REPORT No. 61

The Geology of the

THAINKA LAKE AREA (West Hal f)

Saskatchewan

by

F. KOSTER

1961

I ' .

DEPARTMENT OF MINERAL RESOURCES

Geological Sciences Branch

Precambrian Geology Division

HON. A. C. CAMERON Minister

PROVINCE OF SASKATCHEWAN

J. T. CAWLEY Deputy Minister

CONTENTS

I NTRODUCTION . .

Loca tion and Accessibility .

Physiography

Page

4

4

4

5

5

6

Previous Work ...

Field Work and Acknowledgments ..

References .. . ...

G E NERAL GEOLOGY .. . ......................................................................... .. ...... 8

General Statement 8

Table of Formations......... .... ..................................... ...... .... ................................ ... 9

The Northeastern Basement Complex ( 1, 2 and 3 ) ..... .... .......... 8 The granitic component ( 1 ) ............... ...... ........... ............................ 8 The "quartz dioritic" component (2 ) ............................................ IO Amphibolite (3 ) ................................................................................................. IO Summary .............................................................................................. ...................... .... 10

The Red Gneiss Complex (4, 5, 6, and 7 ) ................................................ 11 Amphibolitic Meta-sedimentary Rocks (4 ) .............................. 12 Granulite (5 )......................................... .................................................................. 12 Red gneiss (6 ). . . ........................................................................... 12 The Eastern Conglomerate (7 ) ................... ........ . . .. .... . 14

The Pegmatitic Migmatite Complex (8) ...... .......... .. ......................... 15

The Western Meta-sedimentary and Meta-volcanic Complex (9 ) ... ................................................................................................. .................................... 16

The Western Conglomerate (10 ) ....................... ..... .... ............... ............ ........ 17

The Western Granodiorite Complex ( 11 ) ............................................. 17

Mafic and Mafelsic Veins ( 12 ) ........................... ....................................... 19

S TRUCTURAL G EOLOGY..... .. ........................................................... 19

Folds ................ .......................................................................... .......................... 19

Faults ...................................................................................................................................... 20

Joints ................. . . ........................................................................... ................... .. 21

Conclusions ........................................ ... ........................................... ................. 21

ECONOMI C G EOLOGY ............. .. . ........... ....... ................................................. 23

ILLUSTRATIONS

Geological Map 61A, Thainka Lake (West Half), Saskatchewan ... ............... .................................................................. ........... ............................................ ..in pocket

PLATE lA : The Eastern Conglomerate (7 ) ; south shore, Tazin River, 700 yards east of longitude 109°45' W. .................... 25

PLATE 18: The Eastern Conglomerate; east shore, T azin River, one mile south of Tessini Gap. Strike is N5°W, dip vertical. Ice grooving is N60°E ..................................................... 25

PLATE 2A: The Western Conglomerate ( 10 ); southeast of Flagon Lake..................................................................................................... .... 26

PLATE 28 : The Western Conglomerate; southeast of Flagon Lake ..................................................................................................... 26

PLATE 3A : Rolled fragment in the Western Conglomerate ; north of Martyn Lake. The pencil points to the north. Drag, due to a right;handed movement, may have caused the S;shape .................................. ............................................................. .......... 27

PLATE 38: 8reccia of the Tessini Fault; east shore of Tazin River, two and one half miles south of la titude 60°N. Blocks and fragments of different rock types and vein quartz and breccia are contained in the chaotic assemblage ... ...................... ................. .................................................................. 27

PLATE 4A: Rodding in the Tazin River Fault ultra;mylonites; west shore of Tazin River, 20 yards north of Prov; incial bounda ry ................................................. ... ........................................ .... 28

PLATE 48: East wall of minor vertical movement plane in Red Gneisses, east shore of Tazin River, two miles south of Provincial boundary. A mineral Jineation with a plunge of 48 degrees to the north intersects crenula; tions at 90 degrees. The east wall moved down, and northward, relative to the west wall. North is to the left side of the photograph ........................................................ .... 28

FIGURE I: Location of the Thainka Lake area in the northwest corner of Saskatchewan. On the west half are indicated the major fauits, the water divide (W;D ) and the northeastern basement ............................................................................. 7

FIGURE 2 : Alternative interpretations of movement on the Tazin Ri\'.er Fault. (a ) The rodding represents the b;direction of move;

ment in which case the fault is a steep thrust; fault;

(b ) The rodding represents the a;direction, in which case faulting is of the wrench;fault type ........................ 22

FIGURE 3: Section across the Kinokamaw Lake Fault.. ..................... 23

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INTRODUCTION

LOCATION ANO ACCESS IBILITY :

The Thainka Lake Area, (West Half) is situated in the northwest corner of the Province of Saskatchewan, on the Alberta and Northwest Territories boundaries. It constitutes the National Topographic system area 74.-N.-13.-W and lies between latitudes 59°45' and 60°00' north and longitudes 109°45' and 110°00' west. The centre of the area lies about 50 miles west.-northwest of Uranium City, Saskatchewan, and 70 miles east.-southeast of Fort Smith, Northwest Territories. Both these com.­munities may be reached by scheduled passenger service flights from either Edmonton, Alberta ; or Prince Albert, Saskatchewan.

The many lakes in the area are easily accessible by float.-equipped aircraft. A canoe route via Thluicho, Tsalwor, and Tazin Lakes, and the Tazin River has been accurately described by Charles Camsell, who passed through the area during the summer of 1914 while carrying out the first exploration between Lake Athabasca * and Great Slave Lake.

PHYSIOGRAPHY:

The Thainka Lake area forms part of the much faulted region north of Lake Athabasca. The topography is one of low relief, although locally rugged; lakes and valleys form pronounced lineaments, separated by ridges and scarps rising as much as 200 feet above the level of the adjacent lowlands and lakes. Stagnant lakes, irregular in outline, occupy depres.­sions in the numerous and widespread sandflats.

A water divide separates the southwestern third of the area, which drains southward into Lake Athabasca, from the rest, which drains via Hughes Lake and the Tazin River into Great Slave Lake (Figure 1 ).

The whole region bears the imprint of the Quaternary glaciation. The topography is a result of differential glacial erosion as controlled by the lithology and structure of the bedrock. Outcrop projects through a Pleistocene cover of glacial drift, minor muskeg and marsh over an estimated 50 per cent of the area.

The direction of ice movement varied from south 30 to 60 degrees west as inferred from glacial striae, crescentic marks, grooving, and roches moutonnees. Deep potholes as much as 18 feet in diameter were observed along canyon.-like valleys, particularly between Chevalier and Kinokamaw Lakes. Perched erratics are found on rounded hill tops.

The glacial drift forms vast, sandy outwash plains with a local "kame.­and.-kettle" topography; the kettles may attain widths of as much as severaJ hundred yards. Eskers are numerous and only the most prominent have been shown on the accompanying map.

Post.-glacial erosion is negligible. A large stream, such as the Tazin River, shows only slight deepening and widening of its bed, even in the outwash plains. The wider part of it forms Thainka Lake, which has steep sandy embankments rising 15 feet above the water level. The sand has been reworked to beaches, shoals, and narrow spits that at several loca.­tions protrude far into the lake. The spits are a result of the interaction of a prevailing northwest wind and currents. The influence of this pre.­vailing wind is also demonstrated by the formation of longitudinal dunes,

*Throughout this report the new approved spelling of Atha basca is employed except where reference is made to specific titles of previous reports which employ the Athabaska spelling.

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trending north 45 degrees west, and " blow,outs", which are especially numerous two miles west of Thainka Lake.

