Uranium in the Earth

Question 1

What is the cosmic origin of uranium?

Answer 1

U in the earth originally came from one or more supernova (<6Ga).
Most U is on terrestrial planets, little in the sun.

Question 2

Where is U predominantly found in the earth?

Answer 2

Enriched continental crust - 1.4ppm

then…
Depleted mantle - 0.004 ppm
Primitive mantle - 0.021ppm
Core - 0ppm

Question 3

Why is U in the core assumed to be 0ppm?

Answer 3

Assumed U was not partitioned into the core during formation as U was incompatible.
U enriched the continental crust.

Question 4

What is earths primary energy source?

Answer 4

Radioactive decay of U, Th and K = 24TW of heat
Drives mantle convection.

U-238 generates >90% of mantle heat.

Question 5

What is earths secondary energy source?

Answer 5

Heat released during progressive solidification of the core.
Drives outer-core convection and generates mantle plumes.

Question 6

Where causes most heat loss from the earth?

Answer 6

Mainly due to plate activity.

But radioactive decay means heat producing material is reducing.

Question 7

How did earths changing conditions effect uranium oxidation states and U ore deposits?

Answer 7

Originally reducing conditions dominated:
- U insoluble, stable at U(IV) in UO2.

2. 5 Ga, conditions shift to oxidising:
   - U oxidised to U(VI), becomes soluble

Since 2.5Ga, U ore deposits formed when U-bearing fluids have been reduced.

Question 8

What is the current U supply and demand?

Consumption, usage, mining

Answer 8

Global consumption: ~60,000 tonnes/year

Usage: expected to last 70 more years

• could extract from seawater
• reuse U by burning up more and enriching
• utilise Th to produce U

Mining:

• Underground mining for U has decreased
• In-situ mining has risen

Question 9

Uranium chemical characteristics

Answer 9

Heaviest naturally occurring element:
- Atomic weight = 238.07g

Isotopes:
U-234 (0.01%)
U-235 (0.71%)
U-238 (99.28%)

4 valences: +3, +4, +5, +6
U(IV) and U(VI) most common.

UO22+ = uranyl ion (+6) mobile.

Question 10

What are the primary uranium ore associated deposits? and their characteristic colours?

Answer 10

Usually U(IV) based minerals e.g., Uraninite and Pitchblende.
Also U(IV) silicate based minerals e.g., Coffinite and Thorite. 

Typically brown/green/black

Question 11

How are secondary uranium minerals formed?

Answer 11

Formed from the weathering and oxidation of UO2 and U3O8 (uraninite and pitchblende).

Question 12

What are the secondary U minerals? and their characteristic colours?

Answer 12

U is in oxidised mobile state, U(VI).
Known as uranyl minerals (+6).

e.g., Carnotite and Torbenite

Tend to be bright yellow/green colours

Question 13

What is the definition of a uranium occurrence?

Answer 13

A naturally occurring, anomalous concentration of uranium.

Question 14

What is the definition of a uranium deposit?

Answer 14

A mass of naturally occurring mineral from which uranium could be exploited either presently or in the future.

Question 15

What are the 5 types of uranium deposits?

Answer 15

1. Vein deposits
2. Intrusive deposits
3. Unconformity related deposits
4. Sandstone deposits
5. Hematite-Breccia complex deposits

Question 16

What are the 3 main uranium deposit associations?

Answer 16

1. Igneous plutonic + volcanic associations (related to primary enrichment)
2. Metamorphic association
3. Sediment + sedimentary basin associations

Question 17

Brief overview of primary enrichment

Answer 17

At Mid-Ocean ridges, get preferential incorporation of U into hydrothermally altered oceanic crust. This is initial U enrichment.

At subduction zones (plate recycling), U, K and Th are preferentially liberated from oceanic crust into hydrous fluids and volatiles.

Volatiles rise in mantle wedge, cause partial melting. Therefore, get U-enriched magma rising to base of overlying crust and intrude crystalline basement rock.

= Partitioned into crust
= Complex fractionation