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DIAMONDS
24
MODERN MINING
October 2016
A split shell design has been
adopted for the open pit.
a detailed two-year mine plan and schedule
was in place.
The Liqhobong property hosts two kimber-
lite pipes, a satellite pipe and the main pipe,
as well as associated blows and dykes, with the
cluster forming part of the Lemphane-Robert
kimberlite belt of northern Lesotho. The two
pipes were discovered in the 1950s but early
efforts to mine at Liqhobong were concentrated
on the 1,6 ha satellite pipe, which has a higher
grade but generally produced poorer quality
diamonds. European Diamonds built a plant
in 2005 to treat the satellite pipe ore and this
was later upgraded and expanded by Kopane
and Firestone. The upgraded facility – or pilot
plant, as it was known – operated from 2011 to
2013, treating ore from a trial mining campaign
conducted on the main pipe. The intention was
to generate some cashflow while the feasibil-
ity study on mining the main pipe was being
completed.
“Around 1,18 Mt was mined and this yielded
over 325 000 carats,” said Brown. “The plant
was, however, not really fit for purpose and the
trial mining exercise actually lost money. A
major problem was that the gap on the primary
crusher was less than 20 mm which resulted
in large stones being broken. We subsequently
did some reconstitution work on the broken
diamonds, analysing ten stones including one
of 74 carats. Our conclusion – and that of the
independent experts we con-
sulted – was that all the stones
were between 100 and 200 car-
ats when mined with two above
200 carats. So clearly there is a
potential for Liqhobong to pro-
duce large stones.”
The pilot plant has now been
dismantled and removed from
site and the new operation now
coming on stream is – to all
intents and purposes – a brand
new ‘greenfield’ mine. It will see
the 8,6 ha main pipe – which
contains a 23 Mct indicated and
inferred resource at a grade of 28
cpht – being mined to a depth of
383 m over a 15-year mine life at
a rate of 3,6 Mt/a – which will
result in 1 million carats a year
being produced.
The process plant is a twin-
stream (2 x 250 t/h) facility
which utilises a conventional
flowsheet comprising scrub-
bing, screening, three stages of
crushing, concentration via DMS
and final recovery using X-ray machines. The
settings on the crushing circuit are such that
any large stones in the 100-carat to 400-carat
range that are liberated by the scrubbers should
make it through to final recovery intact and
undamaged.
As regards the mining, a split shell design
has been adopted for the open pit, with a
14 m bench and 28 m double bench design.
Advantages of this approach include a reduced
risk of ramp failure (once two splits join then
a concentric ramp results), lower capex for the
initial mining fleet (due to lower waste strip-
ping) and less in-pit traffic congestion with
multiple ore and waste faces available. The
projected strip ratio is 2:1 over the life of mine.
In terms of carat production, Liqhobong will
be bigger than Gem Diamonds’ Letšeng, the
current flagship of Lesotho’s diamond mining
industry. Letšeng – which has an extremely low
grade of just under 2 cpht – produces approxi-
mately 100 000 carats a year, which is only a
tenth of Liqhobong’s projected production, but
is highly profitable because of the high quality
of its stones and its proven ability to produce
large diamonds on a regular basis. In 2015, it
received an average of US$2 299 per carat for
its production.
Liqhobong’s diamonds, by contrast, are only
expected to fetch an average price of US$165
per carat escalated over the life of mine. As