Placer Deposits - Mining Fundamentals

A placer deposit, or placer, is a natural accumulation of heavy valuable minerals, which is formed by the gravitational effect during sedimentary processes. Placer mining is critical for gold extraction, and one of the main mining methods used in several gold rushes, including the California Gold Rush. The name was derived from the Spanish word “placer”, which means alluvial sand.

Heavy minerals, that are washed off from their matrix by weathering processes, slowly move down into the streams and settle in the lighter matrix. As a result, these minerals are concentrated in residual, beach, and stream deposits, thereby forming workable ore deposits. Minerals of placer deposits have high weathering resistance, durability and high specific gravity.

Types of Placer Deposits

There are various types of placer deposits, classified based on their formation. Some of them are discussed below:

• Residual placers – It is a type of deposit formed at, or near, the point of release of mineral from the parent rock. The more weathering-resistant minerals will collect near the outcrop, while the less-resistant minerals will be carried away by erosion.
• Stream placers – These placers include valuable minerals, which are concentrated in gravel and sand by the effect of running water. They are generally formed around the obstruction in the water channel or the place at which the water slows down.
• Eluvial placers – These deposits are formed on the hill slopes from weathered deposits. The surface creep slowly moves the mineral and weathered detritus down hill, thereby allowing the light-weight minerals to be removed by wind or rain water.
• Beach placers - Beach placers are formed in gravel and surface near the edge of large water bodies, or along the wave-cut base of a gold-bearing coastal plain.
• Eolian placers - Eolian placers are formed by the action of wind, which blows the lighter rock particles or sand away from a body of low-value material, leaving behind surface veneer containing heavy minerals.

Sources and Further Reading

• Types of Placers – Western Mining History
• Placer Deposit - Britannica