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The geology of Amco Lake, Burnet Creek and Wreck Lake Coppermine River area, N.W.T. Sheng, Cheng-Chun 1958

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THE GEOLOGY OF AMCO LAKE, BURNET CREEK AND WRECK LAKE COPPERMINE RIVER AREA, N.W.T. by CHENG-CHUN SHENG B«,Sc, University of Peking, China, 19h7 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n the Department of GEOLOGY We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA A p r i l , 1958 i ABSTRACT Two separate areas which are located in the southern portion of the Coppermine River area were surveyed by the author during the summer of 1957 • The area under investigation i s underlain by the upper part of the Epworth Series and the lower part of the Coppermine River Series. Because of the lack of fos s i l s i n these series, they are presumed to be of late Precambrian age. The upper part of Epworth Series is represented by dolomite and inter-bedded quartzite. The lower part of the Coppermine River Series i s represented by a series of basalt flows which are typica l tho le i i t e s , and interbedded sandstone in i t s upper part . A monzonite dyke crosses the basalt flows and sandstone at a high angle and is para l le l to the main basaltic dyke swarm seen i n the Takiyuak Lake area. The common structural feature is a series of tension faults trending from N10°E to Nl»5°E and N10°W to N20°VJ, This i s believed to have originated by the compressive force from the north induced by the Caledonian movement* The ore minerals, mainly chalcocite, occur i n quartz-carbonate veins in feeder dykes and flow tops of basalt . Flakes of native copper are occasion-a l l y found in the fractures of the basalt flows* I n p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f the r e q u i r e m e n t s f o r an advanced degree a t the U n i v e r s i t y o f B r i t i s h Columbia, I agree t h a t the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y purposes may be g r a n t e d by t h e Head o f my Department o r by h i s r e p r e s e n t a t i v e . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . SHENG, cheng-chun Department o f if e o l o g y The U n i v e r s i t y o f B r i t i s h Columbia, Vancouver 8, Canada. Date A p r i l , 1958 i i TABLE OF CONTENTS Page ABSTRACT i LIST OF ILLUSTRATIONS i i TABLES i i i ACKNOWLEDGMENTS i v CHAPTER I General Introduction 1 Previous Work . . . . . . . . . . 7 General Geology 8 CHAPTER II Stratigraphy 10 General Statement 10 Epworth Series 10 Lower Coppermine River Series 11 Monzonite Dyke Il; CHAPTER III Petrography . . . . . . . . . . . . . 16 Sedimentary Rocks 16 Volcanic Rocks . . . . . . . . 21 Monzonite dyke CHAPTER IV Structural Geology 3U CHAPTER V Economic Geology. • 37 LIST OF ILLUSTRATIONS Figure Page Feeder dyke of the basalt 1 & 2 LU Inequagranular mosaic of coarse dolomite lense included in fine grain dolomite 3 16 Quartzite composed of grains of quartz showing inclusions due to secondary growth . . . . . . . I; 18 i i i Figure Page Sandstone composed of o o l i t i c chamosite, chalcedonic quartz and iron ore 5> 19 Basalt shows subophitic texture 6 22 Basalt shows microporphitic texture 7 22 Basalt contains microphenocrysts of augite 8 23 Basalt contains olivine . . . . . . . . . 9 2L Araygdaloidal basalt 10 & 11 26 Veinlet of quart-feldspar aggregates in basalt. . . . . . . 12 hO Chalcocite in an amygdule . . . . . . . . . 13 hP Chalcocite i n an amygdule li+ 1(1 TABLES Table Page Strikes of the g lac ia l atriations I f> 2V of pyroxene and the composition of plagioclase of the basalt collected from Amco Lake and Burnet Creek area. II 30 2V of pyroxene and the composition of plagioclase of the basalt collected from Wreck Lake area • III 32 ACKNOWLEDGMENT The writer i s deeply indebted to Prof, K.C. McTaggart, Prof. W.H. White for the i r encouragement i n discussing and c r i t i c i z i n g various geolo-g ica l problems concerned; to Dr. J .V. Ross for his kind leadership i n laboratory work. I. CHAPTER I General Introduction;. The Coppermine River area i s bounded on the north by the A r c t i c Ocean and l i e s w ithin the Ar c t i c C i r c l e between "latitudes 67 degrees west and i s approximately 7,000 square miles i n extent., The part under investigation i s located i n the southern portion of t h i s area, within the region of the Coppermine Mountains which trend N70°W from the west side of the big bend of the Coppermine River. Two separate parts of the area were surveyed by the author during the summer of 1957. One part i s located near Amco Lake,, and has an area of about £U square miles, while the other part l i e s about 9 miles west of Lake Amco, and has an area of approximately 30 square miles., (Map 1 and 2). At present, access to the Coppermine Mountains d i s t r i c t i s best obtained by an a i r c r a f t chartered from Yellowknife... Numerous lakes within the region provide natural ports f o r landing a i r c r a f t . During May of each year, ice on the larger lakes remains thick enough f o r landing a i r c r a f t equipped with skis.. After the l£th of July, when most lakes are free from i c e , aeroplanes which are provided with pontoons can e a s i l y f i n d places to land.. Normally on the 15th day of July the ground i s almost free from snow. Along the south-facing c l i f f s thick b e l t s and patches of snow per s i s t throughout the summer and are s t i l l present i n September when cold wintry weather returns. The weather during the summer i s f a i r * The temperature i s moderate from July to August with frequent showers and few heavy rains., 2 Prospecting conditions in summer are, on the whole, good, but the time is short and prospecting can only be carried on effectively from the beginning of July to the end of August,. The Coppermine River area lies within the Arctic barren lands,. The vegetation of the upland consists of grasses, sedges and low arctic annuals. Trees, mainly tamarac and spruce only grow along the banks of the Coppermine River and other larger rivers© Animals, in general, are not plentiful in this part of the barren lands... Grayish-brown colored squirrels were frequently encountered on the drift covered plain.. Other animals which may be seen occasionally are caribou,, wolves, red foxes and wolverines j grizzly bears are rarely seen.. Seagulls, ptarmigans and wild duck are relatively abundant in the area.. Geese and swans were occasionally observed on the larger lakes and during August, swallows were seen,. Mosquitoes and black fl ies are most numerous during late July and August and constitute a serious nuisance to humans* The only permanent settlement in the area is the Eskimo village of Coppermine. Eskimos, probably from Coppermine, were occasionally seen, during the summer, hunting and fishing along the shores of larger lakes* In general, the area has low relief with elevations varying from 5*00 to 700 feet. The low dips of basalt flows are responsible for the typically flat monotonous topography. The flows dip gently to the north with a series; of characteristic south-facing cliffs that have heavy talus at their base,. The Coppermine River dominates the drainage system of the region,. It starts somewhere near the east or northeast of Point Lake and flows north-wards toward the Arctic Ocean for about four to five hundred miles and enters 3 Coronation Gulf near the town of Coppermine. At t h i s p o i n t , i t s v alley-opens out from a gorge t r a c t i n t o an a l l u v i a l f l a t . From P o i n t Lake, the r i v e r takes a n o r t h e r l y course as f a r as the west end of September Moun-t a i n s j - here i t bends to t h e east between Coppermine Mountains and September Mountains. The n o r t h e r l y course of the r i v e r i s resumed downstream from the southeast end of Coppermine Mountains*. The course of the r i v e r on the east s i d e of Coppermine Mountains and from t h i s p o i n t downstream t o Coppermine i s more or l e s s p a r a l l e l t o the general d i r e c t i o n of one set of f r a c t u r e s i n the rock of the Coppermine R i v e r Series.. This stream cuts across the s t r i k