A Thesis

Presented to

The Graduate Faculty of The University of Akron

In Partial Fulfillment

of the Requirements for the Degree

Master of Science

DEVELOPING A USEFUL SET OF PROXY ELEMENTS FOR THE TARGETING

AND EXPLORATION OF GOLD DEPOSITS, BLACK HILLS, SOUTH

DAKOTA

Michael T. Harp

December, 2010

Approved:

______________________________ Advisor

______________________________Faculty Reader

______________________________Faculty Reader

______________________________Department Chair

Accepted:

______________________________Dean of the College

______________________________ Dean of the Graduate School

______________________________Date

Thesis

ii

Dr. LaVerne M. Friberg

______________________________

Dr. John A. Peck

______________________________

Dr. John P. Szabo

______________________________

Dr. John P. Szabo

Dr. Chand K. Midha

Dr. George R. Newkome

DEVELOPING A USEFUL SET OF PROXY ELEMENTS FOR THE TARGETING

AND EXPLORATION OF GOLD DEPOSITS, BLACK HILLS, SOUTH

DAKOTA

Michael T. Harp

ABSTRACT

iii

X-Ray Fluorescence (XRF), petrography, and Energy-Dispersive X-ray Spectrometry

(EDAX) have been used to determine the element concentration in samples and their

distribution within minerals for 222 metamorphic rock samples from the Black Hills, S.D.

Element concentrations in these samples are compared to sample location and known

gold deposits in the Black Hills.

XRF data of rock chips from whole rock samples were collected using a portable

XRF unit to determine major, minor and trace element abundances. Statistical analyses

of the XRF data indicates a moderate to strong correlation between gold and the elements

Mn (19 to 16,116 ppm), S (1,283 to 79,452 ppm), As (n.d. to 132 ppm), Pb (n.d. to 318

ppm), Cl (625 to 31,277 ppm), Ba (n.d. to 1,101 ppm), and Zn (n.d. to 266 ppm) thus

indicating these elements may serve as proxy indicators of gold. The integration of

elemental data with ArcGIS was used to test the spatial relationship of proxy elements to

known gold deposits in the Precambrian core of the Black Hills.

Sixteen samples having gold concentration greater than 18 ppm were chosen for

more detailed analyses. EDAX raster scans of these samples determined proxy element

variations within individual mineral grains. Petrographic analyses were done to identify

minerals and their textural relationships.

Sample proximity to known gold deposits in the Black Hills can be correlated with

increases in minor and trace proxy element concentrations.

iv

ACKNOWLEDGEMENTS

First and foremost I would like to thank my advisor Dr. LaVerne Friberg. His

guidance and knowledge gave this project life as well as my interest in this field.

Because of him and this research I have discovered my passion in the vast field of

geology. I would like to thank the Department of Geology and Environmental Science

at the University of Akron for the use of the departmental equipment and facilities that

aided in this research, but most especially for allowing me to become part of the graduate

program that allowed me to get this far. I would like to thank Dr. John Szabo and Dr.

John Peck for their willingness to be part of my thesis committee and their guidance

throughout my Masters program. XRF analysis was conducted through an Academic

and Research Relations Grant to The Department of Geology and Environmental Science

provided by Innov-X Systems.

I would like to extend special thanks to Mr. Tom Quick for his expert knowledge,

willingness to help me succeed, his ability to fix all that goes wrong, and for being ever

present throughout my research. To Dr. Kevin Butler for his expert advice and assistance

with all things related to ArcGIS. To Ms. Elaine Butcher for her guidance through the

processes and procedures that come with the Masters program, for helping me to learn

the formatting and the finishing of my thesis, but most importantly for her friendship

and for being there when things were at their best and their worst. Finally I would like

to thank my fellow graduate and undergraduate students, who provided me with their

support, friendship, and their help in keeping my eyes on the horizon when my family

couldnt be there.

vI would like to thank my parents Steve and Cheryl, for giving me a solid base, a good

work ethic, their interest in my research, and for believing in me all the way. I would

also like to thank my grandmother, Dorothy Hoffman for being a mentor and an ever

present figure in my life. I would like to thank my brother Brian and my sister Amanda

for keeping my head up and their support through this project. My son Ayden and my

daughter Delaney for giving me a reason to keep going and to make myself better in

every way. Finally I would like to thank my wife Taryn. She is my biggest fan, my

strongest supporter, and my best friend. Her interest in my life kept the fire burning and

without her none of this could have been possible.

vi

Page

TABLE OF CONTENTS

LIST OF FIGURES viii

LIST OF TABLES x

CHAPTER

I. INTRODUCTION 1

Overview 1

Geologic Setting 2

Previous Works 10

II. METHODS 13

Sample Locations 13

Laboratory Methods 13

X-ray Fluorescence 13

Energy Dispersive X-ray Analysis 17

ArcGIS Analysis 19

Statistical Analysis 20

Petrographic Analysis 20

III. RESULTS 22

X-ray Fluorescence and ArcGIS 22

Statistical Analysis 33

Petrographic Analysis 37

Energy Dispersive X-ray Analysis 47

IV. DISCUSSION 49

vii

Chlorine as a Predictor for Gold 49

Barium as a Predictor of Gold 51

Arsenic as a Predictor of Gold 51

Manganese as a Predictor of Gold 54

Sulfur as a Predictor of Gold 54

Zinc as a Predictor of Gold 57

Lead as a Predictor of Gold 57

Spatial Analysis of the Proxy Elements 57

Petrographic and EDAX analysis 63

Manganese 64

Barium 69

Sulfur72

V. CONCLUSIONS79

REFERENCES 81

APPENDICES 84

APPENDIX A. LATITUDE AND LONGITUDE FOR SAMPLE LOCATIONS 85

APPENDIX B. STATISTICAL DATA FOR ROCK CHIP ORIENTATION 91

APPENDIX C. XRF DATA 95

APPENDIX D. DATA FOR EDAX AND MICROPROBE COMPARISON 136

APPENDIX E. STATISTICAL DATA BASED ON XRF BULK ELEMENT ANALYSIS 144

APPENDIX F. MINERAL ASSEMBLAGE 156

viii

FigurePage

LIST OF FIGURES

1 Generalized diagram showing the geology and geomorphology of the Black Hills, SD 3

2 Geologic cross section of the Black Hills after the Laramide Orogeny, SD (Carter, et al., 2003) 4

3 Geologic map of the Black Hills, SD (Modified after Dahl et al., 2005a) 6

4 Precambrian area and Metamorphic Isograds of the Black Hills, South Dakota 9

5 Precambrian area and Metamorphic Isograds of the Black Hills, South Dakota 14

6 Elemental concentration variance based on sample orientation 16

7 EDAX dot map scan of sample BHMA-27a 18

8 Gold concentration (ppm) contour map of the Precambrian area of the Black Hills, South Dakota using the kriging method 23

9 Arsenic concentration (ppm) contour map of the Precambrian area of the Black Hills, South Dakota, using the kriging method 25

10 Barium concentration (ppm) contour map of the Precambrian area of the Black Hills, South Dakota, using the kriging method 26

11 Chlorine concentration (ppm) contour map of the Precambrian area of the Black Hills, South Dakota, using the kriging method 28

12 Manganese concentration (ppm) contour map of the Precambrian area of the Black Hills, South Dakota, using the kriging method 29

13 Lead concentration (ppm) contour map of the Precambrian area of the Black Hills, South Dakota, using the kriging method 31

14 Sulfur concentration (ppm) contour map of the Precambrian area of the Black Hills, South Dakota, using the kriging method 32

15 Zinc concentration (ppm) contour map of the Precambrian area of the Black Hills, South Dakota, using the kriging method 34

ix

16 Precambrian area and Metamorphic Isograds of the Black Hills, South Dakota. 38

17 Cross plot graph of gold versus chlorine concentration 50

18 Cross plot graph of gold versus barium concentration 52

19 Cross plot graph of gold versus arsenic concentration 53

20 Cross plot graph of gold versus manganese concentration 55

21 Cross plot graph of gold versus sulfur concentration 56

22 Cross plot graph of gold versus zinc concentration 58

23 Cross plot graph of gold versus lead concentration 59

24 Dot map indicating manganese concentration in garnet 65

25 Photomicrograph of sample BHMA-57b 66

26 Dot map indicating iron concentration in garnet 67

27 Dot map indicating tellurium concentration in garnet 68

28 Peak data indicating occurrence and intensity of elements within sample BH-19a 70

29 Photomicrograph of sample BH-19a 71

30 Dot map indicating barium concentration in titanite 73

31 Dot map indicating tellurium concentration in titanite 74

32 Peak data indicating occurrence and intensitiy of elements within sample BH-4 75

33 Dot maps indicating sulfur and iron concentrations in pyrite 76

34 Photomicrograph of sample BH-4 78

xTable Page

LIST OF TABLES

1 Correlation coefficients between paired elements: r is significant when P 0.05 36

1CHAPTER I

INTRODUCTION

Overview

According to Rambeloson (1999), Gold occurs in four main kinds of deposits: 1)

as a diffuse component of crystalline basement rocks, 2) in concordant quartz veins

within the metamorphic rocks of the Precambrian basement, 3) in recent discordant

veins, and 4) in recent and ancient alluvial deposits. Gold in the Black Hills occurs as

four types of deposits: 1) within Precambrian quartz veins that have been injected into

the metamorphic basement rock, 2) in ancient placer deposits within the Deadwood

Formation, 3) as hydrothermal deposits associated with Tertiary igneous activity, and 4)

as modern placer deposits. This study will focus on gold deposited along quartz veins and

as hydrothermal deposits in the Precambrian metamorphic rocks.

