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A SYNGENETIC MODEL FOR THE ORIGIN OF THE
COPPER MINERALIZATION IN THE PRECAMBRIAN

NONESUCH SHALE, WHITE PINE, MICHIGAN

BY

Katherine Balshaw Autra

A THESIS

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

MASTER OF SCIENCE

College of Natural Science

1977

 

ABSTRACT

A SYNGENETIC MODEL FOR THE ORIGIN OF THE
COPPER MINERALIZATION IN THE PRECAMBRIAN
NONESUCH SHALE, WHITE PINE, MICHIGAN

BY

Katherine Balshaw Autra

Copper mineralization in this shale is mainly
restricted to organic-rich, reduced beds in the lower
sixty feet of the unit. A syngenetic model is proposed
for the origin of this mineralization where by Cu would
be introduced into these deltaic sediments by clay—
organic-Cu complexes. The clay (smectite) and Cu ions
would be derived from weathering of Cu-rich volcanic
uplands.

The Cu contents of three clays present in this
shale (cholorite-illite intergrowths, chlorite cement,
and detrital chlorite) are used to test the model. The
intergrowths are alteration products of the original
smectite. With this model, they derive Cu from Cu-
smectite complexes and from sediment pore-water after
alteration, whereas the cement only absorbs Cu from pore-
water. The intergrowths are found to contain .121% Cu;
detrital chlorite has .099% Cu, and chlorite cement has

.087% Cu, which tends to support the syngenetic model.

ACKNOWLEDGMENTS

I am grateful to Dr. Tom Vogel, my advisor and
committee chairman, for his guidance and support on this
thesis. I would also like to express my appreciation to:
Dr. Max Mortland for his assistance with the clay mineral-
ogy; Dr. Duncan Sibley for his constructive criticism of
the method and manuscript; and Dr. John Wilband for his
' help with the instrumentation.

My parents, Robert and Ellen Balshaw, deserve a
special thank you for being staunch supporters of my
educational goals. Their enthusiasm and interest in my
work has been a valuable asset to me.

I am also indebted to Dr. Sam UpChurch, University
of South Florida, for his early guidance and interest in

my geologic career.

ii

TABLE OF CONTENTS

LIST OF FIGURES AND TABLES . . . . . . .
LIST OF PLATES . . . . . . . . . . .
INTRODUCTION . . . . . . . . . . . .
DEVELOPMENT OF THE EPIGENETIC MODEL . . . .
DEVELOPMENT OF THE SYNGENETIC MODEL . . . .
DISCUSSION OF THE SYNGENETIC MODEL . . . .

Burial Diagenesis in the Nonesuch Shale .
Mechanism for Adsorption of Cu by Clay

Particles . . . . .
Mechanism for the Formation of Cu-

Sulfides . . . . . .
Test for the Syngenetic Model . . . .

PROCEDURES FOR EVALUATING THE SYNGENETIC MODEL

Determination of the Copper Content in

the Clays . . . . . .

Clay Matrix Effects on Cu Detection . .
Determination of the Cu Content of the

Organics . . . . . . . . . .

RESULTS AND DISCUSSION . . . . . . . .

Copper in the Clays . . . . .

Matric Effects on Cu Detection . . . .

Cu Content in the Organics . . . . .

CONCLUSIONS . . . . . . . . . . . .

REFERENCES . . . . . . . . . . . .

APPENDIX . . . . . . . . . . . . .

iii

Page

iv

11
ll
l3

14
15

21
22
25
26
29
29
31
32
33
37

42

 

 

LIST OF FIGURES AND TABLES

Figure Page

1. Geographical location of the Nonesuch
Shale O O O O O O O O 0 O O O 2

2. Ore column of the lower Nonesuch Shale at
White Pine (after Alyanak and Vogel,
1974) . . . . . . . . . . . . 3

3. Percent Cu in the clays . . . . . . 30
Table

l. Microprobe and "t" Test data on the Cu
content of the clays . . . . . . . 24

iv

 

Plate

la.

lb.

lc.

2a.

2b.

20.

3a.

3b.

30.

LIST OF PLATES

Chlorite-illite intergrowth matrix sur-
rounding quartz grains (Vogel sample
#46a7) . . . . . . . . . .

Mg content of the chlorite-illite inter—
growths in Plate la . . . . . .

K content of the chlorite-illite inter-
growths in Plate 1a . . . . . .

Detrital chlorite flakes (Vogel sample
#47) . . . . . . . . . . .

Mg content of the detrital chlorite
flakes in Plate 2a . . . . . .

K content of the detrital chlorite
flakes in Plate 2a . . . . . .

Chlorite cement filling pore space between

quartz grains (Vogel sample #E—lZ) .

Mg content of the chlorite cement in
Plate 3a . . . . . . . . .

K content of the chlorite cement in
Plate 3a . . . . . . . . . .

O

Page

43

43

43

44

44

44

45

45

45

INTRODUCTION

The Precambrian Nonesuch Shale is a six hundred
foot thick mineralized unit overlying the Copper Harbor
Conglomerate, in the northwest Upper Penninsula of Michi-
gan (Fig. I). This formation is divided into three
members, the lowest of which is the Parting Shale,
followed by the Upper Sandstone, and finally, the Upper
Shale.

