May 2010

Economic Geology

Poryphyry deposits (Part 5)

By R. J. Cathro


Generalized model of an alkaline Cu-Au porphyry deposit (redrafted from Holliday and Cooke, 2007); courtesy of Claire Chamberlain of Teck Resources Limited

Geologists were very fortunate that Bingham Canyon, the first porphyry deposit to be developed, was also one of the largest, which allowed them to study it continuously for more than a century. During that period, this deposit type has been discovered in many regions of the world, with the North and South American portions of the so-called circum-Pacific “Ring of Fire” being the most productive. “The Ring of Fire is home to more than 75 per cent of the world’s active and dormant volcanoes (450) and about 90 per cent of the world’s earthquakes, including 80 per cent of the strongest” (Wikipedia, 2010).

Bingham Canyon also displays most of the essential characteristics of porphyry deposits, including large size (particularly the horizontal dimensions), the relative uniform distribution of sulphide minerals that are disseminated throughout the deposit, and the low average grades of the ore. Spencer Titley (1966) summarized this deposit type as follows:

“Viewed from the standpoint of a process rather than … the visible results of that process, the porphyry copper deposits are more alike, as a group, than is at first obvious. All are, initially, the result of hypogene processes related to igneous activity. Most are epigenetic although some may be, in part, syngenetic. All are related spatially to igneous rocks, although the genetic associations are not always demonstrable. The associated igneous rocks range from diorite to granite. … The major distinctions which can be made among the various types of porphyry copper deposits are in the mineralogical characteristics of alteration, the composition of the host rock, and the nature of ground preparation. They can be viewed as the variable end result of one hypogene process, influenced by the chemical and structural properties of the rocks in the crust. … One of the most striking characteristics of the porphyry copper deposits seem to be their lack of selectivity for host rock and their capacity to be developed in almost any rock type accessible, … characteristics of occurrence that transect the boundaries of hydrothermal classification.”

Porphyry copper deposits occur in a variety of tectonic settings, typically in the root zones of andesitic stratovolcanoes in subduction-related, continental and island-arc settings. Porphyry Cu-Au deposits, such as those associated with Triassic and Lower Jurassic silica-saturated, alkalic intrusions in British Columbia, formed in an island-arc setting, and possibly during periods of extension.

Porphyry deposits range in age from Archean to recent, although most are Jurassic or younger. On a global basis, the peak periods for development of porphyry deposits were during the Jurassic, Cretaceous and Tertiary (Eocene and Miocene). These ages also correspond to peak periods of mineralization in Canadian deposits, although there are only a few known prospects of Miocene age. Also of note is the fact that the Cu-Au family of porphyry prospects in British Columbia straddles the Triassic-Jurassic boundary.

Porphyry deposits account for about 60 to 70 per cent of world Cu production and more than 95 per cent of world Mo production. They are also major sources of Au, Ag and Sn, and significant producers of byproducts Re, W, In, Pt, Pd and Se. In Canada, they account for more than 40 per cent of Cu production, virtually all Mo production, and about 10 per cent of Au production.

The Cu and Mo porphyries are generally restricted to felsic to intermediate plutons and their immediate wall rocks. Associated igneous rocks vary in composition from quartz diorite-granodiorite to high-silica granite; they are typically porphyritic epizonal and mesozonal intrusions, and commonly subvolcanic. A close temporal and genetic relationship between magmatic activity and hydrothermal mineralization is indicated by the presence of intermineral intrusions and breccias that were emplaced between or during periods of mineralization.

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