A Re-examination of the Context of U-Cu, Cu, and U Mineralization, Duddridge Lake, Wollaston Domain 1

G.D. Delaney

Delaney, G.D. (1993): A re-examination of the context of U-Cu, Cu, and U mineralization, Duddridge Lake, Wollaston Domain; in Summary of Investigations 1993, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 93-4.

During a five week period in the summer of 1993, 1: 1 O 000 scale mapping was completed of a 30 km2 area between Duddridge Lake and Walsh Lake, in the south-central part of the Sandfly Lake mapsheet-area (NTS 730-9). The area comprises a north-northeast­trending strip along the eastern margin of the Wollaston Domain, and flanks the unconformable eastern contact of supracrustal rocks with a heterogeneous granitoid ter­rane containing supracrustal remnants (Figure 1 ). The purpose of this investigation was to re-examine the stra­tigraphic and structural context of the sediment-hosted Thor U-Cu showing as well as other types of mineraliza­tion in the Duddridge Lake area. This work will provide a starting point for a re-evaluation of the stratigraphic and sedimentological context of base metal mineraliza­tion in supracrustals of the Wollaston Domain.

1 . Previous Work

In 1949, Frarey (1950) undertook reconnaissance geo­logical mapping of the lie-a-la-Crosse area. During the summer of 1962, Money (1965) mapped the Duddridge Lake area as part of a three year program of 1 :63,360 mapping of the Sandfly Lake (east half-730-9), Black Bear Island Lake (west half-73P-12), and Eulas Lake (west half-73P-13) map areas. This mapping also formed the basis for Money's Ph.D. dissertation entitled "The Precambrian geology of the Needle Falls area", completed at the University of Alberta in 1967.

On the basis of fieldwork during the summer of 1974, and a compilation of known geological information, Mun­day (1974, 1978) prepared a 1 :100 000 compilation map of the geology of the Precambrian Shield rocks of the eastern part of the lie-a-la-Crosse map-area. In 1975, Haughton (1976), of the Saskatchewan Research Council, completed a multi-media geochemical survey of the area containing the mineralized boulder train as­sociated with the Thor Cu-U showing at Duddridge Lake. In 1976, Bretzlaff completed a B.Sc. thesis enti­tled "Genesis of uranium mineralization at Duddridge Lake, Saskatchewan". In 1978, Coombe and Potter mapped a strip from Duddridge Lake northeast to Mey­ers Lake at a scale of 1 :50 000 (Coombe, 1978b). This work formed part of a study of base metals in the Wol­laston Domain (Coombe, 1977, 1978a, 1979, in prep; Coombe Geoconsultants, 1991 ). Also in 1978, the Sas­katchewan Research Council studied the geochemistry of basal till down ice from the Thor Cu-U showing

(Sopuck and Lehto, 1979). The economic geology sec­tion of this report summarizes several mineral explora­tion programs which have also taken place.

2. Regional Geological Setting

The Duddridge Lake area lies near the eastern side of the Wollaston Domain (Figure 1; Lewry and Sibbald, 1977), a northeast-trending, generally tightly folded lin­ear belt of siliciclastic metasediments and minor meta­volcanics mantled and segmented by interfolded remo­bilized Archean granitoids (Money, 1968; Money et al., 1970; Ray, 1979; Lewry and Sibbald, 1979, 1980; Ray and Wanless, 1980; Lewry, 1981; Stauffer, 1984). The domain has been classified as the easternmost lithos­tructural subdivision of the Cree Lake Zone which is in­terpreted to have "evolved by thermally driven remobili­zation of Archean continental basement and Aphebian shelf-miogeoclinal cover" (Lewry and Sibbald, 1980, p74).

The Wollaston Domain is bounded on the east by the Needle Falls Shear Zone (Munday, 1974; Ray, 1974; Stauffer and Lewry, 1988), a fundamental crustal struc­ture (Lewry and Sibbald, 1980). Although metamorphic grade is lower amphibolite facies in places along the western boundary, the grade increases rapidly to the west to upper amphibolite-granulite facies (Munday, 1978; Lewry et al., 1978). The western boundary is with the Mudjatik Domain, which forms the high-grade core of the Cree Lake Zone, and is marked by a change in structural style from linear to arcuate. Compositionally heterogeneous granitoids predominate and supracrustal rocks, although in structural continuity with those of the Wollaston Domain, are subordinate (Lewry and Sibbald, 1980).

Throughout much of the Wollaston Domain two main episodes of deformation are distinguished (op cit). The first event (01) formed a prominent foliation that is typi­cally parallel to original layering and is interpreted to have paralleled the basement cover contact. The sec­ond major distinguishable event refolded 01 structures into tight northeast-trending, commonly doubly plunging folds. As this corresponds to D3 deformation in the adja­cent Mudjatik Domain, that designation was applied in the Wollaston Domain. Other, generally minor, post 03 deformations are recognized locally such as in the Dud­dridge-Meyers Jakes area (Coombe, 1978b), the Pen-

(1) Saskatchewan Project A.12b was funded in 1993 under the Canada-Saskatchewan Partnership Agreement on Mineral Development 1990-95.

Saskatchewan Geological Survey 73

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Figure 1 - Major /ithostructural elements in the central part of the Precambrian shield of northern Saskatchewan (modified after Macdonald and Broughton, 1980).

74 Summary of Investigations 1993

delton Lake area (Scott, 1973), and the Hidden Bay area (Wallis, 1971).

3. General Geology With the exception of areas to the east of Walsh Lake and west of the narrows on Duddridge Lake, outcrop is generally sparse because of thick glacial cover. The dominant structural grain in the Duddridge Lake area is north-northeasterly. The eastern part of the area is un­derlain by a heterogeneous granrtoid terrane that in­cludes monzodiorite, felsite, granite, and diorite; locally there are abundant xenoliths of biotite granulite and gneiss.

