Uranium in the Earth (Enrichment and deposits) Flashcards
What type of elements are U, K and Th?
Large lithophile elements which do not fit into the high temperature forming mineral structures.
What happens to U during partial melting at subduction zones?
When silicate rocks melt, U is preferentially liberated as it doesn’t fit into the lattice structure well (large lithophile element).
Fractional crystallisation occurs and U is left in melt phase.
As melt cools, silica rich compounds are formed by U, K and Th are left behind due to incompatibility.
What type of rock is primary enrichment of U usually associated with?
Acidic igneous rock
E.g., Granite (but not all granites have high U contents)
What are the 4 types of granite?
- I-type
- S-type
- M-type
- A-type
I-type granite formation and U concentration?
Result from hydrous melting of subducted oceanic crust beneath a continental margin.
Igneous precursor is basaltic.
Low-moderate U-conc (1-3ppm)
S-type granite formation and U concentration?
Results from melting of rock in the deep crust in continental collision zones.
Sedimentary precursor materials.
Low-moderate U-conc (1-3ppm)
M-type granite formation and U concentration?
Result from hydrous melting of subducted oceanic crust beneath an oceanic margin.
Igneous precursor is basic magma
Low-moderate U-conc (1-3ppm)
A-type granite formation and U concentration?
Results from anhydrous melting of sub-continental lithosphere.
Igneous precursor is mantle melt or remelting existing granite rock
High U conc (<20ppm)
Whats the relationship between plate tectonics and U-ore formation?
Granite formation is dictated by plate tectonics and U-ores are typically derived from granitic bodies.
Therefore, plate tectonics dictate U-ore formation.
What is subsequent enrichment?
Subsequent remobilisation and further concentrating of U occurs in sedimentary rock (ore-deposits).
Enrichment factor - 5 x 10^6
When does remobilisation occur?
When primary U(IV) minerals are broken down by weathering and oxidation to produce U(VI), uranyl.
Where uranyl has high solubility and mobility.
When does U(VI) precipitate out and form ores?
In reducing environments.
What does it mean when a mineral becomes ‘Metamict’?
When a mineral loses crystallinity due to internal radiation damage.
Can occur in U and Th.
What happens when granitic rock undergo autogenic alteration?
The mineral crystal structure become ‘vuggy’, easily weathered/broken down.
Which 4 ore formation present the greatest U concentration potential?
- Deep fluids - created in partial melting/dehydration
- Extreme primary enrichment - crystal fractionation
- Hydrothermal circulation
- Sedimentary accumulation
What are the 6 types of U deposits in order of highest U ore grade and extent?
- Unconformity related deposit
- Sandstone deposit
- Quartz-pebble conglomerates
- Vein deposits
- Breccia complex U deposits
- Intrusive U deposits
How do unconformity related deposits form?
- Below the unconformity, metasedimentary rock is faulted and brecciated.
- Allows oxidising metal rich fluid to flow, which can dissolve U out of rock its in contact with
- U deposited at/near the unconformity horizon as the fluid cools.
In unconformity related deposits what are redox fronts and thermal fronts? and role in ore formation?
Redox front:
- Ores form when oxidising metal rich fluid meets a redox front (two fluid concept)
- Redox associated with reducing C-rich fluid
- Mineralls ppt. in pore spaces
Thermal redox front:
- Old cold fluid (groundwater) meets new hot fluid at unconformities
- Get mixing zone
How are U and Th initial deposited in sedimentary basin margins for sandstone deposits to occur?
Granitic rock provides U and Th, which gets washed into sedimentary basin margins and deposited due to high densities.
Then get deposition on top (cap rock)
How are sandstone ore deposits formed at faults at the edge of sedimentary basins?
- Cold, oxidising meteoric water penetrates and heats up
- Reaches natural buoyancy, can move laterally
- Percolates through U and Th containing conglomerates
- Dissolves U and Th, rises as pregnated U
- Hits base of cap rock, get redox front and U ppt. out
High grade U ore deposits
How do role front deposits form? Type of sandstone deposit
- Oxidising meteoric water percolates down-dip through permeable layers
- Picks up U(VI) which is mobile
- ppt. U at redox front
What state is U in in oxidising conditions?
U(VI), mobile, soluble
In UO22+ form
What happens to U oxidation state if you reduce the Eh?
Change the solubility.
Form solid uraninite, UO2 (uranyl in solid form) (+4)
Stable in reducing conditions
Characteristics of quartz-pebble conglomerates?
- Metamorphic rock intruded by granite magma
- Low grade
How are vein deposits formed?
- Fluid-rock interaction partitions U from granitic parent into hydrothermal fluid.
- Hot fluids in veins rise, cool intrusive rock
High grade, narrow extent
Characteristics of breccia complex U deposits?
- Associated with extension of continental lithosphere
- Enormous complexes
- Rich in hematite and magnetite
Characteristics of intrusive U deposits?
- Occur in highly evolved alkaline granites with high fractionation
- large bodies, low grade
