MAICASAGI AREA, ABITIBI-EAST COUNTY PROVINCE of QUEBEC, CANADA Department of Mines Honourable C

RG 060(A) MAICASAGI AREA, ABITIBI-EAST COUNTY PROVINCE OF QUEBEC, CANADA Department of Mines Honourable C. D. FRENCH, Minister A.-O. DUFRESNE, Deputy Minister

GEOLOGICAL SURVEYS BRANCH L W. JONES, Chief

GEOLOGICAL REPORT 60

MAICASAGI AREA

ABITIBI-EAST COUNTY

P.-E. IMBAULT

QUEBEC RÉDEMPTI PARADIS PRINTER TO HER MAJESTY THE QUEEN

1954

TABLE OF CONTENTS

Page

INTRODUCTION 1 Location and means of access 1 Field work 1 Previous work 2 Acknowledgments 2 DESCRIPTION OF THE AREA 3 Topography 3 Drainage 3 Preglacial valleys 4 Flora and fauna 6 GENERAL GEOIOGY 6 General statement 6 Table of formations 7 Volcanic and sedimentary rocks 8 General statement 8 Volcanic rocks 9 Sedimentary rocks 11 Concordant intrusives within the complex 12 Basic sills 13 Acid sills 13 Post-folding intrusive rocks 14 Diorite group 14 Northern granite 18 Nomans stock 20 Southern granite 22 Satellitic intrusives 25 Granitic dykes 25 Lamprophyre dykes 27 Diorite breccia 27 STRUCTURAL GEOLOGY 28 Folding 28 Faulting and shearing 29 ECONOMIC GEOLOGY 30 REEEBENCES 31 ALPHABETICAL INDEX 33

Map No. 971 - Maicasagi Area (in pocket)

LAICASAGI AREA

Abitibi-East County

by P.E. Imbault

INTRODUCTION

Location and Means of Access

The Maicasagi area is bounded by latitudes 49°45' and 50°00'N. and by longitudes 76°15' and 76°40'W. It is about 110 miles north- northeast of Senneterre, a town on the Quebec-Cochrane line of the Canadian National railway. It comprises 320 square miles, and includes parts of the following townships: Urfé, Montviel, Monseignat, La Rouvillière, Meulande, and Johnstone.

Maicasagi lake, the eastern part of which projects into the northwest quadrant of the map-area, may be reached by canoe from Senneterrc or from Rochebaucourt, 25 miles northwest of Senneterre. From Senneterre, the route is north by Bell river to Mattagami lake; from Rochebaucourt, it is by way of Laflamme and Bell rivers to Mattagami lake. From Mattagami lake, the route'continues eastward across Olga and Goéland lakes, and along the short segments of Waswanipi river joining these three lakes. Goéland lake lies southwest of Maicasagi lake, and is connected to it by a wide, shallow arm, Max narrows: The total distance covered in such a trip is approximately 200 miles.

Any paint of the area can be reached in slightly over one hour by hydroplane from bases at Senneterre, Amos, or 'Rouyn. landing facilities are restricted, however, to Maicasagi lake, the longer straighter stretches of Maicasagi river, and, at high water, the lower reaches of Inconnu river.

Field Work

With the exception of a strip along the eastern side, the area was mapped by the writer in the summer of 1949. That eastern strip, comprising about 60 square miles and lying south of Maicasagi river, wag examined the same summer by J.E. Gilbert, also of the Quebec Department of Mines; his findings are incorporated in this report and its accompanying geological map.

The field work was planned so as to cover the area systemat- ically by pace-and-compass traverses at half-mile intervals. Careful -2 - stereoscopic examination of aerial photographs, however, made it possible to modify this plan in many sections of the area. These studies, by revealing the topography of the region to be traversed, permitted avoiding many low,-swampy sections, with the result that more time was available for inspection of the higher ground, where exposures are more plentiful.

As much as possible, the traverses were oriented to inter- sect the regional structure in order to obtain complete cross-sections of the rock units. The exposures along the shores of Maicasagi lake and the banks of the main rivers were examined in more detail than required for mapping on a regional scale.

Previous Work

The main rock groups of the area and their regional trends have been known for more than half a century. The area was visited in 1895 and 1896 by Robert Bell (3)k of the Geological Survey of Canada. In more recent years, much additional information concerning the geology of the general area has been gained as a result of the work of several. observers, including Bancroft, 1912 (1); Cooke, 1927 (6); Lang, 1932 (13); Norman, 1937 (15); and Freeman, 1938 (7).

The map-area lies between the Olga-Goéland Lake area, on the west (10); the Capisisit Lake area, on the east (8); the Waswanipi Lake areas — East Half (4) and West Half (5) — on the south; and part of the Mishagomish area, on the north (16).

Acknowledgments

The party consisted of: W.G. Gillespie, graduate student at the University of Toronto, senior assistant; I.C. Grant and r'. Paquette, third-year students at NcGill University, junior assistants; G. Truchon and U. Therrien, canoemen; and J. Roberge, cook. All discharged their duties in a very satisfactory manner.

The maps used for the field work were prepared by the Surveys Branch of the Department of Lands and Forests of Quebec from vertical air-photographs taken in 1946 by Canadian Pacific Airlines.. The photographs themselves (scale: one inch to one-quarter mile) were used constantly In the field and, as mentioned earlier, were very useful.

xNumbers in parentheses refer to corresponding numbers in the bibliography at.the end Of the report. -3 - Sincere thanks are due to Fecteau Air Services, Senneterre, who supplied the party and rendered many courtesies.

DESCRIPTION OF THE AREA

Topography

The Maicasagi area belongs to the same topographic unit as the region west.of it, to and beyond Mattagami lake. Its relief is low, and its plain-like aspect is broken only by isolated mounds or groups of hills, the elevation of which seldom exceeds a few tens of feet. Through a gradual eastward rise of the level of the whole region, the map-area stands at a higher altitude than the country surrounding Mattagami lake.

The highest hills are found near the northeast corner of the area, north of Maicasagi river, and in Monseignat township, south of Nomans river. Some of these hills reach a height, as estimated visu- ally, of about 150 feet above the surrounding country, which itself stands only a few feet above the nearby rivers.

The local watersheds in the north section of Montviel township and in the northwest part of La Rouvillière are more than 1,000 feet above sea-level. Both areas, however, lack conspicuous hills and their elevations are reached only through a gradual, almost unnotice- able, rise of the level of the plain.

Drainage

All the run-off of the map-area is directed toward Maicasagi lake, excepting a small section, approximately twelve square miles in the south-central part, that drains southward into Waswanipi river and thence to Goéland lake. The area is drained through a well-developed system of. tributaries, arranged in a dendritic pattern that beârs no apparent relationship to the underlying rock structure.

The principal elements of the river system are a main river, Maicasagi, which may be termed a stream of the first order, a tributary of the second order, Inconnu, and a tributary of the third order, Nomans.

Maicasagi river has its source in Monsan lake, 24 miles northeast of the northeast corner of the map-area. From that lake, it flows in a southeast and then southwest direction to a point near the intersection of longitude 76° with latitude 500. Thence the river flows in a general westerly direction toward Maicasagi lake. Within the map-area, the river is about two miles south of the northern boundary. Its width is from four hundred to eleven hundred feet and - 4 - its waters are generally quiet. There are two small rapids in the central part of Monseignat township, and a stretch of rapids and .fast water extends for more than a mile and a half near the western border of the same township. All these rapids can be run by canoe, although at low water great care must be exercised to avoid the numerous boulders strewn in the channel.

Inconnu river is the outlet of Capisisit lake, which lies in the north-central part of Montalembert township, east of La Rouvil- lière. In its westerly course from the lake, it enters the eastern boundary of the map-area one mile north of the southern east-west survey line. About two miles west of the eastern boundary, it turns abruptly northward to follow a comparatively straight course until it. empties into Maicasagi river. Inconnu river is two hundred to three hundred feet wide, and, within the area, is interrupted by three rapids which can be avoided by well-beaten portages, the longest of which is about 1,000 feet.

About three miles before its junction with Maicasagi river, Inconnu river receives the waters from its main tributary, Nomans river. This stream has its source in the granitic terrain near the southwest corner of the map-area. Its main fork is.about half a mile south of post 27 on the southern survey line. Below there, it crosses the region in a general northeasterly direction, maintaining a surpris- ingly consistent width. In early June of 1949, the river was at least eight feet above mid-summer level. It could then be ascended easily with fully loaded canoes up to its main fork. The only portage neces- sary'under those conditions was due to a series of rapids, stretching for a mile and a half, near the mouth of the river. When the spring flood has subsided, however, the general shallowness of the river and the numerous boulders that in many places clutter its bed, make it unadvisable to venture in it with heavy loads.

