Ores

1
Ores

• Principally we discuss ores as sources of metals
• However, there are many other resources bound in
  minerals which we find useful
• How many can we think of?

2
Ore Deposits

• A deposit contains an unusually high
  concentration of particular element(s)
• This means the element(s) have been concentrated
  in a particular area due to some process
• What sort of processes might concentrate these
  elements in one place?

3
Gold ? Au

• Distribution of Au in the crust 3.1 ppb by
  weight ? 3.1 units gold / 1,000,000,000 units of
  total crust 0.00000031 Au
• Concentration of Au needed to be economically
  viable as a deposit few g/t ? 3 g / 1000kg
  3g/ 1,000,000 g 0.00031 Au
• Need to concentrate Au at least 1000-fold to be a
  viable deposit
• Rare mines can be up to a few percent gold
  (extremely high grade)!

4
Ore minerals

• Minerals with economic value are ore minerals
• Minerals often associated with ore minerals but
  which do not have economic value are gangue
  minerals
• Key to economic deposits are geochemical traps ?
  metals are transported and precipitated in a very
  concentrated fashion
• Gold is almost 1,000,000 times less abundant than
  is iron

5
Economic Geology

• Understanding of how metalliferous minerals
  become concentrated key to ore deposits
• Getting them out at a profit determines
  where/when they come out

6
Ore deposit environments

• Magmatic
• Cumulate deposits fractional crystallization
  processes can concentrate metals (Cr, Fe, Pt)
• Pegmatites late staged crystallization forms
  pegmatites and many residual elements are
  concentrated (Li, Ce, Be, Sn, and U)
• Hydrothermal
• Magmatic fluid - directly associated with magma
• Porphyries - Hot water heated by pluton
• Skarn hot water associated with contact
  metamorphisms
• Exhalatives hot water flowing to surface
• Epigenetic hot water not directly associated
  with pluton

7
Geochemical Traps

• Similar to chemical sedimentary rocks must
  leach material into fluid, transport and deposit
  ions as minerals
• pH, redox, T changes and mixing of different
  fluids results in ore mineralization
• Cause metals to go from soluble to insoluble
• Sulfide (reduced form of S) strongly binds metals
  ? many important metal ore minerals are sulfides!

8
Hydrothermal Ore Deposits

• Thermal gradients induce convection of water
  leaching, redox rxns, and cooling create economic
  mineralization

9
Ore deposit environments

• Sedimentary
• Placer weathering of primary mineralization and
  transport by streams (Gold, diamonds, other)
• Banded Iron Formations 90 of worldsiron tied
  up in these (more later)
• Evaporite deposits minerals like gypsum, halite
  deposited this way
• Laterites leaching of rock leaves residual
  materials behind (Al, Ni, Fe)
• Supergene reworking of primary ore deposits
  remobilizes metals (often over short distances)

10
Ore Deposit Types I

• Placer uranium gold
• Stratiform phosphate
• Stratiform iron
• Residually enriched deposit
• Evaporites 
• Exhalative base metal sulphides 
• Unconfornity-associated uranium 
• Stratabound clastic-hosted uranium, lead, copper 
• Volcanic redbed copper 
• Mississippi Valley-type lead-zinc 
• Ultramafic-hosted asbestos  
• Vein uranium 
• Arsenide vein silver, uranium 
• Lode Gold 

11
Ore Deposit Types II

• Clastic metasediment-hosted vein
  silver-lead-zinc 
• Vein Copper
• Vein-stockwork tin, tungsten
• Porphyry copper, gold, molybdenum, tungsten, tin,
  silver
• Skarn deposits 
• Granitic pegmatites 
• Kiruna/Olympic Dam-type iron, copper, uranium,
  gold, silver 
• Peralkaline rock-associated rare metals
• Carbonatite-associated deposits
• Primary diamond deposits 
• Mafic intrusion-hosted titanium-iron 
• Magmatic nickel-copper-platinum group elements 
• Mafic/ultramafic-hosted chromite