The granitoids are overlain unconformably to the west by the Meyers Lake Group, a thick succession of silici­clastic sediments. This sequence includes a basal quartz pebble conglomerate which is overlain by quartz­ite. This is followed by a thick sequence of arkose, con­glomerate, and grit with intercalated pelites. Despite evi­dence of multiple deformations, such as small scale folds and strike parallel high-strain zones, this sedimen­tary sequence appears to essentially young to the west with only minor structural repetition. There is an abrupt rise in metamorphic grade to the west resulting in re­crystallization and obliteration of original textures and structures.

Carbonaceous-bearing lenses in hematitic arkose within the arkose sequence host uranium-copper mineraliza­tion at the Thor showing. Minor copper mineralization also occurs in the same red bed arkosic sequence. Anomalous radioactivity is also associated with the ba­sal quartz pebble conglomerate and some pegmatites.

4. Descriptions of Rock Types

a) Older Supracrustal Rocks

Xenoliths of supracrustal rocks occur in the granitoids that underlie the east part of the area (Figure 2). Among these xenoliths, hornblende blotite granulite and schist are more abundant than biotitic quartzofeldspa­thic gneiss.

Hornblende-biotite Granulite and Schist (unit HBS)

Xenoliths of fine-grained to less commonly medium­grained, dark grey hornblende biotite granulite and schist occur north of Duddridge Lake. Xenoliths range in size from a few centimetres in diameter to several hundred metres in length, such as the conformable body 1.5 km east of the south end of Walsh Lake which is over 100 m wide and several hundred metres long. Unit HBS is composed of 40 to 45 percent biotite, 15 to 30 percent hornblende, 20 percent plagioclase, 10 to 12 percent quartz and minor epidote, sphene, and mag­netite. Locally there are abundant foliated leucocratic quartz-feldspar veins.

Unit HBS is correlated with those parts of Money's (1965) unit 2, consisting mostly of hornblende-biotite gneiss and granulite, that occur in the granitoids and

Saskatchewan Geological Survey

migmatitic derivatives east of the supracrustals. In the northeastern extension of the granitoids in the Hewet­son Lake map area, lenses, layers, and xenoliths of am­phibolite are common (Ray, 1981). The unit HBS xeno­liths are probably the remnants of volcaniclastic rocks.

Quartzofeldspathlc Gneiss (unit QFN)

To the east of the large island at the north end of Dud­dridge Lake, enclaves of fine-grained, pink to buff weathering, compositionally layered, biotitic quartzofeld­spathic gneiss occur in felsite. These xenoliths might be derivatives of immature quartzofeldspathic sediments or volcaniclastics.

b) Younger Supracrustal Rocks: Meyers Lake Group

Quartz Pebble Conglomerate (unit QC)

A sequence containing quartz pebble conglomerate oc­curs at the western contact of the granitoid terrane (Fig­ure 2). Unit QC varies in thickness from 2 to 3 m on the large island at the north end of Duddridge Lake to ap­proximately 40 m east of Walsh Lake. Previous authors traced this unit for another 35 km north-northeast of the current mapping (Money, 1965; Coombe, 1978b). Money (1965) estimated that this unit has an apparent thickness of 30 to 122 m; in contrast, Coombe reported typical thicknesses of 35 to 50 m. At one location on the west limb of a syncline near Webb Lake the appar­ent thickness is about 400 m (Money, 1965) and at other localities the conglomerate is absent. On the ba­sis of the degree of flattening of quartz pebbles Money ( 1965) calculated that the original thickness of this unit was probably one and one half to three times the cur­rent thickness.

The quartz pebble conglomerate is characterized by an intact to disrupted framework of milky to smoky quartz pebbles. Although no granitic clasts were noted, Coombe (1978b) observed that these become progres­sively more abundant to the north. Beds, typically less than one metre thick, are broadly lenticular on a scale of metres to tens of metres; some exhibit apparent nor­mal grading. Locally coarse porphyroblasts of micro­cline are abundant. Intercalated with the quartz pebble conglomerate are thin to medium beds of quartzite, grit, conglomeratic quartzite, and biotite schist. Some of the quartzite and grit beds are trough cross laminated, al­though even, parallel laminations are more common. Both Coombe (1978b) and Munday (1978) observed fin­ing upward cycles defined by beds of quartz pebble con­glomerate overlain successively by cross-bedded quartz­ite, structureless sandstone, and argillite. Cross-bed­ding, scour surfaces, and normal grading consistently in­dicate that tops are to the west. On a formation scale this unit also fines upward. On the southeast side of Duddridge Lake, scarce anhedral porphyroblasts of gar­net occur in the matrix of the conglomerate; in contrast these porphyroblasts are abundant in lenses of interca­lated biotite schist.

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Figure 2 - Geological sketch map of the Ouddridge Lake area.

Summary of Investigations 1993

In exposures on the southeast side of Duddridge Lake and on the large island at the north end of the lake, the contact between the quartz pebble conglomerate and granitoids is covered. To the north of Duddridge Lake the contact is commonly masked by a high-strain zone. Rocks in this high-strain zone typically weather pink to salmon and have high-magnetic susceptibility. At one lo­cality 2.3 km north of Duddridge Lake (UTM co-ordi­nates 430330m E, 6161370m N) the basal quartz peb­ble conglomerate bed displays a scoured contact with a large xenolith of melanocratic biotite-hornblende gneiss in monzodiorite. Although other workers (i.e. Munday, 1978) have postulated that the quartz pebble conglomer­ate marks an unconformity, this observation definitively supports this relationship.

Quartzite (unit Q1)

There is a gradational contact between the uppermost bed of quartz pebble conglomerate and an overlying se­quence of quartzites containing subordinate muscovite schist and laminated argillite (unit 01). This unit ranges in thickness from 37 m on the southeast side of Dud­dridge Lake to about 460 m at the north end of the map area. Regionally, 01, which has a similar geographic distribution as unit QC has been estimated by Money {1965) to be between 460 and 610 m thick, and by Coombe (1978b) to be 300 m thick.

