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 regionnutrients (Loveley et al., 1991). Oxidation <strong>of</strong> ilmenite to rutile and anatase and incorporation <strong>of</strong> U in<strong>the</strong> 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 <strong>the</strong> <strong>ore</strong> zone were not overwhelmingly oxidised and that <strong>the</strong> <strong>ore</strong> fluids containedsignificants amounts <strong>of</strong> sulfur (e.g., as SO 4 2- ?). The presence <strong>of</strong> U- and REE-rich phosphateminerals is consistent with transport <strong>of</strong> uranium by aqueous PO 4 2- complexes, which are one <strong>of</strong> <strong>the</strong>most stable <strong>of</strong> uranium species at low temperature conditions over a wide range <strong>of</strong> pH (see Skirrowet al., 2009 and references <strong>the</strong>rein). O<strong>the</strong>r potential complexes include those <strong>of</strong> carbonate, oxyhydroxide.Destablisation <strong>of</strong> uranium-phosphate complexes, and deposition <strong>of</strong> uranium, is possiblevia reduction, or a dramatic change in <strong>the</strong> activities <strong>of</strong> aqueous calcium and/or phosphate such aswould occur if a phosphate-bearing fluid reacted with a Ca-bearing rock. The replacement <strong>of</strong>kaolinite along cleavages and apparent co-precipitation <strong>of</strong> phosphate minerals with uraninite (e.g.,Figs. 6.9, 6.13) could indicate <strong>ore</strong> fluids out <strong>of</strong> 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 syn<strong>the</strong>sises key results <strong>of</strong> this report as a time-series <strong>of</strong> schematic cross sections andpaleogeographic interpretations <strong>of</strong> a broad region centred on <strong>the</strong> Lake Frome study area. Thepaleogeographic interpretations are redrawn from <strong>the</strong> Paleogeographic Atlas <strong>of</strong> <strong>Australia</strong> (Langfordet al., 1995), and annotated with <strong>the</strong> schematic locations <strong>of</strong> potential uranium mineralisation. Ourresults largely support this earlier work, in particular <strong>the</strong> broadly south-to-north orientation <strong>of</strong> majorpaleochannel <strong>systems</strong> in <strong>the</strong> Lake Frome region during <strong>the</strong> Paleogene and Neogene. The crosssections (not to scale) represent a generic pr<strong>of</strong>ile from a highland area <strong>of</strong> uranium-rich Proterozoicbasement in <strong>the</strong> southwest or south (e.g., Curnamona Province or Mt Painter Inlier), to a basinsetting in <strong>the</strong> north. The section follows <strong>the</strong> general trend <strong>of</strong> uranium transport and deposition in onepart <strong>of</strong> three-dimensional uranium mineral <strong>systems</strong>. The reader may find Figure 3.1 useful inreading Figure 8.1. Note <strong>the</strong> predicted episodic evolution <strong>of</strong> uranium mineral <strong>systems</strong>, in threeperiods from <strong>the</strong> Late Cretaceous-Paleocene to <strong>the</strong> Pleistocene. During each episode <strong>the</strong>re was acoincidence <strong>of</strong> mineral system components: driver <strong>of</strong> fluid flow (topographic uplift driven by farfield tectonic processes), source <strong>of</strong> mobile uranium (e.g., wea<strong>the</strong>red U-rich basement), source <strong>of</strong>oxidised fluids (meteoric waters), permeable fluid pathways (sandy sediment, faults), anddepositional physico-chemical gradients (in-situ or mobile reductants). The intervening periodsgenerally lacked one or m<strong>ore</strong> components.It is emphasised that <strong>the</strong> three episodes <strong>of</strong> uranium mineral <strong>systems</strong> illustrated in Figure 8.1 arehypo<strong>the</strong>tical. We are predicting that uranium mineralisation could have occurred within permeableunits <strong>of</strong> <strong>the</strong> Eromanga Basin, Eyre Formation and Namba Formation during three periods in <strong>the</strong>Cenozoic. This ‘straw man’ model remains to be tested, for example by dating <strong>the</strong> uraniummineralising processes and by m<strong>ore</strong> detailed mapping <strong>of</strong> paleochannel <strong>systems</strong> and redoxarchitecture <strong>of</strong> <strong>the</strong> basin sediments. Indeed, ongoing work by <strong>Geoscience</strong> <strong>Australia</strong> is currentlybeing undertaken to this end.Figure 8.1 (over): Evolution <strong>of</strong> paleogeography and predicted uranium mineral <strong>systems</strong> during <strong>the</strong>Cenozoic, Lake Frome region. Paleogeographic interpretations (panels at left) are redrawn fromLangford et al. (1995), and annotated with generalised locations <strong>of</strong> possible uranium mineralisation (U).The section line A-B corresponds to <strong>the</strong> cross sections to <strong>the</strong> right. Schematic cross sections are not toscale. The projected outline <strong>of</strong> <strong>the</strong> Cooper Basin is also shown.Page 108 <strong>of</strong> 151