Rinfret Area, Electoral Districts of Abitibi-East and Roberval Province of Quebec, Canada

RG 081(A) RINFRET AREA, ELECTORAL DISTRICTS OF ABITIBI-EAST AND ROBERVAL PROVINCE OF QUEBEC, CANADA

DEPARTMENT OF MINES

Honourable W. M. COTTINGHAM, Minister A.-O. DUFRESNE, Deputy Minister

GEOLOGICAL SURVEYS BRANCH

GEOLOGICAL REPORT 81

RINFRET AREA

ELECTORAL DISTRICTS

OF ABITIBI-EAST AND ROBERVAL

W. W. LONGLEY

QUEBEC

REDEMPTI PARADIS

PRINTER TO HER MAJESTY THE QUEEN

1958

Photo Mosaic of Rinfret Area.

TABLE OF CONTENTS

Page

INTRODUCTION 1 General statement 1 Location of area 1 Means of access 2 Acknowledgments 2 Previous work 2 Field work 3 PHYSIOGRAPHY 3 NATURAL RESOURCES 4 Timber 4 Fish and game 4 Water power 4 GENERAL GEOLOGY 4 Table of formations 4 General statement 5 Biotite gneiss group 6 Pegmatite 7 Hornblende gneiss 7 Anorthosite 8 Amygdaloidal dyke S Hornblende schist group 9 Eastern area 9 Northwestern area 11 Anorthosite and gabbro 13 Granite and associated rocks 14 Diorite 15 Diabasic gabbro dykes 15 Pleistocene and recent 16 STRUCTURAL GEOLOGY 16 ECONOMIC GEOLOGY 17 General statement 17 Recommendations 18 BIBLIOGRAPHY 19 ALPHABETICAL INDEX 20

- I - MAP AND ILLUSTRATIONS

Map

No. 1210.- Rinfret area (in pocket)

Plates

Frontispiece - Photo mosaic of the Rinfret area.

I-A.- Caché lake base, Mount Laurier Aviation Company (now Nordair: ed.), showing planes, loading docks, and office building. B.- Roberval base, Mount Laurier Aviation Company (now Nordair: ed.), loading dock and large plane.

II - Rapids on Boisvert river, south of Lapointe lake.

II1-A.- Ridge, north of the western end of Roberge lake, as seen from Nepton bay, Chibougamau lake. B.- Oblique view to the northeast across the northern part of the area. Finger bay, Chibougamau lake in the foreground. Prominent ridge, left of centre, is the ridge north of Roberge lake, shown in Plate III-A.

IV-A.- Oblique of the northern part of the area, view to the east. Taché lake is just to the left of the centre of the photo. The ridge on the horizon, above Taché lake, is the one north of Roberge lake, shown in Plates III-A and B. B.- View to the northeast along Lapointe lake, showing shallow, flat character of lake basin.

V-A.- Rock exposure in Nepton bay, Chibougamau lake, showing angular blocks of anorthosite, gabbro-anorthosite, syenite, and schist, in a matrix that is probably diorite. B.- Close-up of angular blocks of syenite and coarse-grained anorthosite in rock shown in Plate V-A. The anorthosite block shows minor displacement.

VI-A.- Close-up of rock shown in Plate V-A, showing blocks of gabbro- anorthosite, schist and syenite. B.- Biotite gneiss, showing compositional banding and drag folding, east- central part of McCorkill township.

VII-A.- Biotite gneiss, showing compositional banding and small lenticular pegmatitic areas, east-central part of McCorkill township. B.- Hornblende schist blocks, distorted and partially assimilated by pegmatite, east margin, northern part of McCorkill township. VIII-A.- Sharp compositional banding of hornblende gneiss at schist-gneiss contact, south-central part of McCorkill township (X 3). B.- Hornblende schist, at schist-gneiss contact, showing drag folding,

south-central part of McCorkill township (X 21/).

IX-A.- Compositional banding in hornblende schist, west-central part of Rinfret township. B.- Close-up of specimen from the outcrop shown in Plate IX-A, showing detail of compositional banding (X 3).

X-A.- Irregular mass of plagioclase developed along a narrow band of the hornblende schist shown in Plate IX-A. The photograph shows two parallel surfaces that were half an inch apart in the original specimen. B.- Feldspathic band in hornblende schist, showing the general rounded character of the feldspar grains, and the displacement of hornblende grains to accommodate the development of the feldspar crystals. Specimen from the schist-gneiss contact zone in the southwestern corner of the area (X 3).