Although the Tazin River probably existed in pre-glacial times, this is impossible to confirm from the evidence observed in the map-area: most of its course passes through glacial drift, and constrictions like the Tessini Gap are completely scoured out by the ice.

PREVIOUS WORK:

Charles Camsell carried out the first geological reconnaissance in­vestigation over the area; he gave the name Tazin Series to "the oldest rocks encountered, a series of schist, quartzites, conglomerate, narrow beds of limestone, argillite and some volcanic rocks". A report and a map of the route travelled were published in 1916.

F. J. Alcock (I 936 ) considered it possible that more than one series are represented and suggested a "Tazin Group", a term which ever since has become widely accepted and used. His work was published as a report with an accompanying map on a scale of 4 miles to 1 inch.

T. Wilson, (1938 ) and R. Mulligan (1954 ) mapped the areas north of 60°N latitude and west and east of l 10°W longitude respectively. The results were published as maps, with marginal notes, on a scale of 4 miles to 1 inch.

]. D. Godfrey, of the Alberta Research Council, is presently engaged in mapping the Precambrian underlying the northeast corner of Alberta; a preliminary report on the mineralization and a photographic interpre, tation of that region were published in 1958. Reports and maps on a scale of one-half mile to one inch are in press.

Other field work done in the vicinity of Thainka Lake was by W. E. Hale; a preliminary account and maps, with marginal notes, on a scale of 1 mile to 1 inch were published in 1954 and 1955.

FIELD WORK AND ACKNOWLEDGMENTS:

Geological mapping, on a scale of 1 mile to 1 inch, of the Thainka Lake Area (West Half), was carried out by the author during the summer of 1960, with the able help of R. Bugry, assistant geologist, 0. A. Dixon, N . A. Kopperud, and D. B. Clipsham, junior assistants. D. F. Clausen carried out the specialized and meticulous drafting required by this report.

The shorelines of the Tazin River, Thainka, Kinokamaw, Martyn, and Kornash Lakes were mapped in detail. Pace and compass traverses were run, perpendicular to the structure where possible, and were spaced not more than one-half mile apart.

Vertical aerial photographs on an approximate scale of 1800 feet to 1 inch, supplied by the National Air Photographic Library in Ottawa, were used for the preparation of the base map and for mapping control in the field.

Because of the complex structural geology of the area and the resultant deformation of so many of the rock types, the full geological history may not be revealed until detailed mapping is completed in this and adjacent areas. It is hoped that the author will continue work in the Thainka Lake Area (East Half) in 1961 and will publish a report in 1962 which may modify the preliminary observations made in this report.

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The particular nature of the work made it necessary to employ certain special symbols and methods of representation on the map. These are adequately described in the text of the report and in the legend. The schematic, horizontal hatching or colour banding represents areas where no one map unit could justifiably be mapped but where dual components could be recognized as in the following examples :

Units ( I ) and (2 ); an intermittent occurrence of the two units within one locality; neither unit being mappable individualJy; Units (4 ) and (6) ; an area of extensive brecciation where products weakly reminiscent in appearance of both rock units occur; Units (8 ) and ( 11) ; a transitional boundary in which area neither rock unit is dominant.

REFERENCES:

Alcock, F . J . ( 1936) : Geology of Lake Athabaska Region, Saskatchewan; Geo!. Surv., Canada, Memoir 196.

Blake, D. A. W. (1955 ) : Oldman River Map Area, Saskatchewan; Geol. Surv. Canada, Memoir 279.

Camsell, C. (1916 ) : An Exploration of the Tazin and the Taltson Rivers, N .W.T.; Geo!. Surv., Canada, Memoir 84.

Chamberlain, J . A. (1959 ) : Structural History of Beaverlodge Area; Ee. Geo!. Vol. 54, Number 3.

Christie, A. M. (1953 ) : Goldfields-Martin Lake Map Area, Saskatche· wan; Geo!. Surv., Canada, Memoir 269.

Godfrey, J. D . (1958 ) : Aerial Photographs Interpretation of Precambrian Structures North of Lake Athabaska; Res. Council of Alberta, Bulletin 1.

-----: Mineralization in the Andrew, Waugh and Johnson Lakes Area, Northeastern Alberta ; Res. Council Alberta, Prelim. Rept. 58-4.

Hale, W. E . (1954 ) : Black Bay Map Area, Saskatchewan ; Geo!. Surv. Canada, Paper 53-15.

-----: Gulo Lake, Saskatchewan ; Geo!. Surv. Canada, Paper 54-6.

(1955 ) : Forde Lake Map Area, Saskatchewan; Geo!. Surv. Canada, Paper 55-4.

Harker, A. (1950 ) : Metamorphism. Second Edition. London.

Mulligan, R . (1956) : Hill Islands Lake (West Half), District of Mac­kenzie, N.W.T. ; Geo!. Surv. Canada, Paper 55-25.

Wilson, J. T. (1938 ) : Fort Smith, District of Mackenzie, N.W.T.; Map 607A.

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~~ ('-2\

NO'ITHWE ST - TkRfUTORIES - --Mi,oo?j -

Flpre 1: Location of the Thainka Lake Area in the north west corner of Saskatchewan. On the west haH are indicated the major faults, the water divide (W-D) and the northeastern basement.

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GENERAL GEOLOGY

GENERAL STATEMENT:

All the consolidated rocks of the Thainka Lake area are considered to be of Precambrian age.

The geology of the Thainka Lake area is dominated by faulting which caused widespread dynamic metamorphism and makes correlation of rock types and interpretation of historical events exceedingly disputable. For mapping purposes it is difficult to establish the relative importance of the different fault planes and movement zones involved in the "break, ing up" of the area; many of them are poorly exposed; rock units on both sides are, or appear to be, similar; and the width of the zone of cataclasis may be unrelated to the magnitude of the displacement. Considering the differing rock units on both sides of the Tazin River Fault, its continuous character, and the width of the associated breccias and mylonites, it is warranted to divide the description of the geology of the Thainka Lake Area (West Half) into two parts, east and west of this major fault.

East of the Tazin Fault a vast area of red gneisses occurs, apparently lying unconformably upon a "granitic" basement that underlies the northeastern corner of the area (Fig. 1 ). The original character of these red gneisses has been preserved best around Thainka Lake, where they constitute a chaotic assemblage of more or less impure arkoses, conglom, eratic arkoses, and conglomerates. Elsewhere, evidence of the origin of the red gneisses has been obliterated by regional and dynamic meta, morphism.

West of the Tazin River Fault the Red Gneisses (6 ), and the Peg, matitic Migmatite Complex (8 ) are confined to a wedge,shaped area and, towards the south, these units are strongly mylonitized and com, pressed between the Tazin River Fault to the east and the Morris Fault to the west.

The part of the area west of the Red Gneisses is underlain by the Western Meta-sedimentary and Meta,volcanic Complex (9, 10), which is intruded by various phases of the Granodioritic Complex ( 11 ). Lack of outcrop to the north and displacement to the south render the age relationships between these rocks (9, 10, 11 ) and the Red Gneisses un, certain.

THE NORTHEASTERN BASEMENT COMPLEX ( 1, 2, and 3 ):

The topography of the northeastern part of the area lacks the "grain" typical of the rest of the region. The weakly foliated, homogeneous character of the rocks involved, unlike those of other units, is considered to be the cause of their different, more rigid response to deformation, which has resulted in cataclasis being confined to fault planes. These rocks have been grouped in three units forming the Northeastern Base, ment Complex.

