The Great Dyke
The Great Dyke is a geological feature running through the heart of Zimbabwe for about 550 kilometres in a roughly north-south direction. The pgm occur in a layer known as the Main Sulphide Zone, which is typically about 3 metres thick. However, the economic mining width may be as little as one metre, depending on grade, metal prices and the chosen mining method. The pgm content is lower than that of South African ores, with head grades generally below 4 grams per tonne, of which about 55 per cent is platinum. Nickel and copper values are typically higher than those found in South African platinum ores.
Zimbabwe's oldest platinum mine is the Mimosa operation, located in the southern part of the Great Dyke on the Wedza Geological Complex. Ownership is currently split 50:50 between Impala Platinum and Aquarius Platinum.
Mining at Mimosa has a long history: the deposit was exploited briefly in the 1920s, and trial mining was undertaken by Union Carbide Zimbabwe between 1966 and 1975. Zimasco (a ferrochrome mining and smelting company) took over Mimosa in 1992. The pilot plant was refurbished, and mining recommenced in 1994, gradually building up to a rate of just under 30,000 tonnes of ore per month. Although small, the operation was highly successful, and began to attract the attention of the South African pgm producers. A proposed acquisition by Anglo American collapsed in 2000, but the following year Impala Platinum acquired a 35 per cent stake in the mine. In 2002 Impala took a further 15 per cent, with Aquarius Platinum taking the remaining 50 per cent of the company.
Since 2002, output at Mimosa has gradually been expanded, and the mine - which has been among the lowest-cost platinum producers in the world - extracts around 100,000 oz of platinum annually.
During the early 1990s, a second mine, the Hartley Platinum Project, was developed by a joint venture between the Australian companies BHP and Delta Gold. It opened in 1995, but following a string of geological and metallurgical problems, underground operations were suspended in June 1999. BHP's interest in Hartley Platinum was sold to Zimbabwe Platinum Mines (Zimplats), a spin off of Delta Gold's platinum assets, which began to develop a new open-cast mine further south, at Ngezi. Operations here began in 2001, following the acquisition of a share of the project by Impala Platinum and the South African bank Absa. Over the next two years, Impala increased its holding in Zimplats.
In 2011, Ngezi produced about 185,000 oz of platinum from a series of underground "portals" (declines), and an expansion programme is underway which will see output rise to 270,000 oz of platinum annually.
A third platinum mine, Anglo American's Unki project, was commissioned in late 2010 and produced around 50,000 oz of platinum in 2011. The mine is designed to extract and process 120,000 tonnes of ore per month, which should yield around 70,000 oz of platinum annually at full capacity.