Preliminary Report on the Maicasagi Area, Abitibi-East County Province of Quebec, Canada

RP 231(A) PRELIMINARY REPORT ON THE MAICASAGI AREA, ABITIBI-EAST COUNTY PROVINCE OF QUEBEC, CANADA

DEPARTMENT OF' MINES

GEOLOGICAL SURVEYS BRANCH

PRELIMINARY REPORT

THE MAICASAGI AREA

ABITIBI-EAST COUNTY

P. E. IMBAULT

QUEBEC 1949

P. R. No. 231 PRELIMINARY REPART

THE MAICASAGI AREA by

P.E. Imbault

INTRODUCTION

The Maicasagi area, mapped during the summer of 1949, is bounded by longitudes 76° 151 and 76° 401 West and by latitudes 49° 45t and 50° 00t North. It comprises 320 square miles and includes large parts of projected townships Nos. 614, 615, 714, and 715. The greater part of the map-area drains into Maicasagi lake, which lies about 115 miles north-northeast of Senneterre, a town on the Quebec-Cochrane line of the Canadian National Railways. The lake, part of which occupies the northwest corner of the area, is 810 feet above sea-level. It empties into Goéland lake and thence, via Olga and Mattagami lakes and Nottaway river, northward to James bay. The area may be reached from Senneter- re either by canoe ors more conveniently, by aeroplane. The local drainage is effected by three main streams: 1) Maicasagi river, about two miles south of and parallel to the northern boundary of the map-area,, flowing westward into Maicasagi lake; 2) Inconnu river, about one to two miles west of the eastern boundary, flowing northward into Maicasagi river; and 3) Nomans creek, the largest tributary of Inconnu river, crossing the map-area in a northeasterly direction to join the Inconnu about three miles south of the junction of that river and the Maicasagi.

These streams provide relatively easy access to most parts of the map-area. Maicasagi and Inconnu rivers, although interrupted by a few rapids, are navigable all summer. At high water, Nomans creek may be ascended, with heavily loaded çanoes and outboard motor, up to its main fork, half a mi3e south of mile post 27 on the southern east-west surveyed line. The only por- tage necessary along this creek in the spring is due to a series of rapids, one mile and a half long, near the mouth of the creek. When the spring flood has subsided, however, the general shallowness of the creek and numerous boul- der-trains that block its bed make it unadvisable to venture along it with heavy loads.

The greater part of the map-area consists of a flat plain of clay and clayey sand, similar to other areas within the Clay Belt. The plain is dotted with low hills of either glacial debris or of bedrock partly covered by loose deposits. The hills, as a rule, measure only a few scores of feet in height, so that the local relief is gentle. Some of the gabbro hills, especially those near the northeastern corner of the map-area, rise 150 feet or more above the surrounding country. M-2212 The whole region is heavily wooded, mostly with tall. spruce. Jack-pine, poplar, and birch are occasionally seen, especially on sandy hills. Balsam, tamarack, and cedar are rare. Except for restricted patches of windfalls and alders, the underbrush is generally sparse and travel through the bush is quite easy.

GENERAL GEOLOGY

The Maicasagi area offers many more rock exposures than most of the region lying west of it. The consolidated rocks! which are all of Precambrian age, may be subdivided into three main groups which from youngest to oldest are as follows:

Granitic intrusives,

Gabbro-syenite group,

Volcanic-sedimentary rocks.

Some basic dykes and a few offshoots of granite were also noted. These satellitic intrusives are younger than the main rock groups listed above.

Volcanic-Sedimentary Series

The main belt of these rocks is about four and a half miles wide where it enters the northeaAt corner of the area. It crosses the map-area in a`general westerly and southwesterly direction. Six miles west of the eastern boundary of the area, the belt splits into two branches around the eastern apex of a roughly triangular-shaped body of granite. The north branch trends northwestward, becoming gradually wider until it is more than six miles wide where it crosses the northern border of the map-area. The south branch, trend- ing southwestward, maintains a width of two to three miles for a distance of eight miles, after which it apparently widens until it is about nine miles wide where it crosses the western boundary of the map-area. From the direction of the schistosity near the southwest corner of the area, it seems probable that another, but• narrow branch, goes eastward from there and follows the southern edge of the central granitic body. This opinion is strengthened by the pres- ence of volcanic rocks on the southeast and east sides of that intrusive mass.

