(Bloomberg) — BHP Group workers in Chile rejected the company’s wage offer at the close of regular talks, thrusting the process into a final mediation phase in a bid to avoid a strike at the world’s biggest copper mine.

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Members of the main union at the Escondida mine voted against the terms of a new wage contract, according to a statement from the group late Thursday.

In a statement, BHP said that operations continue to function normally and that the company will request the mandatory mediation period allowed under Chile’s collective bargaining rule before a strike can begin. That period includes five business days of mediation, which can then be extended for another five days if both sides agree.

Copper traders, investors and rival producers are paying close attention to the labor talks at Escondida. The mine churns out more than 1 million metric tons a year, about 5% of all the world’s mined copper, easily making it the biggest single supplier.

The negotiations come at a time of global tightness of copper concentrate — the raw material produced at Escondida and used to feed smelters — even though the market for refined metal is well supplied for now. Benchmark copper futures on the London Metal Exchange surged to a record in May as bullish investors placed bets on shortages, but prices have since pulled back by roughly 19%.

Collective bargaining in Chile is often marked by brinkmanship and last-minute agreements, though Escondida has also been the scene of lengthy strikes in the past.

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