“THE CASSIAR STORY”

Geology

Fred G. Hewett, Mine Geologist,
Cassiar Asbestos Corporation Limited,
Cassiar, B.C.

Abstract

The Cassiar orebody is located in a sill-like serpentinite
body intrusive into the Devonian-Mississippian sedimentary
rocks of northern British Columbia. Subsequent intrusion of
the Cassiar batholith formed the McDame synclinorium and
produced chrysotile asbestos.

Present information indicates an orebody of approximately
16,277,000 tons amenable to open-pit mining, but continued

REGIONAL GEOLOGY

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FIGURE 1 — Plan of regional geology.

Fred G. Hewett was born in New Den-
ver, B.C., and graduated from the
University of British Columbia with
a B.Se. in geology. His working back-
ground has been evenly divided be-
tween exploration and mining geology.
He was emploved by Conwest Ex-
ploration Company Limited as a junior
geologist in the Northwest Territories
and Ontario during 1966. In 1967, he
* joined Placer Development Limited as
- a technician and underground diamond
drilling coordinator at Craigmont Mines Limited in
Merritt, B.C., and subsequently transferred to Canex
Placer Limited as a field geologist in 1969. He returned
to Craigmont in 1972 as the exploration geologist, based
in Kamloops.

In 1978, he joined Cassiar Asbestos Corporation Limited
in his current position as mine geologist in charge of the
Cassiar Mine geology department. He is a member of the
CIM and a Fellow of the Geological Association of Canada.

Keywords: Cassiar Mine, Geology, Mine geology, McDame
synclinorium, Sylvester Group, Structural geology, Drill-
ing, Adit exploration, Grade control, Reserve estimation,
Computers.

diamond drilling will be needed to accurately define total
tonnage and grade. A program has been developed at the
mine to provide the required information on a daily basis,
and work is continuing on more effective dollar-value predic-
tion methods.

Geological Setting

REGIONAL

THE CASSIAR MINE is located approximately in the
middle of a northerly trending belt of rocks which
forms the spine of the Cassiar Mountains. This belt is
approximately 45 miles wide, being bounded on the
east by the Liard Plain and on the west by the Stikine
Plateau (Fig. 1).

The Cassiar asbestos deposit occurs within the
Sylvester Group — a thick eugeosynclinal assemblage
of voleanic and sedimentary rocks of late Devonian to
early Mississippian age. This assemblage forms a
major part of the McDame synclinorium, a large pack-
age of predominately sedimentary rocks laid down
from Precambrian to Mississippian time. Regional
rock types are described in Table 1. The syncline was
formed during the upheaval caused by the implace-
ment of the Cassiar batholith in Jurassic-Cretaceous
time. Occurring chiefly along the axis of the syncline,
but also occasionally at other stratigraphic horizons,
lies a string of ultramafic bodies known as the
McDame Intrusives. These Mississippian bodies in-
trude the Sylvester Group intermittently for’ 70 miles
and, although most have been converted to serpentine
and contain chrysotile asbestos, so far only one — the
Cassiar deposit — has proven economically viable.

LOCAL

The Cassiar orebody occurs in a sill-like body of
serpentinite which intruded the west limb of the

TABLE 1 — Regional Rock Types

Age Name Description

Jurassic/Cretaceous Cassiar Intrusions | Quartz-monzonite, grano-

diorite, etc.

McDame Intru-
sives

Mississippian Serpentine and related

rocks.
Devonian-Mississippian

Sylvester Group Interbedded argillites,

quartzites and volcanics.

Platy limestones,
dolomites and shales.

M. to U. Devonian McDame Group black

| Silurian and (?)
Devonian

Sandpile Group Interbedded quartzites,
sandstones, dolomites and
minor phyllites.

M. to U. Cambrian

Kechika Group Black to brown platy

shales; often pyritic.
L. Cambrian Atan Group Interbedded marbles, do-
lomites, argillites and
quartzites.

Late Precambrian

Good Hope Group | Coarsely crystalline lime-

stone and marble.