How do river and beach sands become workable deposits of heavy minerals?

River and beach sands become workable deposits of heavy minerals through a natural process of gravity-driven concentration known as the formation of a placer deposit.1

This process relies on the following key factors:

• Weathering and Release: Minerals that are chemically resistant to weathering and possess high physical toughness and specific gravity are freed from their original rock matrix.12
• Transport and Winnowing: As these heavy particles are washed into streams or moved by wave action along seashores, the energy of the water acts as a sorting mechanism. The moving water carries away lighter, low-density materials (such as quartz) more rapidly than the heavier, durable minerals.314
• Deposition: When the velocity of the water decreases—such as in stream beds or along coastlines—the heavy minerals settle and accumulate. Over time, these accumulations form concentrated layers of "black sand" or other mineral-rich deposits that are economically viable to extract.314

These deposits are then recovered using various mining techniques, including surface excavation or dredging.342