III.

Occurrence

Gold is found in nature in quartz veins or seams, nuggets, flakes, and secondary alluvial deposits as a free metal or in a combined state. There are several chemical and physical processes that may cause these formations, and it is also likely that colonies of soil bacteria and fungi play a part in gold agglomerations. Gold is widely distributed although it is rare, being 75th in order of abundance of the elements in the crust of the Earth. It is almost always associated with varying amounts of silver; the naturally occurring gold-silver alloy is called electrum. Gold occurs, in chemical combination with tellurium, in the minerals calaverite and sylvanite along with silver, and in the mineral nagyagite along with lead, antimony, and sulphur. It occurs with mercury as gold amalgam. It is generally present to a small extent in iron pyrites; galena, the lead sulphide ore that usually contains silver, sometimes also contains appreciable amounts of gold. Gold also occurs in sea water to the extent of 5 to 250 parts by weight to 100 million parts of water. Although the quantity of gold present in sea water is more than 9 billion tonnes, the cost of recovering the gold would be far greater than the value of the gold that could thus be recovered.