- 77 -

NEA/IAEA Test-Area: Basement Geology

by T.I.I. Sibbald

Crystalline bas ement rocks exposed to the eas t of the Athabasca Formation within

the NEA/IAEA tes t -area (Fig . 1) were mapped during the 1979 f ield s eason . This study

i s des igned to provide base-line data for o t her t~st-area project s reviewed in thi s

volume.

o s~ f---,- - - - - --< 0 5M, .

__ ...__ APPRO XIMATE LIMIT Of ATHABA SCA FORMATION

~ .,-,: APPROXIMATE LIMI T Of QUARTZ I TE-AMPH180 LITE .... UNIT {HIDDEN BAY ASSEMBLAGE )

r,JfV N FAULT

• , • • MUOJATIC - WO l lASTO N CO NTACT

~\---"IA LO CATION O f SCHEMATIC STRATIGRAPHIC

SUCCESS ION

• LO CATION O f URANIUM DEPOSIT /PROSPECT

Fi g. 1 - Major geol ogical subdivisi ons and location of unconformity type uranium ueµosi ts/ prospects in t he NEA/ IAEA study area .

55• 12·10"

Mapping was undertaken a t s cales of 1: 31670 , 1:12,000 and 1: 4800 , depending on

outcrop and availability of cut l i ne f or ground control, and compiled at 1:31,680

scale (2 inch t o the mile). Over the area encompas sed by Gulf Mineral s (Canada) Ltd .

mineral dispositions details of outcrop and cut line locations wer e provided by the

Company . This invaluable assistance is grateful l y acknowledged .

- 78 -

Previous geological mapping in the a r ea was carried out by Fahrig (1958) ,

Wallis (1971), Chandler (1978) and Ray (1978). TMs r eport provides a brief des­

cription of lithological, s tructural and metamorphic r ela tionshi.ps of exposed base­

ment rocks. A review of the economic geology may be found e l sewhere (e . g . Hoeve and

Sibbald, 1978; Kirwan, 1978; J ones , 1979) .

Outcrop i s general l y poorly developed, ave r aging only a few per cent, a nd ove r

lar ge trac t s is totally absent, but a r e l a tivel y detailed documentation of strati­

graphy and str ucture has proved possibl e in the vicinity of the Rabbit Lake de pos it

(Hoeve and Sibbald, 1978) .

T~o li t hostructural domains of the Precambrian basement are represented in the

area , the Mudjatik and more easterl y lying Wollaston domain (Sibbald et al, 1977)

(Fig . 1). The Mudjatik domain, typif i ed by (possibly Archean basement) granitoid

gneisses and subordinate (probably Aphebian) supracrustals e l sewhere , is exposed

only i n the north, t o the wes t of Collins Bay as granitoids. By contras t, the

Wollaston domain comprises a wide range of Aphebian s upracrustal lithologies over­

lying a r e latively homogeneous Ar chean granitic basement, which is not exposed in

the area .

The study area straddles the eastern edge of the Athabasca Formation which

attains a thickness close t o 300 m along the western boundary of the area . The

basement contact, wh i ch is in part fault ed, i s only exposed at two locations , eas t

of the south end of Lampin Lake and in the Rabbit Lake open pit. One or more small

outlie r s exist be tween Rabbit La ke and Pow Bay, below the Rabbit Lake thrust fault ,

and a somewhat larger body is present immediately south of the area east of Otter

Bay (Wallis, 1971). The Athabasca Formation within the tes t area i s described by

Ramaekers (this volume ) .

The Precambrian basement is altered by paleoweathering, and a hema tised,

kaolinized and chloritized saprolitic zone commonly from 15 to 30 m thic k is present.

The morphology, mi neralogy and geochemistr y of this zone have been described previ­

ously (Sibbald, 1978; Hoeve and Sibbald, 1978) .

Rocks of t he Mudjatik domain

The granitoid gneisses of the Mudjatik domain are commonly pink to grey ,

medium-grained and comprise quartz (10-40 percent), plagioclase , K-feldspar and

biotite (0-10 percent) . Both layered and massive varieties are recognised the

former apparently be ing invaded and r eplaced by the latter . Layer ing is expressed

by biotite content and by l eucosomal segregation banding . Compositionall y homo­

geneous , mass ive varieties display some variation i n grain size be tween medium and

- 79 -

pegmatitic , and r a rely contain biotite a nd amphibole-rich schlieren and pods.

