Uranium ore-forming systems of the - Geoscience Australia
Uranium ore-forming systems of the - Geoscience Australia
Uranium ore-forming systems of the - Geoscience Australia
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<strong>Uranium</strong> <strong>ore</strong>-<strong>forming</strong> <strong>systems</strong> <strong>of</strong> <strong>the</strong> Lake Frome regionProvided that interconnected permeability existed across faults, <strong>the</strong> numerical modelling implies that<strong>the</strong> Eyre Formation not only in <strong>the</strong> vicinity <strong>of</strong> <strong>the</strong> Four Mile deposit but elsewhere in <strong>the</strong> Lake Fromeregion is prospective for uranium mineralisation. Thus, stacked <strong>ore</strong>bodies in Eyre Formation andoverlying Namba Formation are a possible scenario. The study <strong>of</strong> 3D chemical architectureindicates <strong>the</strong> potential for major north-south trending paleochannel <strong>systems</strong> <strong>of</strong> Paleogene ageextending fur<strong>the</strong>r northwards into <strong>the</strong> Frome Embayment and Callabonna Sub-basin than previouslyrecognised.8.1.4 Depositional gradients for uranium mineralisation3D modelling <strong>of</strong> <strong>the</strong> chemical architecture has revealed important regional variations in <strong>the</strong>oxidation-reduction characteristics <strong>of</strong> Cenozoic units. Drill holes to <strong>the</strong> south <strong>of</strong> <strong>the</strong> Beverleydeposit intersected sediments with overwhelmingly oxidised character even at deeper levels <strong>of</strong>>100m in <strong>the</strong> Namba and Eyre Formations (Fig. 4.4). Similar patterns are observed in drill holesclustered around <strong>the</strong> Namba Paleochannel. O<strong>the</strong>r areas <strong>of</strong> drilling show greater variability fromoxidised to reduced, with a general pattern <strong>of</strong> shallow oxidised and deeper reduced lithologies. Thevoxet model <strong>of</strong> <strong>the</strong> area <strong>of</strong> ELs 5 and 6 (Randell, 1973) east <strong>of</strong> Lake Frome shows a sinuous zone <strong>of</strong>reduced rocks near <strong>the</strong> top <strong>of</strong> <strong>the</strong> Namba Formation, trending roughly north-south in <strong>the</strong> eastern half<strong>of</strong> <strong>the</strong> EL5/6 area (Fig. 4.6). Union Corporation interpreted a north-south trending paleo-sh<strong>ore</strong>line <strong>of</strong>a Cenozoic lake in this area, comprising a string <strong>of</strong> deltas. Alternatively <strong>the</strong> sinuous zone <strong>of</strong> reducedsediments may represent a paleochannel system within <strong>the</strong> Namba Formation. If so, this couldrepresent a younger and nor<strong>the</strong>rly extension <strong>of</strong> <strong>the</strong> Yarramba Paleochannel system in <strong>the</strong> underlyingEyre Formation that hosts <strong>the</strong> Honeymoon and Oban uranium deposits (Fig. 4.11). Fur<strong>the</strong>r work isin progress to test <strong>the</strong> paleochannel and sh<strong>ore</strong>line hypo<strong>the</strong>ses. In ei<strong>the</strong>r case, <strong>the</strong> new 3Dmethodology developed in this study has demonstrated <strong>the</strong> presence <strong>of</strong> regional-scale redoxinterfaces within Cenozoic sediments. 3D mapping <strong>of</strong> redox interfaces is critical in targeting <strong>of</strong>uranium mineralisation in basins, and this method <strong>of</strong>fers a cost-effective means <strong>of</strong> focussingexploration programs.Public domain data indicate that Four Mile East is hosted by <strong>the</strong> Eyre Formation whereas Beverley iscontained within <strong>the</strong> Namba Formation. New petrographic observations <strong>of</strong> <strong>the</strong> Four Mile Eastdeposit reported in Chapter 5 have revealed significant differences with mineralisation at <strong>the</strong>Beverley deposit, despite <strong>the</strong>ir proximity. Although not yet comprehensive, <strong>the</strong> work at Four MileEast indicates that uraninite is <strong>the</strong> dominant <strong>ore</strong> mineral, in comparison with mainly c<strong>of</strong>finite atBeverley. This difference potentially is important from a metallurgical standpoint. M<strong>ore</strong>over, thisand o<strong>the</strong>r mineralogical differences may indicate substantially different <strong>ore</strong>-<strong>forming</strong> chemicalconditions that are not easily reconciled if both deposits formed along <strong>the</strong> same hydrological fluidflow paths. For example, at Four Mile <strong>the</strong>re is textural evidence for quartz and kaolinite dissolutionyet at Beverley <strong>the</strong> silica-bearing c<strong>of</strong>finite indicates saturation <strong>of</strong> <strong>ore</strong> fluids with respect to silica.The presence <strong>of</strong> U-bearing and REE-rich phosphate minerals in uraninite-rich samples from FourMile East is interpreted to indicate that <strong>the</strong> U-bearing <strong>ore</strong> fluids were mobilising <strong>the</strong> REE andphosphorous. Transport <strong>of</strong> some <strong>of</strong> <strong>the</strong> LREE and Eu is redox-sensitive and so <strong>the</strong> presence <strong>of</strong> REEphosphates directly associated with uranium mineralisation is an indication <strong>of</strong> <strong>the</strong> role <strong>of</strong> redoxprocesses during alteration and uranium deposition. Possible reductants present in Eyre Formationsediments prior to mineralisation were carbonaceous matter, biogenic (framboidal) pyrite, andilmenite. M<strong>ore</strong>over, <strong>the</strong> possible role <strong>of</strong> bacteria should not be disregarded, and indeed <strong>the</strong>re arebacteria that thrive by reducing aqueous uranium U 6+ to solid-phase U 4+ in <strong>the</strong> presence <strong>of</strong> organicPage 107 <strong>of</strong> 151