Gold may occur as deposits called lodes, or veins, in fractured rock. It may also be dispersed within Earth's crust. Most lode deposits form when heated fluids circulate through gold-bearing rocks, picking up gold and concentrating it in new locations in the crust. Chemical differences in the fluids and the rocks, as well as physical differences in the rocks, create many different types of lode deposits.

Over millions of years, gold flakes and nuggets worn away from veins are swept into bodies of water. The heavy gold settles in stream-, lake- and riverbeds, and on the sea floor, forming placer ("PLASS-er") deposits.

1. Superheated waters emerge from "smokers," spring-like vents in the seafloor. This occurs where tectonic activity forces the spreading of the oceanic crust. Metal-rich minerals, including small amounts of gold, are deposited as the heated gold-laden water mixes with the cold seawater.

2. Gold-laden water heated by magma-molten rock-in Earth's shallow crust forms a variety of lode gold deposits. Hydrothermal-hot water-fluids rich in sulfur form gold ores in rocks of active volcanoes. The deposits of Summitville, Colorado, are an example.

3. Gold minerals form in hot rocks in and around volcanoes. Low sulfur, gold-bearing hydrothermal fluids form when hot rocks heat ground water. An example of these low-sulfur fluids are hot springs like those at Yellowstone National Park. The ores of Round Mountain, Nevada, are typical low-sulfur deposits.

4. Chemical interactions between hot fluids and sedimentary rocks form deposits of tiny, even invisible, gold particles. The Meikle Mine of Nevada contains such "invisible" gold.

5. Fractures form in Earth's crust as mountains rise. Hydrothermal fluids flow into these spaces and form gold-bearing quartz veins. These fluids are created by hot, deeply buried metamorphic rocks. The Mother Lode and other deposits of the Sierra Nevada gold belt in California are examples.

6. Ancient gold deposits are found in greenstone belts: volcanic belts more than 2.3 billion years old. Although gold deposits continue to form in active volcanic areas, greenstone belts and their gold deposits no longer form on Earth today. Examples include parts of Canada, Zimbabwe and Australia.

7. Placer deposits form at Earth's surface when weathering action exposes gold from other, older lode deposits. The gold is swept into, and settles in, streams, lakes, rivers or the sea floor. Many placer deposits are of recent geologic age, but some are billions of years old.

In most of Earth's crust, gold concentrations are very low. On average, a ton of rock from the crust holds 0.005 grams of gold-compared to 58,000 grams of iron.