Signposts: Tracking Gold

Part of the Gold exhibition.

Finding the Gold

Visible gold deposits are increasingly rare, so new exploration techniques must rely on the physical and chemical characteristics of gold deposits rather than on eyesight alone. Since most gold deposits formed during three main periods of geologic time--2.8-2.6 billion years ago, 1.5 billion years ago, or 400 million years ago--geologists look for rock formations from these periods to increase their chance of finding gold.

Geologists analyze stream and river sediments for naturally occurring elements that are often associated with gold. The magnetic properties of local rocks can help identify hidden gold because some deposits occur with iron-rich rocks. The most productive, but expensive, means of locating gold ore is to drill and analyze rock cuttings for their gold content.

How do geologists find microscopic gold like this? They sometimes look for substances known as pathfinders--elements or minerals that often occur in the same deposits as gold. Arsenic, for example, is easier to find and more widely dispersed than the gold.

Traces of Gold: Easy to Deceive

No other mineral compares to gold, but some come close enough to fool the eye. The mineral pyrite, long known as fool's gold, can form as cubes, like gold, and often occurs in copper, lead or zinc deposits. Sometimes--but not always--it appears along with true native gold. The presence of chalcopyrite in rocks has also fooled gold prospectors in the past.

Natural gold specimens--nuggets, for example--are typically worth much more than the value of the gold by weight. It's not surprising that man-made, forged nuggets surface now and then in an attempt to increase the value of the object.

Squeezing Gold From Rocks

Many of today's biggest mining operations process ores that contain little, if any, visible gold. To obtain trace amounts of gold, these operations remove massive quantities of rock from a mining site, creating large open-pit or underground mines. Pulverized rock is exposed to chemicals such as cyanide and mercury to extract the gold.

Although most gold is mined as a primary commodity, some gold is a byproduct of mining for other ores, including copper, lead and zinc. Byproduct gold accounts for roughly 15 percent of total world gold production.