Copper prices have hit record highs as an ongoing trade war between the U.S. and its key trading partners threatens to squeeze supplies of the vital metal. Buyers in the U.S. have been stocking up on copper ahead of potential tariffs. Future prices for the base metal hit $5.21 per pound on Tuesday. Prices are up about 30% so far this year, following modest gains over the last several years. President Donald Trump has threatened to impose a tariff of up to 25% on all copper imports and has called for increasing U.S. production. China, the world’s largest importer of…