The Occurrence and Petrology of Basic Intrusions

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 southwest.

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), Hume (1954), and Bell (1959).

Bostock (1948) made a comprehensive study of the northern Canadian Cordillera, using trimetrogon air photos taken by the Royal Canadian Air Force and United States Army Air Force between 1941 and 1944. He was thus able to examine large areas of unaurveyed territory, and revise and improve the classification for the physiographic 7 subdivisions. Bostock states (p.20) that "in the northern Mackenzie Mountains ••• no intrusives are apparent in the air photographe". Neverthelesa, ailla are present, so that this statement is not quite true. Of course, one can attri bute it to the fact that it is very difficult to distin guish between a tabular concordant intrusion and a resis tant sedimentary bed, in air photos.

Most of the areas surrounding the Mackenzies have received a good deal of attention by geologists because of economical interests: those to the north and east for the petroleum occurrences in Paleozoic sediments, and those to the southwest for the presence of acidic and basic intrusives and metamorphic rocks, with their associated metalliferous deposits. Many geological publications on theae areas are available. Geological maps 1048 A (Yukon Territory, 1957), and 1055 A (District of Mackenzie, 1958) embody the resulta of the Geological Survey of Canada surveys up to the end of 1956.

Field and Laboratory work: During the summer of 1958 the writer spent 3 months in the northern Mackenzies,and Ogilvie Mountains, as member of a party engaged in structural and stratigraphie mapping. With the use of a helicopter, the Mackenzies were studied on a reconnaissance ecale west of Mountain River, in some places up to 25 miles from the mountain front. Samples from the ailla and the contact zones of the country rock 8 were taken in appropriate places while measuring strati graphie sections, and also on spot landings. The ailla b~t ween Cranswick Riv~r and Ramparta hiver w~re oba~rv~d in

the field. The ~xtent of these east and aoutheast of Ram

parta River, as well as their possible ext~nt towards the south of the map area (see map at end of paper), was deter mined by examination of several hundred air photographe in

the fiel~ and at the National Air Photograph Library in Ottawa.

An area in the vicinity of Blackstona Riv~r,

in the Ogilvie Mountain~ was viaited. Several diorite dikea were found, and specimens taken from thr~e of them.

The basie for the petrology part of this pa per is a suite of lCO samples, of which 58 are from the in truaivea, and the remainder from contact zones of the coun

try rocks. Thin sections and polished sections war~ studied, and a number of phyaical and chemical propertiaa determined.

Acknowledgments: Professer Dr. E.H. Kranck directed this thesia, and gratitude is expresaed for his supervision and guidance. Thanks are alao due to fellow-students and members of the staff, who helped in small but important ways.

Financial assistance, provided in the form of a Solvay Fellowship, is gratefully acknowledged. 9

Terminology:

The following t~rms are used in the sense in which they appear in the Glossary of Geology (Amer. Geel. Inst., 1957).

Diabase: A rock of basaltic composition, oonsisting essentially of labradorite and pyroxeone, and charaoteriz~d by ophitio texture. Tholeiite: A basalt poor in olivine and containing ortho pyroxene and/or pigeonite, and a groundmass of quartz and alkalic feldspar. Ophitic: (=diabasio) A term applied to the texture char acteristic of diabases in whicb euhedral or subhedral cryatals of plagioclase are embedded in a mesostaais of pyroxene. Subophitic:Partly ophitio. Applied to a variety of ophitio texture in which the average length of the plagioclase laths exceeds the diameter of the pyroxene grains and in which the pyroxene grains only partly enclose certain of the feldspar laths. 10

GENERAL GEOLOGY

The geology of the interior of the Mackenzie Mountains is very vaguely known, as most of the field work done so far bas been confined to the accessible peripheral regions. Acoordingly, large blank areas remain on the map of the northern Mackenzies (at end of report).

Stratigraphy: The rocks of the Mackenzie Mountains were de posited in the northern continuation of the geosyncline in which the sediments of the Rocky Mountains wera laid down. The period of accumulation rangea from Cambrian or Precam brien to Tertiary, but the sedimentary record ia not com plete. Canol geologista, and othars, have studied the north ern part, and have establiahed formation namea. The de scription and location of type sections are given by Hume (1954, chap.2). He presents a summary of the stratigraphy in a table, which is incorporated on the next page in a sim plified, and modified, form. As a result of field work car riad out, it was necessary to make some additions and omis sions in order to have a general table that applies to the whole of the map area.

The Cambrian system east of Mountain River is represented by the Katherina group, a series of interbedded quartzites and black platy shales, which is overlain by about 1000 feet of green, grey, and maroon shales, soma ll ·rABU 1 STRATIGRAPHY* Northern Mackenzie Mountains

Agf> Formation Lithology

Recent river gravels, talus, landslides Pleistocene glacial drift Tertiary sands, gravels, clays, lignite un conformi ty . sandstonE", grey, brown, massive, finE" grained, nodular; interbedded with CretacE"ous shale, grey, sandy shalE", dark grey, sandy, with thin ironstone bands, nodules unconformity sandstone, shale, interbedded Imperial massive to thin b&dded, greenish; micaceous; UppE"r marine Devonian and non-marine Fort Creek shale, dark grey to black, platy, bituminous limestone, light grey, massive shalE", black, platy, pyritiferous Middle Ramparts limestonP, light grey, massive to Devonian thin bedded; interbedded with dark shales

Devoniatl or Be ar Rock dolomitE", limestonP, buff to light Silurian greyi gypsum; brecciat?d limestone, dolomite, light grey, 3ilurian che:>rty; somP shale in- tE"rbeds, conglomerate argillite, shalP, limestonf> Ordovician (occurs in western part only) shale, green, grey, rE'Iddish brown, Cambrian interbedded with grey and/or sandstone, orange weathe- older ring dolomite; diabase sills; gypsum sandstone, rPddish brown; grey lime- stonE"

*Hume (1954), with modifications. 12 lim~stone, sandstona, and gypsum beds,b~longing to the

Macdougal group. In the vicinity of Arctic Red River, n~ar tb~ mountain front, the Cambrian (?) rocks are grey lime stone, interbedded black, green, and maroon shales witb orange weathering dolomite, and overlying grey sandstones that are more tban 1200 feet thick. The dolomite-shale se quence is intruded by two diabase ailla aggregating lOO feet in thickn.ess. -( One may note here that the ~rdoviciam succession described by Keele (1910, p.27) on Keele River is very similar to that in the Arctic Red River area. Al though the latter was assigned to the Cambrian by Hume (1954, map lo32A, sheet 3), the sille are not shown, nor mentioned in the text.)- Near Cranswick River the dolomita- shale-diabasa section is overlain by dark brown and reddish brown weathering ferruginous quartzites several hundred feet thick. West of Snake River there are several thousand feet of dolomite, maroon shale, and gypaum beda. No intrusions were seen there. However, in the Knorr Range, to the west, two aoutherly dipping dikes, totalling 9 feet in width, have for.med in a thick succession of bright orange weathering Cambrian (?) dolomite, and dark shale.

Rocks of Ordovician age have not been poai tively identified in the Mountain River area (Hume, p.l3). To the south, on Keele River below Twitya River, Ordovician strata consist of 4000 feet of alternating beda of argil lite, dolomite, and limestone, aucceeded by 1500 feet of sandstone. Just below the sandstone is a lOO foot sill of 13 diabase (Keela 1910, p.27). In the Snake River area,dolo mite and limestones have been observed (G.S.C. map l048A, Yukon Territory, 1957). To the west of the map area, around the upper Peel River, the Ordovician strata consist of shales and argillites 1500 feet thick, and some limastone. Figure 3, p. 14, shows part of the pre-silurian stratigra phy in the fawu Range, on Arctic Red River.

Silurian rocks are widespread in the front ranges of the Mackenzies. They are fossiliferous limestones and dolomites, with soma clastic beda in the western part. In the eastern part they are known as the Ronning group, and have a thickness of 1400 feet (Hume 1954, p.9).

The unfossiliferous Bear Rock formation is a brecciated limestone and dolomite unit, with some gyps~ beda, totalling several hundreù faet in thickness. The age of this formation is not yet definitely established, but it is either Silurian or Devonian.

Overlying the Bear Bock formation is a series of fossiliferous middle Devonian limestones 300 feet thick, the Ramparts formation. This is succeeded by 1500 feet of upper Devonian black bituminous shales,and a limestone reef which produces oil (at Norman Wells); these constitute the Fort Creek formation. It is followed by upper Devonian greenish, micaoeous sandstonea and siltstones of the Impe rial formation, which is about 1500 feet thick. 14

i)iG.bo.5e 1 u p ~· 5 i Il s.. li&tts.-.y ccx"·"' 9"'i.,c lisht src.y ehc.rty, si 1-ty """""*'9-o"'"9«- ye~Jow sts. -si\.: br•w .. ~p·-e.y-lo 10\edi.- ~i" be.dded grc.y 1 IV~~g . rusty ~ro"""

Ss. li~h+ brow"9~'1 "'~diu.., -tt>-tnk.k bcdd~d fe-rugi noul s p«cJ(s ..,.,._...,9. medi

~ts - -5~., So... & .ss. src.y. ""Q.roO" , gree-n """"-·"~ · brow.,

Ss. ligkt brown grey

9 ro.y 5s. liglrt purplish grc.y Ss. gre., 1 g '"-" '1"05~ ~dc.d' fi .... jrQ.ltLC~ ftrru~i t\ov> i"' pla.c.es W«c.-+\>9 . brow.. isl> 9"Y

Ss . li~h+ purpli'\. 9

sts . -sh , So ...., ss . ..._&diu,.. grc.y +ki" b..d~c.q 't'V~~ b"'"'" wc..Jk9 . measured 6 ·5 miles N W of Tawu Lake

measured 1· 8 miles NE of Ta wu Lake

Stratigraphie column of po~t of Combrian C?l in Arctic Red River oreo.

Fig. 3 15

A large break in the sedimentary record marks the time interval between Upper Devonian and Lower Creta ceous. The Crataceoua rocks overlia older formations with angular discordance, and compriaP several thousand feet of dark grey and brownish aandstones and shales. These are the rocks tbat underlie the expansive plains to the north and northeast of the mountains.

Tertiary gravela, aands, and claye, as well as some lignite beda ara known in the Bonnet Fluma basin (Camsall 1906, p.41), which is just to the northwest of the Knorr Range.

