Mining giant Rio Tinto is buying another slug of shares in Canadian mining company Ivanhoe Mines.Rio is paying C$244.7m for 15 million Ivanhoe shares at C$16.31 each to increase its stake in the company to 22.4% from 19.7%.The transaction is a follow-on from a deal inked in 2008 when Rio Tinto lent Ivanhoe US$350m to finance the purchase of equipment for the Oyu Tolgoi copper-gold complex in Mongolia's South Gobi region.'Our further investment in Ivanhoe Mines underlines our confidence in the quality of the world class Oyu Tolgoi deposit and its priority in our project portfolio. We are working with Ivanhoe Mines on finalising the conditions precedent for completion of the Investment Agreement with the Government of Mongolia and are looking forward to moving into the development phase of the project,' said Andrew Harding, chief executive of Rio Tinto's copper division.Rio Tinto has the option to convert all of its $350m loan into shares and if it did so it would raise its shareholding in Ivanhoe to 44%.