Pulacayo Project Feasibility Study - Apogee Silver

Pulacayo 1 000 t/d Phase I Feasibility Study - NI 43-101 Technical Report
090644-3-0000-20-IFI-100
18.9.9 Progressive Slope Reclamation
The working face of the tailings as deposited will be sloped at a relatively gentle 3 horizontal to 1
vertical overall (toe-to-crest), which will reduce the propensity for erosion and allow faster
establishment of native vegetation. Horizontal benches will be provided at each 4 m raise in the
dike, which will help limit erosion during operations. Progressive reclamation will begin in about
the third year of operations, once sufficient final slope area has been established.
The final slope may require some re - contouring, depending on how well operations manages
the slope geometry and the degree of differential settlement that results. The current closure
concept includes light grading of the ultimate constructed slope of the TSF to result in 3 benches
spaced 12 m to 16 m vertically and of nominally 6 m to 8 m in width. The bench face slopes will
be nominally 2.5 horizontal to 1 vertical, and the overall slope (toe to crest) will remain 3:1. Native
species will be used to provide the highest long-term survival rate and in the early years test plots
will be established to find the best combination of plants and methods of planting and starting.
The first plantings will occur on the flat benches, where it is easier to place vegetative soils and
establish the first growths. This, then, will aid in establishing growth on the slopes by both
providing annual seeds and further limiting erosion.
The closure design criteria include more robust storm event (the PMP) and seismic standards
(maximum probable event, MPE). These will require some upgrades to the surface water
diversions. Since the impoundment will be dry in the long term, and the tailings fully consolidated,
long-term stability will be substantially better than that during the operating life, even considering
the more robust standards. The diversion works will either be upgraded to a 500-year return
interval (still a relatively small event, given the small the drainage basin area) or the
impoundment will be contoured to safely store this event (depending on the final configuration of
the tailings, which will depend on how closely they are managed relative to the design, the final
production volume, and the amount of consolidation).
The final layer of tailings, nominally 1 m thick, will also contain 3% cement to provide a firm, low
permeability cap below the growth medium. If this cap is not continuous, or if the actual physical
properties are determined to be less than idea for a cap, then an additional layer of low
permeability soil will be spread and compacted to a thickness of 300 mm. This will then be
capped with a 300 mm thick layer of organic soils (topsoil) recovered during construction to
provide a growth media. Since organic soils are sparse and the recovered topsoil volume will
probably not be adequate for this closure demand, non-organic soils will be upgraded with
nutrients and then mixed with the natural topsoil.
18.9.10 Surface Water Management
Meteorological data was obtained from the national station located in Uyuni, 13 km from the site
and at an elevation of 3,680 m (the TSF average elevation is 4,030 m). The database included
the following years: 1943 to 1948 and 1975 to 2011. This data was then adjusted for elevation to
estimate the climate at the site. This produced the following estimates of rainfall and evaporation
for the TSF site (see Table 18.3 & Table 18.4). Two important results of this are: 91% of the
rainfall occurs between December and March (and thus no dike construction will occur in this
time period); and the average ratio of evaporation to rainfall is 8.5 : 1.
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