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of reduced magnetism owing to the destruction of magnet-
ite by hydrothermal processes and weathering (see
Fig. 3.55 ). Occasionally, magnetite may be concentrated
more in structures such as faults and shear zones than in
the surrounding rocks, so that they exhibit linear positive
responses.
The magnetic responses from nickel and base-metal
sulphides can rarely be de ned, as their responses are
usually weak compared with the responses of their host
stratigraphy. Exploration for these types of deposits using
magnetics is often focused toward locating and mapping
favourable stratigraphy, e.g. the contact between mafic and
ultramafic units. Also, gold mineralisation may be strati-
graphically controlled with iron-rich, and therefore poten-
tially magnetic, units being favourable hosts; and it may
also occur at stratigraphic contacts where adjacent rock
types have contrasting rheological properties.
We have chosen the Kirkland Lake area, part of the
Abitibi Subprovince in Ontario, Canada, to illustrate
the aeromagnetic signatures of an Archaean granitoid-
greenstone terrain, because it has a diversity of lithotypes
and several major structures which are associated with gold
deposits such as the giant Kirkland Lake and world-class
Kerr Addison and Chester eld deposits. There is also sig-
ni cant iron-ore mineralisation in the area, with the
Adams Mine producing magnetite iron ore from
Algoman-type iron formation. In addition, there exists a
significant literature about the geology of the Kirkland
Lake area, e.g. Ispolatov et al.( 2008 ). A concise summary
of the geology is provided by Jackson and Fyon ( 1991 ),
whose stratigraphic terminology has been adopted here.
These data show that remanent magnetism is signi cant in
the area. As expected some of the iron formations have
very high Königsberger ratios, close to 100, but many felsic,
ma c and ultrama c rocks also have ratios greater than 1,
in most cases between 1 and 10. The rocks with values in
excess of 10 may be affected by lightning strikes (see
Section 3.9.8.4 ).
3.11.3.2 Responses from different lithotypes
Virtually the entire spectrum of greenstone lithotypes can
be found in the Kirkland Lake area; metamorphic grade is
greenschist or lower, but may be higher adjacent to intru-
sions. Referring to Figures 3.74a and b , the largest geo-
logical entity is the Round Lake batholith (A), a gneissic
tonalite
granodiorite intrusion with dimensions of tens of
kilometres. Its magnetic response is subdued, allowing the
responses of thin, northwest-trending lamproite dykes (B)
to be seen clearly. In the south of the area, the greenstone
stratigraphy extends around the margins of the batholith,
but further north it strikes dominantly east
-
west.
The Kirkland Lake area has several large syenitic intru-
sions: the Otto (C), Lebel (D) and McElroy (E) stocks. The
magnetic expression of these intrusions is generally sub-
dued, which is consistent with the magnetic property data.
The stocks are surrounded by narrow contact aureoles
where metamorphic grade reaches amphibolite facies.
Based on magnetic fabric studies, the Lebel stock (D) is
thought to be comparatively thin, having a disc-like form,
resulting from southward flow of magma which ascended
along faults near its northern margin (Cruden and Lau-
neau, 1994 ). The Long Lake Fault bisects it, the fault being
clearly visible in the magnetic data as a decrease in the
magnetic anomaly (F). The Otto stock (C) contains a large
roof pendant of basic rocks, at amphibolite facies, in its
west-central area and this is visible in the magnetics (G).
In the east and south of the area, the Archaean rocks are
unconformably overlain by Proterozoic and Phanerozoic
strata (H). These cover units are fairly non-magnetic, but
their presence can be inferred from the subdued responses
of the underlying Archaean rocks (subdued because of the
increased distance between them and the magnetometer).
Similar effects are caused by lakes. The western part of the
survey extends over the southeastern portion of the Wata-
beag Batholith (I), which is weakly magnetic. Intermediate
to felsic igneous rocks of the Watabeag Assemblage occur
within the batholith (J) but, also being poorly magnetic, are
not well resolved in the magnetic data. More obvious are
the
-
3.11.3.1 Magnetic data
The geomagnetic field in the survey area has an inclination
of +74° and a declination of
12°. The aeromagnetic data
were acquired at a nominal terrain clearance of 70 m along
survey lines oriented north
-
south and spaced 200 m apart
(Ontario Geological Survey, 2003 ).
The survey has been very effective in de ning the major
geological entities in the region, mapping a wide variety of
lithotypes, and delineating major structures ( Fig. 3.74 ). An
extensive database of petrophysical data has been compiled
(Ontario Geological Survey, 2001 ) to assist the interpret-
ation of the aeromagnetic data and help in understanding
of the geology. Frequency histograms of magnetic suscepti-
bility and strength of remanent magnetism for selected
lithotypes are shown in Fig. 3.72 and the strength of
remanent and induced magnetism compared in Fig. 3.73 .
-
north
-
south
trending
dolerite
dykes
of
the
 
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