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?
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Pb Isotopes in Metallogenic Studies (Plumbotectonics)
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• 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
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?
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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 ()
206 204 Pb/ Pb
20
7
20
4
Pb
/ P
b
8.0 10.0 12.0 14.0 16.0 18.0 20.0 22.0 8.0
10.0
12.0
14.0
16.0
18.0
Salt Creek (Archaean) HYC (Proterozoic) Woodlawn (Palaeozoic)
Primordial Pb
3 2 1 0 4 Ga
34th International Geological Congress, Brisbane, August 2012
206 204Pb/ Pb
20
72
04
Pb
/P
b
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
34th International Geological Congress, Brisbane, August 2012
Pb/ Pb 206 204
20
7
20
4
Pb
/ P
b
16.00 16.11 16.22 16.33 16.44 16.55 15.38
15.42
15.46
15.50
15.54 Analytical Precision
HYC Orebodies DS - 3 Century DS Mount Isa Pb/ Zn DS Cannington Pop 2 DS Cannington Pop 1 DS
1600 Ma
1700 Ma
1500 Ma
1500 Ma
1400 Ma
1300 Ma
WFB Curve WFB Curve
EFB Curve EFB Curve
Cumming Cumming and Richards Curve and Richards Curve
The Mount Isa Growth Curves
Mount Isa Template
206 204 Pb/ Pb
20
7
20
4
Pb
/ P
b
16.0 16.1 16.2 16.3 16.4 16.5
15.38
15.42
15.46
15.50
15.54 Analytical Precision
Urquhart Shale
Syn Sedimentary
Lady Loretta Formation
Syn Sedimentary
Lawn Hill Formation
Syn 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 Tectonics
1400
1450
1500
1550
1600
1650
1700
Urq
uh
art
Sh
ale
Lad
y L
ore
tta
Fm .
-max
Lad
y L
ore
tta
Fm .
- min
D 1
Law
n H
ill
Fm .
D 2
D 3
Urq
uh
art
Sh
ale
Sy
n -
sed
.
Lad
y L
ore
tta
Syn
- s
ed .
Law
n H
ill
Syn
- se
d .
Earl
y Is
an
LL-
Ce
ntu
ry
Isan
W
FB C
u
Late
Is
an
Law
n H
ill V
ein
Age
Ma
Depositional/Tectonic Age
Hydrothermal Age
Western Fold Belt
CODES Masters 2013
“Other” Deposits
206 204 Pb/ Pb
20
7
20
4
Pb
/ P
b
16.10 16.25 16.40 16.55 15.45
15.47
15.49
15.51
15.53
15.55 Analytical Precision
1500 Ma
1600 Ma
15.53
15.55 Analytical Precision
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
/P
b
16.0 17.0 18.0 19.0 20.015.4
15.5
15.6
15.7
15.8
15.9Analytical Precision
Freytags Gossan IR
500 Ma
1000 Ma
1500 Ma
Proterozoic Fields
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
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The Two Dimensions of Pb
ISOTOPES
Conventional use of Pb isotope discrimination
High Probability!
False Positive
Low Probability
Concentration Threshold
Background Population
Anomalous Population
CO
NC
ENTR
ATIO
N
Isotope Target Signature
Isotope Background
Isotope Mixing
The next Geochemical Frontier?
Very Subtle Anomaly
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Outcrop Shallow basement
Basement depth <500m
Basement depth 500m to 1000m
Basement depth > 1000m
The Problem is Cover
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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.
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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.
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Pb Mixing Model
Target (T)
Iso
top
e R
atio
1
Isotope Ratio 2
Background (B)
Geochemical Sample (S)
Sig = 0.3 +
CB RS RT x RB x
CT CB + CT CB +
CT =
CODES MASTERS 2013
Can be applied to any regolith sample – rock, soil, groundwater, vegetation.
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.
