Archean Geology of the Northern Block 

of the Ishpeming Greenstone Belt, 

Marquette County, Michigan

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Chapter F

Archean Geology of the Northern Block 

of the Ishpeming Greenstone Belt, 

Marquette County, Michigan

By R.C JOHNSON and T.J. BORNHORST

U.S. GEOLOGICAL SURVEY BULLETIN 1904

CONTRIBUTIONS TO PRECAMBRIAN GEOLOGY OF LAKE SUPERIOR REGION

P.K. SIMS and LM.H. CARTER, Editors
U.S. DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, JR., Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

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Library of Congress Cataloging-in-Publication Data

Johnson, R.C.

Archean geology of the northern block of the Ishpeming Greenstone Belt, 

Marquette County, Michigan / by R.C. Johnson and T.J. Bornhorst.

p. cm. (Contributions to Precambrian geology of Lake Superior 

region ; ch. F) (U.S. Geological Survey bulletin ; 1904-F)

Includes bibliographical references.

Supt. of Docs, no.: I 19.3:1904-F

1. Geology, Stratigraphic Archaean. 2. Rocks, Igneous Michigan 

Marquette County. 3. Geology Michigan Marquette County. 

I. Bornhorst, Theodore J. II. Title. III. Title: Ishpeming Greenstone Belt, 

Marquette County, Michigan. IV. Series. V. Series: U.S. Geological Survey 

bulletin ; 1904-F. 

QE75.B9 no. 1904-F 

[QE653.3] 

55.3 s dc20

[551.7'12'0977496] 90-23107

CIP
CONTENTS

Abstract Fl 

Introduction Fl

Present investigation F2 

Previous work F2 

Acknowledgments F4 

Geologic setting F4 

Archean rocks F5

Lighthouse Point Basalt F5 

Distribution F5 

Thickness F5 

Description F6

Nash Creek glomerophyric basalt unit F6 

Reany Lake pyroclastic unit F6 

Fire Center mine iron-formation unit F9 

Hills Lakes pyroclastic unit F10 

Metamorphism F10 

Alteration Fll 

Gabbro F12

Gneissic granitoid rocks F12 

Rhyolite intrusive rocks F12 

Granodiorite near Rocking Chair Lakes F13 

Proterozoic rocks F13 

Structure F14

Bedding, foliations, and lineations F14 

Folds F14 

Faults F15 

Shear zones F16

Dead River shear zone F16 

Willow Creek shear zone F17 

Little Garlic shear zone F17 

Geologic history F17 

Economic geology F18

Base metal occurrence and iron-formation F18 

Gold prospects F18 

Base metal-silver prospects F18 

References cited F19

FIGURES

1. Geologic sketch map showing location of the Ishpeming greenstone belt F2

2. Geologic sketch map of the Ishpeming greenstone belt F3

3. Stratigraphic section showing the Lighthouse Point Basalt and its interflow 

units F5

4. Jensen cation plots of the Lighthouse Point Basalt and selected interflow 

units F8

Contents III
5-7. Geologic sketch maps showing:

5. Location of major folds in the northern block of the Ishpeming greenstone 

belt F9

6. Hills Lakes area F10

7. Distribution of greenschist and amphibolite metamorphic facies in the 

Lighthouse Point Basalt Fll

8. Diagrams of modes of gneissic granitoid rocks and granodiorite near Rocking 

Chair Lakes F13

9. Sketch map showing structures in the east end of the northern block F15

10. Stereographic projections of structures in the northern block F16

11. Schematic cross sections showing inferred structure in the northern 

block F17

TABLES

1. Michigan Geological Survey reports by Michigan Technological University 

students that have contributed to this report F4

2. Means and standard deviations of chemical analyses of rock units in the northern 

block of the Ishpeming greenstone belt F7

IV Contents
CONTRIBUTIONS TO PRECAMBRIAN GEOLOGY OF LAKE SUPERIOR REGION

Archean Geology of the Northern Block of the 

Ishpeming Greenstone Belt, Marquette County, 

Michigan

By R.C. Johnson1 and T.J. Bornhorst1

Abstract

The northern block of the Ishpeming greenstone belt 

occupies 150 square kilometers in north-central Marquette 

County, Michigan. This block, as well as the entire belt, is 

composed of Archean volcanic and volcaniclastic rocks and is 

bounded by Archean gneissic granitoid and Early Proterozoic 

sedimentary rocks. During Archean time, rocks of the belt 

were metamorphosed to greenschist and amphibolite facies 

and were subjected to two deformational events. Recumbent 

folding (D,) was followed by isoclinal to open folding about 

east-west axes (D2).

The northern block of the belt is dominated by the 

Lighthouse Point Basalt, as revised and redefined herein. The 

Lighthouse Point Basalt is composed of tholeiitic pillowed 

basalt flows with minor massive flows and subordinate pyroclastic and sedimentary rocks; it has a thickness of approximately 7,100 meters. The formation contains four informal 

interflow units, from oldest to youngest: Nash Creek glomerophyric basalt unit, Reany Lake pyroclastic unit, Fire Center 

mine iron-formation unit, and the Hills Lakes pyroclastic unit. 

The Nash Creek glomerophyric basalt unit is composed of 

glomerophyric pillowed and massive flows. The Reany Lake 

pyroclastic unit is an ash to lapilli-ash pyroclastic deposit, 

stratigraphically in the middle part of the Lighthouse Point 

Basalt, which is characterized by the occurrence of 

disseminated base metal sulfides. The Fire Center mine ironformation unit lies directly above the Reany Lake pyroclastic 

unit; it is a chert-magnetite iron-formation that contains minor 

pyrite and chlorite-magnetite or epidote-magnetite beds. The 

uppermost Hills Lakes pyroclastic unit grades laterally from a 

lapilli-tuff breccia to a lapilli-ash tuff, near the top of the 

formation. The Lighthouse Point Basalt is intruded by sill-like

Manuscript approved for publication August 30, 1990.

'Department of Geological Engineering, Geology, and Geophysics, 

Michigan Technological University, Houghton, MI 49931.

bodies of gabbro that are chemically similar to the basalts and 

probably were intruded shortly after eruption of the Lighthouse 

Point Basalt. Gneissic granitoid rocks that bound the greenstone belt to the north and east are granite to tonalite in 

composition. They intruded the greenstone belt during D^ and 

metamorphosed the adjacent basalt to amphibolite grade. 

Quartz and quartz-feldspar porphyritic rhyolite dikes intrude 

the Lighthouse Point Basalt in and adjacent to shear zones. The 

granodiorite near Rocking Chair Lakes intrudes the hinge of a 

major fold within the Lighthouse Point Basalt, and is 

interpreted as being comagmatic with the rhyolite intrusive 

rocks. The rhyolite and granodiorite were probably emplaced 

during D2 deformation.

Cold mineralization occurred in sheared, carbonatealtered, and quartz-veined rocks. Anomalous gold commonly 

is present along the brecciated margins of rhyolite dikes. The 

gold mineralization is interpreted as being associated with the 

second (D2) Archean deformational event. Base metal and 

silver mineralization occurred in quartz-carbonate veins 

hosted by brecciated fault zones. This mineralization is paragenetically later than that of the gold: it may not be Archean in 

age.

INTRODUCTION

The northern block of the Ishpeming greenstone belt 

is in north-central Marquette County, Mich., and is approximately 150 km2 in area (figs. 1 and 2). It occupies parts 

of the Marquette, Negaunee, Negaunee SW, Buckroe, 

Negaunee NW, and Silver Lake Basin 71/>-minute quadrangles, and includes the southeastern part of T. 50 N., 

R. 28 W., the southern part of T. 50 N., R. 27 W., the eastern 

part of T. 49 N., R. 28 W., most of T. 49 N., R. 27 W., the 

southwestern part of T. 49 N., R. 26 W., the northeastern 

part of T. 48 N., R. 27 W., and the northwestern part of

Archean Geology of the Ishpeming Greenstone Belt, Michigan F1
isnpeming 

$£ greenstone 

belt

EXPLANATION

MIDDLE PROTEROZOIC

x x x Volcanic, sedimentary, and intrusive 

rocks of the (Keweenawan) Midcontinent rift system

EARLY PROTEROZOIC

Sedimentary and volcanic rocks of 

the Marquette Range Supergroup

ARCHEAN

Granitoid rocks and gneiss

Volcanic rocks

Figure 1. Geologic sketch map showing location of the Archean Ishpeming greenstone belt.

T. 48 N., R. 26 W. A shear zone (Dead River shear zone; 

Bornhorst, 1988, and references therein) that defines the 

southern boundary of the northern block is parallel to the 

Dead River Storage Basin on the southwest and at the east 

end of the basin strikes east-southeast through sees. 8-12 of 

T. 48 N., R. 26 W. and sees. 7, 15-18, 22, and 23 of 

T. 48 N., R. 25 W.

Present Investigation

This investigation is a continuation of mapping of the 

Ishpeming greenstone belt by Michigan Technological 

University personnel begun during the summer of 1984 

under the direction of Bornhorst and continued during 

successive summers; mapping has been at scales ranging 

from 1:2,400 (Owens and Bornhorst, 1985) to 1:24,000 

(Johnson, unpub. data, 1990) and represents approximately 

24 person-months of field work (table 1). Geologic mapping 

has utilized a combination of compass-and-pace along 

traverse lines and direct plotting on topographic sheets 

where topographic features are distinctive.

