Lead Isotope Geochemistry A Brave New World Graham

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 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

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

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

Pb Isotope Variables • Basic Equation: 206 Pb/204 Pb =206 Pb/204 Pb + 238 U /204 Pb(e t i Source Rock Chemistry - 1) Time

The Growth Curve Concept • Growth Curve: Co-variation through geological time of a pair of Pb isotope ratios assuming a common, U/Pb ( ) 18. 0 14. 0 12. 0 Simple (unrealistic) Single Stage Model 207 Pb/ 204 Pb 2 3 4 Ga 16. 0 Primordial Pb 10. 0 8. 0 Salt Creek (Archaean) HYC (Proterozoic) Woodlawn (Palaeozoic) 10. 0 12. 0 14. 0 206 16. 0 Pb/ 204 Pb 18. 0 20. 0 22. 0

The Growth Curve Concept More realistic Two-Stage Model 34 th International Geological Congress, Brisbane, August 2012

The Mount Isa Growth Curves 15. 54 Analytical Precision 1500 Ma WFB Curve 1700 Ma 1600 Ma EFB Curve 1300 Ma 15. 46 207 204 Pb/ Pb 15. 50 1400 Ma Cumming HYC Orebodies DS - 3 Century DS Mount Isa Pb/ Zn DS Cannington Pop 2 DS Cannington Pop 1 DS 15. 42 1500 Ma 15. 38 16. 00 16. 11 and Richards Curve 16. 22 206 16. 33 Pb/ 204 16. 44 16. 55 Pb 34 th International Geological Congress, Brisbane, August 2012

Isan Orogeny Lawn Hill Vein 17 00 M a Analytical Precision 15. 50 Isan Orogeny Century - Lady Loretta Lawn Hill Formation Syn Sedimentary 15 00 Ma 15. 54 16 00 Ma Mount Isa Template Lady Loretta Formation Pb Syn Sedimentary 204 Urquhart Shale Syn Sedimentary 15. 46 Pb/ HYC Syn Sed. Event Mount Isa Orogeny Events (EFB) 207 Soldiers Cap – Dugald River Event Mount Isa Orogeny Cu – Event (WFB) 15. 42 Soldiers Cap – Cannington Event 15. 38 16. 0 CODES Masters 2013 16. 1 16. 2 16. 3 206 Pb/ 204 Pb 16. 4 16. 5

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

“Other” Deposits 15. 55 Analytical Precision 207 Pb/ 204 Pb 15. 53 1500 Ma Grevillea 15. 51 15. 49 1600 Ma 15. 47 Walford Creek Kamarga 15. 45 16. 10 16. 25 16. 40 206 16. 55 Pb/ 204 Pb Different fluids and different source rocks, or…. . a small subset of the same source rocks.

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

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

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

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

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

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

Pb Mixing Model Can be applied to any regolith sample – rock, soil, groundwater, vegetation Isotope Ratio 1 0 n g i S 0 1. 0 e r u at Background (B) 5 2. 0 0. 5 Geochemical Sample (S) . 75 Sig = 0. 3 Target (T) RS = RT x Isotope Ratio 2 CODES MASTERS 2013 CT CT + CB + RB x CB CT + CB

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

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

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

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

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 (Pb. BF) and Pb that has been mobilised by weathering from within the sample or from the surrounding background rocks (Pb. BM). • 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 (Pb. TM)! SMEDG 26 June 2013

Pb In Soils – The Pb Soil Model - 4 • From the generalized mixing model we can derive equations to calculate the concentration of each Pb component of the soil sample: Pb. TM = Sig. Tot x Pb. Tot Pb. BM = Pb. Par _ Pb. TM Pb. BF = Pb. Tot _ Pb. Par Background Ratio Pb. BF Fixed Target Ratio Pb. BM Pb. TM Mobile Partial Total (Am. Ac) (Strong Acid) RPar R Tot • Where Pb. Tot and Pb. Par are the measured total and partial Pb concentrations, Sig. Tot is the Pb isotope signature of the “total” solution SMEDG 26 June 2013

Partial Extraction Geochemistry • 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 SMEDG 26 June 2013

Research Procedure • 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. SMEDG 26 June 2013

Conclusions of Study 1. We have not seen isotopic or trace element anomalies through thick (> 50 m) cover. 2. We have seen clear, very sensitive isotopic anomalies through shallow cover mineralisation with very subtle or no trace element anomalies. 3. 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. 4. We can recognise anomalies associated with anthropogenic contamination. 5. We have not seen anomalies that can be ascribed to vapour transport SMEDG 26 June 2013

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

HYC Deposit SMEDG 26 June 2013

HYC Deposit SMEDG 26 June 2013

HYC – Pb Isotope Data 16. 0 Analytical Precision HYC Base Metal Deposit Northern Territory atu Sign Pb/204 Pb 15. 8 hold s Thre re 0 207 0. 5 15. 6 1 15. 4 16. 0 Background Population ils All so HYC Target 17. 2 18. 4 206 Pb/204 Pb SMEDG 26 June 2013 19. 6

