Processes that form deposits of metalliferous ores

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  • Processes that form deposits of metalliferous ores
    • Hydrothermal Ore Deposits
      • heat & water source from: magma in intrusion
      • hydrothermal vein: example of deposit
        • fracturing of a rock allows hydrothermal solution to deposit ore in open space
          • the vein is younger than the rocks that contain it
            • first minerals crystallise at the edges and later form towards the centre
    • Magmatic deposits
      • syngenetic: ore minerals crystallise from same liquid that produces the silicate minerals
      • pegmatite: example
        • very coarse-grained igneous rock = usually granitic composition
        • fractional crystallisation occurs where rare metals become conc in water-rich residual magma = this magma migrates to form pegmatites
    • Gravity settling in ore deposits
      • Bushveld Igneous Complex is example
      • dense ore minerals form conc layer at base of intrusion to form accumulates
        • as magma rises and cools it differentiates
    • Porphyry Ore Deposits
      • often formed in island arc plate tectonic environments
      • magma cools at depth forming large crystals
        • if magma escapes, water boils to create fractures in overlying rock
          • magma cools & crystallises to give matrix of small crystals around large crystals

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