May 2012

Tested and ready

Cave mining methods attractive as open pits disappear

By Krystyna Lagowski

With some 35 cave mining projects in various stages of study and development around the world, it is only relatively recently that this mass mining method has begun proving its potential. Jarek Jakubec, head of mining and geology at SRK Consulting, is among the champions for cave mining, and he believes the technique’s time has come. According to him, attention has been increasing since the end of 1990s when Rio Tinto’s Palabora copper mine in South Africa began using cave mining for hard rock. “Since then, there has been continuous interest and growing use of cave mining techniques,” he says.

That interest is being spurred by the depletion of ore bodies suitable for open pit operations. According to Jakubec, many companies are turning to cave mining when they need to transition their operations underground to mine deep, large low-grade resources.

The technique

Jakubec defines cave mining as a method based on the principle of undercutting rock, and then naturally letting it collapse.

This method can be used with any type of commodity, since it is the geological context that is important. Jakubec says there are many parameters to consider, but generally the ore body needs to be at least 100 or so metres thick for cave mining to be economical. “Most of the designs which are on the drawing board today have single lifts of 300, 400, 500 or more metres,” he says.

Also, the production rates from caves are ever increasing. Jakubec points to Codelco’s El Teniente project in Chile as a prime example. “This is a supercave, an assembly of several cave mines, which produce over 120,000 tonnes of ore a day,” he says. Also, the Oyu Tolgoi project in Mongolia is one of the largest advanced caving projects outside North America. Both are copper porphyry mines, which are large ore bodies well-suited for this type of mining.

Jakubec says ore bodies with relatively small horizontal footprints can be also mined economically if they have sufficient height. A good example would be the North Parks mine in Australia.

The economics

As economical open pit mines become scarcer, cave mining becomes more and more attractive. Jakubec estimates that if the cost of open pit mining were $2.50 to $3.50 per tonne, the cost of underground cave mining could be about $5.00 per tonne. For comparison, sublevel caving would cost about $15.00 per tonne, and cemented backfill mining would cost $50.00 per tonne or more. “Often, larger ore bodies are low grade – for example, the copper porphyry ore bodies mined today by caving are typically around .7 to 1.2 per cent copper,” Jakubec explains. “So with the current price of copper hovering below $4 a pound, you could get about a $60.00 to $100.00 value in one tonne of ore.”

The disadvantage of cave mining is that large upfront capital investment is required. “You have to do all the preparation, with the access hoisting and ventilation shafts, undercutting and extraction development, before you start making any money,” he notes.

The huge Oyu Tolgoi mine, set to produce more than 1.2 billion pounds of copper and 650,000 ounces of gold each year, is only 73-per cent built. Over eight years, $5 billion have been invested in it.

Jakubec says the other disadvantage of cave mining is a long lead time; it typically takes seven to 10 years from studies to production.

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