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GEOSCIENTIFIC REPORT
GR2011-1
survey
manitoba
ologic
ge
a
Geology and geochemistry of
arc and ocean‐floor volcanic rocks
in the northern Flin Flon Belt,
Embury–Wabishkok–Naosap lakes area,
Manitoba (parts of NTS 63K13, 14)
By
H.P. Gilbert
Geoscientific Report GR2011-1
Geology and geochemistry of arc and ocean-floor
volcanic rocks in the northern Flin Flon Belt,
Embury–Wabishkok–Naosap lakes area, Manitoba
(parts of NTS 63K13, 14)
by H.P. Gilbert
Winnipeg, 2012
Innovation, Energy and Mines
Mineral Resources Division
Hon. Dave Chomiak
Minister
Manitoba Geological Survey
Grant Doak
Deputy Minister
John Fox
Assistant Deputy Minister
E.C. Syme
Director
© Queen’s Printer for Manitoba, 2012
Every possible effort is made to ensure the accuracy of the information contained in this report, but Manitoba Innovation, Energy
and Mines does not assume any liability for errors that may occur. Source references are included in the report and users should
verify critical information.
Any third party digital data and software accompanying this publication are supplied on the understanding that they are for the
sole use of the licensee, and will not be redistributed in any form, in whole or in part. Any references to proprietary software in
the documentation and/or any use of proprietary data formats in this release do not constitute endorsement by Manitoba Innovation, Energy and Mines of any manufacturer’s product.
When using information from this publication in other publications or presentations, due acknowledgment should be given to the
Manitoba Geological Survey. The following reference format is recommended:
Gilbert, H.P. 2012: Geology and geochemistry of arc and ocean-floor volcanic rocks in the northern Flin Flon Belt, Embury–
Wabishkok–Naosap lakes area, Manitoba (parts of NTS 63K13, 14); Manitoba Innovation, Energy and Mines, Manitoba
Geological Survey, Geoscientific Report GR2011-1, 46 p. + DVD.
NTS grid: 63K13, 14
Keywords: back-arc basins; basalts; calc-alkalic composition; Embury Lake; Flin Flon arc assemblage; Flin Flon Belt; geochemistry; island arcs; Manitoba; mid-ocean ridge basalts; Naosap Lake; Paleoproterozoic; rifting; tholeiite; volcanic rocks; volcaniclastics; volcanism; Wabishkok Lake
Published by:
Manitoba Innovation, Energy and Mines
Manitoba Geological Survey
360–1395 Ellice Avenue
Winnipeg, Manitoba
R3G 3P2 Canada
Telephone: (800) 223-5215 (General Enquiry)
(204) 945-4154 (Publication Sales)
Fax:
(204) 945-8427
E-mail: [email protected]
Website:manitoba.ca/minerals
This publication is available to download free of charge at manitoba.ca/minerals
Cover illustration: The northern Flin Flon Belt in the Reindeer Zone of the Trans-Hudson Orogen includes more than 20 geochemically distinct fault blocks containing volcanic rocks of both juvenile-arc and MORB compositional type. Pillowed mafic to
intermediate flows are predominant among these volcanic rocks. Major regional shear zones that locally contain tectonic breccia
may have originated as crustal-scale faults associated with incipient rifting, prior to back-arc basin development. The landscape
view across Wabishkok Lake is located at the northern margin of the Flin Flon arc assemblage, which is tectonically juxtaposed
against back-arc basaltic rocks that extend along the southern margin of the Kisseynew Domain.
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Manitoba Geological Survey
Abstract
After 1880–1870 Ma tectonic amalgamation of the oceanic-arc system, the first (D1) fold event affected juvenile-arc
and back-arc rocks but predated 1870–1840 Ma successor-arc
magmatism and sedimentation. The early (D1) and later (post1840 Ma) D2 deformation events resulted in tight to isoclinal
folds and locally repeated fold patterns in some fault blocks; D1
fold axial traces were locally truncated by reactivation of blockbounding faults during D2. The D3 event resulted in regional
open folds, such as the east-northeast-trending Embury Lake
antiform that dominates the regional structure in the western
part of the NFFB. The Northeast Arm Fault is a crustal-scale
structure that divides the NFFB into western and eastern parts,
distinguished by northwest- to west-trending and northeast- to
east-trending regional faults, respectively. The latest deformation event (D4) resulted in brittle faulting, typically at high
angles to earlier structural trends.
The Flin Flon Belt is characterized by numerous allochthons that are distinguished geochemically and/or stratigraphically and are part of the Amisk Collage. This collage resulted
from a 1.88–1.87 Ga collisional tectonic event that juxtaposed
crustal segments of various types including juvenile arc, backarc ocean-floor, ocean plateau, ocean-island and evolved plutonic arc. The NFFB consists of arc, arc-rift and ocean-floor
(back-arc) fault blocks or slices, together with the products of
subsequent 1.87–1.84 Ga successor-arc magmatism and fluvialalluvial, as well as turbidite, sedimentation. Granitoid plutons
of probable late successor-arc age locally ‘stitch’ the contacts
between juvenile-arc and ocean-floor assemblages, and lensoid
turbidite fault slices are locally emplaced at contacts between
arc or between arc and ocean-floor volcanic rocks. Mafic intrusive sills occur within or at the margins of some fault blocks;
most or all of these intrusions are interpreted as coeval with the
arc/back-arc volcanism or subsequent arc-rifting.
Two north-trending panels of rift-related rock types that
have been identified in the NFFB may have a potential for economic mineralization, based on the association of volcanogenic
massive sulphide mineralization with arc-rifting elsewhere in
the Flin Flon Belt. These two panels are spatially associated
with major, crustal-scale faults that extend across the width of
the Flin Flon Belt (Inlet Arm Fault, Northeast Arm Fault). Several localities of felsic volcanic rocks in the NFFB may also be
prospective for base-metal and/or precious-metal mineralization, based on their geochemical signature. The location of the
Trout Lake mine at the inferred contact between the Cope Lake
and Embury Lake blocks suggests tectonostratigraphic contacts
may, in some cases, also represent promising targets for mineral
exploration.
The Paleoproterozoic Flin Flon Belt in the Reindeer Zone
of the Trans-Hudson Orogen hosts many base- and preciousmetal deposits and has been a focus of mineral exploration and
geological investigations for over a century. Detailed mapping
was initiated in 1979 by the Manitoba Geological Survey in
the south-central part of the Flin Flon Belt, where most of the
mineral deposits have been found. In 1986, the author initiated 1:20 000 scale geological mapping in the northern part of
the Flin Flon Belt (NFFB). The map area extends from latitude 54°48′N northwards to the contact with the Kisseynew
Domain, and from the Manitoba-Saskatchewan border in the
west to Naosap Lake in the east. Map GR2011-1-1 (in back
pocket) is a geological compilation map at 1:30 000 scale that
is derived, in part, from a series of preliminary maps published
by the author between 1986 and 2004.
Geoscientific Report GR2011-1
iii
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Manitoba Geological Survey
TABLE OF CONTENTS
Page
Abstract............................................................................................................................................................................................iii
Introduction.......................................................................................................................................................................................1
Previous work...................................................................................................................................................................................5
Regional geology.............................................................................................................................................................................. 5
Geology of the northern Flin Flon Belt.............................................................................................................................................6
Juvenile arc and arc-rift volcanic suites (units J1 to J15)..........................................................................................................8
Bear Lake suite (unit J1, calcalkaline arc)......................................................................................................................10
Cope Lake suite (unit J2, calcalkaline arc).....................................................................................................................10
Hook Lake suite (unit J3, tholeiitic arc).......................................................................................................................... 15
Tartan Lake suite (unit J4, tholeiitic to calcalkaline arc)................................................................................................ 16
Wabishkok Lake suite (unit J5, tholeiitic arc)................................................................................................................. 16
Manistikwan Lake arc, arc-rift (unit J6, tholeiitic to calcalkaline) and N-MORB–type volcanic suites.......................18
East Mikanagan Lake arc-rift suite (unit J7, tholeiitic)..................................................................................................18
Lac Aimée arc and arc-rift suites (unit J8, tholeiitic)...................................................................................................... 19
Geochemistry of arc and arc-rift rocks in Lac Aimée, East Mikanagan Lake and Scotty Lake blocks................ 20
Sourdough Bay suite (unit J9, tholeiitic to calcalkaline arc)..........................................................................................20
Naosap Lake suite (unit J10, calcalkaline arc)................................................................................................................ 21
Metamorphic history, origin and structural implications of high-grade gneiss (unit J10b)....................................22
Felsic volcanic rocks (units J11–15)............................................................................................................................... 23
MORB-type volcanic suites (unit F)........................................................................................................................................25
Arthurs Lake N-MORB suite (unit F1)............................................................................................................................26
Bluenose Lake N-MORB suite (unit F2).........................................................................................................................26
Animus Lake N-MORB and E-MORB suites (units F3 and F4)..................................................................................... 28
West Mikanagan Lake E-MORB suite (unit F5).............................................................................................................29
Depleted-MORB volcanic suites (unit T)................................................................................................................................30
Dismal Lake depleted-MORB suite (unit T1).................................................................................................................30
Arc volcanic enclaves within the Dismal Lake depleted-MORB assemblage........................................................31
Bluenose Lake depleted-MORB suite (unit T2).............................................................................................................. 31
Sedimentary rocks of uncertain age (unit S)............................................................................................................................32
Successor-arc sedimentary rocks............................................................................................................................................. 32
Missi Group rocks (unit M)...........................................................................................................................................32
Intrusive rocks (units P, X and J)............................................................................................................................................. 32
Discussion.......................................................................................................................................................................................33
Tectonic setting of NFFB arc and MORB-type volcanic rocks...............................................................................................34
Arc-rift propagation, development of intra-arc and back-arc basins and the potential for VMS mineralization....................34
Tectonic setting and ages of N-MORB and E-MORB types in the NFFB..............................................................................36
Tectonic setting of depleted-MORB types in the NFFB..........................................................................................................36
Structural geology...........................................................................................................................................................................38
Structural geology discussion..................................................................................................................................................38
Economic geology..........................................................................................................................................................................40
Conclusions.....................................................................................................................................................................................41
Acknowledgments...........................................................................................................................................................................42
References.......................................................................................................................................................................................42
Geoscientific Report GR2011-1
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APPENDICES
Appendix 1: Geochemical data................................................................................................................................................... DVD
Appendix 2: Field photographs and photomicrographs.............................................................................................................. DVD
TABLES
Table 1: Geological formations, northern Flin Flon Belt..................................................................................................................7
Table 2: Tectonic setting, geochemical affinity and rock types of volcanic suites in the northern Flin Flon Belt...........................8
Table 3: Geochemical data for mafic to intermediate volcanic rock suites and selected felsic volcanic rock types
in the northern Flin Flon Belt...........................................................................................................................................back pocket
Table 4: The εNd isotopic values for selected volcanic rock suites in the northern Flin Flon Belt.................................................17
Table 5: Deformation history of the northern Flin Flon Belt..........................................................................................................39
FIGURES
Figure 1: Regional geology map of part of the Reindeer Zone in the southern Trans-Hudson Orogen........................................... 1
Figure 2: Geology of the northern part of the western Flin Flon Belt, showing the main tectonostratigraphic components...........2
Figure 3: Structural geology of the northern part of the western Flin Flon Belt..............................................................................3
Figure 4: Geology of the western part of the Flin Flon Belt, showing the lateral extent of tectonically defined components
between the northern Flin Flon Belt and south-central part of the Flin Flon Belt............................................................................4
Figure 5: Ternary diagrams (Al2O3–[FeOT+TiO2]–MgO) and ([Na2O+K2O]–FeOT–MgO) for mafic to felsic arc
volcanic rocks in the northern Flin Flon Belt...................................................................................................................................9
Figure 6: Plots of TiO2 versus MgO for mafic to intermediate volcanic rocks in the northern Flin Flon Belt............................... 11
Figure 7: Plots of N-MORB normalized incompatible-elements for mafic to intermediate rocks in arc volcanic suites
in the northern Flin Flon Belt.....................................................................................................................................................11-12
Figure 8: Plots of Th/Nb versus Nb/Y for mafic to intermediate arc volcanic rocks in the northern Flin Flon Belt......................13
Figure 9: Plot of εNd versus Nb/Yb for mafic to felsic volcanic rocks in the northern Flin Flon Belt, showing the
calculated percentage of older crustal Nd in various volcanic rock suites..................................................................................... 14
Figure 10: Plot of Zr versus TiO2 for felsic volcanic rocks and related plagioclase-quartz porphyry intrusions in the
northern Flin Flon Belt...................................................................................................................................................................15
Figure 11: Plot of N-MORB normalized incompatible-elements for a dacitic intrusion, compared with those of andesite
and dacite in the Naosap Lake volcanic suite.................................................................................................................................23
Figure 12: Chondrite-normalized rare earth element plots of Cope Lake rhyolite, Lac Aimée felsic porphyry and other
felsic volcanic and related intrusive rocks in the northern Flin Flon Belt......................................................................................24
Figure 13: Plot of La/Yb(N) versus Yb(N) for felsic volcanic and related intrusive rocks in the northern Flin Flon Belt................25
Figure 14: Plot of Zr/Y versus Y for felsic volcanic and related intrusive rocks in the northern Flin Flon Belt............................25
Figure 15: Plots of N-MORB normalized incompatible-elements for MORB-type mafic volcanic suites in the northern
Flin Flon Belt..................................................................................................................................................................................27
Figure 16: Ternary Th-Zr-Nb plot of mafic to felsic volcanic rocks in the northern Flin Flon Belt, showing arc, arc-rift,
N-MORB, E-MORB and depleted-MORB rocks...........................................................................................................................31
Figure 17: Sequence of events leading to rifting and BAB development in modern oceanic arcs.................................................35
Figure 18: Plot of N-MORB normalized incompatible-elements for two depleted-MORB flows in the Dismal Lake
assemblage and a modern MORB-type flow from the Manus Basin..............................................................................................37
Figure 19: Northeast Arm Fault, northern Flin Flon Belt: restoration of fault blocks to inferred positions prior to the
last major movement.......................................................................................................................................................................40
vi
Manitoba Geological Survey
MAPS
Map GR2011-1-1: Geology of the Embury–Wabishkok–Naosap lakes area, Manitoba (parts of NTS 63K13, 14),
scale 1: 30 000.................................................................................................................................................................back pocket
Map GR2011-1-2: GIS compilation of the geology and geochemistry of the Embury–Wabishkok–Naosap lakes area,
Manitoba (parts of NTS 63K13, 14)...................................................................................................................................... DVD
PRINT/DIGITAL COMPONENTS AND DISTRIBUTION FORMATS
As outlined in the table below, GR2011-1 has digital-only components that are provided on an accompanying DVD for the print
format of this publication. The DVD is also a stand-alone product of the entire publication.
Report GR2011-1
Table 3
Print components
Digital components on accompanying/
stand-alone DVD
√
√
PDF
√
(folded in back pocket)
√
Excel
Appendix 1
√
Excel
Appendix 2
√
Excel
Accompanying Map GR2011-1-1
GIS Map GR2011-1-2
Geoscientific Report GR2011-1
√
(folded in back pocket)
√
PDF
√
ArcReader project
vii
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Manitoba Geological Survey
Introduction
The Flin Flon Belt is one of the largest Paleoproterozoic
volcanogenic massive sulphide (VMS) districts in the world, with
over 162 million tonnes of Cu-Zn-(Au) ore (combined reserves
and production to 2007) in 27 past and currently active mines
(Manitoba Innovation, Energy and Mines, 2010). This belt has
been the subject of geological mapping by provincial and federal
surveys for more than 60 years. Although reconnaissance and
regional mapping has been undertaken across the entire width
of the belt and beyond, detailed mapping has focused largely on
the central and southern parts of the belt where the main VMS
deposits have been found. The northern Flin Flon Belt (NFFB),
on the other hand, had received comparatively little attention
until 1986, when mapping was initiated by the author in the
Embury–Wabishkok–Naosap lakes area. Detailed, 1:20 000
scale mapping was subsequently extended to the northern
margin of the volcanic terrane of the Flin Flon Belt, where it
is bounded by predominantly metasedimentary gneisses of
the Kisseynew Domain, and eastward to the contact with an
extensive granitoid terrane at Naosap Lake (Figures 1, 2 and 3).
The results of this mapping have been documented in various
Manitoba Geological Survey (MGS) reports, preliminary maps
and one open file published between 1986 and 2004 (Gilbert,
1986a, b, 1987a, b, 1988a, b, 1989a, b, 1990a, b, 1996a, b,
1997a, b, 1998, 1999, 2001a, b, 2002a, b, 2003a, b, 2004).
Geological data gathered over the course of eleven field
seasons have been compiled in Map GR2011-1-1 (in back
pocket), which integrates all mapping information and interpretation of the associated geochemical data. This report
contains descriptions of the various tectonostratigraphic components and a discussion of their crustal setting, inferred
magmatic sources, structural history and economic geology.
Analytical data on the geochemistry of 344 rock samples
collected between 1986 and 2004 are provided in Appendix 1, which also includes mineralogical, petrographic and lithological notes for each sample. Appendix 2 contains a comprehensive series of field photographs as well as a less broad
based collection of photomicrographs. Various map legends
for the original 1:20 000 scale preliminary maps in the
Embury–Wabishkok–Naosap lakes area have been integrated
into a single unified legend in Map GR2011-1-1, based on
lithostratigraphic as well as geochemical criteria. This map
complements previous mapping in the Flin Flon–White
Lake area to the south (Figure 4; Bailes and Syme, 1989) by
highlighting lateral stratigraphic and geochemical differences,
and similarities within various tectonic components that extend
from the south-central to the northern part of the Flin Flon Belt.
Map GR2011-1-1 also provides a link to geological data in the
area north of the Flin Flon Belt, continuous with the latter via
a regional basaltic formation that extends northwards from the
NFFB and laterally for over 100 km along the southern margin
of the Kisseynew Domain (Zwanzig, 2010).
55° 33’N
Phanerozoic
Ordovician
Paleoproterozoic
Saskatchewan
Manitoba
KISSEYNEW DOMAIN
Granite, granodiorite
Diorite, enderbite
Southern
Indian
Lake
Arkosic gneiss
Greywacke gneiss
Sherridon
Tonalite,
granodiorite
Map GR2011-1-1
Figure 2
FLIN
Elbow
FLON Lake
DOMAIN
Arc volcanic rocks
Back-arc basalt,
arc-rift basalt
Snow Lake
Domain boundary
Elbow-Athapapuskow
EA ocean-floor assemblage
Figure 4
Athapapuskow
Lake
EA
Reed Lake
97° 30’W
101° 51’W
Flin Flon
54° 30’N
Figure 1: Regional geology map of part of the Reindeer Zone in the southern Trans-Hudson Orogen, west-central Manitoba. The
Flin Flon Domain (Flin Flon Belt) consists of volcanic, sedimentary and intrusive rocks in the Flin Flon–Snow Lake area, south of the
Kisseynew Domain and north of the flat-lying Ordovician rocks that extend southwards from Athapapuskow and Reed lakes. The
northern Flin Flon Belt subarea is shown in Figure 2 (inset), which corresponds approximately to the outline of Map GR2011-1-1.
Figure 4 (inset) shows the lateral extent of tectonically defined components between the NFFB and the south-central part of the
western Flin Flon Belt (Bailes and Syme, 1989).
Geoscientific Report GR2011-1
1
2
Manitoba Geological Survey
Figure 2: Geology of the northern part of the western Flin Flon Belt, west-central Manitoba, showing the main tectonostratigraphic components. Numbered localities of arc rocks
within the Dismal Lake assemblage are characterized by base-metal sulphide mineralization.
54°48’N
Cliff
Lake
Precipice
Lake
IAF
CLA
Embury
Lake
ELA
SBF
Gravel road
Lake
NAF
Mikanagan
Lake
Tartan
Lake
Sourdough Bay Fault
Lac Aimée Fault Zone
Bluenose
LAFZ
Location of fault breccia
Northeast Arm Fault
Inlet Arm Fault
IAF
NAF
Embury Lake antiform
ELA
Fault, shear zone
D3
D2
D1
Cope Lake anticline
CLA
Animus
Lake
LA
FZ
SBF
Alberts
Lake
Lac
Aimée
Lake
Wabishkok
Kisseynew
Lake
0
Kotyk
Lake
Dismal
Lake
1
3
kilometres
2
Naosap
Lake
Blueberry
Lake
4
Ham
Lake
5
54°58’N
Figure 3: Structural geology of the northern part of the western Flin Flon Belt, west-central Manitoba, showing the main faults, shear zones and axial traces of D1 to D3 major folds.
101°53’W
Saskatchewan
Manitoba
Geoscientific Report GR2011-1
101°43’W
Axial trace of major fold (anticline, syncline):
101°21’W
3
Sedimentary rocks
Arkosic sandstone,
conglomerate (Missi Group)
Cope Lake arc
Turbidite sediment
Tartan Lake arc
Wabishkok Lake arc
MORB-like
(Arthurs, Bluenose,
Mikanagan lakes)
Manistikwan Lake arc, arc-rift
MLB Manistikwan Lake Block
CLB Cope Lake Block
EML East Mikanagan Lake Block
ELB Embury Lake Block
WML West Mikanagan Lake Block
SLB
Whitefish Lake–
Mikanagan Lake Block
Gabbro
Sourdough Bay arc
Fault, inferred
Scotty Lake arc-rift
Trout Lake mine
orebody
Grassy Narrows Zone
Tartan Lake gold
mine
Flin Flon arc
HLB Hook Lake Block
WLB
Granodiorite, tonalite
Whitefish Lake–Mikanagan
Lake arc
Hook Lake arc
Volcanic rocks
Depleted MORB
(Bluenose Lake)
Intrusive rocks
East Mikanagan Lake
arc-rift
Bear Lake arc
Bluenose
Lake
Scotty Lake Block
101°53’W
54°53'N
TARTAN LAKE BLOCK
A
CLB
ELB
Mikanagan
Lake
ug
hB
ay
urd
o
So
A Gilbert (1986-1990; 1996-1997; 2001-2004)
au
lt
B Bailes and Syme (1979-1989)
Ba
ugh
rdo
kilometres
SLB
Sou
st
5
thea
4
Nor
3
101°37’30”W
2
Bear
Lake
yF
Schist Lake
(NW arm)
1
Whitefish
Lake
Arm
Inlet
FLIN
FLON
BLOCK
0
lt
Big Island
Lake
WLB
Fau
HLB
Arm
MLB
EML
Fault
B
t
Cliff
Lake
WML
BEAR LAKE
BLOCK
Embury
Lake
ul
B
HLB
Fa
H
ARTHURS Arthurs
Lake
MLB
LAKE
BLOCK
L
Saskatchewan
Manitoba
Tartan
Lake
Athapapuskow
Lake
54°43’N
Figure 4: Geology of the western part of the Flin Flon Belt, west-central Manitoba, showing the lateral extent of tectonically defined
components between the northern Flin Flon Belt and south-central part of the Flin Flon Belt (Bailes and Syme, 1989). Note that Big
Island Lake is known unofficially as Manistikwan lake, also used to name the fault block that contains Big Island Lake.
4
Manitoba Geological Survey
The following text summarizes previous work, describes
the regional geology and provides an outline of the main
tectonostratigraphic components with descriptions of the
various volcanic rock suites in the NFFB. Five arc volcanic
suites in the western and northern parts of the study area are
described first (Bear Lake, Cope Lake, Hook Lake, Tartan
Lake and Wabishkok Lake suites), followed by three arc/arcrift suites (Manistikwan Lake1, East Mikanagan Lake and
Lac Aimée) and two tholeiitic to calcalkaline arc suites at
the eastern margin of the NFFB (Sourdough Bay and Naosap
Lake). Descriptions of the arc volcanic suites and various
back-arc types with mid-ocean-ridge basalt (MORB)-like and
depleted-MORB compositions are followed by a discussion of
their tectonic setting and magmatic origin. A short section on
the structural history of the NFFB is also provided, followed by
a final section on the economic geology of the area.
Map GR2011-1-1 (in back pocket) and this report with
associated figures are presented in both print form and on DVD
(also in back pocket). Additional components that are provided
only on the DVD are as follows:
1) Appendix 1: geochemical data and notes (include mineralogical, petrographic and lithological data for analyzed rock
samples)
2) Appendix 2: field photographs and photomicrographs
3) Map GR2011-1-2: an interactive GIS map with locations
of geochemically analyzed rock samples (listed in Appendix 1) as well as field photographs and photomicrographs
(listed in Appendix 2)
Analytical data and photographic images for specific map
units may be readily accessed either in the appendices (where
the data is referenced and grouped according to the map units),
or directly on the GIS map. In addition, geochemical data are
compiled into averages for individual volcanic rock suites, and
Sm-Nd isotopic data are presented in separate tables within the
text below.
Previous work
The NFFB was mapped previously at a scale of 1 inch to
1 mile (1:63 360) by Tanton (1941) and Bateman and Harrison
(1945). Detailed mapping at a scale of 1:5000 was carried out by
Peloquin and Gale (1985) in the Tartan Lake area. Compilations
of mineral deposit data (Gale and Eccles, 1988a, b, c; Gale
and Norquay, 1996), mineral deposit studies (Gale, 2001) and
detailed mapping (Gale and Dabek, 2002) were carried out in
the NFFB concurrent with mapping by the author.
In 1979, a detailed geological mapping program was
initiated by the MGS in the Flin Flon–White Lake area
(Bailes and Syme, 1989), just south of the area covered by
Map GR2011-1-1. Numerous geological investigations and
targeted research projects have subsequently been carried out
in the central and southern portions of the Flin Flon Belt, both
during and after completion of mapping in the NFFB by the
author, of which only a selection are cited here (Stern et al.,
1995a, b, 1999; Lucas et al., 1996; NATMAP Shield Margin
Project Working Group, 1998; Syme, 1998; Syme et al.,
1999; Ames et al., 2002; Devine et al., 2002; Jonasson et al.,
2002; Mitchinson et al., 2002; Gibson et al., 2003a, b, 2005,
2009; Kremer and Simard, 2007; MacLachlan and Devine,
2007; Simard and Creaser, 2007; DeWolfe, 2008, 2009, 2010;
DeWolfe et al., 2009a, b, 2010; Ordóñez-Calderón et al., 2009,
2011; Simard and MacLachlan, 2009; Babechuk and Kamber,
2010; Rayner, 2010).
