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REPORT No. 107
The Geology
of the
NEMEI LAKE AREA
(West Half)
Saskatchewan
by
R. L. JOHNSON
1966
DEPARTMENT OF MINERAL RESOURCES
Geological Sciences Branch
Precambrian Geology Division
HOM. A. C. CAMERON
J. T. CAWLEY
Minister
Deputy Minister
PROVINCE
OF
SASKATCHEWAN
J
I
CONTENTS
Page
INTRODUCTION ------------- ------------------------------------------------------------- ------
5
Location and Accessibility ----------------------------------------- --- ------------
5
Previous Work ----------------------------------------------------------------------------
5
Field Work and Acknowledgements ___ -·············· ···-· ·· ·········· ·-
5
Physiography and Glaciation ··· ······-···-·---· --·-·---···-------·----· ··-·----·
6
SUMMARY OF THE PRECAMBRIAN GEOLOGY ·····-·····-·····-·····- 6
THE KISSEYNEW GNEISSES ···-········· ········----------------·------··---·········
7
Th~ Garnet-Biotite Gneisses (4) ·-·---------·--------------···--- -· ········-·-··
7
The Biotite Gneisses (1) -·-········-··--·· ···············-·-··-··---··-------------·
The Cale-silicate and Associated Rocks (2 and 3) --··-·----··---·
9
9
THE DIAPIRIC GRANITES (5 and 6) ---·· ············ ·--·· ··-·······--·-··------ 10
STRUCTURE ·-··-------·----------- -- --·- --··········---················· ·· ······-··· ···--········ 13
DISCUSSION -----·--·····--···· ·-- ---·-····· ···············-······················-········-·--- -· 19
ECONOMIC GEOLOGY ···- ·· ······---··------·--------··----················· ··-······-· ·· 21
REFERENCES --·· ··················· -· --·- ·······································--- -·-· -- -----·---- 21
3
ILLUSTRATIONS
Geological Map 107A, Nemei Lake (West Half), Saskatchewan ... ........ in pocket
Structural Map 107B ..................................................................................... in pocket
Figure 1: Typical first folds .................................................................................... 15
Figure 2: Typical second folds ················· ········································· ·· ············· ···· ·· 16
Figure 3: Pi-diagrams for two major second folds ...................... ..................... 17
Figure 4: First fold refolded by second folds ...................................................... 18
Figure 5: Diagram illustrating relationship between metamorphism and
deformation .............................................................................................. 20
Table 1: Summary of the diapiric granites .......................................................... 11
Table 2: Summary of the structural history of the area .................................... 20
Plate 1: Refolded folds ............................................................................................
4
23
INTRODUCTION
This report describes the geology of an area of Precambrian
gneisses in Northern Saskatchewan. The rocks lie within the
Churchill provin'ce of the Canadian Shield and were subjected to
the Hudsonian orogeny circa 1700 m. y. ago (Stockwell, 1961).
LOCATION AND ACCESSIBILITY
·-
The Nemei Lake Area (West Half) (63-M-8-W) comprises about
175 square miles extending from 55° 15' to 55 ° 30' north latitude
and from 102 ° 15' to 102 ° 30' west longitude. The centre of the
area lies just over 40 miles north-northwest of Flin Flon. The
Churchill River Power Company's hydro-electric plant and townsite
at Island Falls and the adjacent Indian reservation at Sandy Bay
lie abo1:1t one-half mile beyond the northern boundary of the
map-she'et.
The larger lakes in the area are accessible by float-equipped
aircraft that may be chartered from Flin Flon or Jan Lake. A number
of canoe routes link the region to Flin Flon, the main ones being
firstly via Mason, Kipahigan and Mari Lakes and secondly via Nemei,
Kakinagimak and Wildnest Lakes. A well-used winter road links
Island Falls to Flin Flon. The area, and in particular the northern
half, will become readily accessible on the completion of the road
linking Island Falls, Sandy Bay, and Pelican Narrows to the Hanson
Lake road south of Jan Lake.
PREVIOUS WORK
Ii
The earliest published description of rocks in the area is that
of Mclnnes (1913) who in 1910 travelled along the Churchill River
and briefly described the gneisses in the northern part of the maparea. The Pelican Narrows quadrangle (63-M), which includes the
present area, was reconnoitred in 1929 and 1930 (Satterley, 1931).
A further survey of this area was carried out by F. C. Taylor in
1957 and the results were published in 1958 as (Map 1-1958) with
marginal notes on a scale of 4 miles to one inch, by the Geological
Survey of Canada. The contiguous 15 minute quadrangles to the
east, southeast, south, and southwest have been mapped on a scale
of one inch to one mile by the Saskatchewan Department of Mineral
Resources: (Pyke, 1965); (Cheesman, 1956); (Pyke, 1961) and
(Pyke, 1966).
FIELD WORK AND ACKNOWLEDGEMENTS
r,
Field work was carried out during the summer of 1965. The
shorelines of all the accessible lakes were mapped in detail and on
land, pace and compass traverses were spaced at roughly one-half
mile intervals. Data were plotted on one-half mile to one inch base
maps compiled from aerial photographs supplied by the National
Air Photographic Library, Ottawa.
The important contribution to the field work made by
R. Netolitski, Senior assistant, is gratefully acknowledged. M. W.
Pyke introduced the party to the area at the beginning of the season.
