The Chuquicamata porphyry copper mine is now in its 85th year and is said to have sufficient reserves to last an additional quarter of a century. On a recent site visit, The Northern Miner observed that the open pit is 4 km long, 2.5 km wide and 750 metres deep. Indeed, the pit is so huge that to drive from surface to the bottom takes about 30 minutes. However, rising stripping and haulage costs in the ever-deepening mine are forcing state-owned Corporacion National del Cobre (Codelco) to rethink its mining plan.
Foremost among the company’s options is the so-called deep sulphide project, which is aimed at examining mineralization directly below the bottom of the pit. Since 1998, 37 holes (4,200 metres) have been drilled beneath the bottom, and results indicate the presence of as much as 1.1 billion tonnes averaging 0.6% copper, plus minor molybdenum. A further 20,000 metres of drilling are planned, with holes ranging in depth from 700 to 1,000 metres.
Eventually, Chuquicamata will shift to underground block-caving, such as is carried out at the Teniente mine, said senior geologist Alejandro Faunes Gutierrez. Teniente, a giant porphyry operation southeast of Santiago, is also owned by Codelco.
To achieve this transformation, Codelco is considering several approaches, all of which are tied to several satellite deposits, which surround the main Chuquicamata porphyry.
One such approach would be to drive a tunnel into the main orebody from the Mansa Mina deposit, about 8 km away. Mansa Mina lies 500 metres below the surface and contains several hundred million tonnes of mineralization averaging 1% copper.
Besides the deep sulphide project, Codelco is attempting to expand the huge satellite deposit known as Mina Sur, which contains an estimated 300 million tonnes grading 1% copper. The mineralization, which occurs as chrysocolla and as several other oxides associated with clays, was derived by the downstream movement of copper-bearing waters, and the company is looking for further downstream extensions to this mineralization.
Surrounding Mina Sur are more than 500 million tonnes of low-grade stockpiles and tailings, which average 0.35% copper. Secondary leaching of this material has proved profitable for Codelco, with cash costs averaging only US20 per lb., said Faunes Gutierrez.
Meanwhile, Codelco is busy evaluating two other targets: Opache, which lies beneath 200 metres of travertine limestone in the nearby Loa River valley; and Chuqui Norte, situated between the main Chuquicamata pit and the recently opened Radomiro Tomic mine, to the north.
Over the next 25 years, the extent of developed copper mineralization could stretch continuously for as much as 16 km, from Mansa Mina to Radomiro Tomic, said Faunes Gutierrez. Controlling this large concentration of mineralization is the West Fissure, a north-trending strike-slip structure. The Fissure cuts off the porphyry mineralization, rendering the western side of the pit entirely waste. Codelco believes the other half to be the Mansa Mina deposit, which demonstrates similar alteration and mineralization patterns.
Mineralization is hosted in late Eocene-aged porphyries and granodiorites intruded into Triassic volcanic rocks and Paleozoic igneous rocks. The oxide mineralization, now largely mined out, consists of chalcocite and numerous oxide minerals, many of which are unique to the dry environment of the Atacama Desert.
The primary sulphide mineralization consists of chalcopyrite, bornite, molybdenite and minor pyrite.
Codelco is currently mining 640,000 tonnes per day, including 180,000 tonnes of ore from Chuquicamata, where head grades remain above 1% copper.
In 1999, Chuquicamata produced more than 600,000 tonnes of refined copper, plus 13,000 tonnes of moly. The cash operating costs, including credits, were US42 per lb.; the total cost, US53 per lb.
In 2000, the mine is expected to crank out 625,000 tonnes of copper and 13,000 tonnes of moly at similar costs.
To date, Chuquicamata has mined more than 1.7 billion tonnes of ore, averaging 1.54% copper, and hauled as much as twice that in waste. Reserves, based on a cutoff grade of 0.5%, stand at 1.49 billion tonnes grading 0.87% copper. The district, as a whole, contains a staggering resource of about 11 billion tonnes grading 0.76% copper, based on a 0.2% cutoff.
History
The modern history of Chuquicamata dates back to 1912, though the Incas knew of high-grade copper veins as early as 1500. Several attempts were made in the early 1900s to develop the property, but it took a Boston-based businessman, Albert Burrage, to see the potential in a mountain of low-grade ore.
