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A petrographic study of granitization in the norite at Dinty Lake, Northern Saskatchewan McLellan, Robert Bryant 1940

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A PetrO'grapiiic Study of Grandtization i n the Norite at Dinty Lake, Northern Saskatchewan. Robert Bryant McLellan A Thesis submitted i n P a r t i a l F u l f i l m e n t of The Requirements for the Degree of MASTER OF ARTS i n the Department of GEOLOGY THE UNIVERSITY OP BRITISH COLUMBIA APRIL 1940. Table of Contents. Page I Introduction 1 (a) General Statement 1 (b) Location and Topography 2 lc) Petrographic Study and Acknowledgements 3 II General Geology 4 (a) P r i n c i p a l Rock Units 4 (b) Structure 5 III Diamond-drill Cores and Thin Sections 71 IV Petrographic description of the Rock Types 9- (a) Regional Granite Gneiss ^ 9 (b) Horite and Related Types 10 (1) Fo r i t e 10 (2) Pyroxenite IL (3) Hybrid Types 12 (c) Pegmatites 14 (d) Lamprophyres 14 V Phenomena of G r a n i t i z a t i o n 15 VI Process cf:Granitiza;tioh-. . .: 17 (a) The Anarthosite Problem 18 (b) G r a n i t i z a t i o n of Horite 19 £c) Accompanying E f f e c t s 21 VII Geologic History 22 VIII Conclusions 24 IX Bibliography 25 I l l u s t r a t i o n s Elate I. Map of the Main Borealis Deposit. Follows page Plate II Section Across Main Borealis Deposit. " " (Both reproduced from the report of Dr. CO. Swanson on the Dinty Lake Nickel Deposits}. A Petrographic Study of Gr a n i t i z a t i o n i n the Norite at Dinty Lake, Northern Saskatchewan, Introduction (a) General Statement The present work i a intended to consider, and propose solutions f o r petrogenic problems a r i s i n g from the examination of the Dinty Lake Nickel Deposits, Saskatchewan, The fundamental problem involves the g r a n i t i z a t i o n of the nickeliferou3 norite body, and divides i t a e l f into three s p e c i f i c problems, namely to determine: ( l ) the degree to which the norite has been alt e r e d , (2) the processes c o n t r o l l i n g the g r a n i t i z a t i o n , and (3) the probable nature of the o r i g i n a l ore body. Although the sedimentary o r i g i n of much of the regional g r a n i t i c ;gneisa was recognized by Alcock 1 before 1936, the known bodies of norite were considered to be intr u s i v e into the granitized sediments. I t was not u n t i l a f t e r the Dinty lake deposits were discovered i n 1936 that th e i r s t r u c t u r a l and mineralogical variations were ascribed to regional metamorphism. The property was examined i n considerable d e t a i l i n June, 1938 by Dr. C O . Swanson. 1 Alcock, P.J., The Geology of the Lake Athabaska Regioni Can. Geol. Survey, Mem. 196, p.10, 1936. Trenching, and diamond-drilling were the sampling metho»ds, and magnetic observations were taken to determine the extent of the ore body. Hia r e p o r t 1 on the depositBs gives conclusive evl&eneemaf g r a n i t i z a t i o n . The group of claims containing the main deposit belongs to the Borealis Mining Syndicate, and adjacent claims are held by the Gonsilidated Mining and Smelting company. (b) Location and Topography Dinty Lake i s situated at the western edge of the Eond-du-Lac map-area of northern Saskatchewan. It i s about 25 miles to the northeast of Goldfields and about 15 miles fromhLake:.Athabaska into which i t drains by way of the Beaver E i v e r . Dinty Lake measures about 2% miles long and -§- mile wide. The area to which the present problem i s confined i s about a quarter mile north of the lake. I t i s the largest norite exposure i n the v i c i n i t y of the lake, having a width of about 300 feet and extending about 1000 feet along i t s north-south elongation. The exposure caps a broad ridge at an elevation of about 110 feet above Dinty Lake. Other s i m i l a r , but smaller, norite bodies l i e along the same zone, the largest being more than a half mile to the north. ^ Swanson, CO., "Report on Dinty Lake Nickel Deposits, Sask.