Preliminary Report on Iserhoff River Area, Abitibi-East County PROVINCE of QUEBEC, CANADA

RP 197(A) Preliminary report on Iserhoff river area, Abitibi-East county PROVINCE OF QUEBEC, CANADA DEPARTMENT OF MINES GEOLOGICAL SURVEYS BRANCH PRELIMINARY REPORT ON ISERHOFF RIVER AREA ABITIBI-EAST COUNTY BY JACQUES CLAVEAU QUEBEC 1947 P. R. No. 197 ISERHOFF RIVER AREA ABITIBI - EAST COUNTY by Jacques Claveau INTRODUCTION Iserhoff River area, which was mapped during the summer of 1946, lies at some ninety miles north of the Canadian National Railway centre Af Senneterre. Comprising about 240 square miles, it is bounded by longitudes 77°00t and 76°451W., latitude 49°45'N., and a line four and a quarter miles south of, and parallel to, latitude 49°$0tN. Two canoe routes lead to the area. One follows Bell river from Senneterre to Mattagami lake, Olga lake, and Goland lake in the northeast corner of the map-area. The other is by way of Bell and Wedding rivers, Duplessis brook and O'Sullivan river to Waswanipi lake into which Iserhoff river flows, four miles east of the map-area. Aeroplane service, available from Senneterre, places the area well within one houris flying time and is greatly preferable to the river and lake routes which necessitate several days of arduous travel. Throughout the earlier half of the stammer, when waters are still fairly high, Iserhoff river can be ascended to within two miles from the western border of the map-area, the operation involving - 2 - only ten small rapids through which fully loaded canoes can be easily waded. Imbault creel_ can be ascended to a point one and a half miles above its confluence with the Iserhoff, whence a portage of three miles and a quarter was cut by the party to give access to Imbault lake. Baptiste lake is ac- cessible only by air or on foot, and trails were cut during the past season leading from it to the chain of lakes to the north. Of the large streams flowing into Ramsay bay, a westward extension of the south end of Goland lake, only the one empty- ing into the deep, narrow, south-trending arm of the bay is navigable over an appreciable length of three miles from its mouth. From the end of its navigable portion, a trail one mile and a quarter long was cut to link the stream with an east west survey line that follows latitude 49035'30"N. The river which, in the northwest corner of the map-area, flows into Goéland lake from the west was ascended by motor-driven canoe to its first small rapid at some six miles from its mouth, from which point it appears navigable by small canoe for some distance farther. TOPOGRAPHY Thvr west-central part of the area, underlain by anorthosite-gabbro, consists of rugged, bare hills rising between 500 and 800 feet above Ramsay bay. A few hills of local prominence and east-west elongation are found in the southern belt of volcanics, especially near the western border of the area, between Baptiste lake and Iserhoff river. The areas underlain by granite and syenite generally stand as plateaus with gently rolling surfaces which are frequently half-bare. Most of the larger streams occupy well- defined valleys which, in part or wholly, have - 3 - been cut into bed-rock. The most important valley of the area is that occupied by Iserhoff river. It is from one to two miles wide with sides gently, but sometimes precipitously, rising 300 to 400 feet above the valley bottom. GENERAL GEOLOGY The consolidated rocks of the map-area are all of Ilrecambrian age. About one-third of the exposed bed-rock consists of volcanic and sedimentary rocks of Keewatin type. Of the remaining two- thirds, a quarter is occupied by rocks of a banded complex of anorthosite and gabbro and the remain- der, by granitic rocks including quartz diorite, syenite, and granite. It appears that the anorthosite-gabbro was intruded, possibly in laccolithic form, before the folding of the Keewatin rocks. The two were then folded together and granitic intrusions followed, starting with quartz diorite and terminating with granite and pegmatite. Keewatin (?) Rocks Volcanic-Sedimentary Series Volcanic rocks, with which are associated some sedimentary rocks and many small bodies of gabbro and diorite, underlie a large part of the map-area in the south and a smaller area in the north. The southern area, bordering but lying most- ly north of Iserhoff river, forms a band running east-west across the entire width of the map-area. Some eight miles wide at its western end, the band becomes narrower eastward until it is hardly more - 4 than two miles in width where it leaves the eastern boundary of the map. The area underlain by the volcanic--sedimentary series in the north is approximately centered about Gotland lake and the eastern part of Ramsay bay. Its northern extent reaches