Geologic and Environmental
Characteristics of Rare Earth
Element Deposit Types Found
in North America
Bob Seal
rseal@usgs.gov
703-648-6290
NARPM
November 27, 2012
Key Points
• Future REE production will be expected to come from a variety
of deposit types.
– Some primary producers and some likely byproduct producers
• Future REE mine development in the United States will likely be
“modest”.
– Occurrence ≠ Prospect ≠ Advanced Exploration Project ≠ Mine
– Mines are defined on the basis of grade, tonnage, and economic
viability
• Understanding of environmental characteristics of REE deposits
is immature compared to that for other commodities (i.e., Cu,
Pb, Zn, Au, Ag, etc.).
Rare Earth Elements
• Less abundant than
common rock-forming
elements
• More abundant than
precious metals
• More or less abundant
than base metals and
metalloids.
Geochemical Controls in Nature
Lanthanide Contraction
• Ionic radius decreases with
increasing number
Oxidation State
• All REEs occur in 3+ state
• Eu also occurs in 2+ state
• Substitutes for 2+ cations like Ca2+
• Ce also occurs in 4+ state
• Even more insoluble
From: Haxel (2005)
Rare Earth Element Ore Processing
Mine
Molycorp’s New Closed-Loop Process
Extrac on/
Purifica on
Mill
REE
Separa on
Finished
Products
Water
Water
HCl/NaOH
Tailings
Waste brine is purified
and converted to reagents
which are recycled into the
process.
Salt
Purifica on
Impuri es
Salt
Pure NaCl
Salt Recovery
Unit
Paste Tailings Impoundment
Chloralkali
Products
Recovery of individual
REEs is complicated by:
• Similar geochemical
behavior of REEs as
a group
• Wide variety of REE
host minerals
Source:
Molycorp (2012)
Source:
Haxel (2005)
Primary and Byproduct REE Deposit Types
• Carbonatites
– Bayan Obo, China
– Mountain Pass, CA
– Bear Lodge Mountains, WY
• Alkaline Intrusion-Related Deposits
– Bokan Mountain, AK
– Thor Lake (Nechalacho), NWT, Canada
• Magmatic Magnetite-Hematite bodies
– Pea Ridge Mine, MO
– Olympic Dam, Australia?
– Kiruna, Sweden?
Potential Byproduct REE Sources
• Monazite-bearing Placer Deposits
– Central Idaho stream placers
– North and South Carolina stream placers
– Virginia, Georgia, and Florida beach
deposits
• Phosphorite Deposits
– Florida, North Carolina
– Phosphoria Formation, ID, MT
Rare Earth Element Deposits of the United States
Pajarito
Rare Earth Element Minerals & Deposit Types
Mineral
Formula
Carbonatite
Alkaline Intrusion
Placer
Phosphorite
Oxides
Aeschynite
(Ln,Ca,Fe)(Ti,Nb)2(O,OH)6
X
Euxenite
(Y,Ln,Ca)(Nb,Ta,Ti)2(O,OH)6
X
YNbO4
X
(Na,Ln,Ca,Sr,Th)(Ti,Nb,Fe)O3
X
Fergusonite
Loparite
X
Carbonates
Bastnäsite
(Ln,Y)CO3F
X
X
Parisite
Ca(Ln)2(CO3)3F2
X
X
Synchisite
Ca(Ln,Y)(CO3)2F
X
X
(Ca,Ln)5(PO4)3(OH,F,Cl)
X
X
Monazite
(Ln,Th)PO4
X
X
X
Xenotime
YPO4
X
X
Allanite
(Ln,Y,Ca)2(Al,Fe3+)2(SiO4)3(OH)
X
Britholite
(Ln,Ca,Th)5(SiO4,PO4)3(OH,F)
X
Eudialyte
Na4(Ca,Ce)2(Fe2+,Mn2+,Y)ZrSi8O22(OH,Cl)2
X
Iimoriite
Y2(SiO4)(CO3)
X
Phosphates
Apatite
X
Silicates
Mosandrite Na(Na,Ca)2(Ca,Ln,Y)4(Ti,Nb,Zr)(Si2O7)2(O,F)2F3
X
Zircon
X
(Zr,Ln)SiO4
X
Ln: Lanthanide (a.k.a. REE)
Carbonatite Deposits
Carbonatites
Carbonatite Deposits
• Carbonatite – an igneous rock with
greater than 50 % carbonate
minerals (calcite, dolomite,
ankerite)
Bastnäsite
(REE)CO3F
• Less than 550 carbonatites known
in world.