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Sources of Pb in a Regolith Sample
U U
U
U
Pb
U U
U
U
Pb
Pb
Pb
Pb Pb
A B
U U
U
U
Pb
Pb
Pb Pb
Pb Pb
C Pb
Pb
Pb
Pb
Pb
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Pb In Soils – The Pb Soil Model - 1
Fixed Mobile
Total (Strong Acid)
Partial (AmAc)
Background Ratio
Target Ratio
Pb BF
Pb BM Pb TM
R Par
R Tot
RPar is the measured isotope ratio of the partial extraction RTot is the measured isotope ratio of the total extraction
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• 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
Pb In Soils – The Pb Soil Model - 2
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• 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)!
Pb In Soils – The Pb Soil Model - 3
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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:
• 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)
Background Ratio
Target Ratio
Pb BF Pb BM Pb TM
R Par
R Tot
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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
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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.
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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
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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.
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HYC Deposit
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HYC Deposit
Line 3400NLine 3400N
Line 7800ELine 7800ELine Line ““WFWF””
MIM LinesMIM Lines
Contamination TestContamination Test
LineLine
McArthur RiverMcArthur River
Outline of Outline of OrebobyOreboby
WozybunWozybun FaultFault
Line 3400NLine 3400N
Line 7800ELine 7800ELine Line ““WFWF””
MIM LinesMIM Lines
Contamination TestContamination Test
LineLine
McArthur RiverMcArthur River
Outline of Outline of OrebobyOreboby
WozybunWozybun FaultFault
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206 204 Pb/ Pb
207
204
Pb
/ P
b
16.0 17.2 18.4 19.6 15.4
15.6
15.8
16.0 Analytical Precision
HYC Target
1
Background Population
HYC Base Metal Deposit Northern Territory
HYC – Pb Isotope Data
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HYC – Test line downslope from a 60 year old drill hole
8183445 8183438 8183431 8183424 8183417 8183410
1.0
0.8
0.6
0.4
0.2
0.0
HYC Contamination Line
Northing
S i g
n a
t u r e
Surface Mid Depth
Total Partial
DD
H
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34th International Geological Congress, Brisbane, August 2012
81834458183436818342881834198183410
40
30
20
10
0
Total Pb Contam Line
Northing
Pb
(p
pm
)
81834458183436818342881834198183410
0.4
0.3
0.2
0.1
0.0
Partial Pb Contam Line
Northing
Pb
(p
pm
)
HYC – Test line downslope from a 60 year old drill hole
34th International Geological Congress, Brisbane, August 2012
HYC – Test line downslope from a 60 year old drill hole
81834458183436818342881834198183410
30
20
10
0
Target Mobile Pb Contam Line
Northing
Pb
(p
pm
)
Surface
Depth
81834458183436818342881834198183410
30
20
10
0
Background Fixed Pb Contam Line
Northing
Pb
(p
pm
)
Surface
Depth
81834458183436818342881834198183410
30
20
10
0
Background Mobile Pb Contam Line
Northing
Pb
(p
pm
)
Surface
Depth
PbTM
PbBM PbBF
0
5
10
15
20
25
30
35
40
1 2 3 4 5 6
Target Mobile
Background Mobile
Background Fixed
Pb
(p
pm
)
HYC Contamination Line Surface Samples
HYC Soil Pb Model Components Surface Samples
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34th International Geological Congress, Brisbane, August 2012
0
5
10
15
20
25
1 2 3 4 5 6
Target Mobile
Background Mobile
Background FixedPb
(p
pm
)
HYC Contamination Line 30 cm Samples
HYC Soil Pb Model Components 30 cm Samples
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!
