GEMOC ARC National Key Centre

Mark C. Pirlo¹ & Angela M. Giblin²
 

1: GEMOC ARC National Key Centre, Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109, Australia
2: CSIRO Exploration and Mining, North Ryde, NSW 2113, Australia
 

Current research has examined the use of groundwater geochemistry (hydrogeochemistry) in the exploration for Tertiary paleochannel uranium targets in the Frome Embayment of South Australia.  The region experiences a semi-arid climate and the uranium mineralisation typically exists in paleochannels that contain saline groundwaters.  The paleochannels are concealed beneath approximately 70m of cover, with no surface indication, making for challenging exploration.   Uranium exploration and production has experienced growing interest in Australia in recent times, particularly in this region.  This is partly due to advancements in in-situ leach production technology being applied to paleochannel roll-front uranium mineralisation.
Thirty six groundwater samples have been collected from monitoring bores in mineralised aquifers.  Samples have been analysed for a suite of major and trace elements.  Equivalent major element concentration plots (Schoeller Plots) have identified a pattern similarity amongst groundwaters sourced from and directly adjacent to sediment hosted uranium mineralisation.  A different pattern is observed for groundwaters in the vicinity of the nearby basement that is inferred to be the source of the uranium now in sediment hosted uranium mineralisation.  A groundwater database maintained by the state government and consisting of 85 samples has been obtained for the region and Schoeller ratios calculated for each of these groundwaters.  These samples exhibit similar Schoeller Plot patterns to the samples collected for this study, supporting the use of Schoeller Plots as an effective means of identifying groundwaters similar to those associated with roll-front uranium mineralisation.  Dominant characteristics of Schoeller Plots from mineralised localities are the Ca:Mg ratios.
The geochemical modeling code REACT (Bethke, 1998) has been used to develop equilibrium models of the groundwater samples collected.  From these, major ion activities and aqueous speciation has been predicted.  Plots of major ion activities against ionic strength have been shown to be in good agreement with those reported for the region by Giblin (1987), although the strong correlation between U and   for groundwaters from mineralised locations is not obvious with all current samples.  However the current groundwater samples plot in the same general fields of the major ion activity-activity plots for dominant carbonate minerals as did the 1987 samples.  The stable equilibrium mineral assemblage predicted by the equilibrium models suggests that uranophane, dolomite and chalcocite control the aqueous U, Ca and SO₄ activities respectively.  All samples are undersaturated with respect to gypsum.  With further development, groundwater geochemistry may be an effective means of detecting economic mineralisation within the paleochannel system.

References

Giblin, A. M.  (1987)  Applications of groundwater geochemistry to genetic theories and exploration methods for Early Tertiary sediment-hosted uranium deposits in Australia.  Uranium, 3:165-186.

Bethke, C. M.  (1998)  The Geochemist's Workbenchì Release 3.0.  A user's guide to Rxn, Act2, Tact, REACT and Gtplot.  University of Illinois Hydrogeology Program.