140

Microscopic examination shows that these bodies are composed almost
entirely of corundum. The only other material, occurring as a patchy
groundmass or as vein-like bands surrounding masses of almost pure
corundum, is a semi-opaque, almost isotropic, white material that probably
contains serpentine, clinozoisite, and perhaps clay-like matter resulting
from the hydrothermal alteration of feldspar.

The corundum-bearing rocks have been found cutting both the serpen-
tinite and the slightly serpentinized pyroxenite and peridotite. No change
in composition or texture of the bodies could be discerned at their borders.
The walls are in all observed places lined by a thin selvedge of dark brown
mica. In one place this selvedge is backed by a zone of coarse black horn-
blende crystals. The wall-rocks of the other bodies either consist of
serpentinized peridotite or grade through a foot or more of grey-green,
very fine-grained, altered material that may be an ageregate of amphibole,
tale (?), and serpentine minerals. The total width of this ‘reaction zone’
in the wall-rock varies from about 1 inch to 3 feet, and was observed to
be almost the same on each side of the body. However, there appears to
be an increase in intensity of serpentinization of the ultramafic rock in a
band 5 to 20 feet wide on each side of the bodies.

Corundum-bearing dyke-like bodies have been found in ultramafic
rocks in many localities, and have given rise to much speculation regarding
their origin. Most of them consist essentially of oligoclase and corundum,
a combination to which the name plumasite has been given. Most of the
plumasites described are associated with, or grade into, rocks consisting
mainly of oligoclase. Larsen (1928) has summarized the descriptions of
many of these occurrences and the hypotheses regarding their origin. He
emphasizes the similarity of the ‘reaction zones’, usually consisting of
concentric layers of dark mica, amphibole, and tale, which frequently bear
no relation to the width of the dykes, and postulates that the corundum-
bearing bodies are not true dykes but deposits formed by alumina-rich
hydrothermal solutions. Du Toit (1918, 1928) and many other workers
have considered that the plumasites represent normal aplitic and pegmatitic
dykes that have been desilicated by the surrounding ultrabasice rock, with
the reaction zones resulting from the progressive transfer of silica, potash,
and fluorine from the dyke to the serpentinized wall-rock. The corundum-
rich ‘white dykes’ of the stock east of Polaris Creek may not be normal
plumasites; their ‘reaction zones’ are more poorly developed than those
described by Larsen and du Toit. The specimens collected are much
richer in corundum than those described from other localities. but the
samples taken may not be representative of the dykes as a whole. Their
narrow (in places lacking) ‘reaction zones’, relatively uniform width and
tabular shape, and restricted occurrence suggest that these bodies are most
probably altered dykes rather than hydrothermal deposits. They may be
similar to some of the ‘white rock’ of the Coquihalla serpentine belt
(Cairnes, 1930), but are more regular in form.

ALTERATION TO SERPENTINE

In common with almost all ultramafic bodies, the peridotites, dunites,
and pyroxenites in the map-area are partly to completely altered to serpen-
tine. The sill-like bodies east of Wasi Lake and in the northwest part of
Lay Range are so completely serpentinized that their original composition