Preglacial Valleys

A few observations are recorded here concerning the probable organization of the preglacial drainage in the map-area. Considering that, in postglacial time, the rivers have not yet had time to trans- port all the unconsolidated material lying in their beds, well-marked valleys entrenched in rock must be of preglacial age. There are three such valleys: Maicasagi, Nomans, and that of a small creek crossing the southern survey line east of post 20 and emptying into Inconnu river at the point where that river begins its northerly course.

Near the eastern boundary of the area, the valley of Maicasagi river is about half a mile wide. It widens westward and is close to two miles wide near its mouth. The ground between the river and the hills, which form the discontinuous walls of the valley, is very low and generally swampy. Its uniformity is reminiscent of a flood plain, but the presence, in many places, of dense clays similar to the deposits of ancient lake Barlow-Ojibxay is taken as evidence that the unconsolidated material forming the"flats" is of lacustrine, as versus river, origin. The glacial débris (till and clay) did not fill the valley, however, and, after the disappearance of the lake,.the present Maicasagi started to remove the loose débris before it could resume the erosional work of its predecessor. •

The valley of Nomans.river is similar to that of Maicasagi. Bordering hills are up to half a mile apart, and between them stretch low plains of sandy clay and numerous swamps. The hills are, for the most part, covered with glacial débris, and outcrops of bed-rock are scarce. A few exposures may be seen in those places where the river has cut against the bases of some of the hills. Throughout most of its course the river flows northeastward, but for the last three miles before it enters the Inconnu it flows slightly south of east. A short distance east of the bend it cascades over a series of rock ledges for a distance of more than a mile and a half, along which the river has cut straight-walled gorges about 20 feet deep. In an almost direct continuation of the northeasterly course lies, first, a northeasterly elongated lake, and beyond that a valley which leads directly to Inconnu river at the point about one mile upstream from where that river enters the Maicasagi. It is possible that, at one time, Nomans river flowed in this northeast valley instead of following the present south-of-east course, which, because of the rapids, appears to be of recent age. It is even possible that, at that time, Nomans river emptied directly into Maicasagi river.

The creek crossing the southern survey line just east of post 20 is clearly a 'misfit' stream. Its valley is marked in many places by steep escarpments, and it is up to one mile wide. The valley can be traced from post 20 continuously eastward to Inconnu river. Indeed, it continues eastward along the west-flowing segment of Inconnu river, upstream from its change from a west to a north course. from this bend northward to its mouth, Inconnu river flows in gently rolling country in which no conspicuous valley is discernable. It seems probable that the north segment represents a disruption in the course of a river that originally flowed across the area in an east-west va?ey.

Thé direction in which the preglacial rivers flowed is not , known. Considering the regional,slopes of the land, it might be suggested that the directions were probably the same as those of the present rivers. If this were true, the easterly course of the small 'misfit' creek would represent a reversal of flow. Such reversals are to be expected in glacially affected regions. The valley of the creek is not traceable near its head because of glacial débris, and this may - 6 - be thick enough in this locality to have blocked the channel, thus causing the reversal.

The widths of the'preglacial valleys are thus of the same order as for those described by the writer in the region west of this map-area (12). They strengthen the idea that, before glaciation, the land had been uplifted and the rejuvenated rivers had carried their erosion work to a stage of early maturity.

Flora and Fauna

.The vegetation consists of the following types of trees: spruce, jack-pine, poplar, birch, balsam, tamarack, and cedar. Spruce, mostly of the black. variety, forms the principal stands and is suf- ficiently abundant in most of the region to warrant commercial exploit- ation. Jack-pine, poplar, and birch are locally abundant, especially on sandy hills.

Flowers and berries are rare. The only flower observed was a delicate wild rose growing in a few places along the shores of . Maicasagi lake. Wild strawberries grow on some of the bare, sandy slopes. They do not ripen until late July. Small patches of goose- berries were seen among alders on the banks of a few creeks. Blue- berries and raspberries are almost non-existant, although they were noted, locally in abundance, in the region lying west of the map-area.

Animal life is not abundant. Among the larger animals, only one bear and two moose were seen by members of the party throughout the summer. There are several beaver huts along the banks of Nomans river, but most of them were unoccupied. Hare and partridge are rare. Pike and pickerel are moderately abundant in Maicasagi lake and river.

GENERAL GEOIAGY

General Statement

Because of an extensive blanket. of drift, rock exposures, although more abundant than in the region lying to the west, are still rather restricted. in the map-area. The better exposed bodies are the intrusives, especially the diorite series and Nomans stock (see accompanying geological map). The lack of exposures is particularly deplor- able in.the pre-intrusive series, where the complex interfingerings of the volcanic and sedimentary rocks and their structure cannot be solved satisfactorily on the basis of the few isolated outcrops available for examination.:

All the consolidated rocks are of early Precambrian age. As inferred in the:preceding paragraph,. they are subdivided into two

-7 - groups separated by a period of intense folding. The rocks formed before folding are chiefly of volcanic and sedimentary origin. They underlie about 40 per cent of the area. The other group, younger than the first one, consists of intrusive rocks, most of which are marginal lobes of large bodies that cover extensive sections outside the area. Small satellitic bodies, younger than the main masses, were also noted. The several rock units are shown in what appears to be their proper sequence in the following table of formations:

Table of Formations

Cenozoic Till and lacustrine clays

Lamprophyre dykes

Dykes of granite, pegmatite and aplite, A genetically associated with the main R granitic bodies C H E Northern granite. Medium- to coarse-grained A quartz syenite with hornblende-biotite N schist facies (due to contamination)

Nomans stock. Fine- to medium-grained R gneissic biotite granite

E Southern granite. Fine- to medium-grained C hornblende biotite granite

A Medium- to coarse-grained, massive diorite M Foldin B R A Sills of gabbro and of porphyry R C Sedimentary rocks: mica and hornblende H schists, impure quartzite, chert E N A N Intermediate lavas: recrystallized andesite with some basalt - 8 -

VOLCANIC AND SgnIMENTARY ROCKS

General Statement

The main belt of volcanic and sedimentary rocks is about four and a half miles wide where it enters the northeast corner of the area. It crosses the map-area in a general westerly and south- westerly direction. Six miles west of the eastern boundary, the belt splits into two branches around the eastern apex of the roughly triangular body of Northern granite. The north branch trends northwest- ward, becoming gradually wider until it is more than seven miles wide where it crosses the northern border of the map-area. The south branch, trending southwestward, maintains a width of two to three miles for a distance of eight miles, beyond which it apparently widens, until it is about nine miles wide where it crosses the western boundary of the area.

The swampy area stretchin3 east-west near the southern border of the area is believed to be partly underlain by another branch of the volcanic-sedimentary series. This opinion is based on the presence of such rocks on the southeast and east sides of the granitic mass (Nomans stock) exposed in the south-central section of the area. Corroborative evidence is supplied by the direction of the schistosity of the lavas and of the gneissic structure along the border of the granite. Furthermore, most of the exposures of the Southern granite show a degree of contamination that would be difficult to explain if such rocks were not present along the northern boundary of this granite mass.

Because of the paucity of exposures throughout this belt of Keewatin-like rocks, particularly in Montviel township, it is impossible to present a complete picture of the relationship between the two main lithological units (volcanic and sedimentary rocks) within the confines of the area. The separation effected on the accompanying geological map is only tentative. The writer has no doubt that the interfingering of the two rock types is more complex than it is possible to visualize. Nevertheless, the presence of lavas in the majority of the exposures in the southwest quadrant of the map-area seems to justify the assumption that the western segment of the southernmost branch of the main belt consists predominantly of volcanic rocks. The northern branch — that underlying the north-central part of the region — contains many intercalated beds of sedimentary rocks. However, it, also, seems to be predominantly volcanic, and, in the absence of sufficient data, all attempts to separate the two rock types have proved unsuccessful.

It may be noted that the volcanics, while relatively more -9 - abundant in the series in the Maicasagi area than in the areas to the east, are apparently less plentiful than in the areas to the west. This feature, that is the relative abundance of distribution, suggests the possibility that there were more centres of volcanic activity to the west than to the east of the Maicasagi area.