One hundred fifty-two whole-rock geochemical 

analyses have been made by X-ray fluorescence on an 

automated Phillips wave-length dispersive spectrometer at 

Michigan Technological University using a modification of 

the technique described by Rose and others (1986). One

hundred seventy-three samples were analyzed for gold 

using fire assay and neutron activation (1 ppb detection 

limit), and 22 samples were analyzed for silver using 

inductively coupled plasma (0.5 ppm detection limit). One 

hundred eighty-six thin sections have been studied. Plagioclase compositions were determined by the Michel Levy 

technique when albite twinning was present and otherwise 

by the relief relative to the mounting epoxy (n=1.54). 

Actinolite and hornblende have been distinguished on the 

basis of color and Z~C (Winchell and Winchell, 1951).

Previous Work

Geologic interest in the greenstone belt was instigated 

by Douglas Houghton, who reported panning gold from a 

stream between Marquette and L'Anse in 1845. Precious 

metal exploration began in the northern block during the 

1860's with the working of base metal- silver veins (Norby, 

1988). In 1880, Julius Ropes discovered gold at the site of 

the Ropes mine in the southwestern block of the Ishpeming 

greenstone belt. During the 1930's, Norgan Mining 

Company explored for gold in the entire greenstone belt and 

drilled in the Fire Center mine area of the northern block 

(Kelly, 1936). From the late 1960's to the present,

F2 Precambrian Geology of Lake Superior Region
87°30'

46°30' Lighthouse Poin ~V!'.'*^>.' ' ' "-- ."" "" -'v'.:

;':'-J;}.'- ';.: ": ' " * "^^ 

V£^:. MarquetteV

EXPLANATION

Sedimentary rocks of Middle and

Early Proterozoic ages 

ARCHEAN 

Gneissic granitoid rocks

Rocks of the Ishpeming greenstone belt

DRSZ 

LGSZ 

WCSZ

Major shear zone that delineates blocks 

Shear zone

Dead River shear zone 

Little Garlic shear zone 

Willow Creek shear zone

Figure 2. Geologic sketch map of the Ishpeming greenstone belt, showing the major lithostratigraphic blocks, 

block-defining shear zones are indicated by the bold shear symbol.

Major

Cleveland-Cliffs Iron Company, Humble Oil, Superior Oil 

Company, Nicor Mineral Ventures, Chevron, St. Joe 

American Corporation, Phelps Dodge, Kerr-McGee 

Corporation, and Callahan Mining Corporation have 

explored for massive sulfides and (or) precious metals in the 

Ishpeming greenstone belt. Chevron geologists provided a 

regional geologic description of the greenstone belt 

(Morgan and DeCristoforo, 1980). In 1985 Callahan Mining 

Corporation resumed production at the Ropes gold mine, 

which continued until late 1989.

Most of the previous geologic work in the greater 

Marquette area concentrated on the geology and mineral 

deposits of Early Proterozoic rocks in the Marquette iron 

range. The work of Van Hise and Bayley (1897) defined the

major stratigraphic features of the area. The geologic reports 

of Gair and Thaden (1968), Puffett (1974), Clark and others 

(1975), and Cannon and Klasner (1977) provide 1:24,000- 

scale geologic maps covering the Marquette, Negaunee, 

Negaunee SW, and southern part of the Diorite and 

Champion quadrangles, respectively. These maps provide 

complete geologic coverage of the southern part of the 

greenstone belt, but they cover only the extreme southern 

part of the northern block, as defined here. Gair and 

Thaden's report includes a summary of prior geologic work 

performed in the southern part of the Ishpeming greenstone 

belt. Large-scale mapping by the Norgan Mining Company 

in the 1930's outlined the extent and some internal detail of 

the geology of the greenstone belt, which was later

Archean Geology of the Ishpeming Greenstone Belt, Michigan F3
Table 1 . Michigan Geological Survey reports by Michigan Technological University students that have contributed to this report

Author Open-file report Land description

Owens, E., and 

Bomhorst, T.J., 

1985.

Johnson, R.C.,

Bomhorst, T.J., and 

Van Alstine, J., 1987.

Baxter, D.A., and 

Bomhorst, T.J., 1987.

MacLellan, M.L., and 

Bomhorst, T.J., 1988.

Small, J.R., and 

Bomhorst, T.J., 1990.

Wilkin, R., and

Bomhorst, T.J., 1990.

Geology and precious metal prospects north of the Dead

River Storage Basin in sec. 35, T. 49 N., R. 27 W. and

north part of sec. 2, T. 48 N., R. 27 W., Marquette

County, Michigan: OFR-85-2. 

Geologic setting of precious metal mineralization in the

Silver Creek to Island Lake area, Marquette County,

Michigan: OFR-87-4. 

Geology, structure, and associated precious metal

mineralization in vicinity of Clark Creek, Marquette

County, Michigan: OFR-87-6. 

Precambrian bedrock geology and associated mineralization

of the Holyoke Trail-Reany Lake area, Marquette

County, Michigan: OFR-88-2.

Precambrian geology of the Penny Lake area, Marquette

County, Michigan: Contributions to Michigan Geology

Report 90-1. 

Geology of the Boise Creek area, Marquette County,

Michigan: Contributions to Michigan Geology Report

90-3.

Sec. 35, T. 49 N., R. 27 W.; and sec. 2, 

T. 48 N., R. 27 W.

Sees. 3, 10-15, 23- 25, T. 49 N., 

R. 28 W.

Sees. 19, 20, 27- 30, 32-34, T. 49 N., 

R. 27 W.

Sec. 36, T. 49 N., R. 27 W.; sec. 31, 

T. 49 N., R. 26 W.; sec. 1, T. 48 N., 

R. 27 W.; and sees. 6 and 7, T. 48 N., 

R. 26 W.

Sees. 1 and 2, T. 49 N., R. 28 W.; 

sees. 25-27, 34-36, T. 50 N., R. 28 W.

Sees. 29-32, T. 50 N., R. 27 W.; 

sees. 4-9, 16, 17, T. 49 N., R. 27 W.

compiled by Bodwell (1972) at a scale of 1:62,500. The 

published geologic maps produced by students at Michigan 

Technological University (table 1), covering approximately 

80 km2 of the northern block, and the more regional field 

studies by Johnson provide the foundation for this report.

Acknowledgments

Geologic research in the northern block has been 

supported by the Michigan Geological Survey, the 

COGEOMAP program of the U.S. Geological Survey, the 

Mineral Institute Program administered by the U.S. Bureau 

of Mines, and Michigan Technological University. Several 

Michigan Geological Survey open-file reports and M.S. 

theses of Michigan Technological University students 

contributed to this paper; these reports are listed in table 1.

Robert Reed and Jack Van Alstine of the Michigan 

Geological Survey are thanked for their support and reviews 

of open-file reports. Our understanding of the geology of the 

northern block has been improved through discussions with 

geologists from Callahan Mining Corporation and KerrMcGee Corporation. Our understanding of structural 

aspects has improved through discussions with R. Bauer 

and S. Nachatilo. S.D. McDowell and J. Kalliokoski 

provided valuable comments on an early draft of this paper.

GEOLOGIC SETTING

The Ishpeming greenstone belt occupies an area of 

325 km2 west of Marquette in Michigan's Upper Peninsula

(figs. 1 and 2). It is a thick accumulation of Archean 

tholeiitic basalts with subordinate felsic volcanic and minor 

sedimentary rocks. It is bounded to the north and west by 

Archean granite and gneissic granitoids (Sims, 1980). To 

the south the Archean rocks of the belt are unconformably 

covered by or are in fault contact with sedimentary rocks of 

the Early Proterozoic Marquette Range Supergroup 

(Cannon and Gair, 1970).

The greenstone belt is divided into three distinct 

lithostratigraphic blocks (Bomhorst, 1988) northern, 

southeastern, and southwestern blocks (fig. 2) which are 

separated by major shear zones. A lithostratigraphic block 

has an internal stratigraphic continuity that differs from 

adjacent blocks and is bounded by major deformational 

zones. The northern and southeastern blocks are dominated 

by subaqueous tholeiitic basalt flows, with subordinate 

felsic volcanics and minor sediments. The southwestern 

block contains both andesite-to-dacite volcaniclastic rocks 

and subaqueous tholeiitic basalt flows.

The northern block occupies an area of approximately 

150 km2, nearly one-half of the entire greenstone belt. The 

stratigraphy of the northern block is dominated by pillowed 

tholeiitic basalt flows with minor felsic volcanic and 

sedimentary rocks. It is bounded to the east and north by 

gneissic granitoid rocks and to the west by Proterozoic 

rocks (fig. 2). On the south it is bounded by the Dead River 

shear zone, which was first recognized by Puffett (1974).

