Gravity and magnetic methods - Geophysics for the Mineral Exploration Geoscientist - page 186

subtle, the folds are de ned by the magnetic data, often as
linear anomalies forming elongated closed loops (L). Felsic
rocks in the extreme east of the image give rise to subdued
responses. The volcanic terrain to the west of the Parkes
thrust has the textured appearance typical of a succession
of variably magnetic igneous rocks. Volcanic lithologies are
predominantly andesitic, but tend towards basaltic in
places. The succession is unmetamorphosed with dips of
30

data containing responses from Palaeozoic volcanic centres
and modern-day drainage. Completely different explor-
ation models could be applied to these data.

3.11.5 Magnetic and gravity responses from
mineralised environments

Data presented elsewhere in this chapter show gravity
responses associated with: the Las Cruces Cu

50°. Two major folds, the Forbes anticline (FA) and
Milpose syncline (MS), have curvilinear traces trending
northeasterly. The sedimentary units are significantly less
magnetic than the volcanics, allowing the broad structure
to be delineated from the magnetic data. The compara-
tively wide survey line spacing of the data in Fig. 3.76a
causes aliasing of the short-wavelength responses which
produces a spurious north

- Au massive

sulphide deposit in the Iberian Pyrite Belt, Seville, Spain;
epithermal Au

-

Te mineralisation in an igneous com-
plex at Cripple Creek, Colorado, USA; gold-bearing
palaeochannel deposits in the Port Wine area, California,
USA; and coal-bearing sequences in the Bonnet Plume
Basin, Yukon, Canada. Also presented are alteration-
associated magnetic responses from the Wallaby Au
deposit, Western Australia, and an iron-ore deposit in
Qian

- Ag -

in the image.
The clastic sediments of the Hervey Group are again asso-
ciated with subdued magnetic responses and lower gravity
(F). The sediments are non-magnetic, but their presence
places the underlying magnetic rocks further from the
magnetometer, so the response of these rocks is attenuated
(see Section 3.10.1.1 ) . The smoothing of responses with
depth is illustrated clearly in the western third of the image
where the non-magnetic cover is present.
Porphyry copper

- south ' fabric '

an District, China. In Section 4.7.3.2 we present the
magnetic responses associated with Au

'

Ag mineralisation
in the Waihi-Waitekauri region in New Zealand, in Section
2.6.1.2 we showed the response from the Elura Zn

-

Ag
volcanogenic massive sulphide deposit in eastern Australia,
and in Section 2.6.4 the response from the Marmora mag-
netite skarn in Ontario, Canada. In Section 2.10.2.3 , we
showed the gravity response from massive nickel sulphide
mineralisation at Pyhäsalmi, Oulu, Finland. Further
examples are shown in Fig. 3.77 . Together these examples
have been chosen to illustrate the three basic types of
potential field responses that can be used for direct detec-
tion of mineral deposits, i.e. responses from mineralisation,
from an associated alteration zone and from the prospect-
ive host lithology.
The mineralisation itself gives rise to the magnetic
response from the magnetite skarn in Figs. 2.12 and
3.77a , the iron-formation hosted iron ore in Fig. 3.64
and the cumulate chromite deposits in Fig. 3.77b . The
gravity anomaly from Las Cruces ( Fig. 3.27 ) is due to
the entire body of mineralisation, i.e. ore minerals and
gangue. The magnetic anomaly associated with the nickel
sulphide mineralisation in the Thompson nickel belt
( Fig. 3.77c ) is due to magnetite and pyrrhotite in the
mineralisation, i.e. non-ore minerals. The epithermal
deposits at Cripple Creek ( Fig. 3.18 ) and Waihi-
Waitekauri (see Fig. 4.25 ) are examples of responses
from the alteration zone produced by the hydrothermal
system that gave rise to mineralisation. The alteration
causes a decrease in density at Cripple Creek and is
magnetism destructive at Waihi-Waitekauri. The Wallaby

- Pb -

gold mineralisation occurs at a
number of sites, all located within a circular structure,
approximately 22 km in diameter, interpreted as a col-
lapsed caldera. The feature is prominent in the magnetic
data, especially where magnetite-bearing monzonites and
diorites form a ring dyke along the northern margin. The
caldera also has a distinct negative gravity anomaly associ-
ated with it, which is thought to be due to an underlying
low-density intrusion at relatively shallow depth. Within
the caldera, circular magnetic features a few kilometres in
diameter are interpreted as high-level stocks. There are
several linears, the most important of which is the Endeav-
our lineament, a structural corridor containing most, but
not all, of the porphyry centres.

-

3.11.4.5 Discussion
Although a very different geological environment to the
granitoid-greenstone terrain of the Kirkland Lake example
(see Section 3.11.3 ) , the aeromagnetic data are again
extremely effective at mapping the geology. The data are
characterised by responses of very different character: the
'

'

response of the volcanics versus the smooth
response from the sediments. The ability of magnetics to
map different aspects of the geology is demonstrated by the

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