By Michael W. Meissner
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Definitions
According
to Wikipedia placer deposit defined as:
·
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, and was the main technique used in the early years of many gold
rushes, including the California
Gold Rush. Types of
placer deposits include alluvium, eluvium, beach placers, and paleoplacers.
·
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. (Plazak, Meissner, Trafford09, & Tide Rolls, 2010)
Etymology of Paleoplacer; Paleo + Placer:
The
word placer derives from the Spanish placer, meaning shoal or
alluvial/sand deposit, from Catalan placer, (shoal), from plassa,
(place) from Medieval
Latin placea
(place) the origin word for "place" and "plaza" in English.[2] The word in Spanish is thus
ultimately derived from placea and refers directly to an alluvial or glacial deposit of sand or
gravel. (Lotje, Smith, Greer, Sactor,
& and Others, 2013)
Syllabification:
plac·er
noun
[often as modifier]
Origin: Early 19th century: from Latin American Spanish,
literally 'deposit, shoal'; related to placel
'sandbank', from plaza 'a place’
(Placer, 2016)
Syllabification:
paleo-
(British palaeo-)
Origin: From Greek palaios 'ancient'.
(Dictionaries, 2016)
Mineral Mined from
Placer Deposits
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Other Industrial Minerals: Gravel, Boulders, Sand, Marble
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Bauxite (Aluminum)
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- To
understand the types of Paleoplacers one must also understand Placers.
- Eluvium
or Residual
- Alluvial
- Beach
- Glacial
- Aeolian
Eluvium or Residual
In
geology,
eluvium,
eluvial, or alluvial deposits
are those geological deposits and
soils that are derived by
in situ weathering
or weathering plus gravitational movement or accumulation.
(Smith &
Altenman, 2006)
Eluvial placers form on hill slopes 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. Examples include the earlier worked gold deposits of Australia
and the cassiterite placers of Malaysia. (Placer Deposits, 2016)
Residual deposits are more common where there has been
weathering on rocks and where there hasn’t been water. They are deposits which
have not been washed away yet or been moved. The residual usually lies at the
site of the lode. This type of deposit undergoes rock weathering.
Alluvial
Alluvial or eluvial deposits sometimes have the largest
gold deposit and are very common. This deposit is created when a force of
nature moves or washes the gold away, but it doesn’t go into a stream bed. It
contains pieces of ore that have been washed away from the lode. Alluvial
deposits are the most common type of placer gold. This type of deposit occurs
mostly in valleys. (Lotje, Smith, Greer, Sactor,
& and Others, 2013)
Stream placers, by far the most important, have yielded the
most placer gold, cassiterite, platinum, and gemstones. Primitive mining
probably began with such deposits, and their ease of mining and sometime great
richness have made them the cause of some of the world’s greatest gold and
diamond “rushes.” Stream placers depend on swiftly flowing water for their
concentration. 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. Examples of stream placers include the rich
gold deposits of Alaska and the Klondike, the platinum placers of the Urals,
the tin (cassiterite) deposits of Malaysia, Thailand, and Indonesia, and the
diamond placers of Congo (Kinshasa) and Angola. (Placer Deposits, 2016)
Bench
Bench deposits are created when gold reaches a stream bed.
Gold accumulations in an old stream bed that are high are called bench
deposits. They can be found on higher slopes that drain into valleys. Dry
stream beds (benches) can be situated far from other water sources and can
sometimes be found on mountain tops. Today, many miners focus their activities
on bench deposits.
Beach
Beach
placers form on seashores where wave action and shore currents shift
materials, the lighter more rapidly than the heavier, thus concentrating them.
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.
(Placer Deposits, 2016)
(Laznicka, 2010)
Glacial
The overall quantity of the gold left behind is generally
much less than you would expect to find in the western United States. Although
much of the deposits that were left behind were likely very rich, they were
distributed so widely across the landscape that very few areas got appreciable
amounts of gold in any one area. There are a few exceptions however, where
significant deposits have been recovered.
Most glacial gold is extremely fine textured. The
occasional “picker” and nugget can sometimes be found, but most gold
prospectors in the Midwest have found that by using techniques that are
specifically designed for
fine
gold recovery will result in the highest yield.