In addition to this main belt, many small remnants of this series are seen within the intrusive bodies. The two largest ones are shown on the accompanying map. One is near the centre of the area, in the northwest corner of township 615, between large bodies of granite and of gabbro-syenite; the other is three miles to the southeast and lies near the border ofa but within, the central granitic mass.

The volcanic members of the series are of intermediate composition, similar to the andesitic and basaltic lavas that have been described in several sections of the broad belt of volcanic and sedimentary rocks - 3 - that extends for more than 200 miles eastward from the Quebec-Ontario boundary, through this area, and on to the Chibougamau Lake region. These rocks occasionally display such features as flow lines, flow breccias, and amygdules. The rare pillows are highly deformed.

The fresher lava is found in a few exposures in the southwest quadrant of the map-area. It is dark-green, fine grained, equigranular, and almost massive. It consists of hornblende and a greyish white feldspar. Most of the lavas, however, have undergone some kind of change: some flows contain phenocrysts of hornblende that seem to be the result of metamorphism; the schistosity is generally strongly developed and the surfaces of schistosity are often glistening with small flakes of mica; in, and near, shear zones, chlorite and epidote are abundant.

It is interesting to note that the volcanics, while relatively more abundant in the series in this Maicasagi area than in the areas to the east that were examined by Gilbert (1), are apparently less plentiful here than in the areas to the west, examined by the writer (2). 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.

The sedimentary rocks are fine-grained, thinly bedded, and consist of recrystallized greywacke, micaceous quartzite, and a few beds of dull grey chert. The greywackes are generally light-coloured and contain sub- stantial quantities of quartz in addition to feldspar, biotite, and (or) hornblende. A few beds are much darker and are distinguishable from the lavas only by their fine bedding. Red garnets, in places occurrirg in clusters more than half an inch in diameter, were seen occasionally, especially in the thin sedimentary beds within the volcanic series on the north shore of Maicasagi river.

Within the arias of volcanic and sedimentary rocks there are some narrow concordant bodies of fine-grained, slightly-schistose, gabbro. They are believed to have been emplaced as sills before folding of that series took place and to be genetically affiliated to the lava rather than to the larger intrusive bodies described below.

Intrusive Rocks

About 65 per-cent of the area is underlain by rocks that are intrusive into the volcanic-sedimentary series. These masses are lentic- ular and, from the direction of their planar structures, it seems that they were emplaced as concordant bodies into the older, previously folded rocks.

(1) GILBERT, J.E., Preliminary Report on Capisisit Lake Area, Abitibi-East County; Que, Dept. of Mines, P.R. 210, 1947. Preliminary Report on Branssat-Kreighoff Area, Abitibi- East County; Que, Dept. of Mines, P.R. 221, 1948.

(2) IMBAULT, P.E., Preliminary Report on Olga Lake Area; Abitibi-East County; Que. Dept. of Mines, P.R. 207, 1947. Preliminary Report on Goéland Lake Area; Abitibi-East ' County; Que. Dept. of Mines, P.R. 218, 1948. - 4

The intrusive rocks are; 1) Gabbro-syenite group, 2) Granitic intrusives, a) Northern body, b) Central stock, c) Southern body. Gabbro-syenite Group

Intrusive rooks of this group occupy three sections of the area. The largest one comprises about 35 square miles in the east-central part of the map-area. It represents the western half of an elliptical mass, the other half of which has been mapped by Gilbert 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 large body by a narrow tongue of volcanic rocks. The third mass lies north of Maicasagi river in the northeastern part of the area. It continues northward beyond• the map-area to form what is probably a small stock (1).

