The Occurrence and Petrology of Basic Intrusions
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THE OCCURRENCE AND PETROLOGY OF BASIC INTRUSIONS IN lHE NORTHhEL~ kAC~NZIE ~OUNTAINS, YUKON A1lJ · ~'tl. T • --------....... _______ ..-- ...... H .J . Hofmann A th~sis submitted to the Faculty of Graduate Studies and Research in partial fulfilment of the requirem~nts for the degree of Master of Science. Department of Geological Sciences ~cGill University N..ontrE>al April, 1959 1 ' TABIJ!: OF CONTENTS INTRODUCTION • • • • • • • • • • • • • • • • • • • • • 1 General Statement . • • • . • • • • o o • o • o o • 1 Location and Access • . • o • • • • • • • o • • 2 Physiography • . • • . • • • • • • • • • • • • • • 4 Previous g~ological work • • • • • • • • • • • 6 Field and Laboratory work • . • • • • • • • . • • • 7 Acknowledgmen.ts • o • • • • • • • • • • • • • • • • 8 Tai'Illinology • • • • • • • • • • • • • • • • • • • • 9 GENBRAL GEOLOGY • . • • • • • • • • • • • . 10 Stratigraphy • • • • • • • • • • • • • • • • . • • lC ::>tructure . • • . 15 TRi.: TAWU ;:>ILL::> • • • . • • . • • • • • . • • • • • • • 18 Occurr~nce and field relations • • • • • • • • 18 Petrology • • • • • • • • • • • • • • • • • • • • • 26 Megaacopic characteristica • • • • • • • • • • 26 Microscopie characteristics • • • • • • • • 30 Mineralogy • • • • • • . • • • • • • • • • • • • • 39 X-ray Fluorescence studies • • • • . • • • • • • • 46 Contact affects •••••••••••••••••• 47 Discussion • • • • • • • • • • • • • • • • • • • • 49 DIKES IN THE KNORR RANGE • • • • • • • • • . • • • • . 56 Occurr~nce and field relations . • • • • • • • 56 Petrology . • . • • • . • • • . • • 56 Mineralogy • • • • • • • • • • • • • • • • • • • • 58 Discussion • ~ ••• . • ••••••••••• • • • 60 DI:K.BS IN BLACKSfONE RIVER AEEA • • • • . • • • • • 6 3 Physiography and General Geology of area • • • • • 63 Local stratigraphy • • • • • • • • • • • • • • • • 65 Local structur~ • . • • • • • • • • . • • • • • • 67 Petrology • • • . • • • • • • • • • • • • • • • • • 68 Mineralogy • • . • • • • • • • • . • • 70 Discussion • • • • . • 73 S~aRY AND CONCLUjiON . • • 76 APPENDIX . • 78 BIBLIOGRAPHY . • 79 LIST OF TABLBS Table 1 Stratigraphy, Northern Mackenzie Mountains •• 11 2 ~odal composition of Tawu Sills •.•••••• 30 3 Modal composition of Blackstone River dikes .• 69 LLST OF ILLU3TRATION3 Figure Page 1 Map ahowing general location of area • 3 2 Canyon Ran.ge a 4 3 3tratigraphic column of part of Cambrian(?) • 14 4 Structure section along Arctic Rad River 17 5 View in southwesterly direction from Tawu Lake 18 6 View in northeaaterly direction from Tawu Lake 20 7 The lower sill, 1.8 miles NE of Tawu Lake • 20 8 Air photo stereopair showing Tawu Sills 21 9 The ailla north of Tawu Lake • 23 10 The ailla 8 miles south of Tawu Lake • 23 11 Fence diagram of sills in Arctic Red River area • 25 12 Spec.ific gravi ty variations of the Tawu Sills • 29 13 Photomicrograph of sill-siltstone contact • • 32 14 Photomicrograph of micropegmatite and inclusions. 36 15 Texture and grain aize variation • • 37 16 Graphs of various vertical variations • 38 17 Plagioclase phenocryst showing oscillatory zoning 39 18 Micrographie intergrowths of quartz and feldspar. 41 19 Olivine phenocryst • 42 20 Magnetite skeletons • 44 21 Pyrrhotite replacing plagioclase phenocryst • 44 22 Pyrite replacement along fracture in diabase • 45 23 Fleckachiefer, showing cordierite-sericite "spots' 49 24 Skeletal pattern of transparent prisms in biotite 58 25 Carbonate pseudomorphs after plagioclase • • 59 26 Map of diorite dikes in Blackstone River area 64 27 Algal reefs and dike • • • 66 28 View showing structural relationshipa • • 66 29 Map of northern Mackenzie Mountains • 84 1 INTRODUCTION G~n~ral Statem~nt: This paper contains the results of the first study of a series of diabase sills and dikes in the north ern Mackenzie ~ountains of northwestern Canada. The Mackenzies are, at this timP, one of the least known r~gions of the Canadian Cordill~ra. F~w re ports are availabl~ becaus~ of the general inaccessibility of the area. The scarcity of geological field observations has been r~sponsible for th~ general opinion that th~r~ are no intrusives within the ~ackenzies, with the excep tion of a diabase sill reported by Keele (1910, p.41) on Keele (Gravel) River. During the last few years, oil com panies have been conducting exploration work in the plains to the north and east of these mountains. Their operations are carried out by parties using helicoptere and float equipped plan~s. It was as a member of such a party that the writer first recognized the presence of diabase intru sions in the Tawu Range ar~a of the Mackenzi~s. Although the main part of the study is con cerned with the intrusive rocks of the northern Mackenzie Mountains, a chapter on newly-discovered diorite dikes in the Ogilvie Mountains is also included here. 2 Location and Access: The area occupied by Mackenzie MountainsM is bounded on the east and north by the Mackenzie Plain and Peel.Plateau, and follows an arch through 90 degrees in 500 miles from ~outh Nahanni to Bonnet Plume River (fig.l). 