Patrick Williamson, PG, a Principal Hydrogeologist at INTERA, recently co-authored a paper with Hernan Beltran, Chris Lane, and Jim Willis, entitled Baseline Investigations and Preliminary Design for an In-Pit Tailings Storage Facility, El Gallo Mine, Sinalo, to be presented at the Tailings and Mine Waste 2019 Conference in Vancouver, Canada in November 2019.
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; and 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 they 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 represents 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.
The El Gallo gold mine in Sinaloa, Mexico provides an excellent example of the evaluation process and testing and design for an IPTSF. The mine has 10 million tonnes of HLM that is undergoing cyanide leaching, but the gold recovery is lower than expected (40%) due to the mineralogy of the ore and method used to place the ore on the leach pad. Sufficient gold remains in the HLM to profitably reprocess the ore through a new 5,000 tonnes/day processing plant, which would include grinding, cyanide in leach, carbon sorption, and cyanide detoxification. Tailings from the plant would be discharged into the mined-out Samaniego pit in such a way to maximize water reclaim and tailings consolidation. An underdrainage system would be installed to capture groundwater entering the base of the pit and water from tailings consolidation. Supernatant water liberated from the tailings slurry would be recovered at the surface of the IPTSF via a pontoon‐mounted pump. The proposed IPTSF would have approximately 8.0 million cubic meters (Mm3) of storage volume. This equates to a tailings storage capacity of 12 to 14.4 million tonnes, assuming a range of tailings in-situ dry densities between 1.5 and 1.8 tonnes per cubic meter (t/m3).
As with any mine development project or significant change in mining operations, a wide range of tasks were performed to evaluate site conditions and develop the technical specifications of the IPTSF and support the permitting process, including:
- Design of a new processing plant.
- Hydrogeologic characterization of the Samaniego pit.
- Geochemical characterization of the tailings and pit walls.
- Design of the tailings fluid recovery and groundwater management system.
- Geotechnical testing of the tailings material.
ADVANTAGES OF AN IPTSF
Recent high-profile tailings dam failures in British Columbia (Mt. Polley), Bello Horizonte, Brazil (Córrego de Feijão) and Chihuahua, Mexico (Cieneguitas) have raised the level of awareness of both mining professionals and the general public regarding the reputational, environmental, financial and operational risks associated with traditional tailings storage facilities (TSF) that employ earthen retaining structures. Alternatives to traditional TSFs include disposal as a paste in underground workings and disposal in mined-out pits. The use of IPTSFs is not new but is gaining increased acceptance for disposal of mine waste (tailings, waste rock and processing solutions), particularly if the material is acid generating. IPTSFs can be an attractive alternative to TSFs, within the following constraints (Arcadis, 2015 and Lane, 2005):
- A locally available pit that will not cover or “sterilize” remaining mineral resources.
- Pit filling above active underground mines are considered unsafe.
- The amount of waste rock and tailings produced from a pit does not usually fit back into the pit, requiring dual disposal scenarios in some cases.
- Local hydrogeologic conditions are a critical factor in the selection and design of an IPTSF.
- The feasibility, cost, and design of an IPTSF depends significantly on the leaching potential of mobile contaminants from the material placed in the pit.
Despite these constraints, an IPTSF can be an attractive engineering solution for tailings storage that can benefit the mine and community during operation and closure of the mine. The associated benefits include (Arcadis, 2015):
- Isolation of potential reactive mine waste in an anoxic environment, which inhibits the formation of acid and metal leaching.
- Reducing or eliminating the necessity of maintaining engineered structures.
- Improved social license and regulatory acceptance of the mining activity by restoring original landform and function.
- Potential for reduced closure costs. In this case, the IPTSF will eliminate the need to close a heap leach facility (by reprocessing the ore) and minimize long-term closure costs.
- In some cases, returning the pit site to its original use (e.g. grazing).
Geochemical studies of the detoxified tailings that will be placed in the Samaniego pit indicate that they have a very low average sulfide content (0.21%) and will not be acid generating or leach metals. Leaching tests and analysis of the supernatant indicate that residual solutions would not impact groundwater quality.
The limited groundwater flow into the pit via fractures (K less than 1E-7 m/s) and lower K of the consolidated tailings (10E-8 to 10E-9 m/s) will minimize potential groundwater flow through the mass of consolidated tailings. The contrast in Ks between the fractures and the tailings would lead to preferential flow around the tailings rather than through it (Arcadis, 2015)
The results of geotechnical and hydrogeological testing clearly demonstrate that a key factor in the operation of the IPTSF is the water recovery system (pumps and piping), which will have sufficient capacity to remove water at a rate which will expedite the consolidation of the tailings and prevent migration of any tailings solutions from the pit by maintaining the cone of depression in the groundwater table. Emplacement of fine-grained tailings in the pit would seal potentially reactive surfaces in the pit walls. The very low permeability of the tailings would limit oxygen diffusion and advection of water, two of the key components in the generation of acid drainage.
To obtain the full paper or to learn more about the conference or to register, visit: https://tailingsandminewaste.com/
Patrick Williamson, PG