INSTITUTE ON LAKE SUPERIOR GEOLOGY

PROCEEDINGSe

36th Annual MeetingMay 9-12, 1990

held atThunder Bay, Ontario

H.L. JAMES VOLUME

VOLUME 36 May 1990

Part 2. Field Trip Guidebook

INSTITUTE ON LAKE SUPERIOR GEOLOGY

PROCEEDINGS . 36th Annual Meeting

May 9-12,1990 held at

Thunder Bay, Ontario

H.L. JAMES VOLUME

VOLUME 36 May 1990

Part 2. Field Trip Guidebook

Organizing Committee, 36th Annual Meeting, ILSG (1990)

The Organizing Committee comprises the following members of the

Department of GeologyLakehead University

Thunder Bay, Ontario P7B 5E1

General Chairman: Manfred M. Kehienbeck

Program Chair and Abstract Editor: Philip W. Fralick

Field Trip Guidebook Editor: Graham J. Borradaile

Volume 36 consists of

Part 1: Abstracts

Part 2: Field Trip Guidebook

Reference to material in Proceedings Volume should follow the example below:

Brown, Bruce A., 1989, Significance of Conglomerates in the Baraboo quartzite ofsoutheastern Wisconsin [abst.I; Institute on Lake Superior GeologyProceedings, 35th Annual Meeting, Duluth, MN, 1989; Houghton, Ml, v. 35,part 1, p. 11-12.

Published and Distributed byInstitute on Lake Superior GeologyJ. Kalliokoski, Secretary/Treasurer

Dept. of Geological Engineering, Geology and GeophysicsMichigan Technological University

Houghton, Michigan 49931

ISSN 1042-9964

Organizing Committee, 36th Annual Meeting, ILSG (1990)

The Organizing Committee comprises the following members of the

Department of Geology Lakehead University

Thunder Bay, Ontario P7B 5E1

General Chairman: Manfred M. Kehlenbeck

Program Chair and Abstract Editor: Philip W. Fralick

Field Trip Guidebook Editor: Graham J. Boiradaile &

Volume 36 consists of

m: Abstracts

m 2 : FW Trip Guktnwk

Reference to material in Proceedings Volume should follow the example below:

Brown, Bruce A., 1989, Significance of Conglomerates in the Baraboo quartzite of southeastern Wisconsin [abst.]; Institute on Lake Superior Geology Proceedings, 35th Annual Meeting, Duluth, MN, 1989; Houghton, MI, v. 35, part 1, p. 11-12.

Published and Distributed by Institute on Lake Superior Geology J. Kalliokoski, Secretary/Treasurer

Dept. of Geological Engineering, Geology and Geophysics Michigan Technological University

Houghton, Michigan 49931

ISSN 1042-9964

TABLE OF CONTENTS

Introduction 1

M. J. Lavigne, Jr.

FIELD TRIP 1

Mafic intrusions, PGE mineralization and granitoidrocks of the Lac des Illes area.

R. H. Sutcliffe 11

FIELD TRIP 2

Geology of the Shebandowan and Quetico ArcheanSubprovinces.

U. J. Borradaile . . . . 43

FIELD TRIP 3

Granitoid-related mineral deposits in the westernLake Superior region.

S. A. Kissin 52

FIELD TRIP 4

Base Metal Mineralization in the ShebandowanGreenstone Belt.

M. J. Lavigne Jr., A. J. Aubut & J. F. Scott 67

11

TABLE OF CONTENTS

. duction ........... ...

M. J. Lavigne, Jr.

Mafic intrusions, PGE mineralization and granitoid rocks of the Lac des Illes area.

....................... R. H. Sutcliffe

FIELD TRIP 2

Geology of the Shebandowan and Quetico Archean Subprovinces.

FIELD TRIP 3

Granitoid-related mineral deposits in the western Lake Superior region.

FIELD TRIP 4

Base Metal Mineralization in the Shebandowan Greenstone Belt.

M. J. Lavigne Jr., A. J. Aubut & J. F. Scott

GEOLOGY OF THE IIAREFIFD REGION

by

Manriee J. Lavigne Jr.Ontario Ministry of Northern Development and Mines

Thunder Bay

S

1

GEOLOGY W THE LAKEIIEAD REGION

Maurice J- Lavigne Jr.

Ontario Ministry of Northern Development and Mines

& Thunder Bay

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—HFigure 18 Lithatectonic ~ul.$provinces of the Superior

Province (a f t er Card and C i e a i e l s k i , 1986)

INTRODUCTION

The Lakehead Region is an area hound to the east byLake Superior, to the south by the international border,

includes Lake Nipigon, and extends to the west as far as the

town of Atikokan. This region is underlain by precambrian

rocks of the Canadian Shield, in particular, Archean rocks

the Superior Province and Proterozoic rocks of the SouthernProvince (Fig 1). Three subprovinces of the SuperiorProvince are represented; Wabigoon, Quetico, and theAbitibi—Wawa. The Southern Province is subdivided into theNipigon Plate and the Port Arthur Homocline, the northern

fringe of the Aninikie basin (enclosed nap)

Stockwell (1964,1970) and Card and Ciesielski (1986)

subdivided the Superior Province on the basis of structure

and lithology. In this region, the subprovinces aresupracrustal-intrusive complexes, the predominance of eithervolcanic or sedimentary rock being a key distinguishing

characteristic. The Ahitibi—Wawa and the Wabigoon

Suhprovinces are arcuate assemblages of volcanic rock

dominated belts, "greenstone belts', and tonalitic to

granodioritic, massive to gneissic intrusions. The Quetico

Subprovince is a linear metasedimentary terrane cored by

granitic intrusions and migmatites. The subprovince

boundaries are broad schist zones and/or distinct faults,

the Seine River—Quetico fault couple, which separate the

Quetico from the Wabigoon, being good examples of both of

these types of strain.

The Nipigon Plate and the Port Arthur Homocline consist

of flat lying sedimentary rock uncomformibly over'ying the

Superior Province. These also include diabase sills and

dykes, late nafic intrusions and v,lcanic rocks.

3

The Lakehead Region is an area bound to the east by

Lake Superior, to the south by the international border,

includes Lake Nipigon, and extends to the west as far as the

town of Atikokan. This region is underlain by precambrian

rocks of the Canadian Shield, in particular, Archean rocks

the Superior Province and Proterozoic rocks of the Southern

Province (Fig 1). Three subprovinces of the Superior

Province are represented; Wabigoon, Quetico, and the

Abitibi-Wawa. The Southern Province is subdivided into the

Nipigon Plate and the Port Arthur Homocline, the northern

fringe of the Aninikie basin (enclosed nap).

Stockwell (1964,1970) and Card and Ciesielski (1986)

subdivided the Superior Province on the basis of structure

and lithology. In this region, the subprovinces are

s~ipracrustal-intrusive complexes, the predoninance of either : '

volcanic or sedimentary rock being a key distinguishing

characteristic. The Abitibi-Wawa and the Wabigoon b

Subprovinces are arcuate assemblages of volcanic rock

dominated belts, 'greenstone belts", and tonalitic to

granodioritic, massive to gneissic intrusions. The Quetico

Subprovince is a linear metasedimentary terrane cored by

granitic intrusions and migmatites. The subprovince

boundaries are broad schist zones and/or distinct faults,

the Seine River-Quetico fault couple, which separate the

Quetico from the Wabigoon, being good examples of both of

these types of strain.

The Sipigon Plate and the Port nithur Homocline consist

of flat lying sedimentary rock unconformibly overlying the

Superior Province. These also include diabase sills and

dykes, late oafic intrusions and vzlcanic rocks

ARCHEAN STRATICRAPHY

High precision U—Pb zircon geochronology, carried out

at the Royal Ontario &tuseun, has resulted in a dramatic

change in the rate of increased understanding of the

archean. Absolute dates corbined with structural

interpretations of well documented lithological distribution

has resolved the stratigraphy within some greenstone belts.

It has also helped resolve the relationships between

plutonism, volcanism, sedimentation, metamorphism,

deformation and mineralization. Many stratigraphic

relationships within subprovinces, and in between

subprovinces have also been resolved. In the Lakehead

Region some fundamental distinctions, and similarities, can

now he drawn in between the Wabigoon and the Abitibi-Wawa

tonalite-greenstone terranes and their relationship to theintervening paragneiss of the Quetico.

In Atilcakan, the Marrnion Lake Batholith, a tonalitecomplex, has been dated at 3004 ma (Davis et al, 1988).

This hatholith is uncomformibly overlain by the Steeprock

Group sedimentary rocks and subsequent volcanic rocks. This

unconformity is well exposed on the east wall of the Roberts

pit. Stone et al.(in press) describe the unconformity and

the overlying Wagita Formation as follows; "The uncomformity

at the base of the Wagita Formation is marked by a

transition from unaltered tonalite to sericitized tonalite

and a sericite—carbonate-quartz 'grit', which becomes well

bedded and clast bearing up section". The ccagita formation

is overlain by the stroriatolitic Mosher Carbonate Formation

and altered iron formation (Jolliffe Ore Zone Formation).

This sedimentary sequence is overlain in ascending order by;

ultranafic pyroclastics, mafic pillow lavas and intermediate

to felsic flows, tuffs and breccias. This sequence is then

overlain by clastic sediments.

Felsic tuffs in the Lumby Lake Creenstone Belt,on the

4

High precision ,. ,- .

at the Royal Ontario Museura, has resulted in&.draraatic <<.r . .

change in the rate. of increased understanding,/of the . v.. * archean. Absolute dates combined with structural

interpretations of well documented lithological distribution

has resolved the stratigraphy within some greenstone belts.

It has also helped resolve the relationships between

plutonism, volcanisn, sedimentation, metanorphisra,

deformation and mineralization. Many stratigraphic

relationships within subprovinces, and in between

subprovinces have also been resolved. In the Lakehead

Region some fundamental distinctions, an2 similarities

now be drawn in between the Wabigoon and the Abitibi-Wawa

tonalite-greenstone terranes and their relationship to-the

interveninq paracmeiss of the Quetico. - - - In Atikokan, the larmion Lake Batholith, a tonalite

omplex, has been dated at 3004 ma (Davis et al, 1988).

his batholith is uncomformibly overlain by the Steeprock

roup sedinentary rocks and subsequent volcanic rocks. This

ncoraformity is well exposed on the east wall of the Roberts

e overlying Wagita Formation as follows; "The uncoraformity

t the base of the Wagita Formation is marked by a

ransition from unaltered tonalite to sericitized tonalite

nd a sericite-carbonate-quartz 'grit', which becomes well added and clast bearing up section". The Wagita formation - ,

altered iron formation (Jolliffe Ore Zone Formation).

his sedimentary sequence is overlain in ascending order by;

o felsic flows, tuffs and bre

north side of the .Marmjcin Lake Batholith, have been dated at

2950 ma aria 2999 ma (Davis and Jackson, 1988). These are

considered to be the stratigraphic equivalents to the felsic

volcanics in the Steeprock nine area. This older, carbonate

sediment bearing, volcariic—sedirnerit:a ry sequence has beenrecognized in many greenstone belts in the northern parts ofthe superior province (Thurstori and Chivers, 1990). Thesesequences, as well as quartz arenites—iron formation—

ultrarnafic lava sequences recognized in other belts, have

shallow water sedimentary features. This is the basis for

the recognition of early archeari platfot-nal sequences which

developed at about 3000 ma (Wood et al, 1986).

These shallow water guartz—arenite and carbonatebearing sequences ('2.85 Ga) are generally overlain by

mafic—ultramafic volcanic sequences (2.740 to 2.700 Ga) and

mafic to felsic volcanic cycles (2.775 to 2.700 Ga) and

comprise what is stratigraphically referred to as

"Keewatin". Keewatin rocks volumetrically dominate all

greenstone belts in the Superior province. The younger of

these Keewatin volcanic sequences are not recognized in the

Atikokan area. They are however, the only recognizable

volcanic sequence older than 2.700 Ga in the Shebandowan

greenstone belt of the Abitibi—Nawa subprovince. Thurston

(1986) interprets two mafic to felsic volcanic cycles.

Corfu and Stott (1986) have generated a date of 2.733 Ga for

felsic volcanics in the Middle Shebandowan Lake area.

Stratigraphic relationships in this belt have been masked by

intense deformation.

A common thread for both the Abitibi-Wawa and the

Vabigoon is that they both contain younger "Temiskaming"-

type sequences, found in many greenstone belts in the

Canadian Shield. These volumetrically insignificant

sequences consist of alluvial—fluvial sedimentary rock,

mainly conglomerate and arkose with or without alkalic and

shoshinitic volcanic rock (Shegelski, 1980). In the

S

orth side of the Marnion Lake Batholith, have been dated at I 950 na and 2999 raa (Davis and Jackson, 1988). These are

onsidered to be the stratigraphic equivalents to the felsic

olcanics in the Steeprock nine area. This older, carbonate

diment bearing, volcanic-sedimentary sequence has been

cognized in many greenstone belts in the northern parts of

he superior province (Thurston and Chivers, 1990). These

equences, as well as quartz arenites-iron fornation-

ultramafic lava sequences recognized in other belts, have

shallow water sedimentary features. This is the basis for

the recognition of early archean platfornal sequences which developed at about 3000 ma (Wood et al, 1986).

1 These shallow water quartz-arenite and carbonate I

earinq sequences (>2.85 Ga) are generally overlain by

afic-ultramafic volcanic sequences (2.740 to 2.700 Ga) and

afic to felsic volcanic cycles (2.775 to 2.700 Ga) and

comprise hat is stratigraphically referred to as

Keewatin". Keewatin rocks volumetrically doninate all

reenstone belts in the Superior province. The younger of

hese Keewatin volcanic sequences are not recognized in the

Atikokan area. They are however, the only recognizable

volcanic sequence older than 2.700 Ga in the shebandowan

greenstone belt of the Abitibi-Kawa subprovince. Thurston

(1986) interprets two mafic to felsic volcanic cycles.

Corfu and Stott (1986) have generated a date of 2.733 Ga for

felsic volcanics in the Middle Shebandowan Lake area.

Stratigraphic relationships in this belt have been masked by

intense deformation.

A connon thread i.ur both the Abitibi-Wawa and the

Wabigoon is that they both contain younger "Temiskaming"-

ype sequences, found in many greenstone belts in the

Canadian shield. These volunetrically insignificant

sequences consist of alluvial-fluvial sedimentary rock,

Atikokan—?line Centre area, a distinctive conglomerate unit,

referred to as the Seine Conglomerate, is found along the

Quetico—Wabigoon subprovince boundary. The contact

relationship with adjoining Quetico sediments and Wahigocn

volcanics is unclear because of poor exposure and shearing.

Poulsen (1984), deduced that the Seine Conglomerates were

younger on the basis of preserved evidence of defornat ion

history. This is supported by U-Pb dates of 2685 ma for a

clast in the conglomerate and an age range of 2700 tO 2795

ma for detrital zircons in the Quetico Metasedinents (Danis

et al, 1990).

In the northern Shehandowan greenstone belt,

interdigitated "Teniskaming' alkaline volcanics,conglomerates and sandstones are dated at 2689 ma and the

underlying "ICeewatin" volcanic-s at 2733 ma (Corfu and Stott,

1986). The contact relationship is obscured by well

developed foliation leading to speculation that the

"Temiskaning" rocks developed in a fault bounded through. A

second belt of Teniskaming rocks a few miles to the south

has a clearly discomfort-uable stratigraphic relationship with

underlying "Keewatin" volcanic rocks.

In summary, the stratigraphy within the volcanic—

tonalite terrane consist of 3 b.y. old tonalite basement,

overlain by a 3 b.y. old platfornal sequence, 2.74 b.y. sub-

alkalic volcanics and 2.68 b.y. old calc—alkalic volcanics

and sedimentary rocks. Recent age dates on detrital zircons

from Quetico greywacke range from 2700 to 2795 ma (Davis et

al, 1990) strongly suggesting a coeval relationship with the

sub—alkalic volcanics. The suggestion by Williams (1987),

that the Quetico represents an accretionary prism is well

accepted. A host to the 3 b.y. tonalite intrusive has yet

to be recognized.

The last major geological event in the tonalite—

greenstone terrane. is represented by the voluminous

tonalites which intruded from 2690 to 2677 Ma (Colvine, et

C)

as the Seine Conglonerate, is found along the

uetico-Mabigoon subprovince boundary. The contact

elationship with adjoining Quetico sediments and Kabigoon

olcanics is unclear because of poor exposure and shearing.

ounger on the basis of preserved evidence of defornati

istory. This is supported by C-Pb dates of 2685 na for a

last in the conglomerate and an age range of 2700 tG 2795

a for detrital zircons in the Quetico Mstasediments (Daris

t al, 1 9 9 0 ) .

In the northern Shebandowan greenstone belt,

nderlying "Keewatin" volcanics at 2733 na (Corfu and Stott,

9 8 6 ) . The contact relationship is obscured by well

eveloped foliation leading to speculation that the

'Teniskaning" rocks developed in a fault bounded through. A

econd belt of Teniskaroing rocks a few miles to the south

as a clearly disconforniable stratigraphic relationship with

nderlying "Keewatin" volcanic rocks.

In suranary, the stratigraphy within the volcanic-

nalite terrane consist of 3 b.y. old tonalite basement,

'erlain by a 3 b.y. old platfornal sequence, 2 . 7 4 b.y. sub-

Ikalic volcanics and 2.68 b.y. old calc-alkalic volcanics

nd sedimentary rocks. Recent age dates on detrital zircons

on Quetico greywacke range from 2700 to 2795 ma (Davis et

at the Quetico represents an accretionary prism is well

e recognized.

The last major geological event in the tonalite-

nstone terrane is represented by the voluminous

analites which intruded from 2690 to 2677 Ma (Colvine, et

al 1988). The. granitic rocks in the Quetico intruded from

2670 tO 2650 Ma (Percival, 1989). Both of these

subprovinces have widespread mafic intrusions which are notwell constrained geochronoloyically.

The isoclinally folded, steeply dipping archean rocksof the Superior Province, are overlain by the shallow

dipping protei-ozoic rocks of the Southern Province. The

Port Arthur Honocline consist of the Gunflint Formation

(taconite, algal chert, limestone, and tuffaceous shale) and

the overlying Rove Formation (shale, greywacke). The age of

these rocks is poorly constrained but is considered to be

older than 1850 Ma. (Morey, 1983). The Nipigon Plate

consist ci the Pass Lake Formation (quartz arenite), the

Passport Fornation ( redbeds" — dolestone, sandstone,

chart, mudstone) and the Kana Fill Formation (nudstone)These three formations comprise the Sibley Group whichuncomfornibly oven ies archean rocks and the Rove Formation.These sedimentary rocks are overlain by the Keweenawanvolcanic rocks of the Osler Group. They have also been

intruded by early Keweenawan diabase, the dominant lithology

in the Nipigon Plate, and late Reweenawan gabbroic stocks

and dykes.

METALLOGENY

The Lakehead Region is host to a wide variety of types

of mineral deposits. The greenstone—tonalite terrane

contain volcanogenic Cu-Zn deposits associated with felsic

volcanic rocks, konatiite (intrusive 7 extrusive 7) hosted

Ni—Cu deposits, nafic intrusive hosted platinum metal group

deposits, iron deposits, and lode gold deposits. The

7

al 1 9 8 8 ) ; T h e g r a n i t i c r o o k s i n t h e Q u e t i c o i n t r u d e d f rom

2670 t 0 2650 Ma ( P e r c i v a l , 1 9 8 9 ) . Both o f t h e s e

s u b p r o v i n c e s have w i d e s p r e a d m a f i c i n t r u s i o n s which a r e n o t : '

well c o n s t r a i n e d C J e 0 ~ h r ~ ~ n 0 l 9 g i ~ a l l y .

The i s o c l i n a l l y f o l d e d , s t e e p l y d i p p i n g a r c h e a n r o c

o f t h e S u p e r i o r P r o v i n c e , a r e o v e r l a i n by t h e s h a l l o w

d i p p i n g p r o t e r o z o i c r o c k s o f t h e S o u t h e r n P r o v i n c e . The

P o r t A r t h u r i lomocline c o n s i s t o f t h e G u n f l i n t Format ion

( t a c c n i t e , a l g a l c h e r t , l i m e s t o n e , and t u f f a c e o u s s h a l e ) a n

t h e o v e r l y i n g Rove Format ion ( s h a l e , g r e y w a c k e ) . The a g e o f

t h e s e r o c k s i s p o o r l y c o n s t r a i n e d b u t i s c o n s i d e r e d t o b e

o l d e r t h a n 1850 Ma. (?Iorey, 1 9 8 3 ) . The X i p i g o n P l a t e

c o n s i s t o f t h e P a s s Lake Format ion ( q u a r t z a r a n i t e ) , t h e

R o s s p o r t F o r n a t i o n ( " r e d b e d s " - d o l e s t o n e , s a n d s t o n e ,

c h e r t , muds tone) a n d t h e Kama H i l l Format ion ( m u d s t o n e ) .

T h e s e t h r e e f o r n a t i o n s c o m p r i s e t h e S i b l e y Group which

unconiforraibly o v e r l i e s a r c h e a n r o c k s and t h e Rove Format ion .

T h e s e s e d i m e n t a r y r o c k s a re o v e r l a i n by t h e Keweenawan

v o l c a n i c r o c k s o f t h e Osier Group. They h a v e a l s o been

i n t r u d e d by e a r l y Keweenawan d i a b a s e , t h e d o n i n a ~ t l i t h o l o g y

METALLOGEN

The ~ a k e h e a d Region is h o s t t

o f m i n e r a l d e p o s i t s . The g r e e n s t o n e - t o n a l i t e t e r r a n e

c o n t a i n v o l c a n o g e n i c Cu-Zn d e p o s i t s a s s o c i a t e d w i t h f e l s i c

v o l c a n i c r o c k s , k o r n a t i i t e ( i n t r u s i v e ? e x t r u s i v e ? ) h o s t e d

Ni-Cu d e p o s i t s , rnafic i n t r u s i v e h o s t e d p l a t i n u m m e t a l g r o u p

Quetico subprovince contains subeconomic rare element

deposits in pegmatites and platinum metal group and Cu-Ni

deposits in naf Ic intrusives. Proterozoic rocks containnumerous veins in sedimentary rocks with varied amounts ofsilver, zinc, lead, harite and fluorite in; native copperdeposits in nafic vulcanic rocks; and platinum metal group-Cu—Ni deposits in mafic intrusives.

S

Quetico subprovince contains sufceconomic rare element

deposits in pegmatitea and platinum metal group and Cu-Xi deposits in nafic intrusive*. Proterozoic. rocks contain

numerous veins in sedimentary rocks with varied amounts a f

silver, zinc, lead, barite and fluorite in; native copper

deposits in mafic volcanic rocks; and platinum neta

CU-xi deposits in i i a f i c intrusives.

REFERENCES

Card, lCD. and Ciesielski, A.1986: Subdivisions of the Superior Province of the

Canadian Shield. Geoscience Canada, 13:5—13.

Colvine, A.C., Fyon, J.A., Heather, N.E., Marmont, S.,Smith, P.M. and Troop, D.G.1988: Archean lode gold deposits in Ontario.

Ontario Geological Survey, Misc. Pap., 139, 136p

Corfu, F. and Stott, G.M.1986: U-Pb ages for late magnatisn and regional

deformation in the Shebandowan Belt, SuperiorProvince, Canada. Can. 3. Earth Scie., 23:1075—1082.

Davis, D.W. and Jackson, M.C.1988: Geochronology of the Lumby Lake Greenstone belt:

a 3 Ga complex within the Wabigoon subprovince,northwest Ontario. Geol. Soc. Am. Bull.,100(6) :818—824.

Davis, D.W., Poulsen, N.H. and Kamo, S.L.1989: New insights into Archean crustal development

from geochronology in the Rainy Lake area,Superior Province, Canada. Journal of Geology,98.

Hyde, R.S.1980: Sedimentary facies in the Archean Temiskaming

Group, and their tectonic implications, Abitibigreenstone belt, northeastern Ontario, Canada.Precambrian Res., 12:141—160.

Percival, J.A. and Williams, H.R.1989: The late Archean Quetico accretionary complex,

Superior Province, Canada. Geology 17:23-25.

Paulsen, N.H.1984b: Archean tectonics and mineralization at Rainy

Lake, northwestern Ontario. Ph.D. Thesis, QueensUniversity, Kingston, Ont., 342pp.(unpublished).

Shegeiski, P.3.1980: Archean cratonization, emergence and red bed

development, Lake Shebandowan area, Canada.Precambrian Research, volume 12, p.331—347.

9

REFERENCES

rd, K.D. and Ciesielski, A. 86: Subdivisions of the Superior Province of the

Canadian Shield, Geoscience Canada, 13:5-13

olvine, A.C., Fyon, J.A., Heather, K.B., Marnont, a., ith, P.M. and Troop, D.G. 988: Archean lode gold deposits in Ontario.

Ontario Geological Survey, Misc. Pap., 139, 136p

Corfu, F. and Stott, G.H. 1986: C-Pb ages for late magnatisn and regional

deforiaation in the Shebandowan Belt, Superior Province, Canada. Can. J. Earth Scie., 23:1075- 1082.

Davis, D.K. and Jackson, M.C. 1988: Geochronology of the Lumby Lake Greenstone belt:

a 3 Ga complex within the Wabigoon subprovince, northwest Ontario. Geol. Soc. Am. Bull., 100(6):818-824.

Davis, D.W., Poulsen, K.H. and Kano, S.L. 1989: Kew insights into Archean crustal development

from geochronology in the Rainy Lake area, Superior Province, Canada. Journal of Geology, 98.

Hyde, R.S. 1980: Sedinentary facies in the Archean Teraiskaning

Group, and their tectonic implications, Abitibi greenstone belt, northeastern Ontario, Canada. Precanbrian Res., 12:141-160.

Percival, J.A. and Williams, H.R. 1989; The late Archean Quetico accretionary complex,

Superior Province, Canada. Geology 17:23-25.

Poulsen, K.H. 1984b: Archean tectonics and mineralization at Rainy

Lake, northwestern Ontario. Ph.D. Thesis, Queens University, Kingston, Ont., 342pp.(unpublished).

Shegelski, R.J. 1980: Archean cratonization, emergence and red bed

development, Lake Shebandowan area, Canada. Precambrian Research, volume 12, p.331-347.

Stockwell, C.H.1964: Fourth report on structural provinces, orogenies

and time-classification of rocks of the CanadianPrecambrian Shield. In Age determinations andgeological studies, part II. Geological Survey ofCanada, Paper 64—17, pp.44—54.

1970: Geology of the Canadian Shield, introduction. InGeology and economic minerals of Canada, part A.Edited by R.J.W. Douglas, Geological Survey ofCanada, Economic Geology Report 1, pp.44-54.

ccillians, H.R.1987c: Structural studies in the Wabigoon and Quetico

subprovinces. Ontario Geological Survey, OpenFile Report 5668.

Wood, J., Thurston, P.C., Corfu, F. and Davis, D.W.1986: Ancient guartzites and carbonates in northwestern

Ontario—-evidence for early (Archean) stability?Geol. Assoc. Can./Mineral Assoc. Can., Progr. withAbstr., 11, p.146.

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S t o c k w e l l , C . H 1 9 6 4 : F o u r t h r e p o r t on s t r u c t u r a l p r o v i n c e s , o r o g e n i e s

nd t i m e - c l a s s i f i c a t i o n o f r o c k s o f t h e Canad ian r e c a m b r i a n S h i e l d . I n Age d e t e r m i n a t i o n s and e o l o g i c a l s t u d i e s , p a r t 11. G e o l o g i c a l Survey o f

n a d a , P a p e r 64-17, pp.44-54.

Geology o f t h e Canad ian S h i e l d , i n t r o d u c t i o n . I n

W i l l i a m s , H . R .

Wood, J . , T h u r s t o n , P . C . , C o r f u , F . and D a v i s , D.W. 1 9 8 6 : A n c i e n t q u a r t z i t e s and c a r b o n a t e s i n nor thw

O n t a r i o ~ e v i d e n c e f o r e a r l y ( A r c h e a n ) s t a b i

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J9ELD TRIP 1

MAFIC INTRUSIONS, PLATINUM-GROUP-ELEMENTS MINERALIZATION ANDGRANITOID ROCKS IN THE LAC DES ILES AREA

Introductory Discussion and Field Guide36th Annual Institute on Lake Superior Geology

Thunder Bay, Ontario

by

R.H. SutcliffeOntario Geological Survey

77 Grenville StreetToronto, Ontario

M7A lW4

11

MAFIC INTRUSIONS, PLATINUM-GROUP-ELEMENTS MINERALIZATION AND GRAMITOID ROCKS IN THE LAC DES ILES AREA

Introductory Discussion and Field Guide 36th Annual Institute on Lake Superior Geology

Thunder Bay, Ontario '

R.H. Sutcliffe Ontario Geological Survey

77 Grenville Street Toronto, Ontario

M7A 1W4

INTRODUCTIONThe Lac des ties region provides an opportunity to

examine a wide range of Late Archean plutonic rock types.This field trip will emphasize: the role of mafic magmatismin the evolution of this Late Archean plutonic terrane; therelationship between mafic xnag-inas and granitoid plutonism;and aspects of platinum—group—elements mineralization.

The field trip is based on surveys done in the Lac deslies area by the Ontario Geological Survey in 1985 and 1986.This work included detailed mapping of the Lac des liesComplex (Sutcliffe and Sweeny, 1986) and Tib Gabbro (Smithand Sutcliffe, 1987), regional mapping of the granitoidrocks (Sutcliffe and Smith, 1988), and a gravity survey ofthe area (Gupta and Sutcliffe, in press). The discussionpresented here makes considerable use of excerpts fromSutcliffe (1989) and Sutcliffe et al (1989) to which thereader is referred for further details on aspects of thegeochemistry of the plutonic rocks. Previous mapping in thearea is primarily the reconnaissance mapping and compilationof Sage et al (1974).

OVERVIEWMafic to ultramafic intrusive racks in the Lac des lies

area form part of an east—northeast trending linear zone ofLate Archean mafic piutons which extends over 200 km fromAtikokan to Lake Nipigon. This zone parallels the boundarybetween the Wabigoon and Quetico Subprcvinces (Stockweli etj 1972). The rocks examined on this field trip are allwithin the Wabigoon Subprovince.

In the immediate region of the field trip, the mafic toultramafic plutons are intruded into gneissic tonalite hostrocks and are distributed in a circular pattern, thediameter of which is approximately 30 km (Figure 1). Theseplutons are composed of serpentinite, wehrlite, minorlherzolite, clinopyroxenite and websterite to magnesiangabbronorite and ferrogabbro (Sutcliffe and Smith 1988).The Lac des Iles Complex (LDIC) is the largest of the maficto ultramafic plutons and displays the most completespectrum of lithologies. The Tib Gabbro is the secondlargest intrusion and consists of predominantlygabbronorite. The marginal zones of larger intrusions andsome of the smaller intrusions consist of hornblendite, andhornblende gabbro to diorite. Most of the mafic toultramafic intrusions have well preserved igneous mineralogyand are not significantly deformed. The form and tectonicsetting of the mafic-ultramaf Ic intrusions, particularly theLIflC, is similar to mafic intrusions associated withorogenic terrains such as the Mesozoic Alaskan complexes(Taylor 1967, Findlay 1969). Preliminary U/Pb zircon dataindicate that the qabbroic rocks of the Lac des Iles Complexand the Tib Gabbro are 2.69 Ga (D.W. Davis, Royal OntarioMuseum, unpublished data), however, an older Sm/Nd age of2,738+/—27 Ma is reported for the ultramafic rocks of theLOIC by Brugmann and Naldrett (1987).

12

INTRODUCTION The Lac des Iles region provides an opportunity to

examine a wide range of Late Archean plutonic rock types. This field trip will emphasize: the role of mafic magmatism in the evolution of this Late Archean plutonic terrane; the relationship between mafic magmas and granitoid plutonism; and aspects of platinum-group-elements mineralization.

The field trip is based on surveys done in the Lac des Iles area by the Ontario Geological Survey in 1985 and 1986. This work included detailed mapping of the Lac des Iles Complex (Sutcliffe and Sweeny, 1986) and Tib Gabbro (Smith and Sutcliffe, 1987), regional mapping of the granitoid rocks (Sutcliffe and Smith, 1988). and a gravity survey the area (Gupta and Sutcliffe, in press). The discussio presented here makes considerable use of excerpts from Sutcliffe (1989) and Sutcliffe (1989) to which the reader is referred for further details on aspects of the geochemistry of the plutonic rocks. Previous mapping in the area is primarily the reconnaissance mapping and compilation of Sage U (1974).

