Mining giant Rio Tinto announced it will acquire an additional 15 million shares in Canadian explorer Ivanhoe Mines, increasing its stake in the company to 22.4%.

According to Australia's Herald Sun, under the deal Rio Tinto has the right to increase its stake Ivanhoe Mines up to 44%.

"Rio Tinto also has, among other things, the right to acquire additional securities so as to maintain its proportional equity interest in Ivanhoe Mines, and the right to acquire additional Ivanhoe Mines securities in certain other circumstances and subject to certain limits," the report quoted a Rio Tinto statement.

Andrew Harding, head of Rio Tinto's copper division, added both companies were working on finalising details of the Oyu Tolgoi project. Oyu Tolgoi, in the Gobi Desert, is believed to be one of the world's largest untapped copper and gold deposits.

The Rio Tinto-Ivanhoe Mines joint venture will be the largest foreign investment ever in Mongolia.