General Geology
Lithology

The rocks of this unit are, for the most part, coarse-grained, equigranular,
and light coloured. Only the body north of Tetachuck River and some border
phases of the body west of Chutanli Lake are at all gneissic. Elsewhere the rocks
are, with few exceptions, uniformly equigranular from the margins to the centres
of the masses. Inclusions or xenoliths are rare. Appreciable shearing or faulting
within the masses has not been found. The common size of feldspar grains is one
eighth to one quarter inch, occasionally one half inch, but rarely are they less
than one eighth inch. Fine-grained phases are neither extensive nor characteristic
of the whole of any body seen in the area. In general the rocks are light coloured
and weathered, in some places, very white. A few diorite bodies and some gab-
broic phases are dark coloured. All rocks mapped weather readily to a depth of
as much as 8 inches. Some of the more deeply weathered rocks may represent
Tertiary weathering from which the cover has recently been stripped.

The batholiths of Fawnie and Nechako Ranges are very similar throughout,
and are composed mainly of a white to pink, coarse biotite granite and grey grano-
diorite. The granites are comprised of 20 to 25 per cent quartz, about 50 per cent
potash feldspar, 10 to 15 per cent oligoclase, 10 to 15 per cent biotite and amphi-
bole, and minor accessory minerals. The quartz content does not vary appreciably
and does not rise much above 25 per cent in any specimen studied. In a few
places the potash feldspar is a deep salmon colour and forms nearly 70 per cent
of the rock; there mafic minerals are less than 5 per cent. The granodiorite is
similar in appearance to the lighter coloured granites, differing mainly in a lower
quartz and feldspar content. Possibly many of the lighter coloured rocks called
granite in the field but not studied under the microscope may actually be quartz
monzonites or granodiorites. Whatever the true proportion may be, nearly all the
rocks of these batholiths are either granites or granodiorites.

The igneous rocks farther west around Tetachuck are more varied in com-
position and range from gabbro to granite. Although no one rock type is typical
of these bodies, diorite does predominate. The diorite bodies are much alike in
composition—about equal amounts of andesine and hornblende, with quartz vary-
ing from none to 10 per cent. Some diorites grade into quartz diorites. The best
exposure of such diorites occur north of Qualcho Lake, on both sides of Bryan
Arm, northwest of Chelaslie Lake, and 10 miles up Chelaslie River. The mass
northwest of Chelaslie Lake is mainly diorite but grades from a quartz diorite to
a dark gabbro that forms a rim near the intrusive contact with basic Mesozoic
lavas. One other gabbro-like mass occurs near the northwest end of Nechako
Range; this may be a neck associated with Mesozoic volcanic rocks. Smaller bodies
of granite, granodiorite, and quartz diorite occur in this general area but one rock
type, particularly the granodiorite, grades into another within a single body or,
in many places, into diorites or Mesozoic volcanic rocks. Many contain inclusions
of the intruded rocks.

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