El Teniente (“The Lieutenant”) is an underground copper mine situated 2,300 m (7,500 ft) above the mean sea level in the Andes in Chile: 44km east of Rancuagua and 75 km south of the capital, Santiago (as demonstrated in Figure 1). El Teniente is owned by CODELCO (Corporación Nacional del Cobre de Chile or, in English, the National Copper Corporation of Chile), a Chilean State-owned copper mining company that is, on a global basis, currently the largest copper producer. El Teniente itself is reputed to be the largest underground copper mine in the world.
1.0 History of El Teniente
The El Teniente orebody has experienced a long history of small scale mining, starting as far back as the 16th century with the operation of a small mine known as the Socavon de los Jesuitas. These small-scale miners were forced out during the 1700s due to the purchase of the property by Don Mateo de Toro y Zambrano y Ureta, but it wasn’t until 1819 that his heirs would enlarge and restart the operation of the mine workings. This attempt, and many attempts thereafter, would not produce anything more than small scale, until the property was acquired by William Braden, an American Mining Engineer with enough capital to initiate the expansion that would lead to its current status as the largest underground copper mine in the world.
Braden formed Rancagua Mines, which later became the Braden Copper Company in association with old Colleagues like Barton Sewell, its first president, after whom the first mining community was named. Between 1904 and 1971, the mine remained under non-Chilean control, until it was nationalized by the Chilean government.
1.1 Sewell, Original Mining Camp
Sewell is the original mining camp founded by William Braden in 1906, which lasted until the late 1980s, when the mine workers were moved down to Rancagua. It was a unique mining community in Chile, defying the inhospitable environment of hard winters, high altitudes, earthquakes and avalanches to eventually reach a peak population of 15,000. The camp’s second name is the City of Stairs, as it appears to have been poured down the mountain (see Figure 2). It holds the status of a UNESCO World Heritage Site.
1.2 Union Disputes
Throughout its history, there have been multiple labour disruptions at the El Teniente Mine, the most significant of those strikes occurring in 1983 and 2008.
In 1983, the arrest of a union leader who called for an end to military rule in Chile sparked a 13,0000 worker strike at the operations of El Teniente and two other Codelco mines, ultimately forcing them to close. Between the three mines, 3,300 workers and 37 labour leaders were fired for their participation in the strike.
Again, in 2008, El Teniente and two other Codelco mines were forced to close due to a strike by contract workers. The contract workers closed access to the mines and threw stones at buses transporting company employees from the mine to the town of Rancagua, causing injury to at least one employee.
2.0 Resource Estimation
Table 1. summarizes the measured, indicated, demonstrated, and inferred resource estimation for the El Teniente deposit in 2008, established with a cutoff grade of 0.2% Copper, as of January 2008 (Codelco).
Table 1: Measured, Indicated, Demonstrated and Inferred resources of the El Teniente Division
The El Teniente mine, a porphyry deposit located in the Andes Cordillera, is currently the largest intrusion-related Cu-Mo deposit in the world. The deposit was formed along the Nazca-South American subduction zone, when tectonic movements caused a shift in the subduction angle of the Nazca Plate (McKinnon, 2003). This shift created a large void along the Teniente fault zone, which resulted in the formation of large amounts of hydrothermal breccias and hypogenic minerals occurring within altered basalts, diabases, andesites, and gabbros (Maksaev, et al., 2003).
In the formation of porphyry deposits, most voids are filled with hydrothermal breccias or some kind of quartz mineralization, but some cavities remain undisturbed by such geological processes. As demonstrated in Figure 3, some of these cavities form spectacular clusters of selenite crystals. In the El Teniente Mine, one of these cavities has produced crystals up to 4 meters long and 1 meter across.
Figure 3: Selenite Crystal formation inside a cavity at El Teniente Mine. (Photo courtesy Ben Schneider)
The deposit is estimated to contain 75 Mt of copper and 1.4 Mt of molybdenum. Though most of the ore is found within the main hydrothermal breccia pipe and in the surrounding alteration zones (as demonstrated in Figure 4), about 20% of the mineralization is found in the altered felsic outgrowths and dikes with biotitic and K-feldspathic alterations surrounding the deposit.