- 25-

Reconna i ssance Geology: Wo l laston l ake (West) Area

(Part of NTS Area 64L) by G. E. Ray

2 This 5050 km ar ea lies appr oximat e l y 400 km nort h-northeast of La Ronge a nd cover s t wo dis t inct geo l ogical r egi mes , t he Mudjatik domain t o the northwest and the Wollas t on doma i n to t he Southeas t (Fig. 1) . The l a tter domain contains upper amphibolite fac ies, nor t heast striki ng me tasedimentary gne i sses of t he Wollaston Cr ou p (Ray 1977) interfolded with folia ted gran itoid bodies . The gr anitoi ds probabl y r epr esent reworked Ar chean basement unconformabl y overla in by the

Aph ebian Wollas ton Gr oup. A va riable sedimenta r y sequence i s r ecogni zed i n tltE' Wo l last on Gr oup and the Aphebi an s tra t igraphy , as e l sewher e , has important contro l s on ur a nium mineral isat ion.

The Mudja t ik doma in i s unde r lai n by gr anito i d r ocks of uncertain orig1n and by several elongat e , narrow be l ts of pelitic / semipel itic met asedimenta r y gneiss. Up pe r amphibol ite t o conditions we r e a ttained during t he r egiona l me t a­ morphism a nd the struc tural trend varies f rom northeast erly t o west-northwest erl y .

To the west, t he rocks of t he Woll aston and Mudjat ik domain s a r e unconformably overla in by sedimen ts of the poorly exposed Athabasca For ma t ion (Rarnaekers 1976).

Rocks of the Mudj atik Domain

The east e rn portion l a r gel y comprises well l ayered qua rtzofel dspa thic migma ­ titic gneisses (C) wh ich in pl aces include t hi n , migma tised horizons of amp hibolite (Cl) or biotit e schist (C2 ). Further west , incr easing r emobilisation causes des­ truc tion of t he original layering t o pr oduce pink, massi ve to we ll fol iat ed grani t ic r ock (B) in which t he or igina l amphibol i t es remain as dis rupt ed xenoliths . The granuli tes (A) ar e mai nly confined to the ar ea be tween Manson and Hatche t Lakes and their r el ationsh i p t o t he other gr a nitoids is unknown .

Pel i t ic a nd s emipe l i t ic me t ased i ment ary gnei sses (D) f orm na r row bel ts up t o 25 km l ong a nd 5 km wide , and a r e distinguis ha ble f rom t he biot ite schist l ayers i n the quartzofelds pathic migma t i t es by t he presence of cordie r ite , garne t and silll­ ma nite wit h spor adic gr aphite, hornblende and pyr ite- pyrrhot ite . The pel i t es ar e spatially associ a t ed wi th bodi es of coarse gr ained, grey "tona lite " (E) which -26-

10 4 • 103° I [\<:J ATHABA SC A FO RMATION

MUDJATIC DOMAIN

PELITI C METASE DI MENTARY GNEI SS

\ I GR ANIT ES , GRANULI TES - \ / J' I ' QUARTZO - FE LDSPATHIC MIGMATITE S

WOLLASTON DOMAIN

WOLLASTON GROUP ~ SUPRACRUSTALS ARCHEAN GRANITOID 0 BAS EMENT

FAULT

JUNCTI O N BETWEEN WOLL ASTON AND MUD JATIK DOMAINS

KILOMETRE S 0 10 20 30 sa·-

Fig. 1 - Major lithological units of the area reconnai ssance mapped in NTS area 64L.

commonly separate the pelite belts from the surrounding gr anitoids . The t onalites a re oft en garnetiferous and i ncorporate metasedimentary xenoliths; they ma y r epr e­ sent a partial melt der ived from the pelites . Northwest of Ross Channe l and west of Scr i me s Lake the pelitic gneisses ar e i nterlayer ed with thin horizons of meta­ a rkose (D4) and quartzite (DS) .

Rocks of t he Wollaston Domain

Archean Basement. The basement inlier s occupy cores of doubly plunging antiforms and are mainly composed of coa r se gr ained, pink to white foliated granitic r ock (1), separable into biotite bearing (la) and biotite-hornblende bearing (lb) -27-

varieties. In places the inliers possess l arge felds par megacrysts (le); elsewhe r e t her e are layer s or xenoliths of amphibolite (ld).

