Ores

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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?

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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?

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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)!

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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

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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

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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

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Ore deposit environments

• Sedimentary
• Placer weathering of primary minerals and
  transport by streams (Gold, diamonds, other)
• Banded Iron Formations 90 of worlds iron
  tied up in these
• 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)

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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
• Sulfides (reduced form of S) strongly binds
  metals ? many important metal ore minerals are
  sulfides!
• Oxides Oxidizing environments form
  (hydroxy)oxide minerals, very insoluble metal
  concentrations (especially Fe, Mn, Al)

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Hydrothermal Ore Deposits

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

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Massive sulfide deposits

• Hot, briny, water leaches metals from basaltic
  ocean rocks
• Comes in contact with cool ocean water
• Sulfides precipitate ?

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Vermont Copperbelt

• Besshi-type massive sulfide deposits
• Key Units
• Giles Mountain formation More siliciclastic,
  including graphitic pelite, quartoze granofels
  (metamorphosed greywacke), hornblende schist,
  amphibolite
• Standing Pond Volcanics mostly a fine grained
  hormblende-plagioclase amphibolite, likely formed
  from extrusive basaltic rocks (local evidence of
  pillow structures in St. Johnsbury). Felsic dike
  near Springfiled VT yielded a U-Pb age of 423 4
  Ma.
• Waits River formation Calcareous pelite
  (metamorphosed mudstone), metalimestone,
  metadolostone, quartzite.

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Minerals associated with economically recoverable
metals

• Elemental forms
• Sulfides
• Oxides
• Carbonates
• Sulfate salt

Cuprite, Cu2O
Elemental copper
Malachite, Cu2CO3(OH)2
Chalcocite, Cu2S
Chalcanthite, CuSO45H2O
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Sulfides Part 1

• Substitution into sulfides is very common
• As and Se substitute for S very easily
• Au can substitute in cation sites (auriferrous
  minerals)
• Different metals swap in and out pretty easily ?
  Cu and Fe for instance have a wide range of solid
  solution materials

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Sulfide Minerals

• Minerals with S- or S2- (monosulfides) or S22-
  (disulfides) as anionic group
• Transition metals bonded with sulfide anion groups

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Iron Sulfides

• Mackinawite FeS
• Greigite FexSy
• Pyrite FeS2 (cubic)
• Marcasite FeS2 (orthorhombic)
• Troilite FeS end member
• Pyrrhotite Fe1-xS (slightly deficient in iron)
• Arsenopyrite FeAsS
• Chalcopyrite CuFeS2

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Other important sulfides

• Galena PbS
• Sphalerite/wurtzite ZnS
• Cinnabar HgS
• Molybdenite MoS
• Covellite CuS
• Chalcocite Cu2S
• Acanthite or Argenite AgS
• Stibnite Sb2S3
• Orpiment As2S3 Realgar AsS

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Sulfides are reduced minerals ? what happens when
they contact O2?

• This is the basis for supergene enrichment and
  acidic mine drainage

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Actively Oxidizing Pyrite

• FeS2 3.5 O2 H2O ? Fe2 2 SO42- 2 H
• FeS2 14 Fe3 8 H2O ? 15 Fe2 2 SO42- 16
  H
• 14Fe2 3.5 O2 14H ? 14 Fe3 7 H2O
• Sulfur species and H generation
• FeS2 2 Fe3à 3 Fe2 ¼ S8 0 H
• FeS2 7 Fe3 3 H2Oà 8 Fe2 0.5 S4O62- 6 H

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AMD neutralization

• Metals are soluble in low pH solutions can get
  100s of grams of metal into a liter of very
  acidic solution
• HOWEVER eventually that solution will get
  neutralized (reaction with other rocks, CO2 in
  the atmosphere, etc.) and the metals are not so
  soluble ? but oxidized S (sulfate, SO42-) is very
  soluble
• A different kind of mineral is formed!

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Ely Mine
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