The granitic component ( 1) : Most of the basement is underlain by a pink, coarse, to medium,

grained granitic component which, in the field, appears to be of very uniform composition and only weakly foliated.

Thin sections show an exceedingly variable composition and an implication fabric due to successive stages of replacement. Both the plagioclase ( albite-ol igoclase ) and the potassium feldspar content may vary from less than 5 per cent to 50 per cent of the total rock volume. Both minerals may form porphyroblasts as much as 1 centimeter in dia,

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The following Table of Formations must be regarded as tentative.

TABLE OF FORMATIONS

Muskeg

Glacial drift, gravel, sand .

------- --- --GREAT UNCONFORMITY---------------

WEST OF THE TAZIN RIVER FAULT EAST OF THE TAZIN RIVER FAULT ----"',---,-------- ------- 11---,---------------

"0 c "O ~

.~l::!1 Md . d hi :._~~-~ v e ium, and fine-grame amp , ., v

..!!l c bolite and feldspathic amphibolite. c ~~ ~~

a] a] • >. Minor pink pegmatitic and granitic • >. ~ ~ material. ~ ~ ~- ~~ ~] ~:B

2:1 ·- )( .... cu .9 0.. "8 E ~8

c.,

Pink, grey, and white, coarse- and medium-grained biotite granodior, ite; locally chloritic and amphibol­itic; locally gneissoseand brecciated.

Western Conglomerate: matrix of fine-grained, green, quartz-chlorite­sericite schist and chlorite,sericite schist; pebbles of fine, and medium, grained chlorite-sericite qua rtzite; minor granitic and quartzitic peb, bles; strongly phyllonitic and brec­ciated.

Fine-grained, dark grey, black, and buff biotite-sericite,quartz schists and impure quartzites; meta-bas, altic flows; commonly phyllonitic, mylonitic, and brecciated.

Dark and light grey, impure quart­zites; dark grey, black, and green biotite-chlorite schists; biotite,seri, cite-quartz-feldspar schists; impure quartz-feldspar gneisses, locally coarse-grained and porphyroblastic; white, pink, and blue pegmatitic granite; mylonites; ultra,mylon, ites; phyllonites; breccia.

Medium- and fine-grained amphi, bolite and feldspa thic amphibolite .

Minor pink pegmatitic and granitic material.

Eastern Conglomerate; matrix of medium, to coarse-grained, red, purple, and green, sericitic meta, arkose; unsorted pebbles of granitic and quartzitic material; brecciated; locally phyllonitic a nd mylonitized.

Red Gneisses: well-foliated, coarse, medium,, and fine-grained, red to green, quartz-feldspar and chlorite­quartz-feldspar gneisses, commonly porphyroblastic and well layered, locally amphibolitic or biotitic; con, tains quartzitic and granitic pebbles; mylonites; ultra, mylonites; breccia.

Massive, and finely layered pink granulite ; locally amphibolitic.

Fine-grained, dark grey, amphiboli, tic, quartz-feldspar gneiss, locally porphyroblastic, locally strongly brecciated.

Red gneisses: well foliated, coarse-, medium, , and fine,grained, red to green, quartz-feldspar and chlorite­quartz-feldspar gneisses; commonly porphyroblastic and well layered; minor quartzitic and granitic peb, bles; mylonite; ultra-mylonites; breccia.

- - - ---SUPPOSED UNCONFORMITY-­

Coarse,grained, weakly to moder, ately foliated feldspathic amphibol, ite.

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Fine-grained, mafelsic, well,foli, a ted, amphibolitic and biotitic quartz diorite.

Coarse- to medium-grained, pink granitic rock; some pegmatitic material.

meter. Quartz constitutes between 20 and 40 per cent, and biotite, muscovite, chlorite, and epidote make up less than 10 per cent of the rock volume. Replacement by microcline, albite (perthitic ), and mus, covite has taken place successively. Later quartz replaces both feldspar and micas. Plagioclase has decalcification rims where it is in contact with the grid,twinned potassium feldspar, and to a lesser extent, with quartz. Incipient and advanced stages of retrograde metamorphism are wide, spread.

The "quartz dioritic" component (2) : A part of the basement consists of a fine,grained, mafelsic, well to

moderately foliated " quartz dioritic" component. In the field this rock weathers dark greenish grey and appears to be uniform in composition. Contacts with the felsic granitic rocks (1) are sharp. In the north this rock forms a considerable part of the basement while in the south only a few layers occur in extensive areas of granitic rocks. The thickness of these layers varies considerably. Unmappable mixtures of (1) and (2) are indicated on the accompanying map by a broad, horizontal hatching in bands of alternate colours. This method of indicating such intimate rock mixtures has been introduced in this report as the most satisfactory symbolic representation of the field relationships.

Thin section examination of the quartz dioritic rocks shows that the plagioclase (andesine,labradorite ) may constitute 30 to 50 per cent and the quartz 15 to 30 per cent of the rock by volume. Other major minerals are amphibole and biotite, forming 5 to 20 and 10 to 20 per cent of the rock volume respectively. Microcline occurs only as thin fracture fillings replacing plagioclase. The plagioclase, which is locally porphyroblastic, shows all stages of replacement by quartz and is commonly altered to an aggregate of sericite and epidote. Epidote also forms porphyroblasts apparently unrelated to the alteration of plagioclase or amphibole. Locally the amphibole may be completely replaced by biotite. A few garnet poikiloblasts occur. Prehnite, which occurs as fracture,fillings, appears to replace plagioclase, amphibole, and biotite.

Amphibolite (3 ) : A dark,green, coarse,grained amphibolite, showing moderate and

locally weak foliation occurs at two localities in the basement: one near the north boundary of the map sheet, the other one mile south of Bari, beau Lake. The amphibolite may contain coarse,grained "pegmatitic", pink feldspar, either throughout or as irregular "nests". Contacts of this rock with other basement rocks were not observed.

Thin sections show a simple mosaic of stubby, anhedral, poikilitic hornblende, which encloses chloritized biotite laths, and epidote. Schiller inclusions are common. Plagioclase occurs as minor, interstitial calcic andesine and also as coarse albite replacing amphibole. The plagioclase is always strongly altered to sericite and epidote and is partly replaced by quartz. This rock was probably of original gabbroic or pyroxenitic com, position.

Summary: The origin and relationships of the basement rocks are unknown. It is

probable that the basement extends eastward and that the mapping of the east half of the Thainka Lake Area will yield further information.

The contact of the granitic basement and the Red Gneiss Complex (4,7 ) is exposed at only two locations, both slightly less than two miles east of Mann Narrows. An unconformity is probably exposed at the southern location. Brecciated and phyllonitic, conglomeratic red gneisses

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and conglomerate (7 ) appear to overlie the granitic rocks which, apart from some cataclasis, in no way differ from rocks of the same unit else, where. The supposed unconformity could not be traced northward, probably because of displacement.

THE RED GNEISS COMPLEX (4, 5, 6, and 7 ) :

The Red Gneiss Complex is dealt with under one heading to stress its unity but may be subdivided as follows: Eastern Conglomerate (7);

Red Gneisses (6 ) ; Granulite (5 ); Amphibolitic Meta,sediment,

ary Rocks (4 ) . The subdivisions are of a very tentative nature, since no adequate thin section work could be done. It is hoped that the 1961 mapping of the easterly adjacent area will permit a more detailed study of the Thainka Lake Red Gneiss Complex.