Regional geology
The Paleoproterozoic Flin Flon Belt is part of the internal
Reindeer Zone within the Trans-Hudson Orogen and consists
largely of juvenile (mantle-derived) crust (Lewry, 1981; Stauffer,
1984; Syme, 1990; Thom et al., 1990; Stern et al., 1995a). The
belt contains a series of tectonostratigraphic assemblages that
are thought to represent components of an island-arc oceanfloor volcanic system (i.e., juvenile arc, back-arc ocean-floor,
ocean plateau, ocean-island basalt and evolved plutonic arc)—
similar, in some respects, to present-day analogues in the
western Pacific Ocean (Stern et al., 1990; Martinez and Taylor,
2003). These assemblages, which range in age from 1.87 to
1.91 Ga (Stern et al., 1999), were amalgamated ca. 1.88–
1.87 Ga into an accretionary tectonic complex (Amisk Collage,
Lucas et al., 1996). The accretionary complex was subsequently
‘stitched’ together by calcalkaline plutons related to 1.87–
1.84 Ga successor-arc magmatism penecontemporaneous with
≤1.87 Ga marine to subaerial volcanism and sedimentation
(‘Schist-Wekusko assemblage’ of NATMAP Shield Margin
Project Working Group, 1998; ‘Schist-Wekusko suite’ of Stern
et al., 1999). Uplift during the waning stage of successor-arc
magmatism resulted in unroofing of the Amisk Collage, leading
to turbidite sedimentation (ca. 1.86–1.84 Ga Burntwood Group,
Machado et al., 1999) in marine basins, and the development
of a paleosol and sedimentation in continental alluvial-fluvial
environments (ca. 1.846–1.842 Ga Missi Group; Ansdell
et al., 1992, 1993, 1999; Heaman et al., 1992; Syme et al.,
1993). Intracontinental tectonic activity ca. 1.84–1.78 Ga
associated with convergence of bounding Archean cratons
resulted in segmentation of the Flin Flon Belt into a series
of strike-slip fault blocks of volcanic and sedimentary rocks
(Lucas et al., 1996). Geochemically distinct volcanic rock
suites of contrasting tectonic affinity (e.g., arc, back-arc oceanfloor) are now juxtaposed or locally separated by intervening
tectonic slices of younger, sedimentary rocks (Missi and
Burntwood groups) of successor-arc age (Lucas et al., 1996).
The various allochthons were locally intruded by 1.83–1.84 Ga
late successor-arc granitic plutons, further deformed and
variably metamorphosed during the final, ca. 1.77 Ga stage of
the Trans-Hudson orogeny.
The Flin Flon arc assemblage consists mainly of tholeiitic
basalt and basaltic andesite, with subordinate heterolithic
breccia, felsic volcanic rocks and epiclastic rock types. All of
the VMS deposits mined to date in the western Flin Flon Belt
Manistikwan lake is an unofficial name for Big Island Lake. In this report, the geological terms Manistikwan Lake Block (Bailes and Syme, 1989) and Manistikwan Lake suite are used for the tectonostratigraphic feature and its associated volcanic rocks, respectively, whereas ‘Big Island Lake’ is used for the geographical
feature.
1
Geoscientific Report GR2011-1
5
are hosted by this juvenile-arc assemblage (Syme and Bailes,
1993). The rocks of the Flin Flon arc assemblage range in age
from 1881 to 1889 Ma (Lucas et al., 1996; Stern et al., 1999;
Rayner, 2010). The Flin Flon mine rhyolite within the Flin Flon
Block is the oldest part of this assemblage; it yielded an age of
1903 +7/–5 Ma (Stern et al., 1999), which has subsequently
been constrained to 1887 ±2 Ma (Rayner, 2010).
In contrast to the Flin Flon arc assemblage, the
compositionally MORB-like ocean-floor assemblage consists
of back-arc basalt that is lithologically simple and virtually
devoid of mineralization. It consists almost exclusively of
subaqueous basalt flows intercalated with synvolcanic gabbro
and ultramafic intrusions. Most of these rocks occur in a broad,
northeast-trending collage (1.90 Ga Elbow-Athapapuskow
ocean-floor assemblage, Stern et al., 1994, 1995b) that extends
along the southern and eastern margins of the Flin Flon arc
assemblage in the western Flin Flon Belt (NATMAP Shield
Margin Project Working Group, 1998, Figure 1b; Figure 1).
Compositionally similar rocks occur as tectonic enclaves
intercalated with fault blocks of arc rocks in the NFFB, and
similar but geochemically more depleted MORB-like rocks
extend along the northern margin of the western Flin Flon Belt.
Postaccretion, successor-arc sedimentary rocks of the
Missi Group occur 1) in a large structural basin in the western
Flin Flon Belt, in the vicinity of the city of Flin Flon; 2) in
several fault-bounded enclaves farther to the east; and 3) at
Athapapuskow Lake to the south (Gilbert, 1986a, 1987a, 1990a,
2002a; NATMAP Shield Margin Project Working Group, 1998,
sheet 2; Bailes and Syme, 1989). A series of turbiditic fault
slices in the NFFB are of uncertain age, but at least one turbidite
deposit is interpreted as part of the (successor-arc) Burntwood
Group. Missi Group rocks were not examined in detail by the
author; for more information on these rocks the reader is referred
to the earliest description by Bruce (1918), and subsequent
publications including those of Mukherjee (1971), Stauffer and
Mukherjee (1971), Bailes and Syme (1989), Stauffer (1990) and
Gale et al. (1999). Burntwood Group turbidites were the main
focus of detailed mapping that resulted in a comprehensive
description of the rocks by Bailes (1980).
Geology of the northern Flin Flon Belt
Map GR2011-1-1 covers an area that is approximately
20 km by 33 km in size and is underlain by more than 20
stratigraphically and tectonically distinct blocks or fault slices
that contain volcanic and/or sedimentary successions. Most
of these fault blocks are dominated by volcanic rocks, which
are grouped into three main compositional types: 1) juvenile
arc and arc-rift (unit J); 2) normal mid-ocean-ridge basalt
(N-MORB) and enriched mid-ocean-ridge basalt (E-MORB;
unit F); and 3) depleted MORB (unit T; Tables 1 and 2). The
rocks with various MORB-like compositions are interpreted as
products of back-arc magmatism as opposed to volcanism in a
mid-ocean-ridge setting.2
Arc rocks are predominant in the NFFB and are part of
the 1881–1889 Ma Flin Flon arc assemblage (Rayner, 2010).
MORB-like volcanic rocks (unit F) are structurally intercalated
with juvenile-arc rocks, and an extensive area of depleted
MORB (unit T) extends along and across the northern boundary
of the Flin Flon Belt, where it is locally in fault contact with
the Kisseynew Domain to the north (Gilbert, 2004; Figure 2).
The age of these volcanic rocks (units F, T) is not known, but
similar MORB-type rocks in the southern part of the Flin Flon
Belt that may possibly be coeval with those in the NFFB are
1901–1904 Ma in age (Stern et al., 1995b).
A series of lensoid turbidite deposits (units S1 to S7)
are intercalated with the various arc- and MORB-type fault
blocks between Embury Lake and Naosap Lake (Figure 2;
Map GR2011-1-1). Most of these sedimentary rocks are fault
bounded and apparently devoid of volcanic interlayers; their
age is uncertain, except for those occupying the Embury
Lake Block that are of successor-arc age (1843 ±9 Ma, age of
youngest detrital zircon, Ordóñez-Calderón et al., 2011; see
‘Cope Lake suite’ below).
Fluvial-alluvial sedimentary rocks of successor-arc age
(Missi Group, units M1 to M3) occupy a 6 km wide sedimentary
basin in the Cliff Lake area, western Flin Flon Belt (Figure 2).
These rocks extend laterally for over 40 km, from Manitoba
westward into Saskatchewan (Figure 1b, NATMAP Shield
Margin Project Working Group, 1998), but the original size
of the sedimentary basin is not known because the rocks are
tectonically attenuated. Missi Group polymictic conglomerate
and sandstone were deposited in the 1846–1842 Ma time
interval (Stern et al., 1999), concurrent with uplift of older
volcanic terranes (units J, T and F) and younger successor-arc
plutonism (unit P).
Granitoid plutons (units P1 to P4) are emplaced in both
arc and MORB-type volcanic rocks along the northern and
eastern margins of the NFFB (Figure 2). Although none of
these intrusions has been dated, their setting suggests they
are stitching plutons of late successor-arc age (ca. 1.84 Ga).
In contrast, smaller granitoid intrusions (unit P1) that occur
sporadically within the Flin Flon arc assemblage (e.g., in Tartan
Lake, Manistikwan Lake and Lac Aimée blocks) are probably
synvolcanic, penecontemporaneous with the 1886–1888 Ma
Cliff Lake plutonic suite (unit J19) within the Hook Lake Block
(Stern et al., 1999; Rayner, 2010).
Mafic and subordinate ultramafic intrusions occupy
over 15% of the area of the NFFB; they are classified as 1)
compositionally layered sills (units J16, J17 and J18), and
2) mafic to ultramafic intrusions of uncertain age (unit X;
Table 1). Batters Lake, Tartan Lake and Mikanagan Lake sills
(units J16, J17 and J18) are lithostratigraphically similar and
probably coeval, synvolcanic intrusions. The 1881 +3/–2 Ma,
Mikanagan Lake Sill (Stern et al., 1999) is emplaced in the
upper part of an arc-rift sequence and has been interpreted as
rift-related (Zwanzig et al., 2001). Other more homogeneous
The terms MORB, ‘MORB-like’ and ‘MORB-type’ in this report denote only compositional equivalence or similarity to MORB; they do not imply a mid-oceanridge setting.
2
6
Manitoba Geological Survey
Table 1: Geological formations, northern Flin Flon Belt, west-central Manitoba.
Paleoproterozoic
SUCCESSOR-ARC AND YOUNGER ROCKS (1.84–1.88 Ga)
L
Late intrusive rocks (may be <1.84 Ga)
Quartz-feldspar porphyry, felsite
P
Granitoid intrusive rocks (may include rocks older than 1.88 Ga)
PISTOL LAKE, KOTYK LAKE, NAOSAP LAKE PLUTONS
Granodiorite, granite, tonalite, quartz diorite, quartz-feldspar porphyry, related gneiss
X
Mafic to ultramafic intrusive rocks (age uncertain, may be >1.88 Ga)
TARTAN LAKE COMPLEX, WABISHKOK LAKE SILL, KOTYK LAKE SILL
Gabbro, quartz gabbro, leucogabbro, anorthositic gabbro, diorite, pyroxenite, hornblendite, diabase
M
Missi Group (1.84–1.85 Ga)
Polymictic conglomerate, sandstone, pebbly sandstone, related gneiss, meta-arenite, metarhyolite
VOLCANIC, INTRUSIVE AND SEDIMENTARY ROCKS (1.88–1.90 Ga)
S
Sedimentary rocks of uncertain age (includes some rocks probably ≤1.86 Ga in age)
BARTLEY LAKE, SMOOK LAKE, TARTAN LAKE, NAOSAP LAKE, SWORDFISH LAKE, EMBURY LAKE
Feldspathic greywacke, siltstone, argillite, schist, conglomerate, chert
J
Juvenile arc and arc-rift rocks
Granitoid intrusive rocks
CLIFF LAKE PLUTONIC SUITE (1886 ±1 Ma, Stern et al., 1999; 1888 ±1 Ma, Rayner 2010)
Tonalite, quartz diorite
Mafic to ultramafic intrusive rocks (compositionally layered sills)
BATTERS LAKE SILL, TARTAN LAKE SILL, MIKANAGAN LAKE SILL (1881 +3/-2 Ma, Stern et al., 1999)
Gabbro, gabbronorite, quartz gabbro, diorite, hornblendite, quartz-eye tonalite, ferrotonalite
Felsic volcanic rocks
BAKER PATTON COMPLEX, COPE LAKE, LAC AIMÉE, BARTLEY LAKE
Rhyolite, dacite flow/sill, related fragmental rhyolite, felsic volcanic breccia, tuff, plagioclase±quartz porphyry
Mafic to intermediate volcanic rocks
BEAR LAKE (1885 ±3 Ma, Stern et al., 1993), COPE LAKE, HOOK LAKE (1882 ±1 Ma, Rayner, 2010), TARTAN LAKE, WABISHKOK
LAKE, MANISTIKWAN LAKE1, EAST MIKANGAN LAKE, LAC AIMÉE, SOURDOUGH BAY, NAOSAP LAKE
Basalt, basaltic andesite, andesite, autoclastic breccia, heterolithic volcanic breccia, tuff
T
Depleted-MORB mafic volcanic rocks
DISMAL LAKE, BLUENOSE LAKE
Basalt, basaltic andesite
F
MORB-type mafic volcanic rocks
E-MORB
ANIMUS LAKE, WEST MIKANAGAN LAKE
Basalt
N-MORB
ARTHURS LAKE, BLUENOSE LAKE, ANIMUS LAKE
Basalt, plagioclase-megaphyric basalt, related gabbro
U
Undivided mafic volcanic rocks
Basalt, andesite
1
Manistikwan lake is an unofficial name for Big Island Lake.
Geoscientific Report GR2011-1
7
Table 2: Tectonic setting, geochemical affinity and rock types of volcanic suites in the northern Flin Flon Belt, west-central Manitoba.
Volcanic rock suite
Tectonic
setting
Map units
Tholeiitic/calcalkaline type
Rock types
(in order of abundance)
Juvenile arc and arc-rift volcanic suites (J)
Naosap Lake
arc
J10a, J10b
calcalkaline
Andesite to dacite flow, autoclastic
breccia, heterolithic volcanic breccia,
related porphyroblastic gneiss
Sourdough Bay
arc
J12, J9
tholeiitic to calcalkaline
Felsic volcanic flow, related breccia and
intrusive rocks; basalt to andesite flow,
autoclastic breccia, heterolithic volcanic
breccia
Lac Aimée
arc, arc-rift
J8a, J8b,
J11, J14
tholeiitic
Basalt to andesite flow, autoclastic
breccia, heterolithic volcanic breccia,
tuff; felsic volcanic flow, related breccia
and intrusive rocks
East Mikanagan Lake
arc-rift
J7
tholeiitic
Basalt, autoclastic breccia
Manistikwan Lake1
arc, arc-rift
J6, J11
tholeiitic to calcalkaline
Wabishkok Lake
arc
J5, J11
tholeiitic
Tartan Lake
arc
J4, J11
tholeiitic to calcalkaline
Hook Lake (rhyolite, south of the
area covered by Map GR2011-1-1,
yields 1882 ±1 Ma)2
arc
J3, J11
tholeiitic
Cope Lake (possibly coeval rocks
at Trout Lake mine yield 1878 ±1.1
Ma)3
arc
J2, J11, J13
calcalkaline
Bear Lake (tuff, south of the area
covered by Map GR2011-1-1, yields
1885 ±3 Ma)4
arc
J1, J11, J15
calcalkaline
Bluenose Lake
back-arc/
ocean floor
T2
tholeiitic
Basalt, basaltic andesite
Dismal Lake
back-arc/
ocean floor
T1
tholeiitic
Basalt, basaltic andesite
Basalt to andesite flow, autoclastic
breccia, heterolithic volcanic breccia;
felsic volcanic flow, related breccia
and intrusive rocks
Depleted-MORB volcanic suites (T)
MORB-type volcanic suites (F)
(synvolcanic gabbro in compositionally equivalent Elbow-Athapapuskow ocean-floor assemblage in the southeast part of the Flin Flon Belt yields
ages of 1901–1904 Ma)5
West Mikanagan Lake (E-MORB)
back-arc/
ocean floor
F5
tholeiitic
Basalt
Animus Lake (N-MORB, E-MORB)
back-arc/
ocean floor
F3, F4
tholeiitic
Basalt
Bluenose Lake (N-MORB)
back-arc/
ocean floor
F2
tholeiitic
Basalt
Arthurs Lake (N-MORB)
back-arc/
ocean floor
F1
tholeiitic
Basalt, plagioclase-megaphyric basalt,
related gabbro
1
Manistikwan lake is an unofficial name for Big Island Lake; 2,3 N. Rayner, 2010; 4 Stern et al., 1993; 5 Stern at al., 1995b
gabbroic intrusions (units X1 to X6), emplaced both within arc
and MORB-type sequences, may also be synvolcanic in age
(e.g., Wabishkok Lake and Kotyk Lake sills and intrusions in
the Dismal Lake assemblage, Figure 2). The tectonic affinity
and age of the heterogeneous Tartan Lake complex (unit X2;
Gilbert, 1990a) are, however, unknown.
Juvenile arc and arc-rift volcanic suites (units J1 to J15)
Subaqueous mafic volcanic and related intrusive rocks of
juvenile-arc affinity are the main components of the NFFB,
where 10 tectonically distinct arc volcanic suites are structurally
juxtaposed with MORB-like volcanic rocks and sedimentary
fault slices (Figure 2). The arc volcanic rocks consist of a wide
range of massive to fragmental types and associated intrusions.
These volcanic suites are typically bimodal, consisting mainly
of basalt or basaltic andesite and dacite-rhyolite3. Based on the
FeO/MgO ratios (Mg#) and the discriminant diagrams of Jensen
(1976) and Irvine and Baragar (1971), the arc volcanic suites
are classified as follows: 1) tholeiitic—Hook, Wabishkok and
Lac Aimée suites; 2) transitional (calcalkaline to tholeiitic)—
Tartan Lake and Sourdough Bay suites; and 3) calcalkaline—
Bear Lake, Cope Lake and Naosap Lake suites (Figure 5a to 5f).
Terminology of volcanic rock types, according to SiO2 content, is as follows: basalt 46–53%, basaltic andesite 53–57%, andesite 57–62%, dacite 62–70%, rhyolite >70%. Geochemical samples were obtained from localities without visible alteration or veining.
3
8
Manitoba Geological Survey
a)
b)
T
FeO +TiO2
T
FeO
Tholeiitic
Tholeiitic arc volcanic rocks
Hook Lake
Wabishkok Lake
Lac Aimée
Manistikwan Lake
(south-central part)
Tholeiitic
Calcalkaline
Komatiitic
Calcalkaline
Al2O3
MgO
c)
Na2O + K2O
MgO
d)
T
FeO +TiO2
T
FeO
Tholeiitic
Transitional (tholeiitic-calcalkaline)
arc volcanic rocks
Tartan Lake
Sourdough Bay
Tholeiitic
Komatiitic
Calcalkaline
Calcalkaline
Al2O3
MgO
e)
Na2O + K2O
MgO
f)
T
FeO +TiO2
Calcalkaline arc volcanic rocks
Bear Lake
Cope Lake
Naosap Lake
T
FeO
Tholeiitic
Tholeiitic
Calcalkaline
Komatiitic
Calcalkaline
Al2O3
MgO
Na2O + K2O
MgO
Figure 5: Ternary diagrams (Al2O3–[FeOT+TiO2]–MgO) after Jensen (1976) and ([Na2O+K2O]–FeOT–MgO) after Irvine and Baragar
(1971), for mafic to felsic arc volcanic rocks in the northern Flin Flon Belt, west-central Manitoba: a) and b) tholeiitic—plots of
Manistikwan Lake suite rocks from Bailes and Syme, 1989; c) and d) transitional (tholeiitic to calcalkaline); and e) and f) calcalkaline
volcanic suites. Abbreviations: FeOT = FeO + (0.8998 x Fe2O3).
Geoscientific Report GR2011-1
9
Basalt, basaltic andesite and related fragmental rocks are the
predominant rock types; felsic volcanic rocks (units J11 to J15)
are generally minor components, except in the southeastern part
of the study area where rhyolitic types form approximately 15%
of the Lac Aimée Block and >50% of the northern part of the
Sourdough Bay Block (Figure 2).
flows (Gilbert, 1986a). Homogeneous gabbro lenses up to
300 m wide, abundant synvolcanic mafic dikes and sills, and
occasional lava tubes are intercalated with the flows. The dikes
and sills, typically 1–5 m thick, are locally amygdaloidal and,
in some cases, display banded margins attributed to chilling of
separate intrusive pulses.
Geochemically distinctive volcanic rocks, interpreted
as products of early arc-rifting, occupy the East Mikanagan
Lake Block (unit J7), and similar rocks are intercalated with
arc types in the Lac Aimée and Manistikwan Lake blocks
(units J8 and J6). Contacts between arc and arc-rift rocks are
commonly faulted, but locally these rock types appear to be
stratigraphically and compositionally gradational.
The two-fold geochemical subdivision of the Bear Lake
suite (lower calcalkaline and upper alkalic, shoshonitic section)
is unique among arc volcanic suites in the Flin Flon Belt, most
of which are uniformly tholeiitic or of transitional, tholeiitic
to calcalkaline affinity (Stern et al., 1995a). In the NFFB,
the lower part of the Bear Lake succession is predominantly
basaltic (13 flows, average of 51.1% SiO2, average Mg# of 0.60,
Table 3, back pocket), whereas stratigraphically equivalent,
least altered rocks in the central Flin Flon Belt consist mainly
of basaltic andesite (9 flows, average of 53.08% SiO2, average
Mg# of 0.55, based on analytical data [including volatiles] in
Bailes and Syme, 1989). One interpretation of this variation
in SiO2 content and Mg# is that the lensoid volcanic edifice is
diachronous; the relatively distal, basaltic part of the volcanic
lens in the NFFB was likely erupted prior to the slightly more
fractionated, basaltic andesite part farther southeast, closer to
the source of the volcanic rocks. The Bear Lake suite has been
interpreted as erupted from a single volcanic centre during
an interval of steadily decreasing water depth, with the upper
section (Vick Lake tuff) rapidly deposited by turbidity currents
in a subsiding basin (Bailes and Syme, 1989).
Bear Lake suite (unit J1, calcalkaline arc)
The Bear Lake suite occupies a lensoid tectonic block that
extends laterally for over 30 km from the central part of the Flin
Flon Belt, north and northwest to the Manitoba-Saskatchewan
boundary (Figure 2). The Bear Lake suite is thickest in the
central Flin Flon Belt where it forms a monoclinal, east to
northeast-facing calcalkaline sequence of basaltic andesite
flows and fragmental rocks up to 3.3 km thick, capped by thin
(25 m) pelagic sedimentary rocks and overlain by an upper
alkalic section of volcaniclastic rocks up to 900 m thick (Vick
Lake andesitic tuff, Bailes and Syme, 1989). A mafic-felsic
bimodal sequence—Two Portage Lake ferrobasalt and rhyolite
tuff (1886 ±2 Ma, Gordon et al., 1990)—occurs between the
basaltic calcalkaline sequence and overlying Vick Lake tuff
(1885 ±3 Ma, Stern et al., 1993). The upper, volcaniclastic
section wedges out northwards in the vicinity of southern
Mikanagan Lake, where small remnants of the tuff persist at the
southwestern shoreline. Up to 1.5 km of subsidence is estimated
to have occurred after the initial Bear Lake mafic volcanism,
mainly during deposition of the upper volcaniclastic sequence
(Bailes and Syme, 1989).
The Bear Lake suite in the NFFB consists of a 3 km
wide sequence of massive to fragmental volcanic rocks
that diminishes in thickness progressively northwestwards
toward the Manitoba provincial boundary, where the width
is less than 150 m. This sequence, which is stratigraphically
equivalent to the lower, calcalkaline part of the Bear Lake
suite in the central part of the Flin Flon Belt, contains aphyric
to plagioclase±hornblende±pyroxene–phyric basaltic flows,
related flow breccia and synvolcanic intrusions. The majority
of flows are characterized by ovoid to bun-shaped pillows,
typically 0.5–2.0 m in diameter. Size variation of individual
pillows reflects their stratigraphic level within the flow; large
‘mega-pillows’ (up to 5 by 13 m) occur in the lowermost
parts of some flow units, whereas small amoeboid pillows are
typical of upper zones. Several features reflect rapid cooling
of the volcanic rocks at shallow to intermediate water depth,
such as widespread vesicularity and amygdaloidal texture, as
well as concentric zoning and cooling fractures within pillows.
Flow brecciation and interpillow hyaloclastic tuff are also
characteristic of the Bear Lake suite. Subordinate, heterolithic
fragmental deposits up to 350 m thick are interpreted as debris
4
The TiO2 versus MgO plot (Figure 6a) shows Bear Lake
mafic to intermediate volcanic rocks occur in a calcalkaline
subfield (Syme et al., 1999) within the field of NFFB arc
volcanic rocks. Compared to tholeiitic and transitional tholeiitic
to calcalkaline arc suites in the NFFB, Bear Lake basalt has a
higher average Mg number (Mg# of 0.60 versus 0.40–0.52),
and lower FeOT 4 (9.8% versus 10.7–12.1%) and TiO2 (0.40%
versus 0.44–0.64%; Table 3, back pocket). Bear Lake basalt Ni
(67 ppm) and Cr (305 ppm) contents are over twice the combined
average values of all other NFFB arc basalts. In incompatibleelement plots, however, Bear Lake calcalkaline volcanic rocks
are not readily distinguished from tholeiitic or transitional
suites (Figure 7a to 7h). Bear Lake rocks display an arc-type,
rare earth element (REE) profile with pronounced negative Nb
and Ti anomalies, elevated Th and light rare-earth elements
(LREE), and depleted middle to heavy rare-earth elements
(HREE), relative to modern N-MORB. The Th/Nb ratios are
significantly higher in arc rocks compared to N-MORB and
E-MORB rock types, which are clearly distinguished in the Th/
Nb versus Nb/Y diagram (Figure 8a). The εNd isotopic ratio for
Bear Lake basalt to basaltic andesite (average +4.1, range from
+3.8 to +4.8 at 1.90–1.86 Ga, Stern et al., 1995a) is in the range
estimated for depleted mantle of the same age (from +3.0 to
+5.5, Lucas et al., 1996) and indicates the source magma for the
Bear Lake suite was juvenile (Figure 9).
