Uranium ore-forming systems of the - Geoscience Australia

Uranium ore-forming systems of the Lake Frome region7.4.3 Modelling granite-water-sandstone interaction (formation of sandstonehosteduranium mineralisation)Geochemical constraints on the formation of sandstone uranium deposits are well known withoxidation-reduction reactions playing a major role in the transport and deposition of uranium. In thismodel the starting composition of fluid is the same as that used above in modelling of granite-waterinteraction, i.e., low-salinity surface water saturated with atmospheric oxygen and CO 2 isequilibrated with a uranium-bearing granite similar in composition to the Mount Neil Granite(Tables 7.2, 7.3) at a 1:1 weigh ratio. This reaction produces fluids containing up to 100 ppm ofdissolved uranium.In this model, the uranium-rich fluid is reacted with fresh sandstone containing pyrite and graphite(to model the effect of reductants). The initial composition of the sandstone is mineralogicallysimilar to that of sandy aquifer units in the Namba Formation or Eyre Formation.To explore the effect of increasing fluid/rock ratio on transport and deposition of uranium, 1 kg ofsandstone in the model is reacted with successive batches (each batch is 50 kg) of uranium-bearingoxidised fluid. Changes in the concentration of dissolved uranium in the fluid and of uranium oxidesdeposited in the sandstone are shown in Figures 7.12 and 7.13, respectively. The figures also showthe changes in the oxidation state of the fluid.As the first batch of fluid reacts with graphite-bearing sandstone dissolved uranium is quantitivelyprecipitated to form uranium oxides, and the uranium concentration in the fluid decreases from ~100ppm to less than 1x10 -9 ppm (Fig. 7.12). The pattern continues until the fluid/rock ratio reaches~2000 when the uranium concentration in the fluid begins to increase. At fluid/rock ratios of lessthan ~2000, deposition of uranium from successive batches of fluids can form a model ore zonecontaining up to ~11 weight percent uranium oxides (Fig. 7.13). Further inflow of oxidised fluid inthe sandstone begins to oxidise the sandstone, and uranium oxides deposited earlier begin dissolving.Above fluid/rock ratios of ~ 3500 all uraninite is dissolved. At each step reduction of the fluidproduces methane which at higher fluid/rock ratios is oxidised to form carbonates. As expectedpyrite is oxidised to hematite.Figure 7.12: Simulationof interaction of oxygensaturated,uranium-richfluid with graphite bearingsandstone (at 25 o C).Uranium concentration inthe fluid dramaticallydecreases as it reacts withinitially reducedsandstone. Introduction ofsuccessive batches of theoxygenated input fluidthrough the samesandstone volume causescomplete oxidation of thesandstone at a fluid/rockratio of ~3500, at whichthe uranium concentrationin the fluid reverts to theoriginal value.Page 99 of 151