Rio Tinto has announced the much anticipated move underground to block caving at Argyle. The company will develop a $760 million block cave underground project at its wholly owned Argyle diamond mine in the Kimberley region of Western Australia. An additional $150 million will be spent on a related open pit cutback to enable continuity of production between the cessation of open pit mining in 2008 and the ramp up of the underground mine. Development work associated with the present exploratory decline will continue, with construction of the underground mine to begin once government approvals have been finalised. The average annual production over the life of the underground mine from 2007 to 2018 is expected be around 60% of Argyle's historical annual average of 34 Mct and of similar quality.
The current open pit is centred on the lamproite AK1 orebody, moving over 80 Mt/y of rock to treat 10 Mt/y of ore. Rio Tinto has much experience in block caving, including the Palabora and Northparkes mines, and has put a great deal of planning into the Argyle block cave. It spent some A$70 million on a feasibility study on underground mining, and sinking an exploration decline into the AK1 pipe. The AK1 and associated mineralization is known to extend some 600 m below the planned pit bottom. The deposit plunges steeply towards the south and dips to the west at approximately 70o. The main area of the AK1 orebody, where a majority of the mine production will come from, thins and bifurcates with depth.
The major technical issues associated with the project, and being addressed, are:
- Ventilation, given hot and humid surface and underground conditions
- Dewatering, given a monsoon environment and mining under an open pit
- Ground conditions associated with numerous structural features and relatively weak host rock units
Caving has probably seen more application in diamonds than any other underground mines, in such operations as De Beers' Finsch and Koffiefontein mines. Rio Tinto chief executive Leigh Clifford said, "This investment springs from Rio Tinto's proven capacity to develop large scale underground mines and to compete successfully in the global diamond business. It will allow the Argyle mine - the world's largest diamond mine- to operate beyond 2008. Furthermore, it will ensure that the mine's high levels of Indigenous employment will continue to deliver economic and social benefits to the Kimberley region."
Argyle is the largest single contributor to the Kimberley economy with 25% of its present full time workforce Indigenous and 50% living locally. "The Western Australian Government's agreement to provide royalty relief and waive some secondary processing obligations has been fundamental to the economics of the expansion and will ensure that Argyle continues to play an important regional role," Mr Clifford said.