Rocks of the Wollaston domain

A wide range of Wollaston Group supracrustals is represented. Hudsonian magmatic

activity i s marked by concordant and discordant micrograni te to granite pegmatite

sheets and lenses, bodies of weakly porphyritic biotite adamellite and by r are mica

lamprophyre dykes. A single diabase dyke , similar to the Grenvillian dykes cutting

the Athabasca Formation, was observed intruding the Wollaston Group .

Graphitic pelites lowermost in the Wollaston Group succession , are recognised

in diamond drill cor es from an unexposed northeasterly t r ending zone in Collins Bay.

This zone of pelites also probabl y overl ies granitoid gneisses of the Mudjatik

domain and underlies the Collins Bay 'A' and 'B' uranium orebodi es . The pelites are

typically grey, medium- t o f ine-grained, finely foliated, poorly layered rocks in

which quartz , feldspa rs, biotite and graphite are major cons tituents. Graphite

content is commonly less than 10 percent, but may locally exceed thi s amount. Minor

garnet is sometimes developed and small amounts of sulphides a re generally always

present . The unit is permeated by quartz-feldspar segr egat ion zones of medium- t o

coarse-grain size, which contain biotite and spo radic tourma line and garnet.

A second pelitic unit, which is in part graphitic, coarser grained and typically

crenulated on a minor scale , occur s higher i n the s tratigraphic success ion and out­

crops east of Rabbit Lake. It is interleaved with a di s tinctive massive white

biotite-sillimanite arkose containing elliptical quartz-fibrolite knots up to l cm

in leng th . The arkose is in part transformed to a white biotite microgranite.

A unique sequence of well foliated, medium-grained, poorly layered , commonl y

rus t y-weather ing garnet-sillimanite pelites with rare cordierite is developed on

Harrison Peninsula . Monotonous poorly layered, medium-grained semipelites with

biotite and rarely garnet also occur in this locality . They are interlayered with

calcareous meta-arkose and calc-silicate unit s . Outcrops a re dominated by medium-

to coarse-grained white granitic segregations, in which the semipelites occur as

rafts. Biotite semipelites are also common as interlayers in calc-silicate sequences.

Medium-grained meta-arkosic to semipelitic gr anulites form the most widespread

compositional unit within the area of investigation. They are typically calcareous

and characterised by clinopyroxene and/or clinoamphibole. Colours vary from grey

to green to orange to brown to pink, and compositional layering is normally well

developed. The only primary sedimentary struc tures encounte red are two questionable

examples of cross bedding. Major constituent minerals include quartz, feldspar,

biotite, c linopyroxene and clinoamphibole. Magnetite is typically present in small

- 80 -

amounts, in places forming euhedral porphyroblasts up to a few millimetres across .

Minor garnet and graphite occur locally, the l atter commonly in association wi th

sulphide minerals. Major compositional variat i ons reflect changes in the quartz­

feldspar r at i o and in concentration of biotite and calcareous componen t s . Calc­

sil i cate , and apati t e-rich phosphatic interlayers of varying thickness a r e sporadi­

cally developed.

Plagioclasites, massive, feldspar rich granulites , are i n terlayered with the

me ta-arkosic to semipelitic granulites. They are gr ey t o pink, medium- grained and

composed essentially of sodic plagiocl ase , with minor c l inopyr oxene and/or clino­

amphibole. The mafic minerals ar e commonly porphyroblastic and aggregat ed into

irregularly or ien t e d planar zones or clots . Relic t pods o f meta-arkose found

commonly in the plagioclasit es lend t he rock a conglome r at i c appearance. The

sodic compos ition and r eplacemen t character of the plagioclasites sugges t s derivation

f r om arkose by i n t roduction of sodium during diagenesis or metamorphism.

Impure dol omitic marbles are recorded in the vicinity of Rabbit Lake and in

Hidden Bay,where they occur as mappabl e units and thin in t erlayer s or pods in calc­

s il icate rocks and plagioclasites. The marbles are white to pink , typically medium­

grained and equiiranula r. Impurity silicate mine r a l s i dentified include d iopside

and scapolite . Irregularly developed layering ar i ses from concentrations of silicate

minerals in the carbonate matrix.

Cale-silica te r ocks outcrop in large, homogeneous i rr egularl y shaped masses severa l

hundr ed metres thick and as thin layers a few me tres thick interlaminated with

calcareous meta-arkoses and semipelites. They are typically green t o grey t o white,

medium- t o coarse-grained , l ocally pegma t i tic , and contain diops ide a nd plagioc l ase

in broadly equal proportions. Scapoli t e , in bladed crys tal s up to several centimetres

l ong ,toge the r with amphibole and biotite are locall y important. Sphen e and apa tite

occur locally.

