. Each ore 

ledge has a defined ore centroid. The number and size of ore 

bodies decrease away from the centroid level, both up and 

down plunge. Distal ends of each ore ledge are 

longitudinally asymmetric, divided into upper (up plunge) 

and lower (down plunge) tail areas (fig. J22, 9 Ledge). 

Lower tail plunge lengths are generally two times greater 

than upper tail lengths. Overall, average ore ledge grade 

remains relatively constant, decreasing slightly as the 

transition is made into upper and lower tail areas. As an 

example, in Main Ledge, geologic grade varies 1.5 g Au/t 

between the core and lower tail areas (2, 100 vertical 

meters). Beyond tail areas, economically mineralized 

Homestake grades into barren Homes take Formation ( <20 

ppb Au), which continues undisrupted along plunge. 

Observed physical changes beyond economic limits include 

a marked decrease in sulfides, chlorite, and quartz veins, all 

intimately associated with ore bodies. Beyond a few 

hundred meters from upper and lower tail limits (not clearly 

defined), trace geochemical signatures for gold, silver, and 

arsenic are near detection limits (not illustrated). 

The metamorphic and physical character of the 

Homestake Formation within an ore ledge varies along 

plunge (fig. J24). The Homestake in upper tail areas is 

siderite dominant, reflecting the effects of upper greenschist 

facies metamorphism. In the core areas, siderite and grunerite generally coexist, indicating a transition zone that 

progresses down plunge into lower amphibolite facies. 

Lower tail areas are grunerite dominant. A volume 

reduction of the Homestake Formation corresponding to the 

transition from upper greenschist to lower amphibolite 

facies is observed along plunge from upper to lower tail 

areas. This volume reduction is due in part to devolatilization of iron-formation during metamorphism and the varied 

nature of deformational strain along plunge. In 9 Ledge, a 

69 percent relative volume decrease of the Homestake is 

observed between upper (1700 level) and lower tail (6800 

level) areas. 

An ore ledge centroid represents the mine level 

containing the largest ore bodies. The ore centroid 

theoretically corresponds to a zone of optimal gold 

deposition. In plan projection, ore ledge centroids generally 

lie on a line that has an average bearing of S. 20° E. and 

varied plunge averaging 26° SE. (fig. J25). This centroid 

trend parallels the latest D 1 a shear zones, lies within the 

metamorphic transition zone, and superimposes early D 1 b 

folds and shears. 

Known ore ledges are elongate and varied in plunge 

length. Larger ore ledges, such as Main or 9 Ledge, display 

general continuity of ore bodies along tremendous plunge 

length. As an example, 9 Ledge has been mined along 

plunge for more than 5 km (fig. J24). Ore ledges are of 

varied sizes (fig. J26) ranging from 75 million t (Main 

Ledge) down to less than 2 million t (19 Ledge). 

With the exception of Main Ledge and Caledonia, all 

other ore ledges at Homestake are blind to the surface even 

though the Homestake Formation fold structures in which 

they occur are well exposed. The eastern ore ledges, except 

13 Ledge, were discovered prior to 1958 (fig. J2). The 

western ore ledges were discovered during the 1960's, the 

last of which was 21 Ledge in 1969. The method of 

discovery during the past 52 years consisted of systematic 

underground drifting and drilling. Exploratory drilling in 

1987 on the 6200 level (Caddey and others, 1990) 

intersected 7 m of mineralized rock averaging 24.4 g Au/t in 

15 Ledge, making this ledge a probable new producer for 

the future (fig. J6). 

Ore Bodies and Ore Mineralization 

Economic gold within an ore ledge is contained 

within tabular to pipelike, relatively undeformed ore bodies. 

Ore bodies occur as segregations of pyrrhotite, arsenopyrite, 

minor pyrite (pyrite content diminishes below the 800 

level), and native gold within a gangue matrix of 

quartz+chlorite+siderite+biotite±garnet. Sulfide content 

varies generally from 5 to 30 percent of the total ore 

volume, averaging 8 percent overall. Ore bodies generally 

occur at or near the Homestake-Ellison or HomestakePoorman contact. Where an ore body is contained entirely 

within the Homestake Formation, the Homestake is 

generally a siderite- or grunerite-dominant biotite-chloritequartz schist. 

Figures J27 and J28 illustrate the varied form of 

folded · Homestake Formation and contained ore bodies 

along plunge. Ore bodies do not conform strictly to the 

Homestake host stratigraphy. They rake across bedding and 

the pre-middle D 1 b folds and foliations. In both figures J27 

and J28, ore bodies lie near either the Homestake-Poorman 

or the Homestake-Ellison contacts, except where the entire 

Homestake Gold Mine, South Dakota J39 
I , 

Composite of upper 

greenschist-lower 

amphibolite facies 

transition zone 

(siderite and grunerite 

coexist in Xh) 

UPPER 

GREENSCHIST 

FACIES 

N 

t 

1000 METERS 

Orientation of 

fold axes: 

LOWER 

AMPHIBOLITE 

FACIES 

Figure )24. Vertical projection of selected horizontal slices of 9 Ledge ore bodies to a horizontal plane, showing geometric 

elements of an ore ledge and the upper greenschist-lower amphibolite facies transition zone. 

volume of Homestake has been mineralized. Commonly 

(fig. J28, 6050 level), individual ore bodies lie transverse to 

compositional layering within the Homestake. Localization 

of ore bodies near formational contacts is observed 

.. 1-troughout the mine and is related to ore-stage ductilebrittle shears (middle D 1 b) along rheologic boundaries. 

J40 Geology and Resources of Gold in the United States 

The number, size, and relative internal continuity of 

individual ore bodies are highly varied depending on the 

position of ore bodies within an ore ledge and on the 

intensity of middle D I b shearing. Ore bodies in figure J27 

represent a transition from the large continuous "core-type" 

ore bodies on the 1850 level to smaller "lower tail-type" ore 
N 

t 

1 KILOMETER 

I 

EXPlANATION 

I Ellison Formation 

:::::::::===::::: 

Xe 

~ Homestake Formation 

Xp I Poorman Formation 

• Ore centroid (projected to horizontal plane; 

not all centroids labeled) 

~ Ore centroid trend-Arrow shows plunge 

Dla STRUCTURAL DOMAINS 

t·:·:-:·:·:-:·:·:·:·:-:·J Early Dla folding/middle D1a fabric (with 

~·:::::::::::.j moderate late D1a sheath overprint) 

I · · · · ·1 Late Dla sheath folding (with strong early 

· . · . · . · . · . D 1 b overprint) 

1· · .. · .. · .. · .. · .. · .. ·. ·) Latest D 1 a ductile shearing (with weak early 

· . . . . . . D1b overprint) 

Xe 

Main Ledge 

(200 level) 

UPPER 

GREENSCHIST 

FACIES 

Xe 

.,_,c--~~~~~-;..........~:.;..;.z;....East 1 7 Ledge 

Composite of upper 

greenschist-lower 

amphibolite facies 

transition zone 

(siderite and grunerite 

coexist in Xh) 

(5300 level) 

13 Ledge 

(5900 level) 

Figure J25. Geology of 2600 level, showing upper greenschist-lower amphibolite facies metamorphic transition zone, 01 a 

structural domains, and ore centroid trend. 

Homestake Gold Mine, South Dakota J41 
CALEDONIA 

MAIN LEDGE 

7 LEDGE 

9 LEDGE L.U 

(!) 

0 

L.U 11 LEDGE ....J 

L.U 

0: 

0 13 LEDGE 

17 LEDGE 

19 LEDGE 

21 LEDGE 

10 20 30 40 50 60 70 80 

METRIC TONS OF ORE X 106 

Figure J26. Histogram of ledge production in the Homestake mine, showing average (prerecovery) grade in grams per short ton. 

Cumulative gold production plus ore reserves (surface and underground) are 147.7 million metric tons averaging 8.36 g Au/t 

through 1988, shown by ore ledge. 

bodies on the 3050 level. Ore bodies in figure J28 occur in 

the lower tail of 9 Ledge; their forms are generally 

characteristic of upper and lower tail ore-body geometry in 

other ore ledges. The detail of figure J28 reveals that 

individual ore bodies are discontinuous along plunge. The 

"D" limb structure of 9 Ledge between the 5600 and 6050 

levels (fig. J28) contains 12 ore bodies; the largest is 65 m 

in plunge length. The 9 Ledge ore system as a whole 

contains 196 individual ore bodies, which range in volume 

from less than 2,000 to greater than 1 million t. 

Ore mineralization took place within and adjacent to 

dilated segments of middle D 1 b shears, which tended to 

dilate preferentially in Homestake Formation. Ore 

mineralization where confined within the shear segments 

formed sharp shear contacts between ore and adjacent 

country rocks (figs. J29, J30). Middle D1b shears 

throughout the mine show reverse movement with northeast 

blocks consistently up, and with vertical to moderate 

northeast dips. 

The type of mineralization that occurred within 

middle D 1 b shear zones resulted in a mineral assemblage 

comparable in the classical sense to a mesothermal vein 

assemblage and referred to herein as "shear ore." "Shear 

ore" commonly shows little if any original fabric and 

generally comprises stage II quartz veins with associated 

chlorite, siderite, arsenopyrite, and minor pyrrhotite (figs. 

