Groundwater geochemistry in the Koongarra ore deposit, Australia (I): Implications for uranium migration

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Groundwater geochemistry at the Koongarra uranium ore deposit was investigated in order to gain a detailed understanding of the migration of uranium in a highly weathered water-rock system. Koongarra groundwaters are quite dilute with the total dissolved solids usually below 200 mg/l. The pH is slightly acidic or neutral, and the major chemical characteristics are dominated by magnesium and bicarbonate. Partial pressures of CO₂ in the deeper groundwaters are substantially elevated relative to those of surface waters. Groundwater in the mineralized zones exhibits elevated levels of uranium up to three orders of magnitude above background levels. Total organic carbon levels are generally low, suggesting that uranium complexation by organic species plays a minor role. Due to the high bicarbonate concentration, uranium appears to be mobile in the weathered zone as uranyl carbonate complexes. Other inorganic uranium complexants are not present at levels sufficient to influence uranium speciation, with the possible exception of phosphate. On the basis of chemical and isotopic evidence, there are two major inputs of groundwater to the system. The first of these is flows from the vicinity of the Koongarra fault into the Cahill formation, which hosts the uranium mineralization. A second major source is infiltrating waters which permeate downward from the surface, and cause a gradual mixing and dilution of the characteristics of groundwaters from the mineralized zone. The migration of uranium in groundwater is not only perpendicular to the fault, but includes a component at an angle to it. In the vicinity of borehole C1 (due south of the ore zone), uranium concentrations are comparatively high, given the distance from the orebody. Moving away from the ore zone to the south-east, there is a gradual decrease of groundwater uranium concentrations to background levels over approximately 200 meters, which coincides with the uranium distribution in the solid phase. Therefore, at Koongarra, uranium seems to have migrated over distances of approximately 200 m toward the south-east over a time period estimated to be 1 to 1.5 million years.