Download Bedrock Geology along the Northern Margin of the Athabasca Basin

Bedrock Geology along the Northern Margin of the Athabasca
Basin West of Fond-du-Lac (NTS 74O-5 and -6), South-central
Beaverlodge Domain, Rae Province, Fond-du-Lac Project
K.E. Ashton, B. Knox 1, K.M. Bethune 1, and J. Marcotte 1
Ashton, K.E., Knox, B., Bethune, K.M., and Marcotte, J. (2006): Bedrock geology along the northern margin of the Athabasca
Basin west of Fond-du-Lac (NTS 74O-5 and -6), south-central Beaverlodge Domain, Rae Province, Fond-du-Lac Project; in
Summary of Investigations 2006, Volume 2, Saskatchewan Geological Survey, Sask. Industry Resources, Misc. Rep. 2006-4.2,
CD-ROM, Paper A-1, 19p.
Abstract
Mapping along the northern shore of Lake Athabasca west of Fond-du-Lac has revealed a western zone dominated
by garnetiferous diatexite and foliated to gneissic granite-granodiorite, a central zone dominated by supracrustal
rocks, and an Eastern Plutonic Complex mainly comprising tonalitic to dioritic rocks. The supracrustal rocks
include quartzite and pyribolite of the ca. 2.3 Ga Murmac Bay Group, and widespread psammopelitic gneiss that
may be dominantly Archean in age and probably represent the precursors of the garnetiferous diatexite. Apparently
intrusive relationships suggest that the psammopelitic gneiss is older than the foliated to gneissic granitegranodiorite; however, heterogeneous biotite-pyroxene gneisses in the central zone are commonly flanked by
Murmac Bay Group quartzite and are thought to represent paleo-weathered equivalents of the foliated to gneissic
granite-granodiorite suite.
Following development of the main regional foliation, the region underwent close to isoclinal, east-southeast–
trending F2 folding tentatively attributed to the ca. 1.93 Ga Taltson Orogen. Subsequent open to close, northeasttrending F3 folding is attributed to ca. 1.90 Ga activity along the Snowbird Tectonic Zone. All three phases of
deformation were apparently accompanied by granulite-facies metamorphic conditions. Late northeast-trending
lamprophyre and minor granitoid dykes are thought to have been emplaced in response to east-west shortening
resulting from the ca. 1.83 Ga Trans-Hudson Orogen and Slave Indentor.
Uranium is concentrated in: 1) anatectic granite sheets derived by partial melting of paragneiss during
metamorphism; 2) late northeast-trending felsite dykes; and 3) locally hematitic, late brittle faults cutting
psammopelitic gneiss or garnetiferous diatexite. The latter probably represent unconformity-type mineralization
related to the nearby Athabasca Group.
Keywords: Beaverlodge Domain, Rae Province, Churchill Province, Fond-du-Lac, Murmac Bay Group,
lamprophyre, Taltson Orogen, Snowbird Tectonic Zone.
1. Introduction
The Fond-du-Lac Project is a new initiative directed towards upgrading knowledge of the geological framework
along the northern margin of the Athabasca Basin. Most of the basement to the west has been recently mapped as
part of the Uranium City Project (Figure 1; Hartlaub and Ashton, 1998; Hartlaub; 1999; Ashton et al., 2000, 2001;
Ashton and Hunter, 2003, 2004) and is currently being compiled at 1:250 000 scale (Tazin Lake map area, NTS
74N). The eastern Beaverlodge and western Tantato Domains, however, have received little attention since the
1950s and 1960s, when the Geological Survey of Canada (GSC) completed 1:63,360 scale mapping spanning the
NTS 74N-74O boundary in the west (Blake, 1955) and 1:250 000 scale mapping of 74O in the east (Baer, 1969). In
the area immediately north of that described in this report, an east-west transect was undertaken by Saskatchewan
Energy and Mines in the mid-1980s (Scott, 1983; Thomas, 1985; Harper, 1986), and some detailed mapping was
completed during studies of the Oldman-Bulyea Shear Zone (Ashton and Card, 1998; Card and Bethune, 1999;
Card, 2001). The Tantato Domain to the east was mapped in the 1990s by the GSC (Hanmer, 1994, 1997) and
continues to be the focus of university-based studies (e.g., Baldwin et al., 2003; Mahan et al., 2003) and magmatic
Ni-Cu exploration (www.reddragonresources.com, accessed 18 Oct 2006). The Grease River Shear Zone, which
delineates the eastern Beaverlodge and Tantato domains, was studied in the 1990s (Slimmon and Macdonald, 1987;
1
Department of Geology, University of Regina, 3737 Wascana Parkway, Regina, SK S4S 0A2.
Saskatchewan Geological Survey
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Summary of Investigations 2006, Volume 2
ZEMLAK
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Figure 1 - Location map showing lithotectonic domains in the Rae Province of northwestern Saskatchewan; area mapped in
2006 outlined by heavy dashed line. OBSZ, Oldman-Bulyea shear zone; SLF, St. Louis fault; and STZ, Snowbird tectonic
zone.
Lafrance and Sibbald, 1997). Within the 225 km2 area covered by this report, however, recent work is limited to a
10-day reconnaissance undertaken in the late 1990s (Ashton and Card, 1998).
a) Access and Exposure
The Fond-du-Lac area is accessible by float plane from Stony Rapids or can be reached by boat from either Stony
Rapids or Fond-du-Lac. Topographic relief reaches about 150 m in the central part of the area but drops to about
50 m towards the eastern and western extremities. Given this relatively high relief, the outcrop exposure is generally
excellent, particularly in the west due to recent forest fires, but decreases in abundance and quality east of Dempsey
Bay. There is a 2 m high zone of lichen-free outcrop above the present waterline of Lake Athabasca further
accentuating the rock exposure, although several islands marked on topographic maps in the central part of the lake
were not found.
b) Previous Work
This part of the southeastern Beaverlodge Domain was previously divided into four main units (Blake, 1955; Baer,
1969; Slimmon, 1989): garnet-feldspar and sillimanite-garnet-feldspar gneisses of inferred supracrustal origin, and
hypersthene-feldspar gneiss and amphibolite/pyribolite, thought to be of igneous derivation. In the subsequent
reconnaissance study (Ashton and Card, 1998), it was suggested that the sillimanite-garnet-feldspar gneisses were
derived from aluminous psammopelites and that, together with the amphibolite/pyribolite and minor associated
quartzite, they were correlative with the Murmac Bay Group, a shelf-type siliciclastic succession best known in the
Uranium City area (Hartlaub, 2004; Hartlaub et al., 2004). Both Paleoproterozoic (O’Hanley et al., 1994) and
Archean (Hartlaub et al., 2004) ages have been suggested for the Murmac Bay Group, but recent work has
confirmed that it was deposited at about 2.3 Ga (R. Hartlaub, pers. comm., 2004). The garnet-feldspar gneiss was
interpreted as a diatexite derived from nearly complete partial melting of the psammopelitic sillimanite-garnetfeldspar gneiss (Ashton and Card, 1998). The origin of pyroxene-biotite gneisses discovered during this
reconnaissance study was unclear.
