A STUDY OF GRAPHITIC SLATES
FROM NQRTHERN MICHIGAK '

Thesis fer Hm Degree 0‘ M. S.
MECHEG‘AN STATE UMVERSET‘Y
James F. Olmsted
1962

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ABSTRACT

A STUDY OF GRAPHITIC SLAIBS
FROM FURTHER! MICHIGAN

by Jamea F. Olmsted

Several occurrencea of so called graphitic éalatea' have been
obaerved in the Precambrian of Northern Michigan. In all ceaea
they are related with upper Precambrian or upper Enronian iron
tormationa. Thia atudy was undertaken to further determine the
nature of theae "alatoe" as well as to better understand their
origin.

It haa been determined from pnbliahed analyaee and poliahed
and thin aectiona atudiea/that the "alatea" are compoaed of pyrite,
carbonaceoua material, clay minerals, eericite and chart. The pyrite
compoeee from 10% to 40% of the rock, and carbon (mostly graphite)
about 10%, although in some caeea, carbon may be aa high as 80%.

The high carbon and pyrite content auggeata a reducing marine environ-
ment.

The Vanaeca member of the Dunn Creek elatea has been atudied
moat intenaivaly as it appeara to be repreaentative of eeveral other
aimilar graphitic ”alatee.” The rock hae actually been determined
to be an argillite, aa the metamorphiem in moat caeea has not proceed-

ed to the extent where alaty cleavage ia dominant. The Waueeca

member haa auffered penecontemporaneoue deformation as well aa

James F. Olmsted

dynamic metamorphism. Slump structures of several types have been
identified.

The pyrite of the Wauseca member, as well as from other areas,
is finely divided and is most frequently found in the form of spheres
and other irregular shapes. Several rather unusual etch patterns
have also been observed. Average size of the pyrite bodies is 3.24
‘microns which is in the range of many bacterial organisms. The
close association of graphite with the pyrite and a comparison of
these rocks to the Precambrian shales of mount Isa, Australia leads
to the suggestion that there is some biogenic relationship in the
origin of the pyrite and the graphite.

The continual relationship between these rocks and carbonate
iron formation bears out the similarity of conditions necessary for
deposition of both rock types, i.e., restricted marine environment
of low oxidation potential. Several samples which are closely
associated with iron formations are peculiarly lacking in iron
minerals but are extremely high in carbon content. This leads to
the suggestion that although biological activity may have been
responsible for deposition of pyrite. It also, under more extreme

conditions may have inhibited pyrite deposition.

A STUDY OF GRAPHITIC SLNTES

ram nonram MICHIGAN
by

James F. Olmsted

A.THESIS
Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of
MASTER OF SCIENCE
Department of Geology

1962

ACKNOWLEDGEMEIT

The writer wishes to thank Dr. Justin Zinn who originally
suggested the problem and under whose guidance the problem
was undertaken. He also expresses gratitude to the other
members of the guidance committee, Dr. J. H. Trow and Dr. R.
B. Stonehouse, who provided many suggestions in both the
research and the writing of the manuscript.

He also wishes to acknowledge Mr. Jack Olson of the
Inland Steel Corporation for providing several drill core
samples of the Wauseca member from the Iron River - Crystal
Falls district, and Dr. C. E. Dutton of the D. 8. Geological
Survey, Madison,‘w1sconsin for information on the location

of many outcrops in the Florence district, Wisconsin.

ii

CONTENTS

Page
ACKNOWLEDGEMENTS . . . . . . . . . . . . . . . . . . . . . it
1181' OF TABLES O C O C 0 0 O O. O O O O O O O O O O O O O 1'
um OF FIGURES O O O O O O O O O O O O O O O O O O O O 0 v
n” or m8 0 O O O O O O .0 O O O O O O O O O O O O O 0 'i
um OF APPWDICES O O O O O O O O O O O O O O O O O O 0 0 vii
1. Imowmm C O O O O O O O O 0 O O O O O O O O O O 1
General
Distribution
Stratigraphy of the Iron River - Crystal Falls
District

Structure of the Iron River - Crystal Falls District
Physical and Chemical Environment of Deposition
Methods Used

11. ROCK DESCRIPTIONS Ann PETROLOGY . . . . . . . . . . . . 18
General Considerations
waueeca‘Member
Non-Pyritic,‘nighly Graphitic Type
Minor Structures

III. ORIGIN OF THE PYRITB AND GRAPHITE . . . . . . . . . . . 30

IV. cmCLUSImOOOCOOOOOOOOOOOOO00....43
Suggestions for Further Study

V. REFERENCBSCITED................... 46

VI 0 mnDIch O O O O O O O O O O O O O O O O O O O O O O ‘8

iii

LIST OF TABLES

Table Page

1 O O O O O O O O O O O O O O O O O O O O O O O O 8

2 eemeeeeeeeeeeeeeeeeeeoee 18

iv

OOOOOOOOOOOOOOOOOOOOOOOOO

OOOOOOOOOOOOOOOOOOOOOOOO

 

LIST OF FIGURES
Figure Page
1 Eh - #3 Diagram for Iron. . . . . . . . . . . . . . . . 14

2 Idealised Drawing Showing Slumping in the Hauseca
M..b.r O 0 O C O O O O O O O O O O O O O O O O O O O O O 28

3 Eh - pa Diagrams for Iron (Marine Environments . . . . 31

4 Stability of Some Organic Compounds with
Temperature . . . . . . . . . . . . . . . . ... . . . 32

5 Size Analysis of Pyrite Spheres from Mount Isa,
Australia and Iron River - Crystal Falls District,
Mir-Chis.“ O O O O O O 0 O O O O O O O O O O O O 0 O o m 37

LIST OF MAPS

up“ Page
1 Loc.tion HIP e e e e e e e e e e e e e e e k
2 LocationMap............... 5

vi

LIST OF APPENDICES

Page

”rum: A O O O O O O O O O O O O O O O O O O O O ‘8

1- List of Collection Sites, Map l
2- List of Collection Sites, Map 2

APPENDIX B

O O O O O O O O O O O O O O O O O O O C 50

Chemical Compositions of Four Graphitic Slates

APPENDIX C

Plate
Plate
Plate
Plate
Plate
Plate

Plate
Plate

0 e O O 0 O O O O O 0 O O O O O O O 0 O 51
I - Megascopic Sections of Graphitic Slates
II _ N N U I
III- I I O! I
Iv _ fl 0‘ I N

V - Photomicrographs of Pyritic Bodies,

VI - Photomicrographs - Thin Sections and
Polished Sections of Pyrite and Other
Included Minerals

VII- Photomicrographs

VIII- X-ray Powder Photographs of Graphite

vii

INTRODUCTION

General

This etudy has been undertaken in an attempt to gain information
concerning the origin and nature of the so-called ”graphitic slates” of‘
lorthern Michigan. Attention is called to them because of their nature
as well as their relationship to surrounding rocks and the bearing they
may have upon the understanding of the environment of iron deposition.
Highly carbonaceous rocks are not rare in Paleozoic and younger rocks
but their paucity of occurrence in Precambrian terranes is notable.

The close association which the "graphitic slates” have with iron
formations is not at all surprising when one considers that iron in the
form of pyrite is commonly found to make up as much as twenty percent
of the rock. An interesting observation has been made that in several
occurrences, iron minerals appear to be lacking, yet, the close
association with iron formations remains. The presence of graphitic
material is strongly suggestive of an organic association. Further, a
comparison of the entire assemblage with rock of known organic associa-
tion gives strong evidence that some life form has played an important
role in the formation of these rocks.

Only recently have the graphitic slates been studied to any
degree despite the close association to many areas where iron formations
have been exploited commercially. United States Giological Survey
monographs thirty-six (Clemments and Smyth, 1899) and fiftybtwo
(Van‘lise and Leith, 1911) only mention the presence of highly graphitic

-1-

-2-

slates in several areas and little descriptive discussion was devoted
to them. Since World War II, the United States Geological Survey has
devoted considerable effort toward working out both the structure and
stratigraphy of several iron bearing areas, particularly the Iron River-
Crystal Falls district. This has been accomplished through the use of
magnetometer surveys, drill core studies, and detailed field mapping.
From these studies, two papers by James (1951 and 1954) have evolved in
which the lithology of the rock types and the environments of deposition
<have been thoroughly discussed. James (1958) has also devoted much
attention to this district in a later paper concerned primarily with the
stratigraphic succession of lorthorn‘Michigan. Kane (1952) has provid-
ed several chemical analyses of these and other similar rock types, but
does not discuss in any detail the environmental factors bearing on
these particular rocks.

Tyler (1957) has studied a highly carbonaceous rock type which
occurs about six miles north of Iron River. This occurrence which is
referred to as the Mbrrison Crook ”coal” in some cases contains as much
as eighty percent carbonaceous material, but it is very low in iron
content. Tyler (1957) has assigned a biogenic relatiomsip to the origin
of this unusual concentration of carbonaceous material. It may be not-
ed here also that highly pyritic rocks are in close association with
the "coal.“ Even though there is much to suggest that the carbon in
graphite bearing rocks to some extent can be attributed to the life
forms present in the water during their deposition; there has been
little enthusiasm for such an explanation. This can be readily under-
stood when one conoiders that life may not have been necessary for the

formation of such rocks, but may have been only contingent to the

-3-

process. Yet, the fact remains that such high contents of carbon are
somewhat difficult to explain by inorganic processes even though the
remainder of the rock could be formed in this manner.