XI-A.- Garnetiferous hornblende schist. The rounded character of the garnet areas indicates that they crystallized after the alignment of the hornblende (X 2). B.- Grains of feldspar (2) with interstitial magnetite, suggesting sedimentary origin, west end of Roberge lake (X 5).

XII-A.- Sedimentary rock showing fine compositional laminations, north of Terry lake (X 2). B.- Compositional banding in sheared and altered greenstone, on an island in Terry lake.

RINFRET AREA

CHIEOUGAMAU REGION

ELECTORAL DISTRICTS OF ABITIBI-EAST AND ROBERVAL

W.W. Longley

INTRODUCTION

General Statement

Rinfret area, Chibougamau, was mapped by the writer in the summer of 1950. It is an area of gentle relief, modified by glaciation and generally mantled by thin glacial deposits. The consolidated rocks are Precambrian in age. In the eastern part of the area the bedrock is mainly biotite gneiss and hornblende gneiss. In the western part, the dominant rocks are hornblende schist, anorthosite, gabbro, diorite, granite, and syenite; some volcanic and sedimentary rocks also are present.

The structural trend is northeasterly, and the dips generally are to the east. Shear zones are common and are assumed to be related to faults in most instances.

The most promising indications of mineral deposits were found near the northwestern corner of the area just north of an intrusion of anorthosite. Here chalcopyrite was observed in several places in a half-mile-wide belt of altered volcanic and sedimentary rocks. This belt is apparently continuous with the O'Leary-Malartic copper prospect a short distance to the west.

Location of Area

The western boundary of Rinfret area lies within a mile of the east- ernmost bay of Chibougamau lake. Lake St. Jean is a little more than 100 miles to the southeast. The area is bounded by longitudes 73°45'W. and 74°00'W., and by latitudes 49°45'N. and 50°00'N., and comprises nearly 200 square miles. It includes the greater parts of Rinfret and McCorkill townships, the west side of the southern half of Thibaudeau, a strip along the west side of Sarrasin, and - 2 -

the northwest corner of Plat township. The boundary between Abitibi-East and Roberval electoral districts extends northeastward across the map-area; about three-quarters of the area is in Roberval electoral district.

Means of Access

The nearest railway depot is at St. Félicien, just northwest of Lake St. Jean. From this point the Chibeugamau district is easily reached by the St. Félicien-Chibougamau highway, a gravelled road some 160 miles long. The Caché Lake hase (P1.I-A) of the Mount Laurier Aviation Company (now, Nordair: ed.), located beside the highway just west of Chibougamau lake, is within 30 miles of several small lakes in the Rinfret area. These lakes provide suitable, though small, landing areas for hydroplanes.

Canoe travel to, and within, the area is difficult because of the many rapids and the small flow of water in the streams. Boisvert river is suitable for canoe travel from the southwest corner of the area to the forks about two miles north of Lapointe lake, a straight-line distance of some 15 miles. Along this route, however, there are numerous rapids (P1.II), and many short portages are necessary. Small lakes extending southeastward from Lyall lake in the southeastern part of the area provide a local canoe route. Nepton river, the upper part of Boisvert river, and the unnamed streams in the area are not suitable for canoe travel.

Cross-country travel is not difficult, as the footing is firm, and there is little underbrush or rugged terrain.

Acknowledgments

Able assistance in geological traversing was rendered by H. Bruce Lyall, a post-graduate student at McGill University. Léo Laurin, junior as- sistant; Armand Lapointe, cook; and Gilbert Roberge, Maurice Dumals, and Maurice Maurice, canoemen, carried out their assignments in an efficient manner.

The pilots of Mount Laurier Aviation Company (now Nordair: ed.) rendered valuable and considerate service.

Previous Work

The area adjoining to the north was mapped by J.E. Gilbert (1958) for the Quebec Department of Mines during the summer of 1950, and that to the west was mapped by Mawdsley and Norman (1935) for the Geological Survey of Canada. - 3 -

Field Work

The field party was in the area for eleven weeks during the summer of 1950, arriving June 19 and leaving September 4. Much time was lost due to bad weather.

The area was mapped by systematic pace-and-compass traverses at half-mile intervals. Many of the traverses were run east or west, from Boisvert river. Exposures of bedrock are rare along streams and the shores of lakes.

