Workers at Canadian-owned Eldorado Gold in northern Greece are protesting in Athens against the government's decision to revoke the approval to build a processing plant at the Skouries project.
In February, Eldorado Gold's Greek subsidiary Hellas Gold received a formal notice from Greece's Ministry of Productive Reconstruction, Energy and Environment regarding the licence.
Thousands of miners protested cancellation of the licence to develop the mine in Skouries, which the company had invested $400m in since 2012.
By 2017, Eldorado Gold planned to spend an additional $700m on constructing a processing plant, in addition to developing two mines in the area.
Eldorado Gold currently employs about 2,000 people at the project and its plans would support hiring another 1,000 by 2020.
The Greek Government said it is investigating whether the company has been working in-line with town planning and environment regulations. Its decision may also force the miner to re-asses its investment plans, Reuters reported.
In March, Eldorado Gold CEO Paul Wright said: "We have 2,000 people working directly for us and our contractors in Greece, and are making a significant contribution to the economy of the country during challenging times.
"We remain committed to the responsible development of our asset base in Greece and announced earlier this year that we plan to invest $310m on the development of Skouries and Olympias in 2015, which will generate additional jobs for the Greek people and taxes for the Greek Government."