129, J30). Chlorite replacement of the host rock (largely of 

biotite) in middle D1b shears has been extensive. "Shear 

ore" in figure J30 is contained within a middle D1b shear 

zone, which lies on reactivated late Fla foliation (late Sla) 

J42 Geology and Resources of Gold in the United States 

entirely in the Homestake Formation. This ore occurrence is 

locally devoid of quartz veins and retains some relict 

original fabric in the form of chert. Gold grades of "shear 

ore" are generally excellent, as illustrated in figure J30. 

Elevated gold values (26.74 and 2.74 g Au/t) in the layered 

pyrrhotite zone left of the middle D1b shear zone represent 

a second type of ore, referred to herein as "replacement 

ore." The "replacement ore" (figs. J29, J30) reflects leakage 

off master middle Dlb shears via early Slb axial-plane 

foliation and earlier foliations. Disseminated pyrrhotite 

replaced certain siderite-rich beds within the Homestake 

and is deposited in discontinuous layers along foliation 

surfaces. Pyrrhotite within the replacement ore zone also 

occurs in minor dilatant zones along quartz, siderite, 

ankerite, and chlorite mineral grain boundaries. The 

replacement process, or sulfidation, was dependent on an 

increase in permeability developed on existing bedding or 

foliation surfaces during middle D 1 b shearing. In addition 

to sulfidation of siderite-bearing parts of iron-formation 

units, moderate chlorite and siderite (scattered coarse grains 

and grain aggregates) alteration occurred within the 

replacement ore zone. The siderite component within the 

replacement and shear portion of the ore body is distinctly 

coarser grained and paragenetically younger than very fine 

grained matrix siderite observed within layers in barren 

carbonate-facies iron-formation. "Replacement ore" as 

currently defined is everywhere spatially associated with 

gold-bearing and chloritically altered middle Dlb shears. 
:r. 

% 

~ 

; 

~ 

C') 

s. 

g. 

~­

~ til 

~ g. 

0 

~ 

~ 

~ 

2450 LeVEL 

2750 LeVEL 

3050 LeVEL 

3200 LeVEL 

\\)\) t.AE'it.RS 

'U: 

<i.-- :t: C1) 

(/) .'2: 

z~ 0 c (/);.::.. 

3 

UJ 

,s50 LeVEL 

Orientation ot 

to\d a)les: 

~ Elll••• for.,atloD 

~ tto"'estak• f•'"'"tlonll>•"•n) 

~ tto"'estak• fonnatlonlor•) 

~ 8 poonn•• f•'"'"""" 

~~ 

--==================~3I0050L£VcL )<..e 

• • t M . L d 1850 2450 and 3050 levels. See ligure )6lor loeatioo ol Maio Ledge relative to Lead anticline. 

\sornetnc """' o• a•O e ge on , ' 

figure )27. 
'i: 

ol:o 

C) 

t!) 

0 

0 

~ 

I» 

:::s 

C)., 

~ 0 

c 

... 

I"') 

t!) 

Ill 

s. 

C') 

0 

~ 

:;· 

.... 

::r 

t!) 

c 

:::s 

~· C)., 

~ I» 

(;' 

Ill 

5600 LEVEL Xp 

~ 

Orientation of 

fold axes: 

\ 

0 500 METERS 

5900 LEVEL 

MINE LEVELS SEPARATED TO AVOID SUPERPOSITION 

EXPlANATION 

I Xe I Ellison Formation 

H?L~H}\J Homestake Formation (barren) 

~ Poorman Formation (ore) 

I Xp I Homestake Formation 

Contact 

Figure j28. Geologic map of "D" limb of 9 Ledge on the 5600, 5750, 5900, and 6050 levels. Configuration of ore bodies is shown enclosed within late F1 a sheath 

fold structures collectively located in hinge of Lead anticline. Tertiary intrusive rocks not shown. 
Late F1 a 

folds 

SOUTHWEST 

"Replacement ore" 

Discontinuous layered 

pyrrhotite zone 

Xp 

(quartz-graphitebiotite phyllite) 

Xh 

(chert-biotite-chloritesiderite schist) 

EXPLANATION 

Ellison Formation 

Homestake Formation 

Poorman Formation 

~Bedding 

,--..... Chert 

~ Stage I quartz vein 

~ Stage II quartz vein 

: * * Arsenopyrite 

Pyrrhotite 

,.r-....J Biotite and (or) chlorite-Dashed 

where discontinuous 

"Shear ore" 

chlorite-arsenopyrite rich 

Middle 01 b shear zone 

(biotite-chlorite-siderite schist) 

2 METERS 

NORTHEAST 

Xe 

(biotite-rich 

quartzite) 

2 METERS 

Figure J29. Face sketch, 7 Ledge, 1550 level, 58-61 stope, Homestake mine. Near-vertical line, shear; heavy line, major shear; 

barbs show relative movement. 

Homestake Gold Mine, South Dakota J45 
NORTHEAST 

" Replacement ore" 

Disseminated and discontinuous layered pyrrhotite 

zone with traces of chlorite and arsenopyrite 

Xh 

(grunerite-chert-biotite schist) 

EXPlANATION 

Xh I Homestake Formation 

~Bedding 

r-- Chert 

~ Stage I quartz vein 

67 .88 

• • • • 

Stage II quartz vein 

Channel sample, showing gold 

assay value (grams/metric ton) 

Arsenopyrite 

Pyrrhotite 

Middle 01 b shear zone 

(chlorite-chert schist) 

2 METERS 

/,-V Biotite and (or) chlorite-Dashed where discontinuous 

SOUTHWEST 

"Shear ore" 

(chlorite-arsenopyrite rich) 

drag folds 

(flat axes) 

Xh 

(grunerite-chert-biotite schist) 

2 METERS 

Transposed So= 

late 51 a= 

middle 51 b 

Figure j30. Face sketch, 13 Ledge, 6350 level, 55 stope, 1west "B" limb, Homestake mine. Heavy line, major shear; barbs show 

relative movement. 

)46 Geology and Resources of Gold in the United States 
In figure J29, the entire shear zone is ore bearing; it 

lies disconformably between a steep northeast-dipping and 

drag-folded Ellison hanging wall and a steep northeastdipping Poorman footwall. The chaotic middle D 1 b 

deformation fabric, Stage II quartz veins (see section, 

"Quartz veins"), and gold mineralization overprinted all 

other observed fabrics. Stage II quartz veins within the 

middle D 1 b shear zone are unfractured, and most were 

emplaced in their present tabular to irregular form; all are 

lined with undisrupted and continuous vein selvages 

consisting of chlorite, arsenopyrite, and minor pyrrhotite. 

The right side of the shear zone (toward northeast) is 

dominated by "shear ore" consisting of arsenopyrite and 

abundant chlorite. The left side (toward southwest) is 

dominated by "replacement ore" consisting of pyrrhotite in 

discontinuous layers with only minor chlorite, a common 

fabric observed in many ore bodies. 

The shear zone in figure J30 is 3 m wide, contains 

"shear ore" consisting of abundant arsenopyrite and chlorite 

with minor pyrrhotite, and dips steeply northeast. It is 

discordant to stratigraphy, contains a transposed and shearinduced foliation, and shows the physical result of middle 

D 1 b reverse shearing contemporaneous with gold 

mineralization. Local disseminated and discontinuous 

layered pyrrhotite, minor arsenopyrite, and ore-grade gold 

overprint late Fla and early Flb folds of the hanging wall; 

both fold sets are truncated by the shear zone. This 

"replacement ore" was also related to the shear zone; it 

terminated beyond 6 m into the hanging wall. 

Mineralization consisted of two styles of coeval 

development (deposition of "shear ore" within the dilating 

middle D 1 b shear zone and "replacement ore" developed 

outward from the shear zone), and these deposits have been 

undeformed since. 

The gold distribution within an ore ledge centers 

around high-grade (> 15.5 g Au/t) zones that form the 

nucleus of individual ore bodies (figs. J31, J32). Lower 

grade material ( <4. 7 g Au/t) lies lateral to and on strike 

extensions of these high-grade zones. The size of high-grade 

parts of an ore body was determined by the intensity of 

middle D 1 b shearing, which in tum was controlled by 

position within the Homestake Formation. The largest 

high-grade ore zones are found in core areas, as illustrated 

by gold-grade distribution in Main Ledge on the 2450 level 

(fig. J31). Also indicated in figure J31 is the extension of 

ore into Poorman and Ellison Formations, a relatively 

common feature throughout the mine. Poorman- and 

Ellison-hosted gold occurs in middle D 1 b shears near the 

Homestake Formation and represents only a small 

percentage of the total gold produced from the mine. 

Gold-grade distribution in lower tail areas is sporadic 

and discontinuous, as illustrated in figure J32. Small zones 

of subeconomic gold-bearing rock exist in upper and lower 

tail areas throughout the mine, similar to the grade 

distribution in figure J32. These zones may contain highgrade (> 15.5 g Au/t) gold-bearing rock that is less than a 

meter wide, or low-grade (1.5-4.7 g Au/t) material that is 

several meters wide. The frequency of occurrence of subeconomic segments decreases with1 

increasing distance from 

the core area. The current mining cutoff is a 5 m width 

assaying 4. 7 g Au/t. 

