Seismic method - Geophysics for the Mineral Exploration Geoscientist - page 401

In this area the Watson Lake Group comprises a dacite
overlain by rhyolite. The deposit is 900

Matagami

1150 m below
surface and dips 50° to the south. The data were acquired
using a Vibroseis source with a 30

-

- 140 Hz sweep. The nal

section has a fold of 120%.
A time-migrated section is shown in Fig. 6.48 . The
response exhibits many short and discontinuous arrivals,
and is typical of a deformed and metamorphosed hard-
rock terrain; compare these data with those shown in
Fig. 6.46 . The strongest reflections were assigned to late
intrusions. Synthetic seismograms suggested that these
would give the strongest responses in the area with the
comparatively large cross-cutting (less deformed) surfaces
of sills favourable for producing good responses. The Key
Tuffite, with a thickness of 1

BA

0 2

Kilometres

Wabassee Watson Lake Group

Mafic volcanics (Cavalier) Bell River Complex

Basalt (Allard River Unit) Gabbroic intrusives

Rhyolite (Dumagami Unit) Peridotite

3 m (corresponding to about
1/30 of the seismic wavelength), can be detected, but its
thickness not resolved. Its location can also be inferred
from changes in the character of the seismic responses
associated with the contrasting lithotypes in the Watson
Lake and Wabassee Groups. The synthetic seismogram
con rmed that lithological contrasts in the felsic and ma c
volcanic units would produce re ections, albeit weaker
than those from the later intrusions. The most distinctive
responses of this type are interpreted as Dumagami rhyo-
lites, which occur in the lower part of Wabassee Group.
The responses suggest they are conformable with the Key
Tuffite. Many faults were interpreted from the displace-
ment of distinctive reflectors. In particular, there appears
to be an older set of faults largely restricted to the Watson
Lake Group that are potential conduits for mineralising
fluids. In addition, there is a post-mineralisation fault set
mostly affecting the Wabassee Group.
Figure 6.49 shows an enlargement of the seismic section
in the vicinity of the Bell Allard deposit with the local
geology derived from drillholes. The weak re

-

Watson Lake Group

Massivesulphide deposits
Anticline axis
Fault
Seismic profile

Rhyolite

Basalt

Figure 6.47 Simplified geological map of the Matagami mining camp.
The location of the seismic section in Fig. 6.48 is also shown. BA -
Bell Allard VMS deposit. Redrawn, with permission, from Calvert
and Li ( 1999 ).

Watson Lake Group, consists mostly of felsic rocks. Over-
lying these are the mainly intermediate and mafic rocks of
the Wabassee Group. The VMS deposits in the area occur
at the contact between the two groups, where there is a
1

3 m thick cherty tuffaceous horizon called the Key
Tuffite. The youngest rocks in the area are tonalite and
gabbro sills. The seismic survey was designed to determine:

-

Whether the economically important Key Tuf te pro-

￿

duces a recognisable response that enables it to be

mapped across the survey area;

Whether the lithological contrasts within the felsic and
ma c units produce recognisable re ections, especially
in the Watson Lake Group (since these would allow
identi cation of faults that potentially acted as conduits
for mineralising fluids); and

￿

ections from
lithological contrasts in the volcanic successions are con-
firmed. There is evidence of a response from the mineral-
isation itself, correlating with a lower and more pyritic lobe
of the orebody. Although this is encouraging, it is obvious
that the response is hard to recognise; and confusion with
responses from faulted sills is likely.

￿ Whether massive sulphide mineralisation produces

identi able seismic responses. A seismic response was
considered more likely from the sharp upper contacts of
the sulphide bodies, with the most pyritic considered the
more likely to be detected because of the increased
acoustic impedance caused by the high velocity of pyrite
(see Fig. 6.35 ) .

6.8 In-seam and downhole seismic surveys

Small-scale seismic surveys at mine and prospect scale may
use tomographic techniques or in-seam methods, usually
to assist in delineating a resource prior to mining (see

A single 2D seismic profile was recorded along a highway
which passes near (100 - 280 m) to the Bell Allard deposit.