• Commonly associated with alkaline
or peralkaline igneous rocks.
• The REE are “incompatible
elements”, such as Th, Nb, and Zr,
that do not tend to participate in
the earlier mineral-forming events.
• Important sources of Nb, Ta, and
REEs.
Mountain Pass, California
Mountain Pass, California
• Located in Mojave Desert.
• Discovered in 1949 by uranium
prospector.
• 1.4 Ga carbonatite complex.
• Stopped mining in 2002.
• Reopened 2010.
From: Haxel (2005)
Sulphide Queen carbonatite
Proven and Probable Reserves
18.4 million short tons of ore
grading 8.0 % REE oxides,
using cut-off grade of 5 %
Bastnäsite
calcite
barite
dolomite
1 mm
Alkaline Intrusion-Related Deposits
Vein and dike REE districts
Pajarito
Alkaline Intrusion-Related Deposits
• Associated with alkaline or peralkaline igneous complexes
Peralkaline: Na2O + K2O > Al2O3
• Structural control on the veins, dikes and breccias
• In zoned alkaline intrusive complexes, REE veins, dikes, and
breccia bodies are late phases.
• The REE are
“incompatible
elements”, such as Th,
Nb, and Zr, that do not
tend to participate in the
earlier mineral-forming
events.
Bokan Mountain, Alaska
Bokan Mountain, Alaska
Ucore Rare
Metals
Thor Lake, NWT, Canada
From: Sheard, E.R., Williams-Jones, A.E., Heiligmann, M., Pederson, C., and Trueman, D.L., 2012, Economic Geology, v.
107, p. 81-104.
Heavy Minerals Sands (Placers)
Ilmenite
• Heavy mineral sands are currently
produced in Florida and Virginia.
• Grades average 4 to 9 % heavy
minerals
– Ilmenite 40 – 70 %
– Zircon trace – 20%
Monazite
– Rutile, leucoxene trace – 20 %
– Garnet, staurolite, kyanite
trace – 40 %
– Monazite trace – 2 %
• Monazite ((Ln,Th)PO4) concentrates are currently not produced because of
concerns about NORMs.
• Monazite production risks: thorium
Iluka Concord Mine, Virginia
Phosphate Deposits
• Phosphate is mined in Florida (~65 %),
North Carolina, Idaho and Utah
•
Current potential and past production
in Montana and Wyoming
• REE concentrations approach 0.5 %.
• REEs currently are not recovered.
• REEs are part of the waste stream from
phosphoric acid production.
• Risks: uranium (radon), selenium
Geologic Summary
• Primary and byproduct deposits
– Carbonatites
– (Per)alkaline intrusive complexes
• Potential byproduct sources (?)
– Phosphate deposits
– Heavy mineral sands (placer)
deposits
• Economic Risks
– Estimated 1 in 10,000 showings
become mines
– Estimated 1 in 2,000 or 3,000
prospects become mines
– Ore processing is very deposit
specific because of variety of
REE minerals
• In other words, the United States has potential, but do not expect a flood of
new REE mines in the near future.
