Uranium ore-forming systems of the - Geoscience Australia

Uranium ore-forming systems of the Lake Frome regionTable 7.3: Composition of fluid/water used in the calculationsComponent (ppm) Surface water Paralana Hot SpringsNa + 4 272K + 0.2 23Ca 2+ 0.4 42Mg 2+ 0.4 15Cl - 6 310Total dissolved sulfur 1.3 136U (ppb) 0 0.07Surface water composition derived from the rainwater analysis at Verdum station, South Australia (Blackburn andMcLeod, 1983). The composition of Paralana Hot Springs water is from Brugger et al. (2005).7.4.2 Granite-water interaction (modelling Paralana Hot Springs)Interaction of oxygen-saturated surface water with uranium-bearing granite is modelled to explorethe possibility of uranium leaching from granites of the Mt Painter Inlier. Recent studies of thegeochemistry of the Paralana Hot Springs show that these surface-derived waters were deeplycirculated within the granite-bearing basement of the Mount Painter Inlier and moved up along morepermeable fault zones such as the Paralana Fault (Brugger et al., 2005; this study). The uraniumcontent of the Paralana Hot Springs is low (0.07ppb, Table 7.3) compared with some of thegroundwaters in the district (Brugger et al., 2005). In this section we seek to explain thisgeochemistry through geochemical modelling, and to determine the implications for uraniummineralising systems.First, a simple model is constructed for granite-water interaction simulating downflow of surface(meteoric) waters into granite. The surface water is saturated with oxygen and carbon dioxide attheir respective partial atmospheric pressures (logfO 2 = -1 and logfCO 2 = -3.5) and is assumed to beof low salinity (11 ppm of total salts, NaCl, KCl, CaCl 2 , MgCl 2 ) with 0.5 ppm of dissolved sulfur(Table 7.3). In the modelling, this 25ºC water is then equilibrated with a rock similar in compositionto the Mount Neil Granite in the Mount Painter Inlier (Table 7.2, Coats and Blissett, 1971) at a 1:1weight ratio. Initial concentration of uranium in the granite is set at 50 ppm in the model. Theresults show that the oxygen saturated water at 25 o C dissolves all the available uranium, and allFe +2 -bearing silicates and oxides are converted to hematite. To model the downward flow of thisfluid into the granite and accompanying heating of the fluid due to the geothermal gradient, the fluidresulting from the initial reaction with granite reacts at equilibrium with the fresh granite atincreasing temperature up to 100 o C. The results show that the fluid which was oxidised at 25 o C anduranium rich is immediately reduced by the granite, and the uranium concentration in the fluid dropssignificantly (from 100 ppm to less than 1x10 9 ppm; Fig. 7.10). Thus the fluid moving downwardthrough the fresh granite cannot dissolve geologically significant concentrations of uranium. Only atvery high fluid rock ratios (of >1000) will the fluid in equilibrium with the granite be sufficientlyoxidised to carry significant concentrations of uranium. However, to attain such high fluid/rockratios, the fresh granite would be required to have unrealistically high permeability. Nevertheless,extremely high permeabilities may be possible in some weathered zones and/or in breccia zones.Second, the chemical changes during upflow of the fluid in the circulation system are modelled. Theproperties of this fluid are initially set to be the same as those derived from the final interaction ofdown-flowing water with granite at 100 o C, modelled above. Upflow is modelled by cooling thefluid to 25 o C at rock-buffered conditions (in equilibrium with the granite, see Fig. 7.3). As the initialconcentration of uranium in the fluid is very low (