Junior assistants were D. Dixon, E. Evans, and R. Wallace. Mr. Bruce
Long, Mining Recorder in Flin Flon, and Mr. 0. C. Christensen and
his staff at Island Falls gave practical help in the course of the
field season.
5
PHYSIOGRAPHY AND GLACIATION
The area has the low relief typical of this part of the shield. In
the south the hills seldom rise more than 100 fe et above the
nearby lakes, but in the west and especially in the north, as the
Churchill River is approached, the relief is greater and hills of the
order 500 feet in height are common.
Although the area lies 200 miles south of the "approximate
southern limit of permafrost" (Glacial map of Canada, Geological
Association of Canada, 1958), permanently frozen ground was
encountered in excavations for the power plant at Island Falls
(Johnston, 1930, p. 34-35). Glacial deposits have been seen only in
the northern part of the region that lies within the limits of glacial
Lake Agassiz, an arm of which extended westward along the
Churchill River (See above-mentioned glacial map).
Fluvioglacial sands and gravels are exposed in workings for
building material near Island Falls. Varved clays are currently
exposed a little way beyond the northern boundary of the map-sheet
in the spillway of "A" dam at Island Falls. These clays, which are
at least 12 feet thick, rest directly on an ice-polished surface of
Precambrian gneisses, and at one locality where the surface is
sloping, they have an initial dip of about 15°. The varves are of
the order 0.75 mm in thickness and each varve grades upwards from
a whitish (when dry) silty base to a grey clay top.
An unusual feature of these clays is the development within
them of concretions. Typically these are lenticular in cross-section
and near circular in plan, though a few have more irregular outlines. Most of them range from two to five centimetres in diameter,
and have the thickness at the centre about one-fifth of the diameter.
They lie with their planes of symmetry parallel to the bedding.
The concretions consist of a brownish-grey porcelaneous rock that
in thin section is seen to consist almost entirely of calcite varying
in grain size from 0.025 mm down to cryptocrystalline dust. At
the base of the clays these nodules frequently come into contact
with the underlying gneiss, to which they tend to adhere after the
clay has been eroded away.
SUMMARY OF THE PRECAMBRIAN GEOLOGY
The map-area is occupied almost entirely by the Precambrian
Kisseynew complex, a group of highly deformed paragneisses that
are older than 1700 m.y. and extend over a wide area of Saskatchewan and adjacent Manitoba, north of Flin Flon (Bruce, 1930;
Harrison, 1951). Garnet-biotite gneisses underlie most of the maparea though biotite gneisses occur in the west. The hornblende
gneisses and associated amphibolites and calc-silicate rocks, that
are an important feature of the region to the south, extend into only
the southern part of the present area.
The biotite and garnet-biotite gneisses represent fine-grained,
predominantly sandy sediments that have been highly deformed
and have reached the sillimanite grade of regional metamorphism,
with the development of veins of granite and granite pegmatite.
In the course of the metamorphism the rocks were deformed twice:
firstly into tight, isoclinal folds; secondly, refolded into more open
structures. The folds that are readily identifiable on aerial photographs belong to the second set.
6
The emplacement of the large bodies of gneissic granite that
are a prominent feature of the region, was associated with the
second phase of folding.
Except where otherwise stated, terminology in this account
follows the usage of the Geological Survey of Canada (Harker,
1965). An exception is the term gneissosity, which, following Moorhouse (1959, p. 409), and in conformity with international usage, is
omitted from this report. The term foliation is restricted to textures
due to the dimensional or lattice orientation of minerals in igneous
or metamorphic rocks.
The term drag-fold, although widely used in this part of the
shield (see for instance Cheesman (1956), Byers and Dahlstrom
(1954), or Pyke (1961) is not employed in this account. "Drag-fold"
as originally defined by Leith (1914, p. 106) is a genetic term with
a specific meaning: "Minor folds are commonly developed in weak
beds by the shearing between two or more competent masses of
rock. These folds are conveniently designated drag-folds. The
position of their axial planes is controlled by the displacement of
the more competent beds adjacent. The term "drag-fold" is -desirable
as emphasizing the differential movement between the controlling
beds. More. recently the designation "drag-fold" has become
synonymous with "asymmetric minor fold" or even "minor fold",
regardless of the mechanism by which the folds have been formed.
There is a strong case for restricting the term "drag-fold" to folds
which have been produced or at least modified (Ramberg, 1963)
by the mechanism proposed by Leith. None of the small-scale folds
examined in the present area are of this type.
In the writer's view the indiscriminate use of the term is
positively harmful in that it tends to obscure the fact that in a
given area in ah orogenic belt, sets of small-scale folds may be of
different ages, and may have been produced by a variety of
mechanisms.
THE KISSEYNEW GNEISSES
THE GARNET-BIOTlTE GNEISSES (4)
I
This group of rocks occupies about two-thirds of the map-area.
The gr\_eisses are essentially psammitic and semipelitic schists that
have been isoclinally folded, metamorphosed to the silliinanite grade,
and in which generally lit-par-lit granitic veins have developed.
There is usually a clear distinction between those parts of the rock
which represent a metamorphosed sediment and the granitic material. In the more psammitic horizons the sedimentary component is
surprisingly fine grained in view of the high metamorphic grade.
Sillimanite occurs over the whole of the map-area, as pale-green
or white acicular grains occurring as sheaves or thin films along
bedding planes. Cordierite is sporadically developed as pale-blue
porphyroblasts as much as 5 mm in diameter. Graphite is another
accessory mineral commonly visible in hand specimen.
