Geology of the Scimitar Lake Area (part of 63M-15), East-central Scimitar Complex

K.E. Ashton, C. Therens 1, and A. Legault 2

Ashton, K.E., Therens. C., and Legault, A. {1996): Geology of the Scimitar Lake area (part of 63M·15). east-central Scimitar complex; in Summary of Investigations 1996, Saskatchewan Geological Survey, Sask. Energy Mines. Misc. Rep. 96-4.

A 1 :20 000 scale remapping project was begun in 1996 to detemiine the lithological makeup and mineral poten­tial of the Scimitar complex and to establish its relation­ship to the Flin Flon, Glennie, and Kisseynew domains. The Scimitar complex was originally temied the Scimitar Lake block and considered a sub-domain of the Glennie Domain (Macdonald, 1981 ), although it has also been depicted as part of the Kisseynew Domain (Saskatchewan Geological Survey, 1994, p36). It was distinguished from the granodiorite-dominated part of the northern Glennie Domain (Laird Lake complex as defined by Macdonald, 1981 , p17) to the west on the basis of an apparently abrupt eastward change to a hornblende-rich supracrustal package. However, this change may relate more to differences in the metamor­phic rock nomenclature than to any real differences in the rocks.

In the earlier work, rocks of the Scimitar complex were mainly divided into biotite and hornblende gneisses with little attempt made to determine protoliths (Pearson, 1973; Kirkland, 1976; Fuh, 1979). In general, they were thought to be paragneisses with a minor volcanic com­ponent. No felsic to intermediate intrusive rocks were distinguished, resulting in the present Laird-Scimitar complex boundary.

The Scimitar complex was distinguished from the domi­nantly pelitic paragneisses of the Kisseynew Domain to the north, east, and south by the presence of abundant hornblende-rich gneisses (Pearson, 1973), although little was known of the boundary relationships.

Economic interest in the Scimitar complex arises be­cause of its possible correlation with rocks of the Attitti Lake region to the south (fomierly the Attitti Block). The two areas comprise similar volcano-plutonic packages, and recent mapping has shown the latter to be the highly metamorphosed northwestern extension of the Flin Flon Domain (Ashton et al., 1987, 1993; Ashton and Leclair, 1991). In a recent regional structural synthe­sis, Lewry et al. (1990) implied that the two may be continuous under a late synform cored by rocks of the Kisseynew Domain. The idea of a single continuous volcano-plutonic protocontinent stretching from the southern Flin Flon Domain to the western Glennie Domain, including the Scimitar complex and the Proterozoic part of the Hanson Lake Block, has recently

been incorporated into a regional tectonic model (Ansdell et al. , 1995).

The Scimitar Lake area was selected for remapping because of its central position within the Scimitar complex (Figure 1) and because it contains known sulphide occurrences. The 90 km2 study area is about 40 km northwest of Sandy Bay and 80 km north of Pelican Narrows, both of which are accessible by road. Topographic relief ranges over 70 m with granitoid rocks dominating the high ground and supracrustal rocks poorly exposed along lakeshores and rivers. The region was last mapped at 1 :63,360 scale (Pearson, 1973). although more recent field observations and preliminary metamo'rphic data are reported from the immediate Scimitar Lake area (Ansdell, 1995, in press).

1 . General Geology

The Scimitar Lake area is dominated by middle to upper amphibolite facies homblende-biotite-plagioclase­quartz gneisses of intemiediate composition. They are divided into gneissic granodiorites and subordinate metavolcanic and metasedimentary rocks, which occur both as infolded packages and screens (Figure 2). The gneissic granodiorites range in colour index from about 10 to 30 and include thin units of biotite-tree quartz monzodiorite(?). Mafic volcanic rocks are rare but inferred telsic to intermediate volcanic and volcaniclastic gneisses under1ie a large area in the south. They host a sulphide occurrence and a possibly related zone of cordierite-anthophyllite-garnet-biotite alteration . Biotite­rich hornblende-bearing rocks were interpreted as paragneisses, as were biotite-garnet±graphite rocks.

