The following information is an extract from The Achmmach Tin Project Small Start Option Definitive Feasibility Study Report completed in 2016.
The shift to the small start (SSO) concept lead to a fundamental change of approach to mine backfill and ventilation strategies. ATS requested Entech abandon the use of paste backfill in favour of selective use of cemented rock fill (CRF), loose rock fill and residual pillars. The outcome of this decision was to remove paste fill capital costs from the mine development costs to significantly reduce cement consumption in CRF use, and make filling operations simpler and less costly in operation.
Entech applied a strategy of utilising dual declines at the EZ and CZ portals to provide for early ventilation to each mining area. This approach substantially reduces the requirement for large-scale raise boring, which is capital intensive, slow and can only be commenced once mine development is sufficiently advanced. A secondary benefit is a concomitant reduction in forest area disturbance.
In January 2016, ATS engaged Entech Pty Ltd of Perth, Western Australia (Entech) to advance its SSO PFS work to DFS status according to the following scope of work:
Entech reviewed fill strategy in consultation with Mining One, who produced a DFS level geotechnical report for the mine. The final DFS mine design produced a 10.5 year mine schedule delivering 6.56Mt at 0.85% Sn.
Basis of Mine Design
ATS and Entech developed the key factors underpinning the mine design through extensive review of capital and operating aspects of the preceding EDFS design. These factors are outlined below and discussed in further detail in the following sections of this report.
ATS recognised backfill strategy needed to move away from the almost exclusive and widespread use of paste backfill proposed in the EDFS. The revised geotechnical report prepared by Mining One was instrumental in enabling Entech to redesign stope dimensions such that the majority of the mine could be developed using loose rock fill and leaving pillars in selected locations.
ATS had identified a zone of broken ground around section 2770 mE during its resource drilling programme, and Entech agreed that a zone 50m either side of this section should be designed to make use of CRF to ensure adequate structural support.
Mine Access Strategy
Entech prepared the mine development schedule on the basis of simultaneous commencement of the EZ and CZ declines. This twin portal approach results in early ore flow from the Upper EZ to the ROM pad via approximately 90m of decline development, while the development of the 500m decline to the CZ is in progress.
An internal 800m straight decline will provide access to the lower EZ, thus obviating the need to construct a spiral decline through poorly mineralised ground between the upper and lower zones of the EZ.
A horizontal drive from the western end of the CZ will provide access to the WZ. This development overcomes topographical constraints on constructing a portal at the Western Zone, while providing opportunity to facilitate resource drilling of the western end of the Meknès Trend and the sparsely drilled Sidi Addi Trend.
Mine Ventilation Strategy
Previous studies relied on the use of raise boring to develop ventilation rises into the EZ, CZ and ultimately the WZ. The construction of ventilation rises up to 4m in diameter requires the importation of substantive equipment with attendant mobilisation and operating costs. Additionally, raise boring can only commence once suitable mine development has been completed to permit establishment of the underground components of the process.
Entech and Minero overcame these limitations by switching to a parallel decline system at each portal, with one designed to act as an exhaust drive. This system offers the opportunity to commence full scale ventilation as soon as strike driving commences. Capital cost reductions are also realised through elimination of raise bore contractor mobilisation and demobilisation charges. Also lower cost decline development can offset raise boring unit costs.
ATS has switched its operations philosophy from owner mining to a contract mining scenario. Whilst the driver for this decision is to eliminate initial capital imposts for the purchase of mine mobile plant, secondary benefits are to be derived from employing a suitably experienced contractor who is recognised in the mining finance industry, and who is able to provide first class training to locally engaged mining personnel.
ATS formed the view that in concert with reducing the scale of the Achmmach Project, a staged approach to the mine schedule would be beneficial to keeping initial project capital costs at a manageable level. Desk top assessments based on modular milling options arrived at approximately 0.5 Mtpa as a suitable minimum annual mine output. Subsequent increase in mine output by 50% upon reaching initial payback will be beneficial in NPV terms. This occurs between 4 and 5 years after first ROM ore delivery.
Table 4‑1 summarises the key aspects of the DFS mine design.
Table 4 1: DFS Mine Design Factors
ATS requested Entech propose an alternative mining method to the top-down sequence with cemented paste backfill assumed in the EDFS, to reduce capital requirements and operational complexity. Entech provided a hybrid mining method based on mechanised longhole stoping but tailored to differing orebody characteristics along the Meknes strike length.
The final mining methods selected by Entech comprise a combination of bottom-up CRF and top-down open stoping methods. The proposed mine design employs CRF in areas of higher grade to minimise metal loss to pillars, with the lower cost open stoping method used in the areas developed later in the life of mine schedule. For scheduling purposes, the Western Zone had both methods applied, with bottom-up CRF above 1015 mRL and top-down open stoping below this point.
Central Zone Mining Method
For the generally thick CZ (in places up to 20 m in width), a bottom-up mining sequence using cemented rock fill (CRF) was proposed. A top-down method with no fill leaving in-situ pillars behind for stability was also analysed but this significantly reduced mined recovered tonnes due to the required widths of the pillars (based on the extrapolated pillar assumptions discussed in Section 4.2.2 below).
