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Gold is a chemical element with the symbol Au (from Latin: aurum) and atomic number 79, making it one of the higher atomic number elements that occur naturally. In a pure form, it is a bright, slightly reddish yellow, dense, soft, malleable, and ductile metal. A relatively rare element, gold is a precious metal that has been used for coinage, jewelry, and other arts throughout recorded history. In the past, a gold standard was often implemented as a monetary policy, but gold coins ceased to be minted as a circulating currency in the 1930s, and the world gold standard was abandoned for a fiat currency system after 1971. A total of 197,576 tonnes of gold exists above ground, as of 2019.

Gold is produced in supernova nucleosynthesis and from the collision of neutron stars, and used in electronics and jewelry. On Earth, gold is found in ores in rock formed from the Precambrian time onward. It most often occurs as native gold, which is sometimes extracted from small shards of gneiss, granite, or quartzite using fire-setting or smelting. Gold also occurs in alluvial placer deposits, bedrock deposits (as veins), and in sea water. However, most of Earth’s gold is inaccessible because it is locked inside safe deposit boxes or jewelry stores.

The metal continued to be mined by pre-Columbian peoples throughout Mesoamerica and South America. The Moors in North Africa are credited with introducing gold mining to Spain during their occupation of the Iberian Peninsula in the 5th century AD. The Spanish conquistadors subsequently brought large scale mining and silver production to much of Central and South America through their sophisticated hydraulic mining techniques by the 15th century. Large scale commercial gold mining began with the California Gold Rush of 1848–1855. While more than 85 percent of California’s gold was extracted from placer mines using simple methods such as panning during this period, lode or vein mining underground became increasingly common as miners sought richer deposits of gold in ever-decreasing areas on the surface. By 1890 over 90 percent of U.S. gold production came from lode mines.

Gold ore processing in placer mines involves screening and separating the gold-bearing gravel from the other material collected during dredging operations. The processing required to recover fine gold from crushed ore is determined by the free-milling or refractory nature of the ore.

While most free-milling ores are easy to process using physical methods such as gravity concentration or flotation, refractory ores require more sophisticated processing techniques involving oxidation or bioleaching.

Heap leaching is an industrial mining process used to extract precious metals such as gold from low-grade ore. The ore is first crushed and then subjected to various processes designed to extract the desired Metal(s) from the ore minerals. Heap leaching is sometimes used in combination with other processes such as milling, crushing or concentrating to speed up the extraction process and increase recovery rates.

Cyanide heap leaching is considered to be a type of gravity concentration because it relies on gravity to transport the ore down through the leaching pad where it can be extracted from the pregnant solution. This type of leaching is usually carried out on low-grade ores that do not respond well to other types of processing such as flotation or cyanidation.

Cyanide heap leaching is very efficient at extracting gold from low-grade ore; however, it also has some drawbacks including environmental concerns about cyanide being released into the environment and potential health risks for workers handling cyanide solutions. There have also been some incidents where cyanide heap leaches have failed causing extensive environmental damage.