Union leaders at the Escondida mine in Chile operated by BHP submitted wage demands late last week, sparking fears of a possible strike.
Failure to reach a labour deal during last year's negotiations led to a 44-day stoppage at the mine, which accounts for 5 percent of global supply. Worries over fresh disruption have helped to lift copper by 6 percent this week.
"This is mostly linked to fears that the wage negotiations at Escondida will turn into a prolonged strike, and therefore there will be some loss in output," said Capital Economics analyst Simona Gambarini.
However, robust stock levels and early indications that talks could be constructive suggest that the current strength in prices may be overdone, she said, raising the risk of a pull-back. "We don't expect this to last," she said. "There is a lot of speculation in the market."
COPPER PRICES: Three-month copper on the London Metal Exchange rallied 1.7 percent on Thursday to hit $7,345 a tonne, its highest since January 2014. By 1420 GMT it was at $7,325 a tonne, up 1.5 percent.
SPREADS: The premium of three-month copper over the cash contract remained depressed at $6.50 a tonne, down from $22.25 two weeks ago. On Monday it touched its lowest since March 2017 at $1.75, signalling tightness in near-term supply.
COPPER STOCKS: On-warrant copper inventories at LME warehouses - representing metal not earmarked for delivery and therefore available to the market - increased by 6,625 tonnes to 237,275, exchange data showed on Thursday.
TECHNICALS: LME copper could test resistance at $7,305 a tonne, Reuters technical analyst Wang Tao said on Thursday, a break above which could lift the price to $7,373.