Radiometric method - Geophysics for the Mineral Exploration Geoscientist - page 225

most sedimentary rocks, but it is comparable to that of
many types of crystalline rocks.
As mentioned previously, coal has low radioelement con-
tent, especially K content, but occasionally U content can be
quite high ( Fig. 4.14b ) . In general coals are amongst the least
radioactive sediments, so composite radiometric ratios may
be required to identify subtle anomalous signatures.

a)

0

10

Kilometres

4.7.3.2 Alteration zones
Hydrothermal alteration is another potential source of
radiometric responses, although for a realistic chance of
detection the alteration zone must be large. Dickson and
Scott ( 1997 ) suggest a width of about 1 km for deposits in
Australia. If it is less than this, as for example in structur-
ally controlled hydrothermal gold mineralisation, detection
from the surface or above is unlikely, although responses
may be detected in

b)

-logs.
There has been success in detecting the potassic alter-
ation haloes associated with porphyry copper deposits (e.g.
Shives et al., 1997 ), epithermal gold

γ

silver deposits (Irvine
and Smith, 1990 ; Feebrey et al., 1998 ; Morrell et al., 2011 )
and to a lesser extent volcanogenic massive sulphide
deposits (e.g. Moxham et al., 1965 and Shives et al.,
2003 ) . Shives et al.( 1997 ) recommend the eTh/K ratio as
the best means of recognising potassic alteration zones.
The depth of erosion is an important control on the
radiometric response since the alteration surrounding
the mineralisation forms zones, not all of which are
radiometric

-

c)

Example - Waihi-Waitekauri epithermal Au-Ag mineralisation
The geophysical responses of epithermal Au

d)

Ag mineral-
isation in the Waihi-Waitekauri region of the North Island
of New Zealand are described by Morrell et al. ( 2011 ) . This
example demonstrates the use of radiometrics for mapping
the surface geology and zones of alteration. These deposits
mostly comprise andesite-hosted quartz veins with alter-
ation haloes, up to 15 km 2 in extent, of pervasive clay
alteration, potassium metasomatism, magnetite destruc-
tion and sulphide mineralisation. The magnetic responses
of the alteration zones are discussed in Section 3.9.5 . Geo-
chemical data were described in Section 4.6.6 . The radio-
metric responses of the alteration zones are shown in
Fig. 4.25 . The K radiometric image ( Fig. 4.25e ) shows high
values in the south which are coincident with outcropping
ignimbrites and high values to the east which are associ-
ated with alluvial deposits. Lower values in the centre and
west of

-

Limit of
orebody

0 2

Kilometres

>1000
500-1000
250-500
<250

c/min
(>1.63 MeV)

Figure 4.24 Radiometric data from the Yeelirrie calcrete U deposit.
(a) Ternary image, (b) eU image, (c) eU/(K+eU+eTh) ratio image,
(d) contour map of ground readings, in the vicinity of the orebody,
of -rays with energies greater than 1.6 MeV. This will include

emissions associated with both U and Th. Rectangle in (a

γ

c) shows
location of (d). Part (d) redrawn, with permission, from Dentith
et al.( 1994 ). Airborne data are used with the permission of
Geoscience Australia.

-

the survey area are associated with unaltered