Grunner (1922) has speculated that many structures found in the
iron formations of Minnesota may have been developed by organisms or may
be due to their influence during deposition. Several of these show
indications of growth structures and for the most part are composed of
highly graphitic material. Many of these forms closely resemble
oolites found in other similar rock types and may well be assigned
inorganic origins. Regardless of other possibilities, the inference re-
mains that primitive organisma played some part in the formation of

these and other similar rock types.

Distribution

There are several known occurrences of highly graphitic rocks
throughout the Lake Superior region. Some of the occurrences in
northern Michigan include horizons within the Michigammo slates of the
Marquette and Menominee districts (Van Rise and Leith, 1911). The Iron
River - Crystal Falls district probably contains the most continous
occurrence of highly graphitic rock in Michigan. Beds of highly gra—
phitic and pyritic rocks are known to occur south of L‘Anse in the
Taylor mine area. Other sporadic occurrences are known throughout
northern Michigan such as that found north of Iron River. Each of these
areas is noted on Map 1. It may be noted here that in each case the
graphitic rocks of Upper nuronian or according to current United States
Geological Survey terminology, either Middle or Upper Animikie in age.

In no case, to the writer's knowledge, are any highly graphitic rocks

 

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-6-

found in the Lower or Middle luronian assemblages.

The sample areas are shown on the accompanying map. As can be
seen, there are three general locations. The area which has been most
frequently referred to in this study is the Iron River - Crystal Falls
district and is shown in some detail on‘Map 2. The second is called
the Taylor mine area and the third rather broad area includes the
Marquette iron mining district.

The samples from.the north side of the Iron River - Crystal Falls
basin were all obtained from drill cores; the remaining ones are from
test pits and outcrops. Because the rapidly weathering pyrite may have
been removed in part, there is some doubt as to the usefulness of
samples obtained from outcrops. Several samples which are of very low
specific gravity were collected from surface outcrops, but in no case
were any such samples found in drill cores. Samples of this type were
collected from both the Florence and Taylor mine areas. In both cases
this rock type is associated with cherty iron formation. Thirty
samples were collected in the field, many of which upon later investiga-
tion were found to be of limited value.

Samples were collected fremIthree areas in the Marquette district.
One of these is a graphitic slate which is associated with the Eijiki
iron formation. The other two, in general, are from the lower Michigamme
slates. These sanples are all good slates showing the typical slaty
fracture cleavage. The pyrite and graphite content vary in these
samples with the pyrite usually occurring in well-defined bands:
weathering effect does not appear to be intensive.

One sample was collected from the Morrison Creek area which has
been cited above. This sample is not the type described by Tyler as

”cool” but is strikingly similar to that found in the cores of the

-7-

Iron River _ Crystal Falls area. Its stratigraphic relationship to'the
Iron River - Crystal Falls district is not known, but some speculation
as to their similarity could be made.

:Although the samples from.the Iron River - Crystal Falls area are
not necessarily representative of all the other samples, they are
representative of most and extensive treatment has been given to them.
‘Much of the structure and stratigraphy of other areas is either
extremely complicated or, as in the case of the Taylor Mine area, not
well known. For these reasons, the Iron River - Crystal Falls district
has been used more or less as a type area and the others may be compared
to it, furthermore, this area has been extensively studied and is well

described.

 

Stratigraph¥ of the Iron River -
Crypts s s D strict

As previously mentioned, the majority of the samples were collect-
ed from the Iron River - Crystal Falls district. As shown on the map,
this area covers the southern half of Iron County and extends south—
eastwsrdly into Florence County, Wisconsin. The district is generally
a three-cornered basin with spices st Iron River on the west, Crystal
Falls on the northeast and Florence, Wisconsin on the southeast
(Button, 19‘9).

The basin is outlined for the most part by the Badwster greenstone
which is a thick succession of altered submarine lava flows which ranges
in thickness from several feet to several miles (James, 1958). In the
area east of Crystal Falls the greenstone is absent and, therefore, the
succession within the basin lies directly on top of the earlier slates

(James, 1958).‘

Above the Bsdwater greenstone is a thick succession of slates and

greywackes known as the Paint River group. They are outlined below.

TABLE 1

( Fortune Lakes Slate
( Stambaugh Formation (overlapped by Fortune

Lakes slates)
( Hiawatha Greywacke (slate south of Alpha)
Paint River (
Group (

( Unconformity
(
( Riverton Iron Formation
( Dunn Creek Slate (upper 50’ - 100', Wauseca
Member)
( Badwater Greenstone
The Dunn Creek slate consists of about 800’ of greywacke and slate

similar in physical appearsnceto much of the Michigan-e slate (Pettijohn,
1946). The upper 50 to 100 feet has been named the Wsuseca member which
is the primary concern of this study. It has been described by James
(1951, 1958) as a black pyritic, graphitic slate. The type area is at the
Wauseca mine located in Mineral Bills just north of Iron River. Due to
its nature, it is very weakly resistant rock and seldom outcrops, but is
commonly encountered in Ideas and test pits throughout the district
(James, 1951). Descriptions will be deferred at this point and a complete
discussion taken up later. It may be pointed out here, though, that
the Wausocs member has been subdivided into two units, the upper laminat-
ed and the lower conglomerstic or ”speckled grey.” The ”speckled grey”
is consistent and distinctive enough to be recognized throughout the
district and is often referred to as a marker unit (Jones, 1958).

Above the Wausecs member and apparently conformable to it is the

Riverton iron formation. It is typical of the Precambrian iron

-9-

formations in the Lake Superior region, being originally composed
mainly of chart and siderite. Its thickness ranges from 100 to 600
feet, the variation being due presumably to the nature of sedimentation
(James, 1958).

The upper three formations are of little interest here so their
description has not been included. These other formations were
deposited above a somewhat important unconformity and they are all for

the most part clsstic in nature.

Structure of the Iron River -

Crystal Falls District

The detailed structure of the district is extremely complex and
at this time is not completely known. Only a general consideration
will be presented here which is believed to be sufficient to fit the
argument that will be presented later. As shown on the U.S.G.S. Map
IMF 225, the district has now been determined to be a three-cornered
basin (Button, 19‘9). Each of the corners are intensely folded into
tight snticlines and synclines. Pettijohn (1952) noted that in the
Paint River outcrop ares therp are four snticlines and synclines within
a distance of two thousand feet. Both Pettijohn (1952) and Dutton
(1949) have expressed the observation that there is little or no ova
idence of faulting in their respective map areas and that the rocks
acted as incompetent bodies in most cases. Some faulting has been
recognised by Dutton (Personal Communication, 1961) in the area near
the southern corner of the basin. The structure in this part of the
district is somewhat confusing. Due to the incompleteness of intensive
study it has not been, as yet, satisfactorily resolved. The faulting

has complicated the picture such that some doubt exists as to the numbers

~10-

of iron formations present in this area, although, it is strongly
felt that there may be two or possibly three (Button, Personal Commu-
nication, 1961). I

In the Iron River corner of the district the folds are generally
east trending with a westerly plunge, but several are doubly plunging.
In several examples, the limbs of folds are nearly sheared out and
several folds are overturned (Button, 1969). From the data presented
on MspMF 225, none of the structures extend over a great distance.
Folds converge with the elimination of one structure giving away to the
beginning of another larger one.

The troughs of synclines are the areas of concentration of the
iron ore. The concentrations are probably due to thickening-as well as
trapping of the oxidised are by the loss permeable footwell formations
(Clemments and Smyth, 1899). Also, it has been pointed out that in
these areas thickening of the footwsll strata has given rise to many
erroneous thickness measurements of the Wsuseca member. Several
authors (Button, 1949 and Janos, 1951) have stated that the Wlusecs
member has behaved almost plastically in its deformation.

Upon examination of the samples, many features are seen that can
be related to the deformation of the rock. This in turn has been super-
imposed upon what appears to be movement that took place during or
immediately subsequent to deposition. James (1958) has suggested that
the lower "speckled grey" phase of the Whusecs member may have been the
result of slumping. Evidence that is believed to support this will be

given later in a more appropriate section.

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-11-

Physical and Chemical
Shiironment of Deposition

It long has been accepted that black shales are deposited under
reducing conditions. Many authors have discussed such environments and
the factors involved in their existence. Most of the literature is
concerned with Paleozoic or later sediments and it is readily accepted
that bacteria of both aerobic and anerobic types play some part in
their formation. In the case of Precambrian rocks, organisms frequent-
ly have been suggested as a means of formation (Greener, 1922 and
James, 1951), but undisputable proof of such origin is difficult to
find.