Cope Lake suite (unit J2, calcalkaline arc)
Massive to fragmental arc volcanic rocks occupy the
elongate Cope Lake Block, up to 1.4 km wide and extending
FeOT = FeO + (0.8998 x Fe2O3)
10
Manitoba Geological Survey
3.0
a)
3.0
Bear Lake arc volcanic
Manistikwan Lake (southcentral part) arc volcanic
2.5
b)
2.5
All NFFB arc volcanic rocks
Bear Lake (south-central part)
calcalkaline arc section
MORB
+
BABB
1.5
1.0
0.5
0.5
2
3.0
4
6
8
MgO
c)
10
12
0.0
14
1.5
0.5
0.5
4
6
8
MgO
10
12
8
MgO
0.0
14
10
12
14
Dismal Lake depleted MORB
Bluenose Lake depleted MORB
MORB
+
BABB
1.5
1.0
2
6
d)
2.0
1.0
0.0
4
2.5
TiO2
2.0
2
3.0
Animus Lake E-MORB
West Mikanagan Lake E-MORB
Arthurs Lake N-MORB
Bluenose Lake N-MORB
Animus Lake N-MORB
Grassy Narrows basalt
MORB
+
BABB
2.5
TiO2
1.5
1.0
0.0
MORB
+
BABB
2.0
TiO2
TiO2
2.0
East Mikanagan Lake arc-rift
Lac Aimée arc-rift
Lac Aimée arc
2
4
6
8
MgO
10
12
14
Figure 6: Plots of TiO2 versus MgO for mafic to intermediate volcanic rocks in the northern Flin Flon Belt (NFFB), west-central
Manitoba: a) field of all NFFB arc volcanic rocks (gray field in all four diagrams) and plots of selected arc types—Bear Lake suite and
southern part of Manistikwan Lake suite (plots of Manistikwan Lake suite rocks from Bailes and Syme, 1989); field of arc volcanic
rocks in the calcalkaline section of the south-central Bear Lake Block after Syme et al. (1999); b) arc and ‘arc-rift’ types (Lac Aimée
and East Mikanagan Lake suites); c) N-MORB and E-MORB suites, and plots of Grassy Narrows basalt (from Bailes and Syme,
1989); d) depleted-MORB types. MORB and BABB compositional field of modern intra-oceanic rocks from Stern et al. (1995b).
Abbreviations: BABB, back-arc basin basalt; MORB, mid-ocean-ridge basalt; E-MORB, enriched mid-ocean-ridge basalt; N-MORB,
normal mid-ocean-ridge basalt.
100
100
10
1
.1
b)
Bear Lake arc
Rock / N-MORB
Rock / N-MORB
a)
100
10
1
.1
Th Nb La Ce Nd Sm Zr Hf Eu Ti Gd Dy Y Er Yb Lu
Cope Lake arc
Th Nb La Ce Nd Sm Zr Hf Eu Ti Gd Dy Y Er Yb Lu
100
10
Geoscientific Report GR2011-1
/ N-MORB
/ N-MORB
c) of N-MORB normalized incompatible-elements for mafic to intermediate
d)
Figure 7: Plots
rocks
in arc volcanic
suites in the northern Flin
Hook Lake arc
Tartan
Lake arc
Flon Belt, west-central Manitoba: a) Bear Lake, b) Cope Lake. Normalizing values from Sun and McDonough (1989). Abbreviation:
N-MORB, normal mid-ocean-ridge basalt.
10
11
.1
100
Rock / N-MORB
10
1
Tartan Lake arc
1
.1
Th Nb La Ce Nd Sm Zr Hf Eu Ti Gd Dy Y Er Yb Lu
100
Sourdough Bay arc
1
100
i)
Scotty Lake basalt
East Mikanagan Lake arc-rift
Lac Aimée arc-rift
1
Th Nb La Ce Nd Sm Zr Hf Eu Ti Gd Dy Y Er Yb Lu
Th Nb La Ce Nd Sm Zr Hf Eu Ti Gd Dy Y Er Yb Lu
h)
Naosap Lake arc
10
1
.1
Th Nb La Ce Nd Sm Zr Hf Eu Ti Gd Dy Y Er Yb Lu
10
.1
.1
100
10
.1
1
Th Nb La Ce Nd Sm Zr Hf Eu Ti Gd Dy Y Er Yb Lu
g)
Lac Aimée arc
10
Rock / N-MORB
.1
f)
Wabishkok Lake arc
Rock / N-MORB
e)
1
Rock / N-MORB
d)
10
Th Nb La Ce Nd Sm Zr Hf Eu Ti Gd Dy Y Er Yb Lu
10
Rock / N-MORB
Th Nb La Ce Nd Sm Zr Hf Eu Ti Gd Dy Y Er Yb Lu
100
100
Rock / N-MORB
100
Hook Lake arc
100
Rock / N-MORB
Rock / N-MORB
c)
.1
.1
Th Nb La Ce Nd Sm Zr Hf Eu Ti Gd Dy Y Er Yb Lu
Th Nb La Ce Nd Sm Zr Hf Eu Ti Gd Dy Y Er Yb Lu
j)
Manistikwan Lake arc-rift
10
1
.1
Th Nb La Ce Nd Sm Zr Hf Eu Ti Gd Dy Y Er Yb Lu
Figure 7 (continued): Plots of N-MORB normalized incompatible-elements for mafic to intermediate rocks in arc volcanic suites in
the northern Flin Flon Belt, west-central Manitoba: c) Hook Lake, d) Tartan Lake, e) Wabishkok Lake, f) Lac Aimée, g) Sourdough
Bay, h) Naosap Lake; arc-rift suites i) Lac Aimée, East Mikanagan Lake and Scotty Lake basalt (data from Bailes and Syme, 1989),
j) Manistikwan Lake. Normalizing values from Sun and McDonough (1989). Abbreviation: N-MORB, normal mid-ocean-ridge basalt.
12
Manitoba Geological Survey
N-MORB:
Arthurs Lake
a)
Bluenose Lake
Animus Lake
Th/Nb
1
E-MORB:
Animus Lake
West Mikanagan Lake
All NFFB arc volcanic rocks
Elbow-Athapapuskow
ocean-floor assemblage
N-MORB types
with arc signatures
.1
E-MORB
N-MORB
.01
Nb/Y
.1
1
Arc-rift:
Manistikwan Lake
Depleted MORB:
Dismal Lake
East Mikanagan Lake
Bluenose Lake
All NFFB arc volcanic rocks
Lac Aimée
Elbow-Athapapuskow
Scotty Lake basalt
ocean-floor assemblage
b)
Th/Nb
1
N-MORB types
with arc signatures
.1
E-MORB
N-MORB
.01
Nb/Y
.1
1
Figure 8: Plots of Th/Nb versus Nb/Y for mafic to intermediate arc volcanic rocks in the northern Flin Flon Belt, west-central Manitoba:
a) arc, N-MORB and E-MORB types, b) arc, arc-rift and depleted-MORB types, and Scotty Lake basalt (after Syme et al., 1999).
Most depleted-MORB rocks do not appear because their Nb content is below the analytical detection limit (applies to 21 out of the
27 Dismal Lake rocks and 6 out of 8 Bluenose Lake rocks, see Appendix 1). Field of Elbow-Athapapuskow ocean-floor assemblage
after Stern et al. (1995b). Compositional fields are from Stern et al. (1995b), based on data in Saunders et al. (1988) and Sun and
McDonough (1989). Abbreviations: MORB, mid-ocean-ridge basalt; N-MORB, normal mid-ocean-ridge basalt; E-MORB, enriched
mid-ocean-ridge basalt; NFFB, northern Flin Flon Belt.
Geoscientific Report GR2011-1
13
5
Nd
(at 1.9 Ga)
3
(% older
crustal Nd)
1%
2%
3%
4%
5%
1
-1
10%
-3
Older
crustal
component
-5
.1
1
10
Nb/Yb
Tartan Lake basalt
Lac Aimée basalt
Lac Aimée rhyolite
Wabishkok Lake basalt
Wabishkok Lake andesite
Arc
volcanic
rocks
Sourdough Bay basaltic andesite
Sourdough Bay rhyolite
Naosap Lake dacite
Bear Lake basalt (south-central part)
Manistikwan Lake basalt (north part)
Scotty Lake basalt
Animus Lake basalt
West Mikanagan Lake basalt
Arc-rift
volcanic rocks
E-MORB volcanic rocks
Animus Lake basalt
Arthurs Lake basalt
N-MORB volcanic rocks
Bluenose Lake basalt
Dismal Lake basalt
Depleted-MORB volcanic rocks
Figure 9: Plot of εNd versus Nb/Yb for mafic to felsic volcanic rocks in the northern Flin Flon Belt (NFFB), west-central Manitoba,
showing the calculated percentage of older crustal Nd (methodology after Stern et al., 1995a) in various volcanic rock suites in the
NFFB, except for Bear Lake basalt (Stern et al., 1995a) and Scotty Lake basalt (Syme et al., 1999), where data are from the southcentral part of the Flin Flon Belt. Manistikwan Lake arc-rift rhyolite samples (εNd 0.7 and –1.7 at 1.90 Ga) would plot close to the line of
10% crustal assimilation (these samples are not plotted due to the lack of Nb analytical data). Abbreviations: MORB, mid-ocean-ridge
basalt; N-MORB, normal mid-ocean-ridge basalt; E-MORB, enriched mid-ocean-ridge basalt.
14
Manitoba Geological Survey
laterally for approximately 14 km along the western side of
Embury Lake and farther south to Big Island Lake (unofficially
known as Manistikwan lake). The Cope Lake suite consists of
predominantly mafic volcanic rocks that are lithologically and
geochemically similar to the lower part of the Bear Lake arc
sequence. The aphyric to porphyritic flows contain up to 30%
plagioclase±hornblende (after pyroxene) phenocrysts, typically
1–5 mm in size. Vesicles, amygdules and localized gas cavities
are common in the massive flows, which are intercalated with
hyaloclastic tuff and related autoclastic breccia. The Cope Lake
suite has been subdivided, on the basis of porphyritic texture,
into six stratigraphic units that are folded in a major northwesttrending syncline (Bailes and Syme, 1989) and a complementary
anticline farther north (Figure 3). The axial trace of the syncline
is truncated by the block-bounding fault at the western margin
of the Cope Lake Block, where it is juxtaposed against the Hook
Lake Block. At the eastern margin of the Cope Lake Block,
sporadic lensoid greywacke units up to 50 m thick, which occur
along the shoreline of Embury Lake, are interpreted as fault
slices derived from the contiguous Embury Lake Block to the
east.
The calcalkaline affinity of the Cope Lake suite is indicated
by ternary discriminant diagrams (Figure 5e, 5f) that reflect
higher Mg# and lower FeOT and TiO2 contents, compared to
arc tholeiitic rocks in the NFFB (Table 3, back pocket). The
Al2O3 content of Cope Lake basalt is relatively higher than in
the tholeiitic volcanic suites, probably due to the widespread
occurrence of plagioclase phenocrysts (stoichiometric Al2O3 in
plagioclase feldspar is approximately 35%, roughly twice the
average Al2O3 content of Cope Lake basalt).
A lensoid, high-silica rhyolite unit within the Cope Lake
Block at the shoreline of Embury Lake (unit J13; ‘Cope Lake
rhyolite’) is compositionally similar to rhyolite that hosts the
Trout Lake mine Cu-Zn ore deposit (Ordóňez-Calderón et al.,
2009), located offshore 0.5 km farther to the east (Figure 4;
Map GR2011-1-1). The Cope Lake rhyolite plots in the field
of ‘extension-related’ rocks in the Zr versus TiO2 diagram
(Figure 10) and is thus identified as prospective for VMStype mineralization (see ‘Felsic volcanic rocks’ below). The
age of a felsic porphyry intrusion (inferred as synvolcanic) at
Trout Lake mine (1878 ±1.1 Ma, N. Rayner, 2010) suggests
that the diverse volcano-sedimentary mine sequence (OrdóňezCalderón et al., 2009) is part of the Flin Flon arc assemblage
(Stern et al., 1995a) and may represent a dismembered part of
the Cope Lake Block (or possibly the Hook Lake Block farther
to the west—see next paragraph). Graphitic greywacke and
argillite (over 0.3 km thick) in the hanging wall, immediately
east of the diverse volcano-sedimentary rocks in the mine
sequence, is considered part of the turbiditic Embury Lake
Block and has a successor-arc depositional age (<1843 ±9 Ma,
age of youngest detrital zircon, Ordóñez-Calderón et al., 2011).
The faulted contact between the Cope Lake and Embury Lake
blocks is thus apparently located within, or immediately east of,
the Trout Lake mine succession.
Hook Lake suite (unit J3, tholeiitic arc)
The Hook Lake Block, located between the Cope Lake
Block to the east and Missi sedimentary basin to the west
(Figure 2), extends laterally for over 30 km from Annabel Creek
at the Manitoba-Saskatchewan provincial boundary, southeast
to the area south of Schist Lake. In the south-central part of the
Flin Flon Belt, the Hook Lake Block has been subdivided by
major faults into five sub-blocks that together have a maximum
stratigraphic thickness of 7.5 km (Bailes and Syme, 1989). More
recently, a two-fold subdivision has been established for the
300
Cope Lake rhyolite
Lac Aimée felsic porphyry
All other NFFB felsic
arc volcanic rocks
250
Zr
200
Extension-related
rhyolite
150
100
50
0
0.0
Arc-assemblage
rhyolite
0.1
0.2
0.3
0.4
0.5
0.6
0.7
TiO2
Figure 10: Plot of Zr versus TiO2 for felsic volcanic rocks and related plagioclase-quartz porphyry intrusions in the northern Flin
Flon Belt (NFFB). Rare earth element–enriched Cope Lake and Lac Aimée felsic rocks (outside the ‘arc assemblage’ field) are
more fractionated than the rest, with higher Zr/TiO2 ratios (as well as higher Th, and negative Eu anomalies). Arc-assemblage and
extension-related rhyolite fields for the Flin Flon Belt are from Syme (1998).
Geoscientific Report GR2011-1
15
Hook Lake Block (‘western’ and ‘eastern’ sequences, Simard et
al., 2010). Geochronological data by Rayner (2010) show that
the older western sequence, which is intruded by a 1888 ±1 Ma
quartz diorite phase of the Cliff Lake plutonic suite, is likely
coeval with ca. 1889 Ma VMS-hosting rocks of the Flin Flon
Block to the west (Simard et al., 2010); whereas the relatively
younger eastern Hook Lake sequence (1882 ±1 Ma) is similar in
age to the 1878 ±1.1 Ma Trout Lake mine sequence to the east.
Conspicuous similarities between part of the eastern Hook Lake
sequence and the Trout Lake mine sequence are consistent with
their possible stratigraphic equivalence and structural repetition
between these two sections (P.D. Kremer, pers. comm., 2009).
In the south-central Flin Flon Belt, the Hook Lake
volcanic suite consists mainly of aphyric to porphyritic or
cumulophyric mafic flows, related flow breccia and subordinate
scoriaceous tuff and breccia (Bailes and Syme, 1989). These
rocks apparently accumulated in a consistently shallowwater (possibly locally subaerial) environment, probably with
associated subsidence. This theory is supported by the presence
of environmental indicators for shallow-water conditions, such
as coarse vesicularity, throughout the sequence. A proximal
environment for this part of the fault block is indicated by such
features as the common thick interbedding of volcanic strata
(10s to 100s of metres) and the occurrence of sporadic ballistic
fragments in some volcaniclastic, resedimented units (Bailes
and Syme, 1989).
Within the NFFB, the northern part of the Hook Lake
Block, like the more proximal south-central part, is dominated
by aphyric to porphyritic basalt and abundant, typically
amygdaloidal autoclastic breccia (Gilbert, 1989a). Subordinate
stratigraphic members include massive to fragmental dacite
and rhyolite (up to 65 m thick), as well as conglomerate (30 m)
containing volcanic and fine-grained, sedimentary clast types.
The synvolcanic Cliff Lake plutonic suite in the central part
of the Hook Lake Block (1886 ±1 Ma, Stern et al., 1999;
1888 ±1 Ma, Rayner, 2010) extends north and northwestward,
between the western and eastern Hook Lake sequences. A major
northwest-trending anticlinal fold occurs within the volcanic
rocks, close and parallel to the western margin of the Cliff Lake
pluton (Figures 2 and 3).
In the south-central part of the Flin Flon Belt, the diverse
mafic to intermediate Hook Lake volcanic sequence contains
rocks of tholeiitic to calcalkaline affinity that were probably
derived from more than one volcanic centre (Bailes and
Syme, 1989). Basalt in the more distal, northern part of the
Hook Lake Block is characterized by relatively higher FeOT
and TiO2 contents and lower Mg# compared to calcalkaline
volcanic types (Table 3, back pocket) and displays tholeiitic to
transitional trends in ternary discriminant diagrams (Figure 5a,
5b). Elevated Al2O3 (18.3%) in Hook Lake basalt, relative to
the average Al2O3 content for all NFFB arc volcanic suites
(15.0%), probably reflects the local abundance of plagioclase
phenocrysts, as in the case of Cope Lake basalt. The average εNd
isotopic ratio for Hook Lake basalt is +3.9 at 1.9 Ga, consistent
with a juvenile magmatic source (Stern et al., 1995a).
5
Tartan Lake suite (unit J4, tholeiitic to calcalkaline
arc)
The Tartan Lake suite is a south-facing, monoclinal
basaltic sequence up to 3 km thick that extends for over 17 km
from Animus Lake in the east to the area beyond the ManitobaSaskatchewan boundary in the west. It is structurally juxtaposed
against MORB-type basalt to the north (unit F2), east (units F3
and F4) and southeast (unit F5). Mafic and felsic intrusions
up to 100 m thick within the basaltic sequence are assumed
to be synvolcanic: felsic intrusions are confined to the west,
whereas gabbro and hornblendite intrusions are most prominent
in the eastern part of the fault block. At the northern margin of
the Tartan Lake Block, a fault sliver of arc rocks is emplaced
within part of the N-MORB Bluenose Lake Block to the north
(Figure 2).
The majority of Tartan Lake mafic flows are pillowed and,
based on top indicators preserved in the central and eastern
parts of the fault block, almost universally south-facing.
Sporadic pillow tops in the area north of the Tartan Lake gold
mine delineate the approximate positions of axial traces of
west-trending folds close to the southern margin of the fault
block (Figures 3 and 4). The mafic volcanic rocks are less well
preserved in the western part of the fault block, where pillows
are more attenuated and partly epidotized. Tectonic strain in
that area is typically localized in discrete, intensely sheared
1–3 m wide zones, which occur within the otherwise relatively
undeformed basaltic sequence.
The Tartan Lake arc volcanic suite is of transitional,
tholeiitic to calcalkaline affinity (Figure 5c, 5d). Incompatibleelement contents are similar to those of other NFFB arc volcanic
rocks (Table 3, back pocket); the N-MORB-normalized plot
(Figure 7d) displays elevated LREE, negative Nb and Ti
anomalies and depleted high-field-strength elements (HFSE),
as described previously for the calcalkaline Bear Lake volcanic
suite. The εNd isotopic ratios for two Tartan Lake basalt flows
are +2.3 and +2.8 at 1.9 Ga (Table 4), indicating a small amount
(1–2%) of recycled older crust was incorporated in the source
magma5 (Stern et al., 1995a; Pearce et al., 1983). Stern at al.
(1995a) noted that Flin Flon Belt arc volcanic rocks locally
provide evidence for intracrustal recycling of older (e.g.,
Archean) crust and speculated that rifted, older crustal remnants
in the basement under the arc volcanic rocks may be implicated
in such a process. The presence of 2.52 Ga, late Neoarchean
basement slivers of tonalite (David and Syme, 1994) within the
Northeast Arm Fault (Figures 3 and 4;‘Northeast Arm shear
zone’ of Lucas, 1993; Syme, 1995) suggest such rocks could
have contaminated the source magma for the arc volcanic
rocks—see ‘Discussion’ below.
Wabishkok Lake suite (unit J5, tholeiitic arc)
Arc volcanic rocks of the Wabishkok Lake suite occupy
an ovoid fault block that extends for approximately 18 km
from Wabishkok Lake to Blueberry Lake (Figure 2). The
predominantly mafic volcanic rocks are intruded by several
granitoid and gabbroic intrusions that constitute over half the
Modelled on rare Archean slivers in the Flin Flon Belt (Stern at al., 1995a).
16
Manitoba Geological Survey
Table 4: The εNd isotopic values for selected volcanic rock suites in the northern Flin Flon Belt, west-central Manitoba.
Geochemical and petrographic data as well as UTM locations for all rock samples are given in Appendix 1.
Sample number
SiO2
(%)
Volcanic rock
suite
32-90-4820-1
50.4
Tartan Lake
Tectonic
affinity
(at 1.9 Ga)
εNd
Sm
(ppm)
Nd
(ppm)
2.3
2.15
8.50
147
Sm/144Nd2
143
Nd/144Nd
0.1531
0.512210
± 2δ
absolute
TCR3
(Ga)
0.000006
N/A
32-04-0232-1
51.1
Tartan Lake
2.8
1.69
6.35
0.1613
0.512338
0.000006
N/A
32-01-0030-1
51.7
Wabishkok Lake
3.4
1.78
6.75
0.1596
0.51235
0.000006
N/A
32-04-0327-1
61.3
Wabishkok Lake
1.7
2.40
8.57
0.1694
0.512383
0.000008
N/A
32-03-0100-1
76.01
Wabishkok Lake
3.0
3.65
15.05
0.1467
0.512198
0.000012
N/A
arc
32-02-0212-1
51.3
Lac Aimée
6.8
2.59
10.29
0.1521
0.512425
0.000007
N/A
32-96-0061-1
71.2
Lac Aimée
-1.5
4.62
20.50
0.1364
0.511846
0.000008
2.58
32-02-0182-1
56.4
Sourdough Bay
-0.8
1.38
5.86
0.1429
0.511927
0.000007
N/A
32-02-0256-1
69.7
Sourdough Bay
0.1
1.28
5.64
0.1375
0.511901
0.000009
2.51
32-02-0140-2
72.8
Sourdough Bay
-0.5
4.01
17.80
0.1362
0.511896
0.000007
2.48
32-97-1531-1
65.3
Naosap Lake
-0.5
1.74
6.46
0.1633
0.512197
0.000009
N/A
32-89-3362-2
46.5
Manistikwan Lake4
1.6
5.32
22.67
0.1418
0.512032
0.000006
N/A
32-88-2217-3
73.1
Manistikwan Lake
32-88-2075-2
77.2
Manistikwan Lake
32-87-1523-4
47.1
Arthurs Lake
32-04-0148-1
49.6
Bluenose Lake
32-97-1395-1
45.9
32-97-1291-1
49.9
arc-rift
-1.7
1.15
3.83
0.1818
0.512362
0.000007
N/A
0.7
3.27
14.21
0.1392
0.511956
0.000006
2.46
3.3
2.60
7.52
0.2086
0.512954
0.000006
N/A
4.0
1.68
4.48
0.2275
0.513228
0.000008
N/A
Animus Lake
5.1
1.21
3.01
0.2422
0.513436
0.000009
N/A
Animus Lake
3.9
3.75
12.54
0.1809
0.512648
0.000008
N/A
3.5
3.81
13.11
0.1760
0.512556
0.000006
N/A
5.2
0.62
1.69
0.2228
0.513215
0.000007
N/A
N-MORB
32-90-4671-1
46.7
West Mikanagan
Lake
E-MORB
32-03-0079-1
51.8
Dismal Lake
depleted
MORB
Estimated SiO2 content. Note that this sample and sample 32-01-0261-1 (listed in Appendix 1) are from the same location.
Estimated error is better than 0.5%.
3
Crustal residence Nd model ages (TCR) calculated according to the model of Goldstein et al. (1984) for samples with 147Sm/144Nd <0.14.
4
Manistikwan lake is an unofficial name for Big Island Lake.
Note: analyses were performed at the University of Alberta Radiogenic Isotope Facility in Edmonton, Alberta, Canada.
Details on the Sm-Nd analytical methods are in Creaser et al. (1997).
1
2
bedrock exposure of the fault block (Pistol Lake and Kotyk Lake
plutons, Kotyk Lake and Wabishkok Lake sills). Emplacement
of the granitoid plutons and subsequent deformation, probably
due to lateral movement along the contact between the Flin Flon
Belt and the Kisseynew Domain to the north (see ‘Structural
Geology’ below), resulted in the irregular, three-branched
configuration of volcanic rocks in the Wabishkok Lake Block.
The volcanic sequence at Wabishkok Lake is up to
2 km wide and provisionally interpreted as a south-facing
monocline. Pillow structures are less well preserved than in the
contiguous Tartan Lake Block and top directions were obtained
at only two localities. Pillows at Blueberry Lake face westsouthwest, whereas at the southern shore of Wabishkok Lake
(UTM 334699E, 6084541N) pillows are south-facing.
The western and northern branches of the Wabishkok Lake
arc volcanic suite consist exclusively of basaltic to andesitic
flows and related gabbro that are largely aphyric; sporadic
plagioclase-phyric flows constitute less than 10% of the
sequence. In contrast, the eastern branch that extends through
southern Kotyk Lake to Blueberry Lake is lithologically more
varied, and the rocks appear to have potential for VMS-type
mineralization (see below; Gilbert 2003a). A 160 m wide inlier
of massive to fragmental rhyolite occurs within the Kotyk Lake
Geoscientific Report GR2011-1
sill (Figure 2; Gilbert, 2001b) and felsic volcanic rocks also
occur east of Blueberry Lake, where they are intercalated with
basalt and andesite flows that are locally silicified. Hornblende
(pyroxene)-phyric pillowed basalt and heterolithic mafic
volcanic breccia occur at islands in the middle of Blueberry
Lake, which is the only known locality of these rock types
within the Wabishkok Lake Block. Numerous electromagnetic
(EM) anomalies are associated with base-metal (±gold)
mineralization at basalt-gabbro contacts south of Kotyk Lake
and in rhyolite flows close to the east shore of Blueberry Lake
(Gilbert, 2001a, b, 2003a, b).
Geochemically, the Wabishkok Lake suite displays a
tholeiitic trend in the ternary discriminant plot (Figure 5a, 5b)
and is characterized by lower average Mg#, and Ni and Cr
contents, compared to calcalkaline suites in the NFFB (Table 3,
back pocket). Average SiO2 for Wabishkok Lake volcanic rocks
(57%) exceeds the combined average of all other NFFB mafic to
intermediate arc rocks (53%). The incompatible-element plot is
correlative with other arc volcanic types, with elevated LREE,
high Th/Nb and La/Yb ratios, negative Nb and Ti anomalies
and depleted HREE (Figure 7e). Variable εNd values (calculated
at 1.9 Ga) for Wabishkok Lake basalt (+3.4), rhyolite (+3.0) and
andesite (+1.7) suggest minor amounts of recycled older crust
17
were incorporated in the source magma (Stern et al., 1995a;
Table 4).
