understanding uranium ore body formation aids in mine closure challenges

© 2013 ARCADISApril 18, 20231

Don CarpenterGeochemist ARCADIS U.S., Inc.Brighton, MI

Understanding Uranium Roll-Front Ore Body Formation Aids in Predicting Mine Closure Challenges


As a Statement of the Obvious “Uranium Mine 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


• Emphasis on uranium roll-front deposits

• Methodology applicable to other ore types

Systematic analysis of the impacted water generation process aids in predicting acid mine drainage potential

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Important for our U.S. clients

Uranium roll-front deposits are an important uranium deposit type in the United States


“Roll-Front” has both a Geochemical and Mining-Related Connotation

Reduced phase uranium in association with iron disulfides

Uranium co-associated with other metals and metalloids

Ore protected from surface oxidation by low permeability

sediment

Open pit mining employed for shallow deposits

“Classic” Roll-Front Uranium Deposit


Roll-front deposits are sufficiently large to represent an important local environmental issue

• Length – A mile or more• Depth – Tens to a few

hundreds of feet• Width – Few to several

hundreds of feet• Potential for billions of

gallons of impacted pit lake water and associated groundwater

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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

= 2

5 °C

, P

=

1.0

13 b

ars

, a

[m

ain

] =

10

–5.0

66,

a [

H2O

] =

1

, a

[H

CO

3- ] =

10

–3;

Sup

pres

sed:

U3O

8(c

,alp

h),

U4O

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) has same approximate area of stability as hydrogen sulfide (H2S and HS-)

• Oxygenated, uranium-bearing groundwater encountering reduced, pyritic sediment would precipitate uraninite

UO2 and H2S or HS-


Uranyl (UO2+2) ions can react with various anions

forming enhanced solubility complexes

• pH and anion activity control complex formation

• UO2+2 is 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

UO2+2 + 3HCO3

- UO2(CO3)3-4 + 3H+


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


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


Reaction of dissolved oxygen with iron disulfides can also result in the formation of ferric oxides

“Initiation Reaction”

4FeS2 + 14O2 + 4H2O → 4Fe+2 + 8SO4-2 + 8H+

4Fe+2 + O2 + 4H+ → 4Fe+3 + 2H2O

4FeS2 + 15O2 + 2H2O → 4Fe+3 + 8SO4-2 + 4H+

Fe+3 + 3H2O → Fe(OH)3 + 3H+

Acidity is buffered by silicate minerals present in aquifer preventing the “Propagation Reaction”

Distinctive coloration of the oxidized sediments


Roll-front formation begins with development of locally reducing conditions

Low Permeability Sediment


H2S or HS-

FeS2

Organic Carbon

Higher 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




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, Se, V

O2 O2

O2

O2

O2

O2


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+


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


Examining Mine Data and Use of Geochemical Modeling

Can Aid in Predicting Acid Potential

1. Assess mine maps and database for rock/alteration types

2. Develop database of iron disulfide, uranium, and other constituent content

3. Identify location(s) of disposal (backfilling, if any)

4. Estimate mine wall and waste dump composition

5. Apply geochemical modeling to assess constraints on acid generation and rate of production


Understanding uranium roll-

front ore body formation aids

in predicting mine closure

challenges

• As a statement of the obvious uranium mine drainage can be problematic

• Uranium roll-front deposits are an important uranium deposit type in the United States

• Oxidation and reduction conditions dominate the geochemical behavior of uranium

• Roll front formation begins with development of locally reducing conditions

• Mining breaches the protective cap allowing surficial oxygenated water to encounter ore and reduced rock

• Acid mine generation can now result from the oxidation of iron disulfides unbuffered by silicates in “pit lakes”


Imagine the result

understanding uranium ore body formation aids in mine closure challenges

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