The core question of whether gold is an ore requires a nuanced answer. Gold is a native element, meaning it frequently exists in its pure, uncombined metallic state within the Earth’s crust. However, the rock or sediment that hosts a high enough concentration of gold to make extraction financially viable is classified as gold ore. Therefore, while gold is not an ore in the strict sense of a compound, the material from which it is mined is treated as such.
Defining Ore and Native Elements
An ore is defined as a naturally occurring rock or sediment that contains one or more valuable minerals, typically metals, concentrated to a degree that makes mining and processing profitable. This definition relies on the presence of a valuable material and the economic feasibility of its recovery. The concentration of the desired substance must exceed a certain threshold, known as the cut-off grade, to be considered an ore body.
Gold is one of about twenty native elements found in nature uncombined with other elements. Native elements, such as gold, silver, and copper, exist as single-element minerals because they are chemically inert and do not easily bond with other substances. Gold’s unreactive nature is why it is often found in metallic flakes or grains rather than in complex mineral compounds. The entire mass of material, whether hard rock or loose sediment, is processed like any other ore because the goal is the profitable separation of the metal from the waste material.
How Gold Occurs in Nature
Gold is found in two primary geological settings: lode deposits and placer deposits. Lode deposits, also known as vein or hard rock deposits, are the original source of the gold. In this setting, the gold is trapped within solid rock, most commonly in quartz veins that formed from hot, mineral-rich hydrothermal fluids moving through fractures in the Earth’s crust.
To recover the gold, this hard rock material must be mined, crushed, and milled, which is characteristic of extracting metal from an ore body. The gold within these veins can be microscopic, or it can appear as visible coarse grains embedded in the quartz. These primary deposits are often associated with other minerals, such as sulfides, which add complexity to the extraction process.
Placer deposits are secondary accumulations of gold that have been freed from their original lode source by natural erosion and weathering. The gold particles, flakes, and nuggets are transported by water and concentrated in riverbeds, alluvial fans, or ancient streambeds. The loose sediment and gravel containing the gold is the material that is mined and processed as a recoverable ore.
Methods for Extracting Gold
The chosen method for gold extraction depends heavily on the type of deposit and the physical state of the gold. For lode or hard rock deposits, the ore must first undergo comminution, which involves crushing and grinding to increase the surface area and liberate the gold particles from the rock matrix. This milled ore is then subjected to chemical treatment.
The most widely used chemical process is cyanide leaching, where a diluted sodium cyanide solution is used to dissolve the gold, forming a soluble gold-cyanide complex. This technique allows for the recovery of up to 96 percent of the gold, even from low-grade ores. The gold is later recovered from the solution, often by adsorption onto activated carbon.
For placer deposits, the method is simpler and relies on the high density of gold compared to the surrounding sand and gravel. Gravity separation techniques, such as using sluice boxes, jigs, or shaking tables, are employed to physically wash away the lighter materials. These physical methods are effective for the coarser, free gold found in these deposits.