Various stages of brecciation and metamorphism render recognition and correlation of the different rock types of the Complex very difficult. The rock changes encountered along strike in the vicinity of Thainka Lake are certainly due in part to structural control; faulting and a northerly plunge are responsible for the disappearance of the petrol, ogically heterogeneous terrane of the southeast corner of the map area. This plunge is in accordance with the plunge of the Kinokamaw Lake anticline. The northeastern limb of this anticline displays no detailed folding and consequently, since the foliation is parallel to the bedding, the core of the anticline is believed to be occupied by the oldest members of the Red Gneiss Complex. Layers of Red Gneiss proper occur inter, calated in this core, but the character is considered sufficiently different to warrant the introduction of the subdivision of the Amphibolitic Meta, Sedimentary Rocks (4 ). The contact with the overlying heterogeneous part of the complex is gradational in the south (indicated on the map by an horizontal hatching ) and abrupt west and southwest of Thainka Lake, where Amphibolitic Meta,Sedimentary rocks are overturned and overlie the younger subdivision of the Red Gneisses (6 ). The Red Gneiss sub, division covers the major part of the area and is composed of a wide variety of rock types with commonly indefinite t ransitions. To the north the Red Gneisses form a lens,shaped belt caught between the Tazin River Fault and the Northeastern Basement Complex and display in, creasing effects of dynamic metamorphism.

In the southeast of the area a poorly layered, fine,grained quartz, feldspar rock occurs. Locally this rock contains pale amphibole. Outcrops could not be traced north and west of Thainka Lake and relationships to the complex are obscure.

The youngest subdivision of the Red Gneiss Complex is formed by the Eastern Conglomerate (7 ). East of Thainka Lake the transition of the Red Gneisses (6 ) to this conglomerate appears to be gradual and, due to brecciation it is commonly impossible to determine the original nature of the rocks. West of Thainka Lake, on the Mann Narrows bluff, con, glomerate has been preserved overlying Red Gneisses with shallow and locally horizontal dips.

Fine, and coarse,grained pink pegmatite occurs throughout the Red Gneiss Complex. The coarse,grained pegmatite displays macroscopic graphic intergrowths.

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Granitic pebbles have been observed in the younger part of the subdivision, around Thainka Lake. Pebbles also occur in the Red Gneisses west of the Kinokamaw Lake Fault, in the vast lens,shaped belt which passes from north to south over the area.

Amphibolitic Meta-sedimentary Rocks (4 ) : Rocks of this subdivision of the Red Gneiss Complex are best pre,

served in the south, where the effects of dynamic metamorphism are least. It forms the core and northeastern limb of an anticline which to the north is overturned and cut off by the Kinokamaw Lake Fault.

The major rock type of the subdivision is a fine,grained, dark, amphi, bolite,bearing, quartz,feldspar gneiss which is finely layered. The layering results from a change in amphibole content and slight changes in colour of the quartz,feldspar component. The amphibole content varies from occasional layers of one crystal in thickness to a foliated, uniform quartz, feldspar rock with dark amphibole disseminated throughout, and forming not more than 15 per cent of the rock. A layering of one,half to one inch in thickness is very common. Locally the rock is biotite,rich and quartzitic. The colour varies from light to dark grey and buff with tinges of green, red, and pink and is dependent upon the mineral constituents. Non ... amphibolitic quartz-feldspar gneisses which resembles the Red Gneisses, although of finer grain, occur intercalated with rocks of unit (4).

A zone of strongly porphyroblastic, coarse,grained, granitic quartz ... feldspar gneiss occurs in the centre of the northeastern limb of the anticline. This rock cannot be differentiated from the Red Gneiss proper but its contact with the amphibolitic meta,sedimentary rocks is arbitrary. The cross-hatching on the accompanying map indicates the gradual increase of the more porphyroblastic character.

At the Kinokamaw Lake Fault, rocks on both sides of the fault show a grada tional transition to a brick,red "breccia,gneiss". The " breccia, gneiss" has been mapped as part of the Red Gneiss subdivision, but where this "breccia ... gneiss" possesses features suggesting its derivation from the Amphibolitic Meta,sedimentary Rocks this feature has been indicated on the accompanying map by a horizontal hatching.

Granulite (5 ) : A fine,grained, pink, quartz ... feldspar rock which Jacks the gneissic

layering characteristic of the other rocks of the Red Gneiss Complex occurs intercalated with the Red Gneiss (6 ) and is indicated by horizontal hatching on the accompanying map since it forms a recognizably different rock type. The term "granulite" is used in the sense of reports previously published by this Department and as described by Harker (1950, pp. 246,8 ) . A pale amphibole is developed in places and minute impurities give rise to a very fine local layering less than two millimeters in thick, ness.

Red Gneisses (6): Rocks mapped as Red Gneisses form a wide variety of types and it is

probable that the differences represent original transverse and lateral changes in the sediments as well as later metamorphic changes. Around Thainka Lake, in what appears to be the youngest part of the sub, division, layering appears less pronounced, isolated quartzitic and granitic pebbles occur, and the contact with the overlying Eastern Conglomerates (7 ) is arbitrary.

In such a structurally complicated area only more extended studies and detailed work may reveal the relative roles of original sedimentary

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differentiation; variation in the regional metamorphism; and the wide, spread dynamic metamorphism.

The part of the area least affected by dynamic metamorphism is the southeast corner, which, in general, is poorly exposed. Good outcrops in this part of the area are situated on the north and south shores of the Tazin River west of the Thainka Lake Fault East. The rocks form an assemblage of pink and red, medium, to fine,grained, quartz,feldspar gneisses, which are irregularly porphyroblastic. A local layering is in, dicated by a chlorite,biotite content which varies from negligible to predominant. The micas are uniformly disseminated and in places the concentration is so high as to give the rock a mafelsic composition, with layering apparently lacking over hundreds of feet.

To the southeast the biotite increases relative to the chlorite and arnphibole is locally abundant. To the north, dynamic metamorphism appears to have had a " homogenizing" influence and resulted in the formation of fine,grained chlorite,quartz,feldspar schists and gneisses, with varying amounts of rolled and angular red feldspar porphyroblasts of medium and coarse grain. The colour of these rocks depends on the relative amounts of chlorite and quartz,feldspar and varies from green to brick red respectively. Continued deformation gave rise to phyllonites, crush breccias, and mylonites which are intermixed with less intensely altered parts of the Red Gneisses. Red Gneisses, similar to those around Thainka Lake, have also been observed to the west, between the Kino, kamaw Lake Fault and the Tazin River Fault. To the north, this vas t belt of Red Gneisses cannot be differentiated due to the effects of dynamic metamorphism previously described. To the south, the Red Gneisses appear to become more coarse grained and porphyroblastic and underlie an extensive terrane of well,layered, reddish, chlorite,quartz,feldspar gneisses with tight folds which usually have shallow plunges. The layering is indicated by the changes in the chlorite and quartz,feldspar proportions. Layers rich in chlorite are regularly less than two inches wide, whilst those of more felsic material attain widths as much as one foot.

The Red Gneisses in the north part of the area, west of the Tazin River Fault, have locally been strongly granitized resulting in the dis, appearance of the layering.

Many extensive occurrences of Red Gneiss in the area may appropri, ately be described as "breccia,gneiss". These rocks are composed of brick,red, strongly brecciated quartz and feldspar of fine and medium grain with or without chlorite and rolled and fractured feldspar porphyro, blasts. Faults in the Red Gneiss Complex are commonly bordered on both sides by this "breccia,gneiss" which may extend for a considerable distance away from the fault. plane. A typical example of these rocks outcrops south of the Tazin River, at the east boundary of the map area, east of the Thainka Lake Fault East. All rocks are strongly brec, dated and their pre,tectonic nature cannot be determined. One small outcrop, indicated by a blue dot on the accompanying map, appears to have been derived from the Eastern Conglomerate (7 ).