Copper mineralization is dominantly restricted
to the organic-rich, dark grey, reduced beds in the lower
Parting Shale and the lower Upper Shale members (Fig. 2;
Alyanak, 1974). The copper occurs principally as dis-
seminated chalcocite, with disseminated native copper
occurring mainly in the Parting Shale. ‘Occasionally
native copper appears along bedding planes and in frac-
tures in the lower Parting Shale and upper most regions
of the Copper Harbor formation. A definite correlation
exists between the location of reduced lithologies and
the occurrence of copper (Ensign et al., 1968). Red
(oxidized) lithologies contain only trace mineralization
relative to the ore content found in reduced areas.

The origin of this mineralization is controver—
sial and has been attributed to both syngenetic and

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ORE COLUMN
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Figure 2.-—Ore column of the lower Nonesuch Shale at
White Pine (after Alyanak and Vogel, 1974).

 

 

 

epigenetic processes. The syngenetic proponents (Jost;
Alyanak & Vogel; Vogel, McBride, & Ehrlich) cite the
following major characteristics_of the ore body as sup-
porting their model: its vast lateral distribution,
restriction of the ore to specific stratigraphic units,
homogeneity of metal content in ore along stratigraphic
horizons,predominance of sulfide mineralization, and the
close association of smectite alteration products (chlor-
ite & illite) with these Cu-sulfides.

Epigenetic proponents (White & Wright; Brown &
Trammell; Ensign et al.; and others) stress different
characteristics of the ore body that lend credence to
their model: the apparent replacement of Fe-sulfides by
Cufsulfides, mineralization in the upper regions of the
Copper Harbor Conglomerate, lowering of the height of
Cu-mineralization over a region underlain by a thick
reduced bed, distribution of the more soluble (Cd, Pb, &
Zn) sulfides in the upper fringe of this mineralized
zone, and the cross-cutting nature (with respect to bed-

ding) of this upper fringe surface.

 

DEVELOPMENT OF THE EPIGENETIC MODEL

Discussion of the origin of Cu mineralization in
the Nonesuch Shale began with the discovery of native
copper deposits, located within this unit, along the White
Pine fault. Workers supported an epigenetic-hydrothermal
origin for this native copper, due, in part, to its halo-
like deposition around the fault zone (Butler & Burbank,
1929; White & Wright, 1954; Amstutz, 1958; and Brady,
1960). Later, when the Cu-sulfide mineralization was
discovered in the siltstones and shales of the Nonesuch
unit, workers also assumed a hydrothermal origin for this
ore (Sales, 1959; Joralemon, 1959, 1963; and Ohle, 1963).
However, there is a definite difference in the ore dis-
tribution between the fault zone deposits and the sul-
fides in the major portion of the Nonesuch Shale. The
Cu-mineralization is patchy and very irregular in dis-
tribution, where the falt zone cuts sandstone beds within
this unit (Lower and Upper Sandstone formations, beds
No. 10 & 30). In contrast, ore horizons away from the
fault zone are fairly stratiform, within reduced horizons,
and individual lithologies are mineralized over large

distances.

 

This discrepancy led White and Wright (1954) to
speculate on other possible mechanismsfor the mineraliza-
tion. One early method they considered, and failed to
develop, was the syngenetic precipitation of the c0pper
sulfides. The explanation they preferred involved the
upward migration of a mineralizing solution from the
Copper Harbor Conglomerate. White & Wright (1966) and
Brown & Trammell (1966) restated this theory to propose
that ascending connate waters passed through the Nonesuch
Shale, after the entire unit had been deposited, and
produced a sequence of mineralization. In this model,
minerals of low Cu content, originally present in the
sediment, were replaced by minerals with a higher per—
centage of Cu, as the mineralizing solution passed upward
through the sediment.

Ensign et al. (1968) stipulated that the mineral-
izing solution passed through the sediment before com-
paction and dewatering occurred because compaction was
visible around knots of chalcocite. They characterized
the tOp of the cupriferous zone as being almost parallel
to bedding where it occurred in "favorable" (grey-black,
lreduced) horizons and steep where it crossed "unfavorable"
(red, oxidized) beds. The authors also documented a deep
depression in the top of the Cu-mineralization fringe
over a relatively thick, highly mineralized, reduced bed

(No. 23), in a lower part of the Nonesuch unit. They

suggested that these two features could best be explained
by the epigenetic model, with an ascending mineralizing
solution.

Brown (1971) and White (1971) reiterated this
theory, that mineralization occurred by replacement of
low Cu minerals along an upward advancing mineralizing
front, but they specified that syngenetic pyrite was the
dominant mineral being replaced. White (1960, 1971) and
Brown (1971, 1974) also noted the very restricted occur-
rence of Cd—Pb-Zn sulfides, which were found at the top
of the cupriferous zone and in pyritic sections of bed
No. 61. Brown (1974) suggested that the presence of
these more soluble sulfides in the upper fringes of the
cupriferous zone indicated their being transported in

the outer regions of the mineralizing solution.

