Unionized workers at the Spence copper mine, located in northern Chile, aren't pleased with management's latest wage offer and are threatening to go on strike in early October, a union leader said Wednesday.

Workers from several local mining companies are closely watching these negotiations as these could set a precedent for other talks scheduled before the year ends.

Current contracts at Spence expire on Sept. 30 and the 560 unionized workers can legally go on strike on the first shift on Oct. 8.

"The way the talks have been evolving, I'd say a strike is imminent," said union spokesman Francisco Aravena.

Earlier Wednesday, the mine, which is owned and operated by BHP Billiton Ltd. (BHP), handed the union its latest wage offer.

"It's practically the same conditions we already have," Aravena said.

He added that talks were now stalled and would likely resume under legally mandated government mediation next week.

Following local labor laws, the union members will vote, by a show of hands at assemblies Thursday and Friday, on whether they accept the wage offer. If rejected, a strike vote, via secret ballot, would be held Sept. 27-29, Aravena said.

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

The union represents about 95% of the workers eligible for union membership and about 60% of the mine's total workforce, according to the union's calculations.

The open-pit Spence mine produced about 165,000 metric tons of copper, in the form of cathodes, last year. This year, it's expected to produce about 200,000 tons of copper.

This is Spence's second collective bargaining process as workers negotiated the about-to-expire contract in 2006 before the mine went into production.

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.

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. Codelco's Andina division is said to be negotiating ahead of schedule. Contracts at the Norte division, which includes the massive Chuquicamata mine, expire on Dec. 31.

In 2006, when copper prices were booming, Escondida workers 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.

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