James (195‘) has thoroughly discussed the pyrite bearing black
slates of the Iron River - Crystal Falls district and has outlined the
environment necessary for its deposition. Garrels (1960) has gone into
great detail to derive diagrams in terms of oxidation reduction potential
(as) and hydrogen ion concentration (pH) to show the stability fields of
several minerals in these terms. Pettijohn (1949) has summarized the
work of several authors in his discussion of euxinic onvirouents and
black shale deposition. Strdh (1939) has studied several lorwegian
fjords in which the dissolved gasses at the bottom are dominantly
hydrogen sulfide. These articles have been used as a guide in the
following discussion of environmental conditions, both physical and
chemical, which may have existed at the time of deposition of these
black slates.

numerous writers have pointed out that barred or restricted
marine basins are conducive to stagnation and the resultant depletion

of the oxygen from the deeper water. In his discussion of lorwegian

-12..

fjords, Strdh has shown how layering of the water will cut off the
circulation to the lower levels, thus likewise, cut off the oxygen
supply. In order to create conditions suitable for the primary
deposition of pyrite there must be a lowering of the oxidation
reduction potential to a point of at least zero on the scale and more
probably even into the reducing range. Therefore, there must be
material to oxidize, thus, consuming the available oxygen. Such
conceivably could be accomplished by two means.

Organic debris is material that finds its way to the bottom of
basins in a state capable of being oxidized. Any inorganic material
that is introduced into a depositional basin is normally in an
oxidized state and therefore is not usually capable of being further
oxidized. The other alternative is the oxidation of material that may
be introduced in a lower valence state through hydrothermal or volcanic
emanations. very likely, the change in organic matter takes place
through the action of anerobic bacteria or is at least aided by their
activity. Thus, there are two possible alternatives to which we might
ascribe the deposition of primary iron sulfides. The former alternative
has been used in part in the formulation of the disputed source bod
concept (Walpole, 1958 and 1960: and Knight, 1957).

The presence of carbon in abundance is not readily explained, at
least, by use of a hypothesis involving volcanic emanations. It
appears that the reduction of carbonates to their respective elements
by purely chemical means would require conditions more extreme than
those normally found in any sedimentary environment. The reduction of
more complex organic material to simpler molecules is typical of the
activity of bacteria. The final step of reducing these substances to

free carbon or graphite would probably require the more severe

conditions provided by the subsequent metamorphism of the rocks.

One of the objectives of this work is to discover evidence as to
which of the above mechanisms was most prevalent in the origin of both
the graphitic material and the pyrite. Love and Zimmerman (1961) have
shown that the Precambrian Mount Isa shale of Australia contains a
great deal of syngenetic pyrite and they have shown that its formation
.is involved with living organisms. Those rocks, from description and
photographs, appear to be somewhat similar to the rocks of this nature
found in the Iron River - Crystal Falls district. The similarity is
very suggestive of an organic relationship in the origin of these black
slates.

As is shown in Fig. 1, the oxidation potential of the water must
'ba near or below zero in order to precipitate pyrite. These conditions
are what James (1954) has referred to as the sulfide facies. The
Riverton iron formation immediately above the Wausecs member is in the
field of carbonate deposition. Near the top of the Wausecs member, one
sample was found showing primary pyrite and siderite in close associa-
tion. There is also abundant carbonaceous material, This is in
agreement with Gsrrel's (1960) diagrams which show that only slight
variations are necessary in 3h in the change from sulfide to carbonate
deposition. Evidently, the fields overlap to some degree. James
(1954) depicts the lower most part of the basin as that of sulfide
deposition; this is in agreement with Strdh (1939) and many others who
have studied such deposits. The conditions necessary for the deposition
of pyrite through inorganic processes should also be necessary for
biogenic deposition. This will be discussed in greater detail in a

later section.

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Fig. 1

Other than carbonaceous material and pyrite, the remainder of the
rock is probably composed of silica and argillaceous material. Chemical
analyses of several samples cited in the literature show that silica
ranges in content from thirty-five to forty-five percent. Little of
this can be seen in thin sections as the rocks for the most part are a

dense opaque black, but where thin sections can be used most of the

-15-

non-opaquomaterial is extremely fine-grained quartz. Alumina is
relatively abundant, showing the presence of some clay minerals which
undoubtably make up the remainder of the mineralogic composition.

Of those samples which are high in pyrite, the total of the
oxides that are most likely to occur in silicate are in the range of
50% to 60%. Considering that all of the.K20 1. in clay minerals,
their average amount can be estimated. From such estimates the clay
minerals make up from 20% to 30% of the rock; the remaining silicate is

than some form of quartz (probably deposited as chart).

Methods used

As originally proposed, this study was undertaken to discover
evidence relating to the origin of the graphitic slates and their
somewhat anomalous high content of carbon. To achieve this, several
methods of study were outlined which included patrographic analysis
and chemical methods which appeared applicable. Neither of these in
the exact sense of their meaning were found useful.

It was found that only in a few isolated cases were thin sections
of any use. The high contents of carbonaceous material and pyrite
rendered the sections opaque. To the writer’s knowledge, there is no
suitable method for removing such material as graphite from a this
section. Polished sections were then turned to as an alternative means
of descriptive study. These have proven to be valuable in the descrip-
tion and elucidation of many of the factors which have been applied to
the final conclusions.

During the study, it was found that the pyrite may have some

bearing upon the solution to the problem of origin of the graphite.

t

-16..

After a search of the literature for methods of studying the pyrite, it
was found that a comparison to the Mount Isa shales of Australia might
be instructive. The theory that the pyrite may have some relationship
to living organisms, as postulated by Love and Zimmerman (1961), was
investigated. Several attempts were made to isolate organisms by
methods outlined by the above authors from the pyrite but were un-
successful. A comparison of shapes and sizes of the pyrite spheres to
those of Mount Isa was resorted to, to show the striking similarities
which the two rock types possess.

It was hoped that a trace element analysis would be instructive to
the understanding of the environmental conditions under which deposition
of the graphitic rocks took place. There are several comparative meth-
ods which are being used which appear to show'marino versus non-marine
deposition. Unfortunately, it has been impossible to complete these
studies at this time. It is hoped that a trace element study can be
completed in the near future.

Several x-rsy powder photographs were made which have been very
helpful in understanding the present state of the graphite. Some of
these have been included in the epbfindix section.

An attempt was also made to isolate any organic material from.the
rock by methods outlined by Tyler (1957). The results of this experi-
ment were apparently negative, which seems to have some bearing upon the
failure to isolate any organisms from the pyrite.

No complete chemical analyses were made due to the complexity of
such work. As an alternative, several chemical analyses were obtain-
ed from litersture,four of which have been included in Appendix B.

'rhese have been instructive as to the mineral assemblage present as

‘9

-17-

well as bringing out the fact of the very high iron and carbon content
of the rock. This has made a basis for the discussion of the conditions

of deposition in relation to the conditions of the iron formation itself.

ROCK DESCRIPTIONS AND PETROLOGY

General Considerations

For descriptive purposes, two types of graphitic slates have been
defined. This division is based upon the pyrite content of the rock
which is manifested in the color and specific gravity. The first is
the highly pyritic type which is almost entirely limited to the
Wausecs member. .A somewhat similar pyritic slate is found in associa-
tion with the Bijiki iron formation north of Champion Station as well as
some lenses of black slate which occur within the lower Michigamme. The
second type recognized is non-pyritic. This is prhnsrily found near
the southern extremity of the Iron River - Crystal Falls district which
has been called the Florence district and at the Taylor Mine area.
Both the pyritic and non-pyritic slate can be further subdivided into
distinct groups depending upon the internal structure of the rock. The

subdivisions are as follow:

TABLE 2
( nodded - often breccisted and folded
Pyritic (
( Massive - containing numerous oonglomeratic
( fragments
( Bedded
Nom-pyrit ic (
( Massive

Although the actual composition of the bedded and massive types
is similar, the subdivision is necessary to describe the different his-

‘tories that may have affected each during and immediately subsequent to
-13-

-19-

deposition. Several structures show evidence of submarine slumping which
introduces what is thought to beimportant evidence upon the question of
primary or a secondary origin of the pyrite. Similar structure is found
in the non-pyritic type, which indicates some similarity of environment

for both.

Wausecs Member

The Wausecs member is high in pyrite and has a specific gravity
ranging fro-.2.3 g 3.1. Color varies somewhat from a light grey to
black, often modified by the typical brassy color of pyrite. Due to the
abundance of pyrite, the rock is much harder than the more graphitic
types and somewhat more brittle. Most of the samples found in the
Wausecs member have a conchoidsl or hackley fracture but some (in partic-
ular, the well-laminated) were found to split along bedding planes.

There are two specific types of structures found in the Wausecs
member: the lower massive and somewhat conglomerstic ”speckled grey”
which has a rather wide distribution (James, 1958) (Plate 1): and the
upper laminated (Plate 3). The ”speckled gray” is a massive homogeneous
rock containing irregular fragments of what appears to be an earlier
laminated slate. Commonly, the fragments are oriented with respect to
their longest axes suggesting the bedding direction. The fragments are
frequently darker in color than the matrix and in some cases much harder.
This was found to be due to quartz_or chart which in some instances made
up meat of the fragments. Others are almost entirely clay minerals and
quartz with enough graphite to give the dark color. Connonly, fragments
can be seen that are almost wholly pyrite.

Under high magnification, the matrix material appears to coatht

~20-

of submicroscopic matter which according to the chemical analyses is
probably clay minerals, quartz and carbonaceous material. In addition,
well-defined blobs of pyrite can be seen which show random shapes, some
being angular while others are well-rounded (Plate 7). It is frequent-
ly impossible to see any relationship between the pyrite and the enclos-
ing material. This form of pyrite will be discussed further in a later
section.