Structure: The rocks of the Mackenzie Mountains were in volved in crustal shortening during late Cretaceous or ear ly Tertiary time. This resulted in a numbar of large, open, parallel trending, but curving folds that generally atrike west or northweat, in the map area. They are accompanied by aome faulting. Associated with the large structures in the mountains is a number of en ~ c h e 1 o n anticlinal folda of Faleozoic rocks in the foreland, to the northeaat, known as Franklin Mountains on the northeast bank of the Mackenzie River. On the southweat aide they form the Imperi al Range. Folding is the predominant type of deformation of the rocks in the central part of the Mackenzies, east of Mountain River. Towards the west, folding ia atill prevalent, 16 but faulting is much more pronounced. On the western aide of the valley of the Arc- tic Red River, where it leaves the mountains, there is an excellent section in which a southerly dipping thrust fault with associated dragfolds is exposed: grey Silurian (Hume, 1954) carbonates have been thrust over the Bear Rock forma tion. In the valley two miles east-northeast of Tawu Lake*, beds have undergone strong deformation and shearing. Fault ing was observed at the mountain front, 26 miles west-north west of Tawu Lake; it can be seen on air photos. Halfway b~tween Arctic Red and Cranswick Rivera there is a repe tition of light grey carbonates and dark quartzites; it is also visible on air photographs. It is possible that this fault continues westward to join with the one just described, and eastward to the one near Tawu Lake. Numerous steeply dipping faults with small separations were sean in the central part of the map area.

One can note that in the southern part of the map sheet (at end of paper, p.84) long northwesterly trending vallays are oontrolled by structure. From air photo examination it was found that in most of them the strata dip steeply, and that in between the valleys thare are broad open folds with flat dips. One has the impression that at least soma of these valleys are along longitudinal faults.

* For the purpose of facilitating the presentation of data in this paper, the name Tawu Lake is applied to an unnamed lake in the Arctic 5ed River valley, 3 miles E of the river, at approximately 65 l7'N 131°07'N, and about 2200' altitude. Locations referred to are with respect tc its northwest cor ner. The lake, 1.0 by 0.4 miles and roughly rectangular, is deep enough for float plane oporations. 17

A large number of miner vertical faults strike N50°W -70°W, paralleling the large valleys. A diagrammatic structure section along Arctic Red River valley (fig.4) shows one of these long, straight valleys in cross section.

îhe mechanics of the deformation, and ether problems concerning the structure of the Mackenzies, have been studied by a number of people, and mostly in conjunc tion with the North American Cordillera as a whole. (e.g. Wilson, 1950; Kay, 1951; Goodman, 1954)

IO ~'\'\il e

Fig.4

Diagrammatic N-3 structure section along Arctic Red River, between mountain front and 65°N. 18

THE TAWU SILLS

Occurrence and field relations: The Tawu Sills are two, or more, tabular concordant intrusives of gabbroic composition in the northern Mackenzies. They are known to occur in a 900 square mile area between latitudes 65°N and 65°25'N, and longitudes 129°30'W and 132°W, and may possibly extend to the west and southeast. In the latter direction lies the sill reported by Keele (1910, p.41) in the Mount Eduni area. If this sill is in any way related to the Tawu Sills, and the cliffs ob-

Fig.5 View in southwesterly direction from Tawu Lake to west side of Arctic Red River valley. Cliff-forming sills have dark color, and show the bread anticlinal structure of the rocks. Ridge at top center of picture is 9 miles distant. Photo taken June 23, 1958. 19 served in the air photographe in the intervening area are also sills, the total intruded region would be about 2300 square miles.

Wherever the sille crop out they form dark brown cliffs up to lOO feet high, and visible for many miles (fig.5). They always have distinct columnar jointing; often there is a set of joints parallel to the bedding, and aometimes one can aee oblique jointing. These non-columnar types produce a atep-like outcrop, as in fig. 10. The ailla weather into large angular blocks that are atrewn along the hill aides. In the river gravels near the mountain front, a fair percentage of well rounded diabase boulders and cobbl~s att~sts to the rapidity aod intensity of physical weatheri~. The cobbles are fresh looking; they have a dark, greenish grey, color, and exhibit the ophitic texture quite wall.

The outcrop characteristics are beat demonstra ted by pictures (figa. 6,7,9,10), and a set of air photo graphe giving stereographie coverage, in which one can notice the general conformable nature of the intruaives. In the air photos they appear as sharp, dark serrate linas of distinct pattern (fig.S).

In many places the effect of tectonic activity on the sills and encloaing sediments can be obaerved. The rocks are disrupted by steeply dipping faults with small vertical separations. Such faulta strike parallel to the large longitudinal structures. No dikes, which could repre sent feeders to the ailla, were sean in the field. However, 20

Fig.6 View looking N1~ from ·J:awu Lake. dills form cliffs showing columnar jointing. Grey rocks abov~ upper sill are sandstone.

Fig.7 The lower sill, 1.8 miles NE of Tawu Lake, in interbedded dolomite-siltstone-shale. A small vertical fault controls the location of th~ gully. The man is holding a 5-foot Jacob's staff. 21 in the air photographe of the area 15 to 20 miles southeast there are at least three thin dark outcrops that appear to have a disconformable relationship to the sediments (fig.8).

------?> North Fig.8 Stereopair showiog outcrop shapes of Tawu dills, and possible dikes. Center of area is 17 miles 3~ of Tawu Lake. (From: R.C.A.F. photos A14405 -1C8,109)

They are steeply dippiog, and have a straight outcrop pat tern; they strike N25°W, making angles of approximately 45° with the northwest trending valleys. If these rocks are dikes, they most probably represent the channelways of the magma producing the sills.

The intrusive sheets dip under rocks strati graphically higher, both at the northern and southern border of their outcrop area, which has a broad anticlinal pattern. Their extent below the surface outside the outcrop belt is 21 in th~ air photographe of the ar~a 15 to 20 mil~s southeast thPre ar~ at least thrPP thin dark outcrops that appear to have a disconformable rPlationship to thP sediments (fig.8).

2 ,,;.les ----?> North Fig.8 Stereopair showing outcrop shapPs of Tawu dills, and possiblP dikPs. CentPr of area is 17 miles ~B of Tawu Lake. (From: R.C.A.F. photos A14405 -1C8,1C9)

They arP st~eply dipping, and have a straight outcrop pat tern; they strike N25°W, making angles of approximately 45° with the northwPst trending vall~ys. If th~s~ rocks are dikPs, they most probably represent thP channelways of the magma producing thP sills.

The intrusivP shPets dip under rocks strati- graphically higher, both at the northern and southern bord~r of their outcrop area, which bas a bread anticlinal pattPrn.

Their extent below the surfac~ outside the outcrop belt is 22 not known, but it is speculated that they do continue for considerable distance because of their extensive and com paratively regular nature in thoae areas where they are visible.

Contacta between sill and country rock are very sharp. Small apophyses penetrate the wall rock, and a few small xenolitha are incorporated in the diabaae near th~ upper and lower margina.

The atrata, into which the aheets are emplaced, conaiat of interbedded Cambrian (?) shal'e, siltstone, and dolomite layera, which are overlain by a thick sandatone sequence. The magma encountered lesa resistance in this thinly bedded unit than it would have in the massive aandatones, aasuming that these exiated at the time of intrusion.

The ailla are conformable locally and generally, but a tranaective attitude is indicated in the area as a whole. At a place 1.8 miles northeaat of Tawu Lake, the lower aill haa a thickneaa of 30 feet (fig.7), the upper 75 feet, and the stratigraphie distance between them amounta to 335 feet (fig.9). In the field, and on air photos, the two ailla were traced to the west. At 6.5 miles northweat of Tawu Lake, the lower and upper ailla have thicknesaes of 35 and 71 feet, respectively, and the distance between them ia 48 feet. Further, from air photo interpretation and field work, it was found that the upper aill ia about 450 feet lower in the stratigraphie section, than at 1.8 miles north east of Tawu Lake. The ailla continue southward, although 23

Fig.9 View north from hPlicopter towards the sills NE of Tawu Lake, showing rPlation to sediments. Note vertical separation of lower sill on the left. r

Fig.lO dills 8 miles south of Tawu Lake. Looking SdW, and up thP vallPy of Arctic Red River. Note columnar, horizontal, and oblique jointing. 24 they are interrupted by a valley, and a !ault zone that runa along it, just north of Tawu Lake. This zone is vis ible at several outcrops on the north aide of the valley, 1.6 miles eaat-northeast of the lake. Here, also, the top 16 feet of a aill for.m the crest of the 250 feet high cliff on th9 north sid9 of the creek. Its basal part is covered by talus, and vegetation. Conaidering the disruption and the thickness of the sill, it is likely that this is a down faulted portion of the thinner, lower sill.

The sheets crop out on the slopes of the first mountain southeast of the lake, and continue to the south for 10 miles, where their dip to the south increases, and they disappear below the surface. On the east aide of Arc tic Red River valley, 7.5 miles south of the lake, the lower and upper ailla are 110 and 57 feet thick, respectively. The vertical distance in between them is 188 feet.

At 20.1 miles in a direction of N80°W from the lake, only one sill was seen, 84 feet thick.

A spot landing was made 17.5 miles S80°E of Tawu Lake. Two sills exist, but only the top of the lower and the base of the ether are exposed. The stratigraphie separation between the two is 15 feet. Both sille are at least 15 feet thick. The interpreted spatial relationship between the sills and the country rock, within a limited area, are pre sented by a fence diagram (fig.ll), on the next page. N

fe et t loo 6.S..,; NW 2ooJi

::'.'-::.:.·..::~ ..:·~: .:,: :·;· lOO ~ zo.t ... i NSO'~>~ Horiz.on+oJ

0 ~ 3 .. ..,;,.,5 Sc.Qies ~ ~

~ To.wlJ ~ ~.. Kc ~ ~ ~

7.5 ..,; s Fig. Il

Fence diagram showing interpreted spatial relationship between Tawu Sills

and sediments in Arctic Red River are a, northern Mackenzie Mountains.

1\) \)1 26

Although the intrusives are shown as continuous- sheets, it is possible that they are not. The convergence of the ailla towards the west may express itself in the appearrance of only one sill. As there was no effect evident, auch as a multiple intrusion would have produced, the single sill at 20.1 miles N80°W of Tawu Lake oould be 1) a separate sill,

2) the west~rn continuation of one of the ailla near Arctic Red River, or 3) one sill which split ap along favourable planes of weakness in the sediments, towards the east, in which case the two would be equivalent in age.

