OUTLOOK GOLD TOM FOX/DALLAS MORNING NEWS/CORBIS FOX/DALLAS TOM Miners walk between elevators en route to the depths of South Africa’s TauTona mine, the world’s deepest gold mine, 4 km below the surface. MINING Extreme prospects High gold prices are making it worthwhile to look for gold in some unusual places. BY BRIAN OWENS ounce in 2003 to almost $1,700 at the end of in the Peruvian Andes, 4,100 metres above 2012. At the same time, gold production has sea level, miners are digging deeper than ever he journey from the surface to the rock seen only marginal increases, with few new before, going to more remote locations and face at the bottom of TauTona, the world’s mines opening. And it’s getting a lot more politically volatile regions. deepest gold mine, takes almost an hour expensive to extract the gold. In 2000, the aver- At the same time, significant amounts of T— even with the lifts that bring the workers age cost of extracting an ounce of gold was just gold can easily be obtained without digging down each of the mine’s three shafts travelling over $200, says Jason Goulden, director of met- into the earth at all — just by recycling the at 58 km per hour. In the dark, hot, cramped als and mining at the SNL Metals Economics gold buried in the growing mountains of dis- tunnels nearly 4 km underground, workers Group in Halifax, Canada. By 2010, he says, it carded electronics. The advent of more effi- excavate a thin dipping vein of gold ore. Rock had risen to more than $850. cient ways to recycle gold from gadgets has is taken to the surface and the gold is extracted Demand has never been higher, but nearly turned scrap into a major source of the pre- using a process that can be traced back to the all the easy gold has already been mined. So, to cious metal. 1880s: the ore is crushed and sprayed with maintain production, mining companies are cyanide to leach out the gold metal. turning to more difficult sources that would DIGGING DEEP Gold has always been a valuable commodity, have been left in the ground if gold prices had There are many factors that influence where but over the past 10 years the price has risen been lower. From the depths of TauTona in the and how deep a mining company will dig for dramatically — from less than US$400 per South African veldt, all the way up to Pierina gold, but in general “as you go deeper it gets S4 | NATURE | VOL 495 | 14 MARCH 2013 | CORRECTED ONLINE 20 MARCH 2013 GOLD OUTLOOK more expensive and time-consuming”, says cases the process doesn’t pick up any gold at Steve McKinnon, a mining engineer at Queen’s all — or so little that it’s almost impossible to University in Kingston, Ontario, Canada, who separate it in solution. Moreover, the activated THINKSTOCK specializes in designing deep mines. It also gets carbon that is used to absorb the gold from the more dangerous. cyanide solution is also swamped by the less Mining at depths such as those of TauTona valuable metal. presents many unique challenges in protecting New ion-exchange resins, developed by the miners, says McKinnon. First of all, it’s hot. Johannesburg-based mineral research organ- The temperature at TauTona’s deepest levels is ization Mintek, are far more selective. These a stifling 58 °C. Air conditioning brings the resins can extract the gold from solutions that temperature down to a toasty but more toler- contain 1,000 times more copper than gold, as able 28 °C. found in the leach solution from the Gedabek Then there is the risk that digging can frac- gold–copper project in Azerbaijan. Although ture the rock around the pit, triggering a seis- the resins are around five times more expen- mic event. “Sometimes that fracture process sive than the conventional carbon used, Mon- can be very violent, because the rock behaves hemius believes that high gold prices will result in a brittle manner,” says McKinnon. “There in this resin technology eventually displacing have been events larger than magnitude 5” — carbon in nearly all forms of gold extraction. equivalent to a moderate earthquake. To minimize the risk, mines use ‘yielding REUSE AND RECYCLE supports’ that are able to deform as the tun- Some companies have turned to a more reli- nel walls move yet still retain their ability to able source: reclaiming and recycling gold that support the structure. There are also networks has already been mined. This ‘urban mine’ of of seismic sensors that constantly monitor the electronic waste — old computers, mobile rock and develop a seismic ‘fingerprint’ for the phones and the like — is far richer than natu- region — any