Manistikwan Lake arc, arc-rift (unit J6, tholeiitic to
calcalkaline) and N-MORB–type volcanic suites
The Manistikwan Lake Block is up to 3 km wide and
extends laterally for over 20 km from northern Embury Lake
to the southern end of Big Island Lake. The west to southwestfacing, lithologically diverse volcano-sedimentary sequence
within this fault block contains three geochemically distinct
volcanic types. Internal faulting (Gilbert, 1989a, b; Bailes and
Syme, 1989) has resulted in the juxtaposition of contrasting
arc, arc-rift and N-MORB geochemical types within the
Manistikwan Lake Block.
Basalt and subordinate flow breccia are predominant in
the northern part of the fault block. The grey-green weathering
flows are locally pillowed, amygdaloidal, and aphyric to
sparsely plagioclase-phyric. Subordinate rock types within
this sequence include heterolithic volcanic breccia, felsic tuff,
conglomerate, siltstone and argillitic siltstone. Abundant felsic
porphyry and quartz-eye tonalite intrusions, and a 160 m thick
extrusive/intrusive rhyolitic unit occur in the northern part of
the Manistikwan Lake Block. Gabbro constitutes approximately
10% of the northern part of the Manistikwan Lake Block, which
also contains at least three fault-bounded ultramafic lenses. A
series of fine-grained sedimentary lenses along the southwestern
margin of the fault block are interpreted as structurally detached
parts of the turbiditic Embury Lake Block.
The central and southern parts of the Manistikwan Lake
Block are dominated by a 600 m thick sequence of basaltic
pillow-fragment breccia with minor interlayered flows of
aphyric basalt (Bailes and Syme, 1989). Many of these rocks
are characterized by conspicuous vesicularity, suggesting a
relatively shallow-water (<700 m) depositional environment
(Jones, 1969; Moore and Schilling, 1973); however, highly
vesicular lava has locally been reported at deeper levels in
modern oceans (e.g., >1150 m, Staudigel and Schmincke, 1984).
Other rock types include buff-brown weathering porphyritic
basalt (600 m), rhyolite and abundant intrusive rocks such
as felsic porphyry, quartz diorite and diorite. Subordinate
deposits include pyritic mudstone (140 m) and well-bedded,
coarse-grained sandstone and conglomerate (350 m) that are
interpreted to represent a subaerial depositional environment
(Bailes and Syme, 1989).
Mafic volcanic flows in the south-central part of the
Manistikwan Lake Block are characterized by low TiO2, Zr
and Y contents (Bailes and Syme, 1989), within the range of
arc basalt compositions, and plot within the field of NFFB
arc volcanic rocks (Figure 6a). These basalts, together with
rhyolitic extrusive/intrusive rocks in the south-central part of
the Manistikwan Lake Block, are characterized by a tholeiitic
trend of magmatic evolution (Figure 5a, 5b).
In the northern part of the Manistikwan Lake Block,
pillow-fragment breccia and porphyritic, amygdaloidal basalt
comparable to the predominant arc volcanic rock types in
the southern part of the block are apparently absent. Instead,
massive basalt sequences with 1) arc-rift and 2) N-MORB–type
18
compositions were identified at two separate localities in the
northern part of the fault block. North of Embury Lake, arcrift mafic flows occur in a >275 m wide section in the inferred
lower to middle part of the stratigraphic sequence. At a different
locality 6 km to the southeast, N-MORB–type basalt occurs in
a >320 m wide section within the inferred middle to upper part
of the sequence. Whereas these two types are geochemically
distinctive, they are lithologically quite similar, that is, massive
to pillowed, grey-green weathering, aphyric and locally sparsely
amygdaloidal.
Compared to the arc volcanic rocks in the south-central part
of the Manistikwan Lake Block, arc-rift basalt in the northern
part of the fault block is characterized by variably elevated
FeOT (7.6–16.4%) and TiO2 (1.2–2.2%) contents, similar to the
compositional range of arc-rift basalt and ferrobasalt in the Lac
Aimée and East Mikanagan Lake blocks (11.69–17.8% FeOT,
0.9–2.0% TiO2, Appendix 1). The profiles of incompatibleelement plots in all three of these arc-rift rock suites are similar
to those of NFFB arc rocks, but overall contents of incompatible
elements are relatively higher, especially Nb and Ti (Figure 7i,
7j). This pattern may reflect a lower degree of partial melting
in the magmatic source compared to arc volcanic types and/or
incorporation of a more fertile mantle component in the source
magma (see ‘Discussion’ below). High average Th content
(1.23 ppm, Table 3, back pocket) of Manistikwan Lake arc-rift
rocks, together with evolved Nd isotopic compositions (εNd ratio
of +1.6 for basalt, +0.7 to –1.7 for rhyolite at 1.9 Ga, Table 4),
suggest greater amounts of older crustal lithosphere (up to 10%,
Figure 9; Stern et al., 1995a) were incorporated in the source
magma, compared to arc volcanic rocks in contiguous fault
blocks in the northwestern part of the Flin Flon Belt (e.g., εNd
values of +2.3, +2.8 at 1.9 Ga for Tartan Lake basalt).
Manistikwan Lake N-MORB is distinguished from the
previously described arc-rift basalt by flat incompatible-element
profiles and moderate depletion (relative to modern N-MORB)
of all elements, except Th. Manistikwan Lake N-MORB is
compositionally very similar to Arthurs Lake and Bluenose
Lake N-MORB (Table 3, back pocket), which are interpreted
to represent a back-arc basin (BAB) setting (see ‘Discussion’
below). The association of compositionally distinct volcanic
rocks characteristic of different tectonic settings (arc, arc-rift
and BAB) within the Manistikwan Lake Block is likely a result
of intrablock faulting during tectonic assembly of the Amisk
Collage, during which fault slices of contrasting geochemical
type were tectonically juxtaposed.
East Mikanagan Lake arc-rift suite (unit J7, tholeiitic)
The East Mikanagan Lake Block consists largely of arc-rift
basalt and related gabbro that occur in a >1 km thick, southwestfacing monoclinal sequence east of Mikanagan Lake. These
rocks are separated from E-MORB–type basalt in the West
Mikanagan Lake Block by the northern part of the Northeast
Arm Fault, which bifurcates farther to the north (Figures 2 and
3). The East Mikanagan Lake Block is shown in Figure 4 as
on strike with rocks in the Whitefish Lake–Mikanagan Lake
Block (Bailes and Syme, 1989), but the sequences in these
two fault blocks differ both geochemically and lithologically,
and they are therefore assumed to be separated by faults. The
Manitoba Geological Survey
arc-type Whitefish Lake–Mikanagan Lake Block consists of a
wide variety of massive and fragmental volcanic and epiclastic
rocks (Bailes and Syme, 1989), in contrast to the simpler
arc-rift, basalt-gabbro sequence in the East Mikanagan Lake
Block. The arc-rift–type Scotty Lake Block farther to the south
(Figure 4), on the other hand, is compositionally similar and
may be stratigraphically equivalent to the East Mikanagan Lake
Block (see ‘Structural Geology’ below).
The volcanic sequence at East Mikanagan Lake contains
massive to pillowed, aphyric basalt that is devoid of subordinate
autoclastic breccia zones and displays minimal seafloor-type
alteration, in contrast to arc sequences found in the contiguous
Lac Aimée Block to the northeast. Hornblende (after pyroxene)
±plagioclase porphyritic mafic dikes of arc composition that
occur sporadically within the arc-rift sequence indicate a
relative age between the arc and arc-rift types. The arc-type
mafic dikes are thus enigmatic if the arc and arc-rift rocks are
related within an evolving magmatic sequence.
The arc-rift basalt weathers grey-green and is grey or
locally blue-grey on fresh surfaces. Sporadic amygdules are
typically sparse and small (1–3 mm) but locally attain 1 cm in
size; polygonal cooling joints are locally preserved. Some finegrained flows contain coarser-grained gabbroic cores; discrete
interflow gabbro sills up to 50 m thick form approximately
10% of the section. Two variolitic6 pillowed flows (20 m and
35 m wide respectively) within the basaltic section contain
pale-weathering varioles (2–20 mm in diameter), which locally
increase in size and abundance (10–90%) and are coalescent
toward the cores of pillows (Gilbert, 1990a). These flows are
sparsely quartz-amygdaloidal and exhibit quench textures in
thin section, characterized by skeletal, branched filaments of
very fine-grained amphibole or epidote, as well as elongate
amphibole prisms (probably clinopyroxene-derived), oriented
in subparallel, pinnate or partly radiating patterns. Variolitic
textures elsewhere have been interpreted variously as due to
liquid immiscibility or spherulitic crystallization during or
after solidification, possibly associated with devitrification of
original volcanic glass (Gélinas et al., 1977; Hughes, 1977;
Fowler et al., 1987; Arndt and Fowler, 2004).
Similar spherulitic and clinopyroxene-quench textures
occur in pillowed basalt approximately 10 km along strike to
the south (Bailes and Syme, 1989) in the possibly related Scotty
Lake basalt (see ‘Geochemistry of arc and arc-rift rocks in Lac
Aimée, East Mikanagan Lake and Scotty Lake blocks’ below).
East Mikanagan Lake arc-rift basalt is also akin to some volcanic
rocks in the Manistikwan Lake and Lac Aimée blocks; these
arc-rift types are geochemically distinguished by higher overall
incompatible-element contents compared to more primitive,
juvenile-arc rocks (Figure 7a to 7h, 7i, 7j). Whereas Th/Nb and
Th/Yb ratios in East Mikanagan Lake basalt are comparable to
those of some arc rocks, HFSE contents are relatively higher,
in particular Th (average of 2.54 ppm), which exceeds that of
rocks in all other NFFB mafic to intermediate volcanic suites
(Table 3, back pocket).
A rhyolite lens in the southeastern part of the fault block
consists mainly of felsic volcanic breccia intercalated with
6
massive felsic volcanic rock. This rhyolite lens, approximately
0.3 km wide and over 0.6 km long, is provisionally interpreted
as conformable with the arc-rift basalt but could alternatively
be an allochthonous fault slice. Several north-trending faults
within the East Mikanagan Lake Block, at or close to the
eastern shore of Mikanagan Lake, are associated with basemetal sulphide mineralization and hydrothermal alteration
(silicification and carbonatization). The north-trending fault
along the eastern margin of the fault block is also locally altered
and mineralized over a 3.5 m wide zone; assays yielded up to
3.4% Cu, 4 ppm Au and 12 ppm Ag.
Lac Aimée arc and arc-rift suites (unit J8, tholeiitic)
The Lac Aimée Block, located between the MORBtype Animus Lake Block to the northwest and arc-type
Sourdough Bay Block to the southeast (Figure 2), is one of
the most lithologically and compositionally diverse blocks
in the NFFB. The Lac Aimée volcanic suite consists mainly
of massive and fragmental mafic extrusive rocks that have
been locally redeposited as heterolithic mass flows; these are
intercalated or laterally gradational with fine-grained turbidite
deposits (unit S1) that range from 1 to >400 m in thickness.
The turbidites, volcaniclastic mass-flow deposits and sporadic
thin (0.5–1 m) chert beds represent ephemeral breaks in the
extrusive magmatic activity. Interpillow chert and silicicalteration zones within basalt flows are interpreted as products
of seafloor volcanic hydrothermal activity. Mafic and felsic
synvolcanic intrusions range from minor dikes and sills (1 m
thick) to larger bodies up to 400 m wide.
Two geochemically distinct volcanic types are recognized:
1) arc and 2) arc-rift. The contact between the arc and arc-rift
rocks has not been observed and the stratigraphic relationship
between these two types is thus unknown. Geochemical data
suggest that the arc-rift types may be fractionated Fe-rich
flows derived from the same magmatic source that produced
the arc rocks (Figures 6b, 7f, 7i). Lac Aimée arc mafic flows
(unit J8a) are typically aphyric, less commonly plagioclasephyric and/or hornblende (pyroxene)-phyric. Quartzamygdules are widespread; densely amygdaloidal basalt
flows typically contain zones of autoclastic breccia. Quartz
amygdules form up to 65% of pillowed basalt flows and flow
breccia in a 900 m wide zone extending northeast through the
northern part of Alberts Lake. Some units display very coarse
amygdaloidal texture, with large spheroids of quartz up to 4 cm
across. The arc flows are commonly pillowed, grey-green to
beige-grey weathering, and locally display concentric thermal
contraction fractures and polygonal cooling joints. Mafic tuff
and heterolithic volcanic breccia (unit J8b) are subordinate
to the mafic flows; the breccia is unsorted and contains both
mafic and felsic clasts as well as sporadic blocks of mafic tuff.
The mixed composition, lack of sorting and subangular to
angular fragment shapes are consistent with a mass-flow mode
of emplacement. Diffuse to well-defined layering and graded
bedding of tuffaceous interlayers in the breccia are attributed to
reworking by ephemeral turbidity currents.
See discussion of terminology in Arndt and Fowler (2004).
Geoscientific Report GR2011-1
19
Arc-rift basalt, ferrobasalt and basaltic andesite extend
through the central part of the Lac Aimée Block in a zone up
to 0.75 km wide and over 9 km along strike. The green-brown
or khaki-coloured ferrobasalt is typically massive, aphyric
and devoid of pillows; polygonal cooling fractures are locally
characteristic. Finely disseminated magnetite grains constitute
approximately 5% of the flows. Plagioclase-phyric mafic rocks
associated with the ferrobasalt are interpreted as synvolcanic
intrusions. Arc flows with up to 14% FeOT occur in close
proximity to the ferrobasalt and probably represent part of an
evolving magmatic series from arc basalt (average of 11.9%
FeOT) to arc-rift ferrobasalt (average of 14.0% FeOT, Table 3,
back pocket).
Felsic volcanic rocks constitute approximately 15% of
the Lac Aimée arc volcanic suite and are thus relatively more
abundant than in other NFFB arc volcanic suites, except for
the Sourdough Bay suite. The felsic rocks are predominantly
rhyolitic and typically plagioclase-quartz–phyric; in many
cases these rocks are homogeneous and their origin (extrusive/
intrusive) cannot be reliably determined. Sporadic, monolithic
autoclastic breccia zones and rare flow-laminated lobes within
massive felsic volcanic units suggest an extrusive environment
of emplacement. A lensoid, 400 m thick plagioclase-quartz
porphyry sill in the southwestern part of the Lac Aimée Block is
coarsely porphyritic, with quartz and plagioclase phenocrysts up
to 1 cm in size (20–30% of the rock) and scattered pyritohedra
(up to 5%); polygonal cooling fractures are locally preserved.
The volcano-sedimentary sequence is deformed by a series
of folds that form a southwest-plunging synclinorial structure
(Figure 3). Gale and Dabek (2002) identified several faults
parallel or subparallel with the stratigraphy in the southern part
of the Lac Aimée Block. The relative age relationships between
the various lithologically defined components in the fault block
are uncertain as a result of this folding and faulting.
Geochemistry of arc and arc-rift rocks in Lac Aimée,
East Mikanagan Lake and Scotty Lake blocks
Lac Aimée arc basalt and basaltic andesite display
incompatible-element profiles characterized by a wider
compositional range than any other NFFB arc volcanic suite
(Figure 7a to 7h). Lac Aimée arc rocks have REE profiles
that are similar to those of arc-rift rocks in both Lac Aimée
and contiguous East Mikanagan Lake blocks, but the arc-rift
rocks are characterized by higher overall incompatible-element
contents, especially Th (0.88 ppm, 2.54 ppm, respectively),
and lower Th/Nb ratios (0.32, 0.38; Table 3, back pocket),
resulting in somewhat less conspicuous negative Nb anomalies
(Figure 7i). The arc-rift volcanic rocks also contain more
TiO2 but less Ni and Cr than Lac Aimée arc rocks. There is
a continuum of increasing, overall incompatible-element
content from Lac Aimée arc, through Lac Aimée arc-rift, to
East Mikanagan Lake arc-rift volcanic suites (Figure 7f, 7i). In
the TiO2 versus MgO diagram, however, these three volcanic
suites plot in separate fields with distinctive TiO2/MgO ratios,
suggesting differences in their tectonic settings (Figure 6b).
7
Lac Aimée arc basalt plots in the field of NFFB arc volcanic
rocks (grey shaded field in Figure 6a to 6d), which corresponds
approximately with that of modern arc magmas (Stern et al.,
1995b); East Mikanagan Lake arc-rift rocks fall in the ‘MORB
+ back-arc basin basalt (BABB)’ field, whereas Lac Aimée arcrift rocks plot in-between these two fields.
Arc-rift volcanic rocks in the Lac Aimée and East
Mikanagan Lake blocks occur within a corridor that extends
from Lac Aimée southwards to the Whitefish Lake–Mikanagan
Lake and Scotty Lake blocks in the central part of the Flin Flon
Belt (Figures 2 and 4). The Fe-rich tholeiite in the Scotty Lake
Block is lithologically and compositionally very similar to the
East Mikanagan Lake arc-rift basalt7 (Table 3, back pocket;
Figure 7i). Both types occur as relatively homogeneous, massive
to pillowed sequences with relatively low vesicularity and
localized quench texture, as well as abundant mafic intrusions.
The εNd isotopic ratios for Scotty Lake basalt (–0.88 to +1.93 at
1.9 Ga, Syme et al. 1999) and Lac Aimée arc rhyolite (–1.5 at
1.9 Ga, Table 4) are similar and among the lowest of all values
for Flin Flon Belt volcanic rocks, suggesting that up to 12% of
older continental crust was recycled during the evolution of the
source magmas (Figure 9; Stern et al., 1995a).
Scotty Lake basalt and arc-rift volcanic rocks in both Lac
Aimée and East Mikanagan Lake blocks could be fractionated
components of a common magmatic source that evolved
concomitant with the onset of rifting and development of an
intra-arc basin. Mixing of primitive, fertile mantle magma
with a depleted arc volcanic source may have contributed to
higher overall incompatible-element contents in the arc-rift
rocks, compared to arc rocks (see ‘Discussion, tectonic setting
of NFFB arc and MORB-type volcanic rocks’ below). The
progressive gradation southward of arc to arc-rift compositions
(from the Lac Aimée Block to the East Mikanagan Lake Block
to the Scotty Lake Block; Figure 7f, 7i) may reflect a concurrent
trend of increasingly more advanced rifting within this structural
corridor. This model implies northward propagation of the rift
axis and widening of an intra-arc basin to the south. However,
this geometry may not correspond to the original orientation
of the intra-arc basin, because the present configuration is
assumed to be a result of tectonic reworking during and after
1.87 Ga assembly of the Amisk Collage (Lucas et al., 1996).
Tectonic movement along the Sourdough Bay and Northeast
Arm faults and related fault splays (Figures 2, 3 and 4) was
likely a major control that resulted in the alignment of the arcrift volcanic suites along a north-northeast-trending structural
corridor across the Flin Flon Belt (see ‘Structural geology
discussion’ below).
Sourdough Bay suite (unit J9, tholeiitic to calcalkaline
arc)
The Sourdough Bay fault block (Figure 2), one of the
largest supracrustal components in the Flin Flon Belt, extends
from the north-northeast to the south-southwest across the belt
for approximately 25 km (Bailes and Syme, 1989). The northern
part of the fault block at Alberts Lake consists largely of felsic
Scotty Lake basalt, average content of FeOT (11.0%), TiO2 (1.3%), Th (2.9 ppm) and La/Yb(ch) ratio (3.0) (Bailes and Syme, 1989).
20
Manitoba Geological Survey
volcanic rocks with subordinate basalt, volcanic fragmental
rocks and rare chert. The southern part of the block at northern
Athapapuskow Lake contains parts of two contrasting volcanic
sequences: 1) felsic, massive to fragmental rocks and associated
intrusions, and 2) mafic flows and breccia, interlayered with
volcaniclastic and turbiditic sedimentary deposits (Bailes and
Syme, 1989). Structural data indicate that the stratigraphic
sequence in the Sourdough Bay Block is a west- to northwestfacing monocline. Felsic volcanic rocks (unit J12), which are
the predominant type in the northern part of the Sourdough Bay
Block, form the felsic ‘Baker Patton Complex’ (Gale, 2001),
which is over 3 km wide and, with an area of approximately
50 km2, is the largest felsic volcanic terrane in the Flin Flon
Belt. The Baker Patton Complex displays evidence of having
undergone intense hydrothermal alteration and is endowed with
one known gold and at least five massive sulphide–type deposits,
as well as over 20 primarily base-metal mineral occurrences
(Gale and Eccles, 1988a; Gale and Dabek, 2002). Substantial
VMS mineralization has, however, yet to be found in the Baker
Patton Complex; the largest known deposit is at Pine Bay and
was reported by the Cerro Mining Company of Canada to
contain 1 340 000 tonnes at an average grade of 1.5% Cu (Gale
and Eccles, 1988a). Two massive Cu-Zn sulphide deposits
that occur farther south in the Sourdough Bay fault block
(Centennial, Sourdough Bay) are hosted by mudstone and felsic
volcaniclastic rocks within a mafic volcanic flow sequence.
The felsic volcanic rocks at Alberts Lake represent the
northern part of the Baker Patton Complex; they are separated
from the main part of this complex by major gabbro and
granodiorite intrusions immediately south of Alberts Lake (Map
GR2011-1-1). Massive rhyolite and related autoclastic breccia
at Alberts Lake appear to be in gradational contact with the finegrained, marginal phase of the granitoid Naosap Lake pluton
(unit P4) to the northeast, suggesting that this part of the pluton,
at least, may be synvolcanic in age. The massive to fragmental
felsic volcanic rocks are typically plagioclase (±quartz)-phyric;
autoclastic breccia contains an unsorted assemblage of rhyolite
fragments that constitute up to 70% of the rock. Small, angular
to irregular or amoeboid fragments (0.5–10 cm) and subordinate
blocks up to 70 cm across are characteristic of the autoclastic
breccia. Sporadic, flow-laminated rhyolite lobes (up to 2 by 3 m)
and elongate tongues in the breccia are locally flow folded
and probably represent extrusions of massive rhyolite within
unconsolidated autoclastic deposits. The majority of the clasts
display fine flow laminae (1–3 mm scale) that are commonly
warped due to contemporaneous flow during emplacement of
the breccia. Massive rhyolite flows, subordinate to the breccia,
also exhibit flow lamination, are locally spherulitic, and contain
pyritohedra and hornblende porphyroblasts.
The felsic volcanic rocks at Alberts Lake are
stratigraphically (and locally tectonically) intercalated with
subordinate basaltic flows and related breccia. These mafic flows
are lithologically similar to arc volcanic rocks in the Lac Aimée
Block. Pillows are typically 1–2 m long, but megapillows (up
to 5 by 1.7 m) occur at several localities in the southeastern part
of Alberts Lake. The mafic flows are aphyric to locally sparsely
plagioclase-phyric and quartz-amygdaloidal; amygdules are
typically smaller and less abundant than in the Lac Aimée arc
basalt. Epidote alteration affects both mafic and felsic flows
Geoscientific Report GR2011-1
in the Sourdough Bay Block but is more pervasive in basaltic
rocks, which locally contain epidosite pods up to 0.5 m across
that form up to one third of some flows. Plagioclase-phyric
diabase dikes interpreted as synvolcanic are abundant in the
northeastern part of Alberts Lake, where they constitute up to
20% of the basaltic section.
In the southeastern part of Alberts Lake, minor components
of the arc volcanic section include heterolithic volcanic breccia,
mafic tuff and chert. Heterolithic breccia contains aphyric basalt
and rhyolite fragments (1–10 cm), and sporadic blocks up to
0.5 by 1 m. The clasts are subangular to angular and the breccia
is unsorted, typical of debris-flow deposits. Sporadic, 1–3 m
thick mafic tuff layers within the felsic volcanic sequence
contain thin (1–4 cm) laminated chert layers. The chert laminae
are locally disrupted and rafted into the mafic tuff due to
synsedimentary deformation. In addition, a thin (0.25 m) chert
deposit occurs at the contact between two basaltic flows near
the southeastern end of Alberts Lake; elsewhere, sporadic white
chert rafts up to 80 cm long within a mafic flow are assumed to
represent reworked interflow chert deposits. These occurrences
of chert are locally magnetiferous and/or mineralized with up to
7% pyrite-chalcopyrite and associated malachite.
Geochemically, Sourdough Bay mafic to intermediate
volcanic rocks are very similar to the arc volcanic rocks in the
Lac Aimée Block, except for a relatively narrower compositional
range. The Mg# and other diagnostic element ratios for the
Sourdough Bay volcanic rocks are similar to average values for
other NFFB arc volcanic rocks (Table 3, back pocket). The εNd
isotopic ratios for Sourdough Bay basaltic andesite, dacite and
rhyolite (–0.8, +0.1 and –0.5 at 1.9 Ga, respectively; Table 4)
suggest up to 9% of older continental crust was incorporated
in the evolving source magma prior to eruption (Stern et al.,
1995a; Figure 9).
Naosap Lake suite (unit J10, calcalkaline arc)
The Naosap Lake calcalkaline suite at the eastern margin
of the NFFB is separated from Lac Aimée arc volcanic rocks
to the west by the northeast-trending Sourdough Bay Fault
(Figures 2, 3); to the east and south, the Naosap Lake Block
is flanked by younger intrusive rocks at the western margin
of an extensive granitoid terrane that extends eastward for
over 30 km to the Elbow Lake area (NATMAP Shield Margin
Project Working Group, 1998, sheet 2). The stratigraphy and
geochemistry of rocks in the Naosap Lake Block are distinctive
and suggest it does not represent a northern extension of the
Sourdough Bay Block to the southeast, as previously inferred
(Gilbert, 2002a). The tholeiitic to calcalkaline Sourdough Bay
suite is dominated by felsic volcanic rocks, whereas the Naosap
Lake suite is calcalkaline and consists mainly of andesite to
dacite, with only minor rhyolite.
Mafic to intermediate flow and fragmental rocks (unit J10a)
are the predominant rock types in the Naosap Lake Block.
These rocks are mainly aphyric and locally contain densely
quartz-amygdaloidal zones, probably coincident with flow tops.