Volcanic Rocks

The lavas are generally dark grey, fine-grained, and equigranular. Amygdules are of common occurrence, whereas pillows are. scarce. Because of the dirtyness of most of the exposures and of de- formation through folding, the thickness of the individual flows could never be determined with any degree of accuracy. In common with other sections of the same belt, these lavas are easily identified, either because they exhibit features typical of volcanic flow or because of their dark colour and uniformity of bedding over many feet, far exceeding the thickness of the sedimentary rocks of the region.

In thin section under the microscope, the majority of the specimens examined show a more or less well developed granoblastic texture. This texture is produced by the mosaic arrangement of hornblende grains in a mass of plagioclase. The hornblende grains range in size from 0.2 mm. to 1.0 mm. and they are more or less eauidimen- sional. Their sides, however, are commonly ragged and, furthermore, many of the larger grains•consist of a non-oriented assemblage of smaller ones. In a few places, this mosaic arrangement is oriented so as to produce a discernible schistosity.

The principal minerals of these lavas are hornblende and plagioclase. The hornblende is pleochroic in light-green shades and constitutes from 40 to 90 per cent of the rock. The feldspar, because of clouding, cannot be identified in all thin-sections. However, enough determinations have been made to indicate that oligoclase is the most common plagioclase. Andesine was found in one slide only.

Accessory minerals are quartz, magnetite, calcite, epidote, and chlorite. The first three are believed to have been introduced through hydrothermal activity, whereas epidote and chlorite appear to have formed by the alteration of the main constituents of the rock.

.The amygdules are generally of quartz and/or calcite, often enclosing small specks of chlorite. In two of the thin sections examined, however, the amygdules have a granitic appearance. They consist of an interlocking assemblage of quartz, feldspar, and slight quantities of hornblende and apatite.

It is considered likely that, at the time of their formation, the lavas, for the most part at least, were of two general - 10 - types - andesite and basalt. Due to metamorphism, however, the'. present minerals of these rocks are not original. Both the hornblende and the plagioclase are products of recrystallization. Because of the uncertainty concerning the original composition of these rocks, it is thought best to use a group name, intermediate lavas, rather - than attempt a petrographic classification based, for example, on the non- critical amount of secondary hornblende. ..furthermore, such a subdivision could not be used on the map because of the erratic distribution of the hornblende-rich and the hornblende-poor lava'flows.

The lavas described above may be considered as the typical • volcanic rock of the map-area. They are easily identified. In a few places, these lavas have undergone structural, and particularly textural, changes. These localities are: close to the contact with the Southern granite in the southwest section of the map-area; the curved triangular block south of Inconnu river near the eastern-border of the area; the inclusion confined between granitic and dioritic rocks in. the northwest sector of La Rouvillière township; and in the few exposures immediately north of that inclusion.

In all these exposures, the lavas are still fine-grained, although the size of the individual minerals is slightly larger than in the non-contaminated flows. While a certain linear structure is visible in most exposures of the rock, that structure results from the alignment of equant grains rather than from the presence of elongated grains. These contaminated rocks are almost everywhere gneissic. The gneissosity is produced by thin injections of feldspathic and granitic material within the lavas. The injections are general» parallel to the faint linear structure as seen on a horizontal surface, whereas the dips are often transgressive.

The composition of the contaminated facies is very similar to that of the typical lavas. The main effect of the intrusives was to cause a recrystallization of the principal minerals, hornblende and plagioclase. While tiffe overall proportions have not changed much, there is a pronounced segregation of minerals in dark and light- coloured bands. Quartz is always present and constitutes five to ten per cent of the rock.

One specimen taken from the block of lavas exposed in the northwest sector-of La Rouvillière township differs appreciably from the main body of the rock: it consists of a fine-grained, dark groundmass in which are set large, black, shiny metacrysts. The thin-section shows a basic pattern of roughly equigranular grains of feldspar and quartz surrounding, and included in; ragged masses of hornblende and of augite partially transformed into hornblende. Both the hornblende'and-the augite, because of the indefinite shape of their grains and the numerous poikilitic inclusions they contain, are believed to - 11 - be secondary (metamorphic). The development of augite was in all probability favoured by the'heat accompanying the cooling of the dioritic intrusive, since, not far south, similar lavas are injected by, and found as inclusions in, such rocks. The formation of horn- blonde is probably due to retrograde metamorphism. To the writer's knowledge, no metamorphic pyroxene has been observed previously in these rocks. The specimen studied would thus represent the highest metamorphic grade known to have been attained by the lavas of this northern belt. Sedimentary Books Sedimentary rocks have their widest distribution in the northeastern quadrant of the map-area. As it crosses the eastern boundary, the belt of these rocks trends southwest and is more than four miles wide. It narrows westward until, six miles from the boundary, it is less than two miles wide. At this locality, the belt is split into two segments by an east-pointing triangular area of lavas. The north branch- is exposed for a length of about five. miles, whereas the south segment, through widely separated but probably related exposures, continues westward for nearly twelve miles.

These rocks form the western end of a long sedimentary belt that extends eastward to:Opémisca lake, a distance of about forty miles. In the east, between La Trève and Opémisca lakes, the sedimentary rocks are believed by Beach (2) and Norman (13) to form two series, Pre-Opémisca and Opémisca, separated from one another by an unconformity. There is no basis for such a subdivision in the Maicasagi area and, accordingly, all the sedimentary rocks are considered to be of the same age, probably interbedded with the lavas.

About one-third of all the exposures seen on the north shore of Maicasagi river are of sedimentary rocks. As mentioned before, it is not possible to define,-within this volcanic-sedimentary belt, any sedimentary pattern of structural constancy or significance. The same is true of the few beds - which probably represent local sites of sedimentation - that are exposed en the volcanic hills a mile and a half south of post 27, on the southern survey line.

. The most abundant detrital rock is a light-grey type that shows good bedding, due to slight colour variations rather than to differences in grain size. Dark minerals form 10 to 15 per cent of these rocks and consist almost exclusively of mica (biotite with subordinate quantities of muscovite). As could be-expected, the beds .that are richer in mica are distinctly schistose. When present, hornblende does not exceed 5 per cent of the total mineral composition.

The light-coloured minerals, quartz and feldspar, constitute from 80 to 90 per cent of the rock. Throughout the area, the relative - 12 - percentages of these two minerals vary widely: quartz from 15 to 70 per cent, feldspar from 75 to 25 per cent. There is no definite trend in these changes, although it seems that feldspar is more abundant in the eastern part of the belt. The common accessories of these mica schists are apatite, epidote, pyrite, magnetite, and hematite.

The next most abundant sedimentary type is a dark (often black) hornblende schist that has a gritty feel and a grain coarser than the light-coloured mica schists. Thicknesses of individual beds are from one-quarter of an inch to many inches. In some outcrops, because of uniformity in colour and grain size, no bedding was seen over widths of many feet.

These dark schists are characterized by a relative abundance of hornblende, which is the main dark mineral and forms 25 per cent to, in certain bands, 80 per cent of the rock. Quartz and feldspar are the only other important constituents. Both are fine-grained and clear, and the feldspar grains do not display any twinning. Thus, in thin section, they appear similar and it is difficult or impossible to estimate the relative amount of each present in the rock. Almost everywhere, the schist contains garnet, though in very minor amount. The red grains of the mineral occur singly and also in clusters up to more than half an inch in diameter. They are particularly abundant in the narrow beds intercalated within the lavas north of Maicasagi river.

A third type of rock of sedimentary origin is a glassy, light-coloured, impure quartzite. Generally, this occurs as narrow beds (rarely exceeding two inches) interbedded with mica schists. Because it is more resistant to weathering than the schists, the quartzite often stands up a fraction of an inch above the general surface, which then takes on a washboard appearance.

The quartzite is more massive than the other sedimentary types. It consists essentially of quartz (60 to 70 per cent) and feldspar (25 to 30 per cent). Dark minerals are biotite, muscovite, garnet, chlorite, and epidote. Generally the dark minerals are concentrated in streaks across the light-coloured rock.

Lenses of chert were observed in a few localities. They, are seldom more than a few feet in length and they commonly show evidence of intense fracturing.

Concordant Intrusives Within the Complex

Associated with the volcanic rocks and, to a lesser extent, with the sedimentary rocks of the map-area are numerous concordant, sill-like, tabular masses. These sills, some basic and others acidic, -13- are believed to have been emplaced before the folding of the volcanic- sedimentary complex. They may be genetically affiliated with the lavas, although doubt may be expressed concerning this association, especially as regards the acid sills.