The laminated type (Plate 111) commonly referred to as being
stratigraphically above the ”speckled grey” (James, 1958) is noticeably
devoid of fragments although some breccistion is found. The individual
laminae are recognized by differences in pyrite, carbon content, as well
as by textural differences. Under the microscope, the highly pyritic
laminae appear similar to the speckled grey, whereas, the dark layers show
much less pyrite and are often entirely black. The boundaries of the
layers are sharp, but frequently show irregularities which might be
caused by contemporaneous deformation. The laminae have thicknesses
ranging from 1 to 4 mm., but in some cases are as much as s centimeter
thick.

In some of the samples from the Wausecs member a rather well
developed foliation may be found and in each case the pyrite appears
to be recrystallized. The pyrite is'thcn usually found in small
veinlets and irregular blobs rather than having an even distribution
throughout (Plate VI). When the rock is broken along a cleavage
plane, the broken surface usually has a glossy sheen similar to that
of graphite and has the characteristic graphite streak. Under the
microscope, this type appears similar to the speckled grey, but the

large veins and cubes of pyrite often contain a darker yellow, soft-9t

mineral believed to be chalcopyrite, showing the presence of copper. It
may be interpreted that this foliated slate has been subjected to a
higher degree of metamorphism due to folding. The possibility that
mineralising solutions accompanied the folding is indicated by the
chalcopyrite. The alternative is recognized that copper may have been

included in minor amounts in the original sediments.

Ron-Pyritic,~§ighly Graphitic Type

The non-pyritic slate commonly occurs in the southern extension
of the Iron River - Crystal Falls district as well as the Taylor Mine
area. The color is very dark bluish-grey to black and the streak of
the rock is graphitic and shiny. Due to the high content of carbon
and lack of pyrite, the specific gravity is much lower than that found
in the Wausecs member. The fracture/of the rock is irregular and
hackley in most instances, but where bedding is more evident it will
cleave along the bedding planes. In one outcrop a definite slaty
cleavage was noted; such occurrences are apparently rare and not
characteristic of the rock. Due tofthe high content of graphite, the
rock is soft and readily scratched by a sharp point. Nevertheless, it
is a very tough rock and is not easily broken by a blow of a hammer.

Commonly seen structures are bedding or laminations, and smell
rounded blebs surrounded by lighter grey weathered material which may
be the alteration product of original pyrite. Small veinlets of quartz
can often be seen. These have been interpreted as products of metamor-
phism. When inspected under a hand lens, the surface of the rock
appears spongy, which may be the result of weathering. In some samples,

folding of the laminae can be seen. This can be on a large scale

,./~.

involving bedding up to a centimeter thick and again on a scale so
small that a hand lens or low power microscope is necessary to see it.

The explanation of the lack of the pyrite in these samples is
somewhat of a problem. In each case, the outcrops are in close
association with iron formations, yet, they are notably low in iron.
There appear to be three choices of explanation: first, the iron may
never have been deposited; second, it may have been removed during the
metamorphism; and three, weathering may have been responsible for its
removal. lone of these are totally satisfactory as an all-inclusive
explanation.

Under the microscope, it appears as an extremely fine-grained
rock. For the most part, it appears_to consist of a randomly oriented,
flaky aggregate. Generally, bedding can be recognised and is usually
somewhat contorted. Some larger fragments are commonly seen which
appear to be composed of material similar to the remainder of the rock.
These are usually randomly distributed and fairly rare. A few flecks of
highly birefringent material is present. In-a few cases, small fragments

or blebs of pyrite can be seen.

Minor Structures

As previously discussed, the environment of deposition of black
shales is presumed to be in a restricted basin. This is well supported
by the presence of sulfide minerals as well as large amounts of carbon.
The shape of such a basin would have a great influence upon the final
characteristics of the material being deposited. The slope of the sides
. of the basin would control any movement of the sediments subsequent to

deposition. That such penecontemporaneous slumping has taken IV“°‘

"l

~23-

becomes apparent upon close inspection of the rock.

The angle of slope necessary for submarine slumping is variable.
depending upon the nature of the material composing the sediments
(Pettijohn, 1937). In some cases where sediments are high in wutor
content, slumping may occur on a slope of only a few degrees. The
mechanisms which cause such slumping may also be variable. The weight
of overlying sediments higher on the slope is a possible cause. A
sudden shock or earthquake is frequently considered as the cause for
such slumping. Regardless of the cause, it will be made evident that
slumping or contemporaneous deformation has taken place in the forma-
tion of many of the rocks of the Wausecs member.

One problem that is somewhat difficult is the determination of
origin of deformational structure found in many of the samples. Several
authors (Button, 1949 and Pottijohn, 1946,.1952) have discussed the
structure of the Iron River - Crystal Falls district and it is evident
that extensive folding has taken place. ‘Hany.of the structures found
in the samples appear to be related to tectonic folding, but several
are also randomly oriented and irregular in shape. The later type
appear to be a result of some type of slumping or movement contempor-
aneous or nearly so with deposition.

Tectonic folding would result in oriented drag folding along the
limbs of structures. Such minor folds are oriented with respect to the
deformational forces and consequently show a relationship to the major
structure as would the cleavage which may result (Pettijohn, 1957).
‘Unfortunately, none of the samples were oriented in the field and in
many cases the structure is not well known so the relationship of minor

to major structure cannot be determined. Few of the samples aha“ “Y

-24-

fracture cleavage, but in isolated cases a regular pattern of veining
can be seen. The typical structure that is associated with such
veining and cleavage is regular and continues over a reasonable distance
(two to three inches). In contrast to these regular structures, several
seem to have little or no continuity within a small area. This type of
structure consists of folding of beds in a random manner, folding of a
group of beds with no translation of the folding into adjacent beds and
folding of beds around fragments (Pettijohn, 1957).

Both types of structures cited above can be recognised in many of
the samples. There is no need here to prove the existence of tectonic
deformation, as this has been readily demonstrated in numerous instances.
The problem at hand is that of showing that contemporaneous movement or
slumping has taken place and is responsible for many of the structures.
found. Some of the criteria frequently used for this determination is
cited below.

l- lotable variations in thickness of the contorted sons (Miller,
1922)

2- A very irregular upper surface of the folded sons and a
rather regular under surface (Miller, 1922)

3- lulging of immediately overlying strata over little
anticlinal folds (Miller, 1922)

4— The distinct evidence of the filling of the depressions on
the upper surface of the corrugated some before the general
layers of overlying materials were laid down (Miller, 1922)

5— Discontinuity of the structures (Pettifiohn, 1957)

6- Breaking or crumpling of beds giving the effect of a
psoudoconglomerate (Dottijohn, 1957). .

Many of the above noted criteria are present in the Wausecs member
and much of the structure is interpreted to be due to slumping. find-

ing and flowing contemporaneous with deposition.

-25-

Some of the mechanisms of slumping and flowing have been noted.
Highly plastic materials as these sediments are presumed to have been
at the time of deposition, would be susceptible to movement from
minor effects. The rocks containing high amounts of pyrite, which was
probably forming during the early stages of burial, were probably
sediments of rather high density. Such heavy material would be subject
to movement or slumping with the slightest disturbance. Some of the
sediments in which chart was being precipitated along with large amounts
of pyrite may have been subject to early lithification. These deposits
may have been included in later slumping which caused their breccistion
or fragmentation. In many examples the results of such breccistion
can be seen and included with what was originally more plastic material.
It is difficult to determine if any such flow features are present in
the graphitic material from other areas. In some cases where bedding
is evident in the highly graphitic rock,folding can be seen. Criteria
for contomporaneous deformation is evident here, also, but is difficult
to show positively. This is due to a lack of contrast between the beds
and the very frequently'massive nature of the rock.

. In the cases where bedding can be observed in the highly graphitic
rocks, several structures can be seen. Stringers of material from the
finer grained beds. have been squeezed into the surrounding more coarse
beds. Small folds in a particular bed are not represented in adjacent
beds and folds in adjacent beds have no relationship to one another.

All of these characteristics are presumed to be typical of contempora-
neous deformation. ,
The presence of abundant fragments in the lower part of the

Vauseca member as described by James (1958) is again evidence of

slumping or submarine flow. Pettijohn (1957) has stated that, ”If the
solid/fluid ratio exceeds some critical value, Newtonian flow ceases
and the deposit is not graded.” This appears to be the case with the
"speckled grey” which shows fragments of several different sizes in

no particular gradational pattern. Thus, the suspension must have been
viscous enough to prevent normal settling according to Stoke's law.
Another factor that is readily noticeable is the orientation of the
fragments into a lineation which probably represents some sort of
“bedding” parallel to the direction of flow.

James (1958) has stated that, "The fragments in the massive type
are of the some rock type as the matrix.” This conclusion deserves
some comment. For the most part, these fragments are of a darker
color than the matrix and the pyrite in them is much more coarse. Up-
on closo examination, it was found that the fragments were high in chert
and clay minerals (some of which appear to be chlorito).

Two samples of the less pyritic andrgraphitic material from.be-
low the Wausoca member were examined and found to be similar in color
and nature to the fragments. They are more like the Dunn Creek slates,
in fact, one sample is from an area mapped as the Dunn Creek formation.

The above descriptions make evident a complex history which may
have some implication on the environment of deposition of the Wauseca
member. Again, reconsider the stratigraphy of the member.