Cal e-silica t e breccia i s found in association with dolomitic marble, scapolite­

rich calc-silicate rocks and plagioclasite on a small island at t he mouth of Hidden

Bay. I t comprises pink angular fragmen ts up to a few centimetres in dimension, set

in a matr i x which inc ludes pale gr een pyroxene and/or amphibol e and fe ldspar. The

fragments are formed by equigranular aggregates of fe lds par (poss ibly albi te ) of

medium-t o fine-grain size. Similar soda-rich breccias have been r epor ted elsewhe re in

t he Wollas ton domain and are tentatively interpr e t ed to be of salt solution collapse

ori gin (Ray , 1978).

White sillimanite quartzites occur in the upper part of the Wollas t on Group

successi on east and south of Rabbit Lake. They a r e medium- gr a ined, poorly layered

- 81 -

to mass ive rocks with up t o 30 percent feldspar, sillimanite faserkiesel and minor

biotite . Coarser grained quartzose segregation zones are usually bluish t o pinkish

col oure d and locally contain black tourma line . A bright green mine r al, possibl y

fuchsite,is a rarely observed minor constituent.

Apparently concordant ~mphibolit es, r anging from a few metres t o several hundred

me tres in thickness, a r e a typical assoc iate of the quartzites . They vary from mass­

ive to irregularly layer ed , and in one out crop contain scattered pale elliptical

felsic clas ts up to 20 cm long. The rocks ar e med ium....grained and compo sed predomi­

nantly of dark green amphibole and plagioclase . Diopside, biotite,garne t and opaque

minerals also occur l ocal l y . Aggregated f e ldspars and white f eldspathic reaction

rims a round garnet generally g ive t hese rocks a spotted appearance . Coarse- grained

garnet-amphibole (anthophylli te- gedri te) r ock somet imes outcrops adj acent to the

amphibolites . The amphibolites were pr obably volcanic i n origin and the coarse­

grained garnet-amphibole r ock possibly represents metamorphosed iron formation.

Intrusive Rocks

Granitic bodies of several generations and varying medium- to pegmat i tic-grain

size occur as concordant and discordant sheets and pods in most supracrustal r ocks ,

in some, for example the semipelites, domi nating the host rock. Colours var y from

pink to white, depending on feldspar composition, in the range orthoclase t o plagio­

clase. Bio t ite , garnet, tourmaline and magne tite are possible minor constituents .

Pink biotite adamellite forms subconcordant sheets and less regular masses a

f ew tens of metres thic k in the vicinity of Rabbit Lake. It i s medium-grained and

typically contains small K- feldspar phenoc r ys t s up to 1 cm long . A weak foliation

marked by an elongation of quartz gra ins and a pre f erred orientation of biotite is

generally apparent.

Discordant maf ic dykes , possibly lamprophyres a f ew metres wide were observed

in three locations, one south of First Link Lake and the other two north of Raven

Lake. They are homogeneous, da rk grey to black r ocks comprising about 30 percent

r andomly oriented biot ite, feldspars and some quartz . The randomly oriented biotite

fabric and presence of rotated blocks of foliated host r ock in the dykes suggests a

late- to post-tectonic age for these bodies.

Structure

Two major followed by at least two minor deformation events are recognised in

the rocks of the Wollaston domain including those in the present area. The f i rst

- 82 -

maj or deforma t i on gave rise to a penetrat i ve laye r-paral lel foliat ion, appa rent ly

without major r epetition of the s tra tigr a phic s uccession by f olding or thrus ting ,

and mobilization of the Archean granitoid basement into mantled gneiss domes

(Sibbald et a l, 1977) . During the second major deformation the early f oliat i on and

the mantled gneiss domes wer e folded or f l attened into doubly- plunging up r ight to

steeply inclined folds with northeasterly trending and southeasterly dipping axial

surfaces . In places , a par a llel penetrative foliation obliterated and t r ansposed

the earlie r plana r f ab ric and a n inte nse r odding developed parallel t o fold axes .

Poorly layer ed calca r eous r ocks (calc-s i l i ca t es and calcareous meta- arkoses) a nd

sillima nite qua rtz ites were most susceptabl e to this pr ocess . For exampl e , conglo­

mer ates , noted by Wallis (1971) on Ashley Peninsula, a r e r e - int erpre t ed as tecto­

nically r uptur e d and rodded cal careous me tasediments .

