Geology of the Courtenay Lake Formation in the Cook Lake Area, Wollaston Domain, Northern Saskatchewan

A. MacNeil 1, G.D. Delaney, and K. Ansdell I

MacNeil. A. , Delaney, G.D., and Ansdell. K. ( 1997): Geology of the Courtenay Lake Formation in the Cook Lake area, Wollaston Domain. northern Saskatchewan; in Summary of Investigations 1997, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 97-4.

The Courtenay Lake-Cairns Lake Fold Be lt (Money, 1968; Kamp-Moller, 1970; Scott, 1970; Coombe, 1994 ), located along the southeast side of the Wollaston Domain (Figure I), is a unique succession of Palcoproterozoic supracrustal rocks. The basal Courtenay Lake Formation comprises arkose, conglomerate, and volcanics deposited in a rift setting (Coombe, 1994 ). These rocks are overlain by a thick rift-fill succession of quartzites, graphitic mudstoncs,

variably calcareous, fine-grained siliciclastic rocks, arkoses, and rare carbonates. Exposure throughout the fold belt is about l percent and quite erratic. The Courtenay Lake Formation has been studied in most de tail in the Courtenay Lake-Brakewell Lake area (Karup-Mollcr, 1970; Scou, 1970; Coombe, 1994; Fossenicr et al .. 1995). To the northeast , it is also well exposed in the Cook Lake and Fordham Lake areas. Although these areas were mapped at a scale of

~ -, Legend

Megac~ys!ic granite -granod1onte Souter Lake Fm

BJ Flasered granite ··1·· -···· -- .... -,,-~i~op~ot;~o;oic

[j Undivided

[~_-_·. ;~ ·..J ~ :l ~:~~J

Causier Creek Fm

Spence Lake Formation

• Iron Formation Member

George Lake Fm

0 2 4 6 8 10 , _1 _ _..1 J . ..J- . ..J.. _.J.._J_j_,.L_J

Km

Courtenay Lake Fm

D Quartz arenite ... .... . . . ·-Paleoproterozoic\Archean

Peter Lake Domain

[UJ Granitoids\paragneiss Archean ~ ~ Granite

Figure I - Geological sketch map of tile Courtenay Lake-Cairns IAke Fold Belt. SLS=Spence Lake Synform (includes data from Coombe, 1994; Lewry et al., 1981).

'Department of Geological Sciences, Universi ty of Saskatchewan, 114 Science Place, Sask:uoon. SK S7N .5E2

Saskatchewan Geological Survey 115

I: I 00 000 by Lewry et al. ( 198 1 ), many details about the character of the Courtenay Lake Formation remain to be resolved. This information is vital lo fully understanding the nature of the early evolut ion of the Wollaston basin. The purpose of th is investigation was to study the stratigraphy of the Courtenay Lake Formation in the Cook Lake area. This work compliments studies of the formation elsewhere in the heh which are reported on by Delaney et al. (this volume).

A 2 km wide by 6 km long strip was mapped at a scale of I: IO 000 on the northwest side of Cook Lake (Figure 2). Magnetic susceptibility was measured for all outcrops mapped and rock samples were collected for age dating, geochemical, and petrographic research. This work was conducted, as part or an Honours dissenation, by the senior author al the University of Saskatchewan.

0 2

1. Local Geological Setting

The Counenay Lake Formation is on the southeast side of the southwest-trending and plunging Spence Lake Syn form (Figure I; Scott . 1970; Coombe, 1994; Lewry et al., 198 1; Delaney et al., this volume). At Cook Lake, there is an unconformity between the formation and a heterogenous suite or paragneiss and grantoids of the Peter Lake Domain (Figures 2a and 2b). Here the Courtenay Lake Formation is estimated to he about 1500 m thick; however, only the lower 700 mis exposed. Locally, along its basal contac t, arc narrow lenticular bodies of andalusite porphyrohlastic magnetite-sericite schist. Most of the exposed part of the formation is conglomerate. Amphibolite derived from mafic to intermediate volcanic and volcaniclastic rocks forms a narrow lens at the base of the formation and a more extensive body about 700 m above the base. Quartz and feldspar porphyries occur as irregular cross­cutting bodies in the conglomerate and as a conformable layer above the upper amphiholitc. These

