President Donald Trump on Wednesday signed on order imposing a universal 50% tariff on copper imports, the latest metal he has targeted with a steep import tax.
U.S. copper prices plunged as much as 18% in after-hours trading as soon as the White House announced the new tariff rate. Copper miners Freeport-McMoRan and Southern Copper were down about 10% and more than 6%, respectively.
The sell-off, if it holds through Thursday's trading session, will mark the metal's largest single-day decline since 1989.
The new, higher copper tariffs go into effect Friday, according to a fact sheet released by the White House.
The Trump administration says that the new duties, which follow 50% U.S. tariffs already imposed on steel and aluminum, are aimed at boosting domestic industries and addressing "trade imbalances."
But experts warn that they could raise prices on all sorts of products, ranging from construction materials to electronics, that utilize the versatile metal.
And Trump's other sector-specific tariffs and ongoing trade investigations, which target imported autos, lumber and pharmaceuticals, could dampen overall U.S. growth, according to the Tax Foundation.
Copper is the third-most-consumed metal, behind iron and aluminum. The U.S. imports nearly half of the copper it uses, most of which comes from Chile, according to data from the U.S. Geological Survey.
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— CNBC's Spencer Kimball contributed to this report.