% July 6, 1938. -3- The r e l i e f of the area i a t y p i c a l of the Laurentian Plateau region, the edge of which i s marked by Lake Athabaska. Howhere i s the r e l i e f greater than 400 f e e t . Plat-topped h i l l s , though modified by g l a c i a t i o n , suggest a former peneplane. Drainage i s disorganized, and countless lakes and swamps occupy the depressions. Dirnty Lake p a r t l y crosses and pa r t l y follows the st r i k e of the bedrock. Since the physiography i s l a r g e l y controlled by g l a c i a t i o n , l o c a l modifications are not necessarily guides to the geological structure. (c) Petrographic Study and Acknowledgments. During the winter months of 1939-40 the writer devoted considerable time to the examination of diamond- d r i l l core samples from the property. A 3 e t of thin sections made from the cores were studied i n d e t a i l by means of the petrographic microscope. TJhe writer wishes to express his indebtedness to Dr. G.O. Swanson for the use of the materials and equipment, and for the many h e l p f u l suggestions and frequent words of advice which were invaluable i n the undertaking of this work. -4- General Geology, (a) P r i n c i p a l Rock Units. The underlying bedrock of the area i s the regional granite gneiss. I t consists of highly metamorphosed sediments varying i n texture from banded q u a r t z i t i e paragneiss to a ^rather basic and .gneissic granite or orthogneisa. The paragneiss is- uniformly banded suggesting: an o r i g i n a l sedimentary bedding:, but there are no horizon markers..The more massive orthogneiss has the c h a r a c t e r i s t i c s of an Intrusive rock i n certain parts but other specimens show a gradation into streaked and f o l i a t e d gneiss with basic c l o t s and i n c l u s i o n s . The largest and most notable inclusions are the masses of pyrrhotrte-bearing n o r i t e . They range i n compos- i t i o n from b i a t i t e - r i e h norite to pyroxenite dense with pyrrhotite and through gneissic portions to highly f e l s p a t h i c types c a l l e d •'"anorthasites1* f o r convenience. These v a r i e t i e s include many altered and hybrid types believed due to processes of g r a n i t i z a t i o n . To the confusing mixtures of granite gneiss and norite types the term "migmatites" i s applied. A great many ir r e g u l a r pegmatite dikes cut the gneiss and n o r i t e . Intruded into a l l three are a few lamprophyre dikes of f r e s h massive material with dark mica phenoerysta. The economic value of the property i s based on the n i c k e l content of the py r r h o t i t e . Assays show that even -5- the r i c h parts of the deposit contain only .2. to ,5 percent n i c k e l , and therefore connot he commercial ore. (h) Structure The regional structure i s shown hy the conformable banding of the paragneias. Ho such extensive i n t e r p r e t a t i o n of the i n t r i c a t e l y folded sedimentary gneiss has yet been worked out, however, fo r mapping, made d i f f i c u l t by wide- spread g l a c i a l deposits and sand p l a i n s , i s s t i l l very d i s j o i n t e d * In the v i c i n i t y of Dinty Lake the structure i s f a i r l y uniform. The banded gneiss assumes a north-south s t r i k e with a variable d i p to the west, and generally conforms to the elongation of the norite zone. The western part of this area has a steep dip and i s generally well banded. Hear the ore bodies and to the east, however, the more massive gneiss i s interpreted aa forming s t r u c t u r a l terraces to which the norite also conforms. This structure' involves f l a t dips to the west. The conformable nature of the main norite body was determined by diamond d r i l l i n g as shown i n Plates I and I I . This leads to the conclusion that i t was o r i g i n a l l y a concordant i n t r u s i v e , probably a s i l l . Numerous cross-faults with small o f f s e t s cut the gneiss and n o r i t e . Most of them are p r a c t i c a l l y v e r t i c a l and s t r i k e west to northwest. Some of these are -6- occupied by the pegmatite dikes which are only a few feet wide, while coarsely brecciated material defines the others. Another set of f a u l t s s t r i k e s 40 to 70 degrees east and dips steeply to the northwest. They are poorly defined and no displacement i a apparent. The lamprophyre dikes follow these fractures which cut a l l other bedrock. Only by the s l i g h t l y magnetic property of the dikes can these i n j e c t i o n s be traced on the surface. The structure of the area i n question i s seen, from this o u t l i n e , to be quite simple, with very few features which might complicate the reconstruction of the o r i g i n a l ore body. In