RP 383(A) PRELIMINARY REPORT ON PERCHE - POITOU AREA, PONTIAC ELECTORAL DISTRICT P. R. NO. 383
DEPARTMENT OF MINES
HON. W. M. COTTINGHAM, MINISTER
GEOLOGICAL SURVEYS BRANCH
PRELIMINARY REPORT
ON
PERCHE - POITOU AREA
PONTIAC ELECTORAL DISTRICT
BY
R. -.A. MARLEAU
QUEBEC 1959
P. R. NO. 383 PRELIMINARY REPORT
on
PERCHE - POITOU AREA
PONTIAC ELECTORAL DISTRICT
by
R.-A. Marleau
INTRODUCTION
The Perche-Poitou area, examined during the summer of 1958, covers approximately 207 square miles and is bounded by longitudes 760301 and 76°451 and by latitudes 46015' and 46030'. It includes parts of Perche, Poitou, Flandre, Isle de France, Gillies and Bourgogne townships in the electoral district of Pontiac. The area is about 70 air-miles northwest of Ottawa and 25 air-miles west of Maniwaki. The easiest access is by float plane from a base on Oblats lake, near Maniwaki. However, good gravel roads afford cheaper means of access to the southern and eastern parts of the area. A road, known locally as the Picanoc, extends from Otter Lake villsr,e to Os- borne lake where it bifurcates to follow John Bull creek and the east branch of Coulorige river. These are private roads owned and operated by the Gillies Brothers Company. Secondary roads branching from these main roads give access to the western half of the area. Abatis lake (Windfall lake) may also be reached by a good gravel - road (Eagle road) which branches from highway No. 11 near Maniwaki. This road also gives access to the south- ea sternmost corner-of the area. The Turtle road extends from the Eagle road to Brodtkorb lake from where, by canoe, there is easy access to Cameronian lake. The area is a part of the Laurentian Uplands of the Canadian Shield. The highest point, immediately south of Hobbs lake is 1,500 feet above sea-level. The lowest elevation, in the south- west corner, is slightly less than 800 feet above sea-level. The surface of the area is gently rolling, with some ridges or hills rising 150 to 400 feet above the general level. The northeastern part of the area is characterized by rounded hills, and the south- western part, by northwesterly-trending hills with, generally, a, scarp on the southwest side and a gentle slope on the northeast. These scarps may extend for miles in a straight line, and possibly result from erosion along fractures. The east branch of Coulonge river flowing south to the Ottawa, receives most of the drainage of the area. In the northeast, the drainage is to Pythonga lake and thence, via Aigle river,. to the Gatineau. Most of the streams are not navigable owing to the many rapids. Swamps and beaver ponds are numerous.
•The area immediately to the west, along Coulonge river, was examined by Retty (1933), and the adjoining.area to the south, by Kretz (1957). Another area 17 miles to the north was studied by Lyall (1957). GENERAL GEOLOGY All the consolidated rocks of the area are crystal- line and are believed to be Precambrian in age. They lie in'the part of the Canadian Shield known as the Grenville subprovince. Crystalline limestone is common only in a belt along the east branch of Coulonge river. The rock is interlayered with various kinds of paragneisses, quartzite, and pyroxene-rich rocks. The paragneisses fall into two distinctive types, one containing and phe other lacking interlayered injections of grey or pink granitic material. Small bodies of gabbro, fine-graines , basic rocks, and some ul•trabasic rocks occur as conformable lenses within the various gneisses. Granitic gneisses and associated injection gneis- .ses predominate in the area. Granite, pegmatite and some syenitic rocks intrude all the gneisses. Diabase dykes, considered as the youngest rocks, occur here and there in the southwestern part. Much of the area is mantled by glacial moraine and exposures are consequently' sparse in places. Other glacial fea- tures are eskers and crescentic cracks or friction cracks. - 3
TABLE OF FORMATIONS
Recent and Sand and gravel Pleistocene
? Diabase dykes Pink granite and pegmatite
VILLE Syenitic rocks N Granitic gneiss and injection gneiss
AN Meta-gabbro and associated basic, and ultrabasic rocks RI POST- GRE AMB
C Paragneiss with lit-par-lit injections of E grey and pink granitic meterial R P Paragneisses: Hornblendic, biotitic, SERIES garnetiferous LE L Impure Quartzite NVI
GRE Crystalline limestone, pyroxene skarn
GRENVILLE SERIES Crystalline Limestone, Pyroxene Skarn The limestones are interbedded with the paragneisses. They occur as lenses and irregularly-shaped masses a few inches up to 30 feet thick. The limestones are crystalline and, except for a few pink layers, are white. The rock is composed mainly of cal- cite, with some phlogopite, diopside, biotite and locally actino- lite. Siliceous inclusions are common. The alignment of the micaceous minerals gives the rock a faint layered structure. In the area, the skarn is a massive medium-grained, light green to deep green rock made up chiefly of diopside, with some calcite, quartz, and biotite. It occurs as small irregular- ]y-shaped masses within or close to the limestone zone. Impure Quartzites
Impure quartzites are interlayered with the various paragneisses and, rarely, with the limestone. They are most com- mon in the east central part of the area, immediately south of Stony creek.
