The deal comes as Rio also announced its first major investment in China since the Stern Hu spy affair disrupted its dealings in China. It is understood the second deal was signed on 25 September. 

Allens, working with Rio's in-house legal team, has been involved in discussions with the Mongolian Government on the copper and gold project.

The signing of the investment agreement and the associated shareholders agreement with the Mongolian Government means Rio Tinto and Canada's Ivanhoe Mines can now proceed with the Oyu Tolgoi copper and gold project.

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Allens partner Igor Bogdanich and senior associate Peter O'Sullivan, with others, worked with Rio Tinto, operating out of Australia and Mongolia. The firm provided 24-hour coverage when required for the negotiations, it said. 

'This was a complex, intense deal, with many stakeholders and with long running negotiations – often through translators. The commercial arrangements had to address the desired stable tax and operating environment for the project, and take into account the local legal landscape," said Bogdanich.

"It is fantastic to be working alongside such a talented client team who worked with such dedication and so enthusiastically over the past number of years – both in Australia and Mongolia – to reach and execute these agreements successfully," he said.

Oyu Tolgoi is the world's largest undeveloped copper and gold resource, and is located in the South Gobi region of Mongolia. Production at Oyu Tolgoi is expected to begin around 2013.