r - - J L ,..c· ·~· Overburden

:........____ ___ _t ___ 1 _____ _

Courtenay Lake Formation

~ Feldspar porphyry

r8"7"3l ~ -~ ~

Mafic to intermediate volcanics and volcaniclastics

Conglomerate - minor arkose

Andalusite magnetite schist

Mafic volcanics

Unconformity lr,-;,·,.1 .., ;,:, ~~ :J Granitoid/Paragneiss

mJ Diorite

a

kilometres

b

• 600

- 400 ~

!!! .; ::, 200

- 0

Figure 2 • a) Geological sketch map af the Cook Lake area; and b) Simplified stratigraphic section for the Cook Lake area.

116 Summary of l11ves1iga1io11s /997

rods have been subjected to upper greenschist to lower amphibo lite facics metamorphism.

2. Description of Formations

a) Peter Lake Domain

These rocks rnmprise a he terogeneous assemblage o f variably granitized quartzofeldspathic paragneiss and granitoids. The southwest end of the study area is underlain by pink Lo buff weathering , fine- grained quartzofeldspathic paragneiss that coniains from IO to 20 percent biotite and magnetite. Locally these rocks are g neissic and contain lenses and cross-cutting veins of fine-to medium-grained granite. Northeastward, through a zone of progressive recrystallization, they become transformed into a pink weathering, massive to lineated, fine-grained g ranite containing from IO to 20 percent disseminated to clotted, fine-grained biolite and mag netite and, in many exposures, scattered 6 mm diameter megacrysts of plagioclase. A minor phase is a pink, medium-grained granite that contains from IO to 20 pcn.:ent hornblende. The granites are cut by sheets of granite pegmatite, fine- to medium-grained aplite, and rnesocratic to rnelanocratic diorite dykes, from I to 2 rn thick. In the northeast part of the study area is a mixture of leucocratic feldspar blastic ·granite ' , fine-lo medium-grai ned leucocratic biotitc granite, granite pegmatile, and fclsitc. In contrast to rocks in the south , there is little or no magnetite and magnetic susceptibilities a re low. Within, and apparentl y intruded by the granitic rocks, is a lenticular body of dark grey weathering, medium-to coarse-grained, mesocratic to melanocratic diorite.

b) Courtenay Lake Formation

Conglomerate

Most of the exposed part of the formation is a 700 m thick, pink to buff grey weathering conglomerate characterized by an intact Lo condensed framework of pebble- to cobble-sized clasts set in a fine - 10 medium­grained matrix of quartz, feldspar, seric ite, and

Figllre 3 - Cobble conglomerate of the Courtenay Lake Formation. Most clasts are of a foliated fine- to medium­grained leucocratic quartzof eldspathic rock.

Saskatchewan Gt'o/ogical Survey

magnetite (Figure 3 ). Clasts are sub- to well-rounded with moderate sphericity. About 80 percent arc of a foliated , line- to medium-grained, leucocratic quartzofeldspathic rock. Subordinate clast types include granite, quartz, and wacke. Typically the conglomerate is a massive, texturally homogeneous looki ng unit with only rare stratification observed. Minor amounts of pebbly to gritty arkose and, near the northeast end of the study area, cross-laminated arkosc are intercalated. Truncation surfaces on the cross beds indicate younging to the northwest. The character o f the conglomerate and rounded exotic nature of most clasts is consistent with depositio n in a proximal braided river or hraidplain setting (Rust and Kostncr, 1984) some distance from the main source area.

Amphibolite

Amphibolitcs derived from mafic to intermediate volcanic and volcaniclastic rocks occur at two stratigraphic posit ions (Figures I and 2). To the north of Cook Lake, at the base of the formation. is a 1.7 km long by I 00 m thick lens referred to as the Lower Cook Lake Volcanic Member. Two main phases are distinguished; a dark green, tine-grained, massive arnphibolite with from 35 to 45 percent hornblende and a second varie ty of amphibolitc with 30 to 40 percent randomly oriented fine- to medium hornblende and/or actinolite in a groundmass of fine-grained plagioclasc and magnetite. Epidotized pillows were identified in one outcrop (Figure 4).