 

DEVELOPMENT OF THE SYNGENETIC MODEL

Most workers suggest that the source rocks for
the Nonesuch deltaic sediments were Middle Keweenawan
basalts and andesites plus metasediments and granites
that outcropped to the south of the White Pine area and
provided sediment to the northward building Nonesuch
delta. The dominant weathering product of these volcan-
ics (assuming a fairly temperate climate because of the
abundant organics present in this unit, Meinschein et al.,
1964) would be smectite, plus lesser proportions of
vermiculite, illite, and kaolinite (Grim, 1968). After
deposition in deltaic muds, smectite and vermiculite
alter to chlorite-illite intergrowths (Hower et al.,
1976; and Grim, 1968). To simplify the following dis-
cussion only smectite will be considered; however,
vermiculite could easily substitute for smectite in all
processes that will be mentioned.

Jost (1968) discussed this clay alteration pro—
cess and concluded, based on texture, that most of the
authigenic chlorite he found in the Nonesuch Shale was
originally smectite. Considering the works of Heydemann
(1959) and Mackenzie (1963), which showed that a smec-

tite (montmorillonite) would selectively absorb Cu from

 

solution and form a strong bond with the ion, Jost sug-
gested that smectite in the Nonesuch system would adsorb
Cu from the weathering solution and retain the metal
until deposition in the delta. He stated that Cu would
have been present in the weathering solution because the
source rocks for the Nonesuch sediment included the Cu-
rich Portage Lake Lave Series. He also proposed that
clays which were deposited in dark, reduced sediments
would have had their adsorbed Cu reduced to form chalco-
cite or native copper, and that the close association of
chlorites with Cu-sulfides, present in Nonesuch samples
on a microscopic level, supported this mechanism, and
thus, the syngenetic model.

Alyanak and Vogel (1974) compared the framboidal
nuclei of chalcocite grains, from the mineralized zone
of the lower Nonesuch Shale, with framboidal pyrite
found immediately above this zone. They showed that the
framboidal chalcocite was morphologically different from
the framboidal pyrite. Also, the pyrite texture, which
was dominantly euhedral in the nonmineralized zone, was
not reflected by the chalcocite grains. These observa-
tions did not support a replacement of pyrite for the
origin of this chalcocite.

Vogel, McBride, & Ehrlich (1976) studied the
copper content of the clay fraction (chlorite-illite

intergrowths) from Cu-rich black shales in the lower

lO

Nonesuch unit. They concluded, based on electron spin
resonance and chemical analysis of the clay size mate-
rial, that Cu and Fe ions had entered octahedral sites

in the clays. The authors stated that the presence of

Fe III indicated that the original unaltered clay (a
smectite) was formed in an oxidizing weathering environ—
ment. They also pointed out that Cu could enter octahed-
ral clay sites during the breakdown of clay—organic-Cu
complexes, after deposition in the deltaic sediments.

The authors proposed that the Cu was adsorbed from the
weathering solution by clay—organic complexes, rather
than by just the clay particles themselves. This assump—
ltion was based on work by Mortland (1965) which demon-
strated the ease with which clay-organic complexes from
in natural systems. Also, works by Ermenko (1966) and
Hodgson et a1. (1966) showed that most Cu in natural
water systems is complexed by organic acids. These three
studies, combined with the well documented, abundant
organics in this Precambrian unit (Meinschein et al.,
1964; Barghoorn et al., 1965; and Jost, 1968) support the
conclusion that clay-organic—Cu complexes were present in

the Nonesuch sedimentary system.

DISCUSSION OF THE SYNGENETIC MODEL

Burial Diagenesis in the Nonesuch Shale

 

Weaver (1967) noted that older (pre—Mesozoic)
shales tend to be enriched in illite and chlorite. Con—
sistent with this observation, the siltstones and shales
of the Nonesuch unit contain abundant chlorite-illite
intergrowths. In other areas, these have been shown to
be alteration products of smectites (Hower et al., 1976).
This process, as documented in modern sediments from
drill cores of Gulf Coast deltaic clays, involves the

following reaction:

Smectite + K-spar + mica = chlorite + illite

+ quartz

Wiese (1973) determined that the Parting Shale
(lowermost zone in the Nonesuch Shale unit) contained at
least 5% K-Spar and 10-15% muscovite (plus other minerals).
Therefore, all the necessary phases and components are
present in the sediment to produce the observed chlorite-
illite association.

The fact that only moderate temperatures con-
trolled the alteration of the Nonesuch clays is

documented by the presence of porphyrins in the organic

ll

 

12

fraction of this sediment (Barghoorn, 1965), and also, by
the presence of djurleite and pink bornite (Brown, 1971).
According to Abelson (1959), porphyrins cannot withstand
temperatures greater than 200°C for 1000 years, and the
temperature tolerance decreases with increasing age of
the sediment. In the case of the two, temperature sensi—
tive, Cu-sulfide minerals, Brown states that djurleite
breaks down to chalcocite and digenite at temperatures
above 93°C, while‘pink bornite exsolves chalcocite at
temperatures above 75°C. Since there is no chalcocite
exsolved from pink bornite in the Nonesuch samples, Brown
suggests that the temperature during and after the forma-
tion of this mineral may have been as low as 75°C.