Unit 01 is characterized by cream, buff or light grey weathering, medium to thick beds of fine-grained quartz­ite commonly in packets of several beds. Many of these beds are massive or parallel laminated. Cross-bedding is rare. Quartzite beds alternate with layers of light greenish grey biotite-muscovite schist containing lami­nae and lenses of quartzite. Coombe (1978b, p101) ob­served that "cyclical units in this part of the succession are 15 to 25 m thick and comprise a basal massive quartzite overlain by quartzite and intercalated schists capped by a thin schist layer". Rare beds of quartz peb­ble conglomerate are intercalated in this sequence. Some thick beds of glassy quartzite occur near the base of 01. Locally actinolite is concentrated along folia­tion planes in the quartzite. Intercalated within the quartzite-biotite-muscovite schist succession are inter­vals of brownish grey weathering. compositionally lay­ered biotite-muscovite argillite-siltrte. One of these inter­vals forms a prominent marker horizon in 01 north of the map area (Coombe, 1978b).

In a few outcrops, some of the more micaceous layers are cut by a northeast-trending crenulation cleavage that dips steeply to the west. Locally the sequence is deformed into tight small-scale folds that generally dip moderately to rarely steeply to the southwest and or rarely to the northeast. Primary structures such as cross-bedding and scour surfaces, common near the base of 01, consistently indicate tops to the west.

The lower contact of unit 01 is placed at the top of the uppermost bed of quartz pebble conglomerate. Rare ex­posures indicate that on the west side 01 typically grades into a sequence of biotite schist and plagioclase porphyroblastic biotite schist.

Saskatchewan Geological Survey

Biotite Schist (unit BS1)

A thick interval of biotite schist underlies the west side of Duddridge Lake. Other than in a few outcrops at the south end of Walsh Lake and one outcrop on the east side of Poulin Lake, this unit is known only from dia­mond drill core (SEM Assessment Files 7309-0012 and -0016). At the south end of Duddridge Lake the biotite schist is apparently intercalated with biotitic quartzofeld­spathic gneiss. The schist is typically dark grey to black, strongly foliated, and locally crenulated. Single grains or clots of magnetite occur locally.

Plagioclase Porphyroblastic Biotite Schist (unit BS2)

Unit BS2, which comprises plagioclase porphyroblastic biotite schist and biotite schist occurs in a northeast­trending belt that extends from the north end of Dud­dridge Lake to the northeast side of Walsh Lake (Coombe, 1978b). Diamond drilling revealed that this unit also underlies the north part of Duddridge Lake (SEM Assessment File 7309-0016). Apparent thickness varies from about 80 to 120 m. Another large belt of 882 has been traced from the Churchill River to 3.3 km north-northeast of Brunning Lake where it pinches out (Money, 1965; Coombe 1978b). This unit also forms a subordinate component of the thick interval of biotite schist (unit BS1) lying west of Duddridge Lake. Other smaller bodies of this unit have also been observed.

The schist weathers grey to dark grey, and contains abundant, foliation flattened and rotated medium- to coarse-grained porphyroblasts of plagioclase in a matrix of biotite and less commonly minor hornblende. Dia­mond drilling under Duddridge Lake, on the strike pro­jection of unit 882 at a location approximately 1 km southwest of the north shore of the lake (UTM co-ordi­nates 428530m E, 6158660m N), intersected two inter­vals of plagioclase porphyroblastic biotite schist flanked and segmented by pyritic biotite schist that includes some carbonaceous material (SEM Assessment File 7309-0016).

Biotite-muscovite Schist (unit MS1)

A large body of biotite-muscovite schist underlies the west side of Duddridge Lake. This unit pinches out on the north side of Duddridge Lake and continues south of the map-area. Biotite-muscovite schist is also interca­lated in arkosic facies (units A1a to A1f) but the layers are generally too thin to be indicated on the map. Al­though MS1 has an apparent thickness of as much as 500 to 600 m, a true estimate is hard to accurately de­termine, because locally pegmatite comprises as much as 50 to 60 percent of the section. Local structural com­plications further hinder accurate thickness determina­tions.

The schist is typically a grey to buff grey, fine- to me­dium-grained rock containing foliae of mica, one or two grains thick, in a quartzofeldspathic matrix. Muscovite and biotite typically compose about 20 percent of the rock. Locally the schist is compositionally layered. This unit is typically crenulated; crenulations are axial planar to small scale folds. Millimetre-thick, medium- to coarse-

77

grained quartz-feldspar veinlets, aligned parallel to folia­tion, are common and are boudinaged locally. Unit MS1 weathers various shades of grey to brownish grey; some foliae and fracture surfaces are coated with limo­nite and or hematite.

Biotite-muscovite Schist with Intercalated Sand­stone (unit MS2)

Within the biotite-muscovite schist (unit MS1) there are intervals that contain thin to medium beds of buff to light grey weathering, fine-grained sandstone that alter­nate with laminae and thin beds of biotite-muscovite schist and siltite. The thicker sandstone beds are typi­cally dissected by numerous grey quartz veins.

Arkose, Conglomerate, Quartzofeldspathic Gneiss and Grit (units A1a to A1f)

Arkose, conglomerate, grit, and metamorphic deriva­tives of these rocks form a significant component of the succession to the west of the quartzite. Many of these units have gradational contacts both laterally and verti­cally. Six subunits, A 1 a to A 1f, are distinguished.

Conglomerate-Heterogeneous Quartzofeldspathic Gneiss (subunit A1a)

Conglomerate and banded quartzofeldspathic gneiss, in­terpreted to be derived from the conglomerate, are ex­posed in small isolated outcrops on the west side of the biotite-muscovite schist. The banded quartzofeldspathic gneiss predominates farther to the west. This unit ap­pears to be intercalated with other facies including grit and biotitic quartzofeldspathic gneiss.

