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
boreholes, it was found that 8 boreholes did not intersect or plot close to the proposed
stopes. These boreholes were excluded from the database and were not used in the design
of the underground workings.
Two rock mass rating systems were used, namely Laubscher’s Mining Rock Mass (RMRL)
System (1990) and Barton’s Q System (1974). The ore body and country rock were found to
be generally in the poor to fair range of rock mass quality. The average RMRL values were
estimated to be 44 (fair) for the hangingwall and 42 (fair) for the footwall. The average Q
values were estimated to be 2.3 (poor) and 1.8 (poor) for the hangingwall and footwall,
respectively. The results of the mapping exercise that was completed in one of the tunnels
(drift OB7) show similar rockmass conditions in the country rock. The Q values average 2.6
(poor) with a minimum of 1.3 (poor) and a maximum of 4.2 (fair). These values translate to
equivalent RMR values of minimum of 47 (fair), average of 53 (fair) and maximum of 57 (fair).
An effort was made to demarcate the different sections of the mine into different geotechnical
zones based on rock mass characterization. This would have enabled design of different
stope dimensions for the different geotechnical zones based on rock mass quality. This
analysis was not conclusive due the limited number of boreholes in some of these areas. It is
therefore recommended that additional geotechnical logging of boreholes that intersect the
proposed stopes should be carried out to increase the database of rock mass quality. For
design purposes, a single geotechnical zone was assumed.
16.5.2 Mining Spans
Analysis of the geotechnical data described above indicate that the recommended stope
dimensions of maximum 30 m (height) by maximum 10 m (length) and maximum 6 m (width)
will be stable. Stopes are expected to remain free standing for a reasonable time with
possible sidewall slabbing issues for stopes that plot in the transition zone on Potvin’s
Stability Graph. The slabbing material will generally come from the mineralized zones thus
the dilution of ore should be minimized, as the diluting material will have some grade in it.
Cemented backfill placed after the broken ore has been drawn out will provide permanent
support to the stope back.
16.5.3 Support of Excavations
16.5.3.1 Pillars
A sill pillar will be required below zero level to protect the San Leon tunnel from the proposed
underground workings and a crown pillar will be required to protect the underground workings
from the planned future open pit mine. The study proposes a crown/sill pillar thickness of
15 m. This was determined based on a Safety Factor of 1.8 and an average Q value of 2.6.
The possibility of having voids in the pillars was also taken into consideration. The design
and stability of the proposed pillar width was assessed using empirical formulae and deemed
sufficient.
Crosscut protection pillars will also be required to support crosscuts during mining. Once all
the stopes being serviced by the crosscut have been mined out, the crosscut pillar will be
recovered on retreat. A 15 m wide crosscut protection pillar is deemed sufficient. This
dimension was checked using numerical modeling code Map3D.
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