Basic Sills

The rocks composing these sills are of gabbroic composition. They are dark, fine to medium grained, and generally equigranular. In view of their similarity to the basic flows, it is probable that different observers would arrive at different conclusions concerning the nature of some exposures. Since practically no criteria were of any use to distinguish certainly between sills and flows, the writer chose to call 'sills' only those concordant masses which had a grain size larger than the lavas and presented no features typical of flow, e.5., pillows, amygdules, flow lines, etc.

The structure of the sills varies considerably from almost massive to higher schistose. In composition, the rocks consist essentially of amphiboles (80-90 per cent) and feldspars (10-20 per cent). l.ccessory minerals are chlorite, sphene, calcite, pyrite, and probably a little quartz in some specimens. Both actinolite and hornblende have been observed in some thin sections. In all cases, the amphibole grains are ragged, and discoloured cores, probably re- presenting remnants of pyroxene, were seen in one slide. The feldspars are usually extensively saussuritized.. In one section, a plagioclase of calcity An9s was determined.

Acid Sills

A few light-coloured sills were found in the volcanic and sedimentary assemblage south and north of Maicasagi.river, in the north-central section of the area. These sills are fine-grained, generally porphyritic rocks, composed mainly of feldspar phenocrysts in a groundmass of quartz, feldspar, mica, chlerite, and epidote. In both phenocrysts and groundmass, the plagioclase is oligoclase (about AnI2). It is in part replaced by later microcline.

According to their composition, these sills could be interpreted as offshoots of the main granitic masses and would thus be much younger than the volcanic series. On the other hand, two observations have prompted placing them in the group of pre-folding rocks. First, the phenocrysts they contain are broken into fragments, in contrast with the well-preserved state of the phenocrysts in the Northern granite. Second, an intrusive breccia on the south shore of Maicasagi river contains fragments that are similar to a nearby acid sill. The matrix of the breccia is a diorite which is believed to have been injected into rocks that had been previously crushed through drag- -14 - folding. This problem will be discussed at greater length in the section on satellitic rocks...

POST-FOLDING INTRUSIVE ROCKS

Intrusive rocks underlie about 60 per cent of the map-area. They form lenticular masses that are generally concordant with the structure of.the•volcanic-sedimentary complex. Geographically, there are six: independent bodies. However, with respect.to their composition, the intrusive -rocks are divisible into two groups: one of an intermediate, and one of an acidic, composition. The intermediate. rocks are diorites.. The acidic rocks are plagioclase granites similar to the common granites of this Waswanipi-Chibougamaubelt. They are herein described under local names: Northern granite, Nomans stock, .and Southern granite.

Satellitic rocks include offshoots of the main granitic masses, lamprophyre dykes,.and an intrusive - breccia. These small bodies 'are of more than one age, although they are all post-folding. The lamprophyres are .the youngest rocks of the area, whereas the:age of the breccia cannot be accurately established.

Diorite Group

:Intrusive rocks of this group occupy three sections of the area. The. largest body underlies about 35 square miles in the.southern part of Monseignat township and the northern part of La Rouvillière. It represents the western half of an elliptical mass, the other half of which has been mapped by Gilbert (8) in the adjoining Capisisit Lake area to the east. The other two bodies are much smaller. One, also elliptical, but measuring only about five square miles, lies near the southeast corner of the map-area and is separated from the larger body to the north by a narrow tongue of volcanic rocks. The third-mass underlies the mirth-central part of Monseignat township. It continues northward: beyond the map-area to form what is probably a:small stock.

The rocks of this series are characterized by a massive appearance and holocrystalline texture. They are medium to coarse grained. .Their colour varies with the relative proportions of the light and dark minerals. The - latter constitute from 20 to 60 per cent of the rocks, the remainder being almost wholly feldspar.

- 15 -

Composition Table

Plagioclase 40-75% Amphibole 15-55J Biotite ' Acc-lOJ Pyroxene 0-20;b Olivine (one section only) -10% Chlorite - 'Acc. 'Potassic-feldspar 0-10%' . 'Quartz . 0-10% Common accessories:'apatite, sphene, iron ' oxides, epidote, calcite

On the basis of the colour of the'feldspar grains as seen in hand specimens, it is possible to subdivide the diorites into three groups: one with pink, one with grey, and one with mauve feldspars. This subdivision, however, had to be abandoned as a result of microscopic studies. Irrespective of the colour of the grains, the calcity of the plagioclase is remarkably. uniform. In all but two of the sections examined, the plagioclase is oligoclase of composition An27- 29• The two exceptions were a pinkish feldspar, which was determined as andesine, Ans5, and a grey feldspar of still higher calcity (An45). It appears that the diversity of shade as seen in hand specimens is due, not to a difference in calcity, but to extra- neous colouring agents, the nature, of which could not be determined.

The plagioclase crystals are slightly zoned and show albite, carlsbad, and pericline twinning. They are well-shaped, although their ends are, in many cases, rounded, either as a result of,inter- ference during growth or by subsequent attrition. The grains do not contain any inclusions of earlier-formed minerals. They are. generally fresh and seldom more than 50 per cent clouded. Ali these,observ- ationspoint toward a primary origin for the plagioclase. However, from considerations to be given later, it might well be concluded that the plagioclase is secondary.

In addition to plagioclase, most of these rocks contain a certain quantity of potassic feldspar in the form of grid-twinned microcline. Microcline is common as small specks within oligoelase or as narrow irregular streaks bordering the larger plagioclase grains. Usually it is present in small amount only, but it forms 10 per cent of two of the thin sections examined. In these, it occurs as large subhedral grains associated with plagioclase, and sends into the latter small salients which demonstrate that the microcline is later than the plagioclase, and replacing it.

Quartz was noted in most of the sections but. in none of -16 - them does it exceed 5 per cent. Its late origin is shown by its constant association with microcline, with which it forms a myrmek- itic intergrowth. It also injects in vermicular fashion some grains of plagioclase in places where the latter is in contact with microcline. Furthermore, it is seen to penetrate and partly replace hornblende, biotite, and chlorite.

The common amphibole is hornblende of the light green variety. Its pleochroic formula is: Z, light bluish-green; Y, pale olive-green; X, straw-yellow. The presence of soda amphibole, probably derived from hornblende, was noted in two sections. In one of these, this amphibole appears to be blue-violet riebeckite. That in the other does not correspond to any mineral described in the liter- ature. Its pleochroic formula is: Z, greenish-blue; Y, brownish-purple; X, greenish-white. It has a negative elongation, the fast ray making an angle of 6° to 9° with the ç-crystallographic axis. The birefringence is about 0.020 and the dispersion (r») is strong. The value of 2V could not be measured, but its sign is negative. These properties place the mineral in the group of soda amphiboles, probably between riebeckite (which has low birefringence) and glaucophane (which has positive elongation).

Hornblende is secondary. The majority of the grains lack the well developed terminal faces that are characteristic of crystallization during the consolidation of a magma. Their ends are ragged, rounded, or pointed and, in many instances, they terminate in irregular shreds that penetrate into the adjoining minerals, including plagioclase. Furthermore, many of the grains have a core that is somewhat paler than the rim. Most of these discoloured cores are peppered with specks of magnetite.

The source of the hornblende seems to have been pyroxene. In- deed, remnants of diallage were identified in four thin sections. The diallage has a creamy tint, good cleavage, and pronounced (010) parting. It occurs in grains which vary in size from'specks to 1.5 mm. Most of the grains are partly or wholly surrounded by uralite. These four sections are believed to represent those parts of the mass that have undergone less than average transformation. From the erratic distribution of the pyroxene-bearing specimens, it is concluded that all of the hornblende was probably derived from pyroxene and that, as a con- sequence, the original composition of the rock was more basic than at present.

Olivine was noted in the one of the four above-mentioned sections that contains the largest grains of diallage. It occurs as small rounded grains completely enclosed within diallage. The para- genetic order of the dark minerals in this specimen is as follows: olivine altered to diallage, which in turn is altered to' uralite. Since -1'7- olivine was seen in only one of the thirteen sections examined, its significance may only be surmised. Considering the instability-of olivine, its preservation in one specimen may be an indication that the mineral was originally quite widespread.

After olivine, pyroxene, and amphibole, the next dark minerals in order.of appearance were chlorite and biotite. Chlorite, often accompanied by epidote, occurs sparsely in these rocks. It is always associated with hornblende or biotite. It is clearly a secondary mineral, although it is not possible to trace its origin definitely to either biotite or hornblende.

