Page 61 - Introduction to Mineral Exploration

44 A.M. EVANS & C.J. MOON

uranium silicate, coffinite. The more contenti- (a) 1.1
ous issue is the associated minerals; in Canada 1.0 n=36
high grade nickel arsenides are common, but by
no means ubiquitous, whereas the mineralogy 0.9
of the Australian deposits is simpler. However,
gold is more common and may be sufficiently 0.8
abundant to change the deposit model to 0.7
unconformity-related uranium–gold. Selenides
and tellurides are present in some deposits, 0.6
although Cox and Singer omit the former but Proportion of deposits 0.5
mention enrichment in palladium.
The genetic model for these deposits in- 0.4
cludes elements of the following: 0.3
1 Preconcentration of uranium and associated
elements in basement sedimentary rocks. 0.2
2 Concentration during the weathering of the 0.1
basement prior to the deposition of the over- 0.0055 0.26 9.6
lying sediments. 0.0
3 Mobilization of uranium and the associated 0.025 0.1 0.4 1.6 6.3 25 100 400
elements by oxidizing fluids and precipitation 0.0004 0.0016 0.0063
in a reducing environment at fault– Million tonnes
unconformity intersections.
4 Additional cycles of oxidation and mobiliza- (b) 1.0 n=36
tion. 0.9
As an example of the differences between the
descriptive and genetic models, the descriptive 0.8
model would suggest exploration for graphitic 0.7
conductors whereas the genetic model would
broaden the possible depositional sites to any 0.6
reducing environment. Descriptive and genetic Proportion of deposits 0.5
models can be combined with the grade-
tonnage curves of Cox and Singer (Fig. 3.12) 0.4
to suggest the probable economic benefits. It 0.3
should be noted that high tonnage does not
necessarily mean low grade, and large deposits 0.2
such as Cigar Lake (1.47 Mt of 11.9% U 3 O 8 ) in
Saskatchewan have grade and tonnage that 0.1 0.18 0.49 2.0
rank in the top 10% of all deposits. 0.0
The history of exploration in the two areas 0.1 0.18 0.32 0.56 1.0 1.8 3.2 5.6 10
clearly illustrates the uses and limitations of 0.032 0.056
models. Unconformity vein deposits were first
discovered as a result of exploration in known Uranium grade in percent U 3 O 8
uraniferous areas of Northern Canada (Fig. 3.13) FIG. 3.12 (a,b) Grade–tonnage curves for
and Australia in the late 1960s. The initial unconformity associated deposits. (From Cox &
Canadian discoveries were made by a French Singer 1986.)
company which was exploring for uranium
veins around the Beaverlodge deposit. These
have no obvious relation to the mid Proterozoic alized (Tona et al. 1985). On this premise the
sandstones and it was only the company’s pre- company (Amok) conducted an airborne radio-
vious experience in Gabon and minor known metric survey which led them to boulders
occurrences near the unconformity which of mineralized sandstone and massive pitch-
suggested that the sandstones might be miner- blende close to the source deposit (Cluff Lake