Gravity and magnetic methods - Geophysics for the Mineral Exploration Geoscientist - page 136

all of which can produce very different magnetic properties
in similar lithotypes. An additional complication arises
because magnetism is a vector property. One or both of
the strength and direction of the rock

mica and iron and manganese carbonates. Economically
signi cant species include pyrite, chalcopyrite, arsenopyr-
ite, marcasite and pure ilmenite.
Ferromagnetic minerals (sensu lato) have signi cant sus-
ceptibility and may carry a remanent magnetism; they can
produce very strong magnetic responses in geophysical
surveys. Within this group are the ferrimagnetic minerals,
which include nearly all the important magnetic minerals like
magnetite, monoclinic pyrrhotite, maghaemite and ilmenite.
The antiferromagnetic minerals have low susceptibilities
similar to those of paramagnetic materials and do not acquire
remanence. Haematite is the most common example.

s magnetism may be
affected, and the affects may pertain to either, or both, of
the remanent and induced magnetisms. Also, it is common
for rocks to contain distinct populations of magnetic min-
erals with different characteristics, and these may be
affected differently by geological events.
The sensitivity of rock and mineral magnetism to the
rock

'

s geological history means that it can be a useful tool
for studying that history (Dunlop and Ozdemir, 1997 ;
Harrison and Freiburg, 2009 ) . Applications relevant to
mineral exploration include studies to constrain the nature
and timing of magmatic, tectonic and mineralising events
(Borradaile and Kukkee, 1996 ; Lockhart et al., 2004 ;
Symons et al., 2011 ; Trench et al., 1992 ).
We present here an overview of the mineralogical con-
trols on rock magnetism followed by a description of the
magnetic properties of different rock types and geological
environments, at the same time considering geological
processes that lead to the creation or destruction of mag-
netic minerals and/or the modi cation of the magnetic
properties of existing minerals.

'

3.9.1.1 Iron-titanium oxide minerals
The ferromagnetic iron titanium oxide minerals comprise

a solid-solution series ( Fig. 3.39a ) that extends from wüs-
tite (FeO) to haematite (Fe 2 O 3 ) through increasing oxida-
tion, and to rutile (TiO 2 ) with increasing titanium content.

-

Titanomagnetites
The titanomagnetite series (Fe 3 - x Ti x O 4 ;0 1), also

known as the spinel group, has magnetite (Fe 3 O 4 )and
ulvospinel (Fe 2 TiO 4 ) as end-members of a solid-solution
series. Magnetite contains a combination of ferric and fer-
rous iron: in its pure form, one-third being ferrous, two-
thirds ferric. As shown in Fig. 3.39b , the more titaniferous
members of the series have signi cantly reduced suscepti-
bility and remanent magnetism, decreasing markedly when
Ti fractional content exceeds 0.8. Moderate amounts of
titanium have little effect on magnetic properties, grain size
being far more significant. Note how susceptibility increases
with grain size, but the opposite occurs for the strength of
the remanence. Ulvospinel is paramagnetic.
Magnetite has the highest susceptibility of all naturally
occurring minerals. Pure magnetite has

< x <

3.9.1 Magnetic properties of minerals

As described in Section 3.2.3.5 , materials can be categor-
ised according to their magnetic characteristics with dia-
magnetic, paramagnetic and ferromagnetic behaviour
important for materials commonly occurring in the geo-
logical environment. It is the ferromagnetic group that
have the strongest magnetic properties and that have the
greatest influence on the magnetic properties of rocks.
Diamagnetic minerals have very weak negative suscepti-
bility (about 10 - 5 SI) and are incapable of carrying reman-
ent magnetism. Rock-forming minerals of this type include
pure phases of quartz, calcite and feldspar. Economically
signi cant examples include pure galena and sphalerite,
graphite, halite, gypsum and anhydrite. Many clay min-
erals and water are diamagnetic.
Paramagnetic minerals have weak, but still potentially
signi cant, positive susceptibilities (about 10 - 3 SI), but
again no remanent magnetism. Paramagnetism is associ-
ated with the presence of iron and, less importantly,
manganese. The susceptibility of paramagnetic species cor-
relates with their total iron content ( Fig. 3.31 ). Minerals of
this type include olivine, pyroxene, amphibole, garnet,

susceptibility

ranging from 13 SI for fine-grained, poorly crystalline,

inhomogeneous or stressed grains, up to about 130 SI for
very coarse, well-crystallised magnetite. Magnetite acco-
unts for about 1.5% of crustal minerals, occurring in igne-
ous, metamorphic and sedimentary environments.

Its

Curie temperature is 578 °C.
An important characteristic of titanomagnetites is the
tendency for exsolution to occur during cooling. This is
best developed in grains formed in the slower cooling
plutonic rocks. At greater than 600 °C, i.e. magmatic tem-
peratures, there is a complete solid solution between ulvos-
pinel and magnetite. At lower temperature there is increasing
immiscibility in the central section of the compositional