Ngezi Platinum Mine, Zimbabwe

Located about 150km southwest of Harare, Ngezi is Zimbabwe's principal platinum-group metals producer. Operated by Makwiro Platinum Mines, owned 70% by Zimbabwe Platinum Mines (Zimplats) and 30% by the South African mining company, Impala Platinum, Ngezi has replaced the failed Hartley operation, which was developed by Australian companies BHP and Delta Gold in the mid-1990s.

An underground operation, Hartley was brought into production in 1997 at a cost of $289m, but shut within two years because of operational problems. Delta spun off its holding into Zimplats, which later bought BHP's share as well. Following further transactions, Zimplats is now 86% owned by Impala, with agreements in place for Zimbabwean citizens to take a 15% holding in Zimplats.

In early 2001, Zimplats announced the development of Ngezi into a 2.2Mt/y open pit operation producing 208,000oz/y of platinum-group metals, plus nickel, copper and cobalt. Ore from Ngezi is treated in the Selous Metallurgical Complex at Selous, thus reviving at least part of the original project, although the refinery section remains on care-and-maintenance. The operation is now producing an output of 3,400t/y of smelter matter containing the precious and base metals.

Great Dyke reserves

Hartley and Ngezi are two of several platinum-bearing deposits hosted in Zimbabwe's 'Great Dyke'. The Great Dyke is a 450km-long mafic and ultramafic intrusive structure that runs through central Zimbabwe, with a maximum width of just 11km. Its platinum potential has long been recognised.

The orebody covers an area of 8km north to south and 1km to 1.5km east to west, and outcrops at surface. Platinum-group metal mineralisation occurs within a 4m vertical interval within the stratiform deposit. The sulphide ore is oxidised at the outcrop, but remains unweathered at depths of 20m or more.

As of March 2009, total proven and probable reserves at Ngezi were estimated to be 228.4Mt grading 1.64g/t platinum (Pt), 1.28g/t palladium (Pd), 0.14g/t rhodium (Rh) and 0.26g/t gold (Au), plus nickel, copper and cobalt. This gave a combined '4E' (Pt+Pd+Rh+Au) grade of 3.31g/t. Overall, the reserves contain 28.4Moz of 4E, including 14Moz of platinum in an orebody that averages 2.7m thick.

Hartley's resource base is 188Mt grading 4.5g/t platinum-group metals equivalent, plus minor copper and nickel.

Ngezi open-pit mining

Ore reserves at Ngezi lie at a depth of between 20m and 50m below surface. After waste stripping, production mining is carried out in 3m-high benches within the 20m to 50m depth zone, with the waste rock being backfilled and worked-out areas rehabilitated on a continuous basis.

During open pit mining, the company used the South African mining contractor, MCC, for all its drilling, blasting, loading and rock hauling operations within the pit, which has four active working areas.

Run-of-mine ore is crushed to minus-300mm, with a 20,000t stockpile on site feeding 100t-capacity road trains that carry feed material to the Selous Metallurgical Complex, 77km away near the old Hartley mine site.

The company began underground mining during 2003, using electro-hydraulic drill rigs and roofbolting jumbos. With the cost of surface mining having increased dramatically after the renewal of its mining contract during 2004, Zimplats is now focusing on phasing out open-pitting and concentrating on the underground operation, which will provide an increasing proportion of its output.

In mid-2006, the company reached agreement with the Zimbabwean government over future expansion of the operation in exchange for relinquishing some of its mineral rights that lie outside its current long-term planning area. Zimplats has released 36% (51 million ounces of platinum) of the company's resource for authorisation.

Costing $258m and scheduled for completion in 2010, this 'Phase 1' expansion project includes the development of two new underground mines as well as the construction of a 1.5Mt/y concentrator at Ngezi. The expansion also includes construction of 715 new staff houses near Ngezi.

Ore processing

The Selous Metallurgical Complex consists of a mill, concentrator, smelter and converter, and on-site laboratories. There is also a base metal refinery that is currently mothballed. Run-of mine ore is ground in a Fuller Traylor SAG mill before flash flotation to recover a coarse, high-grade primary concentrate, followed by regrinding, and rougher and scavenger flotation. Concentrates produced by flotation are smelted, then converted to give a final low-iron matte, containing 44% nickel, 33% copper, 21% sulphur and 1,500g/t precious metals, that is sold to Impala Refining Services for subsequent precious-metals recovery.

Platinum production

In 2008, the Zimplats produced 94,300oz of platinum in matte and the plan to expand its annual production to 180,000oz by 2011 is under progress. Total metal sales for 2008 were 94,300oz platinum, and 1,583t of nickel.