The minor f old eve nts are of little apparent r eg ional s i gnificance .

Outc r op within the Mudjatik domain i n the present ar ea i s too l imited to allow

interpreta tion of structural his t ory. However , t he Mudjatik- Wollaston domain contact

in t he Colli ns Bay a r ea appear s structurally discordant to lithologica l trends in

t he Wollaston domain.

Two gener a t ions of faults are recognised; an earl ier set with l eft-lateral

strike- slip (eg. Dragon La ke fau l t), which f orms part of the north-south trending

Tabber nor Lake Fault sys t em, and a l ater set of r everse faul ts which also cut the

Athabasca Formation (eg . Rabbi t Lake and Col l ins Bay faults) . Out c r ops of quartzit e

intersected by the earlie r set are s trongly fractured, hematised and permeat ed by

microcrys tall ine opalescent quartz . Rocks intersec t ed by r ever se faul ts tend t o be

extensively affec t ed by a rgillic and chloritic alteration . Whi l e the major movement

on the earl ier faults is thought to be late Hudsonian, they may have been reactivated

dur ing the second major f a ul t phase in pos t -Athabasca (possibly Grenville ) times .

Me tamorphism

Me tamorphic mineral parageneses in the Woll aston domain indicate upper amphi­

bolite facies conditions within the low- inte rmediate facies series . Fabrics are

generally random to gr anoblastic , sugge sting tha t c r ystallization out las t ed t he

second major deformation , which was the l ast major penetrative structural ev en t .

Similar fabrics occurring in the Mudjatik doma in wer e presumably produced a t t he

same time .

Stabl e relic t minerals, s uch as e lliptica l fibrol ite knots and co rd ierite

crystals, which def ine the early foliation appear t o demonst r ate the prevalence of

high gr ade conditions during the fi r st episode of de formation. Boudinaged and

- 83 -

f olded gr anit o i d seg r egations indicate anatec tic proces s es occurring prior t o t he

second deforma t i on. The t wo l ates t mi nor f old episodes a ppea r t o have been a ccompa-

ni.ed by r e trogress i ve metamorphi sm.

Discus s ion

A s t r a t i graphic secti on fo r t he Rabbit Lake ar ea propos ed by Hoev e and Sibbald

(1978) is r eproduced he r e (Fi g . 2) . Thr ee major un its a re identi f i ed:

unit, 2) a meta- arkose unit and 3) a quartzi te- amphi bol ite unit.

