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Lead Isotope Geochemistry: A Brave New World?

Lead Isotope Geochemistry: A Brave New World?. Graham R Carr CSIRO Exploration and Mining June 2013. Outline. Traditional Use of Pb Isotopes in Metallogenic Studies Exploration Geochemistry – General Principles Gossans and Residual Soils Groundwaters Partial Extractions of soils

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Lead Isotope Geochemistry: A Brave New World?

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  1. Lead Isotope Geochemistry:A Brave New World? Graham R Carr CSIRO Exploration and Mining June 2013

  2. Outline • Traditional Use of Pb Isotopes in Metallogenic Studies • Exploration Geochemistry – • General Principles • Gossans and Residual Soils • Groundwaters • Partial Extractions of soils • Vegetation • U exploration • The brave new world - • The cost factor – can the commercial labs do better? • The confidence factor – will companies use isotopes? SMEDG 26 June 2013

  3. Pb Isotopes in Metallogenic Studies(Plumbotectonics) • History of the Earth According to my favorite element – Pb. • Really a history of the fractionation of Pb, Th and U in the mantle and crust through geological time. • The key information that Pb isotopes provide are: • Relative contributions of mantle-derived and crustal-derived Pb in rocks and ores • Any evidence of U/Th fractionation as a result of high grade metamorphism or the formation U enriched hydrothermal fluids. • Model age SMEDG 26 June 2013

  4. Pb Isotopes Thesis: • In any geological terrain, mineralisation associated with a major hydrothermal event will have distinctive Pb isotope ratios that can be discriminated from minor mineralisation and from Pb derived from background rocks. • The General Exploration problem is – can we measure and interpret these fingerprints in common regolith geochemical samples – rocks, soils, vegetation, groundwater? SMEDG 26 June 2013

  5. Basic Equation: 206Pb/204Pb =206Pb/204Pbi + 238U /204Pb(et - 1) Pb Isotope Variables Time Source Rock Chemistry

  6. 18.0 2 1 0 3 4 Ga 16.0 14.0 Pb 204 Pb/ 12.0 207 Primordial Pb 10.0 Salt Creek (Archaean) HYC (Proterozoic) Woodlawn (Palaeozoic) 8.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0 22.0 Pb/ Pb 206 204 The Growth Curve Concept • Growth Curve: Co-variation through geological time of a pair of Pb isotope ratios assuming a common, U/Pb () Simple (unrealistic) Single Stage Model

  7. The Growth Curve Concept More realistic Two-Stage Model

  8. The Mount Isa Growth Curves WFB Curve WFB Curve 15.54 Analytical Precision 1500 Ma 1700 Ma 1600 Ma 15.50 EFB Curve EFB Curve Pb 1300 Ma 204 15.46 Pb/ 1400 Ma 207 Cumming and Richards Curve Cumming and Richards Curve 15.42 HYC Orebodies DS - 3 Century DS Mount Isa Pb/ Zn DS 1500 Ma Cannington Pop 2 DS Cannington Pop 1 DS 15.38 16.00 16.11 16.22 16.33 16.44 16.55 206 204 Pb/ Pb

  9. 1600 Ma 15.54 Isan Orogeny Lawn Hill Vein Analytical Precision Isan Orogeny Century - Lady Loretta 1700 Ma Lawn Hill Formation Syn Sedimentary 15.50 Lady Loretta Formation 1500 Ma Syn Sedimentary Pb Urquhart Shale Syn Sedimentary 204 15.46 HYC Syn Sed. Event Pb/ Mount Isa Orogeny Events (EFB) Soldiers Cap – Dugald River Event Mount Isa Orogeny Cu – Event (WFB) 207 15.42 Soldiers Cap – Cannington Event 15.38 16.0 16.1 16.2 16.3 16.4 16.5 206 Pb/ 204 Pb Mount Isa Template CODES Masters 2013

  10. Relating Ores to Tectonics Western Fold Belt 1400 Depositional/Tectonic Hydrothermal Age Age 1450 1500 Age Ma 1550 1600 1650 1700 . . . . 1 2 3 D D min D sed sed Fm sed .-max Shale WFB Cu .- - - - LL-Century Syn Syn Lawn Hill Vein Lawn Hill Urquhart Loretta Fm Isan Loretta Syn Loretta Fm Shale Lawn Hill Isan Isan CODES Masters 2013 Lady Lady Early Lady Urquhart Late

  11. 15.55 15.55 Analytical Precision Analytical Precision 1500 Ma 15.53 1500 Ma 15.53 Grevillea Pb 15.51 204 Pb/ Walford Creek 15.49 207 1600 Ma 1600 Ma 15.47 Kamarga 15.45 16.10 16.25 16.40 16.55 206 Pb/ 204 Pb “Other” Deposits Different fluids and different source rocks, or…..a small subset of the same source rocks.

