Download Lac des Iles-Tib L. area: plutonic rocks

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MARGINAL NOTES
LOCATION AND ACCESS
The Lac des Iles area is centred approximately 80 km northwest of
Thunder Bay. Road access to most of the area is provided by
Great Lakes Pulp and Paper Company logging roads in the Dog
River area The main access road proceeds north from Highway
17, approximately 10km west of Raith, to Great Lakes Pulp and
Paper Company Camp 234. The southeastern part of the area has
no road access and is accessible by helicopter and floai-equipped
aircraft.
An all-weather road from Highway 527 in the easl to Lac des
Iles is currently being built in order to provide access to the
mineralized zone south of Lac des Iles.
MINERAL EXPLORATION
Economic interest in the area was initially for copper-nickel sul
phide mineralization in ultramafic and mafic rocks. More recently,
exploration work has focused on the Lac des Iles Complex which
is the largest intrusion in the area and is host to significant
palladium and platinum mineralization. The Tib Gabbro, which is
the second largest mafic intrusion In the area, has also been a
centre of exploration activity.
Work by Texasgulf Canada Limited and Boston Bay Mines
Limited in gabbroic rocks south of Lac des Iles has revealed a
deposit of 12 000 tons per vertical foot (35 000 tonnes per vertical
metre), grading 0.185 ounce per ton (6.34 g/t) platinum group
elements (PGE), 0.02 ounce per ton (0,62 g/t) gold, and Q.22%
copper-nickel (The Northern Miner, Volume 62, October 21, p.27,
1976). The Canadian Mines Handbook (1976-1977, p.314) reports
an approximate tonnage of 22 500 000 tons (20 400 000 tonnes).
In 1985 and 1986, renewed interest in PGE resulted in exten
sive staking of the mafic and ultramafic rocks in the area. In 1986,
several companies initialed exploration work.
Mineral exploration activity in supracrustal rocks along the
northern margin of the area and in the southeastern part of the
area has not been compiled for the present study. As of February
1987, no properties were held in supracrustal rocks within the
area.
The information on exploration history reported here is, unless
otherwise indicated, taken from the Assessment Files Research
Office, Ontario Geological Survey, Toronto.
LAC DES ILES COMPLEX
In 1958. following an airborne magnetic survey, R.J. Jowsey Min
ing Company Limited acquired a group of 80 claims within ul
tramafic rocks in the northern part of the Lac des Iles complex.
Several conductors indicated by an airborne electromagnetic sur
vey of the claim group in 1958 were further investigated by
ground electromagnetic and magnetic surveys and diamond drill
ing (five holes for 472m) in 1959. The work failed to indicate
significant mineralization.
In 1963, W. Baker and G. Moore discovered widespread
copper-nickel mineralization and staked 175 claims in gabbroic
rocks south of Lac des ties. These claims were acquired by
Gunnex Limited and investigated in 1963 and 1964 by geological
mapping, ground magnetic, electromagnetic, self potential, and soil
geochemical surveys. Eight mineralized zones were discovered
and these were tested by 11 diamond-drill holes (totaling 1516m)
in 1964. In 1966, the property was optioned by Anaconda Ameri
can Brass Limited and an additional 13 diamond-drill holes
(totaling 1900 m) were drilled. No further work was carried out
In 1967, the Canadian Nickel Company Limited diamond
drilled three holes (totaling 1435 m) in the northeastern part of the
Lac des Iles complex within ultramafic rocks. Minor sulphide
mineralization was reported.
In 1969 and 1970, Meridian Mining and Exploration Company
Limited staked 68 claims to the north and south of the property
held by Gunnex Limited. These were investigated in 1970 and
1971 by ground electromagnetic and magnetic surveys. No further
work is reported.
In 1973, K.C. Kuhner staked the mineralized zones outlined by
Gunnex Limited. In 1974, Boston Bay Mines Limited acquired the
property and diamond drilled 35 holes in the mineralized zone
(Dunning 1979). Logs for four drillholes (totaling 487 m) were
submitted lo the assessment files. Texasgulf Canada Limited op
tioned the property in 1975 and, in 1975 to 1976, carried out
airborne magnetic and electromagnetic surveys, ground geo
physical surveys, geological mapping, and diamond drilling. Ap
proximately 19 030 m (119 holes) were diamond drilled. This work
outlined a zone of platinum-palladium mineralization named the
Roby Zone, and tested other occurrences in the gabbro. Texasgulf
Canada Limited dropped the property in mid-1976.
Boston Bay Mines Limited diamond drilled one hole to investi
gate the vertical extent of mineralization in 1976. Significant PGE
values were encountered from 332 to 445 m (Dunning 1979V
In 1986, the Platinum Group Limited, a private federally char
tered company which is 90Vc owned by Boston Bay Mines Limited,
initiated a program of diamond drilling, trenching, and bulk sam
pling to evaluate the feasibility of developing an open-pit mine on
the Roby Zone (The Northern Miner, 1986, Volume 72, Number 4,
p.1). This property is under option to Madeleine Mines Limited.
TIB GABBRO
Economic interest in the Tib Gabbro has been mainly for copper,
nickel, and PGE mineralization. Information reported here is largely
summarized from Smith and Sutcliffe (I987). Exploration activities
on the intrusion reached a peak in the middle 1970s, at the same
time that Texasgulf Canada Limited and Boston Bay Mines Limited
were actively exploring claims in the nearby Lac des Iles Complex
for PGE. Prior to 1972. minor claim staking and trenching were
reported (Kaye 1966).
In 1972, Tex-Sol Explorations Limited carried out electro
magnetic and magnetic surveys on 24 claims occupying the south
western part of the intrusion. The purpose of this program was to
evaluate two nickel-copper sulphide occurrences in gabbronorite.
One of these sulphide mineral occurrences was investigated in
1975 by K.C. Kuhner and J.E. Nelson. A single drillhole (150m
long) was completed on a two-claim group centred on the Kuhner
Occurrence at the southern end of the intrusion. Anomalous PGE,
nickel, copper, and gold values were obtained from assays of
pyroxenite with disseminated sulphide mineralization.