The increase in total thickness of both ailla southward would indicate that the direction of the source of the magma is in that direction, i.e. the interior of the mountain system. Such a conclusion is supported by air photographe: ailla in the headwaters of Ramparts River ap pear to be thicker than those to the northwest.

Petrology:

MeES!,CE_pic_cha_ra~teristi~a: A atriking fe a ture of the ailla is their consistent mineralogical compo sition in lateral directions, and, excepting vertical var iation of grain aize, all samples from the Tawu Sills have easentially the same megascopic properties.

The diabase sheets have a dark brown weath ering color. On fresh surfaces the rocks are dark grey to

~lack, or greeniah grey where there ia a fair amount of 27

chlorite alteration. The upper and lower margina of each sill are porphyritic, containing small phenocrysts in an aphanitic groundmass, with preferred orientation parallel to the contacts. A greyish green selvage zone 2mm. thick for.ms the immediate marginal zone of the diabase.. The grain aize inoreases over a short distance and reaches

3rr~. at the center.

Disse.minated sulfides are distinguishable in several places, notably near the lower and upper contacts of the ailla, and in the contact zones within the country rock. Pyrite is the usual sulfide and coeurs as small specks or fracture fillings, and as small rounded replacement aggregates. Chalcopyrite is in sporadic specks. Pyrrhotite is present in amounts of up to 2 percent near the top of each of the two ailla 7.5 miles south of Tawu Lake.

In many parts the ailla have been sheared and fraotured on a small soale. The fissures are filled with a dark fibrous or soaly mineral with a greenish streak, iden tified as penninite with the aid of the microscope. It ia in the vicinity of auch veinleta where the diabase exhibita the greeniah grey color on a fresh surface. A lem. vein of milky quartz, with calcite at its center, was found in one place; it transects the upper part of the sill perpen dicular to the contact and terminates above the sill, just within the country rock.

Many of the specimens deflect a compass needle a few degrees, attesting to the presence of magnetite in 28 th~m. The magnetic proparty should prove useful in deter mining the lateral extent of the ailla by airborn~ magne tom~t~r, in the event tbat auch a survey is made in the future.

The specifie gravities of the ailla were deter mined by measurements with a beam balance adapted for this purpose. The average value for all samples is 2.94. Although the mean values of the two sills correspond closely to one another, that of the upper sill is slightly higher. Indivi dual vertical variations in four localities are illustrated on the following page (fig.l2). deveral important features are evident, and these have been incorporated on that page in diagram G, which representa the interpretation of the data. The specifie gravity at the contacts has a value of roughly 2.8, which is that of diabase glass. It increases within one foot from the contacts to 3.0, and then decrea ses slightly towards the center. A concentration of dense minerale towarda the bottom was not detected by these deter minations. One can ascribe the lack of gravity differenti ation to the relatively small thickness of the Tawu Sills in the area in which the samples were taken. The time re quired for freazing the magma was not sufficiently long enough to make auch a process important. Some irregulari ti~s in figure 12, notably in diagram A where the smooth specifie gravity variation is absent, are deviations which can be explained by a high content of alteration minerals. Samples showing auch a deviation contain pyroxene tbat is almost completely altered to lesa dense hydrous ferromag- 29

s•·

""'' 71 •

·;;; ...... 0 .:.E Q ..D

o _ 0 0 2-8 Spedfic gravily F 8 D

SY·

JS '

30-

o ,_ ol 0 2.11 2.'9 3'.0 2.9 1., 3.0' 2 -8 2., 3.'0 A c E

Top ·--~

A lowc.-Sill 1 •edion I.S mile& NE of Tawu Lol

8 Upper Si li, sedio11 1.8 miles NE of Ta wu LaKe c lowC!rSill , Se.::.tio11 ~-5 ... iles NW of Towu laKe IAIOD<.f D Upper 5ill , r.e.::.f io11 6-5,..iles NW of To.wu LaKe

7.5 ..,ae.s Tawu I.e Ka E Upper Sill 1 ~oec.fion 5 of

F S ill, &ed·ion 20.1 to~~le~ N so·w of Tawv Lol

t.. terpref-ad vafiation of Ta wu Si lis BoTTo"' G H 3.0 G

Fig. 12

Specifie gravity variations of the Tawu Sills 30 nesian minerals. It is also appare>nt that the average value of

2.94 given her~ is not quite> correct, because calculations were made without regard to the relative volum~s which the> specimens represent: samples from the contacts represent only a thin layer, while those at the ce>ntral portions cor respond to larger volumes. Accordingly, the true mean value for the specifie gravity of the (unaltered) diabaae is somewhat higher than 2.94.

Mi~r~a~oEi~ ~har~c!eEi~tic~: In the following an account of the main characteristics of the sills is pre>aented, rather than a detail description of each apeci- men, eliminating unnecesaary reiterations.

The ailla are tholeiitic diabases made up of labradorite, pigeonite, iron ores, quartz, micropegmatite, and alteration producta. Only one grain of olivine was ob served. Micrometrie measurementa to determine the mode were made with a mechanical stage on thin sections which are comparatively unaltered and not too fine grained. Re sulta are as follows: TABLE 2 MODAL COMPOSITION OF TAWU SILLS (Average> of 20 Rosiwall Analyses) Plagioclase (labradorite) 41.7 % Micropegmatite 8.1 Quartz 2.5 Pyroxene (pigeonite) 27.1 Amphibol~, chlorite, etc. 12.2 Iron ores 8.4 Carbonate 0.4 Olivine one crystal

· ------______...... 31

The fabric varies systematically from marginal

to central portions of th~ ailla. A microporphyritic or glameroporphyritic texture marks the contact zones. Th6

texture grades through intergranular to subophitic in th~ middle. Phenocrysts commomly constitute 6 percent of beth the upper and lower marginal zones, indicating that at least that much of the magma had already crystallized before it reached its present position. They are, as a rule, not longer than 2mm., but one plagioclase crystal seen had attained dimensions of 4.0 by S.Omm. Phenocrysts are in preferred orientation, parallel to the contacts. Two out of

three are euhedral labradorite An63, one out of three is anhedral clinopyroxene. Plagioclase occurring as pheno crysts is often zoned, while that of the matrix is not. Many of the porphyritic crystals coeur as broken and sepa

rated pieces, confirming the theor~ that early frozen aggregates were transported into place.

Contacts between sill and country rock are sharp, even under the microscope (fig. 13). The immediate basal margin of the sills is composed of an approximately lmm. wide olive green, iron rich selvage zone. It has about 70 percent opaque clusters O.Olmrr1. across, most of whicb are altered peripherally to a semi-opaque mineraloid. This zone is in sharp contact with a 0.9mm. thick zone of brown, semi-opaque material that exhibits polygonal structure of a net of opaque mineral grains, in reverse arrangement of that of the immediate border zone. Both structures seem to 32

Fig.l3 Contact between sill and silt stone above. Phenocrysts parallel to contact.

Plain light

0 0.5 sc ale

be due to weathering. Above the second zone, and in fairly sharp contact with it, ia a. 2.4mm. thiok layer composed E?ntirely of opaque ferruginous glass and transparent pheno crysts. This layer grades upward into a part of the sill with slightly transparent felty, microlitic matrix spotted with minute iron ore grains. From there on upward the grain size increases over a short distance. Minute cracks penetrate the sills for a few millimeters; the iron ores have been leaohed out from their immediate vioinity. Small veins are filled with quartz, epidote, ohlorite, or green amphibole.

In the contact zones one can see small quartz grains, which are probably derived from the quartzose sediments, and were incorpora.ted in the magma during intrusion. 33

The inclusion of country rock fragm~nts at the base of a sill can be studied in one sample, where there are two 5mm. slabs of quartzose siltstone, one above the ether. Dark ferruginous glass, and iron ores coeur as described before, but the thickness of opaques is greater because of the vertical displacement caused by the siltstone plates. The xenoliths are made up of rounded tc well rounded quartz grains, O.l5mm. or lesa in diameter, which are coated by silica in optical continuity. Cloudy spaces exist around seme grains, being more plentiful where there is a lack of silica cementation. In between the lower siltstone plate and the ferruginous glass above it, is a green and turbid brown lamina, o.osrr~. thick, and above it, a film of brown limonite. On top of the upper siltstone slab is a 0.2mm. thick lamina of a similar cloudy product, but it contains disseminated specks of iron ores. A serrate boundary sapa rates that lamina from the overlying felty maas of diabase.

~ix inches (15cm.) above the bottom contact the mean lengtb of plagioclase microlites is 0.25mm. Por phyritic plagioclase and clinopyroxene make up 5 percent of the rock, the remainder being an intergranular matrix of plagioclase and interstitial pyroxene and iron ore grains, or alteration minerals. There is a tendency for microlites to form radiating clusters, or aggregates of divergent laths.

One foot above the base the plagioclase laths are 0.6mm. long, and several of them are bent or curved. 34

Phenocrysts, amounting to as muchas 7 p~rcent, are dis tinguishable because they are larger than the grains of the groundmass. The pyroxene is slightly purplish, indicative of the presence of titanium in its crystal lattice. At this interval above the base a crystal of unaltPred olivine, an anhedral grain lmm. long, was found. fhis representa the only olivine recognized in the samples from the Tawu Sills, and is most probably a phenocryst.

Porphyritic crystals constitute 6 percent at 2.5 feet above the base. The original texture is obliterated by the advanced stage of alteration of the samples taken at this interval. However, one can notice the presence of ske letal magnetite.

At 5 feet above the floor of the sills magnetite occurs also as roda that have serrate edges. Phenocrysts are not readily seen aa they ar~ as large as many grains of the matrix. One can distinguish micropegmatite; it for.ms irre gular patches which, und~r crossed nicols, are resolved into checkered black-and-white aggregates. The texture is ophitic to subophitic, and the plagioclase is euhedral and subhedral, while pyroxene is anhedral. Alteration products of pyroxene can be identified; chlorite is common, magnetite is associ ated with it; brown amphibole, ri.mmed by a bluish green variety occurs. fhe green variety is probaoly soda-rich, and due to alkaline enrichment during the course of crystalli zation.

At 10 feet, the samples are medium grained and 35

th~ texture is subophitic. From about this interval to an equal distance measured from the roof of the sill, the dia bases are uniform, except for grain aize which is greatest at the center. There the plagioclase laths have lengths of

1.6 to 2 n~.

