Why does cassiterite collect at the bottom of sluice boxes?

Cassiterite, the primary ore of tin, collects at the bottom of sluice boxes due to the principle of gravity separation, which relies on differences in specific gravity.

As water and placer dirt flow down the inclined sluice box, the riffles mounted on the bottom agitate the current. This agitation prevents lighter materials from settling, while the high density of cassiterite causes it to sink and be retained by the riffles. Because gravity methods use the density of minerals as a concentrating agent, heavier particles naturally separate from lighter gangue, which is washed away as tailings.123