Rio Tinto says the price Chinalco is paying to increase its stake in the company is at a significant premium to market value.
China's state-owned aluminium group is to invest almost $20 billion in Rio Tinto, giving it stakes of up to 50% in nine of Rio Tinto's mining assets.
The deal will help the Anglo-Australian mining company reduce its huge debts.
Rio Tinto's shares in London fell on the news, with analysts saying the company appears to be giving away too much.
But the company's chief executive Tom Albanese says the deal provides excellent value for shareholders and forges a strategic partnership with a major global customer.
He says it is a premier opportunity to get access to resources, new customers and capital in China.
Mr Albanese says China has a bigger part to play in the world's capital market, partly as a result of the world financial crisis.
Last month the company announced it was scrapping more than 14,000 jobs worldwide.
The company's Tiwai Point aluminium smelter at Bluff is offering unpaid leave to staff as a way of avoiding cuts to any of the more than 800 jobs at the plant.