Workers at Antamina, the biggest copper mine in Peru, will proceed with its first indefinite walkout beginning Monday after negotiations broke down, said union reps; no further talks are pending. BHP Billiton and Glencore each own 33% stakes in the project; Teck Resources has a 22% interest, with Mitsubishi Corp. holding the remainder.

Union leaders said they will drop tools over profit-sharing bonus agreement demands after falling output at the asset, which produces 30,000 copper tons monthly. Antamina represents 30% of Peruvian copper, and is the Andean nation’s No. 1 zinc source.

Both sides expressed frustration. “Understanding, beyond demands that cannot be met, is now in their hands,” Silvio Brigneti, company vice president for human resources said in a statement quoted by Reuters News. “The strike will achieve reduction in profits;” a 2009 labor action was averted after management gave workers a one-time bonus.

Lower ore grades, in a phase ending in the medium term, has reduced output, alongside a weak global copper market, according to company officials. Through Q3 2014, they added, the project produced 16% less copper than in the same period last year.

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