Gold is widely distributed throughout the world, normally in very low concentration and generally in native form as metal. It is usually alloyed with silver and often contains small amounts of copper. The only compounds of gold found in nature are the tellurides, typically calaverite (AuTe2), petzite ((AuAg)2Te), sylvanite ((AuAg)Te2), among others. Gold is found in native form in both lode and alluvial deposits. In the largest gold reefs of the world, in South Africa, gold is present as veins and stringers in a quartz matrix accompanied with pyrite and quartz sand. Many pyrite and pyrhotite minerals contain gold from which the metal is recovered during the extraction of copper, silver, lead, zinc and nickel. Although in low concentrations, gold is also present in the sea water, being conservatively estimated that the oceans contain 70 million tons of gold in solution. The major world producers of gold are South Africa, the countries of the old USSR, Canada, the United States and Australia.