The secondary origin of biotite from hornblende is well demonstrated. Biotite is seen in contact with hornblende in all stages of growth. In places it occurs as tiny spots or narrow streaks within and along the border of the hornblende grains. From that stage, there are many transitions to flakes of biotite pseudomorphic after hornblende, particularly well seen in basal sections of that mineral, in which the characteristic cleavages are well preserved.

The present mineral composition of the dioritic rocks is believed to be anomalous. From the study of the dark-coloured minerals, it seems that most, if not all, of the rocks originally contained a pyroxene and, in some cases, olivine. These early crystallizing minerals have been subsequently changed into amphibole, which was later partially altered to biotite and chlorite, accompanied by epidote.

On the other hand, the only important light-coloured mineral of the rocks is a plagioclase close to the oligoclase-andesine boundary, which gives no definite indications of having formed through the alteration of an originally more calcic plagioclase. It is true that some cores or other sections of many grains are saussuritized, but the alteration is not widespread enough to indicate a complete change in the calcity of the plagioclase.

It.thus seems probable that, early in its history, the rock has undergone changes which led to a.complete transformation of the original plagioclase into a more sodic one, while the dark minerals . were affected to a lesser degree. The transformation must have been accomplished through molecular replacement, since the shapes of the. early crystals have been preserved. The original rock may have been a pyroxene gabbro and parts of it probably contained olivine. The present composition, however, is generally that of laugenite, i.e., oligoclase diorite.

Gilbert (8) has observed gradations from gabbro to diorite in the eastern half of the largest of the three intrusive bodies under discussion. Similar gradations were not seen within the confines of -18 - this area, but the 'writer has no doubt that a greater:number of exposures would, through more complete field observations and microscopic studies, reveal a lithological sequence similar-to that described by Gilbert in the Capisisit Lake area. Furthermore, these' rocks are very similar in appearance and composition to the diorite- syenite sequence exposed in the region west of the map-area, particularly between Mattagami and-Olga lakes (10). All these different, bodies•of intermediate rocks are believed to.be genetically associated. They probably belong.to-one•cycle of intrusion, a cycle that would' have followed immediately after the folding of the volcanic and sedimentary complex.

Northern Granite

The Northern granite belongs to'a wide expanse of granite' that extends west, north, and northeast of the Maicàsagi area, and that has been named 'Mattagami gneiss' by Freeman'(7). Within the map- area, it underlies a roughly-triangular section which is more than six miles wide at the western border and which extends eastward across the central part of Urfé township.

The typical facies observed elsewhere is rare within this area. It has been seen in only a few localities, mainly south of Maicasagi lake.' That facies is a light-coloured, pinkish-grey rock with an interlocking texture and a structure that varies from almost massive to'distinctly gneissic. Its-composition is:

Plagioclase (Anio-10) .55-70% quartz 20-40% - Microcline Ace.- 5% Biotite / 5ô Hornblende Accessory minerals: epidote, chlorite, calcite, magnetite, hematite, pyrite.

The plagioclase grains are subhedral, quartz is interstitial, and the rock shows a slight development of a cataclastic -texture: the grains of feldspar are crushed along their borders while the quartz grains are broken into many fragments. 'Microcline has been introduced after the development of that texture, since it penetrates and replaces not only the crystals of plagioclase but their granulated borders as well.

However, the dominant representative of the Northern granite in the present map-area is coarser and much darker than the type described above. Most of the outcrops could be classified between quartz .syenite and diorite. This is particularly true of the exposures on the'northeast shore of Maicasagi lake and of the -inland exposures to -19- the east. Most of these exposures consist of two.kinds of rock, intrusive material and inclusions. The intruded rock,. a fine-grained hornblende-biotite schist, occurs in chunks, in short lenses, or in bands that maintain their width over scores offeet. 'Where these bands are; narrow and alternate with layers of granite of comparable_ thickness, the outcrop has a strong banded appearance.

; The key to the nature and origin of the dark intrusive material is given in three widely.separated exposures. One is on the east shore of the lake, about 3,000 feet southeast of-the northernmost island; the second is about J7,000 feet inland, almost due. east of . the first one; the . thirdis about one mile south of Maicasagi river, -immediately east of_the wide creek that empties into the mouth of the river.

The first exposure is the most enlightening. .,It shows, near the water, a band of dark, fine-grained, hornblende-biotite schist striking southeast and dipping steeply eastward.- On the east, this schist is in contact with a granitic:intrusive, which,.-about five feet away from the contact, is pink, medium-grained, and contains less than 10 per cent dark minerals. Its feldspar (probably Ans ) occurs as. irregular clouded masses, although-occasionally a subhedral crystal stands out and gives the rock a slight porphyritic appearance. Inter- stitial quartz constitutes about 25 per cent of the rock. The only. dark mineral is an almost completely chioritized biotite.

From that point, the intrusive, becomes darker toward the contact. The relative proportion of dark minerals. and.the quantity of phenocrysts increase progressively until, near the schist, the granite has a strongly porphyritic appearance and contains about 25 per cent dark minerals. Examination of a thin section indicates that the plagioclase, partly clouded, is:more calcic (An15)than that in the less:contaminated_ facies. In this section, also, biotite is the only. dark mineral and it is almost completely chioritized. Quartz is. still interstitial,.but constitutes no more than 15 per cent of the rock..

Similar,phenomena, viz, increases in dark minerals and in ,. phenocrysts, mere observed in thin sections of specimens from the other two localities mentioned.above.

Still darker facies have been seen in a few places. These are; truly.dioritic in appearance and contain up to 40 per cent dark minerals, among which hornblende predominates. Their feldspar, however, is not more calcic than Anlef and quartz is present in amounts up, to 25 per cent.. Hornblende occurs.as ragged grains distributed amongst the light-coloured minerals, but in.a few sections small-grains are present as inclusions within the subhedral crystals of plagioclase. These inclusions are believed to be poikilitic, and would thus mean - 20 - that the hornblende was solid while the plagioclase grains were in' the process of developing.

Although the darker intrusive facies have not been seen to grade into light granite, there is a strong suggestion that they represent the highest stage of contamination of the Northern granite. The presence of hornblende in lieu of biotite is compatible with the present beliefs on the behaviour of inclusions. Away from the inclusion, the assimilated material was restricted and the dark minerals had the opportunity to adjust themselves to the new physical'and chemi- cal environment. They were changed into biotite, which was the stable phase for the granite. Nearer the inclusion, on the other hand, the intruded rock was present in increasing quantities, while the magma not only was less abundant, but cooled faster. Under such conditions, hornblende escaped transformation and underwent only a process of recrystallization.

The Northern granite is thus a highly contaminated mass. Its age with respect to the other main formations is not completely known. Observations in the field show only that the granite is younger than the volcanic-sedimentary rocks. From cutting relationships and Other evidence seen in the Goéland lake area (10), it seems permissible to conclude that the granite body of which the Northern granite forms a part was intruded after the consolidation of the diorite group. Rocks of the diorite group are cut by numerous satellitic bodies and dykes of leucocratic grey or pink granite, pegmatite, and some aplites, but no evidence was found in the field to indicate their possible relationship to any of the main intrusive bodies of the area.

Nomans Stock

The name "Nomans stock" has been given to the granitic mass occupying the south-central part of the map-area, astride the boundary between Montviel and La Rouvillière townships. Exposures are numerous in the eastern half of the mass, where the country is hilly. The western half, however, is extensively covered and much doubt arises as to the location of the margin of the mass in that direction. Similarly, the swampy stretch near the southern border of the area renders it impossible to determine whether or not the postulated stock is connected at the bed-rock surface with the Southern granite. For reasons given earlier in this report, the writer inclines to the view that the two granitic masses are separated by a narrow belt of volcanic and sedimentary rocks.

Nomans stock consists of a grey, medium-grained; gneissic biotite'granite. It is very similar in composition and appearance to what- has been described above as the typical facies of the Northern granite: -21-

Plagioclase 50-70% Quartz 20-35% Biotite 1 10-25ô Hornblende Accessory minerals: epidote, apatite, chlorite, microcline, calcite.

The plagioclase is an oligoclase, which is slightly zoned from An20 at the core to An12 along some of the rims. As in.the other granitic masses of the region, the grains of oligoclase are subhedral and to some extent clouded. Biotite is much more abundant and more widely distributed than hornblende, and the latter is always partially altered to biotite. These two minerals are, in turn, locally chloritized.

