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Reconnaissance Geology: Ballentin Island Vicinity, Reindeer Lake (Part of NTS Area 64E-NE) by M.R. Stauffer, S.l. Fumerton, F.F. Langford and O.J. Mossman *

The area is situated in northern Reindeer Lake about 350 to 400 km northeast of the town of La Ronge, . Access is easily obtained by boat from Southend, Saskatchewan via highway 102, as well as by boat via road from Lynn Lake, , and by float plane from La Ronge, Saskatchewan (2~ hours) or Lynn Lake, Manitoba (l hour).

Although the quality of outcrop is excellent along the shores of Reindeer Lake, many other places, notably in the central-west part of the area,contain extensive swamps and Pleistocene sand deposits making mapping of the underlying Precambrian rocks all but impossible.

General Geology

The area lies within the Rottenstone lithostructural domain (Munday , 1974 ; Ray, 1974). The lithological units comprise a variety of intrusive rocks ranging from granite to diorite, some metapsammites, and minor migmatites . The chrono­ logical sequence presented here has been deduced mainly from cross- cutting relation­ ships and xenolith content.

Most of the rocks are deformed, and have been metamorphosed, probably mainly to the amphibolite facies . The dominant foliation is of tectonic origin; it trends generally northeast-southwest and commonly dips moderately to steeply to the northwest.

Lithological boundaries shown on the map are approximate due to poor outcrop, local variation in most of the rock units, similarity in appearance and mineral content of some of the units, and gradational intrusive border zones .

Pink Granitic Gneiss (Unit 1)

This unit comprises mainly fine- to medium-grained granitic gneisses probably derived from either arkose and or granites of similar appearance probably in about equal proportions. Mose r ocks in this unit are moderately- t o well-foliated , pink to locally red or rusty brown. Mafic xenoliths a r e present locally in the granite

* Department of Geology, University of Saskatchewan - 18 -

phases of the unit.

Red Granitic Gneiss (Unit 2)

Many outcrops of this unit resemble those of Unit 1, but they differ mainly in containing a higher proportion of bedded meta-arkose, and in having a generally redder (brick red) colour on both fresh and weathered surfaces, and a generally weaker foliation. Like Unit 1 , the sedimentary and intrusive components are both mainly fine- to medium-grained and have similar mineral composition; as a result it is difficult to distinguish between them at every outcrop.

Quartz-Feldspar-Biotite Gneiss (Unit 3)

Several mappable xenoliths in Unit 8 in Lhe northwest contain fine- to medium­ grained gneisses of probably metasedimentary origin that are tentatively correlated with the Wollaston Group. These primarily include quartz-feldspar-biotite gneisses (meta-arkose) with greater than 5 percent biotite and minor quartz-feldspar-biotite­ amphibole gneiss with 10 to 20 percent total mafics.

Hornblende-Bearing Gneisses (Unit 4)

A single mappable xenolith in Unit 6 in the northwest, also contains rocks of probable metasedimentary origin. These include various pinkish grey to dark grey, fine- to medium-grained mafic gneisses, lesse r amounts of granitic gneisses, and minor granites (some of which may belong to other map units). The mafic content varies from 5 to 30 percent in the gneisses, of which up to one third is hornblende.

Granite t o Granodiorite and Granite Gneiss (Unit 5)

This unit is a complex of felsic intrusive bodies and granitic gneisses that include phases probably derived from all of the previous units (1-4). Most of these rocks are medium-grained, weakly- to moderately-foliated, pink to red on fresh surface (weathering grey to brick red), and contain 5 to 10 percent mafic minerals. More felsic and more mafic varieties are also present.

Whi t e Granite (Unit 6)

Several bodies of a massive to locally weakly-foliated, medium- to coarse­ grained granite crop out i n the nor t heastern corner of the ar ea. This rock i s mainly white on both fresh and weathered surfaces, but i s locally pale pinkish grey on fresh surfaces and pinkish to pale rusty brownish grey where weathered. It - 19 - contains small scattered garnets and small xenoliths of a more mafic weakly-foliated material.

