<P> Porphyry copper deposits are copper orebodies that are formed from hydrothermal fluids that originate from a voluminous magma chamber several kilometers below the deposit itself . Predating or associated with those fluids are vertical dikes of porphyritic intrusive rocks from which this deposit type derives its name . In later stages, circulating meteoric fluids may interact with the magmatic fluids . Successive envelopes of hydrothermal alteration typically enclose a core of disseminated ore minerals in often stockwork - forming hairline fractures and veins . Because of their large volume, porphyry orebodies can be economic from copper concentrations as low as 0.15% copper and can have economic amounts of by - products such as molybdenum, silver and gold . In some mines, those metals are the main product . </P> <P> The first mining of low - grade copper porphyry deposits from large open pits coincided roughly with the introduction of steam shovels, the construction of railroads, and a surge in market demand near the start of the 20th century . Some mines exploit porphyry deposits that contain sufficient gold or molybdenum, but little or no copper . </P>

Porphyry copper deposits are direct evidence for which geologic process
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