THE wealth of its gold mines is one reason why Peru's economy is set to grow by close to 9% this year. But the soaring price of gold, of which Peru is the world's sixth-biggest producer, has also brought a less welcome consequence: wildcat miners are tearing down the country's rainforest and polluting rivers with tonnes of toxic mercury, which they use to separate gold from ore.

Lone prospectors have given way to industrial-scale, but equally illegal, operators, using bulldozers and barges in Madre de Dios, in the south-east. They extract more than 16 tonnes a year (worth more than $690m at today's prices) according to a report published last year by Conservation International, a Washington-based group. Peru's environment minister, Antonio Brack, reckons that the miners have cut down more than 150,000 hectares (370,000 acres) of forest. Researchers have found high levels of mercury in fish caught in Madre de Dios.

The government recently dispatched a unit of anti-terrorist police to tackle the miners, some of whom are armed. But Mr Brack thinks only the army can do the job. Other officials argue that the army cannot be deployed against armed civilians without a new law protecting troops from charges of human-rights abuses.

Those concerns have not stopped other South American governments from using their armies against wildcat miners. Last month Bolivia's president, Evo Morales, sent troops to the lowlands of Santa Cruz department to crack down on gold mining. His government says sovereignty is also at stake: many of the miners are Paraguayans or Brazilians (more than 50 of whom have been arrested by the soldiers).

In Colombia there are fears that left-wing guerrillas and former paramilitaries are profiting from illegal mining. Ecuador and Venezuela have also asked their armies to act against the miners. It may be that the armed forces have found a new role—at least until the price of gold comes down to earth again.