OVERVIEW Mafic to ultramafic in

area form part of an east-n Late Archean mafic plutons Atikokan to Lake Nipigon. between the Wabigoon and Qu

1972). The rocks examined on this field trip are a11 within the Wabigoon Subprovince.

In the immediate region of the field trip, the mafic to ultramafic plutons are intruded into gneissic tonalite host rocks and are distributed in a circular pattern, the diameter of which is approximately 30 tan (Figure 1). These plutons are composed of serpentinite, wehrlite, minor lherzolite, clinopyroxenite and websterite to magnesian gabbronorite and ferrogabbro (Sutcliffe and Smith 1988). The Lac des Iles Complex (LDIC) is the largest of the ma to ultramafic plutons and displays the most complete spectrum of lithologies. The Tib Gabbro is the second largest intrusion and consists of predominantly gabbronorite. The marginal zones of larger intrusions an some of the smaller intrusions consist of hornblendite, and hornblende qabbro to diorite. Most of the mafic to ultramafic intrusions have well preserved igneous mineralogy and are not significantly deformed. The form and tectonic setting of the mafic-ultramafic intrusions, particularly LDIC, is similar to mafic intrusions associated with orogenic terrains such as the Mesozoic Alaskan complexes (Taylor 1967, Findlay 1969). Preliminary U/Pb zircon data indicate that the gabbroic rocks of the Lac des Iles Comp and the Tib Gabbro are 2.69 Ga (D.W. Davis, Royal Ontario Museum, unpublished data), however, an older Sm/Nd age of 2,738+/-27 Ma is reported for the ultramafic rocks of the LDIC by Brugmann and Naldrett (1987).

A major zone of platinum-group-elements (PGE) and Cu—Nisulphide mineralization, known as the Roby Zone, occurs ingabbroic rocks of the LDIC. The property is being developedby Madelaine Mines Limited and this may become the firstprimary producer of platinum group elements (PGE) in Canada(Northern Miner Press 1988). The PGE mineralization in theRoby Zone of the LDIC is associated with suiphides andaltered silicates in gabbroic rocks. This mineralizationexhibits characteristics suggesting the control of bothmagmatic mixing processes (Naldrett and Campbell 1979; Toddet al 1982; Sharpe 1985) and volatile dominated processes(Talkington and Watkinson 1984; Boudreau et al 1986; Stumpf 1and Ballhaus 1986). PGE mineralization also occurs inultramafic rocks of the LDIC and in other intrusions in thearea and these occurrences indicate that PGE's wereconcentrated at several stages in the fractionation of themafic magmas.

Two granitoid plutons consisting of hornblende tonaliteand biotite tonalite intrude older gneissic tonalite. Thegneissic tonalite is dated at approximately 2.77 Ga and theyounger hornblende tonalite at 2.73 Ga by the U/Pb zirconmethod (D.W. Davis, unpublished data). The youngergranitoid plutons contain numerous net—veined mafic dikesand other textures indicating the coexistence of mafic andfelsic magmas. Although the association of mafic magmaswith late Archean granitoids has not been commonlyrecognized, there are other examples of associated LateArchean mafic and felsic magmatism in the WabigoonSubprovince (e.g. Morrison et al 1986). Based on fieldrelationships, Sutcliffe (1989) considered that hornblendetonalite and the mafic—ultramafic intrusions werecontemporaneous. The geochronology does not support thisconclusion and the mixing textures therefore are probablyassociated which an older phase of mafic inagmatism than thatwhich generated the major mafic—ultramafic plutons. Furtherstudies will test this hypothesis.

In this guide, piutonic rocks are named using theclassification of Streckeisen (1976).

LAC DES ILES COMPLEX

Geo 1 oqyMapping by Pye (1968) indicated that the Lac des Iles

Complex (LDIC) has an area of approximately 30 km2. TheLDIC (Figure 2) consists of a predominantly ultramaficsequence centred on Lac des lies and predominantly gabbroicsequences south of the lake (Pye, 1968; Watkinson andDunning, 1979). The description given here is largely basedon mapping by Sutciiffe and Sweeny (1986) and the work ofSutcliffe et al (1989). Details of the mineral assemblagesand textures for rock types are summarized in Table 1.

A gravity study by Gupta and Sutcliffe (in press)indicates that the LOIC is associated with a 30 mgal gravity

13

A major zone of platinum-group-elements (PGE) and Cu-Ni sulphide mineralization, known as the Roby Zone, occurs in gabbroic rocks of the LDIC. The property is being developed by Madelaine Mines Limited and this may become the first primary producer of platinum group elements (PGE) in Canada (Northern Miner Press 1988). The PGE mineralization in the Roby Zone of the LDIC is associated with sulphides and altered silicates in gabbroic rocks. This mineralization exhibits characteristics suggesting the control of both magmatic mixing processes (Naldrett and Campbell 1979; Todd a 1982; Sharpe 1985) and volatile dominated processes (Talkington and Watkinson 1984; Boudreau 1986; Stumpfl and Ballhaus 1986). PGE mineralization also occurs in ultramafic rocks of the LDIC and in other intrusions in the area and these occurrences indicate that PGE's were concentrated at several stages in the fractionation of the mafic magmas.

Two granitoid plutons consisting of hornblende tonalite and biotite tonalite intrude older gneissic tonalite. The gneissic tonalite is dated at approximately 2.77 Ga and the younger hornblende tonalite at 2.73 Ga by the U/Pb zircon method (D.W. Davis, unpublished data). The younger granitoid plutons contain numerous net-veined mafic dikes and other textures indicating the coexistence of mafic and felsic magmas. Although the association of mafic magmas with late Archean granitoids has not been commonly recognized, there are other examples of associated Late Archean mafic and felsic magmatism in the Wabigoon Subprovince (e.g. Morrison a 1986). Based on field relationships, Sutcliffe (1989) considered that hornblende tonalite and the mafic-ultramafic intrusions were contemporaneous. The geochronology does not support this conclusion and the mixing textures therefore are probably associated which an older phase of mafic magmatism than that which generated the major mafic-ultramafic plutons. Further studies will test this hypothesis.

In this guide, plutonic rocks are named using the classification of Streckeisen (1976).

IAC DES ILES COMPLEX

Geoloav Mapping by Pye (1968) indicated that the Lac des lies

Complex (LDIC) has an area of approximately 30 km2. The LDIC (Figure 2) consists of a predominantly ultramafic sequence centred on Lac des lies and predominantly gabbroic seauences south of the lake fPve. 1968: Watkinson and . - . -

~ u i n i n ~ , 1979). The description given here is largely based on mapping by Sutcliffe and Sweeny (1986) and the work of Sutcliffe (1989). Details of the mineral assemblages and textures for rock types are summarized in Table 1.

A gravity study by Gupta and Sutcliffe (in press) indicates that the LDIC is associated with a 30 mgal gravity

high and is modelled as a funnel shaped body part of whichextends to a depth of 4.5 km.

The ultramafic sequence is composed of two coalescingcenters which are defined by the different distributions ofultramafic lithologies and by the attitudes of igneouslayering (Figure 2). Hornblendite to pyroxene hornblenditeand hornblende gabbro occur along the western margin of theLDIC and, with increasing modal pyroxene, grade intopyroxenite. The hornblende-rich lithologies are invaded andbrecciated by veins of hornblende diorite to quartz diorite.

The northern ultramafic center is nearly circular inplan with a diameter of approximately 4 km (Figure 2) andconsists of several cycles of serpentinite and wehrlite,olivine clinopyroxenite and clinopyroxenite, and websteriteto gabbronorite. Lindhardt and Sues (1987) have shown thatthese cycles are typically 100 to 500 m thick in plan andindividual layers can be traced along strike for up to 1.5km. Olivine cumulates are most abundant around the southernand eastern perimeter of the center and the rocks becomemore pyroxene—rich towards the nothwest. The absence ofdistinct marker horizons and extent of outcrop exposureplaces significant limitations on the interpretation of thelateral continuity and thickness of the cycles.

Modal layering due to variation in proportions ofplagiociase and pyroxenes, olivine and clinopyroxene, andorthopyroxene and clinopyroxene is observed. Generally themodal layers are 1 to 20 cm thick and are continuous formeters. Minor disseminated euhedral grains of chromite areenclosed in cumulus olivine and clinopyroxene and arelocally concentrated in layers up to 1 cm wide in thenorthern center. Igneous lamination defined by planaralignment of pyroxenes or plagioclase is also observed atseveral localities. Layer attitudes indicate that thenorthern center has an upright funnel—shaped form. Minordiscordant serpentinite dikes, which contain angularfragments of pyroxenite intrude the northern ultramaficcenter.

The southern ultramafic center is elliptical in planwith an irregularly shaped wehrlite core centered onSoutheast Angle Bay of Lac des lies. The wehrlite core issurrounded by websterite which in some areas containsinclusions of wehrlite. Along the eastern margin of thecenter the websterite grades into gabbronorite. Thesouthern center is predominantly composed of massive rocksand lacks well defined igneous layering.

The ultramafic rocks are partially separated from thegabbroic rocks south of Lac des Iles by a septum oftonalite. Watkinson and Dunning (1979) considered that theultramafics were emplaced later than the gabbroic rocksbased on the presence of gabbroic inclusions in theuitramafics. The only clearly defined gabbroic inclusionsobserved during the current study are troctoiitic and notreadily related to the gabbroic suite.

14

high and is modelled as a funnel shaped body part of which extends to a depth of 4.5 tan.

The ultramafic sequence is composed of two coalescing centers which are defined by the different distributions of ultramafic lithologies and by the attitudes of igneous layering (Figure 2). Hornblendite to pyroxene hornblendite and hornblende gabbro occur along the western margin of the LDIC and, with increasing modal pyroxene, grade into pyroxenite. The hornblende-rich lithologies are invaded and brecciated by veins of hornblende diorite to quartz diorite.

The northern ultramafic center is nearly circular in plan with a diameter of approximately 4 tan (Figure 2) and consists of several cycles of serpentinite and wehrlite, livine clinopyroxenite and clinopyroxenite, and websterit o gabbronorite. Lindhardt and Bues (1987) have shown tha hese cycles are typically 100 to 500 m thick in plan and individual layers can be traced along strike for up to 1.5 tan. olivine cumulates are most abundant around the southern and eastern perimeter of the center and the rocks become more pyroxene-rich towards the nothwest. The absence of distinct marker horizons and extent of outcrop exposure places significant limitations on the interpretation of lateral continuity and thickness of the cycles.

Modal layering due to variation in proportions of plagioclase and pyroxenes, olivine and clinopyroxene, a orthopyroxene and clinopyroxene is observed. Generally the modal layers are 1 to 20 cm thick and are continuous for meters. Minor disseminated euhedral grains of chromite are enclosed in cumulus olivine and clinopyroxene and are locally concentrated in layers up to 1 cm wide in the northern center. Igneous lamination defined by planar alignment of pyroxenes or plagioclase is also observed at everal localities. Layer attitudes indicate that the orthern center has an upright funnel-shaped form. Min iscordant serpentinite dikes, which contain angular

fragments of pyroxenite intrude the northern ultramafic

The southern ultramafic center is elliptical in plan with an irregularly shaped wehrlite core centered on Southeast Angle Bay of Lac des lies. The wehrlite core is surrounded by websterite which in some areas contains inclusions of wehrlite. Along the eastern margin of the center the websterite grades into gabbronorite. The southern center is predominantly composed of massive rocks and lacks well defined igneous layering.

The ultramafic rocks are partially separated from the abbroic rocks south of Lac des lies by a septum of onalite. Watkinson and Dunning (1979) considered that ltramafics were emplaced later than the gabbroic rocks ased on the presence of gabbroic inclusions in the

ultramafics. The only clearly defined gabbroic inclus observed during the current study are t and readily related to the gabbroic suite.

A 50 m wide, composite dike consisting of olivinegabbronorite, olivine websterite to lherzoiite withhornblende diorite margins outcrops on the south—west shoreof Lac des lies and along the west side of the gabbro. Thisdike is olivine—rich (picritic) and is potentially a feederfor the southern ultramafic center

Gabbroic rocks occur largely in the southern part ofthe LDIC (Figure 2) and are host to the Pd, Ptmineralization of the Roby Zone. Based largely on detailedstudies of drill core in the immediate vicinity of the RobyZone, Watkinson and Dunning (1979) indicated that the gabbrocould be divided into two major units known as the EasternGabbro (EG) and Western Gabbro (WG). As defined byWatkinson and Dunning (1979), the WG consists ofinterlayered gabbroic, noritic, pyroxenitic, andanorthositic rocks and the EG consists of oxide—richgabbroic and noritic rocks. This distinction is significantbecause the Roby Zone is interpreted to occur along thecontact of the two units. Macdonald (1985), Sutcliffe andSweeny (1986), and Sweeny and Edgar (1987) suggested thatthe relationships between the units are complex and thatmany of the layers identified in drill core by Watkinson andDunning (1979) are separate intrusive phases.

The gabbroic part of the LDIC consists of a leuco—gabbro to anorthositic gabbro unit and a unit ofpredominantly gabbronorite. In the vicinity of the RobyZone, these units correspond with the EG and WGrespectively, although in detail there is not a simplegeographic subdivision (Figure 3). As defined by Sutcliffeet al (1989), both the EG and WG contain cumulusplagioclase, however, the WG contains cumulus orthopyroxeneor alteration products pseudomorphing orthopyroxene. Thisrepresents a different definition of the units than that ofWatkinson and Dunning (1979), however, because there is animportant geographic connotation to the tens EG and WG inthe Roby Zone, these names have been retained. The EGdisplays igneous lamination and weakly developed wispy modallayering and is pervasively altered obscuring the primaryigneous assemblages. The WG typically consists ofgabbronorite to norite and is fresh to altered. Locally, inthe vicinity of the "D Zone", the WG contains cumulusolivine. Within the WG near the EG/WG contact, pegmatiticgabbro, gabbro breccia, gabbronorite, and clinopyroxeniteform an arc—shaped zone of variable width (Figure 3). Inthis part of the WG, pegmatitic gabbro phases occurprimarily as inclusions, pegmatoidal segregations anddiscordant pegmatite dikes. Pegmatitic phases are rare inthe EG. The WG truncates the igneous lamination and weaklydeveloped modal layering of the EG, indicating that the WGis younger than the EG.

An intrusion of medium—grained hornblende gabbro toleucogabbro with minor coarse—grained hornblende melagabbrooccurs south of the EG and WG. The age of the hornblende

15

A 50 m wide, composite dike consisting of olivine gabbronorite, olivine websterite to lherzolite with hornblende diorite margins outcrops on the south-west shore of Lac des lies and along the west side of the gabbro. This dike is olivine-rich (picritic) and is potentially a feeder for the southern ultramafic center

Gabbroic rocks occur largely in the southern part of the LDIC (Figure 2) and are host to the Pd, Pt mineralization of the Roby Zone. Based largely on detailed studies of drill core in the immediate vicinity of the Roby Zone, Watkinson and Dunning (1979) indicated that the gabbro ould be divided into two major units known as the Eastern abbro (EG) and Western Gabbro (WG). As defined by atkinson and Dunning (1979), the WG consists of nterlayered gabbroic, noritic, pyroxenitic, and northositic rocks and the EG consists of oxide-rich abbroic and noritic rocks. This distinction is significant ecause the Roby Zone is interpreted to occur along the ontact of the two units. Macdonald (1985), Sutcliffe and weeny (1986), and Sweeny and Edgar (1987) suggested that the relationships between the units are complex and that many of the layers identified in drill core by Watkinson and Dunning (1979) are separate intrusive phases.

The gabbroic part of the LDIC consists of a leuco- gabbro to anorthositic gabbro unit and a unit of predominantly gabbronorite. In the vicinity of the Roby Zone, these units correspond with the EG and WG respectively, although in detail there is not a simple geographic subdivision (Figure 3 ) . As defined by Sutcliffe

(1989), both the EG and WG contain cumulus oclase, however, the WG contains cumulus orthopyroxene teration products pseudomorphing orthopyroxene. This

represents a different definition of the units than that of Watkinson and Dunning (1979), however, because there is an important geographic connotation to the terms EG and WG in the Roby Zone, these names have been retained. The EG displays igneous lamination and weakly developed wispy modal layering and is pervasively altered obscuring the primary igneous assemblages. The WG typically consists of gabbronorite to norite and is fresh to altered. Locally, in the vicinity of the 'ID Zone", the WG contains cumulus olivine. Within the WG near the EG/WG contact, pegmatitic gabbro, gabbro breccia, gabbronorite, and clinopyroxenite form an arc-shaped zone of variable width (Figure 3). In this part of the WG, pegmatitic gabbro phases occur primarily as inclusions, pegmatoidal segregations and discordant pegmatite dikes. Pegmatitic phases are rare in the EG. The WG truncates the igneous lamination and weakly developed modal layering of the EG, indicating that the WG is younger than the EG.

An intrusion of medium-grained hornblende gabbro to leucogabbro with minor coarse-grained hornblende melagabbro occurs south of the EG and WG. The aae of the hornblende

gabbro relative to other units of the LDIC has not beendetermined.

Late dikes of microdiorite—gabbro intrude the gabbroicrocks and locally result in intrusion breccia consisting ofangular gabbroic and less common tonalitic fragments in afine—grained igneous matrix. Late leucocratic tonaliticdikes and veins also cross cut the gabbroic rocks.

Petrological and geochemical studies by Sutcliffe et al(1989) indicate that distinct magma sequences can berecognized on a basis of crystallization order, differencesin cumulus minerals, intrusive relations and geochemicaldifferences. These sequences are 1) hornblende gabbro,2)gabbro and gabbronorite and 3) ultramafic. Where there isevidence of intrusive relations the most primitive sequencesare emplaced late and toward the north end of the complex.In the absence of rocks representative of liquids, parentalmagma compositions can only be constrained by thecompositions of cumulus phases. Mineral chemistry andcrystallization sequences indicate that the parental magmasof the gabbroic and ultramafic sequences had tholeiiticbasalt parental magmas with high alumina and picriticaffinities respectively.

MineralizationExploration by Texasgulf Canada Limited and Boston Bay

Mines Limited in 1975 and 1976 outlined 20.4 million tonnesof mineralization grading 6 ppm Pd and Pt and 0.2% Cu + Niin the 600 m long Roby Zone along the EG/WG contact(Northern Miner Press 1976). Pt/(Pt+Pd) is in the range of0.1 (Watkinson and Dunning 1979) to 0.2 (Sutcliffe et al1989). Drill core sections show that the highest grademineralization occurs in the WG immediately adjacent to thecontact with the EQ (Figure 4).

PGE mineralization in the gabbroic rocks is associatedwith 1) finely disseminated sulphide mineralization (1 to 2modal % sulphide) in altered pyroxenite sheets along theWG/EG contact 2) disseminated, bleb-like and net-texturedsulphide mineralization in gabbronorite (1 to 4 modal %sulphide) and 3) coarse interstitial sulphide in pegmatiticgabbro. In addition to mineral assemblages noted in Table1, rare euhedral apatite is observed in mineralized gabbrosin the Roby Zone.

A detailed study of drill core from the Roby Zone(Dunning 1979; Watkinson and Dunning 1979; Cabri andLaflamme 1979) showed that vysotskite and braggite are themost abundant platinum group minerals (PGM) along with minorisomertieite, merenskyite, kotulskite, sperrylite andmoncheite. In addition, electrum and Pt—Fe alloys have beenidentified by Sweeny (1989). The PGE mineralization isassociated with chalcopyrite, pyrrhotite, pentlandite andpyrite interpreted by Watkinson and Dunning (1979) as beingslightly metamorphosed equivalents of primary exsolutionfrom monosulphide solid solution. Millerite and violarite

16

gabbro relative to other units of the LDIC has not been determined.

Late dikes of microdiorite-gabbro intrude the gabbroic rocks and locally result in intrusion breccia consisting of angular gabbroic and less common tonalitic fragments in a fine-grained igneous matrix. Late leucocratic tonalitic dikes and veins also cross cut the gabbroic rocks.

Petrological and geochemical studies by Sutcliffe (1989) indicate that distinct magma sequences can be recognized on a basis of crystallization order, differences in cumulus minerals, intrusive relations and geochemical differences. These sequences are 1) hornblende gabbro, 2)gabbro and gabbronorite and 3) ultramafic. Where there is evidence of intrusive relations the most primitive sequences are emplaced late and toward the north end of the complex. In the absence of rocks representative of liquids, parental magma compositions can only be constrained by the compositions of cumulus phases. Mineral chemistry and crystallization sequences indicate that the parental of the gabbroic and ultramafic sequences had tholeiiti basalt parental magmas with high alumina and picritic affinities resvectivelv. - - .. . >*:..; . . . . . + . . . - . . Mineralization . . 5 .. . . . ;teg*'.: ;. : p ,

Exvloration bv Texasaulf Canada Limited and Boston Bav -

Mines ~imited in 1975 and1976 outlined 20.4 million tonne; of mineralization grading 6 ppm Pd and Pt and 0.2% Cu + Ni in the 600 m long Roby Zone along the EG/WG contact (Northern Miner Press 1976). Pt/(Pt+Pd) is in the range of 0.1 (Watkinson and Dunning 1979) to 0.2 (Sutcliffe a 1989). Drill core sections show that the highest grade mineralization occurs in the WG immediately adjacent to the contact with the EG (Figure 4).

PGE mineralization in the gabbroic rocks is associated with 1) finely disseminated sulphide mineralization (1 to 2 modal % sulphide) in altered pyroxenite sheets along the WG/EG contact 2) disseminated, bleb-like and net-textured sulphide mineralization in gabbronorite (1 to 4 modal % sulphide) and 3) coarse interstitial sulphide in peg-matitic gabbro. In addition to mineral assemblages noted in Table 1, rare euhedral apatite is observed in mineralized gabbros in the Roby Zone.

A detailed study of drill core from the Roby Zonee (Dunning 1979; Watkinson and Dunning 1979; Cabri and -- Laflamme 1979) showed that vysotskite and braggite are the most abundant platinum group minerals (PGM) along with minor isomertieite, merenskyite, kotulskite, sperrylite and moncheite. In addition, electrum and Pt-Fe alloys have been identified by Sweeny (1989). The PGE mineralization is associated with chalcopyrite, pyrrhotite, pentlandite and pyrite interpreted by Watkinson and Dunning (1979) as being slightly metamorphosed equivalents of primary exsolution from nionosulphide solid solution. Millerite and violarite

are present in secondary assemblages (Watkinson and Dunning1979).

PGM in the Roby Zone are most commonly found withpostcumulus hydrosilicates near silicate—suiphide interfaces(Talkington and Watkinson 1984). In this association, thePGM occur both within the hydrosilicates and at thesulphide—silicate interface.

pGE's also occur in the ultramafic rocks with sparselydisseminated to net-textured chalcopyrite, pyrrhotite, andpentlandite (ci modal % sulphide). This type ofmineralization is most commonly associated with websteriteand gabbronoritic phases of the northern and southernultramafic centers.

Field and geochemical relations indicate that the EGand WG cumulates crystallized from a common magma and weresubsequently emplaced as crystal mushes with the morefractionated and hydrous EG emplaced first. Lowerconcentrations of PGE, Au and S in the EG relative to WGsuggests that suiphides were removed from the magma prior tothe crystallization of the EG cumulates or that the parentalmagma was not saturated in sulphides at the time of cumulateformation. Magma mixing may have localized some sulphidemineralization along the EG/WG interface. However, becausethe phases are interpreted to have been emplaced as mushes,there was probably limited opportunity for sulphides tointeract with magma. This would suggest a low R—factor andthat this process may not be the explanation for the highgrade mineralization.

Fe—rich pyroxene cumulate sheets intruded into the EGand WG are another possible source of PGE in the Roby Zone.The close spatial association of the sheets andmineralization suggest that the pyroxene cumulates are alikely source of PGE. As previously noted, the alteredpyroxene cumulate sheet at the EG/WG contact containsparticularly high-grade mineralization. Limited sampling ofthis group of rocks suggests that, in general, they havehigher PGE contents than other samples of the ultramaficsuite. Mixing of magmas which formed the pyroxene cumulatesand the gabbro cumulates may have played a role in causingsulphide saturation in the pyroxene cumulates.

The association of PGM with secondary sulphides andaltered silicates lead Watkinson and Talkington (1984) tosuggest that fluid processes were active in mobilization andconcentration of PGE in the Roby Zone. The role of fluidsin PGE mineralized zones in the Bushveld and StillwaterComplexes is indicated by the occurrence of pegmatoids,volatile-rich phases and hortonolite replacement pipes (Toddet al 1982; Stumpfl and Balihaus 1986; Schiffries 1982;Stumpf 1 and Rucklidge 1982). Fluid-bearing phases and fluidinclusions in both complexes suggest fluids that may be richin Cl (Boudreau et al 1986; Stumpfl and Balihaus 1986).

Several features of the Roby Zone indicate theimportance of a volatile rich magma. These include: theassociation of PGM with postcumulus hydrosilicates near

17

are present in secondary assemblages (Watkinson and Dunning 1979).

PGM in the Roby Zone are most commonly found with postcumulus hydrosilicates near silicate-sulphide interfaces (Talkington and Watkinson 1984). In this association, the PGM occur both within the hydrosilicates and at the sulphide-silicate interface.

PGE's also occur in the ultramafic rocks with sparsely disseminated to net-textured chalcopyrite, pyrrhotite, and pentlandite (<1 modal % sulphide). This type of mineralization is most commonly associated with websterite and gabbronoritic phases of the northern and southern ultramafic centers.

Field and geochemical relations indicate that the EG and WG cumulates crystallized from a common magma and were subsequently emplaced as crystal mushes with the more fractionated and hydrous EG emplaced first. Lower concentrations of PGE, Au and S in the EG relative to WG suggests that sulphides were removed from the magma prior to the crystallization of the EG cumulates or that the parental magma was not saturated in sulphides at the time of cumulate formation. Magma mixing may have localized some sulphide mineralization along the EG/WG interface. However, because the phases are interpreted to have been emplaced as mushes, there was probably limited opportunity for sulphides to interact with magma. This would suggest a low R-factor and that this process may not be the explanation for the high grade mineralization.

Fe-rich pyroxene cumulate sheets intruded into the EG and WG are another possible source of PGE in the Roby Zone. The close spatial association of the sheets and mineralization suggest that the pyroxene cumulates are a likely source of PGE. As previously noted, the altered pyroxene cumulate sheet at the EG/WG contact contains particularly high-grade mineralization. Limited sampling of this group of rocks suggests that, in general, they have higher PGE contents than other samples of the ultramafic suite. Mixing of magmas which formed the pyroxene cumulates and the gabbro cumulates may have played a role in causing sulphide saturation in the pyroxene cumulates.

The association of PGM with secondary sulphides and altered silicates lead Watkinson and Talkington (1984) to suggest that fluid processes were active in mobilization and concentration of PGE in the Roby Zone. The role of fluids in PGE mineralized zones in the Bushveld and Stillwater Complexes is indicated by the occurrence of pegmatoids, volatile-rich phases and hortonolite replacement pipes (Todd

1982; Stumpfl and Ballhaus 1986; Schiffries 1982; Stumpfl and Rucklidge 1982). Fluid-bearing phases and fluid inclusions in both complexes suggest fluids that may be rich in Cl (Boudreau 1986; Stumpfl and Ballhaus 1986).

Several features of the Roby Zone indicate the importance of a volatile rich magma. These include: the association of PGM with postcumulus hydrosilicates near

silicate—sulphide interfaces; the association of PGM withpegmatitic phases and the mineralized breccia zones cored bypeginatitic gabbro which appear to have been fluid channel-ways; and the highly fractionated Pd/Ir. Cl-rich apatite inmineralized gabbro suggest that the fluids associated withthe mineralizing process were Cl—rich.

Brugmann et al (1989) have recently proposed amodification of this hypothesis and suggested that thevolatile content of the residual magma triggered partial re—melting of the gabbro cumulates in a process referred to as"constitutional zone refining".

Occurrences of PGE-bearing sulphides in the ultramaficsequence are hosted mainly by websterite at the top ofcyclic units and by peridotites at the base of overlyingunits. These occurrences have not been studied in detailbut may represent stratifonu mineralization related to theinteraction of fractionated magma with a new magma pulse inthe ultramafic chamber (Sutcliffe and Sweeny 1986; Brugmannand Naldrett 1987). The presence of this mineralization,however, emphasizes that ?GE's were concentrated at severalstages in the fractionation of the LDIC magmas.

TIB GABBRO

GeologyThe Tib Gabbro (Figure 5) is a layered intrusion with

an area of approximately 25 km2 located approximately 15 kmnorthwest of the Lac des Iles Complex. The intrusion wasmapped by Kaye (1966) and Smith and Sutcliffe (1987). Theinformation summarized here is largely from Smith (inpress). Petrological and geochemical studies are inprogress and most of this summary is based in fieldobservations. The gabbro is emplaced into an older suite offoliated to gneissic biotite tonalite which outcrops aroundthe eastern, northern and aouthern margins of the intrusion.A younger suite of megacrystic granodiorite and graniteintrudes the Tib Gabbro and older gneissic tonalites.

The Tib Gabbro is characterized by well preservedprimary igneous mineralogy and moderately well developedlayering. Layer attitudes indicate that the intrusion isfunnel shaped and has been tilted to the northeast. Thesouthwest part of the gabbro therefore represents the baseof the exposed section. A gravity survey by Gupta andSutcliffe (in press) supports this interpretation and modelsthe 16 mgal gravity high associated with the intrusion as atilted basin-shaped body.

The intrusion is divided into four zones by Smith andSutcliffe (1987) which from the southwest to northeast arethe Border, Lower, Middle and Upper Zones. The 50 to 150 mthick Border Zone is defined on a basis of the abundance ofcoarse grained to pegmatoidal gabbro which occurs around thenorthwestern, western and southern margins of the intrusion.The Lower and Middle zones probably represent distinct magmapulses which crystallized from the base upwards. These

18

silicate-sulphide interfaces; the association of PGM with pegmatitic phases and the mineralized breccia zones cored by pegmatitic gabbro which appear to have been fluid channel- ways; and the highly fractionated Pd/Ir. Cl-rich apatite in mineralized gabbro suggest that the fluids associated with the mineralizing process were Cl-rich.

Brugmann et a1 (1989) have recently proposed a modification of this hypothesis and suggested that the volatile content of the residual magma triggered partial re- melting of the gabbro cumulates in a process referred to as "constitutional zone refiningn.

Occurrences of PGE-bearing sulphides in the ultramafic sequence are hosted mainly by websterite at the top of cyclic units and by peridotites at the base of overlying units. These occurrences have not been studied in detail but may represent stratiform mineralization related to the interaction of fractionated magma with a new magma pulse in the ultramafic chamber (Sutcliffe and Sweeny 1986; Brugmann and Naldrett 1987). The presence of this mineralization, however, emphasizes that PGE1s were concentrated at several stages in the fractionation of the LDIC magmas.

TIB GABBRO

Geoloav The Tib Gabbro (Figure 5) is a layered intrusion with

an area of approximately 25 km2 located approximately 15 km northwest of the Lac des Iles Complex. The intrusion was mapped by Kaye (1966) and Smith and Sutcliffe (1987). The information summarized here is largely from Smith (in press). Petrological and geochemical studies are in progress and most of this summary is based in field observations. The gabbro is emplaced into an older suite of foliated to gneissic biotite tonalite which outcrops around the eastern, northern and aouthern margins of the intrusion. A younger suite of megacrystic granodiorite and granite intrudes the Tib Gabbro and older gneissic tonalites.

The Tib Gabbro is characterized by well preserved primary igneous mineralogy and moderately well developed layering. Layer attitudes indicate that the intrusion is funnel shaped and has been tilted to the northeast. The southwest part of the gabbro therefore represents the base of the exposed section. A gravity survey by Gupta and Sutcliffe (in press) supports this interpretation and models the 16 mgal gravity high associated with the intrusion as a tilted basin-shaped body.

The intrusion is divided into four zones by Smith and Sutcliffe (1987) which from the southwest to northeast are the Border, Lower, Middle and Upper Zones. The 50 to 150 m thick Border Zone is defined on a basis of the abundance of coarse grained to pegmatoidal gabbro which occurs around the northwestern, western and southern margins of the intrusion. The Lower and Middle zones probably represent distinct magma pulses which crystallized from the base upwards. These

18

zones record the crystallization of evolving cumulusassemblages which results in a mappable igneous stratigraphywith pyroxene rich cumulates near the base and plagioclase—and magnetite—rich cumulates near the top. The Upper Zoneconsists of amphibole-rich gabbro and is not well layered.