The reaction rate of smectite, altering to chlor—
ite and illite, described by Hower et a1. (1976) is
temperature dependent. In the Gulf Coast sediments, some
smectite is not completely reacted in the loo-175°C tem-
perature interval, and the authors specify that total
conversion of the smectite to illite and chlorite
requires temperatures greater than 175°C or longer reac-
tion times. This reaction time factor explains the
complete alteration of the smectite in the Nonesuch

Shale.

 

l3

Mechanism_fgr Adsorption offiCup§y_
C1ay_Partic1es

 

 

When considering the bonding of Cu to a smectite
,(montmorillonite), Steger (1973) demonstrated that Cu
could be adsorbed on three structurally different sites
on the clay and complexed organics. One adsorption site
is the lattice hydroxyls exposed on edges or crystal
defects. Openings through silicate nets also allow Cu
ions to penetrate to interior lattice hydroxyl positions,
however, Steger found that the Cu ions had to be com-
pletely dehydrated to enter these interior positions.

The final adsorption sites exist on organics complexed to
the clays; they are phenol and carboxylic acid groups.
Steger's results-show that the organic sites on the com-
plexed clays have the highest affinity for Cu. He specu-
ates that this is due to the formation of chelate bonds
with the Cu ions, instead of the formation of complexes.
Chelation has been previously demonstrated to be the
dominant method of adsorption in Cu-organic systems
(Schnitzer & Khan, 1972).

McBride & Mortland (1974), in a later study, also
"documented the migration of Cu ions into hexagonal holes
'and empty octahedral sites within a smectite (montmoril-
lonite). They found, as did Steger, that only dehydrated
Cu ions could penetrate the clay structure. Thus, it is

highly unlikely that smectite could absorb Cu into an

l4

interior position directly from sediment pore water. An
intermediate step, such as adsorption to complexed organ-
ics, would be necessary to dehydrate the ion. After this
dehydration, Cu could enter the clay structure. Vogel,
McBride, & Ehrlich (1976) studied the Cu content of the
clay fraction (<2u) in a Cu-rich black shale from the
lower Nonesuch unit, and found that the .96% Cu in the
clay could not be removed by standard leaching techniques
(NaTPB-NaCl). Their value of .96% Cu was found to be
excessive and probably resulted from the presence of
discrete fine-grain native copper or chalcocite in the

clay film.

Mechanism for_the Formation of Qu-Sulfides

If these clay-organic complexes were deposited
in reduced deltaic sediments, the organics would have
been metabolized by bacteria. This process creates H28,
a strong reducing agent, as a by-product. Cu released
from organic-clay complexes by this bacterial metabolism
would be easily reduced to form the Cu-sulfide, chalcoc-
ite, which could explain the close association of
chalcocite with chlorite—illite intergrowths that Jost
.(1968) found in the Nonesuch samples.

Clay-organic-Cu complexes deposited in oxidized
sediments would have had their organics oxidized and

removed from the system. Copper released from these

 

15

complexes would diffuse through the sediment pore-water
and infiltrate reduced lithologies where the Cu would
react with H28 to form chalcocite (or if the potential
for reduction is high, it could become native copper).
This migration of Cu ions in the pore-water could account
for the solution features that the epigenetic proponents
cite as supporting their mineralization model (these

features are listed in the Introduction).

Test for the Syngenetic Model

A test of the model is to determine whether the
Cu content of the chlorite-illite intergrowths is sig-
nificantly greater than the Cu content of the detrital
chlorite or chlorite cement. 'As previously discussed,
the most probable origin for the intergrowths is from
alteration of smectite deposited in the delta sediments.
With the syngenetic model, these clays could absorb Cu
during the breakdown of their adsorbed organic-Cu com-
plexes. This Cu would be redistributed in the clays as
montmorillonite converted to chlorite and illite.

In natural water systems and soils, clays them-
selves have been shown to be only a minor trace element
sink (Jenne, 1977). Instead, they act as mechanical
substrates for the adsorption of organics and secondary
minerals, which then complex the trace elements. Thus,

only minor amounts of these elements, in this case Cu,

 

16

enter the clay structure. A major limiting factor in this
process could be the fact that the Cu ions must be
dehydrated before they can enter interior clay lattice
positions. Since Cu ions complexed to organics are
dehydrated, the main opportunity Cu has to enter the clay
lattice is when the organic-Cu complexes adsorbed on
exterior and interlamellar clay surfaces are disturbed.
Therefore, Cu can enter the smectite structure after
deposition of the clay-organic-Cu complexes in the None-
such deltaic sediments.

These complexes are attacked soon after deposi-
tion (Degens, 1967), and the organic residues (i.e.,
humic acid, fulvic acid, etc) produced by this microbial
decomposition may become stabilized by recomplexing to
clays. Humic acid is the dominant residue involved in
the recomplexing. It is more abundant and much less
soluble in water than fulvic acid. In addition, its
heavier molecular weight causes humic acid to be more
easily adsorbed on clay surfaces. This is inferred from
work by Mortland (1968) which shows that, within a group
of related organics, those with heavier molecular weights
have a greater tendency to be adsorbed on clay surfaces.
Similarly, Malcolm (1964) notes that fulvic acid is
important in the soluble transport of trace metals,
whereas, humic acid is important in trace element reten-

tion by stream sediments.