Environmental Characteristics of REE Deposits
• Limited Data (Case Studies)
• Few Active, Inactive, or Abandoned Mines
– Bayan Obo
– Mountain Pass
• Few Advanced Exploration Projects
– Thor Lake
– Bear Lodge Mountains
• Phosphate Fertilizer Industry
– The Netherlands (water, sediment and soil guidelines)
– China
• Limited laboratory bioassay studies of REEs
• Geologic Characteristics
Rare Earth Element Minerals & Chemistry
Group-Mineral
Oxides
Aeschynite
Euxenite
Fergusonite
Loparite
Carbonates
Bastnäsite
Parisite
Synchisite
Phosphates
Apatite
Monazite
Xenotime
Silicates
Allanite
Britholite
Eudialyte
Mosandrite
Zircon
Formula
REO Wt%
ThO2 Wt%
UO2 Wt%
(Ln,Ca,Fe)(Ti,Nb)2(O,OH)6
(Y,Ln,Ca)(Nb,Ta,Ti)2(O,OH)6
(Y,Ln)NbO4
(Na,Ln,Ca)(Ti,Nb,Fe)O3
25 - 48
24
46 - 53
30
(Ln,Y)CO3F
Ca(Ln)2(CO3)3F2
Ca(Ln,Y)(CO3)2F
70 - 74
59
49 - 52
0 - 0.3
0 - 0.5
1.6
0.09
0 - 0.3
(Ca,Ln)5(PO4)3(OH,F,Cl)
(Ln,Th)PO4
YPO4
0 -20
35 - 71
52 - 67
0 - 20
0-3
0 - 16
0-5
(Ln,Y,Ca)2(Al,Fe3+)2(SiO4)3(OH)
(Ln,Ca,Th)5(SiO4,PO4)3(OH,F)
Na4(Ca,Ce)2(Fe2+,Mn2+,Y)ZrSi8O22(OH,Cl)2
Na(Na,Ca)2(Ca,Ln,Y)4(Ti,Nb,Zr)(Si2O7)2(O,F)2F3
(Zr,Ln)SiO4
3 - 51
33
1 - 10
33
0 - 0.7
0-3
21
0.1 - 0.8
Ln: Lanthanide (a.k.a. REE)
Acid-Generating Potential - Thor Lake, NWT, Canada
250
Mountain Pass
Tailings
Concentrate
NP kg CaCO3/t
200
• Despite lack of data,
carbonatite-hosted
REE deposits would
be expected to be
strongly net
alkaline.
150
NP/AP = 2
100
NP/AP = 1
50
0
0
5
10
15
20
25
30
35
40
45
50
AP kg CaCO3/t
Thor Lake data: http://www.reviewboard.ca/registry/project.php?project_id=87
Environmental Baseline – Major Elements
1000
Thor Lake SW
Thor Lake GW
Sulfate mg/L
Specific Conductance μS/cm
1000
100
Thor Lake SW
Thor Lake GW
10
Thor Lake HCT
Thor Lake HCT
100
Bear Lodge GW
Bear Lodge SW
Bear Lodge Springs
10
1
Bear Lodge GW
Bear Lodge SW
Bear Lodge Springs
1
5.0
6.0
7.0
pH
8.0
0.1
9.0
5.0
6.0
7.0
8.0
pH
• Data from regional sampling programs
– Some near deposits and others away (km’s)
•
•
•
•
pH is near neutral, ranging from slightly acidic to alkaline.
Specific conductance spans a range of nearly two orders of magnitude.
Sulfate is generally low, reflecting low abundance of sulfide minerals.
Humidity cell tests are near the low end of SC and sulfate.
9.0
Environmental Baseline – Major Elements
1000
1000
Thor Lake SW
Thor Lake SW
Thor Lake GW
Alkalinity mg/L CaCO3
Hardness mg/L CaCO3
Thor Lake GW
Thor Lake HCT
Bear Lodge GW
100
Bear Lodge SW
Bear Lodge Springs
10
Thor Lake HCT
Bear Lodge GW
100
Bear Lodge SW
Bear Lodge Springs
10
1
1
5.0
6.0
7.0
pH
8.0
9.0
5.0
6.0
7.0
8.0
pH
• Hardness and alkalinity are generally moderate to high.
• Humidity cell tests are near the low end of hardness and alkalinity.
9.0
Environmental Baseline – Fluoride
10
Thor Lake SW
Thor Lake GW
Bear Lodge GW
Bear Lodge SW
MCL
F mg/L
Bear Lodge Springs
1
0.1
5.0
6.0
7.0
8.0
9.0
pH
• No surface water ecological criteria available
• Drinking water standard is exceeded in baseline groundwater at
Nechalacho (Thor Lake).