  12. Large, outcropping gossan in Lower Proterozoic rocks of the Mount Isa Western Fold Belt Geochemically highly anomalous with % Zn and Pb Exploration Prospect – Mount Isa Is it worth drilling?? What is its origin? CODES Masters 2013

  13. Pb Isotopes in Regolith Materials • Have greatest value in : • discriminating and eliminating the “False Positive” geochemical anomaly • Having greater sensitivity than absolute abundance data in detecting metal derived from a hidden/buried ore source • Greatest inhibitors to use: • Cost • Anthropogenic contamination SMEDG 26 June 2013

  14. The Two Dimensions of Pb Isotope Target Signature Isotope Background Isotope Mixing Anomalous Population False Positive High Probability! Conventional use of Pb isotope discrimination Concentration Threshold CONCENTRATION Low Probability The next Geochemical Frontier? Very Subtle Anomaly Background Population ISOTOPES SMEDG 26 June 2013

  15. Outcrop Shallow basement Basement depth <500m Basement depth 500m to 1000m Basement depth > 1000m The Problem is Cover SMEDG 26 June 2013

  16. Pb Isotopes in Exploration Through Cover • Problem – detect and discriminate subtle geochemical signals above buried ore systems • Regolith materials that can be used for geochemistry: • Soils – partial extraction geochemistry • Vegetation • Groundwater • Pb isotopes can be used to discriminate “anomalous” from “background” in each of these media – also detect anthropogenic contamination. SMEDG 26 June 2013

  17. Pb Isotopes in Exploration Through Cover • Partial extraction techniques to determine soil metal concentration are commonly used – but the jury is out on their applicability • Pb isotopes are potentially a valuable discriminator to assess partial extraction anomalies • The technique is based on the ability of isotopes to measure the proportion of end member components with distinctive Pb isotope fingerprints in a mixed system. SMEDG 26 June 2013

  18. Pb Mixing Model Can be applied to any regolith sample – rock, soil, groundwater, vegetation. 0 Background (B) 0.25 Signature 0.5 Geochemical Sample (S) Isotope Ratio 1 0.75 1.0 Sig = 0.3 Target (T) CB CT x x + RT = RS RB + + CT CT CB CB Isotope Ratio 2 CODES MASTERS 2013

  19. Pb In Soils – The Pb Soil Model - 1 • The possible sources of Pb in a regolith sample are: • Crystallization Pb – that is, Pb incorporated in the primary mineral lattice at the time of formation • Radiogenic Pb – Pb that have derived from the decay of U and Th in the period since crystallisation • Regolith Pb – labile Pb that has been transported to the sample through regolith processes. SMEDG 26 June 2013

  20. Sources of Pb in a Regolith Sample A U U B Pb Pb Pb U U Pb Pb U U U U Pb U Pb Pb Pb Pb Pb C U Pb U Pb U Pb Pb Pb Pb SMEDG 26 June 2013

  21. Pb In Soils – The Pb Soil Model - 1 Target Background Ratio Ratio Pb Pb Pb TM BM BF Fixed Mobile R Partial Par (AmAc) R Total Tot (Strong Acid) RPar is the measured isotope ratio of the partial extraction RTot is the measured isotope ratio of the total extraction SMEDG 26 June 2013

  22. Pb In Soils – The Pb Soil Model - 2 • The use of Pb isotopes in soil geochemistry requires a knowledge of the target and background isotope populations • In an initial orientation survey both total and partial extractions are required. • Follow up surveys can be based just on partial extractions SMEDG 26 June 2013

  23. Pb In Soils – The Pb Soil Model - 3 • Soil contains “Fixed” and “Mobile” components. The boundary between these will vary for different soils and depends on the strengths of the acid leaches used to liberate the metal. • In any one sampling exercise where the media are similar across the terrain and the analytical procedures standardised, the “Background” population will incorporate a proportion of Pb fixed in the sample (PbBF) and Pb that has been mobilised by weathering from within the sample or from the surrounding background rocks (PbBM). • It may also contain a component of mobile Pb that has been derived from a Target source buried beneath the cover rocks - or through anthropogenic contamination (PbTM)! SMEDG 26 June 2013