beyond the bound- aries of the map. Its southwestern and southern margins are clearly defined west of Gotland lake by a line entering the map-area at its northwest corner and running southeast to Ramsay bay. From there, however, it is impossible to draw a satis- factory boundary between the Keewatin rocks and the surrounding granite which penetrates them in a most intricate fashion. In all probability, the boundary crossess Ramsay bay, about two and a half miles from its mouth, and extends southward a few miles to take in a long, narrow, finger-like body of Keewatin rocks in the granite. Fraa there it follows easterly fairly close to the southern shore of Ramsay bay and of Gotland lake. The predominant representative of the volcanic-sedimentary series is a schistose lava of andesitic composition. Gabbro and diorite, usual- ly in narrow concordant bodies, are corrnonly found in association with the andesite. An amphibolitic gneiss is also of wide-spread occurrence. It is believed to be the recrystallized product of either the lava or the associated gabbro and diorite, but in most cases it is difficult or impossible to determine which of these was the original rock. Sedimentary rocks, interbedded with the volcanics, include impure quartzites, mica schists, greywacke, finely laminated argillaceous and siliceous sedi- ments, and rare beds of pure, fine-grained, white quartzite. A small band of conglomerate with de- formed pebbles of granite, quartz, and argillite was also found in the southern belt of the series. --5- Post-Keewatin Intrusives Anorthosite-Gabbro Comdex A heterogeneous mass of anorthosite and gabbro occupies a rudely semi-elliptical area between Baptiste lake and Ramsay bay. Marked by an extreme variability throughout, this mass may be said to average a gabbroic anorthosite in composition, that is, a rock that could be defined as an anorthosite with as much as 30 to 40 per cent ferromagnesian minerals. The rock of the mass is banded, the banding being either sharp or, more conl,ionly, gradational, and, within a few feet across the banded rock, all intermediate types are likely to occur from a pure white anorthosite to a dark gabbro or, locally, even to a pyroxenite. The banding probably result- ed from differentiation 'in situ' of the original magma and, although it is dipping steeply now, it undoubtedly originated in a horizontal position. The grain is generally quite coarse. The feldspar is white to grey and, for the most part, the dark minerals appear to be hornblende. Pyrite, finely disseminated or in small veinlets, is not rare. Titaniferous magnetite, when present, follows the more gabbroic types and in a few instances is seen in relative abundance (although never in the massive state) in certain bands. Such bands display a rusty appearance throughout. 'Granitic' Intrusives Under this heading are grouped three main types of rocks which bear a close resemblance and are regarded as related members belonging to the great masses of 'granitic' rocks underlying much of the present and neighbouring areas. - 6 - Quartz monzonite to quartz diorite. -The first type, known to be slightly older than the other two, is a medium-grained quartz nonzonite to quartz diorite. It forms a mass in the northwest corner of the area, wedged in between the volcanics to the northeast and the anorthosite- gabbro complex to the south. It is known to ex- tend for a considerable distance in the west beyond the limits of the present map-area. It is also seen in small bodies cutting the Keewatin rocks of Gogland lake,and it is itself repeatedly cut by, and included in, the granite on the south side of the eastern part of Ramsay bay. Small dykes and masses of a rock similar in appearance to the quartz diorite also cut the anorthosite- gabbro complex in several localities. The rock is light grey in colour with an occasional faint pinkish tinge. The ferromagnesian content is rather low,biotite being the domi- nant representative. Plagioclase seems to prevail largely over all other minerals,and quartz is present in varying amounts in irregular grains. Around its border, the rock is commonly finer- grained, dark grey and more typically dioritic. A well-marked to weak gneissosity is found throughout most of the mass and, in some localities, a faint banding is recognizable. Gneissic syenite. - The second type of sgranitic= rock is a syenite which is suspected of being a quartz-free facies of, and gradational into, the granite. It occupies practically all the southern part of the map-area from the southern boundary to Iserhoff river. The rock is nedi- um- to coarse-grained and reddish, carries hornblende and biotite, and is of very uniform composition throughout its whole mass.

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