ABSTRACT: Minera Pangea recently completed a feasibility study (FS) for an in-pit tailing storage facility (IPTSF) that will receive tailings from reprocessing of approximately 10 million tonnes of heap leach material (HLM). The HLM from the existing El Gallo heap leach pad would be reprocessed through a plant and then pumped to the IPTSF. The FS included a range of characterization and engineering design studies to determine the behaviour and potential environmental impacts associated with the IPTSF, including tailings deposition and consolidation; baseline hydrogeology and geochemistry; a groundwater control system and closure design.
The El Gallo gold mine is located in the western foothills of the Cordillera Sierra Madre, east of the city of Guamuchil in the State of Sinaloa. The deposit is a low sulfidation epithermal vein system hosted in Late Cretaceous-Tertiary volcanic rocks, principally andesites. The climate is semi-arid with monsoonal rains during the wet season. Groundwater flow is controlled by fractures that run through and under the pit. The pit is currently a terminal sink, with evaporation exceeding groundwater inflow plus runoff into the pit. Geochemical characterization of the detoxified tailings indicate that are non-acid generating. Analysis of the tailings supernatant and leach solution indicate that neither would have a significant impact on groundwater quality.
A key aspect of the proposed IPTSF will be an underdrain system in the floor of the pit, which will reduce pore pressures and maintain the existing cone of depression in the groundwater surface around the pit during operation and closure, and aid in tailings consolidation by facilitating drainage (McDonald and Lane 2010) . Tailings supernatant will be recovered using a floating pump on the surface of the tailings pond. Tailings solution recovered from the surface of the tailings as well as the mixture of groundwater and tailings solution captured by the pit’s underdrain system will be reused in the process plant.
The El Gallo IPTSF presents an efficient and cost-effective engineered solution for tailings storage that benefits both Pangea and the community by: 1) Mitigating the potential impacts associated with the heap leach facility and evaporative groundwater consumption by the open pit; 2) Eliminating the potential impacts of a surface tailing storage facility (such as dam failures) and associated post closure management of a retention structure; and 3) Providing an economical solution for the storage of process tailings to enable continued operation for Pangea.
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