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Placer Mineral Deposits
Presentation · March 2013
DOI: 10.13140/RG.2.1.3264.0886
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Topic 7: Placer Mineral Deposits
A short series of lectures prepared for the
Fourth year of Geology, Tanta University
2012- 2013
by
Hassan Z. Harraz
hharraz2006@yahoo.com
Prof. Dr. H.Z. Harraz Presentation Placer Mineral Deposits
Outline of Topic 7:
Definition
Concentration of heavy minerals in placer deposits
Steps for Making a Placer Mineral Deposits
Placer Locates
Types (Classification) of Placer Deposits
Placer Minerals:
Categories of resistate minerals
Characteristics of placer minerals
Origin and Mode of Formation of placer mineral deposits
Placer Gold
Model for alluvial gold deposits
Palaeo-placer gold deposit
We will explore all of the above in Topic 7
Prof. Dr. H.Z. Harraz Presentation
2
Placer Mineral Deposits
Placer Mineral Deposits
Definition
Placer deposits, or simply “placers”, are accumulations of valuable minerals concentrated in overburden, in
stream sediments or in beach materials by natural processes.
The minerals are freed from solid rock by mechanical and chemical weathering, and then transported
usually by water or wind action to the final resting place.
Weathering of primary minerals and transport by streams .
where current velocity is low, such as in point bar deposits, between ripple marks are concentrated by
flowing surface waters either in streams or along coastlines.
Sorted and distributed by flow of transporting media such as water, wind and ice .
The velocity of flowing water determines whether minerals are carried in suspension or deposited.
When the velocity of the water slows, large minerals or minerals with a higher density are deposited.
Heavy minerals like gold, diamond, Ti, Cr, Sn, and magnetite of the same size as a low density mineral
like quartz will be deposited at a higher velocity than the quartz, thus the heavy minerals will be
concentrated in areas where water current velocity is low. Mineral deposits formed in this way are
called placer deposits.
Examples:
Concentration of heavy minerals (elements) by streams, waves - Gold, tin, garnet, ilmanite
Witwatersrand, South Africa, a major gold producer - Fossil placers: conglomerate, sst.
California Gold Rush: was after placer gold, Mother Lode was discovered by tracing gold in the
upstream direction.
Most of the placer deposits being mined today are Cenezoic or younger and occur in unconsolidated
materials. However, some ancient placers, or “paleo-placers”, are found in sedimentary rocks as old as
Precambrian in age. In fact, some paleo-placers which are eroded become the source of present day placer
deposits.
Prof. Dr. H.Z. Harraz Presentation
3
Placer Mineral Deposits
Concentration of heavy minerals in placer deposits
1- Source rocks
Sketch illustrating formation of placer gold deposits.
Most gold is transported in hydrothermal fluids
that travel through cracks in the earth. Veins
are cracks filled by gold and the other chemical
elements carried in by the hydrothermal fluids.
Eventually, the veins may be exposed at the
surface of the Earth by weathering and erosion
where running water can concentrate the
heavy gold nuggets in placer (alluvial) deposits.
These deposits form because of gold's high
Figure shows the concentration of heavy minerals in weight and can be prospected with a gold pan
placer deposits occurs by stream action. Gold, diamonds, or mined on a commercial basis with a dredge.
and tin, for example, if eroded from their original
deposits, will accumulate in areas where stream currents
are weak, such as at the base of a waterfall or inside a
meander bend.
Prof. Dr. H.Z. Harraz Presentation
4
Placer Mineral Deposits
Steps for making a Placer Mineral Deposits
1) Weathering removes mineral particles from country rock.
2) Kinetic energy of high velocity stream transports mineral particles.
3) Where kinetic energy drops suddenly, high density particles stop, lower density particles continue.
Factors controlling the deposition of placer minerals:
a) fall velocities.
b) Bed configuration.
c) Grain density.
d) Source rock.
Depositional Sites:
a) Waterfalls.
b) Potholes.
c) Confluence of tributaries with main streams.
d) Point bars of meandering streams (in the mature stage).
