Placer Mineral Deposits

Placer deposits or simply placers are accumulation of valuable minerals concentrated in overburden in stream sediments or in beach materials by natural processes.

Heavy minerals like gold, diamond, Ti, Cr, Sn and magnetite of the same size of low density minerals like 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.

Heavy placer minerals	Resistant place minerals
Cassiterite, Rutile, Gold, Ilmenite, Chromite, Diamond, Garnet, Pt, Ruby, Sapphire, Zircon	Au, pt, Cassiterite, Chromite, Columbite, Cu, Diamond, Garnet, Hematite, Magnetite, Monazite, Ruby, Sapphire, Rutile, Zircon, Xenotime

Steps of making placer deposits :-

1. Weathering removes mineral particles from country rock.
2. Kinetic energy of high velocity streams transport mineral particles.
3. Where kinetic energy drops suddenly, high density particles stops and lower density particles continue.

Examples :

• Witwatersrand, South Africa, a major gold placers : conglomerate, sandstone.
• California gold rush, was after placer gold, mother load was discovered by tracing gold in upstream direction.
• Concentration of heavy elements by streams and waves : gold, tin, garnet, ilmenite etc.

Most of the placers mined today are Cenozoic 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 deposits.

Where to find placers :-

Types of placer deposits :-

• Residual placers :- These placers occurs in flat areas usually on top of the weathered source rock. Ex- Brazilian apatite formed by weathering of carbonatites.
• Eluvial placers :- These placers occur downward slope from the source rock, usually within “pockets or depression in the bed rock surface. Ex – Cassiterite deposits in potholes in Malaysia.
• Alluvial placers :- One of the most common type of placer deposits. Placer minerals in this type of deposits are always much finer grained than the associated lighter minerals. These are formed when the streams slows down. Ex – Sn deposits of Brazil and Malaysia.
• Beach placers :- Commonly exhibit reverse grading. Their formation is strongly influenced by longshore currents and drift. They occurs where the coastline curves. These placers are well presented where the beach was pro-grading. Ex- Black sand – Egypt, Zircon & Rutile deposits along the eastern coast of Australia.
• Aeolian placers :- Types of coastal dunes – foredunes, transgressive dunes, stationary dunes. Ex – Titanomagnetite deposits of the coast of the North island of NZ, Ilmenite, Zircon and Leucoxene deposits of Natal, South Africa.
• Fossil placers / Paleo-placers :- Gold and Uranium in the matrix or cement of quartz-pebble conglomerates along with pyrite and rare hematite. Ex – best example is represented by the Archaean and Proterozoic Au & U deposits of Witwatersrand, South Africa (worlds largest gold field), and the blind river area, Canada.

Implications of Precambrian atmosphere –

1. Uranium is reverse of Iron : Highly oxidized state is soluble
2. Uraninite(UO₂) weathers easily today
3. Detrital Uranium limited to precambrian
4. Detrital pyrite common
5. Evidence of reducing atmosphere

Mode of formation or Factors affecting accumulation :-

1. Less dense mineral surrounding the high one are winnowed with ease leaving the heavy minerals to lag behind.
2. The minerals most likely to accumulates in placers are those which are both resistant and heavy. This is the reason why gold and magnetite (black sand ) are the most common minerals to accumulate in placers.
3. Settling rate is a function of the grain size, shape, sp. gravity, surface roughness and electrostatic charge.
4. Thin flat grains (like gold flacks) tend to catch current and be whisked away more easily than rounded grains ( like gold nuggets ).
5. Surface roughness cause greater friction, inhibiting ease of movement.
6. Some grains are known to carry electrostatic charges which cause them to stick to other minerals.