Breccia Zones - Often major ore carriers in the porphyry system. May be formed by hydrothermal activity, gravitational collapse or later explosive volcanism. The zoning appears to be related to existing or ancient subduction zones with copper deposits nearest the trench while moly and tin deposits lie successively inboard of the trench. It has been suggested that porphyry deposits are associated with small, high level stocks and subaerial calc-alkaline volcanism. Thus, the pluton itself is overlain by a stratovolcano.
Propylitic alteration extends upward into the volcano.
Others alteration zones close on themselves and die out in the subsurface. In general, this model attempts to show that the porphyry copper deposit is part of a larger system that includes higher level epithermal precious metal deposits. Diorite Model Recent studies have shown that there are two distinctly different types of porphyry systems. The second type is characteristic of the Andes and Pacific Islands. The host in this case is usually diorite or more rarely syenite.