Full Answer
What are alluvial placer deposits?
Alluvial placers are formed by the deposition of dense particles at a site where water velocity remains below that required to transport them further. To form a placer deposit, the particles desired must show a marked density contrast with the gangue material, which is able to be transported away from the trap site.
How are eluvial placers formed?
Eluvial placers form on hillslopes from weathered deposits. They are not acted on by streams but by rainfall and wind, which carry away the light materials; thus they may be considered intermediate in the formation of stream placers.
How is a placer deposit formed?
The “placer” deposits are formed by surface weathering and ocean, river, or wind action resulting in concentration of some valuable strongly resistant minerals of economic quantities. The placer can be an accumulation of valuable minerals formed by gravity separation during sedimentary processes.
Which of the following is mined commercially from placer deposits?
Substances commercially mined from placer deposits include: Diamonds. Gold. Garnet. Iron, from ironsands containing high concentrations of magnetite. Platinum group metals.
Which layer of the Shirasu is alluvial?
The upper two layers, fine sand and gravel layers are alluvial deposits. The fine sand layer is totally uniform. This layer has high water content and is soft. The particle size of the gravel layer is 1 to 3 cm and the mixture rate of gravel is 60 to 80 %. The Shirasu layer is classified into the primary Shirasu.
What is the name of the soils that are deposited in riverbeds?
The soils deposited in riverbeds are known as alluvial deposits . Some textbooks use the term, “Fluvial deposits” for the same thing. The size of particles deposited in riverbeds depends on the speed of flow. If the river flow is strong, only large cobble-type material can deposit.
What are river terrace deposits?
River terrace deposits are perhaps the commonest of the alluvial deposits. These materials are eroded from the upper reaches of rivers, carried downstream by the river, particularly at times of flood when it is most energetic, and deposited in the lower reaches typically on a floodplain where the rate of flow is checked and the river can no longer transport its burden of sediment. This sedimentary material builds up in thickness over time. Most rivers have a history that will extend back through the Quaternary Period with its reduced sea levels during the glacial periods, and often well into the Tertiary Period. Such rivers will have had periods of rapid cutting down through the earlier alluvial deposits to adjust to reduced sea levels while at other times at the end of glaciations they would have carried much larger volumes of water than is the case today and have been capable of transporting greater sedimentary loads and larger particles than at present. The idealized block diagram and cross-section in Figure 5.6 illustrates the way in which such processes give rise to river terrace deposits. It should perhaps be noted the deposits of this type are typically only a few metres thick but often are of wide geographical extent.
What is the name of the soil in the ocean?
Soil deposits in ocean beds are known as marine deposits or marine soils . Though oceans can be very violent, seabeds are very calm for the most part. Hence, very small particles would deposit upon seabeds. Texture and composition depends on proximity to land and biological matter.
What is the subsoil sequence in the marsh areas and abandoned paddy areas?
The subsoil sequence in the marsh areas and abandoned paddy areas generally consists of partially decomposed vegetable matter at the top, followed by a layer of peat and organic clay. The peat and organic clay layers are underlain by alluvial silty sandy strata.
Where does jute grow?
Most of the world's supply of jute comes from India and Bangladesh, where it is cultivated in the alluvial soils of the Delta Region of the Ganges and the Brahmaputra rivers (16). The harvest of jute gets ready for gathering in about 3 months, and the correct stage is recognized by the formation of the flower buds. These are not allowed to break and bloom, for that would take too much goodness from the fibers (17). After the plants have fully grown, they are cut down by hand with homemade knives and are ready for retting, which is the operation to make the jute plant ready for the separation of the bast layer from the stem of the plant. This operation requires a large supply of water, since the plants must be completely submerged in water. There are two types of retting: (1) pit water retting and (2) flow water retting. Flow water retting is better, but in a place where flow water for retting purpose is lacking, pit is practiced, i.e., retting in stagnant water is ponds, ditches, etc. (19). In either of the retting processes, bundles of canes are immersed in water and weighted down by planks or stones, bacterial action then separates the outer bast layer of fibers from the canes and from each other. This action may take place in 10–15 days, depending on the weather, the temperature of water, the ripeness of the cane, and the area where it is grown. During immersion, the crop is under constant surveillance to prevent overretting, which produces weak fiber. Underretted fibers are course, gummy, and most difficult to remove from the cane. When the fibrous layer separates freely from the cane, the retting is complete. Then the fiber is washed free from stalk and vegetable debris, finally washed in clean water, dried in the sun, and ready for market (17,20).
