Field Trip Guidebooks Volume 34, Part 2 - Lakehead...

Thirty-fourth Annual MeetingMarquette, MichiganMay l2and 13, 1988

Field Trip GuidebooksVolume 34, Part 2

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Institute on Lake Superior Geology

34th ANNUAL

INSTITUTE ON LAKE SUPERIOR GEOLOGY

Field Trip Guidebooks

Volume 34, part 2

Marquette, Michigan

May 12 and 13, 1988

Organized By

John Hughes, Northern Michigan UniversityJohn Klasner, Western Illinois UniversityKlaus Schulz, U. S. Geological Survey

Edited by Klaus Schulz

Cover: Line drawing of Scandinavian-style headframe and modern head—frame at Cleveland Cliff's Cliffshaft iron mine, Ishpeming,Michigan.

1

34th ANNUAL

INSTITUTE ON LAKE SUPERIOR GEOLOGY

Field Trip Guidebooks

Volume 34, part 2

Marquette, Michigan

May 12 and 13, 1988

Organized By

John Hughes, Northern Michigan University John Klasner, Western Illinois University

Klaus Schulz, U. S. Geological Survey

Edited by Klaus Schulz

Cover: Line drawing of Scandinavian- style headframe and modern headframe at Cleveland Cliff's Cliff shaft iron mine, Ishpeming, Michigan.

TABLE OF CONTENTS

FIELD TRIP 1

An Introduction to Archean Geology and Precious MetalMineralization of the Marquette Greenstone Belt, Michigan.

Geological Overview of the of the Marquette GreenstoneBelt, Michigan

T.J. Borrihorst Al - Al8

Geological Framework of a Part of the MarquetteGreenstone Belt North of the Dead River Storage Basin

D.A. 3axter and M.L. MacLellan A19 - A31

Geology of the Ropes MineR.A. Brozdowski A32 — A53

Geological Field Trip to the Marquette Greenstone Belt:Part I, Day 1 Road Log - Stops 1 to 11

TJ. Bornhorst, D.A. Baxter, M.L. MacLellari, andR.C. Johnson A54 - A64

Geological Field Trip to the Marquette Greenstone Belt:Part II, Day 2 Road Log - Stops A to E

T.J. Bornhorst and D.A. Baxter A65 - A71

Geological Field Trip to the Marquette Greenstone Belt:Part III, Callahan Mining Corporation Ropes MineProperty

R.A. Brozdowski and G.W. Scott A72 A73

FIELD TRIP 2

Marquette mineral district of Michigan, mining historyand geology.

Burton H. Boyum and Robert C. Reed Bi — B15

Marquette mineral district of Michigan with emphasis onMINING HISTORY and GEOLOGY

Burton H. Boyum, Robert C. Reed and Wm. Kangas B16 - B33

FIELD TRIP 3

A structural traverse across a part of the Penokean orogenillustrating Early Proterozoic overthrusting in northernMichigan: text and fieldcuide.

John S. Klasner, Paul K. Sims, Wm. J. Gregg andChristina Gallup Cl - C36

1

TABLE OF CONTENTS

FIELD TRIP 1

An Introduction to Archean Geology and Precious Metal Mineralization of the Marquette Greenstone Belt, Michigan.

Geological Overview of the of the Marquette Greenstone Belt, Michigan

T.J. Borrlhorst ................................... A1 - A18

Geological Framework of a Part of the Marquette Greenstone Belt North of the Dead River Storage Basin

D.A. Baxter and M.L. MacLellan.................... A19 - A31

Geology of the Ropes Mine .................................. R.A. Brozdowski. ~ 3 2 - ~ 5 3

Geological Field Trip to the Marquette Greenstone Belt: Part I, Day 1 Road Log - Stops 1 to 11

T.J. Bornhorst, D.A. Baxter, M.L. MacLellan, and R.C. Johnson...................................... A54 - A64

Geological Field Trip to the Marquette Greenstone Belt: Part 11, Day 2 Road Log - Stops A to E

T.J. Bornhorst and D.A. Baxter ..................... A65 - A71

Geological Field Trip to the Marquette Greenstone Belt: Part 111, Callahan Mining Corporation Ropes Mine Property

R.A. Brozdowski and G.W. Scott....... ............. A72 - A73

FIELD TRIP 2

Marquette mineral district of Michigan, mining history and geology.

........... Burton H. Boyum and Robert C. Reed....... Bl - B15

Marquette mineral district of Michigan with emphasis on MINING HISTORY and GEOLOGY

Burton H. Boyum, Robert C. Reed and Wm. Kangas ...... B16 - B33

FIELD TRIP 3

A structural traverse across a part of the Penokean orogen illustrating Early Proterozoic overthrusting in northern Michigan: text and fieldquide.

John S. Klasner, Paul K. Sims, Wm. J. Gregg and Christina Gallup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cl - C36

FIELD TRIP 1

Geological Overview of theMarquette Greenstone Belt, Michigan

T.J. Bornhorst

Department of Geology and Geological Engineering,Michigan Technological University, Houghton, Michigan 49931

INTRODUCTION

The name Marquette Greenstone Belt is applied to the belt of dominantly

Archean volcanic rocks which underlies an area of about 125 mi2 (325 km2) in

northern Marquette County, Michigan (Figure 1). The belt occupies the eastern

part of the northern complex, an extensive terrane of Archean granite-greenstone

(750 mi2; 1950 km2) north of the Marquette trough. Lower Proterozoic sediments,

which fill the Marquette trough and overlie the complex, bury the Great Lakes

Tectonic Zone, a major regional Archean structural zone described by Sims (1980).

The Marquette Greenstone Belt represents the southwestern extension of the Wawa

Subprovince of the Superior Geologic Province of the Canadian Shield as defined

by Card and Ciesielski (1986). The specific focus of this overview (and of the

accompanying field trips) is the greenstone belt, which occupies about one-fifth of

the entire Archean northern complex.

The Marquette Greenstone belt Consists of several thousand feet of subaqueous

mafic to silicic flows and pyroclastics, and volcaniclastic sediments. These are

intruded by gabbro and rhyolite dikes and by granitoid plutons. The belt also

includes two intrusive peridotite bodies, one in vicinity of the Ropes Mine and

the other at Presque Isle. All these rocks are Archean, about 2700 to 2500 Ma

old (Morgan and DeCristoforo, 1980; Trow, 1979). Their metamorphic grade

ranges from greenschist to amphibolite facies and they have been subjected to

multiple deformation during the Archean. There are numerous gold and base

metal occurrences throughout the belt (Bodwell, 1972) but the Ropes Mine is the

most significant to date (Brozdowski, this volume; Bornhorst and others, 1986;

Brozdowski and others, 1986). The Archean rocks are unconformably overlain by

A-i

FIELD T R I P 1

Geological Overview of the Marquette Greenstone Belt, Michigan

T.J. Bornhorst

Department of Geology and Geological Engineering, Michigan Technological University, Houghton, Michigan 49931

INTRODUCTION

The name Marquette Greenstone Belt is applied to the belt of dominantly

Archean volcanic rocks which underlies an area of about 125 mi2 (325 km2) in \

northern Marquette County, Michigan (Figure 1). The belt occupies the eastern

part of the northern complex, an extensive terrane of Archean granite-greenstone

(750 mi2; 1950 km2) north of the Marquette trough. Lower Proterozoic sediments,

which fill the Marquette trough and overlie the complex, bury the Great Lakes

Tectonic Zone, a major regional Archean structural zone described by Sims (1980).

The Marquette Greenstone Belt represents the southwestern extension of the Wawa

Subprovince of the Superior Geologic Province of the Canadian Shield as defined

by Card and Ciesielski (1986). The specific focus of this overview (and of the

accompanying field trips) is the greenstone belt, which occupies about one-fifth of

the entire Archean northern complex.

The Marquette Greenstone belt consists of several thousand feet of subaqueous

mafic to silicic flows and pyroclastics, and volcaniclastic sediments. These are

intruded by gabbro and rhyolite dikes and by granitoid plutons. The belt also

includes two intrusive peridotite bodies, one in vicinity of the Ropes Mine and

the other at Presque Isle. All these rocks are Archean, about 2700 to 2500 Ma

old (Morgan and DeCristoforo, 1980; Trow, 1979). Their metamorphic grade

ranges from greenschist to amphibolite facies and they have been subjected to

multiple deformation during the Archean. There are numerous gold and base

metal occurrences throughout the belt (Bodwell, 1972) but the Ropes Mine is the

most significant to date (Brozdowski, this volume; Bornhorst and others, 1986;

Brozdowski and others, 1986). The Archean rocks are unconformably overlain by

Figure 1: Regional geological setting of the Marquette Greenstofle Belt which

is shown in solid black.

A—2

Keweenawan

EarIy Proterozoic

::i::Archean granitoids and gneiss

A Archean

0 50I

km

'SSouthern Complex

Paleozoic

Figure 1: Regional geological setting of the Marquette Greenstone Belt which is shown in solid black.

and, in fault contact with, Lower Proterozoic sediments.

The name Ishpeming Greenstone Belt was proposed for these same rocks by

Morgan and DeCristoforo (1980). However, this name is abandoned here in favor

of Marquette Greenstone Belt because these Archean rocks do not crop out in

Ishpeming (Lower Proterozoic sediments crop out in Ishpeming) whereas they do

crop out in the City of Marquette. Further, the belt is located in northern

Marquette County.

The rooks of the belt have been studied and discussed by numerous geologists.

For a historical background the reader is referred to the work of Morgan and

DeCristoforo (1980). This overview is based on work by the author and his

students, and on work by Gair and Thaden (1968), Puffett (1974), Clark and

others (1975), Cannon and Klasner (1977), and on the geological studies summa-

rized by Baxter and MacLellari. (this volume).

STRATIGRAPHY

As documented by the literature from other parts of the Superior Province,

stratigraphic correlations across regional faults or shear zones are quite tenuous and

subject to error. The stratigraphy of the Marquette Greenstone Belt will be

discussed in two separate parts because the belt is traversed obliquely by the Dead

River Shear Zone, a probable major fault (Figure 2). For purposes of description,

the greenstone belt has been subdivided into a northern and a southern part; other

faults necessitate that the southern part be further divided into an "eastern

two-thirds" and a "western one-third".

Northern Part

The rocks of the northern part of the belt have been described by Baxter and

MacLellan (this volume). The rocks are dominated by pillowed and massive basalt

lava flows with interbedded mudflow breccias and iron-formations; this assemblage

has been named the Volcanics of Silver Mine Lakes (Figure 2). These rocks are

cut by dikes and sill-like bodies of gabbro and all are cut by dikes of rhyolite

A-3

and, in fault contact with, Lower Proterozoic sediments.

The name Ishpeming Greenstone Belt was proposed for these same rocks by

Morgan and DeCristoforo (1980). However, this name is abandoned here in favor

of Marquette Greenstone Belt because these Archean rocks do not crop out in

Ishpeming (Lower Proterozoic sediments crop out in Ishpeming) whereas they do

crop out in the City of Marquette. Further, the belt is located in northern

Marquette County.

The rooks of the belt have been studied and discussed by numerous geologists.

For a historical background the reader is referred to the work of Morgan and

DeCristoforo (1980). This overview is based on work by the author and his

students, and on work by Gair and Thaden (1968), Puffett (1974), Clark and

others (1975), Cannon and Klasner (1977), and on the geological studies summa-

rized by Baxter and MacLellar (this volume).

STRATIGRAPHY

As documented by the literature from other parts of the Superior Province,

stratigraphic correlations across regional faults or shear zones are quite tenuous and

subject to error. The stratigraphy of the Marquette Greenstone Belt will be

discussed in two separate parts because the belt is traversed obliquely by the Dead

River Shear Zone, a probable major fault (Figure 2). For purposes of description,

the greenstone belt has been subdivided into a northern and a southern part; other

faults necessitate that the southern part be further divided into an "eastern

two-thirds" and a "western one-third".

Northern Part

The rocks of the northern part of the belt have been described by Baxter and

MacLellan (this volume). The rocks are dominated by pillowed and massive basalt

lava flows with interbedded mudflow breccias and iron-formations; this assemblage

has been named the Volcanics of Silver Mine Lakes (Figure 2). These rocks are

cut by dikes and sill-like bodies of gabbro and all are cut by dikes of rhyolite

LEG

EN

D

PR

OT

ER

OZ

OIC

II

SE

DIM

EN

TS

GRANITOID ROCKS

ARCHEAN

DEAD RIVER PLUTON

AGE RELATION UNCERTAIN

PERIDOTITE

I

KIT

CH

ISCHIST

MONA SCHIST

______

LIG

HT

HO

US

EPOINT MEMER

ARCHEAN

I1

SH

EA

RE

DRHYOLITE TUFF MEMBER

NEALY CREEK MEMBER

LOWER BASALT MEMBER

VOLCANICS OF SILVER MINE LAKES

Q z w 0 Ill -J

0 m E- it: 3 u 1-1 0 n4

a; ca z $

0 n E- Of.

z 0 n

2 1-1 Cd Of.

Of. Ed z Cd z E- z u 0 n. Cd m "3 0 a:

E- E-i 01 x u u a: 1-1 u 1-1 vs

4 z 0 z

ca w Ln s Cd 2:

Ill Ill 3 E-

w E- u 1-1 0 ?-i s Of.

0 Cd

2 Cd a: m

Of. Cd rn z Cd z ^ Cd Cd K u x 2 Cd z

m Cd x 4 1-1

Ed z u z Of. Cd > 1-1 n m

Ill 0

m u n

2 u 1-1

0

Figure 2:

Geology of the Marquette Greenstone Belt, Michigan.

Southern extents of the Dead River

and Carp River Falls Shear Zones are shown.

Numbers and letters represent stops for field

trips (this volume).

Compiled from:

Cannon and Kiasner (1977), Clark and others

(1975),

Puffett (1974), Bodwell (1972) and Gair and Thaclen

(1968).

and plutons of granodiorite. Granitoid plutons intrude the northern part of the

greenstone belt to the northeast (see Day 2 Road Log, Stop E).

The granitoid rocks of the Marquette Quadrangle were mapped as Compeau

Creek Gneiss by Gair and Thaden (1968). This name has also been assigned more

recently to similar-looking crystalline iocks in the region south of the Marquette

trough (Figure 1), and in essence, signifies "undifferentiated plutonic rocks of

Marquette County". The name Compeau Creek Gneiss should be used only in that

context or prhaps, be abandoned.

Southern Part

The rocks of the southern part of the belt, south of the Dead River Shear

Zone, have been mapped by Gair and Thaden (1968), Puffett (1974), Clark and

others (1975), and Cannon and Klasner (1977). Much of the following is based

on their work. The major lithologic units (Figure 2) in the southern part of the

belt are: Mona Schist, Kitchi Schist, Deer Lake Peridotite, Dead River Pluton and

other unnamed granitoid plutons (mapped as Compeau Creek Gneiss and as gra-

nite).

Relative age relations among these major lithologic units are uncertain. The

contact between the Mona and Kitchi Schists was mapped as a shear zone by

Clark and others (1975). In addition, stratigraphic top direction within a few

miles of the contact indicates that Mona Schist faces NE whereas the Kitchi Schist

faces SE. Rossell (1983) has suggested that the contacts of the Deer Lake

Peridotite are structural and not stratigraphic, making age relations uncertain.

Although contacts between the plutonic rocks and volcanics rocks are commonly

faults or shear zones, the plutonic rocks in close proximity to the greenstone belt

can be interpreted to intrude the volcanics, based on smaller intrusive relationships

and contact metamorphic features.

In view of the fact that faults complicate the study of the stratigraphy of

even the southern part of the greenstone belt, it is necessary to resort to

A-7

and plutons of granodiorite. Granitoid plutons intrude the northern part of the

greenstone belt to the northeast (see Day 2 Road Log, Stop E).

The granitoid rocks of the Marquette Quadrangle were mapped as Compeau

Creek Gneiss by Gair and Thaden (1968). This name has also been assigned more

recently to similar-looking crystalline rocks in the region south of the Marquette

trough (Figure l), and in essence, signifies "undifferentiated plutonic rocks of

Marquette County". The name Compeau Creek Gneiss should be used only in that

context or perhaps, be abandoned.

Southern Part

The rocks of the southern part of the belt, south of the Dead River Shear

Zone, have been mapped by Gair and Thaden (1968), Puffett (1974), Clark and

others (1975), and Cannon and Klasner (1977). Much of the following is based

on their work. The major lithologic units (Figure 2) in the southern part of the

belt are: Mona Schist, Kitchi Schist, Deer Lake Peridotite, Dead River Pluton and

other unnamed granitoid plutons (mapped as Compeau Creek Gneiss and as gra-

nite).

Relative age relations among these major lithologic units are uncertain. The

contact between the Mona and Kitchi Schists was mapped as a shear zone by

Clark and others (1975). In addition, stratigraphic top direction within a few

miles of the contact indicates that Mona Schist faces NE whereas the Kitchi Schist

faces SE. Rossell (1983) has suggested that the contacts of the Deer Lake

Peridotite are structural and not stratigraphic, making age relations uncertain.

Although contacts between the plutonic rocks and volcanics rocks are commonly

faults or shear zones, the plutonic rocks in close proximity to the greenstone belt

can be interpreted to intrude the volcanics, based on smaller intrusive relationships

and contact metamorphic features.

In view of the fact that faults complicate the study of the stratigraphy of

even the southern part of the greenstone belt, it is necessary to resort to

describing the stratigraphy of the even smaller "eastern two—thirds" and "western

one-third" areas.

Eastern Two-Thirds. The eastern two-thirds of the southern part of the belt

consists of the Mona Schist (Figure 2). This name should perhaps be modified in

the future to the "Mona Group" because the Mona Schist can be subdivided into

units (currently termed members) at 1:24000 scale which should be elevated to

formation status. In addition, the rocks that are mapped as the Mona "Schist"

have an identifiable protolith and consequently, according to the North American

Code of Stratigraphic Nomenclature, the parent lithology term should be used in

the name instead of schist. Nevertheless, for present purposes the term Mona

Schist will be retained as will the currently recognized four members: Lower

Member, Nealy Creek Member, Sheared Rhyolite Tuff Member, and Lighthouse

Point Member.

The Lower Member of the Mona Schist consists mainly of subaqueous massive

and pillowed tholeiitic basalt lava flows (see Day I Road Log, Stops 1 and 2, this

volume) with minor interbedded volcaniclastic and tuffaceous rocks. The member

crops out in a 25 mi2 (65 km2) area and is over 10,000 feet (3300 m) thick,

assuming there is no structural repetition. Stratigraphic top direction in the Lower

Member is to the north.

The Nealy Creek Member is considered to ove lie the Lower Member. It

consists of a variety of quartz—chlorite-sericite-feldspar schists which crop out in a

7 mi2 (18 km2) area. The schists appear to be mostly greywacke and shale (see

Day 1 Road Log, Stop 11, this volume), but in general the schistose texture

prevents recognition of volcaniclastic or tuffaceous textures. This member

(perhaps 3000 feet (1000 m) thick) is the only significant sedimentary unit in the

greenstone belt.

The Sheared Rhyolite Tuff Member is considered to overlie the Nealy Creek

Member. It consists of light-colored, porphyritic volcanic rocks which crop out in

A- 8

describing the stratigraphy of the even smaller "eastern two-thirds" and "western

one-third" areas.

Eastern Two-Thirds. The eastern two-thirds of the southern part of the belt

consists of the Mona Schist (Figure 2). This name should perhaps be modified in

the future to the "Mona Group" because the Mona Schist can be subdivided into

units (currently termed members) at 1:24000 scale which should be elevated to

formation status. In addition, the rocks that are mapped as the Mona "Schist"

have an identifiable protolith and consequently, according to the North American

Code of Stratigraphic Nomenclature, the parent lithology term should be used in

the name instead of schist. Nevertheless, for present purposes the term Mona

Schist will be retained as will the currently recognized four members: Lower

Member, Nealy Creek Member, Sheared Rhyolite Tuff Member, and Lighthouse

Point Member.

The Lower Member of the Mona Schist consists mainly of subaqueous massive

and pillowed tholeiitic basalt lava flows (see Day 1 Road Log, Stops 1 and 2, this

volume) with minor interbedded volcaniclastic and tuffaceous rocks. The member

crops out in a 25 mi2 (65 km2) area and is over 10,000 feet (3300 m) thick,

assuming there is no structural repetition. Stratigraphic top direction in the Lower

Member is to the north.

The Nealy Creek Member is considered to overlie the Lower Member. It

consists of a variety of quartz-chlorite-sericite-feldspar schists which crop out in a

7 mi2 (18 km2) area. The schists appear to be mostly greywacke and shale (see

Day 1 Road Log, Stop 11, this volume), but in general the schistose texture

prevents recognition of volcaniclastic or tuffaceous textures. This member

(perhaps 3000 feet (1000 m) thick) is the only significant sedimentary unit in the

greenstone belt.

The Sheared Rhyolite Tuff Member is considered to overlie the Nealy Creek

Member. It consists of light-colored, porphyritic volcanic rocks which crop out in

a 1.3 mi2 (3.4 km2) area. The common lithology consists of tabular fragments of

porphyritic rhyolite surrounded by fine-grained matrix of quartz, sericite and

chlorite. Phenocrysts are quartz and feldspar, and make up 10 to 50 percent of

the rhyolite (Puffett, 1974). In some outcrops there are granitic fragments. This

rock was interpreted by Puffett (1975) as an intensely deformed pyroclastic

deposit.

The Lighthouse Point Member lies north of the Nealy Creek Member and

crops out in a 7 mi2 (18 km2) area. This member encompasses two different

lithologies: relatively coarse-grained amphibolite, and layered amphibolitic schist

(see Day 2 Road Log, Stops C and D, this volume). Morgan and DeCristoforo

(1980) have interpreted the coarse-grained amphibolite to be gabbro. The layers

in the layered amphibolite are interpreted as flattened pillows and the material

between the layers is often well-foliated. The flattened pillows may be due to

primary processes or to deformation. This author suggests that much of the

flattening is due to high strain.

Western One-Third. The western one-third of the southern part is underlain

by Kitchi Schist and Deer Lake Peridotite (discussed under Intrusive Rocks)

(Figure 2). The name "Kitchi Schist" should likewise be changed to "Kitchi

Group". The Kitchi Schist crops out in a 12 mi2 (30 km2) area and consists of

three dominant lithologies. To the east of the Deer Lake Peridotite it consists of

quartz-sericite phyllites and schists, commonly containing feldspar phenocrysts and

lithic fragments, (see Day 2 Road Log, Stop A, this volume) and coarse breccias

(see Day 2 Road Log, Stop B, this volume). The phyllites and schists are andesite

to dacite in composition and are interpreted as interbedded pyroclastic fall and

flow deposits, volcaniclastic sediments, and lava flows. The coarse breccias consist

of porphyritic andesite-dacite clasts surrounded by quartz-sericite-chlorite matrix.

The breccias are mostly poorly sorted, but locally along the south shore of Deer

Lake layering is conspicuous (Brozdowski, 1988, personal communication). They

A- 9

a 1.3 mi2 (3.4 km2) area. The common lithology consists of tabular fragments of

porphyritic rhyolite surrounded by fine-grained matrix of quartz, sericite and

chlorite. Phenocrysts are quartz and feldspar, and make up 10 to 50 percent of

the rhyolite (Puffett, 1974). In some outcrops there are granitic fragments. This

rock was interpreted by Puffett (1975) as an intensely deformed pyroclastic

deposit.

The Lighthouse Point Member lies north of the Nealy Creek Member and

crops out in' a 7 mi2 (18 km2) area. This member encompasses two different

lithologies: relatively coarse-grained amphibolite, and layered amphibolitic schist

(see Day 2 Road Log, Stops C and D, this volume). Morgan and DeCristoforo

(1980) have interpreted the coarse-grained amphibolite to be gabbro. The layers

in the layered amphibolite are interpreted as flattened pillows and the material

between the layers is often well-foliated. The flattened pillows may be due to

primary processes or to deformation. This author suggests that much of the

flattening is due to high strain.

Western One-Third. The western one-third of the southern part is underlain

by Kitchi Schist and Deer Lake Peridotite (discussed under Intrusive Rocks)

(Figure 2). The name "Kitchi Schist" should likewise be changed to "Kitchi

Group". The Kitchi Schist crops out in a 12 mi2 (30 km2) area and consists of

three dominant lithologies. To the east of the Deer Lake Peridotite it consists of

quartz-sericite phyllites and schists, commonly containing feldspar phenocrysts and

lithic fragments, (see Day 2 Road Log, Stop A, this volume) and coarse breccias

(see Day 2 Road Log, Stop B, this volume). The phyllites and schists are andesite

to dacite in composition and are interpreted as interbedded pyroclastic fall and

flow deposits, volcaniclastic sediments, and lava flows. The coarse breccias consist

of porphyritic andesite-dacite clasts surrounded by quartz-sericite-chlorite matrix.

The breccias are mostly poorly sorted, but locally along the south shore of Deer

Lake layering is conspicuous (Brozdowski, 1988, personal communication). They

are interpreted as subaqueous mudflows and pyroclastic flow deposits. To the

west of the Deer Lake Peridotite, the Kitchi Schist consists of mafic volcanic and

gabbroic rocks which may underlie the intermediate composition volcanic rocks to

the east (Brozdowski, this volume; Brozdowski and others, 1986). Stratigraphic top

direction in the Kitchi Schist is consistently south (Brozdowski, 1988, personal

communication).

Intrusive Rocks. The Deer Lake Peridotite, now predominantly a serpentinite,

crops out in' a 2 mi2 (5 km2) area (see Field Guide to Callahan Mining

Corporation Ropes Mine Property, this volume), and is surrounded by Kitchi

Schist (Figure 2). Serpentine pseudomorphs after olivine and pyroxene are

common and the rock contains euhedral magnetite. In some serpentinite there are

textures which are indicative of cumulate origin. Foliated serpentinite occurs near

the contacts and at the Ropes Mine. Near the Ropes Mine, the serpentinite is

altered also to a talc-carbonate rock (Bornhorst and others, 1986). Bornhorst and

others (1986) interpret the Deer Lake Peridotite as an discordant intrusive body

with structural contacts whereas Brozdowski and others (1986) agree on the overall

intrusive nature of the body but interpret its contacts as being more concordant

and hence more sill-like.

Dikes of Archean gabbro cut the Mona and Kitchi Schists. Where recognized,

these intrusions were mapped most commonly as metadiabase of uncertain age

relations (Puffett, 1974; Gair and Thaden, 1968). Dikes of rhyolite cut the basalt

and gabbro, but the age relationship between the rhyolite dikes and the granitoid

plutons is less clear; only a few similar-appearing rhyolite dikes cut the plutons.

The textures of the rhyolite dikes are porphyritic (most common), aphanitic, and

granular (least common). Porphyritic rhyolite is composed of phenocrysts of

quartz with or without feldspar set in a very fine-grained groundmass. The

granular rhyolite dikes show a textural continuum to granitoid plutons.

Undifferentiated granitoid plutons (southwest part of Figure 2) and the Dead

A—1O

are interpreted as subaqueous mudflows and pyroclastic flow deposits. To the

west of the Deer Lake Peridotite, the Kitchi Schist consists of mafic volcanic and

gabbroic rocks which may underlie the intermediate composition volcanic rocks to

the east (Brozdowski, this volume; Brozdowski and others, 1986). Stratigraphic top

direction in the Kitchi Schist is consistently south (Brozdowski, 1988, personal

communication).

Intrusive Rocks. The Deer Lake Peridotite, now predominantly a serpentinite,

crops out i d a 2 mi2 (5 km2) area (see Field Guide to Callahan Mining

Corporation Ropes Mine Property, this volume), and is surrounded by Kitchi

Schist (Figure 2). Serpentine pseudomorphs after olivine and pyroxene are

common and the rock contains euhedral magnetite. In some serpentinite there are

textures which are indicative of cumulate origin. Foliated serpentinite occurs near

the contacts and at the Ropes Mine. Near the Ropes Mine, the serpentinite is

altered also to a talc-carbonate rock (Bornhorst and others, 1986). Bornhorst and

others (1986) interpret the Deer Lake Peridotite as an discordant intrusive body

with structural contacts whereas Brozdowski and others (1986) agree on the overall

intrusive nature of the body but interpret its contacts as being more concordant

and hence more sill-like.

Dikes of Archean gabbro cut the Mona and Kitchi Schists. Where recognized,

these intrusions were mapped most commonly as metadiabase of uncertain age

relations (Puffett, 1974; Gair and Thaden, 1968). Dikes of rhyolite cut the basalt

and gabbro, but the age relationship between the rhyolite dikes and the granitoid

plutons is less clear; only a few similar-appearing rhyolite dikes cut the plutons.

The textures of the rhyolite dikes are porphyritic (most common), aphanitic, and

granular (least common). Porphyritic rhyolite is composed of phenocrysts of

quartz with or without feldspar set in a very fine-grained groundmass. The

granular rhyolite dikes show a textural continuum to granitoid plutons.

Undifferentiated granitoid plutons (southwest part of Figure 2) and the Dead

River Pluton cut the greenstone belt. The Dead River Pluton is a composite

pluton and consists of granodiorite, diorite, and syenite (Puffett, 1974). The

southwestern granitoid plutons have been poorly studied, but most are probably

granodiorite. At the Peppin Prospect (3.5 miles NW of the Ropes Mine), Boben

(1986) described a trondhjemite. The Archean granitoid plutons are interpreted as

syn- to post-tectonic.

GEOCHEMISTRY

There aie now over 200 major and trace element analyses from the Marquette

Greenstone Belt. Most trace element data have been determined by XRF and

about 20 by INAA. Geochemically the basaltic lava flows and gabbroic intrusives

are tholeiitic, whereas the andesite to dacite fragmental rocks and rhyolite

intrusives are calc-alkalic. Although variable, the chemical compositions of the

basalts and gabbros overlap to the extent that they must be considered composi-

tionally similar, and to have similar petrogenetic histories.

To demonstrate that the belt is dominantly basalt with a scattering of

compositions towards rhyolite, Bornhorst and Baxter (1987) multiplied the surface

area of each major unit of the greenstone belt (as an indication of volume

relations) by the representative average chemical composition. Of the lesser

number of silicic rocks, more compositions fall near the andesite-dacite boundary

than on either side, i.e., the belt is very mildly bimodal.

The few analyses of granitoid plutonic rocks suggest that the granitoids are

caic-alkalic in affinity. On a variety of compositional diagrams, the rhyolite dikes

that cut the greenstone belt lie on similar trends with the plutonic rocks and

overlap the chemical composition of the more silicic plutons. The rhyolite dikes

are interpreted as late-stage, generally more evolved parts of the plutons that

intrude the greenstone belt.

METAMORPHISM

The rocks of the greenstone belt were metamorphosed from greenschist to

A-li

River Pluton cut the greenstone belt. The Dead River Pluton is a composite

pluton and consists of granodiorite, diorite, and syenite (Puffett, 1974). The

southwestern granitoid plutons have been poorly studied, but most are probably

granodiorite. At the Peppin Prospect (3.5 miles NW of the Ropes Mine), Boben

(1986) described a trondhjemite. The Archean granitoid plutons are interpreted as

syn- to post- tectonic.

GEOCHEMISTRY

There are now over 200 major and trace element analyses from the Marquette

Greenstone Belt. Most trace element data have been determined by XRF and

about 20 by INAA. Geochemically the basaltic lava flows and gabbroic intrusives

are tholeiitic, whereas the andesite to dacite fragmental rocks and rhyolite

intrusives are calc-alkalic. Although variable, the chemical compositions of the

basalts and gabbros overlap to the extent that they must be considered composi-

tionally similar, and to have similar petrogenetic histories.

To demonstrate that the belt is dominantly basalt with a scattering of

compositions towards rhyolite, Bornhorst and Baxter (1987) multiplied the surface

area of each major unit of the greenstone belt (as an indication of volume

relations) by the representative average chemical composition. Of the lesser

number of silicic rocks, more compositions fall near the andesite-dacite boundary

than on either side, i.e., the belt is very mildly bimodal.

The few analyses of granitoid plutonic rocks suggest that the granitoids are

calc-alkalic in affinity. On a variety of compositional diagrams, the rhyolite dikes

that cut the greenstone belt lie on similar trends with the plutonic rocks and

overlap the chemical composition of the more silicic plutons. The rhyolite dikes

are interpreted as late-stage, generally more evolved parts of the plutons that

intrude the greenstone belt.

METAMORPHISM

The rocks of the greenstone belt were metamorphosed from greenschist to

amphibolite facies during the Archean. During the Penokean orogeny, the

Archean rocks were subjected to greenschist facies metamorphism and, in areas of

amphibolite facies, this Penokean metamorphism is denoted by retrograde assem-

blages. Archean regional greenschist facies was elevated to the amphibolite facies

in relatively restricted areas.

STRUCTURE

Archean structural geology of the Marquette Greenstone Belt is dominated by

E-W, NW-SE, to N-S faults, but some data suggest that the belt has been sub-

jected to multiple deformations. For example, from north of the Dead River

Storage Basin, Johnson and others (1987) have described an Archean, steeply-

plunging, synformal anticline with dimensions on the order of miles. This fold

requires at least two periods of Archean deformation. Similarly, the Mona-Kitchi

structural relationship described earlier requires at least two periods of Archean

deformation.

The Archean rocks are overlain by Early Proterozoic supracrustal rocks which

were deformed during the Penokean orogeny (1900 to 1800 Ma). Although reacti-

vation of Archean structures during the Penokean orogeny can be documented

clearly, various lines of evidence suggest that most of the structure in the

greenstone belt represents Archean deformation.

Local Faults and Shear Zones

In the Marquette Greenstone Belt most stratification dips steeply; dips near

vertical (+1_be) are quite common. The rocks are variably foliated, but on both

a local and regional scale there are linear zones along which the foliation is better

developed than in surrounding rocks. These zones of higher strain may be faults

or shear zones. Where stratigraphic units are offset they are mapped as faults.

North of the Dead River Storage Basin many high strain zones are too small

to show at 1:6000 (1 inch equals 500 feet). Faults are of brittle-ductile and

ductile types, and displacement may be mostly vertical. These faults are also of

A-12

amphibolite facies during the Archean. During the Penokean orogeny, the

Archean rocks were subjected to greenschist facies metamorphism and, in areas of

amphibolite facies, this Penokean metamorphism is denoted by retrograde assem-

blages. Archean regional greenschist facies was elevated to the amphibolite facies

in relatively restricted areas.

STRUCTURE

Archean structural geology of the Marquette Greenstone Belt is dominated by

E-W, NW-SE, to N-S faults, but some data suggest that the belt has been sub-

jected to multiple deformations. For example, from north of the Dead River

Storage Basin, Johnson and others (1987) have described an Archean, steeply-

plunging, synformal anticline with dimensions on the order of miles. This fold

requires a t least two periods of Archean deformation. Similarly, the Mona-Kitchi

structural relationship described earlier requires at least two periods of Archean

deformation.

The Archean rocks are overlain by Early Proterozoic supracrustal rocks which

were deformed during the Penokean orogeny (1900 to 1800 Ma). Although reacti-

vation of Archean structures during the Penokean orogeny can be documented

clearly, various lines of evidence suggest that most of the structure in the

greenstone belt represents Archean deformation.

Local Faults and Shear Zones

In the Marquette Greenstone Belt most stratification dips steeply; dips near

vertical (+/-lo0) are quite common. The rocks are variably foliated, but on both

a local and regional scale there are linear zones along which the foliation is better

developed than in surrounding rocks. These zones of higher strain may be faults

or shear zones. Where stratigraphic units are offset they are mapped as faults.

North of the Dead River Storage Basin many high strain zones are too small

to show at 1:6000 (1 inch equals 500 feet). Faults are of brittle-ductile and

ductile types, and displacement may be mostly vertical. These faults are also of

two general ages: older roughly E-W faults truncated by younger N-S faults

(Johnson and others, 1987).

South of the Dead River Storage Basin, Clark and others (1975) interpreted

the contact between the lithologically distinct Kitchi and Mona Schists to be a

shear zone (Figure 2), and contacts of the Deer Lake Peridotite have also been

interpreted as either faults or shear zones (Clark and others, 1975; Rossell, 1983).

Regional Faults and Shear Zones

The Marquette Greenstone Belt is affected by two regional Archean shear

zones (Figure 2). Extending along the entire southern boundary of the greenstone

belt is the Carp River Falls Shear Zone (Puffett, 1974). Rocks along this shear

zone are both intensely foliated and altered (see Day I Road Log, progression

from Stops 1 to 3, this volume). However, the full width of the shear zone is

not known since a part of it was reactivated during the Penokean orogeny,

juxtaposing more altered and sheared Archean rocks against less deformed Early

Proterozoic sediments. The exposed width of this zone is up to 1500 ft (450 m)

as documented by detailed mapping (Brozdowski, 1988, personal communication).

Since this shear zone is cut by much less deformed mafic dikes of Archean age

(Baxter and Bornhorst, 1988), it must be Archean in age. This shear zone may be

part of the Great Lakes Tectonic Zone of Sims (1980).

The second major structural zone cuts through the middle of the greenstone

belt and is termed the Dead River Shear Zone (Puffett, 1974) (Figure 2). As

recognized by Puffett (1974), the shear zone lies within the Sheared Rhyolite Tuff

Member of the Mona Schist (near the east end of the Dead River Storage Basin).

In this area detailed gravity and magnetic data are consistent with the interpreta-

tion of a steeply dipping shear zone (Weeks, 1987). The Dead River Shear Zone

can be extended to the west it was reactivated during the Penokean orogeny,

juxtaposing Archean and Early Proterozoic rocks in a manner similar to the Carp

River Falls Shear Zone. In the west, the existence of the shear zone is indicated

A-13

two general ages: older roughly E-W faults truncated by younger N-S faults


South of the Dead River Storage Basin, Clark and others (1975) interpreted

the contact between the lithologically distinct Kitchi and Mona Schists to be a

shear zone (Figure 2), and contacts of the Deer Lake Peridotite have also been

interpreted as either faults or shear zones (Clark and others, 1975; Rossell, 1983).

Regional Faults and Shear Zones

The Marquette Greenstone Belt is affected by two regional Archean shear

zones (Figure 2). Extending along the entire southern boundary of the greenstone

belt is the Carp River Falls Shear Zone (Puffett, 1974). Rocks along this shear

zone are both intensely foliated and altered (see Day 1 Road Log, progression

from Stops 1 to 3, this volume). However, the full width of the shear zone is

not known since a part of it was reactivated during the Penokean orogeny,

juxtaposing more altered and sheared Archean rocks against less deformed Early

Proterozoic sediments. The exposed width of this zone is up to 1500 f t (450 m)

as documented by detailed mapping (Brozdowski, 1988, personal communication).

Since this shear zone is cut by much less deformed mafic dikes of Archean age

(Baxter and Bornhorst, 1988), it must be Archean in age. This shear zone may be

part of the Great Lakes Tectonic Zone of Sims (1980).

The second major structural zone cuts through the middle of the greenstone

belt and is termed the Dead River Shear Zone (Puffett, 1974) (Figure 2). As

recognized by Puffett (1974), the shear zone lies within the Sheared Rhyolite Tuff

Member of the Mona Schist (near the east end of the Dead River Storage Basin).

In this area detailed gravity and magnetic data are consistent with the interpreta-

tion of a steeply dipping shear zone (Weeks, 1987). The Dead River Shear Zone

can be extended to the west it was reactivated during the Penokean orogeny,

juxtaposing Archean and Early Proterozoic rocks in a manner similar to the Carp

River Falls Shear Zone. In the west, the existence of the shear zone is indicated

by well-foliated and altered Archean rocks along the Archean and Early Protero-

zoic contact (see Day 1 Road Log, Stop 5, this volume). It is proposed that the

eastward extension of this shear zone follows the Lighthouse Point Member of the

Mona Schist which is interpreted as having been subjected to high strain (see

Road Log Day 2, Stops C and D). Thus, the Dead River Shear Zone represents a

NW-SE trending fault, with unknown displacement, that bisects the Marquette

Greenstone Belt.

PRECIOUS METAL MINERALIZATION

Of the numerous base and precious metal occurrences in the belt (Bodwell,

1972), the Ropes Mine is the most significant orebody to date (Figure 2). The

Ropes Mine has been described by Bornhorst and others (1986) and Brozdowski

and others (1986) and in this volume by Brozdowski. It has been mined for Au

and Ag and has a Au/Ag ratio of about 1 to 5. Metallic minerals are dominated

by pyrite, with lesser other sulfides such as tetrahedrite, galena, and chalcopyrite.

Certain parts of the Ropes Mine show a good correlation between abundance of

Au and fine pyrite (Brozdowski, this volume). Non-metallic alteration minerals

are dominated by quartz and carbonate with varying amounts of sericite, chlorite

and talc. The ore host rock for the Ropes orebody is interpreted by Bornhorst

and others (1986) and Brozdowski and others (1986) as a dacitic volcaniclastic rock

within the Deer Lake Peridotite. Both Brozdowski and Bornhorst agree that the

mineralization is epigenetic, but differ in their interpretation of mechanism of

emplacement. Brozdowski and others (1986) suggest that precious metals were

concentrated by hydrothermal fluids related to emplacement of peridotite sills

(Deer Lake Peridotite). Bornhorst and others (1986) suggest that the orebody was

syntectonic and that hydrothermal fluids followed a zone of relatively high strain,

a shear zone. Brozdowski (this volume) describes a structural genetic model for

the Ropes orebody.

Other less important occurrences of mineralization present as disseminations

A- 14

by well-foliated and altered Archean rocks along the Archean and Early Protero-

zoic contact (see Day 1 Road Log, Stop 5, this volume). It is proposed that the

eastward extension of this shear zone follows the Lighthouse Point Member of the

Mona Schist which is interpreted as having been subjected to high strain (see

Road Log Day 2, Stops C and D). Thus, the Dead River Shear Zone represents a

NW-SE trending fault, with unknown displacement, that bisects the Marquette

Greenstone Belt.

PRECIOUS 'METAL MINERALIZATION

Of the numerous base and precious metal occurrences in the belt (Bodwell,

1972), the Ropes Mine is the most significant orebody to date (Figure 2). The

Ropes Mine has been described by Bornhorst and others (1986) and Brozdowski

and others (1986) and in this volume by Brozdowski. It has been mined for Au

and Ag and has a Au/Ag ratio of about 1 to 5. Metallic minerals are dominated

by pyrite, with lesser other 'sulfides such as tetrahedrite, galena, and chalcopyrite.

Certain parts of the Ropes Mine show a good correlation between abundance of

Au and fine pyrite (Brozdowski, this volume). Non-metallic alteration minerals

are dominated by quartz and carbonate with varying amounts of sericite, chlorite

and talc. The ore host rock for the Ropes orebody is interpreted by Bornhorst

and others (1986) and Brozdowski and others (1986) as a dacitic volcaniclastic rock

within the Deer Lake Peridotite. Both Brozdowski and Bornhorst agree that the

mineralization is epigenetic, but differ in their interpretation of mechanism of

emplacement. Brozdowski and others (1986) suggest that precious metals were

concentrated by hydrothermal fluids related to emplacement of peridotite sills

(Deer Lake Peridotite). Bornhorst and others (1986) suggest that the orebody was

syntectonic and that hydrothermal fluids followed a zone of relatively high strain,

a shear zone. Brozdowski (this volume) describes a structural genetic model for

the Ropes orebody.

Other less important occurrences of mineralization present as disseminations

and in quartz veins, are associated with all the major Archean rock types. Most

of the occurrences are interpreted as epigenetic, but at some poorly studied

localities the mineralization may be syngenetic. The description that follows

applies to epigenetic mineralization.

Mineralization is commonly spatially associated with faults and shear zones,

and is interpreted as syn- to post-tectonic. Sulfide minerals are significant

indicators of precious metal mineralization. Pyrite occurs as disseminations and in

quartz and quartz-carbonate veins. Pyrrhotite, chalcopyrite, and arsenopyrite are

less abundant than pyrite. In studies carried out at Michigan Technological

University, no anomalous Au or Ag values were obtained from any sample of rock

or quartz vein that was free of sulfides. Sphalerite and galena are found at some

occurrences, and these minerals tend to be more commonly associated with

anomalous Ag, whereas the yellow sulfides are associated with Au. Abundance of

sulfides in a rock, as measured by abundance of sulfur, is poorly correlated with

the abundance of Au. Quartz, carbonate, chlorite, and sericite occur as wall-rock

alteration minerals; quartz and carbonate also occur as vein minerals associated

with precious metal anomalies.

The present data permit the tentative suggestion that there appear to have

been at least two major pulses of mineralization. An early pulse resulted in

pervasive quartz and carbonate alteration and was accompanied by gold, pyrite and

other yellow sulfide minerals, and abundant chlorite and sericite. Quartz-carbonate

veins are associated with this early pulse. Detailed studies by Callahan Mining

Corporation (Brozdowski, 1988, personal communication) indicate that significant

occurrences of gold mineralization are associated with structures parallel and

conjugate to layering between rock types. A later pulse of mineralization was

dominated by quartz-carbonate veins containing base metal sulfides (pyrite, galena,

sphalerite, with little or no Au), although the relatively abundant galena at the

Ropes Mine may be an exception. The extent of country rock alteration associ-

ated with this latter pulse may have been less than that associated with the earlier

A-15

and in quartz veins, are associated with all the major Archean rock types. Most

of the occurrences are interpreted as epigenetic, but at some poorly studied

localities the mineralization may be syngenetic. The description that follows

applies to epigenetic mineralization.

Mineralization is commonly spatially associated with faults and shear zones,

and is interpreted as syn- to post-tectonic. Sulfide minerals are significant

indicators of precious metal mineralization. Pyrite occurs as disseminations and in

quartz and quartz-carbonate veins. Pyrrhotite, chalcopyrite, and arsenopyrite are

less abundant than pyrite. In studies carried out at Michigan Technological %

University, no anomalous Au or Ag values were obtained from any sample of rock

or quartz vein that was free of sulfides. Sphalerite and galena are found at some

occurrences, and these minerals tend to be more commonly associated with

anomalous Ag, whereas the yellow sulfides are associated with Au. Abundance of

sulfides in a rock, as measured by abundance of sulfur, is poorly correlated with

the abundance of Au. Quartz, carbonate, chlorite, and sericite occur as wall-rock

alteration minerals; quartz and carbonate also occur as vein minerals associated

with precious metal anomalies.

The present data permit the tentative suggestion that there appear to have

been at least two major pulses of mineralization. An early pulse resulted in

pervasive quartz and carbonate alteration and was accompanied by gold, pyrite and

other yellow sulfide minerals, and abundant chlorite and sericite. Quartz-carbonate

veins are associated with this early pulse. Detailed studies by Callahan Mining

Corporation (Brozdowski, 1988, personal communication) indicate that significant

occurrences of gold mineralization are associated with structures parallel and

conjugate to layering between rock types. A later pulse of mineralization was

dominated by quartz-carbonate veins containing base metal sulfides (pyrite, galena,

sphalerite, with little or no Au), although the relatively abundant galena at the

Ropes Mine may be an exception. The extent of country rock alteration associ-

ated with this latter pulse may have been less than that associated with the earlier

pulse. At some individual prospects a paragenetic succession of metallic and

non-metallic minerals can be recognized within a pulse.

SUMMARY


mafic to silicic volcanic flows, pyroclastics, and volcaniclastic sediments. These

are intruded by gabbro and rhyolite dikes and by granitoid plutons, all of Archean

age. The belt also includes two peridotite bodies. All of these rocks have been

metamorphosed from greenschist to amphibolite facies and subjected to multiple

deformation. Precious metal mineralization is dominantly epigenetic. The Archean

geologic history of the belt is obscured by unconformably-overlying Early

Proterozoic sediments and by deformation during the Penokean orogeny.

The potential for discovery of new economic deposits of precious metals in

the Marquette Greenstone Belt is denoted by anomalous Au values, quartz and

carbonate veins, areas of pervasive alteration, areas of relatively abundant faults

and shear zones, and overall geologic setting.

ACKNOWLEDGEMENTS

I thank the present and former graduate students who have spent many hours

working on the geology of the Marquette Greenstone Belt and with whom I have

had many fruitful discussions. They include: D. Baxter, C. Boben, R. Johnson, M.

MacLellan, E. Owens, D. Rossell, and T. Shepeck. I have benefited from numer-

ous conversations with my colleague, J. Kalliokoski. The Michigan Geological

Survey, Department of Natural Resources and the Department of Geology and

Geological Engineering, Michigan Technological University have supported my

program of research on the Marquette Greenstone Belt. D. Schueller and D.

Baxter provided comments on a draft of this paper. This paper was improved by

the reviews of J. Kalliokoski, Michigan Technological University, and K. Schulz,

U. S. Geological Survey.

A-16

pulse. At some individual prospects a paragenetic succession of metallic and

non-metallic minerals can be recognized within a pulse.

SUMMARY


mafic to silicic volcanic flows, pyroclastics, and volcaniclastic sediments. These

are intruded by gabbro and rhyolite dikes and by granitoid plutons, all of Archean

age. The belt also includes two peridotite bodies. All of these rocks have been 1

metamorphosed from greenschist to amphibolite facies and subjected to multiple

deformation. Precious metal mineralization is dominantly epigenetic. The Archean

geologic history of the belt is obscured by unconformably-overlying Early

Proterozoic sediments and by deformation during the Penokean orogeny.

The potential for discovery of new economic deposits of precious metals in

the Marquette Greenstone Belt is denoted by anomalous Au values, quartz and

carbonate veins, areas of pervasive alteration, areas of relatively abundant faults

and shear zones, and overall geologic setting.

ACKNOWLEDGEMENTS

I thank the present and former graduate students who have spent many hours

working on the geology of the Marquette Greenstone Belt and with whom I have

had many fruitful discussions. They include: D. Baxter, C. Boben, R. Johnson, M.

MacLellan, E. Owens, D. Rossell, and T. Shepeck. I have benefited from numer-

ous conversations with my colleague, J. Kalliokoski. The Michigan Geological

Survey, Department of Natural Resources and the Department of Geology and

Geological Engineering, Michigan Technological University have supported my

program of research on the Marquette Greenstone Belt. D. Schueller and D.

Baxter provided comments on a draft of this paper. This paper was improved by

the reviews of J. Kalliokoski, Michigan Technological University, and K. Schulz,

U. S. Geological Survey.

REFERENCES

Baxter, D.A. and Bornhorst, T.J., 1988, Multiple Discrete Mafic Intrusions ofArchean to Keweenawan Age, western Upper Peninsula, Michigan, [abs.]: Proceed-ings and Abstracts, 34th Institute on Lake Superior Geology, Marquette, Michigan,(this volume).

Boben, C.A., 1986, Geological comparison of three precious metal prospects inMarquette County, Michigan: M.S. Thesis, Michigan Technlogical University,Houghton, 77 p.

Bodwell, W.A., 1972, Geologic compilation and non-ferrous metal potential,Precambrian section, northern Michigan: M.S. Thesis, Michigan TechnologicalUniversity, Houghton, 106 p.

Bornhorst, T.J. and Baxter, D.A., 1987, Geochemical character of Archeanrocks from the east half of the Northern Complex, Upper Peninsula, Michigan:Institute on Lake Superior Geology Proceedings and Abstracts, v. 33, part 1, p. 12.

Bornhorst, T.J., Shepeck, A.W. and Rossell, D.M., 1986, The Ropes gold mine,Marquette County, Michigan, U.S.A.: in MacDonald, A. 1., Ed., Proceedings ofGold 86, an International Symposium on the Geology of Gold, Toronto, 1986, p.2 13-227.

Brozdowski, R.A., Gleason, R.J. and Scott, G.W., 1986, The Ropes Mine: Apyritic gold deposit in Archean volcaniclastic rock, Ishpeming, Michigan, U.S.A.:in MacDonald, A. J., Ed., Proceedings of Gold 86, an International Symposium onthe Geology of Gold, Toronto, 1986, p. 228-242.

Cannon, W.F. and Klasner, J.S., 1977, Bedrock geologic map of the southernpart of the Diorite and Champion 7 1/2-minute quadrangles, Marquette County,Michigan: U. S. Geological Survey Miscellaneous Investigation Series Map 1-1058.

Card, K.D. and Ciesielski, A., 1986, DNAG #1. Subdivisions of the SuperiorProvince of the Canadian Shield: Geoscience Canada, v. 13, no. 1, p. 5-13.

Clark, L.D., Cannon, W.F. and Klasner, J.S., 1975, Bedrock geologic map ofthe Negaunee SW quadrangle, Marquette County, Michigan: U. S. GeologicalSurvey Geologic Quadrangle Map GQ- 1226.

Gair, J.E. and Thaden, R.E., 1968, Geology of the Marquette and Sandsquadrangles, Marquette County, Michigan: U. S. Geological Survey ProfessionalPaper 397, 77 p.

Johnson, R.C., Bornhorst, T.J. and VanAlstine, J.L., 1987, Geologic setting ofprecious metal mineralization in the Silver Creek to Island Lake area, MarquetteCounty, Michigan: Michigan Geological Survey Division, Department of NaturalResources, Open-File Report OFR-87-4, Supersedes OFR-86-2, 134 p.

Morgan, P.J. and DeCristoforo, D.T., 1980, Geological evolution of theIshpeming Greenstone Belt, Michigan, U.S.A.: Precambrian Research, v. 11, p.23-41.

