Mponeng

Above-ground operations at Mponeng mine.
Location
Mponeng Location in South Africa
Province	Gauteng
Country	South Africa
Coordinates	26°26′10″S 27°25′50″E / 26.43611°S 27.43056°E / -26.43611; 27.43056
Owner
Company	Harmony Gold
Website	https://www.harmony.co.za

Mponeng is a gold mine in South Africa's Gauteng province, it is an ultra-deep tabular mine of the Witwatersrand Basin. ^[1]Previously known as Western Deep Levels #1 Shaft, the underground and surface works were commissioned in 1986.^[2] It extends over 4 kilometres (2.5 mi) below the surface,^[3] and is considered to be one of the most substantial gold mines in the world.^[4] It is also currently the world's deepest mine from ground level,^[5] reaching a depth of 4 km (2.5 mi) below ground level, with aims to deepen the mine to reach additional reserves.^[6] The trip from the surface to the bottom of the mine takes over an hour.^[3] Mining in South Africa has had a huge effect on the technologies of deep gold mining around the world, South Africa is even considered a pioneer of the trade.^[7]

History and Ownership

Harmony Gold, Africa's largest gold producer, purchased Mponeng from AngloGold Ashanti (AGA) in 2020, Harmony paid approximately $200 million. Harmony Gold also acquired Mine Waste Solutions as they gathered the remainder of AGA's assets in late 2020. ^[8]As of 2022, all-in costs of production were US$1771/oz (US$1614/Troy oz). Even at near record high gold prices, Mponeng is barely breaking even.^[9] Gold is currently worth roughly US$2080/oz (US$1900/Troy oz).^[10] Over 5400 metric tons of rock are excavated from Mponeng each day.^[11]

The most recent publication from Harmony Gold outlines their 2023 year production:

• Produced: 7449kg (239 490 oz) of gold^[12]
• Grade: 8.43g/t^[12]
• Volume of ore milled: 884 000 tons^[12]

History and Operations

Being a part of the Witwatersrand, the largest gold deposit on earth, Mponeng is the result of the discovery of the basin. Beginning in the 1850-70s a series of discoveries were made, Pieter Jacob panning gold from a river and a Henry Lewis finding quartz and gold vein on a farm. ^[13] After shaft sinking for about 5 years (1981) Mponeng officially began mining operations in 1986.^{[14] [15]}Before Mponeng, the mine was known as the Western Deep Levels South Shaft or the No1 Shaft, the name Mponeng came to be in 1999. ^[14]

The mine is estimated to be producing until 2029, when reserves may finally run out, the mine has been running since 1986 and has the possibly to run for 43 years. ^[16] current depths reach roughly 3.8km down, it the coming years and remaining production life of the mine it will likely reach beyond 4.2km depth. ^[16] The mine has not be running continuously since the day of opening in 1986. A seismic event in March of 2020 resulted in a stoppage of operations due to fatalities. ^[16] Like most businesses, ^[17] Mponeng closed its doors due to COVID-19 in May of 2020, but has since returned the producing. ^[16]

Striking in south Africa's mining district was apparent in the 1900s, for example the South African gold mine strike in August of 1946. ^[18] As Mponeng did not start operations until near the turn of the century there has not been many other notable strikes since then, other than 2012. Back in 2012, while AngloGold Ashanti was Mponeng's owner, strikes occurred. ^[19] The strikes were a combined result of gold and platinum industry-worker issues in South Africa. ^[19] AngloGold's position on the matter was to maintain safety, peace, and stability. ^[19] The striking action of 2012 totaled nearly 16% of the total mining workforce of South Africa, not just Mponeng. ^[20] AngloGold specifically experienced nearly 35,000 workers putting down there tools in the illegal strike. ^[21] Although not at Mponeng, the strike of 2012 included a wildcat strike at a nearby platinum mine ended with 34 miners dying from police interactions. ^[20]

Physical conditions

The temperature of the rock reaches 66 °C (151 °F), and the mine pumps slurry ice underground to cool the tunnel air to below 30 °C (86 °F).^[3] A mixture of concrete, water, and rock is packed into excavated areas, which further acts as an insulator.^[3] Tunnel walls are secured by flexible shotcrete reinforced with steel fibers, which is further held in place by diamond-mesh netting.^[3]

In 2008, researchers looking for extremophile organisms discovered the bacterium Desulforudis audaxviator present within groundwater samples from kilometers deep in the mine.^[22] The name 'Audaxviator' comes from the popular novel "Journey to the Center of the Earth" (Jules Verne), which means 'descend, bold traveler, and attain the center of the Earth'.^[23]

