The Mponeng gold mine located in the Gauteng province of South Africa, is the deepest operating mine in the world. It was the last underground operation by AngloGold Ashanti in South Africa.
Harmony Gold, South Africa’s biggest gold producer, became the owner and operator of the Mponeng underground mine by acquiring AngloGold Ashanti’s remaining South African assets in October 2020.
Harmony Gold had entered a definitive agreement to acquire the Mponeng mine and Mine Waste Solutions, an independent surface operation, from AngloGold Ashanti for £154m ($200m) in February 2020.
Harmony Gold produced 175,095 ounces (oz) of gold by processing 683,000 tonnes (t) of Mponeng underground ore during the first nine months of its operatorship from October 2020 to June 2021. The company expects to produce up to 250,000oz of gold a year from Mponeng over an estimated remaining mine life of eight years.
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.
Production disruptions at Mponeng gold mine
Operations at the Mponeng mine were frequently interrupted during AngloGold Ashanti’s operatorship in 2020. Production from the mine was halted due to a seismic event that killed three miners approximately 3.5km below the surface in March 2020.
Although the mine resumed operations running at 50% capacity in April 2020, the production was again halted after several positive COVID-19 cases were reported at the mine in May 2020.
Mponeng mine history
The Mponeng gold mine was originally known as the Western Deep Levels South Shaft or the No. 1 Shaft. The underground mine, along with the nearby gold processing plant, was commissioned in 1986, while the operation was renamed the Mponeng gold mine in 1999.
The mine is part of the West Wits mining district, which also hosted the Savuka and 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
The Mponeng gold project is situated in the West Wits mining district, about 65km west of Johannesburg, South Africa. It lies near the town of Carletonville, on the border between Gauteng and North West provinces.
The gold deposit sits 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 Mponeng ore body underwent a series of deep burials, fracturing, and alterations. 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 of the Mponeng mine were estimated to be 7.7 million tonnes (Mt) grading 8.53g/t gold as of June 2021. The contained gold reserves were estimated to be 2.1 million ounces (Moz).
The measured, indicated, and inferred ore resources are estimated to be 52.7Mt containing 23.5Moz of gold.
Mining and gold processing
The underground mining method is implemented at 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 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. Gold is extracted through the carbon-in-pulp (CIP) process. The extracted gold finally undergoes electro-winning and smelting in induction furnaces at the plant.
The Mponeng gold plant has the capacity to process 165,000t of ore a month. South African electricity utility Eskom supplies the electricity required for the gold operation.