2. Regional Geology
Results from this summer’s 1:20 000-scale mapping (Figure 2) show that the area comprises granitic rocks and
garnetiferous diatexite in the west (‘West-central Plutonic Rocks’), predominantly supracrustal gneisses in the
centre, and a granitic to gabbroic suite in the east (‘Eastern Plutonic Complex’). The West-central Plutonic Rocks
and supracrustal gneisses extend at least 5 km westward to the Dead Man Channel area (Macdonald and Slimmon,
1985; Slimmon, 1989), where psammopelitic gneiss was intruded by the 2.6 Ga ‘Dead Man granite’ (Hartlaub and
Ashton, 1998; Hartlaub et al., 2004), and yielded chemical monazite evidence of metamorphism at about 2.3 Ga
(Hartlaub, 2004). Similar psammopelitic gneiss dominates the central zone of the area mapped in this study, but it
occurs with a quartzite-pyribolite (pyroxene-rich, granulite-facies equivalent of amphibolite) association that is
characteristic of the lower Murmac Bay Group. Therefore, there appear to be two ages of supracrustal rocks
preserved. Since the type locality of the Murmac Bay Group also includes rocks of psammopelitic composition, and
because the main phases of deformation and metamorphism postdate both supracrustal successions, the age of the
psammopelitic gneisses cannot readily be determined.
Saskatchewan Geological Survey
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Dt
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Figure 2 - Simplified geological map of the area west of Fond-du-Lac.
Granite-tonalite±garnet
Garnetiferous quartzofeldsapthic gneiss
Magnetiferous granite-quartz monzonite and gabbro
Garnetiferous diatexite
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West-central Plutonic Rocks
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Saskatchewan Geological Survey
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Summary of Investigations 2006, Volume 2
According to existing maps (Slimmon, 1989), the far eastern part of the area studied is underlain by the same
lithological unit as the far west; however, several significant differences indicate that this may not be the case. Both
areas contain a variety of variably garnetiferous granitoid rocks, but the Eastern Plutonic Complex tends to weather
white, as opposed to pink, and appears to have a broader compositional range between granite and gabbro. It is also
characterized by blue quartz.
Granulite-facies metamorphic conditions appear to have been in place during the first recognizable phase of
deformation, which produced the regional gneissosity. The second phase of deformation resulted in tight folding,
which was probably oriented west-northwest prior to development of more open northeast-trending D3 folds
(Ashton and Card, 1998).
Post-tectonic lamprophyre dykes are abundant and apparently synchronous with minor aphanitic to medium-grained
granitic dykes. Ubiquitous lineaments, resulting from late fractures with little or no offset, are oriented northeast,
north-northwest, and west-northwest.
Ice-flow indicators are widespread and imply a complex glacial history (Campbell et al., this volume).
3. Unit Descriptions
a) Archean and Probable Archean Rocks
Eastern Plutonic Complex
The Eastern Plutonic Complex is dominated by
medium- to coarse-grained granitic to tonalitic rocks
(Figure 3), but also includes minor diorite and gabbro.
Most of the granitoid rocks are pink-white and greybrown, and characterized by blue quartz. They range
from homogeneous to gneissic and locally contain
metre-scale inclusions of fine- to medium-grained
intermediate to mafic gneiss. Most of these generally
non-magnetic granitic-tonalitic rocks contain 10 to
25% combined pyroxene, biotite, and opaque minerals
as well as sporadic hornblende and garnet. They are
commonly injected by abundant late granitic sheets and
rare mafic dykes. Where the injected granitoid material
exceeds 70%, the host rocks occur only as metre-scale
layers and the unit is termed granitic gneiss.2
Figure 3 - Homogeneous granite-tonalite; from island
1.5 km southeast of Lepus Island (UTM 360411E,
6583118N 2).
Near the southwestern margin of the plutonic complex,
the granite-tonalite is interlayered with white to pink
and grey, medium- to coarse-grained garnetiferous
granite-tonalite gneiss having 15 to 20% combined
garnet, biotite, pyroxene, and opaque minerals. These
rocks locally exhibit a garnet-biotite melanosome and
garnetiferous leucosome, as well as decimetre-scale
garnetite layers and pods. Elsewhere, however, they
are more homogeneous, contain blue quartz, and have
an igneous appearance (Figure 4). They may represent
more granitic end members of the plutonic suite that
have compositions better suited to garnet growth,
garnetiferous diatexites, or plutonic rocks that have
been contaminated by interaction with the
psammopelitic gneisses to the west. The unit has been
intruded by garnetiferous granite sheets and mafic
dykes.
Figure 4 - Garnetiferous granite-tonalite gneiss from small
island 2 km east of Lepus Island (UTM 360411E,
6583118N).
2
More heterogeneous zones of garnetiferous
quartzofeldspathic gneiss are found farther east in the
All UTM coordinates are in NAD 83, Zone 12.
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Summary of Investigations 2006, Volume 2
plutonic complex. These include pink, white, or grey, medium-grained rocks that range from homogeneous
garnetiferous leucogranite to garnet-biotite ± orthopyroxene ± graphite gneiss with colour indices of 2 to 20. Pods
of garnetite up to 1 m in size are common. Garnet is generally altered and quartz is typically blue in colour. The
presence of graphite is consistent with their non-magnetic character and suggests that these zones represent enclaves
of metasedimentary rocks, although alternatively some could represent sheared varieties of the mixed garnetiferous
granite gneiss and tonalite.
A distinctive highly magnetic unit (magnetic susceptibility of unit averages 18.5 x 10-3 SI; average of unit core
31 x 10-3 SI) extends southwestward from approximately the eastern end of Dempsey Bay northwards about 6 km
and south-westwards approximately 12 km under Lake Athabasca to Brochet Bay (Figure 2; Geological Survey of
Canada, 1987). The unit is composite, consisting of a pink and grey, coarse-grained, granite-quartz monzonite and
a brown to grey-black, medium-grained, plagioclase-phyric gabbro. The granite-quartz monzonite predominates and
contains blue quartz along with about 25% combined hornblende and magnetite ± orthopyroxene and biotite. It
carries rare centimetre-scale mafic inclusions and is net-veined by pyroxene granite that is commonly aligned axial
planar to the F3 folds (Figure 5). The plagioclase-phyric gabbro is homogeneous and contains variably preserved
plagioclase phenocrysts up to 1 cm long along with 40 to 50% combined pyroxene, hornblende, magnetite, and
minor biotite. Dykes of similar and appearance composition to the plagioclase-phyric gabbro are intrusive into the
garnetiferous quartzofeldspathic gneiss described above and may be co-eval.
