Canada's Baffinland Iron Mines said it has received a hostile bid of C$274m (US$265m) from Energy & Minerals Group.
The offer of C$0.77 a share is at a premium of about 43% to the closing of stocks on 21 September.
Baffinland has been looking for a partner to fund its flagship C$3.7bn Mary River project, which is expected to produce 18 million tons a year.
Baffinland Iron Mines chief executive Richard McCloskey said the company is not up for sale.
The company will form a special committee to review the offer.
Energy & Minerals Group already owns a 6% stake in Baffinland Iron Mines.