AngloGold Ashanti’s Mponeng mine is located in Gauteng province of South Africa. It is mined to an average depth of 2,800m-3,400m below surface and is one of the world’s deepest and richest gold mines with grades at over 8g/t. It is one of three AngloGold projects in the West Witts area apart from Savuka and TauTona mines. The name means ‘look at me’ in the local Sotho language.
Formerly the Western Deep Levels South Shaft, or Shaft No 1, Mponeng is the most recently sunk of the three former Western Deep Levels mines. Sinking of Mponeng shafts began in 1981, and the main shaft was completed in 1986, with the subshaft completed in 1993.
Currently all production is sourced from the VCR (Ventersdorp Contact Reef). The mine has been expanded through many deepening projects with the latest one being the extension from 109 to 120 levels. Work is currently in progress to extract the ore from the Carbon Leader Reef (CLR) below it.
The project will facilitate the development of many other smaller projects and extend the mine life. It is yet to be approved by the company board. The other major project, VCR below 120, entails accessing the mineral reserves below 120 level. AngloGold Ashanti estimates that this project will add 2.5Moz to production for 10 years at a cost of R2.03bn ($252m).
The VCR below 120 project is expected to increase the mine’s life by eight years to 2024. The project was approved by the board in February 2007, following which construction began. On-reef development and thus the start of production is scheduled for 2013 with full production due in 2015.
Geology and reserves
Mponeng is located on the north-western rim of the Witwatersrand Basin. There are seven gold bearing conglomerates within the lease area, of which two are economically viable at present. The VCR is a gold-bearing quartz-pebble conglomerate of intermediate grade, capping the last angular Witwatersrand non-conformity.
The CLR is a 20cm thick tabular auriferous quartz pebble conglomerate.
As of December 2010, proven reserves at the VCR stood at 2.05Mt with an average grade of 9.98g/t Au at 109-120 level and 0.31mt graded at 8.85g/t Au below 120 level.
The yet-to-be-mined CLR below 120 level has probable reserves of 22.52Mt graded at 11.30g/t Au..
The South shaft deepening project commenced in 1996 and as a result Mponeng mines the Ventersdorp Contact Reef (VCR) to the 120 level, which is some 3.4km below datum. Mponeng mines on average at deeper than 2.7km below the surface. The VCR reef that Mponeng mines dips at 22º, and has an average channel width of 78cm.
The deepest operating stope is at a depth of 3.37km below surface. The grade at this operation varies considerably, therefore a sequential grid mining method is used which allows for selective mining and increased flexibility in dealing with changes in grade ahead of the stope. The mine utilises a twin-shaft system housing two vertical shafts and two service shafts.
Exploration drilling for the CLR below 120 project commenced in early 2008. CLR comprises three economical units. Unit 3 is centrally located and is the oldest of the CLR deposits. Unit 2 is a small size complex channel deposit situated in the east of Unit 3. The unavailability of infrastructure to access these units raised the opportunity of containing additional ounces of gold at the mine.
A series of sub-vertical holes were drilled from underground in 2010 to intersect at the CLR depth. The average length of the hole drilled was 900m with the deepest hole being 1,090m
Ore mined is treated and smelted at Mponeng’s gold plant. The ore is initially ground down by means of semiautogenous milling after which a conventional gold leach process incorporating liquid oxygen injection is applied. The gold is then extracted by means of carbon-in-pulp (CIP) technology. The plant conducts electro-winning and smelting (induction furnaces) on products from Savuka and TauTonaas well.
Production in 2007 fell 2% to 587,000 ounces compared with 2006.
The various planned and unplanned work stoppages and safety initiatives conducted towards the end of the year, combined with a decline in grade and reduced face advance, contributed to the decrease in production.
There was a 4% decline in the area mined in 2007, largely as a result of a 3% decrease in face length. Mponeng undertook extensive cost cutting measures throughout the year. Gold Production at the Mponeng mine in 2010 was 532,000oz.