The ear1iest recognizable minor folds (F2) deform a pre­existing tectonic fabric (S1) which is defined by both the gneissosity and main mineral foliation. The F2 folds are tight to isoclinal and have resulted in a structural grain which mainly dips moderately to the east. These fabrics are refolded by tight, gently to moderately north­plunging F3 folds with moderately to steeply east­dipping axial planes. The regional open fold (F4) defining the northeastern boundary of the Scimitar complex (Figure 1) is near upright and plunges gently to the northeast. Open F5 minor folds have gently to moderately north- to northeast-dipping axial planes and plunges which vary from northwest to north to east. A

(1) Department of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon. SK S7N 5E2. (2) Depar1ment of Geology, University of Regina, Regina. SK S4S OA2.

22 Summary of Investigations 1996

poorly developed regional lineation plunges gently north to northeast.

2. Description of Rock Types

a) Mafic Volcanic Rocks (Sm)

Mafic volcanic rocks are fine- to medium-grained, black, and contain about equal proportions of hornblende and plagioclase with rare accessory garnet. Most are inter­layered with other supracrustal rocks in units generally only a few tens of metres thick. Some medium-grained occurrences within the orthogneisses may be either screens or younger gabbroic dykes. Cale-silicate rocks containing up to 20 percent clinopyroxene occur at two distinct localities. They are thought to result from pre- to syn-metamorphic carbonatization of the mafic volcanic rocks.

' ·, . I

55·00< · · '

0 10 20 40 -- ----· Kiiometres

b) Intermediate Volcanlclastic Rocks (Si)

Grey, fine- to medium-grained intermediate rocks are best exposed along the western shore of the east arm of Scimitar Lake, although they also occur as thin units within other supracrustal packages. Typical rocks are well layered on a centimetre scale and contain 15 to 30 percent hornblende, 5 to 15 percent biotite, and locally garnet. Their thinly layered nature and relatively biotite­rich composition suggests volcaniclastic protoliths.

c) Felsic to Intermediate Volcanic and Volcanlclastic Rocks (Sf)

Fine- to medium-grained, white to grey, quartzo­feldspathic gneisses are interlayered with the intermedi­ate volcaniclastic rocks. They include a variety of thinly layered rocks containing 5 to 15 percent biotite, Oto 10 percent hornblende, O to 5 percent garnet, and patchy concentrations of magnetite and sulphides (Figure 3). Pearson (1973) assumed these rocks were psammites based on their composition, but their spatial relationship to other volcanic units, cordierite-anthophyllite altera­

·56°00' . tion, and sulphide occurrences has led to their re-interpretation as felsic volcanic and volcaniclastic rocks. This is supported by their broad similarity to felsic volcanic and volcaniclastic rocks which have been traced from the Flin Flon area into the highly metamor­phosed Attitti Lake region of the northern Flin Flon Domain. In addition, they lack the peralumi­nous phases (e.g. sillimanite) and near-ubiquitous magnetite, which typify the psammitic Missi and Ourom groups.

d) Calcic Wackes (Sc)

Fine- to medium-grained. grey, calcic gneisses containing 10 to 25 percent hornblende, Oto 20 percent biotite, Oto 10 percent garnet, and rare graphite are most common in the east and north. In the absence of graphite, they are difficult to distinguish from the intermediate volcaniclastic and intrusive rocks.

e) Aluminous Wackes (Sw)

The calcic wackes are gradational into more aluminous counterparts in sedimentary packages up to 1 km wide in the north and east. Typical aluminous wackes consist of a fine- to medium-grained, grey, paleosome containing 20 to 30 percent biotite, trace to 15 percent pink subhedral garnet and trace graphite, mixed with 20 to 30

Rgure 1 - Location of the Scimitar Lake area.

Saskatchewan Geological Survey 23

Rgure 2 - Simplified geological map of the Scimitar Lake area. 1, mafic volcanic rocks; 2, intermediate volcaniclastic rocks; 3, felsic to intermediate volcanic suite; 4, sedimentary rocks; 5, gabbro; and 6, gneissic granodiorite.

