Geoscience Reference
In-Depth Information
3.9.5.6
Summary and implications for magnetic data
From a magnetic interpreter
comprise the deeper parts of the regolith are normally non-
magnetic, with magnetism increasing again at the bedrock
interface, depending on the nature of the protolith
(
Fig. 3.36
).
Figure 3.54
shows variations in the strength of
induced and remanent magnetisms through a regolith
pro
le at Lawlers, in the Yilgarn Craton of Western Aus-
tralia. Königsberger ratios are as high as 100 in the iron-
rich laterites, with the strength of remanent magnetism
reaching as much as 100 A/m. This is a very strong mag-
netism, and magnetic responses from these near-surface
layers are often seen in magnetic surveys, often to the
detriment of bedrock responses.
Destruction of magnetism, or demagnetisation (not to
be confused with self-demagnetisation; see
Section 3.2.3.6
),
by alteration and/or weathering is almost ubiquitous in
fault/shear/facture zones. These structural features are
easily seen in magnetic maps and in particular where they
affect strongly magnetised rocks; several good examples are
shown in
Sections 3.11.3
and
3.11.4
.
Figure 3.55
shows the
variations in magnetisation through a fracture zone in a
strength of both the induced and remanent magnetisms
decreases in the fracture zone, but more so for the induced
magnetism, resulting in an increase in the Königsberger
ratio. The country rock contains disseminated almost fresh
magnetite; but where the magnetism decreases, magnetite
has been oxidised to haematite (martite) with very little
magnetite remaining. Iron hydroxides are also present
indicating hydration. There is a clear correlation between
the loss of magnetism and the degree of alteration which
occurred near the ground surface, so it is interpreted to be
the cause of the demagnetisation.
Dendritic anomaly patterns associated with present and
past drainage systems are often seen in aeromagnetic data,
most easily when the bedrock is weakly magnetic (see
Section 3.11.4
)
. The source of these anomalies is magnetic
detritus. In weathered terrains such as Australia, the source
of the anomalies is often maghaemite. In glaciated terrains
it may be magnetic boulders in glacial sediments (Parker
Gay,
2004
)
s perspective, metamorphic
grade is important since it affects the relative magnetism
of different rock types. Changes in metamorphic grade in a
survey area may lead to change in the magnetic character
of the same rock types, e.g. Robinson et al.(
1985
), charac-
terised by variations in both the strength and direction of
the magnetisation. In some instances it may be possible to
identify a magnetic (mineral) isograd. This is most likely in
the case of the high metamorphic gradients associated with
contact metamorphism. Regional metamorphism in high-
grade terrains tends to result in magnetic transitions occur-
ring over hundreds to thousands of metres, and so will not
give rise to a discrete anomaly. At lower grades, appearance
and/or destruction of magnetic minerals has been used to
de
ne
'
'
'
magnetic isograds
3.9.6
Magnetism of the near-surface
Figure 3.53
shows Eh
pH conditions in a variety of near-
surface environments, within the boundaries of water sta-
bility. Outside these boundaries, water disassociates into
hydrogen and oxygen. Also shown are the stability
elds
for various iron oxides, sulphides and carbonates for dif-
ferent levels of total dissolved sulphur (
-
Σ
S) and total dis-
Σ
solved carbonate (
CO
2
). Equilibrium conditions are
assumed, although they may never be attained in the
natural environment. Note how non-magnetic species are
stable in nearly all the natural settings. Weathering tends to
transform iron to the haematitic Fe
3+
state because this is
the stable state of iron in the atmosphere, so weathered
rocks usually have lower magnetism than their fresh
equivalents. Other possible consequences of weathering
are the creation of a CRM, and the formation of strongly
magnetic maghaemite.
Commonly, ferrous iron silicates and crystalline iron
oxide minerals break down in carbonate-rich groundwater,
in which Fe
2+
is slightly soluble. This can oxidise to Fe
3+
,
which is much less soluble and precipitates as the hydrated
ferric oxides goethite and lepidocrosite, which are weakly
and variably magnetic. In a weathered terrain, the low
solubility of Fe
3+
, compared with most other cations, forms
in situ laterites and iron-rich soils which have signi
cant
magnetism. In these environments, strongly magnetic
maghaemite can form. It is extremely stable and occurs
widely in the soil profile. The saprolites and saprock that
3.9.7
Magnetism of mineralised environments
The magnetic responses of various types of mineralisation
magnetism of the mineralisation itself, the alteration zone
and the host lithologies is relatively common and routinely
exploited during exploration.
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