Conglomerate beds as much as a few metres thick, are characterized by an intact to disrupted framework of variably flattened buff to pink quartzofeldspathic clasts of pebble to cobble-size. Conglomerate beds alternate with intervals of homogeneous arkose.

The banded quartzofeldspathic gneiss is a strongly foli­ated, banded, light grey to pinkish light grey, fine- to me­dium-grained rock composed predominantly of quartz and feldspar, and minor muscovite and biotite. Banding is defined by pink to salmon, fine-grained, sharply bounded, feldspar-rich lenses typically a few millimetres thick and up to a few centimetres long. Banded inter­vals alternate with those of homogeneous quartzofeld­spathic gneiss. The banded gneiss commonly weathers a mottled pinkish buff to pinkish light grey. In exposures where foliation is not as strongly developed, the feld­spar-rich lenses are unequivocally leucocratic granitoid clasts forming an intact to disrupted framework. This unit contains disseminated hematite locally.

Biotitic Quartzofeldspathic Gneiss (subunit A 1b)

This unit, which occurs to the west of Duddridge Lake, is intercalated in a variety of the other quartzofeldspa­thic facies and at least locally appears to have a grada­tional contact with biotite schist. The rock is typically strongly foliated with 5 to 1 5 percent biotite dissemi­nated in a framework of quartz and feldspar. In some

78

places the biotite is distributed fairly uniformly on the outcrop scale; elsewhere the biotite content occurs in layers. This is a strongly foliated rock; locally the folia­tion is crenulated and folded. The biotitic quartzofeldspa­thic gneiss typically weathers buff to buff light grey.

Homogeneous Quartzofeldspathic Gneiss (subunit A 1c)

Unit A 1 c is a strongly foliated, salmon coloured to light grey-weathering, fine- to medium-grained quartzofeld­spathic rock which is generally compositionally and tex­turally homogeneous on the outcrop scale. Locally this subunit contains scattered leucocratic granitoid clasts and rare feldspar prophyroblasts. Parts of subunit A 1 c contain disseminated fine- to medium-grained hematite.

Arkose and Feldspathic Quartzite (subunit A td)

A thick interval of this unit is well-exposed to the east of Walsh Lake. The arkose, which is light grey to more typically mottled pinkish buff weathering, includes parts with primary laminae, including rare cross-laminae as well as foliated homogeneous intervals. Locally there are scattered leucocratic granitoid pebbles. Biotite­quartz-feldspar pegmatites, some in excess of 30 m thick measured perpendicular to the main regional folia­tion, are locally abundant. Preliminary thin section ex­amination reveals that the arkose is composed of a fine­grained interlocking mosaic of quartz, microcline, and plagioclase with minor biotite and muscovite.

Grit (subunit A 1e)

Mappable intervals of grit overlie plagioclase porphy­roblastic biotite schist east of Walsh Lake and occur along the western contact of muscovite-biotite schist west of the narrows in Duddridge Lake. Grit also forms beds in subunits A 1 d and A 1 f. The grit overlying the pla­gioclase porphyroblaslic schist is a buffish pink to buf­fish light grey weathering, fine-grained quartzofeldspa­thic rock containing scattered, generally round, grey to dark grey quartz grains typically a few millimetres in di­ameter. Less commonly, medium-grained porphy­roblasts of plagioclase occur in the quartzofeldspathic matrix. Grit along the west contact of the muscovite-bi­olite schist is compositionally similar although quartz granules have been tectonically flattened and locally are extremely ribboned out.

Conglomeratic Argilfaceous Arkose and Grit (subunit A1f)

This subunit comprises pink, wavy thin beds and lami­nae of argillaceous arkose that contain rare dark grey bi­otite-rich rip-up clasts. Within this subunit are thin layers composed of up to 20 percent scattered, pink, fine­grained, leucocratic granitoid pebbles 1 to 3 cm in di­ameter. Beds as thick as 1 m contain about 20 percent scattered, dark grey quartz grains up to 6 mm in diame­ter in a fine-grained quartz-feldspar matrix. Conformable epidote-rich lenses occur locally and some exposures contain disseminated hematite and weather a deep salmon colour.

Summary of Investigations 1993

c) Granitoids

The granitoid terrane east of the quartz pebble conglom­erate is composed of a variety of rocks that locally con­tain abundant supracrustal enclaves.

Feldspar Porphyroblastic Monzodiorite and Quartz Monzodiorite (unit MD)

On the north and east sides of Duddridge Lake, as far as 500 m south of the large island at the north end of the lake, the main granitoid is a salmon to buff col­oured, medium- to coarse-grained monzodiorite to quartz monzodiorite containing coarse-grained porphy­roblasts of microcline and approximately 20 percent me­dium- to coarse-grained clots of hornblende variably re­placed by biotite. Foliation is locally very strong, particu­larly along the contact with the quartz pebble conglomer­ate, but is more typically weak to moderate, and a strong lineation is common. The unit is cut by millimetre­thick shears.

Felsite and Leucogranite (unit fel)

Felsite and leucocgranite are significant in the granitoid terrane east of the quartz pebble conglomerate. To the north of Duddridge Lake, salmon coloured felsite veins and small irregular bodies cut the monzodiorite. To the east of Duddridge Lake salmon to pink, fine-grained fel­site to leucogranite are composed of 40 to 50 percent quartz, 20 to 25 percent potassic feldspar, 20 percent plagioclase, and 5 to 7 percent disseminated fine­grained biotite. This unit, like other granitoid phases on the west side of Duddridge Lake, is so poorly exposed that its form and dimensions are masked.

Diorite (unit D1)

Diorite underlies the east side of the large island in the north end of Duddridge Lake (Figure 1) as well as adja­cent parts of the mainland to the east. The diorite is a massive to weakly foliated, grey, medium- to coarse­grained rock composed of 20 to 35 percent hornblende, variably replaced by biotite, in millimetre-size aggre­gates. Millimetre-thick shear zones cross-cut the weak foliation.