A homogeneous unit of white-cream to pink-brown and
grey foliated pyroxene granite (Figure 6) exposed
north of Wasahaw Bay and ‘Marion Lake’3 probably
extends beyond the limit of mapping to the area east of
Dempsey Bay, where it merges with other phases of the
Eastern Plutonic Complex (Figure 2). The mediumgrained granite contains rarely preserved feldspar
augen up to 1 cm and 10 to 20% combined pyroxene,
hornblende, and biotite. It differs from the granitic
component of the granite-tonalite unit to the east by
being slightly magnetic and having grey rather than
blue quartz; however, unlike the West-central Plutonic
Rocks farther west, both the pyroxene granite and the
granite-tonalite units contain hornblende in addition to
pyroxene and biotite. The pyroxene granite has
commonly been injected by about 25% pale pink-grey
centimetre-scale leucosome. A similar but more
leucocratic unit of homogeneous, foliated, mediumgrained garnet-pyroxene granite is exposed about
0.5 km north of the pyroxene granite. It is pink-brown
to grey and contains 5 to 10% combined pyroxene,
biotite and garnet, the latter of which locally reaches
1 cm in diameter. The garnet-pyroxene granite locally
has a vague gneissosity and rare mafic layers derived
either from dykes or inclusions of the dioritic phase of
the plutonic complex.
Figure 5 - Coarse-grained granite-quartz monzonite
containing small mafic inclusion (black arrow) and en
echelon lenses of granitic melt aligned in S3 foliation plane
(white arrows); from shore of Lake Athabasca 1 km south of
east end of Dempsey Bay (UTM 364674E, 6581708N).
b) Supracrustal Rocks
Psammopelitic to pelitic gneiss underlies much of the
central area (Figure 2). It has a grey-brown to rusty
paleosome and white to pale pink leucosome (Figure 7)
and ranges from fine to coarse grained with garnet
porphyroblasts up to 8 cm in diameter and associated
sillimanite-rich melanosome (Figure 8). The rocks
contain 10 to 30% combined garnet, sillimanite, biotite,
and cordierite, the latter of which is generally not
visible in outcrop, but shows up in thin section as a
late, dominantly retrograde phase replacing garnet and
sillimanite. Graphite is a common accessory, consistent
with the non-magnetic character of the paragneisses.
Decimetre-scale pods and discontinuous layers of
Figure 6 - Foliated pyroxene granite of the Eastern Plutonic
Complex; from 1 km north of Wasahaw Bay (UTM
349600E, 6593638N).
3
Informal nomenclature first appears in single quotation marks; subsequently, the quotation marks are dropped.
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Summary of Investigations 2006, Volume 2
garnetite probably result from garnet-producing melt
reactions, although some may be metamorphosed
silicate-facies iron formations.
The psammopelitic to pelitic paragneisses are locally
intercalated with, and grade into, psammitic to
psammopelitic gneiss. These more quartzofeldspathic
paragneisses are white to pale pink and grey, fine to
medium grained, and well layered at centimetre scale.
Typical samples contain 5 to 15% combined biotite and
minor garnet, sillimanite, cordierite, graphite, or
diopside, and are non magnetic. Cordierite is generally
grey and difficult to recognize in outcrop, but is
preserved as dark blue grains both in the melanosome
and in melt leucosome at several localities (Figure 9).
Thin beds of intercalated quartzite are common and
contacts between mappable units of the two rock types
are generally gradational.
Figure 7 - Compositional layering and melanosomeleucosome development in psammopelitic gneiss; from
eastern shore of Narrow Bay (UTM 341846E, 6588808N).
c) West-central Plutonic Rocks
The western part of the area is dominated by
garnetiferous diatexite/granite and foliated to
gneissic granite-granodiorite. The former is white or
pale pink and grey, medium grained, and homogeneous
(Figure 10) to locally gneissic. It is granitic in
composition and contains 10 to 20% combined biotite
and garnet ± orthopyroxene, but is devoid of graphite
and magnetite. The gneissic varieties exhibit remnant
melanosome-leucosome relationships, and represent an
intermediate stage of melting/homogenization of the
psammopelitic to pelitic gneiss. This is supported by
the local presence of metre-scale schlieren of
psammopelitic to pelitic gneiss in the diatexite.
Although most of the garnetiferous diatexite is thought
to have formed this way, similar rocks locally found at
contacts between psammopelitic gneiss and foliated to
gneissic granite-granodiorite (e.g., Bells Island area and
east of Narrow Bay) contain both garnet and
orthopyroxene, and may result from interaction
between the two.
Figure 8 - Coarse red garnet, grey sillimanite, and white
leucosome in psammopelitic gneiss; from 5 km north of
Lepus Island (UTM 357600E, 6589725N).
Figure 10 - Garnetiferous diatexite showing near-complete
homogenization of psammopelitic layering; from island in
Lake Athabasca 2 km west of Narrow Bay (UTM 337820E,
6589040N).
Figure 9 - Psammitic to psammopelitic gneiss with visible
blue cordierite in both melanosome (white arrow) and melt
leucosome (black arrow); from point between Adair and
Wasahaw bays (UTM 344600E, 6586800N).
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The other common rock type in the west is pink to light
brown, foliated to gneissic granite-granodiorite.
Although generally homogeneous, injection of granitic
sheets and localized strain has produced a gneissosity
in places. The rocks are medium to coarse grained with
rarely preserved feldspar augen up to 2 cm long (Figure
11). They typically contain 10 to 25% combined
pyroxene and biotite, and rare garnet, and are non
magnetic. The foliated to gneissic granite-granodiorite
is intruded by mafic dykes.
The ‘Adair Bay pluton’ in the central part of the area
resembles the foliated to gneissic granite-granodiorite,
but is weakly magnetic throughout. Most is pink and
grey, medium grained, and homogeneous (Figure 12),
but the injection of granitic sheets and mafic dykes,
coupled with differential strain, has locally produced a
significantly more heterogeneous appearance. The unit
ranges from granite to tonalite and contains 10 to 35%
combined orthopyroxene and biotite.
Figure 11 - Relict K-feldspar porphyritic texture in foliated
to gneissic granite-granodiorite; from near shore of Lake
Athabasca 5.5 km west of Narrow Bay (UTM 332983E,
6592490N).
Biotite-pyroxene gneiss is common in the Adair and
western Wasahaw Bay areas, where it is commonly
flanked by Murmac Bay Group quartzite. It is typically
pink brown to green grey, fine to medium grained, and
contains 5 to 30% combined biotite and pyroxene.
Most rocks are gneissic but relatively homogeneous,
medium grained, and resemble the foliated to gneissic
granite and rocks of the Adair Bay pluton (Figure 13).