mapping the area Dr. Swanson1 found no trace of s t r u c t u r a l features such as "'•sedimentation'* structure within the norite i t s e l f . Personal communication. -7- Diamond-drill Gores and Thin Sections. The specimens referred to i n this study were selected as representative samples of the rock types i n the area. They are taken from three of the twelve diamond-drill holes sunk i n and near the main Borealis deposit. A l l three holes (D.D.H.I,2,3; Plate II) l i e i n the same v e r t i c a l east-weat plane, and are arranged as shown hy their numbers. Prom the, s t r u c t u r a l r e l a t i o n s of the norite i t can r e a d i l y be seen that with this arrangement the specimens represent d i f f e r e n t s t r a t i g r a p h i c horizons i n their respective d r i l l - holes and follow no known s t r u c t u r a l or co n s t r i c t i n g features. Prom the 59 specimens at hand 18 thin sections were cut. They were selected to represent the more t y p i c a l rocks, and their d i s t r i b u t i o n i s indicated i n Plate I I . The following table gives the index number, l o c a t i o n , and essential nature of each th i n section. The microscopic examination of theae thin sections constituted the sole means of textural and quantitative determination. The remaining specimens proved h e l p f u l megaseopically, and grains of several were examined under the microscope i n r e f r a c t i o n o i l s as a check on the i r mineral composition. -8- Fa. D.D.H. Rook Type. D.L. 1** 1 356* B i o t i t e granite, ,fcnormaln: granite gneiss. 3 3 33 Gneissic granite, with gar. and q/tz.. 6* 1 506" Gran i t i c gneiss, sedimentary. 7 3: 117£ Andesine rock, with ehlor. and q^tz.. 9* 1 609 Gran i t i c gneiss, c a t a c l a s t i c . 10 2 40 Garnet gneiss, t y p i c a l . 14*' 1 308i Garnet gneiss, near pyr. i n c l u s i o n i n gr.. 18 1 Contact pyr.-pyroxenite and gar. gn.. 21 1 166 Garnet gneiss, with pyr.-pyx. i n c l u s i o n . 24 3 85 Altered n o r i t e , with pyrrhotite and mica. 30 1 215 Altered n o r i t e , with scattered pyrrhotite, etc. 35 1 251 Altered n o r i t e , with amphihole and c h l o r i t e . 37 2. 55 Typical n o r i t e . 40 2 172 Altered pyroxenitej with pyr. and amph.. 46 3 160 Contact pyr.-pyroxenite and gar. gn.. 57 2: 121 Mica lamprophyre, near upper contact. 58 2 125 Mica lamprophyre, center. * Situated well "beyond the l i m i t of the n o r i t e . Aside from the petrographic d i f f i c u l t i e s , the f a c t that the thin sections constitute an i n f i n i t e s i m a l part of the rocks which they represent, indicates that they cannot be accurate quantitative guides to the rock composition. This i s espe c i a l l y true i n the present case where the rocks known to be heterogeneous and i r r e g u l a r l y a l t e r e d . Moreover, the thin sections show wide variations even i n the same type of rock. This feature, however, i s the essential c h a r a c t e r i s t i c upon which this study i s based, f o r the fundamental problem i s the i n t e r p r e t a t i o n of these v a r i a t i o n s . Petrographic Description of the Rock Types, (a) Regional Granite Gneiss As described i n general on a preceding page the granite gneiss varies widely i n texture and composition i n d i f f e r e n t parts. The variations are believed to be due p a r t l y to o r i g i n a l differences i n the composition of the sedimentary rocks but mainly to differences i n the degree of g r a n i t i z a t i o n . As a basis f o r comparisons, the "normal" granite gneiss may be described as the most highly r e c r y s t a l l i z e d rock showing the mineral c h a r a c t e r i s t i c s of an altered rock. Of the four representative t h i n sections of the regional gneiss, section Ho.9 appears to be most highly r e c r y s t a l l i z e d , but quartz, the c h a r a c t e r i s t i c mineral of the sediments, i s not present. Section No.l, then, i s taken as the "normal" granite gneiss. Its composition compares f a i r l y c l o s e l y ta that of Ho.6 i n which a sedimentary bedding i s preserved. These rocks are composed e s s e n t i a l l y of quartz rZ0%) and orthoclase (40-60^) with b i o t i t e (5-2.0#) and garnet 2.