Unionized workers at BHP Billiton Ltd.'s (BHP) Spence copper mine in Chile, currently in wage negotiations with mine management, expect to receive a new offer Sept. 23, a union leader said Wednesday.

Current contracts for the 560 members of the union expire Sept. 30.

"We're currently sitting at the negotiations table and the company will give us its offer on Sept. 23," union spokeman Francisco Aravena told Dow Jones Newswires.

The union would then take the wage and benefit proposal for a union-wide vote, which would likely be held by Sept. 27, Aravena said.

Spence workers are seeking a 5.5% wage increase, social benefits and bonuses linked to international copper prices.

Before workers can go on strike, once their contracts expire, Chilean labor laws include a five-day, government-mediated negotiations period if either party calls for it. Finally, there's an additional five-day mediation period if both parties agree to it.

"The earliest we could go on strike, if we reject the offer, is Oct. 7 or Oct. 10," Aravena said.

The open-pit Spence mine produced about 165,000 metric tons of copper, in the form of cathodes, last year.

In addition to Spence, in Chile BHP owns the Cerro Colorado mine and controls and operates the Escondida copper mine. The latter will begin wage negotiations later this year.

"We're closely watching the Spence negotiations as these will set a precedent for upcoming talks at other mines," said Pedro Marin, the president of the Chilean Mining Federation umbrella union group.

Workers at several divisions of state copper giant Corporacion Nacional del Cobre, or Codelco, will also negotiate new labor contracts at the end of the year.

In 2006, when copper prices were booming, Escondida workers - then led by Marin - went on strike for nearly a month, bringing the world's largest copper mine to a standstill as they sought bigger wages and production bonuses.

A BHP representative wasn't immediately available for comment.

-By Carolina Pica, Dow Jones Newswires; 56-2-820-4244; carolina.pica@dowjones.com