e of the rocks and takes a zig-zag course down the d i p d i r e c t i o n of the rock towards the north.. Thus, the p a t t e r n of the r i v e r course seems t o be the r e s u l t of the i n i t i a l , slope of the l a n d and may be c l a s s i f i e d as a consequent stream.. The numerous t r i b u t a r y creeks which enter the Coppermine R i v e r are i n t e r m i t t e n t i n nature , being s p i l l - w a y s during the summer, when the snow melts on the r e l a t i v e l y h i g h ground. Numerous l a k e s of a l l shapes and s i z e s are c h a r a c t e r i s t i c of t h i s area.. Most of them are shallow and t h e i r shores are g e n e r a l l y f l a t and low. Willow Lake, Amco Lake and Wreck Lake are the l a r g e s t i n the area. Wreck Lake i s f a i r l y deep (over 100 f e e t ) i n r e l a t i o n t o i t s s i z e , and i t s s h o re i s g e n e r a l l y rocky.. This l a k e i s b e l i e v e d t o l i e i n a t e n s i o n a l f a u l t . A s e r i e s of N70°W trending r i d g e s r i s i n g 1,000 f e e t above the r i v e r l e v e l i s known as Coppermine Mountains. This h i g h ground i s bounded, on the southwest, by a narrow b e l t of low ground across which the Copper-mine R i v e r flows between September Mountains and Coppermine Mountains. To the north,, the h i g h ground i s merged i n t o a g e n t l y r o l l i n g p l a i n . k The area was extensively glaciated during the Pleistocene Epoch and g l a c i a l d r i f t covers much of the area* The maximum thickness of d r i f t encountered i n exploration d r i l l i n g was about 30 fe e t * E r r a t i c s are widely scattered through the area.. Large g l a c i a l , e r r a t i c s of carbonate rocks up to 15 feet i n diameter i n places l i e on the dip slopes of the dip slopes of the basalt flows* Straight p a r a l l e l s t r i a t i o n s on the surface of basalt flows were observed i n many places. The s t r i a t i o n s have an average s t r i k e of Nl4.0°S and are w e l l preserved under the t h i n cover of d r i f t . . In a few places, s t r i a -tions which narrow out toward northwest over a short distance ( 1 to 2 feet) were observed. They are probably formed by the scoring action of rock fragments which were carried along by the moving ice sheet. The str i k e s of s t r i a t i o n s recorded i n the f i e l d are given i n Table I . Eskers were observed on the d r i f t p l a i n south of Burnet Creek and along the shore of some of the lakes.. They are more or less winding and long, narrow, steep-sided ridges, having a height of 10 to 30 feet and a width of 20 to 80 f e e t . Sand and gravel are the chief constituents.. I t i s believed that the eskers were formed i n tunnels at the base of the g l a c i e r which had a considerable thickness and was r e l a t i v e l y free from cracks. Many lakes i n the area are the result of g l a c i a t i o n and may be c l a s s i f i e d g e n e t i c a l l y into two types: 1*- Kettle lakes l y i n g within the d r i f t p l a i n * The k e t t l e holes may have resulted from the operation of one of the following processes' ( F l i n t , 19U7, P. 1U8):-(a) The largest and most conspicuous kettles r e s u l t from the melting of r e l a t i v e l y t h i c k projecting ice masses. (b) The melting of buried ice masses. Table I Observations i n the area of Burnet Creek and Amco Lake Number of Location True St r i k e 1*3 N- 85° W 7 N 20° ¥ 19 N 20° ¥ 20 N 35° W 21 N 35° ¥ 2k N 20° ¥ 25 N 2 0 0 ¥ 30 N k0o ¥ iiO N 2 5 0 ¥ lilt N U0 0 ¥ 52 N 3 0 o ¥ 58 N 5 0 o ¥ 61 N 65G ¥ 73 N 5 0 o ¥ 75 N 50 O l ¥. 78 N 1 5 0 w 9 N 5 0 0 ¥ 12 N 55 0w 31 N 1I5O W 35 N 350W 39 N 50 o ¥ hS N 65 U8 •N 65„W 58 N 3 5 ° ¥ 62 N 60 ° ¥ 6k N U 5°W 76 N i i 5 ° ¥ 108 N 75 ¥ Observations i n the area of Wreck Lake Number of Location True Strike 29 N 38° ¥ kS N 35° W 3 N 1 5 ° ¥ 8 N 1 5 ° ¥ 11 N 2li° ¥ 25 N 25° ¥ 6 (c) The melting of ice masses which were floated or dropped into the drift*, Amco Lake seems to be a kettle lake formed by the f i r s t process* 2* Small and shallow lakes were formed by the plucking action of glacial ice on the bed-rock. 1> i s known that there were three continental ice centres i n North America: the Keewatin centre located i n the area between Northern Alberta and and the west side of Hudson's Bay; the Patrician centre located on the height of land north of Lake Superior and the Labrador centre located east of Hudson's Bay. The directions of striations. shown on the Table I, together with the shape of the striations indicate that the dominant movement of ice has apparently been from the southeast to the northwest. By construction of the directions of striations,, i t would seem that the glaciation in this area was related to the Keewatin centre. In the area mapped, the lack of a thick cover of vegetation leaves the rocks exposed to processes of direct mechanical, disintegration and chemical xreathering. Disintegration of rock by frost action appears to be the major process in forming the majority of the south-facing steep c l i f f s of the basalt flows* The efficiency of the frost action is shown by the highly shattered condition of most of the exposed bed-rock and by the large amounts of angular blocks and fragments occurring at the base of the south-facing c l i f f s * . The presence of columnar joints i n the basalt flows tend to f a c i l i t a t e this action.. Frost heaving is much in evidence on the dip slopes of the flows and blocks as large as 10 feet in diameter may be seen heaved from 5 to 10 feet from their original position* 7 Locally, i n parts of the Coppermine River area, a c h a r a c t e r i s t i c feature of frost-heaving i n g l a c i a l d r i f t i s the production of c i r c l e structures which have flat-bottomed, shallow, depressed centres rimmed by low narrow ridges. The expansion of ice which re s u l t e d from the freezing of water within the d r i f t squeezed the d r i f t upward, and thus the loose d r i f t accumu-lat e d around the centre of expansion to make the observed form* These c i r c u l a r structures are different from stone rings (Thoronbury, 19$hf P«88) i n that the material, remains unsorted... Previous Work In 1769, the Hudson's Bay Company at Fort C h u r c h i l l sent Samuel Hearne to explore the Coppermine River where native copper had been reported but there are no accurate descriptions of deposits dating from that expedition or evidence to prove the economic p o s s i b i l i t i e s reported by Hearne, In 1828, S i r John Franklin and other members of his expedition v i s i t e d the area and reported the occurrence of natuve copper i n the v i c i n i t y of the Coppermine River and Bathurst Inlet,. Later, i n 1911 and 1912, the geology along the Coppermine River and i n the v i c i n i t y of Dismal Lake was mapped by George M, Douglas, Lionel Douglas and August Sandberg, The f i r s t s c i e n t i f i c expedition into the area was sent by the Dominion government i n 1913, The geological section, under the d i r e c t i o n of Dr. J . I . O'Neil,: surveyed the coast-line west of the Kent Peninsula and investigated occurrences of native copper on the islands and mainland, i n the v i c i n i t y of Bathurst I n l e t . In August, 1929, a geological party under Thomas Creighton, using two large a i r c r a f t , flew from Baker Lake to Bathurst I n l e t . They were the 8 the f i r s t to traverse this area by aeroplane. From 1929 to 1930, two mining exploration companies sent prospectors and geologists to t h i s region and the results of t h i s survey were reported by Gil b e r t and Duncan (1931 )•-. In the period 19U3-1951, Dr. CP.. Jenney carried out geological reconnaissance and detailed examination of mineral showings i n the area. In the recent years, the i n t e n s i t y of geological exploration i n the area has been increased. During the summer of 1956-1957, Canadian mining companies, notably International Nickle Co. and Pickle Crow Gold Mines Ltd., sent prospectors and geologists to the region,, Modern geophysical methods, and diamond d r i l l i n g were used f o r locating ore bodies. General Geology Knowledge of the geology of