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Definition: Alteration is a mineralogical change at low pressures due to invading fluids or the influence of oxygen.

Alteration may happen near the Earth's surface, where it is considered part of chemical weathering under the name supergene alteration (horn silver forms this way, as do many copper minerals like cuprite). But generally it refers to hydrothermal alteration, the deeper, hotter process that emplaces veins of ore minerals.

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