The central parts offer the beat specimens for analysis under the microscope in the determination of opti cal properties of minerals. There was mutual interference during crystal growth between pyroxene and plagioclase as well as between individual minerals. It auggests that both minerale cryatallized together. Nevertheless, there are some euhedral plagioclase crystals completely enveloped by pyroxene. This requires the early formation of at least some plagioclase. Magnetite is later than these two minerals. Quartz and micropegmatite occupy irregular interstices and constitute two of the last portions to solidify. Where pyro xene is in contact with this crystallized "rest portion" of the magma it is uralitized. fhe micropegmatite mesostasis abounds in tiny magnetite and pyrite specks, amall shapeless amphibole aggregates, and minute colorless needles of amphi bole or apatite. fhe latter are often arranged in rows (fig. 14). Apatite prisms are also contained in the quartz grains.

Twinned carbonate, rimmed by green chlorite, fills a few sporadic vesicles. Amphibole, chlorite, and rarely biotite are alteration or reaction products of the pyroxene. Persis tent cracks are filled with chlorite, while irregular small ones may contain limonite. 36

Fig.l4 Micropegmatite (mp) contains row of apatite or amphibole needles (a) in between plagioclase (pl). Plane-polarized light.

sc ale

The grain size de creas es towards the roof; the vari- at ion from base to top is symmetrical. Ph..,enocrysts in the upper fine grained to aphanitic margins make up 5 to 6 percent of the rock. The amount, composition, and size is prac- tic ally the same as at the bases. One can take this to mean that differentiation of the gravitational settling or roof stoping types did not occur.

The immediate upper contact zone is similar to that at the base; it is a glassy substance containing abundant opaque granules (fig.l3). One thin section studied contains a plagioclase phenocryst that was crushed against the wall rock during intrusion. The granules decrease in number over a distance of 2 mm. From there on downward the fabric becomes felty and plagioclase microlites appear. In the upper parts of the sills at the section 7.5 miles south of Tawu Lake almost all phenocrysts, in both the upper and lower sill,are selectively, but only partially, replaced by pyrrhotite, as in sample HH-68. (dee fig.21).

Illustrations on the following pages summarize vari ous properties of the sills, and their vertical variations. 37 Crossed nicols Plane-polarizPd light

40 fPet abovt:> bast:>

5 fept above basP

1 foot above base

pl plagioclase px pigPonitt:> micropegmatitt:> mp 1/10 foot above base

0 1 ~----~------~ ~m scalt:> of Pach photomicrograph

Fig.l5 TexturP and grain sizP variation of a 71 foot sill. fllisht iMfMt a.bo 38 ., 0 -40 60 100

bO Figure 16

~·;: ~ D .."" .Jt ~ -5 .. ï.::: 'ti ~ .s ~ 0 E 0 ""Q" "i ..8 :31 0 c Oiograms showing the vertical variation e (>.. #, 0 1- ..!< -~ lt < of 2 j x of certain properties si lis. 1

0 Upper sill, 6.5 miles NW of Towu Loke c Lower sill, 6.5 miles NW of Towu Loke 8 Upper sill, 1.8 miles NE of Towu Loke Lpwer sill, 1.8 miles NE of Towu Loke - 35 A

· X·AA'< INSTRUMEI'CT DAT/0. AI'CD SETTINGS! ~ ~

"o i"~.. a\ &1andar4s used.

:1.·7 l-8 2-') 3-0 0 lO 40' ~ 8o lOO ~% ~,.. ICID 160 -2Do IOCID 1100 1200 ... -loo 420 MO 16

~ ... z ~ J! . •: ..... ~ 0 ·;:: E ~ ~ .. B 0 "i 01 "11 0 .. a r ~ 0 .. 0 QI 4 ~ ., .Q .JI . (: -~ 0: ...." ~ {. 0.. E - <

__ __.___

211 ao A I!J 10

0 100 ~ 1200 3SO -400 <420 p--. p y lnTenSITY y·- y y y grov•ty or"piogloë?ase Mn Ti Fe (by *"Y.i'"u"' cxtind"'• <>"91&

olb~ {Kor.pe6l( (.OUOdspu -.o,.ll (k~pee>~(., "'ullb ,_...... s lrlto.Su r'>•"ll'.t" o" ~ (K.. '*'", courrts po• -.....Q 1 obove b•d'Sround ) obove bacl

Mineralogy:

Plagioclase (An44-An64 ) occurs as small porphy ritic crystals and as laths in the matrix. The mineral is colorl~ss exc~pt for cloudy brownish arPas due to altera tions. Most crystals ar~ euh~dral or subhedral. Phenocrysts are fr~quPntly som~what roundPd or broken. In c~rtain s~c- tiens th~ laths of thP matrix ar~ thinner at the middl~ than at the ~xtremiti~s, looking as if th~y had been squ~ezed at thP center. This prop~rty is duP to growth in terf~rPnce with oth~r crystals. Perf~ct crystals are usually small, and are confined to larg~ pyroxene grains.

Fig.l7 Plagioclase phenocryst showing oscillatory zoning.

0 1 '-----'------' Wl M sc ale CrossPd nicols

The reliPf is fairly low. Interf~rence colora are gr~y e.nd whit~. Polys~nth~tic albite twinning is most widespread; a number of crystals arP twinnPd according to the pPricline law, and carlsbad twins cen also be seen. Sorne of the al- bit~ twins are incomplete, leaving individual lamPllaP ta pering into an untwinned part of the crystal. SevPral of th~ phenocrysts are eut parall~l to (010), and thPsP exhibit oscillatory zoning (fig.l7). Maximum extinction angles on albite twins vary from 24° to 36°, but most of th~m arP around 33°, which repres~nts labradorite, An59 . 40

Only one vari~ty of pyroxene is pr~sent: pig~onit~.

It occurs as phenocrysts and in th~ groundmass. Th~ mineral

is colorl~ss or has a very slight purplish tinge, and occurs as anhedral, rarely SUbhPdral, grains showing the Charac

t~ristic pyrox~ne cleavage planes. Some of the crystals ar~ up to 3 mm. long. The pigeonite has a fairly high relief and moderate birefringence; th~ maximum interference color ob served in longitudinal sections was blue of the second order. Extinction angles ( Z/\.c) are a.round 42°. Many grains are twinned on (100). Crystals are biaxial positive with 2V bet ween 10° and 15°, as determined by estimating the distance between isogyres in acute bisectrix figures. The mineral is partly or completely altered to chlorite or amphiboles in a number of samples, while fresh in ethers. Incipient alter ation is along cracks, and at the boundary of grains. The

uralitization at the contacts with microp~gmatite is note worthy (see fig.20); it has been described in papers on tholeiites in other areas.

At least two types of amphibole are represented: a greenish brown variety which for.ms the larger grains, and

a green variety which coats the brown on~, and also occurs in irregular small grains in micropegmatite or as altera

tions of pyroxene. The optical prop~rti~s of th~ gr~en

amphibol~ are difficult to ascertain. It is later than the brown variety, and could be due to soda-enrichment during the late stage of crystallization of the magma. Grains of the brown type are normally anhedral and show amphibole cleavage. The mineral is pleochroic f rom pale brown to 41

dark brown with a gr~enish ting~. ~xtinction anglPs (ZAc) are> of the ord~r of 20°. M inut~ irreogular grains and pris matie colorless crystals in the micropPgmatite mesostasis may beo amphibol~, but could also be apatitP; th~s~ areo too

small for optical id~ntification.

MicropeogœatitP, an intergrowth of quartz in al

kali feldspar, is pl~ntiful, and is ~asily recognizeod undPr

crossed nicols in the coarsPr grainPd sp~cimPns. It is most

abundant in the upp~r parts of the sills. Two main typPs of

intergrowths can b~ distinguishPd: a regular micrographie int~rgrowth wi th angular quartz rods (fig .18) , and a grano-

phyric typP, with irr~gular insPts of quartz in the fPldspar.

Fig.lB

Micrographie int~rgrowths of quartz (white>) in alkali f~ldspar (dark) in betwPPn twinnPd plagioclase (pl). Crossed nicols. o.__..__~~-...... __ o. 5 111 m scalP

Theo quartz is colorl~ss and has higher rPlief tban the enclo sing feoldspar, which has a very slight crPamy color with

respPct to quartz or adjac~nt plagioclase. A wateory appear-

anc~ is charact~ristic of the i~r~gular type in ordinary

light. ·:rh~ alkali feoldspar (n < balsam) do es not show any twinning, and is practically unaltered. It is not known if it is potash-rich or soda-rich. Rarely, alkali feoldspar 42 occurs without quartz intergrowths, but when in contact with it the relationship indicates that the feldspar is earlier. The micropegmatite contains a number of minerals as small grains or needles, and opaque skeletons.

Quartz often fills spaces free from any micropeg matite. As such it forms anhedral colorless grains which frequently contain apatite prisms. The mineral also fills veins near the upper contact of the sills; these could rep resent late stage consolidations in cooling joints. It is present in the contact zones as grains, presumably incorporated from the country rock.

Olivine. The only crystal observed was a color less anhedral phenocryst (fig.l9). It has irregular fractures and is almost unal terf>d. The interference figl.Jr€' is biaxial negative with a large axial angle, about 80 0 , . which are the properties of an iron-rich olivine.

Fig.l9 Part of olivinE" phE"nocryst (ol), surrounded by pyroxen~ and plagioclase laths. Crossed nicols

sc ale

Chlorite is an alteration product of pyroxf>ne, notably of th€' phE"nocrysts near the contacts of the sills. It is colored various shades of green, and is pleochroic. 43

In ordinary light large grains have a watery appearancP. Under crossed nicols they have yPllowish grey or bluish interference colors, parallel extinction, and cleavagP par allel to the slowPr ray. Iron ore specks are associated with this minPral. Penninite and clinochlorP form thin sub vertical vPins in tha diabase, and also fill irregular cracks.

Biotite is rarP. It is pleochroic from light to dark brown, and occurs as subhedra or pseudo hexagonal euhedra. fhe mineral bas parallel extinction.

Twinned carbonate fills veinlets and vesicles that are rimmed by chlorite. Individual grains arP anhPdral to s1..1bh~dral. jrhe carbonate- in veina is cal ci tP; i t pffer vesce-s with application of cold dilute hydrochloric acid.

