How Do Precious Metals Occur in Nature

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How Do Precious Metals Occur in Nature
platinum
gold
silver
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The Precious Metals

• Gold, Silver, Platinum, Rhodium and Palladium are
  considered the precious metals, or the noble
  metals.
• All the precious metals are elements, being pure
  substances, and make up a tiny percentage of the
  earths crust.
• Some geological processes can provide situations
  whereby these precious metals become concentrated
  into economically significant ore bodies.
• Gold occurs in nature in the free state often
  alloyed with Copper and or Silver.
• Gold rarely occurs with Platinum group elements.

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Primary Gold Deposition
Epithermal
Porphyry
Mesothermal
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Tectonic Setting of Epithermal and Porphyry,
Precious Metals deposits
Located in orogenic belts at convergent plate
margins with subduction-related magmatism and
related geothermal systems
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Epithermal Gold Deposits

• Important economic sources of gold, especially in
  the circum-Pacific region
• High-grade vein to low-grade bulk tonnage
  deposits,
• most in Tertiary volcanic terranes
• Two major types
• high sulphidation low sulphidation
  (adularia-sericite)

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Low-sulphidation epithermal gold deposits

• Tectonic setting volcano-plutonic continental
  margin and oceanic arcs, and back arcs
• Geological setting at shallow depths in regional
  faults systems, grabens, calderas.

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• Host rocks commonly Andesite, as noted down the
  western margin of South America.
• Age of host rocks any, most Tertiary and
  Quaternary recent few million years
• Deposit form veins and stockworks grades vary
  from lt1 to gt100 g/t very fine-grained gold
• Ore controls faults, shear zones, permeable
  lithologies, breccias.

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• Ore minerals - electrum (gold and silver
  together), gold and silver.
• Other ore minerals associated to this system
  Zinc, Lead, Copper.

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Schematic cross-section showing the internal
workings of a typical low-sulphidation deposit
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Schematic cross section showing the internal
workings of ahigh-sulphidationepithermal deposit
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Porphyry copper-Gold deposits

• Source material starts at the spreading ridge.
• Sulfides carried up by mantel derived fluids, and
  precipitated when in contact with lower temp and
  sea water.
• As the plate encounters a continental margin it
  is forced down due to isostatic balances.
• The downward movement causes the plate to heat up
  and drive off volatiles which dissolve sulphides
  and take them up, similar to a bubble.

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Porphyry copper-Gold deposits
Rising volatiles dissolve minerals from the mafic
oceanic rocks derived from the spreading ridge
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Porphyry copper-Gold deposits

• The rising volatile soup melts overlying rocks
  which drop into the mix as it rises.
• A point is reached in the ascent where the drop
  in pressure causes the volatiles to boil and
  exert pressure on the confining country rock.
• The result is a sudden fracturing and brecciation
  with the boiling fluids moving into the voids.
• The drop in temperature causes the sulphides
  gold and silver to precipitate, or drop out of
  solution.
• Forms the breccia ores and stock works.

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Breccia ore
Porphyry copper-Gold deposits
Stock work ore
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Mesothermal Gold deposits

• Major sources of gold worldwide, in Archaean
  greenstone belts (Canada, Western Australia)
• Greenstone belts resulting from large slabs of
  oceanic crust at the accretionary wedge, caught
  up in rising granites.
• Generally associated with large regional
  structures, such as transcurrent faults and major
  brittle ductile shear zones.
• Emplaced at depths of 5-10 km, temperatures
  250-350o C

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Mesothermal Gold deposits

• The gold ore bodies are the result of faults
  causing rocks to come apart, dilate, and the void
  then filled with quartz carrying gold and
  sulphides (if present).
• This can happen repeatedly as each fault movement
  during the accretion at the plate boundaries
  causes a new pulse of ore bearing fluids into the
  reactivated fault or void.
• Process known as Seismic Pumping.
• Results in quartz carbonate veins, with
  high-grade ore shoots vertically and laterally
  continuous.

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Placer deposits

• Erosion of gold bearing rocks can accumulate
  enough heavy material to form a placer deposit.
• A mineral with a high specific gravity, such as
  gold and silver, will become concentrated by
  flowing water.
• Deposits of minerals having high specific
  gravities are known as placers.
• Most placers are found in stream gravels that are
  geologically young.

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Some types of placer mechanics in river systems
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• The most important minerals concentrated in
  placers are gold, platinum, cassiterite (Tin
  SnO2), and diamonds.
• More than half of the gold recovered throughout
  all of human history has come from placers.
• Example of large alluvial system is the Tapajos
  region of the Amazon

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The South African placer deposits

• The South African fossil placers are a series of
  gold-bearing conglomerates.
• Nothing like the deposits in the Witwatersrand
  basin has been discovered anywhere else.
• They were laid down 2.7 billion years ago as
  gravels in the shallow marginal waters of a
  marine basin.
• Mining the Witwatersrand basin has reached a
  depth of 3600 m.
• The deposit contains 100s millions of ounces of
  gold.

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Platinum Group Elements or Platinum Group Metals

• Sixty times rarer than gold, platinum is only
  found in a few locations worldwide - Russia's
  Ural Mountains, South Africa's Merensky Reef and
  a few small mines in the US and Canada.

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• The platinum-group metals (PGM) include iridium,
  osmium, palladium, platinum, rhodium and
  ruthenium.
• PGM are among the rarest of elements, and their
  market values particularly for palladium,
  platinum and rhodium are the highest prices of
  all precious metals.

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• PGMs are geochemically similar and tend to
  behave as a one group in geologic processes.
• occur as native metals as well as in sulfide and
  arsenide minerals
• Two important geologic settings associated with
  mafic and ultramafic rocks concentrated in
  chromite layers and sulfides (e.g. Sudbury,
  Bushveld, Stillwater)
• Concentrated in placer deposits are dense and
  corrosion resistant

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Bushveld a model for platinum