GROUNDWATER GEOCHEMISTRY IN THE KOONGARRA ORE DEPOSIT, AUSTRALIA .1. IMPLICATIONS FOR URANIUM MIGRATION

Groundwater geochemistry at the Koongarra uranium ore deposit was inve stigated in order to gain a detailed understanding of the migration of uranium in a highly weathered water-rock system. Koongarra groundwate rs are quite dilute with the total dissolved solids usually below 200 mg/l. The pH is slightly acidic or neutral, and the major chemical cha racteristics are dominated by magnesium and bicarbonate. Partial press ures of CO2 in the deeper groundwaters are substantially elevated rela tive to those of surface waters. Groundwater in the mineralized zones exhibits elevated levels of uranium up to three orders of magnitude ab ove 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 ino rganic uranium complexants are not present at levels sufficient to inf luence uranium speciation, with the possible exception of phosphate. O n the basis of chemical and isotopic evidence, there are two major inp uts of groundwater to the system. The first of these is flows from the vicinity of the Koongarra fault into the Cahill formation, which host s the uranium mineralization. A second major source is infiltrating wa ters which permeate downward from the surface, and cause a gradual mix ing and dilution of the characteristics of groundwaters from the miner alized zone. The migration of uranium in groundwater is not only perpe ndicular 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 conce ntrations are comparatively high, given the distance from the orebody. Moving away from the ore zone to the south-east, there is a gradual d ecrease of groundwater uranium concentrations to background levels ove r approximately 200 meters, which coincides with the uranium distribut ion 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.