The surprise move dragged down US copper prices more than 17 percent on the Comex exchange and unwound a premium over the London global benchmark that had grown in recent weeks, with shipments diverted there in anticipation of higher domestic prices.

"Markets are now busily repricing refined copper much lower after Trump's epic backflip on his own import tariff policy," said Tom Price, an analyst at the London brokerage Panmure Liberum.

"Someone must have finally got through to (Trump) that the US economy simply can't afford this new trade-hit."

Freeport-McMoRan is likely to be among the most harmed by the trimmed tariff, according to RBC Capital Markets, with Hudbay Minerals, Arizona Sonoran and others developing mines in the country also affected.

Trump first teased the tariff in early July, implying that it would apply to all types of the red metal, ranging from cathodes produced by mines and smelters to wiring and other finished products.

Yet in a proclamation released by the White House, the administration said the tariff will apply starting this Friday only to pipes, tubes and other semi-finished copper products, as well as products that copper is heavily used to manufacture, including cable and electrical components.

The move aids manufacturers, but does little to boost the constrained US copper mining industry, which for years has asked Washington for permitting reform or other steps that could fuel growth.

"Copper is being imported into the United States in such quantities and under such circumstances as to threaten to impair the national security of the United States," Trump said in his proclamation.

The tariffs will exclude copper scrap and copper concentrates, mattes, cathodes and anodes, some of the main products of copper mines and smelters.

The move is essentially a boost for Chile and Peru, two of the world's largest copper miners and major suppliers to the United States.

The tariff details sparked a "massive market surprise," said Natalie Scott-Gray, senior metals analyst at the consultancy group StoneX, adding that she expects US copper prices to fall further.

Anant Jatia, founder and chief investment officer at Greenland Investment Management, a hedge fund specializing in commodity arbitrage trading, told Reuters he expects London copper prices to surpass US prices in the short term as American inventories build.

More possible

The measure came after a US investigation under Section 232, which Trump ordered in February. The report was delivered to the White House on June 30 by Commerce Secretary Howard Lutnick, according to the proclamation.

Trump said he may still impose further tariffs, and has asked Lutnick to provide an update on the domestic copper market by June 2026. At that point, Trump will evaluate whether to impose a phased universal import duty on refined copper of 15 percent starting in 2027, and of 30 percent starting in 2028, he said.

Along with tariffs, the order calls for steps to support the domestic copper industry, including requiring 25 percent of high-quality scrap produced in the US to also be sold within the country.

Freeport, the largest US copper producer, said it would comment after it reviewed Trump's order in detail.

Chile's Codelco, the world's biggest copper producer, praised the exclusion of cathodes as a positive for the company and for Chile, which is the top supplier of refined copper to the US.

BHP, which operates the world's largest copper mine in Chile, and Antofagasta, which ships copper from Chile to the US and wants to build a US copper mine, did not immediately reply to requests for comment.