Within a few metres to tens of metres from contacts
with the quartzite, however, the biotite-pyroxene gneiss
may be fine grained and contain garnet, graphite, and
metre-scale boudins of green diopsidite, imparting a
much more heterogeneous appearance (Figure 14). The
biotite-pyroxene gneiss is typically non magnetic and
injected by ~20% pyroxene granite sheets and minor
pyribolite dykes up to 5 m thick.
Figure 12 - Homogeneous gneissic granitoid of the Adair
Bay pluton; from southwestern extent of pluton 0.5 km west
of Adair Bay (UTM 342383E, 6588404N).
Figure 14 - More heterogeneous biotite-pyroxene gneiss
from near contact with adjacent quartzite. Note tight to
isoclinal F2 folds (black arrow) and similarity in orientation
of F3 fold traces (white arrow at bottom of photo) to boudin
necks (white arrow at top of photo); from same outcrop as
Figure 13 (UTM 345900E, 6588820N).
Figure 13 - Biotite-pyroxene gneiss; from island in central
Wasahaw Bay (UTM 345900E, 6588820N).
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The consistent flanking of the biotite-pyroxene gneiss by quartzite suggests that it is part of the basement to the
Murmac Bay Group. The heterogeneous, sedimentary-looking zones adjacent to the quartzite may result from paleoweathering and other processes associated with the unconformity. Following this line of thought, the diopsidite
boudins may represent metamorphosed equivalents of carbonate veins emplaced immediately below the
unconformity, whereas the crosscutting mafic dykes (now pyribolite) may have been feeders to the stratigraphically
overlying Murmac Bay Group basalts. The alternative is that the biotite-pyroxene gneiss is derived from Al-poor
metasedimentary rocks that were locally layered but overall remarkably homogeneous in composition.
d) Murmac Bay Group
In the type locality of the Murmac Bay Group in the Beaverlodge Lake area immediately southeast of Uranium City,
the succession includes basal quartzite with minor, discontinuous, intercalated dolostone, overlying basalt, and
psammopelite (e.g., Macdonald and Slimmon, 1985; Hartlaub et al., 2004). Psammitic paragneiss is commonly
interbedded with the quartzite and patchy iron formation and pelite are locally found at the quartzite-basalt contact.
Psammopelitic gneiss is locally abundant, but generally subordinate to the quartzite and basalt. West of the Black
Bay Fault, in the upper amphibolite-facies Zemlak Domain, the Murmac Bay Group is preserved in narrow belts of
thinly intercalated quartzite, amphibolite, and psammopelitic gneiss, with the former two rock types dominating and
forming the characteristic association (Ashton et al., 2001).
Quartzite-amphibolite outliers are also preserved east of the type locality, including the Reed Bay occurrence
(Hartlaub and Ashton, 1998), so the presence of a structurally preserved quartzite-pyribolite outlier of the Murmac
Bay Group 20 km farther east in the present study area is not surprising. The quartzite varies in apparent thickness
up to about 0.5 km in F2 fold hinges in the Wasahaw Bay and Marion Lake areas, but is more typically about 1 m. It
is nearly ubiquitous at pyribolite contacts, but is generally discontinuous due to boudinage. It may be white, grey, or
pink, fine to medium grained, and almost mono-mineralic, although it may contain up to 10% feldspar, diopside,
biotite, garnet, and/or cordierite. Local metre-scale diopsidite boudins may represent the highly metamorphosed
remnants of interbedded dolostone. Grey-brown, fine- to medium-grained psammitic gneiss is commonly
intercalated with the quartzite. It contains up to 10% combined biotite ± garnet, sillimanite or diopside and is non
magnetic.
Pyribolite (the pyroxene-rich equivalent of amphibolite) is extensive east of Adair Bay, but is also exposed in
numerous narrow synformal keels throughout the area dominated by supracrustal rocks, serving as a useful marker
horizon (Figure 2). Typical rocks are black to brown, medium grained, and homogeneous to layered (Figure 15).
They contain 40 to 60% combined orthopyroxene, clinopyroxene, and locally abundant magnetite with sporadic
hornblende and biotite. The magnetite content is sufficiently high (averages 14 x 10-3 SI) but ranges <1 to 76 x 10-3
SI) to produce a distinctive anomaly on regional aeromagnetic maps (Geological Survey of Canada, 1987), similar
to that associated with the Murmac Bay Group basalt in the Uranium City area (Carson et al., 2001). The
compositional and magnetic similarities, coupled with the nearly ubiquitous adjacent quartzite, strongly suggest that
the pyribolite is a granulite-facies equivalent of the Murmac Bay Group basalt and its intrusive counterpart, the
Lodge Bay Gabbro (Hartlaub et al., 2004).
Iron formation occurs in several settings, but is not
sufficiently thick or continuous to form mappable units
at 1:20 000 scale. It is most commonly observed at, or
within a few metres of, the contacts between quartzite
and pyribolite, where it includes oxide-, sulphide-,
and/or silicate-facies varieties (see accompanying map
separate). Laminated oxide-facies iron formation is best
preserved on an island in southeastern Wasahaw Bay
(UTM 346927E, 6586262), where up to 4 m of
magnetite and chert alternate in 0.5 to 1.0 cm layers
that probably represent relict bedding (Ashton and
Card, 1998, Figure 4). About 1 m of laminated oxidefacies iron formation within quartzite is exposed on a
small island (UTM 347207E, 6583618N) 2 km
northeast of Flat Rock Island. The magnetite has been
completely altered to hematite, presumably due to its
proximity to the once-overlying, sub-Athabasca Group
unconformity. About 5 m of grunerite-magnetite oxidesilicate facies iron formation is locally exposed at the
quartzite-pyribolite contact on the eastern side of Adair
Bay (UTM 342123E, 6591432N). Garnetite occurs in
several settings; where it is situated at or near quartzitepyribolite contacts, as on the island 1 km southwest of
Figure 15 - Layered pyribolite of the Murmac Bay Group;
light-coloured, fine-grained layers are pyroxene rich, dark
layers are hornblende rich, and coarse white material is
injected leucosome with orthopyroxene megacrysts; from
0.5 km north of northeast corner of Adair Bay (UTM
341781E, 6592212N).
Saskatchewan Geological Survey
8
Summary of Investigations 2006, Volume 2
Bells Island (UTM 341900E, 6586050N; see accompanying map separate); it is interpreted as silicate-facies iron
formation. Sulphide-bearing chert or garnetite was observed at contacts between pyribolite and psammopelite at
several localities along the eastern shore of Wasahaw Bay.
e) Syn-tectonic Intrusive Rocks
Garnetiferous leucogranite generally forms metre-scale sheets within the paragneisses, but is the dominant rock
type in the hinge areas of some major folds (see accompanying map separate). It is white to pale pink, generally
medium grained, and homogeneous to gneissic where it contains schlieren of psammopelitic gneiss. It contains up to
10% garnet and minor biotite, is non magnetic, and is thought to be derived by the partial melting of mainly
psammopelitic gneiss.