The rock is fine-grained, light grey on fresh sur- faces and weathers brownish grey. It generally occurs in beds several feet thick marked by laminae with oriented mica flakes. Quartz, feldspar and a little biotite may be recognized in hand specimens.
Paragneisses Mafic paragneisses are abundant in the area, and are everywhere associated with rocks of other types. Thus, their areal extent as shown on the map is merely indicative of domi- nance of paragneisses over other types. For purposes of descrip- tion, they may be divided into paragneisses without injections and paragneisses with injections, but these are not mappable units. The paragneisses without injections have a grey to dark grey colour depending on the quantity of mafic minerals present. They are commonly characterized by a layered structure of primary origin, the different layers being made up of various combinations and proportions of minerals. The layers vary from a fraction of an inch to several feet thick.
These paragneisses are represented by a variety of rocks that can be designated by typical mineral assemblages. The most common varieties are hornblendic and biotitic paragneis- ses. Garnetiferous hornblendic and garnetiferous biotitic gneis- ses are common, whereas sillimanite gneisses are rare. In hand specimens, hornblende, biotite, quartz, feldspar and garnet are the principal minerals identified. The biotitic paragneiss is well laminated. The hornblendic variety exhibits a typical salt and pepper texture. The paragneisses with lit-par-lit injections con- sist in most cases of alternating grey or pink granitic laminae alternating with biotite and/or hornblende rich layers. In gene- ral, the contact between the normal paragneisses and the injec- tions are sharp, but gradational contacts were also observed. Megascopically, these rocks differ from the normal paragneisses in their lighter colour and the presence of pink or grey granitic and pegmatitic layers as well as of secondary layering. 5
POST-GRENVILLE Meta-gabbro and Associated Basic and Ultrabasic Rocks These rocks occur as isolated exposures throughout the area and form relatively small bodies. They are associated with most of the major rock types and, although their relation- ships are not well-known, they are assumed to represent remnants of basic rocks that have been metamorphosed along with the enclo- sing rocks. The meta-gabbros may be correlated with similar and larger bodies to the south which are described as concordant with the local structure. Most of the meta-gabbros are medium-grained, with an ophitic texture easily recognized on the weathered surface, massive and greyish brown. In hand specimens, hornblende, plagio- clase quartz, garnet and calcite were identified. Classification within this group is again arbitrary and is based mainly on the grain size and on the proportion of mafic minerals to plagioclase. A coarse-grained variety of meta- gabbro or amphibolite seems to contain mainly hornblende and bio- tite with some plagioclase. An isolated exposure of ultrabasic rock occurs within the granitic gneiss. The rock is dark grey to black, fine- grained, massive, and is composed chiefly of mafic minerals. Me- gascopically, the rock is similar to ultrabasic rocks of surroun- ding districts. Granitic Gneiss The greater part of the area is Underlain by granitic gneisses, mainly grey but with scattered associations of pink. The two varieties are essentially similar except in colour. Both are fine- to medium-grained .d. cnri(,nly composed of quartz and feldspar with minor biotite and:hornblender. The gneissic structure varies from faint to well developed. In some cases, the intimate introduction of material has produced'a granitic rock of almost homogeneous composition. An exemple is the belt r few miles east of Argent lake, where the rock has the lappearr ce, of a foliated granite. Both gneisses appear to be injective._. the grey variety injected into paragneisses and the pink into the grey variety as well as into paragneisses. The "foliated granite" mentioned above may represent concordant intrusive bodies which were the source of some of the grey injected material. The pink injected material may be correlated with the small bodies of pink granite and pegmatite described below. - 6 -
Pink Granite and Pegmatite, Syenitic Rocks The granite and pegmatite are pink, medium- to coarse-grained, massive rocks, composed mainly of quartz, micro- cline and biotite. Although equigranular types are present, most exposures consist of an inequigranular rock varying from granite to pegmatite, and often intermixed. A gneissic structure may also be observed at the margin. Syenitic rocks are exposed at a feu places along the east branch of Coulonge river. They seem to occur as small lenses within the various gneisses of that belt. A massive and a foliated facies are recognized; in both cases the rock is light grey. The massive type is coarse-grained and may be described as a pegmatitic syenite. The rock is made up chiefly of micro- cline with some plagioclase and very minor amounts of hornblende and sphene. The foliated variety is fine-grained, and the folia- tion is concordant with the local structure.