Puffett, W.P., 1974, Geology of the Negaunee quadrangle, Marquette County,Michigan: U. S. Geological Survey Professional Paper 788, 51 p.

A-17

REFERENCES

Baxter, D.A. and Bornhorst, T.J., 1988, Multiple Discrete Mafic Intrusions of Archean to Keweenawan Age, western Upper Peninsula, Michigan, [abs.]: Proceed- ings and Abstracts, 34th Institute on Lake Superior Geology, Marquette, Michigan, (this volume).

Boben, C.A., 1986, Geological comparison of three precious metal prospects in Marquette County, Michigan: M.S. Thesis, Michigan Technological University, Houghton, 77 p.

Bodwell, W.A., 1972, Geologic compilation and non-ferrous metal potential, Precambrian section, northern Michigan: M.S. Thesis, Michigan Technological University, Houghton, 106 p.

1

Bornhorst, T.J. and Baxter, D.A., 1987, Geochemical character of Archean rocks from the east half of the Northern Complex, Upper Peninsula, Michigan: Institute on Lake Superior Geology Proceedings and Abstracts, v. 33, part 1, p. 12.

Bornhorst, T.J., Shepeck, A.W. and Rossell, D.M., 1986, The Ropes gold mine, Marquette County, Michigan, U.S.A.: in MacDonald, A. J., Ed., Proceedings of Gold 86, an International Symposium on the Geology of Gold, Toronto, 1986, p. 213-227.

Brozdowski, R.A., Gleason, R.J. and Scott, G.W., 1986, The Ropes Mine: A pyritic gold deposit in Archean volcaniclastic rock, Ishpeming, Michigan, U.S.A.: in MacDonald, A. J., Ed., Proceedings of Gold 86, an International Symposium on the Geology of Gold, Toronto, 1986, p. 228-242.

Cannon, W.F. and Klasner, J.S., 1977, Bedrock geologic map of the southern part of the Diorite and Champion 7 1/2-minute quadrangles, Marquette County, Michigan: U. S. Geological Survey Miscellaneous Investigation Series Map 1-1058.

Card, K.D. and Ciesielski, A., 1986, DNAG #I. Subdivisions of the Superior Province of the Canadian Shield: Geoscience Canada, v. 13, no. 1, p. 5-13.

Clark, L.D., Cannon, W.F. and Klasner, J.S., 1975, Bedrock geologic map of the Negaunee SW quadrangle, Marquette County, Michigan: U. S. Geological Survey Geologic Quadrangle Map GQ- 1226.

Gair, J.E. and Thaden, R.E., 1968, Geology of the Marquette and Sands quadrangles, Marquette County, Michigan: U. S. Geological Survey Professional Paper 397, 77 p.

Johnson, R.C., Bornhorst, T.J. and VanAlstine, J.L., 1987, Geologic setting of precious metal mineralization in the Silver Creek to Island Lake area, Marquette County, Michigan: Michigan Geological Survey Division, Department of Natural Resources, Open-File Report OFR-87-4, Supersedes OFR-86-2, 134 p.

Morgan, P.J. and DeCristoforo, D.T., 1980, Geological evolution of the Ishpeming Greenstone Belt, Michigan, U.S.A.: Precambrian Research, v. 11, p. 23-41.

Puffett, W.P., 1974, Geology of the Negaunee quadrangle, Marquette County, Michigan: U. S. Geological Survey Professional Paper 788, 51 p.

Rossell, D.M., 1983, Alteration of the Deer Lake Peridotite near the RopesMine, Marquette County, Michigan: M.S. Thesis, Michigan Technological Univer-sity, Houghton, 83 p.

Sims, P.K., 1980, Boundary between Archean greenstone and gneiss terranes innorthern Wisconsin and Michigan: Geological Society of America Special Paper 182,p. 113-124.

Trow, J., 1979, Final report diamond drilling for geologic information in theMiddle Precambrian basins in the western portion of northern Michigan: MichiganGeological Survey, Department of Natural Resources, Open-File Report UDOEOFR GJBX-162(79), 44 p.

Weeks, V., 1987, Gravity and magnetic investigations in the south-central partof the Ishpeming Greenstone Belt, Marquette County, Michigan: M.S. Thesis,Michigan Tchnological University, Houghton, 61 p.

A-lB

Rossell, D.M., 1983, Alteration of the Deer Lake Peridotite near the Ropes Mine, Marquette County, Michigan: M.S. Thesis, Michigan Technological Univer- sity, Houghton, 83 p.

Sims, P.K., 1980, Boundary between Archean greenstone and gneiss terranes in northern Wisconsin and Michigan: Geological Society of America Special Paper 182, p. 113-124.

Trow, J., 1979, Final report diamond drilling for geologic information in the Middle Precambrian basins in the western portion of northern Michigan: Michigan Geological Survey, Department of Natural Resources, Open-File Report UDOE OFR GJBX-162(79), 44 p.

Weeks, V., 1987, Gravity and magnetic investigations in the south-central part of the Ishpeming Greenstone Belt, Marquette County, Michigan: M.S. Thesis, Michigan ~ tkhno lo~ ica l University, Houghton, 61 p.

Geological Framework of a Part of the Archean MarquetteGreenstone Belt North of the Dead River Storage Basin

D.A. Baxter and M.L. MacLellan

Department of Geology and Geological Engineering,Michigan Technological University, Houghton, Michigan 49931

INTRODUCTION

The Archean Marquette Greenstone Belt underlies approximately 125 mi2 (325

km2) of northern Marquette County in Michigan's western Upper Peninsula

(Figure 1). The greenstone belt is divided into northern and southern portions by

a structural zone which parallels the Dead River Storage Basin (Bornhorst, this

volume). The southern part of the greenstone belt and the metasediments of the

Marquette Supergroup were mapped by the U.S.G.S. The northern part of the

belt was partially mapped by the U.S.G.S. and the remainder was included on the

1:60,000 compilation map of the northern complex by Bodwell (1972). The aim of

this paper is to briefly describe the geology of the northern part of the Archean

Marquette Greenstone Belt based on detailed mapping of the southwest 25 mi2 (65

km2).

Archean rocks of the northern part consist of dominantly subaqueous mafic

lava flows, and minor, interbedded volcaniclastic and pyroclastic deposits of

intermediate composition and iron formation. All of these rocks are assigned to

the Yolcanics of Silver Mine Lakes, an informal formation name used to distin-

guish these strata from the remainder of the Mona Schist which is situated on the

southern side of the Dead River Shear Zone, an Archean structural zone with a

large, but unknown, degree of movement. The volcanic package has been

intruded by gabbro and subsequently by rhyolite and granodiorite of Archean age.

The rhyolite is interpreted as a more evolved equivalent of the granodiorite to

tonalite plutonic rocks (Bornhorst and Baxter, 1987).

A- 19

Geological Framework of a Part of the Archean Marquette Greenstone Belt North of the Dead River Storage Basin

D.A. Baxter and M.L. MacLellan

Department of Geology and Geological Engineering, Michigan Technological University, Houghton, Michigan 49931

INTRODUCTION

The Archean Marquette Greenstone Belt underlies approximately 125 mi2 (325

km2) of northern Marquette County in Michigan's western Upper Peninsula /

(Figure 1). The greenstone belt is divided into northern and southern portions by

a structural zone which parallels the Dead River Storage Basin (Bornhorst, this

volume). The southern part of the greenstone belt and the metasediments of the

Marquette Supergroup were mapped by the U.S.G.S. The northern part of the

belt was partially mapped by the U.S.G.S. and the remainder was included on the

1:60,000 compilation map of the northern complex by Bodwell (1972). The aim of

this paper is to briefly describe the geology of the northern part of the Archean

Marquette Greenstone Belt based on detailed mapping of the southwest 25 mi2 (65

km2).

Archean rocks of the northern part consist of dominantly subaqueous mafic

lava flows, and minor, interbedded volcaniclastic and pyroclastic deposits of

intermediate composition and iron formation. All of these rocks are assigned to

the Volcanics of Silver Mine Lakes, an informal formation name used to distin-

guish these strata from the remainder of the Mona Schist which is situated on the

southern side of the Dead River Shear Zone, an Archean structural zone with a

large, but unknown, degree of movement. The volcanic package has been

intruded by gabbro and subsequently by rhyolite and granodiorite of Archean age.

The rhyolite is interpreted as a more evolved equivalent of the granodiorite to

tonalite plutonic rocks (Bornhorst and Baxter, 1987).

T5ON

T49N

T48N

o 50 100 150I I I 1

Km.

Figure 1. Regional geology and location map of theMarquette Greenstone Belt (modified fromMorgan and DeCristoforo, 1980).

A-20

o 30 60 90• I I

Miles

0 30 60 90 Miles 0 50 100 150

Km.

Figure 1. Regional geology and location map of the Marquette Greenstone Belt (modified from Morgan and DeCristoforo, 1980).


The Volcanics of Silver Mine Lakes (MacLellan and Bornhorst, 1988) are

named for a succession of Archean volcanic units that crop out in the vicinity of

Silver Mine Lakes. Detailed mapping (MacLellan and Bornhorst, 1988; Baxter and

others, 1987; Johnson and others, 1987; Owens and Bornhorst, 1985) has distin-

guished five members: Pillowed Basalt Member, Breccia Member of Bismark

Creek, Breccia Member of Reany Lake, Iron Formation Member, and the Hill's

Lake Pyroclastic Member (Figure 2). Relative ages of the members are based on

the presence of pillows as top indicators in the Pillowed Basalt Member. The

members are too small to be distinguished at 74 minute quadrangle scale

(1:24,000).

Pillowed Basalt Member

This member has been subdivided into three varieties, normal, foliated, and

altered, based on various criteria. The contacts between these varieties are

gradational.

Normal Variety. This variety is commonly dark green to green-black in

color, fine-grained to aphanitic, and massive to moderately foliated (see Day 1

Road Log, Stops 6 and 10, this volume). Visible mineralogy includes very

fine-grained chlorite, very fine-grained plagioclase and sericite, and rarely

fine-grained amphibole. Very fine-grained plagioclase crystals often show relict

igneous microlite textures. Elongate, variably flattened pillows are found locally

and range from 1—8 feet (30-250 cm) in length and 3-30 inches (8-76 cm) in

thickness. Some pillows show small bands of vesicles (1/16-1 inch, 2-25 mm)

along the chilled margins. It is common to find interpillow void space filled with

tan to gray carbonate material. In a few places, stratigraphic tops have been

determined from the cuspate pillows shapes.

Quartz and quartz-carbonate veins from less than 1/16 inch (2 mm) to 9

inches (23 cm) in width can be found throughout this variety. The veins can

A-21


'The Volcanics of Silver Mine Lakes (MacLellan and Bornhorst, 1988) are

named for a succession of Archean volcanic units that crop out in the vicinity of

Silver Mine Lakes. Detailed mapping (MacLellan and Bornhorst, 1988; Baxter and

others, 1987; Johnson and others, 1987; Owens and Bornhorst, 1985) has distin-

guished five members: Pillowed Basalt Member, Breccia Member of Bismark

Creek, Breccia Member of Reany Lake, Iron Formation Member, and the Hill's

Lake Pyroclastic Member (Figure 2). Relative ages of the members are based on

the presence of pillows as top indicators in the Pillowed Basalt Member. The

members are too small to be distinguished at 7+ minute quadrangle scale

(1:24,000).

Pillowed Basalt Member

This member has been subdivided into three varieties, normal, foliated, and

altered, based on various criteria. The contacts between these varieties are

gradational.

Normal Variety. This variety is commonly dark green to green-black in

color, fine-grained to aphanitic, and massive to moderately foliated (see Day 1

Road Log, Stops 6 and 10, this volume). Visible mineralogy includes very

fine-grained chlorite, very fine-grained plagioclase and sericite, and rarely

fine-grained amphibole. Very fine-grained plagioclase crystals often show relict

igneous microlite textures. Elongate, variably flattened pillows are found locally

and range from 1-8 feet (30-250 cm) in length and 3-30 inches (8-76 cm) in

thickness. Some pillows show small bands of vesicles (1/16-1 inch, 2-25 mm)

along the chilled margins. It is common to find interpillow void space filled with

tan to gray carbonate material. In a few places, stratigraphic tops have been

determined from the cuspate pillows shapes.

Quartz and quartz-carbonate veins from less than 1/16 inch (2 mm) to 9

inches (23 cm) in width can be found throughout this variety. The veins can

Correlation of Archean Units

Volcanic andIntrusive Rocks Sedimentary Rocks

Granodiorite of Rocking Chair Lakes

rhyolite Intrusive of

Fire Center Mine

Altered

Variety

Gabbro of

Clark Creek

Altered

Variety

Highly AlteredFoliated Variety

Variety

[Hiils Lake Pyroclastic Member j

Volcanics of Iron Formation MemberSilver Mine Lakes

___________________________

a.

Breccia Member of Reany Lake

Breccia Member of Bismark Creek

Figure 2.

A- 22

Correlation of Archean Units

Intrusive Rocks

Granodiorite of Rocking Chair Lakes c Rhyolite Intrusive of Altered

Fire Center Mine

I Gabbro of I Altered 1 1 Clark Creek 1 Variety 1

Volcanic and Sedimentary Rocks

Volcanics of - Silver Mine Lakes

Foliated Variety

7

Hill's Lake Pyroclastic Member

Iron Formation Member I P-



Figure 2. A - 2 2

carry up to 5% sulfides and there may be up to 2% disseminated sulfides in the

adjacent basalts. Disseminated sulfides occur throughout much of this unit and

are usually > 95% pyrite with minor amounts of chalcopyrite, pyrrhotite, and

arsenopyrite.

Foliated Variety. This variety ranges in color from medium gray-black to

green-black. It has an aphanitic texture, a phyllonitic to schistose fabric, and is

composed dominantly of chlorite with lesser amounts of plagioclase and/or sericite,

quartz and qarbonate. Some exposures contain up to 40% secondary carbonate

(ankerite) as the matrix of brecciated basalt. Other exposures contain alternating

light and dark layers which are interpreted as tectonic layering resulting from

highly elongated pillows. The alternating layers originated from darker pillow

interiors surrounded by lighter colored pillow rinds. Some outcrops may also have

originally been basalt tuffs and the layering may reflect highly elongated lithic

fragments. Disseminated pyrite is found throughout this variçty and can occur

locally in concentrations up to 30%.

Highly Altered Variety. The highly altered variety of the Pillowed Basalt

Member is dark to light gray in color and takes on a green hue near the grada-

tional contact with the normal, less altered variety (see Day 1 Road Log, Stops 4

and 5, this volume). The basalt is aphanitic to very fine-grained and is composed

of varying quantities of chlorite, carbonate, sericite, plagioclase, and quartz. The

rocks are predominantly chioritized, with carbonatization, sericitization, and

silicification occurring in varying degrees. The fabric varies from massive to very

highly foliated. Typically, the degree of foliation in this variety is as high as the

foliated basalt variety. In rare cases, deformed and undeformed pillow structures

can still be observed on weathered surfaces.

Disseminated mineralization is most abundant in this variety of basalt. Sulfide

minerals can reach up to 20 volume percent of the outcrop, but typically average

only 2-6%. Quartz and quartz-carbonate veins, similar to those found in the

foliated variety, also occur in this variety. In general, both types of

A- 23

carry up to 5% sulfides and there may be up to 2% disseminated sulfides in the

adjacent basalts. Disseminated sulfides occur throughout much of this unit and

are usually > 95% pyrite with minor amounts of chalcopyrite, pyrrhotite, and

arsenopyrite.

Foliated Variety. This variety ranges in color from medium gray-black to

green-black. It has an aphanitic texture, a phyllonitic to schistose fabric, and is

composed dominantly of chlorite with lesser amounts of plagioclase and/or sericite,

quartz and carbonate. Some exposures contain up to 40% secondary carbonate

(ankerite) as the matrix of brecciated basalt. Other exposures contain alternating

light and dark layers which are interpreted as tectonic layering resulting from

highly elongated pillows. The alternating layers originated from darker pillow

interiors surrounded by lighter colored pillow rinds. Some outcrops may also have

originally been basalt tuffs and the layering may reflect highly elongated lithic

fragments. Disseminated pyrite is found throughout this variety and can occur

locally in concentrations up to 30%.

Highly Altered Variety. The highly altered variety of the Pillowed Basalt

Member is dark to light gray in color and takes on a green hue near the grada-

tional contact with the normal, less altered variety (see Day 1 Road Log, Stops 4

and 5, this volume). The basalt is aphanitic to very fine-grained and is composed

of varying quantities of chlorite, carbonate, sericite, plagioclase, and quartz. The

rocks are predominantly chloritized, with carbonatization, sericitization, and

silicification occurring in varying degrees. The fabric varies from massive to very

highly foliated. Typically, the degree of foliation in this variety is as high as the

foliated basalt variety. In rare cases, deformed and undeformed pillow structures

can still be observed on weathered surfaces.

Disseminated mineralization is most abundant in this variety of basalt. Sulfide

minerals can reach up to 20 volume percent of the outcrop, but typically average

only 2 4 % . Quartz and quartz-carbonate veins, similar to those found in the

foliated variety, also occur in this variety. In general, both types of

mineralization are related to well foliated structural zones.


The Breccia Member of Bismark Creek is located along a major NW-SE

trending structural zone, the Willow Creek Shear Zone (MacLellan and Bornhorst,

1988) (see Day 1 Road Log, Stop 9, this volume). There ar at least three types

of clasts in this unit: boudinaged porphyritic rhyolite (similar to the Rhyolite

Intrusive of Fire Center Mine), wispy gabbro, and boudinaged granodiorite. The

clasts are lensoidal to elongate in shape and range from 1/2 to 3/4 inches (1-2

cm) for the lensoidal type, and up to 4-6 inches (10-15 cm) in width by 3-6 feet

(1-2 meters) in length for the elongate type. Clasts are supported in an

epidote-rich basaltic matrix. Both the matrix and the rhyolite clasts contain up to

5% disseminated pyrite. This unit is interpreted as a mudflow breccia.


The Breccia Member of Reany Lake, best exposed near Reany Lake, is a

breccia with a strike length of at least 5 miles (8 kilometers) (see Day 1 Road

Log, Stop 8, this volume). The unit is correlative with the Mudflow Member

described by Baxter and others (1987). The breccia clasts are white, gray, or

brown on both weathered and fresh surfaces, whereas the matrix weathers to a

light green-tan color and is a green-black color on fresh surfaces. The lower

portion of the unit contains very few large clasts and was mapped as clast-poor,

whereas the upper portion contains up to 35% clasts of all sizes and was mapped

as the clast-rich horizon. In some exposures, there is second clast-poor zone

above the clast-rich zone.

The clast-poor horizon has equant to oblong, sub-angular to sub-rounded

grains of volcanic rock which rarely exceed 1/3 inch (1 cm) in size. The matrix

is medium to fine-grained, intermediate to mafic in composition, with occasional

1/16 to 1/8 inch (1-3 mm) grains of feldspar. The clast-poor horizon is

interpreted as either a well-sorted mudflow or a reworked tuff deposit.

A- 24

mineralization are related to well foliated structural zones.


The Breccia Member of Bismark Creek is located along a major NW-SE

trending structural zone, the Willow Creek Shear Zone (MacLellan and Bornhorst,

1988) (see Day 1 Road Log, Stop 9, this volume). There are at least three types

of clasts in this unit: boudinaged porphyritic rhyolite (similar to the Rhyolite

Intrusive of Fire Center Mine), wispy gabbro, and boudinaged granodiorite. The

clasts are le&oidal to elongate in shape and range from 1/2 to 3/4 inches (1-2

cm) for the lensoidal type, and up to 4-6 inches (10-15 cm) in width by 3-6 feet

(1-2 meters) in length for the elongate type. Clasts are supported in an

epidote-rich basaltic matrix. Both the matrix and the rhyolite clasts contain up to

5% disseminated pyrite. This unit is interpreted as a mudflow breccia.

Breccia Member of Reany Lake I

The Breccia Member of Reany Lake, best exposed near Reany Lake, is a

breccia with a strike length of at least 5 miles (8 kilometers) (see Day 1 Road

Log, Stop 8, this volume). The unit is correlative with the Mudflow Member

described by Baxter and others (1987). The breccia clasts are white, gray, or

brown on both weathered and fresh surfaces, whereas the matrix weathers to a

light green-tan color and is a green-black color on fresh surfaces. The lower

portion of the unit contains very few large clasts and was mapped as clast-poor,

whereas the upper portion contains up to 35% clasts of all sizes and was mapped

as the clast-rich horizon. In some exposures, there is second clast-poor zone

above the clast-rich zone.

The clast-poor horizon has equant to oblong, sub-angular to sub-rounded

grains of volcanic rock which rarely exceed 1/3 inch (1 cm) in size. The matrix

is medium to fine-grained, intermediate to mafic in composition, with occasional

1/16 to 1/8 inch (1-3 mm) grains of feldspar. The clast-poor horizon is

interpreted as either a well-sorted mudflow or a reworked tuff deposit.

In the clast—rich horizon the clasts are matrix supported. Shapes vary from

highly elongate to sub-equant and rounding from very angular to rounded. The

more equant clasts are generally 1/3-4 inches (1-10 cm) across while the elongated

fragments can reach lengths of up to 10 inches (25 cm). It is important to note

that about 50% of the clasts are white, gray, and black cherts. This, coupled with

the fact that a cherty iron-formation overlies this unit, suggests that these cherts

may have formed in a volcanic vent region and were brecciated during explosive

eruptions. lhe remaining clasts are fine-grained, porphyritic andesites. The

matrix is very similar to the material in the clast-poor horizon. Therefore, we

also interpret the clast-rich horizon as a mudflow breccia deposit.

Iron Formation Member

The Iron Formation Member, which overlies the Breccia Member of Reany

Lake, consists of gray, dark gray, to black banded rock with alternating bands of

micro-crystalline chert and magnetite, or iron-rich chert (see Day 1 Road Log,

Stop 10, this volume). In some localities, the unit is a black, cherty iron

formation containing up to 2-5% disseminated sulfides, mainly pyrite. These

cherty layers are cross-cut by carbonate veinlets which may contain up to 20%

suif ides.

At Clark Creek, the iron formation is gray in color and shows very distinctive

interlayering of micro-crystalline quartz and massive magnetite. This locality also

contains clots of sulfides scattered through the rock, often associated with the

carbonate veins that fill tension gashes. The sulfides comprise a maximum of 5%

of the whole rock.


The Hill's Lake Pyroclastic is a breccia composed of white to tan dacite clasts

in a black schistose basalt matrix. The clasts are flattened and give vertical

surfaces a banded appearance. The metamorphic grade of this unit is lower

amphibolite. The clasts consist of plagioclase and quartz with minor hornblende,

A-25

In the clast-rich horizon the clasts are matrix supported. Shapes vary from

highly elongate to sub-equant and rounding from very angular to rounded. The

more equant clasts are generally 113-4 inches (1-10 cm) across while the elongated

fragments can reach lengths of up to 10 inches (25 cm). It is important to note

that about 50% of the clasts are white, gray, and black cherts. This, coupled with

the fact that a cherty iron-formation overlies this unit, suggests that these cherts

may have formed in a volcanic vent region and were brecciated during explosive

eruptions. The remaining clasts are fine-grained, porphyritic andesites. The

matrix is very similar to the material in the clast-poor horizon. Therefore, we t

also interpret the clast-rich horizon as a mudflow breccia deposit.

Iron Formation Member

The Iron Formation Member, which overlies the Breccia Member of Reany

Lake, consists of gray, dark gray, to black banded rock with alternating bands of

micro-crystalline chert and magnetite, or iron-rich chert (see Day 1 Road Log,

Stop 10, this volume). In some localities, the unit is a black, cherty iron

formation containing up to 2-5% disseminated sulfides, mainly pyrite. These

cherty layers are cross-cut by carbonate veinlets which may contain up to 20%

sulfides.

At Clark Creek, the iron formation is gray in color and shows very distinctive

interlayering of micro- crystalline quartz and massive magnetite. This locality also

contains clots of sulfides scattered through the rock, often associated with the

carbonate veins that fill tension gashes. The sulfides comprise a maximum of 5%

of the whole rock.


The Hill's Lake Pyroclastic is a breccia composed of white to tan dacite c lam

in a black schistose basalt matrix. The clasts are flattened and give vertical

surfaces a banded appearance. The metamorphic grade of this unit is lower

amphibolite. The clasts consist of plagioclase and quartz with minor hornblende,

sericite, and epidote. The matrix is composed of hornblende, plagioclase, biotite,

and garnet, with minor sericite, chlorite, tourmaline, and clinozoisite/epidote


GABBRO OF CLARK CREEK

The Gabbro of Clark Creek (MacLellan and Bornhorst, 1988) is named for

gabbro intrusives that cut the Volcanics of Silver Mine Lakes near Clark Creek

(see Day 1 Road Log, Stops 5, 8, and 10, this volume). The gabbro is medium to

dark green-black on a fresh surface, holocrystalline, medium to very coarse-

grained, and massive to strongly foliated. The mineralogy is blue-green

amphibole, plagioclase, chlorite, and minor pyrite, magnetite, and pyrrhotite. The

more foliated zones within the gabbro have a much higher percentage of chlorite

which causes a deeper green color.

There is also an altered variety of gabbro which is medium to dark gray in

color with plagioclase crystals forming augens surrounded by chlorite, carbonate,

quartz, and sericite. This rock has an undulating, schistose texture on the

foliation surface. The intensity of alteration is greater near zones of intense

foliation.

RHYOLITE INTRUSIVE OF FIRE CENTER MINE

The Rhyolite Intrusive of Fire Center Mine (Johnson and others, 1987) has

three common textural types and cuts the volcanics and the gabbro (see Day 1

Road Log, Stops 5,6, and 7, this volume). The first two, porphyritic and

granular, are grouped together as the normal variety. The third is aphanitic, but

since it is typically altered, it is termed the altered variety.

Quartz-porphyritic rhyolite is the most common textural type, with anhedral

phenocrysts of quartz, 1/16 to 5/16 inches (2-8 mm) across. The other porphy-

ritic type is quartz-feldspar porphyritic rhyolite, with phenocrysts of subhedral

potassium feldspar and/or plagioclase as well as anhedral quartz of similar size.

The matrix in both types is fine-grained to aphanitic and consists of quartz,

A- 26

sericite, and epidote. The matrix is composed of hornblende, plagioclase, biotite,

and garnet, with minor sericite, chlorite, tourmaline, and clinozoisite/epidote


GABBRO OF CLARK CREEK

The Gabbro of Clark Creek (MacLellan and Bornhorst, 1988) is named for

gabbro intrusives that cut the Volcanics of Silver Mine Lakes near Clark Creek

(see Day 1 Road Log, Stops 5, 8, and 10, this volume). The gabbro is medium to

dark green-black on a fresh surface, holocrystalline, medium to very coarse-

grained, and massive to strongly foliated. The mineralogy is blue-green

amphibole, plagioclase, chlorite, and minor pyrite, magnetite, and pyrrhotite. The

more foliated zones within the gabbro have a much higher percentage of chlorite

which causes a deeper green color.

There is also an altered variety of gabbro which is medium to dark gray in

color with plagioclase crystals forming augens surrounded by chlorite, carbonate,

quartz, and sericite. This rock has an undulating, schistose texture on the

foliation surface. The intensity of alteration is greater near zones of intense

foliation.

RHYOLITE INTRUSIVE OF FIRE CENTER MINE

The Rhyolite Intrusive of Fire Center Mine (Johnson and others, 1987) has

three common textural types and cuts the volcanics and the gabbro (see Day 1

Road Log, Stops 5,6, and 7, this volume). The first two, porphyritic and

granular, are grouped together as the normal variety. The third is aphanitic, but

since it is typically altered, it is termed the altered variety.

Quartz-porphyritic rhyolite is the most common textural type, with anhedral

phenocrysts of quartz, 1/16 to 5/16 inches (2-8 mm) across. The other porphy-

ritic type is quartz-feldspar porphyritic rhyolite, with phenocrysts of subhedral

potassium feldspar and/or plagioclase as well as anhedral quartz of similar size.

The matrix in both types is fine-grained to aphanitic and consists of quartz,

carbonate, feldspar, and minor sericite. Color varies from dark gray to light gray

to pink. The granular type is generally light pink with roughly equidimensional

quartz and feldspar grains, 1/16 to 3/16 inches (2-5 mm) across. Disseminated

sulfides are present in amounts of usually less than 1%.

The altered variety is a light green to green-gray, aphanitic, schistose rock.

It is identified in the field primarily by color and lack of visible grains. The

rock is composed of partially recrystallized quartz, feldspar, and interstitial sericite

which may define a crude foliation. There are up to 8% disseminated sulfides in

the altered variety.

GRANODIORITE OF ROCKING CHAIR LAKES

The Granodiorite of Rocking Chair Lakes was named by Johnson and others

(1987) for dominantly granodioritic plutons which intrude the Volcanics of Silver

Mine Lakes and Gabbro of Clark Creek. The intrusive contacts are often sheared

and/or faulted. The Rhyolite Intrusive of Fire Center Mine is interpreted as a

late-stage, more evolved equivalent of the granodiorite plutons (Baxter, 1988;

Johnson others, 1987; Bornhorst and Baxter, 1987).

The Granodiorite of Rocking Chair Lakes is medium gray to light pink,

medium to coarse grained, hypidiomorphic to allotriomorphic granular, and is

composed dominantly of granodiorite with lesser tonalite, quartz monzonite, quartz

monzodiorite, and quartz diorite (using a Streckeisen diagram). The massive

granodiorite is composed of plagioclase, K-feldspar, quartz, amphibole, and minor

sericite, epidote, chlorite, apatite, and carbonate. A more amphibole-rich

granodiorite, owing it's origin to the assimilation of amphibole from the mafic

country rocks, is typically a darker gray in color and medium- to fine-grained.

The altered variety is red to orange, fine- to medium-grained, and is cross-cut by

stockwork quartz veins. The foliated variety, found along intrusive contacts, is a

gray to black, fine-grained to aphanitic, schistose mylonite. The mineralogy of

this variety consists of plagioclase, chlorite, K-feldspar, quartz, and minor apatite,

A-27

carbonate, feldspar, and minor sericite. Color varies from dark gray to light gray

to pink. The granular type is generally light pink with roughly equidimensional

quartz and feldspar grains, 1/16 to 3/16 inches (2-5 mm) across. Disseminated

sulfides are present in amounts of usually less than 1%.

The altered variety is a light green to green-gray- aphanitic, schistose rock.

It is identified in the field primarily by color and lack of visible grains. The

rock is composed of partially recrystallized quartz, feldspar, and interstitial sericite

which may define a crude foliation. There are up to 8% disseminated sulfides in

the altered variety.

GRANODIORITE OF ROCKING CHAIR LAKES

The Granodiorite of Rocking Chair Lakes was named by Johnson and others

(1987) for dominantly granodioritic plutons which intrude the Volcanics of Silver

Mine Lakes and Gabbro of Clark Creek. The intrusive contacts are often sheared

and/or faulted. The Rhyolite Intrusive of Fire Center Mine is interpreted as a

late-stage, more evolved equivalent of the granodiorite plutons (Baxter, 1988; t

Johnson others, 1987; Bornhorst and Baxter, 1987).

The Granodiorite of Rocking Chair Lakes is medium gray to light pink,

medium to coarse grained, hypidiomorphic to allotriomorphic granular, and is

composed dominantly of granodiorite with lesser tonalite, quartz monzonite, quartz

monzodiorite, and quartz diorite (using a Streckeisen diagram). The massive

granodiorite is composed of plagioclase, K-feldspar, quartz, amphibole, and minor

sericite, epidote, chlorite, apatite, and carbonate. A more amphibole-rich

granodiorite, owing it's origin to the assimilation of amphibole from the mafic

country rocks, is typically a darker gray in color and medium- to fine-grained.

The altered variety is red to orange, fine- to medium-grained, and is cross-cut by

stockwork quartz veins. The foliated variety, found along intrusive contacts, is a

gray to black, fine-grained to aphanitic, schistose mylonite. The mineralogy of

this variety consists of plagioclase, chlorite, K-feldspar, quartz, and minor apatite,

sericite, and epidote (Johnson and others, 1987).

STRUCTURE

The rocks in the northern part of the Marquette Greenstone Belt were

affected by several deformational episodes in the Archean and Lower Proterozoic

(Penokean). The first event folded, sheared, faulted, and metamorphosed the

Archean rocks of the Marquette Greenstone Belt in Archean time. A later event

folded, faulted, and metamorphosed the Lower Proterozoic sediments and diabase

of the Marquette Range Supergroup and had a less definite effect on Archean

rocks. Some minor reactivation of Archean faults may have taken place during

the Penokean deformation. Distinguishing between Archean and Penokean meta-

morphism is difficult since they are both greenschist facies, but in the Island Lake

Area where Archean metamorphism reaches amphibolite facies, Penokean metamor-

phism is denoted by retrograde assemblages and fabrics (Johnson and others, 1987).

Foliations in the region are defined by slaty, phyllonitic, and schistose

cleavages and generally trend N70°W. These cleavages are most prominently

displayed in the lepidoblastic textured, mafic rocks. Most foliations in the

Archean rocks are interpreted to have been formed during the Late Archean

tectonic event. The Archean rocks in the area of Silver Mine Lakes, Clark Creek,

and Silver Creek lie on the southern limb of a steeply plunging, synformal

anticline (Johnson and others, 1987). The nose of this regional fold is exposed to

the northwest of Silver Creek where the hinge region of this structure has been

intruded by the Granodiorite of Rocking Chair Lakes.

There are several generations of faults in the region that faults probably

formed during various pulses of the Late Archean deformational event. The oldest

faults in the area generally trend southeast-northwest to east-west, are generally

ductile in nature, and would be termed shear zones in Ontario. A large structural

zone, the Dead River Shear Zone, also trends southeast-northwest, and was

initiated during the Archean. This zone is typified by steeply dipping, phyllonitic

A-28

sericite, and epidote (Johnson and others, 1987).

STRUCTURE

The rocks in the northern part of the Marquette Greenstone Belt were

affected by several deformational episodes in the Archean and Lower Proterozoic

(Penokean). The first event folded, sheared, faulted, and metamorphosed the

Archean rocks of the Marquette Greenstone Belt in Archean time. A later event

folded, faulted, and metamorphosed the Lower Proterozoic sediments and diabase /

of the Marquette Range Supergroup and had a less definite effect on Archean

rocks. Some minor reactivation of Archean faults may have taken place during

the Penokean deformation. Distinguishing between Archean and Penokean meta-

morphism is difficult since they are both greenschist facies, but in the Island Lake

Area where Archean metamorphism reaches amphibolite facies, Penokean metamor-

phism is denoted by retrograde assemblages and fabrics (Johnson and others, 1987).

Foliations in the region are defined by slaty, phyllonitic, and schistose

cleavages and generally trend N70Â¡W These cleavages are most prominently

displayed in the lepidoblastic textured, mafic rocks. Most foliations in the

Archean rocks are interpreted to have been formed during the Late Archean

tectonic event. The Archean rocks in the area of Silver Mine Lakes, Clark Creek,

and Silver Creek lie on the southern limb of a steeply plunging, synformal

anticline (Johnson and others, 1987). The nose of this regional fold is exposed to

the northwest of Silver Creek where the hinge region of this structure has been

intruded by the Granodiorite of Rocking Chair Lakes.

There are several generations of faults in the region that faults probably

formed during various pulses of the Late Archean deformational event. The oldest

faults in the area generally trend southeast-northwest to east-west, are generally

ductile in nature, and would be termed shear zones in Ontario. A large structural

zone, the Dead River Shear Zone, also trends southeast-northwest, and was

initiated during the Archean. This zone is typified by steeply dipping, phyllonitic

to highly schistose volcanic rocks. It originated as an Archean ductile shear zone

which subsequently had a minor portion reactivated as a relatively brittle fault

during the Early Proterozoic.

Earlier Archean faults and shear zones are truncated, and locally offset by

slightly younger, north-south to northeast-southwest trending Archean faults. The

deformational regime for these faults was relatively brittle in contrast to the older

ductile faults. It is likely that movement along these faults took place in the

waning stages' of the Archean deformational event because they offset the zone of

highly altered basalts.

MINERALIZATION

Mineralization in the region exists in two forms: 1) disseminated sulfides

within altered country rocks, and 2) quartz-carbonate-sulfide veins. Disseminated

mineralization is most prominent in the highly altered variety of the Pillowed

Basalt Member and in the Iron Formation Member, but also occurs occasionally in

other Archean rock types. It consists of disseminated pyrite and sporadic minor

pyrrhotite and chalcopyrite. The alteration minerals in the country rocks are

primarily chlorite, sericite, carbonate, and quartz.

The veins of quartz and carbonate, and lesser amounts of chlorite, are often

associated with shear zones and faults. The chief sulfide mineral is pyrite with

much lesser amounts of chalcopyrite, arsenopyrite, and pyrrhotite. Locally, galena

and sphalerite are the dominant sulfide vein minerals.

In general, it appears that two episodes of mineralization have affected the

volcanics and intrusives of the northern part of the Marquette Greenstone Belt.

The early episode produced the majority of the disseminated mineralization and

was likely controlled by the location of the east-west trending faults and shear

zones. This event was also responsible for the production of zones of intense

carbonatization, sericitization, and chioritization. The second episode produced

primarily carbonate-quartz veins with associated galena, sphalerite, and chalcopyrite

A— 29

to highly schistose volcanic rocks. It originated as an Archean ductile shear zone

which subsequently had a minor portion reactivated as a relatively brittle fault

during the Early Proterozoic.

Earlier Archean faults and shear zones are truncated, and locally offset by

slightly younger, north-south to northeast-southwest trending Archean faults. The

deformational regime for these faults was relatively brittle in contrast to the older

ductile faults. It is likely that movement along these faults took place in the

waning stages' of the Archean deformational event because they offset the zone of

highly altered basalts.

MINERALIZATION

Mineralization in the region exists in two forms: 1) disseminated sulfides

within altered country rocks, and 2) quartz-carbonate-sulfide veins. Disseminated

mineralization is most prominent in the highly altered variety of the Pillowed

Basalt Member and in the Iron Formation Member, but also occurs occasionally in

other Archean rock types. It consists of disseminated pyrite and sporadic minor

pyrrhotite and chalcopyrite. The alteration minerals in the country rocks are

primarily chlorite, sericite, carbonate, and quartz.

The veins of quartz and carbonate, and lesser amounts of chlorite, are often

associated with shear zones and faults. The chief sulfide mineral is pyrite with

much lesser amounts of chalcopyrite, arsenopyrite, and pyrrhotite. Locally, galena

and sphalerite are the dominant sulfide vein minerals.

In general, it appears that two episodes of mineralization have affected the

volcanics and intrusives of the northern part of the Marquette Greenstone Belt.

The early episode produced the majority of the disseminated mineralization and

was likely controlled by the location of the east-west trending faults and shear

zones. This event was also responsible for the production of zones of intense

carbonatization, sericitization, and chloritization. The second episode produced

primarily carbonate-quartz veins with associated galena, sphalerite, and chalcopyrite

mineralization. These veins are localized near the areas of younger, north-south,

brittle faults. Anomalous precious metal values can be found associated with both

episodes of hydrothermal mineralization.

GEOLOGIC HISTORY

The earliest event in the region north of the Dead River Storage Basin was

the extrusion of the sub-aqueous, tholeiitic, pillow basalts that form the Pillowed

Basalt Member of the Volcanics of Silver Mine Lakes. The age of these basalts is

about 2.7 Ga based on the radiometric age dating of a rhyolite intrusive which

cuts basalt beneath the Clark Creek Basin (Trow, 1979). During eruption of the

basalts, several thin mafic to intermediate volcanic mudflow breccias that contain

locally abundant iron formation debris were deposited. Continued extrusion of

subaqueous lava flows produced additional pillow basalts. The volcanic pile was

then intruded by dikes and sills of gabbro (Gabbro of Clark Creek). The mafic

rocks were synchronously deformed and intruded by syn- to post-tectonic rhyolite

and granodiorite (2.7 Ga). The rhyolites may represent differentiated portions of

the granodiorite plutons (Bornhorst and Baxter, 1987). An episode of syn- to

post-tectonic hydrothermal alteration resulted in the precious metal mineralization.

The Late Archean deformation and hydrothermal activity also produced the

foliated and altered varieties of the Pillowed Basalt Member of the Volcanics of

Silver Mine Lakes.

ACKNOWLEDGEMENTS

The co-operative research effort which made this study possible was funded

by the Michigan Geological Survey Division of the Department of Natural

Resources and the Department of Geology and Geological Engineering at Michigan

Technological University. This paper has benefited from discussions with Ted

Bornhorst, Rod Johnson, Jo Kalliokoski, and Eric Owens. Careful review of this

paper by Ted Bornhorst, Jo Kalliokoski, and Klaus Schulz are also acknowledged.

A- 30

mineralization. These veins are localized near the areas of younger, north-south,

brittle faults. Anomalous precious metal values can be found associated with both

episodes of hydrothermal mineralization.

GEOLOGIC HISTORY

The earliest event in the region north of the Dead River Storage Basin was

the extrusion of the sub-aqueous, tholeiitic, pillow basalts that form the Pillowed

Basalt Member of the Volcanics of Silver Mine Lakes. The age of these basalts is /

about 2.7 Ga based on the radiometric age dating of a rhyolite intrusive which

cuts basalt beneath the Clark Creek Basin (Trow, 1979). During eruption of the

basalts, several thin mafic to intermediate volcanic mudflow breccias that contain

locally abundant iron formation debris were deposited. Continued extrusion of

subaqueous lava flows produced additional pillow basalts. The volcanic pile was

then intruded by dikes and sills of gabbro (Gabbro of Clark Creek). The mafic

rocks were synchronously deformed and intruded by syn- to post-tectonic rhyolite

and granodiorite (2.7 Ga). The rhyolites may represent differentiated portions of

the granodiorite plutons (Bornhorst and Baxter, 1987). An episode of syn- to

post-tectonic hydrothermal alteration resulted in the precious metal mineralization.

The Late Archean deformation and hydrothermal activity also produced the

foliated and altered varieties of the Pillowed Basalt Member of the Volcanics of

Silver Mine Lakes.

ACKNOWLEDGEMENTS

The co-operative research effort which made this study possible was funded

by the Michigan Geological Survey Division of the Department of Natural

Resources and the Department of Geology and Geological Engineering at Michigan

Technological University. This paper has benefited from discussions with Ted

Bornhorst, Rod Johnson, Jo Kalliokoski, and Eric Owens. Careful review of this

paper by Ted Bornhorst, Jo Kalliokoski, and Klaus Schulz are also acknowledged.

REFERENCES

Baxter, D.A., 1988, Geology, Geochemistry, and Hydrothermal AlterationAssociated with Precious Metal Mineralization in the Clark Creek Region,Marquette County, Michigan: M.S. Thesis, Michigan Technological University,Houghton, Michigan (in preparation).

Baxter, D.A., Bornhorst, T.J., and VanAlstine, J.L., 1987, Geology, Structure,and Associated Precious Metal Mineralization of Archean Rocks in the Vicinity ofClark Creek, Marquette County, Michigan: Michigan Geological Survey Division,Department of Natural Resources, Open File Report OFR-87-8, 54 pp.

Bodwell, W.A., 1972, Geologic Compilation and Nonferrous Metal Potential,Precambrian Section, Northern Michigan: M.S. Thesis, Michigan TechnologicalUniversity, Houghton, Michigan, 106 p.

Bornhorst, T.J, and Baxter, D.A., 1987, Geochemical Character of ArcheanRocks from the East Half of the Northern Complex, Upper Peninsula, Michigan:Institute on Lake Superior Geology Proceedings and Abstracts, v. 33, part 1, p. 12.

Johnson, R.C., Bornhorst, T.J. and VanAlstine, J.L., 1987, Geologic Setting ofPrecious Metal Mineralization in the Silver Creek to Island Lake area, MarquetteCounty, Michigan: Michigan Geological Survey Division, Department of NaturalResources, Open-File Report, OFR-87-4, Supersedes OFR-86-2, 134 p.

MacLellan, M.L. and Bornhorst, T.J., 1988, Geology, Structure, and Mineraliz-ation of the Reany Lake Area, Marquette County, Michigan: Michigan GeologicalSurvey Division, Department of Natural Resources, Open File Report, (in prepara-tion).

Morgan, P.J. and DeCristoforo, D.T., 1980, Geological Evolution of theIshpeming Greenstone Belt, Michigan, U.S.A.: Precambrian Research, v. 11, p.23-41.

Owens, E.O. and Bornhorst, T.J., 1985, Geology and Precious Metal Mineraliz-ation of the Fire Center and Holyoke Mines Area, Marquette County, Michigan:Michigan Geological Survey Division, Department of Natural Resources, Open FileReport OFR-85-2, 105 p.

Trow, J., 1979, Final report on diamond drilling for geologic information inthe Middle Precambrian basins in the western portion of northern Michigan:Michigan Geological Survey Division, Department of Natural Resources, Open FileReport UDOE OFR GJBX-l62(79), 44 p.

A-31

REFERENCES

Baxter, D.A., 1988, Geology, Geochemistry, and Hydrothermal Alteration Associated with Precious Metal Mineralization in the Clark Creek Region, Marquette County, Michigan: M.S. Thesis, Michigan Technological University, Houghton, Michigan (in preparation).

Baxter, D.A., Bornhorst, T.J., and VanAlstine, J.L., 1987, Geology, Structure, and Associated Precious Metal Mineralization of Archean Rocks in the Vicinity of Clark Creek, Marquette County, Michigan: Michigan Geological Survey Division, Department of Natural Resources, Open File Report OFR-87-8, 54 pp.

Bodwell, W.A., 1972, Geologic Compilation and Nonferrous Metal Potential, Precambrian Section, Northern Michigan: M.S. Thesis, Michigan Technological University, Houghton, Michigan, 106 p.

Bornhorst, T.J. and Baxter, D.A., 1987, Geochemical Character of Archean Rocks from the East Half of the Northern Complex, Upper Peninsula, Michigan: Institute on Lake Superior Geology Proceedings and Abstracts, v. 33, part 1, p. 12.

Johnson, R.C., Bornhorst, T.J. and VanAlstine, J.L., 1987, Geologic Setting of Precious Metal Mineralization in the Silver Creek to Island Lake area, Marquette County, Michigan: Michigan Geological Survey Division, Department of Natural Resources, Open-File Report, OFR-87-4, Supersedes OFR-86-2, 134 p.

MacLellan, M.L. and Bornhorst, T.J., 1988, Geology, Structure, and Mineraliz- ation of the Reany Lake Area, Marquette County, Michigan: Michigan Geological Survey Division, Department of Natural Resources, Open File Report, (in preparation).

Morgan, P.J. and DeCristoforo, D.T., 1980, Geological Evolution of the Ishpeming Greenstone Belt, Michigan, U.S.A.: Precambrian Research, v. 11, p. 23-4 1.

Owens, E.O. and Bornhorst, T.J., 1985, Geology and Precious Metal Mineraliz- ation of the Fire Center and Holyoke Mines Area, Marquette County, Michigan: Michigan Geological Survey Division, Department of Natural Resources, Open File Report OFR-85-2, 105 p.

Trow, J., 1979, Final report on diamond drilling for geologic information in the Middle Precambrian basins in the western portion of northern Michigan: Michigan Geological Survey Division, Department of Natural Resources, Open File Report UDOE OFR GJBX-162(79), 44 p.

Geology of the Ropes Gold Mine

R.A. Brozdowski

Callahan Mining Corporation Exploration Dept.25 Industrial Park Rd. Negaunee, Michigan, 49866

INTRODUCTION

The Ropes gold mine is located in the south-west part of the Archean

Marquette Greenstone Belt, 5.8 km northwest of the town of Ishpeming, Michigan,

at latitude 46° 32', longitude 87° 43', in the Sl/2 NW1/4 Section 29, T48N, R27W,

Ishpeming Township, Marquette County, Michigan, USA. The mine can be

reached by traveling northward from US Hwy 41, west via paved County Road

573, then west via unpaved County Road GCL (the Ropes mine access road).

HISTORY

Julius Ropes, a druggist and prospector from Ishpeming, Michigan, discovered

a gold-bearing quartz stringer in 1880 while prospecting for asbestos and

serpentine "marble" in low ground near the east end of the present-day Ropes

mine property. Continued prospecting resulted in discovery of the outcrop of the

Ropes main ore zone approximately 250 m west of the initial discovery (Allen,

1912). The original Ropes mine operated sporadically from 1882 to 1897,

producing 1029 Kg of Au and 2057 Kg of Ag from 195,000 tonnes of ore. The

property remained idle, except for minor cyanidization of tailings, until Calumet

and Hecla Consolidated Copper Co. conducted extensive surface and underground

exploration from 1935 to 1942 and outlined 1,393,000 tonnes of ore averaging 3.98

g/tonne Au (Broderick, 1945). The relatively low grade of the deposit at a time

of controlled gold prices, combined with wartime restrictions on precious metals

mining, precluded any production at that time. Callahan Mining Corporation of

Phoenix, Arizona, purchased the property from Arcadian Copper Mine Tours in

1975, began exploration in 1979, and made a decision to develop the mine in mid

1984. Production began in September 1985, and is now about 1,800 tonnes per

day.

A-32

Geology of the Ropes Gold Mine

R.A. Brozdowski

Callahan Mining Corporation Exploration Dept. 25 Industrial Park Rd. Negaunee, Michigan, 49866

INTRODUCTION

The Ropes gold mine is located in the south-west part of the Archean

Marquette Greenstone Belt, 5.8 km northwest of the town of Ishpeming, Michigan,

at latitude 46' 32', longitude 87' 43', in the Sl/2 NW1/4 Section 29, T48N, R27W, /

Ishpeming Township, Marquette County, Michigan, USA. The mine can be

reached by traveling northward from US Hwy 41, west via paved County Road

573, then west via unpaved County Road GCL (the Ropes mine access road).

HISTORY

Julius Ropes, a druggist and prospector from Ishpeming, Michigan, discovered

a gold-bearing quartz stringer in 1880 while prospecting for asbestos and

serpentine "marble" in low ground near the east end of the present-day Ropes

mine property. Continued prospecting resulted in discovery of the outcrop of the

Ropes main ore zone approximately 250 m west of the initial discovery (Allen,

1912). The original Ropes mine operated sporadically from 1882 to 1897,

producing 1029 Kg of Au and 2057 Kg of Ag from 195,000 tonnes of ore. The

property remained idle, except for minor cyanidization of tailings, until Calumet

and Hecla Consolidated Copper Co. conducted extensive surface and underground

exploration from 1935 to 1942 and outlined 1,393,000 tonnes of ore averaging 3.98

g/tonne Au (Broderick, 1945). The relatively low grade of the deposit at a time

of controlled gold prices, combined with wartime restrictions on precious metals

mining, precluded any production at that time. Callahan Mining Corporation of

Phoenix, Arizona, purchased the property from Arcadian Copper Mine Tours in

1975, began exploration in 1979, and made a decision to develop the mine in mid

1984. Production began in September 1985, and is now about 1,800 tonnes per

day.

GEOLOGY OF THE SW PART OF THE MARQUETTE GREENSTONE BELT

Rocks in the southwest part of the Marquette Greenstone Belt, which host the

Ropes mine, generally have mineral assemblages of the lower greenschist facies,

except in proximity to the granitoid rocks which bound the greenstone belt, where

amphibolite facies assemblages occur. The greenschist facies rocks are char-

acterized by a sub-vertical planer foliation, defined by a weak parallelism of

aphanitic phyllosilicate minerals, which is commonly conformable to flow margins

in volcanic rocks, graded bedding in graywackes, and compositional layering in

units such as chert-magnetite iron formation. There are relict cumulate textures

in serpentinized peridotite, pillows in basalt, and bipyramidal and embayed quartz

grains, and lithic fragments in volcaniclastic rocks. Therefore, rocks with readily

recognizable primary structures and textures are referred to here in by their

sedimentary or igneous names as recommended in the North American Code of

Stratigraphic nomenclature; the prefix "meta-" is impli€d. Hydrothermally altered

rocks in the immediate vicinity of the Ropes mine are referred to by their

dominant mineral assemblages listed in decreasing order of abundance, such as

"quartz—sericite-chlorite rock."

