Geoscience Reference
In-Depth Information
include: the giant alaskite-hosted Rössing deposit in
Namibia (Berning,
1986
); various unconformity-associated
deposits in the Pine Creek Geosyncline in northern Aus-
igneous complex at Poços de Calderas in Brazil, e.g. Cer-
cado (Forman and Angeiras,
1981
); and the calcrete-hosted
Yeelirrie deposit in Western Australia (Dentith et al.,
active U mineralisation in the U-channel data, radiometric
surveys have been widely used to identify terrains with
above-average U content. Within these terrains, areas with
anomalously high eU/K and eU/eTh ratios may be targeted
Figure 4.23
shows radiometric data from the Uranium
City area, in the Athabasca Basin of Saskatchewan,
Canada. Note the similarity between K and TC data,
indicating the former to be the dominant source of
radiation in the area. There are elevated values of U on
this survey line, but the eU/K and eU/eTh pro
les resolve
an anomalous response which is possibly due to increased
concentrations of U. An alternative thesis is that K and
Th have been removed with normal levels of U remaining
to produce anomalously high ratios. The lack of TC
response is quite commonly observed, normally owing
to a reduction in K, and/or U, as Th increases. Data on
eU, eU/eTh and eU/K are important for detecting uran-
ium occurrences. However, we emphasise here the point
made previously in
Section 4.6.1
: that the possibility of
disequilibrium in the
238
U decay series needs to be
considered when interpreting U concentrations obtained
from
-ray radiometric data.
The Yeelirrie U deposit, in the northern Yilgarn Craton
of Western Australia, comprises carnotite mineralisation in
calcrete. The host rocks are Cainozoic sediments within a
palaeochannel, with the uranium leached from surround-
ing radioactive Archaean granitoids. The orebody is a more
or less continuous horizontal lenticular zone approxi-
mately 9 km in length and 0.5 to 1.5 km wide. It averages
about 3 m in thickness and occurs 4 to 8 m below the
surface. Sandy and loamy overburden, ranging in thickness
from zero to a few metres, occurs above the mineralised
calcrete.
The deposit was discovered by a regional airborne mag-
netic and radiometric survey (1600 m line spacing, 150 m
survey height) in 1968.
Figure 4.24
shows recent airborne
data, although still with reconnaissance speci
cations
(400 m line spacing, 80 m survey height). The data were
acquired after the development of a small open pit into the
deposit, and so contain responses related to the resultant
ground disturbance. The host palaeochannel and the
higher-amplitude responses from the mineralised areas
within it are clearly seen. Ground radiometric data,
acquired soon after discovery, clearly delineate the extent
of the orebody (based on a cut-off of 0.1% U
3
O
8
over 2 m
thickness), corresponding with the 400 counts/minute level
which is approximately twice the background response
(
Fig. 4.24d
).
Another possibility for direct detection of mineralisation
using radiometrics results from the stability of monazite
and zircon, which can lead to their accumulation in heavy
mineral sands deposits. Together with magnetic responses,
radiometric data have been successfully used to map Th
carried by outcropping strand lines (Mudge and Teakle,
isation are weak and likely to be masked by other overlying
sediments.
Figures 4.12b
to
4.14b
show that the radioele-
ment content of heavy mineral sands is higher than that of
γ
0
10
Uranium anomaly
highlighted by
ratios
Kilometres
0.4
0.2
0.0
2.0
1.0
0.0
1.0
0.5
0.0
400
200
0
200
100
0
2000
1000
0
2000
1000
0
Figure 4.23
Pro
les of radiometric data, and various channel ratios
from the Uranium City area. The total count data are for an
integration period of 1 s, the other data are counts measured over a
5 s integration period. Anomalies in the uranium channel are clearly
de
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