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.
"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.