Where "breccia,gneiss" have features which suggest derivation from a certain subdivision, this has been indicated on the accompanying map by the horizontal hatching, e.g., Amphibolitic Meta,Sedimentary Rocks (4) east of the Kinokamaw Lake Fault; and the Eastern Conglomerate (7 ) east of the Thainka Lake Fault East.

The "Granulite" has been indicated on the accompanying map by a horizontal hatching because Red Gneisses occur intercalated and outcrop often is scarce.

13

The Eastern Conglomerate (7 ): The Eastern Conglomerate and conglomeratic Red Gneisses are best

preserved east of the Thainka Lake Fault West, in the vicinity ofThainka Lake. The Eastern Conglomerate overlies the locally conglomeratic Red Gneisses around Thainka Lake, and these in turn appear to overlie the heterogeneous part of the Red Gneiss Complex in the southeast portion of the area, but the Eastern Conglomerate also exhibits a vertical contact with the Northeastern Basement (1;3 ).

The southernmost of the contact;outcrops, one and one;half miles east of Mann Narrows, appears to show an unconformity. The conglo; meratic Red Gneisses and Eastern Conglomerate appear to have been preserved in a shallow embayment in the Northeastern Basement and directly overlie granitic basement rocks.

The best exposure of the conglomerate is a small outcrop about two miles southeast of Mann Narrows on the shore of the Tazin River (Plate IA ).

The pebbles, which are as much as five inches in diameter and are rounded to angular, occur unsorted in a coarse; to fine;grained "multi; coloured" matrix. Cataclasis is very strong and though no orientation can be recognized, the whole rock appears to be crushed. Purple ultra; mylonite has been observed in this conglomerate. Pebbles are not stretched and locally show step;Jike offsets due to minor parallel fracturing. The pebbles are composed of white quartz and of pink to red granitic material.

The matrix is variably coloured from dark purple, buff, cream, and olive green, to pink and orange red. The dominant constituents of the matrix are quartz and potassium feldspar, each of which form about 40 per cent of the rock by volume. A few small grains of untwinned and albite;twinned albite, usually sericitized, are commonly present.

The grains of the various constituents appear to be of granoclastic origin; they are locally outlined by ferruginous material and an inter; stitial sericitic "paste" of very fine grain. At least a part of the sericite may have been derived from crushed potassium feldspar and muscovite, the latter resulting from a later replacement. Quartz has been recrystal; lized and remobilized, and forms intragranular implication fabrics in coarse and medium sized, rounded to angular granules. Quartz also fills many minor fractures. Potassium feldspar commonly forms rounded to angular, coarse to fine grains. Apatite and zircon are scarce; well rounded magnetite is abundant.

The Eastern Conglomerate has been designated by one unit number (7 ) on the accompanying map but different varieties of conglomerate occur which may or may not represent the same unit. The conglomerate forming the cap on the Mann Narrows bluff is a strongly crushed, uni; form, unlayered, unorientated, purplish;red quartz;feldspar rock of coarse to fine grain. Quartz fragments, as much as one to two inches in diameter and rounded to angular are dispersed throughout. This variety has also been found at the supposed unconformity, and in a strongly crushed and gneissose form on the scarp a short distance east of Mann Narrows. The crushed gneissose stages resemble the Red Gneisses so closely that it is quite possible that similar occurrences have been overlooked locally while mapping the Red Gneiss terrane.

It is doubtful whether the conglomerate occurring north of Mann Narrows is indeed the same as that forming the cap of the Mann Narrows bluff as has been indicated on the accompanying map. The conglomerate outcropping along the Tazin River (Plate 1B ) contains only a few pebbles

14

)

with a compos1t10n ranging from granitic to white and black quartz. The pebbles occur in a phyllonitic matrix of a buff, green, and olive,green colour.

Contacts of the Eastern Conglomerate with the Red Gneisses are in general gradational. Rocks that are nearly similar to certain parts of the Red Gneisses have been found on the best conglomerate exposures, two miles southeast of Mann Narrows. Identical rocks to those seen inter, mixed with the conglomerate are found in the Red Gneisses, west of the Kinokamaw Lake Fault, on the latitude of central Thainka Lake. The contact of the conglomerate at Mann Narrows is more abrupt.

Around Thainka Lake the Red Gneiss Complex possibly could be interpreted as a chaotic assemblage of metamorphosed conglomerates, conglomeratic arkoses, and arkoses.

THE PEGMATITIC MtGMATITE COMPLEX (8 ) :

The Pegmatitic Migmatite Complex is confined to the western part of the area, where, apart from scattered occurrences north and south of Lamont Lake, it forms a narrow belt one,half to one mile wide, located along the west side of the Tazin River Fault. To the south this belt is compressed between the Western Granodiorites ( II ) and the Red Gneisses (6 ) and becomes increasingly brecciated and mylonitized until ultimately differentiation between the various constituents is very difficult.

The Pegmatitic Migmatite Complex is the southern continuation of the much more extensive zone of "Tazin Group" rocks, as much as four miles in width, in the Hill Island Lake Area (Camsell, 1916, Mulligan, 1956), where the occurrence of limestones and calcareous slates was reported.

The constituents of the Pegmatitic Migmatite Complex consist of a variety of sedimentary rocks: dark,grey to black, more or less impure quartzites, locally lighter coloured; dark,grey to black, rusty weathering, biotite and chlorite schists; biotite,sericite,quartz,feldspar schists; dark, grey, impure meta,arkoses; and light,grey meta,arkoses which can not be distinguished from rocks of the Red Gneiss Complex. The local develop, ment of a gneissic texture, coarser grain size, and porphyroblasts of feld, spar, also tends to obscure the relationships with the Red Gneisses. There appears to be a narrow transition zone in which both units (6 ) and (8 ) occur interlayered and an apparent change along strike into the Red Gneisses northwest of the Tazin River Fault is tentatively indicated on the map by a horizontal hatching east of Martyn Lake.

In some localities north and south of Lamont Lake the rocks are similar to those of the zone confined to the Tazin River Fault. North of Lamont Lake they contain considerably less pegmatitic granite and are more uniform, gneissic and porphyroblastic. The porphyroblasts are commonly rolled and are of calcic andesine composition. They occur in a biotite,quartz,andesine matrix with remnants of a pale amphibole in various stages of replacement by biotite. This has been designated as phase P of the Pegmatitic Migmatite Complex.

The name Pegmatitic Migmatite is appropriate because of the char, acteristic association of the meta,sedimentary rocks with pegmatitic to granitic material, which has been intensively "shot through" the unit. Although mainly restricted to unit (8), the pegmatitic granite also occurs locally in the Red Gneiss Complex (6), which appears to be "impreg, nated" by the granitic material, especially east of Martyn and Waugh Lakes; on the map this has been indicated by a horizontal hatching.

15

Except for the south of the area, where brecciation and mylonitization are extreme, the sedimentary and granitic components are easy to differ ... entiate. Where over 75 per cent of the migmatite consists of the pegmatitic granite this has been indicated on the accompanying map by a " G" overprint. The transitions between the more and less granite ... rich parts of the migmatite are locally abrupt and are indicated on the map as an approximate contact.

The potassium feldspar of the pegmatitic granite component may be white, pink, or blue and forms crystals and cluster of crystals as much as three feet in diameter. They are especially well developed in the rusty, dark quartz ... biotite schists, which locally are strongly contorted by these, and by pods, nests, and bodies of the pegmatitic granite. The plagioclase of the pegmatitic granite is albite-oligoclase and has been replaced by microcline to varying extents. The microcline is perthitic due to replace ... ment by albite. Subsequently introduced muscovite occurs in coarse flakes, as much as one inch in diameter.

