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:
206Pb/204Pb
=206Pb/204Pbi + 238U /204Pb(et - 1)
Source Rock
Chemistry
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
2
3
4 Ga
1
0
14.0
12.0
Simple (unrealistic) Single Stage Model
207
Pb/ 204 Pb
16.0
Primordial Pb
10.0
8.0
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
18.0
3 Ga
2
1
0
14.0
Stage 2
3.7 Ga
12.0
Stage 1
207
Pb/204 Pb
16.0
10.0
8.0
8.0
More realistic Two-Stage Model
Primordial Pb
10.0
Salt Creek (Archaean)
HYC (Proterozoic)
Woodlawn (Palaeozoic)
12.0
14.0
206
16.0
18.0
20.0
22.0
Pb/204Pb
34th International Geological Congress, Brisbane, August 2012
The Mount Isa Growth Curves
15.54
WFB Curve
Analytical Precision
1500 Ma
1700 Ma
1600 Ma
EFB Curve
1300 Ma
204
Pb/ Pb
15.50
207
15.46
1400 Ma
Cumming
15.42
HYC Orebodies DS - 3
Century DS
Mount Isa Pb/ Zn DS
Cannington Pop 2 DS
Cannington Pop 1 DS
1500 Ma
15.38
16.00
16.11
and Richards Curve
16.22
206
16.33
Pb/
204
16.44
16.55
Pb
34th International Geological Congress, Brisbane, August 2012
Mount Isa Template
15.54
Isan Orogeny Lawn Hill Vein
Analytical Precision
Isan Orogeny
Century - Lady Loretta
Lawn Hill Formation
Syn Sedimentary
15.50
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
16.1
16.2
16.3
206
CODES Masters 2013
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 .
D3
D2
Lawn Hill Fm .
1400
D1
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
Walford Creek
15.47
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
Freytags Gossan IR
15.9
Analytical Precision
15.8
207
Pb/204Pb
500 Ma
15.7
1000 Ma
Is it worth drilling??
15.6
15.5
1500 Ma
What is its origin?
Proterozoic Fields
15.4
16.0
CODES Masters 2013
Freytags Gossan IR
17.0
18.0
206
Pb/204 Pb
19.0
20.0
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
Isotope
Mixing
Isotope
Target
Signature
CONCENTRATION
Anomalous
Population
False
Positive
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 <500m
Basement depth 500m to 1000m
Basement depth > 1000m
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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
Background
(B)
Geochemical
Sample (S)
Sig = 0.3
Target (T)
RS = RT x
Isotope Ratio 2
CODES MASTERS 2013
CT
C T + CB
+ RB x
CB
C T + 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
B
Pb
Pb
U
U
U
Pb
U
Pb
Pb
Pb
Pb
U
Pb
U
Pb
U
Pb
Pb
Pb
SMEDG 26 June 2013
Pb
Pb
Pb
U
C
U
Pb
U
Pb
U
Pb In Soils – The Pb Soil Model - 1
Target
Ratio
Background
Ratio
Pb BF
Pb BM
Fixed
Pb TM
Mobile
Partial
(AmAc)
Total
R Par
R 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
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 (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
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:
PbTM = SigTot x PbTot
PbBM = PbPar _ PbTM
PbBF = PbTot
_
PbPar
Background
Ratio
Target
Ratio
PbBF
PbBM PbTM
Fixed
Mobile
Partial
Total
(AmAc)
(Strong Acid)
RPar
R Tot
• Where PbTot and PbPar are the measured total and partial Pb
concentrations, SigTot 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 (> 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
Contamination Test
Line
Line 3400N
Outline of Oreboby
Wozybun Fault
MIM Lines
McArthur River
Line “WF”
SMEDG 26 June 2013
Line 7800E
HYC – Pb Isotope Data
16.0
Analytical Precision
