A placer deposit is a geological concentration of heavy, durable minerals that have been separated from their original source rock and accumulated in a new location. The name originates from the Spanish word placer, meaning “alluvial sand.” Placer formation relies on the principle of gravity separation, where minerals with a significantly higher specific gravity, or density, are concentrated. For a mineral to accumulate, its density must be greater than the common surrounding sediment, such as quartz, allowing natural forces to effectively sort and concentrate the valuable materials.
How Placer Deposits Form
The process of placer formation begins with the physical and chemical breakdown of the primary source rock, known as a lode deposit. Weathering and erosion gradually loosen and free the valuable mineral grains from the less-resistant host rock matrix. These released particles are then transported by natural agents, most frequently by flowing water in streams and rivers.
The concentration of these heavy materials occurs where the velocity of the water current suddenly decreases. Water’s ability to transport larger or denser particles drops sharply as the flow rate slows. This reduction in speed causes the dense mineral particles to settle out of the suspension much faster than lighter sediment like sand and gravel.
These heavy minerals are deposited in natural traps along a stream bed, such as on the inside bends of a river, behind obstructions, or in potholes in the bedrock. The selective removal of the lighter material, often referred to as “winnowing,” leaves behind a concentrated layer of the heavier minerals. To accumulate in a placer, a mineral must also be highly resistant to both mechanical abrasion during transport and chemical breakdown.
Categorizing Placer Deposits
Placer deposits are classified based on the physical environment where the concentration process takes place. There are four main types of placers:
Alluvial Placers
This is the most widespread and economically productive category, sometimes called a stream or fluvial placer. Alluvial placers form within the beds and floodplains of rivers, representing the final stage of transport where running water has sorted and deposited the materials along the drainage system.
Eluvial Placers
This type is a residual deposit found on hillslopes close to the original source rock. Lighter materials are carried away by rainfall runoff and wind, leaving behind a concentration of heavy minerals near the surface. These deposits exist before the minerals are transported into a stream.
Beach Placers
These placers form along coastlines and shorelines from the continuous action of waves and currents. Wave action effectively sorts the material, helping to concentrate the heavier minerals, often forming layers of dense material known as “black sand.”
Eolian Placers
Found in arid regions, Eolian placers use wind as the primary concentrating agent. Wind blows away fine, light particles, leaving the heavier grains behind.
Valuable Minerals and Extraction Techniques
A variety of economically important minerals are found in placer deposits, all sharing the properties of high density and durability. Gold is the most famous placer mineral, known for its high specific gravity (between 15 and 19 grams per cubic centimeter) and chemical stability. Other valuable placer minerals include platinum, cassiterite (tin oxide), diamonds, zircon, rutile, and ilmenite.
The methods used to extract these minerals capitalize on their high density to separate them from the surrounding gravel and sand. Panning is the simplest and oldest technique, where agitation and swirling water in a shallow pan allow the heavy minerals to settle to the bottom while the lighter material is washed over the rim.
On a larger scale, the sluice box is a widely used technique, consisting of a long channel with riffles, or barriers, fixed to the bottom. As water and sediment pass through, the riffles create turbulence that helps the dense minerals fall out of suspension and become trapped, while the lighter material is carried away. Modern operations employ mechanized systems like large-scale dredging and hydraulic mining, which uses high-pressure water to break down the deposit. The material is then fed into sophisticated gravity separation equipment to maximize recovery.