* Rio files official cash offer at C$4.15 per share
* Hathor shares trading well above Rio's offer
* Economic study revised due to payback miscalculation
Oct 27 (Reuters) - Canada's Hathor Exploration said on Thursday that it had received Rio Tinto's formal takeover offer, as the global mining giant looks to gain access to Canada's uranium-rich Athabasca basin.
The friendly deal, which was announced last week and puts Rio Tinto in competition with Canada's largest uranium producer, Cameco, values the exploration company at C$4.15 a share, or C$578 million ($581 million).
Shares of Hathor closed on Wednesday at C$4.56 on the Toronto Stock Exchange, well above Rio's bid as investors eyed Cameco in anticipation of a bidding war.
"This is now a game between two of the titans of the uranium producers," said Dundee Capital Markets analyst David Talbot in a note to clients on Wednesday. "We believe that Cameco could improve on its initial bid for Hathor."
Cameco first floated its C$520 million hostile bid for Hathor in August, after talks on a friendly deal failed. The uranium producer has said it is reviewing Rio's offer.
The attraction of Hathor for both miners is its large exploration-stage Roughrider project in the uranium-rich Athabasca region of Saskatchewan in Western Canada.
Hathor also said it had filed its full preliminary economic assessment for the project, noting that the payback period was miscalculated in a summary released last month. The project is now anticipated to turn a profit about eight months later than previously stated.