Bedding is seen as thin biotite-rich films, and as alternations
of more and less pelitic rock on scales of a few centimetres. In the
latter case the boundaries between the two types are frequently
gradational, but good graded bedding has not been found. The lack
of current and graded bedding may be due to the effects of the
metamorphism and deformation, but the uniformly fine grain-size
and absence of conglomerate must be an original feature of the rock.
7
In general this rock type is poorly exposed, but the mossy ridges
that it underlies have scarp and dip slopes which can readily be
recognized on air photographs. Most of trend-lines shown on the
structural map are of this nature.
In thin section the sedimentary component of the garnet-biotite
gneiss has plagioclase and quartz as the predominant minerals.
Biotite and orthoclase are important accessories, and in some rocks
are present in excess of the quartz. Other accessory minerals include
garnet, graphite, sillimanite, apatite, and occasionally cordierite.
The grain-size of the quartz and plagioclase varies from 0.25 mm to
1 mm; the dimensional orientation of these minerals is weak. The
properties of the minerals in this group of gneisses are as follows:
'
Varies from oligoclase (An25) to andesine (An45).
The foliation is deflected around the larger plagioclase porphyroblasts.
Biotite - Ragged to subhedral tabular grains having c. pale
yellow brown and W medium reddish brown. Although most of the
grains are oriented parallel to one another, defining the foliation,
some grains are oriented at high angles to the foliation and are
undeformed. Probably nucleation took place during the first phase
of deformation, but growth continued later.
Plagioclase -
Garnet - In metamorphic rocks the shape and orientation of
inclusion trails in porphyroblasts, and in particular in garnets, provides important evidence concerning the relationship between the
thermal and kinematic history of the rock (see for instance Zwart
(1962) and Rast (1965) ). In the present case the smaller ( < 0.25 mm)
garnets are generally free from inclusions except where they ophitically enclose biotite laths. The larger garnets, which may be as much
as 8 mm in diameter, frequently contain quartz inclusions. Tn,ese
increase in size from the centres outwards though there is usually
an inclusion-free zone at the margin. In one or two cases inclusions
have a parallel dimensional orientation, forming inclusion trails
which are straight, but oriented at a high angle to the foliation outside. Clearly the growth of the garnets postdated the initiation of
the foliation, but since the inclusion trails are straight the growth
must have taken place under static conditions. The rotation which
is indicated by the angle between the inclusion trail and the foliation
indicates that the garnet was rotated after growth had ceased, and
possibly before or during the growth of the outer clear rim. The
significance of these observations will be discussed in a later section.
Orthoclase - Where much of this mineral is present it occurs
as large irregular poikiloblastic grains.
Cordierite occurs as large anhedral grains larger than or comparable in size with the plagioclase. Polysynthetic rather than interpenetration twins are developed, but the mineral can readily be
distinguished from the plagioclase by its incipient alteration to a
yellow isotropic substance.
Graphite occurs as rather irregular shaped tabular grains which
frequently bifurcate or are bent. In places they are intergrown with
biotite.
Sillimanite forms large sheaves of acicular grains, and also
occurs as groups of parallel noncontiguous needles in the central
parts of cordierite and orthoclase crystals.
8
The granitic component of the garnet-biotite gneiss can be subdivided into two groups which differ in age and composition.
The early foliated quartz-pla,gioclase-biotite pegmatites are veins
and stringers of pegmatite developed parallel to the bedding; they
were folded during the first phase of deformation. The veins vary
in size and in continuity. They range from lines of isolated porphyroblasts of plagioclase through lines of lenticular granite patches to
continuous veins, commonly several centimetres in thickness. These
pegmatites tend to be developed in more pelitic horizons; the matrix
immediately outside them is usually more biotite-rich than elsewhere. The plagioclase is oligoclase or andesine, and is similar in
composition to that in the enclosing rock. Where the enclosing rocks
are graphitic, large flakes of graphite are commonly developed in
the pegmatites.
The foliation due to the parallel orientation of tabular biotite
flakes is normally parallel to the bedding, but at first-fold hinges,
the foliation can be clearly seen to cut across the bedding and be
orie11ted parallel to the axial planes of the folds.
The late unfoliated microcline-bearing pegmatites can be seen
cutting first folds and clearly post-date the first phase of deformation. Their relationship to the .s econd phase is uncertain. Large sills
are developed in places, particularly in the extreme north of the area,
and are well exposed beyond the northern edge of the map-area in
the Churchill River near Island Falls.
These bodies have sharp but irregular contacts with the enclosing rocks, and in places idioblastic "dents de cheval" of microcline
as much as 4 cm across can be seen growing in the enclosing rock.
A notable feature is the irregularity in the grain-size from place to
place. In places the rock is medium grained, in others coarse grained,
and pegmatitic patches of irregular shape and various sizes are
developed. Films of biotite and larger inclusions of psammitic and
pelitic schist are oriented roughly parallel to the foliation outside.
THE BIOTITE GNEISSES (1)
These rocks differ from the garnet-biotite gneiss in minor
respects only: garnet is usually absent; the amount of biotite present
is slightly less; and magnetite is frequently present along bedding
plane~. The lower content of biotite is reflected in the generally
more massive, often granulitic character (Pyke, 1965 p. 9) and rather
paler colour of the rock. In spite of the similarity between the two
types little difficulty was found in separating the two groups in
the field.
