Gold exists in Earth’s crust primarily in its native, uncombined state. Although it is widely distributed, gold is found in extremely low concentrations, often measured in parts per billion. For gold to be economically recoverable, geological processes must concentrate the metal into a deposit. This concentration leads to two main categories of gold deposits: those found in situ within hard rock and those concentrated in loose sediment.
Gold in Primary Hard Rock Deposits
The majority of the world’s gold originates from primary deposits, which are concentrations of the metal found in the rock where they were first deposited. These deposits are commonly known as lode deposits, where gold is often found within a network of quartz veins that cut through the surrounding host rock. The presence of quartz is a strong indicator because gold and silica, the main component of quartz, precipitate from the same mineral-rich solutions. The gold itself can be visible as flakes or nuggets, but it is frequently microscopic, sometimes trapped within sulfide minerals like pyrite or arsenopyrite.
The host rocks for these lode deposits vary widely but are often metamorphic or igneous in origin. Ancient metamorphic rocks, such as schists and greenstone belts, are particularly rich sources globally. Greenstone belts, which are old volcanic and sedimentary sequences altered by heat and pressure, have undergone metamorphism that helped mobilize and concentrate the gold. Igneous rocks like granite and granodiorite can also host gold deposits, typically within veins that formed as the rock bodies cooled.
Disseminated gold is a type of primary deposit where the metal is finely spread throughout the rock matrix rather than concentrated in distinct veins. Carlin-type deposits in Nevada, for example, contain massive amounts of sub-microscopic gold dispersed within sedimentary rocks like silty carbonate and siltstone. The gold is chemically bound or trapped within the crystal lattice of the host rock minerals, meaning the rock itself is the direct ore body.
Gold in Secondary Sedimentary Deposits
Secondary deposits, or placer deposits, form when primary hard rock deposits are broken down by the natural forces of weathering and erosion. The breakdown of the host rock liberates the gold particles, which are then transported by water. Because gold is chemically inert and has an exceptionally high density—about 19.3 times that of water—it resists chemical alteration and settles quickly when water velocity decreases.
The “rock” that hosts secondary gold is generally loose, unconsolidated sediment, such as gravel, sand, and clay. This alluvial gold is concentrated in areas of low energy within river systems, including inside bends of streams, behind large boulders, or in natural depressions and potholes in the bedrock of a riverbed. These accumulations are the result of gravity separating the heavy gold from lighter sediment over geological time.
Ancient, or paleo-placer, deposits represent former river or streambeds that have since been buried and lithified into solid rock, often a conglomerate. The Witwatersrand Basin in South Africa is the most famous example, where gold is found in ancient, cemented quartz-pebble conglomerates. The gold in these deposits was concentrated by water action billions of years ago and now forms a solid sedimentary rock layer.
Geological Processes that Create Gold Deposits
The concentration of gold into viable deposits is driven by hydrothermal processes. These processes involve the circulation of hot, water-rich fluids, often heated by magmatic or metamorphic activity deep within the Earth’s crust. The fluids are mineral solutions, containing sulfur or chlorine compounds that allow them to dissolve and transport trace amounts of gold from surrounding source rocks.
These gold-bearing solutions travel through the crust using pre-existing structural weaknesses, such as fault lines, fractures, and shear zones. These discontinuities act as plumbing systems, channeling the fluids toward shallower, cooler environments. When the fluids encounter a change in temperature, pressure, or chemistry—for instance, by reacting with the host rock or boiling—the dissolved gold precipitates out of the solution.
This precipitation often occurs alongside quartz, forming the characteristic gold-bearing veins targeted by hard rock mining. These processes are associated with large-scale tectonic settings, such as convergent plate boundaries and active metamorphic belts. The forces associated with mountain building create the necessary heat, pressure, and structural pathways for the gold-rich fluids to move and deposit the metal.