Diamonds in the Apple Formation?
Claude Dion and Jean Goutier
Direction de Géologie Québec
Apple Formation and alluvial
diamonds – A comparison
Given the similarity between these different
paleoplacer-type Archean deposits, the possibility of finding diamonds
in conglomerates of the Apple Formation is certainly worth considering.
A few factors support this possibility:
- Sedimentary rocks of the Apple
Formation were deposited on a basement that represents a Mesoarchean
craton. This setting suggests a relatively early lithospheric
root for this part of the Superior craton, which is considered
a favourable criterion for the formation and preservation of diamonds
in the mantle.
- Although no Archean kimberlites
have yet been recognized in the Lac Yasinski area, the fertile
nature of the underlying Archean mantle is indicated by the presence
of diamonds in Wawa-type heterolithic mafic to ultramafic dykes
sampled by Dianor Resources in the same area (PEM 1404 in
map sheet 33F/06 and Lac de l’Astrée in map
sheet 33F/05; Ryder, 2002). These Archean dykes are cross-cut
by Proterozoic gabbro sills, and are interpreted as late to post-tectonic.
Paquette (1998) conducted a detailed
study of uraniferous conglomerates of the Apple Formation, which
did not reveal the presence of diamonds. However, a petrographic
study of thin sections would probably not be sufficient to test
this hypothesis given the restricted size of the samples and the
low diamond count. A study based on a heavy mineral concentrate
from a larger sample (a few tens of kilograms) should be conducted.
The area around the Apple deposit is a good target area of course,
but several other lenses of weakly uraniferous conglomerate are
known elsewhere in the Apple Formation, in map sheets 33F/03
and 33F/06. The same exercise could also be undertaken in conglomerates
and quartz arenites of the Proterozoic Sakami Formation, which are
locally uraniferous as well.