Van Hise and Bayley (1895) named the greenstone 

near Marquette the Mona Schist. Subsequently, Gair and 

Thaden (1968) assigned the name Lighthouse Point 

Member of the Mona Schist to layered amphibole schists

F4 Precambrian Geology of Lake Superior Region
2,700 m

Local unconformity

^^ifi/^^Vjv1'^;^ Hills Lakes pyroclastic unit

Main lithology of 

Lighthouse Point Basalt

Fire Center mine iron-formation unit 

Reany Lake pyroclastic unit

 .'."- *.V> Nash Creek glomerophyric basalt unit

Base unknown

Figure 3. Stratigraphic succession of the Lighthouse Point 

Basalt and its interflow units. Shaded area indicates probable 

correlation of the Lighthouse Point Member of the Mona 

Schist of Gair and Thaden (1968) and Puffett (1974) within 

the revised Lighthouse Point Basalt.

that extend westward from Lighthouse Point, in Marquette, 

Mich. Puffett (1974) interpreted the pillowed basalts about 

8 km west of Lighthouse Point in the Negaunee quadrangle 

also as part of the Lighthouse Point Member of the Mona 

Schist. The volcanic rocks of the northern block include 

large volumes of both pillowed greenschist-facies basalt and 

layered amphibole-rich basalt and are interpreted as part of 

the revised and redefined Lighthouse Point Basalt. However, the strata of the northern block are separated from 

volcanic rocks in the southeastern block by the Dead River 

shear zone, and hence cannot be correlated with these strata 

that have been assigned to the Mona Schist. We propose that 

all the mafic rocks except gabbro in the lithostratigraphic 

block north of the Dead River shear zone, including the 

rocks north of the shear zone previously known as 

Lighthouse Point Member of the Mona Schist, be revised 

and redefined as the Lighthouse Point Basalt. The rocks 

south of the Dead River shear zone that were previously 

interpreted as part of the Lighthouse Point Member are 

considered here to be part of the Mona Schist restricted.

The volcanic pile is intruded by mafic and felsic 

bodies of several ages. The granodiorite near Rocking Chair 

Lakes is a stock-sized intrusion. The remainder of the 

intrusive rocks are discussed following on the basis of the 

dominant lithology: gabbro, gneissic granitoids, and rhyolite intrusions.

Unconformably overlying the volcanic and intrusive 

rocks in the southeastern part of the northern block (fig. 6) 

is a succession of sedimentary rocks named the Reany 

Creek Formation by Puffett (1969). Puffett (1969, p. F5) 

divided the Reany Creek Formation into three units: "(I) a 

basal conglomerate, (2) a middle sequence of graywacke 

and slate with disrupted beds of arkose and dispersed 

pebbles and boulders of granitic rock, and (3) an upper unit

of interbedded arkose, quartzite, slaty graywacke, and 

conglomerate." These sedimentary rocks strike approximately east-west, dip steeply, and young stratigraphically to 

the south. Puffett (1969) and others have assumed the age of 

the Reany Creek Formation to be Early Proterozoic, partly 

because it is apparently not cut by any igneous dikes other 

than Middle Proterozoic (Keweenawan) dikes. The precise 

age of these rocks has been uncertain, however (Small and 

Bornhorst, 1988; Owens and Bomhorst, 1985; Mattson and 

Cambray, 1983; Ojakangas, 1984; Puffett, 1969). Because 

of the structural relationships and resemblance to 

"Timiskaming-type" sequences, Sims (in press) has modified the age of the Reany Creek Formation; it is now 

interpreted as Archean and is no longer considered as the 

basal part of the Marquette Range Supergroup.

ARCHEAN ROCKS 

Lighthouse Point Basalt

The Lighthouse Point Basalt is composed dominantly 

of pillowed and massive tholeiitic basalt flows, and includes 

four informal lithologically distinct and mappable interflow 

units: from bottom to top, Nash Creek glomerophyric basalt 

unit, Reany Lake pyroclastic unit, Fire Center mine ironformation unit, and Hills Lakes pyroclastic unit (fig. 3). 

Stratigraphically, the Nash Creek glomerophyric basalt unit 

is near the base of the exposed Lighthouse Point Basalt, the 

Reany Lake pyroclastic unit is near the middle of the 

formation, and the Fire Center mine iron-formation unit 

immediately overlies the Reany Lake pyroclastic unit. The 

Hills Lakes pyroclastic unit is near the top of the formation.

Distribution

The mafic volcanic rocks of the northern block that 

are assigned to the Lighthouse Point Basalt cover nearly the 

entire block, an area of approximately 150 km2. The 

northwesternmost exposures of the formation are found east 

of Pinnacle Falls in sec. 27, T. 50 N., R. 28 W., and the 

northeasternmost exposures lie east of Blemhuber Lake in 

sec. 4, T. 49 N., R. 26 W. The northern extent is defined by 

the contact between the basalts and the bounding gneissic 

granite and the southern limit by the Dead River shear zone. 

This unit is covered by Early Proterozoic sedimentary rocks 

in the Clark Creek basin in the central part of the northern 

block.

Thickness

Thicknesses are difficult to determine in lithologically monotonous, mafic volcanic piles that have undergone 

polyphase deformation. However, as noted, the Lighthouse 

Point Basalt contains four distinct mappable units. Consequently, it has been possible to decipher some of the

Archean Geology of the Ishpeming Greenstone Belt, Michigan F5
structural complexity. (See section, "Structure.") If the F1 

folds are recumbent and isoclinal and the Sj foliation is 

axial-planar, the minimum stratigraphic thickness is estimated to be 7,100 m (fig. 3).

Description

The Lighthouse Point Basalt is a monotonous pile of 

pillowed tholeiitic basalt lava flows. Pillows are typically 

bun shaped and 30-60 cm in cross section, although 

mattress-shaped pillows as much as 2 m in cross section are 

present. Pillows in greenschist-facies rocks are typically 

ellipsoidal and have classic pillow shapes. Amphibolitefacies basalts are typically well banded or layered, and in 

many places "layers" can be seen as the result of extreme 

flattening of pillows. Flattening tends to obliterate pillow 

cusps that are used to determine stratigraphic top directions, 

and shearing produces pillows with amoeboid shapes and 

feathered margins. Except for a few flows above and below 

the Fire Center mine iron-formation unit, vesicles are absent 

within the pillows throughout the sequence. Where present, 

vesicles are less than 5 mm in diameter.

Massive flows are common but subordinate in 

volume to pillowed flows. The massive flows as well as 

intrusive gabbro bodies have characteristic ophitic textures, 

and accordingly it is difficult to distinguish one from the 

other. For example, the Fire Center mine iron-formation 

unit near Reany Lake was mapped by Puffett (1974) as 

being within a metadiabase intrusion and by MacLellan and 

Bornhorst (1989) as being within Archean gabbro. In 

general, bodies of gabbro-like rock are mapped as intrusions 

barring evidence to the contrary; and in this instance the 

continuity of the gabbro-iron-formation "sandwich" along a 

strike length of approximately 3 km suggests that these 

particular gabbro-like rocks are massive flows rather than 

intrusions. The unit also contains rare pillow breccias, 

hyaloclastites, and hyalotuffs as discontinuous lenses that 

are typically less than 1 m thick and rarely more than 30 m 

long.

The basalts range from light gray to black in color. 

The lighter colored basalts are generally foliated and 

contain carbonate as disseminations and as fine crosscutting 

veinlets; these basalts commonly effervesce with HC1. At 

greenschist facies, most basalts are light grayish green, 

whereas at amphibolite facies they are black. Mineralogy of 

the basalts correlates with metamorphic grade. (See section, 

"Metamorphism.")

Foliated basalts occur along faults, sheared contacts, 

high strain zones, and shear zones throughout the area. This 

variety of basalt has two macroscopically distinct end 

members: (1) dull-black to dull-brown, aphanitic, friable, 

and generally limonite rich schists; and (2) medium-green 

phyllonite. The black end-member is composed dominantly 

of chlorite and albite, but commonly contains abundant 

disseminated pyrite (estimated as much as 15 percent). The

phyllonite is composed dominantly of chlorite and albite 

with minor quartz, and commonly contains lenses of 

porphyroblastic carbonate with quartz. In the phyllonite, 

pyrite occurs as fine disseminations in the chloritic groundmass that surrounds the carbonate-quartz lenses.

The Lighthouse Point Basalt is composed of magnesian to low-iron tholeiitic basalt (table 2; fig. 4). The 

chemical composition of greenschist- and amphibolitefacies basalts is comparable (Small, 1989), suggesting that 

metamorphic mobilization of elements and magmatic 

chemical variability are small. The chemical composition is 

similar to that of other Archean basalts of the Superior 

province (Jensen and Langford, 1985).

Nash Creek Glomerophyric Basalt Unit

Within the eastern exposures of the Lighthouse Point 

Basalt, Puffett (1974) mapped a "felsic augen zone." Our 

recent field and petrographic studies, however, indicate that 

the "felsic augens" within this zone are relict glomerophyric 

plagioclase phenocrysts. Based on lateral continuity, this 

zone is interpreted as representing one or more glomerophyric basalt flows and is informally called Nash Creek 

glomerophyric basalt unit.

