2. Mechanical concentration
(Placer Deposits)
• Mechanical concentration is the natural
gravity separation of heavy from light mineral
by means of moving water or air by which the
heavier mineral become concentrated into
deposits called placer deposite.
• It involve two stages:-
1. The freeing by the weathering of the stable
mineral from their matrix.
2. Their concentration.
3. • Concentration can occur only if the valuable
minerals posses the three properties:-
• High specific gravity ,chemical resistance to
weathering and durability .
• Ex: gold ,platinum ,tinstone , magnetite
, chromite , monazite etc.
4. Source Materials
• The mineral that make up placer deposits may be derived
from:-
1. Commercial lode deposits: such as gold vein , Ex :mother
lode of gold veins of California.
2. Noncommercial lodes : such as small gold quartz stringers or
veinlets of cassiterite , Ex: the tin placers of Indonesia.
3. Sparsely disseminated ore minerals : minute grains of
platinum sparsely disseminated in basic intrusive. Ex : the
Ural Mountains.
4. Rock forming minerals : such as grains of magnetite
, monazite and zircon . Ex : monazite beaches of india.
5. Basic principle of mechanical
concentration
• The operation of mechanical concentration rests on a few
basic principle involving chiefly:-
• The differences in the specific gravity , size , and shape of
particles, as affected by the velocity of fluid.
1. Density contrast :In a body of water , a heavier mineral sinks
more rapidly than a lighter one of the same size.
2. Size of particle : The rate of settling in water is also affected
by the specific surface of particle.Of two sphere of the same
weight but of different size , the smaller , with its lesser
surface and therefore , lesser friction in water , sinks more
rapidly .
3. Shape of particle :The shape of a particle affects its rate of
settling . A spherical pellet has less surface than a thin , platy
disc of the same weight and therefore ,will sink more.
6. • Thus flaky specularite and molybdenite are
difficult to concentrate by gravity despite their
high specific gravity .
Effect of moving water
• The ability of a body of flowing water(or air) to
transport a solid depend upon the velocity and
varies as the square of the velocity.
• Faster moving water increases the difference in
settling rate based on specific gravity .
• If particles of gold and quartz were dropped in
the moving water the gold might drop directly to
the bottom and quartz be swept downstream .
7. • The specific surface again enters in ; of two
equal weight particle ,the one with the larger
specific surface is increasingly rapidly , swept
away from the other with increased water
velocity .
• Thus flaky minerals readily separated from
quartz and fine materials separated from
coarse particle.
8. Effect of Suspension
• The particle suspension is more readily moved by
a flowing fluid than one at rest.
• The swirls and eddies of stream and wave action
simulate the upward pulsation of jigs and table in
ore dressing by which lighter minerals are
bounced higher than heavier ones so that they
can be more easily moved away by flowing water.
• This jigging action enables gold particles
scattered through bottom gravels to become
concentrated at the bottom .
9. • The various factors that I have explained , operate
together to separate the light and fine minerals
from the coarse and heavy ones , and with long
continued action , placer minerals may eventually
become sufficiently concentrated to constitute
workable deposits.
10. Reynolds number
• One of the parameters used to quantify the conditions of
fluid motion is the Reynolds number
• It is a dimensionless ratio identifying fluid flow as either
laminar or turbulent.
• The Reynolds equation is expressed as:
• Re = U*L*δf/η
• where
Re : dimensionless Reynolds number;
U : fluid velocity
L : is the length over which the fluid is flowing
δf : fluid density
η :fluid (molecular) viscosity.
11. (laminar flow ) ( turbulent flow)
• For low Reynolds numbers flow is laminar and vice versa for
turbulent flow .
• Flow in natural stream channels is predominantly turbulent, the
detailed anatomy of which is shown schematically in (Fig 1.1 )
Three layers of flow can be identified.
1. The bottom zone is the non-turbulent viscous sublayer, which is
very thin and may break down altogether in cases where the
channel floor is very rough and turbulence is generated by the
upward protrusion of clasts from the bed load.
12. 2. Above it is the turbulence generation sublayer,
where shear stresses are high and eddies are
generated.
3. The remainder of the stream profile is the outer
or core layer, which has the highest flow
velocities.
• A shear stress (τo in Fig 1.1) is imposed on the
bed load by the moving fluid and is a function of
fluid density, the slope of the stream bed, and
flow depth.
• The curved velocity profile of the channel
section is caused by frictional drag of the fluid
against the bed.
13. Fig 1.1 : Internal structure of turbulant fluid flow in a
natural medium
14. Particle Movement
• A particle or grain will move through a fluid as a
function of its size, shape, and density, as well as the
velocity and viscosity of the fluid itself.
• In water, a particle at any instant will move in one of
three ways:
• the heaviest particles (boulders, gravel) roll or slide
along the channel floor to form the bedload (or
traction carpet).