departure from an established ral deposits: a typical open-pit mine will yield baseline requires that workers be pulled out between 1 and 5 grams of gold per tonne, but until seismic readings return to normal. mobile-phone handsets can contain up to Engineers specializing in rock mechanics, 350 grams per tonne of gold, and computer such as McKinnon, also try to design mines E-scrap has 250–350 grams of gold per tonne, far circuit boards up to 250 grams. in such a way as to minimize and control the more than the 1–5 grams of a typical mine. The explosion in the use of electronics over forces exerted on the rock. Taking into account the past three decades has, in effect, created data on local stresses on the rock, and other at any given time so they can be coordinated another kind of gold mine. Indeed, with leg- factors such as the presence of nearby faults, to move and work in the most efficient way. islation setting targets for the collection and they calculate the most favourable layout and Open-pit mines already use global positioning treatment of electronics, recycling the pre- method of ore extraction to minimize fractur- system (GPS) to move their automated vehicles cious metals from such waste has become a ing. This information helps engineers deter- around in a way that maximizes productivity, lucrative business, says Christian Hagelüken, mine which bits of rock can be safely removed but such satellite signals can’t penetrate deep a mining engineer who specializes in recycling and which need to be left behind to support the underground. Radio signals can’t be used materials from scrap electronics at Brussels- rest — in much the same way as an architect either because they bounce around too much based Umicore, a leading precious-metals decides where to put the pillars in a cathedral inside the tunnels. recycling company. to hold up the enormous roof. This design So Marshall’s group is trying something The process for extracting the gold and modelling is “really at the limit of our techni- different, using laser scanners to construct other precious metals from a pile of circuit- cal ability now”, says McKinnon. detailed three-dimensional maps of all the boards and mobile phones is straightforward. But other new technologies are also mak- tunnels and mounting similar scanners on the The material is shredded and sent to a huge ing deep mining safer and easier. Self-driving vehicles. A robot can then find its location by smelting furnace, where it is melted down at trucks, for example, don’t mind working in comparing the map with the features its scan- 1,250 °C. Two phases form, a metallic layer the baking heat. And while a robot buried in ner can ‘see’ on the tunnel walls. “We don’t rely of mainly copper on the bottom, and a slag a rockfall would be an expensive loss, it’s not on radio signals or trying to penetrate anything layer on top. The precious metals, having a a tragedy. through rock, we just use the features of the high affinity for copper, are dissolved in the “We’ve got robotic vehicles that can drive, environment themselves,” he says. Several bottom layer. Once cooled, the bottom layer but there’s still a lot of missing pieces when mining companies, keen to apply the efficien- is ground into a fine powder and mixed with it comes to integrating them with the min- cies available through GPS above ground, have sulphuric acid to dissolve the copper and leave ing process,” says Joshua Marshall, a robotics already shown an interest, Marshall says. behind the precious metals (the copper itself is engineer at Queen’s who is developing robotic also purified and sold). Then the various met- vehicles. Marshall says these robots will even- WASTE NOT als — gold, silver, platinum and others — are tually be efficient, safe and save money. The The high gold price and new technologies are separated in a series of steps involving precipi- ‘load–haul–dump’ machines that transport also allowing companies to make use of eas- tation, distillation and ion exchange. ore and waste from the rock face to the surface ily accessible ores that were once considered The innovation has come in the develop- are already pretty good at hauling and dump- to be too much trouble, such as those con- ment of sophisticated recycling facilities such ing autonomously, but they still need human taining a mixture of copper and gold. These as Umicore’s plant in Hoboken, Belgium. help to load up. Marshall’s group is developing copper–gold ores are “a pain in the neck” to Such plants are entirely closed-loop systems, a loading algorithm to fix that.