Modern day precious metal extraction technology and processes have largely developed over the past centuries. Although apparently many principles of extraction are focused on gravimetric and cyanidation alternatives, they evolved. Perhaps in the last years the heap leaching process has developed into an efficient method to extract metals from deposits with low gold content that were impossible to treat successfully for many years and that situation made possible to improve other processes.
Although the Earth's crust averages a mere 0.004 grams of gold per ton, commercial concentrations of gold are found in areas distributed widely over the globe. Gold occurs in association with ores of copper and lead, in quartz veins, in the gravel of stream beds, and with pyrites (iron sulphide). Seawater contains astonishing quantities of gold, but the process of recovery is not economical. Gold was probably the first metal known to the early hominids that, on finding it as nuggets and spangles in the soils and stream sands, were undoubtedly attracted by its intrinsic beauty, great malleability, and virtual indestructibility. As tribal development progressed through the Paleolithic, Mesolithic, and Neolithic ages, and as people congregated into civilized centers, the metal appears to have taken on a sacred quality because of its enduring character, being worn initially probably as amulets and later fashioned into religious objects.
The ancients found quantities of gold in Ophir, Sheba, Uphaz, Parvaim, Arabia, India, and Spain. By the time of Christ, written reports were made of deposits in Thrace, Italy, and Anatolia. Gold is also found in Wales, in Hungary, in the Ural Mountains of Russia, and, in large quantities, in Australia. The greatest early surge in gold recovery followed the first voyage of Columbus. From 1492 to 1600, Central and South America, Mexico, and the islands of the Caribbean Sea contributed significant quantities of gold to world commerce. Colombia, Peru, Ecuador, Panama, and Hispaniola contributed 61% of the world's newfound gold during the 17th century. In the 18th century they supplied 80%. Following the discovery (1848) of gold in California, North America became the world's major supplier of the metal. From 1850 to 1875 more gold was discovered than in the previous 350 years. By 1890 the gold fields of Alaska and the Yukon edged out those in the western United States, and soon the African Transvaal exceeded even these. Today the world's unmined reserves are estimated at 1 billion troy oz (31 billion grams), about half in the Witwatersrand area of the Republic of South Africa.
The distribution of gold seems to validate the theory that gold was carried toward the Earth’s surface from great depths by geologic activity, perhaps with other metals as a solid solution within molten rock. After this solid solution cooled, its gold content was spread through such a great volume of rock that large fragments were unusual; this theory explains why much of the world's gold is in small, often microscopic particles. The theory also explains why small amounts of gold are widespread in all igneous rocks; they are rarely chemically combined and seldom in quantities rich enough to be considered an ore. Because of its poor chemical reactivity, gold was one of the first two or three metals (along with copper and silver) used by humans in these metals'' elemental states. Because it is relatively low reactivity, it was found free and required no previously developed knowledge of refining. Gold was probably used in decorative arts before 9000 BC. Even civilizations that developed little or no use of other metals prized gold for its beauty. Pure gold has a melting point of 1,063 deg C and a boiling point of 2,966 deg C.