-35-

Reconnaissance Geology, Area (Part of NTS area 74P)

by C. F. Gilboy

2 The geol ogy of a 3100 km area approximately centred around Grove Lake, 70 km east-northeast of Stony Rapids, has been mapped a t a s cal e of 1:100,000. Access by float-equipped plane is excell ent. Rock exposure i s sparse close to the north­ eastern edge of the Athabasca Forma tion, but generall y improves towards the north.

General Geology

In the western and nor thwest ern parts of the map-area, the NNE- s triking west­ ward dipping Black Lake Shear Zone is defined by cataclastic and mylonitic rocks (2) with a variable thi ckness of from several hundred metres to about 3 km (Fi g . 1). Regional lithological differences exi st eithe r side of this major structure. Although these differences may be due more to primary l at eral variation rather than to large fault movement, the shear zone neverthe l ess forms a convenient break for descriptive purposes .

(a) East of the Bl ack Lake Shear Zone. Lithological features and de formation styl es are broadly comparable with those of t he Mudjatik domain (Lewry and Sibbald, 19 77).

Felsic gne isses (1A _ ) of uncertain origin are dominant. Supracrustal rocks 1 4 which are increasingly abundant t owards the shear zone have been categorized into two distinctive rock assemblages he re termed the western s upracrustals and the eastern supracrustals. The western supracrustals (W prefixes map-unit symbol) are made up of pelitic biotite schists (lJ) and gneis ses (lH), commonly containing garnet and/or sillimanite, cordierite and graphit e, inte rbanded with calc- silicate rocks (lF) and t o a lesser extent with quartzitP- ± s illi manite (lC) and amphibolitic schists and gneisses (lG). They are preserved in synformal keels between felsic gneiss domes . The eastern supracrustals (E prefixes map-unit symbol) chiefly comprise pink or gr ey me ta-arkosic quartzo-feldspathic gneisses~ garnet (1D ) 1 , 2 s illimanite- rich quartzites (lC) and amph ibole-biotite gneisses (lE); cal c-silicate rocks (lF) are generally lacking, and cordierite and graphite are only spar sel y distributed in pelitic to semipelitic i ntercalations. The eastern supr acrustals are in places overlain and unde rlain by s hallow-dipping f e lsic gneisses and form discontinuous concen tric outcrop patterns around dome- and-basin structures . - 36-

LEGEND I:)')• QUATERNARY CJ GLA( IAt DE POSI TS - ·- UNCONF ORMIT Y-- PRECAMBRIAN [ZJ ATHABASCA FORMATION - - UN(ONFORM ITY--

1 Ez;;J ~JifRN t~Nt~D G~m?:m~ ~~m ~b N~L ACK LAKE WEST OF BLACK LAKE SHEA R ZONE ~ MEGACRYSTI C GRANITE [::=) BIOTITE · AMPHIBOL E GNEI SS ES ; POSSIBLE METAVOL CANICS 0 5 E::3 ~1~~1.ScLtA'i.I~~ AS~ ~ MP1W Bbtr..i,5.\u tln~\ 6~~N FElSIC GNE ISSE> EA 5 T OF BLACK LAKE SHE AR ZONE 51 1 lSml ~\ f~tlU~~~R~~NJ,li'iLSM , }tJ8L~iN~r~~EDl~tNJJE ~T CHARLEBOIS LAKE ~ EAS TERN SUPRACRU STAlS AR EA (REPORT 24) r:_~ FE LSTC G NE ISSES BL SZ BLACK LAKE SHEAR ZONE

10 20 Km .

!OS' 11) ~.

Fig. 1 - Sketch of the princi pal geological units ma pped in t he Stony Rapids area (part of NTS area 74P).

Well foliated to massive white trondhjemitic and quartz dior iti c rocks (lK) form both large and small intrusive bodies, those of mappable size apparently having been emplaced only into the western supracrustals. In the southeast, r egionally homogeneous f oliated and/or lineated felsic gneisses (1A ) are of possible magmatic 2 origin. Elsewhere, the felsic gneisses have been extensively injected by minor intrusions of massive equigranular coarse-gra ined syenogranite.

(b) West of the Black Lake Shear Zone. Amphibole-rich rocks, probably of both supracrustal and minor intrusive origin, are widespread and make up a high p ropor­ tion (20 to 60 pe rcent) of the total outcrop. Together wi t h i n tercalated felsic gneisses and subordinate metasediments, they fonn the Hybrid Gneiss Complex (3B) recognized by Johnston (1963, 1964). In the north, mylonitic rocks (3E), the - 37 -

apparent northe ast ern continua tion of the Grease Riv er Shear Zone, s t rike a long the southe rn shor e of Selwyn Lake.