-8$) as the common accessories. Small amounts of magnetite -10- and pyrrhotite are usually present among the dark minerals, A few grains of medium plagioclase and some s e r i c i t e f lakes occur with the orthoclase. The garnet i s a pinkish variety, probably almandite, and commonly contains inclusions of quartz, b i o t i t e , and magnetite. The larger garnet c r y s t a l s are i n v a r i a b l y fractured, and i n both sections the fractures have a common orientation, indicating stress conditions. The texture of the l i g h t minerals i s allotriomorphic m i c r o c r y s t a l l i n e but the accessories usually form large streaks or c l o t s . The other specimens of granite gneiss d i f f e r from the "normal i n mineral proportions and texture. One section shows an abundance of quartz and another very l i t t l e . Gataelaatic textures are common. (b) Norite and Related Types; Cl) Norite The l e a s t altered norite i s a hypidiomorphic f i n e grained to microgranular rock. Hypersthene and a few grains of enstatite make up 65 percent of i t , and labradorite (Ab4sAn55) about 15 percent. Reddish-brown b i o t i t e i s present i n amounts up to 12 percent, and associated with i t are a few grains of t i t a n i t e with magnetite C?) rims. These minerals are a l l primary, for the f i r s t i n d i c a t i o n of a l t e r - a t i o n i s a streak at one edge of the specimen where s e r i c i t e has reduced the plagioclase to a few barely recognizable remnants. -11- An a l t e r e d norite specimen, though d i f f i c u l t to d i s t i n g u i s h from the unaltered rock i n the hand specimen, shows only a textural s i m i l a r i t y under the microscope. No trace of pyroxene remains, but i t i s replaced isomorphously by f i b r o u s u r a l i t e rimmed by c h l o r i t e . The plagioclase i s a sodic andesine and i s i n t e r s t i t i a l between the dark minerals. It i s s l i g h t l y sericitiz.ed. B i o t i t e shows a tendency to be concentrated i n schistose streaks. Each of these minerals compares c l o s e l y i n abundance with the corresponding minerals i n the unaltered n o r i t e . (2.) Pyroxenite The norite grades into pyroxenite by a decrease i n the amount of f e l s p a r . In most specimens labradorite i s completely substituted by p y r r h o t i t e . A small proportion of augite i s also developed* In the fresh pyroxenite b i o t i t e i s the only accessory mineral. A l t e r a t i o n products commonly present are garnet, amphibole, magnetite, and secondary b i o t i t e . A specimen of highly magnetic pyroxenite consists of fibrous u r a l i t e enclosing scattered pyrrhotite and minor amounts of b i o t i t e and c h l o r i t e . C3) Hybrid Types The highly a l t e r e d types of rock within the norite body are represented by the majority of the specimens. I t cannot be assumed from this that there i s necessarily a preponderance of hybrid material i n the body, yet from the f i e l d r e l a t i o n s such types are found to p e r s i s t over large portions of the norite area. Ho estimate of the proportion of unaltered norite and pyroxenite has been made, but the bulk of this frayed s i l l i s probably affected by g r a n i t i z a t i o n . Of the many hybrid types at hand, the most common i s a migmatite c l o s e l y related to the n o r i t e . There i s no t r u l y representative th i n aection of t h i s , but powdered specimens under the microscope were examined. They are r i c h i n both sodic and medium plagioclase with considerable b i o t i t e and garnet. The mineral r e l a t i o n s are shown by a thin section of a more g r a n i t i c type. The sodic f e l s p a r (oligoclase) contains andesine-labradorite as p o i k i l i t i c i n c l u s i o n s . Both reddish and black garnet are present as small rounded grains. Magnetite and b i o t i t e are associated with them forming gneissic c l o t s . A few pyroxene grains also occur with the garnet Another thin section i s about 80 percent andesine with small amounts of quartz and a l b i t e . Chlorite i s present as v e i n l e t s crossing the whole specimen. When describing the fragmental material of the norite exposure, Cdoke x probably referred to. a migmatite type close to the norite composition. He observes that **the rock - — i s rather fine-grained, equigranular, and d i r t y gray, about the texture and g r a i n s i z e of loaf sugar. In hand specimen i t i s r e a d i l y mistaken f o r a fine-grained, rather basic granite, or