1. Mass-Independent Uranium
Fractionation
TM: Origin of sandstone hosted uranium deposits
Christa Placzek
James Cook University, School of Earth and Environmental Sciences
christa.placzek@jcu.edu.au

2. Nuclear and Radiochemistry Division
supported by G.T. Seaborg Foundation; IGPP grant
Jeffrey Heikoop (EES)
Ben Linhoff (EES), now MIT
In collaboration with Uranium Resources Incorporated (URI)
School of Earth and Environmental Sciences
Economic Geology Research Unit (EGRU)
In collaboration with A. Dosseto (U of Wollongong)

3. Mass-Independent Uranium
Fractionation
Isotopic change results from reduction and
oxidation of uranium. These transformations occur
during:
• ore formation
• lSR leaching
• environmental remediation
• weathering of source rocks

4. Uranium Isotopes
% in natural Half-Life (in
Isotope # of Protons # of Neutrons
uranium years)
Uranium-238 99.284 92 146 4.46 billion
Uranium-235 0.711 92 143 704 million
Uranium-234 0.0055 92 142 245,000
Photo credit: silex.com.au

5. Uranium disequilibrium and
fractionation

6. Stable isotope fractionation of uranium
• Heaviest naturally occurring element
• Half lives of 238U and 235U are such that they are
essentially stable
• Fractionation is not mass dependent (occurs in the wrong
direction)
• results during redox transformation
• “nuclear volume” or “nuclear field shift” effect
• Predicted temperature dependence is 1/T, not 1/T2
• Recently able to measure this ratio as a result of advances
in MC-ICP-MS and the availability of a new double spike
(first measurements in 2007)

7. Mass-dependant vs. nuclear volume
% mass difference
18O-16O:12.5 %
56Fe-54Fe: 3.7%
238U-235U: 1.3%

8. Small change in mass…..large change
in volume
limitations to how many protons and neutrons can be bound together
in the nucleus
+
+ 235U
+ + 238U
+ +
+ +
+ +
++
+ + +
+
Oxidation: loss of electrons
Reduction: gain of electrons Photo credit: orbitals from Atom in a Box, Dauger Research

9. Variable 238U/235U in ores
Brennecka et al (2010)

10. Key concept from traditional stable
isotopes-delta notation
(Ratiosample – Ratiostandard)
= Ratiostandard
x 1000 =‰
- 238 235
U/
d
238
U
U=137.88=0‰*
+
oxidized reduced
U(VI) U(IV)
aqueous solid
more 235U more 238U

11. Key concept from traditional stable
isotopes-Raleigh Fractionation
Image credit: Clark and Fritz. Environmental Isotopes in Hydrogeology

12. Ore Formation Model
(‰)
Ore-reduced U(IV)
Favors 238U
238U
Aqueous-oxidized U(VI)
Favors 235U
1 0.8 0.6 0.4 0.2 0
fraction remaining

13. South Texas Location
Rosita

14. Uranium Roll Front
Oxidizing, ore, and reducing zones

15. Rosita site
deposit is very young <2 Ma and currently active
Monitor
Well Ring
Ore • Not yet leached
Natural • Leached
Water Flow
• Active
Oxidizing Reducing • Closed/remedi
ated
Natural
Water Flow
Monitor Wells to Injection Wells = 400’

16. 238U represents value of water from which ore precipitated
Ore-reduced U(IV)
Favors 238U
(‰)
238U
Aqueous-oxidized U(VI)
Favors 235U
1 0.8 0.6 0.4 0.2 0
fraction remaining

17. 238U values in this roll front
0.5
600 ppb U
0
Oxidizing
-0.5 <100 ppb U
(‰)
238U
-1
Ore
-1.5
-2
analytical error ~0.1‰ Reducing
-2.5

18. Mapping ore bodies with 238U
Ore-reduced U(IV)
Favors 238U
(‰)
238U
Aqueous-oxidized U(VI)
Favors 235U
1 0.8 0.6 0.4 0.2 0
fraction remaining

19. ISL is a redox transformation

20. Predicted trend, but 238U values have never been
observed to change during leaching
Aqueous-oxidized U(VI)
Favors 235U
(‰)
238U
Ore-reduced U(IV)
Favors 238U
1 0.8 0.6 0.4 0.2 0
fraction remaining

21. 238U values are altered by ISL?
0.3
0.1
-0.1
(‰) Leached
-0.3
238U
-0.5
-0.7
-0.9 Ore
-1.1
-1.3
-1.5
Provides an integrated picture of U reservoir during leaching
Example of application to understanding multiple ore cycles

22. Examination of multiple events with 238U
(‰)
238U
1 0.8 0.6 0.4 0.2 0
1 0.8 0.6 0.4 0.2 0
fraction remaining

23. Redox transformation during
environmental remediation
Oxidized
U(VI)
Aqueous
• Measures reduction directly
– Not impacted by sorbtion or dilution
– May detect remobilization

24. 238U values are altered by ISL and
subsequent remediation
0.3
0.1
leaching
-0.1
(‰)
-0.3 restored
238U
-0.5
-0.7
-0.9 Ore
-1.1
-1.3
-1.5
Provides a way to distinguish U subjected to ISL from natural U
Provides a way to quantify remediation efforts

25. Oxidizing conditions lead to uranium
weathering in silicates
Courtesy: Bill Deutsch (PNNL)

26. How do we increase chemical
weathering flux?
Climate
-temperature
-precipitation
Supply rate
-uplift
-volcanism
-composition
-high concentration vs. easily weathered

27. Redox during weathering
Mineral U concentration (ppm) weathering redox weathering rate
Apatite high No fast
CaCO3 moderate No fast
Quartz moderate Yes intermediate
Biotite moderate Yes intermediate
Chlorite moderate Yes intermediate
Plagioclase moderate Yes intermediate
Zircon 100 Yes very slow

28. Laboratory leaching of a Tuff

29. 238U values and soil development

30. Are ores with different isotopic
compositions more valuable?
0.3
0.1
-0.1
(‰)
-0.3
238U
-0.5 Leached
-0.7
-0.9
Ore
-1.1
-1.3
-1.5

31. Mass-Independent Uranium
Fractionation
Isotopic change results from reduction and
oxidation of uranium. These transformations occur
during:
• ore deposition
• Leaching
• environmental remediation
• weathering of source rocks
Thank you!

32. Recent uranium mobility
1.80
1.60
(activity)
1.40
1.20
234U/238U
1.00
0.80
0.60