The 1.5 kin wide Lower Zone consists of a sequence oflayered gabbronorite with layers of orthopyroxenite andmelagabbronorite (pyroxene cumulate) near the base. Abovethe orthopyroxenite the lithologies become more leucocraticand anorthosite appears as distinct layers. The top of thezone is marked by magnetite-rich gabbronorite withleucocratic layers. The 1.6 km wide Middle Zone ispredominantly gabbronorite, however, leucocratic units aremore abundant than in the Lower Zone. Conformable pegmatitelayers and discordant dikes are common in the upper part ofthe zone. The top of the zone is again marked by magnetite—rich gabbronorite.

The Upper Zone is characterized by hornblende—rich,massive to weakly layered gabbronorite with cumulus apatite.The layers form a concentric pattern with a shallow inwarddip. Leucocratic quartz—bearing granulite forms adiscontinuous septa between the Upper and Middle Zones andthe unit is interpreted to be metamorphosed tonalite but mayalternatively be a final product of fractionation.

Layering in the Tib Gabbro is comparable to thatdeveloped in the Mulcahy Gabbro (Morrison et al 1986).Planar lamination of feldspars and pyroxene is foundthroughout the intrusion. Modal layering defined bychanging compositions of cumulus minerals is best developedin the upper part of the Lower Zone and occurs in bothcyclic and intermittent layer sequences. Distinctive modallayers include: othopyroxenite (orthopyroxene cumulate)websterite (clinopyroxene — orthopyroxene cumulate)anorthosite (plagioclase cumulate) and magnetite—richcumulate. The modal layers are usually tens of centimetresin width and persist for tens of metres along strike.

MineralizationThe Tib Gabbro is the second largest of the mafic

intrusions in the lac des Iles area and is an exploratiiontarget for PGE and Cu—Ni mineralization. Intermittentexploration on the intrusion since 1966 has outlined anumber of areas of disseminated Cu—Ni sulphidemineralization in the gabbro, some of which containassociated PGE. To date, the most significantmineralization that has been reported is at the KuhnerOccurrence in the Border Zone along the south margin of theintrusion. At this location, net textured sulphidemineralization is present in coarse grained gabbronorite andassay values up to 190 ppb Pt, 390 ppb Pd, 120 ppb Au, 1140ppm Ni and 710 ppm Cu are reported by Smith (in press)

19

assemblages which results in a mappable igneous stratigraphy with pyroxene rich cumulates near the base and plagioclase- and magnetite-rich cumulates near the top. The Upper Zone consists of amphibole-rich gabbro and is not well layered.

The 1.5 km wide Lower Zone consists of a sequence of layered gabbronorite with layers of orthopyroxenite and melagabbronorite (pyroxene cumulate) near the base. Above the orthopyroxenite the lithologies become more leucocratic and anorthosite appears as distinct layers. The top of the zone is marked by magnetite-rich gabbronorite with leucocratic layers. The 1.6 km wide Middle Zone is predominantly gabbronorite, however, leucocratic units are more abundant than in the Lower Zone. Conformable pegmatite layers and discordant dikes are common in the upper part of the zone. The top of the zone is again marked by magnetite- rich gabbronorite;

- .

The Upper Zone is characterized by hornblende-rich, massive to weakly layered qabbronorite with cumulus apatite. The layers form a concentric pattern with a shallow inward dip. Leucocratic quartz-bearing granulite forms a discontinuous septa between the Upper and Middle Zones and the unit is interpreted to be metamorphosed tonalite but may alternatively be a final product of fractionation.

Layering in the Tib Gabbro is comparable to that developed in the Mulcahy Gabbro (Morrison 1986). Planar lamination of feldspars and pyroxene is found throughout the intrusion. Modal layering defined by changing compositions of cumulus minerals is best developed

I in the upper part of the Lower Zone and occurs in both cyclic and intermittent layer sequences. Distinctive modal layers include: othopyroxenite (orthopyroxene cumulate); websterite (clinopyroxene - orthopyroxene cumulate);

I anorthosite (plagioclase cumulate) and magnetite-rich cumulate. The modal layers are usually tens of centimetres in width and persist for tens of metres along strike.

I Mineralization The Tib Gabbro is the second largest of the mafic

intrusions in the lac des lies area and is an exploratiion

I target for PGE and Cu-Ni mineralization. Intermittent exploration on the intrusion since 1966 has outlined a number of areas of disseminated Cu-Ni sulphide

I mineralization in the gabbro, some of which contain associated PGE. To date, the most significant mineralization that has been reported is at the Kuhner Occurrence in the Border Zone along the south margin of the

I intrusion. At this location, net textured sulphide - mineralization is present in coarse grained gabbronori e 3

. p assay values up to 190 ppb Pt, 390 ppb Pd, 120 ppb Au,

I ppm Mi and 710 ppm Cu are reported by Smith (in press)

GRANITOIDS AND RELATED ROCKS

TonaliteGneissic to strongly foliated biotite tonalite gneiss

is the oldest intrusive phase in the Lac des lies area andis the host rock into which most of the younger plutons areemplaced. Discordant intrusive relations between the mafic—ultramafic intrusions and tonalitic gneiss are locallydeveloped on the margins of intrusions such as the TibGabbro, where gabbro apophyses cross-cut the fabric of thegneiss and tonalitic gneiss enclaves occur in the gabbro.

Two late granitoid plutons occupy the center of thecircular structure defined by the mafic intrusions. Theseplutons also discordantly intrude the tonalitic gneiss andcontain tonalitic gneiss enclaves. The plutons include ahornblende—bearing tonalite with an area of 150 km2 and ayounger, foliated to massive biotite tonalite to microclinezuegacrystic granodiorite pluton which locally containsenclaves of the hornblende-bearing tonalite. The hornblendetonalite is intruded by numerous mafic to intermediate dikesand exhibits textures indicating the coexistence of maficand felsic rnagmas (Sutcliffe 1989). These textures are thefocus of the field trip stops in the granitoid rocks.

The association of early tonalitic gneisses and latermassive plutons is common in granitoid terranes of thewestern Superior Province (Schwerdtner et al. 1979).

Modal analyses of representative tonalites and othersamples are shown in figure 6. The tonalite has a medium—to coarse—grained, hypidiomorphic texture and is massive toweakly foliated. The presence of coarse (1 to 2 cm), blockyto prismatic hornblende is a conspicuous feature of thepluton.

The major mineral phases are: 2 to 4 mm subhedral,tabular plagioclase of composition An28—34 (40—58%);anhedral, weakly strained quartz (29—38%) ; 0.5 to 2 mmbrown—green pleochroic biotite (7—16%) and subhedral toprismatic, 2 mm to 2 cm, green pleochroic hornblende (1-15%). Accessory phases are sphene, apatite and zircon.Plagioclase locally displays weak to moderate oscillatoryzoning and minor alteration to sericite and epidote.Although hornblende is locally subordinate to biotite, thislithological unit is referred to as hornblende tonalitethroughout the paper to distinguish it from the youngerbiotite tonalite in which hornblende is absent.

Mela-tonalite containing up to 60% hornblende occurs asirregular patches and intrusive dike-like bodies up toseveral meters in extent which grade into normal tonalite.Mela-tonaiite is the major constituent in 2 to 3% ofoutcrops in the hornblende tonalite pluton. Hornblende inmela—tonalite is typically coarse, occurring as grains up to2 cm of euhedral, prismatic to skeletal habit. Texturalgradations between skeletal hornblende and coarse blockyhornblende are present. The mela—tonalites have complex

20

GRANITOIDS AND RELATED ROCKS

Tonalite Gneissic to strongly foliated biotite tonalite gneiss

is the oldest intrusive ~haSe in the Lac des lies area and is the host rock into which most of the younger plutons are emplaced. Discordant intrusive relations between the mafic- ultramafic intrusions and tonalitic gneiss are locally developed on the margins of intrusions such as the Tib Gabbro, where gabbro apophyses cross-cut the fabric of the gneiss and tonalitic gneiss enclaves occur in the gabbro.

Two late granitoid plutons occupy the center of the circular structure defined by the mafic intrusions. These plutons also discordantly intrude the tonalitic gneiss and contain tonalitic gneiss enclaves. The plutons include a hornblende-bearing tonalite with an area of 150 km2 and a younger, foliated to massive biotite tonalite to microcline megacrystic granodiorite pluton which locally contains enclaves of the hornblende-bearing tonalite. The hornblende tonalite is intruded by numerous mafic to intermediate dikes and exhibits textures indicating the coexistence of mafic and felsic magmas (Sutcliffe 1989). These textures are the focus of the field trip stops in the granitoid rocks.

The association of early tonalitic gneisses and later massive plutons is common in granitoid terranes of the western Superior Province (Schwerdtner et al. 1979).

Modal analyses of representative tonalites and other samples are shown in figure 6. The tonalite has a medium- to coarse-grained, hypidiomorphic texture and is massive to weakly foliated. The presence of coarse (1 to 2 cm), blocky to prismatic hornblende is a conspicuous feature of the pluton.

The major mineral phases are: 2 to 4 mm subhedral, tabular plagioclase of composition An28-34 (40-58%); anhedral, weakly strained quartz (29-38%); 0.5 to 2 mm brown-green pleochroic biotite (7-16%) and subhedral to prismatic, 2 mm to 2 cm, green pleochroic hornblende (1- 15%). Accessory phases are sphene, apatite and zircon. Plagioclase locally displays weak to moderate oscillatory zoning and minor alteration to sericite and epidote. Although hornblende is locally subordinate to biotite, this litholoqical unit is referred to as hornblende tonalite throughout the paper to distinguish it from the younger biotite tonalite in which hornblende is absent.

Mela-tonalite containing up to 60% hornblende occurs as irregular patches and intrusive dike-like bodies up to several meters in extent which grade into normal tonalite. Mela-tonalite is the major constituent in 2 to 3% of outcrops in the hornblende tonalite pluton. Hornblende in mela-tonalite is typically coarse, occurring as grains up to 2 cm of euhedral, prismatic to skeletal habit. Textural gradations between skeletal hornblende and coarse blocky hornblende are present. The mela-tonalites have complex a

internal structures characterized by comb—layering andcolloform structures of mafic material in felsic host.

Skeletal hornblende is best developed in mela-tonaliteadjacent to mafic-felsic interfaces. Euhedral skeletalhornblende grains also occur as wispy schlieren inhornblende tonalite and in hornblendite units of maficintrusions. The skeletal grains have hollow corescontaining biotite or biotite + plagioclase. Theseassemblages probably reflect reaction between amphibole andtrapped liquid.

Large rounded hornblende aggregates as large as 2 cm indiameter locally display a proto—orbicular texture accordingto the classification of Leveson (1966). These are presentin tonalite adjacent to mela-tonalite and mafic dikes. Thecomposite grains have central areas of pale—green pleochroichornblende intergrown with quartz and feldspar producing asieve texture. Their rims are composed of dark—greenpleochroic grains of interlocking prismatic hornblende.

A breccia pipe, approximately 300 m in diameter, isemplaced into the western end of the hornblende—bearingtonalite pluton. The pipe consists of sub—angularhornblende—tonalite fragments and minor hornblenditefragments set in a matrix of fine—grained, comminuted andsilicified tonalite. Tonalite fragments within the brecciaare of the same composition and texture as the host rock butcontain 2 to 3 cm thick silicified, sericitized andepidotized rinds.

Mafic dikes and hybrid texturesA wide range of textures indicative of contemporaneous

mafic and felsic magmatism is developed associated with theemplacement of mafic to intermediate dikes in hornblendetonalite. The dikes range from parallel-sided intrusions, 1to 2 m wide, with sharp contacts which cross cut foliationin the tonalite, to disaggregated linear inclusion swarms inwhich the mafic magma appears to have lost internalcontiguity. These structures are similar to texturesindicative of contemporaneous mafic—felsic magmatism andmixing reported by Blake (1981), Marshall and Sparks (1984),Furman and Spera (1985) and Hyndman and Foster (1988).Mafic dikes are observed in approximately 15% of theoutcrops of the hornblende tonalite pluton.

The dikes are generally fine—grained, lack chilledmargins and are typically back-veined by tonalite whichforms wispy nets separating rounded centimeter— todecimeter-sized mafic globules. This texture is describedas net-veining by Marshall and Sparks (1984).

With increased veining the net-textured dikes gradeinto disaggregated dikes in which the mafic to intermediaterock forms rounded and elongate bodies which resemblepillows of extrusive rocks in the host tonalite. Incontrast to other descriptions of similar structures inTertiary intrusives (Vogel 1982; Brown and Becker 1986;

21

iternal struc iharacterized by comb-layering and colloform structures of mafic material in felsic host.

Skeletal hornblende is best developed in mela-tonalite adjacent to mafic-felsic interfaces. Euhedral skeletal hornblende grains also occur as wispy schlieren in hornblende tonalite and in hornblendite units of mafic intrusions. The skeletal grains have hollow cores containing biotite or biotite + plagioclase. These assemblages probably reflect reaction between amphibole and trapped liquid.

Large rounded hornblende aggregates as large as 2 cm in diameter locally display a proto-orbicular texture according to the classification of Leveson (1966). These are present in tonalite adjacent to mela-tonalite and mafic dikes. The composite grains have central areas of pale-green pleochroic hornblende intergrown with quartz and feldspar producing a sieve texture. Their rims are composed of dark-green pleochroic grains of interlocking prismatic hornblende.

A breccia pipe, approximately 300 m in diameter, is emplaced into the western end of the hornblende-bearing tonalite pluton. The pipe consists of sub-angular hornblende-tonalite fragments and minor hornblendite fragments set in a matrix of fine-grained, comminuted and silicified tonalite. Tonalite fragments within the breccia are of the same composition and texture as the host rock but contain 2 to 3 cm thick silicified, sericitized and epidotized rinds.

Mafic dikes and hvbrid textures A wide range of textures indicative of contemporaneous

mafic and felsic magmatism is developed associated with the emplacement of mafic to intermediate dikes in hornblende tonalite. The dikes range from parallel-sided intrusions, 1 to 2 m wide, with sharp contacts which cross cut foliation in the tonalite, to disaggregated linear inclusion swarms in which the mafic magma appears to have lost internal contiguity. These structures are similar to textures indicative of contemporaneous mafic-felsic magmatism and mixing reported by Blake (1981), Marshall and Sparks (1984). Furman and Spera (1985) and Hyndman and Foster (1988). Mafic dikes are observed in approximately 15% of the outcrops of the hornblende tonalite pluton.

The dikes are generally fine-grained, lack chilled margins and are typically back-veined by tonalite which forms wispy nets separating rounded centimeter- to decimeter-sized mafic globules. This texture is described as net-veining by Marshall and Sparks (1984).

With increased veining the net-textured dikes grade into disaggregated dikes in which the mafic to intermediate rock forms rounded and elongate bodies which resemble pillows of extrusive rocks in the host tonalite. In contrast to other descriptions of similar structures in Tertiary intrusives (Vogel 1982; Brown and Becker 1986;

Marshall and sparks 1984), chilled rinds on "pillows" at Lacdes Iles are generally absent.

Mineralogically the dikes consist of moderately fresh0.5 mm laths of plagioclase (andesine An4o_46) and 1 mmgrains of prismatic hornblende to pale green fibrousamphibole, and minor biotite, quartz, apatite and opaques.The mafic to intermediate dikes locally displaydiscontinuous hornblende—rich reaction rims adjacent to hosttonalite. In addition, mela-tonalite to hornblendite phasesare generally associated with zones of dike emplacement.

Magma MixingField evidence suggests that mixing between magmas has

taken place. Geochemical data does not readily constrain themixing process due to complications of fractionation ofmafic end members and the presence of multiple components.The most likely magma chamber configuration during theemplacment of the late tectonic plutons is that of a zonedchamber with mafic magma underplating felsic magma (Figure7). This schematic configuration has been supported bygravity studies (Gupta and Sutcliffe in press).

An important aspect of the interaction between themafic and tonalitic magmas at Lac des Iles is thedevelopment of hornblende—rich cumulus phases. The skeletaltexture of these phases suggests they may have formed in asupercooled magma during mixing.

Field relations and geochemical considerations suggestthat the open system magmatic processes at Lac des Ilesresulted in a continuum of disequilibrium textures, rangingfrom hybrid tonalite magmas to late net—veined mafic andintermediate dikes. These textures reflect emplacement andmixing of basic magma with varying degrees of fractionationinto felsic magma throughout a large interval of the coolinghistory of the felsic magma. Furman and Spera (1985) havesuggested that features of mixing associated with injectionof mafic magma into a Sierran pluton represent a continuumof mixing states which can largely be related to hostcrystallinity. When the host magma has low crystallinity(<30%) convective velocities are high and mixing featuresoccur on a scale of centimeters or less. At approximately30—70% host crystallinity, mesoscopic schlieren, maficinclusions and inclusion swarms are preserved. At highercrystallinity, outcrop scale textures such as dike swarmsand large inclusion swarms are evident. These stagescorrespond to the observed textures at Lac des Iles. Net—veined dikes and other conspicuous mixing features at Lacdes Iles probably formed at the latest stages of mixing whenthe efficiency of mixing is reduced but outcrop evidence forthe coexistence of two magmas is enhanced. Breecia zoneswithin the tonalite containing fragments of tonalite andmafic rocks with thick alteration selvedges provide evidenceof explosive degassing of the chamber.

In an evaluation of the situation of a zoned mafic—felsic chamber, Rice (1985) has shown that mixing of a

22

Marshall and Sparks 1984), chilled rinds on npillowsm at Lac des Iles are generally absent.

Mineralogically the dikes consist of moderately fresh 0.5 mm laths of plagioclase (andesine An40-46) and 1 mm grains of prismatic hornblende to pale green fibrous amphibole, and minor biotite, quartz, apatite and opaques. The mafic to intermediate dikes locally display discontinuous hornblende-rich reaction rims adjacent to host tonalite. In addition, mela-tonalite to hornblendite phases are generally associated with zones of dike emplacement.

Maama Mixinq Field evidence suaaests that mixina between maamas has - ---

taken place. ~eochemicii data does not readily constrain the mixing process due to complications of fractionation of mafic end members and the presence of multiple components. The most likely magma chamber configuration during the emplacment of the late tectonic plutons is that of a zoned chamber with mafic magma underplating felsic magma (Figure 7). This schematic configuration has been supported by gravity studies (Gupta and Sutcliffe in press).

An important aspect of the interaction between the mafic and tonalitic magmas at Lac des lies is the development of hornblende-rich cumulus phases. The skeletal texture of these phases suggests they may have formed in a supercooled magma during mixing.

Field relations and geochemical considerations suggest that the open system magmatic processes at Lac des Iles resulted in a continuum of disequilibrium textures, ranging from hybrid tonalite magmas to late net-veined mafic and intermediate dikes. These textures reflect emplacement and mixing of basic magma with varying degrees of fractionation into felsic magma throughout a large interval of the cooling history of the felsic magma. Furman and Spera (1985) have suggested that features of mixing associated with injection of mafic magma into a Sierran pluton represent a continuum of mixing states which can largely be related to host crystallinity. When the host magma has low crystallinity (<30%) convective velocities are high and mixing features occur on a scale of centimeters or less. At approximately 30-70% host crystallinity, mesoscopic schlieren, mafic inclusions and inclusion swarms are preserved. At higher crystallinity, outcrop scale textures such as dike swarms and large inclusion swarms are evident. These stages correspond to the observed textures at Lac des Iles. Net- veined dikes and other conspicuous mixing features at Lac des Iles probably formed at the latest stages of mixing when the efficiency of mixing is reduced but outcrop evidence for the coexistence of two magmas is enhanced. Breecia zones within the tonalite containing fragments of tonalite and mafic rocks with thick alteration selvedges provide evidence of explosive degassing of the chamber.

In an evaluation of the situation of a zoned mafic- felsic chamber, Rice (1985) has shown that mixing of a

convecting, density stratified column is possible givenlarge dimensions and a reasonable thermal gradient. Animportant aspect of a zoned chamber is the probability ofroll over and explosive degassing if the magmas containvolatiles (Rice 1985). Evidence for degassing, mixing andviolent emplacement of mixed chambers has also beendocumented by Marshall and Sparks (1984). Densityinstability of mafic magmas at the base of a chamber may beestablished by vesiculation of mafic magma and can lead torapid transfer of mafic magma across a formerly stablemafic/felsic magma interface (Eichelberger 1980; Rice 1985).

Several of the associations reported in this study arecommon to other granitoid terranes in the Superior Provinceand suggest the model presented here may have wideapplicability to late granitoids, particularly thosecontaining hornblende. Xenoliths ranging in compositionfrom hornblendite to microdiorite are a particularlywidespread feature in late Archean plutons (eg. Davis andEdwards, 1985). These inclusions have conventionally beeninterpreted as pre—existing mafic rocks. However, as thisand other studies (eg. Didier 1987) have suggested theinclusions may be chilled mantle—derived magma. Otherfeatures of contemporaneous mafic and felsic magmatismreported elsewhere include the presence of net—veined dikerocks in tonalitic to granodioritic plutons (Davis andEdwards 1985). Recent precise U-Pb zircon geochronologyalso indicates a close temporal association of late Archeangabbro plutons with granitoid rocks in other parts of theWabigoon Subprovince (Morrison et al. 1985). These featuressuggest that mantle—derived mafic magmas are an importantaspect of late Archean granitoid magmatism.

23

convecting, density stratified column is possible given large dimensions and a reasonable thermal gradient. An important aspect of a zoned chamber is the probability of roll over and explosive degassing if the magmas contain volatiles (Rice 1985). Evidence for degassing, mixing and violent emplacement of mixed chambers has also been documented by Marshall and Sparks (1984). Density instability of mafic magmas at the base of a chamber may be established by vesiculation of mafic magma and can lead to rapid transfer of mafic magma across a formerly stable mafic/felsic magma interface (Eichelberger 1980; Rice 1985).

Several of the associations reported in this study are common to other granitoid terranes in the Superior Province and suggest the model presented here may have wide applicability to late granitoids, particularly those containing hornblende. Xenoliths ranging in composition from hornblendite to microdiorite are a particularly widespread feature in late Archean plutons (eg. Davis and Edwards, 1985). These inclusions have conventionally been interpreted as pre-existing mafic rocks. However, as this and other studies (eg. Didier 1987) have suggested the inclusions may be chilled mantle-derived magma. Other features of contemporaneous mafic and felsic magmatism reported elsewhere include the presence of net-veined dike rocks in tonalitic to granodioritic plutons (Davis and Edwards 1985). Recent precise U-Pb zircon geochronology also indicates a close temporal association of late Archean gabbro plutons with granitoid rocks in other parts of the Wabigoon Subprovince (Morrison et al. 1985). These features suggest that mantle-derived mafic magmas are an important asnect of late Archean m-anitoid magmatism.

FIELD TRIP GUIDE

The field trip is designed to be completed in one day,including travel from and return to Thunder Bay. The tripincludes 1 stop on gneissic tonalite host rock, 3 stops onthe Tib Gabbro, 4 stops to examine the granitoids, and onestop at the PGE and Cu-Ni mineralization at the Roby Zone ofMadelaine Mines Limited.

Due to logging and mineral exploration activity in thefield trip area, the road conditions are continuallychanging. If you are doing this trip on your own you willneed an up—to—date map of logging roads in the area.

Proceed west from Thunder Bay along highway 11/17. AtShabaqua, turn north on highway 17 towards Dryden.Turn north on the Dog River Road (also signposted toGreat Lakes Pulp and Paper company Camp 234) which isapproximately 10 km west of the townsite of Raith and 1km west of the Central Time Zone/Eastern Time Zonemarker. Begin recording mileage at the start of theDog River Road. Drive carefully as loaded trucks areusing this road! Proceed north on the Dog River Roadfor 21 1cm, at which point there is a major junctionwith a branch road from the east. The first outcrop isthe large exposure on the southeast side of branch roadapproximately 50 metres from the junction.

Stop 1 - Gneissic tonaliteThis outcrop has not been studied in detail but is

included as a stop to illustrate the host lithology intowhich the mafic—ultratnafic and granitoid intrusions areemplaced. As is typical of other areas of gneissic tonalitein northwestern Ontario, the outcrop is complex and severalgranitoid phases are present.

continue north on the Dog River Road, pass through camp234, at 40 km (distance from highway 17) turn northeast(right) at the major fork. Proceed northeast for 6.2km and then turn east along branch road to Tib Lake.Proceed east along road to Tib Lake for 2.7 km at whichpoint there is a small track that leads to the north.Stop 2 is on the west side of the track approximately100 metres north of the Tib lake Road.

Stop 2 — Tib Gabbro Border ZoneThe Border Zone of the Tib Gabbro is defined based on

the abundant pegmatitic gabbro that occurs around the baseof the intrusion. This well-exposed outcrop displays thetypical textural variation in this zone. Proceeding upwardfrom the lowermost exposed part of the outcrop texturesrange from coarse—grained and pegmatitic gabbro to mediumgrained gabbro and gabbronorite with conformable pegmatiticlayers.

24

FIELD TRIP GUIDE

The field trip is aesigned to be completed in One aay, including travel from and return to Thunder Bay. The trip includes 1 stop on gneissic tonalite host rock, 3 stops on the Tib Gabbro, 4 stops to examine the granitoids, and one stop at the PGE and Cu-Ni mineralization at the Roby Zone of Madelaine Mines Limited.

Due to logging and mineral exploration activity in the field trip area, the road conditions are continually changing. If you are doing this trip on your own you will need an up-to-date map of logging roads in the area.

Proceed west from Thunder Bay along highway 11/17. At Shabaqua, turn north on highway 17 towards Dryden. Turn north on the Dog River Road (also signposted to Great Lakes Pulp and Paper Company Camp 234) which is approximately 10 km west of the townsite of Raith and 1 km west of the Central Time Zone/Eastern Time Zone marker. Begin recording mileage at the start of the Dog River Road. Drive carefully as loaded trucks are using this road! Proceed north on the Dog River Road for 21 km, at which point there is a major junction with a branch road from the east. The first outcrop is the large exposure on the southeast side of branch road approximately 50 metres from the junction.

st00 1 - Gneissic tonalite This outcro~ has not been studied in detail but is

included as a stop to illustrate the host lithology into which the mafic-ultramafic and granitoid intrusions are emplaced. As is typical of other areas of gneissic tonalite in northwestern Ontario, the outcrop is complex and several granitoid phases are present.

Continue north on the Dog River Road, pass through camp 234, at 40 km (distance from highway 17) turn northeast (right) at the major fork. Proceed northeast for 6.2 km and then turn east along branch road to Tib Lake. Proceed east along road to Tib Lake for 2.7 km at which point there is a small track that leads to the north. Stop 2 is on the west side of the track approximately 100 metres north of the Tib lake Road.

Stov 2 - Tib Gabbro Border Zone The Border Zone of the Tib Gabbro is defined based on

the abundant pegmatitic gabbro that occurs around the base of the intrusion. This well-exposed outcrop displays the typical textural variation in this zone. Proceeding upward from the lowermost exposed part of the outcrop textures range from coarse-grained and pegmatitic gabbro to medium grained gabbro and gabbronorite with conformable pegmatitic layers.

Return to Tib Lake Road and proceed east for another1.2 km. Stop 3 is on the north side of the road and isin the bush approximately 10 metres from the road.

Stop 3 — Tib Gabbro 1averjgLayered gabbronorite in the upper part of the Lower

Zone of the Tib Gabbro. Well developed intermittentlayering locally shows modal grading with pyroxene-richbases and plagioclase—rich tops. tJralitic alteration (greencoloured) occurs along fractures which locally cross cut theprimary layering.

Return to Tib lake Road and proceed east for another0.3 km. Stop 4 is on the south side of the Tib LakeRoad, approximately 20 metres from the road, and is onthe east side of a culvert.

Stop 4 - Tib Gabbro Lower ZoneThis outcrop displays wispy layered gabbronorite near

the top of the Lower Zone. The wispy layers consist ofplagioclase or pyroxene rich bands in gabbronorite. Thinpyroxenite dikelets of various orientations cut the layeringand contain inclusions of host gabbronorite. The dikeletsare interpretted to be injections of cumulate material. Asmall hornblende—feldspar dikelet is also present. Greenuralitic alteration of pyroxenes is associated withfractures and is a late feature.

Return to the Tib lake Road and continue east foranother 3.2 km, past the outlet from Tib Lake, to aroad which runs down the east side of Tib Lake. Turnsouth along this road and proceed for 11 km to thejunction with another major logging road which isreferred to here as the "Lac des Iles Road". Take thisroad to the southwest (in the direction of the DogRiver Road) for 6.5 km, until the junction with theRoenicke Lake Road is reached. Take the Roenicke roadnorth for 1.8 km and turn west along an unnamed bushroad. Proceed along this road, taking the left forkafter 2.1 km and reaching the outcrop for stop 5 after5.0 km. Note: The bush road into this stop is becomingovergrown and will probably be difficult to recognizeif you are following this after the spring of 1990.

Stop 5 - Tonalite brecciaThis distinctive breccia zone with dimensions of

approximately 100 by 300 metres cross—cuts the hornblendetonalite east of the Dog River. The breccia consists ofsubrounded fragments of tonalite with 2 to 3 cm thicksericite—epidote-silicif led alteration rinds. The matrixconsists of altered, fine grained and locally comminutedtonalite. Rounded irregular mafic clots up to 0.5 metreslong are locally present interstitial to the tonalite

25

, .

Return to Tib Lake Road and proceed east for another 1.2 tan. Stop 3 is on the north side of the road and is in the bush approximately 10 metres from the road.

stov 3 - Tib Gabbro lavering Layered gabbronorite in the upper part of the Lower

Zone of the Tib Gabbro. Well developed intermittent layering locally shows modal grading with pyroxene-rich bases and plagioclase-rich tops. Uralitic alteration (green coloured) occurs along fractures which locally cross cut the primary layering.

Return to Tib lake Road and proceed east for another 0.3 km. Stop 4 is on the south side of the Tib Lake Road, approximately 20 metres from the road, and is on the east side of a culvert.

Stov 4 - Tib Gabbro Lower Zone This outcrop displays wispy layered gabbronorite near

the top of the Lower Zone. The wispy layers consist of plagioclase or pyroxene rich bands in gabbronorite. Thin pyroxenite dikelets of various orientations cut the layering and contain inclusions of host gabbronorite. The dikelets are interpretted to be injections of cumulate material. A small hornblende-feldspar dikelet is also present. Green uralitic alteration of pyroxenes is associated with fractures and is a late feature.

Return to the Tib lake Road and continue east for another 3.2 km, past the outlet from Tib Lake, to a road which runs down the east side of Tib Lake. Turn south along this road and proceed for 11 km to the junction with another major logging road which is referred to here as the "Lac des Iles Roadw. Take this road to the southwest (in the direction of the Dog River Road) for 6.5 km, until the junction with the Roenicke Lake Road is reached. Take the Roenicke road north for 1.8 km and turn west along an unnamed bush road. Proceed along this road, taking the left fork after 2.1 km and reaching the outcrop for stop 5 after 5.0 km. Note: The bush road into this stop is becoming overgrown and will probably be difficult to recognize if you are following this after the spring of 1990.

Stov 5 - Tonalite breccia This distinctive breccia zone with dimensions of

approximately 100 by 300 metres cross-cuts the hornblende tonalite east of the Dog River. The breccia consists of subrounded fragments of tonalite with 2 to 3 cm thick sericite-epidote-silicified alteration rinds. The matrix consists of altered, fine grained and locally comminuted tonalite. Rounded irregular mafic clots up to 0.5 metres long are locally pr<="=on+ interstitipi <-A +he tonalite

fragments. Several quartz—feldspar porphyry dikes intrudethe breccia.

The breccia is interpreted to be the product of anexplosive degassing of a magma chamber. The process isthought to provide further evidence for underplating ofgranitic magma by mafic magma. Explosive devolatilizationis an expected consequence of the cooling of such a systemif the mafic magma contains volatiles (Rice 1985).

Return to Roenicke Road and then proceed south to the"Lac des Iles Road". Turn southwest along the "Lac desIles Road" (toward Dog River Road) and proceed for 6.5km to the Garden Road. Turn east on the Garden Roadand proceed for 1.8 km. Stop 6 is an outcrop on thesouth side of the road.

Stop 6 — Net-veined dikesMafic dikes are abundant in this area of the tonalite

and range from intrusions with sharp contacts to dikes whichare extensively veined by tonalite, such as the dike in thisoutcrop. The back veining texture is known as net—veinedsince the tonalite forms a wispy net which surrounds roundedcentimetre— to decimetre—sized mafic globules. Thestructure is indicative of emplacement of the mafic magmaprior to the consolidation of the felsic host.