Prisms of apatitP, up to 0.3 mm. long are most often found in quartz and micropegmatite. It can be distin guished by its hexagonal cross section and length-slow ori entation. ~any of the needles are too small to be deter mined optically due to the interference by optical charac teristics of the enclosing min.erals; they may bP amphibole, as has been stated already. These are arranged in rows in many slides (see fig .l4 p.36).

A mineral, believed to be sericite, forma altera tions on plagioclase. It has parallel ectinction, length slow orientation, and second arder interference colors.

Zircon. In one slide (HH-90) a flake of biotitE> contains a colorless grain, 0.01 mm. in diameter, which has 44 a very high rPlief and is surrounded by a pleochroic halo. It is thought to be zircon, especially in view of the prP sPnce of trace amounts of the element zirconium, which was detPcted by X-ray fluorescence analysis.

Titaniferous magnetite is found in amphibole, micropegmatite, or quartz as skeletons (fig.20) and as rods that have serrate edges. It also occurs as abundant minute specks. Secondary magnetite has formed along with chlorite in the alteration process of pyroxene phenocrysts in thP marginal zones of the sille.

Fig.20

~agnetite skeletons. Plane-polarized light. qz quartz mp micropegmatite px pigeonite ur uralite

scale

pyrrhotite is present as minute sporadic specks, but is most abundant in plagioclase phenocrysts (fig.21) as

Fig.21 delective replacement of plagioclase phenocrysts by pyrrhotite (black). Plane-polarized light.

0 ~----~------~ m~ scale 45

replacement bodies in samples from a place 7.5 milas south of ·rawu Lake. In polished sections the color is pinkish cream, and its crystal properties are anisotropie. The mineral is distinctly magnetic.

Pyrite forms irregular replacement aggregates along fractures (fig.22) and small grains in the micropeg- œatite areas or between crystals. In polished section it is isotropie and. has a pale yellow color. fhis mineral also occurs as small replacement bodies along bedding planes of the country rock, close to the margins of the sills.

Fig.22 Polished section of pyrite replacement (white) along fracture in diabase (f).

0 '------' l'DM sc ale

Chalcopyrite. Only occurs in a few sporadic specks, as seen in polished sections.

Limonite fills irregular cracks near the weathered surface. It also forms shapeless dark grains of high relief. The mineraloid gives a brown or yellowish stain to the chlorite wherever both are present together.

In concluding, one may state that it appears that plagioclase and pyroxene crystallized simultaneously, with plagioclase slightly in the lead. Iron ores formed next, and micropegmatite and quartz solidified last. 46

X-ray Fluorescence studies; X-ray fluorescence studies were carried out on a number of powdered samples to determine the presence of trace elem~nts (with atomic numb~rs between 22 and 42), and the r~lative vertical variation of iron, titanium, and man gan~se in two sills.

Traces of Zr and Zn WPre present in all diabas~ samples in amounts detectable with the departmental x-ray unit. Only one sample (HH-27) contained a recognizable trace of vanadium. The zirconium is probably due to minute grains of zircon. Zinc, according to Rankama and Sahama (1950, p. 710), is found in trace amounts in biotite, magnetite, and ilmenite. Vanadium is probably associated with the titani ferous magnetite.

A sample from a chlorite vein (HH-15) was found to have traces of Zn and Ni. Metamorphic carbonate rock from the contact zone (HH-24) was also analyzed. It contains sma11 amounts of Sr and a considerable trace of Mn. The Sr probably replaces Ca diadochically, while th~ Mn may be due to som~ secondary hydrous mangan~se oxides.

The vertical variations of Ti, Mn, and Fe in the sills are incorporated in figure 16 (p.38). It is evident that Ti is most abundant near the contacts, in the products of ear1iest crystallization. This is in agreement with pre vious observations (Rankama and Sahama, 1950 p.56l). The var iation of Fe content i s simi1ar to that of Ti, while that of

Mn is more irregular, and invers~ to the variation of Fe and Ti. 47

Contact Effects:

Three main types of sediments are in contact with the diabase: dolomite, quartzose siltstone, and argillite. The affects on the first two are not as pronounced as an effect on the argillaceous rocks.

Bleaching has taken place in the dolomite, which has been transformed into a dense limestone. In thin section, this rock is composed almost entirely of fine, anhedral grains of carbonate. Miner opaques, and chlorite fill minute interstices. Closest to the contact the grain aize is lar gest, about 0.2 mm. Pyrite was introduced in small quanti ties along bedding planes, and is now partially altered to limonite. Chlorite appears in thin laminae along some of the planes, parallel to bedding, with individual crystals ran domly arranged. This lack of linear structure indicates its secondary origin. However, it also occurs in laminae in which the crystals have orientations parallel to bedding. Subvertical veinlets of twinned carbonate, quartz, chlorite, and miner sulfides traverse the rock. One of these carbo nate grains showed an axial angle of about 3°. A few gypsum veinlets occur in the dolomite, subparallel to bedding. A fibrous mineral (anhydrite ?) is associated with gypsum, with closely spaced needles perpendicular to the walls of the veinlet, while green chlorite forms a thin marginal film.

The quartzose siltstone has undergone compaction without any perceivable change in composition. Platy parting parallel to bedding has developed. Where the siltstone con- 48

tains considerable amounts of argillaceous mate-rial, small specks of opaques, chlorite, and other scaly minerals are visible.

The most pronounced changes have taken place in the argillaceous sediments. Whereas the dolomite and silt stone beda were affected only for a few feet from the dia base, the argillites 20 feet or more above and below the intrusion show incipient crystallization of cordierite. The effect is accentuated in the rocks 6.5 miles northwest of Tawu Lake, where argillaceous rocks occur between the two sills, only 48 feet apart. The pelitic sediments responded to elevated temperature conditions and were metamorphosed to fleckschiefer. The temperature was high enough to induce the formation of cordierite, which occurs as light colored blebs in a very fine grained brown matrix. The aize of the "spots" varies, and depends on several factors, such as proximity to intrusive and relative abundance of argilla ceous constituents; most of them are lesa than 1 mm. across.

Dnder the microscope the spotted shale is semi opaque. With crossed nicols and gypsum plate one can notice a lineation produced by minute- flaky mineral grains along bedding planes; these are length-slow. Quite numerous are quartz grains of silt size which often form laminae. Chlo rite and opaque specks are common. fhe light colored "spots" are resolved into cordierite-serioite (pinite) aggregates, and are in various stages of formation. Least developed grains have usually ellipsoidal shape, while the best de veloped ones are of short prismatic habit and (pseudo) hexa- 49

gonal cross section. A peculiar structure is seen under crossed nicols (fig.23). In vPrtical section, the best formed prisms havP hourglass structure of dark color in aide a rim of radiating sericite (?) crystals with yellowish interference color. Basal sections show a cross.

Fig.23 Thin section of flPckschiefPr under crossed nicols, showing longitudinal sections (hourglass structure) and basal section (cross) of cordierite sericite "spots". These dark outlines are absent in plane polarized light.

0 1 L-----~------~mm scale

The thermal effects on siltstone and dolomite were not as large as those on the pelitic sediments because their minerals are stable at higher temperaturPs than the clay minerals. The incipient formation of cordierite shows that considerable amounts of heat were liberated by the cooling diabase magma.

Discussion: The tabular basic sheets in the northern Mackenzies have been tPrmPd sills. Contact aureoles at both margina, and small apophyses on top of the sills in the country rock, are the best evidence for concluding that the bodies are intrusions rather than flows.

Diabases have bePn subjects of detail studies by investigators in many widely separated places of the worJ.d. 50

At pr~sent, petrologists recognize two kindreds of basal tic magma which produce olivine diabases and quartz diaba ses (tholeiites) (furner and Verhoog~n, 1951, p.l77). Oli vine basalte are the predominant lavas throughout th~ length and breadth of the oceans, and crop out where islands pro ject above the sea. Quartz diabases occur as flood basalts and dike and sill swarms in continental areas. There ar~, nevertheless, provinces such as the Tertiary basalt region in Greenland (Backlund and Malmqvist, 1932), where both types occur together.

Classic examples of tholeiites are the (Jurassic) Karroo diabases of Jouth Africa, and the (Triassic) Newark traps of New Jersey. Other regions in Australia, South Am~ rica, ~urope, and Asia have similar assemblag~s. The chemi cal composition of these rocks is very unifor.m (Washingto~, 1922), and they differ from olivine basalte by their higher Si0 and lower MgO content. 2 Differentiation in place after intrusion of an initially homogeneous mixture (liquid diabase magma + phe nocrysts) has been described from many of the thick ai~ls, such as the Mount Wellington dill in Tasmania (Edwards, 1942) and the Falisade 3ill of New Jersey (Walker, 1940). Two factors dominating the differentiation are fractional crystallization and gravitational differentiation. The vPr tical variations of mineralogy and grain aize of those sills are similar. An olivine-rich layer near th ~ base, succeeded upward by progressively less dense layera, is explained by the sinking of early formed crystals. The upper portions of 51 the sills are rich in micropegmatite, indicating that the fractional crystallization tr~nded towards a granophyric composition.

The Tawu Sills of the Arctic Red River area show a small amount of differentiation by fractional crys tallization, but gravitational differentiation as, for ~xam ple, the gravity settling or roof-stoping of phenocrysts, which should coeur according to dtokes law, was not det~c ted.

Calculations to determine the time for complete solidification cao be mad~ using the formula supplied by Jaeger and Joplin (1955, p.l4). But values thus obtained are very rough approximations, because there are many un known conditions. For a 100 foot sill values ranging from 8 days to 120 days were obtained. (These figures should not be considered too seriously).

3ills totalling thousands of feet in thickness, and thousands of square mil~s in extent, require the supply of colossal volumes of basic magma. Furthermore, since dia bases crystallize at high temperatures, auch a magma can only be generated deep in the earth's crust. The problem of the origin of primary basaltic magmas has stimulated the reasoning of a large number of geologists. Several hypothe ses are summarized by Turner and Verhoogen (1951, pp.l94-200).

According to Bowen (1956, p.315), the production of parental basal tic magma originates by selective fusion of a shell in the earth's crust composed of peridotite, through 52

rel~as~ of pressure. Daly (1933, pp.200-202) advocated th~

origin of olivine basalt and tholeiit~ magmas as being de rived from the same vitreous basaltic substratum, but

tapped from different gravitationally stratifi~d l~vels.

