Gold ore is a naturally occurring rock or sediment containing gold in a concentration that makes mining financially worthwhile. The metal rarely exists in an isolated, pure form when extracted. Instead, it is dispersed as microscopic particles or thin veins, bound within a much larger volume of host material. Understanding gold ore requires defining its composition, the geological environments that create it, and the industrial steps necessary to liberate the metal.
Defining Gold Ore and Its Composition
A rock is classified as gold “ore” only when the gold concentration is high enough to be economically processed. This concentration is the ore’s “grade,” measured in grams of gold per metric tonne of rock (g/t). While high-grade deposits contain 5 to 20 g/t, many modern mines operate profitably on low-grade ore (1 to 4 g/t) due to advanced recovery techniques.
The gold exists in two primary forms. The most straightforward is native gold, the elemental metal, often alloyed naturally with silver to create electrum. A significant portion of gold, however, is chemically bound in mineral compounds, most commonly as gold tellurides.
The surrounding rock, known as the matrix or gangue, dictates the ore’s processing difficulty. Gold is often found embedded in quartz veins formed by hot, mineral-rich fluids. In other cases, gold particles are encapsulated within sulfide minerals, such as pyrite (“fool’s gold”) or arsenopyrite, where the gold is invisible to the naked eye.
Geological Settings and Types of Deposits
Gold ore deposits are categorized based on their formation history. The two major types are lode deposits, the primary source, and placer deposits, which are secondary concentrations.
Lode deposits, also known as primary or vein deposits, form deep underground from hydrothermal activity. This involves superheated water, rich in dissolved gold, circulating through fractures in the bedrock. As these fluids cool, the gold precipitates out, filling cracks to form quartz veins or becoming disseminated throughout the host rock. These hard-rock deposits require deep mining techniques.
Placer deposits are secondary accumulations that form after lode deposits are exposed to weathering and erosion. Primary gold-bearing rock breaks down, and the liberated gold particles are carried away by water. Because gold is denser than most other minerals, gravity causes it to settle and concentrate. These concentrations are found in river gravels, ancient stream channels, or floodplains, where the gold is present as flakes, grains, or small nuggets.
Extracting Gold from Ore
The process of separating gold from its ore is a multi-step industrial operation that begins with reducing the size of the raw material. This initial stage, known as comminution, involves crushing the mined ore into progressively smaller pieces using large crushers. The material is then ground in rotating mills to produce a fine powder or slurry, which liberates the gold particles from the rock matrix.
Once the gold is exposed, various methods are used for separation. For coarse, free gold in placer deposits, gravity separation techniques, which rely on the metal’s high density, are effective. For most modern lode ores, the concentrate is chemically treated using leaching.
The dominant industrial method is cyanide leaching, where a weak sodium cyanide solution is mixed with the ore slurry. The cyanide selectively dissolves the gold by forming a soluble gold-cyanide complex (Au(CN)2-), leaving the inert gangue minerals behind. The gold-bearing solution, referred to as pregnant solution, is separated from the solid waste. The gold is then recovered from the solution, often by adsorption onto activated carbon or by electrowinning, before being melted and cast into impure doré bars for final refining.