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Placer Deposit

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Definitions

Placer deposit is a gold mining term used to describe a concentration of gold particles liberated from a primary source through mechanical or chemical processes, such as weathering and gravity over an extended period of time. The word "placer" derives from Spanish and was an expression used by Spanish miners in both South and North America to describe a gold deposit discovered in sand or stream gravel. In mining, the phrase may be used to describe a zone containing a significant economic concentration of gold. Other terms used by in mining industry to express the same concept include "paystreak," "payload" or "pay dirt."[1]

[edit] Process

Gold can be mined from either a primary or secondary source. The rock bed in which gold is deposited in a vein or lode with other minerals is known as the host rock. The lode vein situated inside the host rock is the primary source of the gold. When gold is mined from the primary source of hard rock it is called lode mining.[2] Over time, the lodes of gold are exposed and broken down by some process of natural erosion and transported and distributed elsewhere. Geological forces such as moving water, glaciers, wind, and the thrashing action of waves along a shoreline are all elements that can contribute to the movement and deposition of gold away from the host source to form a secondary or residual deposit known as a placer.[3] Gravity is another significant force resulting in the formation of all types of placer deposits.[4]

Not all gold travels far from the primary source. The presence of a lode vein can actually signify that a placer deposit is nearby. The reason for this is that gold, 19 times heavier than water, is very dense.[5] Gold particles of a larger size will likely be located in the vicinity of a lode vein trapped in place by their own weight. Sometimes gold is subject to gravitational forces and will move downwards as deep as possible, taking a path of least resistance. If in a stream, for example, it will come into contact with rocks and running water. These forces are exerted upon the gold, breaking it further down until, over time, it becomes smaller and finer. The finer and smaller the particles of gold, the more likely they have traveled a greater distance from the primary source.

[edit] Types

Eluvial or residual placers are overlying placer deposits of gold located very close to the primary source/ lode vein. This type of placer consists of weathered rock in which some of the finer and lighter minerals have been exposed and washed away, thereby yielding a higher concentration of gold.

Colluvial or deluvial placers exist where gold has been transported some great distance from the primary source/lode vein and is not situated in an established river or stream system.

Fluvial or alluvial placers are placer deposits established in a stream or river system. Gold is typically deposited in concentrates upstream of obstructions and obstacles and in spots where the velocity of the water is lower.

Marine placers are placer deposits formed by the natural sorting action of waves and tidal currents. Gold in a marine placer is often deposited with other dense minerals such as iron, titanium, or tin.[6]

[edit] References

  1. Special Gold Deposits. Gold Placers. E-gold Prospecting. 2008-12-03.
  2. Mining the Motherlode. Lode vs. Placer Mining. Wells Historical Society. 2008-12-03.
  3. What is Placer Gold? Goldplacer.com. 2008-12-03.
  4. Special Gold Deposits. Gold Placers. E-gold Prospecting. 2008-12-03.
  5. What is Placer Gold? Goldplacer.com. 2008-12-03.
  6. Marsden, John. The Chemistry of Gold Extraction. SME. 2006. pg.27