A maj or i ntrus ion of richly mega crystic fol iated grani te (3D) lies immediately west of the shear zone in its northern pa rt, but i s separated from the shea r zone at its southern end by about 3.5 km of Hybrid Gnei ss Complex. This megacr yst ic grani te r esembles the Juncti on Gr anite east of the Virgin River Shear Zone (Lewry and Sibbald, 1977) and tha t part of t he Wathaman Ba t holith adjacent t o the Needl e Falls and Parker Lake Shea r Zones (Gilboy , 1975; Ray , 1975) both petr og raphi call y and in its regional setting.

(c) Conglomera t es and s andstones of the Athabasca Forma tion (4), gener ally fla t­ l ying and little me t amor phosed , unconformably overlie t he afor e-descr ibed pre­ Helikian r ocks in the southern part of the map-area. The Athabasca Forma t ion has been mapped by Ramaekers (1976) a nd is not describ ed i n this r eport b eyond recor ding the occurrence of nume rous flakes up to 2 nur. across of white mica in a t hin sha l y unit exposed several me t res above the b ase of the unc onformity in an outcrop several hundre d me tres inland from the south-eastern shore of Newnham Lake. The flakes appeared to be of hydrothe rmal rather than deposition a l origin.

(d) Diabase (5) of simi lar appear ance t o the r ock known t o intrude the Athabasca For­ mation e l sewhere was exposed at several l ocal ities in the mapped a r ea . One out c r op l ies ove r a NW-SE l i near magnetic anomaly, suggesting t hat it may b e pa rt of a dyke.

(e) Glacial fluting recorded throughout mu ch of the northe rn part of t he map-area i ndi cat es that the l a test i ce movements dur i ng t he Quaternary glaciation were from t he northeast (020 0 t o 060 0 , 1 . n particul a r from 0300 t o 050 0 ) . In t he south , whe r e druml inoid ridges a r e a l so well developed , i ce movement is indicate d t o have been from east to west. Esker complexes, several of which are spat ially associ a t ed with s and and grave l deposits, are common . Mos t of t h e area is covered by till which, in the Cypri an- Gauthie r Lakes r egion, cont ains rare boulders of virtually unme t a­ morphosed reddish s h ale and pol ymictic conglomerate wi t h some vesicular l ava clas t s . These b oulders resemble Martin Forma tion s ediments (Ramaekers , per s. comm.) . No sour ce for them f or found. In t h e vicini t y of Young Lake and Herbert Lake, sever a l small (l e ss than 15 cm) rounded white sandstone b oulders were seen; they are possibl y derived from t he Hurwitz Group sediments of the Bake r Lake r egion, No r t hwest Territories .

Structure and Metamorphism

The s truc ture of the a r ea i s complex and i s unlikely to b e clearly under stood - 38-

unt il more detaile d mapping has been carried out. Apart from t he sh ear zones, a notable feature is a swing of structural t r ends i n t he Spre ckley Lake a rea. To the north of this Lake , f oliations and southeastwardly overturned axia l p lanes a r e oriented N- S to NNE-SSW . South and east of Spreckley Lake, general trends a r e E-W or ENE-WSW.

Preliminary analysis of outcrop patterns and struc tural trends suggests the possibility , at l eas t , of early non-cylindrical flat-lying regiona l f olds and gene­ tically related thrust sheets. The ear ly folds deform a gneissic folia tion and s chistosity(S ) produced during a preceding thermotectonic event ; no folds geneti­ 1 cally relate d t o s pla nar fabrics have yet been r ecognized. In the Young- Hawkins 1 Lakes region, these early folds were deformed to produce domes and basins .

Late b rittle fracturing i s responsib le for most of t he wel l developed topo­ graphic lineaments t r ending mai nly at 340° t o 005°, 55° to 075° and 280° to 310° . Pronounced linear aeromagnetic anomalies are coincident with the l ongest and best defined of the approxi mate ly north-south frac tures. Offs e t , if any , is generally too s ma ll a l ong the major ity of frac tures t o affect mapped boundaries. Brecci a has been observed only rare l y .

Me tamorphic gr ades appear to have r each ed middle amphibol ite facies on bot h sides of t he Black Lake Shear Zone in the nort h; they possibly attained upper amphibolite f acies west of the shear zone between Black Lake and t he east end of Stony Lake. La t e static r e trogr essive me tamorphism patchily affected r ocks i n the western part of t he map-area , wher e preliminary petrographic examination i ndicates widespr ead chloritization of biotite and amph ibol e, mo derate t o i n tense sericiti­ za tion of plagioc lase , the abundant gr owth of s ubi dioblastic epidote and l ocalized r ecrys t a l lization of f lasered and mylonitized fel sic mineral s to pr oduce polygonal granoblasti c textures . There does not seem to be any evi dence f or l a t e r etrogres­ s i on farther east, wh ere t her e is a characteristic development of upper amphibolite faci es mineral assemblages and banded migma tites . In the Young-Herbert Lakes region gr anulite facies conditions ar e indicat ed, as the rocks are collllllonl y waxy i n appearance and pyroxenes , i ncl uding probable hypersthene , have been identi fied

Age r elationships

The western s upracrustal s show marked lighological similari ty to t he Wol laston Group as it is devel oped i n the Wollast on Lake area (Ray , this volume) ; by analogy , they may t hus be Apheb ian in age. In the absence of along- strike continui ty and - 39-

o f geochronological data, age relationships between the western and eastern s upra­ crustals are uncertain.

