Rio Tinto Group’s US$2.7 billion offer to buy out Turquoise Hill Resources Ltd. has been rejected, blocking its efforts to gain greater control of a giant copper mine it’s developing in Mongolia.

Rio’s offer doesn’t “fairly reflect the fundamental and long-term strategic value of the company’s majority ownership of the Oyu Tolgoi project,” Turquoise Hill said Monday, after appointing a special committee to review the bid. Shares of the Canadian miner had their biggest drop in more than eight months.

The rejection is a setback for Rio Chief Executive Officer Jakob Stausholm, who has prioritized getting stalled projects moving, while rebuilding the company’s reputation after a series of missteps. Rio owns 51 per cent  of Turquoise Hill, which in turn holds a two-thirds share in Oyu Tolgoi. The London-based miner had offered $34 (US$25.90) a share to Turquoise Hill’s minority shareholders in March, a 32 per cent  premium at the time. 

Rio said in a statement that it was disappointed by the decision of the special committee and reiterated its proposal to buy Turquoise Hill. The offer would deliver “compelling value” for minority investors.

Turquoise Hill fell 13 per cent  to $22.95 as of 9:50 a.m. trading in Toronto, its lowest since March 11. Rio’s stock fell 2.9 per cent  in London.

Copper equities have shed about 27 per cent  over the period since the bid was announced and are now trading within a 5 per cent  valuation range to Turquoise Hill, Morgan Stanley analysts including Alain Gabriel said Monday in a note.

“This dynamic has probably shifted the balance of power in favour of Rio Tinto,” the analysts said.

Rio is also looking to increase its exposure to so-called future facing commodities, the natural resources such as copper and nickel that are key for the green energy transition. Taking a big stake in the Oyu Tolgoi mine was an obvious way to achieve that goal. Earlier this year, the company struck a deal with the government to start work on a long-delayed US$6.9 billion underground expansion of Oyu Tolgoi after agreeing to write off US$2.4 billion owed by the state. 

The mining sector largely turned its back on big deals since a commodity crash in the middle of the last decade. Yet that’s now changing as the biggest miners push to increase their exposure to commodities such as copper. Last week, BHP Group, the biggest miner, had a US$5.8 billion offer for copper miner Oz Minerals Ltd. rejected.

“Engagement between the parties has not resulted in a consensus on value and price or in any improved proposal from Rio Tinto,” Turquoise Hill said in Monday’s statement.