Gold ore refers to gold found in its natural state, often mixed with other minerals and rock.
Appearance of Native Gold
Native gold possesses several unique physical properties. Its most notable characteristic is a bright, shiny, and slightly orange-yellow color, which does not tarnish upon exposure to air or most reagents. The color can vary slightly depending on impurities, with higher silver content leading to a paler yellow or whitish hue.
Gold exhibits a distinct metallic luster. It is also exceptionally malleable, allowing it to be hammered into extremely thin sheets without breaking. A single gram of gold can be beaten into a sheet covering one square meter. It is also ductile, capable of being drawn into fine wires. When found in nature, native gold can appear as irregular nuggets, flat flakes, or even thin wires. Nuggets are typically 75% to 97% pure gold, with the remainder often being silver or copper.
Gold Within Its Geological Context
Gold occurs through various geological processes, often involving hot, mineral-rich fluids. It can appear as tiny specks, flakes, or even visible veins embedded within host rocks.
One common geological context is quartz veins, where gold is deposited as hot fluids circulate through cracks and fissures in the Earth’s crust. The gold in these lode deposits can be present as fine particles or larger visible pieces contrasting with the typically white or milky quartz. Gold is also found in sulfide deposits, where it is intimately associated with sulfide minerals like pyrite or chalcopyrite. In these instances, gold may be microscopic and “locked” within the sulfide crystals.
Beyond lode deposits, gold is also found in placer deposits. These form when gold, weathered out of its original rock, is transported by water and concentrated in stream gravels or riverbeds due to its high density. Placer gold often appears as smooth, rounded nuggets or flakes because of the abrasive action of water during transport.
Identifying Real Gold from Look Alikes
Distinguishing real gold from minerals that resemble it, often called “fool’s gold,” involves several tests and observations. The most common look-alikes include pyrite, chalcopyrite, and mica. Pyrite, or iron sulfide, has a brassy yellow color and a metallic luster, but it is typically a paler, more brassy yellow than real gold. Pyrite often forms cubic or angular crystals, unlike the irregular or rounded shapes of natural gold.
Chalcopyrite, a copper iron sulfide, also has a brassy or bronze-yellow color but is generally softer than pyrite. Mica can appear gold-like due to its reflective, flaky nature. Simple field tests can help differentiate these minerals. A streak test, performed by rubbing the specimen on an unglazed ceramic plate, will yield a golden yellow streak for real gold, while pyrite and chalcopyrite produce a greenish-black or brownish-black streak, and mica leaves a white streak.
Real gold is soft and malleable, allowing it to be scratched with a knife or bent without breaking. Pyrite, chalcopyrite, and mica are brittle and will shatter, crumble, or resist bending when pressed or struck. Gold is also significantly denser than its look-alikes. Pure gold is notably heavier than similarly sized pieces of pyrite or mica. A simple density test by comparing weight for volume can often indicate real gold. Unlike pyrite, which can have a faint sulfur smell when rubbed, gold is odorless.