The offer has a minimum 50% acceptance condition and the major miner is in the running for at least 41.04% of Riversdale’s shares based on the acceptance notifications received under its institutional acceptance facility.

Tata Steel and Brazilian steelmaker Cia Siderurgica Nacional both recently increased their stakes in Riversdale and collectively hold 44.72% of the company.

Riversdale received a letter from Rio today which stated that it was in discussions with one of the company’s major shareholders and "believes that an outcome of those discussions is likely to emerge during the course of this morning".

Rio recently extended its bid for Riversdale to April 6.

Riversdale’s flagship Benga coking coal mine in Mozambique is a joint venture development, with Tata owning 35%.

At full production under stage-two development, the Benga mine is aimed to export 6 million tonnes per annum of prime hard coking coal and 4Mtpa of export thermal coal by 2013.

Riversdale is also advancing a prefeasibility study for its nearby and bigger $2 billion Zambeze hard coking coal project, which holds more than 9 billion tonnes of resources.

Last year the company started investigating the long-term possibility that Zambeze could become a 90Mtpa run-of-mine operation.

Riversdale shares closed at $16.10 yesterday while Rio’s shares are 3% down to $81.80 this morning.