The opportunity and danger of panel caving in the El Teniente copper mine

Table of contents

Abstract
Geology of the Deposits
Genesis of the El Teniente Ore Bodies
The Mine´s History
Existing Mining Infrastructure
El Teniente goes deep

Panel caving
Process of panel caving
Block caving modeling
Advantages and disadvantages of panel caving
Rock Mass Damage of Esmeralda Level

Conclusion

References

Abstract

The El Teniente Copper Mine contains large amounts of chalcopyrite of various grades. As the upper ore bodies have been extracted in the past, mining focuses on large, deep, and low-grade ore bodies. The only mining method capable of extracting these rock masses with high production rates and low production costs is block caving and its variation panel caving. Therefore Codelco started the largest mining operation in the world. The New Mine Level Project covers the entire footprint of the ore deposit, divided in 5 panels (Sur, Andes Sur, Andes Norte, Norte, and Pacifico) and will start to produce in 2017.

Geology of the Deposits

The Chilean subsurface is rich in porphyry-style copper deposits. In total, there are 54 deposits with 455,000,000 t of fine copper combined (Cannell, 2004; after Camus, 2003). The world´s largest known porphyry ore deposits are El Teniente, Los Bronces – Rio Blanco, and Chuquicamata are located in Chile. The El Teniente mine is located about 70 km southeast of Santiago, on the west flank level (masl).

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Figure 1 A: Location map of the 3 giant late Miocene and Pliocene copper deposits – Los Pelambres, Río Blanco- in the Andes of central Chile (After Skewes et al., 2005) 1B: Geologic map of the El Teniente 5 level at 2,284 m above sea (After Skewes et al., 2005)

The deposits found in this location are the largest known copper-molybdenum deposits in the world, containing about 94,400,000 t of copper initially in place in ore with grades >0.67 % and 1,400,000 t fine molybdenum with grades >0.019%. The El Teniente deposit is located in a structural belt called “Dorsal Mioliminar". This belt has a north-south extension of about 150 km (Camus, 1975).

El Teniente is located in middle to late Miocene extrusive and intrusive igneous rocks, which are part of the Farellones Formation. The Rocks contain lavas, volcanoclastic rocks, dikes, sills and stocks of basaltic to rhyolitic composition (Skewes et al., 2002; after Vergara et al., 1988; Rivano et al., 1990). The extrusive rocks of this Formation are locally referred to as the Teniente Volcanic Complex. This complex is dated between 15.2 and 7.5 Ma. These extrusive rocks were intruded by gabbro, diabase, diorite, tonalite, latite, and diacite porphyry plutons (Skewes et al., 2002). About 80 % of the copper mineralization in the El Teniente ore deposit is found in intrusive rocks of the Farellones Formation. These rocks are traditionally referred to as “Andesites of the Mine”. More recent investigation showed that these rocks are more basic than andesites and actually are mafic intrusive rocks including gabbros, diabase, and basaltic porphyries (Skewes et al., 2005; after Villalobos, 1975; Camus, 1975; Skewes and Arévalo, 2000; Skewes et al., 2002). Two felsic plutons intruded the mafic rocks. The large Sewell Tonalite is located southeast of the Braden Pipe and the smaller Teniente Dacite Porphyry north of the pipe. Also mineralized and barren magmatic-hydrothermal breccias are present in the El Teniente area. The Braden Pipe is the largest of them. This barren breccia pipe is the central litho-structural unit in the deposit. The variety of magmatic-hydrothermal breccias reflects a complex sequence of multiple events that caused the formation of copper ore in both, large quantities and high grades (Skewes et al., 2002).

The mafic intrusive rocks of the deposit have grades of about 0.75 % to 1.5 %. The hypogene copper mineralization is associated with pervasive biotite alteration. An irregular pattern of ore with anomalous high copper grades of more than 1.5 % occurs around the Teniente Dacite Porphyry and the Braden Pipe. These High grades are the result of supergene enrichment. The magma chamber needed to cause the amounts of copper mineralized in the El Teniente deposit must exceed 600 Km³ (Skewes et al., 2002).

Genesis of the El Teniente Ore Bodies

The formation of the ore deposit started when an intrusive laccolith formed in extrusive rocks of the Miocene Teniente Volcano Complex. The ore bodies formed above the roof of this large open-system magma chamber, that was fed by mantle-derived mafic magmas. This magma chamber persisted for the period of the multiple episodes of breccias emplacement. The deposit was formed over a period of more than 2 million years (Skewes et al., 2005). Mantle-derived magmas replenished the open-system magma chamber. Exsolution of sulfur and metal-rich aqueous brines and vapor from the crystallizing upper part of the chamber caused alteration, brecciation, and mineralization of roof rocks above the chamber (Skewes et al., 2005, after Burnham, 1985; Cloos, 2001). The alteration of Roof rocks was a multistage development. First magnetite-actinolite alteration occurred, caused by

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Figure 2: Schematic illustration of the sequential stages in the development of the El Teniente deposit. Green represents the Teniente Volcanic Complex and the Coya -Machali volcanic complex (Skewes et al., 2005).

circulation of magmatic fluids or connate formation water. Exsolution of magmatic-hydrothermal fluids caused intensive biotite alteration and copper mineralization in rocks associated with early biotite, igneous and anhydrite breccias complexes. The Sewell Tonalite is the main group of this breccia complexes and contains the highest grade hypogene copper in the deposit. This group is located east and northeast of the Braden pipe. After this breccia complex mineralized and barren tourmaline, anhydrite and rock-flour breccias were emplaced. This stage is associated with sericitic alteration and redistribution of copper mineralization (Skewes et al., 2005). When the magma chamber finally cooled down due to a progressive decrease in the subduction angle, volcanic activity moved eastward (Skewes et al., 2005; after Stern, 1989; Skewes and Stern, 1994, 1995; Stern and Skewes, 1995, 1997). This cool-down led to the crystallization of felsic magma that produced the

Sewell Tonalite, porphyritic felsic apophyses, and the Teniente Dacite Porphyry and latite dikes (Skewes et al., 2005). These dikes and stocks cut the preexisting mafic rocks and redistributed the copper mineralization. The felsic material is poor in copper and sulfur and is not the main source of copper in the deposit (Skewes et al., 2005). In general, mafic magmas contain more copper, sulfur, iron, and calcium than felsic magmas. All these elements are present in anomalous concentrations (Skewes et al., 2005; after Hattori, 1996; Pallister et al., 1996, Kress, 1997; Candela, 1997; Hattori and Keith, 2001).

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Figure 3: Copper grades between levels Teniente 4 and 5 in the El Teniente mine (Skewes et al., 2003, after Arévalo et al., 1998). Copper grades surrounding the Teniente Dacite porphyry, north of the Braden Pipe are elevated due to supergene enrichment.

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