A group of 14 claims in the east-central part of the intrusion
was evaluated in 1975 by Sandhurst Mines Limited. Preliminary
geophysical surveys (ground electromagnetic and magnetic) out
lined a number of conductors and magnetic anomalies. Also in
1975, exploration work was completed by E.W. Bazinet on 12
claims (Bregman-Mourm claim group) located north of Coon's
Lake. A V.L F. electromagnetic survey located a number of con
ductors and a magnetic survey outlined a series of anomalies
corresponding to layering within the intrusion,
Geological mapping was carried out in 1976 by Michael Og
den on 20 claims (held by R.P. Mackay) in the northwestern part
of the intrusion. Mapping served to outline a lew sulphide mineralbearing zones. In the same year, Steppingstone Explorations Limit
ed conducted a drilling program (four diamond-drill holes totaling
320 m) on three claims situated northeast of Coon's Lake.
In 1978, rocks within a single claim located in the south
eastern comer of the Tib Gabbro were tested by a diamond-drill
hole (153m long). This work, completed by J. Kondrat, detected
sulphide mineral-bearing lilholpgies.
The most recent exploration work reported within the Tib
Gabbro was completed by Westmin Resources Limited in 1982.
Fourteen claims located along the southern edge of the intrusion
were evaluated by geological mapping, a soil geochemical survey,
and a magnetic survey. This work delineated a sulphide mineralbearing pegmatitic horizon extending eastward from the Kuhner
Occurrence.
OTHER MAFIC INTRUSIONS
Active exploration in 1975 and 1976 stimulated work on other
mafic intrusions in the area. During 1975 and 1976, Combined
Metal Mines Limited, in conjunction with Goldray Mines Limited,
completed ground magnetic and electromagnetic surveys southwest of Errey Lake. Osisko Lake Mines Limited and Joliet-Quebec
Mines Limited completed ground electromagnetic surveys on a
claim group optioned from Boston Bay Mines Limited in the Shelby
Lake area. Texasgulf Canada Limited completed airborne geo
physical surveys in the area including, and southwest of, the Lac
des Iles Complex. Nomad Mines Limited and White Star Copper
Mines Limited diamond-drilled four holes (totaling 173m) In a
zone of sulphide mineralization in mafic and ultramafic rocks at
Demars Lake.
GENERAL GEOLOGY
Previous mapping of the area is primarily the reconnaissance
mapping and compilation by S age e r a/. (1974). Parts of the area
have been mapped by Pye (1968). Kaye (1966. 1969), Milne
(1964), Sutcliffe and Sweeny (1986), and Smith and Sutcliffe
(1987). Information on the Lac des Iles Complex and the Tib
Gabbro reported here Is summarized from Sutcliffe and Sweeny
(1986) and Smith and Sutcliffe (1987) respectively.
Archean mafic intrusions in the Lac des Iles area form part of
an east-northeast-trending linear zone of mafic plutons which
extends from Atikokan to Lake Nipigon. This zone parallels the
boundary between the Wabigoon and Quetico Subprovinces. With
in the map area, most of the rocks are within trie Wabigoon Subprovince: however, the subprovince boundary passes through the
southeastern part of the area.
The mafic to ultramafic intrusions in the Lac des ties area
form a circular structure approximately 30 km in diameter. The
intrusions, which have broad lithologic and metallogenetic
similarities, are late tectonic, and intrude granitoid host rocks. The
intrusions are tholeiitic in affinity and contain phases ranging trom
ultramafic pen dot i tic and pyroxenitic cumulates to magnesian
gabbronorite and iron-rich gabbro. Hybrid marginal zones, consist
ing of hornblendite intruded by hornblende diorite, are common to
many of the intrusions and are interpreted to be due to contamina
tion of mafic magma by a granitoid component. Most of the
intrusions have well preserved mineralogy and are not signifi
cantly deformed.
Granitoid rocks, into which these intrusions were emplaced,
consist of predominantly gneissic tonalite. The mafic intrusions are
interpreted, on the basis of field relationships, to be the same age
as a suite of late hornblende tonalite to biotite granite occupying
the centre of the circular configuration. Compositions of some of
the younger granitoids reflect magma mixing between granitic melt
and mafic magma.
LAC DES ILES COMPLEX
The initial mapping of the Lac des Iles Complex was by Pye
(1968), at which time the extent of the complex was determined.
Guarnera (1967), Dunning (1979), and Watkinson and Dunning
(1979) studied aspects of the relationship of the gabbroic rocks to
mineralization. A study of the platinum group minerals has been
made by Cabri and Laffamme (1979). Recently, Talkington and
Watkinson (1984) reported on the distribution of PGE in rocks of
the Roby zone.
The Lac des Iles Complex consists of an ultramafic intrusion
centred on Lac des Iles and a gabbroic intrusion south of the lake.
The complex has an area of approximately 30 km2 (Pye 1968;
Dunning 1979). The present work suggests that the mafic and
ultramafic rocks are intruded into gneissic tonalitic granitoids
The ultramafic intrusion is composed of two coalescing cen
tres each of which consists of several intrusive phases. The
centres are defined by the distribution of lithologies and attitudes
of layering The lithological sequence of early peridotite recks,
followed by clinopyroxenite, and finally by late websterite and
gabbronorite, suggests that the ultramafic rocks follow a cry
stallization sequence of olivine, oltvine-clinopyroxene, clino
pyroxene, clinopyroxene-orthopyroxene, and finally clinopyroxeneorthopyroxene-plagioclase.
The northern ultramafic centre is nearly circular in plan with a
diameter of approximately 4 km, and consists of dunite and wehr
lite (olivine cumulates), olivine-clinopyroxenite and clinopyroxenite
(olivine-clinopyroxene and clinopyroxene cumulates), and web
sterite (orthopyroxene-clinopyroxene cumulates). These lithologies
occur in several cycles which probably represent fractional cry
stallization of separate pulses of magma in the chamber. Olivine
cumulates occur primarily around the perimeter of the structure
and the rocks become progressively more pyroxene-rich toward
the core. Attitudes of layering suggest that the northern centre has
an upright, funnel-shaped form. This structure, however, is com
plicated by a later discordant clinopyroxenite phase in the extreme
northwestern part of the intrusion and by the discordant websterite
core. Breccia zones of serpentinite with angular pyroxenite frag
ments are another minor discordant feature in the northern ul
tramafic centre.
The southern ultramafic centre is elliptical in plan with an
irregular wehrlite core centred on the Southeast Angle of Lac des
Iles. The wehrlite core is surrounded by websterite containing
inclusions ol wehrlite a! several locations. In contrast to the
northern ultramafic centre, the southern centre is predominantly
composed of massive rocks, and lacks well defined igneous
layering.