24 Summary of Investigations 1996

Figure 3 - Gamefiferous quartzofeldspathic gneiss interpreted as weakly altered rock of the felsic volcanic suite, western shore of the east arm of Scimitar Lake.

percent medium-grained, dominantly white neosome.

f) Heterogeneous Sedimentary Rocks (Sh)

Where the calcic and aluminous wackes are too thinly interlayered to distinguish at this scale of mapping, they have been assigned to a heterogeneous sedimentary unit. This has also been used for rare sedimentary compositions which fall outside the fairly tight constraints on the calcic and aluminous wackes.

All of the sedimentary rocks are tentatively considered syn-volcanic due to their spatial relationship with the volcanic units. They collectively resemble the Welsh Lake Assemblage (Reilly, 1993) of the Flin Flon Domain and are distinguished from the younger, more alumi­nous Bumtwood Group (Baldwin et al., 1979) of the Kisseynew Domain to the south (Ashton et al., this volume), east, and north based on the relatively high proportion of calcic wackes.

g) Gabbro (Gb)

Gabbroic rocks are rare but a thin weakly foliated unit was observed in the southern Scimitar Lake area. It is homogeneous, white and green-black, medium grained, and contains 40 to 50 percent hornblende and trace sulphides. Contacts were not observed.

h) Gneisslc Granodiorite (Gd)

Granodioritic rocks underlie most of the Scimitar Lake area in units ranging from several kilometres to only a few metres thick. Most are heterogeneous ortho­gneisses (Gd), comprising a medium-grained. grey paleosome and about 20 to 30 percent medium-grained to pegmatilic, white to pink, neosome. The neosome includes hornblende-bearing melt and hornblendite pods which are taken as evidence for an anatectic origin (Ashton et al., 1993). Fine- to medium-grained heterolithic layers, inclusions, and schlieren are common. Most rocks contain 5 to 20 percent horn­blende and 10 to 15 percent biotite.

Saskatchewan Geological Survey

Two compositiooal variations occur within the large bodies of gneissic granodiorite. The first consists of medium-grained, pink-grey quartz monzodioritic(?) rocks (Om) exhibiting about 20 percent centimetre­scale layers of medium- to coarse-grained, pink neosome. Typical rocks contain 20 to 25 percent horn­blende as the sole mafic constituent. Their irregular distribution and gradational contacts make it unclear whether they represent minor compositional variations within dominantly granodioritic plutons or distinct quartz monzodioritic intrusions as observed in the Flin Flon Domain (Ashton, 1990; Ashton and Leclair, 1991 ; Ashton et al., 1993).

The second variation is a more leucocratic granodiorite (GI) containing Oto 10 percent hornblende, 5 to 15 percent biotite, and similar mafic inclusioos and schlieren. The leucosome is also hornblende-bearing but pinker, perhaps reflecting a higher K-feldspar content. A magnetite-bearing, hornblende-free variant, which occurs both within the large granodioritic bodies and as thin sheets in the southern Scimitar Lake area, may represent more granitic compositions which have produced magnetite by the metamorphic breakdown of biotite.

A variant of the leucogranodioritic rocks exposed along the southern shore of northern Scimitar Lake contains about 20 percent combined biotite and hornblende, and O to 3 percent garnet. Many of these rocks are well layered on a centimetre scale, with alternating fine- to medium-grained, intermediate and leucogranodioritic layers. They may have formed by the injection of abundant syn-metamorphic melt into supracrustal or more mafic granodioritic rocks rather than by the partial melting of a leucogranodioritic intrusion.

i) Early Medium-grained to Pegmatltic Granite (Gp)

Small amounts of medium-grained to pegmatitic, pink to white, biotite granite are present in many outcrops as deformed sheets and/or transposed dykes.

j) Late Granitic Pegmatltes

Crosscutting granitic pegmatite dykes are relatively rare and generally less than 1 metre thick. Two orientations are common: east-west and southeast-northwest, although dips range from moderate to steep in either direction . The dykes are pink and contain a few percent biotite. They are tentatively correlated with the ca. 1no Ma (Bickford et al., 1987; Ashton et al., 1992) Jan Lake Suite (Macdonald and MacQuarrie, 1978; Ashton and Shi, 1994).

