Hudbay said it would buy United Copper & Moly's 7.95% stake for $45 million in cash upfront and three annual payments of $10 million from July 2022.
President and CEO Alan Hair said the simplified ownership structure improved Hudbay's financial flexibility to develop Rosemont and it hoped to ultimately own 70% and remain operator.
"We expect there will be substantial interest in a minority stake given recent precedent transactions and Rosemont's unique status as a world-class, shovel-ready copper project in the United States," he said.
Initial capex has been estimated at $1.9 billion for the project with a 19-year mine-life. It is expected to produce about 127,000 tonnes of copper a year at a cash cost of $1.14 per pound, net of by-product credits, over the first 10 years.
The project was granted a Section 404 water permit by the US Army Corps of Engineers last week, which Hair had described as a major milestone as it looked forward to advancing Rosemont to production.
Hudbay this week said it would hold its AGM and special meeting of shareholders on May 7, where it believed 12%-owner Waterton Global Resource Management hoped to replace eight members of the board.
The private equity firm has been ratcheting up the pressure on Hudbay, releasing a 74-page presentation in February saying the miner could be worth about C$19 a share if it changed leadership, reviewed its portfolio and improved capital allocation.
Hudbay shares had fallen to $4.50 in October but are back up to similar levels of a year ago, closing 1c higher yesterday at $9.15 to capitalise it at $2.4 billion.