Proceed for another 2.2 km east on the Garden Road.Stop 7 is a group of outcrops on either side of theroad.

Stop 7 — Mixing texturesIn this series of outcrops, mixing of mafic and

tonalitic maginas has resulted in a variety of igneoustextures. Net-veined and disaggregated mafic dike materialis present in the lower part of the outcrop. Thedisaggregated mafic material could be readily confused withrounded mafic inclusions. Adjacent to the net veined dike,the foliation of the tonalite is destroyed by remobilizationof the felsic material. At the top of the outcrop, thehornblende—rich mela-tonalite is interpreted to be a hybridcomposition resulting from more complete interaction of themafic and felsic magmas. Abundant skeletal textures in thisrock may result from supercooling due to magma mixing.

Turn around and proceed west for 0.7 km. Stop 8 is anoutcrop on the north side of the road.

Stop B - HornblenditeHornblendite occurs as outcrop scale pods within the

tonalite and are gradational with the mela—tonalite. Thehornblendite and mela—tonalite have complex internalstructures such as comb—layering and colloform structures.Comb—layering is defined by the alignment of coarse acicularto skeletal hornblende which is oriented perpendicular to

26

The breccia is interpreted to be the product of an explosive degassing of a magma chamber. The process is thought to provide further evidence for underplating of granitic magma by mafic magma. Explosive devolatilization

an expected consequence of the cooling of such a system the mafic magma contains volatiles (Rice 1985).

Return to Roenicke Road and then proceed south to the "Lac des lies Roadw. Turn southwest along the "Lac des Iles RoadM (toward Dog River Road) and proceed for 6. km to the Garden Road. Turn east on the Garden Road and proceed for 1.8 km. Stop 6 ro south side of the road.

Stop 6 - Net-veined dikes Mafic dikes are abundant in this area of the tonalite

and range from intrusions with sharp contacts to dikes which are extensively veined by tonalite, such as the dike in this outcrop. The back veining texture is known as net-veined since the tonalite forms a wispy net which surrounds rounded centimetre- to decimetre-sized mafic globules. The structure is indicative of emplacement of the mafic magma prior to the consolidation of the felsic host.

Proceed for another 2.2 km east on the Garden Road. on either side of the

In this series of outcrops, mixing of mafic and tonalitic magmas has resulted in a variety of igneous textures. Net-veined and disaggregated mafic dike material is present in the lower part of the outcrop. The disaggregated mafic material could be readily confused with rounded mafic inclusions. Adjacent to the net veined dike, the foliation of the tonalite is destroyed by remobilization of the felsic material. At the top of the outcrop, the hornblende-rich mela-tonalite is interpreted to be a hybrid composition resulting from more complete interaction of the mafic and felsic magmas. Abundant skeletal textures in this rock may result from supercooling due to magma mixing.

Turn around and proceed west for 0.7 km. Stop 8 is an outcrop on the north side of the road. -

Stop 8 - Hornblendite

layering. These rocks have geochemical characteristicswhich suggest that they are cumulates derived from the maficdike suite. The hornblendites also resemble a suite ofrocks known as "appinites" which are described in somePhanerozoic shoshonitic plutonic suites.

Return to the "Lac des Ties Road" and turn back to thenortheast towards Lac des lies. Proceed for 11 km tothe road on the right which provides access to theNadelaine Mines property. Turn east (right) towardsthe Madelaine Mines Property, taking a left fork after3.2 km and another left fork after 6.1 km. At theNadelaine Mines Property there is a gate and we will bemet by staff from the company.

Stop 9 — PGE mineralization and geology in the vicinity ofthe Roby Zone — Madelaine Mines Limited

Note: Due to the ongoing exploration and development work onthe Roby Zone, which includes extensive stripping andblasting, it is not possible to plan the precise featuresthat can be examined at this location prior to arrival atthe site.

Naps of the geology of the gabbroic units in thevicinity of the Roby Zone (Figure 8) show the complexrelations between intrusive units. Figure 3 shows that theRoby Zone occurs along the contact between uniform EC andthe more complex WG. Sheets and minor discordant dikes ofpyroxene cumulate intrude the EG and the EG/WG contact. Thesheets and dikes which are gabbronorite to websterite incomposition, contain cumulus orthopyroxene + clinopyroxeneand intercumulus plagioclase and vary from altered to fresh,even where hosted by altered EG. The highly uralitizedpyroxenite is termed amphibolite in drill logs. The alteredpyroxenite sheet is approximately 5 to 10 m thick andintrudes the EG but appears to grade into gabbronorite ofthe WG and is approximately conformable with the EG/WGcontact.

Figure 8 is a detailed outcrop map from the "C—Zone".This zone is within the WG at the southern end of the RobyZone and illustrates the complexity of the WG unit. Theoutcrop is predominantly fresh to moderately alteredgabbronorite of the WG with several units of alteredleucogabbro which form inclusions and layers within thegabbronorite. Some of the inclusions have cuspate surfacesand gradational contacts suggesting that they were not solidat the time of incorporation. Also present within thegabbronorite are inclusions of pegmatitic gabbro, alteredpyroxenite, and fine—grained, recrystallized, clinopyroxeneamphibolite. The gabbronorite and leucogabbro arediscordantly intruded and disrupted by mineralized gabbropegmatite dikes and a 1 to 4 m wide mineralized intrusionbreccia. The breccia is defined by an abundance of gabbroic

27

ering. These rocks have geochemical characteristics ch suggest that they are cumulates derived from the mat suite. The hornblendites also resemble a suite of

ks known as I1appinitess1 which are described in some nerozoic shoshonitic plutonic suites.

Return to the "Lac des Iles Roadn and turn back to th northeast towards Lac des Iles. Proceed for 11 km t the road on the right which provides access to the Madelaine Mines property. Turn east (right) towards the Madelaine Mines Property, taking a left fork afte 3.2 km and another left fork after 6.1 km. At the Madelaine Mines Property there is a gate and we will b met by staff from the company.

te: Due to the ongoing exploration and development work o e Roby Zone, which includes extensive stripping and asting, it is not possible to plan the precise features at can be examined at this location prior to arrival at

the site.

Maps of the geology of the gabbroic units in the vicinity of the Roby Zone (Figure 8) show the complex relations between intrusive units. Figure 3 shows that the Roby Zone occurs along the contact between uniform EG and the more complex WG. Sheets and minor discordant dikes of pyroxene cumulate intrude the EG and the EG/WG contact. Th sheets and dikes which are gabbronorite to websterite in composition, contain cumulus orthopyroxene + clinopyroxene nd intercumulus plagioclase and vary from altered to fresh, ven where hosted by altered EG. The highly uralitized oxenite is termed amphibolite in drill logs. The altered oxenite sheet is approximately 5 to 10 m thick and rudes the EG but appears to grade into gabbronorite of WG and is approximately conformable with the EG/WG

Figure 8 is a detailed outcrop map from the " C - Z O ~ ~ ~ ~ . his zone is within the WG at the southern end of the Roby e and illustrates the complexity of the WG unit. The crop is predominantly fresh to moderately altered ronorite of the WG with several units of altered

leucogabbro which form inclusions and layers within the gabbronorite. Some of the inclusions have cuspate surfaces and gradational contacts suggesting that they were not solid at the time of incorporation. Also present within the abbronorite are inclusions of pegmatitic gabbro, altered yroxenite, and fine-grained, recrystallized, clinopyroxene amphibolite. The gabbronorite and leucogabbro are discordantly intruded and disrupted by mineralized gabbro egmatite dikes and a 1 to 4 m wide mineralized intrusion reccia. The breccia is defined by an abundance of gabbroic

inclusions and contains a core of gabbro pegmatite. (Note:Unfortunately this outcrop is now partially destroyed byblasting.)

Textures in the WG, such as those shown in Figure 8,suggest that some type of incomplete mixing or contaminationof the WG by the EG took place. The presence of roundedinclusions, particularly of EG in WG and complexinterfingering of the two gabbro types on a decimeter scalesuggests that the EG was not completely solidified when theWG was intruded. Contacts between the two gabbro phases arecommonly cuspate, net—veined and disaggregated. Thesetextures are similar to those described in mixing zonesbetween mafic and felsic magmas by Marshall and Sparks(1984) and would be reflected in the chemistry of the rocksonly by more detailed sampling than that done in this study.The limited development of modal layering, the presence ofcumulate textured dikes of WG intruding EG and the abundanceof inclusion—rich outcrops suggests that the cumulates didnot form in—situ but were emplaced as crystal mushes. Thissituation contrasts with layered intrusions thatcrystallized by upward and inward growth of a cumulate—liquid interface (eg. Irvine et al 1983).

End of trip. Return route to Thunder Bay will be bythe recently opened access route to Lac des Iles fromhighway 527. This is a shorter trip than returning viathe Dog River road.

AcknowledqmentsI would like to thank J.M. Sweeny and A.R. Smith for

capable assistance mapping the Lac des lies area. J.P.Sheridan is thanked for access to map and sample the Lac desIles property of Madelaine Mines Limited. D.W. Davis isthanked for permission to use unpublished geochonologicalresults. K. Gil drafted the figures. Published withpermission of the Director, Ontario Geological Survey.

28

situation contrasts with layered intrusions that crystallized by upward and inward growth of a cumulate

id interface (eg. Irvine 1983).

highway 527. This is a shorter trip than returning the Dog River road.

lished geochonologic s. Published with

REFERENCES

Blake, D.H. 1981. Intrusive felsic-mafic net—veinedcomplexes in north Queensland. BMR Journal ofAustralian Geology and Geophysics, v. 6, pp. 95-99.

Boudreau, A.E., Mathez, E.A., and McCallum, 1.5. 1986.Halogen geochemistry of the Stillwater and BushveldComplexes: Evidence for the transport of the Platinum—group elements by Cl-rich fluids. Journal of Petrology,v.27, pp. 967—986.

Brown, P.E. and Becker, S.M. 1986. Fractionation,hybridisation and magma-mixing in the Kialineg centre,East Greenland. Contributions to Mineralogy andPetrology, v. 92, pp. 57-70.

Brugmann, G.E., and Naldrett, A.J. 1987. Platinum—groupelement abundances in mafic and ultramafic rocksPreliminary geochemical studies at the Lac des IlesComplex, District of Thunder Bay, Ontario. InGeoscience Research Grant Program, Summary of Research1986—1987, Ontario Geological Survey, MiscellaneousPaper 136, p. 99—114.

Brugmann, G.E., Naldrett, A.J. and MacDonald, A.J. 1989.Magma mixing and constitutional zone refining in theLac des Iles Complex, Ontario: Genesis of platinum —group element mineralization. Economic Geology, v. 84,pp. 1557—1573.

Cabri, L.J., and Laflamme, J.H.G. 1979. Mineralogy ofSamples from the Lac des lies Area, Ontario. CANMET,Energy, Mines and Resources, Canada, Report 79—27, 20p..

Davis, D.W. and Edwards, G.R. 1985. The petrogenesis andmetallogenesis of the Atikwa—Lawrence volcanic—plutonicterrain. Geoscience Research Grant Program, Summary ofResearch 1984—1985, Ontario Geological Survey,Miscellaneous Paper 127, pp. 101—111.

Didier, Jean, 1987. Contribution of enclave studies to theunderstanding of origin and evolution of graniticmagmas. Geologische Rundschau, v. 76, pp. 41-50.

Dunning, G.R. 1979. The Geology and Platinum-GroupMineralization of the Roby Zone, Lac Des lies Complex,Northwestern Ontario. Unpublished M.Sc. Thesis,Carleton University, Ottawa, Ontario, 129 p..

Eichelberger, J.C. 1980. Vesiculation of mafic magma duringreplenishment of silicic magma reservoirs. Nature, v.288, pp. 446—450.

Findlay, D.C. 1969. Origin of the Tulameen ultramafic gabbrocomplex, southern British Columbia. Canadian Journalof Earth Sciences. v. 6, pp. 399 — 425.

Furman, Tanya, and Spera, F.J. 1985. Co-mingling of acidand basic magma with implications for the origin ofmafic I-type xenoliths: Field and petrochemicalrelations of an unusual dike complex at Eagle Lake,Sequoia National Park, California, U.S.A. Journal of

29

REFERENCES

Blake, D.H. 1981. Intrusive felsic-mafic net-veined complexes in north Queensland. BMR Journal of Australian Geology and Geophysics, v. 6, pp. 95-99.

Boudreau, A.E., Mathez, E.A., and McCallum, I.S. 1986. Halogen geochemistry of the Stillwater and Bushveld Complexes: Evidence for the transport of the Platinum group elements by Cl-rich fluids. Journal of Petro v.27, pp. 967-986.

Brown, P.E. and Becker, S.M. 1986. Fractionation, hybridisation and magma-mixing in the Kialineq cen East Greenland. Contributions to Mineralogy and Petrology, v. 92, pp. 57-70.

Brugmann, G.E., and Naldrett, A.J. 1987. Platinum-group element abundances in mafic and ultramafic rocks : Preliminary geochemical studies at the Lac des Iles Complex, District of Thunder Bay, Ontario. a Geoscience Research Grant Program, Summary of Research 1986-1987, Ontario Geological Survey, Miscellaneous Paper 136, p. 99-114.

Brugmann, G.E., Naldrett, A.J. and MacDonald, A.J. 1989. Magma mixing and constitutional zone refining in the Lac des lies Complex, Ontario: Genesis of platinum - group element mineralization. Economic ~ e o i o ~ ~ , v. 84, ' UD. 1557-1573. t- - -

Cabri, L.J., and Laflanune, J.H.G. 1979. Mineralogy of Samples from the Lac des Iles Area, Ontario. CANMET, Energy, Mines and Resources, Canada, Report 79-27, 20 P. -

Davis, D.W. and Edwards, G.R. 1985. The petrogenesis and metallogenesis of the Atikwa-Lawrence volcanic-plutoni terrain. Geoscience Research Grant Program, Summ Research 1984-1985, Ontario Geological Survey, Miscellaneous Paper 127, pp. 101-111.

Didier, Jean, 1987. Contribution of enclave studie understanding of origin and evolution of granitic magmas. Geologische Rundschau, v. 76, pp. 41-50.

ning, G.R. 1979. The Geology and Platinum-Group Mineralization of the Roby Zone, Lac Des Iles Comp Northwestern Ontario. Unpublished M.Sc. Thesis, Carleton University, Ottawa, Ontario, 129 p..

Eichelberger, J.C. 1980. Vesiculation of mafic magma during replenishment of silicic magma reservoirs. Nature, v. 288, pp. 446-450.

Findlay, D.C. 1969. Origin of the Tulameen ultramafic gabbro complex, southern British Columbia. Canadian Journal of Earth Sciences. v. 6, pp. 399 - 425.

Furman, Tanya, and Spera, F.J. 1985. Co-mingling of acid and basic magma with implications for the origin of mafic I-type xenoliths: Field and petrochemical relations of an unusual dike complex at Eagle Lake, Sequoia National Park, California, U.S.A. Journal of , .

S

Volcanology and Geothermal Research, v. 24, pp. 151-178.

Gupta, V.1<. and Sutcliffe, RAt. in press. Mafic—ultrmaficintrusives and their gravity field: Lac des lies area,northern Ontario. Geological Society of America,Bulletins.

Hyndmari, D.W. and Foster, D.A. 1988. The role of tonalitesand mafic dikes in the generation of the Idahobatholith. Jounal of Geology, v. 96, pp. 31-46.

Irvine, T.N., Keith, D.W. and Todd, S.G. 1983. The J-MPlatinum—Palladium Reef of the Stillwater Complex,Montana: II Origin by double-diffusive convectivemixing and implications for the Bushveld Complex.Economic Geology, v. 78, pp. 1287—1334.

Kaye, L. 1966. Tib-Jack Lakes area, District of Thunder Bay.Ontario Department of Mines Preliminary Map, P380,Scale 1:31,680.

Leveson, D.J. 1966. Orbicular rocks: A review. GeologicalSociety of America Bulletin, v. 77, pp. 409—426.

Lindhardt, E. and Bues, C.C. 1987. Geology of the northernLac des lies Complex, District of Thunder Bay. ZnSummary of Field Work and Other Activities by theOntario Geological Survey, Ontario Geological Survey,Miscellaneous Paper 137, pp. 281—285.

Macdonald, A.J. 1985. The Lac des lies Platinum-group metalsdeposit, Thunder Bay District, Ontario. Summary ofField Work 1985, Ontario Geological Survey,Miscellaneous Paper 126, pp. 235—241.

Marshall, L.A. and Sparks, R.S.J. 1984. Origin of somemixed magma and net—veined ring intrusions. Journal ofthe Geological Society of London, v. 141, pp. 171-182.

Morrison, D.A., Davis, D.W., Wooden, J.L., Bogard, D.D.,Maczuqa, D.E. Phinney, W.C., and Ashwal, L.D. 1985. Ageof Mulcahy Lake Intrusion, northwestern Ontario, andimplications for the evolution of greenstone-graniteterrains. Earth and Planetary Science Letters, v. 73,pp. 306—316.

Morrison, D.A., Maczuga, D.E., Phinney, W.C., and Ashwal,L.D. 1986. Stratigraphy and petrology of the MulcahyLake layered gabbro: An Archean intrusion in theWabigoon Subprovince, Ontario. Journal of Petrology, v.27, pp. 303—341.

Naidrett, A.J., and Campbell, I.H. 1979. The influence ofthe silicate:sulfide ratio on the geochemistry ofmagmatic sulf ides. Economic Geology, v.74, pp. 1503-1506.

Northern Miner Press. 1976. Deep hole for Boston Bay at Lacdes lies Palladium. Northern Miner Press, v. 62,October 21, p. 27.

Northern Miner Press. 1988. Madelaine Mines on stream forLac des lies production. Northern Miner Press, v. 74,March 14, p. 24.

Pye, E.G. 1968. Geology of the Lac des lies Area, Districtof Thunder Bay. Ontario Department of Mines, Geological

30

Volcanology and Geothermal Research, v. 24, pp. 151- 178.

ta, V.K. and Sutcliffe, R.H. in press. Mafic-ultrmafic intrusives and their gravity field: Lac des lies area, northern Ontario. Geological Society of America, Bulletins. an, D.W. and Foster, D.A. 1988. The role of tonalites and mafic dikes in the generation of the Idaho batholith. Jounal of Geology, v. 96, pp. 31-46.

ine, T.N., Keith, D.W. and Todd, S.G. 1983. The J-M Platinum-Palladium Reef of the Stillwater Complex, Montana: I1 Origin by double-diffusive convective mixing and implications for the Bushveld Complex. Economic Geology, v. 78, pp. 1287-1334.

Kaye, L. 1966. Tib-Jack Lakes area, District of Thunder Bay. Ontario Department of Mines Preliminary Map, P380, Scale 1:31,680.

Leveson, D.J. 1966. Orbicular rocks: A review. Geological Society of America Bulletin, v. 77, pp. 409-426.

Lindhardt, E. and Bues, C.C. 1987. Geology of the northern Lac des Iles Complex, District of Thunder Bay. In Summary of Field Work and Other Activities by the Ontario Geological Survey, Ontario Geological Survey, Miscellaneous Paper 137, pp. 281-285.

Macdonald, A.J. 1985. The Lac des Iles Platinum-group metals deposit, Thunder Bay District, Ontario. Summary of Field Work 1985, Ontario Geological Survey, Miscellaneous Paper 126, pp. 235-241.

Marshall, L.A. and Sparks, R.S.J. 1984. Origin of some mixed magma and net-veined ring intrusions. Journal of the Geological Society of London, v. 141, pp. 171-182.

Morrison, D.A., Davis, D.W., Wooden, J.L., Bogard, D.D., MacZuga, D.E. Phinney, W.C., and Ashwal, L.D. 1985. Age of Mulcahy Lake Intrusion, northwestern Ontario, and implications for the evolution of greenstone-granite terrains. Earth and Planetary Science Letters, v, 73, pp. 306-316.

Morrison, D.A., MacZuga, D.E., Phinney, W.C., and Ashwal, L.D. 1986. Stratigraphy and petrology of the Mulcahy Lake layered qabbro: An Archean intrusion in the Wabigoon Subprovince, Ontario. Journal of Petrology, 27, pp. 303-341.

Naldrett, A.J., and Campbell, I.H. 1979. The influence of the si1icate:sulfide ratio on the geochemistry of magmatic sulfides. Economic Geology, v.74, pp. 1503- 1506. - ..

Northern Miner Press. 1976. Deep hole for Boston Bay at Lac des Iles Palladium. Northern Miner Press, v. 62, October 21, p. 27.

Northern Miner Press. 1988. Madelaine Mines on stream for Lac des Iles production. Northern Miner Press, v. 74, March 14, p. 24.

Pye, E.G. 1968. Geology of the Lac des Iles Area, District of Thunder Bay. Ontario Department of Mines, Geological

Report 64, 47 p., accompanied by maps 2135 and 2136,scale 1:31 680 or 1 inch to 1/2 mile.

Rice, Alan. 1985. The mechanism of the Mt. St. Helenseruption and speculations regarding soret effects inplanetary dynamics. Geophysical Surveys, v. 7, pp.303—384.

Sage, R.P., Breaks, F.W., Stott, G., Macwilliams, G., andBowen, R.P. 1974. operation Ignace-Armstrong,Pashkokogan - Caribou Lakes Sheet, District of ThunderBay, Ontario Division of Mines, Preliminary Map P962,Scale 1:126,720.

Schiffries, C.M. 1982. The petrogenesis of a platiniferousdunite pipe in the Bushveld Complex : Infiltrationmetasomatism by a chloride solution. Economic Geology,V. 77, pp. 1439—1453.

Schwerdtner, W.M., Stone, 0., Osadetz, K., Morgan, 3. andStott, G.M. 1979. Granitoid complexes and the Archeantectonic record in the southern part of northwesternOntario. Canadian Journal of Earth Sciences, v. 16, pp.1965—1977.

Sharpe, M.R. 1985. Strontium isotope evidence for preserveddensity stratification in the main zone of the BushveldComplex, South Africa. Nature, v. 316, pp. 119-126.

Smith, A.R. in press. Precambrian geology of the Tib Gabbro,District of Thunder Bay. Ontario Geological Survey openfile report.

Smith, A.R. and Sutcliffe, R.H. 1987. precambrian geology ofthe Tib Gabbro, Lac des lies area, District of ThunderBay, Ontario Geological Survey Preliminary Map, P3094,scale 1:15,840.

Stockwell, C.H., McGlynn, J.C., Emslie, R.F., Sanford, B.V.,Norris, A.W., Donaldson, J.A., Fahrig, W.F., andCurrie, K.L. 1972. Geology of the Canadian Shield, inDouglas, R.J.w. (editor), Geology and Economic Mineralsof Canada. Geological Survey of Canada, EconomicGeology Report No. 1, 838 p.

Streckeisen, A. 1976. To each plutonic rock its proper name.Earth-science Reviews, v. 12, pp. 1-33.

Stumpf 1, E.F., and Balihaus, e.G. 1986. Stratiform platinumdeposits : New data and concepts. Fortschritte derMineralogie, v. 64, pp. 205—214.

Stumpfl, E.F., and Rucklidge, J.C. 1982. The platiniferousdunite pipes of the Eastern Bushveld complex. EconomicGeology, v. 77, pp. 1419—1431.

Sutcliffe, R.H. 1989. Magma mixing in late Archean tonaliticand mafic rocks of the Lac de Iles area, westernSuperior Province. Precambrian Research, v.44, pp. 81-101.

Sutcliffe, R.H. and Smith, A.R. 1988. Precambrian geologyof the plutonic rocks in the Lao des lies - Tib Lakearea, District of Thunder Bay. Ontario GeologicalSurvey, Map P. 3098. scale 1:50,000. Geology 1986.

Sutcliffe, R.H. and Sweeny, J.M. 1986. Precambrian Geologyof the Lac des lies Complex, District of Thunder Bay.

31

Report 64, 47 p., accompanied by maps 2135 and 2136, scale 1:31 680 or 1 inch to 1/2 mile.

Rice, Alan. 1985. The mechanism of the Mt. St. Helens eruption and speculations regarding soret effects in planetary dynamics. Geophysical Surveys, v. 7, pp. 303-384.

Sage, R.P., Breaks, F.W., Stott, G., MacWilliams, G., and Bowen, R.P. 1974. Operation Iqnace-Armstrong, Pashkokoqan - Caribou Lakes Sheet, District of Thunder Bay, Ontario Division of Mines, Preliminary Map P962, Scale 1:126,720.

Schiffries, C.M. 1982. The petrogenesis of a platiniferous dunite pipe in the Bushveld Complex : Infiltration metasomatism by a chloride solution. Economic Geology, v. 77, pp. 1439-1453.

Schwerdtner, W.M., Stone, D., Osadetz, K., Morgan, if. and Stott, G.M. 1979. Granitoid complexes and the Archean tectonic record in the southern part of northwestern Ontario. Canadian Journal of Earth Sciences, v. 16, pp. 1965-1977.

Sharpe, M.R. 1985. Strontium isotope evidence for preserved density stratification in the main zone of the Bushveld Complex, South Africa. Nature, v. 316, pp. 119-126.

Smith, A.R. in press. Precambrian geology of the Tib Gabbro, District of Thunder Bay. Ontario Geological Survey open file report.

Smith, A.R. and Sutcliffe, R.H. 1987. Precambrian geology of the Tib Gabbro, Lac des lies area, District of Thunder Bay, Ontario Geological Survey Preliminary Map, P3094, scale 1:15,840.

Stockwell, C.H., McGlynn, J.C., Emslie, R.F., Sanford, B.V., Morris, A.W., Donaldson, J.A., Fahrig, W.F., and Currie. K.L. 1972. G ~ O ~ O U V of the Canadian Shield. in - -- Douglas, R.J.W. (editor), Geology and Economic Minerals of Canada. Geological Survey of Canada, Economic Geology Report No. 1, 838 p.

Streckeisen, A. 1976. To each plutonic rock its proper name. Earth-Science Reviews, v. 12, pp. 1-33.

Stumpfl, E.F., and Ballhaus, C.G. 1986. Stratiform platinum deposits : New data and concepts. Fortschritte der Mineralogie, v. 64, pp. 205-214.

Stumpfl, E.F., and Rucklidge, J.C. 1982. The platiniferous dunite pipes of the Eastern Bushveld complex. Economic Geology, v. 77, pp. 1419-1431.

Sutcliffe, R.H. 1989. Magma mixing in late Archean tonalitic and mafic rocks of the Lac de lies area, Western Superior Province. Precambrian Research, v.44, pp. 81- 101.

Sutcliffe, R.H. and Smith, A.R. 1988. Precambrian geology of the plutonic rocks in the Lac des Iles - Tib Lake area, District of Thunder Bay. Ontario Geological

, Survey, Map P. 3098. Scale 1:50,000. Geology 1986. Sutcliffe, R.H. and Sweeny, J.M. 1986. Precambrian Geology

of the Lac des lies Complex, District of Thunder Bay.

Ontario Geological Survey, Map p. 3047, GeologicalSeries—Preliminary Map, scale 1:15 840 or 1 inch to 1/4mile. Geology 1985.

Sutcliffe, R.H., Sweeny, J.M., and Edgar, A.D. 1989. The Lacdes Iles Complex, Ontario: petrology and platinum—group—elements mineralization in an Archean mafiaintrusion. Canadian Journal of Earth Sciences, v. 26,pp. 1408—1427.

Sweeny, J.M. and Edgar, A.D. 1987. The geochemistry, originand economic potential of platinum group elementbearing rocks of the Lac des lies Complex, northwesternOntario. In Geoscience Research Grant Program, Summaryof Research, 1986-1987. Ontario Geological SurveyMiscellaneous Paper 136, pp. 140—152.

Sweeny, J.M. 1989. The geochemistry and origin of the RobyZone, Lac des Iles Complex. MSc thesis, The Universityof Western Ontario, London, Ontario.

Talkington, R.W., and Watkinson, D.H. 1984. Trends in theDistribution of the Precious Metals in the Lac des IlesComplex, Northwestern Ontario. Canadian Mineralogist,v. 22, pp. 125—136.

Taylor, H.P. 1967. The zoned ultramafic intrusions ofsoutheastern Alaska. Ultramafic and related rocks,edited by P.J. Wyllie, 3. Wiley and Sons, pp. 97—121.

Todd, S.G., Keith, D.W., Schissel, D.J., LeRoy, L.L., Mann,E.L., and Irvine, T.N. 1982. The J-M platinum-palladiumreef of the Stillwater complex, Montana : IgneousStratigraphy and Petrology. Economic Geology, v. 77,pp. 1454—1480.

Vogel, T.A. 1982. Magma mixing in the acidic—basic complexof Ardnamurchan: Implications on the evolution ofshallow magma chambers. Contributions to Mineralogyand Petrology, V. 79, pp. 411—423.

Watkinson, D.H., and Dunning, G. 1979. Geology and Platinum-group Mineralization, Lac des lies Complex,Northwestern Ontario. Canadian Mineralogist, v. 17,pp.453—462.

32

Ontario Geological Survey, Map P. 3047, Geological Series-Preliminary Map, scale 1:15 840 or 1 inch to 1/4 mile. Geology 1985.

Sutcliffe, R.H., Sweeny, J.M., and Edgar, A.D. 1989. The Lac des Iles Complex, Ontario: petrology and platinum- group-elements mineralization in an Archean aafic intrusion. Canadian Journal of Earth Sciences, v. 26, pp. 1408-1427.

Sweeny, J.M. and Edgar, A.D. 1987. The geochemistry, origin and economic potential of platinum group element bearing rocks of the Lac des lies Complex, northwestern Ontario. U Geoscience Research Grant Program, Summary of Research, 1986-1987. Ontario Geological Survey Miscellaneous Paper 136, pp. 140-152.

Sweeny, J.M. 1989. The geochemistry and origin of the Roby Zone, Lac des lies Complex. MSc thesis, The University of Western Ontario, London, Ontario.

Talkington, R.W., and Watkinson, D.H. 1984. Trends in the Distribution of the Precious Metals in the Lac des lies Complex, Northwestern Ontario. Canadian Mineralogist, v. 22, pp. 125-136.

Taylor, H.P. 1967. The zoned ultramafic intrusions of southeastern Alaska. In Ultramafic and related rocks, edited by P.J. Wyllie, J. Wiley and Sons, pp. 97-121.

Todd, S.G., Keith, D.W., Schissel, D.J., LeRoy, L.L., Mann, E.L., and Irvine, T.N. 1982. The J-M platinum-palladium reef of the Stillwater complex, Montana : Igneous Stratigraphy and Petrology. Economic Geology, v. 77, pp. 1454-1480.

Vogel, T.A. 1982. Magma mixing in the acidic-basic complex of Ardnamurchan: Implications on the evolution of shallow magma chambers. Contributions to Mineralogy and Petrology, V. 79, pp. 411-423.

Watkinson, D.H., and Dunning, G. 1979. Geology and Platinum- group Mineralization, Lac des Iles Complex, Northwestern Ontario. Canadian Mineralogist, v. 17, pp.453-462.

List of FiguresFigure 1. Generalized geology of the Lac des lies area.

Simplified from mapping by the Ontario Geologicalsurvey (Sutcliffe and Smith 1988). Location of fieldtrip stops are shown.

Figure 2. Geology of the Lac des lies Complex based onmapping by Sutcliffe and Sweeny (1986) withmodifications from Lindhardt and Rues (1987). Locationof field trip stop is shown.

Figure 3. Geological map of the LDIC in the vicinity of theRoby Zone. Location of map is shown on Figure 2.Mineralized zones A to F from Pye (1968).

Figure 4. Section through the Roby Zone showing lithologiesand total PGE values plotted against length of drillhole. Based on diamond drill core logs of TexasgulfCanada Limited.

Figure 5. Geological map of the Tib Gabbro. From Smith andSutcliffe (1987). Location of field trip stops areshown.

Figure 6. Modal analyses of chemically analyzed samplesshowing variation in quartz—plagioclase—alkali feldsparand quartz—feldspar—mafic minerals.

Figure 7. Schematic cross—section of the crust in the Lacdes Iles area showing possible configuration of zonedmagma chamber.

Figure 8. outcrop map showing details of mineralizedbreccia zone and pegmatitic gabbro. Locatedapproximately 200 meters southeast of the Roby Zone inthe "C—zone" (see figure 3). Phase contacts at "A" and"B" are interpreted to be primary igneous layers.Leucogabbro inclusion at "C" has cuspate surfaces. Anintrusion breccia zone cross—cuts igneous layering.Pegmatitic gabbro dikes occur in the core of thebreccia and also disrupt the layering as at location"D". A channel sample at "E" indicates that thehighest values of PGE are associated with thepegmatitic phases.