Environmental Baseline – Trace Elements
100
100
Thor Lake SW
Thor Lake GW
10
Thor Lake HCT
Ni μg/L
Cu μg/L
Acute SW
Chronic SW
Thor Lake SW
1
10
1
Thor Lake GW
Thor Lake HCT
Bear Lodge GW
0.1
0.1
5.0
6.0
7.0
pH
8.0
9.0
5.0
6.0
7.0
8.0
pH
• Cu and Ni concentrations are low.
• Acute and chronic SW criteria assume a hardness of 100 mg/L CaCO3.
9.0
Environmental Baseline – Trace Elements
1000
100
Thor Lake SW
Thor Lake SW
Thor Lake GW
Thor Lake GW
MCL
Bear Lodge GW
10
Thor Lake HCT
100
Mo μg/L
As μg/L
Thor Lake HCT
Bear Lodge SW
Bear Lodge Springs
1
Chronic SW
Bear Lodge GW
Bear Lodge SW
10
1
0.1
0.1
5.0
6.0
7.0
8.0
9.0
5.0
6.0
7.0
8.0
9.0
pH
pH
• As and Mo occur as oxyanions.
• As and Mo concentrations are low.
Mo chronic and acute SW benchmarks from Suter (1996)
Environmental Baseline – Radionuclides
1000
Acute SW
MCL
10
+ 228Ra pCi/L
100
1
Thor Lake SW
0.1
Thor Lake GW
226Ra
U μg/L
Chronic SW
Thor Lake HCT
Bear Lodge GW
0.01
Bear Lodge SW
100
10
MCL
1
Thor Lake SW
Thor Lake HCT
Bear Lodge GW
Bear Lodge SW
Bear Lodge Springs
0.1
Bear Lodge Springs
0.001
0.01
5.0
6.0
7.0
pH
8.0
9.0
5.0
6.0
7.0
8.0
pH
• Uranium concentrations are generally low.
• Combined 226Ra + 228Ra concentrations locally exceed MCL.
• 226Ra is a decay product of 238U; 228Ra is a decay product of 232Th
U chronic and acute SW benchmarks from Suter (1996)
9.0
Rare Earth Elements: Surface Water Guidelines
Yellow River, Baotou, China
Bayan Obo Industrial Complex
MPC*
µg/L
pH
Y µg/L
La µg/L
Ce µg/L
Pr µg/L
Nd µg/L
Sm µg/L
Gd µg/L
Dy µg/L
6.4
10.1
22.1
9.1
1.8
8.2
7.1
9.3
Main
Channel
7.6 - 8.2
Site F
0.103
0.22
0.0304
0.095
0.023
0.028
0.082
140
152
16.08
52.01
6.91
7.22
3.8
Valzinco, VA
Site G
VLZN-10
VLZN-3
3.7 - 6.9
1.1
370
340
870
150
650
180
140
93
3.3
2.0
4.6
9.5
1.2
4.7
0.9
0.7
0.4
988
1149
294
1193
62.35
67.33
9.08
Thor Lake,
NWT,
Canada
Tailings
HCT
7.5
0.183
*From: Maximum Permissible Concentration; Sneller et al. (2000)
•
Surface water guidelines may not be as simple due to the influence of water
chemistry on toxicity…
Thor Lake data: http://www.reviewboard.ca/registry/project.php?project_id=87
Thor Lake Tailings: Leachate (HCT) – Other Elements
Chronic Surface
Water
SO4 mg/L
Y µg/L
Al µg/L
As µg/L
Cd µg/L
Co µg/L
Cr µg/L
Cu µg/L
Fe µg/L
Mo µg/L
Ni µg/L
Pb µg/L
Sb µg/L
Se µg/L
Sn µg/L
U µg/L
V µg/L
Zn µg/L
Zr µg/L
Acute Surface
Water
Drinking Water
250
0.28
750
340
0.3
3.1
570
13
239
470
65
104
73.7
1.87
19.1
120
54.9
6.4
87
150
2
195
74
11
1,000
10,100
52
2.5
985
5
2680
33.5
284
120
982
200
10
5
100
1,300
300
15
50
30
5,000
Thor Lake data: http://www.reviewboard.ca/registry/project.php?project_id=87
Tailings HCT
Leachate
2.9
0.18
20
0.5
0.2
0.1
1.7
1.4
5.8
12.1
1.2
0.15
52
<1
2.2
3.9
0.04
3
0.71
Thor Lake Tailings & Concentrate: Sediment & Soil