CRF is a simple method of backfilling which involves placement of waste rock mixed with cement slurry into the stope void by a loader from a drive at the top of the stope. The sequence has therefore been changed from top-down in the EDFS to bottom-up in the CZ as the CRF method requires top access to stope voids for deposition of the fill. Stoping will be carried out retreating from the ore drive extremities back to a central access.
CRF was selected as the preferred fill method in the CZ rather than paste backfill for the following reasons:
The disadvantages of CRF as opposed to pastefill are as follows;
The quality disadvantage has been partially ameliorated by eliminating the requirement to work directly beneath fill (in-situ sill pillars have been assumed to be left when stoping is being undertaken directly underneath CRF filled stopes).
The use of pastefill could still present a future option for the project should the tin price significantly increase. The driver here would be higher ore extraction available with use of paste which under a high commodity price environment would outweigh the capital and additional operating costs. Figure 4‑1 illustrates the proposed mining method (note that the central access shown in the top level is removed from the lower levels in the diagram for clarity).
Figure 4 1: Bottom-Up CRF Mining Method
This mining method allows multiple concurrent working areas (or panels) to be mined off the same decline as stoping in each panel is advanced upwards underneath the bottom cemented fill stopes in the panel above. This allows stoping to commence after development of only 3 or 4 levels, rather than requiring development to the bottom of the mine before commencement of stoping.
Figure 4‑2 illustrates the panels designed in the Central Zone for the SSO mine plan.
Figure 4‑2: Central Zone Schedule Showing Concurrently Mined Panels (Long-Section Looking North) (Coloured by Panel)
Eastern Zone Mining Method
The Eastern Zone area was assumed to be mined using a top-down no-fill method, leaving behind in-situ pillars for stability; Figure 4‑3. For the purposes of the higher level SSO mine plan, pillar factors were extrapolated across the entire Eastern Zone area from the Eastern Zone Upper assumptions provided in the EDFS (this area was the only part of the EDFS mine plan which was not assumed to be filled with paste but mined as open stopes with in-situ pillars).
Figure 4‑3: Top-Down Open Stoping Method
Western Zone Mining Method
The Western Zone mining method adopted the CRF bottom-up method selected for the Central Zone described in Section 4.2.1 as was designed for the EDFS.
Table 4‑2 summarises the mining methods selected for each mining area in the SSO DFS mine plan.
Table 4‑2: DFS Mining Methods
Figure 4‑4: Mining Methods by Zone (CRF Yellow Highlight) (Long-Section Looking North)
DFS Stope Design
Having prepared raw MSO shapes at the PFS stage, Entech manually reviewed all stopes to ensure practicality of design and optimal geometry for the DFS. Entech returned stope sub-level distances to 25 m as adopted in the earlier EDFS as they were satisfied that the mining shape geometries and development profiles will allow the use of drilling equipment that will provide suitably accurate drill and blast over these heights. By analysing stope sections on 5 m strike lengths, Entech was able to take into account;
A comparison indicated that planned dilution (i.e. material at a grade of <0.55% Sn contained within the stope shapes) increased from 8% in the MSO shapes to 19% in the final designed stopes as lower grade material had to be included for practical reasons.
Figure 4‑5 and Figure 4‑6 illustrate the final stope designs developed in the DFS.
Figure 4‑5: Long-Section SSO DFS Stope Design (Looking North) (Coloured by RL)
Figure 4‑6: Plan View SSO DFS Stope Design (Coloured by RL)
Mine Capital and Operating Costs
Table 4‑9: Achmmach Mine Development Capital Costs
Table 4‑10: ATS Mine Capital Equipment Summary
Mine Operating Cost Estimation
The estimated life of mine operating cost for the mine is USD25.56/t ore. This cost comprises USD32.80/t ore for Stage 1 and USD23.12/t ore for Stage 2.
Entech could find no reason to change its view of employing staged cut off grades in its review of the SSO-PFS mine design and schedule. Consequently, for the purposes of this study, Entech retained 0.8% Sn as a cut off for design of Stage 1, reverting to 0.55% Sn cut off for Stage 2 design.
Entech then completed its rework of the Achmmach mine design and schedule. Coupling this to the final economic analysis presented below, Entech arrived at the SSO July 2016 Ore Reserve.
Table 4‑12 shows the Achmmach July 2016 Ore Reserve.
Table 4‑12 Achmmach July 2016 SSO Ore Reserve
0.55% Sn cut-off; All reported numbers rounded to 1,000t ore, 0.01% Sn, 50t tin metal.
Entech consequently identified “Sterilised” ore in the context of the present mine design, compared with the March 2015 EDFS design that will become inaccessible as a consequence of adopting selective mining during Stage 2.
Figure 4‑20 illustrates the general locations of sterilised ore blocks.
Figure 4‑20: Achmmach SSO Block Model Showing Sterilised Ore Blocks (Long Section, looking North)/