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Uranium
Exploration - The Lead Connection
This page contains a number of diagrams that may take some time to down load. Please be patient....The CSIRO
Division of Exploration and Mining and Rutherford Mineral Resource Consultants participated
in a research project in 1990 to develop more effective geochemical and
mineralogical techniques for the detection of uranium mineralization in regional
exploration in the west Arnhem Land region of tropical northern Australia. Two significant exploration problems were present in this major uranium province that needed to be addressed. The first related to access problems in this remote area and the large tracts of land that needed to be explored (several thousand km2), much of it covered by Tertiary to Recent coarse, largely quartz-rich sand derived from erosion of the prominent Kombolgie Sandstone units that cap much of the province. The second problem was the need to distinguish between uneconomic secondary uranium accumulations and anomalies associated with primary uranium mineralization. The Pb isotope
composition of Proterozoic uranium ores and their weathered equivalents was
found to be distinctive and very different from that of recent secondary uranium
accumulations. Pb isotopes thus provided an objective technique for
discriminating potentially fertile anomalies from "false anomalies". Partial leach procedures
designed to strip only the most mobile Pb from stream sediment samples were
utilised.
This was done to enable collection of Pb precipitated from solution in stream
waters that was released by the radioactive decay of uranium and weathered from
a primary uranium source. The sensitivity of Pb isotope analyses suggested that
the technique could be used in regional-scale geochemical exploration programs. Conventional Pb isotopic analysis is undertaken using thermal ionization mass spectrometers, which give excellent precision, but have a relatively slow throughput and high cost. However, in the search for uranium mineralization, such high precision is not required, and analyses can be carried out at a fraction of the cost using ICP-MS.
The rock units
which make up the provenance of soils and stream sediments in the Arnhem Land
region are the Kombolgie Formation (arenite, volcanic interbeds), the Cahill
Formation (predominantly biotite-muscovite schists), the Nabarlek Granite (and
Tin Camp Granite), the Zamu Dolerite and the Oenpelli Dolerite. The
relationship between the U/Th ratios and m-1 values is as follows: 238U
m-1
= ------- X C
232Th where C
is a function of the decay constants of 238U and 232Th and
the age of the rocks. The variable C is relatively insensitive to even
large age differences as can be seen from Table 1. TABLE 1 VALUES OF C FOR ROCKS OF VARYING AGES. ----------------------------------------------------------------- (Age Ma)
C 1500
3.40 1000
3.31 500
3.22 100
3.15 ---------------------------------------------------------------- A value
of 3.35 is acceptable for Arnhem Land. A broad
estimate of the expected background U and Th compositions can be gained by
reference to accepted global averages for the rock types present. These are
shown in Table 2. TABLE 2 U, Th AND CALCULATED m-1 VALUES FOR VARIOUS
ROCK TYPES ---------------------------------------------------------------------------- Rock Type
U
Th
m-1 ---------------------------------------------------------------------------- Granite
5
22
0.8 Sandstone
1
4
1.2 Basalt
0.4
1.6
0.9 Granite gneiss
4
13
0.9 (Data from
Rogers and Adams, 1969a, b) Data
specific to the Arnhem Land region are also available and these are summarized
in Table 3. TABLE 3 U, Th AND CALCULATED m-1 VALUES FOR ROCK UNITS
IN THE ARNHEM LAND REGION. ----------------------------------------------------------------------------------------------------------------- Unit
No
U(ppm)
Th(ppm)
m-1
Samples
mean
r
mean
r
mean
r ----------------------------------------------------------------------------------------------------------------- Kombolgie
10
5.7
4.9
20.8
14.1 0.9
0.3 Formation
(arenite) Nungbalgarri
3
2.7
-
10.8
-
0.9
- Volc.
