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Reconnaissance Geology: Wollaston Lake

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Reconnaissance Geology: Wollaston Lake - 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 granulite 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 quartz-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 sandstones 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 weathering 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 sandstone 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).
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