In the Black Hills there are many locations where gold mining occurred in the

past. The historic sites of mining are the Lead-Deadwood District, Rochford-Hill City

District, and The Keystone District. The Lead-Deadwood District is encompasses the

town of Deadwood in eastern Lawrence County and the city of Lead in central Lawrence

County, which is also the central area of the mineralized zone. This area also contains

the Homestake Mine. The Rochford-Hill City District is located in the western portion of

Pennington County in the vicinity of Hill City, near the headwaters of Spring Creek and

extending into the city of Rochford to the northwest. The Keystone District is located in

2western Pennington County on the northeastern side of Harney Peak near (Koschmann et

al., 1968).

Using a collection of 222 samples taken from the Black Hills, the purpose of this

study is to assess if minor elements occur within silicate minerals, and if they can be used

as proxies for the presence of gold. Due to the conditions at which gold and these proxy

elements are mobilized and then deposited by metamorphic fluids, it can be hypothesized

that the proxy elements will substitute into the crystal lattices of metamorphic

silicate, oxide, and sulfide minerals in occurrence with gold. Because of the similar

electro-chemical characteristics of the proxy elements and gold, I will show that gold

concentrations are in areas of the Black Hills where Mn, S, As, Ba, Pb, Zn, and Cl are in

higher concentration.

Geologic Setting

The Black Hills is an elliptically domed region in the southwestern portion of South

Dakota that extends into the northeastern portion of Wyoming (Figure 1). The area is 200

km long and about 105 km wide, with its highest point being Harney Peak at an elevation

of 2207 m. The Black Hills is an area that has been subjected to multiple geologic events,

including mountain-building episodes, igneous intrusions, and polymetamorphism related

to the tectonic episodes of the area.

As the Black Hills were uplifted by the Laramide Orogeny and eroded throughout the

late Precambrian and into the Paleozoic, the last rocks to be deposited were sedimentary

and dip away from the granitic core. The Homestake mine, located in the northern part

of the Black Hills, is the location of the first discovery of gold in the Black Hills. The

area is composed of Precambrian schists that are surrounded by steeply outward-dipping

Paleozoic and Mesozoic rocks (Noble, 1950) (Figure 2).

3Figure 1. Generalized diagram showing the geology and geomorphology of the Black Hills, SD. (Modified from Strahler and Strahler, 1987).

4Fig

ure

2. G

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5Exposed Precambrian rock is believed to be the source of paleo-placer deposits that

are present in the Deadwood Formation (Noble, 1950). The Deadwood Formation, which

is mostly sandstone, was another source of gold discovered in the Black Hills as placer

deposits in creeks where the gold was derived from Precambrian rock (Rahn et al., 1996).

As erosion occurred, gold was transported by the river systems and deposited in paleo-

channels within the Cambrian-aged Deadwood Formation.

During the Laramide Orogeny, which occurred in late Cretaceous into the early

Cenozoic, uplift intensified as deformation of the rock continued and another episode of

hydrothermal alteration associated with Tertiary igneous activities occurred, depositing

many economic minerals, including gold (Figure 3). Tertiary intrusive dikes have also

been known to carry high concentrations of Zn and Ba in the northeastern portion of the

Black Hills where remobilization of gold is believed to have occurred (Uzinlar, 2010).

According to Dewitt et al. (1996), XRF analysis indicated that barium occurred in high

abundance within a range of 580-1,700 ppm to the southeast of Deadwood and lower

concentrations centered on the Whitewood Peak pluton to the northeast of Deadwood.

High abundance of barium can be correlated to intrusion of Tertiary dikes that are

prevalent in the northern Black Hills.

Due to igneous, metamorphic and sedimentary processes that have acted on the

Black Hills throughout time, the area has become a location known for its rare minerals

and shows evidence of geological processes that acted upon these rocks. Multiple

episodes of deformation, uplift, and hydrothermal fluid activities have been preserved

in the rocks in the Black Hills area that spans geologic time from the Precambrian to the

present.

The Proterozoic thermotectonic and magmatic history of the Black Hills crystalline

core is associated with arc accretion and continental collision (Redden et al. 1990; Dahl

et al., 2005a, b, 2006; Nabelek et al., 2006). In a study done by Frei et al., (2009), The

6Figure 3. Geologic map of the Black Hills, SD (Modified after Dahl et al., 2005a).

7mode of occurrence of gold at Rochford is strikingly similar to that in the Homestake

Iron Formation of the Lead District (Slaughter, 1968; Bayley, 1972). However the two

intracratonic basins developed independently from each other, in space and time, i.e, ~25

km and ~80-130 Myr apart. Results obtained from this study will identify similarities in

petrogenic origins of the Rochford Iron Formation and deposition of the Homestake Iron

Formation which is constrained within a 2012-1974 Ma time frame (Frei, et al., 2009).

The structural history of this area includes major tectonic rifting and convergence

during the Proterozoic that caused multiple metamorphic episodes in preexisting

basement rock. As the area began to rift, a period of rapid erosion of Archean basement

rock to the west from the Wyoming Craton resulted in deposition of over 3000 m of

sedimentary rock. As rifting stopped, plates were forced back together causing structural

deformation and metamorphism of sedimentary rock (Dahl & Frei, 1998).

In the Precambrian Era, the granitic core of the Harney Peak area of the Black Hills

was formed when magma forced its way into existing rocks during the Trans-Hudsonian

Orogeny (Dahl & Frei, 1998). The Trans-Hudsonian Orogeny was the collision of the

Wyoming and Superior Cratons and accretion of arcs along the southeastern margin of

the Wyoming Craton (Van Boening & Nabelek, 2008).

Intrusion of the Crook Mountain and Harney Peak magmatic bodies are associated

with the Black Hills dynamothermal metamorphic event as well as localized contact

metamorphism. Following closely to emplacement of this large buried pluton in the

northeast, there was continuation of the magmatic event in which large pegmatite bodies

were emplaced around the Harney Peak granite core of the Black Hills. The Harney

Peak granitic core has been dated to 1.75 Ga years with surrounding sedimentary rocks

being dated from 1.8 to 1.9 Ga years. The maximum age for mineralization is 1,746

+/- 10 Ma as indicated from step-leach Pb-Pb dating of garnet from mineralized samples

in the Homestake Mine (Terry et al., 1998). This tectonic episode uplifted, eroded, and

8tilted rocks in the area as the diapir of magma rose through rock layers causing contact

metamorphism with metasedimentary layers giving the area a bulls eye appearance

with all surrounding rock dipping away from the center (Dahl & Frei, 1998).

This area can be broken into areas of equal grade of metamorphism or metamorphic

zones (Figure 4). The highest grade metamorphic rocks are in the sillimanite zone which

extends west of the Harney Peak Granite. Temperature dropped to the north where a

staurolite zone occupies an area a few kilometers wide. Farther north is a broad area

belonging to the garnet zone. Northeast of the garnet isograd, the biotite zone extends

from west of Lead to the southeast (Redden et al., 1975).

Heat and pressure applied to the rocks decreased with distance from the Harney Peak

Granitic intrusion. The area that is preferential to deposition of epigenetic gold is within

the biotite zone of metamorphism. Temperatures in the biotite zone were less than 350

degrees Celsius. Gold deposition is only found within the biotite zone, and as the garnet

isograd is crossed, deposition of gold ceases. The Homestake and Rochford mining

districts, where gold has been actively mined, lie within the biotite zone of the northern

Black Hills.

As hydrothermal fluids move through rock and interact with grains of biotite,

chlorite, and garnet, proxy-element exchange occurs between the fluid and the minerals.

In the case of biotite and chlorite structures, most of the proxy elements in hydrothermal

solution substitute into octahedral sites, whereas in the garnets they will substitute into

cubic or octahedral sites (Klein, 2002). This exchange should occur at the rims in the

highly refractory garnet and penetrate into the interior of the less refractory micas and

opaques along grain boundaries and along fractures and cleavage planes.

