Geoscience Reference
In-Depth Information
Weathering
Rock soil
Other lithotypes
Mafic & ultramafic igneous & metamorphic rocks
a)
6
25
25
5
20
20
4
15
15
3
10
10
2
5
5
1
0
0
0
0
1
2
3
4
5
0
1
2
3
4
5
0
1
2
3
4
5
6
K (%)
K (%)
U (ppm)
b)
25
25
6
(5.3,25)
(5.3,8.2)
5
20
20
(8.2,25)
4
15
15
3
10
10
2
5
5
1
0
0
0
0
1
2
3
4
5
0
1
2
3
4
5
0
1
2
3
4
5
6
K (%)
K (%)
U (ppm)
Figure 4.17
Abundances of K, U and Th compared for bedrock and adjacent overburden. (a) Australia; bedrock compared with soils. (b)
groundwater discharge, U and Th abundance may increase
because of the effects of dissolved Ra. Saturated materials
are denser than unsaturated materials, which affects the
ability of
equivalents, although some local consistencies are likely.
Normally, field studies of the near-surface environment are
required in order to establish local conditions and pro-
cesses, and to properly understand the effects of the cover
on the radiometric response.
-rays to escape into the atmosphere. Permafrost
also subdues the radiometric response, as does snow cover.
Vegetation may also be important since it increases attenu-
ation. This is greatest for woody material, and normally the
vegetation needs to be forest-like to affect the radiometric
response signi
γ
4.6.6
Potassium, uranium and thorium in
mineralised environments
cantly.
The surface drainage system has a signi
cant effect on
the surface distribution of the three radioelements. The
minerals carrying the radioelements can be transported
through the system, forming a useful radioactive tracer of
the drainage. Similarly, wide-area
flooding distributes the
radioelements over large areas, both as mineral grains and
in solution to be redeposited through evaporation. The
effect is to increase the background concentration levels
of the radioelements over large areas, often seen as a
background
Figures 4.12
,
4.13
and
4.14
show that the K, U and Th
minerals, and orebodies containing these minerals, have
substantially higher radioelement content than any kind of
host rock. There are reports in the literature of depletion or
enhancement of virtually every combination of the three
radiometric elements in hydrothermally altered zones,
although U and Th are normally less affected by alteration
than K.
The variation in K and Th through the porphyry Cu
-
Au
'
'
in K, U and Th imagery.
In summary, the effects of cover on the radioelement
distribution are as diverse and as complicated as the pro-
cess of weathering itself. Consequently, there are no uni-
versal
haze
mineralisation at Goonumbla
North Parkes in the Lachlan
Foldbelt of New South Wales, Australia, is shown in
Fig. 4.19
. Potassium content is higher in both the phyllic
and potassium alteration zones. Importantly, weathering
greatly decreases the K content. Uranium content
-
relationships between bedrock and weathered
is
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