  24. Pb In Soils – The Pb Soil Model - 4 Background Target • From the generalized mixing model we can derive equations to calculate the concentration of each Pb component of the soil sample: • Where PbTot and PbPar are the measured total and partial Pb concentrations, SigTot is the Pb isotope signature of the “total” solution Ratio Ratio Pb Pb Pb PbTM = SigTotx PbTot BF TM BM PbBM = PbPar_PbTM PbBF = PbTot_PbPar Fixed Mobile R Partial (AmAc) Par R Total (Strong Acid) Tot SMEDG 26 June 2013

  25. Anomalies appear to form in soils over covered mineralisation via processes that transport target and indicator elements through the covered sequence to the near surface. Possible mechanisms for this transport include: Geogas carrier Electro-chemical potential Interaction of geogas and soil Interaction of soil and groundwater Residual effects Bioturbation Biological migration Partial Extraction Geochemistry SMEDG 26 June 2013

  26. Thesis: Pb isotopes in soils potentially retain “a memory” of their source – thus we can determine whether the Pb has derived from hidden mineralisation or from a non-mineralisation source. We can extract the most mobile Pb from most samples at very low concentrations and differentiate this potentially transported Pb from Pb that is residual in the soil minerals. Procedure: Undertake case histories at sites where there is known covered mineralisation and where there is no anthropogenic contamination Study a range of deposits from shallow to deep burial. Research Procedure SMEDG 26 June 2013

  27. We have not seen isotopic or trace element anomalies through thick (> 50m) cover. We haveseen clear, very sensitive isotopic anomalies through shallow cover over mineralisation with very subtle or no trace element anomalies. We have seen anthropogenic contamination in a variety of situations where it was not expected and which place in doubt the conclusions of many previous studies. We can recogniseanomalies associated with anthropogenic contamination. We have not seen anomalies that can be ascribed to vapour transport Conclusions of Study SMEDG 26 June 2013

  28. The Pb Soil Model – CASE HISTORY HYC • HYC is a sediment –hosted massive sulfide deposit of approximately 400 Mt in the Proterozoic of the Northern Territory. • The host unit sub-crops beneath alluvial sediments but the ore is deep within the stratigraphy. • Numerous attempts have been made to detect the mineralization in the overlying regolith. SMEDG 26 June 2013

  29. HYC Deposit SMEDG 26 June 2013

  30. HYC Deposit SMEDG 26 June 2013

  31. HYC – Pb Isotope Data 16.0 Analytical Precision HYC Base Metal Deposit Northern Territory 0 Threshold Signature 15.8 Pb 0.5 204 Pb/ 207 Background Population 15.6 All soils 1 HYC Target 15.4 16.0 17.2 18.4 19.6 206 Pb/ 204 Pb SMEDG 26 June 2013

  32. HYC – Test line downslope from a 60 year old drill hole DDH HYC Contamination Line 1.0 0.8 e Surface r u Mid 0.6 t a Depth n g i 0.4 Total S Partial 0.2 0.0 8183410 8183417 8183424 8183431 8183438 8183445 Northing SMEDG 26 June 2013

  33. HYC – Test line downslope from a 60 year old drill hole

  34. HYC – Test line downslope from a 60 year old drill hole PbTM PbBM PbBF

  35. HYC Contamination Line Surface Samples HYC Soil Pb Model ComponentsSurface Samples Pb (ppm) SMEDG 26 June 2013

  36. HYC Contamination Line 30 cm Samples HYC Soil Pb Model Components30 cm Samples Pb (ppm)

  37. HYC – The Lesson Learnt • Pb isotopes are very sensitive to labile, “target” Pb that cannot be discriminated by normal geochemistry • This will apply also where the source is geological – not anthropogenic • Case history studies to determine the effectiveness on novel geochemical techniques anywhere near historic mining or exploration is very very problematic! SMEDG 26 June 2013