The lower, primarily elastic Dunn Creek formation shows many
variations in conditions. Pettijohn (19‘6) has mapped much of this
formation on the east side of the basin south of Crystal Falls. no
notes pyritic beds as well as some carbonate iron formation. At the

horison immediately below the Wausecs member, the Dunn Creek formation

-27-

consists of a black slate composed of some pyrite, chert, and
argillaceous material along with small amounts of carbonaceous

material. The only difference here between the Dunn Crook itself and
the Wauseca appears to be the higher content of elastic material in the
Dunn Creek in comparison to the amount of carbonaceous matter and pyrite.

The lower part of the Wauseca member is a massive sometimes rather
choatic assemblage composed of fragments of black slate enclosed in a
typical highly pyritic matrix. The fragments are frequently surround-
ed by a swirl of the matrix material which suggests that the particle
was rolling or twisting as the slurry moved along the slope. The
upper part of the member is well laminated and frequently shows
disturbance of the beds. In some examples, the upper bedded material
shows little or no effects of disturbance. There are few fragments
included, but those which were observed were surrounded by the bedded
material which appeared to flow around them. A sharp transition to the
overlying iron formation is generally typical of the upper part of the
member (James, 1951).

In some cases, breccistion of the upper part of the member was
observed. This disturbance must have been subsequent to lithification
of at least part of the rock. 'The matrix material as well as the
fragments here are frequently similar to cherty iron formation, but
large amounts of pyrite as well as graphite are present. The above
observations suggest a sequence of events that might have been somewhat
as follows.

The inflow of elastic material gradually decreased during the
later stages of the Dunn Creek deposition so that with the beginning

of the deposition of the Wausecs member, only small amounts of clay

-28...

minerals were being introduced, thus causing the relatively high
carbon and pyrite content. As the lower part of the wauseca member
was deposited in a highly viscous state, some mobemsnt due to its
weight would begin to take place. If there were any intorbedding at

all, some of the lower more elastic material would be included to some

degree in the slump.

 

7

. IDEALIZED DRAWING
r SHOWING SLUMPINe
: IN THE
WAUSECA MEMBER

  
  
 

WAUSECA \7-

DUNN CREEK

 

 

 

 

Fig. 2.-'The lower stiplod represents the Dunn Creek and the
upper is the Wausecs. Some intorbedding is possible. The right side
of the figure shows how the Wausecs may have flowed across the basin
completely destroying the bedding, thus, forming the massive ”speckled

grey” phase of the Wausecs.
The first slumping would take place toward the center of the
basin due to overlying weight. With less support, the sediments high-

er on the slope would be included in the slumping. The reverse of

this could also be true with initial slumping higher on the slope

due to a greater angle of the slope. In either case, enough mixing
took place to give a resultant rock which has included fragments from
the underlying beds. At no time did the material become fluid

enough so that differential settling velocities were active.

As material was added to the lower part of the basin, the slump-
ing decreased to a minor amount so that only distortion of the bedding
resulted as opposed to the previous total destruction of any bedding.
Thus, as we go up in the column, very subtle changes take place at least
in some areas and more regular bedding becomes apparent. In the areas
where, as James (1958) describes, there is no bedded phase, rapid
slumping may have continued. In other areas bedding became dominant
in the upper part of the formation.

The fact that in all cases where any bedding is disturbed, the
bedding being shown by changes in pyrite content, the highly pyritic
beds are distorted and folded as well, giving good evidence for the
syngenetic origin of the pyrite. This fact the writer believes is
well displayed in each case and is the basis for much of the reasoning

in the following section.

ORIGIN OF THE PYRITE AND GRAPHITE

The previous discussion makes evident the primary nature of the
pyrite. The presence of large amounts of graphite as well as primary
pyrite is important criteria as to the environment of deposition. The
many primary sedimentary structures and the low amounts of elastic
material in these sediments also indicate the conditions under which
deposition occurred. Further evidence as to the origin of these
graphitic "slates” can be obtained from examination of the rock under
high‘magnification.

Several factors have been established which demonstrate the
primary nature of the pyrite. The shape and size of the individual
grains are believed to be significant (Love and Zimmerman, 1961). The
distribution of the pyrite in relationship to bedding and structure has
been used as criteria for the primary nature of such deposits (Love and
Zimmerman, 1961). The lack of any other sulfides* makes a hydrothermal
origin for the pyrite unlikely. The lack of other sulfides also makes
the theory of volcanic emanations into the depositional basin less
likely. An analysis for such trace elements as chlorine, fluorine
and other volcanic associates might provide valuable information

concerning such deposition.

 

*Some chalcopyrite was determined to be present in secondary
or recrystallised pyrite in association with veining and fissures.

-31—

Bvidence for a suitable environment for the preservation of
organic material during deposition can be obtained by considering the
pyrite. The stability fields of pyrite and siderite are shown in
Fig. 3. This diagram shows only those ranges which might be encounter-
ed under normal marine conditions. A third parameter which controls
the size of the stability fields is the concentration or more
specifically, the activity of the components. Referring to Fig.1,
note the line marked with a (.4) which parallels the hematite field.
This notes the increase of the stability fields when the activity of
iron is changed from It)"6 to 10“. It may be noted that this increase
has its major effect outside of the range of marine environments.

Also, note, that with very small concentrations of both iron and sulfur,

pyrite remains within the range indicated.

 

 
   
    

  

 

 

 

«+C12 i i f
1 + o I T ...
O _ Hemofite __

Fezos
Eh-OJ 4 -+
- 0.2a .n
-O.3‘ --
. SiderHe

-04 x ,L If e

6 7 8 9 l0

Fig. 3.-- Mbdified after Garrels (1960).

—3 2—

From this data, the conclusion may be drawn that it would be
normal for pyrite to occur in rocks deposited under conditions suitable
for the preservation of organic materials. The actual process by
which pyrite was found is difficult to explain, but based upon the
presence or organic material a suggestion may be made that the
association is more than incidental. Even though some organic
activity may be responsible for the formation of the pyrite, the
anerobic conditions would also be necessary for pyrite to be preserved.
Further evidence for such a relationship can be found in the nature of

the presently existing carbon.

 

 
   
   

__ P7 FF: _1
Id 53K‘CAL;M.
/0‘- . / n” - ‘

FJL P55“

4 ”flax/m. 535KC.L/M.

   

—

Ybnrs

Seco ndo

 

 

 

 

Pig. 4.- After Ablesom (1959).

I‘

-33—

The poorly crystalline nature of much of the graphite appears to
be evidence of organic origin of this material. See Plate VIII.
Ableson (1959) has discussed the association of organic material and
carbon in rocks and the stability of such organic compounds. As
shown in Fig. A, many organic compounds are stable to high temperatures,
but with great periods of time even the most stable compounds are
broken down at much lower temperatures. The kinetics of the reactions
which reduce organic materials to carbon indicate that temperatures
encountered in deep burial are sufficient to yield free carbon in a
period of time consistent with the age of these rocks. Tyler (1957)
has pointed out that the severity of metamorphism other than that
caused by folding in the Morrison Creek area is not great. From
evidence presented by the several authors who have worked in the Iron
River - Crystal Falls district, the metamorphism there is also of
limited extent in so far as temperatures are concerned. Yet, it can
readily be imagined that temperatures even as high as 150°C - 2009C
may have been reached, thus, providing the driving energy to dissociate
many of the organic compounds which may have been originally present in
the sediments. Further, it has.been shown that where the grade of
metamorphism has reached the extent where about 70% of the carbon is
jpresent as fixed carbon, very little in the way of organic materials
Inch as spores can be found (Wilson, 1961).

Such evidence as outlined above provides a reasonable basis for
believing that gradual decomposition of organic material produced the
free carbon. The process by which graphite is produced from amorphous
(carbon is unknown for geologic environments. Tyler (1957) has pointed

that that the Morrison Creek occurrences have been graphitised to about

-34...

the same extent as that produced by temperatures of 12509C in the
graphitization of coke. He further notes that there is no evidence

of temperatures of this magnitude found in this area. It is also safe
to say that the same is true of the entire assemblage which constitutes
the sedimentary sequence of the Iron River - Crystal Falls district.
Tyler (1957) further concludes that the graphitization of these
materials is probably more a function of high pressures and geologic
time, rather than of high temperatures. In the light of such processes,
causing the gradual reduction of the complex organic materials to
simpler compounds, ultimately lead to the production of amorphous
carbon and graphite.

