A numerical back-analysis of a series of failures of the east wall of Toquepala mine located on the western slopes of Cordillera Occidental, the southern Andes of Peru, was carried out using the distinct element numerical modeling program UDEC to provide information pertinent to the overall slope expansion design rationale. It included matching of historic movements of the east wall of the pit to understand the mechanisms that caused such movements and calibration the rock mass properties. The results of the back-analyses of slope movement to date indicated that, while toppling initiated in the volcanic rocks in the upper part of the wall at an early stage due to the adverse structure, the main controlling factors were deformation in the diorite of the lower slope and movement along the pebble breccia dike exposed in the lower wall. Because of the height of the slope, elastic compression of the toe could be sufficient to allow further loosening of the rock fabric, exacerbating the toppling action of the upper part of the wall.

1.1 Geological setting

The Toquepala deposit in Peru is located on the western slopes of Cordillera Occidental, the Southern most range of the Andes of Peru. The deposit is part of a mineral district that hosts two more known deposits - Quellaveco and Cuajone. All three deposits are broadly similar in age and are exposed at altitudes of approximately 3000 to 3700 m immediately southwest of the dormant and extinct Pliocene-Quaternary andesitic-dacitic strato-volcanoes, which constitute the main Andean Cordillera.

1.2 Stratigraphy

The Toquepala District exposes several thousand meters of volcanic rocks that have been assigned to the Paralaque Volcanics and Quellaveco Formation of the Upper Cretaceous to Paleocene Toquepala Group. The oldest rocks exposed in the mine area are the Quellaveco quartz porphyry, followed by rocks of the Toquepala Series and the Alta Series. These rocks were intruded by high-level granitoid plutons and subsequently by rocks of the Toquepala Intrusive Centre. The five rock types that characterize the Toquepala Intrusive Centre are dacite porphyry, hydrothermal breccias, dacite agglomerate, latite porphyry, and pebble breccia.

1.2.1 Rocks of the Quellaveco Formation

In the stratigraphic sequence at Toquepala, the layered volcanic rocks of the Quellaveco Formation are easily identifiable, when unaltered, because of their characteristic mineralogy and texture.

1.2.2 Rocks of the Toquepala Intrusive Centre

The most remarkable feature of the Toquepala deposit is the unusually large hydrothermal breccia complex emplaced at the intrusive centre. Large amounts of breccia are contained within a steep-walled, upward-flaring body with a crudely pipe-like form. The breccia complex in the pit is composite in nature, and consists of several breccia units. The breccia bodies formed in succession, each during a separate brecciation and fracturing event that corresponds to a distinct evolutionary stage of the hydrothermal system. The structural fabric and the mineralogy of the individual breccias mirror the stage of the hydrothermal system active at the time of their formation.

1.3 Alteration

The hydrothermal alteration patterns are quite typical of porphyry-copper systems.

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