  38. Pb isotopes – a Vegetation example SMEDG 26 June 2013

  39. Vegetation 6km from an Archaean VMS Mine SMEDG 26 June 2013

  40. Partial Extraction and Vegetation 16.0 16.0 Analytical Precision Analytical Precision 14 SS SA 14 SS SA Out of District Trucked Ore 13 SS SA 13 SS SA 15.6 15.6 14 SS 14 SS AmAc AmAc (72ppb) (72ppb) 14 Twig 14 Twig (390ppb) (390ppb) b b 13 Twig 13 Twig (175ppb) (175ppb) P P 4 4 13 SS 13 SS AmAc AmAc (86ppb) (86ppb) 0 0 14 14 Phyllode Phyllode 2 2 / / 15.2 15.2 (180ppb) (180ppb) b b 13 S SA 13 S SA Phyllode Phyllode P P 14 S SA 14 S SA 7 7 Twig Twig 0 0 13 13 Phyllode Phyllode 2 2 (150ppb) (150ppb) Surface Soil (S) Surface Soil (S) 14.8 14.8 Sub Sub - - surface Soil (SS) surface Soil (SS) 13 S 13 S AmAc AmAc (920ppb) (920ppb) SA SA Strong Acid Strong Acid 14 S 14 S AmAc AmAc (940ppb) (940ppb) AmAc AmAc Ammonium Acetate Ammonium Acetate 14.4 14.4 13.0 13.0 15.5 15.5 18.0 18.0 20.5 20.5 206 206 204 204 Pb/ Pb/ Pb Pb SMEDG 26 June 2013

  41. Groundwater in vicinity of Archaean VMS deposit CODES MASTERS 2013

  42. Mixing Model sensitivity SMEDG 26 June 2013

  43. Theoretical Anomaly Anomaly caused by addition of Pb to soil ) m from target orebody p p ( b P Anomaly Pb TM Background (Pb /Pb ) = K BM BF Distance

  44. 0.01 BM/BF 1.0 BM/BF Signature Model Sensitivity 1.0 0.8 Partial e r u 0.6 t a n g i 0.4 S 0.2 1.02 0.0 1.3 1.0 1.5 2.0 2.5 3.0 Anomaly/Background SMEDG 26 June 2013

  45. Outcrop Shallow basement Basement depth <500m Basement depth 500m to 1000m Basement depth > 1000m Cover = Homogeneous, background Pb SMEDG 26 June 2013

  46. Background Populations Archaean Weld Range – Western Australia 500 times MC-ICPMS Precision 16.0 Analytical Precision 15.6 Background Soils Pb Anomalous Soils 204 15.2 Pb/ 207 14.8 Achaean Target 14.4 13.0 14.4 15.8 17.2 18.6 20.0 206 Pb/ 204 Pb

  47. Background PopulationsProterozoic Bluebush – NW Qweensland 15.85 Analytical Precision 250 times MC-ICPMS Precision 15.70 Proterozoic Target Pb All soil analyses 204 Pb/ 207 15.55 15.40 16 17 18 19 20 206 Pb/ 204 Pb SMEDG 26 June 2013

  48. 15.72 Background Background Analytical Precision Analytical Precision Elura Target Elura Target 0 Ma 0 Ma 15.68 400 Ma 400 Ma Pb 204 15.64 Pb/ Gulson HNO3/HCl 50850N 207 Gulson HNO3/HCl 50740N This study HNO3/HCl 50850N 15.60 This study HNO3/HCl 50740N This study NaAc 50850N 15.56 18.05 18.22 18.39 18.56 18.73 18.90 Pb/ Pb 206 204 Background Populations Palaeozoic 75 times MC-ICPMS Precision

  49. Analytical Precision • MC-ICPMS 16.935+/-0.006 (+/- 0.035%) • Conv. TIMS 16.929+/-0.023 (+/- 0.14%) • HR-ICPMS 16.806+/-0.044 (+/- 0.26%) • Quad-ICPMS ?????? (but probably ~ 0.5%) SMEDG 26 June 2013

  50. How Much precision do we need? 100% 2.24 Analytical Precision Target (HYC) Percent Target Population MC- ICPMS TIMS 50% Pb 206 What is needed? 2.13 Pb/ 208 0% Soils Showing Mixing Background Population 2.02 0.80 0.86 0.92 0.98 • Precision of 1% would represent: • 2.5% of the total expected range of data for Archaean soils, • 5% of the total expected range for Proterozoic soils, • 17% of the total expected range for Palaeozoic soils. 207 Pb/ 206 Pb SMEDG 26 June 2013

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