 

17

An acidic pH is necessary for these organic resi-
dues to recomplex to interlamellar and exterior surfaces
on a smectite (Mortland, 1970). Reduced sediments in the
Nonesuch delta would have maintained an acidic pH by the

production of H S from decomposed organics. This acidity

2
prevents humic acid from complexing Cu because Bondarenko
(1972) demonstrated that Cu-humic acid complexes were
only stable in "neutral and weakly alkaline" solutions.
Therefore, these conditions preclude the formation of
new Cu-organic complexes on smectite in the sediment.
Later, smectite alters to chlorite and illite and
redistributes its Cu content between these clays. Dur-
ing this alteration, the smectite may create an increased
negative interlayer charge by substitutingyAI3 for Si4
in tetrahedral positions and M32 (and possibly Cu) for
F53 in octahedral positions. The smectite's original
lattice charge may arise from either its tetrahedral or
octahedral layers, which will affect the nature of the
ionic substitution necessary to increase the negative
interlayer charge. This facilitates the uptake of K+
necessary for the conversion to illite. Fe and any
excess Mg ions that are expelled from octahedral sites
in the smectite during its alteration from chlorite
layers in the smectite-illite structure. Cu ions could

substitute in the chlorite structure for both octahedral

Fe or Mg and for Al in gibbsite interlayers in these new

 

l8

chlorite layers. Therefore, Cu originally absorbed into
the smectite structure would now be redistributed between
octahedral positions and gibbsite sheets in the newly
formed chlorite-illite intergrowths. However, the chlor-
ite and illite could also absorb Cu ions directly from
the pore—water during their formation. Thus, the
chlorite-—i11ite intergrowths take Cu into their struc-
ture in two stages: first, during the breakdown of Cu-
organic-clay complexes when Cu enters the smectite
structure, and second, during the formation of chlorite-
illite intergrowths when Cu may be absorbed from the
pore-water.

With the epigenetic model, clays would not come
in contact with Cu until a mineralizing solution infil-
trated the sediment from below, before compaction and
dewatering. Smectite, in reduced areas, would possess
the adsorbed humic substances, as previously discussed,
and would be unable to complex Cu ions from the mineral-
izing solution because of the acidic pH that prevails in
the reduced sediments. It has also been shown that
smectite cannot incorporate Cu into its structure
directly from solution because the ions are hydrated.
However, when the smectite alters to chlorite and illite,
the new clays could absorb Cu from the sediment pore—

water.

 

19

Detrital chlorite in the mineralized shale would
have difficulty incorporating Cu because its lattice
was previously formed and is stable in the environment of
deposition. However, it may already contain Cu intro-
duced by its initial environment of formation and may
possibly pickup additional Cu from the weathering solu-
tion or minor organics complexed before deposition.

Chlorite cement from mineralized sands, inter-
bedded with ore-bearing shales, in the lower Nonesuch
unit could incorporate Cu directly from the sediment
pore-water. Its Cu content would be a function of both
the Cu concentration in the adjacent pore-water and the
amount of Cu that the chlorite structure can tolerate.

Therefore, if the Cu contents of the clays are
very similar, it would support the epigenetic model. On
the other hand, if the chlorite-illite intergrowths
contain more Cu than the chlorite cement, it would sup-
port the syngenetic model because the Cu in the chlorite-
illite intergrowths could be derived from the breakdown
of Cu-organic complexes and/or from solution during the
alteration of smectite. However, the Cu in the chlorite
cement could only be derived from the pore-water. A
test of the syngenetic model would be to determine the
Cu content of the chlorite-illite intergrowths and the

chlorite cement in the same sample. Unfortunately,

 

20

chlorite cement only occurs in sandstones, and chlorite-
illite intergrowths appear in shales or siltstones within
the Nonesuch unit. The samples were, therefore, selected

from adjacent, interbedded lithologies.

PROCEDURES FOR EVALUATIONG THE

SYNGENETIC MODEL

Three areas of study have been defined to eval-
uate the viability of the syngenetic model as an origin
for Cu-sulfide mineralization in the lower Nonesuch Shale.
The first involves identification of three types of clay
minerals in the modern shale: detrital chlorite, chlorite
cement, and chlorite-illite intergrowths, (from the Dom-
ino and Junior Shale units and the Upper Sandstone) and
calculation of the amount of Cu each clay contains. To
determine if the variation in Cu content between the
clays is genetic or due to change, the average percentage
of Cu in each clay type is compared using "Student's" t
Test. The electron microprobe is used in the determina-
tion both of the clay types and their Cu content.

The second area of this investigation involves
determining whether the compositions of the clay matrices
inhibit the detection of Cu. Two clays (detrital chlorite
and chlorite cement) are used for the study, and micro-
probe data is collected for six elements (Cu, K, Al, Fe,
Mg, & Si). The data is analyzed by a computer program--

EMPADR--which reports the results as weight percents of

21

 

22

the oxides of the six elements mentioned. If the matrix
of either of these clays inhibits detection of Cu on the
microprobe, the computer program compensates for it. Com-
parison of the weight percent Cu data of detrital chlor-
ite and chlorite cement from the first study with the
corrected data calculated for these clays by the computer
shows the exact dimensions of this matrix effect (if it
does exist).