Non-magnetic granitic sheets and dykes ranging from medium grained to pegmatitic have been injected into all
rock types. Although broadly granitic, with colour indices of less than 10, their mineralogy and colour varies
somewhat due to interaction with the host rocks. In the psammitic and psammopelitic gneisses, they contain garnet
and biotite, whereas those intruding pyribolite and granitoid rocks contain orthopyroxene (Figure 16). Centimetrescale pyroxene ‘sweats’ in the pyribolite do not appear to be the result of partial melting but rather are transposed
granitic apophyses that can be traced back to thicker crosscutting sheets.
A small, outcrop-sized body of medium- to coarse-grained hornblende-biotite-plagioclase gabbro has been
emplaced into sheared garnetiferous diatexite 2 km west of central Oliphant Lake (UTM 333741E, 6592938N). Its
lack pyroxene suggests that it was intruded after the regional granulite facies event, but was affected by subsequent
amphibolite-facies metamorphism.
A set of pink pegmatitic granite dykes is characterized by up to 5% variably deformed, coarse, biotite books
oriented perpendicular to the dyke contacts or randomly. The dykes are straight sided and were apparently emplaced
during the last phase of folding. They locally contain minor garnet, muscovite, and/or tourmaline.
f) Post-tectonic Dykes
Several varieties of late, northeast-trending dykes were mapped. Lamprophyres are by far the most common –
about 240 were measured during this study. They are brown to less commonly pink and fine to medium grained
with 5 to 10% phlogopite and rare pyroxene/amphibole phenocrysts up to 3 mm in diameter. The dykes are straight
sided with sharp contacts, and range from massive to foliated; however, the foliation may be inclined by as much as
20° to the dyke contact, making it unclear whether it is an igneous or tectonic fabric. Most dykes are homogeneous,
but a few contain decimetre-scale, angular to rounded xenoliths of granitoid material and host rocks, along with
minor centimetre-scale mafic-ultramafic inclusions. The lamprophyres are typically 1 to 2 m thick, but can range up
to about 10 m. They contain 20 to 55% combined phlogopite and pyroxene-amphibole, the rest being made up of
dominantly K-feldspar. Rare crosscutting relationships
suggest multiple pulses of emplacement (Figure 17).
Figure 16 - Apophyses of late granite extending
continuously from the sides of thicker sheets into pyribolite
where they are folded by F2; from island at mouth of
Wasahaw Bay (UTM 346517E, 6585978N). Note colour
change in the granite from pink to white adjacent to the host
pyribolite contact in upper left corner.
Saskatchewan Geological Survey
Figure 17 - Lamprophyre dyke cutting across an older phase
of lamprophyre, which in turn intruded garnetiferous
leucogranite; from northeast corner of Neil Bay (UTM
361727E, 6586384N).
9
Summary of Investigations 2006, Volume 2
Three types of late granitoid dykes were observed. The coarsest of these are rare pink and grey, granite dykes,
which are restricted to the West-central Plutonic Rocks in the western part of the area. Most are medium-grained,
although the largest (50 m thick) has a narrow coarse-grained margin. They are internally foliated parallel to the
dyke margin and one was intruded by a co-planar pegmatite dyke. The granite dykes have sharp contacts and are
straight sided, although they tend to splay irregularly at their terminations. Blake (1955) referred to these dykes as
quartz monzonites, although they are more properly termed granites based on his modal analyses and the IUGS
classification of igneous rocks (Streckeisen, 1976). Blake (1955) noted that the granite dykes crosscut the
lamprophyres, although the opposite relationship was observed during the present study, suggesting that the two
dyke sets may be broadly synchronous.
Farther east, several felsite dykes have intruded the Eastern Plutonic Complex in the immediate Lepus Island area.
They are light grey, pale pink, pink red, or zoned with grey margins and pink centres. All are massive and aphanitic,
although very fine-grained quartz eyes were noted in one dyke. They range up to 2 m thick and are straight sided,
but splay irregularly at the ends, similar to the coarser granitic dykes in the east. Pyrite, biotite, and fluorite were
noted on planar to slightly curvilinear joint surfaces and red oxidation spots are common. One dyke is host to a
uranium mineral occurrence (UTM 356229E, 6584591N). At one locality (UTM 356242E, 6584872N), a felsite has
been intruded by a lamprophyre dyke.
South of the eastern end of Dempsey Bay, a pale pink aplite dyke has intruded a thick lamprophyre dyke. It is
massive and homogeneous with sharp contacts, but has a rounded end somewhat similar to a rhyolite tongue. It
contains about 3% biotite ± amphibole, along with rare centimetre-scale mafic to ultramafic inclusions.
The broad compositional similarities of the three varieties of late granitoid dykes suggest that they are co-genetic
and may differ only in the crustal depth at which they were emplaced. Since some of these granitoid dykes intrude
the lamprophyres whereas others are intruded by them, the two apparently diverse dyke sets must be coeval and
potentially related.
4. Structure-Metamorphism
a) D1
The earliest recognizable tectonic fabric is an S1 foliation defined by the leucosomes of partially melted rocks and
by biotite. It is difficult to ascertain whether the peak metamorphic pyroxenes grew during this D1 event due to their
blocky habit, but coarse garnet porphyroblasts have been deformed by the subsequent D2 folding, suggesting that
metamorphic conditions during D1 reached at least upper amphibolite facies and possibly granulite facies.
F1 fold closures were not observed, but are inferred from the repetition of supracrustal units, particularly where they
have been refolded, as in the Marion Lake area (Figure 2). The intense nature of this D1 event can be inferred from
the attenuation and boudinage of competent rock types such as quartzite and diopsidite, which commonly pinch out
only to show up again along strike. The stratigraphic relationships between supracrustal rocks may also have been
modified by thrust faulting during this D1 event. Inferred structural complications in the Marion Lake area may
eventually require more than one pre-D2 phase of deformation to explain.
b) D2
The second regional deformational event produced tight to isoclinal regional folds east of Adair Bay and north of
Natukam Peninsula (Figure 2). Minor, outcrop-scale, close to tight F2 folds are abundant throughout the area
(Figures 14 and 16). Map-scale F2 structures include the isoclinal folding west of Adair Bay and tight to isoclinal
folding in the Marion Lake area (Figure 2). Where not transposed by subsequent folding, they trend east-southeast
and have moderately to steeply dipping axial planes. In areas other than F2 hinge zones, the tight to isoclinal nature
of the F2 folds has resulted in near-parallelism of the earlier S1 and axial planar S2 foliations. This composite S1/S2
fabric is considered correlative with the east-southeast–trending regional fabric in the Mackintosh Bay area
southeast of Uranium City (Macdonald and Slimmon, 1985), which is thought to have formed at about 1.93 Ga
(Ashton et al., 2005).
