Platinum group metals
Located about 150km south-west 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 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 87% owned by Implats.
In early 2001, Zimplats announced the development of Ngezi into a 2.2Mtpa 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. In 2009, the operation produced 190,532oz of smelter matter containing precious and base metals.
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 the 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 June 2009, total proven and probable reserves at Ngezi were estimated to be 217.4Mt grading 1.7g/t platinum (Pt), 1.32g/t palladium (Pd), 0.14g/t rhodium (Rh) and 0.25g/t gold (Au), plus nickel, copper and cobalt. Overall, the reserves contain 23.9Moz of 4E, including 11.9Moz of platinum in an orebody that averages 2.4m thick.
Hartley’s resource base was 218Mt grading 2.04g/t platinum-group metals equivalent, plus minor copper and nickel.
Ore reserves at Ngezi lie at a depth of between 20m and 50m below the 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.
In November 2008, the open pit was closed following a fall in metal prices. The company began underground mining in 2003, using electro-hydraulic drill rigs and roof bolting jumbos. With the cost of surface mining increased dramatically after the renewal of its mining contract during 2004, Zimplats focused on the underground operation, which provides an increasing proportion of its output.
In mid-2006, the company reached an agreement with the Zimbabwean Government over the 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% (51Moz of platinum) of the company’s resource for authorisation.
Costing $340m, this ‘Phase 1’ expansion project included the development of two new underground mines as well as the construction of a 1.5Mtpa concentrator at Ngezi. The expansion also included the construction of 715 new staff houses near Ngezi.
By the end of the third quarter of 2008, the Portal 1 underground mine was completed. By June 2009, full production of 1.2Mt was achieved. The second underground mine, Portal 4, was completed in 2012.
The concentrator was commissioned in July 2009, three months behind schedule. Ramp up to full capacity was achieved by September 2009.
Implementation of the second phase of the expansion began in the first quarter of 2010 with an aim to increase the annual capacity from 180,000oz to 270,000oz. The mine has a current capacity of 6.2Mtpa.
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.
In 2009, the Zimplats produced 190,532oz of 4E metal, including 95,965oz of platinum in matte and the achieved annual production of 185,000oz in 2011.
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