gneiss. — - The feldspar i s andesine, Ahg§An|.g to Ab55An45, and on the whole Is f a i r l y f r e s h • Its proportion v a r i e s , i n d i f f e r e n t fragments, from about 30 percent of the t h i n section to nearly 100 percent'*. These f e l s p a t h i c v a r i e t i e s have been c a l l e d anorthosites. Garnet gneiss Is another variety of hybrid commonly found i n the norite body. I t consists mainly of plagioclase and garnet, with the usual accessory minerals of the regional gneiss. Quartz i s abundant i n certain specimens and forms a mosaic pattern. A l k a l i f e l s p a r may also be present as i s o l a t e d patches. There i s no gradation from the norite or pyroxenite into the garnet gneiss. The contact zones are very narrow, a l t e r a t i o n products such as garnet and s e r i c i t e are confined to a quarter inch width at the edge of the pyroxenite masses. In contrast to the fresh appearance of the minerals i n the t y p i c a l n o r i t e , the b i o t i t e i n hybrid types i s usually bleached, or p a r t l y altered to> c h l o r i t e . S e r i c i t e 1 Cooke, H.C., Goldfields Area, Sask., Geol. Survey. Can., Prelim. Report, Paper 37-3, (1937) p. 19. i s almost; i n v a r i a b l y present among the f e l s p a r s , and amphibole i s not rare as an a l t e r a t i o n product of pyroxene, (c) Pegmatites The veins or dikes which follow east-west cross f a u l t s of the area consist: of quartz, and soda-rich f e l s p a r (oligoclase) with minor amounts of garnet* c h l o r i t e , and magnetite. Except f o r the s l i g h t c h l o r i t i z a t i o n these pegmatites appear to be unaltered. (d) Lamprophyres Thin sections of the lamprophyre dike material show no e f f e c t s of g r a n i t i z i n g solutions. This i s . i n agreement with their crosscutting r e l a t i o n s h i p to a l l other rocks i n the area. The texture and composition of the lamprophyre i s 3hown by two thin sections from a dike within the norite body. The central part has phenocrysts of phlogopite, potash f e l s p a r , and magnetite. Some euhedral f e l s p a r c r y s t a l s are intermediate i n s i z e . Near the contact there i s only a s l i g h t decrease i n the s i z e of the phenocrysts, and the matrix i s somewhat ri c h e r i n hornblende and magnetite. Phenomena of G r a n i t i z a t i o n . The following aection w i l l be devoted to a summary of the petrographic evidence upon which the conclusions of t h i s study are based. The evidence i s not a l l microscopic, however, i n f a c t the main arguments have been observed i n the f i e l d ! . "That the granite gneiss has intruded the norite i s shown by the shape of the n o r i t e , by the development of hybrid rocks due to reaction between the norite and the granite,, and by the presence of i s o l a t e d slabs or fragments of pyrrhotite- bearing norite at some distance from the main contacts 1*. 1 There i s also abundant petrographic evidence of g r a n i t i z a t i o n throughout the area. That the regional granite gneiss was developed by the g r a n i t i z a t i o n of sediments needs no support from the present study, f o r the s t r u c t u r a l r e l a t i o n s and mineral composition, along with the widespread and erratiscoccurencepepreclude any p o s s i b i l i t y of an igneous o r i g i n . In confirmation of the g r a n i t i z a t i o n process, however, th i n sections of the granite gneiss, representing rocks more than 100 feet outside the main contacts, show microscopic evidence. A t y p i c a l specimen of the regional gneiss aa described i n the preceding section has very nearly the same mineral composition as another specimen which s t i l l r e t ains the handed appearance of a thin-bedded sediment. 