On the southwestern contact of the intrusion, a marginal zone
of medium- to coarse-grained hornblendite, pyroxene hornblendite,
and hornblende diorite is present. The diorite commonly forms the
matrix of agmatitic breccia with hornblendite fragments.
Gabbroic rocks within the complex are primarily confined to
an elliptical intrusion south of Lac des Iles, this intrusion can be
subdivided into two major units. The southwestern part of the
intrusion consists of predominantly medium-grained gabbronorite
and has been referred to as the Western Gabbro by Dunning
(1979). The Western Gabbro contains several zones of mineral
ized pegmatitic gabbro with grajn sizes up to 2 cm. Toward the top
of the Western Gabbro, In the northeastern part of this zone, minor
pyroxenite and anorthosite phases are present. The northeastern
pan of the intrusion consists of medium-grained leucogabbro to
gabbro, and is referred to as the Eastern Gabbro (Dunning 1979).
The Eastern Gabbro contains magnetite-rich layers and locally,
has an igneous lamination. Dunning (1979) determined that the
Eastern Gabbro is iron-rich, sulphide-poor (compared to the West
ern Gabbro), and probably not co-magmatic with the Western
Gabbro.
At the top of the Western Gabbro is a 60 to 300 m thick unit of
heterogeneous gabbro which contains coarse-grained to peg
matitic gabbro and sulphide mineralization. The heterogeneous
unit contains numerous gabbroic phases of varying composition
and texture and may be a result of mixing of the Eastern and
Western Gabbros. An altered pyroxenite sill which is probably
related to the ultramafic sequence intrudes the gabbroic rocks
along the contact between the Eastern and Western Gabbros.
West of the gabbro, and outcropping on the southern shore of
Lac des Iles, Is an approximately 50 m wide dike which may be a
feeder dike for the ultramafic rocks. The dike is composite and is
composed of medium-grained olivine gabbronorite, olivine web
sterite, and locally, hornblende diorite.
ssfso-49020'
9000'
49020'
m*******M**MC
In the southern part of the Lac des lies Complex, hornblende
gabbro" is the predominant rock type. This phase is interpreted to
form a separate intrusion since it is llthologically distinct and' is
partially separated from the Western Gabbro by a tonalite septum
Tne hornblende gabbro is medium grained, massive to weakly
foliated, and Is locally intruded and brecciated by quartz diorite.
The hornblende gabbro and quartz diorite appear to be gradational
on the southeastern side of this intrusion, south of Camp Lake.
These two rock types are interpreted to be co-magmatic.
The present mapping indicates that mafic rocks in the Shelby
Lake area south of Lac des Iles are less extensive than previously
indicated and that most of the pronounced magnetic anomaly
which extends south from the Lac des Iles Complex (ODM-GSC
1962) is underlain by dioritic rocks. The hornblende gabbro phase
of the Lac des ties Complex mapped by Sutcliffe and Sweeny
(1986) grades, south of Masson Lake, into hornblende diorite to
quartz diorite with less than 35*y0 ferromagnesian minerals. The
diorite locally contains numerous inclusions and blocks of coarsegrained hornblendite and medium-grained mafic diorite, and is
similar lo dioritic rocks found on the western margin of the
Complex at Lac des Iles. The dioritic rocks intrude gneissic biotite
tonalite on the northwestern side of the structure and grade into
biotite-hornbtende quartz diorite and tonalite on the southeastern
side.
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Ministry of
Northern Development
and Mines
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Mines and Minerals Division
Ontario Geological Survey
MAP P.3098
Geological Series—Preliminary Map
PRECAMBRIAN GEOLOGY
PLUTONIC ROCKS IN THE
LAC DES ILES-TIB LAKE AREA
DISTRICT OF THUNDER BAY
Mile l
TIB GABBRO
The Tib Gabbro is an oval-shaped body with an approximate
exposed area df 25 km2. Layer attitudes and outcrop exposure
suggest emplacement of the gabbro in a funnel-shaped configura
tion into the surrounding granitoids with later tilting to the north
east. The southwestern part of the gabbro, therefore, represents
the base of the exposed section.
The Tib Gabbro can be divided inlo four zones on the basis of
observed lithological characteristics. A Border Zone of coarsegrained mafic rocks forms most of the outer margin of the intru
sion. Other zones, proceeding from southwest to northeast, are
here termed the Lower Zone, Middle Zone, and Upper Zone. The
lower two zones probably represent distinct magma pulses which
crystallized from the base upwards. These zones are character
ized by an evolving cumulate mineral sequence. This sequence
results in a mappable igneous stratigraphy with pyroxene cu
mulates near the base and fractionated gabbro at the top. The
Upper Zone consists of hornblende gabbronorite and is not well
layered.
The Lower Zone attains a maximum width of 1.5 km and has a
maximum strike length of 7.4 km. This zone consists of a se
quence of weakly layered gabbronorite and gabbro grading up
into a well layered sequence with melagabbronorite and ortho
pyroxenite (pyroxene cumulate) layers hosted in gabbronorite.
Narrow conformable pegmatitic lenses and magnetite-rich zones
of short (tens of metres) extent are found in close proximity to the
ultramafic rocks. Above the orthopyroxenite, the lithologies be
come more leucocratic and anorthosite appears as distinct layers.
Intermittent modal layering is well developed at this level. The top
of the Lower Zone is marked by a magnetite-rich gabbronorite
with leucocratic layers.
The 1.6 km wide Middle Zone is simitar in width and overall
strike length (approximately 7 km) to the Lower Zone. Gabbro
norite is again the dominant rock type; however, leucocratic units
are more abundant here than in the Lower Zone. Melanocratic and
ultramafic cumulates occur as conformable horizons approximate
ly 500 m above the base of the zone. These units, however,
appear to pinch out along strike, and can only be traced for tens
of metres. Middle zone gabbroic rocks are hornblende-rich, par
ticularly in the northwestern part of the intrusion. Conformable
pegmatite layers are found within the upper part of the zone;
however, discordant pegmatite dikes are more common. The top
of the Middle Zone Is marked by a well layered magnetite-rich
gabbronorite.