THE CALC-SILICATE AND ASSOCIATED ROCKS (2 AND 3)
This group comprises a series of compositionally layered calcsilicate rocks and associated, often interbedded amphibolites. While
at some localities the amphibolites occur as units ten feet or more in
thickness, at others the amphibolite is interbedded with calc-silicate
rocks on scales of a centimetre or less. In view of this intimate association, and considering the presence of garnet in most of the
amphjpolite there seems little doubt that the amphibolites represent
sedimentary rather than igneous rocks. The calc-silicate rocks
proper, and the associated amphibolites are included in one unit on
the map.
Differential weathering of the layered formation produces the
slabby outcrop which characterizes this group in the field. In the
9
coarser bands diopside grains can be clearly distinguished in the grey
feldspar or scapolite matrix.
In addition to the above-described mappable formation calcsilicate rocks occur within the garnet-biotite gneisses as isolated
lenticular-shaped nodules.
Thin section study confirms that the calc-silicate formation
consists largely of diopside-plagioclase rocks though a wide variety
of accessory minerals are also present. Scapolite and calcite which
are normally present as accessories may in certain bands make up
more than 50 per cent of the rock. The planar structure in this
formation is largely a compositional effect, little lattice or dimensional orientation of the mineral grains is in evidence.
The diopside occurs as subhedral to anhedral grains, whic;h are
very pale green in thin section. They have rounded outlines where
enclosed in calcit~. The extinction angle Z /\ C varies from 30· -42 °.
0
In rocks. that contain a considerable amount of scapolite two
generations of plagioclase can be recognized. The first generation
occurs as a mosaic of twinned grains which are of the order of
0.5 mm - 1 mm in diameter and vary in composition from andesine
with An45 to labradorite with Ano5. The grains are free of inclusions
and though anhedral have fairly regular interfaces with each other
and with the scapolite. The scapolite generally forms polygonal
grains which have straight mutual interfaces and near 120° triple
junctions. They are generally larger than the above-described plagioclase crystals.
The second generation of plagioclase occurs as mosaics of small
(about 0.25 mm diameter) irregular shaped untwinned grains. Within
the mosaics are acicular inclusions of calcite which cross plagioclase
grain boundaries. The mosaics form tongues and lobes which project
into the scapolite grains. In these areas the calcite needles tend to
occur at right angles to the advancing "fronts" of scapolite. The
above-described textural relationships indicate that at an earlier
stage in the metamorphism of the rock both scapolite and plagi6clase
were stable. Later, however, at least the meionite (calcium-rich)
component of the scapolite has become unstable and decomposed
with the production of anorthite and calcite according to the
reaction:
Ca4AloSio024COa _ _ _,... 3CaAhSi20s + CaCOa
Accessory minerals in the calc-silicate rocks include sphene,
actinolite, pyrrhotite, apatite, calcite, quartz, and clinozoisite. The
last three minerals are characteristic of carbonate-rich horizons.
Sphene makes up 5-10 per cent of the rock in some bands. The
amphibolite horizons consist of approximately equal amounts of
hornblende and labradorite. The hornblende is pleochroic with
X-pale olive green; Y-medium green brown, Z-dark olive green and
has the extinction angle Z /\ C circa 20 °. Accessory minerals include
quartz, phlogopite, pyrrhotite, and garnet.
THE DIAPIRIC GRANITES
The diapiric granites are sharply bounded, large bodies of
gneissic granite that deflect the bedding and foliation in the enclosing paragneisses. The diapiric granites have been named for
descriptive convenience after the nearest large lake and their names
10
are shown on the structural map. A number of distinct textural and
compositional types of gneissic granite are developed. The main
features of the various granites are summarized in Table 1. The
north-northwest trending elongate outcrop of granite situated immediately north of the Wuskwi8tic Granite resembles the Oskatim
Granite in texture and composition. It is, however, poorly exposed
and will not be discussed further.
The Wuskwiatic Granite is more resistant to denudation than the
enclosing rocks and tends to outcrop on low rounded hills. The rock,
which is grey in colour, shows no compositional layering, but nevertheless has a distinctive texture. The biotite, which is black in hand
specimen, is segregated into clots, lenticular in cross:section, and
having axes of the order of three, five, and fifteen millimetres in
length. These clots are oriented parallel to one another and define
both the foliation and the lineation. Towards the margins of the
body the foliation is more conspicuous than the lineation and tends
to lie parallel to the outer contact of the granite. Towards the centre,
however, the foliation is less conspicuous and in many places only
a lineation can be seen. These relationships clearly indicate that the
orientation of the internal structures of the granite is due to flow
on emplacement, rather than to the direct effect of tectonic stress
(Balk 1937).
·:
TABLE 1
Summary of the location and composition of the diapiric gneissic granites
NAME OF DIAPIR
LOCATION
ROCK TYPES
Wuskwiatic
Granite
Immediately west of
centre of map-sheet
Coarse-grained homogenous
xenolithic granodioritic
gneiss (6)
N~mei
Granite
Just east of centre
of map-sheet
Mainly as above, but mediumgrained and leucocratic
gneisses also developed,
especially on the southern
flank (6 and 6a)
Oskatim
Granite
Southwest of the
Wuskwiatic
Granite
Very distinctive cataclastic
texture, varies in composition
from granodiorite to
adamellite (6)
McArthur
Granite
Extreme southwest
corner of map-sheet
Two types developed; a fairly
homogenous biotite gneiss (6);
and a more variable biotite
gneiss with included
hornblende-bearing rocks (5).