Member Zamu Dolerite 9 2.3 1.0 8.1 6.2 1.6 1.1 Tin
Camp
2
11
-
63
-
0.6
- Granite Cahill
Formation 38
4.6
2.6
17.1 5.6
0.9
0.3 (unmineralised) (Data from
various papers in Ferguson and Goleby, 1980; part of Kombolgie Sandstone data
from this study). Therefore, the
upper threshold of the background (country rock) population of expected m-1
values for exploration samples on, or derived from, Kombolgie Formation or
Cahill Formation lithologies is 1.5 (mean + 2r). These two units represent the
major provenance of soils and stream sediments in the region. Soils or stream
sediments associated with the Tin Camp Granite should have a similar or slightly
lower threshold, but those associated with the Zamu Dolerite (and probably by
analogy the Oenpelli Dolerite) will have slightly higher thresholds, probably
about 2. Measured
m-1 Values of Unmineralised Rocks
In previous studies (Dickson et al., 1985; Gulson and Mizon, 1980) and for the 1990 study, the Pb isotopic compositions of unmineralised sequences of country rock and associated soils have been determined. The measured m-1 values based on these results are similar to, but are not necessarily exact equivalents of the values calculated from chemical analyses and discussed in the previous section. Differences could result from the partial decoupling of U and Th during metamorphism or incipient weathering, which will not be reflected in the isotopic ratios of the Pb daughter products. However, the
average measured m-1 values shown in Figure 1 are similar to the
calculated values. In comparison to the very high average values for U
mineralization, country rock data for the Drillers Pool radon anomaly and the
Koongarra North Grid have an average m-1 value of 0.6 and country
rock from drill holes into the Kombolgie Formation have an average m-1
of 1.1. The mean m-1 of this data is 0.91 with r = 0.26.
Thus the mean + 2r = 1.43 which is very similar to the values of 1.5
calculated from the chemical data. Based on all
the above data, the upper country rock m-1 threshold for sediments
and soils derived from the Kombolgie Formation lithologies, the Cahill Formation
and granitoids of the region is considered to be 1.5. Thus values of > 1.5
measured in exploration samples have < 5% chance of representing
unmineralised country rock and warrant follow up exploration. Where there has
been a significant input from the Zamu or Oenpelli Dolerites a higher threshold,
probably about 2, should be used. A plot of all
available data from the Arnhem Land region, both mineralized and unmineralised,
with 206Pb/204Pb ratios < 70 are shown in Figure 1. PRESENTATION
OF RESULTS
Results are
usually presented in two forms:
i)
On XY plots
with the ratios 206Pb/204Pb and 208Pb/204Pb
as the axes;
ii)
On XY plots
showing on one axis either distance along a traverse or drainage channel or
depth down a drill hole. On the other axis is shown a comparison of Rn and/or U
content, where available, with the 206Pb/204Pb ratio and
the function m-1. The 206Pb/204Pb
ratio is directly related to the age and U/Pb ratio of the rocks. It will thus
be high in the Proterozoic mineralization of an area and also in associated
secondary dispersion plumes. It will be low in normal country rocks and in Pb
associated with recently formed U accumulations. It may not be
possible to define a threshold 206Pb/204Pb ratio to
distinguish anomalous samples from "background" country rock because
there is a significant overlap as can be seen in Figure 1 on which are plotted
the fields for known U mineralization, including the low grade haloes, and for
country rock. These fields are based on all available data from the Alligator
Rivers region (Gulson and Mizon, 1980; Dickson et al., 1985, Carr and Dean,
1987). The 206Pb/204Pb ratios of samples associated with U
mineralization vary from ≈ 17 to > 1000 and for country rock between 17
and ≈70 and in one instance up to 200. To distinguish
the country rock and U mineralization "signatures" in this important
overlap range, the differences in 208Pb/204Pb ratio must
also be taken into account. This is accomplished by reference, to the function m-1; 206Pb/204Pb
(206Pb/204Pb(S) - 206Pb/204Pb(IR)) m-1
=
------------------- =
----------------------------------------------- 208Pb/204Pb
(208Pb/204Pb(S) - 208Pb/204Pb(IR)) which
represents the inverse of the slope on the 208Pb/204Pb vs 206Pb/204Pb
diagram of a line joining a data point (S) with the assumed initial ratio (IR)
