Tin Ores

natural mineral formations with a sufficiently high tin (Sn) content to make commercial extraction profitable. Cassiterite, which contains up to 78.8 percent Sn, is the most valuable tin ore. Stannite, with 27.5 percent Sn, also contains good industrial concentrations of Sn but is not very widely used because of the numerous difficulties involved in ore dressing and Sn extraction. Nordenskioldine, with 43.5 percent Sn, is a calcium-tin borate of secondary commercial importance. Tin ores often occur as complexes that contain W, Be, Li, and Ta or Cu, Pb, Zn, and Ag. Tin deposits can be native or placer. The high commercial value of tin makes it profitable to develop native deposits that contain as little as 0.1 percent Sn and placer deposits that contain 0.01 percent Sn.

Native tin ore deposits, which form in aluminosilicate rocks, are found in three types of formation: pegmatite, cassiterite-quartz, and cassiterite-sulfide. Pegmatite formations are geologically associated with acid granites and are characterized by irregular aggregates or individual inclusions of large cassiterite crystals. Although these deposits have no primary commercial value, they often serve as a source of complex raw materials, such as Li and Be. Cassiterite-quartz formations are high-temperature hydrothermal deposits that are geologically associated with intermediate and acid granitic magma intrusions mostly in the Transbaikal region and Chukotka in the USSR; in the Ore Mountain in the Czechoslovak Socialist Republic and in the German Democratic Republic; in Nigeria; and in Southeast Asia. The cassiterite in cassiterite-quartz deposits contains W, Ta, Be, and other metals in addition to Sn.

Primary sources of commercial Sn include cassiterite-sulfide formations in the developmental zones of predominantly acidic minor intrusions. These formations are sometimes associated with volcanic formations, for example, in the Primor’e and Transbaikal regions of the USSR and in Bolivia, Great Britain, and Australia. Other stanniferous minerals include stannite, various sulfides, and sulfo salts.

Tin ore deposits that form in carbonate rocks and are spatially associated with subalkaline intrusions are polymorphous structures that contain stanniferous skarns; such deposits are found in Southern China, in the Kinta region of Malaysia, near San Antonio in Mexico, in Alaska, and in Middle Asia in the USSR. Tin ores from carbonate rocks have a high Sn content that can exceed 1 percent, a complex mineral composition, and admixtures of F, W, Cu, Be, Sb, Pb, Zn, and other elements. Examples of stanniferous minerals other than cassiterite that occur in individual deposits include nordenskioldine and other tin borates. Tin ore placers are formed by the destruction of tin ore deposits in predominantly pegmatite and cassiterite-quartz formation in places such as the Transbaikal region in the USSR, Indonesia, and Nigeria.

Open-pit and underground mining operations are carried out to extract tin ores from native deposits, while dredges, excavators, and hydromechanization are used to work placer deposits. Tin ores from native deposits are processed by gravitation in ore-dressing plants. Combined gravitation and flotation methods are used when difficulties are encountered in dressing certain ore varieties. The resultant pure, admixture-free tin concentrates, which contain about 60 percent Sn, are ready for smelting. The fuming process permits extraction of metallic tin from slag that contains over 0.1 percent Sn and from stanniferous copper-lead-zinc ores, which must be subjected to special dressing procedures.

Approximately 70 percent of the world’s tin supply is located in placer deposits, from which up to 75 percent of all commercial tin is extracted. In 1973 the overall tin supply in developing and industrial capitalist countries was estimated at 8.3 million tons. The primary reserves of tin ore are located in Malaysia, Indonesia, Thailand, and Burma in Southeast Asia and in Bolivia, Brazil, Australia, Nigeria, and Great Britain.