the Coppermine River area i s based c h i e f l y on the work of Dr. C.P. Jenney who surveyed the c e n t r a l portion of the D i s t r i c t of Mackenzie during the summers of 19U3-1951* The oldest rock formation i n the d i s t r i c t i s the Tesphierpi granite. I t i s a coarse-grained, gray to pink rock* Sediments, unconformably overlying the granite, consist of sandstone and quartzite of the Lower Epworth Series which are i n turn disconformably overlain by the Coppermine River Series, the lower member of which consist of basalt flows that are estimated by Jenney to be up to 13,000 feet i n t o t a l thickness* This series of basalt flows are described i n d e t a i l i n t h i s paper. The upper member of the Coppermine River Series consists of sandstone, shale and limestone which are intruded by a series of s i l l s of b a s a l t i c composition, Monzonite and ac i d i c dikes have been found i n places to cut the Lower Coppermine River Series* Because of the absence of f o s s i l s i n these rocks, they are assumed 9 to be of late Precambrian age. Palaeozoic dolomites and limestone l i e unconformably above the youngest members of the Coppermine River Series,,. Table of Formations; (Jenney, 1951) Coppermine River Series 1 Up Lo Epworth Series Palaeozoic Dolomite and limestone Late Precambrian Quartz-carbonate veins Monzonite and acid dikes S i l l s , sandstone, scales and limestone. Basalt flows with minor interbedded sandstone and quartzite. -Dis c onf ormity-Dolomite, minor limestone and quartzite Conglomerate Sandstone and quartzite Unconformity Tesphierpi granite For more than a century, occurrence of copper in the Coppermine River area has been known.. Plates of native copper which are as large as 2 feet in diameter are found in fractures which cut the basalt and as surface float.. The chalcocite, bornite and minor amounts of chalcopyrite are the common minerals found in the area.. 10 CHAPTER I I STRATIGRAPHY General Statement The oldest rock i n the Coppermine area i s the Tesphierpi granite but i t i s not exposed i n the area under inve s t i g a t i o n . The Epworth Series overlies unconformably the Tesphierpi granite and i s subdivided into two parts;: a lower part and an upper part. The lower part i s composed of sand-stone and quartzite whereas the upper part i s represented mainly by dolomite and minor amounts of limestone and quartzite; the lower part being separated from the upper part by a conglomerate bed.. Only the uppermost part of the Epworth Series, which i s close to the basalt of the Coppermine River Series, and which i s represented by dolomite interbedded with quartzite was i n v e s t i -gated by the w r i t e r . The Coppermine River Series which disconformably overlies the Epworth Series, i s also subdivided into a lower and an upper part., The lower part i s composed of basalt flows with minor interbedded sandstone and quart-z i t e i n i t s uppermost section. The upper part i s composed predominantly of red sandstone, shale with minor amounts of dark colored limestone, and s i l l s of basalt composition. A monzonite dyke cut across the upper part of the Lower Coppermine Series i n the area under investigation. Epworth Series The upper part of the Epworth Series consists l a r g e l y of dolomite and i s 3,800 to i;,900 feet t h i c k (jenney, 1951). The series crops out along 11. the north side of Dismal Lake. The contact between the dolomite and the overlying basalt i s found at a l o c a l i t y about 12 miles east of Wreck Lake* From t h i s l o c a l i t y the dolomite swings to the south of the September Mountains. In general, the dolomite i s grayish-white j the weathered surface i s white or yellowish-white and i s readily seen es p e c i a l l y on the south-facing c l i f f s . Some beds show dark-coloured streaks. Most of the dolomite i s free of c l a s t i c material and i s f i n e l y c r y s t a l l i n e having a compact uniform texture and smooth conchoidal fractures.. The average dip of the dolomite beds i s 7-10 degrees to the north and the s t r i k e N60°W. The rock i s w e l l bedded, i n d i v i d u a l beds ranging from about 18 feet to about 5 feet t h i c k . Towards the top, the thin-bedded character of the rock i s quite pronounced. Joints i n the rock are w e l l -developed and generally show a columnar pattern. Rectangular blocks of large size are common along the base of the south-facing c l i f f s of the rock. The contact between the dolomite and overlying basalt i s f a i r l y clear and more or less l i n e a r or plane.. No features of thermal metamorphism are present along the dolomite bed close to the basalt. A 3-foot bed of quartzite, s u p e r f i c i a l l y resembling dolomite, occurs as an in t e r c a l a t i o n i n dolomite about 50 feet below the dolomite-basalt contact. The thickness of the quartzite i s quite uniform along the s t r i k e . Lower Coppermine River Series Basalt The basalt flows underlie Copper Mountains and September Mountains and they can be traced northwesterly f o r over one hundred miles. Southeast of 1 2 September Mountains they swing to the northeast and appear to t h i n out as., they approach the A r c t i c coast near longitude ll!* 0 (Jenney, 1 9 5 1 ) * Similar rock has been reported i n Bathurst I n l e t about 200 miles to the east of Copper Mountains (Jenney, 1 9 5 1 ) . On the northern part of V i c t o r i a Island (Washburn) and also about ij.0 miles southeast of Copper Mountains i n the v i c i n i t y of Takiyuak Lake (j.V. Ross, 1 9 5 7 ) , flows of basalt have been found. The rocks as a whole are fine-grained and gray i n color. On the top of some flows they exhibit a reddish-brown color which i s due to oxidation and i s gradually diminished downward.. The tops of flows are characterized by large e l l i p s o i d a l amygdules and vesicules. The long axis of the vesicules seem to have no preferred orientation. The main minerals of the amygdules are dark green epidote, quartz and c a l c i t e , whereas feldspar and zeolites are present i n minor amounts.. Generally, the lower part of the basalt flows i s reddish brown and i s fine-grained i n texture. S l i g h t l y coarse-grained v a r i e t i e s are encountered i n various parts of some flows.. Lath-shaped crystals of plagioclase and short, small prismatic crystals of pyroxene form most of the rock. Irregular grains of native copper are present i n some places. The flow breccia, found i n only one flow i n the Wreck Lake area, i s composed of angular to subrounded reddish-brown rock fragments of b a s a l t i c composition.. Most of the fragments are s i l i c i f i e d . Quartz i s the predominent material f i l l i n g the i n t e r s t i c e s between the fragments. ?he subrounded f r a g -ments i n the breccia seem to be the r e s u l t of abrasion during the moving of flows.. The flows i n the area under investigation are a series of super-imposed flows of basalt, ranging i n thickness from 2 0 to 5 0 f e e t . In some places, flows over 1 0 0 feet t h i c k have been reported i n t h i s series. The thickness of an i n d i v i d u a l flow varies along the s t r i k e . Flows outcrop as 13 south-facing steep c l i f f s , topped by a gentle dip slope toward the north* In general, the flows trend N50°w and dip about 7° to 10° north. At Bathurst Inlet, the basalt flows of the Coppermine River Series overlie Boulburn quartzites and Kanuyak formation which in turn overlie the Epworth dolomites (O'Neil, 192k) •• However, i n the area under investigation the basalt flows.directly overlie the Epworth dolomite in apparent discon-formity. As shown i n the regional geological map of the Coppermine River area (Dr.. J.V.. Ross, unpublished map) between Coppermine Mountains and Takiyuak Lake, there are north trending dyke swarms which cut i n the metamor-phosed shelf sediments of Exmouth Lake Group. These dyke swarm probably the major feeders for the flows occurring in, the Takiyuak Lake and the Lower Coppermine River Series.. In Amco Lake and Burnet Creek area, several dykes crop out; these dykes may also belong to the same group of the dyke swarm above. The best example of the