Although gold is one of the scarcer elements, it is widely distributed in nature and has been found in minute quantities in various kinds of igneous, meta-morphic, and sedimentary rocks in places remote from known ore deposits. Gold is known to concentrate in certain plants and in some marine animals. The ashes of Equisetum (commonly called the “Horsetail” rush) have been reported to contain up to 610 grams of gold per ton (Rankama and Sahama, 1950, p. 707). Sea water has been variously reported to contain from 5 to 65 milligrams of gold per ton (Clarke, 1924, p. 124-125), and on the basis of such assays one estimate of the total content of gold in all the oceans is 10 billion tons.
Gold occurs by far most commonly as the na-tive metal, which always is alloyed with varying amounts of silver, generally 10 to 20 percent. Any alloy in which the silver content exceeds 20 percent is known as electrum. Usually the gold in ores is in particles too small to be seen with the unaided eye; commonly it is of submicroscopic size. Other than the native metal and its alloys, the only natu~ally oc-curring gold minerals are the tellurides, thEy’most com-mon of which are calaverite, AuTe2, con/aining 43.5 percent gold; sylvanite, Au,Ag, Te2, containing 24.5 percent gold; and petzite, Au,Ag3, Te2, containing 25.2 percent gold.
Other gold minerals even less common are the gold-silver-mercury tellurides, kalgoorlite and coolgardite; the silver-gold tellurides, muthmannite and gold-schmidtite; the gold te II uride, krennerite; and the gold-lead-sulpho-telluride, nagyagite. Gold accompanies selenium and particularly tellurium, the latter association being illustrated by the gold tellurides in sulfide ore deposits. In the presence of both sulfides and arsenides or antimonides gold usually is found concentrated in the arsenides or antimonides rather than in the sulfides (Rankama and Sahama, 1950, p. 705). In sulfide deposits gold is usually associated with pyrite and less commonly with arsenopyrite; it may be found also in chalcopyrite, stibnite, pyrrhotite, sphalerite, and other sulfides. The most common gangue mineral is quartz, but other gangue minerals in gold ores include carbonates, flu-orspar, tourmaline, and barite.
Gold deposits are of three main types: veins or other lode ore bodies of hydrothermal origin, ordi-nary placer deposits, and consolidated placer deposits (gold-bearing conglomerates and sandstones).
