Congress has yet to vote on the proposed tax package and it is unclear if they will approve it.

The possible shutdown is part of a prolonged fight over the project that could chill investor appetite in Mexico, the world's biggest silver producer.

Chinese customers are reluctant to sign up for copper supply in 2022 at the highest premium in seven years.

In the longer term, Fitch forecasts prices to decline from an average $155 a tonne in 2021 to $65 a tonne by 2025 and $52 a tonne by 2030.

While the bidding process is still in an initial phase, authorities expect to have news on the contracts later this year.

Most of the investment will go to its giant Minas Rio iron ore mine, located in the mineral-rich state of Minas Gerais.

The Ministro Hales mine produced 144,000 tonnes of copper in the first nine months of the year.