SANTIAGO (Reuters) - Workers at Codelco’s Chuquicamata copper mine in Chile, the state miner’s second largest by output, are threatening to walk off the job soon to protest plans to transform the century-old open pit into an underground mine, a union leader said on Monday.

Four of the Chuquicamata’s six workers’ unions voted to go ahead with strikes, alleging the company’s plans to overhaul the mine - a key cog in its $39 billion 10-year drive to update its aging deposits - had failed to address the concerns of the mine’s nearly 5,664 workers.

“We are going through the worst moment in our labor relationships under {Codelco CEO Nelson} Pizarro,” said Liliana Ugarte, president of union No. 2 at Chuquicamata. “This lack of participation obligates us to take action.”

Ugarte said the unions had not set a date for the walk-off but had begun preparations.

Codelco, the world’s top copper miner, declined to comment on the situation.

The company signed 27-month contracts in December 2016 with six Chuquicamata unions after relatively rapid talks but the labor situation has worsened recently as the mine overhaul confronts delays for technical reasons and rising costs.

The union said the company had announced it would eliminate as many as 1,700 jobs at the mine and said Codelco had not given proper consideration to the “social and economic” impacts of the layoffs.

The tension at Chuquicamata comes amid labor negotiations at Escondida, the world’s largest copper mine, as workers and mine owner BHP seek common ground following an historic strike last year that ended without a real resolution.

Codelco also said last week it had failed to reach agreement in early talks with a union at its much smaller Salvador copper division, pushing off further negotiations until later this year when its contract expires.

Chuquicamata produced 330,900 tonnes of copper in 2017, out of Codelco’s 1.734 million tonnes total.