33

List of Fiaures

Simplified from mapping by the Ontario Geological Survey (Sutcliffe and Smith 1988). Location of field trip stops are shown.

and total PGE values plotted against length of drill hole. Based on diamond drill core logs of Texasgulf

re 6. Modal analyses of chemically analyzed samples showing variation in quartz-plagioclase-alkali feldspar and quartz-feldspar-mafic minerals.

ure 7. Schematic cross-section of the crust in the Lac des lies area showing possible configuration of zoned magma chamber.

ure 8. Outcrop map showing details of mineralized breccia zone and pegmatitic gabbro. Located approximately 200 meters southeast of the Roby Zone in the "C-zone" (see figure 3). Phase contacts at "Aw and 'B" are interpreted to be primary igneous layers. Leucogabbro inclusion at *tC" has cuspate surfaces. An intrusion breccia zone cross-cuts igneous layering. Pegmatitic gabbro dikes occur in the core of the

:TAMNLAxE:sINTRUsIoN:

WITH MAGMA MIXING

+ t + + + + —:--:—..- -.

+••.k•Iomete 4+

Late Mafic to IJltramafic Rocks

I— I mafic to intermediate dikesultramaf Ic

t gabbro to gabbronorite

—— fault• PGE occurrence

— contactbreccia Zone

34

Figure 1.

90°oo' 89030*

49° 15

49°OO'PROTEROZOIC

diabase

AR CH E A N

Late Granitoids

k'+T4 biotite tonalite:—_—:—g hornblende tonalite

! 1 hornblende gabbro, hornblenditehornblende diorite

Early Granitoid Rocks

T1 tonalltic gneissSupracrustal Rocks

[II 11111 matic metavolcanics

t::c:i metasediments

-- fault mafic to intermediate dikes PGE occurrence - contact gabbro to gabbronorite

Figure 1.

PR

OT

ER

OZ

OIC

____

__

diab

ase

AR

CH

EA

N

1gr

anito

ids(

unsu

bdiv

ided

)

Ultr

aroa

f Ic

& R

elat

ed R

ocks

____

__

web

ster

ite, g

abbr

onor

ite

____

__

clin

opyr

oxen

ite, w

elirl

ite, s

erpe

ntin

ite

____

__

horn

blen

dite

, hor

nble

nde

pyro

xeni

te, h

ornb

lend

edi

orite

Gab

broi

c R

ocks

leuc

o ga

bbro

hybr

id z

one

gabb

rono

r its

horn

blen

de g

abbr

o

geol

ogic

al c

onta

ct

faul

t

strik

e &

dip

of l

ayer

ing

Figure 2.

——

a

490

15

LAG

"N)

DES

ILE

S

—L,

\jalTAao\

LI Li -r

+ + + + +

ROSY ZONE

FZONE

ft<r <'A1 '

".1... -

0-ZONE

Figure 3.

+ *+ + +

+ ++ + +

+ + + 4 + ++ I + + + +

4 t + ++ U + + +

+ + ÷ ++ + +

+ + + 4-

÷ ÷ 4

+ + 4 ++ t + t 4

+ + + + 4 ++ + + + + +

+ + + + + ++ + + + ÷ ÷

+ + + + ++ + + + + +

C-,

r—- I Western Gabbro—I -] Pegmatitic gabbro, gabbro breccia, norite,

gabbro, gabbronorjte, clinopyroxeneI Western Gabbro — Gabbronorite,[c:%:::J pyroxenite, gabbro, anorthosite

"J1 Eastern Gabbro—

______

Uralitized leuco—gabbro1 a,L't sj Diabase

0 Approximate location of mineralized zoneprojected to surface

Geological rontact

road

Foliation (strike and dip)

—'--- Igneous layering (strike and dip)

200 metres

+ ÷ + + ++ + + + + +

+ ÷ + + ++ + 4 t 4

+ + + 4 4 $

÷ 4. 4. 4 4-

+ 4- + + +

+ + + + 4+ + + + +

+ + + +÷ + ÷ ÷

+ ++ +

+ +

Pegmatitic gabbro, gabbro breccia,norite, gabbro, gabbronorite, clinopyroxene

+ + + +

+ + + + + + + + + +

+ + + + + + + + + + + +

+ + + + + + + + + + +

+ + + + + + + + + +

+ + + + + + + + +

projected to surface - Geological r-ontact

+ + + + + + + t t + + + + +

+ + + + + +

+ + + + + + + + + +

w E

37

75 drill holenumber

+ elevationreference

Figure 4.

+103 75 106

Section515

0 20 40 60 80metres

0 5 10 15 20

____

Eastern Gabbro

ppm PGE

Western Gabbro

Altered Pyroxenite

-

-

s.

metres

0 5 10 15 20

-t ppm PGE

.... 0 Eastern Gabbro 75 drill hole number 0 Western Gabbro

+ elevation Altered Pyroxenite reference

Figure 4 .

- . ..... - r.. _. .-- - -. --_______

70

0 --,

60 0- -. - - - - - V. -• - - - - . -

-

- - - -. - ;. -. - - - --4 - - -. - -• . -K - - . ,l__

• - :1: - : - ::87.

- :

•- -s-u- - . -

ii. - .. . •. . 0 0.5 1.0

::,!. :• 'I . • .

— I . — . • 'I- . - I'

• . . . .

• . . . . . . . ..

• . . • . . • • . . .

—a' —- .

• .•..•.........

:: :::MIDDLEZaNE.:::::.::. F,

road

LOWER ZONE" .--o

:: •: r+ + +

+ + ++ + + -c—r+ + + + +

P'KUHNER OCCURRENCE

______

diabase

A RC HE A N

foliated to gneissic biotitetonalite

— lithological contact

1+ + + + 4granodiorite (megacrystic)

granodiorite, Hb tonalite- (foliated)

TIB GABBRO

I-,,.: tj hornblende+quartz±biotitegabbro

magnetite-rich gabbronorite

pegmatitic gabbro

quartz granulite

I: - :±j gabbro norite, norite

fault

layering (strike and dip)

r' foliation (strike and dip)

ultramafic cumulates

zone contact

trench site

PROTEROZOIC

+ + + + + + + + ++ + + + + + + +-+

+ .+ + + + + + + +

mafic metavolcanics

A suiphide mineralization with> SOppb POE

0 sulphide mineralization with> 500ppb POE

38Figure 5.

PROTEROZOIC

diabase

ARCHEAN

grand!a~ite tmegacrystk>

granaciiarite~ Hb tanakite (foliated) . . :

TIB GABBRO

h&enW auar%-#2bbtite * * * a

gabbm

magnetiterich gabbronorite

pegmatitic gabbro

quartz granulite

gabbro mite , norite

foliated to gneissic bbttte

/ lithalogical contact

#- fault - layering (strike and dip)

foliation (strike and dip) - ultremafic cumulates

* zone contact

trench site

sulphide mineralization with > SOppb P6E

0 subhide mineralization with > SOOppb PGK

Figure 5 .

FELD

QTZ QTZ

MAFICS PLAG

Figure 6.

• HORNBLENDITE

• MAFIC TOINTERMEDIATE DIKES

O MELA-TONALITE

• HORNBLENDE TONALITE

o BIOTITE TONALITE

o QUARTZ FELDSPARPORPHYRY

•S00

0550

1S

QTZ QTZ + HORNBLENDITE HORNBLENDE TONALITE

MAFIC TO 0 BIOTITE TONALITE INTERMEDIATE DIKES

o MELA-TONALITE a QUARTZ FELDSPAR PORPHYRY

Figure 6 .

.1:-C

Figure 7.

\\ ,1

it

/

//

//channel sample

Itrench I'Li

EG — Ieucogabbro

WG — gabbronorite

gabbroic inclusions

pegmatitic gabbro dikes

gabbroic intrusion breccia

fault60

m

Figure 8.

41

Table L Sunnirv of

'etronraphy ol Litholoures fron the rae Des Ties Connie,.

Rock Tsre

Cunu us nincrals

Tnlercunu 'us Minerals

Alteration

'lest

ut,-

•t'lt

-ain

liar

'C

icer

aliIz

a:rrcr[se:tuer,v-t.

—=

.-—

—

____

serp

t-nt

rr'it

e,wenrirte

ol. ol -cbs

rpn,

nsno

rhb

ol tosc-rp-rrqt,

nesocurrolate

1-4 no

0]

chr,

ci

-

opt t

otalc

-ar

tthr'p

hyin

iate

,O

lClln

orvr

oier

rate,

ol

-op

t,or's

-cp

x,ops. hb,

opt to act,

adcun:rlate

1-4 rwt

opt. opi

Or's

clrnnovroxenite

flinor ph]

ninorc'art,orcate,

to riesi-unhi late

—--

epnd

ote.

chlorite,

Rebstei-t.-,Gabbr000ritecps

opt

cpx. opt, hb. p1, net

-ci

snozoisite

mesocunta.Iate

1—3 nfl

opt cops. pi

iherrolite ,

1—1

nfl

ol

-ch

r.ops 'CPA.

01 Isc-lsterste- dike

eRr •oiscps'npt

p1, Rb, phi

as abeve

nesocuisuiate to

orthocunu late

UI Gabbronorste dike

i—S rIS

opx'cps. pI

-R

b

Hornbiendite,

Rb, Rb

*op

pl. qtz, bio

Pyrosene Hornbiendite

-

epidote,ohiorite.

hvpidiomorphic

ci inozoisite

ire, —

1cm

cpx

•hb

,opt *

hbpl

qtz

iR

io

Norite

oi

p1

+op

t:-

—-

-

-p1

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I

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ite;

phi -

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to-b

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ol it

sqtz—quartz.

FIELD TRIP 2FIELD TRIP 2

1.1.5.0. FIELD TRIP

Geology of the Shebandowan & Quetico Archean subprovinces

0. BorradaileGeology DepartmentLakehead University

Thunder Bay, Ontario P7B 5E1Canada

Proterozoic Rocks

1. Kakabeka Falls. Recessed post—glacial gorge in Proterozoic shale.Soft sediment deformation in Proterozoic shales — car park exposure.

2. Basal conglomerate of Proterozoic sequence. (Care on Highway!)(Kakabeka Formation).

Archean Rocks

3. Pillow lavas and graded tuffs. Structural facing. D1 fabric.(Fig. la, bc).

4. Fourway School. Younger "Timiskaming—like" sequence of Archean. Flapof downward—structural facing sediments. (Fig. 3).

5. Finnmark exposure. "Timiskaming—like" sediments. Intertidal sequence.(General structural style of area, see Fig. 2).

12. Quetico gneisses, Raith. Migmatites, diktyonitic structure, multi-"phase" deformation.

11. Quetico belt gneisses.

6. Shebandowan Mine Road.

"Timiskaming—like" sequence(a) Fragmental red igneous rock (U)(b) conglomerate. Fabrics and strain in region (see Fig. 5,6,7).(c) slate

Older Keewatin—type Archean rocks(d) pillow lava(e) iron formation

1. Swamp Creek Pillow lavas. Strain (see Fig. 1,2)? Younging? Structuralfaci ng?

8. Tuffs and pillow lavas. (Structural facing and folds? see Fig. 8).

9. Increasing metamorphic grade in metasediments (Kashabowie group) towardsbelt boundary.

10. Gneissose rocks of the Quetico Belt.

43

Proterozoic Rocks

1. Kakabeka F a l l s . Recessed post-g lac ia l gorge i n Proterozoic shale. Sof t sediment deformation i n Proterozoic shales - car park exposure.

2. Basal con (Kakabeka

Archean Rocks

3 . P i l l o w lavas and graded t u f f s . S t ruc tura l facing. D, f a b r i c . ' (Fig. l a , be). }.

Fourway School. Younger "Timiskaming-1 i ken' sequence o f Archean. Flap o f downward-structural fac ing sediments. (Fig. 3 ) . i

Finnmark exposure. "Timiskaming-like" sediments. I n t e r t i d a l sequence'. (General s t ruc tu ra l s t y l e o f area, see Fig. 2 ) .

Quet ico gneisses, Raith. Migmatites, d i k t y o n i t i c s t ructure, m u l t i - "phase" deformation. . + - 4

;y. "." . . Q,-T" Quet ico be1 t gneissesi -,. , . ",-

- , c. ^". \- , , P i .~d t ~-! . . Shebandowan Mine Road. , > ,? .: <

! .: . ? ' - , ,. ~. < .... .>: ,

+ . ..> " A . "Timiskaminci-1 i ke" seouence - . ,., (a) Fragmental red igneous rock (!?) .. ..

(b) conglomerate. Fabrics and s t r a i n i n region (see Fig. 5,6,7);" (c) s l a t e

, . ^-$ 4 .E ~ : ' . I Older Keewati n - t v ~ e Archean rocks ",& -..I -'/; ,:>

" 5 ,.:4

Â¥ (d) p i l l o w lava ? . . ..* z . .-. . a ' ', , ' 3 ,,,: (e) i r o n formation ,?+; .- ;~,. .> 6 : c .; ,=

Swamp Creek P i l l o w lavas. S t ra in (see Fig. 1,2)? Younging? St ruc tura l facing?

Tu f f s and p i l l o w lavas. (S t ruc tura l fac ing and fo lds? see Fig. 8 ) .

Increasing metamorphic grade i n metasediments (Kashabowie group) towards be1 t boundary.

1x-'I PARAGNEISS

ARCHEAN LITHOLOGIESI>/-cI GRANITOIDS

"TIMISKAMING" METASEDIMENTS

0 r..— LAC

— Ilr-- Li

IH:.•:1 GREENSTONES

IEI GREYWACKES

"BELT"

7.,

7.—

RAITHLAKE

'' QUETICO "BELT9---,- ,,C()':/

LA

TIMISKAMINGMETASEDIMENTS

c. 2690 my

0 I0 20 KMj MT.

McKAYFORMPIt0H L. -

o EIIIII'long ',

"cij.p-centre lIne

notural pillow the model

An idealised natural lava pillow before straining and thegeometric model of it which is used in this paper.

C)T ka-

/ /__

Fig. la up

structuraltacint I

U.

(a) Undeformed bed with idealised pillow. The cusp-to-centreline of the pillow gives the younging direction, (h) Deformed bed withidealised pillow. The cusp-to-centre line of the pillow now tracks the'stratigraphic thickness' direction, t'. This may approximate to thestructural facing direction, The simple younging direction is no longerperpendicular to the long axis of the pillow in the general case, but, by

definition, it remains perpendicular to bedding.

000cusp peor - like crescent - stuoped

terminations pillow

Fig. btndeformed deformot,

piltow

Schematic sequence of progressively deformed pillows in-ferred from held observations, When the finite strain ellipse ratioreaches about 4.5 the pillows develop pear-like terminations

its a result of heterogeneous strain, and subsequently the ends ofthe pillow become so pointed that it is diftieuls to identify the cusp

45

nolurol pillow the model

An idealised natural lava pillow before straining and the geometric model of i t which is used in this paper.

Fig. l a

(a) Undeformed bed with idealised pillow. The cusp-t-ntre line of the pillow gives the youngingdirection. (b) Deformed bed with idealised pillow. The cusp-to-ccntre line of the pillow now tracks the -stratigraphic thickness' direction, 1 ' . This may approximate to the inidi iml facine direction. The simole vounoinf direction is no longer ~~~.~~~ -~~~ ~~~~~r ~ , eroendicular to the lono axisof the nillow in the ~eneralcase. but.bv

Fig. 1b

pear-like crescent-shaped bmhtims pillow

irdeformed ..... deformation .. ..[/f,4,//1,~-~4.5/ ............... pillow

Schematic sequence of proeressivcly dcformed pillows in- fen-cd from field observations. When the linite str;nin c l t i n ~ ; ratio reaches aboul4.5 the pillows tk-vclnp p~iir.likc tcrniinations

a s ~ i result of heterogeneous strain. :nod suhscqucntly the ends of the pillow become so pointed thiit i t is difficult to identify the cusp

FIG. I. Sketch of a deformed pillow showing the sel'age thick-nesses in the principal directions of strain: , S. S. The ratios ofthese thicknesses give the ratio of the principal strains if the thicknessof the selvage was originally constant. It should be noted that the

principal selvage thicknesses are not normally in the same ratio as thedeformed pilIow's dimensions: one exception is the unlikely event thatthe original, undelbrmcd pillow was spherical.

Fig. ic

46

sy

PtO. 1. Sketch of' a deformed pillow showing the selvage thick- nesses in the principal direcuons of wain S,. 5.. S - . The ratun d these thicknesses give the ratio of the principal strains if the thickness of the selvage was originally constant. It should be noted that the principal selvage thicknesses arc not normally in the ->ame ratio as the deformed ~illow'silin~'n~iuns: oneexccplion is the unlikcly cvcm that Ac original. undefonned pillow was spherical

-.

Fig. 1c

Susc.ptibilitin S maxImum

Fl g. 2 Lower-hemisphere equai-azea stereonets showing the meancleavage orientation (great circle) and mean intersection lineation for(A) the Shebandowan mine area and (B) the eastern Finmark area(east of Fourway School) (see Fig. I). The principal magnetic suscep-tibility orientations for the slates are also shown.

Fly.

47

'downward

structural Interpretatlofacing

Plan of an outcmp surface on the south side of HigiII — 17, about 100 m west of Finmark Road.

Where the glacial polish has been weathered away,vertical cleavage is pronounced; elsewhere the sedimentary detailpreserved. Ripple marks, graded bedding, and sand intrusionscase that the westward-closing fold has older rocks in its core: itanticline. However, the fold plunges to the east: it is a synform. '1this fuss-generation fold is a downward-facing fold that pmbdeveloped when folding affected a locally inverted flap of sedimiThe geometrical interpretation is shown in the sketch at botYounging directions are indicated by Y."

N N

(A) (B)

£ inteonediot. . minimun

X: meon fold plunge / inlerseclion Irneetion

, bed/ orientationcleavage

3

Suç~~tibil it ià :  moxifflum A mfrmdiif m mtftimum

X: i i~fln told plunge / inmfctton llmation

reonets showing the mean n intersection lineation for the eastern Finmark area principal magnetic suscep-

I / b'Zientation I cleavage

downward ,k:.. , strucfural

facing Interpretatic Plan of an outcrop surface on the south side of Hig

11 - 17. about 100 m west of Finmark Road. Where the glacial polish has been weathered away

vertical cleavage is pronounced; elsewhere the sedimentary detai preserved. Ripple marks, graded bedding, and sand intrusions a t e that the westward-closing fold has older rocks in its core: it anticline. However, the fold plunges to the east: it is a synfonn. ' this ht-generation fold is a downward-facing fold that pml developed when folding affected a locally inverted flap of sedim The geometrical interpretation is shown in the sketch at bo Younging directions are indicated by "Y."

clastic meta—[7J sedimentarylJ rocks

F . Stylized regional fold geometry for the Shebandowan gmupdeduced from cleavage—bedding and intenection lineation relation-ships. The fold plunge variation is infened to be quite large, althoughthe principal magnetic susceptibilities of the slates ("max." and"mt.") show relatively little change in orientation within the axial-planar cleavage. Because the susceptibility is due to the grain fabric ofthe rock, we Suggest that the variation in fold plunge occurred duringthe development of the folds rather than later.

fl Granitoidrocks

Fig. Simplified regional geology after Pye and Fenwick (1965)with subprovinces (SW = Shebandowan—Wawa. Q = Quetico. WWabigoon) after Goodwin (1977). Infonnation on boxed areas ispresented in Figs. 2 and 3.

48

volcanic rocks(mainly basic)

Fig. 6 Geological map of northern part of Shebandowan Lakegreenstone structure (SLGS) with horizontal line segment (QT) usefor strain estimate of overall shortening. Strain stations (see Table 2are numbered.

/

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F i y. 8 The structural facing of folds. (a) A general illustration of the concept of structural facing of folds. The structuralfacing direction remains consistently oriented while younging directions are variable. (b) An outcrop sketch of a small F2 foldwhich demonstrates that the S2 cleavage is axial planar. (c) Lower hemisphere stereographic representation of bedding andcleavage and their mutual intersection. ( A schematic profile view of bedding and cleavage. F2 folds face downward sinceyounger beds are encountered downward along the S2 cleavage.

50

(c) (d)

-- . . .. . . . . .

0 ~ t t f - w a ww) toed

I The structural facing of folds. (a) A general illustration of the concept of structural facing of folds. The structu racing directionremainsconsistently oriented white youngingdirectionsare variable. (b)Anoutcropsketch ofa small F fol which demonstrates that the Si cleavage is axial planar. (c) Lower hemisphere stereographic representation of beddi

younger beds are encountered downward along the S; c

References

Borradaile, G.J. 1976. "Structural facing" (Shackleton's rule) and thePaleozoic rocks of the Malaguide Complex near Velez Rablo, SE Spain.Proc. Kon. Nederl. Akad. van Wetens., 8 79, 330—336.

Borradaile, G.J. 1982. Tectonically deformed pillow lava as an indicator ofway up and bedding.J. Struct. Geol., 4, 469—479.

Borradaile, G.J. and Schwerdtner, W.M. 1984. Horizontal shortening of upwardfacing greenstone structures in the southern Superior Province, CanadianShield.Can. J. Earth Sci., 21, 611—615.

Borradaile, G.J. and Brocon, H.G. 1987. The Shebandowan Group: Timiskaminglike" Archean rocks in Northwestern Ontario.Can. J. Earth Sci., 24, 185—188.

Corfu, F. and Stott, G.M. 1986. U—Pb ages for late magmatism and regionaldeformation in the Shebandowan belt, Superior Province, Canada.Can. Jl. Earth Sci., 23: 1075—1082.

Schwerdtner, W.M., Bennett, P.J. and Janes, T.W. 1977. Application of [-Sfabric scheme to structural mapping and paleostrain analysis.Can. J. Earth Sci., 14, 1021—1032.

Schwerdtner, W.M., Stott, G.M. and Sutcliffe, R.H. 1983. Strain patterns ofcrescentic granitoid plutons in the Archean greenstone terrain ofOntario.J. Struct. Geol., 5, 419—430.

Stott, G.M. and Schwerdtner, W.M. 1981. A structural analysis of the centralpart of the Shebandowan metavolcanic—metasedimentary belt.Ontario Geological Survey, Open File Rep. # 5349 44 p.

Stott, G.M. and Schnieders, B.R. 1983. Gold mineralization in the Shebandowanbelt and its relation to regional deformation patterns.Ont. Geol. Survey, Misc. Paper 110, 181—193.

51

References

Borradaile, G.J. 1976. "Structural facing" (Shackleton's ru le ) and the Paleozoic rocks o f the Malaguide Complex near Vdez Rabio, SE Spain. Proc. Kon. Nederl. Akad. van Wetens., B 79, 330-336.

Rorradaile, G.J. 1982. Tectonical ly deformed p i l l o w lava as an ind ica to r o f way up and bedding. J. Struct . Geol., 4, 469-479.

Borradaile, G.J. and Schwerdtner, W.M. 1984. Horizontal shortening o f upward facing greenstone structures i n the southern Superior Province, Canadian Shield. Can. J. Earth Sci., 21, 611-615.

Borradaile, G. J. and Brocon, H.G. 1987. The Shebandowan Group: "Timi skaming- 1 ike" Archean rocks i n Northwestern Ontario. Can. J. Earth Sci., 24, 185-188.

Corfu, F. and S to t t , G.M. 1986. U-Pb ages f o r l a t e magmatism and regional deformation i n the Shebandowan bel t , Superior Province, Canada. Can. J1. Earth Sci., 23: 1075-1082.

Schwerdtner, W.M., Bennett, P.J. and Janes, T.W. 1977. Appl icat ion o f L-S fab r i c scheme t o s t ruc tu ra l mapping and paleostrain analysis. Can. J. Earth Sci., 14, 1021-1032.

Schwerdtner, W.M., S to t t , G.M. and Su tc l i f f e , R.H. 1983. S t ra in patterns of crescentic g ran i to id piutons i n the Archean greenstone t e r r a i n o f Ontario. J. Struct . Geol., 5, 419-430.

S to t t , G.M. and Schwerdtner, W.M. 1981. A s t ruc tu ra l analysis o f the central par t o f the Shebandowan inetavolcanic-metasedimentary be1 t. Ontario Geological Survey, Open F i l e Rep. # 5349 44 p.

1 S to t t , G.M. and Schnieders, B.R. 1983. Gold minera l izat ion i n the Shebandowan be1 t and i t s re1 a t ion t o regional deformation patterns.

I Ont. Geol. Survey, Misc. Paper 110, 181-193.

fiELD

Granitoid-related mineral deposits

of the western Lake Superior region.

Stephen A. KissinLakehead University

Thunder Bay, Ontario P7B 5E1Canada

52

Granitoid-related mineral deposits

of the western Lake Superior region.

Stephen A. Kissin Lakehead University

Thunder Bay, Ontario P7B 5E1 Canada

Introduction

The locations of surface exposures of granitoid rocks of the western LakeSuperior region are illustrated in Figure 1. The Archean granitoids are found in awestern area in the vicinity of Thunder Bay, which is separated by Proterozoicsediments and diabase sills of the Nipigon Plate from various plutons lying south ofLake Nipigon. The western group is associatd with the Shebandowan-Wawagreenstone subprovince and its boundary with Quetico Subprovince to the north. Theeasterly group of granitoids, with the exception of the Black Sturgeon Lake granite (6),lies within the Quetico Subprovince. The plutons in Figure 1 are numbered forpurposes of identification in the following discussion. The names of the bodies, with afew exceptions, are those employed by McCrank.t at. (1981) in their compendium.

The Glacier Lake pluton (3)is a batholith-sized body consisting of two-micaleucogranite in the north grading into biotite granite to the south. The rocks showminor penetrative deformation within a kilometer of their northern contact and arebounded by a weakly defined contact aureole in the intruded metasedimentary rocks,suggesting late tectonic emplacement. The rocks are moderately to stronglyperaluminous, and although bearing no aluminous minerals other than micas, have thegeochemical characteristics of S-type granitoids (A/(CNK).�. 1.1, normative corundum,high &O and high initial 87SR/°6Sr). Textural and geochemical patterns suggest thatthe pluton has been tilted to the north since emplacement (Zayachivsky., 1989).

The MNW stock (1), with essentially identical characteristics, appears to be asatellite of the Glacier Lake Pluton. The llgour Lake Group of Zayachivsky (1985) andKissin and Zayachivsky (1985), equivalent to the Kilgour Lake-Steen Lake metagabbroof McCrank . (1981), isa zoned granitoid intrusive with a core of monzogabbrograding outward to tonalites and granodiorites.

Also present in the Georgia Lake area are numerous tabular intrusions ofleucotonalite (trondhjemite) to granodiorite of thickness ranging from a few meters to afew tons of meters and strike lengths of up to several kilometres. The petrogenicaffinities of these are unclear as is the timing of their emplacement. In fact, contactsamong the Glacier Lake pluton, the Kilgour Lake group and the tabular intrusions havenot been observed in the field.

The Glacier Lake pluton is well exposed to the south of the Georgia Lake area,but its eastern margin is unmapped at present. It is exposed in the west alongHighway 11, but to the west, between Highway 11 and the Nipigon River system, is aregion of thick Pleistocene sediments, deposited in a spiliway between the LakeNipigon and Lake Superior basins. Metasedimentary rocks are intruded by granitoidsof variable size and form; however, poor availability of outcrop makes interpretation ofthe overall geologic picture difficult. Farther to the west, a well-defined intrusion, theChurch Lake quartz monzonite (4), is seen to intrude Archean metasediments and"migmatites". The petrography of these rocks is similar to that of the Glacier Lakepluton (Coates, 1972), and it is possible that they are parts of the same batholithic-sized body.

The Pine Portage intrusion (5) is exposed in a window in Proterozoic rocks at

53

Introduction

The locations of surface exposures of granitoid rocks of the western Lake Superior region are illustrated in Figure 1. The Archean granitoids are found in a western area in the vicinity of Thunder Bay, which is separated by Proterozoic sediments and diabase sills of the Nipigon Plate from various plutons lying south of Lake Nipigon. The western group is associate) with the Shebandowan-Wawa greenstone subprovince and its boundary with Quetico Subprovince to the north. The easterly group of granitoids, with the exception of the Black Sturgeon Lake granite (6), lies within the Quetico Subprovince. The plutons in Figure 1 are numbered for purposes of identification in the following discussion. The names of the bodies, with a few exceptions, are those employed by McCrankad. (1981) in their compendium.

The Glacier Lake pluton (3) is a batholith-sized body consisting of two-mica leucogranite in the north grading into biotite granite to the south. The rocks show minor penetrative deformation within a kilometer of their northern contact and are bounded by a weakly defined contact aureole in the intruded metasedimentary rocks,

. suggesting late tectonic emplacement. The rocks are moderately to strongly peraluminous, and although bearing no aluminous minerals other than micas, have the geochemical characteristics of S-type granitoids (A/(CNK)L^. 1 .l, normative corundum, high <180 and high initial "SR/̂ Sr). Textural and geochemical patterns suggest that the pluton has been tilted to the north since emplacement (Zayachivsky .eta., 1989).

The MNW stock (I), with essentially identical characteristics, appears to be a satellite of the Glacier Lake Pluton. The llgour Lake Group of Zayachivsky (1985) and Kissin and Zayachivsky (1985), equivalent to the Kilgour Lake-Steen Lake metagabbro of McCrankaA. (1981), is a zoned granitoid intrusive with a core of monzogabbro grading outward to tonaliies and granodiorites.

Also present in the Georgia Lake area are numerous tabular intrusions of leucotonaliie (trondhjemite) to granodiorite of thickness ranging from a few meters to a few tons of meters and strike lengths of up to several kilometres. The petrogenic affinities of these are unclear as is the timing of their emplacement. In fact, contacts among the Glacier Lake pluton, the Kilgour Lake group and the tabular intrusions have not been observed in the field.

The Glacier Lake pluton is well exposed to the south of the Georgia Lake area, but its eastern margin is unmapped at present. It is exposed in the west along Highway 11, but to the west, between Highway 11 and the Nipigon River system, is a reaion of thick Pleistocene sediments, deoosited in a soillwav between the Lake ~ i b i ~ o n and Lake Superior basins. ~etkedimentary rocks are intruded by granitoids of variable size and form: however. Door availability of outcroo makes interoretation of the overall geologic picture difficult ' Farther to thewest, a well-defined intrusion, the Church Lake quartz monzonite (4), is seen to intrude Archean metasediments and "migmatites". The petrography of these rocks is similar to that of the Glacier Lake pluton (Coates, 1972), and it is possible that they are parts of the same batholithic- sized body.

The Pine Portage intrusion (5) is exposed in a window in Proterozoic rocks at

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the source of the Nipigon River. It is composed of two-mica leucogranites cut bytabular leucotonatites, similar to those in the Georgia Lake area. The granitoids areclosely associated with lithium-bearing pegmatite dikes, cutting metasedimentary rocksto the east. Geochemical fractionation trends in rare-element and simple pegmatitessuggest that they are derived by fractionation of a leucogranite parent.

The Black Sturgeon Lake granite (6) is exposed in a window of Archean rockseast of Black Sturgeon Lake. It is associated with greenstone-like lithologies and maylie in the Wabigoon Subprovince.

West of the Nipigon Plate lies the Hilma Lake granite (7). The body consistswhite to pink two-mica leucogranite (Tihor, 1973), although textural compositionalvariations of the rock are found, including pegmatitic granite and pegmatite. Therelationship to the Penassen Lakes stock of Scott (1985), equivalent to the Wailer Lakegranite of McCrank.a!. (1981), is unclear owing to a lack of mapping. The twobodies are shown as contiguous by McCrank.eta!., although the different names wereapplied to the separate bodies suggested in Figure 1. This differentiation of the twobodies is supported by the rather different petrography of the Penassen Lakes stock,which is mostly coarse-grained porphyritic granite to quartz monzonite with localhornblende syenite and syenite phases.

The Mackenzie granite (9), equivalent to the Waylan Lake granite of McCrankaL (1981), has a complex outcrop pattern intruding Archean metavoicanics andmetasediments, lying unconformably below Animike Group (Gunflint and RoveFormations) sediments and intruded by Keweenawan diabase sills and dikes.According to Rogers (1979), it is a medium-to-coarse-grained biotite granite. TheMackenzie granite is possibly equivalent to the Kivikoski granite (10), equivalent to theMcintyre granite of McCrank .. (1981), as granite is known to underlie much of thecity of Thunder Bay as basement to the Animike Group. The Gorham granite ofMcCrank.Qta!. (1981) is included on Figure 1 as part of the McKenzie granite (M.M.kehienbeck, pers. comm., 1990).