3. Metamorphism

Rocks of the Scimitar Lake area have been metamor­phosed to upper amphibolite facies. Granodioritic rocks (Gd, GI) and the calcic wackes (Sc) contain about 30 percent partial melt, some of which is the pink to white hornblende-K-feldspar variety noted previously in the region (Ashton et al. , 1993; Ashton and Shi , 1994).

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Mafic rocks (Sm) generally contain the assemblage homblende-plagioclase±garnet. Clinopyroxene is rare and may have formed under conditions of high C02 fugacity rather than high temperature.

The aluminous wackes contain the assemblage garnet­biotite-sillimanite-graphite and appear to exhibit co­existing sillimanite-K-feldspar. They lack the cordierite seen in some Kisseynew Domain paragneisses to the south (Sibbald, 1978), east (Kirkland, 1976), and north (Gilboy, 1980), and generally contain a smaller compo­nent of partial melt, suggesting that the central Scimitar complex was metamorphosed at somewhat lower grade. The presence of K-feldspar-sillimanite, together with the absence of cordierite, orthopyroxene, and kyanite suggests metamorphic conditions in the 660° to 740°C and 4 to 8 kb range (Carmichael et al., 1987; Bucher and Frey, 1994).

4. Structure

Primary features were not observed in the Scimitar Lake area. The main regional foliation (S1) is defined by the peak metamorphic minerals and by gneissic layer­ing, and generally dips moderately to the east.

Minor folds are not common, although lithological contacts and S1 trends define a complex fold interfer­ence pattern. The oldest minor folds are rare, outcrop­scale F2 isoclines, which are variably oriented.

Tight F3 minor folds generally have moderately to steeply east-dipping axial planes and plunge gently to moderately to the north. Minor gently north-northeast­plunging lineations (La) are thought to have developed parallel to, and coeval with, F3 folding. The main north­south map-scale folds are probably F3 structures that have been modified during later folding.

Regional-scale, upright, gently northeast-plunging folds (F4) are common in the southern Reindeer Zone (Lewry et al., 1990). The trace of an F4 antiform transects the study area, causing the regional foliation to change from southeast- to north-dipping in the northeastern Scimitar complex. Their late timing and orientation sug­gests that they result from northwest-southeast shorten­ing related to terminal collision of the Superior craton .

A set of open east-trending folds (Fs), apparent from trend lines defining the main regional foliation, may be related to late outcrop-scale kink folds noted by Pearson (1973). They have axial planes dipping gently to moderately north to northeast and plunge gently northwest, north, or east. They are parallel, and perhaps related, to a set of east-trending lineaments which transect the area. No discernible offsets were recognized.

5. Economic Potential

Several copper showings are known in the Scimitar Lake region (Pearson, 1973), two of which are in the mapped area. The most significant of these is on an island in the east arm of Scimitar Lake where

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limonite-stained, fine-grained, felsic volcanic rocks con­tain trace chalcopyrite and about 3 percent combined pyrrhotite and pyrite. Some of the sulphides have been remobilized during metamorphism and deformation to medium-grained to pegmatitic granitoid rocks. The occurrence is situated immediately south of a previously unmapped, wide zone of cordierite-anthophyllite-gamet­biotite alteration (Figure 4). Such rocks can be pro­duced by amphibolite facies metamorphism of chloritic volcanogenic massive sulphide-type alteration zones and therefore are a worthwhile target for exploration.

A copper occurrence on the eastern shore of the east arm of Scimitar Lake contains up to 2 percent dissemi­nated chalcopyrite in cordierite-anthophyllite-gamet­biotite rocks, which were originally interpreted as "metamorphosed serpentine or a magnesian meta­sediment" (Pearson, 1973, p6). The patchy nature of these rocks , together with their setting in a thin unit of mafic volcanic rocks, strongly argues for a syn-volcanic Mg metasomatic origin. It is worthwhile noting that the regional deformation has not separated either sulphide occurrence from its associated alteration zone.

