The document discusses three main types of uranium deposits in the US - roll front, tabular, and Grant's type humate deposits. It explains the geochemical processes that lead to the formation of each deposit type, noting the presence or absence of acid-generating materials like iron disulfides. The document states that understanding the deposit types aids in predicting and addressing future environmental challenges, as mining can expose acid-generating materials to oxygen and result in acidic, metal-enriched mine drainage if not properly managed based on the deposit's characteristics.

1. © 2012 ARCADIS22 August 20151
Understanding the Formation of
Selected US Uranium Deposit Types
Aids in Predicting and Addressing
Environmental Challenges
Don Carpenter, Geochemist
ARCADIS U.S., Inc.
Brighton, MI
Imagine the result

2. © 2012 ARCADIS22 August 20152
As a Statement of the Obvious
“Uranium Mine and Mill Tailings Drainage Can Be Problematic”
Low pH (< 3.5 [potentially much lower])
Enhanced metal and metalloid mobilization
• Dissolved constituents include: uranium, molybdenum,
selenium, vanadium, sulfate
• Readily detectable (pH)
• Visually apparent
• Adverse concern as to
radionuclides

3. © 2013 ARCADIS22 August 20153
U. S. uranium production was mainly
derived from three different deposit types

4. © 2013 ARCADIS22 August 20154
• Methodology applicable to other uranium and ore deposit types
A systematic approach will be employed to assess
environmental challenges for these deposit types

5. © 2013 ARCADIS22 August 20155
2 4 6 8 10 12 14
–.5
0
.5
1
pH
Eh(volts)
UO2
++
UO2(CO3)2
--
UO2(CO3)3
----
UO2CO3
Uraninite
25°C
JGillow Fri Jun 15 2007
DiagramUO2
++
,T=25°C,P=1.013bars,a[main]=10
–5.066
,a[H2O]=1,a[HCO3
-
]=10
–3
;Suppressed:U3O8(c,alph),U4O9(c)
Oxidation and reduction conditions dominate the
geochemical behavior of uranium
• Uranium exists in two oxidation
states (UO2
+2 and U+4)
• Eh predominates over pH
• UO2
+2 soluble in high Eh
conditions
• U+4 readily precipitates as
Uraninite [UO2]) under low Eh
conditions
• Uraninite (UO2) same
approximate area of stability as
hydrogen sulfide (H2S and HS-)
• Oxygenated, uranium-bearing
groundwater encountering
reduced, pyritic sediment
precipitates uraninite
UO2 and H2S or HS-

6. © 2013 ARCADIS22 August 20156
Uranyl (UO2
+2) ions can react with various
anions forming enhanced solubility complexes
UO2
+2 + 3HCO3
-  UO2(CO3)3
-4 + 3H+
• pH and anion activity control
complex formation
• UO2
+2 typically transformed into an
neutral or negatively charged anionic
complex significantly affecting its
subsequent geochemistry
• For simplicity subsequent
geochemical reactions will be based
on non-complexed ions

7. © 2013 ARCADIS22 August 20157
Reaction with sulfide phases can
lead to uranium precipitation
7UO2
+2 + FeS2 + 8H2O  7UO2 + Fe+2 + 2SO4
-2 + 16H+
4UO2
+2 + H2S + 4H2O  4UO2 + SO4
-2 + 10H+
• Hydrogen sulfide or iron disulfides can be derived from either
biogenic (sulfate reduction) processes or non-biogenic (oil and
gas field brines) sources
• Iron disulfides may be present as either pyrite and/or marcasite

8. © 2013 ARCADIS22 August 20158
Molybdate ion may also precipitate
upon reaction with iron disulfides
• Explains the co-association of this element with uranium
• Also the common presence of elemental sulfur
• Similar reductive precipitation reactions for selenium
and vanadium
3MoO4
-2 + 6FeS2 + 16H+  3MoS2 + 6Fe+2 + 5S + SO4
-2 + 8H2O

9. © 2013 ARCADIS22 August 20159
Uranium roll-front formation begins with
development of locally reducing conditions
Low Permeability Sediment
Low Permeability Sediment
H2S or HS-
FeS2
Organic Carbon
Higher Permeability Sediment

10. © 2013 ARCADIS22 August 201510
Low Permeability Sediment
Low Permeability Sediment
“Oxidized Tongue”
O2 UO2
+2 MoO4
-2
Fe(OH)3
Oxidation of Organic Carbon
“Ore Zone”
UO2 MoS2
FeS2 (Ore Stage)
Incursion of oxygenated uranium bearing water
initiates the “roll-front” process and ore formation

