The company proposing to build the Rosemont Copper mine south of Tucson has agreed to a buyout from one of its largest investors.

Hudbay Minerals has come to terms to acquire Augusta Resource Corporation for $555 million Canadian. Both companies are based in Canada and traded on the Canadian Stock Exchange.

Shareholders who sell to Augusta will receive the equivalent of C$3.56 in Hudbay shares.

That’s a 10 percent premium from the company’s initial offer in February, and 0.12 cents higher than Hudbay’s closing price Monday.

Hudbay representatives have pushed the acquisition, saying they believe their company can more quickly open the Rosemont Copper mine.

An Augusta report gave a low estimate of nearly 6 billion pounds of copper in the proposed mine site.