Science World: All that Glitters
Discover how rocks buried below the ground turn into the sparkling gold that coats shiny awards
What do the Olympic Games, the Academy Awards, and soccer’s World Cup have in common? The most-prized award for each event contains a sparkling element: gold (Au).
But before gold can be added to a trophy or medal, mining engineers must dig deep into the earth to bring gold-containing ore to the surface. Take a dazzling tour to find out how gold is extracted from hidden rocks and is then transformed into a gleaming trophy. Along the journey, James Webster, a geologist and curator of the exhibition Gold at the American Museum of Natural History in New York, will give you the scoop on this valuable element. Let’s start the tour by traveling deep below ground to see where gold is formed.
A treasure trove of gold is hidden miles below Earth’s surface. But the treasure isn’t in the form of a giant heap. Instead, tiny specks of gold are scattered within rocks deep underground. Since it’s impossible to extract the gold from so far underground, mining engineers have to wait for natural processes to move it toward Earth’s surface.
In some areas, the gold-containing rocks are infiltrated by water that has been superheated to a vapor by pockets of magma. The specks of gold dissolve into this sizzling hot water vapor. The vapor, which is less dense than the surrounding rocks, rises—and the gold rides along with it.
Near Earth’s surface, gold’s free ride comes to an end. “When the hot water that contains the gold comes in contact with cooler rocks, it’s going to drop in temperature,” explains Webster. Like most solids, gold’s solubility (measure of how well a solute can dissolve in a solvent) decreases as the temperature drops. Thus, the once-dissolved gold separates from the liquid, forming a solid gold deposit along shallow cracks in the ground. Called a lode deposit, this gold forms swirls, like the fudge in vanilla-fudge ice cream, throughout the rocks.
Now near Earth’s surface, the lode deposits get exposed to the eroding actions of wind and running stream water. These processes can loosen fragments of gold from within cracks. The loosened gold then travels downstream. The gold collects in riverbeds and small depressions in the ground as grains, flakes, and nuggets. These deposits are called placer (PLA-sur) deposits. (See Nuts & Bolts)
GO FOR GOLD
These surface deposits created by erosion are scarce. Most gold is found below the surface. To dig up enough gold to make trophies, awards, and even jewelry, engineers create a mine.
Open-pit mines are often used to collect gold from deposits that are spread out over a broad area, Webster says. First, engineers use giant bulldozers and other earth-moving equipment to scoop out loads of dirt and rocks. The resulting pit can span 1,000 meters (3,280 feet) across: That’s twice as long as the Empire State Building is tall! Then, miners use explosives to blast through the bowl-shaped crater. This round of blasting sends gold-containing rocks, or gold ore, sky-high before they crash to the ground in random piles. Then it’s up to huge dump trucks to collect the golden debris.
MAKE IT SHINE
Passersby wouldn’t be able to see sparkling nuggets piled up in the dump trucks. That’s because gold is rare. The trucks are full of rocks containing just trace amounts of gold scattered throughout. “Once the material’s been brought to the surface, then the next thing is to get the gold out of it,” Webster explains. To extract the gold, mining experts use a toxic chemical: cyanide. “They slowly pump cyanide solutions through this rock, over and over and over again,” says Webster. This chemical mixture of water and cyanide dissolves the gold again, separating it from the other elements in the ore.
Since cyanide can be fatal to humans and wildlife, mines must ensure that none of the noxious mixture seeps into the groundwater. Special pads can be placed under the cyanide and rocks to catch the liquid chemical as it drips out. The chemical is then recycled. However, at times, mines around the world have leaked or spilled the poisonous cyanide. When the chemical enters the environment, it can be devastating to wildlife.
Once the gold is separated from the debris, it is sent to a refinery. There, engineers will use high temperatures and chemicals to remove any remaining impurities, such as other metals, from the gold.
There are a lot of steps involved in making golden awards such as the Academy Awards’ Oscar statuette. Months before the awards ceremony, workers at R.S. Owens in Chicago are filling empty molds with a metal that contains silver. Once the molds cool, workers remove the newly formed metal statuettes and sand them.
Now the metal-plating process begins. Each statuette is dipped into a series of tanks, covering it first with copper, then nickel, then silver—and finally with 24-karat gold (see The Making of an Oscar Statuette).
Gold was chosen as the final coating for the award partly because of its yellowish luster and because it never tarnishes. So the winner’s statuette will stay glowing for years to come.