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Project 2a: Maynard Lake and Sandy Narrows (West) Area

Geological Mapping of 63M-4 and 63M-3(W)

by B. P. Scott

The Maynard Lake Area and Sandy Narrows Area (East Half) lie about half-way

between La Ronge and Pelican Narrows. Mapping in this area was commenced in 1975

(Scott, 1975) and completed by mid-season in 1976. This reports the area as a

whole. Mapping consis t ed of shoreline surveys and land traverse lines at 800 m

to 3200 m spacing.

Precambrian rock is generally well exposed, however, broad areas near the

northern boundary are covered with glacial till and muskeg. Lakes are generally

small and connec ted by shallow or intermittent streams.

General Geology

Two belts of supracrustal rocks, the Brownell Lake belt and Gee Lake belt,

consist of metamorphosed basic volcanic rocks and metamorphosed greywacke, pelite,

arkose and conglomerate . These belts are divided into several branches by felsic

gneiss and intrusive granite and granodiorite. The stratigraphic s e quence is

partly understood.

Intrusive rocks are from oldest to youngest: diorite and gabbro, granodiorite,

granite and pegmatite. Migmatite zones occur at the margins of most intrusive

bodies.

The common occurrence of garnet and staurolite in pelitic rocks indicates

that metamorphic conditions reached amphibolite grade. Retrograde metamorphism

affected most rocks, causing the metamorphosis of basic volcanics and diorite­

gabbro to greenstone.

1ithology

Supracrustal rocks in the area consist of hornblende gneiss and basic meta­

volcanic rock (1), semi-pelite or meta-tuffaceous (?) rock (2), meta-rhyolite (3),

biotite-(garnet)-(muscovit e) -(staurolite) gneiss (4), meta-conglomerate (5) and

meta-arkose (6). Felsic gneiss (7) may consist partly of metamorphosed acidic

supracrustals.

Hornblende gneiss (1) contains minor relict pillowed and fragmental basic

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to intermediate volcanic rocks. Garnetiferous bands occur. Bands rich in garnet,

calcite and diopside constitute a calc-silicate rock.

Semi-pelite (2) is a light grey, fine grained granular rock. Combined biotite,

hornblende and garnet are less than 15 per cent of the rock. Unit 2, which may be

a metamorphosed tuff or agglomerate, is closely associated with hornblende gneiss

and meta-rhyolite.

Meta-rhyolite (3) is a dense, light pink, aphanitic rock with conchoidal

fracture. A weak fracture cleavage is common , parallel to regional foliation.

Locally meta-rhyolite (3) occurs interlayered with semi-pelite (2). Alter­

natively, unit 3 may have originated as a metamorphosed granite or :1rkose .

Biotite gneisses (4) are light grey to brown and locally contain hornblende.

Muscovite is common in a well-banded paragneiss on Vincent Lake, and occurs

sporadically along the northwest arm of Deschambault Lake. Garnet is a common

accessory mineral. Staurolite occurs in biotite-(garnet) gneiss in four

localities, east of Palf Lake , the south shore of Brownell Lake, immediately

west of Brownell Lake, and 5 km north of Horst Lake.

Meta-conglomerate (5) outcrops in a narrow band about 5 km north of Maynard

Lake. Cobbles of hornblende gneiss, biotitic meta-diorite and porphyroblastic

feldspar biotit e gneiss occur in a garnctiferous arkosic matrix. Along strike,

meta-arkose occurs sporadically.

Meta-arkose_J_§l is fine grained and light grey to light pink, with accessory

garnet, hematite and muscovite. Cross-bedding structures occur at Maynard Lake.

Very fine grained magnetite is probably present; meta-arkose occupies an aero­

magnetic high.

Felsic gneiss (7) is light pink to light grey , fine grained and foliated .

Quartz grains are distinctively clear to red. Biotite is commonly less than 5

per cent. Hornblende is a common accessory, north and west of Carroll Lake.

