Uranium ore-forming systems of the - Geoscience Australia

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