Placer locates
1) Behind rock bars
2) In rock holes
3) Below waterfalls
4) Inside meander loops
5) Downstream from a tributary
6) Behind undulations on ocean floor
7) Riffles
8) Ripples—lag deposits
9) Tributary into main stream
Prof. Dr. H.Z. Harraz Presentation
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Placer Mineral Deposits
Placer locates
7- Riffles
2- In rock holes
1- Behind rock bars
8- Ripples- lag Meanders
deposits
3- Below waterfalls 4- Inside meander loops
9- Tributary into
main stream
5- Downstream from a tributary 6- Behind undulations on ocean floor
Prof. Dr. H.Z. Harraz Presentation
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Placer Mineral Deposits
Types (Classification) of Placer deposits:
1) Residual Placers
2) Eluvial Placers gravity
3) Alluvial Placers river water
4) Beach Placers waves
5) Aeolian Placers wind
6) Fossil Placers
Prof. Dr. H.Z. Harraz Presentation
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Placer Mineral Deposits
1) Residual Placers:
Occur in flat areas usually on top of the weathered source rock.
Example: Brazilian apatite formed by weathering carbonatites.
2) Eluvial Placers:
Occur downslope from the source rock, usually within “pockets” or depressions in the bedrock
surface.
Example: Cassiterite deposits in potholes in Malaysia.
3) Alluvial Placers:
One of the most common types of placer deposits.
Placer minerals in this type of deposit are always much finer grained than the associated lighter
minerals
These are deposited when the stream slows down.
Examples: Sn deposits of Brazil and Malaysia.
4) Beach Placers:
Size: 10 m thick, 1 km wide, 5 km long.
Commonly exhibit reverse grading.
Their formation is strongly influenced by longshore currents and drift.
Occur in areas where the coastline curves.
Best preserved where the beach was pro-grading.
Many beach placers are reworked and re-deposited.
Examples: Black sand-Egypt, Zircon & Rutile deposits along the eastern coast of Australia.
Prof. Dr. H.Z. Harraz Presentation
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Placer Mineral Deposits
Beach Placers
Prof. Dr. H.Z. Harraz Presentation
9
Placer Mineral Deposits
5) Aeolian Placers
Types of coastal dunes:
(a) Foredunes,
(b) Trangressive dunes,
(c) Stationary dunes
Examples: Titanomagnetite deposits off of the coast of the North Island of NZ, Ilmenite, zircon,
and leucoxene deposits of Natal, South Africa.
6) Fossil Placers (or Palaeo-placers):
Among the most important types.
Best examples represented by the Archean to Proterozoic Au and U deposits of Witwatersand,
South Africa (world’s largest gold field), and the Blind River area, Canada.
Au and U occur in the matrix/ cement of quartz – pebble conglomerates along with pyrite and rare
hematite. These are overlain by trough cross- bedded arenites (sandstones).
Implications for the Precambrian atmosphere
Uranium is the reverse of iron: highly oxidized state is soluble.
Uraninite (UO2) weathers easily today.
Detrital uranium limited to Precambrian.
Detrital pyrite common.
Evidence of reducing atmosphere.
Located at the mouths of several paleo-river systems all feeding an intracratonic basin. Deposition
aided by repeated episodes of uplift and faulting; sorting by some longshore currents.
Source of Au is different from that of U (fluid inclusion evidence); possibly the hydrothermally
altered granites exposed in the vicinity.
Prof. Dr. H.Z. Harraz Presentation
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Placer Mineral Deposits
Placer Minerals
Two types of minerals form placers:
1)minerals which have high specific gravities (called
“heavy minerals”),
Examples of heavy Placer minerals (also known as heavy minerals):
Cassiterite, Rutile, Gold, ilmenite, chromite, diamond, garnet, Pt, ruby,
sapphire, and zircon.
and
2) minerals which are more resistant to chemical and
mechanical erosion (called “resistate minerals”).
