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

ISERHOFF RIVER AREA

ABITIBI-EAST COUNTY

JACQUES CLAVEAU

QUEBEC 1947

P. R. No. 197 ISERHOFF RIVER AREA

ABITIBI - EAST COUNTY

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 mostly 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 sediments, and rare beds of pure, fine-grained, white quartzite. A small band of conglomerate with deformed 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. It is habitually

49.45' 49°45'

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► ° \\\ ~ r /X X X X X X X_~ X X rM^N Shear zone=Zone de cisaillement gOPj1dh !, L tmbou/t X X x X X X Trail or aorta ge=Sentier ou /oorfoyce. ~ X~q X X X X X X \ o - 4o. . . ..•.,~~`~`~ - ♦ ~ a .y ~ . 7mbè1,/~~x X ~a ~_ \- X X X X _ _~~ ' C;• • \',41111‘ Y . ~ r. . ~ -~~~T E I ••~ ``\ ~ \....1101111. ` \ N ' . `g\ ~6 (' ~\ ~1~ ~~_~ \ 1 ~~ x - ~x ! •r 6 `1 R •X X X ''---->' o ~X X X ~ a renl rho~f Pr, x ~ x x x / / ~: js e /, , ~ s \ 6 F, . X g X X 2X X X X \J 7( 7C / .<4.J X X ~ X X X X X 100 N- "X•rs x X2 X X X X X X X X X.„____ X ; Miles 41i//es1 s Mr~~ dd ,~y~ 3X ~ x x X x x x x X x X x x x x ~ ,Llinisttre des Mines,Qutbec, No618- GEOLOGY eY J.CLAVEAU,i946. éo/o9iePorJ.C/oveou, 4 6 B ISERHOff RIVER AREA REGION DE LA RIVIERE ISERHOff COUNTY OF ABITIBI-EAST COMTE D'ABITIBI —EST Miles Milles 7 - gneissic although sometimes the strike, and frequently the dip, of its gneissosity cannot be re- liably ascertained.

Granite. - The last and most wide-spread type of 'granitic' rock is a pink granite which occupies the whole eastern half of the map-area from Goeland lake southward for nearly twelve miles to the northern border of the volcanic belt, two to four miles north of Iserhoff river. The typi- cal granite is medium-grained, fresh and massive, with a low biotite content, and in many localities it grades to a pegmatitic facies in which the biotite gives place to muscovite. Small, individual masses of muscovite-bearing pegmatite and vein quartz which appear to mark the final expression of the granitic invasion are found abundantly cutting all the rocks of the area.

Basic Dvkes

A few basic dykes which appear to be lam- prophyres cut the rocks of the anorthosite-gabbro complex. They are altered and frequently deformed and are cut by pegmatite in one instance. It seems that these lamprophyre dykes,as they are not found cutting the 'granitic' rocks, are older than the latter.

One, sah.all,fresh diabase dyke cuts granite on one of the islands of Gogland lake.

Unconsolidated Sediments A prominent accumulation of unconsolidated sediments forms a band from one to two miles wide extending diagonally across the map-area from the south of Baptiste lake to the eastern part of Ram- say bay. The material of the band varies greatly - 8- in different localities. South and east of Bap- tiste lake, it consists of yellow clayey sand which forms particularly extensive plains. North of Imbault lake, clay is more abundant and the band there has been deeply dissected into ravines and steep-sided hillocks by a large stream and its tributaries which traverse the band. Farther north, toward Ramsay bay, the material of the band is largely coarse gravel.

STRUCTURAL GEOLOGY

Folding The rocks of the main volcanic belt trav- ersing the southern part of the map-area give evidence of close folding along east-west axes. How. ever, the nature of the folds and the localization of their axes still remain obscure, because of the absence of key horizons and the lack of informa- tion on the attitude of the beds. Pillow structure which could be relied upon for determining the attitude of the lava flows and interbedded sedimentary rocks was noted in one locality only. It is obvious that the intense metamorphism is largely responsible for the obliteration of most of the original features of the volcanic and sedimentary rocks.

The lava flows and sedimentary rocks almost invariably exhibit a strong bedding schistosity in an east-west to northeast-southwest direction, that is, parallel to the trend of the belt and as sumed fold axes. The associated gabbro and diorite bodies are generally more massive, although sometimes schistose especially along the edges, whereas the amphibolitic member of the series is gneissic rather than schistose. The dip of the - 9 -- regional schistosity is, as a rule, consistently to the north at an angle generally exceeding 70 degrees.

