Uranium in the Earth

Question 1

What are the relative concentrations of U in the earth (core, crust, mantle and primitive mantle)?

Answer 1

continental crust = enriched
mantle = depleted
primitive mantle = slightly enriched
core + none

Question 2

What are the primary and secondary energy sources of the Earth?

Answer 2

Primary = radioactive decay of uranium, thorium and potassium 
secondary = heat released during progressive solidification of the core (latent heat of crystallisation)

Question 3

What is fuel ‘burn up’

Answer 3

squeezing more fuel out of recovered materials

Question 4

What are the primary U ore minerals?

Answer 4

Uraninite and pitchblende (predominantly UO2 containing reduced U (U4)

Question 5

What is a uranium occurrence?

Answer 5

A naturally occurring anomalous concentration of uranium

Question 6

what is a uranium deposit?

Answer 6

a mass of naturally occurring mineral from which U can be exploited

Question 7

what are the 5 geological types of U deposits?

Answer 7

1. vein deposits
2. intrusive deposits
3. unconformity-related deposits
4. sandstone deposits
5. hematite breccia complex deposits

Question 8

What are the 3 main associations for U deposits?

Answer 8

1. Igneous plutonic and volcanic association –> primary enrichment
2. metamorphic associations
3. sedimentary and sed. basin associations

Question 9

Describe primary enrichment in the continental crust

Answer 9

• basic magma rises through cont. crust and undergoes - mobilisation, assimilation, storage, homogenisation and fractionation
• preferential incorporation of U in hydrothermally altered ocean crust
• subduction zone fractionation - U and Th and K preferentially removed from slab into hydrous fluids and volatile streams
• partitioned into crust above subjecting zones
• U,K and Th are LILEs (large litho-hole elements) and do not readily fit into the high T forming minerals
• during magma fractionation in the crust, U, K and Th are partitioned into the residual melt

Question 10

How was all cosmic U produced?

Answer 10

During a supernovae rapid capture of neutrons on seed nuclei att rates greater than disintegration through radioactivity produced U

Question 11

Which countries host most of the worlds U deposits?

Answer 11

Australia
Kazakhstan
Canada

Question 12

What is primary enrichment?

Answer 12

certain types of highly evolved granite, rhyolite and alkaline complexes contain exceptionally high levels of radionuclides

Question 13

what are the 4 types of granite with primary enrichment?

Answer 13

I-type = igneous precursor, infracrustal granites
S-type = sedimentary photolithography, supracrustal granites
M-type = Mantle derived
A-type = Anorogenic granites
type of granite formation controls U concentration

Question 14

What are I-type granites?

Answer 14

Results from hydrous melting of subjected oceanic crust beneath a continental margin
e.g. Andes, S America
Igneous precursor is typically basaltic but granites form from progressive fractionation (up to 50%), assimilation
Give low to moderate U concentration 1-3ppm, or high when evolved (<22 ppm )

Question 15

What are S type deposits?

Answer 15

Result from melting of rocks in the deep crust in continental collision zones
e.g., Himalayas - continent/continent collision
Often a sedimentary (supracrustal) precursor material, so U levels are close to average crustal abundances.
Low to moderate U conc: 2-3ppm

Question 16

What are M-type granites?

Answer 16

Result from hydrous melting of subjected oceanic crust beneath an oceanic margin e.g., Bahamas
Igneous precursor is basic magma. Granites form from progressive fractionation and assimilation
Low to moderate U concentrations 1-3 ppm

Question 17

What are A type granites?

Answer 17

Result from anhydrous melting of sub-continental lithosphere occurring ‘within-platte’. related to extension and fracturing (orogenic collapse) e.g., E Africa Rift Valley. Crustal thinning induce melting at base of crust and top of mantle and pushes through fault zones to come to surface.
Igneous precursor are mantle metals at deeper levels or remelting of existing granitic rocks at higher levels.
High U conc: <20ppm

Question 18

Describe subsequent enrichment of uranium bearing rocks

Answer 18

the primary source of U enrichment is in silicic and alkaline igneous rocks.
Subsequent remobilisation and further concentration of U occurs in sedimentary rocks (ore deposits).
Remobilisation occurs when primary U4 minerals are broken down by weathering (wet oxidation) producing uranyl U6 species with high solubility and mobility
U6 does not persist in a reducing environment

Question 19

What does metamict mean?

Answer 19

amorphous because of the disruption of the crystal structure by radiation from contained or nearby radioactive atoms.
lose crystallinity due to internal radiation.
leaves mineral susceptible to alteration and hydration

Question 20

What er the 4 processes with the greatest U concentration potential?

Answer 20

1. Deep fluids - concentration from fluids generated by dehydration/partial melting reactions in lower crustal rocks
2. Extreme primary enrichment - crystal fractionation in alkaline complexes in continental rift systems
3. Hydrothermal circulation – High to low T fluid-rock interactions associated with emplacement of highly evolved leucogranites and rhyolites
4. Sedimentary accumulation – Ground & surface water flow in relation to evolution of certain types of sedimentary basins. Concentration occurs at redox boundaries.

Question 21

What are the 3 main associations of a uranium deposit?

Answer 21

1. Igneous plutonic and volcanic associations –> primary enrichment
2. Metamorphic associations
3. Sediment and sedimentary associations
   In all cases U deposits are attributed to the mobility of the U6 ion.

Question 22

Describe unconformity related deposits

Answer 22

sediment and sedimentary associatio. rocks beneath the unconformity are metasedimentary - often faulted and brecciated
Unconformity: Deposition, uplift, tilting, erosion, deposition
Faults act as channels for fluids to flow. hot fluids –> dissolve rock. fluids get supersaturated in ions and get ppt. under right conditions.
Redox front - two fluid system. pore waters anoxic. GW oxic.
Fault plans concentrate ppt products.
Asymmetrical deposit - elongated in the downs stream direction

Question 23

Describe sandstone deposits

Answer 23

occur in permeable, medium to coarse grained and poorly sorted arkosic and quartzite ssts. occurs in intra-cratonic basins
U associated with organic debris.
Low to medium grade (0.04 -0.4% U)
uses a two-fluid system - toxic/anoxic –> roll front deposits
Use ISL mining - deposits occur in permeable coarse grain rocks
E.g., Oklo Gabon