Rio Tinto said Wednesday it plans to pay US $3.1 billion in cash, or C $40 per share, for a 49 percent stake in Canadian mining company Turquoise Mountain Resources. Turquoise Mountain Resources surged 25% on Wednesday on the news, its biggest intraday gain since March.

The offer is $400m higher than a previous $2.7bn bid from Rio Tinto, which Turquoise Hill Resources formally rejected last week, saying it did not fairly reflect its long-term strategic value.

In March, Rio announced a bid of US $2.7 billion, or C $34 a share, for the 49 per cent of Turquoise Mountain it did not already own, a 32 per cent premium to its share price at the time. Turquoise Hill appointed a special committee to examine Rio’s offer.

Rio already owns 51% of Turquoise Hill and is seeking the remaining 49% to gain more control of the OyuTolgoi copper and gold mine. Turquoise Mountain owns 66 per cent of Oyu Tolgoi, one of the world’s largest known copper and gold mines, in Khanbaogd county in Mongolia’s South Gobi province, with the rest controlled by the Mongolian government.

“Rio Tinto is confident that this offer not only provides full and fair value for Turquoise Hill but is also in the best interests of all stakeholders as we move forward with Oyu Tolgoi,” Jakob Stausholm, Rio’s chief executive, said on Wednesday.

Rio reached a deal with the Mongolian government earlier this year that allowed the long-delayed expansion of Oyu Tolgoi to resume after agreeing to write off $2.4bn in government debt. Once the underground portion of Oyu Tolgoi is completed, it is expected to be the world’s fourth-largest copper mine, with Turquoise Mountain and its partners eventually aiming to produce more than 500,000 tonnes of copper a year.

Since the commodities crash in the middle of the last decade, the mining industry has been wary of acquiring big new mining projects. That is changing, however, as the world transitions to green energy, with mining giants increasing their exposure to green metals such as copper.

Earlier this month, BHP Billiton, the world’s largest mining giant, rebuffed its $5.8 billion bid for copper miner OzMinerals on the grounds that it was also too low.