Extensive granitic sheets and dykes crosscut the S1 fabric, but are axial planar to S2. Apophyses extending from the
sides of these granite sheets are folded by F2 (Figure 16), confirming emplacement during the D2 event. The
ferromagnesian mineral in these granitic sheets and dykes varies with the composition of their host rocks. In
pyribolite and older granitoids, it tends to be pyroxene, indicating that granulite facies conditions were attained or
maintained during D2.
Saskatchewan Geological Survey
10
Summary of Investigations 2006, Volume 2
c) D3
The east-southeast–trending S2 fabric was strongly
overprinted by northeast-trending F3 regional folds
(Figure 2). Open to close, outcrop-scale F3 folds
(Figure 18) generally have steeply southeast dipping
axial planes and plunge to the southwest (Figure 19),
although there are plunge reversals, likely due to the
variable orientation of the composite S1-S2 fabric.
Since D1 and/or D2 produced extensive boudinage of
the composite S1-S2 gneissosity, and D3 structures are
oriented approximately perpendicular to that fabric,
boudin necks can be misinterpreted as F3 folds (Figure
14); however, in places F3 folds have in fact localized
at some of these boudins necks.
An S3 fabric defined by the alignment of granitic melt
leucosome lenses (Figure 5), garnet (Figure 20), biotite,
Figure 18 - Open to close F3 Z folds developed in
and quartz is sporadically developed throughout the
psammopelitic gneiss; from eastern shore of Narrow Bay
area. Many late granite dykes have been emplaced
(UTM 339790E, 6589926N).
parallel to F3 axial planes and subsequently have
facilitated D3 deformation by acting as zones of shear
during fold development. This has locally resulted in F3 folds that were initially symmetrical but then evolved into
asymmetric structures due to mainly dextral drag folding along one limb (Figure 21). The presence of pyroxene in
the S3-aligned granitic melts within host rocks of intermediate to mafic rocks composition indicate that
metamorphic conditions during D3 were in the granulite facies (Figure 5).
Numerous examples of outcrop-scale type 2 fold interference patterns (Ramsay, 1967) have resulted from the
interaction of F2 and F3 (Figure 22). Type 2 fold interference results from the interaction of late, steep to upright
folds with pre-existing folds having more gently dipping axial planes and fold axes near-perpendicular to those of
the early folds. This style implies that the F2 folds originally trended south-eastward and plunged either to the
northwest or southeast. The dominance of moderate southwesterly dips for the composite S1-S2 foliation in the
hinges of the regional F3 folds in the Wasahaw Bay area is consistent with the general southwestward plunge of F3
folding and suggests that the axial planes of the F2 folds were originally southwest dipping.
N
0/
65
N
22
Average
65 —›222
n=472
n=217
a) Poles to F3 Axial Planes
b) F3 Fold Hinge Lines
Figure 19 - Lower hemisphere, equal-area stereonet plots. a) Poles to F3 axial planes; great circle at 220°/65° represents
average orientation of F3 axial planes. b) F3 fold hinge lines; average orientation of 65°Æ 222° is given by maxima in
southwest quadrant. Data are contoured with densities of point data measured as a percentage of total number of points per
1% area of the net; contour interval is 2.
Saskatchewan Geological Survey
11
Summary of Investigations 2006, Volume 2
d) Late Structures
Gentle warping of the regional foliation about a northnorthwest trend was interpreted as evidence for a fourth
phase of deformation. Boudinage of the S1-S2
composite fabric can be mistaken for these F4 folds,
which locally are nucleated in boudin necks, but an
axial planar foliation was recorded in at least one case.
The intensity, relative timing, and orientation of these
F4 folds are similar to those that affect the ca. 1.82 Ga
Martin Group at Uranium City and the region farther
west (e.g., Ashton et al., 2000; Ashton and Hunter,
2004).
Regional joints and late faults displaying little or no
offset are abundant at both map and outcrop scale. The
three dominant sets trend: 1) west-northwest,
2) northeast, and 3) north-northwest to north (Figure
23a). The west-northwest–trending set helps to define
the northern shore of Lake Athabasca. The northeasttrending structures are part of an extensive set that
includes the dextral Grease River Shear Zone (e.g.,
Lafrance and Sibbald, 1997). Orientation data for the
lamprophyre and late granitic dykes (Figure 23b) are
consistent with their emplacement along these
northeast-trending structures, probably at about the
time they were developing. The north-northwest– to
north–trending set is oriented northwesterly in the west
where it is partly controlled by the main S1-S2
composite fabric on the western limb of a regional F3
fold passing through Wasahaw Bay (Figure 2). Farther
east, it takes on a more northerly orientation. All three
sets were probably produced by an east-west–
shortening event resulting from the Slave Indentor
(Gibb, 1978) to the west and terminal collision in the
Trans-Hudson Orogen to the east (Ashton et al., 2004).
Figure 20 - Garnet porphyroblasts defining a crosscutting S3
foliation; from psammopelitic gneiss on island 1 km south of
Natukam Bay; UTM 355937E, 6586345N).
e) Metamorphism and Timing of Deformation
A previous P-T study based of the psammopelitic rocks
in the Neil Bay area (Figure 2) indicated a complex
metamorphic history with peak conditions of 900°C
and 8 kbar (Kopf, 1999); however, such conditions
would normally produce orthopyroxene in natural
aluminous pelitic compositions and garnet in metabasites (Bucher and Frey, 1994), neither of which were
observed in this study. The presence of spinel in some
psammopelitic gneisses implies temperatures in excess
of 770°C, whereas the absence of orthopyroxene in
these aluminous rocks suggests an upper limit of about
850°C (Bucher and Frey, 1994). The persistence of
hornblende in the pyribolite suggests a similar 850°C
upper temperature limit. An upper pressure limit of
about 7 kbar is imposed by the presence of
orthopyroxene together with the absence of garnet in
the mafic rocks (Bucher and Frey, 1994). This is
consistent with the observation that most, if not all, of
the cordierite in the psammopelitic gneisses has formed
during decompression from the breakdown of garnet
and sillimanite. Thus, peak metamorphism appears to
have taken place at temperatures of 770° to 850°C and
pressures of 5 to 7 kbar based on the phases present
today. This is consistent with the 765°C, 7.1 kbar
conditions derived using garnet-biotite
Figure 21 - F3 folds in psammopelitic gneiss with thin
irregular granite emplaced parallel to axial plane; from
island at mouth of Narrow Bay (UTM 340766E, 6586439N).
Note asymmetry developed due to dextral drag.
F3
F2
Figure 22 - Type 2 ‘arrowhead’ fold interference pattern
resulting from interaction between F2 and F3; from
psammopelitic gneiss on island in western Neil Bay (UTM
357978E, 6585983N).
