© 2012 ARCADISMay 1, 20231

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

© 2012 ARCADISMay 1, 20232

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

© 2013 ARCADISMay 1, 20233

U. S. uranium production was mainly derived from three different deposit types

© 2013 ARCADISMay 1, 20234

• Methodology applicable to other uranium and ore deposit types

A systematic approach will be employed to assess environmental challenges for these deposit types

© 2013 ARCADISMay 1, 20235

2 4 6 8 10 12 14

–.5

0

.5

1

pH

Eh

(vol

ts)

UO2++

UO2(CO3)2--

UO2(CO3)3----

UO2CO3

Uraninite

25°C

JGillow Fri Jun 15 2007

Dia

gram

UO

2++, T =

25

°C , P

=

1.0

13 b

ars

, a

[m

ain

] =

10

–5.0

66, a

[H

2O

] =

1

, a

[H

CO

3- ] =

10

–3; Sup

pres

sed:

U3O

8(c

,alp

h), U

4O

9(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-

© 2013 ARCADISMay 1, 20236

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

© 2013 ARCADISMay 1, 20237

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

© 2013 ARCADISMay 1, 20238

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

© 2013 ARCADISMay 1, 20239

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

© 2013 ARCADISMay 1, 202310

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

© 2013 ARCADISMay 1, 202311

Tabular-type U/V deposit formation requires a geochemically reducing brine component

Sulfate-enriched brineOrganic CarbonH2S or HS-

FeS2

Gypsum [CaSO4 * 2H2O]Dolomite [Ca,Mg(CO3)2]

Oxygenated Groundwater (low ionic strength)Dissolved phase Uranium, Vanadium, Molybdenum, Selenium

© 2013 ARCADISMay 1, 202312

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

© 2013 ARCADISMay 1, 202313

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

© 2013 ARCADISMay 1, 202314

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

© 2013 ARCADISMay 1, 202315

Differences in deposit type characteristics affect future environmental challenges

Deposit TypeIron Disulfides (Pyrite and Marcasite)

Acid NeutralizingMinerals

Mode of Mining

In Situ Sediment Permeability

Roll-Front TypeMajor Pre-OreMajor Ore Stage

Absent Open PitIn Situ Leach

High - Preserved

Tabular TypeMinor Pre-OreMinor Ore Stage

Generally Present(Dolomite)

Generally Underground

Moderate - Decreased

Grant’s Type Generally Absent

May be Absent

Generally Underground

Moderate - Preserved

© 2013 ARCADISMay 1, 202316

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 disulfidesSub-ore grade uraniumCo-associated reduced phases of Mo and Se

O2 O2

O2

O2

O2

O2

© 2013 ARCADISMay 1, 202317

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+

© 2013 ARCADISMay 1, 202318

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

© 2013 ARCADISMay 1, 202319

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

© 2012 ARCADISMay 1, 202320

Imagine the result