The southwest part of the greenstone belt comprises, from WNW to ESE, a

sequence of: (1) pillowed to massive tholeiitic basalt, associated hypabyssal gabbroic

sills and dikes, and subordinate mafic-volcanic derived graywackes; (2) an

inter—layered zone of basalt, dacite tuff, and porphyritic dacite sills; (3) dacite tuff

with subordinate tuff breccia, which hosts banded quartz-magnetite iron formation

and quartzose graywacke immediately west of the Ropes mine; (4) serpentinized,

fine grained peridotite, which is carbonatized and talc-altered in proximity to the

Ropes mine; and (5) volcanic conglomerates and tuff breccias of dcitic composi-

tion. The volcanic units strike ENE, dip subvertically, and top consistently to the

southeast, based on pillow facing directions and graded bedding (Callahan Mining

Corporation detailed geologic mapping and drill core logs). The serpentinized

peridotite has a relict, fine grained cumulate texture and was likely a hypabyssal

A- 33

GEOLOGY OF THE SW PART OF THE MARQUETTE GREENSTONE BELT

Rocks in the southwest part of the Marquette Greenstone Belt, which host the

Ropes mine, generally have mineral assemblages of the lower greenschist facies,

except in proximity to the granitoid rocks which bound the greenstone belt, where

amphibolite facies assemblages occur. The greenschist facies rocks are char-

acterized by a sub-vertical planer foliation, defined by a weak parallelism of

aphanitic phyllosilicate minerals, which is commonly conformable to flow margins

in volcanic rocks, graded bedding in graywackes, and compositional layering in

units such as chert-magnetite iron formation. There are relict cumulate textures

in serpentinized peridotite, pillows in basalt, and bipyramidal and embayed quartz

grains, and lithic fragments in volcaniclastic rocks. Therefore, rocks with readily

recognizable primary structures and textures are referred to here in by their

sedimentary or igneous names as recommended in the North American Code of

Stratigraphic nomenclature; the prefix "meta-" is implied. Hydrothermally altered

rocks in the immediate vicinity of the Ropes mine are referred to by their

dominant mineral assemblages listed in decreasing order of abundance, such as

"quartz-sericite-chlorite rock."

The southwest part of the greenstone belt comprises, from WNW to ESE, a

sequence of: (1) pillowed to massive tholeiitic basalt, associated hypabyssal gabbroic

sills and dikes, and subordinate mafic-volcanic derived graywackes; (2) an

inter-layered zone of basalt, dacite tuff, and porphyritic dacite sills; (3) dacite tuff

with subordinate tuff breccia, which hosts banded quartz-magnetite iron formation

and quartzose graywacke immediately west of the Ropes mine; (4) serpentinized,

fine grained peridotite, which is carbonatized and talc-altered in proximity to the

Ropes mine; and (5) volcanic conglomerates and tuff breccias of dccitic composi-

tion. The volcanic units strike ENE, dip subvertically, and top consistently to the

southeast, based on pillow facing directions and graded bedding (Callahan Mining

Corporation detailed geologic mapping and drill core logs). The serpentinized

peridotite has a relict, fine grained cumulate texture and was likely a hypabyssal

sill complex emplaced within the volcanic section, based on its fine grain size and

local complex interlayering with the volcanic rocks. The rocks which most

directly host the Ropes mine represent one of six known localities where volcanic

rocks are interlayered with the serpentinized peridotite (Callahan Mining

Corporation detailed geologic mapping and drill core logs); other volcanic inliers

may exist in poorly to non outcropping low ground throughout the outcrop belt of

the serpentinized peridotite.

The Ropes mine is located to the southeast of the basalt-to-dacite transition

zone and the layers of banded iron formation and graywackes (Figure 1). The

Ropes gold deposit is hosted by quartz—sericite—chlorite rock interpreted as

hydrothermally altered dacite tuff on the basis of the progressive development of

pervasive quartz and sericite alteration. This alteration gradationally replaces and

finally totally pseudomorphs feldspar phenocrysts, matrix feldspar, and felsic lithic

fragments in the tuffs as the Ropes mine is approached from the west. Similar

less mobile trace element abundances and similar chondrite-normalized rare earth

patterns between unaltered (except for greenschist facies metamorphism) dacite tuff

to the west of the Ropes mine and the quartz—sericite-chlorite rock which hosts

the Ropes deposit also provides evidence for a common origin (Callahan Mining

Corp., unpublished data).

GEOLOGY OF THE MINE AREA

There are four main rock types within the immediate mine area (1) Fine

grained quartz-sericite-chlorite rock encloses the ore zones, strikes N70°E and dips

steeply south. This rock type is bounded to the north and south by, (2) fine

grained, carbonate-quartz-chlorite rock which is massive to compositionally layered

on a scale of several millimeters. The quartz-sericite-chlorite rock and the

carbonate-quartz-chlorite rock are locally complexly interlayered, particularly

within and west of the mine (Figures 1, 2, 3). Contacts between the two rock

types are generally sharp. The carbonate-quartz-chlorite rock is flanked

A- 34

sill complex emplaced within the volcanic section, based on its fine grain size and

local complex interlayering with the volcanic rocks. The rocks which most

directly host the Ropes mine represent one of six known localities where volcanic

rocks are interlayered with the serpentinized peridotite (Callahan Mining

Corporation detailed geologic mapping and drill core logs); other volcanic inliers

may exist in poorly to non outcropping low ground throughout the outcrop belt of

the serpentinized peridotite.

The Ropes mine is located to the southeast of the basalt-to-dacite transition

zone and the layers of banded iron formation and graywackes (Figure 1). The ,

Ropes gold deposit is hosted by quartz-sericite-chlorite rock interpreted as

hydrothermally altered dacite tuff on the basis of the progressive development of

pervasive quartz and sericite alteration. This alteration gradationally replaces and

finally totally pseudomorphs feldspar phenocrysts, matrix feldspar, and felsic lithic

fragments in the tuffs as the Ropes mine is approached from the west. Similar

less mobile trace element abundances and similar chondrite-normalized rare earth

patterns between unaltered (except for greenschist facies metamorphism) dacite tuff

to the west of the Ropes mine and the quartz-sericite-chlorite rock which hosts

the Ropes deposit also provides evidence for a common origin (Callahan Mining

Corp., unpublished data).

GEOLOGY OF THE MINE AREA

There are four main rock types within the immediate mine area: (1) Fine

grained quartz-sericite-chlorite rock encloses the ore zones, strikes N70Â° and dips

steeply south. This rock type is bounded to the north and south by, (2) fine

grained, carbonate-quartz-chlorite rock which is massive to compositionally layered

on a scale of several millimeters. The quartz-sericite-chlorite rock and the

carbonate-quartz-chlorite rock are locally complexly interlayered, particularly

within and west of the mine (Figures 1, 2, 3). Contacts between the two rock

types are generally sharp. The carbonate-quartz-chlorite rock is flanked

EX

PLA

NA

TIO

N F

OR

FIG

UR

E 1

JHIH :

Geology

Serpentinized fine-grained peridotite

Carbonatized, serpentinized peridotite

Carbonate-talc rock

Compositionally layered to massive carbonate-quartz

chlorite rock

Graywacke

Quartzite

Symbols

Contact (dotted where

-.

infe

rred

)

Unpaved road

Paved road

Field trip stop

Dacite tuff, tuff breccia and flows, minor andesite

flows; altered to quartz-sericite-chlorite rock in

mine area

Banded quartz-magnetite iron formation

Graywacke and siltstone

Fine-grained mafic intrusions

Basalt (includes pillowed and glomerophyric basalt)

Basalt tuff

Ct, U '-4

LI Ti E

1 IIL±

-

Fault

(dotted where inferred)

rS

haft

JJpdip projection to

surface of orebodies

v Ill N

a

Figure 1.

Geologic Map of the Ropes Mine area.

Explanation on preceding page.

ELEVATION (ft.)

SeaLevel

EXPlANATION

Serpentinized fine-grained peridot ite,locally carbonatized

Carbonate-talc rock

Compositionallylayered to massivecarbonate-quartz-chlorite rock

Quartz-sericite-chlorite rock

Orebody>2 g Au/tonne

Figure 2.Ropes Mine 600 ECross-Sectionlooking S 80° W

800

IH I Ii

LI I

L

61 rn.

600

1400

1200

1000

— 200

400

400

L

Mine Levels

Scale

100 200 ft.

L

200

L0

L

61 m.

0

100

L

200 ft.

A-37

ELEVATIO

Sea Leva

EXPLANAT ION

Serpentinized fine- grained peridotite, locally carbonatized

Carbonate-talc rock

Compositionally layered to massive carbonate-quartz- chlorite rock

Quartz-sericite- chlorite rock

Orebody >2 g Aultonne

Mine Levels

Sca le

100 200 f t .

Figure 2. Ropes Mine 600 E Cross-Section looking S 80' W

Sca

le20

0 ft.

-H 61 m

.

Geologic plan maps of the Ropes Nine.

See Fig. 2 for explanation.

Figure 3.

successively, on both the north and south sides, by, (3) fine grained, massive to

moderately foliated, carbonate-talc rock, and (4) fine grained, totally serpentinized

peridotite which commonly retains a relict cumulate texture after original olivine

and lesser pyroxene. This rock is carbonatized to varying degrees near its contacts

with the carbonate-talc rock. Contacts between the carbonate-quartz-chlorite rock

and the carbonate-talc rock, and between the carbonate-talc rock and the

serpentinized peridotite are generally gradational over one to several meters.

The preservation of relict feldspar and lithic clasts, and the total thickness of

quartz-sericite-chlorite rock, increases gradationally westward from the Ropes main

ore zone (Figure 1) over a distance of several hundred meters into a large body of

dacite that has moderately aligned twinned plagioclase phenocrysts, bipyramidal

quartz, and volcanic rock fragments in an aphanitic quartz-sericite-feldspar-chlorite

matrix characteristic of a crystal—lithic tuff. The dacite locally contains intervals

of lapilli-sized fragments. The carbonate—quartz-chlorite rock is restricted largely

to the mine area (Figure 1), although thin intervals up to 2 m thick occur up to

400 m west of the Ropes main ore zone. The thickest intervals of carbonate-talc

rock occur within the mine area (Figure 1); however, thin intervals of talc-rich

rock occur at numerous localities at the contacts of, and also in, serpentinized

peridotite away from the mine area (Rossell, 1983). Serpentinized peridotite

occurs up to 2 km northeast of the Ropes mine and up to 5 km southwest of the

mine in a northeast trending set of outcrops, approximately 600 m wide (Clark

et.al., 1975).

MAJOR ROCK TYPES IN THE ROPES MINE PROPERTY

Quartz-Sericite-Chlorite Rock

The main host for gold is light green, massive to slightly foliated, fine

grained, quartz-sericite-chlorite rock with relict clasts. This rock is up to 40 m

thick on the 800 Mine Level, but narrows toward the surface and toward the east

to less than 6 m thick. It is complexly interlayered with carbonate-quartz-chlorite

A-39

successively, on both the north and south sides, by, (3) fine grained, massive to

moderately foliated, carbonate-talc rock, and (4) fine grained, totally serpentinized

peridotite which commonly retains a relict cumulate texture after original olivine

and lesser pyroxene. This rock is carbonatized to varying degrees near its contacts

with the carbonate-talc rock. Contacts between the carbonate-quartz-chlorite rock

and the carbonate-talc rock, and between the carbonate-talc rock and the

serpentinized peridotite are generally gradational over one to several meters.

The preservation of relict feldspar and lithic clasts, and the total thickness of

quartz-sericite-chlorite rock, increases gradationally westward from the Ropes main

ore zone (Figure 1) over a distance of several hundred meters into a large body of

dacite that has moderately aligned twinned plagioclase phenocrysts, bipyramidal

quartz, and volcanic rock fragments in an aphanitic quartz-sericite-feldspar-chlorite

matrix characteristic of a crystal-lithic tuff. The dacite locally contains intervals

of lapilli-sized'fragments. The carbonate-quartz-chlorite rock is restricted largely

to the mine area (Figure l), although thin intervals up to 2 m thick occur up to

400 m west of the Ropes main ore zone. The thickest intervals of carbonate-talc

rock occur within the mine area (Figure 1); however, thin intervals of talc-rich

rock occur at numerous localities at the contacts of, and also in, serpentinized

peridotite away from the mine area (Rossell, 1983). Serpentinized peridotite

occurs up to 2 km northeast of the Ropes mine and up to 5 km southwest of the

mine in a northeast trending set of outcrops, approximately 600 m wide (Clark

et.al., 1975).

MAJOR ROCK TYPES IN THE ROPES MINE PROPERTY

Quartz-Sericite-Chlorite Rock

The main host for gold is light green, massive to slightly foliated, fine

grained, quartz-sericite-chlorite rock with relict clasts. This rock is up to 40 m

thick on the 800 Mine Level, but narrows toward the surface and toward the east

to less than 6 m thick. It is complexly interlayered with carbonate-quartz-chlorite

rock, especially west of the Ropes main ore zone (Figure 1) and in the mine

(Figures 2 and 3). The rock has equant quartz grains 50 to 100 microns in

diameter in a randomly oriented to moderately aligned sub-20 micron quartz,

sericite, and minor chlorite matrix. Randomly oriented to moderately aligned, I to

2 mm, angular, rectangular mats of sub-10 micron sericite and minor quartz are

enveloped in this matrix and have an external habit very much like the plagioclase

phenocrysts in the thicker, more massive, lateral equivalent of this rock

immediately west of the mine. This textural change is gradational eastward

toward the orebody over a distance of a few hundred meters and is characterized

by increasing alteration of the plagioclase phenocrysts to sericite and an increase

in the abundance of quartz and sericite in the matrix, with a corresponding

decrease in feldspar. Therefore, the sericite-rich mats in the quartz-sericite-

chlorite rock of the mine are interpreted as pseudomorphs after plagioclase

phenocrysts. The sericite pseudomorphs constitute up to 20 percent of the rock.

Uncommon clasts, I to 2 mm in diameter and composed of microcrystalline aggre-

gates of quartz, may represent pseudomorphs of quartz phenocrysts which now

exhibit subgrain development, or alternatively, some may be chert fragments.

Chlorite-quartz fragments, which may be more mafic volcanic rock fragments or

pseudomorphed mafic mineral phenocrysts, occur less commonly in the rock.

Chlorite is generally a subordinate component of the rock at less than 10% and

occurs both dispersed in the matrix and locally as coarser laths which define a

slight foliation. Ferroan dolomite is locally a minor component of the matrix.

The rock is layered locally with 5 mm thick, planer, sericite-rich and alternate

more chlorite—rich laminae.

Gold abundance is generally greatest where very fine to aphanitic pyrite

comprises 5 to 8% of the rock, quartz and sericite are most abundant, and chlorite

is least abundant (this generality does not apply in the case of the chloritic,

pyritic, farthest eastern rim of the Ropes main ore zone, as discussed in the

subsequent section on "The Orebodies"). Rock with greatest gold abundance

A-40

rock, especially west of the Ropes main ore zone (Figure 1) and in the mine

(Figures 2 and 3). The rock has equant quartz grains 50 to 100 microns in

diameter in a randomly oriented to moderately aligned sub-20 micron quartz,

sericite, and minor chlorite matrix. Randomly oriented to moderately aligned, 1 to

2 mm, angular, rectangular mats of sub-10 micron sericite and minor quartz are

enveloped in this matrix and have an external habit very much like the plagioclase

phenocrysts in the thicker, more massive, lateral equivalent of this rock

immediately west of the mine. This textural change is gradational eastward

toward the orebody over a distance of a few hundred meters and is characterized

by increasing alteration of the plagioclase phenocrysts to sericite and an increase

in the abundance of quartz and sericite in the matrix, with a corresponding

decrease in feldspar. Therefore, the sericite-rich mats in the quartz-sericite-

chlorite rock of the mine are interpreted as pseudomorphs after plagioclase

phenocrysts. The sericite pseudomorphs constitute up to 20 percent of the rock.

Uncommon clasts, 1 to 2 mm in diameter and composed of microcrystalline aggre-

gates of quartz, may represent pseudomorphs of quartz phenocrysts which now

exhibit subgrain development, or alternatively, some may be chert fragments.

Chlorite-quartz fragments, which may be more mafic volcanic rock fragments or

pseudomorphed mafic mineral phenocrysts, occur less commonly in the rock.

Chlorite is generally a subordinate component of the rock at less than 10% and

occurs both dispersed in the matrix and locally as coarser laths which define a

slight foliation. Ferroan dolomite is locally a minor component of the matrix.

The rock is layered locally with 5 mm thick, planer, sericite-rich and alternate

more chlorite-rich laminae.

Gold abundance is generally greatest where very fine to aphanitic pyrite

comprises 5 to 8% of the rock, quartz and sericite are most abundant, and chlorite

is least abundant (this generality does not apply in the case of the chloritic,

pyritic, farthest eastern rim of the Ropes main ore zone, as discussed in the

subsequent section on "The Orebodies"). Rock with greatest gold abundance

contains only trace amounts of carbonate, compared with higher but still minor

concentrations of carbonate in less auriferous parts of the quartz-sericite-chlorite

rock. Fine pyrite, 50 to 100 microns in diameter, is dispersed in the rock matrix

and less commonly within the sericite pseudomorphs after plagioclase and is not

confined only to quartz veinlets or to foliation planes. Coarser pyrite, 0.5 to I

mm in diameter, occurs near quartz veinlets and also along chioritic foliations.

This relatively coarse pyrite is enclosed locally by quartz over-growths, and is

occasionally pulled apart along the foliation. Rock characterized by only coarser

pyrite is not ore. Quartz veinlets less than 1 mm thick comprise up to several

percent of the rock and are randomly oriented. Quartz in the veinlets commonly

has undulatory extinction and local mortar texture. Rock outside of the ore zones

is characterized by <2% very fine pyrite.

Carbonate-Quartz-Chlorite Rock

Carbonate-quartz-chlorite rock envelopes the afore-mentioned quartz-sericite-

chlorite rock and is complexly interlayered with it, particularly toward the west

end of the Ropes mine (Figure 1). The carbonate-quartz-chlorite rock contains

abundant ferroan dolomite, and lesser quartz and chlorite. Minor sericite occurs

in some specimens, and minor talc in others, but these two minerals are generally

mutually exclusive. Contacts with the quartz-sericite-chlorite rock are sharp;

however, minor carbonate minerals are present in the quartz-sericite-chlorite rock

near its contacts with the carbonate-quartz-chlorite rock, and minor sericite is

present in the carbonate-quartz-chlorite rock near its contacts with the quartz-

sericite-chlorite rock. Typically, this rock contains up to 150 ppb Au.

Quartz-rich parts of the carbonate-quartz-chlorite rock are locally slightly pyritic

and contain up to 1 ppm Au.

The carbonate-quartz-chlorite rock is commonly compositionally layered on a

scale of several millimeters, but locally is massive. Layering is defined by

parallel, fine to medium grained, white carbonate laminae, chlorite foliation, and

A-41

contains only trace amounts of carbonate, compared with higher but still minor

concentrations of carbonate in less auriferous parts of the quartz-sericite-chlorite

rock. Fine pyrite, 50 to 100 microns in diameter, is dispersed in the rock matrix

and less commonly within the sericite pseudomorphs after plagioclase and is not

confined only to quartz veinlets or to foliation planes. Coarser pyrite, 0.5 to 1

mm in diameter, occurs near quartz veinlets and also along chloritic foliations.

This relatively coarse pyrite is enclosed locally by quartz over-growths, and is

occasionally pulled apart along the foliation. Rock characterized by only coarser

pyrite is not ore. Quartz veinlets less than 1 mm thick comprise up to several

percent of the rock and are randomly oriented. Quartz in the veinlets commonly

has undulatory extinction and local mortar texture. Rock outside of the ore zones

is characterized by <2% very fine pyrite.

Carbonate-Quartz-Chlorite Rock

Carbonate-quartz-chlorite rock envelopes the afore-mentioned quartz-sericite-

chlorite rock and is complexly interlayered with it, particularly toward the west

end of the Ropes mine (Figure 1). The carbonate-quartz-chlorite rock contains

abundant ferroan dolomite, and lesser quartz and chlorite. Minor sericite occurs

in some specimens, and minor talc in others, but these two minerals are generally

mutually exclusive. Contacts with the quartz-sericite-chlorite rock are sharp;

however, minor carbonate minerals are present in the quartz-sericite-chlorite rock

near its contacts with the carbonate-quartz-chlorite rock, and minor sericite is

present in the carbonate-quartz-chlorite rock near its contacts with the quartz-

sericite-chlorite rock. Typically, this rock contains up to 150 ppb Au.

Quartz-rich parts of the carbonate-quartz-chlorite rock are locally slightly pyritic

and contain up to 1 ppm Au.

The carbonate-quartz-chlorite rock is commonly compositionally layered on a

scale of several millimeters, but locally is massive. Layering is defined by

parallel, fine to medium grained, white carbonate laminae, chlorite foliation, and

quartz-chlorite-rich laminae. The rock is increasingly talcose toward the contact

with carbonate-talc rock. Coarse grained, barren, white dolomite veins up to 5

cm thick cut across all other features and comprise <1% of the rock. Carbonate-

quartz-chlorite rock occurs not only in the mine, but also in a layer up to 60 m

thick on the north side of the mine (Figure 1), where it contains thin layers of

the other three main rock types.

Serpentinized Perdotite and Carbonate-Talc Rock

The interlayered quartz-sericite-chlorite rock and carbonate-quartz-chlorite

rock are bounded on the north and south by sill-like masses of serpentinized

peridotite. There are several smaller serpentinized peridotite bodies within the

carbonate-quartz-chlorite rock on the north side of the Ropes mine. The larger

of these minor serpentinized peridotite masses forms the north wall of the Ropes

mine (Figures 1, 2). The rock is dark gray to green, fine grained serpentine

containing lesser talc and carbonate, and minor chlorite, with accessory chromite

and magnetite. Commonly it has a relict cumulate texture of 1 to 3 mm serpen-

tine pseudomorphs after olivine and pyroxene, which are surrounded by rims of

talc and carbonate, although locally the rock is dark green, felted textured

serpentine. The serpentinized peridotite is increasingly carbonate—rich toward its

borders, although relict texture is commonly continuously preserved. Au

abundance is typically less than 30 ppb in the serpentinized peridotite, even in

close proximity to the mine.

Massive to well foliated, gray to dark green, very fine grained carbonate-talc

rock occurs around the margins of serpentinized peridotite (Figures 1, 2, 3). The

carbonate-talc rock has gradational contacts over one to several meters with

serpentinized peridotite and with talc-rich parts of the carbonate-quartz-chlorite

rock. The carbonate mineral in the carbonate-talc rock is dominantly ferroan

dolomite with minor magnesite. Au abundance ranges from <30 ppb to approxi-

mately 100 ppb in the carbonate-talc rock.

A- 42

quartz-chlorite-rich laminae. The rock is increasingly talcose toward the contact

with carbonate-talc rock. Coarse grained, barren, white dolomite veins up to 5

cm thick cut across all other features and comprise ~ 1 % of the rock. Carbonate-

quartz-chlorite rock occurs not only in the mine, but also in a layer up to 60 m

thick on the north side of the mine (Figure l), where it contains thin layers of

the other three main rock types.

Serpentinized Peridotite and Carbonate-Talc Rock

The interlayered quartz-sericite-chlorite rock and carbonate-quartz-chlorite

rock are bounded on the north and south by sill-like masses of serpentinized

peridotite. There are several smaller serpentinized peridotite bodies within the

carbonate-quartz-chlorite rock on the north side of the Ropes mine. The larger

of these minor serpentinized peridotite masses forms the north wall of the Ropes

mine (Figures 1, 2). The rock is dark gray to green, fine grained serpentine

containing lesser talc and carbonate, and minor chlorite, with accessory chromite

and magnetite. Commonly it has a relict cumulate texture of 1 to 3 mm serpen-

tine pseudomorphs after olivine and pyroxene, which are surrounded by rims of

talc and carbonate, although locally the rock is dark green, felted textured

serpentine. The serpentinized peridotite is increasingly carbonate-rich toward its

borders, although relict texture is commonly continuously preserved. Au

abundance is typically less than 30 ppb in the serpentinized peridotite, even in

close proximity to the mine.

Massive to well foliated, gray to dark green, very fine grained carbonate-talc

rock occurs around the margins of serpentinized peridotite (Figures 1, 2, 3). The

carbonate-talc rock has gradational contacts over one to several meters with

serpentinized peridotite and with talc-rich parts of the carbonate-quartz-chlorite

rock. The carbonate mineral in the carbonate-talc rock is dominantly ferroan

dolomite with minor magnesite. Au abundance ranges from <30 ppb to approxi-

mately 100 ppb in the carbonate-talc rock.

The spatial association of carbonate-talc rock with the margins of serpentin-

ized peridotite, the gradational contacts with serpentinized peridotite, and the

similar less mobile trace element composition of the carbonate-talc rock and

serpentinized peridotite, especially with regard to Cr content, suggests that the

carbonate-talc rock is the altered margins of serpentinized peridotite masses.

SUMMARY OF MINERAL COMPOSITION

Minerals in rock samples from selected traverses across and along the strike of

the Ropes deposit were analyzed by electron microprobe. Although detailed

mineral chemistry is not dealt with here, a brief summary of general trends is

included in this field guide (Callahan Mining Corporation, unpublished data).

Chlorite occurs in nearly all rock types, but its composition and abundance

are variable. In general, the Fe/Mg ratio of chlorite increases toward the center

of the quartz-sericite-chlorite rock and away from serpentinized peridotite. The

chlorite is mostly sheridanite in the quartz—sericite-chlorite rocks and clinochlore in

the other major rock types.

Talc increases in iron content from serpentinized peridotite toward quartz-

sericite-chiorite rock, but is absent in the quartz-sericite-chlorite rock itself. Talc

compositions show a narrow range, with an Fe/(Fe+Mg) cation ratio between 0.0

and 0.11, centered at 0.05.

Sericite is restricted mostly to the quartz-sericite-chlorite rock and is minor in

carbonate—quartz-chlorite rock, where these two rocks are in contact. The sericite

is an Fe- and Mg-bearing muscovite, with FeO and MgO contents of as much as

several wt.% each. The sum of Mg+Fe cations in the sericite structural formula,

on the basis of 22 oxygens, is between 0.4 and 1.2 cations, centered at 0.6 cations.

No consistent trends in sericite composition were recognized in the rocks of the

Ropes mine.

Carbonate minerals are common to all rocks and are dominantly low iron

dolomite and lesser pure dolomite. Low iron dolomite is a minor component in

A— 43

The spatial association of carbonate-talc rock with the margins of serpentin-

ized peridotite, the gradational contacts with serpentinized peridotite, and the

similar less mobile trace element composition of the carbonate-talc rock and

serpentinized peridotite, especially with regard to Cr content, suggests that the

carbonate-talc rock is the altered margins of serpentinized peridotite masses.

SUMMARY OF MINERAL COMPOSITION

Minerals in rock samples from selected traverses across and along the strike of

the Ropes deposit were analyzed by electron microprobe. Although detailed

mineral chemistry is not dealt with here, a brief summary of general trends is

included in this field guide (Callahan Mining Corporation, unpublished data).

Chlorite occurs in nearly all rock types, but its composition and abundance

are variable. In general, the Fe/Mg ratio of chlorite increases toward the center

of the quartz-sericite-chlorite rock and away from serpentinized peridotite. The

chlorite is mostly sheridanite in the quartz-sericite-chlorite rocks and clinochlore in

the other major rock types.

Talc increases in iron content from serpentinized peridotite toward quartz-

sericite-chlorite rock, but is absent in the quartz-sericite-chlorite rock itself. Talc

compositions show a narrow range, with an Fe/(Fe+Mg) cation ratio between 0.0

and 0.1 1, centered at 0.05.

Sericite is restricted mostly to the quartz-sericite-chlorite rock and is minor in

carbonate-quartz-chlorite rock, where these two rocks are in contact. The sericite

is an Fe- and Mg-bearing muscovite, with FeO and MgO contents of as much as

several wt.% each. The sum of Mg+Fe cations in the sericite structural formula,

on the basis of 22 oxygens, is between 0.4 and 1.2 cations, centered at 0.6 cations.

No consistent trends in sericite composition were recognized in the rocks of the

Ropes mine.

Carbonate minerals are common to all rocks and are dominantly low iron

dolomite and lesser pure dolomite. Low iron dolomite is a minor component in

quartz-sericite-chlorite rock, but is a major component in the other three major

rock types at the Ropes mine. Calcite is minor in quartz-sericite-chlorite rock.

Magnesite is commonly a major component of the serpentinized peridotite and the

carbonate-talc rock. Serpentine is major in the serpentinized peridotite.

Disseminated chromite is minor in the serpentinized peridotite and carbonate-talc

rock.

The differences in mineral composition probably reflect, in large part, original

differences in bulk composition between serpentinized peridotite and the dacitic

volcaniclastic protolith of the quartz-sericite-chlorite rock. Talc and chlorite

compositions are more Mg-rich, and Mg-carbonate minerals are also more abun-

dant toward the serpentinized peridotite.

STRUCTURE

The dominant structural element in the Ropes mine area is a N70°E sub-

vertical foliation defined by the alignment of aphanitic phyllosilicate minerals,

which is generally parallel to the layering between rock types. The overall strike

of the Ropes main ore zone also nearly parallels this foliation. A slight,

sporadically present sub-vertical second foliation strikes N45°E and is defined by

generally slightly coarser grained phyllosilicate laths.

Several ore-related structural elements characterize the Ropes deposit: 1) a

series of N55°E to N65°E striking, vertically dipping, steeply westward plunging,

auriferous quartz vein-dominated pods occur progressively further east in

successively deeper levels of the mine. These pods occur at, or south of, the

south side of the Ropes main ore zone; 2) cryptic zones of significantly higher

grade Au mineralization, defined solely on the basis of assay data, are contained

more centrally within the Ropes main ore zone. The middle parts of these higher

grade zones strike across the overall N80°E trend of the orebody in en-echelon

fashion at approximately N55°E, while the extremities of these higher grade

transverse zones have a nearly N70°E trend, thus forming a low angle sigmoidal

A- 44

quartz-sericite-chlorite rock, but is a major component in the other three major

rock types at the Ropes mine. Calcite is minor in quartz-sericite-chlorite rock.

Magnesite is commonly a major component of the serpentinized peridotite and the

carbonate-talc rock. Serpentine is major in the serpentinized peridotite.

Disseminated chromite is minor in the serpentinized peridotite and carbonate-talc

rock.

The differences in mineral composition probably reflect, in large part, original

differences in bulk composition between serpentinized peridotite and the dacitic

volcaniclastic protolith of the quartz-sericite-chlorite rock. Talc and chlorite

compositions are more Mg-rich, and Mg-carbonate minerals are also more abun-

dant toward the serpentinized peridotite.

STRUCTURE

The dominant structural element in the Ropes mine area is a N70Â° sub-

vertical foliation defined by the alignment of aphanitic phyllosilicate minerals,

which is generally parallel to the layering between rock types. The overall strike

of the Ropes main ore zone also nearly parallels this foliation. A slight,

sporadically present sub-vertical second foliation strikes N45OE and is defined by

generally slightly coarser grained phyllosilicate laths.

Several ore-related structural elements characterize the Ropes deposit: 1) a

series of N55OE to N65OE striking, vertically dipping, steeply westward plunging,

auriferous quartz vein-dominated pods occur progressively further east in

successively deeper levels of the mine. These pods occur at, or south of, the

south side of the Ropes main ore zone; 2) cryptic zones of significantly higher

grade Au mineralization, defined solely on the basis of assay data, are contained

more centrally within the Ropes main ore zone. The middle parts of these higher

grade zones strike across the overall N80"E trend of the orebody in en-echelon

fashion at approximately N55OE, while the extremities of these higher grade

transverse zones have a nearly N70Â° trend, thus forming a low angle sigmoidal

pattern across the ore zone (Figure 5). This pattern can best be recognized at the

250 ft. elevation in the mine, toward the east end of the Ropes main ore zone

(Figure 3).

Both of the above structural elements can be regarded as extensional features.

The quartz vein dominated pods are evidence of silica addition in incipient voids.

The higher grade transverse zones within the main ore zone, while not strictly

veins, nonetheless may have been zones of increased permeability to ore fluids. A

comparison with shear zone models presented in Ramsay and Huber (1983) demon-

strates that the above structural elements are best interpreted as extensional

features, and that the rocks containing the Ropes deposit can be considered to

represent a structural setting involving some degree of simple shear, with a

component of positive volume change in the shear zone. Figure 5 compares the

theoretical orientation of extensional vein arrays developed in a shear zone as a

result of positive volume change in a shear zone with the observed geologic

relations of ore related structural elements at the Ropes mine: it is geometrically

correct that en-echelon fissure systems will always make angles of less than 450 to

the trend of the shear zone in cases where positive volume change is involved

(ibid). The observed geometric relationships at the Ropes mine between the two

ore related structural elements discussed above, and the shear zone "walls" (the

nonfoliated serpentinized peridotite), are consistent with the required acute angle

relationship. The required relative sense of shear in the present day horizontal

plan view of the Ropes mine area is, therefore, north side to the WSW and south

side to the ENE, as illustrated in Figure 5. Connecting the midpoints of the

quartz vein dominated pods at the Ropes mine gives the true sense of shear in the

third dimension as lying along a line orien.ed down to the ENE and up to the

WSW (Figure 5). A simultaneous consideration of the plan and long section views

necessitate that the south side of the zone had an ENE and downward net compo-

nent of movement, while the north side of the zone had a WSW and upward net

component of movement.

A-45

pattern across the ore zone (Figure 5). This pattern can best be recognized at the

250 ft. elevation in the mine, toward the east end of the Ropes main ore zone

(Figure 3).

Both of the above structural elements can be regarded as extensional features.

The quartz vein dominated pods are evidence of silica addition in incipient voids.

The higher grade transverse zones within the main ore zone, while not strictly

veins, nonetheless may have been zones of increased permeability to ore fluids. A

comparison with shear zone models presented in Ramsay and Huber (1983) demon-

strates that the above structural elements are best interpreted as extensional

features, and that the rocks containing the Ropes deposit can be considered to

represent a structural setting involving some degree of simple shear, with a

component of positive volume change in the shear zone. Figure 5 compares the

theoretical orientation of extensional vein arrays developed in a shear zone as a

_result of positive volume change in a shear zone with the observed geologic

relations of ore related structural elements at the Ropes mine: it is geometrically

correct that en-echelon fissure systems will always make angles of less than 45' to

the trend of the shear zone in cases where positive volume change is involved

(ibid). The observed geometric relationships at the Ropes mine between the two

ore related structural elements discussed above, and the shear zone "walls" (the

nonfoliated serpentinized peridotite), are consistent with the required acute angle

relationship. The required relative sense of shear in the present day horizontal

plan view of the Ropes mine area is, therefore, north side to the WSW and south

side to the ENE, as illustrated in Figure 5. Connecting the midpoints of the

quartz vein dominated pods at the Ropes mine gives the true sense of shear in the

third dimension as lying along a line oriented down to the ENE and up to the

WSW (Figure 5). A simultaneous consideration of the plan and long section views

necessitate that the south side of the zone had an ENE and downward net compo-

nent of movement, while the north side of the zone had a WSW and upward net

component of movement.

1. Light green to tan quartz-sericite rock, with lenses of lightgray cherty quartz rock containing pyrite-tetrahedriteChalcO

pyrite-galena. Pyrite content of this ore type does notcorrelate with gold grade

2. Light green quartz-sericite-chloritepyrite rock. Good

correlation between fine-grained pyrite content and Au grade.

3. Dark green quartz-chlorite-pyrite rock. Consistently high

fine-grained pyrite content and high gold grade.

Figure 4. Ropes Mine long-section looking N 100 W

A- 46

Major Gold Ore Types

. E Rooes Production Shaft

\ .*l.-Northwes+ 2 Ore Zone

Major Gold Ore Types

1. Light green to tan quartz-sericite rock, with lenses of light gray cherty quartz rock containing pyrite-tetrahedrite-chalcopyrite-galena. Pyrite content of this ore type does not correlate with gold grade

2. Light green quartz-sericite-chlorite-pyrite rock. Good correlation between fine-grained pyrite content and Au grade.

3. Dark green quartz-chlorite-pyrite rock. Consistently high fine-grained pyrite content and high gold grade.

Figure 4 . Ropes Mine long-section looking N lo0 W

A-46

A

B

ELANATION

_____

arrow in planview projectionof plane ofshear

outline of Ropes orebody

zone of higher-grade Aumineralization

vertical projection of quartzvein—dominated pods (extensionveins) onto the plan view

quartz—vein dominated podsprojected onto N80°E longsection

L— contact between serpentinizedperidotite and dacite Cuff (withassociated rocks)

strike of layering in rocks

Figure 5: Schematic structural diagrams of the Ropes MineA is plan view, inset after Ramsay and Huber,1983, Figure 3.21A; B is N80°E vertical long section view

A A

near vertical foliation

(with upcomponent)

(with downcomponent)

7

true sense of shear determinedby connecting midpoints ofextension views

maximum stretch direction(vein opening direction)

A-47

(with up arrow in plan w component) view projection near vertical foliation of plane of

9-> (with down shear outline of Ropes orebody component

zone of higher-grade Au true sense of shear determined mineralization by connecting midpoints of vertical projection of quartz extension views vein-dominated pods (extension veins) onto the plan view

quartz-vein dominated pods projected onto ~ 8 0 ~ long 0 maximum stretch direction

section <\s (vein opening direction)

contact between serpentinized <y

peridotite and dacite Cuff (wi~h - associated rocks)

strike of layering in rocks

Figure 5: Schematic structural diagrams of the Ropes Mine A is plan view, inset after Ramsay and Huber, 1983, Figure 3.21A; B is N80Â° vertical long section view

THE OREBODIES

The Ropes mine main ore zone is steeply dipping, 335 m in maximum strike

length, 12 m in average thickness, and 600 m in presently defined down-dip

extent (Figure 4). It occurs within the quartz-sericite—chlorite rock, and is

characterized by disseminated pyrite with only minor quartz veins. The total

stated property tonnage, including historic production through 1987 and geologic

reserves, is approximately 2.5 million tonnes at 3.25 g/tonne Au. Based on

available information from the upper third of the Ropes main zone, the Ag grade

is estimated at 12 g/tonne.

The Ropes main ore zone can be divided longitudinally into three major ore

types based on the physical characteristics of the ore, as illustrated in Figure 4.

In plan view, these ore types form crescent—shaped, concentric shells, with the

convex side of each ore type toward the east and the concave side toward the

west. The convex-to-the-east geometry of the ore type zones, along with an

overall gradual western cutoff of Au mineralization and a sharp eastern cutoff,

suggests that the flow vector for the mineralizing fluids, relative to the

present-day surface, may have been upwards from the deep, southwest end of the

deposit toward the shallower northeast, high grade keel of the deposit.

The Ropes main ore zone is cut by an east-striking, south- dipping, low

angle reverse fault zone at the 900 mine level (Figure 2). Offset of units across

the fault zone is generally greater at the south side of the mine than at the north

side. The orebody thins down dip toward the fault zone, but thickens below it.

Several other gold-mineralized bodies exist on the Ropes property. The

Northwest ore zone is a small ore zone to the northwest of the Ropes main ore

zone (Figure 4). Significant intersections of gold-mineralized rock, comparable in

grade and width to the Ropes main ore zone, have been encountered in deep

drilling 370 m below the Ropes mine 1284 level (Callahan Mining Corporation

news release, July 20, 1987).

In general, most gold occurs associated with sub-100 micron sized pyrite

A-48

THE OREBODIES

The Ropes mine main ore zone is steeply dipping, 335 m in maximum strike

length, 12 m in average thickness, and 600 m in presently defined down-dip

extent (Figure 4). It occurs within the quartz-sericite-chlorite rock, and is

characterized by disseminated pyrite with only minor quartz veins. The total

stated property tonnage, including historic production through 1987 and geologic

reserves, is approximately 2.5 million tonnes at 3.25 g/tonne Au. Based on

available information from the upper third of the Ropes main zone, the Ag grade

is estimated at 12 g/tonne.

The Ropes main ore zone can be divided longitudinally into three major ore

types based on the physical characteristics of the ore, as illustrated in Figure 4.

In plan view, these ore types form crescent-shaped, concentric shells, with the

convex side of each ore type toward the east and the concave side toward the

west. The convex-to-the-east geometry of the ore type zones, along with an

overall gradual western cutoff of Au mineralization and a sharp eastern cutoff,

suggests that the flow vector for the mineralizing fluids, relative to the

present-day surface, may have been upwards from the deep, southwest end of the

deposit toward the shallower northeast, high grade keel of the deposit.

The Ropes main ore zone is cut by an east-striking, south- dipping, low

angle reverse fault zone at the 900 mine level (Figure 2). Offset of units across

the fault zone is generally greater at the south side of the mine than at the north

side. The orebody thins down dip toward the fault zone, but thickens below it.

Several other gold-mineralized bodies exist on the Ropes property. The

Northwest ore zone is a small ore zone to the northwest of the Ropes main ore

zone (Figure 4). Significant intersections of gold-mineralized rock, comparable in

grade and width to the Ropes main ore zone, have been encountered in deep

drilling 370 m below the Ropes mine 1284 level (Callahan Mining Corporation

news release, July 20, 1987).

In general, most gold occurs associated with sub-100 micron sized pyrite

which is dispersed throughout the mass of the quartz-sericite-chlorite rock, as well

as on fractures, foliations, and along quartz veinlets. However, in certain ore

types the modal abundance of pyrite does not show a strict correlation with gold

grade (see explanation, Figure 4). Native gold, of variable fineness, occurs as

approximately 3 to 10 micron sized grains in the following specific sites: 1)

attached to the surface of the fine grained pyrite, 2) included as fine blebs within

the fine grained pyrite, 3) on fractures within the fine grained pyrite, and 4) at

grain boundaries of fine grained quartz and sericite. A volumetrically minor

amount of coarse free gold occurs on fractures in quartz stringers and lenses.

Silver occurs as electrum, native silver, argentiferous tetrahedrite, argentiferous

galena, and rare dyscrasite.

Pyrite comprises over 95% of the metallic minerals in the flotation mill

concentrate, followed by chalcopyrite at 1%. Argentiferous galena, argentiferous

tetrahedrite, sphalerite, hematite, magnetite, and rutile are present in trace

amounts.

Fine grained, white to light gray sugary-textured auriferous quartz veins are

concentrated at and south of the extreme south side of the Ropes main ore zone.

Individual quartz veins are rarely up to a meter thick, but collectively these veins,

interlayered with quartz-sericite-chlorite rock, form pods up to a maximum of 8

m thick and 30 m in strike and plunge extent. The pods strike approximately

N65°E, compared to the N80°E overall trend of the Ropes main ore zone, plunge

steeply to the west, and occur progressively further east at deeper levels of the

Ropes mine, as discussed in the section on "Structure".

Accessory minerals in these quartz pods include: pyrite, minor argentiferous

tetrahedrite galena, and chalcopyrite, and rare free gold, molybdenite, and

dyscrasite. Trace tourmaline and native Ag were reported by Broderick (1945).

These pods typically contain 4 to 7 g/tonne Au and approximately 20 g/tonne Ag.

Although these pods are higher than average grade ore, the higher grade parts of

the quartz-sericite-chlorite rock ore of the Ropes main ore zone are not zoned

A-49

which is dispersed throughout the mass of the quartz-sericite-chlorite rock, as well

as on fractures, foliations, and along quartz veinlets. However, in certain ore

types the modal abundance of pyrite does not show a strict correlation with gold

grade (see explanation, Figure 4). Native gold, of variable fineness, occurs as

approximately 3 to 10 micron sized grains in the following specific sites: 1)

attached to the surface of the fine grained pyrite, 2) included as fine blebs within

the fine grained pyrite, 3) on fractures within the fine grained pyrite, and 4) at

grain boundaries of fine grained quartz and sericite. A volumetrically minor

amount of coarse free gold occurs on fractures in quartz stringers and lenses.

Silver occurs as electrum, native silver, argentiferous tetrahedrite, argentiferous

galena, and rare dyscrasite.

Pyrite comprises over 95% of the metallic minerals in the flotation mill

concentrate, followed by chalcopyrite at 1%. Argentiferous galena, argentiferous

, tetrahedrite, sphalerite, hematite, magnetite, and rutile are present in trace

amounts.

Fine grained, white to light gray sugary-textured auriferous quartz veins are

concentrated at and south of the extreme south side of the Ropes main ore zone.

Individual quartz veins are rarely up to a meter thick, but collectively these veins,

interlayered with quartz-sericite-chlorite rock, form pods up to a maximum of 8

m thick and 30 m in strike and plunge extent. The pods strike approximately

N65OE, compared to the N8VE overall trend of the Ropes main ore zone, plunge

steeply to the west, and occur progressively further east at deeper levels of the

Ropes mine, as discussed in the section on "Structure".

Accessory minerals in these quartz pods include: pyrite, minor argentiferous

tetrahedrite galena, and chalcopyrite, and rare free gold, molybdenite, and

dyscrasite. Trace tourmaline and native Ag were reported by Broderick (1945).

These pods typically contain 4 to 7 g/tonne Au and approximately 20 g/tonne Ag.

Although these pods are higher than average grade ore, the higher grade parts of

the quartz-sericite-chlorite rock ore of the Ropes main ore zone are not zoned

about the pods; rather, the higher grade parts of the main ore zone, characterized

by dispersed pyrite, occur more centrally within the trend of quartz-sericite-

chlorite rock. It is estimated that the existing and mined out pods constitute, in

total, approximately 5% of the ore at the Ropes mine.

DISCUSSION

The serpentinized peridotite bodies have contact relations with the bordering

and interlayered volcanic rocks, as well as internal relict fine grained igneous

texture, consistent with their intrusion as hypabyssal sills. Serpentinization

preserved igneous textures, and was most like'y at near constant volume (Best,

1982). This constant volume serpentinization process would have necessarily

released large amounts of MgO and Si02 to the surrounding rocks (ibid). The

voluminous carbonate-quartz-chlorite rocks, which bound and are interlayered with

the Ropes gold deposit, are comprised primarily of weakly ferroan dolomite and

lesser quartz. Their origin could conceivably be attributed to such a release of

MgO and Si02 during serpentinization of the peridotite enclosing the trend of

volcanic rocks at the Ropes mine.

The Ropes orebodies are overall tabular, stratiform and stratabound alteration

zones in quartz-sericite-chlorite rock interpreted as dacite tuff on the basis of

relict fragmental crystal-lithic textures preserved in and near ore. These

pseudomorphs are clearly gradational to feldspar phenocrysts and lithic fragments

in tuffs west of the Ropes mine. Additionally, the mine rocks have abundances

of less mobile trace elements indicative of a dacitic composition. Gold

mineralization at the Ropes mine is restricted to these rocks, and it appears that

these originally feldspar-rich rocks provided a chemically receptive host for K20

and Au bearing solutions, which altered the feldspar to sericite. Possibly, either

better permeability in the tuffs relative to the dense crystalline peridotite, or else

a ductility contrast between the volcanic rocks and peridotite, facilitated

preferential focussing of ore bearing solutions. The bounding peridotite near the

A- 50

about the pods; rather, the higher grade parts of the main ore zone, characterized

by dispersed pyrite, occur more centrally within the trend of quartz-sericite-

chlorite rock. It is estimated that the existing and mined out pods constitute, in

total, approximately 5% of the ore at the Ropes mine.

DISCUSSION

The serpentinized peridotite bodies have contact relations with the bordering

and interlayered volcanic rocks, as well as internal relict fine grained igneous

texture, consistent with their intrusion as hypabyssal sills. Serpentinization

preserved igneous textures, and was most likely at near constant volume (Best,

1982). This constant volume serpentinization process would have necessarily

released large amounts of MgO and Si02 to the surrounding rocks (ibid). The

voluminous carbonate-quartz-chlorite rocks, which bound and are interlayered with

the Ropes gold deposit, are comprised primarily of weakly ferroan dolomite and

lesser quartz. Their origin could conceivably be attributed to such a release of

MgO and Si02 during serpentinization of the peridotite enclosing the trend of

volcanic rocks at the Ropes mine.

The Ropes orebodies are overall tabular, stratiform and stratabound alteration

zones in quartz-sericite-chlorite rock interpreted as dacite tuff on the basis of

relict fragmental crystal-lithic textures preserved in and near ore. These

pseudomorphs are clearly gradational to feldspar phenocrysts and lithic fragments

in tuffs west of the Ropes mine. Additionally, the mine rocks have abundances

of less mobile trace elements indicative of a dacitic composition. Gold

mineralization at the Ropes mine is restricted to these rocks, and it appears that

these originally feldspar-rich rocks provided a chemically receptive host for K 2 0

and Au bearing solutions, which altered the feldspar to sericite. Possibly, either

better permeability in the tuffs relative to the dense crystalline peridotite, or else

a ductility contrast between the volcanic rocks and peridotite, facilitated

preferential focussing of ore bearing solutions. The bounding peridotite near the

mine is extensively carbonatized and appears to have been a sink for C02,

possibly contained in the same Au-bearing solutions.

Certain ore—related structural elements such as a series of quartz vein

dominated pods south of the Ropes main ore zone, and transverse zones of higher

gold grade in the Ropes main ore zone, have geometric relations consistent with

an origin as extensional features in a shear zone with positive volume change.

Structural elements indicate that the net component of movement during some

stage of gold mineralization was north side up to the WSW and south side down

to the ENE, relative to the present-day ground surface. Furthermore, the overall

concentric, convex to the east geometries of the major ore types in the Ropes

main ore zone, combined with the gradual western cutoff of gold mineralization

and sharp eastern cutoff in front of the high grade eastern "keel" of the deposit,

suggests that the flow vector for mineralizing fluids, relative to the present day

surface, may have been upwards from the deep southwest part of the deposit

toward the shallow eastern high grade edge.

Timing of gold mineralization can be demonstrated as being later than

deposition of the volcanic rocks which host the Ropes deposit, because alteration

associated with the Ropes deposit affects the serpentinized peridotite, which, on

the basis of relict fine grained igneous texture and interlayering with the volcanic

rocks, appears to be a hypabyssal sill complex, and, therefore, intrusive into the

volcanic section. Emplacement of the peridotite may have been preferentially

controlled by an early-formed structure which appears to have exerted a control

on the original distribution of volcanic and sedimentary units: the peridotite occurs

at or near a major, partially interlayered transition between basalt and dacite

pyroclastic rocks. This transition is accompanied by a zone of sedimentary units

including banded iron formation and quartzose graywackes. This major transition

in the volcanic section, accompanied by sediments, could represent a break or zone

of flexure which controlled the position of the interface between the two partly

contemporaneous major volcanic facies. Such a boundary between contrasting

A-51

mine is extensively carbonatized and appears to have been a sink for CO2,

possibly contained in the same Au-bearing solutions.

Certain ore-related structural elements such as a series of quartz vein

dominated pods south of the Ropes main ore zone, and transverse zones of higher

gold grade in the Ropes main ore zone, have geometric relations consistent with

an origin as extensional features in a shear zone with positive volume change.

Structural elements indicate that the net component of movement during some

stage of gold mineralization was north side up to the WSW and south side down

to the ENE, relative to the present-day ground surface. Furthermore, the overall

concentric, convex to the east geometries of the major ore types in the Ropes

main ore zone, combined with the gradual western cutoff of gold mineralization

and sharp eastern cutoff in front of the high grade eastern "keel" of the deposit,

suggests that the flow vector for mineralizing fluids, relative to the present day

surface, may have been upwards from the deep southwest part of the deposit

toward the shallow eastern high grade edge.

Timing of gold mineralization can be demonstrated as being later than

deposition of the volcanic rocks which host the Ropes deposit, because alteration

associated with the Ropes deposit affects the serpentinized peridotite, which, on

the basis of relict fine grained igneous texture and interlayering with the volcanic

rocks. appears to be a hypabyssal sill complex, and, therefore, intrusive into the

volcanic section. Emplacement of the peridotite may have been preferentially

controlled by an early-formed structure which appears to have exerted a control

on the original distribution of volcanic and sedimentary units: the peridotite occurs

at or near a major, partially interlayered transition between basalt and dacite

pyroclastic rocks. This transition is accompanied by a zone of sedimentary units

including banded iron formation and quartzose graywackes. This major transition

in the volcanic section, accompanied by sediments, could represent a break or zone

of flexure which controlled the position of the interface between the two partly

contemporaneous major volcanic facies. Such a boundary between contrasting

volcanic facies may have been controlled by a zone of crustal weakness which

facilitated intrusion of upper mantle derived peridotite, as well as provided a

conduit for focussing gold bearing solutions. The trend of volcanic rock which

hosts the Ropes deposit is near this transition, and strikes away from the transition

at an acute angle. The Ropes environment may represent a splay or second order

structure off the more major break. The observed zoning of ore types at the

Ropes mine, as discussed previously, is consistent with fluid flow from the

direction of the inferred major break to the west of the Ropes mine, toward the

present-day ENE. Persistent or recurrent movement along the inferred major

structure which controls the distribution of major volcanic fades, could have

provided a seismic pumping action to assist in moving large volumes of mineral-

ized hydrothermal fluid. Such movement could also account for the ore related

structural elements observed in the Ropes main ore zone, which indicate relative

movement during at least part of the gold mineralizing process.

CONCLUSIONS

Gold mineralization at the Ropes mine occurs primarily with fine grained

pyrite dispersed in quartz-sericite-chlorite rock interpreted as altered dacite tuff.

The Ropes environment may represent a splay off a more major early formed

structure which controls the distribution of the two major volcanic facies: basalt

and dacite pyroclastics. The serpentinized peridotite, as well as gold bearing

solutions, may have used this structure as a conduit.

ACKNOWLEDGEMENTS

The author thanks Glen Scott, Ropes mine Chief Geologist, who was instru-

mental in defining mappable units and compiling much of the geology at the

Ropes mine. This paper benefited from editing and revision by Klaus Schulz,

U.S. Geological Survey.

