Copper prices suddenly surged to a near one-month high Monday morning, but several on Wall Street were uncertain about exactly what caused the spike.
Copper futures for July delivery briefly leaped more than 2.5 percent around 9:31 a.m. EDT, hitting $2.6945 a pound, their highest since April 5. Copper traded about 2.3 percent higher as of 10:45 a.m.
It was the kind of move that typically comes when a single big piece of news hits, but traders were scratching their heads.
Copper intraday performance Monday
"I can't see any [headline] in particular," said Dane Davis, commodities research analyst at Barclays. He noted that a strike was set to begin Monday at Freeport-McMoRan's Grasberg mine in Indonesia and a weaker U.S. dollar supported copper's move higher.
Thousands of workers at the world's second-largest copper mine staged a rally on Monday ahead of a planned monthlong strike to protest layoffs, a union leader said in a Reuters report.
The U.S. dollar index traded slightly lower on the day.
Low trade volume was also a likely factor behind the commodity's jump. Key copper trading markets in London and Shanghai were closed Monday for a holiday.
Eric Hunsader,
"The fact that it stayed high tells me something's happening, there's some news," he told CNBC. "This is sudden."
Copper rose 2.76 percent last week ahead of Monday's spike.
— Reuters contributed to this report.