Management at BHP Billiton Ltd.'s (BHP) Spence copper mine in Chile will seek a five-day government-supervisied mediation period in its contract talks with unionized workers, a spokesman said Thursday.

Copper workers at several mines are closely following these talks as they could set a precedent for other negotiations scheduled for later in the year, union leaders have said.

Late Wednesday, the union at the mine unanimously voted in favor of a strike after rejecting the company's latest contract offer.

According to local labor laws, a strike could not begin until Oct. 8, following the end of the mediation period. A strike could still be averted as a second five-day mediation period can be sought, but must be agreed to by both sides.

Contracts for the 560 unionized workers, which represents about 95% of union-eligible employees and 65% of the total mine workforce, expired Wednesday.

Spence workers are seeking a 5.5% wage increase and several other benefits and bonuses. The company offered a cost-of-living increase and smaller benefits than the workers sought.

The open-pit mine, located in northern Chile, produced about 165,000 metric tons of copper cathodes last year.

This is Spence's second collective bargaining process as workers negotiated 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