The region possesses major deposits of iron ore and some other metallurgical ores. Kazakhstan has vast deposits of uranium, precious metals, metals, ores, alloys, ferroalloys and minerals. The earliest brasses may have been natural alloys made by smelting zinc-rich copper ores. These may be "natural alloys" manufactured by smelting zinc rich copper ores in redox conditions. Many metals are geologically most abundant as ionic compounds within ores. The ores of base metals are often sulfides. Bismuth sulfide, , occurs naturally in bismuth ores. Urals contains 48 species of economically valuable ores and minerals. Different estimates exist of the amounts of indium contained within the ores of other metals. The large metallurgical industry was dependent on imports of iron and non-ferrous ores. However, small amounts exist as spontaneous fission products in uranium ores. Some of the deposits, such as the magnetite ores at Magnitogorsk, are already nearly depleted. Gold is also extracted from complex deposits of tungsten ores. However, trace amounts occur naturally as a product of transmutation reactions in uranium ores. Copper extraction refers to the methods used to obtain copper from its ores.