Black sand concentrates

In geology, a placer deposit or placer is an accumulation of valuable minerals formed by deposition of dense mineral phases in a trap site. Geology (from Greek γη gê, "earth" and λόγος Logos, "speech" lit The name is from the Spanish word placer, meaning "alluvial sand". Types of placer deposits include alluvium, eluvium, beach placers, and paleoplacers. Alluvium (from the Latin, alluvius, from alluere, "to wash against" is Soil or Sediments deposited by a river or other running In Geology, eluvium or eluvial deposits are those geological deposits and Soils that are derived by in situ Weathering or weathering

Alluvial placers

Typical locations for alluvial 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, within sand dunes, beach profiles or in gravel beds. "Riverine" redirects here For the use of that term in Maritime geography, see there A stream is a body of Water with a current, confined within a bed and stream-banks In Geomorphology, an escarpment is a transition zone between different physiogeographic provinces that involves a sharp steep Elevation differential characterized A waterfall is usually a geological formation resulting from water often in the form of a Stream, flowing over an Erosion -resistant rock In physical Geography, a dune is a Hill of Sand built by Aeolian processes.

Section of Alluvial Placer Deposit at the Blue Ribbon Mine Alaska

Beach placer deposit of heavy minerals (dark) in a quartz sand (Chennai, India). Quartz (from German) is the most abundant Mineral in the Earth 's Continental crust (although Feldspar is more common in Sand is a naturally occurring Granular material composed of finely divided rock and Mineral particles

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 sought after must show a marked density contrast with the gangue material, which is able transported away from the trap site. Tailings (also known as slimes, gangue, tailings pile, tails, leach residue, or slickens) are the materials left over Only if the deposit is winnowed in this way can the minerals be concentrated to economic levels.

Placer materials must be both dense, and resistant to weathering processes. Weathering is the decomposition of earth rocks, Soils and their Minerals through direct contact with the planet's Atmosphere. Placer environments typically contain black sand, a conspicuous shiny black mixture of iron oxides, mostly magnetite with variable amounts of ilmenite and hematite. Black sand is a heavy glossy partly Magnetic mixture of usually fine sands found as part of a Placer deposit. Magnetite is not to be confused with Magnesite or Maghemite. Magnetite is a ferrimagnetic Mineral with chemical Ilmenite is a weakly magnetic titanium-iron oxide Mineral which is iron-black or steel-gray Hematite, also spelt hæmatite, is the Mineral form of Iron(III oxide (Fe2O3 one of several Iron oxides Valuable mineral components often occurring with black sands are monazite, rutile, zircon, chromite, wolframite, and cassiterite. In Geology, the Mineral monazite is a reddish-brown Phosphate -containing rare earth metals and an important source of Thorium Rutile is a Mineral composed primarily of Titanium dioxide, Ti[[oxygen O]]2 Zircon is a Mineral belonging to the group of nesosilicates. Its chemical name is Zirconium silicate and its corresponding chemical formula is Chromite is iron magnesium chromium oxide (Fe MgCr2O4 It is an Oxide Mineral belonging to the Spinel group Cassiterite is a Tin Oxide Mineral, SnO2. It is generally opaque but is translucent in thin crystals

Exceptionally dense substances such as gold and the platinum group members will accumulate in placers, when they are present. Gold (ˈɡoʊld is a Chemical element with the symbol Au (from its Latin name aurum) and Atomic number 79 The platinum group (alternatively the platinum group metals or platinum metals) is a collective name sometimes used for six Metallic elements Placer mining is an important source of gold, and was the main technique used in the early years of the California Gold Rush. Placer mining (pronounced "plass-er" refers to the mining of alluvial deposits for Minerals This may be done by open-pit (also called open-cast Gold (ˈɡoʊld is a Chemical element with the symbol Au (from its Latin name aurum) and Atomic number 79 The California Gold Rush (1848&ndash1855 began on January 24 1848 when Gold was discovered by James Marshall at Sutter's Mill in Coloma, California

Substances commercially mined from placer deposits:

• Gold
• Platinum group metals
• Tin
• Diamonds
• Thorium
• Uranium (from paleoplacers)

See also

• Heavy mineral sands ore deposits
• Black sand
• Placer mining
• Gold rush
• Sedimentology

External links

• The Blue Ribbon Mine Alaska
• Goldplacer
• Arizona Gold Prospectors