Vertical aerial photographs, taken by the Royal Canadian Air Force, on a scale of approximately two inches to the mile, were used extensively in studying the ground and locating probable rock exposures prior to traversing. They were also used in compiling a detailed field base map, with surveys by the Quebec Department of Lands and Forests serving as controls.

PHYSIOGRAPHY

The surface features of the area have a general north-northeasterly trend. The surface is relatively flat, with scattered, gently rolling hills. The more prominent hills are in the south-central part of McCorkill township, and in the northwestern corner of the area. One of the latter (Pl.III-A) rises about 400 feet above the surface of Roherge lake.

The map-area straddles the height-of-land between the Chibougamau- Nottaway River and the Lake St. Jean-Saguenay River drainage basins. The northwestern sector of the area is drained into Chibougamau lake, chiefly by Nepton river and its tributaries. Boisvert river and its tributaries drain most of the southern and eastern parts of the area southward towards Lake St. Jean. Boisvert river enters, near the northeastern boundary of the map-area, at an elevation of approximately 1,000 feet, and leaves near the southwestern corner at an elevation of approximately 1,350 feet. The height of land between these drainage basins, which trends in a north-northeasterly direction, forms the boundary between Abitibi-East and Roberval counties.

Numerous small lakes, probably of glacial origin, are scattered throughout the area. All are shallow and dotted ✓with rocks and islands. The largest of these lakes are Lyall in the southeast, Laurin and Houde in the central part, and Roberge in the northwest. In general, the lakes lie in flat, shallow basins (P1.IV-B). - 4 -

NATURAL RESOURCES

Timber.- The greater part of the northeastern quarter of the area has been burnt over by forest fires within recent years. Much of the remainder has stands of spruce, and in places jack pine and white birch. Some of these contain a large amount of pulp-wood timber as well as much that is suitable for lumber.

Much of the flat central part of Rinfret township is too swampy and wet for the growth of good timber.

Fish and Game.- Many fur-bearing animals, particularly otter, mink, fox, and muskrat are present. Moose are fairly abundant.

Fish appear to be scarce, pike being the chief species in the lakes and larger streams. Trout were observed in a few of the smaller, clear streams.

Water Power.- Streams within the area do not carry a sufficient flow of water to be of any great significance as a source of water power.

GENERAL GEOLOGY

Table of Formations

Recent Alluvial valley fill Quarternary Pleistocene Glacial moraine, eskers

Great Unconformity

Diabasic gabbro dykes

Diorite, serpentinized diorite with magnetite matrix

Granite, syenite, diorite

Anorthosite, anorthositic gabbro, diorite

Precambrian Biotite gneiss group, hornblende gneiss, inclusions of hornblende schist, anorthositic gneiss, amygdaleidal basalt

Hornblende schist group, garnetiferous hornblende schist, hornblende-chlorite schist, andesitic lava,banded metasediments - 5 -

General Statement

All the consolidated rocks of the area are of Precambrian age. Al- though exposures of bedrock are common, except in the central part of Rinfret township, contact zones between different formations are usually covered by un- consolidated glacial and alluvial deposits.

On the basis of lithology and general structure the area can be di- vided into two more or less equal parts, the dividing line being slightly east of, and nearly parallel to, the height of land. The predominant rocks east of this dividing line are medium- to coarse-grained gneisses, mainly biotite but with some hornblende gneiss, and with scattered inclusions of schist. Small areas of anorthosite are present in the hornblende gneiss. The gneisses resemble some of those generally ;onsidered as belonging to the Grenville sub-province.

West of the dividing line hornblende schists appear to have been the most widespread rocks of any now exposed. These include altered volcanic and sedimentary rocks suggestive of "Keewatin-type". The schists have been intruded by a large wedge-shaped lobe of anorthosite-gabbro, with a core of granite which extends northeastward into the area for a distance of ten miles from the west- central margin. Also,a small lobe of diorite extends eastward for approximately three miles into the northwest corner of the area.

The order of succession of these rocks, as given in the Table of Form- ations, is rather arbitrary. Inasmuch as gabbro dykes cut biotite gneiss and anorthosite gabbro,the relative age of these rocks is established. Lenses and bands of hornblende schist in the biotite gneiss suggest that the belt of hornblende schist and the body of biotite gneiss may be closely related. The hornblende schist cannot be definitely distinguished from rocks of the accepted Keewatin-type. The difference may be only one of degree of metamorphism.

