Porphyry Deposits
Porphyry deposits are typical of the mineral deposits formed by igneous activity. They contain any combination of copper, tungsten, molybdenum and gold, but the classic examples are the porphyry copper deposits of the South American Andes. Porphyry-type deposits are large, and the grade of mineralization is usually low.
Both the intrusion and the country rocks are densely fractured, so the mineralization typically forms vein or breccia bodies in the intrusion itself or in the country rock around it. The fractures are often radial, extending out from the intrusion like the spokes of a wheel, or concentric, with the intrusion at the center. The veins are usually made up of sulphide minerals, plus quartz, calcite, dolomite and fluorite gangue. The metals occur in minerals like chalcopyrite, molybdenite, wolframite and native gold.
The rocks around the deposit are highly altered since large volumes of hydrothermal fluids have circulated through them. In the rocks furthest away from the intrusion, many of the primary minerals are leached. In rocks closer to the intrusion, the primary minerals are completely destroyed and replaced by micas and clays. At the center of the hydrothermal system, where it is hottest, hydrothermal feldspars are formed.
Another process, while it affects many different kind of deposits, is particularly important to porphyry coppers. The process is called supergene enrichment, and occurs after the primary copper deposit has formed. Weathering processes leach copper out of the upper parts of the deposits; the copper-bearing solution travels downward, through fractures and pore spaces, until it meets the water table. Because the copper is no longer in contact with the air, it becomes unstable in solution. It precipitates, forming copper-rich minerals like bornite, chalcocite and cuprite, even native copper. This enriched zone can have substantially higher ore grades than the primary mineralization.
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