A-52

volcanic facies may have been controlled by a zone of crustal weakness which

facilitated intrusion of upper mantle derived peridotite, as well as provided a

conduit for focussing gold bearing solutions. The trend of volcanic rock which

hosts the Ropes deposit is near this transition, and strikes away from the transition

at an acute angle. The Ropes environment may represent a splay or second order

structure off the more major break. The observed zoning of ore types at the

Ropes mine, as discussed previously, is consistent with fluid flow from the

direction of the inferred major break to the west of the Ropes mine, toward the

present-day ENE. Persistent or recurrent movement along the inferred major

structure which controls the distribution of major volcanic facies, could have

provided a seismic pumping action to assist in moving large volumes of mineral-

ized hydrothermal fluid. Such movement could also account for the ore related

structural elements observed in the Ropes main ore zone, which indicate relative

movement during at least part of the gold mineralizing process.

CONCLUSIONS

Gold mineralization at the Ropes mine occurs primarily with fine grained

pyrite dispersed in quartz-sericite-chlorite rock interpreted as altered dacite tuff.

The Ropes environment may represent a splay off a more major early formed

structure which controls the distribution of the two major volcanic facies: basalt

and dacite pyroclastics. The serpentinized peridotite, as well as gold bearing

solutions, may have used this structure as a conduit.

ACKNOWLEDGEMENTS

The author thanks Glen Scott, Ropes mine Chief Geologist, who was instru-

mental in defining mappable units and compiling much of the geology at the

Ropes mine. This paper benefited from editing and revision by Klaus Schulz,

U.S. Geological Survey.

REFERENCES

Allen, C., 1912, Mineral Resources of Michigan with statistical tables ofproduction and value of mineral products for 1910 and prior years: MichiganGeological and Biological Survey, Publication 8, p. 365-366.

Best, M.G., 1982, Igneous and metamorphic petrology: W. H. Freeman andCompany, San Francisco, p. 398-399.

Broderick , M., 1945, Geology of the Ropes Gold Mizie, Marquette County,Michigan: Economic Geology, v. 40, # 2, p. 115-128.

Clark, L.D., Cannon, W.F., and Klasner, J.S., 1975, Bedrock geologic map ofthe Negaunee SW Quadrangle, Marquette County, Michigan: U.S. Geological Sur-vey, Map # GQ-1226, Scale 1:24000.

Ramsay, J.G., and Huber, M., 1983, The techniques of modern structuralgeology, v. 1: Strain analysis: Academic Press, Inc., Orlando, Florida, p. 48-50.

Rossell, D.M., 1983, Alteration of the Deer Lake Peridotite in the vicinity ofthe Ropes Mine, Marquette County, Michigan, (unpublished M.S. thesis): MichiganTechnological University, Houghton, Michigan, 83 p.

A-53

REFERENCES

Allen, C., 1912, Mineral Resources of Michigan with statistical tables of production and value of mineral products for 1910 and prior years: Michigan Geological and Biological Survey, Publication 8, p. 365-366.

Best, M.G., 1982, Igneous and metamorphic petrology: W. H. Freeman and Company, San Francisco, p. 398-399.

Broderick , M., 1945, Geology of the Ropes Gold Mice, Marquette County, Michigan: Economic Geology, v. 40, # 2, p. 115-128.

Clark, L.D., Cannon, W.F., and Klasner, J.S., 1975, Bedrock geologic map of the Negaunee SW Quadrangle, Marquette County, Michigan: U.S. Geological Sur- vey, Map # GQ-1226, Scale 1:24000.

Ramsay, J.G., and Huber, M., 1983, The techniques of modern structural geology, v. 1: Strain analysis: Academic Press, Inc., Orlando, Florida, p. 48-50. %

Rossell, D.M., 1983, Alteration of the Deer Lake Peridotite in the vicinity of the Ropes Mine, Marquette County, Michigan, (unpublished M.S. thesis): Michigan Technological University, Houghton, Michigan, 83 p.

Geological Field Trip to the Marquette Greenstone Belt: Part IDay 1 Road Log - Stops 1 to 11

T.J. Bornhorst, D.A. Baxter, M.L. MacLellan, and R.C. Johnson

Department of Geology and Geological EngineeringMichigan Technological University Houghton, Michigan 49931

*Currently with Kerr-McGee Corporation

NOTE: Additional information on the geologic setting of the field trip stops maybe found in papers by Bornhorst and by Baxter & MacLellan in this volume.

Refer to Figure 1: Route and Stop Map

Mileage

0.0 Start Day 1 - Ramada Inn, Marquette

Assemble at the entrance of the Ramada Inn. From the Ramada Inn turnright (west) to Washington Street and proceed toward junction with U.S. 41. At1.3 mi, turn right (west) to U.S. 41 and proceed toward Negaunee. At 6.4 mi,pull off to the tight shoulder alongside of outcrops.

6.4 Stop 1 - Pillowed Basalt

The dominant volcanic lithologies in the Marquette Greenstone Belt are

massive and pillowed tholeiitic basalt, the massive variety appearing to be more

abundant perhaps because typical mossy outcrops may obscure the pillows. At this

stop are excellent three dimensional views of relatively undeformed pillowed basalt.

The pillows are ellipsoidal in shape with dimensions around 1' X 4' X 5'; attitude

of bedding strikes approximately E-W and dips 85°N. Cusps on pillows indicate

that here stratigraphic top is to the north. There are also characteristic radial

fractures and interpillow void spaces filled with quartz and carbonate.

This locality was mapped most recently by Puffett (1974) as Lower Member

of the Mona Schist. Locaticn: NW+, section 20, T48N, R26W.

Continue west on U.S. 41. At 7.6 mi, pass the road to Marquette CountyAirport. At 8.6 mi, large outcrop on opposite side of highway (south) is Stop 2(for safety proceed 0.1 mi west, make a U-turn just before the Carp River, andproceed east to outcrop; when finished proceed east 0.05 mi, make a U-turn toU.S. 41 west).

A-54

Geological Field Trip to the Marquette Greenstone Belt: Part I Day 1 Road Log - Stops 1 to 11

T.J. Bornhorst, D.A. Baxter, M.L. MacLellan, and R.C. ~ohnson*

Department of Geology and Geological Engineering Michigan Technological University Houghton, Michigan 49931

* Currently with Kerr-McGee Corporation

NOTE: Additional information on the geologic setting of the field trip stops may be found in papers by Bornhorst and by Baxter & MacLellan in this volume.

Refer to Figure 1: Route and Stop Map

Mileage


Assemble at the entrance of the Ramada Inn. From the Ramada Inn turn right (west) to Washington Street and proceed toward junction with U.S. 41. At 1.3 mi, turn right (west) to U.S. 41 and proceed toward Negaunee. At 6.4 mi, pull off to the light shoulder alongside of outcrops.

6.4 Stop 1 - Pillowed Basalt

The dominant volcanic lithologies in the Marquette Greenstone Belt are

massive and pillowed tholeiitic basalt, the massive variety appearing to be more

abundant perhaps because typical mossy outcrops may obscure the pillows. At this

stop are excellent three dimensional views of relatively undeformed pillowed basalt.

The pillows are ellipsoidal in shape with dimensions around 1' X 4' X 5'; attitude

of bedding strikes approximately E-W and dips 85ON. Cusps on pillows indicate

that here stratigraphic top is to the north. There are also characteristic radial

fractures and interpillow void spaces filled with quartz and carbonate.

This locality was mapped most recently by Puffett (1974) as Lower Member

of the Mona Schist. Location: NWi, section 20, T48N, R26W.

Continue west on U.S. 41. At 7.6 mi, pass the road to Marquette County Airport. At 8.6 mi, large outcrop on opposite side of highway (south) is Stop 2 (for safety proceed 0.1 mi west, make a U-turn just before the Carp River, and proceed east to outcrop; when finished proceed east 0.05 mi, make a U-turn to U.S. 41 west).

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A-56

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4-i

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8.6 Stop 2 - Slightly Deformed Pillowed Basalt

The vertical road cut consists of slightly deformed pillowed basalt. Pillow

rinds are not easily visible in the vertical face but are more conspicuous on the

glacially polished surface on the top of the outcrop. Pillows are flattened and

rinds have feathered edges; bedding strikes roughly E-W. Foliation strikes N86°W

and dips 84°S. Quartz and carbonate veins containing a few percent euhedral

pyrite parallel this foliation. Flat-lying fractures are filled dominantly with

carbonate.

This locality was mapped most recently by Puffett (1974) as the Lower

Member of the Mona Schist. Location: SW+, section 28, T48N, R26W.

Proceed west on U.S. 41 and at 8.75 mi cross bridge over Carp River. At9.3 mi, pull off to the right shoulder alongside a number of small outcrops.

9.3 Stop 3 - Deformed .Archean Volcanic Rocks

The rocks in this road cut have a well-developed close-spaced foliation

(N75-88°W, dip 70-89°S) which produces a slate-like appearance. These rocks are

within the Archean Carp River Falls Shear Zone. There are two lithologies in

this outcrop: chlorite schist from a basaltic parent, and quartz—sericite schist from

a rhyolitic parent. The quartz-sericite schist is a tabular shaped body near the

center of the outcrop, and is probably a dike based on analogous lithologies to the

north. Near the extreme eastern end of the outcrop are probable pillow rinds

which suggest the basalt parentage of the chlorite schist. Flat—lying fractures,

similar to Stop 2, are present in the basalts but not in the rhyolites.

This locality was mapped most recently by Puffett (1974) as Undifferentiated

Greenstone. Location: S+, section 29, T48N, R26W.

Proceed west on U.S. 41. At 10.2 mi, enter City of Negaunee and continuethrough Negaunee on U.S. 41 and towards Ishpeming. At 13.8 mi, traffic light atjunction of County Road 573 and U.S. 41, turn right (north) to County Road573. At 14.2 mi, pass cemetery on left, stay on main road, now called Deer LakeAvenue. At 14.9 mi, Deer Lake is visible on right, continue ahead on main road.At 16.7 mi, junction of County Road 573 with Cooper Lake Road on left, stay on573. At 17.5 mi, gravel road to the Ropes Mine on the left (the field trip will

A- 57

8.6 Stop 2 - Slightly Deformed Pillowed Basalt

The vertical road cut consists of slightly deformed pillowed basalt. Pillow

rinds are not easily visible in the vertical face but are more conspicuous on the

glacially polished surface on the top of the outcrop. Pillows are flattened and

rinds have feathered edges; bedding strikes roughly E-W. Foliation strikes N86OW

and dips 84's. Quartz and carbonate veins containing a few percent euhedral

pyrite parallel this foliation. Flat-lying fractures are filled dominantly with

carbonate.

This locality was mapped most recently by Puffett (1974) as the Lower

Member of the Mona Schist. Location: SWi, section 28, T48N. R26W.

Proceed west on U.S. 41 and at 8.75 mi cross bridge over Carp River. At 9.3 mi, pull off to the right shoulder alongside a number of small outcrops.

9.3 Stop 3 - Deformed-Archean Volcanic Rocks

The rocks in this road cut have a well-developed close-spaced foliation

(N75-88OW, dip 70-89"s) which produces a slate-like appearance. These rocks are

within the Archean Carp River Falls Shear Zone. There are two lithologies in

this outcrop: chlorite schist from a basaltic parent, and quartz-sericite schist from

a rhyolitic parent. The quartz-sericite schist is a tabular shaped body near the

center of the outcrop, and is probably a dike based on analogous lithologies to the

north. Near the extreme eastern end of the outcrop are probable pillow rinds

which suggest the basalt parentage of the chlorite schist. Flat-lying fractures,

similar to Stop 2, are present in the basalts but not in the rhyolites.

This locality was mapped most recently by Puffett (1974) as Undifferentiated

Greenstone. Location: S+, section 29, T48N, R26W.

Proceed west on U.S. 41. At 10.2 mi, enter City of Negaunee and continue through Negaunee on U.S. 41 and towards Ishpeming. At 13.8 mi, traffic light at junction of County Road 573 and U.S. 41, turn right (north) to County Road 573. At 14.2 mi, pass cemetery on left, stay on main road, now called Deer Lake Avenue. At 14.9 mi, Deer Lake is visible on right, continue ahead on main road. At 16.7 mi, junction of County Road 573 with Cooper Lake Road on left, stay on 573. At 17.5 mi, gravel road to the Ropes Mine on the left (the field trip will

stop at Ropes on Day 2), continue on 573. At 22.4 mi, at Y-intersection, go leftto the North Dead River Road; road turns to gravel at this point. At 27.7 mi,the hills on the right are Archean rocks of the Clark Creek area. At 28.8 mi,cross bridge over the Dead River; Archean rocks of the Silver Creek area areexposed in the steep topography ahead. At 28.95 mi, intersection of North DeadRiver Road and Silver Lake Road, proceed right (east) on North Dead RiverRoad. At 30.2 mi, outcrops of Michigamme Formation in roadbed. At 30.8 mi,cross over Silver Creek, At 31.1 mi, turn left to dirt road towards Grigg huntingcamp. At 31.4 mi, park at hunting camp and walk about 400 'eet N20-25°Efrom the cabin to the Silver Creek prospect located on the side of the valley.

31.4 Stop 4 — Silver Creek Prospect

The Silver Creek Prospect consists of several shallow shafts and trenches. The

prospect is dominated by a quartz and carbonate vein (up to 4 ft thick) which

contains galena and sphalerite as the major sulfides, lesser amounts of pyrite and

chalcopyrite, and trace amounts of arsenopyrite and pyrrhotite. Pyrite replaces

pyrrhotite. A mineral separate of galena contained 120 ppm Ag and one of

sphalerite contained less than 1 ppm Ag. A yellow sulfide separate (chalcopyrite,

pyrite and pyrrhotite) contained 180 ppb Au. A grab sample of white massive

quartz vein from the rock piles at this prospect contained 38 ppb Au and 4 ppm

Ag. The vein is hosted by altered pillow basalt.

This locality was mapped most recently by Johnson and others (1987) and is

designated as the Highly Altered Variety of the Pillowed Basalt Member of the

Volcanics of Silver Mine Lakes. Location: NE+, section 25, T49N, R2SW.

Proceed back to North Dead River Road and turn left (east) at 31.5 mi. At31.8 mi, turn left and proceed to dirt logging road, turn right and follow loggingroad north, 0.O5mi. At 32.1 mi, park, continue walking on dirt road about 950feet (generally up slope) to Stop 5a; cross altered rhyolite at about 700 feet.

32.1 Stop 5 - Section 30 Alteration Zone

This area is very close to the Dead River Shear Zone, an Archean zone of

high strain and deformation. Alteration intensity varies from outcrop to outcrop,

as does the dominant type of alteration. Chlorite is ubiquitous throughout the

alteration zone, whereas sericite and carbonate are locally abundant alteration

products. Alteration along this zone is interpreted to be synchronous with the

A- 58

stop at Ropes on Day 2), continue on 573. At 22.4 mi, at Y-intersection, go left to the North Dead River Road; road turns to gravel at this point. At 27.7 mi, the hills on the right are Archean rocks of the Clark Creek area. At 28.8 mi, cross bridge over the Dead River; Archean rocks of the Silver Creek area are exposed in the steep topography ahead. At 28.95 mi, intersection of North Dead River Road and Silver Lake Road, proceed right (east) on North Dead River Road. At 30.2 mi, outcrops of Michigamme Formation in roadbed. At 30.8 mi, cross over Silver Creek, At 31.1 mi, turn left to dirt road towards Grigg hunting camp. At 31.4 mi, park at hunting camp and walk about 400 feet N20-25OE from the cabin to the Silver Creek prospect located on the side of the valley.

31.4 Stop 4 - Silver Creek Prospect

The Silver Creek Prospect consists of several shallow shafts and trenches. The

prospect is dominated by a quartz and carbonate vein (up to 4 f t thick) which

contains galena and sphalerite as the major sulfides, lesser amounts of pyrite and

chalcopyrite, and trace amounts of arsenopyrite and pyrrhotite. Pyrite replaces

pyrrhotite. A mineral separate of galena contained 120 ppm Ag and one of

sphalerite contained less than 1 ppm Ag. A yellow sulfide separate (chalcopyrite,

pyrite and pyrrhotite) contained 180 ppb Au. A grab sample of white massive

quartz vein from the rock piles at this prospect contained 38 ppb Au and 4 ppm

Ag. The vein is hosted by altered pillow basalt.

This locality was mapped most recently by Johnson and others (1987) and is

designated as the Highly Altered Variety of the Pillowed Basalt Member of the

Volcanics of Silver Mine Lakes. Location: N E k section 25, T49N. R28W.

Proceed back to North Dead River Road and turn left (east) at 31.5 mi. At 31.8 mi, turn left and proceed to dirt logging road, turn right and follow logging road north, 0.05mi. At 32.1 mi, park, continue walking on dirt road about 950 feet (generally up slope) to Stop 5a; cross altered rhyolite at about 700 feet.

32.1 Stop 5 - Section 30 Alteration Zone

This area is very close to the Dead River Shear Zone, an Archean zone of

high strain and deformation. Alteration intensity varies from outcrop to outcrop,

as does the dominant type of alteration. Chlorite is ubiquitous throughout the

alteration zone, whereas sericite and carbonate are locally abundant alteration

products. Alteration along this zone is interpreted to be synchronous with the

deformation. The stop has been divided into three parts which will be described

separately.

This locality was mapped most recently by Baxter and others (1987) and

lithologies are designated as the Highly Altered Variety of the Pillowed Basalt

Memb'r of the Volcanics of Silver Mine Lakes, Gabbro of Clark Creek, and

Rhyolite Intrusive of Fire Center Mine. Location: NW+, section 30, T49N, R27W.

5a. The Cliff Outcrops

This set of outcrops contains altered gabbroic rocks and intrusives of altered

rhyolite porphyry. The outcrop on the south side of the valley is mainly altered

gabbro with a fault-bounded block of altered rhyolite, which strikes approximately

N50°E and dips 750 to the east, along the eastern contact. The western contact of

this rhyolite block has a similar strike and a dip of about 40° towards the east.

The maximum east-west thickness of the rhyolite block is 50 feet. Mineralization

is concentrated along the faulted contact between the altered gabbro and altered

rhyolite. Pyrite is the dominant sulfide phase, with arsenopyrite and chalcopyrite

occurring in much lesser amounts. The small outcrop on the north side of the

valley is a slightly less altered example of the gabbro.

5b. The Hillside Outcrop (about 225 feet east of 5a)

These outcrops are located along the north side of the valley on a gently

upward sloping hill, and are mainly highly altered basalt. Near the south end of

the outcrop is a rhyolite porphyry contact trending about N10°W and dipping 50°

east. This is a good location to observe quartz-carbonate veining along the contact

between rhyolite and altered basalt.

Between this outcrop and Stop 5c there are several smaller outcrops on the

north and south side of the valley of predominantly altered basalts with rare

rhyolite dikes.

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deformation. The stop has been divided into three parts which will be described

separately.


lithologies are designated as the Highly Altered Variety of the Pillowed Basalt

Member of the Volcanics of Silver Mine Lakes, Gabbro of Clark Creek, and

Rhyolite Intrusive of Fire Center Mine. Location: NWi, section 30, T49N, R27W.

5a. The Cliff Outcrops

This set of outcrops contains altered gabbroic rocks and intrusives of altered

rhyolite porphyry. The outcrop on the south side of the valley is mainly altered

gabbro with a fault-bounded block of altered rhyolite, which strikes approximately

N50Â° and dips 75O to the east, along the eastern contact. The western contact of

this rhyolite block has a similar strike and a dip of about 40Â towards the east.

The maximum east-west thickness of the rhyolite block is 50 feet. Mineralization

is concentrated along the faulted contact between the altered gabbro and altered

rhyolite. Pyrite is the dominant sulfide phase, with arsenopyrite and chalcopyrite

occurring in much lesser amounts. The small outcrop on the north side of the

valley is a slightly less altered example of the gabbro.

5b. The Hillside Outcrop (about 225 feet east of 5a)

These outcrops are located along the north side of the valley on a gently

upward sloping hill, and are mainly highly altered basalt. Near the south end of

the outcrop is a rhyolite porphyry contact trending about NlWW and dipping 50'

east. This is a good location to observe quartz-carbonate veining along the contact

between rhyolite and altered basalt.

Between this outcrop and Stop 5c there are several smaller outcrops on the

north and south side of the valley of predominantly altered basalts with rare

rhyolite dikes.

5c. The Island Outcrop (about 225 feet east of 5b)

This outcrop is in the middle of the valley which is otherwise outcrop-free.

The entire outcrop is highly altered basalt with the exception of the western 1/4

which is cut by rhyolite.

Proceed back to North Dead River Road and turn left (east) at 32.3 mi. At34.3 mi, bridge over Clark Creek, park near bridge. Across the Clark Creekbridge, the road is known as Red Road.

34.3 Stop 6 (optional) - Pillowed Basalt and Rhyolite

The outcrops directly beneath the western side of the bridge are predom-

inantly Archean pillowed basalt. These pillows are vertical and show younging

direction toward S30°W. A fault contact (seen when the water level is low)

juxtaposes a rhyolite porphyry on the east against the pillow basalt on the west.

A few small (inch scale) zones normal to the main fault show shearing of the

basalt into the rhyolite. This structural interleaving gives the rhyolite a

pseudo-pillowed appearance. The large cliff, visible about 1500 feet to the north

of the bridge, is the type locality for the Gabbro of Clark Creek.


contains the Pillowed Basalt Member of the Volcanics of Silver Mine Lakes and

Rhyolite Intrusive of Fire Center Mine. Location: SE+, section 29, T49N, R27W.

Continue north on Red road. At 35.0 mi, pull off to the right shoulder, nearlow relief outcrops.

35.0 Stop 7 (optional) - Granular rhyolite

These outcrops are along the upward slope on the east side of the road.

Even though this rock is holocrystalline, it has been included with the other

rhyolites based on its similar major and trace element chemical composition, e.g.,

calc-alkaline rhyolite on a Jensen diagram. The granular rhyolite Consists of

microcline and albite, but is now moderately altered to sericite, Quartz, and minor

epidote and chlorite. Some sulfide mineralization can be found in localized zones

A- 60

5c. The Island Outcrop (about 225 feet east of 5b)

This outcrop is in the middle of the valley which is otherwise outcrop-free.

The entire outcrop is highly altered basalt with the exception of the western 1/4

which is cut by rhyolite.

Proceed back to North Dead River Road and turn left (east) at 32.3 mi. At 34.3 mi, bridge over Clark Creek, park near bridge. Across the Clark Creek bridge, the road is known as Red Road.

34.3 Stop 6 (optional) - Pillowed Basalt and Rhyolite

The outcrops directly beneath the western side of the bridge are predom-

inantly Archean pillowed basalt. These pillows are vertical and show younging

direction toward S30Â¡W A fault contact (seen when the water level is low)

juxtaposes a rhyolite porphyry on the east against the pillow basalt on the west.

A few small (inch scale) zones normal to the main fault show shearing of the

basalt into the rhyolite. This structural interleaving gives the rhyolite a

pseudo-pillowed appearance. The large cliff, visible about 1500 feet to the north

of the bridge, is the type locality for the Gabbro of Clark Creek.


contains the Pillowed Basalt Member of the Volcanics of Silver Mine Lakes and

Rhyolite Intrusive of Fire Center Mine. Location: SE4, section 29, T49N, R27W.

Continue north on Red road. At 35.0 mi, pull off to the right shoulder, near low relief outcrops.

35.0 Stop 7 (optional) - Granular rhyolite

These outcrops are along the upward slope on the east side of the road.

Even though this rock is holocrystalline, it has been included with the other

rhyolites based on its similar major and trace element chemical composition, e.g.,

calc-alkaline rhyolite on a Jensen diagram. The granular rhyolite consists of

microcline and albite, but is now moderately altered to sericite, quartz, and minor

epidote and chlorite. Some sulfide mineralization can be found in localized zones

of the outcrop, mainly on the south side.

This locality was mapped most recently by Baxter and others (1987) as

Rhyolite Intrusive of Fire Center Mine. Location: NW+. section 28, T49N, R27W.

Continue north on Red road. At 35.05 mi. cross Deer Creek and turn right(east) immediately after the tridge to a dirt road. At 35.35 mi, cross Deer Creekagain. At 35.5 mi, 3-way irtersection at the top of a hill, take road to the right.At 36.0 mi, flat area is part of the Clark Creek basin consisting of LowerProterozoic Michigamme Formation; prominent outcrops on the left are Archeanrocks. At 36.6 mi, pull over to small side road on the right and walk about 200feet southwest to outcrops on small knob.

36.6 Stop 8 - Breccia

This outcrop is the best known exposure of a member which geomorphologi-

cally tends to form a depression. Stratigraphic top is towards the south-southwest,

consistent with nearby pillowed basalt top indicators. This member consists of at

least three lithologic breccia units. The lowest part of this member is interpreted

to be a subaqueous pyroclastic and/or fine-grained mudflow deposit, devoid of

large fragments. Above this is a breccia with a large proportion of fragments

representing a wide range of lithologies. The majority of fragments are

heterolithologic porphyritic volcanic rocks of probable andesite composition, now

extensively altered. These fragments are equant to slightly elongate and

sub-angular to sub-rounded. The other fragments are of iron-rich chert which

occurs in shapes ranging from equant to highly elongate, with angular to

sub-angular edges. This breccia is interpreted as a subaqueous mudflow, of

relatively near-vent facies origin. An unbrecciated cherty iron-formation overlies

this breccia, consequently the chert clasts must have been derived from pre-

existing chert deposits that formed in a less stable environment.

This member has undergone a relatively large amount of strain, demonstrated

by flattened clasts, compared to adjacent members. There are many replacement

pods, consisting of mainly pyrite, throughout the member indicating hydrothermal

alteration.

The large outcrops to the south of the breccia member are of relatively

A-61

of the outcrop, mainly on the south side.

This locality was mapped most recently by Baxter and others (1987) as

Rhyolite Intrusive of Fire Center Mine. Location: NWi. section 28, T49N, R27W.

Continue north on Red road. At 35.05 mi. cross Deer Creek and turn right (east) immediately after the bridge to a dirt road. At 35.35 mi, cross Deer Creek again. At 35.5 mi, 3-way intersection at the top of a hill, take road to the right. At 36.0 mi, flat area is part of the Clark Creek basin consisting of Lower Proterozoic Michigamme Formation; prominent outcrops on the left are Archean rocks. At 36.6 mi, pull over to small side road on the right and walk about 200 feet southwest to outcrops on small knob.


This outcrop is the best known exposure of a member which geomorphologi-

cally tends to form a depression. Stratigraphic top is towards the south-southwest,

consistent with nearby pillowed basalt top indicators. This member consists of at

least three lithologic breccia units. The lowest part of this member is interpreted

to be a subaqueous pyroclastic and/or fine-grained mudflow deposit, devoid of

large fragments. Above this is a breccia with a large proportion of fragments

representing a wide range of lithologies. The majority of fragments are

heterolithologic porphyritic volcanic rocks of probable andesite composition, now

extensively altered. These fragments are equant to slightly elongate and

sub-angular to sub-rounded. The other fragments are of iron-rich chert which

occurs in shapes ranging from equant to highly elongate, with angular to

sub-angular edges. This breccia is interpreted as a subaqueous mudflow, of

relatively near-vent facies origin. An unbrecciated cherty iron-formation overlies

this breccia, consequently the chert clasts must have been derived from pre-

existing chert deposits that formed in a less stable environment.

This member has undergone a relatively large amount of strain, demonstrated

by flattened clasts, compared to adjacent members. There are many replacement

pods, consisting of mainly pyrite, throughout the member indicating hydrothermal

alteration.

The large outcrops to the south of the breccia member are of relatively

unaltered gabbro. This gabbro shows a wide range of grain size, probably due to

the localized fault movement which produced finer-grained zones within the

overall coarse-grained rock.


contains the Breccia Member of Reany Lake of the Volcanics of Silver Mine

Lakes and Gabbro of Clark Creek. Location: SW+, section 27, T49N, R27W.

Retrace route to Red road at first Deer Creek bridge and turn right (north)at 38.0 mi. Continue on the main gravel/dirt road and at 41.3 mi turn right(south) to gravel County Road 510. At 46.3 mi, turn left (east) to dirt road, theHolyoke Trail. Park near intersection and walk 1200 feet east on the road, then160 feet southeast to Stop 9.


These outcrops, located along the extension of the Willow Creek Shear Zone,

consist of a breccia member interbedded with pillowed basalt. The matrix-

supported breccia consists of several types of flattened clasts (1:10 to 1:20) in a

basaltic matrix. The margins of some clasts are feathered. The clasts vary in size

from lensoidal 1/5 inch fragments to 4 inch wide and 6 feet long fragments.

Clast types include rhyolite, gabbro, and granodiorite. The rhyolite clasts are

porphyritic with 20% plagioclase phenocrysts in a fine-grained, quartz, plagioclase,

and chlorite matrix. The granodioritic clasts are porphyritic with 40% plagioclase

phenocrysts in fine-grained, quartz and chlorite groundmass. The gabbro clasts are

medium-grained, sub-ophitic with plagioclase, chlorite and amphibole. The matrix

of the breccia consists of mainly chlorite and epidote with up to 5% pyrite. This

breccia is interpreted as a subaqueous mudflow which has undergone a high

amount of strain.

This locality was mapped most recently by MacLellan and Bornhorst (1988)

and is designated as the Breccia Member of Bismark Creek of the Volcanics of

Silver Mine Lakes. Location: SE+, section 31, T49N, R26W.

Continue south on County Road 510. At 48.4 mi, pull off to right shoulder(outcrops on other side of road). Be careful, this road is heavily traveled attimes.

A-62

unaltered gabbro. This gabbro shows a wide range of grain size, probably due to

the localized fault movement which produced finer-grained zones within the

overall coarse-grained rock.


contains the Breccia Member of Reany Lake of the Volcanics of Silver Mine

Lakes and Gabbro of Clark Creek. Location: SWi, section 27, T49N, R27W.

Retrace route to Red road at first Deer Creek bridge and turn right (north) at 38.0 mi. Continue on the main gravel/dirt road and at 41.3 mi turn right (south) to gravel County Road 510. At 46.3 mi, turn left (east) to dirt road, the Holyoke Trail. Park near intersection and walk 1200 feet east on the road, then 160 feet southeast to Stop 9.


These outcrops, located along the extension of the Willow Creek Shear Zone,

consist of a breccia member interbedded with pillowed basalt. The matrix-

supported breccia consists of several types of flattened clasts (1:lO to 1:20) in a

basaltic matrix. The margins of some clasts are feathered. The clasts vary in size

from lensoidal 115 inch fragments to 4 inch wide and 6 feet long fragments.

Clast types include rhyolite, gabbro, and granodiorite. The rhyolite clasts are

porphyritic with 20% plagioclase phenocrysts in a fine-grained, quartz, plagioclase,

and chlorite matrix. The granodioritic clasts are porphyritic with 40% plagioclase

phenocrysts in fine-grained, quartz and chlorite groundmass. The gabbro clasts are

medium-grained, sub-ophitic with plagioclase, chlorite and amphibole. The matrix

of the breccia consists of mainly chlorite and epidote with up to 5% pyrite. This

breccia is interpreted as a subaqueous mudflow which has undergone a high

amount of strain.

This locality was mapped most recently by MacLellan and Bornhorst (1988)

and is designated as the Breccia Member of Bismark Creek of the Volcanics of

Silver Mine Lakes. Location: SEk section 31, T49N, R26W.

Continue south on County Road 510. At 48.4 mi, pull off to right shoulder (outcrops on other side of road). Be careful, this road is heavily traveled at times.

51.4 Stop 10 - Iron-Formation, Basalt, and Gabbro

This outcrop consists of iron formation in contact to the north and south with

massive basalt. To the west the basalt is cut by gabbro. The contact between the

basalt and gabbro follows the highway. The iron-formation is approximately 5

feet thick and trends N30°W. It is compcsed almost exclusively of fine, black

chert which contains scattered magnetite grains and abundant pyrite, To the west

of this locality, the iron-formation member is generally layered with 1/2 inch

chert and magnetite bands. Although the magnetite content can be highly

variable, the iron-formation produces a strong magnetic signature. The basalt is

dark green to black, fine-grained and relatively non-foliated. The gabbro is

medium-grained, ophitic to sub-ophitic, and is composed of equigranular amphi-

bole, plagioclase and chlorite.

This locality was mapped most recently by Puffett (1974) as the Lighthouse

Point Member of the Mona Schist and Metadiabase of uncertain age. A more

satisfactory correlation is with the Pillowed Basalt and Iron Formation Members of

the Volcanics of Silver Mine Lakes and the Gabbro of Clark Creek as mapped

one mile to the east by MacLellan and Bornhorst (1988). Location: SE+, section 5,

T48N, R26W.

Continue south on County Road 510. At 48.8 mi, intersection with NorthBasin Road and pavement begins, Continue south. At 50.6 mi, pull off to rightshoulder, just before going downhill and across a bridge. Walk east across roadalong an abandoned portion of 510 to small outcrops.

53.6 Stop 11 (optional) - Archean greywacke

This outcrop consists of black, quartz-feldspar-sericite-chlorite schist with

minor disseminated pyrite; foliation trends westerly. This rock is interpreted as

greywacke. This particular member extends eastward for 8 miles and represents

the only major sedimentary unit within the Marquette Greenstone Belt. The

member consists of greywacke and slate.

A- 63

51.4 Stop 10 - Iron-Formation, Basalt, and Gabbro

This outcrop consists of iron formation in contact to the north and south with

massive basalt. To the west the basalt is cut by gabbro. The contact between the

basalt and gabbro follows the highway. The iron-formation is approximately 5

feet thick and trends N30Â°W It is compcsed almost exclusively of fine, black

chert which contains scattered magnetite grains and abundant pyrite. To the west

of this locality, the iron-formation member is generally layered with 1/2 inch

chert and magnetite bands. Although the magnetite content can be highly

variable, the iron-formation produces a strong magnetic signature. The basalt is

dark green to black, fine-grained and relatively non-foliated. The gabbro is

medium-grained, ophitic to sub-ophitic, and is composed of equigranular amphi-

bole, plagioclase and chlorite.

This locality was mapped most recently by Puffett (1974) as the Lighthouse

Point Member of the Mona Schist and Metadiabase of uncertain age. A more

satisfactory correlation is with the Pillowed Basalt and Iron Formation Members of

the Volcanics of Silver Mine Lakes and the Gabbro of Clark Creek as mapped

one mile to the east by MacLellan and Bornhorst (1988). Location: SEi, section 5,

T48N. R26W.

Continue south on County Road 510. At 48.8 mi, intersection with North Basin Road and pavement begins, continue south. At 50.6 mi, pull off to right shoulder, just before going downhill and across a bridge. Walk east across road along an abandoned portion of 510 to small outcrops.

53.6 Stop 11 (optional) - Archean greywacke

This outcrop consists of black, quartz-feldspar-sericite-chlorite schist with

minor disseminated pyrite; foliation trends westerly. This rock is interpreted as

greywacke. This particular member extends eastward for 8 miles and represents

the only major sedimentary unit within the Marquette Greenstone Belt. The

member consists of greywacke and slate.

This locality was mapped most recently by Puffett (1974) as the Nealy Creek

Member of the Mona Schist. Location: Sf, section 10, T48N, R26W.

Continue south on County Road 510. At 50.85 mi, cross bridge over theDead River Storage Basin. At 52.9 mi, bear left and at 53.0 mi, turn left (east)to the east end of County Road 502. At 53.6 mi, turn left (east) to U.S. 41 andreturn to the Ramada Inn in Marquette.

END OF DAY 1

Acknowledgements and References in Day 2 Road Log.

A- 64

This locality was mapped most recently by Puffett (1974) as the Nealy Creek

Member of the Mona Schist. Location: Si, section 10, T48N, R26W.

Continue south on County Road 510. At 50.85 mi, cross bridge over the Dead River Storage Basin. At 52.9 mi, bear left and at 53.0 mi, turn left (east) to the east end of County Road 502. At 53.6 mi, turn left (east) to U.S. 41 and return to the Ramada Inn in Marquette.

END OF DAY 1

Acknowledgements and References in Day 2 Road Log.

Geological Field Trip to the Marquette Greenstone Belt: Part IIDay 2 Road Log - Stops A to E

T.J. Bornhorst and D.A. Baxter

Department of Geology and Geological EngineeringMichigan Technological University, Houghton, Michigan 49931

NOTE: The field trip for Day 2 includes a late morning to afternoon stop at theRopes Mine. There is a separate guide, Part III, authored by Brozdowski andScott of Callahan Mining Corporation. Additional information on the geologicsetting of the field trip stops may be found in the paper by Bornhorst (thisvolume).

Refer to Day 1 Road Log, Figure 1: Route and Stop Map

Mileage


Assemble at the main entrance of the Ramada Inn. Turn right out of theparking lot of the Ramada Inn to Washington Street to U.S. 41 and then westtowards Ishpeming. At 13.8 mi, turn right (north) at traffic light to County Road573. At 14.6 mi, hairpin turn left. At 14.75 mi, pull off to the right shoulderalongside of outcrops.

14.75 Stop A - Schist

These rocks are foliated quartz-sericite schists with angular carbonate clots

scattered throughout them. Foliation strikes N70°W and dips 84°S. The rocks are

intermediate in composition (dacite) and have a calc-alkalic affinity. These rocks

are interpreted to be subaqueously deposited volcaniclastic sediments or pyroclastic

deposits, and are an important component of the southwestern part of the

greenstone belt.

This locality was mapped most recently by Clark and others (1975) as the

Kitchi Schist. Location: NW+, section 34, T48N, R27W.

Continue NW on County Road 573. At 14.9 mi, Deer Lake is on right. At15.55 mi, pull off to the right shoulder near glacially polished surface.

15.55 Stop B - Breccia

The breccia is exposed in a glacially polished outcrop and on the lake shore.

A-65

Geological Field Trip to the Marquette Greenstone Belt: Part I1 Day 2 Road Log - Stops A to E

T.J. Bornhorst and D.A. Baxter

Department of Geology and Geological Engineering Michigan Technological University, Houghton, Michigan 49931

NOTE: The field trip for Day 2 includes a late morning to afternoon stop at the Ropes Mine. There is a separate guide, Part 111, authored by Brozdowski and Scott of Callahan Mining Corporation. Additional information on the geologic setting of the field trip stops may be found in the paper by Bornhorst (this volume).

Refer to Day 1 Road Log, Figure 1: Route and Stop Map

Mileage


Assemble at the main entrance of the Ramada Inn. Turn right out of the parking lot of the Ramada Inn to Washington Street to U.S. 41 and then west towards Ishpeming. At 13.8 mi, turn right (north) at traffic light to County Road 573. At 14.6 mi, hairpin turn left. At 14.75 mi, pull off to the right shoulder alongside of outcrops.

14.75 Stop A - Schist

These rocks are foliated quartz-sericite schists with angular carbonate clots

scattered throughout them. Foliation strikes N70Â° and dips 84OS. The rocks are

intermediate in composition (dacite) arid have a calc-alkalic affinity. These rocks

are interpreted to be subaqueously deposited volcaniclastic sediments or pyroclastic

deposits, and are an important component of the southwestern part of the

greenstone belt.

This locality was mapped most recently by Clark and others (1975) as the

Kitchi Schist. Location: NWL section 34, T48N, R27W.

Continue NW on County Road 573. At 14.9 mi, Deer Lake is on right. At 15.55 mi, pull off to the right shoulder near glacially polished surface.

15.55 Stop B - Breccia

The breccia is exposed in a glacially polished outcrop and on the lake shore.

On the polished surface, the clasts vary from 10" X 14" down to the size of the

matrix grains. Larger clasts, up to 2' X 4' in cross section, are present in the

lake shore outcrops. Clasts greater than 1/2" make up about 20% of the rock and

there is an obvious stratification in clast size. Shape of clasts varies from

very-angular to rounded. The clasts are andesite-dacite (near compositional

division) in composition with small chemical variation between individual clasts

studied to date. This breccia is interpreted as a subaqueously deposited mudflow

breccia.

This locality was mapped most recently by Clark and others (1975) as

agglomerate facies of the Kitchi Schist. Location NE+, section 33, T48N, R27W.

Continue on County Road 573. At 17.5 mi, turn to left on gravel road tothe Ropes Mine. See Field Trip to Callahan Mining Corporation Ropes MineProperty (this volume).

Retrace route back towards Marquette. At 34.2 mi, enter city of Marquetteand at 34.5 mi, make a left turn on Washington Street towards the Ramada Inn.At 34.95 mi, turn left to Rublein Street and at 35.1 mi, turn right to RidgeStreet. Pull over and park at 35.3 mi, near small outcrops on right shoulder.

35.3 Stop C (optional) - Schists

A small roadcut on the south side of Ridge Street exposes well-foliated

chlorite-sericite schists. Foliation strikes roughly E-W and dips vertical. On the

north side, about 200 feet through the trees, there is a clearing with glacially

polished outcrops. On the way to the clearing there are layered amphibolitic

schists (discussed further at Stop D). A banded, tabular body of quartz-sericite

schist is well exposed at the clearing; this is interpreted as a rhyolite dike. The

contorted banding is either due to primary flow banding and/or deformation. We

consider the latter interpretation the most likely.

This locality was most recently mapped by Gair and Thaden (1968) as the

Lighthouse Point Member of the Mona Schist. Location NE+, section 22, T48N,

R25W.

A-66

On the polished surface, the clasts vary from 10" X 14" down to the size of the

matrix grains. Larger clasts, up to 2, X 4' in cross section, are present in the

lake shore outcrops. Clasts greater than 1/2" make up about 20% of the rock and

there is an obvious stratification in clast size. Shape of clasts varies from

very-angular to rounded. The clasts are andesite-dacite (near compositional

division) in composition with small chemical variation between individual clasts

studied to date. This breccia is interpreted as a subaqueously deposited mudflow

breccia.

This locality was mapped most recently by Clark and others (1975) as

agglomerate facies of the Kitchi Schist. Location N E k section 33, T48N. R27W.

Continue on County Road 573. At 17.5 mi, turn to left on gravel road to the Ropes Mine. See Field Trip to Callahan Mining Corporation Ropes Mine Property (this volume).

Retrace route back towards Marquette. At 34.2 mi, enter city of Marquette and at 34.5 mi, make a left turn on Washington Street towards the Ramada Inn. At 34.95 mi, turn left to Rublein Street and at 35.1 mi, turn right to Ridge Street. Pull over and park at 35.3 mi, near small outcrops on right shoulder.

35.3 Stop C (optional) - Schists

A small roadcut on the south side of Ridge Street exposes well-foliated

chlorite-sericite schists. Foliation strikes roughly E-W and dips vertical. On the

north side, about 200 feet through the trees, there is a clearing with glacially

polished outcrops. On the way to the clearing there are layered amphibolitic

schists (discussed further at Stop D). A banded, tabular body of quartz-sericite

schist is well exposed at the clearing; this is interpreted as a rhyolite dike. The

contorted banding is either due to primary flow banding and/or deformation. We

consider the latter interpretation the most likely.

This locality was most recently mapped by Gair and Thaden (1968) as the

Lighthouse Point Member of the Mona Schist. Location NEi, section 22, T48N,

R25W.

Continue east on Ridge Street and at 35.4 mi, turn right to Lincoln Avenue.At 35.5 mi, turn left at traffic light to Washington Street. Continue straight onWashington Street past the Ramada Inn, from 36.1 mi until 36.6 mi, whereWashington Street ends at Lake Street. Turn left on Lake Street. At 37.1 mi,make a right turn at the entrance to the Maritime museum and continue on roadto the Coast Guard Station and park..

37.25 Stop D - Lighthouse Point Member

You must obtain permission from the U.S. Coast Guard to visit this locality.

No rock hammers allowed!!

Excellent shoreline exposures on Lighthouse Point provide a view of Archean

and Proterozoic rocks. We will walk around the point noting the localities shown

on Figure 2. This area was most recently mapped in detail by Gair and Thaden

(1968) as the Lighthouse Point Member of the Mona Schist cut by Early Protero-

zoic and Keweenawan diabase dikes. Location: NW+, section 24, T48N, R25W.

Stop Dl.

Archean, thinly layered, amphibolitic (basaltic) schist is cut by a tabular,

porphyritic rhyolite dike trending roughly parallel to the layering. The layers on

Lighthouse Point, as a whole, strike approximately E-W and dip 70°N. This

layered schist was mapped by Gair and Thaden (1968) as the Lighthouse Point

Member. The layers in the schist are interpreted as flattened pillows (basalt).

The mechanism of flattening could be primary, but we interpret high strain as the

most important means of flattening. The rhyolite is interpreted as an late-tectonic

Archean dike. Some rhyolite dikes in the greenstone belt are more strained than

others.

The Archean rocks are cut by two N-S trending diabase dikes that are

metamorphosed to greenschist facies and are distinctly younger than the layered

amphibolite-grade schist. Gair and Thaden (1968) interpreted these dikes as Early

Proterozoic in age. However, Baxter and Bornhorst (1988) suggest these porphy-

ritic dikes are Archean in age, correlative with the Matachewan dike swarm in

Ontario.

A- 67

Continue east on Ridge Street and at 35.4 mi, turn right to Lincoln Avenue. At 35.5 mi, turn left at traffic light to Washington Street. Continue straight on Washington Street past the Ramada Inn, from 36.1 mi until 36.6 mi, where Washington Street ends at Lake Street. Turn left on Lake Street. At 37.1 mi, make a right turn at the entrance to the Maritime museum and continue on road to the Coast Guard Station and park..

37.25 Stop D - Lighthouse Point Member

You must obtain permission from the U.S. Coast Guard to visit this locality.

No rock hammers allowed!!

Excellent shoreline exposures on Lighthouse Point provide a view of Archean

and Proterozoic rocks. We will walk around the point noting the localities shown

on Figure 2. This area was most recently mapped in detail by Gair and Thaden

(1968) as the Lighthouse Point Member of the Mona Schist cut by Early Protero-

zoic and Keweenawan diabase dikes. Location: NWL section 24, T48N, R25W.

Stop Dl.

Archean, thinly layered, amphibolitic (basaltic) schist is cut by a tabular,

porphyritic rhyolite dike trending roughly parallel to the layering. The layers on

Lighthouse Point, as a whole, strike approximately E-W and dip 70Â°N This

layered schist was mapped by Gair and Thaden (1968) as the Lighthouse Point

Member. The layers in the schist are interpreted as flattened pillows (basalt).

The mechanism of flattening could be primary, but we interpret high strain as the

most important means of flattening. The rhyolite is interpreted as an late-tectonic

Archean dike. Some rhyolite dikes in the greenstone belt are more strained than

others.

The Archean rocks are cut by two N-S trending diabase dikes that are

metamorphosed to greenschist facies and are distinctly younger than the layered

amphibolite-grade schist. Gair and Thaden (1968) interpreted these dikes as Early

Proterozoic in age. However, Baxter and Bornhorst (1988) suggest these porphy-

ritic dikes are Archean in age, correlative with the Matachewan dike swarm in

Ontario.

EXPLANATION

Keweenawan Diabase

ProterOzoiC Diabase

Archean 9 Gabbro

Archean Rhyolite

Figure 2: Geology of Lighthouse Point, Marquette, Michigan.Modified slightly from detailed map by Gair and

Thaden (1968). Numbers at localities described

in text.

A-68

Ala

Ag

Layered Amphlbolitic Schist

— ContactFault

Pd

Ar

If

Ala

0 50 100I I

ft

EXPLANATION

Keweenawan @ Diabase

proterozolc Diabase

Archean ? Gabbm

Archean UhyoIitO

Layered Amphibolitic Schist

- Contact

----- Fault

5 0 100 - f t

Figure 2: Geology of Lighthouse P o i n t , Marquet te , Michigan. Modified s l i g h t l y from d e t a i l e d map by Gair and Thaden (1968) . Numbers a t l o c a l i t i e s d e s c r i b e d i n t e x t .

Walk over ridge to other side of the point. A relatively unmetamorphosed,

E-W trending Keweenawan diabase dike crops out along the crest of the ridge.

Stop D2.

There are bodies of comparatively massive, fine to medium-grained (Archean?)

gabbro within the layered amphibolilic schist. This body must have been

relatively late-tectonic due to the lack of foliation/strain as compared to the

surrounding layered schists. Layered amphibolitic schist and gabbro are cut by a

metamorphosed (Proterozoic?) diabase dike.

Stop D3.

Intense foliation has obliterated layering at this location; interpreted as a

higher strain zone within the layered amphibolitic schist.

Stop D4.

Layered amphibolitic schist is cut by a metamorphosed (Proterozoic?) diabase

dike. The metamorphosed dike and layered amphibolite schist are cut by a thin,

relatively unmetamorphosed, E-W trending diabase dike of Keweenawan age. The

metamorphosed diabase dike is offset by a small NW-SE trending fault.

Retrace route back to Lake Street. At 37.4 mi, turn right to Lakeshore Blvd.and proceed along the lakeshore towards Presque Isle Park. At 38.2 mi, junctionwith Fair Avenue, stay on Lakeshore Blvd. and at 38.7 mi, Lakeshore Blvd. joinsPine Street. At 38.8 mi, excellent view of active iron ore docks and Presque Isle(an Archean, serpentinized peridotite unconformably overlain by JacobsvilleSandstone). At 39.3 mi, turn left to Hawley Street before power generating plant.At 40 mi, turn right (north) to County Road 550. At 43.6 mi, pass turnoff toSugarloaf Mountain overlook (requires climb along a maintained trail with steps).At 43.8 mi, large granitoid gneiss outcrops on both sides of the road. At 44.9mi, turn right towards the lakeshore, opposite pathway to Harlow Lake, into asmall parking area.

44.9 Stop £ - Plutonic rocks at Wetmore Landing

Walk around the gate and down a dirt road to the lakeshore and proceed east.

Walk SE along the road above the beach to large rounded wave-washed outcrops

to view features characteristic of granitoid rocks which intrude the volcanic rocks

A-69

Walk over ridge to other side of the point. A relatively unmetamorphosed,

E-W trending Keweenawan diabase dike crops out along the crest of the ridge.

Stop D2.

There are bodies of comparatively massive, fine to medium-grained (Archean?)

gabbro within the layered amphibolilic schist. This body must have been

relatively late-tectonic due to the lack of foliation/strain as compared to the

surrounding layered schists. Layered amphibolitic schist and gabbro are cut by a

metamorphosed (Proterozoic?) diabase dike-

Stop D3.

Intense foliation has obliterated layering at this location; interpreted as a

higher strain zone within the layered amphibolitic schist.

Stop D4.

Layered amphibolitic schist is cut by a metamorphosed (Proterozoic?) diabase

dike. The metamorphosed dike and layered amphibolite schist are cut by a thin,

relatively unmetamorphosed, E-W trending diabase dike of Keweenawan age. The

metamorphosed diabase dike is offset by a small NW-SE trending fault,

Retrace route back to Lake Street. At 37.4 mi, turn right to Lakeshore Blvd. and proceed along the lakeshore towards Presque Isle Park. At 38.2 mi, junction with Fair Avenue, stay on Lakeshore Blvd. and at 38.7 mi, Lakeshore Blvd. joins Pine Street. At 38.8 mi, excellent view of active iron ore docks and Presque Isle (an Archean, serpentinized peridotite unconformably overlain by Jacobsville Sandstone). At 39.3 mi, turn left to Hawley Street before power generating plant. At 40 mi, turn right (north) to County Road 550. At 43.6 mi, pass turnoff to Sugarloaf Mountain overlook (requires climb along a maintained trail with steps). At 43.8 mi, large granitoid gneiss outcrops on both sides of the road. At 44.9 mi, turn right towards the lakeshore, opposite pathway to Harlow Lake, into a small parking area.

44.9 Stop E - Plutonic rocks a t Wetmore Landing

Walk around the gate and down a dirt road to the lakeshore and proceed east.

Walk SE along the road above the beach to large rounded wave-washed outcrops

to view features characteristic of granitoid rocks which intrude the volcanic rocks

of the Marquette Greenstone Belt.

The dominant rocks are gneissic tonalite to granodiorite. These plutonic rocks

are cut by tonalite dikes of varying age, by mafic dikes (amphibolite), and by

veins of quartz of varying age. The mafic dikes are irregular in plan view and

are deformed (Baxter and Bornhorst, 1988). Ductile to brittle shear zones cut the

rocks; epidote is associated with the brittle shears. These outcrops display the

complex history experienced by the granitoid rocks. These features are often

obscured in more typical moss-covered outcrops.

This locality was most recently mapped in detail by Gair and Thaden (1968).

Location: SE+, section 29, T49N, R25W.

Return to Ramada Inn.

END OF DAY 2

ACKNOWLEDGEMENTS

This field trip would not be possible without support by the MichiganGeological Survey, Department of Natural Resources and the Department ofGeology and Geological Engineering, Michigan Technological University for anon-going program of research on the Marquette Greenstone Belt. This field guidehas benefited from reviews by J. Kalliokoski, Michigan Technological University,and K. Schulz, U. S. Geological Survey.

REFERENCES

Baxter, D.A. and Bornhorst, T.J., 1988, Multiple Discrete Mafic Intrusions ofArchean to Keweenawan Age, western Upper Peninsula, Michigan [abs.]: Proceed-ings and Abstracts, 34th Institute on Lake Superior Geology, Marquette, Michigan(this volume).

Baxter, D.A., Bornhorst, T.J., and VanAlstine, J.L., 1987, Geology, Structure,and Associated Precious Metal Mineralization of Archean Rocks in the Vicinity ofClark Creek, Marquette County, Michigan: Michigan Geological Survey Division,Department of Natural Resources, Open File Report OFR-87-8, 54 p.

