Freeport McMoRan, Southern Copper Stocks Rise. Trump Reveals Start Date for 50% Tariff.

Copper is crucial for industries from construction to energy. (Scott Olson/Getty Images)

Mining stocks were getting a boost on Thursday after President Donald Trump said that a 50% tariff on copper imports would take effect from Aug. 1.

The U.S. currently imports about half the copper it consumes, and the metal is such a key component to industries from construction to energy that it is often seen as a bellwether for the economy.  While the tariff in theory will encourage more domestic production, that shift could take years to happen, and in the meantime supplies may become scarcer.

That is why copper prices have spiked this week. Futures in New York are up 9% over the past five days, bringing gains since the start of the year to almost 40%. Companies will probably want to stockpile supplies now. After the tariffs take effect, it will make prices of the commodity more expensive in the U.S. than elsewhere.

U.S.-listed mining stocks are rising. Freeport-McMoRan, the largest miner by market value, added 22% to $46.50 just after the market opened. Southern Copper gained 2.1% to $101.99. Newmont, which is also a major producer of gold, was up 0.4% to $59.00.

Miners based in other countries were also doing well. Rio Tinto, with dual headquarters in Melbourne, Australia, and London, saw its American depositary receipts climb 2.7%. ADRs for BHP, which also Australian, rose 3.9%.

Write to Brian Swint at brian.swint@barrons.com