1 Swanson, CO., Report on Dinty Lake Nickel Deposits, Sask., July, 1938, p.8. -16- Petrographie features of the norite complex, which support the idea of g r a n i t i z a t i o n and help to explain the s t r u c t u r a l phenomena, include variations i n texture and mineral r e l a t i o n s . At the contacts between the garnet gneiss and pyrrhotite-bearing: pyroxenite c e r t a i n a l t e r a t i o n products develop which are no doubt due to a reaction. The contacts are are quite sharp i n two thin sections. One of these (No. 18) shows garnet and pyroxene grains grouped i n gneissic fashion i n the gneiss, and garnet rims developing around hyperathene grains of the pyroxenite. B i o t i t e and magnetite are extensively developed along the contact zone and among the gneissic c l o t s . Section No. 46 shows a sharp contact between pyroxenite and a coarse band of andesine. Narrow streaks of hypersthene, garnet, and b i o t i t e run p a r a l l e l to and near the contact. The hypersthene i s a highly ferruginous v a r i e t y , some c r y s t a l s containing tabular i n c l u s i o n s . The pyrrhotite i s granular and i n t e r s t i t i a l , and has inclusions and rims of magnetite which appear to be related to the b i o t i t e f l a k e s . There i s considerable s e r i c i t e developed i n the f e l s p a r area, but the process i s nowhere near completion. The f e l s p a r i s basic andesine and contains no accessory minerals. An i n c l u s i o n of pyrrhotite-bearing pyroxenite i n a s i m i l a r f e l s p a t h i c type i s represented by section No. 21. No garnet i s present, but the evidence of a l t e r a t i o n consists of rims of fibrous amphibole between the grains of hypersthene and p y r r h o t i t e . The border of the i n c l u s i o n -cgaî e-. irrlgg&lar and remnant grains of pyroxene are scattered through the gneiss. B i o t i t e i s abundant between the dark minerals, and several flakes of i t i n the thin section are bleached to a pale green with streaks of i r o n oxide deposited p a r a l l e l to the cleavage. Through a l l specimens of t y p i c a l garnet gneiss which are located within the n o r i t e , tiny rounded., grains of pyroxene occur with largerrgarnet c r y s t a l s and b i o t i t e f l a k e s . The b i o t i t e i s more cl o s e l y associated with pyroxene than with garnet. The general character of the migmatites as described i n a preceding section i s evidence of an advanced stage i n the g r a n i t i z a t i o n process. The development of untwinned sodic plagioclase indicates the high degree of regional metamorphism. Process of G r a n i t i z a t i o n An interpretation of the petrographic d e t a i l s described above leads to an acceptable explanation of the processes c o n t r o l l i n g the g r a n i t i z a t i o n . The conclusion Is not arrived a t , however, without investigating more than one p o s s i b i l i t y as to the o r i g i n a l nature of the norite i n c l u s i o n . Omitting any discussion of the g r a n i t i z a t i o n of the regional sediments^- assuming from the fragmentary evidence that the process was a normal one - the discussion w i l l be confined to the norite changes. -18- Ca) The Anorthosite Problem. Prom the i r r e g u l a r d i s t r i b u t i o n of the pyrrhotite, the large tongues of gneissic granite, and masses of hybrid rocks within the ore body, the p o s s i b i l i t y of o r i g i n a l differences i n composition i s suggested. The a r b i t r a r y boundaries between some of the main rock types as shown i n Plate II suggest an o r i g i n a l banded structure. The existence of primary an o r t h o s i t i c layers might be taken as a possible explanation of the p l a g i o c l a s e - r i c h rocks. In describing the norite i n other parts of the region, Alcock noted that "an intere s t i n g feature i n connect- ion with these rocks i s the occurrence of complementary v a r i e t i e s , some dikes consisting of labradorite with only minor amounts of pyroxene and ir o n oxide and others being made up almost completely of orthorhombie pyroxenes'*'.1 I t i s not c l e a r , however, whether both v a r i e t i e s occur i n the same •"dike*. The p o s s i b i l i t y of such a s i t u a t i o n i s worthy of serious consideration i n the present case, for a composite s i l l of complementary v a r i e t i e s i s remotely concievable. The main st r u c t u r a l argument i n favor of the anorthosite bands i s the rough correspondence of the gneissic horizons i n three d r i l l holes, and the i n j e c t i o n of tongues of gneiss p a r a l l e l , i n general to the f l o o r of the s i l l . The 1 Alcock, P.J., Geology of Lake Athabaska Region, Sask., Can. Geol. Survey, Mem. 196, (1936) p. 19. -19- evidence f o r t h i s , however, i s fragmentary, and the corres- pondence may e a s i l y he c o i n c i d e n t a l . There are confusing occurrences of gneiss and norite i n