Clark, L.D., Cannon, W.F., and Klasner, J.S., 1975, Bedrock Geologic Map ofthe Negaunee SW Quadrangle, Marquette County, Michigan: U.S. Geological Sur-vey, Miscellaneous Map Series, GQ-1226.

A-70

of the Marquette Greenstone Belt.

The dominant rocks are gneissic tonalite to granodiorite. These plutonic rocks

are cut by tonalite dikes of varying age, by mafic dikes (amphibolite), and by

veins of quartz of varying age. The mafic dikes are irregular in plan view and

are deformed (Baxter and Bornhorst, 1988). Ductile to brittle shear zones cut the

rocks; epidote is associated with the brittle shears. These outcrops display the

complex history experienced by the granitoid rocks. These features are often

obscured in more typical moss-covered outcrops.

This locality was most recently mapped in detail by Gair and Thaden (1968).

Location: SEi7 section 29, T49N, R25W.

Return to Ramada Inn.

END OF DAY 2

ACKNOWLEDGEMENTS

This field trip would not be possible without support by the Michigan Geological Survey, Department of Natural Resources and the Department of Geology and Geological Engineering7 Michigan Technological University for an on-going program of research on the Marquette Greenstone Belt. This field guide has benefited from reviews by J. Kalliokoski, Michigan Technological University, and K. Schulz7 U. S. Geological Survey.

REFERENCES

Baxter, D.A. and Bornhorst, T.J., 1988, Multiple Discrete Mafic Intrusions of Archean to Keweenawan Age7 western Upper Peninsula7 Michigan [abs.]: Proceed- ings and Abstracts, 34th Institute on Lake Superior Geology7 Marquette, Michigan (this volume).

Baxter, D.A.7 Bornhorst, T.J., and VanAlstine7 J.Le7 19877 Geology7 Structure, and Associated Precious Metal Mineralization of Archean Rocks in the Vic i~ i ty of Clark Creek7 Marquette County, Michigan: Michigan Geological Survey Division7 Department of Natural Resources7 Open File Report OFR-87-8, 54 p.

Clark7 L.D., Cannon7 W.Fe7 and Klasner, J.S., 1975, Bedrock Geologic Map of the Negaunee SW Quadrangle, Marquette County7 Michigan: U.S. Geological Sur- vey7 Miscellaneous Map Series, GQ- 1226.

Gair, J.E. and Thaden, R.E., 1968, Geology of the Marquette and SandsQuadrangles, Marquette County, Michigan: U.S. Geological Survey ProfessionalPaper 397, 77 p.

Johnson, R.C., Bornhorst, T.J., and VanAistine, J.L., 1987, Geologic Setting ofPrecious Metal Mineralization in the Silver Creek to Island Lake Area, MarquetteCounty, Michigan: Michigan Geological Survey Division, Department of NaturalResources, Open File Report OFR-87-4, Supersedes OFR-86-2, 134 p.

MacLellan, M.L. and Bornhorst, T.J., 1988, Geology, Structure, and Mineraliz-ation of the Reany Lake Area, Marquette County, Michigan: Michigan GeologicalSurvey Division, Department of Natural Resources, Open File Report (in prepara-tion).

Puffett, W.P,, 1974, Geology of the Negaunee Quadrangle, Marquette County,Michigan: U.S. Geological Survey, Professional Paper 788, 53 p.

A-71

Gair, J.E. and Thaden, R.E., 1968, Geology of the Marquette and Sands Quadrangles, Marquette County, Michigan: U.S. Geological Survey Professional Paper 397, 77 p.

Johnson, R.C., Bornhorst, T.J., and VanAlstine, J.L., 1987, Geologic Setting of Precious Metal Mineralization in the Silver Creek to Island Lake Area, Marquette County, Michigan: Michigan Geological Survey Division, Department of Natural Resources, Open File Report OFR-87-4, Supersedes OFR-86-2, 134 p.

MacLellan, M.L. and Bornhorst, T-J., 1988, Geology, Structure, and Mineraliz- ation of the Reany Lake Area, Marquette County, Michigan: Michigan Geological Survey Division, Department of Natural Resources, Open File Report (in preparation).

Puffett, W.P., 1974, Geology of the Negaunee Quadrangle, Marquette County, Michigan: U.S. Geological Survey, Professional Paper 788, 53 p.

Geological Field Trip to the Marquette Greenstone Belt: Part IIICallahan Mining Corporation Ropes Mine Property

R.A. Brozdowski and G.W. Scott

Callahan Mining Corporation Exploration Dept.25 Industrial Park Rd., Negaunee, Michigan, 49866

NOTE: Additional information on the Ropes Mine may be found in the paper byBrozdowski in this volume.

STOP 1.

(Outcrop is 225 m west of the intersection of the Ropes mine access road

with the paved highway: Park where access road curves to south and walk the

final 40 m west along the cut line to a small prospect pit). Julius Ropes made

his initial discovery of gold at or near this spot in 1880, and subsequently

discovered the Ropes main ore zone 250 m to the west. The outcrop in the pit is

a fine grained, massive to moderately foliated, quartz-sericite-chlorite rock with

abundant 0.2 mm quartz grains and 1 - 2 mm rectangular mats comprised of

aphanitic sericite in a matrix of aphanitic quartz, sericite, and chlorite. The

sericite mats are interpreted as pseudomorphs after feldspar phenocrysts, because

progressively more sericite altered feldspar phenocrysts are observable in this same

rock type as the Ropes mine is approached from the west. No feldspar remains

within or east of the mine. Toward the north side of the pit is well foliated

quartz-sericite rock in contact with massive, rusty brown weathering carbonate-

quartz rock with white carbonate veins.

STOP 2.

(105 m at azimuth 285 degrees from stop 1). According to a letter written

by Julius Ropes in 1890, these two small quarries were excavated in the late

1880's to provide carbonate flux for a nearby iron ore blast furnace. The north

wall of both quarries is a fine grained, well foliated, light-green carbonate-talc

rock with lenses of blue-green talc and white carbonate veins. Massive,

carbonate-quartz rock occurs toward the south ends of the eastern walls of both

A-72

Geological Field Trip to the Marquette Greenstone B e k Part I11 Callahan Mining Corporation Ropes Mine Property

R.A. Brozdowski and G.W. Scott

Callahan Mining Corporation Exploration Dept. 25 Industrial Park Rd., Negaunee, Michigan, 49866

NOTE: Additional information on the Ropes Mine may be found in the paper by Brozdowski in this volume.

STOP l e

(Outcrop is 225 m west of the intersection of the Ropes mine access road

with the paved highway: Park where access road curves to south and walk the

final 40 m west along the cut line to a small prospect pit). Julius Ropes made

his initial discovery of gold at or near this spot in 1880, and subsequently

discovered the Ropes main ore zone 250 m to the west. The outcrop in the pit is

a fine grained, massive to moderately foliated, quartz-sericite-chlorite rock with I

abundant 0.2 mm quartz grains and 1 - 2 mm rectangular mats comprised of

aphanitic sericite in a matrix of aphanitic quartz, sericite, and chlorite. The

sericite mats are interpreted as pseudomorphs after feldspar phenocrysts, because

progressively more sericite altered feldspar phenocrysts are observable in this same

rock type as the Ropes mine is approached from the west. No feldspar remains

within or east of the mine. Toward the north side of the pit is well foliated

quartz-sericite rock in contact with massive, rusty brown weathering carbonate-

quartz rock with white carbonate veins.

STOP 2.

(105 m at azimuth 285 degrees from stop 1). According to a letter written

by Julius Ropes in 1890, these two small quarries were excavated in the late

1880's to provide carbonate flux for a nearby iron ore blast furnace. The north

wall of both quarries is a fine grained, well foliated, light-green carbonate-talc

rock with lenses of blue-green talc and white carbonate veins. Massive,

carbonate-quartz rock occurs toward the south ends of the eastern walls of both

quarries. Brown weathering, carbonatized, serpentinized peridotite crops out uphill

from the quarries.

STOP 3.

(Park across highway from Ropes mine access road, follow path southeastward

to shore of Deer Lake). The serpentinized peridotite at this locality has a well

developed fine grained cumulate texture of 3-5 mm serpentine pseudomorphs after

olivine and lesser pyroxene. The rock has curvilinear fracture fillings of

chrysotile asbestos.

STOP 4,

Examine drill core and hand specimens of various types of ore and major

types of wall rock from the Ropes mine property. View geologic maps of Ropes

mine area and geologic plans and cross sections of Ropes mine.

A-73

quarries. Brown weathering, carbonatized, serpentinized peridotite crops out uphill

from the quarries.

STOP 3.

(Park across highway from Ropes mine access road, follow path southeastward

to shore of Deer Lake). The serpentinized peridotite at this locality has a well

developed fine grained cumulate texture of 3-5 mm serpentine pseudomorphs after

olivine and lesser pyroxene. The rock has curvilinear fracture fillings of

chrysotile asbestos.

STOP 4.

Examine drill core and hand specimens of various types of ore and major

types of wall rock from the Ropes mine property. View geologic maps of Ropes

mine area and geologic plans and cross sections of Ropes mine.

Field Trip No. 2

Marciuette Mineral District of Michian

Mining History and Geolociy

by

Burton H. BoyumMining Department, Retired

Cleveland-Cliffs, Inc.and

Robert C. ReedMichigan Geological Survey

May, 1988

Field Trip No. 2

Marouette Mineral District of Michicran

Minincr History and Geolocrv

Burton H. Boyum Mining Department, Retired

Cleveland-Cliffs, Inc. and

Robert C. Reed Michigan Geological Survey

May, 1988

MARQUETTE MINERAL DISTRICT OF MICHIGAN

MINING HISTORY AND GEOLOGYIntroduction

Welcome to the Marquette Mineral District situated inMarquette County and the eastcentral part of Baraga County in theUpper Peninsula of Michigan (Figure A)! This district was thefirst of the iron "ranges" of the Lake Superior region to bedeveloped in the mid-1800's, and hosts the only iron mines stillactive in the Upper Peninsula, the Empire and Tilden Mines. TheMarquette Mineral District also contains the only operating goldmine in the U.S. portion of the Lake Superior region, the RopesMine, and has known deposits of copper.

The dominant geologic feature of the district is the Marquettetrough which contains rocks of the Marquette Range Supergroup, anEarly Proterozoic sequence of mostly sedimentary and lesservolcanic rocks deposited unconformably on Archean greenstones andgneisses of the Superior Craton. The Marquette Range Supergroupconsists of four groups (Cannon and Gair, 1970), from oldest toyoungest, the Chocolay, Menominee, Baraga, and Paint River Groups.The three oldest are preserved in the Marquette trough, wereas thePaint River Group is found only in the Iron River-Crystal FallsDistrict to the southwest.The rocks of the Supergroup record aprogressive change intectonic conditions from a stable craton(Chocolay Group) through a sedimentary basin with mild tectonism(Menominee Group) to a highly active tectonic basin (Baraga Group)as the long—stable Superior Craton was reactivated at the onset ofthe Penokean Orogeny. The time of deposition of the Supergroup isstill poorly constrained, but deposition probably commenced about2.1 b.y. and was terminated with the Penokean Orogeny about 1.85b.y. (Morey, 1983). Keweenawan dikes cut rocks of the Supergroup,and the Jacobsville sandstone, of probable Keweenawan age, andlower Paleozoic sedimentary rocks unconformably overlie units ofthe Supergroup locally.

Pleistocene sand, gravel, silt, till, and clay were depositedover the district about 10,000 years ago during retreat of the lastglaciers. Outwash deposits of silt, sand, and gravel preservedQuaternary trees, mostly spruce, in the Tilden Mine Gribben BasinBuried Forest near Palmer. The remains of more than 100 trees arepreserved with their roots in place. Carbon 14 dates on materialfrom these trees, which have as many as 150 growth rings, rangefrom 9545 to 10,230 y.B.P. This indicates the recession of theValders (Great Lakean) glacier (Hughes, 1979). Figure B shows across section of the Gribben Basin forest.Economic Considerations

The principal mineralization of the Marquette Mineral Districthas been iron, with some manganiferous iron ore. Total shipmentsof iron ore through Decnither, 1987 were 590,776,442 long tons. Thetwo active iron mines of the district, the Empire and the Tilden,each mine taconite type ore in open pit operations, and through theuse of concentrators and pellet plants, produce a high gradeproduct. Shipping ports for the taconite pellets are Marquette andEscanaba.

B-2

MARQUETTE MINERAL DISTRICT OF MICHIGAN

MINING HISTORY AND GEOLOGY Introduction

Welcome to the Marquette Mineral District situated in Marquette County and the eastcentral part of Baraga County in the Upper Peninsula of Michigan (Figure A)! This district was the first of the iron "ranges" of the Lake Superior region to be developed in the mid-1800ts, and hosts the only iron mines still active in the Upper Peninsula, the Empire and Tilden Mines. The Marquette Mineral District also contains the only operating gold mine in the U.S. portion of the Lake Superior region, the Ropes Mine, and has known deposits of copper.

The dominant geologic feature of the district is the Marquette trough which contains rocks of the Marquette Range Supergroup, an Early Proterozoic sequence of mostly sedimentary and lesser volcanic rocks deposited unconformably on Archean greenstones and gneisses of the Superior Craton. The Marquette Range Supergroup consists of four groups (Cannon and Gair, 1970), from oldest to youngest, the Chocolay, Menominee, Baraga, and Paint River Groups. The three oldest are preserved in the Marquette trough, wereas the Paint River Group is found only in the Iron River-Crystal Falls District to the southwest.The rocks of the Supergroup record a progressive change intectonic conditions from a stable craton (Chocolay Group) through a sedimentary basin with mild tectonism (Menominee Group) to a highly active tectonic basin (Baraga Group) as the long-stable Superior Craton was reactivated at the onset of the Penokean Orogeny. The time of deposition of the Supergroup is still poorly constrained, but deposition probably commenced about 2.1 b.y. and was terminated with the Penokean Orogeny about 1.85 b.y. (Morey, 1983). Keweenawan dikes cut rocks of the Supergroup, and the Jacobsville sandstone, of probable Keweenawan age, and lower Paleozoic sedimentary rocks unconformably overlie units of the Supergroup locally.

Pleistocene sand, gravel, silt, till, and clay were deposited over the district about 10,000 years ago during retreat of the last glaciers. Outwash deposits of silt, sand, and gravel preserved Quaternary trees, mostly spruce, in the Tilden Mine Gribben Basin Buried Forest near Palmer. The remains of more than 100 trees are preserved with their roots in place. Carbon 14 dates on material from these trees, which have as many as 150 growth rings, range from 9545 to 10,230 y.B.P. This indicates the recession of the Valders (Great Lakean) glacier (Hughes, 1979). Figure B shows a cross section of the Gribben Basin forest. Economic Considerations

The principal mineralization of the Marquette Mineral District has been iron, with some manganiferous iron ore. Total shipments of iron ore through December, 1987 were 590,776,442 long tons. The two active iron mines of the district, the Empire and the Tilden, each mine taconite type ore in open pit operations, and through the use of concentrators and pellet plants, produce a high grade product. Shipping ports for the taconite pellets are Marquette and Escanaba.

Figure A

B—3 Figure A

General Geology ofMichfgan 's Upper Peninsula.General Geology o f Michigan's Upper Peninsula.

Figure A

Figure A

r'±gure B

.rt Lob. SLF

Figure B showing cross. section of Gribben Basin Buried Forest, Palmer

The Reoessiøn of the Valders Glacial Ice, dtring time of Marquette-Munising MoZ'aiCa,ra 9,925 years B P.

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1978- J.M. NMU Car l Ltb. SLF

Figure % showing cross,section of Gribben Basin Buried Forest. Paltner 1

The greater part of the district's iron ore has come from theNegaunee Iron—Formation of the Menominee Group. The Negaunee, oneof the thickest iron-formations of the world, has a maximumstratigraphic thickness of over 1,300 meters (3,500 feet) inNegaunee and Ishpeming. Some iron ore was produced also from iron-formation, of unknown stratigraphic correlation to the NegauneeIron-Formation, in the samller Gwinn District to the south, andfrom the Bijiki Iron-Formation of the Baraga Group.

Today, taconite is the only ore type being mined.. (The termtaconite originated in Minnesota, on the Mesabi Range, after StateGeologist N.H. Winchell proposed, in 1884, that the iron-formationwas of "Taconic Age".) However, in the past, the major iron oreswere direct shipping or natural ores of two main varieties. One was"soft" ore composed of soft, friable, earthy or semi—plastichematite, martite and goethite. The other was "hard" ore composedof dense, compact specular hematite and magnetite. "Ore" isdefined as a mineral or minerals from which a metal or metals canbe mined at a profit.

The natural ores were found in three distinct structuralsettings. The soft ores were found in synclines and in faultstructures at the base of the Negaunee Iron—Formation, and in faultstructures on the intrusive sills in the upper portion of theNegaunee Iron—Formation. The hard ores were found generally at thetop of the Negaunee Iron-Formation. Quantitatively, the totalfootwall soft ores shipped from the district was 186,607,540 longtons, the total sill soft ores shipped was 54,092,030 long tons,and the total hard ores was 60,350,944 long tons. Maximum thicknessof the footwall soft ore was 80 meters (260 feet) normal to thebedding; maximum thickness of the hard ore was 30 meters (100 feet)(see cross sections, Figures C and D).

The Marquette Range is the only iron range in the LakeSuperior Region to have extensive deposits of both hard and softiron ores. The spatial distribution of these two ore types alongthe range appears to correlate with the grade of metamorphism ofthe host rocks. The soft ores are found mostly in the chloritegrade rocks in the Negaunee and Ishpeming area, and the hard oresmostly in higher grade rocks (ie. biotite, garnet, staurolite, tosillimanite grade) of the Republic metamorphic node (James, 1955;see Figure E).

Two principal hypotheses have been advanced for the formationof the direct shipping, natural ores; the "cold-water" hypothesis(VanHise and Bayley, 1897), and the hydrothermal hypothesis(Gruner, 1930). Today, most company geologists favor ahydrothermal origin for both the soft and hard iron ores.Historical Summary

The extensive copper deposits of the Keweenawan Peninsula wereworked by the ancients over 5,000 years ago and were known to theEuropean explorers, missionaries and fur traders. The irondeposits were not known to the Europeans, although some were knownto the native Americans who were very superstitious about them.

The Marquette Range was the first of the region's iron rangesto be discovered. The discovery was made by government surveyorssubdividing the land into townships of 36 square miles. The surveyparty, under Deputy Land Surveyor, William Austin Burt, inventor of

(See Figure E-1)

B- 5

The greater part of the districtts iron ore has come from the Negaunee Iron-Formation of the Menominee Group. The Negaunee! one of the thickest iron-formations of the worldl has a maximum stratigraphic thickness of over 11300 meters (31500 feet) in Negaunee and Ishpeming. Some iron ore was produced also from iron- formationl of unknown stratigraphic correlation to the Negaunee Iron-Formationl in the samller Gwinn District to the southl and from the Bijiki Iron-Formation of the Baraga Group.

Todayl taconite is the only ore type being mined. (The term taconite originated in Minnesota, on the Mesabi Rangel after State Geologist N.H. Winchell proposedl in 1884# that the iron-formation was of ttTaconic Agett. ) Howeverl in the pastl the ma2or iron ores were direct shipping or natural ores of two main varieties. One was ttsofttt ore composed of soft, friablel earthy or semi-plastic hematitel martite and goethite. The other was Ithardt8 ore composed of densel compact specular hematite and magnetite. IfOrett is defined as a mineral or minerals from which a metal or metals can be mined at a profit.

The natural ores were found in three distinct structural settings. The soft ores were found in synclines and in fault structures at the base of the Negaunee Iron-Formationl and in fault structures on the intrusive sills in the upper portion of the Negaunee Iron-Formation. The hard ores were found generally at the top of the Negaunee Iron-Formation. Quantitativelyl the total footwall soft ores shipped from the district was 1861607f540 long tonsl the total sill soft ores shipped was 5410921030 long tonsf and the total hard ores was 6O135OI944 long tons. Maximum thickness of the footwall soft ore was 80 meters (260 feet) normal to the bedding; maximum thickness of the hard ore was 30 meters (100 feet) (see cross sectionsl Figures C and D).

The Marquette Range is the only iron range in the Lake Superior Region to have extensive deposits of both hard and soft iron ores. The spatial distribution of these two ore types along the range appears to correlate with the grade of metamorphism of the host rocks. The soft ores are found mostly in the chlorite grade rocks in the Negaunee and Ishpeming area, and the hard ores mostly in higher grade rocks (ie. biotitel garnetl staurolitef to sillimanite grade) of the Republic metamorphic node (Jamesf 1955; see Figure E) .

Two principal hypotheses have been advanced for the formation of the direct shipping! natural ores; the ttcold-watertt hypothesis (VanHise and Bayleyl 1897)f and the hydrothermal hypothesis (Grunerl 1930). Todayl most company geologists favor a hydrothermal origin for both the soft and hard iron ores. ~htorical summa&

The extensive copper deposits of the Keweenawan Peninsula were worked by the ancient;-over if 000 years ago and were known to the European explorersl missionaries and fur traders. The iron deposits were not known to the Europeansf although some were known to the native Americans who were very superstitious about them.

The Marquette Range was the first of the regionts iron ranges to be discovered. The discovery was made by government surveyors subdividing the land into townships of 36 square miles. The survey partyf under Deputy Land Surveyorl William Austin Burtf inventor of

(See F i g u r e E-1 )

MARQUETTE IRON RANGE — MICHIGAN

REv,5c, lq7c#9

4—

-

FIGURE DGENERALIZED N—S CROSS SECTION

ONE MILE EAST OF WEST LINE OF R.26WLOOKING WEST

MARQUETTE IRON RANGE - MICHIGAN

1 GENERALIZED LONGITUDINAL SECTION FIGURE c

GENERALIZED N - S CROSS SECTION ONE M I L E EAST OF WEST LINE OF R . 2 6 W

FIGURE D

Figure E

MAP SHOWING NODES AND ZONES OF

PENOKEAN REGIONAL METAMORPHISM

Marquette

EXPLANATION

_ _

I...]ChIert• zone Blotite zone Garnet zone

StauroUte zone Sillimanite zone

KEWEENAWAN ROCKS

10 0 tO 20 MILESI I

10 20 KILOMETERS

Grovelond Mine

Modified from James (1955) and Cannon (1973)

FLORENCE

MAP SHOWING NODES AND ZONES OF

PENOKEAN REGIONAL METAMORPHISM

EXPLANATION

Chlorite zone Blothe zone Gornet zone

Silllmanlte zone

KEWEENAWAN ROCKS

Groveland Mim

I0 0 I 1 a * . I

Modified from James (1955) and Cannon (1973) Figure E

Frank G. Matthews, Sr.,Memorial Fund

* HISTORICAL NOTES * NUMBER ONE ********The Historical Notes are prepared to honor the memory of Frank G. Matthews, Sr.,

enthusiastic historian and collector of lore of early iron mining and charcoal iron makingof Michigan's Upper Peninsula.

BURT'S SOLAR COMPASSThe Solar Compass of William Austin Burt revolu-

tionized the early land surveying and land subdivisionby providing a true north-south direction. Prior to thisinvention, the surveyor was completely dependentupon the magnetic compass, which could be unre-liable due to the local magnetic character of the earthin the area being surveyed.

William Austin Burt of Mt. Vernon, Michigan, hadbeen appointed a Deputy Land Surveyor for theFederal Government in 1833. In 1834 he surveyed anarea north of Milwaukee in Wisconsin Territory. Thearea had local magnetic attraction causing great diffi-culty in surveying for Burt and other surveyors. In1835 he worked on a model of his solar compass. thenreferred to as a "variation apparatus" or variationcompass." Variation referred to the amount of differ-ence in degrees between true north and magneticnorth. On February 26, 1836 he received patent

number 9428X for his invention. It was first used thatyear by him and by his son Alvin, also a surveyor. Herewas his original device attached to the standard fieldcompass of the day.

THE SUN DIALTo understand how Burt's Solar Compass works, it

is helpful to think of the sun dial, which most peoplehave seen. The sun dial is an ancient device and hasmany forms. But the simple garden variety has a dialwith the hours marked on it and an arm (called a"gnomon," from the Greek meaning one who knowsor indicates) that casts a shadow read on the hourcircle. If the sun dial has been correctly made andcorrectly oriented, the shadow will tell the time. Atnoon, the sun is in a true north-south line called the"meridian."

So, if one had a portable instrument and knew thetime, it could be oriented to indicate the meridian(provided the sun is shining or at least hazy enough tocreate shadows). This was the heart of Burt's genius.

3-8 Figure E—

FRANK G. MATTHEWS. SR.

Burt's Original SolarAttached to a Compass

Frank G . Matthews, Sr., Memorial Fund

* HISTORICAL NOTES * NUMBER ONE * The Historical Notes are prepared to honor the memoy oj Frank G. Matthews, Sr.,

enthusiastic historian and coilector of lore of early iron mining and charcoal iron making of Michigan's Upper Peninsula.

* * * * * * * FRANK G. MAmHEWS, SR.

BURT'S SOLAR COMPASS The Solar Compass of William Austin Burt revolu-

tionized the early land surveying and land subdivision by providing a true north-south direction. Prior to this invention, the surveyor was completely dependent upon the magnetic compass, which could be unre- liable due to the local magnetic character of the earth in the area being surveyed.

William Austin Burt of Mt. Vernon, Michigan, had been appointed a Deputy Land Surveyor for the Federal Government in 1833. In 1834 he surveyed an area north of Milwaukee in Wisconsin Territory. The area had local magnetic attraction causing great difficulty in surveying for Burt and other surveyors. In 1835 he worked on a model of his solar compass. then referred to as a "variation apparatus" or "variation compass." Variation referred to the amount of differ- ence in degrees between true north and magnetic north. On February 26, 1836 he received patent

number 9428X for his invention. It was first used that year by him and by his son Alvin, also a surveyor. Here was his original device attached to the standard field compass of the day.

THE SUN DIAL To understand how Burt's Solar Compass works, it

is helpful to think of the sun dial, which most people have seen. The sun dial is an ancient device and has many forms. But the simple garden variety has a dial with the hours marked on it and an arm (called a "gnomon," from the Greek meaning one who knows or indicates) that casts a shadow read on the hour circle. If the sun dial has been correctly made and correctly oriented, the shadow will tell the time. At noon, the sun is in a true north-south line called the "meridian."

So, if one had a portable instrument and knew the time, it could be oriented to indicate the meridian (provided the sun is shining or at least hazy enough to create shadows). This was the heart of Burt ' s genius.

the Solar Compass, on September 19, 1844, was running the townshipline between Range 26 West and 27 West south of Teal Lake in whatis now the City of Negaunee. They noted that their magneticcompass pointed south in this area. Burt directed his party tolook around and they found "spathose and magnetic ores abunding".However, news of this discovery was overshadowed by the rush forcopper in the Keweenaw. In fact, the first iron mining company tooperate in the region, the Jackson Mining Company, organized onJuly 23, 1845 in Jackson, Michigan, went to the Upper Peninsulalooking for cooper and other base metals, and was persuaded to lookfor iron by a half-breed, Louis Nolan. Although Nolan could notfind the exposures of iron ore, the local chief "Marji Gesick"finally led them to the east shore of Teal Lake, in what is nowNegaunee. This location was federal permit Number 158, and wassecured by the Jackson Mining Company as Permit Number 593, Section1, Township 47 North, Range 27 West, on October 4, 1845. Theirintent was not to ship iron ore, but to ship iron "blooms" madefrom charcoal forges of the "Catalan" (Spanish) style like thoseused in New York and New England at that time, and used in Europefor over 2,000 years. They built a townsite on the Carp River,along with a dam and the Forge. The first iron blooms were made onFebruary 10, 1848, and "mining" was by gathering high grade lumpsof ore found on the surface at the Jackson Mine.

It was tough going for them and for the other charcoal ironoperations that followed them. It soon became apparent to themanagements of the mining companies, that the future of the ironindustry lay in shipping iron ore rather than the charcoal ironblooms. However, the iron bloom industry limped along in the UpperPeninsula until 1898; over 1.8 million long tons of processed ironhad been shipped from the Upper Peninsula.

The port of Marquette was founded in July, 1849. The firstiron ore shipped, on July 7, 1852, consisted of six barrels. Theopening of the Sault St. Marie locks in June, 1855 was a great boonto both the infant iron and infant copper mining ventures.

All- of the first mines were open pit operations and were inhard iron ores. The hard ores cropped out on hilltops and alsocontained enough magnetite to deflect the magnetic compasses whereconcealed. The Jackson Mine led the way, followed by the ClevelandMines at the east side of Ishpeming (Jasper Knob), the Humboldt,Republic, Champion and Michigannue Mines on the main Marquettetrough, and the Isabella Mine on the Cascade Range.

The first underground mines opened in 1868 as the minersfollowed the iron ore in depth. The soft ores in Negaunee werefound first at the Negaunee Mine in 1873. The soft iron ores didnot crop out because they were easily eroded and were covered byvarying amounts of glacial drift. However, some soft ores came upto ledge surface.

Crude geophysics by magnetic compass and the dip-needle helpedin locating the iron ore bodies. The diamond drill was developedin 1876 and first employed on the Marquette Range in 1879.

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the Solar Compasst on September 19! 1844! was running the township line between Range 26 West and 27 West south of Teal Lake in what is now the City of Negaunee. They noted that their magnetic compass pointed south in this area. Burt directed his party to look around and they found ltspathose and magnetic ores abundingl1. HoweverI news of this discovery was overshadowed by the rush for copper in the Keweenaw. In fact! the first iron mining company to operate in the region, the Jackson Mining Companyf organized on July 23! 1845 in Jackson! Michigsnl went to the Upper Peninsula looking for cooper and other base metals! and was persuaded to look for iron by a half-breed! Louis Nolan. Although Nolan could not find the exposures of iron ore! the local chief tlMarji Gesickt1 finally led them to the east shore of Teal Lake! in what is now Negaunee. This location was federal permit Number 158! and was secured by the Jackson Mining Company as Permit Number 593! Section lt Township 47 North! Range 27 West! on October 4! 1845. Their intent was not to ship iron ore! but to ship iron llbloomsll made from charcoal forges of the tlCatalanll (Spanish) style like those used in New York and New England at that time! and used in Europe for over 2t000 years. They built a townsite on the Carp River! along with a dam and the Forge. The first iron blooms were made on February lol 1848! and tlminingtl was by gathering high grade lumps of ore found on the surface at the Jackson Mine.

It was tough going for them and for the other charcoal iron operations that followed them. It soon became apparent to the managements of the mining companiest that the future of the iron industry lay in shipping iron ore rather than the charcoal iron blooms. However! the iron bloom industry limped along in the Upper Peninsula until 1898; over 1.8 million long tons of processed iron had been shipped from the Upper Peninsula.

The port of Marquette was founded in July! 1849. The first iron ore shipped! on July 7! 18521 consisted of six barrels. The opening of the Sault St. Marie locks in June! 1855 was a great boon to both the infant iron and infant copper mining ventures.

All- of the first mines were open pit operations and were in hard iron ores. The hard ores cropped out on hilltops and also contained enough magnetite to deflect the magnetic compasses where concealed. The Jackson Mine led the way! followed by the Cleveland Mines at the east side of Ishpeming (Jasper Knob)! the Humboldtl Republicl Champion and Michigame Mines on the main Marquette trough! and the Isabella Mine on the Cascade Range.

The first underground mines opened in 1868 as the miners followed the iron ore in depth. The soft ores in Negaunee were found first at the Negaunee Mine in 1873. The soft iron ores did not crop out because they were easily eroded and were covered by varying amounts of glacial drift. However! some soft ores came up to ledge surface.

Crude geophysics by magnetic compass and the dip-needle helped in locating the iron ore bodies. The diamond drill was developed in 1876 and first employed on the Marquette Range in 1879.

Mining towns sprang up around the mines in what were termed"locations". Many towns were developed by the mining companies,with homes and "boarding—houses" subsidized by the companies toattract employees. There was a parallelism between the coppermining companies and their paternalistic efforts to obtainemployees and retain them, and the iron mining companies. Foremostamong the copper companies was Calumet and Hecla ConsolidatedCopper Company while the foremost iron company was the ClevelandIron Mining Company (from 1891 it was The Cleveland-Cliffs IronCompany). C.& H. built the first hospital in Calumet before thecompany paid its first dividend. Cliffs also pioneered companyhouses, hospitals, and visiting nurses. They had the firstpension plan in the Lake Superior Region, the first safetydepartment, and employed the first company geologists.Mining Methods

On the Marquette Range, by the 1880's almost all productioncame from underground mines. In the hard ore mines the principalmethod was open stoping (a stope is a Cornish word for a room) bythe "room and pillar" method. Very little timber was employed forground support. In the soft ore mines there was sublevel stoping,sublevel caving, square set mining, and top slicing. Much later,the concept of block caving was introduced and used extensively.Much timber was used for ground support in the soft ore mines andin the copper mines. One wag said "the best forests of Michiganare underground."

The choice of the mining method, and the use of wood or steelsupports related directly to the geologic conditions. The hardores were "competent" and would support an open span, while thesoft ores did not have that strength. Other factors such asjointing, ground water, and depth from the surface all entered intoboth safety considerations and contamination of leaner materialaffecting the ore grade.

B-lU

Mining towns sprang up around the mines in what were termed fllocations~. Many towns were developed by the mining companies, with homes and wboarding-houses" subsidized by the companies to attract employees. There was a parallelism between the copper mining companies and their paternalistic efforts to obtain employees and retain them, and the iron mining companies. Foremost among the copper companies was Calumet and Hecla Consolidated Copper Company while the foremost iron company was the Cleveland Iron Mining Company (from 1891 it was The Cleveland-Cliffs Iron Company). C.& H. built the firs: hospital in Calumet before the company paid its first dividend. Cliffs also pioneered company houses, hospitals, and visiting nurses. They had the first pension plan in the Lake Superior Region, the first safety department, and employed the first company geologists. - -

~ihincr ~ethods On the Marquette Range, by the 1880fs almost all production

came from underground mines. 1n the hard ore mines the principal method was open stoping (a stope is a Cornish word for a room) by the "room and pillarw method. Very little timber was employed for ground support. In the soft ore mines there was sublevel sloping, sublevel caving, square set mining, and top slicing. Much later, the concept of block caving was introduced and used extensively. Much timber was used for ground support in the soft ore mines and in the copper mines. One wag said "the best forests of Michigan are underground. If

The choice of the mining method, and the use of wood or steel supports related directly to the geologic conditions. The hard ores were NcompetentN and would support an open span, while the soft ores did not have that strength. Other factors such as jointing, ground water, and depth from the surface all entered into both safety considerations and contamination of leaner material affecting the ore grade.

Metallurgical BeneficiationFor two generations, the Marquette Range iron ore that was

mined and shipped, or used in the charcoal iron industry, was rich,natural, direct-shipping materials. Both the mining companyofficials and government geologists recognized that the iron—formations contained rich bands of iron minerals. The questionwas, how to separate these bands from the waste or ganguematerials, essentially the chert or siliceous bands.

Beneficiation (to make "well" from the Latin) is the processor processes used to separate the valuable material from the gangueand generally consists of several steps. First is to crush andgrind the feed material fine enough to "liberate" the valuablematerials, making them essentially free of the gangue. The nextstep is to collect and separate the two materials into a"concentrate" of the valuable material from the worthless material,the latter called the "tails" or "tailings". The "feed" or crudematerial is called the "heads".

This beneficiation process (or processes) is termed"concentration". There are three principal means of concentration:gravity, magnetic, and chemical/physical. The gravity processutilizes the differences in specific gravity of the ore minerals incontrast to the gangue. This might be done with jigs, tables,heavy media, or siphonsizers, to name a few. Magnetic separationapplies principally to magnetite.

Chemical/physical seperation utilizes principallyflotation, but could include electrostatic separation. Flotationuses the surface response of various reagents to various minerals.It is possible, in some instances, to "float" the ore minerals, orto float the gangue minerals. It is also possible, in someinstances, to use a differential float to separate various oreminerals from one another.Negaunee Concentrating Works

The first commercial concentrating plant on the MarquetteRange was the Negaunee Concentrating Works built on the lands ofthe Jackson Mine. It used the jaspilite from the North JacksonMine, paying a royalty of 45 cents a ton. The company, organizedin 1881, built a plant 183 feet long, 116 feet wide and 113 feethigh (nine stories), on a rock ridge towards Teal Lake. It had a600 horsepower boiler, a boiler house, and an engine house. Itused a jaw crusher, "Cornish" rolls, and rotary jigs. In its firstyear of operation, 1882, the plant produced and shipped 1,177 tonsof concentrates. Production increased to 10,394 tons in 1883. Theplant did not operate in 1884 and 1885, and it closed in 1886,years ahead of its time.Edison's Mill, Humboldt

The second concentrating plant was built by Thomas A. Edison.He had patented his electromagnetic separator in 1880 (U.S. PatentNumber 228,329 on June 1, 1880 (see Figure F)), after his researchin 1879. Edison looked around the iron districts of the UnitedStates for an area with magnetic iron ores and heard of the SampsonMine at Humnbolt (also known as the Edwards or the Argyle Mines).He worked with Walter S. Mallory to construct the ConcentratingPlant using magnetic ores and lean ores mined by the Sampsonpersonnel. Edison's separator was different from the separators of

B-li

Metallurqical Beneficiation For two generations, the Marquette Range iron ore that was

mined and shipped, or used in the charcoal iron industry, was rich, natural, direct-shipping materials. Both the mining company officials and government geologists recognized that the iron- formations contained rich bands of iron minerals. The question was, how to separate these bands from the waste or gangue materials, essentially the chert or siliceous bands.

Beneficiation (to make "wellM from the Latin) is the process or processes used to separate the valuable material from the gangue and generally consists of several steps. First is to crush and grind the feed material fine enough to "liberatew the valuable materials, making them essentially free of the gangue. The next step is to collect and separate the two materials into a wconcentrateN of the valuable material from the worthless material, the latter called the "tailsw or "tailingsw. The "feedw or crude material is called the "headsw.

This beneficiation process (or processes) is termed "concentrationw. There are three principal means of concentration: gravity, magnetic, and ~hemical/~hysical~ The gravity process utilizes the differences in specific gravity of the ore minerals in contrast to the gangue. This might be done with jigs, tables, heavy media, or siphonsizers, to name a few. Magnetic separation applies principally to magnetite.

Chemical/physical separation utilizes principally flotation, but could include electrostatic separation. Flotation uses the surface response of various reagents to various minerals. It is possible, in some instances, to "floatw the ore minerals, or to float the gangue minerals. It is also possible, in some instances, to use a differential float to separate various ore minerals from one another. Neaaunee Concentratinq Works

The first commercial concentrating plant on the Marquette Range was the Negaunee Concentrating Works built on the lands of the Jackson Mine. It used the jaspilite from the North Jackson Mine, paying a royalty of 45 cents a ton. The company, organized in 1881, built a plant 183 feet long, 116 feet wide and 113 feet high (nine stories), on a rock ridge towards Teal Lake. It had a 600 horsepower boiler, a boiler house, and an engine house. It used a jaw crusher, "CornishN rolls, and rotary jigs. In its first year of operation, 1882, the plant produced and shipped 1,177 tons of concentrates. Production increased to 10,394 tons in 1883. The plant did not operate in 1884 and 1885, and it closed in 1886, years ahead of its time. Edisonls Mill, Humboldt

The second concentrating plant was built by Thomas A. Edison. He had patented his electromagnetic separator in 1880 (U.S. Patent Number 228,329 on June 1, 1880 (see Figure F)), after his research in 1879. Edison looked around the iron districts of the United States for an area with magnetic iron ores and heard of the Sampson Mine at Humbolt (also known as the Edwards or the Argyle Mines). He worked with Walter S. Mallory to construct the Concentrating Plant using magnetic ores and lean ores mined by the Sampson personnel. Edisonts separator was different from the separators of

(No Model.)

No. 228,329.

T. A. EDISON.Magnetic Ore-Separator.

Patented June 1, 1880.

t:

awn S..r a.

L

B—12 Figure F

y.2.

= —

—

-

(No ~ o d e ' l . )

No. 228.329.

T. A . E D I S O N . K a g n e t i c O r e - S e p a r a t o r .

Patented J u n e 1, 1880.

Figure F

the time in that it had no moving parts. It consisted of a feedhopper, a gate, the unipolar non-contact electromagnet, apartition, and bins for the magnetic concentrate and for the wastmaterial.

The plant was built in 1888 and operated in 1889 as the EdisonIron Concentrating Company. It had a Gates jaw crusher, two 16 by30 Cornish rolls, and screens sizing the feed to 20 mesh, a grainsize for liberation. The iron concentrate analyzed 62 to 68percent iron. The start—up had the usual problems such asbreakdowns of the primary crusher and learning that 20 mesh was notfine enough for liberation. The intermediate product Edison called"mugwump". The plant burned in December, 1890 and was not rebuilt.He said "Well, its all gone, but we had a hell of a good timespending it" (his investment)!

The third mill to open was that of the Republic ReductionCompany in 1888 and 1889; it was sold in 1891. The Michigamme Mineused a Swedish "Wenstroin" electromagnetic separator starting in1889. Although early results were encouraging, the plant closed in1892.Minnesota Concentrating Efforts

After these early efforts at concentration in Michigan, thescene shifted to Minnesota. In 1902, the first concentrating plantwas built at the west end of the Mesabi Range and used the BiwabikIron—Formation for ore. Various plants followed, using gravitymethods on the "wash—ores" . Professor Edward W. Davis, of theMinnesota Mines Experiment Station, pursued a vigorous researchprogram on the magnetic concentration of the magnetic portion ofthe Biwabik Iron-Formation, starting in 1914. The Reserve MiningCompany was organized to mine the magnetic iron—formation and touse Davis' ideas for concentration. The plant operated in the1920's, but closed due to two principal problems: (1) thedifficulty of drilling blast holes, and (2) the high cost ofconcentration while competing with low cost, open pit mined naturalores.

The idea was revived in the 1940 by Professor Davis, who hadcontinued to work on the problem. The Reserve Mining Company wasreorganized and their first plant conuuenced operations after WorldWar II on the "taconite" iron-formation. Shortly after, the finegrained concentrate was agglomerated or pelletized at ReserveMining.Michigan Low Grade Ores

During World War II, Cleveland Cliffs investigated various lowgrade iron—formations that could be amenable to commercialconcentration. The research was directed into two areas: (1) theBijiki Iron—Formation west of Michigamme, and (2) the hard orejaspilite iron-formation at the old hard ore mines.

The Ohio Mine west of Michiganune tested well in terms of heavymedia concentration. The opening of two open pits and theconstruction of a concentrator started in 1951. The firstconcentrates were produced in May, 1952. The plant operated untilSeptember, 1960 producing a total of 747,729 long tons ofconcentrate. This is not significant by today's standards, but itmarked a dramatic shift from underground mining to the new era ofopen pit mining and concentration.

B-13

the time in that it had no moving parts. It consisted of a feed hopper, a gate, the unipolar non-contact electromagnet, a partition, and bins for the magnetic concentrate and for the wast material.

The plant was built in 1888 and operated in 1889 as the Edison Iron Concentrating Company. It had a Gates jaw crusher, two 16 by 30 Cornish rolls, and screens sizing the feed to 20 mesh, a grain size for liberation. The iron concentrate analyzed 62 to 68 percent iron. The start-up had the usual problems such as breakdowns of the primary crusher and learning that 20 mesh was not fine enough for liberation. The intermediate product Edison called nmugwumpw. The plant burned in December, 1890 and was not rebuilt. He said "Well, its all gone, but we had a hell of a good time spending itN (his investment) !

The third mill to open was that of the Republic Reduction Company in 1888 and 1889; it was sold in 1891. The Michigamme Mine used a Swedish "Wenstrom" electromagnetic separator starting in 1889. Although early results were encouraging, the plant closed in 1892. Minnesota ~oncentratina Efforts

After these early efforts at concentration in Michigan, the scene shifted to Minnesota. In 1902, the first concentrating plant was built at the west end of the Mesabi Range and used the Biwabik Iron-Formation for ore. Various plants followed, using gravity methods on the vwash-oresw . Professor Edward W. Davis, of the Minnesota Mines Experiment Station, pursued a vigorous research program on the magnetic concentration of the magnetic portion of the Biwabik Iron-Formation, starting in 1914. The Reserve Mining Company was organized to mine the magnetic iron-formation and to use Davist ideas for concentration. The plant operated in the 19201s, but closed due to two principal problems: (1) the difficulty of drilling blast holes, and (2) the high cost of concentration while competing with low cost, open pit mined natural ores.

The idea was revived in the 1940 by Professor Davis, who had continued to work on the problem. The Reserve Mining Company was reorganized and their first plant commenced operations after World War I1 on the '%aconitet' iron-formation. Shortly after, the fine grained concentrate was agglomerated or pelletized at Reserve Mining. Michicran Low Grade Ores

During World War 11, Cleveland Cliffs investigated various low grade iron-formations that could be amenable to commercial concentration. The research was directed into two areas: (1) the Bijiki Iron-Formation west of Michigamme, and (2) the hard ore jaspilite iron-formation at the old hard ore mines.

The Ohio Mine west of Michigamme tested well in terms of heavy media concentration. The opening of two open pits and the construction of a concentrator started in 1951. The first concentrates were produced in May, 1952. The plant operated until September, 1960 producing a total of 747,729 long tons of concentrate. This is not significant by today's standards, but it marked a dramatic shift from underground mining to the new era of open pit mining and concentration.

The jaspilite, or "jasper" as Cliffs called it, was beinginvestigated at the Humboldt and Republic Mines. The mode ofconcentration was to float the specular hematite and magnetite infatty acids. The Humboldt plant was built in 1952 as a jointventure of Ford Motor Co. and Cliffs, the same year the Ohio Mineopened. The first concentrates were produced in 1954, but no formof agglomeration was used.

Work had gone ahead at the Republic Mine and its firstconcentrates ere produced in 1956, using the same flow sheet asthe Humboldt Mine. The Republic Mine concentrates were shipped,starting also in 1956, to Eagle Mills, east of Negaunee, to aStraight Grate Plant to be pelletized. This plant had its share ofproblems, and Cliffs looked to a different process.

Cliffs worked with Allis-Chalmers Manufacturing Co. ofMilwaukee to adapt the Grate-Kiln process, used in making cement,to produce iron concentrates. This was a first in the world-wideiron ore technology. This process was first applied at the HuiuboltMine with two identical "lines" in 1960. A second plant wasstarted at the Republic Mine in 1962.

The Empire Mine, south of Negaunee and west of Palmer, was a- magnetite rich iron-formation. Test work included the developmentof autogenous grinding (self grinding) mills and a combinationmagnetic separation and gravity methods. Later, flotation wasadded as the final step. The concentrates were pelletized by theACL Grate—Kiln system. The first concentrates and pellets wereproduced in December, 1963.

The Pioneer Pellet Plant was started in 1964 to pelletizeMather Mine underground ores. This improvement in product,appealed to the steel plants and resulted in keeping this lastunderground mine operating until 1979.

The Tilden Mine had been an open pit operation producing aspecial grade of iron—formation called siliceous ore for use in theopen—hearth furnace. Years had been spent on metallurgicalresearch. But a "break through" occurred in April, 1967 from ajoint research—development venture of Cliffs and the U.S. Bureau ofMines at Minneapolis. The mode of concentration developed was"selective flocculation flotation" to recover the non—magnetichematite and goethite. The plant and pit were built in 1972-1974.The first pellets were produced in December, 1974.

The initial pit and plant at the Tilden Mine was sized at 4.0million long tons of pellets per year, and was known as Tilden I.Provision had been made in the design for doubling the capacity to8.0 million tons per year, or even to 12.0 million tons per year.The decision was made to build Tilden II and it commencedoperations in 1979.

The Empire Mine had had a similar growth. Empire I started in1963, Empire II in 1966, Empire III in 1974, and Empire IV in 1980.This brought the annual capacity of each plant to 8.0 million tonsper year. The Republic Mine pit and plant have been on a "standby"basis since 1983.

Pellet shipments for 1987 were:Empire Mine 7,417,703 long tonsTi1n Mine 4,458,620 long tons

B— 14

The jaspilite, or "jasperN as Cliffs called it, was being investigated at the Humboldt and Republic Mines. The mode of concentration was to float the specular hematite and magnetite in fatty acids. The Humboldt plant was built in 1952 as a joint venture of Ford Motor Co. and Cliffs, the same year the Ohio Mine opened. The first concentrates were produced in 1954, but no form of agglomeration was used.

Work had gone ahead at the Republic Mine and its first concentrates were produced in 1956, using the same flow sheet as the Humboldt Mine. The Republic Mine concentrates were shipped, starting also in 1956, to Eagle Mills, east of Negaunee, to a Straight Grate Plant to be pelletized. This plant had its share of problems, and Cliffs looked to a different process.

Cliffs worked with Allis-Chalmers Manufacturing Co. of Milwaukee to adapt the Grate-Kiln process, used in making cement, to produce iron concentrates. This was a first in the world-wide iron ore technology. This process was first applied at the Humbolt Mine with two identical "linesw in 1960. A second plant was started at the Republic Mine in 1962.

The Empire Mine, south of Negaunee and west of Palmer, was a magnetite rich iron-formation. Test work included the development of autogenous grinding (self grinding) mills and a combination magnetic separation and gravity methods. Later, flotation was added as the final step. The concentrates were pelletized by the ACL Grate-Kiln system. The first concentrates and pellets were produced in December, 1963.

The Pioneer Pellet Plant was started in 1964 to pelletize Mather Mine underground ores. This improvement in product, appealed to the steel plants and resulted in keeping this last underground mine operating until 1979.

The Tilden Mine had been an open pit operation producing a special grade of iron-formation called siliceous ore for use in the open-hearth furnace. Years had been spent on metallurgical research. But a "break through" occurred in April, 1967 from a joint research-development venture of Cliffs and the U.S. Bureau of Mines at Minneapolis. The mode of concentration developed was "selective flocculation flotationH to recover the non-magnetic hematite and goethite. The plant and pit were built in 1972-1974. The first pellets were produced in December, 1974.

The initial pit and plant at the Tilden Mine was sized at 4.0 million long tons of pellets per year, and was known as Tilden I. Provision had been made in the design for doubling the capacity to 8.0 million tons per year, or even to 12.0 million tons per year. The decision was made to build Tilden I1 and it commenced operations in 1979.

The Empire Mine had had a similar growth. Empire I started in 1963, Empire I1 in 1966, Empire I11 in 1974, and Empire IV in 1980. This brought the annual capacity of each plant to 8.0 million tons per year. The Republic Mine pit and plant have been on a ftstandbytf basis since 1983.

Pellet shipments for 1987 were: Empire Mine 7,417,703 long tons Tilc'en Mine 4,458,620 long tons

In 1987 the Tilden Mine faced economic necessities and qualityproduct review, and concluded that a new open pit source ofmagnetic ore needed to be developed to replace the present open pitmining of hematite and goethite material. A source of magneticmaterial was known on the Cliffs Drive, and necessary permits arebeing sought.

The Gogebic Range, the last iron range found in the UpperPeninsula, was the first closed, followed by the Menominee Range,leaving today only the Marquette Range active in the UpperPeninsula. Thus, the Marquette Range is still alive andcontinuing, 144 years after its discovery.

3-15

In 1987 the Tilden Mine faced economic necessities and quality product review, and concluded that a new open pit source of magnetic ore needed to be developed to replace the present open pit mining of hematite and goethite material. A source of magnetic material was known on the Cliffs Drive, and necessary permits are being sought.

The Gogebic Range, the last iron range found in the Upper Peninsula, was the first closed, followed by the Menominee Range, leaving today only the Marquette Range active in the Upper Peninsula. Thus, the Marquette Range is still alive and continuing, 144 years after its discovery.

Institute of Lake Superior Geology

FIELD TRIP No. 2

Marquette Mineral District of Michigan withemphasis on MINING HISTORY and GEOLOGY

led by

Burton BoyumuRobert C. Reed

andWin. Kangas

May, 1988

The KEY MAP, Figure 1, provided through the courtesy ofCleveland-Cliffs, Inc.

B- 16

Institute of Lake Superior Geoloay

FIELD TRIP No. 2

Marquette Mineral District of Michigan with emphasis on MINING HISTORY and GEOLOGY

led by

Burton Boyum Robert C. Reed

and Wm. Kangas

May, 1988

The KEY MAP, Figure 1, provided through the courtesy of Cleveland-Cliffs, Inc.

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VIEW NEAR CARP RIVER, LAKE SUPERIOR.

AcJ<ermanlith.3?9.BtOadWay NY.

B-18 Figure 2

••1

VIEW NEAR CART RIVER. LAKE SUPERIOR

Figure 2

I

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/

B-19 Figure 3Figure 3

STOP 1 (Fig. 2 and 3)-- MARQUETTE HARBORThe port city of Marquette was established July 10, 1849, as

Worcester, the name of the home city of Mr. Waterman A. Fisher, atextile mill owner of Worcester, Massachusetts. Figure 2 shows ascene of the harbor in 1851, taken from Foster and Whitney (1851).Figure 3 shows the charcoal bloomery of the Marquette Iron Companyand a part of the community in 1852, as seen from Ripley's Rock.This is the oldest known photograph of Marquette, as the name waschanged in 1851.

