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Reconnaissance Bedrock Geology: Wollaston Lake Northeast Area (Part of NI'S 64L)

by l:l. 11. MacQuarrie

Introducticn

'Ihe Precarrbrian bedrock in the area may be divided into two strikingly different geologi­cal terrains: the Wollaston danain and the Peter Lake danain (Ray, 1979). 'Ille Wollaston danain ccosists of an Aphebian rretasediITentary sequence folded along nearly horizootal north­east-trending axes , with felsic plutcru.c "baserrent" inliers . 'Ihe Peter Lake danain consists alrrost entirely of plutooic rocks, with rennants and )(E!l10li ths of supracrustal rocks , rrost of which appear to be of volcanic origin.

Rodes of the Wollaston Darain

Granitic "Baserrent"_lbcks_jUnit .ll

The granitic "baserrent" rocks include the Johnson River granite and a large plutcnic mass south of Sava Lake . 'Ihese rocks are rrostly well exposed, unlike the rocks of the Wollaston group. They c:xntain magnetite and are well outlined by aeraragnetic maps. The Johnsm River mass consists of uniform pink biotite-bearing granite to granod.iorite. 'Ihe Sava Lake body is Im.lch less uniform, and is cc,np:sea of granocliorite to granite with variable prcportioos of biotite , hornblende, and magnetite. Its central parts are ooly weakly foliated and contain irregular patches of charnockitic rocks .

Enderbi tic_ "Baserrent " Ro:::ks (Unit _11

Very little of the large mass of hypersthene­bearing plutcnic rock in Mani tcba (Weber , et al., 1975) appears to extend into Saskatchewan. The only outcrq:, of enderl>ite encotmtered is a grey , rredium-grained, weakly foliated rock consisting of plagioclase , hornblende , quartz, biotite and hypersthene.

~ibolite {Unit 3)

Both the Johnson River and Sava lake bodies contain narro,,r (lOm) sills of amphibolitic rocks . These arrphilx>lites are rrediun-grained, dark- green rocks made up of plagioclase and hornblende, with minor biotite.

Meta-volcanic_Hornblende Gneiss_JUnit 4t A few small outcrops of neta-volcanic rocks were fotmd east of Gallagher Lake . A dark-

green ma.fie variety (hornblende gneiss) and a pale grey felsic variety (porphyroblastic hornblerrle-biotite quartzofeldspathic gneiss) were recognized, but no ccntacts were seen.

Pelitic Biotite Gneiss_ (Unit_S)

Coarse-grained pelitic gneisses are present at the base of the Wollaston Group through­out the Wollastcn dona.in . Scm:! of the peli tes cx:ntain disseminated graphite, and rrost of the E'.M conductors in the area seem to be assoc­iated with pelitic bands. '!he pelites in sare places a:ntain large porphyrcblasts of potash feldspar, plagioclase, cordierite , or garnet. Cordierite and sillirr.anite are widespread but garnet is restricted.in distri­bution.

Psarnn:irtic Biotite ~SS (Unit 6)

Well-foliated quartzofeldspathic gneisses, including arkoses , quartzites, semipelites, and mi.oor calcareous psanmi tes, are widespread . Sare , including a widespread porphyroblastic cordierite-potassium feldspar semipelite (Unit 6a) , are J!leterogenous, banded rocks that might be highly defamed conglrnerates . The disseminated magnetite occuring in sare psamnitic units may have originated as detritus fran magnetite-bearing "baserrent" pl utooic rocks .

Calcareous Psarrmites_(Unit 7)

calcareous psamnites occupy a large synclinal structure extending fran (l)ack Lake to Kingsley Lake. '!hey are greenish-pink to white gnessic rocks c.arposed of plagioclase, potassium feld­spar, quartz, biotite, hornblende, diopside and magnetite . calcareous psamnite is equiv­alent to the rreta-arkose (Unit U) of Weber et al. , (1975), the calcareous psarrmite (Units Sa and Sb) of Ray (1979) and the calcareous rreta- arkose (Unit 3b) of Ray (1978) .

"Calc-silicate_Rocks"_ (Unit 8)

'lhe ll'C6t ccmron type of "calc-silicate gneiss" is a coarse-graired pink- green or white-green mfoliated rock consisting of plagioclase, diopside , trerrolite-actinolite , zoisite and st*Jere . In sare pl aces it appears to be brecciated. It grades into rredium-graired foliated varieties of approximately the sarre carpositicn. Scapolite, phlogopite , and spinel occur locally . unfoliated varieties resenble

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altered plutcru.c rocks, but their mineralogy and their ca1finerrent to persistent strata suggest that they are rretasedirrentary. The absence of foliaticn seem to result fran post-defornational crystallizaticn or fran a resistance to bulk deforrraticn.

~ta-e:troxenite_cumt 9)

Isolated outcrops of wroxenite occur north­east of Kitely Bay. '!he wraxenite is a dark­green, coar~ained, weakly foliated rock cmsisting of hornblende psei.rlarorphic after pyroxene, and minor plagioclase.