Hornblende Porphyroblastic Diorite (unit D2)

West of Duddridge Lake, the biotitic quartzofeldspathic gneiss is cut by a small body of dark green, medium­grained melanocratic diorite that locally contains coarse­grained porphyroblasts of hornblende.

Granite (unit G1)

Foliated to massive, salmon coloured to pink, medium­grained granite occurs throughout the granitoid terrane east of the quartz pebble conglomerate. This rock typi­cally contains 7 to 12 percent fine-grained biotite, com­monly in clots. Locally there are phenocrysts of potassic feldspar. The most extensive body of granite occurs on the southeast side of Duddridge Lake .

Saskatchewan Geological Survey

Pegmatite (unit P1)

Pegmatite occurs in all granitoid and supracrustal rocks in the area. It is most abundant in the younger su­pracrustal units, particularly in biotite-muscovite schist and meta-arkose. There are both compositional and tex­tural variations of pegmatite and probably more than one generation. In the biotite-muscovite schist, pegma­tite is commonly a pink to grey weathering rock com­posed of two textural components: 1) medium- to coarse-grained equigranular, and 2) very coarse­grained porphyritic. Pegmatite typically occurs in bodies aligned parallel to foliation and range from a few centi­metres to a few tens of metres in width and from a few metres to tens of metres in length. Contacts vary from sharp to gradational and some pegmatites are joined by veins cross-cutting foliation. The pegmatites are com­posed of quartz, potassic feldspar, plagioclase, biotite, and minor muscovite. Some pegmatites in biotite-mus­covite schist contain clots of coarse-grained, black tour­maline. Pegmatites in biotite-muscovite schist at the south end of Duddridge Lake are deformed into boudins and there is commonly a reaction rim between the boudins and adjacent pelites.

Granodiorite (unit GD1)

This unit and granite (unit G1) constitute the two main granitoid phases on the southeast side of Duddridge Lake at the contact with the quartz pebble conglomer­ate. The granodiorite is a grey to buff grey, fine- to me­dium-grained rock composed of 35 to 40 percent smoky quartz, 15 percent potassic feldspar, 35 to 40 percent plagioclase, and 7 percent biotite. This rock is weakly fo­liated and locally contains a strong lineation.

5. Depositional Setting of the Meyers Lake Group

The quartz pebble conglomerate (unit QC) is interpreted to have been deposited in the medial to distal parts of a laterally extensive and unconfined braid plain (Rust and Koster, 1984). Formation of this extensive apron of ma­ture siliciclastic sediment marked the initiation of rifting along a zone that broadly corresponds to the Needle Falls Shear Zone (Coombe, 1979; Ray, 1979). The in­itial phase of uplift, associated with rifting, probably un­roofed previously deposited mature siliciclastic sedi­ments (cf. Money, 1965). The area between Duddridge and Meyers Lake is considered to represent one of a number of initially closed basins that typically develop along extensive rift systems (Jowett, 1989).

The quartzite (unit 01) is interpreted to have been de­posited during pulses of upper flow regime sediment laden surges in broad channels in progressively more distal parts of the braid plain apron. Continued trans­gression resulted in deposition of pelitic sediments, rep­resented by biotitic schist and plagioclase porphyroblas­tic biotite schist (unit BS 1 ), over the braid plain apron. Reactivation of the rift zone is then considered to have produced new pulses of immature coarse elastic sedi­mentation along the rift margin depositing arkose, con­glomerate, and grit in proximal settings, and pelites and

79

semi-pelites farther away from the main sediment dis­persal points. Subsequent alteration of some of the coarse elastic sediments would then have produced red­beds and created local enrichments in uranium and cop­per as well as associated elements (Coombe, 1979).

6. Structural Geology

The structural history of the Duddridge Lake area ap­pears to be similar to that documented for the Dud­dridge- Meyers lakes area by Coombe (1978b). Bed­ding, laminations, and other primary features, such as cross-bedding, grading, and scour surfaces, are well preserved in the quartz pebble conglomerate and quartzites adjacent to the unconformity with granitoids. These structures consistently indicate that the se­quence youngs to the west. To the west, primary struc­tures are progressively obliterated so that only in rare in­stances, such as locally in the arkoses on the southeast side of Walsh Lake, are primary features such as cross­bedding preserved. Where present, these also consis­tently indicate tops to the west.

A strong foliation, S1, developed throughout the area generally trends northeasterly, and parallels bedding. In rare exposures, such as in one outcrop of meta-arkose southeast of Duddridge Lake, S1 was observed to be axial planar to small-scale isoclinal folds.

A second foliation, S2, occurs locally in the siliciclastic supracrustals, but in many places it is hard to distin­guish between S1 and S2. The trend of 82 varies from nort~-~orthwesterly to commonly north-northeasterly, and ti ts apparently axial planar to folds with a maxi­mum wavelength of a few metres that plunge moder­ately to the northeast or southwest. Unlike the docu­mented instances found farther to north (Coombe, 1978b), no large scale examples of these structures were encountered.

North-northeast-trending high-strain zones are also at­tributed to 02 deformation (Coombe, 1978b). Examples of these high-strain zones occur in the grit unit on the west contact with the biotite-muscovite schist west of !he narrows_ on Duddridge Lake and between the quartz­ite and plag1oclase porphyroblastic biotite schist north of Duddridge Lake.

The latest structures recognized in the Duddridge Lake ~rea are ve~ica~ ?renulation cleavages common in peli­t1c and sem1-peht1c rocks. These steeply dipping struc­tures appear to be aligned in two distinct trends: east­erly and north-northeasterly.