Mr. Fisher was the principal financer of the Marquette IronCompany, which was promoted by Mr. Robert Graveraet. Although thecompany had no mines, it built a charcoal iron bloomery on theshore of Lake Superior, using ore from the Jackson Mine.Previously, in 1847, the Jackson Mining Company had constructed abuilding in this area for both a habitation and a storehouse. Theeastern backers of the Marquette Iron Company soon lost interest inthe venture and they sold out to the Cleveland Iron Mining Companyin 1853. The plant was destroyed by fire in 1853 and was notrebuilt because the Cleveland Iron Co. was putting its emphasis onshipping ore promoted by the start of construction of the SaultSte. Marie (SOO) Locks . Agitation had gone on for years to buildlocks at the SOO, because all passengers and freight had to portagearound the rapids each way. On August 26, 1852, U.S. PresidentMillard Fillmore signed a bill authorizing the construction oflocks and a canal.

The first water was let into the system on April 19, 1855, butthere were leaks to be reparied. The first. vessel to lock throughwas the steamer "Illinois", going "upbound", on June 18th. Thebrigantine "Columbia" carried the first cargo of iron ore,amounting to 132 tons, "downbound" in August, 1855. Thus,Marquette became the first iron ore port in the Great Lakes. Atotal of 1447 tons of iron ore was shipped from Marquette Harbor inthat first year, all by the Cleveland Iron Mining Company.

The Cleveland Iron Company built its first ore dock in 1855.The Lake Superior Iron Company, Samuel P. Ely, Agent, built theirore dock next to them in 1857, but used a radically differentdesign. The Lake Superior Iron Co. dock was 25 feet above thewater and had "pockets" to hold the ore and chutes to drop the oreonto the deck or into the hold of vessels. This was the first oredock with pockets built in the world!

STOPS 2 (Fig. 4)--KONA DOLOMITE and COPPER MINERALIZATIONThis is the middle member of the Chocolay Group. It is

essentially a fine to medium grained, pink to gray, crystallinesiliceous dolomite with some interlaminated argillaceous andarenaceou phases. Hematite staining locally gives the rock apinkish to reddish hue. Locally, there are large domal algalstructures in the Kona. One classic locality alongside U.S.Highway 41, in Section 6, T47N, R24W, has been visited by countlessgeologists; however, we have chosen this series of exposures toshow the algal structures.

B-20

STOP 1 (Fig. 2 and 3)-- MARQUETTE HARBOR The port city of Marquette was established July 10, 1849, as

Worcester, the name of the home city of Mr. Waterman A. Fisher, a textile mill owner of Worcester, Massachusetts. Figure 2 shows a scene of the harbor in 1851, taken from Foster and Whitney (1851). Figure 3 shows the charcoal bloomery of the Marquette Iron Company and a part of the community in 1852, as seen from Ripleyts Rock. This is the oldest known photograph of Marquette, as the name was changed in 1851.

Mr. Fisher was the principal financer of the Marquette Iron Company, which was promoted by Mr. Robert Graveraet. Although the company had no mines, it built a charcoal iron bloomery on the shore of Lake Superior, using ore from the Jackson Mine. Previously, in 1847, the Jackson Mining Company had constructed a building in this area for both a habitation and a storehouse. The eastern backers of the Marquette Iron Company soon lost interest in the venture and they sold out to the Cleveland Iron Mining Company in 1853. The plant was destroyed by fire in 1853 and was not rebuilt because the Cleveland Iron Co. was putting its emphasis on shipping ore promoted by the start of construction of the Sault Ste. Marie (SOO) Locks . Agitation had gone on for years to build locks at the SOO, because all passengers and freight had to portage around the rapids each way. On August 26, 1852, U.S. President Millard Fillmore signed a bill authorizing the construction of locks and a canal.

The first water was let into the system on April 19, 1855, but there were leaks to be reparied. The first vessel to lock through was the steamer wIllinoisn, going ttupboundtt, on June 18th. The brigantine flColumbiaH carried the first cargo of iron ore, amounting to 132 tons, ttdownboundtt in August, 1855. Thus, Marquette became the first iron ore port in the Great Lakes. A total of 1447 tons of iron ore was shipped from Marquette Harbor in that first year, all by the Cleveland Iron Mining Company.

The Cleveland Iron Company built its first ore dock in 1855. The Lake Superior Iron Company, Samuel P. Ely, Agent, built their ore dock next to them in 1857, but used a radically different design. The Lake Superior Iron Co. dock was 25 feet above the water and had "pocketsN to hold the ore and chutes to drop the ore onto the deck or into the hold of vessels. This was the first ore dock with pockets built in the world!

STOPS 2 (Fig. 4)--KONA DOLOMITE and COPPER MINERALIZATION This is the middle member of the Chocolay Group. It is

essentially a fine to medium grained, pink to gray, crystalline siliceous dolomite with some interlaminated argillaceous and arenaceoua phases. Hematite staining locally gives the rock a pinkish to reddish hue. Locally, there are large domal algal structures in the Kona. One classic locality alongside U.S. Highway 41, in Section 6, T47N, R24W, has been visited by countless geologists; however, we have chosen this series of exposures to show the algal structures.

JS • JACOBSVILLE SANDSTONE_UflCOflfOrmity —

SS • SIAMO SLATEAQ • AJI8IK QUARTZITEWS • WEWE SCHISTKO • KONA DOLOMITEMQ MESNARD QUARTZITEEL • ENCHANTMENT LAKE FM.

_Unconformity_CCGa COMPEAU CREEK ONEISSMS • MONA SCHIST--—. STRATIGRAPHIC CONTACT—. FAULT CONTACT

FIGURE 4

GENERAL GEOLOGY OFEASTERN PORTION OF THE

MARQUETTE SYNCLINORIUM

After Gair and Thad.n, 968. Toylor p972

IMARQUETTE

SYNCLINORIUN

LokeSuperior

44_, Js —,'.-' __-__— lCD

— . -'. — — —

— j4tc :——

JS JACOBSVILLE SANDSTONE .Unconformity.

SS = SIAMO SLATE '

AQ AJIBIK QUARTZITE WS WEWE SCHIST K O = KONA DOLOMITE MQ = MESNARD QUARTZITE E L = ENCHANTMENT LAKE FM.

AJnconformity. MARQUETTE CCG= COMPEAU CREEK GNEISS

SYNCLINORIUM MS * MONA SCHIST - - STRATIGRAPHIC CONTACT - FAULT CONTACT

FIGURE 4

G E N E R A L G E O L O G Y O F E A S T E R N PORTION O F T H E

M A R Q U E T T E S Y N C L I N O R I U M

STOP 2

0 t 2 k -

After Gatr and Thadan, 1968, Taylor, 1972

Copper Mineralization: A.R. Renfro wrote in Economic Geolociy,(1974): "Recently discovered sedimentary and geochemical processesof coastal Sabkhas provide the foundation for a hypothesis thatsuccessfully explains the genesis of these (copper) deposits"."Coastal sabkhas are evaporite flats that form along the subaeriallandward margins of regressive seas. Because of their uniqueposition, coastal sab]thas are nourished by subsurface flow oflandward-migrating, low Eh-high pH sea water and by seaward-migrating, high Eli-low pH terrestrial water. Commonly they arebordered on the seaward side by intertidal mudflats nad lagoonsthat are carpeted by leather-like mats of sediment-binding, blue-green algae. Fetid ooze consisting of interbedded decaying algaeand detrital sediment occurs immediately beneath the living algalmat." "Coastal sabkhas and their related evaporite facies progradeseaward across adjacent algal-mat facies. Upon banal, the algal-mat facies becomes saturated with hydrogen sulfide generated byanaerobic bacteria. Concurrently, the trailing, landward edges ofcoastal sabkhas are buried by prograding ternigenous clastics ofthe desert. As sabkhas migrate basinward, terrestrial—formationwater eventually must pass upward through the buried, stronglyreducing algal—mat in order to reach the surface of evaporation.""Terrestrial-formation water initially has low pH and high Eh, andthus can mobilize and transport trace amounts of such elements ascopper, silver, lead, and zinc. As terrestrial—formation waterpasses through the hydrogen sulfide-charged algal-mat, its load ofsolute metals is reduced and precipitated interstitially assulf ides."

The suif ides in the Kona dolomite are bornite, chalcopyrite,and pyrite. Estimates suggest a potential of 250 million tons ofmineable material. This potential was not recognized until about1896. Early prospectors in the area included Julian Case, J.M.Longyear, James Wilkinson, and Andrew Pings. Interest in thesulfide mineralization in the Kona died out by 1898, but wasrenewed in the 1970's, with drilling and mapping by several groups.Mineralization is not readily seen in place, but diamond drill coreand hand specimens will be shown.

STOP 3 (Fig. 5)--MORGAN FURNACEAs noted in the Introduction, many of the early developers of

the Marquette Iron Range had the concept of shipping the iron asblooms rather than as ore. As is typical, commercial developmentsprang up along the railroads, as they were developed. The IronMountain Railroad was incorporated February 22, 1855 and commencedoperations in 1857. It was reincorporated February 6, 1857, as theMarquette and Ontonogan Railroad, and later as the Marquette,Houghton and Ontonogan Railroad (M.H.& 0.), followed by the Duluth,South Shore and Atlantic Railroad (D.S.S.& A.); it is now known asthe Soo Line Railroad.

A Mr. Schweitzer started a grist mill along the line someeight miles west of Marquette in a favorable location adjacent tothe Little Carp River (later renamed Morgan Creek). Mr. CorneliusDonkersley, active in the railroad, felt this was a good site for acharcoal furnace. The Ely family of Rochester, New York, wealthyflour milling people, had taken an interest in the Marquette Range

B- 22

Copper Mineralization: A.R. Renfro wrote in Economic Geoloqv, (1974): "Recently discovered sedimentary and geochemical processes of coastal Sabkhas provide the foundation for a hypothesis that successfully explains the genesis of these (copper) depositstt. "Coastal sabkhas are evaporite flats that form along the subaerial landward margins of regressive seas. Because of their unique position, coastal sabkhas are nourished by subsurface flow of landward-migrating, low Fh-high pH sea water and by seaward- migrating, high Eh-low pH terrestrial water. Commonly they are bordered on the seaward side by intertidal mudflats nad lagoons that are carpeted by leather-like mats of sediment-binding, blue- green algae. Fetid ooze consisting of interbedded decaying algae and detrital sediment occurs immediately beneath the living algal mat." "Coastal sabkhas and their related evaporite facies prograde seaward across adjacent algal-mat facies. Upon barial, the algal- mat facies becomes saturated with hydrogen sulfide generated by anaerobic bacteria. Concurrently, the trailing, landward edges of coastal sabkhas are buried by prograding terrigenous elastics of the desert. As sabkhas migrate basinward, terrestrial-formation water eventually must pass upward through the buried, strongly reducing algal-mat in order to reach the surface of evaporation.If wTerrestrial-formation water initially has low pH and high Eh, and thus can mobilize and transport trace amounts of such elements as copper, silver, lead, and zinc. As terrestrial-formation water passes through the hydrogen sulfide-charged algal-mat, its load of solute metals is reduced and precipitated interstitially as sulfides."

The sulfides in the Kona dolomite are bornite, chalcopyrite, and pyrite. Estimates suggest a potential of 250 million tons of mineable material. This potential was not recognized until about 1896. Early prospectors in the area included Julian Case, J.M. Longyear, James Wilkinson, and Andrew Pings. Interest in the sulfide mineralization in the Kona died out by 1898, but was renewed in the 1970fs, with drilling and mapping by several groups. Mineralization is not readily seen in place, but diamond drill core and hand specimens will be shown.

STOP 3 (Fig. 5) --MORGAN FURNACE As noted in the Introduction, many of the early developers of

the Marquette Iron Range had the concept of shipping the iron as blooms rather than as ore. As is typical, commercial development sprang up along the railroads, as they were developed. The Iron Mountain Railroad was incorporated February 22, 1855 and commenced operations in 1857. It was reincorporated February 6, 1857, as the Marquette and Ontonogan Railroad, and later as the Marquette, Houghton and Ontonogan Railroad (M.H.& O.), followed by the Duluth, South Shore and Atlantic Railroad (D.S.S.& A.); it is now known as the Soo Line Railroad.

A Mr. Schweitzer started a grist mill along the line some eight miles west of Marquette in a favorable location adjacent to the Little Carp River (later renamed Morgan Creek). Mr. Cornelius Donkersley, active in the railroad, felt this was a good site for a charcoal furnace. The Ely family of Rochester, New York, wealthy flour milling people, had taken an interest in the Marquette Range

Figurn 5

B-23 Figure 5

Morgan Furnace. Built in 1$83.Morgan Furnace. Built in 1S63.

Figure 5

in the early 1850's. They encouraged Donkersley and financed thedevelopment. The good friend and legal counsel for the family wasLewis Henry Morgan. From the beginning, Morgan had advised them,and he also invested in the development. Starting in 1856,Donkersley had cleared the land and built houses for his workers.The Morgan Iron Company was incorporated July 1, 1863, and had aninitial capitalization of $50,000, made up of 2,000 shares at $25each. The Ely family joined Morgan in this venture. Annualproduction for the company was (in tons):

1863 337 1868 4023 1873 63241864 4023 1869—out of blast 1874 59731865 3489 1870 5952 1875 53771866 3749 1871 4755 1876 32781867 5057 1872 4356 1877 663Total production of 57,573 tonsLewis H. Morgan is noteworthy not only for his role with the

railroads and with the furnace company, but also as a naturalist.He spent a number of summers on the Marquette Range observing thebeaver. His treatise, "The American Beaver", was remarkable, andestablished him as an authority whose works are still respected.

STOP 4--MICHIGAN IRON INDUSTRY MUSEUMThis is the newest of the State of Michigan historical museums

and is the outgrowth of a community effort dating back to 1973. AMichigan non-profit corporation, the Carp River Forge Bi-CentennialPark, organized an effort to honor the Carp River Forge. It raisedmoney and purchased land for this purpose. In time, it joinedforces with the Bureau of History, Michigan Department of State,and had the cooperation of The Cleveland-Cliffs Iron Company. Theresult became this Museum. Ground breaking was in July 1986 anddedication of the museum took place on May 16, 1987.A. Tour of Museum and Rest Stop.B. Convene in Frank G. Matthews, Sr. Memorial Museum

Remarks by Tom Friggens, Director, followed by a briefaudial—visual program.

C. Solar compass demonstration (rain or shine).The Historical Introduction has already commented on William

Austin Burt and his solar compass. Attached is Historical Notes-Number One, which provides more detail. Also demonstrated is a sundial compass, probably more used by field geologists than the SolarCompass.

STOP 5 (Fig. 6)--NEGAtJNEE HIGHWAY CUTThis series of road cut exposures along U.S. Highway 41

contains, over a short distance, the Mesnard Quartzite, basal tothe Chocolay Group; the Ajibik Quartzite, basal to the MenomineeGroup; and the Siamo Formation, the middle member of the MenomineeGroup. To the northwest, along the north side of the hill is athin (one meter) basal conglomerate. The Kona Dolomite pinches outjust to the east of these exposures.

The Mesnard Formation is a thick, vitreous quartzite,generally light gray, but sometimes with shades of pink. Numerousveins of quartz and hematite cut the bedding, and ripple marks andcrossbedding are also present. This is the only member of the

D -

in the early 1850's. They encouraged Donkersley and financed the development. The good friend and legal counsel for the family was Lewis Henry Morgan. From the beginning, Morgan had advised them, and he also invested in the development. Starting in 1856, Donkersley had cleared the land and built houses for his workers. The Morgan Iron Company was incorporated July 1, 1863, and had an initial capitalization of $50,000, made up of 2,000 shares at $25 each. The Ely family joined Morgan in this venture. Annual production for the company was (in tons):

1863 337 1868 4023 1873 6324 1864 4023 1869-out of blast 1874 5973 1865 3489 1870 5952 1875 5377 1866 3749 1871 4755 1876 3278 1867 5057 1872 4356 1877 663 Total production of 57,573 tons Lewis H. Morgan is noteworthy not only for his role with the

railroads and with the furnace company, but also as a naturalist. He spent a number of summers on the Marquette Range observing the beaver. His treatise, "The American Beaveru, was remarkable, and established him as an authority whose works are still respected.

STOP 4--MICHIGAN IRON INDUSTRY MUSEUM This is the newest of the State of Michigan historical museums

and is the outgrowth of a community effort dating back to 1973. A Michigan non-profit corporation, the Carp River Forge Bi-Centennial Park, organized an effort to honor the Carp River Forge. It raised money and purchased land for this purpose. In time, it joined forces with the Bureau of History, Michigan Department of State, and had the cooperation of The Cleveland-Cliffs Iron Company. The result became this Museum. Ground breaking was in July 1986 and dedication of the museum took place on May 16, 1987. A. Tour of Museum and Rest Stop. B. Convene in Frank G. Matthews, Sr. Memorial Museum

Remarks by Tom Friggens, Director, followed by a brief audial-visual program.

C. Solar compass demonstration (rain or shine). The Historical Introduction has already commented on William

Austin Burt and his solar compass. Attached is Historical Notes- Number One, which provides more detail. Also demonstrated is a sun dial compass, probably more used by field geologists than the Solar Compass.

STOP 5 (Fig. 6)--NEGAUNEE HIGHWAY CUT This series of road cut exposures along U.S. Highway 41

contains, over a short distance, the Mesnard Quartzite, basal to the Chocolay Group; the Ajibik Quartzite, basal to the Menominee Group; and the Siamo Formation, the middle member of the Menominee Group. To the northwest, along the north side of the hill is a thin (one meter) basal conglomerate. The Kona Dolomite pinches out just to the east of these exposures.

The Mesnard Formation is a thick, vitreous quartzite, generally light gray, but sometimes with shades of pink. Numerous veins of quartz and hematite cut the bedding, and ripple marks and crossbedding are also present. This is the only member of the

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Chocolay Group to be found this far west. Perhaps the othermembers were eroded here before deposition of the Menominee Group,or they were not deposited this far west from their principal areaof deposition.

The Ajibik Formation is found in the next series of outcropsto the south. It is massive quartzite with minor amounts ofargillaceous and conglomeratic material. In appearance, the Ajibikresembles the Mesnard Quartzite. Present in the quartzite is adistinctive concentric staining similar to Liesegang rings. Alsopresent locally, are distinctive greenish granules.

The Siamo Formation consists of dark gray to light grayargillite, slate, graywacke and quartzite. Slaty cleavage iscommon to conspicuous. Locally, there are carbonate rich layersand beds of conglomerate. Also present are clastic dikes composedof material similar to the graywacke beds. The contact of theSiamo Formation with the Negaunee Iron Formation is not exposedhere, but it lies just south of the Negaunee Branch Bank.

Also of interest is the Miners' Park with the pyramidalmonument located on the west side of U.S. Highway 41. The monumentwas erected originally by the Jackson Iron Mining Company in 1905before the company was sold to, and assimilated into, theCleveland—Cliffs Iron Company. Because the monument was erectedfirst in "Cornish Town", now an area of potential subsidence fromthe deep underground mining of the Mather Mine "B" Shaft, it wastaken apart, each stone marked, moved, and reassembled on thepresent site by Cleveland—Cliffs, at its expense.

STOP 6--BURT SURVEY MARKER (NE CORNER, SECT.1, T47N, R27W)This brief stop is adjacent to the Don Tresedder Insurance

Agency, 109 U.S. 41 W. at the northeast corner of the section whereWilliam Austin Burt commenced his survey of the Town Line betweenRanges 26 and 27. On the morning of September 19, 1844, he and hisparty went south and discovered the first iron ore of the MarquetteRange. The marker was erected by the Marquette County HistoricalSociety in 1987 as part of Michigan's Sesquicentennial.

STOP 7 (Fig. 7)--RAILWAY STREET UNOXIDIZED IRON-FORMATIONThis series of railroad cuts is ideal to examine the

unoxidized, primary, cherty, iron carbonate iron—formation.Structurally, it is located in the faulted section that passedthrough the Negaunee Mine Number Three Shaft. It is on the northdipping limb of the Athens Enginehouse Anticline.

Although this section is unoxidized except for recentalteration, it lies just above a high grade iron deposit. Severalthin sill—like intrusives can be seen in this exposure.

Worthy of note is the westward plunging zone of gypsum; thegypsum was formed after the iron ore. Figure 7 shows the structureof the zone in plan, together with an east-west longitudinalsection, looking north. The ore-enrichment extended down thesyncline from the Regent Group, through the Tracy Mine, the LuckyStar and Athens Mines, to the Bunker Hill Mine. The Lucky Star"Orebody" and the first three levels of the Athens Mine were nevermined because of their high sulfur (gypsum) content. Theenrichment under the railway street was also high in sulfur. This

B-26

Chocolay Group to be found this far west. Perhaps the other members were eroded here before deposition of the Menominee Group, or they were not deposited this far west from their principal area of deposition.

The Ajibik Formation is found in the next series of outcrops to the south. It is massive quartzite with minor amounts of argillaceous and conglomeratic material. In appearance, the Ajibik resembles the Mesnard Quartzite. Present in the quartzite is a distinctive concentric staining similar to Liesegang rings. Also present locally, are distinctive greenish granules.

The Siamo Formation consists of dark gray to light gray argillite, slate, graywacke and quartzite. Slaty cleavage is common to conspicuous. Locally, there are carbonate rich layers and beds of conglomerate. Also present are clastic dikes composed of material similar to the graywacke beds. The contact of the Siamo Formation with the Negaunee Iron Formation is not exposed here, but it lies just south of the Negaunee Branch Bank.

Also of interest is the Miners1 Park with the pyramidal monument located on the west side of U.S. Highway 41. The monument was erected originally by the Jackson Iron Mining Company in 1905 before the company was sold to, and assimilated into, the Cleveland-Cliffs Iron Company. Because the monument was erected first in "Cornish Townw, now an area of potential subsidence from the deep underground mining of the Mather Mine l@Btt Shaft, it was taken apart, each stone marked, moved, and reassembled on the present site by Cleveland-Cliffs, at its expense.

STOP 6--BURT SURVEY MARKER (NE CORNER, SECT.1, T47N, R27W) This brief stop is adjacent to the Don Tresedder Insurance

Agency, 109 U.S. 41 W. at the northeast corner of the section where William Austin Burt commenced his survey of the Town Line between Ranges 26 and 27. On the morning of September 19, 1844, he and his party went south and discovered the first iron ore of the Marquette Range. The marker was erected by the Marquette County Historical Society in 1987 as part of Michigan's Sesquicentennial.

STOP 7 (Fig. 7)-RAILWAY STREET UNOXIDIZED IRON-FORMATION This series of railroad cuts is ideal to examine the

unoxidized, primary, cherty, iron carbonate iron-formation. Structurally, it is located in the faulted section that passed through the Negaunee Mine Number Three Shaft. It is on the north dipping limb of the Athens Enginehouse Anticline.

Although this section is unoxidized except for recent alteration, it lies just above a high grade iron deposit. Several thin sill-like intrusives can be seen in this exposure.

Worthy of note is the westward plunging zone of gypsum; the gypsum was formed after the iron ore. Figure 7 shows the structure of the zone in plan, together with an east-west longitudinal section, looking north. The ore-enrichment extended down the syncline from the Regent Group, through the Tracy Mine, the Lucky Star and Athens Mines, to the Bunker Hill Mine. The Lucky Star ItOrebodytt and the first three levels of the Athens Mine were never mined because of their high sulfur (gypsum) content. The enrichment under the railway street was also high in sulfur. This

FIGURE 7PLAN MAP AND CROSS SECTION - STOP 7

PLAN MAP

LONGITUDINAL CROSS SECTION - LOOKING NORTH

Athens Mine Lucky Star Mine

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B-27Figure 7

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PLAN MAP AND CROSS SECTION - STOP 7

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LONGITUDINAL CROSS SECTION - LOOKING NORTH

Athens Mine Lucky Star Mine

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condition existed also in the upper levels of the Mather Mine. Thepost-ore interstitial fillings of gypsum, calcite, and sometimesdickite and(or) halides have been found on the Gogebic Range, atthe Zenith Mine in the Ely trough at Ely, Minnesota, and at theSteep Rock Mine at Atikokan, Ontario. The latter mine also hadsome arsenic minerals.

STOP 8--JACKSON GROVE: PASTY LUNCH AND REST STOPThe Pasty, as a meal, is a Cornish traditional miner's lunch.

It was developed as a "package" that the miner could take under-ground and even reheat with a candle under his shovel! Itcontinues to be a favorite around the world wherever the Cornishmenhave gone.

STOP 8--SOUTH JACKSON MINE: MAGANESE MINERALSThe South Jackson Mine Pit is in a completely different

geologic setting from that of the North Jackson Mine. The lattermine was in high grade hard ore. The South Jackson Mine was inintermediate (iron content) grade, soft ore. This ore, like thatof the Lucy Mine to the east, was high in manganese, principally asmanganite and pyrolusite. The ores from these mines were the onlyones on the Marquette Range to be high in manganese . There wereexotic manganese minerals found at the Champion Mine, such asmanganiferous garnets, but The Champion Mine was in a differentgeologic setting, both structually and in terms of metamorphicgrade.

STOP 10--JASPER KNOB. ISHPEMINGThis is a wonderful exposure of Jaspilite (the miners' hard

ore jasper). Note the complex folding in the jaspilite and theirregular replacement by the hematite. To the south and west werelenses of high grade hard ore.

To the north are the hard ore open pits of the Incline,Sawmill, Schoolhouse and Little Mountain Mines. The first mininghere was in 1848 by the Cleveland Iron Mining Company. The hardore, which continued along the pitch of the synclinal axis thatpasses under the City of Ishpeming, was followed underground in theCliffs Shaft Mine.

The iron—formation characteristically has alternating thinbeds and lenses of steel-gray, dark-blue-gray, dark-gray, or blackspecular hematite and bright-red hematitic, fine—grained, chertyquartzite. Beds commonly range in thickness from a fraction of amillimeter to about 2 centimeters, and fine internal laminationscan be seen in the thicker layers. Specularite plates are stronglyoriented parallel to bedding in the hematitic layers. Excellentcolored illustrations of this have been published in U.S.Geological Survey Monograph 28.

Folds having drag folds on their limbs and themselves beingdrag folds on the limb3 of a still larger fold are strikingly shownin most exposures. Fold axes are horizontal to gently plunging.In cross section the south limbs of individual anticlinal dragfolds commonly are longer than the north limbs, indicating that themajor synclinal axis passes south of Jasper Knob.

B-28

condition existed also in the upper levels of the Mather Mine. The post-ore interstitial fillings of gypsumf calcitef and sometimes dickite and(or) halides have been found on the Gogebic Rangef at the Zenith Mine in the Ely trough at Elyf Minnesotaf and at the Steep Rock Mine at Atikokanf Ontario. The latter mine also had some arsenic minerals.

STOP 8--JAC?<SON GROVE: PASTY LUNCH AND REST STOP The Pastyf as a mealf is a Cornish traditional minerfs lunch.

It was developed as a ffpackageff that the miner could take underground and even reheat with a candle under his shovel! It continues to be a favorite around the world wherever the Cornishmen have gone.

STOP 8--SOUTH JACKSON MINE: MAGANESE MINERALS The South Jackson Mine Pit is in a completely different

geologic setting from that of the North Jackson Mine. The latter mine was in high grade hard ore. The South Jackson Mine was in intermediate (iron content) gradef soft ore. This oref like that of the Lucy Mine to the eastf was high in manganesef principally as manganite and pyrolusite. The ores from these mines were the only ones on the Marquette Range to be high in manganese . There were exotic manganese minerals found at the Champion Minef such as manganiferous garnetsf but The Champion Mine was in a different geologic settingf both structually and in terms of metamorphic grade. I

STOP 10--JASPER KNOB, ISHPEMING This is a wonderful exposure of Jas~ilite (the minersf hard

ore jasper). Note the complex folding in the jaspilite and the irregular replacement by the hematite. To the south and west were lenses of high grade hard ore.

To the north are the hard ore open pits of the Inclinef Sawmillf Schoolhouse and Little Mountain Mines. The first mining here was in 1848 by the Cleveland Iron Mining Company. The hard oref which continued along the pitch of the synclinal axis that passes under the City of Ishpemingf was followed underground in the - Cliffs Shaft Mine.

The iron-formation characteristically has alternating thin - -

beds and lenses of steel-grayf dark-blue-grayf dark-grayf -

or black specular hematite and bright-red hematiticf fine-grainedf cherty quartzite. Beds commonly range in thickness from a fraction of a millimeter to about 2 centimeters, and fine internal laminations can be seen in the thicker layers. Specularite plates are strongly oriented parallel to bedding in the hematitic layers. Excellent colored illustrations of this have been published in U.S. Geological Survey Monograph 28.

Folds having drag folds on their limbs and themselves being drag folds on the limbs of a still larger fold are strikingly shown in most exposures. Fold axes are horizontal to gently plunging. In cross section the south limbs of individual anticlinal drag folds commonly are longer than the north limbsf indicating that the major synclinal axis passes south of Jasper Knob.

Small breccia zones occur in the iron—formation at thislocality. Along them, the comparatively brittle reddish cherty(jasper) layers , in particular, have been fragmented. Fractureshave been healed mainly by crystalline hematite. Some of thebreccia zones may correspond in orientation to axial planecleavage.

Two principal theories have been advanced for the origin ofthe jaspilitic iron—formation at Jasper Knob. One theory advocatesoxidation of a siderite-chert iron-formation during the post-Negaunee erosional interval and its recrystallization tospecularite—quartz during the Early Precambrian Penokean orogenyand metamorphism. This theory would account for the patches ofjaspilite in carbonate—facies iron—formation which are present nearcontacts between the two rocks as, for example, in the Cliffs ShaftMine about 1/2 mile northwest of here. The second theory advocatesa primary origin for the iron oxide, either as depositionalhematite or as magnetite derived by diagenetic modification ofdepositional oxides. The rock here resembles megascopically,particularly in the very even bedding, magnetite-rich facies ofiron—formation rather than hematitic facies, that ischaracteristically irregular bedded and oolitic.

STOP 11--CLIFFS SHAFT MINE, ISHPEMINGThe distinctive obelisk style headframes of "A" and "B" Shafts

were built in 1919, replacing the original wooden headframeserected when the shafts were sunk in 1873. The shaft sections showthat both shafts cut "oar". Their ultimate depth was 1250 feetfrom surface. "C" shaft was sunk in 1954 to replace the twosmaller shafts for more efficient hoisting of the ore. The Koepehoisting system used in this shaft was the first such installationin the Western Hemisphere. It features an "endless rope" systemwith the hoist in the top of the headframe.

The Cliffs Shaft Mine, closed in December, 1967, had thelongest production life of any mine in the Lake Superior region(1848—1967). It produced 28,960,406 long tons of high grade hardore. Its greatest year of production was 1942 when 747,564 tons ofore were shipped. The "room and pillar" workings present under thetown of Ishpeming, extend to 1250 feet below the collar of theshafts in the deepest (15th) level.

STOP 12--SOUTH PINE STREET, ISHPEMINGAs noted in the Introduction, there are two structural

locations for the soft iron ore. The ores in Negaunee and alongthe north rim in Ishpeming and the North Lake District are all onthe footwall of the Negaunee Iron Formation. The second locationis in fault structures on the big intrusive sills. The mines herewere the Lake, Lake Angeline, Section 16, Holmes, and SalisburyMines. Lake Angeline is a natural expression of the basin in whichthe Lake and Lake Angeline deposits were found. The lake waspumped out in 1890 to permit mining the soft ore on the lakebottom. These ores were very high grade, and had extensivebotryoidal and mammilary hematite and goethite with numerous vugs.The Section 16 Mine, at the west end of Lake Angeline, had both

B- 29

Small breccia zones occur in the iron-formation at this locality. Along themt the comparatively brittle reddish cherty (jasper) layers in particulart have been fragmented. Fractures have been healed mainly by crystalline hematite. Some of the breccia zones may correspond in orientation to axial plane cleavage.

Two principal theories have been advanced for the origin of the jaspilitic iron-formation at Jasper Knob. One theory advocates oxidation of a siderite-chert iron-formation during the post- Negaunee erosional interval and its recrystallization to specularite-quartz during the Early Precambrian Penokean orogeny and metamorphism. This theory would account for the patches of jaspilite in carbonate-facies iron-formation which are present near contacts between the two rocks ast for examplet in the Cliffs Shaft Mine about 112 mile northwest of here. The second theory advocates a primary origin for the iron oxidef either as depositional hematite or as magnetite derived by diagenetic modification of depositional oxides. The rock here resembles megas~opically~ particularly in the very even bedding! magnetite-rich facies of iron-formation rather than hematitic fadesf that is characteristically irregular bedded and oolitic.

STOP 11--CLIFFS SHAFT MINE, ISHPEMING The distinctive obelisk style headframes of ffAff and ftBft Shafts

were built in 191gf replacing the original wooden headframes erected when the shafts were sunk in 1873. The shaft sections show that both shafts cut tgoarff. Their ultimate depth was 1250 feet from surface. ftCtf shaft was sunk in 1954 to replace the two smaller shafts for more efficient hoisting of the ore. The Koepe hoisting system used in this shaft was the first such installation in the Western Hemisphere. It features an Ifendless ropetf system with the hoist in the top of the headframe.

The Cliffs Shaft Minet closed in Decembert 1967# had the longest production life of any mine in the Lake Superior region (1848-1967). It produced 28,96Of4O6 long tons of high grade hard ore. Its greatest year of production was 1942 when 747!564 tons of ore were shipped. The ffroom and pillartf workings present under the town of Ishpemingt extend to 1250 feet below the collar of the shafts in the deepest (15th) level.

STOP 12--SOUTH PINE STREET, ISHPEMING As noted in the Introductionf there are two structural

locations for the soft iron ore. The ores in Negaunee and along the north rim in Ishpeming and the North Lake District are all on the footwall of the Negaunee Iron Formation. The second location is in fault structures on the big intrusive sills. The mines here were the Laket Lake Angelinet Section lGf Holmest and Salisbury Mines. Lake Angeline is a natural expression of the basin in which the Lake and Lake Angeline deposits were found. The lake was pumped out in 1890 to permit mining the soft ore on the lake bottom. These ores were very high gradet and had extensive botryoidal and mammilary hematite and goethite with numerous vugs. The Section 16 Minet at the west end of Lake Angelinel had both

hard ore (above) and soft ore (below) in direct contact with oneanother.

These mines were the first on the Marquette Range to useelectricity for electric for underground locomotives, lights,slushers, and pumps. It was here also that Cliffs voluntarilyintroduced the eight hour work shift in 1892.

STOP 13--HUMBOLDT MINE: EDISON HISTORICAL MARKERThis stop is at the historical marker erected by the Marquette

County Historical Society in 1987 for the Sesquicentennial ofMichigan.

This series of hard ore mines operated from 1865 to 1892 andfrom 1908 to 1917, producing 1,368,546 tons of hard ore from bothopen pit and underground operations. The Humboldt Mine wasMichigan's first taconite type open pit, concentrator and pelletplant operation. It was in production from 1954 to 1970, shipping9,433,305 long tons of concentrates and pellets.

STOP 12 (Fig. 8)--REPUBLIC MINE OPEN PITThe Republic Mine was opened in 1870 as a series of hard ore

open pits, but followed the near vertical dipping beds of theNegaunee Iron Formation into underground operations. The Pascoeshaft was an inclined shaft some 4,000 feet deep (2,910 feetvertically) with high grade, hard hematite and magnetite ore tothat depth, in a host of jaspilite. From 1872 to 1926, theRepublic Mine shipped 8,563,170 long tons of hard ore. It began asa taconite type operation in 1956, but closed in 1983 and is on"stand—by" basis.

The Republic pit is distinctive for its "high wall", thecontact of the upper portion of the iron-formation with themetadiabase sill. The contact of the Negaunee Iron Formation andthe overlying Goodrich Quartzite was marked by an extensiveconglomerate made up of jaspilite. This conglomerate was higher intitanium than the underlying Negaunee Iron Formation.

As the open pit was developed, mining the iron—formation fromthe metadiabase footwall to the conglomerate and Goodrich Quartzitehangingwall, numerous underground workings were encountered. Theseopenings were filled with broken crude ore so the drills, trucks,and shovels could work with safety.

STOP 15--HIGHWAY M-95 ROAD CUTS: MICHIGAMME FORMATIONTightly folded schist of the lower slate member of the

Michigamme Formation is exposed in a roadcut on the east side ofhighway M—95 near the axis of the Republic syncline. The rock hereis iron—rich metasediment, now consisting of biotite—garnet-amphibole schist that contains a few inch-thick layers of impurequartzite. Although the rocks are in the sillimanite zone ofmetamorphism, sillimanite is not present here because of the lackof appropriate aluminous source materials. Minor folds, havingamplitudes much greater than wavelengths and greatly attenuatedlimbs, are common and reflect the geometry of the major synclinewhose preserved keel is deeper than it is wide at this point andbecomes deeper to the northwest. Folding is markedly noncylind—rical at outcrop scale; domains of homogeneous strain are commonly

B- 30

hard ore (above) and soft ore (below) in direct contact with one another.

These mines were the first on the Marquette Range to use electricity for electric for underground locomotivest lightst slusherst and pumps. It was here also that Cliffs voluntarily introduced the eight hour work shift in 1892.

STOP 13--HUMBOLDT MINE: EDISON HISTORICAL MARKER This stop is at the historical marker erected by the Marquette

County Historical Society in 1987 for the Sesquicentennial of Michigan.

This series of hard ore mines operated from 1865 to 1892 and from 1908 to 1917t producing 1t368t546 tons of hard ore from both open pit and underground operations. The Humboldt Mine was Michiganls first taconite type open pitt concentrator and pellet plant operation. It was in production from 1954 to 1970t shipping 9t433t305 long tons of concentrates and pellets.

STOP 12 (Fig. 8)--REPUBLIC MINE OPEN PIT The Republic Mine was opened in 1870 as a series of hard ore

open pitst but followed the near vertical dipping beds of the Negaunee Iron Formation into underground operations. The Pascoe shaft was an inclined shaft some 4t000 feet deep (2t910 feet vertically) with high gradet hard hematite and magnetite ore to that deptht in a host of jaspilite. From 1872 to 1926t the Republic Mine shipped 8t563t170 long tons of hard ore. It began as a taconite type operation in 1956t but closed in 1983 and is on llstand-byll basis.

The Republic pit is distinctive for its *'high wallllt the contact of the upper portion of the iron-formation with the metadiabase sill. The contact of the Negaunee Iron Formation and the overlying Goodrich Quartzite was marked by an extensive conglomerate made up of jaspilite. This conglomerate was higher in titanium than the underlying Negaunee Iron Formation.

As the open pit was developedt mining the iron-formation from the metadiabase footwall to the conglomerate and Goodrich Quartzite hangingwall, numerous underground workings were encountered. These openings were filled with broken crude ore so the drillst trucks, and shovels could work with safety.

STOP 15--HIGHWAY M-95 ROAD CUTS: MICHIGAMME FORMATION Tightly folded schist of the lower slate member of the

Michigame Formation is exposed in a roadcut on the east side of highway M-95 near the axis of the Republic syncline. The rock here is iron-rich metasedimentt now consisting of biotite-garnet- amphibole schist that contains a few inch-thick layers of impure quartzite. Although the rocks are in the sillimanite zone of metamorphismt sillimanite is not present here because of the lack of appropriate aluminous source materials. Minor foldst having amplitudes much greater than wavelengths and greatly attenuated limbst are common and reflect the geometry of the major syncline whose preserved keel is deeper than it is wide at this point and becomes deeper to the northwest. Folding is markedly noncylindrical at outcrop scale; domains of homogeneous strain are commonly

s.

FIGURE 8

GEOLOGY OF THE REPUBLIC MINE AREA000

36 3t

/

./ / - _sFiqure g

FIGURE 8

GEOLOGY OF THE REPUBLIC MINE AREA .-a 530

,000 ,

B-31 E'iqure 8 /

only a few square feet. Most minor folds plunge northwest inaccordance with the regional synclinal axis, but plunges vary fromabout 15° SE to 600 tq

Near the north end of the outcrop, a thin (1 to 3 inches)quartzite bed is repeated many times by ninor folds. Axes ofadjacent folds only a few inches apart have plunges that diverge byas much as 600. This noncylindrical geometry may be a result of anearlier fold set in the sedimentary rocks that formed prior to, andwas strongly overprinted by, the folding which formed the Republicsync 1 me.

STOP 16 (Fig. 9)--GREENWOOD RESERVOIRThe Greenwood Reservoir is a prominent new land feature

created in 1973 by the Tilden Mining Company to provide a constantsource of water for the concentrating plant requirements. Thewater surface approximates 1,400 acres. The Reservoir area is opento the public for fishing, boating, hiking, and other recreationalactivities. The agreement with the Michigan Department of NaturalResources provides that, at all times, the discharge downstream tothe Middle Branch of the Escanaba River will be at least threetimes the minimum low flow observed by stream gauging over manyyears. An interesting feature of the system is the multiportdischarge system in which any combination of four verticallyarranged ports may be utilized to mix waters of varyingtemperatures and oxygen content.

The two principal granitic units forming the Archean SouthernComplex will be seen here. Near point BA (Fig.9) Compeau CreekGneiss is exposed in a large roadcut. The Compeau Creek Gneiss isgenerally medium- to coarse-grained and mostly granodiorite ortonalite. It is characteristically both compositionally andstructurally heterogeneous at outcrop scale. Here, it varies frommassive rocks with faint banding or schleiren expressed byvariations in percentage of mafic minerals, mostly biotite, torather well foliated gneiss. Clots of biotite are common and inplaces are aligned in the foliation. Relict garnet cores can befound in some clots. Toward the northeast end of the road cut, theCompeau Creek Gneiss is cut by a nearly vertical dike of EarlyProterozoic metadiabase.

This ends our trip. We hope that you have found this "sampler" ofthe Marquette Mineral District of interest.

B-32

only a few square feet. Most minor folds plunge northwest in accordance with the regional synclinal axist but plunges vary from about 15O SE to 60Â NW.

Near the north end of the outcropt a thin (1 to 3 inches) quartzite bed is repeated many times by minor folds. Axes of adjacent folds only a few inches apart have plunges that diverge by as much as 60'. This noncylindrical geometry may be a result of an earlier fold set in the sedimentary rocks that formed prior tot and was strongly overprinted byt the folding which formed the Republic sync1 ine . STOP 16 (Fig. 9)--GREENWOOD RESERVOIR

The Greenwood Reservoir is a prominent new land feature created in 1973 by the Tilden Mining Company to provide a constant source of water for the concentrating plant requirements. The water surface approximates lt4O0 acres. The Reservoir area is open to the public for fishingt boatingt hikingt and other recreational activities. The agreement with the Michigan Department of Natural Resources provides thatt at all timest the discharge downstream to the Middle Branch of the Escanaba River will be at least three times the minimum low flow observed by stream gauging over many years. An interesting feature of the system is the multipart discharge system in which any combination of four vertically arranged ports may be utilized to mix waters of varying temperatures and oxygen content.

The two principal granitic units forming the Archean Southern Complex will be seen here. Near point 8A (Fig.9) Compeau Creek Gneiss is exposed in a large roadcut. The Compeau Creek Gneiss is generally medium- to coarse-grained and mostly granodiorite or tonalite. It is characteristically both compositionally and structurally heterogeneous at outcrop scale. Heret it varies from massive rocks with faint banding or schleiren expressed by variations in percentage of mafic minerals, mostly biotite, to rather well foliated gneiss. Clots of biotite are common and in places are aligned in the foliation. Relict garnet cores can be found in some clots. Toward the northeast end of the road cutt the Compeau Creek Gneiss is cut by a nearly vertical dike of Early Proterozoic metadiabase.

This ends our trip. We hope that you have found this ltsamplerll of the Marquette Mineral District of interest.

''f__5_ )

--Wbcg

Xnic

FIGURE 9A PORTION OF THE GREENWO(

7 MINUTE QUADRANGLESHOWING THE LOCATION OF

STOP 17Figure 9

559

- 30 _j54O

FIELD TRIP 3

A STRUCTURAL TRAVERSE ACROSS A PART OF TUE PENOKEAN OROGENILLUSTRATING

EARLY PROTEROZOIC OVERTHRUSTING IN NORTHERN MICHIGAN:TEXT AND FIELD GUIDE

by

JOHN S. KLASNER, Department of Geology, Western IllinoisUniversity and U. S. Geological Survey, Macornb, IL 61455

PAUL K. SIMS, U. S. Geological Survey, Box 25046, Denver FederalCenter, MS 905, Denver, CO 80225

WILLIAM J. GREGG, Department of Geology and GeologicalEngineering, Michigan Technological University, Houghton, MI49931

CHRISTINA GALLUP, Department of Geological and Planetary Science,California Institute of Technology, Pasedena, CA 91125.

C-i

FIELD TRIP 3

A STRUCTURAL TRAVERSE ACROSS A PART OF THE PENOKEAN OROGEN ILLUSTRATING

EARLY PROTEROZOIC OVERTHRUSTING IN NORTHERN MICHIGAN: TEXT AND FIELD GUIDE

JOHN S . KLASNER, Department of Geology, Western Illinois University and U. S. Geological Survey, Macomb, IL 61455

PAUL K. SIMS, U. S. Geological Survey, Box 25046, Denver Federal Center, MS 905, Denver, CO 80225

WILLIAM J. GREGG, Department of Geology and Geological Engineering, Michigan Technological University, Houghton, MI 49931

CHRISTINA GALLUP, Department of Geological and Planetary Science, California Institute of Technology, Pasedena, CA 91125.

INTRODUCTION

The Early Proterozoic Penokean orogen is one of severalmajor orogenic belts in the North American craton that developedbetween about 1.95 and 1.85 Ga (Hoffman, 1988). Rocks of theorogen are exposed in Michigan, Wisconsin, and Minnesota (Fig.

1) and extend eastward into the Lake Huron area (Sims, et al.

1981), and westward in the subsurface into Iowa and Nebraska(Sims and Peterman, 1986). The southern boundary is not exposedinasmuch as it is overlapped by Phanerozoic platform sedimentaryrocks. The Penokean orogen is intruded by 1.76 Ga anorogenicrhyolite granite which is exposed in Wisconsin.

The Penokean orogen consists of a northern assemblage ofsedimentary and bimodal volcanic rocks of the Marquette RangeSupergroup in Michigan (Cannon and Gair, 1970) and equivalants inMinnesota, the Animikie Group and Mills Lacs Group deposited on acontinental Archean margin and a southern terrane composedmainly of Early Proterozoic caic-alkaline plutonic rocks termedthe Wisconsin magrnatic terrane (Sims, 1987 and in press). Themagmatic arc terrane has not been recognized in Minnesota,although it may occur in the subsurface in southeastern Minnesota.

The Niagara fault zone is a brOadly arcurate convexnorthward systems of faults and shears, as much as 10 km wide,that separates the continental margin assemblage and Wisconsinmaginatic terrane (Fig. 2). It contains flattened , steeplydipping rocks that have prominent stretch lirieations parallel todip (Ueng and others, 1984; Sedlock and Larue, 1985; and Sims andothers, 1985), and it is interpreted by the above authors to bethe surface expression of a 1850 My old suture zone. In

northeastern Wisconsin all of the components of an ophioliteassemblage, although dismembered, occur near the suture zone(Schulz, 1987a).

Following the initial suggestion of Van Schmus (1976),several authors (Cambray, 1978; Larue and Sloss, 1980; Greenburgand Brown, 1983; Anderson and Black, 1983; Schulz, 1983, 1984;Schulz and others, 1984; LaBerge and others, 1984; Sims andPeterman , 1984; Klasner and others, 1985; Sims and others, 1985;Klasner and Attoh, 1986; Schulz, 1987a, b; and Sims and others,1987) have proposed a plate-tectonic scenario for the tectonicevolution of the Penokean foldbelt in northern Michigan andWisconsin. Although they differ in detail, these authors suggestthat evolution of the orogeri began with rifting accompanied byformation of basins and troughs (such as the Marquette andRepublic troughs) along the passive margin of the ArcheanSuperior craton in northern Michigan. This was followed bysubduction of oceanic crust to the south, with ultimatecollision of arc-related volcanic and plutonic rocks with theArchean continental margin.

As a consequence of the accretion of arc-related crust to

the Superior craton, widespread deformation of rocks of thecontinental margin as well as of the magmatic arc is to be

C— 2

INTRODUCTION

The Early Proterozoic Penokean orogen is one of several major orogenic belts in the North American craton that developed between about 1.95 and 1.85 Ga (Hoffman, 1988). Rocks of the orogen are exposed in Michigan, Wisconsin, and Minnesota (Fig. 1) and extend eastward into the Lake Huron area (Sims, et al. 1981), and westward in the subsurface into Iowa and Nebraska (Sims and Peterman, 1986). The southern boundary is not exposed inasmuch as it is overlapped by Phanerozoic platform sedimentary rocks. The Penokean orogen is intruded by 1.76 Ga anorogenic rhyolite granite which is exposed in Wisconsin.

The Penokean orogen consists of a northern assemblage of sedimentary and bimodal volcanic rocks of the Marquette Range Supergroup in Michigan (Cannon and Gair, 1970) and equivalants in Minnesota, the Animikie Group and Mills Lacs Group deposited on a continental Archean margin and a southern terrane' composed mainly of Early Proterozoic calc-alkaline plutonic rocks termed the Wisconsin magmatic terrane (Sims, 1987 and in press). The magmatic arc terrane has not been recognized in Minnesota, although it may occur in the subsurface in southeastern Minnesota.

The Niagara fault zone is a broadly arcurate convex northward systems of faults and shears, as much as 10 km wide, that separates the continental margin assemblage and Wisconsin magmatic terrane (Fig. 2). It contains flattened , steeply dipping rocks that have prominent stretch lineations parallel to dip (Ueng and others, 1984; Sedlock and Larue, 1985; and Sims and others, 1985), and it is interpreted by the above authors to be the surface expression of a 1850 My old suture zone. In northeastern Wisconsin all of the components of an ophiolite assemblage, although dismembered, occur near the suture zone (Schulz, 1987a).

Following the initial suggestion of Van Schmus (1976), several authors (Cambray, 1978; Larue and Sloss, 1980; Greenburg and Brown, 1983; Anderson and Black, 1983; Schulz, 1983, 198-2; Schulz and others, 1984; LaBerge and others, 1984; Sims and Peterman , 1984; Klasner and others, 1985; Sims and others, 1985; Klasner and Attoh, 1986; Schulz, 1987a, b; and Sims and others, 1987) have proposed a plate-tectonic scenario for the tectonic evolution of the Penokean foldbelt in northern Michigan and Wisconsin. Although they differ in detail, these authors suggest that evolution of the orogen began with rifting accompanied by formation of basins and troughs (such as the Marquette and Republic troughs) along the passive margin of the Archean Superior craton in northern Michigan. This was followed by subduction of oceanic crust to the south, with ultimate collision of arc-related volcanic and plutonic rocks with the Archean continental margin.

As a consequence of the accretion of arc-related crust to the Superior craton, widespread deformation of rocks of the continental margin as well as of the magmatic arc is to be

EXPLANATION

PHANEROZOIC (600 Ma and younger)

Sedimentary strata

MIDDLE PROTEROZOIC (900—1600 Ma)

Mafic igneous and sedimentary rocks of Midcontinent

rift system (1000—1200 Ma)

Anorogenic anorthosite and rapakivi granite (1480—1500 Ma)

EARLY PROTEROZOIC (1600—2500 Ma)

Granitoid rocks (1760—1800 Ma)

Volcanic and graaitoid rocks of Wisconsin magmacic terrane

(1815—1890 Ma)

Stratified rocks of Animikie basin; Marquette Range Super—

ground and Animikie and Mil].e Lacs Groups

ARCHEAN (2500 Ma and older)

A Greenstone—granite terrane (2600—2750 Ma)

A n Gneiss terrane (2600—3550 Ma)

A Extra—Superior gneiss and schist (2500—3000 Ma)

Lakes tectonic zone; dots, buried trace

C-4

EXPLANATION

PHANEROZOIC (600 Ma and younger)

n Sedimentary strata MIDDLE PROTEROZOIC (900-1600 Ma)

Mafic igneous and sedimentary rocks of Midcontinent

rift system (1000-1200 Ma)

Anorogenic anorthosite and rapakivi granite (1480-1500 Ma)

EARLY PROTEROZOIC (1600-2500 Ma) n

Granitoid rocks (1760-1800 Ma)

Volcanic and granitoid rocks of Wisconsin magmatic terrane

(1815-1890 Ma) -1

Stratified rocks of Animikie basin; Marquette Range Super-

ground and Animikie and Mille Lacs Groups

ARCWAN (2500 Ma and older) - Greenstone-granite terrane (2600-2750 Ma)

Gneiss terrane (2600-3550 Ma)

Extra-Superior gneiss and schist (2500-3000 ~ a )

-.--- Great Lakes tectonic zone; dots, buried trace

480

48°

440

FIGURE 1. Generalized geologic map of the Lake Superior region

showing rock units of the Periokean orogen and adjacent areas.Modified from Morey and others (1982).

