How are alluvial tin deposits formed?

Alluvial tin deposits, a type of placer deposit, are formed through the natural concentration of heavy minerals by gravity in moving water.12

The process begins when weathering releases cassiterite—the primary tin ore—from its original rock matrix. Because cassiterite is chemically resistant, durable, and has a high specific gravity, it is not easily broken down by environmental exposure. As these mineral grains are washed downslope into streams or valleys, the flowing water acts as a sorting agent.12

Because the ability of water to transport material depends on its velocity, heavier particles settle out of the flow more quickly than lighter waste materials when the water speed decreases. Over time, this gravity-based separation causes the dense cassiterite to accumulate in ancient riverbeds, valleys, or stream gravels, creating the concentrated deposits that are later recovered through tin processing methods such as dredging or gravel pumping.13