7. Economic Geology

Mineral occurrences in the Duddridge Lake area in­clude: 1) the Thor U-Cu prospect; 2) U in pegmatites; 3) U-Th concentrations in the quartz pebble conglomer­ate at the base of the Meyers Lake Group; 4) elevated Cu concentrations in a variety of contexts in the arkose of the Meyers Lake Group; and 5) oxide facies iron for­mation float/drift.

80

In this section, a review of exploration history and devel­op'!le.nt for the Duddri~ge Lak~ area is followed by a de­scnpt1on of the geological setting and characteristics of the occurrence types.

a) Exploration History and Development

!n the summer of 1974, Kole Lovang, a prospector work­ing on behalf of Thor Exploration Limited, discovered a train of uraniferous carbonaceous arkose boulders on the west side of Duddridge Lake. Claims in the area were acquired by Thor, Mark V Petroleums and Mines and Brascan. Shortly thereafter, joint venture agree- ' ments were concluded between these companies and Noranda. Subsequent follow-up of the Thor showing in late 1974 and early 1975 included linecutting, geologi­cal mapping, boulder sampling, ground magnetometer and radiometric surveys (SEM Assessment File 7309-0007) and a 21 hole diamond drilling program totalling 2477.4 m. This drilling defined a 61 O m long zone of uranium mineralization of limited width and grade (Fig­ure 2). In 1975, additional geological, geochemical, and geophysical follow-up work was completed in the area o~ _the origin~I showing as well as regionally, and an ad­d1t1onal 11 diamond drill holes totalling 728.7 m in length were drilled on the Thor prospect. On the basis of the combined drilling, two ore zones have been de­fined with a combined tonnage estimated at 357,310 tons grading 2.11 lbs/ton U30s (SEM Assessment File 7309-0012).

Regional work resulted in the discovery elsewhere of anomalous concentrations of uranium and thorium in the basal conglomerate of the Meyers Lake Group. A 0.3 to 0.45 m wide, 45.7 m long pyritic, micaceous sand­stone lens containing anomalous uranium near Sandfly Lake (SEM Assessment File 7309-0010) and a uranifer­ous albite-biotite pegmatite also near Sandfly Lake con­taining patchy concentrations of uranium assayed as much as 1.5 lbs of U30s per ton over a width of 3 m, al­though most were significantly less (SEM Assessment File 7309-0011) .

In ~ 976 the Saskatchew~n Mining Development Corpo­ration became a partner in the Noranda-Brascan joint venture. During that year geophysical and geochemical surveys were focused on the area south of Duddridge Lake. An additional 1 O diamond drill holes totalling 565.7 m in length were drilled to test targets identified during this work. No uranium mineralization was inter­sected. Subsequently the joint venture agreement was terminated and the claim blocks encompassing the showing and adjacent lands reverted to the original own­ers, Thor or Mark V.

In 1978, Thor continued work that included multi-media geochemical surveys over four grids near Duddridge Lake (SEM Assessment File 7309-0019); this was fol­lowed up with a 1980 basal till geochemical survey north of the Thor prospect (SEM Assessment File 7309-0015) and a 1981 program of 10 diamond drill holes to­talling 1610 m in length drilled in the north end of and to the west of Duddridge Lake (SEM Assessment File 7309-0016).

Summary of Investigations 1993

In 1979, Mark V completed a magnetometer survey of the Walsh Lake area (SEM Assessment File 7309-0021 ). Although Thor retained a mineral lease over the U-Cu showing, adjacent claims lapsed until 1993 when Noranda acquired a significant land holding in the area.

b) Descriptions of Mineral Occurrences

Thor U-Cu Prospect

Rock exposure is scarce in the vicinity of the Thor U-Cu Prospect, which is only revealed as a mineralized boul­der train. Consequently, information about the showing and its context has been derived primarily from dia­mond drill holes. The following account is based on in­formation contained in the Mineral Assessment Files of Saskatchewan Energy and Mines, a report by Coombe ( 1978b ), thesis investigations by Bretzlaff ( 1976), and preliminary petrographic and geochemical work by the author.

Mineralization occurs in the structurally lower part of a north-northeast-trending sandstone sequence which dips moderately to steeply to the west. On the basis of primary lithologic variations and alteration mineral as­semblages, a number of subdivisions are recognized in the sandstone. These include striped arkose comprising alternating maroon and greenish light grey colour bands (see Coombe 1978b); a micaceous sandstone contain­ing 10 to 20 percent biotite, muscovite, and chlorite; a hematitic arkose characterized by interstitial dustings of hematite; a vuggy limonitic stained variety; and carbona­ceous arkose.

Carbonaceous arkose, host to the uranium copper min­eralization, is a light grey to buff light grey rock with dark grey carbonaceous laminations and mottled areas. "The uraninite is found either as separate grains dis­persed throughout the sandstone ... or may be concen­trated in aggregates along bedding, fracture or schistos­ity planes" (Bretzlaff, 1976, p14). In addition t~ uraninite, carbonaceous sandstone also contains chalco­pyrite, pyrite, bornite, galena, pyrrhotite, sphalerite as well as very rare molybdenite (op cit.). The carbona­ceous arkose occurs as irregular lenses within hematitic arkose. Two of the carbonaceous arkose lenses are mineralized; others, however, are unmineralized. The sandstone sequence is segmented and locally totally re­placed by pegmatite. At the structural base of the sand­stone sequence is a biotite schist.

Bretzlaff (1976, piii) attributed the Thor occurrence to "epigenetic chemical precipitation of uranium from circu­lating ground waters in the local reducing environment of the carbonaceous arkose". Lewry and Sibbald (1979) noted the similarity between the Thor showing and ura­nium occurrences in sandstone in the Colorado Plateau region. This showing, and the associated copper miner­alization discussed below, are characterized by many features typical of stratiform-hosted copper deposits as­sociated with red beds formed in rift-settings (Kirkham, 1989; Brown, 1992). Coombe (1979) also noted the pos­sible red-bed association of the showing.

