Uranium in the Earth (Enrichment and deposits)

Question 1

What type of elements are U, K and Th?

Answer 1

Large lithophile elements which do not fit into the high temperature forming mineral structures.

Question 2

What happens to U during partial melting at subduction zones?

Answer 2

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.

Question 3

What type of rock is primary enrichment of U usually associated with?

Answer 3

Acidic igneous rock

E.g., Granite (but not all granites have high U contents)

Question 4

What are the 4 types of granite?

Answer 4

1. I-type
2. S-type
3. M-type
4. A-type

Question 5

I-type granite formation and U concentration?

Answer 5

Result from hydrous melting of subducted oceanic crust beneath a continental margin.

Igneous precursor is basaltic.

Low-moderate U-conc (1-3ppm)

Question 6

S-type granite formation and U concentration?

Answer 6

Results from melting of rock in the deep crust in continental collision zones.

Sedimentary precursor materials.

Low-moderate U-conc (1-3ppm)

Question 7

M-type granite formation and U concentration?

Answer 7

Result from hydrous melting of subducted oceanic crust beneath an oceanic margin.

Igneous precursor is basic magma

Low-moderate U-conc (1-3ppm)

Question 8

A-type granite formation and U concentration?

Answer 8

Results from anhydrous melting of sub-continental lithosphere.

Igneous precursor is mantle melt or remelting existing granite rock

High U conc (<20ppm)

Question 9

Whats the relationship between plate tectonics and U-ore formation?

Answer 9

Granite formation is dictated by plate tectonics and U-ores are typically derived from granitic bodies.

Therefore, plate tectonics dictate U-ore formation.

Question 10

What is subsequent enrichment?

Answer 10

Subsequent remobilisation and further concentrating of U occurs in sedimentary rock (ore-deposits).

Enrichment factor - 5 x 10^6

Question 11

When does remobilisation occur?

Answer 11

When primary U(IV) minerals are broken down by weathering and oxidation to produce U(VI), uranyl.

Where uranyl has high solubility and mobility.

Question 12

When does U(VI) precipitate out and form ores?

Answer 12

In reducing environments.

Question 13

What does it mean when a mineral becomes ‘Metamict’?

Answer 13

When a mineral loses crystallinity due to internal radiation damage.
Can occur in U and Th.

Question 14

What happens when granitic rock undergo autogenic alteration?

Answer 14

The mineral crystal structure become ‘vuggy’, easily weathered/broken down.

Question 15

Which 4 ore formation present the greatest U concentration potential?

Answer 15

1. Deep fluids - created in partial melting/dehydration
2. Extreme primary enrichment - crystal fractionation
3. Hydrothermal circulation
4. Sedimentary accumulation

Question 16

What are the 6 types of U deposits in order of highest U ore grade and extent?

Answer 16

1. Unconformity related deposit
2. Sandstone deposit
3. Quartz-pebble conglomerates
4. Vein deposits
5. Breccia complex U deposits
6. Intrusive U deposits

Question 17

How do unconformity related deposits form?

Answer 17

• 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.

Question 18

In unconformity related deposits what are redox fronts and thermal fronts? and role in ore formation?

Answer 18

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

Question 19

How are U and Th initial deposited in sedimentary basin margins for sandstone deposits to occur?

Answer 19

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)

Question 20

How are sandstone ore deposits formed at faults at the edge of sedimentary basins?

Answer 20

• 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

Question 21

How do role front deposits form? Type of sandstone deposit

Answer 21

• Oxidising meteoric water percolates down-dip through permeable layers
• Picks up U(VI) which is mobile
• ppt. U at redox front

Question 22

What state is U in in oxidising conditions?

Answer 22

U(VI), mobile, soluble

In UO22+ form

Question 23

What happens to U oxidation state if you reduce the Eh?

Answer 23

Change the solubility.

Form solid uraninite, UO2 (uranyl in solid form) (+4)

Stable in reducing conditions

Question 24

Characteristics of quartz-pebble conglomerates?

Answer 24

• Metamorphic rock intruded by granite magma

- Low grade

Question 25

How are vein deposits formed?

Answer 25

• Fluid-rock interaction partitions U from granitic parent into hydrothermal fluid.
• Hot fluids in veins rise, cool intrusive rock

High grade, narrow extent

Question 26

Characteristics of breccia complex U deposits?

Answer 26

• Associated with extension of continental lithosphere
• Enormous complexes
• Rich in hematite and magnetite

Question 27

Characteristics of intrusive U deposits?

Answer 27

• Occur in highly evolved alkaline granites with high fractionation
• large bodies, low grade