Gravity and magnetic methods - Geophysics for the Mineral Exploration Geoscientist - page 143

to be distinguished in the field using a portable susceptibility
meter (see Section 3.9.8.1 ) . They can also be identi ed
frommagnetic maps, but the possibility of signi cant reman-
ent magnetism needs to be accounted for in analysing
anomalies.
The characteristics of the magnetite- and ilmenite series
extend over many lithotypes, including gabbroids, syn-,
late- and post-orogenic calc-alkaline granitoids and
alkaline-series anorogenic granitoids. The difference is eco-
nomically important since copper and gold are associated
with the magnetic magnetite-series intermediate I-type
granitoids; molybdenum is associated with more fraction-
ated and oxidised magnetite-series I-type suites; and tin
with the paramagnetic, reduced, fractionated I- or S-type
suites. The relationship is due to redox conditions in the
magma affecting the behaviour of metals such as Cu, Mo,
W and Sn within the melt.
Magnetic properties of granitoids can also be predicted
from the presence of various minerals, which in turn are
indicative of oxidation conditions. For example,
hornblende-biotite granodiorites are predominantly ferro-
magnetic and exhibit moderate susceptibility, whereas the
muscovite-biotite granitoids are usually paramagnetic and
weakly magnetic.
The zoned Alaskian-type igneous complexes have pri-
mary magnetite as the main magnetic mineral. The ultra-
mafic rocks in these intrusions, such as pyroxenites,
hornblendites and serpentinised dunites, generally have
high susceptibility, and the associated mafic and inter-
mediate rocks, such as gabbro, diorite and monzonite have
moderate to high susceptibility. Unaltered komatiitic lavas
and alpine-type peridotites have low susceptibility. How-
ever, serpentinisation of these rocks creates magnetite,
which makes serpentinised ultrama

a)

External
magnetic
field

b)

External
magnetic
field

c)

External
magnetic
field

Figure 3.46 Schematic illustrations ofcommon types of
remanent magnetism. (a) Thermal remanent magnetism (TRM).
(b) Depositional remanent magnetism (DRM); note the
difference in inclination between the DRM and the Earth field.

(c) Crystallisation remanent magnetism (CRM); the white arrow
represents a pre-existing magnetism.

' s

magnetisms. Large intrusive masses often have internal
magnetic zonation related to magnetic mineral fraction-
ation, multiple intrusive phases and changes in remanent
magnetisation related to the cooling period extending
through multiple reversals in the Earth ' s magnetic field

(McEnroe et al., 2009 ) .
The cooling rates of igneous rocks influence their
remanent magnetism, although this can be overprinted
by later thermal or chemical changes. The slowly cooled
plutonic rocks usually have coarse-grained multidomain
magnetite, causing these rocks to have very small Königs-
berger ratios ( Fig. 3.43 ). However, some gabbroic and
dioritic intrusives, which contain very fine single-domain
magnetite within silicate minerals, are notable exceptions
and may have signi cant remanent magnetism. Rapidly
chilled igneous rocks contain fine-grained titanomagne-
tites and consequently can have very large Königsberger
ratios. The largest Königsberger ratios are associated
with the fastest cooling portions of the melt, such as
the sub-aqueous chilled margins and small pillows,
decreasing away from the margin. On the other hand,
slower cooling, thick dolerite sills and dykes can exhibit
high Königsberger ratios. Kimberlites often have ratios
greater than one.

c rocks strongly mag-

netic (see Section 3.9.3 ).

3.9.3.3 Remanent magnetism of igneous rocks
Remanent magnetism of igneous rocks is acquired when
the magnetic mineral grains pass through their Curie tem-
perature, as the lava/magma cools. This primary remanent
magnetism is known as thermoremanent magnetisation
(TRM) ( Fig. 3.46a ) . TRM is a very stable primary magnet-
isation that can exist for long periods on the geological
time scale. It is parallel to the Earth s field and approxi-

mately proportional to its strength at the time of cooling
through the Curie point. As cooling progresses inward
from the outer margins the Earth

'

s field may reverse, so
that inner cooling zones may acquire different remanent

'