Intense t ect oni s m and porphyroblas tesis a long the Black Lake Shear Zo ne has obscured the stratigraphic relationships between the wes t ern supr acrustals t o the e ast and the Hybrid Gneiss Complex to the west. The west ern supracrustals of the Laf ferty-Henry Lakes region near t o the shear zone contain amphibole schis t s abundantly i nte rbanded wi th pelitic and semipe litic biotite s chists. Wh er e they crop out closest to, and locally within, the shear zone, thes e amphibole schists display the s ame delicate mafic and f elsi c layering t hat characterizes exposures of the Hybrid Gneiss Complex in the Bompas Lake area west of the shea r zone. On the assumption that these hornblendic rocks are volcanoc las tic in origin, ther e may t hus be a late ral gra dation within possible Aphebian supr acr ustals from dominant me tasediments east of the shear zone to dominant me t avolcaniclastics west of the s hear zone.

Contacts between all recognizable supr acrustal s and the felsic gneisses are everywhe re highly migmati zed, deformed and me tamorphosed, oblitera ting any evidence as to the prima ry nature of the contact. By analogy with the Wo llas ton domain to the s outh, a possible Archean age for the fels ic gne isses may be assumed . In only one part of the map-area, north of Sereda Lake , however, do the felsic gnei sses petrographically r esemble the distincti ve homogeneous s a lmon pink gr anitoid gneisses of t he postulated Archean inliers of the Wollaston domain (1A ). Rad iome tric s tudies 1 i n the adjoining Charlebois Lake area are i n progress at t he Universit y of Alberta t o try to c larify these a ge problems (Cununing, pe rs. comm.).

As a l l pre-Helikian supracrustal r ocks appear to have s imilar thermot ecton ic histories , and as at l east one s upracrusta l assemblage is probably Aphebian, it is concluded t hat presently recognizable major de formation and metamorphism took place during the Hudsonian orogeny. Post-He likian movement a l ong the Black Lake Shear Zone , and possibly along other fractures , has faul t ed the Athabasca Forma tion.

Economic Geology

(a) Uranium. The po tential of the sub-Athabasca Formati on unconformity as a locus of economically viable uranium deposits has he re, as elsewhere in northe rn Saskat­ chewan, l ed to much expl oratory activity by industry. To the present time , no major finds have been announced in the map-area.

Two r adioact ive s howings close t o the northwest shore of Black Lake were explored by Nis to Mines Ltd . between 1949 and 1953. An e xtensive drilling programme -40- and the driving of two adits reveal ed minera liza tion ass aying up t o 0 . 8 percent occurring as pitchblende in veins along fracture zones and shear s in t he pre­ u3o8 dominant paragne isses of the hanging wall. The best-mineralized fractures are sub­ parallel to the main s hear zone. Re-examina tion of the property by Haymac Mines in 1959 and by Mokta () Ltd . in 1966 produced economi cally discouraging r esul ts . In 1968, Amok Ltd. sank a sing le hole to the s ub-At habas ca Fo rmati on unconformity on the eastern (footwall) side of the Black Lake Shear Zon e . Sited close to the north showing and with a dip of 69° on a strike of 306° , the dri ll­ hole int ersected the unconformity at 140 m; no radioacti vit y was found in ei ther the core or the hole.

Radioactive showings between Elizabeth Falls and Woodcock Rapids a t, or c lose above, the sub-Athabasca Formation unconformity were trenched, s tripped and/or drille d by De e Explorations Ltd. in 1952 and by Mok ta Ca nada Ltd. in 1966. Assayed sampl es gener a lly yielde d u o values of l ess t han 0. 5 percent. 3 8 Particularly f avourable s ites for high-gr ade uranium miner ali zation a re consi­ dered to occur in areas where carbonaceous pelites and s emi peli tes, especially those found a t or clos e t o the base of the Wo llas ton Group, underlie the sub­ Athabasca Formation unconformity (Ray, 1977; Hoeve and Sibbald in press; see a lso Dodson et al, 1974 and Rich et al, 1977). Consequently particular a ttention was pai d to occurrences of graphite-bearing rocks during the current s urvey . Although abundantly developed in the we stern supracrustals south and west of Grove Lake, graphite was seen only rare ly in the eas tern supracrustals where they crop out close to the margin of the Athabasca Formati on east of the Black Lake Shear Zone. The nearest obse rved graphi te occurrence in these s upracrusta ls is in fact situated 6 km from the margin. Only one occurrence of graphite , lying on the eastern shore of Middle Lake, was found west of the Black Lake Shear Zone. Here a few flakes of graphite we re identified in a band of pelitic gneis s in saprolitic felsic gneiss.

