Sept/Oct 2009

Economic Geology

Porphyry deposits (Part 1)

By R. J. Cathro

In the May 27, 1899 issue of the Engineering and Mining Journal [a weekly technical magazine that was the bible of the international mining industry at the time], the respected editor, Richard P. Rothwell, commented on a report by Thomas Weir, manager of the Boston Consolidated Copper & Gold Mining Co., Ltd., which owned claims that became a key part of the Bingham Canyon mine in Utah in 1910. In his report, which was the basis of a prospectus that was being used to raise development funds in London, Weir stated that “a mass of ore 2,000 ft long, 3,500 ft wide and 500 ft deep” … had an average grade of between 0.75 and 2.5% Cu… and contained about 291.7 million tons of copper (ore). In the words of Mr. Rothwell:

“Mr. Weir’s assumptions are certainly on an exceedingly liberal scale, and it may well be questioned whether a good deal more development work is not needed to prove the existence of so great a mass of ore. But even if we accept the statement and admit that the company has a very large body of low-grade ore, it does not seem to better the case …. Even if the greater part — or even the whole — should reach 2 per cent, it would not better the situation. It would be impossible to mine and treat ores carrying 2 per cent or less of copper under existing conditions in Utah …. Allowing for the greatest cheapness of mining, which can be made possible by the large size of the orebody, one cannot figure out anything but a heavy loss on 1.5 and 2 per cent copper ore. Moreover, it is not probable that the price of copper is going to stay permanently at 17 or 18 c.; and with every fall the loss on operations would be greater …. Therefore, the more ore it has of the kind it claims, the poorer it is. Undoubtedly, our London friends who are buying the stock at high prices will realize this a little later.”

The point of major significance, however, is that at that period, 99 engineers out of 100 who had considered the problem would have frankly held that it was impossible to exploit profitably any deposit containing 40 lb of copper per ton …. The editor unquestionably reflected with accuracy the judgement of the mining engineering profession as a whole. Perhaps the most tangible factor that they failed to appraise was the economy that might be affected by operation on a large scale .… But the fact remains that in 1900, the identical deposits that in recent years have contributed 35 per cent of the world’s copper were regarded as so much recalcitrant waste rock!

~ Parsons, 1933, p. 4-7

Porphyry-type mineral deposits are generally thought to have been first recognized and mined at the beginning of the 20th century in the southwestern United States, where the name was first applied to relatively large and low-grade copper deposits containing small amounts of molybdenum, silver and gold. Most people in the mining industry would be surprised to learn that porphyry-type mineralization was actually first discovered around the year 1440 and mined shortly thereafter at Altenberg, in the eastern Ore Mountains (Erzgebirge), about 32 kilometres south of Dresden, on the eastern border of Germany (Kühn and Seltmann, 1993; Altenberg home page, 2009; Cathro, 2006). It is not surprising that early explorers in the United States would not have read about the Altenberg tin mine because the best reference had been published in Latin by Agricola in 1556 (Hoover and Hoover, 1912).

A large part of the Altenberg deposit consists of disseminated cassiterite in a greisenized (altered) granitic porphyry and rholite that is intensely fractured into countless minute, randomly oriented stringers. According to the Hoovers, Agricola called the altered and fractured assemblage zwitter. It occurred in the cupola of a granite stock that was about 350 metres in diameter, with flanks that dipped outward at about 70 degrees. Mining was conducted almost continuously for about 550 years, until the end of March 1991, and totalled about 37 million tonnes. The average grade at closure, when about 60 million tonnes remained, was about 0.3% Sn (Kühn and Seltmann, 1993).

The authors of the earliest geological textbooks attempted to divide mineral deposits into obvious categories but found that the Altenberg deposit was difficult to classify. One of the first compilers, Bernhard von Cotta (1870), created a new category for Altenberg that he called impregnations/disseminations, and stated that it differed from all other metallic deposit types in having undetermined limits that were not sharply defined. In his textbook, Arthur Phillips (1884) placed it in a new category that he called unstratified deposits, and subdivided it into impregnations and stockworks. He defined a stockwork as a network of small veins interlacing one another and traversing the rock in various directions, and generally traversed by numerous thread-like veins containing cassiterite. He also noted that the mineralization was not always confined to the veins and that a considerable portion could occur in the host rock, which was generally porphyrytic and surrounded by granite and different varieties of porphyry.

Parsons (1933) was one of the first to provide a clear definition of porphyry copper deposits and to describe the origin of the title. He pointed out that much of the mineralization can be described as disseminated although, in many cases, most of the ore coats the walls of cracks and minute fractures. The first four porphyry mines to be developed — Bingham, Utah; Morenci, Arizona; Ruth (Robinson/Ely), Nevada; and Braden (El Teniente), Chile — were all associated with porphyritic rocks. The term porphyry refers to a granitic texture that consists of conspicuous phenocrysts in a fine-grained or aphanitic groundmass.

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