of Proterozoic Pb. It thus provides a simple descriptor of the relative amounts
of radiogenic 206Pb and 208Pb and thus the U/Th ratio of
the parent rock (238U decays to 206Pb and 232Th
decays to 208Pb; U/Th = m-1/3.35, see above). Values of less
than 1.5 are considered to have a very high probability of representing the
normal range for unmineralised country rock (Kombolgie Formation, Cahill
Formation, regional granites). Values greater than 1.5 are considered to have a
high probability of being associated, at least in part, with U enrichment. The
derivation of this threshold is discussed above. ORIENTATION
STREAM SEDIMENT SURVEY USING Pb ISOTOPES
The aim of the
survey was to establish the viability of using the Pb isotopic signature of bulk
samples of stream sediments as a regional reconnaissance tool for detection of
primary uranium mineralisation in an analogous way to BLEG or BCL sampling is
used in gold exploration. Analysis of stream sediment samples from an
orientation survey conducted in drainage out from the small Caramel uranium
deposit south of Nabarlek were used to trial the method. Most of the
sediment in drainages of the west Arnhem Land province is dominated by coarse,
predominantly quartz-rich sand and there is little in the way of fine fraction
material. Such material presents problems for sampling in that fine material
(silts and clays), that would be the most suitable fraction for analysis for
uranium, is not always available to collect at a particular site. In addition
multiple sources for uranium, that is both primary Proterozoic age (that
associated with mineralisation) and secondary young, widely transported, 3° and
4° age accumulations in black soil, laterite etc., can not be discriminated by
routine geochemical analysis. A strategy was
therefore needed to discriminate between these and to make use of the sample
materials available. As stated above the Pb isotope composition of Proterozoic
uranium ores and their weathered equivalents has been found to be distinctive
and very different from that of recent secondary accumulations thus providing an
objective technique for discriminating potentially fertile anomalies from
"false anomalies". Because high precision is not required the
relatively cheap ICP-MS technique (rather than thermal ionisation mass spec) was
used to determine the definitive isotopic ratios. The method
easily detected the presence of uranium from the Caramel site by a distinctive
Pb isotopic signature, at the most distant point of sampling, some 1.7 km from
the source. At this point the signature from uranium and Pb by
routine geochemical analysis of the sediment had fallen to below the detection limit of
the analytical method used (XRF). It could be anticipated that the Caramel
site would have been detectable at a much greater distance by the isotopic method. The success of the approach offers scope for the simple collection of bulk sample material and direct detection of primary uranium mineralisation in regional stream surveys. It would be envisaged that catchments of at least 5-15 km2 could be screened with a single sample. The area incorporated in a sample would depend upon the disposition of individual drainage catchments in the area to be sampled and complexity of the geology and regolith within each catchment.
Figure 1. Summary of geology and location of stream sediment samples (grid spacing 500 metres).
Figure 2. Plot of +80# fraction of stream sediments for base metals down drainage from mineralisation. The high Ni and Zn values are related to sheared, altered and weathered dolerite units in the basement; the Pb is sourced locally from the from mineralisation.
Figure 3. Plot of -80# fraction of stream sediments for base metals down drainage from mineralisation.
Figure 4. Plot of both fractions of stream sediments for uranium and isotopic function m-1 down drainage from mineralisation. The " raw isotopic ratio value" needs to be "corrected" for age, rock type and differences in initial lead ratios for 208Pb/204Pb vs 206Pb/204Pb. An "isotopic function" m-1 of >1.5 - 2 and above is considered to be potentially anomalous. Values up to several hundred are generally found associated with local dispersion from primary Proterozoic uranium mineralisation in the Arnhem Land region. Secondary young accumulations of uranium in reducing environments (black soil) or Fe-laterite are typically about 1 or less. Contact: Graham Carr, CSIRO
Division of Exploration and Mining
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