The miner’s share of semi-soft coking coal production was down 14% to 690,000t, while its Australian thermal coal output slipped just 1% to 4.02Mt, as most of this production comes from the Hunter Valley in New South Wales.

Rio’s 80%-owned Kestrel longwall mine in the Bowen Basin produced 970,000t of hard coking coal in the recent quarter, down 9% year-on-year.

But it was still a 36% improvement from the December quarter output of 710,000t, the mine’s lowest quarterly result for the commodity in 2010.

Thermal coal output from the mine was lower at 108,000t in the March quarter compared to 130,000t in the previous three months.

The new Clermont open cut mine, designed to replace Blair Athol which is scheduled to be shut down in 2016, produced just 1.19Mt of thermal coal in the March quarter, nearly half of the previous quarter’s output.

The Blair Athol mine produced 730,000t of this commodity, 40,000t higher than the December quarter, but far off the 2.14Mt the mine produced in the first quarter of 2010 before Clermont was opened.

The big dragline operations in the Hunter Valley associated with Rio subsidiary Coal & Allied all had higher year-on-year thermal coal production for the March quarter.

Most of Rio’s American coal portfolio was sold off after the global financial crisis, apart from the Colowyo mine in Colorado which produced 490,000t in the March quarter, down 15% year-on-year.

Riversdale success

As near-surface tier-one coking coal deposits become harder to find, Rio is pursuing a takeover of emerging African coal producer Riversdale Mining through a $16.50 per share cash offer.

While Riversdale’s flagship Benga coking coal mine in Mozambique is targeting 6Mt per annum of hard coking coal and 4Mtpa of export thermal coal by 2013, the nearby Zambeze hard coking coal project has more promise and already hosts more than 9 billion tonnes of resources.

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

Rio recently secured control of Riversdale but cannot advance far beyond its 52.6% stake of the company.

Brazilian steelmaker Cia Siderurgica Nacional holds a 19.9% stake of Riversdale while Benga mine joint venture partner Tata Steel owns 27.14%, with both companies interested in coking coal supply rather than profits from share transactions.

These stakes leave Riversdale with less than 0.1% of tradeable share equity and brokers are consequently terminating coverage of the stock.

Rio shares are down 68c to $85.03 this morning.