9

Serpentine

A belt of serpentine extends from the headwaters of Eagle river south-
easterly to the eastern edge of the area. It averages 4 miles in width
and attains a maximum width of 8 miles on the east side of Muddy river.
There are smaller bodies near the headwaters of Little Eagle river and in
the neighbourhood of Tanzilla butte. North of the Cassiar batholith ser-
pentine bodies are small and were not mapped.

The main serpentine belt contains a wide variety of rock types. In
addition to serpentine and the partly serpentinized basic rocks, there are
inclusions of highly metamorphosed sediments. Depending upon the
degree of alteration, the associated basic rocks vary from almost pure
serpentine to those with the appearance of comparatively fresh diorite
The serpentine varies somewhat in lithological character from one locality
to another, but is typically massive, and is light to dark green in colour.
Sheared and slickensided varieties are fairly common and in some places
spheroidal weathering of the serpentine simulates pillow structure in lava.

Microscopic examination of the serpentine yields very little informa-
tion as to the original character of the rock. Most of the sections studied
are made up of colourless serpentine and small amounts of magnetite
which follows the original outlines and cleavage cracks of olivine grains.
In spite of their dioritic appearance, most of the associated basic rocks
are almost as badly altered. However, they usually contain a few ragged
crystals of enstatite and actinolite or tremolite in addition to serpentine.
The only section that did not show a comparatively high degree of alter-
ation was of peridotite from the summit of Tanzilla butte. This rock is
made up of fresh olivine and minor amounts of augite.

North of the Cassiar batholith bands of light green to black serpen-
tine occur at the base of the McLeod series and near the top of the Dease
series. The bands are, in most places, about 100 feet thick and contain
thin, discontinuous, dyke-like bodies of white rock. The serpentine con-
sists mainly of the mineral serpentine, but also contains a colourless
pyroxene and chromite. The associated white rock consists of colourless
pyroxene, probably diopside, and a good deal of garnet.

Basic rocks are associated with the serpentine in the northern part of
the area. They are mainly augite porphyrite and occur in bodies varying
in size from a few hundred feet wide to 5 miles or more in diameter.
Stocks of augite porphyrite that are circular or oval in outline are largely
confined to the belt of volcanic rocks, whereas elongate bodies resembling
sills of irregular shape occur in the Dease series. Some of these bodies
are partly altered to serpentine.

The augite porphyrite is very dark grey in colour and is commonly
porphyritic. Most samples examined microscopically contained pheno-
crysts of augite, but a few contained hornblende. The feldspars could
rarely be identified because of alteration to secondary products, but labra-
dorite was recognized.

Basic dykes and sills containing serpentine intrude the argillites and
limestones of the Dease series in the vicinity of Tanzilla butte, and in
many localities immediately adjacent to the main serpentine belt. These
dykes and sills are usually less than 20 feet wide.