dyke was observed on a south-facing c l i f f of a flow i n the Amco Lake area. As shown i n Figure 1, the dyke consists of dark gray angular fragments of basalt cemented by chalcedonic quartz and carbonate.. The exposed length of the dyke ranges from 3 to 12 feet long and 2 to 6 feet wide. The average strike of the dykes is about N30°E to Ni;0oE and i t s dip i s mostly vertical or steep. A Ij-foot long and 2-foot wide, lense-formed dyke of the same kind exposed on the dip slope of a flow in the area south to the Burnet Creek, as shown i n Figure 2 . The basalt country rock surrounding the dyke show no trace of fracture. Since no fractures occur i n the basalt cut by the dykes, the fragments in the dykes can not be explained as the result of the fault,. Thus, the origin of the fragments may be explained by one of the following two processes: 1. The l i q u i d lava receded after most of the dyke had s o l i d i f i e d . This lack of support caused the platy-walls of the dyke to collapse (H.T. Stearns, 19k0, p.l|l).. The quartz aid carbonate l a t t e r l y f i l l e d i n the in t e r s t i c e s between the fragments* 2* The fragments were produced by wedging action of basalt magma along old fractures* Fig.. 1.. Fig.. 2. Sandstone Sandstone, interbedded with basalt flows, i s exposed three miles northeast of Amco Lake,, The sandstone i s medium-grained and reddish-brown i n color and i s composed of ir r e g u l a r to subrounded grains of quartz and iro n ore* The rock i s w e l l bedded, the beds ranging from one inch t o three feet i n thickness. The minimum thickness of the sandstone i s estimated to be 30 feet . The overlying basalt i s grayish green i n color, and has a c h i l l e d contact against the sandstone* However, the sandstone does not seem to be metamorphosed. Monzonite dyke A monzonite dyke i s exposed 3 miles northeast of Amco Lake.. I t forms a prominent, steep ridge about 20 feet above the surrounding d r i f t -15 covered ground. The rock i s grayish, medium-grained, and i s composed mainly of pinkish K-feldspar, gray plagioclase and dark ferromagnesiumminerals. Grains of iron ore and a small amount of quartz are scattered throughout the rock. The grain s i z e of the rock at i t s margin i s f i n e r than at the central portion,, due to c h i l l i n g near the contact of the basalt. The contact between the dyke and the basalt i s covered by the d r i f t . The dyke i s exposed f o r a length of approximately two hundred feet and p a r a l l e l s the main b a s a l t i c dyke swarm to the south. I t s t r i k e s northward across the basalt and sand-stone beds.- The dyke i s approximately 1+0 to 50 feet thick.. 16 CHAPTER I I I PETROGRAPHY Sedimentary Rocks Dolomite In hand specimens, the dolomite is light gray in color and fine grained. Under the microscope, a sample from the lower horizon is seen to be composed entirely of carbonate alternating in bands of fine grain and coarse grain. In the bands of coarse grains, the carbonate i s present as inequagranular mosaic* The boundaries between the bands of the coarse grains and the bands of the fine grains are rather smooth. Stain tests show that the rock consists only of dolomite. At about 10 feet below the dolomite-basalt contact, the rock, as revealed by thin section study, has similar texture with the rock stated above. The sample collected immediately below the contact is mostly composed of fine grains of dolomite of more or less turbid microcrystalline appearance and lenses formed of inequagranular mosaic of coarse dolomite. The lenses may be due to recrystallization resulting from the baking by the basalt above (Figure 3)» Figure 3. Inequagranular mosaic of coarse dolomite lense included in fine grain dolomite* 8ox 17 The dolomite i s believed formed from replacement of c a l c i t e by Mg + + solution or the dolomitic beds may have been magnesium-rich c a l c i t e muds of algal o r i g i n which became contemporaneously or subsequently dolo-mitized from available Mg + + solution (Fairbridge, 1957, p. 130).. The con-centration of Mg + + i n the sea i s , therefore, an important factor f o r dolo-m i t i z a t i o n . Warm shallow n e r i t i c sea favored the growth of algae that may aid the concentration of magnesium (Fairbridge, 1957, p.l5i)-)». On the sea f l o o r or sea shore dolomitization takes place effectively.. No important dolomitization occurs at or near the surface of the sea f l o o r . The presence of minor limestone beds i n the upper Epworth Series may suggest that the dolomite of t h i s series has been formed by dolomitiza-t i o n of the limestone. Quartzite In hand-specimen, the rock i s medium-grained and l i g h t gray. Under Microscope, pockets or layers of f i n e grained quartz are seen to be included i n the coarse grained quartz., Subrounded to irr e g u l a r grains of quartz are present side by side. Many grains of quartz contain c i r c l e s or e l l i p s e s of inclusions. These inclusions mark the mask surface of the o r i g i n a l sand grains. The i n t e r s t i c e s of the quartz grains are mostly f i l l e d by secondary c r y s t a l l i n e quartz grown out from the o r i g i n a l grains. Some of the secondary quartz grains have the same o p t i c a l orientation as the grains from which they grow; others show o p t i c a l discontinuity. Cryptocrystalline quartz i s found between the quartz grainsj i t shows aggregate structure between crossed n i c o l e . Patches of dolomite are present i n te i n t e r s t i c e s of quartz grains or included i n the secondary quartz. I t seems that most of the dolomite i n the rock was introduced into the rock before the r e c r y s t a l -l i z a t i o n took place. (Figure 2|). Figure k* Quartzite composed of grains of quartz showing inclusions due to secondary growth. 80X The o r i g i n a l sediment seems to have been a calcareous sand. The excellent sorting as indicated by the simple mineralogy and w e l l rounded o r i g i n a l grains of quartz r e f l e c t s the thorough washing and reworking of the sands. According to F.J. Pettijohn (1°1 |8) , quartzite of sedimentary o r i g i n i s c a l l e d orthoquartzite and distinguished from metaquartzite of metamorphic o r i g i n . Three genetic types of the high quartz sandstone are distinguished by him: " f i r s t - c y c l e orthoquartzites, which are the product of intense chemical decay of the rock; secondary cycle orthoquartzites which are the product of reworking of the f i r s t cycle type; cleaned graywackes which are the product of washing out of the clay and s i l t portions of graywacke. Intense chemical weathering of the country reck i n a region produces a considerable amount of s i l i c a i n the r i v e r s that drain the region. The f i r s t c i r c l e quartzite, therefore, exhibits intense authigenesis, because, apparently, the sea water was charged with dissolved s i l i c a and a l k a l i e s " . 1 9 From the above considerations, i t i s concluded that the q i a r t z i t e i s a f i r s t cycle quartzite. Sandstone The rock i s composed of large amounts of subrounded to i r r e g u l a r grains of chalcedonic quartz, i r o n ores and minor quartz grains of i r r e g u l a r shape,. Of special interest are the occurrence of abundant grains of o o l i t i c reddish-brown chamosite and small amounts of orthoclase. (Figure 5). Figure Sandstone composed of o o l i t i c chamosite (gray), chalcedonic quartz ( l i g h t gray), i r o n ore (black). 80X The percentage of minerals i n the rock i s estimated as follows: Mineral Percentage Chalcedonic quartz IlO Chamosite Iron ore Calcite Quartz 20 20 12 3 Feldspar The reddish-brown color of the sandstone i s due to the presence of 20 o o l i t i c chamosite. The grains of quartz are clear and free from the coating or crust of red material and are thus, different from those of the red beds i n which the grains of quartz are rimmed with red iron oxide.. The grains of orthoclase are also clear and free from a l t e r a t i o n products resulting from weathering. Chamosite may apparently form either from chemical precipitate or as a replacement of carbonate detritus (PettiJohn, 191*8, p . ! 