9Figure 4. Precambrian area and Metamorphic Isograds of the Black Hills, South Dakota. Black line indicates the Precambrian boundary. Red lines indicate metamorphic isograds boundaries showing level of metamorphism of the area. Brown Area indicates the Harney Peak Granite (HPG).

10

Previous Works

High manganese concentrations have been discovered within black smokers, or

hydrothermal vents that form at spreading centers due to hydrothermal circulation

(Zierenberg et al., 1993). Anomalous concentrations of manganese commonly occur in

or near sulfide ore environments. These anomalies occur as manganiferous limestone

horizons (Russell, 1974; Gwosdz and Krebs, 1977), as manganiferous garnet lithologies

within, above, or beneath metamorphosed massive sulfide deposits (Spry, 1978; Stumpfi,

1979), and as ferro-manganiferous sediments associated with ancient and active mid-

ocean spreading centers (Robertson and Hudson, 1973; Alt et al., 1987). Seafloor

manganiferous sediments can arise from a number of processes, some of which are

not related to sulfide mineralization. These processes include halmyrolysis and occurs

between basalt and sediment, low-temperature precipitation as nodules and crusts, and

diagenetic enrichment in the sediment column. The anomalous manganese and sulfide

occurrence of the Black Hills, as well as gold, may have been deposited at an ancient

spreading center during rifting associated with the Trans-Hudsonian Orogeny (Dahl &

Frei, 1998).

In a study done by Redden (1990), an imprecise upper-intercept 207Pb/206Pb age of

1,884 29 Ma was obtained for bulk zircons in felsic tuff interlayered with the Montana

Mine Formation that underlies the Rochford Formation. Dahl et al., (2008) improved

this age constraint to 1,887 7 Ma U-Pb SIMS age from the same felsic tuff. This age

constrained a maximum depositional age of ~ 1.887 Ga for the Rochford Iron Formation.

A tuffaceous layer within the Ellison Formation, which overlies the Homestake

Formation, was dated at 1,974 8 Ma (Redden et al., 1990) and constrains a minimum

deposition age of ~1.974 Ga for the Homestake Iron Formation. Therefore, gold

11

deposition of this area is constrained by the ages at which the formations were deposited

and gold mobilization began.

A possible mode of gold deposition favors an epigenetic origin for the Homestake

gold deposit, while also inferring a strong genetic association of the gold event with the

late stages of nearby granite magmatism (1.75 Ga) (Caddey et al., 1991). According

to Frei et al. (2009), the timing of Homestake gold mineralization has been estimated

at ~1,730 Ma from Re-Os dating of arsenopyrite (Morelli et al., 2005), which falls

within the known ~1,780-1,715 Ma interval of regional metamorphism and igneous

emplacements.

In a study done by Caddey et al. (1991), gold-sulfide mineralization in the Homestake

Iron Formation and in the Rochford district (Bayley, 1972) was found to be hosted by

quartz veins that were formed during retrograde shearing. Three sequential stages of

quartz veins (stage I, II, and III), associated with ductile, ductile-brittle, and semi-brittle

shear zones, respectively, have been recognized and described in the Homestake Mine

area

In a study by Paige (1924), the timing of sulfide mineralization and gold deposition

has been delineated. All ores of the Homestake lode carry sulfides; and generally,

where sulfides are abundant, the best ore is found. Sulfides that occur are arsenopyrite,

pyrrhotite, and pyrite. Gold is associated with each of these minerals either as inclusions

within them or in gangue minerals that are close by. These sulfides replaced portions

of the carbonate schist and conform to the schistose structure of the rock. Based on

evidence in this study, sulfides were introduced before final stages of metamorphism.

Arsenopyrite was introduced at late stage metamorphism of schists and was partly

deformed; and then shortly after pyrrhotite and pyrite were introduced as gold

mineralization occurred.

12

Previous studies at the Homestake underground mine in the northern Black Hills

show that the manganese content of chlorite increases with proximity to gold-bearing

quartz veins (Armstrong and Friberg, 1998). This study showed that manganese variation

in chlorite was not directly correlated to metamorphic grade or rock type. Manganese

content within the chlorite weakly to moderately correlates with manganese content in

biotite and garnet within a sample, but more strongly correlates with high concentrations

in close proximity to the gold mining districts.

In a similar study by Friberg et al. (1997), chlorite occurs in the greenschist facies

(biotite grade) through the lower amphibolites facies (staurolite grade) rocks having

a wide range of composition. This study showed that chlorite formed during the

dynamothermal event associated with the emplacement of the Harney Peak Granite,

pegmatites, and the late quartz veining and was re-equilibrated with the associated

mineralizing fluids which introduced higher concentrations of manganese along the rims

and cleavage planes in the chlorite.

The chemistry of chlorite appears to be controlled by bulk composition of the host

rock, metamorphic intensity-related chemical exchanges within the coexisting minerals,

as well as mineralizing fluids that are associated with quartz veins (Friberg et al., 1997).

This study also showed that manganese contents of chlorite generally increase with

metamorphic grade. As zones of higher grade metamorphism are crossed, manganese

within chlorite increases. In addition, manganese content in chlorite also increases

toward known gold deposits in the chlorite-biotite grade rock.

13

CHAPTER II

METHODS

Sample Locations

Samples used in this research were collected for previous research across the

Precambrian core of the Black Hills in 1977 by Dr. L.M Friberg and 1998 by M.

Armstong. The 222 samples analyzed in this study are samples collected from all grades

of metamorphism within the Precambrian core of the Black Hills. Samples were taken

and locations of collection marked (Figure 5, Appendix A).

Laboratory Methods

Samples used in this research were both cut rock chips and polished thin sections.

Samples were analyzed using X-ray fluorescence (XRF), energy dispersive X-ray

analysis (EDAX) attached to an environmental scanning electron microscope (ESEM),

ArcGIS; and full petrographic analysis was conducted on 85 samples.

X-ray Fluorescence

Bulk elemental composition was determined using a handheld Innov-X XRF analyzer

(Model Alpha). Standard soil mode was used to obtain elemental compositions present

samples. A standard was inserted in order to calibrate the analyzer at the beginning of

14

Figure 5. Precambrian area and Metamorphic Isograds of the Black Hills, South Dakota. Red dots indicate sample locations collected by L.M. Friberg (1977) and M. Armstrong(1998). Black line indicates the Precambrian boundary. Dashed lines indicate metamorphic isograds showing level of metamorphism of the area. Blue line indicatesouter boundary of Harney Peak Granite. Green lines indicate Rochford and Homestake Mining Districts.

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Figure x. Precambrian area and metamorphic isograds of the Black Hills, South Dakota. Red dots indicate sample locations collected by L.M. Friberg (1977) and M. Armstrong(1998). Black line indicates the Precambrian boundary. Dashed lines indicate metamorphic isograds showing level of metamorphism of the area. Blue line indicatesouter boundary of Harney Peak Granite.Green Lines indicate Rochford and Homestake Mining Districts

15

analysis and was reinserted after 20 consecutive samples had been run to insure that the

analyzer was still operating properly and the system remained clean of foreign particles.

Samples used for the XRF analysis were unpolished rock chips that are were cut into

1x3x1-inch sized chips. Samples were wrapped in plastic wrap in order to keep particles

off the analyzer and the rock chips were placed in the apparatus for 2-minute intervals.

For the first 23 samples, different orientations were used to ascertain whether

placement of the sample into the apparatus affected the elemental concentration

measurement. The orientations chosen were 90, 135, and 225 from the vertical

position. Orientations were graphed using a scatter plot to test for variance between

samples (Figure 6, Appendix B).

For each sample and orientation, concentrations of proxy elements were summed to

obtain proxy element abundance. Proxy element abundance for each of the 3 orientations

was plotted. How well the trend lines aligned with each other determined variance.

Where trend lines were overlapped, variance is considered low; and where they deviated

from one another, variance increased.

Based on the data, three trend lines, each one representing an orientation, indicated

low variance for most samples; and areas where deviation occurred have been labeled

A-F. At point A, all three lines do not coincide. Based on the data, this deviation can

be attributed to changes in sulfur concentration, which vary considerably between the

three orientations, possibly due to bedding planes and foliation in the rock. At point B,

the 90 position trend line is not aligned to the other two trends. This can be attributed

to localized areas of concentration for sulfur. At point C, the 90 position trend line

does not coincide with the other two trends. This deviation can be attributed to a higher

concentration of sulfur. At point D, all lines are not coincident. Based on the data, this

deviation can be attributed to changes in sulfur concentration, which vary considerably

among the three orientations. At point E, the 90 position trend line does not match the

16

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17

other two trends. This deviation can be attributed to a higher concentration of sulfur at

the 90 position. At point F, the 135 position trend line does not align with the other two

trends. This can be attributed to a higher concentration of sulfur at the 135 position.

