Preliminary Report on Gowganda Mining Division District of Nipissing Ontario

by W. H. Collins

HURONIAN.

General Features.

Much of the area mapped, especially the north-western portion, is composed of Huronian rocks. Originally they must have been much more extensive, probably continuous, but erosion has removed them partially or entirely, leaving irregular remnants distributed over the Archæan. The top of the series is gone; consequently complete vertical sections cannot be found. In other respects they are little changed and preserve almost perfectly their bedding and clastic structure. Even their positions assumed at the time of deposition have changed little for the present strata seldom dip more steeply than 30°. They are overlain only by unconsolidated Pleistocene materials.

Basal Conglomerate.

The basal member of this system is a conglomerate, the areal exposure of which is inconsiderable for the whole region or for any part, but erosional sculpturing has brought it to light at many points. Both top and bottom and probably all intermediate portions are visible, but a complete section from which to determine the thickness could not be found. The greatest continuous vertical section observed, in a hill lying one mile south of the 550 foot hill north-west of Duncan lake, is about 200 feet, but neither upper nor lower terminations were seen. In the neighbouring Cobalt district an estimated thickness of 500 feet has been assigned.

From a study of the pebbles contained, this conglomerate appears to be composed largely, or entirely, of Archæan materials, the majority of which are rock types occurring in situ at many places in the district. Both the hornblende and biotite granite and their gneissic phases are abundant. Pegmatite pebbles are sometimes found, but more usually that rock is represented by fragments of feldspar. Pieces of glistening stratiform hornblende gneiss and similar stratiform mica gneiss or schist, greenstone, fine-grained porphyroids and sheared basic rocks represent the Keewatin. Vein quartz is also present, sometimes mineralized. In addition to these some pebbles were observed which closely resembled slate, and one which is undoubtedly conglomeratic. A thin section of this pebble exhibits distinct clastic structure, and the assembled fragments are heterogeneous, so far as distinguishable, including an actinolite rock resembling that found in the Keewatin near Pigeon lake, and a quartzitic type composed of a colourless, microcrystalline mosaic. The cement has been altered largely to chlorite, but small grains of quartz are discernible in it. It would seem, therefore, that sedimentary deposits antedate the basal conglomerate and supply a portion of its materials; most of these, however, are recognizable as igneous members of the Archæan, such as occur in the vicinity.

The pebbles of the conglomerate exhibit remarkable variety in form, size and abundance, as well as composition. Normally they are well rounded and owe their form evidently to water action. But along with these are others which are angular or sub-angular. The exposure near the north end of Shallow lake shows all these forms, the materials also being quite diverse. Ordinarily the pebbles range in size between 2ʺ and 3ʺ diameter, frequently more, and are abundant; but, at points on Duncan lake, on Wapus creek and the West branch, the conglomerate nature is indicated only by occasional well-rounded pebbles, embedded in greywacke cement at intervals of several feet or even yards. These isolated pebbles are often 6ʺ or more in diameter, and in striking contrast with the uniformly fine grain of the matrix. Sometimes the enclosed bodies attain the dimensions of boulders; on an island in Duncan lake, 2¹⁄₂ miles from the foot, a granite boulder nearly 5 feet in diameter was observed, the associated materials being of very much smaller size.

The cementing material also presents some variety. Usually it ranges from coarse grit to greywacke. The coarser material is less abundant than cement of a finer type; near the foot of Pigeon lake, also at the south end of Kenisheong lake the conglomerate is quite porous, the interstices between the pebbles being incompletely filled. In the latter instance these spaces are occupied by hematite. In other cases; a good example of which occurs on the east side of Pigeon lake, half a mile from its north end, the cementing substance is a fine black shale. On the whole there appears to be little relationship between the texture of enclosing and enclosed matter.

Greywacke Slate and Quartzite.

By the disappearance of pebbles the basal conglomerate changes into a greywacke or a shale according as the cement is one or the other. Frequently this transition is marked by an alternation of lenticular beds of conglomerate with the greywacke, probably indicating varying conditions of depth or current in the water in which they were deposited. With the greywacke and shale is associated an impure quartzite or arkose, the whole forming a thick series whose members are not sharply separable one from another and do not occupy definite relative positions. For the well laminated finer grained beds the term slate is in general use, although a secondary cleavage by which this kind of rock is distinguished from shale does not exist. Nearly all the prominent hills in the north-western part of the area are composed of this series.