DFS - Tailings Storage Facility
Bosnian Definitive Feasibility Study
11 September 2021
The 2021 DFS does not include the mining of the Veovaca open pit. This reduces the tonnage of tailings that will require storage in the TSF by 1.91 Mt over life of mine. In addition, mining the Veovaca open pit would have also required stripping waste rock to access the ore, which would also require a dump with a capacity to store 8.64 Mt of waste rock. Total tailings and waste from mining Veovaca would have been 10.6 Mt.
TSF Site Location
The TSF has been designed to international design codes and standards, such as the EU Mine Waste Directive and Best Available Techniques Reference Document for the Management of Waste from Extractive industries and the Global Industry Standard on Tailings Management.
Tailings generated in excess of the PAF requirements will be stored in a dry-stack tailings facility, located in the valley immediately south of the Vares Processing Plant site, as shown on Figure 11. The facility has a total area of 11 hectares and a capacity of 2.5 million cubic metres, which is sufficient to store the tailings generated over LOM.
The site is characterised by a steep sided densely wooded valley with a small stream in the valley bottom. The substrata are characterised by thin topsoils overlying completely to highly weathered siltstones and mudstones which grade in to weathered becoming fresh fractured limestone.
The TSF will be constructed in three phases with an initial starter area and two subsequent expansions providing an initial capacity of 0.97 Mt followed by 1.73 Mt and 2.55 Mt providing a total storage capacity of 5.25 Mt for the LOM. The TSF phasing is summarised below:
Table 21: TSF specification
Each phase will have the trees felled, grubbed and then stripped of topsoil prior to construction. All suitable material, including topsoil, grubbed tree stumps and roots will be stored and used in the restoration process. The near surface weathered rock will be regraded and stripped to supply the low mineral capping layer and improve the subbase profile.
A zoned starter embankment will be constructed at the toe (southern end) of the facility from locally sourced free draining rock fill keyed into the bedrock on the downstream side with an upstream low permeability zone formed from site won material. The embankment will be 10m high at its maximum extent with a 5 m crest, 1 vertical to 2.5 horizontal upstream slope and 1 vertical to 3 horizontal downstream slope.
The impoundment will be lined with an underdrainage geosynthetic layer overlain with 1.5 mm textured HDPE liner to prevent any groundwater seepage coming into contact with the tailings material and prevent seepage from contact water from the tailings into the groundwater.
The small stream in the valley bottom will be culverted for the length of the impoundment and starter embankment to be discharged downstream of the embankment.
Clean surface water will be prevented from entering the impoundment area by cut-off ditches constructed 5m beyond the maximum extent of the impoundment area. The cut-off ditches will direct the upstream surface water around the facility and discharge beyond the downstream side of the embankment.
The contact water within the impoundment will be managed and directed to the toe of the facility and out to a sump located beyond the toe of the embankment. The collected contact water will be pumped back to the process plant for recirculation in the process plant.
From the time of commissioning the Vares Processing Plant the dry filter tailings, not returned to Rupice for backfill, will be transported by truck to the TSF phase 1 area. The tailings will be transported to the TSF at an average rate of 0.4 Mt per year. The tailings will be tipped on the stack then spread by dozer before compaction with a smooth vibrating roller and graded to form lift design. Compaction of the placed tailings will maximise the storage capacity and improve stability. The lifts will be developed in 10 m vertical height with a 5 m bench and an inter bench slope of 1 vertical to 2.5 horizontal and an overall stack slope of 1 vertical to 3 horizontal.
During operation the TSF will be monitored for water quality of both the groundwater and surface water, stability, dust and noise to confirm compliance with the operational management and environmental standards.
TSF Closure & Rehabilitation
The TSF will be progressively closed as each lift is completed with 1 m of low permeability mineral layer sourced from weathered shales and mudstones across the impoundment footprint topped with a 1 m thick sized waste rock cover to prevent erosion of the low permeability layer. The waste rock layer will be covered with a 0.5 m topsoil layer sourced from stockpiled soil and grubbed and chipped tree stumps and roots. The topsoil will be revegetated with local grasses. The capping quantities are summarised in the table below:
Table 22: Capping material quantities used during the closure and rehabilitation of the TSF