U Exploration Using Pb Isotopes : Opportunities In Partial Extraction Geochemistry Graham Carr (CSIRO) And Neil Rutherford (Rutherford Mineral Resource Consultants)
Introduction Research Undertaken in 1990 collaboration between CSIRO and Neil Rutherford (Surtec( Surtec) Motive: to find a low cost broad scale geochemical exploration technique in the Pine Creek based on stream sediments Unreported except to the participants dramatic downturn in U exploration interest
Contents 1. Why Use Pb Isotopes? Brief theory tutorial 2. Sensitivity of isotope systems in geochemical exploration. 3. Examples from the Pine Creek region of the Northern Territory
Pb Isotopes 101 Q: Why use Pb isotopes as a surrogate for U? A: Because: Pb isotope ratios are directly dependent on the amount of U in a system and increase with time, and more importantly In geocehmical explorations samples Pb isotope ratios discriminate between U sources from normal rocks and those from U mineralisation.
Pb Isotopes 101 Why Pb isotopes vary: 238 U decays to 206 Pb (T 1/2 = 4.5 Ga) 232 Th decays to 208 Pb (T 1/2 = 14 Ga) 204 Pb has no parent (Invariate( Invariate) Therefore: If U levels are high, and Th levels are normal, High U/Th as in U Ore then 206 Pb/ 204 208 Pb/ 204 204 Pb will be high, but 204 Pb will be normal
Pb Isotopes 101 Key Messages: Pb isotope ratios are sensitive to the U/Th ratios of the source rocks Simply looking for elevated uranogenic Pb ( 206 Pb/ 204 Pb ratios) is not a viable exploration technique
Conventional Pb Isotope Plot Pine Creek Rocks (Gulson 1979) 200 Analytical Precision (0.05%) Derived from 232 Th Decay 208 204 150 100 50 Two Trends 0 0 300 600 900 1200 1500 206 204 Derived from 238 U Decay
Presentation of Results Derived from 232 Th Decay 208 204 200 Analytical Precision s m = 16.8 m - 1 = 0.06 150 100 50 i m = 0.61 m - 1 = 1.63 m = m = 0.027 m -1 = 38 208 Pb/ 204 206 Pb/ 204 204 Pb(s) 208 204 Pb(s) 206 208 Pb/ 204 206 Pb/ 204 m = 0.0002 m - 1 = 582 0 0 300 600 900 1200 1500 204 Pb(i) 204 Pb(i) Calculated U/Th of source U/Th = 1/3.35*m 206 204 Derived from 238 U Decay
All Pine Creek Data (Gulson, 1979) Background U/Th Derived from 232 Th Decay 208 204 120 100 80 60 40 Analytical Precision Zamu Dolerite Av. Granite Kombolgie Ss Av. Sandstone Threshold (m - 1 = 2) 20 15 25 35 45 55 65 75 206 204 Derived from 238 U Decay Anomalous U/Th
All Pine Creek Data (Gulson, 1979) Background U/Th 120 Analytical Precision Derived from 232 Th Decay 208 204 100 80 60 40 m -1 Range <0.2 1.6 m -1 Range 2 >1000 20 15 25 35 45 55 65 75 206 204 Derived from 238 U Decay Anomalous U/Th
Isotopic Fingerprinting Background Signature Ore Signature N 10 20 Concentration (ppb) Background Population 30 Isotopic Fingerprint Mixing 40 Samples containing a component of ore derived-pb Sensitivity of unmixing background and ore signatures in geochemical samples (particularly partial extractions and biota) is dependent on the isotopic contrast between background and ore
Partial Extraction- Unmixing Ore derived Pb and background Pb m -1 10 Ore Ore m -1 = 1000 1000 Ore m - 1 = 500 Threshold Ore m - 1 = 50 1 0 0.5 1 1.5 2 % Pb Derived From U Ore (Assumes background rocks have m - 1 = 1.5)
Partial Extraction- Unmixing Ore derived Pb and background Pb For U mineralisation isotopic contrast between background and ore is very high and we can in theory distinguish a sample leach where only 0.1% of the Pb derives from ore.
Results 1. Soil and vegetation survey over outcropping ore at Koongarra 2. Partial extractions of stream sediments at Caramel mineralisation 3. Partial extractions of streams sediments recent (secondary) U accumulation 4. Options for analysis
Aqua Regia Leach Soils over Outcropping Koongarra Deposit 50 Analytical Precision 45 208 204 40 35 30 Koongarra Traverse 1 Koongarra Traverse 2 Koongarra Traverse 2 - Vegetation 25 0 300 600 900 206 204
Caramel Caramel
Caramel
Caramel
Caramel Stream Sediments 160 120 Background U/Th Analytical Precision HNO 3 Extraction 208 204 80 40 HCl Extraction 0 0 700 1400 206 204 Anomalous U/Th
Caramel Stream Sediments ppm 300 250 200 150 100 50 0 1000 100 U Th Pb Zn Aqua Regia 2N HCl 0.5N HCl AmAc M -1 10 1 Threshold 0.1-2000 -1500-1000 -500 0 Metres Downstream
Nabarlek Bulk Stream Sed Samples downstream from 800 ppm recent U anomaly 54 Analytical Precision 208 204 48 42 Threshold m - 1 = 2 36 16 20 24 28 206 204
Nabarlek Bulk Stream Sed Samples downstream from 800 ppm recent U anomaly m -1 3 2.5 2 1.5 1 0.5 0 Aqua Regia HCl HNO3 Residue 0 200 400 600 800 1000 Metres Threshold
TIMS v ICPMS Comparison m -1 (ICPMS) 6 5 4 3 2 1 0-1 -2-3 0 0.5 1 1.5 2 2.5 m 1 (TIMS)
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