Ore Mineralogy 

Ore bodies in the Homestake mine are sulfide rich, 

dominated by pyrrhotite. Ore bodies contain varied amounts 

of carbonate and silicate gangue minerals including chlorite, 

siderite, grunerite, quartz, biotite, ±garnet, with minor 

ankerite, muscovite, and albite, in addition to graphite, 

listed in decreasing order of abundance (Noble, 1950; 

present authors). Sulfides include pyrrhotite and arsenopyrite, with subordinate pyrite and chalcopyrite. Pyrrhotite:arsenopyrite ratios in ore bodies are generally estimated 

in the range of 1 :2 to 10: 1. In general, ratios increase from 

eastern to western ore ledges and from upper to lower tails. 

Magnetite, hematite, ilmenite, galena, sphalerite (microscopic), and visible gold are locally present (Noble, 1950; 

McLaughlin, 1933). Ore-body mineralogy at Homestake 

has been described by Gustafson (1930, 1933); McLaughlin 

(1933); Noble (1950); Slaughter (1968); and Chinn (1969). 

The most detailed discussion, that of Noble (1950), is valid 

today even though western ore ledges ( 17, 19, 21) were not 

yet discovered in 1950. Hand specimens and polished 

sections of typical ore specimens are shown in figures J33, 

J34, and J35. 

Pyrrhotite 

Pyrrhotite is present in three textural forms: streaks, 

blebs, and layers. Pyrrhotite is brownish bronze and is either 

magnetic (2M polytype) or nonmagnetic (6H polytype), the 

·magnetic polytype being more common. Pyrrhotite streaks 

are commonly found in iron-formation and phyllite of 

adjacent formations; they are not associated there with 

anomalous gold. In iron-formation, pyrrhotite streaks are 

generally oriented parallel to transposed bedding, are 

crosscut or replaced by pyrrhotite blebs and layers, and are 

locally replaced by arsenopyrite. They are less than 1 mm to 

3 mm thick, 1 mm to 1 em in length (parallel to foliation), 

and flattened in the plane of foliation. This pyrrhotite 

variety is the only sulfide of possible sedimentary 

derivation. 

Pyrrhotite blebs occur in irregular and elliptical 

morphological varieties. Blebs measure less than 1 mm to 

several centimeters in maximum dimension. Irregular blebs 

locally contain fragments of quartz veins and country rock. 

Blebs are common in disseminated lenses and masses 

within and adjacent to Stage I (elliptical blebs) and Stage IT 

Homestake Gold Mine, South Dakota J47 
'i: 

