Rio Tinto has offered £2.2bn (A$3.5bn) for Riversdale Mining, a Sydney-based firm with interests in African coal.Riversdale confirmed Monday it had held talks with the Anglo-Australian mining giant about a deal valuing the business at A$15 a share."The company has had discussions with Rio Tinto concerning a possible transaction at the corporate level for indicative consideration of $15.00 per Riversdale share," said executive chairman Michael O'Keeffe. "These discussions were undertaken in confidence and Rio Tinto advised the company that it is not in a position to submit a proposal for the potential acquisition of the company." If a deal does go through, Rio will get access to projects in Mozambique and South Africa, and a number of mines jointly owned with India's Tata Steel.But the Indians have a 24% stake in Riversdale and might want to take on the whole company, according to reports, while Brazil's Vale, which has coal mines in Mozambique, may also be interested."While discussions with Rio Tinto are ongoing, there is no certainty that Rio Tinto or any other party will proceed with any proposal for the acquisition of Riversdale," the Aussie firm added, suggesting talks with others may already be underway.