1. Yellowstone
Absoraka
The Wyoming Uranium Province
Plateau Black
Hills
A Case Study on the Origin of
Sandstone Uranium Deposits
IAEA Technical Meeting on the Origin of
Overthrust Belt
Sandstone Uranium Deposits: A Global Perspective
Granite Mountains
28 May – 1 June 2012, Vienna,Shirley
Austria
Basin
Greater Great
Green River Divide
Basin Basin
W. William Boberg, Boberg GeoTech International Ltd., Denver, Colorado USA
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3. Wyoming Province Geologic Column
Uranium Production
Uranium Occurrence
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From Boberg, 2010

4. Wyoming Uranium Province
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5. Roll-Front Geology
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6. Geologic Development of the
Wyoming Uranium Province
 Archean intrusion of uraniferous granites derived from partial
melting of pre-existing metamorphic rocks
 Laramide deformation resulting in development of basins and
ranges
 Exposure of Precambrian granitic rocks in cores of mountain
ranges and their extensive weathering and erosion, depositing
thick arkosic sediments in adjacent basins
 Tertiary volcanism throughout the western United States
depositing extensive amounts of rhyolitic volcanic ash across
the region for more than 45 million years
 Formation of the mineralizing fluid at surface and near surface
and transported by paleodrainage systems to ground water
recharge areas where the fluid could enter the subsurface
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7. Late Cretaceous
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8. Early Paleocene
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9. Late Early Paleocene
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10. Late Paleocene
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11. Early Eocene
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12. Early Medial Eocene
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13. Later Medial Eocene
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14. Early Oligocene
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15. Archean Granitic Rocks
As a Source of the Uranium
 Most prominent uranium districts worldwide are associated
with Precambrian rocks
 Major Wyoming uranium districts surround Archean
granitic highlands (Granite Mountains and N. Laramie Range)
 Average U content of Granite Mountains 2-3 times average
 11.5 ppm U in biotite granite
 8.6 ppm U for leucocratic granite
 Granite Mountains granites demonstrate significant loss
 Loss of 10-45% U during 1700-1400 Ma
 Additional loss of ≥70% U during the Laramide orogeny
 Huge volumes of granitic debris deposited in adjacent basins
Central Wyoming Precambrian could easily have generated
sufficient uranium to form the Wyoming uranium districts
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16. Sediment Thickness in Wyoming Basins
Maximum Thickness of Sediments
Basin
Paleocene - Oligocene -
Eocene Pliocene
Powder River Basin 1,500 meters 300 meters
Big Horn Basin 2,600 meters 2,000 meters
Wind River Basin 5,200 meters 1,500 meters
Shirley Basin 200 meters 300 meters
Great Divide Basin 3,000 meters 600 meters
Green River Basin 2,700 meters 600 meters
• Paleocene sediments are predominantly weathered sedimentary rocks from highlands
• Eocene sediments are predominantly weathered granitic and metasedimentary rocks from
highlands
• Oligocene-Pliocene sediments are predominantly volcanic tuffaceous rocks mixed with
weathered granitic and metasedimentary rocks from highlands
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17. Western US Tertiary Volcanism
Great Basin
Marysvale
Volcanism from Mid Eocene (52 Ma) to Quaternary (>1Ma)
White River deposition during Early to Mid Oligocene (37-30 Ma) 17

18. Oligocene White River Formation
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19. Oligocene White River Formation
As a Source of the Uranium
 Area of the Powder River Basin = 31,337 km2
 Covered with 150 m of ash (50% bulk porosity)
 0.4 ppm loss of uranium from ash
 Result - 2.38 M t U released from the ash
 Area of White River outcrop = 452,300 km2
 Current maximum thickness = >300 m
 Result - 68.5 M t U released from the ash
One major ash fall formation could have released 68.5 M t U
(150,000 million pounds U3O8) into the hydrologic system
Wyoming production + resources = 0.25 M t U3O8 or 0.22 M t U
(563 million pounds U3O8)
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20. Age Dates of Wyoming Uranium Deposits
“SB”
“CG”
“GH”
“PRB”
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From Boberg, 2010

21. Formation of Wyoming Uranium Deposits
Host Rock Preparation
 Uplift of mountains, exposure of uranium-rich Precambrian core
 Deep weathering of Precambrian core
 Erosion and deposition of sediments in adjacent downwarping
basins
Modified from Boberg, 1981 21

22. Formation of Wyoming Uranium Deposits
Generation of Mineralizing Fluid
 Intermittent regional volcanism over millions of years
 Ash falls deposited over entire region
 Exotic fluids created by first rainfall after each ash fall
 Uranium leached from uranium-rich Precambrian core rocks
 Uranium leached from various ash falls
Modified from Boberg, 1981 22

23. Formation of Wyoming Uranium Deposits
Emplacement of Uranium Deposit
-
 Uranium enriched fluids carried by streams off the mountains
 Fluids enter recently deposited porous & permeable sediments
 Uranium carried in groundwater until buffering with sediments
exhausts oxygen, changing redox potential causing uranium to
precipitate
Modified from Boberg, 1981 23

24. Late Eocene Drainage and White River
Formation Deposition
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25. Oligocene Drainage and White River
Formation Deposition
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26. Coincident Factors
Change in
Porosity and
Permeability
of Sediments
Over Time
Temperature
Exposure of
Precambrian

27. Wyoming Uranium Province
Summary - Concept of Formation
 Creation of ore-forming fluid as surface or near-surface water
sourced within tuffaceous ash fall units and/or Precambrian rocks
 Transport of uranium within pathways of paleodrainage systems
 Ore-forming fluid enters subsurface in areas of ground water
recharge (recently deposited sediments, older permeable strata
or brecciated zones in other rock types
 Flow of oxygenated ore-forming fluid forming an
oxidized/altered tongue within sedimentary rocks leading to
deposition of uranium at a redox interface as roll-front deposits.
 Repetition of the above process multiple times. Changing
positions of pathways of paleodrainage systems carrying ore-
forming fluid to newly exposed areas of ground water recharge
creating new roll-fronts or adding to existing roll-fronts.
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