The strike-bound Phelps Dodge Corp. copper mine reopened peacefully today as about 500 workers, protected by hundreds of law enforcement officers, crossed picket lines in a convoy.

The company closed the giant mine, mill and smelter complex Aug. 9 after about 1,000 strikers and supporters massed outside the main gate.

More than 200 vehicles today carried workers into the plant past the court-imposed limit of 10 pickets. Union leaders had asked other strikers in the 51-day-old walkout to stay away from the mine while federally mediated contract talks continue in Phoenix. Negotiators met for 3 1/2 hours today before adjourning until 2 p.m. Monday.

Ralph Milstead, director of the Arizona Department of Public Safety, said that 280 officers were assigned to duty at the front gate as the morning shift arrived. Seven National Guard units, dispatched by Gov. Bruce Babbitt, stood just out of sight of the gate.

"This strike will not be settled in a confrontation with the National Guard or the highway patrol," said Jerald Spreitzer, president of Machinists Local 1132. "It will be settled at the bargaining table."

The main issue since a three-year contract expired July 1 has been a company demand that cost-of-living allowances end. The strike originally affected about 2,300 workers.

John Coulter, a Phelps Dodge vice president, said about 80 newly hired employes were among the 500 hourly workers who joined about 300 salaried and supervisory employes at the reopened plant today.