Wollaston Group . The me t asediment ary units out cropping around the basement inliers reveal a consistent lower s tratigraphy t hroughout, whereas the upper portion is variable , probabl y refl ecting a rapid sedimentary facies change across the a r ea (Fig . 2) . I n the southeast (Column D, Fi g. 2) t he str ati graphic s uccession i s s imilar t o the widespread, consistent Aphebian sequence described elsewhere (Gilboy 1975 ; Ray 1975, 1977 ; Potter 1977), compris ing basal pe litic and semipelitic gneisses (2) with graphitic horizons (2f) over lain by a thick unit of me ta-arkose.

A B C D LEGEND RAVEN & HORSESHOE Ol:P051T S AMPH1B0l1TE

APHE61AN QUARTZITE

META - ARKOSE

COLLIN S PELITI C ANO SEMIPEU TI C ROCKS GC.GG G SAY G O G RA PHITIC HORI ZONS DEPOSIT UNCONFORMITY ARCHEAN GRANITIC BASEMENT ARCHEAN

Fig. 2 - Compari son of schema tic stratigraphic successions in the Wollaston Group t hroughout t he Wollaston Lake area. A, Rabbit Lake area (after Hoeve and Sibbald , in press} ; 8, Co ll ins Bay area; C, Fife Island to Grant Island section; D, Wollaston Post Vici nity.

(3) . To the southwest however, near Rabbit Lake (Column A, Fig. 2) , these meta­ arkoses are overla in by t he "Hidden Bay Assemb lage" (Wallis 1971), consisting pre­ dominantly of quartzites (6) and amphibolitic r ocks (5) (Hoeve a nd Sibbald, in pr ess). This upper unit extends northwards (C olumns B, C, Fig. 2) but wedges out to t he eas t . The underlying meta- arkoses present in the Rabbit Lake v icinity (Column A, Fig. 2) also give way northwards into a thick sequence of semipelitic and peliti c rnetasediments (Column C, Fig. 2).

The pelites (2) couunonly contain garne t, cordierit e and sil limanite with sporadic pyrite-pyr r hotit e ; graphite is generally confined to the basal s equence but appears higher up the succession in the Rabbit Lake and Grant Island vicin ities (Column A & c. Fig. 2) . In places the pel itic and semipe litic rocks contain -28-

appreciable amount of hornblende and tremolite-actinolite (2g) and the unit is commonly interlayered with thin horizons of quartzite (2b), meta-arkose ) and calc-silicate or impure calcareous metasediment (2a).

The massive to well banded feldspathic quartzites (6) in the Hidden Bay Assemblage are commonly interlayered with quartzitic meta-arkoses (6b) and both

sporadically contain large -sillimanite faserkeisel (6a). The .;;_...... L.;;=-c'-'--_::::.-=-c:.- (5a) and (Sb) have an intimate association with the quartzites of the "Hidden Bay Assemblage", and this suggests they have a supracrustal origin.

A heterogenous (4) forms thick, persistent horizons throughout the Wollaston Group sequence. These rocks are highly variable in grain­ size, texture and mineralogy, but are characteristically quartz-poor with highly variable amount of tremolite-actinolite and/or diopside, albite, carbonate, epidote, sphene and scapolite. The assemblage includes massive (4a) and well =:::.'.L::-:.::..::.=-..::=.::.:.=::_._:=:::=.:::.:::.:=-..-"::::.:::c==:::;..;::.~ (4b) sporadically containing thin~~~ layers (4c). Other less common types include "plagioclasites" (4d), similar to those in the Rabbit Lake area (Sibbald 1977, Hoeve and Sibbald, in press) and (4e). Most of the assemblage is believed to originate from impure calcareous sediments, although the high sodium content of some types has led Weber et al (1975), Sibbald (1977) and Chandler (1978) to propose that they represent sediments enriched in evaporitic salts. The "breccias" (4e) form thin, essentially oligomyctic layers containing poorly sorted, sub-rounded to angular clasts of albite-oligoclase plagioclase up to 6 cm in dimension. Similar rocks elsewhere in the domain (Ray, in prep.) assayed over 9 percent Na2o, and are tentively believed to represent "solution collapse breccias" like those in the Devonian Prairie Evaporite of southern Saskatchewan (De Mille et al. 1964; Holter 1969).