The Trout Lake granite (11), Barnum Lake quartz monzonite (12) and WhiteliiyLake granite (13) are three similar plutons of circular to elliptical outcrop patternaligned west to east near the Shebandowan-Quetico Subprovince boundary. Therocks are characteristically hornblende and biotite-bearing, porphyritic quartzmonzonites. Kehlenbeck (1977) pointed out their late syntectonic emplacement, asindicated by well defined contact aureoles in the intruded Archean metasedimentaryand metavolcanic rocks.

Mineral Deposits

Pegmatites

The most spectacular granitoid-related mineral deposits of the area are rare-element pegmatites in the Georgia Lake area and a small but similar pegmatite field inthe Pine Portage area (Pye, 1965). These have been studied more recently byZayachivsky (1985) and Archibald (1987), and these and other results are reported byKissin & Zayachivsky (1985), Kissinetal. (1986) and Kissin and Archibald (1988). The

55

the source of the Nipigon River. It is composed of two-mica leucoaranites cut bv tabular leucotonatites,similar to those in the Georgia Lake area. The granitoids are closelv associated with lithium-bearina oeamatite dikes, cuttina metasedimentarv rocks - . - to theeast. Geochemical fractionation trends in rare-element and simple pegmatites suggest that they are derived by fractionation of a leucogranite parent.

The Black Sturgeon Lake granite (6) is exposed in a window of Archean rocks east of Black Sturgeon Lake. It is associated with greenstone-like lithologies and may lie in the Wabigoon Subprovince.

West of the Nipigon Plate lies the Hilma Lake granite (7). The body consists white to pink two-mica leucogranite (Tihor, 1973), although textural compositional variations of the rock are found, including pegmatitic granite and pegmatite. The relationship to the Penassen Lakes stock of Scott (1985), equivalent to the Waller Lake granite of McCrankM. (1981), is unclear owing to a lack of mapping. The two bodies are shown as contiguous by McCrankad., although the different names were applied to the separate bodies suggested in Figure 1. This differentiation of the two bodies is supported by the rather different petrography of the Penassen Lakes stock, which is mostly coarse-grained porphyritic granite to quartz monzonite with local hornblende syenite and syenite phases.

The MacKenzie granite (9), equivalent to the Waylan Lake granite of McCrank a at. (1981), has a complex outcrop pattern intruding Archean metavolcanics and - metasediments, lying unconformably below Animike Group (Gunflint and Rove Formations) sediments and intruded by Keweenawan diabase sills and dikes. According to Rogers (1979), it is a medium-to-coarse-grained biotite granite. The MacKenzie granite is possibly equivalent to the Kivikoski granite (lo), equivalent to the Mclntyre granite of McCrankad. (1981), as granite is known to underlie much of the city of Thunder Bay as basement to the Animike Group. The Gorham granite of McCrankad. (1981) is included on Figure 1 as part of the McKenzie granite (M.M. Kehlenbeck, pers. comm., 1990).

The Trout Lake granite (1 I), Barnum Lake quartz monzonite (12) and Whitelily Lake granite (13) are three similar plutons of circular to elliptical outcrop pattern aliened west to east near the Shebandowan-Quetico Subprovince boundary. The rocks are characteristically hornblende and biotite-bearing, porphyritic quartz monzonites. Kehlenbeck f1977) ~ointed out their late svntectonic em~lacement. as indicated by well defined &nta&aureoles in the intruded Archean metasedimentary and metavolcanic rocks.

Peamatites

The most spectacular aranitoid-related mineral deoosits of the area are rare- element ~e~mat i tes in the ~ e o r ~ i a Lake area and a smail but similar pegmatite field in the Pine Portaae area (Pve. 1965). These have been studied more recently bv . . . . , Zayachivsky (7985) and ~rchibald (1987), and these and other results are reported by

Zayachivsky (1985), Kissin d. (1986) and Kissin and Archibald (1988). The

rare-element pegmatites of the Georgia Lake area are shown in Figure 2. Thepegmatites may be subdivided into three groups, a northern group whose membersshow little difference in relative fractionation of trace element and central and southerngroup whose members show increasing degrees of fractionation from east to west.Barren pegmatites lie to the east of the central and southern group but are not shownin Figure 2.

Most of the pegmatites are unzoned and contain phenocrystic spodumene;however, the more highly fractionated pegmatites of the central and southern groupshow weak to well developed zoning and may contain, as well as spodumene,tantalite-columbite group minerals, beryl, cassiterite or staringite and various Liphosphate minerals. The MNW pegmatite contains spodumene-quartz-intergrowth(SOUl) and is a potential source of ceramic grade spodumene.

An entirely separate group of pegmatites occurs within the Quetico Subprovinceto the west of the Nipigon Plate. Franklin (1978) described uraniferous, white albite-muscovite-biotite-quartz pegmatites containing 60-100 ppm U. The pegmatites havebeen found to contain uraninite, and all such pegmatites containing accessory apatitehave been found to be uraniferous. These pegmatites occur in an east-west trendingzone within the Quetico Subprovince. Their origin is obscure, as they appear to beunrelated to anatectic mobilizate pods in high-grade metamorphic rocks nor to pinksyenitic pegmatites apparently co-magmatic with granitic intrusions in the area. TheHilma Lake granite, however, was mentioned as having an anomalous uranium contentwith respect to other rocks of the area (Franklin, 1978).

Molybdenum

A sub-economic molydenite occurrence is located at Anderson Lake, at thewestern margin of the Hilma Lake granite. Molydenite occurs as coarse aggregrates inquartz veins and within pink pegmatitic granite. It is unclear as to whether themineralization process was pegmatitic or hydrothermal. The quartz veins lackcharacteristics of pegmatites; however, hydrothermal alteration characteristic of vein-or porphyry-style molybdenite deposits is also lacking. The average grade of theoccurrence is reportedly 2.85% Mo; however, the apparent small size of the reserves isresponsible for a lack of exploitation.

Non-magmatic deposits, hosted in granite

Uranium

Uranium occurrences in the western Lake Superior region were summarized byFranklin (1978). Those related to granitoid rocks include uraniferous pegmatites in theQuetico Subprovince and vein-type occurrences in granitic basement or cuffing SibleyGroup sediments. The vein-type occurrences are not of direct hydrothermal derivationfrom granitic magmas, but may be related to uraniferous pegmatites and uranium-richgranites as protore.

The uraniferous pegmatites are prominent in the Greenwich Lake area (U (1) inFigure 4), and exploratory work has indicated that they occur about 40 km to the west

56

rare-element pegmatites of the Georgia Lake area are shown in Figure 2. The pegmatites may be subdivided into three groups, a northern group whose members show little difference in relative fractionation of trace element and central and southern group whose members show increasing degrees of fractionation from east to west. Barren pegmatites lie to the east of the central and southern group but are not shown in Figure 2.

Most of the pegmatites are unzoned and contain phenocrystic spodumene; however, the more highly fractionated pegmatites of the central and southern group show weak to well developed zoning and may contain, as well as spodumene, tantalite-columbite group minerals, beryl, cassiterite or staringite and various Li phosphate minerals. The MNW pegmatite contains spodumene-quartz-intergrowth (SQUI) and is a potential source of ceramic grade spodumene.

An entirely separate group of pegmatites occurs within the Quetico Subprovince to the west of the Nipigon Plate. Franklin (1978) described uraniferous, white albite- muscovite-biotite-quartz peamatites containing 60-100 ppm U. The peamatites have . - been found to contain uianinite, and all such pegmatites containing accessory apatite' have been found to be uraniferous. These oeamatites occur in an east-west trendina. - zone within the Quetico Subprovince. s heir origin is obscure, as they appear to be unrelated to anatectic mobilizate pods in high-grade metamorphic rocks nor to pink svenitic peamatites apparently co-magmatic with granitic intrusions in the area. The thma Likegranite, however,was mentioned as having an anomalous uranium content

(Franklin, 1978).

A sub-economic molydenite occurrence is located at Anderson Lake, at the western margin of the Hilma Lake g quartz veins and within pink pegmat mineralization process was pegmatit characteristics of pegmatites; howeve or porphyry-style molybdenite deposi occurrence is reportedly 2.85% Mo; responsible for a lack of exploitation.

Non-magmatic deposits, hosted in gra

Uranium occurrences in the western Lake Superior region were summarized by Franklin (1978). Those related to granitoid rocks include uraniferous pegmatites in the Quetico Subprovince and vein-type occurrences in granitic basement or cutting Sibley Group sediments. The vein-type occurrences are not of direct hydrothermal derivation from granitic magmas, but may be related to uraniferous pegmatites and uranium-rich granites as protore.

The uraniferous pegmatites are prominent in the Greenwich Lake area (U(1) in Figure 4), and exploratory work has indicated that they occur about 40 km to the west

:/;:m::. NAMA CREEK)••• 7. :NoRTH

NAMA CREEK1SOUTI

2 5 Postogoni

¶I. . :•• . •• :

• . \\\. fr':. NEWK IRK: :'?s .;'9gjz1SALO.: :..Xy SOUTHWESTr

• • • . ..t. 1÷÷_,.Spl 1+ !ANS0NL•.j.j_r+ + 1+ 1fI + + +11- +iI+

Nipigon diabaseFault

0 km

Figure 2. The Georgia Lake Pegniatite Field illustrating the granitoidrocks and rare-element pegmatites. Named pegmatite bodieswere studied by Zayachivsky (1985).

57

+

+

+ 1÷ +1 +

LEGEND• Rare element pegmatite

IMetasedin,ent

I ++I Granifoid 0

_.___— Contact

I 1,1111111 I

L E G E N D Rare element oeamatite -

e- de Fault a Nipigon diabase

Metasediment Contact

Gran~toid 0 10 km I t O ~ P ~ 8 8 9 ~ 1

F igure 2. The Georgia Lake Pegmatite F i e l d i l l u s t r a t i n g t h e g r a n i t o i d rocks and rare-element pegmatites. Named pegmatite bodies were studied by Zayachivsky (1985) .

along Highway 527. As described previously, these pegmatites are contained within arelatively narrow east-west trending zone. The small size of the pegmatite pods andlenses and their low uranium content preclude the possibility of economic deposits.

The vein-type occurrences (Figure 4) at Greenwich Lake (U(1)) and Innes Lake(U(2)) have been described by Franklin (1978) and Yule (1979). Both occurrences aresub-economic but were the source of considerable excitement in the late 1970's. Bothoccurrences are localized in brecciated shear zones cutting all Archean lithologies.The Greenwich Lake occurrence occurs in well defined veins with quartz and pyritegangue and minor pitchblende yielding grab sample grades of 0.5 to 2.0% U.Hydrothermal alteration of wall rock is distinct, and the vein contains altered SibleyGroup fragments indicating its Proterozoic age. The Innes Lake occurrence has norecognisable uranium minerals, although the shear zone is much enriched in apatite,which is thought to be the carrier of uranium. Hydrothermal alteration includingchloritization, sericitization and hematitization is distinctive about the shear zones. Theshear zone yielded values as high as 633 ppm U308.

Franklin (1978) also described minor uranium occurrences (up to 540 ppm U) inthe Enterprise Mine, one of a group of vein-type deposits known as the lead-zinc-bariteveins. These are discussed more fully in the next section; however, both Franklin(1978) and Yule (1979) believed that the Greenwich Lake and lnnes Lake occurrenceswere formed by the same process that formed the sulfide-rich lead-zinc-barite veins.Briefly, the hypothesis favored is circulation of basinal brines through uranium-enrichedsource rocks, the uraniferous pegmatites and anomalous granitic rocks. The brinescirculated through faults marginal to the Sibley Group depositional basin and rose andcooled in response to hydraulic gradients.

Lead-zinc-barite

The lead-zinc-barite veins (Figure 3) are near the margins of the present outcropof the Sibley Group or its inferred former outcrop. The veins were studied in detail byFranklin & Mitchell (1977), who found them to contain galena, sphalerite and barite,with minor chalcopyrite and marcasite in quartz and calcite gangue. The veins occurin three settings: (A) fractures within the Sibley Group, (B) fractures at the Sibley-basement unconformity and (C) fractures within granitic basement. Franklin & Mitchellbelieved that the deposits formed by mixing of bacterially reduced H25 with metal-richbrines derived by leaching of the Sibley basinal fill. Deposition occurred in structuraltraps at or near the basin margin. The basinal character of the brines was confirmedin recent fluid inclusion studies by Haynes (1988).

The differentiation of structural settings by Franklin & Mitchell suggests that type(C) may be a portion of a continuum of deposits extending to the uraniferousoccurrences at Greenwich Lake and Innes Lake. The lead-zinc-barite veins representa sulfide-rich but uranium-poor end-member, whereas the uranium occurrences aresulfide-poor but uranium-rich. The differential factor seems to have been the relativeamount of interaction with basinal fill (base metal source) or granitic basement(uranium source).

58

.&aq Highway 527. As described previously, these pegmatites are containedwithin a :relatively narrow east-west trending zone. The small size of the pegmatite pods and lenses and their low uranium content preclude the possibility of economic deposits.

The vein-type occurrences (Figure 4) at Greenwich Late :mN and fines take [U(2y> have been described by Franklin (1S78) and Yule (1978). Both occurrences are sub-economicbutwarethesourceofconsiderableexcitement;hthe %%¥& occurrences are localized in brecdated shear zones cutting all Arehean ÈaiGitoes The Greenwich ;Lake occurrence occurs in well defined veins with quartz and pyrtte gangue and minor pitchblende yielding grab sample grades of 0.5 to Hydrothermal ateralion of wall rock is disfinct, and the veto contains Group fragments indicating its Proterozoic age. The Innes recognisable uranium minerals, although the shear zone is which is thought to be (he carrier of uranium. Hydrothermal alteration . jndutf i chloritization, sericitizatton and hetnattization is distinctive about the .&ear zones. 'The shear zone yielded values as high as 633 ppm U.4.

Franklin (1978) also described minor uranium (up to 540 ppm U) in the Enterprise Mine, one of a group of vein-type deposits known as the tead-zinc-bante veins. These are discussed more fully in the next section; however, both Franklin (1978) and Yule (1979) believed that the Greenwich Lake and Innes Lake occurrences were formed by the same process that formed the sulfide-rich lead-zinc-barite veins. Briefly, the hypothesis favored is circulation of basinal brines through uranium-enriched source rocks, the uraniferous pegmatites and anomalous granitic rocks. The brines circulated through faults marginal to the Sibley Group depositional basin and rose and cooled in response to hydraulic gradients.

The lead-zinc-barite veins (Figure 3) are near the margins of the present outcrop of the Sibley Group or its inferred former outcrop. The veins were studied in detail by Franklin & Mitchell (1977), who found them to contain galena, sphalerite and barite, with minor chalcopyrite and marcasite in quartz and calcite gangue. The veins occur in three settings: (A) fractures within the Sibley Group, (B) fractures at the Sibley- basement unconformity and (C) fractures within granitic basement. Franklin & Mitchell believed that the deposits formed by mixing of bacterially reduced H2S with metal-rich brines derived by leaching of the Sibley basinal fill. Deposition occurred in structural traps at or near the basin margin. The basinal character of the brines was confirmed in recent fluid inclusion studies by Haynes (1988).

The differentiation of structural settings by Franklin & Mitchell suggests that type 1C) mav be a oortion of a continuum of deposits extendina to the uraniferous occurrences Greenwich Lake and lnnes~ake. The lead-zinc-barite veins represent a sulfide-rich but uranium-poor end-member, whereas the uranium occurrences are sulfide-poor but uranium-rich. The differential factor seems to have been the relative amount of interaction with basinal fill (base metal source) or granitic basement (uranium source).

k1 Diabase Granitic

II Sibley Group

___

MetasedimenfaryGneissic Rocks

I I Animikie Group L1 MetavolcanicRocks

A Pb-Zn—Barite • Amethystdeposit deposit

Figure 3. The Dorion area illustrating the locationsof major amethyst deposits and lead-zinc-barite veins. The star is the locationof the Thunder Bay Amethyst Mine.

59

PROTEROZOIC ARCHEAN

Rocks

PROTEROZOIC ARCHEAN

Diabase Granitic Rocks

. Sibley Group Metasedirnentary . .... Gneissic Rocks

Anirnikie Group Metavo'canic Rocks Pb-Zn-Barite  Amethyst

deposit deposit

Figure 3. The Dorion area illustrating the locations of major amethyst deposits and lead-zinc- barite veins. The star is the location of the Thunder Bay Amethyst Mine.

Amethyst

The largest deposits of amethyst in North America are located in the ThunderBay area (Figure 3). Amethyst is the provincial gemstone of Ontario and is asignificant generator of economic activity in the region. Nearly all the importantdeposits occur in a 10 km-long, east-west trending near the margin of the SibleyGroup outcrop and with Hilma Lake granite as the basement. A few other scatteredoccurrences are spatially related to the Sibley Group outcrop and are invariably hostedin granite.

The amethyst deposits consist mostly of quartz including much of the varietyamethyst with minor calcite, barite and sulfides such as pyrite, chalcopyrite andbornite. The similar geologic selling of these deposits to that of the lead-zinc-bariteveins suggests that they are cogenetic. Fluid inclusion studies by McArthur (1988),reported in McArthur & Kissin (1988), indicate that most amethyst formed near thesurface at temperatures ranging from 60 to 90 C. The deposits appear to be low-temperature derivatives of lead-zinc-barite veins, poor in sulfides and richer in quartz.A crucial factor appears to have been passage of the quartz-depositing solutionsthrough a uranium enriched host rock, e.g. the Hilma Lake granite. A source of silica(from sericitization of feldspar) and radioactivity are probable requirements for theformation of amethyst. The formation of the electronic colour center in quartzproducing the blue of amethyst has been attributed to the coupled electronic transition:

Fe3(substitutional)-+ Fe'(substitutional) + e

Fe3(interstitial) + e-Fe2(interstitial)

The associated Fe4 (substitutional) + Fe2 (interstitial) defect (Cohen & Hassan,1974) is evidently unstable at high temperatures, as older generations of amethystwere observed to be bleached by influx of holler fluids.

REFERENCES

Archibald, D.L.1987: Granitoids and Pegmatites of the Pine Portage Area, Northwestern Ontario;

Unpublished B.Sc. Thesis, Lakehead Unviersity, Thunder Bay, 89 p.

Coates, M.E.1972: Geology of the Black Sturgeon River area, District of Thunder Bay; Ontario

Department of Mines and Northern Affiars, Geological Report 98, 41 p.

Cohen, A.J., and Hassan, F.1974: Ferrous and Ferric Ions in Synthetic a-Quartz and Natural Amethyst; American

Mineralogist, Volume 59, p. 719-728.

60

The largest deposits of amethyst in North America are located in the Thunder Bay area (Figure 3). Amethyst is the provincial gemstone of Ontario and is a significant generator of economic activity in the region. Nearly all the important deposits occur in a 10 km-long, east-west trending near the margin of the Sibley Group outcrop and with Hilma Lake granite as the basement. A few other scattered occurrences are spatially related to the Sibley Group outcrop and are invariably hosted in granite.

The amethyst deposits consist mostly of quartz including much of the variety amethyst with minor calcite, barite and sulfides such as pyrite, chalcopyrite and bornite. The similar geologic setting of these deposits to that of the lead-zinc-barite veins suggests that they are cogenetic. Fluid inclusion studies by McArthur (1988), reported in McArthur 81 Kissin (I=), indicate that most amethyst formed near the surface at temperatures ranging from 60 to 90 C. The deposits appear to be low- temperature derivatives of lead-zinc-barite veins, poor in sulfides and richer in quartz. A crucial factor appears to have been passage of the quartz-depositing solutions through a uranium enriched host rock, e.g. the Hilma Lake granite. A source of silica (from sericitization of feldspar) and radioactivity are probable requirements for the formation of amethyst. The formation of the electronic colour center in quartz producing the blue of amethyst has been attributed to the coupled electronic transition:

The associated Fe4* (substitutional) + Fez* (interstitial) defect (Cohen & Hassan, 1974) is evidently unstable at high temperatures, as older generations of amethyst were observed to be bleached by influx of hotter fluids.

REFERENCES

Archibald, D.L 1987: Granitoias ana Pegmatites of the Pine Portage Area, Nonnwesiern Ontario;

Unpublished B.Sc. Thesis, Lakehead Unviersity, Thunder Bay, 89 p.

Coates, M.E. 1972: Geology of the Black Sturgeon River area, District of Thunder Bay; Ontario

Department of Mines and Northern Affiars, Geological Report 98, 41 p.

Cohen, A.J., and Hassan, F. 1974: Ferrous and Ferric Ions in Synthetic a-Quartz and Natural Amethyst; American

Mineralogist, Volume 59, p. 719-728.

Franklin, J.M.1978: Uranium Mineralization in the Nipigon Area, Thunder Bay District, Ontario;

Current Research, Part A, Geological Survey of Canada Paper 78-lA, p. 275-282.

Franklin, J.M., and Mitchell, R.H.1977: Lead-zinc-barite Veins of the Dorion Area, Thunder Bay District, Ontario;

Canadian Journal of Earth Sciences, Volume 14, p. 1963-1979.

Haynes, F.M.1988: Fluid-inclusion Evidence of Basinal Brines in Archean Basement, Thunder Bay

Pb-Zn-Ba District, Ontario, Canada; Canadian Journal of Earth Sciences,Volume 25, p. 1884-1894.

Kissin, S.A., and Archibald, DL.1988: Genesis of Pegmatites in the Quetico Gneiss Belt of Northwestern Ontario -

Pegmatites and Associated Granitoids of the Pine Portage Area; Grant 225, p. 4-13 Geoscience Research Grant Program, Summary of Research 1987-1988,edited by V.G. Milne, Ontario Geological Survey, Miscellaneous Paper 140, 251p.

Kissin, S.A., and Zayachivsky, B.1985: Genesis of Pegmatites in the Quetico Gneiss Belt of Northwestern Ontario -

Rare-Element Pegmatites and Associated Granitoids of the Georgia LakePegmatite Field; Grant 225, p. 186-199 in Geoscience Research Grant Program,Summary of Research 1984-1985, edited by V.G. Milne, Ontario GeologicalSurvey, Miscellaneous Paper 127, 246 p.

Kissin, S.A., Zayachivsky, B., and Branscombe, L.A.1986: Genesis of Pegmatites in the Quetico Gneiss Belt of Northwestern Ontario -

Granitoids, 'Barren' Pegmatites, and Metasediments, with Additional Data onRare-Element Pegmatites, from the Georgia Lake Pegmatite Field; Grant 225, p.65-78 in Geoscience Research Grant Program, Summary of Research 1985-1986, edited by V.G. Milne, Ontario Geological Survey, Miscellaneous Paper130, 235 p.

McArthur, J11988: Fluid Inclusion and Stable Isotopic Studies on Amethyst, Thunder Bay Amethyst

Mine, Thunder Bay District, Ontario; Unpublished B.Sc. Thesis, LakeheadUniversity, Thunder Bay, 108 p.

McArthur, J., and Kissin, S.A.1988: Stable Isotope, Fluid Inclusion, and Mineralogical Studies Relating to the

Genesis of Amethyst, Thunder Bay Amethyst Mine, Ontario, Canada; p. A40 hiGeological Society of America, October 31-November 3, Denver, Abstracts withPrograms, Volume 20, 423 p.

McCrank, G.F.D., Misuira, J.D., and Brown, P.A.1981: Plutonic Rocks in Ontario; Geological Survey of Canada, Paper 80-23, 171 p.

61

Franklin, J.M. 1978: Uranium Mineralization in the Nipigon Area, Thunder Bay District, Ontario;

Current Research, Part A, Geological Survey of Canada Paper 78-1A, p. 275- 282.

Franklin, J.M., and Mitchell, R.H. 1977: Lead-zinc-barite Veins of the Dorion Area, Thunder Bay District, Ontario;

Canadian Journal of Earth Sciences, Volume 14, p. 1963-1979.

Haynes, F.M. 1988: Fluid-inclusion Evidence of Basinal Brines in Archean Basement, Thunder Bay

Pb-Zn-Ba District, Ontario, Canada; Canadian Journal of Earth Sciences, Volume 25, p. 1884-1894.

Kissin, S.A., and Archibald, D.L. 1988: Genesis of Pegmatites in the Quetico Gneiss Belt of Northwestern Ontario -

Pegmatites and Associated Granitoids of the Pine Portage Area; Grant 225, p. 4- 13jn Geoscience Research Grant Program, Summary of Research 1987-1988, edited by V.G. Milne, Ontario Geological Survey, Miscellaneous Paper 140, 251 P-

Kissin. S.A.. and Zavachivskv. 6. 1985: ~enesis of ~k~mat i tes in the Quetico Gneiss Belt of Northwestern Ontario -

Rare-Element Pegmatites and Associated Granitoids of the Georgia Lake Pegmatite Field; Grant 225, p. 186-199 Jn Geoscience Research Grant Program, Summary of Research 1984-1985, edited by V.G. Milne, Ontario Geological - Survey, ~iscellaneous Paper 127,246 p.

-

Kissin, S.A, Zayachivsky, B., and Branscornbe, LA. 1986: Genesis of Pegmatites in the Quetico Gneiss Belt of Northwestern Ontario -

Granitoids, "Barren" Pegmatites, and Metasediments, with Additional Data on Rare-Element Pegmatites, from the Georgia Lake Pegmatite Field; Grant 225, p. 65-78 jn Geoscience Research Grant Program, Summary of Research 1985- 1986, edited by V.G. Milne, Ontario Geological Survey, Miscellaneous Paper 130,235 p.

McAfthur, J. 1988: Fluid Inclusion and Stable Isotopic Studies on Amethyst, Thunder Bay Amethyst

Mine, Thunder Bay District, Ontario; Unpublished B.Sc. Thesis, Lakehead University, Thunder Bay, 108 p.

McArthur, J., and Kissin, S.A. 1988: Stable Isotope, Fluid Inclusion, and Mineralogical Studies Relating to the

Genesis of Amethyst, Thunder Bay Amethyst Mine, Ontario, Canada; p. A40 in Geological Society of America, October 31-November 3, Denver, Abstracts with Programs, Volume 20, 423 p.

McCrank, G.F. D., Misuira, J. D., and Brown, P.A. 1981: Plutonic Rocks in Ontario; Geological Survey of Canada, Paper 80-23, 171 p.

Pye, E.G1965: Georgia Lake Area; Ontario Department of Mines, Geological Report 31, 113 p.

Scott J.1985: MacGregor Township, District of Thunder Bay; p. 67-70 j Summary of Field

Work and Other Activities 1985, Ontario Geological Survey, edited by J. Wood,O.L. White, RB. Barlow, and A.C. Colvine, Ontario Geological Survey,Miscellaneous Paper 126, 351 p.

Yule, GM.1979: Investigations of the Good Morning Lake Radioactive Fault Breccia: Innes Lake

Area, Dorion Township, Northwestern Ontario; Unpublished B.Sc. Thesis,Lakehead University, Thunder Bay, 93 p.

Zayachivsky, B.1985: Granitoids and Rare-Element Pegmatites of the Georgia Lake Area,

Northwestern Ontario; Unpublished M.Sc. Thesis, Lakehead University, ThunderBay, 234 p.

Zayac/iivsky, 8., Kissin, S.A., and Branscombe, L.A.1989: The Georgia Lake Pegmatite Field, Northwestern Ontario, Part II. Granitoid

Rocks and their Relationship to Rare-Element Pegmatites; p. A20 jj GeologicalAssociation of Canada and Mineralogical Association of Canada, May 15-17,Montreal, Joint Annual Meeting, Program with Abstracts, Volume 14, 135 p.

62

Pye, E.G 1965: Georgia Lake Area; Ontario Department of Mines, Geological Report 31, 113 p.

Scoff, J. 1985: MacGregor Township, District of Thunder Bay; p. 67-70jn Summary of Field

Work and Other Activities 1985, Ontario Geological Survey, edited by J. Wood, O.L. White, R.B. Barlow, and A.C. Colvine, Ontario Geological Survey, Miscellaneous Paper 126,351 p.

Yule, G.R. 1979: Investigations of the Good Morning Lake Radioactive Fault Breccia: Innes Lake

Area, Dorion Township, Northwestern Ontario; Unpublished B.Sc. Thesis, Lakehead University, Thunder Bay, 93 p.

Zavachivskv. B. -

1985: ~ranioids and Rare-Element Pegmatites of the Georgia Lake Area, Northwestern Ontario: Unoublished M.Sc. Thesis. Lakehead Universitv. Thunder - . Bay, 234 p.

Zavachivskv. 8.. Kissin. S.A.. and Branscornbe. LA. 1989: ~ h e ~ e & ~ i a Lake pegmatite Field, ~orthwestern Ontario, Part 11. Granitoid

Rocks and their Relationshio to Rare-Element Peamatites: o. A20 in Geoloaical Association of Canada and~ineralogical ~ssociation of Canada, May 15-17, Montreal, Joint Annual Meeting, Program with Abstracts, Volume 14, 135 p.

Figure 4. Geology of the western Lake Superior region showing numbered field trip stops.U(l) and U(2) indicate the locations of the Greenwich Lake and Innes LakeIIrni urn nrrllrronrcc acnar+4 Ial,

Quetico Subprovince ( meto - sediments + paragneisses )

Shebondowon Subprovince ( undivided )

Plutonic rocks

Province + Subprovince boundories

0 stop locations

Figure 4. Geology o f the western Lake Superior region showing numbered f i e l d t r i p stops. U(1 ) and U ( 2 ) ind icate the locat ions o f the Greenwich Lake and Innes Lake

FIELD TRIP LOG

This field trip will attempt to illustrate as many of the features as possible in oneday. Unfortunately, some of the most interesting ones are the most inaccessible. Thetrip route is illustrated in Figure 4, together with a simplified geological map.

Stop 1: Southeast margin of the Mackenzie granite (LakeshoreDrive near Silver Harbour Road).

In this location the intrusive contact of the Mackenzie granite with Archeanmetavolcanic can be seen. The metavolcanics are amphibolitic near the contactreflecting their contact metamorphism to hornblende hornfels facies, accordingto Rogers (1979). South of Lakeshore Drive the granite is in fault contact withthe Proterozoic Gunflint Formation. A small Gunflint outcrop occurs on thenorth side of Lakeshore Drive, west of the granite contact. The granite is jointedand in places the joints have been mineralized. A small quartz-calcite vein isvisible here, but other veins contain pyrite and small amounts of base metalsulfides. Small veins of amethyst also occur in the area.

A short distance to the west and south toward Silver Harbour are a series ofsilver mines, which operated in the late 19th Century. From north to souththese are the 3A, Beck or Silver Harbour and Algoma Mines. The shafts havebeen capped or sealed and most dump material has been removed in recentyears. Little can be seen of these. These deposits were formed inKeweenawan times (Franklin 1986) and are unrelated to granitic rocks ofthe area.

Stop 2: Main phase, Mackenzie granite (Highway 11-17 nearMackenzie River bridge).

The massive and leucocratic character of the granite can be seen here.According to Rogers (1979), the color index is always s 5, making the granite aleucogranite. The only primary mafic mineral is biotite and the only accessoriesnoted are traces of sphene and apatite. No mineral occurrences are associatedwith this pluton apart from some small but commercial amethyst-bearing veinseast of the Mackenzie River.

64

* .

as possible in one inaccessible. The

Southeast margin of the MacKenzi Drive near Silver Harbour Road).

In this location the intrusive contact of the MacKenzie granite with Archean metavolcanic can be seen. The metavokxnics are amphiboliiic near the contact reflecting their contact metamorphism to hornblende hornfels fades, according to Rogers (1979). South of Lakeshore Drive the granite is in fault contact with the Proterozoic Gunflint Formation. A small Gunflint outcrop occurs on the north side of Lakeshore Drive, west of the granite co and in places the joints have been mineralized. A s visible here, but other veins contain pyrite and small a sulfides. Small veins of amethyst also occur in the are

The massive and leucocratic character of the granite can be seen here. According to Rogers (1979), the color index is always 5 5, making the g leucogranite. The only primary mafic mineral is biotite and the only accessories noted are traces of sphene and apatite. No mineral occurrences are associated with this pluton apart from some small but commercial amethyst-bearing veins east of the MacKenzie River.

Stop 3: Anderson Lake Molybdenum occurrence (East Loon Roadfrom Highway 11-17. Continue 1.8 km to north side of LoonLake. Turn right immediately past bridge over small stream.Continue about 200 m to Hydro transmission lines andclearing. Park and continue north on road by foot for about3 km to Anderson Lake.)

