Placer deposit
 | This article includes a list of general references, but it lacks sufficient corresponding inline citations. (September 2011) |
Placer materials must be both dense and resistant to weathering processes. To accumulate in placers, mineral particles must have a specific gravity above 2.58.[1]
Placer environments typically contain black sand, a conspicuous shiny black mixture of iron oxides, mostly magnetite with variable amounts of ilmenite and hematite.[3] Valuable mineral components often occurring with black sands are monazite, rutile, zircon, chromite, wolframite, and cassiterite.[3] Early mining operations were probably a result of placer deposits as they were easily accessible and potential size.[4] The events known as gold/diamond rushes were caused by placer deposits and have proved to be plentiful.[4]
Characteristics of placer minerals
Density
Placer minerals are defined as having a
Hardness
Hardness allows a placer minerals to resist mechanical breakdown and loss of mass from abrasion during transport.[1] A hardness greater than quartz is desired, however substances such as gold typically deform and create irregularly shaped nuggets when subject to mechanical stress.[1]
Stability
Types of placers
Alluvial placers
Alluvial placers are those formed in river or stream sediments. Another name for alluvial placers are stream placers.[2] Typical locations for alluvial gold placer deposits are on the inside bends of rivers and creeks; in natural hollows; at the break of slope on a stream; the base of an escarpment, waterfall or other barrier. Stream placers are the most economical and common types of placers and have provided many with riches in the past.
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.[1] Only if the deposit is winnowed in this way can the minerals be concentrated to economic levels.
Beach placers
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.[2] Materials collect as beach placers by continuous wave action and currents.[2] Some important examples of beach placers include black sands of Oregon, gold deposits in Nome, Alaska, zircon sands in Brazil and Australia as well as diamond marine gravel in South Africa.[citation needed]
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.[5] Typically beach placers are populated by ilmenite and magnetite, however gold, platinum and chromite are found in varying amounts.[5] Some of the most productive beach placers are considered ancient beaches that are now far inland from the water, these placers no longer shift with storms and tidal action but with wind and rainfall.[6]
Eluvial placers
Eluvial placers are deposits of metal formed on hillsides and slopes weathered by rainfall and wind.[6] Lighter materials are carried away and weathered leaving concentrations of valuable metals.[5]
Eluvial placers are typically not large enough to support large scale mining, however in one case in Nevada there has been large scale gold mining operations based on placer deposits at Round Mountain.[5]
Aeolian placers
Aeolian placers are valuable minerals found in arid regions freed from their source rock by wind actions.[7] Wind erosion leaves a cement like substance in which material like gold are found. Wind action blows sand and dirt away leaving the deposits close to the surface.[8] Historically, aeolian placers have been mined by hand with simple tools by miners due to the proximity to the surface, and small concentration.[5]
Aeolian placers are common in Australia, Western United states, where the climate is arid and the ground is relatively level leaving wind as the primary erosion force.
Paleo-placers
Paleo-placer deposits are deposits of minerals from all of the above placer types that occurred millions of years ago.[9] These deposits are typically very far underground in ancient riverbeds, beaches, or slopes.[9] The Witwatersrand Basin, South Africa is the largest gold deposit in the world and is considered a paleo-placer, it has produced over 1.5 billion ounces of gold.[9] The Witwatersrand Basin is considered an ancient alluvial placer.[9]
Substances mined
Substances commercially mined from placer deposits include:
- Diamonds
- Gold
- Garnet
- Iron, from ironsands containing high concentrations of magnetite
- Platinum group metals
- Rare earth elements, from the mineral monazite
- Ruby
- Sapphire
- Thorium, from the mineral monazite
- Tin, in the mineral cassiterite
- Titanium, from the minerals ilmenite and rutile
- Uranium, from Precambrian paleoplacers
- Zirconium, from the mineral zircon
Source rock
Diamonds
See also
References
- ^ doi:10.1016/j.oregeorev.2005.02.001 – via Elsevier Science Direct.)
{{cite journal}}: CS1 maint: date and year (link - ^ a b c d "Placer deposit | geology". Encyclopedia Britannica. Retrieved 2021-02-11.
- ^ S2CID 126447895– via SpringerLink.
- ^ a b "Placer deposit | geology". Encyclopedia Britannica. Retrieved 2021-04-06.
- ^ a b c d e "Types of Placers". Western Mining History. Retrieved 2021-03-25.
- ^ a b "Placer deposit | geology". Encyclopedia Britannica. Retrieved 2021-03-25.
- ^ "What is Eolian Placer Gold? - Gold Prospecting Mining Equipment Detectors Snake Protection". www.goldrushtradingpost.com. Retrieved 2021-03-25.
- ^ Michaud, David (2016-10-23). "Types of Placers". Mineral Processing & Metallurgy. Retrieved 2021-03-26.
- ^ a b c d "Where Does Placer Gold Come From? – Part 3 Placer | West Coast Placer". Retrieved 2021-04-06.
- ^ "Alluvial Diamond Mining Fact Sheet" (PDF). World Diamond Council. Retrieved August 4, 2019.
External links
