Basic Mechanism of Formation of Placers Deposits

By BADAL DUTT MATHUR

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.

• 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.

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.

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.

• 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 .

• 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.

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 .

• 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.

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.

(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.

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.

Fig 1.1 : Internal structure of turbulant fluid flow in a natural medium

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).

Fig 1.2 :Mechanism of sediment transportation in water and in air.

• 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.

Fig 1.3: Hjulström diagram

• 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.

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.

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.

Fig 1.4 :Eluvial gold ore at San Antonio vein , Chontales District, Nicaragua .

• 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.

Fig 1.5 Famous Australian gold nuggets

(Eluvial gold deposite)

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.

• 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.

Fig 1.6 :Gravel deposition and formation of pay streak in rapidly flowing meandering stream

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).

• Fig 1.7 B : pay stream formed where fast tributary enters slow master stream

•Fig 1.7 A : quartzite ribs , interbedded with slate, which serve as natural riffles for the collection of placer gold(black).

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

• Examples: alluvial deposits of Alaska ,California , British Columbia and Indonesia .

Fig 1.8:Alluvial Gravels at the Blue Ribbon Mine Alaska

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.

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