Comex copper prices rallied to levels not seen since July 1997 on Wednesday when the latest in a series of supply concerns hit the market overnight.
In Chile, miners at BHP Billiton Ltd's Cerro Colorado mine voted against the company's last contract offer, setting the stage for a possible strike that could start as on Friday.
By law, either side can extend negotiations, with government supervision, to avert a strike. The mine produces about 130,000 tonnes of copper per year.
Benchmark March copper set a new closing high at $1.1325 a lb., up 1.85 cent, after notching a new 6-1/2 year peak at $1.11340.
The session low was $1.0270. Other contracts finished up 1.50 to 1.85 cent. Those that actually traded on Wednesday set new contract highs.
"We had Ok Tedi yesterday, we had Cerro Colorado today, (State-owned Chilean copper company) Codelco's already disposed of 100,000 tonnes of their stockpile, premiums up to $130 to $150 a tonne going to China.
Comex estimated final copper volume at 12,000 lots on Wednesday, similar to 11,211 contracts traded on Tuesday. Open interest rose by 1,141 lots on Tuesday to 93,722.
The light volume traded at Wednesday's multi-year peak and the market's reaction to sell on Tuesday's raft of strong fundamentals caused some traders to predict that a lot of bullish news had already been priced into copper and that a selloff would be necessary before seeing another copper rally.
After its two-day meeting, the Fed's communiqué said risks to strengthening growth were about equal, with chances of a price decline about the same as a pick up in inflation.
London Metal Exchange (LME) three-months copper charged up to close the on Wednesday evening kerb at $2,465 per tonne, a $25 increase over on Tuesday's close.
The range extended up to $2,477 from a low at $2,437. Comex is a division of the New York Mercantile Exchange.