9

SEcTION IV, 1940 [21] TRANS. R.S.C.

THE ULTRABASIC ROCKS OF THE FORT FRASER MAP-
AREA (WEST HALF), NORTHERN BRITISH COLUMBIA?

By J. E. ARMSTRONG.

Presented by H. C. Cooks, F.R.S.C.

OME previously unknown areas of ultrabasic rocks in the west

half of the Fort Fraser map-area, northern British Columbia,
were studied and mapped by the writer in 1937. Two of them are
65 and 80 square miles in area respectively, and are thus comparable
in extent to the largest known bodies of such rocks. They have been
partially to completely serpentinized. In addition, in many places
they have undergone later hydrothermal alteration to carbonate-
quartz-mariposite and carbonate-talc rocks (steatitization). In this
paper the writer discusses the implications of the facts gathered
during the study of these rocks with reference to existing theories of
origin, mode of emplacement, serpentinization, and steatitization of
ultrabasic rocks.

The oldest rocks in the area are of Carboniferous or Permian age
and consist of sedimentary and extrusive types, closely folded in a
general north-westerly direction. The ultrabasic and basic rocks
with which this paper deals intrude these late Palaeozoic strata and
are themselves intruded by acidic rocks of Jurassic or Cretaceous age.
They are probably Jurassic in age and to be correlated with the
beginning of the batholithic intrusions which characterize the Jurassic
and Cretaceous periods of British Columbia. Similar rocks occur
elsewhere in British Columbia, south in the Tulameen (Camsell,
1913), Coquihalla (Cairnes, 1930), and Bridge River (Cairnes, 1937)
areas and north in the Eagle-McDame area (Hanson and McNaughton,
1936). In all localities they are regarded as probably Jurassic in age.

The largest body of ultrabasic rocks, 80 square miles in area,
underlies the Mount Williams group of mountains. Another large
mass, 65 square miles in extent, lies between Stuart and Cunningham
Lakes. Both are composed predominantly of peridotite with pyroxen-
ite towards the borders; and in some places pyroxenite is succeeded
by gabbro. A stock of gabbro too small to be mapped cuts the peri-
dotite near the summit of Mount Williams. Dunite forms irregular
masses one-half to two square miles in area within the peridotites.

1Part of a dissertation submitted to the University of Toronto in partial
fulfilment of the requirements for the degree of Doctor of Philosophy.