Inclusions are common within the granite. Two of them are large enough to be shown on the accompanying map, but for the most part they measure only a few square feet, or even inches. Their shapes are angular, or lenticular, or thinly tabular. These inclusions are hornblende schist, commonly containing 50 to 70 per cent dark minerals, with the rest almost exclusively feldspar. In texture and colour, they resemble the contact phases of the volcanic rocks described earlier in this report.

The schistosity of the inclusions is invariably parallel to the gneissic structure of the granite surrounding them. In many places, the contact between schist and granite is perfectly sharp. Some contacts, however, are marked by narrow transitional zones in which schlieren are clearly visible. The schlieren are marked by ir- regularly disposed clots of biotite of varying dimensions: length from half an inch to one foot, width from a quarter of an inch to one inch. The schlieren form a transitional stage from a core (inclusion) rich in hornblende with no visible biotite, to a leucocratic granite with very little or no dark mineral content.

Banded gneisses are another feature of Nomans stock. The best exposures of these gneisses are 4,500 feet northeast of post 19 of the southern survey line, and along the southern contact of the stock, south of the same mile-post. The gneiss consists of alter- nating bands of schistose hornblende rock and of pinkish leucocratic granite that contains no more than 5 per cent biotite. The bands do not exceed one inch in width. It will be observed that both localities are close to areas of older rocks that are large enough to be shown on the.accompanying map. The possibility thus presents itself that the banded gneisses represent a transition on a large scale between the intruded lavas and the gneissic biotite granite. The wide distribution of the inclusions, and the relative abundance of schlieren -22- and banded gneisses, are believed to indicate that the present surface represents very closely the top of the intrusive stock. Had erosion been slightly less extensive, it is probable that a considerable part of the stock would still be covered by remnants or older rocks.

A striking difference between the stock and the Northern granite is the absence in the former of well-developed phenocrysts such as characterize the Northern granite. A partial explanation of that difference may lie in the relative abundance of pegmatites in the two bodies. Pegmatites are rare in the north, whereas, in Nomans stock, numerous dykes and irregular masses have been noted. If, as is commonly believed,.volatiles are influential in the development of large crystals, it is conceivable that, in Nomans stock, the rest-magma was isolated from the crystallizing mass and it concentrated to form the numerous pegmatitic bodies. In the Northern granite, the rest- magma could have been more intimately mixed with the mass and so could have kept it fluid long enough to allow the formation of large crystals.

Nomans stock is younger than both the volcanic-sedimentary complex and the diorite group. Its age with respect to the other granitic masses, however, is unknown. The similarity between the purer types in the Northern granite and Nomans stock may be an indication of a genetic relationship.

Southern Granite

Consequent upon the hypotheses suggested earlier in this report concerning the probable existence of a volcanic belt near the southern limit of the map-area, the scattered granitic exposures south of the postulated branch would not belong to Nomans stock. They would form the northern section of a large granitic mass which,. southeast and east of the area, has been called the "Waswanipi granite",. _ In the area immediately south and southwest, the mass has been referred to as the "pink, massive biotite granite".

The only exposure of pink, massive biotite granite within the present map-area lies 1,600 feet north of the southern boundary and 2,000 feet east of the western limit of Montviel township. This rock is fine-grained, equigranular, and has the following composition:

Plagioclase (Anil) 45% Microcline 20%

Quartz .30% •. Biotite 5%

The plagioclase crystals are subhedral and_they.are cemented by irregular masses of strained quartz: Microoline is later than plagioclase and quartz since it replaces these minerals'and its larger -23- grains contain rounded inclusions of either or both of them. Biotite is brown and dotted with zircons surrounded by pleochroic haloes. It is slightly altered to muscovite.

From that exposure, the granite becomes progressively darker northward and westward in the direction of the contact with the volcanic rocks. A representative transitional stage is seen on the two exposures in the extreme southwest corner of the area. This rock is a medium-grained, pink gneissic granite. Its mineral composition is:

Plagioclase 70% Quartz lz% Microcline 3% Biotite 15%' Accessory minerals: epidote, sphene, apatite.

The calcity of the plagioclase as determined in numerous grains in the thin sections examined ranges from Anlo to Ani9, in- dicating lack of equilibrium. Some of the grains are of appreciably larger size than the average and give the rock a faintly porphyritic appearance. The biotite has pale to deep green pleochroism, and is accompanied by epidote.

The darkest facies are found in the three scattered exposures immediately south of the contact across Montviel township. In those localities, the rocks contain such a high percentage of dark minerals that they have the appearance of a diorite. They are medium- grained and, as seen megascopically, they consist essentially of feldspar phenocrysts surrounded by dark minerals. The composition of a typical specimen is:

Plagioclase 60% Microcline 5% Quartz 10% 'Biotite 15% Hornblende 10% Accessory minerals: sphene, epidote, apatite.

The plagioclase grains are rather well shaped and show slight granulation along their borders. They are oligoclase of composition•Anl4 jwhich, it may be noted,. is intermediate between AnSrof the massive fades and An of the transition stage. Hornblende occurs as trains of grains, all of which are smaller than those of the feldspar or biotite. Biotite occurs as streaks'along the -24 - cleavages of hornblende, as small clusters, and as large flakes containing inclusions of hornblende, sphene, and quartz.

These contact facies are similar. in appearance to the major portion of the "granitic" material constituting the Northern granite described above. Both granites are believed to have originated through a more or less complete assimilation of hornblendic rocks by a biotite-rich magma. Where the intruded rocks were in relatively small volume, their. hornblende was thoroughly digested and changed into biotite. Nearer the contact, because of the abundance of intruded material, the magma could not absorb all the hornblende, which was thus partially preserved and partially transformed into biotite.

Near the northwest corner of the Waswanipi Lake area (West Half), Claveau (5) has mapped the following rock sequence from the shores of Goéland lake eastward: a band of sedimentary rocks, a zone of biotite-hornblende granite, and a wide area of pink, massive biotite granite. He considers this last to be younger than the biotite- hornblende granite. The evidence, both field and microscopic, gathered by the present writer in the Maicasagi area makes it unadvisable to follow Claveau's subdivisions. As explained above, in this area the biotite-hornblende granite is considered as a contaminated border facies of the main intrusive mass, which is the pink, massive biotite granite. The gneissic character of the granite is probably original in the sense that it was not developed through post- consolidation stresses. However, this character is believed to be inherited, that is, it was imposed upon the granite by the previously- folded rocks which were partially digested in the crystallizing magma.

Information concerning the age of the Southern granite is just as meagre as for other members of the acid group. Each of the three main bodies is younger than the diorite group, but their mutual relationships remain a matter of conjecture. Indeed, it is doubtful whether correlation criteria will ever be found that will permit establishing an age sequence for these granitic. masses. The only bases presently available for their comparison are. mineralogical composition and structural character. These two. criteria, according to an hypothesis developed elsewhere (11), would favour the opinion that all the granitic masses, not only of the Maicasagi area but of the whole Waswanipi-Chibougamau region, belong to one and the same period of magmatic activity. This hypothesis suggests that all the separate granite masses represent cupolas of a single, extensive batholith, in which the volcanic belts form huge roof-pendants. The dissimilarities between the various granites of the Waswanipi-Chibougamau belt would represent different elevations in the cupolas. Differences in erosion cannot be considered as a possible factor, since all the masses have been planed down to about the same level. The features.of the gneissic -25 - Northern granite and Nomans stock would be those found near the top of the intrusive, where the.magma would still be in contact with, and would be contaminated by, the invaded rock. The general absence of inclusions within the Southern granite would mean that this cupola had reached a higher level than the other two. Its present eroded surface would represent a deeper level where, except along the sides,' remnants of invaded rocks would not be present. Differences in composition and gneissic character are considered as resulting from differences in the degree of contamination of an originally quite uniform leucocratic, massive, biotite-oligoclase granite.

SATELLITIC INTRIISIVES

The dyke rocks of known age belong to two main groups, granites and lamprophyres.

Granitic Dykes

The granitic dykes include all the satellitic bodies related to the main acid masses. They are found cutting all the rock groups of the map-area. They are pink to grey, and consist essentially of feldspars and quartz, with subordinate quantities of dark minerals. According to their texture, they may be subdivided into fine-grained, equigranular, interlocking, pegmatitic, and aplitic.

The fine-grained, equigranular granite dykes are the most abundant: They arQ found in all parts of the map-area. They are so similar in appearance and composition to the normal biotite granite that no special description of them is warranted. They do not display any characteristics that would help in establishing the relative ages of the several granite masses.