The third study is undertaken to observe whether
any Cu ions may still be complexed to disseminated organ-
ics in the modern shale. An organic—rich sample from the
Domino unit (Vogel sample #7—2-2) was selected for disso-
lution. The dissolved organic fraction is analyzed for
Cu by Atomic Absorption Spectrophotometry.

Determination of the Copper Content
in the Clays

 

The electron microprobe was used for this analy-
sis. Current across the filament was 25 kilovolts (the
amount needed to excite Cu radiation from the sample).
Beam current was adjusted to .4 microamps on a quartz
grain to insure a count rate of 20,000 counts in 10 sec-
onds. The three spectrometers were set to read Cu, K,
and Mg, respectively. Diffraction peaks were maximized
with the chart recorder. Background settings were calcu-

lated by adding .075A to the peak values. Samples were

 

23

polished following the procedure outlined by Woodbury
and Vogel (1970).

A clay film standard of known Cu content (2.9%)
was analyzed at the beginning and end of data collection.
Three peak and three background readings were taken from
different locations on the standard and averaged. The
same method was followed for analysis of detrital chlor-
ite, chlorite cement, and chlorite-illite intergrowths.
Due to the fine grained nature of the sediment, it was
easiest to locate and identify the clays using a sample
current image on the oscilloscope. Polaroid pictures
were taken of this sample current, and K, and Mg X-ray
images (Plates 1-3).

Ten grains of each clay type were analyzed for
Cu, K, and Mg, and the variance and standard deviation
values calculated for the Cu data. "Student's" t Test
(Langley, 1968) was then applied to this Cu data to
determine if the difference between these values was
due to chance or an inherent difference in Cu content
between the clays (Table l).

The following formula was employed for the t

Test:

 

24

 

acoumasmom

 

oma.oa n u omm.m u p Damned mm com:

mmHo. n m oaao. u m maoo.

mnoa.om.omm u mm mloamo oma H mm ouoa.~ Nona
momo. .o>4 ammo. .o>< moma.
oooa. ommo. mmmo.
memo. omoo. mmoa.
ommo. ammo. moHH.
oamo. moaa. omoa.
oooa. oomo. mafia.
Hmoa. omoa. maom.
mmoa. Hmoa. mooa.
emmo. oomo. omaa.
«moo. NHHH. mmoa.
mmoo. Hmoa. ooma.
50 w 50 m :0 w

 

unmfimo mDHHoHEU

 

GusHHOHfiU Hwy. HHfiOQ

 

mopzoumuoucH mamaaoloufluoasu

 

.mxmao ecu mo poopsoo :0 map so spot umoB =p= use mooumouoozll.a mqmde

 

25

t = /H xIM-ml

 

a

n = # of measurements in each sample
M = mean of parent population

m = mean of sample group

S = standard deviation of the sample

(A 98% probability level was used)

Clay Matrix Effects on Cu Detection

 

The electron microprobe was also used for this
study. As before, current across the filament was 25 KV,
and beam current was .4 microamps on quartz. Six ele-
ments were analyzed for: Cu, Si, Al, Fe, Mg, & K. The
standards for these elements were: native copper, quartz,.
corundum, hematite, Mg oxide, and orthoclase. Diffrac-
tion peaks and background readings were determined as
outlined. Only detrital chlorite and chlorite cement
were compared to determine any matrix effects on Cu detec-
tion. Three grains of each clay were probed at five
different locations for peak and background readings of
the six elements. Standards were run at the beginning
and end of data collection to check for current drift.

This probe data was then fed into a computer
program--EMPADR--(Rucklidge and Gasparrini, 1969)
designed to analytically reduce the probe's count data

to weight percents of the elements and oxide percents.

 

26

It also compensates for any reduction in counts for an
element due to the type of matrix being probed. These
computed weight percents of Cu metal for detrital chlor-
ite and chlorite cement were then compared to the data
from Table l to determine the degree to which matrix
inhibition influenced this probe data.

Determination of the Cu_Content
of the Organics

 

 

A Perkin-ElmerAtomic Absorption Spectrophoto-
meter was used in this organic dissolution study. Chloro-
form was selected for the organic solvent, as recommended
by Corless (1968) and Burrell (1965c), since it would
dissolve any organics in the sample without disturbing
Cu from other sources. Methanol was mixed with the '
chloroform in a 3:2 ratio of methanol to chloroform to
give a suitable flame for analysis (Angino & Billings,
1967). A 1:1 ratio (methanol/chloroform) was tried but
did not burn as well as the 3:2 mixture.

The standard stock solution was prepared by dis-
solving 1 gram of Cu metal powder in 10 m1. of HNO3 and
then diluting the Cu solution to 1 liter by adding the
3:2 methanol/chloroform mixture. This forms a 500 ppm
solution. The stock solution was then diluted, through
intermediate steps, to form standard solutions of 6, 5,

3, 1, 0.75, 0.5, and 0.25 ppm. The upper limit of

linearity is 5 ppm, and the lower detection limit of the

 

27

Cu tube is 0.25 ppm. A blank of the 3:2 methanol/
chloroform mixture was also included with the standards.

