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 ON THE MAICASAGI AREA ABITIBI-EAST COUNTY BY P. E. IMBAULT QUEBEC 1949 P. R. No. 231 PRELIMINARY REPART ON 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 pro- jected 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.

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