Freeport McMoRan Copper & Gold's Indonesia mine workers are set to strike from Sept. 15 to Oct. 15 unless the firm meets their demands for a pay raise, Julia Parorongan, the firm's workers union spokeswoman, told Reuters on Tuesday.

The strike would be the second in three months at Grasberg, the world's third biggest copper mine.

The remote Grasberg mine also has the world's largest gold reserves, so the workers will be eyeing a greater share of profits from a metal seeing record prices.

Three-month copper on the London Metal Exchange is hovering just below $9,000 a tonne, having touched a record high above $10,000 in February.

The decision to go on strike comes after pay talks between the firm and the union, which has about 8,000 workers, reached a deadlock on Aug. 26, and follows an eight-day strike in July.

Workers have demanded a pay rise to between $17.5 and $43 per hour, down from hefty initial demands for $30 to $200 per hour, but still much higher than a current $1.5 to $3 per hour rate.

The firm said it has offered a 22 percent rise for the next two years, plus an increased bonus of as much as 230 percent at current prices for copper and gold.   

Freeport Indonesia was unavailable for comment on Tuesday.

Freeport, also facing a two-day copper mine strike in Peru this week, said on Monday, that it's seeking to renew pay talks in Indonesia.

The union held an opinion poll from Friday to Sunday, and found that from about 7,000 ballots received, about 98 percent agreed to go on a strike.

The strike threat at Grasberg comes on the heels of a prolonged strike at Chile's Escondida copper mine, the world's biggest. Escondida lifted a force majeure on concentrate shipments on Sept 2.

In premarket trading, shares of Freeport-McMoRan Copper & Gold were down 2.85 percent to $44.94.