JOHANNESBURG (miningweekly.com) – Southern African diamond mining, development and exploration company DiamondCorp has estimated the total resource tonnage in the main pipe at its 74%-owned Lace mine, in the Free State, to be 38.48-million tonnes to the 920 m level, an increase of 16% on the 33.12-million tonnes estimated in March 2012 to the 855 m level.
This followed an updated technical report carried out by Toronto-based MPH Consulting.
DiamondCorp CEO Paul Loudon noted that the new resource and reserve statements confirmed that the potential for high operating margins increased with depth, which was one of the factors that attracted the miner to the project from the outset.
“The forecast grade numbers are in line with what we are seeing from the UK4 mining ramp-up and the actual carat value achieved will be known when we commence diamond sales in Antwerp in the last week of March,” he added.
The estimated recovered grade for all facies had also now been aligned with the Lace production plant using 1.25 mm slotted bottom screens, compared with 1 mm screens in the original design configuration.
While the increase in bottom screen sizes meant that recoveries would be significantly lower in terms of grade, the carat value of the diamonds recovered would be higher, as the smallest diamonds that would no longer be recovered were also the lowest-value diamonds, lowering economic impact, the company said.
So while the impact on recovered grade resulting from the change can be perceived as significant, the economic impact is minimal.
The change in screen size was also expected to result in considerable operational efficiencies and up to a 50% reduction in water consumption in the processing plant, allowing for mining of the resource at greater than the current planned rate of 1.2-million tonnes a year, once block caving started.
The recoverable diamonds from this resource at the increased bottom screen size was now estimated at 9.39-million carats, up from the March 2012 figures of 13.39-million carats, at 1 mm screen size.
The average value of the Lace diamonds from the stone size frequency distribution achieved with the 1.25 mm bottom screen size had been forecast at $164/ct, with the average grade and carat value equating to revenue of R600/t, compared with forecast mining and processing costs of R238/t for the UK4 Block and R145/t for block caving.
This represented robust operating margins of 60% and 76% respectively.
The whole block could be mined for five years at a rate of 35 000 t/m and generate a positive net present value of R133.3-million, with a robust internal rate of return of 59%.