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 regionpartly due to computational limitations in running large reactive-transport models. However, tobetter understand some <strong>of</strong> <strong>the</strong> chemical processes, a separate chemical modelling exercise wasundertaken (see Geochemical Modelling, below).7.2.3 Building simplified 2D geological models for fluid flow modelling7.2.3.1 Geometrical modelsThe modelling is based on a simplified geological cross-section interpreted from <strong>the</strong> seismicsurvey data close to <strong>the</strong> mineralised area (Fig. 7.2). Details <strong>of</strong> <strong>the</strong> lithostratigraphy andarchitecture are presented in Chapters 2 and 3, and uranium deposit descriptions are given inChapter 5.The model section consists <strong>of</strong> 7 stratigraphic units (Figure 7.4) and up to 3 basement faultspropagating into <strong>the</strong> overlying Mesozoic and Cenozoic units: <strong>the</strong> Paralana Fault system and <strong>the</strong>western and eastern faults that define <strong>the</strong> horst structure within <strong>the</strong> Proterozoic basement and <strong>the</strong>overlying basin. The complex Paralana Fault system was simplified in <strong>the</strong> model as a single faultzone, and hereafter is termed <strong>the</strong> Paralana Fault.The interpreted boundaries <strong>of</strong> <strong>the</strong> geologic units were imported from GoCad into <strong>the</strong> Gmshs<strong>of</strong>tware, a three-dimensional finite-element mesh generator program (Geuzaine and Remacle,2009), and fur<strong>the</strong>r manually edited to simplify <strong>the</strong> initial geometrical model and create itsvariations. The simplified models were subsequently meshed into unstructured triangular finiteelements. Unstructured triangular meshes allow realistic representation <strong>of</strong> geological features witharbitrary shapes, including narrow wedges <strong>of</strong> geological units (Cleverley et al., 2006).Initial model runs have demonstrated that most <strong>of</strong> <strong>the</strong> fluid flow occurred within <strong>the</strong> ~2 x 40 kmblock (Fig. 7.2). Thus, to speed up <strong>the</strong> calculations and focus <strong>the</strong> research effort on <strong>the</strong> arearelevant to mineralisation <strong>the</strong> geometrical model was constrained to this area containingapproximately 50,000 mesh elements. Depending on <strong>the</strong> particular purpose <strong>of</strong> <strong>the</strong> model variation,this basic cross-section was modified fur<strong>the</strong>r (Figs 7.4 to 7.9).7.2.3.2 Physical propertiesThe assigned permeabilities for each rock unit are explicitly shown in Figures 7.4A to 7.9A in logunits. Generally, we have assumed that <strong>the</strong> Mt Painter Inlier block represents <strong>the</strong> mostimpermeable rock unit compared to o<strong>the</strong>r lithologies in <strong>the</strong> modelled section, and we haveassumed a single permeability value for <strong>the</strong> entire MPI. The exception is <strong>the</strong> group <strong>of</strong> models forwhich we specified a layer <strong>of</strong> wea<strong>the</strong>red and fractured granite above <strong>the</strong> fresh coherent rock (Figs.7.8, 7.9), with <strong>the</strong> fractured layer having a permeability two orders <strong>of</strong> magnitude higher than <strong>the</strong>coherent rock. According to Domenico and Swartz (1998) and Freeze and Cherry (1979),permeabilities <strong>of</strong> fractured igneous and metamorphic units (directly proportional to hydraulicconductivities) can be up to four to five magnitudes higher than <strong>the</strong>ir unfractured counterparts.In most <strong>of</strong> <strong>the</strong> models we have presumed that permeabilities <strong>of</strong> <strong>the</strong> fault zones were two to threeorders <strong>of</strong> magnitude higher than permeabilities <strong>of</strong> <strong>the</strong> immediate host units, with <strong>the</strong> exception <strong>of</strong> aspecial case presented in Figure 7.9. The Cadna-owie, Eyre, and Willawortina Formations areconsidered to be largely sand units; thus, <strong>the</strong>ir permeabilities are two orders <strong>of</strong> magnitude higherthan <strong>the</strong> Bulldog Shale and <strong>the</strong> Namba Formation. To compensate for <strong>the</strong> current inability <strong>of</strong> <strong>the</strong>PmdPyRT code to handle unsaturated fluid flow, and to speed up calculations, permeability <strong>of</strong> <strong>the</strong>Willawortina Formation was artificially suppressed relative to <strong>the</strong> permeabilities <strong>of</strong> <strong>the</strong> Cadnaowieand Eyre Formations (Figures 7.4 to 7.9).Page 84 <strong>of</strong> 151