The contact between these masses and the intruded rocks was not observed. Generally, however, as evidenced by the schistosity of both types of rocks, the contacts seem to be concordant. The only exception is at the western nose of the largest body where the southeastern end of the volcanic inclusion seems to be cut off abruptly by the intrusive. These rocks show numerous lithological variations that can not, for the present time at least, be arranged into definite patterns. Hornblende and feldspar are the essential constituents of all the facies. Bi- otite is rare and always subordinate. According to the type of feldspar present, three facies have been recognized, namely gabbro, diorite, and syenite. The gabbro contains a purplish plagioclase, probably labradorite. The feldspar of the diorite is greyish white and less basic. The syenite is characterized by a pink feldspar, probably rich in potash. No age relations between these three facies have been obtained. The northern mass, however, contains a few inclusions of coarse- grained amphibole-rock, poor in feldspar, that may represent the most basic member of this intrusive group. If such is true, it could be concluded that they were from one original magma which, on cooling, followed an order of differen- tiation from basic to acidic, and that the syenite would be the youngest member of the series. Similar relations have already been observed in the Olga Lake area (2).,

(1) SHAW, G., Midhagomish Lake, Abitibi and Mistassini Territories, Quebec; Geol. Surv. Can., Map 689A, 1942.

(2) IMBAULT, P.-E., Preliminary Report on Olga Lake Area, Abitibi-East Count; Que. Dept. of Mines, P.R. 207, 1947. -5- The gabbro-syenite group is older than the granitic intrusives.

Granitic Intrusives

The three masses of granite listed above appear to form separate bodies. Their composition, however, is generally similar. Each body is traversed by dykes of.pegmatite and aplite, but the genetic affiliations of the dykes are unknown. Since no cutting relationships between the different masses have been observed, they are discussed in their order of oe-cturrence from north to south, without implication as to their respective ages. Moreover, it is considered probable that all three masses are closely related.

Northern body. This granite forms a roughly-triangular area which is more than six miles wide at the western boundary of the map-area and which extends eastward across the central part of township 714 to end about a mile and a half inside township 715.

The typical granite, similar to the facies found in the western extension of this mass, is exposed on the south shore of Maicasagi lake and south of Maicasagi river. It is pink to grey, and medium-grained. In places, gneissic structure is strongly developed, whereas in others the dgrk minerals occur as dots giving a coarser, less-defined gneiss. The rook consists mostly of quartz (up to 40 per cent), feldspar, biotite, and (or) hornblende. The ferromagnesian minerals seldom exceed 15 per cent.

The typical granite is seldom seen on the north shore of the lake. In that locality, indeed, no exposure of intrusive rock is free from inclusions. These inclusions occur in the form of irregular and elliptical blocks many feet in diameter and also as persistent bands that may be as thin as a fraction of an inch.

In the extreme northwest corner of the map-area, the light- coloured granite grades eastward, within a few hundred feet, into a dark grey, medium-grained, well-banded granite that consists of subhedral crystals of plagioclase, crushed quartz, biotite (15 to 20 per oent), epidote, and some hornblende. A similar gradation was observed about one mile and a half to the east. Both cases are interpreted as evidence of contamination of the granite by the intruded rocks.

The band of granite gneisses between those too localities is still more basic. Its rocks are grey, medium-to coarse-grained, and in places almost pegmatitic. They are generally low in quartz. Feldspar and hornblende are the main constituents. A white plagioclase and a brownish potassio feldspar ire often found closely associated, the latter having a tendency to occur as phenocrysts. There are many variations in this gneiss, but, on the whole, it seems to have the composition of a monzonite. It is quite likely that this band represents a separate, concordant intrusive, older than the light-coloured granite. It has not yet been separgted, however, from the main mass because it is felt that closer studies must be made before ruling out the possibility of its origin through extensive contamination. -6..

Central stock, Most of the south-central part of the area is occupied by a granitic mass that, from the direction of the gneissic structure along its border, appears to be in the form of a stock. It underlies at least 35 square miles.

This granite is fine-to medium-grained and shows evidence of Brushing. Its main constituents are a white 'feldspar, quartz, and biotite. Hornblende is present in a few localities only. The dark minerals, as a rule, do not exceed fifteen per cent of the rook. The gneissic structure is repre- sented. generally by narrow alternations of dark and light-coloured minerals. In a few exposures, however, the alternating layers have a width of one quarter of an inch or more, giving the gneiss a well-banded appearance.

Southern body. Discontinuous groups of outcrops, found for distances of one to four miles from the southern boundary, mark a northward extension of a large body of granite that has been mapped around Waswanipi lake, south of the present map-area.