Intercalated flow breccia units contain mafic fragments that are
variously aphyric, plagioclase-phyric and/or amygdaloidal.
Epidosite pods and irregular masses (± densely packed,
quartzofeldspathic amygdules) are locally conspicuous. Pillows
21
are rarely preserved due to pervasive tectonic attenuation. The
section also contains sporadic units of heterolithic volcanic
breccia that are interpreted as mass-flow deposits; the breccia
clasts are moderately to strongly attenuated and include both
mafic and subordinate, plagioclase-phyric felsic rock types.
A unique occurrence of monolithic volcanic breccia (>20 m
wide) at the northwestern shore of Naosap Lake consists of
unsorted, angular quartz-phyric rhyolite fragments (1–8 cm)
within a very dark grey, mafic tuff matrix. Subordinate thin beds
of diffusely laminated and graded mafic tuff are intercalated
with the breccia. This fragmental deposit may be the result of
an explosive volcanic event in which felsic volcanic ‘cap rocks’
were shattered by rising magma and subsequently incorporated
as fragments within the mafic tuff matrix.
The predominantly intermediate compositional range of
volcanic rocks distinguishes the Naosap Lake suite from other
arc volcanic suites in the NFFB, which are bimodal basalt and
rhyolite sequences. Relative to other NFFB arc rocks, Naosap
Lake volcanic rocks have higher average light to medium
REE contents, higher contents of HFSE (Nb, Zr and Hf), as
well as higher La/Yb and Th/Yb ratios (Table 3, back pocket).
Average Th content in the Naosap Lake suite (2.4 ppm) exceeds
that of almost all other NFFB mafic to intermediate volcanic
suites and is three times the combined average of other NFFB
arc rocks. These trace element patterns are consistent with
the calcalkaline affiliation of Naosap Lake volcanic rocks, as
indicated by the lack of an iron enrichment trend, and lower
average FeOT content, compared with tholeiitic or transitional
calcalkaline-tholeiitic NFFB arc rocks (Figure 5a to 5f; Table 3,
back pocket).
Felsitic intrusions, possible psammitic to semipelitic paragneiss
and schist occur in the same section; these rocks are interpreted
to be in fault contact with intermediate volcanic rocks
(unit J10a) to the south. The local occurrence of orthopyroxene
in the cordierite-bearing gneiss indicates that metamorphism
was at a higher grade and probably at a different crustal level
compared to that of andesitic rocks immediately to the south.
The orthopyroxene–cordierite–orthoamphibole–biotite mineral
assemblage signifies high-temperature metamorphism (>650ºC)
at medium to high pressure (Winkler, 1974; Akella and Winkler,
1966; Harris and Holland, 1984), whereas the hornblende–
plagioclase–chlorite assemblage in the andesitic rocks to the
south is characteristic of medium-grade metamorphism.
Metamorphic history, origin and structural implications
of high-grade gneiss (unit J10b)
The high-grade gneiss (unit 10b) includes at least four distinct mineral assemblages:
1) cordierite–orthoamphibole–biotite±orthopyroxene
(±andalusite)
2) orthoamphibole–biotite±cordierite±green hornblende
(±chlorite)
3) orthoamphibole–biotite–almandine
4) cummingtonite–biotite–green hornblende–epidote
All assemblages include quartz, plagioclase, K-feldspar and
minor opaque minerals.
The mainly intermediate composition of the Naosap Lake
volcanic suite may be due to modification of the source magma
by either 1) assimilation of contemporaneous (volcanicderived) sedimentary detritus in the subduction zone and/or
older continental crust, or 2) the addition of (or metasomatism
by) subduction-related mantle. Involvement of older crust in
the evolution of the Naosap Lake magma is indicated by the
evolved Nd isotopic composition (εNd of –0.5 at 1.9 Ga; Table 4), which is consistent with a moderate amount of crustal
recycling (Figure 9; Stern et al., 1995a). Incorporation of sialic
lithosphere by assimilation is consistent with the conspicuous
enrichment in Th and overall incompatible-element patterns,
which are characterized by variable amounts of slope (Figure 7h) due to variation in LREE, Th and Nb contents.
The high-grade gneiss contains evidence for two
metamorphic events. High-grade metamorphism resulted in
orthopyroxene and stratabound, ovoid to pseudohexagonal,
1–3 cm cordierite porphyroblasts. Cordierite is locally
overprinted by randomly oriented, postkinematic chlorite
aggregates and anthophyllite, which also occurs as secondary
mantles on relict orthopyroxene porphyroblasts. The typically
random orientation of the secondary minerals suggests they
are products of contact metamorphism, originally assumed
to be due to the granitoid intrusion immediately to the south
(Gilbert, 1997a). However, both the high-grade metamorphism
and subsequent contact metamorphic events probably postdate
emplacement of the Naosap Lake pluton to the south, which
is of successor-arc age (see 2 to 4 below). Thus a different,
relatively younger, intrusion or a late (post-1.83 Ga) intrusive
phase within the Naosap Lake pluton is required to explain the
contact metamorphism.
Porphyroblastic, cordierite-bearing gneiss (unit J10b) is
characteristic of a 400 m wide section of metamorphosed rocks
that underlie an island very close to the south shore of Naosap
Lake, at the southeastern margin of the Naosap Lake Block
(Map GR2011-1-1). The porphyroblastic gneiss and associated
amphibolite are interpreted as derived from metasomatized
volcanic rocks, but a sedimentary precursor cannot be ruled out.
The 400 m wide section with porphyroblastic gneiss was
initially mapped as derived from altered volcanic and finegrained sedimentary rocks of the juvenile-arc assemblage
(Gilbert, 1997a). This section may alternatively be interpreted
as an allochthonous fault slice8, which would explain the
contrasting metamorphic grade between the gneiss and
intermediate volcanic rocks to the south. Geochemical evidence,
An alternative explanation for this metamorphic contrast is to interpret the high-grade gneiss as a fault-bounded remnant of a former granulite terrane that may
have been significantly older than the Naosap Lake arc volcanic rocks. The gneiss is possibly coeval with late Neoarchean (2.52 Ga) fault slivers that have been
identified within the Northeast Arm Fault (David and Syme, 1994) and granulite facies rocks of similar age that have been identified at several other localities in
the Reindeer Zone of the Trans-Hudson Orogen, for example, in Sask craton, west of the Flin Flon Domain (Ashton et al., 1999) and at Southern Indian Lake,
north of the Flin Flon Domain (Corrigan et al., 2001). High-grade gneiss of similar age could also exist at deep crustal levels elsewhere within the southeastern
part of the Trans-Hudson Orogen, in the vicinity of the Flin Flon Belt. Fault slivers of such rocks could have been tectonically intercalated with the Paleoproterozoic volcanic rocks during late collisional tectonism (D4 of Lucas et al., 1996).
8
22
Manitoba Geological Survey
however, does not support this model. The composition of a
>1.2 m wide, probably intrusive dacitic layer (sample 32-971550-1, Appendix 1) within the high-grade gneiss sequence is
very similar to the average composition of rocks of the Naosap
Lake arc volcanic suite (Figure 11), suggesting it could be
associated with Naosap Lake magmatism. These data suggest
that the section containing the porphyroblastic gneiss is not
allochthonous but is derived from the Naosap Lake volcanic
suite, as implied by the original field interpretation. The
implications for this model are as follows:
1) The volcanic and/or sedimentary rocks from which the
gneiss was derived were subjected to pervasive (possibly
synvolcanic) alteration and magnesian metasomatism.
2) High-grade metamorphism that affected the metasomatized rocks was likely associated with ca. 1830–1800 Ma
peak metamorphism in the Flin Flon Belt (Lucas et al.,
1996).
3) The orthopyroxene-bearing, high-grade gneiss to the north
and relatively lower grade volcanic rocks to the south were
tectonically juxtaposed by faulting, possibly during terminal collision of the Reindeer Zone (Trans-Hudson Orogen)
with Archean cratons, following peak metamorphism (Lucas et al., 1996; see ‘Structural geology discussion’ below).
4) Amphibolite-facies contact metamorphism overprinted
the high-grade mineral assemblage in the porphyroblastic
gneiss. If this assemblage is a product of the ca. 1830–
1800 Ma peak metamorphism, the contact metamorphism
thus postdated both the high-grade metamorphism and earlier emplacement of 1838–1876 Ma successor-arc plutons
(Stern et al., 1999).
Felsic volcanic rocks (units J11–15)
Felsic rocks are subordinate (<10%) among all arc rocks
in the NFFB, except in the eastern part (Alberts Lake area),
where the Baker Patton Complex (unit J12) constitutes over
half of the northern Sourdough Bay Block (Figure 2). Although
felsic volcanic rocks are volumetrically only a minor part of
the Flin Flon arc assemblage, they are economically the most
important component because of the close association between
felsic volcanic rocks and VMS ore deposits, both in the Flin
Flon Belt (Syme, 1998) and elsewhere (Franklin, 1996).
Incompatible-element plots of the NFFB rocks are similar to
those of juvenile-arc rhyolite types elsewhere in the Flin Flon
Belt (Syme, 1998; Figure 12a, 12b). Most of these rocks are FII
type in the scheme of Lesher et al. (1986) for the classification
of Archean volcanic rocks (Figure 13)—a system that has been
shown to be, in part, applicable to (Paleoproterozoic) Flin Flon
Belt rocks (Syme, 1998, Bailes and Galley, 1999). The FII-type
felsic volcanic rocks are only rarely associated with economic
mineralization.
The lensoid Cope Lake rhyolite (unit J13), at least 140 m
wide and 0.5 km along strike, is located at the northeastern
margin of the Cope Lake Block, 0.5 km southwest of the Trout
Lake (Cu-Zn) ore deposit (Figure 4, Map GR2011-1-1). The
stratigraphic and structural setting of this rhyolite suggest that
it may have potential for base-metal mineralization, being
situated close to the tectonic contact between the Cope Lake
and Embury Lake blocks as well as the Trout Lake ore deposit,
which is hosted by similar felsic volcanic rocks (OrdóňezCalderón et al., 2009). This VMS-type ore deposit, which is
one of the largest in the Flin Flon area, occurs within a sequence
of felsic volcanic rocks and related porphyry, basalt, gabbro
and argillite (Ordóňez-Calderón et al., 2009). The geochemical
signature of the Cope Lake rhyolite identifies it as different
from most other NFFB felsic volcanic rocks and is consistent
with a possible genetic link to the ore-bearing rhyolite in the
Trout Lake mine sequence.
Compared with most other NFFB felsic volcanic rocks, the
high-silica Cope Lake rhyolite is distinguished by relatively high
REE and HFSE contents, and a more pronounced Eu anomaly
100
Rock / N-MORB
Naosap Lake dacitic intrusion (32-97-1550-1)
Naosap Lake volcanic suite dacite
Naosap Lake volcanic suite andesite
10
1
.1
Th Nb La Ce Nd Sm Zr Hf Eu Ti Gd Dy Y Er Yb Lu
Figure 11: Plot of N-MORB normalized incompatible-elements for a dacitic intrusion (sample 32-97-1550-1), compared with those of
andesite and dacite in the Naosap Lake volcanic suite, west-central Manitoba. Normalizing values from Sun and McDonough (1989).
Geoscientific Report GR2011-1
23
a)
Rock / chondrite
100
10
Cope Lake rhyolite
Lac Aimée felsic porphyry
All other NFFB felsic
arc volcanic rocks
1
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
b)
Rhyolites in the central Flin Flon Belt (Syme, 1998)
Rock / chondrite
100
10
Extension-related rhyolite (Grassy Narrows Zone) n=6
Arc assemblage rhyolite (barren) n=19
1
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Figure 12: Chondrite-normalized rare earth element (REE) plots showing a) REE- and SiO2-enriched Cope Lake rhyolite and Lac
Aimée felsic porphyry, compared with felsic volcanic and related intrusive rocks elsewhere in the northern Flin Flon Belt (NFFB),
west-central Manitoba; and b) juvenile-arc assemblage rhyolites (number of samples, n = 25) in the south-central part of the Flin Flon
Belt (from Syme, 1998). Normalizing values from Sun and McDonough (1989).
(Figure 12; Table 3, back pocket). The extended element plot
shows Cope Lake rhyolite has a relatively flat profile and
incompatible-element contents that exceed those of all other
Flin Flon Belt rhyolites (Syme, 1998). Cope Lake rhyolite
is classified as FIII type (of Lesher et al., 1986; Figure 13)
indicating it may be prospective for VMS mineralization. Such
rocks, which also include FIII rhyolite in the Flin Flon mine
sequence (Syme, 1998), are interpreted as derived from highlevel, subvolcanic magma chambers, representing a heat source
for hydrothermal convection systems that are directly associated
with VMS mineralization (Lesher et al., 1986). The Cope Lake
rhyolite is also compositionally similar to ore-bearing rhyolites
24
in the Archean Abitibi Subprovince and is classified as group
I in the scheme of Barrie et al. (1993)—a rock type that hosts
over 50% of Abitibi VMS ore deposits. Group I rocks consist
of high-silica rhyolite (SiO2 >73%) characterized by elevated
incompatible-element contents, negative Eu anomalies, Zr/Y
ratios <5 and Rb/Sr ratios >1.0 (Figure 14; compare with data
in Table 3, back pocket and Appendix 1).
Lac Aimée felsic porphyry (sample 32-97-1149-1, unit J14;
Appendix 1 and Map GR2011-1-1) is a massive, >10 m wide
unit located in the faulted contact zone between the MORBtype Animus Lake Block and arc-type Lac Aimée Block. It is
interpreted as an intrusive/extrusive formation associated with
Manitoba Geological Survey
100
Cope Lake rhyolite
Lac Aimée felsic porphyry
All other NFFB felsic arc volcanic rocks
La/Yb(N)
FI
10
FII
FIII
1
0
20
40
60
80
100
Yb(N)
Figure 13: Plot of La/Yb(N) versus Yb(N) for felsic volcanic rocks in the northern Flin Flon Belt (NFFB), west-central Manitoba, showing
Cope Lake rhyolite and Lac Aimée felsic porphyry distinct from felsic volcanic and related intrusive rocks elsewhere in the NFFB.
Fields of Archean felsic volcanic rock types (FI, FII and FIII) are after Lesher et al. (1986). Chondrite-normalizing values (N) from Sun
and McDonough (1989).
20
Cope Lake rhyolite
Lac Aimée felsic porphyry
All other NFFB felsic arc volcanic rocks
15
Zr/Y
BARREN
10
5
OREBEARING
0
0
20
40
60
80
100
120
140
Y
Figure 14: Plot of Zr/Y versus Y for felsic volcanic and related intrusive rocks in the northern Flin Flon Belt (NFFB), west-central
Manitoba. Fields of ore-bearing and barren rhyolites in the Abitibi Belt after Barrie et al. (1993); group I types are high-Y (70–150 ppm)
rhyolites within the ‘ore-bearing’ field.
Lac Aimée arc volcanic rocks. This FIII-type quartz-plagioclase
porphyry is more REE-enriched (especially Y) than other
NFFB felsic rocks and is compositionally similar to the Cope
Lake rhyolite (Figures 12, 13 and 14; Table 3, back pocket).
The distinctive geochemical profile suggests that the Lac
Aimée porphyry and/or similar felsic lenses along strike to the
northeast (Map GR2011-1-1) represent potential exploration
targets for base-metal mineralization.
Geoscientific Report GR2011-1
MORB-type volcanic suites (unit F)
Four tectonic enclaves of various MORB-type volcanic
rocks (Arthurs Lake, Bluenose Lake, Animus Lake and West
Mikanagan Lake blocks) are tectonically intercalated with
the numerous arc fault blocks within the NFFB. The MORB
volcanic suites consist almost entirely of basalt and abundant
related gabbro sills that constitute up to 30% of these sequences;
they are thus lithologically and compositionally much less
25
diverse than the arc volcanic suites. The MORB volcanic suites
are interpreted as back-arc rocks derived from source magmas
that were largely unaffected by subduction zone influences; εNd
values range from +3.3 to +5.1 at 1.9 Ga (Table 4), consistent
with a mantle source with little or no crustal recycling of
older lithosphere. Several MORB suites contain aphyric to
plagioclase±hornblende (pyroxene)–phyric diabase dikes
that are compositionally akin to arc rather than MORB-type
magmatism.
Two MORB volcanic types—normal (N-MORB) and
enriched (E-MORB)—are recognized in the NFFB. These two
types are geochemically similar to analogous varieties of MORB
in the Elbow-Athapapuskow ocean-floor assemblage (Stern et
al., 1995b) that is located in the southern and eastern parts of
the Flin Flon Belt (Figure 1). Geochemical variation between
the various formations within the ocean-floor assemblage has
been attributed to mixing of depleted MORB and enriched
mantle together with, in some cases, a subduction-modified
mantle source, but without any crustal assimilation (Stern et al.,
1995b; see ‘Discussion’ below).
Arthurs Lake N-MORB suite (unit F1)
Arthurs Lake N-MORB and abundant related gabbro
intrusions (unit F1) occupy a crescent-shaped tectonic slice
(Arthurs Lake Block) between the Manistikwan Lake and Bear
Lake fault blocks (Figure 4). The southwestern margin of this
tectonic slice is bound by the Inlet Arm Fault (Bailes and Syme,
1989), whereas the northeastern margin consists of a splay of
this same fault that originates in the northeastern part of Big
Island Lake. To the south and along strike from the Arthurs
Lake Block, a heterogeneous assemblage of structurally
intercalated volcanic and subordinate epiclastic rocks occurs
within the Grassy Narrows Zone, between the Manistikwan
Lake and Bear Lake blocks (Bailes and Syme, 1989). To the
northwest, the Arthurs Lake N-MORB rocks extend around the
northern part of Embury Lake and are assumed to wedge out
farther west, beyond the Manitoba-Saskatchewan boundary.
The Arthurs Lake sequence, up to 800 m thick and over
10 km along strike, contains two major folds with axial planes
that trend south to south-southeast and are overturned to the
east (Figures 2 and 3). These structural trends are, in part,
discordant to the block-bounding faults, suggesting the folds
predate tectonic emplacement of the N-MORB fault block.
The margins of the Arthurs Lake Block are characterized
by cataclasis and alteration, as well as localized base-metal
mineralization along the contact with the Bear Lake Block
between Arthurs and Krasny lakes (Map GR2011-1-1). Along
this same contact, fine-grained greywacke-siltstone deposits
(unit S1) up to 60 m wide are interpreted as fault-bounded
lenses of volcanic-derived detritus that is assumed to be either
contemporaneous with volcanic activity in the contiguous Bear
Lake Block or younger, of successor-arc age.
The Arthurs Lake N-MORB suite consists mainly of
aphyric and plagioclase-megaphyric basaltic flows; associated
gabbro intrusions within the flows form approximately one
third of the fault block. Synvolcanic diabase dikes were not
observed within this section, unlike in arc volcanic suites
where such intrusions are common. The N-MORB rocks are
26
characterized by pseudohexagonal plagioclase megacrysts that
occur in approximately half of the flows, as well as in related
gabbro sills. The megacrysts are, within the NFFB, unique to
the Arthurs Lake volcanic suite and are most common in the
area west of Arthurs and Krasny lakes; they have not been
observed within the stratigraphically and compositionally
similar Grassy Narrows basalt to the south (Figure 4; Bailes
and Syme, 1989). The euhedral to ovoid plagioclase crystals
are typically 1–6 cm in size and constitute 5–15% of the
hostrock. The megacrysts are distributed in irregular zones,
in trails of single crystals or, rarely, in diffuse ‘strata’ within
gabbroic intrusions—probably due to sorting by convection
currents during emplacement of the gabbro. The megacrysts are
also present in rare autoclastic breccia zones up to 5 m wide,
within pillowed flows or between massive and pillowed flows.
Pillows are widespread but rarely exceed 1.5 m in length, in
contrast to those in arc volcanic sequences that locally exceed
5 m. Amygdules (quartz±plagioclase) occur in a minority of
flows and are typically sparse (1–5%); variolitic texture was
identified in one basaltic flow at the southern extremity of Ruby
Lake. Polygonal cooling fractures are well preserved at several
localities within both the mafic volcanic and related gabbroic
rocks.
Basaltic flows and synvolcanic gabbro sills are intimately
intercalated and their mutual contacts are variously sharp or
gradational. Contacts are locally very irregular, suggesting the
gabbro permeated the volcanic flows as laccolith-type intrusions,
penecontemporaneous with the volcanism. The gabbros are
compositionally akin to the flows but less fractionated; they
are typically mesocratic to melanocratic (hornblende content of
35–70%), but leucocratic varieties are also present. These rocks
contain subhedral hornblende pseudomorphs after pyroxene
(2–8 mm across) within a medium- to coarse-grained matrix.
Plagioclase megacrysts are widely developed but generally
sparse, rarely exceeding 15% of the gabbroic hostrock. At a few
localities the feldspars are abundant and result in anorthositic
compositions (e.g., 20–50% over 30 m; 70–90% over 6 m).
The Arthurs Lake volcanic suite is compositionally very
similar to modern N-MORB; incompatible-element contents
are equivalent to or slightly less than those of N-MORB
(Figure 15a), except for Th, which shows slight relative
enrichment. One of the synvolcanic gabbro sills was found to be
slightly REE-depleted relative to associated basalt flows, with
moderate negative Nb and Zr anomalies in the incompatibleelement plot. In the TiO2 versus MgO diagram (Figure 6c),
Arthurs Lake N-MORB (as well as Grassy Narrows basalt,
along strike to the south) plot within the ‘MORB + BABB’
field. The +3.3 εNd value (at 1.9 Ga) for Arthurs Lake N-MORB
(Table 4) is equivalent to the projected value for depleted
mantle of that age (DePaolo, 1981), indicating a juvenile origin
for the source magma with little or no crustal contamination.
The lithological and compositional uniformity of the N-MORB
sequence and lack of evidence for subduction zone influences is
consistent with a BAB setting for the Arthurs Lake suite.
Bluenose Lake N-MORB suite (unit F2)
A 1.5 km thick sequence of N-MORB–type volcanic rocks
occurs in the vicinity of Bluenose Lake, where it occupies the
Manitoba Geological Survey
a)
100
Arthurs Lake N-MORB
Rock / N-MORB
Rock / N-MORB
100
10
1
.1
Bluenose Lake N-MORB
10
1
.1
Th Nb La Ce Nd Sm Zr Hf Eu Ti Gd Dy Y Er Yb Lu
100
b)
Th Nb La Ce Nd Sm Zr Hf Eu Ti Gd Dy Y Er Yb Lu
100
c)
d)
Animus Lake E-MORB
West Mikanagan Lake E-MORB
Rock / N-MORB
10
1
.1
10
1
10
1
.1
Th Nb La Ce Nd Sm Zr Hf Eu Ti Gd Dy Y Er Yb Lu
100
g)
Rock / N-MORB
Bluenose Lake depleted MORB
Th Nb La Ce Nd Sm Zr Hf Eu Ti Gd Dy Y Er Yb Lu
100
Gabbro intrusions within/at margin
of Wabishkok Lake Block
Animus Lake E-MORB volcanic rocks
Animus Lake N-MORB volcanic rocks
1
.1
Th Nb La Ce Nd Sm Zr Hf Eu Ti Gd Dy Y Er Yb Lu
f)
Dismal Lake depleted MORB
Rock / N-MORB
Rock / N-MORB
e)
10
1
100
100
.1
10
.1
Th Nb La Ce Nd Sm Zr Hf Eu Ti Gd Dy Y Er Yb Lu
Th Nb La Ce Nd Sm Zr Hf Eu Ti Gd Dy Y Er Yb Lu
h)
Rock / N-MORB
Rock / N-MORB
Animus Lake N-MORB
Diabase dike with arc signature
E-MORB hostrock
(West Mikanagan Lake, Animus Lake)
10
1
.1
Th Nb La Ce Nd Sm Zr Hf Eu Ti Gd Dy Y Er Yb Lu
Figure 15: Plots of N-MORB normalized incompatible-elements for MORB-type mafic volcanic suites in the northern Flin Flon Belt,
west-central Manitoba: a) Arthurs Lake, b) Bluenose Lake, c) Animus Lake, d) West Mikanagan Lake; depleted-MORB suites:
e) Dismal Lake and f) Bluenose Lake; g) gabbro phases within Wabishkok Lake and Kotyk Lake sills, compared with Animus Lake
N-MORB and E-MORB rocks; and h) arc-type porphyritic diabase dikes within E-MORB hostrocks (West Mikanagan Lake and
Animus Lake volcanic suites). Note that in Figures 15e and 15f, a notional value of 0.2 ppm Nb has been used for those rocks
where Nb is below the analytical detection limit (applies to 21 out of the 27 Dismal Lake rocks and 6 out of 8 Bluenose Lake rocks,
see Appendix 1). Abbreviations: MORB, mid-ocean-ridge basalt; E-MORB, enriched mid-ocean-ridge basalt; N-MORB, normal midocean-ridge basalt. Normalizing values from Sun and McDonough (1989).
Geoscientific Report GR2011-1
27
axial zone of a major, east-plunging synclinal fold (Figures 2
and 3; see ‘Structural Geology’ below). Formerly described as
the ‘Wabishkok Lake antiform’ (Gilbert, 2001a, b), this fold is
re-interpreted as synclinal, based on the limited structural data
available. The Bluenose Lake N-MORB volcanic suite consists
largely of aphyric, massive to pillowed flows; related gabbro
intrusions form less than 10% of the section. Quartzofeldspathic
amygdules and/or plagioclase phenocrysts occur in only a few
flows and are sparsely distributed (2–10% of the rock). Pillows
are mostly obliterated due to tectonism, and the rocks are
variously altered to massive or finely laminated amphibolite,
locally with minor epidote alteration. Garnet porphyroblasts
and medium-grained recrystallized zones of mafic gneiss occur
sporadically in the amphibolite.
Conspicuous mineralization occurs within the N-MORB
sequence in the southwestern part of Bluenose Lake; assays
yielded Cu, Zn and traces of Au, hosted by sulphide stringers and
lenses within amphibolite hostrocks. The mineralization occurs
within a 0.5 km wide zone of west-trending EM anomalies that
extend laterally within the N-MORB volcanic suite for at least
1.5 km (Gilbert, 2004). The EM anomalies, which are roughly
conformable with the basaltic flows in which they occur, may be
related to stratabound mineralization or alternatively represent
tectonic zones of faulting and mineralization, postdating the
N-MORB volcanism.
