Porphyry deposits

PORPHYRY DEPOSITS
W.D. SINCLAIR
Geological Survey of Canada, 601 Booth St., Ottawa, Ontario, K1A 0E8 E-mail: [e-mail]

Definition 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 (Kirkham, 1972). The large size and structural control (e.g., veins, vein sets, stockworks, fractures, 'crackled zones' and breccia pipes) serve to distinguish porphyry deposits from a variety of deposits that may be peripherally associated, including skarns, high-temperature mantos, breccia pipes, peripheral mesothermal veins, and epithermal precious-metal deposits. Secondary minerals may be developed in supergene-enriched zones in porphyry Cu deposits by weathering of primary sulphides. Such zones typically have significantly higher Cu grades, thereby enhancing the potential for economic exploitation. The following subtypes of porphyry deposits are defined according to the metals that are essential to the economics of the deposit (metals that are byproducts or potential byproducts are listed in brackets): Cu (±Au, Mo, Ag, Re, PGE) Cu-Mo (±Au, Ag) Cu-Mo-Au (±Ag) Cu-Au (±Ag, PGE)

Au (±Ag, Cu, Mo) Mo (±W, Sn) W-Mo (±Bi, Sn) Sn (±W, Mo, Ag, Bi, Cu, Zn, In) Sn-Ag (±W, Cu, Zn, Mo, Bi) Ag (±Au, Zn, Pb) For deposits with currently subeconomic grades and tonnages, subtypes are based on probable coproduct and byproduct metals, assuming that the deposits were economic. Geographical Distribution Porphyry deposits occur throughout the world in a series of extensive, relatively narrow, linear metallogenic provinces (Fig. 1). They are predominantly associated with Mesozoic to Cenozoic orogenic belts in western North and South America and around the western margin of the Pacific Basin, particularly within the South East Asian Archipelago. However, major deposits also occur within Paleozoic orogens in