Vancouver — Over the past decade, platinum miners have been eager to advance known deposits on Zimbabwe’s Great Dyke, home to the world’s second-largest reserve of platinum. However, a strong nationalist government led by Robert Mugabe prompted the major producers to tread cautiously.
That is, until recently. Today
The latest move has Anglo American Platinum (Amplats) giving the green light to the US$90-million development of the Unki deposit, near Gweru. The company, which is 70%-owned by mining powerhouse
Development of the Unki deposit was suspended in the late 1990s, after the Zimbabwean government declined to offer tax breaks. So far, Amplats has spent more than US$60 million on the project.
The South African-based company has a majority shareholding in Unki and will manage the operation, while the Zimbabwe subsidiary of Anglo American holds a minority stake. Initial development costs will be funded from existing cash resources and borrowing facilities.
Unki, which would be Amplats’ first platinum mine in Zimbabwe, is anticipated to produce 60,000 oz. platinum and 43,000 oz. palladium per year, plus gold and base metal credits. Reserves are in the order of 38 million oz. platinum group metals with an average in situ grade of 5.4 grams combined platinum-palladium-rhodium-gold per tonne.
The project has yet to be approved by Zimbabwean and South African regulators.
Amplats’ decision to move ahead reverses Anglo American’s long-standing trend of cutting investment in Zimbabwe. Over the past three years, it cut its production of ferrochrome from that country and sold businesses, including cement-makers and sawmills. The development also pits Amplats against its main rival, Impala Platinum (Implats), in the race to exploit Zimbabwe’s platinum reserves.
While Anglo American was trimming investments in Zimbabwe, Implats has been developing platinum deposits along the Great Dyke. The world’s second-largest platinum producer has acquired a dominant position in Zimbabwe’s fledgling platinum industry at a time when the country is stuck in a 5-year recession and beset by political crises.
In June 2002, the Zimbabwe African National Union-Patriotic Front, known as ZANU-PF, narrowly won a majority in parliament. The win paves the way for long-standing nationalist leader Robert Mugabe to continue his 20-year reign unchallenged. As always, the worry among international mining houses is that Zimbabwe will nationalize its resource sector, and mining, because it accounts for some 40% of export earnings, would be a likely candidate. However, South Africa may have some influence in the shaping of such policies, as it is both a regional power and as the country’s largest trading partner.
“Our experiences in Zimbabwe so far have been good,” says Implats CEO Keith Rumble. “Even if conditions deteriorate badly, we think that riding through that crisis for two or three years is probably still in Impala’s best interests, having secured a 200-year life-of-mine resource.”
Although platinum was discovered in the Great Dyke in 1925, it was not until 1969 that a company first attempted to develop a platinum deposit — Union Carbide, at Wedza. However, that operation never moved forward, owing to uneconomic platinum prices.
In the following years, several exploration programs and resource evaluations have been undertaken, but nothing of significance was found until 1994, when the Mimosa mine began producing on a small scale.
Mimosa produces platinum group metals in concentrate from underground. The concentrates are sent to Implats’ smelter and refineries in South Africa for metal recovery. The operation produces 30,000 oz. platinum-palladium-gold annually, and this is being expanded to 127,000 oz. Mimosa has proven and probable reserves of 51.7 million tonnes grading 3.92 grams platinum-palladium-gold, resulting in some 6.5 million oz.
Implats and Australia-listed
Hartley
The next platinum development in Zimbabwe occurred in the late 1990s, when BHP, now part of
Zimbabwe Platinum Mines (Zimplats), which is controlled by Implats, picked up AurionGold’s stake in the mothballed project and then added BHP’s 67% interest by paying US$3 million. Hartley’s resource base now stands at 166 million tonnes grading 4.9 grams platinum-palladium-rhodium-gold, plus minor copper and nickel.
In 2001, Zimplats started developing the Ngezi deposit along the southern end of the Great Dyke. The company originally envisioned a 2.2-million-tonne-per-year open-pit operation capable of producing 208,000 oz. platinum-palladium-rhodium-gold, plus nickel, copper and cobalt. Ore from Ngezi is being treated in the Selous metallurgical complex at Hartley. Financing for the project was arranged through Implats, which holds a direct 30% interest in the project, as well as 36% of Zimplats. ABSA, a major South African bank, holds an additional 15% interest in Zimplats.
