Geological origins of economically important minerals

Igneous, Sedimentary and Metamorphic processes

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Igneous processes

 Igneous processes

  • Friction, heat and pressures produced can cause magma to be forced to the surface
  • It may reach the surface as an igneous extrusion where it cools rapidly and forms a fine-grained rock, e.g. basalt
  • Some magma fails to reach the surface so cools slower, producing larger crystals
  • Granite was produced in these deep deposits called batholiths
  • Cracks or fissures form around the batholith that may allow hot solutions containing dissolved minerals to move towards the surface
  • As these solutions cool down, the minerals are deposited in order of solubility, most metals are found as hydrothermal deposits
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Sedimentary processes

Sedimentary processes

  • Materials or processes that involve material being carried by air or water then deposited
  • Alluvial/placer deposits -> Carried by flowing water and deposited when the water slows down. Densest ones settle out first. Mineral solutions may later be deposited in the spaces between rock particles, e.g. sand to sandstone
  • Evaporites -> Formed when water evaporates from mineral rich solutions. Minerals crystallise as the solutions become saturated, each mineral reaching saturation at a different time, e.g. gypsum before halite
  • Biological deposits -> Produced from dead animals and plants, e.g. chalk and many limestones
  • Chemical precipitates -> May be deposited such as the manganese nodules found on some deep ocean beds
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Metamorphic processes

Metamorphic processes

  • Existing rock that is exposed to extreme heat and pressure from nearby igneous activity may change its form without melting, in a metamorphic process
  • Slate was formed form sedimentary shale, often made largely from clay.
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