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Pb ISOTOPES – A VEGETATION EXAMPLE
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Vegetation 6km from an Archaean VMS Mine
206 204Pb/ Pb
207
20
4P
b/
Pb
13.0 15.5 18.0 20.514.4
14.8
15.2
15.6
16.0Analytical Precision
Bark (570ppb)
Core (3ppb)
Intermediate (7ppb)
Whole Twig (170ppb)
Whole Phyllode (380ppb)
Different leaches of soil samples
Archaean Target)
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Surface Soil (S)
Sub - surface Soil (SS)
SA
AmAc
Strong Acid
Ammonium Acetate
Phyllode
Twig
206 204 Pb/ Pb
2 0
7
2 0
4
P b
/ P
b
13.0 15.5 18.0 20.5 14.4
14.8
15.2
15.6
16.0 Analytical Precision
Out of District Trucked Ore
14 S AmAc
13 S AmAc
13 Phyllode
14 Phyllode
14 Twig 13 Twig
14 S SA 13 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
Phyllode
Twig
206 204 Pb/ Pb
2 0
7
2 0
4
P b
/ P
b
13.0 15.5 18.0 20.5 14.4
14.8
15.2
15.6
16.0 Analytical Precision
14 S AmAc
13 S AmAc
13 Phyllode
14 Phyllode
14 Twig 13 Twig
14 S SA 13 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
Groundwater in vicinity of Archaean VMS deposit
206 204Pb/ Pb
207
20
4P
b/
Pb
13.0 15.5 18.0 20.514.4
14.8
15.2
15.6
16.0Analytical Precision
1 & 38 & 6
2
7 (11ppt)
4
9(36ppt Pb, 18ppm Zn)
5 (570ppt Pb, 1ppm Zn)
0.3 - 6ppb Pb5 - 70ppb Zn}
Broken Hill
CODES MASTERS 2013
MIXING MODEL SENSITIVITY
SMEDG 26 June 2013
34th International Geological Congress, Brisbane, August 2012
Theoretical Anomaly
Distance
P b
( p
p m
)
Background (Pb /Pb ) BM BF
= K
Anomaly Pb TM
Anomaly caused by addition of Pb to soil from target orebody
3.0 2.5 2.0 1.5 1.0
1.0
0.8
0.6
0.4
0.2
0.0
Signature Model Sensitivity
Anomaly/Background
S i g
n a t u
r e
0.01 BM/BF
1.0 BM/BF Partial
1.3 1.02
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Outcrop Shallow basement
Basement depth <500m
Basement depth 500m to 1000m
Basement depth > 1000m
Cover = Homogeneous, background Pb
SMEDG 26 June 2013
34th International Geological Congress, Brisbane, August 2012
Weld Range – Western Australia
206 204 Pb/ Pb
207
204
Pb
/ P
b
13.0 14.4 15.8 17.2 18.6 20.0
14.4
14.8
15.2
15.6
16.0 Analytical Precision
Achaean Target
Background Soils
Anomalous Soils
Background Populations Archaean
500 times MC-ICPMS Precision
Bluebush – NW Qweensland
206 204 Pb/ Pb
207
20
4 P
b/
Pb
16 17 18 19 20
15.40
15.55
15.70
15.85 Analytical Precision
All soil analyses
Proterozoic Target
Background Populations Proterozoic
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250 times MC-ICPMS Precision
206 204 Pb/ Pb
20
7
20
4
Pb
/ P
b
18.05 18.22 18.39 18.56 18.73 18.90 15.56
15.60
15.64
15.68
15.72 Analytical Precision
Elura Target Elura Target
Background Background
0 Ma 0 Ma
400 Ma 400 Ma
Analytical Precision
This study HNO3/HCl 50850N
This study NaAc 50850N
Gulson HNO3/HCl 50850N Gulson HNO3/HCl 50740N
This study HNO3/HCl 50740N
Background Populations Palaeozoic
75 times MC-ICPMS Precision
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%)
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207 206 Pb/ Pb
20
8
20
6
Pb
/ P
b
0.80 0.86 0.92 0.98 2.02
2.13
2.24
Background Population
Soils Showing Mixing
Target (HYC)
Analytical Precision
MC- ICPMS
TIMS
What is needed?
100%
50%
0%
How Much precision do we need?
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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.
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