March/April 2010

Economic Geology

Porphyry deposits (Part 4)

By R. J. Cathro

Aerial view looking southwest over the Bingham Canyon Mine (2003), situated at the crest of the Oquirrh Mountains, Utah

“Guggenheim Exploration, with John Hays Hammond and later Pope Yeatman as general managers, was probably the most successful exploration group of all time. Their discoveries and acquisitions read like a litany of the most important copper discoveries of the twentieth century.”

~ Krahulec, 1997, p. 205

By 1910, Daniel Jackling had consolidated the ownership of the Bingham Canyon porphyry deposit with the takeover of the neighbouring Boston Consolidated Mining Company. He had also displayed impressive skill in managing and constructing a vast, integrated mining enterprise in a frontier area, and reaching a profitable level of production in a remarkably short period of time.

By assuming a risk than no other firm was willing to consider, the Guggenheims gained a huge competitive advantage that could be immediately used at several other worthy projects in their pipeline. Among the major copper deposits they controlled were Braden (El Teniente), Chile (1904); Ruth (Ely), Nevada (1904); Kennicott, Alaska (1906); Ray, Arizona (1907); Santa Rita (Chino), New Mexico (1909); and Chuquicamata, Chile (1910; sold to Anaconda in 1922).

In addition to running the Bingham operation, Jackling was given the responsibility for utilizing the operating knowledge gained at Bingham to place the Ruth, Ray and Santa Rita porphyry deposits into production. That was achieved in 1908, 1911 and 1911, respectively. In common with all the early porphyry mines in the southwestern United States, the presence of copper was first noted by prospectors looking for gold or silver but it did not elicit much serious interest until railways were built nearby. Santa Rita had been known for many years before Mexicans began to mine it in 1800, and the other two were discovered about 1870, but the potential of the large porphyry deposits was not recognized until 1900 to 1906. Additional information on the geology and history of those mines can be found in Parsons (1933) and Titley and Hicks (1966).

The Guggenheim family began to restructure their vast holdings around 1915 and chose Kennicott to be the holding company. The mine in Alaska was named after a nearby glacier but the name of the holding company became Kennecott because of a clerical error. Utah Copper Company became a wholly owned subsidiary of Kennecott in 1936 and its name was changed in 1938. Control of Kennecott and the Bingham Canyon Mine was acquired by Standard Oil Company (British Petroleum) in 1981 and by Rio Tinto in 1989.

In the first decade or two of production, geology played a minor role in discovering or exploring the Bingham Canyon deposit. There was no reason for Jackling or his senior executives to know the difference between monzonite or monazite, or to distinguish a gneiss from a gnu. Like any outcropping deposit where it easy to visually distinguish ore from waste, common sense and systematic sampling and assaying were sufficient for production planning. Deep exploration began with nearly 100 churn-drill holes during the period from 1910 to 1919. Nine of the holes were over 300 metres deep. The richest, hole 90, averaged 1.36 per cent copper over a length of 530 metres, proving that the ore persisted to depths that would not be mined during Jackling’s lifetime.

Some significant milestones and pioneering achievements by the company follow.

  • The supergene ore was mostly depleted by 1915.
  • The first flotation tests were conducted in 1914. The forced shutdown caused by the postwar recession in copper prices from 1919 to 1921 was used as an opportunity to double the daily capacity of the mills to 36,000 tonnes and begin flowsheet revisions by converting from a gravity circuit to froth flotation, which was completed by 1926. This increased the copper recovery rate from 61 to 81 per cent and raised the average concentrate grade from 18 to 20 per cent. This was immediately followed by the introduction of selective flotation in 1927, which increased the concentrate grade from about 20 to 30 per cent copper. The daily mill capacity was gradually increased to 98,000 tonnes by 1966.
  • A leach circuit was started during the 1920s to treat over 36 million tonnes of leached cap mineralization averaging between 0.3 and 1.0 per cent copper that had been stockpiled during stripping of the ore body.
  • The mining equipment in the pit was converted from rail-mounted steam shovels to crawler-track electric shovels between 1924 and 1926, and electric trains were introduced into the pit in 1928. By 1930, the main line from the mine to the mills had been equipped with 50-kilogram rails and nine 300-tonne locomotives were in operation, pulling cars with a capacity of 90 tonnes.
  • First aid and safety training were introduced in the late 1920s.

Space limitations only permit the following brief summary of the geology of this remarkable deposit (Krahulec, 2005):

“The Oquirrh Mountains have seen active tectonism throughout much of geologic time, an important factor in the localization of the porphyry-related mineral deposits. Tectonic features include the east-west Uinta axis, the north-south Wasatch line, and the overlap between the Basin and Range extensional terrain and the Cordilleran fold and thrust belt.

The Bingham mining district is centered on a giant, high-K, calc-alkaline porphyry Cu-Au-Mo-Ag deposit. This deposit is largely hosted in a mafic-rich, strongly magnetic, composite monzonite stock dated at about 38 Ma. This stock intrudes a thick, intercalated sequence of Pennsylvanian-Permian marine quartz sandstones and limestones. A 120- to 330-metre-wide, quartz monzonite porphyry dike-like plug and a swarm of later, narrow, east-northeast-trending dikes cut an early monzonite stock. The hypogene porphyry mineralization is concentrically zoned around the quartz monzonite stock from a deep, inner, low-grade core through progressively overlapping hypogene molybdenite, bornite-chalcocite, chalcopyrite, and pyrite zones. These mineral assemblages range from a deep, low-sulfidation bornite-chalcocite-covellite zone through the intermediate-sulfidation disseminated chalcopyrite-pyrite copper shell to local zones of high-sulfidation pyrite-nukundamite in quartzite. The inverted cup- or molar-shaped copper shell is largely coincident with potassic alteration and garnet skarns. The pyrite halo is spatially associated with propylitically altered rocks. Each of three major phases of co-axial porphyry intrusions are followed by cycles of fracturing, veining, alteration and metal introduction. The quartz monzonite porphyry stock and younger dikes are spatially coincident with the highest grades of copper and gold in the ore body. The exposed quartz monzonite porphyry stock is too small (about 0.5 km2) to have provided the fluids and metals for the district. Aeromagnetic data indicates the presence of a large batholith at depth.

The outermost fringe of the pyrite halo is overprinted by the inner margin of a one-kilometre-wide, intermediate-sulfidation, sphalerite-galena±tetrahedrite manto-vein zone where alteration is largely confined to the immediate vein walls. The outer perimeter of the Pb-Zn veins locally contains rhodochrosite and/or barite. The Barneys Canyon and Melco distal disseminated gold deposits lie about seven kilometres north-northeast of the center of the district, completely outside Bingham’s megascopically recognizable sulfide and alteration system, but on the outer fringe of the district’s weak As-Au geochemical halo.”

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