Unit
Al
As
Cd
Cr
Cu
Mo
Ni
Pb
Se
U
Zn
Ce
Dy
Gd
La
Nd
Pr
Sm
mg/kg
mg/kg
mg/kg
mg/kg
mg/kg
mg/kg
mg/kg
mg/kg
mg/kg
mg/kg
mg/kg
mg/kg
mg/kg
mg/kg
mg/kg
mg/kg
mg/kg
mg/kg
Sediment
TEC/NC* PEC/MPC*
9.79
0.99
43.4
31.6
33
4.98
111
149
22.7
35.8
48.6
128
121
256.2
26.2
23.2
83.6
43.2
66.1
30.9
459
18,800
2,200
1,800
4,700
7,500
5,800
2,500
Soil
Residential
77,000
23
70
280
3,100
390
1,600
400
390
230
23,000
Industrial
990,000
160
810
1,400
41,000
5,100
20,000
800
5,100
3,100
310,000
*Negligible Concentration, Maximum Permissible Concentration; Sneller et al. (2000)
Thor Lake data: http://www.reviewboard.ca/registry/project.php?project_id=87
Thor Lake
Tailings
62,000
4.4
6
640
18
73
330
9.9
3.2
9.9
68
1,900
170
210
850
970
250
210
REE Aquatic Toxicity
• Bioluminescence response of bacterium Vibrio fischeri was measured as a
function of the concentration of the free ion Lu3+ (modeled).
• Aquatic toxicity of lutetium varies with the concentration of the free ion (Lu3+).
Modified from Weltje et al. (2004)
REE Aquatic Toxicity
1400
48-hr EC50 (La μg/L)
1200
1000
800
600
400
200
0
0
50
100
150
200
Hardness (mg/L CaCO3)
•
•
•
•
LaCl3 used (0 to 50,000 μg/L).
La precipitated in low hardness sample (concentrations are problematic).
Acute aquatic toxicity of lanthanum varies with hardness for Daphnia carinata.
Free ion responsible for toxicity.
Photo: http://www.sustainabilitymatters.net.au
From: Barry and Meehan (2000)
REE Sulfate Complexes
• Water chemistry greatly affects concentration of free ion.
• Sulfate complexes strongly and influences concentration of free ion.
• Despite sulfide-poor nature of most REE ores, sulfate complexes
may be significant factor.
Modified from Wood (1990)
REE Carbonate/Bicarbonate Complexes
• Carbonate and bicarbonate form strong complexes with REEs.
• Carbonate/bicarbonate complexes influence concentration of
free ion.
• The abundance of carbonate rocks in carbonatite-type deposits
(i.e., Mountain Pass, Bear Lodge Mountains) suggests that
carbonate/bicarbonate complexes may reduce REE toxicity.
Modified from Wood (1990)
Role of pH on REE Complexation
• pH plays an important role in abundance of free ion.
• Background water chemistry is expected to mitigate REE toxicity to
aquatic organisms.
Modified from Wood (1990)
Uranium
• The solubility of uranium depends
on the oxidation state of uranium.
• U(VI) is significantly more soluble
than U(IV) at all pH values.
• Complexation of U(VI), especially
with carbonate, significantly
increases solubility.
• Uranium is significantly mobile
under oxidizing conditions.
• Uranium has potential for
recovery as byproduct.
Modified from Langmuir (1997)
Uranium
• U(VI) dominates in
oxidizing surface
environments at all
pH values.