= 

C) 

~~~ ~ Q.. 

~ 

"' 0 

c 

"" (') 

I'D 

"' s. 

C1 

0 

0:: 

s· 

-

~ 

c 

:::J 

;: 

Q.. 

VI 

~ "' 

Xh 

Xe 

Xp 

EXPLANATION 

Less than 1.5 g Au/t 

1.5 to 4. 7 g Au/t 

4.7 to 15:5 g Au/t 

Greater than 15.5 g Au/t 

l Xe I Ellison Formation 

I Xh I Homestake Formation 

I Xp I Poorman Formation 

----- Contact 

-------- Grade boundary 

Xe 

0 10 20 30 METERS 

I I I I 

~~ 

Figure )31. Geologic map showing gold grades for 2450 level, Main Ledge. This map depicts a core area position in the ore ledge. (See figs. j6 and J27 for relative location.) 
:t 

0 

3 

~ 

S' 

:Ill:" 

I'D 

C'l 

0 

a: 

~ 

s· 

.!D 

~ c 

... 

::r 

0 

I» 

~ 

r-·············1 t??NX?\ 

~ 

Xe 

EXPlANATION 

~Xh Less than 1.5 g Au/t I Xe I Ellison Formation 

1.5 to 4. 7 g Au/t I Xh I Homestake Formation 

I Xp I Poorman Formation 0 10 20 30 METERS ~~ I I I I 

4.7 to 15.5 g Au/t 

Greater than 15.5 g Au/t ----- Contact 

-------- Grade boundary 

Figure )32. Geologic map showing gold grades for "D" limb of 9 Ledge, 5750 level. This map depicts a lower tail position in the ore ledge. (See figs. j6 and j28 for 

l: relative location.) I.C 
(irregular blebs) quartz veins. Locally, irregular blebs in 

Stage II quartz veins are intergrown with arsenopyrite, and 

everywhere associated with low-grade (> 1.0 g/t) to oregrade (>4. 7 g/t) gold. Elliptical blebs are not associated with 

gold. Pyrrhotite blebs in Stage lll quartz veins are also of 

the irregular variety and are locally associated with 

anomalous gold (as much as 300 ppb). 

Pyrrhotite layers occur as 1- to 2-mm-thick seams, or 

1- to 4-cm-thick disseminations in layers alternating with 

parallel transposed beds of chert±biotite±chlorite in 

Homestake Formation. Pyrrhotite layers are found on 

foliation-bedding surfaces as microdilational infilling 

between stratigraphic beds and as replacement 

disseminations in certain strata (siderite beds) near or 

adjacent to Stage IT quartz veins. Pyrrhotite layers are 

confined to ore bodies, contain sparse isolated grains of 

arsenopyrite porphyroblasts, locally contain coarse grains of 

gold, and assay greater than 1.0 g Au/t; indeed, they 

generally assay greater than 4. 7 g Au/t. Pyrrhotite layers and 

blebs are commonly intergrown, and both locally replace 

pyrrhotite streaks. Zones of pyrrhotite layers invariably 

become discontinuous within a few meters from margins of 

JSO Geology and Resources of Gold in the United States 

Figure )33. Photographs of typical ore specimens, Homestake mine; po, pyrrhotite; apy, arsenopyrite; v.g., coarse 

visible gold. A, Pyrrhotite blebs in grunerite- and siderite-rich 

layers interbedded with thin chert beds from drill core in a 15 

Ledge ore body, 8000 level. Minor chlorite is present in the 

grunerite- and siderite-rich strata. Two small grains of coarse 

visible gold border irregular pyrrhotite blebs. A 2 m interval 

assayed 37.3 g Au/t. Modal sulfide contents are 8 percent 

pyrrhotite, <<1 percent coarse gold, and <<1 percent arsenopyrite. 8, Typical layered pyrrhotite (some disseminations and 

blebs) interlayered with biotite-rich chert and siderite in an ore 

body in 21 Ledge, 7400 level. Thin layers that enclose thinner 

pyrrhotite layers contain minor chlorite, and are locally 

replaced by isolated arsenopyrite porphyroblasts. This layered 

texture is restricted to ore bodies. Two single grains of coarse 

visible gold occur within two pyrrhotite layers. Modal sulfide 

contents are 1 2-14 percent pyrrhotite, <1 percent arsenopyrite, and <<1 percent coarse visible gold. C, Coarse-grained 

arsenopyrite within a chlorite-rich siderite matrix from an ore 

body in 7 Ledge, 1550 level. Specimen came from a 2.0-

m-wide Stage II quartz-vein selvage within which coarse 

visible gold is present locally. The gold borders and is within 

arsenopyrite porphyroblasts, and is enclosed as isolated platy 

grains in chlorite. This ore is typically of exceptional grade and 

averages > 16.0 g Au/t. Modal sulfide contents are 15-20 

percent arsenopyrite and <1 percent pyrrhotite. 

Stage IT quartz veins. This texture is exclusively associated 

with Stage IT quartz veins within some ore bodies. Pyrrhotite layers are most common in western ore ledges at lower 

metamorphic grades and where middle D1b shear foliation 

is best developed. In ore bodies, combined pyrrhotite 

content (by volume) varies from 5 percent to more than 40 

percent. 

Arsenopyrite 

Arsenopyrite is an important indicator of gold and is 

at least a minor constituent in parts of all ore bodies. 

Arsenopyrite content (by volume) varies in ore bodies from 

a trace to more than 15 percent. Typical occurrence of 

arsenopyrite is as tin-white, euhedral, pseudo-orthorhombic, 

prismatic crystals with diamond-shaped cross sections; only 

rarely is it deformed into stretched crystals. Crystals are 
Figure )34. Photomicrographs showing typical arsenopyrite occurrence; po, pyrrhotite; apy, arsenopyrite. A, 

Coarse arsenopyrite from the selvage of a Stage II quartz 

vein. Coarse, disseminated, subhedral to euhedral 

arsenopyrite is shown in a chlorite matrix. Field of view 

is 4 mm vertically. 8, Typical layered arsenopyrite, 

which at high magnification is resolved as euhedral 

arsenopyrite crystals whose interstices are filled with 

pyrrhotite. Field of view is 2 mm vertically. C, 

Replacement of arsenopyrite by pyrrhotite with "islands" 

of arsenopyrite in pyrrhotite. Field of view is 2 mm 

vertically. 

generally from 2 mm to 2.5 em long and are commonly 

striated parallel to [001] (Noble, 1950). Arsenopyrite is 

disseminated in the Homestake Formation (and locally in 

the Poorman and Ellison) adjacent to Stage II veins and may 

exist as densely crowded groups of crystals. Chlorite-rich 

and locally pyrrhotite rich rock adjacent to Stage II quartz 

veins, generally near or within ore bodies, contains the most 

abundant arsenopyrite (Noble, 1950; present authors). 

Arsenopyrite is generally, but not everywhere, associated 

with ore-grade (>4.7 g/t) gold. Stage II quartz veins occur 

without exception in the vicinity of abundant arsenopyrite 

concentrations. 

Both coarse- and fine-grained varieties of arsenopyrite are commonly (though not everywhere) associated 

with ore-grade gold (>4. 7 g Au/t). The coarse-grained 

Homestake Gold Mine, South Dakota J51 
)52 Geology and Resources of Gold in the United States 

Figure J35. Photomicrographs showing occurrence of 

gold; apy, arsenopyrite; po, pyrrhotite; Au, gold. A, Gold 

grain in arsenopyrite. Gold grain is 0.1 mm in maximum 

dimension. 8, Gold, pyrrhotite, and arsenopyrite in a 

matrix of siderite, chlorite, and biotite. Field of view is 2 

mm wide. C, Gold intergrown with pyrrhotite. Field of 

view is 1 mm vertically. 

variety is the more common and is found as (1) single 

porphyroblasts or aggregates within Stage II quartz veins, 

(2) single porphyroblasts or aggregates in selvages adjacent 

to Stage II quartz veins, and (3) random grain aggregates 

within or near ore bodies. Coarse-grained arsenopyrite is 

found intergrown with pyrrhotite blebs and layers. The 

fine-grained variety is found as selective replacements of 

biotite-rich layers or introduced along microdilatancies 

along middle D 1 b shear foliations. Arsenopyrite layers, like 

pyrrhotite, are discontinuous, commonly intergrown with 

pyrrhotite layers, and exclusively associated with margins 

of Stage IT quartz veins. Arsenopyrite in these layers 

comprises randomly oriented aggregates of intergrown 

euhedral grains extending in two dimensions in the plane of 

middle Dlb shear foliations or subparallel to compositional 

layering. 

Pyrite 

Early Proterozoic-age pyrite is found within most ore 

bodies. It is more abundant (by volume) in upper eastern ore 
ledges (trace to 10 percent), and sparse in lower eastern and 

western ore ledges (<1 percent). Pyrite is found as mediumgrained (1-5 mm) cubes in the upper mine levels, whereas 

in the lower mine levels it is very fine grained (<1 mm) and 

cubic, and generally requires detection by microscopic 

methods. It occurs as a fine-grained replacement of pyrrhotite streaks, blebs, and layers. Tertiary-age pyrite is coarse 

grained (3 mm to 1 em), is spatially related to Tertiary 

igneous dikes and late vuggy quartz veins, and is clearly 

secondary, locally as a replacement of Precambrian pyrrhotite. 

Gold 

Gold is invariably associated with irregular pyrrhotite 

blebs and layers, and commonly with arsenopyrite. Most 

gold is anhedral, very fine grained (<<1 mm), and 

disseminated throughout the ore bodies; highest 

concentrations are near margins of some Stage II quartz 

veins. As documented by Noble (1950), gold in general is 

spatially associated with the assemblage composed of 

quartz-chlorite-pyrrhotite-arsenopyrite. The gold-bearing 

assemblage largely constitutes alteration selvages found 

along walls of Stage IT quartz veins. Gold grains occur as 

microscopic to submicroscopic blebs or flakes found within 

and along grain boundaries of silicate, sulfide, and 

carbonate gangue minerals, as coarse intergrown networks 

with quartz, and as micrometer-thin smears on chlorite 

parting surfaces. Visible gold is also observed along grain 

boundaries of arsenopyrite and pyrrhotite, and along 

fractures in arsenopyrite, pyrrhotite, and garnet. Main and 9 

Ledges contained the most abundant coarse gold on the 

upper levels; however, visible gold is common throughout 

the mine. 

Gold is reported as 899 fine (based on mill 

recoveries), and the remaining 101 as silver. Based on 

microprobe analyses from 56 gold samples across the 

deposit (R.M. Honea, oral commun., Sept. 9, 1988), average 

gold and silver compositions of gold grains are 86.09 wt. 

percent and 13.91 wt. percent, respectively. High and low 

silver concentrations are 21.20 wt. percent and 6.21 wt. 

percent, respectively. Four samples contained silver 

exceeding 20.00 wt. percent. The gold:silver ratio averages 

6.2. J.C. Groen (oral commun., July 15, 1988) analyzed 36 

selected gold samples from all ore ledges across the mine. 

His gold:silver ratios ranged from 3.0 to 10.0, averaging 4.0. 

The combined data suggest an average gold:silver ratio of 

5:1 for the Homestake deposit. Gold overall at Homestake 

does not contain enough silver to be classified as electrum. 

Gold fineness is calculated at 861 fine based on Honea's 

work, and 838 fine based on Groen's work. Percent of silver 

recovered at Homestake is much lower than that of gold, 

and has varied over time. 

Other Minerals 

Chalcopyrite is common in trace amounts in most ore 

bodies. It occurs as microscopic to <2 mm anhedral grains, 

commonly as streaks or blebs in pyrrhotite. Galena and 

particularly sphalerite are rare, and present only in certain 

quartz veins. The presence of minor l?llingite has b~en 