Some information on age relationships is given by an exposure in Nepton bay, just west of the map-area. This exposure is located just off the extreme southwestern end of the point at the head of Nepton bay, about a mile west of the mouth of Nepton river. The rock consists of angular blocks of coarse-grained anorthosite, anorthositic gabbro, fine-grained pink syenite, and hornblende schist, in a matrix of medium-grained diorite (Plates V-A and B, and VI-A). The feldspar of the matrix is completely altered, and the rock is now chiefly zoisite, epidote, and chlorite. The contacts are sharp and the matrix penetrates small fractures in the included blocks, suggesting intrusive breccia. Much minor movement is shown by displacement along the contacts of the blocks. The matrix may be related to the diorite of the area, indicating that the diorite may be younger than anorthosite, syenite and hornblende schist.

The granite tongue and surrounding anorthosite-gabbro shell are probably - 6 -

contemporaneous in age. Whether their origin is by true magnetic invasion (differentiation of a sill), or the advance of a metamorphic front (variation in composition of a sedimentary series), these bodies are younger than the surrounding hornblende schist.

Biotite Gneiss Group

About 60 per cent of the area, the eastern part, is underlain by rock that generally may be classed as biotite gneiss, although there are many local zones as well as angular inclusions of hornblende schist in the gneiss.

The gneiss varies in appearance from apparently massive to well-banded. The bands are rather uniform in width and character, and are produced by variations in the percentage of dark minerals present. This compositional banding suggests that the gneiss has formed through recrystallization of banded sedimentary rocks. In places the gneiss is lenticular instead of being uniformly banded. As the lenticular areas are pegmatitic in character, it is suggested that they are of secondary origin. Sharp folding or crenulation was also observed in the gneiss. This feature, as well as lenticular areas, are well shown in an exposure three-quarters of a mile west of Boisvert river where it crosses the centre line of McCorkill township (Plates VI-B and VII-A). In this exposure the dark bands contain about 25 per cent biotite, and the light bands are essentially biotite free.

The foliation of the gneiss, evidenced by aligned blades of biotite, strikes north-northeasterly and dips in general eastward, the angle of dip ranging from 20° to vertical. Local sharp variations in the altitude of the foliation were observed. Both strike and dip of the compositional banding conform closely to that of the foliation.

The gneiss is medium- to coarse-grained, and grey to pinkish-grey. Much of it contains brilliant, black biotite, but locally hornblende accompanies, or is present instead of, the biotite. Glassy quartz is a characteristic feature.

Study of a number of thin sections of specimens from locations well dis- tributed throughout the gneiss show a general similarity. Glassy quartz is one of the conspicuous minerals in both hand specimen and thin section, the amount ranging from 20 to 45 per cent, with an average of about 30 per cent. In general, the quartz is in irregular grains, commonly with sutured boundaries and wavy ex- tinction. In some sections, the quartz forms lens-shaped structures composed of small grains.

The plagioclase content ranges from 25 to 70 per cent, the majority of the sections containing about 40 per cent. The range in composition is rather - 7 - restricted. In most specimens the plagioclase is calcic oligoclase (An20-Anso), in a few it is sodic andesine (An32), and in some it is sodic oligoclase (Ani5). In general, the plagioclase is little altered. The orthoclase content ranges from 20 to 50 per cent. In some areas it has been altered, but, in general, it is relatively fresh.

Biotite ranges in amount from 5 to 10 per cent. In most specimens the biotite blades are well-aligned, marking the direction of foliation of the rock. Much of the biotite has feathery margins. It is pleochroic from light-greenish straw to greenish-brown.

A small amount of hornblende is present in some of the sections. It is pleochroic from yellowish-green to bluish-green. Sericite and epidote are common alteration minerals, with some kaolin and chlorite. The common accessory minerals are zircon, apatite, and magnetite. In some sections zircon grains with pleochroic haloes were observed in hornblende and biotite