• U(IV) is most common
in anoxic
groundwaters, anoxic
sediments, and
organic rich soils.
Modified from Langmuir (1997)
Thorium
Modified from Langmuir and Herman (1980)
• Thorium is extremely insoluble, except at low pH.
• Lack of sulfide minerals and commonness of carbonate rocks indicate that pH
associated with REE deposits should be high.
• Thorium is more likely to be a solid-phase concern.
• Thorium currently has limited use and little value as potential byproduct*.
*Commercialization of thorium-based reactors (India, China) would expand market.
Environmental Summary
• Knowledge is extremely limited
– Few mines (active, inactive, abandoned) and advanced exploration
projects
• Environmental risks vary among deposit types, but some risks
are common to most types
• Acid-Generating Potential is Low
– Scarce sulfide minerals
– Carbonatites have abundant carbonate minerals
• Th, U, and F may be biggest concerns
• REEs
– Less mature understanding of toxicity compared to base metals
– High alkalinity and low to moderate sulfate concentrations may mitigate
toxic effects
– Low solubility of REEs
• Other trace elements
– Fluoride (As? Cd? Cr? Cu? Ni?)
Environmental Summary (continued)
• Uranium
– Variable solubility in water (redox sensitive)
– Radium (226Ra): windblown dust; surface water and
groundwater?
– Radon
– Potential for byproduct recovery
• Thorium
– Low solubility in water
– Radium (228Ra): windblown dust; surface water and
groundwater?
– Current liability; domestic market limited
– Future development and construction of thorium-based
nuclear reactors in China, India, or France may represent
future market.
Questions?
References
Haxel, G.B., 2005, Ultrapotassic mafic dikes and rare earth- and barium-rich carbonatite at Mountain Pass, Mojave Desert, Southern
California: Summary and field trip localities: U.S. Geological Survey Open-File Report 2005–1219, 4 p. Available at
http://pubs.usgs.gov/of/2005/1219/.
Haxel, G.B., Hedrick, J.B., and Orris, G.J., 2002, Rare Earth Elements—Critical Resources for High Technology: U.S. Geological Survey Fact
Sheet 087-02, 96 p. Available at http://pubs.usgs.gov/fs/2002/fs087-02/.
Jiang He, Chang-Wei Lu, Hong-Xi Xue, Ying Liang, Saruli Bai, Ying Sun, Li-Li Shen, Na Mi, and Qing-Yun Fan, 2010, Species and distribution of
rare earth elements in the Baotou section of the Yellow River in China: Environmental Geochemistry and Health, v. 32, p. 45-58.
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44, p. 1753-1766.
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http://pubs.usgs.gov/sir/2010/5220/.
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niobium, and the rare earth elements in the Thor Lake rare metal deposit, Northwest Territories, Canada: Economic Geology, v. 107,
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Sneller, F.E.C., Kalf, D.F., Weltje, L., and van Wezel, A.P., 2000, Maximum permissible concentrations and negligible concentrations for rare
earth elements (REEs): Rijksinstituut Voor Volksgezondheid En Milieu (National Institute of Public Health and the Environment), RIVM
Report 601501-011, 66 p. Available at: http://rivm.openrepository.com/rivm/bitstream/10029/9551/1/601501011.pdf
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Environmental Toxicology and Chemistry, v. 15, p. 1232-1241.
Verplanck, P.L., and Van Gosen, B.S., 2011, Carbonatite and alkaline intrusion-related rare earth element deposits–A deposit model: U.S.
Geological Survey Open-File Report 2011–1256, 6 p. Available at http://pubs.usgs.gov/of/2011/1256/.
Weltje, L., Verhoof, L.R.C.W., Verweij, W., and Hamers, T., 2004, Lutetium speciation and toxicity in a microbial bioassay: Testing the freeion model for lanthanides: Environmental Science and Technology, v. 38, p. 6597-6604.
Wood, S.A., 1990, The aqueous geochemistry of the rare-earth elements and yttrium: 1. Review of available low-temperature data for
inorganic complexes and the inorganic REE speciation of natural waters: Chemical Geology, v. 82, p. 159-186.