Examples of Resistate Minerals: Au, Pt, Cassiterite (for Sn), Chromite
(Cr), Columbite (for Nb), Cu, Diamond, Garnet, Ilmenite (for Ti), Magnetite
and Hematite (for Fe), Monazite (for rare earth elements), Ruby and
Sapphire, Rutile (for Ti), zircon (for Zr), and xenotime (for Y)
Prof. Dr. H.Z. Harraz Presentation
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Placer Mineral Deposits
Categories of Resistate Minerals
There are three categories of resistate minerals, including :-
Inert Minerals Maleable Minerals Hard Minerals
including those which are those which are maleable (tend those which have
relatively inert (non-reactive) to bend rather than break) greater hardness
Inert Oxide Minerals: Native Metals: Diamond
Cassiterite (SnO) Gold (S.G. = 19) Corundum
Chromite (Cr2O3) Platinum(S.G. = 19) Garnet
Rutile (TiO2) Bismuth
Magnetite (Fe3O4) (S.G. = 5)
Copper (S.G. = 9)
Ilmenite (FeTiO3) (S.G. = 5)
Monazite (S.G. = 5)
Inert Silicate Minerals:
Wolframite
Zircon (S.G. = 5)
Prof. Dr. H.Z. Harraz Presentation
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Placer Mineral Deposits
Characteristics of Placer Minerals:
1) High specific gravity
2) Chemical resistance to weathering
3) Durability (malleability, toughness, or hardness)
Placer Deposits as an example of resistate minerals:
Tough
Heavy
Chemically resistant
Methods of extraction:
1) Dredging: (e.g. Estrela Do Sul diamond deposits in Brazil).
2) Mining: (e.g. Witwatersand Au deposits in South Africa).
Prof. Dr. H.Z. Harraz Presentation
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Placer Mineral Deposits
Origin and Mode of Formation of Placer Mineral Deposits
Mode of Formation: 1) between ripple marks
1) Decomposition or disintegration
2) Concentration by moving water or air Placer
Stream Direction
Deposit
Placer minerals are concentrated by flowing surface
waters depositing high density minerals either in
streams or along coastlines.
When the velocity of the water slows, minerals with a 2) behind rock bars
Placer
higher density are deposited. Heavy minerals like gold, Stream Direction Deposit
diamond, and magnetite will be concentrated in areas
where water current velocity is low. The lighter
minerals (quartz) are carried away.
Example: Gold originally formed in hydrothermal veins, 3) on the inside of . meandering
is eroded out of the veins and carried in streams where streams
it was deposited in placer deposits. The California gold Placer
rush in 1849 began when someone discovered rich Deposit
placer deposits of gold in streams.
4) in holes on the bottom of a stream
Placer
Deposit
Placer deposits occur in any area where current Stream Direction
velocity is low.
Prof. Dr. H.Z. Harraz Presentation
14
Placer Mineral Deposits
Placer deposits are Concentrated by high density.
Examples of heavy minerals include Gold (Sierra Nevada, Piedmont); Platinum (Russia); Tin
(Malaysia); Diamonds (Namibia, West Africa); Heavy Beach Sands (Australia, Africa): Zircon, Ilmenite,
Monazite, Tungsten; native metals, sulfide minerals (pyrite and galena), magnetite and
scheelite.
The high density of these minerals enables them to be concentrated because they are
less easily mobilized by water currents.
As a result, less dense mineral grains surrounding them are “winnowed” (washed
away) with ease leaving the heavy minerals to lag behind. Winnowing also occurs as a
result of wave action on beaches, and even as a result of wind action.
The minerals most likely to accumulate in placers are those which are both resistates
and heavy. This is the reason gold and magnetite (often called “black sand”) are the
most common minerals to accumulate in placers.
Examples of Placer minerals (also known as heavy minerals):
Cassiterite, Rutile, Gold, ilmenite, chromite, diamond, garnet, Pt, ruby, sapphire, and zircon.
Other factors influence the ability of a mineral to become concentrated in a placer, such as the
settling rate(i.e. Mechanical separation).
The settling rate is a function of the grain size, grains shape, specific gravity, surface
roughness and electrostatic charge.
Larger grains, because they are heavier, settle faster than small ones.
Thin, flat grains (such as gold “flakes”) tend to catch currents and be whisked away
more easily than rounded grains (such as gold “nuggets”).
Surface roughness causes greater friction, inhibiting ease of movement.