Athabasca Formation (Z)

The western part of the area is unconformably overlain by the Athabasca Forma­ tion (Helikian), predominantly comprising and grits with thin conglomer­ ates (Ramaekers 1976). These fluviatile sediments mostly overlie rocks of the Mudjatik domain, but in the Collins Bay vicinity they overlap the Wollaston Group. The Athabasca basin dips gently west and the Formation is probably relatively thin throughout the mapped area since drilling at Mid-West Lake, just west of the present area suggests the Helikian is approximately 185 m thick (Northern Miner, 16th -29-

March, 1978). A rotted, hematite-stained saprolitic zone (regolith) up t o 50 m thick, which presumably formed during the Paleohelikian episode, i mmed­ iately underlies t he Formation. All observed saprolitic outcrops represent original granitic or pegmatic rocks and both liesegang banding and white reduction spots are present in plac.es.

The formation's eastern limi t is poorly defined and is probably extremely irregular; sever a l large and small outliers were located up to 3 km from the main Formation margin, while boulder float suggest outliers may exist under parts of Tromberg Bay (Hatchet Lake) and Cunning Bay (Wollaston Lake). Unexposed basement inliers probably also exist and su spect ed inlier s or areas where the ­ s tone is particularly thin are shown on t he accompanying geological map.

The formation is cut by numerous northerly trending faults, apparently having an easter l y downthrow. Parts of the Formation margin and the disposition of many basement inliers and sandstones outli ers ar e fault controlled. The relatively thin sandstone recorded at Mid-West Lake may partly r eflect the multiple westerly upthrow in many of these faults ,

Economic Geology

The whole r egion, particularl y that underlain by or adjacent to the sub­ Athabasca unconformity is currently be ing explored by numerous private companies and SMDC, primarily for uranium. Uranium mineralization in this and adjacent areas may be categorised as follows :

1. Weak mineralization hosted by pelitic metasediments or adjacent segregation pegmatit es . This widespread association is common throughout t he Wollaston and Mudjatik domains (Mawdsley 1957a, 1957b; Sibbald et a l. 1977) but has little economic significance. Occurrences in t he presently described area include traces of uraninite, c halcopyrite and pyrite in graphitic pe lites 5 km northwest of Topping I s land (Hatchet Lake) and minerali zed pegmatites west of Scrimes Lake, at the north end of Fife Island (Wollaston Lake) and north of Manson Lake. The last mentioned occurrence has been prospected and drilled by SMDC .

2. Sub-economic uranium mineralization associated with cal c- s ilicate rocks. These have been noted elsewhere in the Wollaston and Mudjatik domains (Sibbald et a l. 1977) and there are two showings of this type in the area. At the north end of Spurjack Island (Wollaston Lake) uraninite, apatite and flakes of molybdenite (up to 2 . 5 cm long) are hosted in irregular, coar se grained amphibolitic segrega­ tions within impur e calcareous metase dirnents. The other showing, on the east shore -30- of Harrison Peninsula (Wollaston Lake) occurs in a thin calc-silicate layer in the semipelitic sequence and includes traces of pyrite, pyrrhotite, uraninite, molybdenite and chalcopyrite.

3. High grade, economic "unconformity type" deposits (Beck 1977). Mineralization is generally concentrated at or near the contact between the sub-Athabasca unconfor­ mity and the underlying, highly altered metasediments. While many deposits (eg. Key Lake and Mid-West Lake) are associated with graphitic metasediments of pelitic to semipelitic character, the Raven and Horseshoe occurrences near Rabbit Lake are unique in being hosted by quartzites in the upper part of the Wollaston Group (Hoeve and Sibbald, in press). Similar quartzites are common in the northern and central parts of Wollaston Lake. However, they are considered poor exploration targets because, unlike the Raven and Horseshoes occurrences, they nowhere lie close to the sub-Athabasca unconformity. At Collins Bay (Wollaston Lake), Gulf Minerals Ltd. have discovered two uranium deposits, hosted by Wollaston Group graphitic pelites, close to the base of the succession. It is conjectured that the intensive drilling located approximately 1 km south-southwest of the Gulf Minerals exploration camp may outline the so-called "B zone". Furthermore, the orientation of the drilled strip suggests that part of the ore zone runs along a northerly trending lineament which forms one of many similar fractures crossing Collins Bay.