Late Granodiorite and Tonalite (Unit 10)

Unit 10 inclu::l.es a variety of felsic plutcnic rocks apparently not part of the "basenent" and of uncertain oontact relationship. No clear cut carpositicnal or textural criteria were found to distinguish t:l'ese rocks fran the baserrent plutcnic rocks (Unit 1) . Unit 10 rocks in the vicinity of Kitely Bay and the north are magnetite-bearing grandiori tes . 'lhe poorly exposed area around Kames Lake is uooerlain by a rredium-grained well-foliated granodiori te. Several snaller bodies of felsic plutcnic rock in the northwest part of the map area have been assigned, sarewhat arbitrarily, to Unit 10 rather than to Unit 1.

Pesmatite_ (Unit 11)

Pegmatites occur sporadically throughout the map area, IT'OStly as dikes t,oo SJ!'all to map ~ large sill-like bcx1y of pink pegm:itite cx::curs near Waspison Lake.

Diabase_Di.kes_(Unit_l2l

'Iwo major dikes of diabase strike across the map area in a northwesterly direction. In areas of lo,., magnetic relief they sho,., up as distinctive l:inear anomalies on aeranagnetic maps. '!be only dike of diabase seen, south­west of Lay Lake, is less than 2 m wide and cifPears to be subsidiary, as it is too narro,., to aa::ount for the anomaly. '!be diabase is a dark-green fire-grained to aphanitic rock with a weakly foliated margin.

Rocks of the Peter Lake Domain

'lhe southeastern oorner of the mapped area lies in the Peter Lake danain. Excellent exposures in the vicinity of Zangeza Bay have permitted subdivisim into three major groupings:

III Zangeza Bay granitic ccrrplex II Peter Lake Caiplex I "Early supracrustal gneisses"

I. Early Supracrustal Greisses

'lhe "early supracrustal gneisses" correspond to Shklanka's (1962) Unit 1, "rretano.q:hosed volcanic and sedilrentary rocks'', and to Ray's (1979) Unit s , "possible supracrustal gneisses". Ray has suggested that these rocks may be highly defamed equivalents of the canp:iell River Group (Lewry, 1976) • 'lhree lithological types have been distinguished here: pelite (Unit B) , banded quartzofeldspathic gneiss (Unit X) and mafic rretavolcanic rocks (Unit W).

Pelitic_Biotite Q1eiss (Unit B)

Pelite occurs as a large xenolith in granite on the western shore of Zangeza Bay. 'Il1e rock is a rrediun-grained dai:k-grey banded gneiss cx:l!i)Osed of biotite , feldspar, quartz, garnet and cordierite .

A narrc:M peli tic band occur:ing in a large xenolith of pyrite-bearing rhyolite (Unit X) on the western shore of Zangeza Bay about one kilorcetre no~st of carletcn Island is a soft black rrediun-grained rock ccnsisting alnost entirely of biotite with scattered graphite.

Banded Quartzofelds~thic Biotite Gneiss (Unit_X)

Fine- grained pink to grey quartzofeldspathic gneisses are present throughout the Zangeza Bay area. 'Ibey generally occur as xenoliths in fe lsic plutcru.c rocks, but relatively undisturbed strata are present north of Zangeza Bay. 'lhe rock is well-banded (0.5 to San bands) and carrronly carplexly folded en a small scale .

The quartzofeldspathic gneisses prcbably originated mainly as felsic volcanic (or pyroclastic) rocks, as north of Zangeza Bay they grade into and interfinger with ma.fie rretavolcanic rocks. A volcanic origin is also suggested by the presence of narro,., bands rich in disseminated hornblende or pyrite. A volcanic orig:in is not indicated everywhere, h~ver, and sorre may be psanmitic or defamed plutcnic or hypabyssal rocks.

M:!tavolcanic Hornblende G1eisses (Unit Wl_

Banded and unhanded mafic volcanic rocks occur north of Zangeza Bay and sporadically as xeroliths. They cx:rrprise stroogly foliated fine- t o nedium-grained arrphibolites and horn­blende-plagioclase-quartz gneisses. 'lhe absence of calcareous and markedly heterogeneous varities and the rarity of intemedded sedi­nentary rocks suggests that these rocks originated as flcMs rather than as pyroclastic rocks. No primary volcanic structures have

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been identified, h~ver1 and S(ll);! ot the massive varities nay h.;we ori9i.nated as nafic sills.

II. Peter Lake Conplex

'!be Peter Lake Cbrplex in the area ccnsists of ~ditm1- to ooa.rse-grained plutooic rocks that exhibit a oaiplete range of CX)I'[J)Ositions fran granod.iorite to gabbro. Except in granodiorite, quartz is not aburrlant, and quartz nni.zonites and rronzonites are rrore ccmrcn than tanalites and quartz diorites. Contacts between types are both gradational and intrusive and felsic types generally intrude the rrore nafic. All types range fran foliated to unfoliated, probably doo to local resistance to bulk defonnaticn rather than to any significant age difference.

