The role of vegetation in anomaly formation at Barns Gold Prospect, Eyre Peninsula South Australia

Transcription

1 The role of vegetation in anomaly formation at Barns Gold Prospect, Eyre Peninsula South Australia M.J. Lintern (CSIRO) and E. Rhodes (ANU)

2 The Big Galah Eyre Peninsula

3 Barns DUNE STUDY Eyre Peninsula South Australia 3 km

4 Barns A Gold in calcrete S S S S S S 6-9 S Biota/soil sample sites Mineralization 1 km

5 Plan showing dune detail SECTION Drill holes Biota/soil sample sites Mineralization ~200 m

6 50m 100m (Modified from Drown)

7 Study overview Part 1- biogeochemical survey Part 2 - investigation of geochemical processes occurring in a sand dune a) dating b) bulk element distributions c) isotopes d) LA ICP MS

8 Part 1 Barns traverse at edge of dune

9 Part 1 Melaleuca uncinata (broombush)

10 Part 1 Calcareous rhizomorph Eucalyptus incrassata (lerp mallee)

11 Eucalyptus incrassata Part 1 Au (ppb) bark terminals litter Along dune edge Across dune Easting (metres) Northing (metres) Mineralization

12 Melaleuca uncinata Part 1 Au (ppb) bark terminals litter Along dune edge Across dune Easting (metres) Northing (metres)

13 Eucalyptus incrassata terminals seasonal variation Part Along dune edge end of summer end of winter Across dune 3 Au (ppb) Easting (metres) Northing (metres)

14 Melaleuca uncinata terminals seasonal variation Part Along dune edge end of summer end of winter Across dune 3 Au (ppb) Easting (metres) Northing (metres)

15 Soil and calcrete Part Along dune edge 30 4 Across dune cm soil [LHS] 1.6 calcrete [RHS] 20 Au (ppb) Easting (metres) Northing (metres)

16 IMPLICATION FOR EXPLORATION: THE DILEMMA Calcrete Gold concentration (storage) Soil Litter Vegetation Reliability, confidence, current knowledge Potential usefulness as sample medium in areas of cover, (but uncertainty)

17 (Modified from Butt)

18 Part 2 Plan showing dune detail Drill holes Biota/soil sample sites Mineralization ~200 m

19 Part 2 Before

20 Part 2 During

21 Part 2 After view from top looking into slot

22 Part 2 View looking south looking into slot OSL dating sample BR2/2.4

23 Part 2 How long to form gold anomalies in calcrete? Typical carbonate rhizomorphs containing elevated Au 15 cm 2.5m depth 9.2 ppb Au in calcareous rhizomorph 2.5 m from top of 8 m high dune Sand beneath rhizomorph dated at BP by OSL Collapsed dune

24 Profile in dune Au ppb humic horizon (A) eluvial horizon (O) Depth (metres) illuvial horizon (B) illuvial horizon Bca (calcrete) Gold in basal/interface calcrete = 5-35 ppb

25 Biological mechanism for Au anomaly formation in dune (5-35 ppb Au) CSIRO 2005 Mineralization at 40 m depth

26

27 How long to accumulate this quantity of Au in the sand? [Au] ppb sand Au (mg) in sand Horizon mass (tonnes) NPP kg yr -1 area -2 ( ) = 8600 to years for Au to accumulate in sand [Au] vegetation (1 ppb) < 4000 to 5900 years for Au to accumulate in calcareous rhizomorph

28 What Au concentration in biomass and production rate is required? Gold in veg. vs time for different NPP rates Mallee ecosystem NPP range ( kg-1yr-1area-2) years YEARS NPP=0.5 NPP=1.0 NPP= GOLD CONCENTRATION OF VEGETATION (PPB) NPP=3.0 NPP=4.0 NPP=5.0

29 SUMMARY Anomalous Au in dune sand and vegetation Au particularly concentrated in calcareous rhizomorphs Dual role of roots in Au anomaly formation in the rhizomorphs Vegetation (via roots) capable of suppling all the Au to the sand in ~10000 years in current conditions

30 Acknowledgments John Prescott Adelaide University Andreas Schmidt-Mumm and his students Adelaide University Malcolm Sheard and John Keeling - PIRSA Chris Drown Adelaide Resources Ltd EL holder Dean Walters land owner Dennis Petty District Council of Le Hunte bulldozing and rehabilitation of the dune Angelo Vartesi CSIRO for slide preparation ActLabs CRC LEME and CSIRO Thank you