Confirmation that Mongolia's anti-corruption authority has opened an "investigation" of the historic 2015 $US4.4 billion project financing deal for the Oyu Tolgoi copper mine has sucked Rio Tinto into the vortex of a controversy that started with a Swiss investigation into the contents of a bank account connected to a former Mongolian finance minister.

Ever since it became clear last month that Swiss authorities were taking serious interest in Bayartsogt Sangajav and how he came to fill a Swiss bank account with $US10 million back in September 2008, Rio has been able to take public comfort in the fact that whatever the circumstances were, they occurred at a time before the multinational miner had the capacity to influence negotiations between the government of Mongolia and Oyu Tolgoi's Canadian-listed developer, Ivanhoe Mines.