KEEWATIN.
General Features.
This system is considered as a complex assemblage of metamorphosed igneous rocks whose common and marked characteristics are pronounced alteration and deformation, accomplished in pre-Huronian times. A limited amount of sedimentary material, such as the iron ore formation, is also represented. These old diabases, porphyries and related types are much altered and have developed a more or less uniform schistosity through the secondary development of micaceous minerals, but in other respects the complex shows extreme inconstancy and variety from point to point. For this reason the various localities are separately described. By inspection of the map three fairly well defined areas are distinguishable, in addition to which are portions of several others.
Obushkong area.
Much of the country between Obushkong and Firth lakes is underlain by Keewatin. On the east, south and west sides, these rocks disappear beneath the Huronian or are interrupted by masses of diabase, but on the north they merge into Laurentian gneiss, the contact with which is ill-defined. Although some of them retain much of their original massive character, well defined schistosity is the dominant feature. The schists stand vertically or at angles little less than 90° and trend in a general east and west direction. A series of compass observations made at points over the whole area show the strike to vary from N. 65° E. to S. 75° E.
One of the most abundant rock types is a stratiform, finely speckled hornblende gneiss or schist, the black hornblende cleavage faces giving it a glistening appearance on newly broken surfaces. It is quite fresh, perfectly crystalline and usually eminently fissile, but sometimes grades into a nearly massive dioritic form of undoubted igneous nature. Extensive exposures occur around Gould lake where the gneiss is traversed by numerous stringers of quartz, rusty in colour from the oxidation of pyrite. It is also well exposed near McLaughlin and McIntosh lakes and to the east and south-east of Foot lake. A fine grained chlorite schist of dull greenish black colour is common in this and all the other areas in the district. To the north-east of Serpentine lake it appears as a sheared phase of a weathered diabase, but it has also been derived from porphyry, exposures being seen between Foot and Obushkong lakes, where feldspar phenocrysts appear on weathered surfaces of the schist as pale, oblong spots. What is probably iron formation was observed at points 25 chains south of Gould lake, and 10 chains south of a little pond just east of Serpentine lake. Both outcrops consist of banded, grey quartzite interlaminated with chlorite schist, but magnetite-bearing bands were not found. Occasionally, narrow bands of pale grey, felsitic schists may be seen among the more common darker rocks. From evidence obtained at various points, these appear to have resulted from the decomposition and shearing of granite porphyry dikes probably connected with the Laurentian, and which penetrated the Keewatin during the time of Laurentian intrusion. Serpentine was observed between Foot lake and Obushkong at 20 chains from the latter. The surface is covered by a loose network of fine seams of asbestos which weather white and render the rock somewhat conspicuous. Its recognition is further simplified by the dull green, amorphous appearance of fresh surfaces, the slight translucence of thin edges and the glistening green seams of asbestos which traverse it abundantly. The same rock is more extensively exposed on the south-west of Serpentine lake and on Firth lake half a mile north of the portage leading to the former, also at less than a quarter of a mile south of this portage. It is associated with and derived from a dark green massive rock to which the name wehrlite is applicable, and a more detailed description of which appears later.
Duncan Lake Area.
A Keewatin area of considerable extent lies between Duncan lake and the West branch in the vicinity of L’Africain and Beaverhouse lakes. Unlike the Obushkong area, the prevalent strike of the schists is nearly north and south, the greatest divergence noted being 25° W. In the former case Laurentian lies to the north, while in the present one it occurs on the east; in both cases the schistosity coincides approximately with the direction of the line of contact. On account of the swampy character of the country just west of the river and the scarcity of outcrops, this area was not completely explored, but wherever examined the Keewatin, as in the Obushkong area, consists predominantly of hornblende and chlorite schists, greenstone and decomposed diabase; but serpentine was not found. A nearly black diabase containing small grains of pyrite was observed 20 chains south-east of the southerly extending bay on Duncan lake, and outcrops of the same material were traced for about half a mile northward. Probably the same type was encountered just north of Beaverhouse lake and at some other points. Its unfoliated condition and fresher appearance than the adjacent rocks lead to the opinion that it is really post-Keewatin and intrusive, but the absence of younger rocks with which to correlate it, renders its exact age indeterminable. This rock exhibits enough resemblance to fractured contact edges of the post-Huronian diabase to make their distinction in the field rather difficult. At half a mile east of the south end of L’Africain lake is diabase which from lack of sufficient data has been mapped as Keewatin, although it may be identical with the post-Huronian variety. L’Africain lake lies in glistening hornblende gneiss with which are associated ribbons of a coarser hornblende gneiss belonging to the Laurentian of the area farther east. Greenstone and chlorite schists are the principal rocks around Beaverhouse lake. Associated with them and to the northward, are dikes of light coloured, granite porphyry, usually only a few feet wide, in some of which the original massive structure remains, while others show various gradations toward a felsitic, sericite-bearing schist. The isolated patch of Keewatin indicated on the map as occurring about half a mile east of the wide portion of Duncan lake, is a well foliated green schist in which oval white spots represent squeezed feldspar phenocrysts.
