PLATINUM DEMAND ESTIMATED 17 PCT HIGHER BY 1991 Demand for platinum could reach 3.4 mln ounces by 1991, compared with an estimated offtake of 2.88 mln in 1986, Chris Clark, platinum marketing director for Johnson Matthey Plc said. Clark told a meeting of the Minerals Research Organisation in Milton Keynes he foresaw a 250,000 ounce increase in consumption for use in autocatalysts, currently the largest single application for platinum. Jewellery consumption is set to rise by 70,000 ounces, Clark predicted in his speech, the text of which was released in London today. Clark said his forecast allowed for only a modest further increase in investment buying and may well be cautious. He said South Africa was the most likely source of additional supplies and will need to increase output by about 500,000 ounces to meet increased demand. The capital investment required to produce the additional output would be around one billion U.S. Dlrs and the political climate may make it difficult to raise the money, he said. The Soviet Union, whose exports have declined since the 1970s, might increase sales to the West by about 50,000 ounces, Clark said. "The very probability of a growth in demand set against the massive investment required for expansion - and that expansion only being viable in South Africa or Russia - leads me to conclude that the price of platinum will be substantially underpinned in the medium to long-term," Clark said.