Conformable sheet-like bodies of biotite gneiss and hornblende gneiss, from

several mm to several min thickness, occur within felsic gneiss. Agmatite

zones, consisting of brecciated hornblende gneiss (1) and intrusive granite (10),

occur locally. Adjacent to the meta-conglomerat e (5) occurr <' nce, portions of

felsic gneiss are ~uite granular and rich in clear ellipsoidal quartz grains ,

indicating a sedimentary origin .

Diorite-gabbro (8) is a medium to coarse grained, grey green to dark grey,

weakly foliated rock. Gabbro contains up to 90 per cent hornblende and pyroxene.

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Cranodiorite (9) is light grey , medium to coarse grained, and contains from

15 to 30 per cent mafic minerals . Granodiorite is gradational to granite (10) .

A pluton of gneissic garnetiferous granodiorite occurs east of Palf Lake.

Between this intrusive body and the east boundary of the map area occur agmatites

and migmatites, consisting of hornblende gneiss (1) intruded by garnetiferous

granodiorite (9) and diorite (8).

Granite (10) is light pink and fine to medium grained, with less than 15 per

cent combined biotite a nd hornblende . Small bodies of fine grained intrusive

granite occur within the granitic gneiss (6). The central two-thirds portion

of the Sandy Narrows (West Half) Area consists of coarse grained and porphyritic

granite (10), with phenocrysts of plagioclase up to 3 cm long. This rock is

nearly massive .

Pegmatite (11) is commonly pink, granitic and coarse grained . A light grey

tourmaline- bearing pegmatite outcrops on an island in Deschambault Lake. One

beryliferous pegmatite outcrops in the most southerly bay of Stilborn Lake.

Structural Geology

Early folding (Fl) may have produced axial planes trending east to west.

Small scale folds, with axial planes parallel to m~w - ESE foliations and dipping

NNE, may be relict of Fl. At Gee Lake, these folds plunge gently to the WNW.

At Brownell Lake, plunges are moderate to the east . The dominant regional

foliation dips to the northeast, at shallow to moderate angles at Maynard Lake

and becomes steeper as one progresses northeast. Southwestward dips occur in

the supracrustal belt, west of Gee Lake, giving the appearance of a syncline.

(This structure might be an overturned antiform).

Northerly trending (F2) folds occur in the west , e.g . the Carroll Lake

synform and the Maynard Lake antiform .

by s mall northeast trending (F3) folds.

trendin g folds are possibly syn- F3 .

In the northwest, these are overprinted

Small scale n orthwest - southeast

Post - folding recrystallization parallel to axial planes has generally erased

early fold structures in moderate to tight folds, e . g. in the Fl closure south

of Park Lake and Kvamsing Lake .

A cross- fold (F2 ?) trends north-northwest to south - southeast across the

Sandy Narrows (West Half) area . It extends from north of Trade Lake, south to

the con tact with Paleozoic rocks .

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Directions of stratigraphic tops have been determined at Maynard Lake,

where crossbedded meta-arkose (6) is overturned and apparently rests upon unit 1.

Biotite-staurolite-garnet gneiss (4) is in direct contact with unit 1, but the

direction of tops is not known.

Economic Geology

Low-grade copper-zinc mineralization was found in hornblende gneiss near the

northeast end of Brownell Lake by International Nickel Company of Canada, Limit ed .

The company carried out diamond drilling and trenching. Diamond drilling records

with rock descriptions are available on assessment file.

References

Padgham, W.A. (1968): The Geology of the Deschambault Lake Di strict; Sask. Dept. Min. Res. Rept. 114.

Taylor, F.C. (1958): PPlican Narrows, Saskatchewan; G.S.C. Map 1-1958.

Scott, B.P. (1975): The Maynard Lake and Sandy Narrows (West) area. In Summary of Investigations 197 5 by the Saskatchewan Geological Survey, Sask. Dept. Min. Res., pp. 6-9 .