The Upper Zone is characterized by hornblende-rich, massive
to weakly layered, gabbronorite. Layers form a concentric pattern
with a shallow inward dip. A leucocratic quartz-bearing granulite
unit forms a discontinuous outer rim of variable width around the
Upper Zone. This unit is interpreted to be a metamorphosed
tonalite but alternatively may be a final product of fractionation.
Layering in the Tib Gabbro is comparable lo that developed in
the Mulcahy Gabbro (Sutcliffe and Smith 1985). Planar lamination
of feldspars and pyroxenes is found throughout the intrusion.
Modal layering, defined by changing concentrations of cumulate
phases, is best developed in the upper part of the Lower Zone.
Modal layering occurs both in intermittent and cyclic layer se
quences. Distinct modal layers include orthopyroxenite (ortho
pyroxene cumulate), websterite (orthopyroxene-clinopyroxene cu
mulate), anorthosite (plagioclase cumulate), and magnetite cu
mulate. Ultramafic cumulates occur most frequently in the Lower
Zone. Modal layers are tens of centimetres in width, and persist
for tens of metres along strike.
STRUCTURAL GEOLOGY
The regional structure of the map area reflects the superposition
of the circular configuration of mafic-ultramafic intrusions on older
linear trends associated with the Wabigoon-Quetico Subprovince
^ boundary zone
The subprovince boundary is marked by an east-northeasttrending zone of deformation several kilometres wide which is
characterized by southeast dips and a well developed southeasttrending lineation. The deformation affects both granitoid and
supracrustal rocks in the zone. A similar style of deformation
characterizes narrow fault zones which parallel the subprovince
boundary in the central part of the map area. These zones have
southeast dips ranging from 250 to SO0 and are locally associated
with mylonitization of the granitoid rocks.
A set of faults trending approximately northward is locally
recognized on the basis of offsets of lithological units such as the
Tib Gabbro, and by minor mylonitization and fracturing. This set of
faults may have influenced the course of the Dog River and
Riviere des Iles.
The circular configuration defined by the mafic intrusions is
supported by the distribution of granitoid phases. The gneissic
tonalite phase occurs around the periphery of the structure and
the younger plutons occupy the core. The positions of the individ
ual mafic intrusions, however, appear to be partially controlled by
the east-northeast-trending deformation zones. This fracture set
apparently influences the orientation of mafic dikes and some
larger bodies such as the Buck Lake Intrusion. A preliminary
interpretation of gravity data indicates that the gravity anomaly
maxima in most intrusions are coincident with major rivers and/or
north-trending faults (Vinod Gupta, Geophysicist, Ontario Geologi
cal Survey, Toronto, personal communication, 1986). This suggests
that the locations of mafic intrusions are determined by both the
north-trending fracture set and the east-northeast-trending fault
zones.
As a working hypothesis, it is suggested that the overall
structure of the igneous province reflects underplating of pre
existing crust by mantle-derived magma. Some of this magma rose
to higher levels along fracture sets in the older crust to form the
presently exposed mafic intrusions. In the centre of the structure,
crustal melting and the development of a zoned magma chamber
with magma mixing resulted in the generation of the late granitoid
rocks.
l Mile
l Kilometre
Metres 1000
NTS References: 52 H/4,5
ODM-GSC Aeromagnetic Maps: 2099G, 21 DOG
ODM Geological Compilation Map: 2065
TIB GABBRO
Queen's Printer for Ontario, 1988
Printed in Ontario, Canada
Parts of this publication may be quoted if credit is
given and the material is properly referenced.
This map is published with the permission of V.G.
Milne, Director, Ontario Geological Survey.
4915' J
LOCATION MAP
Scale: 1:1 584 000 or 1 inch lo 25 miles
LEGEND*b
PHANEROZOIC
CENOZOIC
QUATERNARY
PLEISTOCENE AND RECENT
OTHER MAFIC INTRUSIONS
The Demars Lake Intrusion is a small, approximately 1 km wide,
elliptical plug on the southwestern shore of Demars Lake and is
intrusive into gneissic tonalite. The body consists of an ultramafic
core of medium-grained websterite with a marginal zone of horn
blende gabbro and hornblende pyroxenite. The websterite con
tains cumulus orthopyroxene and clinopyroxene with intercumulus
plagioclase and is very similar to the websterite phase of the Lac
des Iles Complex.
Gabbroic rocks at Wakinoo Lake on the southern boundary of
the map area were only briefly investigated. This intrusion appears
to consist predominantly of medium-grained hornblende gabbro,
and is commonly extensively saussuritized and epidotized. The
Legris Lake Intrusion was not systematically investigated during
this study.
Mafic intrusions at Taman Lake and on the Dog River had
previously been defined largely on the basis of pronounced
aeromagnetic anomalies (ODM-GSC 1962). These intrusions con
sist ot hornblende pyroxenite, ranging to hornblende melagabbro
norite with marginal clinopyroxene-hornblende dioritic to gabbroic
phases. In the pyroxenites and melagabbronorite phases, clino
pyroxene and less common orthopyroxene occur as cumulus
grains up to 2 mm in size with intercumulus plagioclase and large
poikilitic hornblende up to 2 cm. The extensive development of
poikilitic hornblende and the presence of marginal zones of horn
blende diorite are considered to be a result of hybridization of
mafic magma with granitoid components. Both of these intrusions
are less extensive than expected from their aeromagnetic expres
sions. Furthermore, the Dog River Intrusion consists of several
Isolated occurrences of mafic rocks surrounded by granitoids.
These relationships suggest that the Dog River and Taman Lake
intrusions may be partially buried by a granitoid cover. This model
is to be investigated by gravity studies.
The Buck Lake Intrusion is a previously unrecognized mafic to
ultramafic intrusion outcropping in the northern part of Buck Lake.
The intrusion is elliptical in plan with a westerly elongation and a
cross-sectional area of approximately 5 km2. A dike-like body
striking at 070C extends east from the intrusion and consists of
medium-grained fresh hornblende gabbro. The main intrusion con
sists of phases ranging from hornblende gabbro and gabbronorite
to clinopyroxenite and hornblende clinopyroxenite. The hornblende
gabbro is the dominant lithology and is typically medium-grained
with local development of pegmatoidal patches containing biotite
and quartz. Ultramafic rocks in the intrusion contain variable
proportions of hornblende and clinopyroxene. Breccia zones are
common in the intrusion, especially along the northwestern shore
of Buck Lake. Melanocratic phases ranging from gabbro to pyrox
enite have been intruded by leucocratic phases to form the brec
cia. These textures are similar to those observed on the margin of
the Lac des Iles Complex. Minor chalcopyrite, pyrite, and pyrrhotite
mineralization was observed in the Buck Lake Intrusion but
anomalous PGE mineralization was not detected in samples col
lected by the field party.
