Porphyry Deposits
by W.D. Sinclair

Contents of this page:

Abstract
Definition
Geographical distribution
Importance
Grade and Tonnage
Geological attributes
Genetic and exploration models
Exploration methods
Knowledge gaps
Areas of high mineral potential in Canada
Acknowledgements
References
Figures
Appendices

Abstract

Porphyry deposits are the world's most important source of Cu and Mo, and are major sources of Au, Ag, and Sn; significant byproduct metals include Re, W, In, Pt, Pd, and Se. They account for about 50 to 60% of world Cu production and more than 95% of world Mo production. In Canada, they account for more than 40% of Cu production, virtually all Mo production, and about 10% of Au production. Porphyry deposits are large, low- to medium-grade deposits in which primary (hypogene) ore minerals are dominantly structurally controlled and which are spatially and genetically related to felsic to intermediate porphyritic intrusions. They are distinguished from other granite-related deposits such as skarns and mantos by their large size and structural control, mainly stockworks, veins, vein sets, fractures, and breccias. Porphyry deposits typically contain hundreds of millions of tonnes of ore, although they range in size from tens of millions to billions of tonnes; grades for the different metals vary considerably but generally average less than 1%. In porphyry Cu deposits, for example, Cu grades range from 0.2% to more than 1% Cu; in porphyry Mo deposits, Mo grades range from 0.07% to nearly 0.3% Mo. In porphyry Au and Cu-Au deposits, Au grades range from 0.2 to 2 g/t Au. Associated igneous rocks vary in composition from diorite-granodiorite to high-silica granite; they are typically porphyritic epizonal and mesozonal intrusions, commonly subvolcanic. A close temporal and genetic relationship between magmatic activity and hydrothermal mineralization in porphyry deposits is indicated by the presence of intermineral intrusions and breccias that were emplaced between or during periods of mineralization. Porphyry deposits range in age from Archean to Recent, although most economic deposits are Jurassic or younger.

Figure 16: Schematic time-depth relations of principal alteration types in Au-rich porphyry Cu systems and other types of porphyry deposits (after Sillitoe, 1993b). Figure 17: Schematic diagram of a porphyry Cu system in the root zone of an andesitic stratovolcano showing mineral zonation and possible relationship to skarn, manto, "mesothermal" or "intermediate" precious-metal and base-metal vein and replacement, and epithermal precious-metal deposits (Kirkham and Sinclair, 1995). Figure 18: Schematic diagram of a convecting magma that is feeding a small subvolcanic intrusion below a porphyry deposit (modified from Shinohara et al., 1995). Fluid separation from the degassing magma occurs near the top of the magma column, forming pockets of magmatic-hydrothermal fluid in which comb-quartz layers grow inward from intrusion margins. Mineralized vein and fracture stockworks form when the fluid pressure exceeds lithostatic pressure and tensile strength of the surrounding rocks. カナダ地質調査所（Geological Survey of Canada）による『Mineral Deposits of Canada　Maps of deposits and resources（world）』から