On Tuesday copper for delivery in March closed more than 2% higher in New York at $2.7085 per pound or just under $6,000 a tonne. It was the highest close since June 2015 and in after hours trade the metal added to gains, last trading at $2.72.
Copper has advanced 40% since hitting near-six year lows this time last year, with most of those gains coming in the last four months.
Copper's latest leg up was spurred by worries over a possible production outage at the Escondida mine in Chile. Majority owner and operator BHP Billiton expects full-year production at Escondida of 1.07 million tonnes, which gives the mine a nearly 5% shares of global mine production.
The current collective agreement with the main union at the mine expires at the end of January and according to a Reuters report workers have rejected BHP's latest revised offer and union leaders have told members "to vote for a strike and prepare for an extended conflict."
The previous labour deal was signed four years ago when copper was trading around $3.40 a pound. Given Escondida's size a prolonged outage could have a meaningful impact on the price.
BHP's copper production for the half year to end December fell 7% to 712,000 tonnes due to a power outage at its Australian Olympic Dam operations in September-October. BHP also cut full year guidance by 40,000 tonnes to 1.62m tonnes.
Chile produces 28% of the world's copper and the country's output dropped by 3.9% in 2016, mainly due to lower production at Escondida and Anglo American Sur.
Production in the South American nation is expected to grow by 4.3% according to the Chilean government forecaster adding that Escondida would account for almost all of the expected increased output.