3elw.yn Mountains and Liard Plateau form the western and southern boundary. The approximatP center of the Mackenzies is at 64°N 128°W, which is about 1000 miles north-northw~st of Vancouver, British Columbia. The region is beat reached by a regularly acheduled air-service to Norman Wells from Edmonton, via Fort McMurray, H~ River, and Fort Simpson. From there, using float planes, bases cao be eatablished on lakes near the mountain front. Helicoptere are then uaed to reach the interior of the mountains. The Canol lioad traverses the Mackenzies, fol lowing a pipeline built as part of the Canol project, which was launched in 1942 to devalop the Norman Wells oil field. It runs for 600 miles from 38 miles southeast of Whitehorse, Yukon, to the Mackenzie River. The road is now abandoned, but if it should be reop~ned in the future, it would for.m a second approach to the ar ea. ~ In this report Mackenzie Mountains is used in the re stricted sens e, according to Bostock (1948, p.l9). Earlier usage of the term included the mountains to the west, now known as 3elwyn Mountains, and Ogilvie Mountains. 3 1 1 0 1 \j 1 , li 1 ' , ~.. 1 . ~""· 1 '-, >···· . 1 ·~- · ' ...... ___ _ \tJtt~ho.:;.',· .. ····< .... '- .... - ' .-- British Q .... / - ' ,_-- 1 1 \ \ \ !30° 1 100 .._~~~~~-'------~• •_• _____...Jt.OO mtles Mackenzie Mountains Areas of diabase dikes and sills' 2 Selwyf1 Mountains first description • @ Ogilvie Mountains previously described • Fig. 1 Map showing general location of area 4 Physiography: Mackenzie Mountains are an area of diversi- fied topography. They have been divided (Bostock 1948) into two units: the Canyon Ranges, which are the front arc of the mountains on the northeast, and the Backbone Ranges, which comprise the main body of the mountains to the south- west. The Canyon Ranges make up a belt of mountains of smooth profiles, plateaus, and widely separated ranges, in the southeast. West of Mountain River the ranges are a rugged, maturely dissected, maas of broadly folded sediments and ailla (fig.2). Fig.2 Canyon Ranges. Looking aout~ at approximately 65°05'N l30°00'W. · A sill forms dark cliffs in foreground. Picture taken on June 4, 1958. 5 Arctic Red River and Mountain River are major atreams that have eut canyons in the floors of their bread valleys. These transsect the fold trends of the mountains. U-shaped valleys and cirques in the higher parts of th~ region point to a history of valley glaciation. Where dips are steep, on the flanks of the ranges, the resistant rocks form rows of flatirons. The Backbone Rangea constitute a compact group of mountains without many bread valleya and almost no remnants of former land surfaces. Dissection of the openly folded sediments haa proceeded to maturity, producing a vast array of jagged peaks and ridges. Elevations exceed 7000 feet. The valleys of Snake and Arctic Red River are large persistent tranches transverse to the mountain trend; they were rounded by alpine glaciers. At this time, a few amall glaciers exist in the interior. All the waters derived from the Mackenzie Mountains drain into th~ Arctic Ocean by way of the Macken zie River. Almost all streams are braided, and many have built large flat alluvial fana where they disgorge from the mountains, a result of the pronounced physical weathering process. The vegetation ia varied; the area lies well within the northern limit of trees, but the mountains are practically barren, as the timberline barely reaohes an al titude of 2000 feet. A very striking feature is the aharp boundary 6 between the front ranges of the mountains and the muskeg covered Peel Plateau to the north. Previous geological work: No previous work has been done on the Tawu Range intrusives. The only mention of an occurrence of a diabase intrusive in thP Mackenzies is that made by Keele (1910, p.41). His report was based on the resulta of a geo logical traverse across the mountains by way of Pelly, Ross, and Keele River during 1907-08. He first obaerved a sill on the alopes of Mount Eduni, near the mouth of Twitya River. However, his description of this occurrence ia restricted to a few lines in field terme, and no petrographie data are given. Up until now, his report is the only published field work on the interior of the Mackenzie Mountains. Several ether reports exiat, but they con tain data obtained in the front rangea, from airplanes, or from air photographe. They are those by Dowling (1922), Hume (1922,1923), Hume and Link (1945), Laudon (1950), Kingston (1951), Goodman (1954),