ABSTRACT:

Uchucchacua mine is located at 4,450 meters above sea level and is ranked the fourth mine producer of silver in the world. The mining method is cut-and-fill. The mine has a production of 2,500 tons per day, with a concentration of 16 once of silver per ton. Between 2003 and 2005 it was the site of several fatalities that occurred, in addition to injuries, as a result of rockbursting accidents. This led the mine to implement a plan of action that required laboratory testing of the limestone and the ore, the two common types of rocks present in the deposit. Additionally, in order to determine the potential for rockbursting, a microseismic monitoring system of was implemented to identify the zones of seismic risk. This allowed both the planning of mine development and regular operations to be carried out with improved safety and enhanced productivity. As a result, no rockbursting accidents occurred in 2006 and 2007.

1. INTRODUCTION

Uchucchacua's deposit is located in the western slope of the Andes and belongs to the district and province of Oyón, in the department of Lima, Peru (Fig. 1). Stress redistribution as a result of mining is responsible for the activation of a complex structural system of faults (e.g., Rita, Ruby, Veronica, Rose). During the last five years, the mine has experienced an increasing level of seismicity and rockbursting associated with these faults. Development activities and ore exploitation have continuously advanced to deeper levels, presently taking place at 1,000 m below surface. Mechanical properties of respective rocks indicated increased susceptibility to rockbursting, which affected both the safety of the mine personnel and productivity.

2. GEOLOGY

The silver deposit of Uchucchacua consists of a principal geological structure that lies under the mine head offices.

Fig. 1 The location of the silver deposit of Uchucchacua (available in full paper)

The opposing rocks are of intermediate acidity. The principal stresses are sub-horizontal, trending NE-SW and EW-NW, respectively. Uchucchacua is a hydrothermal epigenetic deposit of landfill fractures, which were also transportation channels and metasomatic emplacements of mineralization that finally formed the present ore bodies. The principal commercial mineralization consists of silver, but zinc and lead are also extracted as by-products. In the Jumasha formation (Fig. 2) the mineralization is located in calcareous rocks of early Cretaceous. It varies in size and it is associated with irregular and discontinuous emplacement bodies.

Fig. 2 Cross-section of the Jumasha (Jm) formation (available in full paper)

There are two principal types of rocks at Uchucchacua. One type consists of the fragile limestone, whereas the second type is represented by high strength minerals that form the ore, such as proustita, argentita, pirargirita, native silver, esfalerita, marmatita, as well as waste minerals as the pyrite, alabandita, rodocrosita, calcita, pirrotita, in different combinations forming a rocky, hard structure characterized by a high compressive stress. The presence of the Andean tectonics has created a system of principal folding where the minerals to be mined are exposed to zones capable to host rockbursts (Fig. 3).

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