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to intermediate volcanics, so its magnetic response is sub-
dued. Overlying the Skead Assemblage is the McElroy
Assemblage (O), comprising ma c and ultrama c rocks,
and as expected producing more variable and stronger
responses than the Skead Assemblage. However, the dom-
inant magnetic feature here is the Lincoln
a)
10 3
Alkali
Felsic
Intermediate
Mafic
Ultramafic
Non-igneous
10 2
-
Nipissing peri-
10 1
-
dotite
gabbro intrusion (P), which is roughly 20 km in
length and occurs at, or close to, the northern margin of
the McElroy Assemblage. Serpentinisation ensures that it is
strongly magnetic. At the northern margin of the McElroy
Assemblage is the Lincoln
10 0
Nipissing shear zone (Q),
which is associated with a northwest
-
10 -1
southeast trending
linear zone of weaker magnetic response.
The strongest magnetic anomalies in the Kirkland Lake
area are associated with the Boston Assemblage (R). Litho-
types include volcanics and sedimentary rocks, with the
main magnetic units being serpentinised ultrama
-
10 -2
10 -3
10 -3
10 -2
10 -1
10 0
10 1
cs and
iron formations. The petrophysical data show that the iron
formations are signi cantly more magnetic than any other
rock type in the area, which is to be expected. Within this
unit is the Adams magnetite deposit. The stratigraphy
extends around the margin of the Lebel stock (D) with
the local succession forming tight isoclinal folds. These are
too small to be resolved in the aeromagnetic data, but the
general trend of the stratigraphy is clear from its strong
response.
Very low magnetic responses are associated with clastic
metasedimentary rocks (S), whose stratigraphic affinity is
disputed (Hearse Assemblage of Jackson and Fyon, 1991 ).
Although these rocks are apparently easily mapped from
their subdued magnetic responses, it is easy to underesti-
mate their actual extent owing to the stronger magnetic
responses of underlying magnetic units.
The Timiskaming Assemblage (T and U) is the youngest
rock formation in the area and has an unconformity at its
base. It comprises alluvial
Induced magnetism (A/m)
b)
10 3
Iron formation
Other sediments
Non-sedimentary
10 2
10 1
10 0
10 -1
10 -2
10 -3
10 -3
10 -2
10 -1
10 0
10 1
Induced magnetism (A/m)
- uvial and turbidite sediment-
ary rocks, consisting mainly of conglomerate, plus
sandstones and argillites. Associated with the sedimentary
rocks are alkalic flows and pyroclastics. The succession
faces mainly south and has a moderate southerly dip. It is
cut by numerous faults and shear zones, and despite the
consistent facing does not consistently young southwards
because of strike faults and folds. The magnetic response is
variable, the metasedimentary components being mostly
weakly magnetised (T), but the igneous components give
rise to moderate positive anomalies (U). It is the truncation
and displacement of anomalies related to the igneous rocks
that allow faults to be delineated.
Figure 3.73 Plots comparing the strength of the induced and
remanent magnetisms of various lithotypes in the Kirkland Lake
area. Based on data in Ontario Geological Survey ( 2001 ).
the principal control on the magnetic responses, with the
linear anomaly pattern re ecting the interbedded litho-
types, the more ma c rocks being associated with the
stronger magnetic responses. The contrasts in the magnetic
properties of these lithotypes are con rmed by the petro-
physical data. The contact with the overlying Skead
Assemblage (N) is a sheared primary contact. The Skead
Assemblage is composed predominantly of calc-alkali felsic
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