Saskatchewan Geological Survey

U in Pegmatites

Some pegmatites in the Duddridge Lake area contain anomalous concentrations of uranium. For example Bretzlaff (1976) observed that some of the pegmatites in sandstones near the Thor U-Cu showing contain uraninite along veinlets. These pegmatites also contain chalcopyrite locally. Elsewhere, prospecting between Duddridge Lake and Needle Rapids led to the discovery of a dozen outcrops or boulders of radioactive pegma­tite (SEM Assessment Files 7309-0010). No economi­cally significant grades are reported. Radioactive peg-. matites in supracrustal rocks, particularly those associ­ated with graphitic pelites, are common throughout the Wollaston and adjacent Mudjatik Domains (Sibbald et al. , 1977; Lewry and Sibbald, 1979).

U-Th in Quartz Pebble Conglomerate

Some of the beds of quartz pebble conglomerate in the base of the supracrustal sequence contain anomalous concentrations of uranium and thorium (SEM Assess­ment Files 7309-0009, -0010, and -0011 ). In a study of radioactivity in an exposure of the conglomerate at the south end of Duddridge Lake, Munday (1978) observed surface radioactivities of up to 6 times background caused by elevated concentrations of both U and Th. A 23 cm wide bed of quartz pebble conglomerate, that was traced for 37 rn along strike, has 10 times back­ground radiation. Although a grab sample from a trench in this bed yielded 0.09 percent U30a, a bulk sample from the trench contained only 0.005 U30a; Munday also noted elevated concentrations of selenium and va­nadium in the conglomerate (op cit).

Cu in Arkose

In addition to the Thor uranium-copper showing, the sandstone sequence that hosts this mineralization also contains other occurrences of minor copper mineraliza­tion. These include chalcopyrite-bearing fractures, chal­copyrite in quartz veins, pegmatite with stringers of bornite and/or chalcopyrite, disseminated chalcopyrite, and patchy mottled areas of bornite (SEM Assessment File 7309-0016).

Oxide Facies Iron Formation Float

A rusty weathering, tabular, cobble-sized fragment of magnetite-bearing iron formation was discovered during mapping approximately 1 km north of Duddridge Lake along the west side of a prominent valley underlain by unit BS2 (UTM co-ordinates 429605m E, 6160720m N). An assay revealed this rock contains 22.54 percent iron. The friable nature of this boulder suggests it had not been transported a great distance. A follow-up to trace the source of this boulder is warranted.

81

8. Stratigraphy of Wollaston Domain Su­pracrustal Rocks, Duddridge Lake Area

a) Review of Previous Ideas

A variety of stratigraphic classifications have been pro­posed for the supracrustal rocks in the Wollaston Do­main (Saskatchewan Geological Survey, 1987). Even in the southern part of the domain, which encompasses the Duddridge Lake area, several different stratigraphic classifications have been proposed and modified as new data became available.

Money (1965) proposed the name Meyers Lake Group for the basal quartz pebble conglomerate and overlying quartzite sequence with intercalated pelitic schists and suggested that this group unconformably overlies a se­quence of "older metamorphic rocks" which he sub­sequently named the Sandfly Lake Group (Money, 1968). Money included in the Sandfly Lake Group meta­arkose, hornblende-biotite rocks, amphibolite, biotite-pla­gioclase rocks, and acid metavolcanic rocks. The Sand­fly Lake Group thus defined not only comprises most su­pracrustal units west of the quartzite, but also rocks to the east, which Money (1965) postulated to be felsic vol­canics, but which were subsequently identified by Mun­day (1978) as mylonites associated with the Needle Falls Shear Zone.

In the Daly Lake area (Figure 1 ), approximately 110 km north-northeast of Duddridge Lake and 20 km west of the Needle Falls Shear Zone, Money (1966) also pro­posed the name Daly Lake Group for supracrustal rocks containing a basal pelitic succession overlain and, in part, intercalated with arkose, subordinate conglomer­ate and quartzite, and containing calc-silicate lenses. Money noted this succession was regionally extensive in the central part of the Wollaston Domain west of the Needle Falls Shear Zone (Money, 1968; Money et al., 1970).

In 1975, Ray, on the basis of re-mapping in the Foster Lake (NE)-Geikie River (SE) area, concluded that rocks previously mapped as correlative with the Sandfly Lake, Daly Lake and Meyers Lake groups by Scott (1973) comprise a single succession which he termed the Wollaston Group.

On the basis of 1:100 000 scale remapping in 1974, of the lie-a-la-Crosse (East) Map sheet, which includes the Duddridge Lake area, Munday (1978) suggested that Money's (1965) Meyers Lake Group, as well as other siliclastic units to the west including arkoses, pe­lites and their metamorphic derivatives, which had been formerly assigned to the Sandfly Lake Group, are all part of the same sequence; the differences recorded in these rocks are attributed to sedimentary facies changes as well as variations in deformation style and metamorphic grade. Munday also suggested that his new expanded Meyers Lake Group could be consid­ered a lithological variant of the Daly Lake Group (Money, 1966).

82

Coombe, who mapped the area between Duddridge and Meyers Lake area at a scale of 1 :50 000, in 1978, subdivided the supracrustal rocks into two unnamed groups: one included biotitic rocks including biotite schist and plagioclase porphyroblastic biotite schist; and the other included the quartz pebble conglomerate, quartzite, arkose, and biotite-muscovite schist.