The upper amphibolite un it, together with the overlying quarl:t.-feldspar porphyry forms Lhe Upper Cook Lake Volcanic Member, which is about I 00 m thick and has a strike lenglh of al least 5 km. The upper amphibolite is more heterogeneous than the lower volcanic member and includes dark green, line-grained, melanocratic massive amphibolite locally with amygdales; a rock composed of 15 to 25 percent randomly oriented laths of aetinolite in a fine-grained groundmass of amphihole and feldspar; and a rock composed of JO to 20 percent disseminated fine-grained biotite in a fine-grained matrix of quart,. and feldspar. Disseminaled magnetite

Figure 4 - Epidotized pillowed mafic volcanic, Lower Cook Lake Volcanic Member, Courtenay 1.,ake Formation.

117

is common in both amphibolite members.

Preliminary geochemical data from a small sample suite shows the amphibolites to be similar to mafic volcanic units in the Courtenay Lake area (Fosscnier et al .. 1995). The Cook Lake samples are of tholei itic subalkaline basalts and basaltic andesite (Figures 5 and 6) emplaced in a within-plate setting (Figure 7) and they exhibit a chrondrite normalized REE pattern that is enriched in LREE (Figure 8).

Andalusite-magnetite-sericite Schist

A unique unit of andalusitc porphyroblastic magnctite­sericite schist occurs at two places in the study area.

Cale-Alkaline

Na20+K20 MgO

Figure 5 • AF M plot for ma.fie volcanics from the Courtenay Lake Formation, Cook Lake.

70 Rhyodacite-Dacile

N Trachyte

0 60 Andeslte .... 00

Phonolile

50

A Bas-Trach-Neph

40 .(H)l 0.01 0.1

Zr/Ti02*0.0001

10

Figure 6 • Zr/TiO, vs. SiO, classification plot (Winces/er and Floyd, 1977) for volcanics from the Courtenay Lake Formatiofl, Cook Lake.

JUI

One is near the northeast end at the contact between granitic rocks of the Peter Lake Domain and conglomerate of the Courtenay Lake Formation; the other is on the northwest side of an epidotized pillowed mafic volcanic at the top of the lower volcanic member. The schist is grey weathering, foliated, and contains abundant anhcdral porphyroblasts of andalusite, typically about 5 cm in diameter in a matrix of magnetite and sericite. The schist is interpreted to be a metamorphosed latcrite.

Feldspar Porphyry

Maroon-pink- to buff-grey-weathering quartz-fe ldspar porphyry occurs as irregular, narrow, cross-cutting bodies in the conglomerate and as a conformable body at the top of the upper volcanic member. This unit is

10

A • Within Plate Basalts

B - Island Arc Basalts

C • Mid Ocean Ridge Bas

B

) ~~-'----'---'---'-'--L.J..J...L~~ ....... -L--L-....1,..J...U...LJ

10 100

Zr

1000

Figure 7 • Zr/Y vs. Zr discriminatiofl diagram (Pearce and Norry, 1979)/or mafic volcanic rocks of the Courtenay Lake Formation from the Cook Lake area. Hatched area encompasses data from Courtenay Lake area ( Fosseflier et al., 1995).

QI -~ 100 c c

..c

~ -a e 10

"" IJ:J

LaCe PrNdSmEuGdTbDyHoErTmYbLu

Figure 8 • Chondrite normalized rare earth elemml profile for mafic volcanic rocks of the Courtenay !Ake Formatiort, Cook Lake area. The shaded area encompasses data for six samples from the Courtenay /Ake area (Fossenier et al. , 1995).

Summary of lnvestig111ions 1997

characterized by anhedral to subhedral phenocrysts of microcline and subordinate plagioclase, from 3 to 7 mm in diameter, and by quartz phenocrysts from 2 to 4 mm in diameter in a groundmass of line-grained quartz, feldspar, and minor hiotite.