THE WESTERN META ... SEDIMENTARY AND VOLCANIC COMPLEX (9 ) :

The Western Meta ... sedimentary and volcanic Complex is confined to the western part of the area, where it borders the granodiorite masses and bodies which it appears to overlie. Contact relationships with other rock units in the south are obscured by faulting, and are not exposed in the north.

Because much of the movement in this part of the area was taken up by this complex, as shown by its intense folding and dynamic metamor ... phism, the relative amounts of the different components forming the complex are difficult to estimate. Quartzites, schists, meta ... arkoses, meta ... volcanic flows, meta ... conglomerate (IO), amphibolitic veins (12), pegmatitic material and vein ... Jike bodies of the same composition as the protruding granodiorites (11) are present.

The majority of the complex is composed of fine-grained, dark ... grey, black and buff weathering, biotite ... sericite ... quartz schists and more or less impure quartzites which are well bedded and often finely layered.

Porphyritic meta ... volcanic flows have been indicated on the accom ... panying map only where positively identified, and may constitute a much larger part of the complex. West of Martyn Lake the extrusive rocks attain widths as much as 60 feet and occur as several layers intercalated with the conglomeratic schists and intrusive granodioritic veins. The large island and the eastern peninsula in the north end of Martyn Lake are made up wholly of massive extrusive rock. The flows weather dark grey to black and locally buff and rusty and appear to be of uniform composition, weakly foliated in the north and well foliated in the south. Stubby amphibole megacrysts, as much as five millimeters long are numerous southeast of Waugh Lake.

The composition of the flows is basaltic. Quartz is commonly lacking but occurs locally in the matrix. Plagioclase is strongly altered to sericite and epidote, and is the main constituent of the fine ... grained matrix. It forms equidimensional laths, which show a sharp drop in anorthjte content at their rims. The composition of the cores is a calcic Jabradorite, while the rims are of intermediate oligoclase. Rare oscillatory ... zoned plagioclase megacrysts are present. Megacrysts of a pale, weakly pleo ... chroic amphibole, partly altered to chlorite and biotite, may form from less than 5 to 35 per cent of the rock by volume. Remnants of zoning appear to be present in the amphibole megacrysts.

16

The relation between the Western Meta-sedimentary and volcanic Complex (9) and the Pegmatitic Migmatite Complex (8 ) is not clear as outcrops in this part of the area are scarce. Rocks of the two complexes appear to grade into each other, along strike, between Lamont and Martyn Lakes. Minor quartz-feldspar gneisses and blue pegmatitic material, both characteristic of the Pegmatitic Migmatite Complex (8 ), are intercalated in the dark meta-sedimentary rocks of the complex (9), east of the north end of Martyn Lake. In the outcrop area about one-half to one mile southwest of Lamont Lake the rocks are strongly mylonitized but show similarities to those of both uni ts (8 ) and (9 ). It is not known to what extent metamorphism, structure, or original sedimentary com­position are responsible for the lateral change.

THE W ESTERN CONGLOMERATE (10 ) :

The best exposures of the Western Conglomerate (Plates 2A, and 28 ) occur between Martyn and Flagon Lakes where it has been intruded by the granodiorite. Various strongly sheared and less conglomeratic zones are interlayered with flows, schists, and granodiorite veins west of the north end of Martyn Lake.

The matrix of the conglomerate is a green and rusty, quartz-chlorite, sericite schist. Boulders and pebbles of various compositions have been found, including granite and white quartz, but the most predominant type is composed of fine-grained chlorite-sericite quartzite, with some admixture of feldspar. In this fine-grained micaceous quartzite numerous coarse, clear quartz grains are disseminated. A collected pebble, well rounded and about one inch by two inches by two inches in size, is com, posed of a reticular mass of rounded quartz grains of 1.0 millimeter to 1.5 millimeters in diameter, having a granoclastic character. To the north the size and angularity of the pebbles and boulders increases, ultimately resulting in blocks as much as six feet by three feet.

J. D. Godfrey (personal communication ) favours a dynamic origin for this "conglomerate". He considers that parts of quartz and granitic veins in which boudinage was developed became detached from each other and ultimately formed pseudo-boulders.

If the conglomerate is of sedimentary origin, special conditions must have existed to account for the large, and often angular character of the fragments.

THE WESTERN GRANODIORITE COMPLEX (11 ) :

The Western Granodiorite Complex underlies a part of the map-area along the west boundary. The complex forms two main masses, situated in the vicinity of Renwick Lake and Ney Lake respectively, which are the eastern extremities of a more extensive granitic terrane in Alberta, and several smaller bodies, one half mile to one mile wide, that protrude through the Western Meta-sedimentary and volcanic Complex (9).

Like the Northeastern Basement (l-3 ), the granodiorite appears to have better resisted the deformation which is so characteristic of the centre of the map-area. Brecciation is more confined to movement planes. The granodiorite locally contains small bodies or lenses of fine-grained pink felsic material.

The weathered surface is light-grey to pink and is mottled by more or less oriented feldspar megacrysts, as much as one inch in length, which are especially well developed in the most northerly of the two main masses

17

(phase M ). The matrix locally contains much biotite and this, together with some amphibole and chlorite, imparts a darker colour to the rock. The granodiorite is medium to coarse grained. The feldspar megacrysts are poorly developed in places, resulting in a more equigranular phase (phase F ), which is restricted to the smaJ1 bodies. The southern part of the small body, one and one.-half miles east of Morris Lake, is similar to the main mass of granodiorite around Ney Lake.

The contact with the dark Meta.-sedimentary Complex (9) has been observed in several places and, where the influence of deformation was least, west of Lamont Lake, appears to be abrupt. Twelve hundred yards northwest of Lamont Lake the granodiorite is fairly well foliated, rich in megacrysts, and undisturbed and displays a migmatitic contact zone, 15 feet wide, against the dark meta.-sedimentary rocks. Towards this contact zone, the granodiorite contains an increasing number of small inclusions of country rock as much as IO inches long with sharp contacts and typically corroded outlines. At the migmatitic zone the grey grano.­diorite has a pink rim, which is two to three feet wide, and an abrupt, concordant contact, which is locally truncating. The migmatitic zone consists of a gneissic, heterogeneous, medium.- to fine.-grained biotite.­quartz.-feldspar rock, which changes laterally and transversely over short distances. The migmatitic contact zone is only locally developed and is absent one.-half mile to the south.

These rocks are shown on the accompanying map as the "Western Granodiorite Complex" but more laboratory work may reveal important differences between the various small protruding bodies and the more westerly, main masses which are exposed into Alberta. The southeastern, isolated granodiorite body is of a coarse grain size, porphyroblastic, and very similar to the southern main mass around Ney Lake, but most of the smaller bodies are, in contrast, generally equigranular and consist of a medium.- to fine.-grained phase (phase F ). The contacts of the smaller granodiorite bodies resemble those of the northern main mass as pre.­viously described.

The contact of the northern granodiorite body with the Western Conglomerate (IO ) is exposed immediately southeast of Flagon Lake and appears to be parallel to the foliation of the conglomerate, which is "pressed aside" by the granodiorite body. This is also shown a few yards east of the contact, where an oval.-shaped, nine feet long and five feet wide, body of granodiorite protrudes through the conglomerate. Locally at the contact a zone of biotite.-muscovite schist and white vein quartz occurs, which is as much as five feet wide.

