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Equipotential contours of the surface measurements
de
ne the extent of the orebody extremely well, with the
highest potentials coinciding with the shallowest part of
the body (
Fig. 5.66a
)
. Its westerly dip is clearly indicated by
the wider spacing of the contours to the west, and the
northwesterly plunge of the body is also suggested. The
downhole potential measurements very effectively outline
the dipping orebody. The maximum potentials occur in the
vicinity of the intersections, demonstrating the electrical
continuity between them and the intersection containing
the current electrode (
Fig. 5.65b
)
. Note the low gradient in
the surface potentials over the massive mineralisation, and
through it in the downhole data, characteristic of a body
with high conductivity. Compare these data with the ideal-
ised response illustrated in
Fig. 5.64b
.
(a)
50
150
CE
75
W276
200
175
A
W223
100
150
125
W27
A
5.6.9.5
Example - Victoria graphite deposit
This example illustrates the use of hole-to-hole AP data to
investigate the continuity between multiple intersections of
electrically conductive high-grade graphite mineralisation
in a series of drillholes along a strike length of about 200 m
(
Fig. 5.67
). The data are from the Victoria deposit, which
lies within Precambrian rocks of the Grenville Province in
mineralisation occurs within a sequence of highly silici
ed
marbles and paragneisses that have been intruded by
pegmatites. The graphite occurs as massive lenses and as
disseminations.
In this example continuity between high-grade intersec-
tions was established from potential variations within indi-
vidual drillholes, with data from only two holes presented.
In
Fig. 5.67b
data from drillhole D11 are shown which were
recorded with the current electrode in the upper of two
high-grade intersections in drillhole D45. The potential
maximum coinciding with the upper of two graphite inter-
sections in D11 shows this horizon to be electrically con-
nected with that in which the current electrode is located.
The lower intersection has a signi
cantly lower potential,
indicating that it is not electrically connected. Notice that
the potential is roughly constant within the two intersec-
tions because of their high conductivity. Compare these
data with
Fig. 5.65b
. The second dataset (
Fig. 5.67c
)
shows
the potential measured in drillhole D47, which contains
four intersections of high-grade mineralisation. The current
electrode is in the upper of two intersections made by
drillhole D48. Potential maxima coincide with two of the
four intersections, indicating electrical continuity between
these and the intersection in D48. Potential is lower in the
150
125
100
75
Approximate extent of
unweathered massive sulphides
0
100
Metres
(b)
A
A
W276
W27
W223
200
210
160
180
190
170
200
Figure 5.66
Applied potential data from the Woodlawn Cu
Zn
deposit. (a) Surface potential contours (mV/A) with dip direction
of the conductive body indicated by the potential gradient along
section A
-
A
0
. (b) Downhole potential contours (mV/A) on section
A
-
A
0
. CE is the projected location of the buried current electrode.
See text for details. Redrawn, with permission, from Templeton
et al.(
1981
).
-
Pb
-
basic intrusives that have been metamorphosed to greens-
chist facies. The major ore minerals are pyrite, sphalerite,
galena and chalcopyrite. The exploration history of the
Woodlawn deposit is described by McKay and Davies
(
1990
). The current electrode was located in a drillhole
intersecting the orebody near its northern margin. It was
positioned in the centre of a 36 m intersection of massive
sulphides at a vertical depth of 46 m. Potential measure-
ments were made on the surface and down three drillholes.
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