(Bloomberg) -- First Quantum Minerals Ltd. is plunging again, bringing the total decline this week to more than 50% as uncertainty deepens over the future of its flagship copper mine in Panama.

The Canadian metals producer tumbled as much as 15% Wednesday in Toronto, extending the decline to about 51% in the three trading sessions since Panama’s government said it planned to hold a referendum for voters to decide whether to extend or revoke First Quantum’s mining contract. Shares of the Vancouver-based company are trading at their lowest level in three years.

This week’s plunge has wiped out about C$9.6 billion ($6.9 billion) from First Quantum’s market value. The decline comes as investors assess the outlook for Cobre Panama, one of the world’s biggest and newest copper mines and First Quantum’s most important asset.

The company’s biggest shareholders are Capital Group Cos., with about 22%, and Jiangxi Copper Corp., with 18%, according to data compiled by Bloomberg.

Read More: Panama Congress Committee Votes in Favor of Mine Referendum

The future of the mine has been cast into doubt after weeks of civil unrest in the Central American nation over its new operating contract, which would extend the company’s mining license by 20 years. Panama President Laurentino Cortizo said on Sunday that the government plans to hold a referendum on Dec. 17, though it’s still not clear if the vote can take place.

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