Forty-one polished sections were studied in an effort to
characterise the nature of the pyrite and its associations with both
itself and the surrounding matrix. Several features have been noted
which appear significant. Grondijs and Schouten (1957) have, in their
study of the Mount Isa ores, classified the pyrite into five types
according to size, shape and to some extent its mode of occurrence.
These are as follow:

a. ‘Very fine grained ”spherical pyrite,” grain size

0.025 - 0.005 mm., mostly with concentric zonal
structure.

b. Fine grained pyrite in clean cut crystals, idomorphic

or idioblastic, grain size 0.02 - 0.005 mm. Variety

a may pass into b.

c. Very fine grained pyrite with skeleton-like shapes,
mostly smaller than 0.01 mm.

d. Coarse grained. pyrite in sphere like aggregates or in
spherolitic concretions, grain size, 0.25 - 0.05 mm.

o. Coarse grained aggregates of fragments of crystals.
8“. 0.5 - 0.05 -0

-35—

Grondijs and schouten (1937) note that types “a”, ”b" and ”c"
belong in the oldest generation of pyrite and are probably primary.
Each of the first three types are commonly found in the samples from
the Iron River - Crystal Falls district. As can be seen in Plate
VI,the typical concentric structure is seldom observed, but a lack of
homogeneity is evident. Grondijs and Schouten (1957) compared the
Mount Isa pyrite to that of the Meggen, Rsmmelsberg and Mansfield
districts of Germany and concluded, as have many who have studied the
German deposits, that both are syngenetic. However, they have pointed
out some differences between the German occurrences and the Mount Isa
ores. The pyrite in the German occurrences consist either of
aggregates of shell grains or of radial fibrous structure throughout.
The Mount Isa ores as well as the pyrite examined in this study
frequently show a massive core surrounded by a concentric ring.\ It has
also been pointed out that the aggregate type noted in the German
deposits istresent in the Mount Isa deposits (Love and Zimmerman, 1961).
This has also been noted in the present study of the Michigan occurrence.
Grondijs and Schouten also have pointed out that the common presence of
marcasite in the German occurrences is not the case with the Mount Isa
ores. Further, they have assigned a later origin to the small amounts
of marcasite which have been noted in their work. No marcasite has
been encountered in the present study nor has any pyrrhotite been
identified such as that which Grendijs and Schouten have determined
as the predecessor of marcasite in the Mount Isa ores. As noted abovi,
large clusters of pyrite have been observed in the Wausecs member but
Grondijs and Schouten's "type d” is absent as far as can be determined.

”Type e" is rather commonly found and is most frequent in the rocks

d

which more clearly show the effects of metamorphism, ie. show'more
definite fracture cleavage or foliation. This is undoubtedly the result
of recrystallization of earlier pyrite.

'Types ”a”, ”b" and "c" are the forms of pyrite that are of the
greatest concern in this study. They bear important implications
concerning the environment of deposition as well as some of the
mechanisms that were active during and immediately subsequent to
deposition. There are two alternative theories that may be suggested..
The first is the conclusion drawn by Grondijs and Schouten (1937) and
Schouten (1946), that the primary deposition of the pyrite was due to
some inorganic process. The second possibility is that a biogenic
process is responsible for deposition of both the pyrite and the
carbonaceous material.

Referring again to the Mount Isa shales, more intensive work
on the pyrite fraction of the rock than that of Grondijs and Schouten
(1957) was accomplished by Love and Zimmerman (1961). In their study,
intensive work was done to characterize the pyrite more accurately
than previously. Several size and shape analyses were made as well as
descriptions of the particles and the characteristic structures. In
many of their examples, etching proved to reveal a very well-defined
concentric structure as was previously described (Grondijs and Schouten,
1937).' Further, upon oxidation of the pyrite, numerous small, rounded
translucent bodies were freed and these were described as fossil micro-
organisms. The size of the bodies was comparable to that of the pyrite
spheres and in most cases comparable to the shape of the spheres from
which they were removed.

A size analysis of the pyrite from the Wausecs member is comparable

-37-
to that of the Mount Isa pyrite spheres. In a count of two hundred

spheres from two samples, the average size was 5.24 microns with the

mode falling between two and four microns. This consistency of size

can be noted throughout all of the samples from the Iron River -

Crystal Falls district.

 

 

 

 

 

 

10d-
MOUNT ISA PYRITE
90
so WAJSECA PY-RITE —-—-
70‘
60?
l 50*.
40
.w ’
20.
'10-
1‘ 2 3 I: '5 E II 33
SIZE X166”

 

 

 

Fig. 5.- Size analysis of pyrite spheres from Mount Isa,
Australia and Iron River - Crystal Falls district, Michigan. Size
data from.Mount Isa pyrite after Love and Zimmerman (1961).

The shape of the pyrite particles ranges from well-rounded to

sub-angular and random. There appears to be no variation in shape

from one sample to another. Many of the slightly larger, more
irregular shaped particles are the result of coalescing of two or

‘more smaller rounded spheres. This can often be seen much better

-38-

after etching for a short period of time (Plate V).

Two methods were used for etching the pyrite to bring out any
structure that may be instructive. Originally, stO‘ was used, but
it was found to be too fast and tended to over etch the sample or to
completely destroy the pyrite. A solution of'IZSO‘ and KMnO‘ was
found to be slower and produced much better results.

As previously noted, etching made possible the observation that
many of the larger rounded or random shapes were actually clusters of
smaller sphere-like particles. 0n the individual spheres etching
brought out a variety of patterns. The very well-defined concentric
pattern noted in the Mount Isa pyrite was seen in some examples but
more frequently other patterns were observed. Often the entire center.
of the grain would etch leaving only an outer shell. The reverse of
this often was seen where the center remains while most of the outer

“part is etched away. Often one or two wavy lines were observed in
the grain with a slight increase in width of the lines in the center
of the grain. In the more angular grains, the wavy lines were often
observed as well as a dark line around the inner core parallel or
nearly so to the outer edge of the particle.

A definite parallel comparison to the description of the Mount
Isa ores given by Love and Zimmerman (1961) or Grondijs and Schouten
(1957) would be difficult to make, but there are many similarities.
The first and probably most important, is that the spheres and other
irregular shapes are definitely hetrogeneous and subject to differential
etching. The etch patterns seem to follow some pattern not related to
crystal structure. Either the center or the outer part of the frag-

ments will etch away displaying some type of concentric structure.

-39-

as noted by Love and Zimmerman (1961), composite grains often occur
in the Mount Isa ores,this is also true in the Wausecs member.
Recrystallized pyrite was observed in several of the rock samples,
quotably those with a more definite foliation as a result of metamor-
phism. ‘Well-defined cubes were observed in a few cases in addition
to the random shape pyrite of that found in vein or deseminated deposits
of hydrothermal origin. In one case, excellent pressure shadows
composed of feathery quartz were found along the margin of the pyrite
(Plate 111, Fig.1). It appears that this type of pyrite is closely
related to zones of more intense shearing. In such cases, bedding is
no longer evident and the matrix material varies in color from black
to a dark grey. The pyrite is in small veinlets up to a millimeter in
thickness and two or three centimeters long, which parallel the
foliation.
Chalcopyrite is commonly associated with the vein-like pyrite
in small rounded blobs and along fissures (Plate VI, Fig. 2). It
appears to be later than the pyrite, and in most cases replaced it
along the cracks and fractures. Some crystalline quartz (as opposed
to chert) is commonly found in association with the pyrite either as
rounded blobs enclosed in the pyrite or along the edges. Graphite is
also found in small rounded blobs within the pyrite and in a massive
form around the pyrite (Plate VI, Figs. 1,2). It can be identified
easily as it has a tendency to smear over the pyrite during the polish;
ing. No other minerals have been identified with the pyrite other
than the gangue or other primary minerals, is. siderite or chert.
The paucity of such minerals as lead or zinc sulfides to some extent

attests to the primary nature of the pyrite as well as the lack of

mineralising solutions associated with deposition or metamorphimn.

The later pyrite is probably a response of the mineral to the stress
pressures and associated heat which caused the folding of the entire
basin rather than the result of metasomatism.or hydrothermal activity..

In the two areas in the‘Marquette district where samples were
taken, the rock appears to be of slightly higher metamorphic grade in
that it can be called slate. The pyrite is in definite beds which are
usually at some angle to the foliation. The carbon content appears to
be much lower and the typical graphitic streak is not apparent as it is
in the samples from the Wausecs member. ‘Most of the pyrite in these
samples can be classed with the ”type a," It occurs as well-defined
cubes and irregular shapes but almost always with euhedral or ido-
blastic outlines. In a small percentage of cases, spherical and
irregular outlined grains can be seen, but these are minor.

As in the examples of the “type e" pyrite in the Wausecs member,
these samples from other areas appear to show a definite response to
the metamorphic conditions. In each case the quarts that is associated
‘with the pyrite crystals is a coarse crystalline type rather than chert.
Pressure shadows of quartz were frequently observed attesting to the
dynamic metamorphic intensity. As with the Iron River - Crystal Falls
district, intense pressures rather than high temperatures appear to be
the major agent of metamorphism. ‘

As previously discussed, the preservation of carbon in rocks is
due to the lack of oxygen in the depositional environment, it being
one of slightly reducing conditions and near neutral pl values.
Further, the evidence suggests that the pyrite has a biogenic

associated origin. This is supporting evidence that the carbon was

-4 I.

originally in the form of organic materials.

Several x-ray powder patterns have been run which show that
the degree to which the graphite is crystallized is variable. It is
not exactly clear what causes this variation, but in most cases those
rocks which best show the effects of metamorphism.are the samples which
also show the sharpest and most lines on the powder pattern. A direct
correlation between perfection of crystallinity and degree of metamor-
phism.would seem reasonable but at this point would be premature.
Also, these rocks in which the slaty cleavage is best developed may
have been more readily sheared because of the presence of well-
crystallized graphite.