The Knife (Mokoman) Lake copper prospect is situated about 2 km west of the study area. Current reserves are 6 million tonnes averaging 1 percent Cu, with signifi­cant Au, Ag, Ni, and Zn (Saskatchewan Mineral Deposits Index, Report 0406), although new Cu-Au-Ag mineralization has been discovered during an ongoing drill program (press release, July 25, 1996).

Mineralization consists of pyrite, pyrrhotite and chalco­pyrite, which occur in a multiply folded, "crosscutting ... pegmatite-like felsic gneiss" (Pearson, 1973, p12), suggesting that at least some of the sulphides have been remobilized out of the original host rocks during metamorphism, as at the Scimitar Lake occurrence. However, it is noteworthy that the same package of rocks appears to host both mineral occurrences, and that they may represent a single folded horizon.

0

CENTIMETRES

Figure 4 - Cordierite-garnet-anlhophyllite rock from the large alteration zone on the east arm of Scimitar Lake.

Summary of Investigations 1996

6. Discussion Division of Pearson's ( 1973) extensive hornblende gneiss unit into gneissic granodiorite and subordinate supracrustal rocks suggests that the Scimitar complex has a very similar makeup to that of the northeastern Glennie Domain . If these rock types and proportions can be traced westward into the Glennie Domain, then there is no justification for a distinct Scimitar complex sub-domain. Its existence may be nothing more than a result of different mappers using different metamorphic rock nomenclature.

Furthermore , recent work to the south (Tran et al., this volume) suggests that the Ukoop Lake segment con­sists of an early volcano-plutonic package which has been unconformably overlain by Ourom Group aren­aceous sedimentary and interlayered volcanic rocks. This would imply that rocks of the Ukoop Lake seg­ment, as well as those of the Scimitar complex, are actually part of the Glennie Domain. Use of both terms as lithotectonic sub-domains is, therefore, misleading and should be discontinued.

Based on mapping in the Scimitar and Wintego lakes areas (Ashton et al., this volume), it appears that rocks of the Scimitar complex are very similar to those of the Attitti Lake region of the northern Flin Flon Domain. It is, therefore , tempting to speculate that the Scimitar complex and the high-grade northern extension of the Flin Flon Domain (Attitti Block) are continuous beneath the intervening F4 synform cored by migmatitic wackes of the Kisseynew Domain.

7. Acknowledgments

The able and enthusiastic field assistance of J . Monea and C. Card is gratefully acknowledged.

8. References Ansdell , K.M. (1995): Observations in the Scimitar and

Wlntego lakes area, Trans-Hudson Orogen, Saskatchewan; in Summary of Investigations 1995, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 95-4, p162·167 .

Ansdell , K.M. (in press): The Scimitar Lake gneisses: Prelimi­nary data on metamorphic conditions; in Hajnal, Z. and Lewry, J. (eds.), LITHOPROBE Trans-Hudson Orogen Transect, Rep.

Ansdell, K.M. , Lucas, S.B., Connors, K., and Stem, A.A. (1995): Kisseynew metasedimentary gneiss belt, Trans­Hudson Orogen (Canada): Back-arc origin and collisional inversion ; Geol. , v23, p1039-1043.

Ashton, K.E. (1990): Geology of the Snake Rapids area, Flin Flon Domain (parts of NTS 63l-9 and -1 O); in Summary of Investigations 1990, Saskatchewan Geological Survey, Sask. Energy Mines. Misc. Rep. 90-4, p4-12.

Ashton. K.E., Drake, A.J .. and Lewry, J.F. (1993): The Wildnest-Tabbernor Transect: Attitti-Mirond lakes area (parts of NTS 63M-1 and -2}; in Summary of Investigations 1993, Saskatchewan Geological Survey, Sask. Energy Mines. Misc. Rep. 93-4 , p50-66.

Saskatchewan Geological Survey

Ashton, K.E., Hunt, P.A., and Froese. E. (1992): Age constraints on the evolution of the Flin Flon volcanic belt and Kisseynew gneiss belt, Saskatchewan and Manitoba; in Radiogenic Age and Isotopic Studies: Report 5. Geel. Surv. Can .• Pap. 91-2 , p55·69.

