Geoscience Reference
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correction factor can be applied for undersized specimens,
rough surfaces and circular drillcores. High electrical con-
ductivity of samples can disturb the operation of the
sensor. When susceptibility anisotropy is of interest,
oriented rock samples are required and laboratory meas-
urement techniques must be used.
Hand-held susceptibility meters are a means of quickly
and cheaply acquiring data, provided high precision is not
required. Care is required in selecting samples to ensure
that they are representative of the rocks being investigated.
Oxidised samples in particular need to be avoided because
of the likelihood of magnetite being destroyed. Also, the
presence of magnetic veins and the vein/fracture density of
the sample can produce large variations in the measure-
ments. The major mistake is to take too few readings.
When working with drillcore, it is
a)
b)
c)
Susceptibility
(SI)
Susceptibility
(SI)
Susceptibility
(SI)
1
0
-4
10
-2
10
0
10
1
10
3
10
5
0.00
0.02
5
0
m
5
0
m
2
0
m
to take
three equally spaced susceptibility readings around the
perimeter of the core sample, at the same downhole-depth.
For measurements on outcrop, a suf
ciently large dataset
ought to be acquired in order to accurately resolve the
natural spatial variation in susceptibility across the out-
crop. The data can then be analysed in terms of their
frequency distribution (see
Section 3.9.8.3
).
'
good practice
'
Massive/matrix min
Chert (shale)
Iron formation
Unaltered granite
Intermediate
Altered
Highly altered
Disseminated min
Schist
Quartzite
Mafic/ultramafic
Other
3.9.8.2
Magnetic susceptibility logging
Magnetic susceptibility can be measured in situ with a
probe in a drillhole to quickly obtain a continuous suscep-
tibility log. Measurement interval is typically 10 cm. The
logs produce a far larger dataset than can be obtained from
manual measurements. The probe senses roughly a spher-
ical volume of rock a few tens of centimetres in diameter,
so the log shows the variation in susceptibility of the wall-
rock along the drillhole. The small sample volume and the
generally heterogeneous distribution of magnetic proper-
ties mean that susceptibility logs are often locally quite
variable. Normally some kind of mathematical smoothing,
e.g. a median
Figure 3.59
Magnetic susceptibility logs from various geological
environments: (a) Banded iron formation from the Hamersley
(b) Kabanga nickel sulphide deposit, reproduced with the permission
of Barrick (Australia Pacific) Ltd, and (c) alteration zone in a
Canadian granite. Based on a diagram in Lapointe et al.(
1986
).
Figure 3.59c
is the log from a hydrothermally altered
granite in Canada and is the source of the data shown in
alteration correlates with fracture zones.
Magnetic susceptibility logs are used for lithological
discrimination, hole-to-hole correlations and identi
cation
of alteration zones, and are commonly used to characterise
variations in the local stratigraphy. Where mineralisation
has different magnetic properties from the host sequence,
there have been some attempts to use downhole magnetic
susceptibility logs as a fast, low-cost, basis for ore-grade
control, either alone or in association with other physical
property logs.
filter (see Smoothing in
Section 2.7.4.4
), is
applied to the log to resolve relationships with the wall-
rock geology.
Figure 3.59a
shows a magnetic susceptibility log from
the banded iron formation (
) succession in the Dales
Gorge Member of the Brockman Iron Formation,
Hamersley iron-ore province , Western Australia. The
shales are in fact ferruginous cherts and are important
stratigraphic markers within the thick and invariant iron
formation. The log in
Fig. 3.59b
comes from the Kabanga
Ni sulphide deposit, Tanzania. The sulphide mineralisation
has significantly higher susceptibility than the host rocks.
'
shale
'
3.9.8.3
Analysis of magnetic susceptibility data
Magnetic susceptibility data can be displayed as a fre-
quency histogram for each rock type sampled. Mostly,
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