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zone of lower magnetism or as the contact between areas
with different magnetic textures.
Belts of deformed volcanic, volcaniclastic and intrusive
igneous rocks are separated by
to northeast (Emerson et al.,
1979
). Compare these
responses with those from intrusions with dominantly
induced magnetism (K), which have minor negative com-
ponents due to the comparatively high inclination.
Deformation in the Middle Devonian was followed by
molasse sedimentation, comprising the clastic rocks of the
Hervey Group (F) in the study area. These sequences are
non-magnetic and create a characteristic subdued response
in the magnetic data. This is partly due to their lack of
magnetism, but also due to underlying magnetic sources
being at great depth. In the area to the east of
containing sedi-
mentary and volcanic sequences. From the Ordovician to
the early Devonian there was a series of tectonic and
magmatic episodes, often of comparatively localised areal
extent. There is a dominant north
'
troughs
'
-
south trend from folds
and thrust faults, plus some conjugate strike-slip faulting
with northeast and northwest trends. Deformation was
predominantly thin-skinned, above detachments in the
upper and middle crust.
the
Goonumbla
North Parkes porphyry deposits (G-NP), the
change in magnetic character with source depth is quite
clear. Outcropping volcanic rocks have a highly textured
response with many short-wavelength variations. Where
the same rocks underlie sediments of the Hervey Group
(F), the responses coalesce creating a smoother appearance.
Cratonisation in the Early Carboniferous was followed by
erosion which transported magnetic detritus into the
drainage network, delineated by its prominent sinuous
and dendritic anomaly shapes (G).
The magnetic images show several other features of
general interest. The cross-cutting nature of intrusions is
revealed by the truncation of stratigraphical anomalies (H).
Anomaly (J) has long wavelength and does not correlate
with the surface geology. It is probably related to an intru-
sive mass at depth. Most granitoids have comparatively
weak magnetism (M) and there is evidence for a magnetic
halo (N), possibly due to magnetite created by contact
metamorphism, or maybe remanently magnetised zon-
ation related to cooling of the mass. Small intrusive bodies
have a characteristic textured appearance similar to the
Ordovician volcanics they intrude. Linear positive anomal-
ies are caused by dykes (P). There are also good examples
of anomalies truncated by minor faults (O).
-
3.11.4.3
Magnetic responses from lithotypes
and structures
Ordovician quartz-rich turbidites, black shales and chert,
plus mainly shoshonitic igneous rocks (Ov), exhibit the
highly textured magnetic response typical of igneous rocks.
In the Silurian at, and to the east of, the Gilmore Suture (A),
thick sequences of turbidites accumulated in mainly narrow
graben or graben-like troughs, e.g. the Tumult, Cowra
Yass
and Hill End troughs. These rocks have a subdued magnetic
signature although, in places, linear anomalies delineate
structures. The regions between the troughs were sites of
shallow-water sedimentation, mainly clastics, but also some
carbonates. These rocks are very weakly magnetised and so
tend to exhibit lower magnetic intensity.
In the Late Silurian to Early Devonian, the Tumult
Trough was inverted resulting in a zone of folding, thrust-
ing and metamorphism mainly in the region between the
Gilmore (A) and Coolac
-
Narromine sutures (D). Linear
magnetic anomalies in these areas allow some folds to be
recognised (L). Further east, e.g. the Hill End and Cowra
-
-
Yass Zones, deposition continued uninterrupted. During
the Devonian there was also significant magmatism of
ma
c to felsic composition (Di, Dv), e.g. I-type granitoids
and volcanics. Their magnetic responses are similar to the
Ordovician volcanics. Intrusion of gabbro
3.11.4.4
Gravity and magnetic responses in the
North Parkes area
The North Parkes porphyry copper district (NP) contains
several Cu
peridotite or
Alaskan-type intrusions occurred in the west, for example
the Fi
eld Complex (I). Magnetic anomalies associated
with these rocks show evidence for dominance of
remanent magnetism, i.e. they are dipolar anomalies with
the negative component to the southwest of the positive
component. Without remanent magnetism, a response like
that seen in
Fig. 3.8d
would be expected, i.e. a
-
Au deposits, e.g. Endeavour 26 North, Endeav-
our 22 and Endeavour 27 (Heithersay et al.,
1990
). Terrain
is subdued and outcrop sparse, with cover material tens of
metres thick. The base of oxidation varies from zero to
80 m depth. Magnetic surveys have proven to be an effect-
ive means of mapping the bedrock geology in the district.
A geological map and images of the magnetic and Bouguer
gravity data from the North Parkes area are shown in
-
'
'
low
to the
'
'
south of the source with a
north of it. This interpret-
ation is supported by petrophysical data which show mag-
netic vectors with shallow inclinations directed northwest
high
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