Because sulfide deposition occurs within foliations present in the rock (Paige, 1924),

it is believed that variance in sulfur concentrations is attributed to orientation of the

foliation direction in which sulfides were deposited. In samples where sulfur was highly

variable, orientation of foliation affected the concentration of sulfur. Although there was

variance in these samples, orientation was not considered a major factor in the analysis.

For consistency, samples were then placed into the apparatus at 90 from vertical.

Data obtained from XRF analysis are expressed as parts per million (ppm) and are

reported completely in Appendix C.

Energy Dispersive X-ray Analysis

Using XRF data, 16 samples were chosen for detailed analysis using EDAX

(Appendix D). Standards were run on EDAX using known samples that had been

analyzed using an electron microprobe. After standard samples were completed, it was

determined that the EDAX data closely matched data available from microprobe analysis

and would be employed in this research.

Polished thin sections were inserted into the ESEM; and a backscatter image was

taken so that the observed area could be matched up with the slide for petrographic

analysis. Samples were run for an average of 64 frames or 32 minutes at a spot size of

3.5-4.2 m at 25.0 kV at a chamber pressure of 0.60 torr. Sample magnification varied

depending on area being scanned. Bitmap raster scans were created, which show areas

of high elemental concentrations across the thin section (Figure 7). All EDAX data is

stored on the accompanying CD.

18

Figure 7. EDAX dot map scan of sample BHMA-27a.

19

ArcGIS Analysis

Using ArcGIS software version 9.3, sample locations and metamorphic isograds

were included as layers on a digitized map. XRF data of bulk elemental composition

associated with each location was entered into an attribute table. Using the kriging

method of analysis, chosen proxy elements were contoured onto maps showing areas of

high and low concentration.

The kriging method is a technique for interpolating which honors data points exactly.

An output point is calculated as a linear combination of known data points. Kriging

attempts to produce the best linear unbiased estimate (Glossary of Geology, 2005).

Using points that are in proximity to each other, data are extrapolated, and in areas where

data were not present an estimation can be derived to reflect that data more precisely.

Sampling errors, known as edge effects, occur near the edges of an area where sampling

ceases or in areas where sampling coverage is sparse. The result is data that may not

reflect the true concentration value in an area where data has been extrapolated. Areas

where gold has been previously mined, such as the Homestake and Rochford mining

districts, were marked and used to locate and compare proxy element concentration maps

for Au, Mn, Cl, Zn, Ba, As, Pb, and S.

Keystone and Hill City Districts are other areas that will be focal points of this study.

The Hill City District is an area of widely scattered gold deposits in the vicinity of Hill

City, near the headwaters of Spring Creek and around Rochford, northwest of Hill City.

The Keystone District extends 5.5 km northwest of Keystone to 2.5 km southeast and is

northeast of the Harney Peak intrusion (Koschman et al., 1968).

20

Statistical Analysis

Statistical analysis was run to determine if correlations exist between the occurrence

of gold and potential proxy elements. Pearson Product Moment correlation coefficients

(r) were obtained and tested at a 5% significance level to determine if a trend exists.

Variables having significant positive correlation coefficients tend to increase together,

whereas variables having significant negative correlation coefficients tend to decrease

while others increase.

A multiple linear regression was also employed to generate cross plot graphs to see

if a trend emerged. Gold was plotted on the X-axis as the dependant variable and the

proxy elements on the Y-axis as the independent variable. A multiple linear regression

determines whether there is a positive or negative trend between gold and the proxy

elements. Complete data set is included in Appendix E.

Petrographic Analysis

Of the 222 samples used in this project, 16 samples were chosen for detailed

petrographic analysis based on their XRF elemental gold concentrations. Detailed

petrographic analysis of these samples was completed, and photomicrographs were

taken for EDAX analysis of areas of interest. Areas of interest were those that contained

opaque and silicate mineral phases that may contain the proxy elements or areas

indicative of the metamorphic processes on which this study is focused.

Photomicrographs are used to interpret bit maps and backscatter images generated by

EDAX to orient bitmaps to a specified area on the slide. Using chemistry of the mineral

assemblage identified from petrographic analysis, bit maps indicate how much of the

21

proxy element was substituted into the mineral structure. Data for mineral assemblages

are included in Appendix F.

22

CHAPTER III

RESULTS

Analyzed samples can be compared based on their bulk elemental composition,

petrography, statistical trends, and spatial relationships. These factors allow assumptions

to be made on how well groups of samples fit the model for proxy elements and their

ability to predict gold occurrence.

X-ray Fluorescence and ArcGIS

Because sampling in the Homestake Mining District may not reflect the true

concentrations for the proxy elements around the Homestake Mine due to restrictions

in sampling at that area, all interpretations in this area are based on the data directly

surrounding the area to the south. Areas used as focal points for this analysis are the

Homestake Mine near the cities of Lead and Deadwood, the Rochford Mining District

near the city of Rochford, the Keystone District northeast of the Harney Peak intrusion

near the city of Keystone (Koschman, 1968), and the Deerfield Lake area which is about

10 km south of Rochford and 10 km northwest of Hill City.

Gold was mapped using concentrations ranging from 0 to 20 ppm (Figure 8). Areas

containing high concentrations of gold are represented in dark brown and areas of low

concentration appear in yellow. The two areas that serve as a reference for gold are

the Homestake Mine and the Rochford Mining District. Data show that gold is in high

abundance in the Homestake mining area with concentrations centered around 15 ppm

23

Figure 8. Gold concentration (ppm) contour map of the Precambrian area of the Black Hills, South Dakota using the kriging method. Red dots indicate sample locations collected by L.M. Friberg (1977) and M. Armstrong (1998). Black line indicates the Precambrian boundary.Dashed lines indicate metamorphic isograds. Blue line indicates outer boundary of Harney Peak Granite. Green lines indicate Rochford and Homestake Mining Districts

24

to 20 ppm. In the Rochford Mining District the concentration range for gold is between

10 ppm to 15 ppm. The western portion of the mineralized zone indicates that there is a

high concentration of gold south of the Rochford Mining District, in the Deerfield Lake

area, which has concentrations of gold varying from 10 ppm to 20 ppm. This extends

southeast to the Hill City Mining District, but concentrations of gold tend to drop below

10 ppm farther south as one approaches the Harney Peak intrusion.

Arsenic was mapped and has a range of concentrations between 8 and 36 ppm (Figure

9). To the north, in the Homestake Mining District, arsenic hass concentrations between

12 ppm at its outermost extent to 25 ppm near the mine itself. Concentrations around the

Rochford Mining District vary from 10 ppm near the city of Rochford and increase to an

average of 20 ppm to the south toward Hill City.

Betweem the city of Rochford and Hill City the concentrations of arsenic reache a

high of 30 ppm to 36 ppm and then decrease with distance from the district. Near the

Harney Peak intrusion in the south there is an area of highly concentrated arsenic on the

eastern side of Harney Peak that continues north into the central Black Hills. This area

contains concentrations that range from 32 ppm to 36 ppm and can be correlated to the

Keystone Mining District, which is located on the northeast side of the Harney Peak

intrusion

Barium was mapped and varies in concentrations from 0 to 1,750 ppm (Figure 10).

To the north, in the Homestake Mining District, concentrations range from 1,500 ppm

to 1,750 ppm centered around the cities of Lead and Deadwood and decrease sharply as

distance increases from this central point toward the north. As distance increases to the

south, there is a more gradual decrease in concentration to 600 ppm to 800 ppm.

In the Rochford Mining District, concentrations of barium hold fairly consistent,

ranging between 800 ppm and 900 ppm. Southwest of the Rochford Mining District

concentrations increase, ranging from 1,000 ppm to 1,200 ppm toward the western border

25

Figure 9. Arsenic concentration (ppm) contour map of the Precambrian area of the Black Hills, South Dakota, using the kriging method. Red dots indicate sample locations collected by L.M. Friberg (1977) and M. Armstrong (1998). Black line indicates the Precambrian boundary. Dashed lines indicate metamorphic isograds. Blue line indicates outer boundary of Harney Peak Granite. Green lines indicate Rochford and Homestake Mining Districts.

26

Figure 10. Barium concentration (ppm) contour map of the Precambrian area of the Black Hills, South Dakota, using the kriging method. Red dots indicate sample locations collected by L.M. Friberg (1977) and M. Armstrong (1998). Black line indicates the Precambrian boundary. Dashed lines indicate metamorphic isograds. Blue line indicates outer boundary of Harney Peak Granite. Green lines indicate Rochford and Homestake Mining Districts.

27

of the mineralized zone. A trend exists near Keystone, extending from the west to the

southeast and staying to the north of Harney Peak, that increases to 1,450 ppm to 1,750

ppm as you approach the eastern border of the mineralized zone.

In the northeastern portion of the Black Hills, concentrations range from 1,750 ppm at

the edge of the mapped area, and decrease slowly to 900 ppm toward the west and south.

This area is dominated by Tertiary thermal dikes known to have a high occurrence of

barite (Dewitt et al., 1996).