11. © 2013 ARCADIS22 August 201511
Tabular-type U/V deposit formation requires a
geochemically reducing brine component
Sulfate-enriched brine
Organic Carbon
H2S or HS-
FeS2
Gypsum [CaSO4 * 2H2O]
Dolomite [Ca,Mg(CO3)2]
Oxygenated Groundwater (low ionic strength)
Dissolved phase Uranium, Vanadium, Molybdenum, Selenium

12. © 2013 ARCADIS22 August 201512
Diffusive transport of ore and gangue
constituents and their chemical reduction
results in ore formation
H2S or HS-
U
UO2(CO3)3
-4
V2O5
-2 SeO3
-2 MO4
-2
Se°
UO2, V2O3
MoS2

13. © 2013 ARCADIS22 August 201513
Grant’s type humate uranium deposits require
the initial concentration of humic acids
• Oxygenated Groundwater
• Dissolved phase Uranium and Molybdenum
• Dissolved phase organic acids
• Precipitation of geochemically active organic acid mass
• Function of sediment grain size and geochemical conditions

14. © 2013 ARCADIS22 August 201514
Geochemical processes result in
concentration of uranium and molybdenum
• Oxygenated, Groundwater
• Dissolved phase Uranium and Molybdenum
• Dissolved phase organic acids
• Precipitation of geochemically active organic acid mass
• Function of sediment grain size and geochemical conditions
UO2(CO3)3
-4
MO4
-2
UO3
MO3
UO3

15. © 2013 ARCADIS22 August 201515
Differences in deposit type characteristics
affect future environmental challenges
Deposit Type
Iron Disulfides
(Pyrite and
Marcasite)
Acid
Neutralizing
Minerals
Mode of
Mining
In Situ
Sediment
Permeability
Roll-Front Type
Major Pre-Ore
Major Ore
Stage
Absent
Open Pit
In Situ Leach
High -
Preserved
Tabular Type
Minor Pre-Ore
Minor Ore
Stage
Generally
Present
(Dolomite)
Generally
Underground
Moderate -
Decreased
Grant’s Type
Generally
Absent
May be
Absent
Generally
Underground
Moderate -
Preserved

16. © 2013 ARCADIS22 August 201516
Low Permeability Sediment
Low Permeability Sediment
FeS2
Organic Carbon
“Oxidized Tongue”
“Ore Zone”
Mining breaches the protective cap allowing surficial
oxygenated water to encounter ore and reduced rock
Oxidation of: “Pre-Ore” and “Ore-Stage” iron disulfides
Sub-ore grade uranium
Co-associated reduced phases of Mo and Se
O2 O2
O2
O2
O2
O2

17. © 2013 ARCADIS22 August 201517
Acid mine generation can now result from the oxidation
of iron disulfides unbuffered by silicates in “pit lakes”
4FeS2 + 14O2 + 4H2O → 4Fe+2 + 8SO4
-2 + 8H+
4Fe+2 + O2 + 4H+ → 4Fe+3 + 2H2O
4FeS2 + 15O2 + 2H2O → 4Fe+3 + 8SO4
-2 + 4H+
Initiation Reaction(s)
Propagation Reaction (pH <~3.5)
FeS2 + 14Fe+3 + 8H2O → 15Fe+2 + 2SO4
-2 + 16H+

18. © 2013 ARCADIS22 August 201518
Oxidation can also result in dissolution of
uranium and co-associated gangue phases
UO2 + O2 + 2H+ = UO2
+2 + H2O
2Se° + 3O2 = 2SeO3
-2 + 4H+
2MoS2 + 9O2 + 3H2O = 2MoO4
-2 + 4SO4
-2 + 2H+
Result is an acidic, metal-enriched mine water
The resulting geochemistry can be predicted

19. © 2013 ARCADIS22 August 201519
Understanding the Formation of Selected
US Uranium Deposit Types Aids in Predicting
and Addressing Environmental Challenges
• As a statement of the obvious uranium mine and mill tailings
drainage can be problematic
• U. S. uranium production was mainly derived from three
different deposit types
• Roll front formation begins with development of locally reducing
conditions
• Tabular-type U/V deposit formation requires a geochemically
reducing brine component
• Grant’s type humate uranium deposits require the initial
concentration of humic acids
• Differences in deposit type characteristics affect future
environmental challenges

20. © 2012 ARCADIS22 August 201520
Imagine the result