Xenoliths are common and usually consist of a biotite-rich rock
which often contains much magnetite. They are finer grained than
the enclosing granodiorite and tend to be flattened in the plane of
the foliation and elongated in the lineation direction .
. In thin section the granodioritic gneiss is seen to be composed
largely of quartz and oligoclase-andesine (Anao) with potassium
feldspar and biotite the most important accessory minerals. Minor
accessories include hornblende, apatite, zircon, and magnetite. A
curious feature of the plagioclase is the complexity of the twinning;
11
''
.,
)
1
grains showing albite-law twins alone are in a minority. The biotites
occur as rather ragged grains which have only a moderately good
preferred orientation. They are pleochroic with ' £ -pale brown
W -dark brown to black. The hornblende forms very irregular grains
up to 2 mm in length, that in some cases ophitically enclose biotite
and plagioclase crystals. They are pleochroic with X= pale yellow,
Y= dark green brown and Z= medium green. The extinction angle
Z /\ C =20 °. Potash feldspar occurs as antiperthitic intergrowths
with the plagioclase and also as independent grains.
The N emei Granite is the easternmost of the diapiric granites
in the map-area. It underlies a group of low hills, but nevertheless
the quality of the exposure is poor. Two lithological types are distinguished on the map. In the north a coarse granodioritic gneiss is
exposed, which in texture and min~ralogical composition closely
resembles the Wuskwiatic Granite described above. In th e Nemei
Granite the segregation of biotite into clots is even more marked
than at Wuskwiatic Lake. Near the northern margin of the granite
there are isolated outcrops of a coarse-grained rock composed of
roughly equal amounts of quartz and plagioclase with coarse irregular shaped grains of magnetite as an important accessory. The
composition of this rock is not that of a normal igneous rock. It
could, however, represent a feldspathized iron-bearing sedimentary
rock.
The southern part of the Nemei Granite (6a) is a medium- to
coarse-grained granodiorite in which biotite, the chief accessory
mineral, is not segregated into clots but is distributed evenly
throughout the rock. Leucocratic types are also developed in this
area.
A characteristic feature of the southern part of the Nemei
Granite is the wide range of grain size evident in a single thin
section. ~he larger quartz and feldspar grains, as much as 2 mm
across, are enclosed by rims composing mosaics of quartz and
feldspar. Clearly, recrystallization has taken place.
The dimensional orientation of both the quartz and the biotite
is rather weak. The later mineral is pleochroic with £ pale brown,
rn dark brown to black. Microcline is ubiquitous as a minor accessory, but is present in larger amounts in the more leucocratic types.
Small amounts of magnetite and apatite are also present.
The Oskatim Granitic Gneiss has a distinctive texture and differs
from the other granites in the area in containing considerable though
variable amounts of potassium feldspar. The rock is pink in colour
and has a pronounced foliation. In the most characteristic type, lenticular fragments of pink medium-grained leucocratic gneissic granite,
of the order of 5 mm in thickness and several centimetres in diameter, are separated by a fine-grained, more biotite-rich matrix. In
places the biotite is segregated into thin continuous laminae along
which the rock is fissile. Some layers in the rock contain hor~blende.
The strong foliation, and the fact that all the early foliat~d pegmatites have been rotated into the foliation plane indicate that this
rock has undergone intense deformation.
The most important minerals in the rock are quartz, oligoclase
(An23), and microcline. Biotite and hornblende are important accessory minerals in some rocks. Minor amounts of magnetite, apatite,
and red garnet (almandine?) are also present. The rock varies
12
,,
in composition from a granodiorite to a quartz monzonite
(= adamellite). There is little evidence of cataclasis in thin section;
clearly, recrystallization took place contemporaneously with or followed the deformation. The quartz and feldspar grains, which
average about 0.5 mm in diameter in the matrix and 1 mm - 2 mm in
the fragments, show little dimensional orientation. The microcline
is locally replaced by myrmekite. The properties of the biotite and
hornblende in this rock are similar to those in the Wuskwiatic gneiss
described above.
The McArthur Granitic Gneiss has been subdivided into two
groups; a smaller area of more homogenous biotite gneiss is well
exposed along the northern margin, while a larger area of migmatitic
gneiss outcrops extensively on the shores of McArthur and
Robbestad Lakes. Both these types are granodioritic in composition.
The McArthur gneiss extends southwards into the area mapped by
Pyke (1961), where the relative proportions of the two types are
reversed, and in the granitic body, as a whole, the migmatitic gneiss
type is subordinate.
The more homogenous type is a buff-coloured medium-grained
rock, which commonly shows a rather diffuse compositional layering
on a scale of less than a centimetre, due to variation in the amount
of biotite in the rock. In thin section the rock can be seen to consist
largely of quartz and albite-oligoclase (An3o) with biotite and potassium feldspar as the chief accessory minerals. Minor accessories are
apatite, zircon, muscovite, and magnetite. Neither the quartz nor
the feldspar grains show much sign of dimensional orientation. The
quartz grains are polygonised (Voll 1960 p. 511). Perthite and antiperthite are rare. The biotite has the same pleochroism as that in
the Wuswiatic Granite and in some specimens examined it is partly
replaced by chlorite. A few small ragged hornblende grains have
X = pale yellow; Y and Z dark green and the extinction angle
z I\ c =20 °.
