Placer deposit

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, depends on the difference in specific gravity / density.The weathering process allows for the accumulation of placer minerals, while less dense material such as quartz are swept away. Hardness Hardness 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 is important for a placer mineral to resist chemical breakdown such as oxidation, Minerals lacking chemical stability will form less desirable alteration when subject to chemical breakdown. == Types of placers ==

Alluvial placers placer deposit at the Blue Ribbon Mine, Alaska Alluvial placers are those formed in river or stream sediments. Another name for alluvial placers are stream placers. 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. 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. 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 • Gold • Garnet • Iron, from ironsands containing high concentrations of magnetite • Platinum group metals • Rare-earth elements, from the mineral monazite • Ruby • Sapphire • Thorium, from 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 Kimberlites are a major source of diamonds. Kimberlites are long "pipes" of lava brought up from the mantle where diamonds 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. == See also ==