C- 5

FIGURE 1. Generalized cjeologic map of the Lake S u ~ ? r j . ~ > r region showifiq rock units of the Penoksan oropn and adjacent aress. Xodified from Morey znd others (1982).

FIGURE 2. Map of the eastern part of the Lake Superior regionshowing e:tent of the Early Proterozoic contlnental-margLnassemblage and Wisconsin rnagrnatic terrane of the Pnokean oroen.Archean rocks are patternd. Stippled area snows location oftraverse in this field guidee. IM = iOfl Mountain; M =

Marquette.

C-6

expected. In this guidebook, we examine evidence from northernMichigan showing that there was extensive overthrusting on thecontinental margin during the accretionary event. We examine theevidence for overthrusting along a north-south structuraltraverse (Fig. 2) extending from Falls River southward across theMichigamrne basin, Arnasa uplift, and Feich trough-Calumet troughregion to the Niagara fault zone which lies along theMichigan/Wisconsin border.

This guidebook is mainly the outgrowth of field work andrelated synthesis done during 1986 and 1987. Critical areas forexamination were selected from several excellent publishedgeological maps and papers in the south (principally James andothers, 1961; James and others, 1968; Bayley and others, 1966;Dutton, 1971). This was followed, where appropriate, bydetailed structural studies in the north, mainly done by Kiasnerand Gallup. In this part of the area , previously mapped bymainly W. F. Cannon and Kiasner (see for example Cannon andKiasner, 1972; Kiasner, 1972; Cannon, 1973; Kiasner, 1978), fieldwork principally involved refinement and the addition of newdetailed observations. Gregg's work has been in the Baragabasin, primarily northeast of the area covered in this report.Detailed studies at Falls River ( Sikkala, 1987 and Sikkala andGregg, 1987), however, mark the northern end of the traverse in

the area of this report.

GENERAL GEOLOGY

Stratigraphic units of the Marquette Range Supergroup(Cannon and Gair, 1970) are listed in Table 1. The MarquetteRange Supergroup consists of three groups which lie unconforrnablyabove, or in fault contact with, underlying Archean greenstone—granite or gneiss. The rocks occupy broad open basins, such asthe Michigamme basin or relatively narrow, fault—bounded troughssuch as the Marquette and Republic troughs (Fig. 3). Anapparently thick sequence of metavolcanic rocks, the HemlockFormation (see Cannon and Klasner, 1975 and Foose, 1980) liesalong the west side of the Amasa uplift. As discussed below , wetentatively interpret this apparently large thickness of volcanicrocks as resulting, at least in part, from tectonic stackingduring the Early Protrozoic overthrusting event. It should benoted that the Michigamme Formation is informally divided into anupper and lower slate member by the Bijiki Iron Formation Member(see Table 1). The lower slate member is the dominant lithology.Much of the evidence for overthrusting is found in the upperslate member (dominantly graywacke ) member of the Michigamrne

Formation.

The Archean rocks of northern Michigan and Wisconsin havebeen divided by Sims (1980) into a granite-greenstone terrane(2600 - 2700 Ma) on the north and a gneiss terrane (2600 - 3550Ma) on the south. The boundary between these two terranes hasbeen named the Great Lakes tectonic zone. In the study area, it

is oriented eastward and lies roughly along the Marquette trough(Fig. 3). The Penokean foldbelt is localized along the GLTZ,

C- 7

expected. In this gui6ebookf we examine evidence from northern Michigan showing that there was extensive overthrusting on the continental margin during the accretionary event. We examine the evidence for overthrusting along a north-south structural traverse (Fig. 2) extending from Falls River southward across the Michigame basinf Amasa uplift, and Felch trough-Calunet trough resion to .the Niagara fault zone which lies along the fi?ichiqan/~$isconsin border.

This guidebook is mainly the outgrowth of field work and related synthesis done during 1986 and 1987. Critical areas for examination were selected from several exc2llent published geological maps and papers in the south (principally James and othersf 1961; James and othersf 1968; Bayley and othersf 1965; Duttonf 1971). This was followedf where appropriatef by detailed structural studies in the northf mainly done by Klasner and Gallup. In this part of the area previously mapped by mainly W. F. Cannon and Klasner (see for example Cannon and Klasnerf 1972; Klasnerf 1972; Cannonf 1973; Klasnerf 1978), field work principally involved refinement and the addition of new detailed observations. Greggls work has been in the Baraga basinf primarily northeast of the area covered in this report. Detailed studies at Falls River ( Sikkalaf 1987 and Sikkala and Greggf 1987)f howeverf mark the northern end of the traverse i i l

the area of this report.

GENERAL GEOLOGY

Stratigraphic units of the Marquette Range Supergroup (Cannon and Gairf 1970) are listed in Table 1. The Marquette Range Supergroup consists of three groups which lie unconformably abovef or in fault contact withf underlying Archean greenstone- granite or gneiss. The rocks occupy broad open basinsf such as the Michigamme basin or relatively narrowf fault-bounded troughs such as the Marquette and Republic troughs (Fig. 3). An apparently thick sequence of metavolcanic rocksf the Xeinlock Formztion (see Cannon and Klasnerf 1975 and Foasef 1980) lies along the west side of the Amasa uplift. As discussed below , we tentatively interpret this apparently large thickness of volcanic rocks as resultingf at least in part, from tectonic stacking during the Early Prot.~rozoic overthrusting event, It should be noted that the Michigamme Formation is informally divided into an upper and lower slate member by the Bijiki Iron Formation Nember (see Table 1). The lower slate member is the dominant lithology. Much of the evidence for overthrusting is found in the upper slate member (dominantly graywacke ) member of the Michigamme Formation.

The Archean rocks of norkhern PTichigan and Wisconsin have been divided by Sims (1980) into a granite-greenstone terrane (2600 - 2703 Ma) on the north and a gneiss terrane (26CO - 3550 Ma) on the south. The boundary between these two terranes has been named the Great Lakes tectonic zone. In the stuZy areaf it is oriented eastward and lies roughly along the Narquette trough (Fig. 3). The Penokean foldbelt is localized along the GLTZ,

TABLE IFelch-Calumet

Michigamme Basin Iron River syneline trough

paint Fortune Lakes SlateStambaugh Formation

River Hiawatha GraywackeRiverton Iron—

Group FormationDunn Creek Slate

Badwater Greenstone Bedwater Greenstoneichigamme Formatjo, flichigamme Slate Michigamme Slate3ijik. Iron— Ainasa Formation

Saraga Formation Member Hemlock Formation:larksburg

Group Volcanics Member;reenwood Iron—Formation Memberoodrich Quartz ite

___________

LOCAL UNCONFORMITy

__________________

— UNCONFORMITY—I4enominee

Group Negaunee Iron— Vulcan Iron-Marquette Formation Formation

Siamo Slate** ,. Felch Formation

EARLY PROTEROZOIC Range Ajibik Quartzite

Supergroup

____________ ___________________

— UNCONFORMITY —Chocolay Not present Saunders Formation Randville Dolomite

Grou Sturgeon Quartzitep Fern Creek Formation

UNCONFORMITY — — UNCONFORMITY— — UNCONFORMITY—

ARCHEAB Granite — Gneiss Gneiss

greenstone

* Upper slate member of the Michigamme Forma.ion lies above the Bijiki Iron FormationMember and the lower slate member of the Michiganune lies below it.

** Possibly present in the subsurface of the Michigalume basin.

C-B

EARLY PROTEROZOIC

ARCHEAN

Marquette

Range

supergroup

. TABLE I . Michiqamme Basin

Felch-Calumet Iron River svncline trough

Paint

River

Group

Baraqa

Group

Menomine{

Group

Chocolay

Group

ichiqanme Formatit ijiki Iron- Formation Member larksburg '

Volcanics Member reenwood Iron- Formation H e a r oodrich Quartzite

LOCAL UNCONFORMIT' OR DISCONFORMITY

Neqaunee Iron- Formation Siamo Slate** Ajibik Quartzite

Not -present

UNCONFORMITY

ranite - reenstone

Fortune Lakes Slate Stambauqh Formatior Hiawatha Gravwacke Riverton 1ro& Foma tion Dunn Creek Slate

Badwater Greenstone Nichiqannne Slate Amasa Formation Hemlock Formation

Saunders Formation

- UNCONFORMITY -

Gneiss

Bedwater Greenstone Michigam Slate

- UNCONFORMITY -

Vulcan Iron- Formation Felch Formation

- UNCONFORMITY - Randville Dolomite

iturgeon Quartzite 'ern Creek Formation

- UNCONFORMITY -

Gneiss

* Upper slate member of the Michiqamme F0rma:ion lies above the Bijiki Iron Formation Member and the lower slate member of the Michiqame lies below it.

** Possibly present in the subsurface of the Michiqamme basin.

FIGURE 3. Geologic map of the study area showing major structuralfeatures, location of field stops: and location of structuralprofiles A—A' (Fig. 6) and 3—B' Fig. 8); c-c' is part of thecomposite profile (Fig. 9) which lies along a line connecting A

to C'. Features discussed in the text include A4 = ±-rnasa uplift,

BB = Baraga basin, BL = 3ush Lake Fault, CF Canyon Fa)ls, CT =Calumet trough, DD =Dunbar dome, FR = Falls River location, FT =Feich trough, MB = Michigamme basin, MT = Marquette troi;gh, NC =

northern complex, TM = Taylor Mine, P = Peavy metamorphic flO(i,

and PC = Plunjbaqo Creek. RT =RepubliC trough. AA is ti locainof the granitic intrusion dated at 1824 Ma (Z.E. Peterman, pers-onal communication). From Cannon (1983) and Dutton and Linebaugh(1967)

-

C-9 -

Fkld trip stops

\ Thrust fault

& Geologic contact

Fault of unknown dip h u- upthrown block

1 Strike ond dip direction of foliation

Intrusive rocks P= Peavy metamorphic node &= I834 Mo gronlte

;J:;;erentioted Point River Group 0

Undifferentiated Barago Group rocks. Includes undifferentiated Bodwater ond Hemlock volconic units

Undifferentiated Menominee ond Chocoloy Group rocks

@J Undifferentiated qronitoid gneiss ond Dickinson Group metoconglomero

z 0 a a

0 5 0 Miles

0 5 I0 Kilometem

of the granitic intrusion dated at 1824 Ma (z.E. Peterman, personal communication). From Cannon (1983) and Dutton and Linebauqh (1967).

which appears to have played a role in the style of deformationof Early Proterozoic stata. North of the GLT7J, Early Proterozoicstrata are only gently deformed, whereas south of it EarlyProterozoic strata are intensely deformed. The traverse describedin this guidebook lies mostly south of the GLTZ, in the intenselydeformed part of the Penokean foldbelt. Here, Early Proterozoicstrata overlie block faulted Archean rocks (Cannon and Klasner,1972 and Cannon, 1973).

At least four regional metamorphic aureoles (James, 1955)were formed during the Penokean orogenic event (Fig. 4). Attohand Vander Muelen (1984) and Kiasner and Attoh (1986) argue thatthe metamorphic aureoles were caused by crustal thickening,possibly due to tectonic stacking associated with overthrustirig

during the Penokean accretionary event.

PaleozojcRocks

Peavy Node

FItrn2 4. Location of metamorphic aureoles in northern Michian(after James, 1955).

Structural studies (Cannon and Kiasner, 1972; Klasner,

1972; Cannon, 1973; and Klasner, 1978) have shown that EarlyProterozoic rocks in northern Michigan were subjected to at leastthree periods of deformation. The first event (Fl) ws thinskinned and resulted in shortening of Early Proterozo1c strata

and development of a widespread west-northwest-trending

C- 10

C chlorite b biotite

ggarnet S

40 km

WatersmeetNode

Florence Node

which appears to have played a role in the style of deformation of Early Proterozoic stata. North of the GLTZr Early Prt7tsroz~ic strata are only gently deformedr whereas south of it Zarly Proterozoic strata are intensely deformed. The traverse described

. in this guidebook lies mostly south of the GLTZ, in the intensely deformed part of the Penokean foldbelt. Herer Early Proterozoic strata overlie block faulted Archean rocks (Cannon and Klasner, 1972 and Cannonr 1973).

At least four regional metamorphic aureoles (Jamesl 1955) were formed during the Penokean orogenic event (Fig. 4 ) . Attoh and Vander Muelen (1984) and Klasner and Attoh (1986) argue that tke metamorphic aureoles were caused by crustal thickeningr possibly due to tectonic stacking associated with overthrustirq during the Penokean accretionary event.

c chlorite b biotite 40 km - 9 garnet s Staurolite

Paleozoic Rocks

Node

Florence Node -'

F1232E 4. Location of netamorphic aureoles in northsrn bfichiqsn (after Samesl 1955).

Structzral stusies (Cannon and Klasner? 1972; Klssnsr, 1972; Cannon? 1973; and Klasnerr 1978) have sh~i~n that Early ?rotsrozoic rocks in northern Michiqan wers subject& to ~t lsast thrse peri~As of defornatici-i. The first event ( F l ) KZS thin skinned znd resulted in shortening of Early ProtsrazoiL? strata and development of z widespread w e s t - n t ~ r t h ~ c t ? s t - t r ~ f i d i ~ g '

c-10

structural fabric (Fig. 3). The second event (F2) involved blockuplift of Archean basement rocks and formation of structuralfeatures such as the Marquette and Republic troughs and infoiclingof Early Proterozoic strata into the troughs. Larue and Sloss(1980) have shown that the troughs actually started to formearly, during deposition of the sediments, and before Fl

deformation. Kiasner (1972, 1978) noted a third, late stage,brittle deformational event (F3) in the northern part of theMichigamme basin and suggested that it could have been caused bylate block uplift of Archean basement. Petrofabric studies(Kiasner, 1972, 1978) showed that regional metamorphism in thispart of northern Michigan peaked after Fl deformation, about thetime of F2 deformation.

PaleozoicRocks

FIGURE 5. General trends of first and second order structures innorthern Michigan (from Cannon, 1973).

Cannon (1973) divided the regional structures in northernichigan into "firsts' and "second'1 order features (Fig. 5). Firstorder structures have a wide range of orientations and arerelated to block uplift and doming (Sims and others, 1$84a) ofthe Archean crust. Second order structures have a regular west-northwest trend and are related to the Fl shortening of Early

C-li

Uplift Basin

Trends of first order structures

Trends of second

order structures

0 20 miles

str~ctural fabric (Fiq. 3 ) . The second evsnt (F2) inv~lve5 block uplift of Archean basement rocks and formztion of structural features such as the Karquette and Republic troiighs an2 infolding of Early Proterozoic strata into the troughs. Larue and Sloss (1980) have shown that the troughs actually started to form earlyf during deposition of the sediinents, anZ before Fl deformation. Klasner (1972, 1978) notsd a third, late stags, brittle deformational event (F3) in the northern part of the Michigamme basin and suggested that it could have been caused by late block uplift of Archean basement. Petrofakric studies (Klasnerf 1972f 1978) showed that regional metamorphisin in this part of northern Michigan peaked after Fl deformation, about the tine of E'2 deformation.

Uplift Basin

47 + Trends of first order structures

---- Trends of second

0 20 miles L

FIGURE 5 . General trends of first and second or6er structures iz narthern Michigan (from Cannon, 1973).

Cancon (1973) Zivided the regional structures in northern Xichigan into "first" and "secon~" order fsatures ( F i q . 5 ) . First ordsr structxres hs-~e a wide rmqe af arier,tations a n 6 .3rs relste6 to block uplift s n d Samincj (Sins znd others, :-S84a) (2f

ths Archsan crust. S2cond ordsr st.ractu2s have z rsqilai- x?st- northwest trenz an6 are relstd ta the Fl shortsning of Early

Proterozoic strata.

PREVIOUS EVIDENCE FOR THRUSTING OF EARLY PROTEROZOIC ROCKS

The dichotomy in style of deformation between EarlyProterozoic rocks and underlying Archean rocks led Cannon andKiasner (1972), Kiasner (1972), Cannon (1973), and Kiasner (1978)to suggest that a decollement exists between them. Theysuggested that the shortening of Early Proterozoic stratarelative to underlying Archean rocks was caused by gravitysliding off an ancestral Penokean mountain range in the south.They (in particular Kiasner, 1972, 1978) suggested that Fldeformation occurred, in part, while the sediments wereunconsolidated; however, this postulate is no longer tenable,and it appears that the Fl deformational event involved solidrocks.

Recently, Hoist (1982, 1984) identified Early Proterozoicnappe structures in eastern Minnesota. Similarly, Maharidge(1986) proposed that the Felch trough area of northern Michigan(Fig. 2) represents a crystalline-core nappe feature. He basedthis interpretation on reexamination of sedimentary rock outcropsin the Feich trough area, previously mapped by James and others(1961), and on microstructural analysis of hand specimens fromthe region. Later, Sims and others (1987) tentatively suggestedthat the Felch trough area may represent the core of a north-verging nappe thrust structure, but this interpretation remainsequivocal; and, as discussed below, although we cannot rule outthe presence of a nappe, we think that Felch trough area may bepart of a south-verging backthrust. Structural studies underwayin this region should permit resolution of this problem in thenear future.

On the other hand, Sikkala (1987) and Sikkala and Gregg(1987) have identified recumbent folds and a probable nappestructure in slate and graywacke of the Michigamrne Formation atFalls River near L'anse. Similarly, Van Roosendaal (1985)mapped large scale west- northwest trending thrust systems inslates and graywackes of the Baraga basin (Fig. 3) northeast ofFalls River.

Evidence for Early Proterozoic overthrusting has recentlybeen found by Xlasner and others (1988, in press) in severalplaces in northern Michigan. About 120 km west of the traverseshown in Figures 2 and 3, Early Proterozoic slate and graywackeof the Tyler Formation (Baraga Group) near Ironwood, Michigan andmetavolcanic rocks of the Emperor volcanic complex at nearby WolfMountain have a penetrative foliation that strikes roughly east-west and, when corrected for rotation due to deformation in thethe ca 1.1 Ga Midcontinent Rift, dips gently south. Thissuggests that these rocks may have been involved in a thrust, orthrust- nappe event similar to that mapped about 100 km west inMinnesota by Holst (1984) and at Falls River by Sikkala 1987 andSikkala and Gregg (1987). Klasner and others (in press) also

C-12

Proterozoic strata.

PREVIOUS EVIEENCE FOR THRUSTING OF EARLY PROTEROZOIC ROCKS

The dichotomy in style of deformation bstween Early Proterozoic rocks and underlying Archean rocks led Cannon and Klasner (1972) Klasner (1972) Cannon (1973) and Klasner (1978) to suggest that a decollement exists between them. They suggested that the shortening of Early Proterozoic strata relative to underlying Archean rocks was caused by gravity slizing off an ancestral Penokean mountain range in the south. They (in particular Klasnerf 1972, 1978) suggested that Fl deformation occurredf in partf while the sediments were unconsolidated; howeverf this postulate is no longer tenabler and it appears that the Fl deformational event involved solid rocks.

\

Recently, Holst !1982r 1984) identified Early Proterozoic nappe structures in eastern Minnesota. Similarlyf ?.laharidge (1986) proposed that the Felch trough area of northern ?!ichicjan (Fig. 2) represents a crystallice-core nappe feature. He basez this interpretation on reexamination of sedimentary rock outcr~ps in the Felcb. trough areaf previously mapped by Janes znd others (196llf and on microstructural analysis of hand specimens from the region. Later, Sims and others (1987) tentatively sucjgested that the Felch trough area may represent the core of a north- verging nappe thrust structuref but this interpretation remains equivocal; andf as discussed belowr although we cannot rule out the presence of a nappet we think that Felch trough area may be part of a south-verging backthrust. Structural studies underway in this rsgion should permit resolution of this problem in the near future.

On the other handf Sikkala (1987) and Sikkala and Gregg (1987) have identified recumbent folds and a probable nappe structure in slate and graywacke of the Michigame Formation at Falls River near Ltanse. Similarlyf Van Roosendaal (1985) mapped la~ge scale west- northwest trending thrust systems in slates and graywackes of the Baraga basin (Fig. 3) northeast of Falls River.

Evidence for Early Proterozoic overthrusting has recently been found by Klasner and others (l98Bf in press) in several places in northern Xichigan. About 120 km west of the traverse shown in Figures 2 and 3f Early Proteroz~ic slate and graywacke of the Tyler Formation (Baraga Group) near Ironw006~ Xichigan an6 metavolcanic rocks of the Emperor volcanic camplex at nsarby Xolf Xountain have a penetrative foliation that strikes rou~hly 2ast- west and, when corrected for rotati~n due to deforriiation in the the ca 1.1 Ga Midcontinent Riftf dips gently soath. This suggests that these rocks may have been involved in z thrustt or thrust- nappe event similar to that mapped about 100 km west in Ninnesota by Holst (1984) and at Falls River by Sikkala 1987 and Sikkala and Gre~g (1987). Klasner and others (in press) also

presented evidence to show that Archean crust is locally involvedin the overthrust event. A drill hole about six kilometers eastof Canyon Falls in the northern complex (Fig. 3) penetratedgneiss and underlying sheared slate and quartzite, all of whichhave low dipping foliation. The slate and quarzite areinterpreted to be Early Proterozoic in age. Similar carbonaceousslate occurs at Taylor Mine and quartzite at nearby Canyon Falls.These relationships suggest that Archean gneiss was thrust overthe Early Proterozoic slate.

Earlier studies in both Michigan and Wisconsin, nearer tothe Niagara fault zone, support the idea of overthrusting on thecontinental margin. Bayley and others (1966), James and others(1961), and Bayley (1959) mapped local subhorizontal foliationsin the Michigamme Formation, especially in the region betweenstops 9 and 13 (Fig. 3) just north of the Niagara Fault. Uengand others (1984) attributed this foliation to vertical uplift ofArchean basement. However, we attribute the subhorizontalfoliation to thrust faulting, as discussed in some detail below.

Finally, thrusting resulting from the continent-arccollision has also affected rocks south of the Niagara fault.From geologic mapping and geochemical evidence, Sims and others(in press) have concluded that chemically dissimilar rocks in theDunbar area (Sims and others, 1985) have been juxtaposed bythrust faulting and related deformation; and this interpretationaccords with an earlier interpretation of Klasner and Osterfeld(1984) from gravity modelling that the Dunbar dome isallochthonous, either as a consequence of thrusting along thesouth—dipping Niagara fault or a subsidiary fault.

EVIDENCE FOR OVERTHRUSTING ALONG THE STRUCTURAL TRAVERSE

Geologic mapping and detailed structural observations atseveral selected field localities in the study area demonstratethe existence of small to moderate-scale thrust faults andlocally related recumbent folds. These features are particularlywell exposed at Little Mountain (field stop 2) near the northernend of the traverse, and subhorizontal foliation and relatedoverturned to recumbent folds are present at several localities,especially, as mentioned above, at Falls River (field stop 1),and in the broad area encompassed by stops 9, 12, and 13 (seebelow). From these observations, larger—scale thrusts andpossibly nappes can be inferred where permissive structuralfeatures are observed in the rocks and/or where previously mappedfaults are better interpreted as thrusts.

To illustrate the style of Early Proterozoic deformation innorthern Michigan, we have compiled the geologic map shown inFigure 3 and have constructed two interpretive cross sections(Figs. 6, and 8). The thrust system depicted on Figure 3

displays an overall northward sense of structural vergence.

C-13

presented evidence to show that Archean crust is locally involved in the overthrust event. A drill hole about six kilometers east of Canyon Falls in the northern complex (Fig. 3) penetrzted gneiss and underlying sheared slate and quartzitef all of which have low dipping foliation. The slate and quarzite are interpreted to be Early Proterozoic in age. Similar carbonaceous slate occurs at Taylor Mine and quartzite at nearby Canyon Falls. These relationships suggest that Archean gneiss was thrust over the Early Proterozoic slate.

Earlier studies in both Michigan and Wisconsin, nearer to the Niagara fault zonef support the idea of overthrusting on the continental margin. Bayley and others (1966)f James and others (1961)f and Bayley (1959) mapped local subhorizontal foliations in the Michigamme Formationf especially in the region between stops 9 and 13 (Fig. 3) just north of the Niagara Fault. Ueng and others (1984) attributed this foliation to vertical uplift of Archean basement. Howeverf we attribute the subhorizontal foliation to thrust faultingf as discussed in some detail below.

Finallyf thrusting resulting from the continent-arc collision has also affected rocks south of the Niagarz fault. From geologic mapping and geochemical evidence, Sins and others (in press) have concluded that chemically dissimilar rocks in the Dunbar area (Sins and othersf 1985) have been juxtaposed by thrust faultinq and related deformation; and this interpretation accords with an earlier interpretation of Klasner and Osterfeld (1984) from gravity modelling that the Dunbar dome is allochthonousf either as a consequence of thrusting along the south-dipping Niagara fault or a subsidiary fault.

EVIDENCE FOR OVERTHRUSTING ALONG THE STRUCTUXAL TRAVEXSE

Geologic napping and detailed structural observations at several selected field localities in the study area demonstrate the existence of small to moderate-scale thrust faults and locally related recumbent folds. These features are particularly well exposed at Little Mountain (field stop 2) near the northern end of the traversef and subhorizontal foliation and related overturned to recumbent folds are present at several localitiesf especiallyf as mentioned abovef at Falls River (field stop lIf and in the broad area encompassed by stops g f lZf and 13 (see below) . From these observationsf lar9er-scale thrusts aad possibly nappes can be inferred where permissive structural features are observed in the rocks and/or where previously mzpped faults are better interpreted as thrusts.

To illustrate the style of Early Pr~terozoic deformation in northern Michiganf we have compiled the geologic ma? shown in Figure 3 and. have c~nstructed two interpretive cross sections (Figs. 6: and 8). The thrust systen depicted on Fiqure 3 Zisplays an overall northward sense of structural ver9er~ce.

II

III

FIGURE 6.

Structural profile A--At.

Note field stop localities.

See

Figure

3for

location.

Lower

hemisphere

stereoplots

illiitrate orientation of bedding (SO),

foliation (Si), ard fold

axs (black dots) along the traverse.

Directions of each segment

of the profile between stops are shown by arrows at top of figure.

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The style of deformation in the Michigarnme basin is

illustrated on section A—A' (Fig. 6). Structural data in

primarily the upper slate member of the Michigainme Formationalong this profile suggests that the Early Proterozoic rocks weredeformed as part of an irnbricate thrust system. As shown on

Figure 6, there does not seem to be much involvement of the

Archean rocks in the thrusting. Structural relationships at

Taylor Mine, Plumb.go Creek, and Canyon Falls (Fig. 3), however,indicate that there was some involvement of the basement in the

thrusting. At Canyon Falls flat-lying quartzite lies

unconformably on Archean basement and has only minor evidence ofEarly Proterozoic deformation. Yet at Taylor Mine, about 5 km tothe north, Early Proterozoic slates are overturned toward the

north and have steep south dipping foliation as shown in Figure

7. At Plumbago Creek, 1.6 km south of Taylor Mine, penetrativelydeformed Early Proterozoic phyllite has a south-dipping foliation

which projects beneath Archean gneiss. These structural data,

when plotted on the elevation profile, as shown on Figure 6,

indicate that there was some involvement of basement rocks in

Early Proterozoic overthrusting in this region. Because of

difficult access, outcrops at Plumbago Creek will not he visited.Taylor Mine is an alternate stop.

NORTH Field stop area SOUTH

FIGURE 7. Structural cross section in the Ta-la ' - - -(locaLlon 2a on Fia 3)r Jj1fl area

deformat ion . D iamond dri11ho1e data a re f ram Fod tar

C- 15

o 200 feet

• SCLEi:j

I

Keweenawandiabase dike

Cherty ironfortnatiofl

Folded sandylayer

N 'tSlack slate withSi foliation

NOrientationof cleavage

Core hole showingcore dips

The style of deformation in the Kichigamme basin is illustrate6 on ssction A-A' (Fig. 6 ) . Structural 6ata in primarily the u2pr slate member of the Xichigznine Forination along this profile suggests that the Early Proteroz~ic rocks were 6eform~d as p ~ r t of an iribricate thrust systan. As shown ~ r i Figure 6, there d ~ e s not ssem to be much involvement of the Archean rocks in the thrusting. Structural relationships at Taylor Ninef Plumb?-go Creekf and Canyon Falls (Fig. 3If however, indicate that there was some involverent of the b~sernsnt in the thrusting. At Canyon Falls flzt-lying quartzite lies uriconformably on Archean basement and has only minor evisence of Early Proterozoic deformation. Yet at Taylor :4ine, about 5 km to the northf Early Proterozoic slates are overturned towarc? the north and have steep south dipping foliation as shown in Ficpre 7. At Plumbago Creekf 1.6 km south of Taylor Xinet penstratively deforned Esrly Proterozoic phyllite has a south-dipping foliation which projects beneath Archean gneiss. Thess structural datz, when plotted on the elevation profile, as shown on Figure 6, indicate that thsre was sone involvement of bassinent rocks in Early Proterozoic ovsrthr~stinq in this rsgion. 2ecause of difficult zccessf outcrops at ?Li~mb~g~ Creek will not he vtsited. Taylor Mine? is an alternate stop.

Field stop area SOUTH KORTH

\ Orientation

Keweenawan Folded sandy of cleavage diabase dike laver -Y - - -- - - - -. . -

Cherty iron Black slate with core hole ~howinq formation Sl foliation core dips

FIGTJRE 7. Struck~ral cross section in the Taylor 76i~e srsa (location 23 on Fig. 3) showing Sl fcliatici~ snd style c,f dezornation. DiamonS drill-hole data are frmi Ford ~otor C ~ m p a n y ( A f h r Klssrxr, 1 9 7 2 1 .

The main feature illustrated on section B-B' (Fig. 8) is

that the large apparent thickness of Hemlock volcanic rocks (see

Foose, 1980) along the west side of the Amasa uplift may, in

part, be accounted for by tectonic stacking. Structural data atalternate field stops 7a and 7h lend support for the idea of

tectonic thickening of the Hemlock. At stop 7a mafic volcar..crocks are intensely deformed and have a prominent foliation that

is oriented northwestward, as shown on Figure 3. Stretchlineations on the foliation surfaces plunge steeply toward the

west. At station 7b axes of small drag folds in siliceous sletplunge gently northwestward and their Z-shape symmetry indicatesa northeastward sense of vergence. Thinly layered tuff at

altrnate stop 8 has a penetrative foliation that is oriented

N55 W, 60 SW, and other outcrops in the region have similarlyoriented foliations. Taken together, these and other outcrops onthe west side of the Amasa uplift indicate that tectonic stackingcould have significantly thickened the Hemlock volcanic pile.

The nature of the stacking, however, is not resolved at this

time. Profile B-B' (Fig. 8) suggests that it is an imbricate

thrust fan. Alternately, the Arnasa uplift may represent a domedduplex system. Either style of deformation would account fr thethick section of Hemlock volcanic rocks and the apparent.

structural discordance between the Michigamme Formation and the

Hemlock volcanic pile as pointed out by Foose (1980).

B ....N2OE

2000

I' 1000

Rocks offlZ— -

0 I L '—— Baraga Group

IH.m1o BodwatetMichigomme

O - I Rocks of Menominee

I G0 SmIles

L Rondvlile0 OCO ØY rOUOS

_____________

0 5 kilometers

ARCHEAN Basemeift Gneiss

Drag Fold Fault

FIGURE 8. Structural profile B—B'. Note drag fold from stop 7.Rou;hly 20 times vertical exageration. See Figure 3 for location.

C-16

J\

xx xxx xxxx

The main feature illustrated on section B-Bt (Fig. 8 ) Ts that the large a2parent thickness of Hemlock volcanic rocks ( s e Fooser 1980) along the west si6e of the Amsa uplift msyr in partr be accomted for by tectonic stackinq. Structural dsta st alternate field stops 7a and 7b lend support for the idea of tectonic thickening of the Hemlock. At stop 7a mafic volcariic rocks are intsnsely defornsd and have a prominent foliation that is oriented nortkdestward, as shown on Figure 3. Stretch lineations on the foliation surfaces plunge steeply toward the west. At station 7b axes of small drag folds in siliceoas slat.? plunqe gently northwestward and their 2-shape s-pmetry indicates a northeastward sense of vergence. Thinly layere6 tuff at a1Qrnate 8 has a penetrative foliation that is orisnted N55 L?, 60 S7J, and other outcrops in the region have similarly oriented foliations. Taken together, these and other outcrops on the west side of the Amasa uplift indicate that tectonic stackiii~ could have significantly thickened the Hemlock volcanic pile. The nature of the stacking, however, is not resolved at this time. Profile B-8' (Fig. 8 ) suggests that it is an imbricate thrust fan. Alternatsly? the Amzsa uplift zay represent a Zoined Zuplex system. Either style of defornation wo~lZ zccocnt far ths thick szction of Hemlock volcanic rocks sn2. the s~?zre~*. structural discordance between the Nichigxixne Foriaation an3 the :Iemlock volcanic pile as pointed out by Foose (1980).

2000

e 0 - - 0 = I000

Rocks of

o Bamga Group Badwater

Miiigamme Rocks of ~enominea

and Chocolay Groups 0 5 miles Randvilla y

0 5 kilometers

ARCHEAN a Basement Gneiss

* Drag F O I ~ \ F O U I ~

In particular, either system would explain the structuraldiscordance between the Hemlock and the magnetically mapped ironformation that lies just above the contact in the MichigamrneFormation. We interpret the Michigamme Formation to be separatedfrom the Hemlock Formation by a thrust fault. The configurationof the magnetically mapped iron formation mimics theconfiguration of the thrust fault. Inasmuch as the western edgeof the Hemlock volcanic pile is covered, this fault is inferredand was not observed in the field.

The rocks that lie in the region between the Bush Lake faultand the Niagara fault (Fig. 3) are part of the north-vergingthrust system described above, but the exact nature of thethrusting in this region remains problematic. Maharidge's (1986)interpretation that the Feich trough is part of a nappe structurehas not been proven, but, also, it cannot be discounted at thistime. Our recent studies show that both Archean and EarlyProterozoic rocks possess a prominent low-dipping foliation thatis superimposed upon a steeply-dipping foliation in the Archeanrocks (see field stops 10 and 11). Also, in this region there isfirm evidence for southwest verging thrusting in rocks of boththe Feich and Calumet troughs. We tentatively interpret thesouth-verging structure as being formed due to south—directedbackthrusting caused by ramping of the north-verging thrustfaults. Best evidence for this southward vergence is found at

stops 9, 11, 12, and 13. We will visit stops 9, 11, and 12 butwe will most likely not visit stop 13, an alternate stop, becauseof difficult access.

A composite section (Fig. 9) along the length of the

structural traverse summarizes our current, and as yet,

tentative, interpretation of the style of deformation thatoccured in northern Michigan. This interpretation is consistentwith a fore1.nd thrust system as depicted in the model of an

idealized orogen by Hatcher and Williams (1986). The idealizedorogen consists of a foreland overthrust belt, an overthrustmetamorphic core complex that may include ophiolites and thrustedcrystalline basement rocks, and an accreted plutonic/volcanicterrane. Part A-A' of the composite section marks the forelandoverthrust belt which involves thin-skinned tectonism withrelatively little involvement of basement rocks in the thrusting(note these comments apply to the area close to this traverse,and the amount of basement involvement in Early Proterozoicoverthrusting may change along the strike of the Penokeanorogen). Telescoped and foreshortened Early Proterozoic strataare separated from basement rocks along a basal detachment fault.Nd isotope studies by Barovich and others, (1987) indicate thatthe southern half of this terrane was derived from the Wisconsinmagmatic terrane to the south, thus supporting the idea of

northward tectonic transport of at least a part the MichigammeFormation, and explaining the presence of deep water turhiditedeposits on the continental margin.

Section B-B' marks the Amasa uplift area, which is

transitional between thick-skinned basement-involved defamation

In particularr either system would explain the structural discordance between the Hemlock and the magnetically mapped iron fornation that lies just above the contact in the Michigamme Formation. We interpret the Michigamme Formation to be separated from the Hemlock Formation by a thrust fault. The configuration of the magnetically mapped iron formation m.imics th2 configuration of the thrust fault. Inasmuch as the western edge of the Hemlock volcanic pile is coveredr this fault is inferred and was not observed in the field.

The rocks that lie in the region between the 8ush Lake fault and the Niagara fault (Fig. 3) are part of the north-verging thrust system described above, but the exact nature of the thrusting in this region remains problematic. Maharidge's (1986) interpretation that the Felch trough is part of a nappe structure has not been proven, butr alsor it cannot be discounted at this time. Our recent studies show that both Archean and Early Proterozoic rocks possess a prominent low-dipping foliation that is superimposed upon a steeply-dipping foliation in the Archean rocks (see field stops 10 and 11). Alsor in this region there is firm evidence for southwest vercjing thrusting in rocks of both the Felch and Calumet troughs. We tentatively interpret the south-verging structure as beincj formed due to south-Zirectsd backthrusting caused by ramping of the north-verging thrust faults. Best evidence for this southward vergence is found at stops g r llr 12r and 13. We will visit stops g r llf and 12 but we will most likely not visit stop 13r an alternate stopr becauss of difficult access.

A composite section (Fig. 9 ) along the length of the structural traverse summarizes our currentr and as yetr tentativer interpretation of the style of deformation that occured in northern Kichigan. This interpretation is consistent with a forelan6 thrust system as Zepicted in the zodel of an idealized orogen by Hatcher and Williams (1986). The idealized orogen consists of a foreland overthrust beltr an overthrust metamorphic core complex that may include ophiolites and thrusted crystalline basercient rocksr and an accreted plutonic/volcanic terrane. Part A-A' of the composite section marks the foreland overthrust belt which involves thin-skinned tectonisn with relatively little involvement of basement rocks in the thrusting (note these comments apply to the area close to this traverse, and the anount of basement involvenent in Early Proterozoic overthr~sting may change along the strike of the Penokesn orogen). Telescoped and foreshortened Early Proterozoic strata ars separated from basement rocks along a basal detachment fault. Nd isotope studies by Barovich and others, (1937) indicats that the southern half of this terrane was derived from the Wisconsin magmatic terrane to the southr thus supp~rtiricj the idea of northward tectonic transport of at least a part the Michigamme Formationr and explaining the presence of deep xater turbidite deposits on the continental margin.

Section B-B' marks the Amasa uplift arear which is transitional between thick-skinned basement-invalved defamation

81 - Bush Lake Fault

o 10 Miles& a a i.l

0 10 Km

NF - Niag4ra Fault

FIGUflE 9. Composite st.ructural profile 1ong line connectingto C' (in Figure 3. See text for discussion.

A

N

A

Michigomme Basin

Foreland Thrust

MARGINAL BASEMENT ARCH

Amaso Uplift

S

GL XX'S

a'Felch - Calumet Wisconsin

Imbricate Thrust Belt Accreted MagmaticArc

— — —— — —

— — -.- '— — .

—GL

xX X

ARCHEANX x'BaragaGroupRocks

HemlockVolcafliCS

XXX XXX

Menomi nee& ChocolavGroupRocks

AccretedVolcanic& PlutonicRocks

X

Fault

X

GL - Ground level

MARGINAL BASEMENT ARCH A

A LIB B' c i'

I Michlgamme Basin I I Felch - Calumet Wisconsin

Foreland Thrust Amasa Uplift Imbricate Thrust Belt Accreted Magmatic Arc

Baraga Hemlock ~enominee Accreted \ - Fault Group volcanics & Chocolav Volcanic

Group ~ o c k s

& Plutonic GL - Ground level Rocks

BL - Bush Lake Fault

- Niagara Fault

FIGIJRE 9 . C o m p o s i t e st.ruct.ura~1 prof i 1 e <: lt3ng 1 ine connecting A to C ' c,n Figure? 3 . See tsxt for d j . s c u s s i o n .

to the south near the continental margin, and the thin-skinnedtectonism of the foreland thrust belt to the north. Tentatively,we consider the Amasa uplift as having behaved as a stable blockconsisting of an Archean core and a large volcanic pile overwhich less competent rocks of the continental margin were thrust.The thrust system refracted upward from the Archean basement onthe south so that it is largly confined to supracrustal EarlyProterozoic strata north of the uplift.

Part C-C' on Figure 9 marks the crystalline metamorphic corecomplex sytem of Hatcher and Williams (1986). It consists of animbricate, piggy back-type of crystalline thrust sheet which is

bounded on the south by the Niagara fault zone and on the northby the Bush Lake Fault (Fig. 3). The Bush Lake Fault appears tohave been a key feature in evolution of the thrust system. It

marks a boundary between structures on the north which trendnorthwest and those on the south which trend east-west (see Fig.

3). Metamorphic isograds (Attoh and Vander Muelen, 1984 andKlasner and Attoh, 1986) indicate that the south side is

upthrown. We interpret it to be a major fault along whichcrystalline rocks of the continental margin were thrust upwardand over the Amasa uplift.

Rocks of the Wisconsin magmatic terrane, south of theNiagara fault zone, have been accreted to the continental margin.Nd isotope data indicate that the volcanic rocks of the magmaticterrane represent new Early Proterozoic crustal material(Barovich and others, 1987). Klasner and Osterfeld (1984) andSims and others (1984b, 1985) suggested that the magmatic rocksof northeast Wisconsin are, in part, allochthonous. Klasner andothers (1985, page 284) and Klasner and Attoh (1986) inferredthat the actual edge of the continental margin is cryptic, andpossibly buried beneath the allochthonous magmatic terrane in

northern Wisconsin. The Wisconsin magrnatic terrane is equivalentto the accreted volcanic/plutonic terrane of Hatcher and Williams(1986), which can contain slivers of oceanic crust. Schulz(1987b) has identified an ophiolite assemblage in northeastwisconsin.

The geology of the Early Proterozoic continental marginassemblage of northern Michigan is consistent with youngerAtlantic-type (passive) margins (Nelson and others, 1982) as

represented in the Ouachitas, New England Appalachians, TibetanHimalayas, and Oman Mountains. They have in common an inboard,allochthonous foreland thrust belt, and an outboard imbricatethrust system involving crystalline basement rocks.

AGE OF THRUSTING

A minimum age for the overthrusting is 1824 ± 2.6 Ma, whichis a U—Pb zircon intercept age on an undeformed granite plutonthat intrudes Badwater Greenstone at locality A on Figure 5 (Z.

E. Peterman, written communication, 1987). A possible maximunage is 1929 + 17 Ma; two splits of apatite from iron formationunderlying the Michigamme Formation in the Earaga basin yield

to the south near the continental marginf and the thin-skinned tectonism of the foreland thrust belt to the north. Tentatively, we consider the Amasa uplift as having behaved as a stable block consisting of an Archean core and a large volcanic pile over which less competent rocks of the continental margin were thrust. The thrust system refracted upward from the Archean basement on the south so that it is largly confined to supracrustal Early Proterozoic strata north of the uplift.

Part C-C' on Figure 9 marks the crystalline metamorphic core complex sytem of Hatcher and Williams (1986). It consists of an imbricate, piggy back-type of crystalline thrust sheet which is bounded on the south by the Niagara fault zone and on the north by the Bush Lake Fault (Fig. 3). The Bush Lake Fault appears to have been a key feature in evolution of the thrust system. It marks a boundary between structures on the north which trend northwest and those on the south which trend east-west (see Fig. 3). Metamorphic isograds (Attoh and Vander Muelenr 1984 and Klasner and Attoh, 1986) indicate that the south side is upthrown. We interpret it to be a major fsult along which crystalline rocks of the continental margin were thrust upward and over the Amasa uplift.

Rocks of the Wisconsin magmatic terrane, south of the Niagara fault zone, have been accreted to the continental margin. Nd isotope data indicate that the volcanic rocks of the macjmatic terrane represent new Early Proterozoic crustal material (Barovich and others, 1987). Klasner and Osterfeld (1984) and Sims and others (1904b, 1985) suggested that the magmatic rocks of northeast Wisconsin aref in partf allochthonous. Klasner and others (1985, page 284) and Klasner and Attoh (1986) inferred that the actual edge of the continental margin is cryptic, and p~ssibly buried beneath the allochthonous magmatic terrane in northern Wisconsin. The Wisconsin magmatic terrane is equivalent to the accreted volcanic/plutonic terrane of Hatcher and William (1986)r which can contain slivsrs of oceanic crust. Schulz (1987b) has identified an ophiolite assemblage in northeast Xisconsin.

The geology of the Early Proterozoic continental margin assemblage of northern Michigan is consistent with younqer Atlantic-type (passive) margins (Nelson and others, 1982) as represented in the Ouachitas, New England Appalachiansr Tibetan Hiinalayas, and Oman Nountains. They have in common an inboard, allochthonous foreland thrust belt, and an outboard imbricate thrust system involving crystalline basement rocks.

AGE OF THRUSTIEG

A minimum age for the overthrusting is 1824 + 2.6 Ma, which is a U-Pb zircon intercept age on an undeformed-granite pluton that intrudes Badwater Greenstone at locality A on Figure 5 ( Z . E. Peterman, written communication, 1987). A possible imximum age is 1929 + 17 Ma; two splits of apatite from iron formation uriderlying tEe Hichigainme Formation in the Earaga basin yield

lead-lead ages of 1929 ± 17 Ma CR. E. Zartman, writtencommunication, 1987). The thrusting occured within the interval1930 to 1824 Ma. Data from northeastern Wisconsin, within theWisconsin magmatic terrane, is consistent with these constraintsand further refine the age limits. In the Dunbar dome, deformedgranitoid rocks have U-Pb zircon ages of ca 1860 Ma, whereas anundeformed, post-tectonic granite has an age of 1835 ± 6 Ma (Simsand others, 1985). These data suggest collision along theNiagara fault zone occured about 1850 Ma. Probably the thrustingin the continental margin assemblage occurred approximatelycontemporaneous with or slightly later than collision.

FIELD STOPS AND ROAD LOG

The structural traverse across northern Michigan starts inL'Anse, Michigan and ends near Florence , Wisconsin. Eighteenstops are shown on Figure 3 but, because of limited time anddifficult access to several of the localities, only eight to tenof them will be visited. Localities 2b, 7a, and 7b, will not bevisited and localities 2a, 8, and 13 are alternate stops, whichwill be visited only if time permits. Road logs are not given forthe alternate stops, but their location is given by section,township, and range.

ROAD LOG Follow highway U. S. 41 west from Marquette to L'Anse.Proceed west through L'Anse to bridge over Falls River on U. S.41. Park in the pulloff area on the north side of the highwaybetween the RR tracks and the bridge.

STOP 1-FALLS RIVER. Numerous outcrops are present along FallsRiver on both sides of the highway. Structures consist ofnorthward—verging, small-scale1 recumbent folds withsubhorizontal axial-plane foliation indicating nappe—thrust styleof deformation in this area.

ROAD LOG. From the parking lot area for stop 1, follow U. S. 41east for 2.6 miles. Turn west on Golf Course Road and drive 1.75miles to the cul-de-sac at the end of the road. Follow footpathto Little Mountain. It is about a 15 minute walk to the too ofthe mountain; be sure to follow left forks of the main, well wornpath. Path extends around the southwest corner of the mountainwhere it turns sharply toward the east up the south flank of themountain. Follow path to the highest point on the mountain.From this vantage you can see the Keweenaw Peninsula of theMidcontinent Rift System to the north-northwest and the Archeanterrane of the northern complex (Fig. 3) to the east-southeast.The low lying area to the south is underlain by rocks of theMichigamrne basin.

STOP 2-LITTLE MOUNTAIN. Outcrops at the highest point on LittleMountain consist of an unfoliated (ca 1.1 Ga) Keweenawan diabasedike and graded beds of metagraywacke. Grading is expr2ssed bypresence of foliation in the finer grained bed tops. The bedsface toward the north and dip steeply south, indicating thatthey are overturned toward the north.

lead-lead ages of 1929 + 17 Ma (R. E. Zartman, written communication, 1987). het thrusting occured within the interval 1930 to 1824 Ma. Data from northeastern Wisconsin, within the Wisconsin magmatic terrane, is consistent with these constraints and further refine the age limits. In the Dunbar dome, deformed granitoid rocks have U-Pb zircon ages of ca 1860 Ma, whereas an undeformed, post-tectonic granite has an age of 1835 + 6 Ma (Sins and others, 1985). These data suggest collision along the Niagara fault zone occured about 1850 Ma. Probably the thrusting in the continental margin assemblage occurred approximately contemporaneous with or slightly later than collision.

FIELD STOPS AND ROAD LOG

The structural traverse across northern Michigan starts in L'Anse, Michigan and ends near Florence , Wisconsin. Eighteen stops are shown on Figure 3 but, because of limited time and difficult access to several of the localities, only eight to ten of them will be visited. Localities 2b, 7a, and 7b, will not be visited and localities 2a, 8, and 13 are alternate stops, which will be visited only if time permits. Road logs are not given for the alternate stops, but their location is given by section, township, and range.

ROAD LOG Follow highway U. S. 41 west from Marquette to L'Anse. Proceed west through L'Anse to bridge over Falls River on U. S. 41. Park in the pulloff area on the north side of the highway between the RR tracks and the bridge.

STOP 1-FALLS RIVER. Numerous outcrops are present along Falls River on both sides of the highway. Structures consist of northward-verging, small-scale, recumbent folds with subhorizontal axial-plane foliation indicating nappe-thrust style of deformation in this area.

ROAD LOG. From the parking lot area for stop 1, follow U. S. 41 east for 2.6 miles. Turn west on Golf Course Road and drive 1.75 miles to the cul-de-sac at the end of the road. Follow footpath to Little Mountain. It is about a 15 minute walk to the to? of the mountain; be sure to follow left forks of the main, well worn path. Path extends around the southwest corner of the mountain where it turns sharply toward the east up the south flank of the mountain. Follow path to the highest point on the mountain. From this vantage you can see the Keweenaw Peninsula of the Midcontinent Rift System to the north-northwest and the Archean terrane of the northern complex (Fig. 3) to the east-southeast. The low lying area to the south is underlain by rocks of the Michigamme basin.

STOP 2-LITTLE MOUNTAIN. Outcrops at the highest point on Little Mountain consist of an unfoliated (ca 1.1 Ga) Keweenawan diabase dike and graded beds of metagraywacke. Grading is expressed by presence of foliation in the finer grained bed tops. The beds face toward the north and dip steeply south, indicating that they are overturned toward the north.

LITTLE MOUNTAIN, BARAGA COUNTY, Mt.

FIGURE 10 Field sketch mao of the northeast corner of Little

Mountain showing evidence for thrust faulting.

Figure 10, a sketch map, shows the nature of structuraldeformation near the northeast corner of Little Mourt1n. Here,similar overturned graded beds of inetagraywacke strike east--westand dip steeply south, as does Si foliation. In several placesbedding has a convoluted appearance, indicative of irtenedeformation. Deformed zones of concretions crudely definebedding. The long axes of the concretions plunge steeplysouthwest, indicating extension in this direction. old agesplunge gently towardthe west. On a bench at the easternrszedge of the mountain similar graded beds are oriented N(0 W..

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4.,

LITTLE MOUNTAIN. BARAGA COUNTY, MI.

FIGURE 10. Field sketch map of the northeast corner of Littls Mountain showing evidence for thrust faulting.

Figure 10, a sketch map, shows the nature of .?t~ii~tar5l deformation near the northeast corner of Little Mountsin. I T - kcre, sixilar overturned graded beds of netagr&ywacke strike east--west and dip steeply south, as does 31 foliation. In several places bedding has a convoluted appearance, indicative of intense deformation. Deformed zones of concretions crudely define bedding. The long axes of the ~oncretio~is plunge steeply southwest, indicating extension in this direction. Fold axes clung& aently toward the west. O n a bench at the easternrms:. edge of thft mountain similar graded beds are oriented NEC*X:

570NW, and are upright. Looking west from the bench, theintensely deformed beds of an overlying thrust can be seen in thethree-meter-high cliff. Tectonic striations can be found on theunderside of narrow overhangs on the cliff face a few meters tothe north and west. The striations, along with northwardoverturning of bedding indicate that the hanging wall of thethrust moved northeastward. The total thickness of the thrustfault zone at this locality is ameter or less. The thrust faultcan be traced down the east flank of Little Mountain. About 100meters south and two benches down the flank of the mountain aglacially-gouged, upright bed of metagraywacke which lies beneaththe fault observed above is well exposed. It too has deformedconcretions whose long axes plunge southwest.

The main point at this stop is that Little Mountain probablyis an erosional remnant of an allochthonous sheet of rocks thathas been thrust northeastward as part of an imbricate thrustsystem. Further studies at Little Mountain are needed to fullydocument the nature and style of deformation here.

ROAD LOG Return to the vehicle by walking north and west aroundthe north side of Little Mountain to the intersection of thepath. Turn right (east) on the path and return to Golf CourseRoad. Follow the road eastward to the intersection of GolfCourse Road and and U. 5. 41. Turn right (south) on U. S. 41.

ALTERNATE STOP 2a-TAYLOR MINE. Located in the NW 1/4 sec. 9,T49N, R33W, graphitic slate and iron- formation of the MichigammeFormation near the mine have been folded about a penetrativesouth-dipping, axial-plane foliation (see Fig. 7). Keels ofrootless folds in the graphitic slate indicate that bedding hasbeen transposed parallel to foliation, and that the beds areoverturned to the north. An unfoliated Keweenawan diabase dikecuts the graphitic slate. Style of folding in the iron—formation can be seen in a roadcut about 30 meters north of thedike.

