CSN first bought a 16.29% stake of Riversdale in November 2009, and spent millions this February to increase this stake to 19.35% while Rio’s takeover offer was ongoing.

India’s Tata Steel also increased its shareholding of Riversdale during this takeover activity, and holds a 27.14% stake.

Rio further extended its offer to April 29.

Tata owns 35% of Riversdale’s flagship Benga open cut mine in Mozambique, which is targeting 6 million tonnes per annum of hard coking coal and 4Mtpa of export thermal coal by 2013.

Riversdale is also advancing 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.

At the annual general meeting in London last week, Rio chairman Jan Du Plessis indicated there could be more acquisition activity ahead.

“While our focus remains on organic growth, we will also continue to invest in carefully targeted small to medium-sized acquisitions,” he said.

Rio shares are up 48c to $83.60 this morning.