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June 2016
MODERN MINING
31
WEST AFRICA
feature
The Wassa underground
mine is accessed by a twin
decline system. The portals
of the declines are seen here.
Channel sampling under-
ground at Wassa.
ventilation raise and escape way close to the
start of the decline. The main decline is 5,8 m
high by 5,2 m wide and is being developed
using standard trackless mechanised methods.
The upper stopes will be mined using longi-
tudinal longhole open stoping with waste rock
fill. This will enable efficient early produc-
tion before a cemented rock fill preparation
and delivery system is installed. The open pit
will eventually mine down to the top of these
upper stopes, but only towards the end of the
life of mine.
In the deeper, wider areas of the deposit a
transverse longhole open stoping method will
be used. A primary-secondary mining sequence
will be implemented with the primary stopes
filled with cemented rock fill and the secondary
stopes with waste rock fill. The overall stoping
sequence will be bottom-up to reduce the inci-
dence of sill pillar development.
New surface infrastructure to support the
underground mining will be constructed
including electrical power supply from the
grid with backup genset support and surface
mechanical and electrical workshops.
At steady state production, Wassa Under
ground is expected to produce an average of
approximately 2 000 tonnes per day across the
life of mine.
The underground ore will be treated in the
existing processing plant, which is located
just 500 m from the open pit. This relatively
modern facility – GSR constructed it in 2005
– incorporates a four-stage crushing circuit,
grinding, gravity recovery, CIL circuits and a
thickener. It requires only minor upgrades – in
progress and soon to be completed – to serve
the needs of the underground project. Average