Diabase A few diabase dykes, up to several hundreds of feet wide, have been observed in the area. One dyke is particu- larly well exposed in a cut on the road that follows the east branch of Coulonge river. The rock is black, fine- to medium-grained, and massive. The sub-ophitic intergrowth of plagioclase and pyro- xene is well seen on the weathered surface. These diabase dykes cut both the grey and pink injection gneisses, and are thence considered the youngest rocks in the area.
PLEISTOCENE AND RECENT Most of the area is covered by glacial and recent deposits of variable thicknesses. Sand is predominant, and the best sections are seen in the valley of the east branch. of Cou- longe river and around Ward lake. Short esker segments are pre- sent northeast and southeast of Bush lake. The destructive effects of glaciation are practi- cally not apparent in the area. No striae and very few crescen- tic cracks were recorded. This is unusual considering the number of exposures. Recent alluvial deposits consisting of gravel, sand and silt partly fill stream valleys. Deltas are being built at the heads of Ethyl and Ward lakes. STRUCTURAL GEOLOGY Internal structural features, such as lineation and foliation, are conspicuous in most rock types off' the area. Larger structural features, as folds and probably joints, are also common. Folds In detail, the folds are complex and are marked by drags, crenulations, and general undulations of the gneissic structure. On a regional scale, two major structural trends are recognized and are believed to belong to the same major complex fold.. In the southern and western parts of the area the structural trend is generally northwest and the dip is generally easterly at less than 45 degrees. This homocline is probably more complex than shown on the map. Several folds were observed within the homoclinal structure but could not be recognized more than locally because of a lack of distinctive horizon markers. Towards the northeast the structural trend of the homocline changes from northwest to west, and the dip is north. In the northeastern part of the area there is a suggestion of a northwesterly plunging anticline, as outlined by the iso- magnetic lines and some structural features of the rocks. Faults A fault may possibly occur along a depression joining Argent and Abatis lakes, as indicated by the depression itself and by the steep cliffs standing up along this line. Joints The linears shown on aerial photographs indicate that joints are probably abundant in all rocks of the area. However, there is little support for this from field observa- tions. ECONOMIC GEOLOGY Pyrite Pyrite occurs in a number of exposures along the east branch of Coulonge river, within quartzite and talc-sili- cate rocks interbedded with crystalline limestone and other para- gneiss. In places, the amount of pyrite present is as much as 5 per cent. No other economic minerals were seen associated with the pyrite. Pyrite-Graphite A pyrite-graphite association was observed in finely laminated paragneiss immediately east of Billot lake in the east central part of the area. The paragneiss is light grey and is probably quartzitic. The weathered surface is rusty. In places the rock is apparently sheared. Quartz with minor amounts of pyrite, graphite and biotite were recognized. Iron Small pods of magnetite are common within most pegmatitic rocks of the area. A medium-grained, magnetite-rich layer, with an estimated iron content of about 3.0 per cent, was recorded in the southwestern corner of the area. This band is in contact to the east with a highly garnetiferous gneiss. It is followed to the west by an outcrop of biotite gneiss. The occurrence is a few tens of feet wide. A band 10 feet wide of low-grade iron is pre- sent a short distance southeast of McCullough lake within the granitic gneiss. An analysis of a grab sample taken at this locality gave 8.25 per cent iron.
REFERENCES Kretz, Ralph (1957) Preliminary Report on Pontefract-Gillies Area; Quebec Dept. of Mines, P.R. No.357 Lyall, H.B. (1957) Preliminary Report on Hainaut-Champagne Area; Quebec Dept. of Mines, P.R. No.345 Retty, J.A. (1933) Reconnaissance along the Coulonge and Black Rivers, Pontiac County; Ann. Rept. of Quebec Bur. Mines for 1932, pt. D. pp. 83-108.
M-X-30