Rio Tinto Group agreed to buy out Turquoise Hill Resources Ltd. in a deal valued at about US$3.3 billion, securing more control of a giant copper mine in Mongolia.

The $43-a-share deal comes two weeks after Montreal-based Turquoise Hill rebuffed a lower offer from the mining giant and ends almost six months of takeover talks.

Rio, which already owned a 51 per cent stake, has been seeking to add direct exposure to Turquoise Hill’s Oyu Tolgoi asset, which is expected to be the world’s fourth-largest copper mine once an underground expansion is completed.

Rio said the final deal, which is subject to shareholder approval, represented a 67 per cent premium on Turquoise Hill’s share price before Rio’s first offer on March 11. 

Sailingstone Capital Partners LLC, which according to data compiled by Bloomberg as of June 30 holds about 2.2 per cent of Turquoise Hill’s shares, objected to the deal, saying it doesn’t adequately compensate minority shareholders for their economic interest in the project. 

“We are not interested in selling our stake at a massive discount to intrinsic value as we sit on the precipice of a wall of free cash flow,” Sailingstone said in a statement.

“It’s a little bit higher than our base-case valuation,” Saul Kavonic, an analyst at Credit Suisse, said of the deal. “But this clears up the joint venture to a clear pathway to development of what can become a core asset for Rio,” he said, adding that buying the stake “was probably necessary in order to move the project forward.”

Should the deal go ahead, Rio would own 66 per cent of the Oyu Tolgoi mine and the Mongolian government would hold the remaining 34 per cent. 

The deal requires two thirds of shareholders, including Rio Tinto, to vote in support. On top of that, more than 50 per cent of minority shareholders must vote in favor. Rio said it hoped a vote would take place “as early as possible in the fourth quarter of 2022.”

“This agreement represents another significant step following the recent commencement of the underground operations, and will simplify governance, improve efficiency and create greater certainty of funding for the long-term success of the Oyu Tolgoi project,” Rio’s Chief Executive Officer Jakob Stausholm said in a statement.

Shares of Turquoise Hill jumped 14 per cent to $41.16 in Toronto, its highest since March 2018. Rio Tinto fell 3.4 per cent in London, in line with a broader decline in commodities stocks.