Pegmatite.- Pegmatite or granite occurs in many places as lenses or irregular patches ranging in width from a fraction of an inch to a few inches. This is interpreted as being of local origin, resulting from metamorphism. In an exposure on the east side of Boisvert river, about two miles from the northern boundary of the area, corroded and partially assimilated blocks and fragments of hornblende schist are enveloped by an irregular intermeshing mass of pegmatite (Plate VII-B). In general, the schist blocks retain their original position, although some have been more or less folded and some small fragments have been rotated. Coarse grains of recrystallized hornblende are abundant in some of the pegmatite areas in this exposure. Hornblende grains along the corroded margins of the schist are much larger than those in the central part of the blocks. In some blocks, recrystallization has developed coarse-grained hornblende and feldspar throughout the block. There is no general displacement of compositional layers of the schist or other evidence of volume change. Space for the pegmatite has been made through removal of the schist. The character and distribution of the pegmatite in this exposure indicate that it was derived largely from the hornblende schist. The general change that took place in the rock is, at least in part, a phase of metamorphism with only certain constituents supplied from the exterior. Hornblende Gneiss.- Hornblende gneiss predominates in a two-mile-wide zone trending northeasterly through the southeast corner of McCorkill township into Thibau- deau township. The amount of hornblende ranges from 5 to 20 per cent, in general, but locally is as much as 50 per cent. The hornblende blades are brilliant and, commonly, are strongly aligned. Pronounced compositional banding, produced by variations in the relative amounts of light and dark minerals present, is common.

White plagioclase makes up from 25 to 75 per cent of the rock. Two thin sections from the southeast margin of the zone contain oligoclase; the plagioclase - 8 -

in a specimen from the northwest side of the zone is andesine (An40). Up to 30 per cent orthoclase is present in the gneiss, and from 5 to 15 per cent quartz. Epidote, kaolin, sericite and carbonate are the common alteration minerals, and the accessories include zircon, titanite, and apatite.

Anorthosite.- Much anorthosite is present in this zone of hornblende gneiss. This also appears to be a local compositional variation in the main body of gneiss. The anorthosite is definitely gneissic in character, and is not readily distinguishable from the regular gneiss. It appears to have been developed by recrystallization of sedimentary rocks, and in one location where the hornblende is more abundant than in most places it shows a definite compositional banding. The anorthosite is white, and commonly has a sugary or waxy appearance suggesting white marble. The texture is unusual. In places, the rock resembles a poorly- cemented, coarse-grained sandstone, and feldspar grains can be picked from the surface. The maximum size of the plagioclase grains observed is approximately one centimeter. In general, the grains are uniform in size, and in the majority of the specimens examined they are less than a millimeter in diameter. The plagioclase is labradorite, and in some places calcic andesine, ranging in composition from An45 to An68. In general the plagioclase is fresh, but in some of the specimens examined as much as 50 per cent has been altered to zoisite. The dark minerals present include biotite, altered augite, and hornblende.

In an exposure of hornblende gneiss located just west of Boisvert river, a little over a mile south of the Rinfret-McCorkill boundary, the feldspar is white as in the anorthosite. The rock is composed of 60 per cent sodic labradorite (An52), 20 per cent quartz, and 20 per cent hornblende. Compositional banding is well defined, only about 5 per cent hornblende appearing in the feldspar bands. There is marked difference in size of the hornblende grains from dark band to dark band. This exposure is believed to be related to the hornblende gneiss and anorthosite described above. The sharp compositional banding is interpreted as compositional variation in the original sedimentary layers.

Amygdaloidal Dyke.- About half a mile south of the northern boundary of the area, in the bed of Boisvert river, banded hornblende gneiss strikes N.10°E. and dips 45°E. This is cut by a vertical dyke of amygdaloidal basalt that strikes N.55°E. The dyke is four feet wide, and is exposed across the bed of the river. The exposure is covered except at low water, and, as stain from the river gives the entire outcrop a uniform colour, careful examination is necessary to distinguish the dyke and its boundaries. The contacts are sharp, and there is no evidence of contact metamorphism.

The hornblende gneiss here is medium- to fine-grained, and shows pronounced compositional banding. The hornblende grains are not well-aligned. The composition of the gneiss is about 30 per cent hornblende, 30 per cent plagioclase, Plate l

A.-Caché lake base, Mount Laurier Aviation Company (now Nordair), showing planes, loading docks, and office building.

B.-Roberval base, Mount Laurier Aviation Company (now Nordair), showing loading dock and large plane. Plate H

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Rapids on Boisvert river, south of Lapointe lake . Plate III

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A.-Ridge, north of the western end of Roberge lake, as seen from Nepton bay, Chibougamau lake.