The oxide ore at the time ran about 2% copper, then considered barely economic. Undeterred, Burrage employed the just-developed Bradley process of leaching oxide copper ores with sulphuric acid and succeeded in persuading the Guggenheim brothers of New York to help finance construction.
Operations began in 1915 at a modest 10,000 tonnes per day. In 1923, Anaconda Copper acquired a controlling stake in the Chile Exploration Company, which operated the mine. By 1929, the Guggenheims sold their remaining interest to Anaconda in what was then the largest stock deal in market history, valued at US$77 million.
Anaconda began life as a modest producer exploiting porphyry deposits at Butte, Mont. After securing its foothold in the world’s richest copper district, it went on to become the premier copper company in the world. However, with the cold war still raging in Central and Latin America, the company’s presence in Chile eventually became a sensitive political issue.
Chiquicamata and several other large operations soon became the symbols of national sovereignty. Some Chileans believed the foreign companies were exploiting the mines at the expense of the people. With the election of Marxist president Salvador Allende Gossens in 1970, the largest mines, including Chuquicamata, were nationalized.
Though the Allende administration lasted only three years, ending in his overthrow and death in an army assault on the presidential palace, the mines remained under the control of the government, under the auspices of Codelco. This continues today, with 10% of Codelco’s annual revenue going to the military and its net profits helping to fill state coffers. Chile went on to become a prosperous and poltically stable nation, helped by revenues from the red metal.
Anaconda never recovered from its forced exile from Chile. Oil producer Atlantic Richfield bought the company in the late 1970s, only to shut it down a few years later.
New beginnings
Following the nationalization, Codelco controlled the four largest copper mines in the country — Chuquicamata; El Teniente; Andina; and El Salvador — and, for 30 years, the company was content simply to operate the mines. That was to change in the early 1990s with the delineation and development of the Radomiro Tomic mine, 4 km north of Chuquicamata.
Radomiro Tomic (named for an ardent supporter of nationalization) represents the first mine completely developed by Codelco and is a study in contrast to Chuquicamata. The sleek open-pit operation hosts 460 employees, compared with a workforce of 7,200 at Chuquicamata, and is a thoroughly modern mine, producing copper cathode using solvent extraction-electrowinning. The pit measures 700 by 400 metres — only a fraction of Chuquicamata’s dimensions.
Construction and pre-stripping at Radomiro Tomic started in April 1996 and was completed early and under budget at a cost of US$641 million. Production began in May 1998.
Ore is extracted by standard drill-and-blast and truck-and-shovel methods. The material is sent to a 2-stage crushing circuit and subjected to heap leaching on a reusable pad, during which 80% of the copper is removed. A bucket-wheel reclaimer removes the remaining material from the pad and conveys it to the tailings pile.
Radomiro Tomic produces 180,000 tonnes of electrowon copper annually, though Codelco plans to boost production to 250,000 tonnes per year by 2001. (This would raise daily throughput to 135,000 tonnes.) The low stripping costs contribute to low cash operating costs of US33 per lb. Total costs stand at US44 per lb.
Oxide reserves stand at 1 billion tonnes grading 0.55% copper — sufficient for 20 years of operation. Moreover, beneath the oxide material are 3 billion tonnes of sulphide mineralization averaging 0.42% copper.
Discovered in the 1950s by Anaconda geologists, the Radomiro Tomic deposit, then known as Pampa Norte, was covered by as much as 150 metres of gravel. Mineralization consists of secondary sulphides of chalcocite and covellite, and oxide minerals, such as atacamite, chrysocolla and “copper wad.” Underlying that are the primary sulphide minerals: chalcopyrite, bornite and minor pyrite.
With the addition of Radomiro Tomic, Codelco solidified its position as the world’s largest copper producer, though recent mergers have begun to challenge that status. Late last year,
For 2000, Codelco expects to maintain production of 1.5 million tonnes of copper from five operations throughout Chile.
In 1999, the company briefly considered expanding into Zambia. Although it did not commit to any business arrangement, Codelco says it will continue evaluating opportunities in the region.
It also signed a joint-venture agreement with Peoles to explore and develop copper deposits in northern Mexico.
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