The molybdenite occurrence described in the introductory text was diamonddrilled and blasted at surface in the 1960's, giving rise to the chaotic scenehere. Good specimens of coarse-grained molybdenite and pegmatitic granitehost rock may be collected.

Stop 4: Thunder Bay Amethyst Mine (back-track on East LoonRoad to junction with road to Thunder Bay Amethyst Mine.Turn right and continue for approximately 10 km to theMine.)

Note: This is a commercial operation entered bypermission of the owner. There is to be no hammering norcollecting in the pit. Collecting may be done in thedesignated dump area, but material collected here is not tobe broken by hammering.

The amethyst-bearing vein system occupies an east-west striking, near verticalfault zone. The fault is parallel to the trend of occurrence of other amethystdeposits in the area (Figure 3) and may form a major margin to the SibleyGroup depositional basin. The fault is offset by steps of a few meters by aseries of northerly striking en-echelon faults. The easterly and westerly limits ofthe fault have not been determined.

The amethyst reaches its most spectacular development in large vugs wherecrustiform aggregates contain crystals of quartz 30 to 40 cm long and 10 to 15cm in diameter. The crystals display growth zoning, which is consistentthroughout the deposit. The veins contain little apart from quartz, althoughminute sulfide crystals occur in some growth zones. The altered granitecountryrock contains occasional accumulations of pyrite, chalcopyrite, borniteand other minor sulfides. The vein system is surrounded by a well-defined,sericitized, hydrothermal alteration zone.

The vuggy character of the vein indicates that it must have formed near thesurface. However, even more convincing is the presence of breccia fragmentsin the vein consisting of Sibley Group lithologies. The Sibley Group has beenentirely removed from the area by erosion, but its former presence is obvious.On a granite knob north of the vein, a chloritized and hematitic surface possiblymay represent a Precambrian paleoregolith.

65

S t o ~ 3: Anderson Lake Molybdenum occurrence (East Loon Road from Highway 11-17. Continue 1.8 km to north side of Loon Lake. Turn right immediately past bridge over small stream. Continue about 200 m to Hydro transmission lines and clearing. Park and continue north on road by foot for about 3 km to Anderson Lake.)

The molybdenite occurrence described in the introductory text was diamond drilled and blasted at surface in the 1960's, giving rise to the chaotic scene here. Good specimens of coarse-grained molybdenite and pegmatitic granite host rock may be collected.

S t o ~ 4: Thunder Bay Amethyst Mine (back-track on East Loon Road to junction with road to Thunder Bay Amethyst Mine. Turn right and continue for approximately 10 km to the Mine.)

m: This is a commercial operation entered by permission of the owner. There is to be no hammerina nor - kollecting in the pit. Collecting may be done in the designated dump area, but material collected here is not to be broken by hammering.

The amethyst-bearing vein system occupies an east-west striking, near vertical fault zone. The fault is parallel to the trend of occurrence of other amethyst deposits in the area (Figure 3) and may form a major margin to the Sibley Group depositional basin. The fault is offset by steps of a few meters by a series of northerly striking en-echelon faults. The easterly and westerly limits of the fault have not been determined.

The amethyst reaches its most spectacular development in large vugs where crustiform aggregates contain crystals of quartz 30 to 40 cm long and 10 to 15 cm in diameter. The crystals display growth zoning, which is consistent throughout the deposit. The veins contain little apart from quartz, although minute sulfide crystals occur in some growth zones. The altered granite countryrock contains occasional accumulations of pyrite, chalcopyrite, bornite and other minor sulfides. The vein system is surrounded by a well-defined, sericitized, hydrothermal alteration zone.

The vuggy character of the vein indicates that it must have formed near the surface. However, even more convincing is the presence of breccia fragments in the vein consisting of Sibley Group lithologies. The Sibley Group has been entirely removed from the area by erosion, but its former presence is obvious. On a granite knob north of the vein, a chloritized and hematitic surface possibly may represent a Precambrian paleoregolith.

Stop 5: High-grade central portion of the Quetico Subprovince(Highway 11, east side of Lake Helen).

In this area, well developed migmatites are indicative of the highestmetamorphic grades attained in the Quetico Subprovince. Metamorphic gradesdecline rapidly to the north, and much of the metasedimentary terrane in theGeorgia Lake area is lower amphibolite to upper greenschist fades.

Stop 6: McVittie Pegmatite cutting the Postagoni Lake sill (turn offHighway 11 at George Creek and continue fromapproximately 8 km to well-defined clear cut on west side ofroad at Postagoni Lake. Park and proceed west on foot for1/2 km to north side of Dive Lake to McVittie pegmatite.)

The McVittie Pegmatite is a north striking vertical dike approximately 10 m wide,which cuts the east-west striking tonalites of the Postagoni Lake sill. Thepegmatite is typical of the north group pegmatites of the Georgia Lake area inthat it is unzoned and contains no rare-element-bearing minerals other thanspodumene. The spodumene is typical phenocrystic spodumene containingseveral percent iron and forming euhedral, prismatic crystals up to 10 cm long.The spodumene appears greenish owing to its iron content and partialdecomposition to chlorite and sericite.

REFERENCES

Franklin, J.M., Kissin, LA., Smyk, M.C., and Scott, S.D.1986: Silver Deposits Associated with the Proterozoic Rocks of the Thunder Bay

District, Ontario; Canadian Journal of Earth Sciences, Volume 23, p. 1576-1591.

Rogers, J.A.1979: The Southeastern Margin of the Mackenzie Granite, Northwestern Ontario;

Unpublished B.Sc. Thesis, Lakehead University, Thunder Bay, 68 p.

66

In this area, well developed miamatites are indicative of the hiihest metamorphic grades attained the Quetico Subprovince. ~e-morphic grades decline rapidly to the north, and much of the metasedimentary terrane in the Georgia Lakearea is lower amphibolite to upper greenschist fac i .

McViie Pegmatite cutting the Postagoni Lake silt (turn off Highway 11 at George Creek and continue from approximately 8 krn to well-defined clear cut on west side of road at Postagoni Lake. Park and proceed west on foot for 112 km to north side of Dive Lake to McViie pegmatite.)

The McVittie Pegmatite is a north striking vertical dike approximately 10 m wide, which cuts the east-west striking tonalites of the Postagoni Lake sill. The pegmatite is typical of the north group pegmatiies of the Georgia Lake area i that it is unzoned and contains no rare-element-bearing minerals other than spodumene. The spodumene is typical phenocrystic spodumena containing several percent iron and forming euhedral, prismatic crystals up to 10 cm long. The spodumene appears greenish owing to its iron content and partial decomposition to chlorite and sericite.

REFERENCES

Franklin, J.M., Kissin, S.A., Smyk, M.C., and Scott, S.D.

District, Ontario; Canadian Journal of Earth Sciences, Volume 23, p. 1576-1591.

FIELD TRIP 4FIELD TRIP 4

BASE METAL MINERALIZATION

IN THE

SHEBANDOWAN GREENSTONE BELT

Introductory Discussion and Field Guide

36th Annual. Institute on Lake Superior Geology

Thunder Bay, Ontario

by

Maurice J.Lavigne Jr.

Ontario Ministry of Northern Development and Mines

Thunder Bay, Ontario

A. J . AubutInco Exploration and Technical Services Inc.

Thunder Bay, Ontario

John Scott

Ontario Ministry of Northern Development and Mines

Thunder Bay, Ontario

67

BASE METAL MINERALIZATION

IN THE

SHEBANDOWAN GREENSTONE BELT

Introductory Discussion and Field Guide

Maurice J.Lavigne Jr.

Ontario Ministry of Northern Development and Mines

Thunder Bay, Ontario

A.J.Aubut.

Exploration and Technical Services Inc.

John Scott

rio Ministry of Northern Development and Mines

INTRODUCTION

Mineral deposits of the Shebandowan greenstone belt bring

about an extra degree of controversy with respect to theirorigins as the result of the destruction of prerequisite

diagnostic textures and relationships by intensedeformation. This is especially true for deposits which

LD2date deformation, such as synvolcanic mineralization.Although unintended, this field trip highlights the trialnd tribulations of trying to "pigeon hole" highly deformedmineralization. Three styles of base metal mineralizationwill be examined during the cdurse of this trip, and all are

proposed candidates for synvolcanic origins. Allan Aubut

will be presenting arguments which favour that the

Shebandowan nickel-copper mine is komatiite flow hosted,

te., Kambalda type. The other authors will bring forth

arguments that the North Coldstrean Mine is the stringer

zone of a I<uroko type, volcanogenic, polymetallic sulphide

deposit and that the Vanguard prospect is the exhallative

component of such a deposit.

68

Mineral deposits of the Shebandowan greenstone belt bring

about an extra degree of controversy with respect to their

origins as the result of the destruction of prerequisite

diagnostic textures and relationships by intense

deformation. This is especially true for deposits which

::-idate deformation, such as synvolcanic mineralization.

Although unintended, this field trip highlights the trial

and tribulations of trying to "pigeon hole" highly deforced

r!ineralization. Three styles of base metal mineralization

will be examined during the course of this trip, and all are

proposed candidates for synvolcanic origins. Allan Aubut

will be presenting arguments which favour that the

Shebandowan nickel-copper mine is konatiite flow hosted,

ie., Kambalda type. The other authors will bring forth

arguments that the North Coldstream Mine is the stringer

zcne of a Kuroko type, volcanogenic, pclymetallic sulphide

deposit and that the Vanguard prospect is the exhallative

component of such a deposit.

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STOP 1; SHEBANDOWAN MINE

INTRODUCTION

The Shebandowan Mine is a nickel-copper deposit owned by

Inca Limited and is presently being operated by MacIsaac

Explorations. It is located in northwestern Ontario, 75

kilometres west—northwest of Thunder Bay. The mine issituated on the south—western shore of Lower ShebandowanLake (Fig. 1).

It has been in semi-continuous production since 1972 a an

average production rate of about 2000 tons per day.

Presently reserves of broken and developed material stand at

approximately .2 million tons grading 2.10 per cent nickel

and 0.95 per cent copper. Accessory platinum group metals

and gold are also recovered. Concentrate produced is shipped

by truck to Sudbury, Ontario, for smelting and refining.

The Shebandowan Mine has had a iony history with nickel—

copper suiphide mineralization being first discovered in

1913 at what is now Discovery Point, on Lake Shebandowan

(just east of stop 2, Fig 1). The International Nickel

Company of Canada, now Inco Limited, optioned the property

in 1936. The property was purchased for $250,000 in 1937

(Daily Times Journal, 1937). moo explored the property off

and on for the next 28 years. Between 1965 and 1968

exploration was intensified with the collaring of an

exploration shaft in 1966. A production decision was

announced in 1968 and the first shipment of ooncentrate was

r.tade in 1972.

70

The Shebandowan Mine is a nickel-copper deposit owned by

Inco Linited and is presently being operated by MacIsaac

Explorations. It is located in northwestern Ontario, 75

kilorne*'.res west-northwest of Thunder Bay. The nine is

situated on the south-western shore of Lower Shebandowan '~

1). .~ .

It has been in sani-contin~ious production since 1972 at an

average production rate of about 2000 tons per day.

Presently reserves of broken and developed material stand at

approximately 2 nillion tons grading 2.10 per cent nickel and 0.95 per c-snt copper. Accessory platinum group metals

and gold are also recovered. Concentrate produced is shipped

by truck to Sudbury, Ontario, for smelting and refining.

The Shebandcwan line has had a long history with nickel-

spper sulphide '"uneralizaticn being first discovered in

1913 at what is now Discovery Point, on Lake Shebandowan

(just east of stop 2 , Fig 1). The International Sickel

Ccmpany of Canada, now Inco Lirutad , optioned the property in 1936. The property was purchased for $250,000 in 1937

(Daily Tines Journal, 1937). Inco explored the property off

and on for the next 28 years. Between 1965 and 1968

exploration was intensified faith the collaring of an

exploration shaft in 1966. A production decision was

was

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Shebandowan Mine

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24

REGIONAL GEOLOGY

The area in the vicinity of the Shebandowan Mine (Fig. 2) is

underlain by ricewatin tholeiitic volcanics unconfornably

overlain by Tiniskaming type sediments and cab-alkaline

volcanics. This unconformity is indicated by a truncation ofKeewatin axial surface trends, the dissir.ilarity inchemistry (tholeiitic versus calc—alkaline) and the presence

of jasper from Reewatin banded iron formation, as clasts

within Tiniskaminy conglomerates (Borradaile and Brown,1937; Sheyelski, 1930)

MINE GEOLOGY

In the vicinity of the Shebandowan Mine the geology cart bedivided into two domains separated by the Crayfish CreekFault, a regional dextral transcurrent fault. To the south

of the fault the rocks are predominantly tholeiitic basalts,

andesites and some felsic pyroclastios (Figs. 2 and 3).

Intercalated with these are several peridotite bodies

believed to be komatiite flows (Morton, 1982). These

ultranafics are now either serpentinite or talc—carbonate

schist. All are uriconfornably overlain, in close proximity

to the fault, by Timiskaming volcanic breccia.

To the north of the fault a thin wedge of intercalated nafic

volcanics and ultranafics is present between the fault and

'jranitic rocks of the Shebandowan Lake Stock. Within thiswedge are what appear to be two distinct volcanic cyclescharacterized by intercalated nafic flows and ultramafics.

The northern cycle includes the ultramafic unit that hosts

the Shebandowan nickel copper deposit while the southern

cycle includes an ultramafic unit that hosts chronitemineralization (Figs. 3 and 4). The relationship between thenafic vobcanics and the ultramafics is unclear due towidespread deformation associated with the Crayfish CreekFault deformation zone.

-, 'b

?ha area in the vicinity of the Shebandowan Mine (Fig. 2) is

underlain by Keewatin tholeiitic volcanics unconforraably

overlain by Timiskaning type sediments and calc-alkaline

volcanics, This unconformity is indicated by a truncation of

Iiriewatin axial surface trends, the dissimilarity in

chemistry (tholeiitic versus calc-alkaline) and the presence

of jasper from Keewatin banded iron formation, as clasts

'ifchin Tiniskanin~-) conglomerates (Borradaile and Brown

1087; Shegelski, 1930). -.

MIXE GEOLOGY

In the vicinity of the Shebandowan Mine the geology can be

divided into two domains separated by the Crayfish Creek

Fault, a reyional dextral transcurrent fault. To the south

of thz fault the rocks are predominantly tholeiitic basalts,

andesites and some felsic pyroclastics (Figs. 2 and 3 ) .

Intercalated with these are several peridotite bodies :. .,.. - '

believed to be komatiite flows (lorton, 1982). These , -

ultranafics are now either serpentinite or talc-carbonate

schist. All are unconforiaably overlain, 1n close proximity

to the fault, by Timiskaning volcanic breccia.

To the north of the fault a thin wedge of intercalated mafic

volcani.cs and ultramafics is present between the fault and

granitic rocks of the Shebandowan Lake Stock. Within this

wedge are what appear to be twc distinct volcanic cycles

characterized by intercalated mafic flows and ultramafics.

The northern cycle includes the ultramafic unit that hosts

the Shebandowan nickel copper deposit while the southern

cycle includes an ultranafic unit that hosts chronite

mineralization (Figs. 3 and 4 ) . The relationship betwe

mafic- volconics and the ultranafics is unclear due to

widespread deformation associated with the Crayf

nation zone.

—4

l!1 TimiskamingVolcanics

Ultramafics

Figure 3: 1000' Level Plan — Shebandowan Mine

xx xxx xxxx xx xxxx xx xxx-- x x x x x x xxxx xx

1000

F'elsic Intrusives

Level Plan

Metres

Mafic Votcanics

Metres

8;::- . :;.'n#8 Timiskaming 1 - : s l , Volcanics

.... ". .... ". .,.".. ."."". Mafic Volcanics .....,

Figure 3: 1000' Level Plan - Shebandowan Mine

N;IN

________

4N N'C

_______

i N >1N x

_____

:::::::t:;:sN N N

N N1NNNNNNYZZt N NN:;;;:::1NN)CNN

_____

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_____

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_____

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IN NN NJNL N N

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'C

/NJ fy N NS"''*'# N NC'':JxL:;;;:::1 £;:;;I NM""' NC h J..t N I N::1N :::IN a

j Nf :::::.J 'Ci N

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25000 E Section

25000E SectionShebandowan Mine

74

INN' FelsicI x NIIN N IntrusivesIN'.

LC1 TimiskamingI'Tr11 Volcanics

Ultramafics

:ij Mafic9 lcanicsI Ni-CuI Suiphides

___

Chromite

____

Mineralization

0r 25 50

Metres

hebandowan Mine

Figure 4:

STRECTUPAL GEOLOGY

The main structural feature of the Shebandowan Mine is the

Crayfish Creek Fault. This major dextral transcurrent fault

dips steeply to the south. Though a distinct fault breccia

zone up to several metres thick is present a wide zone ofdeformation characterized by extensive shearing is present.

To the south of th fault much of the deformation is hosted

by the Timiskaning volcanic breccia resulting in a highly

foliated quartz sericite schist possessing remnant brecciafragments and relict hornblende-phyric texture.

North of the fault shearing is not as pervasive, likely due

to the more incompetent nature of the ultramafic—mafic

contacts and the ultramafics themselves. This resulted in

deformation being focused into numerous narrow shear zones,

commonly concentrated along contacts with dextral

transposition of slices. This tectonic reworking has had a

major effect on the nickel—copper mineralization Producing

significant sulphide movement. The sulphide zones show

numerous signs of this remobilization such as: disseminated

zones occurring above, below and in blocks within massivesuTphide zones; as sharp contacts between massive or semi—massive ore and disseminated sulphide bearing ultranafic;

and the presence of pentiandite banding within the massivesulphides.

Another factor that has greatly added to the complexity of

the deposit is the presence of numerous apophyses of theShebandowan Lake Stock. These frequently follow along

contacts, shears and sulphide zones as well as crcss—cuttiny

these features. This has made interpretation of stratigraphy

in the mine environment very difficult as well as producing

dilution problems in the ore.

75

STRUCTURAL GEOLOG

The main structural feature of the Shebandowan Mine is the

Crayfish Creek Fault. This major dextral transcurrent fault

dips steeply to the south. Though a distinct fault breccia

zone up to several metres thick is present a wide zone of

deformation characterized by extensive shearing is present.

To the south of the fault much of the deformation is hosted

by the Timiskaning volcanic breccia resulting in a highly

f a t e d quartz sericite schist possessing remnant breccia

fragments and relict hornblende-phyric texture. ,. , ~ ~. ~ , .

" . . . ,*.::<,: . .. , .

North of the fault shearing is' not as pervasive, likely due

to the more incompetent nature of the ultramafic-mafic

contacts and the ultranafics themselves. This resulted in

deformation being focused into numerous narrow shear zones,

commonly concentrated along contacts with dextral

transposition of slices. This tectonic reworking has had a . .

major effect on the nickel-copper mineralization producing

significant sulphide movement. The sulphide zones show

nunerouy signs of this remobilization such as: disseminated

zones occurring above, below and in blocks within riassivn

sulphide zones; as sharp contacts between massive cr semi-

riassive ore and disseminated sulphide bearing ultranafic;

and the presence of pentlandite banding within the massive

sulphides. ~ ~- .~, .~ .!: ."

Another factor that has greatly added to the complexity of

the deposit is the presence of numerous apophyses of the

Shebandowan Lake Stock. These frequently follow along

contacts, shears and sulphide zones as well as cross-cutting

these features. This has made interpretation of stratigraphy

in the mine environment v e r y difficult as well as producing

dilution problems in the ore.

MINERALI ZATIOX

A) Sickel-Copper

The Shebandowan Mine hosts three main types of ore: stringersuiphides, breccia sulphides and massive suiphides. Thoughdisseminated suiphides are present they are not of economicrnortance

The st'ilnger suiphides are confined to nineralized shearzcnes. Suiphides present include chalcopyrite, pyrite and

ninor pyrrhotite and pentlandite.

Breccia suiphides consist of fragments of ultramafic,

volcanic and granite set in a matrix of pyrrhotite,

chalcopyrite and pentlandite. Frequently chalcopyrite coats

many of the inclusions within the breccia suiphide.

Chalcopyrite is also commonly concentrated along the margins

of the breccia zones.

Massive sulphide is composed of pyrrhotite, chalcopyritze and

entIandite. The latter is usually present as discontinuous

layers that are generally parallel to the ore zone contacts.

These layers also wrap around large inclusions.

The average copper to copper-nickel ratio is 0.36 with it

being higher, or more copper rich, in the western portions

of the nine and lower, or more nickel rich, in the eastern

portions of the mine. This variation in copper-nickel ratio

corresponds to the higher proportion of the more

chalcopyrite rich breccia sulphide in the western part of

the ore body.

76

'Â¥1iSERALIZATIO

A) Xickel-Copper

The Shebandowan Mine hosts three main types of ore: stringer

sulphides, breccia sulphides and nassive sulphides. Though

disseminated sulphides are present they are not of economic

q~ortance.

Th? ¥itcinge sulphides are confined to caineralized shear

zones. Sulphides present include chalcopyrite, pyrite and

ninor pyrrhotite and pentlandi'te.

Breccia sulphides consist of fragments of ultramafic,

volcanic and g,ranite set in a matrix of pyrrhotite,

chalcopyrite and pentlandite. Frequently chalcopyrite coats

many of the inclusions within the breccia sulphide.

Chalcopyrite is also commonly concentrated along the margins

,sf tha breccia zones.

Massive sulphide is composed of pyrrhotite, chalcopyrite and

pentlandite. The latter is usually present as discontinuous

layers that are generally parallel to the ore zone contacts.

These layers also wrap around large inclusions.:.. . ' .:. s..,.

The average copper to copper-nickel ratio is 0.36 with it

being higher, or more copper rich, in the western portions

of the mine and lower, or more nickel rich, in the eastern

portions of the mine. This variation in copper-nickel ratio

corresponds to the higher proportion of the more

chalcopyrite rich breccia sulphide in the western part of

the ore bodv.

B) Chromite

Uneconomic chromite mineralization is also present in the

Shebandowan Mihe environment. This chromite is hosted by

ultramafics within the second volcanic cycle, close to the

Crayfish Creek Fault. Chromite is present as finely

disseminated grains in all the ultramafics in the area

(Morton, 1982).

The second nafic volcanic—ultramafic cycle hosts chronitite

(chronite rock) as massive to strongly contorted bands

varying from a few millimetres to several metres in

thickness. On either side of this main zone chromite is

found as brecciated chromitite and chromitiferous

peridotite. Chrornitite mineralization has been traced

discontinuously along strike for a distance of about 3

kilonetres.

Chromium trioxide assays have been reported as high as 44

per cent but are typically much lower. Minor platinum group

metals are associated with the chromite. The chrome—iron

ratio varies from 1.2 To 1.8 although it rarely exceeds 1.6.

GENESIS OF THE NICKEL-COPPER ORE

The ore body has been subjected to intense structural

deformation which has resulted in the remobilization of

nickel—copper sulphides. Primary textures have been obscured

if not erased. Regardless, away from the Crayfish Creek

Fault Deformation Zone there is evidence that many if not

all of these ultramafics are komatiites.

. konatiite is defined simply as being an ultranafic

volcanic rock with 18 or greater MgO (Arndt and Brooks,

1980). These high magnesian flows have a number of physical

77

B) Chromite

Uneconomic chromite mineralization is also present in the

Shebandowan Mine environment. This chromite is hosted by

ultramafics within the second volcanic cycle, close to the

Crayfish Creek Fault. Chromite is present as finely

disseninated grains in all the ultraraafics in the area

(Morton, 1922).

The second nafic volcanic-ultramafic cycle hosts chromitite ' .

(chronite rock) as massive to strongly contorted bands . .

varying from a few millilietres to several metres in

thickness. On either side of this nain zone chromite is

found as brecciated chromitite and chromitiferous

peridotite. chromitite mineralization has been traced . .,,

discontinuously along strike for a distance of about 3

kilometres.

Chromium trioxj.de assays have been reported as high as 44

per cent but are typically much lower. Minor platinum group

cietals are associated with the chrrmite. The chrome-iron

ratio varies from 1.2 To 1.8 although it rarely exceeds 1.6.

GENESIS OF THE SICKEL-C

The ore body has been subjected to intense structural

defor~ation which has resulted in the remobilization of

nickel-copper sulphides. Primary textures have been obscured

if not erased. Regardless, away from the Crayfish Creek

Fault Deformation Zone there is evidence that many if not

all of these ultramafics are komatiites.

a komatiite is defined sinply as being an ultranafic volcanic rock with 18"; or greater >I90 (Arndt and Brooks,

1980). These high raagnesian flows have a number of physical

characteristics which can he used to identify then. A

conclusive feature of komatiite flows is the presence of a

relatively thin layer with spinifex texture. According to

Arndt et ai. (1979) most ultramafic flows are not spinifex

textured but massive with polyhedral jointing. These

Polyhedra are coarse in the centre of the flow and become

finer as the contacts are approached. It is this polyhedral

jointing that is commcn in many of the ultramafic bodies at

S1banJowan.

Another common feature at Shebandowan is the presence of

chemical sediments, as both handed cherty iron formation and

chert, immediately overlying the ultramafics (Morton, 1982).

This feature implies that when the sediments were deposited

the ultramafics must have been present as flows on the ocean

floor.

The Shebandowan Mine ore body shows a lot of physical

similarities with the komatiite hosted Redross deposit, in

the Eanbalda region of Australia. The Bedross deposit is

located 58 kilometres south of Falgoorlie. It has been

affected by regional dynamic metamorphism with faulting and

shearing, primarily along the footwall contact between the

sulphide bearing komatiite flow and nafic volcanics (Barrett

et al, 1977).

The sulphides have undergone major remobilization along the

footwall contact and shear zones. A major ore type consists

of brecciated wall rocks cemented by sulphides. Contacts

between massive or semi—massive ore and disseminated ore are

commonly very sharp with none of the intervening matrix ore

associated with other Ranbalda deposits (ibid.).

The ore zones at Redross possess structures and textures

produced by renobili:ation of both disseminated and more

massive mineralization (ibid.).

78

characteristics which can be used to identify then. A

conclusive feature of komatiite flows is the presence of a

relatively thin layer with spinifex texture. According to

Arndt et a:. (1979) most ultramafic flows are not spinifex

textured but massive with polyhedral jointing. These

polyhedra are coarse in the centre of the flow and become

finer as the contacts are approached. It is this polyhedral

jointing that is common in many of the ultranafic bodies at

Sh.=banJowii.

Another common feature at Shebandowan is the presence of

c5umical sediments, as both banded cherty iron formation and

c'l':~rt, innediately overlying the ultramafics (Morton, 1982).

This feature inplies that when the sediments were deposited

the ultranafic,~ must have been present as flows on the ocean

floor.

The Shebandowan Mine ore body shows a lot of physical

similarities with the konatiite hosted Redross deposit, in

the Kambaida region of Australia. The Redross deposit is

located 33 kilometres qouth of Kalgoorlie. It has been

affected by regional dynamic metanorphism with faulting and

shearing, primarily along the fcctwali contact between the

sulphide bearing komatiite flow and mafic volcanics (Barrett

et al, 1977).

The sulphides have undergone major remobilization along the

footwall contact and shear zones. A major ore type consists

of brecciated wall rocks cenented by sulphides. Contacts

between nassive or semi-massive ore and disseminated ore are

conxcnly very sharp with none of the intervening matrix ore

associated with other Kambalda deposits (ibid.).

The ore zones at Redross possess structures and textures

produced by remobilization of both disserainatad and more

nassive mineralization (ibid.).

.\L the Shehandowan Mine the same chaeacteristics existincluding an affinity of sulphide mineralization for one

side of the ultramafic body. It is therefore suggested that

the Shebandowan nickel-copper deposit is the product of

dynamic metamorphism of a sulphide bearing komatiite flaw

with remobilization and subsequent redeposition of the

sulphides in low strain areas.

FIELD TRIP STOPS

STOP 1; Shebandowan Mine Headframe, Fig. 5

At this stop we will examine strongly deformed Timiskaming

cab—alkaline volcani& breccia as well as the ultranafic

unit hosting the Shebandowan nickel—copper deposit.

The Main structural feature at the Shebandowan Mine is the

Crayfish Creek Fault. South of the fault Tiniskanirg

volcanics have been strongly deformed and are now quartz—

sericite schist with possessing relic textures. North of the

fault deornation has not been as focused into narrow shear

zones separating lenses of serpentinized peridotite. This

deformation has resulted in the remobilization of the Ni-Cu

sulphides and subsequent deposition in lower strain areas

within the host ultranafic.

STOP 1A.

This outcrop consists of strongly deformed hornblende phyric

Tiniskaming type volcanic breccia. Note the highly foliated

nature reflecting its proximity to the Crayfish Creek Fault,approximately 100 metres to the north. Though strongly

foliated note the many similarities with the undeformed

Tiniskaming volcanic breccias seen to the east e long the

road leading to the Shehandowan Mine.

79

At the Shebandowan Mine the sane characteristics exist

including an affinity of sulphide mineralization for one

side of the ultraraafic body. It is therefore suggested that

h e Shebandowan nickel-copper deposit is the product of

dynamic netamorphisn of a sulphide bearing konatiite flow

with remobilization and subsequent redeposition of the

sulphides in low strain areas.

FIELD TRIP STOPS

STOP 1; Shebandowan Mine Headframe, Fig. 5

At this stop we will examine strongly deformed Timiskaming

calc-alkaline volcanic breccia as well as the ultranafic

unit hosting the Shebandowan nickel-copper deposit.

The Main structural feature at the Shebandowan Mine is the

Crayfish Creak Fault. South of the fault Tiniskaning

volcanics have been strongly deforned and are now quartz-

5ericite schist with possessing relic textures. North of the

fault deformation has not been as focused into narrow shear

zones separating lenses of serpentinized peridotite. This

deformation has resulted in the remobilization of the Xi-Cu

sulphides and subsequent deposition in lower strain areas

within the host ultramafic.

STOP 1A.

This outcrop consists of strongly deforned hornblende phyric

Tiniskaming type volcanic breccia. Note the highly foliated

nature reflecting its proximity to the Crayfish Creek Fault,

approximately 100 metres to the north. Though stronaly

foliated note the many similarities with the undeformed

Tiniskaming volcanic breccias seen to the east along the

road leading to the Shebandowan Mine.

Co C

Iiitr

arna

fics

Lake

She

ban

do w

an

Fel

sic

Vol

cani

cs

Met

res

Figure 5:

Stop 1 -

She

band

owan

Mine

Pro

duct

ion

Sha

ft

Ia)

Apo

phys

es o

f She

band

owon

jCLa

ke S

tock

intr

udin

g U

ltra-

mat

ics

host

ing

She

bond

owiru

Ore

Ta

Fel

sic

Intr

usiv

es

Tim

iska

min

g V

olca

nics

Mat

Ic V

olca

nics

Sto

p

Shebandowon Mine

0tO

O20

0

STOP 13.

This outcrop, beside the #2 Shaft headfrarne, consists of

serpentinized peridotite with numerous narrow zones of talc—

carbonate schist that form an anastomosing network. It is

this unit that is host to the nickel-copper sulphides. Note

on the north side of the outcrop a feldspar porphyry dyke,

anapcphysis of the Shebandowan Lake Stock, cubuing acrossthe 2eridotite.

STOP 2, Fig. 6

Approximate1y 3 km w-nw of Stop 1 is a narrow unit ofserpentinized peridotite hosting Ni-Cu suiphides hosted by

banded mafic volcanics. These rock units are exposed in a

small trench on the side of a hill, approximately 75 metres

north of the Crayfish Creek Fault. Points of interest at

this outcrop are: the banding within the nafEc volcanics,

the thin wedge of ultramafic and the breccia sulphides. The

latter consist of numerous nafic to ultranafic volcanic

fragments cemented by pyrrhotite, chaicopyrite and

pentlandite. This suiphide zone averages 1.88% Ni and 1.61%

Cu. Note the porphyry dyke, an apophysis of the Shebandowan

Lake Stock, intruding the sulphides.

STOP 3, Fig. 7

4.5 Km tv—sw of the Shebandowan mine headfrane is a typical

exposure of ultranafic rock, common in the area,

interdigitated with felsic volcanic fragmentals, nafic

volcanics and and intruded by a felsic dyke. With the

exception of portions of the ultrarnafic rock and the dyke,

the entire outcrop is strongly foliated to schistose. Well

developed mineral aggregate lineations and clast elongations

are vertical. Stratigraphic tops are to the south, as

STOP I B .

This outcrop, beside the ? 2 Shaft headframe, consists of

serpentinized peridctite with numerous narrow zones of talc-

carbonate schist that torn an anastonosing network. It is

this unit that is host to the nickel-copper sulphides. Note

or. the north side of the outcrop a feldspar porphyry dyke,

an apophysis of the Shebandowan Lake Stock, cutting across

the peridotite.