1 3 ) . The grains of chalcedonic quarta and feldspar probably derived, p a r t l y , from the flow tops of the basalt i n which the chalcedony occurs as amygdules. The shape of quartz and the presence of the grains of orthoclase indicate the sediment i s poor i n sorting and was transported and deposited i n rapid condition* The presence of the grains of chamosite and ir o n ore i n the rock give the information of the special condition of the deposition. The hydrogen-ion concentration of natural waters i s of great significance i n chemical reactions accompanying sedimentary processes; the pH of the water i s an important con t r o l l i n g f a c t o r f o r the p r e c i p i t a t i o n of hydroxides from solution (Mason, 1951* p.lltO). In the diagram showing the f i e l d of occurrence of t y p i c a l chemical end-member association i n terms of Eh and pH (Krumbein, Garrels, 1952, p . 2 6 ) , the chamosite and iron ore are placed i n the f i e l d l i m i t e d by limestone fence and the organic matter fence;; t h i s f i e l d represents normal marine open c i r c u l a t i o n conditions. The chamosite consists of about Ik per cent S i 0 2 , 7 to 8 per cent A l ^ , around 60 per cent FeO, and about 18 per cent S^O (Twenhofel, 1950, p.Uj.2). The high s i l i c a content as compared with the alumina suggests the water from which the mineral pre-c i p i t a t e d was a l k a l i n i n nature with pH 8-9. 21 Volcanic Rocks; Basalt Samples were collected from various parts of single flows and also from flows at different horizons., Collections of specimens were obtained from two separate areas: one group was collected from Wreck Lake area, another, from Amco Lake and Burnet Creek area. The t o t a l thickness of the flow succession i n the former area i s about 2,800 feet and 1,1|00 feet i n the l a t t e r area. S t r a t i g r a p h i c a l l y , the lavas i n the Wreck Lake area are older than those i n Amco Lake area. The t o t a l thickness of the basalt under study i s about k,2.00 feet . The results of the study of thin-sections made from the separated area are recorded i n the Tables I I and I I I . (See pages 30, 31 and 32). In hand-specimens, the basalts are c r y s t a l l i n e rocks with greyish color, showing small glassy prisms of plagioclase and dark spots of f e r r o -magnesium minerals. No flow structure i s v i s i b l e i n the rock. Under the microscope, the texture of the rocks i s predominently subophitic (Figure 6).. Microporphyritic texture i s also present i n some sections (Figure 7). In few thin-sections the rocks show f a i n t flow texture. From the studies of many t h i n sections i t i s concluded that the mineral composition i s uniform.. The rocks consists mainly cf plagioclase, pyroxene and minor amounts of i r o n ore and apatite. Olivine i s found only i n two thin-sections from Amco Lake and Burnet Creek area; one section con-tains only one grain, another, contains 3 grains. Chlorite, serpentine, epitote, s e r i c i t e and iddingsite are everywhere present i n the sections which show a l t e r a t i o n products* Plagioclase i s one of the main constituents i n the flows;; i t forms 22 Figure 7» Basalt shows microporphitic texture,8 0 X 23 lath-shaped crystals which generally show twinning on the a l b i t e law, but some grains are twinned on the carlsbad law. Most of the plagioclase i s highly altered to give a mottled appearance and contains a l t e r a t i o n products of s e r i c i t e , epidote and serpentine. Composition of plagioclase ranges from labradorite i n the older flows, which are represented i n the section of Wreck Lake shown on the Table I I , to andesine i n the younger flows which are represented i n the section of Amco Lake and Burnet Creek area shown i n the Table I I I . The occurrence of andesine i n the older flows may be the resu l t of d i f f e r e n t i a t i o n of ba s a l t i c magma before i t s extrusion. In a t h i c k flow, about 50 feet thick, the An content of plagioclase i n the upper part i s less than that of the lower part. Two pyroxenes - augite and pigeonite - are present i n nearly every specimen. Augite i s predominant over the pigeonite. In some sections, the augite tends to form microphenocryst and these show twinning p a r a l l e l to (100) (Figure 8). The mineral i s , i n most specinens, allotriomorphic; only Figure 8. Basalt contains microphenocrysts of augite, plagioclase i s highly altered showing mottled appearance. 8OX 2k i n the coarse-grained rock does i t tend to be isomorphic. Measurements, on the universal stage, of the optic a x i a l angles of the augite i n many thin-sections from different l e v e l s of the flows show that the 2V ranges mainly between f>0 to 55 degrees,. S l i g h t l y zoned cry s t a l s of augite are encountered i n some sec-tions showing 2V=52 degrees i n the core and 56 degrees i n the rim. The 2V of pigeonite varies from 20 degrees to 26 degrees. Both minerals are colorless and p a r t l y altered to serpentine and c h l o r i t e . They are c r y s t a l l i z e d out simultaneously from the b a s a l t i c magma as shown by the following f a c t s : i n some thin-sections, the augite p a r t l y encloses the pigeonite but i n others the reverse i s true and i n s t i l l others the two minerals are side by side without encroaching on each other. The pyroxene occurs either p a r t l y enveloping some of the plagioclase grains or p a r t l y i n t e r s t i t i a l between them; t h i s r e l a t i o n suggests the simultaneous c r y s t a l l i z a t i o n of the two minerals. The o l i v i n e i s present i n negligible amounts i n two sections (Figure 9); t h i s mineral i s greenish to greenish-yellow and i s anhedral to Figure 9. Basalt contains o l i v i n e (large grain i n the center of the pi c t u r e ) . 80X 25 subhedral i n shape. I t shows i r r e g u l a r fractures and high r e l i e f . The greenish color i s due to the pseudomorphism of c h l o r i t e after o l i v i n e * In fractures and:along the c r y s t a l outlines of t h i s mineral, iddingsite and i r o n ore are commonly observed*. The minor constituents of the rock consist of i r o n ores and apatite. The i r o n ore occurs as i r r e g u l a r grains and patches molded on the pyroxene or i n the i n t e r s t i c e s between the pyroxene and plagioclase. Apatite which i s present i n small amounts occurs as colorless needles i n the plagioclase. Occasionally patches of c a l c i t e are found i n plagioclase; these may be an alt e r a t i o n product. The serpentine and c h l o r i t e , which are the a l t e r a t i o n products of both pyroxene and plagioclase, are e a s i l y distinguished by t h e i r color and r e l i e f ; the c h l o r i t e shows, i n most cases, B e r l i n blue interference colors, while the serpentine shows gray to yellowish-green colors; both minerals occur as irr e g u l a r patches molded on either pyroxene or plagioclase. Grains of pale greenish and yellowish-green epidote occur as the a l t e r a t i o n products of plagioclase and pyroxene. S e r i c i t e occurs as minute flakes i n plagioclase and i s the a l t e r a t i o n product of the l a t t e r mineral. Iddingsite occurs as ir r e g u l a r grains and patches i n some rock which has a brownish-gray c o l o r . I t i s believed that t h i s mineral i s hydrothermal i n o r i g i n . When found i n rock which contains grains of o l i v i n e , the iddingsite i s undoubtedly the a l t e r a t i o n product of the o l i v i n e . The rocks collected from the flow tops are brownish-gray to dark green color and contain numerous amygdules and ves i c u l e s . The common minerals forming the amygdules are quartz, epidote, c a l c i t e with minor amounts of K— feldspar, prehnite and thomsonite.. The amygdules may consist of one or several of the minerals which are mentioned above.. Epidote or projecting c r y s t a l s of 26 thorasonite and these project toward the center of the amygdules (Figure 10), Figure 10. Amygdaloidal basalt. Amygdule f i l l e d by thomsonite, quartz, feldspar. 