Weathering as a cause of mineral layer formation

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A porphyry copper deposit is composed of different layers of copper minerals. Minerals close to the surface are more oxidated than those deeper down. By courtesy of Anna Bauer and Andor Lips
A porphyry copper deposit is composed of different layers of copper minerals. Minerals close to the surface are more oxidated than those deeper down. By courtesy of Anna Bauer and Andor Lips

Different exposure to oxygen will affect what types of metal bearing materials that are formed in the ground. As the concentration of oxygen is diminishing with depth, layers of differently oxidised minerals will form. Therefore mineral types close to the surface (for example metal oxides) usually have become totally oxidised by the oxygen in the air and are hardly reactive with oxygen any more. Metal sulfides, on the other hand, are mostly found deeper down in more reduced environments. As the concentration of oxygen is low they stay stable there. But, because of anthropogenic activities they may cause ARD.

  • An upper oxidation zone, being in contact with atmospheric oxygen and rain water, which contains secondary minerals formed by oxidation of the primary ore minerals.
  • an underlying cementation zone just below the groundwater level, in which minerals, formed by the reaction of primary ore minerals with the constituents of the leaching solution descending from the oxidation zone, are accumulated.
  • A zone in which the primary ore minerals are unchanged.
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