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How do alluvial gold deposits form in riverbeds and floodplains?

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A worker in safety gear walks beside a conveyor belt carrying ore at the Tanami gold mine in the arid, expansive landscape of the Tanami Desert. — gold processing The Tanami gold mine, Northern Territory, Australia.

Alluvial gold deposits, also known as stream placers, form through the natural concentration of heavy minerals by gravity in moving water. The process begins when weathering breaks down gold-bearing rocks, releasing gold particles from their original matrix.12

Once liberated, these particles are transported downslope by rain and eventually enter streams. Because gold has a high specific gravity, it is highly susceptible to gravitational sorting. As water flows through riverbeds and floodplains, it acts as a winnowing agent: the energy of the moving water carries away lighter materials while allowing the denser gold particles to settle.132

The concentration of these deposits is heavily dependent on water velocity. When the flow rate decreases, the water loses its capacity to transport heavy particles, causing the gold to be deposited much more rapidly than lighter sediment. This process results in the accumulation of gold in riverbeds and floodplains, where it is typically found in its elemental form and often extracted using gravity concentration techniques.21