As shown in Plate VIII, most of the powder patterns show lines
of equal or better perfection than the manufactured graphite, but
there are examples which are less perfect. In any case the recrystalliza-
tion has continued to a point where it is unlikely that much evidence
of the original remains. Further,any unaltered organic material which
might be found in these rocks is most likely material that has
contaminated the rock.at some time subsequent to the metamorphism. One
of the procedures to determine the presence of organic material, used
by Tyler (1957) was followed for several samples and negative results
were obtained. This is not proof of the absence of such materials but
inasmuch as this procedure was the most effective for the Morrison
Creek ”coal" the absence of organic compounds is suggested.

The graphite appears to be evenly distributed throughout the
rock and shows little or no quantative relationship to the pyrite.
Frequently, flecks of strongly anisotropic material can be seen

which are tentatively identified as sericite and clay minerals. In

-42-

most cases the graphite and the associated minerals are so finely
disseminated that little can be seen under the microscope. Rounded
bodies which appear to be composed of graphite can be seen frequently,
but the graphite is predominately in disseminated form. In most .
examples, some parallel orientation can be seen which is probably
foliation.

As shown in Plate VI, Fig, 2, rounded blobs of graphite can be
seen within the massive recrystallized pyrite. The rounded masses are
homogeneous and show no structure. They occur as inclusions within the
recrystallized pyrite which incorporated them during its recrystallization.
The shape of the graphite inclusions again suggests an organic origin

of the presently existing graphite.

CCNCLUSION

0n the basis of the proceeding discussion, several conclusions
have been drawn concerning both the environment of deposition and the
origin of the rather unusual constituents of the graphitic slates.

Few of these conclusions are original, mostly they support previous
work, some of which was more general than this particular study.
Much of this study has been descriptive and comparative in nature
which leads to the fact that the results are somewhat preliminary;
Many questions remain unanswered and in at least one case, a new
question has arisen which poses an interesting problem.‘

The environment of deposition plays an important role in many
of the conclusions that are to be drawn. The minerals present
indicate that the environment must have been one of negative oxidation
reduction potential if the pyrite is syngenetic. Thus, the environment
would be such that the decomposition of organic material would be
negligible. Further, if the pyrite was primary in the rock, a compar-
ison of this environment to presently existing reducing environments
appears to be valid. ‘

From the proceeding evidence, it has been concluded that the
pyrite is syngenetic and that the graphite which is now found included
in the rock is probably all that remains of large quantities of’
original organic material. The x-ray powder photographs of the graphite
show that much of it has been crystallized to a fairly high degree,

although some of these show a rather poor crystalline nature. This

-43-

-4fi— '

variable nature of the graphite is probably the result of either
varying intensity of metamorphism or varying effects of metamor-
phism on slightly different rock types. ‘With the restricted number
of samples used in the study it is difficult to see any definite
correlation with metamorphimn.

The nature of the pyrite in the rock is probably the most
interesting factor to come out of the problem. The many similarities
to the pyrite found in the Mount Isa shales is very suggestive that
both types are the result of similar histories. The writer feels that
the conclusion drawn by Love and Zimmerman should not be accepted with-
out reservation for these samples at this time because of some lack of
evidence. The isolation of fossil micro-organisms has not been
accomplished and until this is done the conclusion must be deferred.
Regardless of this, the many similarities of the pyrite from the two
areas are striking and inference as to the origin of the pyrite in
the Iron River - Crystal Falls district is interesting.

James (1950) has suggested that much of the Wausecs member may
have been subjected to marine slumping. Observations made in this
study appear to support this with one modification. James has implied
a catastropic moeement, thus, making at least the lower part of the
wauseca member a well definedwmarker unit. He has based his reasoning
on the fact that the lower part of the Wausecs member is a choatic
rock containing numerous fragments in a massive matrix and that little
bedding is evident. This is true in many examples but several sections
were also observed in which thinly bedded material was either overlain
or underlain by typical massive rock (Plate II, Fig. 1). Also the

small scale disturbances of the thin beds as shown in Plate I, Fig. 2,

suggests smaller scale movements. The total destruction of bedding is

 

-45.
certainly suggestive of more catastrophic slumping, but again
could be a function of the state of the material when the disturbance
took place. The writer suggests that rather than the occurrence of
one or a few large scale slumps, that many smaller scale slumps may
have occurred.

One question which remains unanswered is the notable lack of
pyrite in several samples from the vicinity of Florence, Wisconsin.

As noted earlier, there are several alternatives which may be taken,
but none of them.appear wholly satisfactory. The massive nature of
many of the samples seems to rule out the possibility that weathering
has been responsible for removal of any pyrite, although the samples
were all taken from.sufface outcrops.

From the diagram showing the stability fields of pyrite, it can
be seen that pyrite could be deposited at extremely low concentrations
of both iron and sulfur. The close relation that most of these
occurrences have with iron formations is not compatible with the
theory that iron was not present in the water at the time of depositon.
A lack of sulfur is equally incompatible in that the high amounts of
carbon suggest that there was nuch organic debris in the basin at the
time of formation. Organic materials should be an adequate source of
sulfur.

This leads to the final alternative that the iron mineral was
removed from the rock at the time of metamorphism of the rock. ‘This is
not wholly acceptable in that the degree of metamorphism in this area
does not appear much greater than other parts of the Iron River -
Crystal Falls district. Further, there is much pyrite present in the

samples from the Marquette district and these appear to have been

-45-

subjected to a high degree of metamorphism.

One other.possibility that may be considered is that of an
inhibiting action taking place in the environment of the highly
graphitic rocks which prevented the'procipitation of pyrite. It is
notable, also, in the samples collected by Tyler in the Morrison Creek
area that iron bearing minerals are negligible, even though the ”coal”
is surrounded by typical pyritic rock. Such a suggestion would need

much investigation and is not offered as a solution to the problem.

Suggestions for Further Study

It has become apparent that the pyrite has provided the best
evidence for the understanding of these rocks. Further intensive
study with a greater number of samples would be required to provide
definite proof of the suggested pyrite-graphite relationship. The
comparison of the pyrite to the Mount Isa pyrite has been instructive
and in the writer's opinion the pyrite may very likely represent fossil
micro-organisms. More work in this light may also have some bearing
upon the deposition of siderite and the other iron minerals found in

the Lake Superior region.

REFERENCES CITED

ABLESOM, P. n.. 1959, Geochemistry of organic substances: Researches
in Geochemistry, New'York, John Wiley and Sons, Inc.

BASSBECKIMG, L. G. M., and MOORE, D., 1961, Biogenic sulfides: Econ.
Geol., vol. 56, p. 259-272.

omens, J. M. and 3mm, u. 1... less, The Crystal 17.11: iron bear-
ing district of Michigan: U.S. Geol. Surv. Mono. #56.

BUTTON, C. 8.. 1949, Geology of the central part of the Iron River
district, Iron County, Michigan, D. S. Geol. Cir. #45.

, 1961, personal communication.

RDUARDS, A. 3., and IAKER, 6., 1951, Some occursnces of supergone
iron sulfides in relation to their environments of deposition:
Journ. of Sod. Pet., v. 21, p. 54.

FLEMIMG, R. E. and REVELLE, R., 1959, Physical processes in the ocean,
recent marine sediments (a symposium)? Am. Assoc. Petrol. Geol.

GARRELS, R. M., 1960, Mineral equilibre: Mew York, Harper and Bros.

GRONDIJS, E. F. and SCHOUTEN, C., 1957, A study of the Mount Isa ores:
Econ. Geol., v. 52, #4.

GRUNER, J. W., 1922, The origin of sedimentary iron formations: the
Biwebik formation of the Mesabi range: Econ. Geol., v. 17,
p. 407.

JAMES, H. L., 1951, Iron formation and associated rocks in the Iron
River district, Michigan: Geol. Soc. America Bull., v. 62,
pp. 251-266.

, 1954, Sedimentary facies of iron formation: Econ. Geol.,
v. 49, #5, pp. 255-295. ‘

JAMES, M. L.. 1958, Stretigraphy of Pre—Keweenewan rocks in parts of
Northern Michigan: 0. S. Geol. Surv. Prof. Paper #514C, p. 58.

JAMES,‘B. L., BOTTOM, C. E., PETTIJOHM, F. J., and WIER, K. L., 1959,
Geologic map of the Iron River-Crystal Falls district, Iron
county, Michigan: U. S. Geol. Surv., Mineral Investigations
Field Studies Map, MP 225.

~46-

‘47-

KNIGHT, C. L., 1957, Ore genesis - the source bed concept: Econ.
Geol., v. 52, pp. 808-917.

KRUMBEIN, W. C. and GARRELS, R. M., 1952, Origin and classification
of chemical sediments in terms of pH and oxidation reduction
potentials: Journ. Geol., v. 60, pp. 1-50.

LOVE, L. G., 1957, Micro-organisms and the presence of syngenetic
pyrite: Geol. Soc. London, Quart. Journ., v. 115, pp. 429-457.

LOVE, L. G., and ZIMMERMAN, D. 0., 1961, Bodded pyrite and micro-organ-
isms in the Mount Isa shale: Econ. Geol., v. 56, #5.

MILLER, W. J., 1922, Intraformational corrugated rocks: Journ. Geol..
v. 50, p. 587.

MANZ, R. E.. 1955, Composition of Precambrian slates with notes on
geochemical evolution of lutites: Journ. Geol., v. 61.