The Nash Creek glomerophyric basalt unit crops out 

in the eastern part of the northern block, in sees. 1 and 2, 

T. 48 N., R. 26 W., and sees. 4, 5, 20, 29, 32, 33, and 34, 

T. 49 N., R. 26 W., and is traversed by Nash Creek in sec. 

29, T. 49 N., R. 26 W. The outcrop pattern has a sinuous 

form which traces out F, folds and mimics the shape of the 

contact with the gneissic granitoid rocks (fig. 5).

In the Negaunee quadrangle, the Nash Creek glomerophyric basalt unit has a map width of 200-300 m 

(Puffett, 1974). If the S, foliation is assumed to be axialplanar with an average dip of 25° and Ft folds are assumed 

to be isoclinal, the unit is approximately 90- 120 m thick.

The glomerophyric basalt unit contains both massive 

and pillowed rocks. The basalt ground-color is dark 

greenish gray to black. In hand samples, the glomerophyric 

phenocrysts are grayish white and have an elongate amoeboid shape. However, relict phenocrysts retaining albite 

twinning have been found in oriented thin sections. The 

phenocrysts are typically replaced by saussurite assemblages of sericite, carbonate, clinozoisite-epidote, and calcic 

plagioclase. The groundmass is composed of dark-green 

hornblende, calcic plagioclase, and clinozoisite-epidote, 

typical of the amphibolite facies in the surrounding Lighthouse Point Basalt. The Nash Creek glomerophyric basalt 

unit is composed of low-iron tholeiitic basalt (table 2; 

fig. 4).

Reany Lake Pyroclastic Unit

The Reany Lake pyroclastic unit is an ash to lapilliash tuff found in the middle stratigraphic part of the 

Lighthouse Point Basalt (fig. 3). This unit crops out south of

F6 Precambrian Geology of Lake Superior Region
Table 2. Means, in weight percent, and standard deviations of chemical analyses of 

rock units in the northern block of the Ishpeming greenstone belt

SiO2

Ti02

A1203

Fe203

MnO

MgO

CaO

Na20

K2O

P205

LOI

Total

SiO2

Ti02

A1203

Fe203

MnO

MgO

CaO

Na20

K20

P205

LOI

Total

Lighthouse Point

Basalt

n=18

Mean Standard

deviation

48.62 1.4

1.07 0.3

12.96 0.9

14.43 2.3

0.18 0.1

8.19 1.1

8.56 3.4

1.87 0.6

1.06 0.8

0.12 0.0

3.05 1.4

100.11

Hills Lakes 

pyroclastic

unit

n=4

Mean Standard

deviation

64.46 3.3

0.47 0.1

16.96 1.2

4.38 0.7

0.07 0.0

1.32 0.5

4.20 0.5

4.55 0.9

1.37 0.2

0.23 0.0

2.05 0.4

100.06

Archean

gabbro

n=12

Mean Standard

deviation

47.74 2.0

1.08 0.3

12.32 1.2

15.49 2.7

0.19 0.1

9.18 1.7

8.14 2.3

1.97 0.5

0.93 0.7

0.14 0.1

2.79 1.3

99.97

Granodiorite 

near Rocking

Chair Lakes

n=10

Mean Standard

deviation

59.90 3.6

0.54 0.1

16.01 1.5

4.91 1.7

0.08 0.0

4.55 2.1

2.77 1.5

4.19 0.8

3.46 1.0

0.26 0.1

2.85 1.2

99.52

Nash Creek

glomerophyric

basalt unit

n=1

49.13

1.08

12.47

15.19

0.22

7.38

9.81

2.80

0.99

0.17

1.54

100.78

Reany Lake

pyroclastic

unit

n=5

Mean Standard

deviation

61.46 4.1

0.61 0.1

14.69 0.9

6.82 1.8

0.06 0.0

4.04 2.2

1.80 1.1

3.60 1.3

2.14 1.3

0.20 0.0

3.72 0.8

99.14

Rhyolite 

intrusions

n=16

Mean

70.87

0.25

15.31

2.09

0.03

1.25

0.91

4.38

3.02

0.09

1.95

100.15

Standard

deviation

3.6

0.2

1.7

1.3

0.0

1.1

0.7

1.7

1.4

0.1

0.8

Reany Lake in sec. 6, T. 48 N., R. 26 W., and has a probable 

strike length of 25-27 km. The surface trace of this unit 

defines a large Z-shaped fold in the northern block (fig. 5). 

Strict continuity is not certain because of the dearth of 

outcrop. This unit has a maximum thickness of about 45 m 

to the west of Reany Lake in the Fire Center mine area.

The Reany Lake pyroclastic unit contains an ashtuff breccia zone of conspicuous angular chert fragments as 

much as 5 cm long that grades upward into bedded 

lapilli-ash tuff and further upward into finely laminated and 

graded tuffs (terminology of Easton and Johns, 1986). The 

matrix is typically a fine-grained quartz-chlorite-sericite 

schist. The bedded lapilli-ash tuff zone consists of lapillisized clasts that are composed of mosaic-textured quartzsericite-carbonate aggregates. The tuffs are fine-grained 

quartz-sericite-chlorite schist. Rocks are typically light gray 

to yellowish tan. The Reany Lake pyroclastic unit is 

composed of calc-alkalic dacite to rhyolite lapilli (table 2; 

fig. 4). Analyses of the ash show calc-alkalic andesite 

compositions.

A unique feature of the Reany Lake pyroclastic unit is 

the association of base metal sulfides, as observed southeast 

of Reany Lake, SE1A sec. 6, T. 48 N., R. 27 W. (Puffett, 

1974) and to the north in sec. 36, T. 49 N., R. 27 W. 

Possibly, the massive sulfide body was incorporated into the 

pyroclastic unit during its turbulent descent from the

Archean Geology of the Ishpeming Greenstone Belt, Michigan F7
FeO+Fe2O3 +TiO3 FeO+Fe 2 O3 +TiO2

AI 2O3

A

A1 2O3

B

FeO+Fe 2O3 +TiO2

FeO+Fe 2 O3 +TiO2

MgO A1 2O3

D

MgO A1 2O3

£

FeO+Fe 2O3 +TiO2

FeO+Fe 2 O3 +TiO2

Rhyoliti

MgO

MgO

A1 2 03

c

MgO A1 2O3

F

MgO

F8 Precambrian Geology of Lake Superior Region
87°30'

46°30' -

10 KILOMETERS 

i

EXPLANATION

Sedimentary rocks of Middle and

Early Proterozoic ages 

ARCHEAN 

Reany Creek Formation

Granodiorite near Rocking Chair Lakes 

Gneissic granite to tonalite 

Lighthouse Point Basalt 

Hills Lakes pyroclastic unit

Fire Center mine iron-formation and

Reany Lake pyroclastic units 

Nash Creek glomerophyric basalt unit

Synform Showing approximate trace of 

axial plane and direction and plunge of axis

-> Antiform Showing approximate trace of

axial plane and direction and plunge of axis

Strike and dip of overturned bedding 

s_ Shear zone

Fault Dashed where inferred

Figure 5. Geologic sketch map showing location of major folds in the northern block of the Ishpeming greenstone belt. The 

width of the interflow units is exaggerated. RCL, Rocking Chair Lakes.

volcano. This base metal association and stratigraphic 

proximity to an iron-formation (described following) provide confidence in stratigraphic correlation of lithologies 

over a considerable distance.

Figure 4 (facing page). Jensen cation plots (Jensen, 1976) of 

A, basalt from the Lighthouse Point Basalt and Nash Creek 

glomerophyric basalt unit (asterisk); B, gabbro; C, Reany Lake 

pyroclastic unit (analyses of lapilli and matrix); D, Hills Lakes 

pyroclastic unit (analyses of lapilli); £, rhyolite intrusions in 

southern part of northern block; F, Jensen cation plot 

classifications.

Fire Center Mine Iron-Formation Unit

The Fire Center mine iron-formation unit (fig. 3) 

crops out in the vicinity of the Fire Center mine in sec. 35, 

T. 49 N., R. 27 W. It is typically less than 9 m thick. 

Outcrop is generally sparse, and ground magnetic surveys 

suggest that the unit, although regionally continuous, may 

pinch and swell and be as much as 23 m thick. This 

iron-formation unit correlates with the "anomaly north of 

Dead River" described by Puffett (1974).

The Fire Center mine iron-formation unit can be 

traced on aeromagnetic maps (Case and Gair, 1965) for 

approximately 27 km, and it defines the major folds in the

Archean Geology of the Ishpeming Greenstone Belt, Michigan F9
EXPLANATION

Sedimentary rocks of Middle and 

Early Proterozoic ages

ARCHEAN

Granodiorite near Rocking Chair Lakes

Gneissic granite to tonalite 

Gabbro

Lighthouse Point Basalt 

Hills Lakes pyroclastic unit

 I * Synformal anticline Showing approximate trace of axial plane and direction 

and plunge of axis

 I * Antiformal syncline Showing approximate trace of axial plane and direction 

and plunge of axis

Figure 6. Geologic sketch map showing location of the 

Hills Lakes pyroclastic unit, granodiorite near Rocking Chair 

Lakes, and axial traces of F, folds.

northern block (fig. 5). The anomaly can be traced under the 

Proterozoic sedimentary rocks of the Clark Creek basin; it is 

segmented, however, by east-west-trending, reversely 

polarized Keweenawan diabase dikes.

