Economic Geology

Question 1

What is an Earth Resource?

Answer 1

A material formed from Earth processes that is used in our daily lives
• Resources: All materials in Earth: discovered or not
o Economic
o Sub economic
o Hypothetical undiscovered deposits

Question 2

What is a reserve? And how does a resource become a reserve?

Answer 2

Discoverable, minable, economical and legal
o Fraction of resources known to exist
• Resource to a reserve?
o Ore: concentration of element above background (enrichment factor)
o Recovery: profitable
o Physical: Water, energy, infrastructure
o Human: Politics, taxation and environment

Question 3

Grade mined?

Answer 3

Percentage in rock to be worth mining

Question 4

Pegmatite formation?

Answer 4

o Molten magma – low water content
o All water from molten magma moves into a smaller area of melt due to crystallisation
o Increases water content
o Cools slower and crystallises slowly = large crystals

Question 5

Example of direct crystallisation?

Answer 5

• Uranium – found in accessory minerals – Zircon
• Ores found in granite body are higher than background volume from melting of crystal rock (shale, mudrock)
• Namibia

Question 6

Formation of Kimberlites?

Answer 6

• Mixture of melt from the mantle and fragments of mantle picked up on pathway to surface
• Xenoliths (rocks) and xeoncryst (crystals)
• Diamonds are xeoncrysts
• Pipeline bodies
• Rapid emplacement
• Located in areas of old, stable lithosphere – Intraplate regions – cratons – Canada and West, Central and South Africa

Question 7

Hydrothermalism on the seafloor?

Answer 7

• Mid Oceanic Ridge
• Black smokers – black because sulphur and metal rich
• Minerals form which settle out based on density
• Expect Cu-Mo, Au, Ag, Pb and Zn

Question 8

Porphory-ore formation?

Answer 8

• Fluids from volcanic arc magmas with high water contents (3 wt %)
• As wet magma cools water becomes concentrated in magma which
o Separates out as bubbles and rises towards the top of the magma chamber
o Water scavenges soluble metals from magma
• Magma ascends through crust:
o Reduces pressure
o Causes water to boil and form steam
o Sudden increases in volume
o Enormous pressure (lithostatic) on enclosing rocks
o Hydrofracture forms a stockwork vein system
• Rapid fluid escape
o Increases crystallisation causing magma to crystallise more rapidly
o Produces porphyritic texture and deposition of ore minerals in fractures
• Repeated process produces altered and mineralised stockwork
o Early mineralisation – magmatic fluids – metal rich
o Later mineralisation – a mixture of magmatic and meteoric fluids

Question 9

Example of diamonds?

Answer 9

• Canada - Glaciation – ice sheets retreated = scowered top surface – moved kimberlite and diamond deposits
• Orange River – Diamonds from Kimberly have moved down Orange river to sea – dredging sea floor