The giant open cut and underground mining complex was set back by an 8-day strike earlier this month which suspended mining, milling and copper concentrate shipments.
The company estimates that the total cost of the industrial action will amount to lost production of 35 million pounds of copper and 60,000oz of gold.
Freeport said negotiations with the union were continuing for the bi-annual renewal of the collective labour agreement, which was due in October.
Copper production for the June quarter fell 5.43% year-on-year to 261,000lb.
However, the average price received was $4.26 per pound - far higher than the $2.95/lb received in the corresponding period of 2010.
The Grassberg mine is expected to notch up 1 billion pounds of copper sales and 1.45 million ounces of gold sales in 2011, compared to the 1.2Blb of copper and 1.8Moz of gold sold in 2010.