The unit is typically banded chert-magnetite ironformation with associated chlorite or epidote schist. In the 

southern part of the northern block, chert layers range from 

less than 1 to as much as 2 cm thick and alternate with 

magnetite layers less than 1 cm thick. Pyrite occurs as 

sparse (<5 percent) thin layers, disseminated euhedral 

grains, and as sparse grains in crosscutting quartz-carbonate 

veins. A red disseminated fine-grained acicular mineral, 

probably an iron amphibole, occurs scattered within the

chert layers. In the northern part of the northern block, the 

iron-formation unit consists of alternating centimeter-wide 

bands of epidote and magnetite with rare chert bands and 

minor amounts of pyrite and hornblende.

Hills Lakes Pyroclastic Unit

The Hills Lakes pyroclastic unit (fig. 3) is a pyroclastic deposit with conspicuous grayish-white pumiceous 

lapilli in a black schistose matrix. It is well exposed on the 

eastern shore of the longer, north-south-trending lake of 

Hills Lakes in sec. 11, T. 49 N., R. 28 W. This unit has a 

strike length of approximately 5 km and a maximum 

thickness of 50 m (fig. 6). The lapilli are strongly elongated, 

giving vertical surfaces a banded appearance.

In the vicinity of Hills Lakes, this unit is a lapilli-tuff 

breccia, and the lapilli and bombs are monomictic pumice of 

rhyolite to dacite composition (table 2; fig. 4). The lapilli 

are composed of calcic plagioclase and quartz with minor 

hornblende, sericite, and clinozoisite-epidote. The lapilli 

and bombs are so deformed that they show a banded 

appearance on vertical faces and an apparent lack of 

deformation on horizontal surfaces. The ash matrix is 

composed of hornblende, calcic plagioclase, biotite, and 

garnet with minor sericite, chlorite, tourmaline, and 

clinozoisite-epidote. The lapilli show calc-alkalic rhyolite 

compositions (table 2; fig. 4).

The Hills Lakes pyroclastic unit grades northward 

along strike into a lapilli-ash tuff where individual lapilli are 

difficult to distinguish because of intense deformation. It is 

a plagioclase-quartz-muscovite-chlorite schist containing 

poikiloblasts of almandine garnet (Small, 1989).

Metamorphism

Metamorphic grade in the Lighthouse Point Basalt 

varies from greenschist to amphibolite facies, increasing 

symmetrically toward the contact with the bounding 

gneissic granitoid rocks (fig. 7). This relationship suggests 

that the amphibolite-facies metamorphism is related to 

higher temperatures associated with the emplacement of the 

granitoid rocks. The width of this metamorphic aureole 

indicates a minimum depth of burial of 2-3 km (Jensen and 

Langford, 1985).

Basalt of lower greenschist facies is characterized by 

chlorite, pale-green pleochroic actinolite, clinozoisiteepidote, and minor sericite, carbonate, and fine-grained 

disseminated pyrite. Relict magmatic textures are preserved. 

Basalt of upper greenschist facies is characterized by 

pale-green pleochroic actinolite, light-green hornblende, 

albite, and clinozoisite-epidote; metamorphic textures are 

present. At lower amphibolite grade, the basalt is characterized by hornblende, calcic plagioclase, clinozoisiteepidote and, in the rare mafic hyalotuffs, by garnet

F10 Precambrian Geology of Lake Superior Region
87°30'

46°30' -

10 KILOMETERS

EXPLANATION

Sedimentary rocks of Middle and

Early Proterozoic ages 

ARCHEAN 

Reany Creek Formation

Granodiorite near Rocking Chair Lakes

Zone of abundant rhyolite intrusions

~ _ - x / I 

i \ C, i ~ H Gneissic granite to tonalite

Lighthouse Point Basalt 

Amphibolite facies

Greenschist facies

Figure 7. Geologic sketch map of the northern block of the Ishpeming greenstone belt, showing distribution of greenschist 

and amphibolite metamorphic facies in the Lighthouse Point Basalt and the zone of abundant rhyolite intrusions.

and biotite; the rock has well-developed metamorphic 

textures. At upper amphibolite grade, the basalt contains 

green to light-brown hornblende, calcic plagioclase, hypersthene, and diopside and has a well-developed schistosity. 

These differences in mineralogy allow the grade of metamorphism to be approximated by examination of thin 

sections.

Alteration

Altered basalt is widely scattered throughout the 

Lighthouse Point Basalt and is commonly associated with 

shear zones or faults. Altered basalts can be distinguished 

from unaltered basalts in being lighter in color (weathered 

surfaces exhibit various shades of light green and yellow).

The altered basalts are typically medium gray to light-gray, 

aphanitic to very fine grained schists composed of chlorite, 

albite, sericite, carbonate, and quartz.

Altered basalt is most prevalent along the southern 

boundary of the northern block. These rocks represent the 

northwesterly continuation of a unit mapped by Puffett 

(1974) as "undifferentiated greenstone." Brecciated rhyolite 

dikes and brecciated margins of rhyolite dikes are 

commonly associated with intense ankerite alteration in this 

area. The overall distribution of alteration indicates that a 

major Archean hydrothermal system may have been 

focused along the Dead River shear zone.

Thin altered zones are common, and in these zones 

the basalt contains variable amounts of chlorite, epidote, 

albite, sericite, and carbonate. The most common sulfide

Archean Geology of the Ishpeming Greenstone Belt, Michigan F11
mineral is pyrite, which occurs along thin shears as fine- to 

medium-grained euhedral masses and as fine-grained euhedral and (or) anhedral disseminations. Less common sulfide 

minerals include pyrrhotite, arsenopyrite, chalcopyrite, 

galena, and sphalerite.

The alteration minerals are retrograde products 

formed after earlier Archean prograde metamorphism. At 

amphibolite facies, the regional metamorphic assemblage 

contains hornblende and calcic plagioclase, whereas the 

altered basalts contain chlorite, albite, and epidoteclinozoisite. Thus, retrograde chlorite-epidote zones are 

found locally within upper amphibolite-facies assemblage. 

Retrograde alteration is not readily evident at lower greenschist facies, where chlorite, albite, epidote, and sericite 

assemblages are typical of the regional metamorphism. 

However, locally in the greenschist facies where actinolite 

is the dominant ferromagnesian mineral, the presence of 

chlorite may indicate retrograde alteration.

Gabbro

The Lighthouse Point Basalt and its interflow units 

have been intruded by mafic rocks of several ages. The 

oldest intrusions are gabbro sills and dikes of probable 

Archean age. The majority of Archean gabbros are sill-like, 

but some are dikes. The gabbro is typically medium 

greenish gray and dark bluish green and medium to coarse 

grained. It is composed of hornblende or actinolite and 

plagioclase with minor sericite, clinozoisite-epidote, and 

sphene. The opaques are magnetite or less commonly pyrite, 

pyrrhotite, and cubanite. Amphibole is typically bluish 

green hornblende; it pseudomorphically replaces ophitic 

pyroxenes. Plagioclase forms aggregates of two types: 

(1) those retaining their magmatic texture and (2) those 

having a mosaic texture. The composition of the plagioclase 

reflects the grade of metamorphism: albite occurs in 

greenschist-facies rocks and calcic plagioclase occurs in 

amphibolite-facies rocks. Chemically, the gabbro intrusive 

bodies are magnesian tholeiites, very similar compositionally to the basalt flows (table 2; fig. 4).

Altered gabbro is associated with faults and shears 

and may be foliated or massive. Fresh surfaces are light gray 

to light green and those that are highly epidotized are a 

distinct pistachio green. Alteration minerals include chlorite, albite, epidote, quartz, sericite, and carbonate with 

chalcopyrite, pyrrhotite, and pyrite.

Gneissic Granitoid Rocks

Archean felsic rocks of several ages intrude the mafic 

pile of basalt and gabbro. The oldest recognizable intrusions 

are gneissic granitoid dikes that are coeval with the large 

surrounding bodies of gneissic granitoid rocks. The dikes

were intruded parallel to the dominant regional foliation and 

are located near the contact of the basalt and gabbro with the 

surrounding gneissic granitoid bodies. The dikes were 

emplaced during D} deformation.

The contact between the Lighthouse Point Basalt and 

the surrounding gneissic granitoid rocks is gradational. As 

the contact is approached, gneissic tonalite and granite with 

thin layers of amphibolite grade into banded amphibolitefacies gneiss in near equal amounts, and finally into basalt, 

where amphibolite is more abundant than felsic gneiss. This 

aureole of mixed amphibolite and felsic gneiss commonly is 

as much as 300 m wide. Thus, the exact location of the 

contact between basalt and gneiss is equivocal. Farther from 

the contact, the basalt contains thin wisps of felsic material 

along deformed pillow margins.