Chlorine was mapped and has concentrations varying from 3,900 ppm to 14,900 ppm

(Figure 11). In the Homestake Mining district, chlorine concentrations range between

8,000 and 11,000 ppm that increase sharply to 14,900 ppm 15 km to the southeast. This

trend continues to the eastern border of the study area where chlorine concentrations

fluctuate between 10,500 ppm and 14,900 ppm across a 20-km2 area

In the Rochford Mining District, concentrations of chlorine range from 11,000 ppm

to 14,900 ppm. This trend continues through the western portion of the study area to

the extent of sampling on the western margin of the Black Hills and continues south for

20 km. As sample points approach the south toward Harney Peak, the concentrations

decreased sharply to averages of 4,000 ppm to 6,000 ppm and eventually declined to

3,900 ppm at the southernmost extent of the study area.

Manganese was mapped and was found to have a concentration variation from 220

ppm to 4,300 ppm (Figure 12). In the Homestake Mining District, concentrations of

manganese have a range of 500-800 ppm. Concentration changes abruptly 10 km to the

southeast to an average of 2,200 ppm and then tapers off to 300 ppm to 400 ppm. This

trend continues to the eastern border of the study area.

In Rochford Mining District, the concentrations of manganese range from 2,200

ppm to 2,400 ppm near the city of Rochford. The concentration of manganese increases

consistently to the west to its upper limit of 4,300 ppm and continues to the south for 35

28

Figure 11. Chlorine concentration (ppm) contour map of the Precambrian area of the Black Hills, South Dakota, using the kriging method. Red dots indicate sample locations collected by L.M. Friberg (1977) and M. Armstrong (1998). Black line indicates the Precambrian boundary. Dashed lines indicate metamorphic isograds. Blue line indicates outer boundary of Harney Peak Granite. Green lines indicate Rochford and Homestake Mining Districts.

29

Figure 12. Manganese concentration (ppm) contour map of the Precambrian area of the Black Hills, South Dakota, using the kriging method. Red dots indicate sample locations collected by L.M. Friberg (1977) and M. Armstrong (1998). Black line indicates the Precambrian boundary. Dashed lines indicate metamorphic isograds. Blue line indicates outer boundary of Harney Peak Granite. Green lines indicate Rochford and Homestake Mining Districts

30

km, having a small variation between 3,900 ppm and 4,000, increasing to 4,300 ppm west

of the Harney Peak intrusion. South of Harney Peak, concentrations fall to the lower

limit of 220 ppm with a slight increase to 350 ppm to the east and west.

Lead was mapped and was found to have concentration variations between 10 ppm

and 125 ppm (Figure 13). In the Homestake Mining District, lead reaches upper limits of

125 ppm near the town of Galena and within the Lead-Deadwood area. Concentrations

of lead drop as one moves to the southeast and drop to their lower limit in the central

portion of the mapped area. A small area in the northeast portion of the map shows

concentrations increase to 80 ppm and taper off slowly in all directions until reaching the

lower limit in the central area.

The Rochford Mining District has average concentrations beteen 20 ppm and 40 ppm.

These concentrations continue to the western extent of the study area and decline to 10

ppm in the central portion of the area. Five km north of the city of Rochford there is a

slight increase in concentration to an average of 50 ppm, which tapers off to the east and

west to 20 ppm.

In the southern portion of the map are two areas of higher concentration of lead

converging near the Harney Peak intrusion. The area that enters from the east extends

westward for 20 km and has a maximum concentration of 125 ppm at the center of

the area and steps down between 90 ppm and 100 ppm near the center of the mapped

area. The area that enters from the west extends eastward for 15 km and has a slightly

lower concentration density that increases to a maximum of 95-100 ppm at its center.

Concentrations tapers off between 70 ppm and 80 ppm before converging with the

eastern area at Harney Peak. The southernmost portion of the study area holds a near

constant concentration of 60 ppm.

Sulfur was mapped and was found to have a concentration varying between 2,300

ppm and 7,400 ppm (Figure 14). In the Homestake Mining District sulfur concentrations

31

Figure 13. Lead concentration (ppm) contour map of the Precambrian area of the Black Hills, South Dakota, using the kriging method. Red dots indicate sample locations collected by L.M. Friberg (1977) and M. Armstrong (1998). Black line indicates the Precambrian boundary. Dashed lines indicate metamorphic isograds. Blue line indicates outer boundary of Harney Peak Granite. Green lines indicate Rochford and Homestake Mining Districts.

32

Figure 14. Sulfur concentration (ppm) contour map of the Precambrian area of the Black Hills, South Dakota, using the kriging method. Red dots indicate sample locations collected by L.M. Friberg (1977) and M. Armstrong (1998). Black line indicates the Precambrian boundary. Dashed lines indicate metamorphic isograds. Blue line indicates outer boundary of Harney Peak Granite. Green lines indicate Rochford and Homestake Mining Districts.

33

ranged between 4,000 ppm to7,400 ppm, having a high concentration of 7,400 ppm

centered near the city of Deadwood and dropping rapidly to 4,000 ppm to the west

toward Lead. This trend continues southeast for 30 km as the concentration of sulfur at

7,400 ppm along the northern border decreases to 5,000 ppm toward the west.

Within the Rochford Mining district, the concentration of sulfur varies between 5,000

ppm and 7,400 ppm. High concentrations continue north to the edge of the study area.

To the south of Rochford, concentrations fluctuate from 3,700 ppm to 5,000 ppm. In the

southern portion of the study area lower concentrations range from 2,300 ppm to 4,000

ppm.

Zinc was mapped and has concentration variations between 20 ppm and 175 ppm

(Figure 15). In the Homestake Mining District, concentrations varied between 70 ppm

and 85 ppm. Moving 5 km southeast, concentrations increase to values between 85 ppm

and 175 ppm and then sharply decrease to 20 ppm.

In the Rochford Mining District the range of concentrations are between 100 ppm

and 120 ppm, centered near the city of Rochford. Moving south to southwest, there is

an increase to 175 ppm until it tapers off northeast of Harney Peak. South of there, it

decreases to a range between 20 and 40 ppm.

Statistical Analysis

Using Sigma Plot and Microsoft Excel software, statistical analysis was conducted

using XRF data for gold, manganese, sulfur, arsenic, chlorine, lead, zinc, and barium.

Full statistical analyses are in Appendix C. Gold is the dependent variable and

manganese, sulfur, arsenic, chlorine, lead, zinc, and barium are the independent variables.

Graphs indicate a positive or negative correlation as well as the variance.

34

Figure 15. Zinc concentration (ppm) contour map of the Precambrian area of the Black Hills, South Dakota, using the kriging method. Red dots indicate sample locations collected by L.M. Friberg (1977) and M. Armstrong (1998). Black line indicates the Precambrian boundary. Dashed lines indicate metamorphic isograds. Blue line indicates outer boundary of Harney Peak Granite. Green lines indicate Rochford and Homestake Mining Districts.

35

The significance (P 0.05) of the correlation coefficient was tested using the

following null and alternative hypothesis:

Ho: r = 0; there is no correlation

H1: r 0; thre is a correlation

In the initial multiple regression analysis of the data, arsenic, barium, and chlorine

exhibited significant positive correlations. Manganese, zinc, and sulfur had positive

correlations that were not significant. This was believed to be attributed to lead, which

had a probability above 0.05 and a poor positive correlation. The multiple regression for

manganese, zinc, sulfur, and lead were recalculated to see if their correlation coefficient

and probability improved without manganese, barium, and arsenic as variables. After

recalculation, the probabilities of the manganese zinc and sulfur improved significantly.

Lead still showed a poor correlation and poor probability after samples were recalculated

(Table 1).

The data can be separated into three groups; a strong positive correlation (> 0.400),

a moderate positive correlation (0.100-0.399), and a weak positive correlation (<

0.099). The elements with strong positive correlations are chlorine, zinc, and barium.

The element that exhibits the best correlation is chlorine with an r-value of 0.514 and a

probability of < 0.001. Zinc was next with an r-value of 0.4034 and a probability of <

0.001. Barium had an r-value of 0.403 and a probability of < 0.001.

The elements that had a moderate positive correlation were arsenic, manganese,

and sulfur. Arsenic had an r-value of 0.269 and a probability of 0.005. Manganese had

an r-value of 0.254 and a probability of 0.001. Sulfur had an r-value of 0.224 and a

probability of 0.029.

36

Tabl

e 1.

Cor

rela

tion

coe

ffici

ents

bet

wee

n pa

ired

ele

men

ts: r

is s

igni

fica

nt w

hen

P

0.05

.

37

Lead was the only element that had a weak positive correlation. It had an r-value of

0.0977 and a probability of 0.237. Because the probability exceeded 0.05, there is no

significant relationship between gold and lead.