PETTIJOHM, F. J., 1946, Geology of the Crystal Falls - Alpha iron
bearing district, Michigan: U. S. Geol. Surv. Stratigraphic
Minerals Investigations Prelim., Map 5-181.

, 1949, Sedimentary rocks: New York, Harper and Bros.

, 1952, Geology of the northern Crystal Falls area, Iron
County, Michigan: U. 8. Geol. Surv. Circu. #155.

SCMOUTEN, C., 1946, the role of sulfur bacteria in the formation of
the so-called sedimentary copper ores andepyrite ore bodies:
Econ. Geol., v. 41, #5.

STRCM, K. M.,l959, Landlocked waters and the deposition of black muds,
Recent marine sediments: A Symposium Am. Assoc. Pet. Geol.

TYLER, J. A., BRAGHOORN, E. S., and BARRETT, L. P., 1957, Anthracite
coal from Precambrian uppor‘Huronian black shale of the Iron
River district, Northern Michigan: Geol. Soc. of Am. Bull.,
v. 68, pp. 1295-1504.

VAN HISE, C. R. and LEITH, C. R., 1911, Geology of the Lake Superior
region: U. S. Geol. Surv. Mon. 52.

WALPOLE, B. P., 1958, Discussion - The source bed concept: Econ.
Geol., v. 55, p. 890-895.

, 1960, Discussion - the source bed concept: Econ. Geol.,
v. 55, pp. 615-617.

WARREN, E. E., 1956, x-ray study of the graphitizstion of carbon
black: Proceedings of the First and Second Conference on
Carbon, University of Buffalo, New York.

WILSON, L. R., 1961, Palynological fossil response to low-grade
metamorphism in the Arkoma basin: Report on Research,
University of Oklahoma.

APPENDIX A

 

LIST OF COLLECTIOR SITES ON MAP 1

Taylor mine area

Champion Station (Bijiki)
Dead River Basin (Michigamme)
Hoist Dam Road (Michigamme)

Menominee District

.fiB-

11.

12.

13.
14.
15.
16.
17.
18.
19.
20.
21.

22.

or

LIST OF COLLECTIM SITES 0N MAP 2

Morrison Creek "Coa1"- SE} of SE}, Sect. 21*

Sherwood Mine

Sunset Lake core- NE} of SE}, Sect. l7, T45M, R54W

Bates Fire Tower Core- SE}, as}, Sect. 27, T45N, 234w
Bristol Mine, horizontal core

Crystal Falls mine dump and test pits-- SE} of ME} Sect. 21
Dunn Creek slate outcrop- SW} of NW}, Sect. 8

Road cut south of Alpha- SW} of NE}, Sect. 15.

Brulo River exposures- SW} of NE}, Sect. 8

Exposures along abandoned RR grade NE of Florence-- NW} of SE}
Sect. 21

Road cut south of Commonwealth (slate) SW} of SW}, Sect. 54

Ridge east of Keyos Lake: slate low in graphito-- NE} of NW}
Sect. 6.

Ridge just north of x.,.. Lake-- SE} of SE}, Sect. 25

Larson exploretion- NW} of NW}, Sect. 55

Roadcut along Wisconsin, Rt. 70-- ME} of ME}, Sect. 54

Section SW} of MW}, Sect. 26

Section SE} of ME}, Sect. 22

Section SW} of NW}, Sect. 25

Section SW} of ME}, Sect. 22

Greek bed 100 yards north of Brulo School-- SE} of SW}, Sect. 14
Road Exposure east of Brulo School-- SE}of SE}, Sect. 14

Section SW} of SW}, Sect. 15

 

*Ranges and townships are noted in Map 2.

_:§9'

1 2 3 4
Si02 47.32 58.05 45.80 36.67
1‘102 0.53 0.64 0.41 0.39
A1205 7.99 15.00 8.26 6.90
3.205 0.24 3.67 2.40 ---.
sec 0.42 5.82 0.91 2.35
a.” 0.07 0.03 12.01 18.031
MnO ---- 0.09 trace trace
ugo 0.32 1.64 --... 0.65
CaO 0.03 0.26 0.15 0.13
Nazo 0.03 ’ 3.52. 0.07 0.26
x20 7 2.19 3.60 2.20 1.81
P205 trace 0.16 0.05 0.20
n20 -10598 13.21 3.46 4.05 1.25
320 -10500 2.11 0.84 0.91 0.55
co2 0.06 0.03 0.07 4----
c 22.68 3.27 10.46 7.60
so3 0.38 0.14 0.89 2.60
s 2.71 0.04 13.77 20.67

APPEEDIX B

 

TABLE 3

 

 

CHEMICAL COMPOSITIOfl

 

*

 

 

*After Mans (1955) Journ. of Cool. Vol. 61.
1- Upper footwell, Judson Mine, Crystal Fells, after Benz
2- Footwall road cut location 14, after lens
5- Footwall just below IF furnished by E. L. James after
Manz
4- Laminated footwall after James

Fe included in pyrite

-50-

APPENDIX C

PLATE I

Fig. 1-- Typical massive "speckled grey.” Note the
suggestion of parallel alignment of the elongated fragments.
x 5.2.

Fig. 2--'Eighly disturbed bedded type containing several
fragments of less pyritic rock. Mote swirls of bedded matrix
around each fragment. x 5.2.

-51-

PLATE I

 

 

‘ PLATE 11

Fig. 1- Bedded type, probably overlying more massive type
showing how beds seem to thin over the fragment and thicken in the
depressions on either side. Large fragment at the bottom of the
picture is almost wholly pyrite. x 5.2.

Fig. 2-- Highly disturbed bedded sample showing several
differences in textured characteristics. x 5.2.

-53... 'i

PLATE II

 

PLATE III

Fig. l.-- Well bedded type exhibiting very sharp undisturbed
contacts between beds. Largo cubes of pyrite have apparently form-
ed from recrystallization of earlier pyrite in those beds where
the pyrite is found. Beds containing pyrite are almost wholly
composed of quartz. Note several pressure shadows of quartz
around several of the crystals. x 5.2.

Fig. 2.- Bedded pyrite showing some results of deformation.
x 5.2. '

-55-

PLATE III

 

PLATE IV

Fig. l.-- Highly graphitic sample showing effects of
deformation which is probably penecontomopaneous. Bedding is
well shown as a result of variation of texture. x 5.2.

Fig. 2.- Highly weathered sample which is well bedded,
Mote numerous small crenulations in the laminae near the
bottom of the photo. x 5.2. ‘

-57- _«

PLATE IV

 

Fig. l

 

Fig. 2

-58-

PLATE V

Fig. 1.-- Unotched fairly well rounded grains. x 1000 oil
immersion.

Fig. 2.-- Unotched. Ragged shapes showing heterogeneous
nature. x 1000 oil immersion.

Fig. 5.- Unotched. Shows variation in shapes from well
rounded to sub-angular. x900 oil immersion.

Fig. 4.- Etched sample. Much of this photo was originally
a large composite grain. x1000 oil immersion.

Fig. 5. and‘Fig. 6.- Slightly etched samples showing several
shapes on grain surfaces. x1000 oil immersion.

-59-

PLATE V

 

 

PLATE VI

Fig. l.-- Metamorphosed slate from Sunset Lake core shows
irregular shapes of recrystallized pyrite (light grey). Variable
color of the matrix material can also be soon. x 5.2.

Fig. 2.-- Same sample as Fig. l in polished section. Light
grey is pyrite. Medium gray in vein-like replacement is chalce-
pyrite. Darker grey rounded blobs are graphite. Note how the
graphite has smeared out along the surface from the rounded areas.
x70.

Fig. 5.-- Crystal of pyrite from.same sample as Plate 111,
Fig. l. Etching has brought out crystallographic structure as
opposed to irregular structures found in the much smaller primary
spheres. x70.

Fig. 4.- Thin section from.the upper part of the Wausecs
member. Note outlines of carbonate rhombs which are being replaced
by chlorite. Black irregular shapes are pyrite. Light grey
ground mass is chert with some finely disseminated graphite includ-
ed. Uncrossed nicols. x70.

PLATE" VI

 

 

Fig. 2

Fig. l

-62.-

 

Fig. 4

Fig. 3

PLATE VII

Fig. 1.-- Thin section showing finely disseminated graphite
included in chert. Circular shapes pose an interesting problem.
x500 oil immersion. '

Fig. 2.- Thin section showing change in content of graphite
(white) giving rise to bedding. x70.

Fig. 5.- Highly graphitic sample containing very little
pyrite. Some regular shapes are seen but much of the material
is in random orientation. x500 oil immersion.

-63-

PLATE VI I

 

PLATE VIII

X-ray powder photographs of several samples of purified
graphite from graphitic slates.

7-

Referenco spectographic graphite
Graphite from lower Michigamme slate
Sunset Lake Core, Wausecs Member
Reference same as # 1

Highly graphitic sample

Wausecs Member from Sherwood Mine

Highly graphitic sample from unknown location
in the Florence, Wisconsin area.

The first three samples were run for three hours, the last

four for two hours.

Note the variation in intensity in the 200 and 201 lines,
i.e., second and third lines outside of the intense line in #4.

-65-

 

-55-

*‘s-

4-

oI
‘t

i

146 2595

1293 03

3

 

iIHIWlllHIIIHW