Zambia revisited

Materials World magazine
,
1 Nov 2008

Michael Forrest talks to Masoud Alikhani, Chairman of Berkeley Mineral Resources, about restarting the mining of lead and zinc deposits in Kabwe, Zambia, while addressing pollution.

The Copper Belt of Central Africa is one of the world’s great metallogenic provinces, extending in an arc through the Democratic Republic of the Congo and Zambia. The scale and extent of the mineralisation is unparalleled and results from mineralised fluids deposited in a series of sediments known as the Katanga Supergroup. These sediments, which include the Roan Group, were metamorphosed and folded in the Lufilian orogenic (mountain-building) event that gives its name to the arc hosting most of the Copper Belt’s mines.

Rich deposits

Immediately south of the eastern end of the arc lies Kabwe, a high-grade lead zinc deposit. The host rocks are dolomites (magnesium limestones) correlated with the Upper Kangomba formation which overlies phyllites and sandstones, resting on a complex of granite, gneiss and amphibolites. The effects of the Lufilian orogeny can be seen at Kabwe as a series of three folding events, with the last northeast–southwest being the dominant.

The main ore-bearing lithology is a massive light grey dolomitic marble that forms the core of the Kabwe syncline. At the edge of the mineralisation the dolomite is silicified, with some parts totally converted to jasper. The folding events and associated faults control the mineralisation, which comprises six major orebodies averaging 10-30m thick. These pipe-like systems consist of a sulphide core of sphalerite, galena and pyrite surrounded by silicate ore (primarily willemite). The Kabwe deposits are characterised by high zinc and lead grades, with values of up to 29% zinc and 16% lead in the 10 mapped orebodies, making these some of the highest grade zinc/lead deposits in the world.

Changing hands

The Kabwe deposits were discovered in 1902 by T G Davey, who was working for the Northern Copper Company. He saw a similarity between the two low hills with lead and zinc mineralisation and the Broken Hill mine in New South Wales, Australia. Consequently when mining began four years later, it was named Broken Hill. Operations continued at Kabwe for the next 88 years, and an estimated 1.8Mt of zinc and 800,000t of lead were recovered.

Until the 1930s Kabwe was the largest mine in the region. Although it became the centre of the region’s railway network, mining was the biggest employer. Underground operations began in 1940 with ore treated in a number of plants, including a gravity and flotation plant and two smelters. The tailings were dumped in a 60-acre site around the mine.

The mine has been owned by several companies, and, since 1971, it had been controlled by the Zambian Government under a programme of nationalisation. By 1982 these interests were merged into ZCCM (Zambia Consolidated Copper Mines), then one of the largest companies in southern Africa. Final closure of the mine came in 1994 after a prolonged period of low metal prices, following which the surface assets were sold off, leaving a landscape with high lead levels, derelict infrastructure and fragmented ownership. A company that formed after the privatisation of ZCCM assets, ZCCM Investment Holdings, retained responsibility for the mining legacy. Since then, ownership and tailings have passed to a number of companies.

Unemployment and pollution legacy

Large-scale unemployment followed the closure of Broken Hill, with 980 workers directly affected and another 5,000 people in companies dependent on ZCCM.

Impacted soils and surface waste materials were left with lead levels many times above health guidelines. Investigation of these metal values revealed high levels in the immediate vicinity of the waste dumps, where lead-laden dust is a problem. Lead has also entered the local drainage, polluting streams and shallow wells. The pollution is so severe that US$15m of the World Bank/Nordic Development Fund Copper Belt Environmental Programme has been allocated to Kabwe.

Berkeley Mineral Resources, based in London, UK, has acquired mining rights for some of the dumps and is preparing to excavate this material and ship it to China for recovery of contained metals.

Some of the dumps licensed to Berkeley are the residues from the imperial smelting furnace process that separated the lead from the zinc, with the latter being captured from zinc vapour at the top of the furnace, and lead tapped from the base. Other dump material is the waste from the Waelz kilns which reduced the zinc ore with coke. This process was characterised by incomplete separation of metals, and entrainment of heavy metals in dusts and exhaust gases. The inefficiency of these processes was shown by a survey and analysis of the waste dump material. In those licensed to Berkeley, there is nearly two million tonnes grading on average 5.7% zinc and 1.4% lead, according to Land and Mine Surveyor, and former ZCCM employee, Stephen Chama.

Deeper excavation have revealed an irregular distribution of particle size and metal content. Masoud Alikhani says, ‘One of the major problems is the metal-laden dust where the ground is disturbed. Many people now work the dumps to produce metal concentrates’.

To reduce toxicity, the waste has to be removed or covered with a benign material. ‘Reprocessing the material to recover valuable metals is a cost-effective way to achieve this end,’ he added.

Back down the mines

Berkeley has commissioned consultants to undertake metallurgical tests to establish slag mineralogy and chemical composition, and to devise a method to increase the lead and zinc content of the material by pre-concentration prior to shipping to a smelter. Analysis has revealed the presence of significant vanadium, germanium and rare earths deposits that could add value to the project. Berkeley plans to sell concentrates directly from site at prices based on London Metal Exchange transactions, and a South African processor has expressed interest. Meanwhile, the dumps’ material is being sold to China.

Berkeley is interested in reviving the Kabwe mine to exploit the underground reserves recorded in the ZCCM mine closure inventory. The company is negotiating with Alberg Mining and Minerals Exploration Ltd, South Africa, to re-open the mine following the successful outcome of a Zambia High Court ruling over mining rights in favour of Alberg.

This work includes a large-scale mining application to cover the No 1 ore body shaft, The Mine Club and Speaks area. Overall, it is estimated that two-thirds of the resources discovered in the Kabwe ore bodies remain unmined.

Further information: Berkeley Mineral Resources