The Ngezi orebody covers an 8-by-1-km area with mineralization outcropping at surface. The sulphide ore is oxidized near the surface but remains unweathered at depths of more than 20 metres. Platinum is hosted in a layer within a 4-metre interval.
Matte
Run-of-mine ore is crushed to minus 300 mm, then carried to the Selous complex, 77 km away, where it is processed into a smelter matte. The matte is then sent to South Africa for processing and refining of final metal products by Implats.
In the year ended June 2002, the operation treated 670,000 tonnes of ore grading 2.9 grams platinum-palladium-rhodium-gold. From this were produced 21,400 oz. platinum, 18,700 oz. palladium, 2,660 oz. rhodium and 1,970 oz. gold, plus nickel and copper.
Reserves to a depth of 50 metres stand at 32 million tonnes grading 1.79 grams platinum, 1.49 grams palladium, 0.15 gram rhodium and 0.27 gram gold per tonne, plus nickel, copper and cobalt credits. The mine life is pegged at 11 years.
By the end of 2002, Zimplats reported a ten-fold increase in attributable reserves to an impressive 34 million oz. combined platinum-palladium-rhodium-gold. Underground mining in areas of known resources allowed a substantial portion of these resources to be converted to reserves. Based on the much-expanded reserve base, Zimplats has completed a scoping study to double capacity to 400,000 oz. combined platinum-palladium-rhodium-gold from the current 200,000 oz. per year. The latest reserves extend the mine life to 20 years.
A bankable feasibility study for the expansion is under way, with completion targeted for the end of 2003.
The newly established Ngezi underground reserves dip at less than 12 and are more than 1.6 metres thick, extending from 50 to 600 metres below surface.
Great Dyke
The 2.5-billion-year-old Great Dyke is a layered intrusive complex similar to that of the Bushveld complex in South Africa or the Stillwater body in Montana. The Great Dyke is a 450-km-long mafic a
nd ultramafic intrusive structure that runs through central Zimbabwe, with a maximum width of just 11 km.
The stratigraphy is broadly divided between the lower ultramafic and overlying mafic sequences. The ultramafic sequence hosts the P1 pyroxenite in the upper cyclic unit, directly below the mafic-ultramafic contact. The P1 pyroxenite is marked by significant lateral variations in thickness, texture and composition and, in turn, hosts the economic platinum-bearing Main Sulphide zone (MSZ).
The MSZ is a discontinuous mineralized layer with a potential strike of some 400 km and is generally 2-3 metres thick. Head grades generally average less than 4 grams combined platinum-palladium-gold per tonne, with 55% of the mineralization attributed to platinum.
The geometry of the sub-chambers are essentially shallow, boat-like, double-plunging synclinal structures. Longitudinally, the layers pitch gently but unevenly toward the centre, and dip moderately at 5-15 from the margins towards the axis.
The Zimbabwean government projects a steady increase in platinum production over the next 10 years. However, as BHP discovered at Hartley, platinum mining in the country is not as straightforward as it may seem.
One of the biggest problems encountered by BHP was grade control. In the Bushveld complex, the main platinum producing reefs, the Merensky and the UG2, are well-defined with geological markers and visible to the naked eye.
The value distribution of platinum mineralization in those reefs is also so consistent that determining where to mine and setting the “mining width” in order to get the highest grade are straightforward.
Unfortunately, this is not the case in Zimbabwe’s Great Dyke, where there are no easily recognizable marker horizons. The mineralization in the Dyke shows no distinction between rock types. The trick is to pick out the section of reef where the platinum group metal content is highest. That layer is slightly offset from where the base metal sulphides occur.
At Mimosa, the platinum-bearing layer is not distinct, but mining has shown that the geologists can pick it out once they have been trained.
At the Ngezi open-pit mine, grade is controlled a different way: holes are drilled ahead of the mining operations on a 50-by-50-metre grid to find the reef, which is then sampled, giving the miners the approximate position of the platinum deposit. The bulk waste is then removed to about 5 metres above the reef and drilled once more in the pit on a 10-by-10-metre grid, pinpointing the platinum reef. The strip waste is selectively removed and finally the reef material is mined.
Assuming Unki and the other deposits live up to expectations, Zimbabwe could become the world’s third-largest platinum producer, cranking out up to 600,000 oz. per year within five years. Implats would have interests in about 80% of that output.
Be the first to comment on "South Africans target platinum in Zimbabwe"