The newly-discovered elongate ore zone within graphitic pelites beneath approximately 185 m of sandstone at Mid-West Lake, just west of the present map sheet, may represent one of Canada's largest uranium deposits (Mining Journal, Sept. 15th, 1978). East of Mid-West Lake, approximately 5 km northwest of Torwalt Lake, Asamera Oil Ltd. have encountered encouraging uranium values; the location of some drill sites is shown on the accompanying geological map. On the basis of the present mapping, the following exploration targets are proposed at localities where graphitic pelites are adjacent to, or continue under the Athabasca Formation: 1. Approximately 3 km southeast of Campbell Lake. 2. Between 8 and 12 km northwest of Smith Bay (Hatchet Lake). 3. At the extreme southern portion of Topping Island (Hatchet Lake). 4. Between 8 and 10 km south of Topping Island (Hatchet Lake). 5, Approximately 5.5 km southeast of the south end of Tromberg Bay (Hatchet Lake). 6. On the western side of Cunning Bay (Wollaston Lake), approximately 10 km southeast of Lasby Lake. 7. Inunediately off the northeastern and northwestern shores of Harrison Island. 8. The channel separating Harrison Island and Harrison Peninsula. -31

Some of the main geological units presumed to underlie the Athabasca Formation which are tentatively outlined on the accompanying geological map, have been extra­ polated from aeromagnetic data on the assumption that low aeromagnetic expression represents metasediments. However, this interpretation is not entirely certain since many tonalitic bodies (E) are also marked by aeromagnetic "lows". Neverthe­ less, the sandstone in this vicinity is probably underlain by granitoid rocks sep­ arated by extensive pelitic belts, similar to those exposed north of Hatchet Lake and these graphitic rocks are considered to represent ideal exploration targets.

Some of the postulated pelitic belts between Lasby and Mallen Lakes run parallel to NE- to ENE-trending lineaments discernible in the overlying sandstones. These lineaments could represent low angle reverse faults similar to that present in the Rabbit Lake Mine (Knipping 1974; Sibbald 1976) and their presence may enhance the prospect for uranium mineralization in the underlying Such reverse faulting is likely to occur both along the graphitic horizons and -metasedimentary contacts.

The presumed junction between the Wollaston and Mudjatik domains in the present map area traces from the west side of Collins Bay, northeastwards to the Fife Island and Wheeler Peninsula vicinities. The rocks either side of this line exhibit con­ trasting lithological and structural styles that probably reflect fundamental differences in the thermotectonic histories of the two domains. In the Wollaston regime, the Archean basement comprises foliated granitoids of remarkably widespread uniformity which are overlain by Aphebian metasediments in which an extensive strati­ graphic succession is recognized. Structural deformation is dominated by early mantled gneiss doming with no evidence of major structural inversion. In contrast, no clear basement-cover relationship or metasedimentary sequence has been identified in the Mudjatik domain, which in this area is marked by the sudden appearance of quartzo-feldspathic migmatites (C) and tonalites (E), both of which are absent in the Wollaston regime. Moreover, the deformational and orientation in the Mudjatik domain is more variable and suggests that the intense migmatisation was accompanied by structural inversion and repetition of the units.

Although the relationship between the lithological units in the Wollaston and Mudjatik regimes is still uncertain, the compositional similarity and common association of U, Mo, Cu mineralized pegmatites suggests the pelite belts in both domains are correlable. This view is supported by lead isotope analyses (Sangster _32-

pers. comm.) which indicate that the Segment Lake Pb occurrence in the Mudjatik domain belongs to the same isotopic population as mineralized Aphebian samples from the Wollaston regime.

Although, the status of the Mudjatik granitoids is unknown, the quartzo-felds- pathic migrnatitic (C) are tentatively believed to represent Archean supra- crustal rocks. This interpretation implies profound differences between the Archean rocks in the Mudjatik and Wollaston domains, with the former representing Archean supracrustals and the latter originating from Kenoran plutons developed on a regional scale.

Mapping by various authors, including Munday (1973), Lewry (1974) and Sibbald (1973; 1974) led Sibbald et al. (1976) to conclude that the Mudjatik domain repre­ sents the core region of a Hudsonian Mobile belt involving the early development of flat lying migmatite lobes and nappes. This model could explain the major differ­ ences between the two domains in this area and suggests the Mudjatik rocks originated as a succession of recumbent nappes or thrust sheets, presumably cored by granitoids, which moved eastwards onto the autochthonous Wollaston domain. The early sliding accompanying nappe development was probably localized along the pelitic zones. Although to date, the identification of individual nappe or thrust sheets has been hampered by the subsequent overprinting of tight northeast trending folds, the junction between the Mudjatik and Wollaston domains in this area presumably repre- sents the edge of a plunging nappe lobe.

Beck, L.S. (1977): History of uranium exploration in Saskatchewan with special reference to changing ideas on metallogenesis, in Dunn, C.E., ed., Uranium in Saskatchewan: Sask. Geol. Soc. Spec. Publ., 3, pp. 1-10.