I HELIKIAN .,

1

I APHEBIAN

I

I

ARCHE AN

I

AI HAi.;CA I , • • , ' I FmMtl iON l=· ~,: ,v<

~

~ ·----. "::_i "°"" I·

~~~~:~l~E r~;j ,oo,.

META­ARKOSE

UNIT

L PEUfE UNIT

l--_-·-J -~~~~ r-~-=~

I}! :...· •\\ K K W

HELIKIAN

\ •.' '. ' : i Al HA3ASC A f0RMAll0N

APHEBIAN

·1 QUARW H IN PJ>RT SILLIMAN ITE llEAOlNG

l-, __ .. _._· ,-1 ( A.LCAREOUS META-ARICO SIC TO SEMI - PE UTIC

META SEDIMENTS . MINOR CA L( -SILICATE S. : . :1 PHOSPH ATES

• AMPH1 60UTE

"' - ~-1 - ·:· -· - -J ~ _. - --

'1 L.

k, -, ,,~ r T"l' "T-, "T'T T T T r·T TT T err r r T

r 1t i,: ; - : · J I :I!! . . )( )I

SILUMANll f MEIA-AA:l(OS~ IN PART TRAN SFO RM ED 'l' O MICROG~ANITE

P£U TIC GN HSSES, IN PART GRAPHITIC

\N TERLA'H RE D CALC- SILIC ATE AN D M ETA ­

ARKOSE !UPPER GNEISSES!

MASSIVE META-ARKOSf (ORE ZONE:)

MARBLE

Pl AGIOCLASITE

CALC-SILICA.Tf

ARCHEAN

GRANI TOrD GN EISSES (MUDJATII( DOMAIN)

1) a pe li te

Fi g. 2 - Schemati c s t r ati gr aphic sect i on , Rabbi t Lake a rea . Serrated l ines bounding pl agiocl as ites indi cate transition al rela t ionshi ps .

The pelite and meta-arkos e unit s form a cha racteri s tic sequence overly ing the

Arc hean ba semen t t hr oughout much of the Wollas ton domain (Ra y , 1977) . The quar tzite-

amphibolite uni t, essent ially synonymous wi th the Hidden Bay assemb l age of Wallis

(1971) a ppear s to be r e stricted to the Wolla s ton Lake area. Ray (1978 , Fig . 2)

implies tha t t his unit passes l a terally i nto me t a - arkoses to the east, rather than

- 84 -

wedging out due to eros i on . However, t he vol canogenic amphiboli t es , which might

transcend sed i mentary facies boundaries , do not appear in the proposed l aterall y

equivalent me t a - a rkoses .

Mapping t o the north of Rabbit Lake also necessi t a tes modification to the lower

par t of the Rabbit Lake, succession in t ha t a rea . Passing northward f r om Rab b i t

Lake , calcareous me ta-a rkoses and plagioclasites of the met a-arkose uni t are i n t er­

l ayered with semipel i tic rocks and i n the north pa rt of Wo l laston Lake the me ta­

arkose s appear to wedge out comple t ely , the quartzite-amphibolite uni t r esting

direct ly on a thick s equence of peli te s and s emi peli tes (Ray , 1978, Fig. 2) .

Graphitic pelites, wh ich dip moder a tely t o the southe ast and par allel t he

Woll as ton-Mudj atik j unction in the Col lins Bay area , appear str ucturall y discordant

t o l ithologi c al trends in the overly i ng meta- arkoses and their semipeli t ic facies

equi valents . The Mudj a tik domain has been inte rpre t ed as the cor e of a Hudsonian

Mobile zone characterized by migmatite l obe ' nappes ' , whe reas t he Wollaston domain

is considered as a regime of mantled gneiss doming (Sibbald et al ., 1977). The

discordant Mudjatik-Wollaston contac t may thus r epresent a nappe s ol e - or upper ­

surface a long which the Mudjatik rocks, granitoid gneisses and graphi tic pel ite,

were transported eastwards over- or under- the authocht honous Woll ast on Group . A

more recent analog i s seen in the Alps of Switzer land where , t o the south of the

zone of ext ernal Pennine nappes along the frontal Pennine thrus t, all ochthonous

me tasediments of the Lower Pennine nappe complex discordant l y overlie essentia lly

authochthonous r ocks of similar age as well as unde rlying basement (Frey, 1967) .

References

Chandler, F.W. (1978): Geol ogy of par t of the Wollaston Lake Fold Bel t , northern Can., Geol. Surv. Bull., 277. Saska tchewan .

Frey , J. D. (1967): Geol ogie des Greinagebeit es . Beier. Geol. Schweiz., N.F. , 131 .

Fahrig , W.F. (1958) : Wollaston Lake, Saskatchewan; Geol . Sur v . Can ., Map 27-1957 .

Hoeve, J . and Sibbald, T.I.I. (1978) : On the genesi s of Rabbit Lake and other unconformity-type uranium deposits in northern Saskatc hewan, Canada . Econ. Geol., 73, 1450-1473.

J ones, B. (197 9) : The geology of the Coll i ns Bay deposit, Saskatchewan. Can. I ns t . Mi n. Me t. , Bull., 72, p. 53, (Abstract).

Kirwan , L.D . (1978): The discovery o f the Midwest Lake uran i um deposit. In Parslow, G.R. ed. , Uranium exploration techniques: Geol . Soc . Saskatchewan, Spec. Pub., 4 .

Ray , G.E . (1977): The geology of the Highrock Lake - Key Lake vi cinity . Sa sk., Dep . Miner. Resour. , Rep . , 197, 29 p.

- 85 -

Ray , G.E. (1978): Reconnaissance geology: Wollas t on Lake (west) area. In Christ opher, J.E. and Macdonald, R., eds . , Summary of Invest igati ons 1978, Sask. Geol. Surv., pp . 25 - 34.

Sibbald, T.I.I. (1978): Uranium metallogenic studies : Rabbit Lake , Geology . In Christopher, J.E. and Macdonald, R., eds ., Summar y of Investigations 1978 , Sask. Geol. Surv., pp. 56-60.

Sibbald, T.I.I., Munday, R.J . C., and Lewry, J.F. (1977) : The geol ogi ca l sett ing of uranium minera lization in northern Saskatchewan. In Dunn, C.E. ed., Uranium in Saskatchewan: Geo l . Soc. Saskatchewan , Spec. Pub ., 3 , pp . 51- 98.

Wallis , R. H. (1971) : The geology of the Hidden Bay area, Saskatchewan. Sask. Dep. Miner. Resour., Rep. 137, 75p .