Pegmatites are widely distributed. They are found in bodies of many forms and sizes: splashes, stringers, tabular masses, and irregular bodies that measure many hundreds of square feet. Exposures large enough to be shown separately on the accompanying map are to be seen in the southern part of Nomans stock and the western section of the Southern granite. The pegmatites consist generally of a pink and a white feldspar, quartz, and sporadic books of. mica. They do not contain any discernable rare minerals.

One pegmatite dyke was found to be of particular interest. An exposure just south of post 20 on the southern east-west survey line consists of gneissic granite in which the biotite has been almost completely chloritized. The gneissic structure strikes N.500E. and dips northwest at the unusually low angle of 20°. The gneiss is cut by a pegmatite dyke striking N.85°W. and dipping south at 80°. On the -26 - north side of the pegmatite, immediately at the contact, the foliated structure of the gneiss has been bent so that it is no longer horizontal (as seen on a vertical plane parallel to the gneissic structure), but is concave upward. This disturbance is an indication that the pegmatite has been forcefully intruded upward into the gneiss. The dyke, which contains large crystals of pink feldspar and blebs of quartz up to two inches by four inches, shows no sign of strain. It seems that this pegmatite was introduced shortly after the emplacement of the gneiss, at a time when the latter had not yet completely solid- ified.

The name aplite has been given to fine-grained leucocratic dykes that possess a granular, sugary texture. They are massive, and either grey or pink. The grey aplites are generally found in Nomans stock, whereas the pink facies is particularly. conspicuous on the shores of Maicasagi lake, where it cuts the Northern granite.

The grey facies has the following composition:

Microcline 75% Plagioclase 4% Quartz 15% Biotite 1%

The microcline is abundantly grid-twinned. It is fresh and many of the grains contain cores of clouded plagioclase, probably An,o, and rounded inclusions of quartz.

In the specimen of pink aplite studied in thin-section, microcline still replaces albite, but it is much less abundant:

Albite 60% Microcline 20% Quartz 15% Hornblende 5%

An important feature of the pink aplite is the presence of fluorite and sphene as the main accessory minerals. Fluorite occurs • as irregular streaks and masses between other minerals and, in a few instances, it seems to have filled voids in the rock. To the writer's knowledge, the only granitic body of this general region that bears evidence of the presence of numerous volatiles is the Goéland batholith underlying the greater part of Goéland lake (10). The pink aplite may thus be related to this batholith. In that case, the Goéland batholith would be younger than the Northern granite, a hypothesis that has already been advanced on the basis of textural evidence. - 27

hamprophyre Dykes

The lamprophyres are the youngest known rocks of the map- area. They intrude both the Northern granite and Nomans stock. They are dark and fine-grained, although generally porphyritic. Biotite, red garnet, and clouded feldspars partly surrounded by garnet and augite, are the common phenocrysts. They range up to three milli- meters in size and are set in a groundmass that is black and consists of biotite, hornblende, garnet, feldspars, magnetite, hematite, and possibly quartz in some specimens.

Diorite Breccia

A puzzling group of exposures was found on the south shore of Maicasagi river, at a point a mile and a quarter east of Urfé township. On the east side of the pronounced bend of the river, vegetation extends close to the water's edge, leaving little opportunity for a complete study of the bed-rock. In numerous restricted places, however, a breccia was seen that consists of a dark matrix enclosing within it fragments of all shapes and up to two square inches in size.

The fragments are exceedingly fine-grained, almost aphanitic. The majority of them are of a feldspar porphyry similar to the acid sills described earlier in this report, although the phenocrysts are generally much smaller than those in the sills. Some fragments are of quartz porphyry, while others contain tiny, round specks of clear quartz that could be either phénocrysts or late quartz 'eyes'.

The matrix of the breccia is a diorite not unlike the volcanic rocks that outcrop nearby. Its composition is essentially hornblende (60-70 per cent) and a completely clouded feldspar. The intrusive nature of the matrix is shown by the fact that small tongues of it penetrate into many of the fragments. This field observation is corroborated under the microscope, where the borders of the inclusions are seen to be highly corroded and many fragments contain local concentrations of hornblende similar to that of the matrix.

Since the contacts of this breccia could not be accurately determined, and in the absence of clear cutting relationships between it and the main rock groups, it is impossible to place the rock in its proper place within the geological column, or to explain its mode of formation. There is evidence of drag folding in this locality, as shown-by abrupt changes in the direction of. the schistosity. The breccia might be related to such secondary folds. Intense drag folding might have caused the crumpling of brittle porphyries along the nose of the fold, thus creating the fragments and, at the same time, providing the channel for the emplacement of the diorite. If this hypothesis -28 - proved true, the fragments would be pre-folding, whereas the matrix would be either post-, or preferably, late- folding in age.

STRUCTURAL GEOLOGY

Folding

The rocks of the volcanic-sedimentary series are intensely folded. Schistosity is .everywhere developed and, except for the devi- ations that are related to the nearby.intrusives, it trends roughly east-west. From numerous observations of bedding, both in the volcanic and in the sedimentary rocks, it is concluded that the bedding and the schistosity are parallel. The beds dip steeply and, in many places, are vertical, even overturned.

The outcrop pattern of the various rock groups, including the main intrusive masses, may well be the expression of a series of plunging folds within the area. Because of the paucity of structural data, the precise nature and attitude of the folds may only be con- jectured, but some ideas may be advanced on the basis of observations both within and outside this area.

West of the map-area, it has been suggested (10) that the volcanic and sedimentary rocks between Olga and Goéland lakes are folded into a huge east-trending syncline plunging eastward at a • rather steep angle (possibly 600). If the axis of this postulated syncline were projected eastward, it mould cross the present area about two miles north of the southern east-west survey line. The soundness of such an extension lies in the fact that, in the Capisisit region (8), the plunge of the folds is still toward the east. There is no reason to believe that the structure is reversed in this area and, therefore, a logical conclusion is that the east-trending syncline is a major structural feature of the volcanic-sedimentary belt, and that it extends for a considerable distance east of Olga lake.

Because the original characteristics of the volcanic and sedimentary rocks have been almost completely obliterated, no positive determinations could be made of the attitudes of the beds and layers in the area. In two places, however, it was possible to make observations concerning the direction in which the beds face, and these, if reliable, would indicate the presence of another fold axis.

The first locality is about four miles east of the western boundary of the area, immediately south of mile posts 33 and 34 of the surveyed line. between Urfé and Montviel townships. Remnants of pillows here show faint curvatures that suggest that the tops of the beds.face toward the south. As the schistosity dips northward, and -29- as schistosity generally parallels bedding, it would seem that these layers are overturned. The volcanic belt of which these pillowed flows are a member terminates eastward in a rather sharp nose that forms a deep salient into a zone of sedimentary rocks. This pattern is definitely that of a plunging fold and, since the pillowéd flows lie on the southern half of the belt, it is probable that these volcanic rocks are folded into an anticline, slightly overturned to the south and plunging eastward.

The second locality is a little over one mile southwest of post 27 on the southern east-west survey line. Grain gradation in two narrow layers in a series of sedimentary beds indicated that the tops of the beds are toward the south. An anticlinal axis would thus lie somewhere north of these beds and, since the dips are also toward the south, the anticline would be in normal position.

It is obviously not possible, solely on the basis of the above observations, to give a complete interpretation of the structure of these highly folded rocks. The available data, however, suggest that within the map-area the volcanic and sedimentary rocks are folded into an easterly-plunging syncline flanked on each side by an anticline, that on one side being slightly overturned.

Faulting and Shearing

No major fault or zone of shearing was encountered in the area. Many small hills terminate in steep escarpments, but in most cases the cliffs may be correlated with planes of jointing. The most conspicuous topographic depression is the valley of the creek that crosses the southern survey line, just east of post 20, to empty into Inconnu river where the latter changes from a west to a north course. The cliffs bordering that valley attain an elevation of more than 100 feet, and some of them are very steep. No indication of shearing, however, has been observed in or along the valley, and the cliffs are interpreted as marking the sides of a- valley-in which widening through normal rivererosion was interrupted by the onset of Pleisto- cene glaciation.

Small faults were observed on the north shore of Maicasagi lake. They strike from N.80°E. to S.50°E., and in all cases the north block has moved west with respect to the south block. The apparent displacement is from one foot to a maximum of three feet. Evidence of stronger movement is seen on the south bank of-Nomans river in the southeast corner of Urfé township, about 1,000 feet north of mile post 39 of the nearby survey. line. Grooves, half an inch deep, follow the plane of schistosity of the rocks and plunge at 350 to the southwest. The direction and amplitude of movement, however, -30 - could not be determined.