The gneissic granitoid rocks are tan to pink, are fine 

to medium grained, and range in composition from granite 

to tonalite (fig. 8). They are composed of quartz, plagioclase, and potassium feldspar with minor amounts of biotite 

and hornblende.

Rhyolite Intrusive Rocks

Rhyolite dikes that postdate emplacement of the 

granitoid rocks intrude the mafic pile in areas of high strain. 

In the northern block, rhyolite dikes are abundant along the 

Dead River shear zone, particularly from Silver Creek to 

east of the Fire Center mine area (fig. 7). East of Fire Center 

mine, this zone of high strain and abundant rhyolite 

intrusions was mapped as undifferentiated greenstone by 

Puffett (1974). Most of the rhyolite intrusions are less than 

6 m thick. In general, they strike N. 60°-70° W., have 

nearly vertical dips, and are well foliated. The rhyolite 

intrusive rocks have a wide range of textures, including 

porphyritic, aphyric, and granular. Of these, porphyritic 

rhyolites are most common and granular rhyolites are least 

common.

Porphyritic rhyolite intrusive rocks are commonly 

light gray to tan schists, composed of quartz, sericite, and 

albite. Porphyritic rhyolite intrusive rocks containing only 

embayed quartz phenocrysts are most common, whereas 

those containing quartz and albite phenocrysts are less 

common. The groundmass is typically composed of granoblastic quartz, albite, lepidoblastic sericite, and disseminated pyrite. Aphyric rhyolite intrusions range in color from 

uniform to mottled pink, orange, red, tan, and white. In hand 

specimen they resemble chert. The aphyric rhyolites are 

typically composed of fine-grained sutured quartz and albite 

with minor disseminated euhedral pyrite. Granular rhyolites 

are gray to dull pink, fine grained, and allotriomorphic with 

a granitic texture. They are composed of albite, quartz, 

microcline, and minor muscovite, epidote, chlorite, apatite, 

and pyrite. The aphyric and quartz porphyritic varieties are 

common in the area of Silver Creek, whereas to the east, 

towards Fire Center mine, quartz-feldspar porphyritic and

F12 Precambrian Geology of Lake Superior Region
Quartz

Alkali 

A feldspar

\ \ 

Plagioclase

Quartz

Alkali 

B feldspar

Plagioclase

Figure 8. Modes of A, gneissic granitoid rocks and B, granodiorite near Rocking Chair Lakes. Basic diagram from 

Streckeisen (1975).

granular varieties are more common. Locally the aphyric 

variety is the oldest and is cut by quartz and quartz-feldspar 

porphyritic varieties. These intrusions are dominantly calcalkalic rhyolite in chemical composition (table 2; fig. 4); 

dacite is subordinate. The chemical variation is small 

among the textural varieties.

Granodiorite Near Rocking Chair Lakes

The youngest felsic intrusions in the northern block 

are granitoid stocks. Representative of these intrusions is a 

large granodiorite body that intrudes the hinge of the 

southern Fj synformal anticline (fig. 6). This intrusion is 

informally called granodiorite near Rocking Chair Lakes.

The granodiorite near Rocking Chair Lakes crops out 

around Rocking Chair Lakes and along Mulligan Creek in 

sees. 10 and 15, along the cliff of Mulligan Creek in sec. 3, 

and as a 300-m-wide body striking subparallel to the basalt

foliation across sees. 13, 14, and 24, T. 49 N., R. 28 W. 

(fig. 6). These stock-sized granodiorite intrusions are 

commonly in fault contact with the surrounding basalt, but 

a sharp intrusive contact has been observed in the SEV4 

sec. 10. Xenoliths of basalt (as much as 3x2 m) are common 

within the granodiorite in the NE]/4 sec. 10. A large basalt 

inlier in sec. 3, T. 49 N., R. 28 W. is interpreted as a roof 

pendant, based on the consistency of its foliation with that 

in the surrounding basalts.

The granodiorite near Rocking Chair Lakes is 

medium gray to light pink, medium to coarse grained, and 

hypidiomorphic to allotriomorphic granular; it has varied 

compositions, including granodiorite, tonalite, quartz monzonite, quartz monzodiorite, and quartz diorite (fig. 8). It 

can be subdivided into three types: massive, amphibolerich, and foliated. The massive granodiorite is composed 

of plagioclase, potassium feldspar, quartz, amphibole, and 

minor sericite, epidote, chlorite, apatite, and carbonate. 

The alteration mineral assemblage is consistent with 

greenschist-facies metamorphism. The amphibole-rich 

variety is found near the margins of the intrusive body and 

probably reflects contamination by the adjoining basalt and 

gabbro. This variety is most common in the southeast corner 

of sec. 3 in a wide zone along the basalt-granodiorite 

contact. It is typically dark gray and fine to medium grained 

and except for the abundance of amphibole (as much as 55 

percent) is similar to the massive type. Foliated granodiorite 

is found along intrusive contacts and faults that cut across 

the granodiorite. It is gray to black, fine-grained to aphanitic 

schist with flaser structures. It is composed of plagioclase, 

chlorite, potassium feldspar, and quartz with minor apatite, 

sericite, and epidote. Locally, this variety hosts as much as 

20 percent of disseminated euhedral pyrite.

The granodiorite near Rocking Chair Lakes has a 

calc-alkalic affinity (table 2). It can be classified as an I-type 

granite according to the criteria of Ferguson and others 

(1980). The rhyolite dikes, discussed previously, are 

interpreted as being a phase of the same magmatic event as 

the granodiorite near Rocking Chair Lakes.

PROTEROZOIC ROCKS

Early Proterozoic sedimentary rocks assigned to the 

Michigamme Formation of the Marquette Range Supergroup (Cannon and Gair, 1970), Early Proterozoic diabase 

dikes, and Middle Proterozoic (Keweenawan) diabase dikes 

occur within the northern block.

The sedimentary rocks of the Michigamme Formation 

crop out in the Mulligan Plains and in the Dead River and 

Clark Creek basins. These rocks include conglomerate, 

slate, gray wacke, and magnetite argillite, which were folded 

and metamorphosed to greenschist facies during the 

Penokean orogeny (Clark and others, 1975). Sparse outcrops of basal quartz-pebble conglomerates are found in 

contact with the Archean Lighthouse Point Basalt around

Archean Geology of the Ishpeming Greenstone Belt, Michigan F13
the margins of the Clark Creek basin. The conglomerate in 

these outcrops is commonly a thin cover over Archean 

rocks, indicating that the present land surface is at or near 

the Early Proterozoic land surface.

Early Proterozoic diabase dikes that are well jointed, 

as much as 18 m wide, and strongly magnetic cut Archean 

and Early Proterozoic rocks in the area. They are dark gray, 

fine to medium grained, and hypidiomorphic granular, and 

are composed of labradorite, hornblende, chlorite, sphene, 

epidote, and skeletal magnetite. They were metamorphosed 

to greenschist facies during the Penokean orogeny and have 

commonly been mapped as metadiabase (for example, 

Puffett, 1974). They are readily distinguished microscopically from the Archean gabbro on the basis of plagioclase composition and texture, whereas macroscopic distinction can be difficult. Where diabase metamorphosed to 

greenschist facies cuts the gneissic granodiorite or the 

amphibolite-facies rocks of the Lighthouse Point Basalt, it 

is readily mapped as Early Proterozoic diabase. Within the 

greenschist-facies volcanic rocks, however, all metamorphosed diabase is mapped as Archean gabbro, failing 

evidence to the contrary.

Keweenawan diabase dikes cut all units in the area 

and are the youngest rocks within the northern block. These 

dikes are of two ages: (1) earlier north-south to northeasttrending, weakly metamorphosed or altered and normally 

polarized diabase and (2) generally east trending nearly 

fresh, reversely polarized olivine diabase (Baxter and Bornhorst, 1988).

STRUCTURE

The northern block has had a long and complex 

structural history that includes at least two major Archean 

deformational events: first, recumbent folding about eastwest axes (D^; second, upright folding about east-west axes 

(D2). These events resulted in folds with amplitudes and 

wave lengths measured in kilometers and a Z-shaped 

pattern, which largely determined the distribution of 

stratigraphic units.

Bedding, Foliations, and Lineations

Interpreting bedding attitudes contributes significantly to an understanding of the deformation of the 

northern block; however, the number of measured attitudes 

is small because of the scarcity of sedimentary and volcaniclastic rocks. Attitudes recorded from pillows in the 

Lighthouse Point Basalt and bedding in the Reany Lake 

pyroclastic, Fire Center mine iron-formation, and the Hills 

Lakes pyroclastic units are shown in figure 5. The attitudes 

indicate that bedding in the area is overturned.

The prevalent foliation (S^ in the northern block is a 

penetrative axial-planar foliation in amphibolite-facies rock

defined by nematoblastic hornblende and an anastomosing 

foliation along pillow margins in greenschist-facies rocks 

defined by lepidoblastic chlorite and (or) nematoblastic 

actinolite. Outcrops of amphibolite-facies basalts are 

banded and layered because of the extreme flattening of 

pillows. The principal flattening direction in greenschistand amphibolite-facies rocks is normal to the S, foliation. 