Petrographic Analysis

Of the 222 samples, 120 samples were analyzed using a polarizing petrographic

microscope. General mineral assemblage and modal estimation for all samples can be

seen in Appendix D. Of the 120 samples, 16 samples were chosen based on XRF data for

their high gold concentrations and subjected to detailed petrographic analysis. Detailed

analysis determines mineral phases present, source rock (protolith), and metamorphic

grade. The locations of the 16 samples can be seen in Figure 16. Order of crystallization

was determined using fabric, inclusion relationships, and cross-cutting relationships.

Sample BH-2 is black to gray with visible hematite staining in the sample. It is fine

to medium grained, holocrystalline, and has subhedral to euhedral crystals. It is non-

foliated with porphyroblasts of garnet grains visible in the sample and show that garnets

were being resorbed by quartz and micas. The sample has an idioblastic texture with

a blastoporphyritic relict texture. The protolith for this sample was a pelite and was

subjected to dynamo-thermo metamorphism. The dominant mineral phases present in

this sample are muscovite, biotite, quartz, and chlorite. This rock is a chlorite, biotite,

muscovite schist.

Hematite staining is present along quartz veins in the sample. Muscovite occurs

as euhedral crystals with an average crystal size of

38

Figure 16. Precambrian area and Metamorphic Isograds of the Black Hills, South Dakota. Red dots indicate sample locations collected by L.M. Friberg (1977) and M. Armstrong(1998). Black line indicates the Precambrian boundary. Dashed lines indicate metamorphic isograds. Blue line indicates outer boundary of Harney Peak Granite.

39

continued late stage crystallization. Quartz grains are subhedral and were most likely

present in the protolith with some recrystallization as metamorphism occurred. Biotite

crystallized along with muscovite. Late stage chlorite and muscovite cross cut all other

minerals and early foliation and was last to crystallize.

Sample BH-4 is greenish black on a fresh surface, medium grained, and is

hypocrystalline. It is non-foliated with visible hematite staining along grain boundaries,

and some quartz veining is also present. It has a heteroblastic texture lacking relict

texture. The protolith for this sample was basalt, and it was subjected to dynamo-thermo

metamorphism. The dominant mineral phases present in this sample are hornblende,

biotite, quartz, plagioclase, and magnetite. This rock is a magnetite, biotite, hornblende

amphibolite.

The first minerals to crystallize were pyrite and biotite. Veins of an opaque mineral

are present in the sample. Hornblende crystallized next and exhibited euhedral to

subhedral crystals indicating that this was most likely the peak of metamorphism because

muscovite, although present in the sample, was in low abundance and cross cuts the

biotite. Late quartz veining is seen within the sample, cross cutting the fabric of the rock

and may have aided in the oxidation of the magnetite and biotite.

Sample BH-5b is black to gray on a fresh surface, with red bands of garnet present. It

is fine to medium grained, hypocrystalline, and contains subhedral to anhedral crystals.

There is evidence of relict bedding in the sample. It is foliated with slight hematite

staining along grain boundaries. It has a lepidoblastic texture with a blastopelitic relict

texture. The protolith for this sample was a pelite, was subjected to dynamo-thermo

metamorphism, and is garnet grade. The dominant mineral phases present in this sample

are muscovite, quartz, biotite, graphite, and ilmenite. This rock is an ilmenite, graphite,

biotite, muscovite schist.

40

Ilmenite formed along with the micas and exhibited subhedral crystals. Biotite

and muscovite crystallized and exhibited subhedral crystals that are sub-parallel to the

foliation of the rock. Quartz was present from the protolith and was also re-crystallized

during metamorphism. Late fluids were introduced causing the oxidation of the iron-

bearing minerals along grain boundaries. Graphite in the sample is present along veins,

indicating that the graphite was a result of the metamorphic fluids intruding into the rock.

The reduction of the carbonates lead to the formation of graphite within the sample.

Sample BH-15 is black to gray on a fresh surface with bands of garnet present. It

is fine grained and occurs as subhedral to anhedral crystals. It is foliated with sub-

parallel alignment of the micas with foliation. The sample has been altered by late fluids,

converting pyrite and ankerite into hematite. It has a mimetic/lepidoblastic texture

with relict bedding present. The protolith for this sample was a marl, was subjected to

dynamo-thermo metamorphism, and is a biotite grade. The dominant mineral phases

present in this sample are muscovite, chlorite, quartz, pyrite, hematite, biotite, and

and iron carbonate. This rock is an iron carbonate, biotite, hematite, pyrite, chlorite,

muscovite banded schist.

Iron carbonate grains are blastoporphyritic, and relict quartz grains are also present

from the protolith. Pyrite crystallization began early and is present as inclusions

within the micas and exhibits euhedral crystals. Biotite was the next to crystallize

followed by muscovite and are both in sub-parallel alignment with foliation. Late stage

fluids infiltrated the sample indicated by quartz veining associated with retrograde

metamorphism. Resorbtion of biotite is shown by chlorite cross cutting the biotite grains.

Sample BH-15b is black to gray on a fresh surface with bands of almandine present.

It is fine to medium grained and has a subhedral to anhedral texture. The sample is

foliated with sub-parallel alignment of the mica grains with late stage cross cutting

porphyroblasts of biotite. It has a lepidoblastic texture with relict bedding present. The

41

sample shows metasomatism occurring between biotite and chlorite, with the biotite

grains showing resorbtion by chlorite. The protolith for this sample was a marl subjected

to dynamo-thermo metamorphism and is biotite grade. The dominant mineral phases

present in this sample are muscovite, quartz, chlorite, pyrite, ankerite, and biotite. This

rock is a biotite, iron carbonate, pyrite, chlorite, muscovite schist.

The iron carbonate grains present are blastoporphyritic and relict quartz grains

are also present from the protolith. Pyrite began to crystallize early, exhibits euhedral

crystals, and is included in the micas. Muscovite and chlorite were the next minerals to

crystallize and are in sub-parallel alignment with foliation. Biotite was last to crystallize

and exhibits subhedral porphyroblasts that cross cut foliation, indicating a reactivation

of dynamo-thermo processes causing prograde metamorphism. Chlorite formed last and

cross cuts all minerals and the foliation.

Sample BH-19a is blackish green with visible quartz crystals. It is fine to medium

grained, holocrystalline, having subhedral to anhedral crystals, and has a heteroblastic

texture. The sample is non-foliated and exhibits hematite staining around the magnetite

and ferroactinolite grains. The protolith for this sample was basalt that was subjected

to dynamo-thermo metamorphism and is biotite grade. The dominant mineral phases

present in this sample were ferroactinolite, plagioclase, iron carbonate, magnetite, and

quartz. This rock is a magnetite, iron carbonate, ferroactinolite metabasalt.

Ferroactinolite and magnetite were the first minerals to crystallize. Plagioclase grains

are bimodal, indicating that some of the plagioclase present are most likely from the

protolith with the smaller grains being a result of metamorphism. Quartz is present as

anhedral crystals

Sample BH-63 is black with visible quartz and almandine bands in the matrix. Garnet

grains exhibit anhedral crystals. It is medium grained, and contains euhedral to subhedral

crystals. The sample is foliated and exhibits hematite staining along the garnet-grain

42

boundaries. It has a poikiloblastic/lepidoblastic/snowball texture. Garnet crystals

have been rolled and inclusions within the crystal have preserved the original fabric

orientation. The protolith for this sample was a pelite that was subjected to dynamo-

thermo metamorphism and is staurolite grade. The dominant mineral phases present in

this sample are muscovite, quartz, biotite, magnetite, and garnet. This rock is a garnet,

biotite, muscovite schist.

Quartz grains present in the sample are relict grains from the protolith because

they are included in the garnet crystals. Magnetite crystallized along with the micas

and exhibits euhedral crystals. Biotite and muscovite crystallized and are included

in the garnet crystals. Garnet was to next to crystallize and exhibits euhedral crystals

having an average crystal size between 2 and 3mm. Inclusions in the garnet crystal

indicate that following crystallization, prograde metamorphism continued, during which

garnet crystals were rolled. Metamorphic fluids and plastic deformation caused the

recrystallization of the micas causing them to be parallel to the new foliation direction.

Biotite, and muscovite were the last to crystallize, forming the current orientation of the

foliation within the rock.

Sample BH-66 is black to gray on a fresh surface with visible hematite staining. It

is fine grained, holocrystalline, and has euhedral to anhedral crystals. The sample is

foliated and contains many quartz veins. It has a lepidoblastic/mimetic texture with a

blastopsammatic relict texture. Garnet crystals have been rolled, and inclusions within

the crystal have preserved the original fabric orientation. The protolith for this sample

was a pelite that was subjected to dynamo-thermo metamorphism and is staurolite grade.

The dominant mineral phases present in this sample were quartz, muscovite, biotite,

magnetite, and garnet. This rock is a garnet, biotite, muscovite schistose quartzite.

