geology Geology (). is a branch of natural science concerned with the Earth and other astronomical objects, the rocks of which they are composed, and the processes by which they change over time. Modern geology significantly overlaps all other Earth ...

, 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 Gold is a chemical element; it has chemical symbol Au (from Latin ) and atomic number 79. In its pure form, it is a brightness, bright, slightly orange-yellow, dense, soft, malleable, and ductile metal. Chemically, gold is a transition metal ...

, and was the main technique used in the early years of many gold rushes, including the

California Gold Rush The California gold rush (1848–1855) began on January 24, 1848, when gold was found by James W. Marshall at Sutter's Mill in Coloma, California. The news of gold brought approximately 300,000 people to California from the rest of the U ...

. Types of placer deposits include

alluvium Alluvium (, ) is loose clay, silt, sand, or gravel that has been deposited by running water in a stream bed, on a floodplain, in an alluvial fan or beach, or in similar settings. Alluvium is also sometimes called alluvial deposit. Alluvium is ...

eluvium In geology, eluvium or eluvial deposits are geological deposits and soils that are derived by ''in situ'' weathering or weathering plus gravitational movement or accumulation. The process of removal of materials from geological or soil horiz ...

, beach placers, aeolian placers and paleo-placers. 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. Placer environments typically contain black sand, a conspicuous shiny black mixture of iron oxides, mostly

magnetite Magnetite is a mineral and one of the main iron ores, with the chemical formula . It is one of the iron oxide, oxides of iron, and is ferrimagnetism, ferrimagnetic; it is attracted to a magnet and can be magnetization, magnetized to become a ...

with variable amounts of ilmenite and hematite. Valuable mineral components often occurring with black sands are monazite, rutile, zircon, chromite, wolframite, and cassiterite. Early mining operations were probably a result of placer deposits as they were easily accessible and potential size. The events known as gold/diamond rushes were caused by placer deposits and have proved to be plentiful.

Characteristics of placer minerals

Density

Placer minerals are defined as having a specific gravity above 2.58. The separation of the valuable minerals from the most common non-economic mineral,

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...

, depends on the difference in specific gravity /

density Density (volumetric mass density or specific mass) is the ratio of a substance's mass to its volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' (or ''d'') can also be u ...

.The weathering process allows for the accumulation of placer minerals, while less dense material such as quartz are swept away.

Hardness

Hardness In materials science, hardness (antonym: softness) is a measure of the resistance to plastic deformation, such as an indentation (over an area) or a scratch (linear), induced mechanically either by Pressing (metalworking), pressing or abrasion ...

allows 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.

Stability

Chemical stability In chemistry, chemical stability is the thermodynamic stability of a chemical system, in particular a chemical compound or a polymer. Colloquially, it may instead refer to kinetic persistence, the shelf-life of a metastable substance or system; t ...

is important for a placer mineral to resist chemical breakdown such as

oxidation Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is ...

, Minerals lacking chemical stability will form less desirable alteration when subject to chemical breakdown. PlacerEN-01

Types of placers

Alluvial placers

Alluvial placers are those formed in river or stream sediments. Another name for alluvial placers are stream placers. Typical locations for alluvial gold placer deposits are on the inside bends of

river A river is a natural stream of fresh water that flows on land or inside Subterranean river, caves towards another body of water at a lower elevation, such as an ocean, lake, or another river. A river may run dry before reaching the end of ...

s and creeks; in natural hollows; at the break of slope on a stream; the base of an escarpment,

waterfall A waterfall is any point in a river or stream where water flows over a vertical drop or a series of steep drops. Waterfalls also occur where meltwater drops over the edge of a tabular iceberg or ice shelf. Waterfalls can be formed in seve ...

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. 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. Materials collect as beach placers by continuous wave action and currents. 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. 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

, however gold,

platinum Platinum is a chemical element; it has Symbol (chemistry), symbol Pt and atomic number 78. It is a density, dense, malleable, ductility, ductile, highly unreactive, precious metal, precious, silverish-white transition metal. Its name origina ...

and chromite are found in varying amounts. 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.

Eluvial placers

Eluvial placers are deposits of metal formed on hillsides and slopes weathered by rainfall and wind. Lighter materials are carried away and weathered leaving concentrations of valuable metals. Eluvial placers are typically not large enough to support large scale mining, however in one case in

Nevada Nevada ( ; ) is a landlocked state in the Western United States. It borders Oregon to the northwest, Idaho to the northeast, California to the west, Arizona to the southeast, and Utah to the east. Nevada is the seventh-most extensive, th ...

there has been large scale gold mining operations based on placer deposits at Round Mountain.

Aeolian placers

Aeolian placers are valuable minerals found in arid regions freed from their source rock by wind actions. 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. Historically, aeolian placers have been mined by hand with simple tools by miners due to the proximity to the surface, and small concentration. Aeolian placers are common in

Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...

and the 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. These deposits are typically very far underground in ancient riverbeds, beaches, or slopes. 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. The Witwatersrand Basin is considered an ancient alluvial placer.

Substances mined

Substances commercially mined from placer deposits include: *

Diamond Diamond is a Allotropes of carbon, solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. Diamond is tasteless, odourless, strong, brittle solid, colourless in pure form, a poor conductor of e ...

Gold Gold is a chemical element; it has chemical symbol Au (from Latin ) and atomic number 79. In its pure form, it is a brightness, bright, slightly orange-yellow, dense, soft, malleable, and ductile metal. Chemically, gold is a transition metal ...

Garnet Garnets () are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives. Garnet minerals, while sharing similar physical and crystallographic properties, exhibit a wide range of chemical compositions, de ...

Iron Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...

, from ironsands containing high concentrations of

* Platinum group metals * Rare-earth elements, from the mineral monazite *

Ruby Ruby is a pinkish-red-to-blood-red-colored gemstone, a variety of the mineral corundum ( aluminium oxide). Ruby is one of the most popular traditional jewelry gems and is very durable. Other varieties of gem-quality corundum are called sapph ...

Sapphire Sapphire is a precious gemstone, a variety of the mineral corundum, consisting of aluminium oxide () with trace amounts of elements such as iron, titanium, cobalt, lead, chromium, vanadium, magnesium, boron, and silicon. The name ''sapphire ...

* Thorium, from monazite * Tin, in the mineral cassiterite * Titanium, from the minerals ilmenite and rutile *

Uranium Uranium is a chemical element; it has chemical symbol, symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Ura ...

, from Precambrian paleoplacers * Zirconium, from the mineral zircon

Source rock

Diamonds

Kimberlites are a major source of diamonds. Kimberlites are long "pipes" of lava brought up from the mantle where

diamond Diamond is a Allotropes of carbon, solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. Diamond is tasteless, odourless, strong, brittle solid, colourless in pure form, a poor conductor of e ...

s form at high temperatures and pressures. These diamonds are then weathered from the source and swept away by alluvial processes (transported by water) to a source that becomes a diamond deposit. Alluvial diamond deposits are mined after removing overburden from the top of the rich, diamond-gravel layer. Roughly 10 percent of diamonds are mined from alluvial diamond placer mines. The Witwatersrand Basin in Africa mentioned above is a recognized alluvial diamond deposit.

References

{{Reflist __FORCETOC__ Ore deposits Economic geology