Lineations (L,) identified in the amphibolite-facies 

rocks are defined by the alignment of hornblende on B! 

surfaces. The attitudes of L^ are similar to the attitude of the 

intersection of bedding and S,, where bedding and lineation 

data are both available, indicating that Lj defines Fj fold 

hinges. Thus, Lj is interpreted to be associated with the 

development of Fj folds.

Folds

The younger folds (F2) control the overall Z-shaped 

pattern of stratigraphic units (fig. 5). As sedimentary and 

volcaniclastic rocks are rare, F2 folds are best defined by the 

folding of Sj (figs. 9 and 1QA). At the northwest end of the 

belt, Lj lineations plunge moderately to the southeast 

(fig. 10£), and S, is deformed by an upright, tight to 

isoclinal southeast-plunging F2 synform verging to the 

southwest (fig. 104). Farther to the east, along the same 

fold, L! plunges shallowly to the east (fig. 10D) and Sj 

forms an upright, open, shallow easterly plunging F2 synform (fig. 10C), whereas bedding (S0) forms an open 

synformal anticline plunging shallowly to the west (fig. 9). 

(Note that the fold plunge direction indicated in fig. 9 is for 

folded Sj foliation.) This apparent contradictory fold 

geometry is produced by folding S0 and Sj surfaces that are 

not parallel. Lj in this area is nearly flat lying; the plunge of 

folded bedding is less than 10° to the west, and the plunge 

of folded S, is less than 10° to the east. The contrary plunge 

of fold surfaces may be exacerbated by scissors motion 

along the Little Garlic shear zone, discussed in the section, 

"Shear zones." If the eastern extreme of the Little Garlic 

shear zone has a more pronounced north-side-up motion 

than segments of the shear zone to the west, this would 

result in an increase in the westerly dip of units north of the 

shear in relationship to units south of the shear zone.

The older generation of folds (F^ can be observed 

northeast and southeast of Rocking Chair Lakes (fig. 6), 

where the outcrop pattern of the Hills Lakes pyroclastic unit 

and gabbro sills define Fl folds. The southern F! fold is a 

synformal anticline and the northern Fj fold is an antiformal 

syncline. Both folds are tight to isoclinal, are upright, and 

plunge moderately to the southeast. In the vicinity of the 

fold hinges, bedding is normal to Sl (figs. 6 and 9), Sj is 

axial-planar, and Lx plunges moderately to the southeast 

(fig. IftB). Farther to the east, L! and Fj folds plunge 

shallowly to the east (fig. 10D). In the southeast are two 

smaller F2 folds that may be controlled by the bounding 

gneissic granitoids.

F14 Precambrian Geology of Lake Superior Region
87°30'

46°30' -

10 KILOMETERS

EXPLANATION

Fire Center mine iron-formation unit 

Trajectory of Si foliation

-> Antiform Showing trace of axial plane and 

direction and plunge of axis

-> Synform Showing trace of axial plane and 

direction and plunge of axis

Fault Dashed where inferred 

Contact

Figure 9. Sketch map showing S, foliation trajectory, F, and F2 folds, outcrop pattern of the Fire Center mine iron-formation 

unit, and major fault in east end of the northern block. A-A'f line of section shown in figure 11.

The development of folds in the northern block is 

shown schematically in figure 11. During D1? large-scale 

recumbent folds developed that are tight to isoclinal Fl folds 

(fig. 1 \A). Later, during D2, F! folds were refolded, forming 

isoclinal to open, upright F2 folds (fig. 1 IB\ and shearing 

occurred along the Little Garlic and Willow Creek shears 

(fig. 11C). This deformational sequence maintained the 

relative S0-Sl angular relationship observed (that is, $! 

typically dips more steeply to the north than S0), which is 

consistent with the exposed lower limb of the Pl recumbent 

fold (fig. 11Q. A similar sequence of deformational events 

has been observed in northern Minnesota (Bauer, 1985; 

Hudleston and others, 1988).

Faults

Several widely spaced subparallel north- to N. 15° E.- 

trending faults occur along the south margin of the northern 

block (fig. 2). These faults are commonly zones of breccia

cemented with quartz and ankerite; some contain disseminated sphalerite, galena, and chalcopyrite. The age of these 

faults is uncertain, but because they cut all other fabrics and 

are cut by Keweenawan diabase they are assumed to be 

either Late Archean or Early Proterozoic.

A northwest-striking fault mapped by Puffett (1974) 

within the gneissic granitoid rocks in sees. 21, 27, and 28, 

T. 49 N., R. 26 W. can be extended into sees. 17 and 20, 

T. 49 N., R. 26 W., where a northwest-trending valley is 

associated with truncation and (or) offset of the Nash Creek 

glomerophyric basalt unit and gabbro sills (fig. 5). 

Projection pushes this fault farther west of the Third Bass 

Lake in sec. 2, T. 49 N., R. 27 W. and then into the 

bounding gneissic granitoids in sees. 34 and 35, T. 50 N., 

R. 27 W. An apparent left-lateral strike-slip component 

along the fault is recognized because of the offset of the 

shallow-dipping Nash Creek glomerophyric basalt unit 

(fig. 6). This fault is of potential economic interest because 

along its southwest projection Puffett (1974) mapped zones

Archean Geology of the Ishpeming Greenstone Belt, Michigan F15
B

Figure 10. Stereographic projections (lower hemisphere) of A, S^ poles from west end of the northern block 

(n=363); 6, L^ from west end (n=17); C, S, poles from east end (n=184); D, L, from east end (n=56). Contours 

at 0.5, 4.0, 8.0, 12.0, and 16.0 percent points per 1 percent of area.

of intense silicification and abundant pyrite. The fault 

offsets the Nash Creek glomerophyric basalt unit and Slf 

suggesting that it postdates D2.

Shear Zones

Three major shear zones within or bounding the 

northern block Dead River, Willow Creek, and Little 

Garlic shear zones (fig. 2) are mappable at 71/2-minute 

quadrangle scale. They are ductile to brittle-ductile zones 

(Ramsay, 1980) and exhibit similar kinematics, with a 

strong north-side-up and right-lateral (dextral) shear sense. 

They are interpreted to have developed late during D2 and 

may have been reactivated during Proterozoic deformation.

Numerous smaller shear zones occur in the belt, for 

example along the contacts of Archean rhyolite intrusions. 

The smaller shears are from several centimeters to a meter 

or two in width. Smaller shears are more abundant in the 

vicinity of larger shears and may be subparallel to or branch 

from the major shear zone.

Dead River Shear Zone

The Dead River shear zone forms the south margin of 

the northern block (fig. 2). The shear zone controlled the 

localization of abundant carbonate alteration and rhyolite 

intrusions from the east end of the Dead River Storage 

Basin in sec. 10, T. 48 N., R. 26 W. to west of Silver Creek 

in sec. 23, T. 49 N., R. 28 W. (fig. 7). C bands in mylonite

F16 Precambrian Geology of Lake Superior Region
Bedding -

B

Figure 11. Schematic cross sections showing evolution of 

folding in northern block and relationships of bedding and S, 

foliation. Bar and ball symbol indicates stratigraphic top. Line 

of section for C shown in figure 9. A, Early recumbent folding 

during D,; S, is axial-planar to F, folds. B, Open folding about 

steep east-west axes during D2 ; at this stage, S-, dips more 

steeply than bedding. C, Present-day configuration, after latestage shearing (S symbols) that occurred during D2 .

and stretching lineations are typically nearly vertical, indicating dominant vertical displacement during at least the 

late stage of D2. West of Silver Creek the relationship of 

flattening and shear foliations (S-C structures) and shallow 

westerly plunging stretching lineations indicate right-lateral 

and north-side-up shear sense.

Willow Creek Shear Zone

The Willow Creek shear zone was named by Puffett 

(1974, p. 45) for a zone of amphibolite containing 

"irregular-shaped streaks of felsic rock" in sec. 4, 

T. 48 N., R. 26 W. and sec. 31, T. 49 N., R. 26 W. (fig. 2 

of this report). Puffett interpreted these felsic streaks as

fragments of a once-continuous body of felsic rock. 

MacLellan and Bornhorst (1989) studied a segment of this 

zone and on the basis of rhyolitic to dacitic and gabbroic 

"clasts" interpreted the lithology to represent a deformed 

volcaniclastic breccia. Further study by Johnson indicates 

the presence of a regionally continuous high strain zone that 

locally contains dikelets of gabbro and rhyolite.

The Willow Creek shear zone is exposed in sees. 21, 

25-28, 35, and 36, T. 49 N., R. 27 W., and forms the north 

margin of the Clark Creek basin in this area. The east 

extension is less well defined, but sheared chloritecarbonate schists in a north-northwest-trending creek in 

sec. 4, T. 49 N., R. 26 W. support extension of the shear 

through that area (fig. 2).