The quartz-feldspar porphyries arc interpreted as fe lsic volcanics or volcaniclastics and feeder dykes. The association of the quartz-feldspar porphyries and the mafic volcanics in the upper volcanic member highlight the bimodal nature of volcanism in the Courtenay Lake Fonnation.

3. Depositional Setting

O n the basis of facic s associations and the nature and geochemistry of the mafic volcanics, previous workers have suggested the Courtenay Lake Formation was deposited in a rift basin (Coombe, 1994; Fossenier et al., 1995). Possible latcri tes at the unconformity between the Courtenay Lake Formation and heterogenous granitoid rocks of the Peter Lake Domain suggest a humid tropical environment prior to and

Courtenay Lake Formation (Paleoproterozoic)

Peter Lake Domain (Archean)

during the early stages of rifting. An early pulse of volcanism emplaced mafic volcanic fl ows on the unconformity locally (Figure 9). Pillows indicate that some of the flows erupted subaqueously in what was probably a shallow lacustrine sett ing. Subsequent emergence and a depositional hiatus during continued humid tropical conditions led to the formation of latcrites on top of the volcanics. Lithospheric extension and uplift created conditions favorable for braided fluvial and braid planes to develop and deposit a thick sequence of conglomerate. The main source of the clasts was a foliated, fine- to medium-grained quartzofeldspathic rock. The rounded to subrounded character of the clasts, their more or less uniform sorting, and their dissimilarity from rock types below the unconformity suggest they were derived some distance from where they were depos ited. A second episode of bimodal volcanism, apparently more extensive than that at the base of the formation concluded the geological history of the exposed lower part of the Courtenay Lake Formation.

[ --] Feldspar porphyry rcT. ':".a L.:ci..:.J ... ~ . .

Conglomerate

Andalusite magnetite sericite schist

Mafic volcanic

Unconformity

Granitoid - paragneiss

Diorite

Figure 9 - Sketch illustrating depositional relationships for the Courtenay Lake Formatio11, Cook Lake area.

Saskatchewan GeoloRical Survey 1/9

4. Acknowledgments

Zoran Jankovich and Joshua McGowan are thanked for the ir assistance in the field with mapping and for their great talents back at camp.

5. References Coombe, W. (1994): Sediment-hosted base metal deposits of

the Wollaston Domain , northern Sa.~katchcwan; Sask. Energy Mines, Rep. 213. 108p.

Fosscnier. K., Delaney. G.D., and Watters . B.R. ( 1995): Lithogcochemistry of volcanic rocks from the Lower Proterozoic Courtenay Lake Formation, Wollaston Domain; in Summary of Investigations 1995, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 95-4. p49-60.

Karup-Mollcr, S. ( 1970): Geology of the Compulsion River Fold Belt; Western Miner, v43, no2. p35-5 I.

Lewry, J.F. . Thomas. D.J .. Recs, C.J., and Roberts, K. ( 198 1 ): Geology of an area around Compubion Bay, Wollaston Lake: Sask. Miner. Resour .. Rep. 205. 27p.

Money, P.L. ( 1968): The Wollaston Lake fo ld-bell sy~tcm, Saskatchewan-Manitoba; Can. J . Earth Sci .. v5, pl 489-1504.

Pearce, J.A. and Norry, M.J . ( l 979): Petrogenetic implications of Ti , Zr, Y, and Nb variations in volcanic rocks; Contrib. Mineral. Petrol.. v69, p33-47.

Rust, B.R. and Kostner, E.H. (1984): Coarse alluvial deposits; i11 Walker, R.G. (ed.). Facic~ Models, second edition, Geosci. Can .. Reprint Se ries I , p53- 69.

Scott , B.P. ( 1970): The Geology of the Combe Lake area, Saskatchewan; Sask. Dep. Miner. Resour .. Rep. 135, 32p.

Wincester. J.A. and Floyd, P .A. ( 1977): Geochemical disc rimination of different magma series and their differentiation products using immobi le clements; Chem. Gcol.. v20, p325-343.

/20 Summary ,f !1111t:.1·ti!,(atiorn /997