Of the few shear zones observed, the main ones are: south of Nomans river, about 3,700 feet southwest of post 39 of the northern survey line; at two places along the south branch of Inconnu. river, just inside the eastern limit of the map-area; two and three- quarter miles north and 2,000.feet east of post 40 of the northern survey line; and 2,300 feet east of post 33, and 1,000 feet east of post 36, of the same line. This last shear zone is in rather massive, fine-grained gabbro, whereas the others are all in volcanic rocks. The two zones along the south branch of-Inconnu river may mark the southwest prolongation of the extensive zone of shearing observed by Gilbert (8) between the two branches of the river, in the Capisisit Lake area.

The zones are all narrow and have no conspicuous topographic expression. It has not been possible to determine.the movement that took place along any of them.

ECONOMIC GEOLOGY

The volcanic and sedimentary rocks are erratically mineral- ized with pyrite and occasionally chalcopyrite. Concentrations of these minerals that are interesting enough to warrant assaying are rare. The main localities are shown on the accompanying geological map, where they are numbered from (1) to (5).

• Locality (1) is 2,200 feet east of post 33 on the northern survey line. The exposure is small and traversed by a shear zone striking parallel to the schistosity of the rocks. On the south side of the shear, the rock is a fine-grained schistose lava similar to that of the nearby exposures: On the north side of the shear, the narrow exposed surface shows a grey feldspar porphyry, the schistosity of which is marked by the alignment of mica, flakes. Pale yellow pyrite, occurring as short lenses and small specks following the planes of schistosity, constitutes about one per cent of the rock.

All the other exposures are of older rocks, probably of sedimentary origin, more or less abundantly injected by granitic material. In localities (2) and (4), the intrusive is restricted to a few small, generally concordant, tabular•bodies. In locality (3), granite is predominant, whereas at (5) it is often difficult to recog- nize the true nature of the rock because of the abundance of granitic material and the coarseness of the granularity following recrystalli- 'zation. Shearing is either absent or very restricted. Pyrite, and less abundant chalcbpyrrite, are found mostly in the schists and appear to be-genetically related to the granites. -31 - Assays of samples from each of these localities showed that gold, silver, and copper were either absent or were present only id negligible amount. The results of this investigation, however, should be considered as preliminary. The favourable structure, the abundance of granitic intrusives, the presence of slight mineralization in widely separated localities, all of these factors favour a degree of optimism concerning the economic potentialities of the map-area.

REFERENCES

(1)Bancroft, J.A., Report on the Geology and Natural Resources of the Drainage Basins of the Harricanaw.and the Nottaway Rivers; Que. Bur. Mines, Report on Mining Ope- rations in the Prov. of Quebec, pp. 131-198, 1912.

(2)Beach, H.H., Michwacho Lake, Abitibi Territory, Quebec; Map No. 623, Geol. Surv. Can., 1941.

(3)Bell, R., Report on the Geology of the Basin of the Nottaway River; Geol. Surv. Can., Ann. Rapt., Vol. XIII, 1900, Pt. K, pp. 1-11 (1902), Map No. 702, (1903).

(4)Blake, D.A.W., Waswanipi Lake Area (East Half), Abitibi-East County; Que. Dept. Mines, G.R. 59, 1953.

(5)Claveau, J., Waswanipi Ia.ke Area (West Half),- Abitibi-East County; Que. Dept. Mines, G.R. 58, 1953.

(21)Cooke, H.C., Nottaway Sheet; Geol. Surv. Can., Map 190A, 1927.

(22)Freeman, B.C., Replacement Shells Around Batholiths in the Waswanipi District, Northwestern Quebec; Jour. Geol., Vol. XLVI, No. 5, pp. 681-699, 1938.

(23)Gilbert, J.E., Capisisit Lake Area, Abitibi East County; Que. Dept. Mines, G.R. 48, 1951, Map No. 849.

(24)Imbault, P.E., Goéland Lake Area, Abitibi-East County; Que. Dept. Mines, P.R. No, 218, 1948.

(25)Imbault, P.E., The Olga-Goéland Area, Abitibi-East County; Que. Dept. Mines, G.R. 51, Map No. 857,:1.952.

(26)Imbault, P.E., Mattagami-Inconnu Region; Can. Min. Jour., June, 1950.

(6)Imbault; P.E., The Olga-Goéland Lake Area, Abitibi East County; 'DUD. Thesis, McGill University, 1949,(unpublished). r-

- 32 (13)Lang, A.H., Waswanipi Lake Area, Quebec; Geol. Surv. Can., Summ. Rept., Pt. D, pp. 36-43, 1932.

(14)Norman, G.W.H., Opémisca Abitibi Territory,Bast Half; Geol. Surv. Can., Paper 37-11, Map No. 401A, 1937; West Half, Paper 38-11, Map No. 602A, 1938. (Both maps revised 1941).

(15)Norman, G.W.H., West Half Waswanipi Map-Area, Quebec; Geol. Surv. Can., Paper 37-8, 1937.

(16)Shaw, G., Mishagomish Lake, Abitibi and Mistassini Territories, Quebec; Geol. Surv. Can., Map 689A, 1942.

- 33 -

ATpuAuETICAL INDFJI

Page Page

Access to area 1 Epidote 9,12,17 Acidic rocks 14 Exposures in area 2,19,25,30 Acid sills 13 Acknowledgments 2 Faulting 29 Actinolite 13 Fauna of area 6 Amphibole 16,17 Fecteau Air Services Andesine 9 Acknowledgment to 3 Andesite 10 Feldspar 11,12,19 Apatite 9,12 Field work 1 Aplite 26 Flora of area 6 Assays of samples 31 Fluorite 26 Augite 10,11 Folding 28 Formations, Table of 7 Bancroft, J.A. - Freeman, B.C. - Ref. to work by 2,31 Ref. to work by 2,18,31 Basalt 10 Base maps 2 Garnet 12 Basic sills 13 Geology - Beach, H.H. - Economic 30 Ref. to work by 11,31 General 6 Bell, Robert - Structural 28 Ref. to work by 2,31 Gilbert, J.E. - Biotite 11,12,17,21,23 Ref. to work by 1,2,6,14 Blake, D.A.W. 17,28,30,31 Ref. to work by 2,31 Gillespie, W.G. - Acknowledgment to 2 Calcite 9,13 Goéland lake 1 Chalcopyrite 30 Gold 31 Chlorite 9,12,13,17 Granite 14,20 Claveau, J. Grant, I.C. Ref. to work by 2,24,31 Acknowledgment to 2 Composition table 15,18 21,22,23,26 Hematite 12 Consolidated rocks 6 Hills of area 3 Cooke, H.C. - Hornblende 9,10,13 Ref. to work by 2,31 16,19,23 Copper 31 Imbault, F.E. Ref. to work by 2,6,18 Description of area 3 20,24,26,28,31 Diorite breccia 27 Inconnu river 3,4 Diorites 14,18,20 Intrusive rocks 14,25 Drainage of area 3 Dykes 25,26,27 Lamprophyre 27 -34-

Page Page

Lang, A.H. - Pyroxene 17 Ref. to work by 2,32 Laugenite ...... 17 Quartz 9,10,11 Lavas 9,10 12,15,18,19 Location of area 1 Quartzite 12 Quartz syenite 18 Magnetite 9,12 Maicasagi lake 1 References 31 Maicasagi river 3 Rivers of. area 3 Mica 11 Roberge, J. Microcline 13,15,18,22,26 Cook for. field party 2 Muscovite .11,12 Satellitic rocks 14,25 Nomans river 3 Sedimentary rocks 8,11,28 Nomans stock 8,14,20,22 Shaw, G. - Norman, G.W.H. - Ref. to work by 2,32 Ref. to work by. 2,13,32 Shearing , 29 Northern granite 14,18,20 Silver 31 Southern granite 14,22 Oligoclase 9,13,21,23 Sphene 24 Oligoclase diorite 17 Olivine 16 Therrien, U. - Canoeman for party , 2 Paquette, F. - .. Topography of area 3 Acknowledgment to - 2 Truchon, G. -...... -. Pegmatites 22,25 Canoeman for party 2 Photographs 2

Plagioclase .... 9,10,13,15,17,18,21 Uralite 16 Porphyry . ... 27 Preglacial valleys. •• 4 Volcanic rocks 8,9 Previous work 2 Pyrite 12,13,30 Waswanipi granite 22