Ashton, K.E. and Leclair, A.O. (1991 ): Revision bedrock geological mapping , Wildnest-Attitti lakes area (parts of NTS 63M-t and ·2) in Summary of Investigations 1991, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 91-4, p29-40.

Ashton, K.E. and Shi, R. (1994): Wildnest-Tabbernor Transect: Mirond-Pelican lakes area (parts of NTS 63M·2 and -3); in Summary of Investigations 1994, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep . 94-4, p27·37.

Ashton, K.E.. Wilcox, K.H., Wheatley, K.J., Paul. D., and de Tombe, J. (1987): The boundary zone between the Flin Flon Domain, Kisseynew Gneisses, and Hanson Lake Block northern Saskatchewan; in Summary of Investigations 1987, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 87-4, p131-134.

Baldwin, D.A., Frohlinger, T.G., Kendrick, G., McRitchie, W.D., and Zwanzig. H.V. (1979): Geology of the Nelson House­Pukatawagan Region (Burntwood Project) ; Manit. Miner. Resour. Div., Geol. Rep. 78-3, 40p.

Bickford, M.E., Van Schmus, W.R., Colterson, K.D., and Macdonald, R. (1987): U-Pb zircon geochronology project: New results and interpretations; in Summary of Investiga­tions 1987, Saskatchewan Geological Survey; Sask. Energy Mines. Misc. Rep. 87-4, p76·79.

Bucher, K. and Frey, M. (1994): Petrogenesis of Metamorphic Rocks; 6th edition, Springer-Verlag, Berlin, 318p.

Carmichael, D.M .• Helmstaedt, H.H., and Thomas, N. (1987): Field trip in the Frontenac Arch with emphasis on stratigra­phy, structure, and metamorphism; Meeting of Friends of the Grenville, 1987.

Fuh. T.M. (1979): Geology of the Gilbert Lake west area; Sask. Dep. Miner. Resour. , Rep. 187, 17p.

Gilboy, C.F. (1980): Geology of the Reindeer Lake south (southeast) area; Sask. Dep. Miner. Resour., Rep. 198, 62p.

Kirkland, S.J.T. (1976): The geology of the Marchel-Wintego­Sandy Bay area; Sask. Dep. Miner. Resour., Rep. 172, 12p.

Lewry, J .F .• Thomas. D.J., Macdonald, R. , and Chiarenzelli, J. (1990): Structural relations in accreted terranes of the Trans-Hudson Orogen, Saskatchewan: Telescoping in a collisional regime?; in Lewry. J.F. and Stauffer, M.A. (eds.). The Early Proterozoic Trans-Hudson Orogen of North America, Geol. Assoc. Can .. Spec. Pap. 37, p75-94.

Macdonald, R. (198 1): Compilation bedrock geology: Pelican Narrows and Amisk Lake areas (NTS 63M, 63L, part of 63N and 63K): Preliminary geological map and report: in Summary of Investigations 1981 , Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 81 ·4, p16-23.

Macdonald. R. and MacQuarrie, R.R. (1978): Geological re­investigation mapping, Jan Lake area (part of NTS area 63M); in Summary of Investigations 1978, Saskatchewan Geological Survey, Sask. Miner. Resour., Misc. Rep. 78-10, p16-24.

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Pearson, D.E. (1973): The geology of the Scimitar Lake area (east half), Saskatchewan; Sask. Oep. Miner. Resour .. Rep. 156, 17p.

Reilly, B.A. (1993): Revision bedrock geological mapping of the northwest Amisk Lake area (parts of NTS 63L-9 and -16) in Summary of Investigations 1993, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 93-4, p12-20.

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Saskatchewan Geological Survey (1994): Geology and Mineral Resources of Saskatchewan; Sask. Energy Mines, Misc. Rep. 94-6, 99p.

Sibbald, T.1.1 . (1978): Geology of the Sandy Narrows (east) area, Saskatchewan (NTS area 63M-3E); Sask. Dep. Miner. Resour .. Rep. 170, 49p.

Summary of Investigations 1996