Quartz grains were bimodal and the larger quartz grains present in the sample are

relict from the protolith, and the smaller quartz grains are the result of recrystallization

43

during metamorphism. This assumption was made because quartz grains are included

within the garnet. Biotite and muscovite crystallized along with quartz, and formed

parallel to the foliation direction. Porphyroblastic garnet was the next to crystallize

with crystals ranging in size from 1 to 4 mm. Garnet crystals had many inclusions

of quartz, muscovite, and biotite, indicating that they formed during and prior to

garnet crystallization. The garnet crystals were slightly rolled, indicating some slight

deformation due to shearing forces.

Sample BHMA-97-27a is black to gray on a fresh surface with visible quartz present.

The rock is fine to coarse grained and contains euhedral to subhedral crystals. The

sample is foliated and has a decussate/heteroblastic texture with a blastopelitic relict

texture. Hematite staining is present along grain boundaries. The protolith for this

sample was a pelite that was subjected to dynamo-thermo metamorphism and is garnet

grade. The dominant mineral phases present in this sample are biotite, muscovite, and

quartz. This rock is muscovite, biotite schist.

Quartz and biotite grains have a bimodal distribution, indicating two growth periods.

Biotite grains not along veins appear to be altered to chlorite but occurs as euhedral

crystals. Muscovite formed next and is parallel to foliation. Late stage fluid infiltrated

the sample causing veining that is predominantly quartz with re-crystallized biotite grains

at the margins of the relict beds that crosscut foliation. Chlorite, garnet and graphite are

also present in the sample in minor abundances. Chlorite was also observed cross cutting

porphyroblasts of biotite.

Sample BHMA-97-31 is black to gray on a fresh surface and is fine to medium

grained. The sample is non-foliated and has an idioblastic texture with a blastopelitic

relict texture. The protolith for this sample was a pelite that was subjected to dynamo-

thermo metamorphism and is biotite grade. The dominant mineral phases present in

44

this sample are muscovite, quartz, biotite, opaques, and garnet. This rock is a biotite,

muscovite schist.

Whole rock crystallization began with the crystallization of pyrite which was included

in the micas and exhibits euhedral crystals with an average crystal size between 1 and

2 mm. Tourmaline was the next to crystallize as euhedral crystals having an average

crystal size between 1 and 2 mm. Muscovite and biotite were the next to crystallize and

are in random orientation within the sample. Quartz crystallized next followed by garnet.

Garnet grains contain many quartz, muscovite, and biotite inclusions. Pyrite crystals

were observed breaking down into hematite which occurs as halos around the pyrite

grains.

Sample BHMA-97-47 is greenish black on a fresh surface and is fine to medium

grained. The sample is non-foliated and has a porphyroclastic/mortar texture with a

blastoporphyritic relict texture. The protolith for this sample was a greywacke that was

subjected to dynamo-thermo metamorphism and is biotite grade. The dominant mineral

phases present in this sample are quartz, muscovite, biotite, and plagioclase. This rock is

a biotite, muscovite metawacke.

Plagioclase and quartz grains present are bimodal, and the larger grains are believed

to be relict grains from the protolith and lack preferred orientation in the rock. Quartz

grains also exhibit a mosaic structure indicating quartz recrystallization as metamorphism

progressed. Muscovite and biotite have average grain sizes < 1 mm. Biotite occurs

within or along quartz boundaries, and muscovite/biotite intergrowths occur within relict

bedding.

Sample BHMA-97-54 is gray to black on a fresh surface and is fine grained,

holocrystalline, and has crystals range from euhedral to subhedral. The sample is

non-foliated and has a lepidoblastic/snowball texture and a blastopelitic relict texture.

The protolith for this sample was a pelite that was subjected to dynamo-thermo

45

metamorphism and is garnet grade. The dominant mineral phases present in this sample

were biotite, quartz, garnet, and chlorite. This rock is a chlorite, biotite schist.

Magnetite crystallized first and exhibits euhedral crystals with an average grain

size of 1 mm. Biotite was next to crystallize followed closely by the crystallization of

chlorite. There is a sub-parallel alignment of biotite grains within the sample indicating

pressure and temperature were beginning to align the minerals.

Sample BHMA-97-57b is greenish-black with relict bedding and probable quartz

veins present in areas rich in garnet. It is medium grained and has euhedral to subhedral

crystals. The sample is foliated and has a porphyroblasts of rotated garnets. The protolith

for this sample was a pelite that was subjected to dynamo-thermo metamorphism and

is garnet grade. The dominant mineral phases present in this sample were muscovite,

garnet, biotite, quartz, and chlorite. This rock is chlorite, biotite, garnet, muscovite schist.

Quartz is bimodal indicating the larger grains are relict quartz grains from the

protolith. Plagioclase grains are also believed to be relict grains from the protolith.

Biotite was the first to crystallize and exhibits euhedral crystals that are in subparallel

alignment with foliation. Chlorite was next to form and shows subhedral crystals.

Muscovite formed late, is observed cross cutting chlorite, and is in subparallel alignment

with foliation. Garnet was last to crystallize and contains many inclusions of quartz and

biotite. The garnet has been rolled, indicating late deformation followed by muscovite

growth.

Sample BHMA-97-63 is black on a fresh surface with garnet porphyroblasts present.

It is fine to medium grained and contains anhedral crystals. The sample is foliated and

has a heteroblastic texture with a blastopsammatic relict texture. The protolith for this

sample was a pelite that was subjected to dynamo-thermo metamorphism. The dominant

mineral phases present in this sample are biotite, plagioclase, garnet, quartz, and

muscovite. This rock is muscovite, garnet, biotite schist.

46

Crystallization began with the formation of biotite and muscovite, which formed

parallel to the foliation. Garnet was next to form, and the crystals present in the sample

exhibit euhedral crystals, contain many quartz, muscovite, and biotite inclusions, and

have been slightly rolled preserving the original foliation. Quartz is present in the sample

as bedding. Hematite staining is present along and within the quartz veins. Biotite is

observed breaking down into chlorite when the grains are in close proximity to the garnet

indicating that as the garnet formed, biotite and garnet were being consumed and chlorite

formed.

Sample BHMA-97-98 is greenish-black with visible quartz and plagioclase. It

is fine to medium grained with crystals that range from subhedral to anhedral. The

sample is foliated and has a heteroblastic/mortar texture with a blastoporphyritic

texture. The protolith for this sample was a greywacke that was subjected to dynamo-

thermo metamorphism. The dominant mineral phases present in this sample are quartz,

biotite, muscovite, plagioclase, and chlorite. This rock is a chlorite, muscovite, biotite

metawacke.

Plagioclase grains present in the sample may be relict grains from the protolith.

Quartz was bimodal, indicating that the larger grains may also be blastoporphyritic before

metamorphism, and the smaller quartz grains recrystallized during metamorphism. The

larger quartz grains appear to be strained as a result of metamorphism. Crystallization

began with the formation of biotite and muscovite which exhibit anhedral crystals that

are parallel to foliation. Chlorite formed next and is found to be either sub-parallel to

foliation or as rims around the biotite grains. Trace amounts of garnet are present in the

sample. Hematite staining is apparent at grain boundaries within the sample.

Sample BHMA-97-101 is gray to black on a fresh surface and is fine to medium

grained and has euhedral to subhedral crystals. The sample is foliated and shows kink

banding near the garnet crystals. It has a lepidoblastic texture with a blastopsammatic/

47

blastopelitic texture. Relict bedding is observed within the sample. The protolith for this

sample was a pelite that was subjected to dynamo-thermo metamorphism and is garnet

grade. The dominant mineral phases present in this sample are biotite, muscovite, quartz,

garnet, and hematite. This rock is a hematite, garnet, muscovite, biotite banded schist.

Biotite and muscovite crystallized early and occur as subhedral crystals that are

parallel to foliation. Quartz was the next to crystallize within the sample as veins.

Quartz veins have hematite staining along the vein boundaries. Hematite is also present

in the sample as free standing crystals. Garnet was last to form, and the grains contain

many inclusions of quartz and biotite.

Energy Dispersive X-ray Analysis

The sixteen polished thin sections used in petrographic analysis were chosen based

on XRF data for their high gold concentrations and were analyzed by energy dispersive

X-ray analysis (EDAX) to obtain the element distribution in the sample, their abundance,

and dot maps which indicated their occurrence across the section (Figure 7). These data

were collected to determine if the elemental concentrations of proxy elements are higher

on the outside rims of the grains than in the central core of the crystal.

The backscatter images are used to match location of the dot maps with

photomicrographs. Petrographically identified minerals can then be matched with

elemental distribution on the dot maps. This will also indicate whether the mineralizing

fluids deposited the proxy elements that were deposited at the rim of the grain. Silicate

minerals capable of including several proxy elements into their structure are chlorite,

biotite, garnet, while sulfur and arsenic partition in the iron bearing opaques.