1. 0 HYC Contamination Line DDH HYC – Test line downslope from a 60 year old drill hole S ig nature 0. 8 Surface Mid Depth 0. 6 0. 4 Total Partial 0. 2 0. 0 8183417 8183424 8183431 Northing SMEDG 26 June 2013 8183438 8183445

HYC – Test line downslope from a 60 year old drill hole 34 th International Geological Congress, Brisbane, August 2012

HYC – Test line downslope from a 60 year old drill hole Pb. TM Pb. BF 34 th International Geological Congress, Brisbane, August 2012

HYC Contamination Line Surface Samples HYC Soil Pb Model Components Surface Samples 40 35 Pb (ppm) 30 25 Target Mobile 20 Background Mobile 15 Background Fixed 10 5 0 1 2 3 4 5 SMEDG 26 June 2013 6

HYC Contamination Line 30 cm Samples HYC Soil Pb Model Components 30 cm Samples 25 Pb (ppm) 20 15 Target Mobile Background Mobile 10 Background Fixed 5 0 1 2 3 4 5 6 34 th International Geological Congress, Brisbane, August 2012

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 problematic! SMEDG 26 June 2013

Pb ISOTOPES – A VEGETATION EXAMPLE SMEDG 26 June 2013

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

Partial Extraction and Vegetation 16. 0 Analytical Precision 14 SS SA Out of District Trucked Ore 14 SS Am. Ac 14 Twig (390 ppb) 13 Twig (175 ppb) 13 SS Am. Ac 14 Phyllode (180 ppb) 13 S SA 14 S SA 13 Phyllode 15. 2 207 P b/204 Pb 15. 6 13 SS SA (72 ppb) (86 ppb) Phyllode Twig (150 ppb) Surface Soil (S) 14. 8 Sub -surface Soil (SS) 13 S Am. Ac (920 ppb) 14 S Am. Ac (940 ppb) 14. 4 13. 0 SA Am. Ac Ammonium Acetate 15. 5 18. 0 206 SMEDG 26 June 2013 Strong Acid Pb/ 204 Pb 20. 5

Groundwater in vicinity of Archaean VMS deposit CODES MASTERS 2013

MIXING MODEL SENSITIVITY SMEDG 26 June 2013

Pb (ppm) Theoretical Anomaly caused by addition of Pb to soil from target orebody Pb. TM Anomaly Background (Pb. BM/Pb. BF ) = K Distance 34 th International Geological Congress, Brisbane, August 2012

Signature Model Sensitivity 0. 01 BM/BF 1. 0 Signature 0. 8 Partial 1. 0 BM/BF 0. 6 0. 4 0. 2 0. 0 1. 02 1. 3 1. 5 2. 0 Anomaly/Background SMEDG 26 June 2013 2. 5 3. 0

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

Background Populations Archaean Weld Range – Western Australia 16. 0 Analytical Precision 500 times MC-ICPMS Precision 15. 6 Pb Background Soils 204 Anomalous Soils 207 Pb/ 15. 2 14. 8 Achaean Target 14. 4 13. 0 14. 4 15. 8 17. 2 206 Pb/ 204 18. 6 20. 0 Pb 34 th International Geological Congress, Brisbane, August 2012

Background Populations Proterozoic Bluebush – NW Qweensland 15. 85 Analytical Precision 250 times MC-ICPMS Precision Pb 15. 70 All soil analyses 207 Pb/ 204 Proterozoic Target 15. 55 15. 40 16 17 18 206 Pb/ 19 204 SMEDG 26 June 2013 Pb 20

Background Populations Palaeozoic 15. 72 Background Analytical Precision Elura Target 0 Ma 400 Ma 15. 64 Gulson HNO 3/HCl 50850 N Gulson HNO 3/HCl 50740 N This study HNO 3/HCl 50850 N This study HNO 3/HCl 50740 N This study Na. Ac 50850 N 207 Pb/ 204 Pb 15. 68 75 times MC-ICPMS Precision 15. 60 15. 56 18. 05 18. 22 18. 39 18. 56 206 Pb/ 204 Pb 18. 73 18. 90

Analytical Precision • • MC-ICPMS Conv. TIMS HR-ICPMS Quad-ICPMS 16. 935+/-0. 006 (+/- 0. 035%) 16. 929+/-0. 023 (+/- 0. 14%) 16. 806+/-0. 044 (+/- 0. 26%) ? ? ? (but probably ~ 0. 5%) SMEDG 26 June 2013

How Much precision do we need? 2. 24 Analytical Precision What is needed? Pb/ 206 Pb TIMS Target (HYC) rc Pe 2. 13 208 MCICPMS t la pu Po t e arg T t 50% en 100% ion 0% 2. 02 0. 80 Soils Showing Mixing Background Population 0. 86 0. 92 207 Precision of 1% would represent: 0. 98 Pb/ 206 Pb • 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. SMEDG 26 June 2013

Pb Isotopes – What we need to do • To develop a robust exploration technology we need to: – Reduce cost of analyses – very large datasets with lower precision rather than small datasets with high precision , <$50 per sample – Undertake case histories to validate the technique in greenfields terrains – minimal to no drilling – no mining. SMEDG 26 June 2013