Some samples contain gold- and quartz-filled veins. Such veins are created when hydrothermal--hot water--fluids from several miles below Earth's surface force themselves through rock. As the gold is deposited, other common minerals such as quartz, calcite and barite are usually precipitated, or drawn out from the fluid, along with the gold. The gold is known as ore and the other minerals are known as gangue (pronounced "gang"). Most crystallized gold samples are recovered from quartz- and calcite-rich veins and lodes.
Gold--the mineral--exists on Earth in veins and placer deposits as native gold.
Gold--the metal--is extracted from rocks and other minerals.
"Slickensides" is a term used for rock surfaces that move against each other along a crack or fault. The surfaces become smoothed, grooved and polished.
Some deposits, known as greenstone belts, are areas of ancient, molten rock, greater than 2.5 billion years old. Geologists think there was a major gold-forming event around that time, possibly linked to the collision of ancient continents. Although gold deposits continue to form in active volcanic areas, greenstone belts and their gold deposits no longer form on Earth today.
Earth's Surprise Package
Although most visible gold exposed at on Earth's surface was found long ago, amazing gold discoveries still happen. The Summitville gold boulder was found by Bob Ellithorpe in October 1975, on the access road to the Summitville mine. While at work for a mining company, Ellithorpe spotted a glinting, yellow streak in a large rock at the side of the road. The 52-kilogram (114-pound) boulder contained an estimated 11 kilograms (29 troy pounds) of gold.