Little Garlic Shear Zone

The Little Garlic shear zone is named for the shear 

that strikes west from the Little Garlic River in sec. 13, 

T. 49 N., R. 27 W. and then along the Little Garlic River 

through sees. 14 and 15, T. 49 N., R. 27 W. The western 

continuation of this shear is buried beneath Early Proterozoic sedimentary rocks and Quaternary alluvium, but it is 

likely that the cliff along the north margin of the Clark 

Creek basin in sees. 17 and 18, T. 49 N., R. 27 W. and a 

fault in sec. 12, T. 49 N., R. 28 W. form the westerly 

extension of this shear zone (fig. 2). Carbonate alteration, 

rhyolite intrusive rocks, and late-stage quartz veins 

characterize this shear zone.

Kinematic indicators in mylonite within the shear 

zone (S-C structures) reveal north-side-up and right-lateral 

shear sense. C bands and stretching lineations in this zone 

dip and plunge steeply to the north. In contrast, kinematic 

indicators in the other two notable shear zones indicate 

near-vertical north-side-up shear sense. This divergence of 

attitude suggests preexisting stratigraphic and (or) structural 

control on the orientation of the shears.

GEOLOGIC HISTORY

The oldest event recorded in rocks of the northern 

block of the Ishpeming greenstone belt is subaqueous 

eruption of tholeiitic basalt lava flows, which accumulated 

to form the Lighthouse Point Basalt. Sporadic deposition of 

felsic pyroclastic material as turbidity and debris flows of 

the Reany Lake and the Hills Lakes pyroclastic units, 

respectively, produced stratigraphically continuous units 

within the basalt flows. During its turbulent descent from 

the volcano, the turbidity flow of the Reany Lake pyroclastic unit possibly incorporated parts of a massive sulfide 

body. The Reany Lake pyroclastic unit is overlain by the 

Fire Center mine iron-formation unit, which was deposited 

during a period of volcanic quiescence. Gabbro sills and 

dikes were emplaced shortly after deposition of the surrounding basalts.

Archean Geology of the ishpeming Greenstone Belt, Michigan F17
Large-scale, north-directed thrusting (Dj) resulted in 

the development of recumbent nappe folds with an axialplanar foliation in the volcanic pile. Granitoid magmas 

intruded the belt as large batholiths and dikes. This 

magmatic event resulted in metamorphism of the volcanic 

rocks near contacts to amphibolite facies. A second northdirected compression (D2) resulted in folding and shearing 

of the Lighthouse Point Basalt and associated rocks and 

deformation of the plutonic granitoid rocks. The rightlateral, north-side-up shear zones suggest that this event 

may be associated with transpression during the development of the Great Lakes tectonic zone to the south. The 

granodiorite near Rocking Chair Lakes then was intruded 

into fold hinges and rhyolite was intruded along zones of 

high strain. Gold mineralization probably occurred late 

during this tectonic event.

A period of extension was associated with the 

deposition of Early Proterozoic sedimentary rocks. Base 

metal quartz veins may have formed during this time. 

North-directed compression during the Early Proterozoic 

Penokean orogeny resulted in thrusting and folding of the 

Early Proterozoic sedimentary rocks and metamorphism of 

these rocks to greenschist facies in the northern block. After 

a considerable hiatus, Middle Proterozoic (Keweenawan) 

dikes intruded all older rocks in the vicinity of the Midcontinent rift system.

ECONOMIC GEOLOGY

No mines are active in the northern block. Historic 

activity includes significant prospecting (trenches and 

shafts) during the 1860's on base metal-silver veins at 

Silver Creek in the NEJ/4 sec. 25, T. 49 N., R. 28 W., the 

Lead pits in the SEV4 sec. 34, T. 49 N., R. 27 W., and the 

Holyoke mine in the NEV* sec. 2, T. 48 N., R. 27 W. The 

Fire Center Mining Company produced gold from the Fire 

Center mine from 1892 to 1893. Descriptions of numerous 

prospects in the northern block are given in Michigan 

Geological Survey reports listed in table 1.

Base Metal Occurrence and 

Iron-Formation

Puffett (1974) described the occurrence of base metal 

sulfides in the Fire Center mine iron-formation unit; 

however, our mapping showed that this occurrence is hosted 

instead by the Reany Lake pyroclastic unit. Pyrite, sphalerite, chalcopyrite, and galena are fine to medium grained, 

disseminated, and crudely bedded in the lithic-tuff horizon. 

The sulfide minerals seem to be present along the entire 

strike length of this unit, but abundances vary greatly from 

place to place. Because the sulfides compose a thin zone and 

are disseminated and variable in concentration, this zone has

a low potential as a source of base metals. Although detailed 

studies of the base metals have not been conducted, we 

presently interpret them as remnants of a massive sulfide 

deposit that was incorporated into the Reany Lake pyroclastic unit during its deposition.

The Fire Center mine iron-formation unit contains a 

significant amount of magnetite (up to 30 percent), but its 

thinness and steep dip (typically over 70°) exclude it as an 

economical source of iron ore at the present time (1990). 

The real potential of this iron-formation may be as host for 

an Archean lode gold deposit, inasmuch as many Canadian 

gold deposits are hosted in iron-formation (Cplvine and 

others, 1988); however, only minor gold anomalies have 

been reported in association with this iron-formation.

Gold Prospects

Several precious metal prospects spatially associated 

with the Dead River shear zone occur along the south 

margin of the northern block. Among these prospects, only 

the Fire Center mine was a producer, yielding about 100 

troy oz of gold.

A typical gold prospect may possess any or all of 

these features: (1) a quartz porphyritic rhyolite dike;

(2) brecciated (cemented with ankerite) margins of the dike;

(3) serialization, silicification, carbonatization, tourmalinization, and sulfidization of the dike; (4) sheared country 

rock immediately adjacent to the dike, with or without 

intense ankerite alteration; (5) late quartz veins that cut both 

the rhyolite dike and the sheared mafic country rock; 

(6) auriferous pyrite in the quartz veins; (7) disseminated 

auriferous pyrite as a halo about the quartz veins; and 

(8) calcite alteration as a halo about the gold occurrence. 

Gold mineralization in the northern block of the Ishpeming 

greenstone belt is closely associated with carbonatized shear 

zones, particularly the Dead River shear zone. The intimate 

spatial relationship between the rhyolite intrusives and the 

gold mineralization in this shear zone suggests that they 

may be synchronous. If so, the age of gold mineralization is 

Archean and syntectonic with D2 deformation.

Base Metal-Silver Prospects

Numerous historic base metal-silver prospects are 

located along the south margin of the northern block, again 

related to the Dead River shear zone. These prospects are 

quartz-carbonate veins bearing galena, sphalerite, chalcopyrite, pyrite, and arsenopyrite. The sulfides occur as fineto coarse-grained disseminations and as masses in the vein, 

and are commonly concentrated along the contact between 

the vein and the country rock. The quartz is white and 

vitreous. The carbonate is coarse grained and grayish white, 

weathering to rusty brown; it is disseminated throughout the

F18 Precambrian Geology of Lake Superior Region
quartz vein and forms late crosscutting veinlets. The base 

metal-silver-bearing quartz-carbonate veins are typically 

less than 1 m wide and hosted in brecciated fault zones. The 

dominant trend of the veins is about N. 15° E.

The base metal silver prospects are too low in grade 

and too small to be considered as likely sources of either 

base metals or silver at the present time (1990). Analyses for 

gold from base metal-silver prospects typically show low 

values (<30 ppb). At the Silver Creek prospect, the base 

metal vein is hosted in a fault breccia that cuts across 

sheared (ductile deformation) carbonate altered basalts that 

have anomalous gold values. This relationship indicates that 

the base metal-silver mineralization postdates gold 

mineralization.

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upright folding across the boundary between an Archean 

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Archean Geology of the Ishpeming Greenstone Belt, Michigan F19
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F20 Precambrian Geology of Lake Superior Region

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the conditions indicated below from the U.S. Geological Survey, Books 

and Open-File Reports Section, Federal Center, Box 25425, Denver, 

CO 80225. (See latest Price and Availability List)

"Publications of the Geological Survey, 1879-1961" may be purchased by mail and over the counter in paperback book form and as a 

set of microfiche.

"Publications of the Geological Survey, 1962-1970" may be purchased by mail and over the counter in paperback book form and as a 

set of microfiche.

"Publications of the U.S. Geological Survey, 1971-1981" may be 

purchased by mail and over the counter in paperback book form (two 

volumes, publications listing and index) and as a set of microfiche.

Supplements for 1982,1983,1984,1985,1986, and for subsequent 

years since the last permanent catalog may be purchased by mail and 

over the counter in paperback book form.

State catalogs, "List of U.S. Geological Survey Geologic and 

Water-Supply Reports and Maps For (State)," may be purchased by mail 

and over the counter in paperback booklet form only

"Price and Availability List of U.S. Geological Survey Publications," issued annually, is available free of charge in paperback booklet form only.

Selected copies of a monthly catalog "New Publications of the U.S. 

Geological Survey" available free of charge by mail or may be obtained 

over the counter in paperback booklet form only. Those wishing a free 

subscription to the monthly catalog "New Publications of the U.S. 

Geological Survey" should write to the U.S. Geological Survey, 582 

National Center, Reston, VA 22092.

Note.-Prices of Government publications listed in older catalogs, 

announcements, and publications may be incorrect. Therefore, the 

prices charged may differ from the prices in catalogs, announcements, 

and publications.