In a 1979 paper, Ray (p24) proposed that the supracrus­tal rocks of the Wollaston Domain are ' separable into two contrasting assemblages, a widely developed 'Wol­laston Group' and a more restricted 'Needle Falls Group' ... The Needle Falls Group is confined as thin strips to the extreme southeastern margin of the Wollas­ton Domain, adjacent to the Needle Falls Shear Zone and includes rocks formerly designated variously as the Meyers Lake, Courtenay Lake, Sandfly Lake or Souter Lake groups (Money, 1965; Money et al. 1970; Lewry, 1977)." These rocks, characterized by coarse elastic fa­cies and locally intercalated mafic volcanics, he inter­preted to have been deposited along a rift margin that broadly corresponds to the Needle Falls Shear Zone (Coombe, 1979; Lewry et al., 1981). In a 1979 paper Coombe adopted Ray's twofold subdivision of the Wol­laston supracrustals into the older Needle Falls Group and younger Wollaston Group (see also Coombe Geo­consultants, 1991 ).

b) Present Author's Conclusions on Regional Stratigraphic Relation of the Ouddrldge Lake Area

Present mapping suggests that stratigraphic relation­ships in the Duddridge Lake area are as established by Munday (1978), that is all supracrustal rocks west of the granitoid terrane belong to one succession . Evi­dence for an unconformity in this sequence is equivo­cal. Similarly, re lationships with supracrustals in the main part of the domain, the "Daly Lake Group", and equivalent st rata are unclear.

Although on a broad scale coarse elastic facies are common along the eastern margin of the Wollaston Do­main (Coombe, 1979), there are sufficient distinctions from area to area which seem to preclude lumping them all into one stratigraphic unit such as the ' Needle Falls Group". For example, in the George Lake area (Figure 1 ). in contrast to the Duddridge-Meyers lakes area, the sequence included in the Needle Falls Group, Courtenay Lake Group (Scott, 1970), and Lower meta­arkose (Coombe, 1978a) consists of a succession of arkoses, conglomerates, and breccias containing two horizons of amphibolite derived at least in part from mafic volcanics. Elsewhere, such as in the Barnett Lake area, rocks that would presumably be included in the Needle Falls Group (Coombe, 1979) have been de­scribed by Money (1961) as arkose with intercalated conglomerate; subsequently Ray (1975) reinterpreted the meta-arkose as metamorphosed felsic volcanics and the conglomerates as tuffaceous derivatives.

A similar argument could be made against including all rocks in the central part of the Wollaston Basin in the Wollaston Group, for again local variations in stratigra­phy have been recognised.

Summary of Investigations 1993

By documenting and then comparing and contrasting re­gional variations in stratigraphy, a more detailed under­standing of the character and evolution of the Wollaston Basin should become apparent. Such an understanding is also critical for understanding metallogenic relation­ships. Consequently I would advocate moving beyond basin-wide stratigraphic classification schemes based upon locally set up assemblage names, such as many of those illustrated in the chart on page 16 of Saskatch­ewan Geological Survey (1987) or that by Coombe Geo­consultants (1991), to a three dimensional scheme of fence diagrams that focuses on lithological regional vari­ations. For the younger supracrustal succession in the Duddridge- Meyers lakes area, I favor, as suggested by Lewry (1981), abandonment of the all-encompassing Needle Falls Group and return to the use of the term Meyers Lake Group in the expanded context as defined by Munday and in this report.

9. Economic Potential of the Wollaston Domain

Many major Proterozoic sedimentary basins. like the Wollaston, are host to a spectrum of economically sig­nificant mineral deposits in a variety of settings. As an example, the rift formed Middle Proterozoic Belt Basin, along the western side of North America, is host, amongst others, to: the 160 million tonne Sullivan Pb­Zn deposit (average grade of 5 percent Pb, 5 percent Zn, and 2 ounces of silver per ton (Hamilton et al., 1982); several sediment-hosted stratiform copper depos­its, such as the 85 Mt Spar Lake deposit (average grade of 0.76 percent Cu and 54 g Ag per tonne (Hayes et al., 1989), and the stratigraphically controlled lead, zinc, and silver veins of the Coeur D'Alene district, which by the end of 1980 had already produced 935,287,000 ounces of silver, 7,679,772 tons of lead, 3,051,743 tons of zinc, 500,553 ounces of gold, and 160,985 tons of copper (Bennett and Venkatakrishnan, 1982).

Although the Wollaston Domain has received but a small fraction of the exploration activity as the Belt Ba­sin, a variety of mineral occurrences have been identi­fied in Wollaston supracrustals. These include the sedi­ment hosted copper in the Pendelton Lake area (Scott, 1973; Coombe, 1977); lead-zinc occurrences in the George and Sito lakes area (Karup-Moller and Brum­mer, 1970; Coombe, 1977; Potter, 1977, 1978, 1980); the Marina lead-zinc showing at Johnson Lake (Coombe, 1977); and U-Cu mineralization at Duddridge Lake (Bretzlaff, 1976; Coombe, 1978b). Although none of these occurrences are presently economic, the diver­sity of mineralization types indicates a significant base metal potential for the Wollaston Domain. This potential has been further underlined by recent significant discov­eries by Noranda (Northern Miner, May 24, 1993).

10. Acknowledgments Kevin Fossenier is thanked for his invaluable assistance during both the field and office portions of this investiga­tion. Pam Schwann helped with mapping for one week.

Saskatchewan Geological Survey

11 . References

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_ _ _ _ _ (1978a): Wollaston base metals project, Spence Lake area; in Summary of Investigations 1978, Saskatche­wan Geological Survey, Sask. Dep. Miner. Resour., Misc. Rep. 78-10, p92-97.

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84

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_ _ __ (1978): Wollaston Belt base metals project, Sito Lake area; in Summary of Investigations 1978, Saskatche­wan Geological Survey, Sask. Miner. Resour., p109-112.

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_ _ _ _ (1975): Foster Lake (NE)-Geike River (SE) area: Reconnaissance geological mapping of 74A-15(E), 16 and 74H-1 and -2, in Summary of Investigations 1975, Sas­katchewan Geological Survey, Sask. Dep. Miner. Resour., p13-18.

_ _ _ _ (1979): Reconnaissance bedrock geology, Wollas­ton Lake east (part of NTS area 64L); in Summary of In­vestigations 1979, Saskatchewan Geological Survey, Sask. Miner. Resour., Misc. Rep. 79-10, p19-28.

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