Chandler, F.W. (1978): Geology of part of the Wollaston Lake Fold Belt, northern Saskatchewnn, Can., Geol. Surv. Bull. 277.

De Mille, G., Shouldice, J.R., and Nelson, H.W. (1964): Collapse structures related to evaporites of the Prarie Formation, Saskatchewan. Bull. Geol. Soc. Amer. v. 75. p. 307-316.

Gilboy, C.F. (1975): Project 8: Foster Lake area, in Summary of Investigations, 1975, eds. J.E. Christopher and R. Macdonald; Sask. Dep. Miner. Resour.

Hoeve, J. and Sibbald, T.I.I. (in press): On the genesis of Rabbit Lake and other unconformity type uranium deposits in northern Saskatchewan, Canada. Uranium issue of Economic Geology, December 1978.

Holter, M.E. (1969): The Middle Devonian Prarie Evaporite, Saskatchewan. Sask. Dep. Miner. Resour. Rep. 123. -33-

Knipping, H.D. (1974): The concepts of supergene versus hypogene emplacement of uranium at Rabbit Lake, Saskatchewan, Canada. (IAEA-SM-183/38); in Formation of Uranium Deposits. Proceedings of a Symposium, Athens, 6-10 May, 1974. Int. Atomic Energy Agnecy, Vienna, pp. 531-549.

Lewry, J.F. (1974): Structural Relations in the La Loche (north) map sheet and adjacent areas; in Summary Report of Field Investigations, eds. L.S. Beck and R. Macdonald. Sask. Dep. Miner. Resour.

Mawdsley, J.B. (1957a): The geology of the Middle Foster Lake Area, northern Saskatchewan; Sask. Dep. Miner. Resour., Rep. 26.

(1957b): The geology of the Charlebois Lake Area, Saskatchewan; • Dep. Miner. Resour. Rep. 24.

Munday, R.J.C. (1973): 74-B-NE: Mudjatik (NE); in Summary Report of Geological Investigations, ed. L.S. Beck. Sask. Dep. Miner. Resour.

Potter, D. (1977): La Ronge-Wollaston Belts Base Metals Project: Sito Lake Area. In Summary of Investigations 1977, eds. J.E. Christopher and R. Macdonald, Dep. Miner. Resour.

Ramaekers, P. (1976): Athabasca Formation; Northeast edge (64L, 74I, 74P): Part I, Reconnaissance Geology, in Summary of Investigations 1976, eds. J.E. Christopher and R. Macdonald; Sask., Dept. Miner. Resour.

Ray, G.E. (1975): Project 5: Foster Lake (NE) - Geikie River (SE) area, in Su11m1ary of Investigations 1975; eds. J.E. Christopher and R. Macdonald; Sask. Dep. Miner. Resour.

(1977): The Geology of the Highrock Lake - Key Lake Vicinity: Sask. Geol. Surv. Rept. 197, 29p.

(in prep.): The Geology of Daly Lake (west) and part of the Middle Foster Lake Areas. Sask. Geol. Surv. Rept. 208.

Sibbald, T.I.I. (1973): 74-B-NW: Mudjatik (NW); in Summary Report of Geological Investigations, ed. L.S. Beck, Sask., Dep. Miner. Resour.

(1974): Project 7: La Loche (North) area; in Summary Report of Investigations, eds. L.S. Beck and R. Macdonald, Sask., Dep. Miner. Resour.

(1976): Uranium metallogenetic studies, I Rabbit Lake, in, topher, J.E. and Macdonald, R. ed., Summary of Investigations 1976, Sask. Geol. Surv., pp. 115-123.

(1977): Uranium metallogenic studies, II Rabbit Lake, Summary Report of Field Investigations, eds. J.E. Christopher, and R. Macdonald, Sask. Dep. Miner. Resour.

Sibbald, T.I.I., Munday, R.J.C. and Lewry, J.F. (1976): The geological setting of uranium mineralisation in northern Saskatchewan. Sask. Geol. Soc • . Publ. no. 3 (ed. C.E. Dunn): in Uranium in Saskatchewan, ed. C.E. Dunn, Proceedings of Symposium, Regina, 10 Nov. 1976, Sask. Geol. Soc., Spec. Publ. no. 3, pp. 51-98. -34-

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

Weber, W., Schledewitz, D.C.P., Lamb, C.F., and Thomas, K.A. (1975): Geology of the Kasmere Lake-Whiskey Jack Lake (North Half) area, Manitoba; Manitoba Min. Res. Div. Geol. Services Branch Publ. 74-2.