verified by X-ray diffraction techniques m some ore bodtes 

by us and by R.M. Honea (oral commun., Sept. 9, 1988). 

Magnetite, hematite, and ilmenite are minor accessory 

minerals; they are generally observed petrographically in 

polished thin sections. 

Tertiary-Age Mineralization 

Tertiary-age mineralization was temporally and 

spatially related to Tertiary igneous dikes and sil~s, a~d 

represents the latest stage of mineralization recogmzed m 

the mine. It was a minor event, noneconomic, characterized 

by minor calcite, quartz, pyrite, dolomite, siderite, sphalerite, galena, molybdenite, chalcopyrite, anhydrite, fluorite, 

gypsum, chlorite, barite, rhodochrosite, realgar, native 

arsenic, and rarely gold-silver tellurides (Noble, 1950; 

present authors). These minerals are found in late Tertiary 

quartz veins, disseminations, and open-space fillings in and 

around fractures cutting all Tertiary intrusive and nearby 

Precambrian rocks. 

Major and Trace Element Geochemistry 

A large geochemical database existing in Homestake 

files is currently being evaluated. A limited amount of new 

geochemical data for selected ore ledges is presented in 

table J8, and analogous data on barren (to weakly 

anomalous) iron-formation were given previously in table 

J4. The reader is referred to Bidgood (1977) and Fantone 

(1983) for other published geochemical data. 

Data from table J8 combined with additional data 

from Homestake files (not shown) allow some 

generalizations that compare and contrast the geochemistry 

of barren and ore-bearing Homestake Formation. The 

formation, in general, can be characterized as having rather 

consistent values for CaO, MgO, Na20, MnO, Ti02, and 

P 20 5• Oxides that exhibit a depletion in ore-bearing Homestake relative to barren Homestake include Si02, FeO, and 

K20. Ore bodies tend to show a general enrichment in 

sulfur, C02, gold, arsenic, and silver with slightly elevated 

values for Fe20 3 and some metals of the first transition 

series (vanadium, chromium, cobalt, nickel, copper) along 

with lead and zinc. Tungsten is barely detectable in barren 

and mineralized rock at an average value of about 2.6 ppm. 

A distinct trend observed for these elements in both barren 

and mineralized iron-formation is as follows: 

Cu::::: Zn > V > Cr > Co > Pb > Ni > W 

Homestake Gold Mine, South Dakota )53 
Table J8. Major, minor, and trace element geochemistry of mineralized Homestake Formation samples, Homestake mine 

[Unpublished Homestake data. Trace element values in parts per million, major and minor element values in weight percent. 13 Ledge sample contains 

visible gold. Leaders ( --), not determined. Samples analyzed by atomic absorption and ICP methods, Skyline Labs, Wheatridge, Colo. C02 determined by 

carbon analyzer. GDS, grunerite-dominant schist; SDP, siderite-dominant phyllite] 

Rock type Si02 Al20 3 Fe20 3 FeO MgO CaO 

1) GDS (ore) 59.40 4.50 4.90 18.40 5.20 0.35 

2) GDS (ore) 47.40 9.00 13.30 15.00 4.80 .63 

3) GDS (ore) 29.20 11.30 11.60 24.80 7.80 1.10 

4) GDS (ore) 32.20 10.00 12.60 16.80 6.40 .94 

5) SDP (ore) 26.20 6.40 17.10 18.20 5.50 1.50 

6) SDP 47.70 3.60 4.00 22.60 5.10 2.90 

7) SDP (ore) 25.50 11.30 15.19 7.30 4.00 5.40 

8) SDP (ore) 54.30 4.00 9.80 14.30 4.20 1.80 

Sample Au Ag Cu Pb Zn Co Ni 

1) Main Ledge 6.80 0.6 40 <5 10 10 10 

2) Caledonia 8.10 6.9 150 85 190 20 10 

3) 11 Ledge 12.68 .6 55 <5 95 20 20 

4) 13 Ledge 67.54 4.1 35 <5 85 15 20 

5) 15 Ledge 5.14 .7 135 <5 75 25 15 

6) 15 Ledge .16<0.2 <5 <5 25 <5 <5 

7) 17 Ledge 8.45 .2 10 125 65 15 

8) 21 Ledge 7.54 1.4 90 <5 50 15 10 

In barren iron-formation, however, values for many 

of these elements are at or below detection limits for atomic 

absorption analytic methods, with the exception of copper, 

zinc, vanadium, and chromium. Other notable trace 

constituents in all Homestake Formation include the 

alkaline-earth components BaO, SrO, and Rb20. BaO is 

more abundant than the other two by an order of magnitude, 

although all three oxides combined rarely exceed 0.05 wt. 

percent. 

In terms of constituents that are enriched in the ore 

bodies (sulfur, C02, gold, arsenic, and silver), sulfur 

correlates best with gold and is present in amounts ranging 

from <0.2 wt. percent totalS to as much as 13.5 wt. percent. 

This correlation corresponds to the direct spatial association 

of ore-grade gold and abundant pyrrhotite in the vast 

majority of ore bodies. A typical example is shown for 21 

Ledge in figure J36. Arsenic (as arsenopyrite) exhibits a 

weak to moderate correlation with gold (+ 0.5), although 

within an ore ledge arsenic is a general indicator of 

ore-grade gold (fig. J36). The presence of abundant arsenopyrite does not always indicate associated ore-grade gold, 

J54 Geology and Resources of Gold in the United States 

Major and minor elements 

Na20 K20 MnO Ti02 P20s 

elemental 

C02 S (tot) 

0.04 0.93 0.381 0.26 0.08 0.86 0.17 

.24 1.40 .465 .21 .04 .42 .14 

.11 .35 .923 .40 .18 1.10 4.03 

.03 .40 .484 .37 .07 4.00 2.94 

.03 .62 .960 .17 .04 4.20 6.53 

.08 .68 .633 .12 .26 <0.02 6.48 

.12 3.50 .910 .40 .04 5.90 8.23 

<0.01 .37 .470 .12 <0.02 4.20 3.57 

Trace elements 

Cr w As v Zr Ba Sr Rb 

15 3 440 60 30 210 <10 30 

25 5 47,000 80 55 190 30 30 

30 4 2,400 100 50 20 

35 <2 60,500 885 80 <10 20 30 

30 4 42,500 90 70 150 30 30 

15 <2 <5 35 20 150 60 30 

<2 53,500 115 30 10 

30 2 1,300 45 30 100 40 20 

especially in extreme upper tail areas of a ledge. Arsenic 

values in ore samples range from below 5 ppm (detection 

limit by atomic absorption with hydride-generation 

analysis) to about 5 wt. percent. Anomalous silver is 

associated with gold. Silver values are generally below 0.2 

ppm in barren Homestake Formation and range from 0.2 

ppm to nearly 7.0 ppm in ore bodies. 

Silver exists in solid solution in gold grains. As 

previously indicated, the silver content rarely exceeds 20 

wt. percent as determined by electron microprobe. Based on 

available data, no systematic trends are recognizable in the 

gold:silver ratio within or between ore ledges. 

Elevated C02 values in ore bodies can be directly 

attributed to introduced carbonate. The introduction of 

carbonate accompanied hydrothermal alteration and gold 

mineralization. 

Other trace metals found in ore-bearing ironformation that have not been detected in barren Homestake 

(AA/ICP methods) include tellurium and selenium in 

single-digit parts per million levels. A few platinum and 
(/) 

a: 

w 

1-

UJ 

~ 

~ 

u) 

20 

40 

ffi 60 

z 

~ 

~ 

:::c 

IIz 

UJ 

a: 

~ a.. 

<{ 80 Q 

z 

<{ 

:::c 

tl: 

w 

Q 

100 

120 

ROCK 

TYPE 

10 5 25 

po apy chi 

VOLUME PERCENT1 

1

Visually estimated 

'\.:.L• ••• • •• • •• ') r ........... , ............ , .. , ..... ~:.,· .. ....:·:·<· ........................................ ·:· ..................................................... -= ~~ .~~ t~ 

~ .. · .. ,....., 

::;::J Gold 

~ 

Xp 

EXPlANATION 

Biotite-sericite-carbonate· 

quartz phyllite of Poorman 

Formation 

Homestake Formation 

t::::==- Arsenic Siderite-rich iron-formation 

0 

I 

1 

I 

2 

T 

3 

I 

4 

I 

5 

Sulfide-rich iron-formation 

Sericite-quartz-biotite phyllite of 

Ellison Formation 

I 

6 

I 

7 

I 

8 

I 

9 

ARSENIC, IN PARTS PER MILLION X 103 

GOLD, IN GRAMS PER TON 

I 

10 

Figure }36. Distribution of gold, arsenic, total sulfide, and chlorite in drill hole 16802A, 21 Ledge, 7400 level. Pynhotite 

is present in Ellison and Poorman Formations in trace amounts (<1 volume percent); po, pyrrhotite; apy, arsenopyrite; chi, 

chlorite. 

Homestake Gold Mine, South Dakota J55 
palladtilm analyses have been performed, but the elements 

have not been detected. 

At the present time, no indicator element, element 

suite, or element ratio, with the possible exception of sulfur, 

shows any practical consistent or systematic trend that can 

be utilized to diagnostically discriminate gold concentrations. As with many gold deposits, the most effective 

geochemical indicator is gold itself. 

Retrogressive Hydrothermal Alteration 

Retrogressive hydrothermal alteration of Early 

Proterozoic rocks within the Homestake mine and Lead area 

took place in response to fluid movement through dilated 

segments of reactivated latest D1a and middle D1b shear 

zones synchronous with renewed thermal activity related to 

emplacement of early phases of the Crook Mountain 

Granite (this report; Bachman and others, 1990). Zones of 

high fluid transport and corresponding intense alteration 

were confined to dilatant parts of shear zones that opened 

preferentially within and adjacent to Homestake Formation 

and Yates unit of the Poorman Formation, owing to 

significant rheologic contrast with bounding phyllites. 

Alteration intensity was related to fluid-rock interaction, in 

tum dependent on fluid volume, transport, and access. In 

general, hydrothermal alteration effects are manifested 

principally by partial to complete mineral replacement that 

commonly entailed mass transfer. Specific alteration 

products are a function of bulk chemistry, pressuretemperature conditions, and fluid composition. Multiple 

fluid and alteration events are recognized and differentiated 

by spatial and temporal relations derived from detailed 

stratigraphic and structural analysis. The composite of the 

mineral alteration events can be related to prograde metamorphic processes, retrograde metamorphism, and a distinct 

retrogressive hydrothermal event that coincided with gold 

introduction. A detailed account of retrogressive hydrothermal alteration related to the latest fluid event (gold 

mineralization) is given in Campbell and others (1990). 

Retrograde metamorphic conditions appear to have 

been short-lived, and to have terminated in latest D1a. 

Retrogressive hydrothermal activity may have been initiated 

in early D1b, but it was most active during gold 

mineralization in middle D 1 b. Extensive fluid movements 

controlled by shearing, and by mineral alteration, were 

associated with both early and middle Dlb but were most 

intense during middle D 1 b. A composite mineral alteration 

history is shown in table J9. 

Chlorite Alteration 

Several generations of chlorite are found throughout 

the mine, the first two of which (Types I and II) were 

described previously in the section, "Prograde metamorphism." A third variety (Type III) is late and related 

J56 Geology and Resources of Gold in the United States 

largely to retrogressive hydrothermal alteration. Some of 

this third-generation chlorite is undoubtedly related to 

retrograde metamorphism but cannot be diagnostically 

distinguished at the present time. 

Type ill chlorite exists as matted aggregates with 

individual chlorite grains developed in random to nearly 

parallel orientation. Similar to Type I, Type ill chlorite has 

