Uranium ore-forming systems of the - Geoscience Australia

Uranium ore-forming systems of the Lake Frome region7.4 GEOCHEMICAL MODELLING OF FROME URANIUM SYSTEMSTo test the chemical implications of predicted fluid-flow patterns for uranium solubility, transportand deposition, a set of auxiliary geochemical modelling calculations have been undertaken. Thesecalculations allow the prediction of uranium solubility in equilibrium with different rock types, aswell as changes in uranium solubility due to fluid-rock reactions at different temperatures andpressures relevant to uranium systems of the Lake Frome region.7.4.1 Methodology and model setupThe geochemical model used for runs with equilibrium-dynamic and reactive-transport modellingcomponents was created using the HCh software for geochemical modelling (Shvarov andBastrakov, 1999). To focus on the most important controls on uranium mobilisation, transport, anddeposition, we have simplified the composition of the rock units. It is assumed that all the rock typesare represented essentially either by granites or arkose sedimentary rocks, and are composed ofquartz, K-feldspar, albite, muscovite, chlorite, biotite, and hematite/magnetite (Table 7.2). Parts ofthe system are modelled to contain uraninite (the MPI) or elemental carbon as a reductant (the EyreFormation).The initial chemistry oof the rocks was defined by volume fractions of minerals, and fluids as molesof the dissolved components. A simple Na-K-Ca-Mg-Cl-S solution with ~12 ppm of dissolved saltswas used as a starting composition for surface waters (Table 7.3).Table 7.2: Rock compositions used in the calculationsMinerals (weight %) Granite SandstoneQuartz 32 80K-feldspar 35 8Plagioclase 9 2Muscovite 7 3Biotite as annite 15 0Epidote 0.6 0Chlorite as chamosite 0.6 0Kaolinite 0 6Calcite 0 0.4Carbon as graphite 0 0.5Ilmenite 0.5 0Magnetite 0.9 0Apatite 0.5 0Fluorite 0.6 0Pyrite 0.02 0.1Uraninite 0.006 0Granite composition based on the composition of Mount Neil Granite Porphyry (Coats and Blissett, 1971).Sandstone composition corresponds to the composition of a sandstone (except for carbon) in Kazakhstan(Fyodorov, 1999).Page 96 of 151