Gedeon mine is a small-scale mining operation in the Nazca-Ocona gold belt in southwestern Peru. It is a gold vein deposit hosted mainly by granodiorites of the Coastal Batholite, in the Rio Grande district, Arequipa region. For this study, the rock masses of Gedeon mine were geomechanically characterized in the underground workings along the Fortuna vein through manual mapping, and Schmidt hammer rebound tests. According to the characterization, three different rock units were identified: host granodiorite with GSI of 55-75, orebody with GSI of 40-65, and a fault zone with GSI of 30-50. Deficiencies in the design and execution of drilling, blasting, and excavation procedures were identified, which can lead to rock mass degradation and overbreak in the walls of the excavations. A series of numerical models were developed based on these data, which were then used to assess the stability of the stopes along the Fortuna vein. In the stopes, we found that the quality of the rock mass of the fault zone controls the stability of the excavations. Also, damage due to poor quality blasting and deviations in the excavation geometry may contribute to instabilities in the hanging wall but they are not the primary reasons for the instabilities.

1. INTRODUCTION

According to official statistics, small-scale and artisanal gold mining in Peru is responsible for around 15% of the country's gold production, although it is estimated that this figure could be twice as large if also counting also the production from informal miners. Part of this gold production comes from the Nazca-Ocona gold belt, in the northern sector of Arequipa region (Loaiza et al., 2008).

Small-scale mines from this region face different challenges than larger underground mines, both because of the relative lack of physical, financial, and knowledge-based resources available to local miners and the unique geotechnical conditions that exist in those mine sites. One of the challenges of these mines is the identification of groundfall risk and the control of ground instabilities in their operations to maintain safe conditions and prevent injuries or fatalities.

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