lead isotope geochemistry: a brave new world?

Lead Isotope Geochemistry:A Brave New World?

Graham R CarrCSIRO Exploration and MiningJune 2013

SMEDG 26 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 IsotopesThesis:• 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?

Pb Isotope Variables

• Basic Equation:

206Pb/204Pb =206Pb/204Pbi + 238U /204Pb(et - 1)Source Rock

Chemistry

Time

The Growth Curve Concept

Simple (unrealistic) Single Stage Model

•Growth Curve: Co-variation through geological time of a pair of Pb isotope ratios assuming a common, U/Pb ()

206 204Pb/ Pb

207

204

Pb/

Pb

8.0 10.0 12.0 14.0 16.0 18.0 20.0 22.08.0

10.0

12.0

14.0

16.0

18.0

Salt Creek (Archaean)HYC (Proterozoic)Woodlawn (Palaeozoic)

Primordial Pb

3 2 1 04 Ga

34th International Geological Congress, Brisbane, August 2012

206 204Pb/ Pb

207

204

Pb/

Pb

8.0 10.0 12.0 14.0 16.0 18.0 20.0 22.08.0

10.0

12.0

14.0

16.0

18.0

Salt Creek (Archaean)HYC (Proterozoic)Woodlawn (Palaeozoic)

Primordial Pb

Stage 1

Stage 23.7 Ga

3 Ga 2 1 0

More realistic Two-Stage Model

The Growth Curve Concept


Pb/ Pb206 204

207

204

Pb/

Pb

16.00 16.11 16.22 16.33 16.44 16.5515.38

15.42

15.46

15.50

15.54Analytical Precision

HYC Orebodies DS - 3Century DSMount Isa Pb/ Zn DSCannington Pop 2 DSCannington Pop 1 DS

1600 Ma1700 Ma

1500 Ma

1500 Ma

1400 Ma

1300 Ma

WFB CurveWFB Curve

EFB CurveEFB Curve

CummingCumming and Richards Curve and Richards Curve

The Mount Isa Growth Curves

Mount Isa Template

1700

Ma

1500

Ma

1600

Ma

206 204Pb/ Pb

207

204

Pb/

Pb

16.0 16.1 16.2 16.3 16.4 16.515.38

15.42

15.46

15.50


Urquhart ShaleSyn Sedimentary

Lady Loretta FormationSyn Sedimentary

Lawn Hill FormationSyn Sedimentary

Isan Orogeny Century - Lady Loretta

Isan Orogeny Lawn Hill Vein

Soldiers Cap – Cannington Event

Soldiers Cap – Dugald River Event

Mount Isa Orogeny Events (EFB)Mount Isa Orogeny Cu – Event (WFB)

HYC Syn Sed. Event

CODES Masters 2013

Relating Ores to Tectonics1400

1450

1500

1550

1600

1650

1700

Urq

uhar

t S

hale

Lady

Lore

tta

Fm.-m

ax

Lady

Lore

tta F

m.-

min D

1

Law

n Hi

ll F

m.

D2

D3

Urq

uhar

t S

hale

Syn

-sed

.

Lady

Lore

tta S

yn-s

ed.

Law

n Hi

ll S

yn-s

ed.

Early

Isan

LL-C

entu

ry

Isan

WFB

Cu

Late

Isan

Law

n Hi

ll Ve

in

Age

Ma

Depositional/TectonicAge

Hydrothermal Age

Western Fold Belt

CODES Masters 2013

“Other” Deposits

206 204Pb/ Pb

207

204

Pb/

Pb

16.10 16.25 16.40 16.5515.45

15.47

15.49

15.51

15.53

15.55 Analytical Precision

1500 Ma

1600 Ma

15.53


1500 Ma

1600 Ma

Grevillea

Walford Creek

Kamarga

Different fluids and different source rocks, or…..a small subset of the same source rocks.

• 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

Freytags Gossan IR

206 204Pb/ Pb

207

204

Pb/

Pb

16.0 17.0 18.0 19.0 20.015.4

15.5

15.6

15.7

15.8


Freytags Gossan IR

500 Ma

1000 Ma

1500 Ma

Proterozoic Fields

Is it worth drilling??

What is its origin?

CODES Masters 2013

SMEDG 26 June 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

The Two Dimensions of Pb

ISOTOPES

Conventional use of Pb isotope discrimination

High Probability!