Mafic to ultramafic intrusions in the Lac des Iles area are
emplaced into older gneissic lonalitic host rocks. Intrusive relation
ships are well developed along the margin of the Tib Gabbro
where gabbro apophyses crosscut the foliation in the tonalite, and
inclusions of gneissic tonalite are present in the gabbro. In gen
eral, the gneissic tonalite occurs around the periphery of the
circular structure defined by the mafic intrusions. The gneissic
tonalite is fine- to medium-grained and is recrystalltzed, with
approximately 5^ biotite as the predominant ferromagnesian min
eral. Hornblende is locally present adjacent to zones of am
phibolite enclaves. The tonalite is strongly foliated to gneissic with
gneissosity primarily defined by layers of ubiquitous leuco-tonaiite
mobilizate. Other phases associated with the gneissic tonalite
include medium-grained hornblende diorite to quartz diorite, and
granite to granitic pegmatite veins and dikes.
Two younger granitoid phases occupy the central part of the
map area and are essentially surrounded by the mafic intrusions.
These granitoids include a foliated biotite-horn blende tonalite
pluton in the southern part of the core and a foliated to massive
biotite tonalite to granite pluton in the northern pan. Both of these
phases contain inclusions of the gneissic tonalite.
The biotite-hornblende tonalite is medium- to coarse-grained
and weakly to strongly foliated. Biolite and hornblende in variable
proportions constitute up to 20 e/.. of the rock; however, coarse
blocky hornblende grains, up to 1 cm across, are a characteristic
feature of the phase. The biotite-horn blend e tonalite is interpreted
to be contemporaneous with mafic magmatism in the Lac des Iles
area. This interpretation is based on textures suggesting the co
existence of two magmas within the pluton. Furthermore, the
textures suggest that the two magmas mixed to produce the
hornblende tonalite. These textures include: a) "net-veined" mafic
to intermediate dikes which discordantly intrude foliated tonalite
and are back veined by the tonalite; b) rounded to cuspate mafic
inclusions in tonalite which can be traced into disaggregated "netvein" dikes; and c) zones of hybrid mafic tonalite characterized
by the development of coarse, 2 to 3 cm, skeletal hornblende
crystals along the interface between the mafic and felsic phases.
The crystallization of skeletal hornblende is interpreted to be a
result of supercooling due to magma mixing. The presence of
skeletal to blocky hornblende throughout the tonalite pluton also
suggests that magma mixing may have produced much of the
mafic material in the tonalite. Similar skeletal textures also occur
in hornblende-rich lithologies around the margin of the Lac des
Iles Complex. These textures provide evidence for contamination
processes through magma mixing at the margins of the major
mafic intrusions and smaller dikes.
The biotite tonalite to granite pluton occupying the northern
part of the circular area rimmed by mafic intrusions is intrusive
into gneissic tonalite and the hornblende tonalite. The pluton is
characterized by medium-grained, equigranular, massive to weakly
foliated tonalite to granodiorite with approximately 5?* biotite. A
border phase of granodiorite to granite, with distinctive 1 to 2 cm
microcline megacrysts, occurs along the northern and southern
margins of the pluton.
Fine- to medium-grained dike rocks, ranging in composition
from mafic amphibolite to diorite, intrude all of the Archean
lithologies in the area including rocks of the Lac des Iles Complex.
These dikes commonly trend at approximately 0700 and are espe
cially common in the hornblende tonalite where they display the
previously described magma mixing textures. The widespread dis
tribution of the dikes in the central part of the area suggests that
mafic magma may have underplated the granitoid rocks.
A distinctive breccia zone, with dimensions of approximately
100 by 300 m, crosscuts the hornblende tonalite east of the Dog
River. The breccia consists of subrounded fragments of tonalite
with 3 to 4 cm thick sericitic and silicified alteration rinds in a
matrix consisting of fine-grained, comminuted and altered felsic
material. Rounded, irregular mafic clots up to 0.5 m are locally
present interstitial to the tonalite fragments. Several quartz-feld
spar porphyry dikes intrude the breccia.
The breccia is interpreted to represent an explosive degassing
of the magma chamber. This process is thought to provide further
evidence for the underplating of granitic magma by mafic magma.
Explosive devolatilization is an expected consequence of the
cooling of such a two magma system if the mafic magma contains
volatiles (Rice 1985).
Archean met a volcanics and metasediments were not exam
ined systematically during the present survey. Information on the
distribution of these rocks is compiled from previous surveys.
Minor metavolcanics in the vicinity of the Tib Gabbro contain calcsilicate, clinopyroxene-hombiende and clinopyroxene-ortho
pyroxene assemblages and are interpreted to be of high metamor
phic grade.
The Archean rocks in the area are intruded by diabase sheets
and dikes of late Proterozoic age. The diabase is typically medium
grained; however, aphanitic potygonally fractured chill margins are
locally observed. Pegmatitic patches are developed toward the top
of diabase sheets, while ophitic textures characterize the lower
parts. A north-trending diabase dike was traced for several
kilometres in the western part of the area in the vicinity of the Dog
River.
j
.
