Augusta Resource Corporation is resisting the efforts of an investor company to buy a controlling interest in Augusta stock.
Both companies are based in Canada, so Hudbay Minerals is petitioning the British Columbia Securities Commission to kill a company policy capping how much stock one shareholder can own.
The current rule is keeping Hudbay from owning more than about 16 percent of Augusta.
In February, Hudbay offered to buy a controlling interest in Augusta from shareholder for $428 million in stock. Augusta officials called it a low-ball bid.
Hudbay officials say their main interest in the deal is acquiring control of the proposed Rosemont mine.
An Augusta report gave a low estimate of nearly 6 billion pounds of copper in Rosemont.
Hudbay and Augusta officials were unavailable for comment.