Field mapping and economic geology

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Field mapping and economic geology

• Lecture 12 - Sedimentary Deposits - Placers

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Placer deposits

• In geology, a placer deposit is a deposit of
  earth, sand, or gravel, containing valuable
  minerals in particles, especially by the side of
  a river, or in the bed of a mountain stream.
  Placers may also be found at the oceanside,
  deposited by the action of waves.
• Placer mining is an important source of gold, and
  was the main technique used in the early years of
  the California gold rush. The name is from the
  Spanish word placer, meaning "pleasure", because
  placer mining is much easier than pick-and-shovel
  mining.
• Placer deposits form due to the differential
  settling of the denser, heavier components of
  moving sand and gravel. Placer materials must be
  both dense, and resistant to weathering
  processes.
• Notably, placer environments typically contain
  black sand, a conspicuous shiny black mixture of
  iron oxides, mostly magnetite with variable
  ilmenite and hematite components. Accessory
  minerals often occurring with black sands are
  monazite, rutile, zircon, chromite, wolframite,
  and cassiterite.
• There are heavy silicates, such as amphibole and
  pyroxene, that will collect in extensive placers
  where they comprise the country rock. Of course,
  exceptionally dense substances like gold, copper,
  silver and the platinum group members will
  accumulate in placers, when they are present.
• http//en.wikipedia.org/wiki/Placer_deposit

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Placer Mining

• Placer mining (pronounced "plass-er") refers to
  the mining of alluvail deposits for minerals.
  This may be done by open-pit or open-cast mining
  or by various forms of tunneling. Excavation may
  be accomplished using water pressure (hydraulic
  mining), surface excavating equipment or
  tunneling equipment.
• The simplest technique to extract gold from
  placer ore is panning. In panning, some mined ore
  is placed in a large metal pan, combined with a
  generous amount of water, and agitated so that
  the gold particles, being of higher density than
  the other material, settle to the bottom of the
  pan. The lighter ore material such as sand, mud
  and gravel are then washed over the side of the
  pan, leaving the gold behind. The same principle
  may be employed on a larger scale by constructing
  a short sluice box, with barriers along the
  bottom to trap the heavier gold particles as
  water washes them and the other material along
  the box. This method better suits excavation with
  shovels or similar implements to feed sediment
  into the device.
• Environmental activists describe the hydraulic
  mining form of placer mining as environmentally
  destructive because of the large amounts of silt
  that it adds to previously clear running streams.
  Most placer mines today use settling ponds, if
  only to ensure that they have sufficient water to
  run their sluicing operations.
• http//en.wikipedia.org/wiki/Placer_mining

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Sorting

• A process of sorting operates which involves
  separation according to grain size and specific
  gravity (SG)
• Important placer mineral include Au, Pt (SG
  17), cassiterite (7), magnetite, monazite,
  ilmenite (5), diamond, garnet (3.5), basic
  silicates (3)
• Compare with feldspar or quartz SG 2.6

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Eluvial Placer Deposits

• Found in the immediate vicinity of primary
  deposits. They are formed from only slightly
  drifted residues with the lighter (eg quartz) and
  more decomposable material (eg feldspars) being
  washed away and is a process of concentration by
  reduction in volume. Eg the platinum deposits of
  the Urals

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Elluvial and Residual Deposits
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Alluvial Placers

• Alluvial placers occur in river beds and form as
  a result of the continual movement of water which
  sorts the detrital grains by their SG
  (winnowing). The heavy minerals move to the
  bottom load of the channel and are the first to
  drop out of suspension
• The ease of extraction of alluvial placers has
  made them an extremely sought after deposit
  they have been the cause of many of the worlds
  greatest gold and diamond rushes

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Alluvial Deposits
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Alluvial Processes

• The heavy mineral fraction of a sediment is much
  finer grained than the light fraction. Several
  reasons
• Many heavy minerals occur in much smaller grains
  than qtz or feldspar in the primary ign or metm
  rocks
• The sorting and composition of a sediment is
  controlled by both the density and the size of
  the particles hydrolic ratio. Thus a large qtz
  grain requires the same current velocity to move
  it as does a small heavy mineral

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Alluvial Processes

• A rapid flow will set all grains into motion but
  with a slackening of velocity the first material
  to be deposited will be large heavy minerals,
  then smaller heavy mins large light grains
  (boulders). If the current does not drop any
  further then a heavy mineral deposit will form.
  This is referred to as an irregular flow and may
  occur at
• The emergence of a canyon
• Projections in a stream bed
• Waterfalls and potholes
• Confluence of a swift tributary with a slower
  master stream
• Most important is deposition in rapidly flowing
  meandering streams. Faster water is on the
  outside curve of a meander slack water on the
  inside, making point bars favourable sites of
  heavy mineral deposition

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Alluvial Deposits
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Alluvial Deposits
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Beach Placers

• Important minerals of beach placers are
  cassiterite, diamond, gold, ilmenite, magnetite,
  monazite (REE), rutile, xenotime and zircon
• Marine placers occur at different topographical
  levels due to Pleistocene sea-level changes
• Beach placers (rutile, zircon) occur along E.
  Australia within Quaternary and Pleistocene sands
  can be 13km wide and 30m thick
• The thickest deposits occur adjacent to the S
  side of headlands due to longshore drift

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Beach Placers
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Fossil Placers

• These are ancient placers that have been
  lithified sometimes metamorphosed eg
  Proterozoic Witwatersrand of SA (largest Au
  district in the world, produces 1/3 of worlds
  gold
• Au and U occurs in several layers of conglomerate
  which extends for over 250km strike along a fault
  scarp