How are collector wells used?
Collector wells sunk in the river bed are widely used in alluvial deposits in valleys in arid areas where the dry weather flow of the river is underground. They take the form of large diameter concrete caissons sunk in the river bed, the caisson having to be of sturdy construction to withstand the force of the flood flow in wet weather. Access by bridge is necessary, the bridge and the top of the caisson being sited above maximum flood level. From the base of the caisson, collector pipes are laid out horizontally usually upstream, though sometimes across the stream bed or fanwise upstream. The collectors usually comprise porous, perforated or unjointed concrete pipes, 200–300 mm diameter, laid in gravel filled trenches cut in the river bed sediments, connected to the wet well. It is important to lay the collectors at sufficient depth to avoid their destruction during floods when some of the stream bed can mobilize into suspension. Where the bed is narrow with deep deposits a simple large diameter well may suffice to intercept flow down the valley. The yield of such wells is limited by replenishment flow down the valley. If ordinary centrifugal pumps (which are the cheapest) are used, they have to be sited above maximum flood level with their suctions dipping into the water, which limits the range of water levels from which water can be pumped. Use of submersible pumps can overcome this difficulty.
How are placer deposits formed?
The “placer” deposits are formed by surface weathering and ocean, river, or wind action resulting in concentration of some valuable strongly resistant minerals of economic quantities. The placer can be an accumulation of valuable minerals formed by gravity separation during sedimentary processes. The type of placer deposits are, namely, alluvial (transported by a river), colluvial (transported by gravity action), eluvial (material still at or near its point of formation), beach placers (coarse sand deposited along the edge of large water bodies), and paleoplacers (ancient buried and converted rock from an original loose mass of sediment). The most common placer deposits are those of gold, platinum group minerals, gemstones, pyrite, magnetite, cassiterite, wolframite, rutile, monazite, and zircon. The California gold rush in 1849 began when someone discovered rich placer deposits of gold in streams draining the Sierra Nevada Mountains. The marine placer deposits of rutile, monazite, ilmenite, and zircon are currently being exploited along the coast of eastern Australia, India, and Indonesia.
What are placer deposits?
The type of placer deposits are namely, alluvial (transported by a river), colluvial (transported by gravity action), eluvial (material still at or near its point of formation), beach placers (coarse sand deposited along the edge of large water bodies) and paleo-placers (ancient buried and converted rock from an original loose mass of sediment). The most common placer deposits are those of gold, platinum group minerals, gemstones, pyrite, magnetite, cassiterite, wolframite, rutile, monazite and zircon. The California gold rush in 1849 began when someone discovered rich placer deposits of gold in streams draining the Sierra Nevada Mountains. Recently formed marine placer deposits of rutile, monazite, ilmenite and zircon are currently being exploited along the coast of eastern Australia, India and Indonesia.
What are the minerals found in beach sand?
Minerals that have been mined throughout the world from beach sands include ilmenite (Ti), rutile (Ti), zircon (Zr), and monazite (REE) (Dardenne and Schobbenhaus, 2001 ). Placer deposits were important until the middle of the past century (mid 1950s), when monazite was used to produce REE compounds in Brazil.
What is placer sediment?