The migmatitic subdivision of this granitic body is essentially
a biotite gneiss, with conspicuous biotite-rich layers. In addition,
however, the rock contains inclusions of amphibolite which may be
continuous )ayers several centimetres thick, or they may be broken
up into sharply bounded lenses or more irregular schlieren. Less
commonly, larger bodies of amphibolite have lit-par-lit granitic
veins developed within them. Where hornblende is developed with
or in place of the biotite in the enclosing gneiss, amphibolite inclusions are also present. The impression was gained that biotite granite
has become contaminated by hornblendic material.
Apart from the local abundance of hornblende, and also of
apatite and sphene, the migmatitic granites are similar in mineralogy
to the more homogenous types.
STRUCTURE
Two approaches can be made to a study of the structure of an
orogenically deformed area. The classical approach involves mapping the various lithologies, determining "the way up" of the succession and then constructing a series of profiles by "looking down"
the plunge of the folds (Wegmann 1929, Stockwell, 1951). In areas
where the stratigraphy can be established or at least marker
horizons mapped, this is an extremely valuable technique, provided
13
one is dealing with a set of homoaxial folds with a reasonably constant direction and amount of plunge. This proviso is an important
one.
It has for many years been realized that the small-scale structures such as cleavage, small-scale folds and lineations are related in
attitude and style to the large-scale structures (Leith 1914). More
recently, however, it has become apparent that in many orogenic
belts successive sets of small-scale structures are related to successive sets of large-scale structures and that structures of different
age may be oriented in various directions (Berthelsen 1960, Ramsay
1962). In these areas early folds became refolded by later structures,
producing complex outcrop patterns, and the attitudes of the fold
hinges vary considerably from place to place. In these circumstances
it is difficult or impossible to draw structural profiles. The modern
techniques of structural geology are concerned to a considerable
extent with unravelling complex structures produced by successive
phases of deformation making full use of evidence from the superimposition of small-scale structures, (Clifford, et al 1957, Sutton,
1960, Ramsay in press). The latter paper provides a valuable summary of the techniques of structural analysis and their application
to an area of economic importance.
In considering the structure of the present area, the following
limitations must be borne in mind:
a. The quality of the outcrops is in general poor. Most of the
small-scale data have been obtained from a very limited number of
clean exposures situated along the power-line and on the shores of
some of the larger lakes;
b. in the absence of good stratigraphical evidence "trend-lines"
taken from aerial-photographs have been used in compiling the
accompanying structural map. Although the validity of individual
lines drawn on this map may be questioned, the resulting pattern
clearly indicates the position, extent, orientation, and style of some
of the larger folds in the area.
'
Examination of the small-scale structures in the area indicate
that two distinct types of fold of different age are developed. In the
first folds, bedding and pegmatite veins are deformed into tight
isoclinal structures with axial planes parallel to the main metamorphic foliation in the rock. Examples of these first folds are illustrated in Figure 1. On the limbs of the folds the bedding and foliation
are parallel, but on the fold hinges the foliation, represented by the
parallel orientations of biotite flakes, can be clearly seen to cut the
bedding and the folded veins. Clearly, the foliation developed during
the first phase of folding. The mineral orientation lineation in the
paragneisses is parallel to the hinges of these folds and both these
structures plunge to the north over the whole map-area. Large-scale·
first folds can be tentatively identified only in the southeast quadrant
of the map-area; (see structural map 107B). These fold-traces are
drawn solely on evidence from the shape of the contact between the
biotite gneiss and the garnet-biotite gneiss on the map. Since the
first folds are isoclinal, the trend of their axial traces is given by the
trend of the foliation and bedding. In those parts of the map-area
where major second folds are not apparent these structures run in
a northwest-southeast direction. The large-scale first folds may be
expected to plunge to the north parallel to the small-scale folds of
the same age.
14
SCALE IN FEET
0
SCALE IN FEET
0
Figure I: First folds outlined by pegmatites (stippled). Note that the foliation
(dashed) is parallel to the axial planes of the fold.
15
SCALE IN FEET
0
Figure 2: Second folds outlined by pegmatites in upper diagram and by bedding (which is parallel to the foliation) in the lower. Note that in
both cases the foliation (indicated by dashes) is folded.
16
N
N
~
axis
Figure 3: Pi-diagrams of poles to bedding in the major second synforms. The
areas covered by these diagrams are marked on the structural map.
The upper diagram refers to the fold lying between the Wuskwiatic
and Nemei Granites, and the lower one to the fold immediately east
of the McArthur Granite. Both diagrams are plotted on the lower
hemisphere of a Schmidt net.
17
0
SCALE IN FEET
- ----AXIAL TRACES OF FIRST FOLDS
---AXIAL TRACES OF SECOND FOLDS
0
SCALE IN FEET
Figure 4: Refolded folds. The upper example is from the Wet claims which are
located about 2 miles east of the eastern boundary of the Area at
latitude 55°26'. The lower example is situated near the dock at
Island Falls, just beyond the northern margin of the map-area.