places where the specimens are more clos e l y spaced, showing that i n t e r p o l a t i o n f o r large distances i s not possible. The more convincing of the possible petrographic arguments are ( l ) that the sharp contact between pyroxenite and andesine rock types i s the r e s u l t of an injected plag- ioclase layer into a s t i l l hot norite body, and (2) that the existence together of c a l c i c and sodic plagioclase i n migma- t i t e i s the r e s u l t of g r a n i t i z a t i o n of the anorthosite. The r e f u t a t i o n of the f i r s t of these arguments i s that i t i s not reasonable to assume a series of such injections of anorthosite into a s i l l . The second i s f a u l t y i n that the basic plagioclase i s not labradorite and i n any case i t could be developed from norite, the pyroxene going to form garnet, c h l o r i t e , and mica. (b) G r a n i t i z a t i o n of Horite There i s now no reasonable argument, from the study of the thin sections, to oppose the idea of an o r i g i n a l norite body which had no anorthositic variations. The p o s s i b i l i t y of c r y s t a l sorting, to cause l a b r a d o r i t e - r i c h layers,has no supporting evidence. There are no labradorite rocks represented, and there i s no gradation between such a type and the n o r i t e . Besides, there i s more than one f e l s p a t h i c band to account f o r . Further evidence supporting the homogeneous nature of the o r i g i n a l s i l l includes the nature of the plagioclase and the d i s t r i b u t i o n of the femic minerals. The most c a l c i c plagioclase i s observed i n the unaltered n o r i t e , and has a composition of Ab^QAngQ* As g r a n i t i z a t i o n e f f e c t s increase, f e l s p a r s of a more sodic composition are formed as shown by the hybrid types i n which the fe l s p a r s range from labradorite, Ab^Angg, to Andesine, Ab^An^g, with a steady gradation. Gneissic hybrids represent a l l stages, i n t r a n s i t i o n , but those more c l o s e l y i n contact with pyroxenite have the more basic, f e l s p a r s . That the gneiss has intruded the norite i s also indicated by the development of gneissic c l o t s of monoclinic pyroxene, b i o t i t e , and garnet by reaction of the g r a n l t i z i n g solutions on the n o r i t e . The a l t e r a t i o n i s d e f i n i t e l y shown by the phenomenon of rimming at the gneissic contacts as previously mentioned. The~granitization of the norite complex involved considerable addition of material. Assuming section No. 37 to be t y p i c a l of the o r i g i n a l n o r i t e , i t can be seen that to produce the migmatite type such as No. 30 i t s composition must have been changed appreciably. To account f o r the formation of garnet of the alraandite v a r i e t y , brown b i o t i t e , and monoclinic pyroxene from the o r i g i n a l combination of hypersthene and labradorite, very l i t t l e gain or loss of material i s necessary. The -21- abundanee of c h l o r i t e and amphibole, however, suggest the addition of s i l i c a - * and water. Since orthoclase i s rare i n the g r a n i t i z e d body, potash i s assumed to have been confined to the sedimentary gneiss. The presence of soda plagioclase i n considerable quantities may be ascribed to r e s i d u a l a l k a l i l i q u i d a f t e r the reaction between the norite constituents. I t i s reasonable to assume, therefore, that the Injection! of aci d solutions from the underlying granite was the only important addition of material i n the g r a n i t i z a t i o n process. (c) Accompanying E f f e c t s The same factors of temperature and pressure which accompanied the regional g r a n i t i z a t i o n , caused a rearrangement of the pyrrhotite. O r i g i n a l l y the sulphide probably occurred as a primary mineral, being disseminated through parts of the norite with a f a i r degree of r e g u l a r i t y . The e f f e c t of the granite i n j e c t i o n was, no doubt, to concentrate the pyrrhotite from granitized parts, i n j e c t i n g then* into the complex i n the form of thin seams. The folding which i s so evident throughout the area was undoubtedly a major factor i n the g r a n i t i z a t i o n process. By way of f i s s u r e s and fracture zones the rearrange- ment of the sulphides which i t caused, was f a c i l i t a t e d . The abundance of pegmatites which follow a d e f i n i t e set of fractures i s conclusive evidence that the movement was -22- contemporaneous with the g r a n i t i z a t i o n process. The coarsely c r y s t a l l i n e quartz;-f elspar pegmatites which are cut hy seams of sulphides can only be ascribed to fr a c t u r i n g and magmatic Injections as a la t e stage i n the g r a n i t i z a t i o n process. Geologic History. From the preceding discussion the sequence of events i n the development of the norite complex at Dinty Lake can be b r i e f l y summarized: The o r i g i n of the ore body dates back to a pre-Cambrian, a time when thick sediments covered the area. The lea s t metamorphosed remnants of these, and the general composition of the r e s u l t i n g gneiss, indicate that they were l a r g e l y composed of coarsely fragraental and highly aluminous material, probably i n the form of argillaceous q u a r t z i t e s . The norite complex was then injested into the sediments as one member i n a s e r i e s of such intrusions i n the region. The nickel-bearing pyrrhotite accompanied the o r i g i n a l magma and was deposited among the minerals of the n o r i t e . Regional f o l d i n g followed this development a f t e r the lapse of an undetermined period of time, and g r a n i t i c i n j e c t i o n s into the sediments and the igneous bodies of the entire region. This transformed the sediments l a r g e l y into a g r a n i t i c gneiss, but much of the o r i g i n a l bedding i s s t i l l -23- preserved by coarse banding. The norite complex was transformed by the g r a n i t - i z i n g solutions into the frayed lenses which remain i n evidence along a d e f i n i t e s t r a t i g r a p h i c horizon of the surrounding gneiss. The remnants of norite which remain show that although the s i l l was not e n t i r e l y assimilated, a considerable part of i t was metamorphosed, with the development of migmatites and other gneissic hybrids. The l e a s t altered norite shows signs of hydrothermal a l t e r a t i o n . Tongues of garnet gneiss penetrated the s i l l to great distances. The regular d i s t r i b u t i o n of the sulphides was disturbed, r e s u l t i n g i n the production of dense pyrrhotite-bearing pyronenite and seams of sulphide along brecciated zones. Pegmatite dike3 were formed as a l a t e phase of the igneous a c t i v i t y , and occupied a set of cross f a u l t s i n the region. Further movements at a l a t e r time caused another se r i e s of fractures which was accompanied by the i n j e c t i o n of lamprophyre dikes. A small, amount of pyrrhotite was again rearranged, and was deposited i n seams which cut the s l i g h t l y sheared lamprophyres. Since the time of these developments, the only changes have been erosion, weathering, and extensive g l a c i a t i o n . -24- Conolusion The nickel-bearing nori.te body at Dinty lake has undergone a r a d i c a l transformation. Part of i t has been assimilated by g r a n i t i c i n t r u s i o n , while the remaining part has suffered various degrees of a l t e r a t i o n . The granite gneiss which has developed within the borders of the norite can be distinguished i n i t s petrographic features from the regional gneiss. The essential processes c o n t r o l l i n g the g r a n i t i z a t i o n were the regional folding and the introduction of acid emanations from the underlying granite. The o r i g - i n a l nature of the deposit was a pyrrhotite-bearing norite s i l l which, except for the d i s t r i b u t i o n of pyrrhotite, was e s s e n t i a l l y homogeneous throughout. - 2 5 - Bibllography. Alcock, F.J., The Geology of the Lake Athabaska Region: Can. Geol. Survey, Mem. 196, CL936^ Cooke, H.C, Goldfields Area, Sask., Geol. Survey. Can., Prelim. Report, Paper 37-5, (1937).;: :=>>-.. Swanson, CO., Report on Dinty Lake Nickel Deposits, Sask., July, (19381 Contours show elevations a b o v e . Dt'wty Lake Plate I MAP OF MAIN BORE A LI5 DEPOSIT I | Mort'fre. comptcx. and Hybrict RocJcs I G-reunite Q-ne-iss «• """" L, eLmproph-yre, »5ccd<2 / inch =lOOf<ie-t Plate E SECTION THROUGH DRILL-HOLES /V03. I,Z,3. MAIN 80REALIS DEPOSIT SHOWING- LOCATION or DRILL CoR£ J/>£c/*/£/VS C 1 Gran CC<z Q-nc/ss O Th in- sec ~tion Sptz a/n e« Sca/e iinch = 200 A e t

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