The Andes Mountains are rich in minerals, including gold. Over millions of years, erosion has carried gold down the eastern slope of the Andes and deposited it along river beds, leaving larger nuggets upstream and finer grains downstream. The tropical rivers have meandered constantly since the Andes began forming about 40 million years ago, spreading the golden wealth over the Amazonian plain in old streambeds long since overtaken by forest. Pre-Columbian settlers discovered the bounty. Gold artifacts date back at least to the Chimu culture (600 BC), and Spanish chronicles describe gold ornaments found in — and plundered from — the Inca capital of Cusco. Some of the gold used to make those objects might well have come from the alluvial deposits of eroded gold, or placers, in the rivers of Madre de Dios.
The earliest Peruvian miners probably stumbled on nuggets on sand bars and developed rudimentary sluices for separating them from silt, panning for gold like the California 49ers did. This kind of mining is known as artisanal mining. Today, however, miners often use gasoline-powered motors to power hoses that suck mud from the river bottom or from a pit in the forest, sending a cascade of cobbles and water over a carpet-lined wooden sluice. The miners work in teams. One runs the motor, another aims a high-pressure stream of water at a wall of mud, a third clears clogs from the suction hose, and the fourth monitors the sluice. After hours of hot, dirty, dangerous work, the miners collect the fine, dark sand that has settled on the sluice carpet, dump it into a barrel and add mercury from a small plastic flask, mixing it with their hands or treading it with their feet until the gold particles separate from the sand are cling to the mercury droplets, forming a “sponge.” They heat the sponge to vaporize the mercury, leaving behind a lump of gold.
This kind of mining causes deforestation, sedimentation of rivers and destruction of palm swamps. It also pollutes water, soil and air with mercury, which can cause neurological, gastric and other health problems. The greatest hazard to the miners comes from inhaling mercury when they head the gold sponge. Mercury in sediment and soil can undergo methylation, converting to a form that accumulates in animal tissue. Mercury levels concentrate more in animals higher up the food chain, so humans, otters or birds that eat fish that have eaten other fish are at greatest risk. This kind of gold mining is practiced by some 15 million people — including an estimated 4 million women and 1 million children — in more than 50 countries. They mine between one-quarter and one-third of the gold produced worldwide,and account for about 10 percent of the world’s atmospheric mercury emissions. In Peru, these small-scale miners produce nearly 10 percent of the country’s gold, most of it in Madre de Dios.
Efforts to control the impacts of artisanal mining involve bringing miners into the formal economy; improve health, safety and working conditions’ and reduce mercury use. In return, miners may be able to receive slightly for their efforts by selling their output as fair trade gold.