Geology and exploration, McDame asbestos deposit, Cassiar, B.C.

CIM Bulletin, Vol. 79, No. 891, 1986

A.A. BURGOYNE, Vice-President, Exploration, Cassiar Mining Corporation, Vancouver, British Columbia

The McDame chrysolite asbestos deposit located near Cassiar in northern British Columbia is found at depth and on trend to the southeast of the currently mined open-pit Cassiar asbestos deposit. The ultramaflc rock hosting the McDame is serpentinized peridotite of the Upper Paleozoic Sylvester Group contained in an ophiolitic sequence preserved within a lower thrust sheet of the Sylvester Allochthon. The Sylvester stratigraphic package is a series of thrust slices composed of argillite, tuff and volcanic flows, diorite and serpentinite. Contacts between units are often fault-controlled. The McDame asbestos-bearing ultramafite is considered to represent a separate interleaved fault-bounded slice of rock distinct from the Cassiar asbestos deposit. Asbestos mineralization of the McDame deposit was discovered in 1978. Exploration during 1980 and 1981 focused on extensive underground drilling of the deposit and in 1983 on geological mapping, airborne and ground magnetic surveys. In 1984 a 622.5 m borehole was drilled with a 242 m intersection of McDame ultramafite of which the lower 151 metres contain potentially economic chrysotile asbestos. The McDame deposit averages 90 m thick, trends north-south, dips easterly and plunges southeasterly. The deposit is estimated to contain geological reserves in the order of 62 million tonnes containing asbestos fibre similar in value to the Cassiar deposit. The McDame is considered a major world-class asbestos deposit. A decision to develop and mine the deposit will require further substantial underground exploration and evaluation. A +1200 m exploration adit driven to the hanging wall of the deposit was undertaken in 1985 in anticipation of bulk sampling and diamond drilling in 1986.
Keywords: Industrial Minerals, Chrysotile asbestos, McDame deposit, Geology, Exploration, Exploration surveys, Drilling.