Gravity and magnetic methods - Geophysics for the Mineral Exploration Geoscientist - page 145

Fine-grained sediments may acquire a post-depositional
remanence (pDRM) where fine-grained magnetic minerals
suspended in the water- lled pore spaces of sediments
align with the Earth

such enormous diversity in the conditions possible, only
generalisations may be made about the magnetic conse-
quences of metamorphism.
In addition to changes in mineralogy, other signi cant
factors in metamorphic environments include heating,
which encourages exsolution of iron- and titanium-rich
phases of titanomagnetite, and deformation causing recrys-
tallisation into coarser grains or altering grain shape, or
causing preferential alignment of grains. The texture of the
rock can also be important since it controls grain size. If
magnetite is locally very abundant, for example in a cumu-
lus phase, it tends to be more stable than if more widely
distributed. This is probably because the large volume of
iron released from the altering magnetite saturates the
metamorphic

is magnetic field. This can occur in
water-logged slurries that

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water
interface. The pDRM is fixed in the sediment by compac-
tion and de-watering, which occur at a depth of about
10 cm; so the pDRM is acquired subsequent to sedimenta-
tion and so is secondary. It is a common mechanism for
remanent magnetisation of pelagic limestones.

form at the sediment -

3.9.4.2 Summary and implications for magnetic data
The magnetic responses of sedimentary rocks are often
featureless owing to their low levels of magnetism; for
examples see Figs. 3.74 and 3.75 . Moderate levels of
induced and remanent magnetism may occur in some
sedimentary rocks owing to the presence of magnetite or
pyrrhotite. Variations in magnetism in a succession are
usually correlated with stratigraphy: see for example
Schwarz and Broome ( 1994 ).
The most magnetic sedimentary rocks are BIF. Magnetic
responses from these units can be extremely strong but
complicated by the effects of strong remanent magnetism,
self-demagnetisation (see Section 3.2.3.6 ) and anisotropic
magnetic properties (Clark and Schmidt, 1994 ) .

fluids to preserve the remaining magnetite.
Also, if magnetic minerals occur as very fine grains within

stable minerals they may survive under conditions outside
their normal stability range.
Metamorphic effects that occur and change over short
distances are much more likely to produce recognisable
magnetic variations in magnetic data. Situations where this
occurs include metamorphic processes that are structurally
controlled, for example retrogressive processes concen-
trated along shear zones; and situations where meta-
morphic conditions may change rapidly, such as contact
aureoles.

3.9.5 Magnetism of metamorphosed
and altered rocks

3.9.5.1 Regional metamorphism of crystalline rocks
There are a number of published studies investigating the
changes in magnetic properties associated with changes in
metamorphic grade, but these tend to be specific to par-
ticular areas and rock types (mostly mafic rocks) and they
cover a relatively small range of metamorphic conditions.
Figure 3.48d is an attempt to summarise how the magnet-
ism of mafic rocks changes as metamorphic grade
increases. There is a general decrease with increasing
grades until the possible creation of secondary magnetite
at granulite facies. At the highest grades, magnetism is
reduced as ferromagnetic species breakdown to form para-
magnetic minerals. Summary conclusions of several studies
follow.
Magnetic properties of sea- oor basalts from the
Atlantic Ocean ranging from fresh to greenschist facies
are shown in Fig. 3.48a . There is much scatter but the
fresh samples are the most magnetic. Note the high
Königsberger ratios of the fresh samples, the reason for
the characteristic striped ocean-floor magnetic anomalies.
Hydrothermal alteration at zeolite facies reduces the

In a geochemical system as complicated as the Earth s

interior, there is an almost infinite range of possibilities
in terms of changes in magnetic mineralogy and the size
and shape of these mineral grains. In general, conditions
become more reducing as depth and metamorphic grade
increases. For this reason, magnetite and ilmenite are the
main Fe-Ti

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Ti oxides in metamorphic rocks. Haematite is
much less common, although it may survive in meta-
morphosed iron formation and oxidised metasedimentary
rocks, and in low- to medium-grade metabasites.
As would be expected, any metamorphic or alteration
process that creates or destroys magnetic minerals is the
primary cause of changes in rock magnetism, especially if
the minerals are ferromagnetic. Obviously, if these are to
occur the rock must contain iron, either primary or intro-
duced by hydrothermal processes, but whether ferromag-
netic species are formed depends on the speci c physical
and chemical conditions (see Section 3.9.3 ) . Metamorph-
ism can significantly change magnetic properties, but, with

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