A 120 m wide fault slice of arc rocks derived from the Tartan
Lake Block is emplaced within the N-MORB volcanic suite in
the southern bay of Bluenose Lake, approximately 350 m south
of the mineralized zone described above (Figures 2 and 3). This
fault slice occurs within a splay of a west-trending fault that
separates arc- and MORB-type volcanic rocks and is continuous
southward as the Northeast Arm Fault, which extends for over
35 km across the Flin Flon Belt (Figures 2, 3 and 4). At the
northern margin of the fault slice, a >17 m wide zone of fault/
intrusion breccia contains angular to tabular, siliceous blocks
within an altered ultramafic matrix containing remnants of
clinopyroxene and disseminated pyrrhotite. The fine-grained
quartzitic fragments contain dark laminae, possibly derived
from bedding in a sedimentary precursor. The laminae are
partly disrupted due to folding that preceded the deformation
episode when they were incorporated into the breccia.
Geochemically, Bluenose Lake N-MORB is very similar
to Arthurs Lake N-MORB but differs from the relatively
more REE-depleted Animus Lake N-MORB (see below). The
compositional range of incompatible elements in Bluenose and
Arthurs Lake N-MORB rocks is virtually the same (Figure 15a,
15b), as are their respective fields in the TiO2 versus MgO
discriminant diagram that distinguishes these rock suites from
more TiO2-depleted arc volcanic types (Figure 6c). Slight Th
and LREE enrichment is characteristic of some flows, but
otherwise incompatible elements are moderately depleted
compared to modern N-MORB.
The +4.0 εNd value for Bluenose Lake N-MORB (at 1.9 Ga, Table 4) indicates the source magma was juvenile and
lacked crustal contamination. Incompatible-element data show
this compositionally uniform N-MORB suite was virtually
unaffected by subduction zone influences, consistent with a
BAB tectonic setting. Stratigraphic and/or structural breaks
28
within the volcanic sequence, inferred from EM anomalies at
southern Bluenose Lake, may be due to rifting associated with
the development of an intra-arc basin.
Animus Lake N-MORB and E-MORB suites (units F3
and F4)
Compositionally distinct, N-MORB and E-MORB volcanic
and related gabbroic rocks occupy the triangular-shaped Animus
Lake Block, immediately south of Wabishkok Lake. The fault
block is a highly deformed tectonic wedge located at a major
structural divide in the NFFB, between northeast-trending rocks
to the east (at Lac Aimée) and northwest-trending formations to
the west (Embury–Tartan–Mikanagan lakes area). The Animus
Lake rocks are juxtaposed against several geochemically
contrasting fault blocks that include arc, arc-rift and E-MORB
types (Figure 2).
Gabbro sills up to 300 m thick constitute approximately
20% of the Animus Lake Block. Subordinate rock types (less
than 2% of the sequence) include: 1) greywacke-siltstone,
volcanic breccia and rhyolite, which occur in several thin
(<20 m) enclaves interpreted as fault slivers within the MORB
rocks (Gale and Norquay, 1996; Gilbert, 1997a, 2002b); and
2) felsic volcanic rocks that contain massive pyrite layers
up to 2 m wide, identified in drillcore 1 km east of Animus
Lake (Gale and Eccles, 1988a). Minor flow-top breccia occurs
locally between basaltic units. The mafic volcanic rocks are
mostly nonvesicular and aphyric, although sparse plagioclase
phenocrysts (0.5–2 mm) and amphibole pseudomorphs after
pyroxene occur in a few flows. Animus Lake E-MORB is
typically medium- to dark-green weathering, in contrast to
generally paler-weathering N-MORB rocks. Pillows are
virtually ubiquitous and provide top determinations that
indicate a complex structural pattern.
The volcanic sequence is characterized by repeated folding,
with at least 11 anticline-syncline fold pairs with northwesttrending axial planes, interpreted as D1 in age. These early folds
were subsequently deformed by a regional northeast-trending
D3 fold (see ‘Structural Geology’ below). The structural pattern
suggests the sequence may originally have consisted of a lower
E-MORB section overlain by N-MORB–type basalts (Gilbert,
1998, 1999). The contact relationships between N-MORB and
E-MORB are uncertain; although there is little evidence in the
field that the contacts are faulted, a conformable stratigraphic
relationship appears unlikely in view of the absence of
any compositional gradation between these geochemically
distinctive types. Within the tectonically similar ElbowAthapapuskow ocean-floor assemblage (Figure 1), intercalated
E-MORB and N-MORB formations are structurally juxtaposed
by faulting (Stern et al., 1995b), and it appears likely that the
contacts between the two MORB types in the Animus Lake
Block are also faulted.
The Animus Lake E-MORB and N-MORB suites are
geochemically distinct, suggesting they are derived from
separate, unrelated magma sources. Relative to modern
N-MORB, Animus Lake N-MORB rocks are conspicuously
depleted in both REE and HFSE. The incompatible-element
plot of Animus N-MORB displays a flat to positive-sloping
profile, in contrast to the negative-sloping pattern for Animus
Manitoba Geological Survey
E-MORB, with no overlap between the two types (Figure 15c).
Animus Lake N-MORB has characteristically low Th content
(0.08 ppm, Table 3, back pocket); such extreme Th depletion
is rare elsewhere in the Flin Flon Belt, matched only by the
N-MORB–type Moen Bay formation within the ElbowAthapapuskow assemblage (Table 3, Stern et al., 1995b).
Virtually all other Flin Flon Belt volcanic rocks display slight
to pronounced Th enrichment relative to modern N-MORB.
Animus Lake N-MORB is also characterized by conspicuous
Zr and TiO2 depletion, resulting in these rocks plotting outside
the fields of typical N-MORB rocks in some discriminant plots
(e.g., TiO2 versus MgO, Figure 6c).
Compared to modern N-MORB, incompatible-element
contents are equivalent or somewhat higher in Animus Lake
E-MORB (Figure 15c). The Nb/Y ratios in the Animus Lake
E-MORB suite (average of 0.21) contrast with much lower
Nb/Y ratios in Animus Lake N-MORB (average of 0.05,
Table 3, back pocket). The TiO2 content serves as a rough guide
to distinguish Animus Lake E-MORB (average of 1.60% TiO2)
from N-MORB (TiO2 is 0.60%) and arc volcanic rocks in the
NFFB (average TiO2 of 0.50%, Table 3, back pocket); these
volcanic types each occupy distinctive fields in the TiO2 versus
MgO plot (Figure 6c). The narrower compositional range of
N-MORB and E-MORB Animus Lake basalts (e.g., 3.7–8.6%
MgO, compared with NFFB arc basalt MgO of 2.1–13.5%)
reflects, in part, more variable fractionation in the arc volcanic
rocks compared to MORB volcanic suites.
The Nd isotope ratios in Animus Lake volcanic rocks are
primitive (εNd at 1.9 Ga: +5.1in N-MORB, +3.9 in E-MORB,
Table 4) and within the range of MORB types of the ElbowAthapapuskow ocean-floor assemblage (εNd at 1.9 Ga in
N-MORB is +3.3 to +5.4, and in E-MORB is +3.1 to +4.5;
Stern et al., 1995b). The isotope data for Animus Lake MORBtype rocks are consistent with a BAB setting in which crustal
contamination was insignificant or absent.
The composition and the structural setting of the Animus
Lake Block, among other geochemically contrasting types,
suggests that this fault block is an allochthonous component
of the NFFB that represents part of a back-arc ocean-floor
assemblage. Animus Lake MORB-type rocks may have been
intercalated with arc components in the Amisk Collage during
tectonic accretion ca. 1.88–1.87 Ga (Lucas et al., 1996), but
final emplacement of the fault block likely occurred after D1
(see ‘Structural Geology’ below). The most likely source for
the Animus Lake rocks appears to be the back-arc assemblage
at the northern side of the Flin Flon Belt (units T1 and T2),
in which sporadic occurrences of N-MORB occur at the
northernmost margin and at Bluenose Lake (Map GR2011-1-1;
Figure 2). Alternatively, the Animus Lake rocks may represent
a dismembered part of the Elbow-Athapapuskow ocean-floor
assemblage in the southern and eastern parts of the Flin Flon
Belt (Figure 1; Stern et al., 1995b; NATMAP Shield Margin
Project Working Group, 1998, sheet 2).
Gabbroic phases within the Wabishkok Lake and
Kotyk Lake sills, in the central and southern parts of the
Wabishkok Lake Block, include two different types that are
compositionally akin, respectively, to N-MORB and E-MORB
Animus Lake rocks (Figure 15g). The two mafic sills may be
Geoscientific Report GR2011-1
synvolcanic and derived locally from magmatic sources that
were also the sources for the Animus Lake MORB-type rocks.
This interpretation implies that the allochthonous Animus Lake
MORB-type rocks were locally derived and possibly related to
back-arc magmatism at the northern side of the Flin Flon Belt,
rather than representing a dismembered part of the more distant
Elbow-Athapapuskow ocean-floor assemblage.
West Mikanagan Lake E-MORB suite (unit F5)
The West Mikanagan Lake E-MORB–type volcanic suite
occupies a fault block flanked by gabbroic rocks to the west and
the East Mikanagan Lake arc-rift suite to the east. The western
margin is, in part, intruded by gabbroic rocks of the Tartan Lake
complex (Gilbert, 1986a); elsewhere the fault block is bounded
by splays of the Northeast Arm Fault (Figures 2 and 3). South
of Mikanagan Lake, the West Mikanagan Lake volcanic suite
is juxtaposed by the same fault against the arc-type Whitefish
Lake–Mikanagan Lake Block (Figure 4).
The West Mikanagan Lake E-MORB suite consists of a
uniform, southwest-facing monoclinal sequence of massive
to pillowed basaltic flows and abundant related gabbro;
fragmental rocks are confined to minor (<1 m) interflow breccia
units. In contrast to typical arc volcanic sequences, alteration
is not widespread in the West Mikanagan Lake section and
pyroclastic/epiclastic detritus is absent. The E-MORB is
typically aphyric, green-grey or blue-grey on fresh surfaces,
and locally sparsely vesicular. Some flows contain minor quartz
or quartzofeldspathic amygdules (typically <5%) and/or minor
plagioclase phenocrysts (<5%); rare polygonal cooling fractures
are locally preserved. Synvolcanic gabbro sills (commonly
2–25 m, rarely up to 100 m thick) are geochemically similar to
the pillowed flows but have slightly lower incompatible-element
contents. Porphyritic diabase dikes with up to 30% hornblende
(after pyroxene) phenocrysts are widely distributed; in contrast
to the interflow gabbros, several of these dikes (and a similar
porphyritic dike within Animus Lake E-MORB hostrock) were
found to have an arc-type composition and are thus not related
to the host, E-MORB–type basalt (Figure 15h).
A northwest-trending zone of variolitic quenched basalt, up
to 165 m thick and extending over 1 km along strike, intersects
the northwestern tip of Swordfish Lake; it occurs approximately
in the centre of the monoclinal West Mikanagan Lake E-MORB–
type sequence (Gilbert, 1987a). The flows contain ovoid,
0.5–1 cm saussuritic varioles that vary in abundance from 10
to 75% of the rock. These rocks are also typically vesicular
and contain 3–25% quartz or quartzofeldspathic amygdules.
Textures derived from quenched clinopyroxene crystals are
defined by delicate trails/filaments/chains of very fine grained
amphibole and/or epidote. The amphibole/epidote texture is
characterized by a wide variety of branched/pinnate/featherlike patterns that occur within quartzofeldspathic domains.
The varioles appear to have ‘overprinted’ the clinopyroxene
quench texture and are interpreted as products of spherulitic
crystallization that occurred during or after consolidation of the
volcanic flow.
The localized clinopyroxene quench texture in variolitic
basalt within West Mikanagan Lake E-MORB (and East
Mikanagan Lake arc-rift rocks) is interpreted as due to
29
rapid cooling of basalt in a quiescent, back-arc/ocean-floor
environment. The controlling factors for this textural feature may
include locally elevated volatile content in the magma, because
basalt with quench texture is invariably quartz amygdaloidal,
especially northwest of Swordfish Lake, where quartz
amygdules are locally more abundant than they are anywhere
else within the E-MORB–type sequence. Clinopyroxene quench
texture is of very limited extent in the NFFB and appears to be
confined to arc-rift or MORB-type settings. It was encountered
elsewhere at only two other localities: 1) within Arthurs Lake
N-MORB (unit F1) at the southern end of Ruby Lake, and 2)
in a silicified diabase dike (unit X1) hosted by Animus Lake
N-MORB (unit F3).
West Mikanagan Lake E-MORB–type rocks are highTiO2 tholeiites that display smooth, negative-sloping profiles
in plots of incompatible elements, characterized by moderately
elevated Th and LREE but lacking HFSE depletion relative to
N-MORB (Figure 15d). The E-MORB plots show no negative
Nb anomalies, unlike those of both arc and arc-rift volcanic
rocks in the NFFB (Figures 7a to 7j). West Mikanagan Lake
basalt, as well as compositionally similar E-MORB volcanic
suites elsewhere in the Flin Flon Belt, is more similar to
modern transitional or plume-related MORB types than to the
depleted mantle source of N-MORB volcanic suites (Stern et
al., 1995b). Both the TiO2 versus MgO plot (Figures 6a to 6c)
and the Th/Nb versus Nb/Y discriminant diagram (Figure 8a)
clearly demonstrate the compositional distinction of West
Mikanagan Lake E-MORB, compared to N-MORB types and
arc volcanic rocks in the NFFB. The +3.5 εNd value for West
Mikanagan Lake E-MORB at 1.9 Ga (Table 4) is within the
+3.1 to + 4.5 range for Flin Flon Belt E-MORB volcanic rocks
elsewhere (Stern et al., 1995b) and is indistinguishable from
the projected value for depleted mantle of that age (DePaolo,
1981), indicating the source magma was juvenile in origin and
lacked crustal contamination.
Depleted-MORB volcanic suites (unit T)
Mafic volcanic rocks that are geochemically transitional
between arc volcanic basalt and MORB extend along the
northern margin of the NFFB in the Wabishkok Lake–Dismal
Lake area (Dismal Lake assemblage, Figure 2). Compositionally
similar, incompatible-element–depleted rocks (Bluenose Lake
depleted-MORB suite, unit T2) occupy a fault slice that extends
east from Bluenose Lake on the limbs of the east-northeasttrending syncline north of the main part of Wabishkok Lake. The
Dismal Lake suite (unit T1) is also geochemically similar to the
Moody Lake basalt that extends along the southern margin of
the Kisseynew Domain, north of the Flin Flon Belt (Zwanzig,
2010). Dismal Lake depleted MORB is laterally continuous
with the Moody Lake basalt, and together these rocks extend
for more than 100 km from File Lake and Moody Lake in the
east (NATMAP Shield Margin Project Working Group, 1998,
sheets 2 and 3) to the Bluenose–Precipice lakes area in the west
(Figure 2). The depleted-MORB type is thus confined to the
northern margin of the Flin Flon Belt and southern margin of
the Kisseynew Domain. Compositionally equivalent types are
absent both in the central part of the Flin Flon Belt and within
30
the Elbow-Athapapuskow ocean-floor assemblage farther south
(Bailes and Syme, 1989; Stern et al., 1995b).
Dismal Lake depleted-MORB suite (unit T1)
The Dismal Lake depleted-MORB succession is up to
6 km wide and consists largely of pillowed, aphyric basalt
flows, derived finely laminated amphibolite and subordinate
related gabbro. These rocks are, in general, more deformed and
attenuated than the supracrustal rocks within the Flin Flon arc
assemblage to the south. Plagioclase-phyric flows extend in a
200 m wide zone along the southern margin of the depletedMORB assemblage, where pillows indicate the sequence is
north facing. There is little evidence elsewhere for top directions;
northeast of Dismal Lake, the sequence is provisionally
interpreted as monoclinal and west-northwest trending, except
for the northernmost part of the depleted-MORB assemblage,
where a fold with a strike-parallel axial trace is delineated by
EM anomalies (Assessment File 73546, Manitoba Innovation,
Energy and Mines, Winnipeg). Heterolithic, arc volcanic rocks
that occur close to both the northern and southern margins
of the Dismal Lake assemblage and in the vicinity of Ham
Lake (Figure 2) may be allochthonous fault slices within the
depleted-MORB rocks; alternatively, they may be conformable
with the depleted MORB and represent periodic changes in the
composition of a heterogeneous mantle source.
Dismal Lake basalt is typically aphyric and pillowed, with
sparsely distributed quartz±plagioclase amygdules. Pillows
are locally well preserved and ovoid (up to 1.5 m long), but
more typically they are deformed and strongly flattened.
Rare autoclastic breccia zones occur sporadically within the
massive flows. Epidote alteration is common but of generally
minor extent (<10% of individual rock outcrops). Gabbro
sills (1–20 m thick) form over 10% of the depleted-MORB
assemblage, but synvolcanic diabase dikes are virtually absent.
Plagioclase-phyric pillowed basalt at the northern shore of
Dismal Lake, and along strike farther to the west, contains
plagioclase phenocrysts (1–4 mm, 5–20% of the rock) as well
as quartzofeldspathic amygdules.
Thinly laminated amphibolite and mafic gneiss are lithologically gradational and intercalated with pillowed to massive
basalt and gabbro at a scale of 1–20 m. They represent highly
strained, incompetent zones within the volcanic sequence.
Epidote±hornblende±chlorite–rich laminae, typically 1–25 mm
thick, are interpreted as structurally transposed features
such as pillow selvages, autoclastic fragments and epidosite
domains. Boudinage and minor folding of the lamination are
characteristic. Both massive and laminated parts of the depletedMORB sequence contain widespread porphyroblastic garnet
(1–4 mm) and hornblende (2–5 mm), and display localized
silicic alteration (±garnet±pyrite).
Geochemically, the Dismal Lake volcanic suite is
characterized by a flat to slightly positive-sloping incompatibleelement profile, typical for N-MORB rocks (Figure 15e).
However, in contrast to both N-MORB and E-MORB types, the
depleted MORB has a conspicuous negative Nb anomaly (Nb is
below the detection limit in most analyzed rocks) and the average
Th/Nb ratio (0.22 ppm) is more than twice that of MORB-type
basalts (Table 3, back pocket). The low average Th content of
Manitoba Geological Survey
0.13 ppm in Dismal Lake basalt is comparable to the 0.12 ppm
average of modern N-MORB (Sun and McDonough, 1989),
in contrast to more enriched NFFB arc basalt with an average
Th content of 0.88 ppm. The negative Nb anomaly in depleted
MORB, on the other hand, is also typical for arc volcanic rocks
and suggests a component of subduction-modified mantle was
present in the source magma. The depleted-MORB rocks plot
in the arc-volcanic field in the Th-Zr-Nb ternary diagram due
to their anomalously low Nb content (Figure 16). The Th/Nb
ratios of Dismal Lake basaltic rocks are mostly equal or greater
than the ‘primitive’ value (0.1) for MORB-type rocks (Stern et
al., 1995b), and most of these rocks therefore plot in the field
of ‘N-MORB types with arc signatures’ in the Th/Nb versus
Nb/Y diagram (Figure 8b). In the TiO2 versus MgO diagram
(Figure 6d), the depleted-MORB rocks overlap the ‘MORB +
BABB’ and arc fields due to their wide range of TiO2 content
(0.23–1.44%). Incompatible-element contents in Dismal Lake
basalt, which exhibit a wide range of values (Figure 15e), are
probably controlled mainly by the amount of partial melting
in the mantle source (Gribble et al., 1998), with the extent of
depletion being directly proportional to the degree of melting
(see ‘Discussion, tectonic setting of depleted-MORB types in
the NFFB’ below). The generally flat REE profile and primitive
Nd isotopic composition of the Dismal Lake depleted-MORB
rocks (εNd value of +5.2 at 1.9 Ga, Table 4) are consistent with
a BAB setting in which there was no assimilation of older crust
by the mantle source magma.
Arc volcanic enclaves within the Dismal Lake depletedMORB assemblage
Mineralized volcanic rocks compositionally akin to
juvenile-arc types occur at four localities within, and at the
margins of, the Dismal Lake depleted-MORB assemblage
Zr/117
(Figure 2): 1) Ham Lake; 2) 1.5 km due south of Ham Lake;
3) Dismal Lake (Gale and Norquay, 1996; Gilbert, 2003a, b;
Assessment Files 73994, 73546, 73348); and 4) the northern
margin of the depleted-MORB assemblage (Gale and Norquay,
1996). These occurrences have been identified largely by
diamond drilling during mineral exploration programs, and are
thus not delineated on Map GR2011-1-1. Sporadic localities of
arc basalt elsewhere within the depleted-MORB assemblage
were identified by geochemical sampling and are indicated on
the map (red diamond symbols).
Arc volcanic enclaves from (1) to (3) are estimated
to be roughly 600–800 m wide and extend laterally for
approximately 1.5–2.5 km; sporadic occurrences along
strike of arc basalt flows suggest their lateral extent may
be greater, as much as 5 km or more. Mineralized arc-type
rocks at the Dismal Lake locality (3) occur at the contact
between the depleted-MORB and younger granitoid rocks
(Kotyk Lake pluton) to the south (Figure 2; Gilbert, 2003b).
At the northern margin of the Dismal Lake assemblage
(locality 4), base-metal sulphide mineralization occurs
sporadically within arc volcanic rocks, along a mineralized
horizon that extends laterally for at least 8 km (Gale and
Norquay, 1996).
B
OR
N-M
These four arc volcanic enclaves contain variously
altered mafic to felsic volcanic rocks (± sulphide-facies iron
formation and fine-grained sedimentary rocks) and derived
gneiss, in which EM conductors have been delineated. Zones
of base-metal (±Au) mineralization are characterized by
pervasive silicic, chloritic and epidote alteration, as well as
conspicuous garnet (±hornblende±anthophyllite) blastesis.
The mineralized zones are typically strongly deformed
and intruded by one or more granitoid, gabbroic and/or
minor ultramafic igneous phases. Contact relationships
of the arc volcanic enclaves are unknown; they may be
structural outliers within the depleted-MORB rocks that
represent remnants of the Flin Flon arc assemblage, or a
relatively older oceanic arc. Alternatively, they may be due
to ephemeral changes in the composition of the magmatic
source for Dismal Lake rocks that reflect the heterogeneous
nature of the mantle source.
Bluenose Lake depleted-MORB suite (unit T2)
Tholeiitic arc
Calcalkaline
arc
B
OR
E-M
Th
Arc volcanic rocks
Arc-rift volcanic rocks
N-MORB volcanic rocks
E-MORB volcanic rocks
Depleted-MORB volcanic rocks
Nb/16
Figure 16: Ternary Th-Zr-Nb plot of mafic to felsic volcanic rocks
in the northern Flin Flon Belt, showing arc, arc-rift, N-MORB,
E-MORB and depleted-MORB rocks. Compositional fields
of modern volcanic rocks after Wood (1980). Abbreviations:
E-MORB, enriched mid-ocean-ridge basalt; N-MORB, normal
mid-ocean-ridge basalt.
Geoscientific Report GR2011-1
A fault-bounded enclave of depleted-MORB–type basalt,
compositionally akin to flows in the Dismal Lake assemblage,
extends for over 12 km between northern Wabishkok Lake
and Bluenose Lake (Figure 2). At Bluenose Lake, this tectonic
enclave is folded by a major, east-plunging synclinal fold
(Figure 3; see ‘Structural Geology’ below). The southern limb
of the fold is a north-facing, apparently monoclinal sequence
up to 1.2 km thick that consists of massive to pillowed aphyric
basalt, derived gneiss and mafic intrusive rocks, intercalated at a
scale of 2–50 m. A conspicuous 300 m wide altered zone, at the
northern (upper) margin of the southern limb, is characterized
by pervasive silicification and epidotization, and contains felsic
porphyry intrusions and sporadic base-metal mineralization.
One 65 m thick plagioclase-porphyry sill is geochemically very
similar to the Kotyk Lake rhyolite (unit J11) in the contiguous
Wabishkok Lake arc volcanic block9, suggesting the felsic
31
porphyry intrusions, as well as the alteration and mineralization,
may be associated with arc magmatism postdating the depletedMORB suite.
Bluenose Lake basalt flows contain ovoid pillows that
are typically 0.5–1.5 m long, but mega-pillows up to 3.5 by
0.8 m occur in several flows. The pillows are commonly quartzamygdaloidal, with up to 15% spheroidal or pipe amygdules,
locally concentrated in upper vesicular zones; rare flat-based
vugs occur sporadically in larger pillows. Garnetiferous gneiss
and laminated amphibolite intercalations within the mafic
volcanic flows are similar to basalt-derived metamorphic rocks
in the previously described Dismal Lake assemblage.
Geochemical plots of incompatible elements (Figure 15f)
show Bluenose Lake depleted MORB has a flat to slightly
positive-sloping profile, similar to that of Dismal Lake basalt,
but with lower average LREE and HFSE contents (Table 3,
back pocket). Conspicuous negative Nb and slight negative Ti
anomalies—features that are typical of modern arc magmas—
suggest a subduction zone influence in the evolution of
the source magma. The depleted-MORB composition with
anomalously low Nb and TiO2 contents is consistent with a
BAB setting for the Bluenose Lake volcanic suite, as suggested
previously for the Dismal Lake depleted-MORB rocks.
Sedimentary rocks of uncertain age (unit S)
Turbiditic rocks of uncertain age are intercalated with arc
and MORB-type volcanic rocks in the area between Embury
Lake and Naosap Lake (Figure 2). These sedimentary enclaves
(units S1–S7) are typically devoid of volcanic interlayers and,
in most cases, appear to be fault bounded; in these respects
they are distinguished from sporadic units of lithologically
similar, fine-grained sedimentary rocks that are interpreted
as conformable with juvenile-arc volcanic rocks. The faultbounded sedimentary enclaves consist largely of greywacke,
siltstone and subordinate argillaceous mudstone and pebble
conglomerate. These rocks locally display graded bedding,
cyclic sequences with Bouma divisions and less commonly,
crossbedding, flame structures, rip-ups and rare sedimentary
folds.