The occurrence of an element can be represented as bitmaps. As the sample is

scanned, X-rays interact with the elements present in the sample. When an element is

48

identified, dots are added to the bitmap where that element occurred. The higher the

concentration of these dots, the higher the abundance of that element. Therefore, a high

density of dots in an area indicates high abundance, and a low density of dots in an area

indicates low concentration for the elements being scanned.

Using photomicrographs taken of the slide and the bit maps generated from EDAX,

occurrence of proxy elements in high concentration can be correlated to the mineral

phases present in the sample. The elements that were chosen for this comparison are

those that correlated best with gold: barium, sulfur, and chlorine. One slide that best

illustrates this relationship was chosen for each element.

EDAX analysis does define elemental occurrence, but does not indicate chemical

zoning between the rim of the grain and its center. Where fluids would have interacted

with the rim of the grain, a higher concentration should be observed. This shows that

dot map scans cannot define chemical zoning and further research using line scans may

delineate chemical zoning.

49

CHAPTER IV

DISCUSSION

The ability of the proxy elements to predict gold is based on their increased

abundance with proximity to known gold deposits. Using ArcGIS, statistical analysis,

EDAX, and petrography, the proxy elements can be rank ordered to predict the location

of gold deposits.

Chlorine as a Predictor for Gold

Chlorine is the proxy element that best correlates with the occurrence of gold. The

correlation coefficient of chlorine with gold was 0.514 (Figure 17). This strong positive

correlation indicates that as concentration of gold increases, concentration of chlorine

increases with it. The r2 value suggests that 26% of the variation in gold can be attributed

to chlorine. The mineral that would likely accommodate chlorine into its structure is

chlorite. Petrographic and XRF analysis indicates that where there is a higher abundance

of gold, there is a corresponding high abundance of chlorine. This occurrence of high

concentrations of chlorine may predict the occurrence of gold.

50

Fig

ure

17.

Cro

ss p

lot g

raph

of

gold

ver

sus

chlo

rine

con

cent

rati

on.

Chl

orin

e is

the

inde

pend

ent v

aria

ble

and

gold

is th

e de

pend

ent v

aria

ble.

y=11

69.8x5

507.9

r=0.264

2

1000

0

1500

0

2000

0

2500

0

3000

0

3500

0

Cl(p

pm)

y=11

69.8x5

507.9

r=0.264

2

0

5000

1000

0

1500

0

2000

0

2500

0

3000

0

3500

0

05

1015

2025

3035

Cl(p

pm)

Au(ppm

)

51

Barium as a Predictor of Gold

Barium is also correlated with the occurrence of gold. This strong positive correlation

indicates that as concentration of gold increases, concentration of barium increases with

it (Figure 18). Thus, variation in barium content can also be responsible for variation in

gold concentration. The mineral that would likely accommodate barium into its structure

is the feldspars. In pagioclase, barium would substitute into the calcium or sodium site

in the structure. Petrographic and XRF analysis indicates that where there is a higher

abundance of muscovite and biotite, there is corresponding high abundance of barium.

Based on statistical data, occurrence of high concentrations of barium may predict

occurrence of gold.

Arsenic as a Predictor of Gold

Arsenic correlates somewhat with gold (Figure 19). This moderate positive

correlation indicates that as concentration of gold increases, concentration of arsenic

increases with it. The minerals that would likely accommodate arsenic into its structure

are pyrite and arsenopyrite. Petrographic and XRF analysis indicates that where pyrite is

present in thin section, there is corresponding high abundance of arsenic. Arsenopyrite is

not present in the slides described in detailed petrographic analysis, so a correlation with

this mineral cannot be made. Based on statistical data, occurrence of high concentrations

of arsenic can possibly predict occurrence of gold.

52

Fig

ure

18.

Cro

ss p

lot g

raph

of

gold

ver

sus

bari

um c

once

ntra

tion

. B

ariu

m is

the

inde

pend

ent v

aria

ble

and

gold

is th

e de

pend

ent v

aria

ble.

y=23

.345

x+37

.074

r=0.162

7

400

600

800

1000

1200

Ba(p

pm)

y=23

.345

x+37

.074

r=0.162

7

0

200

400

600

800

1000

1200

05

1015

2025

3035

Ba(p

pm)

Au(ppm

)

53

Fig

ure

19.

Cro

ss p

lot g

raph

of

gold

ver

sus

arse

nic

conc

entr

atio

n. A

rsen

ic is

the

inde

pend

ent v

aria

ble

and

gold

is th

e de

pend

ent v

aria

ble.

y=1.43

38x+2.32

7r=0.072

3

406080100

120

140

As(pp

m)

y=1.43

38x+2.32

7r=0.072

3

020406080100

120

140

05

1015

2025

3035

As(pp

m)

Au(ppm

)

54

Manganese as a Predictor of Gold

Manganese also correlates with occurrence of gold (Figure 20). This moderate

positive correlation indicates that as concentration of gold increases, concentration of

manganese increases with it. The minerals that would likely accommodate manganese

into its structure are garnet, chlorite, and biotite. Manganese will preferentially substitute

into the garnet structure first, then into chlorite, and if there is any remaining manganese,

into biotite. Petrographic and XRF analysis indicates that where garnet is present in thin

section, there is corresponding high abundance of manganese. Also, when garnet and

chlorite are both found in thin section there is an increase in manganese concentration

indicating that substitution was occurring within both minerals. Based on statistical data,

occurrence of high concentrations of manganese may possibly predict occurrence of gold.

Sulfur as a Predictor of Gold

Sulfur correlates with occurrence of gold (Figure 21). This moderate positive

correlation indicates that as concentration of gold increases, concentration of sulfur

increases with it. The minerals that would likely accommodate sulfur into its structure

are pyrite and arsenopyrite. Petrographic and XRF analysis indicates that where

pyrite was present in thin section, there is corresponding high abundance of sulfur.

Arsenopyrite is not present in the slides. A correlation with this mineral cannot be made,

but arsenic is most likely substituted into the pyrite structure. Based on statistical data,

the occurrence of high concentrations of sulfur may also predict occurrence of gold.

55

Fig

ure

20.

Cro

ss p

lot g

raph

of

gold

ver

sus

man

gane

se c

once

ntra

tion

. M

anga

nese

is th

e in

depe

nden

t var

iabl

e an

d go

ld is

the

depe

nden

t var

iabl

e.

y=17

6.3x10

64.7

r=0.064

4

6000

8000

1000

0

1200

0

1400

0

1600

0

1800

0

Mn(ppm

)

y=17

6.3x10

64.7

r=0.064

4

0

2000

4000

6000

8000

1000

0

1200

0

1400

0

1600

0

1800

0

05

1015

2025

3035

Mn(ppm

)

Au(ppm

)

56

Fig

ure

21.

Cro

ss p

lot g

raph

of

gold

ver

sus

sulf

ur c

once

ntra

tion

. S

ulfu

r is

the

inde

pend

ent v

aria

ble

and

gold

is th

e de

pend

ent

vari

able

.

y=37

7.28

x+10

29.4

r=0.050

2

3000

0

4000

0

5000

0

6000

0

7000

0

8000

0

S(ppm

)

y=37

7.28

x+10

29.4

r=0.050

2

0

1000

0

2000

0

3000

0

4000

0

5000

0

6000

0

7000

0

8000

0

05

1015

2025

3035

S(ppm

)

Au(ppm

)

57

Zinc as a Predictor of Gold

Zinc correlates with occurrence of gold (Figure 22). This moderate positive

correlation indicates that as concentration of gold increases, concentration of zinc

increases with it. The minerals that would likely accommodate zinc into its structure are

the sulfides. Petrographic analysis defined opaques, which most likely are hosts for zinc.

Although correlation between zinc and gold is not as strong an indicator as the other

proxy elements, based on statistical data, occurrence of high concentrations of zinc may

predict occurrence of gold. Extremely high concentrations of zinc may be attributed to

zinc introduction during sample preparation.

Lead as a Predictor of Gold

Lead correlates weakly with occurrence of gold (Figure 23). This weak positive

correlation indicates that as concentration of gold increases, concentration of lead

increases with it. However, the r2 of the correlation suggests that less than 1% of the

variation in gold can be attributed to lead.

Spatial Analysis of the Proxy Elements

Gold has previously been mined from the four mining districts in the Black Hills

Precambrian core. These areas are the Homestake, Rochford, Keystone, and Hill City

Mining Districts in the Black Hills, SD. These districts were used as reference sites

for comparison with concentration levels of the proxy elements within the Black Hills.

58

Fig

ure

22.

Cro

ss p

lot g

raph

of

gold

ver

sus

zinc

con

cent

rati

on. Z

inc

is th

e in

depe

nden

t var

iabl

e an

d go

ld is

the

depe

nden

t var

iabl

e.

y=9.69

64x5

1.30

7r=0.209

200

300

400

500

600