Geology and gold bearing fractures

The mine sits in the Witwatersrand Basin and utilizes 2 horizons; Ventersdorp Contact Reef and Carbon Leader Reef.^[24] In the Mponeng mine the Ventersdorp Contact Reef is mainly interbedded quartzite and coarse conglomerates and constitutes only a meter or two of thickness.^[25] The Witwatersrand Basin has experienced many surges of thrusting, resulting in abundant fractures. ^[26] These small faults often correspond to depositional and sediment contacts.^[26] It is along these faults that mineralizing fluids flow, which lead to the precipitation or mineralization of gold. ^[27] This complicated geologic history associates with gold mineralization to a high degree, the Witwatersrand Basin holds nearly a third of gold reserves and is responsible for over 40% of all gold. ^[26] Mponeng has a proven gold reserve of roughly 46 million ounces (over 1300 tons), that is more than 8x more than the second deepest gold mine 'Driefontein' also located in South Africa.^[28]

The gold mineralization is likely related to hydrothermal activity, and occurs in varying lithology of conglomerates, known as the reefs stated above.^[26] The stratigraphy that the gold is found in ranges in thickness, from around 10cm to a 1-meter. ^[29] Gold is found in these variable thickness layers of pebble lags, stacked fluvial deposits, and other stratigraphy.^{[29] [26]} Mponeng specifically, gold is associated with previously mentioned thrust fracturing and with the following mineralization's:

• Steep quartz/sulphide-bearing fractures
• Sub-horizontal quartz fractures
• Sulphide-bearing fractures
• Ultracataclastics
• Mesophased hydrocarbons.^[26]

As with most mines, Mponeng is limited to the structural control of the geology. The VCR mineralization is governed by thrust-fracture systems discussed above, largely from the Lower Kliprivierberg age. ^[30] The VCR and CLR themselves are around 2.7 billion years old . ^[31][32]

Seismicity

Seismicity in mines is common as removing mass amounts of rock can change stress dynamics, especially if pre-existing faults exist.^[33] This seismicity is termed 'mining-induced seismicity' are still the release of elastic strain but are often low moment magnitude. ^[33][34] Reaching beyond 1000 events a day, events of any size pose serious risk to mining operations and employees. It is not uncommon for equipment to be damaged, or have the collapse of drifts and stopes as a result of events.^[33][34]

On December 27, 2007 a dyke within Mponeng experienced a 1.9 magnitude event. Caused by stress change due to excavation within the mine. In March of 2020 Mponeng experienced a magnitude 2 event, 3 were killed.^[35]In deep mines vertical stress can reach an astounding 80-100 MPa, equivalent to roughly 10km under water. ^{[36][37][38] [39]}Seismicity in deep gold mines is common, and is often induced by mining activities. ^[40][41] 2 events can be considered:^[41]

• Type A - low moment magnitude (<1), clustered in time and space, within 100m of mining surface.
  • Induced by blasting, perturbation of excavating processes, closure of stopes.
• Type B - possible higher moment magnitude (>3), not clustered in time or space.
  • associated with friction dominated existing shear, tectonic earthquakes.

Most earthquakes at depth are mining related (Type A), typically associated with the beginning of a new stope.^[42]

Research

One way to move forward is a better understanding of rock burst and excavation methods. A better understanding can provide better safety protocols. ^[43] A prominent use of deep mines and their associated, often low magnitude, earthquakes is to connect laboratory scale experiments to real world situations. ^[44][45] There is also considerable research trying to understand nucleation of said earthquakes,^[44] and whether they work the same as larger earthquakes. ^[45] In Mponeng, JAGUARS (Japanese-German Acoustic Emissions Research in South Africa)^[46] has emplaced a network of accelerometers and piezoelectric acoustic emissions sensors.^[44] These sensors can record very small moment magnitude earthquakes, capable of recording events with frequencies from 0.7 kHz to 200 kHz (M<0.5).^[47] In a one year period, 2007-2008, nearly 500,000 events were recorded, most of which with low (sub 25 kHz) frequencies.^[47] The JAGUARS network in placed below the Ventersdorp Contact Reef, there are 8 Acoustic Emissions sensor, 2 strainmeters, and one triaxial accelerometer that make up the network. ^[47]

In popular culture

Released in 2010, the American television show 'Build it Bigger' (Powderhouse Productions) hosted by Danny Forster visited Mponeng Gold Mine. ^[48]

Millan Ludeña, an Ecuadorian marathon runner, became the first person to run a half-marathon fully underground in the deepest part of Mponeng Gold Mine. A Guinness World Records adjudicator was on hand to document the race and issued the certificate for the deepest half-marathon.^[49] It was Millan's goal to run as far, and as close to sun as he could. Later, after Mponeng, Millan run up Mt.Chimborazo in Ecuador.^[50] Millan has a movie called 'From Core to Sun' released in 2018 about his achievements. ^[51]

See Also

• Gold Mines
• South Africa
• Witwatersrand
• Gold
• Seismicity
• Earthquakes

References