exogenic mineral deposits

• Placer deposits are concentrated by the action of moving water 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.