Fig. 4. View looking north from middle of Duncan Lake, 550ʹ hill in distance.
Pigeon Lake Area.
West of Pigeon lake and the Montreal river, much of the country examined is underlain by Keewatin. Schistosity is less developed than in either of the preceding areas. The rocks are chiefly diabases and porphyry much decomposed and locally squeezed. Diabase occurs on both sides of Pigeon lake near its central islanded part and along the bay which extends south to Brush lake. An original diabase structure is sufficiently well preserved to show lath-like feldspars in hand specimens, but frequently the rock is altered to a chloritic mass. Around the bay extending toward Brush lake, this old diabase forms a rude wall intersected by a reticulating system of fractures filled with calcite, the resultant structure simulating that of a breccia. Shear zones seen on the east shore exhibit a pseudo-conglomeratic structure, the more resistant pieces of diabase having been partially rounded by the shearing movement and embedded in a matrix of finely pulverized rock matter. The same diabase apparently occurs all along the Montreal river where it follows the Algoma-Nipissing line. About half a mile above Pigeon lake it forms a 90 foot cliff on the west side of the river, near the top of which is a fresher looking, unfoliated eruptive. The form of this body was not ascertained, but microscopic examination of the specimen taken, determines it to be a hornblende lamprophyre, so probably it is a dike or thin sheet. The rock is fine-grained, dark green in colour and characterized by stout prisms of hornblende about one-quarter of an inch long, embedded in a finer, microcrystalline ground mass. Under the microscope it appears much decomposed. This rock was also observed near mile post 67 of the Algoma-Nipissing boundary, near the east end of the 63 chain portage leading to Breese lake and at other points, in all cases the exposures being of small extent.
In the last mentioned locality the predominant rock is an altered porphyry of ash grey colour. Around Porphyry lake it occurs in a fairly massive, easily recognizable condition, the feldspars showing as square white spots one-quarter of an inch in diameter in a grey, ground mass. On Breese lake it has been squeezed to a felsite schist, striking N. 20° W. In small amounts it is associated with post-Huronian diabase on the hills lying near the Montreal river and just south of the 63 chain portage.
The 300 foot hill south-west of Mosher lake is composed of a fine Keewatin greenstone whose surface is curiously weathered so as to suggest a spheroidal structure. It is marked off into round areas a foot or more in diameter by a sinuous network of weathered-out grooves. The main body of the rock is ordinary fine-grained greenstone, but the enclosing grooves have been formed in porphyritic zones about an inch in width. Among other types of less abundant distribution is an actinolite rock seen by the unassisted eye to consist of a felty mass of acicular crystals of actinolite, sometimes half an inch long. This rock was observed 12 chains east of the islanded part of Pigeon lake; also at points west of Montreal river below Pigeon lake.
Unfinished Areas.
The Keewatin formation occupies the space between Near lake and the West branch, only part of which has been mapped. The rocks of this area are well foliated, standing as usual, vertically, and striking about N. 60° E. The exposures on the east shore of Near lake are entirely of fissile, chlorite schist, abundantly traversed by small barren quartz veins. Farther east the chlorite schist gives place to glistening, stratiform (laminated) hornblende gneiss with which are associated bands of a coarser hornblende gneiss, evidently the equivalent of granite. In one locality the stratiform rock contains subangular fragments of the coarser variety, quartz and a greyish eruptive rock, the resultant structure resembling that of a metamorphosed conglomerate or breccia. As some of the brecciated fragments occur near by in continuous bands within the stratiform gneiss, the clastic structure is to be ascribed to deformative movement instead of original sedimentation. A number of fine-grained quartz diabase dikes occur in the immediate vicinity.
A considerable portion of the east shore of Kenisheong lake consists of high, bare Keewatin hills, composed largely of chloritic schists, vertically inclined and striking east and west. Secondary calcite is richly disseminated, causing free effervescence when the rock is treated with acid. Pyrite is also abundant, sometimes segregated to form a lean ore, and at several points weathered superficially to limonite, colouring the cliffs dull red. Much-altered diabase like that on Pigeon lake is also present in subordinate amount.
Fig. 5. West Branch Montreal River, near Mosher Lake.
Keewatin rocks also occur just east of Gowganda lake. A single brief visit was paid for the purpose of observing the iron formation which appears at this place, and further mention of which is made on a subsequent page.