The rock in this area is not as light in colour as the pink, massive, biotite granite that Claveau describes (1) as being typical of this mass. Excepting a few pegmatites, the lighter facies here was found to be a fine-to medium-grained, generally gneissic granite that contains about fifteen per cent of biotite. More than half of the exposures, however, are a darker granite, in which gneissic structure is highly developed. Among the dark minerals, which constitute about 30 per cent of the rock, hornblende is more abundant than biotite. This darker facies possibly resulted from contamination of the light-coloured granite by rocks of the volcanic-sedimentary series along the border of the intrusive mass.

Lykes and Sills

Satellitic intrusives are of restricted occurence. Mention has already been made of the dykes of aplite, pegmatite, and light-coloured granite, that cut the larger granitic masses. Similar rocks are found, as dykes and sills, within the belts of volcanic and sedimentary rocks and in the gabbro-syenite masses.

A dioritic, plug-like intrusive is found in the lavas on the south shore of Maicasagi river, near the head of the long series of rapids about six miles east of Maicasagi lake. The rock is fine-grained and consists mostly of hornblende and feldspar. As well as could be ascertained, this diorite was intruded along the nose of a drag fold in which the older rock had been brecciated. The diorite contains numerous small fragments of the intruded rock.

The granites are cut by some fine-grained dykes that may be conveniently named lamprophyres. They are dark and generally porphyritic, with small phenocrysts of biotite or of hornblende (or both) set in a very fine matrix that appears to consist mostly of dark minerals. One dyke contains tiny red dots that are probably garnet. Most of these dykes are slightly mineralized with light yellow pyrite in small clusters.

(1) CLAVEAU, J., Preliminary Report on the Waswanipi Lake Area, (West Half), Abitibi-East County; Que. Dept. of Mines, P.R. 217, 1948. - 7..,

STRUCTURAL GEOLOGY.

Folding

The rocks of the volcanic-sedimentary series are inten- sely folded. Schistosity is everywhere developed and, except for the devia- tions that are related to the nearby intrusive masses, it trends generally 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 iii many places are vertical.

Positive determinations of the attitudes of the beds and layers of this series could not be made because of the deformation to which the rare pillows have been subjected and because of the fineness of the grain in the sedimentary rocks. Indications concerning the direction in which the tops face were, however, recorded in two places. 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 line between projected townships 614 and 714. Remnants of pillows here show faint curvatures that suggest a top toward the south. As the schistosity dips northward and as schistosity generally parallels bedding or layering, it would seem that these layers would be overturned. The second locality is six miles to the south, where grain gradation in two thin, south- ward-dipping sedimentary layers indicate that the top of the beds is toward the south,

It is evidently not possible, solely on the basis of those two observations, to give an interpretation of the structure of these highly- folded rocks. However, from the outcrop pattern of the volcanic-sedimentary bands and of the intrusive rocks as well, it may be concluded that the map- area is underlain by a series of plunging folds.

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 can be corelated with planes of jointing.

Small faults were observed, mostly on the north shore of Maicasagi Jake. They strike from N. g0° E. to S. 50° E. and in all cases the north block has moved west with respect to the south blocks. The apparent displacement is from one foot to a maximum of three feet. Evidence of stronger movement is seen on the south hank of Nomans creek, in the southeast corner of township 714, about 1,000 feet north of mile post 39 of the nearby surveyed line. Grooves, half an inch deep, follow the plane of schistosity of the rocks and plunge at 35o to the southwest. The direction and amplitude of movemênt, how'- ever, could not be determined.

Of the few shear zones observed, the four main ones are* south of Nomans creek, about 3,700 feet southwest of mile post 39 of the northern one of the two surveyed lines; 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 mile post 40 of the northern surveyed line; and 1,000 feet east of mile post 36 of the same line. The first three are in w 8 - schistose lavas, whereas the last one is within rather massive, fine-grained gabbro. The zones are all narrow and have no conspicuous topographic expres- sion.

ECONOMIC GEOLOGY The volcanic and sedimentary rocks are erratically mineralized with pyrite, and occasionally with chalcopyrite, especially within and in the vicinity of the shear zones. Veins of quartz and stringers of epidote are further evidence that hydrothermal solutions moved through the rocks. No, or negligible, values were obtained, however, from assays made of samples collected during this investigation.