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03 boberg 2012 iaea - wy uranium province

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03 boberg 2012 iaea - wy uranium province

  1. 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 ofOverthrust 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 1
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  3. 3. Wyoming Province Geologic Column Uranium Production Uranium Occurrence 3From Boberg, 2010
  4. 4. Wyoming Uranium Province 4
  5. 5. Roll-Front Geology 5
  6. 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 6
  7. 7. Late Cretaceous 7
  8. 8. Early Paleocene 8
  9. 9. Late Early Paleocene 9
  10. 10. Late Paleocene 10
  11. 11. Early Eocene 11
  12. 12. Early Medial Eocene 12
  13. 13. Later Medial Eocene 13
  14. 14. Early Oligocene 14
  15. 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 basinsCentral Wyoming Precambrian could easily have generated sufficient uranium to form the Wyoming uranium districts 15
  16. 16. Sediment Thickness in Wyoming Basins Maximum Thickness of Sediments Basin Paleocene - Oligocene - Eocene PliocenePowder River Basin 1,500 meters 300 metersBig Horn Basin 2,600 meters 2,000 metersWind River Basin 5,200 meters 1,500 metersShirley Basin 200 meters 300 metersGreat Divide Basin 3,000 meters 600 metersGreen 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 16
  17. 17. Western US Tertiary Volcanism Great Basin MarysvaleVolcanism from Mid Eocene (52 Ma) to Quaternary (>1Ma)White River deposition during Early to Mid Oligocene (37-30 Ma) 17
  18. 18. Oligocene White River Formation 18
  19. 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 systemWyoming production + resources = 0.25 M t U3O8 or 0.22 M t U (563 million pounds U3O8) 19
  20. 20. Age Dates of Wyoming Uranium Deposits “SB” “CG” “GH” “PRB” 20From Boberg, 2010
  21. 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 basinsModified from Boberg, 1981 21
  22. 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 fallsModified from Boberg, 1981 22
  23. 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 precipitateModified from Boberg, 1981 23
  24. 24. Late Eocene Drainage and White River Formation Deposition 24
  25. 25. Oligocene Drainage and White River Formation Deposition 25
  26. 26. Coincident Factors Change in Porosity and Permeability of Sediments Over Time Temperature Exposure of Precambrian
  27. 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. 27

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