anomalously low birefringence. Where it shows nearly 

parallel orientation with respect to bedding-foliation, it is 

difficult to distinguish from Type I chlorite, or Type II 

chlorite related to retrograde metamorphism, particularly in 

barren iron-formation or chloritic Ellison and Poorman 

rocks. Type III is the distinctive chlorite found in all ore 

bodies and is associated throughout the mine with middle 

D 1 b shears containing anomalous gold. Its distribution is 

largely within narrow, planar shear zones. This chlorite 

variety locally is more abundant than Type I or Type II. 

Middle D 1 b shears containing Type III chlorite crosscut 

earlier metamorphic mineral assemblages and overprint 

previously deformed strata. Type III chlorite is believed to 

have been directly related to flow of gold-bearing fluids 

through the ductile-brittle shear system, and thus to Stage II 

quartz veins and to gold mineralization. 

Carbonate Alteration 

Carbonate alteration occurred throughout the Homestake mine area and was especially extensive within the 

iron-formation. Different stages or periods of carbonate 

alteration occurred, but their temporal and spatial relations 

are poorly understood. Carbonate alteration was 

complicated because ( 1) carbonate recrystallization varied 

during prograde metamorphism and associated devolatilization reactions, (2) carbonate alteration took place during the 

retrograde metamorphism, and (3) carbonate introduction 

occurred during a later hydrothermal event related to gold 

mineralization. Fortunately for distinguishing different 

phases, early metamorphic carbonate indigenous to the 

iron-formation and to most of the carbonate-bearing semipelitic rocks is easily characterized; it is intergrown with 

quartz to display a fine-grained heterogranular polygonal 

texture. Any recrystallization or carbonate introduction, at 

least in iron-formation, is largely manifested in the form of 

subhedral to euhedral carbonate porphyroblasts. Most 

carbonate in the semipelitic rocks and iron-formation was 

an early and possibly original component. An introduced 

portion of uncertain volume is marked by abundant coarse 

siderite, and to a lesser extent ankerite, in the ore bodies. An 

elevated carbonate content characterizes a majority of the 

rocks in the Lead window in the Black Hills. Carbonate 

abundance and distribution at Homestake are similar to 

those of the large carbonate haloes associated with the 

major Canadian lode gold deposits, such as in the Abitibi 

region of Quebec and Ontario (Colvine and others, 1988). 

Carbonate alteration associated with most ore bodies 

and subeconomic gold in the Homestake formed 
Table J9. Mineral alteration history related to significant deformation events, Homestake mine 

TIME 

\ 

u 

0 

N 

0 

a: 

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a.. 

~ c::: 

<( 

UJ 

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co 

c 

0 

·- +"' +"' c co Q) E > 

0 Q) 

...... 

Q) 

0 

0, 

c 

·.:::: 

co 

Q) 

..£: 

~ 

..c 

..... 

0 

~ 

'0 

:'Q 

:;?: 

0, 

c 

·.:::: 

co 

Q) 

..£: 

~ 

co 

..... 

0 

+"' 

en 

2 

co 

_J 

O'l 

c 

'0 

;g 

co 

..... 

0 

2 

co 

_J 

Thermal event 

Retrogressive 

hydrothermal 

alteration 

(::::1.72 Ga ?) 

Description of alteration type 

Replacement 

1. Extensive chlorite, siderite, sericite, and minor garnet replacement of 

wall rock in and adjacent to middle 01 b shears. 

2. Sulfidation- arsenopyrite, pyrrhotite, and gold replacement 

of wall rock in and adjacent to middle 01 b shears. 

Fluid generation related to emplacement of subjacent granitic pluton (?) 

Retrograde 

metamorphism(?) 

Prograde 

metamorphism 

Metasomatism 

1. Magnesium± iron and (or) potassium+ aluminum: biotite replacement 

of sericite in phyllites bounding sheared iron-formation; production 

of biotite in iron-formation. 

2. Carbonatization, hydration± potassium metasomatism: calcite and (or) 

dolomite and chlorite and (or) biotite alteration of sheared Yates unit. 

Remobilization 

Mobilization and concentration of graphite in structural traps. 

Fluid generation related to devolatilization during prograde metamorphism 

Prograde 

metamorphism 

(::::1.84 Ga) 

Metamorphism 

Formation of grunerite, garnet, and hornblende. 

siderite+quartZ+H:P=grunerite+C02 

siderite+q ua rtz+ch lo rite=g ru nerite+ga rnet +C02 

chlorite+muscovite+quartz=almandine+biotite+H20 (pelites)l Xp/Xe 

chlorite+biotite+quartz=almandine+biotite+H20 ~ 

actinolite+albite+ankerite=hornblende+andesine+C02+H20 Xpy 

coarse-grained, undeformed siderite (less commonly 

ankerite) porphyroblasts from 2 mm to 1 em in maximum 

dimension that are scattered, generally throughout ore 

bodies, especially in zones of middle Dlb shears. Coarse 

siderite is generally associated with abundant Type III 

chlorite within these shears. The presence of coarse siderite 

(and ankerite) with or without chlorite, however, does not 

necessarily indicate close proximity to ore bodies or subeconomic gold; it occurs also in barren iron-formation 

transected by middle D 1 b shears. 

Homestake Gold Mine, South Dakota J57 
Bleached Zones and Sericite Alteration 

Bleached zones at Homestake are sericite-rich zones 

that transect stratigraphic units, previously formed metamorphic mineral assemblages, and potassium-rich metasomatic haloes (for example, the biotite halo associated with 

iron-formation). These zones are most common (or more 

obvious) in graphite-bearing phyllites of the Poorman than 

in other formations. They generally exceed 5 m wide and 

extend more than 100 m along strike and dip. In most 

instances, contact relations with enclosing phyllite are 

transitional, generally within a width of 1 m. Relict host 

rock features such as faint banding with regular 

stratigraphic spacing are preserved in the bleached rock. 

Fabric in the rock, however, has been transposed by 

shearing into alignment with the associated shear plane, in 

places producing a phyllonite. Mineralogically these 

bleached zones consist dominantly of sericite, carbonate, 

quartz, and plagioclase with minor biotite. Mineralogy is 

related to the original bulk composition of the rock and to 

hydrothermal fluid composition. Chemical details are 

uncertain; however, initial data indicate that carbon, iron, 

and magnesium have been depleted, whereas C02 and 

possibly calcium have been introduced, at least in graphitebearing phyllites. Commonly, these zones are close to ore 

bodies or subeconomic gold-mineralized rock. Bleached 

zones locally contain quartz veins. Most of the quartz veins 

are interpreted as Stage il (see section, "Quartz veins") and 

in some areas are associated with arsenopyrite and pyrrhotite and low but anomalous gold values (<1.0 g Au/t). Some 

zones of bleaching contain abundant Type ill chlorite, are 

located below ore bodies, and have been interpreted by us to 

represent feeder conduits. The bleached zones signify paths 

of fluid flow along middle D 1 b shears and may represent 

small individual shears or splays related to or composing a 

larger shear or shear system generally developed near 

iron-formation. 

Distribution of Hydrothermal Alteration Effects 

Effects of hydrothermal alteration within ore ledges 

and around ore bodies throughout the mine have not been 

fully appreciated owing to (1) limited exposures across the 

ore system, and (2) lack of detailed mineralogic plan maps 

encompassing broad segments of the ore system. Quartz 

veins and associated haloes are local features and do not 

encase ore bodies (Noble, 1950; Wolfgram, 1977). More 

importantly, chlorite that occurs everywhere in and around 

ore ledges and ore bodies is too abundant and widespread to 

be a result of quartz-vein formation alone, but is probably 

related to a broader process that included formation of the 

quartz veins. Specifically, the fluid event that was 

responsible for Stage il quartz veins (see section, "Quartz 

veins") and associated gold mineralization is believed to 

have been responsible for the pervasive latest significant 

stage of chlorite (Type ill) alteration. 

J58 Geology and Resources of Gold in the United States 

Recently, we have begun to examine possible 

widespread hydrothermal alteration effects within and 

peripheral to ore bodies by preparing detailed mineralogic 

plan maps from mine workings and drill-hole data. We have 

completed maps of the 4100 and 5900 mine levels (not 

illustrated) showing ore-body positions and structural 

domains. Partial maps like those shown in figures J37 and 

J38 have been produced for key parts of selected ore ledges 

on other levels. 

Chlorite, Type ill specifically, is a common 

constituent in all ore bodies as recognized by Gustafson 

(1930, 1933), Noble (1950), Slaughter (1968), Chinn 

(1969), Bidgood (1977), and by the present workers. It is 

abundant (from 5 to >30 volume percent) in ore bodies and 

adjacent rocks throughout the ore-producing areas, and 

markedly decreases in abundance outside the ore 

environments where large concentrations are restricted to 

linear shear zones. The chlorite distribution in the mine is 

also confined to ductile-brittle shear zones (figs. J37, J38) 

and in large part represents a replacement of biotite, 

grunerite, and garnet. Restricted but mappable haloes 

encase ore bodies. Within and adjacent to shear zones, 

chlorite replacement affected most Early Proterozoic rock 

types in the mine. Based on recent diamond drilling, shear 

zones encompassing Main and 21 Ledges have been drilled 

in the Poorman Formation to several hundred meters below 

and along strike of the Homestake Formation. Abundant 

chlorite persists to that depth along with local, but 

uncommon, concentrations of pyrrhotite, arsenopyrite, and 

anomalous gold {as much as a few hundred parts per 

billion), indicating preferential shear development near the 

Homestake and leakage from nearby ore bodies. 

Distribution of biotite-, sericite-, and graphite-bearing 

units has been compared to the distribution of chlorite and 

the rock types constituting iron-formation, especially in ore 

zones. In general, biotite- and sericite-bearing units 

represent stratigraphic units and rock types both, chemically 

modified during prograde and retrograde metamorphism, 

and hydrothermal alteration. Intense chloritization (along 

with possible further redistribution of graphite) in bleached 

and graphite-rich phyllites was the result of intense hydrothermal alteration related to gold mineralization and 

controlled by middle D I b shears. Distribution of these rock 

(alteration) types is shown in figures J37 and J38 along with 

the distribution of middle D 1 b ductile-brittle shears. 

On the 2450 level of Main Ledge, grunerite 

dominates iron-formation outside the ore bodies. (Fig. J37 

shows distribution of grunerite, and fig. J31 shows location 