The rock contains about 25 to 30 percent quartz, 70 to 75 percent feldspar (mostly potassium feldspar), and 3 to 5 percent biotite (locally up to 10%). In some places this unit is migmatitic, containing neosome as described above and a fine-grained quartz-feldspar-biotite gneiss paleosome .

Monzodiorite (Unit 7)

Several mappable and numerous smaller xenoliths of mainly medium-grained monzo­ diorite occur within Unit 8. These are various shades of grey on fresh surface and commonly weather spotted creamy white and bro~'tlish grey . They are weakly to modera­ tely foliated and are locally cut by numerous white veins of quartz and feldspar.

Porphyroblastic Granitic Rocks (Unit 8)

The Wathaman batholith (Ray, 1975) extends from the southwest through the Ballentin Island area and into Manitoba, underlying the southeastern ha lf of the area . . Numerous bodies of similar granites occur as dikes and xenoliths elsewhere in the area.

The main r ock type is porphyroblastic granodiorite to quartz monzodiorite containing 10 to 30 percent white to pink orthoclase porphyroblasts mostly about 1.5 by 3 cm in size in a light to medium pinkish grey groundmass. Lesser amounts of slightly porphyroblastic granite with greater than 3 percent porphyroblasts, are present in some outcrops.

Subunit 8c contains more than 10 percent mafic xenoliths and felsic veins. Subunit 8e is a foliated variety occurring mainly in the northwest.

Red Granite (Unit 9)

Red granite, medium-to coarse- grained, and locally foliated but mainly massive, crops out i n several large bodies in the northwestern half of the area. Porphyro­ blasts of orthoclase are locally present in minor amounts.

Mafic Rocks (Unit 10)

These include a wide variety of fine- to coarse -grained (rarely porphyritic) diorites, quartz diorites, and minor anorthosites . Most rocks are dominantly medium to dark grey on fresh s urface and commonly weather spotted whit e and dark brownish grey . They are mainly massive to weakly foliated . - 20 -

White to Pink Felsite (Unit 11)

Numerous small dikes of fine- to locally medium-grained granite cut the Units 1 to 10 and two mappable bodies cut Unit 8 in the south-central part of the area. These rocks are mainly massive to weakly foliated and are generally leucocratic, being white to pale pink on both fresh and weathered surfaces.

Porphyritic Diorite to Quartz Diorite (Unit 12)

Diorite to quartz diorite containing more than 20 percent phenocrysts of pink orthoclase occur in a nearly circular body near the centre of the area. The pheno­ crysts are stubby to highly elongate (up to 2 x 8 cm) and are set in a mid to dark grey matrix of medium-grained dioritic to ultramafic rock apparently composed entirely of hornblende. Small xenoliths of fine-grained dioritic material are common.

Granite Pegmatite (Unit 13)

Numerous small veins of pegmatite occur throughout the area. The only body mapped lies in the southeast and contains scattered red potassium feldspar crystals up to a meter across set in a coarse-grained granitic to pegmatitic groundmass. This body and most of the small veins are pink and white on both fresh and weathered surface, and are non-foliated.

Mylonite (Unit 14)

Numerous narrow zones of highly foliated to mylonitic rock cut most of the units previously described, and some have been indicated on the geological map.

Economic Geology

There has been considerable prospecting activity in the northwestern corner of the area in the last few years. Nothing of apparent direct economic interest was however encountered during mapp ing.

References

Munday, R.J.C. (1974): I l e-a-la-Crosse (E) Area, Saskatchewan; i n Summary of Geo logical Investigations by the Saskatchewan Geologi cal Survey. Sask. Dept . Min. Res. Ray , G.E . (1975): Foster Lake (NE) - Geikie River (SE) Area; in Summary of Geological Investigations by the Saskatchewan Geological Survey, Sask. Dept. Min. Res.