The Mponeng gold mine located in the Gauteng province of South Africa is the deepest operating mine in the world. It is the last remaining underground operation by AngloGold Ashanti in South Africa.
The mine is in the process of changing ownership after Harmony Gold entered into a definitive agreement with AngloGold Ashanti to acquire its South African business for an initial payment of £154m ($200m) in February 2020.
Production at the underground gold mine was halted due to a seismic event that killed three miners approximately 3.5km below the surface in March 2020.
The current operating depth at the Mponeng mine ranges from 3.16km to 3.84km, while the future mining is expected to further deepen the shaft bottom to 4.22km below the surface.
The Mponeng mine produced 244,000 ounces (oz) of gold in 2019 and is expected to be in operation at least until 2027.
History of the Mponeng mine
The Mponeng gold mine was originally known as the Western Deep Levels South Shaft or the No. 1 Shaft. The mine along with the nearby gold processing plant was commissioned in 1986, while the operation was renamed as the Mponeng gold mine in 1999.
The Mponeng mine is part of the West Wits mining district that also hosted the Savuka and the TauTona mines that were closed in 2017.
The ore resources and reserves of the TauTona mine were transferred to the Mponeng mine in the same year.
Location, geology, and mineralisation
Mponeng gold project is situated in the West Wits mining district, approximately 65km west of Johannesburg, South Africa. It lies near the town of Carletonville on the border between Gauteng and North West provinces.
The Mponeng mine is situated on the north-western rim of the Witwatersrand Basin. The main reef horizon mined at the Mponeng mine is the Ventersdorp Contact Reef (VCR), which caps the Witwatersrand Supergroup.
The mine can also access the Carbon Leader Reef (CLR) located approximately 800m below the VCR.
The gold mineralisation at Mponeng mine succeeded a period of deep burial, fracturing, and alteration. The gold and other elements are believed to have precipitated through the reaction of hydrothermal fluids at high temperatures along the reef horizon.
Ore reserves at the Mponeng gold project
The proven and probable ore reserves at the Mponeng mine were estimated to be 36.19 million tonnes (Mt) grading at 9.54g/t gold as of December 2019. The contained gold reserves were estimated to be 345 tonnes or 11.1 million ounces (Moz).
Mining and gold processing
The underground mining method is implemented for the Mponeng gold project. The sequential grid mining technique is used for extracting the ore. The underground mine comprises a twin-shaft system housing two surface shafts and two sub-shafts.
After mining, the waste rock is separated from the ore and is sent to the low-grade stockpile, while the ore material is sent through a conveyor belt to the nearby Mponeng gold plant for processing and smelting.
The ore undergoes crushing and grinding in a semi-autogenous (SAG) mill, followed by a conventional gold leach process incorporating liquid oxygen injection. The gold is extracted through the carbon in pulp (CIP) process.
The extracted gold finally undergoes electro-winning and smelting by means of induction furnaces at the plant. The Mponeng gold plant has the capacity to process 160,000 tonnes of ore a month.
The electricity for the Mponeng gold operation is supplied by South African electricity utility Eskom.