B.- Oblique view to the northeast across the northern part of the area. Finger bay, Chibougamau lake, in the foreground. Prominent ridge left of centre is the ridge north of Roberge lake, shown in Plate ill-A. Plate IV

A.-Oblique view of the northern part of the area, view to the east. Taché lake is just to the left of the centre of the photo. The ridge on the horizon, above Taché lake, is the one north of Roberge lake, shown in Plates ill-A and B.

B.-View to the northeast along Lapointe lake, showing shallow, flat character of lake basin. Plate V

A.-Rock exposure in Nepton Bay, Chibougamau lake, showing angular blocks of anorthosite, gabbro-anorthosite, syenite, and schist, in a matrix that is probably diorite.

B.-Close-up of angular blocks of syenite and coarse-grained anorthosite in rock shown in Plate V - A. The anorthosite block shows minor displacement. Plate VI

A .- Close-up of rock shown in Plate V-A, showing blocks of gabbro- anorthosite, schist and syenite

B.-Biotite gneiss, showing compositional banding and drag folding, east-central part of McCorkill township. Plate VII

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B. - Hornblende schist blocks, distorted and partially assimilated by pegmatite, east margin, northern part of McCorkill township. Plate VIII

4 A.-Sharp compositional banding of hornblende gneiss at schist-gneiss contact, south-central part of McCorkill township-X3.

B.-Hornblende schist at schist-gneiss contact showing drag folding, south central part of McCorkill township-X21. Plate IX

A.-Compositional banding in hornblende schist, west-central part of Rinfret township.

B.-Close-up of specimen from the outcrop shown in Plate IX -A, showing detail of compositional banding-X3. Plate X

A.-Irregular mass of plagioclase developed along a narrow band of the hornblende schist shown in Plate IX-A. The photograph shows two parallel surfaces, that were half an inch apart h the specimen.

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- 15 -

Diorite

A tongue of diorite about a mile wide extends from the western boundary eastward some three miles into the northwestern part of the area. In one place, just north of the western part of Roberge lake, it forms a prominent ridge.

Typically the diorite is dark grey, coarse-grained, and massive. It is composed about equally of plagioclase and dark minerals, chiefly hornblende and chlorite. The plagioclase is andesine (An35); it is generally altered, with the development of paragonite, zoisite, and epidote. Hornblende is in conspicuous grains and blades altered, in general, to chlorite. Some exposures on the shores of Roberge lake have large blades of hornblende in random orientation.

Some of the rock in the southern part of this intrusive body, around Roberge lake and eastward, has not been positively classified. This is identified on the accompanying map as "serpentinized diorite with magnetite matrix". It may be a zone of alteration along a diorite dyke, or along a shear; some of its features suggest a sedimentary origin.

This indeterminate rock consists of white, rounded grains, probably altered feldspar, with magnetite in very fine grains filling the interstices (Plate XI-B). Magnetite makes up to 20 per cent of the rock in some places. Lo- cally, shearing has destroyed the white grains and the rock is black.

Diabasic Gabbro Dykes

Diabasic gabbro dykes were observed in three locations: a mile and a half northeast of Nepton lake, half a mile north of Houde lake, and in the northwest corner of Sarrasin township. The more southern of two occurrences in the last locality forms a ridge about 400 feet long and 50 feet wide, trending northeasterly. However, the dykes do not stand out in prominent ridges, as do the gabbro dykes generally in the Abitibi region.

Northeast of Nepton lake the dianasic gabbro cuts anorthosite-gabbro; elsewhere it intrudes biotite gneiss. The rock, similar in all exposures, is medium- to fine-grained and dark-grey to black. About half the rock is made up of laths of plagioclase (labradorite, An" to An60) up to .5 cm. long; these have the typical diabasic gabbro dimensions and arrangement. They have a brilliant lus- tre and in general show little alteration, although the margins of some of the grains are altered to zoisite. The original rock contained about 50 per cent pyroxene; this has been more or less altered. In some thin sections examined the plagioclase laths are in a matrix of felted chlorite, uralite and magnetite, with relics only of the pyroxene remaining; in other sections, alteration to chlorite, uralite and magnetite is quite minor. - 16 -

Pleistocene and Recent

The surface of the area is marked by the rounded hills characteristic of glaciated regions of low relief. Many of these are ridges that trend in a general south-southwesterly direction owing to the combined effect of un- derlying structure and glacial movement. On some of the hills ice-polished rock surfaces and glacial striae are present. Here and there glacial plucking has produced low escarpments. Many of the lakes show alignment parallel to the direction of movement of the ice, and it is probable that the basins were con- siderably modified by gouging action of the ice. Probably segments of the stream valleys were also modified by glacial action.