The organic shale sample was selected from an
organic-rich lens in the Domino unit. It was prepared by
crushing on a ball mill until the particles were silt
size. Six preparations were made from this sample using
various amounts of rock powder and three different mixing
techniques. The mixing techniques were modified because
the methanol/chloroform solution is unstable. Chloroform
dissolves organics, while methanol precipitates them,
which, depending on the speed of the reaction, could
affect the Cu analysis.

The first two samples contained 0.5 g. and 1.0 g.
of powdered shale, and 100 ml of a previously mixed 3:2
methanol/chloroform solution was added to each. They
were centrifuged within 15 minutes and the supernatant
solution analyzed for Cu on the atomic absorption unit
immediately afterward. The second pair of samples con-
tained 2.0 g. and 5.0 g. of powdered shale. 40 ml of
chloroform was added, and they sat overnight to ensure
dissolution of the organics, before 60 m1 of methanol was
introduced. They were then centrifuged and analyzed,
just as the first samples. The final pair of samples
contained 1.0 g. and 3.0 g. of powdered shale. 40 ml of

chloroform was added first followed 10 minutes later by

 

28

60 m1 of methanol. The solution stood for one hour
. before centrifuging and analysis.

When analyzing each pair of samples, the atomic
absorption unit was zeroed using the 3:2 methanol/
chloroform blank. The Cu wave length was maximized at
326.4A, and the slit setting was 4(.7) nm. All of the
standards were run once, then one sample was analyzed
followed by two standards. The two standards would have
been those that bracketed the sample's absorbance units.
However, since no sample showed any absorbance for Cu,
the standards were randomly selected to check for varia—

tion in the readings from the first standard values.

RESULTS AND DI SCUSS ION

Copper in the Clays

 

When analyzing the data from Table 1, it is appar-
ent that the three different clay types possess different
average Cu contents. Chlorite cement contains the least
with an average Cu content of .087%. Detrital chlorite
contains an intermediate average Cu value of .099%. The
chlorite-illite intergrowths possess the highest average
Cu content at .121% (see Fig. 3). By comparing the
detrital chlorite and chlorite cement values to that of
chlorite-illite, through use of "Student's" T Test,
detrital chlorite is shown to have a "t" value of 5.83
for 10 samples, while the "t" value for chlorite cement
is even larger, t = 16.13.

One makes the basic assumption with the t Test
that there is no significant difference between sample
values (i.e., percent Cu in each clay type). The larger
"t" becomes, the less likely it is that this hypothesis
is true. For a sample size of ten, there is a 98% proba—
bility that the difference between any sample and the
parent population with a "t" value of greater than 4.3
is not due to chance. Both of the calculated "t" values

for these clays exceed 4.3, which indicates that the

29

 

Percent Cu

30

 

 

 

 

 

 

 

 

 

.125. y
.100. .______
,______i
0075-! l
.037 .099 .121
705‘“ Zen Zen ‘ZCu
0025.1
I ‘0
16¢ MK? {6' "\é'q, S
\§'Q? ‘$'<§ {§'§’5’
s? «3' s” 6“ $1.? $°
I$

Figure 3.——Percent Cu in the clays.

 

 

31

difference in Cu content between the clays is not due to
chance variation, but represents a genetic difference

inherent in the clays.

Matric Effects on Cu Detection

 

The concentration of Cu in the two clays deter-
mined by the EMPADR computer program shows very little
variation between these values and those taken directly
from the microprobe analyses mentioned in the first sec-
tion. The Cu values calculated by the computer program,
using six elements, are more accurate than the straight
probe data but the process was too time consuming for the
purposes of this study, and the initial probe data
proved, in comparison, that it did not significantly
deviate from these computed values.

Chlorite cement has a computed average weight
percent for Cu metal of .050%. The initial probe data
gives a Cu content for the chlorite cement of .087%.

The computed average weight percent of Cu in the detrital
chlorite is .096%, whereas the initial probe data has a
Cu content of .099%. The .037% difference in Cu content
between computed and initial chlorite cement data is due
to the effect of Fe. The higher the amount of Fe in the
clay, the greater the amount of Fe radiation absorbed by
Cu ions when the sample is irradiated by an electron

beam. This enhances the production of Cu radiation by

 

32

the sample. The detrital chlorite has a lower percent
Fe and, thus, a much smaller difference between Cu values.
The chlorite-illite intergrowths also possess only a
small amount of Fe and, therefore, would be expected to
show only a minor difference in Cu content between the
computed and initial probe data.

This data comparison shows that, in the present
research, the composition of the clay matrix does not
significantly impair an accurate determination of the

percent Cu present in the Clays.

Cu Content in the Organics

 

The organic dissolution study shows that dissem—
inated organic material, from an ore-bearing section of
the Nonesuch Shale, does not possess complexed Cu ions.
This test was performed mainly to determine the validity
of the initial estimate of .96% Cu made by Vogel, McBride,
& Ehrlich (1976) for the clay fraction of the lower
Nonesuch Shale. This value is approximately eight times
greater than the largest percentage of Cu found in any
clay in the present study. It was believed possible
that the remaining Cu missing from the clays could have
been complexed to organics in the sample studied, and
thus, included in the clay fraction analysis. This
assumption was disproven when no Cu was found in any of

the powdered shale samples.