False Positive

LowProbability

Concentration Threshold

BackgroundPopulation

Anomalous Population

CON

CEN

TRAT

ION

Isotope TargetSignature

IsotopeBackground

Isotope Mixing

The nextGeochemicalFrontier?Very Subtle Anomaly

SMEDG 26 June 2013

SMEDG 26 June 2013

Outcrop Shallow basement

Basement depth <500m

Basement depth 500m to 1000m

Basement depth > 1000m

The Problem is Cover

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.

CODES MASTERS 2013

Pb Mixing Model

Target (T)

Isot

ope

Ratio

1

Isotope Ratio 2

Background (B)

GeochemicalSample (S)

Signature

0.75

0.25

0.5

1.0

0

Sig = 0.3+

CBRS RT x RB x CT CB +CT CB +

CT=

Can be applied to any regolith sample – rock, soil, groundwater, vegetation.

SMEDG 26 June 2013

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

UU

U

U

Pb

UU

U

U

Pb

Pb

Pb

Pb Pb

A B

UU

U

U

Pb

Pb

Pb Pb

Pb Pb

C Pb

Pb

Pb

Pb

Pb

SMEDG 26 June 2013


Fixed Mobile

Total(Strong Acid)

Partial(AmAc)

BackgroundRatio

TargetRatio

Pb BF Pb BM PbTM

R Par

R Tot

RPar is the measured isotope ratio of the partial extractionRTot is the measured isotope ratio of the total extraction

SMEDG 26 June 2013

• 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

• 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


• 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

PbTM = SigTot x PbTot

PbBM = PbPar _ PbTM

PbBF = PbTot _ PbPar

Fixed Mobile

Total(Strong Acid)

Partial(AmAc)

BackgroundRatio

TargetRatio

PbBF PbBM PbTM

RParRTot

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 (> 50m) cover.

2. We have seen clear, very sensitive isotopic anomalies through shallow cover over 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

Line 3400NLine 3400N

Line 7800ELine 7800ELine Line ““WFWF””

MIM LinesMIM Lines

Contamination TestContamination TestLineLine

McArthur RiverMcArthur River

Outline of Outline of OrebobyOreboby

WozybunWozybun FaultFault

Line 3400NLine 3400N

Line 7800ELine 7800ELine Line ““WFWF””

MIM LinesMIM Lines

Contamination TestContamination TestLineLine

McArthur RiverMcArthur River

Outline of Outline of OrebobyOreboby

WozybunWozybun FaultFault

SMEDG 26 June 2013

206 204Pb/ Pb

207

204

Pb/

Pb

16.0 17.2 18.4 19.615.4

15.6

15.8


HYC Target

Signature

1

0.5

0

BackgroundPopulation

HYC Base Metal DepositNorthern Territory

All soils

Threshold

HYC – Pb Isotope Data

SMEDG 26 June 2013

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

818344581834388183431818342481834178183410

1.0

0.8

0.6

0.4

0.2

0.0

HYC Contamination Line

Northing

Sign

atu r

e SurfaceMidDepth

TotalPartial

DDH


81834458183436818342881834198183410

40

30

20

10

0

Total Pb Contam Line

Northing

Pb

(ppm

)

81834458183436818342881834198183410

0.4

0.3

0.2

0.1

0.0

Partial Pb Contam Line

Northing

Pb (p

pm)




81834458183436818342881834198183410

30

20

10

0

Target Mobile Pb Contam Line

Northing

Pb (p

pm)

SurfaceDepth

81834458183436818342881834198183410

30

20

10

0

Background Fixed Pb Contam Line

Northing

Pb (p

pm)

SurfaceDepth

81834458183436818342881834198183410

30

20

10

0

Background Mobile Pb Contam Line

Northing

Pb (p

pm)

SurfaceD epth

PbTM

PbBM PbBF

SMEDG 26 June 2013

1 2 3 4 5 60

5

10

15

20

25

30

35

40

Target MobileBackground MobileBackground FixedPb

(ppm

)

HYC Contamination LineSurface SamplesHYC Soil Pb Model Components

Surface Samples


1 2 3 4 5 60

5

10

15

20

25

Target MobileBackground MobileBackground FixedPb

(ppm

)

HYC Contamination Line30 cm SamplesHYC Soil Pb Model Components30 cm Samples

SMEDG 26 June 2013

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

Pb ISOTOPES – A VEGETATION EXAMPLE

SMEDG 26 June 2013

Vegetation 6km from an Archaean VMS Mine

206 204Pb/ Pb

207

20Pb

/Pb

13.0 15.5 18.0 20.514.4

14.8

15.2

15.6


Bark (570ppb)