K~nnedy'a ~dea (1933) is that of periodic local fusion of a world-wide olivine basalt shell which is succeeded upward, in continental regions, by a tholeiite layer, and a grani tic shell. According to this, the two main types of magma

originate indep~ndently as primary magmas, each with its

own fractional crystallization products. The hypoth~s~s differ in the explanation of derivation of magma types,but

agree in that they assume some sort of basic sh~ll in th~ deep parts of the crust.

Several magmatic cycles can often be r~cog nized in folded mountain regions (deSitter, 1956, p.359; Turner and Verhoogen, 1951, p.201). The first phase is usually represented by basic intrusions (ophiolites) and extrusions of spilitic lavas, the second by ultrabasic and basic plutonic intrusions. Syntectonic migmatization and the formation of granite and granodiorite batholiths are next. Lastly, post-orogenie surface eruptions of basalt,

andesite, and rhyolite occur. According to this sche~e, the Tawu Sills would fall into the first stages, or the last one. However, since they are folded, they would belong to the former.

If one applies the foregoing ~xplanations to

the Mackenzie fuountains, on~ has to postulate a d~ep fracture,

or fractures, which tapp~d the basaltic substratum. Such 53 channelways can conceivably be associated with the large longitudinal faults. Yet, such an explanation is confronted with the following difficulty: The age of emplacement is younger than the deposition of the Cambrian (?) sediments, but antedates the deformation of these rocks. This time delineation is unsatisfactory, as it places the intrusion anywhere in the Paleozoic-Mesozoic, and before the forma tion of these longitudinal faults, if they are of the same age as the deformation. If the faults are older, they may be due to sorne sort of general rupture zone in the base ment, a zone along which recurrent moveme.nts took place, and along which magma could have forced its way upward early in the orogenie history.

The granite and granodiorite plutonic cycle could be represented by intrusives of that composition in the east central 3elwyn Mountains, in the headwaters of Keele River and South Nahanni River, and all through south ern Yukon (Wheeler, 1954; Kingston, 1951; Geol. Surv. Can. maps 1048 A and 1G55 A). While some of these are believed to be Tertiary (?), others are related to the Mesozoic Coast Eange intrusions of the western Cordillera.

Cretaceous (?) basalts and pyroclastics occur in the central Selwyn Mountains (Wheeler, 1954, p.29). Similar rocks are also known in south central Yukon. Ben tonite beda in Upper Cretaceous shales (Hume, 1954, p.47) further attest to the volcanic activity during this time. Probable orogeny during late Cretaceous or early Tertiary 54 time in the Mackenzie Mountains is indicated by the fact that flat-lJing Tertiary terrestial sediments overlie fol ded and eroded Cretaceous for.mations (Hume, 1954, p.54).

Apparently then, igneous activity during late Cretaceous - early Tertiary time was widespread in the Yukon, and i t is possible that the ·:rawu. Sills could have be en intruded during this time, but before orogeny commenced. The relatively unaltered nature of the diabase ailla is a property which supports the hypothesis of auch a young age.

However, the possibility of a very old age (early Paleozoic) should not be discounted, for considera tion has to be given to the fact that the intrusions are emplaced only in pre-Silurian (Cambrian ?) sediments. Wheeler (1954, p.38) mentions the possibility of tectonic activity in the Selwyn Mountains in pre-Ordovicien, probably Cambrian time. Similar indications are also found in areas further west, in the Ogilvie Mountains. These are treated in a separate chapter. Disturbances in thosa areas during the early Pal~ozoic era could have affected the sediments in the Mackenzies.

Recently, White (1959) has inferred no lesa than six major orogenies since the Proterozoic in the Cor dillera of British Columbia: 1} The effects of a fre-Lower Cambrian orogeny are preserved in the Purcell strata that were greatly uplifted and widely folded. These strata contain numerous interbeds of lavas, and sills and dikes of quartz-diorite composition (@eol. 55

Surv. Can., 1957, p.296). In the eastern Cordillera of British Columbia, near Toad River on the Alaska Highway, Williams (1944, p.l3) has observed diabase dikes that eut Precambrian (?) dolomite, slate, and quartzite, but do not eut overlying grey limestone containing Middle Silurian corals. These dikes may or may not be related to the Pur cell lava cycle. MacLaren and Kindle (1950, p.l27) mention them and note that "the occurrence of igneous rocks on the site of the Rocky Mountains is most unusual". 2) White's next orogeny occurred during post-Ordovician - pre-Mississippian time and is characterized by extensive folding and granitization. 3) Folding, faulting, and widespread ultramafic intrusion took place during post-Upper Permian - pre-Upper Triassic time. Peridotites and ether ultrabasics that probably be long to thet cycle are known in the vicinity of Dawson City and southern Yukon (Geol. Surv. Can., 1957, p.320). 4) The Coast Range granodiorite batholiths were intruded during middle Mesozoic time. Tectonism continued from then on, and migrated eastward. 5) By Paleocene time the Rocky Mountains had formed through crustal shortening. 6) Upper Tertiary to Recent vulcanism and intense local folding and faulting express the latest stages of the oro genie history of the Cordillera in British Columbia.

If a similar sequence of tectonic events exista in·the adjacent regions to the north, the Tawu. sills could be of early Paleozoic age. 56

DI~S IN THE :KNORR RANGE

Occurrence and field relations: Two dikes are exposed in a section of bright orange weathering Cambrian (?) carbonates capping the Knorr Range, 5 miles east-northeast of Margaret Lake (see map at end of paper). Looking from Margaret Lake, th.eir location is on the southeasternmost of three peaks, at about 4500 feet elevation. The beda here strike N65°E and dip 20°N. The larger dike is 6 feet thick, has a strike of N75°E, and dips 70°S. The ether dike occurs 500 feet higher in the stratigraphie section; it has a thickness of 3 feet, strikes N85°E, and dips 52°S.

Petrology: Hand specimens of both dikes have similar appearance. The weathered surface is greyish brown, which is due to lichens and a limonite alteration product that forma a ooating 1 to 2 mm. thick. The rock is medium grained, and contains a large amount of a flaky mineral which imparts a greenish grey color to the rock, as seen on a fresh sur face. In soma parts it has a brown mottling of the same color as that on the weathered surface. Pyrite cubes and limonite pseudomorphs after pyrite make up 1 to 2 percent of the dikes. These cubes range up to 3 mm. in aize, but are generally muoh smaller. On the weathered surface they stand out in relief. The rock effervesces slightly with the application of cold dilute hydrochloric acid. 57

From the larger of the two dikes two samples are available which were used to make thin sections. Micro- scopic examination showed that the rock is largely composed of biotite, chlorite, and carbonate. The estimated averag~ composition of the two samples is:

biotite and chlorite 40~ carbonate 25 sericite 15 iron ores 8 quartz, apatite 12 The mean value of the specifie gravity is 2.84, which is lower than that for the Tawu ~ills. An intergranular texture is visible, but commonly it is allotriomorphie-granular. In seme places it is ophitic, with carbonate, or aericite laths penetrating biotite grains. The relation between car bonate and biotite is the sam~ as that between plagioclase and pyroxene in ophitic intergrowths. There are also clus ters with oval and elliptical outlinea, free from opaque minerale, compoaed of sericite and carbonate. Other rounded masses of carbonate, up to 3 mm. in diameter, are tran$- seeted by veinlets and irregular elongated patches of opaque mineral. The whole reaemblea a pattern auch as that found in the irregular fractures of some olivine cryatala, in which aecondary magnetite haa formed at the expanse of olivine. The biotite cryatals terminate abruptly againat these rounded masses.

There is usually a higher concentration of opaque minerale in the border zones around large carbonate masses than in the rest of the matrix. 58

M:ineralogy:

Biotite, with marginal chlorite alt~rations, is present as randomly arranged tabular and lamellar aggregates, up to 0.8 mm. across. Many cr~stals are bent and split along the cleavage planes (fig.25). The biotite is pleochroic from light brown to dark olive brown, and has parallel ex tinction. It is strongly birefringent, with interfer~nee colors ranging up to second order orange. It is optically negative and has a very small optic angle. The marginal chlorite is pleochroic from almost colorless to pale brow nish green. It is optically negative with 2V about 10°. dome of the biotite flakes have minute inclusions, needles of an optically indeterminable mineral; but probably they are rutile. These are regularly arranged, forming a skele tal pattern as_ shown in figure 24.

Fig.24 Skeletal pattern of transparent prisms in biotite. ca carbonate bi biotite cl chlorite 0 0 .\ L-~----J h\wt scale

The carbonate in the rock is mostly calcite, as suggested by the effervescence with cold dilute hydro chloric acid. It is allotriomorphic, with grains up to 0.2 mm. in diameter. Only rarely can one observe twinning, and this is solely in the largest grains. The carbonate fills 59 spaces that have tabular outline (fig.25), a fact which is taken as evidence that it is not primary. The mineral also

Fig.25 Carbonate (ca) pseudomorphs after plagioclase~ penetrating bent biotite (biJ flakes that are marginally altered to chlorite. Note rim of opaques around the pseudomorphs. Plane-polarized light.

ecale

occurs as rounded masses without opaques, which are believed to be small fragments of the carbonate country rock. A cloudy secondary alteration mineral is associated with car bonate grains, but it cannet be identified in thin section.

~ericite is present as colorless tabular aggre gates up to 0.2 mm. long, which taper at the ends. It has low relief and is not readily distinguished in plane-pola rized light. Under crossed nicols it stands out because of its blue, yellow, or red interference colora. The crystals are length-slow and show parallel extinction. They penetrate or cross biotite crystals. Sericite and carbonate are generally side by aide.

Opaques are contained in the rock as euhedral crystals and as shapeless grains. Cubes of pyrite are abun dant, and are most often 0.02 to 0.1 mm. in diameter. Some 60 of these are completely altered to limonite. Crystals are often aligned, especially at the borders of tabular carbo nate areas (fig.25),

Quartz fills cavities of irregular shapes, almost always in the carbonate and sericite aggregates, and, rarely, in between bent biotite flakes. The grains range in aize up to 0.1 mm., but are most often toc small to permit positive identification. The mineral does not have cleavage. It has low relief, with refractive indices higher than that of balsam, has low birefringence, and is uniaxial positive. Soma grains show minute inclusions.

Apatite constitutes an accessory mineral, and forma hexagonal prisme which are up to 0.3 mm. long.

