Shares of US-listed Sigma Lithium Corp rose 21% in extended trading on Friday after Bloomberg News reported that Tesla Inc was considering buying the Canada-based battery metals miner.

Tesla has been discussing a bid with potential advisers, the report said, citing people familiar with the matter, adding that Sigma Lithium is one of many mining options the electric automaker is considering for its own refining.

Sigma is completing the construction of a hard rock lithium mine in Brazil, which is expected to open in April. The mine will produce spodumene concentrate, which can be used to make lithium hydroxide, a metal preferred by some automakers, including Tesla and BMW.

The project will use hydroelectric power, helping to reduce its carbon footprint significantly.

Shares of US-based Sigma Lithium, which has a market capitalisation of $3.21 billion, nearly tripled in value last year.

Chief Executive Elon Musk said last year that Tesla was open to buying a mining company if producing its own metals for electric vehicles could accelerate the global adoption of clean energy technologies.

Tesla and other automakers often discuss potential supplies of lithium and other EV metals with mining companies of all sizes without entering into contracts.

Last month, Tesla signed a deal with Piedmont Lithium to start supplying spodumene concentrate from Quebec later this year.

Tesla also has supply contracts with global suppliers for nickel, lithium, and other electric vehicle metals.