Uranium ore-forming systems of the - Geoscience Australia

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Uranium ore-forming systems of the Lake Frome region4.5 DISCUSSION4.5.1 Reliability of the 3D chemical mapsUncertainties in the spatial distribution of redox characteristics shown in the voxet model arisefrom the following sources. Quality of original observations and descriptions in drill hole logs; Efficiency in the filtering methodology in accurately assigning redox values; Data spacing, determined by the drill hole spacing; Accuracy in the gridding algorithms used in the gOcad program in spatially representingthe redox variation.The broad patterns of oxidation and reduction shown in the 3D models are considered to reflectreal variations in redox characteristics of basin sediments and rocks (e.g., shallower oxidised anddeeper reduced zones). However, in detail the results should be used with caution in targetinguranium mineralisation because of the inherent uncertainties listed above. Areas with close drillhole spacing clearly will yield results with greater reliability than areas of widely spaced drilling.4.5.2 Sediments and redoxVertical sections of the voxet model such as the examples shown in Figure 4.9 show thatsignificant changes of redox state occur at some stratigraphic and lithological boundaries. Inparticular the results show that the Eyre Formation is significantly more oxidised than theunderlying Bulldog Shale in the EL 5-6 area, and the Namba Formation is overall more oxidisedthan the Eyre Formation. The Namba Formation exhibits the greatest internal variation.Figure 4.9: Examples of the redox pattern in vertical sections through the EL 5/6 voxet model.Also evident in comparing Figures 4.5, 4.6 and 4.9 are the complex 3-dimensional shapes ofoxidised and reduced zones within each stratigraphic formation. Perhaps some of the mostsignificant features for uranium mineral systems are the deeper zones of oxidation within the EyrePage 53 of 151
Uranium ore-forming systems of the Lake Frome regionand Namba Formations. In the Eyre Formation zones of ‘mixed’ redox within reduced rocksappear to form elongate north-south domains mainly along and down-slope (westwards) from themedial ridge that is apparent in the upper surface of the Eyre Formation. Given the currentwesterly dip of the Eyre Formation it is plausible that these zones represent oxidation frontsproduced by oxidised waters moving down-dip from east to west within the Eyre Formation. Thisgeometry implies that any uranium mineralisation in the Eyre Formation in the EL 5-6 area thatwas deposited along interfaces between oxidised and reduced rock would occur in north-southoriented zones, mainly in the central-western half of the area. Alternatively, the north-southoriented zones may reflect fluid movement along north-south oriented paleochannels within theEyre Formation. Indeed, paleochannels that involved broadly south to north sediment transport arewell known in the Eyre Formation further south (e.g., Yarramba and Lake Namba Paleochannels).The distributions of sand, gravel and pebbles intersected in drill holes (Fig. 4.8) do notdiscriminate between these alternative scenarios, suggesting that closer spaced drilling and/orother methods of defining paleochannels would be required to further test the 3D model presentedhereinDeeper zones of oxidation within the Namba Formation, like those in the Eyre Formation, alsoappear to be best developed to the west of the ridge in the upper surface of the Eyre Formation.One explanation is that the thicker parts of the Namba Formation to the west of this ridgerepresent a broad northerly trending paleochannel that also carried oxidised groundwaters.However, in detail the complex geometry of the interfaces between oxidised and reduced domainswithin the Namba Formation makes interpretation of potential uranium depositional sites difficultin the EL 5-6 area. It is interesting to note a small zone of oxidation in the Namba Formation tothe east of the ridge in the upper surface of the Eyre Formation. This may also represent oxidisedfluid flow in a subsidiary paleochannel, although its orientation is unknown.In the uppermost Namba Formation, above the small oxidised zone noted above, a zone ofreduced rocks forms a sinuous north-south channel-like geometry. This feature, noted earlier(Figs. 4.6, 4.7) and enlarged in Figure 4.10, apparently aligns with the orientation of knownpaleochannels in the underlying Eyre Formation (Fig. 4.11). There is, however, no obviouscorrelation with the drillhole distributions of sands, gravels and pebbles shown in Figure 4.7.Figure 4.10: Plan of the Namba surface showing the redox pattern in EL 5/6. The reduced areas havethe appearance of paleochannels. The interpreted extent of reduced material is outlined.Page 54 of 151
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