Uraniferous pegrnatites from s everal l ocal ities in t he Charlebois a rea which adjoins the present map-area (Thomas , this volume) have been r epeatedly examine d and test ed f or their economic viability, so far without great r eward . Minerali ­ zation i s mai nly r estrict ed to coarse-gra ine d white pegma t i tic segr egat ions in rocks which resemble the weste rn s upracrustals close t o t he contact be tween t he supracrustals and f e lsic gneisses . As a guide to possible exploration, it i s noted tha t this contact extends into the northern Spre ckley Lake and the Gr ove- Sear cy Lakes regions , and is a lso found i n the Edgar- Jervois Lakes region,

(b) Other me tal s. Triana Explorations Ltd. in 1956 indicated the pres ence of -41- banded iron formation along the contact between the richly megacrystic foliated granite and the Hybri d Gneiss Complex in the r egi on between Black Lake and Middle Lake . The formation has a s trike length of 6.7 km a nd averages about 70 m, but locally ranges up to 150 m, in width. The depos it was re-examined in 1973 as part of the Mineral Evaluation Program (Pearson,1973). I t was estimated that maximum or e reserves recover abl e by open-pit mining to a depth of 150 m mi ght amount t o 19 million t ons , with an average grade of 30-34 percent iron.

A few sulphide showings have been located and e xami ned by s everal mining companies. Gulf Minerals Canada Ltd. dri lled three holes over two EM conductive anomalies l ocated about one kilometre northwest of Herbert Lake. One conductor was ascribed t o almost massive pyrrhotite and magnetite minera liza tion over a total thickness of about 10 m in each of two holes. Similar mineralization over less than one metre is r esponsible for the second weaker conductor. Assays for base and precious metals of samples from the heavi ly mineralized zones yielded low values. Uneconomic occurrences of disseminated sulphides, chiefly pyrite wi th minor chalcopyrite, were located by the same company in the Bompas-Henry Lakes region; f urther occurrences, none considered to be of economic significance , we re found i n this r egion during the pr esent survey .

References

Dodson, R. G.; Needham, R.S.; Wilkes , P.G.; Page, R.W.; Sma rt, P.G . and Watchman, R.L. (1974): Uranium miner alization in the Rum Jungle-Alligator River s Province , Northern Territory, Aus tralia. Formation of uranium or e deposits, pp. 551-568; Interna tional Atomic Energy Agency, Vienna .

Gilboy, C.F. (1975): Foster Lake a r ea. Sask. geol. Surv . Summary of Investigations 1975 , 29- 34 (editor s: Christopher, J. E. and Macdonal d, R.).

Hoeve, J. and Sibbald, T.I.I. (In Press): On t he genesis of Rabbit Lake and other unconformity type uranium deposits in northern , Canada.

Johnston, F.J. (1963): The geology of the Lytl e Lake a r ea (west half) , Saskat­ chewan. Sask. Dept. Min. Res. Rept. 80.

(1964): The geology of the Ly tle Lake area (east hal f), Saskat­ ch ewan. Sask. Dept. Min. Res. Rept. 90.

Lewry, J . F. and Sibb ald, T.I.I. (1977): Variation in lithology and tectonometa­ morphic r e l ationships i n the Precambrian basement of northern Saskat­ chewan. Can. J . Earth Sci., 14, 1453-1467.

Mawdsley, J .B. (1957): The geology of the Charlebois Lake area, northe rn Saskat chewan. Sask. Dept. Min. Res. Rept. 24. -42-

Pearson, W.J. (1973): Mineral Evaluation Program. Summary Report of Geological Investigations conducted in the Precambrian area of Saskatchewan 1973, 60-6 7.

Ramaekers, P. (1976): Athabasca Formation; northeast edge : Part I - Reconnais­ sance geology. Sask. geol. Surv. Summary of Investigations 1976, 73-77 (editors: Christopher, J.E. and Macdonald R.).

Ray , G.E. (1975): Foster Lake (NE) and Geiki e River (SE) areas. Sask. geol. Surv. Summary of Investigations 1975, 13-18 (editors: Christopher, J.E. and Macdonald, R.).

(1977): The geology of the Highrock Lake - Key Lake vicinity , Saskatchewan. Sask. Dept. Min. Res . Rept. 197 .

Rich, R.A., Holland, H. D. and Petersen, U. (1977): Developments in economic geology, volume 6: Hydrothermal uranium deposits. Elsevier Scientific Publishing Company, Amsterdam.