In thin sections of the main masses of granodiorite the mineralogical composition varies within the following limits: quartz 20 per cent; potas.­sium feldspar 20 to 40 per cent; plagioclase 30 to 50 per cent; and biotite and minor amphibole from less than 5 to about 15 per cent.

Late quartz occurs as fracture fillings and intergranular replacement rims and contacts against the other major minerals. Potassium feldspar is commonly coarse grained and displays irregular grid twinning and zoning and is locally perthitic and contains inclusions of amphibole and plagioclase, which it evidently replaces. Post.-microcline myrmekite is developed adjacent to plagioclase, amphibole and biotite. Euhedral to suqhedral plagioclase is locally strongly sericitized and shows oscillatory zoning with a composition ranging from calcic oligoclase at the core to sodic oligoclase in the rim. The outer edges are locally irregularly decal.­cified. Hornblende is partly altered to a strongly pleochroic, dark brown.­ish.-green variety of biotite. Biotite occurs with quartz and epidote in

18

fracture fillings. The plagioclase zoning is best shown in the small body north of Flagon Lake, where the plagioclase cores have a composition of sodic andesine. In the Flagon Lake body plagioclase has better developed crystal faces and appears to be more abundant, forming as much as 50 per cent of the rock. Potassium feldspar is less abundant than in the main granodiorite masses and forms only from 5 to 20 per cent.

Contacts of the southern main mass are not exposed. Contacts of the north end of the northern main mass with phase P of the Pegmatitic Migmatite (8) appear to be locally gradational through an amphibole, bearing transition zone, which has been indicated on the accompanying map by horizontal hatching. Elsewhere it seems likely that the Western Granodiorite has an intrusive nature.

I

MAFIC AND MAFELSIC DYKES (12 ) :

Mafic and mafelsic dykes or veins occur in the Red Gneiss Complex and the Meta,sedimentary and Meta,volcanic Complex but appear to be lacking in the Eastern Conglomerate. These rocks are of medium, to fin~,grain and the composition ranges from amphibolite to feldspathic amphibolite. The feldspathic variety is especially abundant in the Red Gneisses (6 ) . The width varies from a few feet to as much as 300 feet.

The dykes have been indicated schematically on the accompanying map.

STRUCTURAL GEOLOGY

Both faulting and folding are largely confined to the belt of layered rocks between the Northeastern Basement Complex and the Western Granodiorite Complex.

FOLDS:

The rock units involved have been tightly folded in detail although locally the folding may assume a more open character. Most folds are too small to be indicated on the scale of mapping employed, except schemati, cally. The plunge of the fold axes is remarkably consistent at between 0,40 degrees to the north, except for some exceedingly contorted parts where northerly and southerly plunges have been observed.

Structures of regional importance were recognized at two localities. An anticlinal structure with a northerly plunge of 40 to 60 degrees occurs at the south end of Kinokamaw Lake near the southern boundary of the area. To the west and north of this structure detailed folding and extensive brecciation took place, but to the east the finely layered Amphibolitic Meta,sedimentary Rocks ( 4) appear to form an isoclinal series, dipping 35 to 75 degrees to the northeast.

A second structural feature is centered around strongly brecciated, arkosic Red Gneisses (6) at Thainka Lake. The Red gneisses dip at sha11ow angles to the north, and are overlain by conglomerate (7), which forms the cap of the Mann Narrows bluff and is apparently a northerly plunging remnant that became compressed between the arkosic red gneisses further to the north.

19

Dips around Thainka Lake are generally shallow, ranging from Oto 25 degrees to the northeast and in contrast to the rest of the area, folding here is of a larger scale and is more open.

FAULTS:

Faults occur in all the major rock units of the area, but the majority are confined to the belt of metamorphosed, layered, predominantly sedimentary rocks ( 4, 10), which is compressed between the Northeastern Basement Complex (1,3 ) and the Western Granodiorite Complex ( 11 ).

An arc,shaped system of more or less parallel, northerly trending longitudinal faults dominates the structure of the area. South of Thainka Lake parts of these branch to the southeast. A simplified representation of the major fault system is shown in Figure 1.

The dip of the longitudinal faults, as indicated by the dip of the associated ultra,mylonites, mylonites, and breccias, appears to be parallel to the gneissosity, which is in general steeply dipping to the west in the north and west of the area and shallow to the east around Thainka Lake.

Many distinctive features have been observed associated with the faulting: (a) brecciation, which is exposed over vast parts of the layered belt,

and may result in the formation of mylonites and crush breccias over widths of many hundreds of feet, e.g., immediately west of Thainka Lake;

(b) mylonitization, resulting in the formation of ultra,mylonites that may range in width from less than a millimeter to many hundreds of feet. The ultra,mylonite is a dense, cherty, quartzite,like rock, which may have a straight and delicate slaty layering (hartschiefer) and whose colour depends on the original rock. They are in general intercalated with less ultra,mylonitized layers. Ultra,mylonites are especially well developed along the Tazin River Fault, in particular in the south, where the Pegmatite,Migmatite Complex (8) has been caught between the Morris and the Tazin River Faults. Good ex, posures occur also on the Tazin River, immediately north of the Provincial boundary (Plate 4A ) ;

(c) vein quartz, as much as fifteen feet wide, containing rock fragments. (Plate 38);

(d) formation of phyllonites. In the outcrop area a short distance west of Richmond Narrows, phyllonites occur along strike of the more southerly crush breccias;

(e) strongly broken zones, exhibiting a fine "rhombic" parting and attributed to the latest stages of movement;

(f) displacement of rock units; (g) rusty weathering of limited extent has been observed at many locali,

ties, and some leaching and minor mineralization are common; (h) more or less pronounced linear valleys commonly accompany the

faults but exceptions occur. Layers made up solely of ultra,mylonite, as much as 80 feet wide, occur on hill tops. The ultra,mylonite is an exceptionally hard rock and it is evident that the formation of ultra, mylonite itself opposes rather than assists in the formation of linear valleys along the trace of fault planes;

(i) at one locality ultra,mylonites truncate detailed faulting in the red gneisses;

(j ) small scale structures that bear a relation to the relative movement, e.g., rodding, crenulations, slickensides, (Plates 4A and 48 ) .

20

A second system of faults is developed more or less perpendicular to the dominant arc;shaped system. These faults form distinct linear valleys and are, in the north of the area, rather regularly spaced, slightly more than one mile apart. In the centre of the arc they are conspicuously absent. The associated cataclasis, resulting in a brick red, hardened gouge;like material, is entirely confined to the movement plane. For descriptive purposes this set of faults will be described as dip faults.

Two directions of diagonal faults pass at angles of less than 45 degrees through the main arc of strike faults. A northwest trending system seems to be best developed in the south of the area, while northeast trending diagonal faults are more common in the north.

JOINTS:

The dip faults may have originated as tension joints. Signs of catacla; sis are usually lacking along the walls of the associated valleys, which are canyon;like in the south, e.g. , between Oswald and Chevalier Lakes. The sizes of these canyons vary, but their vertical or nearly vertical walls are characteristic. One mile west of the centre of Kinokamaw Lake an east;west canyon, 20 feet deep, three feet wide at the base, and six feet wide at the top displays a few remnants of a parallel crush breccia.

CONCLUSIONS:

A relative displacement between the rigid Northeastern Basement and the Western Granodiorite Complex must have been the cause of the restricted distribution of faults and of the widespread cataclasis developed in the central belt of layered rocks.