The age of these turbiditic rocks (unit S) is unknown except
in two cases. Greywacke within the westernmost sedimentary
enclave (unit S7, Embury Lake Block) has a maximum
depositional age of 1843 ±9 Ma (age of the youngest detrital
zircon, Ordóñez-Calderón et al., 2011). The same greywacke
also contains zircons derived from older, juvenile-arc rocks
(ca. 1881–1889 Ma) but Archean zircons are apparently absent;
the statistical centre of the detrital zircon population is 1860 Ma
(N. Rayner, pers. comm. 2009; see ‘Cope Lake suite’ above).
The Embury Lake greywacke apparently represents a successorarc sedimentary deposit, whereas turbiditic rocks at Bartley
Lake (unit S2) are likely part of the 1885 ±3 Ma Vick Lake tuff
(Stern et al., 1993)—a shoshonitic formation that occupies the
upper part of the Bear Lake arc volcanic sequence in the area
south of Mikanagan Lake (Bailes and Syme, 1989). The Bartley
Lake sedimentary rocks are associated with volcaniclastic
deposits (Gilbert, 1990a) and are interpreted as stratigraphically
equivalent to the Vick Lake tuff. Fault-bounded turbidite
deposits elsewhere in the NFFB (units S1, S3 to S6) may be
affiliated with either successor-arc rocks (i.e., Burntwood
Group age, deposited between 1860 and 1840 Ma, Machado et
al., 1999) or older epiclastic sequences penecontemporaneous
with juvenile-arc volcanism (1889–1881 Ma, Rayner, 2010).
Successor-arc sedimentary rocks
Missi Group rocks (unit M)
Fluvial-alluvial conglomerate and crossbedded sandstone
of the Missi Group are among the youngest known supracrustal
rocks in the Flin Flon Belt. These sedimentary rocks have yielded
detrital zircon ages indicating both Neoarchean and Proterozoic
detrital sources, the youngest of which are approximately
1847–1851 Ma in age (Ansdell et al., 1992; Ansdell, 1993).
Missi Group sedimentary and associated volcanic rocks are
unconformable or in fault contact with volcanic rocks of the Flin
Flon arc assemblage; more recent data indicate that deposition
occurred between 1846 and 1842 Ma (Stern et al., 1999).
Missi Group conglomerate and arkosic sandstone occupy
a large (approximately 12 by 6 km) structural basin at the
western end of the Flin Flon Belt (Figure 4). These rocks,
which were mapped and described in detail by Bailes and
Syme (1989), extend farther west for over 30 km beyond the
boundary between Manitoba and Saskatchewan (NATMAP
Shield Margin Project Working Group, 1998, Figure 1b). Missi
Group rocks also occur in the northern part of Mikanagan
Lake, where a thin fault sliver of conglomerate contains a wide
variety of volcanic, sedimentary and granitoid clasts (Gilbert,
1990a, b). The clasts range up to boulder-size (80 by 40 cm) and
include some types that display spheroidal hematitic veining,
interpreted as evidence for subaerial exposure and weathering
prior to deposition. The >40 m wide conglomerate is part of a
series of structural enclaves of Missi Group rocks that extend
from Athapapuskow Lake, northwards through Whitefish Lake
to Mikanagan Lake (NATMAP Shield Margin Project Working
Group, 1998, sheet 2).
Intrusive rocks (units P, X and J)
Granitoid plutons (unit P) extend along the boundary
between arc volcanic and depleted-MORB rocks, parallel to
the northern margin of the Flin Flon Belt (Figure 2). These
predominantly tonalitic to granodioritic rocks intrude their
volcanic hostrocks and are typically massive to slightly foliated.
A weakly defined foliation in the Pistol Lake pluton (unit P2)
is deformed by a late (D3) east-northeast-trending fold that also
deforms the Kotyk Lake pluton (see ‘Structural Geology’ below).
These granitoid plutons thus stitch contacts between contrasting
volcanic terranes but predate subsequent deformation related
to intracontinental tectonics post-1840 Ma (Lucas et al., 1996;
Gale et al., 1999). In contrast, tonalitic intrusions within the
Flin Flon arc assemblage (e.g., unit P1 in Lac Aimée Block)
Felsic sill (sample 32-01-0391-4) and Kotyk Lake rhyolite (sample 32-01-0261-1) in Appendix 1. Incompatible-element contents of these two rocks vary by an
average of only 5%, resulting in virtually identical chondrite-normalized plots.
9
32
Manitoba Geological Survey
may be synvolcanic and penecontemporaneous with the 1888
±1 Ma Cliff Lake plutonic suite (unit J19; Rayner, 2010). The
southwestern part of the Naosap Lake pluton (unit P4), which
appears to be in gradational contact with the Baker Patton
Complex (unit J12), may also be of juvenile-arc age (see
‘Sourdough Bay suite’ above).
Mafic intrusions, which occupy at least 15% of the area
of the NFFB, are subdivided into 1) compositionally layered
sills, and 2) mafic to ultramafic rocks of uncertain age (units J
and X respectively, Table 1). Batters Lake sill (unit J16) and
Tartan Lake sill (unit J17) are lithostratigraphically similar
to the 1881 +3/–2 Ma Mikanagan Lake Sill (unit J18; Stern
et al., 1999), which has been interpreted as rift-related and
coeval with the youngest products of juvenile-arc volcanism
(Zwanzig et al., 2001). Some gabbroic intrusions of unit X,
emplaced either within arc or MORB-type sequences, may also
be synvolcanic in age; geochemical data are locally consistent
with such a genetic relationship (e.g., Wabishkok Lake sill
(unit X3) and Kotyk Lake sill (unit X4) contain phases that are
compositionally similar to Animus Lake MORB-type rocks; see
Figure 15g). The Tartan Lake complex (unit X2) is a multiphase
heterogeneous gabbroic intrusion that is intruded by younger
granitoid rocks (Peloquin, 1985; Gilbert, 1990a); its tectonic
affinity and age are unknown.
Discussion
The NFFB consists of two main assemblage types—arc
and MORB—that have been mapped in detail and subdivided
primarily on the basis of geochemical affinity. Arc volcanic
suites are tholeiitic, calcalkaline or of transitional (tholeiiticcalcalkaline) composition, and are typically bimodal. An
additional subdivision is based on isotopic data that indicate arc
volcanic rocks in the three fault blocks in the eastern part of the
NFFB (Lac Aimée, Naosap Lake and Sourdough Bay suites;
Figure 2) are distinguished by lower values of εNd (–1.5 to
+0.1), indicating up to 12% assimilation of older crust by their
source magmas, compared to other NFFB arc volcanic rocks
that have higher εNd values (+1.7 to +3.4), indicating relatively
less (<3%) crustal assimilation (Table 4; Figure 9). This pattern
of more conspicuous crustal assimilation in the three eastern
arc-type fault blocks suggests that they may all have originated
in the same part of the original oceanic arc, where the amount
of lithospheric contamination in the mantle source was greater
than elsewhere in the arc system. Similar contamination of the
source for arc volcanic rocks elsewhere in the Flon Flon Belt
has been attributed to Archean crustal remnants (Stern et al.,
1995b). Such rocks have locally been identified as fault slivers
within the Northeast Arm Fault (David and Syme, 1994), and
Archean crust has also been inferred within the basement
underlying arc volcanic rocks in the eastern part of the Flin Flon
Belt at Snow Lake (Stern et al., 1995a).
Arc-rift rock suites that are interspersed with the mainly
juvenile-arc types in the northern and central parts of the Flin
Flon Belt occur in two separate zones. The majority of the
(predominantly basaltic) arc-rift volcanic rocks occur within
the Lac Aimée and East Mikanagan Lake blocks, which are
interpreted as part of a structural corridor that also contains the
(arc-rift) Scotty Lake basalt farther to the south (Figures 2, 3
Geoscientific Report GR2011-1
and 4). This corridor extends southwards across the Flin Flon
Belt for over 30 km (NATMAP Shield Margin Project Working
Group, 1998, sheet 2). Arc-rift basalt also occurs in the
northern part of the lithologically diverse Manistikwan Lake
Block, which includes rhyolite, volcaniclastic and sedimentary
deposits, N-MORB extrusive types and abundant ultramafic
to felsic intrusive rocks; the central and southern parts of the
fault block contain arc volcanic formations (Bailes and Syme,
1989). The Manistikwan Lake Block, together with the MORBtype rocks in the contiguous Arthurs Lake Block and Grassy
Narrows Zone farther to the south (Figure 4; Bailes and Syme,
1989), are interpreted as a second structural corridor, bounded
to the east by the Inlet Arm Fault, a major structural feature
with approximately 11 km of dextral displacement (Bailes
and Syme, 1989). The two structural corridors (associated
with the Northeast Arm and Inlet Arm faults respectively) are
interpreted as accretion-related tectonic zones that may, in part,
be coincident with former zones of rifting within the original
oceanic arc. Subsequent deformation along these corridors
resulted in the development of anastomosing, steeply dipping
faults and the juxtaposition of various fault slices of contrasting
tectonic affinity.
The MORB-type volcanic rocks in the NFFB consist almost
entirely of tholeiitic basalt and related gabbro and include three
geochemically distinctive types: 1) N-MORB and 2) E-MORB
volcanic rocks in the Arthurs Lake, West Mikanagan Lake and
Animus Lake blocks are tectonically interleaved with arc-type
fault blocks, whereas 3) depleted MORB of the Dismal Lake
assemblage extends along the northern margin of the Flin Flon
arc assemblage. The distinctive REE patterns of the N-MORB
and E-MORB suites in the NFFB are comparable to those of
similar formations in the Elbow-Athapapuskow ocean-floor
assemblage (Stern at al., 1995b). The N-MORB rocks in
the ocean-floor assemblage are assumed to be derived from
depleted mantle in an ocean-ridge environment, whereas the
source of the E-MORB formations is interpreted as enriched
mantle, similar to that of plume-related ocean-island basalt.
The combination of MORB- and arc-type compositional
features displayed by both the Dismal Lake and Bluenose Lake
depleted-MORB rocks is interpreted as the result of mixing of
depleted and enriched MORB-like mantle, as well as subductionmodified magmatic sources, as described for some rock types
in the Elbow-Athapapuskow ocean-floor assemblage (‘MORB
types with arc signature’; Stern et al., 1995b). Variations in the
ratio of these different mantle components could account for
the compositional range between the several NFFB volcanic
suites that are interpreted to have been erupted in back-arc
settings (N-MORB, E-MORB and depleted-MORB types).
Similar compositional variation within modern BABB has
been attributed to differences in the depth and amount of partial
melting in the mantle source; the amount of partial melting
would be least in E-MORB, greater in depleted MORB and
most conspicuous in subduction-modified mantle components
(Fryer et al., 1990). In addition, such compositional variation
in modern BABB may reflect the extent of back-arc extension,
such that the magmatic source at the initiation of rifting may
contain more subduction-modified mantle than basalts that
are erupted later in the development of the BAB (Stern et al.,
1990). According to this model, the Dismal Lake and Bluenose
33
Lake depleted-MORB sequences may represent an earlier stage
of extensional BAB development, compared to N-MORB and
E-MORB suites elsewhere in the NFFB.
Tectonic setting of NFFB arc and MORB-type volcanic
rocks
Comparison of the various arc and MORB geochemical
types in the NFFB with modern analogues in the southwestern
Pacific Ocean is the basis for the following discussion of their
tectonic setting10. Juvenile-arc volcanic suites and associated
epiclastic deposits in the NFFB are stratigraphically and
compositionally similar to modern oceanic-arc successions.
In addition, geochemically diverse modern BAB such as the
Lau basin and Mariana Trench contain rock types that can be
equated with the various MORB types in the NFFB (Stern et
al., 1990; Martinez and Taylor, 2003). Tectonism, including
early deformation in the Flin Flon Belt (1886 ±3 Ma; Zwanzig
et al., 2001) and subsequent collisional tectonism, resulted in
a collage of the various crustal components (Amisk Collage,
Lucas et al., 1996). Fault blocks of contrasting geochemical
types have been juxtaposed, and the block-bounding faults
are commonly discordant to the truncated axial traces of their
internal folds; thus the stratigraphic and age relationships
between the various volcanic suites are, in many cases, largely
conjectural. In spite of these constraints, instances of continuity
such as the occurrence of arc-rift volcanic rocks in the structural
corridor containing Lac Aimée and the Scotty Lake blocks
The following synopsis of modern island-arc development (based largely on
Stern et al., 1990; Gribble et al., 1998; Martinez and Taylor, 2003; Sinton et al.,
2003) serves as a base for further discussion of the relationship between arc
and MORB-type volcanic terranes in the NFFB. Arc-rifting of oceanic lithosphere and intra-arc extension are characteristic features of modern island-arc
development that have been attributed to ‘slab roll back’ (Hamilton, 1995). This
occurs when the angle of the subducting lithospheric slab increases, resulting
in a change from a compressive to an extensional tectonic regime in the BAB.
After an extensional regime has been established in the oceanic lithosphere
at a subduction zone, a rift parallel to the arc axis may be initiated anywhere
across the breadth of the arc from the ‘front’ (trench) side to the back-arc area
(Figure 17). The rift will propagate actively, parallel to the elongation of the
arc, and an intra-arc basin will develop behind it, widening progressively on
either side of a central rift. The magmatic regime at the subduction zone continues uninterrupted during this process, but the site of volcanic eruption is
diverted away from the arc axis to the nearby rift axis (extension axis). Thus
initial magmas at inception of arc-rifting may be indistinguishable from the arc
types to which they are related, but subsequent arc-rift magmas incorporate a
component of more primitive, fertile ambient mantle that results in higher overall incompatible-element contents. The intra-arc basin may not develop into
a larger BAB if rifting does not persist. Alternatively, rifting may continue to
propagate parallel to the arc, the rift axis at the same time migrating steadily
away from the arc. Eventually a point is reached where this axis becomes a
‘spreading axis’ and a BAB is established, which may proceed to become more
extensive (e.g., Mariana Trough is over 400 km wide and part of the Izu-BoninMariana arc, initiated at 43 Ma, which extends laterally for 2800 km; Gribble et
al., 1998; Stern et al., 2006).
10
During BAB development, the extension axis becomes steadily more remote
from the arc and there is a change in the way in which the supply of magma
(erupted at the axis) is generated. The mantle melt at the subduction zone
is derived by a combination of decompression melting in rising diapirs and
‘flux melting’ (Pearce and Peate, 1995) in an open system that is largely
controlled by the influence of hydrous fluids rising from the subducted slab.
These fluids 1) lower the melting point of the mantle source; 2) increase the
34
suggest that the associated regional faults/shear zones may, in
part, be conformable with the trend of the original island arc.
Furthermore, interpretation of the tectonic setting of NFFB
volcanic types by comparison with widely documented modern
analogues in western Pacific oceanic arcs indicates how the
arc, arc-rift and back-arc types in the NFFB may be related by
progressive evolution of an arc system, from initial subduction
to subsequent juvenile-arc magmatism, arc-rifting and the
establishment of a mature BAB (Figure 17).
Arc-rift propagation, development of intra-arc and backarc basins and the potential for VMS mineralization
The association of extensional tectonic regimes with
occurrences of VMS mineralization, which has long been
discussed in both ancient and modern arc volcanic terranes
(e.g., Sillitoe, 1982), has been well documented in the Flin
Flon Belt (Syme et al., 1996, 1999; Bailes and Galley, 1999,
2007; Gibson et al., 2009). The presence of arc-rift volcanic
rocks in the NFFB is thus of particular interest, in view of their
potential economic importance. Stratigraphic details within the
Bear Lake arc succession south of the NFFB map area (Syme
et al., 1999) show that two ore deposits (Cuprus and White
Lake) were emplaced at an early stage of arc-rifting, which
was initiated after deposition of the lower, calcalkaline part of
the succession. Uranium-lead zircon dating indicates rifting
in the Flin Flon Belt occurred in the 1886–1881 Ma interval
(Zwanzig et al., 2001); within the Bear Lake Block, the latter
extent of melting (e.g., 28 ±8% in Mariana Arc; Gribble et al., 1998) and thus
the degree of depletion of REE and especially HFSE; and 3) convey selected
elements (large-ion lithophile elements, alkalis, Al) directly (or indirectly via
metasomatism) to the site of melt generation. Flux melting results in characteristic arc magmatic signatures, best shown in plots of incompatible elements
(Figure 7a to 7j). In contrast, melts in mature BAB have a different magmatic
path and reach the rift axis in a stream of upwelling mantle governed by decompression melting, rising directly beneath the spreading axis. The amounts
of melting are relatively less (e.g., 13 ±5% in Mariana Trough; Gribble et al.,
1998) and thus depletion of incompatible elements is much reduced, compared
to a subduction zone setting. The BAB continues to widen in a process similar
to that which generates new lithosphere at mid-ocean ridges. Based on investigations in the Mariana arc system, the interval between initiation of rifting and
onset of spreading is 1–4 m.y., and the spatial extent of rifting is 100–150 km,
which is the distance between the volcanic front of the arc and the rift axis when
spreading would typically be initiated (Gribble et al., 1998). Once the mature
BAB has reached a width of approximately 400 km, the supply of magma may
be exclusively derived from upwelling mantle via decompression melting—the
spreading ridge is then analogous to a mid-ocean-ridge and has a similar magmatic source, with no subduction zone influence. Earlier stages of BAB development are characterized by a wide variety of magmatic types that include arc,
arc-rift and various MORB types. Factors that determine the particular composition of the erupted magma include 1) crustal depth, amount of hydrous fluid
available from the subducted slab and rate of subduction, which control the
temperature, degree of partial melting and the amount of incompatible-element
depletion (inversely proportional to crustal depth); 2) composition of the mantle
source (i.e., REE-depleted versus fertile, more primitive mantle); strongly depleted mantle melts are generated by incorporation of magmatic sources partially depleted by a prior melt cycle or by several cycles; and 3) extent of mixing
of subduction-modified mantle, depleted mantle and undepleted, fertile magma.
The interaction between these various factors in the source magma results in a
gradation, temporally and spatially, between two BABB types (Gribble et al.,
1998)—‘Rift BABB’ (early, at subduction zones) and ‘Spread BABB’ (those in
mature BAB that exhibit little, if any, subduction zone influence).
Manitoba Geological Survey
VF
V
V
V
V
V
MW
V
a)
VF
V
V
V
V
V
b)
V
V
V
V
V
EA
c)
Figure 17: Sequence of events leading to rifting and BAB development in modern oceanic arcs (based largely on Martinez and
Taylor, 2003; Sinton et al., 2003): a) Subduction zone magmatism: partial melting in the mantle wedge (MW) is increased due to
hydrous fluids derived from the subducted slab. The amount of fluid and degree of associated (‘flux’) melting are greatest at shallow
depth. Viscous coupling results in ‘corner flow’ as mantle convection streams down, parallel to the subducting slab, against the
direction of rising mantle diapirs (red lines). Arc volcanism is associated with progressive depletion in the magma of HFSE and HREE
due to partial melting of the mantle source and melt extraction, resulting in a wide compositional range. Fractional crystallization and
the addition of slab-derived components (H2O, LILE, Al and alkalis) increase the diversity of the arc-type rocks; b) Onset of rifting,
development of intra-arc basin and generation of arc-rift basalt: slab rollback (shown by yellow arrow) steepens the subduction
angle and initiates an extensional tectonic regime (white arrows) and the onset of rifting. The rift propogates parallel to the plate
boundary, at the same time migrating progressively away from the volcanic front (VF). Initially, arc-type magmas are redirected to the
site of rifting. Mantle convection adjusts to the new extensional axis, resulting in advection of depleted mantle back to the rift axis,
where it may mix with ambient, more fertile mantle to produce arc-rift basalt; and c) Mature BAB and MORB-type magmatism: the
extensional axis (EA) is now remote from the subduction zone and beyond the influence of the subducting slab. The N-MORB and
E-MORB magma types are generated at the spreading centre by decompression melting in upwelling mantle. During transition from
stage (b) to (c), various ‘MORB types with arc signature’ (including ‘depleted-MORB’ types) may be generated by mixing of different
mantle types, and reflect the waning influence of the subducting slab. A new arc may develop due to ongoing magmatism, as long as
subduction continues at the plate boundary. Abbreviations: BAB, back-arc basin; HFSE, high-field-strength element; HREE, heavy
rare-earth element; LILE, large-ion lithophile element; MORB, mid-ocean-ridge basalt; E-MORB, enriched mid-ocean-ridge basalt;
N-MORB, normal mid-ocean-ridge basalt.
Geoscientific Report GR2011-1
35
part of rifting was accompanied by deposition of the Vick
Lake shoshonitic tuff (1885 ±3 Ma, Stern et al., 1993) within
an actively subsiding rift basin. Economic mineralization is
thought to have been localized at the site of rifting due to a
combination of high heat-flow and abundant hydrothermal
fluids, facilitating the development of high-temperature zones
of alteration and mineralization (Syme et al., 1999). The rifting
is manifested both geochemically (MORB-like ferrobasalt and
associated rhyolite) and structurally (faulting, unconformities
and rift-basins with shoshonitic turbidite).
In the case of the two structural corridors in the NFFB
associated with the Northeast Arm and Inlet Arm faults,
respectively (see previous ‘Discussion’), arc-rifting is inferred
largely from geochemical considerations (see descriptions of
units J6, J7 and J8 above). In both cases, the presence of arc-rift
basalt and juxtaposed arc volcanic rocks (± MORB-type rocks,
± volcaniclastic/epiclastic deposits) suggests that the structural
corridors may coincide, in part, with rifted parts of the original
island arc. Additional support for this model is provided by
the Mikanagan Lake Sill (1881 +3/–2 Ma; Stern et al., 1999)
that extends along the western side of the Northeast Arm Fault
and has been interpreted to be associated with rifting at the end
of juvenile-arc volcanism (Zwanzig et al., 2001). In spite of
numerous exploration programs for base- and precious-metal
mineralization since the early 1950s, no economic mineralization
has been identified, so far, in either the Lac Aimée–Mikanagan
Lake–Scotty Lake corridor or the Manistikwan Lake–Arthurs
Lake–Grassy Narrows Zone corridor.
The presence of the Baker Patton Complex immediately
south of Lac Aimée Block may also be related to rifting of the
Proterozoic arc system. This felsic complex, which represents
by far the largest felsic volcanic terrane in the Flin Flon Belt,
contains one gold and at least five VMS-type mineral deposits
(Gale and Eccles, 1988a). The location of the felsic complex
at the margin of the Lac Aimée–Scotty Lake zone of rifting is
noteworthy, in light of the association of felsic volcanic rocks
with arc-rifting elsewhere, such as 1) rhyolite and rift-related
volcaniclastics at Bartley Lake (units J15 and S2, respectively);
2) elsewhere within the Flin Flon Belt (e.g., Grassy Narrows
rhyolite and basalt, Two Portage Lake rhyolite crystal tuff and
ferrobasalt, Syme at al., 1999); and 3) some sites of active rift
propagation within modern oceanic arcs (Gribble at al., 1998;
Stern et al., 2006).
Tectonic setting and ages of N-MORB and E-MORB
types in the NFFB
The Elbow-Athapapuskow ocean-floor assemblage
(Figure 1) has been interpreted as a back-arc component of
the Paleoproterozoic arc represented by the Flin Flon arc
assemblage (Stern et al., 1995b; NATMAP Shield Margin
Project Working Group, 1998). The ages of several synvolcanic
intrusions within the ocean-floor assemblage (1901–1904 Ma,
Stern at al., 1995b) indicates it is slightly older than the oldest
part of the Flin Flon arc assemblage (1889 Ma, Rayner, 2010)
and thus predates both the juvenile-arc magmatism and the
previously quoted 1886–1881 Ma interval of rifting within the
Paleoproterozoic arc. It is noteworthy that the BAB represented
by the Elbow-Athapapuskow ocean-floor assemblage consists
36
of N-MORB and E-MORB types that show little or no evidence
for admixed subduction-modified mantle, and thus likely
represent a mature stage of BAB development (see ‘Tectonic
setting of NFFB arc and MORB-type volcanic rocks’ above).
If this BAB is analogous to those in the present-day western
Pacific island arcs, associated arc volcanic rocks of similar
or greater age (i.e., ≥1904 Ma) might be expected to occur in
abundance in the Flin Flon Belt. However, rocks of that age that
might represent the onset of juvenile-arc magmatism have not
been documented. It thus seems likely that most of the oceanic
arc within which the ‘Elbow-Athapapuskow BAB’ presumably
began as a rift-basin is not represented in the stratigraphic
record, due to recycling by erosion and/or subduction.
The original site in the Flin Flon Belt from which the
disparate N-MORB and E-MORB components of the NFFB
(Arthurs, Bluenose, Animus and West Mikanagan lakes)
were derived is uncertain. The relative consistency of their
geochemical composition (Figure 15a to 15d) and their
stratigraphic simplicity (basalt and gabbro) suggest that
they may be parts of a mature BAB similar to the ElbowAthapapuskow BAB, and represent allochthonous fault slices
tectonically emplaced within the Amisk Collage. If they
represent dismembered parts of a BAB flanking the arc rocks,
they may well be derived from the MORB-type assemblage at
the (current) northern side of the Flin Flon Belt, at or possibly
close to the present site of the Bluenose Lake N-MORB suite
(see discussion in ‘Animus Lake N-MORB and E-MORB
suites’ above).
Tectonic setting of depleted-MORB types in the NFFB
In contrast to N-MORB and E-MORB types, depleted
MORB in the Dismal Lake assemblage and at Bluenose Lake
is characterized by a wide compositional range and distinctly
different pattern in plots of incompatible elements (Figure 15e,
15f). Although the depleted-MORB sequence is, like other
MORB types, stratigraphically simple (predominantly basalt
and gabbro), the incompatible-element abundances and ratios
indicate the evolution of the magmatic source was more
complex. The incompatible-element pattern suggests that the
source magma was subjected to a substantial amount of partial
melting, based on the petrogenetic principle that incompatibleelement contents are controlled by the amount of melting in
the mantle source of basaltic magmas (Gribble at al., 1998).
In closed systems, the contents of these elements are inversely
proportional to the degree of partial melting; thus the most
strongly depleted-MORB types are likely associated with the
greatest amounts of partial melting.
