understanding uranium roll-front ore body formation aids in mine closure challenges
TRANSCRIPT
© 2013 ARCADISMay 1, 20231
Don CarpenterGeochemist ARCADIS U.S., Inc.Brighton, MI
Understanding Uranium Roll-Front Ore Body Formation Aids in Predicting Mine Closure Challenges
© 2013 ARCADISMay 1, 20232
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
© 2013 ARCADISMay 1, 20233
• 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
© 2013 ARCADISMay 1, 20234
Important for our U.S. clients
Uranium roll-front deposits are an important uranium deposit type in the United States
© 2013 ARCADISMay 1, 20235
“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
© 2013 ARCADISMay 1, 20236
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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© 2013 ARCADISMay 1, 20237
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
JGil low 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-
© 2013 ARCADISMay 1, 20238
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+
© 2013 ARCADISMay 1, 20239
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, 202310
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, 202311
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
© 2013 ARCADISMay 1, 202312
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, 202313
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, 202314
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, Se, V
O2 O2
O2
O2
O2
O2
© 2013 ARCADISMay 1, 202315
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, 202316
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, 202317
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
© 2013 ARCADISMay 1, 202318
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”
© 2013 ARCADISMay 1, 202319
Imagine the result