STOP 2, Fig. 6

Approximately 3 km w-nw ,,f Stop 1 is a narrow unit of

serpentinized peridotite hosting Xi-Cu sulphides hosted by

banded mafic volcanics. These rock units are exposed in a

small trench on the side of a hill, approximately 75 met.res

north of the Crayfish Creek Fault. Points of interest at

this outcrop are: the banding within t-he mafic volcanics,

the thin wedge of ultranafic and the breccia sulphides. The

latter consist cf numerous nafic to ultranafic volcanic

fragments cemented by pyrrhotite, chalcopyrite and

pentlandit?. This sulphide zone averages 1.88% Xi and 1.61%

Cu. Mote the porphyry dyke, an apophysis of the Shebandowan

Lake Stock, intruding the sulphides.

STOP 3, Fig. 7

4.5 Km w-sw of the Shebandowan mine headfrane is a typical

exposure of ultranafic rock, common in the area,

interdigitated with felsic volcanic fragnentals, nafic

volcanics and and intruded by a felsic dyke. Kith the

exception of portions of the ultramafic rock and the dyke,

the entire outcrop is strongly foliated to schistose. Well

developed mineral aggregate lineations and clast elongations

are vertical. Stratigraphic tops are to the south, as

o2.

5S

heba

ndow

an M

ine

Figure 6:

Stop 2

—T

renc

h15200E —

She

band

owan

Mine

1.88

% N

i1.

61 %

Cu

F x

xlx

xxx

N-)

c

'-C,

Fel

sic

Intr

usiv

es

Bre

ccia

Sui

phid

es

Ultr

amaf

ics

Maf

Ic V

olca

nics

Tre

nch

IflO

OE

Sto

p 2

5

Met

res

UJR

AM

AF

Cto

pot

ysut

wed

xtiz

e

'7J1

QU

AR

TZ

-FE

LDS

PA

Ritz

aN P

OR

PH

YR

Y

met

ers

(app

roxi

mat

e)

[.:.Y

C:1

Oc1

AS

TC

-.-

1Lap

tuft,

agr

nera

te

PY

RO

CLA

ST

IC

____

____

Str

ongl

y sh

eare

d

Figure 7:

Stop

3-

Otto

Lake Rd.

interpreted from the occurrence of black chert fragments inthe lower few metres of the felsic fragmental at site A onFig. 7. These clast are interpreted to have been derivedfrom an underlying chert bed, exposed at site on Fig. 7.

The margins of the ultrarnafics are schistose, and more

often than not, covered. The relationship with the

enclosing felsic volcanic rocks is therefore unclear. The

ondeformed interior of the ultranafic is fine grain, and has

polyhedral jointing. It can be argued that these two

textural observations support an extrusive origin for this

unit, although they could be considered equivocal, the

common juxtapcsit4on with chemical sediments also lends some

support to an extrusive origin. Definitive flow features

such as spinnifex and flow top breccia have not been

observed in th ultranafic rocks in this area. The positive

identification of ultramafic flow rocks in the area would

support the contention that the Shehandowan Mine is a

Rambalda type nickel deposit since the host rock at the nine

has had its primary textures destoyed by shearing.

STOP 4; .'orth Ccldstream Mine, Burchell Lake

INTRODUCTION

Copper mineralization was discovered on the north side of

Burchell Lake in 1872. Copper was produced in 1903, 1906,

1916—17, 1957—58 and 1960—67 for a total of 102 000 000

pounds. Most of this production occurred fran 1960 to 1967

when the mine was operated by Noranda at a capacity of 1000

tons per day and was producing 14 000 000 pounds of copper

annually. The mill produced concentrate containing 26¾

copper, 2.65 o.p.t. silver and 0.09 o.p.t. gold. The ore

contained 2¾ copper, 0.012 o.p.t gold and 0.22 o.p,t.

silver. The mined closed as the result of depleted

reserves.

84

i a t e r ~ r e t d fron t h e o c c u r r e n c 3 o f black c h e r t f r a g n e n t s i n

t!>e lower few metres o f t h e E e l s i c f r a ~ n e n t a l a t s i t e ?A on

::ndeformed i n t e r i o r o f t h e u l t r a m f i c is f i n e g r a i n , am3 has

p:-;iybedral j o i n t i n 5 . . I t c a n b e a r g u e d that t h e s e two

t e x t u r a l o i i s e r v a t i o n s s u p p o r t a n e x t r u s i v e o r i g i n f o r t h i s

u a i t , a l t h o u g h t h e y c o u l d b e c o n s i d e r e d e q u i v o c a l . The

cmmon j u x t a p c s i t ; a n w i t h c h e m i c a l s e d i n e n t s also l e n d s some

s u p p o r t t o a n e x t r u s i v e o r i g i n . D e f i n ~ t i v e f l o w f e a t u r e 8

5uch as s p i n n i f e h and f l o w t o p breccia h a v e n o t b e e n

o b s e r v e d i n th,e u l t r a m a f i c r ~ c k s i n t h i 3 a r e a . The p o s i t i v e

i d e n t i f i c a t i o n o f u l t r a m a f i c flow r o c k s in t h e area would

% u p p o r t t h e c o n t e n t ~ o n t h a t t h e Shebandcwan Xine i s a

I < x ~ b a l d a t y p e n i c k e l d e p o s i t s i n c e t h e h o s t ruck at t h e n i n e

2s had i t s p r

2liTRODECTIOX

Copper m i n e r a l i z a t i o n was d i s c o v e r e d on t h e n o r t h s i d e o f

B u r c h e l l Lake i n 1872 . Copper was p roduced i n 1 9 0 3 , 1906,

1916-17, 1957-58 and 1960-67 f a r a t o t a l o f 102 000 000

pcunds . Most o f t h i s p r o d u c t i c m a c c u r r e d f rom 1960 t o 1967

%"he> th:e n i n e w a s o p e r a t e d by Xoranda a t a c a p a c i t y o f I000

t a n s p e r day and w a s p r o d u c i n g 14 000 000 'aunds o f c a p p e r

a n n u a l l y . The m i l 1 p roduced c m c e n t r a t e c o n t a i n i n g 26% c o p p e r , 2 -65 c - p - t . silver and 0 .09 c.$.t.. g o l d * The ope

c a n t a i n e d ?%, c o p p e r , 0.012 0 . p . t g o l d 0.22 ~ ~ . p . t , s i l v e r . The n

REGIONAL GEOLOGY

The North Coldstream Mine is located in the western portion

of the Shebandowan greenstone belt. Stratigraphic and

structural trends are northeasterly. Outcrops have well

developed foliation and are often schistose. According to

Stott and Schwerdtner, 1981, deformation in this area wasdominated by sinistral shear. The nine is on the

souanwestern linh of an anticline with an axial plane

striking northeast (Fig. 2) The anticline is cored with

felsic volcanics which are overlain by nafic volcanics. At a

regional scale, the mine is at this contact, with feisic

volcanics as stratigraphic footwall and mafic volcanics as

hangingwall. Along its entire length, this mafic—felsic

volcanic contact is a 1 km wide schist zone. At the

ninesite, this contact was intruded by a gabbroic stock

which is the immediate footwali to the nine. Foliation

orientation at the minesite shows a departure from the

regional orientation, in that it consist of an enclave of

east trending foliation, with dextral kinematic indicators.

Wether or not this foliation orientation is independent of

feisic plutons, both to the north and south of the nine is

as Of yet undetermined.

MINE GEOLOGY

Copper mineralization at the North Coldstream >line consist

of lenses with a high density network of chalcopyrite and

pyrite veinlets (stringers). Some massive and disseminated

mineralization also exist. As seen on the 500 foot level

plan (Fig. 9), cross-section geometry of these lenses is

variable, with maximum horizontal dimensions of 200 by 200

feet. The greatest dimension of these ore lenses (plunge)

is oriented at 50 degrees to the east. Giblin, (1964)

reported three types of coarse grain veins which crosscut

the orebodies. The most common type of vein consist of

85

?he Xorth Coldstream Nine is located in the western portion

:>f t . 1 ~ Shebandowan greenstone belt. Stratigraphic and

structural trends are northeasterly. Outcrops have well

developed fgliation and are often sc!~istose. According to

Stott and Scl-.werdtner. 1981, deforxation in this area as

dcminated by sinistral shear. The nine is on the

s~>uci~western i i ~ b of an anticline witn an axial plane

striI<ing northeast (Fig. 8 ) . ?ha? 2nticl ine is c<>red with

felsic volcanics which are overlain by nafic volcanics. .At a

regional sca.le, the mine is at- this contact, wit-h felsic

volcanics as stratigraphic footwall and mafic volcanics as

hangingwall. Along its entire length, this mafic-felsic

olcanic contact is a 1 km wide schist zone. At the

inesite, this contact was intruded by a gabbroic stock

hich is the immediate £ootwal to the nine. Foliation

rientation at the minesite shows a departure from the

regional orientation, in that it consist of an enclave of

ast trending foliation, with dextral kinenatic indicators.

ether :>r not this faliation orientati~n is independent of

elsic plutons, both to the north and south of the mine is

C~pper mineralization at the Sorth Coldstream >line consist

>f lenses with a high density network of chalcopyrite and

yrite veinlets (stringers). Sone massive and disseminated

ineralization also exist. As seen on the 5 0 0 foot level

plan (Fig. 9), cross-section geometry of these lenses is

ariable, with maximum horizontal dimensions of 200 by 2 0 0

eet. The greatest dimension of these ore lenses ($lunge)

is oriented at 50 degrees to the east. Giblin, (1964)

reported three types I>Â caarse graic veins which crosscut

the crebodies. The nost conmon type cf vein r2onsist of

Figure 8: Geology Western ShebandowanVolcanic Belt

86

quartz, carbonate, pyrite and chalcopyrite. Carbonate

veinlets constitute the second commonest type. Relatively

rare are veins consisting of quartz, carbonate, barite,

purple fluorite, dark brown sphalerite and chalcopyrite.

The ore is hosted by a massive siliceous lenses, 1000 by 400feet, consisting almost entirely of aphanitic quartz and

historically has been referred to as chert, with or without

;anetc ccnnctatcns. Locally, fabric is discernable

nega.scopically, and generally in these cases the siliceous

rock is white. The most common colour, and always in close

association with the ore, is brownish mauve. A subtle

variant to this last colour is buff, and appears to be

restricted to the south central portion of the mineralized

complex, in close proximity to a non—siliceous buff

alteration of mafic rock.

The dominant lithology surrounding the siliceous rock is a

chlorite schist, then sericitic schist. The northern band

of chlocitic schist separates the ore complex from a

yabbroic stock. At the eastern end of the complex a lens of

sericitic schist separates this chlorite schist from the ore

ccmplex. At the western end, the chlorite schist contains

abundant bluish quartz segrecjations and magnetite. All

contacts between the ore complex and the sericitic schistare sharp, and the smaller siliceous pods are boudin like in

that the sericitic foliation wraps around them. The

southern contact has been described by Scott, (1963) as

sharp, although as described above, there appears to be a

transition locally from buff siliceous to buff alteration,

in less strained rock. The sharp contacts are most likely

the products of shear juxtaposition and the destruction of

gradational contacts. The gradational contact between the

gabhro stock and the chlorite schist indicate a gabhroicprotolith at this location for the schist. Giblin (1964),

noticed an increasing amount of bluish quartz in this

87

rare are v e i n s c o n s i s t i n g o f q u a r t z , c a r b o n a t e , b a r i t e ,

h e o r e is h o s t e d by a m a s s i v e s i l i c e o u s l e n s e s , 100Q by 400

ee t , c c n s i s t i n g a lmosk e n t i r e l y s f a p h a n i t i c q u a r t z and

i s t o r i c a l l y h a s been r e f e r r e d t o as c h e r t , w i t h o r i g i t h o u t

.' -. , - . ,*---tations. L o c a l l y , f a b r i c i s F i i s c e r z 6 5 l 2 .&.. . ~ . , 8 . . . "

n q ~ s c o p i ~ s l l y , and g e n e r a l l y i n t h e s e c a s e s t h e s i l i c e o u s

r o c k i s w h i t e . The most common c o l o u r , and alwa>rs i n c l o s e

a s s o c i a t i o ~ w i t h t h e o r e , is b rownish mauve. A s u b t l e

~ y a r i a n t t o t h i s l a s t c o l o u r i s b u f f , and a p p e a r s t o b e

r e s t r i c t e d t o t h e s o u t h c e n t r a l p o r t i o n o f t h e m i n e r a l i z e d

complex , i n c l o s e p r o x i m i t y

a l t e r a t i o n o f m a f i c r o c k .

The dominant l i t h o l o g y s u r r o u n d i n g t h e s i l i c e o u s r o c k i s a

c h l o r i t e s c h i s t , t h e n s e r i c i t i c s c h i s t . The n o r t h e r n band

r;E c h l o r i t i c s c h i s t s e p a r a t e s t h e o r e complex f rom a

a b b r o i c s t o c k . , A t t h e e a s t e r n end g f t h e c o n p l e s a l e n s o f

s e r i c i t i c s c h i s t s e p a r a t e s t h i s c h l c r i t e s c h i s t f rom t h * o r e

c c m ~ : l e s . . A t t h e w e s t e r n e n d , t h e c h l o r i t e s c h i s t c o n t a i n s

a b u n d a n t b l u i s h q u a r t z s e g r e g a t i o n s and m a g n e t i t e . A l l

c o n t a c t s be tween t h e o r e complex and t h e s e r i c i t i c s c h i s t

a r e s h a r p , and t h e smaller s i l i c e o u s pods a r e boud in l i k e i n

t h a t t h e s e r i c i t i c f o l i a t i o n wraps a round them. The

s o u t h e r n c o n t a c t h a s been d e s c r i b e d by S c o t t , ( 1 9 5 3 ) a s

s h a r p , a l t h o u g h as d e s c r i b e d a b o v e , t h e r e a p p e a r s t o b e a

t r a n s i t i o n l o c a l l y f rom b u f f s i l i c e o u s t o b u f f a l t e r a t i o n ,

i n less s t r a i n e d r o c k . The s h a r p c o n t a c t s a r e n o s t l i k e l y

t h e p r o d u c t s o f s h e a r j u x t a p o s i t i o n and t h e d e s t r u c t i o n o f

g r a d a t i o n d l c o n t a c t s . The g r a d a t i o n a l c o n t a c t be t t seen t h e

s a b b r o s t o c k and t h e c h l o r i t e s c h i s t i n d i c a t e a g a b b r o i c

p r o t o l i t h a t t h i s l o c z t i g n f c r t h e s c h i s t . G i b l i n ( 1 9 6 4 ) ,

n c t i c e d a n i n c r e a s i n g amount o f b l u i s h q u a r t z i n t h i s

Figure 9:

Geology 500' Level

North Coldstream Mine

chlorite schist tcwards the siliceous rook. This may beinterpreted as increasing silicification of the cjabhro.

protolith for the chlorite schist at the southern marginthe ore complex nay be mafic volcanic, although a gabbro

protolitn. is also possible.

The rotolith for the sericitic schist nay be felsic

volcanic as indicated by poorly preserved, questionable,

frajrientai texture. As intense alteration is probable, theprotolith to the sericite schist could also be naficvolcanic or intrusive.

GENESIS OF THE NORTH COLOSTREAM MINE

Several fundanntal questions must be answered in order to

contemplate the origin of the North Col'Jstrearn Mine. What

is the siliceous host? —chert or silicification What is

the host rock? —mafic or felsic volcanic —or gabbro The

nasive nature of the siliceous zone, the lack ofsedimentary features, and other sedimentary lithologies, itsabsence on strike, and the gradational contacts all supportsilicification. Silicification of this intensity is only

known to occur in hydrothermal jo ld deposits (of all ages)

and in the Keiko zone of some Huroko type volcanoyenicmassive sulphide deposits. The former can be negated as a

gold content at the North Coldstream of 450 ppb is much too

low, and is typical of the latter. Siliôification in gold

systems tends to be "late", ie post tectonic, is of the

highest ore grades and is superimposed on fabrics.

Silicification at this site predates deformation as

evidenced by siliceous boudins and inherited fabric. A

volcanogenic origin is also supported by the "stringer"

style of the ore, the piped shaped host silicification and

to some extent, the sequence of alteration. It therefore

appears that the North Coldstream Mine is the stringer zone

of avoloanocjenic massLve sulphide deposit.

89

chlorite schist towards the siliceous rock. This nay be

interpreted as increasing silicification of the gabbro. The

protolith for the chlorite schist at the southern margin of

the ore complex may be mafic volcanic, although a gabbroic

protolit3 is also possible.

The protolith for the sencitic schist nay be felsic

volcanic as indicated by poorly preserved, questionable,

frdyciental texture. As intense alteration is probable, the

protolith to t.he sericite schist could also be nafic

volcanic or intrusive.

GENESIS OF THE NORTH COLDSTREAM MINE

Several fundamental questions must be answered in order to

contemplate the origin of the North Coldstream Mine. What

is the siliceous host? -chert or silicification; What is

the host rock? -mafir: or felsic volcanic -or gabbro: The

nassive nature of the siliceous zone, the lack of

sedimentary features, and other sedinentary litholcgies, its

absence on strike, and the gradational contacts all support

silicification. Silicification of this intensity is only

known to occur in hydrothermal gold deposits (of all ages),

and in the Keiko zone of some Kuroko t.ype volcan::?yenic

massive salphide deposits. The former can be negated as a

gold content at the North Coldstream of 450 ppb is much too

low, and is typical of the latter. Silicification in gold

systems tends to be "late", ie post tectonic, is of the

highest ore grades and is superimposed on fabrics.

Silicification at this site predates deformation as

evidenced by siliceous boudins and inherited fabric. A

volcanogenic origiri is also supported by t.he "stringer"

style of the ore, the piped shaped host silicification and

t some extent, the sequence of alteration. It therefore

appears that the North Coldstrean Mine is the stringer zone

cf a volcanogenic xassive sulphide deposit.

At one iocation there is evidence that the gabbro may be ahust rock. If one considers the apparent increasingsLiciEicatLon or the gahhro to indicate its presence durin

Lh. nneralizng event, then the nineralizing event must berost volcanic. However, the lack of fabric within theyabbro at the east end of the mineralized complex indicates

that the gabbro is post tectonic, and therefore post

c;:!raIrac12n. Thioy this under consideration, thechtoritic schist along the margin of the gabbro may be

releted to emplacement and the blue quartz generated during

feldspar destruction and chioritization.

This mineralized complex appears to be located at a felsic—

mafic volcanic contact. This is certainly true at a

regional scale, but difficult to demonstrate at mine scale.

FIELD TRIP STOPS

STOP 4ct;

This is an outcrop of altered hornblende gabbro on the

northern margin of the mineralized complex. Despite

epidote-chiorite alteration and multiple shear sets, igneous

textures and mineralogy are preserved. This gabbroic stock

has variable composition and texture and this location we

find a medium grain, equigranular gabhro and a perdmatitic

phase. In thin section the hornblende is fibrous and

secondary.

STOP 4b;

Group of outcrops on south side of road, only 2 feet from

stop 4a. These outcrops have rare, ghost like, relic igneous

textures which indicate that the protolith is gabbro. The

yabbro has been sheared and increasingly chloritized at the

90

It one location there is evidence that the gabbro nay be - lust rock. If one considers the apparent increasing

1 . LC ificat ion of the gabbro to indicate its presence durin'j

5.:Â mineralizing event, then the mineralizing event nust be

post volcanic. However, the lack of fabric within the

gabbro at the east end of the mineralized complex indicates

that t'r't.-i gabbro is. post tectonic, and therefore post -...,, ,.Â¥ :.. . 1 - Â . . . L ~ : n . T ~ k i . ~ - j this under consiaaraticn, the

chloritic schist along the margin o the gabbro may be

related to emplacement and the blue quartz generated during

feldspar destruction and Ghloritization.

This mineralized ccoplex appears to be located at a felsic-

rwfic volcanic contact. This is certainly true at a

reyional scale, but difficult to decionstrate at mine scale.

FTELD TRIP STOPS

STOP 4a;

This is an outcrop

northern margin of

of altered hornblende gabbro on the

the mineralized complex. Despite

epidote-chlorite alteration and multiple shear sets, igneous

textures and mineralogy are preserved. This gabbroic stock

has variable conposition and texture and this location we I

f i ~ d a nediun grain, eguigranular gabbro and a pegmatitic

phase. In thin section the hornblende is fibrous and I secondary. I STOP 4b;

Group of outcrop's on south side of road, only 25 feet from

stop 4.3. These outcrop% have rare, ghost like, relic igneous

textures which indicate that the protolith is gabbro. The I

-.̂ t-,-h- has been sheared and increasingly chloritized at the I

expense of epidote. The chlorite is pervasive and alsooccurs as segregation. Blue quartz seyregations andnagnetite are irregularly distributed. Leucoxene andmagnetite are dispersed as sub—millimetre grains, nagnetitealso occurs as acicular crystals up to 5 cm in length. Some

layers in the chlorite schist are in fact, fibrous

hornblende, which are later than the magnetite—leucoxenealteration, and have a more random orientation than

chlorite, indicating post tectonic growth.

The distribution of quartz as irregular blebs, and the

volume of this quartz could be produced in situ during

shearing and chlorite alteration, as opposed to

silicification from an external source.

Those quartz rich portion of the chlorite schist zone are

termed "quartz complex" in mine terminology.

STOP 4c;

This white siliceous zone is the transition from the quartz

complex to the more massive brownish mauve host rock. It

consist of strained quartz and 1Osericxte and leuooxene.

STOP 4d;

This brownish mauve siliceous rock is the ore host. It's

only dissimilarities with the white siliceous unit is that

it unstrained, and as such is an even grain mosaic of fine,

equant quartz crystals.

STOP 4e;

A third variety of the silicification is buff coloured and

only a subtle difference exist with the brownish mauve

variety. This variety is snatially related to a buff

alteration zone along the south central portion of the

mineralized complex. This buff alteration is the

Qi

expense of epidote. The chlorite 1.5 pervasive and also

occurs as se3regation. Blue quartz se::;recjations and

raagnetite are irregularly distributed. Leucoxene and

nagnet-it.e are dispersed as sub-millinetre grains, nagnetite

also occurs as acicular crystals up to 3 era in length. Some

layers in the chlorite schist are in fact, fibrous

hornblende, which are later than the maqnetite-leucoxene

alteration, and hava a ncre random orientation than . . .

l o r i t e , iric-iic~itir'.q post tectonic grcwth.

The distribution of quartz as irregular hlebs, and the

volume of this quartz could be produced in situ during

shearing and chlorite alterati-on, as opposed to . . > :

silicification from an external source.

Those quartz rich portion of the ch1orit.e schist zone are . - , ~ J , - , .

termed "quartz complex" in mine terminology.

: ' , , '

STOP 4c;

This white siliceous zone is the transition from the quartz

complex to the more massive brownish nauve host rock. It

consist of strained quartz and 10 "; sericite and leucoxene.

STOP 4d:

This brownish mauve siliceous rock is the ore host. it's

only dissimilarities with the white siliceous unit is that

it unstrained, and as such is an even grain riosaic of fine,

equant. quartz crystals.

STOP 4e;

ft third variety of the silicification is buff coloured and

only a subtle difference exist iiith the brownish mauve

variety. This variety is spatially relate3 to a buff

alteration zone along the south central portion of the

nip,eralized complex. This buff alteration is the . ' .

-.

sericit Lzatlcn of a felsic rock bearing quarts eyes,L5sLhiV flsic volcanic. Thec dour of the silicificationna reflect the composition of the protolith.

STOP 4f;

This is an outcrop of the buff alteration described in 4e.The rock is massive, fine grain, and textureless.

STOP 4g;

An outcrop of ore in sharp contact with a sericite schist.At the top of the outcrop, fragmental textures of equivocalorigins and a siliceous boudin wrapped in sericitic schist.

STOP 4h

This buff outcrop is included in the sericite schist unit

because of its felsic appearance, although it ray contain

more chlorite than sericite. At this location a fragmental

texture mar helm terpreted as volcanic, hut is ccnsidered an

equivocal texture.

STOP 4i;

This large ridge at the east end of the mineralized complexis gabbro. The lack of fabric indicates that it was notaffected by the deformation that produced the adjacentschist and must therefore be younger and crosscut the orecomplex. It cannot therefore be the host rock.

92

a r i c i t i z a t i c n sf a f e l s i c r o c k b e a r i n g q u a r t s e y e s ,

i s s i b l y f a l s i c v o l c a n i c . The c o l o u r o f t h e s i l i c i f i c a t i o n

. ,.iv r c i f l e c t t h e c o n p o s i t i c n o f the p r o t o l i t h .

TOP 4 f ;

h i s i s a

h e r o c k i s c a s s i v e , f i n e g r a i n , and f e a t u r e l e s s .

An o u t c r o p o f o r e i n s h a r p c o n t a c t w i t h a se r ic i te ¥ichi '- i t

A t t h e t o p o f the o u t c r o p , f r a g m e n t a l t e x t u r e s o f e q u i v o c a l

r i g i n s and a s i l i c e o u s boud in wrapped i n s e r i c i t i c s c h i s t .

. ~

b e c a u s e o f i t s f e l s i c a p p e a r a n c e , a l t h o u g h it. may c o n t a i n

n o r e c h l o r i t e t h a n s e r i c i t e . A t t h i s l o c a t i o n a f r a g m e n t a l

h i s l a r g e r i d g e a t t h e e a s t end o f t h e m i n e r a l i z e d comp

s gabbro . The la.ck o f f a b r i c i n d i c a t e s t h a t it was n o t

f f e c t a d by t h e d e f o r m a t i o n t h a t produced t h e a d j a c e n

s c h i s t and n u s t t h e r e f o r e b e younger and c r o s s c u t t h e

Vanguard Prospec

INTRODCCTTON

The Vanguard Prospect is situated south of the junction

between Highway 802X and Highway 11 at Kashabowie, Ontario

(Fig. 8 ) . A private bush road trends south of this

intersection and accesses the showine; area. The prospect is

situaced on Mining Locations K 5 6 and 271.

The property is private land and permission frcra the owners

is required to visit the prospect. The property is

currently under option to Minnova Inc.

GEOLOGY

The prospect area is underlain by sheared volcanic r

the Shebandowan Greenstone Belt. Anorthositic gabbro suite

rocks intrude the volcanic sequence. The mineralized zone

strikes 060 degrees and dips steeply to the south. The zone

has been traced intermittently along strike for 1310 m ( 4 3 0 0

ft.). The prospect consists of two mineralized sections

about 700 m apart and have been designated the West and East

stop we will be examining the east zone.

The Vanguard East Occurrence forms part of a large outcrop

knoll on the east side of the bush road approximately 800

metres south of Highway 11. The occurrence is very well . exposed by stripping and trenching.

At the Vanguard East location, a massive sulphide lens

containing sphalerite, chalcopyrite, pyrite and pyrrhotite

is hosted by a variolitic to massive mafic volcanics; bedded

silicic tuff caps a portion of the mineralized zone and has

been interpreted by Minnova to be "exhalite". Alteration

associated with the deposit includes silicification and iron

The rocks are chloritized.

With reference to Fig. 10, the sulphide ninsralization is

this zone yielded 2.47% Cu, 6 .92% ZJ, 40.6 grams per ton Ag,

and 1.39 grams per ton Au across an apparent width of 4

~eters. The sulphide ore is finely banded and in the sample

-xanined, consisted of approximately 25'6 sphaierite, 70%

pyrite, and the balance silica, rock fragments, and other

gangue minerals. Chalcopyrite also occurs in the suphide

The two other major rock units exposed near the main trench

ara a silicified variolitic mafic volcanic and a chocolate

brown (weathered surface) carbonate-rich rock. An

alteration zone consisting of massive to bedded

silica/carbonate with disseminated iron sulphides is

situated west of the south end of the main t.rench. The

contact area between this zone and the varioliti:: nafic

volcanic to the west is marked by quartz (veins?).

Sheared but otherwise hell preserved volcanic features such

is varioles can be examined throughout the large flat

stri-pped area north of the sulphide lines.

Most of the faulting trends approximately 270 degrees with

apparent dextral offsets. The intense deformation of the

rocks of the showing area is perhaps explained by the

proximity of the Crayfish Creek Fault. This fault

presumably underlies the low ground to the' east of the

stripped area.

. .

. . Reef a r a n c e s

.:rndt, X . and Brooks , C .

 ¥ W i O K c r a a t i i t e a ; Geo logy , Vol.8, p p 135-156.

" " - A - - '\r-,'Jt, V. , F r a n c i s , D. a n d Hynes, A . J .

1979 : The F i e l d C h a r a c t e r i s t i c s a n d P e t r o l o g y o f Archean

and p r o t e r o z o i c K o m a t i i t e s ; Can. M i n e r a l o g i s t , " ,

y o l 1 7 , ;?p :47-163.

B a c r e t t , P.M., B i n n s , H.A., G r o v e s , 9.1.. M a r s t o n , R . J . a n d

McQueen, K.G.

1977 : S t r u c t u r a l H i s t o r y a n d Metamorphic M o d i f i c a t i o n o f

Archean V o l c a n i c - t y p e N i c k e l D e p o s i t s , Y i l g a r n

B l o c k , Wes te rn A u s t r a l i a ; Economic Geo logy ,

~ 0 1 . 7 2 , pp 1195-1223.

~ o r r a d a i l e , G. and Brown, H.

19B7: The Shebandowan Group: "Tiraiskaming-Like" Archean

Rocks i n S o r t h w e s t a r n O n t a r i o ; CJES, Vo l .24 , pp

185-188.

The D a i l y Times J o u r n a l

1937: I n t . X i c k e l Buys Shebandowan O r e ; p u b l i s h e d i n

F o r t W i l l i a m , O n t a r i o .

G i b l i n , P.E.

1964 ; B u r c h e l l Lake Area; O n t a r i o Depar tment o f Mines ,

G e o l o g i c a l R e p o r t 19 , 39p.

Nor ton , P.

1982: Archean V o l c a n i c S t r a t i g r a p h y , P e t r o l o g y a n d

CLcrais t ry o f Maf i c and L ' l t r a m a f i c Rocks , C h r o m i t e

and t h e Shebandowan Xi-Cu Mine. Shebandowan,

N o r t h n e s t e r n O n t a r i o ; Unpub l i shed PhO T h e s i s ,

C a r l t o n L ' n i v e r s i t y .

.. Pye, E . G . and Fenwick, 3 . G .

1964: Atikokan-Lakehead S h e e t , G e o l o g i c a l C o m p i l a t i o n

S e r i e s ; On t . Dept . o f Mines , >!ap 2065, S c a l e

1 : 2 5 3 , 4 4 0 .

S c o t t , S . 3 .

1963: R e p o r t on t h e N o r t h C o l d s t r e a m Mine L t d .

i dz~::kl LS:::~ :

S h e g e l s k i , R.2.

1980: Archean C r a t o n i z a t i c r n , Emergence a n d Red Bed

Dfivelopraent, Lake Shebandowan A r e a , Canada;

P recambr ian R e s e a r c h , Vo l .12 , pp 3 3 1 - 3 4 7 .

S t o t t , G.M.

1981: A S t r u c t u r a l A n a l y s i s of t h e C e n t r a l P a r t o f t h e

Shebandowan Metavolcanic-Metasedimentary Belt. ;

O n t a r i o G e o l o g i c a l S u r v e y , Open F i l e R e p o r t 3349,

4 4 p . l

PURCHASE OF PROCEEDINGS AND ABSTRACTS AND FIELD GUIDEBOOK

Copies of the Proceedings and Abstracts, Part 1, and the Field Guidebook, Part 2, for the 36th Annual Institute on Lake Superior Geology may be purchased during the meeting.

Proceedings and Abstracts, Volume 36, Part 1 . . . . . . . . . . $5.00 Canadian

Field Guidebook, Volume 36, Part 2 . . . . . . . . . . . . . . . . . $5.00 Canadian

Payable to Institute on Lake Superior Geology

Issues of Proceedings and Abstracts, Part 1, and Field Guidebook, Part 2, from this and the three previous meetings, may be ordered from:

Joe KaSEokoski, .Secretary-Treasurer Department of Geology and Geological Engineering Michigan Technological University Houghtan, Michigan 49931

The cost for each part ordered is $6.00 U.S.

Orders will be filled while supplies last.

..............................................................................

All volumes back to 1955 are available for photocopying at the prevailing rate, from the Michigan Technological University Library,

through Mr. M. S. Spence, Archivist.

Phone 906-487-2505

..............................................................................