Pleistocene glacial and related fluvial sediments of the
Cariboo Mining District in central British Columbia, Canada. (Nicholas Eyles, 1989)
Glacial placers, that overlie the fluvial placers, occur
within lodgment till complexes deposited below the retreating ice sheet, the
basal portions of lodgment tills are commonly enriched in gold as a result of
incorporation from older gravels. Subglacial meltwaters created a highly
effective sluicing system and left lucrative pay zones along meltwater-cut
channels on bedrock benches, within intraformational gravels in lodgment till
and within “lee-side” deposits down-ice of bedrock highs. “Lee-side” deposits
are essentially water-worked talus slopes that accumulated in subglacial
cavities. Finally, postglacial “wandering gravel-bed rivers” have repeatedly
reworked older placers resulting in rich pay zones at the base of extensive bar
platform deposits. (Nicholas Eyles, 1989)
Similar sedimentological controls on gold distribution can
be identified in other glacial placers of late Cenozoic and Paleozoic age in
North America, southern Africa and Australia. A distinction is drawn between
these placers, all characterized by coarse-grained, nuggety gold, and the more
well-known Precambrian and Paleozoic placers where finely-comminuted gold is
dispersed through large thicknesses of rock. Episodes of glaciation typically
occur after long periods of tropical and subtropical weathering when supergene
processes were active and glaciers were able to remove and concentrate coarse
gold. In contrast, gold in non-glacial placers of Precambrian and Paleozoic age
has been through many cycles of erosion and transport and coarse gold is
uncommon. (Nicholas Eyles, 1989)
Aeolian also spelled
eolian or æolian
Eolian placers may form in arid areas where wind, not
water, acts as the concentrating agent, removing fine particles of the lighter
dross. The gold deposits of some parts of the Australian desert are examples. (Placer
Deposits, 2016)
Windblown deposits of mineral-rich dust and silt is called
loess. The term comes from the German
word Loss and from the Alemannic word
losch which means "loose".
A sedimentary rock formed from sand deposits is called a
'Sandstone' or generically a 'Psammite'
Aeolian
processes, also spelled eolian or æolian, pertain to wind activity in
the study of geology and weather and
specifically to the wind's ability to shape the surface of the Earth (or other planets). Winds may
erode, transport,
and deposit materials and are effective agents in regions with sparse vegetation, a lack of
soil moisture and a large supply of unconsolidated sediments. Although
water is a much more powerful eroding force than wind, aeolian processes are
important in arid environments such as deserts.
The term is
derived from the name of the Greek
god Aeolus, the keeper of the winds.
Paleoplacer Types:
Witwatersrand-Type:
Dictionaries, O. (Ed.). (2016, 2 28). Paleo.
Retrieved 2 28, 2016, from Oxford Dictionaries: http://www.oxforddictionaries.com/us/definition/american_english/paleo-
Laznicka, P. (2010). Giant Metallic Deposits:
Future Sources of Industrial Metals (Second ed.). West Lakes South
Australia, Australia: Springer Heidenlberg Dordrecht. Retrieved 2 28, 2016,
from
https://books.google.com/books?id=1tWllDxMkEYC&pg=PA557&lpg=PA557&dq=type+localities+for+paleoplacers&source=bl&ots=ia5FRNZ9Sm&sig=meY5hcva3JABUGfCyO83yDI1vzs&hl=en&sa=X&ved=0ahUKEwiI57vzo53LAhUk4qYKHcO5AgEQ6AEIUTAI#v=onepage&q=type%20localities%20for%20p
Lotje, Smith, Greer, Sactor, & and Others. (2013,
5 3). Wikipedia. Retrieved from Placer Mining:
https://en.wikipedia.org/wiki/Placer_mining
Nicholas Eyles, S. P. (1989). Sedimentological
controls on gold in a late Pleistocene glacial placer deposit, Cariboo Mining
District, British Columbia, Canada. (E. B.V., Producer, & Elsevier
B.V.) Retrieved 2 28, 2016, from Science Direct:
http://www.sciencedirect.com/science/article/pii/0037073889900055
Placer.
(2016, 2 28). Retrieved from Oxford Dictionaries:
http://www.oxforddictionaries.com/us/definition/american_english/placer
Placer Deposits. (2016, 2 29). Retrieved 2 29, 2016, from Encyclopedia Britannica:
http://www.britannica.com/science/placer-deposit
Plazak, Meissner, M. W., Trafford09, & Tide
Rolls. (2010, December 4). Placer Deposits. (Plazic, Editor) Retrieved
April 21, 2015, from Wikipediat: https://en.wikipedia.org/wiki/Placer_deposit
Smith, & Altenman, P. a. (2006, April 19). Eluvium.
(Wikipedia.org) Retrieved 2 29, 2016, from Wikipedia:
https://en.wikipedia.org/wiki/Eluvium
