Placer deposits are placed by streams and rivers. Rocks enriched for heavier minerals settle to the bottom in regions of slower current, such as eddies. Geologica models placer deposits as sand and generates them on the beaches and the sides of rivers. There is cassiterite sand, granitic and basaltic mineral sand, and garnet sand. The general "mineral" sands consist largely of iron oxides and can be smelted to iron. With more complex processing, additional minerals can be isolated, depending on whether the host rock was granitic (felsic) or basaltic (mafic).
Basaltic mineral sand deposited on a gravel beach.
Granitic sand accumulated on the bank of a canyon river.