Gravity and magnetic methods - Geophysics for the Mineral Exploration Geoscientist - page 139

Magnetic susceptibility (SI)

10 -5 10 -4 10 -3 10 -2 10 -1 10 0 10 1 10 2

Haematite Monoclinic pyrrhotite

Minerals

Ilmenite/titanohaematites

Paramagnetic minerals (Titano) magnetites/maghaemite

Disseminated Massive/coarse

Basalt/dolerite
Gabbro/norite

Spilites

Felsic volcanics

Andesite
Granite/granodiorite/tonalite

Ilmenite series Magnetite series

I type

Igneous

S type

Trachyte/syenite

Monzonite/diorite
Pyroxenite/hornblendite

Phonolites

Kimberlites

Peridotites (including dunite)

Serpentinised

Metasediments

Amphibolite/mafic granulite

Metamorphic

Felsic granulite

Skarn

Magnetite skarn

Siliciclastics, shales

Sedimentary

Sedimentary rocks

Carbonates

Banded iron formation

Haematite-rich Magnetite-rich

Laterite

Mineralisation

Haematite-goethite-rich Maghaemite-bearing

Figure 3.42 Susceptibility ranges for
common minerals and rock types.
The darker shading indicates the
most common parts of the ranges.
Redrawn with additions, with
permission, from Clark ( 1997 ) .

Sulphide-oxide
mineralisation

Pyrite-rich Haematite-rich Pyrrhotite-rich Magnetite-rich

Paramagnetic
limit

Steel

10 -5 10 -4 10 -3 10 -2 10 -1 10 0 10 1 10 2

the magnetic grains occupy more than about 20% of the
rock volume the relationship becomes substantially non-
linear and susceptibility increases faster. This is because the
strongly magnetic grains are packed closer together,
increasing interactions between the grains. For weakly
magnetic minerals, such as haematite and paramagnetic
species, susceptibility is essentially proportional to the
magnetic mineral content, right up to 100% concentration.
Figures 3.42 and 3.43 show the variation in susceptibility
and Königsberger ratio (see Section 3.2.3.4 ) for common
rock types. All of the signi cantly magnetic minerals are
accessory minerals, except in certain ore environments.
Their presence or absence is largely disregarded when
assigning a name to a particular rock or rock unit. Since

it is the magnetic minerals that control magnetic
responses, there is no reason to expect a one-to-one cor-
relation between magnetic anomalies and geologically
mapped lithological boundaries. In most cases the overall
magnetism of rocks re ects their magnetite content,

because of magnetite s common occurrence and strong

magnetism. Consequently, magnetic maps are often
described as

'

' magnetite-distribution maps ' , which for most

types of terrain is a valid statement.
Magnetic susceptibility varies widely in all the major rock
groups ( Fig. 3.42 ) . Often the distribution is bimodal, having
members with high and low susceptibilities re ecting the
presence or absence, respectively, of ferromagnetic minerals
along with paramagnetic species. The range in