Program, Abstracts, and Guidebooks - University of Minnesota Duluth

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WIDESPREAD OCCURRENCE OF ALUMINOUS MINERALS IN APH.EBIAN QUA.RTZITES
GRANT M. YOUNG
Department of Geology
University of Western Ontario
ABSIRACT
After the conclusion of the world-wide Kenoran thermo-tectonic events
(Ca. 2.5 b.y. ago) there was development of the first extensively preserved
stable shelf assemblages of the geological record. Rocks of this type
were first studied in Canada in the region north of Lake Huron by Murray.
(1849) and Logan and Sterry Hunt (1855). The Huronian succession includes
several polyrnictic conglomerates which have been interpreted as glacial
deposits. The youngest of these conglomerates (Gowganda Formation) is thick
and extensive and has recently been considered correlative with other
early Proterozoic (Aphebian) tillites in a large area extending from S.E.
Wyoming to the Keewatin District of the N.W.T. (Young, in press).
The upper stratitified unit of the Gowganda Formation is overlain by
a thick (5-6,000 ft.) quartzite formation (Lorrain) that is also very
extensive. Many different subdivisions of this unit have been proposed,
but on a regional scale, a threefold subdivision seems most reasonable.
The lowest subdivision is a varicoloured (red, white and green) succession
of felspathic grits and sandstones. This is followed by a unit charac—
tensed by the presence of quartz and jasper pebble conglomerates and the
uppermost unit is an extremely pure orthoquartzite. In many areas the
middle unit and the lower.part of the upper unit contain aluminous minerals
such as kaolinite, diaspore, pyrophyllite, kyanite and andalusite (Church,
1967; Chandler et al., 1969). These minerals are thought to represent
an in situ weathering (bauxitization) process which occurred shortly
after deposition and gave rise to kaolinite which wa.s later changed by
further diagenesis and metamorphism to the other minerals listed above.
The reasons for invoking this mode of origin rather than origin by deposition
of "primary" kaolinite at the time of sedimentation or by late
post depositional weathering are as follows: -
1. Fresh felspars are abundant in other Huronian outcrops.
2. The kaolinite commonly occurs as "clots" of the same order of
size as the associated quartz grains, suggesting that each clot
represents an altered felspar grain.
3. It is difficult to envisage the conditions under which fine
Jcaolinitic crystals could be sedimented together with coarse
quartz grains (see Ojakangas, 1965, for discussion of the same
problem in Jatulian quartzites of Finland).
4. In some sections metamorphic minerals may be seen developing
from kaolinite.