HYC Base Metal Deposit
Northern Territory
207
Pb/204 Pb
15.8
15.6
Background
Population
1
HYC Target
15.4
16.0
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
8183410
8183417
8183424
8183431
Northing
SMEDG 26 June 2013
8183438
8183445
HYC – Test line downslope from a 60
year old drill hole
Partial Pb Contam Line
40
0.4
30
0.3
Pb (ppm)
Pb (ppm)
Total Pb Contam Line
20
0.1
10
0
8183410
0.2
8183419
8183428
Northing
8183436
8183445
0.0
8183410
8183419
8183428
8183436
8183445
Northing
34th International Geological Congress, Brisbane, August 2012
HYC – Test line downslope from a 60
year old drill hole
Target Mobile Pb Contam Line
Pb (ppm)
30
PbTM
20
Surface
Depth
10
0
8183410
8183419
8183428
8183436
8183445
Northing
Background Fixed Pb Contam Line
Background Mobile Pb Contam Line
Surface
Depth
PbBM
20
10
0
8183410
30
Pb (ppm)
Pb (ppm)
30
8183419
8183428
Northing
8183436
8183445
Surface
Depth
PbBF
20
10
0
8183410
8183419
8183428
8183436
8183445
Northing
34th 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
Background Mobile
Background Fixed
20
15
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
Background Fixed
10
5
0
1
2
3
4
5
6
34th 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 very problematic!
SMEDG 26 June 2013
Pb ISOTOPES – A VEGETATION
EXAMPLE
SMEDG 26 June 2013
Vegetation 6km from an Archaean
VMS Mine
16.0
Different leaches
of soil samples
Analytical Precision
Core (3ppb)
Intermediate (7ppb)
15.2
207
Pb/204Pb
15.6
14.8
Whole Twig (170ppb)
Bark (570ppb)
Whole Phyllode (380ppb)
Archaean Target)
14.4
13.0
15.5
18.0
206
Pb/ 204 Pb
SMEDG 26 June 2013
20.5
Partial Extraction and Vegetation
16.0
Analytical Precision
14 SS SA
Out of District
Trucked Ore
14 SS AmAc
(72ppb)
14 Twig (390ppb)
13 Twig (175ppb)
13 SS AmAc
14 Phyllode
(180ppb)
13 S SA
14 S SA
13 Phyllode
15.2
207
P b/204 Pb
15.6
13 SS SA
(86ppb)
Phyllode
Twig
(150ppb)
Surface Soil (S)
14.8
Sub -surface Soil (SS)
13 S AmAc (920ppb)
14 S AmAc (940ppb)
14.4
13.0
SA
AmAc 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
16.0
Analytical Precision
7 (11ppt)
8&6
1&3
4
Broken Hill
2
}
0.3 - 6ppb Pb
5 - 70ppb Zn
15.2
207
Pb/204Pb
15.6
5 (570ppt Pb, 1ppm Zn)
14.8
9 (36ppt Pb, 18ppm Zn)
14.4
13.0
15.5
18.0
206
Pb/204Pb
CODES MASTERS 2013
20.5
MIXING MODEL SENSITIVITY
SMEDG 26 June 2013
Pb (ppm)
Theoretical Anomaly
Anomaly caused by
addition of Pb to soil
from target orebody
PbTM
Anomaly
Background (PbBM/PbBF ) = K
Distance
34th 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
1.0
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 <500m
Basement depth 500m to 1000m
Basement depth > 1000m
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
34th 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
75 times
MC-ICPMS Precision
Background
Analytical Precision
Elura Target
0 Ma
400 Ma
15.64
Gulson HNO3/HCl 50850N
Gulson HNO3/HCl 50740N
This study HNO3/HCl 50850N
This study HNO3/HCl 50740N
This study NaAc 50850N
207
Pb/ 204 Pb
15.68
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?
Target
(HYC)
What is
needed?
Pb/ 206Pb
TIMS
50%
2.13
208
MCICPMS
100%
2.24
Analytical Precision
0%
Soils Showing Mixing
Background Population
2.02
0.80
0.86
0.92
207
0.98
Pb/ 206 Pb
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.
SMEDG 26 June 2013