Certain zones along the northern border of this southern belt,where the strike of the schistosity differs drastically from the regional trend, may be explained as being the result of a local disturbance, such as 'splitting open' of parts of the belt during the emplacement of later intrusives. Two such prominent zones occur north and northeast of Baptist-lake. Complex movements are also recorded in the narrow band of volcanic rocks in the granite along Imbault creek, one mile below the lake of the same name.

The northern area of Keewatin rocks exhibits a strong schistosity which, as is the case for the southern belt, parallels its contact with the surrounding intrusives. Its strike varies from northwest-southeast,in the western part of the map-area, to nearly east, in the eastern part, and its dip is very steep to the northeast and north. The long, narrow, south-trending zone of volcanic rocks extending south of Ramsay bay appears to be a wedge- like fringe of the northern belt which has been thrust aside to its present position by the invad- ing granite.

The prominent, structural feature of the anorthosite-gabbro complex is its primary banding which roughly parallels its southern and eastern border. As this banding was most probably horizontal in its original state, but is now rudely concordant in strike and dip with the adjacent Keewatin rocks, particularly in that part of the area lying north of Baptiste lake, it would seem that the anorthosite-gabbro complex and the Kee- watin rocks must have been folded at the same time. -10- The quartz diorite occurring between the anorthosite-gabbro complex and the western part of the northern area of volcanics exhibits a mild to strong gneissosity which varies in orientation between west and northwest and dips north 60 to 80 degrees.

The syenite along Iserhoff river displays a fairly persistent gneissic character which parallels its contact vrith the southern border of the belt of volcanic-sedimentary rocks.

Shearing

Only one well-defined shear zone has been observed in the area. It is localized at the contact between granite and volcanics along the east shore of the deep, narrow bay projecting south from the middle part of Ramsay bay. The direction of shear is close to north-south and rarallel to the trend of the volcanics, as well as to that of the contact between them and the granite. Its dip is steep to the east, in the vicinity of 80 degrees. Along its strike, the shear zone has been recognized from the shore of Ramsay bay to a point a little less than one mile to the south, beyond which exposures are lacking. At the contact where the shearing has been most intense, the volcanics have been converted to a crumbly, chlorite schist and the granite, to a schisted rock. Exposures of the chlorite schist have been found as far as two hundred feet east of the contact, and, to the west, a strong gneissosity still persists in the granite on the opposite shore of the narrow bay, almost half a mile distant.

Local shearing of erratic orientation is apparent at many points in the volcanics around Ramsay bay and Goland lake where the rocks have been intimately invaded and crumpled by the granite. Similar deformation is recorded in the Keewatin rocks exposed one mile east of Imbault lake.

At various other points throughout the volcanic-sedimentary belts, intense schistosity and alteration appear to constitute evidence of shearing movements parallel to the general trand of the belts, but thus far no persistent major zones have been uncovered.

A shear cleavage is prominently displayed in the rocks south of Baptiste lake. It runs some thirty degrees west of north across the bedding schistosity. The shear planes are a fraction of an inch or more apart and slippage along them is less than one inch. Quartz veins following the bedding schistosity are cut and displaced by the shear planes.

ECONOMIC GEOLOGY

The rocks of the volcanic-sedimentary series display mineralization at many points throughout the areas where they are exposed. The mineralization, which consists mostly of pyrite, appears to follows certain bands of the series, generally schistose andesite and gabbro and diorite bands or, more rarely, it is in quartz veinlets intro- duced along the planes of schistosity. Locally, sheared and crumpled volcanics very frequently carry mineralization, as is particularly the case east of Imbault lake and at many points around Ramsay bay and west of Gotland lake. It is felt that the southern belt of volcanics should be thoroughly investigated by the prospector, especially throughout its lower half,as it is suspected that the belt is traversed in that region by shear zones parallel to the trend of the belt. - 12 -

In the shear zone extending south from Ramsay bay, pyrite mineralization was observed in the schistose granite at its contact with the volcanics.

In the anorthosite-gabbro conplex, titaniferous magnetite was not seen in appreciable con- lentrations. However, a very strong and persistent magnetic anomaly which was observed two miles southeast of the west end of Ramsay bay may deserve in- vestigation.

M-1433