Introduction to Paleoplacer Deposits

Paleoplacers

By Michael W. Meissner 

#MichaelwMeissner #Michael W Meissner #Michael William Meissner #Geologist #michaelwmeissner.com

Definitions

Placer Deposit Defined. 

  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)

• Placer

Syllabification: plac·er

noun

[often as modifier]

A deposit of sand or gravel in the bed of a river or lake, containing particles of valuable minerals: ‘placer gold deposits’

Origin: Early 19th century: from Latin American Spanish, literally 'deposit, shoal'; related to placel 'sandbank', from plaza 'a place’

(Placer, 2016)

• Paleo

Syllabification: paleo-

 (British palaeo-)

Older or ancient, especially relating to the geological past: Paleolithic paleomagnetism

Origin:  From Greek palaios 'ancient'.

(Dictionaries, 2016)

Economics and Importance of Placer Deposits

Mineral Mined from Placer Deposits

·  Diamonds	·  Quartz Monazite Sands	·  Titanium, from the minerals ilmenite and rutile
·  Gold	·  Rare earth elements, from the mineral monazite	·  Uranium, from Precambrian paleoplacers
·  Garnet	·  Ruby	·  Zirconium, from the mineral zircon
·  Iron, from ironsands containing high concentrations of magnetite	·  Sapphire	Other Industrial Minerals: Gravel, Boulders, Sand, Marble
·  Platinum group metals	·  Thorium, from the mineral monazite	Bauxite (Aluminum)

Types of Paleoplacers and Placers

• 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:

Type Locations of Placers and Paleoplacers

Placers Type Locations

Paleoplacer Type Locations

Rand District
Sierra Nevada
Huronian Supergroup of Ontario		https://journals.lib.unb.ca/index.php/GC/article/viewFile/3370/3884

Understanding the Gold Systems as a Best Practice

Bibliography

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