Saskatchewan Geological Survey
12
Summary of Investigations 2006, Volume 2
N
N
n=250
n=1053
a) Fractures and Joints
b) Late Dykes
Figure 23 - Rose diagrams (class interval of 20°) showing strike orientations of: a) steeply dipping to sub-vertical, outcropscale fractures (largest petal contains 10.8% of data), and b) late lamprophyre (241), aplite (3), and felsite (6) dykes (largest
petal contains 24.8% of data).
thermobarometric techniques on a single psammopelitic gneiss sample from the northern Wasahaw Bay area (Card,
2001). The discrepancy between the inferred metamorphic conditions implied by the present work and those
inferred from the previous P-T study in the Neil Bay area may result from the multiple high-grade metamorphic
events; however, it is more likely for the compositions of existing mineral phases to have been modified than for
apparently ‘missing’ peak metamorphic phases to have been completely replaced during later-stage events. Dating
of metamorphic monazite indicated growth at about 1.9 Ga (Kopf, 1999), but the relationship between this
metamorphic event and the various phases of deformation was not established.
A common feature of this granulite-facies terrain is the presence of garnetite as pods and discontinuous layers up to
about 1 m in size. Although some may have been derived from silicate-facies iron formation (in which case it
should occur with other Fe-Al silicates such as grunerite), most appears to be a product of partial melting. Since
biotite dehydration reactions can produce garnet along with a melt leucosome (Bucher and Frey, 1994), garnetite is
thought to form in situations where this molten leucosome migrates away from the reaction site due to differential
stress, leaving solid-phase garnet behind as a concentrate. The process is analogous to the formation of hornblendite
pods due to the partial melting of granodioritic rocks (e.g., Ashton and Shi, 1994).
Based on work to the west, the east-southeast–trending D1-D2 structures are thought to have resulted from the ca.
1.93 Ga Taltson Orogen, whereas the northeast-trending D3 fabric is thought to have been caused by tectonic
processes taking place along the Snowbird Tectonic Zone (Ashton et al., 2005). The weaker D4 event is considered
a distal response to collisions in the Trans-Hudson Orogen to the east and accretionary tectonics west of the Slave
craton.
5. Economic Potential
a) Uranium
There are several known uranium showings in the area. According to the Saskatchewan Mineral Deposit Index,
most of these are hosted by minor fault/shear zones within psammopelitic gneiss or derived diatexite. Some have
associated sulphides and/or hematitic alteration, the latter suggesting a genetic relationship to the Athabasca Group
unconformity (SMDI 1559, 1564, and 1573).
One U-Cu occurrence (SMDI 1559) is hosted by pegmatite within pyribolite and features yellow gummite-type
alteration. Five new occurrences of yellow gummite staining were found during the course of this work (Figure 24).
Saskatchewan Geological Survey
13
Summary of Investigations 2006, Volume 2
Four of these were in garnetiferous leucogranite sheets
or melt leucosome within psammopelitic gneisses
located on or close to late northwest-trending faults in
the Wasahaw Bay area (UTM 349413E, 6591110N;
UTM 350646E, 6591480N; UTM 345269E,
6591957N; and UTM 346770E, 6589463N). The
fourth was in quartzite proximal to psammopelitic
gneiss (UTM 352029E, 6590006N). This type of
uranium occurrence probably developed by the
concentration of uranium from sedimentary rocks by
partial melting during high-grade metamorphism, and is
analogous to uraniferous pegmatites described from the
Wollaston Domain (Thomas, 1983).
Another uranium showing (SMDI 1565) is hosted by
one of the late, northeast-trending felsite dykes that cut
across Lepus Island, and another of the felsite dykes on
the northern shore of Lake Athabasca 1 km south of the
eastern end of Dempsey Bay (UTM 363936E,
6582175N) has been trenched. These dykes post-dated
the peak metamorphic events and may have been
structural and/or chemical traps for uraniferous fluids
moving along fault zones.
Figure 24 - Gummite uranium staining in melt leucosome of
psammopelitic gneiss; from 1 km south of northeast end of
Wasahaw Bay (UTM 350646E, 6591480N).
b) Other
Rusty sulphide-bearing zones are developed along late
fractures at several localities (Figures 25 and 26; see
accompanying map separates) and pyrite is common
along joint surfaces throughout the area. Late granitic
sheets host most of these sulphides, although this is
probably because they are competent, easily fractured
rocks rather than sources for the sulphides. The
euhedral nature of some of the pyrite and its
emplacement along late fractures indicates that the
mineralization precipitated from fluids and postdates
both metamorphism and the main periods of
deformation. It is not known whether these zones are
radioactive.
Figure 25 - Rusty pyrite-bearing zones developed along late
fractures in tonalitic gneiss; from southwest shore of Lepus
Island (UTM 355774E, 6585084N).
Numerous occurrences of iron formation have been
described in the rock type descriptions above and
placed on the accompanying map separates.
Airborne EM conductors culled from assessment files
mimic the regional structure and are generally
attributed to graphite in the paragneisses and biotitepyroxene gneiss. They are most prominent along the
main pyribolite unit east of Adair Bay and, together
with the strong aeromagnetic anomaly related to the
pyribolite, can be traced southward onto the southern
shore of Lake Athabasca where they extend the length
of the peninsula separating Metos and Brochet bays.
Crosscutting, northeast-trending conductors in that area
are probably related to faults.
6. Discussion
a) Stratigraphic Relationships
Figure 26 - Mass of pyrite developed in rusty pyroxene
granite sheet within pyribolite; from island at mouth of
Wasahaw Bay (UTM 346478E, 6585661N).
Saskatchewan Geological Survey
Psammopelitic gneiss and derived diatexite situated
about 10 km west of the mapped area have been
intruded by the ca. 2.6 Ga Dead Man Granite
14
Summary of Investigations 2006, Volume 2
(Hartlaub, 2004; K. Bethune, pers. comm., 2005), tentatively making them the oldest rocks in the region. These
paragneisses extend continuously eastward into the western and central portions of the study area, where they
appear to have been intruded by the same suite of 2.6 Ga granitoid rocks in the form of the foliated to gneissic
granite-granodiorite. If all of the psammopelitic gneisses are considered Archean, however, then the psammopelitic
component of the ca. 2.3 Ga Murmac Bay Group is conspicuous by its absence.
At the type locality of the Murmac Bay Group at Uranium City, the quartzite generally represents the basal unit with
the basalt overlying it. If this poorly constrained stratigraphic order is employed in the Fond-du-Lac area, it can be
seen from the map that the majority of the psammopelitic gneiss would underlie the quartzite, consistent with the
idea that they are part of the Archean basement. Elsewhere, however, there are hints that this may not be the case for
all of the psammopelitic gneiss. Along the eastern shore of Wasahaw Bay, chert, which is typically precipitated
towards the end of a volcanic event, was found at a pyribolite-psammopelitic contact, implying that the
psammopelitic gneiss is younger. Therefore, although tentative separation of the supracrustal rocks into Archean
psammopelites and Paleoproterozoic quartzites and pyribolites is a convenient working model, future work may
show that psammopelitic gneisses of both ages are present.