Some mineral grains are known to carry electrostatic charges which cause them to stick
to other grains the way a balloon rubbed against your shirt will stick to your hand.
Prof. Dr. H.Z. Harraz Presentation
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Placer Mineral Deposits
Panning: same principle as a placer, but in
a pan.
density contrasts are
substantial:
gold - 19gm/cc
Prof. Dr. H.Z. Harraz Presentation
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Placer Mineral Deposits
Platinum nuggets, 800px Native gold nuggets
A collage of photo, showing 3 pieces of native gold.
The top piece is from the Washington mining district,
California, and the bottom two are from Victoria,
Australia.
Prof. Dr. H.Z. Harraz Presentation
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Placer Mineral Deposits
Garnet Hematite Native Cu
Cassiterite
Au
Prof. Dr. H.Z. Harraz Presentation
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Placer Mineral Deposits
Placer Gold
• Placer gold occurs in many shapes and sizes. Larger
pieces (generally >10 mesh) are called nuggets, and
smaller, flat pieces are called flakes.
• “Colors” are the tiny pieces (generally <0.001 oz)
which are found by panning or sluicing.
• Placer gold is not pure, but instead is a mixture of
gold and other native metals (usually silver, copper
or bismuth).
• The purity of placer gold is referred to as the
“fineness”, which is essentially the volume percent
of gold stated in parts gold per 1000 millileters.
• As gold particles travel further downstream, the
metal impurities are leached out, causing an
increase in gold fineness downstream.
• The texture of gold is also an indication of distance
of downstream transport.
• Rough, angular texture is generally considered to be
an indication of close proximity to the source.
• Example: California Gold Rush: was after placer
gold, Mother Lode was discovered by tracing gold in
the upstream direction.
• Dense, heavy minerals become concentrated in stream
bottoms (e.g. California gold deposits).
Prof. Dr. H.Z. Harraz Presentation
19
Placer Mineral Deposits
The California gold rush in 1849 began when someone discovered rich placer
deposits of gold in streams draining the Sierra Nevada Mountains. The gold originally
formed in hydrothermal veins, but it was eroded out of the veins and carried in
streams where it was deposited in placer deposits.
Klondike Gold Rush: placer miners with sluice box in 1901 (right)
and modern gold mining on same creek in 2002 (left)
Prof. Dr. H.Z. Harraz Presentation
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Placer Mineral Deposits
Placers: Gold is concentrated as a detrital sediment
Prof. Dr. H.Z. Harraz Presentation
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Placer Mineral Deposits
Prof. Dr. H.Z. Harraz Presentation
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Placer Mineral Deposits
Palaeo-placer gold deposit - Witwatersrand Basin, South Africa
Witwatersrand, South Africa, a major gold producer -
Fossil placers: conglomerate, sst.
The Witwatersrand Basin in an ancient (2.8 billion
years old) palaeo-placer deposit, consisting of
multiple stacked and alternating shale, sandstone
and thin conglomerate sedimentary bands.
The gold mineralisation is found in the conglomerate
bands (called “reefs”), typically between 5 to 100
cm thick. The gold was either introduced at the time
the sediments were deposited, or was introduced Geological cross-section through the Welkom
later by gold-bearing hydrothermal fluids (or both). Goldfield. Figure from McCarthy, 2006
Carbon
The sedimentary basin subsequently suffered
extensive deformation, producing folds and faults Pyrite
that disrupt the deposit. Faults impact
Gold
significantly on safe (and efficient) mining.
Quartz
Underground mines operate up to a maximum depth
of about 4,000 meters.
Mineable grades in a deep gold mine operations 1 cm
are of the order of 10 – 20 g/ton.
Gold and carbon nodules with “buckshot” pyrite in
conglomerate reef from the Witwatersrand Basin,
Many of the reefs contain accessory uranium,
South Africa. Figure from McCarthy and Rubidge,
which is processed as by-product on several mines. 2005. Photo credit: Goldfields.
Prof. Dr. H.Z. Harraz Presentation
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Placer Mineral Deposits
Geophysical
2-5 TOPIC
END OF 7 Surveying
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