Gravity and magnetic methods - Geophysics for the Mineral Exploration Geoscientist - page 172

feature which trends north of northwest, and widening to
the south. The Hardscrabble Pit and Brown Shaft workings
lie on the western margin of the wider part of the channel.
Interpretation of the Port Wine channel is as before,
although there is a suggestion that the Iowa Mine occurs
at a distinct bend in the channel.
Figures 3.70g and h show the results for a regional eld
obtained by fitting a second-order polynomial to the Bouguer
anomaly data. The regional variation shows a slight influence
from the shear-zone anomaly and, as such, is intermediate
between the previous two results. The residual image suggests
a radically different interpretation of the course of the palaeo-
drainage, which comprises two roughly north-trending chan-
nels. The Port Wine channel appears to follow a more
northerly course than before and the Iowa Mine is clearly
within a subsidiary feature. The Brown palaeochannel
appears narrower than depicted in the previous residual
datasets, although the increase in gravity on the southeastern
bank is not well de ned. The Hardscrabble Pit and Brown
Shaft workings are located in the centre of the channel.

The Wallaby Au deposit (Salier et al., 2004 ) is located in
a granitoid-greenstone terrain near Laverton in Western
Australia. The Wallaby deposit is associated with a prom-
inent aeromagnetic anomaly caused by a pipe-shaped zone
of actinolite

calcite alteration within a
thick ma c conglomerate. Gold mineralisation occurs
within a series of sub-horizontal

- magnetite - epidote -

lodes largely con ned

within the altered pipe.
Figure 3.71a shows the Wallaby aeromagnetic (TMI)
anomaly after reduction to the pole. Its amplitude is
900 nT at the survey height of 50 m. The anomaly is
slightly elongated north south and the main peak contains

a pair of subsidiary peaks (A). In the north there is a broad
east-northeast-trending zone of higher magnetism which
interferes with the Wallaby anomaly on its northern
margin (B). There is also a suggestion of northwest-
trending features in the data (C), which are probably
associated with faulting.
Figure 3.71b shows the results of 3D Euler deconvolu-
tion applied to the Wallaby magnetics. The source of the
anomaly is shown to be roughly circular in map view with
its top at a depth of about 100 m below the ground surface,
which is consistent with the likely thickness of the sedi-
mentary cover. Note how the Euler solutions only de ne
part of the source

-

3.11.1.1 Discussion
In illustrating the various ways of de ning a regional eld, it
is demonstrated how the nature of the regional field adopted
has major implications for the interpretation of the data.
The various residual gravity data are also likely to be map-
ping the subsurface at different depths and, therefore, pos-
sibly show changes in channel geometry with depth. In
terms of subsequent exploration activity, and depending
on which of the results above is preferred, the known min-
eralisation could be interpreted as occurring in the centre of
the major drainage channels, at locations where subsidiary
channels join the main channel, in the subsidiary channels
themselves, or where channels become wider or change
course. The most important conclusion to draw from this
is that more drilling is required to constrain the interpret-
ation of the geophysical data for various depths, and to
eliminate artefacts introduced by the choice of regional
field. Other geophysical methods, for example electrical or
electromagnetic data, could probably provide additional
information about the shape and depth of the channels.

'

is margins, which is a common outcome
for this type of analysis. The solutions are tightly clustered
except to the north, probably owing to the interference of
the east-northeast-trending anomaly.

3.11.2.1 Forward modelling
Magnetic modelling of the Wallaby anomaly is described
by Coggon ( 2003 ). A very useful component of this project
is the extensive database of downhole magnetic-
susceptibility measurements. These data allow us to use
the Wallaby anomaly to demonstrate various aspects of
magnetic modelling in a situation where much is already
known about

the distribution of magnetism in the

subsurface.
Forward modelling of the southern flank of the anomaly
( Fig. 3.71c ), assuming only induced magnetism, shows that
the source dips to the south. Depending on the source s

depth extent, the dip could be varied, the best- tting model
having the margin dipping at about 45° with the base of the
source at a depth of 700 m. An alternative model, produ-
cing a slightly poorer fit to the observations, comprises a
source with lower susceptibility, steeper dip and much
greater depth extent. Modelling the northern flank of the
anomaly is hampered by interference from other anomalies

'

3.11.2 Modelling the magnetic response
associated with the Wallaby gold deposit

This case study demonstrates forward and inverse model-
ling of a magnetic anomaly and shows the inherent ambi-
guity in modelling potential field data.