The main granitoid units have been separated into an inferred ca. 2.6 Ga granite-granodiorite suite in the west
(West-central Plutonic Rocks) and a granite-gabbro suite of uncertain age (Eastern Plutonic Complex) in the east.
Both suites are in contact with psammopelitic gneiss/diatexite in the western and eastern parts of the map area.
Within the central, dominantly supracrustal zone, however, the biotite-pyroxene gneiss probably represents a paleoweathered equivalent of the granite-granodiorite suite, thereby representing the sub-aerially exposed basement to the
Murmac Bay Group. A unit of tonalitic gneiss adjacent to extensive quartzite and pyribolite northeast of Natukam
Peninsula probably represents a similar unconformity between the Murmac Bay Group and the Eastern Plutonic
Suite. Thus both plutonic suites are probably Archean and part of the basement to the Murmac Bay Group. The
possible sedimentary enclaves in the Eastern Plutonic Complex may indicate that both were also emplaced into
Archean supracrustal rocks.
b) Structural Implications
The curvilinear boundary between the dominantly supracrustal central part of the area and the Eastern Plutonic
Complex appears too open to be a simple F2 fold (Figure 2). Thinning of units on both sides implies that this
boundary is a major discontinuity, although evidence of widespread ductile shear is not overwhelming. North of the
mapped area, the boundary has the same orientation as the Oldman-Bulyea Shear Zone that separates the
Beaverlodge and Train Lake domains (Slimmon, 1989). Both discontinuities are folded by the F2 and F3 folds,
suggesting that they were formed during D1 or early D2 time (Harper, 1986; Card, 2001). They also represent the
eastern and western boundaries of a zone characterized by relatively open northwest-trending folds that are oriented
parallel to the Oldman-Bulyea Shear Zone and would appear to postdate F2 yet predate F3 (Harper, 1986). Thus,
like the Oldman-Bulyea Shear Zone, this discontinuity may be a northeast-verging thrust fault that has been
multiply refolded.
c) Extrapolation of Exposed Units Southward Underneath the Athabasca Basin
Two strong, positive, aeromagnetic anomalies dominate the regional aeromagnetic map (Figure 27; Geological
Survey of Canada, 1987) and have implications for any attempt to extrapolate the geology exposed on the north
shore of Lake Athabasca underneath the Athabasca Basin. The western anomaly is centred on the Murmac Bay
Group pyribolite and extends southward across Lake Athabasca and the along the length of the peninsula west of
Brochet Bay. The eastern anomaly is centred on the composite granite-quartz monzonite – plagioclase-phyric
gabbro unit of the Eastern Plutonic Complex and strikes southwestward to the mouth of Brochet Bay. The
aeromagnetic pattern and measured outcrop-scale structures along the northern shore of Lake Athabasca suggest
that the boundary between the supracrustal-dominated central zone and the Eastern Plutonic Suite turns southward
from eastern Lepus Island. Farther south, this boundary may bend back towards the east, placing the supracrustal
rocks beneath the Athabasca Group along the southern shore of Lake Athabasca, and forming a mirror image of the
broad east-southeast–trending fold to the north.
These results can be used to update the recently released remote predictive map for the basement to the western
Athabasca Basin (Card, 2006). The western positive aeromagnetic anomaly had already been attributed to the
pyribolite derived from Murmac Bay Group basalt, whereas the eastern anomaly was included with a more
extensive moderately magnetic area to the north and south that was accredited to ‘mafic granulite’. It is now known
that the eastern aeromagnetic ‘high’ is derived from the composite granite-quartz monzonite–plagioclase-phyric
gabbro unit and that the moderately magnetic area to the north can be ascribed to the granite-tonalite unit, both
members of the Eastern Plutonic Complex. The area of weak aeromagnetic response surrounding these
aeromagnetic ‘highs’ was attributed to ‘diatexitic psammopelite to pelite’ (Card, 2006). It can be seen from Figure
27 that this interpretation is valid for the area west of Lepus Island and Brochet Bay, but that granitoid rocks of the
Saskatchewan Geological Survey
15
Summary of Investigations 2006, Volume 2
Saskatchewan Geological Survey
16
km
Bp
Eg
Garnetiferous diatexite
Dt
Et
Et
Zp
o
Neil Bay
Em
Eq
s
ea
Gr
Et
iv
eR
Eq
er
Sh
r
ea
e
59o15’
n
Zo
Eq
Average bedrock foliation
Granite-tonalite±garnet
Garnetiferous quartzofeldsapthic gneiss
Magnetiferous granite-quartz monzonite and gabbro
Psammopelitic and psammitic gneiss
Eastern Plutonic Complex
Supracrustal Rocks
Et
Zp
Figure 27 - First vertical derivative aeromagnetic map (Geological Survey of Canada, 1987) with simplified geology superimposed showing continuation of positive anomalies
related to the Murmac Bay Group (MB) pyribolite in the west and the composite granite-quartz monzonite – plagioclase-phyric gabbro unit (Em) of the Eastern Plutonic
Complex in the east across Lake Athabasca and beneath the Athabasca Basin. Heavy dashed line is the extrapolated boundary between the central zone of dominantly
supracrustal rocks and the Eastern Plutonic Complex.
Bp
Foliated to gneissic granite-granodiorite
Zp
Gf
?
MB
Isle
Brochet
Lepus
Island
Lake'
Brochet
Bay
'Marion
Eq
MB
Zp
Et
Adair Bay Pluton
Zp
AB
Flat Rock
Island
MB
Bp
Zp
Em
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Summary of Investigations 2006, Volume 2
Eastern Plutonic Complex are probably responsible for the weakly magnetic response in the area directly adjacent to
the eastern magnetic ‘high’.
7. Acknowledgements
The field work was made possible by the able assistance of Jeanette Marcotte, Brian McEwan, Lindsay Richan, and
Andrew Kaczowka. We were joined for most of the summer by Janet Campbell (SIR) who studied the glacial
history of the area (see Campbell et al., this volume). Thanks to Dan Jiricka of Cameco Corporation for the loan of a
scintillometer to aid in a B.Sc. study of joints, faults, and dykes by Jeanette Marcotte. We also benefited from
discussions with Colin Card, Charlie Harper, and Gary Delaney (SIR) during a one-day visit to the area in late July.
The original manuscript was much improved thanks to constructive reviews by Ralf Maxeiner, David MacDougall,
and Gary Delaney.
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Summary of Investigations 2006, Volume 2
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Summary of Investigations 2006, Volume 2
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