Porphyry Deposits

Porphyry deposits are typical of the mineral deposits which are formed by igneous activity. they contain any combiation of gold, molydenum, tungsten and copper, but the best and classic examples are the porphyry deposits of the Andes of South America. the types of deposits of porphyry are very big, and the level of mineralization is most of the time very low.

both the country rocks and the intrusion rocks are fractured densely, so because of this the mineralization typically forms veins or reccia bodies in the country rock around it or in the intrusion itself. the fractures are radial most of the time, extending out from the intrusion like the spokes of a wheel, or concentric, with the intrusion at the centre of it. the veins very often are made up of minerals of sulphide, including fluorite gangue, dolomite, calcite and quartz. minerals like native gold, wolframite, molybdenite and chalcopyrite are what make the metals occur.

The rocks around the deposit are highly altered due to the fact that large volumes of hydrothermal fluids have circulated through them. many of the primary minerals are leached in the rocks which are the furthest away from the intrusion. the primary minerals are totally destroyed and replaced by micas and clays whe referring to the rocks which are closer to the intrusion. hydrothermal fledspars are formed when they are at the center of the hydrothermal system, this is the area where it is hottest.

Another process, while it affects many different kinds of deposits, is particularly important to prophyry coppers. this process has a name, it is called the supergene enrichmet, this process occurs after the primary deposit of copper has formed. weathering processes leach copper out of the upper parts of the deposit; the solution which bears the copper travels in a downward direction, through pore spaces and fractures, until it meets the water table. due to the fact that copper has contact with the air no longer, it becomes unstable in solution. it precipitates and forms minerals rich in copper like cuprite, chalcocite, bornite, and even native copper. this enriched area can have ore grades which are substantially higher than the primary mineralization.