EDITORIAL & OPINION — GEOLOGY 101 — Porphyry deposits, Pt. 2

Porphyry deposits can be found in orogenic areas such as the Canadian Cordillera, the Andes Mountains of Chile and Peru, and in the southwestern Pacific regions of the Philippines, Indonesia and Papua New Guinea.

These deposits are the most important source of molybdenum, and of rhenium, a platinum group element associated with molybdenite crystal lattice. They are also among the most important sources of copper — reportedly contributing up to half of the metal mined worldwide — and gold. Silver and a number of other metals, including tungsten, tin, lead and zinc, are also recovered from porphyry operations.

These deposits contain hundreds of millions to billions of tonnes of ore grading from 0.2% to more than 1% copper, 0.005% to 0.03% moly, and 0.4 to 2 grams gold per tonne.

As an example, the porphyry copper mine at Bingham, Utah, contains an average of 0.6% copper in more than 2 billion tonnes of ore. Since operations began in 1904, the mine has produced more than 16 million tonnes of copper. Other regions with porphyry deposits include: Butte, Mont., with more than 2 billion tonnes grading 0.85% copper; Chuquicamata, Chile, with more than 10 billion tonnes grading 0.56% copper; and Ok Tedi in Papua New Guinea, with more than 375 million tonnes grading 0.7% copper and 0.66 gram gold.

In Canada, all moly production and roughly half of all copper production are derived from porphyry deposits. With the exception of Quebec’s Gaspe Copper, which is mining a deposit estimated at 150 million tonnes grading 0.37% copper, Canadian porphyry production is limited to British Columbia and the Yukon.

Current and past-producing mines in those areas include: Valley Copper in British Columbia, with 690 million tonnes grading 0.41% copper; Island Copper in British Columbia, with 345 million tonnes grading 0.42% copper and 0.017% moly; Brenda in British Columbia, with 360 million tonnes grading 0.16% copper and 0.039% moly; Mount Polley in British Columbia, with 230 million tonnes grading 0.25% copper and 0.34 gram gold; and Casino in the Yukon, with 162 million tonnes grading 0.37% copper, 0.039% moly and 0.48 gram gold.

Because of their low grades, porphyry mines must be low-cost. To keep costs down, these are mined as open-pit operations, which are less costly to run than underground mines. The size of many of these deposits renders such operations huge. For example, at 800 metres deep and 4 km wide, the pit in Bingham, Utah, is the largest man-made excavation in the world.

Exploration for these deposits focuses on regions with felsic-to-intermediate intrusive rocks, particularly those with a history of multiple intrusions and brecciation or fracturing in the contact zone with country rock. More detailed exploration would zero in on defining alteration halos that grade laterally from the core of the mineralizing system.

A vertical zonation in copper mineralization might also develop in hot, arid regions where surface waters tend to redistribute copper from an exposed porphyry system, concentrating it elsewhere. Such enrichments are called “supergene” and contain higher-grade copper minerals, such as chalcocite and bornite, than found in chalcopyrite. The oxidized surface waters dissolve copper from the original porphyry ore, called protore, and transport it in the water table until such time as the waters encounter a reduced zone and precipitate the copper. The presence of a supergene enrichment indicates the presence of a larger hypogene, or original, porphyry system.

Regional geochemical surveys for both metals and alteration, such as potassium, are useful exploration techniques. Regional airborne geophysical surveys, such as gamma-ray spectrometry, may prove useful in locating and defining alteration halos.

Some porphyry systems in the Andes were first detected through satellite imagery of alteration halos. Ground geophysical surveys useful in exploration include induced-polarization for disseminated and vein sulphides, and magnetic for secondary magnetite content.

— The author is a professor of geology at Memorial University in St. John’s, Nfld.