 

CONCLUSIONS

Evidence presented in this study concerning the
amount of Cu present in three clays from the Nonesuch
Shale tends to support the syngenetic model. The higher
percentage of Cu in the chlorite-illite intergrowths,
over the detrital chlorite and chlorite cement, lends
credence to the proposition that the original smectite
possessed adsorbed organic-Cu complexes which brokedown
after deposition allowing some of this complexed Cu to
enter octahedral positions in the clay structure. The
chlorite cement contained the lowest percent Cu because
it formed after mineralization. The intermediate amounts
of Cu found in detrital chlorite are most likely a com-
bination of Cu acquired during the chlorite's initial
formation and minor amounts of Cu absorbed during weath-
ering or deposition.

An aternate interpretation of the data is that
the difference in Cu composition of the chlorite-illite
intergrowths and the chlorite cement is due entirely to a
difference in the Cu content of the connate waters at the
time of formation of both clays. This interpretation can-
not be completely ruled out because the chlorite-illite

intergrowths occur in organic—rich beds in which the

33

 

34

pore-water would have a lower pH and Eh than the inter-
bedded sandstone in which the chlorite cement occurs.
However, both the organic-rich beds and the sandstone in
which the chlorite cement appears are intimitely asso-
ciated. The chlorite cement sample is from the Upper
Sandstone 2 feet below the Upper Transition shale hori—
zon. The sandstone unit is only 8 feet thick in the
sample region and is surrounded by shale thicknesses
totaling 60 feet. It would be difficult to maintain a
difference in pore-water chemistry between these beds.
By analyzing the microprobe data from chlorite
cement and detrital chlorite, reduced by the EMPADR com—
puter program, it has been determined that these differ-
|ent clay compositions did not inhibit the detection of
Cu by the probe. Therefore, the Cu contents calculated
for the three clay types, using the initial probe data
appear accurate. The calculated clay compositions for
'the chlorite cement and detrital chlorite show that the
detrital chlorite contains approximately 1.5% K20 in its
structure. This suggests the presence of minor illite
interlayers in the chlorite. The detrital chlorite also
possesses twice as much Cu as the chlorite cement. Cu
may have substituted, in the detrital chlorite, for
octahedral ions in the chlorite or illite layers or for

A1 in the chlorite's gibbsite interlayers.

 

35

Atomic absorption work has shown that no Cu
remains complexed to organics in the lower mineralized
section of the shale. The lack of this association
implies either that the reducing environment was strong
enough to precipitate all the Cu as Cu-sulfides or native
copper, or that the Cu—organic complexes broke down With
time. The later is the more probable explanation. Hoer-
ing (1971) determined that fulvic plus humic acids con—
stitute up to 60% of the total organic matter in recent
and ancient sediments, of both continental and marine
origin, and these organic residues have a maximum resi-
dence time in the sediment of only "several thousand
years" (Ellis et al., 1972). The Nonesuch Shale is over
600 million years old.

In this study and similar previous ones, clays
have not proven to be capable of incorporating large
amounts of trace metals in their structures. However,
they do act as transport mechanisms for these ions and
can also concentrate the ions in certain depositional
areas. It was for this second reason that they were
studied in relation to the Nonesuch Shale system. The
present research does not mean to imply that clay-organic-
Cu complexes were the sole source of Cu for this unit.
The possibility that an additional Source of Cu was avail-

able from a connate solution cannot be ruled out.

36

The results of this work do not, by themselves,
prove the syngenetic model. However, if Cu entered the
basin as a smectite-organic complex, it would be expected
that the chlorite-illite intergrowths resulting from the
alteration of this clay would have higher Cu contents

than the chlorite cement.

REFERENCES

37

 

REFERENCES

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39

Butler, B. S., and Burbank, W. S., 1929, The copper
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Ellis, B. G., Erickson, A. E., Knezek, B. D., Wolcott,
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Ermenko, V. Ya., 1966: Gidrokhim. Mater., v. 41,
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Grim, R. E., 1968, Clay Mineralogy, 2nd edition: McGraw-

 

Heydemann, A., 1959, Adsorption aus sehr verdfinnten
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40

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41
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v. 55, p. 2095-2102.

 

APPENDIX

42

 

 

 

 

 

Plate 1a
Chlorite—illite intergrowth matrix surroundingquartzgrains.

(lcm=15u)

 

Plate 1b
Mg content of the chlorite—illite intergrowthsiniPlate 1a.

 

Plate 1c
K content of the chlorite-illite intergrowths in Plate la.

43

 

i;

     

Plate 2a"
Detrital chlorite flakes (1cm==15u)

 

Plate 2b
Mg content of the detrital chlorite flakes in Plate 2a

 

Plate 2c
K content of the detrital chlorite flakes in Plate 2a

 

. "Plate 3a
Chlorite cement filling pore space between quartz grains

(1cm=15u)

 

Plate 3b
Mg content of the chlorite cement in Plate 3a.

 

Plate 3c
K content of the chlorite cement in Plate 3a.

"llllllillllllll“