1-
Scan Conway, Minister olMiras
. ' ^
Till, esker deposits, swamp, stream, and lake
deposits
UNCONFORMfTY
PRECAMBRIAN
MIDDLE PROTEROZOIC
DIABASE DIKES AND SILLS
BUCK LAKE
INTRUSION
11
11a
11 b
11c
11d
DOG RIVER
INTRUSION
Unsubdivided diabase
Aphanitic to fine-grained diabase
Medium-grained diabase
Ophitic diabase
Pegmatitic diabase
INTRUSIVE CONTACT
ARCHEAN
LATE FELSIC DIKE ROCKSC
i
10a Quartz-feldspar porphyry, quartz porphyry
10
LATE FELSIC PLUTONIC ROCKSd
9
9a
9b
9c
9d
9e
9f
9g
9h
9i
9j
j
Unsubdivided
Horr blende-biotite tonalite to quartz diorite
Blotite-hornblende tonalite to quartz diorite
Biotite tonalite to granodiorite
Biotite granodiorite to granite
Biotite-hornblende granodiorite to granite
Microcline megacrystic granodiorite to granite
Mafic tonalite
Granite pegmatite
Leucocratic granite dikes
Amphibolite inclusions, diorite inclusions
MAFIC TO INTERMEDIATE DIKE ROCKS
8
8a
8b
8c
Unsubdivided
Amphibolite dike
Net-veined dike
Fine-grained diorite dike
ULTRAMAFIC ROCKS
7
7a
7b
7c
7d
LAKE
INTRUSION
Unsubdivided
Serpentinite
Wehrlite
Clinopyroxenite
Olivine clinopyroxenite
7e
Olivine melagabbronorite
7f
7g
7h
7i
Websterite
Hornblendite
Pyroxene hornblendite
Olivine hornblendite
GABBROIC ROCKS
05'
6
6a
6b
6c
6d
6e
6f
6g
INTRJSION
6h
6i
6j
6k
6m
6n
60
Unsubdivided
Gabbro
Gabbronorite, norite
Me l a gabbro, melagabbronorite (CI 65 to 90)
Leucogabbro, leucogabbronorite (CI 22.5 to 35)
Anorthosite gabbro (CI less than 22.5)
Oxide-rich gabbro (oxide greater than 5*k)
Hornblende bearing gabbroic rocks (hornblende
10 to 350A)
Hornblende gabbro, hornblende-quartz gabbro,
Hornblende leucogabbro
Hornblende melagabbro
Gabbroic pegmatite
Gabbroic breccia
Altered gabbroic rocks
Ol i vine-bearing gabbro
DIORITIC ROCKS
5
5a
5b
Unsubdivided
Hornblende diorite
Hornblende-quartz diorite
'
INTRUSIVE CONTACT
EARLY FELSIC PLUTONIC ROCKS
4
4a
DEMARS LAKE
4b
INTRUSION
4c
4d
4e
4f
Unsubdivided
Foliated biotite tonalite and horn blende-biotite
tonalite
Gneissic biotite tonalite, and horn blende-biotite
tonalite
Leucocratic tonalite
Tonalitic granulite
Amphibolite inclusions, diorite inclusions
Granite Pegmatite
INTRUSIVE CONTACT
x/C fa
SUPRACRUSTAL ROCKS
METAVOLCANICS AND METASEDIMENTS
METASEDIMENTS
/WAKINOO LAK
3
3a
3b
3c
3d
3e
3f
NTRUSION
Wakinoo
4900'
4900'
9000'
8930'
Unsubdivided
Conglomerate
Feldspathic wacke, quartz wacke
Lithic wacke
Argillite
Biotite schist
Chert magnetite ironstone
METAVOLCANICS
Intermediate to Felsic Metavolcanics
ECONOMIC GEOLOGY
Exploration in the Lac des Iles Complex by Texasgulf Canada
Limited and Boston Bay Mines Limited, in 1975 and 1976, outlined
a zone of PGE mineralization, the Roby Zone, along the contact
between the Eastern and Western Gabbros, A detailed study of
the zone by Dunning (1979) and Watkinson and Dunning (1979)
showed that vysotskite ((Pd, Ni)S) is the most abundant platinum
group mineral and is associated with pentlandite and chalcopyrite.
Most of Ihe sulphide mineralization is contained witrrn the West
ern Gabbro where it occurs as intercumulus sulphide, and less
commonly, as nel-textured sulphide. The sulphide mineralization is
interpreted to be a magmatic phase forming from PGE. copper,
nickel, and sulphur contained in the Western Gabbro liquid
(Watkinson and Dunning 1979; Dunning 1979). Sulphide assem
blages consisting of chalcopyrite, pyrrhotite, pentlandite, and pyriie are interpreied as being slightly metamorphosed equivalents
of primary exsolution from monosulphide solid solution Millerite
and violarite are present in metamorphosed assemblages
(Watkinson and Dunning 1979). Ratios of Pt/fPt+Pd) and
Cu/CCu+Ni) were determined to be approximately 0.07 and 0.5,
respectively (Watkinson and Dunning 1979) Further work by Tal
kington and Watkinson (1984) demonstrated thai later processes
involving a fluid phase have altered the primary silicates, re
distributed the copper-nickel sulphides, and produced the PGE
minerals.
Field work during the present survey supports the previous
conclusion that mineralization occurs near the Eastern GabbroWesiern Gabbro interface. The redistribution of sulphide min
eralization may be related to the emplacement of late pegmatitic
gabbro dikes and associated intrusion breccia zones. The min
eralization appears to be largely confined to the heterogeneous,
gabbro unit, which contains mineralized coarse-grained to peg
matitic gabbro, and to the altered pyroxenite sill which occurs
along the Eastern Gabbro-Western Gabbro interface.
During the field work, up to 2 0/* disseminated to net-textured
chalcopyrite and pyrrhotite mineralization was observed within
ultramafic rocks This type of mineralization is most commonly
associated with websterite and gabbronorite. There appears to
have been a relatively limited investigation of the ultramafic rocks
to date. These rocks warrant further investigation for platinum
group elements Emphasis might be placed on defining and ex
ploring the interfaces between websterite and the more olivine- or
clinopyroxene-rich lithologies. These contacts may represent the
introduction of fresh magma into the chamber and an environment
suitable for sulphide segregation.
Preliminary results from grab samples selected by the field
party reveal anomalously high PGE values in sulphide-bearing
websierite and gabbronorite from the ultramafic intrusion. Further
more, the results suggest that Pt/fPt+Pd) ratios may be substan
tially higher in the ultramafic rocks than in the gabbroic rocks of
the Roby Zone, and that PGE values do not necessarily correlate
with the amount of sulphide present (see Sutcliffe end Sweeny
(1986) for selected values).
In the Tib Gabbro, a zone of sulphide mineralization with PGE
values, known as the Kuhner Occurrence, is located within coarsegrained to pegmatitic rocks on the southwestern margin of the
intrusion. The mineralization occurs in melagabbronorite to norite
and consisls of pyrrhotite, chalcopyrite, and pyrite. The sulphide
mineralization occurs as fine disseminated grains and rounded
aggregates, up to 8 mm in diameter, and as intercumulus neltextured sulphide. One chip sample, taken by the field party
across a 2.4 m interval (the length of the trench), returned values
of 390 ppb palladium, 190 ppb platinum, 120ppb gold, 1140ppm
nickel, and over lOppm copper (analyses by the Geoscience
Laboratories, Ontario Geological Survey. Toronto).