• intermediate sized particles (sand) effectively bounce
along with the current (a process known as saltation)
• The finest or lightest material (silt and clay) will be
carried in suspension by the current (Figure 1.2).
16. • In air the types of movement are similar, but the
lower density and viscosity of air relative to water
dictate that moving particles are smaller, but
their motion is more vigorous.(Figure 1.3).
• The type of particles moving in a fluvial channel
by saltation and suspension is partly a function of
the nature of the fluid flow (as defined by the
Reynolds number)
• Combining fluid Flow (hydrodynamic) and
physical mass transport parameters provides a
useful semi-quantitative indication of the
processes of sedimentation, a technique first
presented diagrammatically by Hjulström.
18. • In this diagram the conditions under which
either erosion, transportation, or deposition
will take place are shown as a function of flow
velocity and grain size.
• Deposition occurs either as flow velocity
decreases or grain size increases (or
both), and these parameters are very relevant
to the formation of placer deposits.
19. Types of placer deposits
1. Eluvial placers: when weathering yields debris
on a hill slope, the heavier particle move
downslope more slowly than lighter ones, giving
a rough concentration into eluvial placers.
2. Stream or alluvial placers: formed by
concentration of minerals in stream during
water transportation.
3. Beach placers: formed when concentration take
place on beaches.
4. Eolian placers: formed when concentration take
place by wind.
20. Eluvial Placer Formation
• Eluvial placer may be considered an intermediate
or embryonic stage in the formation of stream or
beach placers.
• They are formed , without stream action , upon
hill slopes from material released from
weathered lodes that outcrop above them.
• The heavier , resistant, minerals collect below the
outcrop(Fig 1.4) and the lighter nonresitant
product of decay are dissolved or swept downhill
by rainwash or are blown away by the wind.
21. Fig 1.4 :Eluvial gold ore at San Antonio vein , Chontales
District, Nicaragua .
22. • This brings about a partial concentration by
reduction in volume , a process that continues
with continued downslope creep.
• The most important eluvial deposits are gold
ans tin ; minor deposits include manganese
, tungsten , kyanite , barite and gemstones.
• Eluvial gold deposits are mined in
Australia, New Zealand , USA , South America
etc.
• In India wolframite and kyanite are mined as
eluvial deposits.
23. Fig 1.5 Famous Australian gold
nuggets
(Eluvial gold deposite)
24. Stream or Alluvial placer Formation
• Stream placers are by far the most important
type of placer deposits. They have yielded the
greatest quantity of placer gold , tinstone and
platinum
Concentration
• Flowing water is most effective separator of light
from heavy mineral.
• It rushes through canyons sweeping everything
along with it ; it slackens in wide places; it swirls
around the outside of bend, creating back eddies
on the inside ;it laps up over bottom projection ,
forming quiet eddies on the lee side.
25. • In these slack water the heavy substances drop to
the bottom.
• In streams , jigging action is particularly effective
in concentrating placer mineral in the bottom
gravels.
Places of Accumulation
• The most favorable sites for placer accumulation
are:-
1. In a readily flowing meandering stream the
fastest water is on the outside curve of
meanders and slack water is opposite(Fig 1.6).
The junction of the two , where gravel bars
form, is a favorable site for deposition of placer
mineral.
26. Fig 1.6 :Gravel deposition and formation of pay streak in rapidly
flowing meandering stream
27. 2. Where streams cross highly inclined or vertically
layered rocks, such as slates or alternating hard
and soft beds , the harder layers tend to project
upward and softer ones to be cut away. This
form natural “riffles”. Such natural riffles are
excellent traps for placer minerals and may give
rise to bonanzas or exceptionally rich streak (Fig
1.7 A).
3. The mode of entry of the placer minerals into a
stream also determines the site of
accumulation. Where materials are delivered by
swift tributary into a slower master stream
, they accumulate under diminished velocity , as
a pay streak down the near side (Fig 1.7 B).
28. •Fig 1.7 A : quartzite ribs , interbedded with slate, which serve
as natural riffles for the collection of placer gold(black).
• Fig 1.7 B : pay stream
formed where fast
tributary enters slow
master stream
29. 4. If a stream crosses a mineralised lode, and
through its own erosion supplies the placer
minerals , the pay streak will be spread across
the stream channel on the downstream side
of the lode (Fig 1.7 C)
Fig 1.7 C : pay
streak formed
below gold lode
crossed by
stream
30. • Examples: alluvial deposits of Alaska
,California , British Columbia and Indonesia .
Fig 1.8:Alluvial Gravels at the Blue Ribbon Mine Alaska
31. References
• Alan M. Bateman and Mead L. Jensen
(1979), economic mineral deposits 13 , 226-
229.
• Laurence Robb (2005), introduction to ore
forming processes 5, 246- 250.
• Fig 1.1, 1.5, 1.8:Google images.