The bed-rock is, for the most part, covered by a thin mantle of glacial debris. Many eskers are present, some of these extending discontinuously for miles, although generally they are only a few feet in height. They are usually parallel to, and near, streams.

In a few places there is considerable valley fill of gravel, probably of post-Pleistocene age. The most extensive fill of this type is along Boisvert river, in the northeast part of the area, near the boundary between McCorkill and Thibaudeau townships.

STRUCTURAL GEOLOGY

The pronounced north-northeast to northeast structural trend of the rocks of the area is clearly reflected in the topography. The direction of ice movement closely paralleled the structural trend, so it is to be expected that many structural features have been accentuated by glacial action. In contrast to the general trend, the structure northwest of Nepton river trends from northeast to east-west. This conforms with the regional structure west of the map area, immediately north of Chibougamau lake.

Compositional banding in both gneiss and schist is parallel to foliation in all places observed, both striking north-northeast in general. Local folding causes many local variations in strike. In some cases readings taken may have been affected by local magnetic attractions. Dips are generally to the east, ranging from 20° to vertical; there are some steep dips also to the west.

Many escarpments, most of them only a few feet high, but rarely as much as 50 feet or more, were observed. Practically all of these trend in a north-northeasterly direction. Many linear depressions with the same trend were also observed. This parallelism suggests a genetic relationship between escarpments and linear depressions, and it is assumed that they are related to faulting, being fault or shear zones. - 17 -

Little evidence was found to indicate relative direction of movement along the zones of faulting and shearing. Drag folding along the contact between gneiss and schist in the southern part of McCorkill township indicates that the gneiss (eastern side) moved up.

There is much shearing along the contact zone between hornblende schist and anorthosite in the southern part of McCorkill and the northern part of Rinfret township. In many places the rocks are quite fissile. In this zone no evidence to indicate direction of movement was located.

A pronounced structural feature of the area is a lobe of anorthosite and anorthositic gabbro, with a core of granite, that extends northeastward across the northwestern part of the area. The areal distribution of the rocks concerned suggests a northeast-plunging anticline, or a southwest-plunging syn- cline. However, there is no suggestion of such folding in the banding of the surrounding schist, and the true character of the structure is not known.

A narrow band of sedimentary rock occurs northwest of this lobe of anorthosite-gabbro, in the northwest corner of the area. The bedding strikes slightly north of east, and dips steeply to the south. Rock that resembles me- tamorphosed lava was observed north and south of the sedimentary band, and it is assumed that the two rocks are interbedded. Cutting the series of lavas and sedimentary rocks are dykes and tongues of diorite (and possibly gabbro) that trend in an east-northeast direction. The largest of these is about a mile wide and extends into the area for about three miles. Probably there are several small concordant dykes of the diorite or gabbro, rather than one mass. The intrusive rock, being more resistant to weathering, stands out in ridges, with overburden covering the intervening areas, assumed to be made up of Keewatin-type lavas and sedimentaries. ECONOMIC GEOLOGY General Statement

Several mineralized zones were found in the northwestern part of the area, particularly in the greenstone in the northwestern corner. Twenty-four samples, collected from various locations, were assayed for gold and silver. Of these, one contained traces of gold, another 0.003 ounces per ton, and the other 22 samples were barren in gold. Twenty-two of the samples carried traces of silver, but, as the highest value was 0.4 ounce per ton, they are not of much commercial interest.

A considerable amount of disseminated magnetite occurs in the western part of Rinfret township, in the hornblende schist, and,to some extent, in the adjacent anorthosite-gabbro. The magnetite forms coarse grains and, locally, small lenses. Analysis of a specimen of magnetite gave values of 53.12 per cent iron and 7.57 per cent titanium. This zone is probably related to similar deposits in - 18 - the vicinity of Chibougamau lake, that have been known for some time. It would seem that the magnetite here could be separated readily from the schist.

Several quartz veins and some shear zones mineralized with pyrite are located in the hornblende schist adjacent to the gneiss. '',:lever, no base metals were seen, and assays yielded negative values for gemu.

