Rock mass and structural modelling for the large open pit gold mining project in the Northern Andes: The La Colosa project, Colombia

Abstract

The La Colosa gold mining project, located in the Central Cordillera of Colombia, wholly owned and developed by AngloGold Ashanti Colombia, represents the largest gold project in the northern Andes. The project is currently in the pre-feasibility stage (PFS). Rock mass characterisation and the elaboration of a 3D rock mass model and a 3D structural model are required for the geotechnical open pit mine design. Crucial for the interpretation of structural and geotechnical data is the understanding of the tectonic setting of the area, in order to define major faults and zones of increased fracturing. Prior to rock mass characterisation and 3D modelling, a detailed structural geologic survey was carried out, defining tectonic deformation events and the relation to magmatic and ore-forming activity. Geologic-geotechnical data from surface mapping as well as from drill core logs of more than 96,000 m of diamond drilling were analysed. In addition, structural data from oriented drillings and from bore hole scanning were evaluated. Review and analysis of drill core data focused on the quality of information acquired during the various stages of drilling and drill core logging. In particular inconsistent data on fracturing and faulting made a detailed review necessary comparing drill core photos with the geologic and geotechnical drill core logs. The result of the re-logging enabled the elaboration of a structural model, the determination of the depth of weathered and fractured material close to surface, as well as the definition of principal fracture zones. Drill core logging data as well as surface outcrop data were analysed in terms of rock mechanical parameters and were integrated into the existing 3D lithological model. Together with the results from laboratory testing, rock mass types could be defined and modelled in 3D. Seven structural domains were determined using major faults and lithological boundaries as limits, according to the tectonic-structural setting of the deposit.