Core (3ppb)

Intermediate (7ppb)

Whole Twig (170ppb)

Whole Phyllode (380ppb)

Different leaches of soil samples

Archaean Target)

SMEDG 26 June 2013

Surface Soil (S)

Sub - surface Soil (SS)

SA

AmAc

Strong Acid

Ammonium Acetate

PhyllodeTwig

206 204Pb/ Pb

207

204

Pb/

P b

13.0 15.5 18.0 20.514.4

14.8

15.2

15.6


Out of DistrictTrucked Ore

14 S AmAc13 S AmAc

13 Phyllode

14 Phyllode

14 Twig13 Twig

14 S SA13 S SA

13 SS SA

14 SS SA

13 SS AmAc

14 SS AmAc

(940ppb)

(920ppb)

(150ppb)

(86ppb)(180ppb)

(390ppb)(175ppb)

(72ppb)

Surface Soil (S)

Sub - surface Soil (SS)

SA

AmAc

Strong Acid

Ammonium Acetate

PhyllodeTwig

206 204Pb/ Pb

207

204

Pb/

P b

13.0 15.5 18.0 20.514.4

14.8

15.2

15.6


14 S AmAc13 S AmAc

13 Phyllode

14 Phyllode

14 Twig13 Twig

14 S SA13 S SA

13 SS SA

14 SS SA

13 SS AmAc

14 SS AmAc

(940ppb)

(920ppb)

(150ppb)

(86ppb)(180ppb)

(390ppb)(175ppb)

(72ppb)

Partial Extraction and Vegetation

CODES MASTERS 2013

Groundwater in vicinity of Archaean VMS deposit

206 204Pb/ Pb

207

204

Pb/

Pb

13.0 15.5 18.0 20.514.4

14.8

15.2

15.6

16.0 A na lytical Precis io n

1 & 38 & 6

2

7 (11ppt)

4

9 (36ppt Pb, 18ppm Zn)

5 (570ppt Pb, 1ppm Zn)

0.3 - 6ppb Pb5 - 70ppb Zn}

Broken H ill

SMEDG 26 June 2013

MIXING MODEL SENSITIVITY


Theoretical Anomaly

Distance

Pb (p

pm)

Background (Pb /Pb )BM BF = K

AnomalyPbTM

Anomaly caused byaddition of Pb to soilfrom target orebody

SMEDG 26 June 2013

3.02.52.01.51.0

1.0

0.8

0.6

0.4

0.2

0.0

Signature Model Sensitivity

Anomaly/Background

Sign

a tur

e

0.01 BM/BF

1.0 BM/BFPartial

1.31.02

SMEDG 26 June 2013

Outcrop Shallow basement

Basement depth <500m

Basement depth 500m to 1000m

Basement depth > 1000m

Cover = Homogeneous, background Pb


Weld Range – Western Australia

206 204Pb/ Pb

207

204

Pb/

Pb

13.0 14.4 15.8 17.2 18.6 20.0

14.4

14.8

15.2

15.6


Achaean Target

Background Soils

Anomalous Soils

Background PopulationsArchaean

500 times MC-ICPMS Precision

SMEDG 26 June 2013

Bluebush – NW Qweensland

206 204Pb/ Pb

207

204

Pb/

Pb

16 17 18 19 20

15.40

15.55

15.70


All soil analyses

Proterozoic Target

Background PopulationsProterozoic


206 204Pb/ Pb

207

204

Pb/

Pb

18.05 18.22 18.39 18.56 18.73 18.9015.56

15.60

15.64

15.68


Elura TargetElura Target

BackgroundBackground

0 Ma0 Ma

400 Ma400 Ma

Analytical Precision

This study HNO3/HCl 50850N

This study NaAc 50850N

Gulson HNO3/HCl 50850NGulson HNO3/HCl 50740N

This study HNO3/HCl 50740N

Background PopulationsPalaeozoic


SMEDG 26 June 2013


• 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

207 206Pb/ Pb

208

206

Pb/

Pb

0.80 0.86 0.92 0.982.02

2.13

2.24

Background Population

Soils Showing Mixing

Target (HYC)


MC-ICPMS

TIMS

What is needed?

100%

50%

0%

Percent Target Population

How Much precision do we need?

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.

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.

lead isotope geochemistry: a brave new world?

Documents

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