Placer deposits are concentrated by the action of movingwater in rivers or the sea.
Transport sorts sediment by:
Grain size
Hardness
Density
Cleavage
Less resistant minerals worn away or dissolved by water leaving resistant and insoluble minerals as placer deposits.
Minerals found in placer deposits are:
hard with no cleavage
Chemically unreactive
Dense
Placer deposits are found at places along rivers where the water slows down.
Slow water:
Plunge pool
Inside a meander bend
Rapids
Floodplains
Confluence
Storm beach (marine)
Cassiterite is insoluble and unreactive, it occurs in the hydrothermal veins in granite.
Chemical weathering of granite produces clay minerals, unreactive quartz and cassiterite. These products are weathered and move downslope into streams.
Cassiterite has a high density so is deposited sooner in areas of low energy.