Discussion: The modal composition of the dikes given on page 57 is that of a carbonatite. However, they are not considered to be lamprophyres such as, for example, the beforsite and alvikite dikes of the Alno district in east central Sweden, described by von Eckermann (1948, p.llO, p.l24). Those are dikes in which the dolomite and calcite, respectively, are thought to be of magmatic origin. The dikes in the Knorr Range are believed to represent the solidified product of an ordinary basic magma, injected into narrow opening of the argillites and carbonates, which became heavily altered. The source of the large amounts of 61

carbonate most be this country rock. Evidence supporting the hypothesis of secondary carbonate in the dikes is found in the textural relationship of the minerals: 1) The carbonate occurs as small anhedral grains bunched together into aggregates that have tabular outline and of ten penetrate biotite in ophitic fashion, just as plagio clase occurs in pyroxene. Thus, carbonate is pseudomorphic. Biotite may be due to the alteration of pyroxene. 2) The large (up to 3 mm.) piecea of carbonate in the dike, surrounded by truncated and deformed biotite crystals and rima of concentration of opaque minerale, may be small xenoliths of limestone or dolomite broken from the wall rock during the ascent of the magma. In this case, the heat liberated during the crystallization was not sufficient to induce the recrystallization into coarse crystals of calcite.

/ The age of these dikes is not established. From field observations it can only be stated that they are younger than the Cambrian (?) argillite - dolomite sequence. Their relation to the intrusions in the Tawu Range is not

~o~.

The dikes fill fractures that strike approxi mately west, conforming to the structural trend of the area. They dip towards the south at moderate angles, suggesting that the source of the magma is to be sougbt in that direc tion. It is of importance to note that in the Selwyn Moun tains, 60 miles to the south-southeast (see map at end of paper), dioritic intrusives exist (Wheeler, 1954), and that 62

some of these are dikes that strike w~st; the ethers arP irrPgular stocks. Th~ dikes dip steeply to vertioally, and are up to 80 feet thiok. Th~y eut rocks slightly younger than Silurian or Devonian, and antedate the deformation of the Paleozoic rocks. It is possible that these intrusions may be related to those at Margaret Lake in the Knorr Bange, or possibly even to the Tawu Sills. At present, there is no evidence to either support or disprove such a relation. 63

DIKES IN BLACKSTONE RIV~R AREA, OGILVIE MOUNTAINS.

Dikes of dioritic composition were found in the Ogilvie Mountains, about 15 miles northeast of Chapman Lake. Although the region does not belong to the Mackenzie Moun tains as defined by Boatock (1948), and the title of this th~sis implies a restriction to that unit, a description of the dike rocks ie, nevertheless, included here because these rocks have not been describ•d before, and for ressons of possible relations to the Tawu Sille or the diorite dikes of the Selwyn Mountains.

Physiography and general geology of area: The Ogilvies extend eastward from a few miles west of the 14lst Meridian to about Hart River. On the southwest they are bounded by the Tintins Valley - a fea ture similar to the Rocky Mountain Trench. The Selwyn Moun tains and Porcupine Plain for.m the eastern and northern limita. Elevations of the highest peaks are between 5000 and 6000 feet. The mountains are rugged and bare of vege tation. Glaciation in the interior was confined to local ioe fields and valley glaciers.

Basic and granitic intrusions have been reported in the southwestern region (Cairnes, 1912; Cockfield, 1918, 1919), but the main body of the mountains to the northeast is underlain by sedimentary rocks, with a few diorite dikes 64

138"W

c ..0 ..0 ~... o.... ~ ..c 65"N ~... ;:, 1 ..u ...... ;:, ..~ ;:, 1 0 1 ..

1 ~ 1 ..... 1 0 1 z 1 1 1

1 1 \

2.o,ooo 10,000 0 foot

+- ~VI"'c.lihll. ...!.2_ s1'ril

Fig . 2 6 Diorite dikes in Blackstone River area, Ogilvie Mountains, Yukon. 65 described here. Resulta of the writers field work and air photo interpretation are incorporated in figur~ 26.

Local stratigraphy:

In the area visit~d there are three main types of sediments, of which the absolute age is not known. They appear to be unfossiliferous except for certain algal struc tures. The lowermost unit consista of more than 4800 feet of dark grey argillites, shales, siltatones, and some lighter colored sandstone beda. In many plaoes the beda are aheared and slightly metamorphosed to slate or phyllite. Towards the top of this unit are a number of 3-foot beda of bright reddish orange weathering silty dolomite, interbed ded with light grey weathering, medium grained, croas-bed ded sandstones. Overlying the lower unit with apparent conform ity is a series composed of about 2800 feet of bright orange weathering algal dolomite reefa, alternating with grey weathering dolomite and cherty dolomite beda. The algal dolomite forma resistant cliffa up to 100 feet high (fig. 27). Individual algal structures are 3 to 5 feet high col umns of lenses, meniscuses convex upward. They are usually 3 inchea wide at the base of the reefa, while at the top they reach their maximum development and are 3 feet across. In basal section they are polygonal. They resemble Collenia or Cryptozoon. Grey weathering dolomites in between the bioherma are siliceous in many horizons, and in some blaok chert nodules are abundant. i .~ of 65° 138° • on ri t. du o lo

ig.2 66

Fig.27 View southwest from 2 miles SW of 65°N 138°W. Algal reefs for.m cliff on right. Irregular shape of brown dike is due to loose rock.

Fig.28

Airplane view ~s over southern part of area, illustrating structural relationships between the three formations. 67

The third, uppermost unit consista of light gr~y

carbonates, mostly cherty dolomite, with a 40-foot lim~stone layer at the base. At a place about 1 mile northeast of 65°N 138°W these light carbonates lie directly on the dark clastics of the lower formation, separated only by a 70-foot layer of marcon shale; the algal reef sequence is absent.

Local structure: In the map area there is an unconfor.mity between the uppermost formation and the two lower ones (fig.28). Beda of the youngest formation are folded into large open anticlines and synolines that trend west-northwest. Those of the lower formations strike northerly and dip east at moderate degrees; they have been faulted and invaded by ver tical dikes (fig.27).

Twenty-five miles to the east, on the east bank of Hart River at 65°02~137°15'W, a ninety-degree angular discordance is exposed in vertical section. The rock types involved are similar to those at Blackstone River, but in this location a basal conglomerate is present in the almost horizontal upper unit, overlying the northeasterly trending older rocks.

The intrusives follow two directions: one Nl5°E, the ether (main) an easterly direction. Age relationships between the two sets of dikes can not be observed because the place of intersection (near traverse line) is covered by thick talus. Dikes were not aeen to eut the uppermoat unit. The sheared lower formations contain also quartz 68 v~ins up to 2 feet. wid& which strike more or lesa parallel to the northerly trending dike set.

Shear zones are exposed in a few places near the traverse line. Where the 65°N parallel of latitude in tersecta this line, shearing and contortion of beda are well visible on the northeastern aide of the ridge. A fault zone, S@veral tens of feet wide, was also seen 2 miles southwest of 65°N 138°W, where it intersecta a dike. Due to a cover of loose rock the exact relationship is not visible, but from looking at the atrike of the talus of the dark dike rock it aeems that the horizontal separation of the intru sive, if any, is very small.

The width of this dike is approximately lOO feet, while the two ethers exposed along the traverse line are each 45 feet wide. Howev~r, that of the northerly trending one appears larger because of several large, tabular xeno liths of m@tamorphosed dolomite. The dikes can be recognized on air photos by their dark tone, which permits tracing them for aeveral miles along strike. They do not stand out in relief.

Fetrology: As the dikes are almost invariably covered by talus, fresh specimens cannet be obtained. damples of the three dikes look esaentially alike. The weathering color is greyish brown to reddish brown. Fresh fractures show the rocks to be considerably altered, to a greenish to dark grey 69 product. A medium grained, equigranular texture ia visible in the freahar samples. Pyrite specks are aporadically distributed. Chalcopyrite is present as a few irregular graina, and is associated with ita weathering product, malachite. fhe aulfide mineralization is probably due to deposition from hydrothermal solutions, for it occurs in the vicinity of shear zones and quartz veina.

The ye·llowish brown dolomite is metamorphosed to a green carbonate rock which weathers white to a depth of about 0.1 mm. An earthy oder characterizes this white coating. The contact aureoles are not more than a few cen timetera wide. Thermal effects on wall rock fragments vary; soma have undergone completa recrystallization to form large calcite xenoliths, with individual crystala up to 10 cm. across; ethers have remained as dense, fine grained blocks. 'l'hin sections of dike specimens all exbibit heavy alteration of primary minerale. Modal compositions (approximate) of 4 samples from 3 dikes are as follows:

Table 3 Composition (by volume), and specifie gravity of Blackatone River dikes

HH-94 HH-95 HH-97 HH-98 Andesine 30 40 i3aussurite 39 27 46 31 Pigeonite 70 Chlorite 25 46 25 Iron ores 4 2 8 4 Amphibole 1 1 Carbonate 1

S~ecific gravity 2.79 3.01 2.77 2. 72 70

The mark~d differ~nce in composition of the two samples

from the northerly trending dik~ (HH-94, HH-95), and the fact that they are from opposite sides of a 20 f~et wide xenolith (?), suggest multiple intrusion. However, no defi nite contacts are visible b~cause of talus cover.

In view of the presence of large amounts of alteration minerals it is not possible to differentiate distinctly between the three dikes. For this reason they are treated together in the following section.

Mineralogy: Plagioclase (An - An ) crystals are euhedral 33 39 to subhedral, but are most frequently saussuritized with consequent losa of distinct crystal outline. Their length in the specimens is usually 1 mm. Unaltered grains are colorless and have low relief, with refractive indices higher than that of balsam. Albite twinning is beat preserved in section HH-94. The largest extinction angle measured on these twins was 20°, which corresponds to a composition of

Andesine, An39 • Plagioclase from the east trending dike HH-98 is slightly more sodic, An33 •

Saussurite forms cloudy and brownish grey grains of moderate and high relief, and moderate birefringence. Under high power they appear corrugated and somewhat fibrous. Upon insertion of a gypsum plate, under crossed nicols, the elongation of the grains is parallel to the slower ray. Other properties are obscured. 71

Pyroxen~ (Pig~onit~) is pres~nt only in slid~ HH-95, where it is mostly allotriomorphic. Grains, usually 1 mm. across, are colorless, have moderate relief, and show

good pyrox~ne cl~avage. The interference colora in this section range from grey to first order yellow. The mineral is optically positive, with 2V about 15°. Simpl~ twinning on (100) is common, while polysynthetic twinning is present,

but rar~. Zoning was observed on one crystal. Extinction

angl~s measured on it (Z/\c) ranged from 37° n~ar the center to 43° at the rim. Pyroxen~ in the other elides, if present originally, has altered completely to chlorite and other minerale.