18
•
The second folds are characterized by the fact that in addition
to the bedding and the quartz veins, the metamorphic foliation is
folded. They thus post-date the development of the foliation and it
follows, therefore, that they post-date the above-described set of
folds . In general the second folds are more open structures than the
first, though the style varies from place to place and depends on
lithology. In Figure 2, in which small-scale folds are illustrated, the
lower diagram shows more open folds developed in the massive
homogenous b.iotite-gneiss, whereas the upper diagram shows
tighter folds developed in "injected" garnet-biotite gneiss in which
more and less competent bands alternate. There is no visible second
schistosity parallel to the axial plane of these folds . Large-scale
second folds with north-northwest trending axial traces are readily
recognizable on the aerial-photographs (see structural map). One
pair of such structures comprises the synform and complementary
antiform lying immediately to the east of the McArthur Granite.
These folds, like the small-scale structures of the same age deflect
both the bedding and the foliation and are fairly open folds.
The direction and angle of plunge of the above mentioned
synforms have been calculated by plotting poles to the foliation on
diagrams (Sander, 1948) (Fig. (3) ). An outline of this simple method
of determining the style and attitude of folds is given by Byers and
Dahlstrom (1954) pp. 163-169. From the diagrams it can be seen that
both these folds plunge to the north-northeast at shallow angles.
No refolded folds have been seen in the map-area, although
from the above evidence they must exist on both large and small
scales. Small-scale refolded folds have been observed at two localities a little way beyond the boundaries of the present area, (Fig. 4,
Plate 1). In ·e ach case, as might be expected, early tight isoclinal
structures are refolded by later open folds.
Although the north-northwest trending fault marked on the
map is not exposed, and does not offset any beds, its existence and
position can be inferred with a considerable degree of confidence
from the marked topographical lineament along which it is drawn.
It probably belongs to the extensive set of north-northwest trending
sinistral wrench faults, predominantly of Late Precambrian age that
are present in this part of the Shield (Byers, 1962).
DISCUSSION
One of the most striking features of the area is that the suite
of concordant granite and granitic pegmatite veins were folded
during the first phase of deformation. Regional metamorphism must,
therefore, have reached a high grade before deformation began.
Evidence from the inclusion trails in garnets, porphyroblasts, summarized in a previous section indicates that garnets overgrew a
pre-existing foliation under static conditions and were subsequently
rotated. Since the development of the foliation was associated with
the first phase of deformation this would imply that (a) most of
the garnet growth took place between the two phases of deformation
and (b) these two phases were separated by a static interval. The
relationship between the structural and metamorphic history of the
area is summarized in Table 2 and Figure (5).
19
TABLE 2
History of the Hudsonian orogenic cycle in the Nemei Lake area
l. Deposition of Ki ssey new sediments.
(On a basem ent of migm at itic g neiss?)
2. Development of oligoclase-an·desine pegmatites.
3. First phase of deformation ; development of isoclinal fo lds and the regional
foliation.
4. Static interval ; growth of garnets
Emplacement of
5. Second phase of deformation prod ucing
mi crocline-bearing
open folds, and rotating ga rnets;
pegmatites
emplacement of diapiric g ranites.
}
Evidence on the origin and emplacement of the granites is
inconclusive, although a number of interesting . possibilities have
emerged. It is difficult to visualize how the intense migmatization
and granitization, which was involved in the development of the
McArthur gneiss, took place without producing locally more intense
migmatization in the immediately enclosing paragneisses. There is
a strong possiblity that the McArthur granitic gneiss may represent
the basement on which the Kisseynew sediments were deposited. On
this hypothesis the McArthur Gneiss is a mantled gneiss dome
(Eskola, 1948) produced by the mobilization of the (Archaean?)
hasement during the Hudsonian orogeny .
...
Q
:::,
1-
z
I
(l)
<[
I
I
:IE
I
__ .,.,
I
/
, ...
\
I
I
I
\I
', .... _
TIME
Figure 5: Diagrammatic representation of the relationship between metamorphism (solid line) and deformation (dashed line) in the area.
Another curious feature of the diapiric granites is the marked
compositional and textural contrast between the Wuskwiatic and
Oskatim Granites, in spite of their close proximity to one another.
The strong planar structure in the Oskatim granite and the elongate
shape of the body suggests that its ·emplacement may have taken
place, or at least been initiated, during the first phase of deformation.
This hypothesis is supported by the arcuate outline of the body,
which suggests that it was deflected when the Wuskwiatic granitic
gneiss was emplaced.
The relationship between the second phase of folding and the
granite emplacement is problematical. The synform between the
Wuskwiatic and Nemei Granites would appear to be a direct result
of the emplacement of the granite, similarly the synform east of the
McArthur gneiss body has the general form of a rim syncline due
20
,.
f
to the local depression of the enclosing rocks as the granite moved
upwards (Johnson, in press). In the region to the south (Pyke, 1961),
however, northerly trending folds that deflect the foliation and are
similar in size to the second folds in the present area, do not appear
to be due to the emplacement of granite. A possible solution to this
problem is that in the present area embryo second folds localized
the emplacement of granites, but the subsequent development of
these structures was controlled by the rise of the granites.
ECONOMIC GEOLOGY
There is little surface evidence for the presence of economic
mineral deposits, apart from sand and gravel that have been exploited for construction work at Island Falls. Sulphide mineralization is
confined to the sporadic occurrence of pyrrhotite, mainly in the
calc-silicate formations. Green copper carbonate stains were noted
at the contact between meta-hornblendite and a siliceous gneiss
about one mile northeast of Lake Oskatim.