The extensive formation of breccias and crush breccias around Thainka Lake in shallow dipping meta;arkoses and conglomerates (6, 7 ) indicates a considerable horizontal component of the displacement. Parallel, intercalated zones of mylonitization and phyllonitization suggest thrust; type faulting in this part of the area. This is in accordance with crenula; ti.ans observed on the scarp about one mile east of the centre of Thainka Lake, where fold axes plunge from O to 35 degrees southeast, indicating that the hanging wall moved up and south.

This southward component is also indicated by the two northwesterly trending thrusts directly west of the centre of Thainka Lake, close to the shore. The horizontal component in these thrusts is negligible.

The regional arrangement of the Thainka Lake Faults (Figure 1 ) suggests an imbricate type of structure around Thainka Lake, resulting from a directed pressure from the northeast.

The continuous character and length; the different rock units (6 and 8) on opposite sides; and the extensive ultra;mylonitization justify attributing major importance to the Tazin River Fault. The associated ultra;mylonites dip 70 to 80 degrees to the west and northwest. A rodding, best visible on the foliation planes of the ultra;mylonite, (Plate 4A), dips 17 degrees to the north in the northern part of the area and increases to 35 degrees north in the southern part. This rodding is either the a; orb; direction of movement, which in the south, in either case, must have had a considerable horizontal component.

If the Tazin River Fault movement is considered in an areal environ; ment and as being due to the same stress field as the thrusting in the east; then the configuration of the branching longitudinal faults; the dig; tribution of the rock units ( 4; 10 ); and the inferred, southwesterly;

21

directed pressure from the Northeastern Basement, suggest that a right ... handed movement is more probable than a left,handed movement.

This implies the two possibilities as illustrated in figure 2.

,. ,. ;;; ;;; ... ,! ..

u~~o o\ \U

~· ~· Flrure 2: Alternative Interpretations ot movement on the Tazln River Fault

(a ) Rodding represents the b,direction (b) Rodding represents the a,direc, of movement, in which case the fault tion, in which case faulting is of is a steep thrust fault. the wrench-fault type.

The formation of dense, cherty, quartzite ... like, ultra,mylonite layers, as much as 80 feet in width, in relatively undisturbed Red Gneisses suggests that compression took place at some time during the movement.

Many apparently controversial detail,structures have been observed: (l) Crenulations and lineations on a steeply easterly dipping, minor,

longitudinal movement plane, one mile east of the Tazin River Fault and two miles south of the Northwest Territories Boundary, indicate a a relative upward movement of 48 degrees of the west wall with a left, handed horizontal component (Plate 48) ;

(2) In the same area, between the Tesseni and Tazin River Faults, two minor diagonal faults, belonging to the north,northwest system, display a left,handed offset which indicates stretching parallel to the strike;

(3 ) Fracture cleavage in the meta,sedimentary rocks at the southwest shore of Lamont Lake indicates left,handed movement but this is not supported by a rolled pebble in the Western Conglomerate (see Plate 3A ).

Only at Kinokamaw Lake do the breccias and mylonites with westerly dips (characteristic of the western part of the map area) abut those with northeasterly dips ( characteristic of the southeast part of the area) (Figure 3 ). Elsewhere in the map area different attitudes "approach" each other gradually. The movement on the Kinokamaw Lake Fault does not appear to have an important strike,slip component. The thrust, ing that took place from the northeast and from the west appears to be related to the limbs of the Kinokamaw Lake anticlinal structure. North ... easterly dipping thrust faults show changed dips along strike through a vertical attitude to westerly dips further to the northeast when they approach the Kinokamaw Lake Fault. The Kinokamaw Lake Fault appears to be the principal fault of the area and movement is probably as indicated in Figure 3. ·

22

" -I! i:,

-..J

:i l:l

E

J. c, .... 0 c: I:. ·~

:i.r: •

Figure a. Part of Section IV (Map 61A) across the Klnokamaw Lake Fault at Klnokamaw Lake.

Deformation, if related to one main stress field, may still have resulted in faults of different ages, and certainly repeated movement must have taken place along many of the movement planes (Plate 38 ). Mylonitic fragments have been observed to be contained in strongly broken, chaotic, crushed breccias.

The recrystallized, rectilinear ultra;mylonites appear to be more abundant in the west of the area, especially along the Tazin River Fault. Crushing and brecciation evidently played a more important role in the eastern part of the area. These differing features may be due to movement at a shallower depth or to the lower angle of thrusting in the east, or to both factors.

ECONOMIC GEOLOGY

No mineral occurrences of direct economic importance were observed but traces of hydrothermal mineralization occur in many parts of the area.

Minor disseminated pyrite was observed in all rock units encountered and locally constitutes two to three per cent in the Meta;sedimentary and Meta;volcanic Complex. Pyrite also occurs associated with the wide; spread faulting: veinlets as much as one;eighth inch wide occur along fault planes.

Minor galena occurs at the northeastern boundary of the northern granodiorite body, two miles south of Lamont Lake.

Tourmalinization is widespread in the west of the area and occurs in the Western Meta;sedimentary and volcan ic Complex, the Western Conglomerate, and the Granodiorite Complex. The tourmaline is pleo; chroic from dark blue;green to almost colourless and occurs disseminated and as irregular veinlets as much as one inch wide.

J. 0 . Godfrey and R. Y. Watanabe (1958, and personal communica; tion) report the occurrence of"small amounts of pyrrhotite (?) , pyrite,

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and arsenopyrite in quartzite on the east side of Waugh Lake" and " irregular veinlets of arsenopyrite as much as one to two millimeters wide, about one,half mile east of the Alberta boundary at latitude 59° 49' 30"N."

Limited staking for radioactive minerals has been done along the Tazin River Fault southwest of Sheppard Lake and along the Kinokamaw Lake Fault north of Chevalier Lake. Four diamond drill holes each of approximately 100 feet were drilled and a scintillometer ground survey was carried out in both localities. Results were not encouraging and all claims have been allowed to lapse.

The existence of the regional zone of fau lting, and the traces of hydro, thermal mineralization, justifies prospecting in this zone along strike, as far south as Lake Athabasca.

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Plate IA- The Eastern Conglomerate (7) ; South shore, Tazln River, 700 yaTds east of longitude 109° 45'W.

Plate IB- The Eastern Conglomerate: East shore, Tazln River. one mile south of Tesslnl Gap. Strike ls N 5° W, dip vertical. Ice grooving ls N 60 E .

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Plate 2.A- The Western Conglomerate (10) ; southeast of Flagon Lake.

Plate 2B: The Western Conglomerate, southeast of Flagon Lake.

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Plate 3A- Rolled fragD1ent in the Western CongloDlerate, north of Martyn Lake. The pencil points to the north. Drag, due to a right-handed D1ove· Dlent, Dlay have caused the S-shape.

Plate 3B- Breccla of the T essenl Fault; east shore of Tazln River, two and one half .miles south of latitude 60°N. Blocks and fragments of dlfferent rock types, and vein quartz and breccla are contained in the chaotic assemblage.

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Plate 4A- Rodding in Tazin River Fault ultra-mylonites, west shore of Tazin River, 20 yards north of Provincial boundary.

Plate 4B- East wall of minor vertical movement plane in Red Gneisses, east shore of Tazin River, 2 miles south of Provincial boundary. A mineral lineation with a plunge of 48 degrees to the north intersects crenulatlons at 90 degrees. The east wall moved down, and northward, relative to the west wall. North is to the left side of the photosraph.

R EG INA, SASKATCHEWAN :

Printed by LAWRENCE AMON, Printer to the Queen's Most Excellent Majesty 1970

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