Gravity and magnetic methods - Geophysics for the Mineral Exploration Geoscientist - page 179

Kimberlites (Y), some diamondiferous, occur in the
north of the survey area. These are associated with subtle
magnetic anomalies which are only likely to be recognised
in magnetically inactive areas. The aeromagnetic and
ground magnetic responses of kimberlites of the Kirkland
Lake swarm are described in more detail by Brummer et al.
( 1992 ). The various ways of presenting geophysical data
described in Section 2.8 are illustrated using magnetic data
from the Kirkland Lake area which include responses from
the local kimberlites (see Fig. 2.30 ). Note the significantly
different anomaly shapes, probably owing to variations in
remanent magnetism carried by the kimberlites.

Fig. 3.74 . In addition, geological environments which
exploration models predict to be prospective can be iden-
ti ed relatively easily.

3.11.4 Magnetic responses in a Phanerozoic
Orogenic terrain: Lachlan Foldbelt

The Palaeozoic Lachlan Foldbelt extends along much of
the eastern seaboard of Australia. We have chosen a
section of it from central New South Wales to illustrate
the magnetic signatures of a Palaeozoic orogenic belt. The
tectonic history of the region is generally held to involve
the interaction of suspect terranes in an arc/convergent
margin environment experiencing oblique-compression.
Degeling et al.( 1986 ) describe one perspective on its tec-
tonic history, including its implications for metallogeny.
A more recent interpretation emphasising thin-skinned
tectonics is that of Glen ( 1992 ) . Gray and Foster ( 2004 )
provide the most recent synthesis.
The region was chosen because its geology is well docu-
mented (e.g. Scheibner and Basden, 1998 ) , it is only mod-
erately deformed and metamorphosed, and it contains a
variety of sedimentary rocks and both intrusive and extru-
sive igneous rocks. There are also major fault structures
and important porphyry style Cu

3.11.3.3 Responses from major fault zones
The two major fault structures in Kirkland Lake area are
the Larder Cadillac deformation (fault) zone and the Kirk-

land Lake main break (fault). Both are associated with
important gold deposits.
The Larder

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Cadillac deformation zone is a major struc-
ture extending for hundreds of kilometres (Wilkinson
et al., 1999 ) . It dips to the south and has a major reverse-
movement component, and is one of several high-strain
zones in the Abitibi Subprovince. It is characterised by
intense hydrothermal alteration, metasomatism and
veining, which is 500 m wide in places. This major deform-
ation zone has a complex history of reactivation with
different senses of movement in different areas, depending
on their strike, and is generally accepted to mark a suture
zone between major crustal blocks. The deformation zone
is easily recognised in the magnetic data from the associ-
ated linear zones of lower magnetic responses (V), and the
truncation of discordant anomaly trends across it (W).
Mapping its exact position can be difficult when it parallels
the stratigraphy (X).
The Kirkland Lake main break is a system of several
closely spaced faults within the Timiskaming Assemblage
(T and U). It trends east-northeast and has vertical to steep
southerly dip with southerly movement of a few hundred
metres, but no signi cant lateral component. Since its
strike direction is parallel to the local stratigraphic trend,
it is not especially well de ned in the magnetic data.

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- Au deposits, e.g. Cadia -

Ridgeway and Goonumbla - North Parkes.

3.11.4.1 Magnetic data
The aeromagnetic data were acquired at a terrain clearance
of 80 m along survey lines oriented east west and spaced

mainly 250 m apart. Figures 3.75a and b show images of
the TMI and its first vertical derivative for the study area.
The geomagnetic field in the area has an inclination of

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63°
and declination of +11°. The survey has been very effective
in defining the major geological entities, mapping a wide
variety of lithotypes, and delineating major structures.

3.11.4.2 Regional geology
The Lachlan Foldbelt comprises a series of north south-

trending structural domains separated by major fault
structures (which can be seen in the magnetic images in
Figs. 3.75a and b ) along which deformation has been
concentrated, e.g. (E) and the Gilmore (A), Tullamore
(B) and Coodalc

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3.11.3.4 Discussion
Although the Kirkland Lake area dataset would not be
considered of particularly high resolution by modern
standards, it does show just how effective aeromagnetics
can be as a mapping tool. A wealth of stratigraphic and
structural information is evident in the images making up

Narromine (D) sutures and the Parkes
Thrust (C). These structures often have similar trend to the
local stratigraphy, so truncated anomalies are not always
obvious. However, they can be recognised as either a linear

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