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X-Ray Fluorescence (XRF), petrography, and Energy-Dispersive X-ray Spectrometry(EDAX) have been used to determine the element concentration in samples and theirdistribution within minerals for 222 metamorphic rock samples from the Black Hills, S.D.Element concentrations in these samples are compared to sample location and knowngold deposits in the Black Hills.XRF data of rock chips from whole rock samples were collected using a portableXRF unit to determine major, minor and trace element abundances. Statistical analysesof the XRF data indicates a moderate to strong correlation between gold and the elementsMn (19 to 16,116 ppm), S (1,283 to 79,452 ppm), As (n.d. to 132 ppm), Pb (n.d. to 318ppm), Cl (625 to 31,277 ppm), Ba (n.d. to 1,101 ppm), and Zn (n.d. to 266 ppm) thusindicating these elements may serve as proxy indicators of gold. The integration ofelemental data with ArcGIS was used to test the spatial relationship of proxy elements toknown gold deposits in the Precambrian core of the Black Hills.Sixteen samples having gold concentration greater than 18 ppm were chosen formore detailed analyses. EDAX raster scans of these samples determined proxy elementvariations within individual mineral grains. Petrographic analyses were done to identifyminerals and their textural relationships.Sample proximity to known gold deposits in the Black Hills can be correlated withincreases in minor and trace proxy element concentrations.
Citation preview
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
eolo
gic
cros
s se
ctio
n of
the
Bla
ck H
ills
aft
er th
e L
aram
ide
Oro
geny
, SD
(C
arte
r, et
al.,
200
3)
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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Custer
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Lead
Rochford
Deadwood
0 10 205Kilometers
Key_Cities_BH! Sample Locations
Harney Peak GraniteMetamorphic IsogradsMining DistrictsBlack Hills Area
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
Fig
ure
6. E
lem
enta
l con
cent
rati
on v
aria
nce
base
d on
sam
ple
orie
ntat
ion.
X-a
xis
indi
cate
s sa
mpl
e nu
mbe
r.
Y-a
xis
indi
cate
s su
m o
f th
e pr
oxy
elem
ent a
bund
ance
in p
pm.
Are
as w
here
dev
iati
on o
ccur
red
are
labe
led
A-F
. B
lue
line
indi
cate
s 90
or
ient
atio
n tr
end.
Red
line
indi
cate
s 13
5 o
rien
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on tr
end.
Gre
en li
ne
indi
cate
s 22
5 o
rien
tati
on tr
end.
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