Answer:
In geology, a placer deposit or placer is an accumulation of valuable minerals formed by gravity separation during sedimentary processes. The name is from the Spanish word placer, meaning "alluvial sand". Placer mining is an important source of gold.
Minerals commercially mined from placer deposits include:
Types of placer deposits include:
Placer materials must be both dense and resistant to weathering processes. To accumulate in placers, mineral particles must be significantly denser than quartz (whose specific gravity is 2.65), as quartz is usually the largest component of sand or gravel. Placer environments typically contain black sand, a conspicuous shiny black mixture of iron oxides, mostly magnetite with variable amounts of ilmenite and hematite. Valuable mineral components often occurring with black sands are monazite, rutile, zircon, chromite, wolframite, and cassiterite.
Majority of placer deposit are small and often ephemeral as they formon the earth's surface usually at or above the local base level , so that many are removed by erosion before they can be burried.
Placer deposits are mostly of low grade but can be exploited because:
In geology, a placer deposit or placer is an accumulation of valuable minerals formed by gravity separation during sedimentary processes. The name is from the Spanish word placer, meaning "alluvial sand". Placer mining is an important source of gold.
Minerals commercially mined from placer deposits include:
- Gold
- Platinum group metals
- Tin, in the mineral cassiterite
- Diamonds
- Rare earth elements, from the mineral monazite
- Thorium, from the mineral monazite
- Titanium, from the mineral ilmenite
- Uranium, from Precambrian paleoplacers
Types of placer deposits include:
- alluvium placer deposit,
- eluvium placer deposit,
- aeolian placer deposit,
- beach placers deposit, and
- paleoplacers deposit.
Placer materials must be both dense and resistant to weathering processes. To accumulate in placers, mineral particles must be significantly denser than quartz (whose specific gravity is 2.65), as quartz is usually the largest component of sand or gravel. Placer environments typically contain black sand, a conspicuous shiny black mixture of iron oxides, mostly magnetite with variable amounts of ilmenite and hematite. Valuable mineral components often occurring with black sands are monazite, rutile, zircon, chromite, wolframite, and cassiterite.
Majority of placer deposit are small and often ephemeral as they formon the earth's surface usually at or above the local base level , so that many are removed by erosion before they can be burried.
Placer deposits are mostly of low grade but can be exploited because:
- They are loose, easy worked on materials which require no crushing. Relatively semi-mobile separating or hydraulic mining plants can be used. Mining takes the form of dredging, about the cheapest of all mining methods
- Placer deposit tend to concentrate minerals at a shallow depth and may contain valuable minerals such as Gold. Good example is the Witwatersrand in S. Africa.
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