Uranium ore-forming systems of the - Geoscience Australia

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Uranium ore-forming systems of the Lake Frome regionProvided that interconnected permeability existed across faults, the numerical modelling implies thatthe Eyre Formation not only in the vicinity of the Four Mile deposit but elsewhere in the Lake Fromeregion is prospective for uranium mineralisation. Thus, stacked orebodies in Eyre Formation andoverlying Namba Formation are a possible scenario. The study of 3D chemical architectureindicates the potential for major north-south trending paleochannel systems of Paleogene ageextending further northwards into the Frome Embayment and Callabonna Sub-basin than previouslyrecognised.8.1.4 Depositional gradients for uranium mineralisation3D modelling of the chemical architecture has revealed important regional variations in theoxidation-reduction characteristics of Cenozoic units. Drill holes to the south of the Beverleydeposit intersected sediments with overwhelmingly oxidised character even at deeper levels of>100m in the Namba and Eyre Formations (Fig. 4.4). Similar patterns are observed in drill holesclustered around the Namba Paleochannel. Other areas of drilling show greater variability fromoxidised to reduced, with a general pattern of shallow oxidised and deeper reduced lithologies. Thevoxet model of the area of ELs 5 and 6 (Randell, 1973) east of Lake Frome shows a sinuous zone ofreduced rocks near the top of the Namba Formation, trending roughly north-south in the eastern halfof the EL5/6 area (Fig. 4.6). Union Corporation interpreted a north-south trending paleo-shoreline ofa Cenozoic lake in this area, comprising a string of deltas. Alternatively the sinuous zone of reducedsediments may represent a paleochannel system within the Namba Formation. If so, this couldrepresent a younger and northerly extension of the Yarramba Paleochannel system in the underlyingEyre Formation that hosts the Honeymoon and Oban uranium deposits (Fig. 4.11). Further work isin progress to test the paleochannel and shoreline hypotheses. In either case, the new 3Dmethodology developed in this study has demonstrated the presence of regional-scale redoxinterfaces within Cenozoic sediments. 3D mapping of redox interfaces is critical in targeting ofuranium mineralisation in basins, and this method offers a cost-effective means of focussingexploration programs.Public domain data indicate that Four Mile East is hosted by the Eyre Formation whereas Beverley iscontained within the Namba Formation. New petrographic observations of the Four Mile Eastdeposit reported in Chapter 5 have revealed significant differences with mineralisation at theBeverley deposit, despite their proximity. Although not yet comprehensive, the work at Four MileEast indicates that uraninite is the dominant ore mineral, in comparison with mainly coffinite atBeverley. This difference potentially is important from a metallurgical standpoint. Moreover, thisand other mineralogical differences may indicate substantially different ore-forming chemicalconditions that are not easily reconciled if both deposits formed along the same hydrological fluidflow paths. For example, at Four Mile there is textural evidence for quartz and kaolinite dissolutionyet at Beverley the silica-bearing coffinite indicates saturation of ore fluids with respect to silica.The presence of U-bearing and REE-rich phosphate minerals in uraninite-rich samples from FourMile East is interpreted to indicate that the U-bearing ore fluids were mobilising the REE andphosphorous. Transport of some of the LREE and Eu is redox-sensitive and so the presence of REEphosphates directly associated with uranium mineralisation is an indication of the role of redoxprocesses during alteration and uranium deposition. Possible reductants present in Eyre Formationsediments prior to mineralisation were carbonaceous matter, biogenic (framboidal) pyrite, andilmenite. Moreover, the possible role of bacteria should not be disregarded, and indeed there arebacteria that thrive by reducing aqueous uranium U 6+ to solid-phase U 4+ in the presence of organicPage 107 of 151
Uranium ore-forming systems of the Lake Frome regionnutrients (Loveley et al., 1991). Oxidation of ilmenite to rutile and anatase and incorporation of U inthe Ti oxide alteration minerals (Fig. 5.1) suggests but does not prove that this oxidationaccompanied uranium deposition. New pyrite growth with uraninite could be interpreted to indicatethat conditions in the ore zone were not overwhelmingly oxidised and that the ore fluids containedsignificants amounts of sulfur (e.g., as SO 4 2- ?). The presence of U- and REE-rich phosphateminerals is consistent with transport of uranium by aqueous PO 4 2- complexes, which are one of themost stable of uranium species at low temperature conditions over a wide range of pH (see Skirrowet al., 2009 and references therein). Other potential complexes include those of carbonate, oxyhydroxide.Destablisation of uranium-phosphate complexes, and deposition of uranium, is possiblevia reduction, or a dramatic change in the activities of aqueous calcium and/or phosphate such aswould occur if a phosphate-bearing fluid reacted with a Ca-bearing rock. The replacement ofkaolinite along cleavages and apparent co-precipitation of phosphate minerals with uraninite (e.g.,Figs. 6.9, 6.13) could indicate ore fluids out of equilibrium with kaolinite. However, changes influid pH (e.g., acidification) are not normally sufficient to cause uraninite deposition except at verylow pH and high Cl - or F - and elevated temperature conditions (see Skirrow et al., 2009).8.2 PALEOGEOGRAPHIC AND URANIUM MINERAL SYSTEM EVOLUTIONFigure 8.1 synthesises key results of this report as a time-series of schematic cross sections andpaleogeographic interpretations of a broad region centred on the Lake Frome study area. Thepaleogeographic interpretations are redrawn from the Paleogeographic Atlas of Australia (Langfordet al., 1995), and annotated with the schematic locations of potential uranium mineralisation. Ourresults largely support this earlier work, in particular the broadly south-to-north orientation of majorpaleochannel systems in the Lake Frome region during the Paleogene and Neogene. The crosssections (not to scale) represent a generic profile from a highland area of uranium-rich Proterozoicbasement in the southwest or south (e.g., Curnamona Province or Mt Painter Inlier), to a basinsetting in the north. The section follows the general trend of uranium transport and deposition in onepart of three-dimensional uranium mineral systems. The reader may find Figure 3.1 useful inreading Figure 8.1. Note the predicted episodic evolution of uranium mineral systems, in threeperiods from the Late Cretaceous-Paleocene to the Pleistocene. During each episode there was acoincidence of mineral system components: driver of fluid flow (topographic uplift driven by farfield tectonic processes), source of mobile uranium (e.g., weathered U-rich basement), source ofoxidised fluids (meteoric waters), permeable fluid pathways (sandy sediment, faults), anddepositional physico-chemical gradients (in-situ or mobile reductants). The intervening periodsgenerally lacked one or more components.It is emphasised that the three episodes of uranium mineral systems illustrated in Figure 8.1 arehypothetical. We are predicting that uranium mineralisation could have occurred within permeableunits of the Eromanga Basin, Eyre Formation and Namba Formation during three periods in theCenozoic. This ‘straw man’ model remains to be tested, for example by dating the uraniummineralising processes and by more detailed mapping of paleochannel systems and redoxarchitecture of the basin sediments. Indeed, ongoing work by Geoscience Australia is currentlybeing undertaken to this end.Figure 8.1 (over): Evolution of paleogeography and predicted uranium mineral systems during theCenozoic, Lake Frome region. Paleogeographic interpretations (panels at left) are redrawn fromLangford et al. (1995), and annotated with generalised locations of possible uranium mineralisation (U).The section line A-B corresponds to the cross sections to the right. Schematic cross sections are not toscale. The projected outline of the Cooper Basin is also shown.Page 108 of 151
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