When we talk about placer deposits meaning, it is a natural concentration of heavy minerals induced by the effect of gravity on moving particles. When heavy, stable minerals are liberated from their matrix by the mechanism of weathering, they are slowly washed downslope into streams which rapidly winnow the lighter matrix. Therefore, the heavy minerals become concentrated in river, stream, beach, and lag (debris) gravels and include workable ore deposits. Minerals which form placer deposits constitute high particular gravity, are durable and chemically impervious to weathering.
Mineral Found in Placer Deposits
Minerals that form placer deposits include precious deposits like gold, platinum, copper, zircon and various gemstones apart from magnetite, ilmenite, chromite, cassiterite, rutile, and native monazite.
Factors Influencing Placer Development
Placer development is activated by a combination of tectonic activity and oscillations in climate and sea level. Besides that various other factors are responsible for placer developments that are as follows;-
Chemical and Physical weathering that eases all forms of erosion
Release of minerals into sediments that are transported, deposited, sorted, and often reworked.
Placer deposits occupy quite a huge range of geomorphological features, by which they are more simply categorized.
Mineral sands, and in places diamonds, prefer coastal surroundings, whereas the other placer minerals are concentrated often in fluvial deposits.
How to Identify the Type of Placer Formed?
Alterations in energy levels and the associative physical attributes of the minerals and sediments identify the style of placer formed. Lag placers outcomes from an increase of energy that culls out the hydraulically lighter particles (lights) close to Earth’s ground surface. Accumulation placers are created by selective settling of hydraulically heavier particles because of reduction in energy. Both styles are seen in all placer types. Multiphase accumulation, repetition of supply, and reworking are common and generally advantageous. Minerals are favorably concentrated and depleted at different scales, particularly in a fluvial environment, by external physical conditions that form localized energy change. Beach deposits of mineral sands are a type of lag deposit.
Based on the nature of its source, some situations should coincide and events take place to develop a significant placer. The most significant economic characteristics are size and grade (from parts per billion for diamonds to wt percent for minerals), all together with bedrock qualities, sediment particle sizes, and mineral size and morphology. Post depositional alterations, particularly the upgrading of ilmenite by alteration, are significant. The maximum distance isolating a placer from its source varies extensively; from less than a kilometer to thousands of kilometers. Diamond, zircon and rutile placers have formed over hundred to thousand kilometers from their sources. Transport distance is increased by high-energy flow in constricted fluvial channels. An understanding of how placers develop, together with the study of gold particle composition and morphology, aids in the search for primary deposits.
Where to Find Placers?
Placers can be found in virtually any area where gold takes place in hard rock (lode) sediments. The gold is released by weathering and glacier or stream action, transported by gravity and hydraulic action to some preferable point of deposition, and concentrated in the process. Generally, the gold does not travel far from the source, so familiarity with the location of the lode sediments is useful. Gold can also be linked with copper and may form placers in the surrounding area of copper deposits, although this happens less frequently.
Causes of Placer Development
Following are the Causes of Placer Formation:
Geological instances such as subsidence and uplift may cause repeated and prolonged cycles of erosion and concentration, and where these processes have been undergone placer deposits may be enriched.
Antiquated river channels and several river bench deposits are examples of gold-bearing gravels which have been disposed to a number of such events, subsequent to minimum partial concealment by other deposits, including volcanic substances.
Residual placer deposits developed in the immediate vicinity of source rocks are generally not the most productive, though exceptions take place where veins supplying the gold were extraordinarily rich.
Reworking of gold-bearing substances by stream action results in the concentrations required for exploitation.
In desert areas sediments may result from unexpected flooding and outwash of sporadic streams.