Between Welkom and Bothaville, Free State province, South Africa
Harmony Gold (100%)
Precambrian metamorphosed palaeo-placer quartz-pebble conglomerate reef horizons
Native gold plus silver
The Target 1 gold mine is a deep gold mine in South Africa. Officially opened in May 2002, the mine is owned by Harmony Gold, a South African mining company.
The mine had been under development since 1995, with the aim of accessing previously unknown gold resources that lie to the north of the existing Welkom goldfield, 200km south-west of Johannesburg. The total capital cost of bringing the mine into production was estimated at R2.01bn ($215m).
The corporate rearrangement between Avmin, Harmony Gold Mining, and African Rainbow Minerals resulted in Target 1 being transferred from its original owner, Avgold, to Harmony in May 2004.
The gold mine produced 72,146 ounces (oz) of gold, grading 4.13g/t in 2020. The total production in 2021 is expected to be 51,536oz, according to the production guidance. Target 1 has an estimated mine life of seven years.
Target 1 gold mine geology and reserves
The Welkom gold field lies at the southwestern end of the major Witwatersrand basin. The Welkom section was discovered in the late 1940s, leading to the development of several mines, including Anglovaal’s Loraine operation.
The Eldorado Reef mineralisation is contained in a series of ancient alluvial fans, stacked sequentially within the rocks of the Central Rand Group of the Witwatersrand Supergroup. Avgold’s recognition of the periodicity of these sequences led the company to explore the area to the north of its Loraine mine, where it has discovered substantial gold resources contained in the Basal, ‘B’ and Elsburg Reefs, which occur throughout the Welkom area, and the previously uneconomic Big Pebble and Dreyerskuil Reefs.
The proven and probable mineral reserves at Harmony Gold’s Target mine were estimated at 5.1 million tonnes (Mt) grading 4.28g/t gold, as of June 2020.
Target 1 mine development
The Target orebodies, which lie at depths of between 2,200m and 2,500m below surface, were accessed by declines sunk from the existing underground infrastructure at Loraine, where production ended in mid-1999. This substantially reduced the cost of bringing Target into production. Drilling equipment used included Sandvik (Tamrock) twin-boom jumbos.
The declines are equipped with roof-slung Walter-Becker monorails for transporting materials from the Loraine shaft to the Target production areas. The declines also handle crushed ore from the main underground primary crushers to the hoisting skips in Loraine’s No.1 shaft, using belt conveyors that are also used for transporting personnel.
Mining methods at Target 1
The Target underground operations are conducted using the Target 1 shaft, which is operated at a depth of approximately 2.3km below the surface. The mine employs a combination of mechanised mining and conventional open stoping.
The nearby Lorraine mine was closed in August 1998 and Lorraine 1 and 2 shafts were transferred to the Target operation, which became the Target 1 and 2 shafts.
The current Target 1 operation is a single underground mine that was built as an extension to the Loraine mine. Access to the underground mine is provided by shaft one.
From the outset, Avgold planned Target to be highly mechanised, using long-hole stoping where reef widths allow this, and either cut-and-fill or long-hole stoping in narrow reef areas. Broken ore is handled using remote-controlled Sandvik load-haul-dump machines. Drift-and-fill mining is used in flat-lying reef areas.
The depth of the orebodies means that all the stoping areas must first be distressed by developing conventional narrow-reef stopes above them, and once extracted, all stopes are backfilled using cemented mill tailings.
Avgold installed a fully integrated information system – Prostar (Production Systems for Target) – to monitor the mine. Prostar provides real-time information on all aspects of the operation, including geological data, mine operations, maintenance, materials management, production control and other parameters. The network is run from a centralised surface control room. The mine is also equipped with a leaky-feeder radio system for improved communications between the surface and the entire underground operation.
The mine has 24MW of refrigeration capacity for underground ventilation air, helping to maintain the operations at temperatures of below 27.5°C.
Progress in the massive stope areas of the mine has been hampered by high stresses that caused large low-grade rocks to block the draw points. This has been handled by reducing the size of the stopes, increasing the draw point spacing, and mining new destressing slots to increasee mining flexibility. Harmony also commenced an exploration programme to confirm the geological model of Target North, situated on the Witwatersrand Basin.
Ore processing at Target 1 mine
While the initial development material containing ore was treated in the old Loraine concentrator, this has now been partly demolished and a new, highly automated metallurgical plant was completed in 2002.
The ore from Target 1 is milled and processed at the Target plant near the mine. The Target operation shares the processing facility with a Harmony waste rock dump, which is managed by surface sources. Gold recovery is done through gold cyanide leaching.
Multinational engineering company Fluor was engaged in the detailed design of the processes, structures, and mechanical and electrical works, as well as the construction of the processing plant. It was also responsible for the commissioning of the plant.