Uranium ore-forming systems of the - Geoscience Australia

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Uranium ore-forming systems of the Lake Frome region4.4.2 Voxet Model of EL 5 and 6, north-eastern Lake Frome regionThe gross pattern of shallow oxidised and deeper reduced zones observed regionally in Figure 4.4appears also to hold for the EL5-6 area. However, as shown in Figure 4.5, there are importantvertical and lateral redox variations within this area that are potentially important for uraniummobilisation and deposition. It is also clear that there is considerable vertical variability in redoxover narrow intervals of a few metres to 10s of metres (Fig. 4.5).The model shows that the westerly dipping upper surface of the Cretaceous Bulldog Shale ismostly reduced with localised areas of oxidised and mixed redox. The largest oxidised zonesoccur in the upslope areas with smaller zones down slope. The areas of mixed redox show asimilar pattern.The redox pattern on the upper surface of the Cenozoic Eyre Formation shows that this surface isalso mostly reduced with localized mixed redox and oxidised zones. The oxidised areas mostlyoccupy the roughly north-south trending ridge in the centre of the EL5-6 area, with othersoccurring upslope to the east and partway down the westerly slope. The areas of mixed redoxhave a more random distribution.The upper surface of the gently west-dipping Cenozoic Namba Formation shows the greatestvariability in redox values. Roughly half the area is oxidised, a third is of mixed redox and theremainder reduced. The most striking feature of the voxet model is the sinuous broadly northsouthtrending channel-like pattern of the reduced values, coloured blue in Figure 4.6. It ispossible these features represent parts of a paleochannel system within the upper NambaFormation, with reduced material occupying parts of the channel.The Quaternary upper surface dips gently to the west and is almost uniformly oxidised with somescattered areas of mixed redox and reduced values.56.5 km76.5 kmFigure 4.5: Voxet model of redox and topography of each lithological surface in the EL 5/6 area.Page 49 of 151
Uranium ore-forming systems of the Lake Frome regionFigure 4.6: Plan view of the voxet model showing the redox patterns on each lithological surface inthe EL 5/6 area.4.4.3 Distribution patterns of selected materials in ELs 5 and 6In order to better understand the causes of the redox patterns, and relationships between redox,lithology and permeability, information on the occurrence of particular minerals, rock types andsediment grain size sand was extracted from the drill logs. The spatial distribution ofcarbonaceous material, lignite, pyrite, hematite and limonite was mapped in 3D, along with dataon the occurrence of gravel, pebbles and carbonate. Two-dimensional plan projections of theresults are shown in Figures 4.7 and 4.8. Results for the Namba Formation are shown in Figure4.7. At top left of the figure, the redox patterns define a semi-continous zone of reduced materials(blue) trending broadly north-south in the central-eastern part of the area, resembling apaleochannel. However, the distributions of the selected redox-sensitive materials in Figure 4.7(carbonaceous material, lignite, pyrite, hematite and limonite) only partly match the location of thepossible paleochannel. One possible reason for this apparent lack of spatial correlation is that notall materials and criteria listed in Table 4.1 are shown in Figures 4.7 and 4.8. For example, thecolour criteria in Table 4.1 are not illustrated in Figure 4.7. Carbonaceous material occurs in thesouthern part of the inferred paleochannel, whereas limonite appears to negatively correlate withthe location of some of the reduced zones. The distribution of sand versus gravel potentially maycorrelate with the presence of paleochannels, but this is not evident in the data. Silt and clay (notshown in Fig. 4.7) are the dominant constituents of the Namba Formation, whereas sand issporadic and gravel and pebbles are uncommon.Results for the Eyre Formation show a correlation between the redox pattern and the occurrence ofcarbonaceous material (Fig. 4.8). Pyrite and lignite are less widespread but contribute to the broadextent of reduced zones in the Eyre Formation. Restricted oxidised zones correlate partly with thepresence of hematite and limonite. Sand is ubiquitous within this formation and gravel andpebbles are also common. Carbonate materials are mostly restricted to the south-west corner ofthe area.Page 50 of 151
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