The least depleted rocks in the Dismal Lake suite display
flat to slightly positive-sloping patterns (Figure 15e), with
equal or slightly increased contents of some incompatible
elements relative to N-MORB, suggesting partial melting was
not significant in the MORB-type source magma. At the other
extreme, the most depleted types are, in this respect, unequalled
by any other basaltic rocks in the Flin Flon Belt. Theoretical
considerations suggest it is unlikely such extreme depletion
could simply be the result of a single episode of partial melting,
and may be better explained as a result of multiple melting
episodes; in other words, a mantle source that was already
Manitoba Geological Survey
depleted was subjected to additional partial melting prior to
extrusion (Sinton et al., 2003). The site of greatest partial melting
in modern oceanic-arc systems is at shallow crustal levels within
subduction zones, where hydrous (slab-derived) fluids are most
abundant and exert the most influence to facilitate melting of
rising mantle diapirs. Some of the resulting partial melts may,
via ‘corner flow’ within the mantle convection system, be
reintroduced to the rift axis in the developing intra-arc basin
after mixing with ambient mantle at this site (Figure 17). The
BABB that is generated is thus a mixture of a partly depleted
melt (‘subduction-modified mantle’) and a MORB-like mantle
source (Martinez and Taylor, 2003; Sinton et al., 2003). The
subduction-modified mantle, being derived in part from the
descending crustal slab, is also somewhat enriched in LILE,
alkalis and Al. The modest Th enrichment and elevated Th/
Nb values of some Dismal Lake depleted MORB (Figure 15e;
Appendix 1) may be explained as a result of the incorporation
of (or metasomatism by) such partly depleted, subductionmodified mantle melts in the magmatic source. The depletedMORB types are clearly distinguished from N-MORB and
E-MORB by higher Th/Nb ratios but have lower Nb contents
than E-MORB (Figure 15a to 15f; Table 3, back pocket) and as
a result, plot in or close to the fields of arc rocks (Figure 16). In
contrast to arc rocks, however, εNd isotope data for Dismal Lake
basalt (+5.2) show the depleted-MORB magmatic source did
not incorporate any older continental crust (Table 4).
Apart from partly depleted, subduction-modified mantle,
the other main component in the mantle source for Dismal
Lake volcanic rocks is assumed to be N-MORB and/or
possibly a primitive, moderately fertile mantle type. The wide
compositional range of Dismal Lake basalts likely reflects
different ratios of these various mantle components, which
may be a result of the progressive movement of the spreading
centre away from the volcanic front during BAB evolution.
In this model, the youngest and least depleted rocks are
compositionally similar to modern N-MORB with a slight arc
signature. Moderately depleted types, some of which occur very
close to the southern margin of the Dismal Lake assemblage, are
similar to the most primitive basalts in the present-day Manus
Basin (‘M1’ type in Sinton et al., 2003). The incompatibleelement plot of one flow located at the Manus spreading centre
(‘Manus Basin basalt’ in Figure 18) compares very closely with
moderately depleted basalt 50–100 m from the southern margin
of the Dismal Lake assemblage. The most depleted basalts in
the Dismal Lake assemblage are assumed to be the earliest
products, associated with the onset of spreading in the BAB;
these have no systematic distribution geographically.
In addition to depleted MORB, sporadic occurrences of arc
and N-MORB volcanic types are present within the Dismal Lake
assemblage. Contact relationships between these contrasting
rock types are not known, but localized disruption suggests
some arc enclaves may be fault-bounded. In other cases the
contrasting rock types may be conformable with the depletedMORB hostrock—a similar relationship is evident in modern
BAB such as Mariana Trough (Stern et al., 1990; Martinez
and Taylor, 2003), where the presence of a wide variety of
MORB- and arc-type rocks reflects the heterogeneous mantle
and ephemeral changes in the magmatic source during BAB
evolution.
The regional distribution of BAB rock types relative to the
arc volcanic rocks in the Flin Flon Belt (Figures 1 and 2)—
that is, on the northern side (Dismal Lake depleted MORB)
and the eastern and southern sides (Elbow-Athapapuskow
ocean-floor assemblage)—may reflect a formerly less disrupted
combination of two distinct basins within one back-arc terrane.
Geochemically distinct and apparently unrelated, these
two BABB assemblages may, however, have been erupted
contemporaneously, but in different parts of the back-arc basin.
100
Manus Basin basalt
Rock / N-MORB
Dismal Lake depleted MORB
10
1
.1
Th Nb La Ce Nd Sm Zr Hf Eu Ti Gd Dy Y Er Yb Lu
Figure 18: Plot of N-MORB normalized incompatible-elements for two depleted-MORB flows at the southern margin of the Dismal
Lake assemblage (spaced 10 km apart along strike), compared with a modern MORB-type flow from the Manus Spreading Centre,
Manus Basin (M1 type, sample 32-5 in Sinton et al., 2003). Normalizing values from Sun and McDonough (1989). Abbreviations:
N-MORB, normal mid-ocean-ridge basalt.
Geoscientific Report GR2011-1
37
Structural geology
Structural studies in the Flin Flon Belt were initiated over
70 years ago (Ambrose, 1936; Stockwell, 1960; Byers at al.,
1965; Stauffer and Mukherjee, 1971) and mostly focused on the
western part of the belt, where good to excellent preservation
and outcrop exposure facilitated investigation of the tectonic
history in both the juvenile-arc and later successor-arc rocks.
Interpretations of the deformation history vary among these and
more recent studies (Bailes and Syme, 1989; Thomas, 1992,
1994; Fedorowich et al., 1995; Gale et al., 1999; Lafrance et
al., 2007), but the latter group of authors all concur that after
1880–1870 Ma tectonic amalgamation of the oceanic-arc
system, an early deformation event (D1 in this study) occurred
prior to deposition of successor-arc sedimentary rocks; all other
events affected both the juvenile-arc and successor-arc rocks,
and thus were post-1840 Ma (Table 5). After D1 deformation
and the subsequent deposition of successor-arc sediments of the
Burntwood and Missi groups (1860–1842 Ma), the sedimentary
rocks were tectonically intercalated with the juvenile-arc rocks
during a renewed collisional event (D3 and D3a of Lucas et
al., 1996; D2 and D2a in this study) that was characterized by
thrusting and tectonic displacement. The D2 event in the NFFB
is identified where major F2 folds deform an older regional
foliation (S1), as in the Lac Aimée Block (Figure 3). These folds
are typically tight to isoclinal and have moderately to steeply
plunging axes. Axial traces of F2 folds are roughly parallel to
the trend of the fault blocks in which they occur, but they are
locally truncated by block-bounding faults.
Open, east-northeast-trending folds characterize D3, such
as the Embury Lake antiform that dominates the regional
structure in the western part of the NFFB. This antiform has
a moderate easterly plunge, estimated as 30º with azimuth
096º by Stauffer and Mukherjee (1971). A northeast-trending
open D3 fold deforms the internal fabric of the Animus Lake
Block, which is characterized by a series of curvilinear D1
fold pairs that outline the D3 structure. Open, east-northeasttrending folds, also interpreted as D3 in age, deform volcanic
and intrusive rocks east of Wabishkok Lake, as well as the
foliation in the Pistol Lake pluton (unit P2; Figures 2 and 3).
These folds have a moderate easterly plunge, estimated as 17º
with azimuth 080º (Gilbert, 2001a, b), and are associated with a
shallow, east-plunging lineation. The D3 deformation event has
been attributed to sinistral movement at the contact between the
Flin Flon Belt and Kisseynew Domain (Wilcox, 1990; Ashton,
1993).
North-northwest to north-northeast trending, high-angle
faults are products of a final episode of deformation (D4).
Within the NFFB, these late faults are most abundant in the
area between Embury Lake and Tartan Lake, where they are
locally truncated by block-bounding faults, indicating renewed
displacement occurred along some of the older faults. Fault
breccia within the block-bounding faults may be products of
this late reactivation. At one locality along the Sourdough Bay
Fault, close to the western end of Naosap Lake (Figure 3),
a 45 m wide zone of heterolithic tectonic breccia contains a
chaotic assemblage of massive to foliated blocks of volcanic,
sedimentary and gneissic rocks up to 1 m long. Fault breccia that
occupies a >17 m wide zone within the block-bounding fault
38
at southern Bluenose Lake is characterized by an ultramafic
(altered pyroxenite) matrix.
Structural geology discussion
Recognition of most of the major fold structures in the
various fault blocks in the NFFB is based on stratigraphic-facing
indicators and the attitudes of primary layering, which identify
the fold type, style and orientation. The age of deformation is
interpreted from these data, but such an age is provisional unless
constrained by evidence such as the deformation of an earlier
tectonic fabric, or the relationship with younger, successorarc rocks. Recognition of the Cope Lake anticline as a first
generation structure (Figure 3) is based on the interpretation
of a D1 age for the complementary syncline immediately to
the south (Bailes and Syme, 1989). Major folds in other fault
blocks could be contemporaneous with either the D1 Cope Lake
folds or the significantly younger, (post–successor arc) D2 Lac
Aimée synclinorium (Gilbert, 2002a). Most folds have been
provisionally designated as D2 in age. The numerous subparallel
folds in the Animus Lake Block are, like the Cope Lake fold
pair, northwest-trending, discordant to the block-bounding
faults, and deformed by a later (D3) fold; they are interpreted
as products of early (D1) deformation, even though they do not
appear to be overprinted by a later foliation—unlike the Cope
Lake syncline (Bailes and Syme, 1989).
It is unlikely that Animus Lake and Lac Aimée blocks
were contiguous during D1 because there is no evidence (i.e.,
northwest-trending structures) for the early deformation event
in the Lac Aimée rocks. Both fault blocks, however, contain
sporadic southwest-plunging minor folds and lineations that
are attributed to D2 deformation, suggesting that they had been
juxtaposed by tectonic transport during D2a, prior to D2 (Table 5).
These two fault blocks are separated by the Lac Aimée Fault
Zone (Figure 3; Gilbert, 1997a)—a northeast-trending branch
of the Northeast Arm Fault that represents a major structural
break within the Flin Flon Belt.
The Northeast Arm Fault is a regional structure that
extends from the southern part of the Flin Flon Belt northward
for over 35 km (Figures 2, 3 and 4). The fault bifurcates at
southwestern Mikanagan Lake and again at the rapids between
Lac Aimée and Mikanagan Lake; a north-trending branch
extends through Animus Lake, from where several fault splays
continue northwestwards through Bluenose Lake and beyond.
The crustal scale of the Northeast Arm Fault is indicated by the
fact that 1) it corresponds to a discontinuity in the pattern of
reflectors in a regional seismic survey (LITHOPROBE section
line 7A, Lucas et al., 1994; Gilbert, 1997a); 2) late Neoarchean,
2.52 Ga tonalitic slivers within the shear zone are interpreted as
derived from a basement underlying the Flin Flon Belt (David
and Syme, 1994); 3) it juxtaposes a variety of tectonically
contrasting fault blocks in both the NFFB (Figures 2 and 4) and
the south-central part of the Flin Flon Belt (Bailes and Syme,
1989); and 4) it divides the NFFB into two structurally distinct
subareas: west of the Northeast Arm Fault, major faults and
folds trend mainly northwest, whereas east of the fault the main
structural trends are northeast or east. The Northeast Arm Fault
has a long history of repeated movement (Syme, 1986; David
and Syme, 1994; Lucas et al., 1996) and was probably initiated
Manitoba Geological Survey
Table 5: Deformation history of the northern Flin Flon Belt, west-central Manitoba, compared with interpretations of the
sequence of events in the Flin Flon Belt by Bailes and Syme (1989), Lucas et al. (1996) and Gale et al. (1999).
Bailes and Syme (1989),
Flin Flon–White Lake area
Lucas et al. (1996), Flin Flon Belt
Gale et al. (1999), Flin Flon arc
assemblage and Missi Group
This study, northern Flin Flon
Belt (NFFB)
D5
Postcollisional structures: conjugate
set of north-northwest to north-northeast trending faults offsetting metamorphic isograds (1770–1690 Ma).
D4
North-northwest-trending, highangle, oblique-slip faults.
D4
North-northwest to north-northeast
trending, high-angle faults offset
other intrablock and block-bounding
faults.
P5
Open, east-northeast-trending
fold (Embury Lake antiform).
Major fault displacements (all
fault types); foliation not well
developed.
D4
Steeply dipping shear zones, northtrending folds and associated foliation.
Terminal collision with Superior
Province (ca. 1800–1770 Ma).
D3
Upright northwest-trending folds;
spaced cleavage (S3).
D3
Open, east-northeast-trending
folds are defined 1) in Animus
Lake Block, by deformation of F1
fold axial traces; 2) in western
part of NFFB, by deformation of
fault blocks in Cliff Lake–Tartan
Lake area (Embury Lake antiform,
Stauffer and Mukherjee, 1971);
and 3) in eastern part of NFFB, by
deformation of Kotyk Lake pluton
and hostrocks. Subsequent major
fault displacement. Reactivation of
block-bounding faults; truncation of
intrablock fold axial-traces. Brittle
deformation (e.g., fault breccia in
Sourdough Bay Fault, Northeast
Arm Fault).
P4
North-trending moderately
open folds in Missi Group.
Foliation in Missi Group but
rare in Flin Flon arc assemblage. Peak metamorphism.
D3
Southwestward thrusting of Reindeer
Zone over the Sask craton; peak
metamorphism (1830–1800 Ma), S3
foliation.
D2
West-verging folds; foliation (S2)
and lineation (L2); peak metamorphism.
D2
Major, tight to isoclinal folds roughly
concordant with trend of fault
blocks deform S1 foliation (e.g.,
synclinorial structure in Lac Aimée
Block with moderate to steep,
southwest plunge). Tight, synclinal
fold with east-northeast plunge
deformed the Bluenose Lake Block.
Minor southeast-plunging folds in
both Lac Aimée and Animus Lake
blocks. Regional foliation (S2).
P3
North- to northeast-trending
open to tight folds in Flin Flon
arc assemblage. Foliation well
developed.
D3a
Magmatism and thrusting of Kisseynew
Domain (Burntwood Group) southward over the Amisk Collage; collision
of Reindeer Zone with Sask craton
(1840–1830 Ma).
D1
North-northeast-verging folds;
rare cleavage (S1); imbrication
and thrusting of volcanic
basement rocks over cover
sedimentary rocks; age bracket
1842–1820 Ma (1840–1835 Ma,
Connors, 1996).
D2a
Reactivation of Northeast Arm
Fault; juxtaposition of Lac Aimée
and Animus Lake fault blocks.
P2
East- to east-southeast trending open to tight folds in Missi
Group. No foliation.
Boundary intrusion ca. 1842 Ma
Missi Group sedimentation ca. 1845–1835 Ma
P1
North- to northwest-trending
tight isoclinal folds (Burley
Lake, Cope Lake synclines)
predate Mikanagan Lake Sill
type (1.81 Ga) intrusions. No
foliation.
D2
Intrusion of plutons (1860–1840 Ma);
north-trending folds with associated
foliation, shear zones.
D1
Tectonic accretion and regional shear
zones resulting in Amisk Collage
(1880–1870 Ma).
Geoscientific Report GR2011-1
North-northeast-trending folds
within Flin Flon arc assemblage
(Bailes and Syme, 1989).
D1
Major folds roughly concordant with
trend of fault blocks. Northwesttrending Cope Lake anticline. Repeated tight folds in Animus Lake
Block with northwest-trending axial
traces. Foliation (S1).
Tectonic accretion
Tectonic accretion and regional
faults/shear zones (1880–1870
Ma) resulting in Amisk Collage.
Postdates earliest identified folding
that accompanied 1886 ±3 Ma riftrelated Josland Lake sills (Zwanzig
et al., 2001).
39
as a major thrust during tectonic accretion (Table 5), when the
late Neoarchean basement slivers may have been incorporated
(Lucas, 1993; Lucas et al., 1993). In the south-central part of
the Flin Flon Belt, the sense of movement of the fault is not
known but the amount of displacement was probably several
kilometres, based on the truncation of the Bear Lake Block
stratigraphy (Bailes and Syme, 1989).
The north-trending branch of the Northeast Arm Fault
at Animus Lake is interpreted to have been associated with
approximately 3 km of sinistral displacement, based on
stratigraphic offset by the fault. Restoration of the pattern
of fault blocks to the pre-faulting configuration would align
juvenile-arc rock types in the Tartan Lake and Wabishkok
Lake blocks, MORB-type volcanic rocks in the Animus Lake
and West Mikanagan Lake fault blocks, as well as the large
(1 km thick) gabbroic intrusions that extend along the northern
margins of these two MORB-type blocks (Batters Lake sill;
Figures 2, 3, 4 and 19). Farther south on the Northeast Arm
Fault, restoration based on several kilometres of sinistral offset
would move arc-rift basalt in Scotty Lake and East Mikanagan
Lake blocks closer together but not into alignment, possibly
due to additional offset along the Lac Aimée Fault Zone during
emplacement of the Whitefish Lake–Mikanagan Lake Block
(Figures 4 and 19). Much of the lateral displacement on the
Northeast Arm Fault may have occurred during D2a, possibly
corresponding to the interval when Lac Aimée and Animus
Lake blocks were presumably juxtaposed along the Lac Aimée
Fault Zone. Continued deformation (D2) is assumed to have
resulted in folding in Lac Aimée Block as well as various other
fault blocks (e.g., Hook Lake, Arthurs Lake, Tartan Lake).
Economic geology
The potential for particular rock suites in the NFFB to host
base-metal and/or precious-metal ore deposits can be assessed
by investigation of the distribution and setting of known
mineral occurrences in rocks 1) within the NFFB and 2) within
correlative stratigraphic units in the southern and central parts
of the Flin Flon Belt (Gale and Eccles, 1988a, b, c; Syme and
Bailes, 1993; Gale and Norquay, 1996; Syme, 1998; Bailes
and Galley, 1999; Syme et al., 1999; Gale 2001; Gale and
Dabek, 2002). Volcanogenic massive sulphide (VMS)-related
mineralization in the southern and central parts of the Flin Flon
Bluenose
Lake
Batters Lake sill
E
ON
TZ
UL
EE
FA
IM
CA
LA
Lac Aimée
Mikanagan
Lake
Turbidite sediment
MORB-like volcanic rocks
Arc volcanic rocks
NO
R
AR THEA
MF
AU ST
LT
Arc-rift volcanic rocks
Gabbro
Arc volcanic, volcaniclastic
and epiclastic rocks
(Whitefish Lake–Mikanagan
Lake Block)
Block-bounding fault
0
1
2
3
4
5
Geological contact
kilometres
Figure 19: Northeast Arm Fault, northern Flin Flon Belt, west-central Manitoba: restoration of fault blocks on opposite sides of the
fault prior to the inferred last major movement (possibly during D2a), based on an estimated 3 km of sinistral displacement.
40
Manitoba Geological Survey
Belt is typically associated with one or more of the following
features: 1) rifting of arc sequences; 2) felsic magmatism in
arc sequences; and 3) faulted contacts between contrasting
stratigraphic components or distinctive fault blocks. These
three parameters serve as the most obvious guides for assessing
the mineralization potential of localities within the NFFB that
have not so far been targeted for exploration.
Within the NFFB, rhyolite-associated VMS-type mineralization occurs
Two panels extending across the Flin Flon Belt may have
potential for base- or precious-metal mineralization, based on
the interpretation of an arc-rift tectonic setting for some of their
components (see ‘Arc-rift propagation, development of intra-arc
and back-arc basins and the potential for VMS mineralization’
above). The two rift-related panels correspond, respectively,
to 1) Manistikwan Lake Block–Arthurs Lake Block–Grassy
Narrows Zone, and 2) Lac Aimée–East Mikanagan Lake–
Scotty Lake blocks. In spite of ca. 1.88–1.78 Ga tectonic
amalgamation that resulted in the juxtaposition of contrasting
components of the original oceanic-arc system, it is suggested
that these panels are structural corridors that correspond, in
part, to zones of rifting in the original arc.
2) in felsic volcanic rocks within the Baker Patton Complex
(Sourdough Bay Block); and
Mineral exploration has been carried out, beginning in
the 1950s, at numerous localities in the northern part of the
Manistikwan Lake Block, within the first rift-related panel
(locations 44, 45 and 126 to 130 in Gale and Eccles, 1988b).
Similar investigations have been carried out at mineralized
localities farther to the south, at the margins of the Grassy
Narrows Zone (locations 108 and 123 in Gale and Eccles,
1988a). The second rift-related panel also has a long history
of exploration. Precious- and base-metal mineralization has
been prospected in the northern part of Lac Aimée Block and
at stratigraphic/structural contacts immediately to the north
by Esso Minerals, between 1988 and 1992 (Assessment Files
71615, 71818). At the southeastern margin of Lac Aimée
Block, base-metal mineralization was discovered 90 years
ago at the faulted contact between the arc volcanic rocks and
arc-rift volcanic rocks of the East Mikanagan Lake Block
(‘KD zone’, identified in 1921, Gilbert, 1996a); base-metal
mineralization also occurs in north-trending faults within arcrift rocks at northeast Mikanagan Lake (see ‘East Mikanagan
Lake arc-rift suite’ above). In summary, both of the rift-related
panels contain numerous prospects that have been explored
periodically for over five decades and appear to have potential
for VMS-associated mineralization.
Rhyolite-hosted base-metal mineralization at Trout Lake
mine (Figure 4), a Cu-Zn ore deposit of over 20 million
tonnes (Ordóñez-Calderón et al., 2009), occurs at the faulted
contact between a volcano-sedimentary sequence (to the west)
and a greywacke-argillite sequence (to the east), probably
corresponding to the contact between the Cope Lake and Embury
Lake blocks (Gilbert, 1988a, 1990b). This Cu-Zn deposit,
which is the subject of ongoing study (Ordóñez-Calderón et al.,
2009), is hosted by 1878 ±1.1 Ma rocks (Rayner, 2010) and
has a somewhat different stratigraphic setting compared to the
relatively older Flin Flon–777–Callinan VMS deposits farther
southwest, within the Flin Flon Block; the Flin Flon deposits
are hosted by the 1887 Ma mine rhyolite that is part of the ca.
1889 Ma Flin Flon formation (Rayner, 2010).
Geoscientific Report GR2011-1
1) in arc volcanic rocks close to Kotyk Lake and at northeastern Blueberry Lake, less than 100 m from the contact
between arc volcanic and depleted-MORB (back-arc)
rocks of the Dismal Lake assemblage (Gilbert, 2003b;
Assessment Files 93337, 90400, 90401);
3) in arc rocks that occur as enclaves within, or at the margins
of the Dismal Lake assemblage.
These rocks have been targets of mineral exploration for over
50 years, most recently between 1997 and 2002 (Assessment
Files 73994, 73546, 73348; see ‘arc volcanic enclaves within
the Dismal Lake depleted-MORB assemblage’ above). The
Cu-Zn and precious-metal mineralization in these enclaves is
typically hosted by felsic volcanic rocks and is associated with
alteration, metasomatism and pervasive deformation.
In many of the occurrences of VMS-type mineralization
cited above, structural controls have played an important
role in localizing the deposits. Such controls are particularly
important in the Tartan Lake gold mine (Figure 4), an
approximately 600 000 tonne orebody (Peloquin and Gale,
1985) that is currently under redevelopment by St. Eugene
Mining Corporation Ltd. (St. Eugene Mining Corporation
Ltd., 2010). The gold mineralization, which is associated with
quartz veining and carbonate alteration, is located in shears
at the margin of the gabbroic Tartan Lake complex (Figure 2;
Peloquin and Gale, 1985).
Detailed mapping has revealed a hitherto unknown
complexity of structure in the NFFB, and geochemical
investigations have attempted to relate the diverse
tectonostratigraphic components as belonging to various
tectonic types that have been defined in modern oceanic
arcs. As a result, several areas favourable for base-metal and/
or gold exploration have been identified, in addition to areas
likely not to be prospective for such mineralization. This new
information, together with the fact that the Flin Flon district
has an established infrastructure for mineral exploration and
development, may provide a significant incentive for renewed
exploration in the area.
Conclusions
Map GR2011-1-1 represents a substantial upgrade to the
previous geological maps of the NFFB published in the 1940s.
This new map serves as a link between detailed maps recently
published in the area to the north (southern Kisseynew Domain,
Zwanzig, 2010) and the Flin-Flon–White Lake area map
immediately to the south (Bailes and Syme, 1989). Geochemical
investigations that have accompanied mapping of the NFFB
have been key for distinguishing the various allochthons that
make up this part of the belt, as well as for identifying localities
that may represent prospective targets for VMS-associated
mineralization. Trace element, REE and selective Sm-Nd
isotopic data from approximately 300 analyzed volcanic rock
samples in the NFFB has resulted in the recognition of
•
various crustal types (juvenile arc, arc-rift and back-arc
41
ocean-floor) juxtaposed by block-bounding faults;
•
continuity, discontinuity or lateral gradation of map units
between the south-central and northern parts of the Flin
Flon Belt;
•
two structural corridors with evidence for arc-rifting,
bounded in part by the Inlet Arm and Northeast Arm faults,
which may contain promising targets for VMS-associated
mineralization; and
•
felsic volcanic rock map units that may be prospective
for VMS-associated mineralization, based on their geochemical similarity to ore-bearing rhyolites both in the Flin
Flon area and in older (Archean) successions elsewhere.
The most promising scenarios for base-metal or preciousmetal mineralization in the NFFB appear to be stratigraphic and/
or structural discontinuities, rift-related successions and felsic
volcanic hostrocks. The fact that the NFFB has, in general,
received less intensive mineral exploration activity than areas
closer to the various ore deposits in the vicinity of Flin Flon
may be argued in favour of renewed exploration initiatives in
the NFFB. The results of detailed mapping and the associated
geological investigations reported here have indicated several
localities that may profitably be targeted in future exploration
programs. This work has also provided a basis for new programs
of more detailed or focused mapping and associated geological
investigation.
Acknowledgments
Numerous student assistants were employed during the
course of field mapping and their contribution to the project
is gratefully acknowledged here, as well as in the annual
MGS ‘Report of Activities’ that have summarized the results
of mapping at the end of each field season. Logistical support
for field operations was provided by N. Brandson, and staff at
MGS Rock Preparation Lab under G. Benger (and formerly
D. Berk) processed rock samples. Preparation of the map by
digital cartography was by M.E. McFarlane, GIS processing
was by L.E. Chackowsky and B.K. Lenton drafted the figures.
These marginal notes benefited from fruitful discussion with
H.V. Zwanzig and A.H. Bailes, as well as C.O. Böhm and
R-L. Simard, both of whom also edited the first drafts of this
manuscript and the accompanying map. The contribution of
these colleagues at MGS, as well as the meticulous editing of
T. Sifrer is gratefully acknowledged.
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