Placer deposits are masses of unconsolidated and semi-consolidated clastic sediment formed by surface weathering and erosion of primary rocks that are subsequently transported by gravity, water, wind, or ice from their original source and redeposited elsewhere. From: Encyclopedia of Geology (Second Edition), 2021. Download as PDF.
What is the geophysical method used for placer deposits?
The geophysical methods used for aggregate (sand and gravel) mapping are identical to the geophysical methods used for mapping placer deposits. The concentrations of the metals and/or minerals in placer deposits are too low for their direct detection using geophysical methods.
What are the two most important types of placers?
The two most important types of placers are stream (alluvial) and beach placers.
Where does zircon come from?
By contrast, zircon occurs in true placer deposits, concentrated in beach sands. These are primarily Ti deposits (rutile and ilmenite), in which zircon is a by-product, along with minor monazite. The leading producers of zircon in 2009 were Australia and South Africa ( Gambogi, 2010 ).
How are alluvial placers formed?
Alluvial placers are formed by the deposition of dense particles at a site where water velocity remains below that required to transport them further. To form a placer deposit, the particles desired must show a marked density contrast with the gangue material, which is able to be transported away from the trap site.
Where are beach placers formed?
Beach placers are formed in sand and gravel deposited along the edge of large bodies of water and are typically found where streams or rivers flow into a large body of water. Materials collect as beach placers by continuous wave action and currents.
What are the elements in placers?
Substances commercially mined from placer deposits include: 1 Diamonds 2 Gold 3 Garnet 4 Iron, from ironsands containing high concentrations of magnetite 5 Platinum group metals 6 Rare earth elements, from the mineral monazite 7 Ruby 8 Sapphire 9 Thorium, from the mineral monazite 10 Tin, in the mineral cassiterite 11 Titanium, from the minerals ilmenite and rutile 12 Uranium, from Precambrian paleoplacers 13 Zirconium, from the mineral zircon
What is a placer deposit?
In geology, a placer deposit or placer is an accumulation of valuable minerals formed by gravity separation from a specific source rock during sedimentary processes. The name is from the Spanish word placer, meaning " alluvial sand". Placer mining is an important source of gold, and was the main technique used in the early years ...
What is the purpose of hardness in placer minerals?
Hardness. Hardness allows a placer minerals to resist mechanical breakdown and loss of mass from abrasion during transport. A hardness greater than quartz is desired, however substances such as gold typically deform and create irregularly shaped nuggets when subject to mechanical stress.
What is the difference between placer and quartz?
The separation of the valuable minerals from the most common non-economic mineral, quartz, depends on the difference in specific gravity / density .The weathering process allows for the accumulation of placer minerals, while less dense material such as quartz are swept ...
What are gold bearing placers?
Gold bearing beach placers consist of large strips of black sands and are typically constantly changing as a result of storms or sporadic wave action. Typically beach placers are populated by ilmenite and magnetite, however gold, platinum and chromite are found in varying amounts.
What are the different types of placer deposits?
There are several varieties of placer deposits: stream, or alluvial, placers; eluvial placers; beach placers; and eolian placers. Stream placers, by far the most important, have yielded the most placer gold, cassiterite, platinum, and gemstones.
What are some examples of placers?
Among the examples of beach placers are the gold deposits of Nome, Alaska; the zircon sands of Brazil and Australia; the black sands (magnetite) of Oregon and California; and the diamond-bearing marine gravels of Namaqualand, South Africa. Get a Britannica Premium subscription and gain access to exclusive content.
Why do stream placers depend on water?
Because the ability to transport solid material varies approximately as the square of the velocity, the flow rate plays an important part ; thus, where the velocity decreases, heavy minerals are deposited much more quickly than the light ones .
Where are phosphate deposits found?
Large submarine deposits of phosphate rock (phosphorite) are found in the Pacific off the coasts of Peru, eastern Australia, and California and on the Chatham Rise east of New Zealand. Smaller deposits also occur in lagoons of some of the Pacific Islands.
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;-
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.
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.
Causes of Placer Development
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.