of ore bodies and named formations at this level.) Individual 

ore bodies are chlorite rich relative to adjacent ironformation. Chlorite-rich zones are evidence of intense fluid 

flow through grunerite-rich iron-formation along ductilebrittle shear zones, as they show systematic symmetry to the 

shear distribution. A small component of the chlorite is 

present as a result of replacement of biotite. A sericite-rich 
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phyllite occurs as small irregular patches stratigraphically 

below and above the ore bodies in both the Poorman and 

Ellison Formations. Lenticular bodies of graphitic phyllite 

(without pyrrhotite) crosscut bedding in the Ellison adjacent 

to ore bodies. These sericite- and graphite-rich units are. 

interpreted to represent remnants of hydrothermally altered 

rocks. In the transformation to chlorite-rich rocks, minor 

potassium may have been driven out of the Homestake and 

into adjacent formations (Wolfgram, 1977). 

Analogous data are presented in figure J38 for "D" 

limb of 9 Ledge on the 57 50 level. Ore bodies here are 

associated largely with grunerite-rich iron-formation and 

partly with chlorite-rich zones. (See fig. J32 for location of 

ore bodies.) Ore bodies are concentrated above graphitic 

phyllite in the Poorman that contains as much as 60 percent 

pyrrhotite (and consistently as much as 0.5 g Au/t). Parts of 

the Poorman and Ellison Formations contain zones rich in 

sericite. Several of these zones appear to crosscut quartzite 

units and are interpreted to represent hydrothermal 

alteration zones controlled by discordant, middle D 1 b 

shears. In both figures J37 and J38, a direct spatial and 

temporal relation is shown between mineral alteration 

patterns (distributions), ore bodies, and middle D 1 b shears, 

indicating strong structural control. Future research should 

improve the understanding of these relations on a minewide basis. 

Quartz Veins 

Quartz veins occur in all formations in the Homestake 

area, but they are most common in iron-formation of the 

Homestake, and less common in quartzite of the Ellison and 

phyllite of the Poorman or in Ellison semipelitic rocks. 

Quartz veins in amphibolite, specifically the Yates unit of 

the Poorman, are rare. In general, all quartz veins are milky 

white, discontinuous (particularly vertically), and generally 

unzoned. Quartz in veins is consistently coarse grained, 

crystalloblastic, and readily distinguishable from 

sedimentary chert, especially in the eastern ledges. Discordant and concordant quartz veins and irregular quartz masses 

(fig. J39) are found together commonly in all ore ledges and 

parts of all ore bodies (figs. J29, J30). Three distinct stages 

of Early Proterozoic quartz vein development can be 

described based on temporal relations, morphology, relation 

to bedding-foliation, mineralogy, and nature of selvage 

development. The three stages of quartz veins described 

here are listed in order of decreasing age (table J7). 

Stage I 

Stage I quartz veins are largely concordant to 

foliation and transposed bedding. They are folded and 

sheared locally throughout the mine, less than 1 m thick, 

and extend along strike for a few tens of meters. Stage I 

quartz veins are generally boudinaged, drag folded, and 

sheared in high-strain zones, creating irregularly shaped 

detached elongate pods of translucent grayish-white quartz 

exhibiting poorly developed selvages. The long axis of the 

vein pod is parallel to axial fabric produced in latest D 1 a. 

Associated pyrrhotite generally occurs as elliptical blebs 

localized near or on the margins of the veins. Alteration 

selvage mineralogy was largely dependent on host rock 

composition. For example, a vein transecting sericite schist 

may have a fine- to medium-grained sericite selvage. Where 

it traverses a pyrrhotite-bearing graphitic phyllite, the vein 

margin(s) may display a thin (<3 mm) mantle of pyrrhotite. 

In ore bodies, these early veins occur as detached boudins, 

isolated floating fold hinges, discontinuous lenses, and 

rounded fragments (figs. J29, J30). Stage I quartz veins may 

represent coarsened and (or) remobilized chert, but 

probably resulted from large-scale transport and deposition 

of silica during peak prograde metamorphism (table J7). We 

believe that they were produced synchronous with or just 

after peak prograde metamorphic conditions during latest 

D 1 a, when overall strain conditions were dominated by 

ductile shear deformation. 

Stage II 

Stage II quartz veins range from less than 1 em to 

more than several meters thick, and they extend along strike 

from a few meters to greater than 350 m. These veins are 

roughly tabular, are concordant to discordant, and contain 

internal streaks, pods, and aggregates of chlorite and (or) 

biotite, pyrrhotite, arsenopyrite, and commonly coarse 

visible gold. Asymmetric vein selvages comprising chlorite, 

biotite, and minor ankerite are commonly 1 m or more thick, 

and are developed best in Homestake Formation (fig. J29), 

but they also are present in Ellison and Poorman 

Formations. Random, isolated, coarse-grained aggregates of 

irregular pyrrhotite blebs (fig. J33A) or pyrrhotite layers 

(fig. J33B) are commonly found concentrated on one quartz 

vein margin, whereas coarse arsenopyrite porphyroblasts or 

aggregates (fig. J33C; with or without pyrrhotite) 

characterize the other margin (as .described by Wolfgram, 

1977). Ore-grade gold (not in sufficient volume to 

constitute ore) is generally present in both quartz-vein 

selvages (pyrrhotite and arsenopyrite), but it is most 

common adjacent to quartz veins near ore bodies. The 

highest individual gold grades in an ore body combined 

with the most abundantly observed coarse visible gold, as 

noted by Noble (1950), are associated with abundant 

arsenopyrite (figs. J33C, J34A, and J34B) in thick (several 

meters) selvages, developed adjacent to the widest Stage II 

quartz veins. The largest ore bodies with the highest gold 

grades, such as the Main Ledge centroid (figs. J31 and J39), 

contain numerous Stage II quartz veins with well-developed 

vein selvages. This is a common observation in all ore 

Homestake Gold Mine, South Dakota )61 
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EXPlANATION 

[ Xe I Ellison Formation 

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grunerite-rich 

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ledges throughout the mine. Stage IT quartz veins are rarely 

def~rmed, but P?St-ore shearing resulted in local drag 

foldmg and boudmage (fig. J29); the Stage II quartz veins 

consistently truncate Stage I quartz veins. 

Stage II quartz veins in many ore bodies also 

commonly developed into large irregular masses or 

"bl~wouts." Quartz vein "blowouts" generally crosscut 

earlier developed tabular veins and commonly contain 

angular. fragments or blocks of country rock. Arsenopyrite, 

pyrrhottte, and fine to coarse gold are associated and 

~ntergrown with chlorite locally along the margin of 

trregular quartz bodies (a small-scale example of quartz 

vein "blowouts" is illustrated in fig. J29). Tabular Stage II 

quartz veins and "blowouts" are of much larger volume than 

~tage .I qu.artz veins, and we believe (1) that they developed 

m eptsodtc pulses by injection into active shear zones 

during middle D1b, and (2) that they were locally deformed 

(fig. J29) during late D1b shearing (table J7). We also 

suggest that Stage ll quartz veins were introduced in the 

ductile to ductile-brittle transition and were directly related 

to gold mineralization. Irregular Stage IT quartz masses are 

interpreted as a product of extensional hydraulic fracturing 

in dilated parts of shear zones. 

Stage Ill 

Late barren quartz veins of Stage III truncate both 

Stage I and II quartz veins. They are markedly discordant 

and have poorly developed alteration selvages, if any; and 

they are undeformed. These veins average 5 em thick, may 

attain thicknesses as much as 1 m, and extend several tens 

of meters laterally. They are planar, transect foliation, and 

trun~ate ore bodies at large angles (>30° ) in throughgoing 

fashwn. They were emplaced in late D1b (table J7). Stage 

III quartz veins consist primarily of milky quartz and 

c~mmonly ~ontain ~oarse ankerite and pyrrhotite, along 

wtth subordmate albtte. Some veins locally contain narrow 

alteration selvages of biotite and (or) chlorite with sparse 

pyrrhotite, depending on host rock composition. Selvage 

development was dependent on proximity to extensive 

biotite and (or) chlorite alteration in or near ore bodies. 

Commonly, sparse pyrrhotite occurs as irregular blebs as 

much as several centimeters across scattered randomly 

throughout the veins. 

Stage ill quartz veins are more common in 

~reenschist-facies rocks than elsewhere. They are 

mterpreted to have been produced in late D 1 b following 

ore-stage mineralization. Open-space-filling textures 

characterize Stage ill veins, suggesting development in a 

more brittle tectonic regime. 

Interpretation 

. Metamorphic rocks in the Homestake mine span a 

wtde range of pressure-temperature conditions and are 

unevenly traversed by quartz veins. Quartz veins 

undoubtedly represent extensional fractures along which 

fluid flow and mineral deposition were focused. Fluids 

move through rock pervasively along micro-cracks, pores, 

grain boundaries, foliation, bedding, or widely spaced 

fractures. These potential conduits would be held open 

under metamorphic conditions if fluid pressure was greater 

than lithostatic pressure. This condition existed at times in 

or near the Homestake Formation as evidenced by the 

abundance of ductile and ductile-brittle shears and 

associated quartz veins. Devolatilization reactions in ironformation, along with pore fluid liberation, sustained high 

fluid flow and fluid pressure. Devolatilization also was 

responsible for generation of metamorphic mineral 

assemblages that represent specific pressure-temperaturecomposition conditions. Fluid flow was most likely 

channeled by deep-seated ductile shear zones, the conduits 

for the plumbing system, into the Homestake Formation, 

which served as a metamorphic aquifer during devolatilization. Stratigraphically and structurally controlled 

permeability vanatwns strongly influenced fluid 

channeling. Characteristics of pelitic to semipelitic rocks 

(carbonate and fine-grained white mica) that inhibited 

metamorphic reactivity also restricted fluid channeling. 

Carbonates were deformed plastically, reducing porosity. 

Micas served to seal these rocks and restrict fluid flow along 

foliation in the absence of shears. Pelitic to semi pelitic rocks 

characteristically surround the iron-formation and may have 

