(FORTUNE Magazine) – South Africa's political unrest has revived long-standing fears of a disruption in the supply of key minerals. A leading producer of gold and diamonds, South Africa is also a trove of strategic metals essential to industry, which the U.S. obtains mostly from abroad. The troubled country's mines supply 39% of the manganese used by U.S. industry, 44% of the vanadium, 55% of the chromium, and 49% of the platinum group of metals, which includes palladium and rhodium. On the face of it, this suggests a disturbing dependency on an unreliable source. But the worst scenarios -- prolonged shortages or steep price rises stemming from a cutoff -- don't seem likely. Sabotage and strikes, for one thing, probably wouldn't interrupt mining of these metals for long. South Africa's mines are well policed, and fears about labor troubles have dwindled since a miners' strike quickly fizzled in early September. If the U.S. imposed tough economic sanctions, the South African government could of course embargo mineral shipments to the U.S. -- but at a forbidding cost to its own country's economy. Mining accounts for about 26% of South Africa's gross domestic product and 75% of its foreign exchange earnings, and the economy has been in a deep recession for three years. The possibility exists that a future government, white- or black-dominated, would act contrary to South Africa's economic interests. A black-controlled Marxist regime, in particular, might seek to withhold supplies from the U.S. or even try to form a cartel with the Soviet Union -- the other leading producer of most of these metals. But embargoes directed at one country are easily circumvented. South Africa would presumably want to go on selling to other major customers, and U.S. users could buy from them. The Soviets might be tempted to join a cartel, but in the past their hunger for hard currency has ruled their actions; they've been eager sellers in times of shortage. If South Africa nevertheless succeeded in pinching supplies, defense industries could apply to the government's stockpile, which has about a one- year supply of most strategic minerals. Many users could switch to recycled materials or other metals. U.S. vulnerability to a cutoff varies, depending on the metal: VANADIUM. Used to toughen steel, this is the least worrisome. The U.S. meets more than half its needs from domestic mines, which could boost production. MANGANESE. American deposits that can be mined efficiently are negligible. Yet if the U.S. ever became strapped for manganese, essential for steelmaking, it could obtain more from Gabon, China, Brazil, and a big Australian mine. CHROMIUM. Essential for making stainless steel, it also goes into a wide range of other corrosion- and heat-resistant alloys. With little chrome ore, the U.S. relies almost entirely on imports -- from the Soviet Union, Zimbabwe, Turkey, and other countries as well as South Africa. A variety of substitutes -- nickel and molybdenum, for example -- could reduce U.S. chromium needs by one-third. PLATINUM. U.S. vulnerability is greatest in the platinum group of metals, used as catalysts in industry and in converters that curb auto pollution. South Africa accounted for 54% of world production in 1984 and the Soviet Union 42%. A crunch would touch off a worldwide scramble to recycle and to find new sources.