Anomalous PGE values were obtained from a number of
sulphide mineral-bearing layers in the Tib Gabbro discovered
during the current mapping project. One layer, located stratigraphically above an orthopyroxene cumulate in the Lower Zone,
contains sulphide mineral-bearing inclusions in a zone of magma
mixing and brecciation. Mineralization consists of disseminated to
net-textured pyrrhotite, pyrite, and chalcopyrite, in e weakly mag
netic gabbronorite (see Smith and Sutcliffe (1987) for selected
analyses).
Websterite to gabbronorite, with PGE associated with minor
sulphide mineralization, is also present in the Demars Lake intru
sion. A sample from the sulphide mineral showing south of De
mars Lake, with approximately l^/o chalcopyrite and pyrrhotite,
was analyzed and found to contain 1150 ppb palladium, 180 ppb
platinum, and 80 ppb gold. These were the highest values ob
tained for PGE. by the field party, from samples o Jtside of the Lac
des Iles Complex.
In summary, PGE mineralization occurs at several fractionation
stages of the Lac des Iles mafic magmas. Key geological features
in recognizing potential PGE mineralization include: a) the iden
tification of zones oi magma mixing, b) the presence of coarsegrained or pegmatitic rocks; and c) the presence of magmatic
sulphide mineralization In ultramafic intrusions, websterite and
gabbronorite are the phases which are commonly mineralized.
An important consequence of this study relative to mineraliza
tion associated with mafic intrusions is the observation thai many
hornblende-rich lithologies refleci contamination of mafic magma
by granitoid components. Recognition of hornblendite phases im
plies a contribution of mantle-derived mafic magma
Although past and present exploration activity in the area has
focused on PGE and copper-nickel mineralization associated with
mafic intrusions, the present study suggests thai geological envi
ronments favourable for gold mineralization may be present in the
area. In this regard, the breccia zone in hornblende tonalite and
the altered, deformed rocks associated with zones of faulting may
warrant investigation. Smith and Sutcliffe (1987) reported shearing
and quartz veining in altered ga&broic rocks of the Tib Gabbro
along the Dog River. No anomalous concentrations of gold were
found in samples collected by the field party at these locations.
The Wabigoon-Quetico boundary zone in the southwestern
part of the area lies along strike from the Geraioton-Beardmore
area and also has potential for gold mineralization.
REFERENCES
Cabri, L.J., and Laflamme, J.H.G.
1979: Mineralogy of Samples from the Lac des Iles Area, Ontario;
CANMET, Energy. Mines and Resources, Canada. Report
79-27, 20p.
Dunning. G.R.
1979: The Geology and Platinum-Group Mineralization ol the Roby
Zone, Lac Des Iles Complex, Northwestern Ontario: Un
published M.Sc. Thesis. Carleton University, Ottawa, On
tario, 129p.
Guarnera. B.J.
1967: Geology of the McKernan Lake Phase of the Lac Des Iles
Intrusive, Thunder Bay District, Ontario; Unpublished M.Sc.
Thesis. Michigan Technical University, Houghton, Michigan.
Kaye, L.
1966: Tib-Jack Lakes Area District of Thunder Bay: Ontario De
partment of Mines, Preliminary Geological Map P.380, scale
1:31 680 or 1 inch to 1/2 mile. Geology 1956.
ABBREVIATIONS
1969: Geology of the Eayrs Lake-Starnes Lake Area, District of
Thunder Bay; Ontario Department of Mines, Geological Re
port 77, 29p. Accompanied by Map 2172. scale 1:63 360 or
1 inch to 1 mile
Milne. V.G.
1954: Geology of the Garden Lake Area, District of Thunder Bay;
Ontario Department of Mines, Geological Report 25, 21 p.
Accompanied by Map 2058, scale 1:63 360 or l inch lo T
mile.
Au....................................................................... Gold
cr,....,,..,.,..,.,,...,........,,,..,,.......,.,..,. Chromite
cp ............................,.,.....,,......,,.......,..,. Chalcopyrite
Cu..,.,.....,,..,.,,..............,,.......,.,..,.,.,.., Copper
Ni......,.,.,..,.,,,.,....................,.,...........,.,., Nickel
Pd ...............,............................,....,..,,,.,., Palladium
po ,.,.......,.,.,..........,.........,..,....,.,.,...., Pyrrhotite
Pt,.........,,..,.,..,......,...................,.,.,.,.,.. Platinum
py ........,....,,,.,,,..,.,,,,,,.,,..,,.,.,.,,.,,,... Pyrite
ODM-GSC
1962: Nipigon. Ontario; Ontario Department of Mines, Geological
Survey of Canada, Map 7103G. Aeromagnetic Survey Se
ries, scale 1:253 440 or 1 inch to 4 miles.
Pye, E.G.
1968: Geology of the Lac des Iles Area, District of Thunder Bay;
Ontario Department of Mines. Geological Report 64. 47p.
Accompanied by Maps 2135 and 2136. scale 1:31 680 or 1
inch to 1/2 mile.
Rice. A.
1985: The Mechanism of the Mt. St. Helens Eruption and Specula
tions Regarding Sorel Effects in Planetary Dynamics; Geo
physical Surveys. Volume 7. p.303-384.
Sage. R.P.. Breaks, F.W., Stott. G., MacWilliams, G., and Bowen, R.P,
1974: Operation Ignace-Armstrong. Pashkokogan-Caribou Lakes
Sheet, Districl of Thunder Bay: Ontario Division of Mines.
Preliminary Map P.962. Geological Series, scale 1:126720
or 1 inch to 2 miles Geology 1973.
Smith, A.R., and Sutcliffe, R.H.
19B7: Precambrian Geology of the Tib Gabbro, District of Thunder
Bay; Ontario Geological Survey, Map P.3094, Geological
Series-Preliminary Map, scale 1:15840 or 1 inch to 1/4
mile. Geology 1986.
Streckeisen, A.
1976: To Each Plutonic Rock its Proper Name; Earth-Science Re
views, Volume 12, p. 1-33.
Sutcliffe, R.H., and Smith, A.R.
