Workshop Mendoza, Argentina April 11-15, 2016

Transcription

1 Workshop Mendoza, Argentina April 11-15, 2016

2 Geochronology of Uranium Deposits Why is geochronology of uranium deposits important? Source Transport Mechanism Geological Environment or Depositional Mechanism Timing

3 Cumulative discoveries (1000 t U) Ore Deposit Models Purpose: Develop a set of criteria that geologists can use to identify suitable conditions for finding ore deposits Research: Develop tools or techniques that can be used to vector in on a deposit type Cuney and Kyser, s 1980 Present Prospector driven exploration Key Lake Model driven exploration Cigar Lake McArthur River New discovery?? Research/technology driven exploration Cumulative Exploration Expenditures ($Billions, USD)

4 Geochronology Determine the ages of minerals Geochronometers: U- Pb, Rb-Sr etc. U-Pb Method U-series Method Isotopic Methods Thermal ionization Mass Spectrometry (TIMS) Inductively Coupled Mass Spectrometry (ICP-MS) Secondary Ion Mass Spectrometry (SIMS) Chemical Methods Chemical Pb Ages

5 Radioactive Decay The rate of radioactive decay is typically expressed in terms of either the half-life, or the radioactive decay constant. They are related as follows:

6 Generalized Age Equation Decay constant Time D* = D o + N (e lt - 1) Radiogenic daughter Initial daughter element Radioactive parent

7 D*/D s = (D/D s ) o + N/D s (e lt - 1) t = 1/l ln D*/D s - (D/D s ) o N/D s + 1

8 The U-Pb Dating Method Uranium has three naturally occurring isotopes: 238 U, 235 U, 234 U 238 U 206 Pb + 8He + 6b + Q 235 U 207 Pb + 7He + 4b + Q g also emitted

9 Radioactive Decay-U-Th-Pb

10 Common Pb Problem Pb isotopes: 204 Pb, 206 Pb, 207 Pb, 208 Pb Pb is everywhere (known as common Pb) Has a unique isotopic composition (High 204 Pb). Must be measured and subtracted from measurements to obtain proper age. If your mineral is younger than 1 Ma it will have very little radiogenic Pb and you need to use the U-series dating method

11 Concordia Diagram Concordia 3200 Ma 206 Pb/ 238 U Ma 2500 Ma Discordia thermal event, diffusion 3300 Ma Pb/ 235 U

12 Thermal Ionization Mass Spectrometry (TIMS)-Bulk Analysis

13 Geochronology Bulk Thermal Ionization mass spectrometry (TIMS) methods Problems: inherited zircons, zoned minerals Solution: 1990 s single zircon methods-royal Ontario Museum abraded zircons Kober technique Chemical Pb ages Step heated zircon Folded tungsten TIMS filament

14 U-Pb Dating of Uraninite Traditional methods are bulk Concentration of U-bearing phases Dissolution in nitric acid Ion exchange to isolate U & Pb Isotope dilution TIMS Assumes sample is homogenous At the very least only 1 generation present Often not the case Alteration, variable Pb loss, multiple generations The most precise technique

15 In Situ Techniques Preserves mineralogical context Easier sample preparation Instrumentation: Electron Probe Micro Analyzer (EPMA) Inductively-coupled Mass Spectrometry (ICP-MS) Secondary Ion Mass Spectrometry (SIMS) (Böhm et al., 2003)

16 Chemical Pb Ages-EPMA

17 Chemical Pb Ages-EPMA Beam size = spatial resolution 1-50μm Cannot measure isotopes Only major element composition (detection limit 1000 ppm)

18 Chemical Pb ages-epma Measure % U, Pb and Th Use equations to calculate age: t= Pb x7550/(u Th ) Bowles (1990)

19 Chemical Pb ages-epma Use EMPA to obtain wt% U and Pb or PbO and UO 2 Assumes all Pb is radiogenic No way to correct for common Pb Not suitable for very young minerals (no Pb) Young minerals generally have more common Pb compared to radiogenic Pb Errors are ~1-10 Ma Age (Ma) Wt% PbO*

20 U-Pb Dating Using SIMS and ICP-MS Measure many different isotopes 204 Pb +, 206 Pb +, 207 Pb +, 208 Pb +, 230 Th, 234 U, 235 U +, 238 U + Beam size = spatial resolution 1-100μm SIMS ICP-MS

21 U-Pb Dating Using SIMS and ICP-MS Laser and ion beam technology Over 40 years technique development Typical errors spots 1-3% Routine analysis in several laboratories Material consumed during SIMS analysis << ICP-MS Applications to uraniumrich minerals-only the beginning LA-ICP-MS (Böhm et al., 2003) SIMS

22 Coffinite ~2 wt% PbO Ca-U <1 wt% PbO Uraninite ~10 wt% PbO

23 (Sheahan et al, 2016) U3 U4 U1 U3 U3 U1 U4

24 The Athabasca Uranium Deposits U3 U4 855 ± 27 Ma U ± 26 Ma U1 U ± 22 Ma Kianna Deposit, W Athabasca (Sheahan et al. 2016)

25 The Athabasca Uranium Deposits 856 Ma Coffinite 1002 Ma Uraninite Uraninite 1227 Ma 75 mm 100 mm Cigar Lake, E Athabasca Errors ± 15 Ma (Fayek et al. 2002)

26 U-Series Dating

27 No. of Analyses No. of Analyses Oklo-Okélobondo Fission Reactors Pangea (~300 Ma) Benue Rift (~ Ma) 6 5 Pan-African (~0.5 Ga) Rhodinia Rifting (~0.9 Ga) Diagenesis and Tanzinian (~ Ga) Criticality (~1.95 Ga) Accretion: Atlantica (~2 Ga) Age of U Deposits (~2.0 Ga) Athabasca Deposits Pangea (~300 Ma) Grenvillian dike swarm (1.0 Ga) (~ Ga) Rhodinia Rifting: (~ Ga) Post-Pin: dike swarm (~ Ga) Pinwarian (~1.5 Ga) Ryholites & granites (1.45 Ga) Age of U deposits (~1.55 Ga) Rifting (1.54Ga) Age (Ma) Age (Ma)

28 U-Series Dating If your mineral is younger than 1 Ma it will have very little radiogenic Pb and you need to use the U-series dating method When do you use U-Series dating? Wt% UO 2 Wt% PbO Age (Ma) 85% 12% 1500 Ma 88% 2% 150 Ma 88% 0.01%? 206 Pb /204 Pb low < ~400

29 Uranium has three naturally occurring isotopes: 238 U, 235 U, 234 U 238 U 206 Pb + 8He + 6b + Q 235 U 207 Pb + 7He + 4b + Q 206 Pb/ 238 U 207 Pb/ 235 U >1 Ma 234 U/ 238 U 230 Th/ 238 U <1 Ma

30 Activity Ratios [ 234 U/ 238 U] A = ( 234 U/ 238 U) x ( 234 l/ 238 l) [ 230 Th/ 238 U] A = ( 230 Th/ 238 U) x ( 230 l/ 238 l) Rate of decay = decay constant, l,(t -1 ) U deposition Forbidden zone Forbidden zone U removal

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32 No. of Analyses Age of Uranium Minerals Age (ka)

33 Conclusions Isotopic methods are necessary to obtain accurate and precise ages In situ techniques are necessary to obtain accurate ages from zoned minerals Ignoring these two points will give unreliable ages that are difficult to interpret

34 Thank you! Caitlin Sheahan (U of M) Kevin Jones (U of M) Alex Ozaruk (Western)