Uranium ore-forming systems of the - Geoscience Australia

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Uranium ore-forming systems of the Lake Frome regionWe have not considered changes in permeabilities and porosities that might have resulted from thereaction between fluids and rocks (cf. Cleverley, 2008).7.2.3.3 Boundary conditions and model variationsThe model was uniformly saturated with fluid, with uninterruptible fluid supply at the top of themodel. The top of the model was fixed at atmospheric pressure (~1 MPa) and 25°C. The initialpressure-temperature conditions were based on hydrostatic pressures and a geothermal gradient of60°/km (drillhole Paralana 1B recorded 109°C at ~1,807 m; Petratherm, 2007). The base of themodel was maintained at constant temperature, determined by the initial geothermal gradient. Noadditional heat flow was specified at the base or sides of the model. For the MPI we havespecified a radiogenic heat production of 0.00308 x 10 6 W/kg (Neumann, pers. comm., 2008), butthis heat production has not noticeably perturbed the average geothermal gradient for the initialtemperature conditions. The sides of the model were open, and the base of the model was closedwith respect to fluid flow.The main set of models was run without fluid overpressure within the Cadna-owie Formation(Figs. 7.4 to 7.8). However, it is well known that groundwaters in the Cadna-owie Formation arecurrently overpressured, as evident from the presence of active mound springs in the region andartesian water in aquifers of the GAB (e.g., Radke et al., 2000). To illustrate the impact of thisphenomenon on the overall structure of the fluid flow model within Pliocene-Pleistocene epochs, aseparate model was created with fluid overpressure (Fig. 7.9) within the Cadna-owie Formation,accounting for the westward fluid flow at ~1 m y 1 .Routine models were run without reactive-transport but with an added tracer (a non-reactivecomponent dissolved in hydrothermal fluids) to investigate the hydrodynamics of the system inresponse to changes in geometrical (the topography and structure) and permeability parameters. Inthe case of “single-domain” MPI models, the tracer was initially distributed exclusively within thecoherent MPI rocks on both sides of the Paralana Fault (Figs. 7.4D to 7.7D); within the “layereddomain”models, the tracer was introduced exclusively within the weathered layer (Figs. 7.7D,7.8D). In both cases, the initial concentration of tracer within the fluid was specified at the 10mol kg 1 level. The geochemical rationale behind this approach was to trace the fluids from therocks that are the assumed sources of uranium (granitic rocks, and oxidized granitic rocks,respectively).7.2.4 Representation of results – fluid flow modellingTo summarise output results from the numerous modelling runs and to facilitate visualcomparison, we present results as follows (Figs. 7.4 to 7.9). Table 7.1 summarises the variableschanged throughout the presented model set. For each model under discussion, we show plotswith distribution of (a) permeabilities, (b) temperatures, (c) instantaneous Darcy fluxes, and(d) distribution of the tracer. Additionally, plots (a), (b), and (d) show fluid flowlines, but plot (c)shows the instantaneous Darcy fluxes and vectors to clearly identify the regions where the mostfluid flow occurs. All the plots are compiled for the same time slice (or close time slices fordifferent models, around 10,000 years). The notional locations of the Four Mile East and Beverleydeposits are shown in Fig. 7.3.Page 85 of 151
Uranium ore-forming systems of the Lake Frome regionTable 7.1: Values of permeabilities and fluid fluxes changed throughout the presented fluid-flowmodel set.Figure(model)Permeability (m 2 )2 Imposedfluid flux(m y 1 )7.4 7.5 7.6 7.7 7.8 7.9Paralana Fault 1.E-13 1.E-13 1.E-12 n/a 1.E-13 1.E-13Eastern Fault 1.E-13 1.E-13 1.E-13 1.E-16 to 1.E-13 1.E-13 1.E-13Western Fault 1.E-13 1.E-13 1.E-13 1.E-16 to 1.E-13 1.E-13 1.E-131 Willawortina 1.E-15 1.E-15 1.E-15 1.E-15 1.E-15 1.E-15FormationNamba1.E-16 1.E-16 1.E-16 1.E-16 1.E-16 1.E-16FormationEyre1.E-13 1.E-13 1.E-13 1.E-14 1.E-13 1.E-13FormationBulldog Shale 1.E-17 1.E-17 1.E-17 1.E-17 1.E-17 1.E-17Cadna-owie 1.E-13 1.E-13 1.E-13 1.E-13 1.E-13 1.E-13FormationNeoproterozoic 1.E-16 1.E-16 1.E-16 1.E-16 1.E-16 1.E-16AdelaideanMt Painter(weathered)n/a n/a n/a n/a 1.E-15 1.E-15Mt Painter 1.E-17 1.E-16 1.E-16 1.E-17 1.E-1611 Permeability of the Willawortina Formation artificially suppressed to compensate for the current inability of thePmdPyRT code to handle unsaturated fluid flow.2 Westward fluid flow within the Cadna-owie Formation.7.3 RESULTS AND DISCUSSION – FLUID FLOW MODELLING7.3.1 Fluid flow scenarios in the Beverley – Four Mile districtFisher et al. (2008) pioneered the reactive transport modeling of the formation of the sandstonehosteduranium mineralisation inspired by the Frome Embayment scenario. Initially they useda generic geological model with a 2D cross-section cutting across a broad, 8-km wide,paleochannel hosted by basin rocks of similar permeabilities. Once more, the fluid flow within themodel was topographically driven to the east, away from U-enriched granites of the MPI. Thepalaeochannel, hosted by oxidized basin rocks, served as a “local” reduction trap for uraniniferousfluids. Although Fisher et al. (2008) proceeded to more “realistic” layered geometries at theexpense of detailed structure in the paleochannel, their initial approach emphasized the potentialimportance of local chemical heterogeneities within otherwise hydraulically homogeneous rocks.Nevertheless, their 2D approach suffered from the assumption of fluid flow across, rather thanalong, paleochannels.The results of both Fisher et al. (2008) and the current study show that the relative permeabilitiesof the rock units are the fundamental control on fluid flow and fluid fluxes. Potential energyrepresented by topography drives fluid flow in the models in both studies.In our study, despite somewhat different patterns of the fluid flow depicted in Figures 7.3 to 7.8,most of the fluid flow occurs within the Eyre Formation. Overall, the eastward paleo fluid-flowpattern reproduced in the models within Cenozoic sediments is similar to the one suggested byHeathgate Resources (1998) and Brugger et al. (2005) (e.g., Figure 7.3). However, our resultsPage 86 of 151
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