REFERENCES:
Balk, R. (1937): Structural Behaviour of Igneous Rocks; Geo!. Soc. Amer.,
Mem. 5, 177 pp.
Berthelsen, A. (1960): Geology of Tovqussap Nuna. Meddelser om Gronland,
123 pp. 1-226.
Bruce, E. L. (1930): The Kisseynew Gneiss of northern Manitoba and Similar
Gneisses Occurring in northern Saskatchewan; Proc. and
Trans, Roy. Soc. Canada 24, Sec. IV pp. 119-132.
Byers, A. R. and Dahlstrom, C. D. A. (1954): Geology and Mineral Deposits of
the Amisk-Wildnest Lakes Area Saskatchewan; Sask. Dept.
Mineral Res. Rept. No. 14.
Byers, A. R. (1962) : Major Faults in the Western Part of the Canadian Shield
with Special Reference to Saskatchewan. In J . S. Stevenson
(ed.), The Tectonics of the Canadian Shield: Roy. Soc. Can.,
Spec. Pub. No. 4, University ot' Toronto Press.
Cheesman, R. L. (1956): The Geology of the Mari Lake Area, Northern Saskatchewan; Sask. Dept. Mineral Res. Rept. No. 23.
Clifford, P., Fleuty, M., Ramsay, J . G., Sutton, J. and Watson, J . (1957): The
Development of Lineation in Complex Fold Systems; Geo!.
Mag. 94, pp. 1-25.
Eskola, P. E. (1948): The Problem of Mantled Gneiss Domes; Quart. J. Geo!. Soc.
Lond. 104, pp. 461-76.
Harker, P. (1965) (Editor): Guide for the Preparation of Geological Maps and
Reports; Geo!. Surv. Canada.
Harrison, J. M. (1951): Precambrian Correlation and Nomenclature, and Problems of the Kisseynew Gneisses, in Manitoba; Geo!. Surv. Can.
Bull. 20, 53 pp.
Johnson, R. L. (in press): The Western Front of the Mozambique Belt in N. E.
Southern Rhodesia.
Johnston, W. A. (1930): Frozen Ground in the Glaciated Parts of Northern
Canada; Proc. and Trans. Roy. Soc. Canada 24, Section IV,
p. 31-40.
Leith, C. K. (1914): Structural Geology. New York.
Mcinnes, W. (1913): Basins of the Nelson and Churchill Rivers; Geo!. Surv.
Canada, Memoir 30.
Moorhouse, W. W. (1959): The Study of Rocks in Thin Section; Harper Brothers,
New York.
21
Pyke, M. W . (1961): The Geology of the Attitti Lake Area (West Half) Saskatchewan, Sask. Dept. Mineral Res., Rept. No. 54.
Pyke, M. W. (1965) : The Geology of the Nemei Lake Area (East Half) Saskatchewan; Sask. Dept. Mineral Res., Rept. No. 97.
Pyke, M. W . (1966): The Geology of the Pelican Narrows-Birch Portage Area;
Sask. Dept. Mi.neral Res., Rept. No. 93 ,
Ramberg, H. (1963): Evolution of Drag-folds ; Geo!. Mag. 100, pp. 97-105.
Ramsay, J. G. (1962): Interference Patterns Produced by the Superposition of
Folds of Similar Type; Jour. Geo!. 70, pp. 466-481.
Ramsay, J. G. (in press): Structural Investigations in the Barberton Mountain
Land, Eastern Transvaal; Trans. Geo!. Soc. S. Africa.
Rast, N. (1965) : Nucleation and Growth of Metamorphic Minerals . In "Controls
of Metamorphism" (Ed. W. S. Pitcher and G. W . Flinn, Oliver
and Boyd, Edinburgh), pp. 73-102.
Sander, B. (1948): Einfuhrung in die Gefugekunde der Geologischen Korper,
Vienna and Innsbruck.
Satterly, J . (1931): Pelican Narrows Area, Saskatchewan; Geo!. Surv., Canada,
Summ. Rept. 1931 Pt. C.
Schreyer, W . and Schairer, J. F. (1961): Composition and Structural States of
Anhydrous Mg - Cordierites; Jour. Petrology, 3 p. 324.
Stockwell, C. H. (1951): The Use of Plunge in the Construction of Cross-sections
of Folds; Proc. Geo!. Assoc. Canada 3, pp. 1-25.
Stockwell, C. H. (1961): Structural Provinces, Orogenies and Time Classification
of Rocks of the Canadian Precambrian Shield: Geo!. Surv.
Canada Paper 61-17, pp. 108-11 8.
Sutton, J. (1930): Some Structural Problems in the Scottish Highlands; 21st
Int. Geo!. Congr. Pt. 19, pp. 371.
Voll, G. (1960): New York on Petrofabrics; Liverpool & Manchester Geo!. J . 2,
503-567.
Wegmann, 0 . E. (1929): Beispiele Tektonischer Analysen des Graundge-birges
in Finnland; Bull. Comm . Geo!. Finl. 87, 98-127.
Zwart, H. J . (1962) : On the Determination of Polymetamorphic Mineral Associations, and :ts Application to the Bosost Area (Central
Pyrenees): Geo!. Rundschau 52, pp. 38-65.
22
Plate I: Refolded folds (for descriptions and localities see Figure 4).
23
24