Metal zoning and precipitation mechanisms in porphyry systems

A great majority of porphyry districts around the world show a regular pattern of zonal metal distribution both vertically and laterally. Metal zonation commonly consists of a Cu +/- Mol +/- Au core ringed by successive Cu-Zn, Zn-Pb-Ag, Pb-Ag (+/- Mn), and As-Sb-Hg +/- Au zones. If a high-sulfidation epithermal system is developed above the porphyry center, a vertical Cu +/- Au +/- Ag zone may be present in the central upflow conduits. Comparing metal content in natural fluid inclusions from porphyry and associated skarn and epithermal deposits with thermodynamic predictions provides useful constraints on the mechanisms and processes controlling metal transport and deposition in porphyry systems. Results show that decompression and cooling of a magmatic fluid, accompanied by boiling and water-rock interaction, are likely to be the major causes of zonal metal deposition in magmatic-hydrothermal systems. Large-scale fluid mixing is a relatively uncommon phenomenon during metal precipitation and is not expected to be a major cause of ore formation in the porphyry environment.