8OX In some amygdules, chalcedonic quartz occurs which shows concentric layers of progressively finer grain towards the center (Figure 11). Figure 11.. Amygdaloidal basalt, Amygdule f i l l e d by concentric layers of chalcedonic quartz, 80X The basalts, as represented i n the c o l l e c t i o n , are o l i v i n e - f r e e basalt, generally s i m i l a r i n character, and with the texture and composition of t h o l e i i t e s . . The t h o l e i i t i c magma type and the olivine-basalt magma type have been considered as two primary b asalt magmas.. There i s no evidence to indicate conclusively the two primary magmas have originated from a common parent, therefore the o r i g i n of the two magma types i s s t i l l a question., The t h o l e i i t i c magma type i s found predominantly on the continents, while the olivine-basalt magma type occurs mainly i n the ocean b asin and i n the volcanic i s l a n d s . The t h o l e i i t e s were formerly believed to have been developed through contamination of primary o l i v i n e basalt magma by contact with the base of the s i a l i c crust,, Kennedy (1933) considers that there are two great primary basalt magmas, the olivine-basalt type and the t h o l e i i t i c basalt type, each of which gives r i s e normally to i t s own pa r t i c u l a r differentiates;: the former type gives a l k a l i n e d i f f e r e n t i a t e s such as trachyandesite, trachyte and phonolitej the l a t t e r type gives strongly ca l c - a l k a l i n e types,. The two main types of magmatic descent result from the separation of dif f e r e n t pyroxenes from the two different magmas as suggested by him. In the f o r s t e r i t e -d i o p s i d e - s i l i c a system, since the composition of the olivin e - b a s a l t magma l i e s within the s t a b i l i t y f i e l d of f o r s t e r i t e , thus, the dipside-rich or ca l c i c v a r i e t y of pyroxene w i l l c r y s t a l l i z e from the lime r i c h l i q u i d . The removal of lime i n the pyroxene prevents i t from combining with Al^O^ to form anoilhite and the AlgO^ w i l l be forced into combination with a l k a l i e s , giving a l k a l i - f e l d s p a r and eventually, feldspathoids.. The composition of 28 t h o l e i i t i c magma, on the other hand, must l i e either very close to the boun-dary of the f o r s t e r i t e s t a b i l i t y f i e l d or e n t i r e l y within the pyroxene area. The c r y s t a l l i z a t i o n w i l l commence with the separation of monoclinic pyroxenes or lime poor v a r i e t i e s of pyroxene, thus the lime i n the magma i s only removed i n very small amounts during the ferromagnesian c r y s t a l l i z a t i o n : the A I 2 O 3 w i l l combine with most of the lime t o give the anorthite molecule which forms,, therefore, an important constituent (Kennedy, 1 9 3 3 ) • The o r i g i n a l composition of the b a s a l t i c magma and the separation of pyroxene play important roles i n giving r i s e to different descents. In recent research on basalt magma by Dr. H. Yorder and Dr. CT. T i l l e y , different type specimens were selected from the natural representa-t i v e s of three magma types f o r thermal study at atmospheric pressure. The specimens are t h o l e i i t e , a l k a l i n e basalt from Hawaii and high alumina basalt, a non-porphyritic Warner basalt from C a l i f o r n i a . From the study, the following conclusions are derived: " a l l three major phases (pyroxene, o l i v i n e and plagioclase) appear together at about the same temperature ( 1 1 6 0 to 1 1 7 0 ° ) , irrespective of the bulk composition of the basalt; a l l three major phases appear in a short i n t e r v a l of temperature under the experimental conditions; the t o t a l range of c r y s t a l l i z a t i o n i s small of the order of 150°C;; o l i v i n e or plagioclase appears on the liquidus to the exclusion of pyroxene f o r the basalt studies.. The conclusions l e d to the fact that the differences between t h o l e i i t e s , a l k a l i basalt and the high alumina basalt arise as a res u l t of different i n i t i a l bulk composition"* The basalt flows under study are t y p i c a l l y t h o l e i i t e s , the o r i g i n a l composition of the basalt magma seems to l i e within the pyroxene f i e l d i n the system of the f o r s t e r i t e - d i o p s i d e - s i l i c a . . In the area east of Coppermine Mountains, the layers mainly consist of peridotite and monaonite have been reported occurring i n the basalt flows* I t i s evident that here the d i f f e r e n t i a t e d layers and the basalt are geneti-c a l l y related* The layers may resul t from the d i f f e r e n t i a t i o n of the basalt in the deep, magma reservoir* TABLE II The Amco Lake and Burnet Creek Area (Map 1) a,) Section south of Burnet Creek: West Part ^Location No.. .Height ahove base (feet) 2V of pyroxene Composition of plagioclase Texture 0+ 10690 26 50-56 Large An 1+2+-50 Small An 32-39 Micro-porph C3 1061+0 22-23 52-56 Large An 56 Small An 36-1+8 Micro-porph C2 10600 2k 5*2-51* An 32-31+ Suboph. CI 10560 20 5U-58 An i+O-i+8 Suboph, East Part Location No. Height above base (feet) 2V of pyroxene Composition of plagioclase Texture C8 10870 20-22 52-58 An 3^ -38 Suboph. C7 10770 22-21; 51-56 An kd Suboph. C6* 10620 I11 Lc ' . 22, 52-56 3 . 20-28, 52-58 An 31-35 An 1+2-1+9 Suboph.:. c5* 10^ 70 U p < * 2h> 5 0 " 5 2  1 0 i > 7 0 Lo- 22-28, 51-58 i_ An 1+2-1+8 An 1+9 Suboph. # Two samples were collected from the upper part and lower part of a flow which is approximately 50' thick. b) Section of north Burnet Creek: East Part Location No. Height above base (feet) 2V of pyroxene Composition of plagioclase Texture B16 13510 1+2-1+8 An 31+ Suboph. B15 13U00 36, 50-58 An i+2-1+8 Suboph. Bll+ 13320 50-51; Large An 5k Small An 1+2-1+8 Micro-porph. B13 13290 21+, 52 An 36 Suboph. B12 13200 51+-58 Large An 32 Small An 31 Micro-porph. Central Part Location No, Height above base (feet) 27 of pyroxene Composition of plagioclase Texture B l l 13520 51i Large An 32-31; Small An 28 Micro-porph BIO 131*10 22, l|6-52 An 32-56 Suboph. B9 13310 20 Large An 35-36 Small! An 32-35 Micro-porph. B8 13210 26, 514.-58 An 32-36 Suboph. West Part Location Mo, Height above base (feet) 27 of pyroxene Composition of plagioclase Texture B7 1351;0 l;6-57 An 36 Suboph, B6 13380 20 Large An 36 Small* An 30 Micro-porph. B5 13170 21;, li2-i;8 An 3l|-36 Suboph. Bh 13110 50 An k0-kh Suboph, B3 13050 U2-U8 An h0 Suboph. B2 13010 lB~2k, 1;6-U8 An hh Suboph. B l 12860 28, 51-56 An U2 Suboph. c) Section of west Amco Lake: East Part Location No, Height above base (feet) 27 cf pyroxene Composition of plagioclase Texture A9 13170 30 56 Large An 5U-60 Snail An 32-31; Micro-porph. A8 13090 5U-56 An 32 Suboph. A7* 12980 28, 50 - Suboph. A6 12710 28, hh - Suboph. A5 12560 20-21; 5/U-58 Large An 5U Small An I4.6 Micro-porph. •» One grain of chlorite pseudomorphous after olivine. West Part Location No. Height above base (feet) 2V of pyroxene " Composition of plagioclase Texture Al* 131*10 25 1*0 An 1*6-1*8 An 62 Micro-porph. A3 13290 50-52 An 32-35 Suboph. A2 13130 26, 50-56 - Suboph. A l * 13030 50-58 - Suboph. * About three grains of chlorite pseudomorphous after olivine., TABLE i n The Wreck Lake Area (Map 1) Location No. Height above base (feet) 2V of pyroxene Composition of plagioclase Texture W6 2720 22, 56 An 56 Suboph. W5 2320 26, 50-56 An 32 Suboph. Wl* 2100 52 An 56-51* Suboph. W3 1100 50-56 An 1*2 Suboph. W2 50 - An 32 Suboph. Wl 2 - An 52-51* Suboph. 