Unionized workers at BHP Billiton Ltd.'s (BHP) Spence copper mine in Chile unanimously voted in favor of going on strike next week if government mediation fails, a union leader said late Wednesday.

Of the 560 union members at the mine, 523 cast their votes between Monday and Wednesday and all 523 voted for the strike, union spokesman Francisco Aravena said.

Following local labor laws, either the union or mine management can now seek a five-day, government-supervised mediation period.

"The company already told us they'd be seeking the mediation," Aravena said, adding the strike could only begin Oct. 8 after mediation has failed.

Local laws allow for a second five-day mediation period but only if both parties request it. BHP representatives weren't immediately available for comment.

Current contracts expired Sept. 30. Spence workers are seeking a 5.5% wage increase, social benefits and bonuses linked to international copper prices. The company offered a cost-of-living increase and smaller bonuses than what the workers sought.

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

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 higher wages and production bonuses.

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