Gold is a unique element, characterized by its deep yellow color, unmatched resistance to corrosion, and extreme density. As a chemical element, its symbol is Au, derived from the Latin word aurum, and it is classified as a transition metal. Its high specific gravity, roughly 19.3 times that of water, explains how it settles and concentrates in the natural world. Understanding where and how this metal occurs requires looking into the deep geological processes that formed the Earth’s crust.
The Physical Forms of Natural Gold
Natural gold appears in various physical forms based on particle size. The largest and most recognizable forms are nuggets, which are substantial, naturally occurring pieces of native gold. Smaller pieces are referred to as flakes, grains, or fine dust, sometimes called “flour gold,” which can be microscopic. Gold found in nature is almost never 100% pure.
Natural gold is an alloy, typically containing other metals, most commonly silver and copper. Purity is expressed as fineness, usually ranging from 80% to over 90% gold content. The presence of silver can lighten the color, and when silver content exceeds 20%, the alloy is known as electrum. The shape of the particles also indicates their history; rounded, smooth shapes suggest significant transport and erosion.
Primary Deposits (Lode Sources)
Primary gold deposits, or lode sources, are gold still located within the original rock formation where it was first deposited. These deposits originate deep within the Earth’s crust, associated with intense heat, pressure, and the circulation of hydrothermal fluids. These mineral-rich waters travel through cracks and faults, dissolving and transporting gold. As the fluids ascend, a drop in temperature or pressure causes the dissolved gold to precipitate out of the solution.
This precipitation commonly results in the formation of gold-bearing quartz veins, which fill fractures within the host rock. These vein systems can be vertically extensive and require hard-rock mining techniques, such as blasting and crushing, to extract the embedded gold. The deep-seated faults that provide the conduits for these fluids are structurally controlled, making the location of these systems predictable to geologists.
Secondary Deposits (Placer Sources)
Secondary deposits, or placer sources, are created when primary lode deposits are exposed to surface weathering and erosion. Over time, water, ice, and chemical processes break down the hard rock, releasing gold particles from the quartz veins. Once liberated, gold’s high density causes it to behave differently than lighter minerals like quartz during transport. Water carries lighter materials away, but the heavier gold settles quickly, concentrating in specific locations within river systems.
These deposits are found in unconsolidated sediments like gravel, sand, and silt in riverbeds, streams, and ancient floodplains. The concentrated gold accumulates in natural traps where the water flow slows down, such as inside bends, behind large boulders, or in cracks on the underlying bedrock. This process is a form of natural gravity separation. Placer deposits include eluvial placers, concentrated near the source, and alluvial placers, which have been transported significant distances by water.
Gold Found as a Byproduct
A significant portion of the world’s gold supply is recovered as a byproduct during the extraction of other base metals. This gold is often found in disseminated deposits, such as large-scale porphyry copper deposits, where the gold is finely spread throughout a massive volume of rock. Copper remains the main commodity in these operations, but the recovered gold contributes substantial economic value. Because the gold is often invisible to the naked eye, bulk mining techniques are required to process enormous tonnages of low-grade ore.
Gold can also be chemically bound within the crystal structure of sulfide minerals, such as pyrite (“Fool’s Gold”) or arsenopyrite. This makes the gold difficult to recover through simple crushing and gravity separation methods. Gold also forms compounds with the element tellurium, creating gold tellurides like calaverite, which represent a major source of gold in certain significant mining districts.