1985: Precambrian Geology of the Mulcahy Gabbro, District of
Kenora; Ontario Geological Survey. Map P.2826, Geological
Series-Preliminary Map. scale 1:15840 or 1 inch to 1/4
mile. Geology 1984.
Sutcliffe, R.H., and Sweeny, J.M.
1986; Precambrian Geology of the Lac des lies Complex, Districl
of Thunder Bay; Ontario Geological Survey, Map P.3047.
Geological Series-Preliminary Map, scale 1:15 840 or 1 inch
to 1/4 mile. Geology 1985.
Talkington, R.W.', and Watkinson, D.H.
1984: Trends in the Distribution of the Precious Metals in the Lac
Des Iles Complex, Northwestern Ontario: Canadian Mineral
ogist, Volume 22, p. 125-136.
Watkinson, D.H., and Dunning, G.
1979: Geology and Platinum-Group Mineralization, Lac Des lies
Complex, Northwestern Ontario; Canadian Mineralogist, Vol
ume 17, p.453-462.
SYMBOLS
PROPERTIES*
1.
2
3.
4.
5.
6
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
American Platinum Incorporated
Belisle, O.L
Cream Silver Mines Limited
Culbert. R
Heenan Senlac Resources Limited
Imperial Platinum Corporation
International Platinum Corporation
Linkewich, E.
Madeleine Mines Limited
McClelland. M.
Nabigon, A.
Platinum Exploration Canada Incorporated 1^
Salt. H.
Sanders, T.
Sawdo. R.
Schoor, M.
Sheridan. J.P.
J
2
l
2a
2b
Mafic Metavolcanics
1
1a
1b
le
id
1e
1f
1g
16, Tweedle. R.'
Glacial striae
19. Walsten, D.
20. Watts, H.
a)
Small bedrock out
crop
Area of bedrock out
crop
Geological
boundary, position
interpreted
Fault
Lineameni
Schistosity;
(inclined, vertical)
Gneissosity;
(inclined, vertical)
Banding: (inclined,
vertical)
Foliation; (horizontal,
inclined, vertical)
Lineation with
plunge
Jointing; (vertical,
inclined)
Figure 1. Property locations in the Lac des lies-Tib Lake area.
Numbers correspond to those on the list of properties. Shaded
area indicates mafic-ultramafic rocks.
Inclined dike
Inclined fault plane
b)
'
'
Information on ownership is to the best of the author's knowl
edge as of February 28. 1987.
Wholly owned subsidiary of International Platinum Corporation
(Canadian Mines Handbook, 1986-1987).
SOURCES OF INFORMATION
Unsubdivided
Rhyolitic flows
Tuff
'l
Basemap derived from Maps 52 H/4 and 52 H/5 of the National
Topographic System.
ODM-GSC Aeromagnetic Maps 2099G and 21 DOG, scale 1:50 000.
Precambrian Geology of the Tib Gabbro, Lac des Iles Area, Districl
of Thunder Bay; Ontario Geological Survey, Map P.3094, Geologi
cal Series-Preliminary Map, scale 1:15840 or 1 inch to 1/4 mile,
by A.R Smith and R.H. Sutcliffe, 1987.
Precambrian Geology of the Lac des Iles Complex. District of
Thunder Bay; Ontario Geological Survey, Map P.3047. Geological
Series-Preliminary Map, scale 1:15840 or 1 inch to 1/4 mile, by
R.H Sutcliffe and J.M. Sweeny. 1986.
Ttb-JacK Lakes Area. District of Thunder Bay: Ontario Geological
Survey, Map P.380, Preliminary Geological Map, scale 1:31 680 or
2 inches to 1 mile, by L Kaye, 1966.
Eayrs Lake-Starnes Lake Area. District of Thunder Bay: Ontario
Geological Survey. Map 2172. scale 1:63360 or 1 inch to 1 mile,
by L. Kaye, 1969.
Max Lake Sheet. Districl of Thunder Bay; Ontario Geological Sur
vey, Map 2136. scale 1:31 680 or 1 inch to 1/2 mile, by E.G. Pye,
1967.
Lac des Iles Sheet, District of Thunder Bay; Ontario Geological
Survey, Map, 2135, scale 1:31 680 or 1 inch to 1/2 mile, by E.G.
Pye, 1967.
Garden Lake Area, District of Thunder Bay: Ontario Geological
Survey. Map 2058. scale 1:63360 or 1 inch to 1 mile, by V.G.
Milne. 1964.
Geology not tied lo surveyed lines.
'
Magnetic declination approximately 0042'E in 1986.
Metric conversion factor: 1 foot = 0.3048 m.
Unsubdivided
Pillowed flow
Massive flow
Schistose flow
Amphibolite
Gneissic amphibolite
Clinopyroxene amphibolite
Clinopyroxene-orthopyroxene granulite
NOTES:
a) This is a field legend and may be changed as a result of
'
subsequent laboratory investigations.
b) Subdivision of rock units does not imply age relationships.
c) Units 5 through 10 are interpreted to be contemporaneous
and no age relationships are implied by numerical order.
d) Classification of plutonic rocks, ultramafic and gabbroic rocks
follows the I.U.G.S. Subcommission on the Systematics of
Igneous Rocks (Streckeisen 1976).
The letter C proceeding a rock unit indicates data is compiled
from previous work other than Sutcliffe and Sweeny (1986) and
Smith and Sutcliffe (1987).
The letter G preceeding a rock unit indicates Ihe unit is interpreted
from geophysical data
CREDITS
f
Geology by R.H. Sutcliffe, A.R. Smith and assistants. 1985 and
19B6.
Every possible effort has been made to ensure the accuracy of
the information presented on this map; however, the Ontario Min
istry of Northern Development and Mines does not assume any
liability for errors that may occur. Users may wish to verify critical
information; sources include both the references listed here, and
information on file at the Resident Geologist's Office and the
Mining Recorder's Office nearest the map area.
issued 1988
Information from (his publication may be quoted if credit is given.
It is recommended that reference to Ihis map be made in the
following form:
Sutcliffe, R.H., and Smith, A.R.
1988: Precambrian Geology of the Plutonic Rocks in the Lac des
lies-Tib Lake Area, District of Thunder Bay; Ontario Geologi
cal Survey, Map P.3098, Geological Series-Preliminary Map,
-scale 1: 50000. Geology 1985, 1986.