PRECAMBRIAN SHIELD TAKES 51 MLN DLR WRITEDOWN <Precambrian Shield Resources Ltd>, earlier reporting a large loss against year-ago profit, said the 1986 loss was mainly due to a 51,187,000 dlr writedown on its U.S. operations, uneconomic coal and other mineral properties. Precambrian, which is 89 pct owned by <Mark Resources Inc>, said it took the writedown in accordance with new Canadian Insititute of Chartered Accountants guidelines for full cost method accounting by oil and gas companies. Precambrian earlier reported a 1986 loss of 53.4 mln dlrs, compared to profit of 4.5 mln dlrs in the prior year.