4]

ZINC BLENDE DEPOSITS

The zinc blende deposits, some half a dozen in number, are clustered
on McGrath mountain at the edges of an irregularly shaped stock of
augite porphyrite. The deposits occur in sediments, although in places the
sediments are inclusions in the stock. No deposit is known to lie between
walls of augite porphyrite, but at the deposits the rock is in general so
altered that it is not always possible in the field to identify the wall-rocks.
The augite porphyrite of the stock is composed of unaltered augite pheno-
erysts in a greenish grey, altered groundmass. The rock, however, pre-
sents a fresh appearance in hand specimen. Pyrite and pyrrhotite are
common.

The deposits are in part vein-like, in part appear to be replacements
along bedding planes in sedimentary rock, and in part have no demon-
strated form or attitude. Some deposits are probably of large size, as
single exposures more than 20 feet across are not uncommon, but no single
large deposit has been outlined by development work. The two striking
features of the zinc blende deposits are: (1) they are clustered in one
place; and (2) the mineralization is resin-coloured sphalerite with prac-
tically no other sulphide. The gangue is mainly quartz and calcite.
Development work on the deposits has not been at all extensive, but the
results obtained suggest that the deposits farthest from the stock of augite
porphyrite are the most persistent but may not be so wide as those at the
border of the stock or in sedimentary inclusions in the stock.

The vein-like deposits are of quartz and calcite and some contain
small shoots of sphalerite. Some of the veins are well-defined but are in
general of small size. Some deposits are vein-like in that they are roughly
tabular but lack definite boundaries. One such body on being followed
by an adit was found to end abruptly. This body consisted solely of
sphalerite. In one or two places a bed of argillite has apparently been
replaced, in some parts very imperfectly and in others almost completely,
by sphalerite with some calcite and quartz. A very common type of
deposit is a breccia of rock fragments cemented by sphalerite. Such
deposits are evidently broken or crushed zones along which sphalerite has
been Cae chiefly by filling spaces rather than by replacing the coun-
try rock.

The origin of the zine blende deposits like that of the copper deposits
cannot be satisfactorily ascribed to the Coast Range intrusives because of
their extremely localized occurrence and the lack of any zonal relationship
to deposits of other types. The localization is what would be expected if
the augite porphyrite or some unexposed phase was the source of the zine
blende. As the mineral-bearing fractures away from the stock are more
persistent and tend to be narrower than those nearer the edge of the stock
it might be argued: (1) that the fractures are local developments whose
formation was influenced by the presence of the stock; and (2) that more
mineral was available near the stock than farther away. It is conceivable,
however, that the fractures are of regional origin and owe their habit to the
presence of the stock which would not then be the source of the mineraliza-
tion. On the other hand several deposits occur in what appear to be
inclusions of argillite in the stock and if these actually are inclusions then