Chlorite is a major constituent of all dikes. In sample HH-94 it occurs as irregular radiating masses. It

is colorless with a slight greenish tinge in plane-polariz~d light, and lacks pleochroism. Interference colora are vari ous shades of bluish grey. The orientation of fibr~s or scales is length-slow. These are the properties of penninite. In sections from the two easterly trending dikes (HH-97, HH-98) chlorite exista as scales and aggregates of irregular outline, as well as crack fillings in altered plagioclase. It is pleochroic from green to nearly colorless, and has parallel extinction. Its interference colora are of two distinctiv~ kinds, indicative of two separat~ chlorites in the rock. One type is olive green, the other is Berlin blue or bluish grey with a greenish tinge. The olive vari~ty is length-fast, while the blue one is length-slow. The form~r is twice as abundant as th~ latter, but individual grains 72

are, as a rule, only half as larg~. Chlorite contains

minut~ opaQU~ specks in sample HH-97; these are absent in HH-98.

Iron ores are not as abundant as in the Tawu

Sille. Pyrite is in irr~gular specks, while magnetite-ilme nite intergrowths form skeletons. The latter are partially or completely altered to leucoxene.

Amphibole is present in small amounts in the northerly trending dike. Crystals are brown with a thin greenish rim. Pleochroism is moderate, and colora range from pale brown to greenish brown. Extinction (Z/\c) is 25°.

The optic sign is negativ~ and 2V is unusually small, 20°.

Carbonate has formed in minute veinlets and spo radic clusters. Twinning can be detected in the vein carbo nate. A thin section from a non-recrystallized xeno lith (HH-96) is almost completely composed of carbonate grains which are 0.02 mm. across. About 1 percent of the rock ia an opaque mi neral, in grains of similar dimensions, that occurs in laminae, probably along bedding planes of the original dolomite. Along these planes is also a greenish mineral, in elongated crystals which have length-slow orien tation. It is most likely a chlorite. The sample is eut by small veinlets of polysynthetically twinned carbonate.

It may be noted here that the dikes do not carry any quartz. 73

Discussion: The age of emplacement of these dikes is not established because the absolute age of the sediments they intrude is not known. On the basie of the presence of algal reefs alone, the only fossile found, one cannot assign them to any definite period. Consequently, one has to attempt to correlate these assemblages to similar rocks in adjacent or more remote areas.

A geological setting resembling the one at Black stone River is that in an area in the northern Selwyn Moun tains, described by Wheeler (1953, pp.22-23), which has already been mentioned in connection with the dikes at Mar garet Lake. Petrographically, those diorite dikes differ from the ones described in this paper in that many of them are porphyritic and contain calcite amygdules near their margina. Furthermore, their dimensions are larger, and they are asaociated with stocks of the same composition. Pro nounced bleached border zones in the reddish broWn dolomite rocks are as much as Sü feet wide, whereas these are absent, or thin, near Blackatone River, and the bleaching ia con fined to xenolitha. Neverthelesa, the aimilarities between settings of the two regions are significant. In both dis tricts the dikes are of dioritic composition; they trend west and have steep dips; both sets of dikes carry a little chalcopyrite and pyrite, either sparsely disseminated tbrough the rock or occurring in small quartz veina in the diorite. The sediments they intrude have similar properties, parti cularly the reddish brown or orange weathering dolomite 74 containing algal structur~s similar to Cryptozoën or to

Collenia (Fenton and Fenton, 1937). They wer~ assigned to the Cambrian-and -earlier (?) by Wheeler. In his area the diorites eut rocks slightly younger than Silurian or De vonian, and are older than the deformation of these rocks.

At Blackstone River the dikf's are sheared and eut by quartz veina; the~ do not aeem to penetrate the light grey carbonate (uppermost) formation. These carbooa tf's are b~lieved to be ~arly Paleozoic, possibly Ordovici~ on grounds of lithology and stratigraphie position with respect to rocks to the northeast of the area covered by figure 26, which bf'ar Silurian graptolites. If they are Ordovicien, theo the dikes would be older than those in the northern Selwyn Mountains.

Cockfield (1924) bas rf'ported diorites, ande sites, and tuffe in slightly fossiliferous Ordovician Devonian limestone in the upper Beaver River area, near the easternmost extension of the Ogilvies. He considera them to belong to a period of igneous activity not older than the Silurian and perhaps Devonian.

Cockfield (1918, 1919) also found diorites, and other intrusives, in northeasterly trending sediments of undetermioed, but pre-Ordovician age in the Chandindu (Twelvemile) liiver area.

McConnell(l891, pp.l29-133) noted basic intru sions in sediments along the Porcupine River, but did not aasign the sediments to any definite stratigraphie position. 75

Cairnes (1914, pp.48-54) mentions these rocks and corre lates them wi tb his Lower Cambr·ian or pre-Cambrian Tindir group on the Canada-Alaska boundary. The Tindir group is intruded by greenstones which are dominantly diabases and coeur as sills, dikes, and irregular intrusive masses.

Cairnes states (p.53): "Since th~se intrusives were rarely noted intruding the overlying Devono-Cambrian limestones and dolomites, it is concluded that they are dominantly

11 at least older than these rocks •

From all these observations it seems that there was widespread igneous activity in the Cgilvie Moun tain region at seme time during the early Paleozoic. The question as to whether the igneous bodies are the result of only one period of intrusion, or whether they belong to several periode remains open, and cao only be answered by more field study.

The Ogilvies have been investigated by Green (Lord, 1958, p.5) during the summer of 1958, on a recon naissance scale, in anticipation of a helicopter-assisted project to be carried out by the Geological 3urvey of Canada in the near future. No report was published on this reconnaissance work, but after· the completion of the proj e.-ct we shall know considerably more about the intrusions in the Ogilvie Mountains. 76

SUMMARY AND CONCLUSION

Basic dikes and ailla hav~ been found in areas in which no previous geological field work bad been carried out. This th~sis rejects the generally held opinion that no intrusions exiat in theaa areas, and contributes to the knowledge of the history of the Canadian Cordillera.

The name "Tawu Sills" is appliad to tholeiite sills occurring in the Tawu Range of the northern Mackenzie Moun tains. They intrude pre-clilurian (Cambrian 1) sediments, and ara involved in the deformation of these rocks during late Cretace·oua or early Tertiary time. The ailla may have be en emplaced during a magmatic cycle immediately preceding this deformation, but could also have formed at soma time during the early Paleozoic.

To create an (obaerved) outcrop area of 900 square miles and an average total thickness of 100 feet, about 17 cubic miles of basaltic magma are required. This is small compared to the buge volumes which produced similar rocks in ether regions of the world.

Outcrop characteristics, field relationships, and physical and chemical properties are described. They conform to those of the usual quartz-diabases found elsewhere as sill swarms, except for the lack of gravity differentiation, which is due to the smal l thickness of the Tawu Sills.

It is speculated that similar intrusives may occur 77

in the southern ~ackenzie Mountains, but that they have

thus far eluded det~ction.

Dikes in the westernmost Maokenzies and in the Ogilvies are also described. Those in the Ogilvies are be lieved to be ~arly Paleozoio in age, while the ethers oan only be dated as post-Cambrian(?).

The field work for this paper was on a recon.

naissance scale. As a result, th~ trPatise of the intru sions is rather general. It is expected that future inves tigations in the Mackenzie Mountains and Ogilvie Mountains will throw further light on the occurrence of basic intru sions in these areas. 78

APPENDIX

Field location of sampl~s cited in the text, and of samples and thin sections deposited with the Department of Geological Sciences of McGill University.

Sample number Location

HH-5 Metamorphosed carbonate rock, at base of low~r sill, 1.8 miles NE of Tawu Lake. (dee fig.6, p.20) HH-11 Diabase, lower sill, 1.8 miles NE of Tawu Lake, 20 feet above base. (Se~ fig.6, p.20) HH-15 Chlorite vein, lower sill, 1.8 miles NE of Tawu Lake, 25 feet above base. HH-24 Met~orphosed carbonate rock, at base of upper sill, 1.8 miles NE of Tawu Lake. HH-25 Diabase, bottom contact of upper Bill, 1.8 miles NE of Tawu Lake. HH-27 Diabase, upper sill, 1.8 miles NE of Ta wu Lake, 6 inches above base. HH-35 Diabase, upper sill, 1.8 miles NE of Ta wu Lake, 1 inch below top contact. HH-53 Siltstone - shale, 15 feet above top of lower sill, 33 feet below base of upper sill. 6.5 mi. NW of Tawu L. HH-54 Argillite- siltstone, incipient crystallization of cordierite, location same as HH-53. HH-61 Diabase, upper sill (71 feet thick), 6.5 miles NW of Tawu Lake, 40 feet above base. HH-68 Diabase, 1ower sill, 7.5 miles S of Tawu Lake, 2 inches be1ow top. HH-90 Diabase, sill 2o.l miles N80°W of Tawu Lake, 11 feet above bottom. HH-94 Diori~e dikB, norther1y trending, 11,000 feet W of 65 N 138 W (see fig.26, p.64). HH-95 Diorite, same location as HH-94. HH-96 Limestone xenolith in diorite, same location as HH-94. HH-97 East t6ending diorite dike, 11,000 feet W of 65°N 138 W. (see fig.26, p.64) HH-98 Diorite dike, 2 miles SW of 65°N 138°W. 79

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EJ Cretoceous .....,_, Towu Sills , observed c=:J Imperial fm. (U. Dev.) ..... possible sills c==J Fort Creek fm. (U. Dev.) - dikes 'Romports fm. o.)'\J\JVo ~ (M . Dev.) b)'\1\ \.IV fàults: ..> observed, b) infe-red CJ Beor Rock fm. CDev. or Sil.) -Y fold: onticline Ronning fm. .-.- ~ (Silurien) strike 1 dip of bedding ~ Ordovicien MAP OF NORTHERN MACKENZIE MOUNTAINS

CJ Cam brion and ;or older ~ braided stream showi~g outcrops of diobase intrusions, and geology as compiled from Hume (1954) 1

G.S.C. mop 1048 A, Wheeler (1954) 1 and persona! data / 0 10. ) Scale in miles

133° 13 2. 0