Similar steeply south-dipping foliation occurs in EarlyProterozoic phyllite about 2 km to the south at Plumbago Creek(2b, Fig. 3). The phyllite is exposed in a creek bed at the baseof a steep north facing slope that is underlain by Archeangneiss. Thus, it appears that the foliation in the phyllite dipsbeneath the gneiss, suggesting that the gneiss is thrust over thephyllite.

ROAD LOG. From the intersection of Golf Course Road and U. S. 41drive 7.6 mile south on U. S. 41 to Canyon Falls Park. BetweenAlberta on U. S. 41 and Canyon Falls Park the highway passes overthe west end of the northern complex (Fig. 3) which is anuplifted block of Archean basement rock. Follow the nature pathfrom the parking lot at Canyon Falls Park westward for about 10minutes along Sturgeon River to the Falls. Proceed to the north-south oriented cliff face in back of the fence. No need to walkup the steps in the cliff face for the critical information atthis stop is visible in the cliff face.

5 7 ~ ~ ~ and are upright. Looking west from the bench, the intensely deformed beds of an overlying thrust can be seen in the three-meter-high cliff. Tectonic striations can be found on the underside of narrow overhangs on the cliff face a few meters to the north and west. The striations, along with northward overturning of bedding indicate that the hanging wall of the thrust moved northeastward. The total thickness of the thrust fault zone at this locality is a meter or less. The thrust fault can be traced down the east flank of Little Mountain. About 100 meters south and two benches down the flank of the mountain a glacially-gouged, upright bed of metagraywacke which lies beneath the fault observed above is well exposed. It too has deformed concretions whose long axes plunge southwest.

The main point at this stop is that Little Mountain probably is an erosional remnant of an allochthonous sheet of rocks that has been thrust northeastward as part of an imbricate thrust system. Further studies at Little Mountain are needed to fully document the nature and style of deformation here.

ROAD LOG Return to the vehicle by walking north and west around the north side of Little Mountain to the intersection of the path. Turn right (east) on the path and return to Golf Course Road. Follow the road eastward to the intersection of Golf Course Road and and U. S. 41. Turn richt (south) on U. S. 41.

ALTERNATE STOP 2a-TAYLOR MINE. Located in the NW 1/4 sec. 9, T49N, R33W, graphitic slate and iron- formation of the Michigamme Formation near the mine have been folded about a penetrative south-dipping, axial-plane foliation (see Fig. 7). Keels of rootless folds in the graphitic slate indicate that bedding has been transposed parallel to foliation, and that the beds are overturned to the north. An unfoliated Keweenawan diabase dike cuts the graphitic slate. Style of folding in the iron- formation can be seen in a roadcut about 30 meters north of the dike.

Similar steeply south-dipping foliation occurs in Early Proterozoic phyllite about 2 km to the south at Plumbago Creek (2b, Fig. 3). The phyllite is exposed in a creek bed at the base of a steep north facing slope that is underlain by Archean gneiss. Thus, it appears that the foliation in the phyllite dips beneath the gneiss, suggesting that the gneiss is thrust over the phyllite.

ROAD LOG. From the intersection of Golf Course Road and U. S. 41 drive 7.6 mile south on U. S. 41 to Canyon Falls Park. Between Alberta on U. S. 41 and Canyon Falls Park the highway passes over the west end of the northern complex (Fig. 3) which is an uplifted block of Archean basement rock. Follow the nature path from the parking lot at Canyon Falls Park westward for about 10 minutes along Sturgeon River to the Falls. Proceed to the north- south oriented cliff face in back of the fence. No need to walk up the steps in the cliff face for the critical information at this stop is visible in the cliff face.

STOP 3-CANYON FALLS. At this stop subhorizontal EarlyProterozoic quartzite contains prominent interference ripples

•that appear to be unaffected by Early Proterozoic deformation.Close examination of the cliff face, however, reveals that thinpelitic layers intercalated between the uartzite beds have a

penetrative foliation oriented N85°W, 27 SW. Also, tectonicstriations on the quartzite beds are oriented 20 toward NlO°E.

Clearly the quartzite has been transported some unknown distancenortheastward (based on the north sense of vergence found in

previous outcrops). Deformation was taken up by the less

competent shaly layers; the more competent quartzite beds appearto have undergone little, or no, internal deformation, butdetailed petrofabric studies are needed to confirm this.

As shown on Figure 6, the flat-lying quartzite at CanyonFalls must lie beneath one or more thrust faults that intensely

deformed Early Proterozoic strata at Taylor Mine and Plumbago

Creek. Also, elevation data, together with structural data atthese three localities necessitate local involvement of Archeanbasement rocks in the overthrusting.

ROAD LOG Return to Canyon Falls Park parking lot. Drive south on

U. S. 41 for 3.0 miles to the intersection with highway U. S.

141/28. Turn right (west) on highway 141/28 and follow it for

4.1 miles. Turn south on U. S. 141 and drive 4.1 miles to atop

4.

Numerous outcrops of the upper slate member of the

Michigamxne Formation occur along U. S. 141 south of Covington.We have selected a few of the more informative outcrops as field

stops (4, 5, 6, and 7 on Figure 6).

STOP 4-U. S. highway 141. Located on the east side of the

highway, this stop consists of thick beds of graywacke that aregraded and have flame structures at some bed contacts.

Penetrative foliation0refracts across bedding. Bedding (SO) isgenerally oriented N62 W, 20° SW, and cleavage (Si) N75°W, 4cf SW.

Lineations formed by the intersection of cleavage and bedding

plunge 140 toward S83°W. Numerous concretions form strain

ellipses with the axes of maximum elongation that plunge

generally 540 toward S36°E.

At this outcrop bedding (SO) dips only moderately toward the

south, but marked variations in the orientation of bedding as

well as cleavage (Si) can be seen in other outcrops as one

proceeds south along U. S. highway 141. Ne mainly used these

variations in structure to locate the position of thrust faults

shown on Figure 6.

ROAD LOG. Proceed south on U. S. 141 for 2.4 miles.

STOP 5-U. S. 141. A long outcrop of thick-bedded graywacke is

present on the west side of the highway. Graded beds strike.

roughly N67oW, and are nearly vertical; foliation is subparallel

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STOP 3-CANYON FALLS. At this stop subhorizontal Early Proterozoic quartzite contains prominent interference ripples that appear to be unaffected by Early Proterozoic deformation. Close examination of the cliff face, however, reveals that thin pelitic layers intercalated between the quartzite beds have a penetrative foliation oriented N89 W, 27 SW. Also, tectonic striations on the quartzite beds are oriented 2' toward N10 E. Clearly the quartzite has been transported some unknown distance northeastward (based on the north sense of vergence found in previous outcrops). Deformation was taken up by the less competent shaly layers; the more competent quartzite beds appear to have undergone little, or no, internal deformation, but detailed petrofabric studies are needed to confirm this.

As shown on Figure 6, the flat-lying quartzite at Canyon Falls must lie beneath one or more thrust faults that intensely deformed Early Proterozoic strata at Taylor Mine and Plumbaao Creek. Also, elevation data, together with structural data at these three localities necessitate local involvement of Archean basement rocks in the overthrusting.

ROAD LOG Return to Canyon Falls Park parking lot. Drive south on U. S. 41 for 3.0 miles to the intersection with highway U. S. 141/28. Turn right (west) on highway 141/28 and follow it for 4.1 miles. Turn south on U. S. 141 and drive 4.1 miles to 4.

Numerous outcrops of the upper slats member of the Michiganune Formation occur along U. S. 141 south of Covington. We have selected a few of the more informative outcrops as field stops (4, 5 , 6, and 7 on Figure 6).

STOP 4-U. S. highway 141. Located on the east side of the highway, this stop consists of thick beds of graywacke that are graded and have flame structures at some bed contacts. Penetrative foliationrefrac~s across bedding. Bedding (SO) is generally oriented N62 H, 20 SW, and cleavage (Sl) N~!?W, 4CfSH. Lineations formed by the intersection of cleavage and bedding plunge 14 toward S83 W. Numerous concretions form strain ellipses with the axes of maximum elongation that plunge generally 54O toward ~36Â¡~

At this outcrop bedding (SO) dips only moderately toward the south, but marked variations in the orientation of bedding as well as cleavage (Sl) can be seen in other outcrops as one proceeds south along U. S. highway 141. We mainly used these variations in structure to locate the position of thrust faults shown on Figure 6.

ROAD LOG. Proceed south on U. S. 141 for 2.4 miles.

STOP 5-U. S. 141. A long outcrop of thick-bedded graywacke is present on the west side of the highway. Graded beds strike. rouqhly N67oM, and are nearly vertical; foliztion is sabparallel

•to bedding. At the north end of the outcrop bedding is clearlyoverturned toward the north.

ROAD LOG Proceed south on U. S. 141 for 3.4 miles.

STOP 6-TRACY CREEK. Examine outcrops on both sides of thehighway which consist of graded graywacke beds. Folds in be-Jding(SO) have axes that plunge gently toward the 0northwest.Refracted, penetrative Si foliation is oriented N86 E, 85CSE.Concretions tend to lie in the bedding plane. Some concretions,however, have been partly transposed so that their long axes arepartly parallel to bedding and partly parallel to foliation.Many have been deformed so that their long axes are completelyparallel to foliation. This outcrop shows the style ofdeformation in the slates and metagraywackes of the MichigarnneFormation. Some outcrops on this part of the U. S. 141 traversehave two foliations as shown on Figure 6. A completelydifferent, north dipping foliation will be seen at the next stop.

ROAD LOG. Proceed south on U. S. 141 for 1.0 mile.

STOP 7-U. S. 141. Most features in this long outcrop of blackslate and metagraywacke on the east side of the highway indicatea southward sense of thrusting, completely opposite to thatobserved at all previous stops. Figure 11 is a sketchillustrating the types of features observed along the length ofthe outcrop. Most significant are points 5 and 9. At point 5

small drag folds indicate a southward sense of vergence. Thissouthward sense of- vergence is confirmed by several otherfeatures in this outcrop and indicates backthrusting at thislocation. A meter-wide zone of siginoidal quartz veins at point 9is one of the more striking features in this outcrop. Thesigrnoidal shape of the quartz veins suggests down-to-the--northnormal faulting, opposite the sense of movement observed at point5. The structure at point 9 may be a late feature related topost-thrusting uplift of the Aniasa dome. Such uplift would causerotation of pre-existing features, but the amount of rotation isnot known.

We interpret the backthrusting at this locality to be causedby an abrupt change in dip of the thrust sytem as it ramps offthe north edge of the Amasa uplift (see Fig. 6).

ROAD LOG. Proceed south on U. S. 141 for a distance of 23 milesto the junction of U. S. 141 with U. S. highway 2 at the west endof Crystal Falls, Michigan. On this segment of the traverse, thehighway passes over the west flank of the Ainasa uplift. Turn left(east) on U. S. 141/2 and follow it to the center of CrystalFalls, where the highway intersects M 69. This is at theintersection of Superior and Fifth street, near the U. S. PostOffice in Crystal Falls. To proceed to alternate stop 8, turneast on M 69. To continue on to stop 9 proceed south n U. .2/141.

C-24

to bedding. At the north end of the outcrop beddin5 is clearly overturned toward the north.

ROAD LOG Proceed south on U. S. 141 for 3.4 miles.

STOP 6-TRACY CREEK. Examine outcrops on both sides of the highway which consist of graded graywacke beds. Folds in be3ding (SO) have axes that plunge gently toward the northwest. Refracted, penetrative Sl foliation is oriented N86 E, 8 9 SE. Concretions tend to lie in the bedding plane. Some concretions, however, have been partly transposed so that their long axes are partly parallel to bedding and partly parallel to foliation. Many have been deformed so that their long axes are completely parallel to foliation. This outcrop shows the style of deformation in the slates and metagraywackes of the Michigamne Formation. Some outcrops on this part of the U. S. 141 traverse have two foliations as shown on Figure 6. A completely different, north dipping foliation will be seen at the next stop.

ROAD LOG. Proceed south on U. S. 141 for 1.0 mile.

STOP 7-U. S. 141. Most features in this long outcrop of black slats and metagraywacke on the east side of the highway indicate a. southward sense of thrusting, completely opposite t.o that observed at all previous stops. Figure 11 is a sketch illustrating the types of features observed along the length of the outcrop. Most significant are points 5 and 9. At pint 5 small drag folds indicate a southward sense of veqencs. This southward sense of- vergence is confirmed by several other features in this outcrop and indicates backthrusting at this location. A meter-wide zone of sigmoidal quartz veins at point 9 is one of the more striking features in this outcrop. The sigrrioidal shape of the quartz veins suggests down-to-the-north normal faulting, opposite the sense of movement observed at point 5. The structure at point 9 may be a late feature related to post-thrusting uplift of the Amasa dome. Such uplift would cause rotation of pre-existing features, but the amount of rotation is not known.

We interpret the backthrusting at this locality to be caused by an abrupt change in dip of the thrust sytem as it ramps off the north edge of the Amasa uplift (see Fi9. 6).

ROAD LOG. Proceed south on U. S. 141 for a distance of 23 miles to the junction of U. S. 141 with U. S. highway 2 at the west end of Crystal Falls, Michigan. On this seqmit of the traverse, the highway passes over the west flank of tha Amasa uplift. Turn left (east) on U. S. 141/2 and follow it to the center of Crystal Falls, where the highway intersects M 69. This is at the intersection of Superior and Fifth street, near the 3. S . Post Office in Crystal Falls. To proceed to alternate stos 8, turn - - - east on M 69. To continue on to stop 9 proceed south on TJ. S . 2/14l.

STOP 7 ...- NORTH

FIGURE 11. Sketch of roadcut at stop 7, east side of roashowing location and attitude of some key structural features.

ALTERNATE STOP 8-OLD M69. This stop is located in the NW 1/4,

sec. 20, T43N, R31W, about 500 meters east of the bridge over theMicigamme0River on Old M 69. Here thinly layered tuff, orientedN20 W, 50 Sw, has a penetrative Si foliation oriented NG5CW,

79 SW. This northwest—trending foliation is characte;istic ofthe structural fabric in the region (west side of the Amasauplift), as shown on Figure 3.

ROAD LOG. To reach stop 9 from the center of Crystal Falls,proceed south on U. S. 2/141 for roughly 6 miles, to the road onthe east to Horse Race Rapids and Iron County Airport. Turn left(east) and follow this road for 3.5 miles to parking area atHorse Race Rapids. Follow path down slope to plentiful outcropsalong the river.

STOP 9-HORSE RACE RAPIDS. This outcrop area is south of gushLake Fault (see Fig. 3). At this locality graded beds (SO) ofgraywacke stratigraphically face south, and form a system offolds that have a subhorizontal axial—plane foliation (1). Fcldaxes in SO plunge gently westward. Si foliation has beenrefolded in some places on fold axes that are nearly coaxial witlithe fid axes in SO. Syntectonic quartz veins occur in the axes

C-25

•• NC — —Ua 4 g! :0C •• '8 •6

V00

• Vo 0a a• 2U U

C0VV

I-I-I I

0 10 meters

STOP 7 NORTH

0 10 meters

FIGURE 11. Sketch of roadcut at stop 7, east side of road, showing location and attitude of some key structural features.

ALTERNATE STOP 3-OLD M69. This stop is located in the MW 1/4, sec. 20, T43N, R31W, about 500 meters east of the bridge over the Ki~~igammeRiver on Old M 69. Here thinly layered tuff, oriented N2j W, 50 SW, has a penetrative Sl foliation oriented N S ~ W , 79 SW. This northwest-trending foliation is characteristic of the structural fabric in the region (west side of the Amass uplift), as shown on Figure 3.

ROAD LOG. To reach stop 9 from the center of Crystal Falls, proceed south on U. S. 2/141 for roughly 6 miles, to the road on the east to Horse Race Rapids and Iron County Airport. Turn left (east) and follow this road for 3.5 miles to parkinq area at Horse Race Rapids. Follow path down slope to plentiful oiitcr~ps alone the river.

STOP 9-HORSE RACE RAPIDS. This outcrop area is south of 3usl-1 Lake Fault ( see Fig. 3). A t this locality graded beds (SO) of qraywacke stratigraphically face south, and forn a systsm of folds that have a subhorizontal axial-plane foliation (.^1). Fcld axes in SO plunce gently westward. Sl foliation has been refolded in some places on fold axes that are n ~ - i r l y coaxial with the fold axes in SO. Syntectonic quartz veins occur in the axes

of some folds. We interpret this outcrop to be part of thesouth—verging backthrust system, the same as that found in theFeich Trough—Caluinet trough area (see Fig. 3).

ROAD LOG. Return to the intersection of the Horse Race Rapidsroad with U. S. highway 2/141. Turn left onto U. S. 2/141 andproceed east for 18.9 miles tc the intersection with M 95. Turnleft (north) on M 95 and follow 10.2 miles to the intersectionwith County Road 569 which goes eastward to Feich, MI. Follow CO569 east through Feich for 14.9 miles to stop 10 (about 1.5 mileswest of the village of Foster City).

FIGURE 12. Map of east end of Felch trough (adapted from Jamesand others, 1961) showing locations of stops 10 and 11. Smalllower hemisphere stereoplots show orientation of low-dippinç 1foliation; large lower hemisphere stereoplot Shoj steep (Sa)foliation in Archean gneiss. Black dots in the stereoplcts showorientation of lineatior formed by the intersecticn of 3a and 1.Black dots outside the stereoplots show location of outcrops.

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Mile

of some folds. We interpret this outcrop to be part of the south-verging backthrust system, the same as that found in the Felch Trough-Calumet trough area (see Fig. 3 ) .

ROAD LOG. Return to the intersection of the Horse Race Rapids road with U. S. highway 2/141. Turn left onto U. S. 2/141 and proceed east for 18.9 miles to the intersection with M 95. Turn left (north) on M 95 and follow 10.2 miles to the intersection with County Road 569 which goes eastward to Felch.. MI. Follow CO 569 east through Felch for 14.9 miles to stop 10 (about 1.5 miles west of the village of Foster City).

FIGURE 12. Map of east end of Felch trough (adapted from J a w s and others, 1961) showing locations of stqs 10 and 11. Small lower hemisphere stereoplots show orientation of low-dippin5 nl foliation; large lower hemisphere stereoplot shows staep (Sa) foliation in Archean gneiss. Black dots in the stereoplcts show orientation of lineatior formed by the intersectio' o f 3.3 an>-"! r^-- Black dots outside the stereoplots show location of outcrops.

STOP 10:_CO 569. Granitoid gneiss on the north side of the roadhas two distinct foliations. The steeper foliation,0 hereincalled Sa, is an Archean fabric that strikes about N85W, anddips nearly vertical (see large stereoplot on Fig. 12). A sub-horizontal foliation (Si), expressed by0aligned zones and clotsof biotite, is oriented about Nb W, 25 Nfl. Gneissic foliationand fold axes in gneissic foliation in Archeari rocks throughoutthe Felch trough region are steep. The nearly flat lyingpenetrative Si foliation is found throughout the Feich trougharea, as shown on Figure 12, but because the Archean fabric (Sa)is generally more prominent, it is difficult to recognize Si (forexample, climb to the large outcrop area in back of the roadcutand look for Si on this nearly horizontal surface). The smallstereoplots on Figure 12 shows that the Si foliation is irregularin orientation. The next stop shows that Si also exists in EarlyProterozoic rocks.

ROAD LOG. Turn around at stop 10 and proceed west on CO 569 for6.7 miles to the intersection with Old M 69. Proceed south(left) on M69 for only 0.25 miles to the bridge.

STOP 11—OLD M 69. Figure i3A isa geologic map showing asequence from north to south of roughly vertically beddedSturgeon Quartzite and Randville Dolomite sandwiched betweenArchean gneiss blocks. The fact that the stratigraphically olderquartzite lies north of the dolomite indicates that the sectionfaces southward. Also, bedding in the dolomite on the north sideof the river dips steeply north suggesting slight overturn ofbedding toward the south. The dolomite has a subhorizontalfoliation (Si) that is axial planar to small folds in it.

There are two distinct foliations in the Archean gneisssouth of the river. The steep dipping Archean foliation (Sa) iscrosscut by low-dipping Early Proterozoic foliation (SI), thesame subhorizontal foliation as is found in the RandvilleDolomite, but note that the dolomite does not have a steepfoliation.

Our interpretation of the structure at stop 11 is shown incross section A-A' (Fig. 13a). We suggest that the Archeangneiss has been thrust above the Sturgeon Quartzite and RandvilleDolomite, which are exposed as a window in the Archean thrustsheet. Southward facing stratigraphy indicates southwardvergence in the thrust system. The thrusting created the Sisubhorizontal foliation in both Early Proterozoic and Archeanrocks, but the steep Sa fabric occurs only in the Archean gneiss.

Figure 13D shows the thrust system in a broader perspective,and provides additional key evidence for southward vergence inthe Felch trough area. Although the exact configuration of thethrust faults is not known at this time, the drag fold whichoccurs in Sturgeon Quartzite, as diagrarnatically shown on crosssection B-B', unequivocally indicate southward vergence. Thedrag fold actually occurs about a mile west of the profile at thesame structural position along the fault. We will not see it

STOP 10LCO 569. Granitoid gneiss on the north side of the road has two distinct foliations. The steeper foliation, herein called Sat is an Archean fabric that strikes about N85 W, and dips nearly vertical (see large stereoplot on Fig. 12). A subhorizontal foliation (Sl), expressed byaligned zones and clots of biotite, is oriented about N10 W, 25 NE. Gneissic foliation and fold axes in gneissic foliation in Archean rocks throughout the Felch trough region are steep. The nearly flat lying penetrative Sl foliation is found throughout the Felch trough area, as shown on Figure 12, but because the Archean fabric (Sa) is generally more prominent, it is difficult to recognize Sl (for example, climb to the large outcrop area in back of the roadcut and look for Sl on this nearly horizontal surface). The small stereoplots on Figure 12 shows that the Sl foliation is irregular in orientation. The next stop shows that Sl also exists in Early Proterozoic rocks.

ROAD LOG. Turn around at stop 10 and proceed west on CO 569 for 6.7 miles to the intersection with Old M 69. Proceed south (left) on M69 for only 0.25 miles to the bridge.

STOP 11-OLD M 69. Figure 13A is a geologic map showing a sequence from north to south of roughly vertically bedded Sturgeon Quartzite and Randville Dolomite sandwiched between Archean gneiss blocks. The fact that the stratigraphically older quartzite lies north of the dolomite indicates that the section faces southward. Also, bedding in the dolomite on the north side of the river dips steeply north suggesting slight overturn of bedding toward the south. The dolomite has a subhorizontal foliation (Sl) that is axial planar to small folds in it.

There are two distinct foliations in the Archean gneiss south of the river. The steep dipping Archean foliation (Sa) is crosscut by low-dipping Early Proterozoic foliation (Sl), the same subhorizontal foliation as is found in the Randville Dolomite, but note that the dolomite does not have a steep foliation.

Our interpretation of the structure at stop 11 is shown in cross section A-A' (Fig. 13a). We suggest that the Archean gneiss has been thrust'above the Sturseon Quartzita and Ra-ndville Dolomite, which are exposed as a window in the Archean thrust sheet. Southward facing stratigraphy indicates southward vergence in the thrust system. The thrusting created the Sl subhorizontal foliation in both Early Proterozoic and Archean rocks, but the steep Sa fabric occurs only in the Archean gneiss.

Figure 13B shows the thrust system in a broader perspective, and provides additional key evidence for southward vergence in the Felch trough area. Although the exact configuration of thr thrust faults is not known at this time, the drag fold which occurs in Sturgeon Quartzite, as diagramatically shown on cross section B - B ' , unequivocally indicate southward vergence. mu lac drag fold actually occurs about a mile west of the profile at the same structural position along the fault. We will not see it

o ä 2000 feltI-

0 500 meters

Younger sandstone

Randville DolomiteVulcan Iron Fm.

Sturgeon Ouartzlte

______

Archean gneiss

0 1000 feetI I

0 300 meters

ANorth 7,A/

•1

Strike and dip of foliation

Strik, and dip of bedding

FIGURE 13. Geologic maps in area of stop 11 (adapted from Jamesand others, 1961). Map A and geologic cross section shows the

structural relationship between Archean gneiss and Tarly

Prcterozoic Sturgeon Quartzite and andville Dolomit€. Map and

structural cross section is of a larger area (note location of

map A and drag fold). These maps show that low-dipping fnliation(Si) occurs in both Archean and Early Proterozoic rocks, whereas

the steeper foliation (ca) occurs only in the Archean gnei.;s.

The maps and cross sections also present evidence for soi.:thward

structural vergence. See text for further discussion.

C- 28

B B

X X X X X X X X XX X X X X A

Cover

X XXX XXX XX)

A AXXX XXXXX X

XXX X X

X X X X XXXX XXX

• .m45?. .•

Xn XU•V

X XX

X X

X

ounger red

Sondstone Cover

1!'

B

X )C XXX )( XXXXX X

X XX X

30cm0.6 cm

Drag folds'I

Ix I

'aSense of vergence

pP' Thrust fault

Outcrop

Fault and sense of movement

x x x x x x x x x x x x x x x x -

0 500 meters

Younger sandstone

Randviile Dolomite vulcan Iron Fm.

Sturgeon Quartzite

Archean gneiss

0 I 0 0 0 feet A- - 0 300 meters

/ North A

9 7, A'

dm Strike and dip of foliation

> Strike and dip af bedding

V sense of vergence

/ Thrust fault - Outcrop

4 Fault and sense of movement

FIGERE 13. Geologic maps in area and others, 1961). Map A and qeologic Gross seciion shoiis thc structural relationship between Archean gneiss and Ezrly Prctsrozoic Sturgeon Quartzits and Xandville Dolomite. :.:as Z xu? structural cross section is of a largsr area (note l~cction of map A and drag fald). These maps show that low-dig~inq fzliation (Sl) occurs in both Archeari and Early Proterozoic rocks: ~herezs the steeper foliation (Zs) occurs only in the Archean gneiss. T h s nass an2 cross sections also prssent evidence for s~1:thmrZ structural vergeiice. Sse text for further fiiscussion.

because of difficult access to the outcrops.

In summary, several key points can be made from this stop:1) Sa fabric is found only in Archean rocks (and not in EarlyProtérozoic rocks), thereby proving an Archean age for Sa.2)Sub-horizontal Si fabric is found in both Archean and EarlyProterozoic rocks, thereby proving that it is not older thanEarly Proterozoic age. 3) Thrust vergence is toward the south inthe Felch trough area.

ROAD LOG. Turn around and return to intersection of Old 69 andCO 569 (a distance of 0.25 miles). Turn left (west) on 569 andfollow it for 8.2 miles to intersection with M 95. Turn left(south) on M 95 and follow it for 5.4 miles to Steele Road(Swanson Road on right). Turn left on Steele Road and follow itto "THE STEELE FARM" at end of road. After checking withresidents, go through gate to field on the east. Outcrops are inthe southern part of the field about 400 m east of thefarmhouse.

STOP 12-STEELE FARM. This outcrop of Michigamme Formation is inthe Calurnet Trough (Fig. 3). Here graded beds (SO) of.rnetagraywacke, now quartz—sericite-biotite schist, have aprominent subhorizontal foliation (Si) that ref racts acrossbedding. Si is oriented roughly N90 E, 25 SE, and is axialplanar to folds in the metagraywacke that plunge gentlynorthwest. Bedding strikes roughly east and dips steeply south.Graded beds and flame structures in the metagraywacke indicatethat stratigraphic facing is toward the south. Both SO and Sihave been refolded with a steeper foliation (S2) that strikesapproximately N40°E. Also, crinkle folds on the Si surfacesindicate post-Fl folding.

In summary, this outcrop indicates that there is a prominentsubhorizontal foliation in Michigarnme strata in the Calurnettrough. Sedimentary structures indicate that bedding facestoward the south. This implies that SO has been rotated towardthe south and that the Calumet trough is a south—vergingstructure.

ROAD LOG. Return to M 95 and turn left (south). The turn toalternate stop 13 is 1.0 mile south of Steele Road on M 95 at theintersection of M 95 with Sportsman Club Road. Turn left (east)on Sportsman Club Road.

ALTERNATE STOP 13. This is a large outcrop area of theMichigamme Formation in the Calumet trough located in the E1/2,sec 21, T41N, R28W at a sharp bend in the Sturgeon River. Therocks here are metagraywacke, now schist and phyllite. They havea subhorizontal foliation (Si) which is axial planar to numerousdrag folds in bedding (SO), which indicate south vergence. Sifoliation forms a gently undulating surface which, in places, hasbeen folded with an S2 axial plane that strikes northwest nd isteeply dipping. Like the strata at stop 12, this outcrop alsoshows that the Calumet trough is a south verging structure.

because of difficult access to the outcrops.

In summary, several key points can be made from this stop: 1) Sa fabric is found only in Archean rocks (and not in Early Proterozoic rocks)f thereby proving an Archean age for Sa. 2)Sub-horizontal Sl fabric is founG in both Archean and Early Proterozoic rocks, thereby proving that it is not older than Early Proterozoic age. 3) Thrust vergence is toward the south in the Felch trough area.

ROAD LOG. Turn around and return to intsrsection of Old 69 and CO 569 (a distance of 0.25 miles). Turn le2t (west) on 569 and follow it for 8.2 miles to intersection with M 95. Turn left (south) on M 95 and follow it for 5.4 miles to Steele Road (Swanson Road on right). Turn left on Steele Road and follow it to "THE STEELE FARM" at end of road. After checking with residentsf go through gate to field on the east. Outcrops are in the southern part of the field about 400 m east of the farmhouse.

STOP 12-STEELE FARM. This outcrop of Michigamme Formation is in the Calumet Trough (Fig. 3). Here gradsd beds ( S O ) of metagray~acke~ now quartz-sericite-biotite schist! have a prominent subhorizontal foliation (SJ.) tha& refracts across bedding. Sl is oriented roughly N90 E, 25 SE, and is axial planar to folds in the metagraywacke that plunge gently northwest. Bedding strikes roughly east and dips steeply south. Graded beds and flame structures in the metagraywacke indicate that stratigraphic facing is toward the south. Both SO and Sl have been refolded with a steeper foliation (S2) that strikes approximately ~ 4 0 ~ ~ . Alsof crinkle folds on the Sl surfaces indicate post-Fl folding.

In summary, this outcrop indicates that there is a prominent subhorizontal foliation in Michigamme strata in the Calumet trough. Sedimentary structures indicate that bedding faces toward the south. This implies that SO has been rotated toward the south and that the Calumet trough is a south-verging structure.

ZOAD LOG. Return to M 95 and turn left (south). The t u m to alternate stop 13 is 1.0 mile south of Steele Road on M 9 5 at the intersection of 14 95 with Sportsman Club Road. Turn left (east) on S~ortsnan Club Road.

ALTEFUJATE STOP 13. This is a lsrcje outcrop area of the Michigaame Formation in the Calumet trough locatsd in the E1/2, sec 21f T41?Jf R28X at a sharp bend in the Sturgeon Riwr. The rocks here are metagra~wacke~ now schist and phyllits. They have a sub?iorizontal foliation (Sl) which is axial plan~r to numerous draq folds in beddin9 (SO)f which indicate south vergsnce. S l foliation forms a gently undulating surface which, in placzsf has been fol6ed with an S2 axial plane that strikes northvest and is steeply dipping. Like the strata at stop 12f this outcrop also shows that the Calumet trough is a south verging structure.

ROAD LOG. To reach stop 14 from the intersection of Steele Roadand M 95, proceed south on M 95 for a distance of 4.8 miles tothe intersection with U. S. 2/141. Turn right and proceed weston U. S. 2/141 for approximately 11 miles to the center of thetown of Florence, Wisconsin. Turn left (south) on County Road Nin the center of Florence and proceed south for 3.3 miles to theintersection with County Road D. Turn right on CO D and go 1.3miles west to a gravel road labelled "Site 34". Follow thisroad for 3.4 miles to the Pine River dam. From the parking lotnear the dam walk onto the dam embankment to a chain link fencethat blocks access to the discharge flume from the darn. Turnleft and walk down the embankment next to the chain link fencefor about 400 feet (130) meters to outcrops along the river.

STOP 14-PINE RIVER FLOWAGE. This outcrop lies within the Niagarafault zone. The rocks here consist of strongly foliatedchloritic-garnetiferous schist. Primary layering has beentransposed parallel to Si foliation. Dextral drag folds in thetransposed layering have steeply plunging axes, which areparallel to mineral lineations on the foliation surfaces xesof gaximum elongation in deformed concretions plunge 60 towards80 W, parallel to the mineral lineation.

The deformation shown in this outcrop is completelydifferent from that seen in the previous outcrops. Asubhorizontal foliation is lacking as is any evidence for a

southward structural vergence. Rather, these highly strainedrocks have an Si foliation that dips steeply south, as do theelongation (stretching) axes. As defined by Ueng and others(1984), this stop lies within the Florence-Niagara terrane,which is characterized by a west-northwest—trending, steeplysouth-dipping foliation that contains a prominent south—plungingstretch lineation. Similarly oriented structures occur involcanic rocks of the Wisconsin magmatic terrane on the southmargin of the Niagara fault zone, a few miles south of the PineRiver dam (Sims and others, 1985). The structural fabric in thisregion reflects overthrusting of the Wisconsin magmatic terranefrom the southeast onto the continental margin.

ACKNOWLEDGEMENTSThis guide benefited from conversations with W. F. Cannon,

G. L. LaBerge, B. W. Ojakangas, Z. E. Peterman, and K. J. Schulzwho toured with us in the area. It also benefited from criticalreviews by N. Foose and T. Off ield. Z. IL Peterman provided thegeochronological data on the granite. Dale Beaver drafted theillustrations.

0-30

REFERENCES

Anderson, R. fl., and Black, R. A., 1983, Early Proterozoicdevelopment of the southern Archean boundary of the Superiorprovince in the Lake Superior region (abs.): Geological Sco-iety of America Abstracts with Programs, v.15, p.515.

Attoh, K., and Vander Muelen, M. J., 1984, Metamorphictemperatures in the Michigamme Formation compared with ther-mal effect of an underlying intrusion, northern Michigan:Journal of Geology, v.91, pp.417-432.

Barovich, K. M., Patchett, P. J., Peterman, Z. E., and Sims, P.K., 1987, Origin of 1.9 Ga Penokean continental crust of theLake Superior region (abs.): EOS, Transactions of the Amen-can Geophysical Union, v.68, p.1547.

Bayley, R. W., 1959, Geology of the Lake Mary Quedrangle, IronCounty, Michigan: U. S. Geological Survey Bulletin 1077, ll2p.

Bayley, R. W., Dutton, C. E., and Lamey, C. ., l966 neology ofthe Menominee iron-bearing district, Dickinson County, Michi-gan and Florence and Mannette Counties, Wisconsin: U. S.

Geological Survey Professional Paper 513, 96 p.

Cambray, F. W., 1978, Plate tectonics as a model for theenvironment of deposition and deformation of Early Proterozcic(Precambrian X) of northern Michigan (abs.): Geological Soc-iety of America Abstracts with Programs, v.10, p.376.

Cannon, W. F., 1973, The Penokean orogeny in northern Michigan,in G. M. Young (ed.), Huronian stratigraphy and sedimenta-tion: Geological Association of Canada Special Paper 12,pp.253—27l.

0 0Cannon, W. F. 1983, Bedrock Geologic map of the Iron River 1 x 2

quadrangle, Michigan and Wisconsin: U. S. Geological SurveyMiscellaneous Investigation Series Map 1-1360-B, 1:250,000scale.

Cannon W. F., and Gair, J. E., 1970, A revision of stratigraohicnomenclature of Middle Precambrian rocks in northern ichi-gan: Geological Society of America Bulletin, v.31, pp.2843-234g.

Cannon, W. F., and Klasner, J. S., 1972, Guide to Penokeandeformational style and regional metamorphism of the westrriMarquette Range, Michigan: Field Trip descriptions and RoadLogs, W.1. Rose (ed.), 18th Annual Institute on Lake SuperiorGeology, Houghton, MI, pp.B1-B38.

Cannon, W. F. and Klasner, Cr. S., 1975, 3tratgraphicrelationsiips within the r3araga Group of Precainbniaii Fje,central Upper Peninsula, Mchgan: U. S. Geological SurveyJournal of esearch, v.3, pp.47-51.

C-31

REFERENCES

Anderson, R. R . , and Black, R. A., 1983, Early Proterozoic development of the southern Archean boundary of the Superior province in the Lake Superior region (abs.): Geological Scc- iety of America Abstracts with Programs, v.15, p.515.

Attoh, K., and Vander Muelen, M. J., 1984, Metamorphic temperatures in the Michigaxiime Formation compared with ther- mal effect of an underlying intrusion, northern Michigan: Journal of Geology, v.91, pp.417-432.

Barovich, K. M., Patchett, P. J., Peterman, 2. E., and Sims, P. K., 1987, Origin of 1.9 Ga Penokean continental crust of the Lake Superior region (abs.): EOS, Transactions of the Ameri -can Geophysical Union, v.68, p.1547.

Bayley, R. W., 1959, Geology of the Lake Mary Quadrangle, Iron County, Michigan: U. S. Geological Survey Bul.letin 1077, 112p.

Bayley, R. W., Dutton, C. E., and Lamey, C. A . , 1966, Ceolo~y of the Menominee iron-bearing district, Dickinson County, Michi- gan and Florence and ?4annette Counties, Wisconsin: U. S. Geological Survey Professional Paper 513, 96 p.

Cambray, F. W., 1978, Plate tectonics as a model for the environmsnt of deposition and deformation of Early Proter0z~T-c (Precambrian X) of northern Michigan (abs.): Geological Soc- iety of America Abstracts with Programs, v.10, p.376.

Cannon, W. F., 1973, The Penokean orogeny in northern Michiga-n, in G. M. Young (ed.), Huronian stratigraphy and sedi-nenta- tion: Geological Association of Canada Special Paper 12, pp.253-271.

0 0

Cannon, W. F. 1983, Bedrock Geologic ma? of the Iron River 1 x 2 quadrangle, Michigan and Wisconsin: 3. 3. Geological Survey Miscellaneous Investigation Series Map I-1360-B, 1.:250,000 scale.

Cannon W. F., and Gair, J. E., 1970, A revision of strati~raoi~ic nomenclature of Middle Precambrian rocks in northern N'ii:hi- gan: Geological Society of America Bulletin, v.81, rn2843-234E.

Cannon, W. F., and Klasner, J. S., 1972, Guide to Penokean deformational style and regional metamorphism of the W F S ~ , ? ~ ~

Marquette Range, Michigan: Pi2ld Tri? descriptions and Road Logs, W.I. Rose (ed.), 13th Annual Institute on Lal;e Superior Geology, IIoughton, XI, pp.Bl-B38.

Cannon, W. F. and Klasner, J. S., 1975, Stratis:ophic relationships within the Earaga Group of Precainbria~i q e , central Upper Peninsula, Michigan: U. S. Geolocicsl ?urv~-/ Journal of 2esearc3, v.3, pp.47-51..

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Holst, T. B., 1982, Evidence for multiple deformation during thePenokean orogeny in the Middle Precambrian Thompson Formation,Minnesota: Canadian Journal of Earth Sciences, v.19, pp.2O43-2047.

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Greenburg, J. K., and Brown, B. A., 1983, Lower Proterozoic volcanic rocks and their setting in the southern Lake Superior district: Geological Society of America Memoir 160, pp.67-84.

Hatcher, R. D. and Williams, I?. I., 1986, Mechanical model for single thrust sheets, Part I: Taxonomy of crystalline thrust sheets and their relationship to the mechanical behavior of orogenic belts: Geological Society of America Bulletin, v.97, pp.975-985. 1

Holst, T. B., 1982, Evidence for multiple deformation during the Penokean orogeny in the Middle Precambrian Thompson Formation, Minnesota: Canadian Journal of Earth Sciences, v.19, pp.2043- 2047.

Holst, T. B., 1984, Evidence for nappe development during the Early Proterozoic Penokean orogeny, Minnesota: Geology, v.12, pp. 135-138.

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James, H. L., Clark, L. D., Lamey, C. A., and Pettijohn, F. J., 1961, Geology of central Dickinson County, Michigan: U. S . Geological Survey Professional Paper 310, 176p.

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Klasner, J. S., 1972, Style and sequence of deformation and associated metamorphism due to the Penokean Orogeny in the western Marquette Range, northern Michigan: Ph. D. Disserta- tion, Michigan Technological University, Houghton, MI, 132 p.

Klasner, J. S., 1978, Penokean deformation and associated metamorphism in the western Marquette Range, northern Michi-. . gan: Geological Society of America Bulletin, v.89, pp.711-722.

Klasner, J. S., and Attoh, K., 1986, Tectonic implications from metamorphic and gravity data at Peavy and Republic metamorphic nodes, northern Michigan (abs.): Geological Society of America Abstracts with Programs, v.18, no. 6, p. 658.

Klasner, J. S., and Osterfeld, D., 1984, Gravity models of gneiss domes and a granitic pluton in northeastern Wisconsin (abs.): 30th Annual Institute on Lake Superior Geology, Wausau, WI, p.24.

Klasner, J. S., King, E. R., and Jones, w. J., 1985, Geologic interpretation of gravity and magnetic data for northern Michigan and Wisconsin, in W. J. Hinze (ed.). The Utility of gravity and magnetic anomaly maps: Society of Exploration Geophysicists, pp.267-286.

Klasner, J. S., Ojakangas, R. W., Schulz, K. J., and LaBerge, G. L, 1988, Widespread evidence for overthrusting in Early Proterozoic rocks in northern Michigan (abs.): Geological Association of Canada and Mineralogical Association of Canada Programs with Abstracts, 1988 Annual Meeting, in press.

LaBerge, G. L., Schulz, K. J., and Myers, P. E., 1984, The plate tectonic history of north-central Wisconsin (abs.): 30th Annual Institute on Lake Superior Geology, Wausau, WI, pp. 25-27.

Larue, D.K., and Sloss, L.L., 1980, Early Proterozoic sedimentary basins of the Lake Superior region: Geological Society of America Bulletin, v.91, part 1, pp.450-452, part 2, pp.1836- 1874.

Maharidge, A. D., 1986, The structural and tectonic history of a portion of the Felch Trough, central Dickinson County, Michi- gan: Master of Science Thesis, Bowling Green University, Bowling Green, OH, 67 p.

Morey, G. B., Sims, P. K., Cannon, W. F. Mudrey, M. G. Jr., and Southwick, D. L . , 1982, Geologic map of the Lake Superior region: Minnesota, Wisconsin, and northern Michigan: Minnesota Geological Survey State Map series S-13, 1:1,000,000 scale.

Nelson, K. D., Lillie, R. J., deVoogd, B., Brewer, J. A., Oliver,J. E., Kaufman, S., and Brown, L., 1982, COCORP seismicreflection profiling in the Ouachita Mountains of westernArkansas: Geometry and geologic interpretation: Tectonics:v.1, pp. 413—430.

Schulz, K. J., 1983, Geochemistry of volcanic rocks ofnortheastern Wisconsin (abs): 29th Annual Institute on LakeSuperior Geology, Houghton, MI, pp.39-40.

Schulz, K. J., 1984, Volcanic rocks of northeastern Wisconsin.Field Trip 1: Guide to geology of the Early Proterozoic rocksin northeastern Wisconsin: 30th Annual Institute on Lake Su-perior Geology, Wausau, WI, pp.51-93.

Schulz, K. J., 1987a, An Early Proterozoic ophiolite in thePenokean orogen (abs.): Geological Association of CanadaAbstracts with Programs, v.12, p.87.

Schulz, K. J., 1987b, Early Proterozoic evolution of a riftedcontinental margin in the Lake Superior region (abs.): Geo-logical Society of America Abstracts with Programs, v.19,p.243.

Schulz, K. J., LaBerge, G. L., Sims, P. K., Peterman, Z. E., andKiasner, J. S., 1984, The volcanic-plutonic terrane of nor-thern Wisconsin: implications for Early Proterozoic tectonism,Lake Superior region (abs.): Geological Association of CanadaAbstracts with Programs, v.9, p.103.

Sedlock, R. L., and Larue, D. K., 1985, Fold axes oblique to theregional plunge and Proterozoic terrane accretion in southernLake Superior region: Precambrian Research, v.30, pp.249-262.

Sikkala, K. M., 1987, A structural analysis of Proterozoicrnetasediments, northern Falls River, Baraga County, Michigan:Master of Science Thesis, Michigan Technological University,Houghton, MI, 103 p.

Sikkala, K. M., and Gregg, W, J., 1987, A structural analysis ofProterozoic metasediments, northern Falls River, Baraga County,Michigan (abs.): 33 Annual Institute on Lake Superior Geology,v. 33, part 1, pp. 65—66.

Sims, P. K., 1980, Boundary between Archean greenstone and gneissterranes in northern Wisconsin and Michigan: Geological Soc-iety of America Special Paper 182, pp.113-123.

Sims, P. K., 1987, Metallogeny of Archean and Proterozoicterranes in the Great Lakes region -- A brief overview: U. S.U. S. Geological Survey Bulletin 1964, pp. 56-74.

Nelson, K. D., Lillie, R. J., deVoogd, B., Brewer, J. A . , Oliver, J. E., Kaufman, S., and Brown, L., 1982, COCORP seismic reflection profiling in the Ouachita Mountains of western Arkansas: Geometry and geologic interpretation: Tectonics: v.1, pp. 413-430.

Schulz, K. J., 1983, Geochemistry of volcanic rocks of northeastern Wisconsin (abs): 29th Annual Institute on Lake Superior Geology, Houghton, MI, pp.39-40.

Schulz, K. J., 1984, Vol.canic rocks of northeastern Wisconsin. Field Trip 1: Guide to geology of the Early Proterozoic rocks in northeastern Wisconsin: 30th Annual Institute on Lake Su- perior Geology, Wausau, WI, pp.51-93.

Schulz, K. J., 1987a, An Early Proterozoic ophiolite in the Penokean orogen (abs.): Geological Association of Canada Abstracts with Programs, v.12, p.87.

Schulz, K. J., 1987b, Early Proterozoic evolution of a rifted continental margin in the Lake Superior region (abs.): Geo- logical Society of America Abstracts with Programs, v.19, p.243.

Schulz, K. J., LaBerge, G. L., Sims, P. K., Peterman, 2 . E . , and Klasner, J. S., 1984, The volcanic-plutonic terrane of northern Wisconsin: implications for Early Proterozoic tectonisin, Lake Superior region (abs.): Geological Association of Canada Abstracts with Programs, v.9, p.103.

Sedlock, R. L., and Larue, D. K., 1985, Fold axes oblique to the regional plunge and Proterozoic terrane accretion in southern Lake Superior region: Precambrian Research, v.30, pp.249-262.

Sikkala, K. M., 1987, A structural analysis of Proterozoic metasediments, northern Falls River, Baraga County, Michigan; Master of Science Thesis, Michigan Technological University, Houghton, MI, 103 p.

Sikkala, K. M., and Gregg, W, J., 1987, A structural analysis of Proterozoic metasediments, northern Falls River, Baraga County, Michigan (abs.): 33 Annual Institute on Lake Superior Geology, v. 33, part 1, pp. 65-66.

Sims, P. K., 1980, Boundary between Archean greenstone and gneiss terraries in northern Wisconsin and Michigan: Geoloqical Soc- iety of America Soecial Paper 182, pp.113-123.

Sims, P. K., 1987, Metallogeny of Archean and Proterozoic terranes in the Great Lakes region -- A brief overview: U. S. U. S. Geological Survey Bulletin 1964, pp. 56-74.

Sims, P. K., Card, K. D., and Lumbers, S. 13., 1981, Evolution ofEarly Proterozoic basins in the Great Lakes region, inF. H. A. Campbell (ed.), Proterozoic basins of Canada: Geol-Geological Survey of Canada Special Paper 81-10, pp. 379-397.

Sims, P. K., and Peterman, Z. E., 1984, A partisan review of theEarly Proterozoic geology of Wisconsin and adjacent Michigan(abs.): 30th Annual Institute on Lake Superior Geology,Wausau, Wisconsin, pp. 73-76.

Sims, P. K., and Peterman, Z. E., 1986, The Early ProterozoicCentral Plains orogen —— A major buried structure in north-central United States: Geology, v. 14, pp. 488-491.

Sims, P. K., Peterrnan, Z. E., Prinz, W. C., and Benedict, F. C.,1984a, Geology, geochemistry, and age of Archean and EarlyProterozoic rocks in the Marenisco-Watersmeet area, northernMichigan: U. S. Geological Survey Professional Paper 1292—A,pp. Al-PAl.

Sims, P. K.., Peterman Z. E., and Schulz, K. J., 1985, TheDunbar gneiss-granitoid dome; implications for EarlyProterozoic tectonic evolution of northern Wisconsin: Geolo-gical Society of America Bulletin, v.96, pp.1101-1112.

Sims, P. K., Peterman, Z. E., Kiasner, J. S., Cannon, W. F., andSchulz, K. J., 1987, Nappe development and thrust faultingin the upper Michigan segment of the Early ProterozoicPenokean orogen (abs.): Geological Society of America Abs-tracts with Programs, v.19, p. 246.

Sims, P. K., Schulz, K. J., and Peterrnan, Z. E., 1984b, Guide toGeology of the Early Proterozoic rocks in northeastenWisconsin: Field Trip 1, 30th Annual Institute on Lake SuperiorGeology, pp.1-50.

Sims, P. K., Schulz, K. J., Peterman, Z. E., and Van Schmus, W.R., in press, Wisconsin magmatic terrane, Lake Superior region,in J. C. Reed Jr. and others (eds.), Precambrian-conterrninousUniited States: Geological Society of America, The geology ofNorth America, v. C-2.

Ueng, W. L., Larue, D. K., Sedlock, R. L., and Kasper, D. A.,1984, Early Proterozoic tectonostratigraphic terranes of thesouthern Lake Superior region: Field Trip Guide with summary,Field Trip 2. 30th Annual Institute on Lake Superior Geology,23p.

Van Roosendaal, D. J., 1985, An analysis of rock structures andstrain in cleaved pelitic rocks, East Branch of the HuronRiver, Baraga County, Michigan: Master of Science Thesis,Michigan Technological University, Houghton, MI, 82 p.

Sims, P. K., Card, K. D., and Lumbers, S. B . , 1981, Evolution of Early Proterozoic basins in the Great Lakes region, in F. H. A. Campbell (ed.), Proterozoic basins of Canada: Geol- Geological Survey of Canada Special Paper 81-10, pp. 379- 397.

Sims, P. K., and Peterman, Z. E., 1984, A partisan review of the Early Proterozoic geology of Wisconsin and adjacent Michigan (abs.): 30th Annual Institute on Lake Superior Geology, Wausau, Wisconsin, pp. 73-76.

Sims, P. K., and Peterman, Z. E., 1986, The Early Proterozoic Central Plains orogen -- A major buried structure in north- central United States: Geology, v. 14, pp. 488-491.

Sims, P. K., Peterman, Z. E., Prinz, W. C., and Bened-ict, I?. C., 1984a, Geology, geochemistry, and age of Archean and Early Proterozoic rocks in the Marenisco-Watersmeet area, northern Michigan: U. S. Geological Survey Professional Paper 1292-A, pp. A1-A41.

Sims, P. K.., Peterman Z. E., and Schulz, K. J., 1985, The Dunbar gneiss-granitoid dome; implications for Early Proterozoic tectonic evolution of northern Wisconsin: Geolo- gical Society of America Bulletin, v.96, pp.llO1-1112.

Sims, P. K., Peterman, Z. E., Klasner, J. S., Cannon, VJ. F., and Schulz, K. J., 1987, Nappe development and thrust faulting in the upper Michigan segment of the Early Proterozoic Penokean orogen (abs.): Geological Society of America Abs- tracts with Programs, v.19, p. 246.

Sims, P. K., Schulz, K. J., and Peternan, Z. E., 1984b, Guide to Geology of the Early Proterozoic rocks in northeasten Wisconsin: Field Trip 1, 30th Annual Institute on Lake Superior Geology, pp.1-50.

Sims, P. K., Schulz, K. J., Peterman, Z. E., and Van SC~MIIS, W. R., in press, Wisconsin magmatic terrane, Lake Superior region, in J. C. Reed Jr. and others (eds.), Precambrian-conterminous Uniited States: Geological Society of America, The geology of North America, v. C-2.

Ueng, W. L., Larue, D. K., Sedlock, R. L., and Kasper, D. A., 1984, Early Proterozoic tectonostratigraphic terranes of the southern Lake Superior region: Field Trip Guide with sun-iinary, Field Trip 2. 30th Annual Institute on Lake Superior Geology, 23p.

Van Roosendaal, D. J., 1985, An analysis of rock structures and strain in cleaved pelitic rocks, East Branch of the Huron River, Baraga County, Michigan: Master of Science Thesis, Michigan Technological University, Houghton, MI, 82 p.

Van Schmus, W. R., 1976, Early and Middle Proterozoic history ofthe Great Lakes Area, North America: Philosophical Transactionsof the Royal Society of London, A280, pp.605-628.

I Van Schmus, W. R . , 1976, Early and Middle Proterozoic history of

the Great Lakes Area, North America: Philosophical Transactions

I of the Royal Society of London, A280, pp.605-628.

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