Rio has refused Noranda access to the information it needs to conduct a due diligence but has provided it to rival bidder Billiton, which trumped Noranda’s original (C$24.50) bid with a C$27 friendly offer.

Noranda said it based its original offer on public information and its own estimates of available synergies.

The bidding war between Noranda and Billiton is being driven by competition for Rio’s core assets – a 33.75% interest in the developing Antamina copper-zinc project in Peru and a 100% stake in the advanced Spence copper deposit in Chile, as well as its North American aluminium and stainless steel distribution business and significant Canadian copper operations.

Noranda has the backing of Chilean state giant Codelco, which will buy 50% of Rio Algom’s assets if Noranda’s bid is successful.