Genesis of carbonate till in the lee sides of Precambrian Shield uplands, Hemlo area, Ontario: Discussion

Abstract

Hicock proposed an origin for the carbonate till in the Hemlo area of Ontario that is incompatible with the statistical study of attrition of Paleozoic clasts in a carbonate till between Hamilton and London, Ontario; with the occurrences of limestone mapped along Highway 17 north of Lake Superior by M. Tremblay (consulting geologist, personal communication, 1987); and with the occurrence of Paleozoic limestone at the foot of Boot Bay, Kabinakagami Lake, Lizar Township, mapped by Wahl (1954). Vagners (1969) described the attrition in size and shape of Paleozoic carbonate clasts in the till between Hamilton and London. The clasts in this till are composed predominantly of Paleozoic sediments, and the clasts achieve a roundness and become part of the matrix in the matter of a few kilometres. Hicock describes his calcareous till as a dark grey, compact, fissile, stoney, calcareous diamicton with a matrix of up to 35-40% carbonate. The clasts are described as being subrounded to subangular and to consist of Paleozoic and lithologies in equal amounts. It is difficult to understand how Paleozoic and Proterozoic debris could have been encapsulated as a large blob of debris within the ice north of Hearst and transported as a unit more than 150 km without some attrition in size or a change in form. It is especially difficult to understand when one considers the differences in hardness of the two lithologies. Further, it is difficult to understand the origin of the clasts in the till, as the Ontario Geological Survey has mapped only Archean rocks up glacier from Hernlo. However, rocks do occur southeast of Winisk. Surely these rocks are not the source of the clasts. While mapping the magnetite deposit east of Boot Bay in Lizar Township for Neoscope Exploration in 1954, I found Paleozoic limestone underlying the southwest portion of the bay. This bay is approximately 140 km down glacier from the exposed Paleozoic rocks on the edge of the James Bay basin (Wahl 1954). About 1965, M. Tremblay (personal communication, 1987) mapped the heavy minerals in the till between Hearst and Long Lac north of Lake Superior. He reports mapping Paleozoic limestone along Highway 17 north of Lake Superior. On a practical note, drift prospecting is hindered not only by the presence of carbonate till in the Hernlo area, but also by the Paleozoic limestone from which it was derived.