33 Monzonite Dyke The rock of monzonite dyke i s gray, medium-grained and i t contains lath-shaped pinkish or grayish feldspar and dark color ferromagnesium minerals showing shinning cleavage face. The grains of quartz and the golden yellow metallic mineral - probably pyrite - are sparsely scattered i n the rock.. Under the microscope, the minerals occuring in the rock are of allotriomorphic to hypiomorphic form and are typ ica l ly of allotriomorphic-granular texture. Plagioclase, orthoclase, augite and hornblende are present i n approximately equal amountsj. quartz, z ircon, apatite and iron ore are present in minor amounts. The composition of plagioclase ranges from An32 to AnliiJj orthoclase i s mostly turbide i n appearance due to the presence of a considerable amount of alteration products of serci te* The augite has 2V about $0° and bordered by pale green and olive green hornblende. Zircon occurs as short prismatic form i n serpentine, which i s the alteration products of ferromagnesium minerals, and surrounded by pleochroic halos. The monozonite dyke i s probably genetically related to the monzonite found i n the differentiated layers in the basalt.. Thus, the dyke may also be the product of differentiation of the basaltic magma which took place i n the magma reservoir* 3k CHAPTER IV STRUCTURAL GEOLOGY The Coppermine River area i s believed by O'Neill (I92li) to be the southwest border of the so-called Arctic Basin. He states that i t i s an oval , cynclinal basin, with the eastern, southern and western borders occurring on the mainland, and the northern border occurring i n the central part of Vic tor ia Island.. The long axis of the oval basin extends from Cape Lyon, eastwards, to the Boothia Peninsula, a distance of about 600 miles, and the shorter axis pro-bably measures about 300 miles from south to north.. The older formations, from Darnley Bay to the v i c i n i t y of Deas Thompson Point have a general northwesterly strike with small dips to the northeast. Further to the east, the strike of these formations becomes eastern to east-west, with low dips to the north*. In Coronation Gulf, the strike changes to the northeast again, with dips to the north-west. On the west side of Boothia Peninsula, s t ra t i f i ed rocks, older than S i l i u r i a n occur which are reported to dip to the west. Most of the central portion of the main basin i s occupied by Palaeozoic dolomites these occur on the mainland, eastwards, from Tinney Point to Cape Kendall, and form a l l the western and also the eastern side of Victor ia Island. The; dolomite rock exhibits shallow open fo ld with angles of dip much lower than those of the older formations. The southeastern border of the main basin consists of Precambrian formations. At Bathurst Inlet the Epworth dolomites, which immediately overlie the Precambrian granites, were gently warped and deeply eroded before the Kanuyak formation was deposited upon them. The Kanuyak and the Epworth formations after suffering a gentle f o l d along axial l ines trending a l i t t l e east of north, were deeply eroded before the Boulburn quartzites were 35 l a i d down on them. Deep erosion followed the deposition of the quartzites, then a great series of basalt flowed over exposed surfaces of a l l ear l ier formations* The readjustment of the crust, following the extrusion of this great volume of lava, produced a gentle warping, and some normal fau l t ing . In the Coppermine River area, about 300 miles west from Bathurst Inlet , the Kanuyak and Boulburn formations are missing and rocks of Coppermine River Series d i rec t ly overlie the Epworth dolomites. The Kanuyak formation, as described by O 'Ne i l , consists of thin-bedded, gray-to-brown and reddish-brown shaly and sandy limestone*. Thus, the condition! of deposition i n Bathurst Inlet and i n the area between Port Epworth Harbour and the Coppermine River area was changed after the Epworth Series was deposited. The basalt flows i n the Coppermine River area strikes N50°W and dip gently to the north;: i n Takiyuak Lake area and the northern part of Victory Island, s imilar flows have been reported dipping gently to north and south respectively.. Therefore, i t i s l i k e l y that shallow folds with axes trending about N50°W are present i n the Coppermine River area* The Coppermine River area and the Arct ic Archipelago have a very complex geological development. I t i s marked by a major orogenic be l t , 1,1*00 miles long, extending from north Greenland to Beaufort Sea (Keer unpub-l i shed) * The Arctic Archipelago represents the most mobile part i n the outer section of North America. The early tectonic movement which effects this region is the Caledonian movement of the S i lur ian age. Most of the rocks of the Palaeozoic and the Triass ic age in the Arctic Archipelago are f l a t - l y i n g , but the rocks of S i lur ian and the Ordovician age on the west coast of Ellesmere Island have been folded (O'Neil 192k)• According to Armstrong (191*7) i "In the v i c i n i t y of Vendome F iord , on the west coast of Ellesmere Island, folded strata of S i lur ian and the Ordovician age have been observed.. A series of northeast trending and sharp ant ic l ines occurs in th i s region* No folds can be seen on the south side of Baumann Fiord where the folded Si lur ian and the Ordovician strata are either buried beneath the f l a t - l y i n g , younger Devonian and Carboniferous strata or swing to the west beneath the sea* If the former i s the case, the folding i s pre-Devonian and i s probably of the late S i lur ian age". The shallow, open folds i n the Coppermine River area would seem to be induced by the Caledonian movement. The common structure feature i n this part i s a series of tension faults trending from N10°E to N£0°E and from N10°W to N20°W and cutting the strike of the rock i n a high angle,. This development of fractures on the flank of the folding is caused by the compressive force from the north,. The most conspicuous fault i n the mapped area i s the Wreck Lake fault which trends Nii5°W, extending from Wreck Lake southwest to the Dismal Lake, and northeast as far as 5 miles; the to ta l length which can be traced on the a i r photo is about 12 miles. 37 CHAPTER V ECONOMIC GEOLOGY Introduction The pr incipal ore minerals of copper occurring i n the area i s chalcocite and small amounts of chalcopyrite. The deposits can be c l a s s i f i ed as follows according to their occurrence: 1. Chalcocite-quartz-carbonate veins, 2. Chalcocite deposits in feeder dykes of basalt, 3* Chalcocite deposits i n flow tops, li.. Native copper deposits i n the basalt . Pieces of copper-stained f loat are wide-spread here. The f loats mostly occur i n long, narrow belts i n the d r i f t , and have the same trend as the dykes. It i s believed that the fragments are brought up by frost action to the surface from the dykes which are covered with a th in layer of d r i f t . Chalcocite-quartz-carbonate Veins Good examples of chalcocite-quartz-carbonate veins are found 2 miles south-west of Amco Lake. On a south-facing c l i f f of basalt , narrow quartz-carbonate veins, about 1 to 2 inches thick, occupy fractures i n basalt and they trend about 10.5°^ to N20°W. Comb structure i s the common feature;- grains of chalcocite are disseminated in the veins. Under the microscope, chalcocite occurs as irregular grains between the interstices cf quartz grains, a minor amount of chalcedony is present i n addition to quartz and carbonate. Wherever the veins traverse the rock which shows the spheriodal weathering, the grains 38 of chalcocite, quartz and carbonate are seen to be deposited between the con-centric layers of the weathered rock. This kind of occurrence of the chalcocite may be due to the supergene enrichment; since chalcopyrite, which is presented i n small amounts in the vein , i s readily attacked by oxygen to form copper su l -phate (Lindgren, 1933, p.832). Under proper conditions copper sulphate w i l l react with other ore minerals to form copper sulfide (Bateman, 1950, p.277) and redeposited along the fractures* Chalcocite deposits i n feeder dykes Chalcocite deposits i n feeder dykes of the basalt are the main occur-rence of copper i n the Amco and Burnet Creek area* Three samples were collected from a similar dyke at the north side of Burnet Creek, two of them being from the dyke and the th i rd from the wall rock close to the dyke. In the hand-specimen, the wall rock i s reddish-brown and fine-grained, White to reddish-white veinlets , cutting through the rock, contain grains of chalcocite. Under the microscope, the rock i s seen to be of the same composition as the basalt occurring in the v i c i n i t y . The feldspar and pyroxene are highly altered; a l l the pyroxene has been altered to chlorite and serpentine and the feldspar i s highly mottled i n appearance. The veinlets i n