The attackers used petrol bombs and flammable liquid to set fire to machinery, vehicles and containers, police told the Associated Press.
The Hellas Gold site is due to open in 2015 and expected to create new jobs in the recession-hit Halkidiki region.
But it has faced protests from environmentalists who say development would cause irreversible damage.
Citizens’ groups have been trying to halt the project since 2011, when the Greek government allowed Hellenic Gold, a subsidiary of Canadian company Eldorado Gold, to dig in the region.
In January, hundreds of Greeks took to the streets of Athens to demonstrate against the new mine.