The mine was shut down on February 23 due to an ongoing dispute following a three-month strike last year.
After discussions between Freeport, union leaders, members of the workforce and local residents, remobilisation of the workforce began on Monday.
Freeport said mining and milling was ramping up but full production wasn't expected to be reached until the June quarter.
The company's previous production guidance for Grasberg for the March quarter was 210Mlb copper and 300,000oz gold.
Total group guidance for the quarter was previously 875Mlb copper and 425,000oz gold but Freeport revised it down overnight to 795Mlb copper and 300,000oz gold.
The general consensus from analysts was that the guidance cut was less than expected.
"It's not like the lost tonnage is permanently lost. They can recoup it later," Morningstar analyst Daniel Rohr told Reuters in Chicago.
Phoenix-based Freeport stressed that the guidance was dependent on the ramp-up at Grasberg and it would update its annual guidance when its first-quarter results were released.
Freeport shares added 0.6% to $US38.35 on the New York Stock Exchange overnight.