reduced fluid escape, resulting in increased fluid pressure 

that initiated fracturing and led to eventual quartz vein 

formation. If fluid pressure in the Homestake were 

increased to the point of shear failure (ductile mechanics) or 

extensional hydraulic fracturing (ductile-brittle transition), 

permeability would have increased fluid flow into shears or 

fractures. Temperature and pressure gradients would have 

subsequently led to quartz vein development. Deposition of 

quartz veins and ore components depended on continued 

fluid movement and channeling into shear zones or fractures 

where the fluids reacted with favorable host rocks. 

The pressure gradients and chemical instabilities 

between fluids in the veins and fluids and bulk chemistry of 

the wallrock controlled vein selvage development and 

formation of mineral alteration assemblages adjacent to the 

veins. Stage I quartz veins and their associated fractures 

formed during prograde metamorphism and probably had 

lower fluid pressure than corresponding pore fluid pressure 

of the country rock. If so, the pressure would have driven 

fluid from wallrock into the fracture system, preventing 

significant wallrock alteration adjacent to Stage I quartz 

veins. 

Conversely, to explain locally extensive chloritic 

alteration around Stage II and certain Stage ill veins, fluid 

pressure in the veins would have been higher than pore fluid 

pressure in the wallrock. Such a condition may have 

obtained during retrogressive hydrothermal activity 

(cooling) in which younger veins were formed. The fluid 

Homestake Gold Mine, South Dakota J63 
pressure in host rocks may have been reduced below that 

prevailing at hydrostatic conditions as a result of absorption 

of pore water during retrograde metamorphic reactions. 

Fluid pressure in quartz veins would have been higher than 

lithostatic pressure, driving chemically active fluid into vein 

walls to induce chemical reactions· along vein margins. 

These conclusions are taken from Homestake internal 

reports; the mechanisms of vein selvage development are 

analogous to those discussed by Yardley (1986) for the 

Connemara Schists of Ireland. 

DISCUSSION 

The Homestake gold deposit, the largest deposit of its 

type known, is believed to have formed as the result of four 

principal factors: (1) favorable stratigraphy, (2) 

development of a progressive tectonic system with dilational shears, (3) an intensive thermal and fluid system, and 

( 4) a gold source. In recent years progress has been made in 

understanding how stratigraphic, tectonic, and thermal processes interacted in time and space to produce the deposit. 

Our understanding of the source for gold is speculative at 

best and outside the scope of this study. The actual 

gold-mineralizing event, the focus of much of our interest, 

is only the end result of a complex sequence of prior events. 

Understanding these events and related processes is the key 

to definition of ore controls. 

The importance of original stratigraphy as an ore 

control relates to the primary rheologic characteristics of 

individual rock units. Deposition of the voluminous Yates 

unit tholeiite (> 1.97 Ga) is viewed as most significant 

because during later tectonism the competent Yates unit 

acted as a regional "hard knot," buffering strain and 

controlling folding and shearing in the bounding schist and 

phyllite. During the early stages of major "plate-scale" 

transcurrent movement (early Dla) the originally flat lying 

strata were differentially compressed about the Yates unit 

mass as the strata were transposed to nearly vertical. 

Deformational strain was selectively partitioned into 

narrower zones of ductile shearing along the margins of the 

elongated Yates unit mass. The Homestake Formation, 

relatively competent although at district ·scale volumetrically insignificant, was folded and sheared producing a 

near-perfect structural host. The spatially continuous nature 

of the iron-formation in the Homestake and increased 

permeability produced by shearing provided an efficient 

fluid path along the plunging iron-formation and at the 

steeply dipping interface between the metatholeiite and 

schist. 

Contemporaneous with the early transcurrent 

shearing was the development of a regional "hot spot" 

northeast of and below the current mine. This "hot spot" is 

believed to have been the focus of thermal activity in the 

district and also of subsequent fluid generation. Through 

progressive ductile shearing (latest D la), preexisting 

J64 Geology and Resources of Gold in the United States 

foliation and bedding were transposed into parallelism with 

the developing shear zones, producing increased 

permeability. These permeable shear zones became fluid 

release conduits for devolatilization products that buffered 

preexisting metamorphic assemblages. Within the mine 

area, ductile shear zones controlled fluid movement, thereby 

controlling the distribution of thermal energy and metamorphic processes. This series of changes effectively 

produced a structurally controlled isothermal zone surface, 

the upper greenschist-lower amphibolite facies transition 

zone. In this context, we characterize regional prograde and 

retrograde metamorphism as dynamothermal, developing 

synchronously with and controlled by early ductile 

deformation (late and latest Dla) at 1.84 Ga. 

Fluids generated during prograde and retrograde 

metamorphism, although thought to contain no precious 

metals, produced significant district-scale metasomatic 

alteration and local graphite remobilization. This alteration 

and remobilization defined zones of high fluid transport that 

correlate to dilated segments of individual ductile shears. 

The general form and orientation of iron-formation in the 

Homestake after D1a deformation and regional metamorphism appear to have strongly resembled those 

currently observed, but without the gold ore bodies and 

related retrogressive hydrothermal alteration effects. The 

folded and sheared iron-formation map pattern and fold 

plunge were apparently established during Dla, and not 

substantially modified by later events. 

The start of deformation Dlb reflected a significant 

change in the regional stress field that was characterized by 

early, relatively simple northeast-southwest compression 

and which produced upright folds and later semiductile 

high-angle reverse shears. Deformation during D 1 b 

indicates mechanics at or near the ductile-brittle transition, 

semibrittle deformation dominating the later stages of Dlb. 

High fluid pressures, possibly exceeding lithostatic 

pressures, are indicated and may have been important to 

Dlb shear development. Semibrittle deformation mechanics 

suggest lower relative rock temperatures as compared to 

peak metamorphism (540 °C). Synchronous with 

development of D 1 b structures was a diapiric emplacement 

of the Crook Mountain Granite. The granite formed as an 

anatectic response to a "hot spot" which developed 

northeast of the mine; the hot spot coincided· spatially with 

earlier thermal activity that occurred during regional metamorphism. The granite is considered coeval with the 1.72 

Ga Harney Peak Granite in the southern Black Hills. Fluid 

generation associated with "hot-spot" act1v1ty and 

development of the Crook Mountain Granite is considered 

by us to have been substantial. Redden and others (1990) 

suggested significant fluid generation associated with Harney Peak-type granites in general. We postulate that during 

the early stages of "hot-spot" activity, gold-bearing fluids 

(derived deep from within the tectono-stratigraphic section) 

were mobilized and entered a reactivated shear system; gold 
deposition occurred in the Homestake Formation. Gold 

mineralization in the Homestake took place in late-stage 

ductile-brittle shears (middle D 1 b) that appear to have 

dilated largely during reverse movement (northeast sides 

up). These late shears and associated mineralization effects 

crosscut earlier deformational fabrics and metamorphic 

mineral assemblages. The late D 1 b kink folds formed as a 

consequence of the final stages of granite emplacement and 

are postore. These facts are perhaps the most significant and 

compelling line of evidence that may date gold 

mineralization substantially later than peak prograde metamorphism estimated at 1.84 Ga (Zartman and Stem, 1967); 

this line of evidence provides a possible genetic link 

between mineralization and the early development of 

granite before 1.72 Ga. 

S.F. Emmons (in Irving and others, 1904) was the 

first to suggest a Precambrian genesis for the Homestake 

gold deposit. Precambrian hydrothermal or syngenetic 

variations on genesis were shared by Paige (1924), Con~ 

nolly (1927), Gustafson (1930, 1933), McLaughlin (1933), 

Slaughter (1968), Chinn (1969), Rye (1972), Rye and Rye 

(1974), and Rye and others (1974). The latter three 

publications suggested on the basis of initial isotopic studies 

that early gold mineralization was related to a paleo-hot 

spring or sedimentary exhalative origin. Bidgood (1977), on 

the basis of petrographic and trace element studies, 

suggested that introduction of gold and other constituents 

into the Homestake Formation by hot spring activity was 

unlikely. However, he did interpret disseminated gold as 

possibly concentrated by sedimentary or biological processes. Wolfgram (1977) suggested that ore components at 

Homestake were derived from weathering of volcanic rocks 

and became incorporated 'in the iron-formation by biological 

processes during sedimentation. Hosted and Wright (1923), 

and Noble (1950) were the two major proponents of a 

Tertiary age for the Homestake gold deposit, related to 

Tertiary igneous and hydrothermal activity. 

We favor a postpeak metamorphic age (<1.84 Ga) 

and an epigenetic origin for gold-ore mineralization at 

Homestake, an origin which best fits the geologic form of 

the deposit as presently observed. We speculate that sulfidation that accompanied the introduction of sulfur, silica, 

arsenic, gold, and other elements into a structurally prepared 

and chemically reactive, iron-rich, siliceous, carbonatefacies host rock during the late-stage middle D 1 b ductilebrittle shear event was the most likely process to account for 

deposit genesis. Tabular to pipelike ore bodies were 

preferentially localized within dilation zones along a 

network of reactivated ductile-brittle shears. The geologic 

environment of ore deposition presented herein is similar to 

ore environments in Precambrian terrains reported 

elsewhere by Groves and others (1987), Cameron (1989), 

and DeRoo (1989). 

Manuscript received by scientific editors December 1988 

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Homestake Gold Mine, South Dakota J67 
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