A small amount of disseminated chalcopyrite is present in anorthosite on the southwest side of a small lake 11/4 miles east of the western boundary of the area, and 1/3 mile south of the centre line of McCorkill township. The chalcopyrite is in a breccia zone three feet wide trending N.60°E. The mineralization does not appear to follow the shear zone. A shallow prospect pit was dug in this zone some years ago.

The most promising location is a zone, about a mile in width, just north of the anorthosite contact in the extreme northwest corner of the area. This zone appears to be the continuation of the one in which the O'Leary-Malartic showing, a mile and a half to the west, is located. A two-inch band rich in chalcopyrite occurs in this zone at, or slightly west of, the western boundary of the area a half mile north of Terry lake. The band is parallel to bedding in a finely- bedded rock that is either a sedimentary rock or a tuff.

Chalcopyrite is present in several places between the small showing described above and Roberge lake, about three-quarters of a mile to the east. The chalcopyrite occurs in quartz veins, small shears, and as disseminated spots in altered diorite. Chalcopyrite was also found just south of the eastern part of the large island in Terry lake.

Highly carbonatized rock was observed in the western part of Terry lake, and about half a mile north of the lake, just east of the margin of the area. Highly altered and sheared greenstone, probably sedimentary in origin, well-mineralized with pyrite, was found in several places along the zone immediately north of the anorthosite contact.

Recommendations

Although the greater part of the area yielded no indications of the pres- ence of economic mineral deposits, the zone between Terry lake and Roberge lake, and extending northeastward along the anorthosite contact, is regarded as highly promising. The finely-bedded sedimentary or tuffaceous rocks deserve particular attention. - 19 -

BIBLIOGRAPHY

Gilbert, J.E., Bignell Area, Abitibi-East and Roberval Counties; Quebec Dep't.of Mines, G.R. 79, 1958.

Mawdsley, J.B. and Norman, G.W.H., Chibougamau Lake Map-area, Quebec; Geol. Surv. Canada, Mem. 185, 1935. - 20 -

ALPHABETICAL INDEX

Page Page

Acknowledgments 2 General 4 Amygdaloidal dyke 8 Structural 16 Anorthosite 1,8,12,13 Gilbert, J.E. - Apatite 7,8,14 Ref. to work by 2,19 Augite 8 Gneiss 6 Granite 5,7,14 Banding 16 Bibliography 19 Hornblende 7,8,10,14,15 Biotite 6,7,8,10 Hornblende gneiss 7,8 Biotite gneiss 5,6 Hornblende schist 1,5,9,11 Boisvert river 2,3 Bornite 12 Iron 17

Caché Lake base 2 Kaolin 7,8 Canoe travel 2 Kaolinite 14 Carbonate 13,14 Carbonatized rock 18 Labradorite 8,15 Chalcopyrite 1,12,18 Lakes of area 3 Chlorite 5,7,10,14,15 Lapointe, Armand - Contact zones 5 Cook for field party 2 Laurin, Léo - Diabasic gabbro dykes 15 Acknowledgment to 2 Diorite 1,14,15 Location of area 1 Dumais, Maurice - Lyall, Bruce, H. - Canoeman for party 2 Acknowledgment to 2

Epidote 5,7,8,10,14,15 Magnetite 7,10,12,15,17 Maurice, Maurice - Feldspar 5,8,11,13,15 Canoeman 2 Field work 3 Mawdsley, J.B. - Fish in area 4 Ref. to work by 2,19 Folding 17 Means of access 2 Formations, table of 4 Mineralized zones 17

Gabbro 1,12,13 Natural resources 4 Game in area 4 Nepton river 2 Garnet 10,11 Norman, G.W.H. - General statement 1,5,17 Ref. to work by 2,19 Geology - Economic 17 _Orthoclase 7,8,14 - 21 -

Page Page

Pegmatite 7 Royal Canadian Air Force - Plagioclase 6,9,10,13,14,15 Ref. to work by 3 Pleistocene .... 16 Physiography 3 Sericite 7,8,13,14 Previous work 2 Sericite-talc schist 14 Pyrite 13,18 Syenite 1,14 Pyrrhotite 12 Table of formations 4 Quartz 6,8,11,13,18 Timber of area 4 Quartz diorite 14 Titanite 8,10,14,17 Quebec Dept. Lands, Forests - Ref. to work by 3 Uralite 15

Recent 16 Water power of area 4 Recommendations 18 Roberge, Gilbert - Zircon 7,8,10,14 Canoeman for party 2 Zoisite 5,15