Mafic Plutonic Rocks_(UnitM)

Gabbro, diorites, m:nzcnites, and sore quartz diorites occur as large bodies west of Zangeza Bay, as xenoliths in felsic, plutonic rocks, and as lenses in the nafic imtavolcanic rocks and banded quartzofeldspathic gneiss which they appear to intrude. 'lhe rocks are generally equigranular, rrediurn- to coarse-grained, and cx:nposed of hornblende and feldspar. 'lhey are themselves o::mronly int.ruled and brecciated by felsic plutonic rocks.

Felsic_Plutcnic Rocks (Unit G)

Felsic plutcnic rocks form a batholithic carplex that appears to make up for the bulk of the Peter Lake domain. 'lhe rrost comron variety within the nap area is an unfoliated to strongly foliated coarse-grained rnega­crystic quartz rronzonite. The rnegacrysts, carprising microcline crystals up to 20 cm across and smaller plagioclase crystals, are pretectonic, for they have been rotated and do not grow across foliation planes. CUartz m:mzonite locally grades into nore quartz-rich (granodiorite) and less quartz-rich (nonzonite) and less potassic (quartz diorite) variebes.

III. zangeza Bay Cotplex

'lhe Zangeza Bay o::rrplex ccnsists of pink granite or granodiorite plutcnic masses (Unit J), and a few later lanprophyre dikes. '!he dikes range in width fran 1 to 4 m, too srrall to be indicated on the accatpanying map and have a strcng primary foliaticn and are discordant to the regional structure. The lanprophyre is a dark rredi.mrgrained biotite-rich rock, with hornblende and feldspar .

Qfani te (Unit J)

'lhe Zangeza Bay granite is a rredi.tm1-grained pink foliated to unfoliated quartz-rich granite or granodiorite. Although it irregularly intrudes the felsic plutonic rocks (Unit G), it is not always readily distinguished from them. 'lhe granite contains, besides quartz and feldspar, about 5 percent biotite and O to 20 percent hornblende. Greater aJlOU'lts of hornblende (5 to 20 percent) are present only when there are nurrerous nafic xenoliths, and clearly result fran o::ntamina­tion.

Finer-grained, aplitic or alaskitic varietie s are gererally unfoliated. Pegmati tes, which occur as irregular narr CM dikes throughout the Zangeza Bay area, are particularly closely associated with aplitic varieties. The aplitic type grades into sarewhat coarser, greyer, and rrore foliated types.

Foliated granites contain xenoliths that are cxmronly defonred parallel to the foliation, the direction of which is often highly variable over short distances. Although ncne of these elcngated xenoliths appear to have been folded, their "fold axes" are not CO'lsis­tent, and the oaiplex nature of deformation is cerrcnstrated by the occurrence of a few xeno­liths that have been "rolled up" . 'Ihe defonnation and foliation probably resulted fran viscous intrusion of the granitic magma.

1-Etarcorphism and Stroctural Geology

1-Etarrorphism in the Wollastm danain r anges fran upper anphiboli te to lower granuli te grades and the grade seems to increase from southeast to northwest. In detail, h~ver, the netanorphic isograde may be carplex and related to fold structures.

'lhe Wollastcn dona.in seems to be folded on northeasterly subhorizontal axes. Folds in the no.titrn-.estem part of the belt are alrrost isoclinal and have nearly vertical axial planes, whereas those in the southeast seem to be rrore ~n and their axial planes dip nortm.est.

No major faults have been reoognized. A large mrrber of north-south striking fractures prd:lably cross the area, but only a few show up as lireanents, primarily because of the poor bedrock exposure.

Ecalani.c Geology

'!here is sore potential for volcanoqenic

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nassive sulphide mineralization in volcanic rocks of the Peter Lake dcmain, However, the distributioo of volcanic rocks is restricted, and no occurrences of chalccpyrite or sphaler­ite have been found. There is also scree JX)tential for ea::manic mineralization in the plutonic rocks . Chalcx:,pyrite mireralization associated with rmfic plutcnic rocks , like that east of Haqlund Lake described by Potter (1979, undoubtedly occurs elsewhere in the dare.in. Molytdenite was found in apparently subeconanic anounts as widely disseminated flakes in a narrow veinlet of flourite bearing granite northeast of Dauk Lake. '!he two nolybdenite occurrences noted by Shklanka (1962) are in a similar regional setting.

'Ihe Peter Lake domain might be ccnsidered for

inclusion in any broad recormaissanoe evaluation of prospective tungsten, tin and llPlybdenum minaralization.

No rew occurrences of eccnanic mirerals were found in the Wollaston danain . Exploraticn in this part of the domain is haITpered by JX)Or exposures arrl the clayey nature of much of the overburden which tends to inhibit radon anomalies. Nevertheless, urani um and base rretal potential appears to be gcod. 'Ihe calcareous rnetased.irrents may be suitable hosts for Pine Point type lead- zinc mineralization. 'Ihe JX)Or exposure and ncn-conductive nature of targets suggest the need for e~loration by boulder or "micro-boulder" tracing.