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agitation destroys alignment of the dipoles and ferromag-
netism decreases. It disappears at a characteristic tempera-
ture called the Curie point where the material becomes
paramagnetic and, therefore, has low susceptibility and
no remanent magnetism. The Curie point is an intrinsic
property of a ferromagnetic material dependent only on
the material
The direction of the remanent magnetism is de
ected from
parallelism with the external magnetic
field direction.
Where there is preferential alignment of magnetically
anisotropic grains in a rock mass, then an overall magnetic
anisotropy in banded iron formations from the Hamersley
iron-ore province and the Yilgarn Craton of Western Aus-
tralia, where susceptibilities parallel to bedding exceed
those perpendicular to the bedding in the range of 2 to 4
times. In most cases the consequences of susceptibility
anisotropy are insignificant with respect to interpreting
magnetic responses for mineral exploration purposes, but
anisotropy may be exploited for geological purposes, as
reviewed by Hrouda (
1982
). Measurements of anisotropy
of magnetic susceptibility (AMS) are commonly used to
detect planar and/or linear magnetic fabrics that may be
otherwise invisible. Examples of AMS studies with an
economic geology context are those of Scott and Spray
(
1999
), who describe work in the nickel sulphide-rich
Sudbury Basin of central Canada, and Diot et al.(
2003
)
who describe work on the Tellnes ilmenite deposit in
southern Norway.
s composition. The Curie point of the
common magnetic materials is exceeded at mid crustal
depth, the exact depth depending on the specific mineral
and the local geotherm. Magnetic anomalies originating at
greater depths have been detected but their origin is uncer-
'
3.2.3.6
Self-demagnetisation
There exists a magnetic field internal to a body that, like
the external
field, extends from the north pole to the south
pole. It has the effect of reducing the effective magnetism
of the body through a phenomenon known as self-
demagnetisation. The effect is in
uenced by the shape of
the magnetised body. For example, in tabular bodies self-
demagnetisation is greater perpendicular to the plane of
the object than in the plane, so the overall magnetism is
de
ected towards the plane of the object. Self-
demagnetisation produces a shape-related anisotropy that
operates at a range of scales, from individual grains
through to mineralogical layering, to the scale of orebodies
and stratigraphic units. It depends on the strength of the
external field but is only significant in highly magnetic
materials (
3.2.4
Magnetic anomalies
The strength of the Earth
s magnetic
field, which includes
the main geomagnetic
field associated with the Earth
'
'
s core
and the
fields associated with the magnetism of the local
rocks, is measured in magnetic surveys. Modern instru-
ments measure the total field strength, usually referred as
the total magnetic intensity (TMI), which is the resultant,
or vector sum, of the vertical and the two horizontal
components of the field (see
Appendix 1
)
. Unless other-
wise stated, reference to magnetic anomalies invariably
means anomalies in TMI, a convention we use throughout
our description of the magnetic method.
Figure 3.8
shows the variation in TMI across a horizon-
tal surface above a sphere that is more magnetic than its
surrounds. As described in
Section 3.5.1
,
the direction and
strength of the geomagnetic
field varies around the Earth.
Here the source is assumed to have only induced magnet-
ism, and the resulting anomalies for different orientations
(inclinations) of the inducing (geomagnetic)
field are
shown. The inclination is the direction of the
field relative
to the horizontal (see
Section 3.5.1
). Clearly, the resulting
magnetic anomalies are markedly different, even though
the shape of the source is the same. The observed vari-
ations in the magnetic field are the combined effects of the
0.1). Self-demagnetisation is less than the
induced magnetism, but the apparent decrease in magnet-
ism and deflection of the direction of magnetism must be
accounted for when interpreting anomalies caused by
strongly magnetised rocks. Geological materials that are
sufficiently magnetic for self-demagnetisation to be
important include banded iron formations and massive
magnetite mineralisation.
κ
SI
>
3.2.3.7
Magnetic anisotropy
A mineral
s magnetic properties are affected by its mag-
netic domain structure, which can differ along the various
crystal axes. They are also in
uenced by the shape and size
of the magnetic grain (see
Section 3.2.3.6
). The result is
that individual mineral grains are often magnetically aniso-
tropic, and this affects both their induced and remanent
magnetisms. Susceptibility anisotropy de
ects the induced
magnetism away from the direction of the external indu-
cing field towards the direction of highest susceptibility.
'
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