While roaming Chile last summer I had the opportunity to visit the El Teniente copper mine, said to be the largest underground mine in the world. It is located high up in the Andes, but close to civilization, being near the town of Rancagua and only 89 kms south of the capital, Santiago de Chile.
Bus terminal at the mine site
El Teniente is named after an army lieutenant credited with the discovery. It is a classic porphyry copper deposit. Mining began in 1905, first by Braden Copper Co., then by Kennecott Copper Corporation of the USA. In 1968, the Chilean Government bought 51% of the shares and eventually took over the mine in 1971. The state mining company is Codelco which has its head office in Santiago. It also operates Chuquicamata, the huge open-cast copper mine in the Atacama Desert located in the north of Chile.
My Lonely Planet guide book says that the mine is closed to the public, but adds that you may be able to visit the mine if you can show just cause or interest. Nothing ventured, nothing gained! I fronted up at head office and explained I was an Aussie travel writer and former geologist. Open sesame! It was all arranged. Next week I could join a tour of the mine with a group of 30 civil engineering students from the University of Santiago.
This gave me six days in which to explore Valparaiso and Vino del Mar, before returning to Santiago and heading south by bus to Rancagua, where I stayed two nights at the Hotel España.
On the day, we all piled aboard the modern Mercedes bus for the 50 km trip to the mine site, located some 1800 metres above the Central Valley where grapes and fruit trees flourish. The Andean foothills are composed of steeply dipping shales that soon change to volcanic rocks and breccias higher up.
Visitors dressed up and in the cage
The mine site is really a bus and train terminal as the total work force at El Teniente is 3500. About 30 buses were parked waiting for a shift of miners to return home to Rancagua. The train lines were standard gauge and the passenger trains seemed no different from a city metro system. We were each issued with a helmet, electric light, fluorescent jacket and gum boots; then we piled aboard a rail car for our excursion into the mine. This involved a trip of 10 kms into the mountain before we came to the first of the four main shafts that lead to the current workings.
Since 1905, there have been over 2000 kms of tunnels constructed of which some 800 kms are used today. Each year about 64 kms of new tunnels and galleries are excavated.
Our guide explained how things operate. “This is an upside down mine! The main shafts go upwards! We mine in the upper levels and push the ore down shafts to the primary crushers, called chancadores, located underground near the main haulage level. The stopes extend upwards for 1000 metres; there is some subsidence on the high mountain tops but this is normally covered in snow.”
We inspected one of the huge crushers which has a central steel cone that moves in an eccentric fashion breaking up huge lumps of ore to an easily transportable size. Later, outside and further down the hill, fine crushing is done by two ball mills, having 24,000 and 64,000 tonnes per day capacity. The powdered ore is mixed with water and chemicals and is sent to the flotation tanks where a sulphide concentrate containing 31% copper is obtained from a primary ore grading 1.14% copper.
The copper smelter is visible from a lookout on the road up to the mine. Two tall chimneys
belch out fumes that have scorched the mountains for decades. Reverbatory furnaces separate a liquid phase of copper and iron sulphides which is treated in a converter with blasts of oxygen-enriched air to burn off the sulphur, leaving two liquid phases – a scoria rich in iron oxides and white metal containing 75% copper. The latter goes through another converter and air blast to produce blister copper 99.4% pure.
Yearly production at El Teniente is about 375,000 tonnes of refined copper and 2000 tonnes of molybdenum.
Our tour underground was comprehensive. The main haulage shaft and driveways were huge – the ascensor, or cage was necessarily big enough to move any mining equipment used, even a D9 bulldozer. We were taken to the upper levels to see drilling and blasting, and clearing up by bulldozers that pushed the ore down shafts to end up at a crusher hidden deep within the mountain.
Giant selenite crystal in cavern
More highlights of the trip were to come. Next was a visit to “La Caverna de los Cristales”. Only rarely at El Teniente are there encountered crystalline cavities or vugs. Normally the ore is solid porphyry or breccia shot through with blebs of chalcopyrite. When this cavity was opened up it was so spectacular that it was preserved from destruction. Now there is only one entrance with a solid locked door.
The cavity is elongated and slopes up at about 30 degrees for about 12 metres, along which are clustered huge transparent gypsum crystals, or selenite. The one most impressive is 4 metres long and nearly a metre across. Coloured lights are placed behind the crystals thus giving the cavern a fairyland appearance. There was room for only about six people at a time to admire and photograph the magnificent selenites.
Copper smelter belches fumes
Next we went to an underground cafetería where the miners have lunch. Our tour group was expected. We had a delicious casserole meal of spicy beef and chicken with corn and tomato, washed down with soft drink.
At 2 pm we caught the train back out to the open and handed over our borrowed equipment. On the way back down the hill the bus stopped at a lookout point where we could see the smelters and flotation tanks set amid a barren Andean landscape. Rancagua and the Hotel España were reached at 5 pm to conclude an unforgettable trip to an amazing copper mine.
If you want more information about this area you can email the author or check out our South America Insiders page.