the rock are com-posed of ca lc i te , quartz and minor oligoclase. In the portion of the veinlets where the calcite i s predominant, the plagioclase crystals are included in the ca lc i te , and show encroachment by calc i te along i t s crystal outline. In addition to the plagioclase and pyroxene, there i s present a small amount of iddingsite. The sample from the margin of the dyke i s composed of angular frag-ments cemented by quartz. Under the microscope, the thin-section shows the cementing material to be mainly chalcedony, with a minor amount of calcite and grains of chalcocite. Patches of chlor i te , the alteration product of the_ ferromagnesium mineral, are scattered over the section. Small amounts of iddingsite are also present. The sample taken from the highly mineralized portion contains massive chalcocite, quartz, and fragments of basalt. Under the microscope^ the matrix and mineral composition are seen to be similar to those of the rock collected from the margin of the dyke. Chalcocite deposits in flow tops The best example of the chalcocite deposits i n the flow tops i s well i l lu s t ra ted i n the rock occurring at the Wreck Lake area. Boulders and frag-ments of flow tops containing chalcocite and hydrous carbonates of copper are widely spread over the area, though, owing to the glaciat ion, the to ta l number of such fragments i s small. In one l o c a l i t y thin quartz-calcite veins were seen cutting through the flow tops which show heavy copper stain surrounding the veins. The rock a few feet away from both sides of the veins i s free from copper stains and there no trace of copper sulphides has been found in the flow tops. The fragments of the flow tops containing chalcocite are generally dark i n color , with greenish and bluish-green copper stain i n some portions.. The grains of chalcocite are disseminated in the amygdules and i n the flow tops. Malchite and a minor amount of azurite are commonly associated with the chalcocite.. In the th in section (Figure 12), veinlets of quartz-feldspar aggregates are seen cutting through the rock, and the chalcocite i s scattered i n the wall rock on both sides of the veins. The chalcocite which occurs in the amygdules i s founded molded on the projecting crystals of feldspar and quartz or surrounding the l a t t e r minerals (Figure 13, l U ) . In the polished section, i rregular ly shaped patches of chalcocite are scattered i n the basalt. Unsupported fragments of the country rock in the patchds of chalcocite suggest the replacement by the ore solution i n the rock. Figure 12. Veinlet of quartz-feldspar aggregates (white) i n basalt; chalcocite (black) scattered i n the basalt (upper h a l f of the p i c t u r e ) . 80X Figure 13* Chalcocite (black) i n an amygdule of quartz, carbonate and i n the basalt* 80X Figure l i t . Chalcocite (black) molded on the projecting c r y s t a l s of feldspar and q u a r t z * 8 0 X The f i e l d r e l a t i o n between mineralized flow tops and quartz-carbonate veins and the microscopic study of the t h i n sections may suggest that the chal-cocite deposits i n the flow tops are introduced by the quartz-carbonate veins. The malachite and azurite i n the flow tops are believed to have been formed by the action of carbonate waters upon the copper compounds, such as chalco-c i t e . Native copper deposits i n the basalt The native copper has been occasionally found as small grains, plates i n the rock. I t may have d i r e c t l y separated out from the lava during the c r y s t a l l i z a t i o n . Native copper which occurs as minute flakes to flakes about 2 feet i n diameter i n the fractures of the basalt may have precipitated from copper chloride s o l u t i o n * Since no pyrite has been found i n the basalt, kz the origin of the sulpheric acid (H2SO^)j which w i l l react with CugS to cause the deposition of the native copper, i s a question. According to Cornwall. ( 1 9 5 6 ) : "the high chloride content of the present mine waters i n the Michigan d i s t r i c t suggest the pos s ib i l i ty that the ore-toearing solutions may have been chloride; native copper has been precipitated .from copper chloride solutions at 200-250 c in the presence of prehnite, calci te or zeol i tes" . The zeolites and calc i te are the common mineral i n the basalt flows under study. Thus, the native copper precipitated from copper chloride solution seems the reasonable explanation* Origin of ore solution and the c lass i f icat ion of ore deposits Geochemically, copper is mainly chalcophile and showing a high a f f in i ty for sulfur; thus, during the crys ta l l izat ion of the magma, copper w i l l combine with a l l available sulfur to form copper sul f ide . The presence of the native copper in the basalt may suggest that the basalt magma was deficient in sulfur. According to Rankama and Shama (191*9, P»697) • "native copper is found rather regularly as a constituent of the early-separated sulfides which belong chief ly to the pyrrhotite-pentlandite paragenesis. During the separation of the sulfide phase from the s i l i ca te phase, copper becomes considerably en-riched in the sulfide melt. The part of the copper which remains in the s i l i ca te melt after the separation of sulf ide, stays i n the residual magma during the main stage of crys ta l l iza t ion being f i n a l l y separated i n hydrothermal deposits i n which the copper occurs as a constituent of a great number of sulfides".. In the differentiated layers i n the basalt , mentioned under petro-graphy, copper and nickel sulfide ore have been reported. It seems l i k e l y that the sulfide ore is genetically related to the basaltic magma,. Thus, h3 the copper sulphide ore, together with the quartzo-feldspathic veins, would seem dif ferential products of the t h o l e i i t i c magma.. The presence of chalcedonic quartz in the veins and the dykes i n which chalcocite i s deposited, indicates that the deposit was formed at low temperature.. The veins usually show crust i f icat ion, comb structure and irregular wal ls . A l l these features l ed to the bel ief that the ore deposit belongs to the epithermal class,. BIBLIOGRAPHY Armstrong, J.E. (19l;7) The Arctic Archipelago (unpublished). Bateman, A,If. (1950) Economic mineral deposits, 2nd edit. , ed. J. Wiley & Sons, N.Y. Cornwall, H.R. (1956) Origin of native copper deposits, Eco.Geol., vol. 51, no... 71 Duncan, Gordon G.. (1931) Exploration in the Coppermine River area, N.W.T. Can. Min. and Met.,.Bull.. 20. Fairbridge, R.W. (1957) The dolomite question, Soc. of Eco. Paleontologists and Mineralogists, Special publication No. 5« Flint , R.F. (1914-9) Glacial geology and the Pleistocene Epoch, Jith edit. , ed.. J.. Wiley &.Sons, N.Y. Gilbert, G.. (1931) Copper on the Coppermine River area, N.W.T., Eco.. Geol., vol . 26. Jenney,, C P . (1951*) The Coppermine River area, N.W.T., Proc. of the Geol. Assoc. of Can., vol. 6, part II. Kennedy, W.P., (1933) Trends of differentiation in basaltic magma, Amer. Jour. Sc i . , vol . 25-26. Kerr, J.W. Broad structural relationship of the Canadian Arctic (unpublished), Kumbein, W.C.. & Garrels, R.M. (1952-) Origin and classification of chemical sediments in terms of pH and oxidation-reduction potential, Jour., of Geol., vol . 60 . , no. 1.. Lindgren, W. (1933) Mineral deposits, lith edit . , ed.,McGraw-Hill Book Co.N.Y. Mason, B. (1951) Principle of geochemistry, ed. J . Wiley & Sons, N.Y. O'Neill , J.J. (1921;) Report of the Canadian Arctic expedition 1913 to 1918, Geol. and Geog., vol . II. Pettijohn, F.J . (I9I48) Sedimentary rock, 2nd edit. , ed. Harper & Bros., N.Y. Stearns, H.T., (19l;0) Geology and groundwater resources of the islands of Lanai Kahoolane, Hawaii, Hawaii Division of Hydrography, Bull . 6. Thoronbury, W.D. (1951;) Principle of geomorphology, 2nd edit. , ed. J. Wiley & Sons, N.Y* Washburn, A.L. (19U7) Reconnaissance geology of portions of Victory Island and adjacent regions A r c t i c Canada, Geo. Soc. of Amer., Mem 22. Annual report of the director of the Geophysical Laboratory, Carnegie In s t i t u t e of Washington, No. 1277, G.S.A. Mem. 22. 

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